Thermally conductive insulating epoxy resin composite material, and preparation method and application thereof

Abstract

This invention discloses a thermally conductive and insulating epoxy resin composite material, its preparation method, and its application, belonging to the technical field of thermally conductive and insulating material preparation. The thermally conductive and insulating epoxy resin composite material includes an epoxy resin matrix and thermally conductive fillers distributed within the epoxy resin matrix. The thermally conductive fillers include sheet-like fillers and spherical fillers. The sheet-like fillers are at least partially oriented along a predetermined direction within the epoxy resin matrix and at least partially form a layered or stacked structure. The spherical fillers are at least partially distributed in the gaps between adjacent sheet-like fillers, the edge regions of the sheet-like fillers, and / or the interface regions between the sheet-like fillers and the epoxy resin matrix. This invention uses sheet-like and spherical fillers as mixed thermally conductive fillers, combined with the depolymerization, dispersion, and directional rearrangement effects of a segmented variable-frequency bipolar square wave electric field on the thermally conductive fillers, thereby improving the thermal conductivity and insulation properties of the epoxy resin composite material.

Description

Technical Field

[0001] This invention relates to the field of thermally conductive and insulating material preparation technology, and in particular to a thermally conductive and insulating epoxy resin composite material, its preparation method, and its application. Background Technology

[0002] As power electronic devices, motor windings, power modules, and high-voltage insulation components develop towards higher power density, miniaturization, and higher reliability, their high heat generation places higher demands on the heat dissipation performance of encapsulation and insulation systems. If the heat generated during device operation cannot be dissipated in time, it can easily cause problems such as local temperature rise, thermal aging, interface debonding, and decreased insulation performance. Epoxy resin, due to its good electrical insulation properties, adhesion properties, chemical stability, and process adaptability, is widely used in power electronic encapsulation, casting insulation, potting compounds, and thermally conductive insulation components. However, the thermal conductivity of ordinary epoxy resin is typically only about 0.2 W / (m·K), which is insufficient to meet the engineering requirements of efficient heat dissipation.

[0003] Introducing high thermal conductivity insulating fillers such as hexagonal boron nitride, aluminum nitride, alumina, and silicon carbide into epoxy resins can improve the thermal conductivity of composite materials. Among them, lamellar hexagonal boron nitride has high in-plane thermal conductivity and excellent electrical insulation properties, making it a commonly used filler for preparing thermally conductive and insulating epoxy composites. However, simply increasing the filler content significantly increases the viscosity of the system and may cause porosity, agglomeration, and interface defects, thereby weakening the breakdown strength and processing stability. Using an electric field to induce the directional alignment of lamellar fillers during resin curing is an effective method for constructing continuous thermally conductive pathways. This type of method typically applies a single-frequency, single-amplitude alternating current electric field or a bipolar square wave electric field to orient the lamellar fillers along the direction of the electric field, thereby improving the thermal conductivity in the thickness direction. However, a single-frequency electric field is difficult to simultaneously address both filler deagglomeration and strong orientation processes. If a strong electric field is applied directly in the early stages of curing, agglomerates are prone to local enrichment, leading to electric field distortion; if a lower field strength or a higher frequency is used, it is difficult to form a stable oriented framework before resin gelation. Furthermore, after orientation, single-sized lamellar boron nitride still exhibits significant resin barriers and interfacial thermal resistance between adjacent layers, resulting in discontinuous thermophonon transport paths. While nanoparticles can help fill the gaps between layers, their high specific surface area easily leads to secondary agglomeration, making it difficult for simple nanofiller mixing to achieve effective bridging. Therefore, how to control the dispersion, orientation, and gap bridging behavior of fillers within the epoxy curing window, while maintaining good electrical insulation properties alongside high thermal conductivity, is a technical problem that needs to be solved in this field. Summary of the Invention

[0004] To address the problems of filler agglomeration, discontinuous thermal conductivity pathways, limited efficiency induced by a single electric field, and decreased insulation stability in existing epoxy thermally conductive and insulating composite materials, this invention aims to solve at least one of the aforementioned technical problems in the prior art. Therefore, one objective of this invention is to provide a thermally conductive and insulating epoxy resin composite material. This invention uses sheet-like and spherical fillers as mixed thermally conductive fillers, inducing the thermally conductive fillers to form a directional, stacked arrangement and interlayer filling within the resin matrix, creating thermally conductive pathways to reduce thermal resistance and improve the thermal conductivity and insulation properties of the epoxy resin composite material. A second objective of this invention is to provide a method for preparing the aforementioned thermally conductive and insulating epoxy resin composite material. A third objective of this invention is to provide an application of the aforementioned thermally conductive and insulating epoxy resin composite material.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A first aspect of the present invention is to provide a thermally conductive and insulating epoxy resin composite material, comprising an epoxy resin matrix and thermally conductive fillers distributed in the epoxy resin matrix, wherein the thermally conductive fillers include sheet fillers and spherical fillers. The sheet filler is at least partially oriented in the epoxy resin matrix and at least partially forms a sheet overlap or stacked structure; the spherical filler is at least partially distributed in the gaps between adjacent sheet fillers, the edge region of the sheet filler, and / or the interface region between the sheet filler and the epoxy resin matrix.

[0006] This invention uses sheet-like fillers and spherical fillers as mixed thermally conductive fillers, which respectively serve as heat-conducting skeletons and interlayer bridging fillers. This induces the thermally conductive fillers to form oriented stacked arrangements and fill the resin matrix, thereby forming thermally conductive pathways to reduce thermal resistance and improve the thermal conductivity of epoxy resin composites. At the same time, the oriented skeleton and bridging structure help reduce the dispersion of local defects and improve the power frequency breakdown stability of the composite material.

[0007] The thermally conductive filler described in this invention refers to a filler that has electrical insulation and thermal conductivity.

[0008] In some preferred embodiments, the particle size of the sheet-like filler is in the micrometer range.

[0009] In some preferred embodiments, the average particle size of the sheet-like filler is 1-100 μm; in even more preferred embodiments, the average particle size is 10-50 μm; and even more preferably, it is 10-20 μm.

[0010] In some preferred embodiments, the particle size of the spherical filler is in the nanometer range.

[0011] In some preferred embodiments, the average particle size of the spherical filler is 10-100 nm; in even more preferred embodiments, the average particle size is 50-100 nm; and even more preferably, it is 60-80 nm.

[0012] In this invention, unless otherwise stated, the average particle size refers to the median particle size D50, which is the particle size corresponding to the cumulative distribution reaching 50% in the particle size distribution obtained by statistical analysis based on the equivalent diameter of the particles.

[0013] For sheet-like packings, the average particle size refers to the transverse equivalent particle size of the sheet-like packing in the direction of the principal plane of the sheet, excluding the thickness dimension of the sheet-like packing; for spherical packings, the average particle size D50 refers to the equivalent spherical diameter or the diameter of the projected area equivalent circle of the spherical particles. The average particle size can be obtained by statistical analysis of laser particle size analysis, dynamic light scattering, scanning electron microscopy, or transmission electron microscopy images.

[0014] In some preferred embodiments, the mass ratio of the sheet packing to the spherical packing is (3-6):1, and in a more preferred embodiment, the mass ratio is (4-5):1.

[0015] In some preferred embodiments, the thermally conductive filler is present in the mixture at a mass ratio of 1%-30%, for example, any value of 1%, 5%, 10%, 15%, 20%, 25%, 28% or any range between two of these values.

[0016] In some preferred embodiments, the sheet-like filler includes at least one of hexagonal boron nitride, alumina, aluminum nitride, silicon nitride, mica, and silicates.

[0017] The sheet-like filler may be an unmodified sheet-like filler or a sheet-like filler that has undergone surface modification with a silane coupling agent.

[0018] In some preferred embodiments, the spherical packing includes at least one of aluminum nitride, aluminum oxide, magnesium oxide, silicon nitride, silicon dioxide, and boron nitride.

[0019] In some preferred embodiments, the surface of the spherical filler is modified with a silane coupling agent, which is a silane coupling agent containing an organic functional group that can react with or form an interfacial interaction with the epoxy resin curing precursor, wherein the organic functional group includes at least one of epoxy, amino, mercapto, vinyl, or methacryloxy.

[0020] In some more preferred embodiments, the silane coupling agent is an epoxy silane coupling agent, more preferably γ-glycidoxypropyltrimethoxysilane.

[0021] In some preferred embodiments, the modification process includes the following steps: The spherical packing material was dispersed in an aqueous ethanol solution, the pH of the solution was adjusted to 4.0-5.5, a silane coupling agent was added, and the mixture was kept at 60-80℃ and stirred for 2-4 hours. After the reaction was completed, the packing material was separated, washed, and dried to obtain the final product.

[0022] In some preferred embodiments, the amount of silane coupling agent added is 0.5-3.0% of the mass of the spherical packing; in a more preferred embodiment, the amount of silane coupling agent added is 1.0-2.0% of the mass of the spherical packing.

[0023] A second aspect of the present invention is to provide a method for preparing the thermally conductive and insulating epoxy resin composite material described in the first aspect, comprising the following steps: (1) A mixture is obtained by mixing epoxy resin curing precursor with thermally conductive filler; (2) The mixture is cured, and during the curing process, a first bipolar square wave electric field and a second bipolar square wave electric field are sequentially applied to the mixture; (3) The mixture after electric field treatment is cured to obtain the thermally conductive and insulating epoxy resin composite material; The frequency of the first bipolar square wave electric field is greater than the frequency of the second bipolar square wave electric field, and the electric field strength of the first bipolar square wave electric field is less than the electric field strength of the second bipolar square wave electric field.

[0024] The epoxy resin curing precursor includes epoxy resin and curing agent. The epoxy resin may be a bisphenol A or bisphenol F type epoxy resin monomer or its prepolymer. The curing agent may be an acid anhydride curing agent or an amine curing agent.

[0025] This invention uses sheet-like and spherical fillers as mixed thermally conductive fillers, and combines them with a segmented variable-frequency bipolar square wave electric field to achieve deagglomeration, dispersion and directional rearrangement of the thermally conductive fillers, thereby improving the thermal conductivity and insulation of epoxy resin composites. The first bipolar square wave electric field is a high-frequency, low-field electric field, which is used to reduce the agglomeration stability between sheet-like and spherical fillers, improve the dispersion uniformity of filler particles in the epoxy system, and reduce the possibility of local filler enrichment in the subsequent orientation stage. The second bipolar square wave electric field is a low-frequency, high-field electric field, which uses dielectric force and polarization torque to induce the sheet-like fillers to form an orientation skeleton oriented along a predetermined direction, and causes the spherical fillers to tend to be distributed in the gaps between adjacent sheet-like fillers, the resin-rich areas at the edges, and the local interface areas, forming a "surface-point-surface" thermally conductive bridging structure, thereby reducing thermal resistance.

[0026] The predetermined direction described in this invention is a direction in which the main plane of the sheet-like filler is perpendicular to the direction of the bipolar square wave electric field.

[0027] In some preferred embodiments, the epoxy resin curing precursor further includes an accelerator.

[0028] In some preferred embodiments, the mass ratio of the epoxy resin to the curing agent and the accelerator in the epoxy resin curing precursor is 100:(75-90):(0.5-3); in some more preferred embodiments, the mass ratio is 100:(80-85):(1-2).

[0029] In some preferred embodiments, the mixture is further subjected to homogenization and degassing treatment.

[0030] In some preferred embodiments, the homogeneous dispersion is carried out under high shear dispersion at 1800-2200 rpm for 30-60 min in a water bath at 58-63℃; the degassing is carried out under vacuum degassing under gauge pressure ≤ -0.09 MPa or absolute pressure ≤ 10 kPa until no obvious bubbles overflow.

[0031] In some preferred embodiments, the frequency of the first bipolar square wave electric field is 20-70 kHz; more preferably, it is 30-60 kHz; and even more preferably, it is 45-55 kHz.

[0032] In some preferred embodiments, the frequency of the second bipolar square wave electric field is 5-20 kHz; more preferably 5-15 kHz; and even more preferably 8-12 kHz.

[0033] In some preferred embodiments, the electric field strength of the first bipolar square wave electric field is 0.3-0.9 kV / mm; more preferably, it is 0.4-0.8 kV / mm; and even more preferably, it is 0.4-0.6 kV / mm.

[0034] In some preferred embodiments, the electric field strength of the second bipolar square wave electric field is 1.0-3.0 kV / mm; more preferably, it is 1.5-2.5 kV / mm; and even more preferably, it is 1.8-2.2 kV / mm.

[0035] The electric field strength described in this invention is the ratio of the peak output voltage to the electrode spacing.

[0036] In some preferred embodiments, the electrode spacing is 0.5-2.0 mm.

[0037] In some preferred embodiments, the processing time of the first bipolar square wave electric field is 15-30 min; more preferably, it is 15-25 min; and even more preferably, it is 18-22 min.

[0038] In some preferred embodiments, the processing time of the second bipolar square wave electric field is 30-60 min; more preferably, it is 35-55 min; and even more preferably, it is 45-55 min.

[0039] In some preferred embodiments, the duty cycle of the first bipolar square wave electric field and / or the second bipolar square wave electric field is 30-70%; in more preferred embodiments, it is 40-60%.

[0040] In some preferred embodiments, the mixture in step (2) is subjected to an electric field at 78-82°C.

[0041] The temperature of the pre- and post-gel windows of the epoxy resin curing precursor of this invention is 78-82℃. Applying a two-stage electric field within the gel window can satisfy the dispersion and directional migration of the thermally conductive filler.

[0042] In some preferred embodiments, the curing conditions in step (3) are: heat preservation at 78-82℃ for 0.5-1.5h, then heating to 100-110℃ and heat preservation for 1.5-2.5h, and then heating to 110-125℃ and heat preservation for 3.5-4.5h.

[0043] In some preferred embodiments, the curing conditions in step (3) are as follows: heat at 79-81°C for 0.8-1.2h, then heat to 103-108°C and heat for 1.8-2.2h, then heat to 118-122°C and heat for 3.8-4.2h.

[0044] A third aspect of the present invention is to provide an application of the thermally conductive and insulating epoxy resin composite material described in the first aspect, including its application in power electronic packaging, thermally conductive and insulating potting, motor winding insulation, casting insulation, or electrical insulation components.

[0045] The beneficial effects of this invention are: (1) The thermally conductive and insulating epoxy resin composite material of the present invention uses sheet-like filler and spherical filler as mixed thermally conductive fillers. The two serve as heat-conducting skeleton and interlayer bridging filler, respectively, to induce the thermally conductive filler to form oriented stacked arrangement and interlayer filling in the resin matrix, thereby forming a thermally conductive path to reduce thermal resistance and improve the thermal conductivity of epoxy resin composite material. At the same time, the oriented skeleton and bridging structure help to reduce the dispersion of local defects and improve the power frequency breakdown stability of composite material, while maintaining good electrical insulation performance while having high thermal conductivity.

[0046] (2) The present invention also provides a method for preparing the thermally conductive and insulating epoxy resin composite material. Based on the use of sheet-like fillers and spherical fillers as mixed thermally conductive fillers, a segmented variable frequency bipolar square wave electric field is used to achieve depolymerization, dispersion and directional rearrangement of the thermally conductive fillers. This avoids the problem of insufficient depolymerization or orientation efficiency of a single electric field, and transforms the simple “face-to-face” or “face-to-resin-to-face” contact between sheet-like fillers into a “face-to-point-to-face” bridging contact. This reduces the interfacial thermal resistance between sheet layers, improves the continuity of the thermal conduction path in the thickness direction, helps to alleviate local heat accumulation and electric field distortion, and improves the thermal conductivity of the epoxy resin composite material. At the same time, the orientation skeleton and bridging structure induced by the segmented variable frequency electric field help to reduce the dispersion of local defects and improve the power frequency breakdown stability of the composite material.

[0047] (3) The thermally conductive and insulating epoxy resin composite material of the present invention has both high thermal conductivity and good electrical insulation properties. When applied to power electronic packaging, electrical insulation components and other fields, it greatly improves its thermal conductivity and heat dissipation capacity while maintaining good electrical insulation properties, thereby improving the stability of device performance. Attached Figure Description

[0048] Figure 1 This is a schematic flowchart of the preparation method described in Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the two-stage electric field process in Embodiment 1 of the present invention; Figure 3 Example 1 shows the relative permittivity and complex viscosity of the liquid epoxy resin mixture at 25°C and 80°C as a function of applied frequency. Figure 4 This is a comparison chart of the thermal conductivity of the composite materials prepared in Example 1 and Comparative Examples 1-5; Figure 5 This is a line graph showing the thermal conductivity growth rate of insulating and thermally conductive epoxy resin composites prepared under first / second stage bipolar square wave electric field conditions with different frequencies and electric field intensities. Figure 6 This is a Weibull probability distribution of the breakdown strength of the composite materials prepared in Example 1 and Comparative Examples 1-5 under a 50Hz AC electric field.

[0049] Appendix Figure 2 Markings: 1-Bipolar square wave high voltage power supply; 2-Upper electrode plate; 3-Lower electrode plate; 4-Spherical aluminum nitride; 5-Flake hexagonal boron nitride; 6-Epoxy resin curing precursor. Detailed Implementation

[0050] The present invention will be further described in detail below through specific embodiments.

[0051] Unless otherwise specified, the raw materials, reagents, or apparatus used in the following examples and comparative examples are available from conventional commercial sources or can be obtained by existing technical methods. Unless otherwise specified, the raw materials and reagents used have not been further purified before use, and the test or measurement methods are conventional methods in the art.

[0052] The source information of some of the raw materials, reagents and devices involved in the embodiments of the present invention is as follows: Bisphenol A type epoxy resin E51 was purchased from Baling Petrochemical Branch of China Petrochemical Corporation Asset Management Co., Ltd., with the trade name CYD-128; 4-methylhexahydrophthalic anhydride was purchased from Changzhou Sykes Chemical Co., Ltd., with a purity ≥99%; 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30) was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., with a purity ≥95%; flake-shaped hexagonal boron nitride was purchased from Shanghai Gelin Technology Co., Ltd., using flake-shaped powder with a D50 particle size of 15 μm, either selected or customized; nano-sized spherical aluminum nitride was purchased from Shanghai Yunfu Nanotechnology Co., Ltd., using spherical or near-spherical powder with a D50 particle size of 60-80 nm, either selected or customized; γ-glycidoxypropyltrimethoxysilane (KH-560) was purchased from Nanjing Aocheng Chemical Co., Ltd.; anhydrous ethanol was purchased from Sinopharm Chemical Reagent Co., Ltd., and was of analytical grade; deionized water was prepared by a laboratory deionized water system. The bipolar square wave high voltage power supply, high voltage probe, oscilloscope, vacuum degassing device, vacuum drying oven, copper sheet parallel electrode and PTFE insulating mold are all conventional commercially available equipment or processed in accordance with conventional methods in this field.

[0053] Example 1 This embodiment relates to a thermally conductive and insulating epoxy resin composite material, see attached document. Figure 1-2 Its preparation method includes the following steps: (1) Bisphenol A type epoxy resin E51 was mixed with 4-methylhexahydrophthalic anhydride and DMP-30 in a mass ratio of 100:80:1 to obtain an epoxy resin curing precursor. Flaky hexagonal boron nitride with an average particle size of 15 μm and nano-sized spherical aluminum nitride modified with silane coupling agent were added as thermally conductive fillers. The mass fraction of the thermally conductive filler in the mixed system was 25%, of which the mass fraction of hexagonal boron nitride was 20% and the mass fraction of aluminum nitride was 5%, and the mass ratio of hexagonal boron nitride to aluminum nitride was 4:1. The obtained mixed system was subjected to high shear dispersion under a water bath at 60℃, with a dispersion speed of 2000 rpm and a dispersion time of 45 min. Then, vacuum degassing was performed under a gauge pressure ≤ -0.09 MPa until no obvious bubbles overflowed.

[0054] The surface modification method of nano-sized spherical aluminum nitride is as follows: nano-sized spherical aluminum nitride is added to an aqueous solution of ethanol (v / v=19:1), with a material-to-liquid ratio of 1g / 20mL. After ultrasonic dispersion for 30min, the pH of the solution is adjusted to 4.5 with acetic acid. Then, 1.5% KH-560 relative to the mass of the nano-sized spherical aluminum nitride is added. The mixture is stirred and reacted at 70℃ for 3h. After the reaction is completed, the mixture is centrifuged, washed with ethanol, and vacuum dried at 80℃ for 8h to obtain nano-sized spherical aluminum nitride with a silane coupling agent modified on the surface.

[0055] (2) The degassed mixture was poured into a PTFE insulating mold with parallel copper electrodes. The parallel electrodes were connected to a bipolar square wave high-voltage power supply, and the output waveform was monitored by a high-voltage probe and an oscilloscope. The electric field strength was calculated as the ratio of the peak output voltage to the electrode spacing. The square wave duty cycle was 50%, and the electrode spacing was 1.0 mm. The mold was placed in a curing environment at 80°C, and two stages of electric field were applied sequentially in the windows before and after the resin gel.

[0056] The first stage is a high-frequency deagglomeration stage, in which a bipolar square wave electric field with a frequency of 50 kHz and an electric field strength of 0.5 kV / mm is applied for 20 min. This stage is used to reduce the agglomeration stability between nanoscale spherical aluminum nitride and plate-like hexagonal boron nitride, improve the dispersion uniformity of nanoparticles in the epoxy system, and reduce the possibility of local filler enrichment in the subsequent strong orientation stage.

[0057] The second stage is the directional rearrangement stage, which switches to a bipolar square wave electric field with a frequency of 10 kHz and an electric field strength of 2.0 kV / mm, lasting for 45 min. In this stage, the dielectric force and polarization torque are used to induce micron-sized sheet-like hexagonal boron nitride to form an oriented framework along the direction of the electric field, and to cause nano-sized spherical aluminum nitride to be distributed in the gaps between adjacent sheet-like hexagonal boron nitride, the resin-rich areas at the edges, and the local interface regions, forming a "surface-point-surface" thermally conductive bridging structure.

[0058] (3) After the electric field is applied, the epoxy resin composite material is cured in stages according to the procedure of 80℃ for 1 h, 105℃ for 2 h, and 120℃ for 4 h to obtain thermally conductive and insulating epoxy resin composite material, which is denoted as EP / 25%BN / AlN-segmented.

[0059] In this embodiment, micron-sized sheet-like hexagonal boron nitride and nano-sized spherical aluminum nitride are used as mixed thermally conductive fillers. The two serve as the heat-conducting skeleton and the interlayer bridging filler, respectively. At the same time, a segmented variable frequency bipolar square wave electric field is used to achieve depolymerization, dispersion and directional rearrangement of the thermally conductive filler, thereby improving the thermal conductivity and insulation of the epoxy resin composite material.

[0060] Examples 2-43 all involve a thermally conductive and insulating epoxy resin composite material. The difference between the preparation method and that of Example 1 is that the frequency or field strength of the bipolar square wave electric field in the first / second stage is different. The rest is the same as that of Example 1. The specific electric field parameters are shown in Table 1.

[0061] Table 1. Frequency and field strength parameters of the first / second stage electric field in Examples 1-43

[0062] Comparative Example 1 This comparative example relates to an epoxy resin composite material, the preparation method of which differs from that of Example 1 in that the thermally conductive filler is 25% plate-like hexagonal boron nitride with an average particle size of 15 μm; a bipolar square wave electric field of 10 kHz and 2.0 kV / mm is applied for 65 min, specifically including the following steps: (1) Bisphenol A type epoxy resin E51, 4-methylhexahydrophthalic anhydride, and DMP-30 were mixed in a mass ratio of 100:80:1 to obtain an epoxy resin curing precursor. Flaky hexagonal boron nitride with an average particle size of 15 μm was added as a thermally conductive filler, and the mass fraction of the thermally conductive filler in the mixture was 25%. The obtained mixture was subjected to high shear dispersion in a water bath at 60℃, with a dispersion speed of 2000 rpm and a dispersion time of 45 min. Subsequently, vacuum degassing was performed under a gauge pressure ≤ -0.09 MPa until no obvious bubbles overflowed.

[0063] (2) The degassed mixture was poured into a PTFE insulating mold equipped with parallel copper electrodes. The parallel electrodes were connected to a bipolar square wave high-voltage power supply, and the output waveform was monitored by a high-voltage probe and an oscilloscope. The electric field strength was calculated as the ratio of the peak output voltage to the electrode spacing. The square wave duty cycle was 50%, and the electrode spacing was 0.5-2.0 mm. The mold was placed in a curing environment at 80°C, and an electric field was applied in the front and back windows of the resin gel. The electric field was a low-frequency, high-field bipolar square wave electric field with a frequency of 10 kHz and an electric field strength of 2.0 kV / mm, lasting for 65 min.

[0064] (3) After the electric field is applied, the epoxy resin composite material is obtained by step curing according to the procedure of keeping it at 80℃ for 1 h, 105℃ for 2 h, and 120℃ for 4 h. It is denoted as EP / 25%BN-10kHz.

[0065] Comparative Example 2 This comparative example relates to an epoxy resin composite material, the preparation method of which differs from that of Example 1 in that no electric field is applied, and specifically includes the following steps: (1) Bisphenol A type epoxy resin E51 was mixed with 4-methylhexahydrophthalic anhydride and DMP-30 in a mass ratio of 100:80:1 to obtain an epoxy resin curing precursor. Flaky hexagonal boron nitride with an average particle size of 15 μm and nano-sized spherical aluminum nitride modified with silane coupling agent were added as thermally conductive fillers. The mass fraction of the thermally conductive filler in the mixed system was 25%, of which the mass fraction of hexagonal boron nitride was 20% and the mass fraction of aluminum nitride was 5%, and the mass ratio of hexagonal boron nitride to aluminum nitride was 4:1. The obtained mixed system was subjected to high shear dispersion under a water bath at 60℃, with a dispersion speed of 2000 rpm and a dispersion time of 45 min. Then, vacuum degassing was performed under a gauge pressure ≤ -0.09 MPa until no obvious bubbles overflowed.

[0066] (2) The epoxy resin composite material was obtained by step curing according to the procedure of keeping it at 80℃ for 1 h, 105℃ for 2 h, and 120℃ for 4 h, and denoted as EP / 25%BN / AlN-random.

[0067] Comparative Example 3 This comparative example relates to an epoxy resin composite material, the preparation method of which differs from that of Example 1 in that a bipolar square wave electric field of 10 kHz and 2.0 kV / mm is applied for 65 min, specifically including the following steps: (1) Bisphenol A type epoxy resin E51 was mixed with 4-methylhexahydrophthalic anhydride and DMP-30 in a mass ratio of 100:80:1 to obtain an epoxy resin curing precursor. Flaky hexagonal boron nitride with an average particle size of 15 μm and nano-sized spherical aluminum nitride modified with silane coupling agent were added as thermally conductive fillers. The mass fraction of the thermally conductive filler in the mixed system was 25%, of which the mass fraction of hexagonal boron nitride was 20% and the mass fraction of aluminum nitride was 5%, and the mass ratio of hexagonal boron nitride to aluminum nitride was 4:1. The obtained mixed system was subjected to high shear dispersion under a water bath at 60℃, with a dispersion speed of 2000 rpm and a dispersion time of 45 min. Then, vacuum degassing was performed under a gauge pressure ≤ -0.09 MPa until no obvious bubbles overflowed.

[0068] (2) The degassed mixture was poured into a PTFE insulating mold equipped with parallel copper electrodes. The parallel electrodes were connected to a bipolar square wave high-voltage power supply, and the output waveform was monitored by a high-voltage probe and an oscilloscope. The electric field strength was calculated as the ratio of the peak output voltage to the electrode spacing. The square wave duty cycle was 50%, and the electrode spacing was 0.5-2.0 mm. The mold was placed in a curing environment at 80°C, and an electric field was applied in the front and back windows of the resin gel. The electric field was a low-frequency, high-field bipolar square wave electric field with a frequency of 10 kHz and an electric field strength of 2.0 kV / mm, lasting for 65 min.

[0069] (3) After the electric field is applied, the epoxy resin composite material is obtained by step curing according to the procedure of keeping it at 80℃ for 1 h, 105℃ for 2 h, and 120℃ for 4 h. It is denoted as EP / 25%BN / AlN-10kHz.

[0070] Comparative Example 4 This comparative example relates to an epoxy resin composite material, the preparation method of which differs from that of Example 1 in that the thermally conductive filler is 25% lamellar hexagonal boron nitride with an average particle size of 15 μm; specifically, it includes the following steps: (1) Bisphenol A type epoxy resin E51, 4-methylhexahydrophthalic anhydride, and DMP-30 were mixed in a mass ratio of 100:80:1 to obtain an epoxy resin curing precursor. Flaky hexagonal boron nitride with an average particle size of 15 μm was added as a thermally conductive filler, and the mass fraction of the thermally conductive filler in the mixture was 25%. The obtained mixture was subjected to high shear dispersion in a water bath at 60℃, with a dispersion speed of 2000 rpm and a dispersion time of 45 min. Subsequently, vacuum degassing was performed under a gauge pressure ≤ -0.09 MPa until no obvious bubbles overflowed.

[0071] (2) The degassed mixture was poured into a PTFE insulating mold with parallel copper electrodes. The parallel electrodes were connected to a bipolar square wave high-voltage power supply, and the output waveform was monitored by a high-voltage probe and an oscilloscope. The electric field strength was calculated as the ratio of the peak output voltage to the electrode spacing. The square wave duty cycle was 50%, and the electrode spacing was 0.5-2.0 mm. The mold was placed in a curing environment at 80°C, and two stages of electric fields were applied in the front and back windows of the resin gel. In the early stage of curing, a first-stage high-frequency low-field bipolar square wave electric field with a frequency of 50 kHz and an electric field strength of 0.5 kV / mm was applied for 20 min. Then, the second-stage low-frequency high-field bipolar square wave electric field with a frequency of 10 kHz and an electric field strength of 2.0 kV / mm was applied for 45 min.

[0072] (3) After the electric field is applied, the epoxy resin composite material is cured in stages according to the procedure of 80℃ for 1 h, 105℃ for 2 h, and 120℃ for 4 h to obtain thermally conductive and insulating epoxy resin composite material, which is denoted as EP / 25%BN-segmented.

[0073] Comparative Example 5 This comparative example relates to an epoxy resin whose preparation method differs from that of Example 1 in that it does not contain thermally conductive fillers and does not apply an electric field; the product is denoted as EP.

[0074] For the liquid epoxy resin mixture of Example 1, the changes in relative permittivity and complex viscosity with applied frequency at 25°C and 80°C are shown in Appendix. Figure 3 According to the appendix Figure 3Test results show that during the initial curing stage at 80℃, the mixed system exhibits a lower equivalent complex viscosity and a more stable dielectric response near 50kHz, which is beneficial for weakening the agglomeration stability between nano-AlN particles. When the mixed system enters the orientation rearrangement stage, switching to 10kHz can increase the polarization torque of the sheet-like h-BN under a low-frequency strong field, making the sheets more easily oriented along the electric field direction. Therefore, [further details omitted]. Figure 3 The basis for the segmented process of "50 kHz deagglomeration and 10 kHz orientation" in Example 1 was explained from the perspectives of dielectric response and rheological response.

[0075] The thermal conductivity of the composite materials obtained in Example 1 and Comparative Examples 1-5 is shown in the appendix. Figure 4 As attached Figure 4 As shown, with the same total filler content of 25%, the thermal conductivity of the sample in Example 1 was 1.332 W / (m·K), significantly higher than that of the randomly dispersed hybrid sample (0.640 W / (m·K)) and the single h-BN single-frequency electric field oriented sample (1.085 W / (m·K). Compared to the randomly dispersed hybrid sample, the thermal conductivity of the sample in Example 1 increased by 108.1%; compared to the single h-BN single-frequency electric field oriented sample, the thermal conductivity increased by 22.8%. These results indicate that random composite of fillers alone cannot form a continuous thermally conductive network. Although single h-BN orientation can improve the connectivity of the layers, resin barriers still exist between the layers. In contrast, Example 1 further reduced the interfacial thermal resistance through the bridging effect of nano-AlN between the oriented h-BN layers.

[0076] The thermal conductivity growth rate of insulating and thermally conductive epoxy resin composites prepared under first / second-stage bipolar square wave electric field conditions with different frequencies and electric field intensities is shown in the appendix. Figure 5 The thermal conductivity growth rate curves for Examples 1-24 are attached. Figure 5 As shown in the left figure, with the second stage fixed at 10 kHz, 2.0 kV / mm, and 45 min, the thermal conductivity growth rate gradually increased as the first stage frequency increased from 20 kHz to 50 kHz. When the frequency continued to increase to 60-70 kHz, the thermal conductivity growth rate decreased, indicating that the filler polarization response and pre-dispersion effect weakened at excessively high frequencies. When the electric field strength in the first stage was 0.5 kV / mm, the thermal conductivity growth rate was higher than that under the conditions of 0.3 kV / mm and 0.7 kV / mm, indicating that 0.5 kV / mm is more suitable as the field strength for the high-frequency deagglomeration stage. The thermal conductivity growth rate curves for Examples 1 and 25-43 are attached. Figure 5As shown in the right figure, with the first stage fixed at 50 kHz, 0.5 kV / mm, and 20 min, the thermal conductivity growth rate gradually increases as the electric field strength in the second stage increases from 1.0 kV / mm to 2.0 kV / mm. When the electric field strength continues to increase to 2.5-3.0 kV / mm, the thermal conductivity growth rate decreases slightly, indicating that excessively high field strength may lead to local enrichment of filler, increased bubble defects, or increased risk of electric field distortion. Therefore, the combination of 50 kHz / 0.5 kV / mm and 10 kHz / 2.0 kV / mm achieves a better balance between enhanced thermal conductivity and electrical insulation stability.

[0077] The breakdown strength of the composite materials obtained in Example 1 and Comparative Examples 1-5 is shown in the appendix. Figure 6 As attached Figure 6 As shown, under 50Hz AC breakdown conditions, the breakdown data of each group of samples conform to the Weibull probability distribution law. The Weibull characteristic breakdown strength of the pure epoxy sample EP is 184.60 kV / mm, the Weibull characteristic breakdown strength of the randomly dispersed hybrid sample EP / 25%BN / AlN-random is 149.20 kV / mm, the Weibull characteristic breakdown strength of the single h-BN single-frequency electric field oriented sample EP / 25%BN-10kHz is 156.05 kV / mm, the Weibull characteristic breakdown strength of the hybrid filler single-frequency electric field oriented sample EP / 25%BN / AlN-10kHz is 163.80 kV / mm, the Weibull characteristic breakdown strength of the single h-BN segmented frequency electric field oriented sample EP / 25%BN-segmented is 162.35 kV / mm, and the Weibull characteristic breakdown strength of the sample EP / 25%BN / AlN-segmented in Example 1 is 171.30 kV / mm.

[0078] From the appendix Figure 6 It can be seen that the pure epoxy sample has a high breakdown strength due to the absence of filler interface defects; after the addition of filler, the breakdown strength of the randomly dispersed hybrid sample decreases due to filler agglomeration, interface defects, and local electric field distortion. Compared with the randomly dispersed hybrid sample and the single electric field oriented sample, the Weibull probability curve of the sample in Example 1 shifts overall towards the direction of higher breakdown strength, indicating that the orientation skeleton and bridging structure induced by the segmented frequency conversion electric field are beneficial to reducing the dispersion of local defects and improving the power frequency breakdown stability of the composite material.

[0079] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A thermally conductive and insulating epoxy resin composite material, comprising an epoxy resin matrix, characterized in that, It also includes thermally conductive fillers distributed in the epoxy resin matrix, the thermally conductive fillers including sheet fillers and spherical fillers; The sheet filler is at least partially oriented in the epoxy resin matrix and at least partially forms a sheet overlap or stacked structure; the spherical filler is at least partially distributed in the gaps between adjacent sheet fillers, the edge region of the sheet filler, and / or the interface region between the sheet filler and the epoxy resin matrix.

2. The thermally conductive and insulating epoxy resin composite material according to claim 1, characterized in that, The particle size of the sheet-like filler is in the micrometer range; and / or, the particle size of the spherical filler is in the nanometer range.

3. The thermally conductive and insulating epoxy resin composite material according to claim 1, characterized in that, The mass ratio of the sheet-like filler to the spherical filler is (3-6):1; and / or, the mass ratio of the thermally conductive filler in the thermally conductive and insulating epoxy resin composite material is 1%-30%.

4. The thermally conductive and insulating epoxy resin composite material according to claim 1, characterized in that, The sheet-like filler includes at least one of hexagonal boron nitride, aluminum oxide, aluminum nitride, silicon nitride, mica, and silicates; and / or, the spherical filler includes at least one of aluminum nitride, aluminum oxide, magnesium oxide, silicon nitride, silicon dioxide, and boron nitride; and / or, the surface of the spherical filler is modified with a silane coupling agent.

5. A method for preparing a thermally conductive and insulating epoxy resin composite material according to any one of claims 1-4, characterized in that, Includes the following steps: (1) A mixture is obtained by mixing epoxy resin curing precursor with thermally conductive filler; (2) The mixture is cured, and during the curing process, a first bipolar square wave electric field and a second bipolar square wave electric field are sequentially applied to the mixture; (3) The mixture after electric field treatment is further cured to obtain the thermally conductive and insulating epoxy resin composite material; The frequency of the first bipolar square wave electric field is greater than the frequency of the second bipolar square wave electric field, and the electric field strength of the first bipolar square wave electric field is less than the electric field strength of the second bipolar square wave electric field.

6. The preparation method according to claim 5, characterized in that, The frequency of the first bipolar square wave electric field is 20-70kHz; and / or the frequency of the second bipolar square wave electric field is 5-20kHz; and / or the electric field strength of the first bipolar square wave electric field is 0.3-0.9V / mm; and / or the electric field strength of the second bipolar square wave electric field is 1.0-3.0kV / mm.

7. The preparation method according to claim 5, characterized in that, The processing time for the first bipolar square wave electric field is 15-30 min; and / or, the processing time for the second bipolar square wave electric field is 30-60 min.

8. The preparation method according to claim 5, characterized in that, The duty cycle of the first bipolar square wave electric field and / or the second bipolar square wave electric field is 30-70%.

9. The preparation method according to claim 5, characterized in that, The curing conditions described in step (3) are: heat at 78-82℃ for 0.5-1.5h, then heat to 100-110℃ and heat for 1.5-2.5h, then heat to 110-125℃ and heat for 3.5-4.5h.

10. The application of a thermally conductive and insulating epoxy resin composite material according to any one of claims 1-4 in power electronic packaging, thermally conductive and insulating potting, motor winding insulation, casting insulation parts or electrical insulation components.

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