Spark Plasma Sintering of Fine Powders of Proton Conducting Phosphates

Abstract

　Spark plasma sintering (SPS) method was applied to prepare dense compacts of high-temperature proton-conducting phosphates such as rare earth orthophosphates LnPO₄ and rare earth polyphosphates LnP₃O₉ (Ln: rare earth). In the case of Sr-doped LaPO₄, the relative density higher than 98% could be achieved with the SPS method when sintering temperatures above 1050℃ were employed, whereas only 94% at the highest was obtained with the conventional pressure less sintering (PLS) method at 1200-1300℃. Mechanical properties of the SPS samples were considerably improved compared with those of the PLS samples. The SPS samples prepared at 1050-1100℃ exhibited the bending strength of 240 MPa and the Vickers hardness of 6.7 GPa, and these values were much better than those of the conventional high-temperature proton-conducting oxides. Sr- doped LaPO₄ prepared with the SPS method exhibited dominant proton conduction regardless of the sintering conditions, and the conductivities were comparable with those of the PLS samples. These results demonstrated that the SPS method was an effective tool to prepare proton-conducting phosphates with a high density and excellent mechanical properties while keeping a feature of proton conduction.