Additions of 0.5 to 2.0 mol% of CoO or MnO{sub 2} onto SnO{sub 2} promote densification of this oxide up to 99% of theoretical density. The temperature of the maximum shrinkage rate (T{sub M}) and the relative density in the maximum densification rate ({rho}*) during constant sintering heating rate depend on the dopant concentration. Thus, dopant concentration controls the densifying and nondensifying mechanisms during sintering. The densification of SnO{sub 2} with addition of CoO or MnO{sub 2} is explained in terms of the creation of oxygen vacancies.

Effect of cobalt(II) oxide and manganese(IV) oxide on sintering of tin(IV) oxide

SnO 2 green pellets were submitted to ac electric fields at temperatures below 1350 °C. Electric current pulses occurred and a substantial modification was found in the microstructure of the pellets after application of 80 V cm −1 at 900, 1100 and 1300 °C. Similar experiments were carried out in SnO 2 mixed to 2 wt.% MnO 2 . The linear shrinkage of the pellets was monitored with a dilatometer during the application of the electric field. Scanning electron microscopy micrographs of the pellets show the grain structure evolution after the electric current pulses. The larger is the electric current flow through the SnO 2 pellet, the larger are the shrinkage and the average grain size. Even though sintering occurs without significant densification in SnO 2 , the welding of the grains is evident. The apparent density of green pellets of SnO 2 with MnO 2 addition sintered at 1100 °C increased 110% with the application of 80 V cm −1 , 5 A.

Electric field-assisted flash sintering of tin dioxide

Cerium oxide is a very useful base material used as catalyst supports, ion conductors and gas sensors. It is also well known that ceria-based materials are difficult to densify below 1500°C. However, a small amount of Fe doping obviously promotes the densification rate and reduces the sintering temperatures. In this study, the early stage sintering mechanisms of undoped and Fe-doped CeO2 were investigated, based on two sintering models. We confirm that pure CeO2 exhibits volume-diffusion controlled sintering, while a viscous flow mechanism dominates the early stage sintering of Fe-doped CeO2.

Early-stage sintering mechanisms of Fe-doped CeO2

The microstructure development in pure and ZnO-, Nb2O5- or (ZnO + Nb2O5)-containing SnO2 has been studied. Sintering of pure SnO2 proceeds by an evaporation-condensation mechanism and grain and pore sizes increase without densification. The pores are open with no closed pores developed. Grain and pore structures become homogeneous with sintering time. Sintered pure SnO2 is porous and has large grains. Nb2O5 suppresses particle coarsening and pore growth but does not promote densification, leading to porous sintered bodies with small grains. ZnO and (ZnO + Nb2O5) promote densification but do not suppress particle and grain coarsening. The sintered body is composed of large grains with pores located on grain boundaries.

Microstructure development in SnO2 with and without additives

Lanthanum zirconate (La2Zr2O7) was prepared by coprecipitating lanthanum nitrate and zirconyl oxychloride at pH 10, followed by ethanol washing The initial high surface area of 304 m2·g−1 decreased very rapidly with increased sintering temperature and decreased to an immeasurably small value after heating at 1200°C for 15 h The major parameters studied were phase evolution, crystallite size, porosity, surface area reduction, and shrinkage during sintering Three temperature regions were identified based on these studies: below the crystallization temperature, between the crystallization temperature and 1100°C, and above 1100°C The main contribution of surface area reduction in the region 800°–1100°C was due to surface diffusion; the main contribution above 1100°C was due to grain-boundary diffusion coupled with surface diffusion

Sintering of Lanthanum Zirconate

The enhanced sintering of tin dioxide with additives, viz. MnO2, CuO, ZnO, Li2CO3 and Sb2O3 has been studied under isothermal condition. The efficiency of different additives in attaining higher density is discussed in terms of grain growth and pore morphology studied by SEM fractograph.The time exponents of the kinetic equation and activation energies are calculated from isothermal sintering data for various systems and these parameters are used to interpret the sintering mechanism for different additives. It has been found that different mechanisms are predominant in different systems and in some cases it depends on sintering temperature also. From SEM fractograph it appears that due to pore trapping within the grains it is not possible to attain higher density.

A. Paul

Papers

Enhanced Sintering of Tin Dioxide with Additives under Isothermal Condition