"Charcoal Weng Says Carbon" (XII): Carbonaceous Interphase from "Boiling and Stewing"

What is carbonaceous mesophase? How is it still "boiled and stewed"? Is there any particular use for carbonaceous mesophase?

The vast majority of carbon materials and pure carbon materials are obtained by high-temperature heat treatment of carbon-containing organic compounds under the protection of inert gases such as nitrogen to prevent their oxidation and combustion. Depending on the nature of the raw materials, the final product is formed in a very different way. Carbon fiber is made by "burning" non-carbon atoms in high molecular organic matter through solid-phase carbonization (see "carbon ten"), while carbon black and the like are lower hydrocarbons through gas phase carbonization, pyrolyzed into carbon atoms or smaller organic molecules dehydrogenation, and then "grow" again. (See Charcoal Weng said that carbon 11) What changes will occur when organic substances with carbon atoms greater than 20 and boiling point higher than 200 ℃ are heat treated by liquid phase carbonization, and what kind of carbon substance will be formed in the end?

It is said that liquid phase carbonization actually only refers to the initial stage of carbonization below 500 ℃. When higher boiling point organic compounds, heavy oil or asphalt are heat treated below 350 ℃, the whole system will become more and more viscous, as seen when boiling sugar thin, with the increase of temperature and time, it will eventually become a solid with high carbon content. In the process of "boiling and stewing", organic matter will thermally interpret and release various low molecular hydrocarbons, carbon dioxide, hydrogen and other gases. At the same time, organic matter will form aromatic rings through aromatization, and then condense into planar disc-shaped polycyclic aromatic macromolecules (Figure 1).

Figure 1. Formation of discotic polycyclic aromatic macromolecules during liquid-phase carbonization

If there are molecules in the system that can form a large number of planar aromatic hydrocarbons, before the system changes from liquid to solid, the planar discotic aromatic hydrocarbons can be stacked and oriented to form liquid crystals. the schematic model of this liquid crystal is shown in fig. 2.

Figure 2. Schematic model of carbonaceous liquid crystal

When the liquid crystal grows to more than 5 microns, it will become a ball due to surface tension. Under a polarizing microscope, it can be observed that the ball gradually forms and grows up and is optically anisotropic. With the increase of "boiling" temperature and the extension of time, the ball will repeatedly generate, grow up and merge, although there are still different views on the mechanism of how to form a larger ball, but the results are consistent, that is, after the ball will form a streamline, close to the solid whole. Further heating will form a solid semi-coke (Figure 3).

Figure 3. The formation and development of carbonaceous mesophase

This state of transition from liquid to solid state is called intermediate phase (mesophase). Because the term intermediate phase is sometimes used in other fields, they are especially called "carbonaceous intermediate phase". It refers to the mobile phase formed in the process of liquid phase carbonization and can also be considered as pitch containing liquid crystal phase. During the "boiling" process, the low molecular weight gas generated by pyrolysis moves within the high viscosity mesophase to promote the alignment of the aromatic planes along the direction of the gas flow, so that upon final solidification it forms "graphitizable carbon" that is easily converted to graphite upon further high temperature processing ". Depending on the raw materials and processing conditions (whether pressurized, with or without catalyst, with or without non-carbon, other heteroatoms of hydrogen, etc.), some organic compounds may also form "non-graphitizable carbon" that is difficult to form graphite during further high-temperature treatment without passing through the intermediate phase during liquid-phase carbonization ". Because sugar is a carbohydrate (its molecular formula is Cn(H2O)n), the molecule has oxygen atoms, and it is difficult to form planar aromatic molecules when heated. Therefore, an intermediate phase will not be formed when boiling sugar. After water evaporates and the system becomes more viscous, it will be directly carbonized into amorphous carbon with chaotic layer structure.

The processing process of carbonaceous mesophase is not complicated and the manufacturing cost is not high, but the product has high oxidation activity, high carbonization yield and easy graphitization, so it is used as a raw material for a variety of advanced carbon materials. For example, the high modulus carbon fiber made of mesophase pitch, its modulus can reach 1020GPa, almost close to the theoretical modulus of graphite, its thermal conductivity, high electrical conductivity and low thermal expansion rate, is an indispensable material for aviation and aerospace. The mesophase microspheres separated from the mesophase can be prepared by solid-phase carbonization and activation of carbon microspheres with high specific surface area, which can be used as catalyst carriers, chromatographic fillers, anode materials for lithium-ion batteries, and isotropic carbon materials with high density and high strength.

Compared with the ordinary electrode with ultra-high power electrode steelmaking, not only the smelting time can be shortened by 1/3, the power consumption per ton of steel can also be reduced by half, and the production capacity can be increased by 1.3 times. At present, it is almost used in large electric furnace steelmaking. Needle coke made from petroleum or coal tar pitch by liquid-phase carbonization to form mesophase pitch is the main aggregate for the manufacture of ultra-high power electrodes.

The mesophase pitch is foamed at high temperature and high pressure, and can be made into various carbon foams with different properties after carbonization and graphitization. Its thermal properties can be close to thermal insulators, used as heat insulation materials, and its specific thermal conductivity can reach 4 to 6 times that of copper. Become a high thermal conductivity material. Similarly, the resistivity of carbon foam can also vary by 9 orders of magnitude, from 1 × 107cm to 1 × 10-2cm. Carbon foam has low density and high strength and can be formed into various shapes of objects. It can not only be made into isotropic structural composites (carbon fiber composites are anisotropic), but also can be used as functional materials such as wave absorbing and electromagnetic shielding materials. Because carbon foam has a series of excellent properties such as low density, high strength, high thermal conductivity, high conductivity, fire resistance, impact resistance, wave absorption, noise reduction, low thermal expansion coefficient and chemical corrosion resistance, it can not only be applied to small heat dissipation devices of computer devices and heat exchanger systems of large projects, it can also be widely used in aviation, aerospace, and military industries such as thermal transfer systems for aerospace vehicles and satellites, and anti-shock and noise reduction launch platforms for rockets.

This article is from Wang Maozhang Science Net blog.
Link address: http://blog.sciencenet.cn/blog-526286-456017.html