Ultralow expansion ceramics in the HfO2-TiO2 system synthesized by an hydrolysis and polycondensation technique

Ultra-refractory ceramics from the HfO₂-TiO₂ system in the range 30–40 mol% TiO₂, with a near-zero thermal expansion, have been synthesized by hydrolysis and polycondensation of titanium alkoxide and hafnium dichloride alcoholic solutions and sintered at moderate temperature. Thermal stability, crystallization, density and microstructure of these materials have been examined. The as-prepared powder, amorphous at room temperature, crystallized quickly when heated at 500 ° C. Entire crystallization occurred after treatment at 1000 °C. Sintering at 1500 °C on cold-pressed samples led to ceramics with weak porosity (7%), low expansion coefficient <1×10^–6 °C^– with a minimum for 30 mol% TiO₂ content. SEM examination on sintered materials at 1500 °C reveals a grain size from 2–6 m, increasing with TiO₂ content.