Jinliang Song

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.

Effects of FLiNaK infiltration on thermal expansion behavior of graphite

The changes in both the crystallographic coefficient of thermal expansion (CCTE) and the coefficient of thermal expansion (CTE) of FLiNaK-infiltrated nuclear graphite were studied using in situ-heating X-ray diffraction analysis and horizontal push-rod dilatometry. It was found that, at temperatures lower than the melting point of FLiNaK, the CCTE of the d(002) spacing of graphite decreases with an increase in the weight of the graphite sample because of FLiNaK infiltration. On the other hand, the CTE of bulk graphite increases after molten salt infiltration. The CCTEs of the as-prepared FLiNaK salt, and the salt in the graphite pores were compared. The decrease in the CCTE of the FLiNaK salt after it had infiltrated into the graphite pores confirmed that interactions occur between the graphite and the salt. These interactions are probably induced by the difference in the CTEs of graphite and the solidified salt. Further, it is likely that the crystallization pressure also plays an important role here. Thus, both the causes need to be considered when using nuclear graphite in molten salt reactors.

Microstructure and properties of ultrasonic assisted copper coated graphite foams

Abstract Mesophase pitch based graphite foams were electroplated with copper to obtain the improved mechanical and thermal properties and to extend their applications in heat sinks. A nickel interlayer was electroplated prior to copper to improve the wettability and bonding between copper and the graphite foams. An ultrasonic oscillation device and a rotary cathode were applied to enhance the mass transfer and to ensure the more even coatings throughout the foams from inside. Microstructure and properties of the copper coated foams were studied. Results indicated the copper coatings greatly improved the mechanical and thermal properties of the foams. With the prolonged timescale of plating, the foams possessed enhanced mechanical and thermal properties. After 16 min. of plating, the bending strength of the foams increased from 1·1 to 7·6 MPa, and the thermal conductivity of the foams reached 100·2 from 81·6 W m−1 K−1, while the porosity was reduced only by 4·2%.

Microstructure and molten salt impregnation characteristics of a micro-fine grain graphite for use in molten salt reactors

Abstract The microstructure and molten salt impregnation characteristics of a micro-fine grain isotropic graphite ZXF-5Q from Poco Inc. was investigated. The microstructural characteristics of the pores caused by gas evolution, calcination cracks, Mrozowski cracks, and the crystal structure were characterized by optical microscopy, mercury porosimetry, helium pycnometry, transmission electron microscopy, X-ray diffraction and Raman spectroscopy. Results show that the ZXF-5Q has uniformly-distributed pores caused by gas evolution with very small entrance diameters (∼0.4 μm), and numerous lenticular Mrozowski cracks. Molten salt impregnation with a molten eutectic fluoride salt at 650 °C and 1, 3 and 5 atm, indicate that ZXF-5Q could not be infiltrated even at 5 atm due to its very small pore entrance diameter. Some scattered global salt particles found inside the ZXF-5Q are possibly formed by condensation of the fluoride salt steam during cooling.