JP6032128B2 - Thermoelectric generator - Google Patents

Description

  The present invention relates to a thermoelectric power generation element that converts thermal energy into electrical energy.

  As a power generation element that converts thermal energy into electrical energy, there is a thermoelectron power generation element that generates electromotive force by utilizing thermionic emission. For example, Patent Document 1 discloses an example of a thermionic power generation element in which an emitter electrode and a collector electrode are arranged to face each other with a gap therebetween. This thermoelectron power generation element is configured such that when the emitter electrode is heated, thermoelectrons are emitted from the emitter electrode and collected by the collector electrode. And in the state which connected the load between the emitter electrode and the collector electrode, the thermoelectron which reached | attained the collector electrode moves to an emitter electrode through a load, and can supply electric power to a load.

  Also, when a diamond semiconductor is used as the material of the emitter electrode, the surface of the emitter is hydrogen-terminated, so that the emitter electrode has a lower energy level than the lower end of the valence band, so-called negative electrons. It will be in the state which has affinity (NEA: Negative Electron Affinity). Thereby, since it becomes easy to discharge | release a thermoelectron from an emitter electrode, the electric power generation efficiency of a thermoelectron power generating element can be improved more.

JP 2011-29427 A

  However, the emitter made of a diamond semiconductor whose surface is hydrogen-terminated has a problem that, as it continues to be heated, hydrogen on the surface gradually desorbs and the vacuum level gradually rises accordingly. For this reason, when the thermoelectric generator is continuously operated, there is a possibility that thermoelectrons are gradually less likely to be generated from the emitter. As a result, the thermionic current is reduced and the power generation efficiency may be reduced.

  The present invention has been made in view of such a background, and an object of the present invention is to provide a thermionic power generation element having high power generation efficiency and excellent durability.

One embodiment of the present invention includes an emitter that generates thermal electrons;
A collector arranged to face the emitter via a gap and collect the thermal electrons,
The emitter is made of a diamond semiconductor containing hydrogen inside, and the surface of the diamond semiconductor is hydrogen-terminated ,
The diamond semiconductor constituting the emitter is in a thermionic power generation element having a hydrogen concentration of 1 × 10 ²⁰ cm ⁻³ or more .

  The thermoelectric power generation element has an emitter which is made of a diamond semiconductor containing hydrogen inside and whose surface is terminated with hydrogen. Therefore, the emitter has a negative electron affinity (NEA) as described above, and is likely to generate thermal electrons. Thereby, the said thermoelectron power generation element becomes the thing excellent in power generation efficiency.

  Further, since the emitter is made of a diamond semiconductor containing hydrogen, when the emitter is heated, hydrogen diffuses from the inside of the emitter and easily reaches the surface. Therefore, hydrogen reaching the surface is bonded to dangling bonds generated when hydrogen is desorbed from the emitter surface, so that the emitter surface is likely to be in a hydrogen-terminated state again. As a result, the thermoelectric power generation element can maintain the state where the emitter surface is hydrogen-terminated for a longer time, and can easily suppress a decrease in the thermoelectron current generated from the emitter.

  As described above, according to the present invention, a thermionic power generation element having high power generation efficiency and excellent durability can be provided.

Explanatory drawing of the thermoelectric power generation element in an Example. Explanatory drawing of the energy band of the emitter in an Example. The graph which shows the magnitude | size of the thermoelectron current of the emitter produced by changing hydrogen concentration in an experiment example.

In the thermionic power generation element, the diamond semiconductor forming the emitter, the hydrogen concentration is Ru der 1 × ¹⁰ 20 ^{cm -3} or more. For this reason , the hydrogen inside the emitter is more easily diffused, so that the hydrogen easily reaches the emitter surface. As a result, hydrogen termination of dangling bonds caused by hydrogen desorption is more likely to occur. In addition, since the thermoelectron power generation element has a sufficiently large amount of hydrogen contained in the emitter, the state where the emitter surface is hydrogen-terminated can be maintained for a longer time. As a result, the thermoelectron power generation element can easily suppress a decrease in the thermoelectron current generated from the emitter for a longer period of time, and has excellent durability. If the hydrogen concentration is excessively high, the emitter may not have the properties of a diamond semiconductor. Therefore, the hydrogen concentration of the diamond semiconductor constituting the emitter is preferably 1 × 10 ²² cm ⁻³ or less.

  The diamond semiconductor constituting the emitter can be obtained by various methods, but is preferably formed by a microwave plasma CVD method using a hydrocarbon gas and a hydrogen gas as a source gas. A diamond semiconductor manufactured using a microwave plasma CVD method tends to contain more hydrogen in the interior than a diamond semiconductor obtained by another method such as a DC plasma CVD method or a sputtering method. Therefore, in this case, the thermoelectric power generation element tends to have high power generation efficiency and excellent durability.

(Example)
An example of the thermoelectric power generation element will be described with reference to FIGS. 1 and 2. As shown in FIG. 1, the thermoelectric generator 1 includes an emitter 2 that generates thermoelectrons, and a collector 3 that is disposed through a gap d so as to face the emitter 2 and collects thermoelectrons. . The emitter 2 is made of a diamond semiconductor containing hydrogen in the inside 21, and the surface 22 of the diamond semiconductor is hydrogen-terminated.

Emitter 2 in this example, the hydrogen concentration is of n-type diamond semiconductor of 1 × ¹⁰ 21 ^{cm -3,} nitrogen of 1 × ¹⁰ 20 ^{cm -3} is added as the donor. Further, the emitter 2 of this example is formed on the substrate 23 by microwave plasma CVD using molybdenum as the substrate 23 (see FIG. 1). As will be described later, the substrate 23 also serves as an electrode for electrically connecting the external load 4 to the emitter 2.

The film formation conditions of the microwave plasma CVD method for forming the emitter 2 are as follows.
・ Hydrogen gas flow rate: 300sccm
・ CH ₄ gas flow rate: 3 sccm
・ N ₂ gas flow rate: 10 sccm
・ Plasma output: 750W
-Substrate temperature: 800 ° C
-Pressure during film formation: 25 Torr

  Further, after forming the emitter 2 on the substrate 23, the surface 22 was subjected to hydrogen plasma treatment, and the surface 22 was hydrogenated. Further, following the hydrogen plasma treatment, the surface 22 was terminated with hydrogen by placing the emitter 2 in a hydrogen atmosphere.

Although the material which comprises the collector 3 is not specifically limited, In this example, the collector 3 which consists of an n-type diamond semiconductor which contains hydrogen in the inside 31, and nitrogen of 1 * 10 < ¹⁹ > cm < ^-3 > is added as a donor. Used. Further, the collector 3 of this example is formed on the substrate 33 by microwave plasma CVD using molybdenum as the substrate 33. As will be described later, the substrate 33 also serves as an electrode for electrically connecting the external load 4 to the collector 3 like the substrate 23 of the emitter 2.

The size of the gap d between the emitter 2 and the collector 3 is not particularly limited, but in this example, the emitter 2 and the collector 3 are arranged so that the gap d is about 20 to 30 μm. . The space between the emitter 2 and the collector 3 is in a state where the pressure is reduced to 1 × 10 ⁻⁵ Pa or less.

  Next, the operation of the thermoelectric generator 1 will be described. As shown in FIG. 1, the thermoelectric generator 1 connects the substrate 23 of the emitter 2 and the substrate 33 of the collector 3 via an external load 4, and heats the emitter 2 in this state to connect the substrate 2 of the emitter 2 to the external load 4. It is comprised so that an electric current can be sent. Hereinafter, a mechanism for generating an electromotive force between the emitter 2 and the collector 3 by heating will be described in detail.

  FIG. 2 shows an example of the energy band of the emitter 2 whose surface 22 is hydrogen-terminated. The position in the vertical direction in FIG. 2 corresponds to the energy level, and the higher level indicates the higher energy level. Further, the energy band of the inside 21 of the emitter 2 is shown on the left side with respect to the vertical line 200 corresponding to the surface 22, and the position of the vacuum level 201 is shown on the right side with respect to the vertical line 200.

  As shown in FIG. 2, the energy band of the emitter 2 has a negative electron affinity (NEA, see arrow 207) in which the vacuum level 201 is at a lower energy level than the lower end 202 of the conduction band. Yes. Therefore, when the emitter 2 is heated, the electrons 205 thermally excited to the conduction band from the valence band 203, the donor level 204, and the like (see the arrow 208) do not require additional energy, so that the vacuum level 201 is obtained. (See arrow 209). This corresponds to thermionic electrons being emitted from the surface 22 of the emitter 2 to the external space.

  Electrons 205 emitted from the surface 22 of the emitter 2 to the external space pass through the gap d between the emitter 2 and the collector 3 and are collected by the collector 3. Then, as shown in FIG. 1, the electrons 205 collected by the collector 3 flow out from the substrate 33 of the collector 3 to the external circuit (see arrow 101), pass through the external load 4 and return to the emitter 2 (arrow 102). reference).

Next, the function and effect of this example will be described. The diamond semiconductor constituting the emitter 2 of the thermoelectric power generation element 1 has a surface 22 terminated with hydrogen and a hydrogen concentration in the interior 21 of 1 × 10 ²⁰ cm ⁻³ or more. Therefore, as described above, the thermoelectric power generation element 1 has high power generation efficiency and excellent durability.

  The emitter 2 is composed of an n-type diamond semiconductor containing nitrogen as a donor. The Fermi level 206 (see FIG. 2) of the n-type diamond semiconductor is at an energy level closer to the conduction band than the Fermi level of the intrinsic diamond semiconductor. Therefore, thermal excitation to the conduction band is more likely to occur compared to intrinsic diamond semiconductors and p-type diamond semiconductors. As a result, the thermionic power generation element 1 tends to increase the thermionic current generated from the emitter 2 and tends to be more excellent in power generation efficiency.

  The collector 3 is composed of an n-type diamond semiconductor containing nitrogen as a donor. Thereby, compared with the case where a p-type diamond semiconductor or an intrinsic diamond semiconductor is used, it is possible to generate power with the emitter 2 at a lower temperature.

  Further, the collector 3 of this example has a donor concentration of 1/10 or less as compared with the emitter 2. Therefore, the number of electrons thermally excited in the collector 3 is sufficiently smaller than that of the emitter 2. Thereby, the number of thermoelectrons emitted from the collector 3 toward the emitter 2 is sufficiently smaller than the number of thermoelectrons emitted from the emitter 2 toward the collector 3. That is, the thermionic power generation element 1 can suppress so-called back emission, in which thermoelectrons move from the collector 3 to the emitter 2, and can further increase power generation efficiency.

  Thus, the thermoelectric power generation element 1 has high power generation efficiency and excellent durability.

  In this example, molybdenum is used for both the substrate 23 of the emitter 2 and the substrate 33 of the collector 3. However, a substrate made of a material other than this may be used. Examples of materials that can be used for the substrate include silicon semiconductors and diamond semiconductors. When a semiconductor is used as a substrate, an intrinsic semiconductor that does not contain impurities may be used, or a p-type semiconductor or an n-type semiconductor may be used. The substrate 2 of the emitter 2 and the substrate 33 of the collector 3 may be made of the same material or different materials.

  In this example, the collector 3 is made of an n-type diamond semiconductor. However, other materials can be used as long as they can collect the thermoelectrons emitted from the emitter 2. is there. As the material of the collector 3, for example, various metals, intrinsic semiconductors, p-type semiconductors, or n-type semiconductors can be used, and can be appropriately selected according to the position of the Fermi level 206 of the emitter 2 and the dopant concentration. it can.

(Experimental example)
In this example, the durability of each sample manufactured by changing the hydrogen concentration of the emitter 2 in the example was evaluated. Samples of this example (samples No. 1 to No. 5) were produced according to the production method of the emitter 2 in the examples. As an example of a sample preparation method, sample No. 1 and no. 5 shows a production method. Sample No. 2-No. About 4, it produced according to the following method.

・ Sample No. 1
The emitter 2 made of an n-type diamond semiconductor was formed by a microwave plasma CVD method using molybdenum as the substrate 23 and a mixed gas of CH ₄ , H ₂ and N ₂ as a raw material. The flow rate ratio of the mixed gas at this time was CH ₄ / H ₂ = 0.01 and N ₂ / CH ₄ = 1 to 10. The substrate temperature during film formation was 800 to 1000 ° C., and the pressure was 20 to 50 Torr.

After depositing the emitter 2 on the molybdenum, the surface 22 was hydrogenated by performing H ₂ plasma treatment without taking it out of the deposition apparatus. At this time, the substrate temperature was 600 ° C., and the pressure was 20 to 50 Torr. After the completion of the H ₂ plasma treatment, the surface was cooled to 100 ° C. or lower by placing the sample in an H ₂ gas atmosphere at a pressure of 50 Torr or higher to terminate the surface 22 with hydrogen, and then the sample was taken out from the apparatus. As a result, the sample No. The hydrogen concentration inside the emitter 2 in 1 was 1 × 10 ²¹ cm ⁻³ .

・ Sample No. 5
Except that the flow rate ratio of the raw materials used in the microwave plasma CVD method was changed to CH ₄ / H ₂ = 0.005, the sample No. The sample No. 1 was prepared under the same production conditions and production procedure as in Example 1. 5 was obtained. As a result, the sample No. The hydrogen concentration inside the emitter 2 at 5 was 5 × 10 ¹⁹ cm ⁻³ .

The sample obtained as described above was heated at 700 ° C., and the magnitude of the thermionic current at the time when 10 minutes had elapsed from the start of heating was measured. The results are shown in FIG. 3 and Table 1. The sample was heated and the thermoelectron current was measured in a reduced pressure environment at a pressure of 1 × 10 ⁻⁵ Torr. In addition, the vertical axis of FIG. 3 shows the magnitude of the thermoelectron current when 10 minutes have elapsed from the start of heating, and the horizontal axis shows the hydrogen concentration inside the emitter 2 in each sample.

As known from FIG. 3 and Table 1, the sample No. 1 having a hydrogen concentration of 1 × 10 ²⁰ cm ⁻³ or more is used. 1-No. In No. 4, a sufficiently large thermoelectron current flowed even after heating at 700 ° C. for 10 minutes.

  On the other hand, sample No. In No. 5, thermionic current hardly flowed after heating at 700 ° C. for 10 minutes. This is probably because heating at 700 ° C. for 10 minutes was an extremely severe test condition. That is, when the test conditions are lower, the sample prepared without containing hydrogen and the sample No. When comparing the sample No. 5 with the sample no. No. 5 is considered to show higher durability.

DESCRIPTION OF SYMBOLS 1 Thermionic power generation element 2 Emitter 21 Inside 22 Surface 3 Collector d Gap

Claims (5)

Emitters (2, 21, 22) for generating thermal electrons;
A collector (3, 31) arranged to face the emitter (2, 21, 22) via a gap and collect the thermal electrons,
The emitter (2, 21, 22) is made of a diamond semiconductor containing hydrogen inside (21), and the surface (22) of the diamond semiconductor is hydrogen-terminated ,
The thermoelectric power generating element (1) , wherein the diamond semiconductor constituting the emitter (2, 21, 22) has a hydrogen concentration of 1 × 10 ²⁰ cm ⁻³ or more . The thermionic power generation element (1) according to claim 1 , wherein the diamond semiconductor constituting the emitter (2, 21, 22) is an n-type diamond semiconductor. 3. The thermoelectrons according to claim 2 , wherein the diamond semiconductor constituting the emitter (2, 21, 22) is an n-type diamond semiconductor containing any one of nitrogen, phosphorus, sulfur and lithium as a donor. Power generation element (1). Diamond semiconductor constituting the emitter (2,21,22) is claim, characterized in that using a hydrocarbon gas and hydrogen gas as a source gas, and is formed by a microwave plasma CVD method 1-3 The thermoelectric power generation element (1) according to any one of the above. The thermoelectric generator (1) according to any one of claims 1 to 4 , wherein the diamond semiconductor constituting the collector (3, 31) is an n-type diamond semiconductor.

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