Thermal diode

Abstract

To provide a thermal diode with high heat rectification performance and high thermal conductivity.SOLUTION: A thermal diode 1 includes a first member 11 and a second member 12 joined to the first member 11. The first member 11 is formed of a diamond polycrystalline body of which thermal conductivity increases along with a temperature rise in a predetermined temperature region within a range of -200°C or higher and 800°C or lower. Thermal conductivity of the second member 12 decreases along with a temperature rise in the predetermined temperature region. Thus, the thermal diode 1 with high heat rectification performance and high thermal conductivity can be provided.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a thermal diode.

Background Art

[0002] In recent years, from the viewpoints of CO2 reduction and energy conservation measures towards carbon neutrality, effective utilization of thermal energy has been demanded, and technologies for controlling the flow of heat have attracted attention. For example, in Patent Document 1, a first material that is a ceramic material whose thermal conductivity increases in a certain temperature range of -200°C or higher and 1000°C or lower, and a second material whose thermal conductivity decreases in the temperature range, are joined to form a proposed thermal diode.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, in the thermal diode of Patent Document 1, since the first material is a ceramic material, there are limitations in improving the thermal conductivity of the first material and increasing the amount of change in thermal conductivity due to the temperature difference of the first material. For this reason, there are limitations in improving the thermal conductivity and thermal rectification of the thermal diode.

[0005] The present invention has been made in view of the above problems, and an object thereof is to provide a thermal diode having high thermal rectification and thermal conductivity.

Means for Solving the Problems

[0006] Aspect 1 of the present invention is a thermal diode, comprising: a first member formed of a diamond polycrystal in which the thermal conductivity increases as the temperature rises in a predetermined temperature range within the range of -200°C or higher and 800°C or lower; and a second member joined to the first member and having a thermal conductivity that decreases as the temperature rises in the predetermined temperature range.

[0007] Aspect 2 of the present invention is the thermal diode of Aspect 1, wherein the second member is formed of single-crystal diamond.

[0008] Aspect 3 of the present invention is the thermal diode of Aspect 1 or 2, wherein the average particle size of diamond particles in the first member is 5 nm or more and 10 μm or less.

[0009] Aspect 4 of the present invention is the thermal diode of Aspect 1 or 2 (which may be any one of Aspects 1 to 3), wherein the impurity content in the first member is 0.1 mass ppm or more and 5 mass% or less.

[0010] Aspect 5 of the present invention is the thermal diode of Aspect 1 or 2 (which may be any one of Aspects 1 to 4), wherein in the first member, diamond particles are directly bonded to each other without an intervening binder.

[0011] Aspect 6 of the present invention is the thermal diode of Aspect 1 or 2 (which may be any one of Aspects 1 to 5), wherein the first member and the second member are arranged side by side in a predetermined direction with a joining surface therebetween. The thickness of the second member in the predetermined direction is 5 times or more and 1000 times or less the thickness of the first member.

[0012] Aspect 7 of the present invention is the thermal diode of Aspect 1 or 2 (which may be any one of Aspects 1 to 6). The first member and the second member are arranged side by side in a predetermined direction with a joining surface therebetween. When the first end surface, which is the surface of the first member on the side opposite to the joining surface in the predetermined direction, is set to 227°C and the second end surface, which is the surface of the second member on the side opposite to the joining surface in the predetermined direction, is set to 27°C, the heat flow rate at the joining surface is 1.2 times or more and 10 times or less the heat flow rate at the joining surface when the first end surface and the second end surface are set to 27°C and 227°C, respectively.

Effect of the Invention

[0013] In the present invention, a thermal diode with high thermal rectification and high thermal conductivity can be provided.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0015] FIGS. 1 and 2 are diagrams showing the configuration of a thermal diode 1 according to an embodiment of the present invention. The thermal diode 1 includes a first member 11 and a second member 12 coupled to each other. In FIG. 1, a state where the first member 11 side is at a higher temperature than the second member 12 side is shown. In FIG. 2, a state where the first member 11 side is at a lower temperature than the second member 12 side is shown.

[0016] The thermal diode 1 is a member having a thermal diode function in a predetermined temperature range (hereinafter, also referred to as the "operating temperature range") within -200°C or higher and 800°C or lower. The thermal diode function (also referred to as "thermal rectification") is a function that easily transmits heat in a predetermined direction and hardly transmits heat in the direction opposite to the predetermined direction. The operating temperature range of the thermal diode 1 is, for example, room temperature or higher.

[0017] The first member 11 is a member formed of polycrystalline diamond. The polycrystalline diamond constituting the first member 11 has a thermal conductivity that increases with an increase in temperature in the above-described operating temperature range. In this specification, the polycrystalline diamond is a concept including nano-polycrystalline diamond (NPD) and polycrystalline diamond (PCD). Nano-polycrystalline diamond is a dense polycrystal in which diamond particles with a particle size of several tens of nm are firmly directly bonded to each other without an intervening binder or the like. PCD is a polycrystal obtained by sintering diamond powder using a binder such as cobalt (Co). PCD contains inclusions such as a binder at grain boundaries. On the other hand, NPD substantially does not contain a binder, a sintering aid, or the like.

[0018] In the first member 11, the average particle size of the diamond particles constituting the polycrystalline diamond is, for example, 5 nm or more, preferably 20 nm or more. The average particle size is, for example, 10 μm or less, preferably 1 μm or less, and more preferably 100 nm or less. The average particle size is obtained by observing the polished surface of a test piece cut out from the first member 11 with a scanning electron microscope (SEM) at a magnification of 3000 to 30000 times and using the intercept method.

[0019] The impurity content in the first member 11 is, for example, 0.1 mass ppm or more, preferably 0.5 mass% or more. The impurity content is, for example, 5 mass% or less, preferably 2 mass% or less. The impurities are substances other than diamond. The impurities include, for example, graphite generated together with diamond particles when manufacturing the polycrystalline diamond, nitrogen (N), etc. contained in the atmosphere during the manufacture of the polycrystalline diamond. When the first member 11 is PCD, the impurities also include a binder, a sintering aid, or the like.

[0020] The second member 12 is a member whose thermal conductivity decreases as the temperature rises in the above-described operating temperature range. The second member 12 is, for example, a member formed of single crystal diamond. That is, the second member 12 is a member composed of a single crystal of diamond. Note that the second member 12 may be formed of a material other than single crystal diamond as long as it is a material whose thermal conductivity decreases as the temperature rises in the above-described operating temperature range. For example, the second member 12 may be formed of ceramics such as SiC or AlN, or may be formed of a metal or the like.

[0021] The second member 12 is joined to the first member 11, for example, by solid-phase diffusion bonding using spark plasma sintering (SPS), hot pressing, etc., direct bonding in which the surfaces are activated for bonding, or brazing using a metal adhesive or the like.

[0022] In FIGS. 1 and 2, the first member 11 and the second member 12 are shown as rectangles that are long in the left-right direction in the figures. The actual shapes of the first member 11 and the second member 12 are, for example, substantially rectangular parallelepiped shapes that are long in the left-right direction in the figures. In the examples shown in FIGS. 1 and 2, the left end of the second member 12 is joined to the right end of the first member 11. The first member 11 and the second member 12 are in surface contact with each other at the joint surface 13. The first member 11 and the second member 12 are arranged in a predetermined direction (i.e., the left-right direction in the figures) with the joint surface 13 therebetween. In the following description, the direction in which the first member 11 and the second member 12 are arranged is also referred to as the "arrangement direction". Note that the shapes of the first member 11 and the second member 12 are not limited to rectangular parallelepipeds and may be variously changed.

[0023] In the thermal diode 1, the thickness of the second member 12 in the above-described arrangement direction is preferably 5 times or more, and more preferably 50 times or more, the thickness of the first member 11. Also, the ratio is preferably 1000 times or less, and more preferably 500 times or less. Here, when the ratio obtained by dividing the thickness of the second member 12 in the arrangement direction by the thickness of the first member 11 is referred to as the "thickness ratio", the thickness ratio is preferably 5 or more and 1000 or less.

[0024] The thermal conductivity of polycrystalline diamond and single crystal diamond at room temperature (27 °C) is very high compared to ceramics such as silicon carbide (SiC) and aluminum nitride (AlN) which have relatively high thermal conductivity. Therefore, the thermal conductivity of the thermal diode 1 can be increased compared to a thermal diode in which the first member 11 and / or the second member 12 is a ceramic.

[0025] As described above, in the thermal diode 1, the thermal conductivity of the first member 11 increases as the temperature rises in the above-described operating temperature range. On the other hand, the thermal conductivity of the second member 12 decreases as the temperature rises in the above-described operating temperature range. Therefore, when the first member 11 side is at a high temperature and the second member 12 side is at a low temperature, heat easily moves from the high temperature side to the low temperature side (that is, from the first member 11 side to the second member 12 side). On the other hand, when the first member 11 side is at a low temperature and the second member 12 side is at a high temperature, the heat transfer from the high temperature side to the low temperature side (that is, from the second member 12 side to the first member 11 side) is suppressed compared to the case where the first member 11 is at a high temperature and the second member 12 side is at a low temperature. In FIGS. 1 and 2, the magnitude relationship of the heat conduction is indicated by the size of the white arrows. As shown in FIGS. 1 and 2, in the thermal diode 1, the heat conduction when the first member 11 is at a higher temperature than the second member 12 is greater than the heat conduction when the second member 12 is at a higher temperature than the first member 11.

[0026] Note that the case where the first member 11 side is at a high temperature and the second member 12 side is at a low temperature is, for example, when the end face on the side opposite to the joint surface 13 of the first member 11 in the above-described arrangement direction (hereinafter, also referred to as the "first end face 14") is in contact with a heat source, and the end face on the side opposite to the joint surface 13 of the second member 12 in the above-described arrangement direction (hereinafter, also referred to as the "second end face 15") is located in an atmosphere at a lower temperature than the heat source. Also, the case where the first member 11 side is at a low temperature and the second member 12 side is at a high temperature is, for example, when the second end face 15 of the second member 12 is in contact with a heat source, and the first end face 14 of the first member 11 is located in an atmosphere at a lower temperature than the heat source.

[0027] In the following description, the heat flux flowing from the first member 11 to the second member 12 at the joint surface 13 is also referred to as the "first heat flux J1". Further, the heat flux flowing from the second member 12 to the first member 11 at the joint surface 13 is also referred to as the "second heat flux J2". In the heat diode 1, when the first end face 14 is at 227°C and the second end face 15 is at 27°C, the heat flux at the joint surface 13 (i.e., the first heat flux J1) is preferably 1.2 times or more and 10 times or less than the heat flux at the joint surface 13 (i.e., the second heat flux J2) when the first end face 14 is at 27°C and the second end face 15 is at 227°C.

[0028] Here, if the value obtained by dividing the first heat flux J1 by the second heat flux J2 is called the "heat flux ratio J1 / J2", the heat flux ratio J1 / J2 when the first end face 14 and the second end face 15 are at 227°C and 27°C respectively, and when the first end face 14 and the second end face 15 are at 27°C and 227°C respectively, is preferably 1.2 or more and 10 or less.

[0029] Note that the above-mentioned heat flux can be measured, for example, by the method described in "Keisuke Hirata, et al., 'Abnormal Thermal Conductivity of Ag2Ch (Ch = S, Se, Te) and Development of a Solid-State Thermal Rectifier Using It', Journal of the Japan Thermoelectric Society, Vol. 16, No. 1 (2019), pp. 3-7". Specifically, first, a heater is brought into contact with the first end face 14 and the second end face 15 of the heat diode 1 through intermediate members formed of titanium (Ti) or the like, and the heaters on both sides are pressed against the heat diode 1 at 1 MPa toward the heat diode 1 to form an assembly in which each member is preferably in thermal contact. The heaters on both sides of the heat diode 1 can reverse the direction of heat flow under the same thermal contact conditions. Thermocouples are provided on the intermediate members on both sides of the heat diode 1 respectively. Next, the above assembly is placed in a vacuum and the surroundings are covered with an aluminum sheet to constitute a heat flux measuring device. Then, in the heat flux measuring device, with the heater on the first member 11 side of the heaters on both sides of the heat diode 1 at a high temperature, the first heat flux J1 is measured by the steady-state method. Also, with the heater on the second member 12 side of the heaters on both sides of the heat diode 1 at a high temperature, the second heat flux J2 is measured by the steady-state method.

[0030] In the thermal diode 1, in order to increase the heat flux ratio, it is preferable to greatly change the thermal conductivity of the first member 11 with temperature. In other words, by increasing the change rate of the thermal conductivity of the first member 11 with respect to the temperature change, the heat flux ratio of the thermal diode 1 increases. From the viewpoint of greatly changing the thermal conductivity of the first member 11 with temperature, the representative length L of the microstructure of the first member 11 a is set to be "0.1L AMFP ≦L a ≦100L AMFP ".

[0031] The representative length L of the microstructure of the first member 11 a is a typical length for expressing the microstructure of the first member 11. The representative length L a is the average particle size of the diamond particles constituting the first member 11 (that is, the average interval between grain boundaries). L AMFP is the apparent mean free path of phonons at room temperature, and is defined as in Equation (1) below.

[0032]

Equation

[0033] The thermal conductivity, heat capacity, and sound velocity in Equation (1) are the values at room temperature (27°C) of a single crystal of diamond. The sound velocity is the speed of sound propagating within a single crystal of diamond.

[0034] Equation (1) is based on Equation (2) below, which represents the thermal conductivity (κ) in the phonon gas model ("Phonon Transport Analysis of Alloyed Silicon Crystals by Molecular Dynamics", Hori et al., Proceedings of the 49th Japanese Heat Transfer Symposium, B-341 (2012-5)).

[0035]

Equation

[0036] C in Equation 2 is the phonon specific heat, V is the phonon group velocity, Λ is the phonon mean free path, k is the wave number, and s is the branch.

[0037] The representative length L of the microstructure a is the apparent mean free path L of phonons AMFP When it is shorter than a L AMFP than the case where it is L or more, the phonons scattered by the microstructure increase, so the heat conduction by phonons is suppressed and the absolute value of the thermal conductivity becomes smaller. Also, when the representative length L of the microstructure a is L AMFP shorter than a L AMFP than the case where it is L or more, the heat conduction by phonons changes greatly with temperature, and the difference between the thermal conductivity at low temperature and the thermal conductivity at high temperature becomes large. L AMFP becomes shorter as the temperature of the first member 11 increases. For this reason, as the temperature of the first member 11 increases, the phonons scattered by the microstructure of the first member 11 decrease, the absolute value of the thermal conductivity increases, and the rate of change of the thermal conductivity with respect to temperature change becomes small.

[0038] The representative length L of the microstructure a is less than 0.1L AMFP When the temperature of the first member 11 increases and L AMFP becomes shorter, even if L a is L AMFP or more, the proportion is small, so the proportion of phonons scattered by the microstructure does not change much. Therefore, even if the temperature of the first member 11 is changed, the thermal conductivity of the first member 11 does not change much. Therefore, by setting "0.1L AMFP ≤L a <L AMFP ", the absolute value of the thermal conductivity becomes somewhat smaller, but the rate of change of the thermal conductivity with respect to temperature change can be increased. On the other hand, by setting "L AMFP ≤L a ≤100L AMFP ", the absolute value of the thermal conductivity can be increased.

[0039] In the first member 11, as described above, the average particle size of diamond particles constituting the polycrystalline diamond is L a Therefore, by changing the average particle size, the absolute value of the thermal conductivity of the first member 11 and the rate of change of the thermal conductivity with respect to temperature change can be made as desired. As described above, in the present embodiment, the average particle size of the diamond particles of the first member 11 is preferably 5 nm or more and 10 μm or less.

[0040] The L of diamond AMFP is longer than the L of ceramics such as SiC and AlN AMFP Therefore, even if the average particle size of diamond particles (i.e., the representative length L a ) constituting the polycrystalline diamond in the first member 11 increases, a state where L AMFP is longer than L a can be maintained to some extent. As a result, in a wider range of average particle sizes compared to ceramics, the rate of change of the thermal conductivity of the first member 11 with respect to temperature change can be increased, and thermal rectification can be suitably performed. Further, even if the first member 11 is heated and L AMFP becomes smaller, a state where L AMFP is longer than L a can be maintained to some extent. As a result, in a wider temperature range compared to ceramics, the rate of change of the thermal conductivity of the first member 11 with respect to temperature change can be increased, and thermal rectification can be suitably performed.

[0041] As described above, the thermal diode 1 includes the first member 11 and the second member 12 joined to the first member 11. The first member 11 is formed of polycrystalline diamond in which the thermal conductivity increases as the temperature rises in a predetermined temperature range within the range of -200°C or more and 800°C or less. The second member 12 has a thermal conductivity that decreases as the temperature rises in the predetermined temperature range. Thereby, as described above, the thermal diode 1 having high thermal rectification and high thermal conductivity can be provided.

[0042] As described above, the second member 12 is preferably formed of single crystal diamond. Thereby, since the thermal conductivity of the second member 12 can be further increased, the thermal conductivity of the thermal diode 1 can be improved.

[0043] As described above, the average particle size of the diamond particles in the first member 11 is preferably 5 nm or more and 10 μm or less. In this way, by making the range of the average particle size of the diamond particles relatively wide, high thermal rectification and high thermal conductivity of the thermal diode 1 can be realized in a wide temperature range corresponding to the wide range of the average particle size.

[0044] As described above, the impurity content in the first member 11 is preferably 0.1 mass ppm or more and 5 mass% or less. Thereby, in the first member 11, the content rate of diamond having a very high thermal conductivity can be increased, so that the thermal conductivity of the first member 11 can be further increased. As a result, the thermal conductivity of the thermal diode 1 can be further increased.

[0045] As described above, in the first member 11, it is preferable that the diamond particles are directly bonded to each other without using a binder. In other words, the first member 11 is preferably formed by NPD. Thereby, in the first member 11, the content rate of diamond having a very high thermal conductivity can be increased, so that the thermal conductivity of the first member 11 can be further increased. As a result, the thermal conductivity of the thermal diode 1 can be further increased.

[0046] As described above, the first member 11 and the second member 12 are arranged side by side in a predetermined direction (i.e., the arrangement direction) with the joint surface 13 therebetween, and the thickness of the second member 12 in the predetermined direction is preferably 5 times or more and 1000 times or less the thickness of the first member 11. Thereby, the thermal rectification and the thermal conductivity of the thermal diode 1 can be improved.

[0047] As described above, the first member 11 and the second member 12 are arranged in a predetermined direction (i.e., the arrangement direction) with the joint surface 13 therebetween. When the first end surface 14, which is the surface of the first member 11 on the side opposite to the joint surface 13 in the predetermined direction, is set to 227°C and the second end surface 15, which is the surface of the second member 12 on the side opposite to the joint surface 13 in the predetermined direction, is set to 27°C, the heat flow rate at the joint surface 13 is preferably 1.2 times or more and 10 times or less the heat flow rate at the joint surface 13 when the first end surface 14 and the second end surface 15 are set to 27°C and 227°C, respectively. Thereby, the heat diode 1 having high heat rectification can be provided.

[0048] In the above-described heat diode 1, various modifications are possible.

[0049] For example, the thickness of the second member 12 in the above arrangement direction may be less than 5 times the thickness of the first member 11 or may be greater than 1000 times.

[0050] The heat flux at the joint surface 13 (i.e., the first heat flux J1) when the first end surface 14 and the second end surface 15 are set to 227°C and 27°C, respectively, may be less than 1.2 times as long as it is greater than the heat flux at the joint surface 13 (i.e., the second heat flux J2) when the first end surface 14 and the second end surface 15 are set to 27°C and 227°C, respectively, or may be greater than 10 times.

[0051] The impurity content in the first member 11 may be less than 0.1 mass ppm or may be higher than 5 mass%.

[0052] The average particle size of the diamond particles in the first member 11 may be less than 5 nm or may be greater than 10 μm.

[0053] The configurations in the above-described embodiments and each modification example may be appropriately combined as long as they do not conflict with each other.

Industrial Applicability

[0054] Since the thermal diode of the present invention easily conducts heat in one direction, it can be used in various applications as a member for effectively utilizing heat.

Explanation of Signs

[0055] 1 Thermal diode 11 First member 12 Second member 13 Joint surface 14 First end face 15 Second end face

Claims

1. A thermal diode comprising: a first member formed of a diamond polycrystal in which the thermal conductivity increases as the temperature rises within a predetermined temperature range of -200°C or higher and 800°C or lower; a second member joined to the first member and having a thermal conductivity that decreases as the temperature rises within the predetermined temperature range; the thermal diode.

2. The thermal diode according to Claim 1, wherein the second member is formed of single crystal diamond.

3. The thermal diode according to Claim 1 or 2, wherein the average particle size of diamond particles in the first member is 5 nm or more and 10 µm or less.

4. The thermal diode according to Claim 1 or 2, wherein the impurity content in the first member is 0.1 mass ppm or more and 5 mass% or less.

5. The thermal diode according to Claim 1 or 2, wherein diamond particles in the first member are directly bonded to each other without a binder.

6. The thermal diode according to Claim 1 or 2, wherein the first member and the second member are arranged side by side in a predetermined direction with a joining surface therebetween, and the thickness of the second member in the predetermined direction is 5 times or more and 1000 times or less the thickness of the first member.

7. The thermal diode according to Claim 1 or 2, wherein the first member and the second member are arranged side by side in a predetermined direction with a joining surface therebetween, and when the first end face, which is the face of the first member on the side opposite to the joining surface in the predetermined direction, is set to 227°C and the second end face, which is the face of the second member on the side opposite to the joining surface in the predetermined direction, is set to 27°C, the heat flow rate at the joining surface is 1.2 times or more and 10 times or less the heat flow rate at the joining surface when the first end face and the second end face are set to 27°C and 227°C, respectively.

Images