A porous structure insulating paper with low dielectric constant and high thermal conductivity, preparation method and application

Abstract

The invention discloses a porous structure insulating paper with low dielectric constant and high thermal conductivity, a preparation method and an application thereof, comprising the following steps: step 1: treating boron nitride nanosheets (BNNS) with a silane coupling agent to obtain a silane-functionalized boron nitride nanosheet fBNNS dispersion; step 2: adding the fBNNS dispersion to an aramid nanofiber (ANF) dispersion, and performing electrostatic self-assembly to obtain fBNNS@ANF; step 3: constructing a metal organic framework (MOF) on the surface of nanocellulose (CNF) to obtain CNF loaded with the MOF; step 4: mixing the fBNNS@ANF obtained in step 2 and the MOF-loaded CNF obtained in step 3, fully reacting them under oil bath and ultrasonic conditions, vacuum filtering, vacuum heat drying and hot pressing to obtain the desired porous structure insulating paper with low dielectric constant and high thermal conductivity; the insulating paper obtained by the invention can have high mechanical strength while ensuring dielectric constant and thermal conductivity.

Description

Technical Field

[0001] The present invention relates to the technical field of insulating paper, and in particular to a porous insulating paper with low dielectric constant and high thermal conductivity, a preparation method and applications thereof. Background Art

[0002] Power transformers are key equipment for energy transmission and conversion in power systems. Their stable operation is directly related to the safety and stability of the power grid. The continuous increase in grid voltage levels has led to increasing demands for the voltage level, capacity, and reliability of power equipment, posing new challenges to transformer insulation systems. Currently, the oil-paper insulation primarily used in transformers is a key factor in determining the transformer's voltage level and capacity. Because the dielectric constant of cellulose paper is typically more than twice that of insulating oil, the mismatch between the oil-paper dielectric constant and the paper leads to uneven electric field distribution, which can easily cause the insulating oil to break down due to excessive electric field strength. In actual operation, the combined effects of electricity, heat, and force will also accelerate insulation aging.

[0003] Current approaches to reducing the dielectric constant of insulating paper focus on finding low-dielectric-constant fibers within the cellulose matrix. However, this approach disrupts the hydrogen-bonding network between the fiber and cellulose or results in a loose structure caused by micropores, significantly reducing the mechanical properties of the insulating paper. Furthermore, improving the paper's thermal conductivity is difficult. Therefore, finding a way to simultaneously reduce the dielectric constant of insulating paper and improve its thermal conductivity while maintaining mechanical properties is a pressing issue. Summary of the Invention

[0004] In order to solve the problems existing in the prior art, the present invention provides a porous structure insulating paper with low dielectric constant and high thermal conductivity, a preparation method and an application thereof.

[0005] The technical solution adopted by the present invention is: a method for preparing a porous structure insulating paper with low dielectric constant and high thermal conductivity, comprising the following steps:

[0006] Step 1: treating boron nitride nanosheets (BNNS) with a silane coupling agent to obtain a silane-functionalized boron nitride nanosheet (fBNNS) dispersion;

[0007] Step 2: Add fBNNS dispersion into aramid nanofiber ANF dispersion to obtain fBNNS@ANF through electrostatic self-assembly;

[0008] Step 3: Constructing a metal organic framework on the surface of nanocellulose CNF to obtain CNF loaded with a metal organic framework;

[0009] Step 4: The fBNNS@ANF obtained in step 2 and the CNF loaded with metal organic framework obtained in step 3 are mixed, and the mixture is fully reacted under oil bath and ultrasonic conditions. After vacuum filtration, vacuum heat drying, and hot pressing, the desired porous structure insulating paper with low dielectric constant and high thermal conductivity can be obtained.

[0010] Furthermore, the silane coupling agent in step 1 is selected from one of 3-aminopropyltriethoxysilane APTES and 3-aminopropyltrimethoxysilane APTMS.

[0011] Furthermore, the silane coupling agent in step 1 is first subjected to a hydrolysis treatment, and the process is as follows: the silane coupling agent is fully mixed with the alcohol solution and stirred at 60°C.

[0012] Furthermore, the mass ratio of fBNNS to ANF in step 2 is 1:10.

[0013] Furthermore, the metal organic framework in step 3 is a copper metal organic framework; the preparation process of CNF loaded with the metal organic framework is as follows:

[0014] Nanocellulose CNF, trimesic acid H3BTC and copper ion salt are dissolved in solvents to obtain solutions of the three substances;

[0015] The H3BTC solution and the copper ion salt solution are fully mixed, and then added to the CNF solution. After ultrasonication, stirring, centrifugation, and washing, CNF loaded with metal organic frameworks can be obtained.

[0016] Furthermore, the copper ion salt is one or both of Cu(NO3)2 and CuSO4; the molar ratio of H3BTC to copper ions is 2:3; the ultrasonic time is 2 to 3 hours, the stirring time is 24 hours, the centrifugal speed is 16000 rpm, and the centrifugal time is 25 minutes.

[0017] Furthermore, in step 4, the mass ratio of fBNNS@ANF to CNF loaded with metal organic framework is 2:1-4.

[0018] Furthermore, in step 4, the vacuum heat drying temperature is 150° C. and the time is 12 to 36 hours; the hot pressing temperature is 160° C., the hot pressing pressure is 15 MPa, and the hot pressing time is 8 minutes.

[0019] A porous insulating paper with low dielectric constant and high thermal conductivity.

[0020] The invention discloses an application of porous structure insulating paper with low dielectric constant and high thermal conductivity, wherein the insulating paper is used for preparing insulating components of power transformers.

[0021] The beneficial effects of the present invention are:

[0022] (1) The insulating paper obtained by the present invention has a metal organic framework in situ grown on the surface of the nanocellulose, and the covalent cross-linking forms a metal organic framework structure, which is conducive to the transfer of charge; at the same time, the insulating paper has a porous structure, which effectively reduces the relaxation polarization caused by the orientation of polar molecules and interfacial polarization under the power frequency, and can significantly reduce the dielectric constant of the insulating paper;

[0023] (2) Highly thermally conductive Jiyuan boron nitride nanosheets are introduced into the insulating paper obtained by the present invention. The boron nitride nanosheets and nanocellulose are evenly dispersed in the nanocellulose cross-linked network through electrostatic assembly, constructing an effective heat conduction channel and improving the thermal conductivity of the insulating paper;

[0024] (3) The insulating paper obtained by the present invention forms a compact two-dimensional nanofiber cross-linked network with a high aspect ratio through hydrogen bonding between high-strength aramid nanofibers and nanocellulose, overcoming the problem that traditional modification methods easily lead to damage to the mechanical properties of the insulating paper, so that the insulating paper can have high mechanical strength while ensuring dielectric constant and thermal conductivity.

[0025] (4) The insulating paper obtained by the present invention introduces two-dimensional boron nitride nanosheets and three-dimensional metal organic frameworks into the formed fiber cross-linked network through electrostatic self-assembly and covalent bonding. The multi-dimensional system interacts with each other to form a uniform and densely connected network structure, which synergistically improves the comprehensive performance of the insulating paper. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The figure is a schematic flow chart of the method for preparing insulating paper of the present invention.

[0027] Figure 2 This is a schematic diagram of the preparation process of CNF@Cu3(BTC)2 in the present invention.

[0028] Figure 3 These are the dielectric constant test results of the insulating papers obtained in Examples 1, 2, 3, 5, 6 and Comparative Examples 1, 5, 6, and 7 of the present invention at an industrial frequency of 50 Hz.

[0029] Figure 4 Schematic diagram of thermal conductivity test results of insulating paper obtained in Examples 1, 4, 7 of the present invention and Comparative Examples 2, 3, 7.

[0030] Figure 5 Schematic diagram of the tensile strength test results of the insulating paper obtained in Examples 1, 2, 3, 5, and 6 of the present invention and Comparative Examples 1, 4, and 7.

[0031] Figure 6 Schematic diagram of elastic modulus test results of insulating paper obtained in Examples 1, 2, 3, 5, 6 of the present invention and Comparative Examples 1, 4, 7. DETAILED DESCRIPTION

[0032] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0033] like Figure 1 As shown, a method for preparing a porous structure insulating paper with low dielectric constant and high thermal conductivity comprises the following steps:

[0034] Step 1: treating boron nitride nanosheets (BNNS) with a silane coupling agent to obtain a silane-functionalized boron nitride nanosheet (fBNNS) dispersion;

[0035] First, hexagonal boron nitride needs to be exfoliated to obtain boron nitride nanosheets BNNS; ion intercalation is used to assist liquid phase exfoliation.

[0036] The exfoliated boron nitride nanosheets (BNNS) are functionalized with a silane coupling agent to produce positively charged silane-functionalized boron nitride nanosheets (fBNNS). The silane coupling agent is selected from either 3-aminopropyltriethoxysilane (APTES) or 3-aminopropyltrimethoxysilane (APTMS). The weight of the silane coupling agent is 3-5% of the weight of the fBNNS.

[0037] Before functionalizing the silane coupling agent, the silane coupling agent is first hydrolyzed as follows: an alcohol solution of the silane coupling agent is prepared, wherein the mass ratio of alcohol to water in the alcohol solution is 10:1, and the alcohol solution of the silane coupling agent is stirred in an oil bath; the stirring temperature is 60°C, and the stirring time is 30 to 40 minutes. The relative ratio of the silane coupling agent to the alcohol solution can be adjusted according to actual conditions.

[0038] Step 2: Add fBNNS dispersion into aramid nanofiber ANF dispersion to obtain fBNNS@ANF through electrostatic self-assembly;

[0039] Aramid nanofiber ANF is obtained by stripping Kevlar fiber. It has a negative charge and can be electrostatically self-assembled with positively charged fBNNS to obtain the required fBNNS@ANF.

[0040] The peeling process is as follows:

[0041] The Kevlar fibers were stripped by deprotonation in a KOH / DMSO system, and the obtained aramid nanofibers were dispersed in acetone to obtain a dispersion.

[0042] The fBNNS dispersion obtained in step 1 was added to the ANF dispersion in multiple portions. After the dispersion was fully added, the mixture was ultrasonicated at room temperature for 1 to 2 hours. The mass ratio of fBNNS to ANF was 1:10. The fBNNS@ANF was dispersed in acetone to obtain a dispersion.

[0043] Step 3: Constructing a metal organic framework on the surface of nanocellulose CNF to obtain CNF loaded with a metal organic framework;

[0044] The metal organic framework is Cu3(BTC)2, and the specific preparation method is as follows: Figure 2 As shown:

[0045] The method comprises dissolving trimesic acid H3BTC and copper ion salt in dimethylformamide (DMF) to obtain DMF solutions of three substances, wherein the copper ion salt is one or both of Cu(NO3)2 and CuSO4.

[0046] The H3BTC solution and the copper ion salt solution were mixed and stirred at room temperature for 1 to 2 hours, with the molar ratio of H3BTC to copper ions being 2:3.

[0047] The mixed solution was added to the CNF solution, sonicated at room temperature for 2-3 hours, stirred for 24 hours, centrifuged at 16,000 rpm for 25 minutes, and repeatedly washed with deionized water to obtain CNF@Cu3(BTC)2. The relative ratio of CNF to trimesic acid (H3BTC)2 is related to the resulting dielectric constant and can be adjusted as needed.

[0048] Step 4: The fBNNS@ANF obtained in step 2 and the CNF loaded with metal organic framework obtained in step 3 are mixed, and the mixture is fully reacted under oil bath and ultrasonic conditions. After vacuum filtration, vacuum heat drying, and hot pressing, the desired porous structure insulating paper with low dielectric constant and high thermal conductivity can be obtained.

[0049] The fBNNS@ANF CNF@Cu3(BTC)2 mass ratio is 2:1-4. The mixture is stirred in an oil bath at 60°C until the acetone is completely evaporated and ultrasonicated until bubbles disappear. The mixture is then vacuum filtered and dried at 150°C for 12-36 hours. After drying, the mixture is hot-pressed at 160°C and 15 MPa for 8 minutes to produce the desired porous insulating paper with low dielectric constant and high thermal conductivity.

[0050] Unless otherwise specified, the dielectric constant refers to its actual value.

[0051] Example 1

[0052] A method for preparing porous insulating paper with low dielectric constant and high thermal conductivity comprises the following steps:

[0053] Step 1: treating boron nitride nanosheets (BNNS) with a silane coupling agent to obtain a silane-functionalized boron nitride nanosheet (fBNNS) dispersion;

[0054] First, boron nitride nanosheets (BNNS) were obtained by ion intercalation-assisted liquid-phase exfoliation. A 100 mL solution of a 3:1 volume ratio of isopropyl alcohol and deionized water was prepared. 1 g of hBN powder and 1 g of lithium citrate were dispersed in the solution. Ultrasonic exfoliation was performed in an ultrasonic cleaner at 200 W for 12 hours. The resulting solution was placed in a reactor and hydrothermally treated at 180°C for 12 hours. After treatment, the mixture was washed four times with deionized water. The resulting solution was then centrifuged at 1600 rpm for 25 minutes, vacuum filtered, and dried at 100°C for 24 hours to obtain BNNS.

[0055] The BNNS were then silane-functionalized by adding 25 mg of 3-aminopropyltriethoxysilane (APTES) (the weight of the silane coupling agent was 5% of the BNNS weight) to a 50 mL ethanol-water solution with a 10:1 ethanol-water ratio. The mixture was then stirred in an oil bath at 60°C for 30 minutes to complete the hydrolysis of the APTES. 0.5 g of BNNS was then dispersed into the hydrolyzed APTES solution and stirred in an oil bath at 80°C for 6 hours to obtain silane-functionalized BNNS (fBNNS).

[0056] The resulting mixed solution was centrifuged at 1600 rpm for 25 minutes, vacuum filtered, and washed six times with acetone to obtain a 1 mg / mL fBNNS / acetone dispersion. Since the fBNNS surface is covered with amine groups after silane functionalization, the fBNNS in the dispersion are positively charged.

[0057] Step 2: Add fBNNS dispersion into aramid nanofiber ANF dispersion to obtain fBNNS@ANF through electrostatic self-assembly;

[0058] First, Kevlar fibers were exfoliated to obtain aramid nanofibers (ANF). Kevlar fibers were cut into small pieces approximately 1 cm in diameter, placed in acetone, and sonicated for 12 hours. After sonication, the mixture was filtered, and the filtered Kevlar fiber pieces were dried at 65°C for 3 days. After drying, 1 g of Kevlar fibers and 1.5 g of KOH were dispersed in 500 mL of dimethyl sulfoxide (DMSO) and stirred at 800 rpm at room temperature for 14 days. Deprotonation disrupted the hydrogen bonds between the nanofibers, resulting in an ANF / DMSO dispersion with a concentration of 3 mg / mL. 100 mL of the dispersion was filtered, and the ANF was dispersed in acetone. The ANF was washed six times with acetone using a Buchner funnel. Finally, the ANF was dissolved in 300 mL of acetone and sheared at 1400 rpm for 15 minutes to obtain a 1 mg / mL ANF / acetone solution. The ANF in the dispersion was negatively charged.

[0059] The fBNNS dispersion was then mixed with an ANF dispersion for self-assembly: 60 mL of the prepared ANF / acetone solution was placed in a beaker and pipetted into the ANF / acetone solution in 1 mL increments. The mixed solution was sonicated at 200W for 2 hours and then stirred at 300 rpm for 2 hours at room temperature. Self-assembly was achieved under ultrasound and stirring, through electrostatic interactions between the negatively charged ANF and the positively charged fBNNS, as well as hydrogen bonds formed between amine groups on the fBNNS surface and amide, carboxyl, and amine groups on the ANF surface, to form fBNNS@ANF.

[0060] Step 3: Constructing a metal organic framework on the surface of nanocellulose CNF to obtain CNF loaded with a metal organic framework;

[0061] 37.5 mg of CuSO4·5H2O, 21 mg of H3BTC, and 3 g of a 2 wt% CNF reagent were weighed and dissolved in 10 mL of DMF. The mixture was stirred at 400 rpm for 2 h at room temperature. The CuSO4 dispersion was added to the stirring H3BTC solution in multiple portions and stirred at room temperature for another 2 h. The mixed solution was then added to the stirring CNF dispersion in multiple portions. The mixture was then sonicated at 200 W for 3 h and stirred at 400 rpm for 24 h. After stirring, the mixed solution was centrifuged at 16,000 rpm for 25 min, vacuum filtered, and washed six times with deionized water to obtain CNF@Cu3(BTC)2. CNF@Cu3(BTC)2 was dissolved in 100 mL of deionized water.

[0062] Step 4: The fBNNS@ANF obtained in step 2 and the CNF loaded with metal organic framework obtained in step 3 are mixed, and the mixture is fully reacted under oil bath and ultrasonic conditions. After vacuum filtration, vacuum heat drying, and hot pressing, the desired porous structure insulating paper with low dielectric constant and high thermal conductivity can be obtained.

[0063] The fBNNS@ANF / acetone dispersion was added to the CNF@Cu3(BTC)2 solution and stirred in an oil bath at 60°C to fully remove the acetone from the mixture, resulting in an aqueous solution of fBNNS@ANF / CNF@Cu3(BTC)2. The mixed solution was sheared at 14,000 rpm for 15 minutes and then sonicated at 200W for 20 minutes to fully remove bubbles from the mixture. A BNNS@ANF / CNF@CuBTC film was prepared by vacuum-assisted filtration and then vacuum-dried at 150°C for 24 hours. After drying, the film was hot-pressed at 160°C and 15 MPa for 8 minutes to obtain the porous insulating paper with low dielectric constant and high thermal conductivity.

[0064] Example 2

[0065] The other steps of this example are the same as those of Example 1, except that in step 2, the amount of ANF / acetone solution is 80 mL; the amount of fBNNS / acetone solution is 8 mL; and in step 3, the amount of CNF reagent is 2 g.

[0066] Example 3

[0067] The other steps of this example are the same as those of Example 1, except that in step 2, the amount of ANF / acetone solution is 40 mL; the amount of fBNNS / acetone solution is 4 mL; and in step 3, the amount of CNF reagent is 4 g.

[0068] Example 4

[0069] The other steps of this embodiment are the same as those of embodiment 1, except that the silane coupling agent used in step 1 is 3-aminopropyltrimethoxysilane APTMS.

[0070] Example 5

[0071] The other steps of this embodiment are the same as those of embodiment 1, except that the vacuum heat drying time in step 4 is 12 hours.

[0072] Example 6

[0073] The other steps of this embodiment are the same as those of embodiment 1, except that the vacuum heat drying time in step 4 is 36 hours.

[0074] Example 7

[0075] The other steps of this example are the same as those of Example 1, except that the ultrasonic treatment time of the fBNNS and ANF mixed solution in step 2 is 1 h.

[0076] Comparative Example 1

[0077] The other steps of this embodiment are the same as those of embodiment 1, except that vacuum heat drying is not performed in step 4.

[0078] Comparative Example 2

[0079] A method for preparing insulating paper comprises the following steps:

[0080] Step 1: Boron nitride nanosheets (BNNS) were first obtained by ion intercalation-assisted liquid-phase exfoliation. A 100 mL solution of a 3:1 volume ratio of isopropyl alcohol and deionized water was prepared. 1 g of hBN powder and 1 g of lithium citrate were dispersed in this solution. Ultrasonic exfoliation was performed in an ultrasonic bath at 200 W for 12 hours. The resulting solution was placed in a reactor and hydrothermally treated at 180°C for 12 hours. After treatment, the mixture was washed six times with acetone to obtain a 1 mg / mL BNNS / acetone dispersion.

[0081] Step 2: Kevlar fibers were first exfoliated to obtain aramid nanofibers (ANF). Kevlar fibers were cut into small pieces approximately 1 cm in diameter, placed in acetone, and sonicated for 12 h. After sonication, the mixture was filtered, and the filtered Kevlar fiber pieces were dried at 65°C for 3 days. After drying, 1 g of Kevlar fibers and 1.5 g of KOH were dispersed in 500 mL of dimethyl sulfoxide (DMSO) and stirred at 800 rpm at room temperature for 14 days. Deprotonation disrupted the hydrogen bonds between the nanofibers, resulting in an ANF / DMSO dispersion with a concentration of 3 mg / mL. 100 mL of the dispersion was filtered, and the ANF was dispersed in acetone. The ANF was washed six times with acetone using a Buchner funnel. Finally, the ANF was dissolved in 300 mL of acetone and sheared at 1400 rpm for 15 minutes to obtain a 1 mg / mL ANF / acetone solution. The ANF in the dispersion was negatively charged.

[0082] Step 3: Constructing a metal organic framework on the surface of nanocellulose CNF to obtain CNF loaded with a metal organic framework;

[0083] 37.5 mg of CuSO4·5H2O, 21 mg of H3BTC, and 3 g of a 2 wt% CNF reagent were weighed and dissolved in 10 mL of DMF. The mixture was stirred at 400 rpm for 2 h at room temperature. The CuSO4 dispersion was added to the stirring H3BTC solution in multiple portions and stirred at room temperature for another 2 h. The mixed solution was then added to the stirring CNF dispersion in multiple portions. The mixture was then sonicated at 200 W for 3 h and stirred at 400 rpm for 24 h. After stirring, the mixed solution was centrifuged at 16,000 rpm for 25 min, vacuum filtered, and washed six times with deionized water to obtain CNF@Cu3(BTC)2. CNF@Cu3(BTC)2 was dissolved in 100 mL of deionized water.

[0084] Step 4: 60 mL of ANF dispersion and 6 mL of BNNS dispersion were added to the CNF@Cu3(BTC)2 solution. The mixture was stirred in an oil bath at 60°C to fully remove the acetone, yielding a BNNS / ANF / CNF@Cu3(BTC)2 aqueous solution. The mixed solution was sheared at 14,000 rpm for 15 minutes, followed by sonication at 200 W for 20 minutes to fully remove bubbles from the mixture. Vacuum-assisted filtration was used to prepare a BNNS / ANF / CNF@CuBTC thin film, which was then vacuum-dried at 150°C for 24 hours. After drying, the film was hot-pressed at 160°C and 15 MPa for 8 minutes to yield the BNNS / ANF / CNF@Cu3(BTC)2 insulating paper.

[0085] Comparative Example 3

[0086] A method for preparing insulating paper comprises the following steps:

[0087] Step 1: Kevlar fibers were first exfoliated to obtain aramid nanofibers (ANF). Kevlar fibers were cut into small pieces approximately 1 cm in diameter, placed in acetone, and sonicated for 12 hours. After sonication, the mixture was filtered, and the filtered Kevlar fiber pieces were dried at 65°C for 3 days. After drying, 1 g of Kevlar fibers and 1.5 g of KOH were dispersed in 500 mL of dimethyl sulfoxide (DMSO) and stirred at 800 rpm at room temperature for 14 days. Deprotonation disrupted the hydrogen bonds between the nanofibers, resulting in an ANF / DMSO dispersion with a concentration of 3 mg / mL. 100 mL of the dispersion was filtered, and the ANF was dispersed in acetone. The ANF was washed six times with acetone using a Buchner funnel. Finally, the ANF was dissolved in 300 mL of acetone and sheared at 1400 rpm for 15 minutes to obtain a 1 mg / mL ANF / acetone solution. The ANF in the dispersion was negatively charged.

[0088] Step 2: Constructing a metal organic framework on the surface of nanocellulose CNF to obtain CNF loaded with a metal organic framework;

[0089] 37.5 mg of CuSO4·5H2O, 21 mg of H3BTC, and 3 g of a 2 wt% CNF reagent were weighed and dissolved in 10 mL of DMF. The mixture was stirred at 400 rpm for 2 h at room temperature. The CuSO4 dispersion was added to the stirring H3BTC solution in multiple portions and stirred at room temperature for another 2 h. The mixed solution was then added to the stirring CNF dispersion in multiple portions. The mixture was then sonicated at 200 W for 3 h and stirred at 400 rpm for 24 h. After stirring, the mixed solution was centrifuged at 16,000 rpm for 25 min, vacuum filtered, and washed six times with deionized water to obtain CNF@Cu3(BTC)2. CNF@Cu3(BTC)2 was dissolved in 100 mL of deionized water.

[0090] Step 3: 60 mL of the ANF dispersion was added to the CNF@Cu3(BTC)2 solution. The mixture was stirred in an oil bath at 60°C to fully remove the acetone, yielding an ANF / CNF@Cu3(BTC)2 aqueous solution. The mixed solution was sheared at 14,000 rpm for 15 minutes, followed by ultrasonic treatment at 200 W for 20 minutes to fully remove bubbles from the mixture. ANF / CNF@Cu3(BTC)2 thin films were prepared using vacuum-assisted filtration and subsequently vacuum-dried at 150°C for 24 hours. After drying, the film was hot-pressed at 160°C and 15 MPa for 8 minutes to yield the ANF / CNF@Cu3(BTC)2 insulating paper.

[0091] Comparative Example 4

[0092] A method for preparing insulating paper comprises the following steps:

[0093] Step 1: Boron nitride nanosheets (BNNS) were first obtained by ion intercalation-assisted liquid-phase exfoliation. A 100 mL solution of a 3:1 volume ratio of isopropyl alcohol and deionized water was prepared. 1 g of hBN powder and 1 g of lithium citrate were dispersed in this solution. Ultrasonic exfoliation was performed in an ultrasonic bath at 200 W for 12 hours. The resulting solution was placed in a reactor and hydrothermally treated at 180°C for 12 hours. After treatment, the mixture was washed six times with acetone to obtain a 1 mg / mL BNNS / acetone dispersion.

[0094] Step 2: Weigh 37.5 mg of CuSO4·5H2O, 21 mg of H3BTC, and 3 g of 2 wt% CNF reagent and dissolve each in 10 mL of DMF. Stir at 400 rpm for 2 h at room temperature. The CuSO4 dispersion was added to the stirring H3BTC solution in multiple portions and stirred at room temperature for another 2 h. The mixed solution was then added to the stirring CNF dispersion in multiple portions. The mixture was then sonicated at 200 W for 3 h and stirred at 400 rpm for 24 h. After stirring, the mixed solution was centrifuged at 16,000 rpm for 25 min, vacuum filtered, and washed six times with deionized water to obtain CNF@Cu3(BTC)2. CNF@Cu3(BTC)2 was dissolved in 100 mL of deionized water.

[0095] Step 3: 6 mL of the BNNS dispersion was added to the CNF@Cu3(BTC)2 solution. The mixture was stirred in an oil bath at 60°C to remove acetone, yielding a BNNS@Cu3(BTC)2 aqueous solution. The mixed solution was sheared at 14,000 rpm for 15 minutes, followed by sonication at 200 W for 20 minutes to remove air bubbles. A BNNS@Cu3(BTC)2 thin film was prepared by vacuum-assisted filtration and then vacuum-dried at 150°C for 24 hours. After drying, the film was hot-pressed at 160°C and 15 MPa for 8 minutes to yield the BNNS@Cu3(BTC)2 insulating paper.

[0096] Comparative Example 5

[0097] A method for preparing insulating paper comprises the following steps:

[0098] Step 1: Boron nitride nanosheets (BNNS) were obtained by ion intercalation-assisted liquid-phase exfoliation. A 100 mL solution of a 3:1 volume ratio of isopropyl alcohol and deionized water was prepared. 1 g of hBN powder and 1 g of lithium citrate were dispersed in the solution. Ultrasonic exfoliation was performed in an ultrasonic cleaner at 200 W for 12 hours. The resulting solution was placed in a reactor and hydrothermally treated at 180°C for 12 hours. After treatment, the mixture was washed four times with deionized water. The resulting solution was then centrifuged at 1600 rpm for 25 minutes, vacuum filtered, and dried at 100°C for 24 hours to obtain BNNS.

[0099] The BNNS were then silane-functionalized by adding 25 mg of 3-aminopropyltriethoxysilane (APTES) (the weight of the silane coupling agent was 5% of the BNNS weight) to a 50 mL ethanol-water solution with a 10:1 ethanol-water ratio. The mixture was then stirred in an oil bath at 60°C for 30 minutes to complete the hydrolysis of the APTES. 0.5 g of BNNS was then dispersed into the hydrolyzed APTES solution and stirred in an oil bath at 80°C for 6 hours to obtain silane-functionalized BNNS (fBNNS).

[0100] The resulting mixed solution was centrifuged at 1600 rpm for 25 minutes, vacuum filtered, and washed six times with acetone to obtain a 1 mg / mL fBNNS / acetone dispersion. Since the fBNNS surface is covered with amine groups after silane functionalization, the fBNNS in the dispersion are positively charged.

[0101] Step 2: Add fBNNS dispersion into aramid nanofiber ANF dispersion to obtain fBNNS@ANF through electrostatic self-assembly;

[0102] First, Kevlar fibers were exfoliated to obtain aramid nanofibers (ANF). Kevlar fibers were cut into small pieces approximately 1 cm in diameter, placed in acetone, and sonicated for 12 hours. After sonication, the mixture was filtered, and the filtered Kevlar fiber pieces were dried at 65°C for 3 days. After drying, 1 g of Kevlar fibers and 1.5 g of KOH were dispersed in 500 mL of dimethyl sulfoxide (DMSO) and stirred at 800 rpm at room temperature for 14 days. Deprotonation disrupted the hydrogen bonds between the nanofibers, resulting in an ANF / DMSO dispersion with a concentration of 3 mg / mL. 100 mL of the dispersion was filtered, and the ANF was dispersed in acetone. The ANF was washed six times with acetone using a Buchner funnel. Finally, the ANF was dissolved in 300 mL of acetone and sheared at 1400 rpm for 15 minutes to obtain a 1 mg / mL ANF / acetone solution. The ANF in the dispersion was negatively charged.

[0103] The fBNNS dispersion was then mixed with an ANF dispersion for self-assembly: 60 mL of the prepared ANF / acetone solution was placed in a beaker and pipetted into the ANF / acetone solution in 1 mL increments. The mixed solution was sonicated at 200W for 2 hours and then stirred at 300 rpm for 2 hours at room temperature. Self-assembly was achieved under ultrasound and stirring, through electrostatic interactions between the negatively charged ANF and the positively charged fBNNS, as well as hydrogen bonds formed between amine groups on the fBNNS surface and amide, carboxyl, and amine groups on the ANF surface, to form fBNNS@ANF.

[0104] Step 3: Weigh 3g of a 2wt% CNF reagent and dissolve it in 100mL of deionized water. Stir at 400rpm for 30min at room temperature. Add the fBNNS@ANF / acetone dispersion to the CNF solution and stir in an oil bath at 60°C to fully remove the acetone from the mixture, yielding an fBNNS@ANF / CNF aqueous solution. Shear the mixed solution at 14,000rpm for 15min, followed by sonication at 200W for 20min to fully remove air bubbles from the mixture. Vacuum-assisted filtration was used to produce an fBNNS@ANF / CNF film, which was then vacuum-dried at 150°C for 24h. After drying, the film was hot-pressed at 160°C and 15MPa for 8min to yield the fBNNS@ANF / CNF insulating paper.

[0105] Comparative Example 6

[0106] A method for preparing insulating paper comprises the following steps:

[0107] Step 1: Kevlar fiber exfoliation to obtain aramid nanofibers (ANF): Kevlar fibers were cut into small pieces approximately 1 cm in diameter, placed in acetone, and sonicated for 12 h. After sonication, the mixture was filtered, and the filtered Kevlar fiber pieces were dried at 65°C for 3 days. After drying, 1 g of Kevlar fiber and 1.5 g of KOH were dispersed in 500 mL of dimethyl sulfoxide (DMSO) and stirred at 800 rpm at room temperature for 14 days. Deprotonation disrupted the hydrogen bonds between the nanofibers, resulting in an ANF / DMSO dispersion with a concentration of 3 mg / mL. 100 mL of the dispersion was filtered, and the ANF was dispersed in acetone. The ANF was washed six times with acetone using a Buchner funnel. Finally, the ANF was dissolved in 300 mL of acetone and sheared at 1400 rpm for 15 minutes to obtain a 1 mg / mL ANF / acetone solution. The ANF in the dispersion was negatively charged.

[0108] Step 2: Weigh 3g of a 2wt% CNF reagent and dissolve it in 50mL of deionized water. Stir at 400rpm for 2h at room temperature. Add 60mL of the ANF solution to the CNF solution and stir at 400rpm for 1h at room temperature. Shear the mixture at 14,000rpm for 15min, then sonicate at 200W for 20min to remove bubbles from the mixture. Vacuum-assisted filtration is used to prepare an ANF / CNF film, which is then vacuum-dried at 150°C for 24h. After drying, the film is hot-pressed at 160°C and 15MPa for 8min to obtain the ANF / CNF insulating paper.

[0109] Comparative Example 7

[0110] 6 g of a 2 wt% CNF reagent was weighed and dissolved in 50 mL of deionized water. The mixture was stirred at 400 rpm for 2 hours at room temperature. The mixed solution was sheared at 14,000 rpm for 15 minutes, followed by ultrasonic treatment at 200 W for 20 minutes to fully remove bubbles from the mixture. A CNF film was prepared using vacuum-assisted filtration and then vacuum-dried at 150°C for 24 hours. After drying, the film was hot-pressed at 160°C and 15 MPa for 8 minutes to obtain the CNF insulating paper.

[0111] Figure 3 Schematic diagram of the statistical results of dielectric constants of Examples 1, 2, 3, 5, 6 and Comparative Examples 1, 5, 6, 7 at an industrial frequency of 50 Hz.

[0112] As can be seen from the figure, Example 1 and Comparative Examples 5, 6, and 7 demonstrate that the introduction of CNF@Cu3(BTC)2, a metal-organic framework constructed on the CNF surface, can significantly reduce the dielectric constant of the insulating paper. A comparison of Examples 1, 2, and 3 reveals that the dielectric constant of the insulating paper further decreases with increasing proportion of CNF@Cu3(BTC)2.

[0113] from Figure 3It can be seen that by adjusting the ratio of fBNNS@ANF and CNF@Cu3(BTC)2, the dielectric constant of the insulating paper can be controlled. When the mass ratio of fBNNS@ANF to CNF@Cu3(BTC)2 in Example 1 is 1:1, the dielectric constant of the insulating paper obtained is 2.21, which is approximately equal to the dielectric constant of the insulating oil. This can effectively avoid the mismatch of the dielectric constant of the oil-paper, which leads to uneven electric field distribution of the oil-paper and excessive electric field strength of the insulating oil, resulting in breakdown. It can be seen from the results of Examples 1, 5, 6 and Comparative Example 1 that the dielectric constant of the insulating paper decreases with the extension of the vacuum heat drying time; vacuum heat drying can effectively remove water molecules in the metal organic framework to form a highly porous structure. The removal of polar water molecules and the introduction of air with a low dielectric constant can effectively reduce the relaxation polarization caused by the orientation of polar water molecules and interfacial polarization at the power frequency, thereby reducing the dielectric constant of the insulating paper.

[0114] Figure 4 The thermal conductivity test results for the insulating paper obtained in Examples 1, 4, and 7 of the present invention and Comparative Examples 2, 3, and 7 are shown. As can be seen from the figure, the thermal conductivity of the insulating paper obtained in Example 1 is 6.21 W / m·K, while the thermal conductivity of the insulating paper in Comparative Example 7 is 2.17 W / m·K. This represents a 186% improvement in thermal conductivity compared to the conventional nanocellulose insulating paper in Comparative Example 7. The thermal conductivity of the insulating paper obtained in Comparative Example 3 is 1.87 W / m·K, a 232% improvement compared to Comparative Example 3. These results demonstrate that the introduction of the highly thermally conductive nanomaterial fBNNS can create an effective thermal network, significantly improving the thermal conductivity of the insulating paper.

[0115] From the results of Examples 1 and 4, it can be seen that the effect of silane functionalization of fBNNS treated with the coupling agent APTES is better than that of APTMS. From the results of Examples 1 and 7, it can be seen that a longer ultrasonic treatment time can make the electrostatic self-assembly of fBNNS and ANF more complete, avoid the agglomeration of fBNNS, achieve its uniform dispersion in the network, and significantly improve the thermal conductivity of the insulating paper. However, the insulating paper in Comparative Example 2 did not undergo self-assembly of BNNS and ANF. Since BNNS easily agglomerates, adding it alone results in defects in the thermal conductive network, and the thermal conductivity of the insulating paper is significantly lower than that of Example 1.

[0116] Figure 5 The tensile strength test results of the insulating paper were obtained for Examples 1, 2, 3, 5, 6 and Comparative Examples 1, 4, and 7. Figure 6 As can be seen from the figure, the mechanical properties of each embodiment are significantly improved compared with ordinary nanocellulose insulation paper.

[0117] from Figure 5 and Figure 6Comparing the results of Examples 1, 2, and 3 with Comparative Example 4, it can be seen that the mechanical properties of the insulating paper gradually improve with the increase of the fBNNS@ANF content. Because ANF has excellent mechanical strength and a large aspect ratio. At the same time, hydrogen bonds can be formed between ANF and CNF, and ANF forms a nano-network skeleton in the insulating paper, so it can effectively realize the transfer of load and improve the mechanical properties of ordinary cellulose insulating paper. In the range of the mass ratio of fBNNS@ANF to CNF@Cu3(BTC)2 of the present invention is 2: (1 to 4), the content of ANF is positively correlated with the mechanical properties of the insulating paper.

[0118] The results of Examples 1, 5, and 6, as well as Comparative Example 1, show that the mechanical properties of the insulating paper decrease with increasing vacuum heat drying time. This is because vacuum heat drying effectively removes water molecules from the porous structure of the insulating paper, introducing more cavities. While this cavity structure significantly reduces the dielectric constant, it also reduces its mechanical strength somewhat, but still significantly improves compared to traditional cellulose insulating paper. The insulating paper obtained by the present invention significantly reduces its dielectric constant and improves its thermal conductivity while maintaining a high level of mechanical strength.

[0119] The present invention creates a two-dimensional cross-linked ANF-CNF nanofiber network through hydrogen bonding between aramid nanofibers and nanocellulose. This network provides ample active sites for the growth of the Cu3(BTC)2 metal-organic framework (MOF), helping to construct a densely connected porous network and facilitate charge transfer. Furthermore, the MOF's porous structure effectively prevents nanofiber aggregation, facilitating the uniform spatial dispersion of self-assembled high-thermal-conductivity boron nitride nanosheets. This facilitates the construction of an overall stress transfer and thermal conductivity network, improving the mechanical properties and thermal conductivity of the insulating paper.

[0120] The aramid nanofibers and CNFs in the insulating paper obtained by the present invention interact to form a three-dimensional fiber cross-linked network; the Cu3(BTC)2 metal-organic framework also provides a three-dimensional multi-porous structure, and two-dimensional high-thermal conductivity boron nitride nanosheets are uniformly dispersed in the multi-dimensional system structure; the interconnection of multiple substances and the multi-dimensional system structure synergistically improves the dielectric properties, mechanical properties and thermal conductivity of the insulating paper. Vacuum thermal drying is used to remove water molecules in the framework to form a high-porosity structure, reduce relaxation polarization, and significantly reduce the dielectric constant of the insulating paper; the dielectric constant is regulated by adjusting the ratio of fBNNS@ANF and CNF@Cu3(BTC)2, and insulating paper with a dielectric constant approximately equal to that of the insulating oil is prepared. This can effectively avoid the problem of uneven electric field distribution in the oil-paper due to the mismatch of the dielectric constant of the oil-paper and the breakdown of the insulating oil due to excessive electric field intensity.

Claims

1. A method for preparing porous insulating paper with low dielectric constant and high thermal conductivity, characterized in that: The following steps are involved: Step 1: treating boron nitride nanosheets (BNNS) with a silane coupling agent to obtain a silane-functionalized boron nitride nanosheet (fBNNS) dispersion; Step 2: Add the fBNNS dispersion into the aramid nanofiber (ANF) dispersion in multiple times, stir after ultrasonic treatment, and electrostatically self-assemble to obtain fBNNS@ANF. Step 3: Constructing a metal organic framework on the surface of the nanocellulose CNF to obtain CNF loaded with a metal organic framework; the metal organic framework is a copper metal organic framework; the preparation process of CNF loaded with a metal organic framework is as follows: Nanocellulose CNF, trimesic acid H3BTC and copper ion salt are dissolved in solvents to obtain solutions of the three substances; The H3BTC solution and the copper ion salt solution are fully mixed and then added to the CNF solution. After ultrasonication, stirring, centrifugation, and washing, CNF loaded with metal organic frameworks can be obtained. Step 4: The fBNNS@ANF obtained in step 2 and the CNF loaded with metal organic framework obtained in step 3 are mixed, and the mixture is fully reacted under oil bath and ultrasonic conditions. After vacuum filtration, vacuum heat drying, and hot pressing, the desired porous structure insulating paper with low dielectric constant and high thermal conductivity can be obtained.

2. The method for preparing a porous structure insulating paper with low dielectric constant and high thermal conductivity according to claim 1, characterized in that: The silane coupling agent in step 1 is selected from one of 3-aminopropyltriethoxysilane APTES and 3-aminopropyltrimethoxysilane APTMS.

3. The method for preparing a porous structure insulating paper with low dielectric constant and high thermal conductivity according to claim 1, characterized in that: The silane coupling agent in step 1 is first subjected to a hydrolysis treatment, and the process is as follows: the silane coupling agent is fully mixed with the alcohol solution and stirred at 60°C.

4. The method for preparing a porous structure insulating paper with low dielectric constant and high thermal conductivity according to claim 1, characterized in that: The mass ratio of fBNNS to ANF in step 2 is 1:

10.

5. The method for preparing a porous structure insulating paper with low dielectric constant and high thermal conductivity according to claim 1, characterized in that: The copper ion salt is one or both of Cu(NO3)2 and CuSO4; The molar ratio of H3BTC to copper ions is 2:3; the ultrasonic time is 2 to 3 hours, the stirring time is 24 hours, the centrifugal speed is 16000 rpm, and the centrifugal time is 25 minutes.

6. The method for preparing a porous structure insulating paper with low dielectric constant and high thermal conductivity according to claim 1, characterized in that: In step 4, the mass ratio of fBNNS@ANF to CNF loaded with metal organic framework is 2:1-4.

7. The method for preparing a porous structure insulating paper with low dielectric constant and high thermal conductivity according to claim 1, characterized in that: In step 4, the vacuum heat drying temperature is 150° C. and the time is 12 to 36 hours; the hot pressing temperature is 160° C., the hot pressing pressure is 15 MPa, and the hot pressing time is 8 minutes.

8. A porous insulating paper with low dielectric constant and high thermal conductivity obtained by the preparation method according to any one of claims 1 to 7.

9. The use of the porous insulating paper with low dielectric constant and high thermal conductivity as claimed in claim 8, characterized in that: The insulating paper is used for preparing insulating components of power transformers.

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