DE102012022328B4 - Thermoelectric module - Google Patents

Abstract

Thermoelectric module which is formed with a plurality of thermoelectric elements connected in series, which are interconnected by means of at least one substrate, characterized in that the substrate (1) or the substrates of materially interconnected metal fibers or thin metal wires is formed or are each formed and the substrate (1) or the substrates form or form a network structure in which cavities or free spaces are present, and in that there are electrical contacts (5) which produce electrically conductive connections between juxtaposed thermoelectric elements, and in that the substrate (1) or the substrates on a side facing the thermoelectric materials (6) of the thermoelectric elements side and / or on a the electrical contacts (5) for electrically conductive connection between juxtaposed thermoelectric elements side facing a dielektris Surface coating or a dielectric layer (2) comprises or each have.

Description

The invention relates to thermoelectric modules which are formed with electrically interconnected components of thermoelectrically active materials (thermoelectrics). The thermoelectrics are characterized by the fact that the three properties of electrical and thermal conductivity as well as thermoelectricity (Seebeck coefficient) are present in a favorable relationship for the thermoelectric efficiency. The thermoelectric efficiency is described by the dimensionless ZT value, which is defined by the formula ZT = S ² .σ / κ .T _m (S-Seebeck coefficient, σ-electrical conductivity, κ-thermal conductivity, T _m -middle temperature on a thermo leg) and should be> 1 for a technically usable efficiency. In order to produce a technically utilizable electrical voltage with a thermoelectric generator, it is advantageous to construct a module with suitable electrical connection also only of n- or p-type legs, so that p- and n-type legs are combined and one as possible high temperature gradient is applied to the thermoelectric legs, because the height of the achievable thermoelectric voltage results from U _th = S · ΔT. From the mentioned thermoelectric requirements arise a number of thermomechanical requirements. It is not possible with all thermoelectric materials to provide p-type and n-type materials by different doping. Some materials are either only n-type (eg titanium suboxide) or only p-type (eg boron carbide). Such materials can have many advantages over other thermoelectric materials, such as high temperature resistance, high availability, low production costs. In order to be able to use p- and n-type materials, different materials must be used in the construction of a thermoelectric generator / module. However, such materials have different coefficients of expansion (eg TiO _x α = 9 × 10 ^-6 K ^-1 and B _× C α = 4 × 10 ^-6 K ^-1 ). Since these materials are used together in a thermoelectric module / generator and are heated together, it comes to different length expansions when changing the temperature, which can lead to the destruction of the electrical connections or breakage of serving as a generator base substrates.

1. 1. die n- und p-leitenden Schenkelwerkstoffe sollten möglichst gleiche thermische Ausdehnungskoeffizienten aufweisen;
2. 2. Schenkelwerkstoffe und Substratwerkstoffe sollten möglichst gleiche thermische Ausdehnungskoeffizienten aufweisen;
3. 3. der Temperaturgradient im Generator sollte möglichst klein bleiben.

Furthermore, it is necessary to achieve the greatest possible temperature difference at the two legs to generate a thermoelectric voltage. The temperature difference causes the n- and the p-type thermo legs to expand to different extents over the length and therefore different strains occur at the connection positions to the substrate which lead to mechanical stresses. Likewise, the substrate plates on the cold and on the warm side of the module / generator expand to different degrees, so that the legs are mechanically stressed due to this different substrate strain. For these reasons, it is necessary to take the following measures:

1. 1. the n- and p-type leg materials should have the same thermal expansion coefficients as possible;
2. 2. Schenkelwerkstoffe and substrate materials should preferably have the same thermal expansion coefficient;
3. 3. The temperature gradient in the generator should remain as small as possible.

These three requirements are not always feasible at the same time. That's the requirement 3 , the requirement for a high efficiency in energy conversion, which is of course counterproductive.

In previously known technical solutions has been trying to largely comply with the above requirements. With regard to materials, however, this is generally not possible since thermoelectric materials with a high ZT value are present only in small numbers, so that there are only a few options here.

The substrate materials are usually either ceramics ( WO 2008/127017 A1 . JP 2005340559 A ) or plastics ( US 2011/0030754 A1 . JP 2009267316 A ). Metals are not suitable as they would electrically short the generator in direct contact with the thermoelectric materials. Electrically insulating layers can be applied to metals ( KR 10 2008 089 926 A ) would then lead in principle again to the ceramic or plastic-specific problems. Plastics allow only maximum application temperatures of 250 ° C - 300 ° C and are in most cases thermally insulating. There are only a few solutions, such as Kapton substrates ( US Pat. No. 6,096,964 B1 ), which have sufficient temperature resistance and thermal conductivity. Although ceramics can be used at high temperatures and also offer a sufficiently high thermal conductivity in conjunction with electrical insulation. They are however brittle and have high E-modules. Therefore, they are susceptible to breakage when thermo-mechanical stresses occur and can not provide stability for the thermoelectric modules at too high temperature gradients. They are not able to compensate for differences in expansion in thermoelectric materials because of the temperature gradient or different materials.

In order to compensate for this susceptibility to breakage, metallic sliding layers can be applied to the ceramics ( WO 2008/053736 A1 . WO 2007/105510 A1 ). However, their effectiveness is limited and not suitable to ensure long-term stability.

The lack of a solid material connection between the n- and the p-type legs and the substrates and the bond via electrical contacts in the n- and the p-type legs allows a high flexibility of a thermoelectric module / generator ( JP 2009-043752 A ). However, this solution reduces the desirable thermal connection of the thermoelectric materials to the heat transfers of the module / generator and leads to mechanical instabilities and susceptibility to failure of the electrical circuit. Even the use of extremely thin substrates can not overcome the susceptibility of such structures. Only the flexibility of the modules can be increased so far that even curved surfaces can be filled with the module ( US Pat. No. 6,410,971 B1 ).

To reinforce a module during thermal cycling, higher-strength materials have been incorporated into a module instead of thermoelectric materials ( JP 2004-235367 A ). The disadvantage here is that the space in the module can not be completely occupied or exploited by thermoelectric materials, so that the efficiency of such modules is smaller. The use of higher strength ceramic substrates, such as Si ₃ N ₄ substrates, is in principle more favorable for the durability of the modules ( US 2002/0164475 A1 . JP 2002-252379 A ), however, does not completely solve the problem since in this case the thermoelectric materials will fail as the first component.

A thermoelectric conversion element is in WO 99/04439 A1 described. It should be provided as a thermal conductor, a dielectric which is connected to the transducer element.

The DE 10 2006 055 120 A1 relates to thermoelectric elements in which a porous matrix or a porous substrate is used.

Out US 2011/0139207 A1 are known elements which use a coated porous substrate.

At the US 5 022 928 A known technical solution to be used flexible or non-flexible electrically insulating materials for substrates.

The US 2002/0182397 A1 and DE 10 2007 042 494 A1 concern elements with good thermal conductivity, which can also be used anisotropically.

It is therefore an object of the invention to provide thermoelectric modules available that can be used at high temperature differences with increased efficiency with increased life duration and reliability.

According to the invention this object is achieved with a thermoelectric module having the features of claim 1 has dissolved. Advantageous embodiments and further developments of the invention can be realized with features described in the subordinate claims.

An inventive thermoelectric module is formed with a plurality of thermoelectric elements, which are electrically connected in series. In this case, the thermoelectric elements are connected to at least one substrate and held together. The substrate is formed from cohesively interconnected metal fibers or thin metal wires. In a cohesive connection, the metal fibers or wires should be sintered together. In this case, only selective connections between juxtaposed fibers or wires should be made by means of sintered bridges. As a result, cavities or free spaces remain in a correspondingly formed network structure, which allows an elastic deformation of such a substrate.

The proportion of cavities or the porosity can be taken into account in the selection of a usable substrate in the invention. In this case, the desired thermal conductivity, which should be above 10 W / mK, preferably above 20 W / mK, in conjunction with the desired deformability at the mechanical stresses occurring as a result of different thermal expansions, be taken into account. A substrate may have a porosity in the range of 70% to 90% and average pore sizes in the range of 10 μm to 1000 μm.

In order to solve the thermomechanical problems in the construction of a thermoelectric module while allowing a high thermoelectric efficiency, the substrates should be changed so that they compensate for thermo-mechanical stresses while high-temperature resistant and good thermal conductivity. The mentioned features of the substrates can be realized simultaneously by using elements formed of metal fibers or thin metal wires as substrates for thermoelectric modules. The metal fibers or metal wires are while firmly bonded together. Particularly advantageous are mutually sintered metal fibers. The metals can be selected according to the respective conditions of use. In this case, the temperatures occurring, the ambient atmosphere or a medium which comes into contact with the metal, can be taken into account. It is also possible to carry out an optimization, wherein the deformability due to different thermal expansion coefficients and the thermal conductivity of the substrate, particularly advantageous depending on the direction, can be taken into account.

For this purpose, the metal fibers or thin metal wires may at least predominantly be arranged in a plurality of planes aligned parallel to one another and correspondingly aligned such that an anisotropic thermal and electrical conductivity can be utilized. In this case, an increased temperature difference between the hot and the cold side of the module can be maintained.

Elements formed of metal fibers or wires, e.g. Plates or strip-like elements have a sufficiently high thermal conductivity because they inherently contain a network of metal fibers or wires. This network can achieve a relatively high thermal conductivity with a suitable choice of material and suitable network structure. At the same time, such networks are able to elastically deform locally when mechanical stresses are applied, so that the outer geometric shape and dimensioning of a metal fiber or thin wire element can remain unchanged and deformations only in areas where mechanical stresses due to different thermal stresses occur Extension occur, take place. The occurring thermo-mechanical stresses are thereby degraded locally within such a substrate formed and have no effect on the associated with the substrate assemblies or other elements. As a result, different strains of the different thermoelectric and other materials used on a module over the length and over the substrate locally within the substrate can be compensated, so that the total module remains mechanically stable.

The generated mechanical stresses between the cold and the warm side of the module are also minimized by the described local compensation, so that the module can use significantly higher temperature differences without being mechanically damaged or even destroyed.

As a material for metal fibers or metal wires, iron, aluminum or copper or alloys thereof may preferably be used.

A substrate should have on a thermoelectric material and / or an electrical conductor for the electrically conductive connection of juxtaposed thermoelectric elements facing side a dielectric surface coating or a dielectric layer, which is preferably formed from an oxide. This coating or the dielectric layer should be usable at temperatures of at least 400 ° C.

A substrate can additionally be oxidized on this surface in order to form an oxidic coating with sufficient thickness on the metal surface, which is sufficiently electrically insulating. As a result, short circuits or electrical leakage currents can be avoided. The dielectric layers or elements should be as thin as possible in order to maintain a good thermal conductivity with sufficient electrical insulation.

The electrical insulation of the interconnected metal fibers or wires can be ensured by an electrically insulating coating. This coating can be constructed as a multilayer structure of thermally resistant metal / ceramic layers. This can ensure that the electrical insulation between substrate and thermoelectric materials is present, that mechanical stresses between the substrate and thermoelectric materials minimized and that electrical connections between the n- and the p-type single legs of the module can be made. The layer structure increases the reliability of the electrical connections by compensating or minimizing, in the immediate area of the electrical contacts, expansion differences between electrical contact on the respective thermoelectric material and electrical conductor and between the electrical conductor and substrate. For this purpose, metallic and ceramic materials can be combined in order to be able to coordinate thermal expansion coefficients, electrical conductivity, plasticity, elasticity and E-modules.

On a substrate, the metal fibers or metal wires may at least predominantly be interconnected and oriented so that the thermal conductivity from an outer surface towards the thermoelectric elements is greater than in a direction perpendicular thereto. Alone or in addition, the ductility in the direction perpendicular to that of an outer surface in the direction of thermoelectric Elements oriented direction to be larger. By means of these anisotropic properties, the heat conduction from the outside towards thermoelectric materials can be improved or the mechanical stresses which occur in connection areas of the substrate with further elements of the thermoelectric module, which are connected to a substrate as juxtaposed thermoelectric elements, can be better compensated.

1. 1. Metallfaserplatte als Modulsubstrat (z.B. AI-Faserplatte gesintert)
2. 2. Oxidschicht (Aluminiumoxid) zur elektrischen Isolation zwischen Substrat und elektrischem Kontakt
3. 3. thermisch-mechanische Anbindung (Aktivlot) zum Substrat
4. 4. elektrischer Leiter zum Verbinden von thermoelektrischen Schenkeln
5. 5. elektrische Kontaktierung auf dem thermoelektrischen Werkstoff
6. 6. thermoelektrischer Werkstoff (n- oder p-leitend)

A module formed with substrates formed of metal fibers or thin metallic wires and one or more thermoelectric material (s) may be constructed, for example, as follows:

1. 1. Metal fiber plate as module substrate (eg Al-fiber board sintered)
2. 2. oxide layer (aluminum oxide) for electrical insulation between substrate and electrical contact
3. 3. thermal-mechanical connection (active solder) to the substrate
4. 4. electrical conductor for connecting thermoelectric legs
5. 5. electrical contact on the thermoelectric material
6. 6. thermoelectric material (n- or p-type)

The module can be mirrored to the other side and it can be under the points 1 - 5 Repeat described elements, wherein another either n- or p-type thermoelectric material can be used. It is also possible to use another metal or another metal alloy for a substrate arranged there. This allows, for example, other temperatures and external conditions to be taken into account on this side of the module. This concerns, for example, a possible corrosion attack by correspondingly corrosive media, which only occur on one side of the module.

With regard to the layer structure for the oxide layer 2 (Alumina) for electrical insulation between the substrate and the electrical conductor to the thermoelectric material 6 For local voltage compensation in the contact area, various useful variants can be used, which can be selected according to the specific material combinations in the module. It may, for example, a multilayer structure of a metallic layer (eg Al foil) and the oxide layer 2 (Alumina) for electrical isolation between the substrate and electrical conductor can be realized.

The materials used can be coordinated with each other. As thermoelectric materials, established thermoelectric materials, e.g. Bismuth telluride, lead telluride or Si-Ge compounds, as well as other thermoelectric materials, e.g. Skutterudite, Chlatrate, ceramic thermoelectrics (oxides, nitrides, carbides, borides) can be used.

The metals for the substrates can also be chosen according to the conditions of use of the module. The local deformability of the substrates should be considered. This may be accounted for by the metal chosen, the thickness and / or length of metal fibers or thin wires, and / or the manner of the selected interconnection between metal fibers, as well as the porosity or metal to ambient atmosphere per unit volume.

In the invention, the local degradation of thermo-mechanical stresses within the metal fiber or wire structure of the substrate in conjunction with outwardly high mechanical stability of the overall module over the prior art is particularly advantageous. Ensuring good thermal coupling and compensation for thermomechanical and chemical differences offers the possibility of building long-term stable thermoelectric modules for high temperatures or temperature differences (> 400 ° C). With these higher efficiencies can be realized. It is both an electrical use, in which heat can be converted into electrical energy, as well as a tempering use for heating or cooling possible.

The invention will be explained in more detail by way of example in the following.

The single figure in a schematic sectional view shows a structure of an example of a thermoelectric module according to the invention.

In the figure, an example of a thermoelectric module according to the invention is shown in which a plurality of thermoelectric elements are electrically connected in series and arranged side by side. In this example, the module is on two opposite sides of each one substrate 1 edged. The substrates 1 are formed of sintered aluminum fibers. The fibers have a length of 10 mm and a mean circular equivalent diameter of 200 microns. The substrate has a porosity of 81.4%. It is designed in the form of a plate with the dimensions L * B * H (30 x 30 x 5 mm) and achieves a thermal conductivity of 12 W / mK.

Metallic fibers of an alloy consisting of 95% by mass of aluminum and 5% by mass of copper were produced by melt extraction into short fibers having an average length of 6.8 mm and a mean equivalent diameter of 133 μm. The fibers were poured onto a flat surface by means of a sieve drum and sintered under weight to form a 10 mm thick plate with a total porosity of about 80%. From this plate, rectangular strip-shaped elements with a width of 5 mm were cut by laser beam. The substrate is built up from the strips rotated by 90 ° about its longitudinal axis to obtain low stiffnesses in the longitudinal direction of the fiber strips and the plate plane as well as high thermal conductivities perpendicular thereto. The strips are arranged side by side and sintered or soldered on the top and bottom with a very thin (0.05 mm to 0.1 mm) aluminum foil. This film can be a non-conductive, extremely thin ceramic layer 2 (Aluminum oxide) sputtered or deposited by other methods, which is the electrical insulation against the thermoelectric components. Then the thermoelectric components are mounted. The metal fibers are aligned in the strips so that they are aligned in at least nearly the same orientation in several parallel aligned planes and therein again at least predominantly parallel to each other so that they have directional properties, which in particular relates to the thermal conductivity and stiffness. Taking into account a favorable for the application of thermal conductivity they should be aligned in the direction in which a good heat conduction is desired, ie in the direction of the substrate plate to the thermo leg on the hot side and the thermo leg to the substrate on the cold side.

So that can be between the substrates 1 and the thermoelectric material 6 a loss-free thermal line can be realized. The aluminum oxide layer 2 forms an electrical insulator between substrate 1 and electrical contact 5 , The substrate coated in this way 1 can with an a sufficiently high or suitable melting temperature having active solder 3 with strip-shaped elements, which are made of the metal fibers, which have been produced as described above, firmly bonded and thereby ensure a very good thermal connection.

Between the ladders 4 and the thermoelectric material forming the thermoelectric elements 6 are electrical contacts 5 formed over which the thermoelectric material 6 with the electrical conductors 4 are electrically connected. The electrical conductors 4 are made of a metallically conductive material, preferably copper, nickel or silicides, and the electrical contacts 5 are formed of a metallic active solder, such as silver / copper / titanium. As a thermoelectric material 6 In this example, titanium suboxide was used as the n-type leg and boron carbide as the p-type leg.

The structure described is repeated on the side of the thermoelectric material 6 formed opposite elements, ie on the cold side of the module.

• Warmseite: 500°C
• Kaltseite: 100°C

The thermoelectric module was operated with the following parameters:

• Warm side: 500 ° C
• Cold side: 100 ° C

Damage due to thermomechanical or thermal stress could not be determined over a period of 100 h.

The module can be operated so that a substrate 1 with a warm or hot medium and the other substrate is in contact with a colder medium. As a result, an electrical voltage can be tapped off from the electrically series-connected thermoelectric elements of the module to contacts, not shown.

The module can also be operated differently by an electrical voltage is applied to the outer contacts, not shown, and the module can be used on one side via the corresponding substrate for cooling and the respective other substrate for heating; Depending on the direction of the electrical current flow, the respective cold or warm side can be selected.

Claims (10)

Thermoelectric module which is formed with a plurality of thermoelectric elements connected in series, which are interconnected by means of at least one substrate, characterized in that the substrate (1) or the substrates of materially interconnected metal fibers or thin metal wires is formed or are each formed and the substrate (1) or the substrates form or form a network structure in which cavities or free spaces are present, and in that there are electrical contacts (5) which produce electrically conductive connections between juxtaposed thermoelectric elements, and in that the substrate (1) or the substrates on a side facing the thermoelectric materials (6) of the thermoelectric elements and / or on one of the electrical contacts (5) to the electrically conductive Connection between juxtaposed thermoelectric elements facing side, a dielectric surface coating or a dielectric layer (2) comprises or each have. Module after Claim 1 , characterized in that the substrate (1) reaches a thermal conductivity of at least 10 W / mK. Module after Claim 1 or 2 , characterized in that the metal fibers or metal wires from which the substrate (1) is formed, soldered at points, welded together and / or sintered. Module according to one of the preceding claims, characterized in that the metal fibers or thin metal wires, with which the substrate (1) is formed, are arranged and aligned predominantly in mutually parallel planes. Module according to one of the preceding claims, characterized in that the metal fibers or metal wires, with which the substrate (1) is formed, are formed from aluminum, copper, iron or an alloy of these metals. Module according to one of the preceding claims, characterized in that the dielectric surface coating or the dielectric layer (2) can be used at a temperature of at least 400 ° C. Module after Claim 6 , characterized in that the dielectric coating on the surface of the substrate (1) or the dielectric layer (2) is formed of an oxide. Module according to one of the preceding claims, characterized in that a substrate (1) has a porosity in the range of 70% to 90%. Module according to one of the preceding claims, characterized in that two different substrates (1) are provided on opposite sides of the module. Module according to one of the preceding claims, characterized in that on a substrate (1) the metal fibers or metal wires are at least predominantly connected and aligned so that the thermal conductivity of an outer surface in the direction of the thermoelectric elements is greater than in a perpendicular thereto Direction and / or that the deformability is greater in a direction perpendicular to the aligned from an outer surface in the direction of thermoelectric elements direction.

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