Titania phase transition

Unlike melting and the solid-solid phase transitions discussed in the next section, these phase changes are not reversible processes they occur because the crystal stmcture of the nanocrystal is metastable. For example, titania made in the nanophase always adopts the anatase stmcture. At higher temperatures the material spontaneously transfonns to the mtile bulk stable phase [211, 212 and 213]. The role of grain size in these metastable-stable transitions is not well established the issue is complicated by the fact that the transition is accompanied by grain growth which clouds the inteiyDretation of size-dependent data [214, 215 and 216]. In situ TEM studies, however, indicate that the surface chemistry of the nanocrystals play a cmcial role in the transition temperatures [217, 218]. 

Talavera, R.R., Vargas, S., Arroyo-Murillo, R., Montiel-Campos, R., Haro-Poniatowski, E. 1997. Modification of the phase transition temperatures in titania doped with various cations. J Mater Res 12 439 43. 

Pilling-Bedworth ratio of 1 96, anatase phase films can show cracks and fissures with, consequendy, a loss of mechanical stability, however a hydrogen reduction treatment above 600°C leads to phase transition from anatase (101) to rutile (110) [43] with XRD detecting TiH2 upon prolonged hydrogen treatment of titania. As shown in Fig. 4.4, introduction of vanadium increases the intensity of the anatase Ti02 peak above 700°C disappearance of the vanadium (001) peak and the simultaneous appearance of the rutile (110) peak are observed, but anatase continues to dominate even after heat treatment at 800° C. A sharp vanadium (001) peak is observed for heat treatments carried out in air, while no vanadium peak has been seen in the case of heat treatment at 600°C in presence of Ar/H2. 

Au/ZrC>2 catalysts are less active than Au/TiC>2 catalysts, whatever method of preparation is used deposition of colloidal gold,83,91 DP12 or laser vaporisation.70 Activity depends on the method used (Table 6.12), and appears to be due only to the presence of Au°. The reason for the difference between zirconia and titania is not understood Zr4+ is more difficult to reduce than Ti4+, so anion defects may be harder to form. The lattice structures also differ in monoclinic zirconia (baddleyite) the Zr4+ ion is unusually seven coordinate, and phase transitions into tetragonal and cubic structures occur at >1370 and >2570 K, respectively. However, the... 

Thermal stability. Thermal stability of several common ceramic and metallic membrane materials has been briefly reviewed in Chapter 4. The materials include alumina, glass, silica, zirconia, titania and palladium. As the reactor temperature increases, phase transition of the membrane material may occur. Even if the temperature has not reached but is approaching the phase transition temperature, the membrane may still undergo some structural change which could result in corresponding permeability and permselectivity changes. These issues for the more common ceramic membranes will be further discussed here. 

Titania membranes prepared at a temperature lower than about 350 C are essentially amorphous. At 350 or so, phase transition to a crystalline phase of anatase begins to occur. The transformation to anatase (tetragonal in crystallinity) is complete and the new phase of rutile (also tetragonal) begins at a temperature close to 450 0. Transformation to rutile is complete at about 600 C [Xu and Anderson, 1989]. Thus, at a temperature between 450 and bOO C, both anatase and rutile phases are present. It has been suggested that this temperature range may be lower at 350-550 C [Larbot et al., 1986 Burggraaf etal., 1989]. 

Yamanaka T, Fukumori Y (1995) Molecular aspects of the electron transfer system which participates in the oxidation of iron by Thiobacillus ferrooxidans. FEMS Microbiol Rev 17 401-413 Yanagisawa K, Ovenstone J (1999) Crystallization of anatase from amorphous titania using the hydrothermal technique Effects of starting material and temperature. J Phys Chem B 103 7781-87 Yang J, Mei S, Ferreira IMF (2000) Hydrothermal synthesis of nanosized titania powders lirfluence of peptization and peptizing agents on the crystalline phases and phase transition. J Am Ceram Soc 83 1361-1368... 

Titania. Titania powders are used as pigments, catalytic supports, membranes, opacifiers, photocatalysts and fillers in industrial applications . Titania particles have been prepared by a number of methods, such as hydrolysis, sol-gel, microemulsion and hydrothermal synthesis. Titania exists naturally in two tetragonal forms, the metastable phase anatase, and the stable phase rutile. On heat treatment, anatase transforms into rutile. The phase transition temperature depends on the starting materials and the preparation procedure. 

Anosovite (type II) [12065-65-5] P-TijO, M = 223.0070 64.1 wt.% Ti 35.9 wt.% 0 (Oxides and hydroxides) Orthorhombic Distorded pseudobrookite TiO 191-210 pm mC32(Z=4) S.G. C2/m Biaxial ( ) n.a. 4900 Habit acicular cry als. Color bhie-daik. Diaphaneity opaque. Luster metallic. Streak black. Type II can be prepared by the hydrogen r uction of solid TiO, at temperature around 1500 Cwith magnesia as a catalyst. Anosovite type 11 is similar to that found in artiftcial titania dags and it is stabilized at room ten erature with small amount of iron. This oxide is dimorphic with a rapid phase transition from semiconductor to metal occuring at roughly 120 C. 

The Raman spectra of titania-doped Ta20s crystals gave evidence for phase transitions at 327, 397, 487 and 571 °C respectively for 0, 5, 8 and 11% Ti02-... 

Yang J., Mei S., Ferreira J.M.F. Hydrothermal synthesis of nanosized titania powders influence of peptization and peptizing agents on the crystalline phases and phase transitions. J. Am. Ceram. Soc. 2000 83 1361-1368... 

Table 8.2 CTE during phase transitions for titania-doped bismuth vanadates...

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