Method For Obtaining A Material With Giant Magnetocaloric Effect By Ion Irradiation

Abstract

The present invention concerns, in particular, a method for obtaining a product with magnetocaloric effect from a single piece of material having a magnetic phase transition, the method comprising irradiation of at least one part of the material with ions, the irradiation being carried out with a suitable flux so that, after the irradiation, the material has various magnetic phase transition temperatures in the various parts of the material.

Claims

1. Method for obtaining a magnetocaloric product from a single piece of material having a magnetic phase transition, the method comprising irradiating at least part of the material with ions, wherein said irradiating is conducted with a fluence adapted so that the material has, after said irradiating, different magnetic phase transition temperatures in different parts of the material.

2. Method according to claim 1, wherein the single piece of material has a first-order magnetic phase transition.

3. Method according to claim 1, wherein the fluence is adapted so that the magnetic phase transition temperature of the material varies by at least 0.5 kelvin between two different parts of the material.

4. Method according to claim 1, wherein the fluence is adapted so that the material has, after said irradiating, a maximum difference in the magnetic phase transition temperatures of the different parts of the product of value within the range of 0.5 to 150 kelvins.

5. Method according to claim 1, wherein the fluence is adapted so that the magnetic phase transition temperature of the material varies, after said irradiating, monotonously from a first part of the material to a second part of the material.

6. Method according to claim 1, wherein the fluence is adapted so that the magnetic phase transition temperature of the material varies, after said irradiating, continuously from a first part of the material to a second part of the material.

7. Method according to claim 1, wherein the material consists of iron-rhodium.

8. Magnetocaloric product obtainable by the method according to claim 1.

9. Method for implementing a thermal cycle, said method comprising subjecting a product according to claim 8 to a variable magnetic field so that different magnetic phase transition temperatures in different parts of the material are crossed during the thermal cycle.

10. Heat engine configured to implement a thermal cycle, the heat engine comprising: a magnetocaloric product according to claim 8, means for subjecting the product to a variable magnetic field so that the different magnetic phase transition temperatures in different parts of the material are crossed during the thermal cycle.

11. Heat engine according to claim 10, wherein the heat engine is a heat pump or a refrigerator or a thermoelectric generator or an active magnetic generator.

Description

DESCRIPTION OF THE FIGURES

[0045] Other features, aims and advantages of the invention will emerge from the following description, which is purely illustrative and not limiting, and which must be read in conjunction with the appended drawings wherein:

[0046] FIG. 1 shows an Ericsson thermal cycle implemented by a heat engine comprising a magnetocaloric material.

[0047] FIG. 2 shows two curves of the absolute value of the entropy change |S.sub.magn| within three materials as a function of their temperature, for an applied magnetic field change of 0 to 2 tesla.

[0048] FIG. 3 shows a set of curves of entropy change within different materials assembled within a product known from the state of the art, as a function of their temperature.

[0049] FIG. 4 shows a set of Ericsson thermal cycles implemented by a heat engine comprising a plurality of magnetocaloric materials.

[0050] FIG. 5 is a cross-sectional view of a magnetocaloric product, according to an embodiment.

[0051] FIG. 6 shows the atoms of a material in an antiferromagnetic phase and in a ferromagnetic phase.

[0052] FIG. 7 shows two curves of FeRh entropy change as a function of its temperature, depending on whether the material is irradiated or not.

[0053] FIGS. 8, 9 and 10 are three curves of spatial distribution of magnetic phase transition temperature within magnetocaloric products, according to three different embodiments.

[0054] FIG. 11 is a schematic cross-sectional view of a refrigerator according to an embodiment.

[0055] On all figures, similar elements have the same reference signs.

DETAILED DESCRIPTION OF THE INVENTION

[0056] Process for Obtaining a Magnetocaloric Product

[0057] With reference to FIG. 5, a material 1 extends along an axis X. This material 1 has a first edge 2 and a second edge 3 opposite the first edge 2. The two edges 2, 3 have different positions along the axis X (respectively x2 and x3).

[0058] The material 1 has a free surface 4 connecting the first edge 2 to the second edge 3. The free surface 4 is for example flat and parallel to the axis X.

[0059] The material 1 is a single piece. Single piece of material means a single piece of material, with a continuous structure, from a single block. In particular, the material has an identical phase transition temperature at any point in its structure, particularly regardless of its position along the axis X.

[0060] The material 1 is also first-order magnetic phase transition material. Consequently, the entropy change curve of this material 1 as a function of its temperature has a high peak value in its magnetic phase transition temperature.

[0061] The following is a non-limiting example of a material 1 made of an iron-rhodium (FeRh)-based alloy.

[0062] The material 1 will have a composition of type Fe.sub.xRh.sub.1-x with a value of x close to 0.5, comprising about 50% iron and about 50% rhodium by atomic weight.

[0063] The material 1 is single crystal.

[0064] With reference to FIG. 6, at low temperature, iron-rhodium is antiferromagnetic. In this phase, iron atoms have parallel spins, but in opposite directions. More precisely, in this phase, iron-rhodium has a simple cubic configuration (CsCl type): each rhodium atom is at the centre of a cube. At each vertex of the cube, there is a pair of iron atoms with opposite direction spins.

[0065] At higher temperatures, iron-rhodium is ferromagnetic. In this phase, iron-rhodium always has a cubic configuration.

[0066] As shown in FIG. 2, iron-rhodium has a magnetic phase transition temperature from the antiferromagnetic phase to the ferromagnetic phase (or vice versa) of about 380 kelvins.

[0067] The material 1 is placed on a substrate 5, for example an MgO substrate.

[0068] An ion source 6 is used to irradiate the material 1 with ions, for example parallel to an irradiation direction Z.

[0069] For example, the ion source used is the product Supernanogan marketed by Pantechnik.

[0070] The ions projected into the material 1 induce a shift in the magnetic phase transition temperature of the material 1 to a lower value. This phenomenon, known per se, is described in the document Effects of energetic heavy ion irradiation on the structure and magnetic properties of FeRh thin films, by Nao Fujita et al., Nucl. Instrum. Methods B 267, 921-924 (2009).

[0071] The phase transition temperature shift depends on the fluence used during ion irradiation, i.e. the number of ions irradiated in the material 1 per cm.sup.2. FIG. 7 shows, by way of example, two curves of FeRh entropy change as a function of its temperature: a reference curve for unirradiated FeRh, and a second relative curve for FeRh irradiated with Ne.sup.5+ ions with an incidence angle of 60 and a kinetic energy of 25 keV and a fluence of 1.710.sup.13 ions/cm.sup.2.

[0072] The proportionality coefficient between fluence and temperature shift is about 5.10.sup.12 K/(ions/cm.sup.2) under these irradiation conditions. This coefficient depends on the irradiation conditions, particularly the type of ion, its kinetic energy, the angle of incidence and the intrinsic properties of the material.

[0073] The fluence depends on the ion emission parameters of the ion source used. These parameters, well known to the skilled person, include in particular the number of ions impacting the material per unit time and surface area and the irradiation time. By way of example, the above-mentioned conditions produce a fluence between 10.sup.12 and 10.sup.15 ions/cm.sup.2 on a material 1.

[0074] In this case, the kinetic energy of the ions is adjusted (and/or the angle of incidence of the ion beam) to a value suitable for the ions to penetrate the material 1 and possibly to exit it.

[0075] Preferably, the ions used are heavy ions because they generate collisions and defects more efficiently within the irradiated material. It is this number of defects that determines the value of the previously defined proportionality coefficient. The advantage of heavy ions is that they only require irradiation of the material 1 over a relatively short irradiation period to change the phase transition temperature of a given deviation. The energy of the ions must be high enough to penetrate the material. There is no limit on the maximum energy because ions can also pass through the material even if the proportionality coefficient between fluence and temperature shift will depend on it.

[0076] The ions are for example neon ions, typically Ne.sup.5+.

[0077] In an unconventional way, the irradiation of the material 1 with the ions emitted by the ion source 6 is conducted with spatially variable fluence. In other words, the fluence is adapted so that the material 1 has, after irradiation, different magnetic phase transition temperatures in different parts of the material 1.

[0078] Returning to FIG. 5, the ion source 6 is moved and/or oriented relative to the material 1 so that the ions projected by the source scan the free surface 4 of the material 1 from the first edge 2 to the second edge 3 opposite the first edge 2. The scanning direction is for example parallel to the axis X.

[0079] The emission parameters of the ion source are adjusted so that the ion fluence in the material 1 varies monotonously during this scanning (increasing or decreasing). FIGS. 8 to 10 show different phase transition temperature spatial profiles (from the antiferromagnetic phase to the ferromagnetic phase) obtainable by varying the fluence used during ion irradiation of the material 1.

[0080] The spatial profile shown in FIG. 8 can be obtained as follows. The emission parameters of the ion source are set to a first set of values, and the ion source scans a first part of the material 1 with this first set of parameter values. The first part extends from the first edge 2 of position x2 along the axis X to a position line x0 along the axis X, between positions x2 and x3. In this way, the ions emitted by the ion source penetrate into the first part of the material 1 at a first constant fluence. As a result, the magnetic phase transition temperature Tt0 of the material 1 (380 kelvins in the case of FeRh) shifts by a first deviation so that it is lowered to a first value Tt1. At the position of the line x0, the scanning is stopped. The emission parameters of the ion source are then modified and set to a second set of values different from the first set of values. The ion source scans a second part of the material 1 with this second set of parameter values. The second part extends from the position line x0 along the axis X to the second edge 3 of position x3. In this way, the ions emitted by the ion source penetrate into the second part of the material 1 at a second constant fluence different from the first fluence, for example larger. As a result, the magnetic phase transition temperature of the material 1 shifts by a second deviation so that it is lowered to a second value Tt2, lower than the first value Tt1.

[0081] In such an embodiment, the result is a curve of phase transition temperature within the material 1 as a function of the position along the axis X, which is continuous in pieces. At the end of this irradiation step, the material 1 comprises a first part 7 having a first magnetic phase transition temperature Tt1 and a second part 8 having a second phase transition temperature Tt2 different from (for example, lower than) the first magnetic phase transition temperature Tt1.

[0082] It is also possible to irradiate only part of the material 1. In this case, the magnetic phase transition temperature in the unirradiated part will not be modified. In this embodiment, it is also possible to obtain a curve of phase transition temperature within the material 1 as a function of the position along the axis X, which is continuous in pieces. Partial irradiation of the material can be achieved by using one or a series of masks of sufficient thickness to block the ions. The use of a mask has the advantage of very precise control of the edges of irradiated areas that can have complex geometries.

[0083] However, it is preferable to continuously vary the fluence of the irradiated ions in the material 1, from the first edge 2 to the second edge 3 of the material 1. This can be achieved by gradually varying the ion emission parameters during the scanning of the ion radiation emitted by the source from the first edge to the second edge or by varying the local average irradiation time. Consequently, the magnetic phase transition temperature obtained in the material 1, after irradiation, decreases or increases continuously within the material 1 as a function of the position along the axis X, for example linearly, as shown in FIG. 9, or non-linearly, as shown in FIG. 10.

[0084] Alternatively or complementarily, it is possible to spatially vary the transition temperature in the material 1 in a direction parallel to the direction of emission Z of the ions by the ion source 6. For this purpose, one or more ion irradiations are carried out with ions that penetrate more or less deeply into the material in the direction Z. By varying the energy of the emitted ions and/or their angle of incidence, a variable number of collisions, in the material 1 according to the direction Z, can be obtained.

[0085] A continuous magnetic phase transition temperature spatial variation within the obtained product is very advantageous because it increases the cooling power of the product. It is understood that, in both cases, the irradiated material 1 includes an infinite number of phase transition temperatures, the phase transition temperature being maximum in the position x2 (at the first edge 2) and minimum in the position x3 (at the second edge 3 opposite the first edge 2).

[0086] The fluence received in the material 1 is adapted so that the magnetic phase transition temperature of the material 1 varies, after irradiation, by a useful value and for example by at least 0.5 kelvin between two different parts of the material 1.

[0087] Furthermore, the ion fluence is adapted so that the material 1 has, after irradiation, a maximum difference in the magnetic phase transition temperatures of the different parts of the product of value within the range of a few kelvins (e.g. 2 kelvins) to about 150 kelvins.

[0088] The ion fluence is also adapted so that the material 1 has, after irradiation, [0089] a minimum magnetic phase transition temperature within the range of 150 to 280 kelvins, [0090] a maximum magnetic phase transition temperature ranging from 360 to 380 kelvins.

[0091] It should be noted that the single crystal character of the material 1 is advantageous because it allows more precise control of the desired phase transition temperature values in the material as a function of the ion emission parameters.

[0092] Once the irradiation is complete, a product with a giant magnetocaloric effect is obtained that can be used in a heat engine.

[0093] In general, the heat engine comprises the magnetocaloric product 1 obtained after irradiation, and means for subjecting the product to a variable magnetic field so that the different magnetic phase transition temperatures in different parts of the material are crossed during a thermal cycle implemented by the heat engine.

[0094] Magnetic Refrigeration

[0095] With reference to FIG. 11, illustrating a first application of the material 1, the heat engine is a refrigerator 10.

[0096] The refrigerator 10 has a storage element 11 defining an internal storage cavity 12, for example for storing foodstuffs. Instead of a storage cavity, another type of object can be cooled. This cavity 12 constitutes a cold source whose temperature must be maintained at a value T.sub.L.

[0097] The refrigerator 10 also includes a radiator 13 in contact with an environment constituting a hot source at a temperature T.sub.H.

[0098] The general function of the refrigerator 10 is to take heat from the cold source (the cavity) and supply it to the hot source via the radiator 13.

[0099] In the refrigerator 10, the magnetocaloric product 1 is arranged between the cavity 12 and the radiator 13. It is arranged to be in thermal communication with the cavity 12 and the radiator 13.

[0100] The refrigerator includes a first thermal switch 16 configurable in two configurations: a closed configuration, in which the first thermal switch 16 allows thermal communication between the product 1 and the cold source 12, and an open configuration, in which the thermal switch 16 prevents the product 1 and the cold source from being in thermal communication.

[0101] The first thermal switch 16 is typically located near the edge 3.

[0102] Similarly, the refrigerator 10 includes a second thermal switch 18 configurable in two configurations: a closed configuration, in which the second thermal switch 18 allows thermal communication between the product 1 and the radiator 13, and an open configuration, in which the thermal switch 18 prevents the product 1 and the radiator 13 from being in thermal communication.

[0103] The second thermal switch 18 is typically located near the edge 2.

[0104] The two thermal switches 16, 18 are synchronised to be closed and opened alternately (when one is open, the other is closed, and vice versa).

[0105] The refrigerator 10 also includes, as indicated above, means 14 of subjecting the product 1 to a variable magnetic field so that the different magnetic phase transition temperatures in different parts of the material are crossed during a thermal cycle implemented by the heat engine.

[0106] The means of subjection 14 include, for example, a magnet that is movable with respect to the product 1. During a thermal cycle implemented by the refrigerator, the magnet is moved closer and further away from the product 1 to take advantage of its magnetocaloric effect. Alternatively, the means 14 includes a magnetic field generator of variable intensity, for example an electromagnet subjected to a current of variable intensity. Alternatively, the product can be placed in a movable support with respect to one or more fixed magnets.

[0107] The product 1 is oriented so that the edge 2 is closer to the hot source 13 than the edge 3, and the edge 3 is closer to the cold source than the edge 2.

[0108] Of course, all the phase transition temperatures that can be found in the product 1 (two values Tt1 and Tt2 in the case of the profile in FIG. 8, and a continuous range of values between Tt1 and Tt2 in the case of the profiles in FIGS. 9 and 10) are higher than the target temperature T.sub.L for the cavity 12, and lower than the temperature T.sub.H.

[0109] The refrigerator 10 in FIG. 12 uses a magnetic refrigeration method comprising at least one thermal cycle.

[0110] One possible thermal cycle is that of Ericsson, for example. It consists of four steps represented in FIG. 1, except that B1>B2 with B2=0 tesla. The method implemented by the refrigerator 10 comprises the following steps.

[0111] a) The product 1 is initially placed in thermal communication with cold source 12, by closing the first thermal switch 16. The product then cools to the temperature of the cold source T.sub.L.

[0112] b) A magnetic field is applied to the product 1 that absorbs heat from the cold source 12 through the magnetocaloric (inverse) effect, which increases the entropy of the product 1.

[0113] c) The first thermal switch 16 is open, interrupting the thermal communication between the product and the cold source 12. In turn, the second thermal switch 18 is open, which puts the product 1 and the hot source 13 in thermal communication. The product 1 heats up and then takes the temperature T.sub.H of the hot source 13.

[0114] d) The means of subjection 14 of the magnetic field are moved or reconfigured so that the product 1 ceases to be immersed in the magnetic field. The product 1 transfers its heat to the hot source 13 with the effect of reducing the entropy of the product 1.

[0115] a) The second thermal switch 18 is open, interrupting the thermal communication between the product and the hot source 13, and the first thermal switch 16 is closed. The product 1 then cools to the temperature of the cold source T.sub.L. The product is then returned to the starting configuration of the cycle.

[0116] The efficiency of the cycle depends on the increase in the entropy change AS with respect to the variation in the magnetic field to which the product 1 is subjected, when the product is in contact with the hot and cold sources 12, 13. Ion irradiation treatment to have a temperature Tt1 close to T.sub.H and Tt2 and close to T.sub.L maximises the entropy changes AS associated with steps 1 and 3 and results in maximising the exchanged heat.

[0117] The thermal cycle used is for example of the same type as that shown in FIG. 4. Other thermal cycles are possible, such as the Brayton cycle with adiabatic transformations or the Carnot cycle.

[0118] Other Applications

[0119] With a small product, one possible application could be the cooling of microelectronic components. In this case it is possible that the different components of the device described in FIG. 12 may be manufactured by lithography or other microelectronic techniques where the storage cavity 11 is substituted by an electronic element (power diode, micromethodor, etc.) to be cooled. In another application, the irradiated material 1 is used as a magnetocaloric product in a heat pump. The skilled person could, for example, start from the heat pump described in U.S. Pat. No. 8,763,407 or EP2541167A2 or U.S. Pat. No. 2,589,775 and replace the magnetocaloric composite product suggested in this document with the ion-irradiated material 1, which is a single piece.

[0120] In yet another application, the irradiated material 1 is used as a magnetocaloric product in a thermoelectric generator to produce electrical energy. The skilled person could, for example, start from a thermoelectric generator described in document U.S. Pat. Nos. 428,057 or 2,016,100 or 2,510,800, or from an active magnetic generator described in document U.S. Pat. No. 4,332,135, and replace the magnetocaloric composite product suggested in this document with the ion-irradiated material 1, which is a single piece.

[0121] The invention is not limited exclusively to FeRh. Other first-order magnetic phase transition materials can be used instead of FeRh. More specifically, any material that changes its transition temperature when irradiated with ions can be used instead of FeRh.