Zhihai Feng

Thermophysical properties of high-density graphite foams and their paraffin composites

Abstract High-density graphite foams (GFs) were prepared from mesophase pitch with or without mesocarbon microbeads at different foaming temperatures and pressures, followed by carbonization and graphitization at 1 273 and 2 973 K, respectively. In one case, pitch was repeatedly infiltrated into the graphitized foam at 573 K followed by carbonization to increase its density. Paraffin was infiltrated into the GFs to form GF/paraffin composites. Factors determining the thermophysical properties of the GFs and thermal behavior of the GF/paraffin composites were investigated. The microstructure and thermophysical properties of the foams were found to be greatly influenced by the pitch fraction, foaming temperature and foaming pressure. The thermal conductivity of the foams determines the thermal behavior of the GF/paraffin composites. The thermal diffusivity of the GF/paraffin composites investigated can be increased 768 to 1588-fold compared with that of paraffin. The latent heat of the composites has an almost linear relationship with the mass fraction of paraffin in the composites. The composites are suitable candidates for passive cooling of electronics.

Structural evolution of rayon-based carbon fibers induced by doping boron

In the present work, we provide a systematic analysis of the structural evolution of rayon-based carbon fibers (RCFs) induced by doping boron using scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and Raman spectroscopy. For the first time, boron-doped RCFs with tunable amounts of boron were fabricated by exposing the RCFs to a vapor of boron by the decomposition of boron carbide (B4C). SEM and XRD results indicate that at the higher temperatures the strong erosion of boron vapor not only changed the original structure of RCFs, but also produced some flaws. Interestingly, when the temperature of doping boron is higher than 2200 °C, the graphite basal planes of RCFs are perpendicular to the fiber axis. Raman spectra also confirmed the presence of disorders and flaws in graphitic layers because of the displacement and solid solution of boron in the carbon lattice. Further, the chemical environment of boron species was ascertained by 11B nuclear magnetic resonance, indicating that boron atoms exist in three chemical environments, including the substitutional boron (BC3), boron clusters and B4C. Moreover, the TGA data indicated that doping boron greatly improved the oxidation inhibition of RCFs, and is superior to increasing the heat treatment temperature for improving the oxidation resistance. Such a systematic analysis of the structural evolution and oxidation resistance of RCFs induced by doping boron thus provides industrial potential for preparing RCFs with higher oxidation resistance at up to 800 °C.

Structural changes in four different precursors with heat treatment at high temperature and resin carbon structural model

Four precursors (mesophase pitch, condensed polynuclear aromatic resin, polyimide resin, and thermosetting phenolic resin) were heat treated at temperatures from 900 to 3000xa0°C. These products were characterized by X-ray diffraction, transmission electron microscopy, and helium adsorption density instruments. Heterogeneous graphitization was observed above 2200xa0°C in the resin carbons. Various constituents (amorphous, turbostratic, and graphitic) coexisted and transformed from being disordered to ordered with increasing treatment temperature. The molecular structures of the starting materials played important roles in the proportions of various constituents, crystallite sizes, and preferred orientation of the graphitic constituent of the different carbons during high temperature treatment. High-resolution transmission electron microscopy images showed that the structural features of Jenkins and Shiraishi’s model all existed in three resin carbons. Based on these results, we think that their structures are not belong to Jenkins model, also do not belong to Shiraishis model, are a complex of above two models.