EP2250126A2

EP2250126A2 - Doped tin tellurides for thermoelectric applications

Info

Publication number: EP2250126A2
Application number: EP09708165A
Authority: EP
Inventors: Frank Haass
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2008-02-07
Filing date: 2009-02-05
Publication date: 2010-11-17
Also published as: JP5468554B2; KR20110004362A; WO2009098248A3; TW200950165A; WO2009098248A2; RU2010137002A; US8772622B2; JP2011514666A; US20110012069A1; CN101965313A; CA2715040A1

Abstract

The p- or n-conductive semiconductor material contains a compound of general formula (I) Sna Pb1-a-(x1+... +xn) A1 x1...An xn (Te1-p-q-r SepSqXr)1+z, in which 0.05 < a < 1, n = 1, where n is a number of chemical elements that differ from Sn and Pb, and independently 1 ppm = x1... xn = 0.05, A1... An differ from one another and are selected from the group of elements Li, Na, K, Rb, Cs, Mg, Ca, Y, Ti, Zr, Hf, Nb, Ta, Cr, Mn, Fe, Cu, Ag, Au, Ga, In, Tl, Ge, Sb, Bi X F, Cl, Br or l, 0 = p = 1, 0 = q = 1, 0 = r = 0.01, - 0.01 = z = 0.01, with the proviso that p + q + r = 1 und a + x1 +... + xn = 1.

Description

Doped pewter tellurides for thermoelectric applications

description

The present invention relates to semiconductor materials containing tin and usually tellurium and at least one or two further dopants, as well as these containing thermoelectric generators and Peltier arrangements.

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 below 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.

A good overview of effects and materials is available e.g. Cronin B. Vining, ITS Short Course on Thermoelectricity, Nov. 8, 1993, Yokohama, Japan.

At present, thermoelectric generators are used, for example, 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 transport, but only a charge transport. It can be used any fuel from hydrogen over natural gas, gasoline, kerosene, diesel fuel up 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.

A particularly attractive application is the use for the conversion of (waste) heat into electrical energy in motor vehicles, heating or power plants. Although previously unused thermal energy can be recovered by thermoelectric generators at least in part even today, however, previous technologies only achieve efficiencies of well below 10%, so that still a large part of the energy is lost unused. When using waste heat, therefore, significantly higher efficiencies are sought.

The conversion of solar energy directly into electrical energy would also be very attractive. Concentrators such as parabolic troughs can focus solar energy on thermoelectric generators, generating electrical energy.

But also for use as a heat pump higher efficiencies are necessary.

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

S ² - σ

Z =

K

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 Z-factor as high as possible Takes value.

The product S ² σ is called the power factor and serves to compare the thermoelectric materials.

In addition, for comparison purposes, the dimensionless product Z-T is often given. Previously known thermoelectric materials have maximum Z-T values of about 1 at an optimum temperature. Beyond this optimum temperature, Z-T values are often significantly lower than 1.

A more detailed analysis shows that the efficiency η results from T high - T low M - I η = -

T h ₁ and T,

M low

¹ T high

With

M = I ⁺ n, V high + 1 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 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.

For thermoelectric materials with high efficiency preferably further boundary conditions are to be met. Above all, they have to be sufficiently temperature-stable to be able to work under operating conditions 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 components into the adjacent contact materials.

Doped lead tellurides for thermoelectric applications are described, for example, in WO 2007/104601. These are lead tellurides which, in addition to a large amount of lead, also contain one or two further dopants. The respective proportion of dopants, based on the formula (I) given in WO, is 1 ppm to 0.05. Example 5 discloses Pbo, 987Ge _o , oiSn _o , oo3Tei, ooi- This material also contains the lowest lead content of the exemplary compounds. Thus, the materials have very high lead contents and, if any, only very low levels of tin. WO 2007/104603 relates to lead germanium tellurides for thermoelectric applications. These are ternary compounds of lead, germanium and tellurium, which in turn contain very high levels of lead.

For the production of a thermoelectric module n- and p-conductors are always necessary. In order to achieve the highest possible efficiency of the modules, that is to the highest possible cooling capacity in a Peltier arrangement or the highest possible generator power in a Seebeck arrangement, p-type and n-type conductive material must be matched as well as possible be. This applies above all to the parameters Seebeck coefficient (ideally S (n) = -S (p)), electrical conductivity (ideal σ (n) = σ (p)), thermal conductivity (ideal λ (n) = λ (p) ) and thermal expansion coefficient (ideally α (n) = α (p)).

Based on this prior art and the aforementioned material requirements, it is an object of the present invention to provide thermoelectrically active materials which have a high thermoelectric efficiency and show a suitable property profile for different fields of application. Preferably should in this case be materials that in the temperature interval under application conditions (typically between ambient temperature and at least 150 ⁰ C) no change of the conduction mechanism through.

The object is achieved by a

p- or n-type semiconductor material containing a compound of general formula (I)

Sn _a Pbi-a- ₍ χi _{+ + xn)} A ¹ _x1 ... A ⁿ _xn (Tei _-pqr Se _p S _q X _r ) i _{+ z} (I)

with the meaning

0.05 <a <1

n ≥ 1 with n number of chemical elements different from Sn and Pb

each independently

1 ppm <x1 ... xn <0.05 A ¹ ... A ⁿ are different from one another and are selected from the group of the elements Li, Na, K, Rb, Cs, Mg, Ca, Y, Ti, Zr, Hf, Nb, Ta, Cr, Mn, Fe, Cu, Ag, Au, Ga, In, Tl, Ge, Sb, Bi

X is F, Cl, Br or I

0 <p <1

0 <q <1

0 <r <0.01

- 0.01 <z <0.01

under the condition that p + q + r <1 and a + x1 + ... + xn <1.

It has been found according to the invention that Zinntelluride having a tin content of more than 5 wt .-%, preferably of at least 10 wt .-%, in particular of at least 20 wt .-% have very good thermoelectric properties when added with at least one additional dopant are.

It was also found according to the invention that a change in the Leitungsmecha- mechanism, z. B. could be suppressed from p-line to n-line with increasing temperature in the Sn-rich materials. This change is often a problem in the Pb-rich systems, since the p-type samples despite good starting values at room temperature at 300 ^° C reversibly change to the n-type region and therefore are not useful for use at higher temperatures. This problem can be avoided by using the Sn-rich materials of the present invention.

In the compounds of the general formula (I), n indicates the number of chemical elements other than SnPb, and Te, Se, S and X are not considered. The materials may be pure tellurides. In this case, p = q = r = 0. Tellurium may also be wholly or partly replaced by selenium, sulfur or, in minor amounts, halide. Preference is 0 <p <0.2, more preferably 0 <p <0.05. Preferably, 0 <q <0.2, more preferably 0 <q <0.05. Particularly preferred are p = q = r = 0. n is an integer of at least 1. Preferably n has a value <10, more preferably <5. In particular n has the value 1 or 2.

The proportion of tin is according to the invention 0.05 <a <1. Preferably, 0.1 ≤ a ≤ 0.9, particularly preferably 0.15 <a <0.8. In particular, 0.2 <a <0.75.

Each of the mutually different additional elements A ¹ to A ⁿ is present in an amount of 1 ppm <x1 ... xn <0.05. The sum of x1... Xn is preferably 0.0005 to 0.1, particularly preferably 0.001 to 0.08. The individual values are likewise preferably 0.0005 to 0.1, more preferably 0.001 to 0.08.

For example, preference is given to compounds of the general formula (I) where a = 0.2 to 0.75, where the sum x1... Xn is 0.001 to 0.08 and n is 1 or 2 and p = q = r = 0 and z = ± 0.01. The compounds thus contain Sn, Pb and Te.

The dopants A ¹ ... A ⁿ can be selected as desired from the group consisting of the elements Li, Na, K, Rb, Cs, Mg, Ca, Y, Ti, Zr, Hf, Nb, Ta, Cr, Mn, Fe, Cu, Ag, Au, Ga, In, Tl, Ge, Sb, Bi. Particularly preferred are A ¹ ... A ⁿ selected from the group of the elements Li, Na, K, Mg, Ti, Zr, Hf, Nb, Ta, Mn, Ag, Ga, In, Ge. In particular, A ¹ ... A ⁿ are different from one another and are selected from the group of the elements Ag, Mn, Na, Ti, Zr, Ge, Hf.

According to the invention, particular preference is given to p-conductive systems which do not change from the p-conduction to the n-conduction even with increasing temperature.

For the materials according to the invention, Seebeck coefficients were determined, for example, in the range from 70 to 202 μV / K for the p-type systems. The electrical conductivity was, for example, in the range of 1000 to 5350 S / cm. The exemplary power factors were 18 to 54 μW / K ² cm.

The materials of the invention are generally prepared by reactive milling or, preferably, by fusing and reaction of mixtures of the respective constituent elements or their alloys. In general, a reaction time of the reactive grinding or, preferably, melting together of at least one hour has proven to be advantageous.

The melting together and reacting are preferably carried out for a period of at least 1 hour, more preferably at least 6 hours, especially 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, so it is particularly suitable for this purpose, a rotary or tilt oven to ensure the homogeneity of the mixture.

If no mixture is made, longer melting times are generally required to obtain a homogeneous material. If a mixture is made, the homogeneity in the mixture is obtained earlier.

Without additional mixing of the starting mixtures, the melting time is generally 2 to 50 hours, especially 30 to 50 hours.

Melting generally occurs at a temperature at which at least one component of the mixture has already melted. In general, the melting temperature is at least 800 ^° C., preferably at least 950 ^° C. Usually, the melting temperature is in a temperature range from 800 to 1100 ^° C., preferably 950 to 1050 ^° C.

After cooling the molten mixture, it is advantageous, pern the material at a temperature of generally at least 100 ⁰ C, preferably at least 200 ⁰ C lower than the melting point of the resulting semiconductor material to TEM. Usually, the temperature is 450 to 750 ⁰ C, preferably 550 and 700 ⁰ C.

The annealing is carried out for a period of preferably at least 1 hour, particularly preferably at least 2 hours, in particular at least 4 hours. Usually, the annealing time is 1 to 8 hours, preferably 6 to 8 hours. In one embodiment of the present invention, the annealing is performed at a temperature which is 100 to 500 ^° C lower than the melting temperature of the resulting semiconductor material. A preferred temperature range is 150 to 350 ⁰ C lower than the melting point of the resulting HaIb- conductor material.

The preparation of the thermoelectric materials according to the invention is generally carried out in an evacuated and sealed quartz tube. Mixing of the components involved can be ensured by using a rotatable and / or tiltable opening. After completion of the reaction, the furnace is cooled. Thereafter, the quartz tube is removed from the oven and the block-shaped semiconductor material is sliced. These disks are now cut into pieces of about 1 to 5 mm in length, from which thermoelectric modules can be produced. Instead of a quartz tube, it is also possible to use tubes or ampoules of other materials inert to the semiconductor material, for example of tantalum.

Instead of tubes, other containers of suitable shape can be used. Other materials, such as graphite, may also be used as the container material, as long as they are inert to the semiconductor material. A synthesis of the materials can also be carried out by melting / fusing in an induction furnace, for example in crucibles made of graphite.

In one embodiment of the present invention, the cooled material can be ground wet, dry or in another suitable manner at a suitable temperature, so that the semiconductor material according to the invention is obtained in customary particle sizes of less than 10 μm. The milled material of the invention is then extruded hot or cold, or preferably hot or cold pressed into moldings having 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%, than the bulk density of the raw material in the unpressed 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 20 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 350 to 750 ⁰ C, preferably 600 to 700 ⁰ C. It is also spark plasma sintering (SPS) or microwave sintering can be performed.

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. Usually, the sintering time is 0.5 to 5 hours, preferably 1 to 3 hours. In one embodiment of the present invention, the sintering is performed at a temperature which is 100 to 600 ^° C lower than the melting temperature of the resulting semiconductor material. A preferred temperature range is 150 to 350 ⁰ C lower than the melting point of the resulting semiconductor material. The sintering is preferably carried out in a reducing atmosphere, for example under hydrogen, or in a protective gas atmosphere, for example of argon.

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) melting together mixtures of the respective constituent elements or their alloys of the at least quaternary or ternary compound;

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

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

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

The invention also relates to semiconductor materials obtainable or obtained by the processes according to the invention or prepared.

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 process for the production of thermoelectric generators or Peltier arrangements, in which series-connected thermoelectrically active building blocks ("legs") are used with thin layers of the previously described thermoelectric materials.

The semiconductor materials according to the invention can be joined together by methods to 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 systems which 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.

• as a heat pump • for air conditioning of seating, vehicles and buildings

• in refrigerators and (laundry) dryers

• for simultaneous heating and cooling of material streams in processes of substance separation such as

- absorption - drying

- crystallization

- evaporation

- distillation

• as a generator to use heat sources such as - solar energy

- geothermal energy

- Heat of combustion of fossil fuels

- of waste heat sources in vehicles and stationary installations

- of heat sinks during the evaporation of liquid substances - biological heat sources

• for cooling electronic components

• as a generator for converting heat energy into electrical energy, eg. As in motor vehicles, heaters or power plants

Furthermore, the present invention relates to a heat pump, a cooler, a refrigerator, a (laundry) dryer, a generator for converting thermal energy into electrical energy, or a generator for using heat sources, comprising at least one thermoelectric generator according to the invention or a Peltier invention Arrangement. The present invention will be explained in more detail with reference to the examples described below.

embodiments

The synthesis of the materials of the following compositions always took place from the elements or the elemental tellurides. The purity of the materials used was always ≥ 99.99%. The educts were each weighed in the appropriate stoichiometric ratio into a purified quartz ampoule having an inner diameter of 10 mm. The sample amount was 20 g each. The ampoule was evacuated and melted. Subsequently, the ampoule was heated in the oven with a maximum of 500 K h ^"1 to 1050 ⁰ C and held at this temperature for 8 hours., While the contents of the ampoule was continuously mixed by tilting the furnace After the reaction time, the ampoule with a maximum of 100 K. h ^{"1 was} cooled in an upright furnace position to 600 ⁰ C and the material was annealed at this temperature for 24 h. Thereafter, the material was cooled to room temperature.

The samples were always compact, glossy silver Reguli, which were taken from the ampoules and cut with a diamond wire saw in about 1, 5 mm thick slices. The electric conductivity and the Seebeck coefficient were measured on these disks.

The Seebeck coefficient was determined by placing the material to be tested between a hot and a cold contact, the hot contact having a temperature around 300 ° C and keeping the cold side at room temperature. The measured voltage at the respective temperature difference between hot and cold contact provided the respectively specified Seebeck coefficient.

The electrical conductivity was determined at room temperature by a four-point measurement. The process is known to the person skilled in the art.

Table 1 below gives the Seebeck coefficients S, the electrical conductivity σ and the resulting power factor S ² σ for different compositions. Table 1

There are moreover also temperature-resolved measurements of the Seebeck coefficients performed to 300 ⁰ C, which are shown in FIG. 1 The respective Seebeck coefficient is plotted against the temperature. The measurements confirm that the Sn-rich materials do not undergo a change from the p-type to the n-type in the temperature range studied. Single sample disks were measured. To carry out the temperatures of cold and hot side were adjusted to a small interval (.DELTA.T <2 K) and measured in this way the Seebeck coefficient at an average temperature ((T _ka ι _t + T _heιß ) / 2).

For comparison, lead tellurides of high lead content were prepared and the temperature dependence of the Seebeck coefficient was determined. FIG. 2 shows the corresponding results for different materials. The respective Seebeck coefficient is plotted against the temperature. The measurements confirm that materials with a very high lead content show a change from p-type to n-type with increasing temperature. Thus, the systems do not meet the requirements in terms of temperature stability, and the Seebeck coefficient has, depending on the temperature, very low values. In Fig. 2, p-L means p-line and n-L n-line.

Claims

claims

1. p- or n-type semiconductor material containing a compound of general formula (I)

with the meaning

0.05 <a <1

n ≥ 1 with n number of chemical elements different from Sn and Pb

each independently

1 ppm <x1 ... xn <0.05

A ¹ ... A ⁿ are different from one another and are selected from the group of the elements Li, Na, K, Rb, Cs, Mg, Ca, Y, Ti, Zr, Hf, Nb, Ta, Cr, Mn, Fe, Cu, Ag, Au, Ga, In, Tl, Ge, Sb, Bi

X is F, Cl, Br or I

0 <p <1

0 <q <1

0 <r <0.01

- 0.01 <z <0.01

under the condition that p + q + r <1 and a + x1 + ... + xn <1.

2. Semiconductor material according to claim 1, characterized in that A ¹ ... A ⁿ different from each other and selected from the group of elements Li, Na, K,

Mg, Ti, Zr, Hf, Nb, Ta, Mn, Ag, Ga, In, Ge are.

3. Semiconductor material according to one of claims 1 or 2, characterized in that n has the value 1 or 2 and p = q = r = 0.

4. Semiconductor material according to one of claims 1 to 3, characterized in that 0.1 <a <0.9 and / or that the sum of x1 ... xn 0.0005 to 0.1.

5. A method for producing a semiconductor material according to any one of claims 1 to 4, characterized in that the material is prepared by reactive milling or melting together mixtures of the respective constituent elements or their alloys.

6. The method according to claim 5, characterized in that the fusion takes place in an induction furnace.

7. The method according to claim 5 or 6, characterized in that it comprises the following method steps:

(2) milling the material obtained in process step (1); (3) pressing or extruding the material obtained in step (2) into shaped articles; and (4) sintering the shaped article obtained in step (3).

8. Thermoelectric generator or Peltier arrangement, containing a semiconductor material according to one of claims 1 to 4.

9. Use of a thermoelectric generator or a Peltier arrangement according to claim 8 as a heat pump, for air conditioning of seating, vehicles and buildings, in refrigerators and (laundry) dryers, for the simultaneous heating and cooling of streams in processes of separation, as

Generator for using heat sources or for cooling electronic components.

10. Heat pump, radiator, refrigerator, (laundry) dryer, generator for using heat sources, generator for converting heat energy into electrical

Energy containing at least one thermoelectric generator or a Peltier arrangement according to claim 8.