An Al-AlN ceramic composite material, its preparation method and uses

Abstract

The present invention discloses an Al-AlN ceramic composite material, its preparation method and use. The ceramic composite material contains an Al phase, an AlN phase, an Al2O3 phase and a Y3Al5O 12 phase. The Y3Al5O 12 phase is formed by the reaction of Al2O3 generated from AlN during air sintering with Y2O3. The preparation method comprises the following steps: Step (1), mixing aluminum powder, aluminum nitride powder and yttrium oxide powder to obtain a mixed raw material powder; Step (2), adding an aqueous solution of polyvinyl alcohol to the mixed raw material powder and grinding and mixing to obtain a ground mixture; Step (3), cold-pressing and forming the ground mixture to obtain a preform; Step (4), sintering the preform to obtain the product. The Al-AlN ceramic composite material has good compatibility with Al-Si heat storage materials and can be used for the encapsulation of Al-Si heat storage materials. The present invention can solve the technical problems such as poor sintering density at low temperature, unsatisfactory flexural strength and thermal conductivity of existing aluminum nitride ceramics.

Description

Technical Field

[0001] The present invention relates to the technical field of aluminum nitride ceramics. Specifically, it is an Al-AlN ceramic composite material, its preparation method and uses. Background Art

[0002] Solar thermal power generation technology has, to a certain extent, solved problems such as low energy utilization efficiency and spatio-temporal mismatch. However, the current heat storage materials applied in solar heat storage systems generally have problems such as low heat storage density, unstable properties during the heat cycle, and strong corrosiveness to encapsulation materials.

[0003] Al-Si heat storage materials have advantages such as large heat storage density, high thermal conductivity, and stable performance after repeated heat cycles, and are a good phase change heat storage material. However, it is prone to chemical and physical reactions with the encapsulation container at high temperatures, causing corrosion at the interface and even potentially leading to leakage of the heat storage material. Therefore, the performance of the encapsulation material is directly related to whether the Al-Si heat storage material can be promoted and utilized. Currently, the heat storage containers mainly include Al2O3, Si3N4, Al3N4, etc. Although these ceramic containers have strong corrosion resistance, they generally have problems such as poor thermal conductivity or low strength.

[0004] The prior art has used yttrium oxide as a sintering aid, aluminum and alumina as the skeleton materials, and prepared an Al-Al2O3 ceramic with good compatibility with Al-12Si heat storage materials by the cold pressing sintering method. When the addition amount of yttrium oxide is 0.2 wt%, and the addition amount of alumina is 20 wt% of the total amount of the skeleton materials, 8 wt% of polyvinyl alcohol solution (mass fraction 3 wt%) is added and pressed into shape (300 MPa, 100 s), and then sintered at 800 °C for 2 h with a heating rate of 10 °C / min. The prepared Al-Al2O3 ceramic has a low porosity, high flexural strength and thermal conductivity, and the best compatibility with Al-12Si heat storage materials (the wetting angle can reach up to 88°). However, the flexural strength of the alumina ceramic prepared by this method is at most only 76.32 MPa, and the thermal conductivity can also only reach 46.82 W / (m·K) at most. What is more insufficient for it to be used as the encapsulation material of Al-Si heat storage materials is that Si atoms have undergone complete diffusion without heat cycling.

[0005] Al / AlN ceramic composites can be used as the encapsulation materials for Al-Si alloy heat storage materials. In addition to having strong corrosion resistance, they also have high thermal conductivity and flexural strength. However, AlN is a strong covalent bond compound and is difficult to sinter densely at low temperatures. Therefore, it is necessary to find a suitable sintering aid to prepare Al / AlN composites with excellent comprehensive properties at a lower sintering temperature. Summary of the Invention

[0006] For this reason, the technical problem to be solved by the present invention is to provide an Al-AlN ceramic composite material with a small porosity, high flexural strength and high thermal conductivity, and its preparation method and use, so as to solve the technical problems such as poor sintering density at low temperature, insufficient flexural strength and thermal conductivity of the existing aluminum nitride ceramics.

[0007] To solve the above technical problems, the present invention provides the following technical solutions:

[0008] An Al-AlN ceramic composite material contains Al phase, AlN phase, Al2O3 phase and Y3Al5O 12 phase, and the Y3Al5O 12 phase is formed by the reaction of Al2O3 generated from AlN during air sintering with Y2O3. During air sintering, the sintering temperature is controlled at 900 - 1200 °C. When aluminum nitride reacts with oxygen, nitrogen gas overflows and highly active alumina with a loose and porous structure is formed. This loose and porous highly active alumina is different from the low-active dense alumina formed by the reaction of aluminum with oxygen, and it can react with yttrium oxide at 900 - 1200 °C to generate the Y3Al5O 12 phase as a strengthening phase, thus playing a role in bridging and filling pores, and simultaneously improving the strength and thermal conductivity of the Al-AlN ceramic composite material.

[0009] In the above Al-AlN ceramic composite material, the AlN phase accounts for 20 - 30 wt% of the sum of the Al phase, AlN phase, Al2O3 phase and Y3Al5O 12 phase, the Al2O3 phase accounts for 29.4 - 29.6 wt% of the sum of the Al phase, AlN phase, Al2O3 phase and Y3Al5O 12 phase, the Y3Al5O 12 phase accounts for 0.4 - 0.6 wt% of the sum of the Al phase, AlN phase, Al2O3 phase and Y3Al5O 12 phase, and the balance is the Al phase. By controlling the proportions of the Al phase, AlN phase, Al2O3 phase and Y3Al5O 12 phase in the Al-AlN ceramic composite material within the above range, an Al-AlN ceramic composite material with a small porosity, high flexural strength and high thermal conductivity can be obtained.

[0010] A preparation method of an Al-AlN ceramic composite material includes the following steps: Step (1), mixing aluminum powder, aluminum nitride powder and yttrium oxide powder evenly to obtain a mixed raw material powder; Step (2), adding an aqueous solution of polyvinyl alcohol to the mixed raw material powder, grinding and mixing evenly to obtain a ground mixture; Step (3), cold-pressing the ground mixture into a preform; Step (4), sintering the preform, and naturally cooling to room temperature after the sintering treatment is completed to obtain the above Al-AlN ceramic composite material.

[0011] In the preparation method of the above Al-AlN ceramic composite material, in step (1), the mass fraction of aluminum in the aluminum powder is greater than or equal to 99.99 wt%, and the particle size of the aluminum powder is 40 - 100 µm; the mass fraction of aluminum nitride in the aluminum nitride powder is greater than or equal to 99.99 wt%, and the particle size of the aluminum nitride powder is 480 - 550 nm; the mass fraction of yttrium oxide in the yttrium oxide powder is greater than or equal to 99.9 wt%, and the particle size of the yttrium oxide powder is 40 - 100 nm; in the mixed raw material powder, the mass fraction of the yttrium oxide powder is 0.2 - 1.0 wt%, and the mass ratio of the aluminum powder to the aluminum nitride powder is (1.5 - 4) : 1; when the dosage ratio among the aluminum powder, aluminum nitride powder and yttrium oxide powder within the above particle size ranges is within the above range, under the preparation process conditions of the present invention, the Al2O3 phase generated in the Al-AlN ceramic composite material can react with yttrium oxide to generate more Y3Al5O 12 phase, so that the proportions of the Al phase, AlN phase, Al2O3 phase and Y3Al5O 12 phase in the structure of the Al-AlN ceramic composite material are appropriate, which is beneficial to preparing an Al-AlN ceramic composite material with a lower porosity, higher thermal conductivity and flexural strength.

[0012] In the preparation method of the above Al-AlN ceramic composite material, in step (1), the mass fraction of aluminum in the aluminum powder is equal to 99.99 wt%, and the average particle size of the aluminum powder is 48 µm. If the particle size of the aluminum powder is too large, a higher sintering temperature is required during sintering to achieve sufficient bonding between the particles. If the particle size of the aluminum powder is too small, due to the large contact area between the particles, a lower sintering temperature or a shorter sintering time is required to achieve sufficient bonding between the particles, which may lead to poor densification of the prepared composite material or prone to over-sintering phenomenon. The mass fraction of aluminum nitride in the aluminum nitride powder is equal to 99.99 wt%, and the average particle size of the aluminum nitride powder is 500 nm. The aluminum nitride powder with too small particles may aggregate during the processing, increasing the difficulty of mixing and forming. That is to say, if the average particle size of aluminum nitride is too small, it is difficult to disperse evenly in the aluminum matrix due to easy agglomeration phenomenon, which will reduce the flexural strength and thermal conductivity of the composite material. If the average particle size of aluminum nitride is too large, it may reduce the densification and flexural strength of the composite material due to the formation of larger pores in the composite material. The mass fraction of yttrium oxide in the yttrium oxide powder is equal to 99.9 wt%, and the average particle size of the yttrium oxide powder is 50 nm. Too large a particle size of yttrium oxide makes it difficult to distribute evenly in the aluminum matrix, and it is easy to form larger pores during the sintering process, resulting in an increase in the porosity of the material, reducing the density and mechanical properties of the material. If the particle size of yttrium oxide is too small, due to its high reaction activity, the sintering time may be too short and the sintering process is not sufficient, resulting in incomplete elimination of the defects inside the composite material, thus affecting the densification degree of the material. The optimal dosage of each raw material in the mixed raw material powder: the mass fraction of aluminum powder is 70 wt%, the mass fraction of aluminum nitride powder is 29.6 wt%, and the mass fraction of yttrium oxide powder is 0.4 wt%. By controlling the particle sizes of the aluminum powder, aluminum nitride powder and yttrium oxide powder within a certain range, the present invention is conducive to adjusting the sintering temperature and sintering time to a suitable range, enabling each substance in the mixed raw material powder to be fully sintered under this sintering condition, and effectively eliminating the defects generated inside the composite material, so as to be able to prepare an Al-AlN ceramic composite material with good densification, high thermal conductivity and high flexural strength.

[0013] In the preparation method of the above Al-AlN ceramic composite material, in step (2), the mass fraction of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 3-5 wt%, and the relative molecular mass of polyvinyl alcohol is 30,000-100,000; the dosage of the polyvinyl alcohol aqueous solution is 5-10 wt% of the mass of the mixed raw material powder. Adding polyvinyl alcohol within the specific molecular weight range in the above dosage as the binder for aluminum powder, aluminum nitride powder and yttrium oxide powder can make the prefabricated block obtained after pressing have appropriate mechanical strength, which is beneficial to obtaining crack-free and high-mechanical-strength Al-AlN ceramic composite material in the subsequent sintering process; the grinding time is 30-60 min. Through grinding, the aluminum powder, aluminum nitride powder and yttrium oxide powder can be fully mixed, and the material particles can be more evenly dispersed in the matrix, improving the uniformity and performance of the material. If the grinding time is too short, it is difficult for the aluminum nitride powder and yttrium oxide powder to be evenly dispersed in the matrix aluminum powder, which may lead to local aggregation and affect the uniformity and performance of the finally prepared composite material. If the grinding time is too long, not only will the energy consumption increase significantly, the production cost increase, and the production efficiency decrease, but also impurities may be introduced during the long-time grinding, affecting the purity and performance of the product; in step (3), the conditions for cold pressing are: 300-400 MPa, and the pressure is maintained for 60-120 s. If the cold pressing pressure exceeds 400 MPa, cracks will occur in the composite material during the demolding process, affecting the material performance; if the cold pressing pressure is less than 300 MPa, it is difficult to remove the air holes in the composite material, resulting in a decrease in the density of the composite material and affecting the product performance; if the pressure holding time during cold pressing is too long, the raw material particles will be crushed under excessive pressure, especially in the case of more hard particles, and the particle crushing will increase the porosity of the green compact, reducing its density and strength; in addition, a too long pressure holding time means that the mold stays under high pressure for a longer time, which will exacerbate the wear of the mold. When the pressure holding time is too short, the particles may not be fully combined, resulting in uneven density distribution inside the green compact (the particles in some areas are tightly combined, while those in other areas are relatively loose); and, when the pressure holding time is too short, there is not enough time for the particles to form a stable bond under pressure, resulting in a low strength of the green compact; during subsequent handling, processing or sintering, the green compact is prone to breakage or deformation, further affecting the performance.

[0014] [[ID=३]]In the preparation method of the above Al-AlN ceramic composite material, in step (2), the mass fraction of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 3 wt%, the relative molecular mass of polyvinyl alcohol is 50,000-70,000; the dosage of the polyvinyl alcohol aqueous solution is 8 wt% of the mass of the mixed raw material powder; the grinding time is 40 min; in step (3), the conditions for cold pressing are: 300 MPa, and the pressure is maintained for 100 s.

[0015] In the preparation method of the above Al-AlN ceramic composite material, in step (4), the sintering treatment method is as follows: heating from room temperature to 900-1200 °C at a heating rate of 10-15 °C / min and holding for 2-4 h. During the sintering process, if the heating rate is too fast (greater than 15 °C / min), a large temperature gradient will be generated between the inside and the surface of the composite material, resulting in thermal stress; this thermal stress may form stress concentration points inside the material, leading to the initiation and propagation of cracks; moreover, too fast a heating rate will also cause volatile impurities and gases in the material to have no time to escape and remain in the sintered body to form pores, thereby reducing the density and strength of the material; in addition, too fast a heating rate also has a greater impact on the heating equipment, resulting in a shortened service life of the equipment. If the heating rate is lower than 10 °C / min, the sintering time will be significantly prolonged, the production efficiency will be reduced, and the production cost will be increased.

[0016] During the sintering process, if the sintering temperature is lower than 900 °C, the fluidity of Al in the Al-AlN composite material is insufficient, less highly reactive alumina is generated by the reaction of part of aluminum nitride and oxygen, and it is difficult for Y2O3 and the generated Al2O3 to fully react, resulting in less Y3Al5O 12 generated, which cannot play a good connection role, resulting in more defective pores inside the material, so that when the composite material bears a bending load, the stress cannot be well transmitted, and the bending strength decreases; at the same time, more defects will increase the scattering degree of phonons at the interface, reduce the mean free path of phonons, and decrease the thermal conductivity. If the sintering temperature is higher than 1200 °C, the AlN and Al2O3 grains in the Al-AlN composite material will grow rapidly, resulting in a decrease in the number of grain boundaries; since grain boundaries play a role in hindering crack propagation in the material, the decrease in the number of grain boundaries will reduce the bending strength of the material; in addition, during high-temperature sintering, the grain size is too large and the number of grain boundaries decreases. Although the increase in grain size helps heat conduction to a certain extent, the decrease in the number of grain boundaries will make the heat conduction path longer, resulting in a decrease in thermal conductivity.

[0017] During the sintering process, when the holding time is less than 2 h, the Al-AlN composite material cannot be fully densified, and there are more pores and voids inside; these internal defects will reduce the mechanical properties of the material, resulting in a decrease in bending strength; moreover, when the holding time is too short, there are more impurities and defects at the grain boundaries, and these defects will increase the thermal resistance and further reduce the thermal conductivity. If the holding time exceeds 4 h, the AlN phase in the microstructure decreases too much, the amount of alumina phase increases too much, and the AlN and Al2O3 grains will grow excessively, resulting in too large a grain size. Too large a grain size leads to a decrease in the number of grain boundaries, and grain boundaries play a role in hindering crack propagation in the material. The decrease in the number of grain boundaries will reduce the bending strength of the material; in addition, when the holding time is too long, the grain size is too large and the number of grain boundaries decreases. The decrease in the number of grain boundaries will make the heat conduction path longer, resulting in a decrease in thermal conductivity.

[0018] By controlling the heating rate, sintering temperature, and sintering time within the above ranges during sintering, the present invention can make the AlN grains and Al2O3 grain sizes in the structure of the Al-AlN ceramic composite material reasonable and the quantity moderate. And under this sintering condition, the yttrium oxide in the structure of the Al-AlN ceramic composite material can fully react with the generated highly active alumina to generate a relatively large quantity of Y3Al5O 12 phase. These Y3Al5O 12 phases can effectively improve the interfacial bonding effect between the Al phase and the AlN phase in the structure at the sintering temperature and sintering duration of the present invention, enhance the wettability between the Al phase and the AlN phase, thereby significantly improving the density, thermal conductivity, and flexural strength of the Al-AlN ceramic composite material.

[0019] In the preparation method of the above Al-AlN ceramic composite material, in step (4), the sintering treatment method is: heating from room temperature to 1200 °C at a heating rate of 10 °C / min and holding for 2 h.

[0020] In the preparation method of the above Al-AlN ceramic composite material, in step (1), the mass fraction of aluminum in the aluminum powder is equal to 99.99 wt%, and the average particle size of the aluminum powder is 48 µm; the mass fraction of aluminum nitride in the aluminum nitride powder is equal to 99.99 wt%, and the average particle size of the aluminum nitride powder is 500 nm; the mass fraction of yttrium oxide in the yttrium oxide powder is equal to 99.9 wt%, and the average particle size of the yttrium oxide powder is 50 nm; in the mixed raw material powder, the mass fraction of the aluminum powder is 70 wt%, the mass fraction of the aluminum nitride powder is 29.6 wt%, and the mass fraction of the yttrium oxide powder is 0.4 wt%; in step (2), the mass fraction of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 3 wt%, and the relative molecular mass of polyvinyl alcohol is 50,000 - 70,000; the dosage of the polyvinyl alcohol aqueous solution is 8 wt% of the mass of the mixed raw material powder; the grinding time is 40 min; in step (3), the cold pressing forming conditions are: 300 MPa, holding pressure for 100 s; in step (4), the sintering treatment method is: heating from room temperature to 1200 °C at a heating rate of 10 °C / min and holding for 2 h.

[0021] A use of an Al-AlN ceramic composite material, using the Al-AlN ceramic composite material prepared by the above preparation method of the Al-AlN ceramic composite material as a packaging material for an Al-Si heat storage material.

[0022] The technical solution of the present invention has achieved the following beneficial technical effects:

[0023] Preparation method of Al-AlN ceramic composite material, using aluminum nitride powder with a specific particle size, aluminum powder with a specific particle size, and yttrium oxide powder with a specific particle size as the main raw materials. By controlling the ratio among the aluminum nitride powder, aluminum powder, and yttrium oxide powder, and adjusting the heating rate, sintering temperature, sintering time, etc. during the sintering of the preform after cold pressing, so that the aluminum nitride powder, aluminum powder, and yttrium oxide powder can generate a specific amount of Al2O3 phase and Y3Al5O 12 phase during the sintering process under the action of oxygen, making the proportions of Al phase, AlN phase, Al2O3 phase, and Y3Al5O 12 phase in the Al-AlN ceramic composite material within a suitable range; and under the high-temperature sintering conditions of the present invention, the Y3Al5O 12 phase can effectively improve the interfacial bonding between the Al phase and the AlN phase, enhance the wettability between the two phases, make the AlN grains grow and develop, reduce the number of grain boundaries, make the grain boundaries clearer and smoother, and reduce the thermal resistance at the grain boundaries; at the same time, when the composite material bears a bending load, the stress is effectively transmitted, making it not easily damaged, and the thermal conductivity and bending strength of the composite material are both significantly improved. Description of the Drawings

[0024] Figure 1a 、 Figure 1b and Figure 1c are the XRD patterns of Al-40AlN, Al-30AlN, and Al-20AlN composite materials prepared at different sintering temperatures in the examples respectively;

[0025] Figure 2 is the backscattering pattern of the Al-40AlN composite material at 1200 °C in the example;

[0026] Figure 3 is the backscattering pattern of the Al-30AlN composite material at 1200 °C in the example;

[0027] Figure 4 are the backscattering patterns of the Al-20AlN composite material at 1200 °C in the example respectively;

[0028] Figure 5 are the XRD patterns of the Al-30AlN composite materials with different Y2O3 contents in the example;

[0029] Figure 6a and Figure 6b are the backscattering pattern and surface scan pattern of the Al-30AlN composite material without adding Y2O3 in the example respectively;

[0030] Figure 7a 、 Figure 7b are the backscattering pattern and surface scan pattern of the Al-30AlN composite material with added Y2O3 in the example respectively;

[0031] Figure 8 are Figure 7a partial enlarged view and point scan of Y element;

[0032] Figure 9a and Figure 9b are XRD diagrams of Al-30AlN composite material and Al-30AlN-0.4Y2O3 composite material before and after thermal cycling in the examples respectively;

[0033] Figure 10a and Figure 10b are secondary electron image and backscattered image of Al-30AlN composite material after thermal cycling in the examples respectively;

[0034] Figure 11a and Figure 11b are secondary electron image and backscattered image of Al-30AlN composite material after thermal cycling in the examples respectively;

[0035] Figure 12 are Figure 11a partial enlarged view and point scan of Y element;

[0036] Figure 13a 、 Figure 13b are backscattered image and line scan of the interface between Al-30AlN-0.4Y2O3 composite material and Al-12Si alloy under 0 thermal cycle respectively;

[0037] Figure 14a 、 Figure 14b are backscattered image and line scan of the interface between Al-30AlN composite material and Al-12Si alloy under 0 thermal cycle respectively;

[0038] Figure 15a 、 Figure 15b are backscattered image and line scan of the interface between Al-30AlN-0.4Y2O3 composite material and Al-12Si alloy under 15 thermal cycles respectively;

[0039] Figure 16a 、 Figure 16b are backscattered image and line scan of the interface between Al-30AlN composite material and Al-12Si alloy under 15 thermal cycles respectively;

[0040] Figure 17a 、 Figure 17b are backscattered image and line scan of the interface between Al-30AlN-0.4Y2O3 composite material and Al-12Si alloy under 30 thermal cycles respectively;

[0041] Figure 18a 、 Figure 18bBackscattered electron images and line scans of the interface between Al-30AlN composites and Al-12Si alloy under 30 thermal cycles, respectively. Detailed implementation manners

[0042] 1. Experimental method

[0043] In this example, Al powder (purity 99.99 wt%, average particle size 48 µm), AlN powder (purity 99.99 wt%, average particle size 500 nm), and Y2O3 powder (0 - 1 wt%, referring to the mass fraction of Y2O3 powder in the mixed raw material powder, purity 99.9 wt%, average particle size 50 nm) are used as the main raw materials. They are mixed evenly in a certain proportion to obtain the mixed raw material powder. An aqueous solution of polyvinyl alcohol (PVA) with 8 wt% of the mass of the mixed raw material powder (the mass fraction of PVA in the PVA aqueous solution is 3 wt%, and the relative molecular mass of polyvinyl alcohol is 50,000 - 70,000) is added to the mixed raw material powder, and they are mixed and stirred evenly. Then, it is transferred to a mortar and manually ground for about 40 min until evenly mixed. After that, 10 g of the ground mixture is taken each time and placed in a mold with a diameter of φ30 mm × 5 mm, and it is pressed into a preform at 300 MPa, with a pressure holding time of 100 s. The preform is placed in a resistance furnace for sintering, with a heating rate of 10 °C / min. It is heated from room temperature to different sintering temperatures, held for 2 h, and then cooled with the furnace.

[0044] 2. Effects of sintering temperature and Al powder content on the microstructure and properties of the composites

[0045] Table 1 Compositions and sintering processes of composites at different sintering temperatures and different Al powder contents

[0046]

[0047] The Al / AlN composite specimens prepared according to Table 1 at different sintering temperatures and different Al contents have regular shapes and no leakage of aluminum liquid on the surface.

[0048] SEM scans are performed on the specimens prepared according to Table 1 at different sintering temperatures and different Al contents. The results show that as the sintering temperature and Al content increase, the melting amount of Al increases, the fluidity increases, the filling ability increases, the defects decrease, and the density increases.

[0049] Figures 1a to 1c XRD patterns of specimens at different sintering temperatures and different Al contents. It can be seen from the figure that in addition to the main crystal phases of Al and AlN in the Al / AlN composites, an Al2O3 phase also appears. As the sintering temperature and Al content increase, the contact area between the Al phase and the AlN phase and O2 increases, and the chemical reaction proceeds in the direction of the product Al2O3, and the content of the Al2O3 phase gradually increases.

[0050] EBSD scanning and surface scanning were carried out on the specimens prepared at different sintering temperatures and with different Al contents in Table 1. Figure 2 and Figure 3 and Figure 4 are the backscattered images of specimens with different Al contents at 1200 °C. According to the scanning results, it was found that at 1200 °C, the AlN phase was dispersedly distributed in the Al matrix, the bonding between each phase was not firm, the grain size of the AlN phase did not grow significantly, and was all about 5 µm - 6 µm. In addition, O elements were generated on the Al matrix and around the N element, that is, the newly generated Al2O3 phase, with part of it distributed in the Al matrix and the other part covering around the AlN.

[0051] Table 2 shows the porosity, thermal conductivity and flexural strength of specimens at different sintering temperatures and with different Al contents. It can be seen from Table 2 that as the sintering temperature and Al content increase, the porosity of the Al / AlN composite gradually decreases, and the thermal conductivity and flexural strength gradually increase. This is because after increasing the sintering temperature and the content of Al, the melting amount of Al increases, the fluidity enhances, the filling ability enhances, and the defects decrease, resulting in a reduced degree of phonon scattering at the interface, and when the composite material bears a flexural load, no stress concentration occurs, so the thermal conductivity and flexural strength of the composite material increase.

[0052] Table 2

[0053]

[0054] 3. Influence of Y2O3 powder content on the microstructure and properties of the composite material

[0055] The properties of the Al-AlN composite material prepared at 1200 °C are good, but the bonding between each phase is still not firm. When the Al content is too low (60%), the thermal conductivity and flexural strength of the specimen will be low; when the Al content is too high (80%), the content of Al2O3 in the specimen will increase. Therefore, specimens with an Al content of 70% and an AlN content of 30% should be selected, and Y2O3 is added to further optimize the microstructure of the specimen and improve the properties of the specimen. Table 3 shows the raw material ratio and sintering process parameters of the specimen after adding Y2O3.

[0056] Table 3 Raw material ratio and sintering process parameters of the specimen after adding Y2O3

[0057]

[0058] For the Al-30AlN composite material added with different contents of Y2O3 prepared according to Table 3, as the content of Y2O3 increases, the shapes of each specimen are relatively regular, no obvious cracks are generated, and no leakage of aluminum liquid occurs on the surface of the specimen.

[0059] Figure 5 XRD patterns of Al-30AlN composites with different Y2O3 contents. It can be seen from Figure 5 that as the Y2O3 content increases, the contact area between Y2O3 and Al2O3 increases, and the content of the generated Y3Al5O 12 phase increases.

[0060] Figures 6a to 6b And Figures 7a to 7b are the backscattered electron image and surface scan image of the Al-30AlN composites sintered without and with Y2O3 addition, respectively. By comparison, it is found that the Al-30AlN composites without Y2O3 addition have more pores and cracks, and most of them are evenly distributed at the grain boundaries. The bonding between the Al phase and the AlN phase is not tight, the overall structure is relatively loose, the distribution of the AlN phase in the Al matrix is uneven, and Al2O3 phases appear around the AlN phase and the Al phase, and the formed Al2O3 is not dense either. However, in the Al-30AlN composites with Y2O3 addition, there are fewer pores, the bonding between the Al phase and the AlN phase is relatively tight, no obvious cracks are generated between the phases, the distribution of the AlN phase in the Al matrix is also relatively uniform, the AlN grains grow well. It can be seen from Figure 7a that a large number of white particle phases with regular shapes are distributed at the grain boundaries and defects of the Al matrix. By performing point scanning on the white particles, it can be obtained that the atomic ratio of the three elements Y, Al, and O is close to 3:5:12. Therefore, the white particle phase is the newly generated Y3Al5O 12 phase, as marked by the square in Figure 7a and shown in Figure 8 .

[0061] Table 4 shows the test results of the porosity, thermal conductivity, and flexural strength of the Al-30AlN composites with different Y2O3 contents.

[0062] Table 4

[0063]

[0064] It can be seen from Table 4 that as the Y2O3 content increases, the porosity of the Al-30AlN composites shows a trend of first decreasing and then increasing, while the thermal conductivity and flexural strength show a trend of first increasing and then decreasing. This is because after adding an appropriate amount of Y2O3, in addition to improving the thermal conductivity of the composites through the fluidity of Al, Y2O3 can react with Al2O3 to form the Y3Al5O 12 phase. The newly generated Y3Al5O 12 phase is distributed at the grain boundaries, which not only plays a good connecting role, reduces the pores and cracks at the grain boundaries, making the porosity of the composites further decrease (2.2%). In addition, the Y3Al5O12 It also has good chemical stability and thermal stability. During the high-temperature sintering process, it can effectively improve the interfacial bonding between the Al phase and the AlN phase, enhance the wettability between the two phases, promote the sintering of the Al-30AlN composite material, make the AlN grains grow and develop, reduce the number of grain boundaries, make the grain boundaries clearer and smoother, and at the same time reduce the thermal resistance at the grain boundaries. Moreover, when the composite material bears a bending load, the stress is effectively transmitted, making it not easily damaged, and further improving the thermal conductivity and bending strength of the composite material. However, excessive addition of Y2O3 will generate excessive Y3Al5O 12 phase, which will cause the uneven distribution of the Y3Al5O 12 phase in the composite material, forming local enrichment regions, increasing the degree of phonon scattering by the grain boundaries, hindering the conduction of heat, reducing the thermal conductivity of the composite material. At the same time, the phenomenon of uneven distribution of the Y3Al5O 12 phase will also cause stress concentration. When the composite material is subjected to a bending load, cracks are likely to occur at the stress concentration points, and the cracks will rapidly expand, ultimately leading to a decrease in the bending strength of the composite material.

[0065] In view of this, when the Al content is 70%, the Y2O3 content is 0.4%, the pressing pressure is 300 MPa, and the sintering temperature is 1200 °C, the comprehensive performance of the Al / AlN composite material is the best. At this time, the porosity is 2.2%, the thermal conductivity is 84.95 W / (m∙K), and the bending strength is 352.04 MPa; this Al / AlN composite material contains the Al phase, the AlN phase, the Al2O3 phase and the Y3Al5O 12 phase. The AlN phase accounts for 20-30 wt% of the sum of the Al phase, the AlN phase, the Al2O3 phase and the Y3Al5O 12 phase. The Al2O3 phase accounts for 29.4-29.6 wt% of the sum of the Al phase, the AlN phase, the Al2O3 phase and the Y3Al5O 12 phase. The Y3Al5O 12 phase accounts for 0.4-0.6 wt% of the sum of the Al phase, the AlN phase, the Al2O3 phase and the Y3Al5O 12 phase.

[0066] 4. Influence of the number of thermal cycles on the microstructure and properties of Al / AlN composite materials

[0067] The number of thermal cycles has an impact on the microstructure and properties of both the Al-30AlN-0.4Y2O3 composite and the Al-30AlN composite. With the increase in the number of thermal cycles, aluminum liquid leakage occurred on the surface of the Al-30AlN composite without Y2O3 addition, and cracks were generated. However, the shape of the Al-30AlN-0.4Y2O3 composite with Y2O3 addition was regular, and there was no aluminum liquid leakage on the surface. This preliminarily indicates that during the thermal cycling process, adding Y2O3 can effectively prevent aluminum liquid leakage in the specimens.

[0068] It can be seen from Figure 9a and Figure 9b that after thermal cycling, in the Al-30AlN composite without Y2O3 addition, the content of Al2O3 gradually increased. In the Al-30AlN composite with Y2O3 addition, in addition to the increase in the content of Al2O3, the content of Y3Al5O 12 also gradually increased.

[0069] By comparing Figures 10a to 10b and Figures 11a to 11b it can be seen that after thermal cycling, there were more defects in the Al-30AlN composite without Y2O3 addition, mostly distributed at the grain boundaries, as shown by the arrow marks in Figure 10a and Figure 10b . AlN was diffusely distributed in the Al matrix, and the overall structure was relatively loose, and the combination between the phases was not tight. It can be seen from Figures 11a to 11b that after thermal cycling, the number of defects in the Al-30AlN-0.4Y2O3 composite increased slightly, and some cracks were generated at the grain boundaries, as shown by the arrow marks in Figure 11a and Figure 11b . However, compared with the Al-30AlN composite after thermal cycling, the overall structure of the Al-30AlN-0.4Y2O3 composite was still relatively dense, the combination between the phases was relatively firm, and moreover, the Al2O3 generated in the Al-30AlN-0.4Y2O3 composite was denser. In addition, after thermal cycling, in the Al-30AlN-0.4Y2O3 composite, the white particles at the grain boundaries gradually increased. By performing point scanning on the white particles, the atomic ratio of the three elements Y, Al, and O was close to 3:5:12. Therefore, the white particles were still Y3Al5O 12 , as shown in Figure 12 .

[0070] Table 5 shows the porosity, thermal conductivity, and flexural strength of the Al-30AlN-0.4Y2O3 composite and the Al-30AlN composite under different numbers of thermal cycles.

[0071] Table 5

[0072]

[0073] As can be seen from Table 5, with the increase in the number of thermal cycles, the thermal conductivity and flexural strength of the Al-30AlN-0.4Y2O3 composite material and the Al-30AlN composite material gradually decrease. The decrease in the thermal conductivity and flexural strength of the Al-30AlN composite material is relatively fast, while the decreasing trend of the Al-30AlN-0.4Y2O3 composite material finally flattens out. Among them, when the thermal cycle reaches 30 times, the thermal conductivity and flexural strength of the Al-30AlN-0.4Y2O3 composite material are 35.26 W / (m∙K) and 104.21 MPa respectively, which are much higher than the thermal conductivity (15.14 W / (m∙K)) and flexural strength (64.87 MPa) of the Al-30AlN composite material.

[0074] After adding an appropriate amount of Y2O3, the reason why the thermal conductivity and flexural strength of the Al-30AlN composite material during the thermal cycle are much greater than those of the Al-30AlN composite material without addition is that with the increase in the number of thermal cycles, the defects of the Al-30AlN composite material increase and the porosity gradually increases. This will increase the degree of phonon scattering at the grain boundaries and reduce the mean free path of phonons. At the same time, when the Al-30AlN composite material bears a bending load, the defects will weaken the ability of the interface to transfer the load, which makes the composite material more likely to be damaged when bearing a bending load, ultimately resulting in a significant decrease in the thermal conductivity and flexural strength. In the Al-30AlN-0.4Y2O3 composite material, at 750 °C, Y2O3 reacts with Al2O3 to form the Y3Al5O 12 phase. With the increase in the number of thermal cycles, the content of the Y3Al5O 12 phase gradually increases. During the thermal cycle, due to the different thermal expansion coefficients of the Al phase, AlN phase, Al2O3 phase and Y3Al5O 12 phase, this will lead to inconsistent shrinkage rates, resulting in a small number of defects at the interface and a gradual increase in porosity, thus causing the thermal conductivity and flexural strength of the Al-30AlN-0.4Y2O3 composite material to gradually decrease. However, the Y3Al5O 12 phase distributed at the grain boundaries has good thermal stability and chemical stability, which can reduce the defects generated due to different thermal expansion coefficients in the composite material during the thermal cycle to a certain extent, and will not cause a large increase in defects. Instead, it makes the combination between the phases of the composite material still very tight after cycling, and the structure is not loose. Compared with the Al-30AlN composite material without adding Y2O3, it will not cause a significant increase in the degree of phonon scattering at the grain boundaries and a significant reduction in the ability of the interface to transfer the load when the composite material bears a bending load. Therefore, with the increase in the number of thermal cycles, the thermal conductivity and flexural strength of the Al-30AlN-0.4Y2O3 composite material are greater than those of the Al-30AlN composite material.

[0075] As the number of thermal cycles increases, the reason why the decline in the thermal conductivity and flexural strength of the Al-30AlN-0.4Y2O3 composite material tends to level off is that after experiencing multiple thermal cycles, the bonding between the phases remains very tight. At the same time, as the number of thermal cycles increases, the porosity of the Al-30AlN-0.4Y2O3 composite material no longer decreases significantly. This can prevent the interfacial phonon scattering degree from increasing significantly and keep the stress concentration and crack propagation path relatively fixed. Eventually, the decline in the thermal conductivity and flexural strength of the Al-30AlN-0.4Y2O3 composite material tends to level off.

[0076] 5. Compatibility between Al / AlN Composite Material and Al-12Si Alloy

[0077] When the Al content is 70%, the Y2O3 content is 0.4%, the pressing pressure is 300 MPa, and the sintering temperature is 1200 °C, the Al / AlN composite material not only has high thermophysical properties and mechanical properties but also has good thermal cycle stability. Therefore, specimens are prepared based on this raw material ratio and sintering process. In addition, since the Al-12Si alloy has a large heat storage density, the Al-12Si alloy is selected as the phase change heat storage material. Cuboid thin slices with dimensions of 3×3×20 mm are cut from the composite material after 0, 15, and 30 thermal cycles and placed in the molten Al-12Si alloy at 750 °C for a 120-hour compatibility test, and a comparison is made with the Al-30AlN composite material without the addition of Y2O3.

[0078] Comparison Figures 13a to 13b With Figures 14a to 14b It can be seen that the interface between the Al-30AlN-0.4Y2O3 composite material and the Al-12Si alloy is clear and the bonding is relatively tight, and no obvious corrosion layer appears. However, there is a 66.67-µm crack at the interface between the Al-30AlN composite material and the Al-12Si, as Figure 14aArrow marks. In addition, in the compatibility experiment between the Al-30AlN-0.4Y2O3 composite and the Al-12Si alloy, Si atoms did not diffuse, while in the compatibility experiment between the Al-30AlN composite and the Al-12Si alloy, the diffusion of Si atoms was relatively complete. This is because in the Al-30AlN composite without added Y2O3, the combination between the Al phase and the AlN phase is not tight, and the interfacial combination degree between the two is low, ultimately resulting in cracks at the interface. Moreover, compared with the Al-30AlN-0.4Y2O3 composite, the Al-30AlN composite has more pores, which provides more diffusion channels for the diffusion of Si atoms, causing the diffusion of Si atoms. However, in the Al-30AlN-0.4Y2O3 composite, the generation and distribution of the reinforcing phase are at the grain boundaries, playing a good connecting role, strengthening the wettability between the Al phase and AlN, promoting the growth and development of AlN, making the combination between the two relatively tight, and significantly reducing the number of pores and cracks, without providing diffusion channels for the diffusion of Si atoms. Therefore, no cracks are generated at the interface between the Al-30AlN-0.4Y2O3 composite and the Al-12Si alloy, and Si atoms do not diffuse into the Al-30AlN-0.4Y2O3 composite. 12 The generation and distribution of the reinforcing phase are at the grain boundaries, playing a good connecting role, strengthening the wettability between the Al phase and AlN, promoting the growth and development of AlN, making the combination between the two relatively tight, and significantly reducing the number of pores and cracks, without providing diffusion channels for the diffusion of Si atoms.

[0079] Comparison Figures 15a to 15b and Figures 16a to 16b It can be seen that after 15 thermal cycles, the interface between the Al-30AlN-0.4Y2O3 composite and the Al-12Si alloy is still very clear, without cracks and corrosion layers, but Si atoms have diffused, and the farthest diffusion distance is about 148 µm. At the same time, after 15 thermal cycles, the cracks in the Al-30AlN composite and the Al-12Si alloy are further enlarged, and there is also a certain corrosion layer, such as Figure 16a the arrow marks, and Si atoms have also diffused completely. After repeated thermal cycles, the reason for the gradual diffusion of Si atoms into the Al-30AlN-0.4Y2O3 composite is that after adding an appropriate amount of Y2O3, it reacts with Al2O3 to form the Y3Al5O 12 phase, and the amount of this phase increases with the increase in the number of thermal cycles. Y3Al5O 12The reinforcing phase has good chemical and thermal stability, which prevents the bonding degree at the interface from decreasing significantly, and can effectively relieve the stress at the interface caused by the difference in thermal expansion coefficients after repeated thermal cycling. However, it cannot completely avoid this. Therefore, no obvious cracks or oxidation corrosion layer are formed at the interface between the Al-30AlN-0.4Y2O3 composite and the Al-12Si alloy. However, repeated thermal cycling also causes the porosity of the Al-30AlN-0.4Y2O3 composite to gradually increase, and the increase amplitude is lower than that of the Al-30AlN composite, which provides a diffusion channel for the diffusion of Si atoms. Therefore, after 15 thermal cycles, only slight diffusion of Si atoms occurs into the interior of the Al-30AlN-0.4Y2O3 composite.

[0080] Comparison Figures 17a to 17b and Figures 18a to 18b It can be seen that there are still no obvious cracks or corrosion layer at the interface between the Al-30AlN-0.4Y2O3 composite and the Al-12Si alloy. The number of Si atoms on both sides of the interface increases, but the farthest diffusion distance of Si atoms remains basically unchanged, still about 148 µm. After multiple thermal cycles, the cracks at the interface between the Al-30AlN composite and the Al-12Si alloy further expand, and the corrosion phenomenon becomes more serious. As shown by the arrow marks in Figure 18a , the diffusion of Si atoms is still relatively complete. After multiple thermal cycles, the reason why the farthest diffusion distance of Si atoms in the Al-30AlN-0.4Y2O3 composite remains basically unchanged is that in the Al-30AlN composite added with Y2O3, with the increase of the number of thermal cycles, the Y3Al5O 12 phase has good thermal stability, which can enhance the thermal cycling stability of the Al-30AlN-0.4Y2O3 composite, preventing the bonding degree between the Al phase and the AlN phase from decreasing significantly with the increase of the number of thermal cycles. In addition, the porosity of the Al-30AlN-0.4Y2O3 composite increases with the increase of the number of thermal cycles, but the final increase amplitude tends to be gentle. At the same time, after multiple thermal cycles, the Al2O3 film of the Al-30AlN-0.4Y2O3 composite gradually becomes denser, increasing the diffusion resistance of Si atoms and preventing them from continuing to diffuse into the interior of the composite.

Claims

1. Use of an Al-AlN ceramic composite material, characterized in that, The Al-AlN ceramic composite material is used as a packaging material for the Al-Si heat storage material; the Al-AlN ceramic composite material contains an Al phase, an AlN phase, an Al2O3 phase and a Y3Al5O 12 phase, and the Y3Al5O 12 phase is formed by the reaction of Al2O3 generated from AlN during air sintering with Y2O3; the AlN phase accounts for 20-30 wt% of the sum of the Al phase, the AlN phase, the Al2O3 phase and the Y3Al5O 12 phase, the Al2O3 phase accounts for 29.4-29.6 wt% of the sum of the Al phase, the AlN phase, the Al2O3 phase and the Y3Al5O 12 phase, the Y3Al5O 12 phase accounts for 0.4-0.6 wt% of the sum of the Al phase, the AlN phase, the Al2O3 phase and the Y3Al5O 12 phase, and the balance is the Al phase; Preparation method of Al-AlN ceramic composite material, comprising the following steps: Step (1): Mix aluminum powder, aluminum nitride powder and yttrium oxide powder evenly to obtain a mixed raw material powder; the mass fraction of aluminum in the aluminum powder is greater than or equal to 99.99wt%, and the particle size of the aluminum powder is 40-100µm; the mass fraction of aluminum nitride in the aluminum nitride powder is greater than or equal to 99.99wt%, and the particle size of the aluminum nitride powder is 480-550nm; the mass fraction of yttrium oxide in the yttrium oxide powder is greater than or equal to 99.9wt%, and the particle size of the yttrium oxide powder is 40-100nm; in the mixed raw material powder, the mass fraction of the yttrium oxide powder is 0.2-1.0wt%, and the mass ratio of the aluminum powder to the aluminum nitride powder is (1.5-4):1; Step (2): Add an aqueous solution of polyvinyl alcohol to the mixed raw material powder, grind and mix evenly to obtain a ground mixture; the mass fraction of polyvinyl alcohol in the aqueous solution of polyvinyl alcohol is 3-5wt%, and the relative molecular mass of polyvinyl alcohol is 30000-100000; the dosage of the aqueous solution of polyvinyl alcohol is 5-10wt% of the mass of the mixed raw material powder; the grinding time is 30-60min; Step (3): Cold-press the ground mixture into a preform; the conditions for cold-pressing are: 300-400MPa, and the pressure is maintained for 60-120s; Step (4): Sinter the preform, and after the sintering treatment is completed, cool it naturally to room temperature to obtain the Al-AlN ceramic composite material.

2. Use of the Al-AlN ceramic composite material according to claim 1, characterized in that, In step (1), the mass fraction of aluminum in the aluminum powder is equal to 99.99wt%, and the average particle size of the aluminum powder is 48µm; the mass fraction of aluminum nitride in the aluminum nitride powder is equal to 99.99wt%, and the average particle size of the aluminum nitride powder is 500nm; the mass fraction of yttrium oxide in the yttrium oxide powder is equal to 99.9wt%, and the average particle size of the yttrium oxide powder is 50nm; in the mixed raw material powder, the mass fraction of the aluminum powder is 70wt%, the mass fraction of the aluminum nitride powder is 29.6wt%, and the mass fraction of the yttrium oxide powder is 0.4wt%.

3. Use of the Al-AlN ceramic composite material according to claim 1, characterized in that, In step (2), the mass fraction of polyvinyl alcohol in the aqueous solution of polyvinyl alcohol is 3wt%, and the relative molecular mass of polyvinyl alcohol is 50000-70000; the dosage of the aqueous solution of polyvinyl alcohol is 8wt% of the mass of the mixed raw material powder; the grinding time is 40min; in step (3), the conditions for cold-pressing are: 300MPa, and the pressure is maintained for 100s.

4. Use of the Al-AlN ceramic composite material according to claim 1, characterized in that, In step (4), the sintering treatment method is: heat from room temperature to 900-1200°C at a heating rate of 10-15°C / min, and keep the temperature for 2-4h.

5. Use of the Al-AlN ceramic composite material according to claim 1, characterized in that, In step (1), the mass fraction of aluminum in the aluminum powder is equal to 99.99wt%, and the average particle size of the aluminum powder is 48µm; the mass fraction of aluminum nitride in the aluminum nitride powder is equal to 99.99wt%, and the average particle size of the aluminum nitride powder is 500nm; the mass fraction of yttrium oxide in the yttrium oxide powder is equal to 99.9wt%, and the average particle size of the yttrium oxide powder is 50nm; in the mixed raw material powder, the mass fraction of the aluminum powder is 70wt%, the mass fraction of the aluminum nitride powder is 29.6wt%, and the mass fraction of the yttrium oxide powder is 0.4wt%; In step (2), the mass fraction of polyvinyl alcohol in the polyvinyl alcohol aqueous solution is 3 wt%, and the relative molecular mass of polyvinyl alcohol is 50,000 - 70,000; the dosage of the polyvinyl alcohol aqueous solution is 8 wt% of the mass of the mixed raw material powder; the grinding time is 40 min; In step (3), the conditions for cold pressing are: 300 MPa, and the pressure is maintained for 100 s; In step (4), the sintering treatment method is: heating from room temperature to 1200 °C at a heating rate of 10 °C / min and holding for 2 h.

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