Titanium high-temperature phase

Titanium dioxide, Ti02, has three stable allotropic forms the low temperature phases of anatase and brookite and the stable high temperature phase of rutile. Initial reports of CVD Ti02 films using Ti(0-i-Pr)4 in the presence of O2 described the deposition of anatase films over the temperature range of 320-750°C [122], Whereas, studies using Ti(OEt)4 (b.p. = 102°C, 0.05 Torr) as the precursor showed that rutile films are formed at substrate temperatures below 500°C or if high mass flows [123]. 

The a phase of this alloy is similar to that of xm-edloyed titanium. The P phase is the high-temperature phase of titanium, but a considerable amount is stabilized to room temperature by the manganese content. The P phase composition of the Ti-8Mn alloy may be varied by solution treatment above the p transus and below the transus in the a-P region, resulting in the P phase containing 11 and 15% Mn, respectively. 

Mixed-phase oxide pigments are manufactured by high temperature (800-1000 °C) solid state reactions of the individual oxide components in the appropriate quantities. The preparation of nickel antimony titanium yellow, for example, involves reaction of Ti02, NiO and Sb203 carried out in the presence of oxygen or other suitable oxidising agent to effect the necessary oxidation of Sb(m) to Sb(v) in the lattice. 

Apart from the reactions described above for the formation of thin films of metals and compounds by the use of a solid source of the material, a very important industrial application of vapour phase transport involves the preparation of gas mixtures at room temperature which are then submitted to thermal decomposition in a high temperature furnace to produce a thin film at this temperature. Many of the molecular species and reactions which were considered earlier are used in this procedure, and so the conclusions which were drawn regarding choice and optimal performance apply again. For example, instead of using a solid source to prepare refractory compounds, as in the case of silicon carbide discussed above, a similar reaction has been used to prepare titanium boride coatings on silicon carbide and hafnium diboride coatings on carbon by means of a gaseous input to the deposition furnace (Choy and Derby, 1993) (Shinavski and Diefendorf, 1993). 

Operation of cells at higher temperatures such as 80°C, as in membrane fuel cells, is not encouraged here because of the corrosion instability of the hardware, manufactured from titanium or titanium alloy. Even without such constraints, however, this high temperature would be unwelcome as the water produced is present as steam - without the conductive bridge of the liquid phase it would be necessary to bond the catalytic particles to the membrane with all the associated problems of technology and cost. 

Evaporative decomposition erf solutions and spary pyrolysis have been found to be useful in the preparation of submicrometer oxide and non-oxide particles, including high temperature superconducting ceramics [819, 820], Allowing uniform aerosol droplets (titanium ethoxide in ethanol, for example) to react with a vapor (water, for example) to produce spherical colloidal particles with controllable sizes and size distributions [821-825] is an alternative vapor phase approach. Chemical vapor deposition techniques (CVD) have also been extended to the formation of ceramic particles [825]. 

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