EP2467858A1

EP2467858A1 - Polycrystalline magnetocaloric materials

Info

Publication number: EP2467858A1
Application number: EP10744924A
Authority: EP
Inventors: Bennie Reesink
Original assignee: Technology Foundation - STW; Stichting voor de Technische Wetenschappen STW
Current assignee: Technology Foundation - STW; Technische Universiteit Delft
Priority date: 2009-08-18
Filing date: 2010-08-17
Publication date: 2012-06-27
Anticipated expiration: 2030-08-17
Also published as: US20110041513A1; JP2013502510A; EP2467858B1; CA2771669A1; BR112012003818A2; TW201113911A; CN102576587B; RU2012110126A; CN102576587A; KR20120054637A; WO2011020826A1; JP5887599B2

Abstract

The invention relates to polycrystalline magnetocaloric materials of the general formula Mn_aCo_bGe_cA_xwith A, B or C; 0 ≤ x ≤ 0.5; 0.9 ≤ a ≤ 1.1; 0.9 ≤ b ≤ 1.1; 0.9 ≤ c ≤ 1.0, wherein up to 30 mole % of Mn or Co may be replaced with Fe, Ni, Cr, V or Cu, or up to 30 mole % of Mn, Co or Ge may be replaced with vacancies, wherein phases of the orthorhombic TiNiSi structure type and of the hexagonal Ni₂ln structure type are present at temperatures below -40ºC.

Description

The invention relates to polycrystalline magnetocaloric materials, processes for their preparation and their use in coolers, heat exchangers or generators, in particular refrigerators.

Thermomagnetic materials, also referred to as magnetocaloric materials, can be used for cooling, for example, in refrigerators or air conditioners, in heat pumps, or for direct recovery of heat from power without the interposition of mechanical energy conversion.

Such materials are known in principle and described for example in WO 2004/068512. The magnetic cooling techniques are based on the magnetocaloric effect (MCE) and may be an alternative to the known steam-cycle cooling methods. In a material exhibiting a magnetocaloric effect, the alignment of randomly oriented magnetic moments with an external magnetic field results in heating of the material. This heat can be dissipated by the MCE material into the ambient atmosphere through a heat transfer. When the magnetic field is then turned off or removed, the magnetic moments revert to a random arrangement, causing the material to cool to below ambient temperature. This effect can be exploited for cooling purposes, see also Nature, Vol. 415, 10 January 2002, pages 150 to 152. Typically, a heat transfer medium such as water is used for heat removal from the magnetocaloric material.

The materials used in thermomagnetic generators are also based on the magnetocaloric effect. In a material exhibiting a magnetocaloric effect, the alignment of randomly oriented magnetic moments with an external magnetic field results in heating of the material. This heat can be dissipated from the MCE material into the ambient atmosphere by heat transfer. When the magnetic field is subsequently turned off or removed, the magnetic moments revert to a random arrangement, causing the material to cool to below ambient temperature. This effect can be exploited on the one hand for cooling purposes, on the other hand, to convert heat into electrical energy.

Magnetocaloric generation of electrical energy is associated with magnetic heating and cooling. In the times of the first conception that became

Process for energy production described as pyromagnetic energy production. Compared with Peltier or Seebeck type devices, these magnetocaloric devices can have significantly higher energy efficiency.

Research into this physical phenomenon began in the late 19th century when two scientists, Tesla and Edison, applied for a patent for pyromagnetic generators. In 1984, Kirol described numerous possible applications and performed thermodynamic analyzes of them. At that time gadolinium was considered as a potential material for applications near room temperature. A pyromagneto-electric generator is described, for example, by N. Tesla in US 428,057. It is stated that the magnetic properties of iron or other magnetic substances can be partially or completely destroyed or disappear by heating to a certain temperature. Upon cooling, the magnetic properties are restored and return to their original state. This effect can be exploited to generate electricity. When an electrical conductor is exposed to a varying magnetic field, changes in the magnetic field will induce an electrical current in the conductor. If, for example, the magnetic material is enclosed by a coil and then heated in a permanent magnetic field and subsequently cooled, an electrical current is induced in the coil in each case during the heating and cooling. As a result, heat energy can be converted into electrical energy without, in the meantime, converting into mechanical work. In the process described by Tesla, iron is heated as a magnetic substance via an oven or a closed hearth and subsequently cooled again.

For thermomagnetic or magnetocaloric applications, the material should permit efficient heat exchange in order to achieve high efficiencies. Both in cooling and in power generation, the thermo-magnetic material is used in a heat exchanger.

The object of the present invention is to provide magnetocaloric materials having a large magnetocaloric effect.

The object is achieved according to the invention by polycrystalline magnetocaloric materials of the general formula

Mn _a Co _b Ge _c A _x with

A; B or C, d. H. Boron or carbon

0 <x <0.5, 0.9 <a <1, 1

0.9 <b <1, 1

0.9 <c <1, 0 wherein up to 30 mol% of the Mn or Co may be replaced by Fe, Ni, Cr, V or Cu, or up to 30 mol% of the Mn, Co or Ge may be replaced by vacancies may, in which there are ₂ ln-type structure at a temperature below -40 ⁰ C phases of the orthorhombic structure TiNiSi type and the hexagonal Ni.

In one embodiment of the invention, 2.8 <a + b + c <3.2 or a + b + c = 3. A may be boron or carbon.

It has been found according to the invention that polycrystalline magnetocaloric materials in which both phases of the orthorhombic TiNiSi structure type and the hexagonal Ni _{2 In} structure type are present exhibit an unexpectedly high magnetocaloric effect. They are almost intrinsically two-phase magnetocaloric materials. Of the two phases mentioned, the polycrystalline magnetocaloric materials preferably contain at least 5% by weight, more preferably at least 10% by weight, in particular at least 15% by weight. In comparison to the materials according to the invention, those materials which have only one of the stated phases show only slight magnetocaloric effects. This is all the more surprising as it is usually assumed that single-phase materials have more favorable application properties. Two types of magnetocaloric materials exhibit this effect: MnCoGe-type materials which are not stoichiometric and show either vacancies in the Ge sublattice or Fe, Ni, Cr, V, or Cu substitutions in the Co sublattice. In addition, MnCoGe structures formed by boron as interstitial atoms, which are obtained by adding small amounts of boron to stoichiometric MnCoGe, show large magnetocaloric effects. The largest magnetocaloric effects are observed for interstitial alloys.

By adjusting the proportions of the phase transitions can be adjusted, which in turn the magnetic moments and the magnetocalorie effect can be adjusted. Above the Curie temperature, the materials are usually single-phase but below the Curie temperature they are bilateral. The intermetallic compound MnCoGe crystallizes in the orthorhombic TiNiSi structure type with a Curie temperature of 345 K. MnCoGe exhibits a typical second-order magnetic phase transition. Under a magnetic field change of 5 T, the isothermal magnetic entropy change of MnCoGe is about 5 J kg ^"1 K ^{" 1} . It would have been expected that replacing Co with other elements would lower both the magnetic moment and the Curie temperature. According to the invention, however, it has been found that the possible structural transition from the orthorhombic Ti N iS i structure to the hexagonal Ni _{2 In} structure type leads to large magnetocaloric effects in the compounds.

In the magnetocaloric materials of the invention is preferably 0.001 <x <0.1. Particularly preferably, x has the value 0.01 to 0.05.

Preferably, up to 25 mol% of Mn or Co is replaced as indicated, more preferably 1 to 20 mol%, especially 3 to 10 mol%.

The thermomagnetic materials used according to the invention can be prepared in any suitable manner.

The magnetocaloric materials of the present invention can be prepared by solid phase reaction or liquid phase reaction of the starting materials for the material, subsequent cooling, subsequent compression, sintering and annealing under an inert gas atmosphere followed by cooling to room temperature or by melt spinning a melt of the starting or starting alloys.

The preparation of the thermomagnetic materials, for example, by solid phase reaction of the starting elements or starting alloys for the material in a ball mill, subsequent compression, sintering and annealing under inert gas atmosphere and subsequent, z. Slow, cooling to room temperature. Such a method is described, for example, in J. Appl. Phys. 99, 2006, 08Q107.

Processing by melt spinning is also possible. As a result, a more homogeneous element distribution is possible, which leads to an improved magnetocaloric effect, see Rare Metals, Vol. 25, October 2006, pages 544 to 549. In the method described there, the output elements are induction-melted in an argon gas atmosphere and then in molten Condition sprayed via a nozzle on a rotating copper roller. This is followed by sintering at 1000 ^° C. and slow cooling to room temperature. Furthermore, reference can be made to WO 2004/068512 for the production.

Preference is therefore given to a process for the preparation of the thermomagnetic materials, comprising the following steps: a) reaction of chemical elements and / or alloys in a stoichiometry corresponding to the metal-based material, in the solid and / or liquid phase, b) optionally Transferring the reaction product from stage a) into a solid, c) sintering and / or annealing the solid from stage a) or b), d) quenching the sintered and / or tempered solid from stage c) with a cooling rate of at least 100 k / s.

Thermal hysteresis can be significantly reduced and a large magnetocaloric effect can be achieved if the metal-based materials are not slowly cooled to ambient temperature after sintering and / or annealing, but are quenched at a high cooling rate. The cooling rate is at least 100 K / s. The cooling rate is preferably 100 to 10,000 K / s, more preferably 200 to 1300 K / s. Especially preferred are cooling rates of 300 to 1000 K / s.

The quenching can be achieved by any suitable cooling method, for example by quenching the solid with water or aqueous liquids, such as cooled water or ice / water mixtures. The solids can be dropped, for example, in iced water. It is also possible to quench the solids with undercooled gases such as liquid nitrogen. Other quenching methods are known to those skilled in the art. The advantage here is a controlled and rapid cooling.

The rest of the production of the thermomagnetic materials is less critical, as long as the quenching of the sintered and / or tempered solid takes place in the last step with the cooling rate according to the invention. The method can be applied to the production of any suitable thermomagnetic materials for magnetic cooling, as described above. In step (a) of the process, the reaction of the elements and / or alloys contained in the later thermomagnetic material takes place in a stoichiometric amount. metric, which corresponds to the thermomagnetic material, in the solid or liquid phase.

The reaction in step a) is preferably carried out by heating the elements and / or alloys together in a closed container or in an extruder, or by solid-phase reaction in a ball mill. Particularly preferably, a solid phase reaction is carried out, which takes place in particular in a ball mill. Such an implementation is known in principle, compare the above-mentioned writings. In this case, powders of the individual elements or powders of alloys of two or more of the individual elements which are present in the later thermomagnetic material are typically mixed in powder form in suitable proportions by weight. If necessary, additional grinding of the mixture can be carried out to obtain a microcrystalline powder mixture. This powder mixture is preferably heated in a ball mill, which leads to a further reduction as well as good mixing and to a solid phase reaction in the powder mixture. Alternatively, the individual elements are mixed in the chosen stoichiometry as a powder and then melted.

The common heating in a closed container allows the fixation of volatile elements and the control of the stoichiometry. Especially with the use of phosphorus, this would easily evaporate in an open system.

The reaction is followed by sintering and / or tempering of the solid, wherein one or more intermediate steps may be provided. For example, the solid obtained in step a) may be subjected to shaping before it is sintered and / or tempered.

Alternatively, it is possible to feed the solid obtained from the ball mill to a melt spinning process. Melt spinning processes are known per se and described for example in Rare Metals, Vol. 25, October 2006, pages 544 to 549 as well as in WO 2004/068512.

In this case, the composition obtained in step a) is melted and sprayed onto a rotating cold metal roller. This spraying can be achieved by means of positive pressure in front of the spray nozzle or negative pressure behind the spray nozzle. Typically, a rotating copper drum or roller is used which, if desired, may be cooled. The copper drum preferably rotates at a surface speed of 10 to 40 m / s, in particular 20 to 30 m / s. On the copper drum, the liquid composition is cooled at a rate of preferably 10 ² to 10 ⁷ K / s, more preferably at a rate of at least 10 ⁴ K / s, in particular at a rate of 0.5 to 2 x 10 ⁶ K / s. The melt spinning can be carried out as well as the reaction in step a) under reduced pressure or under an inert gas atmosphere. The Meltspinning a high processing speed is achieved because the subsequent sintering and annealing can be shortened. Especially on an industrial scale so the production of thermomagnetic materials is much more economical. Spray drying also leads to a high processing speed. Particular preference is given to melt spinning (middle spinning).

Alternatively, in step b), a spray cooling may be carried out, in which a melt of the composition from step a) is sprayed into a spray tower. The spray tower can be additionally cooled, for example. In spray towers cooling rates in the range of 10 ³ to 10 ⁵ K / s, in particular about 10 ⁴ K / s are often achieved.

The sintering and / or tempering of the solid takes place in stage c), preferably first at a temperature in the range from 800 to 1400 ^° C. for sintering and subsequently at a temperature in the range from 500 to 750 ^° C. for tempering. By way of example, sintering can then take place at a temperature in the range from 500 to 800 ^° C. For moldings / solids sintering is particularly preferably carried out at a temperature in the range of 1000 to 1300 ⁰ C, in particular from 1100 to 1300 ⁰ C. The annealing can then be carried out for example at 600 to 700 ⁰ C. The sintering is preferably carried out for a period of 1 to 50 hours, more preferably 2 to 20 hours, especially 5 to 15 hours. The annealing is preferably carried out for a time in the range of 10 to 100 hours, particularly preferably 10 to 60 hours, in particular 30 to 50 hours. Depending on the material, the exact time periods can be adapted to the practical requirements.

When using the melt spinning method, the time for sintering or tempering can be greatly shortened, for example, for periods of 5 minutes to 5 hours, preferably 10 minutes to 1 hour. Compared to the usual values of 10 hours for sintering and 50 hours for annealing, this results in an extreme time advantage.

The sintering / tempering causes the grain boundaries to melt, so that the material continues to densify. By melting and rapid cooling in stage b), the time duration for stage c) can thus be considerably reduced. This also enables continuous production of the thermomagnetic materials. The magnetocaloric materials of the invention may be used in any suitable applications. For example, they are used in coolers, heat exchangers or generators. Particularly preferred is the use in refrigerators.

The invention will be explained in more detail by examples.

Examples

Polycrystalline MnCoGe-type samples were prepared by arc melting from stoichiometric amounts of the pure elements. In order to obtain a homogeneous phase, the molded samples for 5 days at 500 ⁰ C or 800 ⁰ C were annealed under an argon atmosphere of 500 mbar and then quenched in water at room temperature. The crystal structure was determined by X-ray scattering on a powder sample at room temperature. The DC magnetization was determined in a quantum design MPMS2-type squid magnetometer operating in fields of up to 5 T and in a temperature range of 5 to 400 K. Figure 1 shows the temperature dependence of the magnetization of MnCoGeo.gs, Mno.gFeo .-iCoGe and MnCoo.gCuo.-iGe, determined at a magnetic field of 0.1 T (square, circle or triangle). Only the middle sample was annealed. The values for the Curie temperature for MnCoGe ₀ , 98, Mn ₀ , 9Fe ₀ , iCoGe, and MnCoo.gCuo.-iGe are 325 K, 292 K, and 263 K. A thermal hysteresis is observed at the transition from the ferromagnetic to the paramagnetic state observed, corresponding to a first order magnetic transition.

Figure 2 shows X-ray structural patterns of MnCoGe ₀ , 98, Mn ₀ , 9 Fe ₀ , iCoGe and MnCoo.gCuo.-iGe determined at room temperature. For the sample whose critical temperature is well below room temperature, only the contribution of a single phase of the Ni _{2 In} type is observed since the measurement temperature is above the critical temperature. Intensity is plotted in arbitrary units.

Magnetic properties of non-stoichiometric MnCoGe compounds are summarized in Table 1 below. A strong increase in the magnetocaloric effect is observed with only slightly different magnetic moments. Table 1

The addition of numerous B atoms to the MnCoGe alloy results in a first order phase transition. X-ray diffractograms of compounds MnCoGeB _x with x = 0.01, 0.02 and 0.03 show next to a heat treatment at 500 ⁰ C the simultaneous existence of the hexagonal and orthorhombic structure. From the magnetization curves for MnCoGeB ₀ 0 ₂ , which was annealed at 500 ⁰ C, a clear thermal hysteresis emerges. The sample also shows a virgin effect. On first cooling and initial heating, the hysteresis is 32 K, but only 16 K in subsequent cooling and heating. Very large magnetocaloric effects are observed for different compositions. The largest value of 67.3 J kg ^"1 ^{K" 1} for a magnetic field change of 5T is observed for a sample with x = 0.01, and 3% voids in the Co content are set, and the sample annealed at 850 ⁰ C has been. Table 2 shows the changes in the ordering temperature (T _c), the thermal hysteresis (ΔThys), the change of the magnetic entropy (-ΔSm) and of the magnetic moment for MnCoGeB _x compounds indicated, which were annealed at 850 ⁰ C overall.

Table 2

Claims

claims

1. Polycrystalline magnetocaloric materials of the general formula

Mn _a Co _b Ge _c A _x with A; B or C 0 <x <0.5,

0.9 <a <1, 1

0.9 <b <1, 1

0.9 <c <1, 0 wherein up to 30 mol% of Mn or Co may be replaced by Fe, Ni, Cr, V or Cu or up to 30 mol% of Mn, Co or Ge may be replaced by vacancies in which there at a temperature below -40 ⁰ C phases of the orthorhombic structure TiNiSi type and the hexagonal Ni ₂ in-type structure.

2. Magnetocaloric materials according to claim 1, characterized in that 0.001 <x <0.1.

3. Magnetokalorische Materialie according to claim 2, characterized in that x has the value 0.01 to 0.05.

4. Magnetocaloric materials according to one of claims 1 to 3, characterized in that up to 25 mol% of Mn or Co can be replaced as indicated.

5. Magnetocaloric materials according to claim 4, characterized in that 1 to 20 mol%, preferably 3 to 10 mol% of Mn or Co are replaced as indicated.

6. A process for the preparation of the magnetocaloric materials according to any one of claims 1 to 5 by solid phase reaction or liquid phase reaction of the starting materials or starting alloys for the material, optionally cooling, subsequent compression, sintering and annealing under inert gas atmosphere and subsequent cooling to room temperature, or by melt spinning a Melting of the starting elements or starting alloys.

7. The method of claim 6, comprising the following steps: a) conversion of chemical elements and / or alloys in a stoichiometry corresponding to the metal-based material, in the solid and / or liquid phase, b) optionally converting the reaction product from step a c) sintering and / or tempering the solid from step a) or b), d) quenching the sintered and / or tempered solid from step c) at a cooling rate of at least 100 K / s.

8. Use of the magnetocaloric materials according to one of claims 1 to 5 in coolers, heat exchangers or generators.

9. Use according to claim 8 in refrigerators.