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Rheological Behavior of Nanocrystalline / Submicron Ceramic Powder Dispersions
(Izmir Institute of Technology, 2009) Çağlar Duvarcı, Özlem; Çiftçioğlu, Muhsin
Several rheometric techniques were applied to submicron and nano ceramic powder dispersions systematically in this study. The rheological behavior of the dispersions was determined by steady shear and dynamic shear rheology. Dynamic shear rheological techniques are scarcely used for the characterization of ceramic powder dispersions contrary to polymers.The flow behaviors of the submicron and nano dispersions were found to be dependent on the solids content and fructose concentration. The submicron alumina, nano alumina, and nano titania dispersions in fructose solution showed shear thinning behavior and were fitted to the Herschel-Bulkley model.The dynamic shear rheology measurements showed that the solid part of the dispersions was dominant over the liquid part for both submicron and nano powder dispersions. The elastic modulus was higher than the viscous modulus in stress and frequency sweep measurements. The elastic moduli of the dispersions with solids content lower than 40 vol% were dependent on the angular frequency which indicated a gel-like behavior. However, the elastic moduli of the dispersions with 40 vol% solids were independent of angular frequency which indicated a solid like behavior. Further increase in fructose content had significant effects on both steady shear and dynamic shear rheological behavior of the dispersions regardless of particle size. The submicron and nano ceramic powder dispersions can be prepared by using fructose for the regulation of the rheological behavior of ceramic powder dispersions. The characterization of powder surfaces is essential for the effective adsorption of fructose.
Synthesis and Control of Exciton Dynamics in Cdte, Cdte/Cds and Znxcd1-Xte Colloidal Nanocrystals
(İzmir Institute of Technology, 2012) Eral Doğan, Leyla; Özçelik, Serdar
The aim of this study is to synthesize cadmium-based semiconductor colloidal nanocrystals and to control their exciton dynamics by tuning the size and composition of the nanocrystals (NCs). CdTe, CdTe/CdS binary, and ZnxCd1-xTe ternary semiconductor NCs are prepared by wet chemistry. The reactions are thoroughly optimized to enhance the optical properties. The optical properties of CdTe and CdTe/CdS are tuned by the size of the NCs by adjusting the reaction (the growth) time. Coating CdTe NCs with CdS layer enhances the photoluminescence quantum yields up to 45%. ZnxCd1-xTe ternary nanoalloys were synthesized by varying the initial mole ratios of metals (Zn/Zn+Cd) and the growth time. The size and the composition-tunable ZnxCd1-xTe nanoalloys exhibit highly luminescent optical properties. When the amount of initial Zn precursor is low, the nanoalloys have Cd-rich and Zn-poor internal crystal structure. However, at higher amount of Zn precursor, the nanoalloys have Zn-rich and Cd-poor core exhibiting gradient composition. The exciton interactions and dynamics are investigated as a function of the size of CdTe/CdS, and the composition and the size of ZnxCd1-xTe nanoalloys. The exciton interaction yields amplification in the output signal at the threshold of 1015 photon/cm2s per laser pulse. The exciton lifetimes are in the range of picoseconds to nanoseconds. The decay associated spectra are affected by the laser power, size and composition of the NCs. As the laser power increases new excitonic states are created especially in ZnxCd1-xTe nanoalloys. Multiexcitons were created in the NCs depending on the laser power. Small NCs exhibit stronger exciton-exciton interactions under high laser power compared to larger NCs. However larger NCs have lesser exciton density, therefore reducing the exciton-exciton interactions.

Phd Degree / Doktora

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