Hydrides Systems

One of the problems with early hydride systems was decrepitation of the alloy. Each time the metal hydride storage tank was recharged the particles would break down and eventually the particles became so small that they began to pass through the 5-p.m sintered metal filter which kept the hydride inside the tank. Addition of 0.5% manganese, which caused the decrepitation process to cease once the particles reached a size of about 10 p.m, solved this problem. 

Compared with nickel-cadmium and nickel-metal hydride systems RAM cells exhibit very low self-discharge, making them ideal for intermittent or periodic use without the need to recharge before using, even in hot climates. Figure 6 shows a comparison of the temperature characteristics, for various battery systems in the form of Arrhenius diagrams. 

Nickel-cadmium rechargeable batteries are being researched. Alternatives such as cadmium-free nickel and nickel hydride systems are also being researched, but nickel-cadmium batteries are unlikely to be totally replaced. Nickel-cadmium batteries can be reprocessed to reclaim the nickel. However, currently, approximately 80% of all nickel-cadmium batteries are permanently sealed in appliances. Changing regulations may result in easier access to these nickel-cadmium batteries for recycling. 

Zaluska, A., and L. Zaluski, New catalytic complexes for metal hydride systems, /. Alloys Compd., 404-406, 706-711,2005. 

Other hydride systems do not have such weight penalties and include magnesium nickel alloys, non-metallic polymers, or liquid hydride systems that use engine heat to disassociate fuels like methanol into a mixture of hydrogen and carbon monoxide. 

Improvement in metal hydride hydrogen storage has been slow in achieving the targets needed for several applications. But slow, steady progress is foreseen as material processing in the nano-range improves the kinetics, thermodynamics, and capacity of the metal hydride systems so that they become acceptable for some applications perhaps for stationary systems. 

Vajo, J. J. Salguero, T. T. Gross, A. F. Skeith, S. L. Olson, G. L., Thermodynamic destabilization and reaction kinetics in light metal hydride systems. Journal of Alloys and Compounds 2007,446-447,409-414. 

As mentioned above, the safest method for hydrogen storage is the metal hydride system (Wilhamson and Edeskuty, 1986 Veziroglu, 1975). When hydrogen combines with the metal alloy, an exothermic reaction occurs. The gas is then stored in the metal particles until some applied heat releases the hydrogen and builds up pressure in the tanks. Heat is then emitted when a metal hydride absorbs hydrogen. 

Stasinevich, D.S., Egorenko, G.A. 1968. A thermographie investigation of sodium hydroboiate-sodium hydride system. Russ J Inorg Chem 133 341-343. 

The plateau region on the PCX cnrves in Fig. 2.43a is very flat, showing only a minimal slope (Sect. 1.4.1). However, each absorption-desorption pair of the PCX curves clearly exhibits a pressure hysteresis. This means that the pressure needed for absorption (hydride formation), is always greater than that of hydride decomposition, p. The cause of pressure hysteresis in metal hydride systems is not fnlly nnderstood. A number of models that attempted to explain this phenomenon... 

These developments evolved into a search for novel hydride systems substantially differing from the original mixtnre (LiNHj + LiH). Some sort of a breakthrough was achieved by Luo, and Lno and Ronnebro [143-145]. They replaced LiH with... 

L.L. Shaw, R. Ren, T. Markmaitree, W. Osborn, Effects of mechanical activation on dehydrogenation of the lithium amide and lithium hydride system , J. Alloys Compd. 448 (2008) 263-271. 

J.J. Vajo, T.T. Salguero, A.F. Gross, S.L. Skeith, G.L. Olsen, Thermodynamic destabilization and reaction kinetics in light metal hydride systems , J. Alloys Compd. 446-447 (2007) 409-414. 

Lester et al. [24] have described a robotic system for the analysis of arsenic and selenium in human urine samples which demonstrates how robotics has been used to integrate sample preparations and instrument analysis of a biological matrix for trace elements. The robot is used to control the ashing, digestion, sample injection and operation of a hydride system and atomic absorption instrument, including the instrument calibration. The system, which routinely analyses both As and Se at ppb levels, is estimated to require only... 

Shortened M-Si bonds Long Si-X bonds Si-H contacts of 1.8-2.1A Small Si-M-Si angles in bis(silyl)hydride systems X and H substituents in the apical positions in respect to silicon atom Elongated M-H bonds Elongated M-Si bonds Normal Si-X bonds Si-H contacts of 1.7-1.8 A No regulations No regulations Normal M-H bonds... 

In summary, the tantalum hydride system adds to the few previously reported well-defined organometallic complexes capable of cleaving N-H bonds of ammonia to yield either an amido or an imido complex, and achieves unprecedented dinitrogen N=N triple bond cleavage with dihydrogen on isolated tantalum atoms to yield reduction of both N atoms. 

TABLE 7.1. Theoretical Energies of Dihydrogen Bonding and H- H Distances in Some Transition Metal Hydride Systems... 

Table 7.1 lists energies of dihydrogen bonding and the H- H distances that have been calcnlated for transition metal hydride systems in the gas phase. As shown, the transition metal complexes calcnlated have different ligand environments and interact with different proton donors. 

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