Abstract: YAG-based transparent ceramics are conventionally prepared by vacuum sintering or by a double sintering process, viz. vacuum sintering followed by Hot Isostatic Pressing (HIP). The use of a pressure-assisted process on vacuum pre-sintered ceramics favours the closure of residual porosity, and is therefore suitable for the production of highly transparent ceramics, where pores would otherwise act as light scattering centres. On the other hand, these post-sintering treatments are effective with samples exhibiting a suitable microstructure after vacuum sintering, i.e. no secondary phases and only closed pores with a size smaller that the size of the grains. As an alternative to HIP, a fast post-sintering treatment with Spark Plasma Sintering (SPS) is proposed. In this poster we present a comparison of transparent YAG-based transparent ceramics obtained by vacuum sintering followed by post sintering with HIP and with SPS. Several combinations of vacuum sintering + HIP/SPS conditions were tested on YAG and Yb:YAG samples prepared by reactive sintering of single oxides in order to modify their microstructure, especially grain size and residual porosity. Magnesium oxide (M) or magnesium oxide with TEOS (M+T) were used as sintering aids. 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 YAG ceramics. Acknowledgements: F. P. gratefully acknowledges the financial support of JECS TRUST.The authors from CNR ISTEC gratefully acknowledge the support from the Italian Ministry of Defence under PNRM Contract No. 8731 of 04/12/2019 (CeMiLAP²).

In this article, 5 at.% Yb:Lu2O3 transparent ceramics were fabricated by vacuum sintering combined with hot isostatic pressing (HIP) posttreatment using co-precipitated nano-powders. The influence of precipitant molar ratio, ammonium hydrogen carbonate, to metal ions (AHC/M3+, R value) on the properties of Yb:Lu2O3 precursors and calcined powders was investigated systematically. It was found that the powders with different R value calcined at 1100 degrees C for 4 hours were pure cubic Lu2O3 but the morphologies of precursors and powders behaved differently. The opaque samples pre-sintered at 1500 degrees C for 2 hours grew into transparent ceramics after HIP posttreatment at 1750 degrees C for 1 hour. The final ceramic with R = 4.8 showed the best optical quality with the in-line transmittance of 79.7% at 1100 nm. The quasi-CW laser operation was performed at 1034 nm and 1080 nm with a maximum output power up to 8.15 W as well as a corresponding slope efficiency of 58.4%.

In recent years, oxide materials based on garnet structure are being investigated as very promising candidates in the field of scintillation because of their high density, good chemical stability, optical transparency, and the possibility to easily incorporate luminescent rare-earth ions. Several studies demonstrated that garnet crystals show high light yield and advantageous timing performances, which make them of interest for applications in medical imaging and high energy physics detectors [1]. Among synthetic garnets, Ce-doped gadolinium gallium aluminum garnet (GGAG:Ce) is a relatively new and interesting material. It is a mixed garnet that has displayed very good scintillation and luminescence properties: its high density enhances the interaction with ionizing radiation, and the presence of Gd provides a high cross section for thermal neutron capture [2]. GGAG:Ce preserves a crystalline cubic structure, which allows to produce it in the form of transparent polycrystalline ceramic [3] with favorable characteristics for optical applications such as lasers, LEDs, and scintillators. In this work, ceramic samples were produced by reaction sintering from commercial oxide powders: the mixed powders were pressed into pellets and sintered by a combined process of air sintering and hot isostatic pressing. The sintering process was carefully selected and the use of sintering additives was optimized to eliminate porosity, which is crucial to achieve a good optical transparency. Optical properties were studied by means of optical absorption spectroscopy, steady-state and time resolved photo- and radio- luminescence, and correlated to the fabrication process parameters. Moreover, trapping phenomena caused by the presence of point defects were investigated by wavelength resolved thermally stimulated luminescence in a wide temperature range (10 - 800 K); a significant persistent luminescence signal was also singled out and investigated as a function of temperature. The presence of point defects was also evidenced by the occurrence of a sensitization of the radio-luminescence signal as a function of increasing cumulated X-ray dose, related to a competitive process between traps and Ce recombination centers in free carrier capture. Finally, preliminary results on recently developed layered Y3Al5O12:Pr/Gd3(Ga,Al)5O12:Ce (YAG:Pr/GGAG:Ce) ceramics for particle detection and discrimination will be also reported. This work has been supported by H2020 European Institute for Innovation and Technology (EIT) SPARK project (16290) and H2020 Rise project INTELUM (644260). [1] M. T. Lucchini et al., Nucl. Instrum. Methods Phys. Res. A 816 (2016) 176-183 [2] J. Dumazert et al., Nucl. Inst. Methods Phys. Res. A 882 (2018) 53 [3] Y. Ye et al., Opt. Mater. 71 (2017) 23

Transparent polycrystalline ceramic with magnesium aluminate (MgAl2O4) spinel composition is an attractive material for several optical applications [1,2]. The preferred fabrication approach for large size parts, is a two stage, pressure assisted, sintering process, where a linear pressing step performed up to a level of densification at which open porosity is eliminated, is followed by a hot isostatic stage. Both in this and other fabrication procedures spinel powders are generally used. Starting with oxide mixtures - a procedure able to significantly simplify the fabrication process and reduce its cost - is avoided because MgAl2O4 formation is associated with a noticeable volume change that leads to a consistent reduction of its sinterability. Our preliminary tests showed, however, that when pressure assistance is employed along all the sintering process, the volume change is negligible and does not affect the densification. This work presents the results obtained by double sintering spinel starting from a properly balanced mixture of commercial oxide powders (as opposed to the usual already formed spinel). During the hot pressing cycle at moderate temperature and time the material densify up to the closure of the open porosity. During the hot isostatic pressing cycle at moderate pressure the full closure of the porosity is obtained, leading to a transmittance higher than 75 % for thicknesses larger than 5 mm. The advantage of this process is twofold. It allows to simplify and reduce the cost of the press-feed material preparation and to obtain highly transparent components without the use of extremely high pressures; it also limits the length of polishing operations because the use of hot pressing leads to flat surfaces. 1.A. Goldstein, Correlation between MgAl2O4-spinel structure, processing factors and functional properties of transparent parts (progress review)",J. Eur. Ceram. Soc., 32 [11] 2869-2886 (2012). 2.L. Esposito, A. Piancastelli and S. Martelli, "Production and characterization of transparent MgAl2O4 prepared by hot pressing", J. Eur. Ceram. Soc., 33 (2013) 737-747.