Nd3+-Y3Al5O12 system: Iso-valent substitution driven structural phase evolution and thermo-physical behavior

Abstract

The present work reports structural and thermophysical investigation in Nd3+- substituted Y3Al5O12 (YAG). In addition to being an interesting system for studying the effect of isovalent substitution on phase/structural evolution in a garnet system, it has direct relevance as an inert matrix for burning minor actinides (MA: Am, Cm, and Np) for reducing the long-term radiotoxicity of nuclear wastes. More importantly, garnet has superior thermal conductivity than the zirconia-based systems which are widely being explored for this application. In order to investigate the phase miscibility and resulting thermophysical behavior, fourteen nominal compositions in Y3-xNdxAl5O12 (0.0 ≤ x≤ 3.0) system (Nd3+: chosen as the surrogate for MA) were synthesized by gel combustion followed by variable temperature annealing. The atomistic-level mixing ensured high solubility (33 mol% at 1273 K) of Nd3+ in YAG retaining the garnet-phase but the solubility reduced (13 mol%) upon annealing at 1773 K. Raman spectroscopy supported the phase evolution with composition and temperature. The microstructure of select compositions was investigated using SEM/EDS. Nd3+ caused dilation of garnet lattice, increase in average thermal expansion, and lowering of thermal conductivity. It must be emphasized that Nd-doped garnet still possessed better thermal conductivity compared ZrO2-based systems, a desirable attribute for matrices for MA-transmutation. Other important thermodynamic functions such as molar heat capacity, entropy and molar Gibbs free energy were determined. The study exhibits (a) roles of ionic size, synthesis route, temperature on phase relations upon isovalent substitution in YAG; (b) validation of the thermophysical performance of YAG-type material for MA transmutation.