| The goal of this dissertation is to elastically characterize thermoelectrics and other novel materials using resonant ultrasound spectroscopy (RUS) at elevated temperatures. A "direct-contact" transducer system with test sample in direct contact with the piezoelectric elements was developed. This new transducer system with improved signal-to-noise ratio (SNR) and the conventional buffer-rod system were used for elasticity measurements and phase transition studies. The temperature dependent elastic moduli of thermoelectric materials of four nanostructured polycrystalline silicon-germanium (SiGe) samples were obtained up to 950°C. Abnormal elastic behavior (stiffening) in the temperature range of 350--550°C was observed in the two n-doped SiGe samples, which is associated with dopant (phosphorus) precipitation. The elastic moduli of the complex Zintl phase Yb14MnSb11 were also measured up to 600°C. Using the quick "mode-tracking" method, the various temperature-induced phase transitions in Yb14MnSb 11, transition metal oxide LuFe2O4 and bulk metallic glass Zr50Cu31Pd9Al10 were investigated. In addition to the high-temperature RUS measurements, the continuum elastic model in RUS was also applied to numerically study several lower normal modes of vibration in carbon nanotubes (CNTs). |
