Porous graphite matrix for chemical heat pumps

Expanded graphite powders were prepared by rapid heating of expandable grap hite powders intercalated with sulfuric acid at different heat treatment te mperatures (HTT). Porous graphite matrices with 100-400 kg m(-3) of bulk de nsity were fabricated by pressing expanded graphite powders in order to use as heat conductive media. They were characterized using an C/S analyzer, i nductively coupled plasma spectroscopy, X-ray diffraction, scanning electro n microscopy, Fourier transform infrared (FTIR), nitrogen adsorption, optic al microscopy and helium pycnometer before and after heat treatment. Gas pe rmeability and thermal conductivity were measured for porous graphite matri ces with different HTT and bulk densities. Chemical analysis and FTIR showe d that as the HTT of expandable graphite powders increase, the residual sul fur content decreased remarkably. Nitrogen adsorption experiments for expan ded graphite powders showed that specific surface area and total pore volum e increased with HTT. Helium penetration results showed that porous graphit e matrices with different HTT have noticeably different open porosities whi ch were attributed to the different degrees of expansion of graphite layers . The gas permeability of porous graphite matrices was in the range of 10(- 12)-10(-15) m(2) and exhibited higher values with low HTT. Thermal conducti vity values in the axial and the radial directions were in the range of 4.1 -20.0 and 4.6-42.3 W mK(-1), respectively. A semi-empirical model was devel oped that can be used to correlate with the thermal conductivity of graphit e matrix on the basis of solid conductivity, bulk density and porosity. (C) 1998 Elsevier Science Ltd. All rights reserved.