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Several Methods for Preparing Nano-graphite Powder

(1) Mechanical grinding


The mechanical grinding method takes advantage of the fact that the interlayer force of graphite is much less than that of carbon atoms in the layer, which makes the bonding between layers looser. Therefore, when graphite is subjected to external force, cleavage occurs easily between layers, and the cleavage surface (base surface) appears to prepare flake graphite powder. In ultra-fine comminution equipment, ball mill finishes fine grinding operation by means of impact and grinding of grinding medium under gravity and centrifugal force field. However, due to the increase of surface energy of graphite, electrostatic adsorption easily occurs between the flaky edges of irregular minerals, the tendency of agglomeration among fine particles is obviously enhanced, and the self-lubricating property of graphite increases, which makes the process of graphite fining long and energy consumption strict. The traditional high-energy ball milling method is quite inefficient when graphite is crushed to nanometer scale. When high-energy ball milling is used, adding liquid medium into the grinding tank will protect the grinding object to a certain extent, thus having an important influence on the structure and morphology of the grinding products. According to the different media used, ball milling can be divided into dry milling and wet milling. Dry milling refers to grinding in vacuum or in contact with graphite as air or other protective gas, while wet milling refers to grinding after adding liquid to the grinding tank.
Graphite was grinded by a drum ball mill under the condition that the pressure in the grinding tank was pumped below 0.01 Pa at room temperature, and then grinded for 100 hours to obtain graphite with a thickness of 20 nm and a length of 50 nm. In 2006, using high purity flake graphite powder as raw material, Hentsche et al. B1 put the grinding tank into liquid nitrogen before grinding, changing the ambient temperature to 77K of nitrogen liquefaction temperature, changing the direction of grinding every 30 minutes, and finally obtained graphite sheets with thickness less than 20 nm. The mechanical grinding process is simple and easy to operate, but it not only crushes the graphite layer by using the movement of the moving object inside, but also contains the shear caused by the contact with the wall, which results in relative slip between the layers. It also affects the structure of the graphite when the thickness of the graphite sheet is reduced. ABCABC graphite is transformed from ABABAB graphite. In addition, because graphite itself has lubrication characteristics, grinding is a very long process, which requires a lot of energy, and its operation process is quite complex, including dehydration, drying, secondary grinding and grading.

(2) Detonation cracking method


Detonation cracking uses the properties of graphite that can accommodate external anion layers to form expansible graphite or low-order GICs, in which the ionic layers are called insertion layers. In expansible graphite or low-order GICs, the insertion layers are arranged regularly in graphite sheets. During detonation, the insertion layer decomposes rapidly and releases a large amount of gas, impacts the graphite layers and pushes the adjacent graphite layers apart, thus preparing nano-graphite sheets. In the process of detonation, explosives play two roles at the same time: one is to release a large amount of heat to decompose expansible graphite or low-order GICs; the other is to smash graphite fragments by shock wave generated during detonation, so as to refine graphite, thus producing flaky graphite with small diameter and very thin thickness.

At present, according to the characteristics that graphite can form stable low-order GICs only in strong acid environment, graphite is mixed with strong oxidizing acid to prepare stable GICs, and then the explosive component is added. By detonating the explosive, graphite sheets with diameter of micron and thickness of 40-100 nm can be prepared. Moreover, the graphitization degree of the prepared products is very high and the specific surface area is acceptable, which can increase to 7-9 times of original graphite.

(3) Ultrasound comminution


Ultrasound fragmentation of expanded graphite is an extremely special physical environment, which produces local high temperature and high pressure by ultrasonic cavitation. It can completely separate the graphite layers on expanded graphite and make expanded graphite into completely free nano-graphite micro-sheets. In the process of ultrasonic pulverization of expanded graphite, the solvent can easily enter the pores and crevices of expanded graphite. Under the action of ultrasonic wave, cavitation bubbles are formed and burst in the solvent medium, accompanied by energy release. The instantaneous implosion caused by cavitation has a strong shock wave. The rapid formation and sudden collapse of cavitation bubbles in liquid produce a short high-energy micro-environment. In nanosecond time, the high temperature of 5000K and the high pressure of 500 Atrn can reach 5000K, and the heating and cooling speed is greater than 109 K/s. The high-speed jet produced separates the nano-graphite sheet from the expanded graphite and enters the solvent medium. Therefore, the smashing of expanded graphite by ultrasonic wave is a shock wave mechanism, which has the effect of both cavitation shock wave and micro-jet. Flake graphite powder was obtained by ultrasonic pulverization. Expanded graphite was prepared by a specific process (such as intercalation, washing, drying, thermal shock, etc.). LG was dispersed in 400 ml ethanol aqueous solution (70%) and treated with 100 W ultrasound for 8-12 hours. The products after ultrasonic pulverization were filtered and dried. SEM measurements show that the diameter and thickness of graphite sheets after ultrasonic treatment are 13 drum m and 10-100 nm, with an average thickness of 52 nm.

(4) electrochemical intercalation


In principle, the electrochemical intercalation method is similar to the previous methods. Graphite electrodes are used as raw materials to make some cations move rapidly to the cathode by electrolysis, while some anions move rapidly to the anode. Under the action of the electrolytic gravity, they are inserted into the graphite electrodes, which makes the graphite expand in the c-axis direction, and the expansion results in the increase of the interlayer spacing. More ions are inserted into the graphite layer, and the interlayer force decreases gradually. When the direction of the electrode is changed, the ions will move rapidly in the opposite direction, thus destroying the interaction force between graphite layers and preparing nano-graphite sheets.
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