CN108298525B - Graphene microcrystal and preparation method thereof - Google Patents

Abstract

The invention provides a graphene microcrystal and a preparation method thereof. The graphene microcrystal is a novel glassy carbon, which is a three-dimensional network structure of chemically bonded graphene microcrystals. The crystal is a graphene fragment composed of sp2 state carbon atoms, and the three-dimensional network structure is a long-range disordered and short-range ordered three-dimensional microcrystalline structure composed of sp3 state carbon atoms bonded to graphene fragments. The raw materials for preparing the graphene microcrystals are lignin separated from biomass, including lignin extracted from black liquor produced by papermaking and cellulosic ethanol, crop straw, bagasse and wood. Lignin can be used alone as a raw material, or it can be mixed with some macromolecular substances, such as phenolic resin, furfural resin and epoxy resin. The preparation method uses green renewable resource lignin to prepare graphene microcrystals, which not only reduces the production cost of graphene microcrystals, but is also beneficial to ecology and environmental protection.

Description

A kind of graphene microcrystal and preparation method thereof

technical field

The invention belongs to the branch of carbon materials in the field of material science and technology, and particularly relates to a graphene microcrystal and a preparation method thereof.

Background technique

Carbon materials are an important branch in the field of materials. From the end of the last century to this century, new carbon allotropes and new carbon materials have been emerging in the scientific theoretic and technical fields. Before the 1980s, the only known carbon allotropes were graphite, diamond and amorphous carbon. After the 1980s, carbon allotropes, fullerenes (carbon 60, C ₆₀ ), were discovered one after another. , carbon nanotubes and graphene. In particular, Graphene, which was discovered in 2004, is a two-dimensional planar crystal on the atomic scale of carbon, which breaks the concept of traditional physics and forms new scientific theories and new technologies. Bring a magical new material. Glassy carbon is an allotrope of carbon discovered by British and Japanese scientists in the 1950s and 1960s. It has a microcrystalline structure like ceramics and glass. It integrates the properties of glass, ceramics and graphite. Glass has the same high strength, high elasticity, heat resistance and chemical corrosion resistance as ceramics. Its fractured ballast has sharp edges and rounded sections. Its hardness is much higher than that of graphite, but it has higher conductivity than graphite.

With the continuous progress of carbon science and technology, people's understanding of the structure and properties of glassy carbon is deepening step by step. In the 1960s and 1970s, scientists realized that vitreous carbon is a microcrystalline structure of carbon similar to glass and ceramics, which is not only different from amorphous carbon, such as carbon black and activated carbon; it is also different from crystalline carbon, such as graphite and carbon. diamond. Glassy carbon will not graphitize and diamondize at any temperature and pressure. After the 1980s, fullerenes and carbon nanotubes were discovered successively. Scientists made a new explanation for the structure of glassy carbon, thinking that glassy carbon was microcrystals of fullerenes and/or carbon nanotubes, and some scientists Glassy carbon is considered to be a large fullerene structure. At present, it is generally believed in the scientific community that glassy carbon is a microcrystalline structure of sp2 carbon atoms bonded by sp3 carbon atoms, including fullerene crystallites and carbon nanotube crystallites. A three-dimensional microcrystalline network system of carbon. This peculiar structure brings many magical properties to glassy carbon: low density, high strength, high elasticity, high electrical and thermal conductivity, abnormally high strength at ultra-high temperatures (above 2400°C), resistance to non-oxygen Body and liquid have high resistance to penetration and chemical stability. These superior properties make glassy carbon widely used in many extreme conditions and harsh environments, especially in the military and aerospace fields. The traditional preparation raw materials of glassy carbon are phenolic resin, furfural resin and epoxy resin, which are called precursors of glassy carbon. Under normal pressure or high pressure, in an inert gas environment, these precursors are slowly heated to a high temperature of 800 ° C to 2400 ° C according to a certain temperature control program, and undergo high temperature cracking and carbon atom reorganization to form glassy carbon. In order to obtain high-quality bulk glassy carbon, the heating time of commercial production is as long as 120 hours, or even 400 hours, and the pressure is up to 40GPa.

Some researchers believe that graphene discovered in 2004, like fullerenes and carbon nanotubes, is a crystal structure composed of sp2 carbon atoms, and graphene fragments may also form microcrystals of carbon glass, called graphene microcrystals It is a new type of glassy carbon. The traditional raw materials for the preparation of glassy carbon are all prepared from fossil raw materials such as petroleum, coal and natural gas. The invention uses green, renewable and cheap lignin as raw material to prepare graphene microcrystals. Lignin is separated and purified from biomass waste, which not only turns waste into treasure, reduces production costs, but also reduces the consumption of coal and oil, and protects the ecology and environment. Lignin is a random polymer structure composed of three phenolic monomers. The elemental composition of lignin is about 63.4% carbon, 30% oxygen, 5.9% hydrogen, and 0.7% ash. It is the natural organic polymer substance with the highest carbon content, including a large number of sp2 carbon atoms that form the benzene ring structure. There is also a certain amount of sp3 carbon atoms. When pyrolyzed at high temperature, the sp2 state carbon atoms of lignin are combined into graphene fragments, and the sp3 state carbon atoms bond these graphene fragments together to form the structure of graphene microcrystals. Therefore, lignin is the best raw material for the preparation of graphene microcrystals.

SUMMARY OF THE INVENTION

The "graphene microcrystal" disclosed in the present invention specifically refers to a three-dimensional network microcrystalline structure composed of graphene fragments composed of sp2 state carbon atoms bonded to sp3 state carbon atoms, which is a new type of glassy carbon. For glassy carbon with fullerenes and/or carbon nanotubes as microcrystals. The structure of graphene microcrystals is shown in Figure 1. The parallel short lines in the figure are graphene fragments. At the intersection of graphene fragments, sp3 carbon atoms are chemically bonded to each graphene fragment to form a three-dimensional network structure. The invention prepares graphene microcrystals with green renewable resource lignin, including lignin separated from biomass such as papermaking black liquor, cellulosic ethanol black liquor, crop straw, bagasse and waste wood.

To this, the technical scheme of the present invention is:

A graphene microcrystal, which is a three-dimensional network structure of chemically bonded graphene microcrystals, the graphene microcrystals are composed of sp2 state carbon atoms composed of graphene fragments, and the sp2 state carbon atoms are composed of graphite The alkene fragments are connected by covalent bonds through sp3 carbon atoms to form a three-dimensional network structure. Different from other types of glassy carbon, the graphene microcrystal of the present invention is a carbon microcrystal. The graphene crystallites are graphene fragments composed of sp2 state carbon atoms, and the three-dimensional network structure is a long-range disordered and short-range ordered microcrystalline structure formed by the graphene fragments bonded by sp3 state carbon atoms.

The above graphene microcrystal is a new type of glassy carbon, which is a three-dimensional microcrystal network structure of carbon element, wherein the microcrystal unit is a graphene fragment composed of sp2 state carbon atoms, and the graphene fragments pass through between them. The sp3 state carbon atoms are connected by covalent bonds. The microcrystals in the existing glassy carbon are fullerenes and/or carbon nanotube fragments composed of sp2 state carbon atoms, and the graphene microcrystals of the technical solution of the present invention are the graphene crystals discovered after 2004, Therefore, it is a new type of carbon glass.

As a further improvement of the present invention, it includes a three-dimensional network structure formed by four chemical bonds of sp3 carbon atoms pointing to a tetrahedron connecting graphene microchip segments.

The above-mentioned preparation method of graphene microcrystals adopts lignin melting, thermal cracking, carbonization above 800° C., high-temperature graphitization, annealing and cooling, and the like.

As a further improvement of the present invention, the preparation method of described graphene microcrystal comprises the following steps:

Step S1, extraction of lignin;

Step S2, melting of lignin: the lignin is heated to above 160°C and held for more than 180 minutes; preferably, the temperature is raised to 180°C;

Step S3, thermal cracking of lignin: in an inert gas environment, the lignin is heated to a temperature above 400°C and held for more than 180 minutes; preferably, the lignin is heated to a temperature above 450°C and held for more than 180 minutes;

Step S4, carbonization of lignin: in an inert gas environment, the temperature of the thermally cracked lignin in step S3 is continued to rise to above 800°C, and kept for more than 240 minutes, and the pressure is 0.1MPa to 40GPa;

Step S5, further graphitization of the raw material: the carbonized raw material in step S4 is graphitized at a temperature of 1000°C to 2400°C for more than 240 minutes, and the gas pressure is 0.1MPa to 40GPa;

Step S6, annealing and cooling to room temperature.

Wherein, the atmosphere in step S2 is an air atmosphere or an inert gas atmosphere.

As a further improvement of the present invention, the temperature is raised to above 1000°C in step S4.

As a further improvement of the present invention, the inert gas environment is nitrogen, argon, helium, water vapor, carbon dioxide or a combination thereof.

As a further improvement of the present invention, in step S1, the extraction of lignin includes: adding a solvent of ethanol-water to the raw material containing lignin, and in a sealed reaction kettle under the environment of pH=3-4 Heating to 180～220℃, keeping for more than 60 minutes, separating the insoluble cellulose after cooling, evaporating the solvent of the remaining solution, to obtain lignin containing hemicellulose and hydrolyzed sugar, the lignin containing hemicellulose and hydrolyzed sugar The lignin is put into a dilute sulfuric acid solution, heated at 80 to 100° C. for 80 to 150 minutes, filtered and washed to obtain pure lignin.

The preparation of graphene microcrystals requires specific pure lignin as a raw material, and the commercially available lignin is mostly non-pure lignin such as lignosulfonate, which is not suitable for the preparation of graphene microcrystals. Using this technical solution, pure lignin can be obtained.

As a further improvement of the present invention, the ethanol mass percentage content of the added ethanol-water solvent is 55%, and the mass percentage concentration of the dilute sulfuric acid is 10%.

As a further improvement of the present invention, the raw material of the lignin includes at least one of papermaking black liquor, cellulosic ethanol black liquor, crop straw, bagasse or wood.

Preferably, using bagasse as raw material, using ethanol-water with a mass ratio of 55% as a solvent, in a slightly acidic environment with pH=3 to 4, heating to 200° C. in a sealed reactor for 90 minutes, the lignin and part of hemicellulose are dissolved in ethanol-water solution, the insoluble cellulose is separated after cooling, and the solvent of the remaining solution is evaporated to obtain solid lignin, which contains a small amount of hemicellulose and hydrolyzed sugar. Put the lignin containing a small amount of hemicellulose into a 10% dilute sulfuric acid solution and heat it at 90°C for 120 minutes, in which the hemicellulose is hydrolyzed into soluble sugars, the insoluble lignin is filtered, and washed with deionized water 3 times to Neutral, get pure lignin. Pure lignin can also be obtained by other methods.

By adopting this technical solution, a kind of green, renewable and cheap lignin is used as a raw material to prepare graphene microcrystals, which is more environmentally friendly. Lignin is separated and purified from biomass waste, which not only turns waste into treasure, reduces production costs, but also reduces the consumption of coal and oil, and protects the ecology and environment.

Lignin is a random polymer structure composed of three phenolic monomers. The elemental composition of lignin is about 63.4% carbon, 30% oxygen, 5.9% hydrogen, and 0.7% ash. It is the natural organic polymer substance with the highest carbon content, including a large number of sp2 carbon atoms that form the benzene ring structure. There is also a certain amount of sp3 carbon atoms. When pyrolyzed at high temperature, the sp2 state carbon atoms of lignin are combined into graphene fragments, and the sp3 state carbon atoms bond these graphene fragments together to form the structure of graphene microcrystals. Therefore, lignin is the best raw material for the preparation of graphene microcrystals.

As a further improvement of the present invention, in order to improve the quality of graphene microcrystals, lignin can be copolymerized with phenolic resin, epoxy resin, etc. to form a dense larger block, which is then used to prepare graphene microcrystals.

Compared with the prior art, the beneficial effects of the present invention are:

First, using the technical solution of the present invention, the graphene microcrystals are not only different from crystalline carbon, such as graphite and diamond, but also different from amorphous carbon, such as carbon black and activated carbon, which are long-range disordered, A short-range ordered microcrystalline structure, in which the microcrystals are fragments of graphene composed of sp2 state carbon atoms. These graphene fragments are composed of sp3 state carbon atoms combined with each other by covalent bonds to form a three-dimensional network structure, with a special physical, chemical and electronic properties.

Second, the technical scheme of the present invention uses green renewable resource lignin to replace phenolic resin, furfural resin and epoxy resin with petroleum and coal as raw materials to prepare graphene microcrystals, including papermaking black liquor, cellulose ethanol black Lignin is separated from biomass such as liquid, crop straw, bagasse and waste wood. The raw materials for lignin production are wastes and pollutants from the paper industry, agriculture and forestry. The production cost is low, and it is beneficial to ecology and environmental protection, and has important social benefits, environmental protection and ecological benefits.

Third, the present invention has high economic benefits. Phenolic resin, furfural resin and epoxy resin are all chemically prepared from petroleum, natural gas and coal, and the cost is much higher than lignin prepared from waste. Using lignin as a raw material to prepare graphene microcrystals can greatly reduce the cost of graphene microcrystals.

Fourth, in terms of chemical structure and chemical principle, lignin is the best raw material for preparing graphene microcrystals. Lignin is composed of three elements, carbon, oxygen and hydrogen, and the carbon content is as high as 63%. During thermal cracking, hydrogen and oxygen escape in the form of water molecules, and the resulting glassy carbon does not contain any heteroatoms. In particular, lignin contains a large number of sp2 carbon atoms in the benzene ring structure, and the ratio of sp2 and sp3 carbon atoms is about 24:9, which is very beneficial to the formation of graphene microcrystals.

Description of drawings

Figure 1 Schematic diagram of the structure of graphene microcrystals.

Figure 2 Chemical structure diagram of lignin.

Figure 3. Comparison of graphene microcrystal samples prepared with lignin at different temperatures; Figure 3a is at 800 °C, and Figure 3b is at 1200 °C.

Fig. 4 Scanning electron microscope SEM photo of graphene microcrystalline powder prepared at 1000°C.

Figure 5. Schematic diagram of the preparation of graphene microcrystals using a tubular furnace.

Figure 6 is a schematic diagram of the preparation of graphene microcrystals in a hydrothermal kettle.

7 is a scanning electron microscope SEM image of a graphene microcrystal sample prepared by the one-step method in Example 1 of the present invention.

8 is the Raman spectrum of the graphene microcrystal sample prepared by the one-step method in Example 1 of the present invention.

9 is the XRD pattern of the graphene microcrystal sample prepared by the one-step method in Example 1 of the present invention.

Figure 10 is a photo of the graphene microcrystal sample prepared by the two-step method in Example 2 of the present invention.

Figure 11 XPS spectrum of the graphene microcrystal sample prepared by the two-step method in Example 2 of the present invention.

Reference numerals include:

1- Nitrogen bottle, 2- Tube furnace heater, 3- Temperature controller, 4- Quartz glass tube, 5- Gas washing bottle, 6- Graphite crucible, 7- Lignin, 8- Stainless steel tank, 9- Spiral steel Lid, 10-small beaker; 11-graphite jar, 12-red copper sealing lid, 13-deionized water, 14-lignin.

Detailed ways

The preferred embodiments of the present invention will be further described in detail below.

A graphene microcrystal, a microcrystalline structure composed of graphene crystal fragments composed of sp3 carbon atoms bonded to sp2 carbon atoms, is a new type of glassy carbon, which is different from other types of glassy carbon. , and its structure is shown in Figure 1. The parallel short lines in Figure 1 are graphene segments, and sp3 carbon atoms are chemically bonded to each graphene segment at the intersection of the graphene segments to form a three-dimensional network structure. The invention prepares graphene microcrystals with green renewable resource lignin, including lignin separated from biomass such as papermaking black liquor, cellulosic ethanol black liquor, crop straw, bagasse and waste wood.

It is prepared using the following steps:

1. Extraction of pure lignin

The extraction method of lignin is not the main point of the present invention, but the preparation of graphene microcrystal requires specific pure lignin as raw material, and commercially available lignin is mostly non-pure lignin such as lignosulfonate, which is not suitable for preparing graphite ene microcrystals. For this reason, the present invention uses bagasse as a raw material, uses ethanol-water with a mass ratio of 55% as a solvent, and under the slightly acidic environment of pH=3 to 4, is heated to 200 DEG C in a sealed reactor, kept for 90 minutes, and the wood Lignin and part of hemicellulose are dissolved in an ethanol-water solution, the insoluble cellulose is separated after cooling, and the solvent of the remaining solution is evaporated to obtain solid lignin, which contains a small amount of hemicellulose and hydrolyzed sugars. Put the lignin containing a small amount of hemicellulose into a 10% dilute sulfuric acid solution and heat it at 90°C for 120 minutes, in which the hemicellulose is hydrolyzed into soluble sugars, the insoluble lignin is filtered, and washed with deionized water 3 times to Neutral, get pure lignin. Pure lignin can also be obtained by other methods. The chemical structure of lignin and a photograph of lignin powder extracted from bagasse are shown in Figure 2.

2. Preparation of graphene microcrystals

The preparation process of graphene microcrystals can be divided into the following stages.

(1) Uniform melting of lignin powder

Lignin has no fixed melting point, and the glass point is between 160°C and 180°C. The temperature is slowly raised from room temperature to 180°C, and kept for more than 180 minutes, so that water molecules and other small molecules can escape sufficiently, and no voids are formed in the molten lignin. This process can be carried out in air.

(2) Thermal cracking of lignin

The thermal cracking of lignin should be carried out in an inert gas environment. In the inert gas flow, the lignin is slowly heated from 180°C to 450°C and kept for more than 180 minutes. In this process, the water molecules and small organic molecules that are split out flow out with the inert gas, and the sp2 carbon atoms are recombined into fragments of graphene. This process should be sufficiently slow, and it is best to maintain a certain amount of pressure.

(3) High temperature carbonization of lignin

The preliminarily carbonized lignin is slowly raised from 450°C to a specified high temperature, such as 800°C (or 1000°C, 1200°C, and 2400°C, etc.) in a stream of inert gas, and held for more than 240 minutes. In this process, sp3 state carbon atoms bond the graphene fragments composed of sp2 state carbon atoms with strong chemical bonds to form a stable three-dimensional network.

(4) Annealing and cooling of graphene microcrystals

The graphene microcrystals generated through the above steps are slowly annealed, gradually cooled and cooled to minimize the defects formed by the dangling bonds and free carbon atoms of the graphene microcrystals, and cooled to room temperature to obtain a graphene microcrystal sample.

Stage (2) (thermal cracking of lignin) is a solid-gas reaction, and stage (3) (high-temperature carbonization of lignin) is a solid-phase reaction, which requires a sufficiently long reaction time.

Using lignin as raw material, the quality of graphene microcrystals prepared by the above process, including block size, porosity, uniformity, density, hardness, elasticity, electrical conductivity, etc., has a great relationship with the temperature control program , the slower the temperature rise, the longer the holding time, the higher the temperature and pressure, the better the quality. Commercial production takes 120 to 400 hours, the temperature is as high as 2400 ° C, and a certain high pressure is required. Glassy carbon prepared under normal pressure, shorter time and lower temperature, such as normal pressure, 20 hours and 1200 °C, although the porosity is higher, the ground powder still has the characteristics and properties of graphene microcrystals . Figure 3 shows graphene microcrystal samples prepared with lignin at 800 °C (top) and 1200 °C (bottom), the former has higher porosity and the latter is denser. Figure 4 is a scanning electron microscope (SEM) photo of the graphene microcrystalline powder prepared at 1000 °C. The ground graphene microcrystalline particles are like broken glass slag, with sharp edges and rounded cross-sections, with high The hardness and elasticity are different from the graphene of the prior art.

After the preparation of pure lignin, the quality control of graphene microcrystal preparation mainly depends on the well-designed temperature control program. For the convenience of description, the temperature control procedure and implementation are described below with a preparation process at normal pressure, 1000° C., and 20 hours.

According to the tube furnace device shown in Figure 4, put 10 grams of lignin powder in the graphite crucible, cover the crucible cover, place it in the middle of the quartz tube, use a vacuum pump to evacuate - fill with nitrogen, repeat the operation twice, maintain 50ml/min nitrogen flow, turn on the power, and set the temperature control program as follows.

Step S1: Lignin Melting Stage

At a heating rate of 1 °C/min, the temperature was raised from room temperature 20 °C to 180 °C, and the temperature was maintained at 180 °C for 60 minutes, so that the lignin was fully melted, and water molecules and other small molecules were fully escaped.

Step S2: Lignin Thermal Decomposition Stage

At a heating rate of 1 °C/min, the temperature was increased from 180 °C to 450 °C, and the temperature was maintained at 450 °C for 120 minutes. At this time, the wood polymer structure underwent thermal cracking reaction, and the generated volatile small molecules escaped with nitrogen. The lignin has been partially carbonized at this stage. In order to avoid the appearance of holes, a certain high pressure is required at this stage.

Step S3: Lignin Carbonization and Graphitization Stage

The temperature was increased from 450°C to 1000°C at a heating rate of 2°C/min, and the temperature was maintained at 1000°C for more than 180 minutes to further carbonize and graphitize the lignin. At this stage, the sp2 state carbon atoms begin to be graphitized to form graphene fragments, and the free sp3 state carbon atoms combine with the dangling bonds at the edges of the graphene fragments to form a three-dimensional network structure.

Step S4: Graphene microcrystal annealing cooling stage

At a cooling rate of 3°C/min, the temperature was lowered from 1000°C to 500°C, and then began to cool down naturally until it reached room temperature.

The above process is a total of 1172 minutes, about 19.5 hours.

Example 1

In this example, pure lignin extracted from bagasse by "ethanol-water organic solvent method" is used as raw material, and the "one-step method" preparation scheme is adopted, which is carried out in a quartz tube electric furnace, and the reaction is completed in a nitrogen atmosphere at normal pressure. The device is as follows: Figure 5 shows.

The preparation process includes the following operations:

Step S1: Experimental setup and raw material preparation

According to the tube furnace device shown in Figure 5, put 10 grams of lignin powder in the graphite crucible, cover the crucible cover, place it in the middle of the quartz tube, use a vacuum pump to evacuate - fill with nitrogen, repeat the operation twice, and maintain the 50ml/min Nitrogen flow, power on.

Step S2: Set the temperature control program

The temperature control program of this embodiment is described in Table 1. It is divided into 8 operation stages, excluding the time of natural cooling stage 8, the total operation time of the first 7 stages is 1172 minutes, about 19.5 hours. Set the temperature control program according to Table 1. and stored in the temperature controller.

Table 1 The temperature control program of the "one-step method" preparation scheme of Example 1

Step S3: Run the temperature control program

Turn on the button of the running program and the button of the heating power, and the graphene microcrystal preparation process starts to run automatically. After about 19.5 hours of operation, when the furnace temperature is cooled to room temperature, the tube furnace is turned on, and the graphene microcrystal sample is taken out. The reactions occurring in the above eight stages are described below.

Stage 1: 20°C to 180°C

When the temperature is raised to the glass point of the lignin, the lignin begins to melt.

Stage 2: 180°C to 180°C

The temperature is kept at the glass point of the lignin, the lignin is fully melted, and the water molecules and other small molecules are fully escaped.

Stage 3: 180°C to 450°C

Slowly heating up to the thermal cracking temperature of lignin, the lignin macromolecule begins to crack and partially carbonizes.

Stage 4: 450°C to 450°C

The temperature is kept at the thermal cracking temperature of lignin, the lignin macromolecule is fully cracked and carbonized, and the volatile small molecules produced during the period escape slowly.

Stage 5: 450°C to 1000°C

Slowly heating up to 1000 °C at the pyrolysis temperature of lignin, sp2 carbon atoms begin to be graphitized to form small graphene crystals.

Stage 6: 1000°C to 1000°C

When the temperature is maintained at a high temperature of 1000 °C, the sp2 state carbon atoms are fully combined into graphene crystal fragments, and the free sp3 state carbon atoms are combined with the dangling bonds at the edges of the graphene fragments to form a three-dimensional network structure.

Stage 7: 1000°C to 500°C

The temperature is slowly lowered to 500 °C to minimize the defects caused by free carbon atoms and dangling bonds, and form a stable three-dimensional glassy carbon network structure.

The scanning electron microscope (SEM) photo of the graphene microcrystal powder produced in this example is shown in FIG. 4 ; the Raman pattern of the graphene microcrystal is shown in FIG. 8 ; the XRD pattern of the graphene microcrystal is shown in FIG. 9 . .

Example 2

In embodiment 1, the preparation reaction of graphene microcrystals is always carried out under normal pressure,

Maintaining a certain pressure during the carbonization and graphitization stages is very beneficial to improve the quality of graphene microcrystals. However, the high temperature and high pressure reaction conditions have very strict requirements on the equipment. In order to form a high-pressure reaction environment, this example adopts a "two-step method" scheme to prepare glassy carbon. The first step of the "two-step method" is carried out in a hydrothermal reaction kettle. 50 ml of deionized water and 10 grams of lignin are added to a 100 ml hydrothermal reaction kettle. Preliminary carbonization occurs at this temperature and pressure. In the second step, the initially carbonized lignin was transferred to a tube furnace, and further carbonized and graphitized in a normal pressure and nitrogen environment to prepare graphene microcrystals.

The specific operation steps are described below.

Step 1: Preliminary carbonization in a hydrothermal kettle

Step S1: Installation of the hydrothermal kettle device and adding raw materials

The initial carbonization of lignin was carried out in a hydrothermal kettle, as shown in Figure 6. The schematic diagram of the structure of the hydrothermal kettle. Place a small beaker in the steel barrel of the 100ml stainless steel hydrothermal kettle, add 50ml deionized water and 10g lignin, place a copper sealing ring between the steel barrel and the steel barrel lid, and tighten the steel barrel with a large wrench cover to ensure a reliable seal.

Step S2: Preliminary Carbonization in Hydrothermal Kettle

Put the installed hydrothermal kettle into the muffle furnace, set the temperature controller, and raise the temperature to 350°C at a heating rate of 1°C/min, hold for 120 minutes, and then naturally cool to room temperature.

Step S3: Drying of Preliminary Carbonized Lignin

Take out the small beaker in the hydrothermal kettle, pour out the water, take out the preliminary carbonized lignin, and dry it in a thermostat at 120°C for 2 hours.

The dried carbonized lignin will undergo a second step in a tube furnace for further carbonization and graphitization.

Step 2: Further carbonization and graphitization in a tube furnace

This process is carried out in a tube furnace, and the specific operation is exactly the same as that of the "one-step method" scheme in Example 1, and will not be repeated here. The photo of the graphene microcrystal sample prepared in Example 2 is shown in Figure 10. Due to the churning and rinsing effect of high-pressure steam in the hydrothermal kettle, the graphene microcrystal prepared by the "two-step method" is a shiny and reflective sheet like a lens. , about 3 to 4 mm in length and width. Due to the high pressure in the hydrothermal kettle, the quality of graphene microcrystals prepared by the "two-step method" is significantly better than that of the "one-step method", but the size of the graphene microcrystal samples is smaller. The XPS spectrum of the graphene microcrystals produced in this example is shown in FIG. 11 .

The effectiveness of the technical solution of the present invention is fully confirmed from the Raman pattern ( FIG. 8 ), the XRD pattern ( FIG. 9 ) and the XPS pattern ( FIG. 11 ) of the prepared graphene microcrystals. The D and G peaks in the Raman pattern of graphene microcrystals are completely consistent with those of reduced graphene oxide reported in the upper right corner (Nano Res (2008) 1:273291); in the XRD pattern of graphene microcrystals The diffraction peak at 2θ=25.5° is completely consistent with the reduced graphene oxide reported in the upper right corner (Materials Research. 2017; 20(1):53-61), confirming the existence of graphene crystals in the graphene microcrystal samples . There are tall sp2 state carbon atom peaks and smaller sp3 state carbon atom peaks in the XPS pattern of Figure 11. The peak area of sp2 state carbon atoms is much larger than that of sp3 state carbon atoms, which is related to the composition of graphene microcrystals. Consistent. Graphene microcrystal samples felt very hard when crushed in an agate mortar, exhibiting hardness and stiffness similar to glass and ceramics, and completely different from low-hardness graphite and other carbon materials.

The above content is a further detailed description of the present invention in combination with specific preferred embodiments, and it cannot be considered that the specific implementation of the present invention is limited to these descriptions. For those of ordinary skill in the technical field of the present invention, without departing from the concept of the present invention, some simple deductions or substitutions can be made, which should be regarded as belonging to the protection scope of the present invention.

Claims (8)

1. a graphene microcrystal, is characterized in that: it is the three-dimensional network structure of the graphene microcrystal of chemical bonding, and described graphene microcrystal is made up of sp2 The graphene fragment that state carbon atom forms, described sp2 The graphene fragments composed of state carbon atoms are connected by covalent bonds through sp3 state carbon atoms to form a three-dimensional network structure. 2. Graphene microcrystal according to claim 1, is characterized in that: it comprises the three-dimensional network structure that the tetrahedron of sp3 state carbon atom is directed to connect graphene microcrystal segments. 3. the preparation method of graphene microcrystal according to claim 1, is characterized in that: it comprises the following steps: Step S1, extraction of lignin; Step S2, melting of lignin: the lignin is heated to above 160°C and kept for more than 180 minutes; Step S3, thermal cracking of lignin: in an inert gas environment, the temperature of the lignin is raised to a temperature above 400° C. for more than 180 minutes; Step S4, carbonization of lignin: in an inert gas environment, the temperature of the thermally cracked lignin in step S3 is continued to rise to above 800°C, and kept for more than 240 minutes, and the pressure is 0.1MPa to 40GPa; In step S5, the carbonized raw material in step S4 is graphitized at a temperature of 1000°C to 2400°C, kept for more than 240 minutes, and the air pressure is 0.1MPa to 40GPa; Step S6, annealing and cooling to room temperature. 4. The preparation method of graphene microcrystals according to claim 3, characterized in that: in step S4, the temperature is raised to above 1000°C. 5. the preparation method of graphene microcrystal according to claim 3, is characterized in that: described inert gas environment is a kind of gas or mixed gas environment in nitrogen, argon, helium, water vapor, carbon dioxide. 6. the preparation method of the graphene microcrystal of claim 3, is characterized in that: in step S1, the extraction of described lignin comprises: in the raw material containing lignin, add the solvent of ethanol-water, at pH Under the environment of =3～4, heat to 180～220 ℃ in the sealed reactor, keep more than 60 minutes, separate the insoluble cellulose after cooling, evaporate the solvent of the remaining solution, and obtain hemicellulose and hydrolyzed sugar. The lignin containing hemicellulose and hydrolyzed sugar is put into dilute sulfuric acid solution, heated at 80～100℃ for 80～150 minutes, filtered and washed to obtain pure lignin. 7. The method for preparing graphene microcrystals according to claim 6, wherein the ethanol mass percentage content of the ethanol-water solvent is 55%, and the mass percentage concentration of the dilute sulfuric acid is 10%. 8. the preparation method of graphene microcrystal according to claim 6, is characterized in that: the raw material of described lignin comprises at least one in papermaking black liquor, cellulosic ethanol black liquor, crop straw, bagasse or wood .

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