Improving the mechanical properties and thermal conductivity of mesophase-pitch-based carbon fibers by controlling the temperature in industrial spinning equipment

Abstract

Mesophase-pitch-based carbon fibers (MPCFs) were prepared using industrial equipment with a constant extrusion rate of pitch while controlling the spinning temperature. The influence of spinning temperature on their microstructures, mechanical properties and thermal conductivities was investigated. SEM images of the fractured surface of MPCFs show that the graphite layers have a radiating structure at all spinning temperatures, but change from the fine-and-folded to the large-and-flat morphology when increasing the spinning temperature from 309 to 320 oC. At the same time the thermal conductivity and tensile strength of the MPCFs respectively increase from 704 W·m−1·K−1 and 2.16 GPa at 309 oC to 1 078 W·m−1·K−1 and 3.23 GPa at 320 oC. The lower viscosity and the weaker die-swell effect of mesophase pitch at the outlets of the spinnerets at the higher spinning temperature contribute to the improved orientation of mesophase pitch molecules in the pitch fibers, which improves the crystallite size and orientation of the MPCFs.

1. Introduction

Mesophase-pitch-based carbon fibers (MPCFs) have been widely used in the fields of aerospace, electronic products, nuclear industry and industrial robots[1–5] owing to their high thermal conductivity and high modulus. These excellent properties are ascribed to their highly oriented large-size graphite microcrystal structure which was derived from the orientated structure of the discoid nematic liquid crystal molecules formed in spinneret channels from the mesophase pitch. It is generally believed that the mesophase pitch molecular arrangement formed in the melt-spinning process plays a decisive role in the structures and properties of the resultant MPCFs. In comparison, the subsequent oxidative stabilization, carbonization and graphitization provide just a further modification and improvement[6]. Therefore, it is necessary to investigate systematically the effect of the spinning conditions on the microstructures and properties of the MPCFs.

Tailoring microstructures and properties of MPCFs are immensely complicated by numerous influencing factors such as raw material, spinning temperature, pressure, winding speed, mesh structure and spinneret shape[7–12]. At present, researchers have investigated the effect of melt-spinning temperature on the microstructures and properties of MPCFs, but these studies are mainly based on the melt-spinning process of the "pneumatic single-hole spinning equipment". For example, Yamada et al.[13] found that the cross section structure of MPCFs changed from "onion-skin" shape to "random" shape and then to "radial" shape with the decreasing of the spinning temperature, eventually reaching a "radial-split" shape at the lowest spinning temperature. However, White and Buechler et al.[14] concluded that high viscosity of pitch melt was not conducive to the formation of the radial structure in their experiments. Mochida and Yoon et al. [15–17] prepared the "radial" structure and "onion-skin" structure MPCFs at low and high spinning temperatures, respectively. Nevertheless, Ogale et al.[18] obtained the "circular radial" structure and "radial-split" structure at high and low spinning temperatures, respectively. Recently, Liu et al. [19] studied the effect of spinning temperature on the structure and properties of MPCFs by using a nitrogen pressure-driven single-hole equipment, and found that the structure of MPCFs changed from irregular shape to onion skin with the increase of spinning temperature. In summary, different researchers come to different or even contradictory conclusions using a pneumatic single-hole spinning equipment because of the complicated interaction of extrusion pressure.