Design Principles of Graphitization Furnaces and What is Graphitization?

2025-12-19 09:43:06

We have been promoting our Graphitization furnaces, but many may not fully understand what graphitization entails. Here, Zhuzhou Hechuang offers a brief analysis of what graphitization is and its general research history. Graphitization is the process of using thermal activation to transform thermodynamically unstable carbon atoms from a turbostratic structure into an ordered graphite crystal structure. Therefore, during graphitization, high-temperature heat treatment (HTT) is applied to provide the energy required for atomic rearrangement and structural transformation.

Now, let’s discuss its research history to understand how it was established.

I. Carbide Conversion Mechanism

This theory was proposed by the American Acheson based on the discovery of coarse artificial graphite crystals during the synthesis of silicon carbide. He suggested that the graphitization of carbon materials first involves reactions with various minerals (such as SiO₂, Fe₂O₃, Al₂O₃) to form carbides. These carbides then decompose at high temperatures into metal vapor and graphite. The minerals act as catalysts in the graphitization process.

Since heating in a graphitization furnace progresses from the core outward, the combination of minerals and carbon in the coke first occurs at the furnace center. The metal vapor produced by high-temperature decomposition then reacts with carbon near the outer center of the furnace to form carbides, which subsequently decompose at high temperatures. In this way, a small amount of minerals can convert a large quantity of carbon into graphite. In graphitization furnaces, many silicon carbide crystals can indeed be found, and undecomposed minerals or decomposed graphite are often observed on the surface of artificial graphite products. However, studies have shown that graphite formed from carbide decomposition differs in properties from graphite produced through structural rearrangement of graphitizable carbon. Low-ash petroleum coke can achieve a higher degree of graphitization compared to high-ash anthracite. Pre-demineralization of petroleum coke or anthracite further enhances their graphitizability. In fact, at lower graphitization degrees, certain mineral impurities do catalyze graphitization, but the catalytic mechanism is not limited to carbide formation. At higher graphitization degrees, mineral impurities often induce lattice defects, hindering further improvement in graphitization. Therefore, while the carbide conversion theory holds for decomposed graphite, it does not align with the reality of graphitization for most carbon materials.

II. Recrystallization Theory

With the advent of X-ray diffraction technology, studies on graphite powder diffraction patterns revealed a close relationship between the degree of graphitization and crystal growth. For example, during the graphitization of petroleum coke, lattice changes begin at around 1500°C. As the temperature rises, these changes intensify, particularly between 1600°C and 2100°C, where crystal growth is most rapid. Beyond 2100°C, crystal growth gradually slows and essentially ceases at 2700°C. Since this process closely resembles the recrystallization phenomena observed in metals during high-temperature heat treatment, Tammann extrapolated this into the recrystallization theory of graphitization.

The main points of this theory are as follows:

Carbon raw materials originally contain minute graphite crystals. During graphitization, thermal effects cause these crystals to "weld" together via carbon atom displacement, forming larger graphite crystals.

New crystals form during graphitization. These new crystals grow by absorbing external carbon atoms at the contact interfaces of original crystals, maintaining the orientation of the original crystals.

The degree of graphitization is related to crystal growth but primarily depends on the graphitization temperature, with the duration of high-temperature exposure having limited influence.

The ease of graphitization is related to the structural properties of the carbon material. Porous and loose materials hinder the thermal motion of carbon atoms, reducing opportunities for "welding," thus making graphitization difficult. Conversely, densely structured materials facilitate contact and "welding" of carbon atoms due to less spatial hindrance, making them easier to graphitize.

The size of graphite crystals increases with temperature, but this is a quantitative change rather than a qualitative transformation.

Tammann’s recrystallization theory, to some extent, explains the relationship between crystal growth and graphitization temperature, as well as the influence of raw material properties on the degree of graphitization, representing an advancement over the carbide conversion theory. However, it fails to explain the nature of the minute graphite crystals present in raw materials. Moreover, graphitization is a far more complex multi-stage process than described by recrystallization theory, involving not only increases in crystal size but also qualitative changes such as alterations in atomic bonding and ordered arrangement.

III. Microcrystal Growth Theory

In 1917, Debye and Scherrer, while studying the X-ray diffraction patterns of amorphous carbon, noted similarities with graphite patterns, with some lines overlapping. They concluded that amorphous carbon consists of graphite microcrystals and that the primary difference between amorphous carbon and graphite lies in crystal size. Based on this, Debye and Scherrer proposed the microcrystal growth theory of graphitization.

Subsequent research has enriched and developed this theory, making it widely accepted. The theory posits that the parent substances of graphitizable materials are polycyclic aromatic hydrocarbons. Under thermal influence, these compounds undergo a series of pyrolysis reactions at different temperatures, forming aggregates of large planar molecules—randomly stacked hexagonal carbon networks known as "microcrystals." These microcrystals exhibit order in two-dimensional space but lack long-range order in three-dimensional space, characterizing a turbostratic structure. Thus, microcrystals are not true crystals. However, under graphitization conditions, due to interactions between carbon atoms, the carbon network planes of microcrystals can twist to varying degrees, tending to align parallel to each other. Clearly, microcrystals form the basis for the transformation of amorphous carbon into a graphite structure. Most amorphous carbon contains microcrystals, but not all can transform into graphite under typical graphitization conditions. This is because the aggregation state of microcarbons in amorphous carbon varies depending on the chemical composition and molecular structure of the parent material, leading to different graphitizability. The aggregation states range from essentially parallel orientation to completely random orientation, with intermediate states of varying orientation degrees. For example, petroleum coke and anthracite, with their essentially parallel-oriented microcrystals, are easily graphitizable and are termed graphitizable carbon (or easily graphitizable carbon). In contrast, materials such as sugar carbon, bone char, or wood charcoal, with randomly oriented microcrystals, high porosity, and significant oxygen or hydroxyl groups, are difficult to graphitize.