SYNTHESIS OF ULTRAFINE PURE AND YTTRIA-STABILIZED HAFNIA BY SOLID-STATE REACTION AT RELATIVELY LOW-TEMPERATURE

Highly reactive, ultrafine powders of pure hafnia as well as stabilized hafnia (8 mol % yttria) have been synthesized at relatively low temperature (500 degrees C) by solid-state reaction between standard coarse-grained powders of sodium metaphosphate (NaPO3) and sodium hafnate (Na2HfO3). The reaction product was found to be a two-phase mixture of sodium orthophosphate (Na3PO4) and hafnia. Subsequently, the orthophosphate phase was removed by washing with dilute nitric acid. The as-prepared powders were extremely fine (particle size < 20 nm) and crystalline (cubic fluorite structure). After cold pressing and heating to 1650 degrees C, the materials retained a fine-grained microstructure. Stable cubic HfO2 with Y2O3 addition (8 mol %) was obtained at a temperature which was not too high (1650 degrees C).