WO2014104941A2

WO2014104941A2 - Thermoelectric generator based on samarium sulfide doped with atoms of the lanthanide family and methods of making same

Info

Publication number: WO2014104941A2
Application number: PCT/RU2013/001152
Authority: WO
Inventors: Vladimir Vasil'evich KAMINSKII
Original assignee: Limited Liability Company "Sms Tenzotherm Rus"
Priority date: 2012-12-27
Filing date: 2013-12-23
Publication date: 2014-07-03
Also published as: RU2012157506A; RU2548062C2; WO2014104941A3

Abstract

This invention pertains to a field of conversion of heat energy into electric that by means of semiconductor thermoelectric generators. Gist: Thermoelectric generator comprising at least one layer of semiconducting material Sm_1+xLn_yS based on samarium sulfide SmS doped with atoms of the lanthanide family and current contacts; wherein the aforementioned layer is placed between the said contacts. Technical results of the invention include an increase of generated voltage (thermal electromotive force) to 5 V, increase of maximal electric power to several hundred μW owing to a reduction of the internal resistance in a thermoelectric generator capable of operating without temperature gradient.

Description

THERMOELECTRIC GENERATOR BASED ON SAMARIUM SULFIDE DOPED WITH ATOMS OF THE LANTHANIDE FAMILY AND METHODS OF MAKING

SAME

This invention pertains to a field of conversion of heat energy into electric one and, more specifically, to heat-to-electric energy conversion by means of semiconductor thermoelectric generators. The invention can be practically used in, for example, medicine and nuclear industry.

Thermoelectric generators are used in compact independent electric power suppliers based on conversion of heat energy into electric energy. Thermoelectric generators capable of producing high electric voltage (thermal electromotive force) are in great demand because higher voltage makes it possible to extend a field of applications of such generators. For example, attainment of operating voltage of several volts enables supplying light-emitting diodes and laser diodes as well as various sensors of physical quantities by means of such thermoelectric generator. Simultaneously, enhancement of the voltage output enables increasing a maximal electric power of a device fed by the thermoelectric generator.

Prior art thermoelectric generators based on semiconductor structures, which operating principle relies upon the Seebeck effect (also known as the thermoelectric effect), produce thermal e.m.f. no more than 70 mW per element. Furthermore, for their operation such generators need a considerable temperature gradient between opposite generator ends, which restricts an area of their usage.

Russian Federation Patent No. 2130216 discloses a thermoelectric generator made on basis of a recrystallized semiconductor film of n-type indium antimonide InSb on a mica substrate in form of a monocrystalline matrix comprising inclusions of two-phase system of p- InSb+In. Indium current contacts are placed on one surface of the film. Ends of the film are kept at different temperatures. Because of film inhomogeneity at the matrix-inclusion boundaries a thermal e.m.f. of 10-12 mV is generated in temperature interval of 100-340K. Disadvantages of the prior-art generator are necessity of temperature difference between the film ends and low value of the thermal e.m.f.

Russian Federation Patent No. 2186439 discloses a thermoelectric generator made on basis of a recrystallized semiconductor film of n-type indium antimonide InSb in form of an n- InSb-Si0₂-p-Si heterostructure with contacts placed on the film ends. Because of misfit dislocations and significant difference in workfunctions of contacting materials a thermal e.m.f. of 40-50 mV K is generated in temperature interval from liquid nitrogen to room temperature. Disadvantages of this prior-art generator are necessity of temperature difference between the film ends. In comparison with thermoelectric generators, which operating principle relies upon the Seebeck effect, prior-art thermoelectric generators based on semiconductor structures, which operating principle relies upon the thermovoltaic effect, have the following advantages: such generators do not need a temperature gradient for their operation. Furthermore, they are capable of producing thermal e.m.f. with voltage of about 1 V per element.

A prototype of the invented devices (embodiments) is a thermoelectric generator disclosed in Russian Federation patent No. 2303834 comprising a polycrystalline layer of semiconducting material based on samarium sulfide Sm_1+xS (0<x<0.17) placed between two metallic current contacts, wherein "x" monotonically changes along a direction perpendicular to interfaces of the layer from one current contact to another.

A main disadvantage of the prototype is a moderate value of generated electric voltage (about 1 V) as well as low value of maximal electric power, which can be supplied by this device (several tens of μW).

Russian Federation patent No. 2130216 discloses a method of making a thermoelectric generator involving thermal recrystallization in vacuum of a mica substrate with a layer of indium antimonide to produce a film of n-type InSb which surface thereupon is covered with indium current contacts.

Russian Federation patent No. 2186439 discloses a method of making a thermoelectric generator involving deposition of a polycrystalline film of n-InSb by means of discrete-step evaporation in vacuum at temperature of about 300°C on a substrate of oxidized silicon with subsequent thermal recrystallization thus producing an n-InSb-Si0₂-Si heterostructure, thereupon contacts are soldered to its surface.

A disadvantage of the aforementioned prior-art methods is that a concentration gradient of inclusions, which are responsible for generation of thermal e.m.f., along perpendicular direction can not be created by those methods. This leads to lack of potential difference between boundary interfaces of the film under its heating. As a sequence, a temperature gradient between ends of the film is required to generate thermal e.m.f. Moreover, the aforementioned prior-art methods are not applicable to semiconductor materials based on lantanides.

Russian Federation patent No. 2303834 discloses a method of making a thermoelectric generator involving deposition by means of discrete-step evaporation in vacuum of a polycrystalline film of a semiconducting material based on samarium sulfide SmS out of source powder onto a heated substrate and subsequent connection of current contacts to the film, wherein the substrate temperature during the deposition process is monotonically increases from its initial value to its final value ranging from 250 to 600°C. The aforementioned method is applicable to semiconductor materials based on lanthanides. Moreover, a concentration gradient of excessive atoms of samarium along direction perpendicular to the layer can be created by the method, which makes it possible to generate thermal e.m.f. without a temperature gradient.

A main disadvantage of the said method is impossibility to create by the method a layer (layers) of semiconducting material Sm_1+xLn_yS based on SmS doped with Ln atoms of the lanthanide family. Consequently, the method does not permit controlling values of the concentrations "x" and "y".

A method, which consists with the invented solution in the majority of essential features, thereby referred to as a prototype, has been disclosed in / Didik V.A., Skoryatina E.A., Usacheva V.P., Golubkov AN., Kaminskii V.V. Europium diffusion in single crystal samarium sulfide, Tech. Phys. Lett., 2004, v.30, JN°9, p.756-758. /

The prototype method is characterized by that a monocrystalline boule of samarium sulfide SmS is diced along (100) cleavage planes into plane-parallel wafers. The wafer size is 8 x 5 3 mm. Atoms of europium are applied onto one of the wafer surfaces from an alcoholic solution (¹⁵²Eu radionuclide was used). The sample covered with a layer of europium atoms is placed into a tantalum container, which thereupon is sealed inside a quartz vessel. Diffusion of europium atoms into the samarium sulfide is carried out in vacuum in temperature interval from 950 to 1050°C with duration from 1 to 21 hours. Annealing is performed in a furnace which is capable of providing temperature stability no worse than ± 2°C. During the annealing the temperature is taken by a thermocouple. After the diffusion annealing the vessel is cooled down by air cooling. After that the sample is retrieved out of the vessel.

The prototype method is unable to create layer (layers) of semiconducting materials Sm_1+xLn_yS based on SmS doped with Ln atoms of the lanthanide family as well as it incapable of controlling values of the concentrations "x" and "y". Moreover, the method does not disclose a procedure of making metallic current contacts to the semiconductor material.

The present invention is aimed to develop such a construction and such a method of making a thermoelectric generator being capable of operating without a temperature gradient those make it possible to increase thermoelectric generator voltage (thermal e.m.f.) up to 5 N and to increase maximal electric power up to several hundred μW.

This issue is decided by means of a group of inventions consolidated by a single inventive intension.

As for a generator construction the problem is solved by means of a thermoelectric generator comprising a first and a second metallic current contacts and placed between the said contacts at least one monocrystalline or polycrystalline layer of semiconducting material Sm_1+xLn_yS based on samarium sulfide SmS doped with atoms Ln; wherein Ln refers to any atom of the lanthanide family (i.e. elements of the group III period 6 of the periodic table) with the exception of samarium Sm, wherein a value of at least one of the concentrations "x", "y" in a region joining to the first contact is not equal to a value of the respective concentration in a region joining to the second contact.

Origin of the thermovoltaic effect in samarium sulfide based semiconductor structures are described, e.g., in / Kazanin .M. Kaminskii V.V., Solov'ev S. M. Anomalous thermal electromotive force in samarium monosulfide, Tech. Phys., 2000, v.45, N°5, p.659-661.; Kaminskii V.V., Kazanin M.M. Thermovoltaic effect in thin-film samarium-sulfide-based structures, Tech. Phys. Lett., 2008, v.34, .Nb4, p.361-362. / The thermovoltaic effect in Sml+xS, i.e. samarium sulfide doped with atoms of samarium Sm, is caused by a concentration gradient of samarium Sm atoms located in interstitial positions of the crystal lattice.

The authors of the present invention has discovered that the thermovoltaic effect is also apparent in either samarium sulfide SmS or in Sm_1+xS (samarium sulfide containing excessive atoms of samarium) materials in which a nonuniform concentration of doping atoms Ln of the lanthanide family (i.e. elements of the group III period 6 of the periodic table) is obtained. For example, the authors of the present invention has observed the thermovoltaic effect in such semiconducting materials based on samarium sulfide SmS as SmEu_yS, SmnEu_yS, Sm_! osYb_yS, SmuGd_yS, Sm_{1 15}Ce_yS and other relative materials. By their chemical properties essential for the present invention all the atoms of the lanthanide family can be divided into two groups in accordance with a valence number a given sort of atoms manifests in chemical bonds with sulfur atoms in monosulfides: one group comprises atoms which have a valence of 2 (these include europium atoms Eu and ytterbium atoms Yb); another group comprises atoms which have a valence of 3 (these include other atoms of the lanthanide family, for example, gadolinium atoms Gd and cerium atoms Ce). Samarium atoms can manifest a valence of either 2 or 3 depending on ambient conditions.

The authors of the present invention have revealed a fact, which is essential in view of solving the tasks of the present invention, as follows: doping of Smi_+xS with atoms Ln of the lanthanide family makes it possible to control material characteristics, such as a bandgap enegry, a conductivity, an activation energy of the impurity level, etc., affecting device characteristics of a thermoelectric generator, such as a value of thermal e.m.f. and an internal resistance of the generator. Thus an intentional incorporation of dopants and formation of a semiconducting material of Sm_1+xLn_yS can provide, in comparison with Smi_+xS, an increase of generated voltage and increase of maximal electric power.

The problem is solved by means of the thermoelectric generator, wherein the doping atoms are either gadolinium Gd or cerium Ce, i.e. atoms behaving in chemical bonds with sulfur atoms as elements of valence of 3. Thereby, the thermoelectric generator of claim 2 exploits the semiconducting material of either Smi_+xGd_yS or Smi_+xCe_yS. The authors of the present invention have discovered that a value of internal resistance of a thermoelectric generator based on Smi_+xS, wherein the concentration "x" does not exceed 0.2, decreases as atoms of the aforementioned sort are incorporated into Smi_+xS. In its turn, smaller internal resistance makes it possible to increase a maximal value of electric power, which can be generated by the thermoelectric generator. The discovered phenomenon is presumably associated with the fact, that an additional electron, which appears in the conduction band of samarium sulfide as a trivalent atom is incorporated into it, increases a conductivity of the semiconducting alloy as compared to the initial Sm_1+xS material.

The maximal value of concentration "y" of the doping atoms in the thermoelectric generator of claim 2 is 0.15 being limited by the fact that in alloys with a higher concentration "y" of the doping atoms all the ions of samarium Sm turn into their trivalent state so that the thermovoltaic effect is no longer possible (the thermovoltaic effect is caused by a transition of samarium ion from the bivalent state to the trivalent state).

The above-stated improvement of the operating characteristics of the thermoelectric generator is likely to be found using other sorts of the lanthanides which manifests a valence of 3 in monosulfides.

Furthermore the problem is solved by means of the thermoelectric generator, wherein the doping atoms are either europium Eu or ytterbium Yb, i.e. atoms which have a valence of 2 in chemical bonds with sulfur atoms. Thereby, the thermoelectric generator of claim 3 exploits the semiconducting material of either Sm_1+xEu_yS or Sm_1+xYb_yS. The authors of the present invention have discovered that a value of thermal e.m.f. of a thermoelectric generator based on Sm_1+xS, wherein the concentration "x" does not exceed 0.2, increases as atoms of the aforementioned sort are incorporated into Sm_1+xS. The discovered phenomenon is presumably caused by an increase in energy gap between the 4f-levels and the conductance band edge, i.e. the gap which acts as a forbidden band gap in the forming semiconductor alloy. In particular, the 4f-levels of samarium ions in samarium sulfide are separated by 0.23 eV from the conductance band edge, whereas the 4f-levels of europium ions in europium sulfide EuS are separated by about 1.7 eV from the conductance band edge.

An additional advantage of the thermoelectric generator is that samarium and europium atoms have similar atomic radii. This makes it possible to avoid mechanical strain in a semiconductor structure based on samarium sulfide doped with europium atoms which results, in the long term, in its operational mechanic reliability and stability.

The maximal value of concentration "y" of the doping atoms in the thermoelectric generator of claim 3 is 0.2 being determined by their solubility limit in Sm_1+xS with concentration "x" not exceeding 0.2. Thereby, the thermoelectric generator of claim 3 is characterized by the absence of undissolved metallic fraction of dopants that has a positively impact on its mechanic reliability and stability as well as on thermoelectric performance.

Thermoelectric generator may comprise a single layer of semiconducting material Sm_1+xLn_yS within which a value of at least one of the concentrations "x", "y" monotonically increases along a direction from the first contact towards the second contact.

Thermoelectric generator may also comprise at least two successively arranged layers of semiconducting material Sm_1+xLn_yS, wherein a value of at least one of the concentrations "x", "y" monotonically increases from one layer to another layer along a direction from the first contact towards the second contact, whereas the respective concentration remains approximately constant within each of the said layers.

As for a method of making the generator, the problem is solved by means of formation of a plane-parallel wafer by dicing a monocrystalline boule of samarium sulfide SmS along (100) cleavage planes. The wafer size is in the range of 8...12 mm in length, 3...5 mm in width, 1...5 mm in height. The original monocrystalline boule of samarium sulfide can be grown by one of known methods as described, e.g., in / Golubkov A.V., Goncharova E.I., Juse V.P. et al, Physical properties of chalcogenides of rare earth metals. Leningrad: Nauka, 1973. 304p. / Salts comprising samarium Sm atoms and Ln atoms of the lanthanide family are dissolved in a solvent, e.g. in alcohol, and then the solution is applied into the upper surface of the wafer.

Then the wafer coated with a layer of excessive atoms of samarium and atoms Ln of the lanthanide family is placed into a tantalum container, which thereupon is sealed inside a quartz vessel. Diffusion of the said atoms deep into the wafer is fulfilled in vacuum by means of high- temperature annealing. Temperature and duration of the high-temperature annealing are selected so that a concentration of the doping Ln atoms of the lanthanide family, or a concentration of the excessive samarium atoms, or concentrations of both the doping Ln atoms of the lanthanide family and the excessive samarium atoms near the upper surface of the wafer is higher than a concentration(s) of the respective sort of atoms near the lower surface of the wafer.

During the annealing the temperature is measured by a thermocouple, e.g. by a platinum- rhodium/platinum thermocouple. After the diffusion annealing the vessel is cooled down by air cooling. After that the sample is retrieved out of the vessel. After that metallic current contacts are deposited onto the upper and the lower surfaces of the wafer by means of one of known methods, e.g. nickel contacts are formed by the method of resistive heating evaporation.

The task is solved by means of synthesis of at least two layers of Sm_1+xLn_yS, wherein Ln refers to any atom of the lanthanide family (with the exception of samarium Sm). The synthesis is fulfilled by mixing SmS, Sm and Ln powders taken in a proportion required for achieving preselected atomic concentrations "x" and "y" in a given layer; briquetting the mixture followed by its annealing, e.g. in a quartz vessel. The layer size is in the range of 8...12 mm in length, 3...5 mm in width, 0.05...1 mm in height. At least two layers of Smi_+xLn_yS are formed by the said method, wherein the layers differ from each other in a concentration "y" of the doping Ln atoms, or a concentration "x" of the excessive samarium atoms Sm, or both a concentration "y" of the doping Ln atoms and a concentration "x" of the excessive samarium atoms Sm.

At least two of the layers formed by the aforementioned method are joined to a stack in such manner that a value of the concentration "y" of the doping atoms Ln, or a value of the concentration "x" of the excessive samarium atoms Sm, or values of both the concentration "y" of the doping atoms Ln and the concentration "x" of the excessive samarium atoms Sm monotonically increases from one layer to another layer. The stack is 0.1...5 mm thick. After that the stack is sintered in vacuum or in inert gas environment. As the stack is cooled down, metallic current contacts are formed by one of the known methods (e.g. nickel contacts formed by the method of resistive heating evaporation) onto opposite surfaces of the stack along a direction of variation of the value of the concentration "y" of the doping atoms Ln, or the value of the concentration "x" of the excessive samarium atoms Sm, or the values of both the concentration "y" of the doping atoms Ln and the concentration "x" of the excessive samarium atoms Sm.

By means of deposition by flash evaporation in vacuum onto a substrate with a metallic surface a layer sequence comprising at least two successively arranged polycrystalline layers of Sm_1+xLn_yS semiconducting material, wherein Ln refers to any atom of the lanthanide family (with the exception of samarium Sm). And deposition regimes are varied from one layer to another in such manner that a value of the concentration "y" of the doping atoms Ln, or a value of the concentration "x" of the excessive samarium atoms Sm, or values of both the concentration "y" of the doping atoms Ln and the concentration "x" of the excessive samarium atoms Sm monotonically varies from one layer to another layer. As source materials for the deposition of the said layers a powder comprising samarium sulfide SmS, samarium Sm and lanthanide Ln atoms taken in a required proportion is used. Thus, for successive deposition of N layers of Sm_1+xLn_yS just N mixtures with different content of SmS, Sm and Ln are to be used. A metallic wafer, e.g. of nickel, can be used as the substrate. A dielectric wafer, e.g. of polycore, can be used as the substrate as well provided that one surface of the wafer is precoated by means of a known method, e.g. by the method of resistive heating deposition, with a metallic layer, e.g. of nickel. The metallic surface of the substrate acts simultaneously as a first current contact. A second metallic contact is deposited by a known method, e.g. by the method of resistive heating deposition, onto the last of the stacked Smi_+xLn_yS layers.

The described methods can be used for making a thermoelectric generator with a total thickness of Sm_1+xLn_yS semiconducting material being in the range of 0.1 ...10 μηι or for making a thermoelectric generator with a total thickness of Sm_1+xLn_yS semiconducting material being in the range of 0.1...5 mm.

The authors of the present invention have discovered that an increase in the total thickness of Smi_+xLn_yS semiconducting material leads to increase of a maximal current which can be generated by the thermoelectric generator. As the same time, an increase in the total thickness of Sm_1+xLn_yS semiconducting material leads to decrease of thermal e.m.f. Thus the methods proposed by the authors of the present invention, as described in claims from 6 through 8, enable varying the total thickness of the semiconducting layer within the range of 0.1 μηι - 5 mm and thus make it possible to choose optimally values of thermal e.m.f. and maximal current. In its turn, this makes it possible to increase maximal electric power which can be provided by the thermoelectric generator.

Atoms of gadolinium Gd, atoms of cerium Ce, as well as other atoms of the lanthanide family manifesting a valence of 3 in chemical bonds with sulfur atoms can be used as the doping impurities for making a thermoelectric generator by means of the aforementioned methods. In addition, atoms of europium Eu or atoms of ytterbium Yb can also be used for making a thermoelectric generator by means of the aforementioned methods. Preferred usage of the said types of the dopants is described above when describing different constructions of the thermoelectric generator.

The invention is illustrated by following drawings:

Fig. 1 shows an embodiment of the invented thermoelectric generator;

Fig. 2 shows another embodiment of the invented thermoelectric generator;

Fig. 3 shows still another embodiment of the invented thermoelectric generator;

Fig. 4 shows dependence of output voltage on "y" concentration in operating material of SmS/Sm_)+xGd_yS for x=0.02;

Fig. 5 shows schematic of a thin-film structure based on SmEuo_.nS/SmuS.

Fig. 6 shows dependence of Gd concentration (curve 1) and generated voltage (thermal e.m.f.) (curve 2) on depth in process of thinning SmGd_yS/SmS structure.

Fig. 7 shows temporal variation of generated voltage (curve 1) and temperature (curve 2) for SmGd_yS/SmS structure.

Fig. 8 shows temporal variation of generated voltage (curve 1) and temperature (curve 2) for SmS/Smi_.03Eu_0.iS structure. Fig. 9 shows dependence of maximal generated voltage at T=235°C on "y" concentration in SmS/Sm_{1 03}Eu_yS structure.

The thermoelectric generator of claim 4 shown in Fig. 1 can be fabricated in accordance with the method of claim 6. The generator comprises a first metallic current contact 1, a second metallic current contact 2, and placed between the contacts a single layer 3 of Sm_1+xLn_yS having monotonically increasing concentrations "x" 4 of excessive atoms of Sm 5 and "y" 6 of doping atoms Ln 7.

The thermoelectric generator of claim 5 shown in Fig. 2 can be fabricated in accordance with the method of claim 7. The generator comprises a first metallic current contact 1, a second metallic current contact 2, and placed between the contacts N > 2 layers 3.1, 3.2...3.N of Sm_1+xLn_yS forming a stack 8, wherein in each of the layers concentrations "x" 4 of excessive atoms of Sm 5 and "y" 6 of doping atoms Ln 7 are nearly constant and monotonically increase from layer to layer along direction from the first current contact 1 towards the second current contact 2.

The thermoelectric generator of claim 5 shown in Fig. 3 can be fabricated in accordance with the method of claim 8. The generator comprises a first metallic current contact 1 made in form of a metallic layer deposited onto a dielectric substrate 9, a second metallic current contact 2, and placed between the contacts N > 2 layers 3.1, 3.2...3.N of Sm_1+xLn_yS forming a sequence 8, wherein in each of the layers concentrations "x" 4 of excessive atoms of Sm 5 and "y" 6 of doping atoms Ln 7 are nearly constant and monotonically increase from layer to layer along direction from the first current contact 1 towards the second current contact 2.

Elements-making capability is proved by the following examples.

Example 1

A powder of SmS has been synthesized from elementary substances of Sm and S. Then mixtures have been briquetted comprising the powder of SmS as well as powders of SmS, Sm and Gd taken in proportions, which corresponds to different compositions of Sm_1+xGd_yS, where x=0.02, y=0.03; x=0.02, y=0.06 (2 samples); x=0.02, y=0.08 (2 samples); x-0.02, y=0.09 (3 samples); x=0.02, y=0.13; x=0.02, y=0.28 (2 samples); x=0.02, y=0.38. The prepared briquettes have been annealed at a temperature of 1600°C for 30 minutes. Then each sample of Smi_+xGd_yS has been sintered together with one of the samples of SmS at a temperature of 1300°C for 1 hour. The annealing and sintering procedures have been performed in a molybdenum crucible pumped down to 10^"4 Torr and sealed under such vacuum condition. As a result, 12 double samples (stacks) of different SmS/Smi_+xGd_yS compositions have been fabricated. The material composition at opposite surfaces of the fabricated stack has been measured by means of X-ray diffraction analysis and been found to correspond to the predetermined concentrations of "x" and "y" with an accuracy of not worse than ±0,01.

Contact pads of nickel have been prepared by means of deposition in vacuum onto opposite surfaces of the prepared structures. The contact pads have been connected to output wires, which have been used for acquisition of the output signal. During experiments the structures have been subjected to heating in an electrically heated furnace of a resistive kind. The output signal and the temperature signal measured by a chromel-alumel thermocouple have been detected by an analog-to-binary converter of a personal computer. Maximal values of the output voltage acquired at 230°C are presented in Fig. 4 as a function of composition "y" in the sample. As it follows from the data presented in Fig. 4 the gadolinium concentration affects the value of the output signal under generation of electric voltage. And the voltage increases as "y" decreases below 0.15.

Example 2

A 0.2-μπι thick layer of nickel acting as a first current contact has been deposited onto a substrate of polycore (A1₂0₃) in vacuum of 10^"5 Torr. On top of the nickel layer a 0.35-μηι thick layer of SmEu_0.i₄S followed by a 0.4-μηι thick layer of SmuS have been deposited by flash evaporation in vacuum. As a furnace charge (source materials) for the deposition of the said layers a mixture of SmS, Sm and Eu powders taken in a required mass proportion has been used. The method of flash evaporation has been realized as follows. The charge in form of powder mixtures of SmS and Eu or SmS and Sm has been loaded into a vibrating hopper. During the deposition process the said mixtures (in the following sequence: first SmS and Eu, then SmS and Sm) has gradually poured out of the hopper onto a tantalum boat heated to a temperature of about 2500 °C by an electric current flowing through the boat. As soon as a grain of the charge has fallen onto the boat, the grain has immediately evaporated (flashed out) and the vapors have deposited onto the substrate heated to 475 °C

A 0.27-μπι thick nickel contact has been then formed on top of an ending surface (Smi.iS) by resistive heating evaporation. The formed structure is schematically presented in Fig. 5, where 1 denote a polycore substrate, 2, 5 - metallic contacts (Ni), 3 - SmEu_{0 1}4S layer, 4 - Smi_.iS layer.

Layer thicknesses have been measured by means of a MII-4 microinterferometer. Layer composition of semiconducting materials has been verified by means of a SEM JEOL JSM6610 scanning electron microscope equipped with a spectrometer. Wires have been contacted to the fabricated thermoelectric generator by means of pressed contacts: one to the nickel layer (2) on polycore, another to the nickel layer (5) on surface of SmuS layer (4), see Fig. 5.

Under operation test of the device the substrate of the structure has been placed onto a bulky copper plate heated with an electrically heated furnace of a resistive kind. Temperature of the copper plate and the substrate has been measured by a copper-constantan thermocouple fastened in the copper plate in such a manner that the thermojunction has touched the substrate. Signals acquired from the current contacts and from the thermocouple have been detected by an analog-to-binary converter of a personal computer during heating and cooling steps. Under operation test the structure and the heater have been placed in a reservoir pumped down to 10^"2 Torr. The structure has been heated in a temperature range from 20 to 170 °C. Generation of thermal e.m.f. has started at 160 °C during the heating step and has been finished at 95 °C during the cooling step. A voltage generated by the device has been measured to be 5.1 V.

Example 3.

A 1-mm-thick layer of Gd has been applied (fused) onto a 2.92 -mm thick polycrystalline sample of SmS. Then the sample has been annealed for 30 min. at a temperature T of 1240°C. As a result of diffusion of Sm and S atoms into the Gd layer as well as atoms of Gd into the SmS layer, a SmGd_yS layer with a gradient compositional profile has been formed. Curve 1 in Fig. 6 shows a dependence of Gd concentration (y) on depth in process of thinning the SmGd_yS/SmS structure. The concentration profile has been determined by means of step-by-step polishing of the sample with an abrasive paper; after that X-ray diffraction measurement of chemical composition of a formed surface has been performed. X-ray diffraction measurements have been carried out with a DRON-2 diffractometer.

During experiments the structure has been subjected to heating with in an electrically heated furnace of a resistive kind. An output signal (thermal e.m.f.) and a temperature, which has been measured by a chromel-alumel thermocouple, have been detected by an analog-to-binary converter of a personal computer. Such measurements have been performed repeatedly at each step of the sample thinning. Typical temporal variations as acquired in such experiment are presented in Fig. 7. Curve 1 corresponds to a dependence of the output voltage on time, curve 2 corresponds to a dependence of the sample temperature on time. A dip in curve 1 reflects a process of electric power measurement. This measurement has been performed as follows. A load resistor RL has been connected in parallel to the sample and a voltage U on the load resistor has been detected. Then an electric power P, which the tested thermoelectric generator can provide, has been calculated using an equation P=U²/RL. The resistance of the load resistor was 1 Ohm. At a temperature of 245°C a maximal electric power of 430 μ\ν has been achieved. Curve 2 in Fig. 6 shows a dependence of the maximal electric power on depth in process of thinning the SmGd_yS/SmS structure. As it follows from the data presented, the electric signal, which is generated by the structure under heating, reaches its peak value of 430 μ^^" as a small amount of Gd is incorporated into SmS.

Example 4

A powder of SmS has been synthesized from elementary substances of Sm and S. Then mixtures have been briquetted comprising the powder of SmS as well as powders of SmS, Sm and Eu taken in proportions, which corresponds to different compositions of Sm_1+xEu_yS, where x=0.03, y=0.05; x=0.03, y=0.07; x=0.03, y=0.1 ; x=0.03, y=0.15; x=0.03, y=0.2; x=0.03, y=0.25; x=0.03, y=0.3; x=0.03, y=0.33. The prepared briquettes have been annealed at a temperature of 1600°C for 30 minutes. Then each sample of Sm_1+xEu_yS has been sintered together with one of the samples of SmS at a temperature of 1250°C for 1 hour. The annealing and sintering procedures have been performed in a molybdenum crucible pumped down to 10^"4 Torr and sealed under such vacuum condition. As a result, 8 double samples (stacks) of different SmS/Sm_1+xEu_yS compositions have been fabricated. The material composition at opposite surfaces of the fabricated stack has been measured by means of a SEM JEOL JSM6610 scanning electron microscope equipped with a spectrometer. It should be noted that metallic Eu precipitates have been observed if y>0.2.

Contact pads of nickel have been prepared by means of deposition in vacuum onto opposite surfaces of the prepared structures. The contact pads have been connected to output wires, which have been used for acquisition of the output signal. During experiments the structures have been subjected to heating in an electrically heated furnace of a resistive kind. The output signal and the temperature signal measured by a chromel-alumel thermocouple have been detected by an analog-to-binary converter of a personal computer. Typical temporal variations of the output signal measured for SmS/Srri_LMEuo_.iS and the temperature are presented in Fig. 8. Maximal output voltage at T=235°C, as it is determined from dependences like shown in Fig. 8, is shown in Fig. 9 as a function of concentration "y" in the sample.

As it follows from the data presented in Fig. 9 the europium concentration affects the value of the output signal under generation of electric voltage. And the voltage reaches its maximal value at "y" below 0.2. In addition to the voltage, an electric power has been measured for the prepared SmS/Smi_+xEu_yS samples. This measurement has been performed as follows. A load resistor L has been connected in parallel to the sample and a voltage U on the load resistor has been detected. Then an electric power P, which the tested thermoelectric generator can provide, has been calculated using an equation P=U'^: RL. The peak value of the power of 510 μ\ν has been found for SmS/Sm_{1 0}3Euo.iS sample.

Thus, advantages of the proposed group of inventive solutions have been demonstrated to include possibility of a voltage increase (thermal e.m.f.) to 5 V and a maximal electric power enhancement to several hundred μ\ν owing to a reduction of the internal resistance in a thermoelectric generator capable of operating without temperature gradient.

Claims

1. The thermoelectric generator comprising at least one layer of semiconducting material Sm_1+xLn_yS based on samarium sulfide SmS doped with atoms Ln and current contacts, wherein the said layer is placed between the current contacts.

2. The thermoelectric generator of claim 1, wherein Ln atom of the lanthanide family is either gadolinium Gd or cerium Ce; wherein a value of the concentration "y" of the said sort of atoms does not exceed 0.15; wherein a value of the concentration "x" of the excessive atoms of samarium Sm does not exceed 0.2.

3. The thermoelectric generator of claim 1, wherein Ln atom of the lanthanide family is either europium Eu or ytterbium Yb; wherein a value of the concentration "y" of the said sort of atoms does not exceed 0.2; wherein a value of the concentration "x" of the excessive atoms of samarium Sm does not exceed 0.2.

4. The thermoelectric generator of claim 1 comprising a single layer of semiconducting material Sm_1+xLn_yS within which a value of at least one of the concentrations "x", "y" monotonically increases along a direction from the first contact towards the second contact.

5. The thermoelectric generator of claim 1 comprising at least two successively arranged layers of semiconducting material Sm_1+xLn_yS, wherein a value of at least one of the concentrations "x", "y" monotonically increases from one layer to another layer along a direction from the first contact towards the second contact, whereas the respective concentration remains approximately constant within each of the said layers.

6. A method of making a thermoelectric generator involving: formation of a plane-parallel wafer by dicing a monocrystalline boule of samarium sulfide SmS; coating an upper surface of the wafer with a solution containing excessive atoms of samarium and atoms of the lanthanide family; diffusion of the said atoms deep into the wafer by means of high- temperature annealing in vacuum in such manner that after the said annealing a value of concentration of at least one sort of the said atoms near the upper surface of the wafer is not equal to a value of the respective concentration near the lower surface of the wafer; deposition of metallic current contacts onto the upper and the lower surfaces of the wafer.

7. The method of making a thermoelectric generator of claim 6, wherein Ln atom of the lanthanide family is gadolinium Gd atoms, which concentration does not exceed 0.15, or cerium Ce atoms, which concentration does not exceed 0.15, or europium Eu atoms, which concentration does not exceed 0.2; whereas concentration of the excessive atoms of samarium Sm does not exceed 0.2.

8. A method of making a thermoelectric generator involving: synthesis of layers of materials having composition Smi_+xLn_yS, wherein Ln refers to any atom of the lanthanide family with the exception of samarium Sm, by mixing SmS, Sm and Ln powders taken in a required proportion; briquetting and annealing the mixture; joining at least two of the said layers to a stack in such manner that at least one of the concentrations "x", "y" monotonically increases from one layer to another layer; sintering the stack in vacuum or in inert gas environment; deposition of metallic current contacts onto two opposite surfaces of the stack along a direction of variation of the said atomic concentration.

9. The method of making a thermoelectric generator of claim 8, wherein Ln atom of the lanthanide family is gadolinium Gd atoms, which concentration does not exceed 0.15, or cerium Ce atoms, which concentration does not exceed 0.15, or europium Eu atoms, which concentration does not exceed 0.2, or ytterbium Yb atoms, which concentration does not exceed 0.2; whereas concentration of the excessive atoms of samarium Sm does not exceed 0.2.

10. A method of making a thermoelectric generator involving: deposition by means of flash evaporation in vacuum onto a substrate with a metallic surface, that acts at the same time as a first current contact, a sequence comprising at least two successively arranged polycrystalline layers of semiconducting material Sm_1+xLn_yS, wherein Ln refers to any atom of the lanthanide family with the exception of samarium Sm, wherein at least one of the concentrations "x", "y" monotonically increases from one layer to another layer; joining a second metallic current contact to the last of the said layers; wherein powders of samarium sulfide SmS, samarium Sm and Ln taken in the required proportion are used as source materials for the said deposition.

1 1. The method of making a thermoelectric generator of claim 10, wherein Ln atom of the lanthanide family is gadolinium Gd atoms, which concentration does not exceed 0.15, or cerium Ce atoms, which concentration does not exceed 0.15, or europium Eu atoms, which concentration does not exceed 0.2, or ytterbium Yb atoms, which concentration does not exceed 0.2; whereas concentration of the excessive atoms of samarium Sm does not exceed 0.2.