High temperature hydride

Destabilization of high-temperature hydrides by nanostracturing and compositing of nanohydrides (Sect. 3.5) is in its infancy. At the first look, it seems to be very promising but needs much more vigorous research. 

Figure 7.2 Heat generation ofa 4 kg hydrogen storage tank based on a room-temperature hydride with AH —30kJ (mol H2) and high-temperature hydride with AH rs —75 kj (mol H2) respectively.

A storage device that couples a high-temperature hydride to a phase change material has been tested by Arthur D. Little Inc. This system uses the heat released during hydrogenation to melt the material. In the reverse process, the released solidification energy is taken to liberate the hydrogen from the hydride compound. The material used is a Mg powder coated with Ni by means of chemical vapor deposition. The system was successfully tested as an essentially isothermal and isobaric process [29]. 

The CVD method is a well-studied process, in which materials are yielded by the decomposition or chemical reactions of source gases at high temperatures. Hydride, bromide, chloride, and organometallic compounds are generally used... 

Normally, lithium hydride ignites in air only at high temperatures. When heated it reacts vigorously with CO2 and nitrogen. With the former, lithium formate is obtained. Reaction at high temperature with nitrogen produces lithium nitride. Therefore, dry limestone or NaCl powders are used to extinguish LiH fires. Lithium hydride reacts exothermically with moist air and violently with water. 

At elevated temperatures, CaH2 reacts with halogens, sulfur, phosphoms, alcohols, and ammonia. At high temperatures, it reacts with refractory metal oxides and haUdes. Calcium hydride is substantially inert to organic compounds that do not contain acidic hydrogens. 

The product chunks are hydrided, cmshed, and dehydrided. The resultant powder is blended and pressed into bars which are purified by high temperature sintering. The sintering removes all of the carbon and most of the oxygen and is followed by consoHdation by either arc or electron-beam melting. 

Hydrolysis of primary amides cataly2ed by acids or bases is very slow. Even more difficult is the hydrolysis of substituted amides. The dehydration of amides which produces nitriles is of great commercial value (8). Amides can also be reduced to primary and secondary amines using copper chromite catalyst (9) or metallic hydrides (10). The generally unreactive nature of amides makes them attractive for many appHcations where harsh conditions exist, such as high temperature, pressure, and physical shear. 

Titanium Silicides. The titanium—silicon system includes Ti Si, Ti Si, TiSi, and TiSi (154). Physical properties are summarized in Table 18. Direct synthesis by heating the elements in vacuo or in a protective atmosphere is possible. In the latter case, it is convenient to use titanium hydride instead of titanium metal. Other preparative methods include high temperature electrolysis of molten salt baths containing titanium dioxide and alkalifluorosiUcate (155) reaction of TiCl, SiCl, and H2 at ca 1150°C, using appropriate reactant quantities for both TiSi and TiSi2 (156) and, for Ti Si, reaction between titanium dioxide and calcium siUcide at ca 1200°C, followed by dissolution of excess lime and calcium siUcate in acetic acid. 

An aiyl methane- or toluenesulfonate ester is stable to reduction with lithium aluminum hydride, to the acidic conditions used for nitration of an aromatic ring (HNO3/HOAC), and to the high temperatures (200-250°) of an Ullman reaction. Aiyl sulfonate esters, formed by reaction of a phenol with a sulfonyl chloride in pyridine or aqueous sodium hydroxide, are cleaved by warming in aqueous sodium hydroxide. ... 

Hardness on the Mohs scale is often above 8 and sometimes approaches 10 (diamond). These properties commend nitrides for use as crucibles, high-temperature reaction vessels, thermocouple sheaths and related applications. Several metal nitrides are also used as heterogeneous catalysts, notably the iron nitrides in the Fischer-Tropsch hydriding of carbonyls. Few chemical reactions of metal nitrides have been studied the most characteristic (often extremely slow but occasionally rapid) is hydrolysis to give ammonia or nitrogen ... 

This conclusion was additionally confirmed by Palczewska and Janko (67) in separate experiments, where under the same conditions nickel-copper alloy films rich in nickel (and nickel films as well) were transformed into their respective hydride phases, which were proved by X-ray diffraction. The additional argument in favor of the transformation of the metal film into hydride in the side-arm of the Smith-Linnett apparatus consists of the observed increase of the roughness factor ( 70%) of the film and the decrease of its crystallite size ( 30%) after coming back from low to high temperatures for desorbing hydrogen. The effect is quite similar to that observed by Scholten and Konvalinka (9) for their palladium catalyst samples undergoing the (a — j8) -phase transformation. 

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