WO2018160106A1 - Method for manufacturing a graphene based thermally conductive film - Google Patents

Method for manufacturing a graphene based thermally conductive film Download PDF

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Publication number
WO2018160106A1
WO2018160106A1 PCT/SE2017/050185 SE2017050185W WO2018160106A1 WO 2018160106 A1 WO2018160106 A1 WO 2018160106A1 SE 2017050185 W SE2017050185 W SE 2017050185W WO 2018160106 A1 WO2018160106 A1 WO 2018160106A1
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based film
substrate
range
graphene based
graphene
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PCT/SE2017/050185
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French (fr)
Inventor
Johan LIU
Nan Wang
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Sht Grafilm Ab
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/25—Diamond

Definitions

  • the present invention relates to a method for manufacturing a graphene film having high thermal conductivity to form films for heat dissipation and heat spreading purposes.
  • the heat spreading material have ultrahigh in-plane thermal conductivities, light-weight, flexible and robust structures to match with the complexity and highly integrated nature of high power systems.
  • a graphene based film, GBF comprising:
  • GBF freestanding graphene based film
  • PPS pyrolytic graphite sheets
  • the thermal conductivity of the graphene based film after thermal annealing and press is above than 2500 W/mK for thin GBF with thickness in the range of 0.5-5 pm and the thermal conductivity is in the range of 1600- 2500 W/mK for thick GBF with thickness in the range of 5-500 pm.
  • a concentration of graphene oxide in the aqueous suspension is in the range of 1 -15mg/ml_.
  • an oxygen weight percentage of the graphene oxide is in the range of 10 wt% - 30 wt%.
  • the graphene oxide sheets have a minimum lateral size larger than 50pm and a thickness below 3nm.
  • the method further comprises, after the step of detaching the graphene based film from the substrate, heating the graphene based film to a temperature in the range of 100-200°C in N 2 at a pressure in the range of 1 -10kPa for a time period in the range of 1 -24h to remove bound water from the graphene based film.
  • heating the suspension is performed on the substrate is performed without any airflow at a
  • the substrate has a surface roughness below 1 pm.
  • the substrate has a wetting angle below 40° for the aqueous suspension.
  • the substrate is a reductive metal substrate.
  • the reductive metal for fabricating thin films is one of iron, aluminum, tin and nickel.
  • the substrate is a glass substrate.
  • detaching the graphene based film comprises using liquid nitrogen as detaching agent for peeling off the graphene based film from the substrate.
  • a very thin film may be difficult to detach from a normal substrate.
  • a reductive metal substrate is used to reduce the film first to decrease the bonding between the film and substrate and then use liquid nitrogen to detach the film completely from the substrate. Accordingly, a reductive metal substrate is advantageously used for fabricating films with a thickness in the range of 0.5-5 pm.
  • thermal annealing is performed for a time period in the range of 10-600 min.
  • the inert ambient is Ar or N 2 .
  • a heating rate for the thermal annealing is in the range of 300-1000 °C/h.
  • thermal annealing is performed such that an oxygen/carbon weight ratio of graphene based film after thermal annealing is lower than 0.001 .
  • a cooling rate for cooling down the graphene based film after thermal annealing is in the range of 40-60°C/h, resulting in a total cooling time in the range of 12-72 h.
  • pressing the graphene based film is performed for a time period in the range of 5-180 min.
  • pressing is performed such that a void volume ratio of the graphene based film after pressing is less than 1 % and a density of the graphene based film is higher than 2.1 g/cm 3
  • pressing is performed such that a tensile strength of the graphene based film after pressing is in the range of 70-100 MPa.
  • Fig 1 is a flow chart outlining the general steps of a method according to an embodiment of the invention.
  • Fig. 2 is a graph illustrating thermal conductivity of various materials
  • Fig. 3A is a graph illustrating thermal conductivity of various materials.
  • Fig. 3B is a graph illustrating thermal conductivity of a material according to different measurement methods.
  • Fig. 1 is a flow chart outlining the general steps of a method according to an embodiment of the invention according to the following:
  • the fabrication process of graphene based films is based on the self-assembly of graphene oxide (GO) sheets into highly oriented thin film structures at the gas-solution interface.
  • GO graphene oxide
  • the raw material, GO preferably meet the following requirements:
  • the lateral size of GO is larger than 50 pm. A large lateral size of GO can bring less grain boundary to the final GBF and facilitate the phonon transfer.
  • the thickness of GO is less than 3 nm, meaning that the number of layers of GO should be less than 10.
  • a large thickness of GO can cause instability of the suspension due to the gravity effect and also cause misfit of GO layers during the self-assembling process.
  • the oxygen content of GO is in the range of 10 wt% - 30 wt%. Too low oxygen content can lead to a poor solubility of GO. Too high oxygen content can lead to large weight loss during the thermal annealing.
  • the concentrations of GO suspension is controlled in the range of 1 to 15 mg/mL. Too low concentration can decrease the liquid crystal property of the GO. Too high concentration can increase the viscosity and make uneven thickness of the film.
  • the optimized parameters of GO are
  • the selected substrate needs to have a low wetting angle of GO droplets ( ⁇ 40°).
  • Different substrates are utilized for fabricating GBFs with different thicknesses.
  • the reductive metal substrates need to have high surface flatness (roughness ⁇ 1 pm).
  • the reductive metal substrate together with the GBF film was immersed into liquid nitrogen, which can freeze the water molecule inside the film and further decrease the hydrogen bonds between GBF and reductive metal substrate.
  • the liquid nitrogen molecules that penetrate into the interface between the GBF and substrate can evaporate in the form of nitrogen gas which can easily separate the GBF from the reductive metal substrate.
  • the glass substrate can be assembled into a 10-layer structure inside a big furnace with a size of
  • the obtained GBFs are further dried in nitrogen atmosphere at a temperature of 100-200°C to completely remove water molecules. Water molecules inside the GBFs can cause the formation of cracks during the following high temperature annealing.
  • the GBFs after removing water need to be stored into nitrogen atmosphere to avoid absorbing water from air again.
  • GBFs were fixed between two pieces of polished graphite plates and annealed in an electrical furnace at a temperature between 2800 and 3300oC for 30 minutes in a flow of argon.
  • the heating rate of the furnace was 300-1000 °C/h.
  • the cooling rate of the furnace needs to be very slow and is in the range of 40-60°C/h.
  • the total cooling time is controlled in the range of 12-72 h.
  • the films were pressed by a calender to remove air pockets and obtain the ultimate densified GBFs.
  • the pressure of the calender needs to be at in the range of 50-300 MPa.
  • the pressing time of the film depends on the thickness of the film. Usually, the time for pressing GBF with a thickness of 35 pm is about 0.5-1 h and can vary between 5 minutes to 2 hours.
  • GO was prepared by following the modified Hummers methods.
  • 5 g of expanded graphite flake, 3.75 g of NaNO 3 , and 200 ml_ of concentrated H 2 SO 4 were mixed at 0°C.
  • 15 g of KMnO 4 was slowly added into the mixture within about 1 h, followed by stirring for 1 h in an ice- water bath.
  • the ice water bath was replaced by oil bath in which the temperature was controlled in the range of 32 ⁇ 40°C, and kept stirring for 1 h.
  • 400 ml_ of 5wt% H 2 SO 4 was added in the solution.
  • the resultant mixture was further stirred for 1 h at 98°C.
  • the reaction was terminated by adding 15 mL of 30 wt% H2O2 into the above solution when the temperature was lowered to 60°C.
  • the mixture was precipitated at room temperature and followed by centrifuging and washing with deionized water until the pH value was close to 7.
  • the obtained colloid was dispersed into 1 L of deionized water to obtain a GO solution with a concentration of 5 mg/mL.
  • the exfoliation of GO was carried out by using a L5M high-shear laboratory mixer (Silverson Machines) with a maximum handling volume of 12 L.
  • the proper rotation speed is adjusted in the range of 5000-8000 rpm based on the handling volume. After shear mixing, the obtained GO suspension was centrifuged at 6000 rpm for 10 min to remove all the big particles, giving a homogenous GO dispersion. As a comparison, graphite oxide flakes fabricated from Hummer's method are also exfoliated by ultrasonication with a power percentage of 50% for 20 min.
  • Half-oxidized GO was fabricated by following the same process as the above GO fabrication process with only half amount of oxidants. For example, the amounts of KMnO 4 and NaNO 3 are decreased to 7 g and 1 .5 g respectively.
  • expanded graphite are directly dispersed into 90 wt% dimethylformamide (DMF) aqueous solution and then ultransonicated in water/ice bath for 6 h. The obtained suspension was left for overnight, and only the upper 80 % graphene suspension was collected for the film fabrication.
  • DMF dimethylformamide
  • the GO self-assembly process takes place at the gas-solution interface by following the continuous transition of GO suspensions from isotropic to liquid crystal during water evaporation.
  • graphite oxide flakes fabricated from the Hummer's method are usually too thick to be able to form stable aqueous suspensions, which can affect the self-assembly process. Therefore, it is essential to exfoliate the large graphite oxide to form thin layer GO sheets.
  • most of the exfoliation process, such as ultrasonication would also dramatically decrease the lateral size of GO at the same time of decreasing the thickness. Such small lateral size can bring strong adverse effect to the final thermal properties of GBFs due to the severe phonon scattering induced by the grain boundaries of GO.
  • Pristine graphite oxide flakes present a lateral size with a diameter of 1 10 pm. Such a large lateral size was attributed to the use of expanded graphite as the starting material, instead of commonly used graphite flakes.
  • the lateral size of GO sheets fabricated by the high-speed shear mixing approach (6 pm) is orders of magnitude higher than that of ultrasonication methods (300 nm), showing the great advantages of the high-speed shear mixing approach on protecting the lateral size of GO during the exfoliation process.
  • the thickness of the fabricated GO sheets by the high-speed shear mixing is less than 3 nm ( ⁇ 10 layers), showing the great exfoliation efficiency of the high-speed shear mixing approach.
  • the high-speed shear mixing approach also shows many more advantages than the commonly used ultrasonication method, including low power consumption, minimum temperature increase, high efficiency and mass production potential.
  • Proper GO oxygen contents are essential for both the self-assembly process and the final thermal performance of GBFs.
  • the large amount of oxygen functional groups on the basal plane of GO is the main reason for GO to form stable aqueous suspension.
  • the lower oxygen content the worse stability of the suspension has.
  • the poor stability of the GO suspension can further decrease the alignment of GO sheets.
  • previous studies revealed that the sp3 bonded carbon can transform into stable sp2 bonded carbon upon high temperature annealing. This would cause the coalescence of adjacent graphene sheets and form extended graphene layers, which contributes to a final high thermal conductivity of GBFs.
  • oxygen content is essential for achieving well-aligned structures and high thermal conductivity of the final GBFs.
  • very high oxygen contents can consume large amount of carbon atoms to form CO and CO2 molecules during the thermal reduction process, which can cause large size shrinkage of the GBF and also increase the overall consumption of GO. Therefore, the oxygen contents need to be regulated into a certain range.
  • GBFs based on different graphene flake materials were fabricated by following the process described in the method section, including self- assembly, graphitization at 2850°C and mechanical pressing. All GBF samples have been shown to display perfect long-range honeycomb pattern with a periodicity of ⁇ 0.25 nm in the whole area of the recorded 10 nm ⁇ 10 nm. No atomic defects were detected, indicating good recovery of sp 2 structure in all GBF samples. However, the morphology at micro-scale shows quite big difference among the three samples. GBF-GO shows highly flat surface with large smooth features. As a comparison, the surfaces of GBF- HGO and GBF-G are quite rough and filled with big wrinkles, particles and even cracks.
  • HGO and graphene sheets are the random assembly of HGO and graphene sheets during the film formation. Differently from GO, there is no liquid crystal property detected on HGO and graphene due to their low oxygen contents. Therefore, instead of forming well-aligned layer by layer structures, HGO and graphene tend to aggregate randomly and form big particles after removal of the solvents.
  • Fig. 2 shows thermal conductivity of GBF-GO, GBF-HGO, and GBF-G.
  • GBF-GO shows a high thermal conductivity value of 3214 W/m K, which is about 3 times and 6 times higher than GBF-HGO (964 W/m K) and GBF-G (483 W/m K) respectively.
  • Such a high thermal conductivity of GBF-GO is attributed to its well-aligned structure. Therefore, relatively high oxygen content is essential for achieving well-aligned structures and high thermal conductivity of the final GBFs. However, too high oxygen contents would lead to many other problems.
  • the calculated weight loss for GBF-GO reaches about 50 wt% after the deoxygenation and the graphitization, which could cause the shrinkage of the film and increase the consumption of raw materials.
  • Higher oxygen contents can also increase the amount of defects, which further increase the grain boundaries of final GBFs and lead to lower thermal conductivity.
  • the GBF attaching substrate plays a crucial factor in the film flatness and uniformity due to different wetting angles of the GO solution on different substrate surfaces.
  • GBFs were self-assembled on three different substrates, including polytetrafluoroethylene (PTFE), aluminum, and glass.
  • PTFE polytetrafluoroethylene
  • the poor wetting behavior of PTFE surface could easily cause the folding and uneven spreading of the GO suspension, which would further lead to the formation of wrinkles and inhomogeneous thickness variation of the GBFs.
  • the aluminum substrate can even reduce GO during the self- assembly of GBFs.
  • the top surface exposed to the air shows typical characteristic peaks of oxygen-containing functional groups, such as hydroxide, carbonyl and carboxyl groups, indicating the presence of GO sheets.
  • oxygen-containing functional groups such as hydroxide, carbonyl and carboxyl groups
  • Optical images of the top and bottom surfaces of the GBF fabricated on aluminum substrate were studied. Differently from the black color of the top GO surface, the bottom surface showed a shining metallic luster of graphite due to the reduction of aluminum that increased the visible light reflectivity. The reduction of aluminum substrate would change the originally hydrophilic GO surface to hydrophobic and decrease the stability of the GO suspension, which further lead to a poor alignment.
  • the glass substrate has more advantages, such as low CTE value, flat and chemical inert surface, and strong bonding with GBFs due to the interface hydrogen bonds. All these advantages would benefit to optimize the alignment and flatness of GBFs on the glass substrate.
  • a cross-section view of the GBFs detached from the glass substrate shows a well-aligned layer structure with a uniform thickness In-plane thermal conductivities of GBFs were measured using a well- established self-heating method. As shown in Fig.
  • the untreated GBF shows an extremely low thermal conductivity value of 0.56 ⁇ 0.18 W/mK, which is much lower than the calculated amorphous limit of graphene ( ⁇ 1 1 .6 W/mK), showing the superior thermal insulation property of the untreated GBF.
  • Such an extremely low in-plane thermal conductivity of the untreated GBF is attributed to the significantly magnified phonon scattering effects upon high oxygen coverages.
  • the measured in-plane thermal conductivity values are 2 ⁇ 0.2 W/mK, 214 ⁇ 18 W/mK and 230 ⁇ 25 W/mK respectively.
  • in-plane thermal conductivity values of chemically reduced GBFs increase with the decrease of oxygen contents.
  • such values are still much lower than the commercial PGS and even lower than copper and aluminum, showing the limited thermal performance of GBFs treated by chemical methods.
  • GBF- 1300°C it shows a close in-plane thermal conductivity value as GBF-HI even though a much decreased oxygen content was obtained.
  • the relatively low in- plane thermal conductivity value of GBF-1300°C was mainly caused by a low density of the sample.
  • GBF-2850°C shows an extremely high in-plane thermal conductivity of 3214 ⁇ 289 W/mK, which is almost four-order-magnitude enhancement as compared to the untreated GBF and more than one-order-magnitude higher than the chemically reduced GBFs and GBF-1300°C.
  • GBF-2850°C has a maximum through-plane thermal conductivity of 14.8 ⁇ 1 .5 W/m K that is orders of magnitude lower than its in-plane thermal conductivity, showing a strongly anisotropic thermal property.
  • the thermal anisotropy of the film can further be magnified by the presence of air pockets which can decrease the through- plane thermal conductivities significantly, giving GBF-2850°C great advantages as novel heat spreading materials.
  • In-plane thermal conductivity of GBF-2850°C shows a strong dependence with the thickness variation as illustrated in Fig. 4B.
  • three different methods including self- heating method based on infrared radiation (IR), self-heating method based on temperature dependent resistance (DTR), and Fourier method, were used to measure the change of in-plane thermal conductivity of mechanically pressed GBF-2850 C samples as a function of thickness (detailed
  • Thermal conductivity of 1 pm thick GBF-2850°C measured by the self-heating (IR) and the self-heating (TDR) methods are 3067 ⁇ 229 W/mK and 2942 ⁇ 223 W/mK respectively, showing that the effect of thermal radiation is negligible in both cases due to small temperature increases.
  • thermal conductivity shows a slight decrease to the range of 2800 ( ⁇ 250) - 2549 ( ⁇ 229) W/mK.
  • Further increase of the thickness to 4 - 13 pm leads to a decrease of thermal conductivity below 2500 W/mK (average value in this range is 2234 ⁇ 338 W/mK) and get close to the graphitic film (-2000 W/mK).
  • Molecular dynamics (MD) simulation shows the change of in-plane thermal conductivity of GBF-2850°C as a function of wrinkle density in graphene, and reveals that the decreased thermal conductivity is intimately related to an increased amount of wrinkles/misfit in the thick layers. Also, the simulation shows that the ultra-high in-plane thermal conductivity of GBF- 2850°C is attributed to the generation of graphene with large grain size above 15 pm after thermal reduction at 2850°C, which is consistent with AFM results.
  • the outstanding thermal performance exhibited by GBF-2850°C with an ultra-thin thickness represents more than 100% improvement as compared to the best data of different graphene assembled structures that ever reported in previous studies, such as graphene papers and graphene fibers.
  • the in- plane thermal performance of GBF-2850°C is also superior to most of high thermal conductive materials. For example, it is over 12 times and 8 times higher than that of aluminum and copper respectively, and more than 2 times higher than graphite film produced by compressing natural highly oriented graphite flakes with thermal conductivity varying between 300-1000 W/mK. Notably, the thermal conductivity of GBF-2850°C even outperforms the best commercial PGS sample by 60%, showing the outstanding thermal performance as a novel high thermal conductive material.
  • GBF-2850°C not only shows increased thermal conductivity of 2178 ⁇ 196 W/mK in the thickness range of 8.5-12.5 pm, but also has a relatively stable thermal conductivity around 1900 ⁇ 218 W/mK in the thickness range of 16-35 pm. With the film thickness further increased to 50 pm, thermal conductivity of GBF-2850°C has a small decrease to 1691 ⁇ 172 W/mK.
  • the reason for the significantly increased thermal conductivity difference in the thick film is that commercial PGSs have gradually decreased densities as the increase of the film thickness.
  • GBF-2850°C with different thicknesses were cut by utilizing focused ion beam (FIB).
  • FIB focused ion beam
  • the pristine GBF- 2850°C sample before pressing contains many air pockets which alter the originally well-aligned GO sheets to loose layer structures.
  • the very high crystallinity and strong layer coalescence of individual graphene sheets gives GBF-2850°C very high mechanical strength which prevented the generation of cracks as in the case of GBF-1300°C.
  • Mechanical pressing decreased the film thickness from about 2 pm to 0.8 pm.
  • the compressed film exhibits a highly firm structure without any obvious air pockets.
  • the firm and well-aligned structure contributes significantly for the ultra-high thermal conductivity of GBF-2850°C.
  • GBFs Large-area, freestanding and high thermally conductive GBFs were fabricated, characterized and applied as a novel heat spreading material in this description.
  • the analysis of different factors involved in the GBF formation gives a better understanding on the self-assembly process, and highlights the potential on improving the alignment of GBFs.
  • a scalable GBF fabrication process with an annual production capability of 2400 m 2 was also described, showing the possibility of mass production of GBFs.
  • the reduction effects and mechanisms of different reduction methods were studied and compared by structural characterizations.

Abstract

There is provided a method for manufacturing a graphene based film, GBF. The method comprises: providing graphene oxide sheets in an aqueous suspension; providing a substrate; providing the suspension on the substrate; heating the suspension on the substrate to form a graphene based film by means of self assembly; detaching the graphene based film from the substrate; performing thermal annealing of the graphene based film at a temperature in the range of 2800-3300°C in an inert ambient; and pressing the graphene based film at a pressure in the range of 50-300 MPa.

Description

METHOD FOR MANUFACTURING A GRAPHENE BASED THERMALLY
CONDUCTIVE FILM
Field of the Invention
The present invention relates to a method for manufacturing a graphene film having high thermal conductivity to form films for heat dissipation and heat spreading purposes.
Background of the Invention
Critical thermal dissipation issues are threatening both performance and lifetime of electronics, batteries and many other high power systems due to tremendous heat fluxes generated. Hence, there is a significant need for high performance heat spreading materials to efficiently transport excessive heat away from power components and thereby reduce the working
temperature of the systems. To achieve that, it is required that the heat spreading material have ultrahigh in-plane thermal conductivities, light-weight, flexible and robust structures to match with the complexity and highly integrated nature of high power systems.
Summary
In view of above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide an improved method for manufacturing a film having a high thermal conductivity for use as a heat spreading material.
According to a first aspect of the invention, there is provided a method for manufacturing a graphene based film, GBF, the method comprising:
providing graphene oxide sheets in an aqueous suspension; providing a substrate; providing the suspension on the substrate; heating the suspension on the substrate to form a graphene based film by means of self assembly; detaching the graphene based film from the substrate; performing thermal annealing of the graphene based film at a temperature in the range of 2800- 3300°C in an inert ambient; and pressing the graphene based film at a pressure in the range of 50-300 MPa.
By means of the above described method a large-scale achievable method of producing a freestanding graphene based film (GBF) with an ultra- high thermal conductivity of 3214 W/mK is provided, which is superior to most of the currently existing high thermal conductive materials and even outperforms the best commercial pyrolytic graphite sheets (PGS) by 60%. The scalable fabrication of GBFs is based on an optimized self-assembly process of graphene oxide (GO) sheets followed by graphitization and pressing leading a well-aligned structure, possessing an annual production capability of 2400 m2. The fabricated GBFs demonstrate good flexibility and ultrahigh mechanical strength and can offer new interesting heat dissipation solutions for form-factor driven miniaturized electronics and other power systems.
The thermal conductivity of the graphene based film after thermal annealing and press is above than 2500 W/mK for thin GBF with thickness in the range of 0.5-5 pm and the thermal conductivity is in the range of 1600- 2500 W/mK for thick GBF with thickness in the range of 5-500 pm.
According to one embodiment of the invention, a concentration of graphene oxide in the aqueous suspension is in the range of 1 -15mg/ml_.
According to one embodiment of the invention, an oxygen weight percentage of the graphene oxide is in the range of 10 wt% - 30 wt%.
According to one embodiment of the invention, the graphene oxide sheets have a minimum lateral size larger than 50pm and a thickness below 3nm.
According to one embodiment of the invention, the method further comprises, after the step of detaching the graphene based film from the substrate, heating the graphene based film to a temperature in the range of 100-200°C in N2 at a pressure in the range of 1 -10kPa for a time period in the range of 1 -24h to remove bound water from the graphene based film. According to one embodiment of the invention, heating the suspension is performed on the substrate is performed without any airflow at a
temperature in the range of 60-90°C for a time period in the range of 0.5-24h.
According to one embodiment of the invention the substrate has a surface roughness below 1 pm.
According to one embodiment of the invention, the substrate has a wetting angle below 40° for the aqueous suspension.
According to one embodiment of the invention, the substrate is a reductive metal substrate.
According to one embodiment of the invention, the reductive metal for fabricating thin films is one of iron, aluminum, tin and nickel.
According to one embodiment of the invention, the substrate is a glass substrate.
According to one embodiment of the invention, detaching the graphene based film comprises using liquid nitrogen as detaching agent for peeling off the graphene based film from the substrate. A very thin film may be difficult to detach from a normal substrate. Thereby, a reductive metal substrate is used to reduce the film first to decrease the bonding between the film and substrate and then use liquid nitrogen to detach the film completely from the substrate. Accordingly, a reductive metal substrate is advantageously used for fabricating films with a thickness in the range of 0.5-5 pm.
According to one embodiment of the invention, thermal annealing is performed for a time period in the range of 10-600 min.
According to one embodiment of the invention, the inert ambient is Ar or N2.
According to one embodiment of the invention, a heating rate for the thermal annealing is in the range of 300-1000 °C/h.
According to one embodiment of the invention, thermal annealing is performed such that an oxygen/carbon weight ratio of graphene based film after thermal annealing is lower than 0.001 . According to one embodiment of the invention, a cooling rate for cooling down the graphene based film after thermal annealing is in the range of 40-60°C/h, resulting in a total cooling time in the range of 12-72 h.
According to one embodiment of the invention, pressing the graphene based film is performed for a time period in the range of 5-180 min.
According to one embodiment of the invention, pressing is performed such that a void volume ratio of the graphene based film after pressing is less than 1 % and a density of the graphene based film is higher than 2.1 g/cm3
According to one embodiment of the invention, pressing is performed such that a tensile strength of the graphene based film after pressing is in the range of 70-100 MPa.
There is also provided a graphene based film manufactured according to any one of the preceding embodiments.
Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.
Brief Description of the Drawings
These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing an example embodiment of the invention, wherein:
Fig 1 is a flow chart outlining the general steps of a method according to an embodiment of the invention;
Fig. 2 is a graph illustrating thermal conductivity of various materials;
Fig. 3A is a graph illustrating thermal conductivity of various materials; and
Fig. 3B is a graph illustrating thermal conductivity of a material according to different measurement methods. Detailed Description of Example Embodiments
In the present detailed description, various embodiments of the system and method according to the present invention will be described.
Fig. 1 is a flow chart outlining the general steps of a method according to an embodiment of the invention according to the following:
- providing 102 graphene oxide sheets
- providing 104 a substrate
- providing 106 the suspension on the substrate
- heating 108 the suspension on the substrate to form a graphene based film
- detaching 1 10 the graphene based film from the substrate
- performing thermal annealing 1 12 of the graphene based film at a temperature in the range of 2800-3300°C in an inert ambient
- pressing 1 14 the graphene based film at a pressure in the range of 50-300 M Pa
The fabrication process of graphene based films (GBFs) is based on the self-assembly of graphene oxide (GO) sheets into highly oriented thin film structures at the gas-solution interface. To obtain GBF with very high in-plane thermal conductivity (e.g. a thermal conductivity above 2500 W/mK for thin GBF with thickness less than 5 pm and between 1600-2500 W/mK for thick GBF with thickness between 5-100 pm) the raw material, GO, preferably meet the following requirements:
- (1 ) The lateral size of GO is larger than 50 pm. A large lateral size of GO can bring less grain boundary to the final GBF and facilitate the phonon transfer.
- (2) The thickness of GO is less than 3 nm, meaning that the number of layers of GO should be less than 10. A large thickness of GO can cause instability of the suspension due to the gravity effect and also cause misfit of GO layers during the self-assembling process.
- (3) The oxygen content of GO is in the range of 10 wt% - 30 wt%. Too low oxygen content can lead to a poor solubility of GO. Too high oxygen content can lead to large weight loss during the thermal annealing. (4) The concentrations of GO suspension is controlled in the range of 1 to 15 mg/mL. Too low concentration can decrease the liquid crystal property of the GO. Too high concentration can increase the viscosity and make uneven thickness of the film. The optimized parameters of GO are
approximately 20 pm of lateral size, 3 nm of thickness, 30 wt % of oxygen contents, and 1 0 mg/mL of concentration.
To obtain uniform GBFs, the selected substrate needs to have a low wetting angle of GO droplets (<40°). Different substrates are utilized for fabricating GBFs with different thicknesses. For ultra-thin GBF film with thickness less than 5 pm, they are very difficult to separate from the attached substrate. Therefore, reductive metal substrates with a size of 500x500x2 mm are used due to the reduction effect of reductive metal substrates to the connected GO surface which can decrease the hydrogen bonds of GO with substrate surface. The reductive metal substrates need to have high surface flatness (roughness < 1 pm). During the detachment, the reductive metal substrate together with the GBF film was immersed into liquid nitrogen, which can freeze the water molecule inside the film and further decrease the hydrogen bonds between GBF and reductive metal substrate. The liquid nitrogen molecules that penetrate into the interface between the GBF and substrate can evaporate in the form of nitrogen gas which can easily separate the GBF from the reductive metal substrate.
For thick films (5-500 pm), chemically inert glass substrates can be used since thick GBFs are more easy to separate from the substrate. A plain glass substrate with a size of 500x500x3 mm is cleaned by NaOH and isopropanol solution to completely remove the impurities, and then washed with deionized water. After drying, a dismountable hydrophobic plastic frame with the same size as the glass substrate is fixed onto the glass substrate surface. A GO suspension was uniformly spread onto the glass substrate under mild shaking. The glass substrate is transferred onto a pre-balanced heating board with temperature in the range of 60-90°C to dry the GO solution. During the drying, any air flow is avoided. The drying process lasts about 2h for manufacture of a graphene film with a final thickness of 1 0 pm. After gently peeling off the surrounded hydrophobic plastic frame from the glass substrate, a complete and uniform freestanding GBF was obtained.
To produce the GBFs in a large-scale, the glass substrate can be assembled into a 10-layer structure inside a big furnace with a size of
100*60*60 cm, which can increase the manufacturing speed by a factor of 10. The obtained GBFs are further dried in nitrogen atmosphere at a temperature of 100-200°C to completely remove water molecules. Water molecules inside the GBFs can cause the formation of cracks during the following high temperature annealing. The GBFs after removing water need to be stored into nitrogen atmosphere to avoid absorbing water from air again. For thermal reduction, GBFs were fixed between two pieces of polished graphite plates and annealed in an electrical furnace at a temperature between 2800 and 3300oC for 30 minutes in a flow of argon. The heating rate of the furnace was 300-1000 °C/h. The cooling rate of the furnace needs to be very slow and is in the range of 40-60°C/h. The total cooling time is controlled in the range of 12-72 h. After thermal annealing, the films were pressed by a calender to remove air pockets and obtain the ultimate densified GBFs. The pressure of the calender needs to be at in the range of 50-300 MPa. The pressing time of the film depends on the thickness of the film. Usually, the time for pressing GBF with a thickness of 35 pm is about 0.5-1 h and can vary between 5 minutes to 2 hours.
GO was prepared by following the modified Hummers methods. In a typical experiment, 5 g of expanded graphite flake, 3.75 g of NaNO3, and 200 ml_ of concentrated H2SO4 were mixed at 0°C. 15 g of KMnO4 was slowly added into the mixture within about 1 h, followed by stirring for 1 h in an ice- water bath. After that, the ice water bath was replaced by oil bath in which the temperature was controlled in the range of 32~40°C, and kept stirring for 1 h. Then, 400 ml_ of 5wt% H2SO4 was added in the solution. The resultant mixture was further stirred for 1 h at 98°C. The reaction was terminated by adding 15 mL of 30 wt% H2O2 into the above solution when the temperature was lowered to 60°C. The mixture was precipitated at room temperature and followed by centrifuging and washing with deionized water until the pH value was close to 7. The obtained colloid was dispersed into 1 L of deionized water to obtain a GO solution with a concentration of 5 mg/mL. The exfoliation of GO was carried out by using a L5M high-shear laboratory mixer (Silverson Machines) with a maximum handling volume of 12 L. The mixing head is composed by a four-blade rotor sitting within a fixed stator (D rotor-stator = 100 pm). The diameter of the rotor is 32 mm. The proper rotation speed is adjusted in the range of 5000-8000 rpm based on the handling volume. After shear mixing, the obtained GO suspension was centrifuged at 6000 rpm for 10 min to remove all the big particles, giving a homogenous GO dispersion. As a comparison, graphite oxide flakes fabricated from Hummer's method are also exfoliated by ultrasonication with a power percentage of 50% for 20 min.
Half-oxidized GO (HGO) was fabricated by following the same process as the above GO fabrication process with only half amount of oxidants. For example, the amounts of KMnO4 and NaNO3 are decreased to 7 g and 1 .5 g respectively. For graphene fabrication, expanded graphite are directly dispersed into 90 wt% dimethylformamide (DMF) aqueous solution and then ultransonicated in water/ice bath for 6 h. The obtained suspension was left for overnight, and only the upper 80 % graphene suspension was collected for the film fabrication.
The purpose of fabricating small batch of GO in this study is to research the effect of different material parameters to the thermal property of final GBFs. In future, commercially available GO based on the optimized material properties obtained from this study will be directly used as a raw material as the starting point for the large-scale fabrication of GBFs, instead of fabricating GO by ourselves.
The GO self-assembly process takes place at the gas-solution interface by following the continuous transition of GO suspensions from isotropic to liquid crystal during water evaporation. However, graphite oxide flakes fabricated from the Hummer's method are usually too thick to be able to form stable aqueous suspensions, which can affect the self-assembly process. Therefore, it is essential to exfoliate the large graphite oxide to form thin layer GO sheets. However, most of the exfoliation process, such as ultrasonication, would also dramatically decrease the lateral size of GO at the same time of decreasing the thickness. Such small lateral size can bring strong adverse effect to the final thermal properties of GBFs due to the severe phonon scattering induced by the grain boundaries of GO. Previous studies revealed that even the coalescence of individual GO sheets at the graphitization temperature of carbon is not able to eliminate the grain boundaries that originated from the edges of the individual reduced GO sheets. Therefore, a large lateral size of initial GO sheets is essential to reduce the ratio of interfaces and boundaries within the GBFs, and ultimately, benefit for achieving the high thermal conductivity. In addition to this, GO sheets with a large lateral size show much higher affinity to get well-aligned structures than small sheets due to the high aspect ratio. To obtain GO with a large lateral size, an efficient exfoliation process becomes necessary to decrease the thickness of starting graphite oxide flakes whilst ensuring minimum breakage of the exfoliated GO sheets.
For this work, a high-speed shear mixing approach was developed to exfoliate graphite oxide and fabricate GO with a large lateral size. The mixing head of the shear mixer is composed by a four-blade rotor sitting within a fixed stator (D rotor-stator = 100 pm). During rotation, large graphite oxide flakes are sucked into the mixing head and driven towards the edge of the
rotor/stator where the local shearing rate reaches up to 104 s' Upon such a high shearing rate, it could easily overcome the weak interlayer Van der Waals' force between graphite oxide layers and exfoliate them into individual GO sheets while maintaining a large lateral size due to the much stable sp2 bonded graphenic domains.
Pristine graphite oxide flakes present a lateral size with a diameter of 1 10 pm. Such a large lateral size was attributed to the use of expanded graphite as the starting material, instead of commonly used graphite flakes. After exfoliation, it can be found that the lateral size of GO sheets fabricated by the high-speed shear mixing approach (6 pm) is orders of magnitude higher than that of ultrasonication methods (300 nm), showing the great advantages of the high-speed shear mixing approach on protecting the lateral size of GO during the exfoliation process. In addition, the thickness of the fabricated GO sheets by the high-speed shear mixing is less than 3 nm (<10 layers), showing the great exfoliation efficiency of the high-speed shear mixing approach. In spite of that, the high-speed shear mixing approach also shows many more advantages than the commonly used ultrasonication method, including low power consumption, minimum temperature increase, high efficiency and mass production potential.
To evaluate the effect of concentration to the liquid crystal property of GO, a series of GO aqueous suspensions with varied concentrations in the range of 0.3-10 mg/ml_ were examined by a polarized optical microscope in transmission mode. GO sheets showed the typical birefringence effect of liquid crystals under crossed-polarizers, indicating the spontaneous formation of lyotropic nematic liquid crystals. The liquid crystal phenomenon was observed above a critical GO concentration of 1 mg/ml_. Liquid crystallization occurred at such a low GO concentration is due to the large lateral size of the GO sheets. Therefore, based on the liquid crystal property of GO in a wide range of concentrations, it is possible to regulate the final GBF's thickness from hundreds of nanometers to hundreds of micrometers.
Proper GO oxygen contents are essential for both the self-assembly process and the final thermal performance of GBFs. For example, the large amount of oxygen functional groups on the basal plane of GO is the main reason for GO to form stable aqueous suspension. The lower oxygen content, the worse stability of the suspension has. The poor stability of the GO suspension can further decrease the alignment of GO sheets. Moreover, previous studies revealed that the sp3 bonded carbon can transform into stable sp2 bonded carbon upon high temperature annealing. This would cause the coalescence of adjacent graphene sheets and form extended graphene layers, which contributes to a final high thermal conductivity of GBFs. Therefore, relatively high oxygen content is essential for achieving well-aligned structures and high thermal conductivity of the final GBFs. On the other side, very high oxygen contents can consume large amount of carbon atoms to form CO and CO2 molecules during the thermal reduction process, which can cause large size shrinkage of the GBF and also increase the overall consumption of GO. Therefore, the oxygen contents need to be regulated into a certain range.
To figure out the proper oxygen content, three different graphene materials, including GO fabricated by Hummer's method, HGO by decreasing the amount of oxidation agents in Hummer's method, and graphene directly exfoliated from nature graphite, were used for making GBFs (GBF-GO, GBF- HGO, and GBF-G). The elemental compositions of different graphene materials were characterized by X-ray photoelectron spectroscopy (XPS). The O/C atom ratios of the different graphene materials are 0.43 for GO, 0.1 for HGO, and 0.04 for graphene.
GBFs based on different graphene flake materials were fabricated by following the process described in the method section, including self- assembly, graphitization at 2850°C and mechanical pressing. All GBF samples have been shown to display perfect long-range honeycomb pattern with a periodicity of ~0.25 nm in the whole area of the recorded 10 nm χ 10 nm. No atomic defects were detected, indicating good recovery of sp2 structure in all GBF samples. However, the morphology at micro-scale shows quite big difference among the three samples. GBF-GO shows highly flat surface with large smooth features. As a comparison, the surfaces of GBF- HGO and GBF-G are quite rough and filled with big wrinkles, particles and even cracks. The reason for such rough surfaces is the random assembly of HGO and graphene sheets during the film formation. Differently from GO, there is no liquid crystal property detected on HGO and graphene due to their low oxygen contents. Therefore, instead of forming well-aligned layer by layer structures, HGO and graphene tend to aggregate randomly and form big particles after removal of the solvents.
Fig. 2 shows thermal conductivity of GBF-GO, GBF-HGO, and GBF-G. It can be found that GBF-GO shows a high thermal conductivity value of 3214 W/m K, which is about 3 times and 6 times higher than GBF-HGO (964 W/m K) and GBF-G (483 W/m K) respectively. Such a high thermal conductivity of GBF-GO is attributed to its well-aligned structure. Therefore, relatively high oxygen content is essential for achieving well-aligned structures and high thermal conductivity of the final GBFs. However, too high oxygen contents would lead to many other problems. For example, the calculated weight loss for GBF-GO reaches about 50 wt% after the deoxygenation and the graphitization, which could cause the shrinkage of the film and increase the consumption of raw materials. Higher oxygen contents can also increase the amount of defects, which further increase the grain boundaries of final GBFs and lead to lower thermal conductivity.
The GBF attaching substrate plays a crucial factor in the film flatness and uniformity due to different wetting angles of the GO solution on different substrate surfaces. To study this effect, GBFs were self-assembled on three different substrates, including polytetrafluoroethylene (PTFE), aluminum, and glass. The GO droplet has the highest wetting angle (0C=1 15°) on the very hydrophobic PTFE surface. The poor wetting behavior of PTFE surface could easily cause the folding and uneven spreading of the GO suspension, which would further lead to the formation of wrinkles and inhomogeneous thickness variation of the GBFs. Aluminum substrate shows a better wetting property (0C=55°) than the PTFE. However, due to the large coefficient of thermal expansion (CTE) of the aluminum, small temperature changes on the aluminum substrate surface would cause irregular stress distribution inside the film and form winkles and even cracks on the surface of GBFs. In addition, the aluminum substrate can even reduce GO during the self- assembly of GBFs. In FTIR spectra of the top and bottom surfaces of the GBF fabricated on the aluminum substrate, the top surface exposed to the air shows typical characteristic peaks of oxygen-containing functional groups, such as hydroxide, carbonyl and carboxyl groups, indicating the presence of GO sheets. However, those characteristic peaks of oxygen-containing functional groups almost vanished from the bottom surface that contacted with the aluminum substrate, showing a strong reduction effect of the aluminum substrate. Optical images of the top and bottom surfaces of the GBF fabricated on aluminum substrate were studied. Differently from the black color of the top GO surface, the bottom surface showed a shining metallic luster of graphite due to the reduction of aluminum that increased the visible light reflectivity. The reduction of aluminum substrate would change the originally hydrophilic GO surface to hydrophobic and decrease the stability of the GO suspension, which further lead to a poor alignment.
Compared to PTFE and aluminum substrates, the glass substrate shows the best wetting property (0C=39°), which would benefit the uniform distribution of the GO suspension on the glass surface. Moreover, the glass substrate has more advantages, such as low CTE value, flat and chemical inert surface, and strong bonding with GBFs due to the interface hydrogen bonds. All these advantages would benefit to optimize the alignment and flatness of GBFs on the glass substrate. A cross-section view of the GBFs detached from the glass substrate shows a well-aligned layer structure with a uniform thickness In-plane thermal conductivities of GBFs were measured using a well- established self-heating method. As shown in Fig. 3A, the untreated GBF shows an extremely low thermal conductivity value of 0.56 ± 0.18 W/mK, which is much lower than the calculated amorphous limit of graphene (~1 1 .6 W/mK), showing the superior thermal insulation property of the untreated GBF. Such an extremely low in-plane thermal conductivity of the untreated GBF is attributed to the significantly magnified phonon scattering effects upon high oxygen coverages. For GBF-VC, GBF-HI, and GBF-1300°C, the measured in-plane thermal conductivity values are 2 ± 0.2 W/mK, 214 ± 18 W/mK and 230 ± 25 W/mK respectively. It can be found that in-plane thermal conductivity values of chemically reduced GBFs increase with the decrease of oxygen contents. However, such values are still much lower than the commercial PGS and even lower than copper and aluminum, showing the limited thermal performance of GBFs treated by chemical methods. For GBF- 1300°C, it shows a close in-plane thermal conductivity value as GBF-HI even though a much decreased oxygen content was obtained. The relatively low in- plane thermal conductivity value of GBF-1300°C was mainly caused by a low density of the sample.
The removal of oxygen leads to the expansion of the graphene layer distance and also forms micro-scaled air pockets, which dramatically decrease the film density to 0.2 g/cm3 The following mechanical pressing cannot significantly improve the film density due to a significantly decreased bonding strength between adjacent graphene sheets after removing hydrogen bonds that previously bond GO sheets. Differently, GBF-2850°C shows an extremely high in-plane thermal conductivity of 3214 ± 289 W/mK, which is almost four-order-magnitude enhancement as compared to the untreated GBF and more than one-order-magnitude higher than the chemically reduced GBFs and GBF-1300°C. According to a measurement with the pulsed photothermal reflectance (PPR) method, GBF-2850°C has a maximum through-plane thermal conductivity of 14.8 ± 1 .5 W/m K that is orders of magnitude lower than its in-plane thermal conductivity, showing a strongly anisotropic thermal property. The thermal anisotropy of the film can further be magnified by the presence of air pockets which can decrease the through- plane thermal conductivities significantly, giving GBF-2850°C great advantages as novel heat spreading materials.
In-plane thermal conductivity of GBF-2850°C shows a strong dependence with the thickness variation as illustrated in Fig. 4B. To exclude the error from the testing methods, three different methods, including self- heating method based on infrared radiation (IR), self-heating method based on temperature dependent resistance (DTR), and Fourier method, were used to measure the change of in-plane thermal conductivity of mechanically pressed GBF-2850 C samples as a function of thickness (detailed
measurement descriptions are available in the supplementary material).
Independent samples were fabricated and measured for each single test point to study the repeatability. It was found that GBF-2850°C sample with the smallest thickness of approximately 0.8 pm shows the highest in- plane thermal conductivity value of 3214 ± 289 W/mK (self-heating method (IR)) and 2988 ± 400 W/mK (Fourier method) among all samples. The self- heating (TDR) measurement for thermal conductivity was performed in vacuum to study the effect of thermal radiation on thermal conductivity.
Thermal conductivity of 1 pm thick GBF-2850°C measured by the self-heating (IR) and the self-heating (TDR) methods are 3067 ± 229 W/mK and 2942 ± 223 W/mK respectively, showing that the effect of thermal radiation is negligible in both cases due to small temperature increases. When the thickness increased to 2-3 pm, thermal conductivity shows a slight decrease to the range of 2800 (± 250) - 2549 (± 229) W/mK. Further increase of the thickness to 4 - 13 pm leads to a decrease of thermal conductivity below 2500 W/mK (average value in this range is 2234 ± 338 W/mK) and get close to the graphitic film (-2000 W/mK).
Molecular dynamics (MD) simulation shows the change of in-plane thermal conductivity of GBF-2850°C as a function of wrinkle density in graphene, and reveals that the decreased thermal conductivity is intimately related to an increased amount of wrinkles/misfit in the thick layers. Also, the simulation shows that the ultra-high in-plane thermal conductivity of GBF- 2850°C is attributed to the generation of graphene with large grain size above 15 pm after thermal reduction at 2850°C, which is consistent with AFM results. The outstanding thermal performance exhibited by GBF-2850°C with an ultra-thin thickness represents more than 100% improvement as compared to the best data of different graphene assembled structures that ever reported in previous studies, such as graphene papers and graphene fibers. The in- plane thermal performance of GBF-2850°C is also superior to most of high thermal conductive materials. For example, it is over 12 times and 8 times higher than that of aluminum and copper respectively, and more than 2 times higher than graphite film produced by compressing natural highly oriented graphite flakes with thermal conductivity varying between 300-1000 W/mK. Notably, the thermal conductivity of GBF-2850°C even outperforms the best commercial PGS sample by 60%, showing the outstanding thermal performance as a novel high thermal conductive material.
In addition, in-plane thermal conductivity of GBF-2850°C even
outperforms PGS in larger film thickness between 10-50 pm. According to commercial datasheets, the highest in-plane thermal conductivity of PGS reaches to 1950 W/mK when the film thickness is about 10 pm. With the thickness increase, PGS shows a significant decrease on in-plane thermal conductivity from 1950 to 1300 W/mK. Thermal conductivity of PGSs with different thicknesses was verified by using the self-heating method (IR). The results show good consistency with the datasheet, indicating the high accuracy of our test method. As a comparison, GBF-2850°C not only shows increased thermal conductivity of 2178 ± 196 W/mK in the thickness range of 8.5-12.5 pm, but also has a relatively stable thermal conductivity around 1900 ± 218 W/mK in the thickness range of 16-35 pm. With the film thickness further increased to 50 pm, thermal conductivity of GBF-2850°C has a small decrease to 1691 ± 172 W/mK. The reason for the significantly increased thermal conductivity difference in the thick film is that commercial PGSs have gradually decreased densities as the increase of the film thickness.
Previous studies about the polyimide (PI) pyrolytic process also reported that the orientation of the graphite layer texture became much worse in the case of thick PGS (above than 25 pm) due to the increased amount of curvatures and layer misfit. Therefore, the thickness of PGS fabricated in industry is usually limited within 25 pm when the film density reaches to 2.1 g/cm3 to get a well-oriented graphite layer texture. Differently from the PGS, the GBF-2850°C was pre-assembled by individual GO sheets and has much better orientations in the horizontal direction. Therefore, the thickness increase wouldn't lead to the increase of layer misfit in GBF-2850°C. The well-oriented graphene layer structure and high density of GBF-2850°C contributes significantly to a much higher thermal conductivity in the case of over 25 pm.
To reveal defect regions inside the film, GBF-2850°C with different thicknesses were cut by utilizing focused ion beam (FIB). The pristine GBF- 2850°C sample before pressing contains many air pockets which alter the originally well-aligned GO sheets to loose layer structures. However, the very high crystallinity and strong layer coalescence of individual graphene sheets gives GBF-2850°C very high mechanical strength which prevented the generation of cracks as in the case of GBF-1300°C. Mechanical pressing decreased the film thickness from about 2 pm to 0.8 pm. The compressed film exhibits a highly firm structure without any obvious air pockets. The firm and well-aligned structure contributes significantly for the ultra-high thermal conductivity of GBF-2850°C. In addition to this, it was noticed that the size and the amount of air pockets increased with the increase of the film thickness. Due to the strong air impermeability and robust structure of graphene, the removal of air pockets becomes much more difficult from thick GBF samples. As a result, the irregular shape of air pockets increases the local phonon scattering by causing the folding and misfit of adjacent graphene layers.
These phenomenon become more obvious in the thick samples, and thereby, leading to the gradual decrease of the thermal conductivity of the GBF from 3214 W/m K to 1691 W/m K with the increase of film thickness from 0.8pm to 50pm. The cross-section images of GBF-2850°C are also compared with commercial PGSs from different suppliers to understand the possible reason for the difference in the in-plane thermal conductivity. It can be found that surfaces of all these PGS samples are filled with large amounts of deep gaps which almost run through the top and bottom surfaces. The surface morphology of unpressed PGS reveals that those gaps are mainly located at the boundary of pebble-like structures which were caused by the structural shrinkage of PGS during the pyrolytic process. This presence of deep gaps in the through-plane direction can significantly enhance the phonon scattering in the in-plane direction, and thereby, limiting the in-plane thermal conductivity of PGS. On the contrary, the horizontally aligned air pockets inside of GBF- 2850°C show much lower effect to the in-plane phonon transfer.
Large-area, freestanding and high thermally conductive GBFs were fabricated, characterized and applied as a novel heat spreading material in this description. The analysis of different factors involved in the GBF formation gives a better understanding on the self-assembly process, and highlights the potential on improving the alignment of GBFs. A scalable GBF fabrication process with an annual production capability of 2400 m2 was also described, showing the possibility of mass production of GBFs. The reduction effects and mechanisms of different reduction methods were studied and compared by structural characterizations. The demonstrated superior properties of fabricated large-area and freestanding GBFs, including ultra- high thermal conductivity, good flexibility and robust mechanical strength, highlight the great potential of GBFs as a heat spreading material for high performance, high power density driven and miniaturized electronics and other power systems where form-factor is a critical parameter.
Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Also, it should be noted that parts of the method may be omitted,
interchanged or arranged in various ways, the method yet being able to perform the functionality of the present invention.
Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1 . A method for manufacturing a graphene based film, GBF, the method comprising:
providing graphene oxide sheets in an aqueous suspension;
providing a substrate;
providing the suspension on the substrate;
heating the suspension on the substrate to form a graphene based film by means of self assembly;
detaching the graphene based film from the substrate;
performing thermal annealing of the graphene based film at a temperature in the range of 2800-3300°C in an inert ambient; and
pressing the graphene based film at a pressure in the range of 50-300
MPa.
2. The method according to claim 1 , wherein a concentration of graphene oxide in the aqueous suspension is in the range of 1 -15mg/ml_.
3. The method according to claim 1 or 2, wherein an oxygen weight percentage of the graphene oxide is in the range of 10 wt% - 30 wt%.
4. The method according to any one of the preceding claims, wherein the graphene oxide sheets have a minimum lateral size larger than 50pm and a thickness below 3nm.
5. The method according to any one of the preceding claims, further comprising, after the step of detaching the graphene based film from the substrate, heating the graphene based film to a temperature in the range of 100-200°C in N2 at a pressure in the range of 1 -10kPa for a time period in the range of 1 -24h to remove bound water from the graphene based film.
6. The method according to any one of the preceding claims, wherein heating the suspension is performed on the substrate is performed without any airflow at a temperature in the range of 60-90°C for a time period in the range of 0.5-24h.
7. The method according to claim 1 or 2, wherein the substrate has a surface roughness below 1 pm.
8. The method according to any one of the preceding claims, wherein the substrate has a wetting angle below 40° for the aqueous suspension.
9. The method according to any one of the preceding claims, wherein the substrate is a reductive metal substrate.
10. The method according to claim 9, wherein the reductive metal for fabricating thin films is one of iron, aluminum, tin and nickel.
1 1 . The method according to any one of the preceding claims, wherein the substrate is a glass substrate.
12. The method according to any one of the preceding claims, wherein detaching the graphene based film comprises using liquid nitrogen < detaching agent for peeling off the graphene based film from the substrate.
13. The method according to any one of the preceding claims, wherein thermal annealing is performed for a time period in the range of 10- 600 min.
14. The method according to any one of the preceding claims, wherein the inert ambient is Ar or N2.
15. The method according to any one of the preceding claims, wherein a heating rate for the thermal annealing is in the range of 300-1000 °C/h.
16. The method according to any one of the preceding claims, wherein thermal annealing is performed such that an oxygen/carbon weight ratio of graphene based film after thermal annealing is lower than 0.001 .
17. The method according to any one of the preceding claims, wherein a cooling rate for cooling down the graphene based film after thermal annealing is in the range of 40-60°C/h.
18. The method according to any one of the preceding claims, wherein pressing the graphene based film is performed for a time period in the range of 5-180 min.
19. The method according to any one of the preceding claims, wherein pressing is performed such that a void volume ratio of the graphene based film after pressing is less than 1 % and a density of the graphene based film is higher than 2.1 g/cm3.
20. The method according to any one of the preceding claims, wherein pressing is performed such that a tensile strength of the graphene based film after pressing is in the range of 70-100 MPa.
21 . A graphene based film manufactured according to any one of the preceding claims.
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CN113200534A (en) * 2021-05-19 2021-08-03 重庆交通大学 Preparation method of graphene oxide reduction self-assembly film based on planar base film
CN114702030A (en) * 2022-05-18 2022-07-05 山东大学 Preparation method of ultrathin flexible graphene film
CN116216701A (en) * 2023-03-06 2023-06-06 大连交通大学 Method for preparing graphene oxide film with high reduction rate in situ

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