WO2005109535A2

WO2005109535A2 - A method of manufacturing a thermoelectric device

Info

Publication number: WO2005109535A2
Application number: PCT/IB2005/051434
Authority: WO
Inventors: Clemens J. M. Lasance; Michel M. J. Decre
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-05-06
Filing date: 2005-05-03
Publication date: 2005-11-17
Also published as: WO2005109535A3

Abstract

A thermoelectric device (1) has first and second substrates (2, 3) being separated from each other by a thermoelectric element (4). The element (4) has a p-type semiconductor segment (5) and a n-type semiconductor segment (6) that are connected to each other by an electrode (7) located on the first substrate (2). Other electrodes (8), located on the second substrate (3), connect the segments (5, 6) to an electrical circuit via adjacent elements. At least one of the segments (5, 6) has been made by printing multiple layers (9, 10) on top of each other in consecutive steps. The thermoelectric device (1) could be used for cooling according to the principles of Peltier elements or could be used for heat recovery to produce electricity. The printed layers (9, 10) of the segments (5, 6) provides for high efficiency and low manufacturing costs.

Description

A method of manufacturing a thermoelectric device

FIELD OF THE INVENTION The present invention relates to a method of forming a thermoelectric device having a first substrate and a second substrate being parallel with each other and being separated by at least one thermoelectric element comprising a first segment of a first conductivity type and a second segment of a second conductivity type.

BACKGROUND OF THE INVENTION Thermoelectric devices are used for cooling and heating purposes. The device is used for absorbing heat on a heat absorbing substrate and desorbing the heat at a heat emitting substrate. One common use is the cooling of electronic components such as laser diodes and power devices. The thermoelectric devices have thermoelectric elements, also called Peltier elements, each comprising two semiconductor segments, one p-type segment and one n-type segment. Both segments are in contact with both the heat absorbing substrate and with the heat emitting substrate and are thus connected in parallel with each other with respect to the absorbing substrate and the emitting substrate. The two semiconductor segments are connected to a direct current power source in such a way that the two segments are in series with regard to the electrical current. When an electrical current flows through the two segments heat will be absorbed via the heat absorbing substrate and emitted via the heat emitting substrate. In an alternative mode the thermoelectric element could be installed in a location where a temperature gradient already exists and based on that gradient produce electricity. US 6,300,150 describes a method of producing thin film thermoelectric devices having n- and p-type segments. In the method described the segments are manufactured by first forming a multilayer film by e.g. chemical vapour deposition (CVD) and molecular beam epitaxy (MBE). The multilayer film is then subjected to process steps such as etching and laser ablation to prepare the desired segments. The multilayer structure provides interfaces between the layers which increases the thermal resistance of the segments and thus increases the efficiency of the thermoelectric devices described in US 6,300,150. However the thermoelectric devices described in US 6,300,150 are expensive and not very well suited for mass production.

SUMMARY OF THE INVENTION An object of the present invention is to provide an efficient method of forming a thermoelectric device which is useful as a cooling or heating device or could be used as a waste heat recovery device. This object is achieved by a method of forming a thermoelectric device, the method comprising the steps of providing a first substrate and forming thereon at least one first electrode, forming on said first electrode a first segment by printing thereon a plurality of layers being parallel to the substrate, one layer on top of the other, in a plurality of consecutive steps to form a first segment of a first conductivity type, forming a second segment of a second conductivity type, being opposite to the first conductivity type, on said first electrode, applying a second electrode to each segment and a second substrate for forming a thermoelectric element comprising the first electrode, the two segments and the second electrodes, the second substrate being parallel with the first substrate and separated therefrom by the segments extending between the two substrates. An advantage of this method is that it provides for a much more efficient large scale production of thermoelectric devices compared to the prior art methods. The segments having multiple layers printed on top of each other will provide for an increased efficiency in cooling and heating and heat recovery processes and will make thermoelectric devices made according to the invention competitive in applications where prior art thermoelectric devices were eliminated by other types of products. An advantage of the method according to claim 2 is that the segments need no costly aftermachining to obtain the desired structure but instead are provided with their desired pattern in the printing itself since every layer is printed with a shape suitable for the final product. This makes production costs significantly lower and avoids the risk that the segments are damaged during aftermachining. An advantage of the measure according to claim 3 is that a layer that has been printed is converted to a solid state before a following layer is printed thereon. This provides for a firm base for printing further layers on without the risk of negatively effecting the layers already printed. An advantage of the measure according to claim 4 is that it provides for the easy application of different semiconductor materials that in themselves are in solid state but that could be dispersed in a solvent, a binder or any other medium forming with said materials an ink suitable for the easy application by printing. An advantage of the measure according to claim 5 is that the stamp provides high resolution, high reproducibility printing of multiple layers on top of each other to form segments. The techniques involving bringing a stamp in direct contact with a substrate has proven to provide high accuracy of the multilayer segments and little, or no, requirement for machining. In particular so called wave printing, in which a flexible stamp is brought into direct contact by actuating a part of the stamp with a low pressure, has proven to provide a particularly useful technique for obtaining a good alignment of the multiple layers. In wave printing, which is described in WO03/099463, a flexible stamp has individual portions that can be moved by individual pressure actuators. By actuating the actuators in a certain order the transfer of the pattern can be performed according to a wave moving, e.g., from one side of the stamp to the other. The wave printing provides for a particularly good alignment accuracy, in the range of 2 micrometers or better, which is a large advantage when printing multiple layers, one layer on top of the other, in consecutive steps. An advantage of the measure according to claim 6 is that ink jet printing provides a simple method of large scale production of segments having multiple layers on top of each other. Another object of the present invention is to provide a thermoelectric device which is useful as a cooling or heating device or could be used as a waste heat recovery device and which is cheap and easy to manufacture. This object is achieved by a thermoelectric device having a first substrate and a second substrate being parallel with each other and being separated by at least one thermoelectric element comprising a first segment of a first conductivity type and a second segment of a second conductivity type being opposite to the first conductivity type, the segments extending in parallel with each other between the first substrate and the second substrate, said element further comprising a first electrode being located on the first substrate and electrically connecting the first segment to the second segment at the first substrate, and second electrodes located on the second substrate and connecting the first segment and the second segment respectively to an electrical circuit, at least one of said first and second segments of said element comprising multiple layers being parallel to the substrates and printed on top of each other in consecutive steps. An advantage with the thermoelectric device according to the invention is that it combines a high efficiency in cooling/heating or heat recovery, as the case may be, due to the segment having the multiple layers and a high quality at low cost due to the layers being printed on top of each other. The thermoelectric device according to the invention is thus suitable also for cooling and heating applications and heat recovery applications where thermoelectric devices according to prior art are too expensive. Preferably the device comprises a plurality of thermoelectric elements, the second electrodes of each element connecting the first segment and the second segment to respective segments of the opposite conductivity of adjacent elements. An advantage is that an enhanced efficiency in the cooling or heating or in the heat recovery process is provided. Due to the plurality of elements comprising segments having multiple layers printed on top of each other large thermoelectric devices with high efficiency and quality could be made available at low cost. Preferably both segments of said element comprise multiple layers being parallel to the substrates and printed on top of each other in consecutive steps. An advantage of this measure is that the efficiency of the thermoelectric device is further enhanced when both said first and second segments comprises multiple printed layers. Preferably said at least one of said first and second segments comprises layers of two different materials forming a first type of layer and a second type of layer alternating with each other. An advantage of this measure is that layers printed on top of each other and made of two different materials provides an enhanced thermal resistance to the segment and thereby increases the efficiency of the thermoelectric device. Preferably said at least one of said first and second segments comprises at least 50 layers printed on top of each other. An advantage of this measure is that at least 50 layers printed on top of each other provides a high efficiency heating or cooling or heat recovery at low cost. Preferably the thickness of each layer is less than 1 micrometer. An advantage of this measure is that with a plurality of layers each having a maximum thickness of 1 micrometer a high thermal resistance in the segment can be achieved also with a thermoelectric device having small dimensions. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail with reference to the appended drawings in which: Fig. 1 is a cross section and shows a part of thermoelectric device according to the invention; Fig. 2 is an enlargement and shows a part of a segment shown in Fig 1 ; Figs. 3a to 3i are cross sections and show the different steps of a first method of manufacturing a thermoelectric device according to the invention; Figs. 4a to 4f are cross sections and show the different steps of a second method of manufacturing a thermoelectric device; Figs. 5a to 5b are cross sections and show the different steps of a third method of manufacturing a thermoelectric device; Figs. 6a to 6d are cross sections and show the different steps of a fourth method of manufacturing a thermoelectric device.

DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 is a schematic cross section and shows a thermoelectric device 1 according to a first embodiment of the invention. The thermoelectric device 1 has a first cooling/heating substrate 2 and a second cooling/heating substrate 3. The substrates 2, 3 are parallel with each other and are separated from each other by a number of thermoelectric elements 4. Each thermoelectric element 4 of the thermoelectric device 1 comprises a p-type thermoelectric semiconductor segment 5 and an n-type thermoelectric semiconductor segment 6. Each semiconductor segment 5, 6 extends between the first substrate 2 and the second substrate 3. Each element 4 further comprises an electrode 7, which is attached to the first substrate 2 and which connects the semiconductor segments 5 and 6 to each other. The thermoelectric element 4 further comprises respective electrodes 8 which connect the semiconductors 5, 6 to an electrical circuit EC either directly or, as is shown in Fig. 1, via adjacent thermoelectric elements 4 in such a way that a plurality of thermoelectric elements 4 are arranged between the substrates 2, 3 and are electrically connected with each other via the electrodes 8 to form a plurality of series coupled thermoelectric elements 4 connected to an electrical circuit EC. If a source of electricity is connected to the electrical circuit EC for applying an electrical voltage to the elements 4 of the thermoelectric device 1 heat will be absorbed at one of the substrates 2, 3 and will be emitted at the other substrate 2, 3 thus obtaining a cooling effect at one substrate and a heating effect at the other substrate according to the well known Peltier effect. As alternative the thermoelectric device 1 could be used for producing electrical energy by placing it in a place where a thermal gradient already exists such that one substrate is placed in a cool environment and the other substrate is located in a warm environment, the thermoelectric device thereby producing an electrical current in the electrical circuit EC according to the well known Seebeck effect. Fig. 2 is an enlargement and shows a part of the semiconductor segment 5. The segment 5 comprises a plurality of printed thin first layers 9 made of a first material alternating with a plurality of printed thin second layers 10 made of a second material. The first and the second material are materials suitable for making a p-type segment. The number of printed layers is 50 or more and the thickness t of each layer 9, 10 is preferably less than 1 micrometer and more preferably around 10-50 nm. The thin layers 9, 10 could, as an example, comprise thin-film superlattice or non-superlattice thermoelectric materials, quantum- well and quantum-dot structured materials and materials that are peeled from bulk materials. The many interfaces 11 created between the thin layers 9, 10 of different materials increases the thermal resistance of the segment 5 and increases the efficiency of the thermoelectric device 1. In a similar manner two materials suitable for forming a n-type segment form a multilayered n-type segment 6. The segments 5, 6 shown in Fig. 1 and Fig. 2 are made by printing several consecutive thin layers on top of each other. Figs. 3a to 3i illustrate the respective steps (a) to (i) of a first embodiment of making a thermoelectric device 1 according to the invention. In a first step (a) a monolithic, flexible stamp 12 is provided, the stamp 12 having a pattern corresponding to the intended locations of p-type semiconductor segments 5. Thus the stamp has a number of protrusions 13 each corresponding to a segment 5. In step (b) the stamp 12 is coated by a solid or liquid first material 14. The first material 14 could be a suitable dispersion, solution, paste or similar composition comprising e.g. a solvent or a binder, such as a polymeric binder, and a semiconductor material which may be dissolved in the solvent or binder or be formed as solid particles dispersed in the solvent or binder. Examples of suitable semiconductor materials are those with high Seebeck coefficients, such as compositions of one or several of the materials Si, B C, B9C, Si_xGe_y, SiGeC, BiTe, SbTe, BiTeSe, InGaAs, InGaAsSb, InAlAs, InGaAsP and InP. An example of fluids comprising nanoparticles of semiconductor materials could be found in M. Toprak, "Synthesis of Nanostructured Thermoelectric Materials", ISBN-91-7283-125-1. In step (c) the stamp 12 is aligned above the substrate 2 in such a way that the protrusions 13 of the stamp 12 are located above the electrodes 7 in those positions where segments 5 are intended to be formed. The stamp 12 being provided with a control plate 15 is forced into contact with the substrate 2. The contacting of the protrusions 13 is made e.g. by the application of a pressure in a cavity 16 between the stamp 12 and the control plate 15. An example of such pressure actuated contacting is described in more detail in WO 03/099463 describing a so called wave printing technique. Wave printing includes the use of a flexible stamp and the controlled pressure actuation at low pressures, typically 0, 1 bar or less and only at a portion of the stamp at a time by means of individual actuators, preferably in such a way that portions of the stamp are actuated one after the other from one side of the stamp to the other according to a wave. The wave printing provides a possibility of printing two or more patterns located at some distance from each other without unwanted contact with the substrate between said patterns. On applying the pressure in the cavity 16 the protrusions 13 having provided thereon the first material 14 contacts the electrodes 7, a contact which may be referred to as micro contact printing. When the pressure is relieved the stamp 12 retracts from the substrate leaving thin layers of the first material 14 on the electrodes 7. In step (d) the substrate 2 is subjected to a curing process, which may include exposing the substrate 2 to heat or UV-light to evaporate the solvent and/or cure or bake the binder material as the case may be. After the curing the substrate 2 is provided with electrodes 7 having printed thereon a first layer 9 at the intended locations for the segments 5. In step (e) a stamp 12 is provided for the printing of a second material 18. The stamp 12 may be the same stamp 12 as was used in steps (a) to (c), the stamp being cleaned from the first material 14 to be ready for use, or may be another stamp 12 provided with the same pattern. In step (f) the stamp 12 is coated by a second material 18. The second material 18 is also a material suitable for preparing a p-type segment 5. The second material is however chosen such that the interface formed between layers of the first material and the second material provides for enhanced thermal resistance. In some cases one and the same material could be printed in several consecutive layers, the curing providing for the desired interfaces. In many cases, however, the first and second material need to be different materials to provide interfaces that result in the desired increase in thermal resistance. In the latter case the first and the second materials printed on top of each other may, as an example, constitute superlattices. In step (g) the protrusions 13 of the stamp 12 are aligned with the first layer 9 and a pressure is applied in the cavity 16 to force the protrusions 13 into contact with the previously printed layer 9. On contacting with the layer 9 the second material 18 is printed thereon. In step (h) the second material 18 is cured by e.g. heat or UV-light to form a second layer 10 printed on top of the first layer 9. The steps (a) to (h) are then repeated until a segment 5 has been manufactured, the segment 5 having the desired number of layers comprising first layers 9 and second layers 10 alternating with each other, with a total of e.g. 200 layers or more. In step (i) second segments 6 have been formed as well on the electrodes 7. Preferably the second segments 6 have been formed as printed multilayer segments 6 in accordance with the same principles as described above for the manufacturing of the segment 5. Second electrodes 8 are provided for forming a thermoelectric element 4 comprising the first and second segments 5, 6, the first electrode 7 and second electrodes 8, the second electrodes 8 connecting the element 4 to an electrical circuit via adjacent elements 4. The second electrodes 8 could be applied to the segments 5, 6 followed by the application of the second substrate 3 on top of the second electrodes 8. As alternative the second electrodes 8 could be fixed on the second substrate 3 first, the second substrate 3 then being put on top of the segments 5, 6 such that the second electrodes 8 become properly aligned with the segments 5, 6. It will be appreciated from Fig. 3a that the protrusions 13 have a shape in the horizontal plane, as illustrated in Fig. 3a, which corresponds to the desired shape of the final segment 5. Thus the layers 9, 10 are printed on top of each other in such a way that they have the desired shape in the horizontal plane immediately after printing. Thus no machining is required to provide the correct shape for the segment 5. It will also be appreciated that the segments 5, 6 are made by printing in such a way that a sufficient distance D, the principal location of D being indicated in Fig. 6d, between the segments 5, 6 is obtained for to obtain proper insulation between adjacent segments 5, 6. With the printing technique described with reference to Figs. 3a to 3i the segments 5, 6 could be formed at a mutual distance D being as short as about 4 micrometers or even shorter from each other without any risk of contact. The segments 5, 6 may, as a typical example, have a dimension W in the horizontal plane, as shown in Fig. 3i, of about 10 micrometer, and could be square or rectangular or even circular or oval. The height H of the segments 5, 6 may, as a typical example, be about 10 micrometers. Figs. 4a to 4f illustrate the respective steps (a) to (f) of a second embodiment of the invention. In step (a) a monolithic, flexible stamp 112 having protrusions 1 13 each corresponding to a segment 5 is provided. In step (b) the stamp 112 is lowered towards a plate 1 19 being covered by a first material 1 14, which could be a solid material, a paste or a liquid comprising e.g. a solvent or a binder as described above, the material 114 comprising semiconductor material. In step (c) the stamp 1 12 is brought into contact with the first material 114. This could be made by applying a pressure in a cavity 1 16 formed between the stamp 1 12 and a control plate 1 15 holding the stamp according to the wave printing technique described above and in WO 03/099463. In step (d) the pressure in the cavity 116 has been relieved and the stamp 112 has retracted from the plate 119. The protrusions 113 are covered by the first material 1 14 which thus has been transferred from the plate 119 to the stamp 1 12. In step (e) the stamp 1 12 is brought into contact with electrodes 107 on a substrate 102 in a similar manner as described above with reference to Fig. 3c. Finally, in step (f) the first material 114 is cured to form a first layer 109 on the electrodes 107 of the substrate 102. The steps (a) to (f) are then repeated with a second material to form a second layer. The steps (a) to (f) are then repeated a plurality of times, alternating with a first material and a second material, to from a first multilayer segment of the same type as shown in Fig. 3i. The steps (c) and (e) could be made by the wave printing technique described above, or by another suitable printing technique. Figs. 5a to 5b illustrate the respective steps (a) to (b) of a third embodiment of the invention. In this embodiment a stamp 212 having protrusions 213 is covered by a first material 214. By the application of a pressure in a cavity 216 formed between the stamp 212 and a control plate 215 the protrusions 213 are brought into direct contact with a substrate 202, which is preferably provided with electrodes (not shown in Figs. 5a and 5b) on its surface, in a step (a). As is shown in Fig. 5a the first material 214 is pushed to the spaces formed between the protrusions 213 and away from the places where the protrusions 213 contact the substrate 202. In a step (b) the stamp 215 has been removed and the substrate has been cured leaving a first layer 209 on the substrate 202. Further layers are printed on top of the first layer 209 in a similar manner. Figs. 6a to 6d illustrate the respective steps (a) to (d) of a fourth embodiment of making a thermoelectric device 301 according to the invention. In a first step (a) electrodes 308 are formed on a substrate 303. The electrodes 308 could be formed by masking parts of the substrate 303 followed by spray etching or by means of a printing technique, such as screen printing or any other suitable printing technique which is known in the art. In a second step (b) a first layer 309 is applied by inkjet printing to the electrodes 308 to start forming a segment 305. As is shown in Fig 6b the substrate 303 is moved horizontally at a predetermined speed under an inkjet head 312 having a nozzle 313. At predetermined intervals, corresponding to the locations of the electrodes 308, drops of a first material 314 are ejected by the nozzle 313. The drops of material 314 may comprise a polymer binder, such as curable polymeric resin, and, dispersed therein, a powder of a suitable semiconductor material. The drops of material 314 are ejected towards the electrode 308, contact the electrode 308 and then passes under a UV-curing hood 315 having a UV-lamp 316 and a nitrogen, N , supply 317. In passing under the UV-lamp 316 the polymer binder of the drop is cured to form a first semiconducting layer 309. In a third step (c) drops of a second material 318 are ejected to apply a second layer 310 on the first layer 309 in a similar manner. The steps (b) to (c) are repeated until the desired number of layers 309, 310 have been formed thus forming the p-type segment 305 with a desired height from the substrate

303. In a similar manner the n-type segments 306 are formed. In a final step (d) the substrate 302, having provided thereon electrodes 307 of the desired pattern, are put on top of the segments 305, 306 to connect them to each other and to form the thermoelectric elements

304. The segments 305, 306 are printed at a distance D from each other to avoid contact between them and provide proper insulation. No machining is thus required. It will be appreciated that numerous modifications of the embodiments described above are possible within the scope of the appended claims. Thus other printing techniques could also be used for printing several layers on top of each other to form a printed multilayer semiconductor segment. The most preferred techniques, however, are those where a material is taken from a source of material, such as a stamp 12, 112, a plate 219 or a nozzle 313, and is brought in direct contact with an area, such as an underlying layer, on which it is intended to form a layer as is described in the embodiments above. In particular the techniques involving bringing a stamp in direct contact with a substrate has proven to provide high accuracy of the multilayer segments and little, or no, requirement for machining. Above it is described how one layer is printed in each sequence. It is, however, also possible to print e.g. two layers at the same time. On possible embodiment is to use the stamp 1 12 shown in Fig. 4a and first contact it with a second material such that the protrusions 113 are coated by the second material. The stamp 112 is then brought into contact with the first material 114 in such a way that the protrusions 113 become covered by a film of the second material on which a film of the first material 1 14 is coated. When the stamp 1 12 is brought into contact with the substrate 102 it will print a double layer comprising the first material 114 and, on top of that, the second material. After curing the substrate 102 will have a first layer 109 and, on top of that, a second layer, the two layers being printed in one sequence. On these two layers further layers may then be printed. To summarize a thermoelectric device 1 has first and second substrates 2, 3 being separated from each other by a thermoelectric element 4. The element 4 has a p-type semiconductor segment 5 and a n-type semiconductor segment 6 that are connected to each other by an electrode 7 located on the first substrate 2. Other electrodes 8, located on the second substrate 3, connect the segments 5, 6 to an electrical circuit via adjacent elements. At least one of the segments 5, 6 has been made by printing multiple layers 9, 10 on top of each other in consecutive steps. The thermoelectric device 1 could be used for cooling according to the principles of Peltier elements or could be used for heat recovery to produce electricity. The printed layers 9, 10 of the segments 5, 6 provides for high efficiency and low manufacturing costs. The invention is not restricted to the described embodiments. It can be altered in different ways within the scope of the appended claims.

Claims

CLAIMS:

1. A method of forming a thermoelectric device, the method comprising the steps of providing a first substrate (2) and forming thereon at least one first electrode (7), forming on said first electrode (7) a first segment (5) by printing thereon a plurality of layers (9, 10) being parallel to the substrate (2), one layer (9) on top of the other (10), in a plurality of consecutive steps to form a first segment (5) of a first conductivity type, forming a second segment (6) of a second conductivity type, being opposite to the first conductivity type, on said first electrode (7), applying a second electrode (8) to each segment (5, 6) and a second substrate (3) for forming a thermoelectric element (4) comprising the first electrode (7), the two segments (5, 6) and the second electrodes (8), the second substrate (3) being parallel with the first substrate (2) and separated therefrom by the segments (5, 6) extending between the two substrates (2, 3).

2. A method according to claim 1, wherein each layer (10) is provided with its final shape in a plane parallel to the planes of the substrates (2, 3) immediately on application on the underlying layer (9).

3. A method according to claim 1 or 2, wherein the printing of a layer (10) comprises the application of a material (18) on an underlying layer (9) followed by the curing of said material (18).

4. A method according to claim 3, wherein said material comprises a semiconductor material dispersed in a solvent or a binder.

5. A method according to any one of claims 1-4, wherein the forming of said first segment (5) comprises using a patterned stamp (12), on a surface of which is provided a material (18) and which is then brought into direct contact with an underlying layer (9) to form a layer (10) of said material (18) thereon.

6. A method according to any one of claims 1-4, wherein the forming of said first segment (305) comprises using a nozzle (313) for ejecting a material (318) onto an underlying layer (309) to form a layer (310) thereon.