The effect of Sb2O3 on the densification of ZnO was studied and compared with that for additives such as ZnSb2O6,α-Zn7Sb2O12, and Sb2O4. Addition of up to 2.0 mol% Sb2O3 raised the densification temperature of ZnO from 600° C to 1000° C regardless of doping level and the particle size of Sb2O3. Addition of other antimony-additives, however, slowed down the densification of ZnO, and the densification temperature increased with increasing amount of additives. Results of TG, XRD and SEM showed that the observed densification characteristics in Sb2O3-doped ZnO are explained by the volatile nature of Sb2O3. Thus, Sb2O3 evaporates during oxidation at about 500° C and condenses on the ZnO particle surfaces as a noncrystalline phase of an unknown composition, which checks the material transport across ZnO particles resulting in the retarded densification.

Sintering of Sb2O3-doped ZnO

A detailed analysis of the crystallite shape of zinc oxide produced by thermal decomposition of zinc hydroxynitrate was performed as a function of temperature from 280° to 700°C. The shape and size were analyzed by X-ray line-broadening measurements. Evolution from cylinder to right hexagonal prism shapes is observed during annealing of the loose powder. A homothetic growth of crystallites is observed through the initial stage of annealing. Relations between surface area and X-ray crystallite size are presented and discussed in terms of the packing factor. Results suggest that two parallel growth processes take place during annealing.

Morphological Analysis and Growth of Crystallites During Annealing of ZnO

The effects of SiO2 and Cr2O3 on the formation process of ZnO varistors were investigated. Prior to formation of the Zn2.33Sb0.67O4 spinel phase (Sp-phase), a spinel-like phase forms. However, this phase does not control the varistor microstructure. The Sp-phase and the Bi2O3-phase which were formed by the decomposition of the Bi2(Zn4/3Sb2/3)O6 pyrochlore phase played important parts in the control of the varistor microstructure. That is, the Bi2O3 phase produced in the reaction promotes the initial sintering of the varistor and the Sp-phase inhibits the ZnO grain growth. In this reaction, SiO2 and Cr2O3 play a role in decreasing the decomposition temperature of the pyrochlore phase. Decreasing the decomposition temperature below 900° C (where ZnO grain growth begins) leads to the inhibition of ZnO grain growth.

Effects of SiO2 and Cr2O3 on the formation process of ZnO varistors

This paper discusses a transmission electron microscope investigation of a ZnO powder and low-temperature air-sintered pellets with small additions of Sb{sub 2}O{sub 3}. It was conducted in order to study the thin Sb-rich film on the surfaces of ZnO particles. This film was found to be noncrystalline at temperatures below 700 {degrees} C, and partially crystalline or completely crystalline above this temperature. An oriented overgrowth of the spinel Zn{sub 7}Sb{sub 2}O{sub 12} was observed on the prismatic planes as well as on the basal planes of ZnO crystallites. It is shown that these coherently overgrown spinel films are responsible for the inhibition of grain growth at high temperatures.

Transmission Electron Microscope Study of Antimony‐Doped Zinc Oxide Ceramics

The composition Zn2.33Sb0.67O4 (or Zn7Sb2O12) exists in two polymorphic forms. The thermodynamically stable, low-temperature orthorhombic β form transforms to the high-temperature cubic α-polymorph with a spinel structure at 1225°±25°C. The transformation is fully reversible but slower in the α→β direction and therefore, it is easy to preserve the high-temperature α-polymorph to lower temperatures where it is kinetically stable but thermodynamically metastable. It is also possible to synthesize the α-polymorph directly at low temperatures, e.g., 900°C. This synthesis, of a phase that is thermodynamically stable only at high temperatures, but which has sufficient kinetic stability to exist metastably at low temperatures, represents an example of Ostwald's law of successive reactions in which the first phase to crystallize from a reaction mixture is not necessarily the equilibrium phase of lowest free energy. The crystal structure of the α-polymorph has been confirmed by Rietveld refinement of X-ray powder diffraction data to be an inverse spinel, (Zn)[Sb2/3Zn4/3]O4, in which octahedral sites contain a disordered, random mixture of Zn and Sb and tetrahedral sites are fully occupied by Zn.

D. Kojar

Papers

Sintering and grain growth in doped ZnO