Enhanced densification of pure SnO2 by spark plasma sintering

Abstract

Densification of pure SnO2 by conventional sintering is known [1, 2] to be very difficult due to the evaporative decomposition of SnO2 into SnO(g) and 1/2O2(g) occurring during sintering. The adsorbed impurities on the surface were also suggested to retard the densification process [3]. Therefore, it has been reported [4] that the fully-dense SnO2 specimens could be obtained only by hot isostatic pressing. In this investigation, we tried to achieve a full densification of SnO2 specimen by spark plasma sintering (SPS) which is known to be quite effective for the densification of various materials [5–10]. Although the exact mechanism of the enhanced densification by SPS has not been clarified yet, the surface cleaning by joule heating due to dielectric breakdown [5] would remove the surface impurities. Note also that the SPS is usually carried out at lowtemperatures compared with the conventional sintering, so that the evaporation of SnO2 can be minimized. In this study, therefore, the densification of SnO2 specimens using SPS process was investigated by varying the SPS conditions and starting powders. Commercial SnO2 powders (99.9% purity, Aldrich Chemicals, USA) as well as the powders prepared in the laboratory were used. To prepare the SnO2 powders, 200 ml of 0.2 M SnCl4·5H2O (99.9% purity, Aldrich Chemicals) butanol solution was dripped into 400 ml of 0.5 M (C2H5)2NH (diethylamine, 99.9%, J. T. Baker Co.) butanol solution. Ten milliliters of water was additionally dripped into solution for promoting hydrolysis reaction. The precipitate was washed using ethanol until no Cl− ion was detected. After pulverization, the powders were heat-treated at 250 ◦C for 1 hr to decompose residual (C2H5)2NH, and calcined at 500◦C for 1 hr. A key feature to be noted in the powder processing used in this study is the minimization of water consumption to enhance the dispersion of the powders [11]. Transmission electron microscope (TEM, JEM3000F, JEOL, Japan) was used to observe the agglomerates and particle sizes of each powder. For sintering, the powders were placed in a graphite die (10 mm in diameter) and heat-treated at various temperatures for 5 min under vacuum (∼2.7 Pa) in SPS furnace (Spark Plasma Sintering, ElTek Co., Korea). The pressure and electric current applied during sintering were 37.5 MPa and 1200 A, respectively. The densities