Helen M. Kerch

Small-angle neutron scattering studies of ceramic nanophase materials

Abstract Undisturbed, representative microstructures in nanophase or nanostructured ceramic oxides can be non-destructively characterized using small-angle neutron scattering (SANS). By using samples of sufficient size and uniformity to obtain absolutely-calibrated SANS data, and by utilizing a suitably-flexible microstructural model, quantitative parameters can be extracted for the statistically-significant grain/pore size distributions and surface-areas in a range of nanophase microstructures. It has been found that all of the observed SANS can be accounted for without including a low-density disordered phase. These issues are discussed with reference to sintering studies of nanostructured zirconia and yttria-stabilized zirconia, particularly an assessment of the effectiveness of vacuum-sinter-forging.

Imaging of fine porosity in a colloidal silica: potassium silicate gel by defocus contrast microscopy

Abstract The fine porosity present in a 10:90 colloidal silica:potassium silicate gel was imaged in the transmission electron microscope by the technique of defocus contrast microscopy. With this imaging method it is possible to detect very small pores (≥1 nm) and to obtain an accurate measure of their size and distribution. The technique allows independent measurement of the spatial location, morphology and size of the pores in the gel. Specifically, defocus contrast proves that the colloidal gel exhibits two pore populations in the micro- and mesopore range: a fine porous texture which is found uniformly across the gel particles, and larger pores with an interconnected globular morphology. This result, together with data from previous studies, showed that the gel composition possesses a polydisperse distribution of porosity which extends to many length scales. The influence of the gel structure on the resultant nitrogen sorption data is also discussed.

Sintering studies of nanophase ceramic oxides using small angle scattering

The microstructure evolution during sintering of nanophase ceramics has been followed using small-angle neutron and X-ray scattering. These techniques enable material microstructure parameters and surface areas to be measured nondestructively, and have been applied to study the effects of different sintering temperatures, pressures and additives. Depending on the sintered density, the scattering data from nanophase materials exhibit Porod scattering associated with the high surface area, scattering from the nanometer-size grains and pores, and interparticle interference scattering. By using samples of sufficient size and uniformity to permit absolute calibration of the scattering data, it has been possible to develop a fully quantitative microstructure model. Studies of nanophase yttria (powder, prepared by Nanophase Technologies Corporation, Burr Ridge, IL) and zirconia (powder, prepared at Rutgers University, Piscataway, NJ) are discussed, and it is shown how small-angle scattering, in conjunction with density measurements, electron microscopy and X-ray diffraction, provides a unique probe of the statistically representative microstructures present in these novel materials.

Characterization of Porosity Over Many Length Scales: Application to Colloidal Gels

Processing-microstructure relationships in a silica gel system, based on mixtures of colloidal sol and soluble potassium silicate, have been studied. Quantitative microstructural information regarding colloidal cluster sizes, size distributions, surface areas, and pore-size distribution from the nanopore range to the macropore range was determined via small-angle scattering and transmission electron microscopy. The colloid cluster size distribution varies systematically, with gels fabricated with the least colloidal fraction possessing the most polydisperse microstructure. It is shown that the porosity over the entire range can be tailored by selecting the appropriate starting chemistry; under the same pH conditions, the ratio of the two silicate ingredients controls the average size, the polydispersity of sizes, and the connectivity of the pores. A population of fine (2 nm) uniformly dispersed nanopores, which result from leaching, is responsible for large increases in surface area. The leaching process can be controlled by the surrounding macropore void size, which determines alkali transport. The product material consists of 85% large, open pores, with fine pores within the gel skeleton, making this gel an ideal candidate for controlled-porosity applications such as catalyst supports and magnetic composites.

Small-Angle Scattering Methods for Studying the Sintering of Nanophase Ceramic Compacts

Studies of microstructure evolution as a function of sintering parameters are necessary to better control the microstructure of nanophase ceramic compacts during processing. Small-angle scattering provides a comprehensive probe of the statistically-representative microstructures present in nanophase ceramics. A complete characterization includes complementary density, electron microscopy, nitrogen desorption and x-ray diffraction measurements. Small-angle neutron and ultra-small-angle x-ray scattering have been used to determine the effects of different sintering temperatures, pressures and additives during sintering of nanophase zirconia compacts. The samples were of sufficient size and uniformity to permit absolute calibration of the scattering data, making it possible to confirm that scattering from the pore/grain interface dominates the total scattering cross-section. A quantitative microstructural model was developed for this system, resulting in the determination of specific pore surface area, total pore volume fraction, pore number size distribution and grain size as a function of sintering parameters.