JP2015071795A - Heat storage material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new type of heat storage material which meets the requirements necessary for the heat storage material.SOLUTION: A heat storage material of this invention comprises a substance that makes an electron phase transition, wherein the electron phase transition is a phase transition of multi-degrees of freedom containing the orbital degree of freedom or at least two degrees of freedom among the following three degrees of freedom: the electric charge-, spin- and orbital-degrees of freedom of the internal degrees of freedom that an electron has. The electron phase transition has a transition enthalpy of 5% or more in comparison with the transition enthalpy in the solid-liquid phase transition of HO.

Description

  The present invention relates to a heat storage material.

  Thermal storage means storing heat in a substance, and the substance is called a thermal storage material. With this heat storage, the temperature of the heat storage material itself or the space where the heat storage material is placed can be kept substantially constant. For example, if heat storage (technology) is used, solar energy or exhaust heat is stored in a substance as heat, and the heat is used for heating, or ice is made at night with low power consumption. Heat of melting) can be used for cooling. As described above, since various forms of energy can be converted into heat and stored and reused by heat storage, the heat storage technology plays a part in energy saving that is currently being screamed. Therefore, heat storage technology is an urgent technology that must be further developed in the future.

  The mechanism of heat storage is roughly divided into sensible heat storage and latent heat storage. Sensible heat storage uses a large specific heat of a substance. For example, hot water bottles and the like use a large specific heat of water. The latent heat storage uses the transition enthalpy at the time of phase transition. For example, cooling a drink with ice water utilizes the heat of melting of ice (melting enthalpy).

  Since latent heat storage uses the transition enthalpy at the time of phase transition, the temperature can be kept approximately constant, and heat can be taken in and out at a substantially constant temperature (in sensible heat storage, the temperature change with respect to the external temperature is small. However, the temperature gradually changes). Therefore, at present, development of latent heat storage technology is mainly performed.

  The latent heat storage materials that have been developed so far include inorganic salt hydrates, organic substances, molten salts, and the like, all of which are heat storage materials that utilize a large transition enthalpy of solid-liquid phase transition.

  Certainly, a large enthalpy change due to the solid-liquid phase transition is important for the heat storage material, but there are other properties required for the heat storage material. For example, since it is important to keep the surface temperature of the heat storage material substantially constant for a long time, the heat storage material is required to have a high thermal conductivity. A substance with low thermal conductivity causes a temperature difference between the internal temperature and the surface temperature, and the surface temperature cannot be kept substantially constant (paraffin, which is an organic substance, has low thermal conductivity). In addition, when using a solid-liquid phase transition of a substance having a large volume change (expansion / contraction) due to a phase transition, there is a risk of liquid leakage. Therefore, the volume change is required to be small (if the volume change is large, a container that can withstand the volume change must be selected as a container for the heat storage material). Further, if phase separation or decomposition occurs during the phase transition, the heat storage effect is reduced (in the worst case, it cannot be used as a heat storage material). Therefore, it is required that no phase separation or decomposition occurs during the phase transition.

Journal of Solid State Chemistry, Vol. 6, pp. 258-270, 1973 Journal of the Physical Society of Japan, Vol. 52, no. 11, November, 1983, pp. 11-11. 3953-3959

  Therefore, an object of the present invention is to provide a new type of heat storage material that satisfies the requirements necessary for the heat storage material.

The heat storage material of the present invention is
A heat storage material made of a material that undergoes an electronic phase transition,
The electronic phase transition is an internal degree of freedom of electrons. Among the degrees of freedom of charge, spin, and orbit, the phase transition of double degree of freedom including at least two of the three degrees of freedom orbital degree of freedom. And
The transition enthalpy of the electronic phase transition is 5% or more of the transition enthalpy of the solid-liquid phase transition of H ₂ O.

In the present invention, a substance that undergoes electronic phase transition is used as a heat storage material. Further, in the present invention, a compound including at least two of orbital degrees of freedom or charge / spin / orbital degrees of freedom, which is an internal degree of freedom of electrons, which has not been used in a conventional heat storage material. Use phase transitions with degrees of freedom. Such a phase transition has the following characteristics.

・ Because this is a phase transition that occurs in the solid phase, there is no risk of the heat storage material (liquid) leaking from the container.
-Unlike solid-liquid phase transitions such as inorganic salt hydrates, there is no risk of phase separation or decomposition during phase transitions.
-Volume change during phase transition is small compared to solid-liquid phase transition.

In addition, a substance exhibiting such a phase transition has a high thermal conductivity. Further, among substances exhibiting such a phase transition, a material having a transition enthalpy of 5% or more of the transition enthalpy of the solid-liquid phase transition of H ₂ O is used as a heat storage material. Thereby, the new type heat storage material which satisfy | fills a requirement required for a heat storage material can be provided.

The substances include V _(1-X) W _X O ₂ (0 ≦ X ≦ 0.0650), V _(1-X) Ta _X O ₂ (0 ≦ X ≦ 0.117), V _(1-X). Nb _X O ₂ (0 ≦ X ≦ 0.115), V _(1-X) Ru _X O ₂ (0 ≦ X ≦ 0.150), V _(1-X) Mo _X O ₂ (0 ≦ X ≦ 0) 161), V _(1-X) Re _X O ₂ (0 ≦ X ≦ 0.0964), LiMn ₂ O ₄ , LiVS ₂ , LiVO ₂ , NaNiO ₂ , LiRh ₂ O ₄ , V ₂ O ₃ , V _{4 O 7, V 6 O 11,} Ti 4 O 7, SmBaFe 2 O 5, EuBaFe 2 O 5, GdBaFe 2 O 5, TbBaFe 2 O 5, DyBaFe 2 O 5, HoBaFe 2 O 5, YBaFe 2 O 5, PrBaCo 2 _{O 5.5, DyBaCo 2 O 5.54,} HoB Co ₂ O _5.48, it is preferably any one of YBaCo ₂ O _5.49. By configuring the heat storage material with any of the above substances, a new type of heat storage material that satisfies the requirements for the heat storage material can be provided.

  The value of X is preferably selected according to the target electronic phase transition temperature. In latent heat storage, heat storage is performed at the phase transition temperature (near). Further, since the phase transition temperature can be adjusted by adjusting the value of X in the above-mentioned substance, a substance that undergoes a phase transition at the target temperature can be easily used by selecting the value of X as such. Can do.

  ADVANTAGE OF THE INVENTION According to this invention, the new type heat storage material which satisfy | fills a requirement required for a heat storage material can be provided.

FIG. 1 is a graph showing the relationship between the amount of substitution and the phase transition temperature when part of V in VO ₂ is substituted with tungsten (W). FIG. 2 is a diagram illustrating an example of a measurement result of a powder X-ray diffraction pattern. FIG. 3 is a diagram showing the temperature dependence of the magnetic susceptibility of V _0.977 W _0.023 O ₂ . FIG. 4 is a diagram showing a measurement result of differential scanning calorimetry of V _0.977 W _0.023 O ₂ . FIG. 5 is a diagram illustrating an example of an evaluation result of heat storage characteristics. FIG. 6 is a diagram illustrating an example of a phase transition temperature and a transition enthalpy of a sample. FIG. 7 shows a material exhibiting a multi-degree-of-freedom phase transition including at least two degrees of freedom of orbital or electrons, phase transition temperature, transition enthalpy, and electrons involved in the phase transition. It is a figure which shows a freedom degree.

  Embodiments of the present invention will be described below in detail with reference to the drawings.

  First, the inventors focused on strongly correlated electron materials as materials that undergo phase transitions that cause large enthalpy changes, that is, materials that can be heat storage materials.

  A strongly correlated electron system is a system in which at least one of the degrees of freedom of spin, orbital, and charge possessed by an electron is manifested by strong Coulomb repulsion between electrons. Examples of strongly correlated electron materials include oxides containing transition metal elements. The manifested degrees of freedom of spin, orbital, and charge show large entropy changes accompanying the change in the number of states due to the order-disorder phase transition, respectively. Called the phase transition). The inventors considered that a strongly correlated electron substance can be a heat storage material by using a transition enthalpy that is a product of the amount of change in entropy and the temperature at which the change occurs.

  Unlike the solid-liquid phase transition that has been used for latent heat storage until now, the above-described phase transition with spin, orbital, and charge degrees of freedom is a phase transition that occurs in the solid phase. There is no worry of leaking from. Further, it is not necessary to cover the heat storage material with a container or the like. Furthermore, unlike the solid-liquid phase transition such as inorganic salt hydrate, the phase transition of the strongly correlated electron material as described above has no possibility of causing phase separation or decomposition during the phase transition.

  In addition, a strongly correlated electron substance is easy to handle when used as a heat storage material because the volume change at the time of phase transition is smaller than that of the solid-liquid phase transition.

  Moreover, since the conduction state of a strongly correlated electron substance is a metal or a semiconductor having a relatively small band gap called a Mott insulator, inorganic salt hydrates, organic substances, which have been used as heat storage materials, Compared with an insulator having a large band gap such as a molten salt, it has a high thermal conductivity.

  From the above points, the inventors thought that the problems described in the related art can be solved by using a strongly correlated electron substance as a heat storage material. In addition, the inventors can develop a new heat storage material by utilizing a multi-degree-of-freedom phase transition including at least two degrees of freedom of orbital or spin / orbital / charge. I thought.

<Substance 1>
In consideration of the above points, the inventors tried to develop a heat storage material that can be used near room temperature. Therefore, the inventors first focused on vanadium dioxide (VO ₂ ). VO ₂ is an oxide having a rutile structure and is known to exhibit a metal-insulator transition at 69 ° C., which is higher than room temperature (see Non-Patent Document 1). Further, since the metal-insulator transition is a phase transition of spin and orbital double degrees of freedom, the transition enthalpy is as large as 237 J / cc. The temperature range where the phase transition occurs is also very narrow (first order phase transition).

In VO ₂ , as shown in FIG. 1 (Non-Patent Document 2), by replacing part of vanadium (V) with tungsten (W), the temperature at which the metal-insulator transition occurs (phase transition temperature) decreases. it is known to (FIG. 1 shows the relationship between the substitution amount X and the phase transition temperature T _C in the case of substituting a part of the V tungsten (W)). Therefore, the inventors tried to develop a heat storage material that can be used near room temperature by replacing part of vanadium with another metal. Specifically, a part of V in VO ₂ was replaced with tungsten (W). Below, what substituted 1%, 2.3%, and 5% (molar ratio) of V by W as an example is demonstrated.

(Sample preparation)
Hereinafter, a preparation (synthesis) method of the sample (V _(1-X) W _X O ₂ ) will be described.

First, V ₂ O ₅ powder (manufactured by Kojundo Chemical Laboratory Co., Ltd .: purity 99.99%) was heated to 700 ° C. in a mixed gas of hydrogen and argon (hydrogen 5%, argon 95%), By holding for 48 hours, a precursor V ₂ O ₃ powder was obtained. The V ₂ O ₃ powder, V ₂ O ₅ powder (manufactured by Kojundo Chemical Laboratory Co., Ltd .: purity 99.99%) and WO ₃ powder (manufactured by Furuuchi Chemical Co., Ltd .: purity 99.999%) are combined with vanadium. Mixing was performed so that the molar ratio between tungsten and oxygen was a predetermined molar ratio. Then, the mixture (powder) was put in a quartz tube and vacuum-sealed (degree of vacuum: about 2 × 10 ⁻⁶ torr). Specifically, the quartz tube containing the mixture was connected to an exhaust device, and the quartz tube was evacuated. The quartz tube was sealed while being melted using a gas burner or the like. Thereafter, the mixture was heated to 1000 ° C. together with the quartz tube and held for 48 hours. Through the above steps, a powder sample of V _(1-X) W _X O ₂ was synthesized.

Note that a sintered body sample can be synthesized by pressing and compacting the mixture (powder) at a pressure of about 500 kgf / cm ² to form a pellet, vacuum-sealing it into a quartz tube, and performing the same heat treatment.

(Sample identification)
The synthesized sample was pulverized and placed on a silicon non-reflective plate, and the powder X-ray diffraction pattern at 22 ° C. was measured using an X-ray diffractometer (manufactured by Rigaku Corporation: RINT). An example of the measurement result is shown in FIG. 2 (in FIG. 2, the vertical axis is the diffraction intensity and the horizontal axis is the diffraction angle). From the result of FIG. 2, at 22 ° C., V _0.99 W _0.01 O ₂ is monoclinic rutile type (space group P2 ₁ / c), and V _0.977 W _0.023 O ₂ is tetragonal. It was found that the system rutile type (space group P4 ₂ / mnm) and V _0.95 W _0.05 O ₂ have a tetragonal rutile type (space group P4 ₂ / mnm) crystal structure. Further, no contamination of impurities was confirmed in both samples. That is, it was confirmed that the target sample was obtained by the above synthesis.

(Evaluation of phase transition temperature)
Using a superconducting magnetic flux quantum interferometer, the temperature dependence of the magnetic susceptibility of the synthesized sample was measured. The measurement was performed under a magnetic field of 1000 Oe. As an example, the measurement result of V _0.977 W _0.023 O ₂ is shown in FIG. 3 (in FIG. 3, the vertical axis represents magnetic susceptibility and the horizontal axis represents temperature). About each sample, the abnormality of the magnetic susceptibility accompanying a metal-insulator transition as shown in FIG. 3 was observed in both the temperature rising process and the temperature falling process. From the temperature dependence of the magnetic susceptibility, the phase transition temperature of V _0.99 W _0.01 O ₂ is 44 ° C. during the temperature rising process and 42 ° C. during the temperature falling process, and V _0.977
The phase transition temperature of W _0.023 O ₂ is 11 ° C. in the temperature increasing process, 10 ° C. in the temperature decreasing process, and the phase transition temperature of V _0.95 W _0.05 O ₂ is −67.5 ° C. in the temperature increasing process. In the temperature lowering process, it was confirmed to be −71 ° C. From this experiment, it was confirmed that in V _(1-X) W _X O ₂ , the phase transition temperature linearly decreased with respect to the tungsten substitution amount. Specifically, the relationship between the substitution amount X and the temperature rising process to put the phase transition temperature _{T C} of _tungsten, dT C _{/dX=-27.4K/at.} % W was estimated.

(Evaluation of transition enthalpy)
About the synthesized sample, the transition enthalpy accompanying the phase transition was estimated by differential scanning calorimetry using a differential scanning calorimeter (manufactured by NETZSCH: DSC204F1 / CP Phoenix / μ-Sensor). Measurement was carried out at a rate of temperature rise and a rate of temperature fall of 10 ° C./min. As an example, the measurement result of V _0.977 W _0.023 O ₂ is shown in FIG. 4 (in FIG. 4, the vertical axis is the differential scanning calorific value, and the horizontal axis is the temperature). The differential scanning calorific value represents a temperature difference when a certain amount of heat is applied to the reference material and the sample, or a difference in calorie required to bring both to a certain temperature. For each sample, thermal anomalies associated with the metal-insulator transition as shown in FIG. 4 were observed in both the temperature raising process and the temperature lowering process. Further, from the measurement results of differential scanning calorimetry, the transition enthalpy of V _0.99 W _0.01 O ₂ is 200 J / cc, the transition enthalpy of V _0.977 W _0.023 O ₂ is 151 J / cc, V _0. The transition enthalpy of ₉₅ W _0.05 O ₂ was found to be 66 J / cc. This transition enthalpy is equivalent to the transition enthalpy of a substance that has been used as a heat storage material (for example, the transition enthalpy in the solid-liquid phase transition of H ₂ O (306 J / cc)). It was suggested that it could be used.

Since the electronic phases (spin, orbital, and charge states) of a strongly correlated electron system are generated cooperatively by a group of electrons that interact strongly with each other, there is a risk that physical quantities may change dramatically with a small amount of impurities. is there. In addition, there is no report on the transition enthalpy when V is replaced with another metal, so replacing V with another metal drastically reduces the transition enthalpy and the temperature range where phase transition occurs becomes very wide. (Phase broadening of phase transition). However, by performing the above-described experiment, it is clear that V _(1-X) W _X O ₂ can freely change the transition temperature without causing a drastic decrease in the transition enthalpy or broadening of the phase transition. It was.

Further, from this experiment, it has been clarified that the relationship between the substitution amount of tungsten and the transition enthalpy in V _(1-X) W _X O ₂ is substantially linear. Specifically, the relationship between the substitution amount X and the transition enthalpy [Delta] H _{V tungsten, d (ΔH V) /dX=-34.1 (} J / cc) / at. % W was estimated. From the relationship of d (ΔH _V ) / dX and dT _C / dX, the relationship between the phase transition temperature T _C and the transition enthalpy ΔH _V is d (ΔH _V ) / dT _C = 1.24 (J / cc ) / K.

From the above relationship, the composition range where the transition enthalpy is 5% or more of the transition enthalpy of the solid-liquid phase transition of H ₂ O and the minimum action temperature (the transition temperature when the substitution amount is maximum) are estimated. .

V _(1-X) W _X O ₂ (0 ≦ X ≦ 0.0650): Tmin = −109 ° C.

(Evaluation of heat storage characteristics)
The synthesized sample was evaluated as to whether or not it sufficiently has a function as a heat storage material in a form close to practical use. Specifically, the sample and the thermocouple were placed in a test tube (inner diameter 10 mm, depth 1000 mm) having a vacuum heat insulating structure. And after cooling a sample to -18 degreeC with the test tube, it put in the space of room temperature (22 degreeC), and measured the temperature change of the said sample from the electromotive force of the thermocouple. FIG. 5 shows an example of measurement results (in FIG. 5, the vertical axis is temperature and the horizontal axis is time). In FIG. 5, when nothing is put in the test tube, the dotted line is V _0.977 W _0.023 O ₂ (9.53 g) when H ₂ O (4.43 g) is put in the test tube. ) Is the measurement result.

From the results of FIG. 5, it was confirmed that V _0.977 W _0.023 O ₂ kept the temperature at 11 ° C. for 37 minutes. It was confirmed that H ₂ O kept the temperature at 0 ° C. for 80 minutes. When these times are converted per unit volume, the temperature holding ability of V _0.977 W _0.023 O ₂ and H ₂ O (time during which the temperature can be held at a certain temperature) is 18 minutes and 17 minutes, respectively. It was confirmed that the temperature holding ability of V _0.977 W _0.023 O ₂ was substantially equivalent to H ₂ O under the experimental conditions performed. That is, it was confirmed that V _0.977 W _0.023 O ₂ has a function sufficient for practical use as a heat storage material. Further, when the volume of the sample is V, the thermal resistance with the outside world (heat bath) is R, the transition enthalpy per unit volume is ΔH _V , the outside temperature is T _RT , and the phase transition temperature is T _C , the phase transition temperature is Δt can be expressed as Δt = VRΔH _V / (T _RT −T _C ). As a result of using the transition enthalpy 151 J / cc of V _0.977 W _0.023 O ₂ estimated by differential scanning calorimetry, Δt was estimated to be 34 minutes at 11 ° C., and a result almost equal to the experimental value was obtained. It was. That is, it was confirmed that the result of this experiment was correct.

As described above, a new type of heat storage material that satisfies the requirements for a heat storage material is provided by utilizing the phase transition of V _(1-X) W _X O ₂ orbit and spin double degrees of freedom. Can do.

Further, as described above, the inventors can change the transition temperature without changing the enthalpy of transition or broadening the phase transition by changing the value of X in V _(1-X) W _X O _2. Clarified that it can be changed freely. That is, by adjusting the value of X, the use temperature can be changed as appropriate, and a substance having a phase transition at the use temperature can be easily selected. Moreover, it becomes possible to store heat at a temperature that cannot be held by conventional heat storage materials.

<Substance 2>
Next, an example in which V in VO ₂ is replaced with a substance other than W will be described. Specifically, an example in which V is replaced with Ta, Nb, Ru, Mo, Re, or the like will be described. In addition, about the content similar to the content demonstrated with the substance 1, the description is abbreviate | omitted.

(Sample preparation)
The sample preparation (synthesis) method is the same as that for V _(1-X) W _X O ₂ , and instead of WO ₃ Ta ₂ O ₅ (manufactured by Rare Metallic, Inc .: purity 99.99%), Nb ₂ O ₅ (manufactured by Soekawa Richemical Co., Ltd .: purity 99.9%), RuO ₂ (manufactured by Rare Metallic Co., Ltd .: purity 99.9%), MoO ₂ (manufactured by Soekawa Riken Corporation: purity 99.9%), By using ReO ₃ (manufactured by STREM: purity 99.9%), V _(1-X) Ta _X O ₂ , V _(1-X) Nb _X O ₂ , V _(1-X) Ru _X O _{2, V (1-X)} was synthesized _{Mo X O 2, V (1} -X) Re X O 2.

(Sample identification)
The sample was identified from the powder X-ray diffraction pattern at 22 ° C. An example of the measurement result is shown in FIG. From the results shown in FIG. 2, V _0.92 Ta _0.08 O ₂ , V _0.95 Nb _0.05 O ₂ , V _0.92 Nb _0.08 O ₂ , V _0.9 Ru _{0. 1} O ₂ , V _0.9 Mo _0.1 O ₂ , V _0.96 Re _0.04 O ₂ and V _0.93 Re _0.07 O ₂ are tetragonal rutile types, V _0.98 Ta _{0 0.02} O ₂ , V _0.975 Ru _0.025 O ₂ , and V _0.97
It was found that Mo _0.03 O ₂ has a monoclinic rutile crystal structure. Further, no contamination of impurities was confirmed in all samples. That is, it was confirmed that the target sample was obtained by the above synthesis.

(Evaluation of phase transition temperature)
The temperature dependence of the magnetic susceptibility of the synthesized sample was measured, and the phase transition temperature of the synthesized sample was estimated from the temperature dependence of the magnetic susceptibility. An example of the phase transition temperature (in the temperature rising process and the temperature falling process) of the synthesized sample is shown in FIG. As shown in FIG. 6, during the temperature rising process, the phase transition temperature of V _0.98 Ta _0.02 O ₂ is 49 ° C., V _0.92 Ta _0.08 O ₂ is 4 ° C., and V _0.95 Nb _{0 .05} O ₂ is 16 ° C., V _0.92 Nb _0.08 O ₂ is −6 ° C., V _0.975 Ru _0.025 O ₂ is 37 ° C., and V _0.9 Ru _0.1 O ₂ is −21. .5 ° C., V _0.97 Mo _0.03 O ₂ is 34 ° C., V _0.9 Mo _0.1 O ₂ is −39.5 ° C., and V _0.96 Re _0.04 O ₂ is −4.5. ° C. and V _0.93 Re _0.07 O ₂ was estimated to be −67 ° C. From these results, it was confirmed that the phase transition temperature linearly decreased with respect to the substitution amount of Ta, Nb, Ru, Mo, Re and the like.
Specifically, the relationship between the phase transition temperature _{T C} which put the substitution amount X and the temperature rising process of the Ta _is, dT C _{/dX=-7.98K/at.} % Ta was estimated.
Relationship of the phase transition temperature _{T C} which put the substitution amount X and the temperature rising process of Nb _is, dT C _{/dX=-9.5K/at.} Estimated as% Nb.
The relationship between the substitution amount X of Ru and the phase transition temperature T _C that can be placed in the temperature raising process is as follows: dT _C /dX=−8.76 K / at. % Ru was estimated.
Relationship of the phase transition temperature _{T C} which put the substitution amount X and the temperature rising process of Mo _is, dT C _{/dX=-10.8K/at.} Estimated as% Mo.
The relationship between the substitution amount X of Re and the phase transition temperature T _C that can be placed in the temperature raising process is as follows: dT _C /dX=−19.4 K / at. % Re was estimated.
In addition, it has been clarified from this experiment that even when V is replaced with Ta, Nb, Ru, Mo, Re or the like, the phase transition is not broadened.

(Evaluation of transition enthalpy)
For the synthesized samples, the enthalpy of transition associated with the phase transition was estimated by differential scanning calorimetry. An example of the transition enthalpy of the synthesized sample is shown in FIG. As shown in FIG. 6, the transition enthalpy of V _0.98 Ta _0.02 O ₂ is 194 J / cc, V _0.92 Ta _0.08 O ₂ is 84 J / cc, and V _0.95 Nb _0.05 O _2. Is 135 J / cc, V _0.92 Nb _0.08 O ₂ is 84 J / cc, V _0.975 Ru _0.025 O ₂ is 193 J / cc, V _0.9 Ru _0.1 O ₂ is 87 J / cc, V _0.97 Mo _0.03 O ₂ is 184 J / cc, V _0.9 Mo _0.1 O ₂ is 96 J / cc, V _0.96 Re _0.04 O ₂ is 148 J / cc, and V _{0. 93} Re _0.07 O ₂ was estimated to be 76 J / cc. Therefore, it has been clarified that even when V is replaced with Ta, Nb, Ru, Mo, Re or the like, the transition enthalpy is not drastically reduced.

Further, it was confirmed that the transition temperature and the transition enthalpy change linearly with respect to the substitution amount of Ta, Nb, Ru, Mo, Re and the like.
Specifically, the relationship between the substitution amount X and the transition enthalpy [Delta] H _V of Ta _{is, d (ΔH V) /dX=-18.9 (} J / cc) / at. % Ta was estimated. From the relationship of d (ΔH _V ) / dX and dT _C / dX, the relationship between the phase transition temperature T _C and the transition enthalpy ΔH _V is d (ΔH _V ) / dT _C = 2.37 (J / cc ) / K.
Similarly, when Nb is substituted, d (ΔH _V ) /dX=−19.3 (J / cc) / at. % Nb, d (ΔH _V ) / dT _C = 2.02 (J / cc) / K.
When substituted with Ru, d (ΔH _V ) /dX=−14.8 (J / cc) / at. % Ru, d (ΔH _V ) / dT _C = 1.68 (J / cc) / K.
When substituted with Mo, d (ΔH _V ) /dX=−13.8 (J / cc) / at. % Mo, d (ΔH _V ) / dT _C = 1.28 (J / cc) / K.
When substituted with Re, d (ΔH _V ) /dX=−23.0 (J / cc) / at. % Re, d (ΔH _V ) / dT _C = 1.18 (J / cc) / K.

From the above relationship, the composition range where the transition enthalpy is 5% or more of the transition enthalpy of the solid-liquid phase transition of H ₂ O and the minimum action temperature (the transition temperature when the substitution amount is maximum) are estimated. .

V _(1-X) Ta _X O ₂ (0 ≦ X ≦ 0.117): Tmin = −24.6 ° C.
V _(1-X) Nb _X O ₂ (0 ≦ X ≦ 0.115): Tmin = −40.1 ° C.
V _(1-X) Ru _X O ₂ (0 ≦ X ≦ 0.150): Tmin = −62.2 ° C.
V _(1-X) Mo _X O ₂ (0 ≦ X ≦ 0.161): Tmin = −105 ° C.
V _(1-X) Re _X O ₂ (0 ≦ X ≦ 0.0964): Tmin = −118 ° C.

As described above, even when V of VO ₂ is replaced with Ta, Nb, Ru, Mo, Re, etc., the phase transition of the orbital and spin double degrees of freedom is performed in the same manner as when V is replaced with W. By using it, a new type of heat storage material that satisfies the requirements for the heat storage material can be provided.

  Further, by adjusting the value of X, the use temperature can be appropriately changed, and a substance having a phase transition at the use temperature can be easily selected. Therefore, it becomes possible to store heat at a temperature that could not be held by conventional heat storage materials.

<Substance 3>
Next, another substance exhibiting a multi-degree-of-freedom phase transition including at least two or more of three degrees of freedom of an orbit or an electron will be described. FIG. 7 shows a substance exhibiting such a phase transition, the phase transition temperature, the transition enthalpy, and the degree of freedom of electrons involved in the phase transition (H ₂ O data is also shown for comparison).
In addition, about the substance of * 1 in FIG. 7, Journal of Solid State Chemistry, Vol. 170, pp. 221-226, 2003.
* For the substance of * 2, PHYSICAL REVIEW B, Vol. 31, Num. 12, 15 JUNE 1985, pp. 8143-8147.
* For the substances of * 3, see J.H. Phys. C: Solid State Phys. , Vol. 10, 1977. pp. Reported in L637-L638.
* 4 Substances are described in PHYSICAL REVIEW LETTERS, Vol. 32, Num. 23, 10 JUNE 1974, pp. 1318-1321.
* 5 about the substance of Journal of Solid State Chemistry, Vol. 177, pp. 281-292, 2004.
* For the substance of 6 Phys. : Condens. Matter, Vol. 17, pp. 5813-5820, 2005.

As shown in FIG.
LiMn ₂ O ₄ exhibits a phase transition between electric charge and orbit, its phase transition temperature is 21 ° C., and its transition enthalpy is 37.2 J / cc.
LiVS ₂ exhibits a spin-orbit phase transition with a phase transition temperature of 40 ° C. and a transition enthalpy of 58.3 J / cc.
LiVO ₂ exhibits a phase transition between spin and orbit, the phase transition temperature is 206 ° C., and the transition enthalpy is 326 J / cc.
NaNiO ₂ exhibits an orbital phase transition with a phase transition temperature of 213 ° C. and a transition enthalpy of 107 J / cc.
LiRh ₂ O ₄ exhibits a phase transition between charge and spin, the phase transition temperature is −103 ° C., and the transition enthalpy is 21.6 J / cc.
V ₂ O ₃ shows a phase transition between spin and orbit, the phase transition temperature is −119 ° C., and the transition enthalpy is 63 J / cc.
V ₄ O ₇ shows a phase transition between charge and orbit, the phase transition temperature is −36 ° C., and the transition enthalpy is 66.6 J / cc.
V ₆ O ₁₁ shows a phase transition between charge and orbit, the phase transition temperature is −101 ° C., and the transition enthalpy is 44.9 J / cc.
Ti ₄ O ₇ exhibits a charge, spin, and orbital phase transition with a phase transition temperature of −119 ° C. and a transition enthalpy of 28 J / cc.
SmBaFe ₂ O ₅ exhibits a charge and orbital phase transition with a phase transition temperature of −34.2 ° C. and a transition enthalpy of 23.4 J / cc.
EuBaFe ₂ O ₅ exhibits a charge and orbital phase transition with a phase transition temperature of −16.7 ° C. and a transition enthalpy of 26.3 J / cc.
GdBaFe ₂ O ₅ exhibits a phase transition between electric charge and orbit, a phase transition temperature of −2.7 ° C., and a transition enthalpy of 29 J / cc.
TbBaFe ₂ O ₅ shows a phase transition between charge and orbit, the phase transition temperature is 12.1 ° C., and the transition enthalpy is 34.7 J / cc.
DyBaFe ₂ O ₅ shows a phase transition between charge and orbit, the phase transition temperature is 20.5 ° C., and the transition enthalpy is 34.7 J / cc.
HoBaFe ₂ O ₅ exhibits a phase transition between electric charge and orbit, and its phase transition temperature is 22.87 ° C. and the transition enthalpy is 35.6 J / cc.
YBaFe ₂ O ₅ shows a phase transition between charge and orbit, the phase transition temperature is 36.5 ° C., and the transition enthalpy is 37.2 J / cc.
PrBaCo ₂ O _5.5 exhibits a charge and orbital phase transition with a phase transition temperature of 70.6 ° C. and a transition enthalpy of 69.9 J / cc.
DyBaCo ₂ O _5.54 shows a phase transition between charge and orbit, the phase transition temperature is 44.9 ° C., and the transition enthalpy is 52.4 J / cc.
HoBaCo ₂ O _5.48 shows a phase transition between charge and orbit, the phase transition temperature is 30.8 ° C., and the transition enthalpy is 66 J / cc.
YBaCo ₂ O _5.49 exhibits a charge, spin, and orbital phase transition with a phase transition temperature of 24.1 ° C. and a transition enthalpy of 41.6 J / cc.
All of the substances showed a large transition enthalpy phase transition of 5% or more of H ₂ O, suggesting that they can be used as heat storage materials.

  As described above, in the present embodiment, a phase transition having multiple degrees of freedom including at least two degrees of freedom of an orbit or three degrees of freedom of electrons that has not been used in a conventional heat storage material is used. . Examples of the multiple degrees of freedom include spin and orbit degrees of freedom, spin and charge degrees of freedom, orbit and charge degrees of freedom, and spin and orbit and charge degrees of freedom. By selecting a material exhibiting such a phase transition and having a large transition enthalpy, it is possible to provide a new type of heat storage material that satisfies the requirements for the heat storage material.

The heat storage material of the present invention is
A heat storage material made of a material that undergoes an electronic phase transition (except V ₂ O ₄ ) ,
The electronic phase transition is a double freedom of phase transition that includes two or more even without least Chi freedom sac of internal degrees of freedom der Ru electrostatic load spin orbit with electron,
The electronic phase transition is used for heat storage .

  In the present invention, the transition enthalpy of the electronic phase transition is H ₂ It can be 5% or more of the transition enthalpy of the solid-liquid phase transition of O.

  In the present invention, the substance can be a strongly correlated electron substance.

  In the present invention, the substance may be a substance containing a transition metal element.

Claims (3)

A heat storage material made of a material that undergoes an electronic phase transition,
The electronic phase transition is an internal degree of freedom of electrons. Among the degrees of freedom of charge, spin, and orbit, the phase transition of double degree of freedom including at least two of the three degrees of freedom orbital degree of freedom. And
The heat storage material, wherein the transition enthalpy of the electronic phase transition is 5% or more of the transition enthalpy of the solid-liquid phase transition of H ₂ O. The substances include V _(1-X) W _X O ₂ (0 ≦ X ≦ 0.0650), V _(1-X) Ta _X O ₂ (0 ≦ X ≦ 0.117), V _(1-X). Nb _X O ₂ (0 ≦ X ≦ 0.115), V _(1-X) Ru _X O ₂ (0 ≦ X ≦ 0.150), V _(1-X) Mo _X O ₂ (0 ≦ X ≦ 0) 161), V _(1-X) Re _X O ₂ (0 ≦ X ≦ 0.0964), LiMn ₂ O ₄ , LiVS ₂ , LiVO ₂ , NaNiO ₂ , LiRh ₂ O ₄ , V ₂ O ₃ , V _{4 O 7, V 6 O 11,} Ti 4 O 7, SmBaFe 2 O 5, EuBaFe 2 O 5, GdBaFe 2 O 5, TbBaFe 2 O 5, DyBaFe 2 O 5, HoBaFe 2 O 5, YBaFe 2 O 5, PrBaCo 2 _{O 5.5, DyBaCo 2 O 5.54,} HoB Co ₂ O _5.48, the heat storage material according to claim 1, characterized in that either YBaCo ₂ O _5.49. The heat storage material according to claim 2, wherein the value of X is selected according to a target temperature of electronic phase transition.

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