Aluminum-silicon based phase change material and preparation method thereof

Abstract

The application discloses an aluminum-silicon-based phase change material and a preparation method thereof. In the aluminum-silicon-based phase change material, the mass fraction of silicon is 7-20 wt% of aluminum, the mass fraction of erbium is 0.1-0.7 wt%, the mass fraction of strontium is 0.03-0.05 wt%, and the balance is aluminum. The preparation method comprises the following steps: preparing alloy raw materials; putting the prepared pure aluminum and aluminum-silicon intermediate alloy into a preheated graphite crucible, and then transferring the graphite crucible into an electric resistance furnace for smelting to obtain an aluminum-silicon alloy liquid; sequentially adding aluminum-strontium intermediate alloy and aluminum-erbium intermediate alloy into the aluminum-silicon alloy liquid for smelting to obtain an aluminum-silicon-based phase change material alloy liquid; continuing to heat the aluminum-silicon-based phase change material alloy liquid after cooling, and then pouring the aluminum-silicon-based phase change material alloy liquid into a preheated metal mold. The application can solve the problems of poor high-temperature oxidation resistance, and decreased thermal physical properties after multiple melting-solidification thermal cycles in the existing aluminum-based phase change heat storage materials.

Description

Technical Field

[0001] This invention relates to the field of silicon-based phase change materials technology. Specifically, it relates to an aluminum-silicon-based phase change material and its preparation method. Background Technology

[0002] In the field of high-temperature thermal energy storage for solar thermal power generation, Al-based phase change thermal energy storage materials (PCEs) possess advantages such as strong thermal storage capacity, good thermal conductivity, and stable phase change processes, making them one of the most widely studied metal-based PCEs. Among them, Al-Si alloys exhibit a latent heat of phase change exceeding 400 J / g, demonstrating broad application prospects. Currently, the high-temperature oxidation mechanism of Al-based PCEs during service and the relationship between microstructure changes and thermal energy storage performance changes after multiple melting-solidification thermal cycles remain unclear. The influence mechanism between microstructure changes and thermal energy storage performance changes after thermal cycling has not been effectively elucidated, resulting in unresolved issues such as poor high-temperature oxidation resistance of some Al-based PCEs during service and a decline in thermophysical properties after multiple melting-solidification thermal cycles, thus limiting their further development. Summary of the Invention

[0003] Therefore, the technical problem to be solved by the present invention is to provide a silicon-based phase change material and its preparation method, so as to solve the problems of poor high-temperature oxidation resistance and decreased thermophysical properties after multiple melting-solidification thermal cycles in existing aluminum-based phase change thermal storage materials.

[0004] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0005] An aluminum-silicon-based phase change material is disclosed, comprising 7–20 wt% silicon, 0.1–0.7 wt% erbium, 0.03–0.05 wt% strontium, and the balance being aluminum. The addition of erbium and strontium in this proportion not only effectively refines and modifies the microstructure of the aluminum-silicon alloy but also positively improves the high-temperature oxidation resistance of the aluminum-silicon-based phase change material, thereby enhancing its cyclic thermal stability.

[0006] The aforementioned aluminum-silicon-based phase change material contains silicon at a mass fraction of 7 wt%, erbium at a mass fraction of 0.5 wt%, and strontium at a mass fraction of 0.04 wt%; or: silicon at a mass fraction of 12 wt%, erbium at a mass fraction of 0.3 wt%, and strontium at a mass fraction of 0.04 wt%; or: silicon at a mass fraction of 20 wt%, erbium at a mass fraction of 0.3 wt%, and strontium at a mass fraction of 0.04 wt%.

[0007] The preparation method of aluminum-silicon-based phase change material includes the following steps: Step (1) Prepare alloy raw materials: pure aluminum, aluminum-silicon master alloy, aluminum-strontium master alloy, and aluminum-erbium master alloy; Step (2) Place the prepared alloy raw materials: pure aluminum and aluminum-silicon master alloy into a preheated graphite crucible, and then transfer it to a pit-type resistance furnace for melting. After the pure aluminum and aluminum-silicon master alloy are completely melted, an aluminum-silicon alloy liquid is obtained; Step (3) Add aluminum-strontium master alloy and aluminum-erbium master alloy to the aluminum-silicon alloy liquid in sequence and stir and melt with a graphite rod. After the aluminum-strontium master alloy and aluminum-erbium master alloy are completely melted, an aluminum-silicon-based phase change material alloy liquid is obtained; Step (4) Cool the aluminum-silicon-based phase change material alloy liquid and continue to keep it warm. After the heat preservation is completed, stir it evenly with a graphite rod, and then cast the aluminum-silicon-based phase change material alloy liquid into a preheated metal mold. After casting is completed, the above-mentioned aluminum-silicon-based phase change material is obtained.

[0008] In the preparation method of the above-mentioned aluminum-silicon based phase change material, in step (1), the mass fraction of aluminum in pure aluminum is greater than or equal to 99.9 wt%, the mass fraction of silicon in aluminum-silicon master alloy is 12-20 wt%, the mass fraction of strontium in aluminum-strontium master alloy is 10 wt%, and the mass fraction of erbium in aluminum-erbium master alloy is 20 wt%.

[0009] In the above-mentioned method for preparing aluminum-silicon-based phase change materials, in step (2), the preheating temperature of the graphite crucible is 250-300℃. During melting, the resistance furnace is heated to 750-800℃ at a heating rate of 10-20℃ / min and held for 0.5-1h. During melting, if the temperature rises too quickly, the material temperature will be uneven. If the temperature rises too slowly, the efficiency will be low and resources will be wasted. Under these melting conditions, silicon can exist in the aluminum-based alloy structure in the form of eutectic silicon, which is more conducive to the composite addition of erbium and strontium to refine and modify the alloy structure, and is beneficial to improving the performance of the phase change material.

[0010] In the above-mentioned method for preparing aluminum-silicon-based phase change materials, in step (4), the aluminum-silicon-based phase change material alloy liquid is cooled to 700-720℃ and then kept at that temperature for 10-15 minutes [which can effectively improve the refining and modification effect of alloying elements erbium and strontium, and obtain an aluminum-silicon-based phase change material with a more uniform structure]; during casting, the preheating temperature of the metal mold is 200-250℃ [if the preheating temperature of the metal mold is lower than 200℃ or higher than 250℃, it will lead to changes in the structure and a decrease in the performance of the aluminum-silicon-based phase change material].

[0011] In the above-mentioned method for preparing aluminum-silicon-based phase change materials, in step (1), the mass fraction of aluminum in pure aluminum is greater than or equal to 99.9 wt%, the mass fraction of silicon in aluminum-silicon master alloy is 12 wt% or 20 wt%, the mass fraction of strontium in aluminum-strontium master alloy is 10 wt%, and the mass fraction of erbium in aluminum-erbium master alloy is 20 wt%; in step (2), the preheating temperature of the graphite crucible is 250 ℃, and during melting, the resistance furnace is heated to 750 ℃ ​​at a heating rate of 10 ℃ / min and held for 1 h; in step (4), the aluminum-silicon-based phase change material alloy liquid is cooled to 720 ℃ and then held for 10 min; during casting, the preheating temperature of the metal mold is 200 ℃.

[0012] The technical solution of the present invention achieves the following beneficial technical effects:

[0013] This invention prepares an aluminum-silicon-based phase change material by adding erbium and strontium to an aluminum-silicon alloy and controlling the content of silicon, erbium, and strontium using the smelting and casting process of this invention. The material exhibits good high-temperature oxidation resistance, stable thermophysical properties after thermal cycling, and high latent heat of phase change and thermal conductivity. The phase change temperature of this aluminum-silicon-based phase change material is between 570 and 630°C and remains stable with the increase of thermal cycling.

[0014] While adding erbium alone can effectively improve the latent heat of phase change and thermal conductivity of aluminum-silicon-based phase change materials, a significant increase in these parameters requires a large erbium content. Excessive erbium addition, however, deteriorates the high-temperature oxidation resistance of aluminum-silicon-based phase change materials. This invention, by adding a specific amount of strontium and controlling the ratio of silicon to erbium, not only effectively reduces the deterioration of high-temperature oxidation resistance caused by excessive erbium addition, thus improving its high-temperature oxidation resistance, but also further enhances the latent heat of phase change and thermal conductivity of aluminum-silicon-based materials. Attached Figure Description

[0015] Figure 1 A schematic diagram of the experimental process flow of this invention embodiment;

[0016] Figure 2a , Figure 2b , Figure 2c , Figure 2d and Figure 2e The images shown are metallographic microstructures and magnified views (X = 0, 0.1, 0.3, 0.5 and 0.7) of the Al-7Si-XEr alloy in the examples.

[0017] Figure 3 Statistical results of secondary dendrite arm spacing of Al-7Si-XEr alloy in the examples;

[0018] Figure 4a , Figure 4b , Figure 4c , Figure 4d and Figure 4e The images shown are metallographic microstructures and magnified views (X = 0, 0.1, 0.3, 0.5 and 0.7) of the Al-12Si-XEr alloy in the examples.

[0019] Figure 5a , Figure 5b , Figure 5c , Figure 5d and Figure 5e The images shown are metallographic microstructures and magnified views (X = 0, 0.1, 0.3, 0.5 and 0.7) of the Al-20Si-XEr alloy in the examples.

[0020] Figure 6 , Figure 7 and Figure 8 The following are DSC images of different Al-7Si-XEr alloys, Al-12Si-XEr alloys, and Al-20Si-XEr alloys in the examples;

[0021] Figure 9 The effect curve of Er on the thermal conductivity of Al-7Si alloy in the examples;

[0022] Figure 10a , Figure 10b , Figure 10c and Figure 10d The images shown are SEM images of the eutectic Si after deep etching of the Al-7Si-XEr alloy in the examples. Figure 10a Al-7Si, low magnification; Figure 10b Al-7Si, high magnification; Figure 10c Al-7Si-0.5Er, low magnification; Figure 10d Al-7Si-0.5Er (high magnification)

[0023] Figure 11a and Figure 11b The SEM images and EDS analyses of the Al-7Si-0.7Er alloy in the examples are shown below. Figure 11a This is a backscattering plot. Figure 11b for Figure 11a EDS analysis at the point;

[0024] Figure 12 The effect curve of Er on the thermal conductivity of Al-12Si alloy in the examples;

[0025] Figure 13a , Figure 13b and Figure 13c The images shown are SEM images of the eutectic Si after deep etching of the Al-12Si-XEr alloy in the examples (Al-12Si, Al-12Si-0.7Er, low magnification, Al-12Si-0.7Er, high magnification).

[0026] Figure 14 The effect curve of Er on the thermal conductivity of Al-20Si alloy in the examples;

[0027] Figure 15a , Figure 15b and Figure 15c The images shown are SEM images of primary Si crystals after deep etching of the Al-20Si, Al-20Si-0.5Er, and Al-20Si-0.7Er alloys in the examples, respectively.

[0028] Figure 16a , Figure 16b , Figure 16c and Figure 16d The images shown are SEM images of the eutectic Si after deep etching of the Al-20Si-XEr alloy in the examples (Al-20Si, low magnification, Al-20Si, high magnification, Al-20Si-0.5Er, low magnification, Al-20Si-0.5Er, high magnification).

[0029] Figure 17 , Figure 18 , Figure 19 , Figure 20 and Figure 21 The graphs show the changes in surface morphology of the Al-12Si-XEr alloy after oxidation for 24h, 48h, 72h, and 96h as the Er content increases (X = 0, 0.1, 0.3, 0.5, and 0.7, respectively).

[0030] Figure 22a , Figure 22b , Figure 22c , Figure 22d and Figure 22e The images show the microstructure of the oxide film on the Al-12Si-XEr alloy after 120 hours of oxidation (X = 0, 0.1, 0.3, 0.5, and 0.7).

[0031] Figure 23a , Figure 23b , Figure 23c and Figure 23d The following are surface scan images of the oxide film on the Al-12Si alloy in the example. Figure 23a Here is a diagram of the oxide film morphology. Figure 23b , Figure 23c and Figure 23d (The following are the elemental distribution diagrams for Al, O, and Si, in order.)

[0032] Figure 24a , Figure 24b and Figure 24c The morphology and EDS analysis of the oxide film of Al-12Si-0.7Er alloy after 120 hours of oxidation are shown in the examples. Figure 24b and Figure 24c They are respectively Figure 24a (EDS at the left and right points);

[0033] Figure 25 Isothermal oxidation kinetic curves of different Al-12Si-XEr alloys in the examples;

[0034] Figure 26a , Figure 26b , Figure 26c , Figure 26d and Figure 26e Metallographic microstructures and magnified views (X = 0, 0.1, 0.3, 0.5 and 0.7) of different Al-7Si-0.04Sr-XEr alloys from the examples are shown.

[0035] Figure 27 Statistical results of secondary dendrite arm spacing of Al-7Si-0.04Sr-XEr alloy in the examples;

[0036] Figure 28a , Figure 28b , Figure 28c , Figure 28d and Figure 28e Metallographic microstructures and magnified views (X = 0, 0.1, 0.3, 0.5 and 0.7) of different Al-12Si-0.04Sr-XEr alloys from the examples are shown.

[0037] Figure 29a , Figure 29b , Figure 29c , Figure 29d and Figure 29e Metallographic microstructures and magnified views (X = 0, 0.1, 0.3, 0.5 and 0.7) of different Al-20Si-0.04Sr-XEr alloys from the examples are shown.

[0038] Figure 30 , Figure 31 and Figure 32 DSC images of different Al-7Si-0.04Sr-XEr, Al-12Si-0.04Sr-Xer and Al-20Si-0.04Sr-XEr alloys in the examples;

[0039] Figure 33 The effect of combined Er and Sr on the thermal conductivity of Al-7Si alloy in the example;

[0040] Figure 34a , Figure 34b , Figure 34c and Figure 34dThe images show SEM images of eutectic Si after different alloy depths of corrosion in the examples (Al-7Si, low magnification; Al-7Si, high magnification; Al-7Si-0.04Sr-0.5Er, low magnification; Al-7Si-0.04Sr-0.5Er, high magnification).

[0041] Figure 35a , Figure 35b , Figure 35c , Figure 35d and Figure 35e The images shown are scanned images (microstructure diagrams, distribution diagrams of Al, Si, Sr, and Er elements) of the Al-7Si-0.04Sr-0.5Er alloy after deep etching in the examples.

[0042] Figure 36 The effect of combined Er and Sr on the thermal conductivity of Al-12Si alloy in the example;

[0043] Figure 37a , Figure 37b and Figure 37c SEM images of eutectic Si after different alloy depths of corrosion in the examples are shown below (Al-12Si; Al-12Si-0.04Sr-0.3Er, low magnification; Al-12Si-0.04Sr-0.3Er, high magnification).

[0044] Figure 38 The effect of combined Er and Sr on the thermal conductivity of Al-20Si alloy in the example;

[0045] Figure 39a and Figure 39b SEM images of primary Si crystals after deep etching of Al-20Si and Al-20Si-0.04Sr-0.3Er alloys in Examples 1 and 2, respectively;

[0046] Figure 40a , Figure 40b , Figure 40c and Figure 40d The images show SEM images of eutectic Si after different alloy depths of corrosion in the examples (Al-20Si, low magnification; Al-20Si, high magnification; Al-20Si-0.04Sr-0.3Er, low magnification; Al-20Si-0.04Sr-0.3Er, high magnification).

[0047] Figure 41a and Figure 41b The images show the metallographic microstructures of the Al-7Si and Al-7Si-0.04Sr-0.5Er alloys after thermal cycling, respectively.

[0048] Figure 42a , Figure 42b , Figure 42c , Figure 42d and Figure 42e The images shown are surface scans (backscattered images, and distribution maps of Al, Si, Sr, and Er elements) of the Al-7Si-0.04Sr-0.5Er alloy after thermal cycling in the examples.

[0049] Figure 43a and Figure 43b The images show the metallographic microstructures of Al-12Si and Al-12Si-0.04Sr-0.3Er alloys after thermal cycling, respectively.

[0050] Figure 44a , Figure 44b , Figure 44c , Figure 44d and Figure 44e The images shown are surface scans (backscattered images, and distribution maps of Al, Si, Sr, and Er elements) of the Al-12Si-0.04Sr-0.3Er alloy after thermal cycling in the examples.

[0051] Figure 45a and Figure 45b The images show the metallographic microstructures of Al-20Si and Al-20Si-0.04Sr-0.3Er alloys after thermal cycling, respectively.

[0052] Figure 46a , Figure 46b , Figure 46c , Figure 46d and Figure 46e The images shown are surface scans (backscattered images, and distribution maps of Al, Si, Sr, and Er elements) of the Al-20Si-0.04Sr-0.3Er alloy after thermal cycling in the examples.

[0053] Figure 47a and Figure 47b The images show SEM images and EDS analyses (backscattered image and EDS) of the Al-20Si-0.04Sr-0.3Er alloy from Example 1. Figure 47a EDS analysis at the point;

[0054] Figure 48 DSC images of different Al-Si alloys after thermal cycling in the examples;

[0055] Figure 49 Example: Latent heat change curves of unmodified Al-Si alloy before and after cycling;

[0056] Figure 50 Example: Latent heat change curves of composite modified Al-Si alloy before and after cycling. Detailed Implementation

[0057] 1. Preparation of silicon-based phase change materials

[0058] The experimental process flow for this embodiment is shown below. Figure 1 The preparation method of silicon-based phase change materials is as follows:

[0059] (1) Batching: According to the calculated component ratio, the required alloy raw materials are cut into small pieces and then weighed and batched. The alloy raw materials are pure Al (aluminum content greater than or equal to 99.9 wt%), Al-12Si alloy (silicon content of 12 wt%), Al-20Si alloy (silicon content of 20 wt%), Al-10Sr master alloy (strontium content of 10 wt%), and Al-20Er master alloy (erbium content of 20 wt%).

[0060] (2) Melting: First, select a graphite crucible of appropriate size, clean it and preheat it to 250°C; the metal mold size is Φ30mm×110mm, and preheat the metal mold to 200°C [in some other embodiments, the preheating temperature can be 250~300°C]. Then, using clamps, the weighed pure Al, Al-12Si alloy, and Al-20Si alloy are loaded into a graphite crucible and placed in a pit-type resistance furnace. The temperature is raised to 750°C at a rate of 10°C / min and held for 1 hour. [In some other embodiments, the temperature can be raised to 750-800°C at a rate of 10-20°C / min and held for 0.5-1 hour. During melting, if the temperature rises too quickly, the material temperature will be uneven; if the temperature rises too slowly, the efficiency will be low and resources will be wasted. The melting conditions in this embodiment allow silicon to exist in the form of eutectic silicon in the aluminum-based alloy structure, which is more conducive to the composite addition of erbium and strontium to refine and modify the alloy structure, thereby improving the performance of the phase change material.] After the alloy in the crucible has completely melted, Al-10Sr and Al-20Er master alloys are added, and the mixture is stirred rapidly with a graphite rod. The furnace temperature is then lowered to 720°C and held for 10 minutes.

[0061] (3) Casting: After holding at 720℃ for 10 min [in some other embodiments, the aluminum-silicon-based phase change material alloy liquid is cooled to 700-720℃ and then held for 10-15 min before casting; in this embodiment, holding at 720℃ for 10 min before casting can effectively improve the refining and modification effect of the alloying elements erbium and strontium, resulting in an aluminum-silicon-based phase change material with a more uniform structure], the alloy liquid is stirred again with a graphite rod, and then the alloy liquid is slowly cast into a preheated metal mold [during casting, the preheating temperature of the metal mold is 200-250℃, and 200℃ is selected in this embodiment. If the preheating temperature of the metal mold is lower than 200℃ or higher than 250℃, it will lead to changes in the structure and a decrease in the performance of the aluminum-silicon-based phase change material], to obtain the required cylindrical rod-shaped sample.

[0062] Using the above method, aluminum-silicon-based phase change materials with different silicon, erbium, and strontium contents were prepared by controlling the addition of different intermediate alloys and aluminum. For example, Al-aSi represents an aluminum-silicon-based phase change material with a silicon content of awt% and the balance being aluminum; Al-aSi-bEr represents an aluminum-silicon-based phase change material with a silicon content of awt% and an Er content of bwt% and the balance being aluminum; Al-aSi-cSr-bEr ​​represents an aluminum-silicon-based phase change material with a silicon content of awt%, an Er content of bwt%, and a strontium content of cwt% and the balance being aluminum.

[0063] 2. Evaluation of the properties of silicon-based phase change materials

[0064] High-temperature antioxidant test: Test conditions are as follows Figure 1 After heat preservation, allow to stand in air for 40 minutes. Thermal cycling test: Test conditions are as follows. Figure 1 Each time the upper or lower limit temperature is reached, it is held at that temperature for half an hour. Phase change temperature and latent heat of phase change tests: heating rate is 10℃ / min, heating temperature range is 30℃ to 750℃, under argon atmosphere protection.

[0065] 3. Effects of Er on the microstructure and thermal storage properties of Al-Si alloys

[0066] 3.1 Effect of Er on the microstructure of Al-Si alloys

[0067] (1) The effect of Er on the microstructure of Al-7Si alloy: Figure 2a It can be seen that the unmodified Al-7Si alloy is mainly composed of α-Al and eutectic Si. The α-Al is relatively large and unevenly distributed, with coarse dendrites present in some areas. The eutectic Si is distributed in long needle-like shapes around the α-Al. When the rare earth Er content is 0.1wt%, such as Figure 2b As shown, the α-Al size begins to decrease and the distribution becomes more uniform, but coarse dendrites still exist locally. The size of the long needle-like eutectic Si decreases and transforms into short rod-like shapes. When the rare earth Er content is 0.3wt%, as... Figure 2c As shown, the α-Al size further decreases, the locally coarse dendrites disappear, and the short rod-shaped eutectic Si becomes granular. When the rare earth Er content is 0.5 wt%, as... Figure 2d As shown, α-Al reaches its smallest size and most uniform distribution, and some equiaxed crystals appear, while all eutectic Si transforms into fine particles. Further increasing the rare earth Er content, when the Er addition reaches 0.7 wt%, as... Figure 2e As shown, the α-Al size increases and the shape becomes irregular, with larger columnar crystals appearing, but the size and morphology of the eutectic Si do not change significantly. Figure 3It can be seen that the secondary dendrite arm spacing of the unmodified Al-7Si alloy is approximately 15.70 μm. With the increase of rare earth Er content, the secondary dendrite arm spacing of the alloy gradually decreases. When the rare earth Er addition is 0.5 wt%, the secondary dendrite arm spacing of the alloy is the smallest, approximately 10.92 μm. Further increasing the Er content increases the secondary dendrite arm spacing of the alloy.

[0068] (2) The effect of Er on the microstructure of Al-12Si alloy: Figure 4a It can be seen that the microstructure of the unmodified Al-12Si alloy is mainly eutectic Si, which is distributed in the form of large, long needles on the Al matrix. When the rare earth Er content is 0.1 wt%, such as Figure 4b As shown, the eutectic Si transforms from long needle-like to short rod-like shapes, and its size begins to decrease, but a certain number of long needle-like eutectic Si still exist locally. With the continuous increase of rare earth Er content, the size of the eutectic Si continues to decrease, and its distribution becomes more uniform. When the rare earth Er addition is 0.7 wt%, as shown... Figure 4e As shown, the eutectic Si was completely transformed from long needle-shaped and short rod-shaped into fine granular particles, and α-Al appeared in the microstructure, achieving a good metamorphic effect.

[0069] (3) The effect of Er on the microstructure of Al-20Si alloy: Figure 5a It can be seen that the unmodified Al-20Si alloy mainly consists of primary Si and eutectic Si. The primary Si mainly appears as coarse plates and irregular blocks, with large dimensions and sharp edges, and uneven size distribution. The eutectic Si mainly appears as long needles distributed on the Al matrix. After adding rare earth Er, the morphology and size of the Si phase in the Al-20Si alloy microstructure changed significantly. When the rare earth Er addition amount is 0.1 wt%, such as Figure 5b As shown, the size of primary Si begins to decrease, the coarse, plate-like primary Si disappears, and all becomes irregular blocky primary Si. The edges begin to blunt and become rounded, and the eutectic Si transforms from long needle-like to short rod-like. With the continuous increase of rare earth Er content, the primary Si gradually becomes finer, and the size of the eutectic Si also gradually decreases. When the rare earth Er addition is 0.5 wt%, as... Figure 5d As shown, the primary Si crystals in the Al-20Si alloy microstructure are the smallest, refined into small and regular blocks, while the eutectic Si is distributed as fine particles on the Al matrix. However, further increasing the rare earth Er content, when the rare earth Er addition is 0.7 wt%, as... Figure 5e As shown, the primary Si in the Al-20Si alloy microstructure undergoes a certain degree of coarsening, with large, irregular blocky primary Si appearing in the microstructure. Moreover, the distribution of primary Si on the matrix becomes uneven, with a large number of primary Si distributed in some areas and a small number of primary Si distributed in others.

[0070] 3.2 Effect of Er on the phase transformation temperature and latent heat of Al-Si alloy

[0071] (1) Effect of Er on phase transformation temperature and latent heat of phase transformation of Al-7Si alloy: Figure 6 The DSC curves of Al-7Si alloys with different amounts of rare earth element Er are shown in the figure. It can be seen that the DSC curve of the unmodified Al-7Si alloy exhibits two endothermic peaks, one sharp and the other relatively flat. Combining the data in Table 1, the initial phase transformation temperature of the Al-7Si alloy is 576.29℃, at which point the alloy begins to melt. As the temperature increases, the alloy enters the solid-liquid two-phase region. At 603.2℃, the eutectic structure completely melts, exhibiting a sharp endothermic peak. When the temperature rises to the termination phase transformation temperature of 619.02℃, α-Al completely melts, exhibiting a relatively flat endothermic peak. Furthermore, it can be seen that the peak shape of the Al-7Si alloy remains unchanged after the addition of rare earth element Er, with both endothermic peaks present in all cases. As shown in Table 1, the Al-7Si-0.3Er alloy has the highest latent heat of fusion, which is 11.91 J / g higher than that of the unmodified Al-7Si alloy. The addition of rare earth Er has a moderate effect on improving the latent heat of fusion of Al-7Si alloy. The phase transformation temperatures of Al-7Si-XEr alloy are all above 572℃, and the phase transformation temperature range is between 572-621℃. After adding rare earth Er, the phase transformation temperature range of Al-7Si alloy is relatively stable and does not change significantly.

[0072] Table 1

[0073]

[0074] (2) Effect of Er on the phase transformation temperature and latent heat of Al-12Si alloy: From Figure 7 It can be seen that the DSC curve of the unmodified Al-12Si alloy has only one endothermic peak, which is relatively sharp. Combined with the data in Table 2, the initial phase transformation temperature of the Al-12Si alloy is 584.49℃, at which point the alloy begins to melt. When the temperature rises to the termination phase transformation temperature of 595.91℃, the eutectic structure completely melts, exhibiting an endothermic peak. Furthermore, it can be seen that the peak shape of the Al-12Si alloy does not change after the addition of rare earth element Er; both exhibit only one endothermic peak. Table 2 shows that the Al-12Si-0.7Er alloy has the highest latent heat of fusion, increasing by 46.33 J / g compared to the unmodified Al-12Si alloy. The addition of rare earth element Er has a good effect on improving the latent heat of fusion of the Al-12Si alloy. The phase transformation temperatures of the Al-12Si-XEr alloys are all above 580℃, with a range between 580-596℃. After the addition of rare earth element Er, the phase transformation temperature range of the Al-12Si alloy is relatively stable, without significant changes.

[0075] Table 2

[0076]

[0077] (3) Effect of Er on phase transformation temperature and latent heat of phase transformation of Al-20Si alloy:

[0078] Depend on Figure 8 It can be seen that the DSC curve of the unmodified Al-20Si alloy has only one endothermic peak, which is relatively sharp. Combined with the data in Table 3, the initial phase transformation temperature of the Al-20Si alloy is 575.48℃, at which point the alloy begins to melt. When the temperature rises to the termination phase transformation temperature of 593.90℃, the eutectic structure completely melts, exhibiting an endothermic peak. Furthermore, it can be seen that the peak shape of the Al-20Si alloy did not change after the addition of rare earth element Er, with only one endothermic peak remaining. Table 3 shows that the Al-20Si-0.5Er alloy has the highest latent heat of fusion, increasing by 73.64 J / g compared to the unmodified Al-20Si alloy. The addition of rare earth element Er has the best effect on improving the latent heat of fusion of the Al-20Si alloy. The phase transformation temperatures of the Al-20Si-XEr alloys are all above 571℃, with a range between 571-595℃. After the addition of rare earth element Er, the phase transformation temperature range of the Al-20Si alloy is relatively stable, without significant changes.

[0079] Table 3

[0080]

[0081] DSC testing revealed that the phase transformation temperatures of Al-7Si, Al-12Si, and Al-20Si alloys all decreased to varying degrees. The microstructure of Al-7Si alloy mainly consists of α-Al and long acicular eutectic Si, which are relatively large before metamorphism. After the addition of rare earth element Er, the size of α-Al decreases, and some equiaxed crystals appear. The eutectic Si morphology changes to short rod-shaped and granular, with a significant reduction in size, leading to an increase in the number of internal phase interfaces and a slight decrease in the melting temperature of Al-7Si alloy. Similarly, the microstructure of Al-12Si alloy mainly consists of large, long acicular eutectic Si. After the addition of rare earth element Er, the eutectic Si morphology changes to short rod-shaped or fine granular, with a significant reduction in size, leading to an increase in the number of internal phase interfaces and a slight decrease in the melting temperature of Al-12Si alloy. The microstructure of Al-20Si alloy mainly consists of primary Si and eutectic Si. Before metamorphism, primary Si is in the form of coarse plates and blocks, while eutectic Si is mainly in the form of long needles with relatively large size. After the addition of rare earth Er, the microstructure is refined, the size of primary Si and eutectic Si is significantly reduced, the number of phase interfaces inside the material increases, and the melting temperature of Al-20Si alloy decreases.

[0082] The DSC curve results show that the latent heat of phase transformation in Al-Si alloys is mainly provided by the melting of the eutectic structure, specifically by the melting of eutectic Si within the microstructure. For Al-7Si alloys, the majority of the latent heat of phase transformation is provided by the melting of eutectic Si, with a small portion provided by α-Al. Al-7Si alloys have relatively low Si content and a small amount of eutectic Si in their microstructure. While the addition of rare earth element Er changes the morphology of the eutectic Si from long needle-like to short rod-like and granular, thus increasing its quantity, the change in quantity has little impact on the latent heat of phase transformation due to the low proportion of eutectic Si in the microstructure. Although the amount of α-Al increases, its contribution to the latent heat of phase transformation is small, so the effect of adding rare earth element Er on improving the latent heat of phase transformation in Al-7Si alloys is generally limited. For Al-12Si alloys, the latent heat of phase transformation is mainly provided by the melting of eutectic Si, with a small portion provided by α-Al. The Al-12Si alloy is composed of eutectic Si. After adding rare earth element Er, the morphology of the eutectic Si changes; its size decreases while its quantity increases. This increases the melting entropy provided by the eutectic Si, thus improving the latent heat of phase transformation in the Al-12Si alloy. However, when the Er addition is 0.1 wt%, the latent heat of phase transformation in the Al-12Si alloy decreases significantly. This is mainly because the amount of rare earth added is relatively small, resulting in a large number of long, needle-like eutectic Si particles. Local grain coarsening reduces the amount of eutectic Si per unit area, lowering the melting entropy and consequently decreasing the latent heat of phase transformation. For the Al-20Si alloy, the microstructure consists of primary Si and eutectic Si. After adding Er, the morphology and size of the primary Si change significantly, transforming from large, plate-like pieces to smaller, blocky pieces, increasing in quantity and thus improving the melting entropy of the Al-20Si alloy. The eutectic Si changes from long, needle-like pieces to fine, granular pieces, also significantly increasing in quantity and substantially improving the melting entropy of the Al-20Si alloy. The changes in the morphology and the increase in the amount of Si phase in Al-20Si alloy increase the number of bonds that need to be broken for melting, thus increasing the melting entropy. Therefore, the addition of rare earth element Er has the best effect on improving the latent heat of phase transformation of Al-20Si alloy.

[0083] 3.3 Effect of Er on the thermal conductivity of Al-Si alloys

[0084] (1) The effect of Er on the thermal conductivity of Al-7Si alloy: Figure 9 As shown in Table 4, the addition of rare earth element Er can improve the thermal conductivity of Al-7Si alloy. The thermal conductivity of the unmodified Al-7Si alloy is 154.40 W·m. -1 ·K -1 As the Er content increases, the thermal conductivity of the alloy continuously increases, showing a trend of first increasing and then decreasing. When the rare earth Er content is 0.5 wt%, the thermal conductivity of the alloy is the highest, at 164.46 W·m. -1 ·K -1 Compared to the unmodified Al-7Si alloy, it improved by 10.06 W·m.-1 ·K -1 When the rare earth element Er is added at a concentration of 0.7 wt%, the thermal conductivity of the alloy decreases to 156.00 W·m. -1 ·K -1 However, it still has a higher thermal conductivity than the unmodified Al-7Si alloy.

[0085] Table 4

[0086]

[0087] Depend on Figure 10a and Figure 10b It can be seen that the eutectic Si in the unmodified Al-7Si alloy is in the form of coarse plates, with a size of 20-40 μm. After adding rare earth element Er, it can be observed that most of the coarse plate-like eutectic Si transforms into a fibrous structure. The branches of the fibrous eutectic Si are gaps left by the corrosion of α-Al. A small portion of the eutectic Si transforms into fine plates, with a size of approximately 10 μm. Figure 10c and Figure 10d As shown in the figure. From the three-dimensional morphology of the eutectic Si, it can be seen that the eutectic Si in the unmodified Al-7Si alloy is distributed in coarse, plate-like forms within the Al matrix. This causes scattering of free electrons, hindering electron movement, resulting in a relatively low thermal conductivity of the Al-7Si alloy. After adding rare earth element Er, the spacing between the secondary dendrite arms of the alloy decreases (…). Figure 3The dendritic distribution of eutectic Si narrows, thus reducing the width of the barrier that free electrons need to pass through, making electron passage more efficient and improving the alloy's thermal conductivity. Furthermore, most of the coarse, plate-like eutectic Si transforms into fibrous structures. These fibrous eutectic Si branches contain numerous α-Al atoms, and a lattice distortion layer exists at the interface between α-Al and eutectic Si. The length of the electron transport path through this lattice distortion layer affects the material's thermal conductivity. The spherical ends of the fibrous eutectic Si further reduce the length of the electron transport path through the lattice distortion layer, weakening scattering and increasing electron throughput, thus improving the alloy's thermal conductivity. However, a small portion of small, fine-grained eutectic Si remains, increasing the interface with the Al matrix and increasing electron scattering. While this scattering is weaker than that of plate-like eutectic Si, it is stronger than that of fibrous eutectic Si, thus limiting the improvement in the alloy's thermal conductivity. When rare-earth element Er is added to Al-7Si alloy, a small portion of Er dissolves into the Al matrix. Because the atomic radius of Er is much larger than that of Al, this causes severe lattice distortion, increasing the scattering of free electrons and reducing thermal conductivity. Most of the Er accumulates at the solid-liquid interface front during solidification, precipitating as compounds. Microstructural observation of the Al-7Si-0.7Er alloy reveals... Figure 11a and Figure 11b As shown ( Figure 11b In the alloy, the mass percentages of Al, Si, and Er were 49.18 wt%, 18.06 wt%, and 32.76 wt%, respectively, and the atomic percentages were 68.48%, 24.16%, and 7.36%, respectively. A small amount of bright white second phase was found in the microstructure. EDS analysis of this second phase revealed that it was mainly composed of Al, Si, and Er elements, indicating that it was an Er-containing compound, i.e., an Er-rich phase. The Er-rich phase, distributed in the matrix, directly hinders the movement of free electrons, reducing the alloy's thermal conductivity. Furthermore, it can act as a nucleation site for heterogeneous formation, refining the grains and increasing grain boundaries, further hindering the movement of free electrons and reducing the alloy's thermal conductivity. Additionally, the presence of a small number of pores in the figure also hinders the movement of free electrons, further reducing the alloy's thermal conductivity. Therefore, the addition of excessive rare-earth Er leads to a decrease in the alloy's thermal conductivity.

[0088] (2) The effect of Er on the thermal conductivity of Al-12Si alloy: From Figure 12 As shown in Table 5, the addition of rare earth element Er can improve the thermal conductivity of Al-12Si alloy. The thermal conductivity of the unmodified Al-12Si alloy is 145.28 W·m. -1 ·K -1With increasing Er content, the thermal conductivity of Al-12Si alloy continuously increases. When the rare earth Er content is 0.7 wt%, the alloy exhibits the highest thermal conductivity, reaching 159.52 W·m. -1 ·K -1 Compared to the unmodified Al-12Si alloy, it increased by 14.24 W·m. -1 ·K -1 .

[0089] Table 5

[0090]

[0091] Depend on Figure 13a It can be seen that the eutectic Si in the unmodified Al-12Si alloy is plate-like, cross-distributed, and relatively large in size, ranging from 20 to 45 μm. After the addition of rare earth Er, as... Figure 13b and Figure 13c As shown, the morphology of the eutectic Si did not change significantly, remaining lamellar, but the size was significantly reduced, ranging from 5 to 20 μm. The smaller lamellar eutectic Si particles were distributed in parallel. This type of eutectic Si appears as granular in metallographic images. Figure 4a and Figure 4e In the unmodified Al-12Si alloy, the eutectic Si is plate-like, consistent with the morphology of the Al-7Si alloy, but the size is increased and the distribution is more irregular. Furthermore, due to the increased Si content, the number of plate-like eutectic Si in the Al-12Si alloy also increases, increasing the chances of hindering the movement of free electrons and reducing thermal conductivity. Therefore, the thermal conductivity of the Al-12Si alloy is lower than that of the Al-7Si alloy. After adding rare earth element Er, the size of the layered eutectic Si in the Al-12Si alloy is significantly reduced, which is more conducive to the passage of free electrons, thus improving the alloy's thermal conductivity. At the same time, the layered eutectic Si is more regularly arranged and parallel, reducing the chances of free electrons being blocked by eutectic Si during movement, which also facilitates the passage of free electrons and improves the alloy's thermal conductivity. Additionally, the presence of α-Al( Figure 4a and Figure 4e This provides a fast channel for the movement of free electrons, which significantly improves the thermal conductivity of the alloy.

[0092] (3) The effect of Er on the thermal conductivity of Al-20Si alloy: From Figure 14 As shown in Table 5, the addition of rare earth element Er can improve the thermal conductivity of Al-20Si alloy. The thermal conductivity of the unmodified Al-20Si alloy is 124.17 W·m. -1 ·K -1With increasing Er content, the thermal conductivity of Al-20Si alloy continuously increases, showing a trend of first increasing and then decreasing. When the rare earth Er content is 0.5 wt%, the thermal conductivity of the alloy is the highest, at 136.50 W·m. -1 ·K -1 Compared to the unmodified Al-20Si alloy, it increased by 11.33 W·m. -1 ·K -1 When the rare earth element Er is added at a concentration of 0.7 wt%, the thermal conductivity of the alloy decreases to 128.70 W·m. -1 ·K -1 However, it still has a higher thermal conductivity than the unmodified Al-20Si alloy.

[0093] Table 6

[0094]

[0095]

[0096] Depend on Figure 15a It can be seen that the primary Si crystals in the unmodified Al-20Si alloy are irregularly blocky, with sharp edges and relatively large sizes, ranging from 150 to 200 μm. After adding 0.5 wt% rare earth Er, as... Figure 15b As shown, the primary Si crystals still exhibit a bulk morphology, with a significantly reduced size ranging from 40 to 90 μm, but the edges remain relatively sharp. After adding 0.7 wt% rare earth Er, as... Figure 15c As shown, primary Si coarsens, its size increases, and numerous pores appear within the primary Si. Figure 16a and Figure 16b It can be seen that the eutectic Si in the unmodified Al-20Si alloy is lamellar and irregularly and randomly distributed. After adding rare earth Er, as... Figure 16c and Figure 16d As shown, the morphology of eutectic Si transforms into a dendritic shape, but due to the high Si content, the eutectic Si is more densely packed and the gaps between the dendrites are smaller.

[0097] The microstructure of Al-20Si alloy mainly consists of primary Si and eutectic Si. The presence of large, blocky primary Si crystals severely hinders the movement of free electrons, resulting in lower thermal conductivity compared to Al-7Si and Al-12Si alloys. Adding 0.5 wt% rare earth element Er reduces the size of the primary Si crystals and dendriticizes the eutectic Si, both of which facilitate the passage of free electrons and improve the thermal conductivity of Al-20Si. However, although the size of the primary Si crystals decreases, their edges remain relatively sharp, resulting in weak interfacial bonding between the primary Si and the Al matrix. This increases interfacial thermal resistance and enhances the scattering of free electrons at the interface, thus limiting the improvement in thermal conductivity. Adding 0.7 wt% rare earth element Er causes the primary Si crystals to coarsen, increasing their size and introducing more porosity. This increases the number of phase interfaces between the primary Si and the Al matrix, further enhancing interfacial scattering of free electrons and decreasing the thermal conductivity.

[0098] 3.4 Effect of Er on the High-Temperature Oxidation Resistance of Al-12Si Alloy

[0099] (1) Macroscopic morphological observation: From Figure 17 It can be seen that after 24 hours of oxidation, the oxide films of the Al-12Si alloys were relatively intact. Although some oxide film wrinkles and protrusions appeared around the crucible wall, the overall oxide film did not crack or peel off, demonstrating strong protective ability for the alloys. After 48 hours of oxidation, as... Figure 18 As shown, the oxide films of all alloys remained relatively intact without any damage. However, the Al-12Si alloy with a rare earth Er content of 0.7 wt% exhibited obvious wrinkles and protrusions on its surface. After 72 hours of oxidation, as shown... Figure 19 As shown, the Al-12Si alloy with a rare earth Er content of 0.1 wt% has the most intact oxide film. Bright white spots appeared on the surface of the unmodified Al-12Si alloy, indicating damage to the oxide film surface and partial exposure of the base alloy, but the exposure time was short and it still possessed a metallic luster. Cracks appeared on the surface of the Al-12Si alloy oxide film with a rare earth Er content of 0.5 wt%, indicating oxide film rupture. After 96 hours of alloy oxidation, as... Figure 20 As shown, the Al-12Si alloy oxide film with a rare earth Er content of 0.1 wt% remained the most intact, while the oxide films of other alloys showed varying degrees of damage, cracking, and pore formation. After 120 hours of alloy oxidation, as... Figure 21As shown, the Al-12Si alloy with a rare earth Er content of 0.1 wt% has the most intact oxide film. The surface of the unmodified Al-12Si alloy shows numerous dark gray spots, indicating fine microcracks in the oxide film layer. The base alloy undergoes prolonged oxidation through these microcracks. The Al-12Si alloy with a rare earth Er content of 0.7 wt% suffers the most severe oxide film damage, with partial peeling and loss of protective function. Macroscopic observation of the oxide films of various alloys reveals that a rare earth Er content of 0.1 wt% can enhance the oxide film, while excessive rare earth Er deteriorates it.

[0100] (2) Microscopic morphological observation: from Figure 22a It can be seen that after 120 hours of high-temperature oxidation, the unmodified Al-12Si alloy exhibits large oxide protrusions on its oxide film surface, and the oxide film is damaged with fine pores forming on the surface. This is also... Figure 21 The reason why many dark gray pits can be seen in the macroscopic morphology is due to this. From Figure 22b It can be seen that when the rare earth Er content is 0.1 wt%, the Al-12Si alloy oxide film is best preserved, without cracks or pores, exhibiting the best density and protection for the alloy. Further increasing the rare earth Er content, such as to 0.3 wt%,... Figure 22c As shown, large wrinkles appeared on the surface of the Al-12Si alloy oxide film, surrounded by numerous pores. When the rare earth Er content is 0.5wt%, as... Figure 22d As shown, the oxide film of the Al-12Si alloy exhibits significant damage, and the damaged oxide film is difficult to heal, resulting in the formation of a new oxide film with good density. When the rare earth Er content is 0.7wt%, as... Figure 22e As shown, the oxide film of the Al-12Si alloy peeled off. The peeled oxide film was loose and porous with poor density. After the oxide film peeled off, a new oxide film was formed. However, it can be seen that there were also pores on the surface of the newly formed oxide film, which drastically reduced the protective effect on the alloy.

[0101] from Figures 23a to 23d As can be seen, the main elements of the oxide film are Al, O, and Si. The distribution of Al and O elements indicates that the oxide film is primarily composed of Al₂O₃. The distribution of Si elements reveals that after 120 hours of oxidation, the Al₂O₃ oxide film exhibits poor density, with localized exposure of the Al-12Si alloy matrix. Figures 24a to 24c ( Figure 24b In this mixture, the mass percentages of O, Al, and Er are 49.40 wt%, 29.61 wt%, and 22.99 wt%, respectively, and the atomic percentages are 70.58%, 26.14%, and 3.27%, respectively. Figure 24cIn (where the mass percentages of O, Al, and Er are 49.30 wt%, 31.71 wt%, and 18.99 wt% respectively, and the atomic number percentages are 70.51%, 26.89%, and 2.60% respectively), it can be seen that the left point is the surface of the first-layer oxide film, and the right point is the newly formed oxide film of the matrix alloy. Except for the three elements Al, O, and Si, the element Er was found, indicating that during the formation of the Al2O3 oxide film, the rare earth Er will be enriched on the surface of the alloy.

[0102] (3) Analysis of isothermal oxidation kinetic curves:

[0103] According to the data in Table 7, it can be found that for the five Al-12Si alloys with different rare earth Er contents, as the oxidation time increases, their weight gains become larger and larger, but the weight gains during the same oxidation time interval (24 h) gradually decrease. From Figure 25 it can be seen that the isothermal oxidation kinetic curves of the five Al-12Si alloys with different rare earth Er contents are all approximately parabolic, following the parabolic law of oxidation kinetics. During the oxidation time from 0 to 48 h, the oxidation weight gains of the five Al-12Si alloys with different rare earth Er contents are relatively fast, indicating that the alloy is in the initial stage of oxidation. The surface of the alloy contacts oxygen at high temperature and undergoes an oxidation reaction to form an oxide film. During the oxidation time from 72 to 120 h, the oxidation weight gains of each alloy are relatively slow. This is mainly because a continuous and dense oxide film has formed on the surface of each alloy, protecting the interior of the matrix alloy and blocking its oxidation reaction.

[0104] Table 7 Oxidation weight gain per unit area of different alloys at different oxidation times

[0105]

[0106] From Figure 25 it can be known that within the range where the oxidation time does not exceed 120 h, the oxidation weight gains of each alloy are as follows: Al-12Si-0.1Er < Al-12Si ≈ Al-12Si-0.3Er < Al-12Si-0.5Er < Al-12Si-0.7Er alloy. When the addition amount of rare earth Er is 0.1 wt%, the high-temperature oxidation resistance of the Al-12Si alloy is the best, indicating that adding an appropriate amount of rare earth Er has a positive effect on the high-temperature oxidation resistance of the Al-12Si alloy, while adding an excessive amount of rare earth Er will deteriorate the high-temperature oxidation resistance of the Al-12Si alloy. When the oxidation time of the Al-12Si alloy with a rare earth Er addition amount of 0.7 wt% is within 96 - 120 h, the oxidation weight gain rate increases to a certain extent. Combining the observation of the macroscopic and microscopic morphologies of the alloy oxide film, this is mainly because the oxide film on the surface of the alloy has suffered relatively serious damage and peeling, and the matrix alloy undergoes an oxidation reaction to continue to form a new oxide film.

[0107] (4) Oxidation Mechanism Analysis: High-temperature oxidation resistance tests were conducted at 700℃ on Al-12Si alloys with different amounts of rare earth Er. The results showed that the Al-12Si alloy exhibited the best high-temperature oxidation resistance when the Er content was 0.1 wt%. Further increasing the Er content led to a decrease in the high-temperature oxidation resistance. Therefore, adding an appropriate amount of rare earth Er to Al-12Si alloys has a positive effect on their high-temperature oxidation resistance, while adding excessive amounts of Er will deteriorate their high-temperature oxidation resistance. First, in the initial stage of oxidation of the Al-12Si-XEr alloy, Er, as an active element, accumulates on the alloy surface, undergoing selective preferential oxidation to form Er2O3. The generated Er2O3 is dispersed throughout the oxide film, acting as nucleation sites for Al2O3, increasing the nucleation rate and promoting Al2O3 formation. Simultaneously, it hinders oxide film grain growth, refining the grains and allowing for better release of internal stress within the oxide film through diffusion, thus promoting the formation of a continuous and dense oxide film. Second, due to its large radius, Er has low solid solubility in Al and tends to accumulate at grain boundaries, hindering Al formation. 3+ and O 2- Ion diffusion from grain boundaries hinders the oxidation process. Finally, the addition of rare earth element Er increases the contact area between the oxide layer and the base alloy, reduces porosity at the interface, and improves adhesion between the oxide layer and the substrate. However, excessive addition of rare earth element Er leads to excessive accumulation of this active element on the alloy surface. During prolonged oxidation, the oxide film expands in volume, increasing internal stress. When this internal stress exceeds the adhesion force between the film and the substrate, numerous microcracks form, ultimately causing the oxide film to break down and detach, losing its protective function against the base alloy.

[0108] In summary, rare earth element Er can refine the microstructure of Al-Si alloys and improve the morphology of the Si phase. Adding rare earth Er increases the latent heat of phase transformation in Al-Si alloys, and the phase transformation temperature remains relatively stable without significant changes. Rare earth Er can also improve the thermal conductivity of Al-Si alloys. The increase in thermal conductivity after adding rare earth Er is mainly related to the changes in the Si phase within the microstructure. Adding an appropriate amount of rare earth Er to Al-12Si alloys has a positive effect on their high-temperature oxidation resistance, while adding excessive amounts of rare earth Er will deteriorate their high-temperature oxidation resistance. The experiment found that adding a certain amount of strontium to aluminum-silicon-based phase change materials can effectively reduce the deterioration of the high-temperature oxidation resistance of aluminum-silicon-based phase change materials caused by excessive erbium addition. In particular, when the amount of strontium added is 0.04 wt%, it can not only effectively reduce the deterioration of the high-temperature oxidation resistance of aluminum-silicon-based phase change materials caused by excessive erbium addition, but also has a positive effect on improving the high-temperature oxidation resistance of aluminum-silicon-based phase change materials.

[0109] 4. Effects of combined Er and Sr addition on the microstructure and thermal storage properties of Al-Si alloys

[0110] 4.1 Effect of combined Er and Sr addition on the microstructure of Al-Si alloy

[0111] (1) Effect of combined addition of Er and Sr on the microstructure of Al-7Si alloy

[0112] Figure 26a The microstructure is that of an unmodified Al-7Si alloy. When 0.1 wt% Er and 0.04 wt% Sr are added, the size of α-Al in the Al-7Si alloy decreases significantly, the distribution becomes more uniform, and the coarse dendrites in the microstructure disappear. From the magnified local image, it can be seen that all the long needle-like eutectic Si has been transformed into fibrous structures. With the continuous increase of rare earth Er content, the size of α-Al continuously decreases, and the refining effect continuously improves. When 0.5 wt% Er and 0.04 wt% Sr are added, such as... Figure 26d As shown, α-Al has the smallest size, the most uniform distribution, and the best refining effect; eutectic Si is distributed in a fibrous manner around α-Al. When 0.7wt% Er and 0.04wt% Sr are added, as... Figure 26e As shown, the α-Al size increases and its distribution becomes more uneven. A magnified view reveals coarse, irregular dendrites formed by interconnected partial dendrites within the microstructure. Figure 27It can be seen that the average secondary dendrite arm spacing of the unmodified Al-7Si alloy is approximately 15.70 μm. After the combined addition of Er and Sr, the secondary dendrite arm spacing of the alloy decreases significantly. When 0.5 wt% Er and 0.04 wt% Sr are added, the average secondary dendrite arm spacing of the alloy is the smallest, approximately 10.46 μm. With further increasing the Er content, the secondary dendrite arm spacing increases slightly to approximately 11.87 μm.

[0113] (2) Effect of combined Er and Sr addition on the microstructure of Al-12Si alloy

[0114] When 0.1 wt% Er and 0.04 wt% Sr are added, such as Figure 28a As shown, α-Al appears in the microstructure, but its size is relatively large, and all eutectic Si transforms from long needle-like structures to fibrous structures. When 0.3 wt% Er and 0.04 wt% Sr are added, as... Figure 28c As shown, α-Al has the smallest size and a significantly increased quantity, indicating a better metamorphic effect. Further increasing the content of rare earth Er, such as... Figure 28d and 28e As shown, the size of α-Al increases, and local coarsening occurs.

[0115] (3) Effect of combined Er and Sr addition on the microstructure of Al-20Si alloy

[0116] Figure 29a The microstructure is that of an unmodified Al-20Si alloy. When 0.1 wt% Er and 0.04 wt% Sr are added, as... Figure 29b As shown, the size of the primary Si crystals is significantly reduced, the coarse, plate-like primary Si crystals disappear, and they are replaced by irregular, fine, blocky primary Si crystals. The eutectic Si crystals change from long needle-like to fibrous. When 0.3 wt% Er and 0.04 wt% Sr are added, as shown... Figure 29c As shown, the primary Si crystals are the smallest in size, exhibiting a regular blocky morphology and uniformly distributed within the matrix. The eutectic Si crystals are all transformed into fibrous structures, achieving a better modification effect. Further increasing the content of rare earth Er, such as... Figure 29d and Figure 29e As shown, the primary Si crystals undergo varying degrees of coarsening, becoming larger in size, with sharper edges and corners, and exhibiting uneven distribution.

[0117] 4.2 Effect of combined Er and Sr addition on phase transformation temperature and latent heat of Al-Si alloy

[0118] (1) Effect of combined addition of Er and Sr on phase transformation temperature and latent heat of phase transformation of Al-7Si alloy

[0119] Depend on Figure 30It can be seen that the peak shape of the Al-7Si alloy did not change after the composite addition of Er and Sr. The DSC curves of the five alloys all showed two endothermic peaks, one sharp and the other flat. The sharper endothermic peak is due to the complete melting of the eutectic structure, while the flatter endothermic peak is due to the complete melting of α-Al. Table 8 shows that the Al-7Si-0.04Sr-0.5Er alloy has the highest latent heat of fusion, which is 16.22 J / g higher than that of the unmodified Al-7Si alloy. The composite addition of Er and Sr has a slightly greater effect on improving the latent heat of fusion of Al-7Si alloy than adding rare earth Er alone. The phase transformation temperatures of the Al-7Si-0.04Sr-XEr alloy are all above 572℃, and the phase transformation temperature range is between 572-622℃. The phase transformation temperature range of Al-7Si alloy is relatively stable after the composite addition of Er and Sr, without significant changes.

[0120] Table 8

[0121]

[0122]

[0123] (2) Effect of combined addition of Er and Sr on phase transformation temperature and latent heat of phase transformation of Al-12Si alloy

[0124] Depend on Figure 31 It can be seen that the peak shape of the Al-12Si alloy did not change after the composite addition of Er and Sr. The DSC curves of the five alloys all showed only one sharp endothermic peak, which is the result of complete melting of the eutectic structure. As shown in Table 9, the Al-12Si-0.04Sr-0.3Er alloy has the highest latent heat of fusion, which is 58.65 J / g higher than that of the unmodified Al-12Si alloy. The composite addition of Er and Sr has a better effect on improving the latent heat of fusion of Al-12Si alloy than the effect of adding rare earth Er alone. The phase transformation temperature of the Al-12Si-0.04Sr-XEr alloy is above 580℃, and the phase transformation temperature range is between 580-598℃. The phase transformation temperature range of Al-12Si alloy is relatively stable after the composite addition of Er and Sr, and no significant changes have occurred.

[0125] Table 9

[0126]

[0127] (3) Effect of combined addition of Er and Sr on phase transformation temperature and latent heat of phase transformation of Al-20Si alloy

[0128] Depend on Figure 32It can be seen that the peak shape of the Al-20Si alloy did not change after the composite addition of Er and Sr. The DSC curves of the five alloys all showed only one sharp endothermic peak, which is the result of complete melting of the eutectic structure. As shown in Table 10, the Al-20Si-0.04Sr-0.3Er alloy has the highest latent heat of fusion, which is 90.02 J / g higher than that of the unmodified Al-20Si alloy. The composite addition of Er and Sr has a significantly better effect on improving the latent heat of fusion of Al-20Si alloy than the effect of adding rare earth Er alone. The phase transformation temperature of the Al-20Si-0.04Sr-XEr alloy is above 572℃, and the phase transformation temperature range is between 572-594℃. The phase transformation temperature range of Al-20Si alloy is relatively stable after the composite addition of Er and Sr, and there is no significant change.

[0129] Table 10

[0130]

[0131] Melting generally begins at the interface, and the initial melting temperature is related to the number of phase interfaces within the microstructure. After the combined addition of Er and Sr, the phase transformation temperatures of Al-7Si, Al-12Si, and Al-20Si alloys all decreased to varying degrees. For Al-7Si alloys, after composite modification, the size of α-Al in the microstructure decreases, and the eutectic Si morphology completely transforms into a fibrous structure, with a significant decrease in size and a significant increase in quantity, leading to an increase in the number of internal phase interfaces and a slight decrease in the melting temperature of Al-7Si alloys. For Al-12Si alloys, after composite modification, a large number of α-Al particles appear in the microstructure, and the eutectic Si morphology completely transforms into a fibrous structure, significantly increasing the number of internal phase interfaces and causing a decrease in the melting temperature of Al-12Si alloys. For Al-20Si alloys, after composite modification, the coarse lamellar primary Si in the microstructure transforms into fine blocky particles, with a significant decrease in size and a significant increase in quantity, and the eutectic Si morphology completely transforms into a fibrous structure, increasing the number of internal phase interfaces and causing a decrease in the melting temperature of Al-20Si alloys.

[0132] DSC testing results show that the combined addition of Er and Sr significantly improves the latent heat of phase transformation in Al-Si binary alloys, and both effects are superior to the improvement effect of adding rare earth Er alone. This is attributed to the better modification effect achieved by the combined addition of Er and Sr on Al-Si alloys. The latent heat of phase transformation is generally related to the strength and number of interatomic bonds within the crystal. The Si phase in Al-Si alloys, as a small planar phase, has a high fusion entropy and can provide more latent heat during melting. For Al-7Si alloys, the latent heat provided by the melting of eutectic Si is much greater than that provided by α-Al. After composite modification, although the modification effect is good, the size of α-Al in the microstructure decreases and its quantity increases. The eutectic Si completely transforms into a fibrous structure, increasing the amount of eutectic Si. These factors all increase the number of bonds broken during melting. However, because the area ratio of eutectic Si in Al-7Si alloys is relatively low, the increase in the amount of eutectic Si in the microstructure is not significant. Therefore, the improvement effect of composite modification on the latent heat of phase transformation in Al-7Si alloys is only moderate. For Al-12Si alloy, after composite modification, all the long acicular eutectic Si in the microstructure transforms into fibrous structures, significantly increasing their quantity and increasing the alloy's fusion entropy. Therefore, composite modification effectively improves the latent heat of phase transformation in Al-12Si alloy. For Al-20Si alloy, after composite modification, the primary Si crystals change from coarse lamellar to fine blocky structures, and all the long acicular eutectic Si transforms into fibrous structures. This significantly increases the amount of Si phase in Al-20Si alloy and its fusion entropy, resulting in the best improvement in latent heat of phase transformation in Al-20Si alloy after composite modification.

[0133] 4.3 Effect of combined Er and Sr addition on the thermal conductivity of Al-Si alloy

[0134] (1) Effect of combined addition of Er and Sr on thermal conductivity of Al-7Si alloy

[0135] Depend on Figure 33 As shown in Table 11, the combined addition of Er and Sr significantly improves the thermal conductivity of the Al-7Si alloy, and is significantly better than the effect of adding rare earth Er alone. The thermal conductivity of the unmodified Al-7Si alloy is 154.40 W·m. -1 ·K -1 As the Er content increases, the thermal conductivity of the alloy continuously increases, showing a trend of first increasing and then decreasing. When 0.5 wt% Er and 0.04 wt% Sr are added, the thermal conductivity of the alloy reaches its maximum of 175.30 W·m. -1 ·K -1 Compared to the unmodified Al-7Si alloy, it increased by 20.90 W·m. -1 ·K -1When 0.7 wt% Er and 0.04 wt% Sr are added, the thermal conductivity of the alloy decreases to 168.93 W·m. -1 ·K -1 However, it still has a higher thermal conductivity than the unmodified Al-7Si alloy.

[0136] Table 11

[0137]

[0138] Depend on Figures 34a to 34d It can be observed that after the composite addition of Er and Sr, the morphology of the eutectic Si in the Al-7Si alloy changed significantly. The eutectic Si transformed entirely from a plate-like structure to a fibrous structure, with finer branch diameters and nearly spherical ends. The fibrous eutectic Si was densely distributed, with numerous voids left from the corrosion of α-Al between the branches. Figures 35a to 35e The results show that most of the eutectic Si is distributed around α-Al, with a small amount of uncerased α-Al remaining between the branches. The distribution of Si and Sr elements is basically consistent, indicating that the mechanism of Sr element modification is to hinder Si phase growth by adhering to the Si phase surface. The distribution of Er elements reveals that some Er elements are distributed at the interface between α-Al and eutectic Si. Due to the low solid solubility of Er elements in Al and Si, they accumulate at the interface where eutectic Si grows during alloy solidification, hindering eutectic Si growth.

[0139] The three-dimensional morphology of the Al-7Si-0.04Sr-XEr eutectic Si shows that, compared to Al-7Si alloy where Er is added alone and most of the eutectic Si transforms into fibrous and a few fine lamellar structures, the composite addition of Er and Sr results in a better modification effect, transforming the eutectic Si entirely into finely branched, spherically shaped fibers. This morphology of eutectic Si has a weaker ability to scatter free electrons, which is more conducive to electron passage, thus significantly improving the thermal conductivity of the Al-7Si alloy. Therefore, the thermal conductivity of the Al-7Si alloy with the composite addition of Er and Sr is better than that of the Al-7Si alloy with Er added alone. Furthermore, after composite modification, the α-Al size of the Al-7Si alloy is significantly reduced, and the secondary dendrite arm spacing is significantly decreased. Figure 27 The width of the barrier that free electrons need to pass through decreases significantly; the fibrous eutectic Si is tightly packed and has many gaps between branches, which increases the number of α-Al passing through during the movement of free electrons and reduces the barrier they encounter, thus improving the thermal conductivity of the alloy.

[0140] (2) Effect of combined addition of Er and Sr on thermal conductivity of Al-12Si alloy

[0141] Depend on Figure 36As shown in Table 12, the combined addition of Er and Sr significantly improves the thermal conductivity of the Al-12Si alloy, and is significantly better than the effect of adding rare earth Er alone. The thermal conductivity of the unmodified Al-12Si alloy is 145.28 W·m. -1 ·K -1 With increasing Er content, the thermal conductivity of Al-12Si alloy continuously increases. The alloy exhibits the highest thermal conductivity, reaching 164.39 W·m, when 0.3 wt% Er and 0.04 wt% Sr are added. -1 ·K -1 Compared to the unmodified Al-12Si alloy, it increased by 19.11 W·m. -1 ·K -1 As the content of rare earth element Er continues to increase, the thermal conductivity of the alloy decreases, but it is still higher than that of the unmodified Al-12Si alloy.

[0142] Table 12

[0143]

[0144] Depend on Figures 37a to 37c It can be seen that after the composite addition of Er and Sr, the morphology of the eutectic Si changed significantly, mostly transforming into a fibrous structure, similar to the Al-7Si alloy. However, the arrangement differs from the Al-7Si alloy; besides a portion of the eutectic Si being closely packed, another portion is distributed in a parallel state, with a relatively regular distribution of voids between the branches. The void distribution indicates a large number of α-Al atoms and a relatively uniform distribution. The three-dimensional morphology of the Al-12Si-0.04Sr-XEr eutectic Si shows that after the composite addition of Er and Sr, the morphology of the eutectic Si mostly changed from plate-like to fibrous. Compared to the lamellar morphology of the Al-12Si alloy eutectic Si after Er modification alone, the morphology after composite modification was greatly improved. The fibrous eutectic Si is more conducive to the passage of free electrons, thus significantly improving thermal conductivity. Furthermore, from the metallographic photographs (… Figures 28a to 28e It can be seen that after composite modification, a large amount of α-Al appears in the Al-12Si alloy microstructure, and a portion of the eutectic Si in the microstructure is distributed in parallel. Figures 37a to 37c This makes the α-Al distribution between branches more uniform, thereby increasing the number and channels of free electrons for effective heat transfer, increasing the mean free path of free electrons, and improving the thermal conductivity of Al-12Si alloy.

[0145] (3) Effect of combined addition of Er and Sr on thermal conductivity of Al-20Si alloy

[0146] Figure 38As shown in Table 13, the combined addition of Er and Sr significantly improves the thermal conductivity of the Al-20Si alloy, and is superior to the effect of adding rare earth Er alone. The thermal conductivity of the unmodified Al-20Si alloy is 125.17 W·m. -1 ·K -1 With increasing Er content, the thermal conductivity of Al-20Si alloy continuously increases. The alloy exhibits the highest thermal conductivity, reaching 142.98 W·m, when 0.3 wt% Er and 0.04 wt% Sr are added. -1 ·K -1 Compared to the unmodified Al-20Si alloy, it increased by 17.81 W·m. -1 ·K -1 As the content of rare earth element Er continues to increase, the thermal conductivity of the alloy decreases, but it is still higher than that of the unmodified Al-20Si alloy.

[0147] Table 13

[0148]

[0149] Depend on Figures 39a to 39b It can be seen that after the composite addition of Er and Sr, the morphology of the primary Si crystals did not change significantly, remaining blocky, but the size was significantly reduced, and the edges became blunted and rounded. From Figures 40a to 40d As can be seen, after the composite addition of Er and Sr, the morphology of eutectic Si changed significantly, transforming from lamellar to fibrous and coral-like structures. The fibrous eutectic Si constituted a higher proportion, with finer branches and a denser arrangement. The coral-like eutectic Si was less abundant and larger in size, with a maximum size between 10-15 μm. The three-dimensional morphology of the primary and eutectic Si in Al-20Si-0.04Sr-XEr alloys shows that after the composite addition of Er and Sr, the size of the primary Si decreased significantly to 15-40 μm, smaller than that of the primary Si in Al-20Si alloys modified by Er alone. Furthermore, the edges of the primary Si were significantly blunted after the composite modification, compared to the sharp edges of the primary Si in the Er-modified Al-20Si alloy. This enhanced the interfacial bonding with the Al matrix, reduced interfacial thermal resistance, weakened the scattering of free electrons, and improved thermal conductivity. The transformation of the eutectic Si from lamellar to fibrous and a small amount of coral-like structures facilitated the passage of free electrons, further improving the alloy's thermal conductivity.

[0150] 4.4 Effect of combined Er and Sr addition on the cyclic thermal stability of Al-Si alloys

[0151] (1) Microstructure analysis of Al-7Si alloy after thermal cycling

[0152] Figure 41a and Figure 41bThe images show the metallographic microstructures of Al-7Si alloy and Al-7Si-0.04Sr-0.5Er alloy after 100 thermal cycles, respectively. Before thermal cycling, the microstructure of the unmodified Al-7Si alloy consisted of α-Al and long needle-like eutectic Si. After Er and Sr composite modification, the α-Al size decreased and the distribution became more uniform, while the morphology of the eutectic Si changed from needle-like to fibrous. Figure 41a It can be seen that after thermal cycling, the α-Al size in the unmodified Al-7Si alloy increases and its distribution becomes uneven. The morphology of the eutectic Si does not change significantly, remaining elongated needle-like, but its size increases significantly, exhibiting localized coarsening. Figure 41b It can be seen that after thermal cycling, the α-Al in the Al-7Si alloy that underwent composite modification also grew. Most of the eutectic Si remained fibrous, while a small portion of the eutectic Si changed morphology, transforming into long needle-like structures. This indicates that although the added Er and Sr elements were burned off after multiple melting-solidification thermal cycles, the modification effect was still well maintained. Furthermore, it can be observed from the figure that after thermal cycling, both the unmodified and composite-modified Al-7Si alloys showed irregularly shaped and sized pores on their surfaces. This is mainly because the Al content of the Al-7Si alloy is relatively high. Although the crucible was sealed during the thermal cycling test, a small amount of air still entered the crucible, causing oxidation of the alloy. The resulting Al2O3 particles easily detached during repeated thermal cycles, forming pores. Figure 42a This is a backscattered image of the Al-7Si-0.04Sr-0.5Er alloy after thermal cycling. Figure 42b and Figure 42e The figure shows the distribution of elements in the alloy. As can be seen from the figure, the alloy is mainly composed of eutectic Si, and the phase composition remains stable after thermal cycling. Most of the eutectic Si is distributed in granular form in the Al matrix, indicating that the Al-7Si alloy with Er and Sr composite additions can still maintain a certain degree of modification effect after thermal cycling. However, almost no Sr element is visible in the surface scan results, because the amount of Sr added is small, and it was burned off during multiple melt-solidification thermal cycles, leading to a gradual weakening of the modification effect. The Er element in the alloy microstructure is mainly distributed at the interface between Al and eutectic Si. In addition, a large number of bright white second phases can be found around the eutectic Si. Figure 42e It can be seen that the bright white second phase contains a large amount of Er element.

[0153] (2) Microstructure analysis of Al-12Si alloy after thermal cycling

[0154] Figure 43a and Figure 43bThe images show the metallographic microstructures of Al-12Si alloy and Al-12Si-0.04Sr-0.3Er alloy after 100 thermal cycles, respectively. Before thermal cycling, the microstructure of the unmodified Al-12Si alloy mainly consists of long acicular eutectic Si. After Er and Sr composite modification, the microstructure of the Al-12Si alloy mainly consists of α-Al and fibrous eutectic Si. Figure 43a It can be seen that after thermal cycling, the eutectic Si morphology in the unmodified Al-12Si alloy remains elongated needle-like, but its size becomes larger, and irregular blocky primary Si crystals appear locally. Figure 43b It can be seen that after thermal cycling, α-Al disappears in the Al-12Si alloy microstructure after composite modification, and the morphology of eutectic Si changes from fibrous to long needle-like and granular. No pores were found to form in the Al-12Si alloy, indicating that the alloy did not undergo severe oxidation. The Al-12Si alloy maintains a dense microstructure and exhibits high stability during multiple thermal cycles. Figures 44a to 44e It can be seen that the alloy is mainly composed of eutectic Si, and the phase composition remains stable after thermal cycling. The eutectic Si is distributed in the Al matrix in the form of granules or long needles, and Sr is not visible in the microstructure, indicating a weakening of the modification effect. Er elements in the alloy microstructure are mainly distributed at the interface between Al and eutectic Si, which is similar to the composite modified Al-7Si alloy after thermal cycling. The bright white second phase surrounding the eutectic Si also contains a relatively large amount of Er elements.

[0155] (3) Microstructure analysis of Al-20Si alloy after thermal cycling

[0156] Figure 45a and Figure 45b The images show the metallographic microstructures of Al-20Si alloy and Al-20Si-0.04Sr-0.3Er alloy after 100 thermal cycles, respectively. Before thermal cycling, the microstructure of the unmodified Al-20Si alloy mainly consists of lamellar primary Si and long acicular eutectic Si. After Er and Sr composite modification, the microstructure of the Al-20Si alloy mainly consists of smaller blocky primary Si and fibrous eutectic Si. Figure 45a It can be seen that after thermal cycling, the unmodified Al-20Si alloy underwent significant segregation. The primary Si crystals became significantly larger, reaching 350-400 μm, while the morphology of the eutectic Si remained unchanged, still being elongated needle-like. Figure 45bIt can be seen that after thermal cycling, no segregation was observed in the Al-20Si alloy after composite modification. The morphology of primary Si changed from blocky to elongated, and the eutectic Si changed from fibrous to needle-like and a small amount of granular. Furthermore, it can be observed from the figures that after thermal cycling, both the unmodified and composite-modified Al-20Si alloys showed the formation of pores around the primary Si. The unmodified Al-20Si alloy showed a larger number of irregularly shaped pores after thermal cycling. This is mainly because Al-20Si alloy is a hypereutectic Al alloy. During the solidification process of the alloy melt, after reaching the liquidus temperature, a portion of the Si phase precipitates first. As the remaining Al and Si solidify, the alloy undergoes volume shrinkage. After multiple thermal cycles, pores are formed inside the alloy. Since the density of Si and Al differs significantly, during solidification, the difference in Si and Al densities causes Si to segregate at the pores, resulting in severe segregation in the Al-20Si alloy. Figures 46a to 46e It can be seen that the alloy mainly consists of primary Si and eutectic Si, and the phase composition remains stable after thermal cycling. Primary Si is elongated, while eutectic Si is distributed in the Al matrix as long needles and a small amount of granules, without segregation. Furthermore, Sr is not visible in the microstructure, indicating a weakened modification effect. Er is mainly distributed at the interfaces between Al, primary Si, and eutectic Si; the bright white secondary phase surrounding the Si phase also contains a significant amount of Er. Figure 47a As shown, a small number of bright white particles can be found distributed around the Si phase in the alloy microstructure. EDS analysis of these bright white particles reveals... Figure 47b As shown, the bright white particles contain a large amount of Er element. When the alloy melt solidifies, Er aggregates, and when its concentration reaches the eutectic composition, the Al3Er phase precipitates. Figure 47b (The mass percentages of Al, Si, and Er are 25.45wt%, 25.51wt%, and 49.04wt%, respectively, and the atomic percentages are 43.98%, 42.35%, and 13.67%, respectively.) This shows that the molar ratio of Er to Al is close to 1:3, which suggests that the bright white particles are Al3Er phase.

[0157] After thermal cycling, most of the eutectic Si in the composite-modified Al-7Si alloy retained its fibrous morphology; in the composite-modified Al-12Si alloy, some eutectic Si existed in a granular morphology; and in the composite-modified Al-20Si alloy, no segregation occurred, with primary Si existing in an elongated morphology and a small amount of eutectic Si existing in a granular morphology. Therefore, after multiple melt-solidification thermal cycles following the addition of Er and Sr, the Al-Si alloys maintained a certain degree of modification effect and structural stability. EDS analysis revealed that with increasing thermal cycling, Sr was burned off, gradually reducing the modification effect; while Er existed stably as the Al3Er phase, distributed around the Si phase, hindering the segregation and growth of the Si phase during alloy solidification, thus maintaining structural stability.

[0158] (4) Analysis of DSC results of Al-Si alloy after thermal cycling

[0159] Figure 48 The figure shows the DSC curves of the Al-Si alloys after 100 thermal cycles. As can be seen from the figure, the Al-7Si alloy exhibits two endothermic peaks after thermal cycling, while the Al-12Si and Al-20Si alloys each have only one endothermic peak. Compared to before thermal cycling, the peak shapes of the DSC curves have not changed, indicating that the phase composition of the alloys remains stable after thermal cycling, which is consistent with the results obtained from microstructure analysis. Table 14 lists the phase transformation temperature and latent heat parameters of the Al-Si alloys after 100 thermal cycles. Combining the data from Tables 8, 9, and 10, we can plot the changes in latent heat before and after cycling for the unmodified Al-Si alloy and the composite modified Al-Si alloy, as shown below. Figure 49 and Figure 50 As shown in the figure, the latent heat of phase transformation of Al-Si alloys decreased to varying degrees after thermal cycling, but the latent heat of phase transformation of the composite modified Al-Si alloy was still higher than that of the unmodified Al-Si alloy. In addition, Tables 8, 9, 10 and 14 show that the phase transformation temperature of Al-Si alloys increased slightly after thermal cycling, but remained generally stable.

[0160] Table 14

[0161]

[0162] Regarding the phase transformation temperature, after multiple thermal cycles, the Si phase in Al-7Si, Al-12Si, and Al-20Si alloys all exhibited aggregation and growth, and severe oxidation occurred within the alloys. This reduced the number of phase interfaces within the alloys, thus increasing the phase transformation temperature. The composite-modified Al-Si alloy, even after multiple thermal cycles, still showed some refinement effects. The smaller Si phase size and the presence of a small amount of rare earth phases in the microstructure resulted in a greater number of phase interfaces within the alloy. Therefore, after thermal cycling, the phase transformation temperature of the composite-modified Al-Si alloy was slightly lower than that of the unmodified Al-Si alloy.

[0163] Regarding the latent heat of phase transformation, after multiple thermal cycles, the latent heat of phase transformation of Al-Si alloys decreased to varying degrees. In Al-7Si alloys, the α-Al grains coarsen and the eutectic Si size increases after thermal cycling. This reduces the number of bonds that need to be broken for melting, leading to a decrease in the fusion entropy and thus a decrease in the latent heat of phase transformation. However, in composite-modified Al-7Si alloys, most of the eutectic Si remains fibrous, resulting in a relatively large amount of eutectic Si. Therefore, after thermal cycling, the latent heat of phase transformation in composite-modified Al-7Si alloys is higher than that in unmodified Al-7Si alloys. The latent heat of phase transformation in Al-12Si alloys decreased more significantly after thermal cycling, mainly because its microstructure is primarily composed of eutectic Si. This eutectic Si aggregates and grows after thermal cycling. Furthermore, in unmodified Al-12Si alloys, the Si phase segregates to form primary Si, reducing the amount of eutectic Si and the fusion entropy, thus lowering the latent heat of phase transformation. In the composite-modified Al-12Si alloy, the eutectic Si transforms from fibrous to long needle-like and granular forms after thermal cycling, significantly reducing the amount of eutectic Si and lowering the fusion entropy. This results in a lower latent heat of phase transformation (LTU) compared to the unmodified Al-12Si alloy, but it remains higher. The Al-20Si alloy exhibits significant segregation after thermal cycling. On one hand, the primary Si crystals become significantly larger. Due to the short holding time during thermal cycling, the segregated primary Si crystals are too large and do not completely melt, resulting in insufficient latent heat provided by the unmelted primary Si. On the other hand, the eutectic Si also coarsens, further reducing the fusion entropy and thus lowering the LTU. In the composite-modified Al-20Si alloy, no segregation occurs. The primary Si crystals are elongated, while the eutectic Si is long needle-like and contains a small amount of granular material. The amount of eutectic Si is relatively abundant, therefore, the LTU of the composite-modified Al-20Si alloy is higher than that of the unmodified Al-20Si alloy after thermal cycling.

[0164] In summary, the combined addition of Er and Sr effectively refines the microstructure of Al-Si alloys, significantly improving the morphology of the Si phase and transforming the eutectic Si from long needle-like to fibrous. The combined addition of Er and Sr significantly increases the latent heat of phase transformation (LTH) of Al-Si alloys while maintaining a stable LTH temperature. The combined addition of Er and Sr also significantly improves the thermal conductivity of Al-Si alloys. Furthermore, the combined addition of Er and Sr enhances the cyclic thermal stability of Al-Si alloys. After 100 thermal cycles, the eutectic Si in the Al-Si alloys coarsens and increases in size. The LTH of phase transformation in the Al-Si alloys decreases to varying degrees after thermal cycling, while the LTH slightly increases, but remains generally stable. The composite-modified Al-Si alloys still exhibit a certain degree of modification effect after thermal cycling, with a higher LTH than the unmodified Al-Si alloys, thus improving the cyclic thermal stability of Al-Si alloys.

Claims

1. An aluminum-based phase change material, characterized in that, The silicon content is 7–20 wt%, the erbium content is 0.1–0.7 wt%, the strontium content is 0.03–0.05 wt%, and the balance is aluminum; Aluminum-based phase change materials are prepared using the following steps: Step (1): Prepare alloy raw materials: pure aluminum, aluminum-silicon master alloy, aluminum-strontium master alloy, and aluminum-erbium master alloy; the mass fraction of aluminum in pure aluminum is greater than or equal to 99.9 wt%, the mass fraction of silicon in aluminum-silicon master alloy is 12-20 wt%, the mass fraction of strontium in aluminum-strontium master alloy is 10 wt%, and the mass fraction of erbium in aluminum-erbium master alloy is 20 wt%. Step (2): Place the prepared alloy raw materials, pure aluminum and aluminum-silicon master alloy, into a preheated graphite crucible, and then transfer it to a pit-type resistance furnace for melting. After the pure aluminum and aluminum-silicon master alloy are completely melted, an aluminum-silicon alloy liquid is obtained. The preheating temperature of the graphite crucible is 250-300℃. During melting, the resistance furnace is heated to 750-800℃ at a heating rate of 10-20℃ / min and held for 0.5-1h. Step (3): Add aluminum strontium master alloy and aluminum erbium master alloy to the aluminum-silicon alloy liquid in sequence and stir and melt with a graphite rod. After the aluminum strontium master alloy and aluminum erbium master alloy are completely melted, an aluminum-based phase change material alloy liquid is obtained. Step (4): Cool the aluminum-based phase change material alloy liquid to 700-720℃ and continue to keep it at that temperature for 10-15 minutes. After the temperature is kept at that temperature, stir it evenly with a graphite rod. Then, pour the aluminum-based phase change material alloy liquid into a preheated metal mold. Once the casting is completed, the aluminum-based phase change material is obtained. During casting, the preheating temperature of the metal mold is 200-250℃.

2. The aluminum-based phase change material according to claim 1, characterized in that, The mass fraction of silicon is 7 wt%, the mass fraction of erbium is 0.5 wt%, and the mass fraction of strontium is 0.04 wt%.

3. The aluminum-based phase change material according to claim 1, characterized in that, The mass fraction of silicon is 12 wt%, the mass fraction of erbium is 0.3 wt%, and the mass fraction of strontium is 0.04 wt%.

4. The aluminum-based phase change material according to claim 1, characterized in that, The mass fraction of silicon is 20 wt%, the mass fraction of erbium is 0.3 wt%, and the mass fraction of strontium is 0.04 wt%.

5. A method for preparing an aluminum-based phase change material as described in any one of claims 1-4, characterized in that, Includes the following steps: Step (1): Prepare alloy raw materials: pure aluminum, aluminum-silicon master alloy, aluminum-strontium master alloy, and aluminum-erbium master alloy; the mass fraction of aluminum in pure aluminum is greater than or equal to 99.9 wt%, the mass fraction of silicon in aluminum-silicon master alloy is 12-20 wt%, the mass fraction of strontium in aluminum-strontium master alloy is 10 wt%, and the mass fraction of erbium in aluminum-erbium master alloy is 20 wt%. Step (2): Place the prepared alloy raw materials, pure aluminum and aluminum-silicon master alloy, into a preheated graphite crucible, and then transfer it to a pit-type resistance furnace for melting. After the pure aluminum and aluminum-silicon master alloy are completely melted, an aluminum-silicon alloy liquid is obtained. The preheating temperature of the graphite crucible is 250-300℃. During melting, the resistance furnace is heated to 750-800℃ at a heating rate of 10-20℃ / min and held for 0.5-1h. Step (3): Add aluminum strontium master alloy and aluminum erbium master alloy to the aluminum-silicon alloy liquid in sequence and stir and melt with a graphite rod. After the aluminum strontium master alloy and aluminum erbium master alloy are completely melted, an aluminum-based phase change material alloy liquid is obtained. Step (4): Cool the aluminum-based phase change material alloy liquid to 700-720℃ and continue to keep it at that temperature for 10-15 minutes. After the temperature is kept at that temperature, stir it evenly with a graphite rod. Then, pour the aluminum-based phase change material alloy liquid into a preheated metal mold. Once the casting is completed, the aluminum-based phase change material is obtained. During casting, the preheating temperature of the metal mold is 200-250℃.

6. The method for preparing the aluminum-based phase change material according to claim 5, characterized in that, In step (1), the mass fraction of aluminum in pure aluminum is greater than or equal to 99.9 wt%, the mass fraction of silicon in aluminum-silicon master alloy is 12 wt% or 20 wt%, the mass fraction of strontium in aluminum-strontium master alloy is 10 wt%, and the mass fraction of erbium in aluminum-erbium master alloy is 20 wt%. In step (2), the preheating temperature of the graphite crucible is 250℃. During melting, the resistance furnace is heated to 750℃ at a heating rate of 10℃ / min and held for 1 hour. In step (4), the aluminum-based phase change material alloy liquid is cooled to 720°C and then kept at that temperature for 10 minutes; during casting, the preheating temperature of the metal mold is 200°C.

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