Some Properties of Hafnium Oxide, Hafnium Silicate, Calcium Hafnate, and Hafnium Carbide

<jats:p> The behavior of hafnium oxide was studied particularly in the temperature range 1500° to 18OO°C. Properties of HfO <jats:sub>2</jats:sub> at these temperatures and its reactions with ZrO <jats:sub>2</jats:sub> , SiO <jats:sub>2</jats:sub> , and CaO are given in terms of lattice and other physical measurements, many of which are new. Mono‐clinic hafnium oxide is stable to 1700°C., which is 600° higher than the corresponding inversion temperature of zirconia. Otherwise HfO <jats:sub>2</jats:sub> closely resembles ZrO <jats:sub>2</jats:sub> (a) in its lattice dimensions and sintering behavior, (b) in forming a high‐temperature tetragonal phase closely resembling tetragonal ZrO <jats:sub>2</jats:sub> , <jats:italic>(c)</jats:italic> in forming a continuous series of solid solutions with ZrOz, <jats:italic>(d)</jats:italic> in forming with silica a single compound (HfO <jats:sub>2</jats:sub> .SiO <jats:sub>2</jats:sub> ) similar to zircon, <jats:italic>(e)</jats:italic> in forming a carbide, (f) in uniting with up to 40% CaO to form cubic solid solutions; thereafter a compound CaO.Hf O <jats:sub>2</jats:sub> appears which is very similar to the corresponding zirconia compound. </jats:p>