Dehydriding

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. 

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. 

Figure 6. Schematic representation of the reaction paths for hydriding and dehydriding LaNis [331.

All AB, alloys are very brittle and are pulverized to fine particles in the hydrid-ing-dehydriding process (see Sec. 7.2.1). 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, before coating the resulting particles with Ni by chemical plating. The powder is mixed with a Teflon dispersion to obtain 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],... 

The alloy powder (used in hydrogen-storage systems) may ocasionally be pyrophoric after hydriding-dehydriding operations, igniting when placed on a combustible surface (e.g. weighing paper). 

Figure 6. Typical comparison on the dehydriding (DH)/rehydriding (RH) profiles between the samples that were prepared by mechanically milling NaH/AI with (a) 4 mol % TiH2, (b) 4 mol % metallic Ti under Ar atmosphere for 10 h.

Figure 7. XRD patterns of hydrogen storage composite Mg/MWNTs with (a) Without ball milling (b) Ball milling for 0.5h (c) Ball milling for 3 h (d) After hy driding and dehydriding cycles.

Aoki, M. Noritake, T. Kitahara, G. Nakamori, Y. Towata, S. Orimo, S., Dehydriding reaction of Mg(NH2)2-LiH system under hydrogen pressure. Journal of Alloys and Compounds 2006,428, (1-2), 307-311. 

S. Orimo, Y. Nakamoii, G. Kitahara, K. Miwa, N. Ohba, S. Towata, A. Ziittel, Dehydriding and rehydriding reactions of LiBH, J. Alloys Compd. 404-406 (2005) 427-430. 

A. Karty, J. Grunzweig-Genossar, P.S. Rudman, Hydriding and dehydriding kinetics of Mg/ MgjCu eutectic alloy Pressure sweep method, J. Appl. Phys. 50 (1979) 7200-7209. 

A. Yonkeu, I.P. Swainson, J. Dufour, J. Huot, Kinetic investigation of the catalytic effect of a body centered cubic-aUoy TiV, Mn, (BCC) on hydriding/dehydriding properties of magnesium, J. Alloys Compd. 460 (2008) 559-564. 

H. Chi, C. Chen, Y. An, T. Ying, X. Wang, Hydriding/dehydriding properties of La Mg i alloy prepared by ball milling in different miUing environments, J. Alloys Compd. 373 (2004) 260-264. 

They found that the amounts of the hydrogen desorbed from the mixtures with = 6, 8, and 12 on a unit mass basis slightly decreased with increasing n ( 5.4, 5.1 and 4.5 wt%, respectively). However, the molar ratios of the desorbed hydrogen to the mixtures were almost equal and the PCT isotherms were similar to each other. The plateau pressure for desorption of the (3Mg(NH2)2 + 12LiH) mixture was equal to 8-10 MPa, 3.5 MPa and 2 MPa at 250°C, 225 and 200°C, respectively. The desorption/absorption PCT curve at 250°C exhibited only very small hysteresis which means that the plateau pressures at this temperature are nearly identical. The Li Mg(NH)2 and LiH phases were observed in XRD profiles of all the mixtures after PCT measurements. These results suggest that the dehydriding reaction of the... 

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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