Rutile ceramics

Titania occurs in three crystalline modifications anatase, brookite and rutile. Because above approximately 800 °C both anatase and brookite have transformed to rutile it is the only form of significance in the ceramics context and attention here is limited to it. 

The rutile structure (Fig. 5.27) is based on nearly close-packed oxygen ions with Ti4+ ions occupying half the octahedral sites. The tetragonal unit cell contains two formula units, and the Ti4+ is at the centre of a distorted oxygen octahedron. 

Rutile is anisotropic, with the values of er at room temperature being approximately 170 and 90 in the c and a directions respectively. In the polycrystalline ceramic form er averages to intermediate values with a 

An important feature of Ti02 is the extent to which it can be chemically reduced above approximately 900 °C with accompanying significant changes in electrical conductivity, as shown in Fig. 5.30. The fall in resistivity is accompanied by a loss of oxygen and the movement of Ti ions onto interstitial sites, probably the empty octahedral sites in the rutile structure  

Measurements of conductivity at 1000°C and oxygen pressures below 101 mPa (10 10atm) have confirmed Eq. (5.20) (see Section 2.6.2). 

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Rutile Ceramic Pigments. Structurally, all rutile pigments are derived from the most stable titanium dioxide structure, ie, rutile. The crystal structure of rutile is very common for AX2-type compounds such as the oxides of four valent metals, eg, Ti, V, Nb, Mo, W, Mn, Ru, Ge, Sn, Pb, and Te as well as halides of divalent elements, eg, fluorides of Mg, Mn, Fe, Co, Ni, and Zn. 

The system BaO — T1O2 comprises 5 compounds, three of which have an incongruent melting point. The lowest eutectic melts at 1317 °C. Only two of the compounds mentioned find practical applications BaTi03 and BaTi409. The former is especially significant and will be discussed in detail below. The other compound is one of the correcting phases used in rutile ceramics (see above). In addition, it constitutes abase for linear dielectrics with a low temperature dependence of permittivity. The properties of some of the materials dealt with above are listed in Table 27. 

The materials mentioned in the previous sections had permittivities approximately equal to 6. Values exceeding 10 are rare among standard substances. The first exception discovered was the permittivity of 86 to 170 exhibited by Ti02 (rutile) as a function of crystallographic orientation. This discovery was exploited in the manufacture of rutile ceramics in the thirties. The theoretically attainable value for polycrystalline materials is about 105. In practice, the permittivity of rutile ceramics is always lower owing to the presence of pores and foreign phases. 

The initial mix for the manufacture of rutile ceramics usually contains about 90 % T1O2 in the form of anatase or rutile. Anatase is converted above 900 °C to rutile, which is then stable over the entire temperature range. Only rutile is therefore present in a sintered product. The polymorphic inversion involves a considerable volume contraction, since the density of anatase is 3.9 while that of rutile is 4.25 g/cm. This is why cracks occur on firing in anatase-containing bodies. This drawback can be eliminated by pre-calcining at least some of the Ti02 at temperatures exceeding 1000 C, to convert it to rutile. 

Ceramics based on Ti02 (rutile ceramics) have already been dealt within Section 6.2. 

Gargori, C, Cerro, S., Galindo, R., and Monros, G. (2010) In situ synthesis of orange rutile ceramic pigments by non-conventional methods. Ceram. Int, 36,... 

In one of the most significant observations, small amounts of recrystallized material were observed in rutile at shock pressure of 16 GPa and 500 °C. Earlier studies in which shock-modified rutile were annealed showed that recovery was preferred to recrystallization. Such recrystallization is characteristic of heavily deformed ceramics. There has been speculation that, as the dislocation density increases, amorphous materials would be produced by shock deformation. Apparently, the behavior actually observed is that of recrystallization there is no evidence in any of the work for the formation of amorphous materials due to shock modification. Similar recrystallization behavior has also been observed in shock-modified zirconia. 

A. Navrotsky and O.J. Kleppa, Enthalpy of the anatase-rutile transformation, J. Am. Ceram. Soc. 

Titanium (TV) dioxide (TiO ), also known as rutile, is one of the best-known compounds used as a paint pigment. It is ideal for paints exposed to severe temperatures and marine climates because of its inertness and self-cleaning attributes. It is also used in manufacture of glassware, ceramics, enamels, welding rods, and floor coverings. 

Liu H, Gao L (2004) (Sulfur, Nitrogen)-codoped rutile-titanium oxide as a visible-light-activated photocatalyst. J Am Ceram Soc 87 1582-1584... 

We have included here, for comparison, the results of a study of zirconolite-rich Synroc nominally composed of 80 wt% Ce- or Pu-doped zirconolite plus 10 wt% hollandite and 10 wt% rutile (Hart et al. 1998). Inclusion of this study in this section is significant because the two additional phases are both highly durable in their own right and the experiments were conducted at two different temperatures (90 and 200 °C) and in three different aqueous solutions (pure water, silicate, and brine). The authors found no major differences in the release rates of Ca, Ce, Hf, Ti, Zr, Pu, and Gd apart from those for Ce and Ti, which appeared to be somewhat higher in the brine. On average, for all elements, the increase in temperature caused the release rates to increase by a factor of approximately seven. Release rates were generally below 10 2 g/m2/d for Ca, 10 3 g/m2/d for Ce and Gd, and 10 4 g/m2/d for Ti, Zr, Hf, and Pu (except for the brine at 200 °C, which gave a Ti release rate of 2 x 10 3g/m2/d). Hart et al. (2000) also determined the release rate of Pu in an LLNL-type zirconolite ceramic. After nearly one year in pure water at 90 °C the release rate of Pu decreased from 2 x 10-3 g/m2/d to less than 10-5 g/m2/d (Fig. 7). 

Titanium dioxide [13463-67-7], Ti02, Mr 79.90, occurs in nature in the modifications rutile, anatase, and brookite. Rutile and anatase are produced industrially in large quantities and are used as pigments and catalysts, and in the production of ceramic and electronic materials. 

On heating to higher temperatures, no crystalline phases are observed until anatase crystallizes at 1000 °C. At 1400 °C, anatase, rutile and crystobalite are the only products. No single-phase material is obtained. The lack of correspondence between the TGA ceramic yield and the theoretical ceramic yield calculated for TiSi04 presages this problem. The exact reasons for the formation of a mixed-oxide phase are unknown at the moment, but they clearly contrast with the behavior of the Zr and Hf analogs. 

Titanium dioxide is found in nature in three crystal forms anastase, brooldte, and rutile. Its extreme whiteness and brightness and its high index of refraction are responsible for its widespread use as a white pigment in paints, lacquers, paper, floor covering, plastics, rubbers, textiles, ceramics, and cosmetics, see also Aluminum Catalysis and Catalysts Copper Fertilizer Haber, Fritz Iron Steel. 

Samples of the ceramic polycrystalline Ti02 (rutile) doped electrodes of the VxTi . x02 composition were studied at different vanadium content (0.001 < x < 0,05) in [128, 129]. It was shown that at x < 0,003 the EPR spectra perform a well resolved hyperfine structure (hfs) typical of V4+-doped rutile (Fig. 8.10), in which V4+ ions substituted Ti4+ ions in the crystal lattice. At 0.003 < x < 0.01, the dipolar broadening of the individual lines 8H occurred. At x > 0.01, in parallel with continuing broadening of the hfs lines, a broad single line appears (Fig. 8.11). Its part in the spectrum increased with the increase of vanadium content. 

Pure rutile is an excellent insulator at room temperature with an optical band gap between the filled O 2p valence band and the empty Ti 3d conduction band probably in the range 3.5-4.0eV. A thermal energy of approximately 1.7-2.0eV can transfer electrons from the valence to the conduction band leading to semiconductivity. Figure 5.29 shows typical conductivity data for a high-purity titania ceramic (> 99.95 wt% Ti02) measured in oxygen at 1 atm. 

Another circumstance is when deterioration becomes apparent under fields in excess of 0.5 MVm-1 at temperatures above 85 °C, and occurs more rapidly the higher the field or the temperature. The fall in resistance has been observed in single crystals of rutile and barium titanate and so must be assumed to be a bulk rather than a grain boundary effect, although there is evidence that grain boundaries play a part in degradation processes in ceramics. 

Titanium raw-material utilization can be broken down as illustrated in Figure 9. About 4% of the titanium mined is used as metal, 94% is used as pigment-grade Ti02, and 2% as ore-grade rutile for fluxes and ceramics. In 1995, the estimated U.S. Ti02 pigment production was valued at 2.6 billion and was produced by five companies at 11 plants in nine states. About 47% was used in paint, 18% in plastics, 24% in paper, and 18% in other miscellaneous applications (56). 

Anatase and rutile thin films can be generated from Ti(thd)2(OPr-/)2 (26b) by CVD and photoassisted CVD (glass, qnartz and ceramic substrates substrate temperature 360-600 °C deposition pressure 9-76 mTorr). The photocatalytic activity of these films was studied towards the photodegradation of fenarimol. In contrast to the films obtained from Ti(OPr-i)4 with 26b as starting material, no photocatalytic effect was observed. 

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