ZnO Grains Research Articles

Formation of two-dimensional ZnO nanosheets by rapid thermal oxidation in oxygenated environment.

Crystalline 2-D (dimensional) ZnO nanosheets were formed by rapid thermal oxidation of etched Zn foil in oxygen at 300 degrees C and 400 degrees C. Short oxidation time was varied from 10, 20 and 30 min. The morphologies and optical properties of the ZnO nanosheets evolved with the oxidation temperature and time. At 300 degrees C, ZnO nanosheets with thickness ranging from 32 nm to 80 nm were obtained while at 400 degrees C, the thickness of the nanosheets increased from 88 nm to approximately 200 nm after 10 and 30 min of oxidation, respectively. The surface roughness of the ZnO nanosheets and grain size increased with oxidation time and temperature. Photoluminescence of the oxidized samples shows ultraviolet (UV) and visible emissions indicating good crystallinity of ZnO which was further confirmed by high-resolution transmission electron microscope observation of ZnO wurtzite interplanar spacing. Photocatalytic activity of ZnO was also investigated by using degradation of methyl orange (MO) and all the samples exhibit photocatalytic activity. The sample oxidized at 400 degrees C for 10 min show better MO degradation after 2 h of exposure due to higher surface area and better crystallinity of the ZnO nanosheets obtained.

Effect of heating rate on microwave sintering of nanocrystalline zinc oxide

In this study, the effect of heating rates on microwave sintered ZnO was investigated at heating rates ranging from 50 to 100°C/min. The relative density and heating rate of microwave sintered samples were found to be independent, and faster heating rates were found to result in smaller ZnO grain sizes. The activation energy of grain growth was calculated and showed a tendency to decrease with increasing heating rates. The main microwave sintering mechanism that prevails during ZnO sintering is grain boundary diffusion, and the nonthermal effect is believed to occur particularly at the beginning of sintering.

Microstructure and Electrical Properties of Y<sub>2</sub>O<sub>3</sub> and Sb<sub>2</sub>O<sub>3</sub> Co-Doped ZnO-Based Varistors

Through a conventional ceramic process, Y2O3 and Sb2O3 co-doped ZnO-based varistors were prepared. The microstructure and electrical properties of the as-prepared varistors were investigated. Y2O3 could act as an inhibitor to the growth of ZnO grains when working with Sb2O3. The mean size of ZnO grains in the Sb2O3 and Y2O3 co-doped samples was smaller than those of the samples only added with Y2O3 or Sb2O3. And with appropriately increased ratio of Y:Sb, it would result in increased sample densification. When the doping level of Y2O3 was small, the nonlinear coefficientand breakdown voltage of the varistors would increase with increasing doping amounts of Y2O3, and the leakage current would decrease. However, when Y2O3 was doped without Sb2O3, both the nonlinear coefficientand breakdown voltage of the varistors would decrease sharply, thus the leakage current increase dramatically. The electrical properties of the Sb2O3 and Y2O3 co-doped varistors would be better than those of the samples only added with Y2O3 or Sb2O3, and when the Y:Sb atom ratio was 5, the nonlinear coefficients, breakdown voltages and leakage current of the varistors reached their optimum values of 777 V/mm, 23 and 0.17 mA/cm2, respectively.

Micro four-point probe investigation of individual ZnO grain boundaries in a varistor ceramic

The varistor effect in ZnO ceramics is triggered by the behavior of the grain boundaries. To understand this effect in detail we have electrically characterized individual grain boundaries. Here, we apply the micro four-point probe method to measure the electrical properties of individual grain boundaries in a polycrystalline ZnO varistor ceramic with a mean grain size of 10 μm. The investigation revealed a wide spread in electrical properties, like nonlinearty exponents from 10 to 150 and switching voltages from 2.3 V to 3.6 V.

Al掺杂浓度对ZnO陶瓷释能电阻材料性能的影响

Al-doped ZnO conductive ceramics were fabricated in order to determine the effects of Al doping concentration on ZnO energy absorption materials.The experimental results show that Al doping concentration has great effects on the linearity,conductivity,and energy absorption density of ZnO energy absorption materials.The doping of Al2O3can improve the linearity of ZnO conductive ceramic by reducing the barriers of ZnO grains,and the nonlinear coefficient can be as low as 1.02.When the doping mole fraction of Al is 1%,the resistivity of ZnO conductive ceramic is the lowest( 0.54 Ω·cm).Al doping can improve the uniformity and density of ZnO conductive ceramic,as a result,the energy absorption density reaches to 720 J / cm3,2 ~ 3 times higher than that of metal energy-release materials.

H/Al共掺杂对ZnO基透明导电薄膜光电性质和晶体结构的影响

By incorporating suitable amount of H dopants and lowering the Al contents,the conflicts between low resistivity and high transmission in transparent conducting films have been successfully solved. The reduced resistivity of ZnO∶ Al films by hydrogen doping is attributed to the increased carrier density and carrier mobility. Hydrogen behaves as a shallow donor in ZnO and can provide plenty of electrons. Most importantly,it can increase the carrier mobility effectively by lowering the potential barrier between ZnO grains due to the passivation of O-defects around grain boundaries. The carrier mobility can also be increased due to the less impurity scattering induced by the lowering of Al dopants in ZnO films. With hydrogen doping,low resistivity(6. 3 × 10- 4Ω·cm) ZnO∶ Al samples with only 1 /10 of Al contents compared to conventional AZO films can be got. The optical transmittance in near infrared region(1 200 nm) increases from 64% to 90% by comparing samples without and with H-doping is shown. This kind of high conductivity and high transmittance ZnO thin film will be excellent transparent conducting oxide candidate for various types of thin-film solar cells to improve the efficiency of the device.

Solution-processed high-haze ZnO pyramidal textures directly grown on a TCO substrate and the light-trapping effect in Cu2O solar cells

We have prepared pyramidally textured ZnO (PT-ZnO) films with a high light-scattering effect using a simple low temperature (75 °C) solution process that involved no annealing and vacuum processes. The PT-ZnO films, which were composed of dense ZnO columnar grains with a (101)-faceted pyramidal tip, were formed directly on a TCO substrate by electrodeposition. The electrodeposition was performed within 10 min from Zn(NO3)2 aqueous solutions that contained NaCl; during deposition, a two-step electrolytic current was applied to separately control the nucleation density and grain size of ZnO grains. Light-scattering effects from the pyramid structure were examined by measuring haze values in the visible region and comparing the results with those from planar and nanorod structures. The PT-ZnO films showed a haze value of ∼52%, which is 3.7 and 2.2 times higher than those of planar and nanorod ZnO films, respectively. On the basis of total transmittance and haze, the optimal thickness of the pyramidal textures was determined to be 0.7–1.2 μm. Theoretical simulation of light scattering on the PT-ZnO was performed using a simple geometric model which indicated that the refraction angle, path length and intensity of transmitted light vary significantly depending on the outer refractive index of ZnO. The light-trapping effect derived from the PT-ZnO was demonstrated by fabricating ZnO/Cu2O thin-film solar cells. The Cu2O solar cell using the PT-ZnO showed efficient light absorption, achieving a 1.3 times higher short circuit current density than that using the planar ZnO.

Optimization of Bi2O3, TiO2, and Sb2O3 doped ZnO-based low-voltage varistor ceramic to maximize nonlinear electrical properties.

In ZnO-based low voltage varistor, the two essential features of microstructure determining its nonlinear response are the formation Bi-enriched active grain boundaries as well as a controlled ZnO grain size by secondary spinel-type phases. Besides, the microstructure and phase composition are strongly affected by the dopant concentration during sintering process. In this study, the optimal dopant levels of Bi2O3, TiO2, and Sb2O3 to achieve maximized nonlinear electrical property (alpha) were quantified by the response surface methodology (RSM). RSM was also used to understand the significance and interaction of the factors affecting the response. Variables were determined as the molar ratio of Bi2O3, TiO2, and Sb2O3. The alpha was chosen as response in the study. The 5-level-3-factor central composite design, with 20 runs, was used to conduct the experiments by ball milling method. A quadratic model was established as a functional relationship between three independent variables and alpha. According to the results, the optimum values of Bi2O3, TiO2, and Sb2O3 were obtained 0.52, 0.50, and 0.30, respectively. Under optimal conditions the predicted alpha (9.47) was calculated using optimal coded values from the model and the theoretical value is in good agreement with the value (9.43) obtained by confirmation experiment.

Photoluminescence of nanocoral ZnO films

Photoluminescence studies of nanocoral ZnO nanostructures, fabricated using magnetron sputter deposition of porous Zn films with subsequent oxidation by RTP annealing at 400–800°C is presented. Structural characterization of the films performed using electron microscopy techniques show the nanocoral morphology of the films with individual coral branches constituting of bunched single crystal ZnO grains of 30–100nm in dimensions. High resolution images confirm the high quality of individual grains. The features in the photoluminescence spectra are identified using temperature-dependent and power-dependent measurements, with rich features observed in the excitonic region at 5K, as well as zinc vacancy lines and surface-state recombination lines being present. It is found, that the spectra significantly depend on the annealing temperatures of the samples, with the spectra acquired for the sample annealed at 800°C exhibiting stronger excitonic emission by 2 orders of magnitude than any other features, thus proving the highest structural quality of the nanostructures.

Characterization of electrospun Pr-doped ZnO nanostructure for acetic acid sensor

Pure and Pr-doped (2wt%, 8wt%) ZnO nanostructures are synthesized by an electrospinning method and followed by calcination. The as-synthesized nanostructures are characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and static gas sensor testing device. Pure and Pr-doped ZnO nanostructure exhibited hexagonal ZnO lattice structure and another Pr7O12 phase is detected in the 8wt% Pr-doped ZnO nanofibers. The surface morphology of pure ZnO and 8wt% Pr-doped ZnO is nanofiber linked by nanosized ZnO grains. The surface morphology of 2wt% Pr-doped ZnO is spherical nano-ZnO crystal-like island. Comparing with pure ZnO nanofibers, Pr-doped ZnO nanostructure exhibit improved acetic acid sensing properties at 380°C. 8wt% Pr-doped ZnO nanofiber shows a high selectivity in detecting acetic acid, acetone and methanal. Pr-doped ZnO sensors show different optimal operating temperature with different concentration of Pr.

Influence of incorporation of Al3+ ions on the structural, optical and AC impedance characteristics of spin coated ZnO thin films

Aluminium doped zinc oxide thin films were deposited onto glass substrate using spin coating technique. The effects of Al doping on structural, optical and electrical properties of these films were investigated. X-ray diffraction analysis showed that all the thin films were of polycrystalline hexagonal wurtzite structure with (002) as preferential orientation except 2 at.% of Al doped ZnO films. The optical band gap was found to be 3.25 eV for pure ZnO film. It increases up to 1.5 at.% of Al doping (3.47 eV) and then decreased slightly for the doping level of 2 at.% (3.42 eV). The reason for this widening of the optical band gap up to 1.5 at.% is well described by Burstein–Moss effect. The photoluminescence spectra of the films showed that the blue shift and red shift of violet emission were due to the change in the radiative centre between zinc vacancy and zinc interstitial. Variation in ZnO grain boundary resistance against the doping concentration was observed through AC impedance study.

Low-temperature preparation of (002)-oriented ZnO thin films by sol–gel method

Zinc acetate was used as the starting material to prepare ZnO thin films on Si(100) and fused quartz glass substrates by a sol–gel method. The effects of baking temperature (170–350°C), solution concentration (0.1–0.5molL−1) and annealing temperature (250–400°C) on (002) orientation and morphologies of the ZnO thin films were investigated. The ZnO nuclei could be formed by the hydrolysis and/or thermal decomposition of zinc acetate during the baking process. The (002) orientation of ZnO thin films was mainly affected by the baking temperature. As the wetting film was baked at 210°C, the hydrolysis and thermal decomposition of zinc acetate simultaneously occurred to form the ZnO nuclei, and the oriented attachment dominated the ZnO grain growth that resulted in the (002)-oriented ZnO nuclei. With the further annealing at 250°C for 60min, the (002)-oriented ZnO thin film was obtained due to growth of the (002)-oriented ZnO nuclei. With increasing the Zn-solution concentration and annealing temperature, the ZnO grain size significantly increased. As the ZnO thin film using 0.4molL−1 Zn-solution was baked at 210°C for 10min and annealed at 400°C for 60min, the highly (002)-oriented ZnO thin film was obtained. These results indicate that it is possible to prepare the highly (002)-oriented ZnO thin film at low temperature owing to the solid-state oriented aggregation of adjacent ZnO nuclei.

Work function engineering of ZnO electrodes by using p-type and n-type doped carbon nanotubes

Transparent electrodes in organic electronic devices are strongly needed in order to replace indium tin oxide (ITO). Some of the best candidates are ZnO films, which have shown both good electronic properties and solution processability compatible with roll-to-roll production of the devices. We present the possibility to engineer the work function of ZnO by blending it with carbon nanotubes (CNTs). B-doped (p-type), N-doped (n-type) and undoped CNTs as well as their blends with ZnO have been characterized by atomic force microscopy (AFM), scanning Kelvin probe microscopy (SKPM) and Raman spectroscopy. The results of Raman spectroscopy demonstrate the substitutional doping of carbon nanotubes, which preserves their covalent structure although increasing the disorder within the nanotubes. The roughness and average shape of grains of ZnO when blended with the doped nanotubes have been measured by AFM. Finally, SKPM shows that the work function of the blends can be engineered from 4.4 ± 0.1 to 4.9 ± 0.1 eV according to the kind of nanotube that is blended even if only a small amount of nanotubes is added to the blend (0.08 wt%).

Effects of Eu2O3 doping on microstructural and electronic properties of ZnO-Bi2O3-based varistor ceramics prepared by high-energy ball milling

ZnO-Bi2O3-based varistor ceramics doped with Eu2O3 in a range from 0 to 0.4% were obtained by high-energy ball milling and fired at 900–1 000 °C for 2 h. XRD and SEM were applied to determine the phases and microstructure of the varistor ceramics. A DC parameter instrument was applied to investigate the electronic properties and V-I characteristics. The XRD analysis of Eu2O3-doped ZnO-Bi2O3-based varistor ceramics shows that the ZnO, Eu-containing Bi-rich, Zn7Sb2O12-type spinel and Zn2Bi3Sb3O14-type which is the pyrochlore phase are present. With increasing Eu2O3 content, the average size of ZnO grain firstly decreases and then increases. The grain boundary defect model was particularly used to explain the excellent nonlinearity of ZnO-Bi2O3-based varistor ceramics with the addition of 0.1 % Eu2O3 and sintered at 950 °C.

Effect of WO<sub>3</sub> Doping on Microstructural and Electrical Properties of ZnO-Pr<sub>6</sub>O<sub>11</sub> Based Varistor Materials

The effect of WO3doping on microstructural and electrical properties of ZnO-Pr6O11based varistor materials was investigated. The doped WO3plays a role of inhibitor in ZnO grain growth, resulting in decreased average grain size from 2.68 to 1.68 μm with increasing doping level of WO3from 0 to 0.5 mol%. When the doping level of WO3was lower than 0.05 mol%, the nonlinear current-voltage characteristics of the obtained varistors could be improved significantly with increasing amount of WO3doped. But when the doping level of WO3became higher, their nonlinear current-voltage performance would be dramatically deteriorated when more WO3was doped. The optimum nonlinear coefficient, varistor voltage, and leakage current of the samples were about 13.71, 710 V/mm and 13 μA/cm2, respectively, when the doping level of WO3was in the range from 0.03 to 0.05 mol%.

Microstructure and electrical properties of Dy2O3-doped ZnO–Bi2O3 based varistor ceramics

The Dy2O3-doped ZnO–Bi2O3 based varistor ceramics were prepared by a solid state reaction route, and the microstructure and electrical properties were also investigated. The results show that a multiphase composition exists in all samples. With increasing content of Dy2O3, the average size of ZnO grains decreases and the threshold voltage greatly increases. The nonlinear coefficient of the ceramics is in the range of 4.7–60.2, the threshold voltage is in the range of 364.6–884V/mm, and the leakage current is between 1.77μA/cm2 and 159.1μA/cm2. Typically, the varistor ceramics with 0.60mol.% Dy2O3 sintered at 1050°C exhibit excellent electrical properties with the threshold voltage of 872.5V/mm, the nonlinear coefficient of 60.2 and the leakage current of 5.46μA/cm2. The results show doping Dy2O3 is a promising route for the production of the higher threshold voltage of ZnO–Bi2O3 based varistor ceramics.

Bi4Ti3O12 modified low voltage ZnO-based varistors: microstructure, electrical properties and aging characteristics

The effects of Bi4Ti3O12 addition on the microstructure, electrical properties and aging characteristics of ZnO-based varistors were investigated. The addition of Bi4Ti3O12 can reduce the formation of Zn2TiO4 spinel phase and promote the growth of ZnO grain. Bi4Ti3O12 doped ZnO-based varistors sintered at 1,130 °C possessed excellent performance of I L = 2.9 μA, E 1mA = 29.7 V/mm, α = 30.2, and good surge absorption capability with %△E 1mA = −3.5 % for 500 A 8/20 μs impulse current. In addition, the varistors exhibited the most stable accelerated aging characteristics with %△E 1mA = −0.3 % for DC accelerated aging stress of 125 °C/7 h. These results revealed that Bi4Ti3O12 addition is highly beneficial to attain enhanced varistor properties of ZnO ceramics.

Quality assessment of ZnO-based varistors by 1/f noise

Noise has been used as a diagnostic tool of surge arrester varistor structures comprising of ZnO grains of various type and size. The physical and electrical properties of the measured samples have been described. In the experimental study, the applied measurement system and the results of noise measurements for the selected structures of varistors designed for the continuous working voltage 280V, 440V and 660V have been presented. Noise properties are related to electrical characteristics of the measured specimens giving more distinctive results than their voltage–current characteristics. It is suggested that the proposed procedure can be applied as an effective non-destructive testing method focused on defects and structural heterogeneity detection in the tested objects to assess their preparation processes.

Fabrication and Characterization of n-ZnO/n-Si Heterojunction

The n-ZnO/n-Si heterojunction are fabricated by depositing ZnO films on n-Si (111) films substrate using magnetron sputtering Al doped ZnO ceramic target. The structures of n-ZnO films, analyzed by X-ray diffraction (XRD) spectroscopy, and the preferential orientation of the ZnO grains is observed along the (101) and (100) axis aligning with the growth direction. The photoelectric properties, charge carrier transport properties and conductive mechanism were studied by testing the I-V, C-V characteristics with illumination and without illumination. Current-voltage (I-V) measurements of n-ZnO/n-Si heterojunctions show good diode characteristics and photovoltaic effects with illumination. The forward conduction is respectively determined by carrier recombination in the space charge region, defect-assisted tunneling and exponential distribution trap-assisted space charge limited current mechanism with the increase of forward voltage. Also, a band diagram of n-ZnO/n-Si heterojunctions has been proposed to explain the transport mechanism. As the conduction band and valence band offset in the ZnO/n-Si heterojunction is too big, the current transport mechanism is dominated by the space-charge limited current (SCLC) conduction at the forward voltage exceed 0.8 V. The results suggest the existence of a large number of interface states in ZnO/n-Si heterojunction, and the interface states can be reduced and the photoelectric properties can be further improved. DOI: http://dx.doi.org/10.11591/telkomnika.v11i9.3268

Field failure mechanism and reproduction due to moisture for low-voltage ZnO varistors

This paper presents an investigation of the insulation resistance/leakage current failure of low-voltage ZnO varistors in the field. Failed varistors resumed normal operation after surface polishing, and the fault site was located in the Si coating layer and the outermost side of the bulk ZnO. To reproduce the field failure phenomenon, an effective stress that causes failure was suggested. Accelerated reproduction tests were conducted to determine if the chosen stress caused the field failure. To accelerate the formation of the defect, a water precipitation/reflow precondition was applied. A biased HAST after preconditioning could reproduce the same field failure phenomenon. The failure mechanism and root cause were determined by comparison with a sample that used an improved coating method. A failure mechanism was proposed in which the insulation resistance degradation was caused by Zn(OH)2, which is generated by moisture absorbed into the surface of the varistor. Further, the ZnO and leakage current degradation was caused by H+ ions generated by dissociation of the diffused H2O to the ZnO grain boundaries on the surface of the bulk ceramic. Finally, the effective factors for field failure of the ZnO varistor were defined as the weakness of the Si coating, moisture ingress, and bias.