Heat storage material and method for producing same

Abstract

Provided is a heat storage material that is unlikely to break or deteriorate in performance even after having undergone repeated increase and decrease in temperature. This heat storage material comprises heat storage particles (1) each having a core (10) that is formed of a zinc alloy containing aluminum, and a shell (20) that is formed of a coating film containing a metal oxide and that covers the core (10). The cores (10) each have voids (30), and have a porosity, which is the proportion of the voids (30) in the heat storage particles (1), of more than 1.4%. The porosity is the area ratio calculated by dividing the total value of areas of portions having the voids (30) in cross sections of the heat storage particles (1) by the areas of the cross sections of the heat storage particles (1).

Description

Heat storage material and its manufacturing method

[0001] The present invention relates to a heat storage material and a method for producing the same.

[0002] Phase-change heat storage materials capable of storing and releasing heat by utilizing solid-liquid phase changes are known. For example, Patent Document 1 discloses a heat storage material in which the surface of a core 100 made of a zinc-aluminum alloy is covered with a shell 200 made of a metal oxide coating (see FIG. 4). The shell 200 is provided to prevent the core 100 from oxidizing and thereby deteriorating its performance as a heat storage material.

[0003] International Publication No. 2022 / 176746

[0004] However, the heat storage material disclosed in Patent Document 1 leaves room for improvement in durability. Specifically, when the heat storage material is in use, the temperature of the heat storage material repeatedly rises and falls. As shown in FIG. 5 , the expansion of the core 100 at high temperatures may destroy the shell 200, causing the core 100 to leach out of the heat storage material. If the core 100 leach out of the heat storage material, zinc oxidizes, potentially reducing the performance of the heat storage material. In FIG. 5 , the stress caused by the expansion of the core 100 is indicated by arrows. An object of the present invention is to provide a heat storage material and a method for manufacturing the same that are resistant to destruction and performance degradation even when the temperature repeatedly rises and falls.

[0005] A heat storage material according to one embodiment of the present invention is a heat storage material comprising heat storage particles having a core made of a zinc alloy containing aluminum and a shell made of a coating containing a metal oxide and covering the core, wherein the core has voids, and the porosity, which is the proportion of voids in the heat storage particles, exceeds 1.4%, and the porosity is an area ratio calculated by dividing the total area of ​​the voids in the cross section of the heat storage particle by the cross section area of ​​the heat storage particle.

[0006] Furthermore, a method for manufacturing a heat storage material according to another aspect of the present invention is a method for manufacturing a heat storage material according to the above-mentioned aspect, and is characterized by comprising a cutting step of cutting a metal lump made of a zinc alloy to obtain cutting chips, a crushing step of crushing the cutting chips to obtain crushed particles, and a shell formation step of coating the surface of the crushed particles with a film containing a metal oxide to obtain heat storage particles.

[0007] According to the present invention, it is possible to provide a heat storage material that is resistant to breakdown and performance degradation even when temperature increases and decreases repeatedly.

[0008] Fig. 1 is a conceptual diagram showing the structure of a heat storage material according to one embodiment of the present invention. Fig. 2 is a conceptual diagram explaining expansion of a core at high temperatures in a heat storage material according to one embodiment of the present invention. Fig. 3 is a graph explaining a pulverization step in a method for manufacturing a heat storage material according to one embodiment of the present invention. Fig. 4 is a conceptual diagram showing the structure of a conventional heat storage material. Fig. 5 is a conceptual diagram explaining expansion of a core at high temperatures in a conventional heat storage material.

[0009] An embodiment of the present invention will be described below. Note that this embodiment is merely an example of the present invention, and the present invention is not limited to this embodiment. Furthermore, various modifications and improvements can be made to this embodiment, and such modifications and improvements can also be included in the present invention.

[0010] The heat storage material according to this embodiment comprises heat storage particles having a phase-change type heat storage substance capable of storing and releasing heat by utilizing a solid-liquid phase change. The heat storage material according to this embodiment may comprise one heat storage particle, or may comprise a plurality of heat storage particles. Furthermore, the heat storage material according to this embodiment may be composed of only heat storage particles, or may be composed of heat storage particles and components or members other than the heat storage particles. Examples of components or members other than the heat storage particles include a holding material that holds the heat storage particles.

[0011] The heat storage particle 1 will be described with reference to Figure 1. The heat storage particle 1 has a core 10 made of a zinc alloy containing aluminum, and a shell 20 made of a coating containing a metal oxide and covering the core 10. The zinc alloy containing aluminum is the phase-change heat storage material described above. The shell 20 may cover the entire surface of the core 10, or may cover only a portion of the surface of the core 10, but it is preferable that the shell 20 cover the entire surface of the core 10.

[0012] The core 10 has voids 30 therein. There are no particular limitations on the number and size of the voids 30, but the porosity, which is the proportion of the voids 30 in the heat storage particle 1, exceeds 1.4%. This porosity is an area ratio calculated by dividing the total area of ​​the voids 30 in the cross section of the heat storage particle 1 by the cross section area of ​​the heat storage particle 1.

[0013] The heat storage material according to the present embodiment having such a configuration is less likely to break even when the temperature rises and falls repeatedly, and its performance as a heat storage material (e.g., heat storage performance, heat dissipation performance) is less likely to deteriorate. In conventional heat storage materials, the core 100 does not have voids. As described above, the expansion of the core 100 at high temperatures can destroy the shell 200, causing the core 100 to leach out. In contrast, as shown in FIG. 2 , in the heat storage material according to the present embodiment, even if the core 10 expands at high temperatures, the voids 30 absorb the expansion, thereby alleviating the stress caused by the expansion and suppressing the destruction of the shell 20, i.e., the leach-out of the core 10 from the heat storage particles 1. Therefore, the heat storage material according to the present embodiment has excellent durability, and is less likely to break even when the temperature rises and falls repeatedly, and its performance as a heat storage material is less likely to deteriorate. Note that in FIG. 2 , the stress caused by the expansion of the core 10 is indicated by arrows.

[0014] The heat storage material according to this embodiment can be manufactured as follows: That is, the manufacturing method of the heat storage material according to this embodiment is a method for manufacturing the heat storage material according to this embodiment, and includes a cutting step of cutting a metal block made of a zinc alloy to obtain cutting chips, a crushing step of crushing the cutting chips to obtain crushed particles, and a shell forming step of coating the surfaces of the crushed particles with a film containing a metal oxide to obtain heat storage particles.

[0015] The pulverized particles obtained in the pulverization step become the cores 10 of the heat storage particles 1, and the coatings formed on the surfaces of the pulverized particles in the shell formation step become the shells 20 of the heat storage particles 1. When the cutting chips are pulverized to obtain pulverized particles, the pulverized particles re-aggregate to form pulverized particles, which is thought to form voids inside the pulverized particles. Both the cutting step and the pulverization step are preferably performed dry. When the cutting step and the pulverization step are performed dry, the particles are more likely to aggregate, making it easier for voids 30 to be formed.

[0016] The use of the heat storage material according to this embodiment is not particularly limited, but it can be used, for example, to suppress a decrease in the temperature of a catalyst. An example of an application is given below. A catalyst for purifying exhaust gas is installed in an automobile, and the temperature of the catalyst drops when the engine is stopped. To suppress this, the heat storage material according to this embodiment may be used. By heating the catalyst installed in the automobile by heat dissipation from the heat storage material according to this embodiment, the temperature of the catalyst can be maintained at a temperature at which the catalyst is activated (for example, 300 to 400°C) without using electrical energy. The heat storage material according to this embodiment can be used not only for automobile exhaust gas purification, but also for suppressing a decrease in the temperature of a catalyst for purifying exhaust gas from an internal combustion engine.

[0017] The heat storage material and its manufacturing method according to this embodiment will be described in more detail below. (1) Regarding the Core The core is made of a zinc alloy containing aluminum. This zinc alloy may be an alloy of zinc and aluminum, or may further contain other components, as long as zinc is the predominant component. The type of zinc and aluminum alloy is not particularly limited, but examples include zinc alloys with a zinc content of 83% by mass to 98% by mass and an aluminum content of 2% by mass to 17% by mass. If the zinc and aluminum contents are as described above, the effect of rapidly dissipating absorbed heat when solidifying and facilitating a temperature rise in the catalyst coating layer is more likely to be achieved. When the zinc alloy further contains components other than zinc and aluminum, the type of other components is not particularly limited, but examples include magnesium, tin, indium, and lead.

[0018] (2) Regarding the Shell: The shell is made of a coating containing a metal oxide. Because metal oxides are resistant to further oxidation, the shell can prevent the core from oxidizing and deteriorating its performance as a heat storage material. This coating may be formed solely of a metal oxide, or may further contain components other than the metal oxide. The type of metal oxide is not particularly limited, but examples include at least one of aluminum oxide (Al2O3) and zinc oxide (ZnO). When the coating further contains components other than the metal oxide, the type of the other components is not particularly limited, but examples include phosphorus pentoxide (P2O5), silicon dioxide (SiO2), and titanium oxide (TiO2).

[0019] (3) Regarding voids As mentioned above, the number and size of the voids are not particularly limited, but the porosity, which is the proportion of voids in the heat storage particles, must exceed 1.4%, and is preferably 2% or more. Furthermore, the upper limit of the void ratio is not particularly limited, but is preferably 16% or less, and more preferably 12% or less.

[0020] This porosity is an area ratio obtained by observing the cross section of the heat storage particle. The method for calculating the porosity is not particularly limited, but can be calculated by the following method, for example. The heat storage particle is cut and the exposed cross section is observed with a scanning electron microscope (SEM), and elemental analysis is performed. By performing image processing on the SEM image, the portion containing zinc in the cross section of the heat storage particle and the void portion are distinguished. Then, the porosity can be calculated as an area ratio by dividing the total area of ​​the void portion in the cross section of the heat storage particle by the cross section area of ​​the heat storage particle. When calculating the porosity, it is preferable to observe the cross sections of multiple heat storage particles and calculate the average value to obtain the porosity. The number of heat storage particles used to calculate the porosity is not particularly limited, but can be, for example, 80 or more.

[0021] (4) Regarding heat storage particles The diameter of the heat storage particles is not particularly limited, but the smaller the diameter, the better. For example, the diameter of the heat storage particles is preferably 50 μm or less, and more preferably 30 μm or less. The smaller the diameter of the heat storage particles, the faster the melting rate and solidification rate of the heat storage particles, so that heat storage and heat release by the heat storage material according to this embodiment can be performed in a short time (i.e., the heat storage rate and heat release rate tend to be high). In addition, the type of heat storage particles may be one type, or two or more types. In other words, a mixture of multiple types of heat storage particles differing in at least one of the type and diameter of the phase-change heat storage material may be used in the heat storage material.

[0022] (5) Regarding the Cutting Process The cutting process is a process in which a metal block (e.g., an ingot) made of a zinc alloy is cut to cut out, for example, fibrous cutting chips. The type of tool (cutting blade) used in the cutting process is not particularly limited, and there are no particular restrictions on the material, hardness, or shape of the tool, as long as cutting chips can be obtained by cutting a metal block made of a zinc alloy. Furthermore, there are no particular restrictions on the conditions for the cutting process, and there are no particular restrictions on the relative speed between the metal block and the tool or the force (pressing force) with which the tool is pressed against the metal block, as long as cutting chips can be obtained by cutting a metal block made of a zinc alloy.

[0023] (6) Regarding the Pulverization Process The pulverization process is a process in which the cutting chips obtained in the cutting process are pulverized into fine particles to obtain pulverized particles. The pulverization method is not particularly limited, but examples thereof include impact pulverization. The impact pulverization method is a pulverization method in which pulverization is performed using the shear force and impact force of rotating pulverization blades. The type of pulverization blade used in the pulverization process using the impact pulverization method is not particularly limited, and there are no particular restrictions on the material, hardness, or shape of the pulverization blade, as long as the cutting chips can be pulverized to obtain pulverized particles. Furthermore, there are no particular restrictions on the pulverization conditions using the impact pulverization method, and there are no particular restrictions on the rotation speed of the pulverization blade, pulverization time, etc., as long as the cutting chips can be pulverized to obtain pulverized particles.

[0024] However, when the particles resulting from the grinding of the cuttings re-agglomerate to form ground particles, it is thought that voids will form inside the ground particles, so the type of grinding blades and grinding conditions must be set so that voids will form inside the ground particles. For example, when the grinding process is carried out using the impact grinding method, as shown in Figure 3, the particle size (median size in Figure 3) of the particles produced by grinding becomes smaller as the grinding time increases, but if re-agglomeration occurs, the particle size will increase, so it is preferable to continue grinding until the particle size stabilizes after it has increased.

[0025] (7) Shell Forming Step The shell forming step is a step of obtaining heat storage particles by coating the surfaces of the pulverized particles obtained in the pulverization step with a film containing a metal oxide. The method of forming the film containing a metal oxide is not particularly limited, but examples thereof include hydrothermal treatment. By subjecting the pulverized particles obtained in the pulverization step to hydrothermal treatment, the surfaces of the pulverized particles can be coated with a film containing aluminum oxide to obtain heat storage particles.

[0026] (8) Method of Using the Heat Storage Material The method of using the heat storage material according to this embodiment is not particularly limited, but can be, for example, by the following method: The heat storage material according to this embodiment is heated to a temperature equal to or higher than the melting point of the zinc alloy that forms the core of the heat storage particles. At this time, the core of the heat storage particles melts, causing heat absorption (i.e., the heat storage material stores heat). Next, the heated heat storage material according to this embodiment is cooled to a temperature equal to or lower than the melting point of the zinc alloy that forms the core of the heat storage particles. At this time, the molten core of the heat storage particles solidifies, causing heat to be generated (i.e., the heat storage material dissipates heat).

[0027] Next, a more specific description will be given of how to use the heat storage material according to this embodiment, taking as an example a case where the material is used to suppress a temperature drop in a catalyst for purifying automobile exhaust gas. The heat storage material according to this embodiment is installed near a catalyst mounted on an automobile. When the automobile engine is operated, the temperature of the catalyst rises, and so does the temperature of the heat storage material. At this time, the cores of the heat storage particles melt, and the heat storage material stores heat. Next, when the automobile engine is stopped, the temperatures of the catalyst and the heat storage material drop, but when the cores of the heat storage particles solidify, the heat storage material releases heat. This heat release suppresses a temperature drop in the catalyst, and the temperature of the catalyst can be maintained at a temperature at which the catalyst is activated, without using electrical energy.

[0028] The present invention will be explained in more detail below with reference to examples and comparative examples. [Example 1] An ingot of zinc alloy Zn-5Al was prepared. Zinc alloy Zn-5Al is an alloy of zinc and aluminum, with a zinc content of 95% by mass and an aluminum content of 5% by mass. The melting point of zinc alloy Zn-5Al is 381°C.

[0029] A zinc alloy Zn-5Al ingot was cut to obtain fibrous cutting pieces (cutting process). The cutting machine used was a Scratch Race (product name) cutting machine manufactured by Sugiyama Heavy Industries Co., Ltd. The standard cutting blade of the Scratch Race was used. This cutting blade has a sawtooth cutting edge, and the pitch of the multiple protrusions formed on the cutting edge is 400 μm. In the cutting process, the cutting time was 0.2 to 1 min. -1The cutting was performed by pressing the ingot against a cutting blade rotating at a rotation speed of 1000 rpm in an inert atmosphere.

[0030] Next, the cuttings were crushed to obtain crushed particles (crushing step). The crushing method used was an impact crushing method. The crusher used was an impact type fine crusher, Bevel Impactor (product name), manufactured by Sugiyama Heavy Industries Co., Ltd. In the crushing step, the rotation speed of the crushing blades of the impact type fine crusher was set to 4500 min -1 After impact crushing for 3 minutes, the rotation speed was increased to 5000 min -1 The mixture was subjected to impact pulverization for 5 minutes.

[0031] This impact milling process pulverized the chips into particles, and the particle size gradually decreased over time. However, after reaching a certain size, the particles aggregated, causing the particle size to increase slightly (see the graph in Figure 3). After the particle size increased slightly, it stabilized at a roughly constant value, and the milling process was terminated at this point. Next, the milled particles obtained in the milling process were subjected to hydrothermal treatment, and the surfaces of the milled particles were coated with a film containing metal oxide to obtain heat-storage particles (shell formation process). The hydrothermal treatment conditions were 60°C for 3 hours.

[0032] The obtained heat storage particles were then dried and then fired. The drying conditions were that they were dried at room temperature overnight, followed by drying at 60°C for 24 hours. The firing conditions were that they were heated from room temperature to 300°C at a heating rate of 1.25°C / min, and then fired at 300°C for 2 hours. The heat storage particles of Example 1 obtained in this manner have a core made of zinc alloy Zn-5Al, and a shell made of a coating containing aluminum oxide that covers the core. The following tests were conducted on the heat storage particles of Example 1.

[0033] (Measurement of porosity) After embedding a plurality of heat storage particles in a liquid resin, the liquid resin was solidified. The solidified resin was then polished to expose the cross section of the heat storage particle. The exposed cross section of the heat storage particle was observed with a scanning electron microscope and subjected to elemental analysis. By performing image processing on the SEM image, the zinc-containing portion and the void portion of the cross section of the heat storage particle were distinguished. The porosity was calculated as an area ratio by dividing the total area of ​​the void portion of the cross section of the heat storage particle by the cross section area of ​​the heat storage particle.

[0034] In measuring the porosity, only heat storage particles whose core surfaces were entirely covered with a shell were measured, and heat storage particles whose core surfaces or voids were exposed were excluded from the porosity measurement. In measuring the porosity, the porosity of each of the 80 heat storage particles was calculated from the cross sections of the heat storage particles, and the average value of these was taken as the porosity of the heat storage particles of Example 1.

[0035] (Heat Cycle Test) A heat cycle test was performed on the heat storage particles of Example 1. That is, the heat storage particles of Example 1 were heated to 300°C and held there for a certain period of time, then heated to 600°C (a temperature above the melting point of the zinc alloy) and held there for one hour, and then cooled to 300°C (a temperature below the melting point of the zinc alloy) and held there for one hour. This cycle was repeated 48 times. Next, differential thermal analysis (DTA) was performed using a differential thermal analyzer on the heat storage particles before and after this heat cycle to measure the amount of heat released during temperature reduction. That is, the heat storage particles were heated to 600°C in the differential thermal analyzer, and then cooled at a rate of 10°C / min, and the amount of heat released during this process (when the zinc alloy solidified) was measured. Then, the ratio of the amount of heat released by the heat storage particles after the heat cycle (remaining rate of heat release) was calculated, assuming that the amount of heat released by the heat storage particles before the heat cycle was 100%.

[0036] Comparative Example 1 Particles made of zinc alloy Zn-5Al were produced from a molten zinc alloy Zn-5Al using an atomization method. These particles were subjected to hydrothermal treatment, drying, and firing in the same manner as in Example 1 to obtain heat storage particles. The heat storage particles of Comparative Example 1 have a core made of zinc alloy Zn-5Al and a shell made of a coating containing aluminum oxide that covers the core. For the heat storage particles of Comparative Example 1, the porosity was measured in the same manner as in Example 1, and a heat cycle test was conducted to calculate the residual heat release rate.

[0037] The porosity of the heat storage particles of Example 1 was measured as described above and was 2%. The residual heat release rate of the heat storage particles of Example 1 was measured as described above and was 100%. The porosity of the heat storage particles of Comparative Example 1 was measured as described above and was 1.4%. The residual heat release rate of the heat storage particles of Comparative Example 1 was measured as described above and was 50%. These results show that when the porosity of the heat storage particles exceeds 1.4%, the heat storage particles are less likely to be destroyed and their performance as a heat storage material is less likely to deteriorate even when the temperature is repeatedly increased and decreased.

[0038] 1... Heat storage particle 10... Core 20... Shell 30... Void portion

Claims

1. A heat storage material comprising heat storage particles having a core made of a zinc alloy containing aluminum and a shell made of a coating containing a metal oxide that covers the core, The core has multiple voids inside the surface, and the porosity, which is the ratio of the voids in the heat storage particles, is greater than 1.4%. The porosity is an area ratio calculated by dividing the total area of ​​the void portion in the cross-section of the heat storage particle by the area of ​​the cross-section of the heat storage particle. The aforementioned zinc alloy is a heat storage material comprising zinc and aluminum, wherein the zinc content is 83% by mass or more and 98% by mass or less, and the aluminum content is 2% by mass or more and 17% by mass or less.

2. (delete)

3. A method for manufacturing a heat storage material, The heat storage material comprises heat storage particles having a core made of a zinc alloy containing aluminum and a shell made of a coating containing a metal oxide that covers the core. The core has voids, and the porosity, which is the proportion of the voids in the heat storage particles, is greater than 1.4%. The porosity is an area ratio calculated by dividing the total area of ​​the void portion in the cross-section of the heat storage particle by the area of ​​the cross-section of the heat storage particle. A cutting process to obtain a cutting piece by cutting a metal block made of the aforementioned zinc alloy, A grinding step of grinding the aforementioned cutting pieces to obtain ground particles, A shell forming step to obtain the heat-storing particles by coating the surface of the pulverized particles with a film containing a metal oxide, A method for manufacturing a heat storage material that includes [the specified features].

4. The method for manufacturing a heat storage material according to claim 3, wherein both the cutting step and the crushing step are performed by a dry method.

5. The method for manufacturing a heat storage material according to claim 3 or claim 4, wherein the zinc alloy is an alloy consisting of zinc and aluminum, the zinc content is 83% by mass or more and 98% by mass or less, and the aluminum content is 2% by mass or more and 17% by mass or less.

6. The heat storage material according to claim 1, wherein the core is formed by the aggregation of zinc alloy particles, and the void is located inside the aggregated particles.

7. A heat storage material according to claim 1 or claim 6, for heating a catalyst installed in an automobile.