Doctor of Philosophy (PhD)


Mechanical Engineering

Document Type



Glass microballoons have high strength, low thermal and electrical properties, and provide closed cell porosity to reduce the density of composites. On the other hand, filamentous carbon nanostructures have excellent mechanical, thermal, and electrical properties that make them naturally multifunctional. This work presents a method of developing low-density multifunctional nanocomposites utilizing glass microballoons and carbon nanostructures. Two different approaches are investigated. In the first approach, carbon nanotubes (CNTs) are used as a filler material to fabricate nanocomposites containing glass microballoons (CNT-syntactic foams). The weight percent of CNTs is varied from 0 to 0.8 wt%. In this method, CNTs were grown on few microballoons and mixed with plain microballoons before they were added into epoxy matrix to fabricate CNT-syntactic foams. Transmission electron microscopy studies indicate that the method is effective in avoiding CNT cluster formation in a matrix. The compressive properties, dynamic mechanical properties, and electrical properties of the nanocomposite foams have been analyzed and the results are compared with their neat counterparts. Significant improvements in compressive modulus and damping coefficient are obtained. Elastic modulus and glass transition temperature of the foams also showed a slight increment. In the second approach, a paper like structure formed from hollow glass microballoons and carbon nanofibers (CNFs) is fabricated. A layer of nickel (Ni) coated glass microballoons is first formed on a silicon wafer by a process similar to dip coating. This technique comprised of immersing a wafer in ethanol suspension of Ni coated microballoons and lowering the level of suspension by draining from the bottom. CNF networks are then generated by growing them on the surfaces of the self-assembled microballoons using thermal chemical vapor deposition method. The self-assembled microballoons are bonded together with CNF networks to form a paper like structure in approximately 20 minutes of growth time. An I-V characteristic of the structure indicates formation of conductive electrical path ways. Nanocomposites fabricated from this structure, using vacuum infiltration technique, are investigated for their mechanical, electrical, and strain sensing properties. A curve fitting method is also developed to relate the change in resistance of the nanocomposite to an applied strain.



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Committee Chair

Woldesenbet, Eyassu