WO2008028852A2

WO2008028852A2 - Doped bi-te compounds for thermoelectric generators and peltier arrangements

Info

Publication number: WO2008028852A2
Application number: PCT/EP2007/059007
Authority: WO
Inventors: Hans-Josef Sterzel; Frank Haass; Patrick Deck
Original assignee: Basf Se
Priority date: 2006-09-05
Filing date: 2007-08-29
Publication date: 2008-03-13
Also published as: WO2008028852A3; TW200822404A

Abstract

A p-conductive or n-conductive semiconductor material contains a Ge-, Co-, Fe- and/or Ni-doped bismuth telluride of the general formula (I) or (II) Bi2-xDopuSeyTez (I) with the meaning Dop = Ge, Co, Fe, Ni or mixtures thereof u, x = independently of one another 0.001 to 0.06 y = 0.01 to 1.0 y+z = 3.00 to 3.2 Bi2-xSb yDopuSezTev (II) with the meaning Dop = Ge, Co, Fe, Ni or mixtures thereof u, x = independently of one another 0.001 to 0.4 y = 0 to 1 z = 0 to 1 z+v = 3.00 to 3.3.

Description

Doped Bi-Te compounds for thermoelectric generators and Peltier arrangements

description

The present invention relates to semiconductor materials containing dopants, bismuth and tellurium (doped bi-tellurides) and thermoelectric generators and Peltier devices containing them.

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 10% 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.

At present, thermoelectric generators are used, for example, in modules for controlling the temperature of microprocessors or optoelectronic components, in space probes for generating direct currents, for the cathodic corrosion protection of pipelines, for the power supply of illuminated and radio buoys and for the operation of radios and television sets. The advantages of the thermoelectric generators are in their utmost reliability. So they work regardless of atmospheric conditions such as humidity; there is no fault-susceptible mass transfer, but only a charge transport; the fuel is burned continuously - also catalytically without a free flame -; 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. Thus, the thermoelectric energy conversion fits extremely flexibly into future needs such as hydrogen economy or energy production from renewable energies.

Thermoelectrically active materials are evaluated essentially on the basis of their efficiency. Characteristic of thermoelectric materials in this regard is the so-called Z factor (figure of merit):

zz - ^s2 K- ^σ

with the Seebeck coefficient S, the electrical conductivity σ and the thermal conductivity K. Preference is given to thermoelectric materials which have the lowest possible thermal conductivity, the highest possible electrical conductivity and the largest possible Seebeck coefficient, so that the figure of merit as high as possible Takes value.

For comparison purposes, moreover, the dimensionless product Z ^■ T is often given. Previously known thermoelectric materials have maximum values of Z ^■ T of about 1 at an optimum temperature. Beyond this optimum temperature, the values of Z • T are often lower than 1. Materials such as Bi ₂ Te3, PbTe, and antimony ZnSb ₃ and CoSb _{3 are} currently the best known in the art.

A more detailed analysis shows that the efficiency η results from

^ high ~ * low M ~ \ η =

T h ₁ och low

M +

¹ high

With

M = ^ - ~ "~ ~ \ high ^" "^""* low)

(See also Mat. Sci and Eng. B29 (1995) 228). The aim is thus to provide a thermoelectrically active material which has the highest possible value for Z and a high temperature difference that can be achieved. From the point of view of solid-state physics many problems have to be overcome:

A high σ requires a high electron mobility in the material, ie electrons (or holes in p-type materials) should not be strongly bound to the atomic hulls. Materials with high electrical conductivity σ usually have at the same time a high thermal conductivity (Wiedemann - Franz's Law), which means that Z can not be favorably influenced. Currently used materials such as Bi ₂ Te ₃ , PbTe or SiGe are already compromises. Thus, the electrical conductivity is reduced by alloying less than the thermal conductivity. Therefore, it is preferable to use alloys such as (Bi ₂ Te ₃ ) 9o (Sb ₂ Te ₃ ) ₅ (Sb ₂ Se ₃ ) ₅ or Bi ₁₂ Sb ₂₃ Te ₆ S, as described in US Pat. No. 5,448,109.

Germanium-lead tellurides for thermoelectric generators and Peltier arrangements are described, for example, in DE-A-10 2004 043 787.

In the foreground of existing applications is the bismuth telluride, because of its inherent low thermal conductivity. Typical n-type bismuth telluride materials have compositions corresponding to Bi ₂ Seo _, sTe _2.5 , and p-type such as Bi _o , 7Sbi, ₃ Te ₃ .

For thermoelectric materials with high efficiency preferably further boundary conditions are to be met. Above all, they must be temperature-stable in order to be able to work at operating temperatures of up to 1,000 K or 1,500 K for years without significant loss of efficiency. This requires a high temperature stable phase per se, a stable phase composition and a negligible diffusion of alloying constituents into the adjacent contact materials.

Based on this prior art, the object of the present invention is to provide semiconductor materials (thermoelectrically active materials) which have a high degree of efficiency and exhibit a suitable property profile for different fields of application.

A preferred object of the invention is to provide a thermoelectrically active material based on Bi ₂ Te ₃ with better ZT values than in the prior art. The object is achieved according to the invention by a p-type or n-type semiconductor material comprising a bismuth telluride doped with Ge, Co, Fe and / or Ni of the general formula (I) or (II)

Bi ₂ -χDopuSe _y Te _z (I)

with the meaning

Dop = Ge, Co, Fe, Ni or mixtures thereof

u, x = independently from 0.001 to 0.06

y = 0.01 to 1.0

y + z = 3.00 to 3.2

Bi ₂ -χSbyDopuSe _z Te _v (II)

with the meaning

Dop = Ge, Co, Fe, Ni or mixtures thereof

u, x = independently 0.001 to 0.4

y = 0 to 1

z = 0 to 1

z + v = 3.00 to 3.3

It has been found according to the invention that bismuth tellurides of the formula (I) doped with Ge, Co, Se, Ni or mixtures thereof and Se are advantageous n-doped semiconductor materials which have an improved property profile as thermoelectrically active materials. It has also been found that bismuth tellurides of the formula (II) doped with Ge, Co, Fe, Ni or mixtures thereof and optionally Sb and / or Se give advantageous p-doped semiconductor materials which exhibit an improved property profile for thermoelectrically active materials.

For the formulas (I) and (II), the radicals or indices indicated have the meanings given for the respective formula. This means that the indices for the compounds of the formula (I) and (II) can have different meanings. Particularly preferred semiconductor materials are characterized in that in the doped bismuth telluride of the general formula (I)

u, x are independently 0.01 to 0.03,

y 0.1 to 0.7 and

y + z is 3.01 to 3.1.

Particularly preferred semiconductor materials of the formula (II) are characterized in that in the doped bismuth telluride of the general formula (II)

u, x are independently 0.005 to 0.25.

In this case, according to the invention, one of the preferred ranges can be combined with the general ranges of the formulas (I) and (II) given above. This means that, according to the invention, individual ones of the preferred areas can also be realized, while the other areas remain in the generally defined area.

Dop means Ge, Co, Fe, Ni or mixtures thereof. Preferably, dop means only one of these metals in a compound of general formula (I) or (II). Particularly preferred are combinations of elements, as indicated in the examples below. The proportions can be freely selected according to the above limits or preferred limits.

It should also be noted that the sum of Bi and Dop need not necessarily be 2. Accordingly, the sum of y and z in formula (I) or the sum of y, z and v in formula (II) does not necessarily have to be 3.

The materials of the invention are generally made by fusing together mixtures of the respective constituent elements or their compounds / alloys. In general, a reaction time of melting together of at least one hour has proven to be advantageous.

The melting together is preferably carried out for a period of at least 1 hour, more preferably at least 5 hours, in particular at least 10 hours. The melting process can be carried out with or without mixing of the starting mixture. When the starting mixture is mixed, it is suitable for this purpose in particular a rotary kiln to ensure the homogeneity of the mixture. If no mixture is made, generally longer melt times of 2 to 100 hours, especially 30 to 100 hours, are required to obtain a homogeneous material. If a mixture is made, the homogeneity in the mixture is obtained earlier.

Melting generally occurs at a temperature at which at least one component of the mixture has already melted and the material is already in the molten state. In general, the melting temperature is at least 700 ⁰ C, preferably at least 1000 ⁰ C. Usually, the melting temperature in a temperature range 700-1500 ⁰ C, preferably 1000-1300 ⁰ C. This ensures that the dopants are distributed homogeneously.

The production of the thermoelectric materials according to the invention is generally carried out in a heatable quartz tube. A mixing of the components involved can be ensured by using a rotatable and / or tiltable furnace. After completion of the reaction, the furnace is cooled. Subsequently, the quartz tube is removed from the oven and the z. B. sliced in the form of blocks present semiconductor material. These discs are z. B. now cut into pieces of about 1 to 5 mm in length, from which thermoelectric modules can be produced.

Instead of a quartz tube, tubes made of other materials, for example tantalum, can also be used. This is preferred because the thermal conductivity of this material is higher than that of quartz.

Instead of tubes, other containers of suitable shape can be used. Other materials, such as graphite, can be used as container material.

It is also possible to quench the melt by discharging the quartz tube with the melt into a liquid such as water or oil. Higher quenching rates are obtained by allowing the melt to flow directly into the quenching liquid under a protective gas atmosphere (N ₂ , Ar). Do not use water as it is not inert enough. Preference is given to using a paraffin oil.

With the quenching of the melt gives the advantage that the material during cooling can not separate with respect to the dopants, if a multi-phase phase region is undergone during the cooling process. You get a high seamless front material, which can then be subjected to yet a temperature treatment of, preferably, up to 100 ⁰ C below the melting point.

In one embodiment of the present invention, the cooled material can be ground at a suitable temperature, so that the semiconductor material according to the invention is obtained in conventional particle sizes smaller than 50 μm. The milled material according to the invention is then preferably pressed into shaped parts which have the desired shape. The bulk density of the shaped parts pressed in this way should preferably be greater than 50%, particularly preferably greater than 80%, of the bulk density of the raw material in the unpressed and preferably unmilled state. Compounds which improve the densification of the material according to the invention can be added in quantities of preferably 0.1 to 5% by volume, more preferably 0.2 to 2% by volume, based in each case on the powdered material according to the invention. Additives which are added to the materials according to the invention should preferably be inert to the semiconductor material and preferably dissolve out of the material according to the invention during heating to temperatures below the sintering temperature of the materials according to the invention, if appropriate under inert conditions and / or vacuum. After pressing, the pressed parts are preferably placed in a sintering furnace in which they are heated to a temperature of preferably at most 100 ⁰ C below the melting point.

The pressed parts are 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. Typically, the sintering temperature is 200 to 500 ⁰ C, preferably 300 to 450 ⁰ C.

The sintering is carried out for a period of preferably at least 0.5 hours, particularly preferably at least 1 hour, in particular at least 2 hours. Normally, the annealing time is 0.5 to 5 hours, preferably 1 to 3 hours. In one embodiment of the present invention, the sintering is carried out at a temperature which is 100 to 300 ^° C. lower than the melting temperature of the resulting semiconductor material. A preferred temperature range is 150 to 250 ⁰ C lower than the melting point of the resulting Halbleiterma- terials. The sintering is preferably carried out under hydrogen or a protective gas atmosphere, for example of argon.

It is also possible to carry out the pressing in the hot-pressing process under the specified sintering temperatures and simultaneous pressurization and generally obtains higher densities than during cold pressing and subsequent pressureless sintering. Thus, the pressed parts are preferably sintered to 95 to 100% of their theoretical bulk density.

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

(1) fusing together mixtures of the respective constituent elements or their alloys of the doped bismuth telluride; or to a Ge, Co, Fe and / or Ni doped Bismuttellurid

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

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

(4) sintering of the molded articles obtained in process step (3).

It is also possible to extrude the inventive ground materials 50 to 300 ⁰ C below its melting point in an extrusion process continuously stan- genförmigen profiles. This type of processing leads to extrudates with a particularly low pore content and thus particularly high conductivity.

The materials according to the invention are outstandingly suitable for applications in thermoelectrics as active semiconductors in thermoelectric modules. Particularly advantageous is their use in modules for Peltier arrangements in tumble dryers, air conditioners or for cooling seats, as well as in electronic devices (such as CPU coolers) or mobile refrigerators.

In thermoelectric products, p- and n-type materials are electrically connected in series to avoid thermal losses.

The present invention further relates to a p- or n-type semiconductor material of a compound of the general formula (I) or (II) which is prepared according to the method described above.

Another object of the present invention is the use of the above-described semiconductor material and the semiconductor material obtainable by the method described above as a thermoelectric generator or Peltier arrangement. Another object of the present invention are thermoelectric generators or Peltier arrangements, which contain the semiconductor material described above and / or the semiconductor material obtainable by the method described above.

Another object of the present invention is a method for producing thermoelectric generators or Peltier arrangements, in which electrically connected in series thermoelectrically active blocks ("legs") are used with thin layers of the previously described thermoelectric materials.

In a first embodiment of this method, the production of the thermoelectric generators or Peltier arrangements is carried out as follows:

The semiconductors of the invention according to a first conductivity type (p- or n-doped) are applied to a substrate by means of conventional semiconductor fabrication techniques, in particular CVD, sputtering technique or molecular beam epitaxy.

The semiconductors according to the invention are likewise applied to a further substrate by means of sputtering technique or molecular beam epitaxy, but the conductivity type of this semiconductor material is inverse to the semiconductor material used first (n- or p-doped).

The two substrates are now sandwiched on top of each other, so that thermoelectrically active building blocks ("legs") of a different charge type are alternately arranged.

The individual thermoelectrically active building blocks ("legs") have a diameter of preferably less than 100 .mu.m, more preferably less than 50 .mu.m, in particular less than 20 microns and a thickness of preferably 5 to 100 .mu.m, more preferably 10 to 50 .mu.m, in particular 15 to The area occupied by a thermoelectrically active building block is preferably less than 1 mm ² , particularly preferably less than 0.5 mm ² , in particular less than 0.4 mm ² .

In a second embodiment, the production of the thermoelectric generators or Peltier arrangements takes place in such a way that layers of inventive semiconductor materials of different charge type (p- and n-doped) are produced alternately on a substrate by suitable deposition methods, for example molecular beam epitaxy. The layer thickness is in each case preferably 5 to 100 μm, more preferably 5 to 50 μm, in particular 5 to 20 μm. The semiconductor materials according to the invention can also be joined by methods to form thermoelectric generators or Peltier arrangements, which are known per se to the person skilled in the art and are described, for example, in WO 98/44562, US Pat. No. 5,448,109, EP-A-1 102 334 or US Pat. No. 5,439,528.

The thermoelectric generators or Peltier arrangements according to the invention generally expand the available range of thermoelectric generators and Peltier arrangements. By varying the chemical composition of the thermoelectric generators or Peltier arrangements, it is possible to provide different Liehe systems that meet different requirements in a variety of applications. Thus, the thermoelectric generators or Peltier arrangements according to the invention expand the range of applications of these systems.

The present invention also relates to the use of a thermoelectric generator according to the invention or a Peltier arrangement according to the invention in a tumble dryer.

Furthermore, the present invention relates to a dryer comprising at least one thermoelectric generator according to the invention or a Peltier arrangement according to the invention, directly or indirectly heated via the or a material to be dried and directly or indirectly via the or the obtained during the drying water or solvent vapor is cooled indirectly.

In a preferred embodiment, the dryer is a clothes dryer and the material to be dried is laundry.

The present invention will be explained in more detail with reference to the examples described below.

Examples

To prepare the materials shown in the following table, the components were filled in the indicated molar ratio in quartz tubes with 10 mm inner diameter. The total mass was 5 - 9 g. Subsequently, the quartz tubes were 5 min. heated to about 100 ⁰ C in vacuo and then sealed in vacuo. The elements were used as granular granules with a purity of 99.995%. In a tube furnace, the quartz tubes were heated from room temperature to temperatures of 1000 ⁰ C within 5 h. This temperature was maintained for 7 h. During the entire heating period, the oven was in one period 2 minutes about a drive about the longitudinal axis tilted to achieve a good mixing of the melt.

The Seebeck coefficients found in the temperature range of 30 to 130 ⁰ C and the average electrical conductivity in this temperature range are given in the table below.

The materials of the invention were compared to bismuth tellurides of the prior art. For this purpose, specimens were a temperature difference of 100 K.

(cold side 30 ⁰ C, hot side 130 ⁰ C) exposed. The high-ohmic voltage results in 100 K divided by a mean Seebeck coefficient over the applied temperature difference. Finally, the short-circuit current was measured at very low impedance.

The product of no-load voltage and short-circuit current corresponds to a "power factor." The bismuth tellurides according to the prior art have anisotropic properties, the best combination of open circuit voltage and short-circuit current was selected as standard and the anisotropy is promoted by cooling the melt in a temperature gradient However, the isotropic property average is measured instead, which means that with the same relative "power factor" the material according to the invention is better.

Another advantage of the "isotropic" material of the invention is that it is mechanically much more stable than the known anisotropic, fracture-prone material.

table

Claims

claims

1. p- or n-type semiconductor material containing a bismuth telluride doped with Ge, Co, Fe and / or Ni of the general formula (I) or (II)

Bi ₂ -χDopuSe _y Te _z (I)

with the meaning

Dop = Ge, Co, Fe, Ni or mixtures thereof

u, x = independently from 0.001 to 0.06

y is 0.01 to 1, 0

y + z = 3.00 to 3.2

Bi ₂ -χSbyDopuSe _z Te _v (II)

with the meaning

Dop = Ge, Co, Fe, Ni or mixtures thereof

u, x = independently 0.001 to 0.4

y = 0 to 1

z = 0 to 1

z + v = 3.00 to 3.3.

2. Semiconductor material according to claim 1, characterized in that in the doped bismuth telluride of the general formula (I)

u, x are independently 0.01 to 0.03,

y 0.1 to 0.7 and

y + z is 3.01 to 3.1.

B06 / 0655EP final 5 September 2006

3. Semiconductor material according to claim 1, characterized in that in the doped bismuth telluride of the general formula (II)

u, x are independently 0.005 to 0.25.

4. A method for producing a semiconductor material according to any one of claims 1 to 3, characterized in that a Ge, Co, Fe and / or Ni doped bismuth telluride is prepared by melting together mixtures of the respective constituent elements or their alloys.

5. The method according to claim 4, characterized by the following method steps:

(1) melting together mixtures of the respective constituent elements or their alloys into a Ge, Co, Fe and / or Ni doped bismuth telluride;

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

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

(4) sintering of the molded articles obtained in process step (3).

6. The method according to claim 5, characterized in that in process step (3) the pressing to moldings takes place cold.

7. Semiconductor material, obtainable by a process according to one of claims 4 to 6.

8. Use of a semiconductor material according to one of claims 1 to 3 or 7 as a thermoelectric generator or Peltier arrangement.

9. A thermoelectric generator or Peltier arrangement comprising a semiconductor material according to one of claims 1 to 3 or 7.

10. A dryer, comprising at least one thermoelectric generator or a Peltier arrangement according to claim 9, via which or a material to be dried material directly or indirectly heated and cooled directly or indirectly over the or the incurred during the drying water or solvent vapor becomes.