DE102005008865A1 - Modified bismuth sulfide composition, useful in semiconductor material, which is used in e.g. thermoelectric modules and photovoltaic cells is new - Google Patents

Abstract

Modified bismuth sulfide composition (I) is new. Modified bismuth sulfide composition (I) of formula (Bi xE yS zMg u) is new. E : germanium and/or silicon; x : 1.9-2.1; y : 0.001-0.08; z : 2.95-3.05; and u : 0-2y. Independent claims are also included for: (1) a semiconductor material (II), comprising (I); and (2) the preparation of (II) comprising reaction of mixtures of the respective elements or their alloys at at least 800[deg]C.

Description

The The present invention relates to modified bismuth sulfides, these containing semiconductor materials for thermoelectrics and photovoltaics, Process for the preparation of these semiconductor materials and their Use.

That sulfides, such as Cu ₂ S, PbS, CdS or Bi ₂ S ₃ , are semiconductors, has been known for a long time. Their properties in this regard have been intensively studied. The use of in particular Cu ₂ S or PbS as rectifier diodes or CdS as active material in solar cells has long been known.

In isolated cases, investigations of complex sulphides in the application as thermoelectrically active material are described as in "Thermoelectric Properties Of The Family Of Cubic Compounds Ag Pb Bi Q ₃ (Q = S, Se, Te). Very Low Thermal Conductivity Materials ", S. Sportouch et al., Thermoelectric Materials 1998, Materials Research Society, Symposium Proceedings Vol. 545, p. 123-130, ISBN 1-55899-451-3.

thermoelectric Generators and Peltier arrangements as such have long been known. p- and n-doped semiconductors that are heated on one side and cooled on the other side electric charges are transported through an external circuit. Through these thermoelectric generators can be connected to a consumer electrical work is performed in the electric circuit. This can be thermoelectric Materials for the conversion of heat energy to be used in electricity. Also the reverse process, which is referred to as Peltier effect is by thermoelectric Materials under application of an electric current for generation possible from temperature differences. Such a Peltier arrangement operates as a heat pump and is therefore suitable for cooling of apparatus parts, vehicles or buildings. Also the heating over the Peltier principle is cheaper as a conventional one Heating, because more and more heat is transported is supplied as the energy equivalent equivalent.

a good overview of themoelectric Effects and materials are z. B. Cronin B Vining, ITS Short Course on Thermoelectricity, 08.11.93, Yokohama, Japan, 1997 Proceedings, Sixteenth International Conference on Thermoelectrics (ICT), CRC Handbook of Thermoelectrics, CRC-Press 1995, and Materials Research Society Symposium Proceedings Volume 545: Thermoelectric Materials 1998 - The next generation materials for small-scale refrigeration and power generation applications.

Becoming present Thermoelectric generators in modules for controlling the temperature of microprocessors or opto-electronic components, in space probes and satellites for the generation of direct currents, for cathodic corrosion protection of pipelines, for energy supply of lights and radio buoys, to the operation of radios and TVs as well as a cooling unit in cool bags used. The advantages of thermoelectric generators and Pettier arrangements are in their high reliability. That's how they work independently from atmospheric Conditions such as humidity; there is no problematic material transport, but only one charge transport; the fuel becomes continuous - also catalytic without a free flame - burned, resulting in only small amounts of carbon monoxide, nitrogen oxides and unburned fuel get free; any fuels can be used, from hydrogen to natural gas, Gasoline, kerosene, diesel fuel up to biologically produced fuels like rapeseed oil methyl ester.

In order to The thermoelectric energy conversion adapts very flexibly in future Needs, such as hydrogen economy or energy production from regenerative Energies, one.

mit dem Seebeck-Koeffizienten S (μV/grad), der elektrischen Leitfähigkeit σ (Ω^–1 cm^–1) und der Wärmeleitfähigkeit k (mW/cm·grad). Bevorzugt werden thermoelektrische Materialien, die eine möglichst geringe Wärmeleitfähigkeit, eine möglichst große elektrische Leitfähigkeit und einen möglichst großen Seebeck-Koeffzienten aufweisen, sodass der figure of merit einen möglichst hohen Wert annimmt.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)

with the Seebeck coefficient S (μV / grad), the electrical conductivity σ (Ω ^-1 cm ^-1 ) and the thermal conductivity k (mW / cm · grad). Preference is given to thermoelectric materials which have the lowest possible thermal conductivity, the highest possible electrical conductivity and the largest possible Seebeck coefficients, so that the figure of merit assumes the highest possible value.

For comparison purposes, moreover, the dimensionless product Z · T is often stated. Previously known thermoelectric materials have maximum values of Z · T of about 1 in an optima low temperature. Beyond this optimum temperature, the values of Z · T are often lower than 1. The optimum state of the art is currently embodied by materials such as Bi ₂ Te ₃ , PbTe and the antimony Zn ₄ Sb ₃ and CoSb ₃ .

The aim is therefore to provide a thermoelectric semiconductor material with the highest possible value for Z and high temperature difference achievable. From the point of view of solid-state physics many problems have to be overcome:
Materials with high electrical conductivity usually have at the same time a high thermal conductivity (Wiedemann-Franz's law), whereby the figure of merit Z is adversely affected. 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. B. (Bi ₂ Te ₃ ) ₉₀ (Sb ₂ Te ₃ ) ₅ (Sb ₂ Se ₃ ) ₅ or Bi ₁₂ SB ₂₃ Te ₆₅ , as described in the US 5,448,109 to be discribed.

For thermoelectric High-efficiency materials are preferably still others To meet boundary conditions. So have to They are temperature stable to work at temperatures of 700 to 1000 K over Years to work without significant loss of efficiency. This requires both high temperature stable phases per se, a stable Phase composition, as well as a negligible diffusion of alloying constituents in the adjacent contact materials.

As from the reference cited above "Thermoelectric Properties ..." decreases the electrical conductivity in the order of telluride about the selenide to the sulfide in the expected order. It will be 300, 67 and 25 S / cm. Conversely, the band gap increases between Valence band and conduction band of 0.28 eV for telluride over 0.48 eV of selenide at 0.54 eV for the sulfide. It is generally expected that with larger bandgap one lower electrical conductivity is obtained. In general, a thermoelectrically active semiconductor yes by heat be excited, giving low band gaps of 0.1 to usually 0.4 eV required.

For this reason, corresponding semiconductors with low band gaps are used in thermoelectric applications, essentially Bi ₂ Te ₃ (by 0.3 eV) and PbTe (0.25 eV).

On the other hand, one would like to use semiconductors with larger band gaps in solar cells, because one uses the higher-energy light quanta to lift charge carriers from the valence band into the conduction band. For this purpose, sulfides such as CdS have been investigated with a band gap of 2.4 eV or CuInS ₂ with 1.2 to 1.5 eV. Because lower electrical conductivities occur with higher band gaps, CdS was not used, and the band gap was too large. In contrast, thin-film solar cells with CuInS _{2 are} already manufactured as p- and n-semiconductors.

task The invention was a thermoelectric semiconductor material with one as possible high value for Z and provide high realizable temperature difference, the for different application areas a suitable property profile demonstrate.

The Task was inventively characterized solved, that used as semiconductor material modified bismuth sulfides become.

Articles of the invention are modified bismuth sulfides which have a composition according to the following general formula (1) Bi _x E _y S _z Mg _u (1) where E is germanium and / or silicon and the indices x, y, z and u correspond to one of the following values:
x = 1.9 to 2.1,
y = 0.001 to 0.08,
z = 2.95 to 3.05,
u = 0 to 2y,
and a semiconductor material containing these modified bismuth sulfides, and the use of such semiconductor material in thermoelectric modules and photovoltaic cells or modules.

Objects of Invention are also a method for producing a semiconductor material based on modified bismuth sulfides.

Surprisingly It has been found that per se known high band gap sulfides in before be modified so that they both in thermoelectric applications as well as in photovoltaic applications can be used. They are both of heat as well as excited by light and are capable of both temperature differences in the range of -100 up to + 600 ° C as well as to convert visible light directly into electrical energy.

Bi ₂ S ₃ has a very low grating thermal conductivity of 16 mW / cm at room temperature for thermoelectric applications.

Advantageous is also that for the production of semiconductors according to the invention commercially available Sulfur, mainly from the desulphurisation of oil and natural gas comes from and is very pure, can be used. Also the used Bismuth falls in large-scale Processes in high purity.

Bi ₂ S ₃ obtains completely new properties when it is modified according to the invention according to the following general formula (1) Bi _x E _y S _z Mg _u (1).

there E means germanium and / or silicon.

The germanium is preferably used in elemental form. In a further advantageous embodiment, Mg ₂ Ge is used.

The silicon can also be used in elemental form, but is preferably its use in the form of Mg ₂ Si.

Of the Index x corresponds to a value of 1.9 to 2.1, preferably of 1.95 to 2.05.

Of the Index y corresponds to a value of 0.001 to 0.08, preferably from 0.005 to 0.03.

The Values of the indices x and y must do not compliment each other.

Of the Index z corresponds to a value of 2.95 to 3.05, preferably of 2.98 to 3.02.

Of the Index u corresponds to values from 0 to 2y, preferably from 0 to 0.06.

The modified according to the invention Bismuth sulfides have in the temperature range from room temperature to 130 ° C one Seebeck coefficients of more than 300 μV / grad and an electric conductivity of more than 150 S / cm.

So For example, at a temperature of from room temperature to 130 ° C average Seebeck coefficient of up to 500 μV / degree and an electric conductivity of up to 450 S / cm.

These Modified bismuth sulfides are ideal for use as semiconductor material, in particular as n-type materials.

Advantageously usable modified bismuth sulfides are z. B. Bi _1.98 Ge _0.022 S ₃ and Bi _1.99 Ge _0.02 S ₃ . Further exemplified in the examples below.

The materials of the present invention are usually prepared 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 the starting mixture. If mixed, then this is particularly suitable for a furnace which can be tilted about its longitudinal axis in order to ensure the homogeneity of the mixture. If no mixture is made, generally longer melting times are used from 2 to 100 hours, especially 30 to 100 hours, required to obtain a homogeneous material.

The melting together is usually carried out at a temperature of at least 800 ° C (melting point of Bi ₂ S ₃ is about 770 ° C), preferably at least 850 ° C. In particular, the melting temperature is in a range of 850 to 950 ° C.

The Production of the material according to the invention generally takes place in a heatable quartz tube. A mixture the involved components can by using a rotary and / or tiltable furnace or an induction furnace where the magnetic field for the Mixing the melt ensures, be guaranteed. After completion the reaction is cooled down the furnace. Following is the quartz tube removed from the oven and present in the form of blocks semiconductor material cut in slices. These discs are now in pieces of approximately 1 to 5 mm in length cut, from which thermoelectric modules can be produced.

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 quench liquid under an inert 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 the advantage that the material during cooling relative to the Dopants can not demix. This gives a homogeneous material, that afterwards still a temperature treatment of preferably up to 100 ° C below of the melting point can be subjected.

Instead of of the quartz tube can also tubes made of other materials, such as graphite used become. Instead of pipes can also other containers suitable form can be used.

In an embodiment According to the present invention, the cooled material can be at a suitable temperature be ground, so that the semiconductor material according to the invention in conventional Particle sizes smaller than 50 um is obtained. The milled material of the invention is then preferably pressed to the desired moldings. The bulk density of the pressed moldings should preferably be greater than 50%, more preferably greater than 80%, as the raw density of the raw material in the unpressed state be. Compounds which the compaction of the material according to the invention can improve in amounts of preferably 0.1 to 5 vol .-%, more preferably 0.2 to 2% by volume, based in each case on the powdered material according to the invention, be added. Additives which contribute to the material according to the invention should be added, preferably inert to the Semiconductor material, and preferably during the heating on Temperatures below the sintering temperature of the material according to the invention, optionally under inert conditions and / or vacuum the material of the invention detach. After pressing, the pressed parts are preferably in one Given sintering furnace in which they are at a temperature of preferably maximum 20 ° C heated below the melting point become. The pressed parts are kept at a temperature of Generally at least 100 ° C, preferably at least 200 ° C, lower than the melting point of the resulting semiconductor material sintered. Usually is the Sintering temperature 350 to 750 ° C, preferably 400 to 670 ° C.

The Sintering is during a period of preferably at least 0.5 hours, more preferably at least 1 hour, in particular at least 2 hours. Usually is the sintering time 0.5 to 5 hours, preferably 1 to 3 hours.

In an embodiment In the present invention, sintering is carried out at a temperature carried out, the 100 to 600 ° C is lower than the melting temperature of the resulting semiconductor material. A preferred temperature range is 150 to 350 ° C lower is the melting point of the resulting semiconductor material. Preferably, the sintering under hydrogen or a protective gas atmosphere, for example Argon, performed.

you can in the hot pressing process the pressing also under the specified sintering temperatures and at the same time Apply pressure and receives generally higher Dense than cold pressing and subsequent pressureless sintering.

The pressed parts are preferably at 95 to 100% of their theoretical Bulk density sintered.

• (1) Zusammenschmelzen von Mischungen der jeweiligen Elementbestandteile oder deren Verbindungen/Legierungen,
• (2) Mahlen des in Verfahrensschritt (1) erhaltenen Materials,
• (3) Pressen des in Verfahrensschritt (2) erhaltenen Materials zu Formkörpern und
• (4) Sintern der in Verfahrensschritt (3) erhaltenen Formkörper.

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 compounds / alloys,
• (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).

In a preferred embodiment become the semiconductor materials according to the invention for example, prepared by reaction of Bi, Ge and / or Si and sulfur at temperatures of up to 1100 ° C, preferably from 850 to 950 ° C, for example in evacuated quartz vessels.

It is also possible first to produce Mg ₂ Ge and / or Mg ₂ Si, e.g. B. by reaction of Mg and Ge or Mg and Si at temperatures of preferably 1150 to 1250 ° C, and then admixing Bi and sulfur and the mass again to temperatures above 800 ° C, in particular up to 950 ° C, to heat.

The materials according to the invention have increased Seebeck coefficients compared to unmodified bismuth sulfides and electrical conductivity in an order of magnitude as stated above on. They are thus excellently suited for applications in thermoelectrics and in photovoltaics, in particular as active semiconductors in thermoelectric modules and in photovoltaic Cells or modules. Particularly advantageous is their use in modules for solar cells as well as thermoelectric generators and Peltier arrangements, in clothes dryers or air conditioners.

In Thermoelectric methods become p- and n-type materials connected in series to avoid thermal losses. In the Photovoltaic becomes a p-n junction used, wherein in the boundary layer, the charge separation.

As a combination with the semiconductors used according to the invention predominantly as n-type material, it is possible to use all p-type materials commonly used in thermoelectrics and photovoltaics. Such suitable p-type materials are Zn ₄ Sb ₃ and p-type lead or bismuth tellurides in thermoelectrics and p-type ZnTe and p-type CuIn chalcogenides in photovoltaics.

It has proved to be particularly advantageous to use modified copper sulfides as p-type material, which likewise have a Seebeck coefficient of more than 300 μV / degree and an electrical conductivity of more than 150 S / cm in the temperature range from room temperature to 130 ° C. , Such modified copper sulfides have, for example, a composition according to general formula (2) (Cu _x Me _y ) ₂ S _z (Mg ₂ Si) _v (2) where Me denotes Co and / or Bi and the indices x, y, z and v correspond to one of the following values:
x = 0.95 to 1.00, preferably 0.98 to 0.998,
y = 0 to 0.05, preferably 0.002 to 0.01, wherein
x + y is an amount of 0.98 to 1.02, preferably 0.99 to 1.01,
z = 0.95 to 1.05, preferably 0.98 to 1.02,
v = 0 to 0.01, preferably from 0 to 0.005.

In a preferred embodiment, these modified copper sulfides are prepared for example by reaction of Cu, Co and / or Bi, sulfur and optionally Mg ₂ Si at temperatures of up to 1300 ° C, preferably from 1200 to 1250 ° C, for example in evacuated quartz vessels.

It is also possible to first produce Cu ₂ S, z. B. by reaction of Cu or Cu ₂ O with sulfur or hydrogen sulfide at temperatures of 400 to 1200 ° C, and then mix the additives necessary for the modification and the mass again to temperatures above 1100 ° C, in particular up to 1300 ° C, to heat.

The Invention will be explained with reference to the following embodiments, without However, this is to make a corresponding limitation.

The in the following table were given component weights in quartz tubes filled with 10 mm inner diameter. Subsequently were the quartz tubes 5 min to about 100 ° C heated in vacuo and then melted in vacuo. It came to Use: bismuth with 99.999% purity and sulfur from a tanker delivery liquid Sulfur with a purity of 99.99%. In a tube furnace were the quartz tubes heated from room temperature to 1000 ° C within 10 h. This temperature was kept for another 5 hours. While the entire heating time was the oven with a period of about 2 min over one Drive around the longitudinal axis tilted to achieve a good mixing of the melt.

After that one left cool the oven, the quartz tubes were opened and the material samples taken.

The found Seebeck coefficients in the temperature range of 30 to 130 ° C as well the mean electrical conductivity in this temperature range are given in Table 1.

Table 1

In comparison, the pure Bi ₂ S _{3 shows} a Seebeck coefficient of 100 to 160 μV / degree with an electrical conductivity less than 0.01 S / cm.

Claims (8)

Modified bismuth sulfides, characterized in that they have a composition according to the following general formula (1) Bi _x E _y S _z Mg _u (1) where E is germanium and / or silicon and the indices x, y, z and u correspond to one of the following values: x = 1.9 to 2.1, y = 0.001 to 0.08, z = 2.95 to 3.05, u = 0 to 2y. Modified bismuth sulfides according to claim 1, characterized in that the germanium as Mg ₂ Ge and / or the silicon are used as Mg ₂ Si. Modified bismuth sulfides according to claim 1 or 2, characterized in that they are in the temperature range of room temperature up to 130 ° C a Seebeck coefficient of more than 300 μV / degree and an electrical conductivity of more than 150 S / cm. Semiconductor material containing modified bismuth sulfides according to claim 1. Process for producing a semiconductor material according to claim 4, characterized in that the compound by reaction of Mixtures of the respective constituent elements or their alloys at temperatures of at least 800 ° C takes place. Use of a semiconductor material according to claim 4 as active semiconductor in thermoelectric modules. Use of a semiconductor material according to claim 4 as active semiconductor in photovoltaic cells or modules. Use of a semiconductor material according to claim 4 in modules for thermoelectric generators and Pettier arrangements, solar cells and tumble dryer.