Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
  • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
  • H01F1/012—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials adapted for magnetic entropy change by magnetocaloric effect, e.g. used as magnetic refrigerating material
  • H01F1/015—Metals or alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/07—Alloys based on nickel or cobalt based on cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C22/00—Alloys based on manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C30/00—Alloys containing less than 50% by weight of each constituent
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
  • H01F1/01—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials
  • H01F1/012—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of inorganic materials adapted for magnetic entropy change by magnetocaloric effect, e.g. used as magnetic refrigerating material
  • H01F1/017—Compounds

Definitions

• 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.
• the magnetic cooling techniques are based on the magnetocaloric effect (MCE) and may be an alternative to the known steam-cycle cooling methods.
• MCE 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.
• 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.
• a heat transfer medium such as water is used for heat removal from the magnetocaloric material.
• thermomagnetic generators are also based on the magnetocaloric effect.
• 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.
• 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.
• heat energy can be converted into electrical energy without, in the meantime, converting into mechanical work.
• iron is heated as a magnetic substance via an oven or a closed hearth and subsequently cooled again.
• 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.
• A may be boron or carbon.
• 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.
• 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.
• 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.
• 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.
• magnetocaloric materials of the invention is preferably 0.001 ⁇ x ⁇ 0.1.
• x has the value 0.01 to 0.05.
• Mn or Co is replaced as indicated, more preferably 1 to 20 mol%, especially 3 to 10 mol%.
• 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.
• 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.
• a method is described, for example, in J. Appl. Phys. 99, 2006, 08Q107.
• thermomagnetic materials 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.
• thermomagnetic materials 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.
• 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.
• a solid phase reaction is carried out, which takes place in particular in a ball mill.
• 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.
• 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.
• reaction is followed by sintering and / or tempering of the solid, wherein one or more intermediate steps may be provided.
• the solid obtained in step a) may be subjected to shaping before it is sintered and / or tempered.
• 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.
• 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.
• 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.
• 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).
• 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.
• 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.
• 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.
• 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.
• Polycrystalline MnCoGe-type samples were prepared by arc melting from stoichiometric amounts of the pure elements.
• 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.

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• 2010
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