Synthesis and Characterization of Gd5Si2Ge2-Al Composite for Automobile Applications

This thesis research synthesizes a new class of composite materials and investigates their properties, performance, and potential applications. The new materials that are multi-phase and multifunctional are considered for use in car cooling systems, internal combustion engine waste-heat-power generators, and engine crack healing which are major problems plaguing the auto industry. This research uses primarily experimental approaches to study the magnetocaloric compound, Ge5Si2Ge2 (GSG), that has large strain effects. Such a material was formed into a composite using Al as a substrate. The newly developed composite, GSG-Al, is the first material of its kind that possesses self-healing effects in cracks. X-ray diffraction was used to determine the crystal structures that existed within the material. It is found that the sintering process used to create the composite caused the formation of GdAlGe that is a magnetic compound with a high Curie temperature. The GSG-Al has a wide variety of crystal structures, ranging from face centered cubic for aluminum phases to monoclinic and orthorhombic phases for GSG. The discovery of GdAlGe confirmed that alpha-ThSi-type tetragonal and YAlGe-type orthorhombic crystal structures existed. Transmission electron microscopy (TEM) was used to analyze the wear debris collected during tribo-testing. The debris were also analyzed using energy-dispersive X-ray spectroscopy (EDS) for chemical analysis. The GSG-Al was put through tribological studies at several different temperatures to determine the thermal effects on the composite. The GSG-Al, although found to be ductile, showed high resistance to wear when compared to a common aluminum alloy, Al 6061-T651. The wear rate decreased with increasing temperature when the temperature was increased from the room temperature to 150 degrees C. Results showed that with GSG, the composite did not show cracking common in Al alloys. This was due to the unique thermal expansion properties of the GSG-Al. The phase transformation induced a significant volume expansion in the material and thus a giant strain effect. This research opens new approaches in energy conversion and improving efficiency of automobile engines. The composite developed here is important for future scientific investigation in the area of multifunctional materials as well as materials that exhibit self-healing tendencies.