Regenerator material for extremely low temperatures and regenerator for extremely low temperatures using the same

A cold heat accumulating material for extremely low temperatures which comprises cold heat accumulating granular bodies in which a rate of particles, which are destroyed when a compressive force of 5 MPa is applied thereto by a mechanical strength evaluation die, out of the magnetic cold heat accumulating particles constituting the magnetic cold heat accumulating granular bodies is not than 1 wt. %. In this magnetic cold heat accumulating granular bodies, a rate of magnetic cold heat accumulating particles having more than 1.5 form factor R expressed by L2/4.pi.A, wherein L represents a circumferential length of a projected image of each magnetic cold heat accumulating particle, and A a real of the projected image, is not more than 5%. Such a cold heat accumulating material for extremely low temperatures is capable of providing excellent mechanical properties with respect to mechanical vibration with a high reproducibility. A cold heat accumulator for extremely low temperatures is formed by filling a cold heat accumulating container with a cold heat accumulating material for extremely low temperatures comprising the above-mentioned magnetic cold heat accumulating granular bodies. Such a cold heat accumulator for extremely low temperatures can display excellent performance for a long period of time.

TECHNICAL FIELD 
The present invention relates to a regenerator material for extremely low 
temperatures for use in refrigerators and such like and a regenerator for 
extremely low temperatures using the same. 
BACKGROUND OF ART 
In recent years there have been notable developments in superconducting 
technology, and along with expansion in relevant fields of application the 
development of compact and high performance refrigerators has become 
essential. Such refrigerators demand light weight, compactness and high 
efficiency. 
For instance, refrigerators with freezing cycles such as the Gifford 
MacMahon system or the Sterling system have been used in superconducting 
MRI and cryopump and the like. In addition, high performance refrigerators 
are indispensable for magnetic levitation trains. In such refrigerators, 
an operating medium such as compressed He gas flows in one direction 
through a regenerator filled with regenerator material and supplies the 
resulting thermal energy to the regenerator material, and the expanded 
operating medium then flows in the opposite direction and receives thermal 
energy from the regenerator material. In this process, as the regenerative 
effect is improved, thermal efficiency of the operating medium cycle is 
increased and it becomes possible to achieve even lower temperatures. 
Cu or Pb and the like have conventionally been used as regenerator material 
in the above-mentioned refrigerators. However, specific heat of such 
regenerator material becomes noticeably low at extremely low temperatures 
below 20 K and consequently the above-mentioned regenerative effect does 
not function sufficiently making it difficult to achieve extremely low 
temperatures. 
Therefore, in order to achieve temperatures closer to absolute zero, the 
use of magnetic regenerator materials which exhibit substantial specific 
heat in extremely low temperatures such as Er--Ni type intermetallic 
compounds such as Er.sub.3 Ni, ErNi, ErNi.sub.2 (See Japanese Patent 
Laid-Open Application No. Hei 1-310269) or ARh type intermetallic 
compounds (A: Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb) (See Japanese Patent 
Laid-Open Application No. Sho 51-52378) such as ErRh is recently being 
considered. 
However, during operation of the above-mentioned regenerators, the 
operating medium such as He gas passes at high pressure and high speed 
through gaps in the regenerator material with which the regenerator is 
filled and consequently the flow direction of the operating medium changes 
at frequent intervals. As a result, the regenerator material is subject to 
a variety of forces such as mechanical vibration. Stress is also applied 
when filling the regenerator with the material 
Though the regenerator material is subject to the various forces, magnetic 
regenerator material of the intermetallic compounds described above such 
as Er.sub.3 Ni or ErRh is generally brittle and consequently is prone to 
pulverization as a result of mechanical vibration during operation or 
pressure during filling or such like. The particles generated by this 
pulverization influence harmfully the performance of the regenerator, such 
as obstructing the gas seal. Moreover, there is also the problem that the 
degree of deterioration in the performance of the regenerator when using a 
magnetic regenerator material of the intermetallic compounds as described 
above varies widely depending the manufactured batches of magnetic 
regenerator material and the like. 
It is therefore the object of the present invention to provide a 
regenerator material which have excellent mechanical properties for 
mechanical vibration and filling stress and such like with a high 
reproducibility, a regenerator which have excellent refrigerating 
performance in extremely low temperature over a long period of time with a 
high reproducibility by using such a regenerator material, and a 
refrigerator using such a regenerator for extremely low temperatures. 
DISCLOSURE OF THE INVENTION 
Having considered various means for achieving the objectives described 
above, the present inventors have discovered that the mechanical strength 
of magnetic regenerator material particles of intermetallic compounds and 
such like containing rare earth elements is highly dependent on the 
precipitation volume, the precipitation situation, the form and such like 
of rare earth carbides and rare earth oxides, which exist in the grain 
boundary. The precipitation volume and precipitation situation and such 
like of these rare earth cabides and rare earth oxides are complexly 
related to the amount of carbon and oxide impurities, atmosphere in the 
rapid solidification process, cooling velocity, melt temperature and such 
like, and therefore they alter greatly depending the manufactured batch of 
the magnetic regenerator material particles. It was discovered that the 
mechanical strength of the magnetic regenerator particles therefore varies 
greatly with each manufactured batch and that it would be extremely 
difficult to predict mechanical strength from manufacturing conditions and 
such like alone. 
In order to improve the mechanical reliability of magnetic regenerator 
particles, following detailed consideration of the mechanical properties 
of magnetic regenerator particles, it was learned that mechanical 
reliability of magnetic regenerator particles can be estimated by 
considering the mechanical strength of not an individual magnetic 
regenerator particle but an aggregation of magnetic regenerator particles, 
concentration of stress when a force is applied to aggregation of magnetic 
regenerator particles. With regard to the form of magnetic regenerator 
particles, it was further discovered that it is possible to increase the 
mechanical reliability of magnetic regenerator particles by selectively 
using magnetic regenerator particles with a form having few protrusions. 
The present invention is based on these new knowledges. 
In other words, a first regenerator material for extremely low temperatures 
of the present invention is characterized in that it comprises aggregation 
of magnetic regenerator particles, in which a rate of the particles which 
are fractured is not more than 1 wt. % when a compressive stress of 5 MPa 
is applied thereto. 
A first regenerator for extremely low temperatures of the present invention 
comprises a regenerator container filled with the above-mentioned first 
regenerator material for extremely low temperatures. 
Furthermore, a second regenerator material for extremely low temperatures 
of the present invention is characterized in that it comprises aggregation 
of magnetic regenerator particles, in which a rate of the particles 
satisfying that form factor R is more than 1.5, wherein R is expressed by 
L.sup.2 /4.pi.A, L represents a perimeter of a projected image of the 
individual regenerator particle and A represents an area of the projected 
image, is not more than 5%. 
A second regenerator for extremely low temperatures of the present 
invention comprises a regenerator container filled with the 
above-mentioned second regenerator material for extremely low 
temperatures. 
Moreover, a refrigerator of the present invention includes the 
above-mentioned first regenerator for extremely low temperatures or the 
second regenerator for extremely low temperatures. 
A regenerator material for extremely low temperatures of the present 
invention consists of magnetic regenerator particles, namely an aggregate 
of magnetic regenerator particles. For instance, intermetallic compounds 
including rare earth elements expressed by RM.sub.Z (R represents at least 
one rare earth element chosen from Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, 
Dy, Ho, Er, Tm and Yb; M represents at least one metallic element chosen 
from Ni, Co, Cu, Ag, Al and Ru; z represents a number between 
0.001.sup..about. 9.0) or intermetallic compounds including rare earth 
elements expressed by ARh (A represents at least one rare earth element 
chosen from Sm, Gd, Tb, Dy, Ho, Er, Tm and Yb) are appropriate as the 
magnetic regenerator material in the present invention. 
When the magnetic regenerator particles described above have almost 
spherical form and are uniform in size, they can smooth out the flow of 
the gas. Consequently, not less than 70 wt. % of the whole magnetic 
regenerator particles can suitably be constituted with magnetic 
regenerator particles each having a shape such that the ratio of the major 
diameter to the minor diameter (aspect ratio) is not greater than 5, and 
with a diameter of 0.01.sup..about. 3.0 mm. 
When the magnetic regenerator particle aspect ratio exceeds 5, it becomes 
difficult to fill to make gaps uniform. Consequently when such particles 
exceed 30 wt. % of the whole magnetic regenerator particles, the 
regenerator performance and the like may deteriorate. The aspect ratio 
should preferably be not more than 3 and ideally not more than 2. 
Furthermore, the rate of magnetic regenerator particles with a particle 
aspect ratio of not more than 5 should preferably be not less than 80 wt. 
% and ideally not less than 90 wt. %. 
Moreover, when the diameter of the magnetic regenerator particles is less 
than 0.01 mm, the packing density becomes too much, thereby the pressure 
loss of working medium such as helium is likely to increase. On the other 
hand, when the particle size of the magnetic regenerator particles is more 
than 3.0 mm, the area of heat transfer surface between the magnetic 
regenerator particles and the working medium becomes small, thereby heat 
transfer efficiency deteriorates. Accordingly, when the percentage of such 
particles is more than 30% by weight of the magnetic regenerator 
particles, the regenerator performance etc. is likely to deteriorate. The 
particle size is preferably in a range of 0.05.sup..about. 2.0 mm, more 
preferably in a range of 0.1.sup..about. 0.5 mm. The percentage of the 
particles having a diameter ranging 0.01.sup..about. 3.0 mm in the whole 
magnetic regenerator particles is preferably not less than 80% by weight, 
more preferably not less than 90% by weight. 
A regenerator material for extremely low temperatures of the present 
invention comprises magnetic regenerator particles in which the rate of 
particles which are fractured when a compressive stress of 5 MPa is 
applied to an aggregate of magnetic regenerator particles with the 
above-mentioned form is not more than 1 wt. %. As described above, the 
present invention considers the mechanical strength of an aggregate of 
magnetic regenerator particles in which the mechanical strength of each 
regenerator particle for extremely low temperatures is complexly related 
to the volume of carbon and oxide impurities, atmosphere during the rapid 
solidification process, cooling velocity, melt temperature and such like, 
and wherein a complex concentration of stress occurs when stress is 
applied to an aggregate of these particles. By measuring the rate of 
particles fractured when a compressive stress of 5 MPa is applied to such 
aggregates of magnetic regenerator particles, it is possible to evaluate 
the reliability of the magnetic regenerator particles with respect to 
mechanical strength. 
In other words, when the rate of particles fractured when a compressive 
stress of 5 MPa is applied to an aggregate of magnetic regenerator 
particles is not more than 1 wt. %, hardly any magnetic regenerator 
particles are pulverized as a result of mechanical vibration during an 
operation of refrigerator or by stress and such like when filling the 
regenerator container with these particles, even if the manufacturing 
batches and manufacturing conditions are different. Therefore, the 
problems such as obstruction of gas seals in refrigerators and the like 
can be prevented by using magnetic regenerator particles with these 
mechanical properties. The reliability cannot be evaluated, since most 
magnetic regenerator particles, irrespective of their internal morphology, 
are not fractured by the application of a compressive stress of less than 
5 MPa. 
The above-mentioned reliability evaluation of magnetic regenerator 
particles is carried out as follows. First, a fixed amount of magnetic 
regenerator particles is extracted randomly from each manufacturing batch 
which comply with a specified aspect ratio, particle size and such like. 
Second, as FIG. 1 shows, the extracted magnetic regenerator particles 1 
are filled within a die 2 for the mechanical strength evaluation and a 
stress of 5 MPa is applied thereto. The stress needs to be increased 
gradually; for instance, crosshead speed in these tests is roughly 0.1 
mm/min. Furthermore, the die 2 material is die steel and such like. After 
stress has been applied, fractured magnetic regenerator particles are 
sorted by sieving and shape separation, and the reliability of the 
aggregate of magnetic regenerator particles is evaluated by measuring the 
weight of the fractured particles. An extraction of around 1 g of magnetic 
regenerator particles from each manufacturing batch is sufficient. 
The rate of particles fractured when a compressive stress of 5 MPa is 
applied to magnetic regenerator particles should preferably be not more 
than 0.1 wt. % and ideally not more than 0.01 wt. %. In addition, for a 
reliability evaluation of magnetic regenerator particles, the rate of 
particles fractured when a compressive stress of 10 MPa is applied thereto 
should preferably be not more than 1 wt. % and should ideally satisfy the 
same conditions when a compressive stress of 20 MPa is applied. 
A regenerator material for extremely low temperatures of the present 
invention can basically prevent the generation of pulverization of 
particles by satisfying the above-mentioned mechanical strength of 
aggregates of magnetic regenerator particles when a compressive stress is 
applied thereto, and mechanical reliability can be further improved in 
order to be capable of preventing more effectively the chipping and such 
like by the use of magnetic regenerator particles with a form as described 
below. 
In other words, regenerator particles should preferably have a spherical 
form as explained above and when this form is more precisely spherical and 
the size of the particles is more uniform, the flow of the gas can be 
smoothed out and extreme stress concentration occurring when a compressive 
stress is applied to these particles can be restricted. Mechanical 
vibration during refrigerator operation or stress applied when the 
regenerator is filled with regenerator material are conceivable as the 
above-mentioned compressive stress. The stress is most likely to 
concentrate when particles with a less spherical form are subjected to a 
compressive stress. 
Conventionally, only the ratio of the major diameter to the minor diameter 
(i.e. the aspect ratio) has been used when evaluating the spherical form 
of magnetic regenerator particles (for instance, see Japanese Patent 
Laid-Open Application No. Hei 3-174486). However, the aspect ratio tends 
to be a lower value when the roundness of an ellipse is evaluated although 
it is valid as a parameter for evaluating the whole particle form, even if 
there are protrusions on the particle surface for example these 
protrusions have little influence on the aspect ratio. 
When the magnetic regenerator particles used as regenerator material for 
extremely low temperatures comprise particles with complex surface forms 
such as protrusions, stress concentrate on the protrusions and such like 
when a compressive stress is applied, and the mechanical strength of the 
magnetic regenerator particles is thereby adversely affected. Therefore in 
the present invention, a rate of regenerator particles satisfying that 
form factor R is greater than 1.5, wherein R is expressed by L.sup.2 
/4.pi.A, L represents a perimeter of a projected image of the individual 
magnetic regenerator particles and A represents an area of the projected 
image, is preferably not more than 5%. 
As FIG. 2 shows, when protrusions are present on the particle surface, even 
a particle with a highly spherical form will have a high form factor R 
value (high partial shape irregularity). Furthermore, as FIG. 3 shows, a 
particle with a comparatively smooth surface will have a low form factor R 
value even if its form is rather unspherical. In contrast, the aspect 
ratio described above tends to be a lower value for particles such as that 
shown in FIG. 3 (aspect ratio=b/a) and a higher value for particles with 
surface protrusions and the like such as shown in FIG. 2. 
In other words, a low form factor R indicates that the particle surface is 
comparatively smooth (low partial shape irregularity) and R is an 
effective parameter for evaluating partial form irregularity of particles. 
Therefore, by using particles with a low form factor R it is possible to 
achieve improvements in the mechanical strength of magnetic regenerator 
particles. In fact, even particles whose aspect ratio exceeds 5 do not 
adversely affect the mechanical strength of magnetic regenerator particles 
substantially provided that the particle surface is smooth. On the other 
hand, when particles with the projections and such like have high partial 
form irregularity and their form factor R exceeds 1.5, the projections are 
liable to chip and consequently such particles have poor mechanical 
strength. Therefore, when the rate of such particles with high partial 
form irregularity exceeds 5%, the mechanical strength of the magnetic 
regenerator particles is adversely affected. 
Based on the reasons described above, the rate of particles with a form 
factor R exceeding 1.5 should preferably not be more than 5%, more 
preferably not more than 2% and ideally not more than 1%. Furthermore, the 
rate of particles with a form factor R exceeding 1.3 should preferably not 
be more than 15%, more preferably not more than 10% and ideally not more 
than 5%. However, since the aspect ratio is important for evaluating the 
degree of sphericity, having satisfied form factor R provisions, not less 
than 70 wt. % of the magnetic regenerator particles should preferably have 
an aspect ratio of not more than 5 as described above. 
The manufacturing method of magnetic regenerator particles described above 
is by no means restricted and a variety of manufacturing methods can be 
employed. For instance, melt of a designated composition can be rapidly 
solidified using methods such as centrifugal atomization, gas atomization 
and rotational electrode method. In addition, magnetic regenerator 
particles in which a rate of particles satisfying that form factor R is 
greater than 1.5 is not more than 5%, can be obtained by for instance 
optimizing manufacturing conditions and carrying out shape separation such 
as inclined vibrating plate method. 
A regenerator for extremely low temperatures of the present invention uses 
magnetic regenerator particles having mechanical properties as described 
above, namely magnetic regenerator particles with a rate of particles 
fractured when a compressive stress of 5 MPa is applied of not more than 1 
wt. %. Moreover a regenerator for extremely low temperatures of the 
present invention can be composed of magnetic regenerator particles with a 
rate of particles satisfying that form factor R is greater than 1.5 of not 
more than 5%. A regenerator for extremely low temperatures wherein a 
regenerator has been filled with magnetic regenerator particles satisfying 
both mechanical properties and form is especially preferable. 
Since magnetic regenerator particles used in a regenerator for extremely 
low temperatures of the present invention contain hardly any magnetic 
regenerator particles which are pulverized as a result of mechanical 
vibration during a refrigerator operation or compressive stress when 
filling the container of a regenerator, and such like, obstruction of gas 
seals in refrigerators and such like can be prevented. Therefore, a 
regenerator for extremely low temperatures capable of steadily maintaining 
refrigerating performance over a long period of time and moreover a 
refrigerator capable of steadily maintaining refrigerating performance 
over a long period of time can be obtained with high reproducibility.

MODE FOR EMBODYING THE INVENTION 
The preferred embodiments of the present invention will next be explained. 
Embodiment 1 
First, an Er.sub.3 Ni mother alloy was prepared by high frequency fusion. 
This Er.sub.3 Ni mother alloy was melted at approximately 1373 K and the 
melt thereby obtained was poured onto a rotating disc in Ar atmosphere 
(pressure=approximately 101 kPa) and rapidly solidified. The particles 
obtained were sieved and classified according to form and 1 kg of 
spherical particles with diameters of between 0.2.sup..about. 0.3 mm was 
selected. Particles with an aspect ratio of not more than 5 constituted 
not less than 90 wt. % of all the particles in these particles. This 
process was carried out repeatedly and 10 batches of spherical Er.sub.3 Ni 
particles were obtained. 
Next, 1 g of particles was randomly extracted from each of the ten batches 
of spherical Er.sub.3 Ni particles. These extracted particles were each 
filled within a die 2 for mechanical strength evaluation shown in FIG. 1 
and a compressive stress of 5 MPa (crosshead speed=0.1 mm/min) was applied 
using an Instron-type testing machine. Following the test, all particles 
were sieved and classified according to form and the weight of the 
fractured spherical Er.sub.3 Ni particles was measured. The batch in which 
the fractured particle rate was 0.004 wt. % was selected as magnetic 
regenerator particles for this embodiment. When the form factor R of these 
magnetic regenerator particles in this batch was evaluated by image 
analysis, the rate of particles having a form factor R of more than 1.5 
was not more than 5%. 
Magnetic regenerator spherical particles comprising Er.sub.3 Ni selected in 
the manner described above were filled in a regenerator container at a 
packing factor of 70% to construct a regenerator for extremely low 
temperatures. A two-stage GM refrigerator, which is shown schematically in 
FIG. 4, was constructed using this regenerator for extremely low 
temperatures and refrigerator testing was carried out. Test results showed 
an initial refrigeration capacity of 320 mW was obtained at 4.2 K and 
stable refrigeration capacity was obtained throughout 5000 hours of 
continuous operation. 
The two-stage GM refrigerator 10 shown in FIG. 4 has a vacuum chamber 13 
provided with a large-diameter first cylinder 11 and a small-diameter 
second cylinder 12 which is cocentrically connected thereto. A first 
regenerator 14 can reciprocate in the first cylinder 11 and a second 
regenerator 15 can reciprocate in the second cylinder 12. Seal rings 16 
and 17 are provided respectively between the first cylinder 11 and the 
first regenerator 14 and between the second cylinder 12 and the second 
regenerator 15. 
The first regenerator 14 contains a first regenerator material 18 such as 
Cu mesh. The second regenerator 15 is configured according to a 
regenerator for extremely low temperatures of the present invention and 
contains a regenerator material for extremely low temperatures 19 of the 
present invention as a second regenerator material. The first regenerator 
14 and the second regenerator 15 have passages for an operating medium 
such as He gas provided in the gaps and such like of the first regenerator 
material 18 and the regenerator material for extremely low temperatures 19 
respectively. 
A first expansion space 20 is provided between the first regenerator 14 and 
the second regenerator 15. A second expansion space 21 is provided between 
the second regenerator 15 and the cold stage of the second cylinder 12. A 
first cooling stage 22 is formed in the lower portion of the first 
expansion space 20 and a second cooling stage 23 at a lower temperature 
than the first cooling stage 22 is formed in the lower portion of the 
second expansion space 21. 
A compressor 24 supplies a high pressure operating medium (e.g. He gas) to 
the above-mentioned two-stage GM refrigerator 10. The supplied operating 
medium passes through the first regenerator material 18 contained in the 
first regenerator 14 and reaches the first expansion space 20, then passes 
through the regenerator material for extremely low temperatures 19 (the 
second regenerator material) contained in the second regenerator 15 and 
reaches the second expansion space 21. In this process, the operating 
medium cools by supplying thermal energy to both regenerator materials 18 
and 19. Having passed through regenerator materials 18 and 19 the 
operating medium expands and absorbs heat in the first and second 
expansion space 20, 21 and both cooling stages 22 and 23 are cooled. The 
expanded operating medium now flows in reverse direction through both 
regenerator materials 18 and 19. After receiving thermal energy from the 
regenerator materials 18 and 19, the operating medium is exhaused. This 
process increases the cooling efficiency of the operating medium cycle and 
achieves even lower temperatures, as the regenerator efficiency improves. 
Embodiment 2 
As in the embodiment 1, 10 batches were produced of spherical Er.sub.3 Ni 
particles with particle diameters of between 0.2.sup..about. 0.3 mm of 
which particles with an aspect ratio of not more than 5 constituted not 
less than 90 wt. %. Next, 1 g of particles was randomly extracted from 
each of the ten batches of spherical Er.sub.3 Ni particles. These 
extracted particles were each filled within the die 2 for mechanical 
strength evaluation shown in FIG. 1 and a compressive stress of 5 MPa 
(crosshead speed=0.1 mm/min) was applied thereto using an Instron-type 
testing machine. Following the test, all the particles were sieved and 
classified according to form and the weight of the fractured spherical 
Er.sub.3 Ni particles was measured. The rate of fractured particles is 
shown in Table 1. 
The magnetic regenerator spherical particles consisting of Er.sub.3 Ni from 
each of the 10 batches were respectively filled in regenerator containers 
at a packing factor of 70% and then put in a two-stage GM refrigerator and 
refrigerating testing was carried out as in the embodiment 1. The test 
results are also shown in Table 1. 
COMATIVE EXAMPLE 1 
A batch in which the rate of spherical Er.sub.3 Ni particles fractured when 
a compressive stress of 5 MPa was applied thereto was 1.3 wt. % was 
selected from the 10 batches of spherical Er.sub.3 Ni particles produced 
in the embodiment 1. The selected magnetic regenerator spherical particles 
of Er.sub.3 Ni were filled in a regenerator at a packing factor of 70%, 
respectively, and then put in a two-stage GM refrigerator and 
refrigerating testing was carried out as in the embodiment 1. The test 
results are shown in Table 1. 
TABLE 1 
______________________________________ 
Rate of particles 
fractured by Refrigeration 
compressive capacity (mW) 
stress test of Initial 
After 5000 
Test No. 5 MPa (wt. %) Value hours 
______________________________________ 
Embodiment 2 
1 0.001 321 320 
2 0.007 325 325 
3 0.840 327 305 
4 0.014 326 321 
5 0.001 322 320 
6 0.110 325 318 
7 0.021 329 326 
8 0.008 330 328 
9 0.045 324 320 
10 0.216 321 314 
Comparative 
1.3 320 270 
Example 1 
______________________________________ 
As Table 1 clearly shows, all the regenerators using magnetic regenerator 
particles in which the rate of particles fractured when a compressive 
stress of 5 MPa was applied was not more than 1 wt. % maintained excellent 
refrigeration capacity over a long period of time. 
COMATIVE EXAMPLE 2 
As in the embodiment 1, 10 batches were produced of spherical Er.sub.3 Ni 
particles with diameters of between 0.2.sup..about. 0.3 mm of which 
particles with an aspect ratio of not more than 5 constituted not less 
than 90 wt. %. Next, 1 g of particles was randomly extracted from each of 
the ten batches of spherical Er.sub.3 Ni particles. These extracted 
particles were each filled within the die 2 for the mechanical strength 
evaluation shown in FIG. 1 and a compressive stress of 3 MPa (crosshead 
speed=0.1 mm/min) was applied using an Instron-type testing machine, but 
hardly any particles were fractured. Since hardly any particles are 
fractured by a compressive stress of less than 5 MPa, reliability cannot 
be evaluated. 
EMBODIMENT 3 
First, an Er.sub.3 Co mother alloy was prepared by high frequency fusion. 
This Er.sub.3 Co mother alloy was melted at approximately 1373 K and the 
melt thereby obtained was poured onto a rotating disc in Ar atmosphere 
(pressure=approximately 101 kPa) and rapidly solidified. The particles 
obtained were sieved and classified according to form and 1 kg of 
spherical particles with diameters of between 200.sup..about. 300 .mu.m 
was selected. Particles with an aspect ratio of not more than 5 
constituted not less than 90 wt. % of all the particles. This process was 
carried out repeatedly and 10 batches of spherical Er.sub.3 Co particles 
were obtained. 
Next, 1 g of particles was randomly extracted from each of the 
above-mentioned 10 batches of spherical Er.sub.3 Co particles. These 
extracted particles were each filled within a die 2 for mechanical 
strength evaluation shown in FIG. 1 and a compressive stress of 5 MPa 
(crosshead speed=0.1 mm/min) was applied thereto using an Instron-type 
testing machine. Following the test, all particles were sieved and 
classified according to form and the weight of the fractured spherical 
Er.sub.3 Co particles was measured. The rates of particles fractured are 
shown in Table 2. When the form factor R of each of these magnetic 
regenerator particles was evaluated by image analysis, all rates of 
particles in which R was more than 1.5 were not more than 5%. 
The above-mentioned magnetic regenerator spherical particles of Er.sub.3 Co 
were filled in a regenerator at a packing factor of 70%, respectively, put 
in a two-stage GM refrigerator identical to that in the embodiment 1 and 
refrigerator testing was carried out. Test results are also shown in Table 
2. 
TABLE 2 
______________________________________ 
Rate of particles 
fractured by refrigeration 
compressive capacity (mW) 
stress test of Initial 
After 5000 
Test No. 5 MPa (wt. %) Value hours 
______________________________________ 
Embodiment 3 
1 0.002 306 305 
2 0.003 309 308 
3 0.109 302 297 
4 0.021 305 302 
5 0.007 308 308 
6 0.030 302 299 
7 0.004 306 304 
8 0.005 300 298 
9 0.043 306 303 
10 0.007 309 309 
______________________________________ 
As Table 2 clearly shows, all the regenerators using magnetic regenerator 
particles in which the rate of particles fractured when a compressive 
stress of 5 MPa was applied was not more than 1 wt. % maintained excellent 
refrigeration capacity over a long period of time. 
Furthermore, it was confirmed from this embodiment 3 and from embodiments 1 
and 2 described above that irrespective of the composition and such like 
of the magnetic regenerator material, when magnetic regenerator particles 
in which the rate of particles fractured when a compressive stress of 5 
MPa was applied was not more than 1 wt. % are used, excellent 
refrigerating capability can be maintained over a long period of time. 
EMBODIMENT 4, COMATIVE EXAMPLE 3 
An ErAg mother alloy was prepared by high frequency fusion. This ErAg 
mother alloy was melted at approximately 1573 K and the melt thereby 
obtained was poured onto a rotating disc in Ar atmosphere 
(pressure=approximately 101 kPa) and rapidly solidified. The particles 
obtained were sieved and classified according to form and 1 kg of 
spherical particles with diameters of between 0.2.sup..about. 0.3 mm was 
selected. Particles with an aspect ratio of not more than 5 constituted 
not less than 90 wt. % of all the particles. This process was carried out 
repeatedly and 5 batches of spherical ErAg particles were obtained. 
Next, 1 g of particles was randomly extracted from each of the 
above-mentioned 5 batches of spherical ErAg particles. These extracted 
particles were each filled within a die 2 for mechanical strength 
evaluation shown in FIG. 1 and a compressive stress of 5 MPa (crosshead 
speed=0.1 mm/ml) was applied using an Instron-type testing machine. 
Following the test, all particles were sieved and classified according to 
form and the weight of the fractured spherical ErAg particles was 
measured. The rates of particles fractured are shown in Table 3. 
The above-mentioned magnetic regenerator spherical particles of ErAg were 
filled in regenerator at a packing factor of 64%. These regenerators were 
then put in a two-stagte GM refrigerator as a second regenerator 
respectively and refrigerator testing was carried out to measure the 
lowest temperatures attained by the refrigerators. Initial values of 
lowest temperatures attained and lowest temperatures achieved after 5000 
hours of continuous operation are shown respectively in Table 3. 
TABLE 3 
______________________________________ 
Rate of particles 
Lowest 
fractured by Temperature 
compressive Attained (K) 
stress test of Initial 
After 5000 
Test No. 5 MPa (wt. %) Value hours 
______________________________________ 
Embodiment 4 
1 0.031 6.3 7.6 
2 0.003 6.7 7.4 
3 0.107 6.6 8.3 
Comparative Example 3 
4 1.259 6.7 15.4 
5 2.117 6.5 23.8 
______________________________________ 
EMBODIMENT 5, COMATIVE EXAMPLE 4 
First, an ErNi mother alloy was prepared by high frequency fusion. This 
ErNi mother alloy was melted at approximately 1473 K and the melt thereby 
obtained was poured onto a rotating disc in Ar atmosphere 
(pressure=approximately 101 kPa) and rapidly solidified. The particles 
obtained were sieved and classified according to form and 1 kg of 
spherical particles with diameters of between 0.25.sup..about. 0.35 mm was 
selected. Particles with an aspect ratio of not more than 5 constituted 
not less than 90 wt. % of all the particles. This process was carried out 
repeatedly and 5 batches of spherical ErNi particles were produced. In 
addition, 5 batches of spherical Ho.sub.2 Al particles were produced. 
Next, 1 g of particles was randomly extracted from each of the 
above-mentioned 5 batches of spherical ErNi particles and the 5 batches of 
spherical Ho.sub.2 Al particles. The extracted particles were each filled 
within a die 2 for mechanical strength evaluation shown in FIG. 1 and a 
compressive stress of 5 MPa (crosshead speed=0.1 mm/min) was applied 
thereto using an Instron-type testing machine. Following the test, all 
particles were sieved and classified according to form and the weight of 
the fractured particles was measured. The rates of particles fructured are 
shown in Table 4. 
The magnetic regenerator spherical particles of ErNi and Ho.sub.2 Al were 
filled in regenerator in a 2-layered structure in which ErNi particles 
occupied the lower temperature half side and Ho.sub.2 Al particles 
occupied in the higher temperature half side at a packing factor of 64%, 
respectively. Each of these regenerators was then put in a two-stage GM 
refrigerator as second regenerators and refrigerator testing was carried 
out to measure the lowest temperatures attained by the refrigerator. 
Initial values of lowest temperatures attained and lowest temperatures 
achieved after 5000 hours of continuous operation are shown respectively 
in Table 4. 
TABLE 4 
______________________________________ 
Rate of particles 
Lowest 
fractured by 
Temperature 
compressive Attained (k) 
stress test of 
Initial 
After 5000 
Test No. 5 MPa (wt. %) 
Value 
hours 
______________________________________ 
Embodiment 5 
1 ErAg 0.003 3.4 3.7 
Ho.sub.2 Al 
0.005 
2 ErAg 0.005 3.6 4.1 
Ho.sub.2 Al 
0.048 
3 ErAg 0.016 3.4 3.9 
Ho.sub.2 Al 
0.009 
Comparative Example 4 
4 ErAg 1.600 3.7 7.3 
Ho.sub.2 Al 
1.233 
5 ErAg 1.706 3.9 8.3 
Ho.sub.2 Al 
1.727 
______________________________________ 
EMBODIMENT 6, COMATIVE EXAMPLE 5 
An HoCu.sub.2 mother alloy was prepared by high frequency fusion. This 
HoCu.sub.2 mother alloy was melted at approximately 1373 K and the melt 
thereby obtained was poured onto a rotating disc in Ar atmosphere 
(pressure=approximately 101 kpa) and rapidly solidified. The particles 
obtained were sieved to adjust diameters 0.2.sup..about. 0.3 mm, shape 
separation was carried out using an inclined vibrating plate method and 1 
kg of spherical particles was selected. Particles with an aspect ratio of 
not more than 5 constituted not less than 90 wt. % of all the particles. 
This process was carried out repeatedly and 5 batches of spherical 
HoCu.sub.2 particles were produced. The roundness of each batch of 
spherical HoCu.sub.2 particles was then altered by adjusting shape 
separation conditions such as for instance an angle of inclination and 
vibration power. 
The perimeter of a projected image L and the area of the projected image A 
of each particle of the 5 batches of spherical HoCu.sub.2 particles 
obtained were measured by image analysis and a form factor R expressed by 
L.sup.2 /4.pi.A was evaluated. Results are shown in Table 5. 
In addition, 1 g of particles was randomly extracted from each of the 
above-mentioned 5 batches of spherical HoCu.sub.2 particles. These 
extracted particles were each filled within a die 2 for mechanical 
strength evaluation shown in FIG. 1 and a compressive stress of 5 MPa 
(crosshead speed=0.1 mm/min) was applied thereto using an Instron-type 
testing machine. Following the test, all particles were sieved and 
classified according to form and the weight of the fractured spherical 
HoCu.sub.2 particles was measured. The rates of particles fractured are 
shown in Table 5. 
The magnetic regenerator spherical particles of HoCu.sub.2 were filled in 
regenerator, respectively, at a packing factor of 64%. These regenerators 
were then put respectively in two-stage GM refrigerators as second 
regenerator and refrigerator testing was carried out to measure the lowest 
temperatures attained by the refrigerators. Initial values of lowest 
temperatures attained and lowest temperatures achieved after 5000 hours of 
continuous operation are also shown respectively in Table 5. 
TABLE 5 
______________________________________ 
Rate of Rate of particles 
Lowest 
particles fractured by 
Temperature 
each of which compressive Attained (K.) 
R is more stress test of 
Initial 
After 5000 
Test No. 
than 1.5 (%) 
5 MPa (wt. %) 
Value hours 
______________________________________ 
Embodiment 6 
1 0.6 0.012 5.1 5.6 
2 1.5 0.007 5.3 5.9 
3 6.6 0.040 5.5 6.6 
4 5.6 0.307 6.7 8.2 
Comparative Example 5 
5 7.9 1.474 6.5 13.8 
______________________________________ 
EMBODIMENT 7 
First, an Er.sub.3 Ni mother alloy was prepared by high frequency fusion. 
This Er.sub.3 Ni mother alloy was melted at approximately 1373 K and the 
melt thereby obtained was poured onto a rotating disc in Ar atmosphere 
(pressure=approximately 101 kPa) and rapidly solidified. The particles 
obtained were sieved and particles with diameters of 0.2.sup..about. 0.3 
mm were obtained. Furthermore, shape separation using inclined vibrating 
plate method was carried out to the particles thereby obtained, to remove 
particles with high partial irregularity and to select Er.sub.3 Ni 
spherical particles with low partial irregularity. 
The perimeter of a projected image L and the area of the projected image A 
of each particle of obtained the Er.sub.3 Ni spherical particles were 
measured by image analysis and a form factor R expressed by L.sup.2 
/4.pi.A was evaluated. The result showed that the rate of particles with a 
form factor R more than 1.5 was 0.6% and that the rate of particles with a 
form factor R more than 1.3 was 4.7%. The aspect ratio for all particles 
was not more than 5. 
Magnetic regenerator spherical particles of Er.sub.3 Ni selected by the 
method described above were filled in a regenerator at a packing factor of 
70%. This regenerator was then put in a two-stage GM refrigerator and 
refrigerator testing was carried out. As a result, an initial 
refrigeration capacity of 320 mW was obtained at 4.2 K and stable 
refrigeration capacity was obtained over 5000 hours of continuous 
operation. 
EMBODIMENT 8 
An Er.sub.3 Ni mother alloy was prepared by high frequency fusion. This 
Er.sub.3 Ni mother alloy was melted at approximately 1300 K and the melt 
thereby obtained was poured onto a rotating disc in Ar atmosphere 
(pressure=approximately 30 kPa) and rapidly solidified. The particles 
obtained were sieved and particles with diameters of 0.2.sup..about. 0.3 
mm were obtained. Furthermore, shape separation using inclined vibrating 
plate method as in the embodiment 7 was carried out to the particles 
thereby obtained, to remove particles with high partial irregularity and 
to select Er.sub.3 Ni spherical particles with low partial irregularity. 
The perimeter of a projected image L and the area of the projected image A 
of each particle of the Er.sub.3 Ni spherical particles obtained were 
measured by image analysis and a form factor R expressed by L.sup.2 
/4.pi.A was evaluated. The result showed that the rate of particles with a 
form factor R more than 1.5 was 4% and the rate of particles with a form 
factor R more than 1.3 was 13%. However, particles with an aspect ratio 
more than 5 constituted 32 wt. % of all particles. 
Magnetic regenerator spherical particles of Er.sub.3 Ni selected by the 
method described above were filled in a regenerator at a packing factor of 
70%, placed in a two-stage GM refrigerator and refrigerator testing was 
carried out. As a result, an initial refrigeration capacity of 310 mW was 
obtained at 4.2 K and refrigeration capacity after 5000 hours of 
continuous operation was 305 mW. 
COMATIVE EXAMPLE 6 
Shape separation of particles produced and sieved as in the embodiment 7 
was carried out using a inclined vibrating plate with a comparatively 
smaller angle of inclination than in the embodiment 7 and Er.sub.3 Ni 
spherical particles were selected. When the aspect ratio of the Er.sub.3 
Ni spherical particles obtained was measured, the aspect ratio of all 
particles was not more than 5. Furthermore, evaluation of the form factor 
R of the Er.sub.3 Ni spherical particles as in the embodiment 7 revealed 
that the rate of particles with a form factor R more than 1.5 was 7% and 
the rate of particles with a form factor R more than 1.3 was 24%. 
The above-mentioned Er.sub.3 Ni spherical particles were filled in a 
regenerator at a packing factor of 70%, placed in a two-stage GM 
refrigerator and refrigerator testing was carried out. The result was that 
an initial refrigeration capacity of 320 mW was obtained at 4.2 K but 
after 5000 hours of continuous operation refrigeration capacity had 
deteriorated to 280 mW. 
COMATIVE EXAMPLE 7 
An Er.sub.3 Ni mother alloy was prepared by high frequency fusion. This 
Er.sub.3 Ni mother alloy was melted at approximately 1273 K and the melt 
thereby obtained was poured onto a rotating disc in Ar atmosphere 
(pressure=approximately 101 kPa) and rapidly solidificated. The particles 
obtained were sieved and particles with diameters of 0.2.sup..about. 0.3 
mm were obtained. Furthermore, shape separation using inclined vibrating 
plate method as in the Comparative Example 6 was carried out to the 
particles obtained and spherical particles were selected. 
When the aspect ratio of the Er.sub.3 Ni spherical particles obtained was 
measured, particles with an aspect ratio more than 5 constituted 34 wt. % 
of all particles. In addition, when the form factor R of the Er.sub.3 Ni 
spherical particles was evaluated by the same method as in the embodiment 
7, the rate of particles with a form factor R more than 1.5 was 11% and 
the rate of particles with a form factor R more than 1.3 was 27%. 
The above-mentioned Er.sub.3 Ni spherical particles were filled in a 
regenerator at a packing factor of 70%, placed in a two-stage GM 
refrigerator and refrigerator testing was carried out. The result was that 
an initial refrigeration capacity of 320 mW was obtained at 4.2 K but 
after 5000 hours of continuous operation refrigeration capacity had 
deteriorated to 270 mW. 
EMBODIMENT 9 
An Er.sub.3 Co mother alloy was prepared by high frequency fusion. This 
Er.sub.3 Co mother alloy was melted at approximately 1373 K and the melt 
thereby obtained was poured onto a rotating disc in Ar atmosphere 
(pressure=approximately 101 kPa) and rapidly solidificated. The particles 
obtained were sieved and particles with diameters of 0.2.sup..about. 0.3 
mm were obtained. Furthermore, shape separation using inclined vibrating 
plate method was carried out to the particles obtained, to remove 
particles with high partial irregularity and to select Er.sub.3 Co 
spherical particles with low partial irregularity. 
The perimeter of a projected image L and the area of the projected image A 
of each particle of the Er.sub.3 Co spherical particles obtained were 
measured by image analysis and a form factor R expressed by L.sup.2 
/4.pi.A was evaluated. The result showed that the rate of particles with a 
form factor R more than 1.5 was 0.2% and the rate of particles with a form 
factor R more than 1.3 was 3.3%. Furthermore, the aspect ratio of all 
particles was not more than 5. 
Magnetic regenerator spherical particles of Er.sub.3 Co selected by the 
method described above were filled in a regenerator at a packing factor of 
70%, placed in a two-stage GM refrigerator and refrigerating testing was 
carried out. As a result, an initial refrigeration capacity of 250 mW was 
obtained at 4.2 K and stable refrigeration capacfity was obtained over 
5000 hours of continuous operation. 
INDUSTRIAL APPLICABILITY 
As the above embodiments clearly show, according to a regenerator material 
for extremely low temperatures of the present invention, excellent 
mechanical properties for mechanical vibration can be obtained with a high 
reproducibility. Therefore, a regenerator for extremely low temperatures 
of the present invention using such regenerator material is capable of 
maintaining excellent refrigerating performance for a long period of time 
with a high reproducibility.