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The pore structures of microporous carbon fibers having less-crystalline structures with different activation extents were examined by small-angle X-ray scattering (SAXS), N2 adsorption, and density measurements.The different kinds of porosities were associated with observable quantities and a new concept of an inaccessible pore was introduced. Inaccessible pores include closed pores and ultrapores which are accessible by He at 303 K but not by NZ at 77 K. The surface area of samples was determined by both the high-resolution a, plot of the N2 adsorption isotherm and the Debye-Bueche plot of the SAXS profile. The surface area a, from SAXS was greater than that a, from N2 adsorption. The difference a, -a, was associated with the volume and size of the inaccessible pores. The possibility for a general application of this inaccessible pore

The adsorption isotherms of He at 4.2 K and N2 at 77 K on pitch-based activated carbon fiber (ACF) and ACF treated at 1473 K in Ar (ACF-1473) were determined. Also their small angle X-ray scattering (SAXS) spectra were measured in air. The amount of He adsorption expressed in terms of the adsorbed volume using Steele's theoretical density of the adsorbed He layer was compared with that of N2 adsorption.The He adsorption isotherm had much steeper uptake below PIP0 = 0.02; the amount of He adsorption below PIP0 = 0.02 was greater than that of Nz adsorption by more than 50%; heat-treatment of ACF increased the difference between He and N2 adsorption at low pressure. Both He and N2 adsorption isotherms were analyzed by the Gaussian distribution. The NZ adsorption provided completely different peak positions of the micropore size distribution of ACF and ACF-1473, while He adsorption led to slight difference in the mean micropore width. The presence of necked micropores in ACF-1473 was evidenced by the distinct adsorption behaviors of He and N2. The distribution of the Guinier gyration radius was detdrmined from SAXS data and the mean Guinier gyration radius was obtained from the Maxwellian distribution. The mean gyration radius and the width of the slit-shaped scattering entity by the thickness plot of the SAXS data gave an average geometry of micropores. The slit width from the thickness plot was very close to that from molecular adsorption. The heat treatment did not change the micropore geometry of ACF from the SAXS analysis. The difference between molecular adsorption and SAXS results on the heat treatment was attributed to the necked pore structure.

Porous silicons (PSs) were prepared from a lightly doped (boron; 7 × 1014/cm3) crystal silicon (c-Si) wafer by anodization in an aqueous hydrofluoric acid (HF) solution of different concentrations (25−55%). The porosity of these PSs was examined by nitrogen adsorption at 77 K. The relationship between the pore structure and the HF concentration upon anodization was examined. All the PSs investigated showed the type IV isotherm with a type H2 desorption hysteresis, indicating mesoporosity. The pore size distribution was derived from nitrogen adsorption isotherms with the BJH method. The total pore volume of the PSs decreased gradually with increasing HF concentration, but the specific surface area increased with concentration above 37 wt %. The pore size distribution varied with the HF concentration. The profile of the distribution became sharper and shifted to a smaller size with the increase in the HF concentration.