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Understanding the growth mechanisms of nanomaterials usually leads to the successful preparation of targeted micro-structures with excellent properties. In this paper, flower-like and roughly spherical ZnO nanocrystals have been successfully synthesised through facile wet chemical methods, i.e. hydrothermal pathways and homogeneous precipitation method. The photocatalytic activities of the prepared photocatalysts were evaluated by degradation of rhodamine B (RhB) and phenol under ultraviolet (UV) irradiation. Remarkably, the ZnO powder via homogeneous precipitation (urea/Zn2+ = 2.0) exhibited better photocatalytic performance and photostability than those of flower-like ZnO substrate. The enhanced photocatalytic properties could be attributed to more active catalytic sites and effective separation of carriers, which were confirmed by low-temperature nitrogen adsorption, photocurrent responses, and photoluminescence spectral analysis. A possible mechanism for the excellent photocatalytic activity of ZnO substrates was proposed. [ABSTRACT FROM AUTHOR]

Based on previous work, this study further investigated the high-voltage varistor materials prepared from chemically aided method. Microstructural characterization and electrical measurement results showed that the sample possessed more homogeneous microstructure and ultrahigh drop voltage per grain boundary ( Vgb) than the conventional sample. The obtained ultrahigh value of Vgb is connected with the much lower donor concentration ( Nd) and density of interface states ( NIS) of the samples, which resulted from the increased number of active grain boundaries per unit volume. The improved homogeneous microstructure, together with the smaller grain size and narrower size distribution are essential to obtain high-voltage ZnO varistor. [ABSTRACT FROM AUTHOR]

ZnO is a promising thermoelectric material for high-temperature applications; however, the strong correlation between the electrical and thermal transport properties has limited their simultaneous optimization to achieve superior thermoelectric performance. In this work, defect engineering was applied to solve this problem. The results revealed that by eliminating the intrinsic acceptor defects at the grain boundaries, the Schottky barrier disappeared, which led to a huge increase in the Hall mobility. Meanwhile, an increased solid solution of the trivalent dopant Al was achieved to increase the carrier concentration. The increased Hall mobility and carrier concentration gave rise to a maximum electrical conductivity ( σ 310K ) of 1.9 × 10 5 S m −1 , showing a metallic-like behavior. Owing to the ultrahigh σ with moderate Seebeck coefficient, a maximum power factor of 8.2 × 10 −4 W m −1 K −2 was obtained at 980 K. Moreover, by introducing large numbers of lattice defects in the grains, the lattice thermal conductivity was simultaneously decreased. Therefore, the multiple-doped ZnO ceramic with defect engineering of both grains and grain boundaries optimized the electrical and thermal transport properties in a relatively independent way which provided a new and effective route to optimize the performance of the ZnO-based thermoelectric materials. [ABSTRACT FROM AUTHOR]

The phase evolution, microstructure, and electrical properties of WO3-doped ZnO-Bi2O3-based varistors were investigated for different amounts x (0 ≤ x ≤ 1.60 mol%) of the dopant. When x was less than 0.40, the dissolved W6+ in the β-Bi2O3 acted as a donor in the grain boundaries and reduced the electrical properties of the ZnO varistors. However, when x was 0.40 mol%, which meant an amount of WO3 equal to that of Bi2O3, the electrical properties dramatically increased, which means the W6+ donor effect is removed at the grain boundaries because a new Bi2WO6 phase was formed in the grain-boundary regions. The Bi2WO6 phase has high oxygen conductivity at high temperatures; it transfers more oxygen to the grain boundaries in order to further enhance the electrical properties. For x values higher than 0.40 (i.e., an addition of WO3 that is greater than the content of Bi2O3), the electrical properties were steadily reduced in comparison to the composition with x = 0.40. This could be explained by the reduced amount of Co, Mn, and Al at the grain boundaries and in the ZnO grains as a result of their incorporation into the ZnWO4 phase. The electrical properties of the ZnO grains and the grain boundaries were in agreement with the results of the impedance spectroscopy analysis. [ABSTRACT FROM AUTHOR]

Large size needle-like ZnO microrods with diameters around 100 μm and length of up to 2.5 mm have been achieved by a simple method of thermal evaporation of ZnO powder discs in the open air. Metallic catalysts and vacuum condition are not necessary. Both the micro-sized and nano-sized ZnO rods were grown vertically at different sites of the substrate. The as-synthesized microrods and nanorods are pure ZnO without any other element, and both of them consist of single crystalline cores with hexagonal cross sections. As determined by the electron diffraction and high-resolution TEM image, the growth direction of ZnO micro/nanorods is [0001] and most of them are free from defects and dislocations. The formation mechanisms for the microrods and nanorods in different sizes have been preliminary presented. The technique developed in this work makes it possible to fabricate the large size ZnO microrods. [ABSTRACT FROM AUTHOR]

ZnO–Bi 2 O 3 –Co 2 O 3 based varistors doped with SnO 2 , from 0 to 0.3 mol% were prepared by conventional ceramic processing. The effect of SnO 2 on the microstructure and the electrical characteristics of varistor samples were investigated. Results showed that the addition of small amounts of SnO 2 inhibited the grain growth and induced a change of grain morphology. With increasing SnO 2 doping level up to 0.3 mol%, Bi 2 O 3 reacted with SnO 2 to form a cubic crystalline phase Bi 2 (Sn 2 O 7 ) at 950 °C and a pyrochlore phase Bi 2 Sn 2 O 7 at 1100 °C. Moreover, small amounts of SnO 2 produced higher nonlinear coefficients and reduced leakage currents of ZnO–Bi 2 O 3 –Co 2 O 3 based varistors. Meanwhile, the breakdown fields increased noticeably with the addition of SnO 2 , which was due to the decrease of average grain sizes. It was found that the addition of SnO 2 as well as the sintering temperature had an obvious effect on the insulating level at grain boundary region, which was indicated from the changes of relative dielectric constants. [ABSTRACT FROM AUTHOR]

Abstract: The microstructure and electrical properties of ZnO Bi2O3–MnO2-based varistor ceramics doped with different Cr2O3 content were investigated. The addition of Cr2O3 caused a transformation of minor secondary phases from Bi7.72Mn0.28O12.14 to Bi2O3 dissolving different amount of Cr. The average grain size of sample with 0.2mol% Cr2O3 increased remarkably from a range of 6.38–6.62μm to 9.79μm, and the barrier voltage decreased to 1.31V. With the increase of Cr2O3, the nonlinear coefficients of sintered samples decreased to 25.0 at 0.2mol%, and then it further increased up to 40.0, a comparable value with the one of the undoped sample, at 0.8mol%. The results showed small amounts of Cr2O3 promote the grain-growth at a relatively low sintering temperature, and thus result in the shift of breakdown voltages to a lower value. [Copyright &y& Elsevier]

Abstract: The infiltration of molten Bi2O3 into ZnO varistors was carried out. The sintered varistor disks doped with Bi2O3 were painted by Bi2O3 paste on the surfaces and then were reheat-treated. The DC degradation test was carried out to demonstrate the electrical stability of varistors. The influence of the Bi2O3 infiltration on the electrical nonlinear characteristics and stability of varistors was analyzed by capacitance–voltage (C–V) measurements and impedance spectroscopy. The results showed that the Bi2O3 infiltration process at temperatures from 800 to 900°C reduced the donor density in ZnO grains and inhibited the decrease of grain boundary resistance of varistors caused by degradation. The samples infiltrated with Bi2O3 at 850 and 900°C exhibited an excellent electrical stability and maintained relatively high nonlinear characteristics. The enhancement in the electrical stability may be attributed to the diffusion of oxygen into the body of ceramics facilitated by the infiltration of molten Bi2O3. [Copyright &y& Elsevier]

Abstract: Er3+, Eu3+ single-doped several glasses with various composition xB2O3–(80−x)TeO2–10ZnO–10Na2O (x increases by a step of 10) were prepared by a melt-quenching technique. The effects of composition on density, molar volume, refractive index, and phonon sideband spectra were investigated. The dependence of glass structure properties on the composition was discussed. The influence of composition on the radiative and non-radiative relaxation rate, lifetime, and quantum efficiency of the level 4I13/2 of Er3+ in these various glasses were studied. Upon 980nm excitation, the composition dependence of up-conversion and down-conversion emission was studied. [Copyright &y& Elsevier]