HK1258368B - Rational method for the powder metallurgical production of thermoelectric components - Google Patents

Description

Rational method for the powder metallurgical production of thermoelectric elements

The invention relates to a powder metallurgical production method for a thermoelectric element or at least a semifinished product thereof.

A thermoelectric element is an energy converter that utilizes the thermoelectric effect described by Peltier and Seebeck to convert thermal energy into electrical energy. Since the thermoelectric effect is reversible, any thermoelectric element can also be used to convert electrical energy into thermal energy: an element called a peltier element is used to cool or heat an object while consuming electric power. Thus, a peltier element is also considered a thermoelectric element in the context of the present invention. Thermoelectric elements for converting thermal energy into electrical energy are commonly referred to as thermoelectric generators (TEGs).

An industrial implementation of a thermoelectric element comprises at least one thermocouple of a thermoelectrically active material formed by two thermocouple branches (thermoelements), and a housing which carries and surrounds the thermocouple and electrically insulates it from the outside.

Various thermoelectrically active materials are described in the literature. For commercial use, alloys of the bismuth telluride (especially with additional selenium and/or antimony moieties) class, such as semiconductors, are suitable from which doping, first p-conducting and then n-conducting-can establish the thermocouple.

Further classes of thermoelectrically active substances are: semi-heusler materials, various suicides (in particular magnesium, iron), various tellurides (lead, tin, lanthanum, antimony, silver), skutterudites, various antimonides (zinc, cerium, iron, ytterbium, manganese, cobalt, bismuth, occasionally also referred to as Zintl phases), TAGS, silicon-germanides, clathrates (in particular based on germanium). In addition to these semiconductor materials, the thermoelectric elements can also be made of a combination of most standard metals, for example conventional thermocouples for temperature measurement, for example Ni — CrNi. However, the achievable "figure of merit" (thermoelectric "efficiency") is significantly lower than for the cited semiconductor materials.

Conventional thermoelectric elements are generally composed of a solid block of thermoelectrically active semiconductor and a hard, usually ceramic, electrically insulating housing. Where solid blocks are used, the blocks are typically sawn from solid ingots.

Since ingots typically contain defects or shrinkage holes, they are typically first ground into a powder and the powder sintered as needed to form highly compacted flakes (wafers). The block TE branch is then sawed from the compact low-cavity wafer as needed.

WO 2008061823 a1 discloses the production of semi-finished thermoelectric elements by introducing thermoelectric materials as powders into a flat porous substrate. The thermocouple branches of the resulting element extend perpendicular to the plane of the substrate.

DE102012205087a1 discloses another powder metallurgical process for producing thermoelectric elements. The compaction of the active material, provided in powder form, occurs within the pores of a perforated template that becomes part of the resulting thermoelectric element, i.e., the substrate.

A disadvantage of this method can be seen in that the template must be composed of a thermally and electrically insulating material, since it remains in the TEG as a substrate. At the same time, the template must withstand high mechanical loads during pressing of the green body, which limits the choice of thermally and electrically insulating substrate materials.

WO2015/043824a1 discloses an improved method for powder metallurgical production of thermoelectric elements. In this method, a powdered active material is pressed in a mold disposed outside a substrate to form a green body, which is forced out of the mold into a hole disposed in the substrate and sintered therein to form a thermocouple branch.

A disadvantage of this method is that the die, which is disposed outside the substrate and in which the active material is pressed to form the green body, must be arranged in alignment with the hole of the substrate into which the green body is forced. This orientation and the transfer of the green body from the mold into the substrate requires a special machine that must be separately developed and produced for this purpose. This adds significantly to the capital cost of an assembly line created using this approach. Furthermore, the green body is subjected to sintering shrinkage, i.e. volume reduction during the sintering operation. As a result, the thermocouple branches are shortened within the substrate, and thus the thermocouple branches may be difficult to contact. Typically, the protruding substrate material must be ground to achieve a flush finish of the thermocouple branches to the substrate surface, a prerequisite for reliable electrical contact. This process step increases the manufacturing cost.

Another fundamental disadvantage of the method described in WO2015/043824a1 is that it does not allow individual weight control of the branches. This makes it more difficult to maintain narrow specification limits for branch density uniformity.

Starting from this prior art, it is an object of the present invention to provide a powder-based method for producing thermoelectric elements or corresponding semi-finished products, which can be carried out using standardized machinery in order to reduce capital costs and increase process stability. In addition, post-processing steps for equalizing the substrate and thermocouple branches are desirably avoided. To promote quality assurance, the individual thermocouple branches and/or their green bodies can be weight-controlled.

This object is achieved by a method having the following steps:

a) providing a substantially planar substrate made of an electrically and thermally insulating substrate material, the through-holes through the substrate extending substantially perpendicular to the substrate plane;

b) providing a powdered thermoelectrically active material;

c) compressing the active material to form a green body, wherein the compressing is performed in a mold different from the substrate;

d) inserting the green bodies into the through-holes of the substrate such that one green body extends through the substrate along its axis within each through-hole;

e) disposing the substrate with the green body inserted therein between two substantially flat electrodes such that the two electrodes and the substrate are oriented substantially parallel to each other;

f) contacting the end of the green body with an electrode such that a connection is provided between the two electrodes through the green body, the connection not only transferring electrical current but also transferring mechanical pressure;

g) exposing the green body to an electrical current flowing between the electrodes such that heat is induced within the thermoelectrically active material;

h) exposing the green body to a pressure acting between the electrodes such that the thermoelectrically active material is under pressure;

i) sintering the green body under the action of pressure and heat to form thermocouple branches;

k) the base plate and thermocouple branches contained therein are leveled by bringing them close to the electrodes while maintaining their parallelism so that the thermocouple branches are flush-trimmed with the base plate, wherein any axial offset of the green body in the base plate and any sintering shrinkage are compensated for.

The present invention relates to such a method.

The basic concept of the invention is to perform the process steps of pressing, inserting and sintering on different machines in order to be able to use standardized machines for as many method operations as possible.

An essential aspect of the invention is that sintering and equalization take place in one working step, since a planar sintering electrode is used, arranged with its plane parallel to the substrate, which moves towards the substrate after sintering shrinkage. The green body is accordingly provided in an oversized dimension to compensate for sintering shrinkage. At the end of the sintering operation, the spacing between the electrodes not only defines the thickness of the substrate, but also the length of the thermocouple branches. This ensures a flush arrangement of the thermocouple branches with the substrate without further post-processing.

Another advantage of rationalization is that it is possible to obtain that, because a plurality of substrates with inserted green bodies are combined to form a stack, wherein the substrates extend parallel to one another within the stack, and in each case a substantially flat separator plate is placed between two adjacent substrates within the stack, which separator plate extends parallel to the substrates and produces an electrically conductive and force-transmitting connection between the green bodies of the adjacent substrates, and because the entire stack is arranged between two electrodes. One sintering operation was performed on one sintering machine using a divider plate, producing a plurality of semi-finished products (i.e., substrates with inserted thermocouple branches).

In the above-described assembly method, a plurality of substrates with the inserted green bodies may be individually arranged or stacked in a plane between two electrodes. As a result, the manufacturing is further rationalized.

The planar electrodes and/or separators are preferably composed of graphite because this material readily conducts electrical energy, withstands high sintering temperatures, and does not adhere to the thermoelectrically active material. Graphite has the property that its mechanical strength increases with increasing temperature. To take advantage of this effect, the development of the process of the present method provides that the green body for contacting is exposed to a first pressure, then the green body is exposed to an electric current under the action of the first pressure until the electrode and/or separator plate composed of graphite has reached a temperature at which the electrode and/or separator plate composed of graphite has an increased load-bearing capacity or breaking load above the first pressure, and the green body is then exposed to a second pressure which is higher than the first pressure and lower than the increased load-bearing capacity and/or breaking load. Therefore, when the electrode and/or the separator have sufficient strength due to temperature, the second pressure required for pressing is first established. The exposure is performed using only the first, lower pressure before the temperature is reached.

The method is used for processing thermoelectrically active materials selected from the group consisting of bismuth telluride, lead telluride, zinc antimonide, silicides, skutterudites, and hemi-cobblestonesAlloys of the class of materials. Nanoscale silicon can also be used as the active material. Bismuth telluride (Bi) is particularly preferable₂Te₃)。

During the compaction of the powdered active material to obtain a green body, the powdered active material is compacted. Preferably, however, the powder is not compressed to a (theoretical) true density, but only to a first compressed density, which corresponds to between 75% and 85% of the true density of the active material. True density refers to the density of the ideal solid made from the active material used. The true density of the commercial grade bismuth telluride alloy is, for example, 6.9g/cm³. The green body is porous because the first compressed density is lower than the true density. The first compression density is logically necessarily greater than the bulk density of the powdered active material because the powder is compacted during compaction. The bulk density of the powder used is preferably 30% to 50% of the true density.

In order to obtain the first compression density, the particle size distribution of the powdered active material and the compression force in the tablet press must be properly selected. Generally, in the case of bismuth telluride as the active material, it should have a median particle diameter d of between 3 μm and 30 μm before compression₅₀(ii) a The pressure at which the thermoelectrically active material is compressed to form the green body should then be between 541MPa and 955 MPa. According to Mie theory, the particle size distribution is determined by static laser scattering. The analytical method is specified in DIN-ISO 13320; by wettingAnd (6) measuring. A suitable measurement instrument is the laser light scattering spectrometer HORIBA LA 950 from Retsch Technology GmbH, Haan (germany). The pressure applied by the tablet press during compression of the powder can be read on the tablet press. For this purpose, the force is determined at the punch, for example using a load cell (extension measuring strip) and the associated punch area.

The length of the green body should correspond to 105% to 150% of the thickness of the substrate in the leveled state. This means that the green body inserted into the substrate protrudes from the substrate, which allows good mechanical and electrical contact with the electrodes and/or separator plate. The sintering shrinkage then shrinks the thermocouple branches to the substrate thickness, so that subsequent equalization is not required.

The temperature set in the green body during sintering should correspond to 50 to 70% of the melting temperature of the active material. The melting temperature depends on the active material used. In the case of bismuth telluride, the melting temperature is 573 ℃. If alloys from the bismuth telluride class are used, the optimum sintering temperature is therefore between 287 ℃ and 401 ℃, depending on the particular alloy.

The temperature can hardly be measured in the green body itself. Alternatively, the temperature is measured using a thermometer on the electrode during sintering. The sintering temperature is controlled by the current to which the green body is exposed.

If bismuth telluride is used as the active material, the green body during sintering should be exposed to an electrical current of the order of:

current density based on green cross-sectional area: 10 kA/m²To 100 kA/m²

Exposure time: 600 seconds to 1100 seconds

Initial weight of electrical energy input/active material: 150kJ/g to 250kJ/g

The current may be applied as an alternating current with a frequency in the range of 20Hz to 100 Hz. Alternating current with a standard grid frequency of 50Hz or 60Hz is suitable. Direct current may also be used.

The electrode and green body are heated to a temperature between 300 ℃ and 400 ℃ as a result of exposure to the electrical current. The temperature can be measured in the electrodes and used as a control parameter. High temperatures affect sintering of the active material. The sintered green body corresponds to the thermocouple branch. As a result of sintering, the resistance of the active material decreases and the thermoelectric activity of the sintered thermocouple branches increases compared to the porous green body.

For each thermocouple, two thermoelectrically active materials conducting in different ways are required, which are electrically connected to each other, wherein the first active material is, for example, p-conducting and the second active material is n-conducting, or vice versa. By "different" is meant herein that the two active materials have different seebeck coefficients. The p-conductive semiconductor and the n-conductive semiconductor are particularly preferable as the active material because their seebeck coefficients have different signs (negative for the n-conductor and positive for the p-conductor), and thus the difference in the numerical value of the seebeck coefficient is particularly large. This improves the efficiency of the thermoelectric element.

In a first variant of the preparation method, two active materials are compressed one after the other to form the respective green bodies and used. This means that the first green body is made of a p-conductive material and inserted into the substrate, and then the n-conductive active material is processed. Thereby reducing machine changeover time. Of course, it is also possible to process the n-conductive material first and then the p-conductive material.

However, a second and preferred variant provides for the use of two tabletting machines, the first for the p-conductive active material and the second for the n-conductive active material. By reducing the changeover time, one can quickly recapture the doubled capital cost compared to the first variant: since the n-and p-conducting materials cannot mix in the thermocouple branch, the machine must be thoroughly cleaned during the material change. In particular, when complex rotary presses are used, the changeover time becomes very long. When two machines are used, each machine must operate in a single kind of manner, eliminating changeover time and significantly extending machine maintenance time. Furthermore, the use of a special press for the n-conductive and p-conductive active materials leads to an improvement in quality, since contamination of the thermocouple branches with foreign substances is completely precluded.

The pressure at which the green body is loaded during sintering is significantly lower than the pressure previously during pressing of the powder to form the green body.

Preferably, the green body is compressed to a second compressed density during exposure to the pressure acting between the electrodes, the second compressed density corresponding to between 90% and 97% of the true density of the active material. Thus, the green density immediately prior to sintering is further increased, but the theoretical true density (as defined above) is not determined.

In the case of bismuth telluride, the green body should be exposed to a pressure that results in a pressure in the thermoelectrically active material of between 10MPa and 50 MPa.

Cold graphite typically has a compressive strength of 60MPa to 120 MPa. Thus, the graphite electrode/separator plate should first be heated to a temperature of about 300 ℃ before the final pressure is applied. At this temperature, the graphite element has reached the necessary fracture strength to withstand the sintering pressure without being damaged. The heating rate for a given temperature should be about 50K/min.

The green bodies used in the method may have different geometries. First, a general cylindrical shape is considered. This need not be based on a circular base but could be elliptical, rectangular, square or hexagonal. Cylindrical green bodies based on regular or irregular n-polygons may also be used. However, it is particularly preferred that the green body has a cylindrical shape.

In addition to being cylindrical, the green body may also be slightly conical. This means that the cross-sectional area decreases in the axial direction and the green body tapers. The advantage of a tapered green body is that it is firmly clamped in the through hole of the substrate and does not fall off when processing the substrate with the inserted green body. The cone angle should be chosen such that a sufficient clamping force is generated between the base plate and the inserted green body. Also, the taper angle must not be too steep to avoid splitting the substrate under the wedge action. The appropriate taper angle also depends on the coefficient of friction and thus on the material pairing.

The same effect can also be achieved by a tapering through hole or by a green body which is inserted radially oversized (press fit) into the through hole.

In order to be able to easily insert the cylindrical green bodies into the base plate, they should each have a chamfer on the end side. This applies even more when the green body has a radial oversize. Due to the chamfer, there is also no material splitting (sharp edges are easily broken) when inserting the green body. The green body is also less susceptible to damage from impact. Finally, the risk of developing protruding "mushroom heads" (or "rivet formations") during sintering is reduced.

The green bodies may also have a smooth surface so that they easily slide into the pores of the substrate. The surface quality of the green body is predetermined by the shape, the gauge mass and the pressure during pressing. Preferably, the side surface of the green body has an average roughness value Ra, specified in DIN 4766T2, between 12 μm and 24 μm.

An important advantage of the method described herein is that the green body can be pressed outside the substrate on a standardized machine. Surprisingly, existing tablet presses are suitable for this, as used in the pharmaceutical industry for powder drug compression. This finding is surprising because tableting machines are used to process pharmaceutical formulations that are chemically and physically significantly different from thermoelectrically active materials. For example, bismuth telluride has an extremely high density as compared with conventional drugs.

However, eccentric or rotary type tablet presses achieve the compressive force required to compress thermoelectrically active materials to form green bodies without modification, and large quantities of green bodies can be produced completely automatically from the active materials at high rates. The tablet press is not charged with a charge but with a powdered thermoelectrically active material. The press force should be set in such a way that the desired pressure (in Bi) is achieved in the die₂Te₃In the form of 700MPa to 1200 MPa). Therefore, a high load punch is selected. Thus, the complex new development of a separate machine can be bypassed by purchasing an immediately available tablet press.

A suitable rotary tablet press is a tablet press of the type 1200i from Fette Compacting GmbH, Schwarzenbek (germany). As mentioned above, two tablet presses are preferably purchased to produce n-doped and p-doped green bodies, respectively.

Another advantage of pharmaceutical tablet presses is that they are equipped with precisely operating metering devices from the beginning; thus, the active material is weighed into the mold with a high degree of "out of the box" accuracy. Metering is performed by volume measurement.

Another advantage of using separate green bodies produced ex situ on a tablet press is that defective green bodies can be more easily removed and their active material recycled by grinding. If the green body is pressed in situ (that is to say in the substrate) or transferred en masse directly from the mould into the substrate, individual defective pressed green bodies can penetrate into the substrate and reduce the quality of the subsequent TEG.

The tablet press randomly pops out the green body. For the individual insertion of the green body into the base plate, commercially available isolation and transport equipment can be used, as is standard for inserting tablets into blister packs. Preferably, a conveyor with a piezoelectric actuator or an oscillating conveyor is used. A suitable machine is Harro from Allmers bach im Tal, GermanyVerpackungsmaschinen GmbH. Other automated placement machines may be used, or the green body may be manually inserted into the substrate.

Basically, the method according to the invention makes it possible to choose from a wide range of possible substrate materials. In order to increase the efficiency of the thermoelectric generator, the substrate material should be selected to be as thermally and electrically insulating as high as possible. However, the substrate material must also be cost effective to ensure economic viability. In addition, the substrate material must have temperature (change) stability suitable for further fabrication routes and subsequent use in thermoelectric elements. It must also have a certain mechanical strength.

An example of a particularly economically advantageous substrate material is a composite material composed of an inorganic raw material and a binder. The inorganic raw material is preferably mica, perlite, phlogopite or muscovite. As the adhesive, silicone resin and/or epoxy resin is preferably used. With these materials, it is possible in particular to laminate substrates constructed as layered materials. Most suitable as substrates are Miglasil available from von Roll AG, Switzerland andthe insulating panel obtained. These are laminates composed of silicone-bonded muscovite mica. Such temperature-stable insulating materials may be used in accordance withThe process of the present invention is processed significantly.

When using laminated substrate materials made of inorganic raw materials and binders, it is important to observe suitable machine parameters during machining to avoid damaging the material. Therefore, in the case of cutting the perforations of Pamitherm boards with solid carbide bits, cutting rates in the range of 0.3m/s to 1.5m/s should be maintained. For a bit diameter of 4mm this means that the rotational speed is about 1500/min to 7500/min. The forward speed should be in the range of 50 mm/min to 250 mm/min. Drills and milling drills developed specifically for laminates may also be used.

As an alternative to drilling, it is also possible to perforate the substrate without cutting, for example using a punch.

The substrate is used as a planar material with a thickness between 1mm and 10 mm. Preferably, the thickness is between 1.5mm and 4mm, very particularly preferably between 2mm and 3 mm.The plate is provided in this layer thickness.

The method will now be explained in more detail with reference to the schematic drawings. To this end, the figure shows:

FIG. 1: providing a substrate;

fig. 2a to 2 f: providing an active material and compressing the active material to form a green body;

FIG. 3: inserting the green body into a substrate;

FIG. 4: disposing a substrate between two electrodes;

FIG. 5: contacting the end side of the green body with an electrode;

FIG. 6: exposing the green body to electrical power and pressure to sinter the green body to form thermocouple branches;

FIG. 7: leveling the substrate and thermocouple branches by approaching the electrodes;

FIG. 8: semi-finished products;

FIG. 9: a thermoelectric element;

FIG. 10: variants: a stacked form arrangement of a plurality of substrates using separators between two electrodes;

FIG. 11; the stack is sintered.

First, a substrate 1 is provided. The substrate 1 is composed ofA in the finished flat panel. This is a thermally and electrically insulating laminate consisting of silicone-bonded muscovite mica. The surface size and shape depend on the intended use of the TEG at a later time. For example, a rectangular plate having dimensions of 52mm × 52mm may be used.The material thickness of the plate was 2 mm. The dimensional ratios in the drawings are not to scale.

The substrate 1 is provided with a plurality of through holes 2, which through holes 2 extend through the substrate perpendicular to the plane of the substrate 1. Since the Pamitherm boards are blank, through holes 2 must be drilled through them. This is formed using a cemented carbide drill bit. Correspondingly, the through-hole has a circular cross-section with a diameter of 4.07 mm. Nevertheless, other cross-sectional shapes, such as hexagonal, may be provided for the vias in order to increase the packing density. For a platform width of 2mm, the median packing density of circular holes with a diameter of 4.07mm is 2 to 3 through holes per square centimeter of substrate surface. In the drawings, eight through holes 2 are shown for simplicity.

Fig. 2a to 2f show step by step the cycle sequence for producing the green body 3 on the tablet press 4. The tablet press is simplified to an eccentric type.

To produce the green body 3, first a powdery thermoelectrically active material 5 is provided. This is a bismuth telluride powder having a particle diameter d₅₀Is about 8 μm. The powder is arranged in a charging hopper 6 of the tablet press 4; refer to fig. 2 a.

The tablet press 4 meters a preset amount of bismuth telluride powder 5 of about 200 mg into a die 7; refer to fig. 2 b.

The die 7 is part of the tablet press 4 and is commonly referred to as a "die plate" by the manufacturer of the tablet press. This expression is not intended to be used here because some thermoelectric generator manufacturers refer to the portion of the TEG designated herein as the substrate as the template. In the terminology used herein, the expression "die" is always part of the tablet press and the "base plate" is always part of the TEG.

The die 7 is closed at the bottom by a bottom punch 9. The die 7 is cylindrical with a diameter corresponding substantially to the diameter of the through-hole 2. The length of the green body varies between 2.2mm and 2.6 mm. This therefore corresponds to 110% to 135% of the substrate thickness. Thus, the green body has an axially oversized dimension compared to the substrate. Alternatively, the diameter of the die 7 may be slightly larger than the diameter of the through-hole 2 in order to produce a green body with a radial oversize.

After the mold is filled with the active material, the charging hopper 6 is placed on one side; refer to fig. 2 c.

A linearly conducting top punch 10 compacts the powder 5 within the die 7 to form a cylindrical green body 3 (fig. 2 d). The impulse is about 8.5 kN. This produced a pressure of 676MPa in the mold.

The top imprint 10 is then inverted. The bottom imprint 9 follows it and in doing so forces the green body 3 away from the mold 7 (fig. 2 e).

Finally, the hopper 6 is returned to its previous position on the die and in so doing ejects the green body 3 from the tablet press 4. The ejected green bodies 3 are randomly collected in a collection container 11 (fig. 2 f). The bottom punch 9 is moved downwards and the machine is thus returned to the starting position shown in fig. 2 a. The charging hopper 6 is filled again with the active material 5.

Steps 2a to 2f are repeated on the tablet press 4 at high speed, so that a plurality of green bodies can be continuously produced. Since the same die 7 is used each time and the powder can be metered accurately, the green body has a constant quality in terms of dimensional stability, density and surface quality. Any defective pressing is eliminated.

To increase the production rate, a rotary press may be used instead of the schematically shown eccentric press. The rotary press has a plurality of top punches, bottom punches and dies arranged in a circle around a rotating flow passage. The top and bottom punches are conducted along fixed links to produce axial lifting movement of the punches relative to the die. Pressure is applied to the punch by a pressure roller. Such rotary presses are used in the industrial production of pharmaceutical tablets and are optimized for high throughput speeds.

Since the two branches of the thermocouple should have seebeck coefficients which are as different as possible, two different types of thermoelectrically active material, namely an n-doped bismuth telluride and a p-doped thermoelectrically active material, are pressed in order to generate a high thermoelectrically voltage. Thus, two different types of green bodies are produced one after the other on the same press, once made of n-doped active material and once made of p-doped active material. In order to find no residue of n-doped active material in the p-doped green body, the machine must occasionally be thoroughly cleaned. To avoid this, the p-doped and n-doped active materials can also be processed on different machines.

Fig. 3 shows two collecting containers 11p and 11n, which are filled with a plurality of p-doped green bodies 3p and n-doped green bodies 3n, respectively. The green bodies 3n, 3p are located as a single type in the respective collection containers 11n, 11p, but are not geometrically classified.

The green bodies 3n, 3p are removed from the respective collection containers 11p, 11n and inserted into the through-holes 2 of the base plate 1, respectively, such that the green bodies extend axially through the through-holes so as to be perpendicular to the plane of the base plate. A pharmaceutical robotic placement machine (not shown) is used for this purpose. Alternatively, the green bodies 3n, 3p may be inserted into the substrate 1 by hand.

The p-and n-doped green bodies 3p, 3n are arranged alternately next to one another. Each adjacent p-doped and n-doped green body then forms a thermocouple. It is not important whether all green bodies of one type are inserted first and then of the other type, either alternately or continuously, or whether any other desired pattern is present.

Then, the substrate 1 with the inserted green body 3 is arranged between two substantially flat electrodes 12a, 12 b; refer to fig. 4. It is important that the substrate 1, the first electrode 12a and the second electrode 12b are each oriented parallel to each other.

In contrast, it is not important whether the end faces of the green bodies 3 used lie in a common plane. As can be seen from fig. 4, the green body is introduced into the base plate 1 with a non-uniform axial offset, so that the end faces do not lie in one plane. The reason for this is that automatic chip mounters cannot operate so precisely, which in turn operates quickly.

According to the invention, the end sides of the green body are placed in one plane using electrodes. For this purpose, the two electrodes 12a, 12b are each moved towards each other along a path Δ x, while maintaining their parallelism. In this case, the planar electrodes contact the end sides of the green body and align them in the plane of the contact end sides of the respective electrodes 12a, 12 b. As can be seen in fig. 5, the green bodies 3n, 3p now both lie in one plane.

Since the end faces of all green bodies 3n, 3p are situated directly on the electrodes 12a, 12b, it is possible to close not only the circuit by the green bodies between the electrodes, but also the mechanical pressure.

This is the case in fig. 6. The two electrodes are each exposed to a force F acting in the direction of the substrate 1, which force F is axially directed to the green body. The dimensions of which take into account the contact end face of the green body, so that a mechanical pressure of 30MPa acts on the active material. Furthermore, an alternating voltage AC of 50Hz is applied between the electrodes, the magnitude of which is such that the current density is 50kA/m, taking into account the contact end face of the green compact²The alternating current of (2) flows through the green body.

The ohmic resistance results in the green body being heated to a temperature between 300 ℃ and 400 ℃ in the region where the optimum sintering temperature is also located. Under the action of mechanical forces, the individual particles of the green body sinter together, thus producing a solid sintered body from the pressed powder material. The sintered green body is the thermocouple branch 13.

During sintering, the green body is compacted, so that the thermocouple branches have a correspondingly smaller volume (sintering shrinkage). In order to maintain electrical and mechanical contact between the green body 3 and the electrode 12, the two electrodes 12a, 12b are each repositioned along the direction of the substrate 1 by the sintering shrinkage, although the sintering shrinkage proceeds, each repositioning along the channel Δ y while maintaining their parallelism (fig. 7).

The length of the green body 3 is chosen in view of sintering shrinkage so that the sintered thermocouple legs 13n, 13p are flush with the substrate 1; refer to fig. 8. In the material combinationIn the case of bismuth telluride, of green bodiesThe oversize should be about 15% of the thickness of the substrate so that the green body shrinks to the thickness of the substrate during sintering. In this way, no subsequent separate levelling of the workpiece is required. Furthermore, the restriction path Δ y prevents the substrate 1 from being wedged and compressed between the two electrodes 12a, 12 b. Otherwise, any elasticity of the substrate material may cause the substrate to return through the end face of the thermocouple branch, which makes subsequent contact of the thermocouple branch more difficult. In contrast, the latter semi-finished version 14 of the thermoelectric element is obtained directly from a sintering process which is planar on both sides and in which the end faces of the thermocouple legs 13 are flush with the surface of the substrate on both sides. Semi-finished version 14 is shown in fig. 8.

In order to manufacture the thermoelectric element 15 from the semi-finished product, the thermocouple branches must be combined in pairs to form the thermocouples 16. In each case one thermocouple branch 13p made of p-doped active material and one thermocouple branch 13n made of n-doped active material form a thermocouple 16. Furthermore, the thermocouples 16 must be connected in series. This is achieved using contact bridges 17, which contact bridges 17 are good electrical and thermal conductors, which are welded on alternating sides to the end faces of the thermocouples 13n, 13p (fig. 9).

Thus, the resulting thermoelectric element 15 has functioned: when the substrate 1 is placed between the heat source and the heat sink, heat flows from one side of the substrate to the other side through the contact bridges and through the thermocouple branches 13, a thermoelectric voltage can be tapped on the free contact bridges 17+, 17-on one side. The thermoelectric element 15 then operates as a thermoelectric generator. Vice versa, by applying a voltage to the free contact bridges 17+, 17-on one side, a thermal pressure (peltier element) can be induced from the cold side to the hot side of the substrate. In order not to form an electrical short circuit, the total thermoelectric element 15 should also be provided with a thermally conductive and electrically insulating sheath, which also protects it from mechanical damage (this is not shown).

In fig. 10, a particularly rational method variant is shown, in which a plurality of substrates 1i, 1ii and 1iii with inserted green bodies are sintered simultaneously. For this purpose, three substrates 1i, 1ii and 1iii are arranged between the two electrodes 12a and 12 b. A flat separating plate 18a, 18b is placed in each case between the inner substrate 1ii and the two outer substrates 1i and 1 iii. Forming a stack 19. The two flat separators 18a, 18b are made of graphite, as are the electrodes 12a, 12 b. All of the substrates 1i, 1ii and 1iii, the electrodes 12a, 12b and the two separators 18a, 18b are oriented planes parallel to each other and form a stack 19.

The stack 19 is sintered in one operation, resulting in three semifinished products at the same time (fig. 11). The stack 19 is sintered as described in figure 6.

Multiple substrates with inserted green bodies may also be subjected to the sintering process simultaneously without stacking them. For this purpose, the substrate is placed between electrodes in one plane adjacent to each other. This saves separator plates, but requires electrodes with a larger surface area than a single substrate. This variant of the invention is not shown.

A combination of both arrangements is likewise possible.

Regardless of the arrangement and number of substrates between the electrodes, and the sintering process may be controlled such that the green body is first exposed to a force F1 that is less than the force R_coldWhich corresponds to a temperature in the cold state (room temperature T)₀) Breaking load of the lower electrode 12a, b or the separator 18a, b. An alternating voltage is then applied, as a result of which the green body is heated to a temperature T_limitAt a temperature lower than the sintering temperature T_sinterHowever, at this sintering temperature the breaking load of the graphite element increases to R_hot. The force is increased to F only after a higher mechanical strength is obtained₂Thereby creating the required mechanical sintering pressure. After the sintering pressure is reached, the temperature is raised to the desired sintering temperature T_sinterAnd the sintering operation is carried out under these conditions. Thus, the following two relationships apply:

F₁<R_cold<F₂<R_hot (1)

T₀<T_limit<T_sinter (2)

this method may be employed when sintering the respective substrates (fig. 6), when a plurality of substrates are adjacent to each other without a separator, or when substrates with separators are stacked (fig. 11). The lowest breaking load of the graphite elements used at all times is important.

REFERENCE SIGNS LIST

1 substrate

1i first substrate (exterior)

1ii second base plate (inner)

1iii third substrate (exterior)

2 through hole

3 green body

3n n-doped green body

3p p-doped green body

4 tablet press

5 powdered thermoelectrically active material (Bi)₂Te₃)

6 charging hopper

7 mould

8 not allocated

9 bottom punch

10 Top punch

11 collecting container (in general)

11n collecting container for n-doped green bodies

11p collecting vessel for p-doped green bodies

12a first electrode

12b second electrode

Path of electrode when Deltax contacts

Paths of electrodes during Delta y sintering

Force F

AC alternating current

13 thermocouple branch

13n thermocouple branches made of n-doped active material

13p thermocouple branch made of p-doped active material

14 semi-finished product

15 thermoelectric element

16 thermocouple

17 contact bridge

17⁺Free-sided contact bridge

17^-Free-sided contact bridge

18a first divider plate

18b second partition plate

19 Stack

Claims (16)

1. Method for producing a thermoelectric element or at least a semifinished product thereof, having the following steps:

a) providing a substantially planar substrate made of an electrically and thermally insulating substrate material, through-holes extending through the substrate in a direction substantially perpendicular to the plane of the substrate;

b) providing a powdered thermoelectrically active material;

c) compressing the active material to form a green body, wherein the compressing is performed in a mold different from the substrate;

d) inserting the green bodies into the through-holes of the substrate such that one green body extends through the substrate along its axis within each through-hole;

e) disposing the substrate with the green body inserted therein between two substantially flat electrodes such that the two electrodes and the substrate are oriented substantially parallel to each other;

f) contacting the end of the green body with an electrode such that a connection is provided between the two electrodes through the green body, the connection not only transferring electrical current but also transferring mechanical pressure;

g) exposing the green body to an electrical current flowing between the electrodes such that heat is induced within the thermoelectrically active material;

h) exposing the green body to a first pressure acting between the electrodes such that the thermoelectrically active material is under pressure;

i) sintering the green body under a second pressure and heat to form thermocouple branches;

k) leveling the base plate and the thermocouple branch by bringing the base plate and the thermocouple branch accommodated therein close to the electrode while maintaining their parallelism such that the thermocouple branch is done flush with the base plate, wherein any axial offset of the green body in the base plate and any sintering shrinkage are compensated, wherein the green body is provided in an oversize to compensate for the sintering shrinkage.

2. Method according to claim 1, characterized in that a plurality of substrates with inserted green bodies are combined to form a stack, wherein the substrates extend parallel to one another within the stack and in each case a substantially flat separator plate is placed between two adjacent substrates within the stack, which separator plate extends parallel to the substrates and produces an electrically conductive and force-transmitting connection between the green bodies of the adjacent substrates, and in that the entire stack is arranged between two electrodes.

3. The method according to claim 1 or 2, characterized in that a plurality of substrates with inserted green bodies are individually arranged or stacked in a plane between two electrodes.

4. The method according to claim 2, wherein the electrode and/or the separator consists of graphite, characterized in that the green body for contacting is exposed to a first pressure, characterized in that the green body under the effect of the first pressure is then exposed to an electric current until the electrode and/or the separator consisting of graphite has reached a temperature at which the electrode and/or the separator consisting of graphite has an increased load-bearing capacity above the first pressure, and in that the green body is then exposed to a second pressure above the first pressure and below the increased load-bearing capacity.

5. The method of claim 4, wherein the active material is compacted to a first compressed density corresponding to between 75% and 85% of a true density of the active material when the powdered active material is compacted to form the green body.

6. The method according to any one of claims 1, 2, 4-5, wherein the green body is sintered at a temperature corresponding to between 50% and 70% of the melting temperature of the active material to form the thermocouple branch.

7. A method according to claim 5, characterized in that the green body is compacted to a second compressed density, which corresponds to between 90% and 97% of the true density of the active material, during the exposure of the green body to the pressure acting between the electrodes.

8. The method of any one of claims 1, 2, 4-5, 7, wherein said green body has a cylindrical shape.

9. The method of claim 8, wherein the green bodies each have a chamfer at an end face.

10. The method according to claim 8, wherein the green body has an average roughness value Ra on its side surface of between 12 μm and 24 μm as specified in DIN 4766T 2.

11. A method according to claim 1 or any one of claims 2, 4, 5, 7, 9, 10, wherein the green body can be clamped into the through hole.

12. The method of claim 11, wherein the green body is a conical green body, the through-hole is a tapered through-hole, and the green body has a radial oversize compared to the through-hole.

13. A method according to claim 1 or any of claims 2, 4, 5, 7, 9, 10, 12, characterized in that the powdered thermoelectrically active material is dry prepared in a tablet press, characterized in that dies, which are pressed to form a green body, of the active material are arranged in the tablet press, and in that the green body is randomly ejected from the tablet press.

14. The method of claim 13, wherein the green bodies are collected, isolated and inserted into the through-holes of the substrate in an ordered manner, either manually or by a conveying device.

15. A method according to claim 1 or any one of claims 2, 4, 5, 7, 9, 10, 12, 14, characterized in that the substrate material is a composite material made of inorganic raw materials and a binder.

16. The method according to claim 15, characterized in that the composite material is made as a laminate, characterized in that the inorganic raw material is selected from mica, perlite, phlogopite, muscovite and the binder is a siloxane or a silicone or an epoxy resin.