JP2010192780A - Thermoelectric conversion element - Google Patents

Abstract

To improve the thermoelectric conversion efficiency of a thermoelectric conversion element.
A composite structure 53 having a structure in which graphene 53a and a carbon nanotube 53b are joined, having high electron mobility and low thermal conductivity is used as a thermoelectric conversion material, and a pair of ends is provided at both ends thereof. The thermoelectric conversion element 50 is configured by electrically connecting the electrodes 54 and 55.
[Selection] Figure 9

Description

  The present invention relates to a thermoelectric conversion element.

  In electronic parts and electronic devices, the heat generation often becomes a problem. If thermoelectric conversion elements that convert thermal energy into electrical energy by the Seebeck effect are used, heat generation of electronic components and electronic devices can be suppressed and they can be operated more efficiently, which can also contribute to energy saving. it can. As the thermoelectric conversion element, one using a metal, a semiconductor or the like as a thermoelectric conversion material is known (for example, see Patent Document 1).

JP 2005-129627 A

  In the thermoelectric conversion element, in order to obtain high thermoelectric conversion efficiency, it is desired to use a material having high carrier mobility and low thermal conductivity. However, until now, there has been a problem that it is difficult to realize a material having such two properties of high carrier mobility and low thermal conductivity.

  According to one aspect of the present invention, it includes at least one first structure in which a plurality of carbon atoms are arranged in a sheet shape, and at least one second structure in which a plurality of carbon atoms are arranged in a cylinder shape. There is provided a thermoelectric conversion element having a carbon material and a pair of electrodes electrically connected to the carbon material.

  According to the disclosed thermoelectric conversion element, it is possible to improve the thermoelectric conversion efficiency.

It is a figure which shows the example of a graphene. It is a figure which shows the example of a composite structure. It is a figure which shows the dispersion | distribution relationship of the electron of a composite structure, Comprising: (A) is the case of a 5-3 period composite structure, (B) is the case of a 7-4 period composite structure, (C) is 9 In the case of a composite structure having a −5 period, (D) is a case of a composite structure having an 11-6 period. It is a figure which shows the dispersion | distribution relationship of the phonon of a composite structure, Comprising: (A) is the case of the composite structure of 5-3 period, (B) is the case of the composite structure of 7-4 period, (C) is 9 In the case of a composite structure with a −5 period, (D) is the case of a composite structure with a 19-10 period. It is a graph for explaining the characteristics of graphene, in which (A) is a graph showing the electron dispersion relationship of graphene, and (B) is a graph showing the phonon dispersion relationship of graphene. It is a figure which shows the relationship between the figure of merit ZT and thermoelectric conversion efficiency (eta). It is a figure which shows the relationship between carrier concentration, thermal conductivity (kappa), and figure of merit ZT. It is a figure which shows the relationship between the period of a composite structure, and the frequency and temperature which can suppress the excitation of phonon. It is a schematic block diagram of the thermoelectric conversion element which concerns on 1st Example. It is a cross-sectional schematic diagram of the foundation | substrate layer and catalyst layer formation process which concerns on 1st Example. It is a cross-sectional schematic diagram of the composite structure formation process which concerns on 1st Example. It is a cross-sectional schematic diagram of the electrode formation process according to the first embodiment. It is a schematic block diagram of the thermoelectric conversion element which concerns on 2nd Example. It is a cross-sectional schematic diagram of the foundation | substrate layer formation process which concerns on 2nd Example. It is a cross-sectional schematic diagram of the catalyst layer formation process which concerns on 2nd Example. It is a cross-sectional schematic diagram of the composite structure which concerns on 2nd Example, and a graphene electrode formation process. It is a schematic block diagram of the thermoelectric conversion element which concerns on 3rd Example. It is explanatory drawing of the catalyst layer and base layer formation process which concerns on 3rd Example, Comprising: (A) is a cross-sectional schematic diagram, (B) is a plane schematic diagram. It is a cross-sectional schematic diagram of the composite structure formation process which concerns on 3rd Example. It is a cross-sectional schematic diagram of the electrode formation process which concerns on 3rd Example. It is a schematic block diagram of the thermoelectric conversion element which concerns on 4th Example. It is a cross-sectional schematic diagram of the 2nd foundation | substrate layer and catalyst layer formation process which concerns on 4th Example. It is a cross-sectional schematic diagram of the 2nd composite structure formation process which concerns on 4th Example. It is a cross-sectional schematic diagram of the electrode formation process which concerns on 4th Example. It is a figure which shows the example of application of a thermoelectric conversion element.

First, the carbon material will be described.
Examples of the carbon material include a sheet-like structure in which a plurality of carbon atoms are arranged and bonded in a sheet shape, a cylindrical structure in which the carbon atoms are arranged and bonded, and a spherical shape in which the carbon atoms are arranged and bonded in a spherical shape. There are structures. Examples of the sheet-like structure include graphene in which carbon 6-membered rings are bonded in a network, graphite in which a plurality of graphenes are stacked, and the like. Examples of the cylindrical structure include carbon nanotubes in which graphene is cylindrical. Examples of the spherical structure include fullerenes containing a carbon 6-membered ring and a 5-membered ring.

  The cylindrical structure includes not only single-walled or multi-walled carbon nanotubes but also a structure in which carbon nanotubes include different carbon structures such as fullerene (peapod structure). Moreover, even if a carbocyclic ring other than a 6-membered ring is partly included in the sheet-like structure, it should be included as long as it is in the form of a sheet as a whole.

  Here, as the carbon material, a composite structure in which a sheet-like structure and a cylindrical structure are combined will be described. Graphene is taken as an example of the sheet-like structure, and carbon nanotubes are taken as an example of the cylindrical structure.

FIG. 1 is a diagram illustrating an example of graphene. FIG. 2 is a diagram showing an example of a composite structure.
As shown in FIG. 1, the graphene 10 has a structure in which a plurality of carbon atoms 11 are arranged and bonded in a sheet shape. FIG. 1 exemplifies graphene 10 in which seven carbon atoms 11 having the same bonding environment surrounded by a circle are arranged in a sheet shape so that seven carbon atoms 11 exist in the x direction and four in the y direction. ing. Here, a structure in which a plurality of carbon atoms 11 are arranged in this way is referred to as a 7-4 period. Such a 7-4 period graphene 10 has a square planar shape.

  A single-layer carbon nanotube is obtained by forming a single-layer graphene having a structure as shown in FIG. 1 into a cylindrical shape with a diameter of nanometer size. 2A to 2C, the composite structures 30a, 30b, and 30c each have a structure in which the graphenes 10a, 10b, and 10c are joined to the ends of the carbon nanotubes 20a, 20b, and 20c, respectively. Have.

  The composite structure 30a shown in FIG. 2A has a predetermined size on a graphene 10a having a planar size in which five carbon atoms having the same bonding environment are arranged in the x direction and three in the y direction. Carbon nanotubes 20a having a diameter are joined (5-3 period). The composite structure 30b shown in FIG. 2B has a predetermined size on a graphene 10b having a plane size in which seven carbon atoms having the same bonding environment are arranged in the x direction and four in the y direction. The carbon nanotubes 20b having a diameter are joined (7-4 cycle). The composite structure 30c shown in FIG. 2C has a predetermined size on a graphene 10c having a plane size in which nine carbon atoms having the same bonding environment are arranged in the x direction and five in the y direction. Carbon nanotubes 20c having a diameter are joined (9-5 cycles).

  In the composite structures 30a, 30b, and 30c shown in FIGS. 2A to 2C, the arrangement of the carbon atoms of the graphenes 10a, 10b, and 10c and the carbon nanotubes 20a, 20b, and 20c uses the first principle calculation. Have been optimized. Here, each junction part of graphene 10a, 10b, 10c and carbon nanotube 20a, 20b, 20c is made into the carbon-carbon covalent bond. In addition, it is assumed that the graphenes 10a, 10b, and 10c have the above-described period (planar size), and the carbon nanotubes 20a, 20b, and 20c have the same diameter (about 1 nm).

  The graphene 10 simple substance and the carbon nanotube simple substance as shown in FIG. 1 each have unique properties. For example, graphene 10 alone exhibits high electron mobility, while carbon nanotube alone exhibits high mechanical strength as well as high current density resistance and thermal conductivity. Each of the composite structures 30a, 30b, and 30c has a structure in which the graphenes 10a, 10b, and 10c having different characteristics are bonded to the carbon nanotubes 20a, 20b, and 20c.

Here, the characteristics of such a composite structure will be described.
3A and 3B are diagrams showing the electron dispersion relationship of the composite structure, in which FIG. 3A is a composite structure with a 5-3 period, and FIG. 3B is a composite structure with a 7-4 period. ) Is for a composite structure with 9-5 periods, and (D) is for a complex structure with 11-6 periods. 4A and 4B are diagrams showing the phonon dispersion relationship of the composite structure, in which FIG. 4A is a composite structure having a 5-3 period, FIG. 4B is a composite structure having a 7-4 period, and (C ) Is for a composite structure with 9-5 periods, and (D) is for a composite structure with 19-10 periods. For comparison, the graphene characteristics are shown in FIG. 5A and 5B are graphs for explaining graphene characteristics, in which FIG. 5A is a graph showing the electron dispersion relationship of graphene, and FIG. 5B is a graph showing the phonon dispersion relationship of graphene.

  3 and 4 illustrate the electron and phonon dispersion relationship for the composite structure in which graphene is bonded to both ends of the carbon nanotube as described above. The diameters of the carbon nanotubes in the composite structure of each period are about the same. The phonon dispersion relation is calculated using a spring model between nearest atoms based on the optimized micro atomic structure.

First, the electron dispersion relationship between the composite structure and the graphene is described with reference to FIGS.
In the case of a single graphene, as shown in FIG. 5A, there is a band crossing the Fermi level E _F (0 eV). In graphene, thus Fermi level E slope of the band is large across the _F, are enabled fast electron transfer. The electron mobility of graphene is about 10 times higher than that of silicon (Si).

On the other hand, in the case of a composite structure having a 5-3 period, a band crossing the Fermi level E _F (0 eV) exists as shown in FIG. The mobility of is relatively low.

In the case of a composite structure having a period of 7-4, as shown in FIG. 3B, there is a band that crosses the Fermi level E _F (0 eV), the inclination of the band is relatively large, and the mobility of electrons Is relatively high.

In the case of a 9-5 period composite structure, as shown in FIG. 3C, there is no band crossing the Fermi level E _F (0 eV), and there is a band gap. That is, the 9-5 period composite structure itself is a semiconductor. However, a band with a large inclination is seen. For example, if a 9-5 period composite structure is doped with a predetermined impurity to generate a band that crosses the Fermi level, a material having high electron mobility. Is possible.

In the case of an 11-6 period composite structure, as shown in FIG. 3D, there is a band crossing the Fermi level E _F (0 eV), and the inclination of the band is also shown in FIG. It can be said that the mobility of electrons is very high as in the case of graphene.

  Thus, a composite structure in which graphene and carbon nanotubes are joined becomes a metal or a semiconductor depending on the period. In such a composite structure, the mobility of electrons becomes as high as that of graphene at a certain period or more, or can be increased by doping or the like.

Next, the phonon dispersion relationship between the composite structure and the graphene will be described with reference to FIGS. 4A to 4D and FIG.
In the case of a single graphene, as shown in FIG. 5 (B), the frequency rises from 0 THz. Thus, graphene is likely to cause phonon excitation (lattice vibration) from a low frequency, that is, a low energy state, and thus exhibits high thermal conductivity. The thermal conductivity of graphene is about 10 times higher than copper (Cu), which is generally considered to have high thermal conductivity.

  On the other hand, in the case of a composite structure having a 5-3 period, as shown in FIG. 4A, the characteristic that the frequency rises from 0 THz, which is seen in graphene, is not seen. That is, there is a certain gap G (about 40 THz here) up to a frequency (energy) at which the phonon is excited with a certain probability. In addition, the phonon band lies on its side, indicating that the heat transfer rate is slow.

  The characteristic similar to that shown in FIG. 4A is a composite of other periods such as 7-4 period, 9-5 period, and 19-10 period as shown in FIGS. 4B to 4D. As seen in the structure, the gap G tends to decrease as the period increases.

  Thus, unlike a graphene simple substance, a composite structure in which graphene and carbon nanotubes are joined requires a certain amount of energy for excitation of phonons, and the heat transfer rate is also slow. Considering the above-described electron dispersion relationship, such a composite structure can be a material having both high electron mobility and low thermal conductivity depending on the period, the presence or absence of doping, the temperature environment where the composite structure is placed, and the like. That is, such a composite structure is suitable as a thermoelectric conversion material. On the other hand, graphene alone has high electron mobility and high thermal conductivity. For this reason, graphene alone is not suitable as a thermoelectric conversion material.

In general, the thermoelectric conversion efficiency η is expressed by the following equation (1) using a high temperature side temperature T _h (K) and a low temperature side temperature T _c (K).
η = {(T _h −T _c ) / T _h } × {(M−1) / (M + T _c / T _h )} (1)
In this formula (1), (T _h −T _c ) / T _h represents the Carnot efficiency η _c , and (M−1) / (M + T _c / T _h ) represents the material efficiency. M is represented by the following formula (2).

M = (1 + ZT) ^1/2 (2)
In this formula (2), ZT is called a figure of merit, is a dimensionless number, and is expressed by the following formula (3).

^{ZT = α 2 T / ρκ ···} (3)
In this formula (3), α is the Seebeck constant (μVK ⁻¹ ), T is the average temperature of T _h and T _c (= (T _h + T _c ) / 2), ρ is the electrical resistivity (Ωm), and κ is Thermal conductivity (Wm ⁻¹ K ⁻¹ ). Furthermore, as shown in the following equation (4), the thermal conductivity κ is a carrier thermal conductivity κ _e (= LT / ρ (L: Lorentz factor)) indicating heat transport by carriers (mainly electrons) and phonon. It is represented by the sum of lattice thermal conductivity κ _l indicating heat transport by (lattice vibration).

κ = κ _e + κ _l (4)
FIG. 6 is a diagram showing the relationship between the figure of merit ZT and the thermoelectric conversion efficiency η.
FIG. 6 shows the relationship between the figure of merit ZT and the thermoelectric conversion efficiency η when T _c / T _h = 0.5, that is, the Carnot efficiency η _c = 0.5. As shown in FIG. 6, the thermoelectric conversion efficiency η increases as the figure of merit ZT increases and approaches the Carnot efficiency η _c . The thermoelectric conversion material currently in practical use has a figure of merit ZT of about 1 and a thermoelectric conversion efficiency η of about 0.1 (10%). As apparent from FIG. 6, if the figure of merit ZT can be improved, the thermoelectric conversion efficiency η can be improved.

  Therefore, a composite structure in which graphene and carbon nanotubes as described above are joined will be considered. In such a composite structure, as described above, at least a certain level of energy is required for phonon excitation (FIGS. 4A to 4D). In other words, even if heat is applied, phonon excitation is suppressed up to a certain temperature, and therefore heat transport by phonons is suppressed.

FIG. 7 is a diagram showing the relationship between the carrier concentration, the thermal conductivity κ, and the figure of merit ZT.
As described above, the thermal conductivity κ of the entire material can be expressed by the sum of the carrier thermal conductivity κ _e and the lattice thermal conductivity κ _l (formula (4)). As shown in FIG. 7, the carrier thermal conductivity κ _e increases as the carrier concentration increases.

Assuming that the lattice thermal conductivity κ _l is 0.8 Wm ⁻¹ K ⁻¹ , the figure of merit ZT shows a transition as shown by the curve A in FIG. 7 with respect to the carrier concentration. Here, for example, by using a composite structure as described above, and were able to reduce the lattice thermal conductivity kappa _l from 0.8Wm ^-1 K ^-1 to 0.2Wm ^-1 K ^-1 (other parameters Shall not change). In that case, the figure of merit ZT shows a transition as shown by the curve B in FIG. 7 with respect to the carrier concentration.

The figure of merit ZT of the curve B shows a value higher than the figure of merit ZT of the curve A in a certain carrier concentration range. For example, at the carrier concentration X (the figure of merit ZT of the curve A is the maximum), the figure of merit ZT decreases as the lattice thermal conductivity κ _l decreases from 0.8 Wm ⁻¹ K ⁻¹ to 0.2 Wm ⁻¹ K ^−1. Can be increased up to about 2 times. Further, for example, even when the carrier concentration Y (the figure of merit ZT of the curve B is the maximum), the lattice thermal conductivity κ _l decreases from 0.8 Wm ⁻¹ K ⁻¹ to 0.2 Wm ⁻¹ K ⁻¹ , The index ZT can be increased up to about twice. The fact that the figure of merit ZT can be improved by about 2 times indicates that the thermoelectric conversion efficiency η can be improved by about 2 times according to the relationship shown in FIG.

  For this reason, it is possible to realize a thermoelectric conversion element having a high thermoelectric conversion efficiency η by using the above-described composite structure that can suppress heat transport by phonons as the thermoelectric conversion material.

  As shown in FIGS. 4A to 4D, the frequency gap G at which the excitation of phonons is suppressed differs depending on the period of the composite structure. The relationship between the period of the composite structure and the frequency at which phonon excitation can be suppressed is as shown in FIG.

FIG. 8 is a diagram showing the relationship between the period of the composite structure and the frequency and temperature at which phonon excitation is suppressed.
In FIG. 8, the vertical axis represents frequency and temperature, and the horizontal axis represents the period of the composite structure. Here, the period of the composite structure shown on the horizontal axis is the period expressed by the number of carbon atoms having the same bonding environment such as the 5-3 period, the 7-4 period, and the 9-5 period as described above. This is expressed in terms of the length in the plane direction of graphene (length of one side). Further, the frequency shown on one vertical axis is a frequency at which phonon excitation is suppressed, and is the frequency of the gap G in the case of FIGS. 4A to 4D as an example. This frequency converted into temperature is shown on the other vertical axis.

From the phonon dispersion relationship of the composite structure of various periods, when the frequency corresponding to the gap that suppresses the excitation of phonons and the converted temperature are obtained, as shown in FIG. There is a tendency to decrease as the period of the structure increases. Based on the knowledge shown in FIG. 8, the figure of merit ZT is improved by suppressing the temperature range in which phonon excitation can be suppressed, that is, the lattice thermal conductivity κ _l , in the composite structure of each period. It is possible to estimate the temperature range that can be used.

  However, in practice, phonons are excited about 10% up to about twice the temperature given to the composite structure, and therefore it is preferable to set the temperature in consideration of this point. If the period of the composite structure is about 5 nm, in order to sufficiently suppress the excitation of phonons, the temperature should be set lower than the temperature shown in FIG. 8 (about 500 ° C.), for example, about 100 ° C. Is preferred. Similarly, when the period of the composite structure is about 3 nm, in order to sufficiently suppress the excitation of phonons, the temperature is lower than the temperature shown in FIG. 8 (about 1100 ° C.), for example, about 300 ° C. It is preferable to set.

  As described above, a high-performance thermoelectric conversion element can be formed by using a composite structure in which graphene and carbon nanotubes are bonded as described above. In addition, since such a composite structure can be formed using only carbon or carbon as a main component, the material cost of the thermoelectric conversion element can be reduced. Furthermore, as will be described later, such a composite structure can be formed by a process at a relatively low temperature, so that the manufacturing cost of the thermoelectric conversion element can be reduced.

Hereinafter, an example in which a composite structure including graphene and carbon nanotubes is applied to a thermoelectric conversion element will be specifically described.
[First embodiment]
FIG. 9 is a schematic configuration diagram of the thermoelectric conversion element according to the first embodiment.

  In the thermoelectric conversion element 50 shown in FIG. 9, a composite structure 53 including graphene 53 a and carbon nanotubes 53 b is formed on a substrate 51 via an underlayer 52 so as to cover both ends of the composite structure 53. Electrodes 54 and 55 are formed on the substrate.

  Examples of the substrate 51 include a semiconductor substrate such as Si, an insulating substrate such as silicon oxide (SiO), a substrate in which an insulating layer such as SiO is formed on the semiconductor substrate, and a substrate in which a semiconductor layer such as Si is formed on the insulating substrate. For example, various substrates can be used.

  As the base layer 52, a titanium (Ti) layer, a titanium nitride (TiN) layer, a laminate of a Ti layer and a TiN layer, or the like can be used. These layers can be formed by vapor deposition, sputtering, or the like.

  The composite structure 53 can be formed by forming a catalyst layer (not shown in FIG. 9) on the base layer 52 and using it by a thermal CVD (Chemical Vapor Deposition) method or the like. In this case, a transition metal such as iron (Fe), nickel (Ni), or cobalt (Co), or a binary metal or alloy containing such a transition metal can be used for the catalyst layer. For example, as shown in FIG. 9, the composite structure 53 is formed at a front end portion of a plurality of single-walled or multi-walled carbon nanotubes 53 b standing on the base layer 52 by a thermal CVD method under predetermined conditions. The graphene 53a can be joined. Each carbon nanotube 53b is not limited to any one of the multiple layers of graphene 53a, and may be bonded to two or more layers.

For the electrodes 54 and 55, a laminate of a Ti layer and a gold (Au) layer can be used. Such a layer can be formed by vapor deposition, sputtering, or the like.
Then, an example of the formation method of the thermoelectric conversion element 50 which has such a structure is demonstrated.

  10 is a schematic cross-sectional view of the underlayer and catalyst layer forming step according to the first embodiment, FIG. 11 is a schematic cross-sectional view of the composite structure forming step according to the first embodiment, and FIG. 12 is an electrode according to the first embodiment. It is a cross-sectional schematic diagram of a formation process.

  First, as shown in FIG. 10, a base layer 52 such as a Ti layer is formed on a predetermined region (region where the composite structure 53 is formed) of a substrate 51 such as Si. Further, a catalyst layer 56 such as Co is formed on the formed underlayer 52 with a film thickness of 2 nm to 10 nm, for example.

  Next, using the formed catalyst layer 56, a composite structure 53 including a plurality of layers of graphene 53a and a plurality of carbon nanotubes 53b is formed by thermal CVD, as shown in FIG. The catalyst layer 56 is granulated by heating, and is taken into the interior or surface of the composite structure 53 as it is formed, or remains on the base layer 52. In FIG. 9 and FIG. 11, the illustration of the catalyst layer 56 and the catalyst particles generated from the catalyst layer 56 are omitted.

Formation of the composite structure 53 using the thermal CVD method uses, for example, a mixed gas of acetylene (C ₂ H ₂ , 10% Ar dilution) and argon (Ar) as a reaction gas, and this reaction gas is placed in a vacuum chamber. It is introduced and carried out under conditions of a pressure of 1 kPa and a substrate temperature of about 350 ° C. to 450 ° C. As a result, a composite structure 53 in which a plurality of carbon nanotubes 53b stands on the base layer 52 and a plurality of layers of graphene 53a are formed at the tip thereof is obtained.

  After the formation of the composite structure 53, as shown in FIG. 12, for example, Ti layers 54a and 55a are first formed so as to cover both ends thereof, and Au layers 54b are respectively formed on the formed Ti layers 54a and 55a. , 55b. Thereby, the electrode 54 in which the Au layer 54b is laminated on the Ti layer 54a and the electrode 55 in which the Au layer 55b is laminated on the Ti layer 55a are formed at both ends of the composite structure 53.

  With such a flow, the thermoelectric conversion element 50 including the composite structure 53 can be formed. By using the composite structure 53 having both high electron mobility and low thermal conductivity, the thermoelectric conversion element 50 having excellent thermoelectric conversion efficiency η can be realized. In addition, since the composite structure 53 which is a thermoelectric conversion material is mainly made of carbon, the material cost can be kept low, and further, it can be formed using a relatively low temperature thermal CVD method. The manufacturing cost of the element 50 can be kept low.

[Second Embodiment]
FIG. 13 is a schematic configuration diagram of a thermoelectric conversion element according to the second embodiment.
In the thermoelectric conversion element 60 shown in FIG. 13, the graphene 63 a is formed on a substrate 61 such as a semiconductor substrate or an insulating substrate via a base layer 62 using a Ti layer, a TiN layer, a laminate of a Ti layer and a TiN layer, or the like. A composite structure 63 including carbon nanotubes 63b is formed. Each carbon nanotube 63b of the composite structure 63 is not limited to any one of the multiple layers of graphene 63a, and may be bonded to two or more layers.

  At both ends of the composite structure 63, catalyst layers 64 and 65 such as transition metals are formed on the base layer 62, and graphene electrodes 66 and 67 (portions formed immediately above the catalyst layers 64 and 65) are formed thereon. Is formed.

  As described above, graphene alone exhibits good conductivity. Therefore, in this thermoelectric conversion element 60, graphene electrodes 66 and 67 including at least one layer of graphene are used as electrodes provided at both ends of the composite structure 63.

Such a thermoelectric conversion element 60 can be formed as follows, for example.
14 is a schematic cross-sectional view of the underlayer forming process according to the second embodiment, FIG. 15 is a schematic cross-sectional view of the catalyst layer forming process according to the second embodiment, and FIG. 16 is a composite structure and graphene according to the second embodiment. It is a cross-sectional schematic diagram of an electrode formation process.

  First, as shown in FIG. 14, the base layer 62 is formed on the substrate 61. Next, as shown in FIG. 15, a transition metal is used to form a catalyst layer 68 having a thickness of 2 nm to 10 nm on the region where the composite structure 63 of the base layer 62 is formed, and the graphene electrode 66 of the base layer 62 is formed. , 67 are formed to be thicker than the catalyst layer 68 on the region where the. The thin catalyst layer 68 and the thick catalyst layers 64 and 65 can be separately formed using patterning. Further, the material of the thin catalyst layer 68 and the thick catalyst layers 64 and 65 may be the same or different.

After the formation of the catalyst layers 68, 64, 65, as shown in FIG. 16, a composite structure 63 including a plurality of layers of graphene 63a and a plurality of carbon nanotubes 63b, and graphene electrodes 66, 67 are formed by thermal CVD. Form. Formation of the composite structure 63 and the graphene electrodes 66 and 67 uses, for example, a mixed gas of C ₂ H ₂ (10% Ar dilution) and Ar as a reaction gas, and introduces this reaction gas into a vacuum chamber, and a pressure of 1 kPa. The substrate temperature is about 350 ° C. to 450 ° C.

  When such a thermal CVD method is used, the composite structure 63 is selectively formed from the thin catalyst layer 68. The catalyst layer 68 is granulated by heating and is taken into the interior or surface of the composite structure 63 or remains on the underlayer 62. In FIG. 13 and FIG. 16, the catalyst layer 68 and Illustration of the catalyst particles generated from the catalyst layer 68 is omitted. On the other hand, the graphene electrodes 66 and 67 are selectively formed from the thick catalyst layers 64 and 65.

  Thus, by controlling the film thicknesses of the catalyst layers 68, 64, 65, the form of the carbon material formed from the catalyst layers 68, 64, 65 can be controlled. Therefore, in forming the catalyst layers 68, 64, 65, the catalyst layer 68 is formed with a film thickness capable of selectively forming the composite structure 63, and the graphene electrodes 66, 67 are formed with a film thickness capable of being selectively formed. Catalyst layers 64 and 65 are formed.

  Through such a flow, the thermoelectric conversion element 60 including the composite structure 63 and including the graphene electrodes 66 and 67 as electrodes can be formed. By using the composite structure 63 having both high electron mobility and low thermal conductivity, a thermoelectric conversion element 60 with excellent thermoelectric conversion efficiency η can be realized, and such a thermoelectric conversion element 60 can be manufactured at low cost. Can be formed.

[Third embodiment]
FIG. 17 is a schematic configuration diagram of a thermoelectric conversion element according to the third embodiment.
In the thermoelectric conversion element 70 shown in FIG. 17, the first layer is sandwiched between a substrate 71 such as a semiconductor substrate or an insulating substrate, using a Ti layer, a TiN layer, or a laminate of a Ti layer and a TiN layer. , Second composite structures 73 and 74 are formed.

  In the first composite structure 73 formed between the substrate 71 and the base layer 72, a plurality of graphenes 73a are formed on the substrate 71 side, and a plurality of carbon nanotubes 73b are formed on the base layer 72 side. On the other hand, in the second composite structure 74 formed on the underlayer 72, a plurality of carbon nanotubes 74b are formed on the underlayer 72 side, and a plurality of layers of graphene 74a are formed at the tip portions thereof.

  Here, a case where a plurality of such laminated structures including the first and second composite structures 73 and 74 are juxtaposed on the substrate 71 is illustrated. Electrodes 75 and 76 are formed so as to cover both ends of the first and second composite structures 73 and 74, respectively.

Thus, by forming a plurality of composite structures as the thermoelectric conversion material, it becomes possible to further increase the thermoelectric conversion efficiency η.
Such a thermoelectric conversion element 70 can be formed as follows, for example.

  18A and 18B are explanatory views of the catalyst layer and underlayer forming process according to the third embodiment, in which FIG. 18A is a schematic sectional view and FIG. 18B is a schematic plan view. FIG. 19 is a schematic cross-sectional view of the composite structure forming step according to the third embodiment, and FIG. 20 is a schematic cross-sectional view of the electrode forming step according to the third embodiment.

  First, as shown in FIG. 18, a first catalyst layer 77, a base layer 72, and a second catalyst layer 78 are sequentially formed on a predetermined region of the substrate 71. The first and second catalyst layers 77 and 78 are formed with a film thickness of 2 nm to 10 nm using a transition metal.

Thereafter, as shown in FIG. 19, first and second composite structures 73 and 74 are formed by thermal CVD. The first and second composite structures 73 and 74 are formed by using, for example, a mixed gas of C ₂ H ₂ (10% Ar dilution) and Ar as a reaction gas, introducing the reaction gas into a vacuum chamber, It is carried out under conditions of 1 kPa and a substrate temperature of about 350 ° C. to 450 ° C.

  When such a thermal CVD method is used, the graphene 73a and the carbon nanotube 73b are formed from the first catalyst layer 77 between the substrate 71 and the base layer 72, and the second catalyst layer on the base layer 72 is formed. From 78, graphene 74a and carbon nanotube 74b are formed. As a result, a plurality of carbon nanotubes 73b and 74b were erected on the upper and lower surfaces of the underlayer 72, respectively, and a plurality of layers of graphene 73a and 74a were formed at their tip portions (side opposite to the underlayer 72). The first and second composite structures 73 and 74 are obtained. The first and second catalyst layers 77 and 78 are granulated by heating and are taken into the inside and the surface of the first and second composite structures 73 and 74 or remain on the base layer 72. In FIG. 17 and FIG. 19, the first and second catalyst layers 77 and 78 and the catalyst particles generated from them are not shown.

  After the formation of the first and second composite structures 73 and 74, for example, Ti layers 75a and 76a and Au layers 75b and 76b are formed so as to cover both ends thereof as shown in FIG. 75, 76 are formed.

  With such a flow, the thermoelectric conversion element 70 including the first and second composite structures 73 and 74 formed with the base layer 72 interposed therebetween can be formed. By using the first and second composite structures 73 and 74 having both high electron mobility and low thermal conductivity, a thermoelectric conversion element 70 having excellent thermoelectric conversion efficiency η can be realized. The conversion element 70 can be formed at low cost.

[Fourth embodiment]
FIG. 21 is a schematic configuration diagram of a thermoelectric conversion element according to the fourth embodiment.
In the thermoelectric conversion element 80 shown in FIG. 21, a first base layer 82 using a Ti layer, a TiN layer, or a laminate of a Ti layer and a TiN layer is disposed above a substrate 81 such as a semiconductor substrate or an insulating substrate. A first composite structure 83 is formed. Further, in the thermoelectric conversion element 80, the second composite structure is disposed above the first composite structure 83 via the second underlayer 84 using a Ti layer, a TiN layer, or a laminate of a Ti layer and a TiN layer. A body 85 is formed.

  The first composite structure 83 formed between the first underlayer 82 and the second underlayer 84 has a plurality of carbon nanotubes 83b on the first underlayer 82 side and a plurality of layers on the second underlayer 84 side. Graphene 83a is formed. On the other hand, in the second composite structure 85 formed on the second underlayer 84, a plurality of carbon nanotubes 85b are formed on the second underlayer 84 side, and a plurality of layers of graphene 85a are formed at the tip portions thereof. Electrodes 86 and 87 are formed so as to cover both end portions of the first and second composite structures 83 and 85, respectively.

Thus, by forming a plurality of composite structures as the thermoelectric conversion material, it becomes possible to further increase the thermoelectric conversion efficiency η.
Such a thermoelectric conversion element 80 can be formed as follows, for example. In the fourth embodiment, the processes up to the formation of the first composite structure 83 are the same as those shown in FIG. 10 (formation of the base layer 52 and the catalyst layer 56) and FIG. Since this can be performed in the same manner as each step of (formation), the subsequent steps will be described here.

  22 is a schematic cross-sectional view of the second underlayer and catalyst layer forming step according to the fourth embodiment, FIG. 23 is a schematic cross-sectional view of the second composite structure forming step according to the fourth embodiment, and FIG. 24 is the fourth embodiment. It is a cross-sectional schematic diagram of the electrode formation process which concerns on an example.

  First, in the same manner as shown in FIGS. 10 and 11 of the first embodiment, a first underlayer 82 and a catalyst layer are formed on a substrate 81, and a plurality of layers of graphene 83a and a plurality of layers are formed by thermal CVD. A first composite structure 83 including the carbon nanotubes 83b is formed. The catalyst layer used for forming the first composite structure 83 is granulated by heating and taken into the inside or surface of the first composite structure 83, or remains on the first underlayer 82. In FIG. 22, illustration of the catalyst layer and the catalyst particles generated therefrom is omitted.

  After the formation of the first composite structure 83, as shown in FIG. 22, a second underlayer 84 is formed thereon in the same manner as the first underlayer 82, and further on the second underlayer 84. The catalyst layer 88 having a film thickness of 2 nm to 10 nm is formed using a transition metal.

  Next, as shown in FIG. 23, a second composite structure 85 including a plurality of layers of graphene 85a and a plurality of carbon nanotubes 85b is formed by thermal CVD using the catalyst layer 88 thus formed. The catalyst layer 88 used to form the second composite structure 85 is granulated by heating and taken into the inside or surface of the second composite structure 85 or remains on the second underlayer 84. In FIG. 23, illustration of the catalyst layer 88 and catalyst particles generated therefrom is omitted.

The formation of the first and second composite structures 83 and 85 using the thermal CVD method uses, for example, a mixed gas of C ₂ H ₂ (10% Ar dilution) and Ar as a reaction gas, and the reaction gas is evacuated. It introduce | transduces in a chamber and implements on the conditions of a pressure of 1 kPa, and substrate temperature 350 degreeC-about 450 degreeC. As a result, a plurality of carbon nanotubes 83b and 85b stand on the first and second base layers 82 and 84, respectively, and a plurality of layers of graphene 83a and 85a are formed at the tip portions thereof. Composite structures 83 and 85 are obtained.

  After forming the first and second composite structures 83 and 85 in this way, as shown in FIG. 24, for example, first, Ti layers 86a and 87a and Au layers 86b, 87b is formed, and electrodes 86 and 87 are formed.

  Through such a flow, the thermoelectric conversion element 80 including the first and second composite structures 83 and 85 on the first and second base layers 82 and 84 can be formed. By using the first and second composite structures 83 and 85 having both high electron mobility and low thermal conductivity, a thermoelectric conversion element 80 having excellent thermoelectric conversion efficiency η can be realized. The conversion element 80 can be formed at low cost.

The first to fourth examples have been described above, but the configuration of the thermoelectric conversion element using the composite structure including graphene and carbon nanotubes is not limited to the above example.
For example, in the first embodiment, a plurality of composite structures 53 can be formed side by side above a single substrate 51. In that case, a plurality of composite structures 53 can be formed side by side between the pair of electrodes 54 and 55 so that both ends are connected to the pair of electrodes 54 and 55. Alternatively, a plurality of composite structures 53 are formed between the plurality of pairs of electrodes 54 and 55 so that both end portions are connected to the pair of electrodes 54 and 55, respectively. Each electrode 55 can also be connected using a conductor.

  Similarly, in the second embodiment, a plurality of composite structures 63 can be formed side by side above one substrate 61. Furthermore, it is possible to stack the composite structure on the composite structure 63 formed above the substrate 61 by using, for example, the method described in the third and fourth embodiments.

Also in the third embodiment, another composite structure is laminated on the upper layer of the first and second composite structures 73 and 74 by using, for example, the method described in the third and fourth embodiments. Is possible.
Also in the fourth embodiment, a plurality of first and second composite structures 83 and 85 are formed side by side above a single substrate 81, or a set of first and second composite structures 83 and 85 is formed. For example, the composite structure can be laminated on the upper layer using the method described in the third and fourth embodiments.

By making such a change, it is possible to realize a thermoelectric conversion element having a higher thermoelectric conversion efficiency η.
Here, as the composite structure, an example in which graphene is bonded to one end of a carbon nanotube is illustrated, but the formation condition of a graphene bonded to both ends of a carbon nanotube is appropriately controlled. By doing so, it can be formed. Even when the composite structure having such a configuration is used, a thermoelectric conversion element having a high thermoelectric conversion efficiency η can be realized as described above.

  Thermoelectric conversion elements are widely used for thermoelectric conversion from various heating elements such as CPU chips, high-power / high-frequency power amplifiers, electronic vehicle drive modules, thermal power plants, server systems, and body temperature. It is possible.

FIG. 25 is a diagram illustrating an application example of a thermoelectric conversion element.
FIG. 25 illustrates an electronic device 100 including a chip 101 such as an LSI (Large Scale Integration) serving as a heating element, and a thermoelectric conversion element 102 that converts thermal energy generated from the chip 101 into electrical energy. Yes.

  The chip 101 is electrically connected to a wiring substrate 104 such as a build-up substrate via bumps 103 such as solder. The wiring board 104 is further electrically connected to a wiring board 106 such as a mother board via bumps 105 such as solder. In addition, a micro heat pipe 107 for transferring heat generated from the chip 101 is mounted on the wiring board 104. In the micro heat pipe 107, a predetermined amount of a working fluid 107b such as water is sealed inside a wick 107a, and one end portion (heat receiving portion) 107c is heated with the chip 101 via the heat dissipation sheet 108. Connected. In addition, the thermoelectric conversion element 102 is thermally connected to the other end (heat radiation part) 107d, and the heat sink 109 is thermally connected to the thermoelectric conversion element 102.

  As the thermoelectric conversion element 102, for example, an element including one or two or more thermoelectric conversion elements 50 to 80 having the above-described form can be used. In this case, different types of thermoelectric conversion elements 50 to 80 can be used in combination.

  In such an electronic device 100, when the heat generated from the chip 101 is transferred to the heat receiving portion 107c of the micro heat pipe 107, the working fluid 107b evaporates and the vapor flows to the heat radiating portion 107d. Then, the heat of the working fluid 107b is transferred to the thermoelectric conversion element 102, the working fluid 107b is condensed, and the liquid travels through the wick 107a and returns to the heat receiving portion 107c.

  The thermoelectric conversion element 102 is supplied with heat on one side and cooled on the other side by the heat sink 109, thereby giving a temperature difference between both ends, thereby generating an electromotive force. The electrical energy converted from the heat energy in this way is stored in the power storage unit 110. It is also possible to electrically connect the power storage unit 110 to the wiring substrate 104 via the bump 103 and supply the stored electrical energy to the chip 101 or the like.

  According to such an electronic device 100, since the heat generated in the chip 101 is not simply dissipated to the outside but is converted into electric energy using the thermoelectric conversion element 102, the chip 101 is efficiently operated while being cooled. , Can contribute to energy saving.

  As described above, as a thermoelectric conversion material, a structure in which a plurality of carbon atoms are arranged in a sheet shape like graphene, and a structure in which a plurality of carbon atoms are arranged in a cylinder shape like carbon nanotubes Use carbon materials. According to such a carbon material, it is possible to achieve both high electron mobility and low thermal conductivity. Therefore, a thermoelectric conversion element with high thermoelectric conversion efficiency η can be realized. In addition, since such a carbon material can be formed using only carbon or carbon as a main component, the material cost of the thermoelectric conversion element can be reduced. Furthermore, since such a carbon material can be formed by a process at a relatively low temperature, the manufacturing cost of the thermoelectric conversion element can be reduced.

  In addition, by using such a thermoelectric conversion element with various heating elements such as mounting on an electronic device including a chip, it is possible to effectively use the generated heat and further contribute to energy saving. become.

10, 10a, 10b, 10c, 53a, 63a, 73a, 74a, 83a, 85a Graphene 11 Carbon atoms 20a, 20b, 20c, 53b, 63b, 73b, 74b, 83b, 85b Carbon nanotubes 30a, 30b, 30c, 53, 63 Composite structure 50, 60, 70, 80, 102 Thermoelectric conversion element 51, 61, 71, 81 Substrate 52, 62, 72 Underlayer 54, 55, 75, 76, 86, 87 Electrode 54a, 55a, 75a, 76a , 86a, 87a Ti layer 54b, 55b, 75b, 76b, 86b, 87b Au layer 56, 64, 65, 68, 88 Catalyst layer 66, 67 Graphene electrode 73, 83 First composite structure 74, 85 Second composite structure Body 77 First catalyst layer 78 Second catalyst layer 82 First underlayer 84 Second underlayer 100 Electronic device 01 chips 103 and 105 bumps 104, 106 wiring board 107 micro heat pipes 107a wick 107b working fluid 107c heat receiving portion 107d radiating portion 108 dissipation sheet 109 heat sink 110 power storage unit

Claims (7)

A carbon material including at least one first structure in which a plurality of carbon atoms are arranged in a sheet shape, and at least one second structure in which a plurality of carbon atoms are arranged in a cylindrical shape;
A pair of electrodes electrically connected to the carbon material;
The thermoelectric conversion element characterized by having.   The carbon material has a gap in which excitation of phonons is suppressed between a frequency of zero and a predetermined frequency in a predetermined wave number region in a dispersion relationship indicating a relationship between a phonon wave number and a frequency. 1. The thermoelectric conversion element according to 1.   The thermoelectric conversion element according to claim 1, wherein the first structure is joined to an opening end of the second structure.   4. The thermoelectric conversion element according to claim 1, wherein the plurality of second structures are periodically joined to the first structure. 5. The carbon material is disposed above the substrate,
The first structure extends in a planar direction of the substrate, the second structure extends in a vertical direction of the substrate;
5. The thermoelectric conversion element according to claim 1, wherein the pair of electrodes are disposed above the substrate with the carbon material interposed therebetween. Forming a carbon material including at least one first structure in which a plurality of carbon atoms are arranged in a sheet shape and at least one second structure in which a plurality of carbon atoms are arranged in a cylinder shape;
Forming a pair of electrodes electrically connected to the carbon material;
The manufacturing method of the thermoelectric conversion element characterized by having. A carbon material including at least one first structure in which a plurality of carbon atoms are arranged in a sheet shape, and at least one second structure in which a plurality of carbon atoms are arranged in a cylindrical shape;
A pair of electrodes electrically connected to the carbon material;
An electronic apparatus comprising a thermoelectric conversion element having

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