A study of the porosity of nuclear graphite using small-angle neutron scattering

Small angle neutron scattering (SANS) measures porosity in nuclear graphites, including both open pores, caused by escaping decomposition gases, and internal cracks (in coke particles) generated by anisotropic thermal contraction along the c-direction (Mrozowski Cracks). Porosity changes on the length scale observable by SANS must control the development of internal stresses and hence of cracking in AGR graphite due to irradiation (both fast neutron displacements of carbon atoms and radiolytic corrosion by CO2). Such cracking may cause premature reactor shutdown. SANS measurements show that porosity is fractal on a length scale between ~0.2-300 nm, presumably due to Mrozowski cracks – because the fractal index of the SANS signal depends only on the porosity of the graphitic filler. We report here two novel uses of the SANS technique as applied to reactor graphite – contrast matching with D-toluene (to measure the fraction of the porosity open to the surface) and the temperature dependence of the scattering (to measure pore width changes up to 2000 °C). These results provide important new information on AGR graphite porosity and its evolution during irradiation

The use of small angle neutron scattering with contrast matching and variable adsorbate partial pressures in the study of porosity in activated carbons

The porosity of a typical activated carbon is investigated with small angle neutron scattering (SANS), using the contrast matching technique, by changing the hydrogen/deuterium content of the absorbed liquid (toluene) to extract the carbon density at different scattering vector (Q) values and by measuring the p/p0 dependence of the SANS, using fully deuterated toluene. The contrast matching data shows that the apparent density is Q-dependent, either because of pores opening near the carbon surface during the activation processor or changes in D-toluene density in nanoscale pores. For each p/p0 value, evaluation of the Porod Invariant yields the fraction of empty pores. Hence, comparison with the adsorption isotherm shows that the fully dry powder undergoes densification when liquid is added. An algebraic function is developed to fit the SANS signal at each p/p0 value hence yielding the effective Kelvin radii of the liquid surfaces as a function of p/p0. These values, when compared with the Kelvin Equation, show that the resultant surface tension value is accurate for the larger pores but tends to increase for small (nanoscale) pores. The resultant pore size distribution is less model-dependent than for the traditional methods of analyzing the adsorption isotherms

Production and characterisation of molecular hydrogen storage materials

The subject of the present work is the characterization of potential materials for molecular hydrogen storage with a stress on the use of Small Angle Neutron Scattering (SANS).This research is dedicated mainly to a study of the porous structure of activated carbon. Employment of the contrast matching technique together with SANS led to an understanding of the process of pore filling resulting from an increase of partial pressure of contrast matching liquid (deuterated toluene). The Author has designed a special Al alloy sample cell for the variable vapour pressure SANS experiments. The accessible empty pores fraction at each p/po is calculated using the Porod Invariant. The data obtained matches the results from gravimetric measurements of D-toluene adsorption. The fractal nature of activated carbon is determined via application of the neutron scattering technique. The density of activated carbon - an important characteristic affecting the total hydrogen uptake - is found to be Q-dependent with an average saturation limit of 1.85 g/cc. The effect of the carbon activation process, i.e. the formation of micropores, is illuminated by SANS. Finally, a novel model, implying exponential decay of the pore size distribution with a lower cut-off, is proposed and the minimum pore radius is calculated for each experimental partial pressure of wetting liquid. These results are compared with those derived from the standard Guinier approximation. The proposed model yields the exact value of D-toluene surface tension when the derived pore radii are associated to the corresponding pressures via the Kelvin equation (within the range of its applicability), whereas the Guinier approximation gives an average value of the D-toluene surface tension approximately twice the tabulated value. Thus, it is concluded that the novel model presented in this thesis is an improved approximation to the porous structure of activated carbon.Additionally, a complimentary double-Gaussian pore size distribution model is suggested. It highlights the presence of ultramicro- and microporosity and shows a good agreement with the SANS data for dry activated carbon

A study of the porosity of nuclear graphite using small-angle neutron scattering

Small angle neutron scattering (SANS) measures porosity in nuclear graphites, including both open pores, caused by escaping decomposition gases, and internal cracks (in coke particles) generated by anisotropic thermal contraction along the c-direction (Mrozowski Cracks). Porosity changes on the length scale observable by SANS must control the development of internal stresses and hence of cracking in AGR graphite due to irradiation (both fast neutron displacements of carbon atoms and radiolytic corrosion by CO2). Such cracking may cause premature reactor shutdown. SANS measurements show that porosity is fractal on a length scale between ~0.2-300 nm, presumably due to Mrozowski cracks – because the fractal index of the SANS signal depends only on the porosity of the graphitic filler. We report here two novel uses of the SANS technique as applied to reactor graphite – contrast matching with D-toluene (to measure the fraction of the porosity open to the surface) and the temperature dependence of the scattering (to measure pore width changes up to 2000 °C). These results provide important new information on AGR graphite porosity and its evolution during irradiation