Method for producing thermoelectric elements using additive manufacturing

Abstract

The invention relates to a method for manufacturing a thermoelectric element (10), comprising the following steps: - producing, by additive manufacturing, an intermediate element (50), the production comprising depositing successive layers in a stacking direction (X), each layer comprising: a powder of thermoelectric material and a binder, inert with respect to the thermoelectric material; then - subjecting the intermediate element to a high-temperature treatment, so as to remove the binder and densify the thermoelectric material in order to lead to the thermoelectric element (10); the thermoelectric material comprising at least 90% by weight of an alloy of at least two of the following elements: Si, Mn, Fe, Mg, Ge, Bi, Te, Pb, Sn, Sb, Ag, Cu, Se.

Description

[0001] DESCRIPTION

[0002] Method for manufacturing thermoelectric elements by additive manufacturing

[0003] The present invention relates to a method for manufacturing a thermoelectric element.

[0004] Seebeck-based thermoelectricity allows for the generation of electricity from a temperature gradient. Thermoelectric modules can be constructed by assembling n-type and p-type thermoelectric elements, or thermoelectric leads. These thermoelectric elements exhibit a high Seebeck coefficient, good electrical conductivity, and low thermal conductivity.

[0005] To improve the temperature gradient between two ends of a thermoelectric element, it is advantageous to give the element a shape that promotes heat dissipation between those ends by increasing the exchange surface area. Such a shape is preferably non-cylindrical, for example flared, equipped with fins, or with lateral cavities.

[0006] It is known that such thermoelectric elements can be produced by cutting or machining, which lengthens production times, increases costs and generates waste.

[0007] Additive manufacturing of thermoelectric elements by laser sintering is notably described 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).

[0008] 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.

[0009] The aim of the invention is therefore to propose a method for manufacturing thermoelectric elements, which is easy to implement and allows for good heat dissipation between the ends of said elements.

[0010] 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 to 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 selected from: Si, Mn, Fe, Mg, Ge, Bi, Te, Pb, Sn, Sb, Ag, Cu, Se.

[0011] According to other advantageous aspects of the invention, the method comprises one or more of the following characteristics, taken individually or in all technically possible combinations:

[0012] - the thermoelectric material comprises at least 90% by mass, preferably at least 97% by mass, of a silicide of formula M x If y ; M being chosen from Mn, Fe, Mg, Ge, Bi, Te, Pb, Sn, Sb, Ag, Cu, Se ; x and y being decimal numbers, a y / x ratio being between 0.5 and 2;

[0013] - M is chosen from Mn, Fe, Ag, Cu; and the y / x ratio is between 1.73 and 1.77;

[0014] - the silicide is chosen from Mg2Si, FeSi2, CrSi2 and MnSi 1 7 5;

[0015] - the production of the intermediate element includes a step of cooking the binder after the deposits of successive layers, at a temperature preferably below 200 °C;

[0016] - the binder includes an organic resin, water-based or not;

[0017] - the high-temperature treatment of the intermediate element includes a sintering step of said intermediate element at a first temperature, preferably greater than or equal to 500°C, so as to densify the thermoelectric material;

[0018] - the sintering step is implemented by simultaneously applying mechanical pressure to the intermediate element, along a compression direction;

[0019] - during the sintering stage, the intermediate element is arranged so that the compression direction is perpendicular to the stacking direction;

[0020] - during the sintering stage, the intermediate element is embedded in a bed of inert powder on which mechanical pressure is applied;

[0021] - the sintering stage is carried out under an electric field;

[0022] - before the sintering stage, the high-temperature treatment of the intermediate element includes a heating stage at a second temperature, so as to eliminate the binder; said second temperature being lower than the first temperature;

[0023] - the intermediate element is configured such that: the thermoelectric element extends along an axis, between a first and a second end, and has 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 end; - the intermediate element is configured such that the thermoelectric element comprises at least two fins;

[0024] - the intermediate element is configured so that the thermoelectric element has a cavity, extending preferentially along the axis.

[0025] 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:

[0026] - Figure 1 is a perspective view of thermoelectric elements according to embodiments of the invention;

[0027] - Figure 2 schematically represents a manufacturing process for a thermoelectric element from Figure 1; and

[0028] - Figures 3 and 4 schematically represent steps in the process of Figure 2.

[0029] Figure 1 represents a first 10 and a second 110 thermoelectric elements according to embodiments of the invention.

[0030] Thermoelectric elements 10 and 110 are specifically designed to serve as thermoelectric legs in a thermoelectric module. These thermoelectric elements will be described simultaneously below.

[0031] 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.

[0032] 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.

[0033] 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 precisely, 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.

[0034] 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 precisely, 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.

[0035] This complex shape allows, in particular, for improved heat dissipation through the lateral surface 16, 116, resulting in a high temperature gradient between the first 14, 114 and second 15, 115 ends. The Seebek effect is thus enhanced in the thermoelectric element 10, 110.

[0036] For example, the thermoelectric element 10 comprises: a main body 20, substantially parallelepiped in shape extending along the axis 12; and a plurality of 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.

[0037] In the embodiment shown, the main body 20 and the fins 22 have a rectangular outline perpendicular to the axis 12. In an alternative not shown, said outline is for example triangular, circular or oval.

[0038] 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.

[0039] In the embodiment shown, the thermoelectric element 110 has a triangular base. In an alternative not shown, this shape is, for example, quadrilateral, circular, or oval.

[0040] 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.

[0041] 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 100S / 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.

[0042] 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.

[0043] Preferably, the thermoelectric material 30 comprises at least 90% by mass, more preferably at least 97% by mass, of a silicide of formula M x If y Preferably, M is chosen from Mn, Fe, Mg, Ge, Bi, Te, Pb, Sn, Sb, Ag, Cu, and Se; and x and y are decimal numbers, with a y / x ratio between 0.5 and 2. For example, the alloy is chosen from Mg₂Si, FeSi₂, and CrSi₂. Aluminum- or cobalt-doped p-FeSi₂ is notably used. In one embodiment, M is a transition metal chosen from Mn, Fe, Ag, and Cu; and the y / x ratio is between 1.73 and 1.77. Germanium-doped MnSi₂.₇₅ 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 of an antimony-based alloy.

[0045] Figure 2 schematically represents a process 200 for manufacturing the thermoelectric element 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 Figure 4, is a precursor to the thermoelectric element 10 described above. The process 200 will be described below for the production of the thermoelectric element 10 but is similar to a process for the production of the thermoelectric element 110.

[0048] The first step 202 includes 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 Figure 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] Powder 68 is formed from thermoelectric material 30 chosen for the manufacture of thermoelectric element 10, 110.

[0053] Preferably, 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 suitable for projecting drops 72 of a binder 74 onto the powder layers 68. The second distribution device 65 includes, 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 that forms the powder 68. The binder 74 preferably comprises an organic resin, either aqueous or organic. For example, the binder 74 chosen is the product marketed under the name AquaFuse®. In one alternative, 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 precisely, the electronic module 66 stores a segmentation of said digitized image 76 into a plurality of substantially flat 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 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 includes: the deposition of a layer of powder 68 on a surface of the platform 62, by the first application device 64; then the projection, onto 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, for example, has 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, and 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 production step 202 of the intermediate element 50 includes 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 allows for easy handling of the intermediate element. Preferably, after the first substep 206 and the optional second substep 208, the step 202 includes a third substep 210 of depowdering, so as to extract the intermediate element 50 from the remaining powder layers 68.

[0064] The intermediate element 50 is thus obtained. As can be seen in Figure 4, said intermediate element has a shape similar to that of the thermoelectric element 10 but with different dimensions, which will be detailed below.

[0065] Several intermediate elements 50 can be carried out simultaneously during the same first step 202 described above.

[0066] The second step 204 of the process 200 includes at least a fourth sub-step 212 of sintering, so as to densify the intermediate element 50.

[0067] Substep 212 of sintering 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.

[0068] 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.

[0069] According to a first embodiment (not shown), the fourth substep 212 of sintering is carried out at atmospheric pressure, for example in a furnace.

[0070] According to a second embodiment, the fourth substep 212 of sintering is carried out under mechanical pressure or load. The mechanical pressure is applied in particular uniaxially, multiaxially or isostatically.

[0071] The sintering techniques under load that can be used include: hot pressing; hot isostatic pressing; and field assisted sintering technology (FAST).

[0072] Field-assisted sintering refers to 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 "EGAS" for Electric Current Assisted Sintering.

[0073] Preferably, the fourth substep 212 of sintering is implemented according to the SPS technique.

[0074] Figure 4 shows a second installation 85 for implementing the fourth substep 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.

[0075] Preferably, the second installation 85 further includes 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.

[0076] 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).

[0077] Preferably, the inert powder 90 has a particle size smaller than the size of possible patterns of the thermoelectric element 10 and smaller than the cavity 126 of the thermoelectric element 110.

[0078] An implementation of the second step 204 of process 200 will now be described.

[0079] First, the intermediate element 50 is placed in the compartment 89 and the inert powder bed 90 is made up so as to cover said intermediate element.

[0080] Preferably, the intermediate element 50 is arranged in compartment 89 so that the compression direction Z is perpendicular to the stacking direction X implemented during the manufacture of said intermediate element.

[0081] 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.

[0082] 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.

[0083] Preferably, in the case where the thermoelectric element 110 includes a central cavity 126, said central cavity extends along the compression direction Z.

[0084] 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.

[0085] 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. As an example, substep 212 of sintering allows, for a density of 50% of the intermediate element 50, to obtain a density of 95% for the thermoelectric element 10.

[0086] According to one embodiment, prior to the fourth sintering substep 212, the process 200 includes a fifth binder removal substep 214. This removal substep 214 is carried out by heating the intermediate element to a temperature below the sintering temperature. For example, the removal substep 214 is performed at a temperature between 500°C and 600°C.

[0087] Such a substep 214 of debinding allows the binder to be removed at a lower temperature than that of sintering. Debinding at too high a temperature could trap gas bubbles in the material, which would weaken the resulting thermoelectric element 10.

[0088] 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 214 removal substep is carried out at a temperature of 500 to 600°C; then the fourth 212 sintering substep is carried out as described above, with the temperature rising to at least 500°C. Preferably, for the silicides FeSi2 and MnSi 1 75 said temperature rises to at least 900 °C.

[0089] Thus, despite the elimination of binder 74 after the fifth substep 214, the intermediate element is not deteriorated before the fourth substep 212 of sintering.

[0090] 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.

[0091] 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.

[0092] The thermoelectric modules derived from the aforementioned thermoelectric elements 10, 110 therefore lead to good electrical efficiencies.

[0093] EXAMPLE

[0094] The first substep 206 of additive manufacturing, described above, is implemented. Powder 68 is a spherical MnSi powder. 1 75 Ge-doped, 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®. The printing time is, for example, approximately 5 hours, depending on the number of parts and their dimensions.

[0095] 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.

[0096] The intermediate elements 50 are then dedusted by blowing. Their porosity is approximately 50%. The size of the intermediate elements 50 can range from 1 mm to several centimeters.

[0097] The second processing step 204 described above is then implemented: 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 X axis 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 implement the binder 74 removal substep 214 described above.

[0098] 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.

[0099] The temperature was then lowered to ambient temperature, the mold was opened, and the resulting thermoelectric element(s) were recovered. The BN powder can be recycled.

[0100] The thermoelectric elements 10 are depowdered by blowing and then by ultrasonic cleaning in ethyl alcohol.

[0101] The final sizes of the resulting parts can range from 1 mm to several centimeters. The part resolution is 75 µm in the construction plane. The part size is reduced by up to 50% in the Z direction of the SPS pressure application. Conversely, the size varies little in the plane orthogonal to the pressure; a slight increase in dimensions may be observed.

[0102] Thermoelectric elements 10 with fins 22 exhibit greater temperature gradients than parallelepiped pieces (simple shape) when a hot temperature is applied to their base.

[0103] 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.

[0104] The finned geometry dissipates heat more effectively. Consequently, this heat generates higher voltages due to the Seebeck effect. A 16% voltage increase (2.2mV vs. 1.8mV) was observed for parts with two fins compared to a parallelepiped-shaped part of identical volume.

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 lead to 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 M x If y ; M being chosen from Mn, Fe, Mg, Ge, Bi, Te, Pb, Sn, Sb, Ag, Cu, Se ; x and y being decimal numbers, a y / x ratio being between 0.5 and 2.

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

77.

4. Method according to claim 2, wherein the silicide is selected from Mg2Si, FeSi2, CrSi2 and MnSi2.

75.

5. A method according to any one of the preceding claims, wherein the realization (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. A method according to any one of the preceding claims, wherein the binder (74) comprises an organic resin, whether aqueous or not.

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. 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. 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. 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. 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. 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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