Defect analysis during vacuum sintering of large Nd: YAG laser ceramics by FEM

Large-sized Nd: YAG laser transparent ceramics have great potential as the gain medium in the high-power laser devices for military and industry due to the superior thermal conductivity and optical property. However, some defects of large-sized ceramics such as deformation and fracture sometimes occur during sintering process. The real-time characterization of thermodynamic properties in the high-temperature sintering (> 1750 °C) is very difficult. It leads to unclear reasons of sintering defect. Therefore, in this work, combined with sintering systems and experiments, the thermal distribution, stress, and strain of sintered bodies were analyzed by finite element method (FEM) with non-uniform density and temperature field assumptions. For Φ15 × 220 mm ceramic rod, the order of factors affecting the thermal stress was the temperature inhomogeneous (28.29 MPa) > density gradient (15.34 MPa) > the cooling rate (11.7 MPa). The order of factors affecting the strain was also the same: inhomogeneous temperature in furnace cavity > uneven density > the cooling rate under our sintering system, and the maximum deformation was at the middle part of ceramic rod. In addition, the fracture at random position could be considered as the microcrack inside the green body. Therefore, the quality of green body directly affected the formation of ceramic defects in later sintering. These results are crucial for controlling the defects in large-sized laser ceramics preparation.