Method for manufacturing thermoelectric elements by additive manufacturing

Abstract

Method for manufacturing thermoelectric elements by additive manufacturing. The invention relates to a method for manufacturing a thermoelectric element (10), comprising the following steps: - manufacturing by additive manufacturing an intermediate element (50), said manufacturing comprising the deposition of successive layers in a stacking direction (X), each layer comprising: a powder of thermoelectric material; and a binder, inert with respect to said thermoelectric material; then - high-temperature treatment of the intermediate element, so as to eliminate the binder and densify the thermoelectric material to produce the thermoelectric element (10); the thermoelectric material comprising at least 90% by mass of an alloy of at least two elements selected from: Si, Mn, Fe, Mg, Ge, Bi, Te, Pb, Sn, Sb, Ag, Cu, Se. Figure for the abstract: Figure 4

Description

Title of the invention: Method for manufacturing thermoelectric elements by additive manufacturing

[0001] The present invention relates to a method for manufacturing a thermoelectric element.

[0002] Thermoelectricity based on the Seebeck effect makes it possible to generate electricity from a temperature gradient. Thermoelectric modules can be formed by assembling n-type and p-type thermoelectric elements, or thermoelectric leads. These thermoelectric elements exhibit a good Seebeck coefficient, good electrical conductivity, and low thermal conductivity.

[0003] In order to improve the temperature gradient between two ends of a thermoelectric element, it is advantageous to give said element a shape that promotes heat dissipation between said ends by increasing the exchange surface area. Such a shape is preferably non-cylindrical, for example flared, fitted with fins or lateral cavities.

[0004] It is known to produce such thermoelectric elements by cutting or machining, which lengthens production times, increases costs and generates waste.

[0005] An additive manufacturing of thermoelectric elements by laser sintering is described in particular by H. Zhang et al., Laser Additive Manufacturing Process Development for Bismuth Telluride Thermoelectric Material, Journal of Materials Engineering and Performance volume 31, pages 6196-6204 (2022).

[0006] However, the low thermal conductivity of the materials used (<10 W / m / K) makes them unsuitable for laser sintering. In particular, cracks and phase-shift zones can be observed within the resulting elements, which degrades their efficiency.

[0007] The object of the invention is therefore to propose a method for manufacturing thermoelectric elements, easy to implement and allowing good heat dissipation between the ends of said elements.

[0008] To this end, the invention relates to a process of the aforementioned type, comprising the following steps: production by additive manufacturing of an intermediate element, said production comprising the deposition of successive layers in a stacking direction, each layer comprising: a powder of thermoelectric material; and a binder, inert with respect to said thermoelectric material; then high-temperature treatment of the intermediate element, so as to eliminate the binder and densify the thermoelectric material to produce the thermoelectric element; the thermoelectric material comprising at least 90% by mass, preferably at least 97% by mass, of an alloy of at least two elements chosen from: Si, Mn, Fe, Mg, Ge, Bi, Te, Pb, Sn, Sb, Ag, Cu, Se.

[0009] According to other advantageous aspects of the invention, the method comprises one or more of the following features, taken individually or in all technically possible combinations:

[0010] - the thermoelectric material comprises at least 90% by mass, preferably at least 97% by mass, of a silicide of formula MxSiy; M being chosen from Mn, Fe, Mg, Ge, Bi, Te, Pb, Sn, Sb, Ag, Cu, Se; x and y being decimal numbers, a ratio y / x being between 0.5 and 2;

[0011] - M is chosen from Mn, Fe, Ag, Cu; and the y / x ratio is between 1.73 and 1.77;

[0012] - the silicide is chosen from Mg2Si, FeSi2, CrSi2 and MnSii.75;

[0013] - the production of the intermediate element includes a cooking step of the bonding after the deposition of successive layers, at a temperature preferably below 200 °C;

[0014] - the binder comprises an organic resin, whether aqueous or not;

[0015] - the high-temperature treatment of the intermediate element comprises a step sintering of said intermediate element at a first temperature, preferably greater than or equal to 500°C, so as to densify the thermoelectric material;

[0016] - the sintering step is implemented by simultaneously applying pressure mechanical action on the intermediate element, along a direction of compression;

[0017] - during the sintering step, the intermediate element is arranged so that the compression direction should be perpendicular to the stacking direction;

[0018] - during the sintering step, the intermediate element is embedded in a powder bed inert material on which mechanical pressure is applied;

[0019] - the sintering step is carried out under an electric field;

[0020] - before the sintering stage, the high-temperature treatment of the element intermediate includes a heating step to a second temperature, so as to eliminate the binder; said second temperature being lower than the first temperature;

[0021] - the intermediate element is configured such that: the thermoelectric element extends along an axis, between a first and a second extremity, and comprises an external lateral surface, extending around said axis; the lateral surface has dimensions, perpendicular to the axis; and said dimensions are variable between the first and the second extremity;

[0022] - the intermediate element is configured so that the thermoelectric element includes at least two fins;

[0023] - the intermediate element is configured so that the thermoelectric element includes a cavity, extending preferentially along the axis.

[0024] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0025] [Fig-1] [Fig.1] is a perspective view of thermoelectric elements according to embodiments of the invention;

[0026] [Fig.2] [Fig.2] schematically represents a manufacturing process for a thermoelectric element of [Fig.1]; and

[0027] [Fig.3] [Fig.4] Figures 3 and 4 schematically represent steps in the process of [Fig.2].

[0028] Fig. 1 represents a first 10 and a second 110 thermoelectric elements according to embodiments of the invention.

[0029] The thermoelectric elements 10, 110 are intended in particular to serve as thermoelectric legs in a thermoelectric module. The thermoelectric elements 10, 110 will be described concurrently below.

[0030] The thermoelectric element 10, 110 extends along an axis 12, 112, between a first 14, 114 and a second 15, 115 end. The thermoelectric element 10, 110 has an external lateral surface 16, 116, extending around the axis 12, 112.

[0031] Preferably, a length of the thermoelectric element 10, 110 along the axis 12, 112 is on the order of a few millimeters, for example between 5 and 10 mm.

[0032] According to a first embodiment of the invention (not shown), the lateral surface of the thermoelectric element has a cylindrical shape along the axis 12, 112. More specifically, according to said first embodiment, the lateral surface of the thermoelectric element is defined by generators parallel to the axis 12, 112. Such a cylindrical shape may have a circular, oval, polygonal or any other base.

[0033] According to a second embodiment of the invention, the lateral surface 16, 116 of the thermoelectric element 16, 116 has a so-called complex shape. More specifically, according to the second embodiment, the lateral surface 16, 116 has dimensions 17, 18, perpendicular to the axis 12, 112. The dimensions 17, 18 vary between the first 14, 114 and the second 15, 115 ends.

[0034] Such a complex shape makes it possible, in particular, to improve heat dissipation through the lateral surface 16, 116, so as to obtain a high temperature gradient between the first 14, 114 and second 15, 115 ends. The Seebek effect is thus improved in the thermoelectric element 10, 110.

[0035] For example, the thermoelectric element 10 comprises: a main body 20, substantially parallelepiped in shape extending along the axis 12; and a plurality fins 22, projecting radially from the main body 20. The fins 22 extend substantially in planes parallel to each other and perpendicular to the axis 12.

[0036] In the embodiment shown, the main body 20 and the fins 22 have a rectangular contour perpendicular to the axis 12. In an alternative not shown, said contour is for example triangular, circular or oval.

[0037] According to another example, the lateral surface 116 of the thermoelectric element 110 has substantially flat faces 124 inclined with respect to the axis 112. Thus, the thermoelectric element 110 has a flared shape between the first 114 and the second 115 ends.

[0038] In the embodiment shown, the thermoelectric element 110 has a triangular base. In an alternative not shown, said shape is, for example, quadrilateral, circular or oval.

[0039] According to a first embodiment, the thermoelectric element 10 forms a solid. According to a second embodiment, the thermoelectric element 110 has a tubular shape and comprises a central cavity 126, open at the first 114 and second 115 ends.

[0040] The thermoelectric element 10, 110 is made of a thermoelectric material 30, preferably a material having a Seebeck coefficient greater than or equal to 100pV / K and / or an electrical conductivity greater than or equal to 1000S / m and / or a thermal conductivity less than 10 W / m / K. The thermoelectric material 30 comprises an alloy of at least two elements selected from: Si, Mn, Fe, Mg, Ge, Bi, Te, Pb, Sn, Sb, Ag, Cu, Se.

[0041] The above alloy forms at least 90% by mass, preferably at least 97% by mass, of the thermoelectric material 30. The remainder by mass of said thermoelectric material 30 may comprise doping elements, as will be described below.

[0042] Preferably, the thermoelectric material 30 comprises at least 90% by mass, more preferably at least 97% by mass, of a silicide of formula MxSiy. Preferably, M is selected from Mn, Fe, Mg, Ge, Bi, Te, Pb, Sn, Sb, Ag, Cu, Se; and x and y are decimal numbers, with a y / x ratio between 0.5 and 2. For example, the alloy is selected from Mg2Si, FeSi2, and CrSi2. Aluminum- or cobalt-doped [3-FeSi2] is notably used.

[0043] According to one embodiment, M is a transition metal chosen from Mn, Fe, Ag, Cu; and the y / x ratio is between 1.73 and 1.77. Germanium-doped MnSiI.75 is notably used.

[0044] Alternatively, the thermoelectric material 30 comprises at least 90% by mass, more preferably at least 97% by mass, of bismuth telluride Bi2Te3 or an antimony-based alloy.

[0045] Figure 2 schematically represents a method 200 for manufacturing the element thermoelectric 10, 110. Said process 200 will now be described.

[0046] The process 200 comprises: a first step 202 of making an intermediate element 50; then a second step 204 of high-temperature treatment of said intermediate element 50, so as to obtain the thermoelectric element 10, 110.

[0047] The intermediate element 50, visible in [Fig. 4], is a precursor of the thermoelectric element 10 described above. The process 200 will be described below for the fabrication of the thermoelectric element 10 but is similar to a process for the fabrication of the thermoelectric element 110.

[0048] The first step 202 comprises at least one first substep 206 of additive manufacturing. A first installation 60, for the implementation of the first substep 206, is schematically represented in [Fig.3].

[0049] The first installation 60 comprises: a platform 62; a first 64 and a second 65 application devices; and an electronic control module 66.

[0050] The platform has a horizontal surface 67. The first application device 64 is capable of depositing, on said horizontal surface 67, successive layers of powder 68 in a stacking direction X.

[0051] The first application device 64 includes, for example, a hopper 70 and a device (not shown) for moving said hopper parallel to the horizontal surface 67.

[0052] The powder 68 is formed from the thermoelectric material 30 chosen for the manufacture of the thermoelectric element 10, 110.

[0053] Preferably, the powder 68 is composed of at least 90% by mass, more preferably at least 95% by mass, of substantially spherical particles. Preferably, the diameter of said substantially spherical particles is between 5 pm and 100 pm.

[0054] The second application device 65 is capable of projecting drops 72 of a binder 74 onto the powder layers 68. The second distribution device 65 comprises, for example, a distribution head 75, a device (not shown) for moving said distribution head parallel to the horizontal surface 67, and a device (not shown) for supplying binder 74 to said distribution head.

[0055] The binder 74 is inert with respect to the thermoelectric material 30 forming the powder 68. The binder 74 preferably comprises an organic resin, either aqueous or organic. For example, the chosen binder 74 is the commercially available product under the name AquaFuse®. According to one variant, the binder 74 chosen is a phenolic resin.

[0056] The electronic control module 66 is connected to the first 64 and second 65 application devices. Said electronic module 66 stores a digitized image 76 of the intermediate element 50. More specifically, the electronic module 66 stores a segmentation of said digitized image 76 into a plurality of substantially planar slices 77, 78, 79 adjacent to each other.

[0057] According to one embodiment, the first installation 60 is a binder jet 3D printing installation, as marketed by the company ExOne.

[0058] An implementation of the first substep 206 of the process 200 will now be described. This implementation is carried out by means of a program stored in the electronic module 66.

[0059] Successive layers 52, 53, 54 are formed on the platform 62 according to the stacking direction X, by the first 64 and second 65 application devices.

[0060] The formation of each layer comprises: the deposition of a layer of powder 68 on a surface of the platform 62, by the first application device 64; then the projection, on an area 82, 83, 84 of said layer of powder, of a quantity of binder 74 by the second application device 66.

[0061] Each layer of powder has, for example, a thickness of approximately 20 to 100 pm. Each zone 82, 83, 84 has a shape corresponding to that of a slice 77, 78, 79 of the digitized image 76 of the intermediate element 50.

[0062] Zones 82, 83, 84, formed from a mixture of powder 68 and binder 74, adhere to each other. The binder 74 polymerizes, thus forming the intermediate element 50.

[0063] Preferably, after the first additive manufacturing substep 206, the manufacturing step 202 of the intermediate element 50 comprises a second substep 208 of curing the binder 74, so as to improve the polymerization of said binder. More preferably, the second substep 208 of curing the binder 74 is carried out at a temperature less than or equal to 200°C, for example, around 160°C. The complete polymerization of the binder 74 makes it easy to handle the intermediate element 50.

[0064] Preferably, after the first substep 206 and the possible second substep 208, step 202 includes a third substep 210 of depowdering, so as to extract the intermediate element 50 from the rest of the powder sheets 68.

[0065] The intermediate element 50 is thus obtained. As can be seen in [Fig.4], said intermediate element has a shape similar to that of the thermoelectric element 10 but with different dimensions, which will be detailed below.

[0066] Several intermediate elements 50 can be carried out simultaneously during the same first step 202 described above.

[0067] The second step 204 of the process 200 includes at least a fourth substep 212 of sintering, so as to densify the intermediate element 50.

[0068] The sintering substep 212 includes a high-temperature treatment of the intermediate element 50. Preferably, said treatment is carried out at a temperature greater than or equal to 950°C.

[0069] At such a temperature, the porosities located between the powder grains 68 migrate towards the outside of the intermediate element 50, which becomes densified. The thermoelectric element 10 is thus obtained.

[0070] According to a first embodiment (not shown), the fourth substep 212 of sintering is carried out at atmospheric pressure, for example in a furnace.

[0071] According to a second embodiment, the fourth substep 212 of sintering is carried out under mechanical pressure or under load. The mechanical pressure is in particular applied uniaxially, multiaxially or isostatically.

[0072] The sintering techniques under load that can be used include: hot pressing; hot isostatic pressing; or field assisted sintering technology (FAST).

[0073] “Field-assisted sintering” means sintering assisted by an electric or magnetic field. Electric current-assisted sintering is also known as “flash sintering” and by the English acronyms “SPS” for Spark Plasma Sintering and “ECAS” for Electric Current Assisted Sintering.

[0074] Preferably, the fourth substep 212 of sintering is implemented according to the SPS technique.

[0075] Figure 4 shows a second installation 85 for the implementation of the Fourth sub-step 212. The second installation 85 comprises: a chamber 86; and a device 87 for applying uniaxial pressure along a compression direction Z. The device 87 comprises, for example, two pistons 88 arranged opposite each other and capable of sliding along Z relative to the chamber 86. Said chamber 86 and the pistons 88 define a compartment 89.

[0076] Preferably, the second installation 85 further comprises an inert powder bed 90, suitable for receiving in compartment 89. The inert powder bed 90 is intended to receive the intermediate element 50, so as to encase it, during the fourth sub-step 212 of sintering.

[0077] The inert powder 90 is non-reactive with respect to the thermoelectric material 30 and the sintering conditions. The inert powder used is, for example, boron nitride (BN).

[0078] Preferably, the inert powder 90 has a particle size smaller than the size of possible motifs of the thermoelectric element 10 and smaller than the cavity 126 of the thermoelectric element 110.

[0079] An implementation of the second step 204 of the process 200 will now be described.

[0080] First, the intermediate element 50 is placed in the compartment 89 and the inert powder bed 90 is formed, so as to cover said intermediate element.

[0081] Preferably, the intermediate element 50 is arranged in the compartment 89 so that the compression direction Z is perpendicular to the stacking direction X implemented during the manufacture of said intermediate element.

[0082] Next, the fourth sub-step 212 of sintering is implemented. The conditions of hot sintering according to the SPS technique are applied, while the pistons apply a mechanical pressure along Z to the powder bed 90.

[0083] The intermediate element 50 is thus densified, leading to the thermoelectric element 10. For example, in the embodiment shown, the compression direction Z is parallel to the axis 12 of the thermoelectric element 10 and perpendicular to the fins 22.

[0084] Preferably, in the case where the thermoelectric element 110 includes a central cavity 126, said central cavity extends along the compression direction Z.

[0085] The general shape of the intermediate element 50 is preserved in the thermoelectric element 10, but the second processing step 204 leads to a reduction in the dimensions of said element. In particular, the dimensions of the thermoelectric element 10 along the compression direction Z are smaller than the dimensions of the intermediate element 50 along said compression direction Z.

[0086] Better densification is obtained when the compression direction Z is perpendicular to the stacking direction X than when said compression direction Z is parallel to said stacking direction X.

[0087] By way of example, the sintering substep 212 allows, for a density of 50% of the intermediate element 50, to obtain a density of 95% for the thermoelectric element 10.

[0088] According to one embodiment, prior to the fourth sintering substep 212, the process 200 comprises a fifth binder removal substep 214. Such a removal substep 214 is carried out by heating the intermediate element to a temperature lower than the sintering temperature. For example, the substep 214 elimination is implemented at a temperature between 500°C and 600°C.

[0089] Such a substep 214 of removal makes it possible to remove the binder at a lower temperature than that of sintering. Debinding at too high a temperature could indeed trap gas bubbles in the material, which would weaken the thermoelectric element 10 obtained.

[0090] Preferably, the fifth 214 and fourth 212 substeps are linked together without the intermediate element 50 being moved between said substeps. For example, when the second installation 85 described above is used, the intermediate element 50 is first placed in the compartment 89 and in the powder bed 90; then the fifth elimination substep 214 is carried out at a temperature of 500 to 600°C; then the fourth sintering substep 212 is carried out as described above, with the temperature rising to at least 500°C. Preferably, for the silicides FeSi2 and MnSi75, said temperature rises to at least 900°C.

[0091] Thus, despite the elimination of the binder 74 after the fifth substep 214, the intermediate element is not deteriorated before the fourth substep 212 of sintering.

[0092] Several intermediate elements 50 can be arranged in the second installation 85, so as to simultaneously produce several thermoelectric elements 10 during the same second step 204 described above.

[0093] The process 200 described above makes it easy to obtain thermoelectric elements 10, 110 formed from a dense and homogeneous material. In particular, no phase shift of the thermoelectric material 30 was observed in the thermoelectric elements 10, 110 produced according to said process.

[0094] The thermoelectric modules derived from said thermoelectric elements 10, 110 therefore lead to good electrical efficiencies. EXAMPLE

[0095] The first substep 206 of additive manufacturing, described above, is implemented. Powder 68 is a spherical MnSi₂ 75 Ge-doped powder with a particle size centered on 25 µm. 2 kg of said powder 68 are inserted into a hopper of a Binder Jetting additive manufacturing machine (ExOne innovent+). The binder 74 used is AquaFuse® binder. The printing time is, for example, approximately 5 hours, depending on the number of parts and their dimensions.

[0096] The second sub-step 208 of curing the binder 74 is then carried out. The printing platform 62 is placed in an oven at 185°C in ambient air for 4 hours.

[0097] The intermediate elements 50 are then depowdered by blowing. Their porosity is approximately 50%. The size of the intermediate elements 50 can range from 1 mm to several centimeters.

[0098] The second processing step 204 described above is then carried out: One or more intermediate elements 50 are arranged in an SPS mold surrounded by a Boron Nitride powder (D50 = 50pm), so that the SPS pressure Z axis is oriented perpendicular to the stacking axis X of the intermediate elements 50. The mold is then placed in the SPS and brought to a first temperature plateau of 600°C for 5 min, in order to carry out the substep 214 of removing the binder 74 described above.

[0099] The mold is then subjected to a second temperature step at 900°C for 15 min, while applying a pressure of 25MPa to the mold, in order to implement sub-step 212 of sintering described above.

[0100] The temperature was then lowered back to ambient temperature, the mold was opened, and the thermoelectric element(s) 10 obtained were recovered. The BN powder can be recycled.

[0101] The thermoelectric elements 10 are depowdered by blowing and then by ultrasonic cleaning in ethyl alcohol.

[0102] The final sizes of the parts obtained can range from 1 mm to several centimeters. The resolution of the parts is 75 µm in the construction plane. The size of the parts is reduced by up to 50% in the Z direction of the SPS pressure application. Conversely, said size varies little in the plane orthogonal to the pressure; a slight increase in dimensions can be observed.

[0103] Thermoelectric elements 10 having fins 22 exhibit greater temperature gradients than parallelepiped pieces (simple shape) when a hot temperature is applied to their base.

[0104] For example, for a temperature of 75°C applied to the first end 14, 114, the temperature at the second end 15 of the thermoelectric element 10 having fins 22 is 62°C, compared to 65°C in the case of a parallelepiped piece.

[0105] The finned geometry dissipates heat more effectively. Consequently, this heat generates higher voltages due to the Seebeck effect. A voltage increase of 16% (2.2mV versus 1.8mV) was achieved for parts with two fins compared to a part of identical volume and parallelepiped shape.

Claims

Demands

1. A method for manufacturing a thermoelectric element (10, 110), said method comprising the following steps: - fabrication (202) by additive manufacturing of an intermediate element (50), said fabrication comprising deposits of successive layers (52, 53, 54) in a stacking direction (X), each layer comprising: a powder (68) of thermoelectric material (30); and a binder (74), inert with respect to said thermoelectric material; then - high-temperature treatment (204) of the intermediate element, so as to eliminate the binder and densify the thermoelectric material to produce the thermoelectric element (10); the thermoelectric material (30) comprising at least 90% by mass, preferably at least 97% by mass, of an alloy of at least two elements selected from: Si, Mn, Fe, Mg, Ge, Bi, Te, Pb, Sn, Sb, Ag, Cu, Se.

2. A method according to claim 1, wherein the thermoelectric material (30) comprises at least 90% by mass, preferably at least 97% by mass, of a silicide of formula MxSiy; M being selected from Mn, Fe, Mg, Ge, Bi, Te, Pb, Sn, Sb, Ag, Cu, Se; x and y being decimal numbers, a ratio y / x being between 0.5 and 2.

3. Method according to claim 2, wherein M is selected from Mn, Fe, Ag, Cu; and the y / x ratio is between 1.73 and 1.

77.

4. A method according to claim 2, wherein the silicide is selected from Mg2Si, FeSi2, CrSi2 and MnSii.

75.

5. A method according to any one of the preceding claims, wherein the fabrication (202) of the intermediate element (50) comprises a step (208) of baking the binder (74) after the deposition of successive layers, at a temperature preferably below 200 °C

6. V-. A method according to any one of the preceding claims, wherein the binder (74) comprises an aqueous or non-aqueous organic resin.

7. A method according to any one of the preceding claims, wherein the high-temperature treatment (204) of the intermediate element (50) comprises a step (212) of sintering said intermediate element at a first temperature, preferably greater than or equal to 500°C, so as to densify the thermoelectric material.

8. A method according to claim 7, wherein the sintering step (212) is carried out by simultaneously applying mechanical pressure to the intermediate element (50), along a compression direction (Z).

9. A method according to claim 8, wherein, during the sintering step (212), the intermediate element (50) is arranged so that the compression direction (Z) is perpendicular to the stacking direction (X).

10. A method according to claim 8 or 9, wherein, during the sintering step (212), the intermediate element (50) is embedded in a bed of inert powder (90) on which mechanical pressure is applied.

11. A method according to any one of claims 7 to 10, wherein the sintering step (212) is carried out under an electric field.

12. A method according to any one of claims 7 to 11, wherein, prior to the sintering step (212), the high-temperature treatment (204) of the intermediate element (50) comprises a heating step (214) to a second temperature, so as to eliminate the binder (74); said second temperature being lower than the first temperature.

13. A method according to any one of the preceding claims, wherein the intermediate element (50) is configured such that: the thermoelectric element (10, 110) extends along an axis (12, 112), between a first (14, 114) and a second (15, 115) end, and comprises an external lateral surface (16, 116), extending around said axis; the lateral surface has dimensions (17, 18), perpendicular to the axis (12, 112); and said dimensions are variable between the first (14, 114) and the second (15, 115) end.

14. A method according to claim 13, wherein the intermediate element (50) is configured so that the thermoelectric element (10) comprises at least two fins (22).

15. A method according to claim 13, wherein the intermediate element is configured so that the thermoelectric element (110) has a cavity (126), extending preferably along the axis (112).

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