Impurity elements easily escape from carbon fiber insulation felts in high-temperature environments. They can pollute crystalline silicon during its growth in a monocrystalline silicon production furnace when such felts are used for insulation. Consequently, it needs further graphitization at high temperatures above 2200 ℃ to discharge non-carbon impurity elements, causing its carbon content (mass fraction) to increase to over 99%. The traditional electric heating graphitization technology is associated with several issues, including a long equipment preheating cycle, high energy consumption, harsh requirements for furnace materials, and the inability to sustain high temperatures. Laser heating technology has the characteristics of direct heating, a fast heating rate, and high energy utilization; therefore, it can effectively replace the traditional electric heating method. However, laser graphitization of carbon fiber insulation felts currently faces two problems, namely, an uneven distribution of the power density in the Gaussian beam irradiation area and large temperature gradient in the depth direction of the carbon fiber insulation felt. This study presents the use of laser graphitization technology for carbon fiber insulation felts. The influence of the laser process parameters on the chemical properties and microstructure of carbon fiber insulation felts is revealed, providing theoretical and technical guidance for using laser ultra-high-temperature graphitization to prepare these felts.

Polyacrylonitrile (PAN)-based carbon fiber insulation felts are selected for this study. First, the basic principle of the laser graphitization of carbon fiber thermal insulation felt is introduced. The heat transfer model of the carbon fiber thermal insulation felt is analyzed, and the Gaussian beam emitted by a laser is shaped using a diffractive optical element (DOE). Second, characterization methods for different depth graphitization degrees and carbon contents of the carbon fiber insulation felts are proposed. Then, an experimental device for the laser graphitization of the carbon fiber insulation felt is set up, and a shaped flat-top laser is used to quickly heat the PAN-based carbon fiber felt. Finally, the carbon and sulfur analysis, Raman spectroscopy, and scanning electron microscope are used to analyze the carbon content, graphitization degree, and surface morphology of carbon fiber insulation felts, respectively. The results are used to explore the effects of the laser power density and irradiation time on the graphitization degree and carbon content of the carbon fiber insulation felts at different depths.

The Gaussian beam emitted by the laser has a flatness of 92.46%, and the energy conversion efficiency is 88.94% after passing through the DOE shaping system, which can be approximately evenly distributed (Fig. 3). The laser graphitization process ensures that there is no ablation on the surface of the carbon fiber, while simultaneously obtaining a carbon fiber insulation felt with a carbon content of more than 99% and graphitization degree (R value) of 0.07. Generally, with an increase in the laser power density, the carbon content increases, and the difference in the radial carbon content decreases. When the laser power density is increased to 1120 W/cm² (i.e., the irradiation temperature is 2210 ℃), the overall carbon content of the carbon fiber insulation felt reaches 99% (Fig. 7). The temperature of the carbon fiber insulation felt, as well as the order and graphitization degrees of the carbon material structure, increases with increasing laser power density. In addition, the distribution law of the radial R value of the carbon fiber insulation felt is essentially the same under different laser power densities, indicating that increasing the laser power density cannot improve the uniformity of the radial graphitization degree of the carbon fiber insulation felt (Fig. 9). When the irradiation duration is increased to 120 s, the graphitization degree of the carbon fiber insulation felt reaches its highest value, the R value is 0.07, and the graphitization effect is significant. Increasing the laser irradiation duration can alleviate the issue of the uneven radial graphitization of the carbon fiber insulation felt (Fig. 11).

To ensure uniform graphitization degree and surface morphology during the preparation of carbon fiber insulation felts, this study examines the carbon contents, chemical structures, and surface morphologies of the felts under different laser power densities and irradiation time. The experiments entail laser irradiation on PAN-based carbon fiber insulation felts. The results show that the laser graphitization process can produce a carbon fiber insulation felt with a carbon fiber content of more than 99% and graphitization degree (R value) of 0.07, while obviating ablation on the surface of the carbon fiber. In addition, the radial graphitization degree of the carbon fiber insulation felt becomes more uniform with an increase in the laser irradiation time. The findings of this study can provide theoretical and practical basis for the use of laser irradiation to prepare carbon fiber insulation felts.