Littérature scientifique sur le sujet « Nanostructured heterophasic material »

The paper is dedicated to the study of the interdependence of structure and properties of sintered heterophase ceramic composition based on ultra-fine (nanosized) powders. The compositions based on brittle oxygen-free high-strength compounds under certain conditions are able to demonstrate micro-plasticity. Microstructure of such materials typically features homogeneous fine grains with certain critical maximum grain size. The substantial contribution of interphase boundaries and their dislocation structure to the non-trivial mechanical properties is discussed. Special features of interphase boundaries are demonstrated to contribute to the change in rupture resistance of the heterophase ceramics. Correlations between strength and elastic properties and results of tribological tests using «pin and disk» arrangement were studied. Increased fracture resistance, wear resistance, and micro-plasticity of heterophase ceramics is found to be due to the modification microstructure size parameters and distinctive features of interphase boundaries. The generally formulated purpose of the present study is a search for the solution to the problem of production of the advanced structural ceramic materials based upon oxygen-free refractory compounds with improved physical and mechanical properties.

The paper gives a brief review of properties and applications of developed extra-hard nanostructured composite materials and coatings based on them. The presentresearch suggestsaerospace applications of nanostructured composite materials based on carbides, carbonitrides and diboridesof transition and refractory metals. To improve the technical and economic performance of gas turbine engines, it is advisable to use new composite structural materials whose basic physicomechanical properties are several times superior to traditional ones. The greatest progress in developing new composites should be expected in the area of materials created on the basis of polymer, metal, intermetallic and ceramic matrices. Currently components and assemblies of gas turbine engines and multiple lighting power units with long operation life and durability will vigorously develop. Next-generation composites are studied in all developed countries, primarily in the United States and Japan.

Cobaltite-based perovskite materials such as La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) which have rapid surface activation and mixed ionic and electronic ionic conductivity (MIEC) usually mixed with ionic conducting electrolyte material to lower cathodic polarization resistance. Moreover, decreasing the particulate size of the cathode at nanoscale can effectively enhance the ORR activity of the cathode by extending the electrochemically active sites due to its high surface area. However, due the high surface energy, the nanoparticles easily aggregate before mixing into homogeneous heterophase nanoparticles, resulting in significant coarsening between adjacent particles during high-temperature sintering. In this study, GDC and LSCF-GDC composite powders were synthesized via advance sol-gel method. The physicochemical properties such as sintering behavior, phase stability, etc. of the synthesized nanopowders were investigated. The optimal sintering conditions of GDC nanopowders were evaluated based on the relative density and total conductivity of the sintered GDC pellets. Furthermore, the electrochemical performances of the LSCF-GDC nanocomposite cathodes were measured by the electrochemical impedance spectroscopy (EIS). The microstructural characteristics of the LSCF-GDC nanocomposite cathode were quantified via a 3D reconstruction techniques and related to electrochemical performance of the Ni-YSZ anode supported SOFC single cell.

The novelty of this work was to prepare a series of defect-rich colored TiO2 nanostructures, using a peroxo solvothermal-assisted, high-pressure nitrogenation method. Among these solids, certain TiO2 materials possessed a trace quantity of anatase–rutile heterojunctions, which are beneficial in obtaining high reaction rates in photocatalytic reactions. In addition, high surface area (above 100 m2/g), even when utilizing a high calcination temperature (500 °C), and absorption of light at higher wavelengths, due to the grey color of the synthesized titania, were observed as an added advantage for photocatalytic hydroxyl radical formation. In this work, we adopted a photoluminescent probe method to monitor the temporal evolution of hydroxyl radicals. As a result, promising hydroxyl radical formations were observed for all the colored samples synthesized at 400 and 500 °C, irrespective of the duration of calcination.

2010/2011
This thesis describes: (i) synthesis and characterization of colloidal nanocrystals of II-VI semiconductor compounds; (II) development of two novel materials using such nanocrystals as “building blocks”: (IIa) a nanocrystals/polymer composite, to be used as phosphor in LED-based lighting devices; (IIb) an inorganic, nano-structured multiphase material, showing a promising geometry as an electronic intermediate band material. Different typologies of nanocrystals (single-phase, alloyed or core-shells) were successfully synthesized using air-stable, safe reagents. Their optical properties (absorption spectrum, fluorescence wavelength and fluorescence quantum yield) were mapped as function of different parameters. Good results in engineering optical properties were achieved by: (a) changing size and/or composition in single-phase nanocrystals; (b) tuning shell composition and thickness and/or mutually diffusing one material into the other in multi-phase nanocrystals. The influence of different surface ligands on optical properties and on solubility in different media was also studied. Nanocrystal/polymer composite lenses were obtained from nanocrystals with desired fluorescence wavelength and quantum yield, mixed in an appropriate solvent with polymer pellets. The mixture was drop casted or tape casted on a solid substrate, obtaining solid, transparent lenses after solvent evaporation. A nano-structured, all-inorganic material (composed of semiconducor nanocrystals embedded into a wider bandgap semiconductor) was obtained through self-assembly and densification of colloidal core-shells nanocrystals. The realization of this composite supracrystal was achieved via a multi-step process: (i) colloidal synthesis of core-shell nanocrystals; (ii) surface ligands exchange; (iii) assembly; (iv) heat treatment. Evolution of the optical properties during heat treatment suggests that it is possible to sinter the shell material without altering the internal nano-heterostructure, if temperature and time of the treatment are controlled properly.
In questa tesi sono descritti: (I) la sintesi colloidale e la caratterizzazione di nanocristalli di semiconduttori II-VI; (II) lo sviluppo, utilizzando i suddetti nanocristalli quali “unità da costruzione”, di due materiali innovativi: (IIa) un composito nanocristalli/polimero, da usare come fosforo in dispositivi per illuminazione basati su LED; (IIb) un materiale inorganico nano-strutturato multifase, con una geometria promettente quale materiale a banda elettronica intermedia. Differenti semiconduttori II-VI sono stati sintetizzati in forma di nanocristalli (monofasici, in forma di lega o in struttura di tipo “core-shell”) usando reagenti sicuri e stabili in atmosfera. Le loro proprietà ottiche (spettro di assorbimento, lunghezza d’onda di fluorescenze e resa quantica di fluorescenza) sono state mappate in funzione di numerosi parametri. Sono stati raggiunti ottimi risultati nel controllo delle proprietà ottiche sia in nanocristalli a fase singola (modificandone le dimensioni o la composizione chimica) che in nanocristalli multifase (regolandone la composizione e lo spessore della “shell”, nonché mutualmente diffondendo un materiale nell’altro). È stata anche studiata l’influenza di differenti leganti superficiali sulle proprietà ottiche e sulla solubilità dei nanocristalli in differenti solventi. Lenti composite di nanocristalli/polimero sono state ottenute a partire da nanocristalli aventi la lunghezza d’onda e la resa quantica di fluorescenza desiderate, mescolandoli con pellet di polimero in solventi appropriati. La miscela è stata depositata su un supporto, tramite drop casting o tape casting, ottenendo lenti solide trasparenti dopo l’evaporazione del solvente. Un materiale inorganico nano strutturato (costituito da nanocristalli di semiconduttore racchiusi all’interno di un secondo materiale semiconduttore a bandgap maggiore) è stato ottenuto tramite l’autoassemblaggio e la densificazione di nanocristalli core-shell sintetizzati con procedure di chimica colloidale. La realizzazione di suddetto sovra-cristallo si è svolta in più fasi: (i) sintesi colloidale; (ii) sostituzione dei leganti superficiali; (iii) assemblaggio; (iv) trattamento termico. I risultati derivanti dallo studio dell’evoluzione delle proprietà ottiche durante il trattamento termico suggeriscono che sia possibile sinterizzare il materiale della shell senza alterare la nano-eterostruttura interna, se la temperatura e il tempo del trattamento sono scelti opportunamente.
XXIV Ciclo
1983