POROD SCATTERING OF NEUTRONS BY AN ANISOTROPIC MICROSTRUCTURE. G. G. Long¹, J. Ilavsky^1,2, A. J. Allen¹, ¹Ceramics Division, NIST, Gaithersburg, MD 20899, ²Plasma Spray Laboratory, SUNY/Stony Brook, Stony Brook, NY 11794

The specific surface area of the pores and cracks that are found in complex microstructures, such as plasma-sprayed ceramic deposits, was investigated by means of small-angle neutron scattering as a function of an array of processing variables. The prevailing orientation of microcracks, which presumably form upon cooling, depends upon the spray direction. Voids formed between the lamellae, however, are independent of spray angle. The amount of microcracking that takes place depends upon the chemistry of the deposit and the spraing conditions.

The above results were derived from large-Q ( | Q | = (4[[pi]]/[[lambda]])sin[[theta]]) small-angle neutron scattering data, described by the single-particle diffraction Porod law. When the shapes are isotropically distributed, the Porod law is independent of the scattering particle shape, and independent of direction in the material. The preferentially-oriented polydisperse systems of pores and cracks in a plasma-sprayed microstructure, however, will lead to anisotropic determinations of an "apparent" Porod constant. For these specific materials, there are two anisotropic directions and one isotropic direction, which makes possible a direct quantitative determination of specific surface areas as a function of orientation, when data from three orthogonal planes within the material are available.