What Color Is Graphite?

In life and industry, graphite is extremely common and important, from pencil leads to aerospace fields have its figure. When it comes to the color of graphite, most of us think it is black. In fact, its color is affected by a variety of factors, and it has a unique meaning in different scenes.

The natural color of graphite

In essence, pure graphite appears dark gray, close to black. This is determined by its internal atomic structure and electron cloud distribution. Graphite has a typical layered crystal structure in which the carbon atoms are covalently bonded and the layers are bonded by weak van der Waals forces. This structure makes the carbon atoms have a special distribution of electrons, and the electrons can move relatively freely within the layer. When light hits the surface of the graphite, most of the visible light is absorbed. And only a small amount of light is reflected back, which gives the graphite a dark gray to black appearance.

In nature, after simple mining and preliminary purification of graphite ore, the graphite raw materials we see basically show this typical dark gray. It has uniform color, fine texture, and greasy feeling when touched. This is also an important feature of graphite, further indicating its special layered structure.

The factors affecting the color of graphite

Influence of impurities

Graphite is easily mixed with impurities during mining, processing and application, and then changes color. Like the metal impurity iron, its atomic electronic structure is different from that of carbon atoms. And it has a unique absorption of specific wavelengths of light. As the iron content increases, the graphite will appear brown or dark brown. Because the iron interferes with the graphite’s original absorption and reflection of light.

Organic impurities also have an impact. They form films or inclusions inside and outside the graphite, resulting in dark graphite color, with other tones. For example, when it contains trace humus, the graphite is slightly grayish dark brown. This is the result of the organic impurities’ own color and interaction with graphite. The impurities change the optical properties of the graphite and eventually change its color.

Influence of processing method

The processing method has a prominent effect on the color of graphite. When grinding, the graphite becomes a fine powder and the color is darker. Because the specific surface area of the powder is large, the light is scattered and absorbed many times between the particles. And the reflected light is less. For example, in the production of pencil lead, the color of the ground graphite powder is darker than the original raw material. It is easy to leave black marks.

High temperature can regulate the internal structure of graphite, remove impurities or trigger chemical reactions, make the color more pure. However, if the temperature is too high and the time is too long, the graphite will be oxidized. And the oxide film will be generated on the surface, the color will become grayish black or even grayish white. Different processing methods achieve color change by changing the state of graphite and adjusting its effect on light.

Microstructure changes affect

The change of graphite microstructure will also change the color. When the layered structure is distorted or deformed by external forces or the layer spacing changes, the color will change. For example, the preparation of less layers of graphene by mechanical stripping. Due to the reduction of the number of layers and the change of microstructure, the interference color such as blue and green will appear on the specific substrate. This is caused by the interference of light in the multi-layer film structure.

Chemical intercalation can also change the microstructure and color. Potassium atoms are inserted between graphite layers to form intercalation compounds, which become bronze or golden in color. Because intercalation atoms change the electron and band structure of graphite, they affect the absorption and emission characteristics of light. It essentially changes the interaction between graphite and light, and achieves color change.

The embodiment and significance of graphite color in different applications

Pencil manufacturing

In pencil manufacturing, graphite color is a key performance indicator. Pencil lead is made of graphite mixed with clay, the ratio of which determines hardness and color. Graphite is more dark color, writing marks are obvious, like 2B pencil, suitable for painting card, sketch. More clay hardness, light color, HB pencil proportion is moderate, suitable for daily writing. To meet people’s different writing and painting needs.

Battery field

In the field of batteries, the color of graphite can reflect the performance quality. You can commonly use graphite as a negative electrode material for lithium-ion batteries. And the high-quality graphite negative electrode is uniformly black, indicating high crystallinity, less impurities, and stable structure. This is conducive to lithium ion embedding and removal, ensuring battery charging and discharging efficiency and cycle life. If the color is gray or uneven, there may be impurities, structural defects and other problems, resulting in poor battery performance.

Material coating

When using graphite for material coating, the color is significant. In the anticorrosive coating, the graphite coating can be anticorrosive, and the black appearance also has a decorative effect. In the aerospace field, the color of graphite matrix composite coating is related to beauty and thermal radiation performance. And the black coating can effectively adjust the surface temperature of the aircraft. So that it can operate stably in extreme environments.

Color of special graphite materials

Nano graphite

Nanometer graphite refers to the size of the nanometer order of graphite materials. Its color performance is different from the traditional graphite. Due to the influence of quantum size effect and surface effect, nano graphite usually presents brownish black or dark brown in solution. It is obviously different from the dark gray or black of bulk graphite. This is because the nanoscale graphite particle size is close to the wavelength of visible light. And light is strongly scattered and absorbed on the surface of the nanoparticle, resulting in a change in its color. In addition, the surface of nano graphite is usually accompanied by some active groups. This also affects its absorption and reflection of light, further changing its color.

Modified graphite

Modified graphite is a material obtained by modifying graphite through physical or chemical methods, and its colors are equally diverse. For example, when graphite is modified by surface oxidation, a large number of oxygen-containing functional groups are introduced to the surface. This makes it lighter in color, which may appear grayish black or light gray. The color of the modified graphite deposited on the surface of the graphite by electroless plating and other methods depends on the deposited material. If silver is plated on the surface of the graphite, the modified graphite will show a silver gray. This color change not only enrichis the appearance of the graphite material, but also gives the it new properties. So that it has a wider range of applications in electronics, optics and other fields.

Conclusion

The color of graphite is not a single, the natural color is dark gray and nearly black. But it is affected by impurities, processing methods, microstructure and other factors. In different fields, its color is associated with material properties and practical significance. And special graphite materials also show unique color characteristics.