ZnO Grains Research Articles

Enhanced electrical properties of ZnO varistor ceramics by spark plasma sintering: Role of annealing

ZnO varistor ceramics are synthesized by spark plasma sintering (SPS), and then annealed in a conventional furnace in air atmosphere. Dense microstructure with more than 98% of theoretical density is achieved, and homogenous ZnO grains with typical second phases such as Zn7Sb2O12 spinel, Zn2Bi3Sb3O14 pyrochlore are observed from X-ray diffraction spectra (XRD) and scanning electron microscopy (SEM). In addition, a metallic Bi phase is also observed, which is resulted from the reduction of Bi2O3 because of inherent low oxygen partial pressure of SPS, and it can be reoxidized after a post-annealing process. The breakdown field of the optimum ZnO varistor ceramic is 725 V/mm with a nonlinear coefficient of 53.5, which stems from its high Schottky barrier height of 0.60 eV. In analysis of dielectric spectra from 173 K to 293 K, two relaxation peaks with activation energy of 0.22 eV and 0.48–0.54 eV can be observed, which are attributed to the intrinsic zinc interstitial and defect of grain boundary or intergranular phase, respectively. The grain boundary resistances are discussed according to Cole-Cole plot, from which the activation energy controlling resistance at the grain boundary is determined to be 0.87 eV.

Loading of Zn/ZnO particles in the precursor feedstock affects the characteristics of liquid plasma sprayed nano-ZnO coatings for photocatalytic applications

It is known that ZnO is an n-type semiconductor with photocatalytic performances under ultraviolet light irradiation. Constructing a superior structure for a modified electron band has been one of the major research goals for photocatalytic ZnO. Here we report a new technical route for making nano-ZnO coatings with a porous topographic morphology. The coatings were fabricated by plasma spraying the mixture of suspension and solution liquid precursors. Pre-loading of ZnO and Zn powders in the precursor was carried out for the purpose of tailoring the structure of the coatings. The coatings in micron thicknesses showed a porous skeleton and a fluffy top layer consisting of ultrafine ZnO grains. Photocatalytic testing by measuring the degradation of methylene blue revealed significantly enhanced activities of the coatings deposited using the ZnO/Zn loaded precursor. The hybrid-structured ZnO coatings exhibited a narrowed band gap and modified oxygen defects as compared to those deposited from the single liquid feedstock. The results shed light on a one-step easy thermal spray fabrication of polytropic nanostructured functional coatings by employing solid powder-loaded liquid as the starting feedstock.

Defects and oxygen vacancies tailored structural, optical, photoluminescence and magnetic properties of Li doped ZnO nanohexagons

In this study, Zn1-xLixO (x = 0.02, 0.04, 0.06, 0.08 and 0.10) nanohexagons were prepared by sol-gel combustion route and characterized using standard techniques for understanding their crystallography, surface morphology, optical, photoluminescence and magnetic properties. Single phase wurtzite type hexagonal crystal structure of the synthesized Li doped ZnO nanoparticles was identified from the Rietveld profile refinement of the powder XRD patterns and HRTEM & SAED analysis. HRTEM micrographs and XRD peak profile analysis confirmed formation of highly crystalline Li doped ZnO nanohexagons (ZnO:Li NHs) with average crystallite size in the 20–40nm range. Formation of functional Zn(Li)–O bonds in ZnO:Li NHs was confirmed by FTIR. SEM micrographs of the prepared samples showed random shaped, uniform distribution and well dispersed grains of ZnO:Li NHs. All the ZnO:Li NHs absorbs significant light in the UV band and the band gap energy monotonically decreases with increasing Li concentration in ZnO:Li NHs. Photoluminescence spectra exhibited two characteristic emission bands in all the ZnO:Li NHs and the UV emission band showed red shifting with increasing Li concentration, which is ascertained with the optical absorbance spectrums. Visible band spectral analysis of the PL data revealed formation of neutral, mono, and divalent oxygen vacancies along with the other defects like Zn vacancy in the ZnO lattice by Li doping and synthesis mechanism. All the ZnO:Li NHs exhibited weak ferromagnetism at RT. Lattice defects and oxygen vacancies created by Li1+ ions in the host ZnO lattice play significant role in band gap narrowing, photoluminescence in orange-red region and weak ferromagnetic ordering in ZnO:Li NHs.

Characterization of ZnO/AlO x /benzene thin-film heterostructures grown through atomic layer deposition/molecular layer deposition

Multilayer thin-film structures are promising for many future high-tech applications. We investigate the structure of polycrystalline ZnO thin films with sub-nanometer amorphous inorganic (AlO x ) and organic (benzene) layers grown by atomic/molecular layer deposition. Small quantities of aluminium are typically introduced in ZnO films for doping, while one of the intended functions of the organic layers is to block thermal conductivity. We apply the AlO x and benzene layers both simultaneously and separately, and investigate the resultant superlattice films with transmission electron microscopy, x-ray reflectivity and x-ray diffraction measurements. The study reveals that both AlO x and benzene form distinct layers in the ZnO matrix even down to one atomic/molecular layer. Furthermore, we demonstrate that despite the clear layering, the ZnO grains can penetrate through thin (below ca. 2 nm) benzene and AlO x layers.

Improvement of surge current performances of ZnO varistor ceramics via C3N4-doping

ZnO-Bi2O3 based varistor ceramics doped with C3N4 were fabricated via solid state method. Experimental results show that C3N4 additive acts as an inhibitor in ZnO grain growth, and the average grain size decreases from 10.2–7.1 μm. The varistor breakdown voltage gradient increases from 222.3–282.3 V/mm, and the nonlinear coefficient increases from 51.9–58.2 with the increase of C3N4 content from 0 to 3.0 wt%. The C3N4-added samples exhibited smaller residual voltage ratio and better surge current withstanding capability. It is proposed that the C3N4-doping leads to substitution of nitrogen for oxygen in the grain boundary region, forming acceptor type defects. The acceptor type defects act as electron traps, increasing the barrier height from 1.31 to 1.50 eV and the depletion layer width from 54.9–61.8 nm, which increases the nonlinearity, and the surge current performances of the C3N4 doped ZnO varistor ceramics are improved.

Deposisi ZnO Doping Ag pada Substrat Alumunium Foil untuk Degradasi Methylene Blue

This research has successfully deposited ZnO: Ag on aluminum foil substrates with variations in deposition temperature. The purpose of this study was to obtain the surface morphology of ZnO:Ag on aluminum foil substrates with variations in deposition temperature and obtain optimization of the photocatalytic activity of ZnO: Ag in degrading Methylene Blue dyes. ZnO:Ag deposition on the aluminum foil substrate was carried out by the sol-gel method and spray coating deposition technique. Surface morphology characterization and photocatalytic activity with SEM and with UV-Vis. The surface morphology results obtained from ZnO:Ag on aluminum foil substrate with deposition temperatures of 250 ℃, 300 ℃, 350 ℃, and 400 ℃ are the forms of ganglia with indications as ZnO and Ag grains attached to ZnO. Obtained surface area and roughness level ZnO:Ag decreases with an increase in temperature from 250 ℃ - 300 ℃, and surface area and roughness increases at 350 ℃ -400 ℃. The highest level of roughness is found in the ZnO layer: Ag temperature 400 ℃. The most optimum photocatalytic activity is indicated by the largest percentage of degradation. The highest percentage of degradation is shown by the ZnO: Ag layer with a temperature of 350 ℃ of 87.33%. This is due to the modification of silver at low temperatures which is <400 ℃ effective for photocatalytic activity.

Mechanically tuned conductivity at individual grain boundaries in polycrystalline ZnO varistor ceramics

Mechanically induced changes in electrical properties have gained increasing interest over the last decade. The research field piezotronics, which describes the change in potential barrier height (e.g., at Schottky contacts) by piezoelectric charges, could lead to promising applications such as sensors or strain trigged transistors. In this contribution, the changes of potential barriers by mechanical stress at several grain boundaries of polycrystalline ZnO have been quantified. Rationalization is provided by concurrent measurement of crystal orientations of both neighboring grains. The highest changes in barrier height could be observed at ZnO–ZnO grain boundaries exhibiting varistor properties. In this case, the barrier height can be almost completely reduced by positive piezoelectric charges. Furthermore, an increase in barrier height is observed with negative piezoelectric charges. The developed physical model suggests an equivalent impact of externally applied voltage and induced positive piezoelectric charge on the barrier properties. In turn, not only the barrier height is modified by piezoelectric charges but also the breakdown voltage. This has, nevertheless, been only indirectly verified in experiments. These allow rationalizing the proposed model. Furthermore, a statistical study reveals a change in the distribution of breakdown voltages with changing stress conditions in the varistor ceramic.

Nanometric morpho-structural characterization of mesoporous metal oxide semiconductors for chemo-resistive gas sensors

SnO2 and ZnO are among the most studied semiconductor oxides for applications as gas sensing devices for detecting and monitoring the presence of toxic gases such as CO, NO, NO2. Enhanced sensing properties have been recently reported on gas sensor based on SnO2-ZnO composites due to the formation of hetero-junction between ZnO and SnO2 grains and the contribution of a depleted layer at the intergrain ZnO-SnO2 interfaces. The sensing characteristics can be further improved by adding metallic nanoparticles with a catalytic activity, like Pt or Pd. Along with the electrical properties of such a complex system, morphology plays an essential role in facilitating and enhancing the interaction with the surrounding gas. In our work, ZnO doped SnO2 mesoporous system for gas sensors has been prepared using solvothermal methods in various experimental conditions. Aiming for a proper optimization of the gas sensing properties, the above mentioned system has been decorated with Pd using wet spray method. This work presents an in depth structural and morphological study by combined techniques of analytical electron microscopy on the mesoporous SnO2-ZnO systems decorated with Pd in function of the synthesis conditions. Electron tomography has been employed for a complete, 3D investigation of the Pd clusters distribution within the mesoporous SnO2-ZnO matrix. By a proper image segmentation Pd clusters have been successfully isolated in the surrounding ZnO-doped SnO2 matrix, thus allowing more complex correlations between the micro/nano-structure and the gas sensing properties.

Doping Effect of Yb2O3 on Varistor Properties of ZnO-V2O5-MnO2-Nb2O5 Ceramic Semiconductors

This study describes the doping effect of Yb2O3 on microstructure, electrical and dielectric properties of ZnO-V2O5- MnO2-Nb2O5 (ZVMN) ceramic semiconductors sintered at a temperature as low as 900°C. As the doping content of Yb2O3 increases, the ceramic density slightly increases from 5.50 to 5.54 g/cm3; also, the average ZnO grain size is in the range of 5.3-5.6 μm. The switching voltage increases from 4,874 to 5,494 V/cm when the doping content of Yb2O3 is less than 0.1 mol%, whereas further doping decreases this value. The ZVMN ceramic semiconductors doped with 0.1 mol% Yb2O3 reveal an excellent nonohmic coefficient as high as 70. The donor density of ZnO gain increases in the range of 2.46-7.41×1017 cm−3 with increasing doping content of Yb2O3 and the potential barrier height and surface state density at the grain boundaries exhibits a maximum value (1.25 eV) at 0.1 mol%. The dielectric constant (at 1 kHz) decreases from 592.7 to 501.4 until the doping content of Yb2O3 reaches 0.1 mol%, whereas further doping increases it. The value of tanδ increases from 0.209 to 0.268 with the doping content of Yb2O3.

34.2: Invited Paper: ZnO based transparent conductive oxides for to‐date flat panel displays

Promising approaches to improve electrical characteristics, chemical resistance, and thermal stability of the resistivity of ZnO‐based TCO thin films are investigated. The following ways were tested: a) gradient doping of ZnO grains into the synthesized TCO films by indium in order to create passivating coatings on the surface of the grains; b) preventing the oxygen diffusion into the films by suppressing the columnar structures formation; c) methods combining the above ones. It was found that in the ZnO‐based TCO films deposited by magnetron sputtering at temperature above 150°C under the presence of excess zinc in the reagent flow, a decrease in the porosity of the columnar ZnO films is observed after subsequent heat treatments in air. The introducing of In, Ga, Sn metals to the composition of the ZnO films contributes to the formation of thin surface oxide sub‐layers with passivation properties on the surface of the ZnO grains. The results of research clearly show that preventing the formation of a columnar structure in combination with the passivation of grain surfaces creates real prospects for expanding the areas of application of ZnO‐based transparent electrodes.

Low‐temperature sintering and electrical properties of BBSZ glass‐doped ZnO‐based multilayer varistors

AbstractThe effect of Bi2O3‐B2O3‐SiO2‐ZnO glass (denoted as BBSZ) content on the densification, microstructure, and electrical properties of ZnO‐based multilayer varistors (MLVs) has been investigated. In ZnO‐based (MLVs), BBSZ acted as a promoter in ZnO grain growth, which increased the grain size of ZnO, resulting in a decrease in breakdown voltage. Results showed that the adding of BBSZ glass had an improvement in densification and overall properties of MLVs. It was found that a maximum value of 5.45 g/cm3 of the bulk density and the optimum properties (V1mA = 27.28 V, α = 40.33, IL = 0.75 μA, Ip = 35 A) of ZnO‐based MLVs were achieved with 3 wt% BBSZ glass sintered at 925°C for 3 hours.

Grain growth kinetics of Er2O3 doped ZnO-V2O5 based varistor ceramics

Control of grain coarsening during sintering of oxide mixtures remains a challenge to manufacture high-performance varistors. This article reports microstructural evolution, grain growth kinetics and nonlinear electrical properties of Er2O3 (0–1.0 mol%) doped ZnO-V2O5 based varistors sintered at 900–1300 °C for a duration ranging from 30 to 480 min. Microstructural characterizations revealed that raise in sintering temperature and time enhanced coarsening of ZnO grains while Er2O3 doping inhibited the grain growth substantially with marginal slowdown of the densification process. Phenomenological grain growth kinetics analyses inferred that the magnitudes of grain growth exponent (3–5), as well as apparent activation energy (263–774 kJ mol-1), increased monotonically with Er2O3 concentration establishing its effectiveness as a strong grain growth inhibitor. In Er2O3 doped sintered pellets, the development of Er-rich and ErVO4 spinel phases along the intergranular layers of ZnO was confirmed by microstructural characterizations like FESEM, EDS and XRD studies. These phases found to exert pinning effects causing retardation of grain coarsening. Judicial selection of Er2O3 doping concentration (0.5 mol%) and sintering parameters (900 °C and 120 min) generated ZnO-V2O5 varistor ceramic with excellent electrical (nonlinear exponent: 80 and breakdown field: 8715 V cm−1) properties.

Effect of Pr6O11 doping on the microstructure and electrical properties of ZnO varistors

ZnO-Pr6O11 based varistors have anticipated application prospect in electrical system, but the study of the detailed microstructure is still poor. In this paper, the effect of Pr6O11 doping on the microstructure (such as element distributions and grain boundary characteristics) and electrical properties of the ZnO-Pr6O11–Co2O3–Cr2O3–Y2O3 varistors manufactured by the conventional solid-state sintering procedure was studied detailedly. Without Pr6O11 doping, the elements have the similar distributions in the interior and grain boundaries of ZnO phase, and the double Schottky barrier cannot be formed. With Pr6O11 doping, Pr6O11 liquid phase dissolving a large number of Zn, Cr, and Y forms during sintering and Pr6O11 solid phase forms in the ZnO grain boundaries during cooling, so the double Schottky barriers form. The voltage gradients E1 mA and nonlinearity coefficients α of the samples with Pr6O11 contents of 0.25–1 mol% are much larger than those of the sample without Pr6O11 doping, and the values of α and E1 mA increase with Pr6O11 content increasing from 0 to 1 mol%. The values of α and E1 mA simultaneously depend on the double Schottky barrier height ϕb and the CPE exponent αCPE. Finally, the ZnO varistor with a voltage gradient of 333.6 V/mm and a nonlinearity coefficient of 24.4 was obtained by Pr6O11 doping of 1 mol%. The detailed study of the microstructure and related mechanism is of great importance for preparing ZnO-Pr6O11 based varistors with excellent electrical properties.

CuO–ZnO nanocomposite films with efficient interfacial charge transfer characteristics for optoelectronic applications

CuO–ZnO nanocomposite films were synthesized through the oxidation of nanostructured Cu–Zn films deposited by vacuum deposition technique at a relatively high pressure of an inert gas. XRD patterns showed that the film samples were nanocomposites containing nanograins of CuO and ZnO and that the size of the CuO grains decreased with the increase in the Zn atom %. Optical absorption and Raman spectra confirmed the samples to be nanocomposites. The decrease in the activation energy for DC conduction and the enhancement in the photocurrent in the CuO–ZnO nanocomposite films with the increase in the atom % of Zn were attributed to an efficient interfacial charge transfer between the CuO and ZnO grains.

Rapid Hydrothermal Growth of ZnO Nanorods on a Magnetron Sputtered Thick ZnO Seed Layer

In this work, we report rapid hydrothermal growth of ZnO nanorods on a magnetron sputtered thick ZnO seed layer. The ZnO seed layer on the glass substrarte is monocrystalline and formed by 600 °C annealing for 1 hour after magnetic sputtering. The morphology of the ZnO grain in the ZnO seed layer plays a critical role in the growing of the ZnO nanorods, and the slant ZnO grain results in the slant ZnO nanorod and connected ZnO nonrods. It is found that the average growth of the ZnO nanorods is ~75 nm/minute. The rapid grow rate may be owing to the monocrystallie and the pure water solution of the growth solution.

Influence of processing on microstructure and electrical characteristics of multilayer varistors

The paper reports on the influence of processing on microstructure and electrical properties of multilayer varistors based on zinc oxide doped with Bi2O3, Sb2O3, Co2O3, MnO, Cr2O3, B2O3, and SiO2. 0.5–1 wt% of AlF3-CaB4O7 was used as a new effective sintering aid. The behavior of green laminates during heating was characterized using differential thermal analysis and a heating microscope. As revealed by XRD, SEM, and EDS methods, the varistor layers are composed of ZnO grains of 1–5 µm size, submicrometer spinel and pyrochlore grains situated at the ZnO grain boundaries, and nanometric Bi2O3-rich films surrounding ZnO grains. Complex impedance studies carried out in the frequency range of 0.01 Hz-2 MHz at temperatures changing from −30 to 150 °C imply the formation of semiconducting grains and insulating grain boundaries. Frequency dependence of dielectric permittivity shows a high plateau at lower frequencies, typical for barrier layer capacitance effect. The fabricated multilayer varistors show nonlinear current-voltage characteristics with a high nonlinear coefficient of 26–38. The breakdown voltage was found to decrease within the range of 66–130 V with sintering temperature increasing from 1000 to 1100 °C. Good surge current capability of the varistors was confirmed by the tests using 8/20 µs pulses.

Nonlinear Electrical Properties of ZnO-V2O5 Based Rare Earth (Er2O3) Added Varistors

The densification, microstructure and nonlinear electrical properties of ZnO-V2O5-MnO2-Nb2O5 varistor ceramics have been investigated with different amounts (0–2 mol.%) of Er2O3 addition. The ball milled powder mixtures have been sintered at 900°C, 1100°C and 1300°C for 1 h. The microstructure shows the presence of ZnO grains as the primary phase with secondary phases like Zn3(VO4)2, ErVO4, Zn4V2O9, Er- and Mn-rich in the intergranular layers. Generation of ErVO4 and Er-rich secondary phases at triple points and grain boundaries is found to inhibit grain growth resulting in the significant reduction of ZnO grain size, although this slightly diminishes the densification process. At a particular sintering temperature, the size of ZnO grains gets reduced over ten times when Er2O3 addition is increased from 0 mol.% to 2 mol.%. The nonlinear electrical property of the varistors improves with Er2O3 addition up to 0.5 mol.% and further addition (1–2 mol.%) does not appear to be beneficial in the chosen sintering temperatures. For varistors sintered at 1100°C, average ZnO grain size reduces from 10.8 μm in Er2O3 free sample to 7.2 μm in 0.5 mol.% Er2O3 added sample; this, in turn, significantly improves nonlinear electrical properties. The breakdown field increases from 1588 V cm−1 to 2585 V cm−1 with concurrent enhancement of nonlinear exponent value from 16 to 26 when Er2O3 concentration is raised from 0 mol.% to 0.5 mol.%.

Facile and Versatile Sol-Gel Strategy for the Preparation of a High-Loaded ZnO/SiO2 Adsorbent for Room-Temperature H2S Removal.

Preparation of well-dispersed ZnO nanograins is necessary to improve their reactivity toward room-temperature H2S removal. However, the challenge to design such a ZnO-based adsorbent with high ZnO loading is yet to be fulfilled. Herein, a facile sol-gel strategy is reported for the preparation of ZnO/SiO2 adsorbents for efficient H2S removal, by innovating a gel-drying method and simultaneously controlling ZnO grain formation through optimizing the molar ratio of ethylene glycol (EG)/nitrates in its precursors. The fabricated adsorbent embedded well-dispersed ZnO nanograins, of approximately 10-15 nm, into a SiO2 matrix (57 wt % ZnO loading) and thus yielded a high H2S removal capacity of 108.9 mg S/g sorbent. Therein, EG was used as a modifier for inhibiting the formation of a denser SiO2 network during the gel drying process and was used as a fuel for promoting the decomposition of nitrates and increasing the surface area of the composites in the subsequent calcination. Modulating the molar ratio of EG/nitrates ≤ 2 in precursors or traditional drying of the gel in an oven should be avoided because these would lead to the oxidation of EG by metallic nitrates and form carboxylate complexes during the gel-drying process. Although the produced ZnO grains had a very small size of less than 5 nm, a layer of monodentate ZnCO3 impurity was formed on the ZnO surface, which will drastically decrease the reactivity of ZnO toward H2S. According to the encouraging results from CuO and Co3O4, this strategy has proved to be versatile for the preparation of other metal oxide/SiO2 adsorbents.

69‐2: Invited Paper: Improved ZnO based materials for to‐date flat panel displays

Promising approaches to improve electrical characteristics, chemical resistance, and thermal stability of the resistivity of ZnO‐based TCO thin films are investigated. The following ways were tested: a) gradient doping of ZnO grains into the synthesized TCO films by indium in order to create passivating coatings on the surface of the grains; b) preventing the oxygen diffusion into the films by suppressing the columnar structures formation; c) methods combining the above ones. It was found that in the ZnO‐based TCO films deposited by magnetron sputtering at temperature above 150°C under the presence of excess zinc in the reagent flow, a decrease in the porosity of the columnar ZnO films is observed after subsequent heat treatments in air. The introducing of In, Ga, Sn metals to the composition of the ZnO films contributes to the formation of thin surface oxide sub‐layers with passivation properties on the surface of the ZnO grains. The results of research clearly show that preventing the formation of a columnar structure in combination with the passivation of grain surfaces creates real prospects for expanding the areas of application of ZnO‐based transparent electrodes.

Novel stainless steel based, eco-friendly dye-sensitized solar cells using electrospun porous ZnO nanofibers

Herein, we report a new type of eco-friendly, cost-effective stainless steel mesh-based flexible quasi-solid dye-sensitized solar cell (DSSC) using electrospun ZnO nanofibers as the photoelectrode. The electrospun ZnO nanofibers showed enhanced surface to volume ratio due to the high porosity of the fibers composed of ZnO grains having 12–20 nm size with high interconnectivity. For DSSCs fabrication, commonly available natural dye chlorophyll was extracted from the Neem plant and used as the dye sensitizer. An overall solar cell conversion efficiency of 0.13 % was observed for the assembled DSSC with a short-circuit photocurrent, open-circuit voltage and fill factor of 28 μA, 0.321 mV and 32.77 %respectively. The present approach to develop cost-effective and eco-friendly DSSCs would open up enormous possibilities in effective harvesting of solar energy for commercial and rural area applications