HYFROGEN STORAGE ALLOY Technical Field The present invention relates to a hydrogen storage alloy which is applicable to fields of hydrogen storage, heat pumps, and compressors, and, more particularly, to a Ti-Zr-Mn-Cr group Laves phase hydrogen storage alloy which has a hydrogen storage capacity larger than a related art hydrogen storage alloy, and has excellent slopping, and hysteresis characteristics.

Background Art It is in general known that AB2 type Laves phase hydrogen storage alloys (A: Zr, Tr, B, V, Cr, Mn), having a larger hydrogen storage capacity, and a fast hydrogen chemical reaction, are applicable to fields of hydrogen storage, heat pump, and the like.

However, scientists D. O. Northwood et. al. reports in"Storing Hydrogen in AB2 Laves phase type Compounds" [Z. Phys Chem. N. F., pl47, pl91-209, 1986] that, as a result of study of hydrogen reaction behavior of the alloy, though the alloy has excellent in view of a hydrogen storage capacity, and reaction speed, a resultant hydrogen compound is too stable, with a tendency very conservative in emigration of hydrogen, to apply to practical applications, in which reversible hydrogen emigration is required. This is caused by a very low plateau pressure below the atmospheric pressure at a room temperature, and current researches are focused on elevating the plateau pressure.

As one of examples of such researches, Shaltiel et. al. [J. Less-Comm. Metals, p73, p369-376,1980], Northwood [J. Less-Comm. Metals, pl47, pl49-159, 1989], et. al. report three element alloy in which Ti instead of Zr, and Fe instead of Cr, are substituted, and Wallace [U. S. Pat. No. 4,556,551], and Jai-Young Lee [U. S. Pat. No.

5,028,389], et. al. report four element alloy. The four element hydrogen storage alloy reported by Jai-Young Lee [U. S. Pat. No. 5,028,389] is advantageous in that the plateau pressure can be varied as required within a range of 0.1-10 atmospheres which is common for many field of applications by varying Cr, and Fe contents. However,

though the alloy [a four element hydrogen storage alloy having composition of Zrl- xTiCrl+yFel+y, where O. 05<x 1, OSy<0. 4] disclosed in U. S. Pat. No. 5, 028,389 has a hydrogen storage capacity (1. 6wt%) greater than the present alloy (1. 3wt%), the hydrogen storage capacity is still too small. Moreover, the great hysteresis, a pressure difference between absorption, and discharge of hydrogen, is an obstacle for putting into commercial use, and developing high performance systems in the fields of applications due to a great energy loss in the absorption and discharge of hydrogen.

Accordingly, there have been many researches for developing alloys having great hydrogen storage capacities, inclusive of Y Moriwaki et. al. [J. Less-Comm.

Metals, pl72-174, p1028-1035, 1991] reporting Til_xZrxMn2-yCry alloy, and T. Gamo et. al. [Int. J. Hydrogen Energy, 10 (1985) 39] reporting Tio. lMnl4cro4vo2 alloy.

Though the alloy reported by prof. Moriwaki, employing Ti as a major element instead of Zr, has a good hydrogen storage capacity of approx. 2wt% because Ti with an atomic weight approx. 47 g/mol is lighter than Zr with an atomic weight approx. 91 g/mol, the alloy of prof. Moriwaki has a disadvantage of significant increase of slopping when a Zr content is increased for reduction of the plateau pressure. Therefore, it is very important to develop an alloy that allows to drop the plateau pressure without change of hydrogen storage capacity, and still has small hysteresis and slopping.

Disclosure of the Invention Accordingly, the present invention is directed to a hydrogen storage alloy that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.

An object of the present invention is to provide a hydrogen storage alloy which has a large hydrogen storage capacity, a low plateau pressure, and small hysteresis and slopping.

Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the

written description and claims hereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the hydrogen storage alloy of Laves phase in Ti-Zn-Mn-Cr group has composition of (Til-xZrx) 1+AMn2-yCry, where 'A'is greater than'0', to have non-stoichiometry composition.

The'A'is preferably in a range greater than 0'and smaller than approx. 0.2.

Preferably, the'X'is in a range ofO<X0. 3, and'Y'is in a range of 1. 0<Y1. 2.

The Cr is preferably substituted with'M'having at least one of V and Cu, to have composition of (Til-xZrx) l+AMn2-ycry-BMBS wherein the'B'is in a range greater than 0, and smaller than approx. 0.4.

In other aspect of the present invention, there is provided a hydrogen storage alloy of Laves phase in Ti-Zn-Mn-Cr group wherein the Cr is substituted with 'M'having at least one of V and Cu, to have stoichiometry composition of Til xZrxMn2 yCry-BMB.

The'B'is preferably in a range greater than 0, and smaller than approx. 0.4.

Preferably, the'X'is in a range of 0#X#0. 3, and the'Y'is in a range of 1.0#Y#1. 2.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

Brief Description of Drawings The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention: In the drawings: FIG. 1 illustrates a P-C-T graph comparing a hydrogen storage alloy in accordance with a first preferred embodiment of the present invention and a related art hydrogen storage alloy;

FIG. 2 illustrates a P-C-T graph comparing a hydrogen storage alloy in accordance with a second preferred embodiment of the present invention and a related art hydrogen storage alloy ; FIG. 3 illustrates a P-C-T graph comparing hydrogen storage alloys in accordance with a third preferred embodiment of the present invention and a related art hydrogen storage alloy; FIG. 4 illustrates a P-C-T graph comparing one of hydrogen storage alloys in accordance with a third preferred embodiment of the present invention and a related art hydrogen storage alloy; FIG. 5 illustrates a graph showing an XRD analysis of the hydrogen storage alloy of the present invention; FIG. 6 illustrates a graph showing a hydrogenation reaction rate of the hydrogen storage alloy of the present invention; FIG. 7 illustrates a P-C-T graph of the hydrogen storage alloy of the present invention for different non-stoichiometry composition ratios; FIG. 8 illustrates a P-C-T graph of the hydrogen storage alloy of the present invention varied as some of a Cr content is substituted with other element; --FIG. 9 illustrates a P-C-T graph of the hydrogen storage alloy of the present invention varied as some of a Zr content is substituted with other element; and, FIG. 10 illustrates a table showing a comparison of performance of the hydrogen storage alloys of the present invention and the related art.

Best Mode for Carrying Out the Invention Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. A hydrogen storage alloy in accordance with a first preferred embodiment of the present invention will be explained, with reference to FIG. 1.

The hydrogen storage alloy in accordance with a first preferred embodiment of the present invention is a Ti-Zr-Mn-Cr group Laves phase hydrogen storage alloy with composition of (Tii. xZrx) i+AMn2-yCry, where'X"denotes a Ti content substituted with

Zr, Y denotes an Mn content substituted with Cr, and'A'denotes an extent of deviation of a sum of contents of Ti, and Zr from stoichiometry. The hydrogen storage alloy of the present invention, with'A'greater than'0', has non-stoichiometry composition.

Referring to FIG. 1, it can be noted that the non-stoichiometry hydrogen storage alloy in accordance with a first preferred embodiment of the present invention has a larger hydrogen storage capacity, and smaller plateau pressure, slopping, and hysteresis in comparison to the related art stoichiometry hydrogen storage alloy Tio. 75Zro 2sMno 8Cri. 2. It can also be noted that the greater the'A', i. e., the greater the deviation from stoichiometry composition, various characteristics become the better.

Referring to FIG. 7, it is preferable that'A'is smaller than approx. 0.2. When 'A'is greater than 0.1, because the plateau pressure becomes higher, and a section of the plateau pressure becomes smaller, the hydrogen storage alloy shows a slight decreasing tendency of hydrogen movement which can be used in a reversible reaction.

Therefore, it is more preferable that'A'is in a range greater than'0', and smaller than approx. 0.1.

A hydrogen storage alloy in accordance with a second preferred embodiment of the present invention will be explained, with reference to FIG. 2.

The hydrogen storage alloy in accordance with a second preferred embodiment of the present invention is a Ti-Zr-Mn-Cr group Laves phase hydrogen storage alloy with composition of (Ti1-xZrx)1+AMn2-yCry-BMB, where 'M' denotes at least one of elements of'V', and'Cu, and'B'denotes a range of'M'content substituted for'Cr.

The hydrogen storage alloy of the present invention has'A'equal to'0', and'B'at least greater than'0'. That is, the second embodiment hydrogen storage alloy of the present invention is of stoichiometry composition, in which Cr is substituted with Cu, and/or V.

Referring to FIG. 2, it can be noted that the Cr substitution with Cu, and/or V from the Cr in the related art stoichiometry hydrogen storage alloy Tio. 75Zro. 25Mno. 8Cri. 2 improves various characteristics.

Next, a hydrogen storage alloy in accordance with a third preferred embodiment

of the present invention will be explained, with reference to FIG. 3.

The hydrogen storage alloy in accordance with a third preferred embodiment of the present invention is a Ti-Zr-Mn-Cr group Laves phase hydrogen storage alloy with composition of (Til-xZrx) +AMn2-yCry-BMB, where'A', and'B'are at least greater than '0', respectively. That is, the third embodiment hydrogen storage alloy of the present invention is of non-stoichiometry composition, in which Cr is substituted with Cu, and/or V Referring to FIG. 3, it can be noted that, if the Cr is substituted with Cu and V, for non-stoichiometry composition, the slopping is improved significantly while the hydrogen storage capacity is maintained. It can also be noted that, if the non- stoichiometry composition, and Cu, and V are substituted at a time, the same substitution effect exhibits.

The effect of the third embodiment hydrogen storage alloy [ (Tio. 7sZro. 2s) t. o5Mno. 8Cri. o5Vo. o5Cuo. i] of the present invention will be explained, with reference to FIGS. 4,5, and 10. FIG. 4 illustrates a P-C-T graph comparing one of hydrogen storage alloys in accordance with a third preferred embodiment of the present invention and a related art hydrogen storage alloy reported by Y Moriwaki [J. Less- Comm : Metals, pl72-174, pl028-1035, 1991], wherein it can be noted that the hydrogen storage alloy of the present invention has a very large hydrogen storage capacity of approx. 2wt%, and slopping and hysteresis better than the related art alloy.

FIG. 5 illustrates a graph showing an ERD analysis of the hydrogen storage alloy of the present invention, wherein it can be noted that the hydrogen storage alloy of the present invention maintains the Laves phase even in a case Cu and V are substituted at a time in a non-stoichiometry composition state.

FIG. 6 illustrates a graph showing a hydrogenation reaction rate of the hydrogen storage alloy of the present invention, wherein it can be noted that approx.

90% of hydrogenation reaction is finished within two minutes, which is very excellent.

FIG. 10 illustrates a table showing a comparison of hydrogen storage capacities, hydrogen absorption/discharge pressures, and slopping and hysteresis characteristics of

the hydrogen storage alloy [ (TiO. 75Zro. 25) 1. o5Mno. 8Cri of the present invention, and the hydrogen storage alloy [Ti0. 7Zr0. 3mol. 2Cr0. 8Tio 7Zro. 3Mn0. 8Crl. 2] of the related art reported by Y. Moriwaki et. al. [J. Less-Comm. Metals, p172-174,p1028- 1035,1991], and the hydrogen storage alloy [Tio. 9Zro. Cro. 6Fe1.4] of the related art reported by Jai-Young Lee [U. S. Pat. No. 5, 028,389], et. al., wherein it can be noted that the hydrogen storage alloy of the present invention has a very excellent hydrogen storage capacity as well as very excellent slopping, and hysteresis characteristics in comparison to the related art hydrogen storage alloy.

In the meantime, referring to FIG. 8, Cu substitution more than 0.4 shows a reduction of the hydrogen storage capacity. Therefore, it is preferable that'B'is less than approx. 0.4. Also, there is tendency that, even if the slopping is improved when 'B'is less than 0.3, the hydrogen storage capacity is reduced, and the slopping is increased on the contrary when'B'is greater than 0.3. Therefore, it is more preferable that'B'falls on a range greater than 0'and smaller than approx. 0.3.

In the meantime, referring to FIG. 9, it is preferable that'X'is in a range of 0X0. 3.

In summary, the hydrogen storage alloy of the present invention has composition of (Til-xZrx) i+AMn2-yCry-BMB, where'M'denotes at least one element of V and Cu. Also, it is the most preferable that OAO. 1, 0#B#0. 3, 0#X#0. 3,1.0<Y<1.2.

For reference, a process for preparing the Ti-Zr-Mn-Cr group Laves phase hydrogen storage alloy of the present invention will be explained.

An amount of each of the elements in (Til. xZrx) 1+AMn2-yCry-B Mb ('M' denotes at least one of V and Cu) is fixed, to fall atom ratios of element contents within ranges of 0#A#0.1, 0#B#0.3, 0#X#0. 3, 1. 0<Y<1. 2, and to amount in a range of 5g in total, which is then subjected to plasma arc melting under an argon atmosphere. In order to enhance a uniformity of a specimen, a process of turning over, and re-melting the specimen is repeated for a few times (for an example, 4-5 times) after the specimen is solidified. Then, the melted specimen is crushed, and only specimens with 100- 200mesh are put into a Sievert's automatic P-C-T (Pressure-Composition-Temperature)

curve measuring equipment, and hydrogenation reaction characteristics is measured.

Then, for activation, an inside of a reaction tube is maintained at approx. 10-2 Torr for approx. lOmin., heated for approx. 5min. with an alcohol lamp, and cooled down with cold water in a state hydrogen with approx. 20 atmospheres is applied. In this instance, a hydrogen reaction is completed within approx. 5-lOmin., the reaction tube is maintained at vacuum, to release all hydrogen in the specimen, and such a hydrogen absorption/releasing processes are repeated for a few times (for an example, 2-3 times) such that the hydrogen absorption/releasing processes can be completed within a few minutes. Then, a hydrogen storage alloy (Tií-xZrx) l+AMn2-ycry-BMs ('M'denotes at least one of V and Cu) with ratios of element contents within ranges of OSAS0. 1, O<B<0. 3, OSX<0. 3, l. O<Y<1. 2, i. e., a hydrogen storage alloy which can drop the plateau pressure, and has smaller hysteresis and slopping characteristics, without change in the hydrogen storage capacity, can be prepared.

Industrial Applicabilitv As has been explained, the hydrogen storage alloy of the present invention can improve slopping, and hysteresis characteristics significantly while the hydrogen storage capacity is maintained in a range of 1. 9wt%, and the plateau pressure is maintained below 10 atmospheres, permitting to provide a system stability, and improve an output in application fields of hydrogen storage, heat pump, and compressor, and the like.

It will be apparent to those skilled in the art that various modifications and variations can be made in the hydrogen storage alloy of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.