Щоб переглянути інші типи публікацій з цієї теми, перейдіть за посиланням: High temperature characterisations.
Дисертації з теми "High temperature characterisations"
Оформте джерело за APA, MLA, Chicago, Harvard та іншими стилями
Ознайомтеся з топ-50 дисертацій для дослідження на тему "High temperature characterisations".
Біля кожної праці в переліку літератури доступна кнопка «Додати до бібліографії». Скористайтеся нею – і ми автоматично оформимо бібліографічне посилання на обрану працю в потрібному вам стилі цитування: APA, MLA, «Гарвард», «Чикаго», «Ванкувер» тощо.
Також ви можете завантажити повний текст наукової публікації у форматі «.pdf» та прочитати онлайн анотацію до роботи, якщо відповідні параметри наявні в метаданих.
Переглядайте дисертації для різних дисциплін та оформлюйте правильно вашу бібліографію.
1
Audouard, Lisa. "Conception et caractérisation de matériaux ultra haute température à gradient de propriétés." Electronic Thesis or Diss., Bourgogne Franche-Comté, 2023. http://www.theses.fr/2023UBFCA019.
Повний текст джерелаАнотація:
Le développement d’un nouveau prototype d’ergol vert destiné aux moteurs de repositionnement de satellites implique des conditions thermiques et environnementales plus sévères pour les matériaux de la chambre de combustion, par rapport aux conditions actuelles. De ce fait, des matériaux alternatifs dits Matériaux à Gradient de Propriétés (MGP) sont développés depuis plusieurs années dans le cadre d’une étude ONERA-CNES-ICB. Cette thèse a pour objectif de poursuivre le développement de ce type de matériau à gradient céramique/métal afin d’optimiser sa conception et d’assurer ainsi sa mise en œuvre jusqu’à 2400 °C en présence de vapeur d’eau. Premièrement, différentes configurations de MGP élaborés par projection thermique plasma sous air (APS) ont été testées sous flux thermique laser sous vide jusqu’à 2350 °C. La mise en place d’une modélisation de la fissuration de ces matériaux soumis à un choc thermique a permis de mieux faire le lien entre les dégradations observées et les configurations de MGP. En particulier, il a été montré que l’augmentation de l’épaisseur de la céramique à la surface du MGP est responsable de l’apparition et de la propagation de fissures plus profondes et déviées.Dans un second temps, la possibilité d’utiliser les MGP élaborés dans une ambiance oxydante à ultra haute température a été étudiée au moyen de deux bancs d’essais expérimentaux. Le premier d’entre eux est un banc laser qui a permis de tester les matériaux face à des chocs thermiques répétés jusqu’à 1800 °C et en présence de vapeur d’eau. Les matériaux testés ont présenté une bonne résistance et les mécanismes de dégradation relatifs à l’oxydation du MGP ont pu être identifiés et reliés à aux différentes configurations de MGP et aux conditions d’essai testées. Dans ces conditions, l’augmentation de l’épaisseur de la couche céramique assure une meilleure protection contre l’oxydation. Le second moyen d’essai a permis de qualifier les MGP dans la flamme H2/O2 d’une chambre de combustion. De ce fait, les conditions d’essais étaient relativement proches des conditions réelles visées. Aucune dégradation majeure n’a été relevée à la suite de ces essais en chambre de combustion, ce qui démontre le potentiel de ce type de MGP pour l’application visée.En parallèle, un travail a été mené sur l’amélioration de la partie en céramique du MGP. En effet, le métal utilisé a un coefficient de dilatation thermique deux fois inférieur à celui de la céramique choisie. De ce fait, et malgré la présence du gradient, de fortes contraintes thermomécaniques s’exercent au niveau des interfaces entre les différentes couches du MGP. Ainsi, un point clé de cette étude a consisté à comprendre l’influence de la composition de la céramique et en particulier du taux et de la nature de l’oxyde de terre rare utilisé sur le coefficient de dilatation thermique. De plus, des mesures de conductivité ionique et de conductivité thermique ont permis de rendre compte du rôle de barrière thermique et environnemental de la couche en céramique pure à la surface du MGP. Il a été montré que des compositions à base de forts taux de Lu2O3 étaient les plus prometteuses. Enfin, une dernière partie de cette thèse était consacrée à étudier la possibilité de cicatriser les fissures observées au sein de la couche céramique, apparues à la suite du traitement thermique ou à la suite d’un essai sous flux thermique. Pour cela, un disilicate d’yttrium a été introduit dans la couche en céramique pure du MGP directement au cours de l’élaboration par APS. Son influence sur la résistance des échantillons dans des conditions sévères de température et d’atmosphère a été reportée. En particulier, la présence de ce disilicate est responsable de transformations chimiques au sein du MGP au cours des essais à haute températureThe development of a new green ergol prototype for satellite repositioning engines requires more severe thermal and environmental conditions for combustion chamber materials than is currently the case. As a result, alternative materials known as functionally graded materials (FGM) have been developed for several years as part of an ONERA-CNES-ICB study. The aim of this thesis is to pursue the development of this type of ceramic/metal gradient material, in order to optimize its design and ensure that it can be used up to 2400 °C in the presence of water vapor. Firstly, different configurations of FGM developed by air plasma thermal spraying (APS) were tested under vacuum laser heat flux up to 2350 °C. By modelling the cracking of these materials when subjected to thermal shock, the link between the observed degradations and the FGM configurations was better established. In particular, it has been shown that increasing the thickness of the ceramic on the FGM surface is responsible for the appearance and propagation of deeper, deviated cracks.Secondly, the possibility to use such FGM under an oxidising atmosphere at ultra-high temperature was studied through two experimental set ups. The first one is a laser test bench which allowed to assure the resistance of the materials submitted to repeated thermal schocks up to 1800 °C in presence of water vapour. The tested materials presented an appropriate behaviour under the tested conditions. The degradation mechanisms related to FGM oxidation have been identified and compared from one FGM configuration to another and linked to the tested conditions. The second one permits to qualify the behaviour of FGM in the H2/O2 flame of a combustion chamber. Thus, the tested conditions were relatively close to the ones of the intended application. No major degradation was observed after the combustion chamber tests, which demonstrates the potential of this type of FGM for the application.In parallel, a study was carried out about the improvement of the ceramic part of the FGM. Indeed, the thermal expansion coefficient of the chosen metal is twice lower than the one of the chosen ceramic. Thus, and despite the presence of graded layers in-between the metal and the ceramic, high thermomechanical stresses occur at the interfaces between the different layers of the FGM. Thus, a key point of this study consisted in the understanding of the influence of the ceramic composition, and in particular of the amount and nature of the rare earth oxide, on the thermal expansion coefficient. In addition, ionic conductivity and thermal conductivity measurements most accurately reflect the role of thermal and environmental barrier coating of the pure ceramic layer upon the FGM. It has been shown that high content Lu2O3 based compositions are the most promising to be used for the ceramic composition of the FGM. The last part of this thesis was dedicated to study the possibility to heal the cracks observed in the ceramic, which came either from the thermal treatment, either from the thermal tests. Thus, an yttrium disilicate was introduced in the pure ceramic layer of the FGM directly during the elaboration process with APS. Its influence on the resistance of FGM under harsh thermal and environmental conditions was finally reported. In particular, the presence of this disilicate is responsible of chemical transformations in the FGM during high temperature tests
2
Yasmin, Asma. "Characterisation and evaluation of high temperature composites." Thesis, University of Birmingham, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.391379.
Повний текст джерела
3
Georgiades, Alexis N. "Synthesis and characterisation of novel high temperature polyimides." Thesis, University of Surrey, 2001. http://epubs.surrey.ac.uk/733/.
Повний текст джерела
4
Lee, Elaine Tse Ching. "Synthesis and characterisation of high-temperature superconducting materials." Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.624888.
Повний текст джерела
5
Littleton, C. J. "Characterisation of high temperature corrosion products using Raman microscopy." Thesis, University of Newcastle upon Tyne, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.378937.
Повний текст джерела
6
Frost, A. J. "The characterisation of high temperature superconductors for power engineering." Thesis, University of Oxford, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239287.
Повний текст джерела
7
Hole, Christopher. "Pulsed magnetic field characterisation of technological high temperature superconductors." Thesis, University of Oxford, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.294363.
Повний текст джерела
8
Davies, Michael I. "High temperature nanoindentation characterisation of P91 and P92 steel." Thesis, University of Nottingham, 2013. http://eprints.nottingham.ac.uk/13233/.
Повний текст джерелаАнотація:
Modern demands in power generation call for higher efficiencies from every area of the power plant. One aspect of this is a drive to increase plant operating temperatures placing higher demand on structural materials. P91 and P92 are two steels commonly used in steam pipes. In order to accurately predict the service lifetime of components, mechanical properties at operating temperatures are critical. In particular properties of material around weld fusion joints are of interest as it is in these regions where failures occur. Conventional techniques such as Vicker’s hardness testing and uniaxial tensile testing are used to characterise the mechanical properties and creep behaviour of bulk materials. These techniques are often used to determine the properties of P91 and P92 parent and weld materials, the limitation of these techniques is that they require large volumes of material. They are therefore unable to determine differences in properties through the heat affected zone of the parent material which is typically only a few millimetres across. Nanoindentation is a technique which offers a potential solution to this problem. It was developed in order to examine the properties of thin films and small material volumes. In recent years several approaches have been developed to perform nanoindentation experiments at elevated temperature. These approaches have been examined in order to establish which provides the best thermal stability for high temperature nanoindentation measurements. This technique has then been used to perform high temperature nanoindentation experiments to determine the mechanical properties and creep behaviour of P91 and P92 steel. The correlation between nanoindentation measurements on bulk materials and those obtained using conventional methods is examined. In particular the significance of creep stress exponents calculated from nanoindentation dwell data is discussed. Nanoindentation is then used to characterise the heat affected zone of a weld, giving clear indications of the effects of microstructural differences on the material properties.
9
Gribben, F. "Development and characterisation of high temperature elastomeric polymer blends." Thesis, Queen's University Belfast, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.437869.
Повний текст джерела
10
Manton, Stephen John. "Fabrication and characterisation of high temperature superconducting bulk YBCO." Thesis, University of Southampton, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.396128.
Повний текст джерела
11
Jenkins, Delyth Myfanwy. "The characterisation of chemical species in high temperature lamps." Thesis, University of Southampton, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239677.
Повний текст джерела
12
Mason, Amy. "Single-Channel Characterisation of Potassium Channels with High Temperature Studies." Thesis, University of Oxford, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.491373.
Повний текст джерелаАнотація:
Potassium channels control the conduction of K+ across cell membranes, down their electrochemical gradient. This rapid and highly selective movement of K+ is essential to many biological processes. K+ channels are largely alpha helical, tetrameric proteins that span the lipid bilayer. Diversity in K+ channels arises primarily in the mechanism of gating by various ligands or voltage; however, the basic structural elements, notably the selectivity filter are conserved within the family. Studies ~n this thesis focus on the single-channel behaviour of the K+ channels KcsA and Kcv. KcsA is a proton-activated channel from the bacterium Streptomyces lividans and its crystal structure was the first of a K+ channel to be solved. Kcv expressed by the Paramecium bursaria Chlorella virus is the smallest known K+ channel and thus represents the minimal structural entity necessary to form a functional and selective pore. Studies on the bacterial inward rectifying channels, KirBacs, have also been initiated. The KirBacs are a superfamily of prokaryotic channels homologous to eukaryotic Kir channels. In this work, KcsA and Kcv were found to form stable tetramers, which can be expressed by coupled in vitro transcription and translation and purified by polyacrylamide gel electrophoresis. The purified tetramers were reconstituted into planar lipid bilayers and studied at the single-channel level. Through single-channel recordings the ionic selectivity, gating behaviour, functional effects of site-directed mutagenesis and the interaction between Kcv and blockers have been studied. In addition, single-channel studies at elevated temperatures have revealed the remarkable thermostability of Kcv, as well as insight into the transport of ions through a narrow and selective pore. Through temperature studies, it has been possible to obtain the thermodynamic and kinetic parameters describing Kcv activity.
13
Martin, Lucy Claire. "Characterisation of silicon carbide CMOS devices for high temperature applications." Thesis, University of Newcastle upon Tyne, 2015. http://hdl.handle.net/10443/3030.
Повний текст джерелаАнотація:
In recent years it has become increasingly apparent that there is a large demand for resilient electronics that can operate within environments that standard silicon electronics cease to function such as high power and high voltage applications, high temperatures, corrosive atmospheres and environments exposed to radiation. This has become even more essential due to increased demands for sustainable energy production and the reduction in carbon emissions worldwide, which has put a large burden on a wide range of industrial sectors who now have a significant demand for electronics to meet these needs including; military, space, aerospace, automotive, energy and nuclear. In extreme environments, where ambient temperatures may well exceed the physical limit of silicon-based technologies, SiC based technology offers a lower cost and a smaller footprint solution for operation in such environments due to its advantageous electrical properties such as a high breakdown electric field, high thermal conductivity and large saturation velocity. High quality material on large area wafers (150 mm) is now commercially available, allowing the fabrication of reliable high temperature, high frequency and high current power electronic devices, improving the already optimised silicon based structures. An important advantage of SiC is that it is the only wide band gap compound semiconductor that can be thermally oxidised to grow insulating, high quality SiO2 layers, which makes it an ideal candidate to replace silicon technologies for metal-oxide-semiconductor applications, which is the main focus of this research. Although the technology has made a number of major steps forward over recent years and the commercial manufacturing process has advanced significantly, there still remains a number of issues that need to be overcome in order to fully realise the potential of the material for electronic applications. This thesis describes the characterisation of 4H-SiC CMOS structures that were designed for high temperature applications and fabricated with varying gate dielectric treatments and process steps. The influence of process techniques on the characteristics of metal-oxide-semiconductor (MOS) devices has been investigated by means of electrical characterisation and the results have been compared to theoretical models. The C-V and I-V characteristics of both MOS capacitor and MOSFET structures with varying gate dielectrics on both n-type and p-type 4H-SiC have been analysed to explore the benefits of the varying process techniques that have been employed in the design of the devices. The results show that the field effect mobility characteristic of 4H-SiC MOSFETs are dominated at low perpendicular electric fields by Coulomb scattering and at high electric fields by low surface roughness mobility, which is due to the rough SiC-SiO2 interface. The findings also show that a thermally grown SiO2 layer at the semiconductor-dielectric interface is a beneficial process step that enhances the interfacial characteristics and increases the channel mobility of the MOSFETs. In addition to this it is also found that this technique provides the most beneficial characteristics on both n-type and p-type 4H-SiC, which suggests that it would be the most suitable treatment for a monolithic CMOS process. The impact of threshold voltage adjust ion implantation on both the MIS capacitor and MOSFET structures is also presented and shows that the increasing doses of nitrogen that are implanted to adjust the threshold voltage act to improve the device performance by acting to modify the charge at the interface or within the gate oxide and therefore increase the field effect mobility of the studied devices.
14
Trimble, Rachel Jane. "Material and process characterisation of PolyEtherKetone for EOSINT P800 high temperature laser sintering." Thesis, University of Exeter, 2017. http://hdl.handle.net/10871/33116.
Повний текст джерелаАнотація:
Laser Sintering (LS) is a powder based Additive Manufacturing (AM) technology capable of producing near-net shape objects from 3D data. The benefits of LS include almost unlimited design freedom and reduced material waste, however the number of commercially available materials are limited, with materials traditionally being optimised for the process using a trial and error method and material development being led by previous research into polyamide (PA). There is a desire for greater material choice in LS, particularly high performance polymers. The EOSINT P800 by AM systems manufacturer EOS GmbH is the first commercially available high temperature laser sintering (HT-LS) system capable of working high performance polymers; a PolyEtherKetone (PEK) known by the trade name HP3 PEK is the first material offered by EOS for use with the system. This research project undertakes to characterise the EOSINT P800 and HP3 PEK material with different thermal histories. Experimental work focusses on establishing material properties such as size and shape, crystallinity and decomposition. Characterisation of coalescence behaviour and comparison with theoretical models for viscous sintering is presented as a less experimentally intensive method of understanding how a material will behave during the LS process. A map of temperatures inside the powder bed in the EOSINT P800 is created for the first time and compared with output from on-board temperature sensors in the system, demonstrating the thermal distribution within the bed during building, and explaining differences between as-received and used powder. The results demonstrate that material and process characterisation methods are useful for understanding how and why a high temperature laser sintering material behaves the way it does. The behaviour of HP3 PEK observed during experimental work indicates that guidelines based on LS of PA are too restrictive as indicators of suitability for LS and newer systematic approaches are potentially better suited for qualification of HT-LS polymers. The novel method for mapping thermal distribution inside the LS system documented here shows the limitations of current hardware to effectively process high performance polymers. Overall, the finding of this research project is that understanding of material and process cannot be considered in isolation but combined have the potential to reduce the amount of trial and error required during qualification of new materials and increase the range and variety of polymers available for LS and HT-LS.
15
McDowell, John Christopher. "Preparation characterisation and stability of selected high critical temperature ceramic superconductors." Thesis, University of Ulster, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.260854.
Повний текст джерела
16
Al-Mosawi, Maitham Khazal. "The fabrication and characterisation of High Temperature Superconducting tapes and coils." Thesis, University of Southampton, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.287318.
Повний текст джерела
17
Lasfargues, Mathieu. "Nitrate based high temperature nano-heat-transfer-fluids : formulation & characterisation." Thesis, University of Leeds, 2014. http://etheses.whiterose.ac.uk/7531/.
Повний текст джерелаАнотація:
This work relates to the development of high temperature heat-transfer-fluid with enhanced specific heat capacity using nano-particle additives. A eutectic mixture of nitrate (60 wt% NaNO3 & 40 wt% KNO3) was produced through ball-milling and characterised on DSC, TGA, Rheometer. The results obtained showed that the salt mixture melted at 221°C with a heat of fusion of 97 J/g. Onset of melting was seen at 215°C whilst crystallisation started at 219°C, reaching a solid state below 217°C with an enthalpy of 97 J/g. Displaying very little overcooling, the salt showed specific heat capacity of 1.41 J/[°C*g] at 260°C to 1.44 J/[°C*g] at 440°C with viscosity values changing from 4.8 cP at 250°C to 1.7 cP at 450°C for this Newtonian fluid. Thermal decomposition of the salt showed that it was stable up to 550°C. The addition of nano-particles displayed an overall positive effect toward the specific heat capacity enhancing the latter whilst reducing the onset of melting due to increased entropy. The addition of 0.1, 0,5 and 1.0 wt% copper oxide gave the best results with increase of 10.5%, 9,2% and 8,5% in specific heat capacity respectively. SEM analysis of the samples showed that the nano-particles clearly disrupted the crystallisation structure showing a rougher organisation. Rheological tests on 0.1 wt% CuO demonstrated a slight rise in viscosity due to the addition of nano-particles. The stability of 0.1 wt% CuO was tested in large scale rigs (>1.0 kg) and demonstrated that sedimentation of nano-particles did occur. Different manner of dispersion were tested and revealed that they each affected the specific heat capacity differently with some causing negative enhancements whilst others were positive. The method of production did not affect the specific heat capacity values, and current theories point toward the formation of liquid nano-layers as a reason toward this increase.
18
Pullinger, Mark A. "Synthesis and characterisation of novel polymeric materials for high temperature composite applications." Thesis, University of Surrey, 2002. http://epubs.surrey.ac.uk/843754/.
Повний текст джерелаАнотація:
Attempts have been made to synthesise a novel siloxane-containing dicyanate ester, 1,3-bis(4-cyanatophenyl)-1,1,3,3-tetramethyldisiloxane, commencing from 4-bromophenol or 4-methoxyphenol. Most of these syntheses have involved the use of a Grignard reaction to form a disiloxane group, followed up subsequent cleavage of an alkyl-protecting group to yield a free phenol. Several different ethers have been attempted, but none was successful. The extreme susceptibility of the aryl silicon-carbon bond to cleavage under acidic conditions has been demonstrated by these reactions. A novel phosphazene-containing dicyanate ester, 1,3-bis(4-cyanatophenyl)-1,3,5,5-tetraphenoxycyclotriphosphazene has been successfully synthesised in a four-step procedure with an overall yield of ca. 35%. The cyanate ester has been characterised by differential scanning calorimetry (DSC), thermogravimetric (TG) analysis, thermomechanical analysis (TMA), 1H, 13C and 13P NMR spectroscopy, and Fourier-Transform Infra-red (FT-IR) and Raman spectroscopy. The thermal data collected showed a surprisingly low glass transition temperature (59 °C) and enthalpy of polymerisation (ca. 36-5 kJ mol-1 of cyanate). The former figure was confirmed by TMA and modulated DSC. However, the cured cyanate showed reasonably high thermal stability: no mass loss was observed before 300 °C. Total decomposition did not occur until about 650 °C.
19
Cedillos, Barraza Omar. "Process development and characterisation of (Ta,Hf)C ultra-high temperature ceramics." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/32138.
Повний текст джерелаАнотація:
Tantalum carbide (TaC), hafnium carbide (HfC) and compounds in the TaC-HfC system have extremely high melting points (>3700°C) making them potential candidates for thermal protection structures in hypersonic space vehicles. Information regarding mechanical and thermal properties of these compounds and their solid solutions is scarce. Synthesis and sintering of 4TaC-1HfC compounds was conducted using efficient reactive routes using self-propagating high temperature synthesis (SHS) and a spark plasma sintering (SPS) furnace. Reactive one-step reactive spark plasma sintering (RSPS) and a combination of SHS+SPS were used as the processing routes to produce TaC-HfC ceramics. Relative density >98% was achieved by the SHS+SPS method without sintering aids at 2100°C for 20 min and 60 MPa. Product conversion of the reactants after SHS and after sintering was characterised by XRD. Analysis of microstructures was conducted by SEM and EDS. TaC, HfC and different TaC-HfC compounds were sintered using SPS at temperatures up to 2450°C using commercial powders of TaC and HfC. Microstructural evolution and solid solution formation was analysed in 4TaC-1HfC ceramics fabricated using SPS from 2050-2450°C. XRD, SEM and EDS were used to analyse the formation of (Ta,Hf)C solid solutions. TEM was conducted and the diffusion mechanisms during sintering were analysed. Single-phase solid solutions were formed at sintering temperatures ≥2350°C for 20 min and 30 MPa. In addition, TaC, HfC, 1TaC-1HfC and 1TaC-4HfC ceramics were sintered by SPS at 2350°C. Measurements of mechanical properties (hardness, elastic modulus and fracture toughness) and thermal properties (thermal diffusivity, thermal conductivity and coefficient of thermal expansion) are reported. Melting temperatures (Tm) were reassessed using a laser melting technique with a 4.5 kW, 1064 nm Nd:YAG CW laser programmed to deliver pulses with time ranging from 100 to 1000 ms and power up to 3980 W. HfC showed the highest melting temperature at 3959 ± 50 °C, and the highest melting temperature for any known compound. Tm for TaC was measured at 3768 ± 40 °C and the solid solutions fall in between the single member carbide values with 4TaC-1HfC at 3905 ± 40 °C. Microstructural characterisation using SEM and TEM on samples after the laser testing experiments is reported.
20
Jarvis, Andrew John Christopher. "Simulation of ultrasonic monitoring data to improve corrosion characterisation within high temperature environments." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/39351.
Повний текст джерелаАнотація:
Practical applications which involve analyzing how waves scatter from objects with complex shapes span countless scienti c and engineering disciplines. Having been the focal point of much research over the past century, many di erent techniques for simulating such interactions are in common use throughout literature; however there is still an opportunity to improve upon the balance between accuracy and e - ciency o ered by the most commonly implemented methods. A simulation based on the scalar wave distributed point source method is proposed, exhibiting a large im- provement in computational e ciency when compared to the nite element method, and providing greater accuracy than the Kirchho approximation by including phe- nomena such as multiple scattering, surface self-shadowing and edge di raction. The technique is applied to the problem of simulating how ultrasonic pulses re- ect from rough surfaces; the practical application being wall thickness monitoring in high temperature and corrosive environments. Results show that the re ected pulse can take any number of forms, depending on the speci c shape of the scat- tering surface, which can have a dramatic impact on the accuracy of the thickness measurement. Conclusions are drawn about the stability of various time of ight algorithms under conditions of increasing surface roughness. Potential thickness er- ror metrics are also proposed with the aim of estimating measurement uncertainty based on signal shape change. The great e ciency of the simulation technique is further demonstrated by applying it to three dimensional scattering scenarios which would be impossible to carry out using any other method, leading to the proposal of a correction procedure capable of converting results gained in two dimensional geometries to more closely resemble three dimensional results based on the speci c transducer and rough surface characteristics. Simulation validation is carried out by comparison to experimental results in both two dimensional and three dimensional scattering scenarios, showing agreement within the experimental error bounds of the shear horizontal ultrasonic waveguide transducers used by the wall thickness sen- sor. Alternative high temperature structural degradation monitoring applications are also proposed and experimentally veri ed using an array of waveguide transduc- ers, providing monitoring solutions for thermal fatigue crack growth and hydrogen attack.
21
Day, Trevor Neil. "Characterisation of high temperature molecules by matrix isolation infrared spectroscopy and mass spectrometry." Thesis, University of Southampton, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.240576.
Повний текст джерела
22
Duncan, Fiona Hazel. "Characterisation of superconducting Nd123 solid solutions and related phases." Thesis, University of Aberdeen, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.297558.
Повний текст джерелаАнотація:
The stoichiometry of the Nd1+xBa2-xCu3O7-δ solid solution has been investigated using XRD and EPMA. At 980°C in air, an essentially continuous solid solution forms with limiting compositions xmin = 0.03(1) and xmax = 0.92(2). The solid solution limits are independent of temperature over the range 300 to 1050°C, i.e. stoichiometric Nd123 does not form. Preliminary studies show that annealing in an Ar atmosphere does not affect xmin. Three structurally distinct polymorphs of Nd123ss exist - tetragonal Nd123ss, orthorhombic Nd123ss and orthorhombic Nd123ss. The stability range of each in air has been determined. Quenched samples with 0.03 ≤ x ≤ 0.6 have the tetragonal Nd123ss structure. On oxygenation, samples with 0.03 ≤ x <0.2 are orthorhombic. The orthorhombic-tetragonal phase transition is second order, both with increasing temperature and increasing x. Samples with 0.7-0.9 have the orthorhombic Nd213ss structure at all oxygen contents. Tetragonal Nd123ss is isostructural with tetragonal Y123 and orthorhombic Nd123ss is isostructural with orthorhombic Y123. Orthorhombic Nd213ss has the ideal stoichiometry Nd2BaCu3O7-δ and is based on a 2a x b x 2c superstructure of the Nd123ss structure. The supercell is due to ordering of the Nd and Ba atoms, which leads to ordering of the oxygen atoms. Melting temperatures decrease with x. Two distinct regions of melting behaviour are observed; the first for 0.03 ≤ x ≤ 0.6 with a thermal minimum at x0.4, and the second for 0.7 ≤ x ≤ 0.9. Oxygen contents increase with x. Samples with larger x values have a smaller range of oxygen contents. High pressure oxygen annealing results in a constant Cu valence state of 2.35 for all values of x. Average copper valence states <2 are only obtained readily for x ≤ 0.3. Tc decreases with x and samples become non-superconducting at x0.5. For samples annealed in 1 bar O2, 'double plateau' behaviour is observed.
23
Viklund, Peter. "High temperature corrosion during waste incineration : characterisation, causes and prevention of chlorine-induced corrosion." Licentiate thesis, Swerea KIMAB AB, Stockholm, Sweden, 2011. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-32412.
Повний текст джерелаАнотація:
Waste-fired boilers suffer severely from corrosion of critical components such as superheater tubes. In this work the high temperature corrosion of candidate superheater alloys have been investigated by detailed laboratory studies and controlled field exposures in full-scale boilers. In a laboratory study the detrimental effect of gaseous hydrochloric acid (HCl) on three different ground surface and preoxidised austenitic stainless steels was investigated. Exposures were conducted in an environment comprising N2-10O2-5H2O-0.05HCl at both 400 °C and 700 °C. A positive effect of preoxidation is evident when the alloys are exposed at 400 °C. Oxide layers formed during preoxidation effectively suppress chlorine ingress and lower the corrosion rate for all three materials while accelerated corrosion and chlorine accumulation at the metal/oxide interface is detected for ground surface specimens. The positive effect of preoxidation is lost at 700 °C and corrosion resistance is dependent on alloying level. At 700 °C metal chloride evaporation contributes significantly to the material degradation. Based on the results, high temperature corrosion in the presence of gaseous HCl is discussed in general terms. In two different waste-fired boilers measures for counteracting superheater corrosion were investigated. In a grate-boiler the deposit formation and high temperature corrosion of some candidate superheater materials were studied. Metal loss measurements showed unacceptably high corrosion rates for the lower alloyed ferritic steels 13CrMo44 (Fe-1Cr-0.5Mo) and HCM12A (Fe-11Cr-2W), as well as for the austenitic Super 304 (Fe-18Cr-9Ni-3Cu). The corrosion attack for these alloys was manifested by the formation of mixed metal chloride/metal oxide scales. A different type of behaviour was seen for the higher alloyed austenitic steels and nickel-base alloys, which were able to form a chromium-enriched oxide next to the metal. However, the alloys suffered from localised pitting attack. Since analyses of the deposit revealed appreciable amounts of low melting salt mixtures such as ZnCl2-KCl, PbCl2-KCl, FeCl2-KCl and NaCl-NiCl2, oxide dissolution in these molten salts is the probable reason for pitting attack. In a waste-fired boiler ammonium sulphate solution was added to the flue gas and the effect on flue gas and deposit composition was evaluated. It was evident that the sulphur-rich additive reduced the amount of alkali chlorides in both the flue gas and the deposit. Results also indicated that the initial corrosion rates were lowered with the use of ammonium sulphate. It was concluded that using the additive could be a possible strategy for changing the flue gas chemistry so that superheater corrosion is mitigated.QC 20110414
24
Heath, David John. "Characterisation of waxy gas-condensates by high temperature capillary gas chromatography and oxidative degradation." Thesis, University of Plymouth, 1995. http://hdl.handle.net/10026.1/460.
Повний текст джерелаАнотація:
High molecular weight (HMW) hydrocarbons (defined herein as C35+ compounds) are difficult to characterise by conventional analytical methods. Very few studies have reported precise and reproducible quantification of such compounds in fossil fuels. Nonetheless, such components have important effects on the physical and biological fate of fossil fuels in the geosphere. For example, the phase behaviour of waxy gas condensates is significantly affected by the varying proportions of HMW compounds. Similarly HMW compounds are amongst the most resistant petroleum components to biodegradation. The current study reports the development of reproducible quantitative high temperature capillary gas chromatography (HTCGC) methods for studying both these aspects of the chemistry of HMW hydrocarbons. In addition those hydrocarbons which remain unresolved when analysed by gas chromatography (so called unresolved complex mixtures UCMs) are also studied. UCMs may account for a large portion of the hydrocarbons in many fossil fuels yet very little is known about their composition. Knowledge of these compounds may be important in enhancing the prediction of phase behaviour. Oxidative degradation and GC-MS is used to elucidate the types of structures present within the UCM. The concentrations of C3S4h. ydrocarbons in two unusually waxy gas condensates from high temperature wells in the North Sea were determined by HTCGC. The whole C, 5+ fraction comprised about 20% of the total hydrocarbons and consisted of compounds with carbon numbers extending up to and beyond Coo. By paying particular attention to sample dissolution and injection, good reproducibility and precision were obtained. For example, for authentic n-C, to n-C60 alkanes a relative standard deviation of under 5% for manual injection, linear response factors (1.01 Cm to 0.99 C6), and a linear calibration for 5 ng to 25 ng on-column were found. Limits of detection are reported for the first time for HMW n-alkanes. The limits were found to be as low as 0.8 ng for Cto to 1.87 ng for C60. Tristearin is proposed as a suitable HTCGC internal standard for quantification since the FID response factor (1.1) was close to that of the HMW n-alkanes and response was linear. Importantly, when co-injected with the two waxy North Sea condensates, tristearin was adequately separated from the closest eluting alkanes, n-C59 and n-C60 under normal operating conditions. Qualitative characterisation of the HMW compounds in the waxy gas condensates and in synthetic wax blends (polywax 1000) using HTCGC-EI MS and HTCGC-CI MS produced molecular ions or pseudo molecular ions for n-alkanes up to n- C6o. The spectra of some HMW compounds contained fragment ions characteristic of branched compounds but detailed characterisation was very limited. This study has also shown, for the first time, the significance of the unresolved complex mixture in gas condensatesU. CM hydrocarbonsa ccountedf or over 20% of the total hydrocarbons in a waxy North sea condensateT. he condensatew as first distilled and the distillate UCMs isolated. Thesew ere found to be between 64 to 97 % unresolved after molecular sieving (5A) and urea adduction. The UCMs were oxidised using CrO3/AcOHw hich produced5 -12% C02, and 55-83% dichloromethane-solublep roducts. Thus 65-94% of the original UCMs were accounted for as oxidation products. The remainder were thought to be water soluble acids which could not be determined in the presence of the AcOH reagent. Of the recovered oxidised products, 27- 81 % were resolved and these comprised mainly n-monocarboxylic acids (19-48 %). The average chain length was found to be C12 indicating the average length of alkyl groups. Branched acids, ketones, ketoacids, ndicarboxylic acids, branched dicarboxylic acids, lactones, isoprenoid acids, alkylcyclohexane carboxylic acids and toluic acids accounted for the majority of the remaining resolved products. The distillate UCMs all showed variations in amountso f productsb ut not in composition. Retro-structurala nalysis suggestedth at the UCM in the gasc ondensatew as mainly aliphatic and branched.T he numbero f isomerso f simple brancheda lkaneso ver the UCM molecular weight range (determined by cryoscopy) was calculated to be over 15000. Overall, oxidation provided structural information for about half of the UCM. HTCGC was also used to measure the biodegradability of HMW alkanes in a waxy Indonesian oil. Traditional alkane isolation techniques (TLC and CC) discriminated against HMW compounds above C40 whereas adsorption onto alumina in a warm cyclohexane slurry provided an aliphatic fraction still rich in HMW compounds and suitable as a biodegradation substrate. A waxy Indonesian oil was subjected to 136 day biodegradation by Pseudomonas fluorescens. Extraction efficiencies of over 90 % (RSD <5 %) were obtained for n-alkanes up to C6o using continuous liquid-liquid extraction. Over 80 % of the oil aliphatic fraction was degraded within 14 days. After 136 days only 14% of the original aliphatic fraction remained, yet surprisingly no decreases in the concentrations of compounds above C45 were observed. However, the use of a rapid screening biodegradation method proved conclusively that Pseudomonasfluorescens was capable of utilising n-alkanes up to C60 once the bacteria had acclimated to the HMW alkanes. This is the first report of bacterial utilisation of an n-alkane as large as C.
25
Pal, Suchandan. "Characterisation and high-temperature sensing potential of fibre Bragg gratings in specialised optical fibres." Thesis, City University London, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.407547.
Повний текст джерела
26
Spittle, Peta Jean. "Characterisation of high temperature metal halides by mass spectrometry and matrix isolation infrared spectroscopy." Thesis, University of Southampton, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.242391.
Повний текст джерела
27
Mogire, Evans O. "Microstructural characterisation of high temperature oxidation of boiler materials for coal fuelled power plants." Thesis, Loughborough University, 2013. https://dspace.lboro.ac.uk/2134/13189.
Повний текст джерелаАнотація:
Due to the projected increase in global electricity demand, it is estimated that nearly 1400GW of new coal fired power plants will be built providing about 38% of global electricity demand in 2030. This growth will have a negative impact on the environment through the emission of CO2, a greenhouse gas detrimental to the climate, unless stringent emission targets for the coal fired power plants are put in place. This has resulted in placing more emphasis on the need for adopting the best available technologies for ‘new built' plants or through retrofit of existing plants or the construction of high efficiency power plants with CO2 capture and storage technologies. The high efficiency power plants are mainly achieved by operating the boilers at higher temperatures of up to 700°C compared to the conventional power plants operating at ~ 565°C. This expected temperature increase will have an effect on the material degradation mechanisms of both the existing and proposed new alloy materials for the critical components of the boilers, that is, headers, superheaters, reheaters and wall membranes. This thesis explores the material degradation mechanisms associated with the high temperature oxidation and/or corrosion of the alloy materials found in these components.
28
Qian, Lingyi. "Fabrication, characterisation and oxidation behaviour of Pt-based bond coats for high temperature applications." Thesis, University of Nottingham, 2017. http://eprints.nottingham.ac.uk/39531/.
Повний текст джерелаАнотація:
Over the years, platinum modified aluminide (PtAl) bond coats have been used in high temperature protection systems for commercial aircraft engines to protect turbine blades from high temperature oxidation and corrosion. In the process of operation, the oxidation and rumpling behaviours of the bond coats are the key factors in determining the lifetime of the thermal barrier coating. The latest development in aircraft engines has given rise to the need for bond coats with better oxidation and rumpling resistance at high temperature. This investigation aims to improve the oxidation and rumpling resistances of general PtAl bond coats by introducing ZrO2 nanoparticles, Ir and Pd additions: ZrO2 nanoparticles were incorporated into PtAl bond coats by co electrodeposition in an attempt to enhance their performance by exploiting the effect of reactive element (Hf, Zr, etc.) oxides. PtAl coatings with and without ZrO2 nanoparticles were deposited onto three commercially available Ni-based superalloys, namely, Mar-M-247, Mar-M-246 and Inconel 718. Thermal cycling oxidation tests were performed to evaluate the influence of ZrO2 addition and substrate composition. The addition of ZrO2 particles to PtAl coatings on Mar M 246 and Inconel 718 appeared to accelerate the growth of thermally grown oxide and reduced the rumpling of thermally grown oxide. However, when depositing PtAl coatings with ZrO2 nanoparticles on Mar-M-247, there was no changes, as mentioned above, in thermally grown oxide. Through the analysis of the coatings on different substrates, the interactions among Hf, Al and ZrO2 have been revealed and clarified, which offers a better understanding of the reactions of ZrO2 and the influence of substrate composition on bond coats. Ir and Pd coatings were deposited by means of electroplating and electroless plating respectively to explore the effects of other platinum group metals. Ir-modified PtAl coatings were investigated on Mar-M-246 with different vacuum annealing durations before the cyclic thermal test, which presented a significant reduction in growth rate of TGO and inward diffusion of Al in the oxidation test. Instead of oxygen diffusion barrier, Ir exhibited a diffusion barrier to Al possibly by forming a high temperature solid solution with Al, which has a higher melting point than general β-NiAl phase. As a result, it restrained the inward diffusion of Al and slowed down the outward diffusion of Al, thus improving the oxidation resistance. In addition, Ir concentration produced an effect on the diffusion barrier of Ir. When it is reduced lower than 15 wt.%, the barrier effect of Ir begins to degrade, which provides a better comprehension of the diffusion barrier effect of Ir. However, Pd-modified PtAl coating did not strengthen oxidation resistance, which may be caused by the phosphorus precipitates generated from the electroless plating process. ZrO2 and Pd-modified PtAl bond coats were further investigated with 8 wt.% yttria stabilised zirconia top coat. Comparison between conventional PtAl and high velocity oxygen fuel sprayed NiCoCrAlY bond coats was made as well. Isothermal and cyclic thermal oxidation tests were carried out to evaluate the prepared bond coats. ZrO2 modified PtAl coating showed the best oxidation and rumpling resistance in cyclic thermal oxidation, while Pd-modified PtAl coating was worse than conventional PtAl coating and better than the NiCoCrAlY coating. It has been observed that ZrO2- and Pd-modified PtAl bond coats are affected by different methods of oxidation test. Isothermal oxidation has a greater impact on ZrO2- and Pd-modified PtAl bond coats. The difference is considered to be a consequence of long term annealing in isothermal oxidation, which leads to a fast grain boundary diffusion of Al.
29
Pedrazzini, Stella. "Characterisation and mechanical properties of bulk nanostrictured Al-based composites for high temperature applications." Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:4b8b77b7-a522-403a-b1a3-73d3399486a4.
Повний текст джерелаАнотація:
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.
30
Saleh, Paul Matthew. "Characterisation of practical high temperature superconductors in pulsed magnetic fields and development of associated technology." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.365786.
Повний текст джерела
31
Pandiman, Dien. "Synthesis and characterisation of novel water-soluble, electrically conducting high temperature polyimides and their precursors." Thesis, University of Huddersfield, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.363238.
Повний текст джерела
32
Dudley, Richard Anthony. "The fabrication, thermomechanical processing and characterisation of high temperature superconducting Bi-2212 tapes for applications." Thesis, University of Southampton, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.409756.
Повний текст джерела
33
Wang, Jing. "Electrodeposition and characterisation of nickel-niobium-based diffusion barrier metallisations for high temperature electronics interconnections." Thesis, Loughborough University, 2016. https://dspace.lboro.ac.uk/2134/20590.
Повний текст джерелаАнотація:
The control of interfacial microstructural stability is of utmost importance to the reliability of liquid solder interconnects in high temperature electronic assemblies. This is primarily due to excessive intermetallic compounds (IMCs) that can form and continuously grow during high temperature operation, which practically renders conventional barrier metallisations inadequate. In this study, electrically conducting, NbOx containing Ni coatings were developed using electrodeposition. Their suitability as a solder diffusion barrier layer was assessed in terms of the electrical conductivity and barrier property. The present work explores a novel electrochemical route to produce Ni-NbOx composite coatings of good uniformity, compactness and purity, from non-aqueous glycol-based electrolytes consisting of NiCl2 and NbCl5 as metal precursors. The effects of cathodic current density and NaBH4 concentrations on the surface morphology, composition and thickness of the coatings were examined. A combined study of Scanning Transmission Electron Microscopy (STEM) and Electrochemical Quartz Crystal Microbalance (EQCM) was conducted to understand the fundamental aspects of this novel electrodeposition process. The composite coatings generally exhibited good electrical conductivity. The reaction behaviour between a liquid 52In-48Sn solder and Ni-NbOx, with Nb contents up to 6 at.%, were studied at 200°C. The results indicate that, Ni-NbOx with sufficient layer thickness and higher Nb content, offered longer service lifetime. Nb enrichment was generally observed at or close to the reaction front after high temperature storage, which suggests evident effectiveness of the enhanced diffusion barrier characteristics.
34
Li, Aihua. "A study of the fabrication and characterisation of high temperature superconductor YBa₂Cu₃O₇ thin films." Access electronically, 2006. http://www.library.uow.edu.au/adt-NWU/public/adt-NWU20060928.143000/index.html.
Повний текст джерела
35
Moussa, Mohamed A. A. "Fabrication and characterisation of Nd-123 high-temperature superconducting thick films on yttria stabilized zirconia substrates." Thesis, University of Birmingham, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.288886.
Повний текст джерела
36
Smith, J. J. "Microstructural characterisation of duplex 316 weld metals : The effects on the mechanical and high temperature properties." Thesis, University of Southampton, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.384218.
Повний текст джерела
37
Marsh, James W. "In silico and functional characterisation of the high temperature requirement a (HtrA) protease from Chlamydia trachomatis." Thesis, Queensland University of Technology, 2015. https://eprints.qut.edu.au/83818/1/James_Marsh_Thesis.pdf.
Повний текст джерелаАнотація:
This project used modelling, biochemical, and genetic approaches to investigate the physiological function of the HtrA protease in C. trachomatis. This organism is responsible for several human diseases, however our understanding of how it causes these diseases is limited. By focusing on a single C. trachomatis protein, HtrA, we were able to demonstrate the protein is a unique protease/chaperone which is important for the replicative phase of the organism. This project has identified HtrA as one of the key components for the pathogenesis of C. trachomatis and will guide the generation of new therapeutics that target this protein.
38
Bonfils, Laure. "Characterisation of the high strain rate deformation behaviour of α-β titanium alloys at near-transus temperature". Thesis, University of Oxford, 2017. https://ora.ox.ac.uk/objects/uuid:e2507c22-6478-4461-be57-347382a5ee0c.
Повний текст джерелаАнотація:
The aim of this thesis is to provide microstructural and mechanical characterisation of α-β titanium alloys exposed to a range of thermo-mechanical conditions, in particular under-going high rate deformation at elevated temperatures, representative of the Linear Friction Welding (LFW) manufacturing process. Three α-β titanium alloys provided by Rolls-Royce are studied: Ti-64 blade, disc and Ti-6246 disc. Ti-64 and Ti-6246 show complex deformation behaviour with strain, strain rate and temperature, especially near the transus temperature, where the low temperature α phase is transformed into the high temperature β phase. The microstructure and mechanical properties evolve in an interconnected fashion, and understanding this mutual influence is necessary to better predict the behaviour of these alloys. Characterisation of the mechanical properties was performed through uniaxial compression tests at strain rates from 0.001 to 3000 s-1, using an Instron screw-driven machine at quasi-static rates, a servo-hydraulic machine at medium rates and a Split-Hopkinson Pressure Bar and a drop-weight tower at high strain rates. The tests were performed over a range of temperatures from room temperature to 1300 °C. The main focus was on high strain rate and high temperature tests, with the development of a gravity driven direct impact Hopkinson bar, referred as a drop-weight system, which is intended to evaluate the mechanical response of metals to high strain rate loading at temperatures up to c. 1300 °C. The design and principles of operation of the system are presented, along with calibration and validation data. Preliminary tests were performed on stock Ti-64, heated at two rates: 1 and 20 °C s-1. The evolution of the mechanical properties was analysed, focussing on the strain rate, temperature and phases dependencies. Characterisation of the microstructure was realised by performing interrupted compression tests, first at room temperature, three plastic strains, 4%, 10% and 20%, and two different strain rates, 0.001 and 2000 s-1; then at 4% plastic strain, a strain rate of 2000 s-1 and three elevated temperatures, 700, 900 and 1100 °C. A better understanding of the microstructure evolution with strain, strain rates and temperature, including the macrotexture and microtexture of the specimens, was obtained using Electron Backscatter Diffraction (EBSD) to characterise the texture of the undeformed and deformed materials. The better understanding of the flow stress and microstructural evolution of both Ti-64 and its individual α and β phases with various strain rates and temperatures is intended to be used in the development of more accurate models representing the behaviour of these alloys. Predicting the microstructure evolution and then the mechanical properties of a material is essential to optimise the final mechanical properties of the alloys when welded by manufacturing processes such as the LFW process.
39
White, Richard James. "Synthesis and characterisation of complex sulfide materials with potential use as high temperature inorganic sulfide-ion conductors." Thesis, Loughborough University, 2006. https://dspace.lboro.ac.uk/2134/7824.
Повний текст джерела
40
Palai, Ratnakar. "Growth, optimisation and characterisation of pulsed laser deposited Nd-based high temperature superconducting thin films for device applications." Thesis, University of Strathclyde, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.405145.
Повний текст джерела
41
Bennett, Marc. "Synthesis of high temperature superconductors HgBaâ†2CuOâ†4â†+â†#delta# and YBaâ†2Cuâ†3Oâ†7â†-#delta# and characterisation by 1/f noise." Thesis, University of Bristol, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.297841.
Повний текст джерела
42
Vikhareva, Anna. "Tribological characterisation of additively manufactured hot forming steels." Thesis, Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-80588.
Повний текст джерелаАнотація:
Over the last decade, the application of ultra-high strength steel as safety components and structural reinforcements in automobile applications has increased due to their favourable high-strength-to-weight ratio. The complex shaped components are widely produced using hot stamping. However, this process encounters problems such as galling and increased wear of the tools due to harsh operating conditions associated to the elevated temperatures. Moreover, quenching is a critical step that affects the hot formed components. Slow cooling rates results in inhomogeneous mechanical properties and increased cycle time. Therefore, fast and homogeneous quenching of the formed components in combination with reduction of wear rates during hot forming are important targets to ensure the quality and efficiency of the process. The use of additive manufacturing (AM) technologies opens up potential solutions for novel tooling concepts. The manufacturing of complex shape cooling channels and integration of high-performance alloys at the surface could benefit the tribological performance in the forming operation. However, the research into high temperature tribological behaviour of AM materials in hot forming applications is very limited. The aim of this work is to study the tribological performance of additively manufactured materials. Two steels were used – a maraging steel and modified H13 tool steel. The hot work tool steel H13 is commonly applied for dies in metal forming processes. In this thesis it was used to study additive manufacturing as the processing route instead of conventional casting. The choice of a maraging steel is motivated by a possible application of high-performance alloys as a top layer on dies. The materials were post-machined and studied in milled, ground and shot-blasted conditions. The different post-machining operations were applied to study the effect of surface finish on the tribological behaviour and also to evaluate different methods of post-machining an AM surface. As fabricated dies are usually manufactured with milled surface. During its use, the dies undergo refurbishment after certain number of cycles and the surface condition is changed to a ground surface. These surface finishes are commonly tested for hot forming applications. The shot blasted operation was chosen as alternative surface finish. The process allows to prepare large sized tools easily and the surface has beneficial compressive stresses. The tribological behaviour of AM steels was studied using a hot strip drawing tribometer during sliding against a conventional Al-Si coated 22MnB5 steel. The workpiece temperature during the tests was 600 and 700°C. The results of the tribological performance of AM materials were compared to conventionally cast tool steel QRO90.The results have shown that the friction behaviour of both maraging and H13 steels at 600°C was stable and similar whereas at 700°C the COF was more unstable and resulted in an earlier failure of the tests due to increased material transfer of Al-Si coating from the workpiece surface.The main wear mechanisms for AM materials were galling and abrasion at both temperatures. Abrasion is more severe for the AM steels in comparison to cast tool steel QRO90. The galling formation on milled and ground surfaces showed similar behaviour to cast steel and it increased with higher workpiece temperatures. The shot-blasted surfaces showed less build-up of transferred material on the surface but folding of asperities and entrapment of Al-Si particles within surface defects generated during shot-blasting.
43
Kenningley, Scott David Peter. "High temperature thermal and mechanical load characterisation of a steel fibre reinforced aluminium metal matrix composite (AlMMC) for automotive diesel pistons." Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/high-temperature-thermal-and-mechanical-load-characterisation-of-a-steel-fibre-reinforced-aluminium-metal-matrix-composite-almmc-for-automotive-diesel-pistons(5cc789fc-d64e-4905-bc1c-beb0e3b9c0df).html.
Повний текст джерелаАнотація:
In modern automotive engines, the vast majority of light vehicle diesel (LVD) pistons are made from gravity die cast monolithic AlSi based alloy systems. Presently, the market drivers for reduced emissions, more efficient fuel consumption and increased specific power output are providing cyclic thermal and mechanical fatigue loading above the safe life durability threshold for the current AlSi based alloy systems. Peak temperatures in the diesel piston’s fatigue critical combustion bowl region are presently 420 °C for the AlSi based alloys, which represents a homologous TH value in excess of 0.8. In combination with peak temperatures of 420 C, the pistons are subject to cylinder pressures up to 220 bar, inducing mechanical stress amplitudes 15-20% greater than the allowable component fatigue strength for 1x108 cycles, in some applications. This durability deficit naturally leads to a requirement for new material and process solutions aimed at improving thermal and mechanical fatigue resistance at temperatures in excess of 420 C.One solution to this problem is to locally reinforce the pistons combustion bowl edge with a metal matrix composite (MMC) system. In this study, an aluminium based metal matrix composite (AlMMC) has been investigated and has shown some promise with increases in iso-thermal high cycle (1x 107) fatigue strength of 50 % compared to standard monolithic piston alloys. The AlMMC consists of a premium AlSi based LVD piston alloy matrix reinforced with 0.15 Vf of an interconnected network of 2-4 mm long Fe based fibres. The AlMMC is manufactured by pressure assisted infiltration of a sintered metallic fibre preform with as cast materials having a pore density of 0.2 %. In contrast to the use of ceramic fibre reinforcement systems generally requiring high pressure infiltration techniques, preform infiltration is considered possible with a comparably inexpensive manufacturing route. The Fe based fibre preforms can be infiltrated at lower pressure due to the reactivity between the Fe based fibres and the AlSi based matrix alloy. Unfortunately, this increased reactivity, although an advantage for preform infiltration, can result in (FeAlXX)Si(+X) interfacial reaction products forming between the fibre and matrix at operating temperatures of greater than 440 °C. These interfacial reactions result in a 15-20 m interfacial intermetallic layer after prolonged periods of exposure (>500 hrs), resulting in depleted fibre Vf and void formations on the matrix side of the interface.
44
Sneary, Adrian Bernard. "The fabrication of a high temperature superconducting magnet and critical current characterisation of the component Bi₂Sr₂Ca₂Cu₃Oₓ tapes and filaments in high magnetic fields". Thesis, Durham University, 2000. http://etheses.dur.ac.uk/4517/.
Повний текст джерелаАнотація:
The transport critical current density (J(_c)) of a 37 filament Bi-2223/Ag tape has been measured as a function of field and temperature from 4.2 K up to 90 K. Data have been obtained over a large current range from 10 mA up to 100 A and in fields up to 23 T with the tape in 3 orientations with respect to field. These comprehensive data have been used to test the predictions of the flux creep and weak link models used to explain J(_c) in Bi-2223 tapes. The J(_c)(B,T) dependence of optimised Bi-2223 tapes has been calculated using a curved film model. The model assumes perfect grain connectivity and that the local superconducting properties are equivalent to those in the best reported thin films. A comparison between the calculations and measured J(_c)(B,T) dependencies suggest that in high fields at 20 K, J(_c) in presently available industrially processed tapes is only a factor of 8 below the performance of ideal fully optimised tapes. Transport measurements have been made on Bi-2223 single filaments extracted from an alloy sheathed multifilamentary tape in liquid nitrogen at 77 K in fields up to 300 mT with the field aligned parallel and perpendicular to the a-b planes. Further Jc(B,T) data have been taken in a variable temperature insert at temperatures between 60 to 90 K in fields up to 15 T. In a study of the electric field-current density {E-J) characteristics of the c-axis orientated data at 77 K, negative curvature is observed in traces below 280 mT. However, the 280 mT trace exhibits both positive and negative curvature in different current regimes in contrast to the predictions of standard theory. A laboratory scale Bi-2223 superconducting magnet producing a maximum field of 1.29 T at 4.2 K has been designed and fabricated. The magnet comprises 6 resin impregnated double wound pancakes with a 40 mm bore fabricated via the react and wind route. Critical current density measurements have been made as a function of magnetic field, angle and strain at 4.2 K and 77 K on short samples of the constituent tape. The E-J characteristics of all component coils have been measured and a comparison with short sample data shows that minimal additional damage occurred beyond that produced by the bending strain on the tape and the long length variation in J(_c). Sufficient detail is provided for the non-specialist to assess the potential use of brittle superconducting tapes for magnet technology and construct a laboratory scale magnet.
45
Dini, Hoda. "As-cast AZ91D magnesium alloy properties : Effects of microstructure and temperature." Doctoral thesis, Tekniska Högskolan, Högskolan i Jönköping, JTH, Material och tillverkning, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:hj:diva-38148.
Повний текст джерелаАнотація:
Today, there is an essential need for lightweight, energy-efficient, environmentally benign engineering systems, and this is the driving force behind the development of a wide range of structural and functional materials for energy generation, energy storage, propulsion, and transportation. These challenges have motivated the use of magnesium alloys for lightweight structural systems. Magnesium has a density of 1.74 g/cm3, which is almost 30% less than that of aluminium, one quarter of steel, and almost identicalto polymers. The ease of recycling magnesium alloys as compared to polymers makes them environmentally attractive, but their poor mechanical performance is the primary reason for the limited adoption of these alloys for structural applications. The Mg-Al-Zn alloy AZ91D exhibits an excellent combination of strength, die-castability, and corrosion resistance. However, its mechanical performance with regard to creep strength, for example, at evaluated temperatures is poor. Moreover, very little is known about the correlation between its mechanical properties and microstructural features. This thesis aims to provide new knowledge regarding the role played by microstructure in the mechanical performance of the magnesium alloy. The properties/performance of the material in relation to process parameters became of great interest during the investigation. An exhaustive characterisation of the grain size, secondary dendrite arm spacing (SDAS) distribution, and fraction of Mg17Al12 was performed using optical and electron backscatter diffraction (EBSD). These microstructural parameters were correlated to the offset yield point (Rp0.2), fracture strength, and elongation to failure of the material. It was proposed that the intermetallic phase, Mg17Al12, plays an important role in determining the mechanical and physical properties of the alloy in a temperature range of room temperature to 190°C by forming a rigid network of intermetallic. The presence of this network was confirmed by studying the thermal expansion behaviour of samples of the alloy containing different amounts of Mg17Al12. A physically based constitutive model with a wide validity range was successfully adapted to describe the flow stress behaviour of AZ91D with various microstructures. The temperature-dependent variables of the model correlated quite well with the underlying physics of the material. The model was validated through comparison with dislocation densities obtained using EBSD. The influence of high-pressure die-cast parameters on the distortion and residual stress of the cast components was studied, as were distortion and residual stress in components after shot peening and painting. Interestingly, it was found that intensification pressure has a major effect on distortion and residual stresses, and that the temperature of the fixed half of the die had a slight influence on the component's distortion and residual stress.Numera finns det ett väsentligt behov av lätta, energieffektiva och miljövänliga tekniksystem. Detta behov är drivkraften för utveckling av ett brett utbud av material för energigenerering, energilagring, framdrivning och transport. Dessa utmaningar motiverade användningen av magnesiumlegeringar för lättviktskonstruktioner. Magnesium har en densitet på 1,74 g/cm3, vilket är ca 30% lägre än för aluminium, en fjärdedel av densiteten för stål och nästan i nivå med många polymerer. Då magnesiumlegeringar dessutom är lätta att återvinna, jämfört med polymerer, gör det dem miljömässigt attraktiva. Låga mekaniska egenskaper är den främsta orsaken till begränsad användning av dessa legeringar för lastbärande tillämpningar. Mg-Al-Zn-legeringen AZ91D uppvisar en utmärkt kombination av styrka, gjutbarhet och korrosionsbeständighet. Dess mekaniska egenskaper vid förhöjd temperatur, som tex kryphållfasthet, är låga. Dessutom är korrelationen mellan mikrostruktur och mekaniska egenskaper oklar. Denna avhandling syftade till att ge ny kunskap om mikrostrukturens roll för magnesiumlegeringars mekaniska egenskaper. Slutligen var materialets egenskaper i förhållande till processparametrar vid tillverkningen av stort intresse. En omfattande karaktärisering av kornstorleks-, sekundära dendritarmavstånds (SDAS)-fördelning och fraktion av Mg17Al12 utfördes med hjälp av optisk mikroskopering och diffraktion av bakåtspridda elektroner (EBSD). Mikrostrukturen korrelerades till sträckgränsen (Rp0.2), brottstyrkan och brottförlängningen. Det föreslogs att den intermetalliska fasen, Mg17Al12, spelar en viktig roll vid bestämning av legeringens mekaniska och fysikaliska egenskaper vid temperaturintervall från rumstemperatur upp till 190°C genom att bilda ett styvt nätverk av intermetaller. Uppkomsten av ett sådant nätverk stöddes genom en studie av den termiska expansionen av legeringen för olika fraktioner av Mg17Al12. En fysikalisk konstitutiv modell med ett brett giltighetsområde användes framgångsrikt för att beskriva det plastiska flytbeteendet hos AZ91D för olika mikrostrukturer. De temperaturberoende variablerna i modellen korrelerade ganska väl med materialets underliggande fysik. Modellen validerades genom att jämföra dislokationstätheten som predikterades av modellen och den med EBSD uppmätta dislokationstätheten. Påverkan av pressgjutningsparametrar på geometrisk tolerans och restspänning hos de gjutna komponenterna studerades. Vidare studerades geometrisk tolerans och restspänning av komponenter efter pening och målning. Intressant nog hade eftermatningsfasen en stor effekt på geometrisk tolerans och restspänningar. Dessutom hade temperaturen på den fasta formhalvan av verktyget även ett visst inflytande på komponentens geometriska tolerans och restspänning.
46
Schröders, Sebastian J. [Verfasser], Sandra [Akademischer Betreuer] Korte-Kerzel, and W. J. [Akademischer Betreuer] Clegg. "Plasticity of topologically close packed phases : characterisation from high temperature to high resolution in a case study of the Fe$_7}$Mo$_6}$-µ-phase / Sebastian J. Schröders ; Sandra Korte-Kerzel, W. J. Clegg." Aachen : Universitätsbibliothek der RWTH Aachen, 2018. http://d-nb.info/1181108780/34.
Повний текст джерела
47
He, Man [Verfasser], Björn [Akademischer Betreuer] [Gutachter] Winkler, and Eiken [Gutachter] Haussühl. "Mechanochemical activation assisted solid state synthesis, self-propagating high-temperature synthesis, and characterisation of Ti-containing solid solutions with spinel-, pseudobrookite- and Mn5Si3-type structures / Man He ; Gutachter: Björn Winkler, Eiken Haussühl ; Betreuer: Björn Winkler." Frankfurt am Main : Universitätsbibliothek Johann Christian Senckenberg, 2016. http://d-nb.info/111860721X/34.
Повний текст джерела
48
Li, Mengya. "Caractérisation expérimentale et modélisation du panneau composite bois-ciment." Thesis, Université de Lorraine, 2018. http://www.theses.fr/2018LORR0302/document.
Повний текст джерелаАнотація:
Les bétons légers, formés des fibres de bois et d’une pâte de ciment Portland, constituent une nouvelle alternative à explorer pour réduire l’impact environnemental des bâtiments. Ils sont utilisés dans la construction durable, comme des éléments secondaires, pour leurs performances thermiques, hydriques et mécaniques. Cependant, la généralisation de leur utilisation dans le bâtiment ne sera rendue possible sans résoudre certains verrous scientifiques liés à leur caractérisation et à leur formulation. Le présent travail s’inscrit dans cet objectif. Il s’agit de contribuer à la caractérisation de ces bétons légers à base des fibres de bois à travers l’expérience et la modélisation. Le module d’Young et la résistance à la rupture ont été mesurés par des tests de flexion et de compression. Un modèle numérique a été également développé pour prédire le comportement des éprouvettes en flexion et la réponse structurale des systèmes de coffrage permanent. La méthodologie numérique permet ainsi d’aider dans le choix des paramètres optimums pour une meilleure conception des panneaux de coffrage destinés à la construction. L’étude du comportement hygrothermique du matériau de construction bois-ciment a été abordée en s’appuyant sur l’expérience et la simulation. Les équations des transferts couplés de chaleur et d’humidité d’un milieu poreux ont été implémentées dans le logiciel Comsol Multiphysics®. En dernier, le modèle développé a été appliqué et validé sur plusieurs réponses dynamiques issues des tests hygrothermiques réalisés en interne. Les mesures des propriétés physico-thermique du matériau composite bois-ciment ont été ensuite intégrées dans le code Abaqus via une routine utilisateur Umatht dans l’objectif de simuler le comportement thermique à hautes températures des panneaux composites bois-ciment. Les profils des températures sont évalués et comparés à ceux des tests de carbonisation réalisés, à l’aide d’un panneau rayonnant, sur des échantillons exposés à un flux de chaleur uniforme de 6kW/m2. Les simulations montrent que le modèle développé est capable de prédire les profils de températures, la zone et la profondeur de la couche du charbon durant l’exposition au feuLightweight concretes made from wood fibres and Portland cement paste are a new alternative for the reduction of the environmental impact of buildings. They are used in sustainable constructions as secondary elements for their thermal, hydric and mechanical performance. However, the generalisation of their use is not possible without resolving certain scientific obstacles related to their characterisation. Hence the aim of the present work, which is to contribute towards their characterisation through experimentation and numerical simulation. The Young's modulus and tensile strength were measured through flexural and compression tests. A numerical model has also been developed to predict the behaviour of specimens under bending test as well as their structural response when used as permanent formwork. In particular, the model helps to choose the optimum parameters for a better design of the formwork system. The study of the hygrothermal behaviour of the wood-cement material was carried out using both experimental work and simulation. The equations of coupled heat and moisture transfers for a porous medium have been implemented in the Comsol Multiphysics® software. The developed model has been applied and validated on several dynamic responses resulting from hygro-thermal tests carried out in the laboratory. The obtained physico-thermal properties of the wood-cement composite material were then incorporated into the Abaqus code via a Umatht user subroutine to simulate its high temperature behavior. The temperature profiles are evaluated and compared with the charring tests performed using a radiant panel on samples exposed to a uniform heat flux of 6kW/m². The simulations show that the developed model is able to predict the temperature profiles, the area and the depth of the charred layer during fire exposure
49
Hamieh, Youness. "Caractérisation et modélisation du transistor JFET en SiC à haute température." Phd thesis, INSA de Lyon, 2011. http://tel.archives-ouvertes.fr/tel-00665817.
Повний текст джерелаАнотація:
Dans le domaine de l'électronique de puissance, les dispositifs en carbure de silicium (SiC) sont bien adaptés pour fonctionner dans des environnements à haute température, haute puissance, haute tension et haute radiation. Le carbure de silicium (SiC) est un matériau semi-conducteur à large bande d'énergie interdite. Ce matériau possède des caractéristiques en température et une tenue aux champs électriques bien supérieure à celles de silicium. Ces caractéristiques permettent des améliorations significatives dans une grande variété d'applications et de systèmes. Parmi les interrupteurs existants, le JFET en SiC est l'interrupteur le plus avancé dans son développement technologique, et il est au stade de la pré-commercialisation. Le travail réalisé au cours de cette thèse consiste à caractériser électriquement des JFET- SiC de SiCED en fonction de la température (25°C-300°C). Des mesures ont été réalisé en statique (courant-tension), en dynamique (capacité-tension) et en commutation sur charge R-L (résistive-inductives) et dans un bras d'onduleur. Un modèle multi-physique du transistor VJFET de SiCED à un canal latéral a été présenté. Le modèle a été développé en langage MAST et validé aussi bien en mode de fonctionnement statique que dynamique en utilisant le simulateur SABER. Ce modèle inclut une représentation asymétrique du canal latéral et les capacités de jonction de la structure. La validation du modèle montre une bonne concordance entre les mesures et la simulation.
50
Tonmukayakul, Narongsak. "Rheological characterisation of low-rank coal ash at high temperatures." 2004. http://hdl.handle.net/2440/59643.
Повний текст джерелаАнотація:
Title page, summary and table of contents only. The complete thesis in print form is available from the University of Adelaide Library.Ash deposition is a problem in power generation when coal with high ash and alkali contents are utilised. The problem is more severe in fluidised bed combustion where the ash deposition can causes agglomeration of the bed material, may lead to defluidisation of the unit. The successful operation of fluid bed combustion with coal high in ash and alkali content will depend on the ability to control ash deposition. The rheological properties of coal ash under furnace conditions are important in controlling the stickiness and mobility of the molten ash deposition. Therefore, a good knowledge of the rheological properties of coal will improve the understanding of the mechanisms associated with ash deposition, and may assist in controlling the deposition and agglomeration of fluid bed material. At present, a good deal of information about coal ash rheology under conditions similar to those found in fluidised bed combustion is not known, and greater understanding is required. This is primarily due to a lack of reliable instruments and measurement techniques. In this work, a new high temperature rheometer has been developed based on the principle concepts of visco metric flow. The developed rheometer allows fundamental rheological properties, such as shear stress and shear rate, to be obtained without relying on calibrations with materials of known properties. With this instrument the flow characteristics of the tested samples can be determined directly, without assuming a particular fluid model. The new rheometer has the capability to measure the rheological properties of materials at temperatures ranging from 500°C to 1300°C and under different processing conditions. Rheological characteristics and properties of a range of low rank Australian coal ashes have been carried out using the newly developed high temperature rheometer, equipped with a cone and plate measuring geometry. It has been found that coal ash samples exhibit thixotropic and visco plastic flow behaviours. SEM and XRD analyses have revealed that during high temperature rheological measurements the coal ash sample is basically a suspension of colloidal mineral solids in a molten eutectic liquid. The solid phase is mainly silica (SiO₂,), and the liquid phase is a mixture of alkali sulphates mainly CaS0₄, MgS0₄ and Na₂S0₄ compounds. The equilibrium visco metric data of coal ash samples is found to be satisfactorily described using the Herschel-Bulkley model. The equilibrium rheological properties are strongly affected by the concentration levels of CaS0₄, MgS0₄ and Na₂S0₄ . The operating temperature and chemical composition of the surrounding gas phase were also found to affect the rheological properties of the coal ash samples. In order to obtain a better understanding and to model the rheological properties of the coal ashes, a series of synthetic ash mixtures were examined. The synthetic mixtures contained the key chemical components that represent the solid and the liquid phases. The solid phase is represented by silica (SiO₂), while a mixture of CaS0₄, MgS0₄ and Na₂S0₄ compounds represented the liquid phase. In this work, the rheological characteristics of mixtures of synthetic ash were investigated using a factorial experimental design. Using the synthetic ash mixtures together with the statistical design experiment, the effect of key chemical compounds on the rheological properties could be systematically investigated. The rheological results showed that the synthetic mixtures exhibited thixotropic and viscoplastic behaviours. It was also found that mixtures predominantly high in CaS0₄and MgS0₄ had a high degree of thixotropy behaviour, while those mixtures predominantly high in Na₂S0₄ showed a lower degree of thixotropy behaviour. The statistical analysis also showed that Na₂S0₄ is the most significant chemical compound causes a high yield stress and high viscosity. In contrast, CaS0₄and MgS0₄ were found to decrease the value of the yield stress and the viscosity. The rheological behaviour of the synthetic ash mixtures can be used to describe rheological behaviour of the coal ash samples. Relationships between equilibrium flow properties and chemical compounds, and temperatures are developed using a linear regression method. The statistical analysis has shown that CaS0₄, MgS0₄ and Na₂S0₄ , and their interactions are all significant compounds that have effects on the yield stress and viscosity of the synthetic mixtures. It was also found that the yield stress and viscosity decreased with increasing concentration level of either CaS0₄or MgS0₄ . Yield stress and viscosity are increased with increases in the concentration of Na₂S0₄ . The statistical models can successfully predict rheological properties of ash with high concentrations of CaS0₄, MgS0₄ and Na₂S0₄ , but it fails to predict the rheological properties of ashes that also high concentrations of either Fe₂0₃ or Al₂0₃, or a combination of both. The relationship between ash rheology and fluidised bed agglomeration has been established. The yield stress of a coal ash may be used to describe the tendency of the molten ash to deposit on surface of the fluid bed particles. Yield stress also determines the tendency of stickiness of the molten ash deposit to adhere the fluid bed particles during fluidised bed combustion process. The viscosity of the molten ash describes the ability of the molten ash layer to adhere the fluid bed particles after a collision. High viscosity ash tend to hold the colliding particles together longer than a low viscosity ash. Shear thinning behaviour of the ash samples (decreasing viscosity with increasing shear rate) suggests that the operating conditions could be arranged so as to minimise the chance of agglomeration. For example, in order to avoid agglomeration a high viscosity coal ash would benefit from operating the fluidised bed combustion at a high velocity, this is because a high velocity means a higher shear rate and this causes a reduction in the viscosity of the molten ash. Thus, particles agglomerated by a low viscosity ash would be easily broken by the hydrodynamic forces present during the fluidised bed process. Finally, information about ash rheology has formed a basic knowledge for estimating tendency of fluid bed agglomeration when coal obtained from different source is being used.
http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1141958
Thesis (Ph.D.) -- University of Adelaide, School of Chemical Engineering, 2004