Microcapsules for latent heat storage materials, a method for producing the same, a powder containing the microcapsules for latent heat storage materials, and a heat storage device containing the powder

Abstract

To provide a microcapsule for a latent heat storage material with high repetition durability.SOLUTION: A microcapsule for a latent heat storage material comprises a metal core containing Zn and Al and a shell that covers the metal core. The shell of the microcapsule contains a first oxide film containing Zn and O, and a second oxide film that is adjacent to the inside of the first oxide film and contains Al and O. The microcapsule is produced by a method including the following steps: providing metal particles containing Zn and Al; subjecting the alloy particles to chemical conversion in an Al-containing solution; performing crystallization after the chemical conversion; and performing oxidation after the crystallization.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a microcapsule for a latent heat storage material, a method for manufacturing the same, a powder containing the microcapsule for the latent heat storage material, and a heat storage device containing the powder.

Background Art

[0002] Technologies for storing heat include, for example, sensible heat storage and latent heat storage. Sensible heat storage utilizes the temperature change of a heat storage body. On the other hand, latent heat storage utilizes, for example, the phase change of a heat storage body from a solid phase to a liquid phase.

[0003] Patent Document 1 discloses a latent heat storage body, where the latent heat storage body is composed of core particles of an Al-Si alloy and a shell of an Al oxide film covering the core particles. Regarding the shell, Patent Document 1 discloses that the core particles can be subjected to a chemical conversion film treatment, further subjected to a thermal oxidation treatment, and thereby an oxide film can be formed.

[0004] Patent Document 2 discloses that the core particles of the latent heat storage body are an alloy (A-B alloy) of at least one alloy component A selected from the following Group A and at least one alloy component B selected from the following Group B. Group A: Ca, Si, Bi, Mg, Sb, In, Sn, Al, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Pd, Ag, Au, Pb Group B: Al, Cr, Mn, Si, Mg, Co, Ni Furthermore, Patent Document 2 discloses that the following relationship is satisfied. ΔG A 0 ≧ΔG B 0 (ΔG A 0 ): The standard free energy of oxide formation of the alloy component A (ΔG B 0 ): The standard free energy of oxide formation of the alloy component B

[0005] Patent Document 3 discloses a latent heat storage material aimed at reducing the occurrence of leakage in the core part, and discloses that the BET specific surface area is 10 m 2 / g or more. It also discloses the use of a specific amine compound during production, particularly during boehmite treatment.

[0006] Non-Patent Document 1 discloses the following three types of alloys. Zn 84 Al 8.7 Mg 7.3 、 Zn 88.7 Al 11.3 、 Zn 92.2 Mg 7.8 (Each number represents at.%) And Non-Patent Document 1 discloses that it evaluated the possibility of whether these alloys can be used as phase change materials for latent heat storage. Furthermore, according to the document, the melting points are 344 °C, 382 °C, and 371 °C respectively, and the latent heats of fusion are 132 J / g -1 、118 J / g -1 、106 J / g -1 respectively.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0008]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0009] The temperature range of automobile exhaust is wide. For example, it is about 200°C during idling and about 800°C during full-load operation. The exhaust system of an automobile is equipped with an exhaust purification catalyst. However, if the exhaust temperature is too high, it may cause a decrease in performance and / or deterioration of the exhaust purification catalyst. Therefore, for the purpose of suppressing an excessive rise in exhaust temperature, it is conceivable to install a latent heat storage body in the exhaust system of an automobile to perform heat exchange. On the other hand, if the exhaust temperature is too low, the performance of the exhaust purification catalyst cannot be fully exerted. From the above viewpoints, the inventor considered that it might be possible to suppress excessive fluctuations in the exhaust temperature of an automobile by using a latent heat storage body having an appropriate operating temperature of about 300 to 550°C and an excellent latent heat amount.

[0010] And the inventor considered using a combination of Al and Zn as a core material suitable for such a temperature range. The reason for this is that by adjusting the ratio of Al and Zn, it is possible to adjust the melting point of the binary alloy of Al-Zn to the above temperature range.

[0011] However, the characteristics required for the microcapsules for latent heat storage materials are not only the temperature range of the melting point of the core part, but also the characteristics (repeated durability) that can maintain the structure even when repeated severe temperature changes occur. What is important for realizing such repeated durability is the characteristics of the shell. The shell is generated by a chemical reaction or the like occurring in a part of the components of the core.

[0012] The present invention has been created in view of the above circumstances, and an object thereof is to provide microcapsules for a latent heat storage material having excellent repeated durability.

Means for Solving the Problems

[0013] As a result of intensive studies by the present inventors, when a crystallization treatment was carried out as part of the process for forming the shell, it was found that the finally obtained shell gave excellent results in the repeated durability test. The present invention has been completed based on such findings and, in one aspect, includes the following inventions.

[0014] (Invention 1) A microcapsule for a latent heat storage material comprising a metal core containing Zn and Al and a shell covering the metal core, The shell of the microcapsule comprises an oxide film containing Zn and O and an oxide film containing Al and O adjacent to the inside of the oxide film, A microcapsule for a latent heat storage material produced by a method including the following steps: · A step of providing metal particles containing Zn and Al, · A step of performing a formation treatment on the alloy particles with a solution containing Al, · A step of performing a crystallization treatment after the formation treatment, and, · A step of performing an oxidation treatment after the crystallization treatment. (Invention 2) The microcapsule for a latent heat storage material according to Invention 1, wherein the step of performing the oxidation treatment includes raising the temperature to a holding temperature under the condition that the rate of temperature increase is 10 ° C / min or more and holding in the holding temperature and an oxygen-containing atmosphere. (Invention 3) The formation treatment includes performing at a temperature of 80 ° C or higher for 3 hours or more, The crystallization treatment includes performing at a temperature of less than 80 ° C for 10 hours or more, The microcapsule for a latent heat storage material according to Invention 1 or 2. (Invention 4) When the total mass of Al and Zn in the core in the microcapsule is 100 parts by mass, the microcapsule for a latent heat storage material according to any one of Inventions 1 to 3, wherein Zn is 60 to 95 parts by mass and Al is 5 to 40 parts by mass. (Invention 5) The microcapsule for a latent heat storage material according to any one of Inventions 1 to 4, wherein the average thickness of the oxide film containing Al and O is 100 to 500 nm. (Invention 6) The microcapsule for a latent heat storage material according to any one of Inventions 1 to 5, wherein the average thickness of the oxide film containing Zn and O is 100 to 1500 nm. (Invention 7) The microcapsule for a latent heat storage material according to any one of Inventions 1 to 6, wherein the mass ratio of ZnAl2O4 obtained by analyzing with XRD (X-ray diffractometer) and performing quantitative analysis using the RIR (Reference Intensity Ratio) method on the result is more than 4%. (Invention 8) The microcapsule for a latent heat storage material according to any one of Inventions 1 to 7, wherein the volume expansion ratio when the core of the microcapsule melts is 5 to 9%. (Invention 9) A powder containing a plurality of microcapsules for a latent heat storage material according to any one of Inventions 1 to 8. (Invention 10) The powder according to Invention 9, having an average particle size of 20 to 80 μm. (Invention 11) The powder according to Invention 10, having an average particle size of 20 to 38 μm. (Invention 12) A heat storage device including the powder according to any one of Inventions 9 to 11. (Invention 13) The heat storage device according to Invention 12, which is installed on the outer periphery of an exhaust passage of an automobile. (Invention 14) A method for manufacturing a microcapsule for a latent heat storage material, comprising a metal core containing Zn and Al and a shell covering the metal core, wherein the shell of the microcapsule includes an oxide film containing Zn and O and an oxide film containing Al and O adjacent to the inside of the oxide film. A method comprising the following steps: ·Providing metal particles containing Zn and Al; ·Performing a formation treatment on the alloy particles with a solution containing Al; ·Performing a crystallization treatment after the formation treatment, and ·Performing an oxidation treatment after the crystallization treatment. (Invention 15) The method according to Invention 14, wherein the step of performing the oxidation treatment includes heating to a holding temperature under the condition that the heating rate is 10 °C / min or more and holding in the holding temperature and an oxygen-containing atmosphere. (Invention 16) The formation treatment includes performing at a temperature of 80 °C or higher for 3 hours or more, The crystallization treatment includes performing at a temperature lower than 80 °C for 10 hours or more, The method according to Invention 14 or 15. [Advantages of the Invention]

[0015] On one hand, the microcapsules for latent heat storage material of the present invention are produced by performing a crystallization treatment. As a result, defects are generated relatively uniformly in the shell. The existence of such uniformly dispersed defects can prevent the force from concentrating at specific locations and breaking the shell. Therefore, the repeated durability is improved. [Brief Description of the Drawings]

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0017] <1. Microcapsules for Latent Heat Storage Material> (1-1. Overall Composition of Microcapsules) In one embodiment, the microcapsules for latent heat storage material according to the present invention contain Al and Zn. The melting point of Al is 660.3 °C, and the melting point of Zn is 419.5 °C. Therefore, as the ratio of Zn combined with Al is increased, the melting point of the microcapsules can be lowered. Also, even though the melting point decreases, on the other hand, the microcapsules containing Al and Zn also have the characteristic that the latent heat per unit volume during melting is large. Furthermore, since Al has a large volume expansion rate during melting, by adding Zn with a small volume expansion rate during melting, the volume expansion during the phase change from the solid phase to the liquid phase can be alleviated. However, if the ratio of Zn added to Al becomes too high, an AlOOH film as a precursor of the aluminum oxide film is not sufficiently formed during the chemical conversion treatment. For this reason, it is considered that it becomes difficult to form a sufficient aluminum oxide film in the subsequent oxidation treatment. Therefore, there is an appropriate content ratio of Al and Zn.

[0018] Therefore, in the core of the microcapsules for latent heat storage material according to a preferred embodiment of the present invention, the Zn content is 60 to 95 parts by mass, and the Al content is 5 to 40 parts by mass (here, the total mass of Al and Zn in the core of the microcapsules is 100 parts by mass). In the core of the microcapsules for latent heat storage material according to a more preferred embodiment of the present invention, the Zn content is 60 to 90 parts by mass, and the Al content is 10 to 40 parts by mass. In the core of the microcapsules for latent heat storage material according to an even more preferred embodiment of the present invention, the Zn content is 60 to 80 parts by mass, and the Al content is 20 to 40 parts by mass.

[0019] The metal core may contain one or more third elements (e.g., Sn, Bi, Cu, In, Ni, etc.). However, for the purpose of preventing unexpected property changes, the microcapsules preferably contain a total of 70% by mass or more, more preferably 90% by mass or more, and even more preferably 95% by mass or more of Zn and Al, based on the total mass of the microcapsules. The microcapsules may be composed of only Zn and Al, excluding oxygen (O) and inevitable impurities.

[0020] In relation to the above-described ratio of Al to Zn, in one embodiment, the core volume expansion rate of the microcapsules for latent heat storage materials according to the present invention during melting is 5 to 9%, preferably 6 to 9%, and more preferably 7 to 9%. Within these ranges, even if a physical force acting on the shell is applied when melting is repeated, the oxide film constituting the shell is less likely to be damaged. Note that the expansion rate of the core volume changes according to the ratio of the constituent elements. For example, it is possible to obtain the volume expansion rate between the solidus temperature and the liquidus temperature by Factsage (manufactured by the Computational Mechanics Research Center, Ver. 7.3). For example, by setting the conditions within the following ranges, the volume expansion rate of a binary alloy of Al-Zn at a predetermined composition ratio can be obtained. · Database: FTLite · Elements: Al, Zn · Calculated composition: 5 to 40 wt% Al (in 5 wt% increments) · Temperature: 0 to 600 °C (in 1 °C increments) · Pressure: 1 atm

[0021] In this specification, the volume expansion rate of the microcapsules for heat storage latent heat materials during melting means the predicted result of software (Factsage), not the measured value.

[0022] (1-2. Structure of Microcapsules) In one embodiment, the microcapsules for latent heat storage materials according to the present invention have a metal core containing Zn and Al and a shell covering the metal core.

[0023] The metal core contains Zn and Al. In one embodiment, Zn and Al in the metal core can exist in the form of a Zn-Al alloy (for example, a binary alloy of Zn-Al).

[0024] In one embodiment, for the purpose of preventing unexpected property changes, the metal core can contain a total of 70% by mass or more of Zn and Al, preferably 90% by mass or more, and more preferably 95% by mass or more. The metal core may be composed of only Zn and Al, excluding inevitable impurities.

[0025] In one embodiment, the shell has at least two membranes (for example, a double membrane). These membranes may be composed of an oxide film containing Zn and O (for example, an oxide film containing Zn, Al, and O, an oxide film of ZnAl2O4, or a combination of an oxide film of ZnO and an oxide film of ZnAl2O4, etc.), and an oxide film containing Al and O adjacent to the inside of the oxide film (for example, an oxide film containing Al2O3, an oxide film containing Al and O and not containing Zn, etc.). Furthermore, the oxide film containing Zn and O may be a combination of an oxide film composed of Zn and O and an oxide film composed of Zn, Al, and O. In the case of such a combination, typically, the oxide film composed of Zn, Al, and O may be located inside the oxide film composed of Zn and O. And the oxide film may form a triple membrane of the oxide film containing Al and O described above, in addition to the oxide film composed of Zn and O and the oxide film composed of Zn, Al, and O. The presence of these membranes can be detected by analyzing the cross-section of the microcapsule by AES (Auger electron spectroscopy). For example, by performing AES analysis on the shell of the microcapsule, an oxide film containing Zn and O and an oxide film containing Al and O adjacent to the inside of the oxide film are detected. The metal core is protected doubly or triply by the shell having a double membrane or a triple membrane. Therefore, leakage of phase change substances such as Zn and Al in the metal core is prevented.

[0026] In one embodiment, the mass ratio of ZnAl2O4 in the microcapsules for latent heat storage materials of the present invention is more than 4%. The mass ratio of ZnAl2O4 is obtained by analyzing with an XRD (X-ray diffractometer) and performing quantitative analysis using the RIR (Reference Intensity Ratio) method on the results. Specifically, when the total mass of each crystal phase present in the microcapsules is taken as 100% and quantitative analysis is performed using the RIR method, it is a microcapsule in which the mass ratio of ZnAl2O4 is more than 4%. There is a certain relationship between the mass ratio of ZnAl2O4 and the amount of defects in the oxide film (or the amount of surface defects). Whether it is a correlation or a causal relationship is unclear, but for example, when the mass ratio of ZnAl2O4 is large, there is a tendency for more defects in the oxide film.

[0027] From the perspective of leakage of the core component, having many defects in the oxide film is not preferable. Therefore, when a person skilled in the art obtains such knowledge, they may attempt to manufacture microcapsules for latent heat storage materials with a low mass ratio of ZnAl2O4. However, unexpectedly, the present inventor has found that by introducing appropriate defects (cracks) in the oxide film into the shell in a highly uniform manner, the repeated durability is improved.

[0028] The following explanation is not intended to limit the scope of the invention, but it is considered that the uniform dispersion of an appropriate amount of defects in the oxide film leads to preventing the concentration of force at specific defect sites. In other words, when the temperature change is repeated, the expansion and contraction of the core are repeated, and physical force is applied to the shell. Here, if there are defects in the shell only at specific locations, it is considered that the physical force from the core will concentrate and ultimately lead to the leakage of the core component.

[0029] However, in the microcapsules for latent heat storage materials of the present invention in one embodiment, appropriate defects exist in the shell, and furthermore, it is considered that, by the manufacturing method described later, the defects are dispersed in a highly uniform form. Thereby, the physical force from the core can be dispersed, and it is considered that the repeated durability is improved. The upper limit of the mass ratio of ZnAl2O4 is not particularly limited, but if it is too high, an extreme amount of defects will be introduced and the durability of the shell structure will be impaired. Therefore, typically, it may be 15% or less, or may be 11% or less.

[0030] Note that the RIR (Reference Intensity Ratio) method is a method for obtaining the mass ratio of a crystal phase from the ratio of the RIR value and the value of the strongest peak intensity of each crystal phase obtained from the XRD (X-ray diffractometer) result. When crystal phases A, B, C, ··· are included in the microcapsules, the mass ratio X A of crystal phase A is calculated by the following formula. X A =I A k A / (I A k A +I B k B +I C k C +···) Here, I represents the intensity of the strongest peak of the X-ray of each crystal phase, and k represents the RIR value of each crystal phase. As the RIR value, the value described in the powder diffraction file (PDF) database of the International Center for Diffraction Data can be used (for example, ICSD No. 01-070-8186).

[0031] The average thickness of the oxide film containing Al and O can be, for example, 100 to 500 nm, and typically can be 300 to 500 nm. The average thickness of the oxide film containing Zn and O can be, for example, 100 to 1500 nm, and typically can be 500 to 1500 nm. Further, when the oxide film containing Zn and O includes an oxide film composed of Zn and O and an oxide film composed of Zn, Al, and O, the oxide film composed of Zn and O may be 0 to 1000 nm, preferably 0 to 900 nm, and the oxide film composed of Zn, Al, and O may be 500 to 1500 nm. Thereby, most of Al and Zn constituting the microcapsule are present in the core and contribute to the latent heat storage function by the solid-liquid phase change. Therefore, the advantage of being able to increase the latent heat amount per unit volume is obtained.

[0032] In this specification, the average thickness constituting the shell (specifically, the average thickness of the oxide film containing Al and O and the average thickness of the oxide film containing Zn and O) is measured by the following methods, respectively. Observe the cross section of the microcapsule by SEM, measure the thickness of the oxide film at three or more positions for one microcapsule, and calculate the average value of these measured values. Perform the same operation for three or more microcapsules, and calculate the average value of the thickness of the oxide film of each microcapsule. Then, calculate the overall average value of the oxide film of these three or more microcapsules, and adopt this value as the average thickness. The type of the oxide film can be specified by AES (Auger electron spectroscopy) analysis.

[0033] In one embodiment, the microcapsule for latent heat storage material according to the present invention has such an extremely thin shell on the order of nanometers and at the same time has the characteristic that surface defects exist. Such a microcapsule for latent heat storage material can be a microcapsule for latent heat storage material excellent in latent heat amount and durability.

[0034] (1-3. Melting point of microcapsule) In one embodiment, the melting point of the microcapsules for latent heat storage material according to the present invention is 300 to 550 °C, preferably 420 to 520 °C.

[0035] In this specification, the melting point of the microcapsules refers to the melting start temperature when differential scanning calorimetry (DSC) is performed.

[0036] (1-4. Size of Microcapsules) In one embodiment, the microcapsules for latent heat storage material according to the present invention are provided in the form of a powder containing a plurality of microcapsules for latent heat storage material. In one embodiment, the average particle size of the powder is 20 to 80 μm. From the viewpoint of shell formation with few surface defects, the average particle size of the powder is preferably 20 to 50 μm, and more preferably 20 to 38 μm.

[0037] The average particle size described in this specification (for example, the average particle size of the raw material powder and the average particle size of the powder containing microcapsules for latent heat storage material) is the value measured by a laser diffraction particle size distribution analyzer (for example: HORIBA LA-920). More specifically, the volume distribution of the particle group is measured by a laser diffraction particle size distribution analyzer, and the value of the cumulative 50 volume% diameter (D50) is regarded as the average particle size.

[0038] (1-5. Latent Heat of Microcapsules) In one embodiment, the latent heat of the microcapsules for latent heat storage material according to the present invention is 70 to 120 J / g, and in a preferred embodiment, the latent heat is 100 to 115 J / g.

[0039] In this specification, the latent heat of the microcapsules refers to the heat flow change accompanying the solid-liquid phase change when differential scanning calorimetry (DSC) is performed.

[0040] (1-6. Applications of Microcapsules) According to an embodiment of the present invention, there is provided a heat storage device including a powder containing microcapsules for a latent heat storage material. In the microcapsules for a latent heat storage material according to an embodiment of the present invention, the melting point can be easily adjusted to a range of about 300 to 550°C, which can be achieved by changing the Zn content. Further, in the microcapsules for a latent heat storage material according to an embodiment of the present invention, the latent heat amount is large. Therefore, the microcapsules for a latent heat storage material can be suitably used, for example, in a latent heat storage device, and the latent heat storage device may preferably be installed in an exhaust system of an automobile for the purpose of adjusting the exhaust temperature. In one embodiment, a heat storage device including a powder containing microcapsules for a latent heat storage material can be installed on the outer periphery of an exhaust passage of an automobile. Further, the heat storage device can store not only the exhaust heat of an automobile but also various unused heats generated in the above temperature range.

[0041] <2. Method for manufacturing microcapsules for latent heat storage material> Hereinafter, a method for manufacturing microcapsules for a latent heat storage material according to the present invention will be exemplarily described.

[0042] (2-1. Raw materials) First, metal particles containing Zn and Al are provided. For example, Zn-Al binary alloy particles are prepared as raw materials. The content ratios of Al and Zn in the Zn-Al binary alloy particles can be appropriately adjusted according to the required characteristics. Typically, the Zn-Al binary alloy particles are provided in the form of powder. In one embodiment, the average particle size of the raw material powder is 20 to 80 μm. From the viewpoint of shell formation with few surface defects, the average particle size of the powder is preferably 20 to 38 μm. Further, the composition ratio of Zn-Al in the raw material may be set to a value close to the composition ratio required in the final product. Although there are some fluctuations in the composition ratio through the manufacturing process described later, there are no significant fluctuations. Therefore, by preparing and manufacturing several raw materials having a close composition ratio, microcapsules with a desired composition ratio can be easily obtained.

[0043] (2-2. Chemical conversion treatment) Next, the alloy particles are subjected to a chemical conversion treatment with a solution containing Al. For example, chemical conversion treatment (boehmite treatment) is performed on the binary alloy particles of Zn-Al as raw materials to form a film. By performing the chemical conversion treatment, a precursor of the oxide film constituting the shell can be formed. Specifically, the raw Zn-Al alloy particles are placed in hot water to generate a film on the alloy surface. At this time, Al (more specifically, aluminum hydroxide Al(OH)3, Al2O3, Al, AlOOH, etc., preferably aluminum hydroxide Al(OH)3) is added to the water. Al in the water plays a role of forming defects in the film on the particle surface by coming into contact with the particle surface during the chemical conversion treatment and the crystallization treatment described later. Therefore, if the amount is too small, sufficient defects cannot be formed. On the other hand, if the amount is too large, there is a possibility that excessive defects are introduced or the oxidation inside the core particles progresses and the latent heat storage amount decreases. For these reasons, the preferable addition amount (the addition amount converted to aluminum hydroxide) is 1.0 to 10.0 g / L, and more preferably 1.0 to 8.0 g / L.

[0044] The temperature and time are not particularly limited, but it is preferable to carry out the treatment at a temperature of 80°C or higher for 3 hours or more. For example, the Zn-Al alloy particles can be chemically converted under the conditions of a water temperature of 80 to 100°C and 3 to 12 hours. The chemical conversion treatment is preferably carried out with stirring.

[0045] (2-3. Crystallization treatment) After the chemical conversion treatment, the temperature is lowered and the crystallization treatment is carried out. Thereby, aluminum hydroxide Al(OH)3 is precipitated on the surface of the Zn-Al alloy particles. Also, as described above, by coming into contact with the particle surface, defects are formed in the film on the particle surface.

[0046] The temperature, time, etc. are not particularly limited, but it is preferably carried out at a temperature of less than 80°C, more preferably at a temperature of from room temperature (e.g., 25°C) to less than 80°C, still more preferably at a temperature of from 70 to less than 80°C for 10 hours or more. For example, the crystallization treatment can be carried out under the conditions of a water temperature of from 70 to less than 80°C and 12 to 20 hours. By carrying out the crystallization treatment over a sufficient period of time, uniform defect formation by the above-described aluminum hydroxide can be promoted. In addition, the defects can be more uniformly dispersed.

[0047] After the crystallization treatment, it may be allowed to cool naturally to lower the liquid temperature to room temperature. Then, the particles after the crystallization treatment are collected, suction-filtered, and dried. The reason for drying the particles after the crystallization treatment is to remove excess moisture on the particle surface.

[0048] (2-4. Oxidation treatment) Next, an oxidation treatment is carried out. For example, the Zn-Al binary alloy particles subjected to the crystallization treatment are oxidized. More specifically, a high-temperature treatment is carried out in an oxygen-containing atmosphere. By carrying out the oxidation treatment after the boehmite treatment (chemical conversion treatment), a shell having a double oxide film, a triple oxide film, or the like can be formed.

[0049] Although conditions such as the holding temperature, heating rate, and time of the oxidation treatment may affect defect formation, the influence of aluminum during the crystallization treatment is greater. Therefore, the holding temperature, heating rate, time, etc. of the oxidation treatment are not particularly limited.

[0050] Typically, the heating rate may be 10°C / min or more, preferably 40°C / min or more. The upper limit value may be, for example, 60°C / min or less. The oxygen-containing atmosphere may be any atmosphere containing oxygen. For example, an oxygen atmosphere in which oxygen with a purity of 99.5% is supplied at a flow rate of 200 mL / min may be used, or an air atmosphere may also be used. The holding temperature may be, for example, from 700°C to 910°C, preferably from 750°C to 850°C.

[0051] Note that the heating rate here is calculated by the following formula. Heating rate = (holding temperature - starting heating temperature) / (time required to reach the holding temperature after starting heating)

[0052] On the other hand, when lowering the temperature from the holding temperature, the cooling rate is not particularly limited and may be any rate. For example, the cooling rate may be -40 °C / min to -60 °C / min.

[0053] The time for maintaining at the holding temperature can be, for example, 30 minutes to 5 hours, preferably 1 hour to 5 hours.

Examples

[0054] Examples of the present invention are shown below together with comparative examples. These examples are provided to better understand the present invention and its advantages and are not intended to limit the present invention.

[0055] <Example 1 and Comparative Example 1> (1. Preparation of powder containing microcapsules for heat storage latent heat material) Powders containing microcapsules for heat storage latent heat material of Example 1 and Comparative Example 1 were prepared by the following procedure.

[0056] (1-1. Raw material powder) First, a binary alloy (Zn-30 mass% Al) powder of Zn-Al with a Zn mass ratio of 70% and an Al mass ratio of 30% was prepared by the rotating disk atomization method. When the average particle size of the powder was measured using a laser diffraction particle size distribution analyzer (manufactured by HORIBA, model LA-920), it was 20 to 38 μm.

[0057] (1-2. Conversion treatment) Put 300 mL of distilled water into a beaker and bring the distilled water to a boil. At that time, while stirring at a rotation speed of 500 rpm using a hot stirrer, bring it to a boil. In Example 1, aluminum hydroxide was added to the boiling water so that it became 3.3 g / L. In Comparative Example 1, boiling water was used without adding aluminum hydroxide. Add 3 g of the powder prepared above into 300 mL of boiling water at 100 °C. Then, carry out a formation treatment for 3 hours while continuing stirring. During the formation treatment, the amount of water in the beaker was maintained by adding distilled water.

[0058] (1-3. Crystallization treatment) After the formation treatment, crystallization treatment was performed on the sample of Example 1. For the sample of Comparative Example 1, crystallization treatment was not performed. Specifically, the temperature function of the hot stirrer was adjusted to lower the temperature of the water to 75 °C and maintain it for 16 h. After 16 h elapsed, the water in the beaker was allowed to cool to room temperature. Then, the powder was taken out from the beaker, subjected to suction filtration, and dried.

[0059] (1-4. Oxidation treatment) After that, the powder after the crystallization treatment was put into the sample pan of TG-DSC (manufactured by Mettler Toledo, model TGA / DSC 3+ ). Next, under the atmosphere condition of supplying oxygen instead of air at a flow rate of 200 mL / min, oxidation treatment was carried out in a stationary state. The conditions during the oxidation treatment were as follows. Heating rate: 50 K / min Holding temperature: 800 °C Holding time: 3 h Cooling rate: -50 K / min The heating rate and the cooling rate were calculated according to the following formula.

[0060] Heating rate = (holding temperature - 25 °C) / (time required to reach the holding temperature from 25 °C) Cooling rate = (25 °C - holding temperature) / (time required to reach 25 °C from the holding temperature after the start of cooling)

[0061] (2. Property evaluation) For the powders containing microcapsules of the heat storage latent heat material of Example 1 (MEPCM03) and Comparative Example 1 (MEPCM0) obtained by the above procedure, the following characteristic analyses were performed.

[0062] (2-1. Cross-sectional observation by SEM) Grooves were formed on the surface of the carbon piece with a cutter. A sample of the powder containing microcapsules of the heat storage latent heat material was embedded in the grooves. Next, the surface of the carbon piece and the particles were shaved by CP (Cross section polisher) (more specifically, by an Ar ion beam) to expose the particle cross section. Ar ion etching was performed on this particle cross section (etching conditions are as follows: beam energy 3000 eV, ion current 4.0 μm, gas pressure 8.0×10 -2 Pa for 30 seconds). Thereby, the natural oxide film on the alloy surface in the particle cross section was removed. Then, the particle cross section was observed with an SEM (manufactured by JEOL, model JSM-7001FA).

[0063] As shown in Figure 1, for the particles in both Example 1 (MEPCM03) and Comparative Example 1 (MEPCM0), it was confirmed that a double film (shell) covering the metal core was formed on the particle surface.

[0064] (2-2. AES (Auger electron spectroscopy) analysis) After forming the particle cross section by the method described in the previous item "(2-1. Cross-sectional observation by SEM)", AES analysis (manufactured by JEOL, model JAMP-9500F) was performed.

[0065] As a result, as shown in Figure 1, in Example 1 (MEPCM03), an oxide film containing Al, Zn, and O was formed, and it was shown that an oxide film containing Al and O was formed inside. Also, for the core part, O was not detected. On the other hand, in Comparative Example 1 (MEPCM0), an oxide film containing Zn and O was formed, and it was shown that an oxide film containing Al and O was formed inside. Also, for the core part, O was not detected.

[0066] (2-3. Surface Observation by SEM (Scanning Electron Microscope)) The surface of the microcapsules for heat storage latent heat materials was observed by SEM (JEOL, model JSM-7001FA). SEM images of the particle surfaces of Example 1 (MEPCM03) and Comparative Example 1 (MEPCM0) are shown in Fig. 2. As can be seen from Fig. 2, almost no cracks were found in the particles of Comparative Example 1. On the other hand, many cracks were found in the particles of Example 1.

[0067] Also, for the particles of Example 1, SEM photographs of the sample at the time of crystallization treatment (photo number 1) and the sample at the time of oxidation treatment (photo number 4) are shown in Fig. 3. It is considered that by performing the oxidation treatment, the defects caused by aluminum hydroxide during the crystallization treatment became apparent in the form of cracks.

[0068] (2-4. Latent Heat Amount) The latent heat amount of the microcapsules for heat storage latent heat materials was determined by differential scanning calorimetry (DSC) (METTLER TOLEDO, model DSC823e). The results were as shown in Table 2.

[0069] (2-5. Durability Test) The temperature increase and decrease were repeated 100 times at a rate of 50 K / min. The maximum temperature was 600 °C and the minimum temperature was 350 °C. It was carried out in an air atmosphere. A photograph of the surface of the particles after the test observed by SEM is shown in Fig. 4. In Example 1 (MEPCM03), almost no particle disintegration was observed, indicating that it has durability against repeated temperature changes. On the other hand, in Comparative Example 1 (MEPCM0), particle disintegration was observed as shown in the part circled by a circle. Thus, although there are cracks on the surface of the particles of Example 1, in the durability test, Example 1 was superior to Comparative Example 1. Here, the number of microcapsules existing in the SEM image of ×300 after the repeated test was counted. Furthermore, the number of microcapsules existing in the same image and having a damaged shell was counted. The retention rate after the durability test was calculated from the ratio of the two. The retention rate of Comparative Example 1 was 87%. On the other hand, the retention rate of Example 1 was 97%.

[0070] (2-6. Film thickness measurement) The cross-section of the microcapsules for the heat storage latent heat material was observed by the method described in "(2-1. Cross-section observation by SEM)". Then, the average thickness of the oxide film composed of Al and O, the average thickness of the oxide film composed of Zn, Al, and O, and the average thickness of the oxide film composed of Zn and O were measured and calculated by the methods described above, respectively. The results were as shown in Table 1.

[0071] (2-7. XRD (X-ray diffraction method) analysis) The microcapsules for the heat storage latent heat material were crushed so that the core / shell structure was destroyed. The crushed sample was analyzed by an X-ray diffractometer (manufactured by Rigaku, model MiniFlex600). Then, the RIR (Reference Intensity Ratio) method was applied to the results for quantitative analysis to determine the mass ratio of ZnAl2O4. In addition to the above compound, the mass ratio of Zn and Al in the microcapsule core was also determined. Here, the mass ratio of Zn and Al described means the mass ratio of pure Zn and pure Al. In other words, it does not include Zn compounds (for example, oxides) and Al compounds (for example, oxides). Although the object to be crushed and analyzed includes the shell and the core, the pure Zn and pure Al detected here are substantially derived from the core.

[0072] For the RIR value, the value described in the powder diffraction file (PDF) database of the International Center for Diffraction Data was used (ICSD No. 01-070-8186). The analysis conditions were set as follows. · X-ray source: CuKα ray · Measurement range: 2θ = 3° to 90° · Step: 0.01° · Scan speed: 1.0° / min · Detector: High-speed one-dimensional detector D / teX Ultra2 · Tube voltage: 40 kV · Tube current: 15 mA The results were as shown in Table 2.

[0073] (2-8. Melting point) The melting point of the microcapsules for heat storage latent heat material was determined by differential scanning calorimetry (DSC) (manufactured by METTLER TOLEDO, model DSC823e). The results were as shown in Table 2.

[0074] (2 - 9. Volume expansion rate) Regarding the core volume expansion rate during melting of the microcapsules for heat storage latent heat material, based on the composition ratio of the raw material particles, the volume expansion rate between the solidus temperature and the liquidus temperature was determined by Factsage (manufactured by the Center for Computational Mechanics Research, Ver. 7.3, conditions are as follows). The results are shown in Table 2. · Database: FTLite · Elements: Al, Zn · Calculated composition: 5 - 40 wt% Al (in 5 wt% increments) · Temperature: 0 - 600 °C (in 1 °C increments) · Pressure: 1 atm

[0075] <Examples 2 - 4> Microcapsules were manufactured and evaluated in the same procedure as in Example 1 above. However, the addition amount of Al(OH)3 during the formation treatment was set as shown in Table 1.

Table 1

[0076] The evaluation results are shown in Table 2.

Table 2

[0077] From the above results, it was shown that by crystallizing Al through the crystallization treatment, changes occurred in the characteristics of the microcapsules, specifically, the repeated durability was improved.

Claims

1. A method for manufacturing microcapsules for latent heat storage materials, comprising a metal core containing Zn and Al, and a shell covering the metal core, wherein the shell of the microcapsules comprises an oxide film containing Zn and O, and an oxide film containing Al and O adjacent to the inside of the oxide film, the method comprising the following steps: - a step of providing metal particles containing Zn and Al, - a step of performing a conversion treatment on the metal particles with a solution containing Al, - a step of performing a crystallization treatment after the conversion treatment, and - a step of performing an oxidation treatment after the crystallization treatment, wherein the step of performing the conversion treatment includes forming a film on the alloy surface by immersing raw Zn-Al alloy particles in water at a temperature of 80°C or higher with Al added, the step of performing the crystallization treatment includes lowering the temperature to precipitate aluminum hydroxide Al(OH)3 on the surface of the Zn-Al alloy particles, the step of performing the oxidation treatment includes performing a heat treatment on the Zn-Al alloy particles subjected to the crystallization treatment at a temperature of 700°C or higher in an oxygen-containing atmosphere.

2. The method for manufacturing microcapsules for latent heat storage materials according to claim 1, wherein the step of performing the oxidation treatment includes raising the temperature to the holding temperature under the condition that the heating rate is 10°C / min or higher, and holding at the holding temperature in an oxygen-containing atmosphere.

3. The conversion treatment includes performing the treatment at a temperature of 80°C or higher for 3 hours or more, the crystallization treatment includes performing the treatment at a temperature lower than 80°C for 10 hours or more, The method for manufacturing microcapsules for latent heat storage materials according to claim 1 or 2.

4. When the total mass of Al and Zn in the core of the microcapsules is 100 parts by mass, Zn is 60 to 95 parts by mass and Al is 5 to 40 parts by mass. The method for manufacturing microcapsules for latent heat storage materials according to any one of claims 1 to 3.

5. The method for manufacturing microcapsules for latent heat storage materials according to any one of claims 1 to 4, wherein the average thickness of the oxide film containing Al and O is 100 to 500 nm.

6. The method for manufacturing microcapsules for latent heat storage materials according to any one of claims 1 to 5, wherein the average thickness of the oxide film containing Zn and O is 100 to 1500 nm.

7. ZnAl obtained by analyzing with an XRD (X-ray diffractometer) and performing quantitative analysis on the results using the RIR (Reference Intensity Ratio) method 2 O 4 The method for producing microcapsules for a latent heat storage material according to any one of claims 1 to 6, wherein the mass ratio of is more than 4%.

8. The method for manufacturing microcapsules for latent heat storage materials according to any one of claims 1 to 7, wherein the volume expansion rate when the core of the microcapsules melts is 5 to 9%.

9. A method for producing a powder containing a plurality of microcapsules for a latent heat storage material, using the method for producing microcapsules for a latent heat storage material according to any one of claims 1 to 8.

10. The method for producing a powder according to claim 9, wherein the average particle size is 20 to 80 μm.

11. The method for producing a powder according to claim 10, wherein the average particle size is 20 to 38 μm.

12. A method for producing a heat storage device including the powder, using the method for producing a powder according to any one of claims 9 to 11.

13. Use of the heat storage device according to claim 12, for installation on the outer periphery of an exhaust passage of an automobile.

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