Ceramics, sintered titania

Edeison L H and Giaeser A M 1988 Roie of particie substructure in the sintering of monosized titania J. Am. Ceram. See. 71 225... 

Nadaud N., Nanot M., Boch P. Sintering and electrical properties of titania- and zirconia-containing In203-Sn02 (fTO) ceramics. J. Am. Ceram Soc. 1994 77 843-46. 

High purity binary oxides such as BeO, MgO, AI2O3, Ti02, And stabihzed zirconia are sintered under pressure to produce high density ceramic ware, particularly cracibles and other containers for use in nonferrous metallurgy and other specialized applications where temperature resistance and corrosion are a problem. Titania is also formed in honeycomb arrays for use as catalyst supports. 

Mixed ion and electronic conducting ceramic membranes (e.g. yttria-stabilized zirconia doped with titania or ceria) can be slip cast into a tubular form from the pastes containing the constituent oxides in an appropriate proportion and other ingredients and the cast tubes are then subject to sintering at 1,200 to 1,500X to render them gas impervious [Hazbun, 1988]. 

As heated alkalies readily attack stainless steel, it was found necessary to use liners or crucibles in the above bombs. Borosilicate glass was found satisfactory up to about 450 °C. for short periods of time, but it could not be used above this temperature. Various ceramic materials were tried for possible use as containers for the sodium peroxide-superoxide mixtures. Sintered aluminum oxide and magnesium oxide were satisfactory up to 450 °C., but above this temperature the sodium oxides penetrated the sintered material and corroded the stainless steel bomb. Commercial ceramic coatings containing (as the principal components) alumina, magnesia, and titania were also tried without success at a temperature above 450 C. 

Figure 5.10. Densification during sintering of two titania compacts. Ceramic A is sol-gel made and ceramic B is made of commercially available milled powder. The former sinters at lower temperature and to a higher final density. With permission from Engineered Materials Handbook, Vol. 4, Ceramics and Glasses (1991). ASM International, Materials Park, OH.

Ceramic foams are produced from organic precursor foams such as polyurethane or polyolefins. Their pores are then filled with an aqueous slurry of the ceramic typically containing 20 wt.% of ceramic particles in the size range of from 0.1 to 10 pm [461]. Wetting agents, dispersion stabilisers and viscosity modifiers are added to the slurry. Suitable ceramics are alumina, alumina silicates, zirconia, stabilised zirconia and titania, amongst others. The pores ofthe precursor foam may be filled completely or only coated on their surface by the ceramic particles. The foam is then dried and calcined at 1000 °C, which removes the polymer and sinters the ceramic. Metallic foams have similar properties compared with ceramic foams, but superior mechanical stability and improved heat conductivity. 

Fig. 8. Average X-ray crystallite size evolution upon sintering of the alumina ceramics, doped with (a) - titania, AH, (b) - magnesia, AMI, (c) - AI2Q3 without dojjants constant heating rate 10°C/ min.

In order to exploit the application of titania in electronic ceramics, Nair et al. (1999) studied sintering of nanostructured pure and doped titania samples. An aqueous sol containing titania particles of 10 nm or less size was used for the preparation ofpure and Cu, Ni, La-doped gel samples. The densification temperature and properties of some selected titania compositions are also shown in Table 6-2. Figures 6-2(a) and (b) show the field emission scanning electron micrographs (FE-SEM) ofpure titania sintered at 750°C and 800°C for 8 h. It may be noted that the 750°C calcined gel samples, though porous. 

From the sintering studies oftitania, it was clear that sintering and phase transformation are interrelated and both contribute to the densification of the ceramic. Kumar et al. (1992) reported that phase transformation precedes sintering, and thus enhances the densification of nanostnictured titania, whereas Nair et al. (1999) observed that sintering precedes phase transformation in some cases and vice versa in other cases. 

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