Dense Molten Salt Method for Large-Scale Synthesis of Rare Earth Hafnate Powders

Rare earth hafnates (RE2Hf2O7; RE = La, Gd, Nd, Eu, etc.) with low thermal conductivity and excellent high-temperature stability are indispensable key materials in extreme environments and high-tech fields. In this work, a dense molten salt method (DMS) was developed for mass preparation of hafnate powders including La2Hf2O7, Nd2Hf2O7, Gd2Hf2O7, Eu2Hf2O7, and even (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Hf2O7, which only allowed the trace salt volatilization, while the internal salt “micro-pools” significantly promoted the in-situ formation of target products at relatively lower temperatures. Using La2Hf2O7 as an example, it could be successfully prepared at 1100 °C with 1:1 mass ratio of salt to reactant, both of which are much lower than those of traditional “powdery” molten salt method. Furthermore, only ~6 wt.% of salt loss was detected in current dense route, while it was as high as ~80 wt.% in the traditional one. A large-scale synthesis of RE2Hf2O7 powder by DMS may be achievable by stacking these dense blocks in a tunnel kiln, suggesting its potential applicability and scalability toward industrial production.