Étude du frittage non-conventionnel de céramiques de type YAG:Nd en présence d’ajout de silice

L’objectif de cette étude était de cerner l’influence du procédé de frittage non conventionnel employé pour la mise en forme des pièces (frittage SPS ou post-frittage HIP) sur l’activation des processus densifiants. Il s’avère qu’un traitement SPS des poudres initiales permet d’abaisser la température de début de frittage et conduit dans certaines conditions à des pièces translucides ou transparentes. L’influence de la silice, introduit comme ajout de frittage, et du néodyme, introduit comme dopant, est également discutée dans ce travail. Le post-frittage HIP, quant à lui, permet l’élimination de la porosité résiduelle dans les échantillons, ce qui conduit à la transparence des pièces réalisées

Grain Boundary Mobility of BaTiO₃ Doped with Aliovalent Cations

The effect of three aliovalent cations, Nb5+, La3+ and Co2+, on the grain growth kinetics of nearly fully dense BaTiO3 (Ba/Ti atomic ratio = 1.001) was measured in O2 at 1300 °C and for dopant concentrations of up to 1.25 atomic per cent (at%). For the donor cation Nb5+, the boundary mobility initially increased with cation concentration but then decreased markedly above a doping threshold of 0.3-0.5 at%. The boundary mobility of the BaTiO3 doped with the acceptor cation Co2+ decreased monotonically with dopant concentration. At a cation concentration of 0.75 at%, the boundary mobility was reduced by a factor of approximately 25, 10 and 50 times by Nb5+, La3+ and Co2+, respectively. A major role of the dopants is seen to be their ability to influence the boundary mobility. The effects of the dopants on the boundary mobility are discussed in terms of the defect chemistry and the space-charge concept

Sol-gel Processing and Sintering of Yttrium Aluminum Garnet (YAG) Powders

Gels of yttrium aluminum garnet (YAG) with the stoichiometric composition 3Y2O3·5AI2O3, were prepared by a sol-gel technique and dried by supercritical extraction with CO2. Powders were produced by lightly grinding the dried gels. Crystallization of the powder occurred at ≈900°C and within the limits of detection, the X-ray diffraction pattern of the crystallized material was identical to that of the stoichiometric composition. Powder compacts with a green density of ≈0.50 of the theoretical were sintered to nearly full density in O2 during constant heating rate sintering at 5 °C min−1 to 1600 °C. This is better than the density obtained with powders from a similar gel dried conventionally (by evaporation of the liquid) and considerably better than that obtained with powders prepared by solid state reaction. The room temperature flexural strength and fracture toughness of the material fabricated from the supercritically dried gels were 190 MPa and 2.2 MPa.m1/2, respectively. These strength and fracture toughness values are higher than those reported in other studies for YAG produced by the sintering route