Graphite and graphite composite materials are of great importance in various applications; however, they have been widely used in nuclear applications. Primarily in nuclear applications such, as a moderator where its primary aim is to stop the fast neutrons to thermal neutron. The composite graphite (HTR-10) has potential applications as a moderator and other applications including in aerospace field. Structurally the composite shows stable hexagonal form of graphite and no traces of the unstable Rhombohedral patterns. Thermal conductivity indicates the same trends observed and known for nuclear graded graphite. The composite was made as a mixture of 64 wt% of natural graphite, 16 wt% of synthetic graphite binded together by 20 wt% of phenolic resin. The resinated graphite powder was uni-axially pressed by 19.5 MPa to form a disc shaped specimen. The disc was then cut and annealed to 1800 °C. The composite was further cut into two directions (parallel and perpendicular) to the pressing direction. For characterization the samples were cut into 2.5 x 2.5 x 10 mm3. There were exposed to proton irradiation for 3 and 4.5 hrs respectively and characterized both structurally and thermally. Through the study what we have observed was that as the composite is exposed to proton irradiation there is an improvement structurally. Thus, the D peak in the Raman spectroscopy has decreased substantially with the irradiated samples. XRD has indicated that there is no un-stable Rhombohedral phase pattern in both the pristine and the irradiated samples. However this was further confirmed with that thermal conductivity is also increasing with irradiation exposure. This is anomalous to irradiated graphite in which defects are supposedly induced. Looking into the electrical resistivity we have noted that pristine samples have higher resistivity as compared to the irradiated samples. Seebeck coefficient indicates that there is some form of structural perfection and the samples have a phonon drag dip at the known graphite temperature of 35 K. This has shown us there are no impurities induced by irradiation of the samples. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Physics / Unrestricted

Metal foams are lightweight structures and have large use in many components acting as impact energy absorbers. They have exceptional mechanical, thermal and acoustic properties. The design or selection of foam for packaging is done on the basis of impact loads to be sustained or energy to be absorbed. For transportation of nuclear material, metal foams can be used as a packaging material. It may be noted that apart from other qualification requirements, a package containing nuclear material, has to be certified for drop test. Foam can serve the purpose by providing proper cushioning. Metal foams are still under development and need to be accurately characterized in terms of their mechanical properties as well as cell morphology. The aim of this work is to develop, characterize and model foam using experiments and analysis. Aluminum foam has been developed by powder metallurgy technique and the effect of addition of varying amounts of Mg and Alumina on the strength and energy absorption has been studied. Foams of varying densities have also been developed. The reason for going for higher density is to obtain higher plateau stress. If a package is designed with lower density foam, it may become very bulky and even impractical. The characterization part of the work includes study of porosity distribution, cell wall structure, microscopy, SEM images, etc. Mechanical testing (uniaxial compression) was performed on foam samples to get load deflection curve of foams. Area under a given curve i.e. energy absorbed per unit volume has been compared for various compositions and densities. The analysis part of the work presents effect of specimen size on bulk properties of foam. 2D honeycomb and 3D cases have been discussed. To model the porosities, spherical cavities have been assumed. Uniaxial compression cases with different combinations of porosities have been analyzed. The properties like Young’s modulus, plateau stress, Poisson’s ratio, tangent modulus, etc. have been evaluated. The effect of variation in yield strength and tangent modulus of base material on foam has been studied. It appears that if the model is based on uniform porosity distribution, it may lead to lower bound values of physical properties and give conservative result. Although some of these trends have been observed in published literature, the current numerical study has generated additional information and insight.