Magnetic refrigerant

Abstract: A magnetic refrigerant having a composition represented by Ln a A b M c wherein Ln is at least one element selected from Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb; A is any one of elements of Al and Ga; M is at least one element selected from Fe, Co, Ni, Cu and Ag; each of a, b and c is atomic %, with the proviso that a + b + c = 100 atomic %, 20 atomic % ≦ a ≦ 80 atomic %, 5 atomic % ≦ b ≦ 50 atomic %, 5 atomic % ≦ c ≦ 60 atomic %, and having an amorphous structure with a difference ΔT of 10 K or more between a glass transition temperature Tg and a crystallization temperature Tx.

Description: This invention generally relates to a novel magnetic refrigerant or magnetic refrigeration working substance for use in a magnetic refrigerator, and particularly, to a magnetic refrigerant having an amorphous structure and to processes for producing the same. The magnetic refrigerator utilizes a magnetic calorie effect of the magnetic refrigerant and has an advantage of its high cooling capability per unit volume, as compared with a gas refrigerator and hence, it is used in the production of liquid helium. Magnetic refrigeration is based on the principle of alternate repetition of two heat-exchange steps: a heat exhausting step of magnetizing the magnetic refrigerant, wherein heat generated thereby is released to the outside, and a heat absorbing step of abstracting heat from an object such as helium by the magnetic refrigerant cooled by adiabetic demagnetization. In the case of Ericsson cycle as a refrigeration cycle, a work W performed by a magnetic material is represented by W = ΔS M (T 1 - T 2 ), wherein ΔS M is a magnetic entropy; T 1 is a high temperature in the cycle; and T 2 is a low temperature in the cycle. The magnetic refrigerant is required to have characteristics such as a large magnetization in a range of operation, a high coefficient of thermal conductivity in a range of operation, and be a large-sized block. In general, the magnetic refrigerant is classified broadly into a type used in a range of low temperature of less than 20 K, and a type used in a range of high temperature of 20 K or more. GGG (Gd 3 Ga 5 O 12 ) belongs to the former, and compounds containing a rare earth element or elements belong to the latter. The magnetic refrigerant according to the present invention belongs to the latter. There is a conventionally known magnetic refrigerant having an amorphous structure and containing a rare earth element or elements, as disclosed in Japanese Laid-open Patent Application No. 37945/86. This magnetic refrigerant is produced by a melting process such as a single-roll process, or by a spattering process. However, a ribbon produced by the melting process usually has a thickness of 10 to 40 µm and therefore, in order to produce a block larger in size than this ribbon, e.g. a thick plate, a larger number of thin plates cut from a ribbon must be secondarily laminated and press-bonded to one another. However, the resulting thick plate has a problem in that each of the large number of thin plates contains an oxide film on their surface. Hence, the thick plate has a low coefficient of thermal conductivity, resulting in a reduced cooling efficiency. Various amorphous alloys containing rare earth elements are also known from JP-A-62-30840 and Materials Transactions, JIM, 31(2): 104-109 (1990) and 31(5): 425-428 (1990). It is an object of the present invention to provide a magnetic refrigerant which is capable of being primarily formed into a large-sized block and secondarily formed into a further large-sized block and which has a high coefficient of thermal conductivity. It is another object of the present invention to provide a magnetic refrigerant which has an excellent toughness and an excellent resistance to oxidation and whose electric resistance can be increased to reduce a power loss due to an eddy current. It is a further object of the present invention to provide a magnetic refrigerant producing process, wherein a magnetic refrigerant which is a large-sized block and has an amorphous structure can be cast by use of a rotary metal mold. It is a yet further object of the present invention to provide a magnetic refrigerant producing process, wherein a magnetic refrigerant can be produced to have a desired shape by a plastic working. To achieve the above objects, according to the present invention, there is provided a magnetic refrigerant, which has a composition represented by Ln a A b M c wherein Ln is at least one element selected from Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb; A is Ga; M is at least one element selected from Fe, Co, Ni, Cu and Ag; each of a, b and c is atomic %, with the proviso that a + b + c = 100 atomic %, 20 atomic % ≤ a ≤ 80 atomic %, 5 atomic % ≤ b ≤ 50 atomic %, 5 atomic % ≤ c ≤ 60 atomic %, and which has an amorphous structure with a difference ΔT of 10 K or more between a glass transition temperature Tg and a crystallization temperature Tx, with the proviso that Tx > Tg. The magnetic refrigerant used in the range of high temperature utilizes an internal magnetization by a ferromagnetic interaction. Therefore, in order to enlarge the range of cooling temperature as wide as possible, it is required that the effective magnetic moment is large in a wide range of temperature, and that the Curie point can be arbitrarily selected. In the above-described composition, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb (Ln, rare earth element) are essential as an element for magnetization. If the content a thereof is less than 20 atomic % (a < 20 atomic %), the magnetization is small. On the other hand, if the content a thereof is more than 80 atomic % (a > 80 atomic %), it is difficult to produce the amorphous structure. Unlike a structure formed by a crystalline intermetallic compound, the amorphous structure enables an enlargement of the range of temperature in which a high effective magnetic moment can be provided and also enables a wide selection of Curie points, and the like. Ga (A) acts to stabilize the amorphous structure and to improve the wettability of a metal mold with a molten metal to accelerate the cooling. Therefore, Ga (A) is an element which is essential for producing the amorphous structure for the magnetic refrigerant by a casting process. In addition, Ga functions to produce an extremely thin and firm oxide film that provides the magnetic refrigerant with a characteristic for restraining a loss or wear of the material due to oxidation by air so that it can be stored for a long period. If the content b of Ga is less than 5 atomic % (b < 5 atomic %), ultra-quenching means must be used for producing the amorphous structure. On the other hand, if the content b of Ga is more than 50 atomic % (b > 50 atomic %), the magnetization is significantly reduced. Fe, Ni, Co, Cu and Ag (M) are essential elements for producing a magnetic refrigerant having an amorphous structure clearly exhibiting a glass transition temperature Tg by co-addition along with Ga. The present invention utilizes the fact that the larger the difference ΔT between the glass transition temperature Tg and the crystallization temperature Tx for an amorphous alloy, the lower the cooling rate of the molten metal can be and still produce an amorphous structure. From Materials Transaction, JIM, Vol. 31, No. 2 (1990), pp 104-109, it is known that if the difference ΔT between the glass transition temperature Tg and the crystallization temperature Tx is large, an amorphous alloy can be produced even at a slow cooling rate. From Material Transaction, JIM, Vol. 31, No. 5 (1990), pp 425-428, it is known that as the difference ΔT becomes larger, the thicker an amorphous alloy can be made by casting. To effectively utilize the above facts, it is necessary to use an amorphous alloy which clearly shows a glass transition temperature Tg. If the content c of Fe or the like is less than 5 atomic % (c < 5 atomic %), the above facts cannot be effectively utilized, and as a result, a thick magnetic refrigerant having an amorphous structure cannot be cast. On the other hand, if the content c of Fe or the like is more than 60 atomic % (c > 60 atomic %), the magnetization is significantly reduced. It should be noted that unavoidable impurities in the magnetic refrigerant is of 1 atomic %. The above-described difference ΔT is required to be at least 10 K in order for the magnetic refrigerant to have an excellent amorphous structure forming capability. The difference ΔT depends upon a correlation of individual chemical constituents Ln, A and M. However, Ln has a nature that it raises the glass transition temperature Tg and the crystallization temperature Tx; A has a nature that it lowers the glass transition temperature Tg and the crystallization temperature Tx; and M has a nature that it raises the glass transition temperature Tg and the crystallization temperature Tx. Therefore, in view of these natures, the contents of individual chemical constituents should be adjusted. In produce a magnetic refrigerant having an amorphous structure, a process is employed which comprises ejecting a molten metal having the above-described composition and an amorphous alloy composition with a difference ΔT of 10 K or more between the glass transition temperature Tg and the crystallization temperature Tx, onto an inner peripheral surface of a drum type rotary metal mold, and continuously solidifying the ejected molten metal at a cooling rate of 10 2 K/sec or more. This process enables a magnetic refrigerant as thick as 3 to 20 mm to be cast, because the alloy solidified on the inner peripheral surface of the rotary metal mold is accumulated thereon. In this case, the molten metal is solidified under a pressurized condition. This delays the crystallization of the molten metal and hence, is advantageous for producing the amorphous structure for the magnetic refrigerant. The rotary metal mold is formed from a material having a good thermal conductivity, eg. a Cu alloy or the like. The rotary metal mold need not be forcibly cooled. A cooling rate of 10 2 K/sec or more is required for producing the amorphous structure. If a molten metal is cooled and solidified on an outer peripheral surface of a rotor, the cooling rate can be further increased, but in this method, a thick magnetic refrigerant cannot be produced. A cylindrical magnetic refrigerant is produced by the above-described casting process. When this cylindrical magnetic refrigerant is to be placed into a container of the magnetic refrigerator, it may be subjected to a predetermined working as required. For example, when a magnetic refrigerant thicker than that produced by the casting process and having a predetermined size, the following procedure is employed: The magnetic refrigerant produced by the casting process is used as an intermediate product and is cut into a proper size and then subjected to a setting or rectifying treatment for removal of a warpage. The resulting flat plates are laminated and press-bonded, thereby providing a magnetic refrigerant having a proper thickness and size and a density of 99% or more. The press-bonding is a hot working conducted at a temperature between a glass transition temperature Tg and a crystallization temperature Tx. This is for the purpose of increasing the workability by utilizing a phenomenon that a material having an amorphous structure becomes an ultraplastic when it is heated to its glass transition temperature Tg or higher. However, if the working temperature exceeds the crystallization temperature Tx, the worked material will crystallize. Therefore, the working temperature should be set at a value lower than the crystallization temperature Tx. The magnetic refrigerant according to the present invention has the following effects: (a) it has a large magnetization and thus a high cooling efficiency, because it has been formed into a large-size block by use of the casting process; (b) it is has a high coefficient of thermal conductivity, because there is little bore; (c) it has a uniform surface, which is slow to oxidize, because it has an amorphous structure even if it contains a large amount of Ln added thereto; (d) it has a large electric resistance and thus its power loss due to eddy currents is small, because it is of amorphous alloy; and (e) it is easily formed into a large-sized block by a hot-working, because it has an excellent toughness and a large difference ΔT between the glass transition temperature Tg and the crystallization temperature Tx. The above and other objects, features and advantages of the invention will become apparent from a consideration of the following description of the preferred embodiments, taken in conjunction with the accompanying drawings. Fig. 1 is a view of a casting apparatus; and Fig. 2 is a view of a single-roll apparatus. Example 1 Fig. 1 illustrates a casting apparatus for producing a magnetic refrigerant or magnetic refrigeration working substance. The apparatus is constructed in the following manner: A bevel gear type supporting plate 2 is horizontally mounted on an upper end of a vertical rotary shaft 1, and a drum-like rotary metal mold 3 made of a Cu alloy is mounted on an upper surface of the supporting plate 2. A bevel gear 6 of a driving shaft 5 connected to a motor or the like is meshed with a toothed portion 4 of an outer peripheral surface of the supporting plate 2. A crucible 7 of quartz is inserted into the rotary metal mold 3, and is provided at a leading end of the crucible with a nozzle 8 which is opposed to a lower portion of an inner peripheral surface of the rotary metal mold 3. The crucible 7 is liftable, and a heater 9 having a high-frequency induction coil is disposed around an outer periphery of the crucible 7 outside the rotary metal mold 3. The following is a description of a process for preparing and testing an alloy having the composition Ln a A b M c wherein A is Al, and is included to further illustrate the invention. Alloys having such a composition are not intended to form part of the invention. First, an ingot having an amorphous alloy composition represented by Gd 50 Al 20 Cu 30 (wherein each of numeral values is an atomic %) was produced using an arc furnace. Then, the ingot was placed into the crucible 7 and heated by heater 9 to prepare a molten metal, and the rotary metal mold 3 was rotated at a peripheral speed of 10 to 40 m/sec. The crucible 7 was raised while ejecting the molten metal through the nozzle 8 of the crucible 7 onto the inner peripheral surface of the rotary metal mold 3. In this case, the amount of molten metal ejected was set such that the thickness of the solidified alloy became 50 µm or less upon one rotation of the rotary metal mold. The cooling rate for the molten metal was set at 10 2 K/sec. A cylindrical magnetic refrigerant having an outside diameter of 50 mm, a thickness of 3 mm and a length of 10 mm was produced through the above-described steps. A test piece fabricated from the magnetic refrigerant was subjected to X-ray diffraction, thereby examining the metallographic structure of the magnetic refrigerant. As a result, it was confirmed that the metallographic structure was an amorphous structure. The test piece was also subjected to various measurements, thereby providing the following results: Glass transition temperature Tg 536 K Crystallization temperature Tx 575 K Difference ΔT between the temperatures Tx and Tg 39 K Curie temperature Tc 68 K Magnetic moment 7.9 µB Close-contact bending test Close-contact bendable at 180° Oxidation resistance No oxidation increment The measurements of the glass transition temperature Tg and the crystallization temperature Tx were conducted by a differential scanning calorimeter (DSC). The Curie temperature Tc and the magnetic moment were calculated by VSM. The close-contact bending test was conducted by bending the test piece while bringing it into close contact with an outer peripheral surface of a round rod having a diameter of 0.3 mm. In the oxidation resistance test, the test piece was heated in the atmosphere at 100°C for 1 hour, and the weights of the test piece before and after the heating thereof were compared with each other to estimate the degree of oxidation. The following experiment was carried out in order to examine whether or not the magnetic refrigerant produced by the above-described casting process had physical properties equivalent to those of a ribbon produced by a single-roll process and having an amorphous structure. An ingot having the same composition as the above-described composition was placed into a quartz crucible 10 of a single-roll apparatus shown in Fig. 2. Atmosphere in the crucible 10 was evacuated to a high vacuum and then the crucible 10 was filled with argon gas to produce an argon gas atmosphere. Then, the ingot was heated by a heater 11 having a high-frequency induction coil which is disposed around an outer periphery of the crucible 10, thereby preparing a molten metal. Thereafter, the molten metal was ejected through a nozzle 12 having a diameter of 0.3 mm and located in a bottom wall of the crucible 10 onto an outer peripheral surface of a roll 13 of a Cu alloy rotating at a peripheral speed of 15 m/sec and was quenched and solidified, thereby providing a ribbon 14 having a thickness of 10 µm, a width of 1 mm and a length of 5 mm. A test piece fabricated from the ribbon was subjected to an X-ray diffraction to examine the metallographic structure. As a result, it was confirmed that the metallographic structure was an amorphous structure. The test piece was likewise subjected to various measurements to give the following results: Glass transition temperature Tg 535 K Crystallization temperature Tx 573 K Difference ΔT between the temperatures Tx and Tg 38 K Curie temperature Tc 67 K Magnetic moment 8 µB Close-contact bending test Close-contact bendable at 180° Oxidation resistance No oxidation increment It was confirmed from the above results that a magnetic refrigerant having substantially the same physical properties as those produced by the single-roll process could be produced even by the casting process. Example 2 Using the casting apparatus shown in Fig. 1, a cylindrical magnetic refrigerant having the above described various compositions and an outside diameter 50 mm, a thickness of 2 mm and a length of 10 mm was produced in the same manner as in Example 1. The relationship among the compositions, metallographic structures, differences ΔT between the crystallization temperature Tx and glass transition temperature Tg, and Curie temperatures is as given in Table I. In this table, amo means an amorphous structure, and cry means a crystalline structure.

Claims: A magnetic refrigerant, which has a composition represented by Ln a A b M c wherein Ln is at least one element selected from Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb; A is Ga; M is at least one element selected from Fe, Co, Ni, Cu and Ag; each of a, b and c is atomic %, with the proviso that a + b + c = 100 atomic %, 20 atomic % ≤ a ≤ 80 atomic %, 5 atomic % ≤ b ≤ 50 atomic %, 5 atomic % ≤ c ≤ 60 atomic %, and which has an amorphous structure with a difference ΔT of 10 K or more between a glass transition temperature Tg and a crystallization temperature Tx, with the proviso that Tx > Tg. A magnetic refrigerant as claimed in claim 1 having a thickness of from 3-20mm. A process for producing a magnetic refrigerant having an amorphous structure, comprising the steps of: preparing a molten metal which has a composition represented by Ln a A b M c wherein Ln is at least one element selected from Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb; A is Ga; M is at least one element selected from Fe, Co, Ni, Cu and Ag; each of a, b and c is atomic %, with the proviso that a + b + c = 100 atomic %, 20 atomic % ≤ a ≤ 80 atomic %, 5 atomic % ≤ b ≤ 50 atomic %, 5 atomic % ≤ c ≤ 60 atomic %, and which has an amorphous alloy composition with a difference ΔT of 10 K or more between a glass transition temperature Tg and a crystallization temperature Tx, with the proviso that Tx > Tg; and ejecting said molten metal onto an inner peripheral surface of a drum-shaped rotary metal mold, thereby continuously casting said molten metal through solidification under pressure at a cooling rate of 10 2 K/sec or more. A process as claimed in claim 3 further comprising subjecting the resulting product to a hot working at a temperature between the glass transition temperature Tg and the crystallization temperature Tx.

Inventors: HORIMURA HIROYUKI, INOUE AKIHISA, KITA KAZUHIKO, MASUMOTO TSUYOSHI, YAMAGUCHI HITOSHI, HORIMURA, HIROYUKI, INOUE, AKIHISA, KITA, KAZUHIKO, MASUMOTO, TSUYOSHI, YAMAGUCHI, HITOSHI, HORIMURA, HIROYUKI, C/O KABUSHIKI KAISHA HONDA, Yamaguchi, Hitoshi, c/o Teikoku Piston Ring Co.Ltd
Applicants: MASUMOTO TSUYOSHI, YKK CORP, MASUMOTO, TSUYOSHI, YKK CORPORATION
Application year: 1992
Main classification: H01F1
IPC codes: C22C45, C22C1, H01F1, B22D27, B22D13, C22C28