Key Engineering Materials Vols. 280-283

Abstract: The electrical resistivity (r), the dielectric constant (e), and the dielectric loss (tan d) were accurately measured in the temperature range 60-200oC for the samples (BaTiO3-La2O3-Nb2O5) with (Nb2O5 = 0.1) wt% and La2O3 = 0.0, 0.1, 0.2, 0.3 and 0.4). It is found that the addition of La content cause increase in resistivity and decrease the dielectric constant.

Abstract: Effect of Li+ and Mn2+ doping on the electrical properties of SrTiO3-based varistor ceramics was investigated. The results showed that the varistor ceramics cannot possess ideal electrical properties when only Li+ or Mn2+ was doped. However, when Li+ and Mn2+ were properly doped together, the samples acquired excellent electrical properties: V1mA = (5.7 ~ 11.7)V/mm; α = 6.8 ~ 11.0; er = (2.4 ~ 4.4) × 105; tan d = (7.1 ~ 10.4)%. When the contents of Li2CO3 and MnCO3 were 1.00 mol% and 0.05 mol% respectively, the nonlinear coefficient α reached the largest value of 11.0. Compared with only Mn2+ doping, the addition of Li+ increases the α value significantly.

Abstract: This paper made a thorough investigation on (Ba,Sr,Pb) TiO3-BaPbO3 system compound PTC ceramic. The optimum compound method is determined by lots of experiments in different compound schemes. Also the formula and process are optimized and improved. The best properties acquired are: ρroom-temperature=18.8Ω·cm, ρmax/ρmin >103. In the end, the phenomena in the experiment are illustrated with electron trap model.

Abstract: Impedance spectroscopy is suitable to evaluate the electric structure and behavior by probing the relaxational processes during the charge transport within this kind of electrically inhomogeneous PTCR ceramics. This technique has been used to characterize the change of structure and behavior in sensing process of La-doped PTC-type BaTiO3 as a CO gas sensor. The kinetic behavior of PTCR-CO ceramics in sensing process was detected. The sensitivity of the PTCR-CO comes from the change in the grain-boundary resistance of the ceramics with different temperatures in surrounding atmosphere. An important indirect evidence for the creation of new electron traps in grain boundaries of the semiconducting PTCR ceramics have been found using by impedance spectrum. And the creation of the new defects and electron traps in grain boundaries, [VO •], [e¢], and/or [Vo ×], still plays a dominate role.

Abstract: A number of paramagnetic defects and defect complexes in BaTiO3 have been detected by the Electron Paramagnetic Resonance (EPR) technique. The signal intensity, the g factors and the hyperfine splitting constants A of Mn2+ at the different temperatures were distributed with temperatures in a U-shape. In this paper, a new direct evidence for the presence of manganese with an electron spin of S=5/2 (Mn2+) in all four phases (R, O, T and C) and the high spin state of the electron configuration in MnTi² at the high temperature cubic phase is presented by EPR method in La-Mn codoped BaTiO3 PTCR ceramics.

Abstract: TiO2 photocatalyst were obtained from the hydrolysis product of TiOSO4 with ammonia and the photocatalytic activities were investigated by the photocatalytic degradation of methyl orange aqueous solution under UV-light and visible-light. The results showed that the absorption intensity in visible-light was dependent on the concentration of the materials, pH value and heat-treatment temperature. The color of the TiO2 varied from yellowish to vivid yellow when the TiO2 was calcined at different temperatures from 300°C to 500°C for 1h. The photocatalytic activity of TiO2 under visible light was determined by the photo-absorption feature in visible light region, which was related to the heattreatment temperature. The TiO2 calcined at 400°C shows stronger photo-absorption in visible light region and exhibits higher visible-light activity than the TiO2 calcined at other temperature.

Abstract: CeO2-doped WO3 ceramics were fabricated by using nanometer WO3 and CeO2 powders as raw materials. The microstructure and electrical properties were studied. The ceramics have relatively low breakdown voltage and high dielectric constant. The nonlinear coefficient does not decrease with the increase of the ambient temperature. The electrical conductivity decreases with increasing temperature, indicating that the ceramics have metallic behavior instead of semiconducting behavior.

Abstract: The ZrNiSn/C60 thermoelectric composite was prepared by a spark plasma sintering (SPS)technique. The obtained sample was fully dense and homogeneous. Slices of the sample were characterized by electrical resistivity, Seebeck coefficient and thermal conductivity up to 850K. It was shown that the composite has a higher electrical conductivity, a lower Seebeck coefficient and a higher thermal conductivity than the ZrNiSn matrix alloy. XRD pattern and EPMA analysis of the composite revealed that the C60 nano particles reacted with ZrNiSn matrix to form ZrC inclusions.

Abstract: Ternary compounds (PbTe)1-x(SnTe)x with nanocrystallines were synthesized at a temperature of 180oC by conventional solvothermal method. XRD analyses revealed that relatively pure compounds can be synthesized using ethylenediamine as a solvent only when a reaction time of at least 24h is applied. The XRD results are in agreement well with those from EMPA analysis. The grain sizes of the materials, ranging from 50~70nm, are slowly increased with reaction time. Measurements showed that the maximum power factor of about 6.0×10-4 W.m.K-2 for the synthesized materials are higher than that of similar material prepared by vacuum melting.

Abstract: The n-type Bi2(Te,Se)3 thermoelectric materials doped with different contents of TeI4 were fabricated through the spark plasma sintering (SPS) technique at 693K under a pressure of 60MPa. The sintered materials were rather dense and showed preferred grain orientation. Electrical conductivity (s), Seebeck coefficient (a), and thermal conductivity (k) were measured in the temperature range of 300-500K. The optimal figure of merit ZT (ZT=a2sT/k) of the sintered material in the direction perpendicular to the pressing direction showed a value of 0.75 for the sample containing 0.14wt% TeI4.