Thermoelectric conversion element, method for manufacturing same, thermoelectric conversion system, and power generation method

Abstract

The present invention provides: a thermoelectric conversion element in which first and second semiconductor members of the same conductivity type among p-type and n-type are joined and in which a band gap of the second semiconductor member is smaller than a band gap of the first semiconductor member; a method for manufacturing the same; a thermoelectric conversion system; and a power generation method.

Description

Thermoelectric conversion element and manufacturing method thereof, thermoelectric conversion system, and power generation method

[0001] The present invention relates to a thermoelectric conversion element that generates electricity when heated, a method for manufacturing the same, a thermoelectric conversion system, and a power generation method.

[0002] There is currently a growing global movement toward decarbonization. As of November 2021, when COP26 concluded, 154 countries and one region have announced their intention to achieve carbon neutrality by 2050 or another deadline.

[0003] Looking at the fossil fuel dependency rates of major countries, Japan's rate is 88.9%, which is extremely high, compared to 46.9% in France, where nuclear power generation is popular, and 76.7% in Germany, which is actively introducing renewable energy. In thermal power generation using fossil fuels, more than 50% of the thermal energy converted into electrical energy is discarded as waste heat (see Non-Patent Document 1). For this reason, thermoelectric materials that can convert the discarded thermal energy of waste heat into electrical energy have attracted attention.

[0004] The present inventors have proposed several novel thermoelectric conversion elements (see Patent Documents 1 to 3).

[0005] International comparison of fossil power efficiency and CO2 intensity, ECOFYS, (2018)

[0006] Japanese Patent No. 6551849 Japanese Patent No. 6875733 Japanese Patent Laid-Open No. 2020-88028

[0007] An object of the present invention is to provide a novel thermoelectric conversion element, a method for manufacturing the same, a thermoelectric conversion system, and a power generation method.

[0008] The present inventors have discovered a novel thermoelectric conversion element in which semiconductor members of the same conductivity type, p-type or n-type, but different bandgaps are joined, rather than a typical junction such as a junction of a p-type semiconductor member and an n-type semiconductor member (pn junction), and have completed the present invention.

[0009] That is, the present invention is as follows. [1] A thermoelectric conversion element characterized by first and second semiconductor members of the same conductivity type, p-type or n-type, being joined together, and the band gap of the second semiconductor member being smaller than the band gap of the first semiconductor member. [2] The thermoelectric conversion element according to [1] above, characterized in that the second semiconductor member is a degenerate semiconductor. [3] The thermoelectric conversion element according to [1] or [2] above, characterized in that the first and second semiconductor members are silicon doped with a dopant. [4] The thermoelectric conversion element according to [3] above, characterized in that the doping amount of the dopant in the second semiconductor member is greater than the doping amount of the dopant in the first semiconductor member.

[0010] [5] The doping amount of the dopant in the first and second semiconductor members is 1.0 × 10 １７ atom / cm ３ [6] The thermoelectric conversion element according to any one of [3] to [5] above, characterized in that the dopant of the first and / or second semiconductor member is boron or aluminum. [7] The doping amount of the dopant of the first semiconductor member is 1.0 × 10 １７ ~5.0 x 10 １９ atom / cm ３ and the doping amount of the dopant in the second semiconductor member is 5.0×10 １７ ~1.0 x 10 ２２ atom / cm ３ The thermoelectric conversion element according to [6] above,

[0011] [8] The thermoelectric conversion element according to any one of [3] to [5] above, wherein the dopant of the first and / or second semiconductor member is phosphorus. [9] The doping amount of the dopant of the first semiconductor member is 1.0 × 10 １７ ~7.0 x 10 １９ atom / cm ３ and the doping amount of the dopant in the second semiconductor member is 5.0×10 １７ ~1.0 x 10 ２２ atom / cm ３ The thermoelectric conversion element according to [8] above,

[0012]

[10] A method for manufacturing a thermoelectric conversion element, comprising: preparing a p-type or n-type first semiconductor material powder; and a second semiconductor material powder that has the same conductivity type as the first semiconductor material powder and a band gap smaller than that of the first semiconductor material powder; placing one of the first semiconductor material powder and the second semiconductor material powder in a sintering device; placing the other semiconductor material powder in the sintering device so as to be in contact with the one semiconductor material powder; and sintering the first semiconductor material powder and the second semiconductor material powder using the sintering device, thereby manufacturing a thermoelectric conversion element in which a first semiconductor member and a second semiconductor member are joined.

[0013]

[11] A thermoelectric conversion system comprising the thermoelectric conversion element according to any one of [1] to [9] above and a heating means for heating the thermoelectric conversion element.

[12] The thermoelectric conversion system according to

[11] above, wherein the heating means heats the thermoelectric conversion element to 50°C or higher.

[13] The thermoelectric conversion system according to

[11] or

[12] above, wherein the heating means heats the first and second semiconductor members of the thermoelectric conversion element at the same temperature.

[14] The thermoelectric conversion system according to

[11] or

[12] above, wherein the heating means heats the first and second semiconductor members of the thermoelectric conversion element at different temperatures.

[0014]

[15] A power generation method characterized by heating the thermoelectric conversion element according to any one of [1] to [9] above to generate power.

[16] A power generation method according to

[15] above, characterized by heating the thermoelectric conversion element to 50°C or higher.

[17] A power generation method according to

[15] or

[16] above, characterized by heating the first and second semiconductor members of the thermoelectric conversion element at the same temperature.

[18] A power generation method according to

[15] or

[16] above, characterized by heating the first and second semiconductor members of the thermoelectric conversion element at different temperatures.

[0015] According to the present invention, it is possible to provide a novel thermoelectric conversion element, a manufacturing method thereof, a thermoelectric conversion system, and a power generation method.

[0016] (a) shows the p-Si single layer sample and p ＋ (b) is a diagram showing the band energy of a single-layered silicon sample; (c) is a diagram showing the band energy of a single-layered silicon sample; (d) is a diagram showing the band energy of a single-layered silicon sample; ＋ 10A and 10B are diagrams showing the band energies of the p-Si / p-Si bilayer samples heated at different temperatures. ＋ (b) Schematic of the band-to-band transition in the p-Si / p-Si bilayer sample heated at different temperatures. ＋ 10A and 10B are explanatory diagrams showing the suppression of the bipolar effect by the energy barrier in the p-Si / p-Si bilayer sample. ＋ (b) is a diagram illustrating the band-to-band transition in a p-Si / p-Si bilayer sample heated at the same temperature. ＋ 10A and 10B are explanatory diagrams showing the suppression of the bipolar effect by the energy barrier in the n-Si / p-Si bilayer sample. ＋ (b) is an explanatory diagram showing the band energy of a single-layered silicon sample; ＋ 10A and 10B are diagrams showing the band energies of the n-Si / n-Si bilayer sample. ＋ (b) is a diagram illustrating the band-to-band transition in a n-Si / n-Si bilayer sample heated at the same temperature. ＋1 is an explanatory diagram showing the suppression of the bipolar effect by the energy barrier of a -Si / n-Si bilayer sample. It is a schematic diagram of a spark plasma sintering apparatus used to manufacture the sample. It is an explanatory diagram showing the circuit of a Seebeck coefficient mapping device for measuring and visualizing the electromotive force of a sample. It is an explanatory diagram showing the circuit of a line measurement device for performing a one-dimensional profile of the voltage drop of a sample. It is a schematic diagram showing the cross section of a sintering die used to manufacture the sample. (a) and (b) are schematic diagrams explaining the extraction of a measurement sample from a manufactured bulk sample. (a) and (b) are explanatory diagrams showing the results of Seebeck coefficient mapping of a bilayer sample (p-type), and (c) is an explanatory diagram showing the results of Seebeck coefficient mapping of a single-layer sample. (a) and (b) are graphs showing the voltage drop profile of a bilayer sample (p-type). (a) and (b) are graphs showing the evolution of thermoelectric power in a bilayer sample (p-type) and a single-layer sample (p-type). Graphs (a) and (b) show the evolution of thermoelectric power in a bilayer sample (p-type) and a single-layer sample (p-type) when no temperature difference is applied. An explanatory diagram showing the circuit of an electromotive force measuring device for bilayer samples (p-type) connected in series via a metal plate. A graph showing the evolution of thermoelectric power in bilayer samples (p-type) connected in series via a chromel plate. The graphs show, from top to bottom, the chromel series connection, Sample 2, and Sample 1. A diagram showing the results of Seebeck coefficient mapping of a bilayer sample (n-type). A graph showing the voltage drop profile in a bilayer sample (n-type). A graph showing the evolution of thermoelectric power in a bilayer sample (1.1-7.4) (n-type) and a single-layer sample (n-type). A graph showing the evolution of thermoelectric power in a bilayer sample (5.3-7.4) (n-type) and a single-layer sample (n-type).

[0017] The thermoelectric conversion element of the present invention is characterized in that first and second semiconductor members of the same conductivity type, either p-type or n-type, are joined together, and the band gap of the second semiconductor member is smaller than the band gap of the first semiconductor member.

[0018] The first and second semiconductor members are semiconductor members of the same conductivity type, in which a semiconductor main material (intrinsic semiconductor) is doped with a p-type or n-type dopant (additive). The semiconductor main materials used in the first and second semiconductor members may be the same or different materials, and the dopants used in the first and second semiconductor members may also be the same or different materials. Note that it is preferable that the semiconductor main materials used in the first and second semiconductor members are the same material, as this can reduce the contact resistance at the bonding interface and reliably reduce the bonding interface resistance.

[0019] Here, the semiconductor main material is not particularly limited as long as it is a substance having semiconducting properties. Examples of the semiconductor main material include elemental semiconductors such as silicon (Si), germanium (Ge), and selenium (Se), compound semiconductors such as gallium arsenide (GaAs), gallium nitride (GaN), gallium phosphide (GaP), indium phosphide (InP), and silicon carbide (SiC), and organic semiconductors such as polyacetylene. In particular, elemental semiconductors and compound semiconductors are preferred because they are easy to dope, and inexpensive silicon and germanium that are commonly used as semiconductor main materials are more preferred.

[0020] Examples of p-type dopants for elemental semiconductors and compound semiconductors include elements with fewer outermost electrons than the semiconductor main material. For example, when the semiconductor main material is silicon, examples include boron (B), aluminum (Al), gallium (Ga), indium (In), palladium (Pd), etc., and any one or more of these can be used in combination, with boron and aluminum being particularly preferred. Examples of p-type dopants for organic semiconductors include electron-withdrawing elements and molecules such as halogens.

[0021] Examples of n-type dopants for elemental semiconductors and compound semiconductors include elements with more outermost electrons than the semiconductor main material. For example, when the semiconductor main material is silicon, examples include phosphorus (P), antimony (Sb), arsenic (As), titanium (Ti), etc., and any one or more of these can be used in combination, with phosphorus being particularly preferred. Examples of n-type dopants for organic semiconductors include electron-donating elements and molecules such as alkali metals.

[0022] In the thermoelectric conversion element of the present invention, the band gap of the second semiconductor member is smaller than the band gap of the first semiconductor member. For example, when the first and second semiconductor members are semiconductor members made of the same semiconductor main material doped with the same dopant, the doping amount of the dopant in the second semiconductor member can be made larger than the doping amount of the dopant in the first semiconductor member, thereby making the band gap of the second semiconductor member smaller than the band gap of the first semiconductor member. In particular, it is preferable that the second semiconductor member is a degenerate semiconductor doped with a high concentration of dopant, and the first semiconductor member is a non-degenerate semiconductor. Note that when the semiconductor main material is silicon, the band gap of the second semiconductor member is 2.6 × 10 １９ atom / cm ３ When doped with more than 2.6 × 10 １９ atom / cm ３ When doped with less than 1000 dopants, it becomes a non-degenerate semiconductor.

[0023] Here, the doping amount of the dopant in each of the first and second semiconductor members is 1.0×10 １７ atom / cm ３ It is preferable that the ratio is 5.0×10 or more. １７ atom / cm ３ More preferably, it is 1.0 × 10 or more. １８ atom / cm ３ More preferably, it is 5.0 × 10 or more. １８ atom / cm ３ It is particularly preferable that the value is 1.0 × 10 or more. １９ atom / cm ３It is most preferable that the doping amount is equal to or greater than this range. By using a doping amount within this range, a thermoelectric conversion element with an appropriate resistance value can be obtained. In the thermoelectric conversion element of the present invention, the resistivity at the second semiconductor member is greater than the resistivity at the first semiconductor member, and is preferably 0.5 to 100 mΩ cm, more preferably 0.5 to 50 mΩ cm, and even more preferably 0.5 to 25 mΩ cm throughout the thermoelectric conversion element.

[0024] In addition, when both the first and second semiconductor members are silicon doped with boron or aluminum (semiconductor members of the same conductivity type, p-type), the doping amount of the dopant in the first semiconductor member is 1.0 × 10 １７ ~5.0 x 10 １９ atom / cm ３ It is preferable that the １７ ~5.0 x 10 １９ atom / cm ３ More preferably, it is 1.0 × 10 １８ ~5.0 x 10 １９ atom / cm ３ More preferably, it is 5.0 × 10 １８ ~5.0 x 10 １９ atom / cm ３ It is particularly preferable that １８ ~3.0 x 10 １９ atom / cm ３ It is most preferable that:

[0025] At this time, the doping amount of the dopant in the second semiconductor member is greater than the doping amount of the dopant in the first semiconductor member, and is 5.0×10 １７ ~1.0 x 10 ２２ atom / cm ３ It is preferable that the １８ ~5.0 x 10 ２１ atom / cm ３ More preferably, it is 5.0 × 10 １８ ~1.0 x 10 ２１ atom / cm ３ More preferably, it is 1.0 × 10 １９ ~1.0 x 10 ２１atom / cm ３ It is particularly preferable that １９ ~5.0 x 10 ２０ atom / cm ３ It is most preferable that:

[0026] In addition, when both the first and second semiconductor members are silicon doped with phosphorus (semiconductor members of the same n-type conductivity), the doping amount of the dopant in the first semiconductor member is, for example, 1.0 × 10 １７ ~7.0 x 10 １９ atom / cm ３ is 5.0 × 10 １７ ~7.0 x 10 １９ atom / cm ３ More preferably, it is 1.0 × 10 １８ ~7.0 x 10 １９ atom / cm ３ More preferably, it is 5.0 × 10 １８ ~7.0 x 10 １９ atom / cm ３ It is particularly preferable that １８ ~3.0 x 10 １９ atom / cm ３ It is most preferable that:

[0027] At this time, the doping amount of the dopant in the second semiconductor member is greater than the doping amount of the dopant in the first semiconductor member, and is 5.0×10 １７ ~1.0 x 10 ２２ atom / cm ３ It is preferable that the １８ ~5.0 x 10 ２１ atom / cm ３ More preferably, it is 5.0 × 10 １８ ~1.0 x 10 ２１ atom / cm ３ More preferably, it is 1.0 × 10 １９ ~1.0 x 10 ２１ atom / cm ３ It is particularly preferable that １９ ~5.0 x 10 ２０ atom / cm ３ It is most preferable that:

[0028] The principle of the thermoelectric conversion element of the present invention will be described below by taking as an example a case where both the first and second semiconductor members are boron-doped silicon (p-type semiconductor members).

[0029] Silicon has the property that the band gap narrows as the dopant concentration increases. Therefore, there are two types of silicon: lightly doped p-type Si (p-Si) and heavily doped degenerate p-type Si (p ＋ The energy band diagram of the conduction band of silicon (Si) is shown in Figure 1(a), and when these two materials are bonded together, the energy band diagram is thought to be as shown in Figure 1(b). This creates an energy barrier at the junction interface of the conduction band.

[0030] Here, as shown in FIG. 2(a), the energy required for interband excitation is small. ＋ When the first and second semiconductor members are heated at different temperatures (using the Seebeck effect) so that p-Si is on the high temperature side and p-Si is on the low temperature side, ＋ It is believed that band-to-band transitions occur actively on the -Si side. Normally, when band-to-band transitions occur, electrons diffuse in the conduction band from the high-temperature side to the low-temperature side due to the bipolar effect. However, when first and second semiconductor members with different band gaps are joined, the energy barrier at the joining interface suppresses the diffusion of electrons, as shown in Figure 2(b). This suppresses the reduction in charge separation due to the bipolar effect, improving the electromotive force (suppressing the decrease in electromotive force).

[0031] On the other hand, when the first and second semiconductor members are heated at the same temperature (when the Seebeck effect is not utilized), in order for the thermoelectric conversion element to generate electricity, it is necessary for the band gap size to change within the material (the band gap of the second semiconductor member must be smaller than the band gap of the first semiconductor member). ＋ The band structure and power generation mechanism in the bilayer sample of -Si / p-Si are shown in Figures 3(a) and (b). As shown in Figure 3(a), the energy required for the band transition is p ＋ The -Si side is smaller than the p-Si side. ＋As shown in Figure 3(b), holes (positive holes) diffuse, while the diffusion of these excited electrons is suppressed by the energy barrier at the junction interface, resulting in a p ＋ An electromotive force is generated such that the -Si side is the negative electrode and the p-Si side is the positive electrode.

[0032] Next, the concept of adjusting the band gap will be explained using the example of the case where the first and second semiconductor members are both silicon doped with phosphorus (n-type semiconductor members). However, since the basic concept is the same as that of the p-type semiconductor members of the same conductivity type described above, a brief explanation will be given.

[0033] As mentioned above, silicon has the property that the band gap narrows as the dopant concentration increases. Therefore, there are two types of silicon: lightly doped n-type Si (n-Si) and heavily doped degenerate n-type Si (n ＋ The energy band diagram of the valence band of silicon (Si) is shown in Figure 4(a), and when these two materials are bonded together, the energy band diagram is thought to be as shown in Figure 4(b). This creates an energy barrier at the junction interface in the valence band.

[0034] As shown in FIG. 5(a), the energy required for band-to-band transition is n ＋ The -Si side is smaller than the n-Si side. Therefore, the band gap is narrower. ＋ At this time, as shown in FIG. 5(b), electrons can diffuse to the n-Si side, but holes are difficult to diffuse to the n-Si side due to the energy barrier at the junction interface, resulting in an increase in electromotive force (n ＋ It is thought that an electromotive force is generated (when the temperatures on the -Si side and the n-Si side are different) or when the temperatures are the same.

[0035] A method for manufacturing a thermoelectric conversion element of the present invention, which can produce the thermoelectric conversion element of the present invention described above, will now be described. The method for manufacturing a thermoelectric conversion element of the present invention includes the steps of: preparing a first semiconductor material powder of p-type or n-type and a second semiconductor material powder having the same conductivity type as the first semiconductor material powder and a band gap smaller than that of the first semiconductor material powder (raw material preparation step); arranging one of the first semiconductor material powder and the second semiconductor material powder in a sintering device (first powder arrangement step); arranging the other semiconductor material powder in the sintering device so as to contact the one semiconductor material powder (second powder arrangement step); and sintering the first semiconductor material powder and the second semiconductor material powder in the sintering device to produce a thermoelectric conversion element in which the first semiconductor member and the second semiconductor member are bonded together (sintering step).

[0036] The first and second semiconductor material powders may be, for example, semiconductor material powders doped with a dopant that satisfy predetermined conditions, but powders obtained by pulverizing a molding material (wafer) that satisfies the predetermined conditions may also be used. The particle size of the powder is not particularly limited, but from the viewpoint of producing thermoelectric conversion elements of uniform quality, a powder with a uniform particle size and a narrow particle size distribution is preferred. The particle size of the powder is, for example, about 1 to 50 μm, and preferably 1 to 30 μm.

[0037] In the sintering process, for example, a pressure sintering method or a pressureless sintering method (PLS method) can be used. Here, examples of the pressure sintering method include a solid compression method and a gas compression method. Examples of the solid compression method include a spark plasma sintering method (SPS method), a hot press sintering method (HP method), and an ultra-high pressure sintering method. Examples of the gas compression method include a hot isostatic pressure sintering method (HIP method), and a high-pressure gas reaction sintering method. Examples of the pressureless sintering method include a reaction sintering method and an atmosphere sintering method. Specific examples include a thermal plasma sintering method, a microwave / millimeter wave sintering method, and the like.

[0038] In the method for manufacturing a thermoelectric conversion element of the present invention, spark plasma sintering is preferably used because it can reduce the interfacial resistance between the first semiconductor member and the second semiconductor member, thereby enabling the production of thermoelectric conversion elements of uniform quality with good productivity and low cost. As a sintering apparatus used in the spark plasma sintering method, for example, a sintering apparatus having a cylindrical sintering die made of graphite (as shown in FIG. 6) can be used. First and second semiconductor material powders are sequentially (in any order) filled into the sintering die, and the filled powders are pressed by sintering punches inserted from both axial sides of the sintering die. Pulse current and voltage are applied to sinter the powders, thereby bonding the first semiconductor member, which is a sintered body of the first semiconductor material powder, and the second semiconductor member, which is a sintered body of the second semiconductor material powder. The thermoelectric conversion element can then be manufactured by molding into a predetermined shape as needed.

[0039] Next, a thermoelectric conversion system of the present invention including the thermoelectric conversion element of the present invention will be described. The thermoelectric conversion system of the present invention is characterized by having, for example, the above-described thermoelectric conversion element of the present invention and a heating means for heating the thermoelectric conversion element. In this specification, "heating" means supplying heat so that the temperature is higher than room temperature (25°C).

[0040] One or more thermoelectric conversion elements can be used, and for example, a large electromotive force can be obtained by electrically connecting them in series. This is preferable because a large electromotive force can be obtained without increasing the temperature at which the thermoelectric conversion elements are heated. Examples of methods for electrically connecting the thermoelectric conversion elements in series include a configuration in which multiple thermoelectric conversion elements are electrically connected by wiring, a configuration in which multiple thermoelectric conversion elements are stacked via a metal plate or metal block, and a configuration in which first semiconductor members and second semiconductor members are alternately stacked. Examples of metals for the metal plate or metal block include copper, nickel, chromel (Ni—Cr alloy), platinum, and brass, with chromel being preferred.

[0041] The heating means is not particularly limited as long as it can heat the thermoelectric conversion element to a temperature high enough to generate electricity. For example, a means for heating the thermoelectric conversion element to 50°C or higher is preferred. Since a large electromotive force can be generated by increasing the temperature, for example, 100°C or higher is more preferred, 150°C or higher is even more preferred, 200°C or higher is even more preferred, 300°C or higher is particularly preferred, and 500°C or higher is most preferred. On the other hand, the upper limit of the temperature is not particularly limited, but in order to suppress deterioration of the thermoelectric conversion element or thermoelectric conversion system, 1300°C or lower is preferred, 1000°C or lower is more preferred, 800°C or lower is even more preferred, 700°C or lower is particularly preferred, and 600°C or lower is most preferred. Furthermore, in a configuration in which multiple thermoelectric conversion elements are stacked via a metal plate or metal block, heating is preferably to 250°C or lower, and preferably to 200°C or lower. The heating medium of the heating means for heating the thermoelectric conversion element may be gas, liquid, or solid.

[0042] The heating means may heat the first and second semiconductor members of the thermoelectric conversion element at the same temperature or at different temperatures. That is, the thermoelectric conversion element of the present invention can generate an electromotive force even when the first and second semiconductor members are heated at the same temperature, and when they are heated at different temperatures, a larger electromotive force can be generated by utilizing the Seebeck effect. Furthermore, even under conditions where there is a transition between a state with no temperature difference and a state with a temperature difference, thermoelectric conversion can be performed in both cases, and an electromotive force can be generated efficiently.

[0043] The manner in which the first and second semiconductor members are heated at the same temperature includes a manner in which the first and second semiconductor members are heated in separate spaces of the same temperature, and a manner in which the first and second semiconductor members are heated in a space of the same environment. In the latter case, the first and second semiconductor members do not need to be individually temperature-controlled, which provides good operability.

[0044] Furthermore, when heating at different temperatures, the first semiconductor member with a smaller band gap is the high-temperature side, and the second semiconductor member with a larger band gap is the low-temperature side. Only the first semiconductor member needs to be heated, and the second semiconductor member may be cooled. The temperature difference between the first and second semiconductor members is, for example, preferably 10 ° C or more, more preferably 30 ° C or more, even more preferably 70 ° C or more, and most preferably 100 ° C or more. On the other hand, the upper limit is not particularly limited, but is, for example, preferably 900 ° C or less, more preferably 700 ° C or less, and even more preferably 400 ° C or less.

[0045] Here, the heat source for the heating means preferably utilizes waste heat generated from, for example, an internal combustion engine, a motor, a battery, an inverter, a factory, a power plant, etc. Examples of the internal combustion engine include engines such as gasoline engines and diesel engines. Furthermore, examples of the motor, battery, and inverter include motors, batteries, and inverters of electric vehicles or hybrid vehicles, etc.

[0046] Finally, a power generation method of the present invention using the thermoelectric conversion element of the present invention will be described. The power generation method of the present invention is characterized by heating the thermoelectric conversion element of the present invention described above to generate power. In the power generation method of the present invention, the thermoelectric conversion element is preferably heated to 50°C or higher, and the first and second semiconductor members may be heated at the same temperature or at different temperatures. Detailed conditions are the same as those described for the thermoelectric conversion system described above, and therefore will not be described here.

[0047] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples.

[0048] First, we will provide an overview of the spark plasma sintering method used in the manufacture of the thermoelectric conversion element of the present invention, Seebeck coefficient mapping using the thermal probe method to evaluate the electrical properties, voltage drop line measurement, and thermoelectric power measurement to evaluate the thermoelectric performance.

[0049] (Spark Plasma Sintering) Spark plasma sintering is a type of powder sintering method. This sintering method requires short heating, holding, and cooling times, and can produce uniform, dense samples, making it suitable for sintering a variety of materials. As shown in Figure 6, a powder sample is filled into a cylindrical graphite sintering die, and sintering is performed by mechanical pressure from a sintering punch and pulse current heating. Thermal and mechanical energy, as well as discharge plasma energy generated by a spark discharge phenomenon created by passing a large pulsed current at a low voltage between the compacted powder particles under pressure from the sintering punch, are used as the driving force for sintering. The sintering conditions used here were a sintering temperature of 1000°C, a pressure of 50 MPa, a holding time of 5 min, and a maximum sintering current of 780 A.

[0050] (Seebeck Coefficient Mapping Using the Thermal Probe Method) To measure and visualize the local electromotive force within a sample, the Seebeck coefficient was mapped using the thermal probe method. As shown in Figure 7, a sample was placed on an electrode (sample stage) and a heated probe was brought into contact with the sample, generating a local temperature difference within the sample directly below the probe. The temperature difference that causes the Seebeck effect is equal to the temperature difference between the heated probe and the room temperature at the bottom of the sample. The local electromotive force was measured from the potential difference between the probe and the electrode, and the local Seebeck coefficient was calculated from this electromotive force and temperature difference. Here, the probe was moved at 0.25 mm intervals under computer control, and the Seebeck coefficient of the sample surface was mapped.

[0051] (Voltage Drop Line Measurement) As shown in Fig. 8, the potential difference between the probe and the potential measurement electrode was measured while a constant current was passed through the sample. Here, computer control was used to move the probe in a straight line at intervals of 0.25 mm, and a one-dimensional profile of the voltage drop within the sample was obtained.

[0052] (Measurement of Thermoelectromotive Force) Both ends of the sample were heated using heaters, and the temperature was increased while maintaining a constant temperature difference. The potential difference between the samples at each temperature was measured, thereby calculating the thermoelectromotive force.

[0053] [Example 1] (Production of thermoelectric conversion element) As a raw material, p＋ -Si wafer (B doping amount: 3.5 × 10 ２０ atom / cm ３ ) and two types of p-Si wafers (B doping amount: 1.1 × 10 １９ atom / cm ３ , 3.0 × 10 １９ atom / cm ３ The amount of B doped was 1.1 × 10 １９ atom / cm ３ The p-Si wafer is made of a non-degenerate semiconductor, and the B doping amount is 3.0×10 １９ atom / cm ３ p-Si wafer and B doping amount: 3.5 × 10 ２０ atom / cm ３ No. ＋ -Si wafers consist of degenerate semiconductors.

[0054] (1) p ＋ -Si / p-Si bilayer sample (thermoelectric conversion element of the present invention) The wafer was pulverized to obtain a semiconductor material powder having a particle size of 30 μm or less, and p ＋ The -Si semiconductor material powder and the p-Si semiconductor material powder were packed sequentially into a sintering die as shown in Figure 9, and sintered using a spark plasma sintering method under the conditions of a sintering temperature of 1000°C, a pressure of 50 MPa, a holding time of 5 minutes, and a maximum sintering current of 780 A. The sintered body obtained by this sintering was cut into two-layer samples as shown by the dotted lines in Figure 10(a), and the two-layer sample (1.1-35) and two-layer sample (3.0-35) of the present invention shown in Table 1 were manufactured.

[0055]

[0056] (2) Single-layer sample (comparative thermoelectric conversion element) The wafer was pulverized to obtain semiconductor material powders having a particle size of 30 μm or less, and each semiconductor material powder was packed into a sintering die as shown in Fig. 9 and sintered using a spark plasma sintering method under the conditions of a sintering temperature of 1000°C, a pressure of 50 MPa, a holding time of 5 minutes, and a maximum sintering current of 780 A. The sintered body obtained by sintering was cut as shown by the dotted lines in Fig. 10(b) to produce three types of single-layer samples shown in Table 2.

[0057]

[0058] The results of various measurements on the prepared sample (thermoelectric conversion element) are shown below.

[0059] (Seebeck coefficient mapping by thermal probe method) After setting the probe temperature to 65°C and the sample temperature to room temperature (25°C), resulting in a temperature difference of 40°C, the probe was moved at intervals of 0.25 mm to map the Seebeck coefficient of the sample surface (see Figure 7). The results for the two-layer sample are shown in Figures 11(a) and 11(b), and the result for the single-layer sample is shown in Figure 11(c).

[0060] As shown in Figure 11, all regions showed a positive Seebeck coefficient, confirming that the fabricated sample was a p-type semiconductor. Furthermore, as shown in Figures 11(a) and 11(b), a junction interface perpendicular to the longitudinal direction of the sample was confirmed in the center of the two-layer sample. The Seebeck coefficient was high on the p-Si side with a low doping amount, and was low on the p-Si side with a high doping amount. ＋ Since the Seebeck coefficient is low on the -Si side, the upper side is p-Si and the lower side is p ＋ -Si, p ＋ It is believed that a -Si / p-Si bilayer sample has been successfully fabricated.

[0061] As shown in FIG. 11, the p-Si side and p-Si side of the bilayer sample ＋ The Seebeck coefficient of the -Si side was similar to that of each single-layer sample. Therefore, it is thought that interdiffusion is not actively occurring within the bilayer sample. Furthermore, as shown in Figures 11(a) and 11(b), the bilayer sample (3.0-35) had a lower Seebeck coefficient on the p-Si side than the bilayer sample (1.1-35) because the doping amount on the p-Si side was higher.

[0062] (Voltage Drop Line Measurement) In the two-layer sample, the electrical resistance at the layer interface, which leads to a decrease in thermoelectric power generation output, was investigated, and in order to calculate the resistivity of each layer, a constant current (two-layer sample (1.1-35): 50 mA, two-layer sample (3.0-35): 51 mA) was passed through the sample in a room temperature environment, and the voltage drop in the longitudinal direction of the sample was measured. The results are shown in Figure 12.

[0063] As shown in FIGS. 12(a) and 12(b), no large voltage drop was observed at the layer interface.

[0064] (Thermoelectric Power Measurement) The thermoelectric power measurement was carried out for the two-layer sample and the single-layer sample. The temperature difference between the heaters placed at both ends of the sample was 3°C, and the measurement was carried out in the temperature range of 150 to 550°C. ＋ In the -Si / p-Si bilayer sample, p ＋ The temperature on the -Si side was set to a high temperature. The results are shown in FIG.

[0065] The bilayer sample consisted of p-Si powder and p ＋ Since the samples were fabricated by filling equal amounts of Si powder into the sintering die, the thermoelectric power is expected to be the average value of the single-layer samples if the influence of the energy barrier is not taken into consideration. However, the thermoelectric power of the double-layer sample is ＋ The thermoelectric power of the p-Si single layer sample exceeded that of the -Si single layer sample at temperatures above 300°C. This is thought to be due to the suppression of bipolar diffusion by an energy barrier at high temperatures where band-to-band transitions occur.

[0066] The thermoelectric power of the bilayer sample (1.1-35) exceeded that of the bilayer sample (3.0-35) across the entire temperature range, particularly at high temperatures. The reason for the bilayer sample (1.1-35) exceeding that of the bilayer sample (3.0-35) across the entire temperature range is thought to be due to a higher Seebeck coefficient resulting from a lower boron doping level on the p-Si side. The reason for the significant increase at high temperatures is thought to be due to a larger difference in the boron doping level at the interface of the bilayer sample (1.1-35) than that of the bilayer sample (3.0-35), resulting in a larger energy barrier and stronger suppression of the bipolar effect.

[0067] Example 2 Subsequently, the thermoelectric power of the double-layered sample and the single-layered sample used in Example 1 was measured without applying a temperature difference, and power generation without a temperature difference was verified.

[0068] (Thermoelectric power measurement) Figure 14(a) shows the results of the two-layer sample (1.1-35), p ＋ Fig. 14(b) shows the thermoelectric power measurement results for the single-layered (3.0-35) and single-layered (1.1) p-Si samples. ＋ The thermoelectric power measurement results of the -Si single layer sample (35) and the p-Si single layer sample (3.0) are shown below. The measurements were carried out in the temperature range of 150 to 550°C.

[0069] As shown in Figures 14(a) and (b), the generation of thermoelectric power was confirmed in the bilayer sample (1.1-35) and the bilayer sample (3.0-35) as the temperature increased. No clear thermoelectric power was observed in the single-layer sample. In the bilayer sample, the energy required for the band-to-band transition was p ＋ Since the -Si side is smaller than the p-Si side, ＋ It is believed that the electromotive force is generated because the electron excitation probability increases on the -Si side, and the diffusion of electrons is suppressed by the energy barrier at the layer interface.

[0070] 14(a) and 14(b), the thermoelectric power of the bilayer sample (1.1-35) is greater than that of the bilayer sample (3.0-35). This is thought to be because the amount of boron doped on the p-Si side of the bilayer sample (1.1-35) is smaller, resulting in a larger band gap on the p-Si side than that of the bilayer sample (3.0-35).

[0071] [Example 3] Power generation in water was verified using a two-layer sample (1.1-35) manufactured in the same manner as in Example 1. Specifically, the sample was submerged in a water bath equipped with a heater, and then water was added and heated. After the water temperature stabilized, the electromotive force was measured every 0.1 seconds for 150 seconds.

[0072] When the average water temperature was 86°C (almost no temperature difference between the electrodes), a maximum electromotive force (Vmax) of 0.132 mV and an average electromotive force (Vave) of 0.057 mV were obtained. Similarly, when the average water temperature was 97.4°C (almost no temperature difference between the electrodes), a maximum electromotive force (Vmax) of 0.16 mV and an average electromotive force (Vave) of 0.081 mV were obtained.

[0073] Example 4 As shown in FIG. 15, two two-layer samples (1.1-35) manufactured in the same manner as in Example 1 were connected in series via a metal plate, and the electromotive force was measured.

[0074] Figure 16 shows a graph showing the evolution of thermoelectric power when a chromel plate (0.15 mm thick) was used as the metal plate. Samples 1 and 2 are measurements of two-layer samples connected via a metal plate. As shown in Figure 16, a maximum electromotive force of approximately 0.84 mV was obtained. Furthermore, a clear improvement in thermoelectric power due to sample bonding was confirmed at relatively low temperatures below 200°C.

[0075] Furthermore, when a copper plate (thickness 0.10 mm) was used as the metal plate, a maximum electromotive force of approximately 0.90 mV was obtained; when platinum foil (thickness 0.02 mm) was used, a maximum electromotive force of approximately 0.90 mV was obtained; and when a brass block (thickness 1.70 mm) was used, a maximum electromotive force of approximately 0.72 mV was obtained.

[0076] [Example 5] (Production of thermoelectric conversion element) As a raw material, n ＋ -Si wafer (P doping amount: 7.4 × 10 １９ atom / cm ３ ) and two types of n-Si wafers (P doping amount: 1.1 × 10 １９ atom / cm ３ , 5.3 × 10 １９ atom / cm ３ The amount of B doped was 1.1 × 10 １９ atom / cm ３ The n-Si wafer is made of a non-degenerate semiconductor, and the B doping amount is 5.3×10 １９ atom / cm ３ n-Si wafer and B doping amount: 7.4 × 10 １９ atom / cm ３ n ＋ -Si wafers consist of degenerate semiconductors.

[0077] The wafer was pulverized to obtain a semiconductor material powder having a particle size of 30 μm or less, and the two-layer sample (1.1-7.4) and the two-layer sample (5.3-7.4) shown in Table 3 were manufactured in the same manner as in Example 1.

[0078]

[0079] For comparison, three types of single layer sample (1.1), single layer sample (5.3), and single layer sample (7.4) shown in Table 4 were similarly produced.

[0080]

[0081] Various measurements were carried out on the prepared sample (thermoelectric conversion element).

[0082] (Seebeck Coefficient Mapping by Thermal Probe Method) The Seebeck coefficient of the surface of the two-layer sample was mapped in the same manner as in Example 1. The results are shown in FIG.

[0083] As shown in Figure 17, all regions showed a negative Seebeck coefficient, confirming that the fabricated samples were n-type semiconductors. Furthermore, in the two-layer sample (1.1-7.4), a junction interface perpendicular to the longitudinal direction of the sample was clearly visible in the center. The Seebeck coefficient was low on the n-Si side (top), where the doping amount was low, and was low on the n-Si side (top), where the doping amount was high. ＋ Since the Seebeck coefficient is high on the -Si side (bottom), the upper side is n-Si and the lower side is n ＋ -Si, n ＋ It is believed that a -Si / n-Si bilayer sample has been successfully fabricated.

[0084] (Voltage Drop Line Measurement) As in Example 1, the electrical resistance at the layer interface in the two-layer sample, which leads to a decrease in thermoelectric power generation output, was investigated, and in order to calculate the resistivity of each layer, a constant current was passed through the sample in a room temperature environment and the voltage drop in the longitudinal direction of the sample was measured. The results are shown in Figure 18.

[0085] As shown in FIG. 18, no large voltage drop was observed at the layer interface.

[0086] (Thermoelectric Power Measurement) As in Example 1, the thermoelectric power measurement was carried out for the two-layer sample and the single-layer sample. The temperature difference between the heaters placed at both ends of the sample was 3°C, and the measurement was carried out in the temperature range of 150 to 550°C. ＋ In the -Si / n-Si bilayer sample, n ＋ The temperature was set high on the -Si side. The results are shown in Figures 19 and 20.

[0087] The two-layer sample consisted of n-Si powder and n ＋Since the samples were fabricated by filling equal amounts of Si powder into the sintering die, the thermoelectric power is expected to be the average value for the single-layer samples if the influence of the energy barrier is not taken into consideration. However, the thermoelectric power of the double-layer sample is ＋ The thermoelectric power of the bilayer sample (1.1-7.4) exceeded that of the n-Si single layer sample at high temperatures above 250°C (see Figure 19), and the bilayer sample (5.3-7.4) exceeded that of the n-Si single layer sample over the entire temperature range (see Figure 20). This is thought to be due to the suppression of bipolar diffusion by the energy barrier.

[0088] The thermoelectric power of the bilayer sample (1.1-7.4) exceeded that of the bilayer sample (5.3-7.4) across the entire temperature range, particularly at high temperatures. The reason for the bilayer sample (1.1-7.4) exceeding that of the bilayer sample (5.3-7.4) across the entire temperature range is thought to be due to a lower Seebeck coefficient resulting from a lower phosphorus doping level on the n-Si side. The reason for the significant increase at high temperatures is thought to be due to a larger difference in the phosphorus doping level at the interface of the bilayer sample (1.1-7.4) than that of the bilayer sample (5.3-7.4), resulting in a larger energy barrier and stronger suppression of the bipolar effect.

[0089] INDUSTRIAL APPLICABILITY The present invention is industrially useful because it can provide a novel thermoelectric conversion element, a manufacturing method thereof, a thermoelectric conversion system, and a power generation method.

Claims

1. A thermoelectric conversion element characterized by first and second semiconductor members of the same conductivity type, either p-type or n-type, joined together, and the band gap of the second semiconductor member being smaller than the band gap of the first semiconductor member.

2. A thermoelectric conversion element according to claim 1, wherein the second semiconductor member is a degenerate semiconductor.

3. A thermoelectric conversion element according to claim 1, wherein said first and second semiconductor members are silicon doped with a dopant.

4. A thermoelectric conversion element according to claim 3, wherein the amount of dopant in said second semiconductor member is greater than the amount of dopant in said first semiconductor member.

5. The doping amount of the dopant in the first and second semiconductor members is 1.0 × 10 １７ atom / cm ３ 5. The thermoelectric conversion element according to claim 4, wherein:

6. A thermoelectric conversion element according to claim 5, wherein the dopant of said first and / or second semiconductor member is boron or aluminum.

7. The doping amount of the dopant in the first semiconductor member is 1.0 × 10 １７ ~5.0 x 10 １９ atom / cm ３ and the doping amount of the dopant in the second semiconductor member is 5.0×10 １７ ~1.0 x 10 ２２ atom / cm ３ 7. The thermoelectric conversion element according to claim 6, wherein 8. A thermoelectric conversion element according to claim 5, wherein the dopant of said first and / or second semiconductor member is phosphorus.

9. The doping amount of the dopant in the first semiconductor member is 1.0×10 １７ ~7.0 x 10 １９ atom / cm ３ and the doping amount of the dopant in the second semiconductor member is 5.0×10 １７ ~1.0 x 10 ２２ atom / cm ３ 9. The thermoelectric conversion element according to claim 8, wherein 10. A method for manufacturing a thermoelectric conversion element, comprising: preparing a first semiconductor material powder of p-type or n-type; and a second semiconductor material powder that has the same conductivity type as the first semiconductor material powder and a band gap smaller than that of the first semiconductor material powder; placing one of the first and second semiconductor material powders in a sintering device; placing the other semiconductor material powder in the sintering device so as to contact the one semiconductor material powder; and sintering the first and second semiconductor material powders using the sintering device to manufacture a thermoelectric conversion element in which a first semiconductor member and a second semiconductor member are joined.

11. A thermoelectric conversion system comprising the thermoelectric conversion element according to any one of claims 1 to 9 and a heating means for heating the thermoelectric conversion element.

12. The thermoelectric conversion system according to claim 11, wherein the heating means heats the thermoelectric conversion element to 50° C. or higher.

13. A thermoelectric conversion system according to claim 11, wherein said heating means heats said first and second semiconductor members of said thermoelectric conversion element to the same temperature.

14. A thermoelectric conversion system according to claim 11, wherein said heating means heats said first and second semiconductor members of said thermoelectric conversion element at different temperatures.

15. A method for generating electricity, comprising heating the thermoelectric conversion element according to any one of claims 1 to 9 to generate electricity.

16. The method for generating electricity according to claim 15, wherein the thermoelectric conversion element is heated to 50°C or higher.

17. The method for generating electricity according to claim 15, wherein the first and second semiconductor members of the thermoelectric conversion element are heated to the same temperature.

18. The power generating method according to claim 15, wherein the first and second semiconductor members of the thermoelectric conversion element are heated at different temperatures.

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