Polyimide composite film for flexible cover plate of space solar cell

Abstract

The invention discloses a polyimide composite film for a flexible cover plate of a space solar cell. Belongs to the technical field of polyimide composites.A novel diamine monomer is synthesized by introducing a fluorine-containing group, then the novel diamine monomer and 4, 4 '-(4, 4'-isopropyl diphenoxy) bis (phthalic anhydride) are subjected to solution polymerization, a polyimide film is formed on a glass plate in a curtain coating mode, and then the polyimide film is prepared by adopting a four-layer structure. The silicon dioxide layer, the magnesium fluoride layer, the polyimide substrate layer and the silicon dioxide layer are sequentially arranged from top to bottom. The silicon dioxide layer is prepared through a sol-gel method, and the magnesium fluoride layer is subjected to vacuum coating through electron beam evaporation. The polyimide composite film obtained by the method has excellent thermal properties, mechanical properties and irradiation resistance, and is suitable for a solar cell cover plate in a complex space environment; the preparation process is simple and easy to implement, and industrial production can be realized.

Description

Technical Field

[0001] The invention belongs to the technical field of polyimide composite materials and relates to a polyimide composite film for a flexible cover sheet of a space solar cell. Background Art

[0002] As a clean energy conversion device, the efficiency and durability of solar cells directly affect their application scope and lifespan. As a protective layer of solar cells, the cover sheet needs to have high light transmittance, anti-reflection performance, and good mechanical and chemical stability. Although traditional radiation-resistant glass is widely used in space solar cell covers, it has many defects such as large mass, high cost, easy breakage, and complex process. In order to reduce the launch cost, the glass cover sheet is developing in the direction of lightness and thinness, but it is limited by the processing equipment and the brittleness of the glass itself, and 50 microns is close to the manufacturing limit. In addition, during the installation, launch and operation process, the glass cover sheet is prone to breakage due to vibration, posing a serious threat to solar cells. At the same time, the continuous bombardment of space particles will cause the glass transmittance to decrease, the optical performance to degrade, and then reduce the photoelectric conversion efficiency of the battery. Therefore, lightweight, radiation-resistant, high light transmittance and good flexibility materials have gradually become the core direction of solar cell cover sheet research. Although ultra-thin glass has high transparency and excellent thermal properties, its manufacturing difficulty and cost increase significantly as the thickness decreases, limiting its practical application. Polyimide (PI) films are widely used in flexible solar cell devices due to their excellent heat resistance, mechanical strength and chemical stability. However, the optical properties of PI films, such as transmittance and anti-reflection properties, still have room for improvement. Summary of the invention

[0003] In view of the above problems, the present invention aims to provide a method for synthesizing a fluorinated diamine monomer 4,4'-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) (TPFA), and a method for preparing a novel transparent polyimide film, so as to achieve a polyimide film with high transmittance in the visible light region. Subsequently, a method for preparing a polyimide composite film by combining electron beam evaporation vacuum coating and sol-gel immersion coating is provided, so that it has ultra-high transmittance in the visible light region and excellent radiation resistance.

[0004] The technical solution of the present invention is: a polyimide composite film for a flexible cover sheet of a space solar cell according to the present invention, the structure of the polyimide composite film comprises a magnesium fluoride layer, a polyimide base layer and a magnesium fluoride layer, and silicon dioxide layers on both sides of the polyimide base layer;

[0005] Wherein, the structure of the polyimide base layer is shown in the following formula:

[0006]

[0007] Where: 50 <n<1000。

[0008] Furthermore, the preparation steps of the polyimide base layer are as follows:

[0009] Step (1): 2,6-difluoroaniline and 3,4,5-trifluorobenzaldehyde are added to a reaction container, and after passing an inert gas, trifluoromethanesulfonic acid is added dropwise at 60-80° C., the temperature is raised to 140-160° C. and stirred for 18-22 hours, and then cooled to 50-70° C., sodium bicarbonate solution is added for neutralization, and the diamine monomer 4,4'-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) is obtained by extraction and purification;

[0010] Step (2): adding the prepared diamine monomer 4,4'-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) and dianhydride monomer 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride), m-cresol and isoquinoline into a reaction container, introducing an inert gas for a period of time, heating to 110-130° C. to dissolve, and reacting for 3-5 hours until a transparent viscous liquid is obtained;

[0011] Step (3): heating the obtained viscous liquid to 190-205° C. and reacting for 18-24 hours to obtain a light brown transparent polyimide solution; then pouring the obtained light brown transparent polyimide solution into 400-800 ml of methanol, precipitating a light yellow fibrous polyimide solid by suction filtration to remove the methanol;

[0012] Step (4): dissolving the obtained light yellow fibrous polyimide solid in N,N-dimethylacetamide, heating to 55°C and stirring for 3h to obtain a light yellow transparent polyimide solution, pouring the light yellow transparent polyimide solution into 400-800ml methanol, precipitating a white fibrous polyimide solid, filtering with suction, and vacuum drying at 110°C for 24h;

[0013] Step (5): dissolving the purified white fibrous polyimide solid in N,N-dimethylacetamide, stirring at 55°C for 12 hours, casting it into a film on a glass substrate by a casting method, and placing it in deionized water for peeling after the heating program is completed.

[0014] Furthermore, in step (1), the molar ratio of the 2,6-difluoroaniline and 3,4,5-trifluorobenzaldehyde is 2.5:1;

[0015] The molar ratio of trifluoromethanesulfonic acid to 3,4,5-trifluorobenzaldehyde is 0.01-0.02:1;

[0016] The inert gas is nitrogen or argon.

[0017] Furthermore, in step (2), the molar ratio of the dianhydride monomer 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) to the diamine monomer 4,4'-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) is 1:1;

[0018] The amount of isoquinoline added is 3 to 5 drops;

[0019] The solid content of the diamine monomer 4,4'-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) and the dianhydride monomer 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) in m-cresol in the reaction system is 10-15wt%;

[0020] The inert gas is nitrogen or argon;

[0021] The period of time is 10 to 30 minutes.

[0022] Furthermore, in step (6), the solid content of the white fibrous polyimide solid dissolved in N,N-dimethylacetamide is 5-8 wt %.

[0023] Furthermore, in step (6), the temperature rising procedure includes three stages: the first stage is a non-vacuum state, the temperature is 60-80°C, and it is maintained for 5-7 hours; the second stage is a vacuum state, the temperature is 90-110°C, and it is maintained for 3-5 hours; the third stage is a vacuum state, the temperature is 190-210°C, and it is maintained for 11-14 hours.

[0024] Furthermore, the magnesium fluoride layer is obtained by depositing magnesium fluoride on its upper surface by electron beam evaporation vacuum coating; its thickness is 120-150nm.

[0025] Furthermore, the silicon dioxide layer is prepared by a sol-gel method, and the preparation steps are as follows:

[0026] (1): Acidic silica sol: 80-100 g ethanol, 8-10 g methyltriethoxysilane and 0.34-0.36 g hydrochloric acid are added into a reactor, stirred at 25-30°C for 2-4 h, and then placed in an environment at 3-5°C away from light for aging for 7-12 days;

[0027] (2): Alkaline silica sol: 130-150 g ethanol, 16-20 g tetraethyl silicate, 1-3 g ammonia water and 4-5 g deionized water are added into a reactor, stirred at 25-30°C for 2-4 h, placed in a 3-5°C environment away from light for aging for 7-12 days, and placed at 80°C for condensation and reflux for 24-36 h;

[0028] (3): Acid-base composite silica sol: alkaline silica sol and acidic silica sol are mixed in a mass ratio of 10-60:1, and stirred at 25-30°C for 2-4h;

[0029] (4): The prepared polyimide composite film with magnesium fluoride deposited on the surface is placed on top of the acid-base composite silica sol, and a silica layer with a thickness of 100-120 nm is obtained on the upper and lower surfaces of the polyimide composite film with magnesium fluoride deposited on the surface by an immersion pulling method.

[0030] Furthermore, the method for preparing a polyimide composite film for a flexible cover sheet of a space solar cell comprises depositing a magnesium fluoride layer on the upper surface of the prepared polyimide substrate by electron beam evaporation vacuum coating, and then immersing the obtained polyimide composite film with magnesium fluoride deposited on the surface in the prepared silica sol by an immersion pulling method to obtain silica layers on the upper and lower surfaces, and finally obtaining a polyimide composite film having a structure including a magnesium fluoride layer, a polyimide substrate layer and a magnesium fluoride layer, and silica layers on both sides of the polyimide substrate layer.

[0031] The beneficial effects of the present invention are as follows: 1. The polyimide composite film of the present invention has good flexibility and processability. At the same time, since the fluorine-containing electron-withdrawing group introduced into the polyimide composite film can effectively inhibit the formation of charge transfer complexes, the prepared polyimide film itself has a high optical transmittance; 2. The SiO 2 The refractive index of the anti-reflection film can be adjusted by selecting different sol ratios in the acid-base composite sol, and the thickness can be adjusted by the pulling speed during immersion and pulling, which can be applied to the anti-reflection and anti-transmission of polyimide films with different repeating units; 3. The polyimide composite film of the present invention has an ultra-high transmittance in the visible light region, exceeding 97%, and the composite film also has excellent flexibility and heat resistance (can withstand 300°C heat treatment); 4. The polyimide composite film of the present invention has an ultra-high transmittance in a wide range of wavelengths, and is used as a cover film on solar cells to effectively improve the photoelectric conversion efficiency; 5. The polyimide composite film of the present invention has high radiation resistance. The composite film is subjected to an energy of 50KeV and a dose of 2×10 15 e / cm 2 After electron irradiation, it still has a high transmittance in the visible light region, exceeding 95%, while maintaining good heat resistance (can withstand 300°C heat treatment). BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is the H NMR spectrum of the diamine monomer 4,4'-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) (TPFA) obtained in Example 1 of the present invention;

[0033] Figure 2 is the ultraviolet-visible light spectrum of the polyimide film obtained in Example 7 of the present invention;

[0034] Figure 3 The polyimide composite film obtained in Example 7 of the present invention was subjected to an energy of 50 KeV and a dose of 2×10 15 e / cm 2 The UV-visible spectrum of electron irradiated

[0035] Figure 4 It is a schematic structural diagram of the polyimide composite film of the present invention. DETAILED DESCRIPTION

[0036] The specific technical scheme of the present invention is further described in detail below with reference to specific examples.

[0037] The polyimide composite film for a flexible cover sheet of a space solar cell according to the present invention comprises a magnesium fluoride layer (MgF 2 layer), polyimide base layer (PI base layer) and magnesium fluoride layer (MgF 2 layer), silicon dioxide layer (SiO 2 );

[0038] Among them, the structure of the PI base layer is:

[0039]

[0040] Of which: 50 <n<1000。

[0041] The composite film structure comprises, from top to bottom, a silicon dioxide layer, a magnesium fluoride layer, a polyimide film substrate, and a silicon dioxide layer, wherein the silicon dioxide layer is prepared by a sol-gel method with a thickness of 100-120 nm; the magnesium fluoride layer is prepared by a magnetron sputtering method with a thickness of 120-150 nm.

[0042] The present invention also provides a method for preparing the above-mentioned polyimide film substrate, comprising the following preparation steps:

[0043] (1) 2,6-difluoroaniline and 3,4,5-trifluorobenzaldehyde are added to a reaction container, and after passing an inert gas, trifluoromethanesulfonic acid is added dropwise at 60-80° C., the temperature is raised to 140-160° C. and stirred for 18-22 hours, and then cooled to 50-70° C., sodium bicarbonate solution (5 wt %) is added for neutralization, and the diamine monomer 4,4′-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) (TPFA) is obtained by extraction and purification.

[0044] (2) adding a diamine monomer 4,4'-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) (TPFA) and a dianhydride monomer 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) (BPADA) as well as m-cresol and a small amount of isoquinoline into a reaction vessel, introducing an inert gas for a period of time, and then heating to 110-130° C. to dissolve, and reacting for 3-5 hours until the mixture becomes transparent and viscous;

[0045] (3) heating the viscous liquid obtained in step (2) to 190-205° C. and reacting for 18-24 hours to obtain a light brown transparent polyimide solution;

[0046] (4) Pour the light brown transparent polyimide solution obtained in step (3) into 400-800 ml of vigorously stirred methanol to precipitate a light yellow fibrous polyimide solid, which is filtered to remove the methanol.

[0047] (5) The light yellow fibrous polyimide solid obtained in step (4) is dissolved in N,N-dimethylacetamide (DMAc), and the temperature is raised to 55° C. and stirred for 3 h. The light yellow transparent polyimide solution is then poured into 400-800 ml of vigorously stirred methanol described in step (3) to precipitate a white fibrous polyimide solid, which is then filtered and dried in vacuo at 110° C. for 24 h.

[0048] (6) The purified white fibrous polyimide solid obtained in step (5) was dissolved in N,N-dimethylacetamide (DMAc), stirred at 55° C. for 12 h, cast into a film on a glass substrate by a casting method, and placed in deionized water for peeling after the heating program was completed.

[0049] Preferably, in the above method for preparing a polyimide film substrate, the solid content of the reaction system is 10-15 wt %.

[0050] Preferably, in the above-mentioned film casting process, the solid content of the white fibrous polyimide solid dissolved in the organic solvent is 5-8 wt %.

[0051] Preferably, in the above-mentioned heating program, it is divided into three stages. The first stage is a non-vacuum state with a temperature of 60-80°C and maintained for 5-7 hours. The second stage is a vacuum state with a temperature of 90-110°C and maintained for 3-5 hours. The third stage is a vacuum state with a temperature of 190-210°C and maintained for 11-14 hours.

[0052] In the method for preparing the polyimide film substrate of the present invention, the molar ratio of the dianhydride monomer to the diamine monomer is 1:1.

[0053] The preparation method of the diamine monomer 4,4'-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) (TPFA) of the present invention is characterized in that the molar ratio of 2,6-difluoroaniline to 3,4,5-trifluorobenzaldehyde used is 2.5:1, and the molar ratio of trifluoromethanesulfonic acid to 3,4,5-trifluorobenzaldehyde used is 0.01-0.02:1.

[0054] In the preparation method of the polyimide film substrate of the present invention, the molar ratio of the dianhydride monomer 4,4'-(4,4'-isopropyldiphenyloxy)bis(phthalic anhydride) (BPADA) to the diamine monomer 4,4'-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) (TPFA) is 1:1.

[0055] The present invention also provides SiO 2 The method for preparing the layer comprises the following specific steps:

[0056] (1) Acidic silica sol: 80-100 g of ethanol, 8-10 g of methyltriethoxysilane (MTES) and 0.34-0.36 g of hydrochloric acid (0.01 mol / L) were added into a reactor, stirred at 25-30°C for 2-4 h, and then placed in an environment at 3-5°C and away from light for aging for 7-12 days.

[0057] (2) Alkaline silica sol: 130-150 g of ethanol, 16-20 g of tetraethyl silicate, 1-3 g of ammonia water (28-30 wt%) and 4-5 g of deionized water are added into a reactor, stirred at 25-30°C for 2-4 h, placed in a light-proof environment at 3-5°C for aging for 7-12 days, and placed at 80°C for condensation and reflux for 24-36 h.

[0058] (3) Acid-base composite silica sol: alkaline silica sol and acidic silica sol are mixed in a mass ratio of 10-60:1 and stirred at 25-30° C. for 2-4 hours.

[0059] (4): The prepared polyimide composite film with magnesium fluoride deposited on the surface is placed on top of the acid-base composite silica sol, and a silica layer is obtained on the upper and lower surfaces of the polyimide composite film with magnesium fluoride deposited on the surface by an immersion pulling method.

[0060] As a preference, the SiO2 coated on both sides of the polyimide film substrate 2 The anti-reflection film is prepared by an immersion pulling method with a pulling speed of 1000-9000μm / s.

[0061] Preferably, the mass ratio of the alkaline silica sol to the acidic silica sol is 10-60:1.

[0062] Specifically, the preparation method of the diamine monomer 4,4'-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) (TPFA) is as follows:

[0063] Example 1

[0064] (1) Add 2,6-difluoroaniline (25.8 g, 0.2 mol) and 3,4,5-trifluorobenzaldehyde (12.5 g, 0.078 mol) into a 100 ml three-necked flask, heat to 70 °C under nitrogen protection, and continue stirring for 30 min; then, slowly add 4 mL of trifluoromethanesulfonic acid dropwise into the three-necked flask within 1 h, stir evenly, and then heat the reaction system to 150 °C and continue stirring for 20 h;

[0065] (2) When the reaction solution of step (1) is cooled to 60°C, dichloromethane is slowly added dropwise for extraction, and sodium bicarbonate solution (5%, 300 ml) is added for stirring and neutralization. The organic phase is washed with deionized water, and the mixture is repeated three times. The organic phase is dried with anhydrous sodium sulfate to remove water. After standing for a period of time, the organic phase is filtered out and eluted with a mixture of ethyl acetate and petroleum ether (1:10, V / V).

[0066] The agent was purified by silica gel column chromatography to obtain a light yellow solid, which was then recrystallized from ethanol to obtain 24.8 g of a colorless transparent solid.

[0067] Example 2

[0068] (1) Add 2,6-difluoroaniline (25.8, 0.2 mol) and 3,4,5-trifluorobenzaldehyde (12.5 g, 0.078 mol) into a 100 ml three-necked flask, heat to 70°C under nitrogen protection, and continue stirring for 30 min; then, slowly add hydrochloric acid (37 wt%, 4 ml) dropwise into the flask within 1 h, stir evenly, and then heat the reaction system to 155°C and continue stirring for 24 h;

[0069] (2) When the reaction solution of step (1) is cooled to 60°C, dichloromethane is slowly added dropwise to extract, and aqueous ammonia (28-30wt%, 10ml) is added dropwise to stir and neutralize, and the organic phase is washed with deionized water, and the washing is repeated twice and then dried with anhydrous magnesium sulfate to remove water; after standing for a period of time, the organic phase is filtered out and purified by chromatography on a silica gel column using a mixed eluent of ethyl acetate and petroleum ether (1:20, V / V) to obtain a light yellow solid; then, the product is recrystallized in ethanol to obtain 7.23 g of a colorless transparent solid.

[0070] Example 3

[0071] The preparation process of the polyimide film substrate in this embodiment is as follows:

[0072]

[0073] (1) Under the protection of flowing nitrogen, TPFA (0.8002 g, 2 mmol), BPADA (1.0410 g, 2 mmol) and 14.4 ml of m-cresol were poured into a 50 ml three-necked flask, mechanically stirred and condensed under reflux. After the system was heated to dissolve, 3 drops of isoquinoline were added dropwise, and the temperature was raised to 120°C and maintained for 3 h, and then raised to 190°C and maintained for 20 h;

[0074] (2) The viscous liquid of step (1) was cooled to room temperature and poured into hot methanol with vigorous stirring to precipitate a white fibrous polyimide solid. Subsequently, the PI fibrous solid was placed in a forced air oven at 80° C. and dried overnight;

[0075] (3) Dissolve the white fibrous polyimide solid in step (2) using DMAc, heat to 55°C, stir for 3 hours, pour into hot methanol again and reprecipitate for washing, repeat this step twice; finally, place the PI solid in a vacuum oven and dry in a vacuum environment at 110°C for 24 hours;

[0076] (4) Dissolve polyimide in DMAc (solid content of 6 wt%) and stir at 55°C to obtain a uniform solution, and use a 0.7 μm organic filter to filter out impurities; then cast it into a film on a clean glass substrate by a casting method, place it in a vacuum oven at 70°C (6 h), and then evacuate to 70°C (1 h), 100°C (4 h), and 200°C (12 h). After the temperature drops below 60°C, place the glass substrate in deionized water and wait for natural peeling to obtain a polyimide film.

[0077] Example 4

[0078] (1) Under the protection of flowing nitrogen, DTFA (0.8002 g, 2 mmol), 6FDA (1.0410 g, 2 mmol) and 20.5 ml of m-cresol were poured into a 50 ml three-necked flask, mechanically stirred and condensed under reflux. After the system was heated to dissolve, 3 drops of isoquinoline were added dropwise, and the temperature was raised to 120°C and maintained for 3 h, and then raised to 190°C and maintained for 20 h;

[0079] (2) The viscous liquid of step (1) was cooled to room temperature and poured into hot methanol with vigorous stirring to precipitate a white fibrous polyimide solid. Subsequently, the PI fibrous solid was placed in a forced air oven at 80° C. and dried overnight;

[0080] (3) Dissolve the white fibrous polyimide solid in step (2) with DMAc, heat to 55°C, stir for 3 hours, pour into hot methanol again and reprecipitate for washing, repeat this step twice. Finally, place the PI solid in a vacuum oven and dry it in a vacuum environment at 110°C for 24 hours;

[0081] (4) Dissolve polyimide in DMAc (solid content of 6 wt%) and stir at 55°C to obtain a uniform solution, and use a 1.2 μm organic filter to filter out impurities; then cast it into a film on a clean glass substrate by a casting method, place it in a vacuum oven at 70°C (6 h), and then evacuate to 70°C (1 h), 100°C (4 h), and 200°C (12 h). After the temperature drops below 60°C, place the glass substrate in deionized water and wait for natural peeling to obtain a polyimide film.

[0082] Example 5

[0083] (1) The polyimide film obtained in Example 3 was washed with ethanol and deionized water, and then placed in a vacuum drying oven at 100° C. for 8 h; the film was fixed on a glass substrate in a clean room, and a magnesium fluoride layer was deposited on the surface of the polyimide by an electron beam evaporation coating machine for later use;

[0084] (2) Preparation of alkaline silica sol: weigh 138 g of ethanol, 17 g of tetraethyl silicate and 0.9 g of ammonia water (28-30 wt%) into a 250 ml round-bottom flask and stir vigorously at room temperature for 3 h; then, seal and protect from light in a refrigerator for aging for 7 days; place under condensation reflux at 80° C. for 24 h; after the sol is cooled, filter the sol with a 0.7 μm glass fiber filter for later use;

[0085] (3) placing the film obtained in step (1) on top of the sol, and using an immersion and pulling machine for immersion and pulling, setting the descending speed to 5000 μm / s and the pulling speed to 1000 μm / s;

[0086] (4) The film obtained in step (3) was suspended in the air and allowed to stand for 10 minutes.

[0087] Example 6

[0088] (1) The polyimide film obtained in Example 3 was washed with ethanol and deionized water, and then placed in a vacuum drying oven at 100° C. for 8 h; the film was fixed on a glass substrate in a clean room, and a magnesium fluoride layer was deposited on the surface of the polyimide by an electron beam evaporation coating machine for later use;

[0089] (2) Preparation of acidic silica sol: weigh 81 g of ethanol, 8.4 g of methyltriethoxysilane (MTES) and 0.34 g of hydrochloric acid (0.01 mol / L) into a 250 ml round-bottom flask, and stir vigorously at room temperature for 3 h; then, seal and protect from light in a refrigerator for aging for 7 days;

[0090] (3) Preparation of alkaline silica sol: weigh 138 g of ethanol, 17 g of tetraethyl silicate and 0.9 g of ammonia water (28-30 wt%) into a 250 ml round-bottom flask and stir vigorously at room temperature for 3 h; then, seal and protect from light in a refrigerator for aging for 7 days; place under condensation reflux at 80° C. for 24 h; after the sol is cooled, filter the sol using a 0.7 μm glass fiber filter and set aside;

[0091] (4) Preparation of acid-base composite silica sol: weigh the aged alkaline silica sol and acidic silica sol into a 250 ml beaker, mix them in a mass ratio of 60:1, and stir at room temperature for 2-4 hours;

[0092] (5) placing the thin film substrate obtained in step (1) on the sol, and performing immersion pulling using an immersion pulling machine, setting the descending speed to 5000 μm / s and the pulling speed to 1000 μm / s;

[0093] (6) The film obtained in step (4) was suspended in the air and allowed to stand for 10 minutes.

[0094] Example 7

[0095] (1) The polyimide film obtained in Example 3 was washed with ethanol and deionized water, and then placed in a vacuum drying oven at 100° C. for 8 h; the film was fixed on a glass substrate in a clean room, and a magnesium fluoride layer was deposited on the surface of the polyimide by an electron beam evaporation coating machine for later use;

[0096] (2) Preparation of acidic silica sol: weigh 81 g of ethanol, 8.4 g of methyltriethoxysilane (MTES) and 0.34 g of hydrochloric acid (0.01 mol / L) into a 250 ml round-bottom flask, and stir vigorously at room temperature for 3 h; then, seal and protect from light in a refrigerator for aging for 7 days;

[0097] (3) Preparation of alkaline silica sol: weigh 138 g of ethanol, 17 g of tetraethyl silicate and 0.9 g of ammonia water (28-30 wt%) into a 250 ml round-bottom flask and stir vigorously at room temperature for 3 h; then, seal and protect from light in a refrigerator for aging for 7 days; place under condensation reflux at 80° C. for 24 h; after the sol is cooled, filter the sol using a 0.7 μm glass fiber filter and set aside;

[0098] (4) Preparation of acid-base composite silica sol: weigh the aged alkaline silica sol and acidic silica sol into a 250 ml beaker, mix them in a mass ratio of 30:1, and stir at room temperature for 2-4 hours;

[0099] (5) placing the thin film substrate obtained in step (1) on the sol, and performing immersion pulling using an immersion pulling machine, setting the descending speed to 5000 μm / s and the pulling speed to 1000 μm / s;

[0100] (6) The film obtained in step (4) was suspended in the air and allowed to stand for 10 minutes.

[0101] Example 8

[0102] (1) The polyimide film obtained in Example 3 was washed with ethanol and deionized water, and then placed in a vacuum drying oven at 100° C. for 8 h; the film was fixed on a glass substrate in a clean room, and a magnesium fluoride layer was deposited on the surface of the polyimide by an electron beam evaporation coating machine for later use;

[0103] (2) Preparation of acidic silica sol: weigh 81 g of ethanol, 8.4 g of methyltriethoxysilane (MTES) and 0.34 g of hydrochloric acid (0.01 mol / L) into a 250 ml round-bottom flask, and stir vigorously at room temperature for 3 h; then, seal and protect from light in a refrigerator for aging for 7 days;

[0104] (3) Preparation of alkaline silica sol: weigh 138 g of ethanol, 17 g of tetraethyl silicate and 0.9 g of ammonia water (28-30 wt%) into a 250 ml round-bottom flask and stir vigorously at room temperature for 3 h; then, seal and protect from light in a refrigerator for aging for 7 days; place under condensation reflux at 80° C. for 24 h; after the sol is cooled, filter the sol using a 0.7 μm glass fiber filter and set aside;

[0105] (4) Preparation of acid-base composite silica sol: weigh the aged alkaline silica sol and acidic silica sol into a 250 ml beaker, mix them in a mass ratio of 20:1, and stir at room temperature for 2-4 hours;

[0106] (5) placing the thin film substrate obtained in step (1) on the sol, and performing immersion pulling using an immersion pulling machine, setting the descending speed to 5000 μm / s and the pulling speed to 1000 μm / s;

[0107] (6) The film obtained in step (4) was suspended in the air and allowed to stand for 10 minutes.

[0108] Example 9

[0109] (1) The polyimide film obtained in Example 3 was washed with ethanol and deionized water, and then placed in a vacuum drying oven at 100° C. for 8 h; the film was fixed on a glass substrate in a clean room, and a magnesium fluoride layer was deposited on the surface of the polyimide by an electron beam evaporation coating machine for later use;

[0110] (2) Preparation of acidic silica sol: weigh 81 g of ethanol, 8.4 g of methyltriethoxysilane (MTES) and 0.34 g of hydrochloric acid (0.01 mol / L) into a 250 ml round-bottom flask, and stir vigorously at room temperature for 3 h; then, seal and protect from light in a refrigerator for aging for 7 days;

[0111] (3) Preparation of alkaline silica sol: weigh 138 g of ethanol, 17 g of tetraethyl silicate and 0.9 g of ammonia water (28-30 wt%) into a 250 ml round-bottom flask and stir vigorously at room temperature for 3 h; then, seal and protect from light in a refrigerator for aging for 7 days; place under condensation reflux at 80° C. for 24 h; after the sol is cooled, filter the sol using a 0.7 μm glass fiber filter and set aside;

[0112] (4) Preparation of acid-base composite silica sol: weigh the aged alkaline silica sol and acidic silica sol into a 250 ml beaker, mix them in a mass ratio of 10:1, and stir at room temperature for 2-4 hours;

[0113] (5) Place the thin film substrate obtained in step (1) on the sol and use an immersion pulling machine for immersion pulling. Set the descending speed to 5000 μm / s and the pulling speed to 1000 μm / s.

[0114] (6) The film obtained in step (4) was suspended in the air and allowed to stand for 10 minutes.

Claims

1. A polyimide composite film for a flexible cover sheet of a space solar cell, characterized in that: The structure of the polyimide composite film includes a magnesium fluoride layer, a polyimide base layer, a magnesium fluoride layer, and silicon dioxide layers on both sides of the polyimide base layer; Wherein, the structure of the polyimide base layer is shown in the following formula: Where: 50 <n<1000。 2. The polyimide composite film for a flexible cover sheet of a space solar cell according to claim 1, characterized in that: The preparation steps of the polyimide base layer are as follows: Step (1): 2,6-difluoroaniline and 3,4,5-trifluorobenzaldehyde are added to a reaction container, and after passing an inert gas, trifluoromethanesulfonic acid is added dropwise at 60-80° C., the temperature is raised to 140-160° C. and stirred for 18-22 hours, and then cooled to 50-70° C., sodium bicarbonate solution is added for neutralization, and the diamine monomer 4,4'-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) is obtained by extraction and purification; Step (2): adding the prepared diamine monomer 4,4'-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) and dianhydride monomer 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride), m-cresol and isoquinoline into a reaction container, introducing an inert gas for a period of time, heating to 110-130° C. to dissolve, and reacting for 3-5 hours until a transparent viscous liquid is obtained; Step (3): heating the obtained viscous liquid to 190-205° C. and reacting for 18-24 hours to obtain a light brown transparent polyimide solution; then pouring the obtained light brown transparent polyimide solution into 400-800 ml of methanol, precipitating a light yellow fibrous polyimide solid by suction filtration to remove the methanol; Step (4): dissolving the obtained light yellow fibrous polyimide solid in N,N-dimethylacetamide, heating to 55°C and stirring for 3h to obtain a light yellow transparent polyimide solution, pouring the light yellow transparent polyimide solution into 400-800ml methanol, precipitating a white fibrous polyimide solid, filtering with suction, and vacuum drying at 110°C for 24h; Step (5): dissolving the purified white fibrous polyimide solid in N,N-dimethylacetamide, stirring at 55°C for 12 hours, casting it into a film on a glass substrate by a casting method, and placing it in deionized water for peeling after the heating program is completed.

3. The polyimide composite film for a flexible cover sheet of a space solar cell according to claim 2, characterized in that: In step (1), the molar ratio of 2,6-difluoroaniline to 3,4,5-trifluorobenzaldehyde is 2.5:1; The molar ratio of trifluoromethanesulfonic acid to 3,4,5-trifluorobenzaldehyde is 0.01-0.02:1; The inert gas is nitrogen or argon.

4. The polyimide composite film for a flexible cover sheet of a space solar cell according to claim 2, characterized in that: In step (2), the molar ratio of the dianhydride monomer 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) to the diamine monomer 4,4'-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) is 1:1; The amount of isoquinoline added is 3 to 5 drops; The solid content of the diamine monomer 4,4'-((3,4,5-(trifluoromethyl))phenyl)methylene)bis(2,6-difluoroaniline) and the dianhydride monomer 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) in m-cresol in the reaction system is 10-15wt%; The inert gas is nitrogen or argon; The period of time is 10 to 30 minutes.

5. The polyimide composite film for a flexible cover sheet of a space solar cell according to claim 2, characterized in that: In step (6), the solid content of the white fibrous polyimide solid dissolved in N,N-dimethylacetamide is 5-8 wt %.

6. The polyimide composite film for a flexible cover sheet of a space solar cell according to claim 2, characterized in that: In step (6), the temperature rising procedure includes three stages: the first stage is a non-vacuum state, the temperature is 60-80°C, and it is maintained for 5-7 hours; the second stage is a vacuum state, the temperature is 90-110°C, and it is maintained for 3-5 hours; the third stage is a vacuum state, the temperature is 190-210°C, and it is maintained for 11-14 hours.

7. A polyimide composite film for a flexible cover sheet of a space solar cell according to claim 1, characterized in that: The magnesium fluoride layer is prepared by depositing magnesium fluoride on its upper surface through electron beam evaporation vacuum coating.

8. The polyimide composite film for a flexible cover sheet of a space solar cell according to claim 1, characterized in that: The thickness of the magnesium fluoride layer obtained is 120-150 nm.

9. A polyimide composite film for a flexible cover sheet of a space solar cell according to claim 1, characterized in that: The silicon dioxide layer is prepared by a sol-gel method, and the preparation steps are as follows: (1): Acidic silica sol: 80-100 g ethanol, 8-10 g methyltriethoxysilane and 0.34-0.36 g hydrochloric acid are added into a reactor, stirred at 25-30°C for 2-4 h, and then placed in an environment at 3-5°C away from light for aging for 7-12 days; (2): Alkaline silica sol: 130-150 g ethanol, 16-20 g tetraethyl silicate, 1-3 g ammonia water and 4-5 g deionized water are added into a reactor, stirred at 25-30°C for 2-4 h, placed in a 3-5°C environment away from light for aging for 7-12 days, and placed at 80°C for condensation and reflux for 24-36 h; (3): Acid-base composite silica sol: alkaline silica sol and acidic silica sol are mixed in a mass ratio of 10-60:1, and stirred at 25-30°C for 2-4h; (4): The prepared polyimide composite film with magnesium fluoride deposited on the surface is placed on top of the acid-base composite silica sol, and a silica layer with a thickness of 100-120 nm is obtained on the upper and lower surfaces of the polyimide composite film with magnesium fluoride deposited on the surface by an immersion pulling method.

10. A method for preparing a polyimide composite film for a flexible cover sheet of a space solar cell according to any one of claims 1 to 9, characterized in that: The prepared polyimide substrate is subjected to electron beam evaporation vacuum coating to deposit a magnesium fluoride layer on its upper surface, and then the polyimide composite film with magnesium fluoride deposited on the surface is immersed in the prepared silica sol by an immersion pulling method to obtain silica layers on its upper and lower surfaces, and finally a polyimide composite film having a structure including a magnesium fluoride layer, a polyimide substrate layer and a magnesium fluoride layer, and silica layers on both sides of the polyimide substrate layer is obtained.

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