Oxygen Vacancies in Pure Tetragonal Zirconia Powders: Dependence on the Presence of Chlorine during Processing

Abstract

We have used several experimental methods to study how a large extrinsic oxygen vacancy density in pure tetragonal ZrO 2 powders depends on details of how those powders are made. Samples were made from oxychloride and nitrate precursor solutions. We used perturbed angular correlation spectroscopy to determine in situ phase structure and the density of oxygen vacancies at 1200°C, XRD and SEM to determine the grain size and morphology of samples annealed at temperatures ranging from 200°-1200°C, and neutron activation analysis (NAA) to investigate purity of samples. NAA results showed that samples contain cation impurities at levels <<100 ppm. The XRD and SEM measurements showed that grains were nanometer-size, had a broad distribution, and grew from ϳ10 nm at 200°C to ϳ1 m at 1200°C. The most striking process dependence is on presence of chlorine during processing. The grain size and phase above 600°C, and both the morphology and the density of oxygen vacancies at 1200°C were strongly affected by presence of chlorine-containing vapor during annealing. Samples processed in a chlorine-free atmosphere had large well-sintered grains and large (>500 ppm) oxygen vacancy concentrations at 1200°C, whereas samples processed in flowing H 2 O/HCl vapor had smaller grains, porous morphology, and small (<100 ppm) vacancy density. All samples were loose powders consisting of single grain particles at <1000°C and multiple-grain particles at 1200°C.

Three‐Dimensional Assemblies of Zirconia Nanocrystals Via Shape‐Preserving Reactive Conversion of Diatom Microshells

“…The enhanced formation of relatively large zirconia crystallites, and of monoclinic zirconia, at 6501C with an increase in the amount of the ZrCl 4 reactant was consistent with prior reports on the effect of chlorine on the tetragonal-to-monoclinic zirconia transformation at temperatures below 10001C. 48,49 Karapetrova et al 48 reported that zirconia specimens with relatively high initial chlorine contents (up to several percent, synthesized from oxychloride precursors) exhibited rapid grain growth between 6001 and 8001C, and transformed more rapidly to monoclinic zirconia over this temperature range, than specimens containing little initial chlorine (on the order of 200 ppm, synthesized from nitrate precursors). 48 Pasquevich et al 49 found that the growth rate of isolated crystallites, and the tetragonal-to-monoclinic transformation, occurred at faster rates in Ar-Cl 2 gas mixtures (with a chlorine partial pressure of 300 Torr) than in pure argon at 9401C.…”

“…48,49 Karapetrova et al 48 reported that zirconia specimens with relatively high initial chlorine contents (up to several percent, synthesized from oxychloride precursors) exhibited rapid grain growth between 6001 and 8001C, and transformed more rapidly to monoclinic zirconia over this temperature range, than specimens containing little initial chlorine (on the order of 200 ppm, synthesized from nitrate precursors). 48 Pasquevich et al 49 found that the growth rate of isolated crystallites, and the tetragonal-to-monoclinic transformation, occurred at faster rates in Ar-Cl 2 gas mixtures (with a chlorine partial pressure of 300 Torr) than in pure argon at 9401C. 49 The present work demonstrates, for the first time, that a series of gas/solid displacement reactions may be used to exchange the silica in biologically reproducible, 3-D microtemplates with a first oxide and then a second oxide, without a loss of the original microtemplate shape.…”

Synthesis of zirconia nanopowders from various zirconium salts via polyacrylamide gel method

“…by increasing chloride impurity content, oxygen vacancies density decreases significantly. 25 The possibility remains that oxygen vacancies may act as point defects, inhibiting the martensitic t → m transformation. 27 …”

Mechanisms of room temperature metastable tetragonal phase stabilisation in zirconia

“…The importance of oxygen ion vacancies in stabilising the tetragonal structure, at room temperature, within the nanocrystalline ZrO 2 has been suggested by different research groups. [75][76][77][78][79][80][81][82][83][84] It is to be noted that the stability of tetragonal phase in the bulk ZrO 2 has been attributed to the presence oxygen ion vacancies…”