Metal treatment – Stock – Magnesium base
Reexamination Certificate
1999-11-22
2001-12-11
King, Roy (Department: 1742)
Metal treatment
Stock
Magnesium base
C420S402000, C420S405000, C420S407000, C420S900000, C075S255000
Reexamination Certificate
active
06328821
ABSTRACT:
FIELD OF THE INVENTION
The instant invention relates generally to hydrogen storage alloys and more precisely alloys which are useful as a hydrogen supply material for powering internal combustion engine or fuel cell vehicles. Specifically the invention relates to modified Mg based hydrogen storage alloys. The inventors have for the first time produced Mg based alloys having both hydrogen storage capacities higher than about 4 wt. %, a plateau pressure equivalent to Mg
2
Ni, and a low heat of formation by atomic engineering the Mg alloy and tailoring the local atomic environment thereof.
BACKGROUND OF THE INVENTION
In the past considerable attention has been given to the use of hydrogen as a fuel or fuel supplement. While the world's oil reserves are rapidly being depleted, the supply of hydrogen remains virtually unlimited. Hydrogen can be produced from coal or formed by the electrolysis of water. Moreover hydrogen can be produced without the use of fossil fuels, such as by the electrolysis of water using nuclear or solar energy. Furthermore, hydrogen, although presently more expensive than petroleum, is a relatively low cost fuel. Hydrogen has the highest density of energy per unit weight of any chemical fuel and is essentially non-polluting since the main by-product of burning hydrogen is water.
While hydrogen has wide potential application as a fuel, a major drawback in its utilization, especially in mobile uses such as the powering of vehicles, has been the lack of acceptable lightweight hydrogen storage medium. Storage of hydrogen as a compressed gas involves the use of large and heavy vessels. In a steel vessel or tank of common design only about 1% of the total weight is comprised of hydrogen gas when it is stored in the tank at a typical pressure of 136 atmospheres. In order to obtain equivalent amounts of energy, a container of hydrogen gas weighs about thirty times the weight of a container of gasoline.
Hydrogen also can be stored as a liquid. Storage as a liquid, however, presents a serious safety problem when used as a fuel for motor vehicles since hydrogen is extremely flammable. Liquid hydrogen also must be kept extremely cold, below −253 degree C., and is highly volatile if spilled. Moreover, liquid hydrogen is expensive to produce and the energy necessary for the liquefaction process is a major fraction of the energy that can be generated by burning the hydrogen.
Storage of hydrogen as a solid hydride can provide a greater percent weight storage than storage as a compressed gas or a liquid in pressure tanks. Also, hydrogen storage in a solid hydride presents fewer safety problems than those caused by hydrogen stored in containers as a gas or a liquid. A desirable hydrogen storage material must have a high storage capacity relative to the weight of the material, a suitable desorption temperature, good kinetics, good reversibility, resistance to poisoning by contaminants including those present in the hydrogen gas and be of a relatively low cost. If the material fails to possess any one of these characteristics it will not be acceptable for wide scale commercial utilization.
A high hydrogen storage capacity per unit weight of material is an important consideration in applications where the hydride does not remain stationary. A low hydrogen storage capacity relative to the weight of the material reduces the mileage and hence the range of the vehicle making the use of such materials impractical. A low desorption temperature is desirable to reduce the amount of energy required to release the hydrogen. Furthermore, a relatively low desorption temperature to release the stored hydrogen is necessary for efficient utilization of the available exhaust heat from vehicles, machinery, or other similar equipment.
Good reversibility is needed to enable the hydrogen storage material to be capable of repeated absorption-desorption cycles without significant loss of its hydrogen storage capabilities. Good kinetics are necessary to enable hydrogen to be absorbed or desorbed in a relatively short period of time. Resistance to contaminants to which the material may be subjected during manufacturing and utilization is required to prevent a degradation of acceptable performance.
The prior art metallic host hydrogen storage materials include magnesium, magnesium nickel, vanadium, iron-titanium, lanthanum pentanickel and alloys of these metals. No prior art material, however, has all of the required properties required for a storage medium with widespread commercial utilization.
Thus, while many metal hydride systems have been proposed, the Mg systems have been heavily studied ever since hydrogen storage was first reported in Mg
2
Ni. The Mg systems are of high interest because of their large hydrogen storage capacity. However, as discussed above, Mg hydrogen storage systems have not been used commercially to date because of the disadvantages inherent therein.
While magnesium can store large amounts of hydrogen, it's primary disadvantages are high absorption-desorption temperature, low plateau pressure, extremely slow kinetics and large heat of hydride formation (~75 kJ/mol). For example, magnesium hydride is theoretically capable of storing hydrogen at approximately 7.6% by weight computed using the formula: percent storage=H/H+M, where H is the weight of the hydrogen stored and M is the weight of the material to store the hydrogen (all storage percentages hereinafter referred to are computed based on this formula). While a 7.6% storage capacity is suited for on board hydrogen storage for use in powering vehicles, magnesium's other hydrogen storage characteristics make it commercially unacceptable for widespread use.
Magnesium is very difficult to activate. For example, U.S. Pat. No. 3,479,165 discloses that it is necessary to activate magnesium to eliminate surface barriers at temperatures of 400° C. to 425° C. and 1000 psi for several days to obtain a reasonable (90%) conversion to the hydride state. Furthermore, desorption of such hydrides typically requires heating to relatively high temperatures before hydrogen desorption begins. The aforementioned patent states that the MgH
2
material must be heated to a temperature of 277° C. before desorption initiates, and significantly higher temperatures and times are required to reach an acceptable operating output. The high desorption temperature makes the magnesium hydride unsuitable for many applications, in particular applications wherein it is desired to utilize waste heat for desorption such as the exhaust heat from combustion engines.
Mg-based alloys have been considered for hydrogen storage also. The two main Mg alloy crystal structures investigated have been the A
2
B and AB
2
alloy systems. In the A
2
B system, Mg
2
Ni alloys have been heavily studied because of their moderate hydrogen storage capacity, and lower heat of formation (64 kJ/mol) than Mg. However, because Mg
2
Ni has a storage capacity of only 3.6 wt. % hydrogen, researchers have attempted to improve the hydrogenation properties of these alloys through mechanical alloying, mechanical grinding and elemental substitutions. For instance, in U.S. Pat. Nos. 5,506,069 and 5,616,432 (the disclosures of which are incorporated herein by reference), Ovshinsky et al, have modified non-Laves phase Mg-Ni alloys for electrochemical work.
More recently, investigators have attempted to form MgNi
2
type alloys for use in hydrogen storage. See Tsushio et al, Hydrogenation Properties of Mg-based Laves Phase Alloys,
Journal of Alloys and Compounds,
269 (1998), 219-223. Tsushi et al. determined that no hydrides of these alloys have been reported, and they did not succeed in modifying MgNi
2
alloys to form hydrogen storage materials.
Finally some work has been done on high Mg content alloys or elementally modified Mg. For instance, in co-pending U.S. application Ser. No. 09/066,185, now U.S. Pat. No. 5,976,216, Sapru, et al have produced mechanically alloyed Mg-Ni-Mo materials containing greater than about 80 atomic percent Mg, for thermal storage o
Ovshinsky Stanford R.
Young Rosa T.
Energy Conversion Devices Inc.
King Roy
Schumaker David W.
Siskind Marvin S.
Wilkins, III Harry D.
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