Key Engineering Materials Vols. 336-338

Abstract: In this paper, the effect of Cr2O3 addition on the electrical properties and microstructures of WO3-MnO2 ceramics was studied. The samples were fabricated by the conventional solid phase reaction techniques. The current-voltage curves and impedance spectra of sintered samples were measured at room temperature. The samples without Cr2O3 doping exhibit the max nonlinear coefficient α above 8 in current region from 1mA to 10mA and an unstable characteristic of current-voltage curve for repeat measuring cycle, while the samples doped with Cr2O3 display lower nonlinearity with α about 5 and an improved stability of current-voltage curve at the same conditions. The impedance measurements indicate that the influence on electrical properties comes from grain boundary as well as grain. SEM shows the doping of Cr2O3 into WO3-MnO2 ceramics suppresses obviously the growth of WO3, and XRD analysis reveals the coexistence of WO3 phases with some manganese tungstates and chromium tungstates. The problems related to electrical transporting mechanism were discussed simply.

Abstract: For the first time, it was found that low-valence additives could be employed to improve the conversion efficiency of dye sensitized solar cell. It was experimentally discovered that by forming nonstoichiometric compound, Ti3+ was located in the lattice of TiO2 film, generating surplus electrons within the film and affecting the morphology of TiO2 particle. The improvement of the conversion efficiency of the solar cell was mainly due to the increase of short circuit current along with the content of Ti3+. The surface of the TiO2 particle became more coarsely after TiCl3 added and the absorbed dye molecules was increased. It was another reason of the improvement of conversion efficiency.

Abstract: The reaction process and kinetics of Al-TiO2-C-Ti-Fe system were investigated by differential scanning calorimetry (DSC) analysis, X-ray diffraction (XRD) analysis and scanning electron microscope (SEM). In order to obtain the information of reaction process for complicated system, the reaction characteristics of Al-TiO2, Al-TiO2-C and Al-TiO2-C-Ti systems are explored firstly. The results show that the reaction process varies with temperature in Al-TiO2-C-Ti-Fe system. At the lower temperature, the dominating reaction in Al-TiO2-C-Ti-Fe system is that between Al and Ti, Al and Fe, and so TiAlx, FeAlx, and Ti2Fe intermetallic compounds form. With the temperature increasing, the intermetallic compounds are decomposed. Then the decomposed Ti and Al react with C and TiO2 respectively and the stable TiC, Al2O3 and Fe three phases form in the final product.

Abstract: The reaction mechanism of silicon and iron composite powders was clarified during the fabrication of high silicon iron sheet with the Si-content of 6.5wt% by Direct Powder Rolling (DPR) technique. The changes of phase composition and structure evolvement were mainly studied. It is found that a local graded structure, Fe-Fe(Si)-Fe3Si(Si)-Si, forms when sintering at 950-1000oC, which plays an important role in the DPR process. Fe3Si(Si) phase keeps higher content of Si, and Fe(Si) phase remains the state with much lower Si-content, thus provides good mechanical proprieties of rolling and cutting. Then, the subsequent sintering at about 1200oC improves the density and makes the distribution of Si homogeneous in the final high silicon iron sheets.

Abstract: Al2(1-x)MgxTi1+xO5(x=0.05-0.3) composite powder was prepared by the method of chemical coprecipitation and subsequent sintering using TiCl4, MgCl2 and AlCl3 solution as the raw materials, and ammonia and ammonium carbonate as the solvent. Thermal dynamics and kinetic dynamics analysis of the precursor during the heat treatment were explored in detail, and the reaction process of Al2(1-x)MgxTi1+xO5 (x=0.3) composite powder was confirmed. Results show that, as the temperature increases MgO reacts with TiO2 of anatase phase to form MgTi2O5. At about 650°C, anatase transfers into rutile. Then MgTi2O5 reacts with Al2O3 to produce MgAl2O4 at 900°C. When the temperature is above 1100°C, the desired Al2(1-x)MgxTi1+xO5(x=0.3) composite powder is synthesized by the reaction of MgAl2O4, Al2O3 and TiO2 of rutile phase.

Abstract: TiO2 sol containing nano anatase crystalline grains was prepared using Ti(OC4H9)4 as a precursor in a water-excessive reaction system. The processes of hydrolytic-polymeric reactions and the structures of the products were analyzed. It was found that the -OC4H9 radicals previously linked to Ti atom in Ti (OC4H9) 4 molecular are broken, and there is no organic radicals linked to Ti atoms in the final sol. The structure of particles in the sol changes along with time from tetrahedral [TiO4] to octahedral [TiO6], with the coordination number changing from 3.14 to 5.18, accordingly. Therefore, the structure becomes closer to an anatase, which is octahedral [TiO6] with a coordination number of 6.0. Transmission electron microscope micrograph also reveals the foursquare projection shape of crystalline grains with a size of 200 nm in the sol. As the particles in the sol have a quasi anatase structure, the TiO2 sol can directly photodegrade Rhodamine B under Ultraviolet illumination with a rate of 62.35%.

Abstract: SiC/Cu composites were prepared by spark plasma sintering under different uniaxial pressure. X-ray diffraction (XRD), SEM techniques were used to characterize the sintered samples. It was found that higher pressure led to the transformation of Cu into Cu2O. The microhardness of the composites was improved by SiC reinforcements. The optimised pressure during the spark plasma sintering was about 50 MPa with the maximum hardness of 1.36 GPa.

Abstract: Sintering kinetics of the α-Al2O3 powders with average particle size 0.1 μm and ca.1% ZrO2 impurity was investigated. Several two-step sintering methods were tried, and the results were compared with the conventional (one-step) process. The temperature (<1600oC), holding time (up to 20 h), heating rate (2 oC /min to 90 oC/min), initial grain size, and porosity were controlled. The optimal results of two-step sintering, a smaller grain size (0.57 μm) and a good density (>98% TD) were achieved by heating to 1450oC, then holding at 1350oC for 20 h. The sintering mechanism was determined, belonging to lattice diffusion in the temperature region of 1400 to 1600oC. The activation energy was 688 kJ/mol, which was interpreted as a volume diffusion of Al3+ ions in polycrystalline matrix.

Abstract: A series of MnO-doped ZnO with different grain sizes and grain morphologies were prepared by sintering the samples at different temperatures for different holding times. The grain size distribution for each sample was determined. It was found that, although the grain size increases and the grain morphology varies with the sintering temperature and/or the holding time, the normalized grain size distribution keeps invariable.