Solid thermal storage materials and composites with adjusted thermal conductivity

Abstract

The solid thermal storage material of the present invention is a composite material in which a high thermal conductivity material having a higher thermal conductivity than vanadium dioxide is dispersed in vanadium dioxide, and the vanadium dioxide and the high thermal conductivity material are densely bonded together, wherein the volume fraction of the high thermal conductivity material is 0.03 or more.

Description

Technical Field

[0001] This invention relates to a solid thermal storage material and composite with adjusted thermal conductivity, and more specifically, to a solid thermal storage material with adjusted thermal conductivity composed of a conjugate of vanadium dioxide and a material with high thermal conductivity, capable of adjusting heat dissipation and heat storage over a wide range. Background Technology

[0002] In electronic devices, such as those using power semiconductors, heat dissipation issues have become increasingly apparent as chips and other components are miniaturized. Common countermeasures include two methods: directly placing a thermally conductive component (such as a heat sink) beneath the chip to improve heat dissipation, and increasing the heat capacity to suppress temperature rise. In the latter case, to simultaneously achieve miniaturization and increased heat capacity, latent heat is utilized (Patent Document 1, Patent Document 2).

[0003] Traditional latent heat storage materials mostly utilize the heat of fusion that occurs during the transition from a solid to a liquid phase, making heat storage density (latent heat per unit weight or unit volume) a crucial characteristic. On the other hand, thermal conductivity, a vital factor in heat exchange, has not been adequately studied. For example, paraffin, a representative heat storage material, has a thermal conductivity of approximately 0.2 W / mK, which is clearly low for rapid heat dissipation / absorption. Therefore, the design of heat exchangers, such as structures that increase the contact area between the heat storage material and the sealed container, or the selection of materials with high thermal conductivity for the container, is inherently important. This leads to a tendency towards structural complexity.

[0004] Recently, solid heat storage materials utilizing solid-solid phase change are being developed (Patent Document 3). The material described in Patent Document 3 does not melt because the latent heat originates from the phase change of the electronic system, and it maintains its shape before and after the phase change. Therefore, this material can be used as a structural material with latent heat storage properties (Patent Documents 4, 5, and Non-Patent Document 1). That is, it can be directly used as a material in heat exchangers, etc. Since leakage is not a concern and there is no increase in thermal resistance with encapsulation, vanadium dioxide has attracted attention as a solid heat storage material that can solve the problems of molten heat storage materials.

[0005] However, vanadium dioxide has a thermal conductivity of about 6 W / mK (Non-Patent Literature 2, Non-Patent Literature 3), which is insufficient as a heat dissipation material and has the problem of low suppression effect on excessive temperature rise.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent document 1: International Publication WO2017 / 081833.

[0009] Patent document 2: International Publication WO2017 / 073010.

[0010] Patent document 3: Japanese Patent No. 568825 (Japanese Unexamined Patent Publication No. 2009-163510).

[0011] Patent document 4: Japanese Patent Application Publication No. 2018-128190.

[0012] Patent document 5: Japanese Patent Application Publication No. 2016-79351.

[0013] Patent document 6: WO2019 / 026773.

[0014] Non-patent literature

[0015] Non-patent document 1: Press release “Development of a phase change heat storage component that simultaneously achieves high heat storage density and robustness” National Institute of Advanced Industrial Science and Technology, 2019 / 03 / 01 (https: / / www.aist.go.jp / aist_j / press_release / pr2019 / pr20190301 / pr20190301.html).

[0016] Non-patent literature 2: CN Berglund and HJ Guggenhein, “Electrical properties of VO2 near the semiconductor-metal transition”, Physical Review B 185, 1022-33 (1969).

[0017] Non-patent literature 3: S. Lee, K. Hippalgaonkar, F. Yang et al., “Abnormally low electronic thermal conductivity in metallic vanadium dioxide”, Science 355, 371-374 (2017). Summary of the Invention

[0018] The problem that the invention aims to solve

[0019] The objective of this invention is to address the problems of the prior art and provide a solid thermal storage material with adjusted thermal conductivity that has high thermal conductivity, strong suppression of excessive temperature rise, and the ability to adjust heat dissipation and heat storage properties over a wide range.

[0020] Methods for solving problems

[0021] According to the present invention, in order to solve the above-mentioned problems, the following solid heat storage material is provided.

[0022] [1] A solid thermal storage material, characterized in that the solid thermal storage material is a composite material in which a high thermal conductivity substance with a higher thermal conductivity than vanadium dioxide is dispersed in vanadium dioxide, and the vanadium dioxide and the high thermal conductivity substance are densely bonded together, wherein the volume fraction of the high thermal conductivity substance is 0.03 or more.

[0023] [2] A solid heat storage material, wherein, in the above-mentioned [1] invention, the material with high thermal conductivity is copper.

[0024] [3] A solid heat storage material, wherein, in the above-mentioned [1] invention or [2] invention, vanadium dioxide with excess oxygen is used as raw material.

[0025] [4] A solid heat storage material, characterized in that, in any of the inventions of the above [1] to [3], there is no diffusion layer and reaction phase at the interface between the vanadium dioxide and the high thermal conductivity material.

[0026] [5] A solid heat storage material, characterized in that it is stable against oxidation and corrosion in any of the inventions described in [1] to [4].

[0027] [6] A solid heat storage material, wherein, in any of the inventions described in [1] to [5] above, the material with high thermal conductivity is oriented parallel to the heat transfer direction.

[0028] [7] A solid heat storage material, wherein, in any of the inventions described in [1] to [6] above, vanadium dioxide is contained in which the transition temperature is adjusted by doping.

[0029] [8] A composite material, characterized in that the solid heat storage material of any one of the inventions of [1] to [7] is bonded to copper.

[0030] Invention Effects

[0031] According to the present invention, due to the dense bonding of a material with excellent thermal conductivity to vanadium dioxide, it exhibits excellent endothermic / exothermic characteristics with excellent time response, and can effectively utilize high heat storage density based on latent heat. Therefore, a solid heat storage material with tunable thermal conductivity, high suppression effect on excessive temperature rise, and adjustable heat dissipation and heat storage properties over a wide range has been achieved. Attached Figure Description

[0032] Figure 1 This is an electron microscope photograph showing the bonding state of vanadium dioxide (VO2) with electrolytic copper powder (Cu) in the sample of Example 1.

[0033] Figure 2This is a graph showing the temperature dependence of the thermal conductivity (k) of the vanadium dioxide / copper powder composite in the sample of Example 1. Here, V Cu This represents the volume fraction of copper.

[0034] Figure 3 The volume fraction (V) of copper is shown to represent the latent heat (L) and thermal conductivity (k) at room temperature of the sample in Example 1. Cu Dependency diagram.

[0035] Figure 4 This is a graph showing the temperature changes (solid lines) of chips mounted on various substrates in the sample of Example 4. The substrate temperature is also represented by dashed lines. The substrates were copper (Cu), copper volume fraction 0.50 vanadium dioxide volume fraction 0.50 (Cu / VO2), and vanadium dioxide (VO2).

[0036] Figure 5 This is an electron microscope photograph showing the bonding state of vanadium dioxide (VO2) with copper plate (Cu plate) in the sample of Example 5.

[0037] Figure 6 This is an electron microscope photograph showing the bonding state of untreated vanadium dioxide (VO2) with copper plate (Cu plate) in the sample of Comparative Example 2.

[0038] Figure 7 This is the result of the EDX composition distribution map of the sample in Example 5.

[0039] Figure 8 The results are the EDX composition distribution diagrams of the sample from Comparative Example 2.

[0040] Figure 9 This is a scanning electron microscope image of the bonding interface of the composite of Example 11. Detailed Implementation

[0041] The present invention will now be described in detail based on its embodiments.

[0042] To improve thermal responsiveness, it is necessary to increase the thermal conductivity of the material. Since single crystals have fewer defects in scattering thermal vibrations, their thermal conductivity can be considered to be at its maximum. It has been reported that the thermal conductivity of vanadium dioxide single crystals is around 6 W / mK at room temperature (Non-Patent Literature 2, Non-Patent Literature 3). Significant improvements beyond this value cannot be expected through methods such as improving the crystallinity of vanadium dioxide or controlling the crystallization orientation of polycrystalline materials. Therefore, to improve thermal conductivity, densely mixing materials with high thermal conductivity (e.g., copper, aluminum, carbon fibers, etc.) is effective. However, since the latent heat decreases linearly with increasing volume ratio of high thermal conductivity materials, large-scale mixing is limited. On the other hand, when the volume ratio is too small, no improvement in thermal responsiveness has been observed.

[0043] The thermal conductivity of a mixture of different materials is maximized when the highly thermally conductive material is oriented in one direction, requiring the minimum amount of material dispersed. In the case of a two-phase system, the thermal conductivity k is as follows.

[0044] k = (1-V) highk )×k VO2 +V highk ×k highk (1)

[0045] Here, V highk k is the volume fraction of a material with high thermal conductivity. VO2 k is the thermal conductivity of vanadium dioxide. highk The thermal conductivity of a material with high thermal conductivity.

[0046] Therefore, in order to achieve the desired thermal conductivity (k limit The volume fraction required to achieve high thermal conductivity is as follows.

[0047] V highk ≥(k limit -k VO2 ) / (k highk -k VO2 (2)

[0048] For heat sinks and similar applications requiring high thermal responsiveness, the thermal conductivity needs to be above that of typical metallic materials. Specifically, it needs to be above 15 W / mK for stainless steel. Since vanadium dioxide has a thermal conductivity of 6 W / mK, and typical high thermal conductivity materials (copper, silver) have a thermal conductivity of around 400 W / mK, the volume fraction of the high thermal conductivity material needs to be above 0.03 to achieve a thermal conductivity of 15 W / mK or higher.

[0049] Thermal conductivity can be determined using steady-state and transient methods. Due to the influence of latent heat, the steady-state method is preferred for measurements near the transition temperature; however, the method can be appropriately selected based on factors such as sample shape. Furthermore, differential scanning calorimetry (DSC) is preferred for determining latent heat.

[0050] Typically, oxides have poor wettability with metals or carbon, making high-density bonding difficult. Therefore, material selection is crucial. Bonding oxides to metals can be achieved through indirect bonding using solder or direct bonding, but the latter is preferred due to its ability to produce a stronger bond and lower interfacial thermal resistance.

[0051] Since direct bonding integrates materials through the same process as sintering, it is important in this study to determine the appropriate process temperature based on the sintering temperature of each material.

[0052] First, regarding the sintering temperature of vanadium dioxide, 550℃-1050℃ is a suitable range. When this temperature range coincides with the sintering temperature of materials with high thermal conductivity, uniform bonding can be achieved over a wide range of volume fractions. For example, the sintering temperature of copper is in the range of 500-800℃, aluminum is 500-600℃, and carbon is difficult to sinter within the sintering temperature range of vanadium dioxide.

[0053] Considering copper's melting point of 1085°C, its processing window is approximately 550-1050°C, allowing for processing over a wide temperature range and making it suitable for combination with vanadium dioxide. On the other hand, aluminum forms an oxide film on its surface and has a low melting point of 660°C, resulting in a narrow processing window of approximately 550-600°C. Therefore, achieving good bonding tends to be difficult. Furthermore, carbon materials require processing at temperatures above 900°C. Additionally, in the case of carbon, the reaction with vanadium dioxide becomes problematic.

[0054] In bonding, the ease with which an interface forms between different materials is important, and copper is a suitable material from the perspective of forming a good interface with vanadium dioxide. In particular, since a well-adhesive interface can be obtained by utilizing the reaction of vanadium dioxide with oxygen, it is suitable to have an excess of oxygen on the surface of vanadium dioxide.

[0055] As a method for mixing oxides and high thermal conductivity materials, besides physically mixing the respective raw materials, another method is to coat the high thermal conductivity material by depositing or sputtering the oxide powder. The latter has the advantage of achieving uniform mixing even with small amounts added. On the other hand, physical mixing methods are suitable for situations where the volume fraction of the mixture is increased. The appropriate method can be selected based on the desired mixing amount / dispersion pattern.

[0056] Regarding the morphology of high thermal conductivity materials, typical forms include particles, lines, and plates. The appropriate form should be chosen based on the heat transfer direction. Specifically, when heat dissipation / absorption is isotropic, a particle form is suitable; when the heat dissipation / absorption direction is unidirectional, a line or plate-shaped high thermal conductivity material can be oriented parallel to that direction. In the latter case, even a small amount of high thermal conductivity material exhibits high thermal conductivity because the volume fraction of vanadium dioxide can be maintained at a high concentration, thus suppressing the reduction of latent heat in the high thermal conductivity material.

[0057] The phase transition temperature of vanadium dioxide can be adjusted by doping with atoms such as tungsten or chromium. The sintering behavior of doped vanadium dioxide is not significantly different from that of undoped vanadium dioxide, and they can be sintered under the same conditions. That is, bonding can also be directly achieved using the methods described above.

[0058] Since the interface of the bonding materials is the cause of interfacial thermal resistance, its control is important. In particular, when a reactive phase forms at the interface, the low thermal conductivity of the reactive phase due to the solid solution effect leads to an increase in interfacial thermal resistance. Therefore, from the viewpoint of thermal resistance, it is preferable to prevent the formation of a reactive phase or diffusion layer at the interface.

[0059] Furthermore, vanadium oxide has been shown to have environmental resistance issues (Patent Document 6). Since it essentially forms hydrates, careful consideration must be given to the environment in which it is used. In particular, it is prone to corrosion in acidic environments, so its use in acidic environments should generally be avoided. However, from the viewpoint of electrocorrosion protection, corrosion can be prevented by bonding it to a metal (copper or aluminum) with a work function lower than that of vanadium dioxide, thereby significantly improving environmental resistance. In this case, due to electrocorrosion protection, it is not necessary to completely coat vanadium dioxide as described in Patent Document 6; electrical conductivity is sufficient.

[0060] The solid thermal storage material of the present invention is characterized in that it is a composite containing a high thermal conductivity material having a higher thermal conductivity than vanadium dioxide and the two being densely bonded together, wherein the volume fraction of the high thermal conductivity material is 0.03 or more.

[0061] Examples of materials with higher thermal conductivity than vanadium dioxide include copper, silver, aluminum or alloys containing these metals, and carbon materials, with copper being particularly preferred.

[0062] The bonding assembly constituting the solid thermal storage material of the present invention requires vanadium dioxide to be densely bonded to a material with a higher thermal conductivity than vanadium dioxide. Here, "dense" means bonded at 90% or more, more preferably 95% or more of the theoretical density. To improve the bonding, it is preferable to pre-treat in air at 250°C for 20 minutes. The heat treatment temperature and heat treatment time can be appropriately set according to the type of high thermal conductivity material, the porosity achieved, etc.

[0063] For accurate determination of porosity, the Archimedes method is preferred for density measurement.

[0064] In this invention, to improve thermal responsiveness, the lower limit of the volume fraction of the high thermal conductivity material is 0.03 (3% by percentage). Furthermore, since latent heat decreases linearly with increasing volume fraction of the high thermal conductivity material, its upper limit is approximately 0.75 (75% by percentage). The volume fraction can be appropriately determined within this range depending on the application, but due to the negative correlation between latent heat and thermal responsiveness, the volume fraction is adjusted to 0.03-0.40 (3-40% by percentage) for the case prioritizing latent heat, 0.40-0.60 (40-60% by percentage) for the case achieving both, and 0.60-0.75 (60-75% by percentage) for the case prioritizing thermal responsiveness.

[0065] As mentioned above, since the ease of interface formation between different materials during bonding is important, and since a well-adhesive interface can be obtained by utilizing the reaction of vanadium dioxide with oxygen, it is also preferable to have an excess of oxygen on the surface of vanadium dioxide. Here, excess oxygen refers to an excess containing VO₂. 2+x The oxygen in the description is approximately x = 0.01.

[0066] The solid thermal storage material of the present invention can be bonded and used with high thermal conductivity materials such as copper plates. Specifically, the composite of the present invention is characterized by bonding the solid thermal storage material of the present invention with copper. As bonding methods, brazing and diffusion bonding can be used, but diffusion bonding is preferred to reduce interfacial thermal resistance.

[0067] The solid thermal storage material and composite of the present invention are not limited to the above embodiments.

[0068] Example

[0069] The present invention will be further described in detail below with reference to embodiments and comparative examples, but the present invention is not limited to the following embodiments at all.

[0070] [Example 1]

[0071] Copper powder (99.9% purity, particle size passing through a 45μm mesh, electrolytic copper powder) was bonded with vanadium dioxide powder (99.9% purity, average particle size 1μm). To improve adhesion, the vanadium dioxide powder was pre-heat-treated in air at 250°C for 20 minutes to produce an excess of oxygen. The powders were weighed with copper volume fractions of 0.00, 0.25, 0.50, 0.75, and 1.00, and mixed using a planetary mixer. Subsequently, the mixture was treated in vacuum using an electro-sintering apparatus at 550°C for 30 minutes and a molding pressure of 30 MPa to obtain a dense bond. All samples achieved more than 95% of the theoretical density. Here, in the case of vanadium dioxide-copper, assuming no reactants, the theoretical density was determined according to the composite law as follows.

[0072] (Theoretical density) = (Density of vanadium dioxide) × (Volume fraction of vanadium dioxide) + (Density of copper) × (Volume fraction of copper)

[0073] Figure 1 An electron microscope image of the polished surface of the sample prepared in this embodiment is shown. The image shows that vanadium dioxide and copper are bonded together without any voids.

[0074] In X-ray diffraction analysis, diffraction patterns caused by copper and vanadium dioxide were detected, and no other impurity phases were found.

[0075] The thermal conductivity (k) of the above-prepared specimens was determined by laser flash method (NETZSCH LFA447). The results are shown below. Figure 2 .Depend on Figure 2 It can be seen that as the volume fraction of copper increases, the thermal conductivity increases significantly across all temperature ranges.

[0076] In addition, the latent heat (L) of these samples was determined by differential thermal analysis (Bruker DSC3300SA). Figure 3 The figure shows the dependence of latent heat (L) and thermal conductivity (k) on the volume fraction of copper. As can be seen from the figure, latent heat (L) increases with the volume fraction of copper (V). Cu On the one hand, the thermal conductivity (k) decreases proportionally, while on the other hand, the thermal conductivity (k) increases as a quadratic function.

[0077] [Example 2]

[0078] Aluminum powder (purity ≥ 99%, particle size ≥ 300 μm, atomized powder) was bonded to vanadium dioxide powder. To improve adhesion, the vanadium dioxide powder was pre-heat-treated in the same manner as in Example 1. The powder was weighed with an aluminum volume fraction of 0.50 and mixed using a planetary mixer. Subsequently, it was heated in a vacuum using an electrically sintering apparatus at a forming pressure of 30 MPa and a temperature of 600 °C for 30 minutes. The result was a dense bond with a density of 93% relative to the theoretical density. Its thermal conductivity was confirmed to be 13 W / mK, an increase compared to 4 W / mK for pure vanadium dioxide.

[0079] [Example 3]

[0080] The copper powder described in Example 1 was mixed with vanadium dioxide powder (heat-treated) and carbon fibers (pitch-based, chopped fibers, 6 mm in length) in a mortar. The volume fractions were 0.25% copper, 0.5% vanadium dioxide, and 0.25% carbon fibers. Subsequently, the mixture was heated in a vacuum using an electric sintering apparatus at a forming pressure of 30 MPa and a temperature of 700°C for 30 minutes. The result was a dense bond with a density of 96% relative to the theoretical density. Its thermal conductivity was confirmed to be 27 W / mK, an increase compared to that of pure vanadium dioxide.

[0081] [Comparative Example 1]

[0082] Aluminum powder (purity ≥ 99%, particle size ≥ 300 μm, atomized powder) or carbon fiber (asphalt-based, chopped fiber, 6 mm length) was bonded with vanadium dioxide powder (purity 99.9%, average particle size 1 μm). To improve adhesion, the vanadium dioxide powder was pre-heat-treated as in Example 1. The aluminum and carbon fiber were weighed with a volume fraction of 0.50 and mixed using a planetary mixer. It should be noted that the carbon fiber was pre-dispersed ultrasonically in ethanol. Subsequently, the mixture was treated in a vacuum using an electrically sintering apparatus at a molding pressure of 30 MPa. The aluminum was heated at 550°C for 30 minutes, and the carbon fiber was heated at 950°C for 30 minutes. As a result, the aluminum-vanadium dioxide bonded sample had 87% of the theoretical density, and the carbon fiber-vanadium dioxide bonded sample had 75% of the theoretical density; a dense bond could not be obtained. Furthermore, in the carbon fiber-vanadium dioxide bonded sample, the two materials underwent a chemical reaction to generate V₂O₃.

[0083] [Example 4]

[0084] The influence of substrate material on chip heating was investigated. The substrates consisted of three types: 1) copper volume fraction 1.00 (hereinafter referred to as copper substrate); 2) copper volume fraction 0.50 and vanadium dioxide volume fraction 0.50 (hereinafter referred to as copper / vanadium dioxide substrate); 3) vanadium dioxide volume fraction 1.00 (hereinafter referred to as vanadium dioxide substrate). The substrate material preparation was the same as in Example 1. The substrates were machined into 30×30×5mm sheets and used as substrates.

[0085] In the experiment, a 10×10×1mm ceramic microheater was bonded to the center of a substrate using silver paste with a chip mounter, and the heater output was used to simulate the heat dissipation of the chip. The surface temperature of the chip (i.e., the heater) and the surface temperature of the substrate were measured using a thermal imager (measured at a distance of 5mm from the chip). The heating conditions were 9W heater output for 180 seconds.

[0086] The results are shown in Figure 4 The chip temperatures on each substrate are represented by solid lines, while the substrate temperatures are represented by dashed lines. Chips mounted on copper substrates experience a slow temperature rise over time, but chips mounted on vanadium dioxide substrates maintain a rapid initial temperature rise, reaching higher temperatures than those on copper substrates. This is because the vanadium dioxide substrate has low thermal conductivity, resulting in uneven heating of the substrate and only heating the area around the chip mounting location. Therefore, although the substrate temperature is kept below the transition temperature, the latent heat of the substrate cannot contribute to suppressing the chip temperature rise. On the other hand, the copper / vanadium dioxide substrate, with its improved thermal conductivity, can also suppress the chip temperature rise when the substrate reaches its transition temperature, providing better temperature suppression than the copper substrate.

[0087] [Example 5]

[0088] A 0.1 mm thick copper plate (99.96% purity) was laminated with vanadium dioxide powder and treated in a vacuum using an electric sintering apparatus at 550°C for 30 minutes and a forming pressure of 30 MPa. The vanadium dioxide powder was pre-heat-treated in the same manner as in Example 1.

[0089] Electron micrographs of the interface of the sample from Example 5 are shown below. Figure 5 As shown in the figure, the copper plate and vanadium dioxide are bonded together without any gaps.

[0090] The joints of the specimens from Example 5 were subjected to thermal cycling tests across the metal-insulator transition temperature. The temperature profiles were obtained by repeated heating / cooling at 5°C / min between 45°C and 90°C. No peeling occurred during the 100-cycle test.

[0091] [Example 6]

[0092] The interface of the sample in Example 5 was analyzed using electron microscopy (STEM) and energy-dispersive X-ray fluorescence (EDX) analysis. Figure 7 The diagram shows the composition distribution of EDX. (From...) Figure 7 It can be seen that no diffusion layer or reactive phase was observed near the interface, forming a clean interface.

[0093] [Comparative Example 2]

[0094] A 0.1 mm thick copper sheet (99.96% purity) was laminated with vanadium dioxide powder and treated in a vacuum using an electric sintering apparatus at 550°C for 30 minutes and a forming pressure of 30 MPa. The vanadium dioxide powder was not heat-treated. Electron micrographs of the bonding interface of the sample from Comparative Example 2 are shown below. Figure 6 .Depend on Figure 6 It can be seen that the copper plate and vanadium dioxide have poor adhesion and many gaps exist.

[0095] [Comparative Example 3]

[0096] The interface of the sample in Comparative Example 2 was analyzed using electron microscopy (STEM) and energy-dispersive X-ray fluorescence (EDX) analysis. Figure 8 The composition distribution of EDX is shown. The result is as follows: Figure 8 As shown, a Cu₂O phase exists at the interface, and an amorphous Cu-VO diffusion layer is clearly observed near the grain boundaries of VO₂ at the interface. It is believed that the formation of these reactive phases hinders the formation of a dense interface.

[0097] [Example 7]

[0098] Multiple layers of 0.1 mm thick copper plates and vanadium dioxide were stacked at equal intervals under the same conditions as in Example 4. The volume fraction of copper was 0.29. The heat transfer direction of the laminate in Example 6 was aligned with the face of the copper plates (perpendicular to the copper), and the thermal conductivity was measured. The results are shown in Table 1. For comparison, the data from Example 1 are also shown. As can be seen from Table 1, even with the same copper content at the same volume fraction, the thermal conductivity is significantly improved. It should be noted that, as shown in Example 1, the latent heat is determined by the volume fraction of vanadium dioxide. Therefore, it can be seen that the orientation of copper is effective in increasing thermal conductivity while maintaining latent heat.

[0099] [Table 1]

[0100] Thermal conductivity [W / mK] Copper / vanadium dioxide laminate (vertical orientation) 103±2 Copper / vanadium dioxide powder bonding material (copper volume fraction 0.29) / according to Figure 3 Value 24 Copper / vanadium dioxide laminate (vertical orientation) after thermal cycling test 102±1

[0101] Thermal cycling tests were conducted on this laminate across the metal-insulator transition temperature. Heating / cooling was repeated at 10°C / min between 45°C and 90°C. No delamination occurred during the 100-cycle test. Furthermore, no degradation in thermal conductivity was observed after the thermal cycling test (Table 1).

[0102] [Example 8]

[0103] Copper powder (99.9% purity, particle size passing through a 45μm mesh, electrolytic copper powder) was bonded with vanadium dioxide powder with added tungsten (phase transition temperature 10℃, average particle size 1μm). To improve adhesion, the vanadium dioxide powder with added tungsten was pre-heat-treated in air at 250℃ for 20 minutes to produce an excess of oxygen. The powders were weighed with a copper volume fraction of 0.50 and mixed in a mortar. Subsequently, the mixture was sintered in vacuum at 550℃ for 30 minutes under a molding pressure of 30MPa using an electric sintering apparatus. The resulting sample was a dense bond with a theoretical density of 96%.

[0104] [Example 9]

[0105] The copper powder described in Example 1 was bonded with chromium-added vanadium dioxide powder (phase transition temperature 120°C, average particle size 1 μm). To improve adhesion, the chromium-added vanadium dioxide powder was pre-heat-treated to an excess of oxygen. The powders were weighed with a copper volume fraction of 0.50 and mixed in a mortar. Subsequently, the mixture was treated in vacuum using an electric sintering apparatus at 700°C for 30 minutes and a molding pressure of 30 MPa. The resulting sample was a dense bond with 97% of its theoretical density. Its thermal conductivity was confirmed to be 31 W / mK, an increase compared to that of pure vanadium dioxide.

[0106] [Example 10]

[0107] The vanadium dioxide-copper conjugate (copper volume fraction 0.50) prepared under the conditions of Example 1 was impregnated in 2 mol / L dilute sulfuric acid for 24 h. As a result, the sample retained its shape before impregnation. Furthermore, the dilute sulfuric acid did not stain.

[0108] [Comparative Example 4]

[0109] Vanadium dioxide (copper volume fraction 0.00) prepared under the conditions of Example 1 was impregnated in 2 mol / L dilute sulfuric acid for 24 h. As a result, the sample completely dissolved in the dilute sulfuric acid, which turned blue.

[0110] [Example 11]

[0111] Under the conditions of Example 1, a dense sintered body (solid heat storage material) of vanadium dioxide powder and copper powder (volume fraction 0.50) was pre-prepared and processed into a plate shape. This dense sintered body was overlapped with a copper plate (purity 99.96%, thickness 1 mm), and after applying a pressure of 30 MPa, heated to 600°C and held for 30 min. The environment was under vacuum. The resulting sintered body and copper plate joint was cut perpendicular to the interface using a cutter. After grinding the cut surface, it was processed using an ion polishing machine, and the joint interface was observed using a scanning electron microscope. Figure 9 As shown, a tight interface is formed, confirming that a good composite of sintered body and copper sheet can be obtained through diffusion bonding.

Claims

1. A solid heat storage material, characterized in that, The solid thermal storage material is a composite containing a thermally conductive material with a higher thermal conductivity than vanadium dioxide dispersed in vanadium dioxide, and the vanadium dioxide and the thermally conductive material are tightly bonded together at more than 90% of their theoretical density. The volume fraction of the thermally conductive material is 0.03 or higher. There is no diffusion layer or reaction phase at the interface between the vanadium dioxide and the thermally conductive material.

2. The solid thermal storage material as described in claim 1, characterized in that, The thermally conductive material is copper.

3. The solid heat storage material as described in claim 1 or 2, characterized in that, The solid thermal storage material uses vanadium dioxide with excess oxygen as raw material.

4. The solid thermal storage material according to any one of claims 1 to 3, characterized in that, The solid heat storage material is stable against oxidation and corrosion.

5. The solid thermal storage material according to any one of claims 1 to 4, characterized in that, The thermally conductive material is oriented parallel to the heat transfer direction.

6. The solid thermal storage material according to any one of claims 1 to 5, characterized in that, The solid thermal storage material contains vanadium dioxide whose transition temperature has been adjusted by doping.

7. A composite material, characterized in that, The solid thermal storage material according to any one of claims 1 to 6 is bonded to copper.

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