Papers by Author: Shu Ping Gong

Abstract: High-quality SnO₂ thin-film materials capable of detecting H₂S gas of low concentrations at room temperature was demonstrated in this paper. We employed aerosol-assisted chemical vapor deposition process for the deposition of SnO₂ thin films on alumina substrates with pre-patterned electrodes. The gas-sensing performances of the films prepared under different deposition conditions were systematically compared and analyzed. When SnCl₂·2H₂O was used as the precursor, a response sensitivity of 98.4 toward 50 ppm of H₂S at room temperature was achieved. At room temperatures, the resistance upon the H₂S gas exposure could recover to 90% of the initial resistance of the sensor when the H₂S gas flow was turned off.

Abstract: The macroporous SnO₂ thin film has been successfully prepared on alumina substrate with printed Ag interdigital electrodes by sol-gel dip-coating method. The carbonaceous polysaccharide microspheres synthesized by hydrothermal method were used as pore-forming agent. The SnO₂ thin film prepared without using carbon microspheres was also synthesized for comparison. X-ray diffraction and scanning electron microscopy were taken to study the micro-characteristics of samples. The influences of operating temperature, gas concentration and structure feature on the H₂S sensing performance of SnO₂ thin film samples were systematically studied. Compared with SnO₂ thin film prepared without using carbon microspheres, the macroporous SnO₂ thin film showed a considerably reduced recovery time and good response-recovery properties.

Abstract: As a lead-free positive temperature coefficient of resistivity (PTCR) material, [Ba_0.95-x (K_0.5Bi_0.5)_0.05Ca_x]_1-yNb_yO₃ system was prepared by the conventional solid-state reaction method. All samples sintered in air at 1300°C possess PTC characteristics as well as semi-conductivity characteristics, especially they show high Tc(130°C~160°C) value and the jump of the resistivity (maximum resistivity ρ_max / minimum resistivity ρ_min ) is four orders of magnitude. Samples with the composition of 0.3mol% Nb₅₊ have low room-temperature resistivity (ρ25°C) of ~10³Ω.cm.

Abstract: In this article, nanocomposites of BaTiO₃/CoFe₂O₄ have been prepared by sol-gel technique. The samples have been calcined at various temperatures ranging from 500 to 900 °C and then the microstructure of the composite nano-particles was studied. XRD, SEM analysis showed the powders can form the two phase composite system of BaTiO₃ and CoFe₂O₄ and the average particle size of the crystalline phases is 50nm at sintering temperature of 800 °C. In addition, we have studied the effects of different solvents and mixed solvents on the stability of the nanocomposites suspensions. Results of sediment volumes and conductivity showed under ultrasonic agitation for at least 25 min, suspensions of the BaTiO₃/CoFe₂O₄ in ethanol-acetylacetone (1:1 in volume) were most stable (the sediment volumes is 0.6cm3/0.4g, the conductivity is 9μS/cm). Electrophoretic deposition (EPD) was utilized for preparation of magnetoelectric (ME) composite films, the impact of deposition time and deposition voltage on electrophoretic deposition process was investigated. The obtained ME composite films exhibited good ferroelectric and ferromagnetic properties, which can meet the demand of ME devices.

Abstract: Electrical properties, positive temperature coefficient of resistivity (PTCR), and microstructures of (Ba_m-0.007Sm_0.007)TiO₃ (BST) with different Ba-site/Ti-site (A/B) ratio sintered in a reducing atmosphere and reoxidized in air are investigated. The results reveal that the room temperature resistivity of the semiconducting BST ceramics first decreases and then increases with increasing of A/B ratio (m), particularly when m is equal to 1.006, the semiconducting BST ceramics which have been sintered in a reducing atmosphere and reoxidized at 800°C exhibit significant PTCR effect with a resistance jumping ratio of 3 orders magnitude, and achieve a lower room temperature resisitivity of 80.8 Ω∙cm, in addition, the grain size distribution of the Ti-excess specimens is much better than that of the Ba-excess ones.

Abstract: The positive temperature coefficient of resistivity (PTCR) behavior of semiconductive BaTiO3 ceramics is often explained by the Heywang model. However, Heywang model couldn’t explain some experimental phenomena of jump range more than 106. This paper considered that the migration of donors, electrons and holes has important influence on grain boundary effect. A differential equation about Fermi level was established on the base of Heywang model. By solving the equation the jump range can be calculated quantitatively. It was found that a potential well exists on the edge of grain due to the donor ionization, and the experimental phenomena of PTC jump range more than 106 could be explained.

Abstract: Semiconducting barium titanate powderss with an average particle size less than 40 nm were used for tape casting. Green tapes with a thickness of 100-500 μm were tape-cast from aqueous slurry of barium titanate powders using polyvinyl alcohol (PVA) as binder and polyelectrolyte ammonium salt as dispersant. The solid loading, viscosity and rheological properties of the ceramic slurries were investigated. Slurries with low viscosity were obtained when the concentration of dispersant was 1.5-2.0 wt% (based on ceramic powders weight) with fixed 75 wt% solid loading. The highest solid loading and optimum dispersion were determined through the proper viscosity to cast. Microstructure and PTC effect of the ceramic chips were studied as a function of firing temperature. Yttrium ions were homogeneously diffused into barium lattices at a low temperature of 1240 °C, and fine-grained PTC ceramics with grain size of 1-2 μm were developed successfully. These results can be explained by the higher rate of reactivity of finer powders and stable behavior of the suspension.

Abstract: Dense PTC ceramics were prepared with BaTiO3 nanopowders synthesized by hydrothermal method. BaCO3 and Ti(OC4H9)4 were used as barium and titanium sources, and Y(NO)3·6H2O as the donor dopant respectively. The average grain size of the powders obtained after hydrothermal treatment at 160°C for 9h was about 30nm with cubic structure. Mn(NO3)2 was introduced to the as-prepared nanopowders in order to improve the PTC effect. After sintered at 1280°C, the PTC ceramic samples exhibited sufficient resistance jump ratio(1.086×103) around Curie temperature, the density of which was 5.81g/cm3(96.5% of the theoretical density).

Abstract: BaTiO3 nanopowders prepared by sol-gel process were used for multilayer PTCR ceramics in order to utilize grain boundary effect and lower sintering temperature. The precursor gel was calcined at different temperatures and the powders were characterized by XRD and TEM. The average grain size was about 26nm when calcined at 800°C for 2h. Effects of acceptor/donor concentration and sintering temperature on PTCR ceramics were also investigated. The optimal concentration of the donor was found to be 0.6mol with the acceptor concentration being 1/8 of the donor. Multilayer PTCR elements were fabricated by tape-casting technique. The jump ratio of PTCR chips sintered at 1240°C was above 103 with the average grain size smaller than 1~2 μm, which is suitable for the multilayer PTCR elements.

Abstract: Nanocrystalline thick-film gas sensor was fabricated by screen printing method with CuO-doped tin oxide powder synthesized by hydrothermal method. Average grain size of the CuO-doped tin oxide powders was typically below 10 nanometers and the thick-films had a narrow grain size distribution typically below 50 nanometers. Effect of the amount of CuO on the sensing properties was investigated and the optimal value was found to be 3 wt%. The nanocrystalline CuO-SnO2 thick-film gas sensors were more sensitive to H2S than those based on commercial micro SnO2 powders, which were attractive to the detection of low concentrations of H2S gas at relative low temperatures.