Papers by Author: Hua Tang

Abstract: The YBa₂Cu₃O_x/Ag and Bi₂Sr₂CaCu₂O_x/Ag self-lubricating composites were prepared using powder metallurgic method. The crystal structure and morphology of the as-synthesized samples were characterized by XRD and SEM. The YBa₂Cu₃O_x/Ag and Bi₂Sr₂CaCu₂O_x/Ag self-lubricating composites were found to compose of superconductor phase and Ag phase. The tribological properties from ultra-low temperature to room temperature of the composites were studied by pin-on-disk friction test. It was found that the friction coefficients of pure YBa₂Cu₃O_x (YBCO) and Bi₂Sr₂CaCu₂O_x (BSCCO) were both dropped abruptly when the temperature cooled below the superconducting transition temperature. At room temperature, the friction coefficient of pure YBa₂Cu₃O_x is 0.68~0.95, when mixing 15wt% Ag, the friction coefficient of the sample decreased to the lowest value 0.11. The friction coefficient of pure Bi₂Sr₂CaCu₂Ox is 0.15~0.17, When Ag content reach 10wt%, the coefficient was lowest (average value is 0.13). The addition of appropriate amount of Ag obviously improve the tribological property of YBCO, while only slightly meliorate that of BSCO. On the other hand, the YBCO/Ag composites exhibit better tribological properties than BSCCO/Ag composites at higher load under the same experimental condition.

Abstract: In this paper, calcium carbonate particles with unusual morphologies could be successfully synthesized by a precipitation reaction of sodium carbonate with calcium chloride in the presence of a novel polymer-surfactant aggregate consisting of poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEG-PPG-PEG) and sodium dodecyl sulfonate (SDS). The as-prepared products were characterized with scanning electron microscopy and X-ray diffraction. The results showed that the SDS concentrations, the molar ratio of CO₃^2- to Ca²⁺ (R), and the reactive temperature in the mixed system turned out to be important parameters for control of morphologies of the as-prepared CaCO₃ particles. Various unusual crystal morphologies, such as hollow microsphere, cubic, and rombehedra, etc., can be obtained depending on the experimental conditions. Moreover, the formation mechanisms of calcium carbonate with different morphologies were discussed.