Key Engineering Materials Vols. 512-515

Abstract: Using iron nitrate, bismuth nitrate, citric acid and glacial acetic acid as the raw materials, the BiFeO₃ thin films in crystalline state were prepared on FTO substrate with the self-assembled monolayers by liquid phase deposition after graded induction and annealing at 550°C for 30min. The physical phase composition, the surface morphology and dielectric properties of the thin films were characterized respectively by XRD, SEM, and Precision LCR Meter. This paper studied that the deposition temperature and the number of film layers had the effects on the thin films. The results show that the as-prepared thin films show the random orientation and good crystalline. When the deposition temperature is 70°C, the surface of the as-prepared thin film is smooth and uniform. The size of grain is 100nm. The thin film has a dense structure without the apparent pore phase. When the test frequency is between 1kHz and 1MHz, the loss of the thin films is decreased as the increase of the number of the film layers. When the number of the layers is 15, the dielectric constant of the thin films is 44 and the loss is 0.02 when the test frequency is 10kHz.

Abstract: BiFeO₃ thin films co-doping Nd and Co were prepared on FTO/glass substrate by sol-gel method with Bi(NO₃)₃•5H₂O, Fe•(NO₃)₃•9H2O, Nd(NO₃)₃•6H₂O and Co(NO₃)₂•6H₂O as raw materials, 2-methoxyethanol together with acetic anhydride as a solvent. XRD, FE-SEM, Agilent E4980A Precision LCR Meter and TF 2000 Ferroelectric Analyzer were used to characterize the structure, morphology, dielectric property and ferroelectric property of the BiFeO3 thin films. The results show that after Nd and Co co-doping, the BiFeO₃ thin films still keep the perovskite structure. The crystal structure turns square or orthogonal from rhombus. The thickness of the BiFeO₃ thin films is about 500nm and the grain size is 80nm to 30nm. BiFeO₃ thin films co-doping Nd and Co have the larger dielectric constant and the lower dielectric loss compared with Nd doping. BiFeO₃ thin films co-doping Nd10% and Co1% have the dielectric constant of over 170 and the dielectric loss of below 0.03. Both have the better frequency stability. Co-doping Nd and Co could decrease the coercive electric field of BiFeO₃ thin films.

Abstract: The atomic structure and electronic properties of Cr adsorbed on polar ZnO surfaces is studied using first-principles calculations based on density functional theory. It is found that the Cr atom at the on top of O forms a strong ionic bond, the electrons transfer from Cr to O atoms. The Cr atom is adsorbed at the hcp-hollow site on Zn-terminated surface forms metallic bonding with the surface Zn atom, shows a free-electron-like behavior. The adsorbed atoms could not effect more atoms in ZnO due to a strong screening of ZnO to the outside metal, the character of adsorption surfaces is only decided by the atoms near the surface.

Abstract: A method using first principles and pseudopotentials based on density functional theory is applied to calculate the electronic structure and the density of states of ZnO doped with Cr, Mn and Co. Portion of Zn atoms in ZnO crystal randomly substituted by Mn, Cr or Co elements, the electronic structure of Cr ²⁺ , Mn ²⁺ and Co ²⁺ change into 3d⁴, 3d⁵ and 3d⁷, which result in giving rise to localized magnetic moments in ZnO. It was concluded that electronic property of ZnO is not only related with levels of electrons, but also associated with spin, spin-dependent scattering and spin-dependent hopping conductivity are maybe two important mechanism.

Abstract: A novel fabricated technique, by feeding two sets of different ZnO formulations powder in a die by parts, molded only once to produce layered structure(including layer A and layer B) low-voltage ZnO varistor. The samples are examined by using energy dispersive X-ray spectroscopy (EDS), electron probe microanalysis (EPMA), scanning electron microscope (SEM) and DC electrical measurements. EDS and EPMA data indicate that doped elements only exists in layer A, The results of SEM indicate that secondary phases are formed at grain boundaries in layer A, not found in layer B. It is found that the electrical properties of low-voltage varistor are improved without reducing thickness and changing energy absorption capabilities. The higher nonlinearity coefficients, lower breakdown fields and leakage currents of layered structure low-voltage ZnO varistor, as compared to those of ZnO varistor fabricated from the conventional route. The improved current-voltage properties are attributed to the band structure difference in both sides grains, due to the different ion concentration and species in both sides of grain boundary. Layered structure varistor also has more simpler prepared technology than multilayer chip varistor.

Abstract: Mixtures of ITO and SnO2 powders were prepared by a solid-state reaction method in order to produce porous composite materials. X-ray diffraction, scanning electron microscope and gas sensing testing system have been used to analyze the microstructure and properties of it .It has been concluded that when the sintering temperature is 1573K,it has better gas-sensing properties. The pore structure and CO gas sensing property of porous SnO2-In2O3 composite materials can be improved with the increase of SnO2 contents.

Abstract: In this paper, the effect of sintering temperature on microstructure and donor solid solubility of Nb-doped TiO₂ varistor ceramic is investigated. SEM and EDXS were carried out to study the microstructure and chemical compositions of TiO₂ grain. The results show that the TiO₂ sample sintered at 1350°C have better microstructure and properties. The grains size of the TiO₂ sample is about 15μm and solid solubility of donor Nb ⁵⁺ cation solute in TiO₂ grains is 1.49mol%.

Abstract: B2O3-doped ZnO-Pr6O11 based varistor ceramics were fabricated by sintering samples at 1350 °Cfor 2h with conventional ceramic processing method. The microstructure and electrical properties of the as-prepared samples were investigated. X-ray diffraction analysis showed that after the addition of B2O3 with the amount designed in this study, no new phase was examined in the detection limit. Through scanning electron microscopy it was found that the doping of B2O3 can promote the growth of ZnO grains, and the sintering of the samples. The result of electrical properties indicated that the nonlinear coefficient and varistor voltage of the samples could be improved to some extent with appropriate doping amount of B2O3, resulting in the highest nonlinear coefficient 6.7, lowest leakage current 329 μA/cm2, and highest varistor voltage 92.4 V/mm, respectively.

Abstract: In this work, orthorhombic vanadium pentoxide (V₂O₅) fibres have been prepared by electrospinning from a sol-gel precursor and the following calcination at 723 K. Structure, composition and morphology of the obtained V2O5 fibres are characterized. The Raman results show that the obtained V₂O₅ fibres don’t undergo a semiconductor-metal transition. And their high temperature electrical properties are discussed according to a thermally activated small-polaron hopping process.

Abstract: Failure behavior of PZT 95/5 under direct current, pulsed electric field and shock compression is studied by experimental and theory analysis in this paper. The electrothermal coupled failure behavior is a key mode when the PZT 95/5 is subjected to direct current. But when the pulsed electric field is applied to the PZT 95/5, the resonance effect and the relation between energy and frequency have been considered, the vibrant energy shifts to high frequency and the possibility of electromechanical-coupled failure behavior is increased with decreasing the pulsed duration. When the pulsed duration is increased, the failure behavior transfers from mechanical coupled with electric failure to direct current mode step by step. The Failure behavior of PZT 95/5 under compression is complex that not only all of failure mechanism in static must be considered but also the microstructure’s evolvement under shock compression should be considered.