EP2459762A2

EP2459762A2 - Method for sintering thermoelectric materials

Info

Publication number: EP2459762A2
Application number: EP10742116A
Authority: EP
Inventors: Madalina Andreea Stefan; Frank Haass
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2009-07-27
Filing date: 2010-07-23
Publication date: 2012-06-06
Also published as: CA2768979A1; WO2011012548A2; US20110020164A1; US20140353880A1; WO2011012548A3

Abstract

The method for producing, processing, sintering, pressing or extruding thermoelectric materials under heat treatment using an inert gas, or at a reduced pressure at temperatures ranging from 100 to 900 °C is characterised in that the producing, processing, sintering, pressing or extruding is carried out in the presence of oxygen scavengers which form thermodynamically stable oxides under the producing, processing, sintering, pressing or extrusion conditions in the presence of free oxygen and thus keep free oxygen away from the thermoelectric material.

Description

Method for Sintering Thermoelectric Materials Description The invention relates to methods of sintering thermoelectric materials resulting in thermoelectric materials with improved properties.

Thermoelectric generators and Peltier devices as such have long been known, p-type and n-type doped semiconductors, heated on one side and cooled on the other, carry electrical charges through an external circuit, electrical work being done to a load in the circuit can be performed. The achieved conversion efficiency of heat into electrical energy is thermodynamically limited by the Carnot efficiency. Thus, at a temperature of 1000 K on the hot and 400 K on the "cold" side, an efficiency of (1000 - 400): 1000 = 60% is possible. To date, however, only efficiencies up to 6% are achieved.

On the other hand, if a direct current is applied to such an arrangement, heat is transferred from one side to the other side. Such a Peltier arrangement operates as a heat pump and is therefore suitable for cooling equipment parts, vehicles or buildings. The heating via the Peltier principle is cheaper than a conventional heating, because more and more heat is transported than the supplied energy equivalent corresponds. A good overview of effects and materials are z. See, for example, S. Nolan et al., Recent Developments in Book Thermoelectric Materials, MRS Bulletin, Vol. 31, 2006, 199-206.

Currently, thermoelectric generators are used in space probes for generating direct currents, for cathodic corrosion protection of pipelines, for powering light and radio buoys, for operating radios and televisions. The advantage of the thermoelectric generators lies in their extreme reliability. So they work regardless of atmospheric conditions such as humidity; there is no fault-susceptible material transport, but only a load transport; The fuel is burned continuously - even without catalytic free flame -, whereby only small amounts of CO, NO _x and unburned fuel are released; It can be used any fuel from hydrogen to natural gas, gasoline, kerosene, diesel fuel to biologically produced fuels such as rapeseed oil methyl ester. As a result, thermoelectric energy conversion adapts extremely flexibly to future needs, such as hydrogen economy or energy generation from regenerative energies. For an advantageous mode of operation of a thermoelectric module, high requirements are placed not only on the module structure itself but, above all, on the thermoelectric material. This must be as homogeneous as possible, free of cracks and holes, of high specific gravity and of high mechanical stability. Therefore, after a synthesis of the thermoelectric material, a metallurgical processing step usually follows to meet these requirements.

In the procedure known to those skilled in the art, therefore, the material is first comminuted (eg by grinding) and then compacted again. The compaction can be done by uniaxial or isostatic cold or hot pressing, extrusion, spark plasma sintering, etc. The material is homogenized in the course of comminution on the one hand, and in the course of compaction, the other properties required above are achieved. It is essential, especially during cold pressing, to add another sintering step. During sintering, further densification and intimate bonding of the crystal structure occurs, so that the sintered bodies end up having a high density and high electrical conductivity, as desired for thermoelectric materials.

Not all materials can be processed easily. Especially with oxygen-sensitive materials, special care must be taken during manufacture. So it can easily happen that oxygen is already adsorbed on the surface during powder production, which reacts with the material during the later heat treatment (hot pressing, sintering). The following effects can be observed:

(1) On the one hand, the sintering behavior is changed by the formation of superficial oxide layers, since the oxide layers, for example, behave like an inert protective layer, which is difficult to sinter. This leads to material bodies with lower density or powders which are no longer sinterable at the actually desired temperatures.

(2) Furthermore, these oxide layers can act as an electrical insulator and thus as a barrier. This results in a massive reduction of the electrical conductivity the original bulk material, and the sintered body loses its good thermoelectric properties.

(3) Third, oxygen can cause a chemical change in the material, not only superficially, but, depending on the chemical nature, also in the bucket. This is the case, for example, when the thermoelectric material contains dopants which react easily and quickly with oxygen and thus are removed as an oxide from the thermoelectric material and are no longer available as a dopant.

For this reason, the thermal treatment of thermoelectric materials from powders or high surface area grains poses a challenge when an oxygen effect, e.g. B. from the ambient air, can not be excluded meticulously.

The object of the present invention is to provide a method for sintering thermoelectric materials by heat treatment under inert gas or at reduced pressure, which avoids the disadvantages of the existing methods and, in particular, largely prevents oxygen contact with the thermoelectric material.

The object is achieved by a method for producing, processing, sintering, pressing or extrusion of thermoelectric materials under heat treatment under inert gas or at reduced pressure at temperatures in the range of 100 to 900 ⁰ C, in which the manufacturing, processing, sintering, pressing or extruding in the presence of oxygen scavengers, which form thermodynamically stable oxides in the production, processing, sintering, pressing or extrusion conditions in the presence of free oxygen and thus keep free oxygen from the thermoelectric material.

It has been found according to the invention that, by adding an oxygen scavenger to the thermoelectric material, oxygen which is still present during sintering is adequately trapped and can no longer develop a damaging effect. During sintering, the oxygen scavenger catches residual oxygen residues quickly and largely completely so that they can no longer react with the thermoelectric material.

The oxygen scavenger forms thermodynamically stable oxides in the sintering conditions in the presence of free oxygen and thus keeps free oxygen away from the thermoelectric material. A thermodynamically stable oxide forms in the With the unoxidized thermoelectric material, the lowest possible oxygen partial pressure prevails. On the other hand, this means that the oxides formed do not decompose to any appreciable extent again at sintering temperatures. The oxygen scavenger reacts by oxidation reaction with the oxygen still present in the gas space during sintering and binds it, so that the oxygen can not react with the thermoelectric material. The oxygen scavenger is more easily oxidized than the thermoelectric material or at lower temperatures.

As oxygen scavengers, inorganic materials, preferably metals, metal alloys and semimetals and their alloys are used. Typical examples are titanium, zirconium, hafnium, silicon, aluminum, vanadium, scandium, yttrium, rare earth metals (eg lanthanum or cerium), lithium, sodium, potassium, magnesium, calcium, strontium, barium, manganese, iron, cobalt, nickel, Copper, zinc, cadmium, but also non-metals such as phosphorus, graphite and mixtures thereof.

Gaseous oxygen scavengers may be selected from H ₂ , CO, CO / CO ₂ mixtures, H ₂ / H ₂ O mixtures or inert gas / H ₂ mixtures.

Oxygen scavengers may also be selected from hydrides, carbonyls, lower valent oxides, sulfides, phosphides of metals, preferably metals above, sulfur or phosphorus containing compounds in general, sulfur, phosphorus or mixtures thereof. Low-valent oxides are those oxides which can be oxidized to higher valent oxides in the presence of free oxygen. As the oxygen scavenger, a material containing no chemical elements of the thermoelectric material can be used. It can also be a dopant material.

The amount of oxygen scavenger to be used can be adjusted according to the practical requirements. These depend on the remaining oxygen content in the inert gas during sintering and on the oxygen affinity of the constituents of the thermoelectric materials. As a rule, based on the amount of the thermoelectric material, below 25 wt .-%, preferably 0.05 to 15 wt .-%, in particular 0.05 to 1 wt .-% of oxygen scavenger used.

The surface of the solid oxygen scavengers may be pretreated to increase their effectiveness, for example by roughening, mechanical, chemical or electrochemical removal of an already existing oxide layer, or by mechanical, chemical or electrochemical activation of the surface. The solid oxygen scavenger can be used in any form, for. As a powder, wire, sheet, strip, chunks, spheres, moldings, sponge or mesh or supported on an inert material. The thermoelectric material may be used in any suitable form for sintering. Often, a green body is sintered, but it is also possible to sinter a powder or granules of the thermoelectric material under pressure and molding. According to one embodiment of the invention, a green body made of a thermoelectric material which has been subjected to shaping is sintered in direct contact with the oxygen scavenger.

According to a further embodiment of the invention, the green body of a thermoelectric material, which has been subjected to a shaping and the oxygen scavenger during sintering spatially separated from each other, but connected via a common gas space.

According to a further embodiment of the invention, the sintering takes place under pressure and shaping of a powder of the thermoelectric material. The sintering under pressure as hot pressing, isostatic pressing or hot pressing or spark plasma sintering can take place. In this procedure, the oxygen scavenger can be arranged in the pressing tool in contact with the thermoelectric material or be pressed in the form of a sandwich with the powder of the thermoelectric material.

It is also possible to carry out the sintering directly in an extrusion.

By means of the sintering according to the invention, any shaped bodies of the thermoelectric material can be produced. The sintering is preferably carried out for the direct production of thermoelectric material legs.

When manufacturing or processing z. As powders, granules or melts of the thermoelectric materials or components thereof can be used. They are not in direct contact with the oxygen scavenger, which is e.g. B. may be connected via a common gas space with them.

The thermoelectric material used in the method according to the invention is not subject to any restrictions. The materials may be p-type or n-type and have corresponding dopants. Preferably, the underlying thermal electrical material selected from PbTe, Bi ₂ Te 3, Zintl phases, skutterudites, clathrates and zinc antimonides, Heusler compounds, silicides, oxides and mixtures thereof. Suitable materials are for. As mentioned in the cited font of S. Nolan.

The thermoelectric materials are generally prepared by reactive milling or, preferably, by fusing and reaction of mixtures of the respective constituent elements or their alloys, which steps may be carried out in the presence of oxygen scavengers.

The thermoelectric material (green body, legs, powder, granules) is sintered at a temperature of generally at least 100 ^° C., preferably at least 200 ^° C., lower than the melting point of the resulting semiconductor material in the presence of the oxygen scavengers. Usually, the sintering temperature is 350 to 900 ^° C., preferably 500 to 800 ^° C. Spark plasma sintering (SPS) or microwave sintering may also be carried out.

The sintering is carried out for a period of preferably at least 0.5 hours. Usually, the sintering time is 1 to 24 hours. In one execution of the present invention approximate shape, the sintering is carried out at a temperature which is 100 to 500 ⁰ C lower than the melting temperature of the resulting semiconductor material. The sintering can be carried out under a protective gas atmosphere, for example of argon, hydrogen or inert gas / hydrogen. Thus, the pressed parts are preferably sintered to 90 to 100% of their theoretical bulk density.

Overall, this results in a preferred embodiment, a method which is characterized by the following process steps:

(1) fusing together mixtures of the respective constituent elements or their alloys of the thermoelectric material;

(2) crushing the material obtained in process step (1);

(3) pressing the material obtained in process step (2) into moldings and

(4) sintering of the shaped body obtained in process step (3) with oxygen scavenger.

There is no limit to the pretreatment of the material and powder production. The material for sintering, for example, by grinding a Melting body or directly in powder form by rapid solidification (MeIt spinning) or corresponding synthesis methods (precipitation, spraying, etc.) are produced. The pre-compression of the powder to the green body is carried out according to the techniques known to those skilled in the art. It is preferred not to densify the green body already close to 100% in order to allow a gas exchange with the environment during sintering. The actual compaction takes place only in the sintering step.

There are several options for sintering according to the present invention. The oxygen scavenger can be directly enclosed with the green body in an ampoule. In this case, the oxygen scavenger can either be in direct contact with the green body, or spatially separated. The direct contact can z. B. by wrapping with wire, placing on a mesh, embedding in a powder bed, etc. take place.

The spatial separation can be achieved by a partition wall (eg quartz wool), but also in an ampoule with several compartments (eg in dumbbell shape). Several compartments have the advantage that the oxygen scavenger and the material in a multi-zone oven can also be exposed to different temperature levels, if desired and necessary.

The ampoule can be made of quartz glass, for example, but also directly from the material of the oxygen scavenger. The sintering should then be carried out in the oven under inert conditions to prevent oxidation of the outside or permeation of oxygen through the container wall. However, it is also possible to subsequently coat the ampoule of the catcher material with an inert layer on the outside, or, conversely, to register a layer of the catcher material on the inside of the ampoule. It is also possible to use the oxygen scavenger in open sintering. Open sintering can be done, for example, in a conventional furnace in a graphite, quartz or metal crucible. Possibly. can also serve the crucible material itself as an oxygen scavenger. The oxygen scavenger can simply be added into the crucible to the green body, either in solid form (wire, net, etc.), or as a powder. The green body can then be placed on the powder, or be directly in the powder. Possibly. the oxygen scavenger can also be "diluted" as a powder by an additive such as graphite, quartz sand, inert ceramic or the like.

Alternatively, it is also possible to place the oxygen scavenger spatially in front of the green body in the gas stream. The sintering can be carried out under an inert gas stream. leads (He, Ar, N ₂ ), the reduction effect of the oxygen scavenger can be supported by a reducing gas (H ₂ , CO), which is added to the inert gas stream or completely replaced. Alternatively, it may be sintered under reduced pressure. The sintering can be effected by electric, inductive, microwave or combustion heating.

Also in the sintering under pressure (hot pressing, spark plasma sintering), an oxygen scavenger can be used. For this purpose, for example, the following embodiments are conceivable.

First, it is possible to integrate oxygen scavenger materials into the jacket of the press die, rather than using a pure graphite or a pure steel die. It is also possible to coat the inside of the press die with the oxygen scavenger. Same precautions can be taken in the manufacture or processing of the thermoelectric materials.

Furthermore, a common compression of the thermoelectric material powder and the oxygen scavenger in a form of a sandwich is possible. The thermoelectric material powder is layered on the oxygen scavenger and both are densified together by hot pressing or SPS. The oxygen scavenger can also be used as a powder, but also as a pre-pressed or solid molded body. The same applies to the thermoelectric material, which may also be pre-compressed before the hot pressing or SPS step to a green body. After compaction, oxygen scavengers and material bodies are mechanically separated from one another (cutting, sawing, etc., the corresponding processes are known to the person skilled in the art).

It is also possible to extrude the materials into dense moldings. This is usually carried out at elevated temperature for thermoelectric materials and can be carried out as described in WO 01/17034, see also US Pat. No. 3,220,199 and US Pat. No. 4,161,111. Here, an oxygen scavenger can be used by a suitable capsule material in the course of encapsulated extrusion (US Pat. the basic method is known to those skilled in the art), wherein either the entire capsule can be made of the relevant material, which is then rendered inert to the outside by an additional coating, or the capsule is coated on the inside with the oxygen scavenger material.

An analogous capsule process is also used in hot isostatic pressing. The compaction by hot pressing, SPS or extrusion etc. may be followed by another sintering step. For this, the same possibilities and conditions apply as already described above for cold pressing / sintering. The sintered bodies can either already be produced directly in the leg geometry necessary for the thermoelectric module, or limbs can be cut out of the sintered bodies in the required geometry. This can be done by the methods known in the art. The invention also relates to a method for increasing the long-term stability of thermoelectric legs, in which the legs are operated in a thermoelectric module in the presence of oxygen scavengers.

The invention is further illustrated by the following examples.

Examples

Example 1 PbTe n-doped bulk material was minced and compressed in air at 60 kF with 30 kN into a compact pill. The pill was removed from the press and wrapped with Ti wire 0.25 mm in diameter and 3 cm in length and sintered in a closed quartz ampule at 600 ^° C. for 72 hours. After sintering, the power factor at 300 ⁰ C was determined.

For the body wrapped with Ti wire, a power factor of 21, 5 μW / K ² cm resulted. If the Ti wire was omitted for comparison, the power factor was 0.45 μW / K ² cm.

This makes it clear that the sintering process according to the invention leads to significantly improved power factors.

Example 2

Doped PbTe bulk material was crushed and ground and mixed with 0.1 wt% TiH ₂ . The mixture was submerged into a compact pill for 1 second with 15 kN force Air pressed. The pill was removed from the cold press and sintered in an ampule at 700 ^° C. for 3 hours.

Claims

claims

1. A method for producing, processing, sintering, pressing or extruding thermoelectric materials under heat treatment under inert gas or at reduced pressure at temperatures in the range of 100 to 900 ⁰ C, characterized in that the manufacturing, processing, sintering, pressing or extrusion in The presence of oxygen scavengers takes place, which form thermodynamically stable oxides in the production, processing, sintering, pressing or extrusion conditions in the presence of free oxygen and thus keep free oxygen from the thermoelectric material.

2. The method according to claim 1, characterized in that the oxygen scavengers are selected from Ti, Zr, Hf, Si, Al, V, Sc, Y, rare earth metals, Li, Na, K, Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Cu, Zn, Cd, P or mixtures thereof.

3. The method according to claim 1, characterized in that the oxygen scavengers are selected from H ₂ , CO, CO / CO ₂ mixtures, H ₂ / H ₂ O mixtures or inert gas / H ₂ mixtures.

4. The method according to claim 1, characterized in that the oxygen scavengers are selected from hydrides, carbonyls, niedervalenten oxides, sulfides, phosphides, of metals, sulfur or phosphorus-containing compounds, sulfur, phosphorus or mixtures thereof.

5. The method according to any one of claims 1 to 4, characterized in that the thermoelectric material is selected from PbTe, Bi ₂ Te ₃ , Zintl phases, Skut teruditen, clathrates and zinc antimonides, Heusler compounds, silicides, oxides or Mixtures thereof.

6. The method according to any one of claims 1 to 5, characterized in that a green body of a thermoelectric material which has been subjected to shaping, is sintered in direct contact with the oxygen scavenger.

7. The method according to any one of claims 1 to 5, characterized in that a green body of a thermoelectric material which has been subjected to shaping, and the oxygen scavenger during sintering spatially separated from each other, but are connected via a common gas space.

8. The method according to any one of claims 1 to 5, characterized in that the sintering is carried out under pressure and shaping of a powder of the thermoelectric material.

9. The method according to claim 8, characterized in that the sintering is carried out under pressure as hot pressing, hot isostatic pressing or spark plasma sintering.

10. The method according to claim 8 or 9, characterized in that the oxygen scavenger is arranged in the pressing tool in contact with the thermoelectric material or in the form of a sandwich with the powder of the thermoelectric material is pressed.

1 1. A method according to claim 8, characterized in that the sintering is carried out under pressure as extrusion.

12. The method according to any one of claims 1 to 1 1, characterized in that the sintering is carried out for the direct production of thermoelectric material legs.

13. The method according to any one of claims 1 to 12, characterized in that the solid oxygen scavenger is used in the form of a powder, wire, sheet, strip, granules, shaped body, mesh or in spherical form.

14. The method according to any one of claims 1 to 13, characterized in that the oxygen scavenger contains no element which is present in the thermoelectric material.

15. A method for increasing the long-term stability of thermoelectric legs, in which the legs are operated in a thermoelectric module in the presence of oxygen scavengers.