Patent Application
20130263907
The present application claims priority from Japanese patent application JP 2012-089104 filed on Apr. 10, 2012 the content of which is hereby incorporated by reference into this application.
The present invention relates to a thermoelectric conversion material, a thermoelectric conversion device, and a thermoelectric conversion module.
Currently, there is a demand for actively utilizing natural energy such as sunlight, wind power, and geothermal heat, which is not accompanied by generation of greenhouse effect gas without depending on fossil fuel due to the environment and energy problem or resource depletion. Sunlight generation or wind power generation having a low environment load is spreading, and an effective utilization of heat energy is receiving attention. In practice, heat energy is emitted in a great quantity from an incineration plant, a subway, or an electric power substation around us. A temperature of waste heat emitted from the incineration plant or the like is 300 to 600° C., which is high, and a temperature of waste heat from the subway or the electric power substation is 40 to 80° C., which is low. The total quantity of waste heat energy having the relatively low temperature (200° C. or less) is great, but an effective energy recovery technology is not established. A thermoelectric conversion device has been known from old times as one of utilization methods of waste heat energy. In thermoelectric conversion, electricity is directly generated due to a temperature difference without a driving portion, and thus a loss is small as compared to a method of generating electricity by generating steam from heat of thermal power or atomic power to rotate a turbine. Further, since wastes are not generated, the thermoelectric conversion is environmentally friendly. Further, if a voltage is applied to both ends of a thermoelectric conversion device, a temperature difference occurs, and a Seebeck effect of the thermoelectric conversion, which was found in 1821, is obtained, but there is a problem in that conversion efficiency is low. Currently, Bi2Te3 is commercialized as a thermoelectric conversion material having relatively good efficiency at a temperature of 200° C. or less. Further, the thermoelectric conversion material such as Bi—Te, which has good conversion efficiency at around room temperature, is a Peltier device, and can be used as a cooling device and can be used in a cooling apparatus having a small environment load which does not use a cooling medium.
Performance of the thermoelectric conversion material is evaluated by a dimensionless performance index (ZT).
Herein, σ is electric conductivity, S is a Seebeck coefficient, κ is thermal conductivity, and T is a temperature. A unit of Z itself is K−1.
A thermoelectric conversion material including Fe2VAl that is a Full-Heusler alloy as a base material has been reported as a material system having a low environment load in Japanese Patent Application Laid-Open (JP-A) No. 2004-253618. Fe2VAl is constituted by elements having a low environment load and a relatively low cost, such as Fe, V, and Al, and toxic rare metal is not used unlike the Bi—Te-based material, and thus Fe2VAl is a material system having industrial applicable value. However, a thermoelectric conversion characteristic surpassing that of the Bi—Te system at the temperature range of 200° C. or less is not obtained, and thus further researches and developments subsequently are needed.
Further, a CdI2 type material having a laminate structure including particularly Ti has been reported as a thermoelectric material having excellent conversion efficiency in JP-A-2002-270907. JP-A-2002-270907 shows a material having the same structure as a crystal structure of TiS2 and high thermoelectric conversion efficiency at the temperature range around room temperature, which are results from an n type, but does not show a p-type material system having good efficiency. Accordingly, there is a demand for a material system having a low environment load and low costs, exhibiting a p type and having high thermoelectric conversion efficiency.
Further, sulfides of transition metal such as Fe or Ni have been reported as a thermoelectric conversion material, which enables a low environment load and a reduction in cost, in IEEE 22nd International Conference on Thermoelectrics pp. 376-379 (2003). However, since a kind, concentration dependency, or a carrier density of an element doped in sulfides of transition metal is not controlled in IEEE 22nd International Conference on Thermoelectrics pp. 376-379 (2003), an optimum doping element needs to be selected and the carrier density needs to be controlled in order to exhibit a high thermoelectric conversion characteristic.
Henceforth, it is considered that environment and energy problems become more important and transition to a clean power generation system not depending on fossil fuel is performed. Above all, there is a need to utilize an energy source rarely used until now, such as geothermal heat or waste heat. However, a thermoelectric conversion device using toxic rare metal, such as a Bi—Te system, commercialized as a thermoelectric conversion material having relatively low temperatures (200° C. or less), cannot be cheaply and stably provided to the market in a great amount, and thus a possibility of extensively spreading the thermoelectric conversion element is generally low. Further, in the material system described in JP-A-2004-253618, JP-A-2002-270907, or IEEE 22nd International Conference on Thermoelectrics 376 (2003), it is difficult to allow a low environment load, a reduction in cost, a high Seebeck coefficient, and a high p-type carrier density to be compatible.
The present invention has been made in an effort to provide a p-type thermoelectric conversion material in which a low environment load and a reduction in cost are feasible and a high Seebeck coefficient and a high carrier density are compatible, and a thermoelectric conversion device and a thermoelectric conversion module having high conversion efficiency.
An embodiment of the present invention for accomplishing the object provides a thermoelectric conversion material having a pyrite structure, in which a composition is represented by Fe1-xMxS2-yTy, element M is at least one kind of element selected from V, Cr, Mn, Zr, Nb, Mo, Hf, Ta, and W, element T is at least one kind of element selected from B, C, Al, Si, Ge, Sn, N, O, P, and Bi, x and y that are total composition values of the individual elements are each in the range of 0<x<0.5 and 0<y<1, and a conductive type is a p type.
Further, another embodiment of the present invention provides a thermoelectric conversion device including a thermoelectric conversion material layer, and a first upper electrode and a first lower electrode installed by interposing the thermoelectric conversion material layer therebetween, in which the thermoelectric conversion material layer is a p-type material layer having a pyrite structure whose composition is represented by FeS2, and including at least a portion of Fe and S which is substituted by an addition element.
Further, yet another embodiment of the present invention provides a thermoelectric conversion module where a plurality of p-type thermoelectric conversion material layers spaced apart from each other and a plurality of n-type thermoelectric conversion material layers spaced apart from each other, which are arranged to be adjacent to each other, on an insulating substrate are connected in series, in which the p-type thermoelectric conversion material layer is a p-type material layer having a pyrite structure whose composition is represented by FeS2 and including at least a portion of Fe and S which is substituted by an addition element.
There may be provided a p-type thermoelectric conversion material in which a low environment load and a reduction in cost are feasible and a high Seebeck coefficient and a high carrier density are compatible, and a thermoelectric conversion device and a thermoelectric conversion module having high conversion efficiency.
The present inventors examined a material system in which a low environment load, a reduction in cost, a high Seebeck coefficient, and a high p-type carrier density can be compatible. Hereinafter, the results and the obtained opinions will be described.
High thermoelectromotive force depends on an electron state of a material, and a material having a rapid change in state density around a Fermi level is preferable. Since the material having the large change in state density needs to be in a localized electron state, a material system where electrons at a d orbit contribute to an electron state around the Fermi level, such as transition metal, becomes a candidate.
Examples of cheap and non-toxic transition metal may include iron (Fe). If the material system has the material system where a state derived from 3d of Fe is around the Fermi level as a mother phase, a large amount of earth-crust deposits exists, and it is possible to manufacture the thermoelectric conversion material having a low environment load. Accordingly, FeS2 of the pyrite structure was conceived.
Next, a lower drawing of
Temperature dependency of the Seebeck coefficient at each hole carrier density obtained by calculation is illustrated in
Appropriate doping needs to be performed in order to control the carrier density to the aforementioned carrier density, and a valence electron number density (VEC) can be used as a design guideline. The total valence electron number VEC in a stoichiometric composition of FeS2 is obtained from the valence electron number nFe=8 of Fe and the valence electron number ns=6 of S so that VEC=nFe+2ns=20. Further, it is possible to control the VEC by forming Fe1-xMxS2-yTy into which M and T that are elements different from Fe and S are introduced. The valence electron number of each element is described in Table 1.
It is possible to express a p-type property by setting the VEC to 20 or less. A relationship between a change in VEC (ΔVEC) and the Seebeck coefficient at room temperature obtained by the calculation is illustrated in
Next, elements to be added to these pyrites will be described. It is possible to control the Fermi level by adjusting the VEC as described above. In Table 1, V, Cr, and Mn that are 3d transition metal have the valence electron number that is smaller than that of Fe, and thus are effective to substitute Fe and reduce the VEC. It is known that MnS2 has the pyrite structure, and thus Mn is easily substituted by Fe. Since a difference in valence electron number from Fe is increased in order of Mn, Cr, and V, the smaller the difference in valence electron number of the element from Fe is, the better the element is. In Table 1, likewise, Zr, Nb, and Mo that are 4d transition metals have the valence electron number that is smaller than that of Fe, and thus are effective in reducing the VEC.
Further, since a difference in valence electron number from Fe is increased in order of Mo, Nb, and Zr, the smaller the difference in valence electron number of the element from Fe is, the better the element is. In addition, 4d transition metals have masses that are largely different from that of Fe, and thus is effective in controlling the VEC and reducing thermal conductivity. In Table 1, likewise, Hf, Ta, and W that are 5d transition metals have the valence electron number that is smaller than that of Fe, and thus are effective in reducing the VEC. Further, since a difference in valence electron number from Fe is increased in order of W, Ta, and Hf, the smaller the difference in valence electron number of the element from Fe is, the better the element is. In addition, the 5d transition metals have masses that are largely different from that of Fe, and thus has an effect of reducing thermal conductivity, which is higher than the effect of the 4d transition metal. Next, doping of a typical element will be described. FeS2 can control the VEC by substituting S as well as Fe. Among the typical elements, examples of the relatively cheap and non-toxic or low-toxic elements may include B, C, N, O, Al, Si, P, Ge, and Sn. Among the examples, B, C, N, and O can control the VEC according to the valence electron number, but have atomic radiuses that are largely different from that of S, and thus cannot be added in a great amount. However, B, C, N, and O are light elements having the masses that are largely different from that of S, and thus can be expected to have an effect of inhibiting lattice thermal conductivity. Since a difference in valence electron number from S is increased in order of P, Si, and Al, the smaller the difference in valence electron number of the element from S is, the better the element is. Sn has the mass that is larger than that of S, and thus can be expected to have an effect of reducing lattice thermal conductivity. Further, Bi is a relatively rare material, but is the heaviest element in Table 1 and has the valence electron number that is different from that of S by only 1, and thus is effective to control the VEC and reduce lattice thermal conductivity. Further, since pyrite has low thermal stability at high temperatures, there is a high possibility of decomposing the pyrite by FeS and S gases of the NiAs structure at 750° C. or more, and thus it is preferable to use the pyrite under an environment of 700° C. or less. In addition, it is preferable to use the pyrite at room temperature or more in view of thermoelectric conversion efficiency.
The crystal structure of the thermoelectric conversion material having the pyrite structure may be easily confirmed by an X-ray diffraction (XRD). Further, a lattice image may be observed by an electron microscope such as a TEM (transmission electron microscope), or a monocrystal or a polycrystal crystal structure may be confirmed from a spot type pattern or a ring type pattern in an electronic beam diffraction phase. The composition distribution may be confirmed by using an EPMA (electron probe microanalyzer) such as an EDX (energy dispersive X-ray spectroscopy), a SIMS (secondary ionization mass spectrometer), or methods such as X-ray photoelectron spectroscopy and ICP (inductively coupled plasma). In addition, information on state density of the material may be confirmed by a UV photoelectron spectroscopy, the X-ray photoelectron spectroscopy, or the like. The electric conductivity and the carrier density may be confirmed by electric measurement and hole effect measurement using a four-point-probe method. The Seebeck coefficient may be confirmed by setting a temperature difference at both ends of a sample and measuring a voltage difference between both ends. Thermal conductivity may be confirmed by a laser flash method.
The present invention is deduced from the examination results and new opinions obtained therefrom, and is characterized in that an excellent thermoelectric conversion characteristic is expressed by performing appropriate doping on the compound having the pyrite structure to introduce elements having different valence electron numbers and masses, and controlling the carrier density, electric conductivity, and thermal conductivity. Specific examples thereof include a material system where FeS2 of the pyrite structure having Fe and S as main components is used as a main component.
According to the present invention, it is possible to change an electron occupying number of a d-band, modulate an electron state at a Fermi level, and adjust a carrier type and carrier density according to the purpose by adding elements having different valence electron numbers by doping and controlling a valence electron number of a compound. Further, it is possible to reduce thermal conductivity by doping a lighter element and a heavier element than Fe and S.
Hereinafter, the embodiments will be described.
A first embodiment illustrates an example of sample preparation. Herein, the Preparation Example is just an example, and, of course, is not limited to the corresponding preparation condition.
After metal Fe powder, Mn powder, and S powder having purity of 99.9% were mixed at a compositional ratio of 99:1:200 and the alloy was prepared by a known mechanical alloying method, if the VEC was calculated from the prepared composition Fe0.99Mn0.01S2, the VEC became 19.99 (Mn addition amount: 1%).
Metal Fe powder, Mn powder, and S powder having purity of 99.9% were mixed at a compositional ratio of 7:3:20, put into the quartz tube, and subjected to heat treatment under a vacuum atmosphere at 700° C. for 24 hours, and thereafter, the sample was pulverized by using the ball mill. The structure was analyzed by performing X-ray diffraction of the powder sample, and as a result, the structure was the pyrite structure. If the VEC was calculated from the powder sample of Fe0.7Mn0.3S2 by the aforementioned process, the VEC became 19.7 (Mn addition amount: 30%).
Metal Fe powder, P powder, and S powder having purity of 99.9% were mixed at a compositional ratio of 100:1:199, put into the quartz tube, and subjected to heat treatment under a vacuum atmosphere at 700° C. for 24 hours, and thereafter, the sample was pulverized by using the ball mill. The structure was analyzed by performing X-ray diffraction of the powder sample, and as a result, the structure was the pyrite structure. If the VEC was calculated from the powder sample of FeS1.99P0.01 by the aforementioned process, the VEC became 19.99 (P addition amount: 0.05%).
Fe0.99V0.01S2, Fe0.99Cr0.01S2, Fe0.99Zr0.01S2, Fe0.99Mo0.01S2, Fe0.99Hf0.01S2, Fe0.99Ta0.01S2, and Fe0.99W0.01S2 were prepared by substituting Fe by V, Cr, Zr, Mo, Hf, Ta, and W by the same method as Sample Preparation Example 2. If the VECs were calculated from the aforementioned compositions, the VECs became 19.97, 19.98, 19.96, 19.98, 19.96, 19.97, and 19.98 (each addition amount: 1%).
The thin film having the film thickness of about 300 nm was prepared on the Si substrate having the thermal oxide film by performing sputtering using the target where Fe and S were mixed at a composition ratio of 1:2, and subjected to heat treatment under a nitrogen atmosphere at the condition of 600° C. for 1 hour. The structure was analyzed by performing X-ray diffraction of the thin film, and as a result, the peak of the pyrite structure could be observed.
The temperature of the samples prepared in Preparation Examples 1 and 2 were made different from room temperature by 20° C. and the Seebeck coefficient was measured. As a result, the high Seebeck coefficients of 350 μV/K and 100 μV/K were obtained. Accordingly, it could be confirmed that in the material system, the Seebeck coefficient was modulated and the high thermoelectromotive force as the p type was ensured by preparing the sample having the doping amount changed by the method of the present Example 1 to change the VEC. Further, thermal conductivities were each 10 mW/Kcm and 15 mW/Kcm.
The sample preparation method may be a vacuum deposition method such as molecular beam epitaxy other than the present embodiment, or chemical vapor deposition using a transition metal complex or the like may be used. Further, the sample preparation method may be a method of heating sulfur to vaporize sulfur, sending the vaporized sulfur to the reaction room where the iron plate was charged with carrier gas that was inert gas such as argon, and reacting the vaporized sulfur with the iron plate.
According to the present embodiment, an electron occupying number of a d-band may be changed and an electron state at a Fermi level may be modulated by adding elements having different valence electron numbers by doping and controlling a valence electron number of a compound, thus implementing high thermoelectromotive force. Further, it is possible to significantly reduce a cost as compared to the Bi—Te system by combining cheap materials with few anxieties of depletion.
As described above, according to the present embodiment, it is possible to provide a p-type thermoelectric conversion material where a low environment load and a reduction in cost are feasible and a high Seebeck coefficient and a high carrier density can be compatible.
A second embodiment will be described by using FIGS. 7A and 7B. Further, a matter described in the first embodiment 1 but not described in the present embodiment can be applied to the present embodiment unless there is an unavoidable reason.
As illustrated in
As illustrated in
In the structures illustrated in
So far, according to the present embodiment, it is possible to provide the thermoelectric conversion device having high conversion efficiency even at about room temperature (<200° C.)
A third embodiment will be described by using
A desired voltage and current may be obtained by connecting the desired number of n-type thermoelectric conversion devices of the second embodiment in series or in parallel as illustrated in
So far, according to the present Example, it is possible to provide the thermoelectric conversion module having high conversion efficiency even at about room temperature (<200° C.). Further, the desired voltage and current can be obtained by arranging the thermoelectric conversion devices in series or in parallel.
Further, the present invention is not limited to the aforementioned embodiments, but includes various modifications. For example, the aforementioned embodiments are described in detail to easily understand the present invention but do not essentially have all constitutions described above. In addition, a portion of the constitution of an embodiment can be substituted by the constitution of another embodiment, and the constitution of another embodiment can be added to the constitution of the embodiment. Further, another constitution can be added to, removed from, and substituted for a portion of the constitution of each embodiment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-089104 | Apr 2012 | JP | national |
