Method for producing latent heat storage material

Abstract

To provide a method for producing, with high productivity, an alloy-based latent heat storage material that can be used at relatively high temperatures and has excellent heat storage density and thermal conductivity.SOLUTION: The present invention provides a method for producing an alloy-based latent heat storage material that comprises a core particle composed of an Al-Si alloy coated with an aluminum oxide coat. The method includes a coat formation step for forming the aluminum oxide coat on the core particle, in which the core particle is added into water containing an inorganic acid salt of an alkaline-earth metal and / or an alkali metal of 0.015 mass% or more and 1 mass% or less. The production method can increase a charge ratio to a reaction container, leading to improved productivity.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a latent heat storage material, and more particularly to a method for producing a micro-sized latent heat storage material that can be used at relatively high temperatures and has excellent heat storage density and thermal conductivity. [Background technology]

[0002] Known methods for storing heat include sensible heat storage that utilizes temperature changes (for example, Patent Document 1) and latent heat storage that utilizes phase changes of substances (for example, Patent Document 2).

[0003] Of these, sensible heat storage technology can store heat at high temperatures, but because it uses only the sensible heat generated by temperature changes in a substance, it has the problem of low heat storage density. A latent heat storage technology that stores heat using the latent heat of molten salts, etc., has been proposed as a way to solve this problem.

[0004] Various types of heat storage bodies have been proposed for use in latent heat storage technology. For example, Patent Document 3 discloses inventions such as a latent heat storage capsule characterized by having one, two, or three layers of metal coating formed on the surface of a latent heat storage material, and a method for manufacturing a latent heat storage capsule characterized by coating a metal coating on a latent heat storage material by electroplating.

[0005] Furthermore, Patent Document 4 discloses that in a latent heat storage medium consisting of a core portion and a shell covering the core portion, the core particles are subjected to a chemical coating treatment and then subjected to a thermal oxidation treatment to form an oxide coating on the shell. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 6-50681 [Patent Document 2] Japanese Patent Application Publication No. 10-238979 [Patent Document 3] Japanese Patent Application Publication No. 11-23172 [Patent Document 4] International Publication No. 2015 / 162929 Brochure Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the production of alloy-based latent heat storage microcapsules (Micro-Encapsulated Phase Change Material: hereinafter abbreviated as MEPCM) disclosed in Patent Document 4, foaming occurs violently during the chemical conversion coating treatment, and there is a risk of the reaction liquid overflowing from the reaction vessel. In practice, it is necessary to carry out the reaction with less than half the amount charged to the reaction vessel, which results in extremely low productivity.

[0008] The present invention has been made in consideration of the above-mentioned conventional problems, and aims to provide a highly productive method for manufacturing a latent heat storage material that can be used even at relatively high temperatures and has excellent heat storage density and thermal conductivity. [Means for solving the problem]

[0009] The present inventors have conducted various studies to achieve the above-mentioned object and have arrived at the present invention, which is a method for producing an alloy-based latent heat storage material in which core particles made of an Al-Si alloy are coated with an aluminum oxide film, wherein the coating step of forming the aluminum oxide film on the core particles comprises adding the core particles to water containing 0.015 mass % or more and 1 mass % or less of an inorganic acid salt of an alkaline earth metal and / or an alkali metal. [Effects of the Invention]

[0010] The production method of the present invention allows the charging ratio to the reaction vessel to be increased compared to conventional methods, thereby improving the productivity of the alloy-based latent heat storage material, and thereby enabling the supply of molded bodies of the latent heat storage material at lower cost. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will be described in detail below. It should be noted that a combination of two or more of the individual preferred embodiments of the present invention described below is also a preferred embodiment of the present invention. [Method of manufacturing the latent heat storage material of the present invention] The manufacturing method of the present invention is a method for manufacturing an alloy-based latent heat storage material in which core particles made of an Al-Si alloy are coated with an aluminum oxide coating, and is characterized in that the coating formation step of forming an aluminum oxide coating on the core particles comprises adding the core particles to water containing 0.0015 mass % or more and 1 mass % or less of an inorganic acid salt of an alkaline earth metal and / or alkali metal.

[0012] The coating formation process of the present invention is a process of forming an aluminum oxide coating on the Al-Si alloy core particles by boehmite treatment, and is a method of generating a coating on the surface of the core particles in high-temperature water containing inorganic acid salts of alkaline earth metals and / or alkali metals.

[0013] The content of the inorganic acid salt is 0.0015% by mass or more, preferably 0.002% by mass or more, more preferably 0.005% by mass or more, even more preferably 0.008% by mass or more, and particularly preferably 0.01% by mass or more. When the content of the inorganic acid salt is 0.0015% by mass or more, a foaming suppression effect can be expected, but the higher the content, the greater the foaming suppression effect. The upper limit of the content of the inorganic salt is 1% by mass or less, more preferably 0.5% by mass or less, even more preferably 0.3% by mass or less, and particularly preferably 0.2% by mass or less. The higher the content of the inorganic acid salt, the greater the foaming suppression effect, but even if the content exceeds 1% by mass, the foaming suppression effect does not change significantly. By including the inorganic acid salt within the above range, foaming when core particles are added is suppressed, allowing for efficient production.

[0014] The alkaline earth metals include magnesium, calcium, strontium, and barium, with magnesium and calcium being preferred.

[0015] The alkali metals include lithium, sodium, potassium, rubidium, and cesium, with sodium and potassium being preferred.

[0016] Examples of the inorganic acid salts include sulfates, carbonates, hydrochlorides, hydrobromides, hydroiodides, phosphates, borates, sulfites, nitrates, perchlorates, chlorates, chlorites, hypochlorites, and phosphites. Sulfates, carbonates, hydrochlorides, and phosphates are preferred, sulfates, carbonates, and phosphates are more preferred, and sulfates are most preferred.

[0017] The Al-Si alloy may be any alloy made of aluminum and silicon, with the silicon content preferably in the range of 4% to 40%, more preferably 10% to 30%, and particularly preferably 12% to 25%.Within this range, large heat absorption and heat generation occur at the eutectic temperature of 580°C, so that the material functions as a heat storage material with a large heat storage capacity, while the volume expansion rate during the phase change from solid to liquid can be kept low, thereby increasing the durability of the MEPCM.

[0018] In the coating formation step, aluminum hydroxide is preferably added before adding the core particles to prevent sintering and agglomeration of MEPCM particles during the thermal oxidation treatment step. The amount of aluminum hydroxide to be added is 0.5 to 25 parts by mass, more preferably 1 to 20 parts by mass, per 100 parts by mass of core particles. Adding 0.5 parts by mass or more can prevent agglomeration, and the greater the amount added, the more agglomeration can be prevented during the thermal oxidation treatment step. However, adding 25 parts by mass or more does not significantly affect the effect on agglomeration. On the other hand, adding 25 parts by mass or more increases the amount of aluminum oxide that forms in the alloy after the thermal oxidation treatment step, thereby reducing the amount of heat stored, so the above range is preferable.

[0019] The crystal structure of the aluminum hydroxide is not particularly limited, and examples thereof include bayerite, gibbsite, and boehmite. Gibbsite and boehmite are preferred, and boehmite is particularly preferred because it has a small weight change during the thermal oxidation treatment step and can reduce defects.

[0020] The coating formation step may be carried out by adding a small amount of ammonia, etc. It has been confirmed that the higher the pH value of the solution used in the boehmite treatment, the better the quality of the resulting aluminum oxide coating, and it is particularly preferable to set the pH value in the range of 6.0 or more to less than 11.0, more preferably 7.0 to 10.0, and most preferably 8.5 to 9.5.

[0021] The reaction temperature in the coating formation step is preferably 60°C to 100°C, more preferably 70°C or higher, even more preferably 80°C or higher, and particularly preferably 90°C or higher. The upper limit of the temperature is the boiling point of the aqueous solution, which is 100°C under normal pressure. The reaction time is preferably 0.25 to 24 hours, more preferably 0.5 to 5 hours. A higher temperature and a longer reaction time allow for the formation of a better coating, and therefore the aluminum oxide coating after the thermal oxidation treatment step will also be better, and a highly durable coating can be obtained.

[0022] The manufacturing method of the present invention preferably further comprises a thermal oxidation treatment step, which further oxidizes the aluminum oxide coating formed in the coating formation step, thereby converting it into MEPCM, which is crystalline Al2O3.

[0023] The temperature of the thermal oxidation treatment process is preferably higher than the melting point of the Al-Si alloy that constitutes the core. For an alloy containing 25 wt% silicon, the melting point is 580°C, and heating at a higher temperature (e.g., 700°C or higher) is preferred. Treatment at 800°C or higher is more preferred, and 900°C or higher is even more preferred. This is because the aluminum oxide coating formed by heat treatment has a γ-Al2O3 crystal structure at relatively low temperatures below 800°C, while a coating with the chemically stable α-Al2O3 crystal structure is obtained at relatively high temperatures above 880°C. While there are no particular limitations on the upper limit, it is preferably 1300°C or lower. If the temperature of the thermal oxidation treatment process exceeds 1300°C, the thickness of the aluminum oxide coating increases, reducing the proportion of the Al-Si alloy and the heat storage capacity of the MEPCM. The heat treatment time is preferably 0.5 to 12 hours, more preferably 2 to 5 hours. By carrying out the thermal oxidation treatment process, it is possible to obtain a MEPCM having high repetitive stability.

[0024] The strength of the thermal oxide coating obtained in the above process may be further enhanced by forming a coating thereon. For example, a metal coating and / or a ceramic coating may be formed by chemical or physical treatment. For example, chemical methods include the sol-gel method, CVD, electroplating, and electroless plating, while physical methods include PVD. The mechanical strength of the capsule can be enhanced by applying a metal coating and / or a ceramic coating over the coating. [Example]

[0025] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified, "parts" means "parts by mass" and "%" means "% by mass."

[0026] <Measurement of average particle size> The measurement was carried out using a laser diffraction particle size distribution analyzer LA-950V2 manufactured by HORIBA. Specifically, an Al-Si alloy was dispersed in an aqueous solution in which 0.2% of sodium pyrophosphate (manufactured by Fuji Film Wako Pure Chemical Corporation) was dissolved, and the measurement was performed with a particle size distribution analyzer. The value of the cumulative 50% volume diameter was taken as the average particle diameter.

[0027] <Al-Si alloy> Core particles composed of an Al-Si alloy (Al-25wt%Si) with a weight ratio of Al of 75% and a weight ratio of Si of 25% were prepared. The diameters of these core particles were less than 38 μm, and the average diameter was 36.3 μm.

[0028] [Example 1] Calcium sulfate (manufactured by Fuji Film Wako Pure Chemical, calcined gypsum: CaSO4·1 / 2H2O) was put into a flask with a diameter of 8.5 cm and a height of 25 cm so that the calcium sulfate was 0.02% by mass with respect to water, and then 500 g of water was added. It was heated using an oil bath while stirring at 150 rpm with a stirring blade. After measuring the water temperature and reaching 100 °C, 5 g of aluminum hydroxide (boehmite) was added and dispersed. Then, ammonia water adjusted to 1 M was added, and the pH of the dispersion was adjusted so that it was in the range of 9.0 to 9.5 when measured at room temperature. 50 g of an Al-Si alloy was added to the dispersion after pH adjustment. After addition, it was continuously stirred for 2 hours while adjusting the pH, and then cooled. Regarding the rise of the liquid level due to the generated bubbles after adding the Al-Si alloy, the difference between the highest value: 14.5 cm and the height of the liquid level in the state of standing still after cooling: 10.5 cm, which was 4.0 cm, was taken as the foam height.

[0029] [Examples 2 to 8, Comparative Examples 1 to 3] In Example 1, the treatment was carried out in the same manner as in Example 1 except that magnesium sulfate, potassium sulfate, sodium sulfate, calcium chloride, sodium chloride, calcium carbonate, sodium metaphosphate, sodium acetate, sodium stearate, and sodium dodecyl sulfate were used instead of calcium sulfate, and the foam height was examined. The foam height was evaluated as follows: 5 cm or less: ⊚, 5 cm to 10 cm: ◯, and over 10 cm: ×. The results are shown in Table 1.

[0030] [Table 1]

[0031] [Example 9] The treatment was carried out in the same manner as in Example 1, except that the calcium sulfate concentration was adjusted to 0.01% by mass, and the foam height was examined. The foam height was 6.5 cm, and the evaluation was good.

[0032] [Example 10] The treatment was carried out in the same manner as in Example 9 except that magnesium sulfate was used instead of calcium sulfate, and the foam height was examined. The foam height was 6 cm and the evaluation was good.

[0033] [Example 11] The treatment was carried out in the same manner as in Example 1, except that the calcium sulfate concentration was adjusted to 0.01% by mass and the magnesium sulfate concentration was adjusted to 0.01% by mass, and the foam height was measured. The foam height was 3.5 cm and was evaluated as excellent.

[0034] [Example 12] The treatment was carried out in the same manner as in Example 1, except that magnesium sulfate was used instead of calcium sulfate to adjust the concentration to 0.2 mass %, and the foam height was examined. The evaluation was ⊚.

[0035] Comparative Example 4 The treatment was carried out in the same manner as in Example 1, except that the calcium sulfate concentration was adjusted to 0.001% by mass, and the foam height was examined. The foam height exceeded 10 cm, and the evaluation was poor.

[0036] [Example 13] The calcium sulfate described in Example 1 was added to a 3 L reactor so that the amount was 0.02% by mass relative to the amount of water added, and then 2.4 kg of water was added. The mixture was heated in an oil bath while stirring at 150 rpm with a stirring blade. After the water temperature was measured and reached 80°C, 24 g of aluminum hydroxide (boehmite, manufactured by Taimei Chemical Industry Co., Ltd.) was added and dispersed. Ammonia water adjusted to 1 M was then added, and the pH of the dispersion was adjusted to a range of 9.0 to 9.5 when measured at room temperature. After the pH adjustment, 240 g of Al-Si alloy was added to the dispersion liquid. After the addition, the water temperature was raised to 100°C, and stirring was continued for 2 hours while adjusting the pH, and then the dispersion liquid was cooled. When the Al-Si alloy was poured in, no rise in the liquid level due to foaming was observed.

[0037] Comparative Example 5 The treatment was carried out in the same manner as in Example 13 except that calcium sulfate was not used. However, after the Al-Si alloy was added, the generated bubbles overflowed from the 3 L reactor, so the treatment was interrupted. As a result, the treatment could not be completed.

[0038] <Heat storage measurement> The heat storage capacity was measured using a TG-DSC (TA Instruments SDT650). First, the sample was placed in an alumina sample pan and oxidized under air flow at 200 ml / min, heating rate at 10 °C / min, and firing conditions at 1150 °C for 3 hours to produce MEPCM. The sample was then allowed to cool to room temperature and again heated to 700 °C at a heating rate of 10 °C / min under nitrogen flow at 20 ml / min, and the heat storage capacity (latent heat / endothermic heat) was measured (first run). The temperature was then lowered to below 100 °C at a heating rate of 5 °C / min, and the heat release capacity (latent heat / heat release capacity) was measured (first run). The sample was again heated to 700 °C at a heating rate of 10 °C / min, and the heat storage capacity (latent heat / endothermic heat) was measured (second run). Thereafter, the temperature was lowered at a rate of 5°C / min until it reached 100°C or below, and the amount of heat released (latent heat / heat release amount) was measured (second time). The results are shown in Table 2.

[0039] [Table 2]

[0040] The MEPCM obtained from Example 1 had a first heat storage capacity of 179 J / g and a heat release capacity of 176 J / g, and a second heat storage capacity of 180 J / g and a heat release capacity of 176 J / g, confirming that it functions as a heat storage material and can function repeatedly. Similarly, it was confirmed that the MEPCMs obtained from Examples 2 and 13 also function repeatedly.

Claims

1. A method for producing an alloy-based latent heat storage material in which core particles are coated with a coating, the method comprising the step of forming an aluminum oxide coating on core particles made of an Al-Si alloy, the coating forming step includes adding core particles to water containing 0.015% by mass or more and 0.2% by mass or less of an inorganic acid salt of an alkaline earth metal and / or an alkali metal; the inorganic acid salt of an alkaline earth metal and / or an alkali metal is any one selected from calcium sulfate, magnesium sulfate, potassium sulfate, and sodium metaphosphate; In the coating forming step, the core particles are added to the water when the temperature of the water is 70°C or higher, The core particles have a volume average particle size of less than 38 μm, A method for producing an alloy-based latent heat storage material, characterized in that in the coating formation step, 1 to 20 parts by mass of aluminum hydroxide is added to 100 parts by mass of core particles.

2. 2. The method according to claim 1, wherein the film-forming step is carried out at a pH of 8 or higher.

3. 3. The method according to claim 1, wherein aluminum hydroxide having a crystal structure of boehmite or gibbsite is used as the aluminum raw material for forming the aluminum oxide coating.

4. 4. The method according to claim 1, further comprising the step of: subjecting the substrate to a thermal oxidation treatment at a temperature of 900° C. to 1300° C. after the film forming step.

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