Titanium hydride, decomposition

Intense milling has been shown to cause a decrease in the titanium hydride decomposition temperature. 

Zirconium i dride. Zirconium hydride [7704-99-6] ZrH2, is a britde, metaUic-gray soHd that is stable in air and water, and has a density of 5.6 g/cm. The chemical properties of ZrH2 closely resemble those of titanium hydride. Thermal decomposition in vacuum (1 mPa (7.5 x 10 //mHg)) begins at 300°C and is nearly complete at 500—700°C. It is prepared in the same manner as T1H2. 

The theoretical hmit of 5.4% (NaAlH4+2 mol% TiN) for the two subsequent decomposition reactions is in both cases only observed in the first cycle. The reason for the decrease in capacity is stiU unknown and litde is known about the mechanism of alanate activation via titanium dopants in the sohd state. Certainly, the ease of titanium hydride formation and decomposition plays a key role in this process, but whether titanium substitution in the alanate or the formation of a titanium aluminum alloys, i.e., finely dispersed titanium species in the decomposition products is crucial, is stiU under debate [41]. 

The decomposition of titanium hydride in vacuum between 523 and 773 K was slower than the rate predicted by diffusion calculations and the controlling step was identified [12] as the surface combination of hydrogen atoms. The rate of reaction was sensitive to traces of gaseous Hj, but not to Oj. The inhibiting effect exerted by the presence of helium was ascribed to opposition to the diffusive dispersal of product from the vicinity of the desorption interface. The rates of decomposition of the hydrides of four related metals [13] (TiHj, ZrHj, NbH and TaH) studied between 343 and 973 K pass through a temperature maximum. This was explained by the occurence of two consecutive reactions first-order decomposition of the hydride, followed by second-order combination of the hydrogen atoms before desorption. 

Titanium hydride is well suited for the production of larger quantities of very pure hydrogen. It has a relatively low decomposition temperature (400-900°C), a relatively high hydrogen content, and is easily regenerated. Aside from this, titanium oxide and nitride are completely stable at the required decomposition... 

Condensation of metal vapors followed by deposition on cooler surfaces yields metal powders as does decomposition of metal hydrides. Vacuum treatment of metal hydrides gives powders of fine particle size. Reaction of a metal haHde and molten magnesium, known as the KroU process, is used for titanium and zirconium. This results in a sponge-like product. 

These methods deal with specific cases. The list of examples is not exhaustive. The low-T (200-300°C) decomposition of the transition-metal borohydrides M(BH4> , e.g., leads to titanium, zirconium, halfnium, uranium and thorium borides . Alternatively, the uranium diboride may be obtained by reacting uranium hydride with diborane in hydrogen at 200-400°C. 

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