Cooling device comprising a paramagnetic garnet ceramic

Abstract

A magnetic cooling device includes a magnetocaloric element, the magnetocaloric element being a paramagnetic garnet ceramic. The density of the paramagnetic garnet ceramic is preferably greater than or equal to 90%. The garnet ceramic is preferably a gadolinium gallium garnet ceramic or an ytterbium gallium garnet ceramic.

Claims

1. A magnetic cooling device comprising a magnetocaloric element, the magnetocaloric element comprising a paramagnetic garnet, the paramagnetic garnet being a ceramic; a heat source and a cold source, the paramagnetic garnet being arranged between the cold source and the heat source; and a field generator designed to produce magnetic fields external to the field generator; the magnetocaloric element and the field generator being positioned and configured such that a magnetic field is applied to the magnetocaloric element; wherein the ceramic is selected from oxide ceramics and is a sintered ceramic comprising pores, and wherein a relative density of the sintered ceramic is greater than or equal to 90% and less than 100%.

2. The device according to claim 1, wherein the sintered ceramic is formed from grains with different sizes, the grains being separated from each other by grain boundaries, the grain boundaries delimiting zones with different crystalline orientations.

3. The device according to claim 1, comprising a first thermal switch located between the heat source and the magnetocaloric element; and a second thermal switch located between the cold source and the magnetocaloric element, wherein the first and second thermal switches are configured to have open and closed positions.

4. The device according to claim 1, wherein the relative density of the sintered ceramic is greater than or equal to 95% and less than 100%.

5. The device according to claim 1, wherein the magnetocaloric element is composed of the paramagnetic garnet ceramic.

6. The device according to claim 1, wherein the sintered ceramic contains gallium.

7. The device according to claim 6, wherein the sintered ceramic is a gadolinium gallium garnet ceramic.

8. The device according to claim 6, wherein the sintered ceramic is an ytterbium gallium garnet ceramic.

9. The device according to claim 1, wherein a volume of the magnetocaloric element varies from 1 mm.sup.3 to 500 dm.sup.3.

10. The device according to claim 1, wherein a volume of the magnetocaloric element varies from 1 cm.sup.3 to 100 cm.sup.3.

11. The device according to claim 1, wherein the magnetocaloric element is fixed to a metallic element.

12. The device according to claim 11, wherein the metallic element is bonded onto the magnetocaloric element.

13. The device according to claim 11, wherein the metallic element is brazed onto the magnetocaloric element.

14. The device according to claim 11, wherein the magnetocaloric element is a thermal bus.

15. The device according to claim 11, wherein the metallic element is an enclosure.

16. A magnetic cooling device comprising a magnetocaloric element, the magnetocaloric element comprising a paramagnetic garnet, the paramagnetic garnet being a ceramic, wherein the ceramic is a sintered oxide ceramic comprising pores which is an ytterbium gallium garnet ceramic, and wherein a relative density of the sintered ceramic is greater than or equal to 90% and less than 100%.

17. The device according to claim 1, wherein the sintered ceramic is formed from grains with first and second sizes, the first size is larger than the second size, the grains with the first size are coalesced to form grains with a third size larger than the first size, and the grains with the second size are not coalesced.

18. The device according to claim 1, wherein a volume of the magnetocaloric element varies from 50 cm.sup.3 to 100 cm.sup.3.

19. The device according to claim 1, wherein a volume of the magnetocaloric element varies from 70 cm.sup.3 to 100 cm.sup.3.

20. The device according to claim 1, wherein the magnetocaloric element is in the form of a cylindrical rod.

21. The device according to claim 1, wherein the paramagnetic garnet ceramic is obtained by a method comprising a sintering step.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) This invention will be better understood after reading the description of example embodiments given purely for information and that is in no way limitative, with reference to the appended drawings on which:

(2) FIG. 1 described above represents a temperature-entropy diagram for a magnetic refrigeration cycle made with a paramagnetic material,

(3) FIG. 2a diagrammatically represents a cooling device according to one particular embodiment of the invention.

(4) FIG. 2b diagrammatically represents a cooling device according to another particular embodiment of the invention.

(5) FIG. 3 diagrammatically represents a cooling device according to another particular embodiment of the invention,

(6) FIG. 4 is a graph representing temperature curves as a function of time, for a cooling device like that represented in FIG. 3,

(7) FIGS. 5a and 5b diagrammatically represent sectional elevation and top views respectively of part of a cooling device according to one particular embodiment of the invention.

(8) FIG. 6 diagrammatically represents a cooling device according to another particular embodiment of the invention,

(9) FIG. 7 diagrammatically represents a cooling device according to another particular embodiment of the invention.

(10) FIG. 8 is a photographic plate of a machined bar of a YbGG garnet ceramic according to one particular embodiment of this invention, and

(11) FIG. 9 is a plate obtained by scanning electron microscopy of a YbGG garnet ceramic sintered at 1700 C. under air, according to one particular embodiment of the invention.

(12) The different parts shown on the figures are not necessarily all at the same scale, to make the figures more easily understandable.

(13) The different possibilities (variants and embodiments) must be understood as not being mutually exclusive and can be combined with each other.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

(14) In the following, the paramagnetic garnet ceramic is described for use in a cooling device, and more particularly for cryogenics, and particularly in an adiabatic demagnetization refrigerator (ADR). Use of the ceramic is particularly advantageous for 100 mK to 10 K applications.

(15) The paramagnetic garnet ceramic could also be used for magnetic refrigeration at ambient temperature.

(16) The paramagnetic garnet ceramic can also be used in an active magnetic regenerative refrigeration (AMRR) system.

(17) The paramagnetic garnet ceramic can be used in any type of thermal regulation system.

(18) Cooling Device:

(19) Refer firstly to FIGS. 2a, 2b and 3 that represent cooling devices 100, and more particularly in adiabatic demagnetization refrigerators (ADR). These magnetocaloric devices 100 comprise at least;

(20) a paramagnetic element 110, also called magnetocaloric element,

(21) a heat source 120 and a cold source 121, the paramagnetic element 110 being arranged between the cold source 121 and the heat source 120,

(22) a field generator 150 designed to produce an external magnetic fields.

(23) Magnetocaloric Element:

(24) The magnetocaloric element 110 is a paramagnetic material. It comprises, and is preferably composed of, a paramagnetic garnet ceramic. The use of a garnet ceramic, and particularly a GGG garnet ceramic or a YbGG garnet ceramic, opens up new prospects due to the very attractive paramagnetic properties of these materials and their mechanical robustness.

(25) The magnetocaloric element 110 may be in the form of a cylindrical rod, a disk, or a more complex shape.

(26) The dimensions of the magnetocaloric element 110 vary from a few mm.sup.3 to several hundred dm.sup.3, and preferably from a few cm.sup.3 to 100 cm.sup.3.

(27) Only one magnetocaloric element 110 is used in the embodiment illustrated in FIGS. 2a and 2b. According to one variant embodiment, the cooling device 100 may include several magnetocaloric elements (for example two, three or four), arranged in series or in parallel. The different magnetocaloric elements can be separated by thermal switches 140. Larger temperature differences can then be obtained, or a continuous stage at an intermediate temperature can be obtained.

(28) For illustrative purposes, FIG. 3 represents a device 100 with two magnetocaloric element stages 110a and 110b arranged in series, and comprising two exchange gas thermal switches 140. Such a device 100 can obtain a large temperature difference, for example with the first element 110a being used to obtain cooling between 4 K and 1.2 K and the second element 110b obtaining cooling between 1.2 K and 0.4 K. This device can also be used to obtain continuous cooling, for example to 400 mK with power of the order of 50 W. FIG. 4 represents the temperature curve as a function of time corresponding to such a device. The temperature of the heat source 120 and the paramagnetic elements 110a and 110b mounted in series is represented as a function of magnetisation/demagnetisation cycles. Thus, the temperature of the paramagnetic element 110b, located on the side of the cold source 121, is kept constant at the required cold temperature. The temperature of the paramagnetic element 110a, located on the side of the heat source 120, alternates between a temperature higher than the heat source 120, allowing transfer of heat to the heat source and a temperature lower than the cold source 121, to extract heat from the paramagnetic element.

(29) This cooling device 100 nay be completed by other ADR type cooling stages, the other stages possibly including (independently of each other), a garnet ceramic or another magnetocaloric material, not necessarily in ceramic form, to also provide cooling below 200 mK.

(30) According to one advantageous embodiment represented on FIGS. 5a and 5b, at least one thermally conducting metallic material 160 is fixed on the magnetocaloric element 110 to improve the thermal conductivity. This is an element with a high thermal conductivity that carries heat exchanges between the interfaces (element to be cooled and heat source) and the magnetocaloric element 110. This element is also called the thermal bus.

(31) The thermal bus 160 maybe in the form of wires. It may also be a plate. In all cases, the thermal bus has at least one interface 161 to which it will be thermally connected (to the heat source 120 or to the device to be cooled (cold source 121)). The thermally conducting element may for example be made of copper or stainless steel.

(32) According to a first variant, the metallic element 160 is bonded to the magnetocaloric element 110, for example with a conducting epoxy type glue (for example Stycast).

(33) According to another particularly advantageous variant, the metallic element 160 is brazed onto the magnetocaloric element 110.

(34) In one variant, the possibility of having one or several metallic elements inside the ceramic could perfectly well be envisaged.

(35) The magnetocaloric element 110 can be maintained by thermally insulating suspension means 170, typically kevlar wires, or carbon fibre tubes cable of fixing said element to a mechanical support, not shown.

(36) Thermal Switch:

(37) According to one particular embodiment, the ADR device also comprises at least one thermal switch 140 (or valve), arranged between the cold source 120 and the paramagnetic element 110 and/or between the heat source 121 and the paramagnetic element 110. The thermal switches may for example be exchange gas switches.

(38) As represented on FIGS. 2a and 3, the thermal switches 140 are located between the heat source 120 and the cold source 121. The so-called hot switch is located between the heat source 120 and the paramagnetic element 110. The so-called cold switch is located between the cold source 121 and the paramagnetic element 110. The hot and cold switches are closed during the adiabatic demagnetisation and during adiabatic magnetisation. During the isothermal demagnetisation, the hot switch is open and the cold switch is closed. During the isothermal magnetisation, the hot switch is closed and the cold switch is open.

(39) According to another variant embodiment shown on FIG. 2a, the part to be cooled, for example a detector, is thermally connected to the paramagnetic element and does not require a cold switch. The device comprises a single thermal switch 140, namely the hot switch.

(40) For some applications, the thermal switches 140 can be replaced by a fluid called the cycle fluid that can circulate around and/or through the paramagnetic material. This embodiment is advantageous to obtain high powers and/or to perform fast cycles with large heat exchanges.

(41) To confine the fluid, an enclosure 180, for example made of stainless steel, is advantageously fixed on the magnetocaloric element 110 (FIG. 6). It may be fixed by bonding or preferably by soldering.

(42) For example for an AMRR type device, the magnetisation and demagnetisation steps are alternated with the placement of a fluid flow that passes through the paramagnetic material in one direction and then in the other direction in order to create a temperature gradient inside the paramagnetic material. The four steps are: adiabatic magnetisation, fluid flow from hot to cold, adiabatic demagnetisation, fluid flow from cold to hot.

(43) Field Generator:

(44) The magnetic element 110 and the field generator 150 are positioned and configured such that a magnetic field is applied to the paramagnetic element 110 to produce heat or cold depending on the magnetisation/demagnetisation cycles. More particularly, an adiabatic demagnetisation, an isothermal demagnetisation, an adiabatic magnetisation and finally an isothermal magnetisation of the paramagnetic element 110 can be provoked successively and cyclically.

(45) This field generator 150 can be formed from any element capable of creating a variable magnetic field. For example, it may be a coil of conducting wire or one or several permanent magnets that can be displaced relative to the paramagnetic element 110.

(46) Magnetic screening means 130 can surround the field generator 150, so as to limit parasite magnetic fields outside the device.

(47) According to a first embodiment, the position of the paramagnetic element is fixed relative to the field generator and the magnetic field is variable (FIGS. 2 and 3).

(48) According to another embodiment, the position of the paramagnetic element 110 varies relative to the field generator 150 and the magnetic field is fixed (FIG. 7). Preferably, the device 100 comprises two fixed field generators between which the paramagnetic field is displaced alternately. Weak field generators 151 can be added into the device. The device does not comprise a thermal switch; the cooling device can use alternating helium flows.

(49) According to another embodiment, not represented, the position of the paramagnetic element is fixed relative to the field generator and the magnetic field is fixed. There is no thermal switch, gravity and the different convection or condensation regimes are used.

(50) For example, the field generator 150 comprises a magnetic element that will generate the magnetic field.

(51) The field generator may for example be a superconducting coil.

(52) Heat Source and Cold Source:

(53) The magnetocaloric element 110 is thermally connected to the heat source 110 and to the cold source 121.

(54) Advantageously, the cold source 121 and the heat source 120 are arranged along the same axis. According to one variant, the cold source 121 and the heat source 120 can be arranged arbitrarily.

(55) Advantageously, the heat source 120 and the cold source 121 are arranged in the immediate environment of the magnetocaloric element 110 so as to easily and/or quickly recover at least part of the heat and/or cold that it emits. They can form a heat transfer fluid circuit.

(56) Advantageously, helium will be used as the heat source 120 and as the cold source 121. Advantageously, the helium is a mixture of liquid and gas at different pressures and temperatures for the cold source 121 and the heat source 120.

(57) The refrigeration device may also comprise: fluid circulation means, an electrical power supply, for example, slaved by a control unit, and a heat sensor to determine the temperature of the heat transfer fluid.

(58) The cooling device 100 can be used for high power applications (100 mW to several kW) or for refrigeration applications at ambient temperature.

(59) The cooling device 100 can also be used for space applications: coolers below 100 mK and/or a stage to about 400 mK can be added for astrophysical missions.

(60) For land applications, the same magnetocaloric garnet ceramic material would also be particularly suitable because, in addition to having low cost and being easy to integrate, it is capable of cooling to temperatures of less than one Kelvin. It can also be used for studies on quantum electronics.

(61) Method of Elaborating Garnet Ceramic

(62) The method of fabricating a garnet ceramic comprises several steps: a) synthesis of a fine power of precursors, for example by reaction in solid phase, b) fine grinding of the precursors powder, c) forming of the material, for example in the form of a bar, the unsintered material having a density of less than 70%, preferably between 55% and 65%, for example of the order of 60% (unbaked part), and d) heat treatment of the unbaked part below the melting temperature of the material to obtain a sintered ceramic with the required density.

(63) The ground particles of synthesised powder preferably have a diameter of less than 10 m, preferably from 1 m to 7 m and even more preferably from 3 m to 5 m. Such diameters facilitate obtaining a dense material at the end of the process. Particles forming the powder are advantageously monodispersed.

(64) During step c), the particles are compacted.

(65) During the sintering step (step d), the material is heated to below the melting temperature to make the grains coalesce, to reduce the internal porosity and thus density the material.

(66) More particularly, during sintering, some grains coalesce together to become larger grains reducing the free space between the initial grains, which increases the density. Necks form between grains and grains interpenetrate until open and closed porosities are almost entirely eliminated. During sintering, a majority of pores became clogged, but some residual gas spaces remain: the density of the ceramic will be less than 100%.

(67) Finally, the ceramic is formed from a set of grains of different sizes. A first part of the grains (the largest grains) is coalesced and the other part of the grains (the smallest grains) is not coalesced or is only very slightly coalesced.

(68) Process parameters will be chosen so as to obtain, for example, a density as close to 100% as possible, and preferably more than 95%.

(69) The sintering temperature depends on the method of preparing the powder, the particle size and any additives that can have been added. Conventionally, the sintering temperature can be evaluated by dilatometry.

(70) Illustrative and Non-Limitative Example of One Embodiment

(71) A YbGG ceramic is obtained from Ga.sub.2O.sub.3 and Yb.sub.2O.sub.3 powders.

(72) The powders are ground and mixed using a planetary mill (zirconium oxide bowl and balls). The presynthesis is made in an alumina crucible at 1050 C. under air. Forming is achieved by isostatic pressing in a natural rubber bladder at 2500 bars. The final temperature of the heat treatment is 1700 C. under air (temperature lower than the melting point of the YbGG compound).

(73) The material thus produced has a final density of more than 95%, a smaller grain size that confers good mechanical properties and optimised conduction of heat.

(74) The YbGG ceramic obtained is a 50 cm3 cylinder (FIGS. 8 and 9).

(75) The garnet ceramic is placed in a superconducting coil. The heat source is at 2 Kelvin and cooling is done at 400 mK.

(76) A paramagnetic garnet ceramic like that described above could also be used in a heating device.