Patent Application
20240381773
The present disclosure relates to a thermoelectric conversion module and a thermoelectric conversion system.
When a temperature difference occurs between both ends of a thermoelectric conversion material, an electromotive force proportional to the resulting temperature difference is generated. The phenomenon that thermal energy is converted into electrical energy is known as Seebeck effect. Thermoelectric power generation is a technology that uses the Seebeck effect to directly convert thermal energy into electrical energy. In contrast, when an electric current is passed through a thermoelectric conversion material, a heat flow is generated corresponding to the current value, causing a temperature difference between the both ends of the thermoelectric conversion material. This phenomenon is called a Peltier effect. Thermoelectric cooling is a technology that uses the Peltier effect to cool objects by consuming electrical energy.
As known in the field of thermoelectric conversion material, performance of a thermoelectric conversion material used in a thermoelectric conversion device is evaluated by a dimensionless figure of merit ZT. The ZT is an index made dimensionless by multiplying a figure of merit Z by an absolute temperature T. The ZT is expressed as ZT=S2σT/κ using a Seebeck coefficient S of a substance, electric conductivity σ, and thermal conductivity κ. When the ZT is higher, the thermoelectric conversion efficiency becomes higher.
JP 6127281 B2 describes an n-type thermoelectric conversion material represented by Mg3+mAaBbD2−eEe. The element A represents at least one selected from the group consisting of Ca, Sr, Ba, and Yb, and the element B represents at least one selected from the group consisting of Mn and Zn. The element D represents at least one selected from the group consisting of Sb and Bi, and the element E represents at least one selected from the group consisting of Se and Te. This thermoelectric conversion material has a La2O3-type crystal structure.
WO 2020/003554 A1 describes a thermoelectric conversion element equipped with a thermoelectric conversion layer formed of a thermoelectric conversion material including Mg, at least one selected from the group consisting of Sb and Bi, and at least one selected from the group consisting of Se and Te. This Literature describes further a thermoelectric conversion module equipped with this thermoelectric conversion element and a known p-type thermoelectric conversion element.
An attempt to improve the thermoelectric performance of a GeTe-based thermoelectric conversion material is described in W. Liu, et al., “High-Performance GeTe-Based Thermoelectrics: from Materials to Devices”, Adv. Energy Mater. 20, 2000367 (2020) (Non Patent Literature 1).
It is described that doping GeTe with Mg and Sb improves the thermoelectric conversion performance in a GeTe-based thermoelectric conversion compound, in T. Xing, et al., “Superior performance and high service stability for GeTe-based thermoelectric compounds”, Natl. Sci. Rev. 6, 944-954 (2019) (Non Patent Literature 2).
An attempt of achieving zT of 2.3 in Ge1-x-ySbxIny Te is described in M. Hong et al., “Realizing zT of 2.3 in Ge1-x-ySbxInyTe via Reducing the Phase-Transition Temperature and Introducing Resonant Energy Doping”, Advanced Materials, Vol. 30, Issue 1705942 (2018) (Non Patent Literature 3).
Thermoelectric performance of Ge1-x-yTixSbyTe is described in M. Li et al., “Crystal symmetry induced structure and bonding manipulation boosting thermoelectric performance of GeTe”, Nano Energy, Vol. 73, Issue 104740 (2020) (Non Patent Literature 4).
It is described that Ge0.84Pb0.1Sb0.06TeB0.07 has ZT of 2.2 at 773 K, in G. Bai et al., “Boron Strengthened GeTe-Based Alloys for Robust Thermoelectric Devices with High Output Power Density”, Adv. Energy Mater. 11, 2102012 (2021) (Non Patent Literature 5).
It is described that a Ge1-x-yCrxSbyTe alloy can exhibit ZT of more than 2.2, in M. Hong et al., “Computer-aided design of high-efficiency GeTe-based thermoelectric devices”, Energy Environ. Sci. 13, 1856-1864 (2020) (Non Patent Literature 6).
A general formula for the efficiency of thermoelectric power generation is described in G. J. Snyder, “Thermoelectric Power Generation: Efficiency and Compatibility”, in Thermoelectrics Handbook: Macro to Nano, ed. D. M. Rowe, CRC Press, ch. 9 (2006) (Non Patent Literature 7).
An electrical resistivity, a Seebeck coefficient, a power factor, and a figure of merit of Mg3.2Sb1.5Bi0.49Te0.01 are described in T. Kanno et al., “Enhancement of average thermoelectric figure of merit by increasing the grain-size of Mg3.2Sb1.5Bi0.49Te0.01”, Applied Physics Letters, Vol. 112, 033903 (2018) (Non Patent Literature 8).
A conversion efficiency of a thermoelectric generation module using n-type Pb0.93Sb0.05S0.5Se0.5 and p-type Na-doped PbTe is described in B. Jiang et al., “Realizing high-efficiency power generation in low-cost PbS-based thermoelectric materials”, Energy Environ. Sci., 13, 579-591 (2020) (Non Patent Literature 9).
The present disclosure provides a novel thermoelectric conversion module that is advantageous from a viewpoint of practical utility.
A thermoelectric conversion module of the present disclosure includes:
The present disclosure provides a novel thermoelectric conversion module that is advantageous from a viewpoint of practical utility.
A thermoelectric conversion module having a π-type structure is known as a component of a thermoelectric conversion device. In this thermoelectric conversion module, a P-type thermoelectric conversion element and an N-type thermoelectric conversion element having Seebeck coefficients of signs different from each other are arranged in series electrically and in parallel with regard to a heat flow. This allows an efficient thermoelectric conversion. For thermoelectric conversion modules with a practical π-type structure, durability against a stress caused by applied pressure or a thermal stress caused by temperature differences is important. Therefore, it is important that the P-type thermoelectric conversion material and the N-type thermoelectric conversion material have sufficient mechanical strength. In addition, it is important that the difference in thermal expansion coefficient between the P-type thermoelectric conversion material and the N-type thermoelectric conversion material is small, and that the generation of local thermal stress itself is prevented/reduced.
According to studies by the present inventors, an N-type thermoelectric conversion material including Mg and at least one selected from the group consisting of Sb and Bi can exhibit a ZT of more than 1.5. On the other hand, a P-type thermoelectric conversion material that has a thermal expansion coefficient close to that of the N-type thermoelectric conversion material and that can exhibit a high ZT in the operating temperature range of the N-type thermoelectric conversion material has not yet been found. For this reason, it has been difficult to generate electricity with high power generation efficiency in a thermoelectric conversion module using such an N-type thermoelectric conversion material. For example, a P-type Mg3Sb2-based compound is compatible with an N-type Mg3Sb2-based compound from the viewpoint of thermal expansion coefficient and operating temperature range, but its ZT is about 0.5 at most. MgAgSb-based compounds are promising from the viewpoint of thermal expansion compatibility and the ZT level. However, since a MgAgSb-based compound exhibits a structural phase transition with rapid structural changes at around 300° C., it is difficult to use the compound at a high temperature that exceeds 300° C.
As for the P-type GeTe-based compound, as described in Non Patent Literature 1, substitution of its Ge site by Sb or any other element allows this compound to exhibit ZT exceeding 1.5 and reaching 2.0. In the meantime, a problem for a GeTe-based compound is that the linearity of the thermal expansion coefficient tends to collapse at a high temperature since the compound undergoes a phase transition from a rhombohedral phase to a cubic phase at around 300° C. According to Non Patent Literature 2, by doping GeTe with Mg and Sb, the temperature change in the thermal expansion coefficient becomes linear, and the properties of an 8 mm long element are maintained in a cycle test in which the temperature raising step and temperature lowering step are repeated. In addition, it has been disclosed that the thermal expansion coefficient dL/L0 increases to approximately 8×10−3 [K−1] when the temperature rises from 30° C. to 430° C. In other words, it can be comparable to the typical thermal expansion coefficient of an N-type Mg3Sb2-based compound. Therefore, it is conceivable to configure a thermoelectric conversion module by combining an N-type Mg3Sb2-based compound and a P-type GeTe-based compound doped with Mg and Sb.
On the other hand, according to studies by the present inventors, Ge0.85Mg0.05Sb0.1Te disclosed in Non Patent Literature 2 is easily broken during cutting and is difficult to process. In addition, this material has low mechanical strength and is brittle, resulting in poor impact resistance, and thus, cracks are likely to occur in the thermoelectric conversion material due to the forces applied to the thermoelectric conversion module during use of the thermoelectric conversion module. As a result, it was discovered that the thermoelectric conversion module is prone to failure. By adjusting x in the composition of GexMgySbzTe to less than 0.83, the breaking stress of the compound is likely to increase. However, compared to Ge0.85Mg0.05Sb0.1Te, the output factor S2σ is difficult to increase.
In view of these circumstances, the present inventors have conducted extensive studies on a P-type thermoelectric conversion material that can configure a novel thermoelectric conversion module advantageous from the viewpoint of practice utility, when combined with an N-type thermoelectric conversion material including Mg and at least one selected from the group consisting of Sb and Bi. And the present inventors have found that a certain material including Ge, at least one selected from the group consisting of In and Ti, at least one selected from the group consisting of Sb and Bi, and Te, has desired properties from the viewpoint of practical utility. Based on this new knowledge, the present inventors have completed the thermoelectric conversion material according to the present disclosure.
Embodiments of the present disclosure will be described below with reference to the attached drawings.
As shown in
The P-type thermoelectric conversion elements 10 include a P-type thermoelectric conversion material. The P-type thermoelectric conversion material includes Ge, at least one selected from the group consisting of In and Ti, at least one selected from the group consisting of Sb and Bi, and Te. The P-type thermoelectric conversion material satisfies a requirement (1): α+β+γ<1.00. In the requirement (1), α is a molar ratio of a content of Ge to a content of Te. β is a molar ratio of a sum of a content of In and a content of Ti to the content of Te. γ is a molar ratio of a sum of a content of Sb and a content of Bi to the content of Te. The contents of Ge, In, Ti, Sb, Bi, and Te in the thermoelectric conversion material can be determined, for example, in accordance with SEM-EDX that is a combination of scanning electron microscopy (SEM) and energy dispersive-type X-ray spectroscopy (EDX).
When the P-type thermoelectric conversion material satisfies the requirement (1), it will make it easier to sufficiently decrease the difference between the thermal expansion coefficient of the P-type thermoelectric conversion element 10 and the thermal expansion coefficient of the N-type thermoelectric conversion element 20 including the N-type thermoelectric conversion material in the temperature range from 30° C. to 400° C. In addition, the P-type thermoelectric conversion materials is more likely to have a desired breaking stress. Furthermore, the P-type thermoelectric conversion material is more likely to exhibit desired thermoelectric conversion performance. Therefore, the thermoelectric conversion module 1a is advantageous from a viewpoint of practical utility.
The P-type thermoelectric conversion material may satisfy a requirement α+β+γ≤0.99, may satisfy a requirement α+β+γ≤0.98, or may satisfy a requirement α+β+γ≤0.97.
The P-type thermoelectric conversion material, for example, satisfies a requirement (3): β+γ<α. Thereby, the P-type thermoelectric conversion material is more likely to have desired properties from the viewpoint of thermal expansion coefficient, breaking stress, and thermoelectric conversion performance.
In the P-type thermoelectric conversion material, α−(β+γ) is not limited to a specific value. The P-type thermoelectric conversion material, for example, satisfies a requirement 0.45≤α−(β+γ)<1. In this case, the P-type thermoelectric conversion material is more likely to have desired properties from the viewpoint of thermal expansion coefficient, breaking stress, and thermoelectric conversion performance.
The P-type thermoelectric conversion material, for example, satisfies a requirement (4): α+β+γ>0.85. Thereby, the P-type thermoelectric conversion material is more likely to have desired properties from the viewpoint of thermal expansion coefficient, breaking stress, and thermoelectric conversion performance.
The P-type thermoelectric conversion material may satisfy a requirement a +β+γ>0.86, may satisfy a requirement α+β+γ≥0.87, may satisfy a requirement α+β+γ≥0.88, or may satisfy a requirement α+β+γ≥0.89.
The P-type thermoelectric conversion material may satisfy a requirement 0.85≤α+β+γ≤0.99, may satisfy a requirement 0.86≤α+β+γ≤0.99, or may satisfy a requirement 0.87≤α+β+γ≤0.99. The upper limit of α+β+γ may be 0.99, may be 0.98, or may be 0.97. The lower limit of α+β+γ may be 0.85, may be 0.86, may be 0.87, may be 0.88, or may be 0.89. The range of α+β+γ can be defined by any combination of the upper and lower limits.
The P-type thermoelectric conversion material, for example, has a first constitution represented by GeαInxTiβ-xSbyBiγ-yTe. In this first constitution, requirements 0≤x≤β and 0≤y≤γ are satisfied. In this case, the P-type thermoelectric conversion material is more likely to have desired properties from the viewpoint of thermal expansion coefficient, breaking stress, and thermoelectric conversion performance.
In the first constitution, a requirement α≥0.75 may be satisfied, a requirement α≥0.77 may be satisfied, a requirement α≥0.78 may be satisfied, a requirement α≥0.80 may be satisfied, or a requirement α≥0.82 may be satisfied. In the first constitution, a requirement α≤0.99 may be satisfied, a requirement α≤0.98 may be satisfied, or a requirement α≤0.96 may be satisfied. In the first constitution, a requirement α≤0.94 may be satisfied, a requirement α≤0.92 may be satisfied, or a requirement α≤0.90 may be satisfied.
In the first constitution, a requirement β≥0.001 may be satisfied, a requirement β≥0.002 may be satisfied, a requirement β≥0.003 may be satisfied, a requirement β≥0.004 may be satisfied, or a requirement β≥0.005 may be satisfied. In the first constitution, a requirement β≤0.09 may be satisfied, a requirement β≤0.08 may be satisfied, a requirement β≤0.07 may be satisfied, a requirement β≤0.06 may be satisfied, or a requirement β≤0.05 may be satisfied. The first constitution, for example, satisfies a requirement 0.001≤β≤0.04. Thereby, the P-type thermoelectric conversion material is more likely to have desired properties from the viewpoint of thermal expansion coefficient, breaking stress, and thermoelectric conversion performance.
In the first constitution, a requirement γ≥0.01 may be satisfied, a requirement γ≥0.02 may be satisfied, a requirement γ≥0.03 may be satisfied, or a requirement γ≥0.04 may be satisfied. In the first constitution, a requirement γ≤0.19 may be satisfied, a requirement γ≤0.18 may be satisfied, a requirement γ≤0.17 may be satisfied, a requirement γ≤0.16 may be satisfied, or a requirement γ≤0.15 may be satisfied. The first constitution, for example, satisfies a requirement 0.04 ≤γ≤0.15. Thereby, the P-type thermoelectric conversion material is more likely to have desired properties from the viewpoint of thermal expansion coefficient, breaking stress, and thermoelectric conversion performance.
The crystal structure of the P-type thermoelectric conversion material is not limited to any particular crystal structure. The P-type thermoelectric conversion material, for example, has a NaCl-type crystal structure. In this case, the P-type thermoelectric conversion material is more likely to have desired properties from the viewpoint of thermal expansion coefficient, breaking stress, and thermoelectric conversion performance. The P-type thermoelectric conversion material may have a crystal structure belonging to at least one of a space group Fm-3m and a space group R-3m. Similarly in this case, the P-type thermoelectric conversion material is more likely to have desired properties from the viewpoint of thermal expansion coefficient, breaking stress, and thermoelectric conversion performance.
The P-type thermoelectric conversion material is, for example, a polycrystal including a plurality of crystal grains. Each of the crystal grains has, for example, a NaCl-type crystal structure. Thereby, the P-type thermoelectric conversion material is more likely to have desired properties from the viewpoint of thermal expansion coefficient, breaking stress, and thermoelectric conversion performance. For example, the crystal structure of the crystal grains can be confirmed by analyzing the thermoelectric conversion material in accordance with the X-ray diffraction method.
The P-type thermoelectric conversion material has a breaking stress BS that is not limited to any particular value. In the thermoelectric conversion material, for example, a requirement 40 N/mm2≤BS≤5000 N/mm2 is satisfied. In the requirement, BS indicates the breaking stress BS of the P-type thermoelectric conversion material. The breaking stress BS of the P-type thermoelectric conversion material can be determined, for example, by a three-point bending test. The three-point bending test is performed using a test piece that is prepared from a P-type thermoelectric conversion material for the three-point bending test. The breaking stress of the P-type thermoelectric conversion material can be determined by dividing the breaking load of the test piece in this test by the area of the fracture surface of the test piece. The shape and dimensions of the test piece can be determined from the viewpoint of the area of the fracture surface, the distance between fulcrums, and easiness in calculation of the point of effort. The test piece is rectangular parallelepiped, for example. The three-point bending test is preferably performed with two or more test pieces for each P-type thermoelectric conversion material. The three-point bending test, more desirably, is performed with four or more test pieces. The three-point bending test is performed at 25° C., for example. For example, a three-point bending test can be performed by a testing machine conforming to Japanese Industrial Standard JIS B7721 Class 1, ISO 7500-1 Class 1, EN10002-2 Grade 1, or ASTM E4.
A P-type thermoelectric conversion material with a breaking stress BS satisfying the aforementioned requirement has high toughness, and for instance, a sintered body including the P-type thermoelectric conversion material can be cut easily. The P-type thermoelectric conversion material desirably satisfies a requirement BS≥45 N/mm2. It may satisfy a requirement BS≥50 N/mm2, may satisfy a requirement BS≥55 N/mm2, or may satisfy a requirement BS≥60 N/mm2. The thermoelectric conversion material may satisfy a requirement BS≤4000 N/mm2, may satisfy a requirement BS≤3000 N/mm2, may satisfy a requirement BS≤2000 N/mm2, or may satisfy a requirement BS≤1000 N/mm2. The thermoelectric conversion material may satisfy a requirement BS≤500 N/mm2.
The N-type thermoelectric conversion material, for example, includes a Mg3(Sb, Bi)2-based N-type thermoelectric conversion material. In this Description, (Sb, Bi) indicates that at least one selected from the group consisting of Sb and Bi is included. The Mg3(Sb, Bi)2-based N-type thermoelectric conversion material includes Mg3(Sb, Bi)2and a material where an element of a part of Mg3(Sb, Bi)2 is substituted by another element. In a case where the Mg3(Sb, Bi)2N-type thermoelectric conversion material is a material where an element of a part of Mg3(Sb, Bi)2 is substituted by another element, the content of the other element is smaller than the content of Mg on the basis of a molar ratio, for example. Further, the content of the other element is less than the sum of the content of Sb and the content of Bi on the basis of a molar ratio. Examples of the other element to substitute the element of a part of Mg3(Sb, Bi)2 include Ca, Sr, Ba, Nb, Zn, Al, Sc, Y, La, Ce, Mn, Si, Cr, Se, and Te.
The constitution of the N-type thermoelectric conversion material is not limited to any particular constitution. The N-type thermoelectric conversion material, for example, has a second constitution represented by Mg3+m−a−bAaBbC2−c−eDcEe. In the second constitution, A is at least one element selected from the group consisting of Ca, Sr, Ba, Nb, Zn, and Al. B is at least one element selected from the group consisting of Sc, Y, La, and Ce. C is at least one element selected from the group consisting of Sb and Bi. D is at least one element selected from the group consisting of Mn, Si, and Cr. E is at least one element selected from the group consisting of Se and Te. In the second constitution, for example, requirements −0.1≤m≤0.4, 0≤a≤0.1, 0≤b≤0.04, 0≤c≤0.1, and 0.001≤e ≤0.06 are satisfied. Thereby, a thermoelectric conversion module 1a is likely exhibit a desired generation efficiency.
The crystal structure of the N-type thermoelectric conversion material is not limited to any particular crystal structure. The N-type thermoelectric conversion material, for example, has a La2O3-type crystal structure.
In the thermoelectric conversion module 1a, the thermal expansion coefficient of the N-type thermoelectric conversion material and the thermal expansion coefficient of the P-type thermoelectric conversion material are not limited to any particular relationship. In the thermoelectric conversion module 1a, the N-type thermoelectric conversion material and the P-type thermoelectric conversion material, for example, satisfy a requirement (2): |(Xn−Xp)/Xn|≤0.3. Here, Xn indicates the thermal expansion coefficient of the N-type thermoelectric conversion material at a specific temperature in the temperature range from 30° C. to 400° C. Xp is the thermal expansion coefficient of P-type thermoelectric conversion material at a specific temperature in the temperature range from 30° C. to 400° C. The thermal expansion coefficient of the P-type thermoelectric conversion material at 400° C. is, for example, 6.5×10−3[K−1] or more and 7.3×10−3[K−1] or less. In this case, in the thermoelectric conversion module 1a, the difference between the thermal expansion coefficient of the N-type thermoelectric conversion material and the thermal expansion coefficient of the P-type thermoelectric conversion material is small, and thus, cracks in the thermoelectric conversion material, which is caused by the thermal stress in the thermoelectric conversion module 1a, is likely to be prevented or reduced. Therefore, the thermoelectric conversion module 1a is likely to have a high reliability. The thermal expansion coefficient of the N-type thermoelectric conversion material and the thermal expansion coefficient of the P-type thermoelectric conversion material can be determined, for example, by a thermomechanical analysis (TMA). As an example of the thermomechanical analysis for determining the thermal expansion coefficient, the method described in Examples can be referred to. In the thermoelectric conversion module 1a, a requirement |(Xn−Xp)/Xn|≤0.25 may be satisfied, or a requirement |(Xn−Xp)/Xn|≤0.20 may be satisfied.
As shown in
The material of the electrodes 30 is not limited to a particular material as long as the electrodes 30 are capable of electrically connecting the P-type thermoelectric conversion element 10 and the N-type thermoelectric conversion element 20. The electrodes 30 may include, for example, at least one selected from the group consisting of, silver, copper, and gold.
As shown in
In the thermoelectric conversion module 1a, for example, each of the P-type thermoelectric conversion elements 10 and each of the N-type thermoelectric conversion elements 20 are joined to the substrates 40 by predetermined joining layers that compose the electrodes 30. The joining layers are formed of, for example, a silver-containing paste, a silver-containing brazing material, a copper-containing paste, or a copper-containing brazing material. The joining layers may be formed of a solder material with a high melting point, such as a Pb alloy and an Au alloy.
As shown in
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The second layer 21 may include a simple substance of Cu, or may include a Cu alloy containing a component such as Zn, Mg, or Ca. The second layers 21 may include a simple substance of Fe or may include a Fe alloy such as stainless steel.
An example of a method of manufacturing the thermoelectric conversion module of the present disclosure will be explained. A raw material of the P-type thermoelectric conversion material is melted at a temperature of 700° C. or higher, thereby obtaining a melt. The melt is then quenched, thereby obtaining a first ingot. The raw material for the P-type thermoelectric conversion material is, for example, a simple substance. The first ingot is pulverized to produce a first powder, and the obtained first powder is sintered by a method such as hot pressing or spark plasma sintering (SPS). In this way, a high-density second ingot can be obtained. The first layer 11 is formed by sintering, if necessary, the raw material powder for the first layer 11 together with the first powder. Alternatively, the first layer 11 may be formed by any method such as integral sintering, plating, or thermal spraying, after cutting the second ingot into a predetermined shape.
A second powder is produced from a raw material for the N-type thermoelectric conversion material by a method such as mechanical alloying, melting, or a solid phase reaction. The second powder is sintered by a method such as hot pressing or SPS to produce a high-density third ingot. The second layer 21 is formed by sintering, if necessary, the raw material powder for the second layer 21 together with the second powder. The second layer 21 may be formed by any method such as integral sintering, plating, or thermal spraying, after cutting the third ingot into a predetermined shape.
The second ingot including the P-type thermoelectric conversion material and the third ingot including the N-type thermoelectric conversion material are, for example, flat plates. Each flat ingot is diced into a square or rectangular shape in a planar view, whereby the P-type thermoelectric conversion element 10 and the N-type thermoelectric conversion element 20 are obtained. On the substrate 40 or on a predetermined surface, the P-type thermoelectric conversion element 10 and the N-type thermoelectric conversion element 20 each is arranged in contact with the electrode 30. The P-type thermoelectric conversion element 10 and the N-type thermoelectric conversion element 20 each is joined to the electrode 30 via a binder. The binder may be a brazing medium, a solder compound, or a sinterable metal-containing paste. Heating the binder causes melting of the binder or sintering of the component in the binder, thereby forming the joining layers. In this way, the P-type thermoelectric conversion element 10 and the N-type thermoelectric conversion element 20 each is joined to the electrode 30 by the joining layers. For example, the thermoelectric conversion module of the present disclosure is manufactured in this way.
In the thermoelectric conversion system 2, the heat source 50 may include a heat transfer tube, and a predetermined heating medium is introduced into the heat transfer tube. The heating medium may be a gas such as exhaust gas, or a liquid such as water or oil. The heat source 50 may include a plate for collecting radiant heat.
As shown in
Hereinafter, the present disclosure will be described in detail with reference to Examples, though the thermoelectric conversion material of the present disclosure is not limited to the specific embodiments shown below.
In a glove box of an argon atmosphere, granular Ge, granular In, granular Ti, granular Sb and granular Te were weighed. The weighing was carried out so that the mole ratio Ge: In: Sb: Te was 0.85:0.01:0.09:1. The thus weighed Ge, In, Sb and Te were introduced into a quartz tube. The quartz tube had an inner diameter of 8 mm and an outer diameter of 10 mm. Next, the interior of the quartz tube was brought into a vacuum state of 3×10−2 Pa using a turbo molecular pump, and the quartz tube was sealed. This quartz tube was placed inside a table-top electric furnace F-1404P manufactured by Tokyo Garasu Kikai Co., Ltd., and the temperature inside the table-top electric furnace was maintained at 900° C. for 12 hours. In this way, Ge, In, Sb, and Te inside the quartz tube were melted, whereby a melt was obtained. The quartz tube including the liquid-phase melt was then taken out from the table-top electric furnace, the quartz tube was placed in a water tank filled with sufficient water, thereby quenching the melt to obtain a solidified body. With the solidified body inside the quartz tube, the quartz tube was placed again inside the table-top electric furnace, and the temperature inside the table-top electric furnace was maintained at 600°° C. for 96 hours. In this way, a composition for a thermoelectric conversion material according to Sample 1 was obtained.
Next, the composition for a thermoelectric conversion material according to Sample 1 was pulverized on a mortar to obtain a powder according to Sample 1. The resulting powder was sintered by a spark plasma sintering (SPS) method to obtain a dense sintered body. The powder (2.0 g) was filled into a cylindrical die made of carbon. The die had an outer diameter of 50 mm and an inner diameter of 10 mm. The powder according to Sample 1 was sintered by SPS under conditions of applying a pressure of 60 MPa and heating with electricity for 10 minutes at 550° C. In this way, a thermoelectric conversion material according to Sample 1 was obtained. This thermoelectric conversion material was a P-type material with a constitution of Ge0.85In0.01Sb0.09Te.
Compositions for thermoelectric conversion materials according to Sample 2 to Sample 7 were prepared in the same manner as Sample 1 except that the addition amount of each raw material was adjusted so that the constitutions of the materials are as shown in Table 1. Thermoelectric conversion materials according to Sample 2 to Sample 7 were produced in the same manner as Sample 1 except that the respective compositions for thermoelectric conversion materials according to Sample 2 to Sample 7 were used. These thermoelectric conversion materials were P-type materials.
Granular Sb and granular Bi were melted at temperatures in a range from 1000° C. to 1500° C. using an arc melting method, whereby an alloy of Sb and Bi was obtained. Next, the obtained alloy was ground in a mortar to prepare a SbBi powder.
Next, powders of Mg and Te were added to the SbBi powder. Afterwards, these powders were thoroughly mixed. The mole ratio Mg:Sb:Bi:Te in the powder mixture was 3.2:1.5:0.49:0.01. The mixed powder was then fed to a tablet press to form a tablet. Next, the tablet was placed in a carbon crucible. The carbon crucible was filled with argon gas. The tablets were then heated for 10 seconds at a temperature in a range from 800° C. to 1000° C. The tablet melted due to heating, whereby an ingot was obtained. Next, an ingot was introduced into a mortar placed in a glove box filled with argon gas. The ingot was ground in the mortar to obtain MgSbBiTe powder. The obtained powder had a particle size of 100 μm or less.
Next, the MgSbBiTe powder was sintered using the SPS method to obtain a sintered body. Sintering using SPS was performed as follows. First, MgSbBiTe powder was filled into a cylindrical graphite die. The die had an outer diameter of 50 mm and an inner diameter of 10 mm. This filling was carried out in a glove box filled with argon gas. Next, the die was placed in the chamber of the spark plasma sintering equipment. The chamber was adjusted in advance to an argon atmosphere. Next, while a pressure of 50 MPa was applied to the powder filled in the die, a pulsed current was applied to the die by a sintering device. By applying the current, the temperature of the die increased at a rate of approximately 50° C./min. After the die temperature reached the sintering temperature of 850° C., the die temperature was maintained at 850° C. for 5 minutes. Heating of the die was then discontinued by stopping the current. After the temperature of the die lowered to room temperature, the cylindrical sintered body was removed from the die, and the thermoelectric conversion material of Sample 8 was obtained. This thermoelectric conversion material was N-type with a constitution of Mg3.22Sb1.52Bi0.47Te0.01.
An X-ray diffraction measurement was performed on specimens produced from the thermoelectric conversion materials according to Samples 1-3 and 8, using an X-ray diffractometer Aeris manufactured by Malvern Panalytical. In this measurement, a Cu-Kα ray was used for the X-ray.
Rectangular parallelepiped specimens for a thermomechanical analysis (TMA) were prepared from the thermoelectric conversion materials according to Samples 1, 3, 7, and 8. The lengths of the three mutually orthogonal sides of each specimen were about 8 mm, about 2 mm, and about 2 mm. The specimen was placed on a thermomechanical analyzer Thermo plus EVO2 TMA8311 manufactured by the Rigaku Corporation. To prevent deterioration of the specimen due to oxidation, the atmosphere inside the analyzer was replaced with argon at 0.5 atm. The thermal expansion coefficient (linear expansion coefficient) dL/L0 was measured in a direction parallel to a side of about 8 mm in a temperature range of 30° C. to 500° C. with a compressive load of 100 mN applied while sweeping the temperature inside the device at a rate of 10° C./min.
On the other hand, in Samples 1 and 3, the linearity of the thermal expansion coefficient is maintained in the temperature range from 30° C. to 400° C. According to the comparison of Samples 1, 3 with Sample 8, |(Xn−Xp)/Xn| was 0.14 or less in the temperature range from 30° C. to 400° C., that is, the difference between the thermal expansion coefficient of the P-type thermoelectric conversion material and the thermal expansion coefficient of the N-type thermoelectric conversion material is small.
Test pieces S for the three-point bending test were prepared by cutting the thermoelectric conversion materials according to Samples 1 to 6. Each test piece S was a rectangular parallelepiped with a length of 4 mm or more, a thickness of about 2 mm, and a width of about 2 mm.
Specimens for thermoelectric property evaluation were prepared from the thermoelectric conversion materials according to Samples 1 to 3 and 8. The specimens each had a rectangular parallelepiped shape, and the lengths of the three mutually orthogonal sides were 6 mm or more, about 2 mm, and about 2 mm. Both end faces in the longitudinal direction of the specimen were polished using #8000 abrasive paper. This specimen was placed inside the thermoelectric property evaluation device ZEM-3 manufactured by Advance Science and Engineering. The interior of the device was adjusted to a helium atmosphere of 0.5 atm or 1 atm. The temperature inside the device was swept from room temperature to 500° C., and the Seebeck coefficient S and the electric conductivity o were measured at 50° C. intervals. The electromotive force was measured at the time of adjusting the temperature difference between the upper block and lower block to 20° C., 30° C., and 40° C. at the base temperature, whereby a regression line was determined for the plots showing the relationship between the electromotive force and the temperature difference between the upper and lower blocks. The Seebeck coefficient S at the base temperature was determined from the slope of this regression curve.
Specimens for heat conductivity measurements were prepared from the thermoelectric conversion materials according to Samples 1 to 3 and 8. The specimens each was disk-shaped with a thickness of about 1 mm and a diameter of about 10 mm. A carbon coating was formed on the surface of the specimen using a carbon spray. This specimen was placed inside a table-top laser flash analyzer LFA 457 MicroFlash manufactured by NETZSCH-Gerätebau. While flowing argon through the device at a flow rate of 300 mL/min, the temperature inside the device was swept from room temperature to 500°° C., and the heat conductivity K of the sample was measured at 50° C. intervals.
Based on the thus determined Seebeck coefficient S, electric conductivity σ, and heat conductivity κ, the dimensionless figure of merit ZT of the thermoelectric conversion materials according to Samples 1 to 3 and 8 were determined.
Based on the results regarding the thermoelectric conversion properties of the aforementioned thermoelectric conversion materials, a prediction was made about the maximum thermoelectric conversion efficiency of the thermoelectric conversion module prepared as a combination of a P-type thermoelectric conversion material according to Sample 3 and an N-type thermoelectric conversion material having a constitution of Mg3.2Sb1.5Bi0.49Te0.01. This prediction was made using the following procedure with reference to the description in Non Patent Literature 7.
The hot side temperature and the cold side temperature of the thermoelectric conversion element of the thermoelectric conversion module are expressed as Th and Tc, respectively. At this time, the temperature section from temperature Th to temperature Tc is divided into N parts, and the temperature at each point is set as Tn. Here, n takes an integer value of 0 or more and N or less. The relationship T0=Th and TN=Tc holds true. The current density at each point of the thermoelectric conversion element is expressed as J. The Seebeck coefficient, the thermal conductivity, and the electrical resistivity of the thermoelectric conversion material corresponding to the temperature Tn are expressed as αn, κn, and ρn, respectively. The relative current density un at each point in the thermoelectric conversion element is expressed by the following Equation (1).
The relative current density un is expressed by the recurrence formula in Equation (2) below. In Equation (2), a relationship ΔT=Tn−Tn−1 holds true. ρ, κ, and T each is the average in the interval from Tn−1 to Tn.
By using the recurrence formula in Equation (2), the value of un at n32 1, . . . ,N can be obtained for both the P-type thermoelectric conversion element and the N-type thermoelectric conversion element, with u0 as a variable. The thermoelectric conversion efficiency η of a π-type structure thermoelectric conversion module prepared by combining a P-type thermoelectric conversion element and an N-type thermoelectric conversion element is expressed as a ratio P/Qh of the power generation amount P to the passing heat flow rate Qh on the high temperature side of the thermoelectric conversion element. Therefore, a thermoelectric conversion efficiency n can be expressed as shown in Equation (3) below, using the above variables. The subscripts p and n in Equation (3) indicate values related to P-type thermoelectric conversion element and N-type thermoelectric conversion element, respectively.
The thermoelectric conversion efficiency η is a function of u0, and the maximum conversion efficiency ηmax can be determined as the maximum value of η obtained when u0 is varied. In other words, the temperature dependence of the maximum conversion efficiency ηmax can be predicted by deriving the optimal u0 value in accordance with the requirement of the temperature difference ΔT=Th−Tc given to the thermoelectric conversion module with a π-type structure. Furthermore, the optimal value of the ratio Ap/An of the cross-sectional area Ap of the P-type thermoelectric conversion element to the cross-sectional area An of the N-type thermoelectric conversion element can be determined based on the following Equation (4). With the optimal value, the thermoelectric conversion efficiency may be maximized.
Based on Equations (1) to (4), a prediction was made for a maximum thermoelectric conversion efficiency of a thermoelectric conversion module with a π-type structure, where the thermoelectric conversion module was prepared by combining a P-type thermoelectric conversion material according to Sample 3 and an N-type thermoelectric conversion material having a constitution of Mg3.2Sb1.5Bi0.49Te0.01. In this prediction, the above evaluation results of thermoelectric conversion performance were referred to as the values of the Seebeck coefficient, the electrical resistivity, and the thermal conductivity of the P-type thermoelectric conversion material according to Sample 3. In addition, the description in Non Patent Literature 8 was referred to for the values of the Seebeck coefficient, the electrical resistivity, and the thermal conductivity of an N-type thermoelectric conversion material having a constitution of Mg3.2Sb1.5Bi0.49Te0.01. And the temperature on the low-temperature side of the thermoelectric conversion element was set at 27° C.
As described in Non Patent Literature 9, the thermoelectric conversion efficiency of a thermoelectric conversion module in which the P-type thermoelectric conversion element and the N-type thermoelectric conversion element each is made of a single material is 8% under the condition of ΔT=565 K. In this thermoelectric conversion module, an N-type PdS-based thermoelectric conversion material and a P-type PbTe-based thermoelectric conversion material are used. In addition, according to Non Patent Literature 9, a thermoelectric conversion module having a P-type thermoelectric conversion element and an N-type thermoelectric conversion element each formed of a plurality of materials has a thermoelectric conversion efficiency of 11.2% under the condition of ΔT=585 K. This thermoelectric conversion module uses an N-type PdS-based thermoelectric conversion material, a P-type PbTe-based thermoelectric conversion material, an N-type Bi2 Te3-based thermoelectric conversion material, and a P-type Bi2 Te3-based thermoelectric conversion material.
In the thermoelectric conversion module according to the present disclosure, toxic Pb is not used and a single thermoelectric conversion material can be used for each of the P-type thermoelectric conversion element and the N-type thermoelectric conversion element, whereby the manufacturing process can be simplified easily. In addition, the thermoelectric conversion module according to the present disclosure is capable of exhibiting high thermoelectric conversion efficiency.
A thermoelectric conversion module of the present disclosure can be used in a variety of applications, including the applications of conventional thermoelectric conversion modules.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-012178 | Jan 2022 | JP | national |
This application is a continuation of PCT/JP2022/047479 filed on Dec. 22, 2022, which claims foreign priority of Japanese Patent Application No. 2022-012178 filed on Jan. 28, 2022, the entire contents of both of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2022/047479 | Dec 2022 | WO |
| Child | 18782978 | US |
