Papers by Author: Qiu Sun

Abstract: A series of polycrystalline (Ca1-xBix)3Co4O9 ( x = 0.0 ~ 0.075 ) powders were synthesized rapidly by a polyacrylamide gel method. The dense ceramics were fabricated using the spark plasma sintering ( SPS ) technique. Effects of Bi substitution on high temperature thermoelectric properties of Ca3Co4O9 were evaluated. Both the electrical conductivity and Seebeck coefficient increased with increasing Bi content up to x = 0.05, thus leading to an enhanced thermoelectric power factor. The Bi substituted sample with x = 0.05 obtained in this study has the highest thermoelectric power factor in the measured temperature range. It reaches 4.810-4 Wm-1K-2 at 700 °C, which is 26 % higher than that of Ca3Co4O9 without Bi substitution, and is by up to 15 % larger as compared to the Bi substituted sample synthesized by the solid state reaction method and the SPS technique due to the high chemical homogeneous powder prepared by the polyacrylamide gel method.

Abstract: Rare earth Yb-doped bismuth titanate Bi4-xYbxTi3O12 (BYT) nanocrystalline powders were obtained by a modified sol-gel method. TG-DTA, FI-IR and XRD were used to determine the lowest heat treatment temperature. Phase changes in the formation of BYT crystalline powders were discussed by XRD. The effect of Yb3+ cation substitution for Bi3+ cation on the microstructure of BYT was also developed with XRD. The grain size of BYT nanopowders is about 40 nm determined by TEM.

Abstract: A modified polyacrylamide precursor method is presented to synthesize BaSm2Ti4O12 microwave dielectric ceramics. Under an alkaline circumstance, all the cations together with the monomer and bridging agent were dispersed into a hydrous solution with the aid of chelating agent EDTA. This mixture was initiated at about 90°C, dried, calcined at 1000°C for 2hs and finally turned into well-crystallized and single-phased BaSm2Ti4O12 ceramic powder, which was modeled and sintered at 1250°C for 2hours into well-sintered ceramics. Differential thermal analysis, thermogravimetric analysis, X-ray powder diffraction and transmission electron microscope were used to characterize the process. Microwave dielectric properties measured by a resonating cavity method contain a dielectric constant (εr) about 77.0 and quality factor (Q×f) lager than 11000.

Abstract: BaTi4O9 - BaPr2Ti4O12 microwave dielectric ceramic composite powders were prepared by a polyacrylamide gel route using ethylenediaminetetraacetic acids (EDTA) as a chelating agent. This method provides a fast, cheap, reproducible and easy to scale-up way to prepare homogeneous mixed phases compounds. A light green, molecular-level, homogeneously mixed gel was prepared by the gelification of acrylamide. The precursors and derived oxide powders were characterized by thermal analysis (TG-DSC) and X-ray powder diffraction (XRD). Two phases and well-crystallized BaTi4O9 - BaPr2Ti4O12 composite powders were obtained at 1200 °C for 4 h.

Abstract: The Pb(Zr0.52Ti0.48)O3 thin films with 0-2at.%Gd dopants (denoted as PGZT) were prepared on Pt/Ti/SiO2/Si substrates by a sol-gel technique and a rapid thermal annealing process. The structures of PGZT films were characterized and the ferroelectric properties such as P–V loop, C–V and I–V characteristics were investigated. Improved polarization (2Pr = 46.373 μC/cm2) and the low leakage current (J = 1.5×10-9 A/cm2 at the electric field of 400 kV/cm) were obtained in the PZT thin film with 1at.% Gd dopant, which was better than that of the pure PZT thin film (2Pr = 39.099 μC/cm2, J = 4.3×10-8A/cm2). With the Gd contents up to 2at.%, a decreased remanent polarization was found.

Abstract: Ferroelectric lead zirconate titanate(PZT)thin films have attracted great attention because of their potential applications in memory devices[1] due to their unique properties, for example, hysteresis loop and high dielectric constant. To realize these memory devices, it is necessary to overcome the reliability problems such as fatigue, retention and imprint. It is well known that lead base perovskite family ferroelectric thin films with donor dopant such as La3+ and Nb5+ have improved the electrical properties of PZT thin films effectively [2-3]. And it is proposed that the cation substitution could reduce the number of defects such as oxygen vacancies, which could promote electrical fatigue and leakage current of PZT ferroelectric thin films obviously. In the present study, rare earth Yb-doped lead zirconium titanate (PYZT) nanocrystalline powders with a composition near the morphotropic phase boundary (Zr/Ti=52/48) were prepared by a modified sol-gel method. DTA/TG and XRD were used to determine the thermal and phase changes in the formation of PYZT crystalline powders. The effect of Yb3+ cation substitution for Pb2+ cation on the microstructure of PZT was developed with XRD. The grain size of PYZT nanopowders is about 40 nm determined by TEM.