Method for producing a magnetocaloric heat exchanger element

Abstract

The invention relates to an economical, resource-efficient, easy-to-perform and fault-tolerant method which is as safe as possible and by which a long-lasting, robust, chemically and mechanically stable heat exchanger element having advantageous thermohydraulic and magnetic properties can be produced. According to the invention, the method comprises the following steps: - providing a magnetocaloric substrate (10) having a plurality of channels or pores, which preferably form continuous channels (12), - immersing the substrate (10) into a temperature-controlled aqueous reaction solution (6) which leads to an autocatalytic chemical deposition of a metal coating (40) on the surface of the substrate (10).

Description

[0001] Description

[0002] Method for producing a magnetocaloric heat exchanger element

[0003] The invention relates to a method for producing a magnetocaloric heat exchanger element. It also relates to a heat exchanger element produced by such a method, as well as to a heat pump or cooling or heating device comprising such a heat exchanger element. The invention further relates to a nickel-based plating solution and a copper-based plating solution, particularly for use in said method.

[0004] The economically viable exploitation of the theoretically expected energy efficiency gains from the use of magnetocaloric materials in heat pumps and cooling devices requires not only the selection of a high-performance functional material tailored to the desired temperature range and reasonably cost-effective, but also a suitable design of the magnetocaloric heat exchanger. This applies to the composition, structure, and architecture of the magnetocaloric heat exchanger. For example, properties such as low flow resistance, a high volume density of magnetocaloric material, a high entropy change during the magnetic phase transition, production from inexpensively available raw materials, effective heat transfer, and a long service life are desirable. The latter requires high mechanical and chemical stability under operating conditions, i.e.in the presence of the heat exchange medium and the forces occurring during device operation. These include, in particular, the flow of the heat exchange medium, magnetic attraction, and the volume changes caused by repeated (de)magnetization and the associated phase transformations.

[0005] The focus on inexpensive, low-criticality raw materials makes magnetocaloric materials, which are primarily composed of readily available transition or main group metals and the relatively uncritical rare earth metal lanthanum, particularly promising. Notable material classes include Heusler compounds, Fe2P-based magnetocaloric compounds, and lanthanum-iron-silicon-based alloys. Hydrogenated variants are particularly interesting for cooling applications under ambient conditions due to their elevated Curie temperature and high power density. By adjusting the composition, the ideal application range of magnetocaloric compounds such as lanthanum-iron-silicon-based alloys can be significantly reduced, making such material classes also relevant for low-temperature applications.

[0006] The required combination of the numerous properties mentioned above cannot be satisfactorily achieved with solid magnetocaloric materials. For example, due to their composition of oxophilic, moderately to highly base metals (e.g., lanthanum) and poorly passivating metals (e.g., iron), these materials are intrinsically susceptible to corrosion. The bonding structure of intermetallic materials such as lanthanum-iron-silicon-manganese alloys and Heusler compounds or iron phosphides makes them intrinsically brittle and susceptible to fracture. Lanthanum-iron-silicon-manganese alloys that have been hydrogenated to adjust their magnetic properties lose hydrogen through outdiffusion even at moderately elevated temperatures below 100 °C, leading to an undesirable change in their magnetic functionality.Around the Curie temperature, which for the primary applications lies at room temperature and below, even partial dehydrogenation leads to a drastically accelerated migration of the hydrogen remaining in the lattice, which precludes the commercial use of partially hydrogenated lanthanum-iron-silicon alloys due to accelerated aging and performance loss. Therefore, such substrates are incompatible with deposition methods or modification processes that rely on elevated temperatures, such as sintering treatments used to reduce coating stresses, promote interlayer diffusion, and improve adhesion or bond granular starting materials.

[0007] Due to their high chemical reactivity, damage-free processing and modification of such materials is challenging. Corrosion in the presence of organic or water-based cooling media leads to the loss of active material and, depending on the ambient pH value, to the buildup of oxidic, hydroxydic, or soluble oxidation products. Solid corrosion products can migrate into the cooling circuit or adhere, causing a loss of thermal conductivity and a deterioration of flow properties. In wet-chemical coating processes such as electrodeposition, the desired coating reaction competes with the same corrosion processes, which in this scenario can negatively impact coating quality and adhesion.

[0008] The object of the invention is to provide a cost-effective, resource-efficient, easy-to-implement, as safe as possible, and fault-tolerant process for producing a durable, robust, chemically and mechanically stable heat exchanger element with advantageous thermohydraulic and magnetic properties. In particular, low flow resistance for a heat transfer medium flowing through it, a high volume density of magnetocaloric material, a high entropy change during the magnetic phase transition, effective heat transfer, and high material and corrosion stability under application conditions, particularly in water-based heat exchange media, while avoiding hydrogen embrittlement or loss, are desirable. Furthermore, a deposition solution suitable for carrying out the process, one based on nickel and the other based on copper, is to be specified.The object is achieved according to the invention by a method having the features of claim 1. The corresponding heat exchanger element is the subject of claim 19.

[0009] The method according to the invention for producing a magnetocaloric heat exchanger element therefore comprises the following steps:

[0010] • Providing a magnetocaloric substrate having a plurality of channels or pores, which preferably form continuous channels,

[0011] • Immersing the substrate in a tempered aqueous reaction solution, which leads to an autocatalytic chemical deposition of a metal coating on the surface of the substrate.

[0012] The substrate is also called the matrix. The reaction solution is also called the deposition solution. The term deposition bath is also used synonymously here.

[0013] The heat exchanger element is also called a heat transfer element or regenerator element. Abbreviations such as heat exchanger, heat transfer element, or regenerator are also common.

[0014] In this application, the term "magnetocaloric" refers, in accordance with the common understanding of the art, to the property that the substrate exhibits a technically exploitable magnetocaloric effect. This refers to the phenomenon of a temperature change in a magnetizable material when exposed to a changing magnetic field.

[0015] Continuous or through-channels are to be understood as passing through the heat exchanger element from one end to the other, in particular completely. The respective channel therefore has two open ends, one opening being located in a first surface area of ​​the heat exchanger element and the second opening being located in a second surface area different from the first surface area. A fluid, in particular a liquid, can therefore flow through the respective channel from one opening (inlet opening) to the other opening (outlet opening), the flow direction also being reversible depending on the pressure conditions. The channels formed by the pores are preferably arranged as a whole in such a way that they enable directed flow through the heat exchanger element along a main flow direction or opposite thereto.

[0016] Given the preferred dimensions mentioned below, the pores or channels are also called microchannels.

[0017] "Surface coating" here refers to the coating of the entire inner and outer surface of the substrate that may come into contact with the heat transfer medium during subsequent use as a heat exchanger element. In particular, the walls of the pores or channels should be coated.

[0018] The invention is based on the consideration that solid magnetocaloric regenerators are poorly suited for effective heat exchange with the surrounding medium due to their moderate thermal conductivity. Instead, the present invention pursues the approach of creating a readily accessible and large internal surface area by incorporating a suitable pore architecture.While this intensifies heat exchange, it simultaneously increases the challenges in operation and processing: For example, an enlarged surface in contact with the cooling medium exacerbates the corrosion problem, the increased geometric complexity of porous magnetocaloric materials places high demands on the conformity and homogeneity of the coating techniques in question, in addition to avoiding undesirable side reactions, and due to their dimensions, the finely structured channels are particularly susceptible to blockage by solid corrosion products.

[0019] As recognized within the scope of the invention, effective coating techniques capable of coating porous versions of magnetocaloric materials with a secondary material in a controlled, degradation-free, and uniform manner are desirable as a technological solution for creating high-performance, cost-competitive, and robust magnetocaloric heat exchangers. This secondary material, through its surface-applied functionality, complements the properties of the substrate and compensates for its weaknesses. Thus, encasing a magnetocaloric material in a rigid and plastically deformable coating increases its fracture resistance and reduces the risk of phase-change-induced pulverization, while completely covering the surface with a more chemically resistant material improves its corrosion resistance and aging stability.In order not to unnecessarily increase the thermal mass of the system and not to jeopardize the effective heat transfer from the magnetocaloric material into the surrounding cooling medium as a key performance parameter, thin and poorly thermally insulating coatings are desirable.

[0020] In summary, the required properties (in particular, high corrosion resistance under operating conditions, mechanical protection of the magnetocaloric material, ductility, high thermal conductivity) can be achieved primarily through metal coatings. Applying these coatings in a surface-conforming, defect-free, and well-adhering manner to complexly shaped, chemically and physically sensitive magnetocaloric substrates with large internal surfaces is challenging, but can be achieved with tailored autocatalytic chemical metal deposition processes that take into account the specific reactivity of magnetocaloric materials, their geometry, and the desired combination of target properties, as detailed below.

[0021] The substrate treated according to the invention is particularly suitable for magnetocaloric applications, especially in cooling devices, with water-based heat exchange media. Such heat exchange media are ecologically advantageous, possess good physical and chemical performance data (very high heat capacity, high thermal conductivity, relatively low viscosity), and are available cost-effectively in large quantities. However, unlike aprotic organic solvents or comparable non-aqueous cooling media, these are corrosive, which justifies the need for the inventive protection of the magnetocaloric functional material against chemical attack. Advantageous embodiments of the invention, some of which have an independent inventive character, are the subject of the dependent claims in conjunction with the following description.

[0022] Advantageously, the substrate is monolithic (in the sense of one-piece, integral, coherent) and preferably uniformly permeated by continuous pores. The pores are preferably substantially rectilinear and aligned parallel to one another, so that a plurality, in particular a multiplicity of channels connected in parallel flow directions is formed. Curved channels that redirect the flow direction to intensify heat exchange at the expense of friction are also encompassed by the invention.

[0023] It is preferred if the pores have a diameter between 200 and 1000 pm, are preferably monodisperse, and preferably have a constant diameter along their length, and / or wherein walls are formed between the pores whose wall thickness is less than 1000 pm, preferably less than 600 pm. In the case of pores with a non-circular cross-section, the term "diameter" is to be understood in a general sense as the extent along a cross-sectional direction. However, the diameter of the channels can also vary in the longitudinal direction. In particular, steps or offsets can be present.

[0024] Advantageously, the volume fraction of the pores of the substrate is in the range of 10 to 60%, preferably 20 to 55%, of the total volume.

[0025] In a variation of the concept, the substrate, prior to immersion in the reaction solution, is in the form of a loose or compressed bed of, for example, granular, powdery, grainy, or other particles. These particles are bonded together (cohesively bonded or "glued") during the coating process by the developing metal coating, so that the previously loose pore structure solidifies and becomes permanent. This means, in particular, that the bed only becomes a monolithic-porous structure, shape, or body during the deposition or coating process. The individual particles of the bed are essentially the building blocks or precursors / precursor particles of the subsequent solid. In general, the substrate can be poured / stacked / assembled / etc. from smaller, originally loose or non-cohesively bonded units or building blocks such as particles, spheres, tubes, or structured layers.which are then bonded together through metallization. These variations are possible with both the nickel plating and copper plating described below.

[0026] The substrate preferably consists of a material based on a metal or metal alloy from the group of transition or main group metals or on the rare earth metal lanthanum. The substrate is particularly preferably a Heusler alloy, a magnetocaloric compound of the iron phosphide type, or a lanthanum-iron-silicon-based alloy, in particular a hydrogenated lanthanum-iron-silicon-manganese alloy.

[0027] Advantageously, the deposition bath has a temperature in the range of 20 to 90°C, preferably less than 60°C. The metal coating is preferably carried out in such a way that the protective effect and stability of the deposited layer does not depend on treatments at elevated temperatures, in particular above 90°C (such as sintering treatments). In other words, at no point during the metal coating process and during any pretreatment before coating and / or during a post-treatment after coating of the substrate is the temperature of the substrate exceeded. Preferably, even 60°C is not exceeded.

[0028] It is preferred if the entire surface of the substrate, in particular the inner surfaces of the pore walls, is constantly in contact with fresh reaction solution during the deposition process, realized by a passive flow through the pores caused by microconvection and / or active flow through the pores with the reaction solution caused by pumps or the like, wherein the substrate is aligned at least temporarily, preferably most of the time, during the deposition process in such a way that hydrogen bubbles forming during the deposition are automatically removed from the pores. This is achieved in particular by a preferred vertical or oblique alignment of the pores, or by preferably constant rotation of the substrate in the solution. In the outer part of the substrate, contact surfaces which, for example,by resting or fastening, preferably minimized in both their extent and their durability by selecting the smallest possible number and size and / or by regular turning or moving. In other words, constant contact points, a build-up of gas bubbles, and stagnation of the reaction solution inside the substrate are avoided. Thus, a uniform coating across the entire substrate surface is ensured by constant contact with fresh, unused reaction solution. A preferred embodiment of this implementation is represented by flatly connected mesh baskets into which individual microchannels are placed, the positioning of which is changed by regular, not necessarily constant, shaking.

[0029] In a preferred special case of the rotation method, the substrate is stored and rotated during the deposition process in a receiving cylinder mounted in the deposition bath. The receiving cylinder preferably has a peripheral surface provided with flow openings for the reaction or deposition solution.

[0030] In a first advantageous variant, the plating bath is a nickel-phosphorus plating bath, which preferably comprises the following ingredients:

[0031] • a Ni(II) source, preferably nickel sulfate

[0032] • Ligands, preferably citrate and / or other carboxylic, hydroxycarboxylic or aminopolycarboxylic acids, preferably in a molar ratio of about 6:1 to 10:1, measured in relation to the number of Ni(II) binding sites provided by the ligand, based on Ni(II)

[0033] • alkalis, preferably ammonia and / or ammonium salts such as ammonium sulfate, particularly preferably ammonia supplemented by an ammonium salt • a catalytically convertible reducing agent, preferably hypophosphites such as potassium, sodium and / or ammonium hypophosphite, and / or hydrazine, borohydride and / or aminoboranes, preferably in a molar ratio of about 2:1 to 3:1, based on Ni(II)

[0034] • optional stabilizers, preferably Pb(II) salts such as lead acetate

[0035] • optionally surface-active compounds, preferably anionic surfactants such as sodium and / or potassium soaps, organosulfonates and / or sulfates, and / or neutral surfactants such as polysorbates

[0036] In a second advantageous variant, the plating bath is a copper plating bath, which preferably comprises the following ingredients:

[0037] • a Cu(II) source, preferably copper sulfate

[0038] • Ligands, preferably tartrate and / or other carboxylic, hydroxycarboxylic or aminopolycarboxylic acids, preferably in a molarity ratio of about 5:1 to 10:1, measured in relation to the number of Cu(II) binding sites provided by the ligand, based on Cu(II)

[0039] • Alkalis, preferably alkali metal hydroxides and carbonates or mixtures thereof, such as sodium hydroxide, potassium hydroxide or sodium carbonate

[0040] • a catalytically convertible reducing agent, preferably formaldehyde, and / or glyoxylic acid and other aldehydes, preferably in a molar ratio of about 2:1 to 3:1, based on the number of molar equivalents required for the complete reduction of the Cu(II)

[0041] • optionally surface-active, preferably anionic surfactants such as sodium or potassium soaps, organosulfonates and / or sulfates, and / or neutral surfactants such as polysorbates

[0042] • optional additives, preferably cyanides such as sodium cyanide and / or nitrogen-based heterocycles such as polypyridines and / or azoles

[0043] • optional stabilizers, preferably oxo anions, sulfur compounds and / or heterocycles

[0044] • optionally Ni(II) salts, preferably nickel sulfate, preferably in a molarity ratio of about 1:4 to 1:8, based on Cu(II). To support the deposition reaction, the deposition solution is advantageously enriched with oxygen during the deposition process, in particular flushed with introduced air bubbles (compressed air) or other oxygen-containing gases or gas mixtures or corresponding gas bubbles.

[0045] This also advantageously initiates or supports circulation of the solution in the separation vessel.

[0046] A particular advantage of the method according to the invention is that the metal deposition on the substrate begins spontaneously, in particular without prior introduction of catalytically active components such as palladium particles, in the submicrometer range, uniformly covering and within less than 10 minutes, preferably within less than 2 minutes, particularly preferably even in less than 30 seconds.

[0047] To promote the onset of the deposition reaction, the deposition process can be optionally assisted by applying an electrical voltage to the substrate or by galvanic contact of the substrate with a metal, in particular copper. Although such assistive measures are advantageously omitted, which reduces the equipment required and simplifies the process, the method according to the invention is in any case compatible or acceptable with them, if desired or advantageous in specific cases.

[0048] Advantageously, the deposition time is selected such that a layer thickness of the metal coating in the range of 3 to 50 pm, preferably 10 to 30 pm, is achieved.

[0049] Advantageously, the substrate is cleaned in / with an organic solvent, in particular N-methylpyrrolidone, isopropanol, ethanol, or acetone, before the deposition process and then, if necessary, dried. Furthermore, no further pretreatment of the starting workpiece is preferably required. Advantageously, the coated substrate is simply washed with water after the deposition process and then, if necessary, dried in air. Furthermore, no further post-treatment of the coated workpiece is preferably required. Rather, it can be used directly as a heat exchanger element or the like; in particular, thermal post-treatments are not required.

[0050] Advantageously, the deposition is carried out electrolessly, i.e., non-galvanically, without applying an (external) galvanic voltage. Therefore, no electrodes are required, which simplifies metallization. Furthermore, the contour accuracy of the deposition is particularly high due to the autonomy of the chemical reaction, which occurs simultaneously at all accessible catalytic interfaces, which is necessary for the uniform coating of the large internal surfaces of delicately structured magnetocaloric heat exchangers.

[0051] Furthermore, the direct wet-chemical growth of the metal film at comparatively low temperatures avoids thermal stresses that lead to material degradation, such as dehydrogenation, which are common in gas-phase reactions such as CVD. Vacuum processes such as sputtering, which, due to shadowing effects, are also incapable of coating hidden, only indirectly accessible internal surfaces and highly selective pores, and which are instrumentally complex, are also avoided. Bonding or coating with comparatively low-melting alloys such as solder is also eliminated.Such processes subject the workpieces to higher thermal stresses and are difficult to achieve in the form of the thin and very homogeneous metal films desired here (several pm to several tens of pm thick), since molten metals are poorly suited to producing uniform wetting films due to their high viscosity and surface tension, especially in thin layers. This problem is further exacerbated by the structural complexity and small-scale dimensions of the components outlined here. In a possible further development of the process, the coated substrate is immersed in a second temperature-controlled deposition bath containing a second autocatalytic chemical reaction solution after the deposition process and, if necessary, after washing and drying. This is done to form a second metal coating on the surface of the previously obtained metal coating.

[0052] The magnetocaloric heat exchanger element produced by the process according to the invention is preferably used in a heat pump or a magnetocaloric cooling or heating device, where it is preferably flowed through by an aqueous or water-based heat exchange medium during operation.

[0053] In addition to or instead of magnetocaloric heat exchanger elements, the method according to the invention can also be used to provide other objects made of corrosion-sensitive magnetocaloric materials with a mechanically stabilizing and / or chemically protective metal layer. Such a coating is particularly suitable for all objects that come into contact with aqueous solutions during operation or that are to be protected from oxidation upon contact with air. In this respect, the present invention generally also encompasses a method for coating corrosion-sensitive objects made of magnetocaloric materials, which can be shaped as desired due to the contour accuracy of the coating, wherein the object forms a substrate that is immersed in an aqueous reaction solution, and wherein the reaction solution leads to an autocatalytic chemical deposition of a metal coating on the surface of the substrate.The process is particularly suitable for objects or substrates that are permeated with continuous pores or microchannels, or that have a fine, porous, granular, or powdery structure. Even internal, curved, or partially concealed surfaces are coated as completely and homogeneously as possible. However, coating the walls of macrochannels is also possible in principle.

[0054] A further aspect of the present invention, which is considered to be inventive in its own right, consists in the provision of an aqueous reaction solution for producing a metal coating on a metallic substrate by autocatalytic chemical deposition, in particular for use in a process of the type described above for producing a magnetocaloric heat exchanger element, comprising the following ingredients:

[0055] • a Ni(I) source, preferably nickel sulfate

[0056] • Ligands, preferably citrate and / or other carboxylic, hydroxycarboxylic or aminopolycarboxylic acids, preferably in a molar ratio of about 6:1 to 10:1, measured in relation to the number of Ni(II) binding sites provided by the ligand, based on Ni(II)

[0057] • Alkalis, preferably ammonia and / or ammonium salts such as ammonium sulfate, particularly preferably ammonia supplemented by an ammonium salt

[0058] • a catalytically convertible reducing agent, preferably hypophosphites such as potassium, sodium and / or ammonium hypophosphite, and / or hydrazine, borohydride and / or aminoboranes, preferably in a molar ratio of about 2:1 to 3:1, based on Ni(II)

[0059] • optional stabilizers, preferably Pb(II) salts such as lead acetate

[0060] • optionally surface-active compounds, preferably anionic surfactants such as sodium and / or potassium soaps, organosulfonates and / or sulfates, and / or neutral surfactants such as polysorbates

[0061] In addition to such a nickel-based coating solution, the present invention also teaches a corresponding copper-based reaction solution comprising the following ingredients:

[0062] • a Cu(II) source, preferably copper sulfate

[0063] • Ligands, preferably tartrate and / or other carboxylic, hydroxycarboxylic or aminopolycarboxylic acids, preferably in a molarity ratio of about 5:1 to 10:1, measured in relation to the number of Cu(II) binding sites provided by the ligand, based on Cu(II)

[0064] • Alkalis, preferably alkali metal hydroxides and carbonates or mixtures thereof, such as sodium hydroxide, potassium hydroxide or sodium carbonate

[0065] • a catalytically convertible reducing agent, preferably formaldehyde, and / or glyoxylic acid and other aldehydes, preferably in a molar ratio of about 2:1 to 3:1, based on the number of molar equivalents required for the complete reduction of the Cu(II) • optionally surface-active compounds, preferably anionic surfactants such as sodium or potassium soaps, organosulfonates and / or sulfates, and / or neutral surfactants such as polysorbates

[0066] • optional additives, preferably cyanides such as sodium cyanide and / or nitrogen-based heterocycles such as polypyridines and / or azoles

[0067] • optional stabilizers, preferably oxo anions, sulfur compounds and / or heterocycles

[0068] • optionally Ni(II) salts, preferably nickel sulfate, preferably in a molar ratio of about 1:4 to 1:8, based on Cu(II)

[0069] In summary, the invention describes autocatalytic chemical deposition processes, particularly for nickel-phosphorus and copper layers, tailored to magnetocaloric materials, especially thermally and chemically sensitive compounds such as hydrogenated lanthanum-iron-silicon-manganese alloys. Such processes enable gentle (particularly avoiding undesirable corrosion side reactions and not subjecting the material to thermal stress), uniform, and effective coatings with metal films. Due to their excellent deposition conformity, these coatings can also be applied to complex substrate geometries such as the technologically particularly relevant porous-monolithic configurations described above. This is ensured by process control and deposition solution compositions tailored to the specific substrate reactivity.

[0070] The advantages achieved by the invention are in particular the following:

[0071] (i) Demonstration of a reliable corrosion-protective autocatalytic chemical coating reaction, in particular the first demonstration of a copper plating reaction, for magnetocaloric, temperature-sensitive materials. Linear sweep voltammetry demonstrated a shift in the corrosion potential of the substrate material consisting of a hydrogenated lanthanum-iron-manganese-silicon alloy by more than 600 mV; with this value, the corrosion potential lies in a range corresponding to that of the pure coating metal copper (FIG. 5), indicating reliable shielding of the substrate material from the surrounding electrolyte. Accompanying scanning electron microscopy investigations confirm the excellent freedom from defects, contour fidelity, and homogeneity of the copper coating (FIG. 6, FIG.7) and the high quality of the interface between the magnetocaloric lanthanum-iron-manganese-silicon substrate and the copper coating, which shows no noticeable traces of oxidation, but an intimate bond between substrate and coating (FIG. 7).

[0072] (ii) Due to the spontaneous and reliable onset of the metallization reactions simply by immersion in the deposition solution(s), aggressive pretreatment steps (e.g. etching, pickling, sensitization or activation of the substrate surface) that can attack the substrate and create weak spots where corrosion can start prematurely (e.g. oxidic precipitates that impair the adhesion of the metal film or that are not completely covered and cause pores in the coating, or chemically attacked areas that have an increased sensitivity to corrosion or are already corroded) can be completely dispensed with.

[0073] (iii) The aqueous deposition solutions have only a weak corrosive effect on the magnetocaloric substrates, and the rapid deposition of metal, which quickly leads to covering films, minimizes the time during which the substrate surface is in direct contact with the solution and can corrode in undesirable side reactions.

[0074] (iv) Due to direct metallization without pretreatment steps, the process control is simplified: savings in production time, costs, material and waste; higher process reliability due to fewer interlocking work steps that depend on each other to ensure product quality.

[0075] (v) By omitting aggressive pretreatment steps in combination with the suppression of corrosion by the bath composition and the rapid coverage of the sensitive surface of the magnetocaloric material, higher quality coatings are achieved and material damage and weak points are avoided.

[0076] (vi) Uncontrolled cementation reactions between the metal ions in the plating baths (in particular Ni(II), Cu(II)) and the base substrate, which result in poorly adhering coatings, corrosion and / or the formation of powdery or voluminous reaction products (reduced Ni or Cu, oxidised substrate components), are avoided.

[0077] (vii) The deposition baths allow a high reactivity even at low deposition temperatures, ideally below 60 °C, which are significantly lower than the industrially usual temperatures of autocatalytic-chemical Ni-P and Cu deposition reactions, but are necessary for the integrity of sensitive magnetocaloric substrates. This is reflected in a rapid and spontaneous onset of the deposition reaction and in high deposition rates of typically 4-10 pm h -1 on these substrate materials. Metal deposition on the substrate begins spontaneously (especially without the use of catalytically active nuclei, such as palladium) within less than 10 minutes, preferably less than 2 minutes. However, the deposition time chosen to achieve the desired layer thickness is usually significantly longer than the onset time and typically amounts to several hours, for example, 2 to 4 hours.

[0078] (viii) The process according to the invention enables the realization of surface-accurate and uniform (contour-accurate) metal depositions on magnetocaloric substrates using simple wet-chemical means. Complex instrumentation, high energy consumption, high control requirements, and the need for a controlled reaction atmosphere (as in the case of gas-phase reactions) are avoided. The thermal stress on the substrates is kept low.

[0079] (ix) Compared to alternative designs such as compacted and sintered granules, the architecture proposed here ensures better flow through the heat exchanger and a more favorable surface-to-volume ratio.

[0080] An embodiment of the invention is explained in more detail below with reference to the accompanying drawings, in which:

[0081] FIG. 1 shows an exemplary, highly simplified view of a porous, monolithic magnetocaloric substrate, here with a multitude of continuous, straight, parallel and monodisperse pores,

[0082] FIG. 2 shows a flowchart-like overview of the process sequence for coating such a substrate,

[0083] FIG. 3 is a schematic representation of an apparatus used for coating the substrate, here with a microchannel substrate stored upright in a reaction solution, and

[0084] FIG. 4 is a schematic representation of a rotating receiving cylinder that holds a plurality of substrates during coating, as an example of an alternative substrate metallization with automated sample movement,

[0085] FIG. 5 shows an electrochemical characterization of the corrosion behavior of an untreated, hydrogenated lanthanum-iron-manganese-silicon alloy substrate, such a substrate that has been copper-plated according to the present invention, and a pure Cu reference sample,

[0086] FIG. 6 is a scanning electron micrograph at two different magnifications showing a plan view of a copper-coated platelet made of a hydrogenated lanthanum-iron-manganese-silicon alloy according to the present invention; no exposed alloy substrate is visible, but only the contour-accurate and compact copper film covering the entire surface, and

[0087] FIG. 7 shows a scanning electron micrograph of the fracture edge of a copper-coated plate made of a hydrogenated lanthanum-iron-manganese-silicon alloy according to the present invention. Due to its granular structure, the plate (upper half of the image) is easily distinguishable from the fine-crystalline copper film, which is compact, homogeneous, and intimately bonded to it. Furthermore, no traces of corrosion are visible at the interface, confirming the integrity of the coating process.

[0088] The invention comprises the realization of a monolithic porous magnetocaloric architecture, mechanically reinforced by an external metal coating and protected against corrosion, for use as a long-term stable and effective temperature conversion element in magnetocaloric heat pumps and cooling devices by means of an autocatalytic chemical deposition process.

[0089] For this purpose, a preferably monolithic magnetocaloric substrate is provided, which consists, for example, of hydrogenated lanthanum-iron-silicon-manganese alloys, Heusler compounds, or iron phosphide-based magnetocaloric materials and has continuous, ideally parallel, and monodisperse pores. An example structure is shown schematically in FIG. 1. The starting product or precursor for the desired heat exchanger element formed by the untreated substrate is, in this example, cuboid-shaped with a first end facing the observer and, opposite, a second end facing away from the observer (not visible).The pores extend through the cuboid from the first to the second end face, forming continuous, parallel (micro)channels with intermediate walls made of substrate material, thus enabling directed flow through the cuboid by a fluid heat transfer medium, also known as a coolant, in or against the direction of the arrow. After an optional cleaning step with one or more organic solvents (e.g., N-methylpyrrolidone, isopropanol, ethanol, or acetone), but otherwise preferably without any further pretreatment steps, the substrate is autocatalytically coated with a metal by immersion in a deposition bath of suitable reactivity. A precise specification of the characteristics preferred specifically for this application can be found below. Targeted average layer thicknesses are typically between 3 and 50 pm, ideally between 10 and 30 pm.In addition to a single coating, a coating with one or two metals can also be applied in two separate deposition steps. The flow chart of the entire process is summarized in FIG. 2.

[0090] Favorable architectures utilize the magnetocaloric material in maximum (wall) thicknesses of approximately 1000 pm, ideally below 600 pm, to ensure good heat transfer from the magnetocaloric functional material. They exhibit sufficiently high porosity for easy flow and uniformly distributed, continuous, straight, and monodisperse pores with diameters of approximately 200–1000 pm, while at the same time possessing a sufficiently high volume fraction of magnetocaloric material to provide good power density. For this purpose, global porosities of 10% to 60% are desirable, ideally 20% to 55%. The pores do not need to have circular cross-sections; other cross-sections are also possible. In particular, elongated or slit-like pores are permissible if they are limited in one dimension to the above-mentioned value of approximately 200–1000 pm. Viewed in cross-section, an area of ​​1 cm 2in the flow direction, preferably a number of many 10 to several 100, in particular 100 to 800 microchannels, wherein the areal density of pores results primarily from the dimensioning of the pores and their distribution in the magnetocaloric matrix.

[0091] Suitable monolithic-porous substrates for the coating can be realized, for example, by stacking and connecting (structured) plates or by 3D printing, or subtractively, for example, by waterjet cutting or wire erosion of solid materials. These details are to be understood as examples. The process described here assumes the availability of suitable substrates, preferably with a chemically reduced or at least not noticeably oxidized high-quality surface. Feasibility has been empirically proven, and corresponding samples are commercially available. However, details of the production of the starting substrates are not the subject of this description.

[0092] Particularly noteworthy among plating baths are those used for the production of nickel-phosphorus and copper coatings. While nickel-phosphorus stands out primarily for its high corrosion resistance, copper is particularly recommended for this application due to its plasticity and excellent thermal conductivity.

[0093] The nickel-phosphorus plating bath preferably contains:

[0094] (i) a Ni(II) source (preferably nickel sulfate) providing the precursor ions required for the metal coating (preferably in the range of 0.03-0.4 mol L' 1 , especially 0.06-0.3 mol L' 1 )

[0095] (ii) Ligands to ensure the solubility of the metal salts used in the alkaline medium, as well as to complex the ions and to adjust their reactivity (preferably citrate or other carboxylic, hydroxycarboxylic or aminopolycarboxylic acids) (preferably in the range of 0.04-0.55 mol L' 1 , especially 0.09-0.4 mol L' 1 )

[0096] (iii) alkalis to adjust the pH to a sufficiently high value to reduce the corrosiveness of the solution to the substrate (preferably ammonia (preferably in the range of 0.6-2.4 mol L' 1 , especially 1.0-2.0 mol L -1 ) and ammonium salts such as ammonium sulfate (preferably in the range of 0.3-

[0097] 1.2 mol L' 1 , especially 0.5-1.0 mol L' 1), particularly preferably ammonia supplemented by an ammonium salt to build up a buffer, with subsequent addition of ammonia, either continuously or in batch mode halfway through the reaction time, to keep the pH stable

[0098] (iv) a sufficiently strong, catalytically convertible reducing agent (preferably hypophosphites such as potassium, sodium or ammonium hypophosphite, but also hydrazine, borohydride or aminoboranes) (preferably in the range of 0.1 -0.5 mol L' 1 , especially 0.2-0.35 mol L' 1 )

[0099] (v) optional, but strongly recommended stabilizers to prevent homogeneous nucleation (for example Pb(II) salts such as lead acetate) (exemplarily in the range of 2-25 ppm, especially 4-9 ppm)

[0100] (vi) optional, but strongly recommended, surface-active compounds to improve the wettability of the substrate surface, to accelerate the detachment of gas bubbles from the substrate to counteract deposition inhomogeneities, and to optimize the solution reactivity, but avoiding the formation of excessively stiff foams or the precipitation of poorly soluble metal soaps, which can close the substrate pores and counteract uniform metallization (preferably anionic surfactants such as sodium or potassium soaps, organosulfonates and sulfates, but also neutral surfactants such as polysorbates) (for example up to 2 mmol L' 1 , especially 0.01-0.7 mmol L -1 )

[0101] The copper plating bath preferably contains:

[0102] (i) a Cu(II) source (preferably copper sulfate) providing the precursor ions required for the metal coating (preferably in the range of 0.02-0.2 mol L' 1, especially 0.03-0.09 mol L' 1 )

[0103] (ii) Ligands to ensure the solubility of the metal salts used in the alkaline medium, as well as to complex the ions and to adjust their reactivity (preferably tartrate or other carboxylic, hydroxycarboxylic or aminopolycarboxylic acids) (preferably in the range of 0.06-0.6 mol L' 1 , especially 0.1 -0.3 mol L' 1 ) (iii) alkalis to adjust a sufficiently high pH (preferably alkali metal hydroxides and carbonates or mixtures thereof, such as sodium hydroxide, potassium hydroxide or sodium carbonate) (preferably in the range of 0.15- 0.6 mol L' 1 , especially 0.2-0.4 mol L -1 )

[0104] (iv) a sufficiently strong, catalytically convertible reducing agent (preferably formaldehyde, but also glyoxylic acid and other aldehydes) (preferably in the range of 0.15-1.0 mol L' 1 , especially 0.2-0.5 mol L -1 )

[0105] (v) optional, but strongly recommended, surface-active compounds to improve the wettability of the substrate surface, to accelerate the detachment of gas bubbles from the substrate to counteract deposition inhomogeneities, and to optimize the solution reactivity, but avoiding the formation of excessively stiff foams or the precipitation of poorly soluble metal soaps, which can close the substrate pores and counteract uniform metallization (preferably anionic surfactants such as sodium or potassium soaps, organosulfonates and sulfates, but also neutral surfactants such as polysorbates) (for example up to 2 mmol L -1 , especially 0.01-0.7 mmol L' 1 )

[0106] (vi) optional, but strongly recommended additives to improve ductility and stabilize the deposition solution (preferably cyanides such as sodium cyanide or nitrogen-based heterocycles such as polypyridines or azoles) (for example up to 3 mmol L' 1 , especially 0.02-0.8 mmol L -1 )

[0107] (vii) optional, but strongly recommended additional stabilizers to prevent homogeneous nucleation under deposition conditions (for example, oxo anions, sulfur compounds, heterocycles) (exemplarily up to 1 mmol L -1 )

[0108] (viii) optional, but strongly recommended Ni(ll) salts for reactivity optimization (preferably nickel sulfate) (exemplarily in the range of 0.003-

[0109] 0.03 mol L' 1 , especially 0.006-0.018 mol L -1) To initiate the reaction, the substrate is simply immersed in the deposition bath, which has been heated to a reaction temperature of below 90 °C, ideally below 60 °C. Deposition begins spontaneously and evenly over the entire substrate surface within less than 10 minutes, ideally in less than 2 minutes (indicated by vigorous gas evolution), but can be further stimulated by applying an electrical voltage, electrical contact of the substrate with a less noble metal such as aluminum, or by contact with metallic copper. During heating and deposition, the bath is preferably flushed with air bubbles to enrich it with oxygen and circulate it. Blowing in other oxygen-containing gases or pure oxygen is also possible.

[0110] When positioning the substrates stationary in the deposition solution, it is preferable to ensure a diagonal to vertical arrangement of the pores so that the hydrogen bubbles formed during deposition are quickly and independently removed from the pores by buoyancy. This ensures both continuous wetting of the entire substrate surface with deposition solution (and thus a uniform coating) and constant renewal of the deposition solution inside the substrate. This can be achieved, for example, by tilting the substrates sideways against the vessel wall, by placing half of them on a raised surface on the vessel bottom, by using a concavely curved vessel bottom, or by placing the substrates on a grid suspended in the solution or similar holder.

[0111] After a deposition time corresponding to the desired layer thickness, the substrate is removed from the solution, preferably thoroughly washed with water, dried in air, and is ready for integration into a heat pump or cooling device. Advantageously, no further post-treatment steps are required.

[0112] An example of the reaction procedure described above is shown in FIG. 3. To avoid permanently inaccessible contact points between the substrate to be metallized and the holder, which could cause weak spots or pores in the coating, it is also advisable to rotate the samples regularly. Ideally, this is done automatically, maintaining as open access as possible to the deposition bath and preventing substrates from falling out of the sample holder. Likewise, the rubbing of multiple substrates against each other and damage to the brittle substrate material or the coating resulting from friction or impact with the moving sample holder, e.g., due to excessively hard surfaces, should be avoided.

[0113] In summary, FIG. 3 shows a deposition bath 2 used in the process according to the invention. A deposition solution 6 or liquid of the above-mentioned type is located in a suitable vessel 4, preferably accessible from above and preferably in a trough-like manner. The vessel 4 contains one or more holders 8 for the substrate 10 to be coated, which, as described above, is preferably a monolith made of a magnetocaloric material and is traversed by parallel pores or (micro)channels 12. The substrate 10 is preferably oriented such that the microchannels 12 are aligned vertically. In this example, the holders 8 are mounted on the bottom of the vessel 4 and support the inserted or loosely placed substrate 10 from below. By suitable shaping, the contact between the holders 8 and the substrate 10 is minimized to a few point contacts.The fill level of the deposition solution 6 is dimensioned such that the substrate 10 is completely immersed in the deposition solution 6. Due to the selected storage, the substrate 10 is in contact with the deposition solution 6 on all sides. In particular, the underside and top of the substrate 10, with the inlet and outlet openings of the microchannels 12 located there, are easily accessible. The deposition solution 6 is adjusted to a desired process temperature by a temperature control device (not shown here).

[0114] Through an air hose 14 immersed in the deposition solution 6, an air stream is introduced into the deposition solution 6 at its outlet opening with the aid of an associated compressor pump or the like. The air bubbles 16 released in this way and ultimately rising upwards enrich the deposition solution 6 with oxygen and circulate it within the vessel 4, thus effectively functioning as a stirrer. In this way, during the deposition process, a directed flow of the deposition solution 6, indicated by the flow arrows 42, exists from bottom to top through the pores or microchannels 12 of the substrate 10 (and along the outer walls). As a result, the entire inner and outer surface of the substrate 10, including the pore walls, is coated with a uniform, thin metal coating 40.Hydrogen bubbles 18 released during the autocatalytic coating reaction rise upwards in the microchannels 12 and escape at the surface of the deposition solution 6 into the surrounding atmosphere.

[0115] Of course, in a suitably large tank with multiple holders, several substrates can also be coated simultaneously, side by side. It is preferable to ensure sufficient distances between the workpieces (substrates) to be coated, so that they do not touch each other during handling.

[0116] A possible modified configuration is the individual storage of the substrates to be coated in the compartmentalized cells of an apertured cylinder rotating in the deposition bath. The cylinder is made of a material that is not chemically attacked by the bath and that is soft enough, at least in the areas in contact with the substrate (inner walls of the cells), to prevent mechanical damage to the substrates during the coating process (e.g., silicone-coated metal).

[0117] Such a receiving cylinder 20 is shown schematically in FIG. 4. The receiving cylinder 20 is designed as a hollow cylinder with a shell surface 22 enclosing the interior and end-face closure caps or end walls 24. The cylindrical interior is divided longitudinally by partition walls 26 into individual cells 28 or compartments, each of which is designed to accommodate a single workpiece (substrate 10). For the insertion and subsequent removal of the respective monolith or workpiece, each cell is provided with an access opening 30 of sufficient size, which can be closed by a lockable closure flap 32 for the deposition process. This prevents the workpieces from falling out during coating.To ensure sufficient flow through the individual cells 28 during coating, the outer surface 22 of the receiving cylinder 20 has a plurality of, for example, slot-like flow openings 34 distributed over the circumference and in the longitudinal direction, which are kept so small that the workpieces cannot pass through. At the end faces, the receiving cylinder 20 is connected to a drive shaft 36 that is mounted so as to rotate in the deposition bath 2 and is preferably motor-driven, so that the receiving cylinder 20 can be rotated in the deposition bath 2 during the deposition or coating process. The axis of rotation is preferably aligned horizontally, so that the individual workpieces tumble around in the lower region of the receiving cylinder 20 at a not too high rotation speed and are constantly turned.

[0118] Through direct autocatalytic wet-chemical coating of magnetocaloric substrates, effective corrosion protection has actually been demonstrated in experiments, for example by the reduction of the corrosion potential to the value of the coating metal measured in linear voltametric measurements, which indicates an excellent defect-free coating.

[0119] For example, reference is made to FIG. 5, which shows a diagrammatic representation of the corrosion potential measured on various samples, namely an untreated, hydrogenated lanthanum-iron-manganese-silicon alloy substrate, a substrate copper-plated according to the present invention, and a pure Cu reference sample. The measurement is based on a three-electrode setup with a platinized Ti mesh as the counter electrode, various metal plates as the working electrode, and a mercury-mercury sulfate reference electrode. Other parameters are: Electrolyte: ultrapure (>18.2 MO cm) water, mixed with 100 mM TRIS buffer and 20 mM citric acid; scan rate: 0.25 mV / s; control via a Gamry Reference 600 potentiostat.Even complex-shaped magnetocaloric workpieces such as the microchannel monoliths mentioned above could be metallized simply, reliably, quickly and uniformly using the developed deposition reactions with scalable, material-friendly processes based on inexpensive basic chemicals.

[0120] The high quality of the deposited metal film was also demonstrated by scanning electron microscopy (see FIGS. 6 and 7), as well as the successful, homogeneous and covering metallization of the inner surfaces of microchannel monoliths.

[0121] All deposition and processing steps actually take place at comparatively low temperatures. More thermally activated methods for deposition (e.g., chemical or physical vapor-phase synthesis, immersion in molten metals) or for enhancing coating quality (e.g., improving layer adhesion, sintering a coating, or stress relief through thermal treatment), which cannot be readily applied to temperature-sensitive magnetocaloric substrates, are not required.

[0122] The mechanical toughness of the copper film was investigated in fracture tests, which demonstrated an increase in the mechanical stability and fracture resistance of the intrinsically brittle magnetocaloric monoliths due to the metal coatings.

[0123] The improved quality of metallized magnetocaloric microchannel substrates in terms of coating homogeneity, defect-freeness, and corrosion protection by avoiding aggressive process steps was also demonstrated: The inclusion of otherwise common pretreatment methods such as etching the substrate surface with dilute acids (e.g., hydrochloric or sulfuric acid) or immersion in sensitization and activation solutions (e.g., hydrochloric acid solutions of SnCl2 or PdCl2), which are common for the preparation of chemical metallizations, led to impaired adhesion of the metal films and produced visible and noticeable traces of corrosion, in contrast to the direct metallization proposed here. The need to apply a catalyst is unnecessary in this case due to the spontaneous initiation of the reaction upon immersion of the substrate in the deposition solutions.

[0124] List of reference symbols

[0125] 2 Deposition bath

[0126] 4 vessels

[0127] 6 Reaction or deposition solution

[0128] 8 holders

[0129] 10 Substrat

[0130] 12 (micro) channel

[0131] 14 Air hose

[0132] 16 air bubbles

[0133] 18 hydrogen bubbles

[0134] 20 receiving cylinders

[0135] 22 lateral surface

[0136] 24 End wall

[0137] 26 Partition wall

[0138] 28 Cell

[0139] 30 access opening

[0140] 32 Closing flap

[0141] 34 Flow opening

[0142] 36 Drive shaft

[0143] 40 Metal coating

[0144] 42 Flow arrow

[0145] 44 underflow area

Claims

Claims 1. Method for manufacturing a magnetocaloric heat exchanger element comprising the following steps: • Providing a magnetocaloric substrate (10) with a plurality of permeable channels (12) or pores, which preferably form continuous channels (12), • Immersion of the substrate (10) in a tempered aqueous reaction solution (6), which leads to an autocatalytic chemical deposition of a metal coating (40) on the surface of the substrate (10).

2. Method according to claim 1, wherein the substrate (10) is monolithic and preferably uniformly permeated by continuous pores.

3. Method according to claim 1 or 2, wherein the pores have a diameter between 200 and 1000 pm, are preferably monodisperse and preferably have a constant diameter along their course, and / or wherein walls are formed between the pores, the wall thickness of which is less than 1000 pm, preferably less than 600 pm.

4. Method according to one of the preceding claims, wherein the volume fraction of the pores of the substrate (10) is in the range of 10 to 60%, preferably 20 to 55%, of the total volume.

5. Method according to claim 1, wherein the substrate (10) is present before immersion in the reaction solution (6) as a loose or pressed bulk or stack of smaller building units such as particles, spheres, tubes or structured layers, which are joined together during the coating process by the forming metal coating.

6. Method according to any one of the preceding claims, wherein the substrate (10) is a Heusler alloy, a magnetocaloric compound of a iron phosphide type or a lanthanum-iron-silicon-based alloy, in particular a hydrogenated lanthanum-iron-silicon-manganese alloy.

7. Method according to one of the preceding claims, wherein the reaction solution (6) has a temperature in the range of 20 to 90 °C, preferably less than 60 °C.

8. A method according to one of the preceding claims, wherein the entire surface of the substrate (10), in particular the interior surfaces of the pore walls, is constantly in contact with fresh reaction solution (6) during the deposition process, realized by a passive flow through the pores with the reaction solution (6) effected by microconvection and / or by pumps or the like, and wherein preferably the substrate (10) is oriented during the deposition process at least temporarily, preferably for most of the time, such that hydrogen bubbles (18) formed during deposition are automatically removed from the pores.

9. Method according to any of the preceding claims, wherein the reaction solution (6) acts as a nickel-phosphorus deposition bath.

10. The method of claim 9, wherein the reaction solution (6) comprises the following ingredients: • a Ni(ll) source, preferably nickel sulfate • Ligands, preferably citrate and / or other carboxylic, hydroxycarbon or aminopolycarboxylic acids, preferably in a molarity ratio of about 6:1 to 10:1, measured in terms of the number of Ni(ll) binding sites provided by the ligand, based on Ni(ll) • Alkalis, preferably ammonia and / or ammonium salts such as ammonium sulfate, particularly preferably ammonia supplemented by an ammonium salt • a catalytically reactive reducing agent, preferably hypophosphites such as potassium, sodium and / or ammonium hypophosphite, and / or Hydrazine, borohydride and / or aminoboranes, preferably in a molarity ratio of about 2:1 to 3:1, based on Ni(ll) • Optional stabilizers, preferably Pb(II) salts such as lead acetate • Optionally, surfactants, preferably anionic surfactants such as sodium and / or potassium soaps, organosulfonates and / or sulfates, and / or neutral surfactants such as polysorbates 11. Method according to any one of claims 1 to 8, wherein the reaction solution (6) acts as a copper deposition bath.

12. The method of claim 11, wherein the reaction solution (6) comprises the following ingredients: • a Cu(ll) source, preferably copper sulfate • Ligands, preferably tartrate and / or other carboxylic, hydroxycarboxylic or aminopolycarboxylic acids, preferably in a molarity ratio of about 5:1 to 10:1, measured in terms of the number of Cu(ll) binding sites provided by the ligand, based on Cu(ll) • Alkalis, preferably alkali metal hydroxides and carbonates or mixtures thereof, such as sodium hydroxide, potassium hydroxide or sodium carbonate • a catalytically reactive reducing agent, preferably formaldehyde, and / or glyoxylic acid and other aldehydes, preferably in a molarity ratio of about 2:1 to 3:1, based on the number of molar equivalents required for the complete reduction of Cu(ll) • Optionally, surfactants, preferably anionic surfactants such as sodium or potassium soaps, organosulfonates and / or sulfates, and / or neutral surfactants such as polysorbates • Optional additives, preferably cyanides such as sodium cyanide and / or nitrogen-based heterocycles such as polypyridines and / or azoles • Optional stabilizers, preferably oxo anions, sulfur compounds and / or heterocycles • optional Ni(ll) salts, preferably nickel sulfate, preferably in a molarity ratio of about 1 :4 to 1 :8, based on Cu(ll) 13. Method according to one of the preceding claims, wherein the metal deposition on the substrate (10) occurs spontaneously, in particular without prior introduction of catalytically active components such as palladium particles, uniformly covering in the submicrometer range and within less than 10 minutes, preferably within less than 2 minutes, particularly preferably less than 30 seconds.

14. Method according to one of the preceding claims, wherein the coated substrate (10) is immersed in a second temperature-controlled deposition bath containing a second autocatalytic chemical reaction solution after the deposition process and, if necessary, after washing and drying, to form a second metal coating on the surface of the previously obtained metal coating.

15. Method according to any of the preceding claims, wherein no pretreatment steps from the group consisting of etching, pickling, grinding, polishing, sensitization and / or activation of the substrate surface are carried out before immersing the substrate (10) in the reaction solution (6).

16. Method according to claim 15, wherein, prior to immersion of the substrate (10) in the reaction solution (6) for pretreatment, the substrate (6) may be cleaned with water and / or an organic solvent and may be subsequently dried.

17. Method according to any of the preceding claims, wherein the deposition is carried out without current.

18. Method according to one of the preceding claims, wherein the channels (12) or pores are permeable to a heat transfer fluid, in particular water or an aqueous solution, after metal coating has been carried out.

19. Magnetocaloric heat exchanger element, manufactured according to a method according to one of the preceding claims.

20. Heat pump or cooling or heating device with a heat exchanger element according to claim 20.

Images