JP2007165463A - Thermoelectric converting element and power generating module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermo-electric converting element for power generation having improved performance index, and also to provide a thermoelectric module for generation of electrical power. <P>SOLUTION: A ratio (x=I<SB>B</SB>/I) of the length l<SB>B</SB>for the total length l of a thermoelectric element is set to 0.83 or less by assembling a thermoelectric material after plating the front surface thereof in contact with a heat source. In the case where the ratio is further set to 0.20 or less, the high performance thermoelectric converting element remarkably increases in the performance index of the thermo-electric converting element and exceeds 7 in its performance index when (x) is equal to 0.15. The maximum energy converting efficiency under the temperature difference of Tc=300 K and Th=350 K becomes equal to about 13% using the value of ZT=7. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a thermoelectric conversion element for power generation and a Bi-Te-based thermoelectric material used for a thermoelectric module for power generation, and relates to a thermoelectric conversion element and a power generation module with significantly improved performance index.

  Thermoelectric conversion elements are already used for temperature control of electronic coolers, optical communication equipment, measuring equipment, incubators, etc. If further improvements are made in the future, refrigerators that do not use CFCs and automotive air conditioners will be commercialized. It becomes possible. Thermoelectric conversion elements are devices that are expected to be put into practical use from the viewpoint of effective utilization of exhaust heat energy, which is highly demanded in recent industries. For example, systems that convert waste heat into electrical energy, It can also be used for small portable power generators for easily obtaining electricity, flame sensors for gas appliances, and the like. Higher performance will also create applications for recovering thermal energy from automobile exhaust gas and controlling the temperature of fuel reformers for in-vehicle fuel cells.

The thermoelectric conversion element has a configuration in which, for example, a P-type and an N-type semiconductor are PN-bonded by plating, soldering, silver brazing, or the like to form an element. In addition to high-performance chalcogen compounds such as Zn ₄ Sb ₃ , IrSb ₃ , Bi ₂ Te ₃ , and PbTe as thermoelectric conversion materials for forming these elements, FeSi is abundant in resources but has low thermoelectric properties. _2. Silicides such as Si-Ge alloys are known.

  Note that the thermoelectric conversion element directly converts heat into electricity by providing a temperature gradient at both ends of the P-type and N-type thermoelectric conversion materials, or conversely converts electricity into heat by applying a voltage to the material. The latter is well known as a Peltier element. The feature of this element is that no moving parts are required.

A thermoelectric conversion element (Peltier element) generates a thermoelectromotive force (temperature difference) using a temperature gradient (potential difference) applied to the material, and its conversion efficiency is a figure of merit (ZT) of the thermoelectric (electrothermal) conversion element. = TS ² / ρκ, where T is the absolute temperature, S is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the thermal conductivity), and at present ZT = 1 or so, and its conversion efficiency is several The percentage was low and not enough. Since this conversion efficiency increases as ZT increases, a material or a thermoelectric conversion element having a performance index as high as possible is required.

At present, most Bi-Te-based thermoelectric materials are produced in a crystal state close to a single crystal by using the single crystal technology by Bridgeman method, Czochralski method, zone melt method, or powder metallurgy method by hot pressing It is produced as a polycrystal using For example, Patent Document 1 describes that a thermoelectric element is produced by sintering powder, and Se and Sb are added as a main material of Bi ₂ Te ₃ as a technique related to a method for manufacturing a thermoelectric conversion element. However, it is contained in the form of Bi ₂ Se ₃ , Sb ₂ Te _3, etc., and does not destroy the basic structure of Bi ₂ Te ₃ .
JP2002-33525

  The conversion efficiency of electrothermal or thermoelectric conversion elements is very low compared to solar cells (about 17%), and it is only a few percent at present, which is why the applications of thermoelectric conversion elements or Peltier elements are narrowing. There is also a reason why thermoelectric conversion elements are not widespread.

  A single crystal of a Bi-Te thermoelectric material is manufactured by melting a raw material sealed in a quartz tube with vacuum or argon gas in an electric furnace having a temperature gradient, and then pulling up the quartz tube to grow a crystal.

  In particular, single crystals have the property of being prone to cracks along the (001) cleavage plane (c-plane). On the other hand, the polycrystalline body has a problem that the electrical resistivity increases due to the grain boundary scattering of the carrier and the figure of merit decreases. For this reason, when producing a module having a high figure of merit (ZT), a single crystal is used, but when giving priority to mechanical strength, a polycrystal is used.

  In general, when Bi-Te thermoelectric materials are used as Peltier modules in a state close to a single crystal, the direction of the current flowing through the module is perpendicular to the crystal pulling direction, that is, the crystal c-axis, in order to prioritize this mechanical strength. It is assembled to become.

  In order to satisfy both the figure of merit and mechanical strength, various studies were made on the composition of Bi-Te, the type of additive, the amount added, the production method, etc., but the figure of merit at room temperature does not greatly exceed 1. It was. The Seebeck coefficient is not sensitive to the orientation of the single crystal and there is almost no crystal anisotropy, but the thermal conductivity is lower in the c-axis direction than in the direction perpendicular to the c-axis, and conversely, the electrical resistivity is in the c-axis. Since the figure of merit was higher in the c-axis direction than in the vertical direction, the figure of merit changed depending on the crystal orientation, but the figure of merit at room temperature did not greatly exceed 1.

  An object of the present invention is to provide a thermoelectric conversion element for power generation with improved performance index, a thermoelectric module for power generation, and a manufacturing method thereof.

In order to solve the above problems, it is most effective to dramatically increase the Seebeck coefficient. Recently, it was pointed out by Balmush et al. (II Balmush et al) that in a PN-bonded semiconductor, when a vertical temperature gradient is applied to the bonding surface, a rapid temperature gradient is applied in the vicinity of the bonding interface and the thermoelectromotive force increases. .,
Semiconductors 29 (1995) 937).

  More recently, the present inventors have found that Seebeck is dramatically increased by bonding the Bi-Te-based thermoelectric material with a metal of a predetermined thickness due to the interface effect of the bonded boundary (Japanese Patent Application 2005- 092061, Japanese Patent Application 2005-194040).

  The inventors have conducted various studies on the application method of the temperature gradient applied to the Bi-Te thermoelectric conversion element, the length of the Bi-Te thermoelectric material and the metal, and the Bi-Te thermoelectric conversion element, with the aim of further increasing such interfacial effects. The inventors have found that a high-performance thermoelectric conversion element for power generation can be obtained.

The method of manufacturing the thermoelectric conversion element (FIG. 1) of the present invention is a method in which the surface of the thermoelectric material in contact with the heat source is plated and then assembled by soldering with metal, and the length l of the thermoelectric material relative to the total length l of the thermoelectric element The ratio of _B (x = l _B / l) is 0.83 or less. A method for producing a high-performance power generation module (FIG. 2) is characterized in that P-type and N-type thermoelectric elements having x = 0.45 or less are alternately connected in series.

An ingot of a Bi-Te-based thermoelectric material is characterized in that a thermoelectric conversion material is dissolved in a quartz tube and then the quartz tube is pulled up at a pulling speed of 1 to 20 cm / hr. Since Bi ₂ Te ₃ -based materials have Van der Waals bonds between the c-planes of the crystal, there is a drawback that the bonding force between the c-planes is weak. Generally, the mechanical strength of the material decreases as the crystal grain size increases. Although the size of the average crystal grain size of the thermoelectric material of the present invention is not specified, when the crystal grain size is such that the mechanical strength of the thermoelectric conversion material is a concern, the four sides of the assembled element are thermally conductive. By coating with a resin having a very low rate, it is possible to make it stronger rather than a normal thermoelectric conversion element.

The thermoelectric conversion material of the present invention uses Bi ₂ Te ₃ or Sb ₂ Te ₃ as a main material, and particularly in the case of a P-type semiconductor, the additive element is at least one of Group 4, Element 6, and Group 7 elements. Containing 12 wt.% Or less of seeds or more, and in the case of an N-type semiconductor, containing 0.25 wt.% Or less of at least one of group 6 element, group 7 element and halide as an additive element It may be. The electrode metal is preferably a material having low electrical resistivity, high thermal conductivity, and excellent corrosion resistance. In the present invention, the thermoelectric conversion material is a material used for a thermoelectric conversion element for power generation.

The thermoelectric conversion module currently on the market has a ratio of the length l _B of the thermoelectric material to the total length l of the thermoelectric element (x = l _B / l) exceeds x = 0.85, and the production of the thermoelectric conversion element of the present invention The thermoelectric module manufactured by this method is also novel in that x is 0.83 or less.

Furthermore, when the ratio of the thermoelectric material length l _B to the total length l of the thermoelectric element l _B (x = l _B / l) is 0.20 or less, the performance index of the thermoelectric conversion element greatly increases. It exceeded 7 at x = 0.15. Using the value of ZT = 7, the maximum energy conversion efficiency under the temperature difference of Tc = 300K and Th = 350K is calculated to be about 13%. This conversion efficiency is lower than the solar cell energy conversion efficiency of 17%, but more than three times that of the conventional module.

  It was found that by using such a conversion element to produce a power generation module and using it under a temperature gradient that changes periodically, the output power is improved to about three times that when the temperature gradient is constant. The module according to the present invention is a power generation module that applies a temperature gradient that changes periodically.

The present invention relates to a compound in which a part of Bi atom in a Bi ₂ Te ₃ compound having a rhombohedral crystal structure is substituted with a group 5 element or a compound in which a part of Te atom is substituted with a group 6 element. The present invention relates to a Bi ₂ Te ₃ thermoelectric material in which a group 4, element 6 or group 7 element is added alone or in combination. By assembling a thermoelectric module using a thermoelectric conversion element in which the ratio of the length of the thermoelectric material to the total length of the thermoelectric conversion element is a predetermined ratio, the Seebeck coefficient S increases dramatically due to the interface effect between the metal and the thermoelectric material. They found that the figure of merit of thermoelectric conversion elements and the power generation efficiency of thermoelectric modules can be greatly improved. The present invention has been made based on these findings, and the embodiments thereof will be described in detail below.

The Bi-Te-based thermoelectric conversion material is melted in a quartz tube, and then the quartz tube is pulled up at a pulling speed of 1 to 20 cm / hr so that the c-planes of the crystal grains are aligned in the pulling direction. In order to improve mechanical strength, an ingot may be pulverized and a polycrystalline body may be produced by a hot press method. Alternatively, a Bi-Te thermoelectric material can be produced by depositing a thin film on a metal plate by vapor deposition or sputtering, and then multilayering the metal and thermoelectric material so that the thickness ratio is a predetermined ratio. good. Either method can be used to obtain a Bi ₂ Te ₃ thermoelectric conversion element having a high figure of merit.

Bi ₂ Te ₃ based thermoelectric materials originally show P-type thermoelectric properties, but Bi ₂ Te ₃ has a hole carrier concentration that is too high, so when entering the crystal to reduce the hole carrier concentration. The figure of merit is increased by adding a group 6 chalcogen element that emits electrons. On the other hand, in an embodiment in which the present invention is applied to an N-type semiconductor, the polarity is changed from positive to negative by adding a group 6 chalcogen element, a group 7 halogen element, or a halide of a metal element, and the addition amount thereof To improve the figure of merit by adjusting the carrier concentration.

The additive amount of additive elements and additives to the Bi ₂ Te ₃ system thermoelectric material must be at least one in the P type in order to make a semiconductor having the desired polarity and carrier concentration (˜10 ¹⁹ cm ⁻³ ). It is desirable to add 3 wt.% Or more, and if it exceeds 15 wt.%, The electrical resistivity may increase due to the impurity effect, and therefore it is preferably less than 15 wt.%, Particularly preferably 3 to 12 wt.%. When adding in combination, the total addition amount is preferably 4 to 13 wt.%. In addition, N type requires addition of at least 0.01 wt.% Or more, and if it exceeds 0.10 wt.%, The electrical resistivity may increase due to the impurity effect. It is preferable that When adding in combination, the total addition amount is preferably 0.09 to 0.25 wt.%.

  The additive element is Si, Ge, Sn, Pb as the Group 4 element, S, Se, Te as the Group 6 element, Br, I as the Group 7 element, and compounds of these various elements when added in combination Alternatively, a compound with a main component element may be used.

In general, the operating temperature range of the Peltier element and the thermoelectric conversion element varies depending on the application, so that a material having a high figure of merit (ZT) in the required temperature range is required. For this reason, the main component of the Bi ₂ Te ₃ series thermoelectric material is appropriately selected according to the application. For example, in the case of P type when used at a low temperature, Bi is contained more than Sb, and when used near room temperature, Bi is selected to be less than Sb.

  When the crystal pulling speed exceeds 20 cm / hr, the degree of c-axis orientation of the crystal decreases, the thermal conductivity in the direction perpendicular to the pulling direction of the ingot increases, and the figure of merit decreases, but the pulling speed decreases. If it is less than 2 cm / hr, most of the additive elements precipitate at the grain boundaries, the electrical resistivity in the direction perpendicular to the pulling direction increases, and the figure of merit decreases as well. Accordingly, the pulling speed is preferably 2 to 20 cm / hr.

  As a method for producing a polycrystalline ingot from a molten metal, any method of the Bridgeman method and the zone melt method may be used. When a polycrystalline body is produced by powder metallurgy, the ingot may be finely pulverized and the powder may be press-molded and sintered, or may be molded by hot press. The material may be a single crystal or a polycrystal, but a composition giving low thermal conductivity is preferable. The lower the thermal conductivity, the greater the temperature gradient generated inside the thermoelectric material close to the metal, and a thermoelectromotive force proportional to the temperature gradient is generated.

  Ingots produced at a high crystal growth rate have a large amount of additive elements and additives segregated or precipitated in the grain boundaries, and they do not disperse very much within the crystal grains, but crystals of ingots produced at a slow growth rate The structure has a large crystal grain size, and the additive elements and additives are considerably dispersed within the crystal grains. When the additive element is appropriately dispersed in the crystal grains, the thermal conductivity due to the phonon is lowered, and at the same time, the additive element is ionized to release carriers, so that the electrical resistivity tends to be lowered. However, when the additive element is segregated, the opposite tendency is exhibited. Therefore, since the electrical resistivity is influenced by the crystal growth rate, if the crystal growth rate is controlled so that the additive element is in an appropriate dispersed state in the crystal grains, the increase in the electrical resistivity can be suppressed considerably.

  When the powder metallurgy method is used, the additive element is appropriately dispersed in the crystal grains by controlling the sintering conditions or the molding temperature during hot pressing, so that the increase in electrical resistivity can be suppressed considerably. . The production conditions for the polycrystalline body by the powder metallurgy method are affected by the powder particle size, the composition of the material, and the additive elements, so the production conditions may be selected according to the application.

The growth temperature of the ingot may be any number of degrees Celsius as long as the Bi ₂ Te ₃ compound is dissolved, but the atmosphere in the molten metal is preferably a vacuum or an inert gas.

In the case of a thermoelectric element used in a module that generates electricity with a constant temperature gradient direction, the ratio of the length l _B of the thermoelectric material to the total length l of the element (x = l _B / l) is 0.83 or less, By making the length l _M 1 mm or less and the thermoelectric material length l _B 3 mm or less, the performance index of the device can be increased to 1.3 or more. This corresponds to about twice the figure of merit of commercially available thermoelectric elements. In particular, when x is 0.20 or less and l _B is 0.1 mm or less, the figure of merit of the thermoelectric element exceeds 7, especially when x = 0.143 and l _B = 0.05 mm, the figure of merit becomes 13.2 and Tc = 300K. The calculated value of maximum energy conversion efficiency under the temperature difference of Th = 350K reached 15%. This is an energy conversion efficiency slightly lower than the energy conversion efficiency 17% of the solar cell.

  Furthermore, the output power of the power generation module assembled using these thermoelectric elements has been dramatically improved along with the improvement of the performance index of the thermoelectric elements so as to support the above results. Such a high figure of merit was obtained because of the dramatic increase in Seebeck coefficient due to the extremely low thermal conductivity and interface effect of the thermoelectric material used.

  Furthermore, the module of the present invention has a property that the figure of merit improves as the element length is shortened as long as the length of the thermoelectric material and the metal maintains a predetermined ratio. This is due to the relative expansion of the region where the Seebeck coefficient increases due to the decrease in the electrical resistance of the thermoelectric material and the interface effect within the thermoelectric material. Therefore, the module of the present invention has the advantage that it can be made lighter, thinner and smaller than the conventional module.

On the other hand, in the case of a thermoelectric element used for a module that generates electricity by periodically changing the direction of the temperature gradient, the metal length l _M is 0.5 mm or less and the thermoelectric material length l _B is 3 mm or less. By making the ratio of the length l _B of the thermoelectric material to the total length l of the element (x = l _B / l) 0.15 or less, the performance index of the element can be increased to 1.3 or more. Especially when the l _B below in l _M is 0.3mm or less 0.1mm, the performance index of the thermoelectric elements was greater than 7.

Unlike the case where the temperature gradient is constant, the output power when the periodically changing voltage (Fig. 3) is applied to the power generation module assembled using these thermoelectric elements is x Rather, it strongly depends on the length l _{B of the} thermoelectric material and the metal thickness l _M , and high output power can be obtained when l _B is 0.5 mm or less and l _M is 4 mm or less. By applying a periodic voltage as shown in Fig. 3 to the heating and cooling modules and applying a periodic temperature gradient, the output voltage of the module becomes higher than when a DC voltage is applied, T = 240 sec The output voltage was about 3 times that of the DC voltage applied.

  The reason why the thermoelectromotive force is increased by applying the periodic temperature gradient is that a rapid temperature gradient is generated in the thermoelectric material near the interface between the metal and the thermoelectric material, and the Seebeck coefficient is drastically improved. In other words, when the thermal conductivity of the thermoelectric material is extremely low, the heat flow from a metal with extremely high thermal conductivity does not flow instantaneously into the thermoelectric material, and the heat flow does not reach the inside of the thermoelectric material. This is because heat easily accumulates only in the surface layer portion of the material. This is the reason why a steep temperature gradient was created in the thermoelectric material close to the interface. Therefore, the Seebeck coefficient is considered to be improved by the interface effect and the metal heat sink effect.

The electrical resistivity of the metal M constituting the thermoelectric element is low and the thermal conductivity is high, the Seebeck coefficient of the thermoelectric conversion material B is high and the thermal conductivity is low, and each constituent material has predetermined l _B and l _The element figure of merit increases most when it has _M. As the thermoelectric conversion material, the performance index of the device increases when a material having a high performance index is used. In particular, when the constituent materials have appropriate dimensions, the maximum figure of merit exceeds 10 and increases by more than 10 times that of the Bi ₂ Te ₃ series thermoelectric material itself.

  In this way, the high-performance thermoelectric conversion element (ZT> 7) with dramatically improved figure of merit has a total element length of 0.5 mm or less, so it is reduced from one-half to one-third of the thickness of the conventional element. There is an advantage that can be thinned. This is suitable not only for attaching a module to a heat source in a narrow space and cooling the outside of the module to generate electricity, but also for reducing the cost of raw materials used and reducing the weight of the thermoelectric module. Furthermore, since the conversion efficiency of the proposed module exceeds 10% under the temperature difference of Tc = 300K and Th = 350K, the exhaust heat energy can be recovered and various applications can be developed. Become. It turns out that it is particularly suitable for in-vehicle use.

  The Seebeck coefficient S of a sandwich-type thermoelectric conversion element in which a thermoelectric material is sandwiched between metals is remarkably increased from the original Seebeck coefficient due to the interface effect between the metal electrode and the thermoelectric material, and the figure of merit is dramatically improved. The increase rate of the Seebeck coefficient also varies depending on the joining method of the thermoelectric material and the metal. That is, it is necessary to prevent mutual diffusion of atoms constituting both materials when soldering. This is because when the atoms are interdiffused, the thermal conductivity of the thermoelectric material is increased, the temperature gradient in the thermoelectric material near the interface is significantly decreased, and the increase rate of the Seebeck coefficient is decreased.

  A metal having low electrical resistivity and high thermal conductivity is preferable, and a thermoelectric material is preferably a material having low thermal conductivity so that a rapid temperature gradient is generated near the interface. Since the metal to be used needs to be a material excellent in corrosion resistance, a simple metal or an alloy of noble metals such as Cu, Ag and Au is preferable. The thermoelectric conversion material is not particularly defined, but the performance index of the device is dramatically improved by using a thermoelectric material having a high performance index and a low thermal conductivity.

  The conventional thermoelectric generation using the Seebeck effect has a constant temperature gradient direction. This is because the positions of the heat source and the cold source are determined. However, it has been found by the present invention that by periodically changing the direction of the temperature gradient applied to the thermoelectric element, the temperature gradient at the interface between the thermoelectric material and the metal becomes abrupt and the Seebeck coefficient due to the interface effect can be increased. It was.

  By making the thermoelectric material length less than one-third of the metal material length and applying a moderate periodic temperature gradient to the module, the module output power was improved to about 3 times. Although there are actually few heat sources that alternate between the high temperature side and the low temperature side in this way, it is possible to create it.

  As shown in Fig. 4, as a method to improve the thermoelectric conversion efficiency by creating a temperature gradient that changes periodically, as shown in Fig. 4, the power generation module is rotated at a certain period, and the module is a heat pipe from a high-temperature heat source and a low-temperature heat source. If the heat pipes are alternately sandwiched between the heat pipes, a temperature gradient that periodically changes even from a fixed heat source can be applied to the module. Since the time interval for switching the heat source between high and low temperatures exceeds 1 minute, there is enough time to rotate the module, and less electric energy is required to move, so it can be put to practical use.

  When joining the thermoelectric material to the metal, if the joining metal does not dissolve or react with the thermoelectric material or metal, either solder after soldering or melt the Bi or Bi-Sb alloy as a joining material. May be used. If the plating film thickness or the thickness of the bonding material is reduced, there is no significant effect on the characteristics by either method.

Example 1
Examples of the thermoelectric material and the manufacturing method thereof according to the present invention will be described. Table 1 shows various combinations of main components and additive elements used to produce N-type and P-type Bi-Te thermoelectric materials.

  After compounding elements and compounds in a predetermined ratio in this way, vacuum sealed in a 20φ quartz tube and melting at high frequency (purity of raw material used is 99.99% or more), and after melting the material, the cylindrical ingot The measurement sample was cut from the center and the thermoelectric properties were measured at 25 ° C. Table 2 shows the preparation conditions of these samples, and Table 4 shows the measurement results of the thermoelectric properties in the pulling direction.

  A sintered body having a fine crystal grain size was prepared by hot pressing at the temperatures shown in Table 3 after coarsely grinding the above ingot and then finely grinding with a jet mill. The average crystal grain size of the prepared samples is shown in Table 3, and the thermoelectric characteristics in the direction perpendicular to the pressing direction are shown in Table 4.

Example 2
The assembly of the thermoelectric conversion element was cut into a 5 mm × 5 mm shape and then cut into lengths shown in Table 5. Cu and Ag used as electrodes were also processed to the length shown in the table. P-type and N-type thermoelectric elements as shown in FIG. 1 were fabricated by Ni plating on both ends of the thermoelectric material and then joining with Cu or Ag by eutectic solder.

  Table 5 shows the results of measuring the thermoelectric properties of the fabricated P-type and N-type thermoelectric conversion elements at 25 ° C. Table 6 shows the measurement results of the electrical resistance R, effective thermoelectromotive force Veff, and output power W of a pair of modules manufactured by π-bonding the P-type and N-type thermoelectric elements in Table 5 as shown in FIG.

The output power of the module was calculated from the measured electromotive force V and electrical resistance R using W = Veff ² / 4R (W). The thermoelectromotive force is measured when a temperature difference is generated by applying a constant DC voltage of 1.7 V to the heating and cooling modules connected in series (internal resistance 0.55 Ω / month) (corresponding to T = ∞) And a case where a temperature difference is generated by changing a voltage having an amplitude of 1.7 V at a constant period T. The pass / fail criterion for the output power of the module was 25 (mW), and more than that was acceptable.

In the case of the combination of Bi ₂ Te ₃ thermoelectric material and metal according to the present invention, by assembling the thermoelectric conversion element with the length of the metal and the thermoelectric material being a predetermined ratio, the metal heat sink effect with good heat conduction As a result, the Seebeck coefficient S increases dramatically due to the interface effect between the metal and the thermoelectric material, and the figure of merit can be greatly improved.

In the case of the combination of the Bi ₂ Te ₃ thermoelectric material and metal according to the present invention, the power generation efficiency can be greatly improved by periodically changing the direction of the temperature gradient applied to the thermoelectric module.

It is a perspective explanatory view showing the composition of a thermoelectric conversion element. It is explanatory drawing which shows the assembly structure of a P-type and an N-type thermoelectric conversion element. It is a graph of the voltage and time which show the voltage application method to a thermoelectric conversion element and a module. It is perspective explanatory drawing which shows the application method of the temperature gradient which changes periodically.

Claims (3)

The Bi ₂ Te ₃ with a single crystal or polycrystal of the thermoelectric conversion materials composed primarily, the thermoelectric material (length: l _B) across the metal (length: l _M) of the joining of the thermoelectric material A thermoelectric conversion element characterized in that a ratio x = l _B / (l _B + 2l _M ) between the length (l _B ) and the total length (l _B + 2l _M ) of the thermoelectric conversion element is 0.83 or less. Using a thermoelectric conversion element in which metal (length: l _M ) is bonded to both ends of the thermoelectric material (length: l _B ), the length of the thermoelectric material (l _B ) and the total length of the thermoelectric conversion element (l _B + 2l power generation module, wherein the ratio of _M) x = l _B / a (l _B + 2l _M) and 0.45 or less, thereby generating under the direction of the temperature gradient does not change. Using a thermoelectric conversion element with metal (l _M ≤ 4 mm) bonded to both ends of a thermoelectric material (l _B ≤ 0.5 mm), the length of the thermoelectric material (l _B ) and the total length of the thermoelectric conversion element (l _B +2 l _M ) Ratio x = l _B / (l _B + 2l _M ) is 0.63 or less, and a power generation module is configured to generate power by applying a periodically changing temperature gradient.