To further enhance the mechanical properties of carbon fibre, particularly its modulus, and to meet the requirements of specialised applications, it is often necessary to subject the carbon fibre to high-temperature treatment, namely graphitization. Graphitization is typically conducted within high-temperature furnaces under inert gas protection, subjecting the carbon fibres to temperatures of 2000–3000°C. This process facilitates the further removal of non-carbon constituents from the fibres while concentrating the carbon content, ultimately achieving a carbon purity of 99%–100%. Concurrently, structural transformations within the fibre occur: the diameter (La) of graphitic microcrystalline units increases, interlayer spacing decreases, and microcrystals exhibit enhanced axial orientation along the fibre. This significantly elevates the elastic modulus of the carbon fibre. High-temperature graphitization is an essential process step for producing both high-modulus carbon fibres (e.g., Toray M40) and high-strength, high-modulus carbon fibres (e.g., Toray MJ series).
The high-temperature graphitisation process can be categorised into two-step and one-step methods depending on the implementation approach. The two-step graphitisation process typically involves subjecting wound carbon fibre products to further high-temperature energy treatment to produce graphite fibres. This two-step process generally encompasses stages such as yarn laying, degumming, passivation, high-temperature graphitisation, surface treatment, sizing, and rewinding. The advantage of the two-step graphitisation method lies in its independence from carbon fibre production processes, offering considerable process flexibility. Its disadvantages include a greater number of process steps, with degumming and passivation significantly impacting the strength properties of carbon fibres, thereby hindering improvements in fibre strength. Additionally, production and preparation costs are relatively high. The two-step process currently represents the primary method for producing M40-grade high-modulus carbon fibres in China.
Unwinding and laying involves the uniform release of pre-wound carbon fibre bundles at a constant speed, followed by the placement of multiple fibre strands onto a flat surface to prepare for subsequent processes. Unwinding typically employs dedicated equipment fitted with damping mechanisms, where both unwinding speed and fibre tension are meticulously controlled. Maintaining a continuous and uniform unwinding rate is fundamental to achieving stable, continuous graphitization treatment. Laying entails the orderly and even distribution of multiple fibre strands across a flat plane, with careful regulation of the inter-fibre spacing. Excessively narrow spacing causes inter-fibre friction during processing, leading to fraying that disrupts the normal, stable graphitization process and compromises the fibers’ mechanical properties. Conversely, overly wide spacing reduces the equipment’s production efficiency.
Degumming is the process of pre-treating carbon fibers in an air furnace at temperatures between 300 and 500°C. As finished carbon fibers typically carry a sizing agent protecting the fibers, degumming is required prior to high-temperature graphitization to prevent damage to the fibers caused by the vigorous decomposition of the sizing agent during the high-temperature treatment. The degumming process must be stabilised to match the carbon fibre sizing agent. The principle is to completely remove the sizing agent from the fibre surface while minimising damage to the filaments caused by oxidation. As the oxidation reaction rate of carbon fibres increases sharply above 500°C, degumming temperatures rarely exceed this threshold. For sizing agents with higher decomposition temperatures, the degumming duration may be appropriately extended to achieve complete removal.
Passivation is a treatment process conducted at 800–1200°C under nitrogen atmosphere. Following degumming, carbon fibers exhibit decomposed sizing agents that leave reactive oxygen-containing functional groups on their surfaces. These oxygen-containing groups generate oxygen during subsequent high-temperature heat treatment, which can adversely affect the structure and properties of the treated fibers. Consequently, degummed carbon fibers must undergo further passivation treatment to eliminate the reactive functional groups remaining on the fibre surface after degumming.
Graphitization is the process of heat treating carbon fibers at temperatures exceeding 1800°C. Graphitization must be conducted under inert gas protection. At temperatures below 2600°C, high-purity nitrogen gas with an oxygen content below 1×10⁻⁶ can serve as the protective atmosphere. However, at temperatures exceeding 2600°C, nitrogen in the atmosphere may react chemically with the carbon (C) in the fibers. Consequently, argon gas must be employed as the protective atmosphere at these elevated temperatures. Temperature, duration, and tension constitute the three primary process parameters governing the coarse-grained graphitization of carbon fibers, collectively determining the structural properties of the final fibre product.
To enhance the composite properties between graphite fibers and matrix materials such as resins, surface treatment of the graphite fibers is typically required. The surface treatment of graphite fibers is similar to that of carbon fibers, employing anodisation. However, as the surface of graphite fibers is more inert than that of carbon fibers, the process conditions for surface treatment must be correspondingly adjusted.
The one-step graphitization process involves subjecting carbon fibers to high-temperature graphitization immediately after high-temperature carbonization. As this method eliminates the need for repeated processes such as degumming and passivation, it streamlines the production workflow, thereby enhancing fibre strength properties and reducing manufacturing costs. The one-step high-temperature graphitization process also offers considerable flexibility in process adjustment. The pre-oxidation and carbonization steps can be tailored to meet the structural requirements of carbon fibres during high-temperature graphitization, effectively emulating the capabilities of a two-step process. This approach demands higher technical proficiency, as the stability of the pre-oxidation and carbonization stages directly influences the stability of high-temperature graphitization and ultimately affects the performance of the final fibers. The one-step graphitization process represents an inevitable trend in the production of high-strength, high-modulus carbon fibers.
Carbon fiber is a brittle material, exhibiting tensile elongation at break typically between 1% and 2% at ambient temperatures. Owing to its high modulus, it generally undergoes elastic deformation under tensile stress at room temperature prior to fracture, rendering it difficult to stretch. However, at elevated temperatures of 1800°C, carbon fiber demonstrates a degree of plasticity. Research indicates that at 1800°C, carbon fiber strain can reach 6%, while at 2000°C, maximum strain exceeds 10%. Theoretically, this means carbon fiber can be stretched by 5%-10% when temperatures exceed 1800°C.
The high temperatures required for carbon fibre graphitization entail significant energy consumption at elevated temperatures and shorten the service life of high-temperature equipment, resulting in substantial production costs. This severely restricts the application of graphite fibers in industrial sectors. Accelerated graphitization enables reduced graphitization temperatures (without diminishing graphitization degree) while meeting performance requirements. This simplifies equipment specifications, minimises thermal stress, shortens processing times, achieves energy savings, and lowers manufacturing costs. Consequently, accelerated graphitization remains a primary research focus for scholars worldwide within the carbon fibre sector.
The catalysed graphitization of carbon materials dates back to the late 19th century, with research activity peaking during the 1960s and 1970s. Scholars from Japan and Germany conducted extensive studies on the influence of metallic or mineral additives during the graphitization process. This catalysed graphitization involves a complex interplay of both physical and chemical transformations. Two primary mechanisms currently explain its action: firstly, the dissolution-reprecipitation mechanism. The catalyst dissolves carbon, and when disordered carbon reaches saturation, the graphite becomes supersaturated. Under the energy difference between ordered and disordered carbon, the dissolved carbon crystallises from the liquid phase as low-energy graphite. The second is the carbide transformation mechanism. The element first combines with carbon to form carbides. Upon further heating, the carbides decompose to produce graphite or carbon that readily graphitises.
Additionally, in carbon fibre graphitization research, fibre performance can be enhanced through alloy precipitation/coating, increased graphitization pressure, application of strong magnetic fields, and radiation exposure. Studies indicate that coating/depositing nickel-iron alloy onto carbon fibers achieves graphitization effects equivalent to 2400°C at 1400°C. Elevating pressure or applying strong magnetic fields during graphitization can moderately improve the tensile strength of carbon fibers. Recent studies have employed γ-ray irradiation treatment to enhance carbon fibre graphitization, thereby increasing the modulus of carbon fibers with minimal reduction in fibre strength.
Although catalysed graphitization can effectively enhance the modulus of carbon fibers during graphitization, no ideal method suitable for continuous production has been established to date. Consequently, catalysed graphitization remains largely confined to the research stage.