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 (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)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.