Recyclable ZIF-67 coated hydroxylated BNNS high-thermal-conductivity composite material and preparation method thereof

Abstract

The invention discloses a recoverable ZIF-67 coated hydroxylated BNNS high-thermal-conductivity composite material and a preparation method thereof, and belongs to the technical field of composite materials. According to the preparation process of the composite material, hydroxylated boron nitride nanosheets HO-BNNS are obtained through a high-energy ball milling technology, the average particle size of HO-BNNS is 1.04 microns, no large defect is generated in the stripping and modifying process, HO-BNNS prepared through the high-energy ball milling technology can serve as a heat conduction additive of nanofluid, and the heat conductivity of a base material is effectively improved; bN is stripped and hydroxylated by using a high-energy ball milling technology and then compounded with ZIF-67, an efficient, simple and convenient method for rapidly preparing the high-quality and high-yield HO-BNNS coated ZIF-67 composite material is developed, and a new feasible scheme is provided for practical application of the HO-BNNS coated ZIF-67 composite material in the field of thermal management.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of composite materials, in particular to a recyclable ZIF-67-coated hydroxylated BNNS high-thermal-conductivity composite material and a preparation method thereof. BACKGROUND

[0002] With the miniaturization, integration, high frequency and intelligentization of electronic components and electrical equipment, many high-tech devices such as microelectronic devices, micro robots, biomedical instruments and precision test instruments are often in the nanometer scale, so the problems related to heat dissipation in micro machines increase sharply, and the heat transfer problem of nanoscale devices is increasingly prominent. Developing new functional composite materials for heat conduction of microelectronic devices is an efficient solution to this problem.

[0003] ZIF-67 is a typical metal-organic framework (MOF) material, which is formed by coordination of Co 2+ As a porous material, ZIF-67 has high specific surface area, adjustable pore size, controllable metal center and excellent physical and chemical stability, which provides a basis for its application in thermal management. Through pyrolysis or carbonization, ZIF-67 can be converted into a composite material containing cobalt, which usually has high thermal conductivity and magnetic properties, can effectively transfer heat and recycle, thereby improving the thermal management effect of the material and being more conducive to the green development of energy. ZIF-67 is durable and stable in air, aqueous solution and strong alkali solution, and has good high-temperature stability. This means that in thermal management applications, ZIF-67 and its derivatives can maintain stable performance at high temperatures and are not prone to decomposition or failure, thereby ensuring the long-term stable operation of the equipment. The advantages of ZIF-67 in thermal management mainly include its high thermal conductivity, good thermal stability, easy processing and molding, composite with other materials and environmental friendliness. These advantages make ZIF-67 have broad prospects in the thermal management applications of electronic devices, new energy vehicles, aerospace and other fields.

[0004] Hexagonal boron nitride (h-BN) as a typical III-V compound, also a non-oxide ceramic material, its crystal structure is very similar to graphite, is one of the best thermal conductivity of ceramic materials, its in-plane (001 plane) thermal conductivity coefficient reaches 180~200 W / (m·K). At the same time, the thermal expansion coefficient of hexagonal boron nitride is very low, which can maintain the shape without change at high temperature, and because of its high temperature thermal conductivity, lubricity and wear resistance, hexagonal boron nitride material becomes an ideal heat transfer material. Boron nitride nanosheet (BNNS) has a two-dimensional sheet structure and a large specific surface area, compared with hexagonal boron nitride, the thermal resistance of the interlayer interface is eliminated, and it is easier to form a heat conduction network, so it has more advantages in heat conduction. But the chemical inertness of BNNS surface seriously hinders its dispersion in the matrix, and the preparation technology of functionalized BNNS has not yet realized high efficiency mass production, which limits the development of BNNS composite materials.

[0005] Mechanical ball milling is a common technique for preparing nanomaterials, but traditional ball milling methods such as planetary and vibration type mechanical ball milling will cause pollution of the grinding medium, low efficiency and poor effect. It is of great significance to seek a kind of ZIF-67 and HO-BNNS composite to obtain a kind of high thermal conductivity, no pollution to the environment, and meets the current green and environmental protection development trend of recyclable composite material. SUMMARY

[0006] In view of the problems existing in the prior art, the present application discloses a recyclable ZIF-67 coated hydroxylated BNNS high thermal conductivity composite material and a preparation method thereof. The preparation process of the composite material is as follows: first, hydroxylated boron nitride nanosheet (HO-BNNS) is obtained by high-energy ball milling technology, then ZIF-67 is coated on the surface of HO-BNNS by one-step solution method, and finally carbonization treatment is carried out, to obtain a composite thermal conductivity material with core-shell structure. The high thermal conductivity material prepared by the present application uses low-cost chemical raw materials, has higher synthesis efficiency by one-step synthesis, and the prepared composite material has a thermal conductivity efficiency of 0.9 W·(m·K) -1 and stable state; in a relatively wide and lower calcination temperature range of 500~700℃, a composite material with high thermal conductivity can be obtained, and the recovery efficiency is higher and has no obvious effect on the thermal conductivity of the material, and the obtained ZIF-67 coated hydroxylated boron nitride nanosheet (HO-BNNS) composite material has higher heat transfer performance and recovery efficiency, which has important significance in the field of electronic packaging.

[0007] The preparation method of the recyclable ZIF-67 coated hydroxylated BNNS high thermal conductivity composite material provided by the present application comprises the following steps: Step 1, high-energy ball milling method is used to ball mill boron nitride (BN) and PVA, then the ball-milled product is washed by centrifugation in a mixture of water and ethylene glycol, and finally HO-BNNS is obtained by drying; Step 2, the methanol solution of HO-BNNS and the methanol solution of cobalt nitrate hexahydrate are respectively ultrasonicated and then mixed to obtain a mixed solution A; the methanol solution of 2-methylimidazole is added to the mixed solution A, stirred and then left to stand to obtain a mixed solution B, which is washed by centrifugation in methanol, and finally the composite precursor is obtained by drying; Step 3, the composite precursor is calcined in nitrogen atmosphere to obtain a high-thermal-conductivity composite material of ZIF-67 coated HO-BNNS.

[0008] The high-energy ball milling method in step 1 is specifically: using a plasma ball mill to ball mill boron nitride (BN) and PVA; the ball milling process of the plasma ball mill is 6-10 cycles of discharge, and each cycle process is 20-40 min of working time and 10-30 min of intermittent time; the rotation speed of the plasma ball mill is 1000-1500 rpm, and the frequency is 8-10 KHz; the mass ratio of the boron nitride (BN), the PVA and the steel balls in the plasma ball mill is 1: (5-10): 70; The boron nitride (BN) in step 1 is hexagonal boron nitride (h-BN), and the particle size of the hexagonal boron nitride (h-BN) is 5-10 μm; The volume ratio of water to ethylene glycol in step 1 is 11: 9-10; The rotation speed of the centrifugal washing in step 1 is 1000-3000 rpm, and the centrifugal washing time is 5-10 min; The drying temperature in step 1 is 60-100℃; The average particle size of the HO-BNNS in step 1 is 1.04 μm, the thickness of the HO-BNNS is 1-3 μm, the yield of the HO-BNNS is 48.31-55.12%, and the Zeta potential of the HO-BNNS is -30 mV; The amount ratio of HO-BNNS to methanol in the methanol solution of HO-BNNS in step 2 is 1g: 100-200ml; the amount ratio of cobalt nitrate hexahydrate to methanol in the methanol solution of cobalt nitrate hexahydrate is 1g: 50-100ml; The ultrasonic time in step 2 is 30-60 min, and the ultrasonic frequency is 30-50 kHz; The mass ratio of HO-BNNS to cobalt nitrate hexahydrate in the mixed solution A in step 2 is 1: 10-20; The ratio of 2-methylimidazole and methanol in the methanol solution of 2-methylimidazole described in step 2 is 1g:10-20ml; The volume ratio of the methanol solution of 2-methylimidazole described in step 2 and the mixed solution A described is 1:3-5; The stirring time described in step 2 is 1-3h, and the standing time is 24-48h; The centrifugal washing speed described in step 2 is 6000-8000rpm / min, and the centrifugal washing time is 2-5min; The drying temperature described in step 2 is 60-80℃, and the drying time is 4-6h; The calcination temperature described in step 3 is 500-700℃, the calcination time is 2-3h, and the calcination heating rate is 5-10℃ / min.

[0009] A high-thermal-conductivity composite material of ZIF-67 coated hydroxylated BNNS prepared by the above-mentioned preparation method.

[0010] Compared with the prior art, the present application has the following advantages: 1. The present application uses high-energy ball milling technology to exfoliate and hydroxylate BN, and develops a high-thermal-conductivity composite material preparation method of ZIF-67 coated hydroxylated BNNS which can be recycled repeatedly. The advantages of using high-energy ball milling technology to prepare HO-BNNS material are that a series of chemical reactions can occur during the ball milling process to form a unique functional structure of the material, pollution during the ball milling process can be reduced, synthesis problems during the material preparation process can be reduced, ball milling efficiency can be improved, and common problems caused by traditional ball milling methods can be avoided. HO-BNNS prepared by high-energy ball milling technology is characterized by transparency, ultra-thin and overlapping planar structure, and exhibits a stacking structure due to the addition of -OH groups. The average particle size of HO-BNNS is 1.04μm, and no large defects are generated during the exfoliation and modification process. HO-BNNS prepared by high-energy ball milling technology can be used as a thermal conductivity additive of nanofluid to effectively improve the thermal conductivity of the matrix material. After BN is exfoliated and hydroxylated by high-energy ball milling technology, the present application is combined with ZIF-67, and a high-efficiency and simple method for rapidly preparing high-quality and high-yield ZIF-67 coated HO-BNNS composite material is developed, which provides a new feasible scheme for the practical application of the material in the field of thermal management.

[0011] 2. The present application prepares a high-thermal-conductivity composite material of ZIF-67 coated hydroxylated BNNS by one-step solution method, which has higher synthesis efficiency. ZIF-67 has obvious magnetic attraction after carbonization, has higher recovery efficiency and has little effect on the performance of the material, which expands the advantages of material recycling and reuse.

[0012] 3. The synthesis of ZIF-67 and the coating of HO-BNNS in the present application are carried out simultaneously. In the one-step solution method, ZIF-67 grows directly with HO-BNNS as the core during the generation process, can be more uniformly wrapped on the surface of HO-BNNS, and the combination between ZIF-67 and HO-BNNS is formed during the growth process, which can exist chemical bond and strong physical adsorption, so that the combination of the two is more firm. The one-step solution method does not need to carry out the synthesis, separation and purification of ZIF-67, and then carries out the coating operation, but all raw materials are mixed in one reaction system for reaction, which reduces the intermediate link, simplifies the process flow, reduces the operation difficulty and cost, and also reduces the impurities possibly introduced by the intermediate operation, can save the time cost, and also reduces the production cost. Finally, through the one-step solution method, the growth rate of ZIF-67 and the coating degree can be more accurately controlled by adjusting the reaction conditions, so as to realize the accurate control of the morphology and structure of ZIF-67 coated HO-BNNS material.

[0013] The method for preparing the ZIF-67 coated hydroxylated BNNS high thermal conductivity composite material can obtain the functionalized composite material with high thermal conductivity at a calcination temperature of 500-700℃ for 2-3h, which is compared with the calcination temperature (800-900℃) and the calcination time (5-8h) for preparing the related functionalized composite material in the art, is not only beneficial to reduce the production cost, but also has wider applicability. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 is the XRD pattern of HO-BNNS obtained in Example 1; Figure 2 is the SEM pattern of HO-BNNS obtained in Example 1 and Comparative Example 1, wherein, Figure 2 (a) is the SEM pattern of HO-BNNS obtained in Example 1, Figure 2 (b) is the SEM pattern of HO-BNNS obtained in Comparative Example 1; Figure 3 is the XRD pattern of the composite material precursor obtained in Example 1 and the composite material BN-O@ZIF-67 obtained in Examples 2-4; Figure 4 is the SEM pattern of the composite material obtained in Examples 1-4, wherein, Figure 4 (a) is the SEM pattern of BN-O@ZIF-67 obtained in Example 1, Figure 4 (b) is the SEM pattern of BN-O5@ZIF-67 obtained in Example 2, Figure 4 (c) is the SEM pattern of BN-O6@ZIF-67 obtained in Example 3, Figure 4(d) is the SEM image of BN-O7@ZIF-67 obtained in Example 4; Figure 5 Micro-morphology and EDS area scan element distribution map of the composite material obtained in Example 4, wherein, Figure 5 (a) is the micro-morphology map of BN-O7@ZIF-67, Figure 5 (b) is the EDS area scan element C distribution map, Figure 5 (c) is the EDS area scan element O distribution map, Figure 5 (d) is the EDS area scan element Co distribution map, Figure 5 (e) is the EDS area scan element N distribution map, Figure 5 (f) is the EDS area scan element P distribution map. Figure 6 The cooling curve diagram of the composite material precursor obtained in Example 1 and the composite material BN-O@ZIF-67 obtained in Examples 2-4; Figure 7 The thermal conductivity diagram of the composite material precursor obtained in Example 1 and the composite material BN-O@ZIF-67 obtained in Examples 2-4. DETAILED DESCRIPTION

[0015] The BN used in the preparation of HO-BNNS in the application is hexagonal boron nitride (h-BN), which is purchased from Macklin, with a purity of 99.9% and a particle size of 5-10 μm.

[0016] The plasma ball mill used in the ball milling process in the preparation of HO-BNNS in the application is a PBMS plasma ball mill. Cold field discharge plasma is introduced into mechanical vibration ball milling, and near-constant pressure gas forms high-energy non-equilibrium plasma and mechanical ball milling in the ball mill tank. Under the action of cold plasma in the sealed ball mill system, the molecules of the material are converted into atomic state and excited state for recombination, which promotes the refinement of the powder organization, alloying, activity activation, chemical reaction and accelerates the in-situ gas-solid phase reaction, etc. The ball milling efficiency is greatly improved, the ball milling pollution is significantly reduced, and under the limited addition ratio of BN, PVA and steel balls in the application, a unique structure is formed, which has high yield and good dispersion stability, thereby significantly improving the performance of the material. The steel balls can provide stronger impact force and grinding force during the ball milling process, and the limited steel ball ratio can strengthen the peeling efficiency. The limited addition ratio of PVA optimizes the dispersion and hydroxylation, and the limited ratio of BN can ensure the uniformity of the product. Under the synergistic action, the final HO-BNNS has multi-dimensional advantages in preparation efficiency, dispersion stability and functional performance.

[0017] The HO-BNNS is completely dried in the drying process of preparing the HO-BNNS, and the drying time of the HO-BNNS in the following examples of the present application is more than 72 hours.

[0018] In the examples of the present application, the methanol solution of the HO-BNNS and the methanol solution of the cobalt nitrate hexahydrate need to be mechanically stirred when mixing the two solutions in the preparation of the mixed solution A, and the mechanical stirring time is 45-60 minutes.

[0019] Example 1

[0020] Step 1, preparation of HO-BNNS by PVA assisted ball milling based on high-energy ball milling method The hexagonal boron nitride (h-BN) and PVA were ball milled by using a PBMS plasma ball mill, the mass ratio of h-BN:PVA:steel ball was 1:10:70, and the ball milling discharge was 8 cycles during the ball milling process, the working time was 30 minutes and the intermittent time was 20 minutes in each cycle; the rotation speed of the PBMS plasma ball mill was 1000 rpm, and the frequency was 8 KHz; then the ball-milled product obtained was centrifugally washed by a mixed solution of water and ethylene glycol with a volume ratio of 11:9 for 5 minutes at a rotation speed of 1000 rpm, and finally dried at 80℃ for 72 hours to obtain HO-BNNS, the average particle size of the HO-BNNS was 1.04μm, the thickness of the HO-BNNS was 1μm, the yield of the HO-BNNS was 48.31%, and the Zeta potential of the HO-BNNS was-30 mV; Step 2, synthesis of composite material precursor of HO-BNNS coated with ZIF-67 The methanol solution of HO-BNNS with a concentration of 1g:100ml and the methanol solution of cobalt nitrate hexahydrate with a concentration of 1g:50ml were respectively ultrasonically treated for 30 minutes at a frequency of 30 kHz, then the methanol solution of HO-BNNS and the methanol solution of cobalt nitrate hexahydrate were mixed in a mass ratio of HO-BNNS to cobalt nitrate hexahydrate of 1:10 and mechanically stirred for 45 minutes to obtain a mixed solution A; then a methanol solution of 2-methylimidazole with a concentration of 1g:10ml was added to the mixed solution A and stirred for 1 hour, and then the mixture was left to stand for 24 hours to obtain a mixed solution B; wherein the volume ratio of the methanol solution of 2-methylimidazole to the mixed solution A was 1:3; the obtained mixed solution B was centrifugally washed with methanol at a rotation speed of 6000 rpm / min for 2 minutes, and finally dried at 60℃ for 4 hours to obtain a composite material precursor BN-O@ZIF-67.

[0021] Figure 1The XRD pattern of the prepared HO-BNNS powder obtained from step 1 of example 1 is shown in the figure, it can be seen from the figure that the intensity of the characteristic peaks of the prepared HO-BNNS is obviously reduced and the width is obviously widened compared with the original h-BN, which indicates that the structure order of BNNS along the c direction is successfully exfoliated in the exfoliation process.

[0022] Example 2

[0023] Step 1, preparation of HO-BNNS based on high-energy ball milling method using PVA assisted ball milling The hexagonal boron nitride (h-BN) and PVA were ball milled by PBMS plasma ball mill, the mass ratio of h-BN: PVA: steel ball was 1:5:70, the ball milling discharge was 6 cycles during the ball milling process, the working time was 20 min and the intermittent time was 10 min in each cycle; the rotation speed of the PBMS plasma ball mill was 1500 rpm and the frequency was 10 KHz; then the ball-milled product obtained was washed by centrifugation for 10 min with a mixture of water and ethylene glycol at a volume ratio of 11:10, the rotation speed was 3000 rpm, and finally dried at 60℃ for 80h to obtain HO-BNNS, the average particle size of HO-BNNS was 1.04μm, the thickness of HO-BNNS was 3μm, the yield of HO-BNNS was 49.76%, and the Zeta potential of HO-BNNS was-30 mV; Step 2, synthesis of ZIF-67 coated HO-BNNS composite precursor The HO-BNNS methanol solution with a concentration of 1g:200ml and the cobalt nitrate hexahydrate methanol solution with a concentration of 1g:100ml were respectively ultrasonically treated for 60min at a frequency of 50kHz, then the HO-BNNS methanol solution and the cobalt nitrate hexahydrate methanol solution were mixed in a proportion of HO-BNNS and cobalt nitrate hexahydrate at a mass ratio of 1:20, and then mechanically stirred for 45min to obtain a mixed solution A; then the 2-methyl imidazole methanol solution with a concentration of 1g:10ml was added to the mixed solution A and continuously stirred for 1h, and then left to stand for 24h to obtain a mixed solution B; wherein the volume ratio of the 2-methyl imidazole methanol solution to the mixed solution A was 1:3; the obtained mixed solution B was centrifuged with methanol at a rotation speed of 6000rpm / min for 2min, and finally dried at 60℃ for 4h to obtain the composite precursor BN-O5@ZIF-67.

[0024] Step 3, calcination and carbonization of BN-O5@ZIF-67 composite The composite precursor BN-O5@ZIF-67 was calcined, the calcination temperature was 500℃, the heating rate was 5℃ / min, the holding time was 2h, the protective gas was nitrogen, and the high-thermal-conductivity composite BN-O5@ZIF-67 of ZIF-67 coated hydroxylated BNNS was obtained after cooling in the furnace.

[0025] Example 3

[0026] Step 1, preparation of HO-BNNS by PVA assisted ball milling based on high-energy ball milling method h-BN and PVA were ball milled by PBMS plasma ball mill, the mass ratio of h-BN:PVA:steel ball was 1:5:70, and the ball milling discharge was 10 cycles during the ball milling process, the working time was 40 min and the intermittent time was 30 min in each cycle; the rotation speed of the PBMS plasma ball mill was 1500 rpm and the frequency was 10 KHz; then the ball-milled product obtained was centrifuged and washed by a mixture of water and ethylene glycol with a volume ratio of 11:9 for 5 min at a rotation speed of 3000 rpm, and finally dried at 100℃ for 72h to obtain HO-BNNS, the average particle size of HO-BNNS was 1.04μm, the thickness of HO-BNNS was 2μm, the yield of HO-BNNS was 55.12%, and the Zeta potential of HO-BNNS was-30 mV; Step 2, synthesis of composite material precursor of ZIF-67 coated HO-BNNS The HO-BNNS methanol solution with a concentration of 1g:100ml and the cobalt nitrate hexahydrate methanol solution with a concentration of 1g:100ml were respectively ultrasonically treated for 60min at a frequency of 30kHz, then the HO-BNNS methanol solution and the cobalt nitrate hexahydrate methanol solution were mixed in a mass ratio of HO-BNNS to cobalt nitrate hexahydrate of 1:15, and then mechanically stirred for 45min to obtain a mixed solution A; then a 2-methylimidazole methanol solution with a concentration of 1g:10ml was added to the mixed solution A and continuously stirred for 3h, and then left to stand for 48h to obtain a mixed solution B; wherein the volume ratio of the 2-methylimidazole methanol solution to the mixed solution A was 1:5; the obtained mixed solution B was centrifuged and washed with methanol at a rotation speed of 8000rpm / min for 5min, and finally dried at 80℃ for 6h to obtain the composite material precursor BN-O@ZIF-67.

[0027] Step 3, calcination and carbonization of the BN-O@ZIF-67 composite material The composite material precursor BN-O@ZIF-67 was calcined, the calcination temperature was 600℃, the heating rate was 10℃ / min, the holding time was 3h, the protective gas was nitrogen, and the high-thermal-conductivity composite material BN-O6@ZIF-67 of ZIF-67 coated hydroxylated BNNS was obtained after cooling in the furnace.

[0028] Example 4

[0029] Step 1, preparation of HO-BNNS by PVA assisted ball milling based on high-energy ball milling method The PBMS plasma ball mill is used for ball milling of hexagonal boron nitride (h-BN) and PVA, the mass ratio of h-BN:PVA:steel ball is 1:8:70, the ball milling discharge is 10 cycles during the ball milling process, the working time is 35 min in each cycle, and the intermittent time is 10 min; the rotation speed of the PBMS plasma ball mill is 1200 rpm, and the frequency is 8 KHz; then the ball milling product obtained after ball milling is centrifugally washed by a mixed solution of water and ethylene glycol in a volume ratio of 11:9 for 5 min at a rotation speed of 3000 rpm, and finally dried at 80 ℃ for 72 h to obtain HO-BNNS, the average particle size of HO-BNNS is 1.04 μm, the thickness of HO-BNNS is 3 μm, the yield of HO-BNNS is 54.10%, and the Zeta potential of HO-BNNS is -30 mV; Step 2, synthesis of ZIF-67 coated HO-BNNS composite material precursor The methanol solution of HO-BNNS with a concentration of 1 g:200 ml and the methanol solution of cobalt nitrate hexahydrate with a concentration of 1 g:100 ml are respectively ultrasonically treated at a frequency of 30 kHz for 60 min, then the methanol solution of HO-BNNS and the methanol solution of cobalt nitrate hexahydrate are mixed in a mass ratio of HO-BNNS to cobalt nitrate hexahydrate of 1:20, and then mechanically stirred for 45 min to obtain a mixed solution A; then the methanol solution of 2-methylimidazole with a concentration of 1 g:10 ml is added to the mixed solution A and continuously stirred for 3 h, and then left to stand for 48 h to obtain a mixed solution B; wherein the volume ratio of the methanol solution of 2-methylimidazole to the mixed solution A is 1:3; the obtained mixed solution B is centrifugally washed with methanol at a rotation speed of 8000 rpm / min for 2 min, and finally dried at 60 ℃ for 4 h to obtain a composite material precursor BN-O@ZIF-67.

[0030] Step 3, calcination and carbonization of the BN-O@ZIF-67 composite material The composite material precursor BN-O@ZIF-67 is calcined, the calcination temperature is 700 ℃, the heating rate is 5 ℃ / min, the holding time is 2 h, the protective gas is nitrogen, and the high-thermal-conductivity composite material BN-O7@ZIF-67 coated with ZIF-67 and hydroxylated BNNS is obtained after cooling in the furnace.

[0031] Figure 5 The micro-morphology and EDS area scanning element distribution map of the material obtained in Example 4 further prove the existence of various elements and the uniform distribution of various elements.

[0032] Figure 3The XRD pattern of the material BN-O@ZIF-67 obtained in Example 1-4, the XRD results determine the crystal structure of the HO-BNNS coated ZIF-67 composite material, the composite material has similar diffraction peaks with ZIF-67 crystal, indicating that the preparation process has no effect on the crystal integrity of ZIF-67.

[0033] As shown in Figure 4 , Figure 4 (a) is the SEM image of the material BN-O@ZIF-67 obtained in Example 1, from which it can be seen that the prepared ZIF-67 is regular rhombohedron shape, and the powdery ZIF-67 is accumulated by granular ZIF-67, and the morphology of each particle is clear, and the HO-BNNS is tightly coated by ZIF-67; Figure 4 (b), Figure 4 (c) and Figure 4 (d) are respectively the SEM images of the materials BN-O5@ZIF-67, BN-O6@ZIF-67 and BN-O7@ZIF-67 obtained in Examples 2-4; from the figures it can be seen that the HO-BNNS is tightly coated by ZIF-67, and the composite material maintains the polyhedral structure after high-temperature carbonization, but the surface presents wrinkles, the outline becomes blurred, and the surface roughness increases. This change in morphology is attributed to the partial collapse of the ZIF-67 framework and the reconstruction of the carbon matrix during high-temperature carbonization; Preparation of ZIF-67 The preparation process is as follows: 2-methylimidazole and methanol are prepared into a methanol solution of 2-methylimidazole at a ratio of 1 g:20 ml, and cobalt nitrate hexahydrate and methanol are prepared into a methanol solution of cobalt nitrate hexahydrate at a ratio of 1 g:40 ml, the methanol solution of 2-methylimidazole and the methanol solution of cobalt nitrate hexahydrate are respectively ultrasonically mixed at a volume ratio of 10:1 to obtain a mixed solution, then the mixed solution is ultrasonically treated for 60 min, and is left to stand for more than 24 h, after standing is completed, the mixed solution is centrifuged and washed, the centrifugal speed is 6000 rpm / min, and the time is 5 min, and finally the ZIF-67 is obtained by drying at 60°C for 4 h.

[0034] Figure 6 and Figure 7 h-BN, ZIF-67, HO-BNNS, BN-O@ZIF-67, BN-O5@ZIF-67, BN-O6@ZIF-67 and BN-O7@ZIF-67 as shown in the above table represent respectively water-glycol fluids each containing 0.12 wt% of h-BN, ZIF-67, HO-BNNS, BN-O@ZIF-67, BN-O5@ZIF-67, BN-O6@ZIF-67 and BN-O7@ZIF-67 by mass fraction, wherein the volume ratio of water and glycol is 11:9; Figure 7The water-glycol shown in the middle is a mixture of water and glycol with a volume ratio of 11:9.

[0035] From Figure 6 It can be seen from the figure that the cooling rate of the composite material obtained in Example 1-4 is obviously improved after being dispersed in the water-glycol solution compared with h-BN / water-glycol fluid and ZIF-67 / water-glycol fluid; it can be seen from the figure that the cooling rate of the BN-O6@ZIF-67 composite material obtained in Example 3 is the fastest after being dispersed in the water-glycol solution, which is obviously improved compared with h-BN / water-glycol matrix, ZIF-67 / water-glycol fluid and BN-O@ZIF-67 / water-glycol fluid material, and the heat conduction capacity is the strongest.

[0036] From Figure 7 It can be seen that h-BN is peeled off and forms HO-BNNS after hydroxylation, and because the product has fewer layers, it can establish weak interaction with water-glycol in the case of smaller volume and mass, therefore, HO-BNNS can be effectively dispersed in the water-glycol solution and form a heat conduction network, which also improves the compatibility of HO-BNNS with the base solution, reduces phonon scattering and enhances the thermal conductivity.

[0037] After the precursor BN-O@ZIF-67 is compounded with ZIF-67, the density of the precursor material increases, and under the same mass fraction, the formation of the heat conduction network of the composite precursor BN-O@ZIF-67 and the thermal conductivity are lower compared with HO-BNNS.

[0038] Figure 7In Example 1, the thermal conductivity of the composite precursor BN-O@ZIF-67, after dispersion in a water-ethylene glycol solution, was significantly higher than that of water-ethylene glycol fluid and h-BN / water-ethylene glycol fluid. Furthermore, the thermal conductivity of the composite material did not decrease significantly after magnetic recovery and redispersement. Similarly, in Example 2, the thermal conductivity of the composite material BN-O5@ZIF-67, after dispersion in a water-ethylene glycol solution, was significantly higher than that of the water-ethylene glycol matrix, h-BN / water-ethylene glycol fluid, ZIF-67 / water-ethylene glycol fluid, HO-BNNS / water-ethylene glycol fluid, and BN-O@ZIF-67 / water-ethylene glycol fluid. Moreover, the thermal conductivity of the composite material did not decrease significantly after magnetic recovery and redispersement. The thermal conductivity of the composite material BN-O6@ZIF-67 obtained in Example 3, after dispersion in water-ethylene glycol solution, was significantly improved compared to all other materials, and the thermal conductivity of the composite material did not decrease significantly after magnetic recovery and redispersement; the thermal conductivity of the composite material BN-O7@ZIF-67 obtained in Example 4, after dispersion in water-ethylene glycol solution, was significantly improved compared to water-ethylene glycol matrix, h-BN / water-ethylene glycol fluid, ZIF-67 / water-ethylene glycol fluid, HO-BNNS / water-ethylene glycol fluid, and BN-O@ZIF-67 / water-ethylene glycol fluid materials, and the thermal conductivity of the composite material did not decrease significantly after magnetic recovery and redispersement.

[0039] Comparative Example 1 By changing the mass ratio of BN, PVA and steel balls in step 1 of Example 1 to 1:15:70, while keeping other contents unchanged, HO-BNNS is obtained.

[0040] like Figure 2 As shown, Figure 2 (a) is a SEM image of the HO-BNNS powder prepared in step 1 of Example 1. It can be seen from the image that the prepared HO-BNNS powder is similar to h-BN in that it is a transparent, ultrathin and overlapping planar structure. The original h-BN is effectively peeled into several layers with relatively uniform particle size, which is conducive to the smooth progress of subsequent ZIF-67 coating of HO-BNNS. Figure 2 (b) is the HO-BNNS obtained in Comparative Example 1. As can be seen from the figure, after the ball-to-material ratio increases, the friction between the balls and the material increases during the ball milling process, which makes the surface of BNNS rougher and causes more unevenness and wrinkles. Excessive mechanical action causes the shape of BNNS to become irregular, with more fragments and irregular edges, which has an adverse effect on the performance of BNNS.

[0041] Comparative Example 2 The planetary ball mill in step 1 in example 3 is replaced by a high-energy ball mill, and other contents remain unchanged to obtain HO-BNNS. The planetary ball mill can only perform high-speed impact and shear through pure mechanical force to achieve the exfoliation and hydroxylation of BNNS. Compared with the high-energy ball mill, there is no assistance of high-energy particles and electric field, which leads to a significant decrease in the yield of the material and a large number of multi-layer agglomerates. The surface bond of h-BN is limited during the mechanical ball milling hydroxylation process, and the number of active sites is small, so the hydroxylation is not sufficient. The combination of hydroxyl and surface also depends on physical adsorption and weak chemical action, which leads to the easy falling off of hydroxyl in the subsequent treatment, resulting in poor dispersibility. The high-speed impact of the planetary ball mill can cause interlayer exfoliation and transverse crushing at the same time, leading to uneven transverse size and jagged defects on the edge of the product, resulting in high defect degree and low purity of the material.

[0042] Comparative Example 3 The amount ratio of HO-BNNS and methanol in the methanol solution of HO-BNNS in step 2 in example 3 is changed to 1g:50ml, and the amount ratio of cobalt nitrate hexahydrate and methanol in the methanol solution of cobalt nitrate hexahydrate is changed to 1g:20ml, and other contents remain unchanged to obtain BN-O@ZIF-67 composite material. Due to the insufficient amount of methanol, the concentration of the solution is significantly increased, and the dispersibility of the solvent is weakened, leading to an increase in the diffusion resistance of the substance, and the uniform coating of HO-BNNS by ZIF-67 becomes local agglomeration coating, and the coating effect is significantly deteriorated, and the structural integrity is reduced. The collision frequency of Co ions and ligands increases at high concentration, the nucleation rate is greater than the growth rate, the crystal nucleus is small, and the diffusion is blocked and cannot grow fully, which is easy to cause crystal nucleus agglomeration, leading to uneven crystal size and significantly reduced crystallinity. The agglomeration of the product during preparation leads to the close combination of particles through van der Waals force and covalent bond, which is difficult to disperse.

[0043] Comparative Example 4 The amount ratio of 2-methylimidazole and methanol in the methanol solution of 2-methylimidazole in step 2 in example 3 is changed to 1g:5ml, and other contents remain unchanged to obtain BN-O@ZIF-67 composite material. In example 3, ZIF-67 can be grown in an orderly manner to form a uniform dodecahedron, but in comparative example 4, the volume of the methanol solution is reduced, leading to an increase in the concentration of the reactants, and the dispersion is not timely, and the independently grown crystals have irregular blocky morphology, blurred edges, rough surfaces, and low crystal quality. The reduction of the volume of methanol leads to an increase in the viscosity of the system, and the stability after dispersion is significantly reduced.

[0044] Although the embodiments of the present application have been shown and described, it can be understood by those of ordinary skill in the art that various changes, modifications, replacements and variations can be made to these embodiments without departing from the principles and spirits of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a recyclable ZIF-67-coated hydroxylated BNNS high thermal conductivity composite material, characterized in that, The method comprises the following steps: Step 1, high-energy ball milling is used to mill boron nitride (BN) and PVA, and then the ball-milled product is washed by centrifugation in a mixture of water and ethylene glycol, and finally HO-BNNS is obtained by drying; The boron nitride (BN) is hexagonal boron nitride (h-BN), and the particle size of the hexagonal boron nitride (h-BN) is 5-10 μm; The average particle size of the HO-BNNS is 1.04 μm, the thickness of the HO-BNNS is 1-3 μm, the yield of the HO-BNNS is 48.31-55.12%, and the Zeta potential of the HO-BNNS is -30 mV; Step 2, the methanol solution of HO-BNNS and the methanol solution of cobalt nitrate hexahydrate are respectively ultrasonically treated and then mixed to obtain a mixed solution A; the methanol solution of 2-methyl imidazole is added to the mixed solution A, stirred and then left to stand to obtain a mixed solution B, the mixed solution B is washed by centrifugation in methanol, and finally a composite material precursor is obtained by drying; Step 3, the composite material precursor is calcined in nitrogen atmosphere to obtain a high-thermal-conductivity composite material of ZIF-67 coated hydroxylated BNNS.

2. The method of claim 1, wherein the method of preparing the recyclable ZIF-67 coated hydroxylated BNNS high thermal conductive composite material is characterized by, The high-energy ball milling method in step 1 is specifically: an inductive ball mill is used to mill boron nitride (BN) and PVA; the ball milling process of the inductive ball mill is carried out for 6-10 cycles of discharge, and the working time of each cycle is 20-40 min, and the intermittent time is 10-30 min; the rotating speed of the inductive ball mill is 1000-1500 rpm, and the frequency is 8-10 KHz.

3. The method of claim 2, wherein the method of preparing the recyclable ZIF-67 coated hydroxylated BNNS high thermal conductive composite material is characterized by, The mass ratio of the boron nitride (BN), the PVA and the steel balls in the inductive ball mill is 1: (5-10) :

70.

4. The method of claim 1, wherein the method of preparing the recyclable ZIF-67 coated hydroxylated BNNS high thermal conductive composite material is characterized by, The volume ratio of water to ethylene glycol in step 1 is 11: 9-10; the rotating speed of the centrifugal washing is 1000-3000 rpm, and the centrifugal washing time is 5-10 min; and the drying temperature is 60-100 DEG C.

5. The method of claim 1, wherein the method of preparing the recyclable ZIF-67 coated hydroxylated BNNS high thermal conductive composite material is characterized by, The amount ratio of HO-BNNS to methanol in the methanol solution of HO-BNNS in step 2 is 1 g: 100-200 ml; and the amount ratio of cobalt nitrate hexahydrate to methanol in the methanol solution of cobalt nitrate hexahydrate is 1 g: 50-100 ml.

6. The method of claim 1, wherein the method of preparing the recyclable ZIF-67 coated hydroxylated BNNS high thermal conductive composite material is characterized by, The ultrasonic treatment time in step 2 is 30-60 min, and the ultrasonic frequency is 30-50 kHz.

7. The method of claim 1, wherein the method of preparing the recyclable ZIF-67 coated hydroxylated BNNS high thermal conductive composite material is characterized by, The mass ratio of HO-BNNS to cobalt nitrate hexahydrate in the mixed solution A in step 2 is 1: 10-20; the amount ratio of 2-methyl imidazole to methanol in the methanol solution of 2-methyl imidazole is 1 g: 10-20 ml; and the volume ratio of the methanol solution of 2-methyl imidazole to the mixed solution A is 1: 3-5.

8. The method of claim 1, wherein the method of preparing the recyclable ZIF-67 coated hydroxylated BNNS high thermal conductive composite material is characterized by, The stirring time in step 2 is 1-3 h, and the standing time is 24-48 h; the rotating speed of the centrifugal washing is 6000-8000 rpm / min, and the centrifugal washing time is 2-5 min; the drying temperature is 60-80 DEG C, and the drying time is 4-6 h.

9. The method of claim 1, wherein the method of preparing the recyclable ZIF-67 coated hydroxylated BNNS high thermal conductive composite material is characterized by, The temperature of the calcination in step 3 is 500-700℃, the time of the calcination is 2-3h, and the heating rate of the calcination is 5-10℃ / min.

10. The ZIF-67 coated hydroxylated BNNS high thermal conductive composite material prepared by the preparation method of any one of claims 1-9.

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