Défense de thèse de doctorat en sciences physiques, par Rosa Moronta Perez

The purpose of this work was to elaborate Yttrium Aluminium Garnet (YAG) and Barium Titanate (BT) ceramics with a high level of transparency and to investigate to what extent functional properties (optical and dielectric) were linked to the powder characteristics and the sintering parameters.

Three different YAG powders (two commercial and a home-synthesized one) were densified by Spark Plasma Sintering (SPS). The role of the physico-chemical characteristics of the powders (purity, agglomeration state and stoichiometry) on the sintering and the optical transmission was investigated. Depending on the powder type, different pre-treatments and/or post-treatments were necessary to increase both homogeneity and optical transmission of densified parts. In the case of agglomerated powders, dispersion by ultrasonication was efficient and led to better homogeneity and higher optical transmission. Post-treatments such as annealing and post-hipping in air were helpful to reduce oxygen vacancies and residual porosities and improved the optical transmission of the ceramics. The detrimental effect of impurities (sulphur), even in a quantity as low as 50 ppm, on optical transmission, was evidenced. The YAG powder with the lowest agglomeration degree and the highest purity resulted in the highest values of optical transmission. 70% real in-line transmission at the wavelength of 600 nm was obtained under SPS conditions of 50 MPa at 1500°C and after addition of 0.2% Lithium Fluoride.

Concerning barium titanate and barium strontium titanate powders, black and opaque samples were obtained after Spark Plasma Sintering. However, optimisation of SPS parameters (with prior dispersion by ball mixing in the case of BT powder) led to fully dense samples with grain size under 1 μm. These two conditions were necessary to achieve translucent ceramics after a suitable air annealing treatment. These samples presented the highest values of transmission obtained so far: total forward transmission values reached 35% and 41% at 600 nm, for barium titanate and barium strontium titanate, respectively. Ceramics with full density and homogeneous grain size around 1 μm provided the highest permittivity values with low dielectric losses, after air annealing.