| Functionally Gradient Materials (FGMs) are inhomogeneous materials whose compositions vary from one phase to another. By tailoring the inhomogeneous properties, FGMs can be used to reduce the stresses that are caused by severe thermal gradients. Thermal gradient loading can further be compensated by heat transfer into a cooling fluid circulating in a network of channels and manifolds. In an envisioned application, heat from a localized source is transferred to the cooling fluid, easing sharp thermal loads while minimizing the unwanted spread of heat energy to the ambient surroundings.; This study reports on the fabrication of functionally gradient ceramics and the embedding of simple internal channels within these ceramics. Functional gradiency (variation of composition) is built in via the layering of different components across the thickness of a plate sample. Traditional powder processing techniques are applied to fabricate the test pieces, and recently developed methods of joining are used to build assemblies from individually sintered plate layers.; For a well-formed FGM to be made, materials parameters need to be selected based on mechanical, thermal and chemical properties. As a class, ceramics are hard, wear-resistant, refractory, electrically and thermally insulative, nonmagnetic, chemically stable, and oxidation-resistant. However, because of their brittleness, ceramics with minute channels are difficult to machine. Instead, for this study, a graphite fugitive phase is used as a spacer to support channel volumes within a ceramic powder compact; during pre-sintering, the graphite burns out to expose a network of channels. Full sintering fixes the final shape. At the operating temperatures of the ovens used in our fabrication study, sintering of alumina, partially stabilized zirconia, fully stabilized zirconia and hydroxyapatite have been successful, and these ceramic powders form the basis of the present fabrication studies.; Inhomogeneities inherent in the composition of layered FGMs and non-uniformities introduced in the fabrication process (possibly during powder compaction) give rise to non-uniform densification behavior, which leads to warping, cracking, and the development of residual stress both within the layers and along layer boundaries. As a remedy, powders of different particle size are mixed within component layers, and the effect of particle mixture on final specimen curvature is shown. The optimal mixture ratio gives a flat layer, wherein the effects of thermal expansion and densification are balanced. Different pressing procedures and layer-stacking orders have also been used to minimize warping; these are viewed as optional, complementary steps to particle mixing.; A study of the difference between conventional and microwave heating is also presented. The main interest has been to quantify the different sintering behavior associated with conventional heating and microwave heating.; Finally, scanning electron microscopy and energy dispersive spectroscopy are used to illustrate the gradual changes in element composition across the thickness of the produced functionally gradient ceramic plates. |
