Metal treatment – Stock – Magnetic
Reexamination Certificate
1999-06-17
2002-01-01
Sheehan, John (Department: 1742)
Metal treatment
Stock
Magnetic
C062S003100, C062S006000
Reexamination Certificate
active
06334909
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a cold accumulating material and a cold accumulating type refrigerator using the same, and more particularly to a cold accumulating material which exhibits significant refrigerating performance at an extremely low temperature region of 10 K or less, and a cold accumulation refrigerator using the cold accumulating material.
BACKGROUND ART
Recently, superconductivity technology has been progressed remarkably and with an expanding application field thereof, development of a small, high performance refrigerator has become indispensable. For such a refrigerator, light weight, small size and high heat efficiency are demanded.
For example in a superconductive MRI apparatus, cryopump and the like, a refrigerator based on such refrigerating cycle as Gifford MacMahon type (GM refrigerator), Starling method has been used. Further, a magnetic floating train absolutely needs a high performance refrigerator. Further, in recent years, a superconductive power storage apparatus (SMES) or a in-magnetic field single crystal pull-up apparatus has been provided with a high performance refrigerator as a main component thereof.
In the above described refrigerator, the operating medium such as compressed He gas flows in a single direction in a cold accumulating unit filled with cold accumulating materials so that the heat energy thereof is supplied to the cold accumulating material. Then, the operating medium expanded here flows in an opposite direction and receives heat energy from the cold accumulating material. As the recuperation effect is improved in this process, the heat efficiency in the operating medium cycle is improved so that a further lower temperature is realized.
As a cold accumulating material for use in the above-described refrigerator, conventionally Cu, Pb and the like have been used. However, these cold accumulating materials have a very small specific heat in extremely low temperatures below 20 K. Therefore, the aforementioned recuperation effect is not exerted sufficiently, so that even if the refrigerator is cyclically operated under an extremely low temperature, the cold accumulating material cannot accumulate sufficient heat energy, and it becomes impossible for the operating medium to receive the sufficient heat energy. As a result, there is posed a problem of that the refrigerator in which the cold accumulating unit filled with aforementioned cold accumulating material is assembled cannot realize the extremely low temperatures.
For the reason, recently to improve the recuperation effect of the cold accumulating unit at extremely low temperature and to realize temperatures nearer absolute zero, use of magnetic cold accumulating material made of intermetallic compound formed from a rare earth element and transition metal element such as Er
3
Ni, ErNi, ErNi
2
, HoCu
2
having a local maximum value of volumetric specific heat and indicating a large volumetric specific heat in an extremely low temperature range of 20 K or less has been considered. By applying the magnetic cold accumulating material to the GM refrigerator, a refrigerating operation to produce an arrival lowest temperature of 4 K is realized.
However, as aforementioned refrigerators are concretely reviewed to be applied to various systems, a technical demand for cooling a large-scaled object under a stable state for a long time is increased, so that it is required to further improve the refrigerating performance (capacity).
By the way, in a cold accumulating unit of the final cooling stage for the refrigerator having a plurality of cooling stages, i.e., in a cold accumulating unit of the second cooling stage for a two-staged expansion type refrigerator, a temperature gradient is formed such that a temperature of a high-temperature side end portion into which the operating medium flows is about 30 K while a temperature of a low-temperature side (downstream side) end portion is about 4 K.
There exist no cold accumulating material of which volumetric specific heat is large at entire region of the broad temperature range. Therefore, in actual, various cold accumulating materials each having a suitable specific heat for the respective temperature regions corresponding to the temperature distribution in the cold accumulating unit are filled in the unit. Namely, a lower temperature side of the cold accumulating unit is filled with cold accumulating materials such as, for example, HoCu
2
having a large volumetric specific heat at a broad temperature range of low temperature side, while a higher temperature side of the cold accumulating unit is filled with cold accumulating materials such as, for example, Er
3
Ni having a large volumetric specific heat at a broad temperature range of high temperature side.
In this regard, a main factor having a great influence on a capacity (performance) of a cold accumulating type refrigerator operated at an extremely low temperature of about 4 K is a kind of cold accumulating material to be filled in the lower temperature side of the cold accumulating unit. Up to now, as the cold accumulating material to be filled in the lower temperature side of. the cold accumulating unit, the cold accumulating materials having various compositions such as ErNi
2
, ErNi
0.9
Co
0.1
, ErNi
0.8
CO
0.2
, ErRh and HoCu
2
are investigated and tried to be applied to the actual refrigerator. When these cold accumulating materials are used in the cold accumulating unit of the second stage of the ordinary two-expansion type GM refrigerator, HoCu
2
results in a particularly high refrigerating performance at a temperature of 4 K. However, the volumetric specific heat of HoCu
2
is still insufficient, so that a remarkable improvement in the refrigerating performance cannot be attained.
In addition, when the cold accumulating materials composed of ferromagnetic substances such as ErNi
2
ErNi
0.9
Co
0.1
, ErNi
0.8
Co
0.2
are applied to refrigerators for superconduction systems, such cold accumulating materials are liable to be affected by leakage magnetic field from the superconducting magnet, so that there may be posed a problem of causing a fear, for example, that magnetic force is applied to component parts of the refrigerator thereby to cause a biased wear and deformations to the component parts.
On the other hand, the cold accumulating materials composed of ErRh is antiferromagnetic substance, so that the cold accumulating material has an advantage of being hardly affected by the leakage magnetic field. However, rhodium (Rh) as a constituent is extremely expensive, so that there may be posed a problem that it is extremely difficult to industrially utilize rhodium as a cold accumulating material for a refrigerator in which rhodium is used at an amount of several hundreds grams order.
The present invention has been achieved to solve the above described problems and an object of the invention is to provide a cold accumulating material capable of exhibiting a significant refrigerating performance at an extremely low temperature for a long period of time in a stable condition, and a cold accumulation refrigerator using the same. In addition, another object of the present invention is to provide an MRI apparatus, a superconducting magnet for magnetic floating train, a cryopump and an in-magnetic field single crystal pull-up apparatus capable of exerting an excellent performance for a long period of time by using the aforementioned cold accumulation refrigerator.
DISCLOSURE OF THE INVENTION
To achieve the above objects, the inventors of this invention had prepared a lot of cold accumulating materials having various compositions and specific heat characteristics, and filled the cold accumulating material into cold accumulating unit of a refrigerator. Then influences of the compositions and specific heat characteristics of the materials on a refrigerating performance of the refrigerator, life and durability of the material are comparatively investigated through experiments.
As a result, the following findings and knowledges were obtained. Namely, when a cold a
Arai Tomohisa
Hashimoto Keisuke
Okamura Masami
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