n-TYPE THERMOELECTRIC MATERIAL, METHOD FOR PRODUCING SAME, AND THERMOELECTRIC POWER GENERATION ELEMENT

Abstract

Provided are: an n-type thermoelectric material which has a dimensionless performance index (ZT) of more than 1.3 at at least 723 K; a method for producing same; and a thermoelectric power generation element.　An n-type thermoelectric material according to the present invention includes an inorganic compound that contains: magnesium (Mg); antimony (Sb) and / or bismuth (Bi); indium (In); and an element M (the element M is at least one element selected from the group consisting of selenium (Se) and tellurium (Te)), and is represented by the formula MgaSb2-b-cBibMcInd, wherein the parameters a, b, c and d respectively satisfy the relationships 3≤a≤3.5, 0≤b≤2, 0<c<0.04, 0<d≤0.15 and b+c≤2.

Description

N-type thermoelectric material, its manufacturing method, and thermoelectric power generation element

[0001] The present invention is ３ Sb ２ The present invention relates to an n-type thermoelectric material of the above-mentioned system, a method for producing the same, and a thermoelectric power generation element.

[0002] Even in Japan, where energy conservation has progressed particularly well compared to other countries in the world, approximately three-quarters of the primary energy supply is currently discarded as thermal energy in waste heat recovery. Under these circumstances, thermoelectric power generation elements have attracted attention as solid-state elements that can recover thermal energy and directly convert it into electrical energy.

[0003] Thermoelectric power generation elements are elements that directly convert energy into electricity, and have the advantage of being easy to maintain due to the lack of moving parts, as well as being scalable. For this reason, active research is being conducted into thermoelectric semiconductors as materials for waste heat power generation, which contributes to carbon neutrality.

[0004] For waste heat power generation applications that contribute to carbon neutrality, practical use is expected at temperatures between room temperature (300K) and around 723K (450°C), but the thermoelectric material with the best performance between room temperature and around 573K (300°C) is Bi. ２ Te ３ However, there are few materials other than these Te compounds that have relatively high performance between room temperature and 723K, which has been a problem. ３ Sb ２ A material based on ZnO has been proposed as one of the candidates (see, for example, Non-Patent Document 1).

[0005] Non-Patent Document 1 describes an indium-doped n-type Mg ３ Sb ２ According to Non-Patent Document 1, Mg ３．５－ｘ In ｘ Sb １．７ Bi ０．２６ Te ０．０４ (x=0, 0.01, 0.02, 0.03, 0.04) were synthesized, and n-type Mg ３ Sb ２We report that the thermoelectric material based on this material exhibited a dimensionless figure of merit (ZT) of 1.27 at 773 K (500 °C). However, considering applications such as waste heat power generation and IoT power generation that contribute to carbon neutrality, it is expected that thermoelectric materials with an average dimensionless figure of merit (ZT) of more than 1.3 from room temperature to 723 K will be developed.

[0006] Wenyan Zhu et al., Journal of Electronic Materials, 2022, 51, 1591-1596

[0007] In view of the above, an object of the present invention is to provide an n-type thermoelectric material having a dimensionless figure of merit (ZT) of more than 1.3 at least at 723 K, a method for producing the same, and a thermoelectric power generation element.

[0008] The n-type thermoelectric material according to the present invention includes an inorganic compound containing magnesium (Mg), antimony (Sb) and / or bismuth (Bi), indium (In), and an M element (wherein the M element is at least one element selected from the group consisting of selenium (Se) and tellurium (Te)), and the inorganic compound is Mg ａ Sb ２－ｂ－ｃ Bi ｂ M ｃ In ｄ The parameters a, b, c, and d satisfy the following conditions: 3≦a≦3.5, 0≦b≦2, 0<c<0.04, 0<d≦0.15, and b+c≦2, respectively, thereby solving the above problem. The parameters b, c, and d may satisfy the following conditions: 0.2≦b≦0.7, 0.002≦c≦0.025, and 0.001≦d≦0.1, respectively. The parameters b, c, and d may satisfy the following conditions: 0.4≦b≦0.55, 0.005≦c≦0.02, and 0.003≦d≦0.1, respectively. The inorganic compound is La ２ O ３ The structure may have a symmetry of the space group P-3m1. The In may be present as a substitutional solid solution in the Mg site. The lattice strain (%) calculated by the Williamson-Hall method is 3.5×10 －１Above 5.0 x 10 －１ The thermoelectric material may be in a form selected from the group consisting of powder, sintered body, and thin film. The thermoelectric material may be in the form of a powder or sintered body, and the inorganic compound may be composed of crystal grains having an average grain size of 4 μm to 20 μm. The thermoelectric material may be in the form of a thin film, and the inorganic compound may be composed of crystal grains having an average grain size of 4 μm to 20 μm, and may further contain an organic material. A method for producing the n-type thermoelectric material according to the present invention includes mixing a raw material containing magnesium (Mg), a raw material containing antimony (Sb) and / or a raw material containing bismuth (Bi), a raw material containing indium (In), and a raw material containing an M element (wherein the M element is at least one element selected from the group consisting of selenium (Se) and tellurium (Te)) to prepare a mixture, and sintering the mixture, thereby solving the above-mentioned problem. The sintering may be spark plasma sintering. The spark plasma sintering may be performed at a temperature of 723 K to 1173 K under a pressure of 30 MPa to 100 MPa for 5 minutes to 30 minutes. The method may further include pulverizing the sintered body obtained by sintering. The method may further include mixing the powder obtained by pulverization with an organic material. The method may further include performing physical vapor deposition using the sintered body obtained by sintering as a target. A thermoelectric power generation element according to the present invention includes at least an n-type thermoelectric material, which is the n-type thermoelectric material described above, thereby solving the above-mentioned problem. The element may also include a p-type thermoelectric material alternately connected in series with the n-type thermoelectric material. The p-type thermoelectric material may be selected from the group consisting of BiSbTe-based, MgAgSb-based, and AgSbSe-based materials.

[0009] The thermoelectric material of the present invention functions as an n-type thermoelectric material because it contains an inorganic compound containing magnesium (Mg), antimony (Sb) and / or bismuth (Bi), indium (In), and an M element (wherein the M element is at least one element selected from the group consisting of selenium (Se) and tellurium (Te)). ａ Sb ２－ｂ－ｃ Bi ｂ M ｃ In ｄ Since the parameters a, b, c, and d each satisfy the above-mentioned specific ranges, the material exhibits excellent thermoelectric performance and can achieve a dimensionless figure of merit (ZT) of more than 1.3 at 723 K. If such an n-type thermoelectric material is used in a thermoelectric power generation element, it can provide a highly efficient thermoelectric power generation element, particularly in the medium-temperature range from room temperature to 723 K.

[0010] The method for producing a thermoelectric material of the present invention is highly versatile because it can obtain the above-mentioned thermoelectric material by mixing a raw material containing magnesium (Mg), a raw material containing antimony (Sb) and / or a raw material containing bismuth (Bi), a raw material containing indium (In), and a raw material containing an M element (wherein the M element is at least one element selected from the group consisting of selenium (Se) and tellurium (Te)) to prepare a mixture, and then sintering this mixture.

[0011] FIG. 1 is a flowchart showing the steps of producing the n-type thermoelectric material of the present invention; FIG. 2 is a schematic diagram showing a thermoelectric power generation element using the n-type thermoelectric material of the present invention; FIG. 3 is a schematic diagram showing another thermoelectric power generation element using the n-type thermoelectric material of the present invention; FIG. 4 is a diagram showing the appearance of the sample of Example 2; FIG. 5 is a diagram showing SEM images of the samples of Examples 2, 4, and 6; FIG. 6 is a diagram showing STEM and EDS images of the sample of Example 2; FIG. 7 is a diagram showing the XRD pattern of the sample of Example 2; FIG. 8 is a diagram showing the XRD pattern of the sample of Example 4; FIG. 9 is a diagram showing the temperature dependence of the electrical conductivity of the samples of Examples 2 and 4 to 7;

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that like elements are designated by like numbers and their description will be omitted.

[0013] (Embodiment 1) The n-type thermoelectric material of the present invention includes an inorganic compound containing magnesium (Mg), antimony (Sb) and / or bismuth (Bi), indium (In), and an M element (wherein the M element is at least one element selected from the group consisting of selenium (Se) and tellurium (Te)).

[0014] The inorganic compound is Mg ａ Sb ２－ｂ－ｃ Bi ｂ M ｃ In ｄThe parameters a, b, c, and d satisfy the following conditions: 3≦a≦3.5, 0<b≦2, 0<c<0.04, 0<d≦0.15, and b+c≦2, respectively. This results in a composition in which the M element and In are added to an inorganic compound with a matrix of Mg and Sb and / or Bi, and the material functions as an n-type thermoelectric material with electrons as carriers. In particular, by controlling the amount of Te added in addition to the addition of In, the thermal conductivity can be reduced, achieving a dimensionless figure of merit (ZT) of greater than 1.3 at least at 723 K, and exhibiting excellent thermoelectric performance over the entire temperature range from 300 K to 723 K.

[0015] The inorganic compound matrix is ​​preferably Mg ３ Sb ２ It is a La-based crystal. ２ O ３ It has a P-type structure and belongs to the P-3m1 space group (164th in the International Tables for Crystallography). In the present specification, "-3" represents "3 with an overbar." In the present invention, the inorganic compound is obtained by adding an M element and In to the above-mentioned parent phase.

[0016] In is La ２ O ３ The addition of In delocalizes localized charge carriers. This means that the addition of In suppresses the loss of Mg and effectively suppresses the formation of disordered structures caused by Mg defects due to Mg loss. As a result, the carrier mobility increases, improving electrical conductivity in the medium temperature range (room temperature to 723 K), especially at 723 K.

[0017] M element is La ２ O ３ By adding Se and / or Te as the M element, the material becomes an n-type thermoelectric material with electrons as carriers.

[0018] Mg ３ Sb ２ The system is not particularly limited as long as it is made of the above elements and has the above crystal structure and space group. ３＋ｘ Sb ２ , Mg３＋ｘ (Sb, Bi) ２ (x is 0≦x≦0.6) When written as (Sb, Bi), it means that Sb and Bi can be placed in the position where Sb and Bi can be placed, without distinguishing between them.

[0019] The M element forms a substitutional solid solution in the sites where Sb and Bi are substituted, and In forms a substitutional solid solution in the sites where Mg is substituted, resulting in (Mg, In) ３＋ｘ ((Sb, Bi), M) ２ In this case, In indicates that Mg and In occupy the Mg site without distinction, and M element indicates that Sb and / or Bi and M element occupy the Sb and / or Bi site without distinction, where M element is at least one element selected from the group consisting of Se and Te.

[0020] (Mg, In) ３＋ｘ ((Sb, Bi), M) ２ Exemplary compositions include Mg ３．２ Sb １．５ Bi ０．４９ M ０．０１ In ０．００５ , Mg ３．２ Sb １．５ Bi ０．４９５ M ０．００５ In ０．００５ These may all have the above-mentioned crystal structure and space group, and the so-called excess components are thought to constitute defects in a part of the crystal structure, imparting n-type properties to the inorganic compound as a whole.

[0021] In the present invention, if the lattice constants obtained by Rietveld analysis of the results of X-ray diffraction or neutron diffraction in the space group P-3m1 are within ±5% of the theoretical values ​​(a = 4.582 Å, b = 4.582 Å, c = 7.244 Å), it is determined that the crystal is Mg. ３ Sb ２ The added In substitutes for the Mg site, and the M element substitutes for the Sb / Bi site, changing the lattice constant, but the atomic positions given by the crystal structure, the sites occupied by the atoms, and their coordinates do not change so much that the chemical bonds between the skeletal atoms are broken.

[0022] The parameter a is the Mg content, and may be 3 or more and 3.5 or less. ３ Sb ２ The resulting material becomes a bismuth-type crystal, resulting in a stable crystal structure. Parameter b is the Bi content and may be 0 or more and 2 or less. Parameter c is the M element content and may be greater than 0 and less than 0.04. This results in the M element being substituted into Sb and / or Bi to form a solid solution, resulting in a stable crystal structure and excellent n-type thermoelectric properties. In particular, the present inventors have found that the dimensionless figure of merit ZT is improved by precisely controlling the M element content.

[0023] The parameter d is the In content and is in the range of greater than 0 and equal to or less than 0.15. By setting it in this range, the crystal structure becomes stable and excellent thermoelectric properties are exhibited. The added In remains at the Mg site, and the balance between In substitution and Mg vacancy formation improves the electrical conductivity in the medium temperature range, particularly around 723 K, reduces the thermal conductivity, and can improve the thermoelectric performance.

[0024] The parameters b to d preferably satisfy the following: 0.2≦b≦0.7, 0.002≦c≦0.025, and 0.001≦d≦0.1. By using such a composition, the electrical conductivity in the medium temperature range is further improved, the thermal conductivity is further reduced, and in particular, the dimensionless figure of merit ZT at 723K can be improved.

[0025] More preferably, the parameters b to d satisfy the following: 0.4≦b≦0.55, 0.005≦c≦0.02, and 0.003≦d≦0.1. By using such a composition, the electrical conductivity in the medium temperature range can be further improved, the thermal conductivity can be further reduced, and in particular, the dimensionless figure of merit ZT at 723K can be further improved.

[0026] In the inorganic compound, the lattice distortion (%) calculated by the Williamson-Hall method is preferably 3.5×10 －１ Above 5.0 x 10 －１ The following ranges are satisfied: The lattice distortion can further improve the electrical conductivity and further reduce the thermal conductivity in the medium temperature range, and in particular, can further improve the dimensionless figure of merit ZT at 723K.

[0027] The Williamson-Hall method is a simplified integral width method that uses the peak width as a function of 2θ to estimate the extent to which the peak has become broadened due to strain. In this specification, the lattice strain ε (%) is calculated using the following formula: β ｈｋｌ × cos θ = (K × λ) / D + {4 × (ε / 100) × sin θ} where β ｈｋｌ is the half-width of the diffraction peak of (hkl), D is the crystal size, K is the shape factor (0.9), λ is the wavelength of CuKα radiation (1.54056 Å), and θ is the X-ray diffraction angle. (hkl) uses the diffraction peaks of (100), (101), (110), (103), (104), and (203), eliminating the need for the D value, and β ｈｋｌ × cos θ was plotted on the y-axis and 4 × (ε / 100) × sin θ on the x-axis, and the average lattice strain ε (%) was calculated from the gradient of a straight line fitted to the plot.

[0028] Controlling the amounts of In and M added and the sintering time causes changes in the microstructure, which are expressed by the lattice strain ε. In particular, by setting the lattice strain ε within the above range, the lattice thermal conductivity decreases in the medium temperature range, the dimensionless figure of merit ZT improves, and an excellent n-type thermoelectric material can be provided.

[0029] The n-type thermoelectric material of the present invention may be in a form selected from the group consisting of powder, sintered body, and thin film, and can be applied to various thermoelectric conversion elements having high conversion efficiency in the medium temperature range from room temperature to 723 K.

[0030] Generally, powder may include crushed particles or powder. A powder compact can be formed by compressing powder using a press such as a powder compactor. A powder compact is a powder compact formed by compressing powder into a predetermined shape. When heated at a temperature below the melting point of the powder components, the contact surfaces of the powder particles adhere to each other, and the powder compact shrinks and densifies with increasing heating time. This phenomenon is called sintering, and the product obtained by sintering is called a sintered body. A thin film is a thin film and may include a layer formed by condensation of a gas phase on a solid surface.

[0031] When the n-type thermoelectric material is a powder or a sintered body, the inorganic compound may be composed of crystal grains having an average grain size in the range of 4 μm to 20 μm. This reduces grain boundary scattering of charge carriers and improves electrical conductivity in the medium temperature range. The average grain size is determined by randomly selecting images of particles observed with a scanning electron microscope (e.g., FESEM, Hitachi SU8000), measuring the grain size (major axis) of 100 particles using Image J (ver. 1.53k: open-source, public domain image processing software), and averaging the results.

[0032] The thermoelectric material of the present invention may be in the form of a thin film, which may be a crystalline thin film formed by physical vapor deposition, as described below, or a thin film containing the above-mentioned powder.

[0033] When the thermoelectric material of the present invention is a film containing an inorganic compound powder, the powder is mixed with an organic material and processed into a film. In this case, the organic material can be at least one selected from the group consisting of poly(3,4-ethylenedioxythiophene)polystyrenesulfonate (PEDOT:PSS), poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT), polyaniline (PANI), tetrathiafulvalene (TTF), and benzodifurandione paraphenylenevinylidene (BDPPV). These organic materials can provide a flexible thermoelectric material film.

[0034] In this case, the content of the powder is not particularly limited as long as a film can be formed, but the powder is preferably contained in the range of 4% by mass to 80% by mass, more preferably 4% by mass to 50% by mass, even more preferably 4% by mass to 10% by mass, and even more preferably 4% by mass to 7% by mass, relative to the organic material. This allows for a film that has flexibility and thermoelectric performance.

[0035] Next, an exemplary method for producing such an n-type thermoelectric material of the present invention will be described. Figure 1 is a flowchart showing the steps for producing the n-type thermoelectric material of the present invention.

[0036] Step S110: A mixture is prepared by mixing a raw material containing magnesium (Mg), a raw material containing antimony (Sb) and / or a raw material containing bismuth (Bi), a raw material containing indium (In), and a raw material containing an M element (wherein the M element is at least one element selected from the group consisting of selenium (Se) and tellurium (Te)). Step S120: The mixture obtained in step S110 is fired.

[0037] The n-type thermoelectric material of the present invention is obtained by the above-mentioned steps S110 and S120. Each step will be described in detail below.

[0038] In step S110, the Mg-containing raw material may be Mg metal alone, or a silicide, oxide, carbonate, nitride, oxynitride, chloride, fluoride, or oxyfluoride of Mg. The Sb-containing raw material may be Sb metal alone, or a silicide, oxide, carbonate, nitride, oxynitride, chloride, fluoride, or oxyfluoride of Sb. The Bi-containing raw material may be Bi metal alone, or a silicide, oxide, carbonate, nitride, oxynitride, chloride, fluoride, or oxyfluoride of Bi. The In-containing raw material may be In metal alone, or a silicide, oxide, carbonate, nitride, oxynitride, chloride, fluoride, or oxyfluoride of In. The M-containing raw material may be M metal alone, or a silicide, oxide, carbonate, nitride, oxynitride, chloride, fluoride, or oxyfluoride of M. The raw materials are preferably in the form of powder, granules or small lumps from the viewpoint of mixability and handling.

[0039] In step S110, the metal elements in the raw materials are selected from the group consisting of Mg ａ Sb ２－ｂ－ｃ Bi ｂ M ｃ In ｄ Here, the parameters a, b, c, and d satisfy the following: 3≦a≦3.5, 0≦b≦2, 0<c<0.04, 0<d≦0.15, and b+c≦2. Note that the preferred parameters are as described above, and therefore will not be described here.

[0040] In step S120, sintering may be performed by any method, such as spark plasma sintering (SPS), hot press sintering (HP), hot isostatic pressing (HIP), cold isostatic pressing (CIP), or pulse current sintering, but is preferably performed by spark plasma sintering (SPS). This allows a sintered body with suppressed grain growth to be obtained in a short time without using a sintering aid.

[0041] SPS is preferably carried out at a temperature of 723 K to 1173 K under a pressure of 30 MPa to 100 MPa for a time of 1 minute to 30 minutes. Under these conditions, the thermoelectric material of the present invention, which is the above-mentioned sintered body, can be obtained with a high yield. The sintering time is more preferably 5 minutes to 30 minutes, and even more preferably 10 minutes to 20 minutes. This generates lattice distortion, further improving electrical conductivity in the medium temperature range, and providing an n-type thermoelectric material with improved thermoelectric performance.

[0042] The resulting sintered body may then be pulverized by mechanical milling such as a ball mill, thereby obtaining the thermoelectric material of the present invention in the form of a powder.

[0043] The thermoelectric material of the present invention, which is a powder obtained in this manner, can be mixed with an organic material to provide a flexible thermoelectric material. In this case, the organic material and mixing ratio described above can be used.

[0044] Alternatively, the obtained sintered body may be used as a target to carry out physical vapor deposition, thereby providing a thin film made of the thermoelectric material of the present invention.

[0045] Second Embodiment In a second embodiment, a thermoelectric generating element using the thermoelectric material of the present invention described in the first embodiment will be described.

[0046] FIG. 2 is a schematic diagram showing a thermoelectric power generating element using the n-type thermoelectric material of the present invention.

[0047] A thermoelectric power generating element 200 according to the present invention includes an n-type thermoelectric material 210 and electrodes 220, 230 at each end of the n-type thermoelectric material 210. The n-type thermoelectric material 210 is the n-type thermoelectric material of the present invention described in the first embodiment.

[0048] The electrodes 220, 230 may be made of a common electrode material, illustratively Fe, Ag, Al, Ni, Cu, or the like.

[0049] FIG. 2 shows a state in which a chip made of n-type thermoelectric material 210 is joined by solder or the like to an electrode 230 on the low-temperature side, and an opposite end of the chip of n-type thermoelectric material 210 is joined by solder or the like to an electrode 220 on the high-temperature side.

[0050] When the thermoelectric generating element 200 of the present invention is placed in an environment where the electrode 220 is at a high temperature and the electrode 230 is at a lower temperature than the electrode 220, and the end electrodes are connected to an electric circuit or the like, a voltage is generated due to the Seebeck effect, and a current flows in the order of the electrode 230, the n-type thermoelectric material 210, and the electrode 220, as shown by the arrows in Figure 2. In more detail, the current flows based on the principle that electrons in the n-type thermoelectric material 210 obtain thermal energy from the electrode 220 on the high-temperature side, move to the electrode 230 on the low-temperature side, and release the thermal energy there. Such a thermoelectric generating element 200 may be called a single-element generating element.

[0051] Since the n-type thermoelectric material of the present invention described in embodiment 1 is used as the n-type thermoelectric material 210, even the thermoelectric power generation element 200 of a single element module exhibits excellent energy conversion efficiency in the medium temperature range from room temperature (300 K) to 723 K.

[0052] FIG. 3 is a schematic diagram showing another thermoelectric power generating element using the n-type thermoelectric material of the present invention.

[0053] Another thermoelectric power generating element 300 according to the present invention includes a pair of n-type and p-type thermoelectric materials 310 and 320, and electrodes 330 and 340 at their respective ends. The electrodes 330 and 340 electrically connect the n-type and p-type thermoelectric materials 310 and 320 in series.

[0054] The n-type thermoelectric material 310 is the thermoelectric material of the present invention described in embodiment 1. The n-type thermoelectric material of the present invention exhibits excellent performance over the entire temperature range (300 to 723 K) and is effective for converting waste heat into energy.

[0055] Here, the p-type thermoelectric material 320 is not particularly limited, but preferably has high thermoelectric performance (for example, ZT of 0.4 to 1.6) in the medium temperature range from room temperature to 723 K. Illustrative examples of the p-type thermoelectric material 320 include BiSbTe-based, MgAgSb-based, and AgSbSe-based materials. An exemplary composition of a BiSbTe-based material is, for example, Bi ０．５ Sb １．５ Te ３ , Bi ０．４ Sb １．６ Te ３ Exemplary compositions of the MgAgSb system include, for example, MgAgSb, MgAg ０．９６５ Ni ０．００５ Sb ０．９９ An exemplary composition of the AgSbSe system is, for example, AgSbSe ２ Note that these are examples and are not limiting.

[0056] The electrodes 330, 340 may be made of a common electrode material, illustratively Fe, Ag, Al, Ni, Cu, or the like.

[0057] When the thermoelectric power generation element 300 of the present invention is placed in an environment where the electrode 330 is at a high temperature and the electrode 340 is at a lower temperature than the electrode 330, and the end electrodes are connected to an electric circuit or the like, a voltage is generated by the Seebeck effect, and a current flows in the order of the electrode 340, the n-type thermoelectric material 310, the electrode 330, and the p-type thermoelectric material 320, as shown by the arrows in FIG. 3 .

[0058] The high-temperature side temperature of the thermoelectric power generation element 300 of the two-pair module is controlled by the stability of the p-type thermoelectric material 320, so that, for example, by using a p-type thermoelectric material (e.g., MgAgSb) that has high performance in a low-temperature range of 573 K or less, it is possible to provide a thermoelectric power generation element that operates in a temperature range from room temperature to a low temperature range. Such a thermoelectric power generation element 300 may be called a two-pair power generation element.

[0059] In addition, as a thermoelectric material, the n-type thermoelectric material of the present invention is Mg ３ Sb ２When the thermoelectric material of the present invention uses a powder made of an inorganic compound having the system as a parent phase to which the M element and the In element are added, a film containing the powder, or a thin film obtained by targeting a sintered body made of the above inorganic compound, it is possible to provide a flexible thermoelectric power generation module as an IoT power source.

[0060] 3, the n-type thermoelectric material of the present invention may be used in a U-type thermoelectric power generation element (not shown). In this case as well, the n-type thermoelectric material of the present invention and a known p-type thermoelectric material are alternately electrically connected in series.

[0061] The present invention will now be described in detail using specific examples, but it should be noted that the present invention is not limited to these examples.

[0062] [Raw Materials] In the following examples, Mg (powder, purity 99.99%, manufactured by Sigma-Aldrich Japan LLC), Sb (powder, purity 99.99%, manufactured by Sigma-Aldrich Japan LLC), Bi (powder, purity 99.99%, manufactured by Sigma-Aldrich Japan LLC), Te (powder, purity 99.99%, manufactured by Sigma-Aldrich Japan LLC), and, if necessary, In (powder, purity 99.99%, manufactured by Sigma-Aldrich Japan LLC) were used.

[0063] [Examples 1 to 10] In Examples 1 to 10, as shown in Table 1, the compounds of the composition formula Mg ａ Sb ２－ｂ－ｃ Bi ｂ Te ｃ In ｄ The raw materials were mixed so as to satisfy the above conditions, and an n-type thermoelectric material was produced.

[0064] Each raw material powder was weighed to satisfy the composition shown in Table 1, filled into a stainless steel ball mill container in a glove box, and mixed in a ball mill for 5 hours. The mixture was then filled into a graphite sintering die (inner diameter 10 mm, height 30 mm) and sintered under the sintering conditions shown in Table 1 using a spark plasma sintering apparatus (SPS, manufactured by SPS Syntex, Inc., SPS-1080 system).

[0065] The appearance of the obtained sintered body was observed. The microstructure and composition of the sintered body were evaluated using a scanning electron microscope (FESEM, Hitachi SU8000) and a scanning transmission electron microscope (STEM, JEM-ARM300F) equipped with an energy dispersive spectrometer (EDS, XFlash FlatQUAD 5060F). The accelerating voltage of the EDS was 300 keV.

[0066] The obtained fired body was wet-pulverized using ethanol in an agate mortar. After pulverization, the particles of the fired body were sieved through a mesh (opening 45 μm), and only particles with a particle size of 45 μm or less that passed through the mesh were extracted. The particles were identified by powder X-ray diffraction (Rigaku Corporation, Rigaku SmartLab 9 kW). The θ of the diffraction peaks (100), (101), (110), (103), (104), and (203) in the XRD pattern were respectively determined using the Williamson-Hall method described above, and the β ｈｋｌ × cos θ was plotted on the y-axis and 4 × (ε / 100) × sin θ on the x-axis, and the average lattice strain ε (%) was calculated from the gradient of a straight line fitted to the plot.

[0067] The sintered body was cut into a 1.5 mm x 1.5 mm x 9 mm rectangular parallelepiped using a high-speed cutter (see Figure 4), and electrical conductivity and thermoelectric property measurements were performed. Electrical conductivity was measured using a DC four-terminal method. Thermoelectric properties, such as the Seebeck coefficient and thermal conductivity, were measured using a steady-state temperature difference method using a thermoelectric property measurement and evaluation device (ZEM-3, manufactured by Advance Riko Co., Ltd.) and a thermal conductivity evaluation device (Hyperflash 467, manufactured by Netsch GmbH), respectively. Measurements were performed under helium gas atmosphere over a temperature range from room temperature to 723 K. The electrical power factor was calculated from the thermoelectric power obtained from the electrical conductivity or electrical resistivity and the Seebeck coefficient, and the dimensionless figure of merit ZT was calculated from the Seebeck coefficient, electrical conductivity, and thermal conductivity.

[0068] For the sake of simplicity, the manufacturing conditions for the samples of Examples 1 to 10 are summarized in Table 1, and the results will be explained below.

[0069]

[0070] FIG. 4 is a diagram showing the appearance of the sample of Example 2.

[0071] As shown in Figure 4, the sample of Example 2 was a disk-shaped sintered body with a diameter of 10 mm and a thickness of 2 mm. Although not shown, other samples had similar shapes. Figure 4 also shows a sample machined into a rectangular parallelepiped for measuring electrical conductivity and thermoelectric properties.

[0072] FIG. 5 shows SEM images of the samples of Examples 2, 4 and 6.

[0073] 5(A) to 5(C) are SEM images of the samples of Examples 2, 4, and 6, respectively. Morphological observations of the samples by fracture showed similar grain sizes, and no secondary phases were observed. The average grain sizes were calculated and were all in the range of 4 μm to 20 μm. For example, the sample of Example 2, which was added with In and sintered for 20 minutes, had an average grain size of 10 μm, and the same was true for the sample of Example 7, which was added with In and sintered for 30 minutes. It was found that the grain size was independent of the sintering time.

[0074] The micron-sized pores seen in the figure are generated by the loss of Mg due to SPS sintering. Comparing Figures 5(A) and 5(B), the number of pores in the sample of Example 1, to which In was added, was smaller than that in the sample of Example 4, to which In was not added. This indicates that the addition of In effectively suppresses the loss of Mg due to sintering. Although not shown, the samples of Examples 2, 3, 5, and 7 to 10 also had similar EDS images.

[0075] FIG. 6 shows an STEM image and an EDS image of the sample of Example 2.

[0076] According to the STEM image in Figure 6, the grain boundaries of the sample of Example 2 are clear, and no segregation phases were observed in the triangular region. The EDS image is shown in grayscale, with bright regions indicating the distribution of elements. The EDS image revealed that the elements Mg, Sb, Bi, Te, and In were uniformly distributed. Although not shown, the samples of Examples 1, 3, and 5 to 10 also had similar EDS images.

[0077] Composition analysis by EDS revealed that the composition of the sample of Example 2 was the same as the designed composition. ａSb ２－ｂ－ｃ Bi ｂ Te ｃ In ｄ It was confirmed that the parameters a, b, c, and d respectively satisfied the following: 3≦a≦3.5, 0≦b≦2, 0<c<0.04, 0<d≦0.15, and b+c≦2. The samples of Examples 1, 3, and 5 to 10 also satisfied the above composition formula.

[0078] Figure 7 shows the XRD pattern of the sample of Example 2. Figure 8 shows the XRD pattern of the sample of Example 4.

[0079] The results of Rietveld refinement are also shown in Figures 7 and 8. Both XRD patterns are of the trigonal α-La crystal with the P-3m1 space group. ２ O ３ The lattice parameters could be indexed by the type structure. According to Figures 7 and 8, the addition of In slightly increased the lattice parameter. This indicates that In was substitutionally doped at the Mg site, and is caused by the difference in atomic radius between Mg (1.45 Å) and In (1.56 Å). Although not shown, the XRD patterns of the samples of Examples 1, 3, and 5 to 10 were also similar. Note that Te is a crystalline form of Mg. ３ Sb ２ It is commonly used to adjust the carrier concentration in the system and is known to substitute into the Sb site.

[0080] From the above, the samples of Examples 1 to 3 and Examples 5 to 10 are inorganic compounds containing magnesium (Mg), antimony (Sb) and / or bismuth (Bi), indium (In), and tellurium (Te) as the M element, and Mg ａ Sb ２－ｂ－ｃ Bi ｂ M ｃ In ｄ It has been shown that the parameters a, b, c and d satisfy the following conditions: 3≦a≦3.5, 0≦b≦2, 0<c<0.04, 0<d≦0.15, and b+c≦2, respectively.

[0081] In addition, the samples of Examples 1 to 3 and Examples 5 to 10 were La ２ O３ It was shown that the material has a P-3m1 structure and symmetry in the space group P-3m1. It was also confirmed that In was substitutionally solid-solved at the Mg site and Te was substitutionally solid-solved at the Sb / Bi site.

[0082]

[0083] As shown in Table 2, the addition of In reduces the lattice distortion ε (%) of the inorganic compound to 3.5×10 －１ It was found that the lattice strain ε (%) increased as the sintering time increased.

[0084] FIG. 9 is a graph showing the temperature dependence of the electrical conductivity of samples of Example 2 and Examples 4 to 7. FIG. 10 is a graph showing the temperature dependence of the weighted mobility of samples of Example 2 and Examples 4 to 7. FIG. 11 is a graph showing the temperature dependence of the Seebeck coefficient of samples of Example 2 and Examples 4 to 7. FIG. 12 is a graph showing the temperature dependence of the electrical output factor of samples of Example 2 and Examples 4 to 7. FIG. 13 is a graph showing the temperature dependence of the total thermal conductivity of samples of Example 2 and Examples 4 to 7. FIG. 14 is a graph showing the temperature dependence of the lattice thermal conductivity of samples of Example 2 and Examples 4 to 7. FIG. 15 is a graph showing the temperature dependence of the dimensionless figure of merit ZT of samples of Example 2 and Examples 4 to 7. FIG. 16 is a graph showing the Te content dependence of the dimensionless figure of merit ZT.

[0085]

[0086]

[0087]

[0088] 9, comparing Example 2 with Example 4, the addition of In improved the electrical conductivity in the low temperature range. Furthermore, comparing Example 2 with Examples 5 to 7, the electrical conductivity was further improved by increasing the sintering time, reaching approximately 5×10 at room temperature (300 K). ４ (Ωm) －１ achieved an electrical conductivity of .

[0089] 10, comparing Example 2 and Example 4, the weighted mobility in the low temperature region was significantly improved by adding In. This is thought to be because the addition of In and the control of the amount of Te increased the structural order of defects, thereby weakening the localization caused by the disorder of the electronic state in order to obtain high mobility.

[0090] 11, it was confirmed that all samples had n-type conductivity, with a large absolute Seebeck coefficient of 180 μV / K or more. All of the samples with In added exhibited a lower Seebeck coefficient than the sample of Example 4 without In added, but showed high electrical conductivity (FIG. 9) over the entire temperature range. This is thought to be because the addition of In suppressed the loss of Mg and the decrease in carrier concentration.

[0091] 12, the electric power factor of the sample doped with In was high across the entire temperature range, but was particularly improved in the room temperature to low temperature range (300K to 400K). As shown in Table 3, comparing Example 2 and Example 4, the addition of In improved the electric power factor (300K) by approximately 60%. This indicates that the n-type thermoelectric material of the present invention can be applied to thermoelectric power generation elements, and is suitable for recovering waste heat in various thermoelectric cooling applications and as a power source for IoT operation, and can provide thermoelectric power generation elements for consumer use.

[0092] 13, it was found that the total thermal conductivity decreased by adding In and by extending the sintering time. The Lorentz number L was calculated, and the lattice thermal conductivity was calculated by subtracting the electronic thermal conductivity from the total thermal conductivity. As shown in FIG. 14, the lattice thermal conductivity decreased by adding In and by extending the sintering time over the entire temperature range.

[0093] 15, the dimensionless figure of merit ZT of the sample doped with In was improved over the entire temperature range compared to the sample not doped with In, exceeding 0.4 at room temperature and easily exceeding 1.3 at 673 K and 723 K, as shown in Tables 4 and 5. For example, it was found that a technical effect (e.g., a dimensionless figure of merit ZT of 1.5 or more at 723 K) can be obtained when the parameters a, b, c, and d in the parameter expression satisfy the following, respectively: 3.05≦a≦3.25, 0.475≦b≦0.51, 0.0045≦c≦0.025, and 0.0045≦d≦0.15.

[0094] According to FIG. 16, it is shown that the dimensionless figure of merit ZT depends on the amount of Te. ３．５－ｘ In ｘ Sb １．７ Bi ０．２６ Te ０．０４ n-type Mg (x=0.03) ３ Sb ２ However, the present inventors have not suggested the relationship between the dimensionless figure of merit (ZT) and the amount of Te. It was only through experiments that the present inventors discovered that a dimensionless figure of merit (ZT) well exceeding 1.3 at 773 K can be achieved simply by controlling the Te parameter to less than 0.04, preferably between 0.002 and 0.025.

[0095] The n-type thermoelectric material of the present invention has excellent thermoelectric performance over the entire temperature range of 300 K to 723 K. ２ Te ３ These materials can function as alternative materials for thermoelectric cooling devices and power generation devices used in various electrical devices. In particular, if they are thinned, they can be used to provide flexible thermoelectric power generation elements as IoT power sources.

[0096] 200, 300 Thermoelectric power generation element 210, 310 n-type thermoelectric material 220, 230, 330, 340 Electrode 320 p-type thermoelectric material

Claims

1. An inorganic compound containing magnesium (Mg), antimony (Sb) and / or bismuth (Bi), indium (In), and an M element (wherein the M element is at least one element selected from the group consisting of selenium (Se) and tellurium (Te)), wherein the inorganic compound is Mg ａ Sb ２－ｂ－ｃ Bi ｂ M ｃ In ｄ The n-type thermoelectric material is represented by the following formula: where the parameters a, b, c, and d satisfy the following conditions: 3≦a≦3.5, 0≦b≦2, 0<c<0.04, 0<d≦0.15, and b+c≦2, respectively.

2. The n-type thermoelectric material according to claim 1, wherein the parameters b, c, and d satisfy the following conditions: 0.2≦b≦0.7, 0.002≦c≦0.025, and 0.001≦d≦0.1, respectively.

3. The n-type thermoelectric material according to claim 2, wherein the parameters b, c, and d satisfy the following conditions: 0.4≦b≦0.55, 0.005≦c≦0.02, and 0.003≦d≦0.1, respectively.

4. The inorganic compound is La ２ O ３ The n-type thermoelectric material according to any one of claims 1 to 3, having a structure and space group P-3m1 symmetry.

5. The n-type thermoelectric material according to claim 4, wherein the In is substitutionally solid-solved in the Mg sites.

6. The lattice strain (%) calculated by the Williamson-Hall method is 3.5 x 10 －１ Above 5.0 x 10 －１ The n-type thermoelectric material according to any one of claims 1 to 5, wherein the n-type thermoelectric material has a molecular weight of 1000 or more and a molecular weight of 1000 or more.

7. The n-type thermoelectric material according to any one of claims 1 to 6, wherein the thermoelectric material is in a form selected from the group consisting of powder, sintered body, and thin film.

8. The n-type thermoelectric material according to claim 7, wherein the thermoelectric material is in the form of a powder or a sintered body, and the inorganic compound is composed of crystal grains having an average grain size in the range of 4 μm to 20 μm.

9. The n-type thermoelectric material according to claim 7, wherein the thermoelectric material is in the form of a thin film, the inorganic compound is composed of crystal grains having an average grain size in the range of 4 μm to 20 μm, and further contains an organic material.

10. A method for producing the n-type thermoelectric material according to any one of claims 1 to 9, comprising: mixing a raw material containing magnesium (Mg), a raw material containing antimony (Sb) and / or a raw material containing bismuth (Bi), a raw material containing indium (In), and a raw material containing an M element (wherein the M element is at least one element selected from the group consisting of selenium (Se) and tellurium (Te)) to prepare a mixture; and sintering the mixture.

11. The method of claim 10, wherein said sintering is spark plasma sintering.

12. The method according to claim 11, wherein the spark plasma sintering is performed at a temperature in the range of 723K to 1173K under a pressure of 30MPa to 100MPa for a period of 5 minutes to 30 minutes.

13. The method according to any one of claims 10 to 12, further comprising pulverizing the sintered body obtained by sintering.

14. The method of claim 13, further comprising mixing the powder obtained by grinding with an organic material.

15. The method according to any one of claims 10 to 12, further comprising carrying out physical vapor deposition using the sintered body obtained by sintering as a target.

16. A thermoelectric power generating element comprising at least an n-type thermoelectric material, wherein the n-type thermoelectric material is the n-type thermoelectric material according to any one of claims 1 to 9.

17. The thermoelectric generating element according to claim 16, comprising p-type thermoelectric materials connected in series alternating with the n-type thermoelectric materials.

18. The thermoelectric power generating element according to claim 17, wherein the p-type thermoelectric material is selected from the group consisting of BiSbTe-based, MgAgSb-based, and AgSbSe-based materials.

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