Synthesis of monolithic nanoparticulate ZrO2 in a new polymorph of orthorhombic crystal structure at ambient pressure

Abstract

A new polymorph of self-stabilized o-ZrO2 of orthorhombic crystal structure has been synthesized at ambient pressure. It has the lattice parameters a = 0.3340, b = 0.5535, and c = 0.6364 nm in the Pmnb space group. A single phase of o-ZrO2 forms in 2 h of heating of a porous ZrO(OH)2 · xH2O precursor (amorphous) at 500 °C in air. The kind of precursor used in this novel o-ZrO2 synthesis is derived by a simple hydrolysis of aqueous ZrOCl2 · 8H2O in reaction with ammonia liquor at a controlled temperature of ∼5 °C. At this temperature, ZrO(OH)2 molecules as soon as they form (ZrOCl2 + 2NH4OH → ZrO(OH)2 + 2NH4Cl) recombine with one another through H-bonded H2O to form a ZrO(OH)2 · xH2O polymer gel. In flowing air at 20–40 °C, some of the H2O evaporates leaving behind porous ZrO(OH)2 · xH2O with NH4Cl impurities. The o-ZrO2 sample, ∼15 nm average crystallite size, transforms to monoclinic m-ZrO2, with a concomitant growth of ∼23 nm crystallites at temperatures as high as 800 °C. A kinetically different process operates in the direct heating of ZrO(OH)2 · xH2O gel, forming a tetragonal ZrO2 or m-ZrO2. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)