**Zinc-Tin-Oxide-Based Porous Ceramics: Structure, Preparation and Properties** Zinc-Tin-Oxide-Based Porous Ceramics: Structure,

DOI: 10.5772/intechopen.71581

Tamara Ivetić

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76 Recent Advances in Porous Ceramics

snb.2016.01.015

Additional information is available at the end of the chapter Tamara Ivetić

http://dx.doi.org/10.5772/intechopen.71581 Additional information is available at the end of the chapter

Preparation and Properties

#### Abstract

Zinc-tin-oxide-based ceramics have been extensively investigated especially regarding the synthesis of zinc stannate (Zn2SnO4), a spinel structure ternary compound with a wide range of possible applications. Among all of those, the best-known use of this material is in the combustion gas and moisture sensors. This chapter presents the research results review on the structure, morphology, and properties of mechanochemically synthesized Zn2SnO4 ceramics with large open porosity, as well as the results obtained during its solid-state processing optimization. Also shown is the review of the results obtained in the study of the influence of addition of the small amounts of bismuth oxide (Bi2O3) on the obtained Zn2SnO4 structure, microstructure, and electrical properties, as it provides the condition for the liquid phase sintering and creates a new dynamics in the zinc-tin-oxide reaction sintering process.

Keywords: ZnO-SnO2 ceramics, spinels, zinc stannate, mechanical activation, sintering

## 1. Introduction

Zinc stannate is a non-toxic transparent n-type semiconducting oxide material whose electrical conductivity is sensitive to the changes of oxygen stoichiometry and environmental atmosphere, so it is mostly known for its gas-sensing (detection of combustion gases, CO, H2, NO, NO2, and moisture) applications [1–3]. However, the unique electrical and optical properties of the zinc stannate, which has been manufactured so far in different forms (thin and thick films, polycrystalline powders, composite and porous sintered ceramics) [4–7], makes it a suitable material for the various other applications as well (as functional coatings, flat panel displays, thin film solar cells, windows coatings, transparent conducting electrodes, as anode material in Li-ion batteries, as various photoelectrical devices, in photocatalysis) [8–20]. Zinc stannate is a spinel compound with a general formula Zn2SnO4 and a band gap of 3.6 eV. The spinel

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© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

Zn2SnO4 in the bulk form is stable in the inverse spinel structure, with a face-centered cubic unit cell (space group Fd3m), where Zn2+ occupy the 8a sites, and both Zn2+ and Sn4+ cations occupy the 16d sites, while O is placed in the 32e sites. Its unit cell parameter has a value of a = 8.6574 Å (JCPDS PDF 24-1470). The spinel-type structures can have big cation disorders in the crystal lattice and certain nonstoichiometry. Nevertheless, the disorders in spinels are not conventional so there is no change in the symmetry. The most general spinel formula is AB2O4, where A is two valent or four valent metal ions, and B are two valent or tree valent metal ions. To this day, there have been synthesized over 200 different types of spinel oxide compounds. Some spinel compounds are known to have the characteristic sensor and catalytic properties, like Zn2SnO4, and exhibit complex disordering phenomena related to the redistribution of cations over (B) and [A] sublattices in their structure. The partly inverse spinels like Zn2SnO4 have four valent ions on octahedral positions [A] and two valent ions in some ration distributed over tetrahedral (B) and octahedral coordination [A]. The zinc stannate compound may then be presented as (Zn2+)[Sn4+Zn2+]O4 to emphasize the site occupancy at the atomic level.

The previous solid-state synthesis investigations established that complete Zn2SnO4 formation needs prolonged mechanical activation of the starting reaction precursors (ZnO and SnO2 powders) and considerable high temperature of sintering (in the range from 1000 to 1280C). The solid-state chemical reaction between the ZnO and SnO2 starts relatively slow at 1000C while the monophase polycrystalline zinc stannate is formed at 1280C.
