Rotary magnetic refrigerator

A rotary magnetic refrigerator comprising two rotors which are packed with portions of a working substance at equal spacing along the outer circumference thereof, and which are made of a material having a low thermal conductivity, openings that are formed in the outer surfaces of said rotors so as to communicate with a high-temperature cooling medium, housings having cooling portions provided on sides thereof opposite to said openings, means which establishes an intense magnetic field near said openings and which establishes a weak magnetic field near said cooling portions, and means for driving said rotors.

BACKGROUND OF THE INVENTION 
The present invention relates to a magnetic refrigerator which produces 
refrigeration by applying and removing a magnetic field to a working 
substance for magnetic refrigeration, and particularly to a rotary 
magnetic refrigerator adapted to produce superfluid helium. 
As is well known, the principle of the magnetic refrigerator is based upon 
the magnetocloric effect of a magnetic material which generates heat when 
a magnetic field is applied thereto, and which absorbs heat when the 
magnetic field is removed therefrom. Superfluid helium can be produced by 
magnetic refrigeration, i.e., by using a rotary magnetic refrigerator. The 
rotary magnetic refrigerator is effective for solving the problems 
inherent in the existing rotary and reciprocal refrigerators which have so 
far been announced as magnetic refrigerators for producing superfluid 
helium. They, however, still do not utilize effectively the intense 
magnetic fields established by electromagnets. For instance, the rotary 
refrigerator disclosed in U.S. Pat. No. 4,033,734 is of a wheel type in 
which magnetic material is arranged over the entire outer peripheral 
surface of a pillar. According to this refrigerator, however, when a 
magnetic material which has a very good heat conductivity, such as 
gadolinium gallium garnet (Gd.sub.3 Ga.sub.5 O.sub.12) is used as the 
working substance, there are large thermal losses and the efficiency is 
low because of the heat conduction of the working substance in the 
circumferential direction. 
The refrigerator of the rotary wheel type developed by W. A. Steyert (J. 
Appl. Phys. 49(3), 1978), on the other hand, has a problem with regard to 
maintaining seals, since a working fluid is allowed to flow into a rotor. 
It is also necessary to provide means for circulating the fluid, making 
the equipment very complex. 
In a refrigerator of a reciprocating type in which a magnetic material is 
reciprocally inserted into an intense magnetic field and is removed 
therefrom, proposed by R. Beranger et al. (Advances in Cryogenic 
Engineering, Vol. 27, p. 703, 1982, Plenum Press, New York), a mechanism 
is needed to convert rotary motion into linear motion, when it is driven 
by a motor. In addition, when the stroke is large, the construction of the 
drive portion must be complicated and large. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a rotary magnetic 
refrigerator which is free from the above problems. 
The feature of the present invention resides in that two cylindrical 
housings are positioned so that openings thereof communicating with a 
high-temperature cooling medium surface each other, each of the 
cylindrical housings contains a cylindrical rotor which has a working 
substance packed into a plurality of portions thereof, an electromagnet is 
positioned so that it applies an intense magnetic field to the two 
openings, and the intense magnetic field established by the electromagnet 
is simultaneously applied to the plurality of working substance portions 
in the two cylindrical rotors located in the openings communicating with 
the high-temperature cooling medium. 
According to the present invention, the two rotary magnetic refrigerators 
can be operated by commonly using the intense magnetic field. Namely, the 
intense magnetic field is efficiently utilized, and the construction of 
the refrigerator is simplified.

DESCRIPTION OF THE EMBODIMENT 
An embodiment of the present invention will be described below with 
reference to FIGS. 1, 2, 3 and 4. A plurality of portions of a 
heat-insulating cylinder 2 are filled with a working substance 1 for 
magnetic refrigeration, in such a manner that the outer surfaces thereof 
are exposed, to form a rotor 3. The working substance 1 can be a magnetic 
material such as Gd.sub.3 Ga.sub.5 O.sub.12 (gadolinium gallium garnet). 
The cylinder 2 is made of a material such as alumina, crystallized glass, 
or the like which has an expansion coefficient comparable to that of the 
working substance, and which has a small value of heat conductivity. The 
exposed surfaces of the working substance 1 have the same diameter as the 
cylinder 2. Reference numeral 4 denotes a shaft about which the rotor 3 
rotates. Two such rotors 3a, 3b are contained in cylindrical housings 5a, 
5b. The working substances 1a, 1b packed in the rotors 3a, 3b are arranged 
with their phases offset, as shown in FIG. 4. In the example of FIG. 4, 
phases of the working substances are offset by .delta..sub.1 and 
.delta..sub.2 relative to the line connecting centers O.sub.1, O.sub.2 of 
the shafts 4a, 4b, where .delta..sub.1 +.delta..sub.2 =45.degree.. When 
the working substance fills six portions, .delta..sub.1 +.delta..sub.2 is 
approximately 30.degree.. The inner diameter of the cylindrical housings 
5a, 5b is 10 to 100 .mu.m larger than the outer diameter of the rotors 3a, 
3b. The cylindrical housings 5a, 5b are made of a material which has a 
small value of heat conductivity, and which has an expansion coefficient 
comparable to that of the rotor material. Openings 7a, 7b are so formed 
that the working substances 1a, 1b of the rotors 3a, 3b are able to come 
into contact with ordinary liquid helium 6 (at 4.2 K., 1 atm) surrounding 
them. The cylindrical housings 5a, 5b are so disposed that the openings 
7a, 7b face each other, and are also provided with cooling chambers 8a, 8b 
at positions virtually symmetrical to the openings 7a, 7b. The cooling 
chambers 8a, 8b are filled with superfluid helium 9a, 9b (at 2.2 K. or 
lower, 1 atm) which acts as a low-temperature cooling medium. The 
superfluid helium 9a, 9b is able to communicate, via heat-insulating tubes 
10a, 10b, with a cooling chamber 11 which is thermally insulated by a 
vacuum. In the cooling chamber 11, a material 12 to be cooled is immersed 
in the superfluid helium 13. 
A main superconductive coil 14 is arranged so as to apply an intense 
magnetic field to the working substances 1a, 1b in the rotors 3a, 3b 
facing the openings 7a, 7b. In this case, a magnetic field (intense 
magnetic field 7) generated by the main superconducting coil 14 is applied 
to the working substances 1a, 1b in the rotors facing the cooling chambers 
8a, 8b. Superconducting compensation coils 15a, 15b generate a magnetic 
field in the direction opposite to that of the intense magnetic field 7, 
so that a low magnetic field 15 is applied to the proximity of working 
substances 1a, 1b in the rotors facing the cooling chambers 8a, 8b. In 
this way, the working substances 1a, 1b in the intense magnetic field 7 
are able to come into direct contact with the liquid helium 6 acting as 
the high-temperature cooling medium through the openings 7a, 7b, and the 
working substances 1a, 1b in the low magnetic field 15 are able to come 
into direct contact with the superfluid helium 9a, 9b acting as the 
low-temperature cooling medium in the cooling chambers. Under conditions 
in which the magnetic field is distributed as described above, the rotors 
3a, 3b are driven by a motor (not shown) positioned in a room temperature 
portion (300 K.), via a drive shaft 16 and gears 17, 18. 
The rotors are supported by bearings 19a, 19b, 20a, and 20b and rotate in 
the cylindrical housings 5a, 5b in a non-contact manner, maintaining a 
small gap therebetween which is filled with liquid helium 6. As the rotors 
3a, 3b rotate, the working substances packed into the cylinders 
alternately pass through the intense magnetic field portion 7 and the low 
magnetic field portion 15. 
It is desirable to increase the number of portions containing the working 
substance to, for example, 12 to 36 per rotor, from the standpoint of 
reducing variations in the driving force required and also reducing the 
absolute value of the driving force required. In this case, the members 
with the small heat conductivity disposed between the neighboring working 
substances should be at a distance of between 5 to 30 mm to reduce the 
thermal losses due to the heat transfer. 
The rotors 3a, 3b may be rotated in the same direction when there are a 
large number of working substance portions. When the number of working 
substance portions is small, however, the rotors should be roated in 
opposite directions relative to each other. That is, when the number of 
working substance portions is small, the working substance is attracted by 
the magnetic field with a relatively large force. By rotating the rotors 
in the opposite directions to cancel the attractive force, variations in 
the torque required for driving the rotors can be reduced. 
The working substance should be rotated at a frequency f of between about 
0.1 to 1 Hz. 
The working substance generates heat in the high magnetic field 7 and 
releases the heat to the liquid helium 6. In the low magnetic field 15, 
the temperature of the working substance drops so that it absorbs the heat 
from the superfluid helium 9a, 9b. By continuously rotating the rotors, 
the working substance releases heat and absorbs heat repetitively. That 
is, the heat the heat generated by the material 12 being cooled is 
absorbed through the superfluid helium 13, 9a, 9b, so that the material 12 
being cooled is maintained at a low temperature. 
In the foregoing description, liquid helium (at about 4.2 K., 1 atm) was 
specified for use as the high-temperature cooling medium used to produce 
superfluid helium (at about 2.2 K. or lower) on the low-temperature side. 
However, the low-temperature side can be maintained at a temperature at 
least below the temperature of the high-temperature cooling medium, so 
that superfluid helium need not necessarily be produced on the 
low-temperature side. 
The rotors may also be contained in two cylindrical portions that are 
positioned in a single housing.