YAG transparent ceramics by double sintering - A comparison between HIP and FAST post-sintering treatments
PICELLI F. 1,2, BOULESTEIX R. 3, HOSTAÜA J. 1, ESPOSITO L. 1, MA╬TRE A. 3, PIANCASTELLI A. 1, BIASINI V. 1
1 Istituto di Scienza, Tecnologia e SostenibilitÓ per lo Sviluppo dei Materiali Ceramici ISSMC-CNR (former ISTEC), Via Granarolo 64, Faenza, Italy; 2 UniversitÓ degli studi di Parma, Via universitÓ 12, Parma, Italy; 3 UniversitÚ de Limoges, IRCER UMR CNRS 7315, 12 rue atlantis, Limoges, France
YAG-based transparent ceramics are conventionally prepared by vacuum sintering or by a double sintering process, for example vacuum sintering followed by Hot Isostatic Pressing (HIP). The use of a pressure-assisted densification on vacuum pre-sintered ceramics favours the closure of residual porosity, leading to high transparency.
On the other hand, these post-sintering treatments are effective with samples exhibiting a suitable microstructure after vacuum sintering, characterized by the absence of secondary phases and by closed pores only at grain boundaries with a size smaller than that of the grains.
FAST (Field Assisted Sintering Technique or SPS), usually applied directly on powders, can also be used as an alternative densification technique to HIP, which represents a novel approach for the production of transparent ceramics.
In this presentation we will show the results obtained by using HIP and FAST for the production of transparent Yb:YAG ceramics. Different pre-sintering cycles under vacuum were tested to obtain the desired microstructure for the second treatment, and many combinations of vacuum-sintering and HIP or FAST were tested. Samples were prepared starting from a mixture of stoichiometric single oxide powders. TEOS and MgO were used as sintering aids, alone and in combination.
SEM and optical microscopy analyses were used to characterise the microstructure of the samples after vacuum sintering and after post-sintering, and to identify correlations between the microstructure and optical properties of transparent Yb:YAG ceramics.
We show that with the use of FAST as a post-sintering step we obtained a complete pore closure at lower temperature and pressure compared to HIP. However, during the FAST densification, carbon contaminated the samples, turning them dark. It was possible to reduce the carbon contamination by annealing the samples in air at high temperature.
Selected samples of high optical quality were LASER-tested and LASER emission was obtained.
F. P. gratefully acknowledges the financial support of JECS TRUST contract no 2021280.