The effect of Li-salt additions on the densification of tin oxide
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Sintering of pure SnO 2 to high densities is difficult due to its high vapor pressure, and hence, additives are typically used to enhance densification. In this study, the effects of two lithium compounds, LiF and LiNO 3 , on the densification behavior of SnO 2 were evaluated. While LiF resulted in only a modest improvement in densification, LiNO 3 additions resulted in densities of 5=95% theoretical at 1500 °C in air. Thermal, x-ray, and SEM/TEM microstructural analyses indicated no liquid phase formation. From these studies we attribute the enhanced sintering behavior to the ionic-compensation of Li + as an acceptor dopant, i.e., 3[Li's'^] = 2[Vo], which in turn increased the diffusivity of oxygen.
I. INTRODUCTION Polycrystalline tin oxide (SnO2) is difficult to densify by traditional powder-processing techniques because of its high vapor pressure at high temperatures. Mass transport in pure SnO 2 occurs primarily by the evaporation-condensation mechanism, which not only leads to neck growth without shrinkage, but also increases the grain and pore size.1'2 Due to this problem, many researchers have studied the effect of minor additives on the sintering behavior of SnO 2 . Matthews and Kohnke3 prepared Zn-doped polycrystalline SnO 2 and achieved 95% theoretical density at 1460 °C. Unfortunately, no details of the resulting microstructure and possible densification mechanisms were elaborated. Kimura et al.4 examined microstructure development of Zn-doped SnO 2 at temperatures from 1000 to 1300 °C and concluded that rapid bulk and/or grain boundary diffusion due to the presence of the additive led to the enhanced densification. Torvela et al.5 reported that A12O3 and SiO 2 additives inhibit grain and pore growth in the sintering of SnO 2 . However, dilatometric results and sintered microstructures showed no significant densification when fired at 1300 °C. Duvigneaud and Reinhard6 showed that the addition of CuO and ZnO enhanced sintering and attributed their observations to the influence of Cu 2+ and Zn 2+ on the defect structure of SnO 2 . These studies have shown that the addition of acceptor dopants to SnO 2 improves the densification behavior. To date, no studies have appeared that evaluate the potential of liquid phase sintering. From studies on the densification of BaTiO3,7'8 it is well known that Li salts, such as LiF, can lower the sintering temperature
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Present address: Department of Ceramic Engineering, 222 McNutt Hall, Rolla, University of Missouri, Missouri 65401. J. Mater. Res., Vol. 8, No. 7, Jul 1993
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several hundred degrees. The reason for this effect is the formation of a eutectic liquid, which not only results in extensive particle rearrangement during initial stages of sintering due to capillary forces, but also results in enhanced mass transport during later stages due to solution-reprecipitation processes. The present study was initiated to evaluate the potential of LiF or LiNO 3 to lower the sintering temperature of SnO 2 .
II. EXPER
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