JP2013149883A - Production method for large magnetocaloric material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method for large magnetocaloric material, and to provide a large magnetocaloric material produced by this method.SOLUTION: The production method for large magnetocaloric material where ferromagnetism and feeble magnetism coexist in alternate atomic layers or at clear positions of whole material comprises: (a) a step of selecting 50 atom% or more of at least one kind of magnetic ions from Cr and Mn, Fe, Co, Ni; (b) a step of selecting less than 50 atom% of at least one kind of stabilization chemical element which is selected from P, As, Sb, Bi, Si, Ge, Sn, B, Al, Ga, In, Se; (c) a step of performing electronic structural calculation of a material thus selected; and (d) a step of determining whether or not a ferromagnetic ion which loses the magnetic order only at the Curie temperature and a feeble magnetic or meta-magnetic ion which loses at least 80% of magnetic moment at a temperature higher than the Curie temperature coexist in the alternate atomic layers of the material or on the clear positions of the whole material.

Description

  The present invention relates to a method for producing a large magnetothermal material, a large magnetothermal material obtained by this method, and its use in a magnetothermal heat pump, magnetothermal power converter, actuator or magnetic switch.

  Due to the limited resources and better prosperity, efficient use of energy is required. The UN Advisory Group on Energy and Climate Change recommends the goal of improving utilization efficiency by 40% by 2030. Material research is extremely important to reach this goal. Magnetic refrigeration has the potential to achieve 50% higher energy efficiency than vapor compression refrigeration. For this reason, magnetic refrigeration has become a technology that attracts attention in recent years. Magnetic refrigeration is due to the magnetocaloric effect (MCE). When an external magnetic field is applied or removed, the solid magnetic material is heated or cooled by the entropy movement ΔS between the magnetic system and the crystal lattice. In the opposite process, heat is converted to electricity by a thermomagnetic generator. Kirol, L.M. D. and J. et al. I. Mills, “Numerical analysis of thermomagnetic generators”, J. Am. Appl. Phys. 56, 824-828 (1984). This device can be used to generate electricity from "exhaust heat", that is, heat that is released into the environment without currently being used. Therefore, an efficient magnetothermal material contributes to a significant reduction in energy consumption.

  Efficient coupling between lattice and spin degrees of freedom in magnetic materials can be used for refrigeration and energy conversion. This coupling is large for materials exhibiting a large magnetothermal effect.

After Gd ₅ (Si, Ge) ₄ was found to have a large MCE, magnetothermal materials exhibiting a number of first order magnetic phase transitions (FOMT) were eagerly sought. In these materials, this FOMT enhances the magnetothermal effect near the magnetic phase transition. Thus, the maximum isothermal entropy change is often much greater than the value of the reference material Gd that exhibits a second order magnetic phase transition. Combining materials with different _Tc in series resulted in higher efficiencies and greater temperature spans than Gd. For optimal performance, materials used in such composite heat storage materials need to have very similar magnetothermal properties in order to exhibit a constant entropy change as a function of temperature. Rowe, A.M. & Tura, A.A. “Trial study of three-layer laminated active magnetic regenerator”, Int. J. et al. See Refrigeration 29, 1286-1293 (2006). The large thermal hysteresis often found in FOMT is a major obstacle to use in the refrigeration cycle. Thermal hysteresis and magnetic field hysteresis are inherent properties of primary materials. However, the magnitude of this hysteresis can be highly dependent on the microstructure or strain of the system.

  Therefore, there is still a search for magnetothermal materials that still exhibit large magnetothermal effects and that can be used in magnetothermal heat pumps, magnetothermal power converters, actuators or magnetic switches.

Kirol, L.M. D. and J. et al. I. Mills, “Numerical analysis of thermomagnetic generators”, J. Am. Appl. Phys. 56, 824-828 (1984). Rowe, A.M. & Tura, A.A. “Trial study of three-layer laminated active magnetic regenerator”, Int. J. et al. Refrigeration 29, 1286-1293 (2006)

  The objective of this invention is providing the manufacturing method of the large magnetothermal material which can be preferably used for the said use.

An object of the present invention is a method for producing a large magnetocaloric material in which strong and weak magnetism coexist in alternating atomic layers or in a clear position of the entire material,
a) selecting at least one magnetic ion selected from the group consisting of Cr and Mn, Fe, Co, Ni in an amount exceeding 50 atomic%;
b) selecting at least one stabilizing chemical element selected from the group consisting of P, As, Sb, Bi, Si, Ge, Sn, B, Al, Ga, In, and Se in an amount of less than 50 atomic% ( The total atomic% of magnetic ions and stabilizing chemical elements is 100 atomic%),
c) performing an electronic structure calculation of the selected material;
d) Ferromagnetic ions that lose their magnetic order only at the Curie temperature in alternating atomic layers of the material or on well-defined positions of the whole material and weak magnetism or metamagnetism that loses at least 80% of the magnetic moment above the Curie temperature This is achieved by a production method comprising the step of determining whether or not ions coexist.

  It is to be noted that a large magnetothermal effect can be obtained by the coexistence of these two ion species.

  This object is further achieved with a large magnetocaloric material produced by the above method.

  This object is also achieved by a large magnetocaloric material in which strong and weak magnetism coexist in alternating atomic layers or at well-defined positions throughout the material.

  This object is also achieved by the use of these large magnetothermal materials in magnetothermal heat pumps, magnetothermal power converters, actuators or magnetic switches.

  A corresponding paper by the inventor on mixed magnetism for refrigeration and energy conversion is described in Adv. Energy Mater. 2011, 1, 1215-1219.

  According to the present invention, it has been clarified that a large magnetothermal material material in which strong magnetism and weak magnetism coexist in alternating atomic layers or in a clear position of the entire material has a large magnetothermal effect.

  This is particularly true for materials in which there is at least one magnetic ion selected from the group consisting of Cr and Mn, Fe, Co, and Ni. This first type of magnetic ion is present in the magnetothermal material in an amount greater than 50 atomic percent.

  The stabilizing chemical element is selected from the group consisting of P and As, Sb, Bi, Si, Ge, Sn, B, Al, Ga, In, and Se. These stabilizing chemical elements are present in the magnetothermal material in an amount of less than 50 atomic percent.

  In any case, the total atomic% of magnetic ions and stabilizing chemical elements is 100 atomic%.

  The electronic structure calculation is preferably a band structure calculation.

  This electronic structure calculation is performed as described below.

  Commercially available software can be used for the electronic structure calculation. A typical software program is Wien2k.

  This electronic structure calculation gives a ferromagnetic ion that loses its magnetic order only at the Curie temperature in alternating atomic layers of the material or on well-defined positions of the whole material and a magnetic moment of at least 80% above the Curie temperature. It can be determined whether there is a weak magnetic or metamagnetic ion to lose.

  If this last condition is satisfied, the material exhibits a large magnetothermal effect.

FIG. 1 shows the local density of states of manganese (top) and iron (bottom) in a ferromagnetic configuration. FIG. 2 shows the local density of states of manganese (top) and iron (bottom) in antiferromagnetic configuration in the z direction. 3, some typical _{Mn x Fe 1.95-x P 1} -y Si y compound 0~1T under (open symbols curve) and 0~2T lower (closed symbols of the magnetic field change in the magnetic field change in Curve) shows the isothermal magnetic entropy change. Include Gd metal data for comparison.

  In certain embodiments of the invention, this distinct position is distinguished by its symmetry and arrangement. That is, they have different symmetry and arrangement.

  Preferably these distinct positions are the tetrahedral and octahedral positions of the crystal lattice.

  A preferred large magnetocaloric material exhibits a yarn-teller effect.

  The change in the band structure can be detected usually by resonant spin-polarized photoemission.

  The method for producing a large magnetothermal material can also be regarded as a method for determining or discovering a large magnetothermal material. In step d), the requirements for the large magnetothermal effect are determined, investigated and confirmed.

  In step a), at least one magnetic ion is selected, preferably one or two of the above metals. It is particularly preferred to select two of the above metals.

  In step b), at least one stabilizing chemical element is selected. Preferably one or two stabilizing chemical elements are selected.

In step c) “electronic structure calculation” is understood in a broad sense. This electronic structure calculation is performed with standard software, eg Wien2k, or Phys. Rev. B 41, 5613 to 5626 (1990), J. MoI. Phys, C: Solid State Phys. 4, 2064 to 2083 (1971), J. MoI. Appl. Crystallogr. 41, 653 to 658 (2008) and Phys. Rev. B 54, 11169 to 11186 (1996).

  In step d), ferromagnetic ions and weak magnetic or metamagnetic ions are detected by analyzing the local density of states of the ferromagnetic and antiferromagnetic configurations as shown in FIGS.

  “Ferromagnetic” means that only at the Curie temperature (ferromagnetic) ions lose their magnetic order. “Weak magnetism” means that an ion (weak magnetism or metamagnetism) loses at least 80% of its magnetic moment at temperatures above the Curie temperature. Therefore, substantially all local magnetic moments disappear at the Curie temperature.

“Large magnetothermal material” refers to a material that exhibits a large magnetothermal effect of the type first found in Gd ₅ (Si, Ge) ₄ . Phys. Rev. Lett. 78, 4494-4497 (1997).

  “Alternate atomic layer” refers to an atomic layer arrangement in which ferromagnetic ions occupy every two layers and weak magnetic ions occupy every other two layers.

  “A clear position of all materials” means that a specific position in the crystal lattice is occupied by ferromagnetic ions and a specific position is occupied by weak magnetic ions. For example, this distinct position is distinguished by its symmetry and arrangement, such as the tetrahedral position and octahedral position of the crystal lattice.

  Steps a) and b) are first performed in order to make a new large magnetocaloric material. The electronic structure calculation of step c) is then carried out and the results are analyzed for ferromagnetism and weak magnetism in alternating atomic layers. If ferromagnetism and weak magnetism are observed in such alternating atomic layers in step d), the material is regarded as a large magnetothermal material in the sense of the present invention.

  If in step d) there is no alternating atomic layer with ferromagnetism and weak magnetism or a clear position in the whole material, the material is not considered a large magnetothermal material.

  Thus, the method of the present invention allows the identification and generation of large magnetocaloric materials. The change in the band structure can be detected by a physical method such as resonant spin-polarized photoemission.

  In the first-principles electronic structure calculation, for example, a calculation related to hexagonal MnFe (P, Si) shows an unknown magnetic form, that is, coexistence of ferromagnetism and weak magnetism in alternating atomic layers. The weak magnetism of Fe layer (disappearance of local magnetic moment at Curie temperature) brings about strong bond with crystal lattice, and the ferromagnetism of adjacent Mn layer keeps Curie temperature high enough at room temperature or higher. Can be operated. By changing the composition of these magnetic sublattices, it is possible to finely adjust the operating temperature and strongly suppress undesired thermal hysteresis. In this way, new materials can be designed from widely available elements that have properties that meet the requirements of efficient refrigeration or energy conversion cycles.

  Here, we report a novel mechanism in which the coexistence of weak magnetism and ferromagnetism on adjacent lattice planes induces a large magnetothermal effect. By this mechanism, exceptionally excellent magnetothermal performance can be obtained, for example, in a wide region of Mn—Fe—P—Si system.

When more than 10% of P is substituted with Si in Fe ₂ P, the hexagonal crystal lattice is converted to an orthorhombic crystal lattice. MnFeP _0.5 Si _0.5 with FOMT crystallizes in a hexagonal Fe ₂ P structure with four distinct lattice positions near room temperature, but the thermal hysteresis ΔT _{hys of} 35K makes it difficult to use. Cam Thanh, D.C. T. T. et al. Brook, E .; Trung, N .; T. T. et al. , Klaasse, J .; C. P. Buschuw, K .; H. J. et al. , Ou, Z .; Q. Tegus, O .; & Caron, L, “Structure, magnetism and magnetothermal properties of MnFePt _1-x Si _x compounds”, J. et al. Appl. Phys. 103,07B318 (2008). In contrast to many other magnetothermal materials, the volume change in this material is rather small, and mainly changes in the c / a ratio of the hexagonal lattice are observed. In order to elucidate the cause of the observed magnetothermal effect, electronic structure calculations are performed for the ground state of ferromagnetism, and the Curie temperature behavior is doubled for the unit cell (allowing antiferromagnetic configuration) Model with the resulting superlattice. These calculations show that the layers occupied by manganese are ferromagnetic and that the magnetic order is lost only at the Curie temperature. The size of the Mn moment decreases from 2.8Myu _B in a ferromagnetic phase 2.6Myu _B in paramagnetic phase. The iron layer exhibits weak itinerant magnetism. Fe moment in the phase of the ferromagnetic are 1.54μ _B, which is lost in the paramagnetic phase (± 0.003μ _B). The nonbonding electron density at the Fe position at temperatures below _Tc shifts to a strongly hybridized distribution with Si / P at temperatures above _Tc . The disappearance of the moment at the iron position is also evident from the partial density of states as a function of energy. In sharp contrast to manganese, above the Curie temperature, it shows the same polarity in the two spin directions.

Such a combination of ferromagnetism and weak magnetism in one compound is unexpected. Weak magnetism is rare; it is found in materials such as ZrZn ₂ or Ni ₃ Al. The Curie temperature is low (eg, ZrZn ₂ : 33K; Ni ₃ Al: 23-58K, depending on the composition). Mixed magnetism is directly related to the large magnetothermal effect because the presence of magnetic moments competes with chemical bonds in solids. This can best be explained for a half-filled d-shell. Non-magnetic allows maximum chemical bonding (as with all half-filled shells), but in high spin states, the majority and minority subbands are either completely filled or empty, so bonding Not shown. When the Curie temperature is exceeded due to the disappearance of the magnetic moment of iron, strong coupling to the lattice becomes possible, and the c / a ratio becomes discontinuous, resulting in FOMT. On the other hand, the ferromagnetism of the manganese layer can have a Curie temperature near room temperature.

Experimentally, we investigated the effect of changing the element at the lattice position. Both the Fe substitution into the Mn sublattice and the Mn substitution into the Fe sublattice, and the increased Si content, lead to a decrease in ΔT _hys . When the Mn content increases, T _c decreases, and when the Fe content increases, T _c increases. From this tendency, it was considered that a large ΔS _m coupled with a small ΔThy can be obtained by balancing the Mn: Fe ratio and the P: Si ratio. Also, in order to maintain both a large ΔS _m and a small ΔT _hys , T _c can be adjusted by simultaneously changing both the Mn: Fe ratio and the P: Si ratio. This trend also applies to somewhat non-stoichiometric compounds. _{Mn x Fe 1.95-x P 1} -y Si y compound in Mn: Fe ratio and P: changing Si ratio at the same time, the operating temperature can be controlled between 210 and 430K (respectively, x = 1.35, y = 0.46 and x = 0.66, y = 0.42), but this transition was steep and ΔT _hys remained small (1-1.5K).

FIG. 3 shows the entropy change as a function of temperature obtained from the magnetic isothermal curve from Maxwell's relational expression. Under a magnetic field change of 0 to 2T, the absolute value of ΔS _m approaches 18 Jkg ⁻¹ K ⁻¹ at both 215K and 350K. These peak values are rather stable (between 12.8-18.3 Jkg ^-1 K ^-1 ) over the entire temperature range of 220-380K. At feasible temperatures, these values are about 4 times greater than Gd (see data in FIG. 3). In order to obtain the same effect, it should be noted that MnFe (P, As) requires a magnetic field change larger than twice, that is, 0 to 5T. Thus, this material allows much cheaper magnets to be used for the magnetothermal refrigerator. Since a large effect can be obtained with a wide range of compositions, a large MCE can be obtained in a wide temperature range by changing various alloys having slightly different compositions in the active magnetic regenerator material.

Since this effect is great even at temperatures exceeding the boiling point of water, these materials can be used in magnetothermal power generators that convert a large amount of exhaust heat into electricity. In order to utilize the entire temperature range from room temperature to the temperature of the heat source, the generator needs to contain a variety of different materials. Even with a magnetic field change of 0 to 1T, the observed entropy change is rather large, so this simple generator operates efficiently. This is due to the magnetic field induced transition with very small magnetic hysteresis that occurs in these compounds at low magnetic fields. This very small magnetic hysteresis is consistent with the observed small ΔT _hys , indicating a low energy barrier to FOMT nucleation.

The coexistence of weak magnetism and ferromagnetism in a single material has the potential to effectively couple spin and lattice degrees of freedom. In this way, a large magnetothermal effect and a small thermal hysteresis were simultaneously achieved in a Mn—Fe—P—Si compound having a hexagonal Fe ₂ P structure by changing the Mn: Fe ratio and the P: Si ratio. At the same time, the operating temperature can be controlled to 210-430K by changing the Mn: Fe ratio and the P: Si ratio. By combining many alloys having slightly different compositions in a single active magnetic regenerator, efficient magnetic refrigeration is possible over a wide temperature range. In particular, a Mn—Fe—P—Si compound can be produced by a non-toxic element present on the earth in large quantities, industrially mass-produced raw materials, or a simple powder metallurgy production method. The discovery of these high-performance, low-cost magnetic coolants opens up new avenues for the practical application of magnetic refrigeration and magnetothermal power generation. In addition, the discovery of the importance of the coexistence of ferromagnetism and weak magnetism makes it possible to search for new magnetothermal materials.

  The following examples further illustrate the present invention.

  First-principles electronic structure calculations were performed using the Local Spherical Wave Method (LSW) and the Abinitio Total Energy Molecular Dynamics Program (VASP) described below.

Example 1 Samples of adjusting Mn-Fe-P-Si compound, Mn (99.9%) chip and Si (99.999%) chips and binary compounds Fe ₂ P (99.5%) and red phosphorus A starting material consisting of (99.7%) powder was prepared by performing a solid-phase reaction if it was crushed with a ball mill. The powder obtained after crushing for 10 hours was pressed into tablets. The tablet was enclosed in a quartz ampule under Ar, fired at 1373K for 2 hours, heat treated at 1123K for 20 hours, and slowly cooled to room temperature in a furnace. The powder X-ray diffraction pattern was obtained on a PAN analysis X part pro diffractometer equipped with a second flat crystal monochromator and X ′ serator RTMS detector system using CuKα rays. Magnetic measurements were made using the RSO mode in a SQUID (superconducting quantum interference) magnetometer (Quantum Design MPMS-5XL). When measuring a Gd sample (3N Alfa Aesar) placed in the same manner as the other samples above, a thermal delay of about 0.4 K was observed with a temperature change of 1 K / min near room temperature. .

Example 2 Band Structure Abinitio band structure calculations were performed according to van Leuken, HM Rodder, A. et al. , Czyzyk, M.M. T. T. et al. Springelkamp, F .; & De Groot, R.D. A. “Ab initio electronic structure calculation of Nb / Zr multilayer system using scalar relativistic Hamiltonian”, Phys. Rev. B. 41, 5613-5626 (1990), and performed using the local spherical wave method (LSW). In the space filling model, Hedin, L & Lundqvist, B.M. I. , “A clear local exchange correlation potential”, J. Am. Phys. C: Solid State Phys. 4, 2064-2083 (1971), using the local density exchange correlation potential, and the sphere overlap around the atomic components. When the local partial charge change in the atomic sphere was smaller than 10 ⁻⁵ , it was considered self-consistent. In constructing the LSW base, the spherical wave is added with the solution of scalar relativity calculation indicated by atomic symbols 4s, 4p and 3d for transition metals, and 3s and 3p for silicon and phosphorus, and the internal angle Momentum was added to compensate for the Hankel function at adjacent positions including 4f electrons for transition metals and 3d states for silicon and phosphorus. In the simulation calculation at a temperature exceeding the Curie temperature, a supercell in the xy and z directions and having a starting magnetic configuration of antiferromagnetism (z) and ferrimagnetism (xy) was used. The magnitude of the initial moment is obtained by calculation in the case of ferromagnetism. The density of states with spin polarization in the first case is shown in FIGS. A small amount of mixing prevented the ferromagnetic or ferrimagnetic starting configuration from falling to the ground state.

  The change in electron density was visualized by inputting VASP data using the program VESTA. Momma, K .; & Izumi, F.A. , “Three-dimensional visualization system for electronic structure analysis”, J. Appl. Crystallogr. 41, 653-658 (2008) and Kresse, GM FurthmuHer, J. “Ab initio total energy calculation using plane wave foundation set Efficient Iterative Method for ", Phys. Rev. B 54, 11169-11186 (1996).

Phase transition and hysteresis The effect of changing the lattice position element can best be explained as a graph. When the hexagonal Fe ₂ P alloy MnFe (P, Si) was produced, almost complete occupation of Fe at the tetrahedron 3f position and occupation of Mn at the pyramid 3g position were observed. Both Fe substitution into the Mn sublattice and Mn substitution into the Fe sublattice were performed. Surprisingly, T _c decreases if the Mn content increases, T _c is increased by increasing the Fe content (Figure 4). Si and P occupy the 1b and 2c positions almost randomly, increasing the Si content causes an increase in the deviation of T _c from the complete occupation of the Mn and Fe sublattices, and increasing the Si content causes a decrease in ΔT _hys . Brought.

Claims (8)

A method for producing a large magnetocaloric material in which strong magnetism and weak magnetism coexist in alternating atomic layers or in a clear position of the whole material,
a) selecting at least one magnetic ion selected from the group consisting of Cr and Mn, Fe, Co, Ni in an amount exceeding 50 atomic%;
b) selecting at least one stabilizing chemical element selected from the group consisting of P, As, Sb, Bi, Si, Ge, Sn, B, Al, Ga, In, and Se in an amount of less than 50 atomic% ( Note that the total atomic% of magnetic ions and stabilizing chemical elements is 100 atomic%)
c) performing an electronic structure calculation of the selected material;
d) Ferromagnetic ions that lose their magnetic order only at the Curie temperature, in alternating atomic layers of the material or on well-defined positions of the whole material, and weak magnetism or meta that lose at least 80% of the magnetic moment above the Curie temperature. A production method comprising a step of determining whether magnetic ions coexist.   The method according to claim 1, wherein the distinct position is distinguishable by its symmetry and arrangement.   The method according to claim 1 or 2, wherein the distinct positions are a tetrahedral position and an octahedral position of the crystal lattice.   The method according to claim 1, wherein the large magnetothermal material exhibits a yarn-teller effect.   The method according to claim 1, wherein the change in the band structure can be detected by resonant spin-polarized photoemission.   A large magnetocaloric material produced by the method according to claim 1.   A large magnetocaloric material that exhibits the coexistence of ferromagnetism and weak magnetism in alternating atomic layers or at distinct locations in all materials.   A magnetothermal heat pump, a magnetothermal power converter, an actuator or a magnetic switch comprising the large magnetothermal material according to claim 6.

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