Method for producing thermoelectric components by powder metallurgy

Abstract

The invention relates to a method for producing a thermoelectric component or at least one semi-finished product of same, in which a multiplicity of thermolegs made of a thermoelectrically active material are introduced into an essentially planar substrate made of an electrically and thermally insulating substrate material such that the thermolegs extend through the substrate essentially perpendicular to the substrate plane, and in which the active material is provided in pulverulent form, is pressed to give green bodies and is then sintered within the substrate to give thermolegs. It is based on the object of refining the method of the generic type mentioned in the introduction so as to increase the freedom of choice of the thermally and electrically insulating substrate material. The object is achieved in that the pulverulent active material is pressed, in a mould arranged outside the substrate, to give green bodies, the green bodies are pushed out of the mould and into holes provided in the substrate, where they are sintered to give thermolegs.

Claims

1. A method for producing a thermoelectric component or at least one semi-finished product of same, in which a multiplicity of thermolegs made of a thermoelectrically active active material are introduced into an essentially planar substrate made of an electrically and thermally insulating substrate material such that the thermolegs extend through the substrate essentially perpendicular to the substrate plane, and in which the active material is provided in pulverulent form, is pressed to give green bodies and is then sintered within the substrate to give thermolegs, characterized in that the pulverulent active material is pressed, in a mould arranged outside the substrate, to give green bodies, the green bodies are pushed out of the mould and into holes provided in the substrate, where they are sintered to give thermolegs.

2. The method according to claim 1, characterized in that pressing the pulverulent active material to give green bodies and pushing the green bodies into the holes in the substrate is performed with the aid of the same tools.

3. The method according to claim 2, characterized in that the tools are at least one pair of punches which are inserted from both sides into the mould and of which one engages through the hole provided in the substrate for the respective green body.

4. The method according to claim 3, characterized in that a plurality of moulds are combined to give a planar template, and in that, at least during pressing of the active material and insertion of the green bodies, the template lies areally on the substrate such that holes and moulds are in line with each other.

5. The method according to claim 1, wherein the holes and the mould are of circular cylindrical shape and have essentially the same diameter.

6. The method according to claim 1, wherein the holes are introduced into the substrate in a chip-removing manner, in particular by drilling and without the aid of cooling lubricants.

7. The method according to claim 6, wherein the holes are blown clean after chip-removing machining, in particular using an inert gas.

8. The method according to claim 6, wherein the substrate is held on both sides, in particular a really, by planar clamping means during introduction of the holes.

9. The method according to claim 4, wherein one of the two clamping means is used as a template after introduction of the holes.

10. The method according to any claim 1, wherein the substrate material is a composite material made of inorganic raw materials and binders.

11. The method according to claim 10, wherein the composite material is constructed as a laminate, in that the inorganic raw materials are selected from the group comprising mica, perlite, phlogopite, muscovite and in that the binders are silicone or silicone resin.

12. The method according to claim 10, wherein the thickness of the substrate is between 1 and 10 mm, preferably that it is between 1.5 and 4 mm and very preferably that the thickness is between 2 and 3 mm.

13. The method according to claim 1, wherein the active material is an alloy which is chosen from the class of bismuth tellurides, zinc antimonides, silicides, semi-Heusler materials, and in that the particle size distributiondetermined by means of laser diffraction methodsof the active material has an average particle size d.sub.50 of between 1 and 50 m, and in that, to set this particle size distribution, the active material is ground at a maximum temperature of between 30 C. and 50 C.

14. The method according to claim 1, wherein the pulverulent active material is acted upon with vibration within the mould prior to pressing.

15. The method according to claims 1, wherein the substrate, with introduced green bodies, is placed for sintering into an autoclave in which the sintering process takes place at elevated pressure and elevated temperature in an inert atmosphere, wherein in particular the gas pressure within the autoclave is lower than the pressure exerted on the pulverulent active material during pressing of the green bodies.

Description

DESCRIPTION OF FIGURES

(1) The present invention will now be explained in more detail on the basis of exemplary embodiments. The figures show, in schematic form:

(2) FIG. 1: drilling through the substrate;

(3) FIG. 2: preparing the pulverulent active material within the mould;

(4) FIG. 3: pressing the powder to give green bodies;

(5) FIG. 4: pushing the green bodies into the holes in the substrate;

(6) FIG. 5: substrates with green bodies in the autoclave.

(7) An essentially planar substrate 1 in the form of a platen made of an electrically and thermally insulating substrate material is placed between two clamping means 2, 3 and is clamped areally therebetween. The clamping means 2, 3 are made of steel and are provided with a multiplicity of holes 4, wherein the holes in the upper clamping means 2 are aligned with those in the lower clamping means 3. A drill 5 enters through the holes 4 in the upper clamping means 2 and drills through-holes 6 in the substrate 1. During drilling, a clamping force is exerted by the clamping means 2, 3 on both sides of the substrate 1 in order to prevent breakup of the holes 6.

(8) The pierced substrate 1 is then clamped between two templates 7, 8, see FIG. 2. The two templates 7, 8 are also provided with holes 4 which are aligned with the holes 6 in the substrate 1. The templates 7, 8 can be, but are not necessarily, identical to the clamping means 2, 3. The planar templates 7, 8 are made of steel, lie areally on the substrate 1 and exert a clamping force on the substrate 1. The holes 4 in the lower template 8 serve as a mould for receiving pulverulent thermoelectric active material 9. To that end, the holes must be closed on their lower side. To that end, in each case a lower punch 10 is moved into the holes 4 of the lower template 8 such that there results a cavity which is open only in the direction of the substrate 1, which cavity is filled from above with pulverulent active material 9. Two sorts of active material are introduced in alternation, corresponding to the subsequent n-legs and p-legs. In the drawing, no distinction is made here between the two sorts of active material.

(9) In addition, a foil 11 of a barrier material such as nickel can be placed over the upper template 7. A multiplicity of upper punches 12 are combined to give a pressing tool.

(10) As shown in FIG. 3, the pressing tool with the upper punches 12 is moved downwards such that the upper punches 12 reach through the upper template 7 and the holes 6 in the substrate 1. When plunging into the upper template 7, the upper punches 12 stamp out, at the cutting edge of the upper template 7 serving as a die, a round of barrier material from the foil 11 and press it against the active material 9. In similar fashion, it is possible for a round of barrier material to be pressed against the pulverulent material from below, in order to also cover the underside with barrier material. This is however not shown in the drawings.

(11) Where necessary, the pulverulent active material 9 can be acted upon with vibration within the lower template 8. This is effected by vibrating the lower template 8 or the lower punch 10 or by means of a vibrating needle (not shown) plunged into the powder fill from above. The introduced vibration causes the active material to settle.

(12) The pulverulent active material 9 is now pressed within the lower template 8 to give green bodies 13. The transverse forces resulting therefrom are absorbed by the lower template 8. Pressing is effected by loading the punch pairs 10, 12 via the respective pressing tools.

(13) The pressing tools 10, 12 then move upwards such that the lower punches 10 push the green bodies 13 into the holes 6 in the substrate 1 (FIG. 4). In that context, the upper punch 12 withdraws at the same speed as the lower punch 10 advances in order that the green bodies 13 move into the substrate 1 without being destroyed. In the process, the templates 7, 8 exert a pressure on the substrate 1.

(14) Once the green bodies 13 have arrived at their intended place, the punches 12 and 10 withdraw from the templates 7 and 8, the templates 7, 8 move apart and a semi-finished version 14, comprising the substrate 1 with the inserted green bodies 13 and, where relevant, the applied diffusion barriers 15, demoulds.

(15) The production process has up to this point been carried out as far as possible as a cyclical, continuous process. In order to sinter the green bodies 13, these are gathered on a pallet 16 and are placed into an autoclave 17; see FIG. 5. There, the semi-finished versions 14 are subjected to a thermal sintering process at elevated atmospheric pressure and elevated temperature. In the process, the green bodies 13 sinter to form finished thermolegs 18.

(16) After the end of the sintering process, the individual thermolegs 18 must still be connected to form thermocouples. This is carried out for example using a soldering process which is known per se and which will not be discussed further here. Connecting the thermolegs 18 to form thermocouples and possibly connecting the thermocouples to one another produces a functional thermoelectric component.

EXAMPLE

(17) First, suitable semiconductor powders (n-and p-doped) must be produced. Table 1 shows the composition of the ingots used as starting material.

(18) TABLE-US-00001 TABLE 1 Composition of the starting materials p-type n-type Elements wt % wt % Sum Be . . . Fe 0.16 34 + Se 2.64 51 + Sb 26.88 52 + Te 56.68 43.12 Sum La . . . Lu 0.12 0.27 83 + Bi 16.38 54.23

(19) The compositions have been determined by means of semi-XRF analysis (maximum relative deviation +/5%)

(20) In that context, the grinding procedure is as follows for all of the above-mentioned semiconductor materials: Inerting: All work performed under nitrogen (5.0) in a glovebox Mill: Fritsch Pulverisette 6 classic line Grinding container: Zirconium oxide, gas-tight closure Grinding media: 20 balls (diameter 2 cm) made of zirconium oxide Speed: 650 rpm Powder filling: 225 cm.sup.3 (coarsely broken, d50<5 mm) Sequence: 10 grinding periods of 10 min each, 60 min pause inbetween for cooling (in order to limit the thermal load on the grinding stock) Analysis: Particle size distribution using HORIBA 920-L, powder using ultrasound dispersed in demineralized water, maximum pump circulation rate Target value: d50<8 m (otherwise further grinding periods).

(21) A 51 mm square is sawn from 2 mm-thick Pamitherm.

(22) This substrate is placed between two clamping means (steel block 515115 mm) and is secured therebetween with a clamping pressure of 20 kPa. The clamping means have a multiplicity of holes of diameter 4.1 mm, with a minimum lateral distance of 1.9 mm between any two holes. The holes in both clamping means are in each case identically placed and are thus aligned with one another.

(23) Now, a drill is passed through each of the holes of the first clamping means, creating a through-hole in the substrate, in line with the holes in the two clamping means. The drill diameter is 4 mm, rate of advance 200 mm/min, speed 1600 rpm, drill type: solid carbide drill product Miller Mega-Drill-Inox, shank form HA, MxF-coated, type M1703-0400AE.

(24) The substrate thus obtained is secured between two clamping means similar to those first mentioned. The only difference with respect to the first clamping means is that here the through-holes have a nominal diameter of 4 mm, identical to the holes in the substrate.

(25) This sandwich is secured in a hydraulic press acting on both sides. This press has two hydraulic punches with a nominal diameter of 4 mm and a length of 30 mm which lie on the same vertical central axis. Both punches can be moved independently of one another on this central axis, the punch faces being opposite one another. In that context, one punch acts from below while the other acts from above. The two punches, and the holes in the two clamping means, are produced in accordance with DIN 7157, with a H7/g6 fit with respect to one another (or possibly alternatively: H8/h9).

(26) The lower punch is moved upwards through a hole in the lower clamping means until its upper side is still at a distance of 6 mm from the substrate.

(27) This hole in the upper clamping means is then filled from above with a quantity of 0.186 g of the ground bismuth telluride powder (n-doped). A vibrating needle (diameter 0.5 mm, length 100 mm) is now inserted from above into the powder fill and is vibrated for 1 second (frequency 100 Hz, amplitude at the free needle tip 0.5 mm). This settles and homogenizes the powder fill.

(28) The upper punch is now moved from above into this hole (rate of advance 1 mm/s) until between the two punches a green body of height 2 mm (tolerance +/0.1 mm) is generated and a pressure of approximately 830 MPa is reached. The punches remain in this position for 5 seconds.

(29) Then, both punches move synchronously with a rate of advance of 1 mm/s in the opposite direction such that the green body is pushed upwards into the substrate, the pressing force acting on the green body being essentially maintained but in no circumstances increased. The green body is now embedded in the substrate and its upper and lower sides are essentially flush with the two surfaces of the substrate.

(30) The two punches now withdraw completely from the substrate and the clamping means.

(31) An analogous procedure is now used to produce all of the other n-legs in the substrate (half of all the holes in the substrate). Then, the same procedure is repeated for the p-legs such that, in the end, all of the holes in the substrate are filled with green bodies made of n- or p-doped bismuth telluride. The only differences in the case of the p-legs are the mass of powder used for each green body (0.162 g) and the maximum pressing force (approximately 800 MPa).

(32) The filled substrate is now placed into a glovebox, flushed with nitrogen 5.0 and a residual oxygen content of <100 ppm. In this glovebox there is an autoclave which is preheated to a surface temperature of the internal wall of 290 C. The filled substrate is now placed into this autoclave. The interior of the autoclave is also flushed with nitrogen 5.0 (at least 20 complete gas exchanges per hour). The pressure in the autoclave is now raised to 90 bar=9 MPa within 2 minutes, after which the gas temperature inside the autoclave rises to 285-290 C. within a further 3 minutes. This pressure and this temperature are maintained for 5 minutes. Then, the pressure is reduced to normal pressure within 1 minute, the now-sintered semi-finished product is removed and is left in the glovebox to cool to room temperature.

(33) Subsequently, the operational steps required for completing a TE-component can be carried out on the sintered semi-finished product: Cleaning the end faces of the TE-legs (polishing, plasma treatment or the like) Applying diffusion barriers (e.g. nickel by means of atmospheric pressure plasma coating) Applying contact-promoting layers (e.g. tin by means of atmospheric pressure plasma coating) Soldering with contact bridges Applying casing layers

LIST OF REFERENCE NUMERALS

(34) 1 substrate

(35) 2 upper clamping means

(36) 3 lower clamping means

(37) 4 holes

(38) 5 drill

(39) 6 hole

(40) 7 upper template

(41) 8 lower template

(42) 9 pulverulent active material

(43) 10 lower punch

(44) 11 foil

(45) 12 upper punch

(46) 13 green body

(47) 14 semi-finished product

(48) 15 diffusion barrier

(49) 16 pallet

(50) 17 autoclave

(51) 18 thermoleg