The influence of condition of the monazite structured La$_{0.9}$Y$_{0.1}$PO$_4$ nanocrystals sintering on thermal and mechanical properties of the material

A lanthanum orthophosphate- and yttrium-based monazite structured La$_{0.9}$Y$_{0.1}$PO$_4$ nanocrystalline material was synthesized. The influence of the thermal treatment temperature on its thermal and mechanical properties has been determined. It was shown that the hydrothermal treatment of coprecipitated phosphates at 230$^\circ$C for 110 minutes yielded monazite structured nanoparticles with the crystallite size of about 16 nm. Nanopowder thermal treatment for 20 min in the annealing-quenching mode at 1000–1400$^\circ$C resulted in obtaining compact materials, the porosity of which varied from 52 to 27% with the grain growth from $\sim$20 to $\sim$100 nm. Active sintering occurred at temperatures above 1300$^\circ$C. Nanocrystalline materials obtained by heat treatment at 1200$^\circ$C with a grain size of $\sim$40 nm have a porosity of 42%, microhardness of $H_v$ (25$^\circ$C) = 4 $\pm$ 0.5 GPa, thermal conductivity $\lambda$ (25$^\circ$C) = 0.4 W$\cdot$m$^{-1}\cdot$K$^{-1}$, and can be used as thermal insulation material.