Some strategies to (co)-sinter refractory functional oxides at low temperature by spark plasma sintering

The sintering at high temperatures (1000-1400°C) of refractory oxides widely used in electronic devices, raises several issues related to defects, chemistry, microstructure and interface control. Reducing sintering temperatures below 900°C is a major challenge in particular when high relative densities, optimal microstructure and the control of reactivity at interfaces (grain boundaries, multi-materials) are mandatory. In this context, we propose to highlight some strategies focused on interfaces and phases control through two different illustrations of our recent works. The first one is focused on Micro-Electromechanical System (MEMS) energy harvesters (EH) using piezoelectric materials[1]. We will show the potentiality of SPS to co-sinter in one step and below 900°C complex devices such as screen-printed PbZrTiO3 in sandwich between two gold electrodes and supported on a stainless steel substrate. Here, the sintering aids in the pastes should be removed if possible or adapted for good adhesion, and delamination and bending of the multilayer EH must be avoided. We also intent to get rid of the annealing process after the SPS sintering. The second illustration reports on the ambitious goal to sinter zirconia ceramics at temperatures below 400°C. Recently, the exploration of non-equilibrium sintering, through transient liquid phase, hydrated precursors, or by using solvent assisted sintering, Flash sintering and Spark Plasma Sintering has been investigated to sinter ZnO and thermodynamically unstable materials at very low temperature [2-5]. Here, our approach is based on the use of specific precursors and deals with the control of transient non-equilibrium phases to find the driving force to establish the most favorable pathway for enhanced densification. [1]. M. I. Rua-Taborda, O. Santawitee, A. Phongphut, B. Chayasombat, C. Thanachayanont, S. Prichanont, C. Elissalde, J. Bernard, H. Debéda, Printed PZT Thick Films Implemented for Functionalized Gas Sensors , Key Engineering Materials, 777,158, 2018 [2].B. Dargatz, J. Gonzalez Julian, M. Bram, P. Jakes, A. Besmehn, L. Schade, R. Röder, C. Ronning and O. Guillon, “FAST/SPS sintering of nanocrystalline zinc oxide—Part I: Enhanced densification and formation of hydrogen-related defects in presence of adsorbed water, J. Eur. Ceram Soc. 36, 1207, 2016 [3]. S. Funahashi, J. Guo, H. Guo, K. Wang, A. L. Baker, K. Shiratsuyu, and C. A. Randall, Demonstration of the cold sintering process study for the densification and grain growth of ZnO ceramics, Journal of the American Ceramic Society, 100, 546, 2017. [4]. Luo J., “The scientific questions and technological opportunities of Flash sintering: from a case study of ZnO to others ceramics”, Scripta Mater., 146, 260, 2018 [5]. T. Herisson de Beauvoir, A Sangregorio, I. Cornu, C. Elissalde and M. Josse, “Cool-SPS: an opportunity for low temperature sintering of thermodynamically fragile materials” J. Mater. Chem. C, 6, 2229, 201

Sur l'élaboration de nouveaux carbonitrures (des variétés graphitiques aux structures peu compressibles)

En Chimie des Matériaux, la recherche de performances optimales pour la matière condensée a toujours été un challenge. Au début des années 90 Liu et Cohen proposèrent par des calculs ab initio, un modèle empirique permettant de calculer le module de compressibilité des matériaux covalents ainsi qu un nouveau matériau appelé C3N4-b isostructural de Si3N4-b pouvant posséder des propriétés mécaniques proches de celles du diamant. Dès lors plusieurs autres variétés (quatre tridimensionnelles et une bidimensionnelle) furent envisagées pour ce carbonitrure de composition C3N4. Notre démarche a consisté à synthétiser un carbonitrure bidimensionnel afin de le convertir en une phase dense par analogie avec la synthèse du diamant, par conversion directe grâce à l utilisation de hautes pressions et hautes températures ou par conversion assistée par flux. L élaboration du carbonitrure 2D a été réalisée par la décomposition thermique de la thiosemicarbazide sous flux d azote. Sa caractérisation par différentes techniques physico-chimiques et des essais de modification de sa composition chimique nous ont permis de mettre en évidence une relation entre la composition et la cristallinité. Par ailleurs, il a été constaté au sein de cette phase 2D la présence de nanoparticules amorphes à l intérieur desquelles il a été remarqué l existence de nanodomaines cristallisés de symétrie cubique composés exclusivement de carbone et d azote. La mesure du module de compressibilité de cette phase cubique grâce à l équation du troisième ordre de Birch Murnaghan a permis de mettre en évidence une valeur égale à 76% de celle du diamant et à 96% de celle du nitrure de bore cubique (second matériau le plus dur). Ainsi cette étude a donc permis de mettre en évidence l élaboration d une variété tridimensionnelle de carbonitrure à pression ambiante et température modérée et de confirmer la faible compressibilité des carbonitrures 3D.In Materials Chemistry, the optimal performance research in condensed matter has always been a challenge. At the begin of the 90 s LIU and COHEN have been proposed an empirical model by ab-initio method in the aim to calculate the bulk modulus in covalent materials and a new material called C3N4- isostructural of Si3N4- which would have mechanical properties similar to those of diamond. From this time, other polymorphs (four in 3D and one in 2D) were predicted for this carbon nitride with C3N4 composition. In the first step, our approach consisted to synthesize one bidimensionnal carbon nitride in order to convert it in second step to a dense phase by analogy with the diamond synthesis. For it, two processes could be used: the direct conversion (HPHT process) or the flux assisted conversion. The 2D carbon nitride was elaborated by thermal decomposition of the thiosemicarbazide under nitrogen flux. The characterization by different physico-chemical methods and several tests to modify its composition and its crystallinity led to underline a relation between the composition and the crystallinity. Thus, it was see the presence through this 2D phase of some amorphous nanoparticles into which, it was noticed the existence of some crystallized-nanodomains with cubic symmetry and composed only by nitrogen and carbon. The bulk modulus of this phase was measured thanks to the 3rd Birch-Murnaghan equation and it value has been shown equal to 76% of diamond one s and 96% of cubic boron nitride one s (second hardest material). Thereby, this study permitted to elaborate one 3D carbon nitride polymorph at ambient pressure and at moderate temperature and to confirm the low compressibility of 3D carbon nitrides.BORDEAUX1-BU Sciences-Talence (335222101) / SudocPESSAC-ICMCB (333182204) / SudocSudocFranceF

State of art and recent trends in bulk carbon nitrides synthesis

Light element-based materials are of relevant interest because of their specific physico-chemical properties mainly dependent on the involved strong chemical bonds. Among them, diamond and c-BN are still today the most representative materials especially concerning super hardness property. Nevertheless, since the prediction by Liu and Cohen (on the basis of ab initio calculation) of theoretical materials with C3N4 composition exhibiting low compressibility, many efforts have been devoted to the synthesis of carbon nitrides. The first part of this review deals with the brief description of the different predicted C3N4 polymorphs exhibiting either a dense network or a graphitic one. In addition, predicted carbon nitrides with different stoichiometry are also described..

Solvothermal synthesis of a-quartz powder doped with germanium

Experimental investigations concerning the synthesis of Si1−xGexO2 solid solutions with quartz structure have been carried out under solvothermal conditions from sol-gel preparations. The cell parameters of the powders have been determined by X-ray powder diffraction. The amount of germanium inside the SiO2 structure was measured also by electron probe micro-analysis (EPMA)

Gallium nitride bulk crystal growth processes : a review

Optoelectrical and microelectronic devices involving gallium nitride have become a challenge but their development is limited because of a lack of suitable substrates. This paper deals with the crystal growth of gallium nitride, the processes leading to GaN bulk crystals being significantly expanded during the last decade (the ones involving GaN thin films or nanocrystallites are not studied in this review). The main reviewed routes are: (i) the high pressure nitrogen solution growth (H.P.N.S.G.), (ii) the Na flux method, and (iii) the ammonothermal crystal growth

Solvothermal synthesis of a-quartz powder doped with germanium

Experimental investigations concerning the synthesis of Si1−xGexO2 solid solutions with quartz structure have been carried out under solvothermal conditions from sol-gel preparations. The cell parameters of the powders have been determined by X-ray powder diffraction. The amount of germanium inside the SiO2 structure was measured also by electron probe micro-analysis (EPMA)

Perovskites de manganèse nanométriques (vers des applications biomédicales)

Les nanoparticules seront sans doute les outils diagnostiques et thérapeutiques de demain. Si ellessont magnétiques, elles sont promises à des applications telles que le renforcement du contraste enIRM, la thermothérapie et la libération contrôlée de médicaments. Les nanomatériaux La1-xSrxMnO3ont été sélectionnés car leur température de Curie (TC) peut être ajustée dans la gamme detempérature thérapeutique. Des particules calibrées en taille et désagrégées ont été élaborées par leprocédé glycine-nitrate (GNP). Les caractérisations chimiques et structurales ont permis de mieuxcomprendre les résultats contradictoires de la littérature concernant la soi-disant dépendance de TCavec la taille des nanoparticules. L adaptabilité de ces nanoparticules pour des applications enhyperthermie ou en IRM a été confirmée. Enfin, la capacité des nanoparticules à s échauffer a étéutilisée pour réticuler autour d elles une couronne de macromolécules thermosensibles selon leconcept nouveau de chimie localement stimulée.Nanoparticles may be the next generation of diagnostic and therapeutic tools. If they are magnetic,they are dedicated to applications such as MRI contrast agent, thermotherapy and controlled drugrelease. La1-xSrxMnO3 nanoparticles were selected because their Curie temperature (TC) may be tunedwithin the range of therapeutic temperature. Size sorted and disaggregated particles weresynthesized by the Glycine-Nitrate Process. Chemical and structural characterizations allowed abetter understanding of conflicting results found in the literature about the particle size-dependenceof TC. The possibility to use these nanoparticles for hyperthermia and MRI applications has beenconfirmed. Finally, their ability to heat has been used to crosslink thermosensitive macromoleculesall around them according to the new concept of locally stimulated chemistry.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF