Preparation of monophasic YAG raw material by multi-stage process

Abstract

The paper deals with the study of the features of the process of YAG phase formation via the Solid-State Reaction (SSR) method according to the proposed multi-stage scheme, including three stages of various mechanical and thermal processing (pre-milling in a ball mill for 24, 48 and 72 h, as well as firing at 1150 °C, 1250 °C, and 1450 °C with intermediate grinding step). The commercial micrometer-sized Y2O3 and Al2O3 powders were used as starting materials. The phase transformation was monitored by the X-ray diffraction in each synthesis stage. The microstructure and elemental distribution of samples were studied by scanning electron microscopy and energy-dispersive X-ray spectrometry. During the work, we found the dual effects of partially reacted mixture grinding, which can accelerate or slow down the synthesis rate depending on the YAG content in the preceding stage. We have proposed a simple model including the concept of “critical YAG concentration”. When the lower limit of this parameter (over 60 %) is reached, grinding has a negative effect on the further course of the SSR. Carrying out the synthesis according to a three-stage scheme makes it possible to obtain a product with a YAG content above 90 %, regardless of the pre-milling time, and the use of a two-stage synthesis scheme provides a monophasic raw material (99 % of YAG) suitable for further single crystals growth. The increased efficiency of the multi-stage YAG synthesis scheme was demonstrated in comparison with the conventional one-stage obtaining process that yielded only up to 60% of the target phase in the temperature range up to 1500 °C.