Epoxy resin castable for solid-state transformer and preparation method of epoxy resin castable

Abstract

The invention provides a method for preparing an epoxy resin castable for a solid-state transformer. The method comprises the following steps: providing 100 parts of epoxy resin, 80-120 parts of an anhydride curing agent, 150-300 parts of a functionalized heat-conducting filler, 0.1-0.5 part of a click reaction catalyst and 0.5-2 parts of a curing accelerator; the preparation method comprises the following steps: carrying out vacuum defoamation on a functionalized heat-conducting filler, a click reaction catalyst and epoxy resin at 50-60 DEG C, and then heating to 70-80 DEG C, so that a click reaction is carried out on sulfydryl and alkenyl to obtain a premix; and adding an anhydride curing agent and a curing accelerator, mixing, defoaming, pouring and curing. According to the method, a three-dimensional rigid network in isotropic distribution is formed through a step-by-step design of firstly constructing a covalent skeleton between fillers and then anchoring the covalent skeleton on resin, so that the thermal expansion behavior of a resin matrix is restrained, the thermal expansion coefficient is reduced, the anisotropy is reduced, and the castable has good thermal cycle stability.

Description

Technical Field

[0001] This application relates to the field of polymer composite materials technology, specifically to an epoxy resin casting material for solid-state transformers and its preparation method. Background Technology

[0002] Epoxy resin castings are key materials for the insulation encapsulation of solid-state transformers due to their excellent insulation, adhesion, and processability. As device power density continues to increase, heat dissipation has become a bottleneck restricting their reliability. A common method to improve thermal conductivity is to fill with a high proportion of thermally conductive fillers, but this usually disrupts the continuity of the resin matrix, leading to problems such as a sharp increase in viscosity, molding difficulties, high internal stress, and a significant decrease in mechanical strength.

[0003] However, for devices like solid-state transformers that undergo long-term temperature cycling (-40℃ to 150℃), the matching and stability of the coefficient of thermal expansion (CTE) are more critical reliability indicators than simply pursuing high thermal conductivity. The CTE of cured epoxy resin is typically as high as 50-80 ppm / ℃, while the CTE of copper conductors is only about 17 ppm / ℃, a significant difference. Under frequent temperature fluctuations, this CTE mismatch can generate enormous shear stress at the interface, leading to debonding, microcrack initiation, and even failure.

[0004] Therefore, how to effectively reduce the coefficient of thermal expansion of epoxy castables and optimize their isotropy while improving thermal conductivity has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This application provides an epoxy resin casting material for solid-state transformers and its preparation method. The epoxy resin casting material uses epoxy resin as the main material and is optimized with functionalized thermally conductive fillers, which helps to constrain the thermal expansion behavior of the resin matrix, thereby reducing the coefficient of thermal expansion and anisotropy, and giving the casting material good thermal cycling stability.

[0006] In a first aspect, this application provides a method for preparing an epoxy resin casting material for solid-state transformers, comprising the following steps: M1: providing 100 parts of epoxy resin, 80-120 parts of anhydride curing agent, 150-300 parts of functionalized thermally conductive filler, 0.1-0.5 parts of click reaction catalyst, and 0.5-2 parts of curing accelerator as raw materials, wherein the functionalized thermally conductive filler includes mercapto-modified one-dimensional thermally conductive filler, alkenyl epoxy-co-modified two-dimensional thermally conductive filler, and epoxy-modified zero-dimensional thermally conductive filler; M2: mixing the functionalized thermally conductive filler, click reaction catalyst, and curing accelerator... The curing agent and epoxy resin are vacuum stirred and degassed at 50~60℃ for 10~30 min, and then stirred and heated to 70~80℃ for 10~20 min to allow the thiol groups of the thiol-modified one-dimensional thermally conductive filler to undergo a click chemical reaction with the alkenyl groups on the surface of the alkenyl epoxy co-modified two-dimensional thermally conductive filler to obtain a premix; M3: An anhydride curing agent and curing accelerator are added to the premix and stirred and mixed under vacuum to remove air bubbles to obtain a mixture; M4: The mixture is poured into a preheated solid transformer winding mold and cured to obtain an epoxy resin casting material.

[0007] According to this application, the castable uses epoxy resin as the continuous phase and constructs a three-dimensional network within the composite material by introducing multi-scale thermally conductive fillers with specific reactive functional groups on their surfaces. This method, through a step-by-step construction mechanism of "connecting the filler first, then fusing it with the matrix," helps form a rigid filler skeleton linked by covalent bonds. This skeleton can, to some extent, constrain the free expansion of the resin matrix under temperature changes, thereby reducing the coefficient of thermal expansion of the composite material. Simultaneously, due to the uniform distribution of the filler skeleton, the thermal expansion behavior of the composite material tends to be consistent in the X, Y, and Z directions, helping to reduce internal stress concentration caused by anisotropic expansion, thus improving the thermal cycling stability of the castable.

[0008] Specifically, in the initial curing stage, the click reaction catalyst enables the mercapto groups on the surface of the one-dimensional filler and the alkenyl groups on the surface of the two-dimensional filler to preferentially undergo click chemical reactions during the preheating stage at 70-80℃, establishing covalent bonds between filler particles and initially forming a filler skeleton network bonded by chemical bonds. This skeleton network formation is mainly completed during the preheating stage, helping to reduce uneven filler dispersion or network structure defects that may result from resin gelation. Subsequently, during the temperature-curing stage, the epoxy groups on the surfaces of the two-dimensional and zero-dimensional fillers undergo copolymerization reactions with epoxy resin and anhydride curing agents, chemically anchoring the pre-constructed filler skeleton into the three-dimensional cross-linked network of the resin matrix. The resulting rigid filler skeleton penetrates the resin matrix, providing a certain degree of constraint on the free expansion behavior of the resin under temperature changes in three-dimensional space, thereby reducing the overall thermal expansion coefficient of the composite material. Simultaneously, based on the uniformity and isotropic distribution of the filler skeleton, the thermal expansion behavior of the composite material tends to be consistent in the X, Y, and Z directions, reducing the internal stress generated during temperature cycling and, to a certain extent, inhibiting interfacial debonding and the initiation and propagation of microcracks. Therefore, by optimizing the coefficient of thermal expansion and its isotropy, the structural stability and long-term reliability of a material under temperature cycling can be indirectly reflected.

[0009] In some embodiments, the thiol-modified one-dimensional thermally conductive filler is prepared by the following method: 5-15 parts of γ-mercaptopropyltrimethoxysilane are dispersed in 100-150 parts of an aqueous ethanol solution and pre-hydrolyzed at 40-60°C for 20-40 min; then 100 parts of one-dimensional thermally conductive filler are added and reacted at 70-90°C for 4-8 h to obtain the thiol-modified one-dimensional thermally conductive filler.

[0010] In some of the above embodiments, the surface of the one-dimensional thermally conductive filler is modified with γ-mercaptopropyltrimethoxysilane, providing reaction sites for the one-dimensional filler to participate in the subsequent "stepwise construction" process. During the preheating stage, the thiol group can undergo a click chemical reaction with the alkenyl group on the surface of the two-dimensional filler, preferentially forming a one-dimensional-two-dimensional covalently linked framework. This lays the structural foundation for the construction of the entire three-dimensional network, enabling the one-dimensional filler to establish a stable chemical connection with the two-dimensional filler before resin curing. This facilitates the formation of a uniformly distributed three-dimensional rigid network during subsequent curing, thereby effectively constraining the expansion behavior of the resin matrix under temperature changes. Therefore, the thiol-modified one-dimensional thermally conductive filler prepared by this method provides a basis for optimizing the coefficient of thermal expansion and thermal cycling stability of castables.

[0011] In some embodiments, the one-dimensional thermally conductive filler is a needle-like or wire-like silicon nitride whisker or silicon carbide whisker.

[0012] In some of the above embodiments, silicon nitride or silicon carbide whiskers are selected as one-dimensional thermally conductive fillers. Their high aspect ratio allows for the construction of a continuous framework, and γ-mercaptopropyltrimethoxysilane is grafted onto their surface hydroxyl groups to introduce thiol functional groups. During the preheating stage, these thiol groups can undergo click chemistry reactions with the alkenyl groups on the surface of the two-dimensional filler to form a long-range covalent framework that penetrates the matrix, laying the foundation for the main structure of the three-dimensional network.

[0013] In some embodiments, the alkenyl epoxy co-modified two-dimensional thermally conductive filler is prepared by the following method: 3-8 parts of vinyltriethoxysilane and 3-8 parts of γ-glycidyl etheroxypropyltrimethoxysilane are dispersed in 100-150 parts of an aqueous ethanol solution, the pH is adjusted to 4-5, and pre-hydrolyzed at 40-60°C for 20-40 min; then 100 parts of the two-dimensional thermally conductive filler are added, and the mixture is reacted at 70-90°C for 4-8 h to obtain the alkenyl epoxy co-modified two-dimensional thermally conductive filler.

[0014] In some of the above embodiments, vinyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane are used to co-modify the two-dimensional thermally conductive filler, which can simultaneously introduce both alkenyl and epoxy reactive functional groups onto the same filler surface. The alkenyl groups provided by vinyltriethoxysilane can undergo a click chemical reaction with the mercapto groups on the surface of the one-dimensional filler during the preheating stage, allowing the two-dimensional filler to act as a "bridging unit" connecting adjacent one-dimensional filler trunks, forming a stable filler skeleton network. The epoxy groups provided by γ-glycidoxypropyltrimethoxysilane undergo a copolymerization reaction with the epoxy resin matrix during the subsequent curing stage, chemically anchoring the formed filler skeleton into the resin crosslinking network. This allows the two-dimensional filler to participate in the construction of the inter-filler covalent network and to deeply integrate with the resin matrix, contributing to the formation of an isotropically distributed three-dimensional rigid skeleton, thereby uniformly constraining the expansion behavior of the resin matrix under temperature changes. Therefore, the alkenyl-epoxy co-modified two-dimensional thermally conductive filler prepared by this method provides a structural basis for optimizing the thermal expansion coefficient and thermal cycling stability of castables.

[0015] In some embodiments, the two-dimensional thermally conductive filler is a sheet-like boron nitride nanosheet.

[0016] In some of the above embodiments, boron nitride nanosheets are selected as two-dimensional thermally conductive fillers. Their sheet-like structure can connect adjacent one-dimensional filler backbones after the click reaction, expanding the linear skeleton into a network structure. At the same time, their own low thermal expansion characteristics help to enhance the overall network's constraint effect on resin expansion.

[0017] In some embodiments, the epoxy-modified zero-dimensional thermally conductive filler is prepared by the following method: 5-15 parts of γ-glycidyl etheroxypropyltrimethoxysilane are dispersed in 100-150 parts of an aqueous ethanol solution, the pH is adjusted to 4-5, and pre-hydrolyzed at 40-60°C for 20-40 min; then 100 parts of zero-dimensional thermally conductive filler are added, and the mixture is reacted at 70-90°C for 4-8 h to obtain the epoxy-modified zero-dimensional thermally conductive filler.

[0018] In some of the above embodiments, the surface of the zero-dimensional thermally conductive filler is modified with γ-glycidyl etheroxypropyltrimethoxysilane, enabling the zero-dimensional filler to participate in the copolymerization reaction of epoxy resin and anhydride curing agent during the subsequent curing stage. Since the epoxy group remains inert during the preheating stage, it does not interfere with the click chemistry reaction between the one-dimensional and two-dimensional fillers, thus allowing the stepwise construction mechanism of "first building the filler skeleton, then anchoring it to the resin" to be implemented. During the high-temperature curing stage of the resin, the epoxy groups on the surface of the zero-dimensional filler covalently bond with the resin matrix, anchoring the filler in the network voids formed by the one-dimensional and two-dimensional skeleton, thereby effectively filling and locking the entire three-dimensional network. Therefore, the epoxy-modified zero-dimensional thermally conductive filler prepared by this method provides a structural basis for optimizing the coefficient of thermal expansion and thermal cycling stability of the castable.

[0019] In some embodiments, the zero-dimensional thermally conductive filler is spherical alumina.

[0020] In some of the above embodiments, spherical alumina is selected as a zero-dimensional thermally conductive filler. Its spherical morphology can fill the network voids formed by the one-dimensional and two-dimensional skeleton. During the curing stage, the zero-dimensional filler is anchored in the voids by copolymerizing with the resin matrix through surface-grafted epoxy groups. This synergistic structure of "trunk-bridging-filling" helps to form an isotropically distributed three-dimensional network, thereby uniformly constraining the thermal expansion behavior of the resin matrix.

[0021] In some embodiments, the mass ratio of the mercapto-modified one-dimensional thermally conductive filler, the alkenyl epoxy-modified two-dimensional thermally conductive filler, and the epoxy-modified zero-dimensional thermally conductive filler is 1:(0.7~0.9):(0.4~0.6).

[0022] In some of the above embodiments, epoxy resin castings prepared using one-dimensional, two-dimensional, and zero-dimensional thermally conductive fillers with a mass ratio of 1:(0.7~0.9):(0.4~0.6) can effectively optimize thermal expansion behavior. This may be because the one-dimensional thermally conductive filler serves as the main load-bearing skeleton for achieving high thermal conductivity in the longitudinal direction and high strength. This mass ratio allows a sufficient number of two-dimensional sheet-like fillers to effectively span the average spacing between adjacent one-dimensional fillers, forming stable two-dimensional thermally conductive "bridges" through click chemical bonding. This connects isolated one-dimensional skeletons into an integrated three-dimensional mesh. Furthermore, the zero-dimensional thermally conductive filler can effectively fill the pores in the three-dimensional network skeleton formed by the interweaving of one-dimensional and two-dimensional fillers, allowing the fillers to be isotropically distributed in the matrix. This uniformly constrains the thermal expansion behavior of the resin matrix in three-dimensional space, helping to reduce the overall thermal expansion coefficient of the composite material and making the thermal expansion in the X, Y, and Z directions more consistent. Therefore, epoxy resin castings prepared using this filler ratio have an optimized thermal expansion coefficient.

[0023] In some embodiments, the anhydride curing agent is methyltetrahydrophthalic anhydride; the click reaction catalyst is triphenylphosphine; and the curing accelerator is an imidazole accelerator.

[0024] In some of the above embodiments, methyltetrahydrophthalic anhydride is selected as the anhydride curing agent. It not only reacts fully with the epoxy resin matrix to form a cross-linked network, but also copolymerizes with the epoxy groups modified on the surface of the thermally conductive filler, chemically anchoring the filler network into the resin matrix. During the filler skeleton construction stage, triphenylphosphine is used as a click reaction catalyst to promote covalent bonding between one-dimensional and two-dimensional fillers. Subsequently, in the high-temperature curing stage, imidazole accelerators effectively accelerate the main curing reaction of epoxy-anhydride, allowing the resin matrix and the epoxy groups pre-reserved on the filler surface to cure simultaneously, fusing the pre-fabricated filler network with the resin cross-linker. The synergistic effect of the click reaction catalyst, curing agent, and curing accelerator helps the filler network effectively constrain the expansion behavior of the resin matrix, thereby optimizing the coefficient of thermal expansion of the castable.

[0025] In a second aspect, this application provides an epoxy resin casting material for solid-state transformers, prepared according to the method described in any one of the first aspects.

[0026] According to this application, the epoxy resin casting material prepared according to any one of the methods of the first aspect has the beneficial effects of the first aspect, and the obtained epoxy resin casting material has an optimized coefficient of thermal expansion.

[0027] Compared with the prior art, the beneficial effects of this application are at least as follows: By using epoxy resin as the main material of the casting material and performing surface functionalization treatment on multi-scale thermally conductive fillers, specific reactive functional groups are acquired on the surface of fillers of different dimensions. Based on this, this application designs a step-by-step construction process: the thiol groups on the surface of one-dimensional fillers and the alkenyl groups on the surface of two-dimensional fillers undergo a click chemical reaction during the preheating stage, initially forming a covalent network between fillers; then, a curing agent is added for high-temperature curing, causing the epoxy groups on the surfaces of two-dimensional and zero-dimensional fillers to copolymerize with the resin matrix, anchoring the filler network in the cross-linked structure. This process design, which first constructs the filler skeleton and then anchors it to the resin, is beneficial for forming a three-dimensional rigid skeleton that is isotropically distributed and connected by covalent bonds. This skeleton can uniformly constrain the free expansion behavior of the resin matrix under temperature changes in three-dimensional space, helping to reduce the coefficient of thermal expansion of the composite material and making the thermal expansion in the X, Y, and Z directions more consistent. This optimization of thermal expansion behavior reduces, to some extent, the internal stress concentration and interface failure caused by anisotropic expansion, thereby improving the thermal cycling stability of the casting material and meeting the long-term reliability requirements of solid-state transformers. Detailed Implementation

[0028] The various embodiments or implementation schemes in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments.

[0029] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with an embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0030] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0031] In this specification, unless otherwise specified, "parts" refers to "parts by weight".

[0032] The following describes embodiments of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0033] Epoxy resin: Bisphenol A type epoxy resin, model E-51, epoxy value 0.51; One-dimensional thermally conductive filler: silicon nitride whiskers, with a diameter of 1~3μm, a length of 5~20μm, and an aspect ratio of 5~10; Two-dimensional thermally conductive filler: boron nitride nanosheets, with a diameter of 0.5~10μm and a thickness of ≤100nm; Zero-dimensional thermally conductive filler: spherical alumina, D50 particle size 6.0±0.5μm; Silane coupling agents: γ-mercaptopropyltrimethoxysilane (KH-590), γ-glycidoxypropyltrimethoxysilane (KH-560), γ-aminopropyltriethoxysilane (KH-550), and vinyltriethoxysilane, all with a purity >97%.

[0034] Preparation Example 1 Preparation of mercapto-modified silicon nitride whiskers 100 parts of silicon nitride whiskers were added to the reactor; 12 parts of KH-590 were dispersed in 120 parts of 95% ethanol aqueous solution and stirred at 50℃ for 30 min to hydrolyze the solution; the hydrolyzed silane solution was added to the packing material and stirred at 80℃ for 6 h; after the reaction was completed, the mixture was filtered, the filter residue was washed three times with anhydrous ethanol and dried to obtain mercapto-modified silicon nitride whiskers.

[0035] Preparation Example 2 Preparation of alkenyl epoxy-modified boron nitride nanosheets 100 parts of boron nitride nanosheets were added to the reactor; 6 parts of vinyltriethoxysilane and 6 parts of KH-560 were dispersed in 120 parts of 95% ethanol aqueous solution and stirred at 50℃ for 30 min to hydrolyze the mixture, and acetic acid was added to adjust the pH to 4; the hydrolyzed mixed silane solution was added to the packing material and stirred at 80℃ for 6 h; after the reaction was completed, the mixture was filtered, the filter residue was washed three times with anhydrous ethanol, and dried to obtain alkenyl epoxy-modified boron nitride nanosheets.

[0036] Preparation Example 3 Preparation of alkenylsilane-modified boron nitride nanosheets 100 parts of boron nitride nanosheets were added to the reactor; 12 parts of vinyltriethoxysilane were dispersed in 120 parts of 95% ethanol aqueous solution and stirred at 50℃ for 30 min to hydrolyze it. The hydrolyzed silane solution was added to the packing material and stirred at 80℃ for 6 h. After the reaction was completed, the mixture was filtered, and the filter residue was washed three times with anhydrous ethanol and dried to obtain alkenylsilane modified boron nitride nanosheets.

[0037] Preparation Example 4 Preparation of epoxy-modified spherical alumina 100 parts of spherical alumina were added to the reactor; 12 parts of KH-560 were dispersed in 120 parts of 95% ethanol aqueous solution and stirred at 50℃ for 30 min to hydrolyze the alumina; acetic acid was added to adjust the pH to 4; the hydrolyzed silane solution was added to the packing material and stirred at 80℃ for 6 h; after the reaction was completed, the mixture was filtered, the filter residue was washed three times with anhydrous ethanol and dried to obtain epoxy-modified spherical alumina.

[0038] Comparative Preparation Example 1 Preparation of KH-550 thermally conductive filler 100 parts of silicon nitride whiskers were added to the reactor; 12 parts of KH-550 were dispersed in 120 parts of 95% ethanol aqueous solution and stirred at 50℃ for 30 min to hydrolyze it; the hydrolyzed silane solution was added to the packing material and stirred and mixed at 80℃ for 6 h; after the reaction was completed, the mixture was filtered, the filter residue was washed three times with anhydrous ethanol and dried to obtain KH-550 treated silicon nitride whiskers.

[0039] 100 parts of boron nitride nanosheets were added to the reactor; 12 parts of KH-550 were dispersed in 120 parts of 95% ethanol aqueous solution and stirred at 50℃ for 30 min to hydrolyze it; the hydrolyzed silane solution was added to the packing material and stirred at 80℃ for 6 h; after the reaction was completed, the mixture was filtered, the filter residue was washed three times with anhydrous ethanol and dried to obtain KH-550 treated boron nitride nanosheets.

[0040] 100 parts of spherical alumina were added to the reactor; 12 parts of KH-550 were dispersed in 120 parts of 95% ethanol aqueous solution and stirred at 50℃ for 30 min to hydrolyze it; the hydrolyzed silane solution was added to the packing material and stirred and mixed at 80℃ for 6 h; after the reaction was completed, the mixture was filtered, the filter residue was washed three times with anhydrous ethanol and dried to obtain KH-550 treated spherical alumina.

[0041] Example 1 100 parts of E-51, 86.7 parts of the mercapto-modified silicon nitride whiskers obtained in Preparation Example 1, 69.3 parts of the alkenyl epoxy-modified boron nitride nanosheets obtained in Preparation Example 2, 44.0 parts of the epoxy-modified spherical alumina obtained in Preparation Example 4 (total 200 parts, ratio 1:0.8:0.5) and 0.3 parts of triphenylphosphine were vacuum stirred at 50°C for 30 min to remove bubbles, and then the temperature was raised to 75°C and stirring was continued for 20 min. Add 90 parts of methyltetrahydrophthalic anhydride and 1 part of 2-ethyl-4-methylimidazole, degas under vacuum, pour into a mold preheated to 80°C, preheat at 80°C for 2 hours, cure at 110°C for 3 hours, and then cure at 150°C for 5 hours to obtain epoxy resin casting material.

[0042] Example 2 The preparation method is largely the same as Example 1, except that the mass fractions of the three thermally conductive fillers are different: 83.3 parts of the mercapto-modified silicon nitride whiskers obtained in Example 1, 50.0 parts of the alkenyl epoxy-modified boron nitride nanosheets obtained in Example 2, and 66.7 parts of the epoxy-modified spherical alumina obtained in Example 4 (total 200 parts, ratio 1:0.6:0.8). Example 3 The preparation method is largely the same as Example 1, except that the alkenyl epoxy-modified boron nitride nanosheets obtained in Preparation Example 2 are replaced with the alkenyl silane-modified boron nitride nanosheets obtained in Preparation Example 3.

[0043] Example 4 Similar to Example 1, except that the total mass fractions of the three thermally conductive fillers are different: 78.3 parts of the mercapto-modified silicon nitride whiskers obtained in Example 1, 62.6 parts of the alkenyl epoxy-modified boron nitride nanosheets obtained in Example 2, and 39.1 parts of the epoxy-modified spherical alumina obtained in Example 4 (total 180 parts, ratio 1:0.8:0.5). Comparative Example 1 100 parts of E-51, 86.7 parts of untreated silicon nitride whiskers, 69.3 parts of untreated boron nitride nanosheets, 44.0 parts of untreated spherical alumina (total 200 parts, ratio 1:0.8:0.5) and 0.3 parts of triphenylphosphine were vacuum stirred at 50°C for 30 min to remove bubbles, and then the temperature was raised to 75°C and stirring was continued for 20 min. Add 90 parts of methyltetrahydrophthalic anhydride and 1 part of 2-ethyl-4-methylimidazole, degas under vacuum, pour into a mold preheated to 80°C, preheat at 80°C for 2 hours, cure at 110°C for 3 hours, and then cure at 150°C for 5 hours to obtain epoxy resin casting material.

[0044] Comparative Example 2 100 parts of E-51 and 86.7 parts of KH-550-treated silicon nitride whiskers obtained in Comparative Preparation Example 1, 69.3 parts of KH-550-treated boron nitride nanosheets obtained in Comparative Preparation Example 1, 44.0 parts of KH-550-treated spherical alumina obtained in Comparative Preparation Example 1 (total 200 parts, ratio 1:0.8:0.5), 90 parts of methyltetrahydrophthalic anhydride, and 1 part of 2-ethyl-4-methylimidazolium were mixed at one time, vacuum stirred to remove bubbles, and then poured into a mold preheated to 80°C. The mixture was preheated at 80°C for 2 hours, cured at 110°C for 3 hours, and post-cured at 150°C for 5 hours to obtain epoxy resin casting material.

[0045] Comparative Example 3 100 parts of E-51, 86.7 parts of mercapto-modified silicon nitride whiskers obtained in Preparation Example 1, 113.3 parts of epoxy-modified spherical alumina obtained in Preparation Example 4 (total 200 parts, ratio 1:0:1.3) and 0.3 parts of triphenylphosphine were vacuum stirred at 50°C for 30 min to remove bubbles, and then the temperature was raised to 75°C and stirring was continued for 20 min. Add 90 parts of methyltetrahydrophthalic anhydride and 1 part of 2-ethyl-4-methylimidazole, degas under vacuum, pour into a mold preheated to 80°C, preheat at 80°C for 2 hours, cure at 110°C for 3 hours, and then cure at 150°C for 5 hours to obtain epoxy resin casting material.

[0046] Comparative Example 4 100 parts of E-51, 86.7 parts of the mercapto-modified silicon nitride whiskers obtained in Preparation Example 1, 69.3 parts of the alkenyl epoxy-modified boron nitride nanosheets obtained in Preparation Example 2, and 44.0 parts of the epoxy-modified spherical alumina obtained in Preparation Example 4 (total 200 parts, ratio 1:0.8:0.5) were mixed with 0.3 parts of triphenylphosphine and vacuum stirred at 50°C for 30 min to remove bubbles, and then stirred at 50°C for another 20 min (without the heating to 75°C holding step). Add 90 parts of methyltetrahydrophthalic anhydride and 1 part of 2-ethyl-4-methylimidazole, degas under vacuum, pour into a mold preheated to 80°C, preheat at 80°C for 2 hours, cure at 110°C for 3 hours, and then cure at 150°C for 5 hours to obtain epoxy resin casting material.

[0047] Test section Coefficient of thermal expansion (CTE): Tested according to standard ASTM E831-19. A thermomechanical analyzer (TMA) was used to prepare 5 mm × 5 mm × 5 mm cubic specimens of the epoxy resin castings prepared in each example and comparative example. Tests were conducted in the X, Y, and Z directions. The test temperature range was -40℃ to 150℃, with a heating rate of 5℃ / min. The coefficient of thermal expansion in each direction was recorded, and the difference between the maximum and minimum CTE values ​​(ΔCTE) in the three directions was calculated. The average value of three specimens in each test group was taken.

[0048] Table 1

[0049] According to the test results in Table 1, the epoxy resin casting material prepared in this application has a better coefficient of thermal expansion and anisotropy than the comparative examples, indicating that the epoxy resin casting material provided in this application has a better effect on optimizing thermal expansion behavior. The possible reasons are: in Comparative Example 1, none of the fillers underwent surface functionalization treatment, resulting in extremely poor interfacial bonding between the fillers and the resin matrix, making it impossible to form an effective rigid network to constrain the resin's expansion behavior; in Comparative Example 2, although KH-550 treatment was used to form a strong chemical bond between the fillers and the resin matrix, the lack of pre-constructed covalent connections between the fillers resulted in isolated filler particles. Dispersed within the matrix, it cannot form a three-dimensional rigid framework that runs through the entire structure. The constraint on resin expansion behavior relies only on local dispersion, making it difficult to achieve overall synergistic constraint. In Comparative Example 3, the absence of two-dimensional sheet-like fillers as "lateral connecting bridges" prevents the formation of a complete three-dimensional covalently cross-linked network structure. The isotropic distribution of the filler framework is disrupted, leading to increased CTE anisotropy. In Comparative Example 4, the crucial preheating and insulation step was omitted, preventing the mercapto groups on the surface of the one-dimensional thermally conductive filler from fully undergoing click chemical reactions with the alkenyl groups on the surface of the two-dimensional thermally conductive filler. This resulted in the failure to pre-construct a robust filler framework network, leading to a decrease in the constraint effect on resin expansion.

[0050] Comparing Examples 1 and 2, it can be seen that within the preferred ratio range, different specific filler proportions will affect the final performance. Example 1 uses a ratio of 1:0.8:0.5, which can enhance the synergistic effect of fillers in each dimension, and the constructed network is more uniform and efficient, thus achieving better overall performance.

[0051] Comparing Examples 1 and 3, it can be seen that using alkenyl and epoxy silane co-modified two-dimensional thermally conductive fillers, compared with using monofunctional silanes containing only alkenyl groups, introduces epoxy groups that covalently bond with the matrix during the resin curing stage, deeply integrating the pre-constructed filler network into the resin crosslinking network, forming a more stable rigid skeleton, thereby constraining the expansion behavior of the resin to a certain extent, resulting in a lower CTE value and better isotropy.

[0052] Comparing Examples 1 and 4, it can be seen that when the filler type, ratio and surface treatment are exactly the same, a moderate reduction in the total filler content will slightly weaken the restraining effect on resin expansion, but it is still better than each of the comparative examples, which reflects the advantages of the network structure.

[0053] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method for preparing epoxy resin casting material for solid-state transformers, characterized in that, Includes the following steps: M1: Provides 100 parts epoxy resin, 80-120 parts acid anhydride curing agent, 150-300 parts functionalized thermally conductive filler, 0.1-0.5 parts click reaction catalyst, and 0.5-2 parts curing accelerator as raw materials, wherein the functionalized thermally conductive filler includes mercapto-modified one-dimensional thermally conductive filler, alkenyl epoxy-modified two-dimensional thermally conductive filler, and epoxy-modified zero-dimensional thermally conductive filler; M2: The functionalized thermally conductive filler, the click reaction catalyst, and the epoxy resin are vacuum stirred and degassed at 50-60°C for 10-30 min, and then stirred and heated to 70-80°C for 10-20 min to allow the thiol groups on the surface of the thiol-modified one-dimensional thermally conductive filler to undergo a click chemical reaction with the alkenyl groups on the surface of the alkenyl epoxy-modified two-dimensional thermally conductive filler to obtain a premix. M3: Add anhydride curing agent and curing accelerator to the premix, stir and mix under vacuum conditions, remove air bubbles, and obtain a mixture; M4: The mixture is poured into a preheated solid transformer winding mold and cured to obtain epoxy resin casting material.

2. The method according to claim 1, characterized in that, The thiol-modified one-dimensional thermally conductive filler was prepared by the following method: 5-15 parts of γ-mercaptopropyltrimethoxysilane were dispersed in 100-150 parts of an aqueous ethanol solution and pre-hydrolyzed at 40-60℃ for 20-40 min; then 100 parts of one-dimensional thermally conductive filler were added and reacted at 70-90℃ for 4-8 h to obtain the thiol-modified one-dimensional thermally conductive filler.

3. The method according to claim 2, characterized in that, The one-dimensional thermally conductive filler is a needle-shaped or wire-shaped silicon nitride whisker or silicon carbide whisker.

4. The method according to claim 1, characterized in that, The alkenyl epoxy co-modified two-dimensional thermally conductive filler was prepared by the following method: 3-8 parts of vinyltriethoxysilane and 3-8 parts of γ-glycidyl etheroxypropyltrimethoxysilane were dispersed in 100-150 parts of an aqueous ethanol solution, the pH was adjusted to 4-5, and the mixture was pre-hydrolyzed at 40-60℃ for 20-40 min; then 100 parts of the two-dimensional thermally conductive filler were added, and the mixture was reacted at 70-90℃ for 4-8 h to obtain the alkenyl epoxy co-modified two-dimensional thermally conductive filler.

5. The method according to claim 4, characterized in that, The two-dimensional thermally conductive filler is a sheet-like boron nitride nanosheet.

6. The method according to claim 1, characterized in that, The epoxy-modified zero-dimensional thermally conductive filler was prepared by the following method: 5-15 parts of γ-glycidyl etheroxypropyltrimethoxysilane were dispersed in 100-150 parts of an aqueous ethanol solution, the pH was adjusted to 4-5, and the solution was pre-hydrolyzed at 40-60℃ for 20-40 min; then 100 parts of zero-dimensional thermally conductive filler were added, and the solution was reacted at 70-90℃ for 4-8 h to obtain the epoxy-modified zero-dimensional thermally conductive filler.

7. The method according to claim 6, characterized in that, The zero-dimensional thermally conductive filler is spherical alumina.

8. The method according to claim 1, characterized in that, The mass ratio of the mercapto-modified one-dimensional thermally conductive filler, the alkenyl epoxy co-modified two-dimensional thermally conductive filler, and the epoxy-modified zero-dimensional thermally conductive filler is 1:(0.7~0.9):(0.4~0.6).

9. The method according to claim 1, characterized in that, The anhydride curing agent is methyltetrahydrophthalic anhydride; the click reaction catalyst is triphenylphosphine; and the curing accelerator is an imidazole accelerator.

10. An epoxy resin casting material for solid-state transformers, characterized in that, Prepared by the method according to any one of claims 1 to 9.

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