Titanate phases

Experimental studies of the polyphase ceramics noted above demonstrate that hollandite is one of the most durable titanate phases in aqueous solutions. Pham et al. (1994) carried out experimental work on synthetic Ba-hollandite doped with Cs and containing Al on the B-site for charge balance. These authors suggested that, following the initial release of Cs and Ba from reactive surface sites the first few monolayers of the structure rapidly dissolved due to the release of Al and consequent precipitation of Al-OH species, driving solution pH to lower values. However, the alteration process was mediated via the formation of a continuous Al- and Ti-rich surface layer. Further evidence for selective removal of Ba and enrichment of Al and Ti on the surface of hollandite tested at 250-300 °C was presented by Myhra et al. (19886). These conclusions were largely based on the different release rates of Ba (10 g/m2/d), Al (7 x 10 3 g/m2/d), and Ti (<8 x 10 4 g/ m2/d) after 14 days of dissolution testing, combined with XPS analyses of the altered surfaces. 

A second-generation immobilization material, synroc, is in development. This synthetic rock, based on mixed titanate phases such as zirconolite, hollandite, or perovskite, incorporates the HLW elements into its crystal structure, yielding excellent chemical stability. Synroc features leach rates more than an order of magnitude lower than borosilicate glass. 

TS-1 can be synthesised by hydrothermal methods using a variety of silica and titania sources, structure-directing agents and mineralisers. Alkali metal hydroxides cannot be used in the syntheses, however, because their presence results in the precipitation of separate titanate phases. It is only possible to... 

As an alternative to ZnO, Lew et al. investigated the adsorbent Zn0-Ti02 [33]. The sulfidation reactions that may occur with the various zinc titanate phases are... 

The situation becomes complex when various intermediates occur during a reaction between powders. Let us consider the formation of BaTiOa from BaCOa and Ti02, which is still a relatively simple synthesis reaction. Owing to the heterogeneity of the initial state and according to the phase equilibria, BaO-rich and Ti02-rich titanate phases occur. Figure 6.71 indicates the course of compoimd formation, which is... 

Reduction of sulfur dioxide by methane is the basis of an Allied process for converting by-product sulfur dioxide to sulfur (232). The reaction is carried out in the gas phase over a catalyst. Reduction of sulfur dioxide to sulfur by carbon in the form of coal has been developed as the Resox process (233). The reduction, which is conducted at 550—800°C, appears to be promoted by the simultaneous reaction of the coal with steam. The reduction of sulfur dioxide by carbon monoxide tends to give carbonyl sulfide [463-58-1] rather than sulfur over cobalt molybdate, but special catalysts, eg, lanthanum titanate, have the abiUty to direct the reaction toward producing sulfur (234). 

Fluoroall l-SubstitutedTitanates. Tetraliexafluoroisopropyl titanate [21416-30-8] can be prepared by the reaction of TiCl and hexafluoroisopropyl alcohol [920-66-17, in a process similar to that used for TYZOR TPT (7). Alternatively, it can be prepared by the reaction of sodium hexafluoroisopropoxide and TiCl ia excess hexafluoroisopropyl alcohol (8). The fluoroalkyl material is much more volatile than its hydrocarbon counterpart, TYZOR TPT, and is used to deposit titanium on surfaces by chemical vapor-phase deposition (CVD). 

Spherical, Fine-Particle Titanium Dioxide. Spherical, fine-particle titanium dioxide that has no agglomeration and of mono-dispersion can be manufactured by carrying out a gas-phase reaction between a tetraalkyl titanate vapor and methanol vapor in a carrier gas to form an initial fine particle, which can then be hydrolyzed with water or steam (572). 

Barium carbonate also reacts with titania to form barium titanate [12047-27-7] BaTiO, a ferroelectric material with a very high dielectric constant (see Ferroelectrics). Barium titanate is best manufactured as a single-phase composition by a soHd-state sintering technique. The asymmetrical perovskite stmcture of the titanate develops a potential difference when compressed in specific crystallographic directions, and vice versa. This material is most widely used for its strong piezoelectric characteristics in transducers for ultrasonic technical appHcations such as the emulsification of Hquids, mixing of powders and paints, and homogenization of milk, or in sonar devices (see Piezoelectrics Ultrasonics). 

Historically, materials based on doped barium titanate were used to achieve dielectric constants as high as 2,000 to 10,000. The high dielectric constants result from ionic polarization and the stress enhancement of k associated with the fine-grain size of the material. The specific dielectric properties are obtained through compositional modifications, ie, the inclusion of various additives at different doping levels. For example, additions of strontium titanate to barium titanate shift the Curie point, the temperature at which the ferroelectric to paraelectric phase transition occurs and the maximum dielectric constant is typically observed, to lower temperature as shown in Figure 1 (2). 

R. D. Roseman, "Stmcture and Phase Development in Donor Doped Barium Titanate," in print, 1992. 

Fig. 5. A 90° polished cross section of a production white titania enamel, with the microstructure showing the interface between steel and direct-on enamel as observed by reflected light micrography at 3500 x magnification using Nomarski Interface Contrast (oil immersion). A is a steel substrate B, complex interface phases including an iron—nickel alloy C, iron titanate crystals D, glassy matrix E, anatase, Ti02, crystals and F, quart2 particle.

Methode 1 lm Reaktionskolben legt man 40 ml Diathylather und 5 mMol Sulfoxid vor und kiihlt mit einem Wasserbad. Unter Riihren gibt man rasch das Titan(IV)-chlorid und anschlieBend in kleinen Portionen den Zink-staub zu. Nach 1 Min. gibt man 50 ml Wasser zu, trennt die organ. Phase ab, wascht sie mit Wasser, trocknet, engt ein und chromatographiert den Riickstand mit Hexan oder Pentan an Kieselgel. 

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