Processing and Properties Of Polycrystalline Nd:YAG Ceramics

Abstract

Polycrystalline Neodymium doped Yttrium Aluminum Garnet (Nd:YAG) ceramics are potential solid state laser gain media with consist of optical transparency and combination of thermal and mechanical properties. The most common scattering sites in the polycrystalline ceramics are residual porosity, grain boundary thickness, and grain boundary with second phases that reduce the laser performance. In order to develop such a material, the present work demonstrates the systematic study on the nanoscale powder synthesis, coarsening of nanoparticles, pressureless sintering in presence of additives (SiO2) and dopant (Nd), and eventually measure the mechanical, thermal, and optical properties to some extent. In this respect, YAG nanopowders with ~40 nm average particle size was prepared via co-precipitation synthesis and characterized to establish their purity, crystallinity, and morphology. The nanoparticles were calcined in the temperature range of 900°C-1500°C with an equal interval of 100°C for 1, 6, and 12 h. at the peak temperatures to optimize the coarsening phenomenon of pure YAG nanoparticles for further consolidation and sintering study. Silica (SiO2) was added in different proportions in the range of 0-2000 ppm to the optimum coarsened pure YAG (~0.7 μm) particles in order to understand its influence on the densification behaviour and sintered microstructure. Pressureless sintering at 1700°C for 2 h. comprises the uniform grain size (~3.9 μm) and high relative density (~97.2%) in the presence of optimum concentration of 1000 ppm SiO2 compared to without SiO2 (~ 96.1%). Despite SiO2 optimization, Nd was doped in different proportions in the range of 0-2 at.% to the optimum coarsened YAG powders containing 1000 ppm SiO2 in order to fabricate highly dense Nd:YAG ceramics with fine and homogeneous microstructure. Interestingly, a reversible phase transition of garnet (YAG) to perovskite (YAP) composition noticed at an Nd concentration beyond 1.5 at.%. Nd:YAG ceramics with a high relative density (~99.2%) and grain size (~5.3 μm) were obtained at an optimum concentration of 1.5 at.% Nd. The mechanical properties of the Nd doped YAG ceramics that include Vickers hardness and flexural strength were measured as 13.1 GPa and 202 MPa, respectively. The thermal properties in specific average linear thermal expansion coefficient from 30°C to 1000°C was 9.124 x 10-6 /°C and thermal shock resistance parameters R1 and R2 at 1000°C were 51°C and 548 W/m, respectively. The in-line transmittance was ~20% in the visible region. Apart from the achieved properties, one can envisage further transparency increment in the perspective of SSL application of Nd:YAG.