Browsing by Author "Bhattacharyya, Somnath"

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Investigation of intergranular glassy films in polycrystalline ceramics using Transmission Electron Microscopy
(2006) Bhattacharyya, Somnath; Rühle, Manfred (Prof. Dr. Dr. Ing. h.c.)
The presence of intergranulaur glassy films (IGFs) in ceramics such as Si3N4 and SiC strongly influences their material properties, including fracture toughness and creep resistance. The aim of this PhD work is to apply a variety of conventional as well as newly developed transmission electron microscopy (TEM) techniques in the detailed characterization of these films. Fresnel fringes hidden in high-resolution micrographs can be used to objectively demarcate the glass-crystal interface and to measure the thickness of IGFs. A method to objectively extract Fresnel fringe spacings from bright field TEM images is presented. Difficulties in using the standard Fresnel fringe extrapolation technique are pointed out and an alternative method based on zero-defocus (in-focus) bright field images is suggested. This discussion of the Fresnel fringe technique is intended to be a practical toolkit for measuring the width of IGFs by taking experimental difficulties into account and comparing different techniques. Since Si3N4 is used for high temperature applications it is important to examine the effect that temperature has on IGFs in this material. In-situ heating experiments within different microscopes have been conducted while taking care to separate beam irradiation from temperature effects. In contrast to previous literature the IGF width was found to be greater at 950ºC than at room temperature. The tendency to restore the thickness of room temperature during cooling was also observed. Possible mechanisms and sources of artifacts in the observed data are discussed. This work shows that certain material transport processes may be active at rather low temperatures in Si3N4. An iterative method for reconstructing the exit face wave function using series of line profiles across interfaces and grain boundaries extracted from TEM images of varying defocus has been developed. This method allows the reconstruction from images across a large defocus range, including their lateral alignment. From the reconstructed wave function the projected electrostatic as well as absorptive potential profiles across interfaces are determined using the strong phase object approximation. This method is finally applied to IGFs and grain/triple pocket interfaces in different rare earth oxide and MgO doped Si3N4 ceramics. Using energy-filtered TEM the specimen thickness has been measured, allowing relative mean inner potential profiles across interfaces and grain boundaries to be determined from the reconstructed exit-face wave functions. However, using the experimentally obtained mean inner potential of the Si3N4 grain next to the interface (from vacuum-Si3N4 grain interface), the potential profiles across interfaces and grain boundaries are placed in absolute scales. For La2O3-MgO doped Si3N4 the potential profile across the grain / triple pocket interface was observed to be very similar to that across the intergranular glassy film.