Synthesis of Germanium-Gallium-Tellurium (Ge-Ga-Te) Ceramics by Ball-Milling and Sintering

International audienceIn this paper, we present the preparation of a bulk material with a composition of 80GeTe2-20Ga2Te3 by combining mechanosynthesis and sintering. This composition cannot be prepared by conventional melt/quenching technique. The progressive evolution of the powder during ball milling is followed by XRD and DSC analysis. The final powder obtained is highly crystalline but a glass transition temperature is observed, indicating the presence of some amorphous phase remaining, allowing for its efficient sintering. By hot pressing, a dense bulk material with a fine microstructure and a high electrical conductivity is obtained. The synthesis method described represents a simple and cost effective way to produce tellurium-based materials of desired dimension with potential applications for optical storage or thermoelectric devices

Vitrocéramiques infrarouges pour application à la vision nocturne

Chalcogenide glasses are used as optics for the IR cameras thanks to their transparence in the two atmospheric windows [3 – 5 µm] and [8 – 12 µm]. In order to reduce their price and to increase the panel of compositions which may be produced, a new method of synthesis has been elaborated in the Glass and Ceramics group. Thus, this manuscript presents the development of the new way of synthesis of chalcogenide glasses and glass-ceramics by mechanical milling and SPS sintering, and the study of the structure and mechanical properties of glass-ceramics. The different milling and sintering parameters have been studied and the possibility to produce bulk samples with a structure and properties similar to those of glasses synthesized by melt-quenching method has been demonstrated. Also, it has been shown that the generation of crystalline particles in the glassy matrix increases mechanical properties of the samples without spoiling their optical transmission. Finally, a theoretical study, based on the DFT method, has been initiated in order to access more precise information concerning glass and glass-ceramic structure and mechanical properties.Les verres de chalcogénures sont utilisés en tant qu'optiques pour les caméras IR grâce à leur transparence dans les deux fenêtres atmosphériques [3 – 5 µm] et [8 – 12 µm]. Afin de diminuer leur prix et d'augmenter la gamme des compositions qui pourraient être produites, une nouvelle méthode de synthèse a été élaborée au laboratoire Verres et Céramiques. Les travaux présentés dans ce manuscrit ont ainsi porté sur le développement de la technique de synthèse des verres et vitrocéramiques de chalcogénures par mécanosynthèse et frittage flash, ainsi que sur l'étude de la structure et des propriétés mécaniques des vitrocéramiques. Les différents paramètres de broyage et frittage ont été étudiés et la possibilité de produire des matériaux massifs, avec une structure et des propriétés similaires à celles des verres obtenus par voie classique de fusion-trempe, a été démontrée. Egalement, il a été constaté que la génération des particules cristallines dans la matrice vitreuse permet d'améliorer les propriétés mécaniques sans altérer la transmission optique des échantillons. Finalement, une étude théorique, basée sur la méthode DFT, a été initié pour accéder à des informations plus précises concernant la structure et les propriétés mécaniques des verres et vitrocéramiques de chalcogénures

Infrared glass-ceramics for night vision applications

Les verres de chalcogénures sont utilisés en tant qu'optiques pour les caméras IR grâce à leur transparence dans les deux fenêtres atmosphériques [3 – 5 µm] et [8 – 12 µm]. Afin de diminuer leur prix et d'augmenter la gamme des compositions qui pourraient être produites, une nouvelle méthode de synthèse a été élaborée au laboratoire Verres et Céramiques. Les travaux présentés dans ce manuscrit ont ainsi porté sur le développement de la technique de synthèse des verres et vitrocéramiques de chalcogénures par mécanosynthèse et frittage flash, ainsi que sur l'étude de la structure et des propriétés mécaniques des vitrocéramiques. Les différents paramètres de broyage et frittage ont été étudiés et la possibilité de produire des matériaux massifs, avec une structure et des propriétés similaires à celles des verres obtenus par voie classique de fusion-trempe, a été démontrée. Egalement, il a été constaté que la génération des particules cristallines dans la matrice vitreuse permet d'améliorer les propriétés mécaniques sans altérer la transmission optique des échantillons. Finalement, une étude théorique, basée sur la méthode DFT, a été initié pour accéder à des informations plus précises concernant la structure et les propriétés mécaniques des verres et vitrocéramiques de chalcogénures.Chalcogenide glasses are used as optics for the IR cameras thanks to their transparence in the two atmospheric windows [3 – 5 µm] and [8 – 12 µm]. In order to reduce their price and to increase the panel of compositions which may be produced, a new method of synthesis has been elaborated in the Glass and Ceramics group. Thus, this manuscript presents the development of the new way of synthesis of chalcogenide glasses and glass-ceramics by mechanical milling and SPS sintering, and the study of the structure and mechanical properties of glass-ceramics. The different milling and sintering parameters have been studied and the possibility to produce bulk samples with a structure and properties similar to those of glasses synthesized by melt-quenching method has been demonstrated. Also, it has been shown that the generation of crystalline particles in the glassy matrix increases mechanical properties of the samples without spoiling their optical transmission. Finally, a theoretical study, based on the DFT method, has been initiated in order to access more precise information concerning glass and glass-ceramic structure and mechanical properties

Short and medium range structures of 80GeSe2-20Ga2Se3 chalcogenide glasses

International audienceThe short and medium range structures of 80GeSe-20GaSe (or GeGaSe) chalcogenide glasses have been studied by combining ab initio molecular dynamics (AIMD) simulations and experimental neutron diffraction studies. The structure factor and total correlation function were calculated from glass structures generated from AIMD simulations and compared with neutron diffraction experiments showing reasonable agreement. The atomic structures of ternary chalcogenide glasses were analyzed in detail, and it was found that gallium atoms are four-fold coordinated by selenium (Se) and form [GaSe] tetrahedra. Germanium atoms on average also have four-fold coordination, among which Se is 3.5 with the remaining being Ge-Ge homo-nuclear bonds. Ga and Ge tetrahedra link together mainly through corner-sharing and some edge-sharing of Se. No homo-nuclear bonds were observed among Ga atoms or between Ge and Ga. In addition, Se-Se homo-nuclear bonds and Se chains with various lengths were observed. A small fraction of Se atom triclusters that bond to three cations of Ge and Ga were also observed, confirming earlier proposals from Se solid state nuclear magnetic resonance studies. Furthermore, the electronic structures of ternary chalcogenide glasses were studied in terms of atomic charge and electronic density of states in order to gain insights into the chemical bonding and electronic properties, as well as to provide an explanation of the observed atomic structures in these ternary chalcogenide glasses

Incorporation of Ga into the structure of Ge-Se glasses

International audienceThe structure of Ga-containing Ge-Se glasses is studied by high-resolution X-ray photoelectron and extended X-ray absorption fine-structure spectroscopies. The results show that in GaxGeySe100-x-y glasses both Ga and Ge atoms are 4-fold coordinated, whereas Se atoms are 2-fold coordinated. For Se-deficit samples with Se content less than 67 at.%, Ge-Ge(Ga) bonds appear. First, Ge-Ge bonds are formed indicating preference for the formation of (Se)2 > Ga < (Se)2 units over the regular GeSe4/2 tetrahedra. Metal-Ga bonds appear only in the most Se-deficit sample. Addition of Ga facilitates more uniform Se distribution in comparison to binary Ge-Se or ternary As(Sb)-Ge-Se systems. The observed structural features are correlated with some basic thermo-mechanical properties of the investigated glasses

Synthesis of GeSe4 glass by mechanical alloying and sintering

International audienceThe possibility to synthesize GeSe4 glass by mechanical alloying followed by sintering has been demonstrated. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analysis have been performed on the powder as a function of milling duration in order to follow the amorphization process of raw metallic germanium and selenium elements. Completely amorphous powder has been sintered using a homemade hot press. Optical transmission and the density of the final bulk material have been studie

Positron Annihilation in IR Transmitting GeS2-Ga2S3 Glasses

International audiencePositron annihilation lifetime spectroscopy combined with Doppler broadening of annihilation radiation was applied to study free-volume entities in GeS2-Ga2S3 glasses affected by Ga additions. It is shown that Ga-related void sub-system plays a decisive role in positron trapping process, while the overall density variation is defined mainly by Ge-related sub-system. These results serve as basis for new characterization route for inner free-volume structure of these glasses

CsCl effect on the optical properties of the 80GeS(2)-20Ga(2)S(3) base glass

International audienceOptical properties of chalcogenide glasses belonging to the series (80GeS2-20Ga2S3)100−x (CsCl) x with x=0;5;10;15;20 were investigated. The linear refractive indices (n 0) were determined by prism measurements at four wavelengths: 633 nm, 825 nm, 1311 nm, and 1511 nm. Z-scan experiments were performed at 800 nm to measure the non-linear indices (n 2) and the absorption coefficients (β). CsCl additions in the base glass (80GeS2-20Ga2S3) are characterized by a white shift of the transmission in the visible range and a strong decrease of both n 0 and n 2. As the same time, β is also decreasing and this results in a figure of merit FOM=2βλ/n 2 that remains relatively low at 800 nm, meaning that this series of highly non-linear glasses should be very suitable for optical switching applications at telecommunication wavelengths

Structural study by Raman spectroscopy and 77Se NMR of GeSe4 and 80GeSe2–20Ga2Se3 glasses synthesized by mechanical milling

International audienceAmorphous powders of GeSe4 and 80GeSe2-​20Ga2Se3 compns. have been prepd. by mech. alloying of raw Germanium, Selenium, and Gallium. The amorphization process and structural evolution of the powder during mech. alloying have been studied by XRD, DSC, Raman spectroscopy and 77Se NMR. The role of Gallium in the amorphization process has been pointed out. Similar structures and thermal properties of amorphous powders and resp. glasses synthesized by melt-​quenching method have been obsd. (ISNOG 2014

Regioselectivity of Al–O Bond Hydrolysis during Zeolites Dealumination Unified by Brønsted–Evans–Polanyi Relationship

We determine the mechanism of the initiation of Al–O­(H) bond breaking for zeolitic structures mordenite (MOR), faujasite (FAU), MFI, and chabazite (CHA) with high Si/Al ratio occurring during dealumination. Periodic density functional theory calculations demonstrate that water adsorption on the Al atom takes place in anti position to the Brønsted acid site, via a penta- or tetra-coordinated Al species. A subsequent 1,2-dissociation of water on adjacent framework oxygen atoms leads to the first Al–O­(H) bond breaking (with activation energies of ∼76–125 kJ/mol). A Brønsted–Evans–Polanyi relationship to estimate transition states (TS) is established and opens the door to predictions of which crystallographic sites are able to initiate dealumination