Dissertations / Theses on the topic 'Consumption of bulk materials'

Актуальність На данний момент вимірюванню витрати сипучих матеріалів прилділяється значно менше уваги ніж вимірюванню витрати рідин та газів. В наслідок цього досить гостро стоїть питання обліку витрат сипучих матеріалів в таких сферах як харчова промисловість(наприклад перетворення зерна в борошно), металургія (наприклад при виготовленні сталі). Тому дана тема є акутальною для задоволення потреб технологічного процесу. Об’єктом дослідження даної роботи є витратомір сипких матеріалів заснований на тахометричному методі вимірювання. Магістерська дисертація складається з пояснювальної записки, що містить вступ, 4 розділи, список літератури, 48 малюнків , 30 таблиць. Загальнний обсяг 128 сторінок. Також до магістрестької дисертації входить графічна частина, що містить 2 аркуші А1 графіків, 2 аркуші А1 схем, 1 А1 складальних креслень та презентаційний лист.
At present, much less attention is paid to measuring the flow of bulk materials than to measuring the flow of liquids and gases. As a result, the issue of accounting for the cost of bulk materials in such areas as the food industry (eg, conversion of grain into flour), metallurgy (eg in the manufacture of steel) is quite acute. Therefore, this topic is urgent to meet the needs of the technological process. The subject of study of this work is a flow meter of bulk materials based on the tachometric method of measurement. The master's dissertation consists of an explanatory note containing an introduction, 4 sections, a list of references, 48 figures, 30 tables. The total volume is 128 pages. The master's dissertation also includes a graphic part containing 2 sheets of A1 graphs, 2 sheets of A1 diagrams, 1 A1 of assembly drawings and a presentation sheet.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2003.
Vita.
This thesis explores the spectroscopic properties of phonon-polaritons, which are admixtures of coupled electromagnetic and mechanical vibrations in polar crystals. An in-depth theoretical treatment supplemented with simulations of experimental results of a four-wave mixing impulsive stimulated Raman scattering (ISRS) method to generate and probe polaritons with arbitrary wavevectors is developed. A novel method to generate phonon-polaritons with high amplitudes via focusing is also presented. The motivation for this work is ultimately the generation of lattice oscillations with high amplitude that will permit exploration of the potential energy surface of collective vibrational motion beyond its linear regime. Femtosecond laser machining has been used to fabricate microstructures in lithium niobate and lithium tantalate. Phonon-polaritons propagation has been extensively characterized in a number of functional elements, including waveguides, resonators, and various diffractive, reflective, and focusing elements. The experimental results are supplemented by two-dimensional finite-difference time-domain simulations of polariton generation and propagation in arbitrary two-dimensional patterned structures. The phonon-polaritons studied have THz frequencies and propagate at lightlike speeds. The motivation for this research is the development of a versatile terahertz spectroscopy platform, in which phonon-polaritons are used as a source of THz radiation. Furthermore, these fabricated microstructures can serve as the basic building blocks of an intergrated platform in a single crystal where phonon-polaritons are used for ultrafast signal processing.
by Nikolay Staykov Stoyanov.
Ph.D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2001.
Includes bibliographical references.
by Michael J. Gleason.
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2001.

Piezoelectric voltage transformers (PTs) can be used to transform an input voltage into a different, required output voltage needed in electronic and electro- mechanical systems, among other varied uses. On the macro scale, they have been commercialized in electronics powering consumer laptop liquid crystal displays, and compete with an older, more prevalent technology, inductive electromagnetic volt- age transformers (EMTs). The present work investigates PTs on smaller size scales that are currently in the academic research sphere, with an eye towards applications including micro-robotics and other small-scale electronic and electromechanical sys- tems. PTs and EMTs are compared on the basis of power and energy density, with PTs trending towards higher values of power and energy density, comparatively, indicating their suitability for small-scale systems. Among PT topologies, bulk disc-type PTs, operating in their fundamental radial extension mode, and free-free beam PTs, operating in their fundamental length extensional mode, are good can- didates for microfabrication and are considered here. Analytical modeling based on the Extended Hamilton Method is used to predict device performance and integrate mechanical tethering as a boundary condition. This model differs from previous PT models in that the electric enthalpy is used to derive constituent equations of motion with Hamilton’s Method, and therefore this approach is also more generally applica- ble to other piezoelectric systems outside of the present work. Prototype devices are microfabricated using a two mask process consisting of traditional photolithography combined with micropowder blasting, and are tested with various output electri- cal loads. 4mm diameter tethered disc PTs on the order of .002cm

3 , two orders smaller than the bulk PT literature, had the followingperformance: a prototype with electrode area ratio (input area / output area) = 1 had peak gain of 2.3 (± 0.1), efficiency of 33 (± 0.1)% and output power density of 51.3 (± 4.0)W cm

-3 (for output power of80 (± 6)mW) at 1M? load, for an input voltage range of 3V-6V (± one standard deviation). The gain results are similar to those of several much larger bulk devices in the literature, but the efficiencies of the present devices are lower. Rectangular topology, free-free beam devices were also microfabricated across 3 or- ders of scale by volume, with the smallest device on the order of .00002cm

3 . These devices exhibited higher quality factorsand efficiencies, in some cases, compared to circular devices, but lower peak gain (by roughly 1/2 ). Limitations of the microfab- rication process are determined, and future work is proposed. Overall, the devices fabricated in the present work show promise for integration into small-scale engi- neered systems, but improvements can be made in efficiency, and potentially voltage gain, depending on the application

Thin coatings and other surface engineering techniques are widely used to improve the friction and wear properties of surfaces. However, current understanding of the behaviour of surfaces is relatively poor. Production methods are therefore to a large extent based upon empiricism. Many well-established materials characterisation tests are inadequate, and so novel techniques for the examination of surface layers have emerged, with the eventual aim of predicting the tribological performance of surfaces. One such technique is the micro-scale abrasive wear test, in which a ball is rotated against a specimen in the presence of a slurry of fine abrasive particles, producing a well-defined crater whose volume may be measured geometrically, allowing the wear coefficient of the specimen to be determined. It has been shown that the existing understanding of test is inadequate; further characterisation of the test has been performed. The effects of various parameters on the mechanism and severity of wear have been identified and explained by adaptation of existing models. Recommendations have been made for the optimisation of the accuracy and reproducibility of the test. The capability of the test to characterise thin coatings has been extended by shallow ball-crater testing which does not penetrate though the coating, thereby eliminating any influence of the substrate wear resistance. A number of different formulations of the wear equation, and a number of data analysis methods have been discussed, with the aim of minimising error. Since many surface coatings are exposed to high temperatures in service, the micro-scale abrasion test and the commonly used scratch test have been adapted for use at elevated temperatures in order to investigate changes in coating properties under these conditions. A new scratch test apparatus was designed and constructed for these tests. The behaviour of various PVD coatings has been investigated by these methods.

Moisture measurements play an important role in many material preparations and industrial processes. Microwave techniques have been used for several decades for such measurements. Shortcomings associated with the method have been addressed. Graphical solutions used hitherto to obtain the permittivity from waveguide measurements have been replaced by computer programs. The accuracy of measurement have been improved by the development of waveguide standards and the implementation of calibration procedures. Permittivity measurements in through and short circuited rectangular waveguides are reported on a wide range of solid, granular and liquid materials important to the food industry. Techniques to suppress standing wave effects have enabled accurate plane wave measurements of permittivity. For bulk materials, new measurement probes have been developed for on line measurements and associated with these probes lower cost instrumentation has been considered. The· main outcome of the study is the improvement in permittivity measurements of sample quantities of material. Moisture measurements in bulk materials have been facilitated by novel non invasive probes.

This thesis is partly concerned with investigating news ways of applying microwaves in a chemical synthesis; it is also concerned with the mechanism of such processes, including microwave-assisted diffusion, structural changes induced by microwave fields and the effect of pressure on microwave-driven reactions. In some cases we also studied the mechanism of reactions driven by more traditional sources of heat. A novel microwave reactor was designed and constructed to enable us to perform mechanistic studies of the hydrothermal synthesis of iron oxide particles by small angle neutron scattering (SANS); this was performed in parallel with small angle X-ray scattering (SAXS) and EXAFS on samples heated conventionally. SANS measurements showed that after an initial burst of nucleation, particles of 50 Å mean diameter were formed in the first 30 minutes, evolving to a final size of 100 Å after 7.5 hours. The SAXS measurements reported a burst in particle growth as the sample was heated above 50-60°C, producing particles up to 250-300 Å in diameter. However, wide-angle X-ray scattering data showed that the latter samples did not exhibit any Bragg scattering, implying that they had no medium-long range crystalline order when suspended in the growth medium. EXAFS data revealed the presence of very small particles of haematite in solution, which seems to add weight to the hypothesis that haematite particles are produced first through the growth of small particles which then aggregate to larger clusters. Studies were also performed of the microwave-assisted diffusion of cations into channelled iron and manganese oxides, and showed that the choice of experimental conditions -particularly the nature of the host material - have a significant effect upon the final product. In several cases, microwave heating accelerated diffusion and could also lead to novel routes to insertion compounds. Further studies on the influence of microwave heating on the structure and dynamics of materials involved the construction of further pieces of apparatus to perform in situ diffraction during microwave irradiation. Neutron diffraction was performed on aspirin to probe proton disorder, and high-resolution powder X-ray diffraction was conducted on the fast-ion conductor beta alumina to look at thermal excitation of the relatively mobile sodium ions during microwave heating. In both cases we demonstrated that the sample could be heated and studied in situ, and structural changes were revealed through changes in the Debye-Waller factors of particular atoms in both materials. The experiments on aspirin indicated that the sample had been subjected to a localised heating effect.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.
Includes bibliographical references (p. 95-99).
Thermoelectric materials are materials which are capable of converting heat directly into electricity. They have long been used in specialized fields where high reliability is needed, such as space power generation. Recently, certain nanostructured materials have been fabricated with high thermoelectric properties than those of commercial bulk materials, leading to a renewed interest in thermoelectrics. One of these types of nanostructured materials is nanocomposites, which are materials with either nanosized grains or particles on the nanometer scale embedded in a host material. Nanocomposites present many challenges in modeling due to their random nature and unknown grain boundary scattering mechanisms. In this thesis we introduce new models for phonon and electron transport in nanocomposites. For phonon modeling we develop an analytical formula for the phonon thermal conductivity using the effective medium approximation, while for electron modeling and more detailed phonon modeling we use the Boltzmann equation to calculate the thermoelectric properties. To model nanocomposites we incorporate a grain boundary scattering relaxation time. The models allow us to better understand the transport processes in nanocomposites and help identify strategies for material selection and fabrication.
by Austin Minnich.
S.M.

This work is a comprehensive summary of research projects the author conducted when attending the PhD program at the University of Arizona. Research topics cover the structural chemistry of lanthanide-amino-acid clusters, optical up-conversion properties of lanthanide based nanomaterials, magnetic and luminescent properties of lanthanide metal-organic frameworks (MOFs), as well as two research projects focusing on transition metal MOFs, which were derived from lanthanide metal-organic framework projects. In chapter 2, the discovery of halide anion templated lanthanide-histidine hydroxide cluster and a comprehensive study of the influence of anionic size on cluster nuclearity are discussed. Both Cl⁻ and Br⁻ were able to serve as template anions assisting the formation of pentadeca-nuclear lanthanide hydroxide clusters for Nd, Gd and Er. When I⁻ was used as the template anion, pentadeca-nuclear hydroxide cluster only formed in neodymium case. In erbium case a dodeca-nuclear hydroxide cluster formed when I⁻ was used as template ion. However I⁻ was not effective in assisting the formation of high nuclearity gadolinium hydroxide cluster. In chapter 3, doping Er³⁺ ion into CuₓSe nanoparticles for the purpose of making efficient optical up-conversion materials is discussed. Er³⁺ ion was successfully doped into CuₓSe nanoparticles. However no up-conversion luminescence was detected, possibly due to the in-direct bandgap nature of CuₓSe. Chapter 4 discuses attaching CuxSe nanoparticles on the surface of NaYF₄:Gd, Er,Yb nanorod. The purpose is to increase up-conversion efficiency of NaYF₄:Gd, Er,Yb nanorod through surface plasmon resonance enhancement property of CuₓSe nanoparticle. The CuₓSe nanoparticles were successfully attached onto NaYF₄:Gd, Er,Yb nanorod surface through exposing the suspension containing CuₓSe nanoparticles and NaYF₄:Gd, Er,Yb nanorods to UV irradiation. The up-conversion efficiency of NaYF₄:Gd, Er,Yb nanorods was increased after CuₓSe nanoparticle attachment. Chapter 5 discussed the synthesis and characterization of functional Ln(BDC)(1.5)∙DMF (Ln = Eu, Tb, Gd) metal-organic frameworks (MOFs). The absence of OH group containing species within this MOF rendered them ideal substrates as luminescent material because luminescence quenching caused by OH groups could be avoided. A series of MOF with luminescent color ranging from red, orange, yellow and green were obtained by adjusting the relative Eu and Tb content in the MOF lattice. The magnetocaloric effect of Gd(BDC)1.5∙DMF was also studied. Chapter 6 discussed doping Co²⁺ ion into pyrochlore-like Zn(INA)₂ (INA = isonicotinate) MOF lattice for the purpose of making magnetically active pyrochlore-like MOF structures. The highest Co²⁺ doping concentration of 57% was successfully achieved. However, no significant magnetic frustration was observed, possibly due to the far separation between doped Co²⁺ ions. Chapter 7 discussed the etching of Zn(INA)₂ MOF crystal to increase microporous exposure. When Zn(INA)₂ MOF crystals were immersed in Co(NO₃)₂∙6H₂O acetonitrile solution, defined effective etching, which could effectively increase microporous exposure, took place. When Zn(INA)₂ MOF crystals were immersed in Co(NO₃)₂∙6H₂O N,N-dimethylformamide solution, defined ineffective etching, which could not increase microporous exposure, took place dominantly. Increasing etching temperature resulted in similar but more severe etching. However, new cobalt dominant MOF phases formed when etching was performed under elevated temperature.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 131-134).
We describe the fabrication and study of bulk heterojunction solar cells composed of PbS quantum dots and TiO2. In particular, we study the effects that bulk heterojunction composition and structure have on resulting device performance. We find that PbS and titania are extremely evenly distributed throughout our bulk heterojunction devices, such that charge carriers generated anywhere within the blend are well within a carrier collection length of the charge separating driving force required to separate them and enable their collection. Of the compositions we studied, we found that devices with a TiO2 rich bulk heterojunction composition outperformed devices employing other compositions. As a result of the size difference between the PbS quantum dots and the titania nanocrystals which compose the blends, the likelihood of forming a truly complete, bicontinuous bulk heterojunction network is maximized at a TiO2-rich blend composition. We find that diffuse interfaces exist between adjacent layers of our devices as a result of interfacial surface roughness. Rather than being deleterious, this increased interfacial area extends the spatial extent of the depletion region over a greater volume of our devices. Our bulk heterojunction blends form well packed, high density binary particle mixtures, particularly at a TiO2-rich composition. Device efficiency was maximized for bulk heterojunctions employing the smallest titania nanocrystals, an indication that at constant volume fractions, larger titania nanocrystals decrease the total number of titania particles available to form complete and continuous pathways through the depth of the bulk heterojunction. Furthermore, a peak in device performance was observed at intermediate blend layer thicknesses. This results from the balance between two opposing effects: an increase in light absorption and photocurrent with thicker bulk heterojunctions and an increased likelihood of charge carrier recombination with thicker bulk heterojunctions. Finally, enhanced light absorption and current generation was observed at red and infrared wavelengths, validating the ability of bulk heterojunctions to spatially extend the reach of the charge separating driving force, such that the previously missed red and infrared photons may be captured.
by Kevin J. Huang.
Ph. D.

Spray forming with eo-injection of a solid particulate phase to form a metal-metal composite has been studied as a new route for manufacture. Two Al-based matrices were investigated: AI-12Si for testing the feasibility of the new manufacturing route and Al-4Cu for providing better mechanical performance. For both composite types, Ti was chosen as the particulate phase and the processing-microstructure-property relationships then studied. At Peak Werkstoff GmbH, Germany 12 wt%Ti particles were eo-injected into an atomised Al alloy droplet spray and eo-deposited to form a rv300 kg billet. The microstructure comprised refined equiaxed a-AI grains (rv5fLm), spherical Si particles (rv5fLm) and uniformly distributed Ti particles (rv80fLm). Sections of the billet were extruded under a range of conditions into long strips 20mm wide and 6mm, 2.5mm and 1mm thickness. At high strains, the Ti particles were deformed into continuous fibres of a few microns in thickness. Accumulative roll bonding was then performed to higher total strains, while maintaining a constant cross-section, reducing the Ti fibres to sub-micron thickness. The fibres were studied by extraction after selective dissolution of the a-AI matrix. There was no interfacial reaction between a-AI and Ti or any measurable oxide formation, thus providing encouragement for the manufacture of metal-metal composites by eo-spray forming. A powder injection pump was successfully integrated and commissioned on the spray forming facility at Oxford University. The pump was calibrated to optimise powder flow rates. Three AI-4Cu+ Ti composite billets were processed with each containing Ti powder with a different processing history. Up to 20vol%Ti was successfully incorporated, however due to the cooling effect from powder injection, porosity was significant. The quenching effect provided a finer AI-4Cu grain structure in the region of Ti injection, and also promoted precipitation of O'-AbCu precipitates. A Ti/ Al-4Cu interfacial reaction was more prominent in the billet spray formed at 850°C than those spray formed at 750°C. Angular Ti processed by a hydride-dehydride route had better deformation characteristics than spherical gas atomised Ti. Deformation processing by extrusion and rolling was investigated for Al-4Cu+20vol%Ti using SEM, EBSD and FIB. After extrusion to a strain of 5, the composite contained elongated reinforcing fibres characteristic of metal-metal composites. The microstructure studied by EBSD revealed equiaxed polygonal Al-4Cu matrix grains. Rolling was not as efficient as extrusion in producing elongated Ti fibres and was attributed to a lower deformation processing temperature. The rolled composites consisted of elongated Al-4Cu grains 1-5J1m in thickness. An UTS of 339MPa at a strain of 3 was attributed to texture strengthening in the Q- AI.

A partir dels anys seixanta, els vidres metàl·lics han estat objecte d'un gran número d'investigacions, des de llavors s'han realitzat avanços molt significatius en la comprensió de la seva estructura i algunes de les seves propietats. Com el seu nom indica, els vidres metàl·lics són aliatges metàl·lics que no presenten ordenament atòmic a llarg abast. Aquesta falta d'ordre els confereix propietats i comportaments considerablement diferents respecte als aliatges cristal·lins. Per exemple, es comporten generalment com materials magnètics tous (baixa coercitivitat i elevada permeabilitat) i s'han comercialitzat com a bases de transformadors, capçals de lectura magnètics i protectors magnètics [1]. A partir de certs tractaments tèrmics o d'altres tècniques, és possible controlar la seva total o parcial cristal·lització. En alguns casos, precipiten nanocristalls repartits uniformement obtenint un material magnètic dur amb aplicació industrial [2]. A part de les seves propietats magnètiques, s'ha demostrat que algunes de les seves propietats mecàniques difereixen significativament dels materials cristal·lins, ja que mostren un elevat límit elàstic, una elevada deformació en règim elàstic, deformació plàstica heterogènia i homogènia i també l'aparició de material fos en les superfícies de fractura [3,4].
La combinació d'un elevat límit elàstic juntament amb la possibilitat d'obtenir vidres metàl·lics massissos ha obert un nou interès en la utilització d'aquests com a materials estructurals [5]. No obstant això, els vidres metàl·lics mostren una clara localització de la deformació plàstica en bandes de cisalla al ser deformats a temperatura ambient [6,7]. A més, en lloc d'experimentar enduriment per deformació, els vidres metàl·lics s'ablaneixen a causa de la formació de bandes de cisalla que a més impedeixen l'elongació estable del material quan és deformat en tensió. Així doncs, la millora de la ductilitat d'aquest tipus de materials s'ha convertit en l'objectiu de molts treballs d'investigació.
Recentment, s'ha estudiat l'enduriment intrínsec dels vidres metàl·lics [8,9]. S'ha demostrat que existeix correlació entre l'energia de fractura i el quocient entre el mòdul de cisalla (G) i el mòdul de compressibilitat (B). En aquest estudi s'ha conclòs que una bona forma d'augmentar la plasticitat dels vidres metàl·lics és escollir els elements que constituiran l'aliatge amb baix G/B o el que és equivalent, elevat coeficient de Poisson.
El considerable increment de ductilitat que acompanya l'aparició de múltiples bandes de cisalla, indica que la seva proliferació, independentment de com tingui lloc, hauria de ser un poderós mecanisme d'enduriment i ductilització en metalls amorfs [10].
Això obre clarament una oportunitat per a dissenyar microestructures que endureixin els vidres metàl·lics massissos a partir de diferents mètodes. Per exemple, s'ha vist que la presència d'una segona fase (amorfa o cristal·lina) amb propietats mecàniques diferents de la matriu promou la nucleació de múltiples bandes de cisalla, al mateix temps que impedeix la propagació de les mateixes. El resultat final és l'augment de la plasticitat d'aquests materials en compressió [11,12].
Així doncs, en aquesta tesi s'han estudiat els fonaments de la deformació de diverses famílies de vidres metàl·lics i materials nanocomposats a partir d'assajos de compressió i nanoindentació.
Els mecanismes de deformació elàstica, anelàstica i plàstica dels vidres metàl·lics influencien la resposta obtinguda en els experiments de nanoindentació de forma fonamental. Les observacions i la discussió realitzades en el treball presentat ajuden a diferenciar els tres mecanismes de deformació en les gràfiques obtingues en els experiències de nanoindentació realitzades.
S'han estudiat els mecanismes de deformació de diferents materials nanocomposats:
- A partir de vidres metàl·lics basats en Cu s'ha aconseguit la formació d'un aliatge de matriu amorfa amb una dispersió homogènia de cristalls de grandària nanomètrica. Així doncs, la cristal·lització, l'estabilitat tèrmica i les propietats mecàniques dels vidres Cu60ZrxTi40-x (x = 15, 20, 22, 25, 30) han estat estudiades. A partir dels coneixements obtinguts s'ha procedit a la obtenció d'un material nanocomposat provocant la cristal·lització primària dels vidres metàl·lics estudiats anteriorment. S'ha observat que la matriu amorfa domina les propietats mecàniques del compost, però que la precipitació d'una fase intermetàl·lica endureix l'aliatge.
- S'han obtingut cintes en el sistema Ni58.5Nb20.25Y21.25 (at%) formades per dues fases amorfes, degut a la immiscibilitat que presenta el sistema Nb-Y tant en estat sòlid com en estat líquid. S'ha observat que la deformació plàstica d'aquest aliatge és clarament diferent al d'un vidre metàl·lic monolític. Així doncs, la seva plasticitat i duresa només es poden explicar degut a la interacció entre les bandes de cisalla formades en la matriu i la segona fase precipitada en forma globular.
- S'ha dut a terme l'estudi de l'evolució microestructural i els mecanismes d'enduriment després de deformar plàsticament per torsió un aliatge basat en Ti format per una matriu eutèctica nanomètrica combinada amb dendrites de grandària micromètrica. Abans de la deformació plàstica, les dendrites són més dures que la matriu eutèctica ja que sofreixen un enduriment per solució sòlida. Després de la deformació, tant la matriu com les dendrites s'endureixen a diferent ritme fins arribar a la mateixa duresa en ambdues fases. Els mecanismes d'aquest enduriment són diferents a cada fase degut a la seva diferent naturalesa.
Les investigacions realitzades durant la tesi han permès comprendre millor algunes de les rutes proposades per millorar les propietats mecàniques dels vidres metàl·lics, com ara el desenvolupament de nanocomposats o la separació en dues fases amorfes. La nanoindentació encara permet estudiar en molts casos la deformació dels vidres metàl·lics. Encara que els estudis de la deformació d'aquests materials utilitzant nanoindentació no són molt abundants, les avantatges d'aquesta tècnica s'han mostrat clarament en aquest treball, com per exemple en l'observació directe de l'enduriment de les diferents fases constituents d'un material nanocomposat. Per tant, els mecanismes subjacents que governen la deformació plàstica dels materials nanocomposats (per exemple per assajos de compressió o deformació plàstica severa) s'han pogut comprendre millor. A més a més, la utilització de tècniques complementàries, com la microscòpia electrònica tant de rastreig com de transmissió, ha aportat informació molt valuosa per investigar els mecanismes microscòpics que governen al deformació plàstica en els vidres metàl·lics i materials nanocomposats.
Els mecanismes de deformació i les aplicacions dels vidres metàl·lics i materials nanocomposats són encara un camp actiu d'investigació. El treball presentat en aquesta tesi motivarà nous estudis en aquest camp científic, des dels punts de vista teòric i tecnològic. Així doncs, aquesta tesi ajudarà en la interpretació de fenòmens com l'efecte de grandària de la indentació, processos de relaxació, deformació cíclica i deformació durant la indentació de vidres metàl·lics. Finalment, cal dir que s'ha d'investigar molt més en aquests temes per tal d'optimitzar les propietats mecàniques dels vidres metàl·lics i així poder ser utilitzats en aplicacions tecnològiques.
Referències:
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[3] Pampillo CA, Polk DE: Acta Metall 1974; 22:741.
[4] Masumoto T, Maddin R: Mater Sci Eng 1975; 19:1.
[5] Hufnagel TC: On Mechanical Behavior of Metallic Glasses, Scripta Mater 2006; viewpoint nº37.
[6] Spaepen F: Acta Metall 1977; 25:407.
[7] Argon AS: Acta Metall 1979; 27:47.
[8] Lewandowski JJ, Greer AL, Wang WH: Philos Mag Lett 2005; 85:77.
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Metallic glasses have been the subject of widespread research over the past four decades with significant advancement in their understanding. As the name suggests, they are metallic alloys with no long-range order. The lack of long-range atomic order makes their properties and behaviour considerably different from those of crystalline alloys. For example, they typically behave as very soft magnetic materials (low coercitivity and high permeability) and have led to commercial applications such as transformer cores, magnetic read-heads and magnetic shielding [1]. By some specific treatments or techniques, it is possible to control the total or partial crystallization of metallic glasses. In some cases very fine, uniform microstructures have been exploited for their hard magnetism [2]. Furthermore, early work already pointed out that their mechanical behaviour showed unique properties, i.e. high strength, large elastic limit, homogeneous and inhomogeneous modes of deformation, and the novel "molten" appearance of fracture surfaces [3,4].
The combination of their high yield strength together with the possibility of casting metallic glasses in bulk form has triggered the interest in using them as structural materials [5]. However, metallic glasses show a distinctive localization of the plastic deformation into shear bands when loaded under ambient conditions [6,7]. Instead of work-hardening, metallic glasses soften due to the shear band formation which prevents stable plastic elongation in tension. Therefore, enhancement of the ductility of this type of materials has been the aim of much research work.
Recent works have studied the instrinsic toughening of metallic glasses [8,9]. The competition between flow and fracture relates the resistance to plastic deformation, proportional to G, to the resistance to dilatation that occurs in the region of a crack tip, which is proportional to B. The results of these works on metallic glasses indicate that exceeding a critical value of G/B (i.e. in the range of 0.41-0.43) produces an amorphous/annealed glass that approaches the ideal brittle behaviour associated with oxide glasses. Therefore, the correlation between fracture energy and elastic moduli indicates that the intrinsic toughness in metallic glasses may be enhanced by selection of elements with low G/B (or, equivalently, high Poisson ratio, ?) as constituents.
The tremendous toughness increase that accompanies multiple shear banding indicates that proliferation of shear bands, regardless of how it is accomplished, should provide a powerful toughening mechanism in amorphous metals [10]. This clearly provides the opportunity for microstructural design of extrinsically toughened BMGs via a variety of techniques. The presence of a secondary phase (amorphous or crystalline) has been shown to promote multiple shear band nucleation sites via mismatch in various mechanical properties, while also providing barriers to shear band propagation. The result of the promotion of shear bands and hindering their propagation finally results in macroscopic compressive ductility [11,12].
The fundamentals of deformation behaviour of several families of metallic glasses and composite materials have been investigated by means of compression tests and nanoindentation experiments.
- The mechanisms of elastic, anelastic and plastic deformation of metallic glasses influence the response of the material during a nanoindentation test. The observed and discussed results on the deformation behaviour of a Pd-base BMG will help to differentiate the deformation mechanisms in the load-displacement curve obtained in an indentation test.
The fundamentals of deformation behaviour in different composite materials have been studied:
- Cu-based metallic glasses have been used to obtain a homogeneous dispersion of nanocrystalls in an amorphous matrix. Therefore, the crystallization behaviour, thermal stability and mechanical properties of Cu-Zr-Ti metallic glasses have been extensively studied. The influence of relaxation and the precipitation of secondary phases on the mechanical response of the studied alloys have been analysed. The precipitation of nanocrystals does not change the main deformation mechanism of these materials and therefore, shear bands form and propagate across the amorphous matrix. Fracture strength and Young's modulus increase with increasing crystalline volume fraction.
- Ribbons of the composition Ni58.5Nb20.25Y21.25 (at%) have been obtained and show phase separation due to the immiscibility gap in the Nb-Y system. The mechanical behaviour of a two-phase metallic glass, consisting of a Y-rich softer matrix and a globular harder Nb-rich phase, is clearly different from a monolithic glass. The plasticity and the hardness of the two-phase alloy are enhanced with respect to the single softer amorphous alloy composing the matrix, due to deflection of the shear bands in the vicinity of the hard globular phase.
- The microstructure evolution and the mechanisms of mechanical hardening after high pressure torsion in a Ti-based dendrite/eutectic nanostructured alloy have been investigated. The dendrites are found to be harder than the eutectic matrix. The structural refinement that occurs in all phases during the severe plastic deformation imposed by HPT strengthens the material. Interestingly, this hardening is more pronounced for the eutectic regions, probably due to the bending effect observed in the lamellae which causes a concomitant loss in their directionality, thus hindering the interlamellar glide.
The work has shed some light into the recently proposed routes to increase mechanical toughness of metallic glasses, such as the development of nanocomposites or phase separation into two amorphous counterparts. Nanoindentation can still be vastly used to study the deformation behaviour of metallic glasses. Although studies using nanoindentation in composite materials are still not widely carried out, the power of this technique is clearly shown in this work enabling a distinction to be made between the hardening of the constituent phases. Hence, the underlying mechanisms governing the property changes in a composite material during plastic deformation (i.e. compression tests or severe plastic deformation) can now be better understood. The use of complementary techniques, such as SEM or TEM, has shown to provide valuable information for the in-depth investigation of the microscopic mechanisms governing plastic flow in metallic glasses and their composites.
The deformation mechanisms and the applications of metallic glasses and composite materials are still under investigation. The work presented in this thesis is likely to motivate new studies on the subject, from both fundamental and technological points of view. The obtained results can help in the interpretation of phenomena, like the indentation size-effect, relaxation processes, cyclic deformation and deformation during indentation in metallic glasses. Finally, more work has to be done in the optimization of ductilization procedures of metallic glasses and nanocrystalline alloys which may enhance their performance and widen their applicability as structural materials.
References:
[1] Masumoto T, Egami T: Mater Sci Eng 1981; 48:147.
[2] Croat JJ, Herbst JF, Lee RW, Pinkerton FE: J Appl Phys1984; 55:2078.
[3] Pampillo CA, Polk DE: Acta Metall 1974; 22:741.
[4] Masumoto T, Maddin R: Mater Sci Eng 1975; 19:1.
[5] Hufnagel TC: On Mechanical Behavior of Metallic Glasses, Scripta Mater 2006; viewpoint nº37.
[6] Spaepen F: Acta Metall 1977; 25:407.
[7] Argon AS: Acta Metall 1979; 27:47.
[8] Lewandowski JJ, Greer AL, Wang WH: Philos Mag Lett 2005; 85:77.
[9] Xi XK, Zhao DQ, Pan MX, Wang WH, Wu Y, Lewandowski JJ: Phys Rev Lett 2005; 94:1255510.
[10] Schroers J, Johnson WL: Phys Rev Lett 2004; 93:255506.
[11] Hays CC, Kim CP, Johnson WL: Phys Rev Lett 2000; 84:2901.
[12] Ott RT, Sansoz F, Molinari JF, Almer J, Ramesh KT, Hufnagel TC: Acta Mater 2005; 53:1883.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2013.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 52-56).
Bulk luminescent solar concentrators (LSC) cannot make use of Forster resonance energy transfer (FRET) due to necessarily low dye concentrations. In this thesis, we attempt to present a poly-vinylalcohol (PVA) waveguide containing dye-aggregate polystyrene nanospheres that enable FRET at concentrations below that required for the bulk LSC due to dye confinement. In the aqueous state, the maximum achieved energy transfer efficiency of the dye-doped nanoparticles was found to be 8 7% for lwt%/lwt% doping of Coumarin 1 (C1) and Coumarin 6 (C6). In the solid state, however, energy transfer is lost, reducing to 32.8% and 20.1% respectively for the C1(lwt%)/C6(lwt%) and C1(0.5wt%)/C6(lwt/ ) iterations, respectively. Presumably, the dyes leach out of the polystyrene nanospheres and into the PVA waveguide upon water evaporation during drop casting.
by Ron Rosenberg.
S.B.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Vita.
Includes bibliographical references (p. 175-188).
(cont.) The self consistent defect model established the defect chemistry of langasite, enabling important parameters describing reduction (Er = 5.70± -0.06eV and 6.57±-0.24eV for acceptor and donor doped langasite respectively) and oxidation (Eo = 2.18±0.08eV), intrinsic electron-hole generation (Eg [approx. equals] 4.0-4.4eV) and defect ionization (ED-ion = 52±0.06eV for Nb ionization), to be extracted. The predictive defect model was used to calculate the dependence of the partial ionic and electronic conductivities and mass change as functions of temperature, dopant level and pO₂. Given that the magnitudes of conductivity and mass change directly affect the resolution and sensitivity limits of langasite resonators, their predictions allowed for the definition of acceptable operating limits and/or the design of properties for optimum resolution and sensitivity. Two high temperature applications of resonant sensors were studied. Praseodymiumcerium oxide was selected for oxygen partial pressure monitoring and is representative of films which change mass upon absorption or desorption of gaseous species. Barium carbonate film was selected for NO₂ sensing and is representative of films which change mass upon reaction with the gas phase to form a new product phase. Both sensors showed sensitivity to their respective target chemicals and demonstrated the feasibility of high temperature sensor applications. The performance of each sensor was discussed and suggestions for improving sensor performance were presented.
The high temperature transport properties of langasite, La₃Ga₅SiO₁₄, were investigated with special attention focused on their potential impact on the utilization of langasite as a mass sensitive resonant platform for high temperature sensor applications. The electrical properties of acceptor and donor doped langasite were examined at temperatures ranging from 700 to 1000 ⁰C, and pO₂ of 1 to 10-25atm. Acceptor doped langasite was shown to exhibit mixed ionic-electronic conductivity behavior, with predominant ionic conduction due to mobile oxygen vacancies at high pO₂, and n-type electronic conduction due to electrons at low pO₂. Increasing acceptor level resulted in the appearance of p-type hole conduction at high pO₂ and increased ionic conductivity, while the n-type electron conduction was depressed. Donor doped langasite was shown to be electronic at all temperatures and pO₂. The electron mobility of langasite was found to be activated (polaron hopping) with an activation energy of 0.15(±0.01)eV, whereas the holes were assumed to be quasi free carriers. The activation energy for oxygen vacancy migration was estimated to be 0.91(±0.01)eV under dilute solution conditions and 1.27(±0.02)eV for 1% Sr level under concentrated solution conditions. Both values of activation energy of ionic conductivity-temperature product are consistent with activation energy of oxygen self-diffusivity in the respective materials. The electrical properties were related to the underlying defect and transport processes using defect modeling.
by Huankiat Seh.
Ph.D.

In the past decade, ultrashort laser sources have had a decisive impact on material processing for photonic applications. The technique is usually restricted to the elemental association of an ultrashort source with a focusing lens. It is thus limited in the achievable bulk modifications. Accompanying studies of material modifications in space and time, we propose here that automated spatio-temporal tailoring of the laser pulses is an efficient manner to overcome these limitations. More precisely, we demonstrate the generation of multiple processing foci for synchronous photomachining of multiple devices in the bulk. Thus, we report on the parallel photowriting of waveguides, light couplers, light dividers in 2D/3D in fused silica glass. We show that the domain of photowriting can be extended to deep focusing. We indicate that this can be achieved by wavefront shaping or temporal profile tailoring conducted by an evolutionary optimization loop. We also have unveiled a singular interaction regime where regular structuring takes place before the focal region. For the first time, the dynamics of the energy coupling to the glassy matrix is evaluated for various temporal pulse profiles. Enhanced energy confinement in the case of picosecond pulses is confirmed by characterization of the transient electronic gas and of the subsequent pressure. These pump-probe studies were carried out with a self-build time-resolved microscopy system with temporally shaped pump irradiation. We also developed a new method based on the Drude model to differentiate the electronic and matrix contributions to the contrast of the microscopy images.

La presente tesis doctoral versa sobre el transporte de calor llevado a cabo por los fonones en sólidos cristalinos semiconductores. La motivación de este trabajo es doble. En primer lugar, se pretende contribuir a entender mejor cómo funciona el transporte de calor a distintas escalas de tamaño: desde semiconductores con tamaño bulk (del orden de milímetros o mayores) hasta semiconductores nano-estructurados, como por ejemplo nanocables o láminas finas, cuyos tamaños característicos están en la escala nanométrica. La intención es describir dicho transporte de calor en estas escalas en un amplio rango de temperaturas, prestando especial atención a las colisiones entre fonones, pues son la causa intrínseca de la propagación del calor en los sólidos cristalinos semiconductores. En segundo lugar, se pretende mejorar la capacidad de predicción a la hora de describir el comportamiento de la conductividad térmica de los semiconductores más comunes por su implicación en procesos termoeléctricos, como son el silicio, el diamante, el germanio y el bismuto de telurio. Para lograr alcanzar estos objetivos, es necesario formular un nuevo modelo que nos permita superar las dificultades asociadas a los modelos ya existentes, con el objetivo de cumplir dos condiciones muy deseables. Por un lado, obtener una expresión general para la conductividad térmica válida para diferentes materiales, que pueda ser aplicada a muestras de dichos materiales con diferentes composiciones isotópicas, diferentes tamaños (desde la macro hasta la nano-escala) y con diferentes geometrías. Por otro lado, dicha expresión deberá tener el menor número posible de parámetros ajustables para asegurar la fiabilidad del modelo. La potencialidad de dicho modelo radicaría en servir como herramienta a la hora de guiar el diseño de dispositivos termoeléctricos más eficientes. La presente tesis se organiza en 8 capítulos ordenados de la siguiente manera: En el capítulo 1 se contextualiza el tema en el que está enmarcado el presente trabajo de investigación y se presentan los conceptos físicos necesarios para trabajar con el transporte fonónico. En el segundo capítulo se desarrolla la dinámica de la red para los distintos materiales que serán objeto de estudio en el presente trabajo, en particular se aplica el modelo Bond-charge para obtener las relaciones de dispersión y la densidad de estados de los semiconductores del grupo IV (silicio, germanio, diamante y estaño gris) y análogamente se aplica el modelo Rigid-ion sobre el bismuto de telurio para obtener sus relaciones de dispersión y densidad de estados. Los tiempos de relajación apropiados para dichos materiales se discutirán en detalle en el capítulo 3, proponiendo nuevas expresiones empíricas para describir las interacciones fonón-fonón. En el capítulo 4 se introducen y discuten los modelos de conductividad térmica más representativos de la literatura y a continuación se presenta un nuevo modelo para predecir la conductividad térmica: el modelo Kinetic-collective, cuya principal característica consiste en interpretar el transporte de calor en dos regímenes diferentes, el primero de ellos de tipo cinético donde los fonones son tratados como partículas libres y el segundo de tipo colectivo donde todos los fonones que participan en el transporte pierden su individualidad y se comportan como una colectividad de partículas. En el capítulo 5 el modelo Kinetic-collective se aplica a silicio bulk con diferentes composiciones isotópicas, y a varias muestras de silicio nanoestructuradas con diferentes geometrías y tamaños efectivos. Se obtienen predicciones de la conductividad térmica en un amplio intervalo de temperaturas que concuerdan satisfactoriamente con las medidas experimentales y se discuten diversos aspectos novedosos sobre el transporte fonónico. En el capítulo 6 el modelo Kinetic-collective se aplica al resto de materiales componentes del grupo IV de semiconductores y se obtiene una relación teórica que nos permite predecir los valores de los parámetros libres asociados a los tiempos de relajación de dichos materiales y así poder predecir sus conductividades térmicas sin la necesidad de añadir nuevos parámetros. En el capítulo 7 vamos un paso más allá y aplicamos el modelo a bismuto de telurio, obteniendo predicciones de la conductividad térmica para nanocables con diferentes diámetros y discutimos los resultados en vista a posibles aplicaciones termoeléctricas. Finalmente, el capítulo 8 está dedicado a recoger las principales conclusiones de este trabajo de investigación y a indicar posibles líneas futuras de trabajo surgidas a consecuencia de los resultados obtenidos.
The aim of this theoretical work is twofold. First, to contribute to a better understand- ing of phonon heat transport in bulk and nanostructured semiconductors, like thin-films or nanowires, in a wide range of temperatures, paying special attention to phonon-phonon col- lisions. Second, to improve the prediction capability of the thermal conductivity of the most common semiconductors. To achieve this, it becomes necessary the formulation of a new model allowing us to overcome the diculties associated to the existing models, with the aim to fulfill two desirable conditions: to provide a general expression for the thermal conduc- tivity, valid for several materials with di↵erent size-scales and geometries in a wide range of temperatures, and to have the smallest number of free adjustable parameters to assure the reliability of the model. The potentiality of such model would be to serve as a useful tool to design more ecient thermoelectric devices. The fruit of our study is the Kinetic-collective model which is developed in the framework of the Boltzmann transport equation as a natural generalization of the Guyer-Krumhansl model. Since phonon interactions are the source of thermal resistance, they deserve a special discussion in any thermal conductivity study. Precisely, the keystone in our work is the treatment of phonon-phonon collisions regarding their di↵erent nature. The prediction capability of the model need to be tested on several materials. In particular, we study five materials with thermoelectric interest. In first place, silicon, because it is an ideal test material due to the considerable amount of experimental data available in the literature, and because of its inherent scientific and technological importance. Secondly, we extend our study to other materials with the same lattice structure as silicon, that is the family of group IV element semiconductors (germanium, diamond, silicon and gray-tin), which also have been object of intense study, specially germanium, due to the recent and fast development of SiGe alloys and superlattices. Finally, we finish our study with a more complicated material regarding its lattice structure, bismuth telluride, which is known to be a very ecient thermoelectric material due to its high figure of merit. The Thesis is arranged in eight Chapters. The lay out is as follows: Chapter 1 con- textualizes the topic of the work and briefly introduces the basic physics related to phonon transport. In Chapter 2 the fundamental quantity necessary for considering any thermal property, the phonon dispersion relations, have been obtained for the materials under study. For this purpose, two lattice dynamics models are used: the Bond-charge model for group-IV semiconductors (silicon, germanium, diamond and gray-tin), and the Rigid-ion model for bismuth telluride (Bi2Te3). Along with their corresponding phonon dispersion relations, phonon density of states and specific heat results are also presented. The phonon relaxation times that suit these materials are discussed in Chapter 3, where new expressions to account for the phonon-phonon collisions are also presented. In the first part of Chapter 4 the most represen- tative thermal conductivity models to date are introduced and discussed, in the second part, a new model to predict the thermal conductivity, the Kinetic-collective model, is presented and its conceptual di↵erences and advantages with respect to previous similar models are discussed. In Chapter 5 the Kinetic-collective model is applied to silicon bulk samples with di↵erent isotopic composition and several nanostructured samples with di↵erent geometries (thin-films and nanowires) obtaining predictions for their thermal conductivity in a wide in- terval of temperatures. Some novel aspects of phonon transport arising from these results are discussed. In Chapter 6 the Kinetic-collective model is applied to the other group-IV materials using theoretical expressions to predict their relaxation times and, eventually, their thermal conductivity. Results for several samples with di↵erent isotopic compositions in a wide range of temperature are presented and discussed. In Chapter 7 the Kinetic-collective model is applied to Bi2Te3, providing thermal conductivity predictions for nanowires with several diameter values, and the results are discussed in view of possible applications in ther- moelectricity. Finally, in Chapter 8 the main conclusions of this Thesis are summarized and possible future lines of work stemming from its several results are discussed.

Research on organic photovoltaic devices (OPV) has developed during the past 30 years, but especially in the last decade it has attracted scientific and economic interest triggered by a rapid increase in power conversion efficiencies. Thanks to the indtroduction of the bulk heterojunction (BHJ) concept, today BHJ OPV efficiencies are exceeding 9%. This thesis gives an overview on the different possible strategies that could be adopted for a further. improvement of BHJ OPV devices performances. The accurate analysis of the chemical, energetic and physical criteria governing the solar cells functioning allowed to individuate some critical aspects and apply possible solutions by a fine tuning of the materials chemical structures, device processing techniques and device architecture engineering. Even though noit in all cases the applied strategy successfully led to device efficiency improvements, the fundamental understanding of some of the efficiency limiting factors could serve as useful scientific basis for future developments.

From pharmaceutical powders to agricultural grains, a great proportion of the materials handled in industrial situations are granular or particulate in nature. The variety of stesses that the matierals may experience and the resulting bulk behaviours may be complex. In agricultural engineering, a better understanding into agricultural processes such as seeding, harvesting, transporting and storing will help to improve the handling of agricultural grains with optimised solutions. A detailed understanding of a granular system is crucial when attempting to model a system, whether it is on a micro (particle) or macro (bulk) scale. As numerical capabilities are ever increasing, the Discrete Element Method (DEM) is becoming an increasingly popular numerical technique for computing the behaviour of discrete particels for both industrial and scientific applications. A look into the literature shows a lack of validation of what DEM can predict, specifically with respect to bulk behaviour. In addition, when validation studies are conducted, discrepancies between bulk responses in physical tests and numerical predictions using measured particles properties may arise. The aire of this research is to develop a methodology to calibrate DEM models for agricultural grains using data meaured in bulk physical tests. The methodology will have a wider application to granular solids in general and will advance understanding in the area of DEM model calibration. A contrasting set of granular materials were used to develop the methodology including 3 inorganic solids (single and paired glass beads, and polyethylene terephthalate pellets) and two organic materials (black eyes beans and black kidney beans). The developed methodology consists of three steps: 1. The development of bulk physical tests to measure the bulk responses that will be used to calibrate the DEM models, 2. The creation of the numerical dataset that will describe how the DEM input parameters influence the bulk responses , and 3. The optimisation of the DEM parameters using a searching algorithm and the results from Step 1 and 2. Two laboratory devices were developed to provide calibration data for the proposed methodology: a rotating drum and an confined compression test. These devices were chosen as they can produce bulk responses that are repeatable and easy to quantify, as well as generate discriminating results in numerical simulations when DEM parameters are varied. The bulk response determined from the rotating drum device was the dynamic angle of repose Ør formed when the granular material in a 40% filled drum is rotating at a speed of 7 rpm. the confined compression apparatus was used to determine the bulk stiffness of a system by monitoring the change in void ratio from the stress applied during a loading and unloading cycle. The gradient of the loading and unloadng curves termed λ and κ respectively were chosen as the bulk responses to calibrate the DEM models. The experimental results revealed that the dynamic Ør was significantly influences by the particle aspect ration and boundary conditions. The stiffness parameters were found to be predominantly influences by the initial packing arrangement. The numerical dataset describing how the DEM input parameters influence the numerical bulk responses was created by simulating the bulk physical tests, varying selected DEM parameters and monitoring the effects on bulk parameters. To limit the number of simulations required, design of experiment (DOE) methods were used to determine a reduced factorial matrix of simulations. In additions, an extensive parametric investigation on the non-optimised parameters as well as a scaling sensitivity study was carried out. The final step in determining the optimised parameters is to use a searching algorithm to infer the DEM parameters based on the numerical dataset and used the experimental results as calibration data. To perform a comparative study, tow searching algorithms were explored: the first was a simple method based on Microsoft Excel's Solver algorithm coupled with a weighted inverse distance method. The second made used of the statistical analysis program Statistica. It was shown that the Excel Solver algorithm is simpler and quicker to use but for the present first implementation, could only perform an optimisation based on two bulk responses. Statistica required the creation of a staistical model based on the numerical dataset before using the profiling and desirability searching technique, but was able to optimise the parameter using all three bulk responses. A verification and validation of the optimisation methodology was conducted using the optimised parameters for the black eyed beans. A verification was cnducted by simulating the two calibration experiments using the optimsed parameters and comparing these with the experiments. In addition, a validation was peformed by predicting the response of ta shallow footing penetration on a bed of black eyed beans. It was found that DEM simulations using optimised parameters predicted vertical stress on the footing during penetration to an acceptable degree of accuracy for industrial applications (<10%) at penetration depths up to 30mm.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 133-138).
Thermal conductivity is an important transport property that plays a vital role in applications such as high efficiency thermoelectric devices as well as in thermal management of electronics. We present a first-principles approach based on density-functional perturbation theory (DFPT) to predict the thermal conductivity of semiconducting materials. Heat in these materials is conducted by lattice vibrations (phonons). The most important ingredients in the prediction of thermal conductivity in such materials are the second- and third-order derivatives of energy with respect to atomic displacements. Typically, these are derived using empirical potentials which do not produce the correct harmonic and anharmonic behavior, necessary to accurately compute phonon frequencies and relaxation times. We obtain these derivatives from quantum mechanics through DFPT, and use them along with the solution of the phonon Boltzmann transport equation to predict thermal conductivity. We apply the approach to isotopically pure silicon and germanium as well as materials with disorder such as silicon-germanium alloys and show how this leads to excellent agreement between computed and experimentally measured values. The approach is also applied to predict thermal transport in nanostructured materials such as superlattices. In isotopically pure silicon and germanium, phonons scatter only through the three-phonon anharmonic scattering processes. Using the single-mode relaxation time approximation and estimating the scattering rate of these processes based on the force constants derived from DFPT, excellent agreement is obtained between computed and measured values of thermal conductivity. The approach predicts that in isotopically pure silicon, more than 90% of the heat is conducted by phonons of mean free path larger than 40 nm, providing avenues to lower thermal conductivity through nanostructuring. To predict thermal transport in disordered silicon-germanium alloys of any composition, we make use of the phonon modes of an average crystal which has the two atom unit cell and average mass and force constants appropriate for that composition. The disorder is taken to lead to elastic two-phonon scattering in addition to the three-phonon scattering present in pure materials. The idea was first proposed by Abeles in 1963; however we are able to compute all the ingredients from firstprinciples. The force constants for the composition Sio.5 Geo.5 are obtained by using the virtual crystal where the atomic potential at each site is an average of the silicon and germanium potentials. We demonstrate how this approach can be used to guide design of nanostructured materials to further lower thermal conductivity. In superlattices, we again use the virtual crystal to obtain the second-order and third-force constants. Computed thermal conductivity is found to lower with increase in superlattice period; however, the predicted values are higher than experimentally measured values, and we discuss the cause of this discrepancy. In the limit of very small period superlattice, we find that thermal conductivity can increase dramatically and can exceed that of isotopically pure silicon. This cause of this unexpected result is discussed, and its implications for high thermal conductivity materials, important for applications in thermal management of electronics.
by Jivtesh Garg.
Ph.D.

Thesis advisor: Zhifeng Ren
SiGe alloys are the only proven thermoelectric materials in power generation devices operating above 600 °C and up to 1000 °C in heat conversion into electricity using a radioisotope as the heat source. In addition to radioisotope applications, SiGe thermoelectric materials have many other potential applications, for example, solar thermal to electricity energy conversion and waste heat recovery. However, traditional SiGe alloy material shows low ZT values of about 0.93 at 900 °C, thus, 8% is the highest device efficiency for commercial SiGe thermoelectric devices. Recently, many efforts have been made to enhance the dimensionless thermoelectric figure-of-merit (ZT) of SiGe alloys. Among them, the nano approach has been recognized as an effective mechanism to obtain thermoelectric materials with good performance. In this approach, dense bulk samples with random nanostructures with high interface densities are synthesized through ball milling and a direct current hot press, leading to an enhancement ZT through reduced phonon thermal conductivity. Such a practical technique produced samples of nanostructured p-type dense bulk bismuth antimony telluride with a peak ZT of 1.4 at 1000 °C from either alloy ingot or elemental chunks. However, the generality of this approach has not been demonstrated. Here, we applied the same technique in SiGe system in order to fabricate a nanostructured n-type SiGe alloy with enhanced thermoelectric properties. In this thesis, numerous nanostructured n-type SiGe alloy samples were successfully pressed. The structure of these nanostructured samples was investigated via XRD, EDS, and TEM. It has been confirmed that many nano grains exist in our nanostructured samples
Thesis (PhD) — Boston College, 2009
Submitted to: Boston College. Graduate School of Arts and Sciences
Discipline: Physics

In the field of Photovoltaic technologies the organic solar cells are particularly attractive because of their ease of processing, mechanical flexibility and potential low cost production techniques. So far, the reported efficiencies are not high enough to allow to be competitive in the market, however with the introduction of new photoactive materials, device architectures and light management structures, the power conversion efficiencies, at laboratory scale, has rapidly reached the 12%, showing a great potential and a bright future for organic solar cells. Nevertheless, in view of commercial products, two main problems are still unresolved: the relatively low performance of the modules and their lifetime. In sight of this, the present Ph.D thesis has a double goal: 1) a better understanding of the relationship between devices performance and photoactive materials structures 2) a deep investigation on device degradation processes, with particular attention on the effects induced by temperature and incident light. As a result, promising approaches to further optimize the polymer’s optoelectrial properties, and thus the corresponding device performance, are proposed. About the lifetime, first the thermal degradation mechanisms involved on the active layer was investigated and it has been demonstrated the role of the other layers and interfaces in the solar cell thermal stability. In this contest an innovative fast capacitance based thermal test has been developed in order to obtain information regarding the limit operating temperature above which the device becomes thermally unstable. In the end, a preliminary study on the photostability issue was carried out demostrating that the photodegradation of organic solar cells not depends just on the photostability of the donor polymer, but is connected also with the composition of the active layer solution and on the interaction with the adjacent layers. Solutions to limit or prevent the devices degradation processes are proposed.

Thesis (M.S.)--Boston University
Fundamental understanding of the structure-property relationship of pi-conjugated poly- mers is critical to predictive materials designs of bulk heterojunction solar cells. In this thesis, the adapted Su-Schrieffer-Heeger Hamiltonian is implemented as the computational tool to systematically explore the opto-electronic properties of nearly 250 different kinds of pi-conjugated systems. New physical insights on the structure-property relationship are extracted and transformed into practical guiding rules in optical bandgap designs. For the most power efficient donor-acceptor copolymer structures, we find that the energy variation of frontier orbitals, in particular the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO), can be controlled either independently or collectively, depending on their specific donor or acceptor structures. In particular, we find that having five-membered conjugated carbon rings in the acceptor units is essential to break the electron-hole charge conjugation symmetry, so that the LUMO levels of the copolymer can be reduced dramatically while holding the HOMO energy levels in the donor units constant. On the other hand, by incorporating heteroatoms into the donors units, we can vary the HOMO levels of the copolymers independently. Predicted optical bandgaps of a total of 780 types of these copolymers constructed by using 39 different types of donor and acceptor units are tabulated in this thesis. In addition, the effects of introducing various side groups(-R, -0, -CO, -COO, and thiophene) on the primitive donor and acceptor structures are investigated and their results are discussed in details. Finally, unexpected localized states are found, for the first time, in our calculations for a few special co-polymer structures. These localized states, with electrons localized on one end of the copolymer chain and holes on the other end, contain large dipole moments and therefore may be treated as a new design dimension when these copolymers are placed in polar and non-polar solvent environments.

Atomically thin Molybdenum disulfide, MoS2, a star member of the group VI transition metal dichalcogenide(TMDC) family has been attracting rising interests for its potential applications in emerging electronics and optoelectronics. Bulk MoS2is a semiconductor with an indirect gap located between the top of valence band at Γ points and the bottom of conduction band in mid of K and Γ points in its Brillouin zone. Atomically thin MoS2 films including monolayers and multilayers, being chemically inert, present a class of intrinsic 2D semiconductors which are widely regarded as a platform for ultimate electronics. As yet tremendous efforts focus on the optical properties and electric transport study. In this thesis, we report the experimental study of photocurrent measurements on MoS2thin films. The sample preparation, device fabrication, optical and electric characterizations are introduced. The experiments have been carried out on a field effect transistor (FET) structured MoS2 device. The photocurrent spectroscopy reveals the interband excitonic transitions at spin-split bands around K valleys. The results demonstrate that MoS2has potential applications in optoelectronics.
published_or_final_version
Physics
Master
Master of Philosophy

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 96-100).
An accelerated sintering method called 'nano-phase separation sintering' is developed, with specific applicability to nanostructured tungsten alloys. Nanocrystalline tungsten alloys containing minority additions of chromium are produced by high-energy ball milling and then consolidated. Such alloys exhibit the onset of sintering at a very low temperature around 950 °C and a very rapid rate of densification. The mechanism of this accelerated sintering is established through understanding the role of nano-scale, solid second phase precipitation during the sintering cycle, as analyzed by thermomechanical analysis, electron microscopy and x-ray diffraction. In addition, control experiments are used to establish that the accelerated sintering is apparently accomplished from the combination of two features of the powders: (i) nanocrystallinity and (ii) alloy supersaturation. In addition to accelerating sintering, the incorporation of alloying elements and second phases are also beneficial for mitigating grain growth during a thermal cycle, so nanophase separation sintering is thus naturally appropriate to the production of fine-grained bulk materials. Sintered compacts achieved through nano-phase separation sintering display 10~30 times smaller grain sizes at comparable densities than those produced by conventional accelerated sintering methods such as solid-state activated sintering and liquid phase sintering. The thermodynamic features and conditions for nano-phase separation sintering are further explored based on the binary phase diagram in order to generalize the concept to other alloy systems. After presenting a series of proposed alloy design rules, the consolidation of chromium with an addition of nickel is accelerated. Prospects of the technique for the development of full density bulk products in more complex alloy systems are also discussed.
by Mansoo Park.
Ph. D.

Al93Fe3Cr2Nb2 (at.%) nanoquasicrystalline alloys have been shown to have the potential to push the applications of aluminium alloys to more elevated temperatures, by maintaining a high strength. They also have more thermally stable microstructures than previous nanoquasicrystalline alloys from similar systems (the most studied of which is Al93Fe3Cr2Ti2 (at.%)). Al93Fe3Cr2Nb2 (at.%) alloys have never previously been produced in samples on a scale larger than melt-spun ribbon. This study examines the production parameters of bulk nanoquasicrystalline Al-Fe-Cr-Nb alloys. Firstly an attempt was made to reduce the melting temperatures of thermally stable nanoquasicrystalline alloys through additional alloying. The melting processes of binary, ternary, quaternary and quinary nanoquasicrystalline alloys was analysed though DTA, with endothermic reactions up to 1034oC observed. Rapidly solidified Al-Fe-Cr-Nb alloys were then produced in kilogram quantities through gas atomisation at an industrial scale. The smallest atomised powder particles contained fine scale microstructures consisting of nano-scale quasicrystals embedded in an aluminium matrix. As the cooling rate of the powder particles decreased new phases, including the theta phase (Al13(Fe,Cr)2-4) and Al3Nb were produced. 0-25μm, 25-50μm and 50-75μm (diameter) size fractions of atomised powder were each consolidated through extrusion to produce nanoquasicrystalline Al-Fe-Cr-Nb bars. Composite bars of the nanoquasicrystalline alloy mixed with 10(vol.)% and 20(vol.)% pure aluminium were also produced. The consolidation of the nanoquasicrystalline atomised powders through extrusion led to precipitation of intermetallics including (Al13(Fe,Cr)2-4) and Al3Nb, particularly in the smallest powder size fractions with the most metastable microstructures. Finally the effects of the atomisation and extrusion conditions on the microstructure and its mechanical properties were studied. Improved strength, coupled with reduced ductility was observed with decreases in the elemental aluminium composition of the Al-Fe-Cr-Nb bars and the powder size fraction they were produced from. There was however improvements in toughness of the extruded composite bars, over the nanoquasicrystalline alloy bars.

Rapidly solidified nanoquasicrystalline Al₉₃Fe₃Cr₂Ti₂ at% alloy has previously shown outstanding mechanical performance and microstructural stability up to elevated temperatures. Despite this, no in-depth study had previously been performed assessing the active strengthening mechanisms, the long term microstructural stability and the effect of plastic deformation at elevated temperature to simulate the production methods utilised for engineering applications. The current project analysed eight bars consisting of a nanoquasicrystalline Al₉₃Fe₃Cr₂Ti₂ at% alloy matrix with varying amounts of pure Al fibres, produced through gas atomisation and warm extrusion. Microstructural characterisation and thermal analysis of the as-atomized powder was carried out to assess whether microstructural changed were likely to occur at the extrusion temperature. A microstructure made primarily of nanometre-sized icosahedral particles in an FCC-Al matrix was observed through a combination of SEM, TEM (and CBDP), EDX, XRD. Thermal analysis of the powders performed by DSC showed that no change was expected to occur at the extrusion temperature. Five bars were extruded during the course of this project: one bar of pure Al-Fe-Cr-Ti alloy, two composite bars with 10 vol% added pure Al and two bars with 20 vol% added Al. Three more bars were received from a previous project and analysed. Warm extrusion caused the powder particles to become well bonded and elongated in the extrusion direction introducing a preferred orientation in the FCC-Al grains. A bimodal distribution of grain size was observed after extrusion. Several low angle (5-15 °) grain boundaries were also identified by EBSD along the extrusion direction. No obvious change in size or shape was observed by TEM in the icosahedral phase (a bimodal distribution of hard, incoherent precipitates was observed after extrusion), or any change in the amount of solutes in solid solution in the Al matrix. Mechanical properties at room temperature were tested by Vickers microhardness, quasi-static tensile tests, dynamic tensile tests and dynamic compression tests. A theoretical model correlating the microstructures observed with the various active strengthening mechanisms was applied in order to predict an estimate of the yield strength of the material produced. It was found that the strength of the Al₉₃Fe₃Cr₂Ti₂ alloy came primarily from a combination of the effect of the hard, incoherent nanoparticles, the small grain size and work hardening. The fibre addition to this alloy caused a linear decrease in mechanical strength with increasing vol% pure Al. This work represents the first quantitative estimate of which strengthening mechanisms are active and how much they influence the mechanical strength of Al₉₃Fe₃Cr₂Ti₂ alloy and composites. An understanding of the yield strength is essential as engineering components would only be safe to use within the elastic region. To investigate the thermal stability of the alloy and composites, thermal analyses involving DSC and long heat treatments (up to a maximum of 1000 hours) were performed at various temperatures along with microstructural characterisation by XRD, SEM and TEM and microhardness tests. No microstructural change was detected, however a 2-5% decrease in microhardness was observed. Compression tests were performed across a range of temperatures and strain rates to simulate the behaviour of these composites under typical conditions necessary to process them into useful engineering components. Phase changes occurring during plastic deformation at high temperature were investigated by XRD. The measured yield strength at 350 °C was over 3x that of high strength 7075 T6 Al alloy showing outstanding thermal stability and mechanical performance. However, the microstructure was shown by XRD to undergo a phase transformation which resulted in the decomposition of the icosahedral phase at 500 °C into more stable intermetallic phases. Serrated flow was also observed in some of the tests. The high temperature compressive data was then used for the first time in a semi-quantitative analysis to determine which species in solid solution (Fe, Cr or Ti) was likely to cause the serrations. A dynamic strain ageing model, which calculates the diffusion coefficients at the minimum in ductility and strain rate sensitivity, suggested that the Ti in solid solution in the matrix could be the most likely candidate.

This thesis presents the results of an experimental study into the nonlinear optical properties of novel nonlinear materials at infrared regions of the electromagnetic spectrum for the realisation of nonlinear optical devices in the near- and mid-infrared. Because of its exceptional nonlinear optical properties and its promise of implementation in a range of mid-infrared applications graphene had a prominent place in this research. Extensive investigations in the nonlinear optical properties of single and multilayer chemical vapour deposition (CVD) graphene are presented. This study revealed that graphene presents a nonlinear phase shift due to a negative, irradiance-dependent nonlinear refraction. The high peak powers available enabled the study of both saturable absorption (SA) and two-photon absorption (2PA), identifying the irradiance limits at which the contribution of two-photon absorption exceeded that of saturable absorption. Moreover, the nonlinear optical properties of graphene-polyvinyl alcohol (G-PVA) composite films were studied. The results indicate the thermal damage of the host polymer due to graphene heating and temperature transfer. Studies in the third order nonlinear optical properties of chalcogenide glasses with the perspective of integration with graphene for the development of mid-infrared devices and applications are also performed. Of all the glasses investigated, gallium lanthanum sulphide (GLS) was found to have the most interesting nonlinear optical properties. Its optical Kerr nonlinearity was found to be approximately 35 times higher than silica and the upper limit of its two-photon absorption coefficient was the lowest of all the chalcogenide glasses analysed, implying that GLS would be an excellent candidate for ultrafast all-optical switching. Subsequently GLS was chosen as the host material for optical waveguide and device fabrication via ultrafast laser inscription (ULI). Near- and mid-infrared waveguides were successfully fabricated; fundamental features such as, refractive index profiles and material dispersion were investigated. The Zero Dispersion Wavelength (ZDW) of GLS was experimentally measured for the first time; the ZDW was determined to be between 3.66-3.71 μm for the waveguides and about 3.61 μm for the bulk. Single mode directional couplers at 1550 nm were also developed and their ultrafast all-optical switching properties were investigated, leading to the assessment of the nonlinear refractive index n2 of the ULI modified area. Furthermore, waveguides in Er3+ doped GLS were successfully fabricated and the infrared transitions at 1550 and 2750 nm were detected opening the potential for GLS waveguide lasers.

By focusing an ultrashort pulse of light in the volume of a transparent dielectric, free carrier generation takes place via nonlinear ionization. A substantial part of the laser energy is deposited into the free carrier gas, transferred into the bulk, and a material with new optical properties emerges upon energy relaxation. Phase contrast (PCM) and optical transmission microscopy (OTM) techniques are employed to characterize the morphology of the laser-induced refractive index change in the bulk of amorphous silica and N-BK7. The experimental results are correlated with a theoretical estimation of the energy deposited in the vicinity of the focal plane based on the resolution of the nonlinear Schrödinger equation. In fused silica, the formation of a void is connected with the appearance of a high energy exposure upon nonlinear propagation. A time-resolved study of the laser generated refractive index modifications with sub-picosecond and sub-micrometer resoltion is performed. This analysis points out the importance of the thermal mechanisms and of the subsequent thermochemical transformations in laser modification of bulk dielectrics. By using an adaptative pulse shaping apparatus, we demonstrate that optical structures that do not normally appear in standard ultrafast irradiation conditions can be generated. In particular, we report the onset of large positive refractive index regions in BK7. Finally, the flexibility offered by temporal pulse manipulation is exploited for microprocessing purposes. We demonstrate writing of embedded waveguiding structures at optical frequencies in the bulk of BK7
En focalisant une impulsion lumineuse ultra brève dans la masse d'un matériau diélectrique transparent, un mécanisme d'ionisation non-linéaire peut conduire à la création de porteurs libres. L'énergie lumineuse est alors efficacement déposée. Après relaxation de l'énergie, un matériau avec de nouvelles propriétés optiques est obtenu. Les propriétés optiques de ce matériau transformé ainsi que la morphologie de la zone altérée sont caractérisés en microscopie à contraste de phase et en microscopie optique classique. Les échantillons étudiés sont principalement la silice pure et le N-BK7. Les observations expérimentales sont corrélées avec une estimation théorique de la densité d'énergie déposée obtenue en résolvant l'équation de Schrödinger non-linéaire. Dans la silice pure, l'apparition d'une micro-cavité est ainsi associée à une région de forte exposition à l'énergie lumineuse. Une étude basée sur un dispositif de microscopie de phase et de microscopie classique caractérisée par une résolution spatiale submicrométrique et une résolution temporelle subpicoseconde est également présentée. Cette analyse révèle l'importance des phénomènes thermiques et des effets thermomécaniques. En optimisant la forme temporelle de l'impulsion, nous démontrons la possibilité de conduire le matériau de manière permanente dans des états inaccessibles lorsqu'on se limite à une irradiation ultra brève classique. En particulier, nous montrons l'existence de régions de densités élevées dans le BK7 après irradiation. Enfin, la souplesse offerte par la mise en forme temporelle est employée afin de réaliser l'écriture de guides d'ondes enterrés dans le BK7

Thermoelectric materials have been intensively investigated during the last years for energy harvesting applications. The main drawback of this technology is the low efficiency of the current materials. Significant advances in this respect have been recently achieved by nanostructuring, to mainly reduce the thermal conductivity. In this approach a bulk sample is milled into a nanostructured powder that is then compacted to form a nanobulk sample. The main objective of this thesis is to explore and introduce a new technique, ultrasound milling, based on the crushing of bulk samples by means of ultrasound effects taking place in a liquid medium, for the preparation of nanostructured bulk materials. Bismuth telluride alloys are the industrial standard in thermoelectrics and has been chosen as the material to perform this investigation. The most suitable conditions for the preparation of the nanostructured powders by the ultrasound milling technique have been identified. The optimised powders were used to prepare compacted nanobulk samples. The thermoelectric properties of these samples were finally characterised at room temperature and their performance related to their microstructure. Extraordinarily low thermal conductivity was obtained for both n- and p-type samples prepared (0.5 and 0.35 W/Km respectively), which are within the lowest reported values for any thermoelectric alloy. This reduction was accompanied with a significant decrease in electrical conductivity which led to a non-significant improvement in the figure of merit (Z). However, high ZT values (1 and 1.4 for n- and p-type bismuth telluride respectively) were identified in non-treated samples after a very simple grinding process which was employed as pre-treatment. The figure of merit of these two materials prepared by this simple methodology is close to the best reported values for Bi2Te3. Our results identify ultrasound milling and the simple crushing method as promising tools for the fabrication of nanostructured bulk thermoelectric materials.

The Discrete Element Method (DEM) has been applied to numerical modelling of the bulk compression of low modulus particulates. An existing DE code for modelling the contact mechanics of high modulus particles using a linear elastic contact law was modified to incorporate non-linear viscoelastic contact, real containing walls and particle deformation. The new model was validated against experimental data from the literature and physical experiments using synthetic spherical particles, apple and rapeseed. It was then used to predict particle deformation, optimum padding thickness in a handling line and bulk compression parameters during oilseed expression. The application of DEM has previously been limited to systems of hard particles of high compressive and shear modulii with relatively low failure strain. Material interactions have therefore commonly been modelled using linear contact law. For high modulus particles, particle shape change resulting from deformation is a not a significant factor. Most agricultural particulates however deform substantially before failure and their interaction is better represented with non-linear hysteretic viscoelastic contact relationship. Deformation of geometrically shaped particles in DEM is usually treated as "virtual" deformation, which means that particles are allowed to overlap rather than deform due to contact force. Change to particle shape has not previously been possible other than in the case of particles modelled as 2-D polygons or where each particle is also modelled concurrently with an FE mesh. In this work a new approach has been developed which incorporates a non-linear deformation dependent contact damping relationship and a shape change while maintaining sufficient geometrical symmetry to allow the problem to be handled by the same DE algorithms as used for true spheres. The method was validated with available experimental results on impact behaviour of rubber and the variations with different damping coefficients were simulated for a selected fruit. A fruit handling process dependent on the impact process was then simulated to obtain data required in the design of a fruit processing line. Changes in shape of spherical synthetic rubber particles and rapeseeds under compression were predicted and validated with physical experiments. Images were taken and analysed using image processing techniques with 1: 1 scaling. The method on shape change entails a number of simplifying assumptions such as uniform stress distribution and homogeneous material properties and uniform material distribution when deformed, which are not observed in real agricultural materials and will tend to overestimate the true contact area between particles. In reality for fruits and vegetables, material redistribution is a complex process involving a combination of compaction and movement. However with the new method a better approximation of bed voidage (which standard DEM approaches underestimate) and stress were obtained in the compression of a synthetic material. This is a significant improvement on existing methods particularly with respect to stress distribution within a bulk particulate system comprising deforming elements where the size and orientation of contact surface between particles has a strong influence on the bulk modulus. The new model was used for prediction of mechanical oil expression in four oilseed beds. Similar patterns in the variation of the characteristic parameters were obtained as observed in existing experimental data. The data could not be matched exactly as the quantity and arrangement of seeds in the initial seedbeds were not the same as those used in the experimental work. However the DE model gave approximate oil point data for seedbeds with the same physical properties and initial conditions as in the experiment. This suggests that the new model may be a useful tool in the study of mechanical seed-oil expression and other agricultural particulate compression processes.

Bulk nanostructured silver components were fabricated from nano-sized powder using a shockwave consolidation technique. The grain size evolution during compaction, the mechanical properties of the bulk components, and the effect of surface finish on the mechanical behavior were studied. X-Ray diffraction, transmission electron microscopy (TEM), atomic force microscopy (AFM), microhardness, compression testing and shear punch testing at room temperature were used to characterize the materials. Upon consolidation, the average grain size calculated from image analysis of the TEM micrographs was 49+/-22 nm, showing the feasibility of maintaining a nanostructure upon dynamic consolidation. The hardness of the bulk nanostructured components was constant across the diameter with an average of 83+/-1 HV. Compression results showed strength about 390+/-10 MPa and ductility of 23+/-2%, which is well above strength level obtainable from strain hardened Ag components. The AFM results show that samples possessing a surface roughness of 267 nm exhibited a brittle behavior and a reduction in strength of 35% when compared to the smoother surfaces. Dimples were observed for the samples exhibiting plasticity, while an intergranular pattern was identified for the brittle materials. Fracture toughness of 0.2 MPa m was calculated, which confirms the strong relationship between fracture toughness and defects observed in nanomaterials.