Method for manufacturing a tbco2 magnetocaloric alloy in ribbon form for use in magnetic refrigeration

Abstract

The present invention relates to the field of magnetocaloric materials of importance for magnetic refrigeration. Specifically, it provides a method of manufacturing a TbCo2 magnetocaloric alloy in ribbon form which can be used as a magnetic refrigerant in magnetocaloric devices. In this way, it is possible to obtain the TbCo2 magnetocaloric alloy in ribbon form with improved magnetocaloric properties by means of a simple and expeditious manufacturing method.

Description

TECHNICAL FIELD OF INVENTION

[0001] The present invention relates to the field of magnetocaloric materials for applications in magnetic refrigeration. Specifically, it provides a method of manufacturing for obtaining a TbCo 2 magnetocaloric alloy in ribbon form that can be used as a potential refrigerant in the development of new magnetic refrigeration technologies.BACKGROUND OF THE INVENTION

[0002] In the technical aspect of manufacture, the TbCo 2 magnetocaloric alloy has been obtained by conventional metallurgy techniques, such as electric arc or induction melting, followed by heat treatment at elevated temperature for long periods of time (approximately one week).

[0003] In this regard, different magnetocaloric alloys manufactured by different methods have been developed that seek simpler and higher-scale production.

[0004] For example, document US 2007 / 137732 describes a magnetic alloy material obtained by a method comprising the steps of: preparing a melt of a predetermined composition; rapidly cooling and solidifying said alloy by a quenching process to obtain a rapidly solidified alloy that includes a phase having a NaZn 13 -type crystal structure and a composition: Fe 100-a-b-c RE a A b TM c where RE is a rare-earth element that is selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er and Tm and that includes at least about 90% of La; A is an element that is selected from the group consisting of Al, Si, Ga, Ge and Sn; TM is a transition metal element that is selected from the group consisting of Sc, Ti, V, Cr, Mn, Co, Ni, Cu and Zn; and mole fractions a, b and c satisfy at 5%≤a≤10, 4.7%≤b≤18% and 0%>c≤9%, respectively.

[0005] On the other hand, US 2006 / 213580 describes a magnetic refrigeration material, comprising a chemical composition including one element selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Ho, Er, Tm and Yb in a total of 4 to 15 atomic percent, one element selected from the group consisting of Fe, Co, Ni, Mn and Cr in a total of 60 to 93 atomic percent, one element selected from members of the group constituting of Si, C, Ge, Al, Ga and In in a total of 2.5 to 23.5 atomic percent; and B of 0.5 atomic percent to 1.5 atomic percent; and material phases comprising NaZn 13 -type crystal structure phase; and BCC crystal structure phase comprising Fe as the main constituent element with a phase region size of 20 µm or less. Additionally, the invention provides a method for manufacturing a magnetic refrigeration material comprising a melting process that melts a raw material composition and a forced cooling process that forcefully cools and solidifies the molten metal and obtains a rapidly cooled alloy comprising a NaZn 13 -type crystal structure phase.

[0006] Consequently, a magnetocaloric alloy in ribbon form with improved magnetocaloric properties and obtained by a simpler and more expeditious manufacturing method is required.SUMMARY OF THE INVENTION

[0007] The present invention provides a TbCo 2 magnetocaloric alloy in ribbon form characterized in that it features a Laves phase with a cubic MgCu 2 (C15)-type structure, Fd-3m space group.

[0008] In a preferred embodiment, the TbCo 2 magnetocaloric alloy in ribbon form is characterized in that it has a cell parameter a of 0.7207 nm (7.207 Å).

[0009] In another preferred embodiment, the TbCo 2 magnetocaloric alloy in ribbon form is characterized in that it has a Curie temperature of 236 K.

[0010] In a further preferred embodiment, the TbCo 2 magnetocaloric alloy in ribbon form is characterized in that it has a magnetization of 155 Am 2< / Kg at 5 K for an applied magnetic field of 5 T.

[0011] The present invention further provides a method of manufacturing TbCo 2 magnetocaloric alloy ribbons for application in magnetic refrigeration, characterized in that it comprises the steps of melting a TbCo 2 alloy, and solidifying said molten alloy by ultrafast cooling.

[0012] In a preferred embodiment, the manufacturing method is characterized in that said TbCo 2 magnetocaloric alloy ribbons have an average thickness of 34 µm.

[0013] In another preferred embodiment, the method of manufacturing the TbCo 2 magnetocaloric alloy ribbons is characterized in that it is performed by melt-spinning.

[0014] In a further preferred embodiment, the manufacturing method is characterized in that said melting of said TbCo 2 alloy is carried out at a temperature of 1300 °C.

[0015] In a preferred embodiment, the manufacturing method is characterized in that the difference of pressure between the melt-spinning chamber and the auxiliary tanks during the manufacture of the TbCo 2 magnetocaloric alloy in ribbon form is 30 kPa (0.3 bar).

[0016] In another preferred embodiment, the manufacturing method is characterized in that the spinning speed of the wheel for obtaining the TbCo 2 magnetocaloric alloy in ribbon form is 20 m / s.

[0017] In another preferred embodiment, the manufacturing method for obtaining the TbCo 2 magnetocaloric alloy ribbons is characterized in that the distance between the crucible and the wheel is 0.8 mm.

[0018] In a further preferred embodiment, the manufacturing method for obtaining the TbCo 2 magnetocaloric alloy ribbons is characterized in that said process is carried out in argon atmosphere.BRIEF DESCRIPTION OF THE FIGURES

[0019] FIG. 1 shows an X-ray diffraction pattern for finely powdered ribbons of the TbCo 2 magnetocaloric alloy according to the present invention. FIG. 2 shows magnetization curves as a function of temperature for the TbCo 2 magnetocaloric alloy in ribbon form according to the present invention. FIG. 3 shows the magnetization isotherms measured over a temperature range from 150 K to 300 K for the TbCo 2 magnetocaloric alloy in ribbon form according to the present invention. FIG. 4 shows the curves of magnetic entropy variation as a function of temperature at 2 T (a) and 5 T (b) for the TbCo 2 magnetocaloric alloy in ribbon form according to the present invention. FIG. 5 shows the cooling capacity, estimated in its three criteria RC-1, RC-2 and RC-3 as a function of the applied magnetic field µ 0 H for the TbCo 2 magnetocaloric alloy in ribbon form according to the present invention. FIG. 6 shows the working temperature range (δT FWHM ) as a function of the applied magnetic field µ ο H for the TbCo 2 magnetocaloric alloy in ribbon form according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention provides a TbCo 2 magnetocaloric alloy in ribbon form that can be used as a magnetic refrigerant in magnetic refrigeration technologies.

[0021] In the context of the present invention, a ribbon will be understood as a substantially two-dimensional structure, wherein the length of one of its dimensions, which will be referred to as thickness, is much smaller than the length of its other two dimensions, which will be referred to as width and length. For example, and without limiting the scope of the present invention, said thickness may be less than 0.5 times the lesser of its other two dimensions, more preferably, less than 0.1 times the lesser of its other two dimensions, and even more preferably less than 0.05 times the lesser of its other two dimensions.

[0022] In a preferred embodiment, without limiting the scope of the present invention, the TbCo 2 magnetocaloric alloy ribbons have a thickness of between 10 µm and 60 µm. In a more preferred embodiment, the TbCo 2 magnetocaloric alloy ribbons have a thickness of between 20 µm and 50 µm. In an even more preferred embodiment, the TbCo 2 magnetocaloric alloy ribbons have a thickness of between 30 µm and 40 µm. In an exemplary embodiment, without limiting the scope of the present invention, the TbCo 2 magnetocaloric alloy ribbons have a thickness of 34 µm.

[0023] In a preferred embodiment, without limiting the scope of the present invention, the ribbons of the TbCo 2 magnetocaloric alloy that is the subject matter of the present invention may have a length whose length is between 4 mm and 100 mm. Moreover, in a preferred embodiment, and without limiting the scope of the present invention, the ribbon of the TbCo 2 magnetocaloric alloy that is the subject matter of the present invention may have a width whose length is between 2 mm and 4 mm.

[0024] In the context of the present invention, without limiting the scope of the same, a magnetocaloric material will be understood as one which exhibits changes in its temperature in response to the application or withdrawal of an external magnetic field. This allows, for example and without limiting the scope of the present invention, to define cooling cycles by varying the external magnetic field.

[0025] The TbCo 2 magnetocaloric alloy in ribbon form which is the subject matter of the present invention has a Laves phase and a cubic MgCu 2 (C15)-type structure, (Fd-3m space group).

[0026] FIG. 1 shows an X-ray diffraction pattern obtained at room temperature, corresponding to a sample of finely powdered TbCo 2 magnetocaloric alloy ribbons according to the present invention. All diffraction peaks observed in FIG. 1 correspond to the TbCo 2 phase and were correctly indexed based on the cubic MgCu 2 (C15)-type structure (Fd-3m space group) of the Laves phases. The Miller indices corresponding to the different crystallographic planes of the crystal structure of the TbCo 2 magnetocaloric alloy ribbon, which is the subject of the present invention, are also identified.

[0027] Furthermore, from the X-ray diffraction pattern it is possible to obtain the lattice parameter for the crystalline structure of the TbCo 2 magnetocaloric alloy ribbon which is the subject of the present invention. In the case of the TbCo 2 magnetocaloric alloy ribbon which is the subject of the present invention a cell parameter a of 0.7207 nm (7,207 Å) was obtained.

[0028] In a preferred embodiment, and without limiting the scope of the present invention, the TbCo 2 magnetocaloric alloy ribbon has a Curie temperature of between 220 K and 240 K. In an even more preferred embodiment, the TbCo 2 magnetocaloric alloy ribbon has a Curie temperature of 236 K. In the context of the present invention, Curie temperature will be understood as the temperature at which a material transitions from the ferromagnetic state to the paramagnetic state. In this sense, it will be understood that said material exhibits a ferromagnetic behavior below its Curie temperature and a paramagnetic behavior above said Curie temperature.

[0029] In the context of the present invention, magnetization will be understood as the process in which the magnetic moments of a material are oriented or tend to be oriented according to the direction of the applied magnetic field. Said magnetization depends, among other aspects, on the temperature of the material. It is possible to measure said magnetization at a temperature close to that of liquid helium (approximately 4 K). In a preferred embodiment, and without limiting the scope of the present invention, the TbCo 2 magnetocaloric alloy ribbon has a magnetization at 5 K of between 150 Am 2< / Kg and 160 Am 2< / Kg. In an even more preferred embodiment, the TbCo 2 magnetocaloric alloy ribbon has a magnetization of 155 Am 2< / Kg at 5 K for a magnetic field value of 5 T.

[0030] The present invention further provides a method of manufacturing a TbCo 2 magnetocaloric alloy in ribbon form for magnetic refrigeration comprising, essentially, the steps of melting a TbCo 2 alloy, and solidifying said molten alloy by ultrafast cooling; wherein said ultrafast cooling is obtained by melt-spinning technique. In the context of the present invention, without limiting the scope thereof, ultrafast cooling will be understood as a process in which the cooling rate is greater than 10 3< K / s, more preferably greater than 10 4< K / s.

[0031] The melting and solidification process by ultrafast cooling of the TbCo 2 alloy is performed by a technique known as melt-spinning. Said technique involves providing a metal wheel or drum that is internally cooled, usually with water, and rotated at high speed. A squirt of liquid material, for example and without limiting the scope of the present invention, a molten alloy, is thrown onto the outer surface of the wheel and cooled, causing extremely rapid solidification. This technique is used to produce materials requiring extremely high cooling rates, such as, but not limited to, metallic glasses or amorphous ribbons. In the melt-spinning technique, cooling rates of the order of 10 5< K / s are possible.

[0032] In a preferred embodiment, without limiting the scope of the present invention, the method which is the subject matter of the present invention comprises the steps of: providing said cooled metal wheel and a crucible containing the TbCo 2 alloy inside a manufacturing chamber; purging said manufacturing chamber with an inert gas; rotating said cooled metal wheel; bringing the temperature of said crucible up to a temperature at which said TbCo 2 alloy is in a liquid state; and dropping a squirt of said alloy in liquid state onto said cooled metal wheel.

[0033] In a preferred embodiment, without limiting the scope of the present invention, said crucible containing the TbCo 2 alloy is positioned inside an induction coil. In this way, without limiting the scope of the present invention, said induction coil allows the melting of said alloy by means of induced currents in the material. In another preferred embodiment, without limiting the scope of the present invention, the manufacturing system comprises a vacuum pump and an inert gas supply operatively connected to said manufacturing chamber. In this way, for example and without limiting the scope of the present invention, it is possible to purge and clean the manufacturing chamber by means of said vacuum pump and said inert gas supply. In a further preferred embodiment, without limiting the scope of the present invention, it is possible to provide one or more auxiliary gas tanks and one adjustable pressure connection of inert gas, coming from said auxiliary tanks, which is connected to the crucible. In this way, without limiting the scope of the present invention, it is possible to apply an overpressure necessary to expel the molten alloy onto the surface of the spinning metallic wheel.

[0034] Optionally, without limiting the scope of the present invention, it is possible to perform the melt-spinning process in an inert atmosphere. In the context of the present invention an inert atmosphere will be understood as one which is formed by one or more gases that do not react chemically with the material to be melted. In a preferred embodiment, without limiting the scope of the present invention, the gases forming said inert atmosphere may be chosen from the group formed by noble gases. In an even more preferred embodiment, said inert atmosphere is formed by argon. The use of inert atmosphere is advantageous in that the material from which the ribbons will be formed will be the material of the alloy contained in the crucible, since no reactions occur between the atmosphere and the molten material.

[0035] On the other hand, the temperature at which the TbCo 2 magnetocaloric alloy is melted does not limit the scope of the present invention. Additionally, the material to be melted may be provided as a previously obtained alloy, or be obtained in the crucible by melting an appropriate amount of Tb and an appropriate amount of Co, without limiting the scope of the present invention. In a preferred embodiment and without limiting the scope of the present invention, the melting temperature of the TbCo 2 alloy is between 1180 °C and 1310 °C. In an even more preferred embodiment, the temperature at which the melting of the TbCo 2 magnetocaloric alloy is carried out is 1300 °C.

[0036] Additionally, and without limiting the present invention, the gas pressure in the melt-spinning chamber may be between 70 kPa (0.7 bar) and 90 kPa (0.9 bar). In an even more preferred embodiment, the gas pressure may be 80 kPa (0.8 bar).

[0037] In a preferred embodiment, and without limiting the scope of the present invention, the difference of gas pressure between the melt-spinning chamber and the auxiliary tanks may be between 15 kPa (0.15 bar) and 40 kPa (0.4 bar). In an even more preferred embodiment, the difference of gas pressure may be 30 kPa (0.3 bar).

[0038] Another aspect that is possible to vary in the melt-spinning process is the spinning speed of the wheel or drum, without limiting the scope of the present invention. In the context of the present invention the spinning speed will be understood as the tangential speed of the portion of the wheel or drum that is in contact with the squirt of molten material. The spinning speed influences the microstructural characteristics of the TbCo 2 magnetocaloric alloy ribbons obtained by the method which is the subject of the present invention. In a preferred embodiment, without limiting the scope of the present invention, the spinning speed of the wheel may be between 10 m / s and 50 m / s. In an even more preferred embodiment, the spinning speed of the wheel is 20 m / s.

[0039] Moreover, without limiting the scope of the present invention, it is possible to adjust the distance between the crucible and the spinning wheel. Said distance, for example and without limiting the scope of the present invention, may influence the possibilities of obtaining the TbCo 2 magnetocaloric alloy ribbons. A person ordinarily skilled in the art will realize, without limiting the scope of the present invention, that there may be distances at which it is not possible to manufacture the TbCo 2 magnetocaloric alloy ribbons. In a preferred embodiment, and without limiting the scope of the present invention, the distance between the crucible and the wheel may be between 0.5 mm and 5 mm. In a more preferred embodiment, said distance may be between 0.6 mm and 1 mm. In an even more preferred embodiment, said distance may be 0.8 mm.

[0040] According to the method that is the subject of the present invention, it is possible to obtain TbCo 2 magnetocaloric alloy ribbons with magnetocaloric properties superior to those reported for the same TbCo 2 alloy in bulk form in the prior art.

[0041] Different aspects of the previously described embodiments may be combined with each other or with other options envisaged by a person ordinarily skilled in the art, in any manner without limiting the scope of the present invention.

[0042] Hereinafter, examples of embodiments of the present invention will be presented. It should be understood that said examples are intended to provide a better understanding of the present invention, but in no way limit the scope of the present invention.

[0043] Furthermore, technical features described in different examples may be combined with each other, or with previously described technical features, in any manner envisaged by a person ordinarily skilled in the art without limiting the scope of the present invention.Example 1: Magnetization curves as a function of temperature for the TbCo 2 magnetocaloric alloy in ribbon form.

[0044] FIG. 2 shows the magnetization curves as a function of temperature for the TbCo 2 magnetocaloric alloy measured during cooling for a constant magnetic field of 5 mT, as well as at 5 T during heating. The Curie temperature (T c ), or transition temperature from the ferromagnetic to the paramagnetic state, T c = 236 ± 1 K was estimated from the inflection point of the M(T) curve in the Field Cooled (FC) regime at 5 mT. For this, the minimum of the dM / dT curve as a function of temperature was obtained (see inset in FIG. 2). Furthermore, for a temperature of 5 K and an applied magnetic field of 5 T, a saturation magnetization M s of 155 ± 2 Am 2< kg -1< was obtained.Example 2: Magnetization isotherms measured for the TbCo 2 magnetocaloric alloy in ribbon form.

[0045] FIG. 3 shows the magnetization isotherms measured over a temperature range from 150 K to 300 K, with a temperature increment between one isotherm and the subsequent one of 5 K. It can be observed that, above the Curie temperature, 236 ± 1 K, the isotherms gradually tend to linearity with increasing temperature. This behavior is characteristic of the transition from the ferromagnetic to the paramagnetic state.Example 3: Magnetic entropy variation curves for the TbCo 2 magnetocaloric alloy in ribbon form.

[0046] The evaluation of the magnetocaloric effect (MCE) was carried out from the magnetization isotherms described in Example 2. FIG. 4 shows a comparison of the magnetic entropy variation curves as a function of temperature ΔS M (T) for a field variation µ o ΔH of 2 T and 5 T, respectively, for the TbCo 2 magnetocaloric alloy in ribbon form which is the subject matter of the present invention. In both cases, it is observed that the peak of the curve is around the Curie temperature.Example 4: Estimation of the cooling capacity for the TbCo 2 magnetocaloric alloy in ribbon form .

[0047] FIG. 5 shows the dependence of the cooling capacity on the applied magnetic field; estimated according to the following criteria: (a) by the product |ΔS M peak< | × δT FWHM (referred to in the literature as RC-1), (b) by obtaining the area under the curve ΔS M (T) between the cold and hot focus temperatures, T cold and T hot , respectively (referred to as RC-2). By the calculation of area of the rectangle with the largest area that can be inscribed under the curve ΔS M (T) (RC-3; Wood and Potter criterion), for the TbCo 2 magnetocaloric alloy in ribbon form that is the subject matter of the present invention.Example 5: Characterization of the working temperatures of the TbCo 2 magnetocaloric alloy ribbons.

[0048] FIG. 6 shows the dependence of the working temperature range, (δT FWHM ) as a function of the applied magnetic field (µ ο H) for the TbCo 2 magnetocaloric alloy in ribbon form that is the subject matter of the present invention.

Claims

1. A TbCo2 magnetocaloric alloy in ribbon form CHARACTERIZED in that it corresponds to a single-phase material having a Laves phase with a cubic MgCu2(C15)-type structure with Fd-3m space group.

2. The TbCo2 magnetocaloric alloy in ribbon form according to claim 1, CHARACTERIZED in that it has a cell parameter a of 0.7207 nm.

3. The TbCo2 magnetocaloric alloy in ribbon form according to claim 1, CHARACTERIZED in that it has a Curie temperature of 236 K.

4. The TbCo2 magnetocaloric alloy in ribbon form according to claim 1, CHARACTERIZED in that it has a magnetization of 155 Am2 / Kg at 5 K for a magnetic field of 5T.

5. A method of manufacturing a TbCo2 magnetocaloric alloy in ribbon form for magnetic refrigeration CHARACTERIZED in that it comprises the steps of: - melting a TbCo2 alloy; and - solidifying said molten alloy by ultrafast cooling; wherein said ultrafast cooling is obtained by melt-spinning technique.

6. The manufacturing method according to claim 5, CHARACTERIZED in that the melting of said TbCo2 magnetocaloric alloy is carried out at a temperature of 1300 °C.

7. The manufacturing method according to claim 5, CHARACTERIZED in that said ultrafast cooling by melt-spinning technique is performed in an inert atmosphere.

8. The manufacturing method according to claim 7, CHARACTERIZED in that said inert atmosphere has a pressure of 80 kPa.

9. The manufacturing method according to claim 7, CHARACTERIZED in that said inert atmosphere is argon.

10. The manufacturing method according to claim 5, CHARACTERIZED in that said melt-spinning technique comprises contacting said molten alloy with a spinning metal wheel.

11. The manufacturing method according to claim 10, CHARACTERIZED in that said metal wheel has a spinning speed of 20 m / s.

12. The manufacturing method according to claim 10, CHARACTERIZED in that said melt-spinning technique comprises: providing a crucible containing said molten alloy; and dropping said molten alloy onto said metal wheel.

13. The manufacturing method according to claim 12, CHARACTERIZED in that said crucible and said metal wheel are at a distance of 0.8 mm from each other.

14. The manufacturing method according to claim 5, CHARACTERIZED in that it comprises the steps of: - providing a metal wheel and a crucible containing said TbCo2 alloy inside a manufacturing chamber; - purging said manufacturing chamber with an inert gas; - spinning the metal wheel; - bringing the temperature of said crucible up to a temperature at which the TbCo2 alloy is in a liquid state; and - dropping the alloy in liquid state onto the metal wheel.

Images