Method for producing a working component for magnetic heat exchange

Abstract

A method for producing a working component for a magnetic heat exchanger, comprising: Selecting for each of the two or more sections of the working component of amounts of La, Fe, M and of one or more of the elements of T and R which are suitable to form a La 1-a R a (Fe 1-x-y T y M x ) 13 H z-phase, where T is one or more of the elements from the group consisting of Mn, Co, Ni and Cr, M is silicon and optionally aluminum, and R is one or more of the elements from the group consisting of Ce, Nd, Y and Pr, where the amount of silicon is within the range of 0.05 ≤ x ≤ 0.2 and the amount of one or more of the elements R and T is selected within the ranges 0 ≤ a ≤ 0.5 and 0.003 ≤ y ≤ 0.2, where the amount of the one or more elements of T and R and the amount of Si are selected for each of the two or more sections to provide the two or more sections with different Curie temperatures and similar sintering activities, Mixing the amount of the selected elements of T, R and M with La, Fe or alloys thereof in amounts suitable to produce the La 1-a R a (Fe 1-x-y T y M x ) 13 H z-phase to produce two or more precursor powder mixtures, Producing a green body from the two or more precursor powder mixtures, Heat treatment of the green body at a temperature T sinter and producing a working component having two or more sintered sections, each having a different T c and a density, d, within a range of ±5% or ±2% of the mean density, d av , a total number of sintered sections, wherein a single monolithic green body is formed from two or more precursor powder mixtures and heat treated to form a single monolithic working component having two or more sintered sections.

Description

[0001] The present application relates to a method for producing a working component for magnetic heat exchange.

[0002] Practical magnetic heat exchangers, such as those disclosed in US Pat. No. 6,676,772 B2, may comprise a recirculation pumping system, a heat exchange medium and a cooling fluid, a chamber packed with particles of a working material exhibiting the magnetocaloric effect, and a means for applying a magnetic field to the chamber. The working material may be referred to as magnetocalorically active.

[0003] The magnetocaloric effect describes the adiabatic conversion of a magnetically induced entropy change to heat generation or heat absorption. Therefore, applying a magnetic field to a magnetocalorically active material can induce an entropy change that results in heat generation or heat absorption. This effect can be used to provide cooling and / or heating.

[0004] US 2010 / 0 047 527 A1, DE 11 2009 000 060 T5, DE 11 2008 003 830 T5, DE 11 2007 003 321 T5 and US 7 063 754 B2 each disclose materials that exhibit the magnetocaloric effect.

[0005] Magnetic heat exchangers are, in principle, more energy-efficient than gas compression / expansion cycle systems. They are also considered more environmentally friendly than chemicals such as chlorofluorocarbons (CFCs), which are believed to contribute to ozone depletion and should not be used.

[0006] In practice, a magnetic heat exchanger requires magnetocalorically active material that exhibits several different magnetic phase transition temperatures to provide cooling over a broad temperature range. In addition to a variety of magnetic phase transition temperatures, a practical working fluid should also exhibit a large entropy change to provide efficient cooling and / or heating.

[0007] A variety of magnetocalorically active phases are known that have magnetic phase transition temperatures in a range available for providing domestic and commercial air conditioning and refrigeration equipment. One such magnetocalorically active material, disclosed, for example, in US Pat. No. 7,063,754 B2, has a NaZn 13 -type crystal structure and can be represented by a general formula La (Fe 1-x-y T y M x ) 13 H2, where M is at least one element from the group consisting of Si and Al, and T can be one or more of the transition metal elements, such as Co, Ni, Mn, and Cr. The magnetic phase transition temperature of this material can be adjusted by adjusting the composition.

[0008] Consequently, magnetic heat exchanger systems are being developed to practically realize the potential benefits provided by these magnetocalorically active materials. However, further improvements are desirable to enable more extensive applications of magnetic heat exchange technology.

[0009] According to the invention, a method for producing a working component for magnetic heat exchange is provided, the method comprising a selection, for each of two or more sections of the working component, of amounts of La, Fe, M and one or more elements T and R which are suitable for producing a La 1-a R a (Fe 1-x-y T y M x ) 13 H z-phase. T is one or more of the elements from the group consisting of Mn, Co, Ni and Cr, M is silicon and optionally also aluminum and R is one or more of the elements from the group consisting of Ce, Nd, Y and Pr, wherein the amount of silicon is within the range of 0.05 ≤ x ≤ 0.2 and the amount of one or more of the elements R and T is selected within the ranges 0 ≤ a ≤ 0.5 and 0.003 ≤ y ≤ 0.2. The amount of the one or more elements T and R and the amount of Si is selected for each of the two or more sections in order to provide two or more sections with different Curie temperatures and similar sintering activities. The amount of the selected elements T and R are mixed with La, Fe and M or alloys thereof in amounts suitable to produce the La 1-a R a (Fe 1-x-y T y M x ) 13 H z-phase to produce two or more precursor powder mixtures. A green body is produced from the two or more precursor powder mixtures and at a temperature T sinter heat treated to produce a working component having two or more sintered sections. Each of the two or more sintered sections has a different T c and a density d within a range of ± 5% or ± 2% of an average density d av the total number of sintered sections. A single monolithic green body is formed from two or more precursor powder mixtures and heat-treated to form a single monolithic working component having two or more sintered sections.

[0010] According to this method, the amounts of the elementary components of the La 1-a R a (Fe 1-x-y T y M x ) 13 H z-Phase selected for each of the two or more sections of the working component, so that each section has a different T c but has a similar sintering activity, so that the two or more sections of the working component have a similar density and in particular a density, d, within a range of ± 5% or ± 2% of an average density, d av , of a total number of sintered sections. The T c the sections may differ from each other by, for example, one or more centigrades Celsius.

[0011] The similar sintering degree activities allow the two or more sintered sections of the working component to be produced by a heat treatment at the same temperature, namely at the same T sinterTherefore, despite the different compositions, mechanically stable sintered bodies can be produced at the same temperature. At the same time, the two or more sections can have different Curie temperatures, allowing the production of a working component that can be used for magnetic heat exchange over a wider temperature range.

[0012] In a non-inventive embodiment, a separate green body is formed from each of the two or more precursor powder mixtures, so that the working component has two or more separate, sintered sections. In this embodiment, two or more green bodies are formed, each having a different composition according to the two or more precursor powder mixtures. These two or more separate green bodies can be sintered at the same sintering temperature T sinterheated to produce two or more separate sintered sections which, when placed together, can form the working component.

[0013] By heat-treating two or more separate green bodies together at the same temperature, production time can be reduced compared to a process in which the two or more green bodies are heated sequentially. By heat-treating two or more green bodies together, costs can be saved compared to a process in which each green body is heated at the same time but in a separate furnace set to a different temperature.

[0014] According to the invention, a single, monolithic, green body is formed from two or more precursor powder mixtures and is heated at a temperature T sinterheat-treated to produce a single monolithic working component having two or more sintered sections. In this context, the term monolithic is used to describe a single article having integral sections of different composition and different Curie temperatures. However, the different sections are macroscopically large and are arranged in the single monolithic working component in such a way that the T c in one direction of the work component. Excluded from this definition of the monolithic work component is a multiphase material in which a single plane of an object has a plurality of phases that have different T c Each section can have at least one dimension greater than 5 mm.

[0015] In one embodiment, the two or more sections of the working component have different silicon contents. The different silicon contents can be used to adjust the sintering activity of the two or more sections so that the sintered sections of the working component have a similar density, as described above.

[0016] The two or more sections of the working component can also have different values ​​of a and y. The amount of elements a and y can be selected to determine the Curie temperature of the two or more sections. Therefore, the two or more sections have different elements T and / or R and / or values ​​of a and y. For example, replacing the elements Nd, Pr and / or Ce for La and / or Mn, Cr, V and Ti for Fe leads to a reduction in the Curie temperature. The Curie temperature can also be increased by replacing Fe with Co and / or Ni.

[0017] Different values ​​of a and y for a particular element can result in different sintering activities. In this case, the silicon content, x, can be adjusted to make the sintering activities of the sections more similar, so that the sintered sections have a density as required above.

[0018] In addition to silicon, aluminum may also be present.

[0019] In one embodiment, the element T is Mn. Increasing the Mn content results in a decrease in T c and increasing the density in the working component for a given silicon content. Therefore, for an increasing Mn content, the silicon content is increased. In one embodiment, the amount of manganese Mn m to set the desired Curie temperature T c to produce, according to T c (°C) = 80.672 - 26.957 × Mn m selected, where Mn m is the metallic weight fraction of manganese.

[0020] As used herein, the subscript m is used to denote the metallic weight fraction. The metallic weight fraction is defined herein as the result of a calculation that separates and removes the rare earth content, RE, bound in the form of RE oxides and RE nitrides, from the total RE composition according to the following formulas for RE = La: La2O3=6.79*O LaN=10.9*N f=100100−La2O3−LaN

[0021] Consequently, Lam=(La−5.8*O−9.9*N)*f Sim=Si*f Com=Co*f Mnm=Mn*f where the index m denotes the metallic weight fraction and La, O, N, Si, Co and Mn etc. indicate the percentage weight fraction of these elements.

[0022] In a first approximation, the metallic RE content can also be calculated for La-rich alloys as: REm=(RE−5.8*O−9.9*N)×100100−6.8*O−10.9*N

[0023] For Si, Co, Mn, etc., the metallic contents are close to the total content, with the factor f being approximately 1.02. However, for the Re element, there is a larger difference. For example, in embodiments described here, a content of approximately 18 wt.% La is used to produce a metallic content of 16.7 wt.%, which corresponds to the stoichiometry of the 1:13 phase.

[0024] In one embodiment, T is Mn and optionally Co and the amount of silicon is determined according to Si m = 3.85 - 0.0573 × Co m - 0.045 × Mn m 2 + 0.2965 × Mn m selected, where Si m the metallic weight fraction of silicon, Mn m the metallic weight fraction of manganese, Co mthe metallic weight fraction of cobalt is to provide two or more sections comprising the element Mn and optionally cobalt as a substituting element, which have a density d within a range of ± 5% or within ± 2% of the average density d av the total number of sintered sections.

[0025] The sintered sections include a silicon content Si, Si act , which in one embodiment is within ± 5% of Si m or within ± 2% of Si m lies.

[0026] The La 1-a R a (Fe 1-x-y T y Si x ) 13 H z -Phase has a NaZn 13 -type structure and is magnetocalorically active. If hydrogen is present, it is interstitially trapped within the NaZn 13 -structure installed.

[0027] A magnetocalorically active material is defined herein as a material that undergoes a change in entropy when exposed to a magnetic field. The entropy change can be a result of a change from ferromagnetic to paramagnetic behavior, for example. The magnetocalorically active material can exhibit an inflection point in only a portion of a temperature range, where the sign of the second derivative of the magnetization with respect to an applied magnetic field changes from positive to negative.

[0028] A magnetocalorically passive material is defined herein as a material that shows no significant change in entropy when exposed to a magnetic field.

[0029] A magnetic phase transition temperature is defined herein as a transition from one magnetic state to another. Some magnetocalorically active phases exhibit a transition from antiferromagnetic to ferromagnetic, which is associated with a change in entropy. Magnetocalorically active phases such as La 1-a R a (Fe 1-x-y T y M x ) 13 H z exhibit a transition from ferromagnetic to paramagnetic, which is associated with a change in entropy. For these materials, the magnetic transition temperature can also be referred to as the Curie temperature.

[0030] As described above, the Curie temperature of the working component can be adjusted by adjusting the amount of the substituting elements R and T. In one embodiment, T is Mn and the Curie temperature T c of the working component is within ± 10 K of the value of the Curie temperature T c(calc), derived from the relationship T c(calc) (°C) = 80.672 - 26.957 × Mn m , where Mn m is the metallic weight fraction of manganese. In a further embodiment, T c within ± 5 K of T c(calc) .

[0031] In further embodiments, the amount of element M can be adjusted depending on the type and amount of the substituting elements R and T in order to achieve a larger entropy change in the La 1-a R a (Fe 1-x-y T y M x ) 13 H z -phase. In one embodiment, M is Si and the metallic weight fraction of Si, Si act , is within ± 5% of the value of the metallic weight fraction of silicon, Si m , which is from the relationship Si m = 3.85 - 0.0573 × Co m - 0.045 × Mn m 2 + 0.2965 × Mn m where Mn mthe metallic weight fraction of Mn and Co m is the metallic weight fraction of Co. In another embodiment, Si act within ±- 2% of Si m .

[0032] The working component can be provided in a number of physical formulas, for example as a sintered block or a reactively sintered block.

[0033] The term "reactively sintered" describes an article in which crystal grains are bonded together to form congruent crystal grains through a reactive sintered composite. A reactive sintered composite is produced by heat-treating a mixture of precursor powders of different compositions. The particles of the different compositions react chemically with each other during the reactive sintering process to form the desired final phases or products. The composition of the particles therefore changes as a result of the heat treatment. The phase formation process also causes the particles to bond together to form a sintered body that exhibits mechanical integrity.

[0034] Reactive sintering differs from conventional sintering in that conventional sintering involves particles that exhibit the desired final phase prior to the sintering process. The conventional sintering process causes the diffusion of atoms between neighboring particles to bond the particles together. The composition of the particles therefore remains unchanged as a result of the conventional sintering process.

[0035] The working component may further comprise a magnetocalorically passive phase. This magnetocalorically passive phase may provide a matrix in which magnetocalorically active phases are embedded. Alternatively, the magnetocalorically passive phase may be a coating of a solid magnetocalorically active block. In both cases, the magnetocalorically passive phase may have a corrosion-resistant coating to prevent corrosion of the magnetocalorically active phase.

[0036] The precursor powder mixture can be pressed to form one or more green bodies before heat treatment. Isostatic or compression molding can be used. This embodiment can be performed to produce the working component in the form of a reactively sintered block.

[0037] In another group of embodiments, the working component is further hydrogenated.

[0038] The La 1-a R a (Fe 1-x-y T y M x ) 13 H z -Phase has a NaZn 13 -type structure and, when it includes hydrogen, the hydrogen atoms occupy interstitial lattice sites in the NaZn 13 -type structure. The hydrogen can enter these interstitial lattice sites after the formation of the La 1-a R a (Fe 1-x-y T y Si x ) 13 H z -phase will be introduced. The T ca substantially fully hydrogenated ternary La(Fe,Si) 13 H z -phase can be approximately + 85°C. The T c in the La 1-a R a (Fe 1-x-y T y M x ) 13 H z -phase can be adjusted by adjusting the hydrogen content as well as by substituting the metallic elements of La and Fe.

[0039] Hydrogenation can be carried out by heat-treating the working component under a hydrogen partial pressure of 0.5 to 2 bar. The hydrogen partial pressure can be increased during the hydrogenation heat treatment. The hydrogenation can comprise a heat treatment at a temperature in the range of 0°C to 100°C, and preferably in the range of 15°C to 35°C. A final heat treatment at temperatures of less than 100°C in a hydrogen atmosphere, preferably at 1.5 to 2 bar, has been found to be reliable for producing working components with a hydrogen content, z, of at least 90% of the hydrogen saturation value, z sat .

[0040] In further embodiments, the hydrogenation comprises a residence time at a temperature T hyd , where 400°C ≤ T hyd ≤ 500°C and can have a residence time at a temperature T hyd in the range of 400°C ≤ T hyd≤ 500°C, followed by cooling to a temperature of less than 100°C.

[0041] In further embodiments, the working component is only exposed to hydrogen gas above a threshold temperature. In one embodiment, the hydrogenation comprises heating the working component from a temperature of less than 50°C to at least 300°C in an inert gas atmosphere and introducing hydrogen gas only when a temperature of at least 300°C is reached. The working component is maintained in a hydrogen-containing atmosphere at a temperature in the range of 300°C to 700°C for a selected period of time and cooled to a temperature of less than 50°C to provide a second working component. This process has been found to result in second working components having a hydrogen content, z, of 90% or more of the hydrogen saturation content, Z sat, and also leads to mechanically stable second working components. The hydrogenation process can be used to produce second working components in the form of a sintered block or a reactively sintered block.

[0042] In further embodiments of a method in which the working component is exposed to hydrogen only at a temperature above a threshold temperature, the working component may be cooled to a temperature of less than 50°C in the hydrogen-containing atmosphere.

[0043] In particular, it has been found that if hydrogen is first introduced at a temperature below approximately 300°C, the working component will disintegrate into pieces or at least lose its previous mechanical strength. However, these problems can be avoided by only introducing hydrogen when the working component is at a temperature of at least 300°C.

[0044] Alternatively or additionally, hydrogen is introduced only when a temperature of 400°C to 600°C is reached. After hydrogenation, the working component can contain at least 0.18 wt.% hydrogen.

[0045] To form the work component that a La 1-a R a (Fe 1-x-y T y M x ) 13 H z -phase with z = 0, the precursor powder mixture can be heated at a temperature T sinter heat treated, where 1050°C ≤ T sinter ≤ 1200°C.

[0046] A multi-step heat treatment process may also be used to heat the powder mixture and produce the working component. In one embodiment, the multi-step heat treatment includes a first residence time at T sinter for a time t1 in vacuum and a time t2 in argon, followed by cooling to a temperature T1, where T1 < T sinterfollowed by a second residence time at T1 for a time t3, followed by rapid cooling. Typical parameter ranges for such a multi-step heat treatment can be: 1000°C ≤ T1 ≤ 1080°C and / or 0.5 h ≤ t1 ≤ 10 h and / or 0.5 h ≤ t2 ≤ 10 h and / or 1 h ≤ t3 ≤ 20 h and / or rapid cooling at a rate of 5 to 200°C / min.

[0047] Mixing of the precursor powder can be performed using steel balls and, optionally, isopropanol to more thoroughly mix the elements. The mixing time can be limited to a maximum of 1 hour.

[0048] The monolithic working component comprising two integral sections may be provided in the form of a block, whether a sintered block or a reactively sintered block. It may be desirable to machine the working component by removing at least one section to change its external dimensions. For example, it may be desirable to separate the working component into two or more separate parts and / or adjust the external dimensions, and / or it may be desirable to introduce channels or through-holes into the working component through which a fluid of the heat exchange medium can flow.

[0049] The at least one portion may be removed from the working component by one or more operations, such as machining, mechanical grinding, mechanical polishing, chemical mechanical polishing, electric discharge cutting, wire discharge cutting, laser cutting and laser drilling or water jet cutting.

[0050] However, it has been found that the magnetocalorically active phase is difficult to machine due to its mechanical instability. Therefore, a number of alternative measures can be taken to remove one or more sections of the working component to reliably achieve the desired external dimensions.

[0051] In one group of embodiments, the at least one portion of the working component is removed while maintaining the working component at a temperature above the Curie temperature or below the Curie temperature. This has been found to prevent undesirable cracking of the working component.

[0052] Heating or cooling of the object can be carried out by using a heated or cooled working fluid such as water, organic solution or oil for example.

[0053] Without being bound by any theory, if the temperature of the object changes during processing so that the object undergoes a phase change, this phase change can lead to the formation of cracks within the object.

[0054] The magnetocalorically active phase may exhibit a temperature-dependent transition in length or volume. In this case, at least one section may be removed at a temperature above or below the transition to avoid a transition in length or volume during removal of the section(s). The temperature at which this transition in length or volume occurs may roughly correspond to the Curie temperature.

[0055] The transition can be characterized by (L 10% -L 90% )×100 / L(T) > 0.35, where L is the length of the object at a temperature below the transition, L 10% is the length of the object at 10% of the maximum change in length and L 90% at 90% of the maximum length change. This region characterizes the largest change in length per unit of temperature T.

[0056] Performing machining of the object by removing one or more sections while maintaining the object at a temperature where the phase change does not occur prevents the phase change from occurring during machining of the object and avoids any stress associated with the phase change that occurs during machining. Therefore, the object can be machined reliably, increasing the production rate and reducing production costs.

[0057] A combination of these processes can also be used on a single part. For example, the part can be separated into two or more separate pieces by removing a portion of the part using wire EDM cutting, and then the surfaces are subjected to mechanical grinding, removing a further section to achieve a desired surface finish or, more precisely, to define the external dimensions.

[0058] Typically, removing sections of the working component, for example, by grinding or sawing, generates heat in the working component due to friction between the tool and the working component. Therefore, active cooling at a temperature sufficient to compensate for the heat generation prevents the magnetocalorically active phase from undergoing a phase change, allowing the working component to be reliably formed to the desired external dimensions.

[0059] In another group of embodiments, the working component is heat-treated to decompose the magnetocaloric phase to produce an intermediate article. This intermediate article can then be processed, for example, to remove at least one portion, and the intermediate article or article can be heat-treated again after processing to recover the magnetocalorically active phase. By removing portions of the article that do not contain a magnetocalorically active phase, such as a layer 1-a R a (Fe 1-x-y T y M x ) 13 H z -Phase, in a substantial amount, the intermediate object can be reliably processed without undesirable cracks in the intermediate object.

[0060] In particular, in the case of processing objects containing a magnetocalorically active phase La 1-a R a (Fe 1-x-y T y M x )13 H z and which have large dimensions, for example blocks having dimensions of at least 5 mm or several tens of millimeters, the inventors have recently observed that undesirable cracks were formed in the articles during machining, which limited the number of smaller articles with the desired dimensions that could be manufactured from the large article.

[0061] Furthermore, the inventors observed that these undesirable cracks can be largely avoided by heat-treating the article to form an intermediate article containing a permanent magnet. This intermediate article comprises a coercive field strength greater than 10 Oe (approximately 795.77 A / m) according to the definition of a permanent magnet used herein.

[0062] Without being bound by any theory, it is assumed that the observed objects with cracks exhibiting the magnetocalorically active phase during processing may be caused by a temperature-dependent phase change occurring in the magnetocalorically active phase. The phase change may be a change in entropy, a change from ferromagnetic to paramagnetic behavior, a change in volume, or a change in linear thermal expansion.

[0063] Performing machining of the object while the object is in a non-magnetocalorically active machining state avoids any phase change that occurs in the object during machining and avoids any stress associated with the phase change that occurs during machining. Therefore, the object can be machined reliably, increasing the production rate and reducing production costs.

[0064] In one embodiment, the working component is heat treated at a temperature T2 to form an intermediate article having at least one permanent magnetic phase, where T2 <T sinter T2 can range from 600°C to 1000°C.

[0065] The working component may be heat treated under conditions selected to ensure the 1-a R a (Fe 1-x-y T y M x ) 13 Hz -phase, which is a NaZn 13 -type crystal structure and forms at least one α-Fe-type phase in the intermediate article. The heat treatment conditions can be selected to produce an intermediate article having an α-Fe content of greater than 50 vol%. The intermediate article can then be machined at room temperature.

[0066] After the intermediate article has been processed by removing at least a portion of the intermediate article, the intermediate article may be heat treated to produce a final product of the working component comprising at least one magnetocalorically active layer. 1-a R a (Fe 1-x-y T y M x ) 13 H z -phase. The intermediate article can be heat-treated at a temperature T3 to produce the final product containing at least one magnetocalorically active layer 1-a R a (Fe1-x-y T y M x ) 13 H z -phase, where T3 > T 2. In one embodiment, T3 < T sinter . T3 can be about 1050°C.

[0067] The composition of the working component can be chosen to ensure reversible decomposition of the phase with the NaZn 13 -type crystal structure at T2 and to prevent the regeneration of the NaZn 13 -type crystal structure at T3.

[0068] In one embodiment, the composition of the at least one layer 1-a R a (Fe 1-x-y T y M x ) 13 H z -Phase was selected to show a reversible phase of the decomposition reaction. This allows the La 1-a R a (Fe 1-x-y T y M x ) 13 H z-phase in a first step, which is decomposed to provide the working component and then reformed in a further heat treatment once the machining is completed.

[0069] The composition of at least one La 1-a R a (Fe 1-x-y T y M x ) 13 H z -Phase can be selected to exhibit a reversible phase decomposition reaction into at least one α-Fe-based phase and La-rich and Si-rich phases.

[0070] In a further embodiment, the composition of the at least one layer 1-a R a (Fe 1-x-y T y M x ) 13 H z -Phase must be selected so that at least one La 1-a R a (Fe 1-x-y T y M x ) 13 H z -phase through

[0071] Melt-phase sintering is feasible. This allows for the production of a high-density object and also the production of a high-density object in an acceptable time.

[0072] In one embodiment, the intermediate article comprises a composition as a whole where a = 0, T is Co and M is Si and z = 0, and in another embodiment, 0 < y ≤ 0.075 and 0.05 < x ≤ 0.1, where a = 0, T is Co and M is Si and z = 0.

[0073] In further embodiments, the intermediate article comprises the following magnetic properties: B r > 0.35 T and H cJ > 80 Oe (approximately 6366.20 A / m) and / or B s > 1.0 T

[0074] The intermediate object may have a coercive field strength of greater than 10 Oe (approximately 795.77 A / m) but less than 600 Oe (approximately 47746.48 A / m). Objects with such a coercive field strength are sometimes referred to as semi-hard magnets.

[0075] The intermediate article may have a composition structure comprising a non-magnetic matrix and a plurality of α-Fe inclusions distributed throughout the non-magnetic matrix. As used herein, non-magnetic refers to the matrix condition at room temperature and includes paramagnetic and diamagnetic materials, as well as ferromagnetic materials with a very small saturation polarization.

[0076] A magnetic heat exchange article is provided comprising a monolithic working component having two or more sections. The two or more sections comprise amounts of La, Fe, Si, and one or more elements T and R suitable for forming a La 1-a R a (Fe 1-x-y T y Si x ) 13 H z -phase, where T is one or more of the elements from the group consisting of Mn, Co, Ni, and Cr, and R is one or more of the elements from the group consisting of Ce, Nd, Y, and Pr. The amount of the one or more elements T and R and the amount of Si is selected for each of the two or more sections to produce two or more sections with different Curie temperatures.

[0077] Therefore, a single unit is provided having two or more sections with different Curie temperatures, which are integrally arranged within this single unit, so that the monolithic working component, when used in a magnetic heat exchanger, is capable of exchanging heat over a wider temperature range than is possible using a single unit containing a random mixture of two phases having different Curie temperatures. For example, the two sections may be arranged within the monolithic working component such that two sublayers are formed within the monolithic working component, each sublayer having a different T c than the others. Each sublayer can have a thickness of 5 mm or greater.

[0078] The monolithic working component, which has two or more different Curie temperatures (T c) has the advantage of being easier to arrange in a practical magnetic heat exchanger. The problems associated with powders, such as flocculation from filters and pumps, are avoided. Furthermore, only a single item needs to be arranged, as opposed to a multitude of items, which is the case when a variety of differently sintered or reactively sintered blocks are provided.

[0079] The T cThe sintering activity for each section can be adjusted by adjusting the type of metallic substitution element R and T and / or the amount of the included element. However, different elements and different amounts of the same element differ and can lead to changes in sintering activity. In this case, if two sections with different metallic contents are heated at the same sintering temperature, one section may sinter to form a stable block, while the other section has not reached the optimal sintering temperature and therefore remains mechanically fragile.

[0080] A monolithic working component exhibiting two or more different Curie temperatures can be fabricated because the amount of silicon, which depends on the amount or type of metallic elements, can be adjusted so that the different phases exhibit similar sintering activity. Therefore, the two or more integral sections of the monolithic working component can have a similar density, allowing the working component to remain intact despite including macroscopic sections exhibiting different Curie temperatures and different metallic constituents.

[0081] In one embodiment, the two or more components have a density d within a range of ± 5% or ± 2% of the average density d av of a total number of sections.

[0082] This enables a mechanically stable, monolithic working component with different Curie temperatures and therefore a wider working temperature range can be produced.

[0083] The two or more sections of the working component may have different silicon contents and may, for example, comprise increasing or decreasing silicon content for increasing amounts of one or more of the elements R and T.

[0084] In one embodiment, the amount of silicon is in the range of 0.05 ≤ x ≤ 0.2.

[0085] The sintered bodies can have a silicon content Si, Si act , which is within ± 5% or ± 2% of Si m where Si m = 3.85 - 0.0573 × Co m - 0.045 × Mn m 2 + 0.2965 × Mn m is, where Si m is the metallic weight fraction of silicon, Mn mthe metallic weight fraction of manganese, Co m is the metallic weight fraction of cobalt.

[0086] The monolithic working component may include sequentially arranged sections having sequentially increasing Curie temperatures or having sequentially decreasing Curie temperatures.

[0087] Each of the two or more sections has one or more of the elements R and T in the following amounts: 0 ≤ a ≤ 0.5, 0.003 ≤ y ≤ 0.2, 0.05 ≤ a ≤ 0.5, 0.003 ≤ y ≤ 0.2.

[0088] Embodiments will now be described with reference to the accompanying drawings: Fig. Figure 1 shows a schematic diagram of a non-inventive article for a magnetic heat exchanger having five separate working components, Fig. Figure 2 shows a schematic diagram of a non-inventive article for magnetic heat exchange comprising a monolithic working component having five integral working component sections, Fig. Figure 3 shows a graph showing the density as a function of sintering temperature for a third embodiment, Fig. Figure 4 shows a graph showing the α-Fe content as a function of sintering temperature for the third embodiment, Fig. Figure 5 shows a graph showing density as a function of sintering temperature for a fourth embodiment, and Fig. Figure 6 shows a graph showing the density as a function of manganese content with a constant silicon content and a varying silicon content.

[0089] Fig. Figure 1 shows a non-inventive article 1 for a magnetic heat exchanger comprising five individual working components 2, 3, 4, 5, 6, each in the form of a reactively sintered block. Each of the working components 2, 3, 4, 5, 6 comprises a magnetocalorically active phase, which is La 1-a R a (Fe 1-x-y T y M x ) 13 H z M may comprise one or more of the elements selected from the group consisting of Al and Si, T may comprise one or more of the elements selected from the group consisting of Co, Ni, Mn, Cr, Cu, Ti, and V, and R may comprise one or more of the elements selected from the group consisting of Ce, Nd, Y, and Pr.

[0090] In this embodiment, the hydrogen content, z, of each of the working components is 90% or higher of a hydrogen saturation value, Z sat .

[0091] For a sample with selected values ​​of a, x, and y, the hydrogen content can be determined experimentally by heating a hydrogenated sample in a hydrogen-containing atmosphere at a temperature in the range of 20°C to 100°C for at least 1 hour. The hydrogen-containing atmosphere may have a hydrogen partial pressure in the range of 0.5 bar to 2.0 bar. The sample may be preheated in a hydrogen atmosphere to a temperature between 200°C and 500°C before being held at a temperature of 20°C to 100°C for at least 1 hour. The preheating step helps avoid activation difficulties.

[0092] If the hydrogen content of the sample does not increase measurably, the sample can be assumed to be fully hydrogenated and the hydrogen saturation content, Z satThe hydrogen content of the sample can be measured using techniques such as hot gas extraction. Alternatively or additionally, the change in hydrogen

[0093] Content can be determined by measuring the Curie temperature before and after heat treatment.

[0094] The values ​​of a, x and y are chosen to give a different Curie temperature T for each working component 2, 3, 4, 5, 6 c The different Curie temperatures are not achieved to a substantial extent by partial dehydrogenation of the working components, but by selecting appropriate amounts of the elements R, T, and M.

[0095] T cmax is the Curie temperature at the corresponding working component with La 1-a R a (Fe 1-x-y T y M x ) 13 H z -phase, which has a hydrogen content z = z satand the selected values ​​a, x and y for each working component 2, 3, 4, 5, 6. The working components 2, 3, 4, 5, 6 are at least 90% completely hydrogenated, so that the Curie temperature T c for each of the working components within 20 Kelvin of T cmax In other words (T cmax - T c ) ≤ 20 K. In this particular embodiment, for each of the working components 2, 3, 4, 5, 6, the element M is Si, the element T is Mn and the element R is omitted.

[0096] The Curie temperature T c for the working components 2, 3, 4, 5, 6 is within ± 10 K of the value of the Curie temperature, T c(calc) , which depends on the relationship T c(calc) (°C) = 80.672 - 26.957 × Mn m is derived, where Mn mis the metallic weight fraction of manganese. By adjusting the amount of manganese in the working component 2, 3, 4, 5, 6, the Curie temperature of the working component can be selected to lie within a range of + 80°C to - 90°C. The T c decreases with increasing Mn content.

[0097] It has been observed that increasing manganese content increases sintering activity, leading to an increase in the density of the working component. To counteract this increase in density, the silicon content can be increased.

[0098] The values ​​of Si, x and Mn, y, satisfy the following dependencies on each other for the working component of the metallic weight fraction of Si, Si act , is within ± 5% of the value of the metallic weight fraction of silicon, Si m , which is from the relationship Si m = 3.85 - 0.0573 × Co m - 0.045 × Mn m 2 + 0.2965 × Mn mBy adjusting the silicon content in relation to the amounts of the substitute metal Mn, the sintering activity of the La 1-a R a (Fe 1-x-y T y Si x ) 13 H z -Phase can be adjusted so that after sintering at a temperature T sinter , all working components 2, 3, 4, 5, 6 have a density d which is similar for example within a range of ± 5% or ± 2% of an average density d av a total number of sintered sections.

[0099] In this embodiment, each of the working components 2, 3, 4, 5, 6 is manufactured by reactive sintering of the elements or precursor alloys to form a working component in the form of a reactive sintered block.

[0100] The working components 2, 3, 4, 5, 6 can also be provided as a composite which further comprises a magnetocalorically passive phase such as copper as a matrix in which the magnetocalorically active phase is embedded.

[0101] The working components 2, 3, 4, 5, 6 are arranged in the object 1 so that T c of the working components increases sequentially along a longitudinal direction of the object 1. This arrangement produces better overall cooling quality when the object 1 is used in a magnetic heat exchanger.

[0102] Fig. Figure 2 shows an article 1' according to a second embodiment. The article 1' contains five working components 2', 3', 4', 5', 6' as in the first embodiment, which are shown in Fig. 1. These working components 2', 3', 4', 5', 6' also include combinations of the five working components 2, 3, 4, 5, 6 of the first embodiment.

[0103] However, in the second embodiment, the five working components 2', 3', 4', 5', 6' are formed in the form of a monolithic working component 7. Therefore, the monolithic working component 7 comprises five integral sections 2', 3', 4', 5', 6', each of which has a different T c different values ​​of a and / or y and different silicon contents. However, the sintered density of the five sections 2', 3', 4', 5', 6' is similar and within a range of ± 5% or ± 2% of an average density d av a total number of sections, as in the first embodiment. Again, the density of the sections 2', 3', 4', 5', 6' within this monolithic working component 7 is controlled by adjusting the silicon content in relation to the type and amount of the metallic substituting elements, which in the second embodiment are Mn.

[0104] Therefore, the monolithic working component 7 of a Tc which varies from one end 8 to the other end 9 and which increases sequentially or gradually from one end 8 to the other end 9. By carefully selecting the silicon content required for the composition of the metallic element to produce the desired Tc for a particular section, the monolithic working component 7 has a similar density throughout its entire volume. This provides a monolithic working component 7 with mechanical integrity that is substantially constant throughout its entire volume.

[0105] In a first embodiment, the silicon content of the five working components 2, 3, 4, 5, 6 of the first embodiment and the five working components 2', 3', 4', 5', 6' of the second embodiment can be Si act which is between ± 5% of the value of the metallic weight fraction of silicon, Si m, and which is derived from the relationship: Sim=3.85−0.0573×Com−0.045×Mnm2+0.2965×Mnm.

[0106] The objects 1, 1' of the first and second embodiments can be manufactured using one or the following embodiments.

[0107] To produce separate working components, as in the first embodiment, each powder composition is used separately. For example, each powder composition is used to form separate green bodies by pressing. To produce a monolithic working component with integral sections of the different compositions, the powders of the different compositions can be coated on top of each other to form a single green body.

[0108] In the third embodiment, the following relationships were used as starting points to select appropriate amounts of the elements La, Mn and Si to produce working components having different T c in a 1-a R a (Fe 1-x-y Mn y- Si x ) 13 H z -System: Tc=75.57−23.72×Mnm where T c measured in °C, Mn m is the metallic weight fraction of Mn and the relationship represents fully hydrated samples.

[0109] The relationship between the metallic weight fraction of silicon Si m and the metallic weight fraction of manganese Mn m The following equation was used to calculate the amount of silicon for a given content of manganese and a for a given T c to select: Sim=3.86+0.176×Mnm

[0110] Using these two equations, three compositions were selected and three fine powders were prepared from a manganese-containing melt. The compositions and calculated and expected T c,hyd Temperatures for fully hydrated samples are summarized in Table 1.

[0111] Each of these powders was isostatically pressed for 30 minutes in 60 g batches to form a green body. One green body of each composition was heated to three sintering temperatures before cooling to 1050°C in 1 hour, with the temperature held at room temperature for 6 hours before cooling. The three sintering temperatures were 1100°C, 1120°C, and 1140°C. At each temperature, the samples were held in a vacuum for 3 hours and in argon for 1 hour before cooling to 1050°C.

[0112] The density and α-Fe (alpha iron) were measured after this heat treatment and the results are shown in Table 2 and in the Fig. 3 and Fig. 4. The samples sintered at a temperature of 1100°C and 1120°C have an α-Fe content of less than 5 volume percent, which shows that a large volume fraction of the desired La 1-a R a (Fe 1-x-y T y Si x ) 13 -phase was generated. For the samples heated to 1140°C, the α-Fe content is higher, between 7 and 8 volume percent.

[0113] As these results show, increasing the silicon content with increasing manganese content according to the equation above did not produce the desired constant or essentially constant for the sintered density as desired.

[0114] In particular, the density of samples sintered at 1100°C decreases with increasing manganese content. This indicates that the silicon content should have been increased to compensate for the effect of the increasing manganese content on density.

[0115] The samples sintered at 1120°C were hydrogenated using the following process.

[0116] The samples were heated to 500°C under vacuum and held at this temperature for one hour in an argon atmosphere. The hydrogen partial pressure was then increased to 0.5 bar, 1.0 bar, 1.5 bar, and 1.9 bar, and held at each of these pressures for 15 minutes. Once the hydrogen partial pressure reached 1.9 bar, the furnace was turned off, and the samples were allowed to cool to room temperature in the furnace.

[0117] The magnetocaloric properties of these hydrogenated samples are summarized in Table 3. The Curie temperature T c of the samples lies within a narrow range of the expected T c according to the above equation. However, it was observed that the entropy change ΔS m,max decreases with increasing manganese content.

[0118] In a fourth embodiment, another set of samples is prepared to further reduce the difference in sintered density and also to achieve an increase in the entropy change, especially for samples having higher manganese contents.

[0119] Two samples were prepared with different manganese contents, which were expected to have a T cof 3.5°C and 26.5°C, respectively, when the samples are fully hydrogenated. The compositions are summarized in Table 4. In this embodiment, the silicon content was kept constant.

[0120] A sample of each composition was heated to one of three sintering temperatures (TS): 1100°C, 1120°C, and 1140°C, where it was held for three hours in a vacuum and then for one hour in argon. In each case, the samples were cooled to 1050°C in one hour. This temperature was maintained for six hours before the samples were cooled to room temperature.

[0121] The density of all the samples was measured and the α-Fe content for the samples sintered at 1120°C was measured. The results are summarized in Table 5 and shown in the graph of Fig. 5 shown.

[0122] Fig.Figure 6 shows a graph comparing the density of the samples as a function of manganese content for constant and variable silicon content. By comparing these results for the four embodiments in which the silicon content was kept constant with those of the three embodiments in which the silicon content was increased with increasing manganese content, an improved relationship between the silicon content and the manganese content can be formulated as follows: Sim=3.85−0.045×Mnm2+0.2965×Mnm

[0123] It was found that this relationship produces a more similar sintered density for samples having different manganese contents with respect to the starting relationship.

[0124] The samples were hydrogenated using the previously described procedures, and before performing the magnetocaloric measurements, the samples were heated in flowing nitrogen at 150°C for 10 minutes. The magnetocaloric properties of the hydrogenated powder are summarized in Table 6.

[0125] These results show that mixing the powder with steel balls (shown in Table 6) rather than without steel balls (shown in Table 6) results in an increase in the entropy change (ΔS m,max ), so that an entropy change of at least 8 J / kgK is achieved, even for the higher manganese contents.

[0126] These results also suggest that the following modified description between the peak temperature at which the largest entropy change is observed (denoted as T peak in Table 6), which corresponds to the Curie temperature Tc, and manganese content Mn m more precisely: Tc=80.672−26.957×Mnm

[0127] In a third set of embodiments, the two revised equations 3 and 4 were used to prepare two samples having the following composition and expected Curie temperature, summarized in Table 7. The two metallic compositions are: 16.7 wt% La, 4.33 wt% Si, 2.86 wt% Mn, balance Fe, and 16.7 wt% La, 4.26 wt% Si, 2.02 wt% Mn, balance Fe. The expected T c for a fully hydrogenated composition is 3.5°C and 26.3°C.

[0128] These powders were mixed with steel balls, and 60 g of each powder was isostatically pressed to form green bodies. A green body of this composition was heated under three different heat treatments.

[0129] In the first heat treatment A, the samples were heated to 1100°C for 3 hours in vacuum and 1 hour in argon, followed by cooling to 1050°C for 1 hour, followed by a residence time of 6 hours at 1050°C and cooling to room temperature.

[0130] In the second heat treatment B, the green bodies were heated to 1080°C for 4 hours in vacuum, followed by cooling to 1000°C for 1 / 2 hour, followed by heating to 1080°C for 1 hour. The samples were held for 3 hours in vacuum and 1 hour in argon before being cooled to 1050°C for 24 hours. The samples were held at 1050°C for 24 hours before being cooled to room temperature.

[0131] In a third heat treatment C, the samples were heated to 1090°C and held at this temperature for 4 hours in a vacuum before being cooled to 1000°C in 1 / 2 hour. This was followed by heating to 1090°C for 1 hour, which was held under vacuum for 3 hours, followed by 1 hour in argon. The samples were then cooled to 1050°C in 6 hours, and this temperature was held for 6 hours before cooling to room temperature.

[0132] The results of density and α-Fe content measured for these three samples are summarized in Table 8.

[0133] As can be seen, the α-Fe content is less than 2 volume percent for almost all samples. Furthermore, the density of the three samples at a given sintering temperature varies by less than the previous embodiments.

[0134] The samples were hydrogenated as previously described and the magnetocaloric properties were measured. Before measuring the magnetocaloric properties, the samples were heated to 150°C for 10 minutes. The results are summarized in Table 9. As can be seen in Table 9, the entropy change, -ΔS m,max , increased compared to the previous two embodiments and in some cases almost 12 J / kgK.

[0135] Therefore, the Curie temperature T c fully or substantially fully hydrogenated La 1-a R a (Fe 1-x-y T y Si x ) 13 H z -based samples can be selected by substituting an appropriate amount of manganese according to equation 4: Tc=80.672−26.957×Mnm

[0136] Furthermore, the silicon content of the powder can be adjusted as a function of the manganese content according to the following relationship of equation 3: Sim=3.85−0.045×Mnm2+0.2965×Mnm

[0137] The sintered density of the samples, which have different manganese contents and therefore different T c , can be made substantially constant.

[0138] This nearly constant sintered density can be used to produce a variety of separate working components that have different manganese contents and therefore different T c , at a single sintering temperature. Therefore, the multitude of separate working components desirable for a particular article for a particular magnetic heat exchanger can be produced in a single furnace step.

[0139] In further embodiments, these relationships can be used to produce a monolithic working component that includes integral sections having different manganese contents and thus different T c , but has similar sintered densities. The similar sintered densities make it possible to produce different compositions within a single monolithic working component.

[0140] If the sintering activities and resulting densities were not similar, sections of the monolithic working component exhibiting lower sintering activity would not sinter sufficiently at a given temperature and would be softer than other sections exhibiting increased sintering activity at that temperature, resulting in increased density. Therefore, a monolithic working component comprising integral sections of different compositions would possess regions that are mechanically unstable due to low sintering activity and low density in these sections, or would fracture in these mechanically unstable regions, preventing a monolithic working component from being formed at all.

[0141] This problem can be avoided by adjusting the silicon content to the La 1-a R a (Fe 1-x-y T y Si x ) 13H z -phase as a function of the substitution elements R and T and the amount of substituted elements R and T.

Claims

[1] Method for producing a working component for a magnetic heat exchange, comprising: Select for each of the two or more sections of the work component sets of La, Fe, M and of one or more of the elements of T and R that are suitable for a La 1-a R a (Fe 1-x-y T y M x ) 13 H z-phase to generate, wherein T is one or more of the elements from the group consisting of Mn, Co, Ni and Cr, M is silicon and optionally aluminum, and R is one or more of the elements from the group consisting of Ce, Nd, Y and Pr, wherein the amount of silicon is within the range of 0.05 ≤ x ≤ 0.2 and the amount of one or more of the elements R and T is selected within the ranges 0 ≤ a ≤ 0.5 and 0.003 ≤ y ≤ 0.2, wherein the amount of one or more of the elements T and R and the amount of Si are selected for each of the two or more sections to provide the two or more sections with different Curie temperatures and similar sintering activities, Mixing the quantity of the selected elements T, R and M with La, Fe or alloys thereof in quantities suitable to achieve the La 1-a R a (Fe 1-x-y T y M x ) 13 H z-phase to produce two or more precursor powder mixtures, Producing a green body from the two or more precursor powder mixtures, Heat treatment of the green body at a temperature T sinter and producing a working component that has two or more sintered sections, each with a different T c and a density, d, within a range of ±5% or ±2% of the mean density, d av , has a total number of sintered sections, wherein a single monolithic green body is formed from two or more precursor powder mixtures and heat-treated to form a single monolithic working component having two or more sintered sections. [2] Method according to claim 1, wherein the two or more sections of the working component have different silicon contents. [3] Method according to claim 1 or claim 2, wherein the two or more sections of the working component have different values ​​for a and y. [4] Method according to any one of claims 1 to 3, wherein T is Mn and optionally Co and the amount of silicon is selected according to Si m = 3.85 - 0.0573 × Co m - 0.045 × Mn m 2 + 0.2965 × Mn m , where Si m The metallic fraction of silicon by weight is Mn. m The metallic fraction of manganese by weight is Co m The metallic weight fraction of cobalt is... [5] Method according to claim 4, wherein the sintered sections have a silicon content Si, Si act exhibiting values ​​within ±5% of Si m lies. [6] Method according to claim 5, wherein Si act within ±2% of Si m lies. [7] Method according to any one of claims 1 to 6, wherein the set of one or more of the elements R and T is selected within the ranges 0.05 ≤ a ≤ 0.5 and 0 ≤ y ≤ 0.

2. [8] Method according to any one of claims 1 to 6, wherein the set of one or more of the elements R and T is selected within the ranges 0.05 ≤ a ≤ 0.5 and 0.003 ≤ y ≤ 0.

2. [9] Method according to any one of claims 1 to 8, wherein 1050°C ≤ T sinter ≤ 1200°C. [10] Method according to any one of claims 1 to 9, wherein a multi-step heat treatment process is used to heat treat green bodies. [11] Method according to claim 10, wherein a multi-step heat treatment includes a first residence time at T sinter for a time t1 in vacuum and a time t2 in argon, followed by cooling to a temperature T1, where T1 < T sinteris followed by a second residence time at T1 for a time t3, followed by rapid cooling. [12] Method according to claim 11, wherein 1000°C ≤ T1 ≤ 1080°C and / or 0.5 h ≤ t1 ≤ 10 h and / or 0.5 h ≤ t2 ≤ 10 h and / or 1 h ≤ t3 ≤ 20 h and / or rapid cooling is carried out at a rate of 5 to 200°C / min. [13] Method according to any one of claims 1 to 12, further comprising hydrogenation of the sintered sections, which includes heat treatment under an H2 partial pressure of 0.5 to 2 bar. [14] Method according to claim 13, wherein the H2 partial pressure is increased during hydrogenation. [15] Method according to claim 13 or claim 14, wherein the hydrogenation comprises the heat treatment in the range of 0°C to 100°C. [16] Method according to claim 15, wherein the hydrogenation comprises heat treatment at a temperature in the range of 15°C to 35°C. [17] Method according to any one of claims 13 to 16, wherein the hydrogenation involves a residence time at a temperature T hyd exhibits, where 300°C ≤ T hyd ≤ 700°C. [18] The method of claim 17, wherein the hydrogenation involves a residence time at a temperature T hyd exhibits, where 300°C ≤ T hyd ≤ 700°C, followed by cooling to a temperature below 100°C. [19] Method according to any one of claims 13 to 18, wherein the hydrogenation comprises: Heating the working component from a temperature of less than 50°C to at least 300°C in an inert gas atmosphere, Hydrogen gas should only be introduced when a temperature of at least 300°C has been reached. Holding the working component in a hydrogen-containing atmosphere at a temperature in the range of 300°C to 700°C for a selected duration, and Cooling the working component to a temperature of less than 50°C in order to provide a second working component. [20] Method according to claim 19, wherein the working component is cooled at a temperature of less than 50°C in a hydrogen-containing atmosphere. [21] Method according to claim 19 or claim 20, wherein the hydrogen gas is only introduced when a temperature of 400°C to 600°C is reached. [22] Method according to any one of claims 19 to 21, wherein after hydrogenation the second working component has at least 0.18 wt% hydrogen. [23] Method according to any one of claims 1 to 22, wherein the mixing is carried out using steel balls and optionally isopropanol. [24] Method according to any one of claims 1 to 23, wherein the precursor powder mixtures are pressed to form the green body.

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