Date of Award

8-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Mechanical Engineering

Advisor

Li, Gang

Committee Member

Biggers , Sherrill B.

Committee Member

Joseph , Paul F.

Committee Member

Thompson , Lonny L.

Abstract

Nanocomposites are composite materials which incorporate nanosized particles, platelets or fibers. The addition of nanosized phases into the bulk matrix can lead to significantly different material properties compared to their macrocomposite counterparts. Due to their extraordinary properties, nanocomposites promise new applications in many fields such as ultra-high strength and ultra-light automotive parts, non-linear optics, biomedical applications, sensors and actuators, and thermoelectric devices. The design and fabrication of nanocomposite structures, devices and systems can be accelerated by developing accurate and efficient computational tools that can describe the properties and behavior of the nanocomposites. However, the development of such tools is challenging due to the multiscale nature of the materials. In addition, many devices where nanocomposites are employed are multiphysics systems with interactions of the mechanical, thermal and electrical energy domains. In such systems, while mechanical deformation is dependent on the temperature change, the thermal and electrical transport properties are functions of mechanical strain. In this work, we develop theoretical and computational models to address these issues and investigate the strain effect on the thermal and electrical transport properties in Si/Ge nanocomposites.
We model strain effect on the phonon thermal conductivities in the Si/Ge nanocomposite materials by combining the strain dependent lattice dynamics and the ballistic phonon Boltzmann transport equation (BTE). The Seebeck coefficient and electrical conductivity of the Si/Ge nanocomposites are calculated by using an analytical model derived from the BTE under the relaxation-time approximation. The effect of strain is incorporated into the analytical model through strain induced energy shift and effective mass variation calculated from the deformation potential theory and a degenerate kp method at the zone-boundary X point. By using the models, strain effect on the thermoelectric figure of merit is investigated for n-type Si/Ge nanocomposite materials. Our calculations reveal that in the 300 − 800 K temperature range, uniaxial tensile strain along < 100 > direction increases dimensionless figure of merit parallel to the tension, and biaxial tensile strain along [100] and [010] directions decreases it at low temperatures and increases it at high temperatures in the tension directions. Shear strain and compressive uniaxial and biaxial strains decrease the figure of merit. At 800K with an electron concentration of 10^19/cm^3, 1% uniaxial tensile strain can increase the figure of merit of Si(0.8)Ge(0.2) nanocomposites by as much as 14%.
In light of nanocomposites' high electrical to thermal conductivity ratio, we propose to use Si/Ge nanocomposite materials to improve the performance of micro thermal actuators. The high electrical to thermal conductivity ratio of Si/Ge nanocomposites is utilized to facilitate a rapid temperature change within a short distance, enabling a high temperature increase in a large region of the actuator beams. The total structural thermal expansion and consequently the actuation distance can be increased significantly. A top-down quasicontinuum multiscale model is presented for computational analysis of the nanocomposite based thermal actuators. Numerical results indicate that incorporating Si/Ge nanocomposites in thermal actuators can significantly increase their energy efficiency and mechanical performance. In addition, parametric studies show that the size of the nanocomposite region and atomic percentage of the material components have significant effects on the overall performance of the actuators.

Share

COinS