Key Engineering Materials Vols. 368-372

Abstract: This paper systematically analyzes the microstructural nonuniformity of ZnO varistors when Al2O3 dopants are added. The introduction of Al2O3 dopants leads to a decrease of ZnO grain growth rate and correspondingly an increase of the microstructural uniformity of ZnO varistors. Both the average value and the standard deviation of ZnO grain sizes decrease with the increase of Al2O3 content, approximately in a log-linear relation. With detailed experimental results, this study provides quantitative references for the practice of adding Al2O3 dopants to give ZnO varistors desired microstructural and electrical characteristics.

Abstract: The effect of Ho2O3 doping on the electrical properties and microstructure of ZnO base varistor was investigated. It was found that Ho2O3 is an effective dopant for increasing the breakdown electric filed. The Ho2O3 doping can also improve the nonlinear performance both in low and high current area. But excessive doping of Ho2O3 will decrease the withstanding surge current. With 0.8mol% Ho2O3 doping, the varistor samples exhibit a breakdown voltage of about 400V/mm, a nonlinear coefficient of 80 and the withstanding surge current of 8/20μs, waveshape is higher than 5kA. Ho2O3 dopant can hinder ZnO grain growth and make the crystal grains more uniform.

Abstract: The numerical simulation is an effective method for researching the relationship between macroscopic properties of ZnO varistors and their microstructural characteristics. However, the computing efficiency is a big problem for further researches. In this paper, a quick piecewise linear method for solving the complicated nonlinear resistor networks of ZnO varistors is proposed, which can significantly improve the computing efficiency by a hundred times. The simulating models for the microstructural characteristics of ZnO varistors are improved based on actual conduction mechanism of grain boundaries. The simulations of more bulk characteristics of ZnO varistors become possible, such as alternating current (AC) and surge properties.

Abstract: ZnO varistors with higher voltage gradients are expected in the power system, and the key to this property is to diminish the grain size. In this paper, nano-sized ZnO particles are used as the raw material in order to obtain small grains. Pellets are fabricated by two sintering methods: the conventional pressureless sintering and the spark plasma sintering (SPS). For the samples fabricated by the conventional sintering, the average grain size varies from 1 μm to more than 10 μm when the sintering temperature changes from 900 oC to 1200 oC. The voltage gradient can reach as high as 2020 V/mm. The grain size of the SPS samples is less than 1 μm, and the voltage gradients can reach 963 V/mm.

Abstract: Dense tin oxide based ceramics are a new type of varistor materials. To further understand the electrical properties of SnO2 varistors doped with CoO, Nb2O5, and Cr2O3, the techniques of capacitancevoltage (C-V) measurement and deep level transient spectroscopy (DLTS) were used to investigate the electron traps in the SCN samples (doped with 1.0 mol% CoO and 0.05mol% Nb2O5) and SCNCr samples (doped with 1.0 mol% CoO, 0.05mol% Nb2O5 and 0.05mol% Cr2O3). Two electron traps were detected: trap T1 is located at Ec - 0.30 ± 0.01eV and trap T2 is located at Ec – 0.69 ± 0.03eV for both SCN and SCNCr samples. The variations in the donor density and trap density could be related to the addition of chromium oxide. The features of these traps are discussed based on the defect theory related to the SnO2 varistors.

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.

Abstract: Co-doped tin oxide thin films were prepared using spin-coating method. Variation of doping content on the magnetic property and optical property were measured. XRD and magnetic measurement shows that Co solubility limit in SnO2 is less than 3%. When the doping content is lower than 3%, the films show good optical transmittance. When the doping content is reached to 10%, the optical transmittance became lower. When the solution is diluted, the optical transmittance drops more slowly.

Abstract: Multistep electrochemical anodization is an electrochemical experiment orderly conducting in different electrolytes. In this paper, TiO2 nanotube arrays have been firstly anodic grown in aqueous electrolyte (H3PO4/HF) and later anodic grown in organic electrolyte (glycerol/NH4F). Compared with separately anodizing in aqueous and organic electrolyte, the morphology of the resulting nanotube arrays can be optimized. SEM images showed that the obtained nanotubes have a length of more than 2 μm and single-pore diameter ranging from 120 to 150 nm. The current work indicates that the multistep electrochemical anodization has a contribution to the optimization of the morphology of TiO2 nanotube arrays.

Abstract: The self-organized nanotube arrays were fabricated in glycerol electrolyte containing different additives, such as deionized water or ethylene glycol. The effects of anodization conditions, including anodic voltage, electrolyte viscosity, additives and ion concentration on the pores size and morphology were studied. Combined with preoxidation process, high aspect ratio titania nanotube arrays were formed rapidly in glycerol and saturated NaF solutions at 60 V for 1 h. SEM observation showed that a film with the thickness of about 10μm was obtained. A possible mechanism of the nanotubes growth was presented.

Abstract: Thin Mn2(NO3)2/PVA precursor nanowire was fabricated via electrospinning technology at a low temperature in this paper. After dried at 150°C for 10 hours and then sintered at 450°C for 2 hours, Mn2O3 nanowire was successfully obtained. The nanowire was characterized using X-ray diffraction and scanning electron microscopy. The results showed that the nanowire was composed of pure Mn2O3 phase. The nanowire exhibited smooth surface and continuous sharp with length of tens of micrometer. The diameter of Mn2O3 nanowire was measured and calculated through SEM, which showed most probable value of about 110 nm and 270 nm. The reason of the diameter separation was also discussed.