An in situ small angle neutron scattering study of expanded graphite under a uniaxial stress

The present article studies the in situ evolution of the pore structure of compressed expanded graphite under a uniaxial stress up to 1000 bar using small-angle neutron scattering (SANS). The uniaxial stress was applied in the direction of the average c-axis of the graphite crystallites composing the sample. Ex situ characterization by electrical resistivity and mercury intrusion porosimetry was performed on the compressed samples. The anisotropic SANS pattern indicates the presence of spheroidal pores in the 4-100 nm detectable range. The stress dependence of the different extractable parameters (fractal dimension, apparent specific surface area and apparent porosity) was related to the meso and macro pore structure evolution. In particular, the fractal dimension increases irreversibly with the applied stress. We propose a model of evolution under uniaxial load in which the irreversible collapse and splitting of larger pores into smaller size ones provides a coherent description of the experimental observations.