B.E.T. Surface Area and Porosity

Solid materials interact with their environment, whether chemically or physically, only at their surfaces. Solids with larger surface areas would therefore be subject to greater effects from outside influences, when compared with identical materials which have smaller exposed surface areas. The relationships between surface area, particle volume, and porosity are well understood.

In the general case, the surface area of a solid varies as the square of its dimensions, whereas the volume of a solid varies as the cube of its dimensions. Therefore, smaller particles will tend to have larger surface area to volume ratios than do larger particles. In contrast, two solids of similar dimensions, but with differing porosities, may have dramatically different surface areas, dependent entirely on the size and number of the voids themselves. The critical determinant of the magnitude of this effect is the pore size and number, as these area/volume relationships apply equally to pore size as they do to particle size. Only when particle sizes, surface areas, and pore/void measurements are available can truly meaningful comparisons between different samples of the same material be made. An extensive literature search did not reveal B.E.T. surface area (after Brunauer, Emmett, and Teller (1938)) or porosity data which could be directly compared to the data reported here, arising from analysis of the +65 mesh Miller hydrothermal graphite flotation concentrate.

B.E.T Surface Area and Porosity Results

In 2011, Oak Ridge National Laboratory published some characterization results for candidate graphites being submitted for consideration in an ongoing pebble bed nuclear reactor research program, under the title: "Analysis of Natural Graphite, Synthetic Graphite, and Thermosetting Resin Candidates for Use in Fuel Compact Matrix". ( See: http://info.ornl.gov/sites/publications/files/Pub32010.pdf) Three graphites were identified as (statistical) "outliers", with relatively low B.E.T. surface areas. These include: synthetic graphite KRB2000 (thought to be no longer available; B.E.T. = 1.33 m²/g); synthetic graphite (Graftech) GTI-D (B.E.T. = 2.77 m²/g); and, a natural graphite, Asbury 3482 (B.E.T. = 1.32 m²/g). KRB2000 was thought to be a resin-impregnated synthetic material, which treatment would have reduced the porosity significantly. The Asbury sample was thought to be of low B.E.T. due to large particle size. No explanation for the low B.E.T. for the Graftech material was suggested therein. The range of B.E.T. values for the synthetic graphites was 1.33 – 19.38 m²/g (13 samples), whereas the range of B.E.T. values reported for the natural graphites ranged from 1.32- 7.56 m²/g (7 samples). Porosity values were not reported therein. The Oak Ridge report also provides numerous GD-MS assays for these candidate nuclear graphites, which can be compared with those published by Canada Carbon Inc.

Graphite porosity is typically reduced by means of resin impregnation or surface coating technologies; the Miller graphite was untreated, here.

Laboratory Procedures, and Facilities

B.E.T. surface area measurements and porosity determinations were conducted by Porous Materials, Inc., Analytical Services Division, Ithaca, NY. ASTM Standard Method C 1274-12, "Standard Test Method for Advanced Ceramic Specific Surface Area by Physical Adsorption", was specifically adapted for the determination of the B.E.T. surface area and porosity of graphitic materials. The adsorbent was nitrogen gas, with the test conducted at -195.76 C.

References:

Brunauer, P., Emmett, H., and Teller, E. (1938). "Adsorption of Gases in Multimolecular Layers". Journal of American Chemical Society, 60, 309-319.

Trammell, M.P., and Pappano, J. (2011). "Analysis of Natural Graphite, Synthetic Graphite, and Thermosetting Resin Candidates for Use in Fuel Compact Matrix", Oak Ridge National Laboratory, Carbon Materials Technology Group. 66pp. http://info.ornl.gov/sites/publications/files/Pub32010.pdf