Thermally conductive composite filler and preparation method of thermally conductive composite material

Abstract

The application provides a preparation method of a heat-conducting composite filler and a preparation method of a heat-conducting composite material. First, hexagonal boron nitride is ultrasonically peeled off, and then is blended with interface-modified gallium-indium alloy to obtain a high-heat-conducting gallium-indium alloy and boron nitride nanosheet composite heat-conducting filler. The composite heat-conducting filler can effectively improve the heat-conducting efficiency of boron nitride, and has excellent heat-conducting effect. Meanwhile, in the deformation process of the heat-conducting composite material in a tensile state, the heat-conducting capacity of the composite material in a deformed state can be improved, and the tensile stress can be effectively dissipated. The interface modification of the gallium-indium alloy microspheres can improve the compatibility between the heat-conducting composite filler and the base material of the heat-conducting composite material.

Description

Technical Field

[0001] The present invention belongs to the field of preparing thermal conductive fillers using boron nitride, and specifically relates to a method for preparing a thermal conductive composite filler by combining a high thermal conductive gallium-indium alloy with boron nitride nanosheets, and a method for preparing a thermal conductive composite material. Background Art

[0002] With the improvement of the performance of high-power microelectronic devices, the integration of components and the miniaturization of size, the waste heat generated by the devices during operation not only affects the performance and life of the devices, but also inevitably interferes with the functions of other nearby devices. Therefore, the heat dissipation problem of electronic components is an important issue that needs to be solved urgently. The demand for the development of electronic heat dissipation materials is increasing, and boron nitride (BN)-based polymer composites have become advanced composite materials due to their high temperature and good electrical insulation properties. Hexagonal boron nitride (BN) has become a promising thermal conductive filler due to its high thermal conductivity, excellent electrical insulation properties and high-quality mass production. Hongyu Niu et al. prepared BN film-filled silicone rubber composites by a simple stacking-cutting method. This method maintained the high orientation of the BN film, thus obtaining a power of 19.1 W m -1 K -1 The high pass surface and low compression modulus of 5.42MPa. The low BN content (37vol1%) ensures the softness and elasticity of the preparation.

[0003] Gallium-based liquid metal has attracted more and more attention as a thermal interface material due to its advantages such as flexibility, relatively high thermal conductivity and low resistance. Liu Han et al. combined AIN with liquid metal (LM, Ga 68.5 In 21.5 Sn 10 ) and polydimethylsiloxane (PDMS), one of the most commonly used silicone oils, to enhance interfacial heat transfer, resulting in the synthesis of a new composite thermal interface material (TIM). Gallium-based liquid metals have inherent drawbacks, such as their excellent fluidity, which can easily lead to leakage, and their tendency to corrode with contacting metals, hindering their large-scale application.

[0004] Gallium-based liquid metal and boron nitride (BN)-based polymer composites represent an emerging material structure, exhibiting unique mechanical responsiveness and thermal functionality, improving their reliability and possessing enormous potential in the field of thermal conductivity. Therefore, the preparation and thermal conductivity research of liquid metal and boron nitride (BN)-based polymer composites is of great significance. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method for preparing a thermally conductive composite filler and a method for preparing a thermally conductive composite material in response to the deficiencies of the above-mentioned prior art. The thermally conductive composite filler is prepared by a composite of gallium-indium alloy liquid metal and boron nitride nanosheets, has high thermal conductivity, effectively improves the thermal conductivity efficiency of boron nitride, and has excellent thermal conductivity effect. At the same time, the thermally conductive composite material is prepared by the thermally conductive composite filler. During the deformation process of the thermally conductive composite material under the tensile state, the thermal conductivity of the composite material under the deformation state can be improved, and the tensile stress can be effectively dissipated. Thanks to the interface modification of the gallium-indium alloy microspheres, the compatibility between the thermally conductive composite filler and the thermally conductive composite material substrate can be improved; the thermally conductive composite filler and the thermally conductive composite material have high reliability in use and have huge potential value in the field of thermal conduction.

[0006] In order to achieve the above technical objectives, the technical solution adopted by the present invention is:

[0007] A method for preparing a thermally conductive composite filler comprises the following steps:

[0008] Step (1), preparation of boron nitride nanosheets BNNSs:

[0009] (1.1) Hexagonal boron nitride (BN) is placed in a mixed solvent of N,N-dimethylformamide (DMF) and distilled water in a certain ratio, an interfacial intercalation agent is added, and ultrasonic exfoliation is performed. Afterwards, the ultrasonically exfoliated boron nitride nanosheets (BNNSs) are collected by filtration, rinsed, and dried for later use;

[0010] Step (2), blend preparation:

[0011] (2.1) Gallium-indium alloy LM is placed in distilled water, and an interface modifier is added to obtain a blend;

[0012] (2.2) Ultrasonicate the blend for a period of time, remove a certain volume of the supernatant from the blend while stopping the ultrasound, and then inject distilled water into the blend in the same volume as the removed supernatant;

[0013] (2.3) Repeat step (2.2) several times until a certain volume of the blend is retained;

[0014] Step (3), preparation of LM-BNNSs composite thermal conductive filler of gallium-indium alloy and boron nitride nanosheets:

[0015] (3.1) The last mixture retained in step (2.3) is mixed with boron nitride nanosheets BNNSs in a container in proportion, and the mixture is mixed and stirred in an aqueous phase under a certain temperature environment to promote the gallium-indium alloy LM to adhere to the surface of the boron nitride nanosheets BNNSs in the form of microspheres. Finally, after the water evaporates, a high thermal conductivity gallium-indium alloy and boron nitride nanosheet composite thermal conductive filler LM-BNNSs, referred to as a thermal conductive composite filler, is obtained.

[0016] As a further improved technical solution of the present invention, the step (1) is specifically as follows:

[0017] 5 g of hexagonal boron nitride (BN) was placed in 100 mL of a mixed solvent of N,N-dimethylformamide (DMF) and distilled water in a certain proportion, 4 mL of an interfacial intercalant was added, and ultrasonic exfoliation was performed at room temperature for 4 hours at a power of 500 W. Afterwards, the boron nitride nanosheets (BNNSs) after ultrasonic exfoliation were collected by filtration, rinsed with distilled water, and dried at 60°C for later use.

[0018] As a further improved technical solution of the present invention, in step (1), the co-blended solvent is a mixture of N,N-dimethylformamide (DMF) and distilled water in a volume ratio of 1:1-3.

[0019] As a further improved technical solution of the present invention, in step (1), the interfacial intercalant is one of diol solvents such as hexanediol.

[0020] As a further improved technical solution of the present invention, the step (2) is specifically as follows:

[0021] (2.1) 1 g of gallium-indium alloy LM was placed in 20 mL of distilled water, and 1.5 g of interface modifier was added to obtain a blend;

[0022] (2.2) Ultrasonicate the blend for 30 minutes using a 600W probe at 100% power, specifically in a 15-second on / 15-second off mode; while ultrasonicating, remove 10 mL of the upper layer of the blend and inject 10 mL of distilled water into the blend;

[0023] (2.3) Repeat step (2.2) 3-4 times, and finally retain 10 mL of the blend.

[0024] As a further improved technical solution of the present invention, in step (2), the gallium-indium alloy LM is gallium-indium or gallium-indium-tin, wherein the mass ratio of gallium is above 60%.

[0025] As a further improved technical solution of the present invention, in step (2), the interface modifier is one of polythioureas such as 2,5-dithiodiurea and thiourea.

[0026] As a further improved technical solution of the present invention, the step (3) is specifically as follows:

[0027] The last retained blend in step (2.3) is mixed with boron nitride nanosheets BNNSs in a container in proportion, and the mixture is stirred in an aqueous phase at 60°C for 2 hours to promote the gallium-indium alloy LM to adhere to the surface of the boron nitride nanosheets BNNSs in the form of microspheres. Finally, after the water evaporates, a highly thermally conductive gallium-indium alloy and boron nitride nanosheet composite thermal conductive filler LM-BNNSs is obtained.

[0028] As a further improved technical solution of the present invention, in the step (3), the gallium-indium alloy LM and the boron nitride nanosheets BNNSs in the blend are blended in proportion to a mass ratio of 1:2-3, wherein the mass of the gallium-indium alloy LM in the blend is the gram weight of the gallium-indium alloy LM in step (2.1).

[0029] In order to achieve the above technical objectives, another technical solution adopted by the present invention is:

[0030] A method for preparing a thermally conductive composite material prepared from a thermally conductive composite filler, comprising:

[0031] Dissolve polyurethane in a benign solvent and stir at a certain temperature until the polyurethane is completely dissolved to obtain a polyurethane solution;

[0032] Adding the gallium-indium alloy and boron nitride nanosheet composite thermal conductive filler LM-BNNSs to the polyurethane solution, stirring, then transferring to a planetary disperser for dispersion, and taking out to obtain a dispersed and mixed mixture;

[0033] Deionized water is added to the dispersed and mixed mixture under stirring to promote the precipitation of the thermal conductive polyurethane composite material. The precipitated thermal conductive polyurethane composite material is dried and hot-pressed to obtain a formed thermal conductive polyurethane composite material, referred to as a thermal conductive composite material.

[0034] The beneficial effects of the present invention are:

[0035] The present invention provides a method for preparing a thermally conductive composite filler and a method for preparing a thermally conductive composite material, wherein the thermally conductive composite filler is prepared by compositely preparing gallium-indium alloy liquid metal and boron nitride nanosheets, belongs to a thermally conductive composite filler, has high thermal conductivity, effectively improves the thermal conductivity efficiency of a single boron nitride nanosheet, and has an excellent thermal conductivity effect. At the same time, the thermally conductive composite material is prepared from the thermally conductive composite filler. During the deformation process of the thermally conductive composite material under a tensile state, the thermal conductivity of the composite material under a deformed state can be improved, and tensile stress can be effectively dissipated. Thanks to the interface modification of the gallium-indium alloy microspheres, the compatibility between the thermally conductive composite filler and the thermally conductive composite material substrate can be improved; the thermally conductive composite filler and the thermally conductive composite material have high reliability in use, have huge potential value in the field of thermal conduction, and are of great significance.

[0036] At the same mass ratio of thermal conductive filler, the thermal conductivity of the composite thermal conductive filler containing gallium-indium alloy liquid metal and boron nitride nanosheets BNNSs of the present invention is significantly higher than that of a single boron nitride nanosheet BNNSs filler. The gallium-indium alloy droplets have a significant bridging effect between BNNSs, optimizing the phonon transmission channel. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1These are the SEM and SEM-mapping images of the thermally conductive composite filler prepared in Example 4 of the present invention.

[0038] Figure 2 This is a comparison chart of the in-plane thermal conductivity coefficients of the thermally conductive polyurethane composite material prepared from 400-micron boron nitride nanosheets (BNNSs) in Comparative Example 1 of the present invention and the thermally conductive polyurethane composite material prepared from a composite thermal conductive filler of gallium-indium alloy and boron nitride nanosheets (referred to as thermally conductive composite filler, LM-BNNSs) in Example 4. DETAILED DESCRIPTION

[0039] The specific embodiments of the present invention are further described below with reference to the accompanying drawings:

[0040] Example 1:

[0041] (1) 5 g of hexagonal boron nitride (BN) was placed in 100 mL of a mixture of N,N-dimethylformamide (DMF) and distilled water in a ratio of 1:3 (volume ratio), 4 mL of intercalant hexanediol was added, and ultrasonic stripping was performed at room temperature for 4 h at a power of 500 W. The boron nitride nanosheets (BNNSs) after ultrasonic stripping were collected by filtration, rinsed with distilled water, and dried at 60 °C for later use;

[0042] (2) 1 g of gallium-indium alloy (gallium-indium, gallium mass ratio is 62%) is placed in 20 mL of distilled water, and 1.5 g of interface modifier 2,5-dithiodiurea is added to obtain a blend.

[0043] The blend was sonicated for 30 minutes at 100% power using a 600W probe ultrasound (15 seconds on, 15 seconds off mode). 10 mL of the supernatant on the blend was removed while the ultrasound was stopped, and 10 mL of distilled water was injected into the blend. This process was repeated 3-4 times, leaving approximately 10 mL of the blend.

[0044] (3) The mixture retained in step (2) is mixed with BNNSs in a container in proportion (the specific mixing in proportion is that the mass ratio of gallium indium to boron nitride nanosheets BNNSs in the mixture is 1:2.2, wherein the mass of gallium indium in the mixture is the gram weight of gallium indium in step (2), i.e., 1 g). The mixture is mixed and stirred in an aqueous phase at 60°C for about 2 hours to cause gallium indium to adhere to the surface of BNNSs in the form of microspheres. Finally, after the water evaporates, a high thermal conductivity gallium indium alloy and boron nitride nanosheet composite thermal conductive filler (LM-BNNSs) is obtained, which is referred to as a thermal conductive composite filler.

[0045] The prepared gallium-indium alloy and boron nitride nanosheet composite thermal conductive filler were mixed with polyurethane in a ratio of 3:7 to prepare a thermally conductive polyurethane composite containing 30 wt% of the thermally conductive composite filler, referred to as the thermally conductive composite. The preparation method was as follows: 7 g of polyurethane was dissolved in a benign solvent (DMF) with the amount of solvent no greater than approximately twice the volume of the polyurethane. The polyurethane was stirred at 60°C until the polyurethane was completely dissolved. 3 g of the gallium-indium alloy and boron nitride nanosheet composite thermal conductive filler LM-BNNSs was added to the polyurethane solution and stirred for approximately 30 minutes. The mixture was then transferred to a planetary disperser and dispersed at 500 rpm for 30 minutes before removal. Deionized water, approximately four times the volume of the polyurethane solvent, was added to the dispersed and mixed mixture while stirring at 300 rpm to promote the precipitation of the thermally conductive polyurethane composite. The precipitated thermally conductive polyurethane composite was dried and hot-pressed for later use. (Other thermally conductive composites containing different mass fractions of thermally conductive composite fillers were prepared in the same manner as described above, with only the mass of the corresponding thermally conductive composite filler being varied.)

[0046] Example 2:

[0047] (1) 5 g of hexagonal boron nitride (BN) was placed in 100 mL of a mixture of N,N-dimethylformamide (DMF) and distilled water in a 1:1 ratio (volume ratio), and 4 mL of an intercalating agent, hexanediol, or other diol solvents were added. Ultrasonic exfoliation was performed at room temperature at a power of 500 W for 4 h. The boron nitride nanosheets (BNNSs) after ultrasonic exfoliation were collected by filtration, rinsed with distilled water, and dried at 60 °C for later use.

[0048] (2) 1 g of gallium-indium alloy (gallium-indium, gallium mass ratio is 62%) is placed in 20 mL of distilled water, and 1.5 g of interface modifier 2,5-dithiodiurea, thiourea or other polythiourea is added to obtain a blend.

[0049] The blend was sonicated for 30 minutes at 100% power using a 600W probe ultrasound (15 seconds on, 15 seconds off mode). 10 mL of the supernatant on the blend was removed while the ultrasound was stopped, and 10 mL of distilled water was injected into the blend. This process was repeated 3-4 times, leaving approximately 10 mL of the blend.

[0050] (3) The mixture retained in step (2) is mixed with BNNSs in a container in proportion (the proportion mixing is specifically that the mass ratio of gallium indium to boron nitride nanosheets BNNSs in the mixture is 1:2, wherein the mass of gallium indium in the mixture is the gram weight of gallium indium in step (2), i.e., 1 g). The mixture is mixed and stirred in an aqueous phase at 60° C. for about 2 h to cause gallium indium to adhere to the surface of BNNSs in the form of microspheres. Finally, after the water evaporates, a highly thermally conductive gallium indium alloy and boron nitride nanosheet composite thermal conductive filler (LM-BNNSs) is obtained, referred to as a thermal conductive composite filler.

[0051] The prepared gallium-indium alloy was mixed with boron nitride nanosheet composite thermal conductive filler and polyurethane in a ratio of 3:7 to prepare a thermal conductive polyurethane composite material containing 30 wt% of thermal conductive composite filler, referred to as thermal conductive composite material. The preparation method was the same as that of Example 1.

[0052] Example 3:

[0053] (1) 5 g of hexagonal boron nitride (BN) was placed in 100 mL of a mixture of N,N-dimethylformamide (DMF) and distilled water in a ratio of 1:2.5 (volume ratio), 4 mL of intercalant hexanediol was added, and ultrasonic stripping was performed at room temperature for 4 h at a power of 500 W. The boron nitride nanosheets (BNNSs) after ultrasonic stripping were collected by filtration, rinsed with distilled water, and dried at 60 °C for later use;

[0054] (2) 1 g of gallium-indium alloy (gallium-indium, gallium mass ratio is 65%) is placed in 20 mL of distilled water, and 1.5 g of interface modifier 2,5-dithiodiurea is added to obtain a blend.

[0055] The blend was sonicated for 30 minutes at 100% power using a 600W probe ultrasound (15 seconds on, 15 seconds off mode). 10 mL of the supernatant on the blend was removed while the ultrasound was stopped, and 10 mL of distilled water was injected into the blend. This process was repeated 3-4 times, leaving approximately 10 mL of the blend.

[0056] (3) The last remaining blend in step (2) is mixed with BNNSs in a container in proportion (specifically, the proportion of the blend is that the mass ratio of gallium indium to boron nitride nanosheets BNNSs in the blend is 1:3, wherein the mass of gallium indium in the blend is the gram weight of gallium indium in step (2), i.e., 1 g). The blending and stirring are carried out in an aqueous phase at 60°C for about 2 hours to cause LM to adhere to the surface of BNNSs in the form of microspheres. Finally, after the water evaporates, a highly thermally conductive gallium indium alloy and boron nitride nanosheet composite thermal conductive filler (LM-BNNSs) is obtained, which is referred to as a thermal conductive composite filler.

[0057] The prepared gallium-indium alloy was mixed with boron nitride nanosheet composite thermal conductive filler and polyurethane in a ratio of 3:7 to prepare a thermal conductive polyurethane composite material containing 30 wt% of thermal conductive composite filler, referred to as thermal conductive composite material. The preparation method was the same as that of Example 1.

[0058] Example 4:

[0059] (1) 5 g of hexagonal boron nitride (BN) was placed in 100 mL of a mixture of N,N-dimethylformamide (DMF) and distilled water in a ratio of 1:2 (volume ratio), and 4 mL of intercalant hexanediol was added. Ultrasonic exfoliation was performed at room temperature at a power of 500 W for 4 h. The boron nitride nanosheets (BNNSs) after ultrasonic exfoliation were collected by filtration, rinsed with distilled water, and dried at 60 °C for later use.

[0060] (2) 1 g of gallium-indium alloy (gallium-indium-tin, the mass ratio of gallium-indium-tin is 62:15:13) is placed in 20 mL of distilled water, and 1.5 g of interface modifier thiourea is added to obtain a blend.

[0061] The blend was sonicated for 30 minutes at 100% power using a 600W probe ultrasound (15 seconds on, 15 seconds off mode). 10 mL of the upper layer of the blend was extracted while the ultrasound was stopped, and 10 mL of distilled water was injected into the blend. This process was repeated 3-4 times, leaving approximately 10 mL of the blend.

[0062] (3) The mixture retained in step (2) is mixed with BNNSs in a container in proportion (specifically, the mass ratio of gallium indium tin to boron nitride nanosheets BNNSs in the mixture is 1:2.2, wherein the mass of gallium indium tin in the mixture is the gram weight of gallium indium tin in step (2), i.e., 1 g). The mixture is mixed and stirred in an aqueous phase at 60° C. for about 2 h to cause the gallium indium tin to adhere to the surface of the BNNSs in the form of microspheres. Finally, after the water evaporates, a highly thermally conductive gallium indium alloy and boron nitride nanosheet composite thermal conductive filler (LM-BNNSs) is obtained, referred to as a thermal conductive composite filler. Figure 1 The SEM and SEM-mapping images of the thermally conductive composite filler prepared in Example 4 of the present invention are as follows: Figure 1 As shown, GaInT is implanted as nanoparticles on the surface of boron nitride nanosheets (BNNSs), and is dispersed evenly and stably.

[0063] The prepared gallium-indium alloy was mixed with boron nitride nanosheet composite thermal conductive filler and polyurethane in a ratio of 3:7 to prepare a thermal conductive polyurethane composite material containing 30 wt% of thermal conductive composite filler, referred to as thermal conductive composite material. The preparation method was the same as that of Example 1.

[0064] Comparative Example 1: Commercially available 400-micron boron nitride nanosheets (BNNSs) and polyurethane were mixed in a ratio of 3:7 to prepare a thermally conductive polyurethane composite containing 30 wt% boron nitride nanosheets (BNNSs). The thermally conductive polyurethane composite was prepared similarly to the thermally conductive polyurethane composite in Example 1. The following steps were used to prepare the thermally conductive polyurethane composite containing 30 wt% boron nitride nanosheets (BNNSs): 7 g of polyurethane was dissolved in a benign solvent (DMF) (the amount of solvent was no more than approximately twice the volume of the polyurethane) and stirred at 60°C until the polyurethane was completely dissolved. 3 g of boron nitride nanosheets (BNNSs) were added to the polyurethane solution and stirred for approximately 30 minutes. The mixture was then transferred to a planetary disperser and dispersed at 500 rpm for 30 minutes before removal. Deionized water (approximately 4 times the volume of the polyurethane solvent) was added to the dispersed and mixed mixture while stirring at 300 rpm to promote the precipitation of the thermally conductive polyurethane composite containing the boron nitride nanosheets (BNNSs). The precipitated thermally conductive polyurethane composite was dried and hot-pressed for later use. (The preparation methods of other thermal conductive polyurethane composites containing different mass fractions of BNNSs are the same as above, only the corresponding BNNSs filler mass is changed).

[0065] The performance comparison of Examples 1-4 and the comparative example is shown in Table 1.

[0066] Table 1, performance comparison:

[0067] <![CDATA[Thermal conductivity w.m －1 k －1 > Example 1 4.05 Example 2 3.96 Example 3 4.07 Example 4 4.01 Comparative Example 1 2.25

[0068] As can be seen from Table 1, the thermally conductive polyurethane composite materials containing gallium-indium alloy and boron nitride nanosheet composite thermal conductive filler (LM-BNNSs) in Examples 1-4 have better thermal conductivity than the thermally conductive polyurethane composite materials containing boron nitride nanosheets (BNNSs) in Comparative Example 1.

[0069] Figure 2 The in-plane thermal conductivity coefficients of the thermally conductive polyurethane composite material prepared by a single 400-micron boron nitride nanosheet (BNNSs) filler in Comparative Example 1 are compared with the thermally conductive polyurethane composite material prepared by a high thermal conductivity gallium-indium alloy and boron nitride nanosheet composite thermal conductive filler (LM-BNNSs) in Example 4. Figure 2 The horizontal axis is the mass ratio of filler (BNNSs or LM-BNNSs) to the total mass of the corresponding filler and polyurethane, and the vertical axis is the thermal conductivity of the corresponding thermally conductive polyurethane composite material. Figure 2The results show that, for different loading amounts of boron nitride nanosheets (BNNSs) and gallium-indium alloy and boron nitride nanosheet composite thermal conductive fillers (LM-BNNSs), the thermal conductivity of the thermally conductive polyurethane composite prepared with the gallium-indium alloy and boron nitride nanosheet composite thermal conductive filler (LM-BNNSs) is much greater than that of the thermally conductive polyurethane composite prepared with boron nitride nanosheets (BNNSs) alone. In other words, at the same thermal conductive filler mass ratio, the thermal conductivity of the composite thermal conductive filler (LM-BNNSs) is significantly improved due to the significant bridging effect of the gallium-indium alloy droplets between the BNNSs, which optimizes the phonon transmission channel.

[0070] The protection scope of the present invention includes but is not limited to the above embodiments. The protection scope of the present invention is based on the claims. Any replacement, deformation, and improvement of the technology that can be easily thought of by those skilled in the art fall within the protection scope of the present invention.

Claims

1. A method for preparing a thermally conductive composite filler, characterized in that: The steps include: Step (1), preparation of boron nitride nanosheets BNNSs: (1.1) Hexagonal boron nitride (BN) is placed in a mixed solvent of N,N-dimethylformamide (DMF) and distilled water in a certain ratio, an interfacial intercalant is added, and ultrasonic exfoliation is performed. Afterwards, the ultrasonically exfoliated boron nitride nanosheets (BNNSs) are collected by filtration, rinsed, and dried for later use; the interfacial intercalant is hexanediol; Step (2), blend preparation: (2.1) Gallium-indium alloy LM is placed in distilled water, and an interface modifier is added to obtain a blend; the interface modifier is 2,5-dithiodiurea or thiourea; (2.2) Ultrasonicate the blend for a period of time, remove a certain volume of the supernatant from the blend while stopping the ultrasound, and then inject distilled water into the blend in the same volume as the removed supernatant; (2.3) Repeat step (2.2) several times until a certain volume of the blend is retained; Step (3), preparation of LM-BNNSs composite thermal conductive filler of gallium-indium alloy and boron nitride nanosheets: (3.1) The last mixture retained in step (2.3) is mixed with boron nitride nanosheets BNNSs in a container in proportion, and the mixture is mixed and stirred in an aqueous phase under a certain temperature environment to promote the gallium-indium alloy LM to adhere to the surface of the boron nitride nanosheets BNNSs in the form of microspheres. Finally, after the water evaporates, a high thermal conductivity gallium-indium alloy and boron nitride nanosheet composite thermal conductive filler LM-BNNSs, referred to as a thermal conductive composite filler, is obtained.

2. The method for preparing a thermally conductive composite filler according to claim 1, wherein: The step (1) is specifically as follows: 5 g of hexagonal boron nitride (BN) was placed in 100 mL of a mixed solvent of N,N-dimethylformamide (DMF) and distilled water in a certain proportion, 4 mL of an interfacial intercalant was added, and ultrasonic exfoliation was performed at room temperature for 4 hours at a power of 500 W. Afterwards, the boron nitride nanosheets (BNNSs) after ultrasonic exfoliation were collected by filtration, rinsed with distilled water, and dried at 60°C for later use.

3. The method for preparing a thermally conductive composite filler according to claim 2, wherein: In the step (1), the co-blended solvent is a mixture of N,N-dimethylformamide (DMF) and distilled water in a volume ratio of 1:1-3.

4. The method for preparing a thermally conductive composite filler according to claim 2, wherein: The step (2) is specifically as follows: (2.1) 1 g of gallium-indium alloy LM was placed in 20 mL of distilled water, and 1.5 g of interface modifier was added to obtain a blend; (2.2) Ultrasonicate the blend for 30 minutes using a 600W probe at 100% power, specifically in a 15-second on / 15-second off mode; while ultrasonicating, remove 10 mL of the upper layer of the blend and inject 10 mL of distilled water into the blend; (2.3) Repeat step (2.2) 3-4 times, and finally retain 10 mL of the blend.

5. The method for preparing a thermally conductive composite filler according to claim 4, wherein: In the step (2), the gallium-indium alloy LM is gallium-indium or gallium-indium-tin, wherein the mass ratio of gallium is greater than 60%.

6. The method for preparing a thermally conductive composite filler according to claim 4, wherein: The step (3) is specifically as follows: The last retained blend in step (2.3) is mixed with boron nitride nanosheets BNNSs in a container in proportion, and the mixture is stirred in an aqueous phase at 60°C for 2 hours to promote the gallium-indium alloy LM to adhere to the surface of the boron nitride nanosheets BNNSs in the form of microspheres. Finally, after the water evaporates, a highly thermally conductive gallium-indium alloy and boron nitride nanosheet composite thermal conductive filler LM-BNNSs is obtained.

7. The method for preparing a thermally conductive composite filler according to claim 6, wherein: In the step (3), the gallium-indium alloy LM and the boron nitride nanosheets BNNSs in the blend are blended in proportion to a mass ratio of 1:2-3, wherein the mass of the gallium-indium alloy LM in the blend is the gram weight of the gallium-indium alloy LM in step (2.1).

8. A method for preparing a thermally conductive composite material, characterized in that: include: Dissolve polyurethane in a benign solvent and stir at a certain temperature until the polyurethane is completely dissolved to obtain a polyurethane solution; adding the thermally conductive composite filler prepared by the preparation method according to any one of claims 1 to 7 to the polyurethane solution, stirring, then transferring to a planetary disperser for dispersion, taking out, and obtaining a dispersed and mixed mixture; Deionized water is added to the dispersed and mixed mixture under stirring to promote the precipitation of the thermal conductive polyurethane composite material. The precipitated thermal conductive polyurethane composite material is dried and hot-pressed to obtain a formed thermal conductive polyurethane composite material, referred to as a thermal conductive composite material.

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