Graphene-coated hollow glass bead and preparation method thereof

Abstract

The invention belongs to the field of modification treatment of hollow glass beads, and particularly relates to a graphene-coated hollow glass bead and a preparation method thereof. The method comprises the following steps: coating the surfaces of hollow glass beads with polyimide acid, and then carrying out imidization treatment to form hollow glass beads coated with polyimide films; after the hollow glass beads coated with the polyimide film are tiled into layers, laser scanning is used under the protective atmosphere, the laser penetrates through the tiled layers of the hollow glass beads to induce the polyimide film to form graphene, and the hollow glass beads coated with the graphene are obtained; and annealing treatment. Layering is performed after the hollow glass beads coated with the polyimide film are prepared, laser is regulated and controlled to penetrate through the hollow glass bead tiling layer, damage to the hollow glass beads is avoided, and full coverage of the hollow glass beads can be achieved; and further annealing treatment can reduce the defects of the generated graphene and improve the conductivity of the product.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of modification treatment of hollow glass microspheres, and particularly relates to a graphene-coated hollow glass microsphere and a preparation method thereof. BACKGROUND

[0002] Hollow glass microspheres, as an inorganic low-density and high-strength material, have an irreplaceable position in the field of material lightweight. However, the addition of hollow glass microspheres will lead to a decrease in material strength. Graphene, as a nanomaterial, has excellent reinforcing effect. Meanwhile, graphene has high surface energy and is difficult to disperse in materials alone. Hollow glass microspheres are easy to disperse in materials due to their large particle size. Therefore, coating graphene on the surface of hollow glass microspheres can solve the dispersion problem of graphene and also play a reinforcing role in materials. Meanwhile, the hollow glass microspheres are endowed with the performance of electrical conductivity, which plays a key role in scenarios that require lightweight and electrical conductivity of materials.

[0003] A graphene-coated hollow glass microsphere is disclosed in Chinese Patent Application No. CN112569516A, published on March 30, 2021. The graphene-coated hollow glass microsphere is obtained by fully mixing hollow glass microspheres and graphene oxide solution through rapid stirring to obtain a mixed solution, then adding a reducing agent sodium borohydride to the mixed solution and continuing to stir to obtain a mixed solution of graphene-coated hollow glass microspheres, and finally collecting graphene-coated hollow glass microspheres floating on the surface of the liquid. The essence of this method is to realize the coating of graphene by surface adsorption. Since the surface of hollow glass microspheres is a relatively smooth sphere, the adsorption is low. Therefore, this method can only coat a small amount of graphene on the surface of hollow glass microspheres, and the coating is uneven and easy to fall off.

[0004] Chinese Patent Application No. CN113929315A, published on January 14, 2022, discloses a method for inducing a graphene coating layer on the surface of glass fibers by laser ablation. After surface treatment of the glass fibers, a polyimide film-coated glass fiber (i.e., graphene glass fiber precursor) is prepared. Then, the graphene glass fiber precursor is uniformly laid on a flat plate in a single layer, and the graphene glass fiber precursor is required to be uncovered from each other. After laser ablation, the graphene glass fiber precursor is turned over 180 degrees, and the other side is ablated by laser. Finally, the polyimide film is converted into a graphene film. This method is not suitable for hollow glass microspheres. First, the glass fiber is a solid structure, while the hollow glass microsphere is a thin-walled hollow structure, and laser ablation can easily damage the hollow glass microsphere. Second, the hollow glass microsphere is a powder material, and it cannot be ablated on one side by laser and then ablated on the other side by laser after turning over 180 degrees, like the graphene glass fiber precursor described above. SUMMARY

[0005] The application aims to provide a preparation method of graphene-coated hollow glass microspheres to solve the problems of the existing graphene coating method.

[0006] The second object of the application is to provide graphene-coated hollow glass microspheres prepared by the above method.

[0007] To achieve the above objects, the application adopts the following technical scheme:

[0008] A preparation method of graphene-coated hollow glass microspheres comprises the following steps:

[0009] (1) coating polyimide acid on the surface of hollow glass microspheres, and then performing imidization treatment to form polyimide film-coated hollow glass microspheres;

[0010] (2) placing the polyimide film-coated hollow glass microspheres in layers, and using laser scanning under a protective atmosphere, the laser penetrating the layer of hollow glass microspheres to induce the polyimide film to form graphene, thereby obtaining graphene-coated hollow glass microspheres;

[0011] (3) annealing the graphene-coated hollow glass microspheres obtained in step (2) at 300-400℃ to obtain graphene-coated hollow glass microspheres with improved electrical conductivity.

[0012] The application is an improved application. After the polyimide film-coated hollow glass microspheres are prepared, they are placed in layers, and the laser is controlled to penetrate the layer of hollow glass microspheres to avoid damaging the hollow glass microspheres and achieve full coverage of the hollow glass microspheres. Further annealing treatment can reduce the defects of the generated graphene and improve the electrical conductivity of the product. The graphene is directly generated on the surface of the hollow glass microspheres, so the coating is more uniform and less likely to fall off.

[0013] Preferably, the thickness of the layer of hollow glass microspheres is not greater than 1mm; the laser uses a femtosecond laser with a power of 20-40W; the scanning speed is 200-300mm / s, and the spot diameter is 75-100μm. Under the above parameters, the laser can penetrate the layer of hollow glass microspheres without damaging the hollow glass microspheres, and the graphene coating of the hollow glass microspheres can be completed at one time.

[0014] Preferably, the annealing treatment is performed under a protective atmosphere, and the treatment time is 1-2h. Further preferably, the graphene-coated hollow glass microspheres obtained in step (2) are cleaned in a solvent to remove loose carbon fragments, and then the annealing treatment of step (3) is performed.

[0015] Preferably, the polyimide acid coating in step (1) is spraying 5-10% polyimide acid solution to the rotating hollow glass microspheres in a sugar-coating machine or a coating machine, and then in-situ imidization. If polyimide acid is coated on the surface of hollow glass microspheres directly by using the immersion method, it is easy to cause the hollow glass microspheres to be caked in the drying process; while using the above-mentioned coating method or sugar-coating method, the coating is more uniform, and it will not cause the hollow glass microspheres to be caked, and the coating and imidization of polyimide acid can be continuously completed in the same equipment, the process is simple, and the production efficiency is high.

[0016] Further preferably, the in-situ imidization comprises first drying at 70-90℃ for 1-2h, and then heating to 250-270℃ for 1.5-2h.

[0017] Further preferably, the mass ratio of the hollow glass microspheres to the polyimide acid solution is 1:4-4.5; and the polyimide acid solution is sprayed onto the surface of the hollow glass microspheres for more than twice. For example, the spraying times can be 2-3 times.

[0018] Preferably, the hollow glass microspheres are pretreated to improve the adhesion of polyimide acid before the polyimide acid is coated in step (1), and the pretreatment comprises first ultrasonic cleaning and impurity removal of the hollow glass microspheres in a solvent, and then argon plasma treatment. Further preferably, the power of the plasma treatment is 30-50W, and the treatment time is 5-10min.

[0019] A graphene-coated hollow glass microsphere prepared by the above-mentioned method.

[0020] The graphene-coated hollow glass microsphere prepared by the above-mentioned method has more uniform graphene coating, and the electrical conductivity is further improved. Compared with the unmodified hollow glass microspheres which are not conductive, the electrical conductivity of the graphene-coated hollow glass microspheres reaches 0.061-0.068S / m, and can be well applied to the scenes which require both light weight and electrical conductivity of the material. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 The infrared spectrum of graphene-coated hollow glass microspheres and graphene of Example 1 of the present application;

[0022] Figure 2 The SEM image of unmodified hollow glass microspheres;

[0023] Figure 3 The SEM image of graphene-coated hollow glass microspheres of Example 1 of the present application. DETAILED DESCRIPTION

[0024] (I) Preferred embodiment description of graphene-coated hollow glass microspheres and the preparation method thereof of the present application

[0025] Currently, there are two methods for coating graphene on the surface of hollow glass microspheres, one is surface adsorption method, and the other is chemical grafting method. For the first method, the hollow glass microspheres are smooth spheres, and the adsorption is very low, so the amount of graphene adsorbed on the surface is very small. For the second method, the hollow glass microspheres are formed at high temperature, so the content of hydroxyl groups on the surface is also small, and the active sites provided for chemical grafting are also small, so the amount of graphene coated on the surface by chemical grafting is also small.

[0026] Currently, the preparation methods of graphene include mechanical exfoliation method, chemical vapor deposition method, oxidation-reduction method, liquid phase exfoliation method, epitaxial growth method, electrochemical exfoliation method, carbon nanotube cutting method, biosynthesis method and laser induction method. Among them, the in-situ growth method of graphene is only chemical vapor deposition method and laser induction method, and the growth substrate of chemical vapor deposition method is copper and nickel. It is difficult to uniformly plate these two metals on the surface of hollow glass microspheres, and the decomposition temperature of the carbon source is also relatively high, about 500-1000℃, which exceeds the softening point of hollow glass microspheres, so this method is also not suitable.

[0027] The present application is based on the method of laser induction, and the carbon source is polyimide. The precursor of polyimide, polyimide acid, can be prepared into a solution, which is uniformly coated on the surface of hollow glass microspheres, and then the polyimide is carbonized by laser to prepare a graphene film.

[0028] The preparation method of graphene-coated hollow glass microspheres of the present application adopts the following steps:

[0029] S1. Coating polyimide acid on the surface of hollow glass microspheres, and then treating by imidization to form polyimide (PI) film-coated hollow glass microspheres.

[0030] The preparation of polyimide acid in this step is a prior art. The diamine monomer and dianhydride monomer are stirred in a solvent to obtain a polyimide acid solution.

[0031] The diamine monomer can be 4,4'-diamino diphenyl ether (ODA), and the dianhydride monomer can be pyromellitic dianhydride (PMDA). The low-temperature reaction is carried out at 0-5℃ for more than 4h to obtain a polyimide acid PAA solution (mass concentration of about 5-10%) with η≥1.5dL / g (corresponding to a weight average molecular weight of >50000). For example, the PAA solution with η=1.5-1.8dL / g.

[0032] The molar ratio of ODA to PDMA can be (1-1.2):(1-1.2). The solvent can be N,N-dimethylacetamide, and the water content in the solvent is preferably <50ppm. The amount of ODA corresponding to 1800g of solvent can be (0.4-1.2)mol.

[0033] In the specific reaction, ODA can be dissolved in a solvent, and then PDMA can be added in multiple times with an interval of 30-40 min between two adjacent additions. After the addition of PDMA is completed, the reaction is carried out at 0-5°C for more than 4 h.

[0034] In addition, the hollow glass beads can be pre-cleaned by ultrasonic cleaning and treated by argon plasma to improve the adhesion.

[0035] The ultrasonic cleaning is carried out in a solvent such as ethanol, and the treatment time can be 10-15 min.

[0036] The argon plasma treatment has a power of 30-50 W and a treatment time of 5-10 min.

[0037] The coating of polyimide acid on the hollow glass beads can be carried out in a sugar-coating machine or a coating machine. The above-mentioned 5-10% PAA solution is sprayed on the pretreated hollow glass beads in 2-3 times, and then gradient heating is carried out: first dried at 80°C for 1 h, then heated to 250°C for 2 h to realize imidization, and finally a uniform PI film is formed on the surface of the hollow glass beads.

[0038] Taking the sugar-coating machine as an example, unlike traditional large-size materials, the hollow glass beads have a micron-level hollow structure. During the coating of polyimide acid by the traditional dipping method, the hollow glass beads are prone to clumping during the drying process. When using the sugar-coating machine for coating, the microbeads are in a rotating state, and the polyimide acid is atomized and sprayed on the surface of the hollow glass beads, so that the solvent in the polyimide acid solution volatilizes rapidly, and the polyimide precursor remains on the surface of the hollow glass beads. Then, through the gradient heating in the rotation process of the sugar-coating machine, the dried polyimide film is coated on the hollow glass beads, which does not cause clumping of the hollow glass beads.

[0039] S2. The polyimide film-coated hollow glass beads are laid into a layer, and then scanned using a laser under a protective atmosphere. The laser penetrates the hollow glass bead layer to induce the formation of graphene from the polyimide film, thereby obtaining graphene-coated hollow glass beads.

[0040] This step is the process of laser-induced formation of a graphene film from a PI film.

[0041] The PI film-coated hollow glass beads are laid on a heat-resistant substrate, and the layer thickness is not more than 1 mm to ensure that the laser can penetrate the entire hollow glass bead layer. A femtosecond laser is used for scanning under a protective atmosphere of N2, Ar, etc. The control power is 20-40 W, the scanning speed is 200-300 mm / s, and the spot diameter is 75-100 μm. Thus, graphene-coated hollow glass beads are obtained.

[0042] S3. Annealing the graphene-coated hollow glass microspheres obtained in step S2 at 300-400℃ to obtain graphene-coated hollow glass microspheres with improved conductivity.

[0043] In this step, the graphene-coated hollow glass microspheres obtained in step S2 are washed in ethanol, and then annealed at 300-400℃ under N2 atmosphere for 1-2h, which can reduce the defects of graphene and improve its conductivity to obtain the final product. The washing can be combined with ultrasonic cleaning to thoroughly remove the loose carbon fragments.

[0044] The above preferred modes are illustrated below in conjunction with specific examples. In the following examples, the raw materials involved are all commercially available conventional raw materials, and the related processing methods are all conventional methods, unless otherwise specified. The "%" is the mass percentage, unless otherwise specified.

[0045] Example 1

[0046] The preparation method of graphene-coated hollow glass microspheres in this example adopts the following steps:

[0047] S1. Coating polyimide acid on the surface of hollow glass microspheres, and then performing imidization treatment to form polyimide (PI) film-coated hollow glass microspheres.

[0048] 1.1 Preparation of polyimide precursor

[0049] Dehydration of solvent DMAC (N,N-dimethylacetamide): mix 1800g of DMAC with 50g of molecular sieves , stand for 24h, and remove residual moisture by vacuum distillation (temperature 80℃, negative pressure 50mbar) before use to ensure water content <50ppm.

[0050] Put 95.73g of ODA into a dry 3L three-necked flask, then inject 1800g of dehydrated DMAC, and mechanically stir (speed 200rpm) until the ODA is completely dissolved (about 1h).

[0051] Put the three-necked flask in an ice bath at 0-5℃, protect it by passing nitrogen gas at a flow rate of 50ml / min, add 104.27g of PMDA in 4 portions with an interval of 30min each time, ensure that the reaction heat is completely absorbed by the ice bath, and ensure that the temperature is below 10℃ during each addition, continue to stir at low temperature for 4h after the addition of PMDA is completed, measure the intrinsic viscosity (η) of a small amount of solution with an Ubbelohde viscometer after 4h, stop the reaction when η≥1.5dL / g, filter the reacted solution with a 0.2μm PTFE filter membrane to remove unreacted monomers and gel particles, and prepare a PAA solution with a mass fraction of about 10%, seal it in the dark and store it at -20℃ for later use.

[0052] 1.2 Plasma treatment of hollow glass microspheres

[0053] The 50 g of hollow glass microspheres were ultrasonically cleaned using anhydrous ethanol at a power of 100 W for 10 min, and then dried and treated with argon plasma at a power of 50 W for 8 min.

[0054] In other embodiments, the plasma treatment power was 30 W and the treatment time was 10 min, or the plasma treatment power was 40 W and the treatment time was 5 min, and substantially the same treatment effect was achieved.

[0055] 1.3 Coating of hollow glass microspheres with polyimide

[0056] The 50 g of treated hollow glass microspheres were placed in a 5 L closed coating machine, and before the microspheres were added, the interior of the coating machine was evacuated using nitrogen. During the rotation of the coating machine, 200 g of 5% PAA solution was sprayed three times, and then the temperature was gradually increased, i.e., the hollow glass microspheres were dried at 80 °C for 1 h, then imidized at 250 °C for 2 h, and finally a uniform PI film was formed on the surface of the hollow glass microspheres.

[0057] In other embodiments, the hollow glass microspheres were dried at 70 °C for 2 h, then imidized at 270 °C for 1.5 h, and substantially the same effect was achieved.

[0058] S2. The hollow glass microspheres coated with the polyimide film were laid in a layer, and then scanned using a laser under a protective atmosphere, the laser penetrated the layer of hollow glass microspheres to induce the polyimide film to form graphene, thereby obtaining hollow glass microspheres coated with graphene.

[0059] Specifically, 10 g of the hollow glass microspheres coated with PI were laid on a heat-resistant base material quartz sheet, and the thickness of the layer was about 1 mm to ensure that the laser could penetrate the entire layer of hollow glass microspheres. The hollow glass microspheres were scanned using a femtosecond laser under a nitrogen atmosphere at a power of 20% of the rated power (100 W), a scanning speed of 300 mm / s, and a spot diameter of 80 μm. The PI on the surface of the hollow glass microspheres was carbonized to form graphene, thereby forming hollow glass microspheres coated with graphene.

[0060] In other embodiments, the scanning speed was set to 200 mm / s and the spot diameter was 100 μm, or the scanning speed was 250 mm / s and the spot diameter was 75 μm, and substantially the same treatment effect was achieved.

[0061] S3. The graphene-coated hollow glass microspheres were washed with ethanol three times to remove loose carbon fragments, and were ultrasonically cleaned in ethanol for 1 min at a power of 100 W. Then, the graphene-coated hollow glass microspheres were annealed in a tube furnace at 350°C under a N2 atmosphere for 1 h to reduce defects in the graphene and improve the electrical conductivity of the graphene, thereby obtaining graphene-coated hollow glass microspheres with good performance.

[0062] In other embodiments, the graphene-coated hollow glass microspheres can be annealed at 400°C under a N2 atmosphere for 1 h or at 300°C under a N2 atmosphere for 2 h, both of which can achieve substantially the same treatment effect.

[0063] The graphene-coated hollow glass microspheres of this embodiment were prepared by the method described above.

[0064] Example 2

[0065] The method for preparing the graphene-coated hollow glass microspheres of this embodiment is different from that of Example 1 only in that, in step S1-1.3 of coating the hollow glass microspheres with polyimide, 200 g of a 10% PAA solution was sprayed three times, and then the hollow glass microspheres were dried at 80°C for 1 h and imidized at 250°C for 2 h, thereby forming a uniform PI film on the surface of the hollow glass microspheres.

[0066] Example 3

[0067] The method for preparing the graphene-coated hollow glass microspheres of this embodiment is different from that of Example 1 only in that, in step S3, a femtosecond laser was used to scan the hollow glass microspheres at a power of 30% of the rated power (100 W) and a scanning speed of 300 mm / s and a spot diameter of 80 μm, thereby carbonizing the PI on the surface of the hollow glass microspheres to form graphene and thereby forming graphene-coated hollow glass microspheres.

[0068] Example 4

[0069] The method for preparing the graphene-coated hollow glass microspheres of this embodiment is different from that of Example 1 only in that, in step S3, a femtosecond laser was used to scan the hollow glass microspheres at a power of 40% of the rated power (100 W) and a scanning speed of 300 mm / s and a spot diameter of 80 μm, thereby carbonizing the PI on the surface of the hollow glass microspheres to form graphene and thereby forming graphene-coated hollow glass microspheres.

[0070] Comparative Example

[0071] Comparative Example 1

[0072] In Comparative Example 1, the coating of the hollow glass microspheres with polyimide in step S1-1.3 of Example 1 was replaced with a conventional dipping method, i.e., the hollow glass microspheres were dipped in a 5% polyimide acid solution for 4 h (generally 3-6 h), were filtered and dried, and then were subjected to subsequent operations under the same conditions.

[0073] Comparative Example 2

[0074] The power used by the femtosecond laser in Example 3 was changed to 80% of the rated power, and other operating conditions remained unchanged.

[0075] Comparative Example 3

[0076] In Example 3, step S3 was not annealed, and other operating conditions remained unchanged.

[0077] (III) Experimental Examples

[0078] Experimental Example 1 Infrared Characterization

[0079] The infrared analysis of graphene and graphene-coated hollow glass microspheres obtained in Example 1 was performed, and the results are shown in Figure 1 .

[0080] Figure 1 In the infrared response curves of graphene (GO curve) and graphene-coated hollow glass microspheres (GO-S curve), the curves are basically consistent, and there is a larger difference at 1100 cm -1 , which represents the characteristic absorption peak of Si-O-Si bond in hollow glass microspheres. Through the comparison of the two curves, it is shown that the hollow glass microspheres have been successfully coated with graphene.

[0081] Experimental Example 2 SEM Characterization

[0082] The SEM analysis of the original hollow glass microspheres and the graphene-coated hollow glass microspheres obtained in Example 1 was performed, and the results are shown in Figure 2 and Figure 3 .

[0083] As can be seen from Figure 2 and Figure 3 , the outer surface of the original hollow glass microspheres is smooth, the surface of the coated hollow glass microspheres is rough, and the graphene grows in situ on the surface of the hollow glass microspheres, and the combination of the two is relatively tight.

[0084] Experimental Example 3 Comprehensive Performance Comparison

[0085] The PI coating effect, graphene coating effect and electrical conductivity of the products obtained in each example and comparative example were compared, and the results are shown in Table 1. Among them, the electrical conductivity test was tested using a four-probe tester, and the detailed test method referred to standard DB32 / T4027-2021.

[0086] Table 1 Comparison of comprehensive performance of each example and comparative example

[0087]

[0088]

[0089] As can be seen from the results of Table 1, the method of the embodiments can effectively prevent the microbeads from caking and avoid destroying the microbead structure in the process of laser-induced generation of graphene. At the same time, the conductivity of the unmodified hollow glass microbeads is 0, and after the hollow glass microbeads are coated with graphene, the hollow glass microbeads are endowed with conductivity, and the graphene-coated hollow glass can further improve the conductivity of the graphene-coated hollow glass after annealing treatment.

[0090] Finally, it should be noted that: the above only for the preferred embodiments of the present application, and not for limiting the present application, although the foregoing embodiments of the present application are described in detail, for those skilled in the art, it still can be modified, or part of the technical features of the equivalent replacement, within the spirit and principles of the present application, any modification, equivalent replacement, improvement, etc., should be included within the scope of the present application.

Claims

1. A method for preparing graphene-coated hollow glass microspheres, characterized in that: The following steps are involved: (1) coating the surface of the hollow glass microspheres with polyimide acid, and then performing imidization treatment to form hollow glass microspheres coated with a polyimide film; (2) After the hollow glass microspheres coated with polyimide film are laid flat into a layer, laser scanning is performed under a protective atmosphere, and the laser penetrates the flat layer of hollow glass microspheres to induce the polyimide film to form graphene, thereby obtaining graphene-coated hollow glass microspheres; (3) annealing the graphene-coated hollow glass microspheres obtained in step (2) at 300-400° C. to obtain graphene-coated hollow glass microspheres with improved conductivity.

2. The method for preparing graphene-coated hollow glass microspheres according to claim 1, wherein: The thickness of the hollow glass microsphere flat layer is not greater than 1 mm; the laser uses a femtosecond laser with a power of 20 to 40 W; the scanning speed is 200 to 300 mm / s, and the spot diameter is 75 to 100 μm.

3. The method for preparing graphene-coated hollow glass microspheres according to claim 1, wherein: The annealing treatment is carried out under a protective atmosphere for 1 to 2 hours.

4. The method for preparing graphene-coated hollow glass microspheres according to claim 3, wherein: The graphene-coated hollow glass microspheres obtained in step (2) are washed in a solvent to remove loosely attached carbon fragments, and then annealed in step (3).

5. The method for preparing graphene-coated hollow glass microspheres according to claim 1, wherein: The polyimide acid coating in step (1) is achieved by spraying 5-10% polyimide acid solution onto the rotating hollow glass microspheres in a sugar coating machine or a coating machine, followed by in-situ imidization.

6. The method for preparing graphene-coated hollow glass microspheres according to claim 5, wherein: The in-situ imidization comprises first drying at 70-90° C. for 1-2 hours, and then heating to 250-270° C. for 1.5-2 hours.

7. The method for preparing graphene-coated hollow glass microspheres according to claim 5 or 6, wherein: The mass ratio of the hollow glass microspheres to the polyimide acid solution is 1:4-4.5; the polyimide acid solution is sprayed onto the surface of the hollow glass microspheres for more than two times.

8. The method for preparing graphene-coated hollow glass microspheres according to claim 1, wherein: Step (1) pre-treating the hollow glass microspheres before coating with polyimide acid to improve the adhesion of the polyimide acid, wherein the pre-treatment comprises first ultrasonically cleaning the hollow glass microspheres in a solvent to remove impurities, and then performing argon plasma treatment.

9. The method for preparing graphene-coated hollow glass microspheres according to claim 8, wherein: The power of the plasma treatment is 30 to 50 W, and the treatment time is 5 to 10 minutes.

10. Graphene-coated hollow glass microspheres prepared by the method according to any one of claims 1 to 9.

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