Of Pencils and Diamonds – Everything About Graphite

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Structure and properties of graphite

Graphite has a layer structure. In each layer each carbon atom is bound to three others. This results in a two-dimensional network of hexagons. Within each layer there are strong bonds, but between the different layers the bonds are very weak. Thus, the layers can easily be shifted against each other and even separated. This structure is the reason graphite is very soft and is even used as a lubricant. But graphite has other special properties as well:

Electrical conductivity of graphite

The fact that graphite is electrically conductive results from its atomic structure. Each carbon atom in a graphite crystal has four valence electrons, also called outer electrons, which can form bonds with neighboring atoms. However, only three of the four valence electrons enter into a bond, while the fourth electron remains freely mobile and thus allows electricity to be conducted.

Thermal conductivity of graphite

Graphite has excellent thermal conductivity combined with high temperature resistance. Graphite does not have a melting point; it changes from the solid state directly into the gaseous state. This process is called sublimation. In an inert gas atmosphere, graphite becomes plastically deformable starting at a temperature of °C. At temperatures above °C graphite sublimates even without the presence of oxygen.

Chemical resistance of graphite

Graphite is one of the most chemically resistant materials. It is resistant to almost all media of organic chemistry. These typically include the intermediate and/or end products in the petrochemicals, coal refining, plastics industry, the production of paints, coatings, refrigerants and antifreeze, but also in the cosmetics and food industries. Graphite is also resistant to most inorganic media, such as non-oxidizing acids, alkalis, aqueous salt solutions and most technical gases.

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Download: Chemical resistance of natural graphite

3.1.1. Graphite Particle Characterization

90 and d10 slightly decrease over time (90 ' d10)/d50. The (d90 ' d10)/d50-ratio remains nearly constant at 0.43 over time due to the same operating parameters of the classifier and the cyclone (90-value of 36.74 µm. The sharpness of the particle size distribution is increased by spheroidization and classification. In general, the spheroidization of the graphite flakes results in a decrease of the average particle size by 5 µm and by nearly 50% of the d90-value. A sharp particle size distribution with a high yield is conclusively obtained after 15 min. The pancake-shaped graphite particles exhibit a broader particle size distribution with 0.69 and a slightly increased mean particle size. While the d10-value stays the same, the d90-value increases to 21.42 µm (

NFG was treated by the GyRHO system from Netzsch in a scalable spheroidization process. During stressing the NFG in the classifier system, the graphite flakes were formed into rounded particles by impact. Product quality and the intrinsic properties of the spheroidized graphite were characterized by different criteria. The spherical graphite is compared with the natural graphite starting material (Gr_flake) and the shaped pancake morphology (Gr_pancake). Figure 2 a illustrates the yield of the spheroidization process for different treatment times. While the other operating parameters stayed the same, the yield declined with increasing treatment time. A treatment time of 15 min results in a yield of 55%. After 30 min, the yield reduces to 38%. With higher treatment times, the amount of smaller particles rises. Smaller graphite particles for the demanded product particle size of 8 µm < d < 36 µm are screened out of the system by the internal classifier wheel, reducing the overall yield. Gr_pancake in the up-scaled CSM 360 results in a yield of 14% after 20 min and requires further optimization. Characteristic particle sizes dand dslightly decrease over time ( Figure 2 and Table S1 ). However, the particle size distribution remains nearly constant for the different treatment times ( Figure 2 b). The sharpness of the particle size distribution can be characterized by the ratio of (d' d)/d. The (d' d)/d-ratio remains nearly constant at 0.43 over time due to the same operating parameters of the classifier and the cyclone ( Table S1 ). Gr_flake has a broad particle size distribution with 1.36 and a d-value of 36.74 µm. The sharpness of the particle size distribution is increased by spheroidization and classification. In general, the spheroidization of the graphite flakes results in a decrease of the average particle size by 5 µm and by nearly 50% of the d-value. A sharp particle size distribution with a high yield is conclusively obtained after 15 min. The pancake-shaped graphite particles exhibit a broader particle size distribution with 0.69 and a slightly increased mean particle size. While the d-value stays the same, the d-value increases to 21.42 µm ( Table S1 ).

SEM images illustrate the spheroidization of graphite. Figure 3 a'd provide an optical comparison between the spherical samples, the flake graphite, and the Gr_pancake. The NFG has clear edges and shows the natural flake-like structure of graphite ( Figure 3 a). SEM images of the rounded graphite show distinct morphological changes ( Figure 3 b,c). The particles exhibit a more rounded shape and a much smoother surface with increasing treatment time. While edges are still present after 15 min, they disappear with increased treatment time. Cracks and step-like undulations are visible on the surface of the spherical particles in Figure S1a in the Supplementary Materials . Gr_pancake particles evince more of a pancake-like morphology than a potato-like shape. Edges were rounded by the comminution process as well, and a plane surface was observed. Thus, the spheroidization process eliminated the edges of the graphite flakes and formed rounded particles. Smaller flakes could be distinguished to a low extension on the surface and were most likely due to renewed attachment of graphite fragments onto the surface of larger spherical particles. After 30 min, the flake character of the graphite was no longer evident, and the graphene layers were rounded on the surface ( Figure S1b ).

2 g'1 (

The increasing treatment time of the graphite particles resulted in an increase in the specific surface area of graphite ( Figure 4 a). While a smaller specific surface area with more spherical particles is expected, the opposite trend is observed. Biber et al. also determined a higher surface area of their rounded graphite particles depending on the applied process energy [ 34 ]. Figure S2 illustrates the link between the volumetric specific surface area, which is calculated by the mean particle size, and the measured specific surface area. A correlation with the decreasing mean particle size can be determined. Fine particles have a larger specific surface area and the possibility to reattach onto coarser particles. While the ratio of particles greater 17 µm decreases and the ratio of spherical graphite particles smaller 10 µm increases, the specific surface area increases at the same time. Figure 4 a shows the link between the ratio of particles <10 µm and >17 µm and the specific surface area. Although Gr_flake has the highest proportion of larger particles, the proportion of particles smaller than 10 µm is also high. The result is a smaller specific surface area than the spheroidized graphite particles. After 30 min of spheroidization, the surface area has more than doubled in comparison to the NFG. This is a result of an interaction between the ratio of small particles and the formation of new micropores and cracks caused by the spheroidization process. Mundzinger et al. [ 32 ] determined an increase in the proportion of open pores accessible to nitrogen molecules by evaluating FIB tomograms. New pores and cracks were created due to rounding and an increase in energy impact. Cracks within the spherical graphite particles coated with an internal SEI film were also located by Zhang et al. [ 44 ]. Newly created pores result in higher adsorption of nitrogen molecules and, thus, a greater specific surface area. The specific surface area of the pancake-shaped graphite is 12.5 m Figure 4 b) and significantly higher compared to spherical graphite. In addition to the particle ratio and inner porosity, the morphology of the graphite particles also influences the specific surface area [ 9 ]. The different spheroidization processes and operation times result in an increase in the specific surface area of the graphite particles ( Figure 4 b).

'3 for spherical graphite; flake graphite reached a coating density of 2.02 g cm'3. Point-to-point interfaces due to the shape and increased mechanical stability are the reason for inhibition of the electron transfer with the spherical graphite material. However, the smaller and more spherical graphite particles generate a closer packing at low pressure and, therefore, a higher density in comparison to the coarser graphite flakes ('3 with the minimal roller gab. Due to their pancake-like shape, the particles can rearrange and be compressed more easily with less force.

The intrinsic properties of graphite were further investigated by measuring the electric powder conductivity as a function of density or pressure, respectively ( Figure 4 c,d). With increased pressure and, thus, increased compression, the electric powder conductivity of graphite increases ( Figure 4 d). At the same time, the packing density of the graphite rises ( Figure 4 c). Flake graphite (Gr_flake) shows significantly higher conductivity at lower densities than spherical graphite particles. For the spherical graphite, the conductivity increases significantly with increasing density, whereas the change in density is relatively low. Essentially, the conductivity increases with greater contact areas between the particles. Parameters such as particle shape, interfacial forces between particles, electron transport mechanism, and packing density have a significant influence on the contact areas between particles [ 45 46 ]. Heo et al. described electrical conductivity by two mechanisms: inter-particle conductivity and intra-particle conductivity [ 46 ]. Since Gr_flake is a planar flake graphite, wider surface-to-surface contact points are formed. Spherical graphite particles generally form point-to-point contacts rather than surface-to-surface contacts. In the first compression stage, the density is controlled by the rearrangement and fragmentation of agglomerates. A second stage is determined by the elastic and plastic deformation of the particles [ 45 ]. Gr_flake converges, and the contact areas become greater when it is compressed under high pressure. The high compression pressure causes individual graphite flakes to break, refold, and compress, thereby increasing the density. A higher density of graphite leads to higher electrical conductivity because conductive paths are shorter (inter-particle conductivity) [ 47 ]. Conduction by point-to-point interfaces of the rounded graphite particles (Gr15'Gr30) limits inter-particle conductivity. The decrease in electrical conductivity with increasing treatment time occurs due to the stability of the spherical graphite particles. The graphite particles become rounder and smaller, resulting in a more stable shape. Particles deform less with increasing compression and remain connected by point-to-point interfaces. A higher line load for calendering with a minimal roller gap of 25 µm exhibits the rising stability of the particles in Table S2 . The spherical graphite particles become more stable with increasing treatment time, and the force to deform increases. The density of the coating layer was limited to ~1.91 g cmfor spherical graphite; flake graphite reached a coating density of 2.02 g cm. Point-to-point interfaces due to the shape and increased mechanical stability are the reason for inhibition of the electron transfer with the spherical graphite material. However, the smaller and more spherical graphite particles generate a closer packing at low pressure and, therefore, a higher density in comparison to the coarser graphite flakes ( Figure S3 and Figure 4 c). Electrodes with Gr_pancake require a lower line load for a density of 1.9 g cmwith the minimal roller gab. Due to their pancake-like shape, the particles can rearrange and be compressed more easily with less force.

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