JP4372276B2 - Clathrate compound, high-efficiency thermoelectric material, manufacturing method thereof, and thermoelectric module using high-efficiency thermoelectric material - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to a clathrate compound, a high-efficiency thermoelectric material using the clathrate compound, a production method thereof, and a thermoelectric module, and relates to a technique capable of providing a thermoelectric material having excellent conductivity and Seebeck coefficient and low thermal conductivity. .
[0002]
[Prior art]
Conventionally, various methods of using thermoelectric materials have been studied, but conventional thermoelectric materials have poor thermoelectric conversion efficiency, and are limited to applications that require reliability or special applications. Accordingly, it has been considered difficult to provide thermoelectric materials for the purpose of waste heat power generation, which is considered as a general use.
In order to improve the efficiency of this type of thermoelectric material and use it as a high-efficiency thermoelectric material,
1) low thermal conductivity,
2) High Seebeck coefficient
3) High electrical conductivity,
It is necessary to satisfy the conditions.
[0003]
However, with conventional thermoelectric material development techniques, material development has been carried out by selecting the composition empirically. For this reason, an example in which the dimensionless index (ZT), which is the maximum value of the performance coefficient at a temperature of 700 K or higher, exceeds 1 in a currently developed thermoelectric material is GeTe-AgSbTe known as a p-type thermoelectric material.₂It was only.
[0004]
[Problems to be solved by the invention]
Considering the conventional thermoelectric material in such a background, there is no situation that completely satisfies the conditions for becoming a highly efficient thermoelectric material. For example, a metal has a large electrical conductivity as an advantage, but has a large thermal conductivity as a disadvantage and a small Seebeck coefficient. In the case of a semiconductor, although it has a small thermal conductivity and a large Seebeck coefficient as advantages, it does not hold as a high-efficiency thermoelectric material because the electrical conductivity is small. BiTe is known as a thermoelectric material, but it is inefficient and cannot be practically used.
[0005]
In addition, in order to use for general power generation as a thermoelectric material, a p-type thermoelectric material and an n-type thermoelectric material must be combined to form a power generation system. However, the conventional GeTe-AgSbTe is used.₂In this case, it is said that there is no n-type.
[0006]
The present invention has been made in view of the above circumstances, and a highly efficient thermoelectric material having a figure of merit exceeding 1 to satisfy the three conditions of high thermal conductivity, high Seebeck coefficient and high electrical conductivity. One of the objects is to provide a clathrate compound which is expected to be applied to the above.
Furthermore, it is an object of the present invention to provide a highly efficient thermoelectric material having a figure of merit exceeding 1 in order to satisfy the three conditions of high thermal conductivity, high Seebeck coefficient, and high electrical conductivity. .
An object of the present invention is to provide an excellent thermoelectric material and thermoelectric module that can be used for general uses such as waste heat power generation.
An object of the present invention is to provide a method capable of easily producing an excellent clathrate compound and a high-efficiency thermoelectric material as described above.
[0007]
[Means for Solving the Problems]
  The clathrate compound of the present invention was made in view of the above circumstances, and Si orMade of GeA clathrate lattice and a mass larger than the constituent atoms of the clathrate lattice contained in the lattice gap of the clathrate latticeBa'sSubstituted with doping atoms and at least some of the atoms constituting the clathrate latticePIt is characterized by comprising mainly substituted atoms.
[0008]
  Si or GeBasically has semiconducting or insulating properties and has a high Seebeck coefficient, but has low electrical conductivity and high thermal conductivity.Specific gravityBig ofBaBy suppressing the vibration of the clathrate lattice by lowering the thermal conductivity, metal conductivity is imparted and electrical conductivity is improved. Furthermore, it was allowed to penetrate the clathrate latticeBaIn order to weaken the addition of metallic properties byPBy substituting the metal properties closer to those of semi-metals, i.e., semiconductors, synergistically balance the appropriate thermal conductivity, Seebeck coefficient, and electrical conductivity, and have an excellent performance coefficient. obtain.
[0010]
  The clathrate compound of the present invention is, in the above configuration, the clathrate lattice is a silicon clathrate 46 which is a mixed lattice of Si20 clusters composed of dodecahedrons of Si atoms and Si24 clusters composed of tetrahedra of Si atoms. It is characterized by. In the clathrate compound of the present invention, the clathrate lattice is a silicon clathrate 34 which is a mixed lattice of Si20 clusters composed of dodecahedrons of Si atoms and Si28 clusters composed of sixteen-sided Si atoms. It is characterized by.
  Ba doping atoms may enter into the 2a site of the Si20 cluster or the 6d site of the Si24 cluster.
[0011]
  Of the present inventionn-typeHigh efficiency thermoelectric material is Si orMade of GeA clathrate lattice and a mass larger than the constituent atoms of the clathrate lattice contained in the lattice gap of the clathrate latticeBa'sSubstituted with doping atoms and at least some of the atoms constituting the clathrate latticePIt is characterized by comprising mainly substituted atoms.
[0012]
  In the high-efficiency thermoelectric material of the present invention, the clathrate lattice may be a silicon clathrate 46 that is a mixed lattice of Si20 clusters composed of Si atoms dodecahedrons and Si24 clusters composed of Si atoms tetrahedrons.
  In the high-efficiency thermoelectric material of the present invention, the clathrate lattice may be a silicon clathrate 34 that is a mixed lattice of Si20 clusters composed of dodecahedrons of Si atoms and Si28 clusters composed of sixteen-sided Si atoms.
  Furthermore, in the high efficiency thermoelectric material of the present invention, Ba doping atoms may enter into the 2a site of the Si20 cluster or the 6d site of the Si24 cluster.
[0013]
  The method for producing the clathrate compound of the present invention comprises Si orMade of GeA clathrate lattice and a mass larger than the constituent atoms of the clathrate lattice contained in the lattice gap of the clathrate latticeBa'sSubstituted with doping atoms and at least some of the atoms constituting the clathrate latticePWhen producing a clathrate compound mainly composed of substituted atoms, the atoms for constituting the clathrate lattice, the doping atoms, and the substituted atoms are mixed at a predetermined ratio, and after pressure forming into a desired shape, It is characterized by heat treatment and further sintering by pressure sintering.
[0014]
  The method for producing a high-efficiency thermoelectric material of the present invention includes Si orMade of GeA clathrate lattice and a mass larger than the constituent atoms of the clathrate lattice contained in the lattice gap of the clathrate latticeBa'sSubstituted with doping atoms and at least some of the atoms constituting the clathrate latticePMainly with substitution atomsn-typeWhen manufacturing a high-efficiency thermoelectric material, the atoms for constituting the clathrate lattice and the doping atoms are mixed at a predetermined ratio, pressed into a desired shape, subjected to pre-heat treatment, and further subjected to pressure sintering. It is characterized by sintering by the method.
[0015]
In the thermoelectric module of the present invention, one or more pairs of p-type thermoelectric elements and n-type thermoelectric elements are combined via electrodes, a heat exchanging portion is formed on the connection portion side of both thermoelectric elements, and faces the connection portion. An electric circuit is connected to the side, and the high-efficiency thermoelectric material according to any one of claims 5 to 8 is applied as a constituent material of the n-type thermoelectric element.
[0016]
  In the present invention, the constituent material of the p-type thermoelectric element of the thermoelectric module described above is Si orMade of GeA clathrate lattice and a mass larger than the constituent atoms of the clathrate lattice contained in the lattice gap of the clathrate latticeBa'sSubstituted with doping atoms and at least some of the atoms constituting the clathrate latticeAlA thermoelectric material mainly composed of substituted atoms is used.
  In the thermoelectric module of the present invention, the figure of merit (ZT) of the constituent material of the p-type thermoelectric element can be 1.05, and the figure of merit (ZT) of the constituent material of the n-type thermoelectric element is 1. 01.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
The clathrate compound according to the present invention is doped with clathrate lattice (basic constitutional unit of clathrate) composed of at least one kind of group IVB elements such as C, Si, Ge, Sn and the like and doped inside this clathrate lattice It is comprised from a doping atom and the substituted atom which substituted a part of several atoms which comprise the said clathrate lattice.
[0018]
“First Embodiment”
FIGS. 1 and 2 show a first embodiment in which a silicon clathrate lattice is applied to the present invention. The silicon clathrate lattice 1 of the first embodiment is formed from a dodecahedron of Si atoms shown in FIG. Si₂₀Si composed of cluster 2 and tetrahedron of Si atoms_{twenty four}A plurality of structural units 4 combined with the clusters 3 are combined to form a silicon clathrate lattice 1 as shown in FIG.
Of the silicon clathrate lattice 1 shown in FIG.₂₀2a site of cluster 2 or Si_{twenty four}At least one of the atoms of Group IA, IIA, IIIA, IB, IIB, and IIIB having a mass greater than Si is doped in at least a part of at least one of the 6d sites of cluster 3 Has been.
[0019]
As the most preferable doping atom used here, Sr, Ba, and Ra among the elements of Group IIA of the periodic table can be exemplified. When these group IIA elements enter the previous site of the Si cluster, the group IIA atoms are divalent, and the two electrons in them move to the atoms constituting the clathrate. Therefore, the clathrate compound as a whole has metallic properties. In addition, since these elements have a mass larger than that of Si constituting the clathrate lattice 1, the vibration of atoms in the clathrate lattice 1 is suppressed, and the lattice vibration is scattered to reduce the heat conduction.
FIG. 3 shows the lattice structure of a silicon clathrate lattice, and Si is present at the apex of the basic unit structure of each lattice. FIG. 4 shows the 2a site (Si₂₀FIG. 5 shows the 6d site (Si) of the silicon clathrate lattice._{twenty four}A state in which Ba atoms have invaded the inside of the cluster.
[0020]
In addition to the group IIA atom as the atom doped in the clathrate lattice 1, the group IA atom may be exemplified by Li, Na, K, Rb, Cs, Fr, and the group IIIA atom may be Sc, Y La, lanthanoid elements such as Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, U, Np, Pu, Am, Cm, Examples include Bk, Cf, Es, Fm, Md, No, and Lr.
Further, Cu, Ag, Au can be exemplified as the group IB doping atom, Zn, Cd, Hg can be exemplified as the group IIB element, and B, Al, Ga, In can be exemplified as the group IIIB doping atom. , Tl can be exemplified.
[0021]
  Next, among the plurality of Si atoms constituting the silicon clathrate lattice 1, at least some of the Si atoms are any of VB group, VIB group, VIIB group, VA group, VIA group, VIIA group, and VIII group in the periodic table. It is substituted with any atom. Replacing a part of Si atoms with these atoms is due to the metallic properties (high thermal conductivity among metallic properties) of the silicon clathrate lattice 1 made metallic by the previous penetration of doping atoms. This is done to suppress or eliminate, and this element substitution gives semiconducting properties and thermal conductivity.LowerAt the same time, the Seebeck coefficient is increased. Of the above-mentioned substitutional elements in each group, the VB group atom, which is considered to easily form a compound, is most preferable, and at least one or more of N, P, As, Sb, and Bi are preferable.
[0022]
Other substituent atoms may include O, S, Se, Te, and Po as group VIB atoms, and F, Cl, Br, I, and At as group VIIB atoms. Further examples of the substitution atom may include Fe, Co, Ni, Ru, RhIr, Ni, Pd, Pt, and Os of group VIII atoms, and examples of V, Nb, and Ta as group VA atoms. Cr, Mo, W can be exemplified as the VIA group atoms, and Mn, Tc, Re can be exemplified as the VIIA group atoms.
[0023]
Next, the material development guideline which this inventor considers for making the said clathrate compound suitable as a thermoelectric material is described. Conventionally, since no clear development guideline has been known in the material development of this type of thermoelectric material, this guideline is originally proposed by the present inventor.
1). Reduction of thermal conductivity
In order to lower the thermal conductivity of the thermoelectric material of the clathrate compound, it is considered possible to scatter the lattice vibration of the clathrate compound. Specifically, an atom heavier than the mass of the atoms constituting the clathrate skeleton is selected as a doping atom and introduced into a site free of the clathrate skeleton. As a result, the doping atom suppresses vibrations of atoms constituting the skeleton. As a result, phonon scattering occurs and the thermal conductivity can be lowered.
[0024]
Next, according to the calculation of the first principle pseudopotential method using the plane wave basis, the intrinsic energy of Ba (E_Ba)
E_Ba= (1 / x) ・ (Ba_x@Si₄₆-Si₄₆) -Ba₁ ^{(isolate-atom)}It can be expressed by the following formula.
Where x is the number of atoms of solid solution barium, Ba_x@Si₄₆Is the total energy of the electron system in the case where x barium atoms are dissolved in the silicon clathrate 46, Si₄₆Is the total energy of the electronic system of silicon clathrate 46 alone, Ba₁ ^{(isolate-atom)}Is the total energy of an electron system with one barium isolated atom.
The barium solid solution energy when 2, 6, and 8 barium atoms enter the silicon clathrate is shown below.
▲ 1 ▼ Ba₂@Si₄₆(2a site, 12 Hedra)
Barium solid solution energy: -91.150 / (eV / atom)
▲ 2 ▼ Ba₆@Si₄₆(6d site, 14 Hedra)
Barium solid solution energy: -92.805 / (eV / atom)
▲ 3 ▼ Ba₈@Si₄₆(2a site + 6d site)
Barium solid solution energy: −93.628 / (eV / atom)
[0025]
From this result, since it is more energetically stable that many barium atoms are dissolved in the silicon clathrate, Ba can easily enter all the void sites of the silicon clathrate lattice according to the present invention, and the lattice vibration The thermal conductivity can be lowered by scattering.
[0026]
2) Improvement of Seebeck coefficient
In order to increase the Seebeck coefficient, it is necessary to obtain an intermediate property between a metal and a semiconductor. First, it is an insulator in a single structure of a silicon clathrate lattice, but at least of atoms of VB group, VIB group, VIIB group, VA group, VIA group, VIIA group, and VIII group having valence electrons of 5 or more. By introducing one type as a donor, it can be converted to an n-type semiconductor and the Seebeck coefficient can be improved.
[0027]
3) Improvement of electrical conductivity
Since the group IVB atoms constituting the silicon clathrate lattice 1 originally have semiconductor or insulating properties, the electrical conductivity cannot be increased by the group IVB atoms alone. Therefore, by introducing any of the group IA, IIA, IIIA, IB, IIB, and IIIB atoms having metallic properties into the vacant sites of the silicon clathrate lattice 1, the electrical conductivity is increased. be able to.
[0028]
As a result of these, the silicon clathrate compound of the first embodiment has characteristics that combine a low thermal conductivity, a high Seebeck coefficient, and a high electrical conductivity that are not found in conventional thermoelectric materials. Has a better figure of merit. The silicon clathrate compound of the first embodiment is an n-type or p-type thermoelectric material.
[0029]
Next, an example of a method for producing the silicon clathrate compound having the above-described configuration will be described.
In the case of using a silicon clathrate 46 lattice, Si, a doping atom powder, and an atomic powder for substitution of clathrate lattice constituent elements are weighed and mixed so as to have a target composition ratio, and these mixed powders are arced. It melt | dissolves by melt | dissolution and melts and obtains the ingot of the target composition. The raw material powder used here may be either a pure powder or a compound powder of each element, but it is preferable to use a pure powder that contains as few impurities as possible.
Next, the ingot is pulverized, and the pulverized powder is inspected by X-ray analysis to determine whether or not the desired composition ratio is obtained. That is, if the target composition is obtained, the process proceeds to the next step. If the target composition is not obtained, the ingot is melted again from the powder mixture, and the newly prepared ingot is analyzed. Moreover, it is preferable to use the powder of the pulverized product to be used with a fine particle size as much as possible.
[0030]
Once the powder of the pulverized product having a good composition ratio is obtained, heat treatment (calcination treatment) is performed in an Ar gas atmosphere or a vacuum atmosphere to remove unnecessary components in a gas state, and the powder after the calcination treatment is further removed. After pulverizing, aligning the particle size, analyzing with X-rays, checking whether the composition ratio is correct, selecting the one with the desired composition ratio, aligning the particle size, and discharging the pulverized product into a discharge plasma device Can be used to obtain a desired thermoelectric element composed of a thermoelectric material of a silicon clathrate compound in a desired shape, for example, a columnar sintered product.
In spark plasma sintering, a mixture of powders is pressed with a pair of punches at a pressure of several MPa to several tens of MPa, and at the same time, an electric current is applied and heated to about 1000 ° C. for about several minutes to several hours. It is a kind of pressure sintering method.
[0031]
In addition, when performing the said heat processing, when manufacturing the silicon clathrate 46, it is preferable to heat-process in Ar gas atmosphere, and when manufacturing the silicon clathrate 34, it is preferable to heat-process in a vacuum atmosphere. Moreover, when the composition ratio has deviated from the target composition by the previous X-ray analysis, the same process is repeated by returning to the process of obtaining the ingot from arc melting again.
[0032]
By the way, in the above-mentioned manufacturing process, instead of arc melting, a mechanical alloying treatment can be performed to obtain a mixed powder having a desired composition ratio. Mechanical alloying is a process in which powder mixed in a hollow attritor containing a large number of metal balls such as stainless steel balls is mixed, then the attritor is rotated at high speed to mix the powder, and the powder is pulverized between the metal balls. In this method, mixed powder is obtained by mixing. The thermoelectric element of the silicon clathrate compound according to the present invention can also be obtained by molding the mixed powder obtained by performing the mechanical alloying treatment in this way, preliminarily heat-treating, and further sintering.
[0033]
Further, in the above manufacturing method, when Ba is used as a doping atom and P is selected as a substitution atom, Ba becomes a nucleus during powder mixing, first attracts Si, and a large number of clusters are generated. Clusters are considered to form a cluster lattice. Therefore, a clathrate compound can be produced by an extremely simple method of mixing necessary elements, performing a calcining treatment, and then sintering. Therefore, the clathrate compound of the present invention has a simple production method as compared with conventional thermoelectric materials, and a highly efficient compound having a high figure of merit at 700 K or higher can be obtained.
[0034]
“Second Embodiment”
FIG. 6 is a schematic diagram of the crystal structure of the silicon clathrate 34 constituting the clathrate lattice of the second embodiment of the silicon clathrate compound according to the present invention. The crystal structure 10 of the silicon clathrate 34 is composed of Si dodecahedron as shown in FIG.₂₀Si composed of clathrate 11 and Si icosahedron₂₈A plurality of basic structural units 13 composed of the class rate 12 are combined to form a silicon class rate 34.
[0035]
Si constituting the silicon clathrate 34 of the second embodiment₂₀At the 2a site of clathrate 11 or Si_{twenty four}At the 6d site of clathrate 12, atoms in the periodic table IA, IIA, IIIA, IB, IIB, IIIB having a mass larger than that of Si as in the first embodiment For example, Ba is doped.
Further, as in the case of the first embodiment, a part of Si constituting the silicon clathrate 34 is in the periodic table VB group, VIB group, VIIB group, VA group, VIA group, VIIA group, VIII group. It is substituted with any atom, for example P.
In the silicon clathrate compound of the second embodiment, excellent characteristics as a thermoelectric material can be obtained in the same manner as the silicon clathrate compound of the first embodiment.
[0036]
“Third Embodiment”
FIG. 8 shows an embodiment of a thermoelectric module constructed using a silicon clathrate compound thermoelectric material according to the present invention. The thermoelectric module 30 of this embodiment is insulated in the vertical direction so as to be opposed to each other. A plurality of thermoelectric elements 33 made of p-type columnar thermoelectric materials and a plurality of thermoelectric elements 34 made of n-type columnar thermoelectric materials are alternately arranged between adjacent substrates 31 and 32 and adjacent to each other. When the lower end portions of one set of thermoelectric elements 33 and 34 are intermittently connected by the electrode plate 35, and the upper end portions of other thermoelectric elements 33 and 34 adjacent to each other are intermittently connected by the electrode plate 36. At the same time, a plurality of electrode plates 35, 36 are connected so that the ends of the adjacent p-type thermoelectric elements 33 and the ends of the n-type thermoelectric elements 34 are alternately connected and all the thermoelectric elements are connected in series. Connected with. Of the plurality of thermoelectric elements 33 between the upper and lower substrates, the connection wiring 37 is joined to the thermoelectric element 33 at one end, and the connection wiring 38 is joined to the thermoelectric element 34 at the other end. Has been configured.
Here, the n-type thermoelectric element 34 is composed of a thermoelectric material made of the clathrate compound described above.
[0037]
The thermoelectric module 30 having the configuration shown in FIG. 8 has a resistor 39 as a load connected between the connection wires 37 and 38 by heating the upper electrode plate 36 side as shown in FIG. 9A. If the electric circuit 41 is configured, the electrode plate 36 side is heated by another heat source, and the electrode plate 35 side is radiated to cause a potential difference between the connection wirings 37 and 38 so that a current flows. Can be used for thermoelectric power generation.
Further, the thermoelectric module 30 shown in FIG. 8 incorporates a power supply 40 in the connection wirings 37 and 38 as shown in FIG. 9B, and flows current in the direction indicated by the arrow, so that the upper electrode plate 36 side is provided. Therefore, the lower electrode plate 35 side can be used as a thermoelectric cooler by the above endothermic action.
[0038]
Next, a thermoelectric material of a clathrate compound suitable for constituting the p-type thermoelectric element 33 will be described.
In the silicon clathrate compound described above, a part of the silicon clathrate lattice is substituted with atoms such as P, and a part of the atoms of the silicon clathrate lattice is substituted with atoms having a valence of 5 or more. However, since the atoms constituting the silicon clathrate lattice are IV valences, 1 to 3 valence atoms, for example, IA group, IIA group, IIIA group, IB group having 1 to 3 valence electrons, When substitution is made with either an IIB group or an IIIB group atom, the 1 to 3 valence electrons therein move to the group IVB atom side constituting the clathrate. Therefore, the silicon clathrate lattice can be converted into p-type semiconductor properties as a whole.
[0039]
Of the above-mentioned substitutional elements in each group, the group IIIB atom, which is considered to easily form a compound, is most preferred, and at least one or more of B, Al, Ga, In, and Tl are preferred.
Other substituent atoms can include Cu, Ag, and Au as group IB atoms, and Zn, Cd, and Hg as group IIB atoms.
Furthermore, as an outer substituent atom, Li, Na, K, Rb, Cs, Fr of Group IA, Be, Mg, Ca, Sr, Ba, Ra, Group IIIA Sc, Y of Group IIIA, La, and lanthanoid element La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, as actinoid elements, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr can be exemplified.
[0040]
A p-type silicon clathrate compound can be obtained by allowing the above-described elements to penetrate the silicon clathrate lattice and substituting a part of the elements of the silicon clathrate lattice with the above elements. Here, for example, Ba is penetrated into the lattice of the silicon clathrate 46 by the same method as in the examples described later, and a part of Si is replaced with Al, so that the obtained silicon clathrate compound has a p-type. Moreover, it has been found by experiments of the present inventors that 1.01 can be obtained at 700 K in the performance coefficient.
Therefore, the thermoelectric element 33 can be manufactured from the p-type thermoelectric material having the above composition. As a result, the p-type thermoelectric element 33 can be composed of a thermoelectric material having a performance coefficient of 1.01, and the n-type thermoelectric element 34 can be composed of a thermoelectric material having a performance coefficient of 1.05 as in the embodiments described later. In this case, a thermoelectric module having extremely excellent power generation efficiency or heat absorption efficiency can be obtained.
[0041]
FIG. 10 shows an example of a power generation stack using the thermoelectric material of the present invention. The power generation stack 50 in this example is formed on the outer peripheral portion of an inner shell 51 made of a flat multi-hole tube through which exhaust gas or the like flows. The six power generation modules 52 are mounted, and a flat outer shell 54 is provided on the outside of the power generation module 52 so as to cover them, and the heat of the exhaust gas flowing inside the inner shell 51 is utilized. It is comprised so that electric power generation can be performed.
Also in the power generation stack of this embodiment, the thermoelectric module 52 can be used for power generation by configuring the power generation module 52 similarly to the thermoelectric module 30 of the previous embodiment.
[0042]
Here, the figure of merit (ZT) frequently used as a performance index of the thermoelectric material will be described below.
The performance of thermoelectric materials is as follows: α is thermoelectric power, κ is thermal conductivity, and ρ is specific resistance.
Z = α²/ Κρ (1) It is shown by the formula (1).
In addition, the figure of merit of the thermoelectric module shown in FIG.
Z_pn= (Α_p-Α_n)²/ (Κ_pρ_p+ Κ_nρ_n)²    ... indicated by equation (2)
The subscript corresponds to each value of the p-type molded body and the n-type molded body.
The maximum power generation efficiency of the thermoelectric material is the temperature at the high temperature end and the low temperature end respectively._hAnd T_cThen,
η_max= (T_h/ T_h-T_c) ・ {(M−1) / (M + T_h/ T_c)} ... (3) formula,
M = (1 + Z (T_h+ T_c) / 2)^1/2  ... expressed by equation (4).
[0043]
On the other hand, the maximum coefficient of performance for thermoelectric cooling is
φ_max= (T_h/ T_h-T_c) ・ {(MT_h/ T_c) / (M + 1)} (5), and in thermoelectric heating, φ_maxGiven by +1. In addition, if the heat absorption part is completely insulated and there is no inflow of heat, T_cBecomes the most lowered state, φ_max= 0, the maximum cooling temperature difference is
ΔT_max= (T_h-T_c )_max = (1/2) · ZT² _c  ... The relationship of equation (6) is obtained.
[0044]
Here, in general, thermoelectric cooling / heating is performed using a temperature difference (T_h-T_c) Since it is used in the range of ≦ 100K, a higher Z becomes a necessary condition, and now it is 3.4 × 10^-3K^-1More than that is required.
On the other hand, T_hHowever, it is required to be a heat-resistant material that is chemically stable at high temperatures. In general, the figure of merit Z has a temperature dependency inherent to the material, but the temperature at which the maximum value varies depends on the material.
Materials whose maximum value ZT of the figure of merit Z exceeds 1 are currently GeTe-AgSbTe.²And most other materials show ZT values lower than 1.
FIG. 11 and FIG. 12 show the absolute temperature dependence of the figure of merit of the p-type and n-type thermoelectric materials known in the past. As is clear from these figures, the maximum figure of merit A value exceeding ZT = 1 indicates that in the case of p-type GeTe—AgSbTe in the temperature range of 600 to 900K.²Only, and there is no outside. In the n-type, only SiGe (GaP) is promising in a specific narrow temperature range, and does not exist at room temperature to 700K.
[0045]
On the other hand, if it is a thermoelectric material according to the present invention, it is possible to provide an n-type thermoelectric material having a ZT exceeding 1 as will be apparent from Examples described later. Therefore, if a thermoelectric module is configured using the thermoelectric material of the present invention, a highly efficient thermoelectric module that is far superior to that of conventional thermoelectric materials can be provided.
[0046]
Next, the Seebeck coefficient will be described.
In the thermoelectric module shown in FIG. 9B, when a current flows from the power source 40 in the direction of the arrow, the current I flows through the electric circuit 42 and Peltier heat is generated on the upper electrode plate 36 side, and the Seebeck coefficient of the n-type thermoelectric element 34 -Α_n, The Seebeck constant of the p-type element_p, Α of the Seebeck coefficient of the electrode plate_mThen, the upper electrode plate and the PN element Peltier heat absorption Q_cpIs
Q_cp  = (-Α_n-Α_m ) T_cI + (α_m -Α_p) T_cI = − (α_n+ Α_p) T_cI
And α_m Can be ignored. Where T_cIs the temperature of the junction. α_e= Α_n+ Α_pAs the absolute value Q of the endothermic quantity._cpQ_cp= Α_eT_cI.
Thus, since the Seebeck coefficient is an index when the heat calculation of the thermoelectric module is performed, this numerical value is widely known as having a great influence on the performance improvement of the thermoelectric module.
[0047]
【Example】
The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples.
Each powder was weighed so that the composition ratio (atomic%) of Ba, Si, and P was Ba: Si: P = 8: 26: 20, and a mixed powder was prepared by mechanical alloying treatment. This mixed powder is subjected to a preliminary heat treatment in which the mixed powder is heated to 800 ° C. for 24 hours in an Ar gas atmosphere. Plasma sintering was performed at 40 ° C. and a pressure of 40 MPa for 30 minutes.
As a result of X-ray diffraction of the obtained columnar sintered product,
space group: Pm3m origin at 43m (In the crystallographic analysis, Pm3Usually overlined above 3 of m, but here about overline of 33A silicon clathrate 46 Ba having a lattice constant of 10.5１０₈@ (Si₂₆, P₂₀) Was confirmed. Moreover, as a result of measuring the figure of merit (ZT) at 700 K of this sintered product, it was confirmed that it was 1.05, a value exceeding 1.
[0048]
In addition, the same process is performed, each powder is weighed so that the composition ratio (atomic%) of Ba: Si: Al = 8: 26: 20, and the thermoelectric power of the silicon clathrate compound is started from this mixed powder. Sintered material “silicon clathrate 46 Ba₈@ (Si₂₆, Al₂₀) ”. When the figure of merit of this sintered body at 700 K was measured, an excellent value of the figure of merit 1.01 shown in FIG. 11 in terms of the SiBaAl clathrate compound was shown.
From the above results, silicon clathrate 46 Ba₈@ (Si₂₆, P₂₀) To form an n-type thermoelectric element, and a silicon clathrate 46 Ba₈@ (Si₂₆, Al₂₀It was found that a thermoelectric module having excellent thermoelectric efficiency can be obtained by forming a thermoelectric module by forming a p-type thermoelectric element with the thermoelectric material.
[0049]
【The invention's effect】
  As explained above, according to the clathrate compound of the present invention, the basic lattice is Si orGeIt is a clathrate lattice, and its specific gravity is larger than the constituent elements of the clathrate lattice inside the clathrate latticeBaTherefore, the vibration of the clathrate lattice is suppressed and the thermal conductivity is lowered, and at the same time the electrical conductivity is improved, and some of the atoms constituting the clathrate lattice have 5 or more valence electrons.PA clathrate compound having an improved Seebeck coefficient can be provided as a result of substitution and substitution to n-type semiconducting properties. In the present invention, Si or Ge is selected as the clathrate lattice constituent element, Ba is selected as the doping atom, and P is selected as the substitution atom, so that the thermal conductivity, electrical conductivity, and Seebeck coefficient are well balanced. be able to. In the present invention, the silicon clathrate 46 or the silicon clathrate 34 can be selected as the clathrate lattice.
[0050]
  Next, according to the thermoelectric material of the present invention, the basic lattice is Si orGeIt is a clathrate lattice, and its specific gravity is larger than the constituent elements of the clathrate lattice inside the clathrate latticeBaTherefore, the vibration of the clathrate lattice is suppressed and the thermal conductivity is lowered, and at the same time the electrical conductivity is improved, and some of the atoms constituting the clathrate lattice have 5 or more valence electrons.PAn n-type thermoelectric material with an improved Seebeck coefficient can be provided as a result of substitution and substitution with n-type semiconducting properties. In the present invention, Si or Ge is selected as the clathrate lattice constituent element, Ba is selected as the doping atom, and P is selected as the substitution atom, so that the thermal conductivity, electrical conductivity, and Seebeck coefficient are well balanced. be able to. In the present invention, the silicon clathrate 46 or the silicon clathrate 34 can be selected as the clathrate lattice.
[0051]
In producing the clathrate compound or thermoelectric material according to the present invention, if the constituent elements are mixed, after pressure forming, preheated, and pressure sintered, the above-described good thermal conductivity and electrical conductivity An excellent clathrate compound or thermoelectric material exhibiting a Seebeck coefficient can be obtained.
[0052]
Furthermore, if it is the thermoelectric module of the present invention in which an n-type thermoelectric element is configured from the thermoelectric material having the above-described configuration, as described above, an excellent thermoelectric device having low thermal conductivity, electrical conductivity, and a good Seebeck coefficient. Performance can be obtained and a thermoelectric module with good thermoelectric efficiency can be provided.
[0053]
  Furthermore, in the present invention, as a constituent material of the p-type thermoelectric element,Si or GeA clathrate lattice,BaDoping atoms ofAlIf a thermoelectric material mainly composed of substitution atoms is used, the thermoelectric efficiency of the p-type thermoelectric element can be made excellent in addition to the good thermoelectric efficiency of the n-type thermoelectric element. A thermoelectric module can be provided.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a crystal structure including a clathrate lattice and an intruding atom of a silicon clathrate compound according to the present invention.
2 is a dodecahedron of Si serving as a partial structural unit of the silicon clathrate compound shown in FIG.₂₀Cluster and Si tetrahedral Si_{twenty four}The schematic diagram which shows a cluster.
FIG. 3 is a plan view showing a basic crystal lattice structure of a silicon clathrate compound according to the present invention.
FIG. 4 is a plan view showing a structure in which Ba atoms have entered the 2a site of the basic crystal lattice of the silicon clathrate compound according to the present invention.
FIG. 5 is a plan view showing a structure in which Ba atoms have penetrated into the 6d site of the basic crystal lattice of the silicon clathrate compound according to the present invention.
FIG. 6 is a schematic view showing a crystal structure of a silicon clathrate 34 for constituting a silicon clathrate compound according to the present invention.
7 is a Si dodecahedron of Si serving as a partial structural unit of the silicon clathrate 34 shown in FIG.₂₀Cluster and Si 16-hedron Si₂₈The schematic diagram which shows a cluster.
FIG. 8 is a cross-sectional view showing one embodiment of a thermoelectric module configured using the thermoelectric element according to the present invention.
9 shows an example of how the thermoelectric module shown in FIG. 8 is used. FIG. 9 (a) is a configuration diagram in the case where a thermoelectric reaction is caused, and FIG. 9 (b) is a case in which an endothermic effect is caused. Diagram.
FIG. 10 is a cross-sectional view showing an example in which the thermoelectric module according to the present invention is used in a power generation stack.
FIG. 11 is a diagram showing the absolute temperature dependence of the figure of merit of a conventional p-type thermoelectric material and the thermoelectric material according to the present invention.
FIG. 12 is a diagram showing the absolute temperature dependence of the figure of merit of a conventional n-type thermoelectric material and the thermoelectric material according to the present invention.
[Explanation of symbols]
1 ... silicon clathrate compound, 2 ... Si₂₀Cluster, 3 ... Si_{twenty four}Cluster, 10 ... silicon clathrate 34, 11 ... Si₂₈Cluster, 30 ... thermoelectric module, 33 ... p-type thermoelectric element, 34 ... n-type thermoelectric element, 35, 36 ... electrode plate, 37, 38 ... connection wiring, 41, 42 ... electric circuit, 50 ... power generation stack, 52 ... thermoelectric module.

Claims (13)

A clathrate lattice made of Si or Ge; a Ba doping atom contained in the lattice gap of the clathrate lattice and having a mass larger than that of the constituent atoms of the clathrate lattice; and at least a part of the atoms constituting the clathrate lattice And a substituted atom of P substituted. 2. The clathrate according to claim 1, wherein the clathrate lattice is a silicon clathrate 46 that is a mixed lattice of a Si20 cluster composed of a dodecahedron of Si atoms and a Si24 cluster composed of a tetrahedron of Si atoms. Compound. 2. The clathrate according to claim 1, wherein the clathrate lattice is a silicon clathrate 34 that is a mixed lattice of a Si20 cluster composed of a dodecahedron of Si atoms and a Si28 cluster composed of a sixteen-faced Si atom. Compound. The clathrate compound according to claim 2, wherein a Ba doping atom is penetrated into the 2a site of the Si20 cluster or the 6d site of the Si24 cluster. A clathrate lattice made of Si or Ge, a doping atom of Ba contained in a lattice gap of the clathrate lattice and having a mass larger than that of the constituent atoms of the clathrate lattice, and at least a part of the atoms constituting the clathrate lattice An n-type high-efficiency thermoelectric material characterized in that it mainly comprises a substituted atom of P. 6. The high efficiency according to claim 5, wherein the clathrate lattice is a silicon clathrate 46 which is a mixed lattice of a Si20 cluster composed of a dodecahedron of Si atoms and a Si24 cluster composed of a tetrahedron of Si atoms. Thermoelectric material. 6. The high efficiency according to claim 5, wherein the clathrate lattice is a silicon clathrate 34 that is a mixed lattice of a Si20 cluster composed of a dodecahedron of Si atoms and a Si28 cluster composed of a dodecahedron of Si atoms. Thermoelectric material. The high-efficiency thermoelectric material according to claim 6, wherein Ba doping atoms are intruded into the 2a site of the Si20 cluster or the 6d site of the Si24 cluster. A clathrate lattice made of Si or Ge; a Ba doping atom contained in the lattice gap of the clathrate lattice and having a mass larger than that of the constituent atoms of the clathrate lattice; and at least a part of the atoms constituting the clathrate lattice When the clathrate compound mainly composed of the substituted atoms of P substituted with the above, the atoms for constituting the clathrate lattice, the doping atoms, and the substituted atoms are mixed at a predetermined ratio to obtain a desired shape. A method for producing a clathrate compound, characterized in that after pressure molding, preliminary heat treatment is performed, and further sintering is performed by pressure sintering. A clathrate lattice made of Si or Ge; a Ba doping atom contained in the lattice gap of the clathrate lattice and having a mass larger than that of the constituent atoms of the clathrate lattice; and at least a part of the atoms constituting the clathrate lattice When the n-type high-efficiency thermoelectric material mainly composed of the substituted atoms of P and the substituted atoms is mixed with atoms for forming the clathrate lattice and the doping atoms in a predetermined ratio, and a desired shape A method for producing a high-efficiency thermoelectric material, characterized in that after pressure forming, pre-heat treatment is performed, and further, sintering is performed by pressure sintering.   One or more pairs of p-type thermoelectric elements and n-type thermoelectric elements are combined via electrodes, a heat exchange part is formed on the connection part side of both thermoelectric elements, and an electric circuit is connected to the side opposite to the connection part. A high-efficiency thermoelectric material according to any one of claims 5 to 8 is applied as a constituent material of the n-type thermoelectric element. As a constituent material of the p-type thermoelectric element, a clathrate lattice made of Si or Ge, a Ba doping atom contained in a lattice gap of the clathrate lattice and having a mass larger than that of the constituent atoms of the clathrate lattice, The thermoelectric module according to claim 11, wherein a thermoelectric material mainly comprising at least part of atoms constituting the clathrate lattice and substituted Al atoms substituted is used. The figure of merit (ZT) of the constituent material of the p-type thermoelectric element is 1.05, and the figure of merit (ZT) of the constituent material of the n-type thermoelectric element is 1.01. The thermoelectric module according to 12.

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