JP7656911B2 - Thermoelectric material, its manufacturing method and thermoelectric generation element - Google Patents

Description

The present invention relates to a thermoelectric material, its manufacturing method, and its thermoelectric generating element.

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

In addition to excellent thermoelectric properties, thermoelectric materials applied to such thermoelectric conversion elements are required to be made of low-cost and harmless elements, and to have mechanical properties and chemical stability. Among such thermoelectric materials, research on transition metal silicides, represented by chromium silicide (CrSi ₂ ), has been active. According to Non-Patent Document 1, it is known that hexagonal chromium silicide single crystals have a higher power factor in the direction along the c-axis compared to the a-axis. However, in chromium silicide polycrystals, the advantageous properties of the c-axis orientation cannot be utilized, and the thermal conductivity is high due to phonon scattering at the grain boundaries, so the Seebeck coefficient is small, and improvement is required.

F. Yu. Solomkin et al., Technical Physics, 2013, Vol. 58, No. 2, pp. 289-293

The objective of the present invention is to provide a thermoelectric material made of polycrystals containing chromium silicide with excellent thermoelectric properties, a manufacturing method thereof, and a thermoelectric generating element.

The thermoelectric material according to the present invention is made of a sintered body made of grains containing chromium silicide (CrSi ₂ ) as a main component, and the grains are oriented in the c-axis direction, thereby solving the above-mentioned problems.
The sintered body may have an _fL value in the thickness direction (Lotgering factor in the 0003 direction) in the range of 0.5 or more and 1 or less.
The _fL value (Lotgering Factor in the 0003 direction) may be in the range of 0.53 to 0.95.
The particle size may be in the range of 0.5 μm to 1.5 μm.
The density of the sintered body may be 95% or more.
A portion of the silicon in the chromium silicide may be substituted with an element selected from the group consisting of germanium (Ge), vanadium (V), aluminum (Al), zirconium (Zr), niobium (Nb), and copper (Cu).
A portion of the chromium in the chromium silicide may be substituted with an element selected from the group consisting of manganese (Mn) and rhenium (Re).
The particles may further contain a second phase which is a chromium silicide.
The second phase may have a cubic crystal structure and belong to the _P213 space group.
The second phase may be contained in an amount of more than 0 mass % to 6 mass %.
The method for manufacturing the above-mentioned thermoelectric material according to the present invention includes dispersing a raw material powder mainly composed of chromium silicide ( _CrSi2 ) in a solvent to prepare a raw material slurry, solidifying and molding the raw material slurry while applying a magnetic field to form a molded body, and sintering the molded body, thereby solving the above-mentioned problems.
The solvent may be an aliphatic hydrocarbon selected from the group consisting of pentane, hexane, heptane and octane, or an alcohol selected from the group consisting of methanol, ethanol and propanol.
The particle size (median diameter D50) of the raw material powder may be in the range of 350 nm to 550 nm.
The magnetic field may be in the range of 8T to 14T.
The forming of the compact may be performed by slip casting.
The density of the compact may be in the range of 40% to 70%.
The sintering may be performed at a temperature in the range of 1373K or more and 1773K or less.
The method may further include pressing the green body prior to or simultaneously with the sintering.
The thermoelectric power generation element according to the present invention includes p-type thermoelectric materials and n-type thermoelectric materials alternately connected in series, and the p-type thermoelectric materials and/or the n-type thermoelectric materials are the above-mentioned thermoelectric materials, thereby solving the above-mentioned problems.

The thermoelectric material of the present invention is made of a sintered body made of grains mainly composed of chromium silicide ( _CrSi2 ), and is characterized in that the grains are oriented in the c-axis direction. Because of the c-axis orientation, it is possible to achieve a power factor equivalent to that of a single crystal of chromium silicide. A thermoelectric material made of such a sintered body is applied to a thermoelectric conversion element.

The method for producing a thermoelectric material of the present invention includes dispersing a raw material powder mainly composed of chromium silicide ( _CrSi2 ) in a solvent to prepare a raw material slurry, molding the raw material slurry while applying a magnetic field to form a compact, and sintering the compact. By applying a magnetic field, the particles are oriented in the c-axis direction parallel to the direction of the magnetic field application. This method does not require single crystal growth technology, and a sintered body having thermoelectric properties equivalent to those of a single crystal can be obtained simply by applying a magnetic field, which is advantageous in that it reduces production costs.

Schematic diagram showing a thermoelectric material of the present invention. Flowchart showing the manufacturing process of the thermoelectric material of the present invention Schematic diagram showing a thermoelectric conversion element of the present invention. Diagram showing a micrograph of an ingot FIG. 1 shows an XRD pattern of the prepared raw material powder. FIG. 1 shows an SEM image of the raw material powder. Particle size distribution of raw powder Procedure showing part of the manufacturing process of the sintered bodies of Examples 1 and 2 FIG. 1 shows XRD patterns of sintered bodies of Examples 1 to 4. 1 shows an SEM image and an EDX image of the sintered body of Example 1. FIG. 1 shows the crystal orientations from electron backscatter diffraction (EBSD) on the ND planes of the sintered bodies of Examples 1, 3, and 4. FIG. 1 shows the temperature dependence of electrical resistivity of sintered bodies of Examples 1 to 4. FIG. 1 shows the temperature dependence of the Seebeck coefficient of the sintered bodies of Examples 1 to 4. FIG. 13 is a graph showing the temperature dependence of the power factor of the sintered bodies of Examples 1 to 4. FIG. 1 shows the temperature dependence of thermal conductivity of sintered bodies of Examples 1 to 4. FIG. 13 is a graph showing the temperature dependence of the dimensionless figure of merit ZT of the sintered bodies of Examples 1 to 4. FIG. 1 is a graph showing the relationship between the dimensionless figure of merit ZT and the _fL value of the sintered bodies of Examples 1 to 4.

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

(Embodiment 1)
In the first embodiment, a thermoelectric material and a method for producing the same according to the present invention will be described.
FIG. 1 is a schematic diagram showing a thermoelectric material of the present invention.

The thermoelectric material of the present invention is made of a sintered body 100 made of particles 110 mainly composed of chromium silicide (CrSi ₂ ). Chromium silicide is known as a thermoelectric material made of a low-cost and harmless element. Furthermore, in the thermoelectric material of the present invention, the particles 110 are oriented in the c-axis direction as shown in FIG. 1 . Thus, even though it is a sintered body 100, since the particles 110 are oriented in the c-axis direction, it is possible to achieve a power factor equivalent to that of a bulk single crystal of chromium silicide. This will be described in detail below.

Here, the chromium silicide crystal belongs to the hexagonal crystal system, belongs to the P6 ₂ 22 space group (the 180th space group of the International Tables for Crystallography), and has the crystal structure parameters (atomic coordinate positions) shown in Table 1.

The particles 110 may be a single crystal of a single phase of chromium silicide, or may be a single crystal in which a dopant is added to a parent phase of chromium silicide. If the composition of Cr and Si is slightly deviated from the stoichiometric ratio due to defects or the like, or if a part of chromium (Cr) or silicon (Si) is replaced with another element, the lattice constant changes, but the crystal structure and the atomic positions given by the sites occupied by the atoms and their coordinates do not change so much that the chemical bonds between the skeletal atoms are broken. Therefore, if the length of the Cr-Si chemical bond (nearest atom distance) calculated from the _lattice constant and atomic coordinates obtained by La Bail analysis of the results of X-ray diffraction or neutron diffraction of the target sintered body in the P6 2 22 space group is within ±5% of the length calculated from the lattice constant and atomic coordinates of the crystal shown in Table 1, the sintered body 100 may be determined to be made of particles 110 containing chromium silicide as the main component.

When the composition deviation or the amount of addition (substitution amount) is small, it can be easily determined that the crystal structure is the same as that of _CrSi2 when the lattice constant calculated from the X-ray diffraction result measured on the target sintered body and the diffraction peak position (2θ) calculated using the crystal structure data in Table 1 match for the main peaks (e.g., eight peaks).

When the particles 110 are made of undoped chromium silicide single crystals, they become p-type semiconductors and function as p-type thermoelectric materials, but in order to adjust the carrier concentration, some of the Si in the chromium silicide may be replaced with an element selected from the group consisting of germanium (Ge), vanadium (V), aluminum (Al), zirconium (Zr), niobium (Nb), and copper (Cu). This can increase the Seebeck coefficient. Among these, Ge is preferable. The amount of addition is appropriately adjusted within a range that does not change the crystal structure, and may be, for example, in the range of 0.5 at% to 5 at%. Within this range, it is easy to adjust the carrier concentration.

On the other hand, a part of the Cr in the chromium silicide may be replaced with an element selected from the group consisting of manganese (Mn) and rhenium (Re). This makes the particles 110 an n-type semiconductor and functions as an n-type thermoelectric material. Of these, Mn is preferred. The amount of addition is appropriately adjusted within a range that does not change the crystal structure, but may be, for example, in the range of 15 at% to 25 at%. Within this range, it becomes an n-type semiconductor and is easy to obtain a high Seebeck coefficient.

The particles 110 are preferably composed of a single phase of doped/undoped chromium silicide, but may contain a second phase to the extent that the thermoelectric properties are not impaired. Such a second phase may be a chromium silicide containing Cr and Si, and is preferably CrSi, which is a space group _P213 having a cubic crystal structure. CrSi is preferred because it does not impair the thermoelectric properties of chromium silicide. In this way, chromium silicide is permitted as a second phase generated during manufacturing, which is advantageous in terms of easing manufacturing conditions and improving yields.

The content of the second phase contained in the particles 110 is preferably more than 0 mass% and not more than 15 mass%. This range does not impair the thermoelectric properties. The content of the second phase is more preferably more than 0 mass% and not more than 10 mass%, even more preferably more than 0 mass% and not more than 6 mass%, and even more preferably 1 mass% or more and not more than 5 mass%. In this specification, the content is calculated by Rietveld analysis of the X-ray diffraction pattern. From this perspective, in this specification, the amount of chromium silicide as the main component in the sintered body 100 is more than 85 mass%, preferably more than 90 mass%, and more preferably more than 94 mass%.

The particle size of the particles 110 is not particularly limited, but is preferably in the range of 0.5 μm to 1.5 μm. In this range, the particles 100 can be efficiently c-axis oriented. More preferably, the particle size of the particles 110 is in the range of more than 0.85 μm to 1.0 μm. Even more preferably, the particle size of the particles 110 is in the range of 0.88 μm to 0.92 μm. In this specification, the particle size is calculated as the equivalent circle diameter from the area of the crystal grain by electron backscatter diffraction (EBSD) using an electron microscope such as a scanning electron microscope. The area of the crystal grain is calculated from the step width during measurement and the number of data points within the crystal grain.

The sintered body 100 of the present invention is characterized in that the particles 110 are c-axis oriented. In this specification, c-axis orientation means that the peak intensity of the peak corresponding to the (0003) plane is the highest in the X-ray diffraction pattern obtained when X-ray diffraction measurement is performed on the polished sintered body.

In the sintered body 100 of the present invention, the _fL value (Lotgering factor in the 0003 direction) in the thickness direction of the sintered body 100 is preferably in the range of 0.5 to 1. When the particles 110 are c-axis oriented so as to satisfy this range, the power factor is large and excellent thermoelectric properties are obtained. More preferably, the _fL value in the thickness direction of the sintered body 100 is in the range of 0.53 to 0.95. Within this range, the power factor is large and particularly excellent thermoelectric properties are obtained.

The _fL value is calculated by the following formula:
f _L = (P-P ₀ )/(1-P ₀ )
P ₀ = (ΣI ₀ (000m))/(ΣI ₀ (hklm))
P=(ΣI(000m))/(ΣI(hklm))
Here, ΣI ₀ (000m) is the sum of the measured peak intensities of the (0003) and (0006) diffraction planes among all diffraction planes in the XRD measurement range of the sintered body of interest, where 2θ is in the range of 20 to 100°, and ΣI ₀ (hklm) is the sum of the measured peak intensities of all diffraction planes of the sintered body of interest. ΣI (000m) is the calculated peak intensities of the (0003) and (0006) diffraction planes among all diffraction planes of the standard sample, and ΣI ₀ (hklm) is the sum of the calculated peak intensities of all diffraction planes of the standard sample. The standard sample may be a powder sample or an ICDD card. For example, if it is a polycrystalline powder, f _L =0 (P=P ₀ ), and if it is a single crystal, f _L =1 (P=1), but even if it is a polycrystalline, if the deviation of the c-axis of all crystal grains is small (for example, within 10 degrees), f _L =1 can be achieved. However, even in the case of a polycrystal that satisfies f _L =1, the XRD pattern measured from another direction differs from that of a single crystal, and therefore it is easy to distinguish between the polycrystal and the single crystal.

In this way, the inventors of the present application have found that even if the chromium silicide ( _Cr2Si ) bulk single crystal is not used, a chromium silicide sintered body having a c-axis orientation within the above-mentioned range can provide excellent thermoelectric properties comparable to those of a bulk single crystal. This makes it possible to eliminate the need for advanced techniques and expensive equipment required for bulk single crystal growth, making the thermoelectric material made of the sintered body 100 of the present invention extremely effective.

The sintered density of the sintered body 100 of the present invention is not particularly limited, but is preferably 95% or more. This reduces the number of voids and holes between the particles 110, resulting in high thermoelectric properties. The sintered density is more preferably 96% or more, and even more preferably 98% or more. Note that the sintered density in this specification is measured by the Archimedes method and is expressed as a percentage (relative density) of the true density.

Next, a method for producing a thermoelectric material made of the sintered body 100 of the present invention will be described.
FIG. 2 is a flow chart showing the steps of manufacturing the thermoelectric material of the present invention.

A thermoelectric material made of the sintered body 100 of the present invention is obtained by the following steps S210 to S230.
Step S210: A raw material powder containing chromium silicide (CrSi ₂ ) as a main component is dispersed in a solvent to prepare a raw material slurry.
Step S220: The raw material slurry is solidified and molded while a magnetic field is applied to form a molded body.
Step S230: Sinter the compact.
In step S220, a magnetic field is applied, so that the raw material powder consisting of single crystal particles is oriented so that their c-axes are parallel to the direction of the applied magnetic field. The compact thus obtained is sintered in step S230, so that the above-mentioned sintered body 100 is obtained. Each step will be described in detail below.

In step S210, the raw powder mainly composed of chromium silicide has each particle composed of a single crystal, and is the chromium silicide described with reference to FIG. 1. The raw powder may also be chromium silicide in which some of the Si has been replaced with other elements such as Ge, or in which some of the Cr has been replaced with other elements such as Mn. The amount of addition (substitution amount) is as described above.

In step S210, the content of the raw powder in the raw slurry is preferably in the range of 10 mass% to 30 mass%. In this range, the raw powder can be dispersed well. The content of the raw powder is more preferably in the range of 15 mass% to 25 mass%.

The particle size (median diameter D50) of the raw material powder is preferably in the range of 350 nm to 550 nm. In this range, the raw material powder can be well dispersed in the solvent, and the c-axis orientation is easily achieved in step S220 described below. The particle size (median diameter D50) of the raw material powder is more preferably in the range of 380 nm to 425 nm.

In step S210, the solvent is not particularly limited as long as the raw material powder can be dispersed in it, but a solvent with low viscosity and no reactivity with chromium silicide is selected. The solvent is preferably an aliphatic hydrocarbon selected from the group consisting of pentane, hexane, heptane, and octane, or an alcohol selected from the group consisting of methanol, ethanol, and propanol. These solvents can promote the dispersion of the raw material powder. Among them, alcohol such as ethanol is preferred.

In step S210, a polycationic polymer or a polyanionic polymer may be further added. This causes the particles of the raw material powder to be coated with the polymer, and the particles are well dispersed by electrostatic repulsion. As a result, in step S220, the rotation of the particles is promoted by application of a magnetic field, and a c-axis-oriented sintered body with an _fL value of 0.5 or more can be obtained.

Examples of polycationic polymers include trialkylamines (PAM), polyethyleneimines (PEI), polylysines (PL), polypeptides, chitosan, polysaccharides, or copolymers thereof. Examples of polyanionic polymers include ammonium polycarboxylates, polystyrene sulfonates, polyacrylates, polymethacrylates, polyvinyl sulfates, polyphosphates, carrageenans, gellan gums, carboxymethyl celluloses, carboxymethyl agaroses, carboxymethyl dextran, carboxymethyl chitins, carboxymethyl chitosans, polymers modified with carboxymethyl groups, and alginates.

The amount of the polycationic polymer or polyanionic polymer added may be, for example, in the range of more than 0 mass% to 5 mass% or less, preferably in the range of more than 0 mass% to 1 mass% or less, and more preferably in the range of more than 0 mass% to 0.5 mass% or less. This allows a sintered body to be obtained whose _fL value satisfies the range of 0.53 to 0.95.

In step S210, an acid or alkali may be added to the raw slurry. This allows the particles to disperse even better.

In step S220, the raw material slurry is solidified and molded in a magnetic field to obtain a molded body. The magnetic field applied is preferably in the range of 8 T (tesla) or more and 14 T or less. This range can promote rotation of the particles. The magnetic field applied is more preferably in the range of 10 T or more and 14 T or less. This range causes the particles to rotate, further promoting c-axis orientation.

In step S220, the direction of the magnetic field application can be selected from any direction, such as a direction parallel to gravity or a direction perpendicular to gravity. This allows the orientation direction to correspond to the method of applying the magnetic field, without being limited by the molding direction or shape.

In step S220, various methods may be appropriately employed for solidification. Methods for solidification include slip casting, pressure filtration, tape casting, and electrophoretic deposition. Among these, slip casting is preferred because it produces a molded body with good c-axis orientation and there are no limitations on the shape. By employing slip casting, a magnetic field can be applied in any direction, even for complex shapes, so that a sintered body with c-axis orientation in any direction can be obtained.

In step S220, the density of the compact is preferably in the range of 40% to 70%. As a result, a sintered body having a sintered density of 96% or more is obtained by sintering in the subsequent step S230. The density of the compact is measured by the Archimedes method.

Following step S220, the compact may be pressurized. For example, a cold isostatic press (CIP) may be used for the pressurization. This allows a high-density sintered body to be obtained by the subsequent sintering step S230.

In step S230, the sintering of the compact is preferably carried out in a temperature range of 1373K or more and 1773K or less. In this range, the particles grow and a high-density sintered body is obtained. More preferably, the sintering is carried out in a temperature range of 1423K or more and 1623K or less. In this range, the particles grow and a high-density sintered body is obtained.

In step S230, the sintering method is not particularly limited, and spark plasma sintering (SPS), atmospheric sintering, hot press (HP) sintering, and hot isostatic pressing (HIP) sintering can be appropriately used. The sintering time varies depending on the sintering method, but may be, for example, 5 minutes or more and 30 minutes or less when spark plasma sintering (SPS) is used.

In step S230, the compact may be sintered under pressure so that HP sintering or HIP sintering can be adopted, whereby a sintered body with higher density can be obtained.
In this manner, a thermoelectric material made of the sintered body 100 of the present invention is obtained.

(Embodiment 2)
In the second embodiment, a thermoelectric conversion element using the thermoelectric material of the present invention will be described.
FIG. 3 is a schematic diagram showing a thermoelectric conversion element of the present invention.

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

Here, the p-type thermoelectric material 310 and/or the n-type thermoelectric material 320 are the thermoelectric material 100 of the present invention described in embodiment 1. When using the thermoelectric material of the present invention for both the p-type and n-type, it is sufficient to use materials with different carrier types, or materials with the same carrier type but different Seebeck coefficients.

Alternatively, when the thermoelectric material of the present invention is used as the p-type thermoelectric material 310, an n-type thermoelectric material such as MgSi-based, ZnSb-based, or ZnO-based, _which has high thermoelectric performance at _300K or _{higher and 773K ,} can be used. Exemplary compositions of MgSi-based materials are, for example, _{Mg2Si0.4Sn0.6} and _{Mg2Si0.37Sn0.6B0.03} . Exemplary compositions of SiGe are, for example, _Si0.8Ge0.2 . Exemplary compositions of ZnO are, for example _{, Zn0.98Al0.02O} .

Alternatively, when the thermoelectric material of the present invention is used as the n-type thermoelectric material 320, p-type thermoelectric materials having high thermoelectric performance at 300K to 773K, such as BiSbTe-based, MgAgSb- _based , and AgSbSe- _based materials _, can be used. Exemplary compositions of the BiSbTe-based materials are, for example, _{Bi0.5Sb1.5Te3} and _{Bi0.4Sb1.6Te3 .} Exemplary compositions of the MgAgSb-based materials are, for example, MgAgSb _{and MgAg0.965Ni0.005Sb0.99} . Exemplary compositions of the AgSbSe-based materials are, for example, _AgSbSe2 . It should be noted that these are merely examples _and are not limiting.

The electrodes 330, 340 can be made of a normal electrode material, illustratively Fe, Ag, Al, Ni, Cu, etc. Here, the electrodes 330, 340 are arranged perpendicular to the c-axis direction of the thermoelectric material of the present invention. This allows the sintered body to exhibit excellent thermoelectric performance of chromium silicide comparable to that of a bulk single crystal.

In FIG. 3, a chip made of n-type thermoelectric material 310 is joined by solder or the like to electrode 340 on the low-temperature side, and the opposite end of the chip of n-type thermoelectric material 310 is joined by solder or the like to electrode 330 on the high-temperature side. Similarly, a chip made of p-type thermoelectric material 320 is joined by solder or the like to electrode 330 on the high-temperature side, and the opposite end of the chip of p-type thermoelectric material 320 is joined by solder or the like to electrode 340 on the low-temperature side.

When the thermoelectric generating 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 low temperature compared to the electrode 330, and the end electrodes are connected to an electric circuit, a voltage is generated by the Seebeck effect, and as shown by the arrows in Figure 3, 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. In detail, the electrons in the n-type thermoelectric material 310 obtain thermal energy from the high-temperature electrode 330, move to the low-temperature electrode 340, and release the thermal energy there, while the holes in the p-type thermoelectric material 320 obtain thermal energy from the high-temperature electrode 330, move to the low-temperature electrode 340, and release the thermal energy there. This is the principle by which a current flows.

In FIG. 3, a π-type thermoelectric generating element is used for explanation, but the thermoelectric material of the present invention may also be used in a U-type thermoelectric generating element (not shown). In this case as well, n-type thermoelectric material and p-type thermoelectric material made of the thermoelectric material of the present invention are alternately connected electrically in series.

The present invention will now be described in detail using specific examples, but please note that the present invention is not limited to these examples.

[Preparation of chromium silicide powder]
Chromium silicide ( _CrSi2 ) powder was synthesized by arc melting a mixture of chromium chips (Sigma-Aldrich, purity 99.995%) and silicon pieces (Sigma-Aldrich, purity 99.95%) in a stoichiometric ratio. The resulting ingot was repeatedly remelted and homogenized. The weight loss was less than 0.2 mass%. The ingot was observed with a tabletop microscope (Hitachi High-Technologies Corporation, TM3000). The results are shown in Figure 4.

The ingot was crushed and powdered in a wet ball mill using a planetary ball mill (Fritsch, PULVERISETTE 7). The coarsely crushed ingot and crushing balls were placed in a crushing vessel (yttria-stabilized zirconia) so that the mass ratio of crushing balls (yttria-stabilized zirconia, φ = 5 mm) to powder was 5, and an aqueous ethanol solution (15% by volume) was added. The mixture was milled at a rotation speed of 200 rpm for 8 hours. The resulting powder was dried overnight at 373 K.

The resulting powder was identified by X-ray diffraction. For X-ray diffraction, a θ-2θ Rigaku TTRAXIII diffractometer (Rigaku Corporation) equipped with a rotating Cu-X-ray tube (λ _Kα1 = 1.5406 Å, λ _Kα2 = 1.5444 Å, λ _Kα2 /λ _Kα1 = 0.5) and a DTex detector was used. The size of the incident beam was set to a diameter of about 500 μm, and the diffraction patterns were measured in the range of 2θ from 5° to 130° with a step width of 0.02°. MAUD (Materials Analysis Using Diffraction, an open source and public domain software) was used to analyze the XRD patterns. The results are shown in Figure 5.

The morphology of the obtained powder was observed using a field emission scanning electron microscope (JSM-7000F, manufactured by JEOL Ltd.). The observation results are shown in Figure 6. The particle size distribution of the obtained powder was determined by dynamic light scattering (DLS) using a Microtrac Nanotrac ULTRA device. The results are shown in Figure 7.

The above results are summarized below.
FIG. 4 is a micrograph of the ingot.

According to Figure 4, the ingot was mostly composed of macro-sized crystals larger than 100 μm.

Figure 5 shows the XRD pattern of the prepared raw material powder.

According to Fig. 5, all the diffraction peaks were indexed to the _CrSi2 crystal (space group _P6222 , number 180) shown in Table 1, and the La Bail analysis revealed that the lattice constants a(=b) and c were a=4.42724(6) Å and c=6.36194(9) Å, respectively. This confirmed that the obtained raw material powder was _CrSi2 .

FIG. 6 is a diagram showing an SEM image of the raw material powder.
FIG. 7 is a diagram showing the particle size distribution of the raw material powder.

As shown in Fig. 6, the raw powder was well pulverized by the ball mill. As shown in Fig. 7, the particle size distribution showed an average particle size (D50) of 405 nm and a standard deviation of 132 nm. The pulverized powder was composed of _CrSi2 single crystal grains.

[Examples 1 to 3]
In Examples 1 to 3, the prepared chromium silicide powder was used, and a sintered body was produced by slip casting and sintering the molded body while applying a magnetic field.
FIG. 8 is a procedure showing a part of the manufacturing process of the sintered bodies of Examples 1 and 2.

The prepared chromium silicide powder 810 was dispersed in absolute ethanol 820, and the solid phase concentration was adjusted according to Table 2 (step S210 in FIG. 2). Also, according to Table 2, polyethyleneimine (PEI, Sigma-Aldrich, average molar mass (g/mol) = 10,000, linear) was added as a polycationic polymer to the raw material slurry, and the acidity was adjusted with 0.1 vol% acetic acid. The raw material slurry was ultrasonically treated to disperse the aggregates.

The raw material slurry was then solidified and molded while applying a magnetic field to form a molded body (step S220 in FIG. 2). The raw material slurry was poured into a slip cast setup consisting of a cylindrical polyethylene mold 850 (φ=12 mm, height=40 mm) placed on a porous alumina plate 840 covered with a membrane filter 830 (porosity 200 nm). The polyethylene mold 850 was covered with a plastic film 860 to prevent evaporation of the ethanol.

The slip cast setup was placed in a 12 T superconducting magnet, and a 12 T magnetic field was applied in a direction parallel to the direction of gravity (arrow B in FIG. 8). At this time, ethanol slowly flowed out through the pores of the membrane filter 830 and the porous alumina plate 840 (arrow SC in FIG. 8). In this way, a compact 870 was obtained. The compact 870 was then pressed at 394 MPa for 10 minutes using a cold isostatic press (CIP). The density of the compact at this time was 55%.

The pressed compact was sintered (step S230 in FIG. 2). For sintering, the compact was placed in a graphite die (φ=10 mm) and subjected to spark plasma sintering (SPS) at 1523 K for 10 minutes.

The sintered body of Example 3 was produced in the same manner as Example 1, except that the magnetic field was applied in a direction perpendicular to the direction of gravity.

The sintered density (relative density) of the sintered bodies of Examples 1 to 3 thus obtained was determined by the Archimedes method in absolute ethanol. The results are shown in Table 4. The XRD patterns of the sintered bodies of Examples 1 to 3 were measured by powder X-ray diffraction. At this time, the φ angle was measured in the range of 0° to 180° with a step width of 5°. The results are shown in FIG. 9. In addition, the _fL value (Lotgering factor in the 0003 direction) was calculated according to the above-mentioned method. The XRD patterns were subjected to Rietveld analysis to calculate the second phase. The results are shown in Table 3.

Backscattered electron images and elemental energy dispersive spectroscopy (EDS) analysis of the sintered bodies of Examples 1 to 3 were performed using a tabletop microscope. Electron backscattered diffraction (EBSD) mapping was performed using a field emission scanning electron microscope. The sintered bodies were successively polished with silicon carbide, diamond, and colloidal silica to prepare samples for EBSD. The results are shown in Figures 10 and 11. Grain sizes were calculated from the EBSD images. The results are shown in Table 4.

Next, the thermoelectric properties of the sintered bodies of Examples 1 to 3 were measured. The thermal diffusion coefficient (D) was measured using a laser flash analyzer Netzsch 467 HyperFlash. Graphite electrodes were attached to the surface of the sintered bodies (φ = 10 mm, thickness = 1 mm) to prepare the measurement samples.

The thermal conductivity κ was calculated using the following formula.
κ=D× _Cp ×d
Here, the specific heat _Cp was calculated using the Dulong-Petit law Cp=3×N×R/M=0.69 Jg ⁻ 1K− ¹ , where N is the formula weight per unit cell, R is the gas constant, and M is the molar mass. The density d was calculated by Archimedes' method in absolute ethanol.

The Seebeck coefficient and electrical resistivity were simultaneously measured in a He atmosphere using a thermoelectric property measurement and evaluation device (Advance Riko Co., Ltd., ZEM5). The sintered body was processed into a rod shape of 9 × 1 × 2 ^mm3 to be used as a measurement sample. Electrodes were attached to the surface of the sintered body. The Hall coefficient R _H was measured using the same sample. The Hall coefficient was measured in AC mode using a physical property measurement system (PPMS, Quantum Design Co., Ltd.) by sweeping the magnetic field from -5 T to +5 T. The carrier concentration n was calculated from the relational equation n _D = 1/eR _H. The power factor PF and dimensionless figure of merit ZT were calculated from the obtained Seebeck coefficient, electrical resistivity, and thermal conductivity. The results are shown in Figures 12 to 17 and Table 5.

[Example 4]
In Example 4, the prepared chromium silicide powder was used, and a slip cast compact was sintered without applying a magnetic field to produce a sintered body. The sintered body of Example 4 was produced in the same manner as in Example 1, except that a magnetic field was not applied.

The XRD pattern, EBSD mapping, thermoelectric properties, density, grain size, _fL value, and second phase were determined for the sintered body of Example 4 in the same manner as in Examples 1 to 3. The results are shown in Fig. 9, Fig. 11 to Fig. 17, and Tables 3 to 5.

The above will be explained in summary.
For the sake of simplicity, Table 2 shows a list of the manufacturing conditions for the sintered bodies of Examples 1 to 4.

FIG. 9 is a diagram showing XRD patterns of the sintered bodies of Examples 1 to 4.
FIG. 10 shows an SEM image and an EDX image of the sintered body of Example 1.

According to FIG. 9, all the diffraction peaks were indexed to _CrSi2 . The sintered bodies of Example 1 and Example 2, to which a magnetic field perpendicular to the surface of the sintered body (pellet) shown in FIG. ₉ was applied, showed strong diffraction peaks of (0003) at 2θ=about 42° and (0006) at 2θ=about 96°, with the intensity of the diffraction peak of (0003) being the highest. This is because the c-axis of CrSi2 is the easy magnetization axis, and the diamagnetic constant |χ _c | is superior to |χ _a |. From this, it was found that the sintered bodies of Example 1 and Example 2 have c-axis orientation in the thickness direction.

The XRD pattern of the sintered body of Example 2 showed the highest intensity of the (0003) diffraction peak, but unlike the sintered body of Example 1, it also showed diffraction peaks other than (000m). This indicates that the degree of c-axis orientation of the sintered body of Example 2 is lower than that of the sintered body of Example 1.

On the other hand, in the XRD pattern of the sintered body of Example 3, to which a magnetic field parallel to the pellet surface was applied, the intensity of the (000 m) diffraction peak was smaller than that of (112-0) ("-" indicates an overbar of 2). This is because the sintered body of Example 3 has a c-axis orientation perpendicular to the thickness direction (parallel to the pellet surface), but the crystals on the surface of the sintered body are in a random state. The XRD pattern of the sintered body of Example 4 is similar to that of the powder in Figure 5, and it was found that the crystals in the sintered body of Example 4 are in a random state throughout.

In the XRD patterns of the sintered bodies of Examples 1 to 4, diffraction peaks indicating chromium silicide (CrSi) (indicated by * in Figure 9) were confirmed. Chromium silicide was also confirmed, albeit in small amounts, as shown in Figure 10.

According to the _fL value calculated from the XRD pattern in FIG. 9, it was found that the sintered body of Example 1 had a good c-axis orientation. Although the sintered body of Example 2 had a c-axis orientation, other orientations were also confirmed. The _fL value of the sintered body of Example 3 was a negative value, but this is because the c-plane in the measurement surface was reduced compared to the random body (Example 4). In addition, all of the sintered bodies of Examples 1 to 4 contained 4 mass% chromium silicide (CrSi) as a second phase.

Figure 11 shows the crystal orientations from electron backscatter diffraction (EBSD) on the ND planes of the sintered bodies of Examples 1, 3, and 4.

Figure 11 also shows the ND inverse pole figure and RD inverse pole figure. Figures 11(a) to (c) show the accumulation of the (000l) plane with grayscales of different densities. Figure 11(a) shows the texture of the (000l) plane accumulated on the ND plane, which is mainly represented by a dark grayscale, and the inverse pole figure shows that the a-axis and b-axis are random in the ND plane. Figure 11(b) shows the texture shown by a light grayscale, which shows that the planes that are 90 degrees to the (000l) plane are accumulated. The inverse pole figure shows that the c-axis is oriented perpendicular to the RD plane. Figure 11(c) shows the texture shown by grayscales of various densities, and the inverse pole figure shows that no accumulation of specific crystal planes was observed, and it was found that the crystals in the sintered body of Example 4 are random.

According to Table 4, the sintered bodies of Examples 1 to 4 all had a relative density exceeding 96%. When the grain size was calculated from Figure 11, it was found that the average grain size of the sintered body of Example 1 was 0.90 μm, which fell within the range of 0.88 μm to 0.92 μm.

FIG. 12 is a graph showing the temperature dependence of the electrical resistivity of the sintered bodies of Examples 1 to 4.
FIG. 13 is a graph showing the temperature dependence of the Seebeck coefficient of the sintered bodies of Examples 1 to 4.
FIG. 14 is a graph showing the temperature dependence of the power factor of the sintered bodies of Examples 1 to 4.
FIG. 15 is a graph showing the temperature dependence of the thermal conductivity of the sintered bodies of Examples 1 to 4.
FIG. 16 is a graph showing the temperature dependence of the dimensionless figure of merit ZT of the sintered bodies of Examples 1 to 4.
FIG. 17 is a graph showing the relationship between the dimensionless figure of merit ZT and the _fL value for the sintered bodies of Examples 1 to 4.

As described above, the sintered bodies of Examples 1 to 4 have electrodes attached to the surface of the sintered body. Therefore, it should be noted that in the sintered bodies of Examples 1 and 2, in which the c-axis is oriented in the thickness direction, the direction of the electrode plane and the c-axis are approximately perpendicular, while in the sintered body of Example 3, in which the c-axis is oriented in the surface direction of the sintered body, the direction of the electrode plane and the c-axis are approximately parallel.

According to Figure 12, the electrical resistivity of the sintered bodies of Examples 1 to 4 all tended to increase with increasing temperature, but the electrical resistivity of the polycrystalline sintered body of Example 4 was the least temperature-dependent, and the electrical resistivity of the sintered body of Example 3 was the most temperature-dependent. It is believed that the electrical resistivity can be selectively reduced by the morphology of the sintered body, added elements, etc.

According to Figure 13, the Seebeck coefficients of the sintered bodies of Examples 1 and 2 were up to twice as large as those of the sintered bodies of Examples 3 and 4 in the measured temperature range. In particular, the sintered body of Example 1 showed a large value of 160 μV/K at 500 K to 600 K. In addition, the Seebeck coefficients of all the sintered bodies were positive values, confirming that the sintered bodies of Examples 1 to 4 are p-type semiconductors.

According to Fig. 14, the power factors of the sintered bodies of Examples 1 and 2 were up to 7 times larger than those of the sintered bodies of Examples 3 and 4 in the measured temperature range. This shows that the sintered body made of grains mainly composed of chromium silicide (CrSi ₂ ) and oriented in the c-axis direction functions as a thermoelectric material. In particular, the power factor is dramatically increased in the low temperature range of 350K to 600K, and the thermoelectric material of the present invention can be said to be suitable for recovering waste heat as a power source for various thermoelectric cooling applications and IoT operation, and can provide a thermoelectric power generation element for civilian use.

According to Figure 15, the thermal conductivity of the sintered bodies of Examples 1 to 4 decreased with increasing temperature. The thermal conductivity of the sintered bodies of Examples 1 and 2 was slightly higher than that of the sintered bodies of Examples 3 and 4 in the measured temperature range, but no significant increase was observed.

According to FIG. 16, the ZT of the sintered bodies of Examples 1 and 2 was higher than that of the sintered bodies of Examples 3 and 4 in the measured temperature range. In particular, as shown in Table 5, at 773 K, the ZT of the sintered body of Example 1 was comparable to that of a bulk single crystal. Furthermore, if the thermal conductivity or electrical resistivity of the sintered body is selectively reduced by the morphology or added elements of the sintered body, a further increase in ZT can be expected.

FIG. 17 shows the relationship between the ZT and the _fL value of the sintered bodies of Examples 1 to 4 at 773K. According to FIG. 17, the closer the _fL value is to 1, the higher the ZT. However, referring to Table 5, by making the sintered body c-axis oriented, the ZT becomes comparable to that of the bulk single crystal, even though the electrical conductivity, Seebeck coefficient, power factor, or thermal conductivity values are different from those of the bulk single crystal. In particular, it was found that a sintered body having a high ZT value can be obtained if the _fL value is in the range of 0.5 to 1. This is extremely advantageous in terms of the ease of manufacturing a c-axis oriented sintered body, compared to the bulk single crystal growth, which requires advanced control and expensive equipment.

The thermoelectric material of the present invention is a sintered body, but has thermoelectric performance equivalent to that of a bulk single crystal, and is therefore used in thermoelectric cooling devices and power generation devices used in various electrical devices. For example, it is possible to generate electricity using the exhaust heat from cars and trucks, or from nuclear power plants, thermal power plants, industrial furnaces, production lines, etc. It can also be used in radioisotope thermoelectric converters for space applications.

REFERENCE SIGNS LIST 100 Sintered body 110 Particles 300 Thermoelectric generating element 310 n-type thermoelectric material 320 p-type thermoelectric material 330, 340 Electrode 810 Chromium silicide powder 820 Absolute ethanol 830 Membrane filter 840 Porous alumina plate 850 Polyethylene mold 860 Plastic film 870 Molded body

Claims (18)

The ceramic material is made of a sintered body made of particles mainly composed of chromium silicide (CrSi ₂ ),
The grains are oriented in the c-axis direction,
A thermoelectric material, wherein the _fL value (Lotgering factor in the 0003 direction) of the sintered body in the thickness direction is in the range of 0.5 to 0.95 . The thermoelectric material according to claim 1 , wherein the f _L value (Lotgering factor in the 0003 direction) is in the range of 0.53 to 0.95. The thermoelectric material according to claim 1 , wherein the particles have a particle size in the range of 0.5 μm to 1.5 μm. The thermoelectric material according to any one of claims 1 to 3 , wherein the density of the sintered body is 95% or more. 5. The thermoelectric material according to claim 1, wherein a portion of silicon in the chromium silicide is substituted with an element selected from the group consisting of germanium (Ge), vanadium (V), aluminum (Al), zirconium (Zr), niobium ( Nb ), and copper (Cu). 5. The thermoelectric material according to claim 1 , wherein a portion of the chromium in the chromium silicide is substituted with an element selected from the group consisting of manganese (Mn) and rhenium (Re). The thermoelectric material according to claim 1 , wherein the particles further contain a second phase which is a chromium silicide. The thermoelectric material of claim 7 , wherein the second phase has a cubic crystal structure and belongs to the P2 ₁ 3 space group. The thermoelectric material according to claim 7 or 8 , wherein the second phase is contained in an amount in the range of more than 0 mass% to 6 mass%. A raw material powder containing chromium silicide (CrSi ₂ ) as a main component is dispersed in a solvent to prepare a raw material slurry;
solidifying and molding the raw material slurry while applying a magnetic field to form a molded body;
The method for producing a thermoelectric material according to claim 1 , further comprising: sintering the molded body. The method of claim 10 , wherein the solvent is an aliphatic hydrocarbon selected from the group consisting of pentane, hexane, heptane, and octane, or an alcohol selected from the group consisting of methanol, ethanol, and propanol. The method according to claim 10 or 11 , wherein the particle size of the raw material powder is in the range of 350 nm to 550 nm. The method according to claim 10 , wherein the magnetic field is in the range of 8 T to 14 T. The method according to any one of claims 10 to 13 , wherein forming the compact involves slip casting. The method according to any one of claims 10 to 14 , wherein the density of the molded body is in the range of 40% or more and 70% or less. The method according to any one of claims 10 to 15 , wherein the sintering step sinters the compact at a temperature in the range of 1373K or more and 1773K or less. The method according to any one of claims 10 to 16 , further comprising pressing the green body prior to or simultaneously with the sintering. A thermoelectric generating element including p-type thermoelectric materials and n-type thermoelectric materials alternately connected in series,
A thermoelectric power generation element, wherein the p-type thermoelectric material and/or the n-type thermoelectric material is the thermoelectric material according to any one of claims 1 to 9 .