Production and use of coolant in cryogenic devices

Abstract: A coolant for use in cryogenic devices of closed cycle system which comprises a mixture of a liquid nitrogen and a fluorocarbon of the formula: CnF 2n+2 , wherein n is an integer of 2 to 4. The coolant, when used to cool heat generating semiconductor devices and other devices to a cryogenic temperature, exhibits an increased heat flux and thus an increased cooling capability, in addition allows a simple and small structure of the cooling system, and a prevention of environmental pollution.

Description: The present invention relates to a coolant having high cooling capability for use in cryogenic devices. The present invention also relates to a method for the production of the coolant, a method of cooling an article to the cryogenic temperature, and a cryogenic device using the coolant. There is a particular requirement, currently, for coolants which are suitable, in particular, as cooling medium or a heat exchange medium for cooling semiconductor devices such as those which can exhibit a high speed switching at a cryogenic temperature of about 123 K or less such as 77.3 K, i.e., the boiling point of liquid nitrogen (LN₂), or other devices such as those using a superconducting materials which are cooled to a working temperature thereof. Note, any cryogenic temperature around the boiling point of liquid nitrogen is referred to herein as a liquid nitrogen temperature. As is well known in the art, the mobility of a carrier in a semiconductor can be increased by reduction of the temperature to which the semiconductor is subjected, and based on this temperature characteristic, high speed switching devices which are operable at the liquid nitrogen temperature, such as high electron mobility transistors (HEMTs) or complementary metal oxide semiconductor (CMOS) devices, have been developed. For example, M. Abe, T. Mimura and M. Kobayashi, "Ultra-High-Speed HEMT LSI Technology", FUJITSU Sci. Tech. J., 24, 4, PP. 271-283 (December 1988) teach that HEMTs developed by Fujitsu Limited have achieved a 5.8 ns switching speed, and T. Vacca, D. Resnick, D. Frankel, R. Bach, J. Kreilich and D. Charlson, "A Cryogenically Cooled VLSI Supercomputer", VLSI SYSTEMS DESIGN, June 1987, pp. 80-84 teach that the first cryogenic computer using a 77K CMOS was shipped in the previous year. To operate these devices at the liquid nitrogen temperature with satisfactory results, it is necessary to use a high power coolant or cooling medium which can constantly maintain a desired cryogenic temperature such as the liquid nitrogen temperature, and also enables an effective dissipation of the heat generated in the devices during operation thereof to be obtained. Hitherto, to cool the semiconductor devices and other devices to a cryogenic temperature of about 123K or less and to maintain such a temperature, it has been proposed to arrange a heat generating part of the device in contact with a cold head of a refrigerator to dissipate the heat generated from the device through a heat conduction, or to immerse the device in a cryogenic fluid such as a liquefied gas, for example, liquid nitrogen, to dissipate the heat from the device through a boiling heat transfer. Particularly, the latter cooling method based on the boiling heat transfer can be advantageously used when the devices to be cooled have a large heat generation density and a complex structure or configuration. For example, this cooling method can be applied to the cooling of central processing units (CPUs) of a computer. Further, although a variety of liquefied gases is available, the liquefied gas usable as the cryogenic fluid in this cooling method is limited in view of the toxicity and reactivity of such gases. Suitable liquefied gases having a stable and simple composition are liquefied nitrogen, described above, and liquefied helium. Liquefied nitrogen is obtained by liquefying air and separating the resulting fluid. Use of liquefied nitrogen as the cryogenic fluid is described in, for example, F.H. Gaensslen, V.L. Rideout, E. J. Walker and J.J. Walker, "Very Small MOSFET's for Low-Temperature Operation", IEEE TRANSACTIONS ON ELECTRON DEVICES, MARCH 1977, in which FET's are cooled by direct immersion in an open pool of the liquefied nitrogen. Further, the use of liquefied fluorocarbons as the cryogenic fluid is described in, for example, "Cooling a Superfast Computer", ELECTRONIC PACKAGING & PRODUCTION, JULY 1986, in which the entire computer is immersed in a non-conductive circulating fluorocarbon liquid. Furthermore, the use of a mixture of liquefied nitrogen and liquefied fluorocarbon (CF₄) as the cryogenic fluid is described in, for example, T. Amano and M. Nagao, "Boiling Heat transfer Characteristics of Mixed Coolant", 1988 Cryogenic Society of Japan Spring Meeting, C1-4, May 1988, in which a superconducting material having a high Tc is directly immersed in a mixed coolant consisting of the liquefied nitrogen and liquefied CF₄ in an open vessel. The cooling method taught by Amano et al is illustrated in Fig. 1. To ascertain the effectiveness of the mixed coolant when cooling the high Tc superconducting material, they used the illustrated experimental apparatus in which a Pt wire 1 is disposed in a mixed coolant 2 in an open vacuum vessel 3. A thermocouple 4, as a temperature sensor, connected with a recorder 8 is also disposed in the mixed coolant 2. In the apparatus, 5 is a balance-scale used to determine a change of the weight of the coolant 2, 6 is a DC supply, and 7 is a resistor. Surprisingly, the experiments using this apparatus showed that the mixed coolant of the liquefied nitrogen and 8 mol % of the liquefied CF₄ can increase the cooling capability to about twice the cooling capability obtained when liquefied nitrogen is used alone. Although this paper does not teach the use of the mixed coolant (LN₂ plus liquefied CF₄) when cooling semiconductor devices and related devices, it is understood that such a use will be accompanied by many drawback, for example: (1) Since an open system is employed, atmospheric air or moisture in the atmosphere can be taken up by and frozen in the mixed coolant; (2) Since the coolant is boiled and vaporized, the gas generated can cause atmospheric pollution and an additional coolant must be supplied to the open vessel to compensate for vaporization loss of the coolant. To avoid the drawbacks (1) and (2), use of the mixed coolant in a closed system has been considered, but since the mixed coolant must contain a higher amount of the liquefied CF₄, i.e., as high as 8 to 20 mol %, the composition of the mixed coolant will be changed upon liquefaction of the vaporized coolant, thereby causing variations in the cooling capability of the coolant. Further, in the vaporized coolant, the high boiling components thereof tend to be condensed and separated in a liquefier or refrigerator used to again liquefy the vaporized coolant, because of a high concentration of the high boiling components. The separated components will cause clogging of the pipes, stoppage of the apparatus and other unavoidable drawbacks. Furthermore, not all fluorocarbons except for CF₄, which are considered to be appropriate for admixture to the liquefied nitrogen, provide a solution after they are directly admixed with the liquefied nitrogen in accordance with conventional mixing methods; in practice, they are solidified and precipitated in a mixing apparatus. It is of note, furthermore, that the present inventors also tried to use a mixed coolant consisting of liquefied nitrogen and liquefied argon or liquefied krypton, but could not obtain satisfactory cooling effects. On the other hand, to obtain an increased cooling capability, it has been suggested to use a combination of a cryostat or closed cryogenic container and a liquefier, particularly a refrigeration unit, a heat exchanger pipe of which liquefier is disposed in an upper inner portion of the cryostat. An article to be cooled is immersed in a coolant or cooling medium in the cryostat, and the vaporized coolant is liquefied as a result of heat exchange between a vapor of the coolant and a heat exchange medium circulated through the heat exchanger pipe. In this cryogenic cooling system, liquefied nitrogen is generally used as the coolant, and helium gas is used as the heat exchange medium. This system, however, suffers from the following drawbacks: (1) To avoid undesirable effects on the article to be cooled due to vibration of the liquefier, the liquefier must be disposed far from the cryostat; (2) Since helium gas is used as the heat exchange medium, the resulting cooling capability is not satisfactorily high, as apparently the heat exchange is based on gas-gas contact; (3) To increase the cooling capability of the gas-gas contact system, it is possible to increase the diameter of the heat exchanger pipe used. This, however, will result in an increase of the power and equipment area of the apparatus and thus increase operation costs. Therefore, it is desired to provide an improved coolant or cooling medium having a high cooling capability for use in cryogenic devices, and not having the drawbacks of the prior art coolants. An object of the present invention is to provide a novel coolant for use in cryogenic devices using a closed cycle or loop system. The coolant should exhibit an increased cooling capability, and also should not have the problem of the prior art as described above. According to one aspect of the invention, there is provided a coolant, for use in a cryogenic device having a closed cycle system, which comprises a mixture of a liquid nitrogen and a fluorocarbon of the formula: CnF 2n+2 , wherein n is an integer of 2 to 4, the fluorocarbon being present in said mixture in an amount of 0.1 to 3 mol %. Unexpectedly, an increased cooling capability was obtained with an admixture of even a small amount of the fluorocarbon. In a second aspect of the present invention, there is provided a method of producing a coolant comprising a mixture of a liquid nitrogen and a fluorocarbon of the formula: CnF 2n+2 , wherein n is an integer of 2 to 4, which method comprises the step of cooling a mixed gas of nitrogen and fluorocarbon of the formula: CnF 2n+2 , wherein n is as defined above, under pressure to liquefy the mixed gas, the fluorocarbon being present in said mixture in an amount of 0.1 to 3 mol %. In another aspect of the present invention, there is provided a method of producing a coolant comprising a mixture of a liquid nitrogen and a fluorocarbon of the formula: CnF 2n+2 , wherein n is an integer of 2 to 4, the fluorocarbon being present in said mixture in an amount of 0.1 to 3 mol %, which method comprises the step of introducing a gaseous fluorocarbon of the formula: CnF 2n+2 , wherein n is as defined above, into a liquid nitrogen to obtain the liquid nitrogen having the fluorocarbon dissolved therein. In a further aspect of the present invention, there is provided a method of cooling an article to a cryogenic temperature of 123K or less, which method comprises the step of immersing the article in a coolant which comprises a mixture of a liquid nitrogen and a fluorocarbon of the formula: C n F 2n+2' wherein n is an integer of 2 to 4, the fluorocarbon being present in said mixture in an amount of 0.1 to 3 mol %. In a yet further aspect of the present invention, there is provided a cryogenic device in a closed cycle System which comprises a closed cryostat containing, as a cooling medium, a mixture of a liquid nitrogen and a fluorocarbon of the formula: C n F 2n+2' wherein n is an integer of 2 to 4, the fluorocarbon being present in said mixture in an amount of 0.1 to 3 mol %, an article to be cooled being immersed in the cooling medium in the cryostat, and cooled to a cryogenic temperature of 123K or less. As described hereinafter in detail, according to the present invention, an unexpectedly increased cooling capability can be obtained with the use of a small amount of the fluorocarbon in the liquefied nitrogen. In comparison with use of the liquefied nitrogen alone, about twice the cooling capability can be obtained in the present invention, without a separation of high boiling point components in a liquefier and other prior art drawbacks. For a better understanding of the invention and to show how the same can be carried into effect reference will now be made, by way of example only, to the accompanying drawings, wherein: FIGURE 1 is a diagrammatic cross-sectional view of the cooling apparatus in accordance with a prior art method and already described herein; FIGURES 2 and 3 are diagrammatic cross-sectional views of apparatus used in the production of the coolant according to the present invention; FIGURE 4 is a diagrammatic cross-sectional view of a cryostat used in the practice of the present invention, FIGURE 5 is a diagrammatic cross-sectional view of a cooling apparatus according to one aspect of the present invention; FIGURE 6 is a diagrammatic cross-sectional view of a cooling apparatus according to another aspect of the present invention, FIGURE 7 is a graph showing the relationship between the liquefied nitrogen with and without added C₂F₆ and the cooling capability, with regard to the temperature of a LSI chip; and, FIGURE 8 is a graph showing the relationship between the concentration of the fluorocarbon and the cooling capability, with regard to different types of the coolants. A coolant according to the present invention, comprises a mixture of liquid nitrogen and a fluorocarbon CnF 2n+2 , wherein n is as defined above. Preferably, the coolant comprises liquid nitrogen having the fluorocarbon dissolved therein. The fluorocarbon used is C₂F₆ , C₃F₈ , C₄F₁₀ or a mixture thereof. The use of CF₄ must be avoided because as described above, it must be added in a remarkably increased amount to the liquid nitrogen, and thus produces drawbacks originating from such an addition of CF₄. On the other hand, the use of C n F 2n+2 wherein n is 5 or more must be also avoided, because such fluorocarbons having a higher number of carbon atoms have an increased boiling point, and can solidify when mixed with the liquid nitrogen. According to the present invention, a smaller amount of the fluorocarbon is added to the liquid nitrogen, in comparison with the prior art coolant of Amano et al mentioned hereinabove in which the fluorocarbon, CF₄, is added in an amount of about 8 to 20% by moles or more to the liquid nitrogen, to obtain a satisfactory cooling capability of the order of about 40 W/cm² The fluorocarbon C n F 2n+2 (n = 2, 3 or 4) is added in an amount of 0.1 to 3% by mole to the liquid nitrogen, to obtain the coolant of the present invention. For C₂F₆ , an increase of the cooling capability begins at a concentration of 0.1% by mole and a maximum cooling capability is obtained at a concentration of 0.5% by mole. A saturated cooling capability is obtained at a concentration of more than 0.5% by mole. When C₂F₆ is added in an amount of more than 3% by mole, the resultant coolant is clouded and has a reduced saturated cooling capability. Similar cooling characteristics can be observed for other fluorocarbons C₃F₈ and C₄F₁₀ except that an increased cooling capability can be obtained by an addition of an amount of 0.1 to 0.5% by mole for C₃F₈ and of 0.1 to 0.3% by mole for C₄F₁₀. The details of the mechanism of the increase of the cooling capability in the coolant of the present invention have not been clarified, but it is assumed from the inventors' experience that the cooling capability is increased for the following reasons: 1) A reduction of the volume of the coolant due to the mixing of two components. This reduction of the volume occurs as when mixing of water and alcohol. Namely, when the mean free path diameters of molecules greatly differ, the volume of the mixture of the molecules is decreased because a close packing of the molecules occurs. When a suitable amount of the fluorocarbons C n F 2n+2 (n = 2, 3 or 4) is used in the liquid nitrogen, the thus caused reduction of volume of the coolant will result in an additional increase of the cooling capability. 2) A reduction of the surface tension of the coolant. Namely, in the immersion cooling method of the present invention, a limit of the cooling capability at which an article can be stably cooled, because of a small increase of the temperature of the article, is considered to be a layer or film boiling transition temperature or less of the coolant, i.e., a cooling capability at which the coolant is transited to a layer boiling state. The layer boiling transition temperature, which is a limit for a stable cooling, can vary depending upon specific properties of the liquid components of the coolant, such as vaporization heat and surface tension. The fluorocarbons C n F 2n+2 have a low surface tension, and thus if the fluorocarbons are dissolved in the liquid nitrogen, the resulting mixture or coolant exhibits a remarkably reduced surface tension. Next, if an article to be cooled or a heat generating body is cooled in the coolant through cooling by boiling, bubbles of the vaporized coolant formed on a surface of the article are released from the surface of the article at an initial stage of the formation thereof, thereby retarding the transition to the layer boiling state. Further, the generated bubbles destroy the boundary layer of the coolant at the article, and as a result, the cooling capability is increased. Note, fluorocarbons having a higher molecular weight can induce an increased cooling capability, even if used at a relatively low concentration. 3) A dissolving condition of the fluorocarbons is that if liquid fluorocarbons are slowly frozen, they are thickened and become gelatinous. In the liquid nitrogen, the dissolving of the fluorocarbons is also assumed to produce a thin gelatinous liquid, which may account for the increased heat of the vaporization and cooling capability, due to the energy required for cutting gelatinous bonds between the fluorocarbon molecules. Note, the inventors tried to replace the fluorocarbons with other substances such as chlorocarbons, but better cooling effects could not be obtained, and thus they consider that one of the reasons why the fluorocarbons provide remarkable cooling effects is because of the above-described reduction of the surface tension. The coolant according to the present invention can be advantageously produced in accordance with the following methods. One production method comprises the step of cooling a mixed gas of nitrogen and fluorocarbon of the formula: C n F 2n+2 , wherein n is as defined above, under pressure to liquefy the mixed gas. In this production method, preferably,the ratio of fluorocarbon to nitrogen in the mixed gas is substantially the same as that of the resultant coolant, and an increased pressure of 1 to 2 atms is applied to the mixed gas during the cooling step, and further the mixed gas in a high pressure vessel is introduced into a liquefier where the gas is cooled with a compressed and adiabatically cooled gas. A second production method comprises the step of introducing a gaseous fluorocarbon of the formula: C n F 2n+2 , wherein n is as defined above, into a liquid nitrogen to obtain the liquid nitrogen having the fluorocarbon dissolved therein. In this production method, the gaseous fluorocarbon in a high pressure vessel is preferably introduced into the liquid nitrogen in a Dewar vessel, and more particularly, gaseous fluorocarbon from a high pressure vessel is passed through liquid nitrogen in a first Dewar vessel before introduction into the liquid nitrogen in a second Dewar vessel to cool the gaseous fluorocarbon to a temperature beyond which the fluorocarbon is liquified. In the method of cooling an article to a cryogenic temperature of 123K or less, and with an apparatus for carrying out such a cooling, or cryogenic device, according to the present invention, the article is immersed in the coolant consisting of the liquid nitrogen and fluorocarbon as described above. In a preferred embodiment of the present invention, the coolant is placed as a cooling medium in a closed cryostat of the cryogenic device. In this embodiment, the cooling medium boiled and vaporized in the cryostat is preferably liquefied in a separate liquefier, and the thus produced cooling medium in the form of a liquid is returned to the cryostat. In another preferred embodiment of the present invention the cooling medium boiled and vaporized in the cryostat is liquefied within the same cryostat as a result of heat exchange between a vapor of the cooling medium and a heat exchange medium in a heat exchanger pipe disposed at a suitable position in the cryostat so that the heat exchange is made therein. In this embodiment, the heat exchange medium circulated through the heat exchanger pipe is preferably a liquid nitrogen. Of course, other heat exchange media may be circulated, if desired. In the cryogenic device of the present invention, a wall of the cryostat is preferably provided with inlet and outlet openings communicated through pipes with a separate liquefier, so that the cooling medium boiled and vaporized as a result of heat generation by the article in the cryostat is guided into and liquefied in the liquefier, and the thus produced cooling medium in the form of a liquid is returned to the cryostat. Alternatively, an upper inner portion of the cryostat has a heat exchanger pipe disposed at a suitable position therein and communicated with a separate liquefier, so that the cooling medium boiled and vaporized as a result of heat generation in the cryostat is liquefied within the same cryostat, as a result of a heat exchange between vapor of the cooling medium and a heat exchange medium in the heat exchanger pipe. The liquefier used may have any conventional structure, but preferably the liquefier comprises a Stirling cycle refrigerator with a compressor. As described above, any heat exchange medium may be used in the practice of the present invention, but to obtain a satisfactory heat exchange effect, it is necessary to circulate a heat exchange medium having a lower boiling point than that of the cooling medium used simultaneously in the cryostat, through the heat exchanger pipe of the liquefier. If the above requirements are satisfied, a heat transfer by boiling occurs at the portion of the heat exchanger pipe, and thus a higher efficiency heat transfer, compared with that of the convection heat transfer obtained by using a helium gas in the heat exchanger pipe, is ensured. This is because, as described above, the coolant of the present invention is preferably used as the cooling medium in the cryostat, and the liquid nitrogen as the heat exchange medium in the heat exchanger pipe. The present invention can be applied to the cooling of any articles which are to be cooled to a cryogenic temperature of 123 K or less. Typical examples of such articles, although not limited thereto, are semiconductor devices such as high electron mobility transistors or complementary metal oxide semiconductor devices, devices using a superconducting material which exhibits a superconductivity at the cryogenic temperature, or electronic equipment which is operable at the cryogenic temperature. Surprisingly, according to the present invention, large-scale computers can be cooled, with a high efficiency, in a cryogenic device having a simple structure. The present invention will be further described with reference to the accompanying drawings in which Figs. 2 and 3 illustrate the production of the coolant according to the present invention. In Fig. 2, a coolant was prepared by using a liquefier 13 which consists of a Stirling cycle refrigeration unit 14 and a high-pressure compressor 15. A mixed gas of nitrogen and fluorocarbon (C₂F₆ was used here ) had been charged to a steel gas cylinder 11 and the gas was supplied therefrom through a conduit 12 to the refrigeration unit 14. The gas in a container 17 of the refrigeration unit 14 was cooled under an applied pressure of about 1.3 to 1.5 atm. to form the coolant, i.e., mixture of the liquid nitrogen (LN₂) and C₂F₆ , 18. The gas was cooled by cooling a bottom portion of the container 17 with a cold head (not shown) of the refrigeration unit 14. Namely, a specific cooling gas having, for example, a temperature of about 70K, such as liquid helium of 11 to 13 kg/cm² was compressed in the compressor 15, and the compressed gas was adiabatically cooled with a piston 16, and as a result of an indirect contact of the thus cooled gas in the cold head with the mixed gas from the gas cylinder 11, the coolant 18 was produced in the container 17. For the illustrated production of the coolant, the coolant or liquefied gas of LN₂ and C₂F₆ having a predetermined ratio of LN₂ to C₂F₆ can be prepared with a good reproducibility and controllability, because such a ratio of LN₂ to C₂F₆ is defined depending upon the specific composition of the mixed gas filled in the gas cylinder. In Fig. 3, a coolant was prepared by bubbling a fluorocarbon gas (C₂F₆ was used here ) into liquid nitrogen 28 in a Dewar vessel 27. The C₂F₆ gas in a steel gas cylinder 21, before bubbling into the liquid nitrogen 28 in the vessel 27, was initially cooled to a temperature beyond that at which the C₂F₆ gas is liquefied. Namely, the C₂F₆ gas was supplied through a conduit 22 to a cooling pipe 23 dipped in a liquid nitrogen 26 in a Dewar vessel 25, and after cooling, the cooled gas from the cooling pipe 23 was supplied through a conduit 24 to the vessel 27 and bubbled into the liquid nitrogen 28. As a result of this bubbling, the coolant, i.e., mixture of the liquid nitrogen and C₂F₆, was prepared in the vessel 27. The mixing ratio of C₂F₆ in the resulting coolant could be freely controlled by suitably changing the pressure applied to the C₂F₆ gas in the gas cylinder and the flow rate of the C₂F₆ gas in the conduit 24. In the above production procedures described with reference to Figs. 2 and 3, C₂F₆ was used as the fluorocarbon, but, it should be noted that similar satisfactory results can be obtained if C₂F₆ is replaced with a fluorocarbon C₃F₈ or C₄F₁₀, or if any combination of these fluorocarbons is used. Further, in the production procedure of Fig. 3, the C₂F₆ gas in the cylinder 21 may additionally contain nitrogen gas, if necessary. For the cryogenic cooling of an article to be cooled to the cryogenic temperature, the coolant of the present invention is placed in a closed cryostat. The cooling using the cryostat will be described hereinafter with reference to Fig . 4. Note, a heat exchanger portion was omitted from Fig. 4, to simplify understanding of the present invention. In Fig. 4, a closed cryostat 33 is charged with the coolant or cooling medium 34 of the present invention. The cooling medium 34 used consists of the liquid nitrogen and fluorocarbon. A circuit board 32 of 100 mm x 100 mm in size having 4 x 4 LSI chips 31 of 10 mm x 10 mm in size mounted thereon is dipped in the cooling medium 34. With an application of an electric voltage through an electric cable (not shown) to the LSI chips 31, the boiling of the cooling medium 34 begins and small bubbles 35 of the vapor of the medium 34 are generated on the surface of the heated LSI chips 31. The chips 31 were thus cooled as a result of removal of vaporization heat from the chip surface. The cooling capability of the used cooling medium is evaluated by defining it as the heat flux at the shift from nucleate boiling to layer or film boiling. The shift point is decided by measuring the relationship between the chip temperature and power dissipation. The chip temperature is measured at diodes formed on the chip surface. The cryogenic cooling of the present invention was described above with reference to Fig. 4. More particularly, the cryogenic cooling can be advantageously carried out by using a cryogenic device of Fig. 5 or Fig. 6. The cryogenic device of Fig. 5 is of the closed loop or cycle system which comprises a cryostat 33 and a liquefier 13. The liquefier 13, as has been described with reference to Fig. 2, consists of a refrigeration unit 14 and a high-pressure compressor 15. The cooling medium 34 occupying the cryostat 33 is a mixture of the liquid nitrogen and 0.5% by mole of C₂₋₄ fluorocarbon (C₂F₆ was used here ). A circuit board 32 of 100 mm x 100 mm in size and having 4 x 4 LSI chips 31 of 10 mm x 10 mm in size mounted thereon was directly immersed in the cooling medium 34. The supply of an electric voltage to the chips 31 was made using a power cable (not shown) from an electric source, and boiling of the cooling medium 34 begun upon application of the electric voltage to the chips 31, whereby small bubbles 35 of the vapor of the medium 34 were generated on the surface of the heated chips 31. The chips 31 were thus cooled by the vaporization of heat therefrom. The vapor of the cooling medium thus produced was fed through a feed pipe 36 to a container 17 of the refrigerator 14. The vapor was liquefied in the container 17 by cooling a bottom portion of the container 17 with the cold head (not shown) of the refrigeration unit 14. This cooling, as described with reference to Fig. 2, was carried out by using the compressor 15 and the attached piston 16. Thus, the function of the liquefier 13 of Fig. 5 is substantially the same as that of Fig. 2. The produced fluid of the cooling medium in the container 17 was returned through a feed pipe 37 to the cryostat 33. The cooling capability of the used cooling medium was evaluated as described above and the results of the evaluation were plotted in Fig. 7 as a function of the temperature of the LSI chips immersed in the cooling medium. Also, for comparison, the above procedure was repeated by using the liquid nitrogen as the cooling medium, and the results of the evaluation of the cooling capability were plotted in Fig. 7. The results plotted in Fig. 7 showed that the mixing of 0.5% by mole of C₂F₆ in the liquid nitrogen increases the cooling capability of the cooling medium to about 40 W/cm², i.e., about two times higher than that of the cooling medium consisting of only liquid nitrogen (about 15 to 20 W/cm²). Note, 0.5% by mole of C₂F₆ is a remarkably reduced concentration which does not cause any coagulation and separation problem in the liquefier of the closed system. The cryogenic device of Fig. 6 is a modification of the cryogenic device described above with reference to Fig. 4 or Fig. 5. As illustrated, a cryostat 33 was disposed apart from a liquefier 13, but a heat exchanger pipe 38 from the liquefier 13 was disposed in an upper inner portion of the cryostat 33 to effect cooling and liquefication of the vaporized cooling medium. A detailed description of the cryostat 33, the cooling medium 34 in the cryostat 33, the circuit board 32 with the LSI chips 31 and the liquefier 13 are omitted herein, as they correspond to those of Fig. 5 and have been already explained. Note, the heat exchange medium 39 circulated through the heat exchanger pipe 38 was liquid nitrogen. After the circuit board 32 was immersed in the cooling medium 34, an electric voltage was applied through an electric cable (not shown) to the LSI chips 31. The chips 31 were thus heated and caused boiling of the surrounding cooling medium 34, whereby small bubbles 35 of the medium vapor were generated on the surface of the chips 31. The medium vapor was then subjected to an indirect heat exchange with liquid nitrogen 39 in the heat exchanger pipe 38, and as a result, the medium vapor was liquefied and dropped back into the still liquid cooling medium 34. As will be appreciated, the chips 31 were cooled as a result of a removal of vaporization heat from the chip surface during the generation of the bubbles 35. The cooling capability of the used cooling medium was evaluated as described above, and the results were satisfactory and were comparable to those obtained by using the cryogenic device of Fig. 5. Note, for the illustrated device, since the heat exchange medium is not a gas but a liquid such as liquid nitrogen, it is possible to reduce a diameter of the heat exchanger pipe, without reducing the cooling capability. For example, when cooling of 1 KW is intended, the heat transfer can be increased to 10⁵ W/m²K (10³ W/m²K for the use of He gas), and the diameter of the heat exchanger pipe can be reduced to 25 mm⊘ (50 mm⊘ for He gas). In addition to the cryogenic devices of Figs. 5 and 6, the cryogenic device of Fig. 2, which was used to produce the coolant may be used in the practice of the present invention if desired. Although not shown, an article to be cooled may be immersed in the produced coolant 18 in the container 17 of the refrigerator 14 (see, Fig. 2). Further, in Figs. 2, 5 and 6, although not shown, a heater may be attached to a bottom portion of the container 17. The heater can be advantageously used to control temperature of the cooling medium to an approximate boiling point thereof, or to control the speed of the heat exchange reaction. Figure 8 is a graph plotted to summarize experimental data and from which the present invention will be further understood. The graph of Fig. 8 has three lines: a first solid line for LN₂ + CnF 2n+2 (n = 2 or 3); a second solid line for LN₂ + C₄F₁₀; and a third dotted line for LN₂ + CF₄; and one point for LN₂. For the cooling medium of the present invention (mixture of LN₂ and C n F 2n+2 wherein n is 2, 3, or 4), a satisfactory cooling capability was obtained by addition of a small amount of CnF 2n+2 to LN₂. The dotted line in the range of 0.5 to 1% by mole of CnF 2n+2 means that the CnF 2n+2 was not dissolved in LN₂. For the prior art cooling medium (LN₂ alone or mixture of LN₂ and CF₄), a satisfactory cooling capability could not be obtained, and, for a mixture of LN₂ + C₅F₁₂ (not shown), no increase of the cooling capability could be obtained; rather the mixture showed a tendency to solidify.

Claims: A coolant, for use in a cryogenic device having a closed cycle system, which comprises a mixture of a liquid nitrogen and a fluorocarbon of the formula: CnF 2n+2 , wherein n is an integer of 2 to 4, the fluorocarbon being present in said mixture in an amount of from 0.1 to 3 mol %. A coolant according to claim 1, in which the liquid nitrogen has the fluorocarbon dissolved therein. A coolant according to claim 1, which is located, as a cooling medium, in a closed cryostat of a cryogenic device, an article to be cooled being immersed in the cooling medium. A method of producing a coolant comprising a mixture of liquid nitrogen and a fluorocarbon of the formula: CnF 2n+2 , wherein n is an integer of 2 to 4, which method comprises the step of cooling a mixed gas of nitrogen and fluorocarbon of the formula: CnF 2n+2 , wherein n is as defined above, under pressure to liquefy the mixed gas, the fluorocarbon being present in said mixture in an amount of from 0.1 to 3 mol %. A method according to claim 4, in which the ratio of fluorocarbon to nitrogen in the mixed gas is substantially the same as that of the resultant coolant. A method according to claim 4, in which an increased pressure of 1 to 2 atms is applied to the mixed gas during the cooling step. A method according to claim 4, in which the mixed gas in a high pressure vessel is introduced into a liquefier where the gas is cooled with a compressed and adiabatically cooled gas. A method of producing a coolant comprising a mixture of a liquid nitrogen and a fluorocarbon of the formula: CnF 2n+2 , wherein n is an integer of 2 to 4, the fluorocarbon being present in said mixture in an amount of from 0.1 to 3 mol %, which method comprises the step of introducing a gaseous fluorocarbon of the formula: CnF 2n+2 , wherein n is as defined above, into a liquid nitrogen to obtain the liquid nitrogen having the fluorocarbon dissolved therein. A method according to claim 8, in which the gaseous fluorocarbon in a high pressure vessel is introduced into liquid nitrogen in a Dewar vessel. A method according to claim 9, in which the gaseous fluorocarbon from the high pressure vessel is passed through liquid nitrogen in another Dewar vessel, before introduction thereof into said liquid nitrogen, to cool the gaseous fluorocarbon to a temperature beyond which the fluorocarbon is liquefied. A method of cooling an article to a cryogenic temperature of 123K or less, which method comprises the step of immersing the article in a coolant which comprises a mixture of liquid nitrogen and a fluorocarbon of the formula: CnF 2n+2 , wherein n is an integer of 2 to 4, the fluorocarbon being present in said mixture in an amount of from 0.1 to 3 mol %. A method according to claim 11, in which the liquid nitrogen has the fluorocarbon dissolved therein. A method according to claim 11, in which the coolant is placed as a cooling medium in a closed cryostat of a cryogenic device. A method according to claim 13, in which the cooling medium boiled and vaporized in the cryostat is liquefied in a separate liquefier and the thus produced cooling medium in the form of a liquid is returned to the cryostat. A method according to claim 13, in which the cooling medium boiled and vaporized in the cryostat is liquefied within the same cryostat as a result of heat exchange between vapor of the cooling medium and a heat exchange medium in a heat exchanger pipe which is disposed at a suitable position in the cryostat so that the heat exchange is made therein. A method according to claim 15, in which the heat exchange medium circulated through the heat exchanger pipe is liquid nitrogen. A method according to any one of claims 11 to 16, in which the article to be cooled is a semiconductor device which is designed to be operated at the cryogenic temperature. A method according to claim 17, in which the semiconductor device is a high electron mobility transistor. A method according to claim 17, in which the semiconductor device is a complementary metal oxide semiconductor device. A method according to any one of claims 11 to 16, in which the article to be cooled is a device using a superconducting material which exhibits a superconductivity at the cryogenic temperature. A method according to any one of claims 11 to 16, in which the article to be cooled is electronic equipment which is designed to be operated at the cryogenic temperature. A cryogenic device having a closed cycle system, which comprises a closed cryostat containing, as a cooling medium, a mixture of a liquid nitrogen and a fluorocarbon of the formula: CnF 2n+2 , wherein n is an integer of 2 to 4, the fluorocarbon being present in said mixture in an amount of from 0.1 to 3 mol %, an article to be cooled being immersed in the cooling medium in the cryostat, and cooled to a cryogenic temperature of 123K or less. A device according to claim 22, in which the liquid nitrogen has the fluorocarbon dissolved therein. A device according to claim 22, in which a wall of the cryostat is provided with inlet and outlet openings communicated through pipes with a separate liquefier, so that the cooling medium boiled and vaporized as a result of a heat generated from the article in the cryostat is guided into and liquefied in the liquefier and the thus produced cooling medium in the form of a liquid is returned to the cryostat. A device according to claim 22, in which an upper inner portion of the cryostat has a heat exchanger pipe disposed at a suitable position therein and communicated with a separate liquefier, so that the cooling medium boiled and vaporized as a result of heat generation in the cryostat is liquefied within the same cryostat as a result of heat exchange between a vapor of the cooling medium and a heat exchange medium in the heat exchanger pipe. A device according to claim 25, in which the heat exchange medium circulated through the heat exchanger pipe is liquid nitrogen. A device according to any one of claims 24 to 26, in which the liquefier comprises a Stirling cycle refrigerator with a compressor.

Inventors: KAMEHARA NOBUO, NIWA KOICHI, YAMADA MITSUTAKA, YOKOUCHI KISHIO, KAMEHARA, NOBUO, NIWA, KOICHI, YAMADA, MITSUTAKA, YOKOUCHI, KISHIO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1989
Main classification: C09K5
IPC codes: C09K5, H01L23, F25D3