CN115011315B

CN115011315B - Preparation and application of flexible electronic biomass/polyimide-based heat dissipation material film

Info

Publication number: CN115011315B
Application number: CN202210667998.5A
Authority: CN
Inventors: 陶劲松; 贾冬梅; 刘浩
Original assignee: South China University of Technology SCUT
Current assignee: South China University of Technology SCUT
Priority date: 2022-06-14
Filing date: 2022-06-14
Publication date: 2023-09-26
Anticipated expiration: 2042-06-14
Also published as: CN115011315A

Abstract

The invention belongs to the field of flexible electronic products, and discloses preparation and application of a flexible electronic biomass/polyimide-based heat dissipation material film with high heat dissipation. The invention selects lignin and polyimide to prepare biomass-based graphite material. The high carbon content of lignin and good dispersibility of lignin in an organic solvent are utilized, the lignin is added into a polyimide acid solution polymerized by dianhydride and diamine monomers, the lignin is crosslinked with the polyimide acid in a high-temperature imidization stage to reconstruct a substrate, and then the biomass graphite heat dissipation film is obtained through carbonization and graphitization treatment. The graphite film exhibits excellent heat dissipation performance (thermal conductivity 691.322 W.m) ^‑1 ·℃ ^‑1 ) Good flexibility, and a low density (1.65 g/cm ³ ) And the lignin loading is high (up to 30 wt%) and the cost of the composite graphite film can be reduced.

Description

Preparation and application of flexible electronic biomass/polyimide-based heat dissipation material film

Technical Field

The invention belongs to the field of flexible electronics (Flexible Electronics, FE), and particularly relates to preparation and application of a flexible electronic biomass/polyimide-based heat dissipation material film with high heat dissipation.

Background

Heat sink materials have been widely used in the consumer electronics and electronics packaging fields for decades to ensure good operation of the device. For electronic equipment, the heat dissipation film can cool high-power electronic devices rapidly, so that the devices can be operated at high speed and high efficiency, and long-term reliability is ensured. However, with the rapid decrease in size of electronic devices, the integration density and power density are rapidly increasing, and heat dissipation has become a key issue that hinders device performance and reliability, especially the high power density of current 5G consumer electronic devices presents a greater challenge to the heat dissipation capability of the devices. Secondly, the rise of intelligent and flexible devices brings new challenges to the development of flexible heat sink materials. In addition, the high price of graphene and graphite heat dissipation films also makes the graphene and graphite heat dissipation films only applicable to the field of high-end electronics, and the graphene and graphite heat dissipation films are difficult to apply to middle-end and low-end electronic products. Therefore, developing a flexible lightweight heat sink material with high heat dissipation properties and low cost has become a primary task in the consumer electronics field.

Composite graphite heat dissipation films are the most common heat dissipation materials for electronic devices, and have been widely studied for their excellent heat conduction properties, flexibility, deformability, light weight, removability, and considerable price. Composite graphite heat dissipation films are generally divided into two categories: 1) Directly compounding epoxy resin or alloy serving as a filler with a graphite sheet to obtain a graphite composite heat-dissipating material; 2) And adding a filler in the stage of polycondensation of dianhydride and diamine monomers to prepare the polyimide-based composite graphite material, and carbonizing and graphitizing to prepare the graphite-based composite heat dissipation material. The first type of composite graphite heat dissipation film is generally prepared by coating resin or metal on the surface of graphite, and impregnating the resin or metal into a graphite sheet layer by a hot press curing molding method. However, these materials have a common disadvantage in that the orientation of the materials in the hot pressing process is not controlled, so that the heat dissipation performance of the in-plane texture of the composite material is greatly different, and therefore, the materials are difficult to be widely applied. The heat dissipation defect of the graphite composite material is difficult to solve, and the requirement of uniform heat dissipation of the most basic heat dissipation material cannot be met. Therefore, researchers focus on turning to a second type of composite graphite heat dissipation film: polyimide-based composite graphite film. Polyimide-based composite graphite films are a hot point of research in recent years of flexible electronic heat dissipation materials, and are generally prepared by adding a high-carbon material or a catalyst into a polyimide monomer polymerization stage or a polyimide acid solution, imidizing to obtain a polyimide-based composite material, and carbonizing and graphitizing. Common high carbon fillers include: graphene oxide, carbon nitride, carbon fiber, and the like. The graphene oxide is used as a filler, and the prepared graphite-based composite heat dissipation film has excellent heat conductivity coefficient, low thermal expansion rate and isotropic heat dissipation performance in the in-plane and vertical directions, but the dispersibility of the graphene oxide in polyimide acid is poor, so that the mechanical performance of the graphite film is poor. Therefore, the common pretreatment method is to carry out carboxylation and amination modification on graphene oxide so as to improve the dispersibility of the graphene oxide, but the problem of poor flexibility of the graphite film is still difficult to solve. And the expensive price of the graphene oxide also limits the production and application of the graphene oxide/polyimide graphite film. Secondly, silicon carbide, carbon nanotubes and carbon nitride are also often used as fillers to enhance the graphitization of polyimide, but nanoparticles are extremely easy to agglomerate and often require silanization modification. In addition, the metal particles and pyridine have induction and catalysis effects on polyimide graphitization, so that the graphitization process can be accelerated, and energy consumption required by high-temperature firing is saved, but the catalysis effect cannot improve heat dissipation performance or flexibility. Therefore, despite the indistinct advantages of polyimide-based graphite heat dissipation films, there are inevitably drawbacks of inherent poor mechanical flexibility, high production cost, and the like. Therefore, how to prepare polyimide-based composite graphite films for high performance flexible electronics, including good heat dissipation factor, good flexibility, and lower cost, remains a challenge.

Disclosure of Invention

In order to overcome the defects and shortcomings of the prior art, the primary purpose of the invention is to provide a preparation method of a flexible electronic biomass/polyimide-based heat dissipation material film.

The invention further aims to provide the flexible electronic biomass/polyimide-based heat dissipation material film prepared by the method.

The invention also provides application of the flexible electronic biomass/polyimide-based heat dissipation material film in heat dissipation of a computer Central Processing Unit (CPU).

The aim of the invention is achieved by the following scheme:

a preparation method of a flexible electronic biomass/polyimide-based heat dissipation material film with high heat dissipation comprises the following steps:

(1) Preparation of polyimide acid (PAA) solution: adding aprotic polar solvent into a reaction container, placing the reaction container in an environment of 6-8 ℃, adding diamine monomer into the reaction container, stirring and dissolving, adding dried dianhydride monomer into the reaction container under the protection of nitrogen after the diamine monomer is completely dissolved, stirring and reacting, pouring out the generated polyimide acid (PAA) solution after the reaction is finished, filling the solution into a brown glass bottle, and preserving at a low temperature (4 ℃) for later use;

(2) Preparation of lignin/polyimide (LA/PI) composite membrane: adding lignin (dealkalized) into a PAA solution to prepare lignin/polyimide (LA/PAA) mixed solution, coating the mixed solution on a high-temperature-resistant glass sheet by a tape casting method, placing the glass sheet in a vacuum drying oven, heating in a gradient way, and imidizing polyimide acid heat to obtain a lignin/polyimide (LA/PI) composite film;

(3) Preparation of lignin/polyimide (LA/PI) graphite film: fixing the composite film obtained in the step (2) by using a graphite clamping piece, introducing nitrogen into a continuous carbonization furnace to protect and heat up for carbonization, and obtaining a carbonized film; transferring the carbonized film into a horizontal graphitizing furnace, introducing argon and heating to graphitize to obtain the graphite film.

The aprotic polar solvent in the step (1) is at least one of DMAC (N, N-dimethylacetamide), DMF (N, N-dimethylformamide), DMSO (dimethyl sulfoxide) and NMP (N-methyl-2-pyrrolidone);

the diamine monomer in the step (1) is at least one of 2, 2-bis (3-amino-4-hydroxyphenyl) hexafluoropropane (6 FAP), 4 '-diamino-2, 2' -bistrifluoromethyl biphenyl, 2 '-bis (trifluoromethyl) -4,4' -diaminophenyl ether, 2-bis [4- (4-aminophenoxy) phenyl ] propane, 1, 3-bis (4-aminophenoxy) benzene and the like, preferably 2, 2-bis (3-amino-4-hydroxyphenyl) hexafluoropropane (6 FAP); the dianhydride monomer is at least one of 1,2,4, 5-benzene tetracarboxylic acid dianhydride (PMDA), cyclobutane tetracarboxylic acid dianhydride, hexafluorodianhydride, 3', 4' -biphenyl tetracarboxylic acid dianhydride, 4' -biphenyl ether dianhydride, bisphenol A type diether dianhydride and the like, and preferably 1,2,4, 5-benzene tetracarboxylic acid dianhydride (PMDA).

The diamine monomer and dianhydride monomer used in the step (1) are as follows: the molar ratio of diamine monomer to dianhydride monomer is 1:1-1.015; the amount of the aprotic polar solvent satisfies the mass fraction of the obtained polyimide acid solution of 75-90%.

The dried dianhydride monomer in the step (1) refers to dianhydride monomer after being dried in vacuum at 150 ℃ for 10 hours; the dianhydride after drying is preferably added in portions so as to be more rapidly dissolved in the aprotic polar solvent;

the stirring in the stirring reaction in the step (1) is only for sufficient contact between the raw materials, and thus the stirring speed is not necessarily limited; the stirring reaction time in the step (1) is 10-12h; preferably, after 2 hours of reaction, the nitrogen feed may be stopped and the reaction continued for 8-10 hours.

The lignin dosage in the step (2) satisfies that the lignin mass content in the lignin/polyimide (LA/PI) composite membrane is 0-30wt% and is not 0;

preferably, in order to uniformly disperse lignin in the polyimide acid solution, after lignin (dealkalized) is added to the PAA solution, bubbles in the mixed solution are removed by ultrasonic for 30-60min with an ultrasonic cleaner.

The gradient heating in the vacuum drying oven in the step (2) means that the temperature is kept at 80 ℃ for 2h,120 ℃ for 1h,150 ℃ for 1h,200 ℃ for 1h and 235 ℃ for 1h.

The carbonization in the step (3) is to heat up to 1000-1300 ℃ and preserve heat for 3-5h; graphitization means heating to 2800-3000 ℃ and preserving heat for 6 hours.

The flexible electronic biomass/polyimide-based heat dissipation material film with high heat dissipation, namely the lignin/polyimide (LA/PI) graphite film, is prepared by the method.

The flexible electronic biomass/polyimide-based heat dissipation material film (lignin/polyimide graphite film) with high heat dissipation is applied to the heat dissipation of electronic elements, in particular to the application of the flexible electronic biomass/polyimide-based heat dissipation material film serving as a heat dissipation material of a CPU (central processing unit) of a computer.

The mechanism of the invention is as follows:

the flexible electronic heat dissipation material is required to be light in weight, and has high heat dissipation, high flexibility and low cost. The current polyimide-based graphite film has difficulty in achieving both high performance and low cost. Lignin has the characteristics of nature, abundance, low price, high carbon content, good organic solvent dispersibility and the like. The invention selects lignin and polyimide to prepare the flexible electronic heat dissipation film. Polyimide films have a great influence on the performance of composite films and subsequent graphite films, however, the types of polyimide monomers and the preparation method have a direct influence on the prepared composite films. According to the invention, PMDA (1, 2,4, 5-benzene tetracarboxylic dianhydride) and 6FAP (2, 2-bis (3-amino-4-hydroxyphenyl) hexafluoropropane) are selected to prepare a polyimide film with high strength and easy stripping from a glass sheet through low-temperature polycondensation, and pulping and cooking byproducts lignin (dealkalization) are selected. The polyimide is used as a framework to provide mechanical strength of the material, the polyimide framework is filled with lignin, excellent heat dissipation performance and good flexibility are obtained, and finally, a novel light flexible electronic heat dissipation material with high heat dissipation, high flexibility and low cost is hopeful to be obtained.

Compared with the prior art, the invention has the following advantages:

(1) According to the invention, lignin is added into polyimide acid solution polymerized by dianhydride and diamine monomers, and is crosslinked with polyimide acid at a high-temperature imidization stage to reconstruct a substrate, and then carbonization and graphitization treatment are carried out to obtain the biomass graphite heat dissipation film.

(2) Firstly, lignin and polyimide acid solution show excellent compatibility, and secondly, the high carbon content and typical pi-pi conjugated structure of lignin are also beneficial to further graphitization and filling up the defects, so that the heat conductivity, flexibility and integrity of the heat dissipation film are improved.

(3) The biomass-based graphite film shows excellent heat dissipation performance (heat conductivity coefficient 691.322W.m) ^-1 ·℃ ^-1 ) Good flexibility, and a low density (1.65 g/cm ³ ) And the lignin loading is high (up to 30 wt%) and the cost of the composite graphite film can be reduced.

Drawings

FIG. 1 is a flow chart of a process for preparing PAA from dianhydride monomer and diamine monomer and compounding with LA to obtain a heat sink material that can be used in a computer CPU.

FIG. 2 is a high-definition physical photograph of the pure PI and the LA/PI composite films with different lignin addition contents prepared in example 1.

FIG. 3 is a FT-IR spectrum of a LA/PI composite membrane having a lignin addition of 10wt% and pure PI prepared in example 1.

FIG. 4 shows the thermogravimetric and slightly commercial thermogravimetric values of the LA/PI composite membrane with an addition of 10wt% of pure PI and lignin prepared in example 1.

FIG. 5 shows stress strain curves and elastic moduli of the LA/PI composite films prepared in example 1 and having different lignin addition levels.

FIG. 6 is a sectional scanning electron micrograph of a composite membrane, a carbonized membrane, and a graphite membrane of a LA/PI composite membrane with pure PI and a lignin addition of 10 wt%.

FIG. 7 is an XRD spectrum of a composite LA/PI film with 10wt% of pure PI and lignin added, and a commercial graphite heat-dissipating film and a composite LA/PI film with 10wt% of lignin added, prepared in example 1.

FIG. 8 is a Raman spectrum of the LA/PI graphite film prepared in example 1 with a lignin addition of 10 wt%.

Fig. 9 shows the thermal conductivity of the LA/PI graphite film with 10wt% added pure PI and lignin prepared in example 1, and the commercial PI graphite heat dissipation film.

FIG. 10 is a graph showing the heat dissipation performance of LA/PI graphite film with a lignin addition of 10wt% prepared in example 1, compared with aluminum sheets and copper sheets.

FIG. 11 is a demonstration of flexibility of LA/PI graphite films with a lignin addition of 10wt% prepared in example 1, and density comparisons of commercial PI graphite heat sink films, pure PI graphite films, LA/PI graphite films with a lignin addition of 10 wt%.

Fig. 12 is a graph showing heat dissipation performance of LA/PI graphite film, aluminum sheet, and copper sheet with lignin addition of 10wt% on a computer CPU prepared in example 1, and comparison of heat dissipation performance.

Detailed Description

The present invention will be described in further detail with reference to examples and drawings, but embodiments of the present invention are not limited thereto. The specific conditions are not noted in the examples and are carried out according to conventional conditions or conditions recommended by the manufacturer. The reagents or apparatus used were conventional products commercially available without the manufacturer's attention. Lignin (abated) (company ala Ding Shiji); PMDA (1, 2,4, 5-benzene tetracarboxylic dianhydride, tianjin mass material science, ltd); 6FAP (2, 2-bis (3-amino-4-hydroxyphenyl) hexafluoropropane, tianjin Zhongtai Material technologies Co., ltd.); DMAC (N, N-dimethylacetamide).

In this example, a mechanical property test was performed using a universal material tester (Instron, 5565, USA) with a draw speed of 1mm/min; the substrate cross section was measured and observed using SEM (Zeiss, EVO 18, germany) with a test voltage of 10kV; the functional groups were analyzed by FTIR (Bruker, vector-22, germany) in the wavelength range 400-4000cm ^-1 The method comprises the steps of carrying out a first treatment on the surface of the The thermal performance is measured by a thermogravimetric analyzer (TG-2099F1, NETZSCH, germany), the weight of the thermogravimetric sample is 5-10mg, the temperature range is 45-800 ℃, the nitrogen flow is 25mL/min, and the heating rate is 10 ℃/min; performing crystallinity test by using a multi-position automatic sample injection X-ray diffractometer (X' pert Powder, PANalytical), wherein the angle range is 10-80 degrees; molecular structure test with confocal laser micro-Raman spectrometer (HJY LabRAM Aramis, horiba Jobin Yvon, germany) with wavelength range of 800-4500cm ^-1 The method comprises the steps of carrying out a first treatment on the surface of the Continuous high-temperature carbonization treatment is carried out by adopting a continuous carbonization furnace (XR-LXL-60X 15X1500, hunan Enrui, china); graphitizing the material by a horizontal graphitizing furnace (JR-SML-55X 55X160, hunan En Rui, china).

The reagents used in the examples are commercially available as usual unless otherwise specified.

(1) Preparation of polyimide acid (PAA) solution: 8.8818g of PMDA (1, 2,4, 5-benzenetetracarboxylic dianhydride) was weighed into a vacuum oven and dried under vacuum at 150℃for 10 hours. 94.13g of DMAC (N, N-dimethylacetamide) was weighed into a 250ml three-necked flask, the three-necked flask was placed in a cold water bath at 6℃and 14.65g of 6FAP (2, 2-bis (3-amino-4-hydroxyphenyl) hexafluoropropane) was then weighed into DMAC and dissolved by stirring. After the 6FAP was completely dissolved, the three-necked flask was purged with nitrogen to protect (remove water vapor in the flask), and PMDA taken out of the vacuum oven was added to the flask three times to start the reaction. Stopping introducing nitrogen after reacting for two hours, stopping stirring after reacting for about 8 hours, pouring out the generated polyimide acid (PAA) solution, and filling the solution into a brown glass bottle, and preserving at a low temperature (4 ℃) for later use;

(2) Preparation of lignin/polyimide (LA/PI) composite membrane: lignin (dealkalized) was added to the PAA solution to prepare a lignin/polyimide (LA/PAA) mixed solution. And removing bubbles in the mixed solution by using an ultrasonic cleaner, and performing ultrasonic treatment for 30min to ensure that LA is uniformly dispersed in the PAA. The mixed solution is coated on a high temperature resistant glass sheet of 6cm multiplied by 6cm by a tape casting method, and the glass sheet is placed in a vacuum drying box and gradually heated. The lignin/polyimide (LA/PI) composite film with the diameter of 40mm and the thickness of about 60 mu m is finally obtained, wherein the lignin (dealkalized) content (namely, the lignin (dealkalized) mass accounts for the lignin/polyimide (LA/PI) composite film mass fraction) is respectively 0, 5, 10, 15, 20, 25 and 30 weight percent. As in (c) of fig. 1;

(3) Preparation of lignin/polyimide (LA/PI) carbonized film: cutting the composite film in the step (2) into a specification of 2cm multiplied by 4cm, placing the composite film in a crucible, and smoothly placing the composite film in a constant temperature area of a tube furnace. Nitrogen was introduced at a flow rate of 20ml/min. After nitrogen is introduced for half an hour, heating is started, the heating rate is set to be 5 ℃/min, the temperature is raised to 600 ℃, 700 ℃ and 800 ℃ and is respectively kept at constant temperature for 1 hour for carbonization treatment, then the temperature is lowered to room temperature, and the carbonized film is taken out;

(4) Preparation of lignin/polyimide (LA/PI) graphite film: fixing the composite film obtained in the step (2) by using a graphite clamping piece, introducing nitrogen for protection in a continuous carbonization furnace for carbonization, and heating to 1300 ℃ for 4 hours to obtain a carbonized film; transferring the obtained carbonized film into a horizontal graphitizing furnace, introducing argon, heating to 2800 ℃, preserving heat for 4 hours, and graphitizing to obtain the graphite film.

(5) Computer CPU heat dissipation demonstration

Taking an aluminum sheet and a copper sheet as comparison samples, folding the bottom of the aluminum sheet, the copper sheet and the LA (10 percent)/PI composite graphite film (namely, the graphite film prepared by the step (4) of the LA/PI composite film with the lignin content of 10 percent) into a right angle at about 1.5cm, and pasting the right angle on a CPU (Central processing Unit) by using commercial heat-conducting silicone grease as shown in figure 12. The computer is started, and the real-time temperature is photographed for 5s, 10s and 30s by a thermal infrared camera so as to test the heat dissipation performance.

Performance test:

(1) Characterization of relevant properties of composite films

A high-definition physical photographic photograph of the pure PI film (namely, lignin content of the lignin/polyimide (LA/PI) composite film is 0%) prepared in the example 1 and LA/PI composite films with different lignin contents is shown in FIG. 2. The pure PI film is yellow and transparent, and after lignin is added, the color is darkened, the pure PI film is yellow brown and is no longer transparent, but the pure PI film has flat and smooth surfaces and no obvious holes.

FIG. 3 shows the IR spectrum of a LA/PI composite film with a pure PI film and a lignin addition of 10%. The upper curve shows the spectrum of the original PI with the polyimide characteristic absorption peak C-N stretching peak appearing at 1380cm ^-1 At which a c=o symmetric stretch peak occurs at 1720cm ^-1 1500cm at ^-1 The characteristic spectral band at this point is designated as the "ring breathing" vibrational mode of the aromatic amine in 6 FAP. The lower curve is the spectrum of the LA (10%)/PI composite membrane, and after lignin is added, the characteristic vibration peak of the aromatic ring skeleton (C=C) appears in 1605 cm ^-1 Here, the aromatic amine has a "ring respiration" vibration stretching peak of 1500cm ^-1 Offset to 1515cm ^-1 Here, the alkoxy stretching vibration peak appears at 1119cm ^-1 At 1248cm ^-1 The epoxy stretching vibration peak at the position becomes weak.

FIG. 4 shows the thermogravimetric curve and the differential thermogravimetric curve of a pure PI film and a LA/PI composite film with a lignin addition of 10%. For pure PI films, thermal decomposition starts at around 485 ℃, ends at around 750 ℃, and weight loss is most severe at around 586 ℃. After adding 10wt% lignin, two steps appear in the thermal decomposition process, the thermal decomposition starts at 280 ℃, the thermal decomposition speed is slowed down to some extent at 473 ℃, then the thermal decomposition speed is increased, the weight loss is most severe at about 569 ℃, and finally the thermal decomposition ends at about 750 ℃, because the lignin in the LA (10%)/PI composite film starts to decompose at 280 ℃, the composite film has 13% weight loss at 473 ℃, the lignin has mostly thermally decomposed, polyimide starts to thermally decompose, the composite film has greatly lost weight, and the addition of lignin leads to the advance of the thermal decomposition temperature of the composite film.

FIG. 5 shows the stress-strain curves and elastic moduli of the pure PI film and LA/PI composite films prepared with different lignin additions, respectively. The ultimate tensile strength and elastic modulus of the pure PI film were 114.37M Pa and 1908.26MPa, respectively. The ultimate tensile stress and the elastic modulus of the LA (10%)/PI composite film were 99.92MPa and 2067.30MPa, respectively. As the lignin content increases, the ultimate tensile stress gradually decreases, but the elastic modulus changes from increasing to decreasing. The lignin is used as an inorganic filler, and when a small amount of lignin is distributed in a polyimide matrix, stress concentration can be generated to trigger polyimide to generate silver marks, and part of deformation work is absorbed in the stretching process, so that destructive cracking is prevented from being formed, the toughening effect is achieved, the elastic modulus is increased, and meanwhile, the ultimate tensile stress of the composite film is reduced. However, when the lignin content (20 wt%) is too large, too much lignin loosens the composite material, and macro-cracking is likely to occur, and the elastic modulus of the composite die is rather lowered. When the LA content was increased to 30wt%, an ultimate tensile stress of 39.87MPa and an elastic modulus of 1676.89MPa were still obtained, indicating that the high LA loaded composite still had considerable strength. The excellent mechanical properties of our composite are attributed to: i) Polyimide has excellent mechanical strength; ii) excellent bond strength contributed by outstanding compatibility. The polyimide film is used as a precursor of the graphite heat dissipation film, and can keep the film form from being broken in the subsequent high-temperature calcination process, which is critical to the final graphite film whether to keep the integrity and whether to have good mechanical flexibility. Therefore, the stronger mechanical property of the polyimide composite film is beneficial to the mechanical flexibility of the subsequent graphite heat dissipation film and has important influence.

(2) Microcosmic shape test of composite film, carbonized film and graphite film

The microstructure of the composite film, the carbonized film and the graphite film has important influence on different performances such as heat conduction performance, flexibility, surface smoothness and the like. The microstructure of the matrix is affected by a number of factors, such as the lignin content and the dispersibility of the additives in the polyimide acid solution. FIG. 6 shows a sectional scanning electron microscope image of a LA/PI composite film, a carbonized film (referred to as the carbonized film prepared in step (3), and carbonized films appearing later also referred to as the carbonized film prepared in step (3)) and a graphite film with a pure PI and lignin added amount of 10%. From the first row it can be observed that the pure PI film is in the form of a tightly structured, rough-surfaced rock. Compared with a pure PI film, the lignin can be uniformly dispersed in the PI after being added by 10%, the composite film shows lamellar structure arrangement, the surface roughness is slightly increased, and obvious micropore defects are avoided. This is due to the good dispersion of lignin in PAA, the excellent compatibility leading to high loading of lignin (up to 30 wt%). It is worth mentioning that high lignin loadings are beneficial to significantly reduce the cost of graphite heat dissipation films.

The second row of FIG. 6 is a cross-sectional SEM image of the carbonized film after carbonization of the pure PI and LA (10%)/PI composite film. It can be seen that the structure of the carbonized PI film is still compact, but the surface roughness is greatly reduced, because the molecular structure is reconstructed after pyrolysis reaction brought by high temperature, more stable groups are generated, the carbonized PI film has a smooth and flat surface, and the orientation degree is higher. After pure PI was added to 10% lignin, the surface roughness of the carbonized film was reduced, but a shallow layered structure was still seen.

The third line of FIG. 6 is a section scanning electron microscope image of a graphite film graphitized by a pure PI graphite film and a LA/PI composite film with a lignin addition of 10 wt%. As shown, the graphitized PI film exhibits a highly ordered lamellar structure due to the transition of the carbonized film to the graphitized structure after the temperature is raised to 2000 c, graphite crystal formation, and reduction of structural defects. The graphite film added with 10wt% of lignin also shows a lamellar structure with higher order, the order is lower than that of a pure PI graphite film, but the graphite crystal structure is obvious, and the overall defect is less. This shows that biomass with high carbon element content has graphitization potential, which provides possibility for preparing biomass-based composite graphite film with low cost.

(3) Graphitization degree test of graphite film

The heat dissipation performance of the graphite film depends on the graphitization degree of the composite film to a great extent, the order degree of crystals of the graphite heat dissipation film and the crystal defects of the graphite film, and a high-crystallization and defect-free graphite structure is a key factor for controlling the heat conduction performance of the material. To investigate the crystal structures of the obtained biomass composite carbonized film and graphite heat dissipation film, the crystallinity of the carbonized film and graphite film was tested using X-ray diffraction (XRD) in combination with Cu internal standard method, as shown in fig. 7. The left graph of FIG. 7 shows the X-ray diffraction pattern of carbonized film after carbonizing pure PI and LA (10%)/PI composite film, the diffraction peak of carbonized film appears at about 23.5 deg. but the peak shape is not obvious, thus it is seen that carbonized film obtained by 800 deg. C treatment is still amorphous structure, but the order degree is different. The PI carbonized film has obvious diffraction peak shape and higher structural order, and is favorable for converting the amorphous carbon structure into a graphite structure in the subsequent graphitization process. The peak intensity of the diffraction peak was slightly weakened by adding a 10wt% lignin composite film, but the peak shape was consistent with the PI carbonized film. This is due to the uniform dispersion of lignin in PI, excellent compatibility, lower molecular rearrangement of lignin in the composite membrane at the thermal decomposition stage than polyimide, and difficult achievement of structural order to pure PI carbonized membrane.

To further explore the degree of order of the graphitized lignin/polyimide composite film, the graphitized PI film and the graphitized LA (10%)/PI film were further tested for crystallinity, as shown in the right graph of fig. 7. There is an obvious (002) peak at 26.5 °, which is characteristic of graphite, and the diffraction peak of the two graphitized films becomes sharp in shape, indicating that the crystallinity is very high, but the (002) peak after graphitization of the pure PI film is significantly stronger and sharper than the (002) peak after graphitization of LA (10%)/PI film, indicating that the graphitization degree of LA (10%)/PI film is lower than that of the pure PI film after graphitization treatment. Nevertheless, the graphitization degree is still considerable for composite membranes with high lignin loadings (10 wt%). Secondly, the d002 spacing after graphitization of the LA (10%)/PI film is very close to the d002 spacing after graphitization of the pure PI film, which indicates that the graphitization calcination process completely removes lignin and oxygen-containing functional groups of polyimide in the composite film. Consistent with the results on the right hand side of fig. 7, the raman spectrum of fig. 8 also shows the highly crystalline graphite structure of LA (10%)/PI composite films and pure PI films. As shown in the figure, the D peak is hardly detected, and the ratio of the D peak to the G peak (ID/IG) is low, which indicates that the structure of the LA (10%)/PI composite film and the pure PI film after graphitization has almost no defects. These results all indicate high crystallinity and high order of the LA (10%)/PI composite graphite film, which also indicates that the LA (10%)/PI composite graphite film has considerable heat dissipation properties, and the potential for the use of biomass-based composite graphite film as a heat sink material.

(4) Heat dissipation and flexibility of graphite film

The heat dissipation performance is the most visual and important index of the heat dissipation material, in order to represent the heat dissipation performance of the graphite film, the heat dissipation coefficients of the commercial graphite heat dissipation film, the PI graphite film (namely, the graphite film is prepared when the lignin content in the LA/PI composite film is 0 percent), and the LA (10 percent)/PI graphite film (namely, the graphite film is prepared when the lignin content in the LA/PI composite film is 10 percent) are respectively tested, and the heat dissipation effect of the LA (10 percent)/PI graphite film is tested by taking an aluminum sheet and a copper sheet as comparison. In addition, we have conducted related tests on the density and flexibility of graphite films.

Fig. 9 shows the thermal conductivity of different graphite films tested using the steady state thermal flow method. The principle of the steady-state heat flow method is that a composite graphite film with known thickness is placed between two flat plates, flow and pressure are applied to the film at the same time, the heat flow passing through the film and the temperature difference between the two flat plates are measured, and therefore linear data fitting is carried out according to the heat flows with different thicknesses to obtain the heat conductivity coefficient of the film. As shown in FIG. 9, the heat conductivity of a commercially available polyimide-based graphite heat dissipation film (Guangdong spring New Material Co., ltd.; synthetic graphite coiled material) is typically 1200 W.m ^-1 ·℃ ^-1 The heat conductivity coefficient of the pure polyimide graphite film prepared by our experiment is 832.479 W.m ^-1 ·℃ ^-1 When the lignin addition amount is 10wt%, the lignin/polyimide composite graphite film has a thermal conductivity of 691.322 W.m ^-1 ·℃ ^-1 . As can be seen from the combination of XRD spectrum and raman spectrum, the addition of lignin decreases the thermal conductivity,lignin cannot be completely converted into an graphite structure during graphitization. As the amount of lignin added increases, lignin also tends to form aggregates, and graphitization is more difficult. The component that does not undergo graphitization is still an amorphous structure, thereby reducing the thermal conductivity and affecting the thermal diffusion effect thereof. The heat conductivity coefficient of the lignin/polyimide composite film is higher than that of a metal heat dissipation material and a resin or metal composite graphite heat dissipation material, although the heat conductivity coefficient of the lignin/polyimide composite film is difficult to reach a commercial graphite heat dissipation film, and the heat dissipation requirement of an electronic element with high heat dissipation requirement is met.

In order to observe the heat transfer rate of the biomass-based composite graphite film in practical application, a thermal infrared camera is adopted to shoot the heat dissipation effect of the LA (10%)/PI composite graphite film (namely, the LA/PI composite film with the lignin content of 10% is prepared by graphitization), as shown in FIG. 10. We used commercial aluminum flakes, copper flakes of the same size and thickness as a control. The heating stirrer is heated to 95 ℃, and an iron block is placed on the heating stirrer, and after waiting for 30min to ensure that the temperature of the heater and the iron block are stable. The edges of the aluminum sheet, the copper sheet and the LA (10%)/PI composite graphite film are respectively pressed between the heater and the iron block, and the pressed edges have the width of 2cm. Thermal infrared images were taken at the time of heating for 30s, 1min and 5min, respectively, and the results are shown in fig. 10. From the thermal infrared image, the temperature difference between the temperature of the three samples at 30s and the temperature of the three samples at 1min and 5min is not great, which indicates that the three heat dissipation materials can quickly dissipate heat. The aluminum sheet area keeps purple all the time, the real-time temperature keeps about 29.5 ℃, the copper sheet is similar, and the difference between the image color and the real-time temperature is small. Meanwhile, the LA (10 percent)/PI composite graphite film area always presents blue color, and the real-time temperature is kept at about 45 ℃ and is far higher than that of aluminum sheets and copper sheets. This indicates that the biomass-based composite graphite heat dissipation film has heat dissipation properties superior to those of the metal foil.

Table 1 shows the thickness variation of the LA (10%)/PI composite graphite film during the preparation process, and the original LA (10%)/PI composite film thickness was about 120. Mu.m. After carbonization, the composite film is thermally decomposed, and elements such as oxygen, nitrogen and the like escape, and the thickness is thinned to 100 mu m. After graphitization, the composite film is further thermally decomposed, the amorphous structure is converted into a highly ordered structure, larger interlayer spacing and micropores are generated, the composite film is foamed, the thickness is changed to 150 mu m, and the thickness is reduced to 90 mu m after stamping, so that the thickness meets the thickness standard of a commercial graphite heat dissipation film.

TABLE 1

	Composite membrane	Carbonized film	Foaming graphite film	Calendered graphite film
					Thickness (μm)	120	100	150	90

The left graph of fig. 11 shows a high-definition physical photograph of the LA (10%)/PI composite graphite film, the graphite film is entirely black, has a relatively bright surface, can be rolled into a roll, shows good flexibility and structural stability, and can adapt to external bending under relatively large extension and folding deformation. The right panel of FIG. 11 shows the density of different graphite films, commercially available graphite heat sink film densities of 2g/cm ³ The density of the polyimide graphite film prepared by the invention is 1.68g/cm ³ The density of the LA (10%)/PI composite graphite film is 1.65g/cm ³ Lower than commercial graphite heat dissipation films, and lighter than metal heat dissipation materials such as copper and aluminum.

In summary, the biomass-based composite graphite heat dissipation film has good heat dissipation performance, lower density and excellent flexibility, is an excellent candidate material for the high-performance heat dissipation material of the next-generation commercial consumer electronics, and provides a promising method for preparing the light heat dissipation material with high performance, low cost and application to the 3C consumer electronics.

(5) Exemplary applications of graphite film

Based on the above excellent performance, we have made a heat dissipation demonstration on the CPU of an idle computer to demonstrate its potential application in the consumer electronics field (fig. 12). We folded the bottom of the aluminum sheet, copper sheet and LA (10%)/PI composite graphite film at about 1.5cm to a right angle and attached the CPU with a commercially available thermally conductive silicone grease as shown in fig. 12. The computer is started, and the real-time temperature is photographed for 5s, 10s and 30s by using the thermal infrared camera. The results show that after the electronic equipment is started, three heat dissipation materials can quickly dissipate heat, and the real-time temperature at 5s is not greatly different from the real-time temperature at 10s and 30 s. However, when the heat dissipation is stable, the heat dissipation rates of the three materials are different, and the real-time temperatures of the aluminum sheet, the copper sheet and the LA (10%)/PI composite graphite film are 35.9 ℃, 36.89 ℃ and 50.61 ℃ respectively at 30 s. Therefore, the heat dissipation effect of the aluminum sheet and the copper sheet is smaller, and the heat dissipation performance of the LA (10 percent)/PI composite graphite film is far better than that of the other two metal sheets. Therefore, the biomass-based composite graphite film has excellent heat dissipation performance, good flexibility and acceptable cost, and is particularly attractive for current consumer electronic products.

The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications that do not depart from the spirit and principles of the present invention should be made in the equivalent manner, and are included in the scope of the present invention.

Claims

1. The preparation method of the flexible electronic biomass/polyimide-based heat dissipation material film is characterized by comprising the following steps of:

(1) Preparation of polyimide acid solution: adding an aprotic polar solvent into a reaction vessel, placing the reaction vessel in an environment of 6-8 ℃, adding diamine monomer into the reaction vessel, stirring and dissolving, adding dried dianhydride monomer into the reaction vessel under the protection of nitrogen after the diamine monomer is completely dissolved, stirring and reacting, pouring out the generated polyimide acid solution after the reaction is finished, and filling the polyimide acid solution into a brown glass bottle for standby;

(2) Preparation of lignin/polyimide composite film: adding dealkalized lignin into a polyimide acid solution to prepare lignin/polyimide mixed solution, coating the mixed solution on a high-temperature-resistant glass sheet, and placing the glass sheet in a vacuum drying oven for gradient heating to thermally imidize the polyimide acid to obtain a lignin/polyimide composite film;

(3) Preparation of lignin/polyimide graphite film: fixing the composite film obtained in the step (2), and introducing nitrogen into a continuous carbonization furnace to protect and heat for carbonization to obtain a carbonized film; transferring the carbonized film into a horizontal graphitizing furnace, introducing argon and heating to graphitize to obtain a graphite film;

the diamine monomer in the step (1) is 2, 2-bis (3-amino-4-hydroxyphenyl) hexafluoropropane; the dianhydride monomer is 1,2,4, 5-benzene tetracarboxylic dianhydride;

the diamine monomer and dianhydride monomer used in the step (1) are as follows: the molar ratio of diamine monomer to dianhydride monomer is 1:1-1.015;

the stirring reaction time in the step (1) is 10-12h;

the amount of the dealkalized lignin in the step (2) is 0 to 30 weight percent of the lignin in the lignin/polyimide composite membrane, and is not 0;

the gradient heating in the vacuum drying oven in the step (2) means that the temperature is kept at 80 ℃ for 2h,120 ℃ for 1h,150 ℃ for 1h,200 ℃ for 1h and 235 ℃ for 1 h;

the carbonization in the step (3) is to heat up to 1000-1300 ℃ and preserve heat for 3-5h; graphitization refers to heating to 2800-3000 ℃ and preserving heat for 6h.

2. A flexible electronic biomass/polyimide-based heat sink material film prepared according to the method of claim 1.

3. Use of the flexible electronic biomass/polyimide-based heat dissipation material film according to claim 2 for the preparation of heat dissipation of electronic components.

4. Use of the flexible electronic biomass/polyimide-based heat sink material film according to claim 2 as a heat sink material for a computer CPU.