Hydride/dehydride process

Zirconium is a highly active metal which, like aluminum, seems quite passive because of its stable, cohesive, protective oxide film which is always present in air or water. Massive zirconium does not bum in air, but oxidizes rapidly above 600°C in air. Clean zirconium plate ignites spontaneously in oxygen of ca 2 MPa (300 psi) the autoignition pressure drops as the metal thickness decreases. Zirconium powder ignites quite easily. Powder (<44 fim or—325 mesh) prepared in an inert atmosphere by the hydride—dehydride process ignites spontaneously upon contact with air unless its surface has been conditioned, ie, preoxidized by slow addition of air to the inert atmosphere. Heated zirconium is readily oxidized by carbon dioxide, sulfur dioxide, or water vapor. 

Above 40 wt % hydrogen content at room temperature, zirconium hydride is brittle, ie, has no tensile ductiHty, and it becomes more friable with increasing hydrogen content. This behavior and the reversibiHty of the hydride reaction are utilized ki preparing zirconium alloy powders for powder metallurgy purposes by the hydride—dehydride process. The mechanical and physical properties of zirconium hydride, and thek variation with hydrogen content of the hydride, are reviewed in Reference 127. 

The conversion rate (recovery rate) of the mechano-chemical process reaches close to 100 %, but that of the dynamic hydriding/dehydriding process reaches a maximum of 70 %. The conversion rate can be improved considerably by reducing the size of the Mg particles. 

Hydride-dehydride process (HDH) Ti(s)-HH,(g)->TiH,(s) TiH,(s)->Ti(s)-m,(g) Hydriding of titanium followed by crushing and dehydriding at 700 C under reduced pressure Oremet, HC Starck, Kennametal, Sumitomo, Toho 90 Angular Oxide free, chlorine less than 10... 

All AB5 alloys are very brittle and are pulverized to fine particles in the hydriding-dehydriding process (see Section 9.3.1, Alloy Activation). Thus, electrodes must be designed to accommodate fine powders as the active material. There are several methods of electrode fabrication Sakai et al. [35] pulverize the alloy by subjecting it to several hydrogen absorption-desorption cycles, then coat the resulting particles with Ni by chemical plating. The powder is then mixed with a Teflon dispersion to get a paste, which is finally roller pressed to a sheet and then hot pressed to an expanded nickel mesh. The fabrication of a simple paste electrode suitable for laboratory studies is reported by Petrov et al. [37]. 

X. Liu, Y. Zhu, L. Li, Hydriding and dehydriding properties of nanostructured Mg Ni aUoy prepared by the process of hydriding combustion synthesis and subsequent mechanical grinding , /. Alloys Compd 425 (2006) 235-238. 

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