Synthesis of Monolithic ZrO2 in a New Polymorph of Orthorhombic Crystal Structure with a Mesoporous Precursor

Abstract

A high energy amorphous ZrO(OH)2.xH2O precursor (porous) is obtained by a controlled hydrolysis of dispersed Zr4+ cations in an aqueous solution in reaction with NH4OH at 5°C followed by aging and drying of the recovered gel at room temperature. At 20° to 40°C in air, the excess H20 evaporates leaving behind a porous ZrO(OH)2.xH2O powder with φ ∼ 37% porosity. On heating, a reconstructive ZrO2 phase transformation occurs from a refined amorphous state at 500°C to form a new orthorhombic structure in a mesoporous structure. Average crystallite size is D ∼ 15 nm. X-ray diffractogram determines the lattice parameters of a = 0.3340, b = 0.5535 and c = 0.6364 nm, with V0 = 0.1177 nm3 lattice volume or ρ = 6.96 g.cm−3. A high-pressure phase of Pnma o-ZrO2 is known of a similar value of ρ = 6.78 g.cm−3. Other known c-, t- or m-ZrO2 polymorphs of the cubic, tetragonal, or mono-clinic structures have somewhat lower ρ-values of 6.21 g.cm−3or still smaller. The o-ZrO2 irreversibly transforms to m-ZrO2, D = 23 nm, at as early a temperature as 800°C.