Nanocrystalline Ni-based alloys

Details Nanocrystalline Ni-based alloys Nanocrystalline Ni-based alloys

1999-12-02

2001-08-21

Mai, Ngoclan (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Producing or purifying free metal powder or producing or...

Reexamination Certificate

active

06277170

ABSTRACT:

BACKGROUND OF THE INVENTION
a) Field of the Invention
The present invention relates to new Ni-based alloys hereinafter called “nanocrystalline alloys”, which distinguish over the known alloys of the same composition in that they consist of crystallites having a grain size lower than 100 nm and a crystalline structure allowing hydrogen absorption.
The invention also relates to a process for preparing these new nanocrystalline alloys, and to their use for the transportation and/or storage of hydrogen.
b) Brief Description of the Prior Art
It is known that some metallic alloys are capable of absorbing hydrogen in the reversible manner. By way of examples of alloys that can form hydrides reversibly, reference can be made to FeTi, LaNi
5
and Mg
2
Ni.
Thanks to their ability to absorb hydrogen, these alloys in a powder form are particularly useful for storing hydrogen. Indeed, they have the following advantages:
(1) a very large hydrogen storage capacity, which is even higher than the one of liquid hydrogen, because the volume density of hydrogen within these alloys is higher than the one of liquid hydrogen due to the formation of hydrogen-metal bonds that make it possible for the hydrogen atoms to be at a distance lower from each other than the distance at which these atoms are within liquid hydrogen;
(2) a complete reversibility of the mechanism of hydride formation;
(3) an endothermical release of the hydrogen from the alloys, which reduces to a maximum extend the safety problems; and
(4) a non-requirement for an advanced technology to obtain hydrogen transfer to or from the alloys.
In spite of these advantages, the known alloys capable of absorbing hydrogen in a reversible manner have never been used on an industrial scale, because of the following difficulties.
(1) First of all, some alloys like Mg
2
Ni are not easy to prepare inasmuch as the phase diagram of the metals that constitute the same does not allow direct preparation of the same by mere cooling of a molten mixture of their constituting metals;
(2) Secondly, these alloys are efficient for storing hydrogen only if they are in a crystalline form. Accordingly, one must first convert these alloys into a crystalline form if there are amorphous, and then activate the same. This calls for annealing the alloys at high temperature under vacuum and/or a high hydrogen pressure, several times in order to obtain the requested reproducible absorption/desorption characteristics.
(3) Moreover, during the absorption/desorption cycle, the known crystalline alloys usually fragment into small particles and loose their structure integrity. This results in a deterioration of the hydrogen absorption kinetic and in a heat transfer problem.
(4) Last of all, because of oxides that are formed on their surface when they are in contact with ambient air, the known alloys, even when they are in a crystalline form, must be activated in order to absorb hydrogen. The activation treatment consists in heating several times at high temperature the alloys under a high pressure of hydrogen. This treatment must be repeated every time the alloy is exposed to air.
During a research program carried out over the last three years on the FeTi alloy and on its ability to reversibly absorb hydrogen, the present inventors have discovered that, if a powder of FeTi or a mixture of powders of Fe and Ti in preselected amounts are intensely ground for several hours under an inert atmosphere, a crystalline powder of FeTi can be obtained directly, which is made of particles having a grain size lower than 100 nm.
The present inventors have also discovered that this nanocrystalline powder of FeTi is useful as an absorbing medium for storing hydrogen without requiring an activation treatment, contrary to the conventional polycrystalline powders of FeTi.
Continuing their research, the present inventors have now discovered in a very surprising manner that it is possible to obtain others of Ni-based alloys directly in a nanocrystalline form, including, in particular, Mg
2
Ni and LaNi
5
alloys that are known to absorb hydrogen, if one proceeds to an intense grinding of a powder of such alloys or of a suitable mixture of powders of the metals forming these alloys for several hours.
This is a priori very surprising inasmuch as, on the one hand, the synthesis of any alloy or intermetallic compound by grinding is not always possible and, on the other hand, such a synthesis, when possible, often leads to an alloy that is in an amorphous state rather than in a crystalline form.
It has also been discovered that if such a grinding is carried out in such a manner as to reduce the size of the particles to a few nanometers, the nanocrystalline powder Mg
2
Ni, LaNi
5
or any other nickel-based alloy having a crystalline structure capable of absorbing hydrogen, can subsequently be used as an absorbing medium for storing hydrogen in the forms of hydrides, without requiring any activation treatment.
OBJECTS AND SUMMARY OF THE INVENTION
Therefore, a first object of the present invention is to provide a nanocrystalline powder of an alloy of Ni with another metal selected from the group consisting of Mg, La, Be and Li, wherein this powder consists of crystallites having a grain size lower than 100 nm and a crystalline structure allowing hydrogen absorption.
This powder may consist of crystallites of LaNi
5
having a grain size lower than 100 nm, or of crystallites of alloys of Ni with Be or Li having a grain size lower than 100 nm and a crystalline structure allowing hydrogen absorption.
According to a particularly preferred embodiment of the invention, which forms another object of the present invention, the powder preferably consists of crystallites of the formula Mg
2−x
Ni
1+x
, x ranging from −0.3 and +0.3, wherein these crystallites have a grain size lower than 100 nm and preferably lower than 30 nm.
A further object of the present invention is to provide a method for storing and/or transporting hydrogen, consisting in reversibly absorbing hydrogen in a nanocrystalline powder like those that have just been disclosed. Indeed, it has been discovered that these Ni-based nanocrystalline powders, like those of FeTi, require either no or only one simple activation treatment at low temperature. It has also been discovered that the kinetic of absorption and diffusion of hydrogen within said alloys is much faster. This can be explained by the fact that the nanocrystalline powder has a very great number of grain boundaries.
Still another object of the invention is to provide a process for preparing the nanocrystalline powders mentioned thereinabove, which comprises the steps of intensely grinding a powder of this alloy until the grain size of the crystallites reaches requested value, or of intensely grinding a mixture of a powder of Ni with a powder of the other metal in such amounts so as to obtain the requested alloy, said grinding allowing the preparation in a mechanical way of the crystallites of the requested alloy from the powders of Ni and of the other metal, and simultaneously allowing the reduction of the grain size of these crystallites to the requested value. This process is particularly advantageously in that it is very simple and can be carried out at ambient temperature under atmospheric pressure. However, it is necessary that the grinding step be carried out under an inert atmosphere in order to avoid oxidation of the alloy that is being formed.
According to a preferred embodiment of the invention, at least one additional metal can be added to Ni and the other metal at the beginning of the grinding steps. This additional metal must be selected so as not to modify the crystalline structure of the intermetallic alloy that will be obtained. This additional metal can be aluminum, cobalt, lanthanum, palladium or preferably copper.
According to another preferred embodiment of the invention, once the grinding step is completed and the requested crystallites particles are obtained, a given amount of a hydrogen dissociation catalyst material can be added to the particles

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