METHOD FOR METALIZING A THERMOELECTRIC STRUCTURE

Description

TECHNICAL FIELD

[0001] The present invention relates to the general field of thermoelectric modules. The invention concerns a method for manufacturing thermoelectric structures. The invention finds applications in numerous industrial fields, and in particular for applications requiring thermoelectric generators where a thermal gradient is available (e.g., transportation, industry, etc.), radioisotope thermoelectric generator applications, Peltier applications, and thermal sensor applications.

[0002] The invention is particularly interesting because it allows the formation of thermoelectric structures / devices exhibiting low resistivities. PREVIOUS STATE OF THE ART

[0003] Generally, thermoelectric (TE) modules comprise a set of first pads made of a thermoelectric material with a first type of conductivity and a set of second pads made of a thermoelectric material with a second type of conductivity. For example, the first material is an N-type material (i.e., with N-type conductivity) and the second material is a P-type material (i.e., with P-type conductivity).

[0004] The pads are electrically connected in series and thermally connected in parallel. The pads are linked together by metallic elements. Thermoelectric junctions are also called NP junctions. The pads are held in place by ceramic substrates arranged on either side of the pad assemblies.

[0005] The electrical performance of a TE device in generator mode is given by: an internal electrical resistance R int defined according to (1): R int = N × ρ np × L / A + R c + R met with N the number of junctions np, ρ np the electrical resistivity of the materials NP, L the length of a line or thickness of a pad, A the cross-section of a line or pad, R c the total resistance of the contacts and R met the total resistance of the metallic junctions, a useful electrical power Pu defined according to (2): Pu = V oc 2 / 4 R int with V oc the voltage generated by the TE module.

[0006] Thus, to have high power, it is necessary to have a low internal electrical resistance, and therefore to reduce the contribution of the total resistance of the contacts R c and the total resistance of the metallic junctions Rmet.

[0007] Typically, the manufacturing of TE modules is generally carried out from the following steps: manufacturing of TE materials (sintering), shaping of the pads, metallization of the pads, assembly with the substrates.

[0008] The metallic connections are made directly on the substrates, for example by the so-called direct copper (or DBC for 'Direct Bonding Copper') technique, then brazing and pressing with the pads.

[0009] However, this manufacturing process is relatively complex and greatly limits the geometry and modularity of the thermoelectric device manufactured.

[0010] There are also substrate-free TE modules, also called "skeleton modules." These modules therefore do not have heat loss due to substrates. However, they cannot be in contact with electrically conductive surfaces.

[0011] It is also possible to use TE pads made from several TE materials. These so-called segmented pads can accommodate a greater temperature difference at the ends of the module because the materials used are generally optimized for different temperature ranges.

[0012] Document WO 2020 / 066948 A1 describes a method for manufacturing a thermoelectric module that facilitates the arrangement of thermoelectric elements, a method that can be carried out in a limited number of steps. DESCRIPTION OF THE INVENTION

[0013] One aim of the present invention is to propose a method for manufacturing thermoelectric structures, simple to implement, and enabling the manufacture of thermoelectric structures having good electrical properties (in particular, low contact resistance) and / or good thermal properties.

[0014] To this end, the present invention proposes a method for manufacturing a thermoelectric structure comprising the following steps: a) provide a substrate, covered completely or locally, by a metallic layer, b) form a thermoelectric element in a thermoelectric material, on the metallic layer, by additive manufacturing, preferably by selective laser sintering (SLS) or laser powder bed fusion (FLLP).

[0015] The process according to the invention further includes a subsequent step c) in which the substrate is removed, thereby obtaining a thermoelectric structure comprising the metallic layer and the thermoelectric element.

[0016] The invention differs fundamentally from the prior art by the implementation of a step in which the functionalization (metallization) of the thermoelectric element (for example a thermoelectric pad) is carried out during the additive manufacturing process.

[0017] This leads not only to a significant reduction in the number of steps and therefore to a simplification of the process compared to prior art processes, but also to considerable time savings and a reduction in costs.

[0018] The resulting metallization exhibits good mechanical strength and good electrical and / or thermal conductivity properties.

[0019] Preferably, the additive manufacturing technique is powder bed fusion (PBF) or selective laser sintering (SLS). The process includes a subsequent step (c) in which the substrate is removed, resulting in a thermoelectric structure comprising the metal layer and the thermoelectric element.

[0020] Advantageously, the thermoelectric element is a part having the shape of a comb delimiting a base and a plurality of branches, substantially parallel to each other, extending substantially orthogonally from the base, the plurality of branches having a first end and a second end, the first end being connected to the base, and the second end of the plurality of branches being in contact with the metallic layer.

[0021] Advantageously, the thermoelectric element is a pad, having a base and a height.

[0022] According to this advantageous variant, the metallic layer can have the same surface area as the base of the pad. Also according to this advantageous variant, the metallic layer can have a larger surface area than the base of the pad. Advantageously, in step a), the metallic layer locally covers the substrate so as to form a plurality of islands, and one or more thermoelectric elements are deposited on each island in step b). Advantageously, the thermoelectric material is Si, SiGe, Bi₂Te₃, Half-Heusler, or Skutterudites.

[0023] Advantageously, the substrate is 316L steel, aluminum, titanium, a CuZr alloy, a ceramic or graphite.

[0024] Advantageously, the metallic layer is made of a material chosen from Al, Ti, Cu, Au and Ni.

[0025] The process offers numerous advantages: It has a small number of steps, it is simple and economical, it allows a great modularity in the shape and design of the thermoelectric device manufactured, the pads can have complex shapes.

[0026] The thermoelectric structure obtained by such a process comprises successively: possibly a substrate, a metallic layer, for example in Al, Ti, Cu, Au or Ni, one or more thermoelectric elements, preferably pads, arranged on the metallic layer.

[0027] Other features and advantages of the invention will become apparent from the supplementary description that follows.

[0028] It goes without saying that this additional description is given only as an illustration of the object of the invention and should in no way be interpreted as a limitation of this object. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The present invention will be better understood upon reading the description of exemplary embodiments given by way of illustration only and in no way limiting, with reference to the accompanying drawings in which: THE Fig. 1A, Fig. 1B and Fig. 1C schematically and in cross-section, they represent the different stages of a manufacturing process for a thermoelectric structure according to a first embodiment of the invention. Fig. 2A, Fig. 2B , Fig. 2C and Fig. 2D They represent, schematically and in cross-section, the different stages of a manufacturing process for a thermoelectric device according to an embodiment outside the scope of this invention. Fig. 3A, Fig. 3B, Fig. 3C , Fig. 3D and Fig. 3E schematically and in cross-section, they represent the different stages of a manufacturing process for a thermoelectric device according to a second embodiment of the invention. Fig. 4A, Fig. 4B, Fig. 4C And Fig. 4DThey represent, schematically and in cross-section, the different stages of a manufacturing process for a thermoelectric device according to an embodiment outside the scope of this invention. Fig. 5A, Fig. 5B and Fig. 5C schematically and in cross-section, they represent the different stages of a manufacturing process for a thermoelectric structure according to a third embodiment of the invention. Fig. 6A, Fig. 6B,Fig. 6C and Fig. 6D represent, schematically and in cross-section, the different stages of a manufacturing process for a thermoelectric structure according to a fourth embodiment of the invention.

[0030] The different parts represented in the figures are not necessarily shown on a uniform scale, in order to make the figures more legible.

[0031] The different possibilities (variants and modes of implementation) should be understood as not being mutually exclusive and can be combined with each other.

[0032] In addition, in the description below, orientation-dependent terms such as "above", "below", etc. of a structure apply assuming that the structure is oriented as illustrated in the figures. DETAILED DESCRIPTION OF SPECIFIC IMPLEMENTATION METHODS

[0033] Although not limiting in any way, the invention is particularly interesting for applications requiring thermoelectric generators (TEGs) where a thermal gradient is available (e.g., transportation, industry, etc.), radioisotope thermoelectric generator (RTG) applications, especially for SiGe, Peltier applications, and thermal sensor applications. As shown in the Fig. 1A to 1C , 3A to 3C , 5A to 5C , 6A to 6D The manufacturing process for a thermoelectric structure according to the invention comprises the following steps: a) provide a substrate 100, covered completely or locally, by a metallic layer 300, b) form a thermoelectric element 200 of a thermoelectric material, on the metallic layer 300, by additive manufacturing, preferably by SLS or FLLP, thereby obtaining a thermoelectric structure comprising the substrate 100 covered successively by the metallic layer 300 and by the thermoelectric element (200), and c) remove the substrate 100, thereby obtaining a thermoelectric structure comprising the metallic layer 300 and the thermoelectric element (200).

[0034] The substrate 100 provided in step a) can be a tray or a top tray.

[0035] In additive manufacturing machines, build plates are attached directly to the build plates, allowing not only for thinner thicknesses (between 200 µm and a few millimeters), but also for a wider range of usable materials. The build plate is advantageously made of ceramic.

[0036] The substrate 100 is, for example, a metal (e.g., Al, Ti, Cu, Au, or Ni), a metal alloy, a semiconductor material, a ceramic, or graphite. For example, it is 316L steel, aluminum, titanium, CuZr, ceramic, or graphite.

[0037] The substrate 100 can have a thickness ranging from a few hundred micrometers to a few centimeters, and preferably from a few hundred micrometers to a few millimeters.

[0038] The substrate 100 supplied in step a) is covered by a metallic layer 300 (also called the bonding layer) in a third material.

[0039] The 300 bonding layer can be deposited, for example, by Physical Vapor Deposition (PVD), evaporation, or spraying. The 300 bonding layer can locally cover ( Fig. 1A, 2A , 3A , 4A , 6A ) or completely ( Fig. 5A ) the substrate 100. For example, the metallic bonding layer 300 forms islands on the surface of the substrate 100.

[0040] To obtain a metallic layer 300 locally covering the substrate 100, it is possible to perform a localized deposition of this layer. Alternatively, it is possible to perform a full-plate deposition of a continuous layer followed by a step in which part of the continuous layer is removed, for example by etching, to form the islands.

[0041] The metallic bonding layer 300 is made of a different material than the substrate 100.

[0042] The 300 bonding layer is, for example, made of metal or a metal alloy. Preferably, the metal is chosen from Al, Ti, Cu, Au, and Ni. Several layers can be superimposed; for example, it can be a bilayer or a trilayer. A trilayer consisting of a copper layer, a nickel layer, and a gold layer could be chosen. By way of illustration and not limitation, a trilayer consisting of Cu (for example, 200 nm) + Ni (for example, 5 µm) + Au (for example, 10 nm) could be chosen.

[0043] After the formation of the 300 bonding layer, an annealing step can be carried out.

[0044] In step b), the thermoelectric element 200 is formed on the metallic tack layer 300. It is in direct contact with this tack layer 300. The thermoelectric element 200 is made of a second material. The second material is preferably chosen from Si, SiGe, MnSi, Bi₂Te₃, Half-Heusler, and Skutterudites. Skutterudites are mineral species composed of cobalt and nickel arsenide with the formula (Co,Ni)As₃-x and traces of S, Bi, Cu, Pb, Zn, Ag, Fe, and Ni.

[0045] The thermoelectric element 200 can be of type N conductivity to favor the movement of electrons (i.e. the material which composes it has a strictly negative Seebeck coefficient) or of type P conductivity to favor the movement of holes (i.e. the material which composes it has a strictly positive Seebeck coefficient).

[0046] For example, N-type doped thermoelectric material is a phosphorus-doped silicon-germanium (SiGe) alloy or N-type doped polysilicon. The N-type dopant can be phosphorus or arsenic.

[0047] For example, P-type material is a boron-doped silicon-germanium (SiGe) alloy or P-type doped polysilicon. The P-type dopant is preferentially boron.

[0048] The doping agent is advantageously integrated directly into the base powder.

[0049] A thermoelectric element 200 ( Fig. 1B, 2B , 3B , 6B ) or several thermoelectric elements 200, 201 can be formed on each island ( Fig. 4B ).

[0050] The thermoelectric element 200 deposited in step b) is obtained by additive manufacturing. The process consists of depositing the material in several successive passes onto the adhesion layer 300. After these successive depositions, the thermoelectric element is obtained. The substrate can be a tray or a sub-tray.

[0051] The additive manufacturing technique is preferably a powder bed fusion laser technique (FLLP or PBF for "Powder Bed Fusion" in Anglo-Saxon terminology) or a Selective Laser Sintering technique ("SLS" which is the acronym for "Selective Laser Sintering") according to Anglo-Saxon terminology.

[0052] FLLP processes involve melting certain regions of a powder bed, for example using a laser beam.

[0053] In the SLS process, the powders are sintered. The materials in the powders do not pass into the liquid phase.

[0054] However, other additive manufacturing techniques can be considered, such as "Cold Spray", "Electron Beam Melting", etc.

[0055] The thermoelectric element 200 deposited in step b) can take several forms. According to a first advantageous embodiment, the thermoelectric element 200 is a comb-shaped part ( Fig. 5B ).

[0056] The part is shaped like a comb, delimiting a base and a plurality of branches, substantially parallel to each other, extending substantially orthogonally from the base. The plurality of branches has a first end and a second end. The first end is connected to the base, and the second end is in contact with the metallic bonding layer 300.

[0057] By "approximately orthogonal", we mean "orthogonal" or "orthogonal within a tolerance of plus or minus 10°".

[0058] By "substantially parallel", we mean "parallel" or "parallel within a tolerance of plus or minus 10°".

[0059] According to another embodiment, the thermoelectric element 200 is a stud ( Fig. 1B, 2B , 3B , 4B , 6A The plot has a base with a surface area and a height.

[0060] The 300 metallic layer can have the same surface area as the surface area of ​​the base of the stud or the second end of the comb arms.

[0061] The metallic layer 300 can have a surface area greater than the surface area of ​​the base of the stud. Advantageously, in step b), several studs 200, 201 are deposited on the metallic layer 300 ( Fig. 4B ).

[0062] At the end of step b), a thermoelectric structure is obtained comprising a substrate 100, a continuous or discontinuous bonding layer 300, for example in the form of islands, on which one or more thermoelectric elements 200 are formed.

[0063] After step b), thermal annealing can be carried out.

[0064] Advantageously, between step b) and step c), the process includes an additional step in which an intermediate metallization layer 400 is deposited on the thermoelectric element 200 ( Fig. 6B ) then an additional thermoelectric element 500 in a fourth material ( Fig. 6C The fourth material is different from the second material.

[0065] During step c), substrate 100 was removed.

[0066] The substrate 100 can be removed by laser cutting, water jet, wire saw, etc.

[0067] At the end of step c), a thermoelectric structure is obtained comprising a metallic bonding layer 300 on which one or more thermoelectric elements 200 are formed.

[0068] Thus, at the end of step c), a first thermoelectric structure is obtained. Advantageously, the manufacturing process described above is implemented to manufacture a second thermoelectric structure ( Fig. 3D ).

[0069] At the end of step b), the structure successively comprises a substrate 110, a metallic layer 310 and one or more thermoelectric elements 210, 211. The thermoelectric material of this structure has a different doping level than that of the first structure. The substrate 110 is then removed to obtain a second thermoelectric structure comprising a metallic layer 310 and one or more thermoelectric elements 210, 211 ( Fig. 3D ).

[0070] The two structures obtained are advantageously assembled and electrically connected to form a thermoelectric device ( Fig. 3E ).

[0071] The devices can be connected in series and / or parallel. Combining series and parallel connections is advantageous for optimizing the electrical output performance of the thermoelectric device.

[0072] The materials of the metallic layers 300 and 310 of the two structures may be identical or different. The materials of the substrates 100 and 110 used may be identical or different.

[0073] The invention is particularly interesting for manufacturing conventional thermoelectric modules, DBC substrates, so-called skeleton thermoelectric modules or even segmented thermoelectric pads.

[0074] The resulting thermoelectric device can operate in Seebeck mode (i.e., the thermoelectric device is then an electrical energy generator) or in Peltier mode (i.e., the thermoelectric device is then a thermal energy generator).

[0075] We will now describe in more detail different embodiments with reference to the attached figures.

[0076] According to a first embodiment represented in Fig. 1A to 1C The process according to the invention comprises the following steps: a) provide a substrate 100 in a first material, locally covered by a metallic layer 300 forming islands in a third material, b) deposit a thermoelectric pad 200 in a second material on each island of the metallic layer 300, by additive manufacturing, preferably by SLS or FLLP, the surface of the base of the pads advantageously being the same size as the surface of the islands, c) remove the substrate 100, thereby obtaining a thermoelectric structure comprising thermoelectric pads 200 covered by a metallic layer 300.

[0077] This first method of implementation is advantageous because it allows for the simple metallization of the pads by depositing a 300 metallic layer on the platform of the additive manufacturing machine.

[0078] According to an embodiment outside the invention shown on the Fig. 2A to 2D A process is described that includes the following steps: a) provide a substrate 100 in a first material, locally covered by a metallic layer 300 forming islands in a third material, b) deposit a thermoelectric pad 200 in a second material on each island of the metallic layer 300, by additive manufacturing, preferably by SLS or FLLP, the surface area of ​​the base of the pads being advantageously smaller than the surface area of ​​the islands, thereby obtaining a first thermoelectric structure comprising a substrate 100 locally covered by a metallic layer 300 forming islands, a thermoelectric pad 200 being disposed on each island, d) repeat steps a) and b) to form a second thermoelectric structure comprising a substrate 110 locally covered by a metallic layer 310 forming islands, each island being covered by a thermoelectric pad 210,the thermoelectric material of the thermoelectric pads of the second structure being of opposite conductivity to that of the second material of the first structure (, Fig. 2C ), e) assemble and electrically connect the first thermoelectric structure and the second thermoelectric structure ( Fig. 2D ).

[0079] This embodiment, outside the scope of the invention, allows for the creation of skeleton modules.

[0080] According to a second embodiment, represented in Fig. 3A to 3E The process according to the invention comprises the following steps: a) provide a substrate 100 of a first material, locally covered by a metallic layer 300 forming islands of a third material, b) deposit a thermoelectric pad 200 of a second material on each island of the metallic layer 300, by additive manufacturing, preferably by SLS or FLLP, the surface area of ​​the base of the pads being advantageously smaller than the surface area of ​​the islands, c) remove the substrate 100, thereby obtaining a first thermoelectric structure comprising thermoelectric pads 200 covered by a metallic layer 300, d) repeat steps a) to c) to form a second thermoelectric structure ( Fig. 3D ), e) assemble and electrically connect the first thermoelectric structure and the second thermoelectric structure ( Fig. 3E ).

[0081] More specifically, step d) includes the following steps: provide a substrate 110 of a material which may be identical or different from that of the substrate 100 of the first structure, the substrate 110 being locally covered by a metallic layer 310 forming islands, deposit a thermoelectric pad 210 of a fourth material on each island of the metallic layer 310, by additive manufacturing, preferably by SLS or FLLP, the surface of the base of the pads being advantageously less than the surface of the islands, the pads of the second structure being of a thermoelectric material of conductivity opposite to the conductivity of the thermoelectric material of the first thermoelectric structure, remove the substrate 110, thereby obtaining a thermoelectric structure comprising thermoelectric pads 210, 211 covered by a metallic layer 310.

[0082] According to this second embodiment, skeleton modules can thus be formed ( Fig. 3E ).

[0083] According to one embodiment (not part of the invention), shown in Fig. 4A to 4D A process is described that includes the following steps: a) provide a substrate 100 in a first material, locally covered by a metallic layer 300 in a third material, forming islands, b) deposit several thermoelectric pads 200, 201, in a second material on each island of the metallic bonding layer 300, by additive manufacturing, preferably by SLS or FLLP, thereby obtaining a structure comprising a substrate 100 in a first material covered by a metallic layer 300 forming islands, on which are arranged several thermoelectric pads 200, 201.

[0084] The same process is used to manufacture an additional structure comprising a substrate 110, locally covered by a metallic layer 310 forming islands on which several thermoelectric pads 210, 211 are formed by additive manufacturing ( Fig. 4C The thermoelectric pads 210, 211 of the second structure have a conductivity opposite to that of the pads 200, 210 of the first structure. The substrates 100, 110 of both structures are advantageously ceramic overlays, having, for example, a thickness between 200 µm and a few millimeters (for example, 3 millimeters).

[0085] The two structures are then assembled ( Fig. 4D ).

[0086] This embodiment, outside the scope of this invention, allows for the combination of series and parallel connections, thereby optimizing electrical output performance. Indeed, it is possible to connect the various thermoelectric terminals in a customized electrical configuration. Typically, all terminals are connected in series. However, this can lead to high output voltages (several volts), which is incompatible with the associated electronics (power management unit) where voltages are generally only a few volts. This embodiment allows for connecting certain terminals in parallel while reducing the output voltage and maintaining the generated power.

[0087] According to a third embodiment shown on the Fig. 5A to 5C The process according to the invention comprises the following steps: a) provide a substrate 100 in a first material, covered by a metallic tack layer 300 in a third material, b) form a comb-shaped thermoelectric part 200 in a second material on the metallic tack layer 300, by additive manufacturing, preferably by SLS or FLLP, c) remove the substrate 100 and cut the metallic tack layer 300 to the size of the second end of the branches, thereby obtaining a comb 200 having branches metallized by a metallic layer 300.

[0088] The same process is used to manufacture another comb from a material with conductivity opposite to that of the first comb. The two combs are then assembled. According to a fourth embodiment shown in the Fig. 6A to 6D The process according to the invention comprises the following steps: a) provide a substrate 100 in a first material, locally covered by a metallic layer 300, so as to form metallic islands, b) deposit a thermoelectric pad 200 in a second material on each metallic island covering the substrate 100, by additive manufacturing, preferably by SLS or FLLP, then a layer of metallization 400 and another thermoelectric element 500 in another thermoelectric material, c) remove the substrate 100, thereby obtaining a thermoelectric structure comprising a metallic layer 300 covered by a first thermoelectric pad 200, a layer of metallization 400 and then a second thermoelectric pad 500.

[0089] This method of embodiment is particularly advantageous for manufacturing segmented thermoelectric pads.

Claims

1. Method for manufacturing a thermoelectric structure comprising the following steps: a) providing a substrate (100), covered completely or locally with a metal layer (300), b) forming a thermoelectric element (200) made from a thermoelectric material on the metal layer (300), by additive manufacturing, preferably by selective laser sintering or by powder bed laser fusion, characterised in that it includes a subsequent step c) during which the substrate (100) is removed, whereby a thermoelectric structure is obtained comprising the metal layer (300) and the thermoelectric element (200).

2. Method according to claim 1, characterised in that the thermoelectric element (200) is a pin, having a base and a height.

3. Method according to claim 1 or 2, characterised in that the metal layer (300) has the same surface area as the surface area of the base of the pin.

4. Method according to claim 1 or 2, characterised in that the metal layer (300) has a surface area greater than the surface area of the base of the pin.

5. Method according to any one of claims 1 to 4, characterised in that, in step a), the metal layer (300) locally covers the substrate (100) so as to form a plurality of islands and in that a thermoelectric element or a plurality of thermoelectric elements (200) are deposited on each island during step b).

6. Method according to claim 1, characterised in that the thermoelectric element (200) is a part in the form of a comb delimiting a base and a plurality of arms, substantially parallel to each other, extending substantially orthogonally from the base, the plurality of arms having a first end and a second end, the first end being connected to the base, and the second end of the plurality of arms being in contact with the metal layer (300).

7. Method according to any one of the preceding claims, characterised in that the thermoelectric material is selected from Si, SiGe, Bi2Te3, Half-Heusler and Skutterudites.

8. Method according to any one of the preceding claims, characterised in that the substrate (100) is 316L steel, aluminium, titanium, a CuZr alloy, a ceramic or graphite.

9. Method according to any one of the preceding claims, characterised in that the metal layer (300) is made from a material selected from Al, Ti, Cu, Au and Ni.