ZnO Grains Research Articles

Microstructure and suppressed thermal depolarisation behaviours of lead-free Na0.5Bi0.5TiO3:ZnO ferroelectric composite ceramics

ABSTRACTA 0–3 type 0.8Na0.5Bi0.5TiO3(BNT):0.2ZnO lead-free ferroelectric composite ceramic structure was prepared by a solid-state oxide route. The X-ray diffraction analysis and scanning electron microscopy observation indicate that ZnO grains were scattered in the matrix consisting of BNT grains to form a (0–3) type composite structure, and a third phase Zn2TiO4 was formed due to the reaction ability of nano-sized ZnO with NBT compositions. The temperature-dependent electrical responses show that the dielectric anomaly at around 200°C inherited from NBT itself was suppressed as ZnO is composited, and the Pr of the specimen sintered at 1000°C keeps almost unchanged as the poling temperature is 175°C, while its retained piezoelectric strain d33 value maintains 77% of the initial as exposed to 125°C annealing. These results suggest that the thermal depolarisation of BNT is suppressed due to the introduction of ZnO, even though the third phase Zn2TiO4 exists.

Mössbauer spectroscopy of 119Sn probe cations on the surfaces of ZnO crystallites: Valence state, local environment, and hyperfine interactions of tin additives

A synthesis procedure that allows us to obtain ZnO:0.3 at % 119Sn samples with Sn2+ ions on the surfaces of oxide particles containing structure-forming cations on tetrahedral sites is developed for the first time. Analysis of the 119Sn Mossbauer spectra of obtained samples does not confirm the existence of the localized magnetic moments at ZnO grain boundaries recently suggested in the literature.

Understanding on the selective carbon monoxide sensing characteristics of copper oxide-zinc oxide composite thin films

In the present work we have demonstrated CuO-ZnO composite thin film with optimized CuO content selectively sense carbon monoxide gas. The ‘n’-type gas sensing characteristics of these composite films are changed to ‘p’-type beyond 8.0mol% CuO contents. Through the analyses of photoluminescence spectra in conjunction with XPS we have demonstrated that up to 33.0mol% CuO contents the gas sensing characteristics of the composite films are controlled by the type of point defects (oxygen vacancies, zinc interstitials, and copper vacancies) in CuO and ZnO grains. We have hypothesized that these point defects help to chemi-absorb oxygen and CO preferentially on ZnO and CuO grains respectively to yield selective carbon monoxide sensing of ZnO-CuO composite film with 7.0mol% CuO contents. Beyond 33.0mol% CuO contents the effect of point defects diminishes and the gas sensing characteristic are grossly controlled by the nature of the CuO grains covering ZnO grains in the composite films. Our work provides comprehensive insight towards the understanding of the gas sensing characteristics of hetero-composite thin films deposited using a mixed precursor sol.

Processing, characterization and properties of Er2O3 added ZnO based varistor ceramics

This article reports densification process, microstructural evolution and nonlinear electrical properties of Er2O3 (0–2.0 mol%) added ZnO–V2O5–MnO2–Nb2O5 varistor powder prepared by high energy ball milling followed by conventional (950–1350 °C, 1 h) sintering. Ball milled powder and sintered pellets have been characterized by density measurement, XRD analysis, TEM and SEM examinations coupled with EDS and image analyses for morphological modifications. Addition of Er2O3 is found to retard the ZnO grain growth significantly due to the formation of Er-rich spinel phases in the intergranular layer that reduces the grain boundary migration. Measurements of nonlinear electrical properties of sintered pellets reveal an enhancement in the breakdown field (E 1mA ) and corresponding nonlinear exponent (α) of Er added varistor samples. A typical varistor sample with 1 mol% Er2O3 sintered at 1200 °C exhibits ultrafine grain structure (grain size: 1.8 µm and sintered density: >96% theoretical density) with excellent nonlinear properties (E 1mA = 6714 V cm−1 and α = 45).

Influences of lithium on the defect structures and electrical properties of ZnO–Bi2O3 based varistors

The role of alkali metal elements in ZnO varistor is of interest because of the small ion radius and valence state. They are expected to form acceptor state and convert the n-type ZnO grain to p-type. However, researches to manufacture p-type ZnO semi-donductor by using alkali metal elements have failed. In the present work, the effects of lithium element on the defect structures, microstructures and corresponding electrical performances of ZnO varistors is investigated. According to the obtained results, the influences of lithium element on the defect structures of ZnO varistors will vary with the amount of lithium. Lithium ion occupies the interstitial site of ZnO grain at first and then occupies the substitutional site. Trace amount of lithium (0.0005–0.002 wt%) can reduce the residual voltage of ZnO varistors while the E–J characteristics at low current region remain stable.

Local photocurrent mapping and cell performance behaviour on a nanometre scale for monolithically interconnected Cu(In,Ga)Se2 solar cells.

The local efficiency of lamellar shaped Cu(In,Ga)Se2 solar cells has been investigated using scanning near-field optical microscopy (SNOM). Topographic and photocurrent measurements have been performed simultaneously with a 100 nm tip aperture. The lamellar shaped solar cell with monolithic interconnects (P scribe) has been investigated on a nanometre scale for the first time at different regions using SNOM. It was found that, the cell region between P1 and P2 significantly contributes to the solar cells overall photocurrent generation. The photocurrent produced depends locally on the sample topography and it is concluded that it is mainly due to roughness changes of the ZnO:Al/i-ZnO top electrode. Regions lying under large grains of ZnO produce significantly less current than regions under small granules. The observed photocurrent features were allocated primarily to the ZnO:Al/i-ZnO top electrode. They were found to be independent of the wavelength of the light used (532 nm and 633 nm).

Influence of SiO2 on electrical properties of the highly nonlinear ZnO-Bi2O3-MnO2 varistors

The ZnO-Bi2O3-MnO2-xSiO2 (ZBMS) varistor was prepared at a low sintering temperature of 880°C via the conventional solid state method. The phase transformation, microstructure, and electrical properties of the ZBMS varistor were studied as a function of doping amount of SiO2. It is showed that the growth of ZnO grain is restrained by the introducing of SiO2; and the grain size decreases from 4.68μm to 2.98μm. The breakdown voltage E1mA exhibits a simultaneous variation from 608.11V/mm to 1232.88V/mm. It is also revealed that SiO2 has a significant effect on the Schottky barrier structure. As a result, the highest barrier height φb of 5.34eV is attained at a composition of x=2.0wt%, which contributes to the highest nonlinear coefficient α of 73.68. Moreover, all the samples show low leakage current of IL<0.1μA.

Effect of interfacial charge relaxation on conducting behavior of ZnO varistors under time varying electric fields

The conducting mechanism of ZnO varistors under the steady state has been well documented, but the corresponding time-dependent property was rarely studied. In this work, we proposed a physical model for analyzing the conducting behavior of a single ZnO grain boundary under pure ac fields. The established model mainly deals with the time-delay effect of the interfacial charge and its influence on the barrier height as well as the conducting process, which leads to a phase shift of the conduction current in advance of the applied voltage and an enhancement of the effective capacitance. To validate the model, the nonlinear conducting characteristics of ZnO varistors under pure ac fields were measured and the result agreed well with the model.

The Influence of the multilayer ZnO-HfOx structure on the characteristics of a light-emitting device

A MOS structure device with a ZnO-HfOx layer was fabricated on a p-type silicon substrate. Current (i.e., conductive) paths formed after the breakdown of the dielectric layer under a voltage and thermal-induced emission of broadband light, including visible and near-IR wavelengths. To investigate the influence of the ZnO-HfOx layer on a device’s light-emitting characteristics, we prepared devices with one, two and three layers of ZnO-HfOx thin film on a p-type silicon substrate. We then measured the light emission, the emission spectrum, the I–V curves and the microstructure. The results show that the range and peak wavelength of the emission spectra from devices with different numbers of ZnO-HfOx layers are the same. Under the condition of a fixed total thickness of HfOx, luminous intensity increases with increasing number of ZnO-HfOx layers. Multilayer ZnO-HfOx can cause the ZnO grains to diffuse more into the HfOx layer, which is beneficial for the formation of current paths and increases the luminous intensity.

Photocatalytic Cr(VI) reduction by mixed metal oxide derived from ZnAl layered double hydroxide

Mixed metal oxide (MMO) is one of the most important photocatalyts intensively investigated for water remediation. In this study, ZnO/ZnAl2O4 (ZnAl-MMO) nanocomposite was readily prepared via one-step calcination of ZnAl-layered double hydroxide (ZnAl-LDH) at 900°C.·The obtained MMO composite was thoroughly characterized by powder X-ray diffraction (XRD), Scanning electron microscopy (SEM), High-resolution transmission electron microscopy (HRTEM), UV–Vis diffuse reflectance spectroscopy (DRS). ZnAl-MMO was composed of nano-scaled ZnO and ZnAl2O4 crystal grains of high crystallinity and exhibited strong UV light adsorption with a well-developed interface between the two components of the composite. ZnAl-MMO showed high photocatalytic Cr(VI) reduction activity under UV light irradiation and its rate constant was as high as 0.8134h−1. The photoreduction activity of ZnAl-MMO photocatalyt was ascribed to the synergistic effect between two components of the nanocomposite.

Further improvements in conducting and transparent properties of ZnO:Ga films with perpetual c-axis orientation: Materials optimization and application in silicon solar cells

Technologically appropriate device friendly ZnO:Ga films have been prepared at a low growth temperature (100°C) by changing the RF power (P) applied to the magnetron plasma. Structurally preferred c-axis orientation of the ZnO:Ga network has been attained with I〈002〉/I〈103〉>5. The c-axis oriented grains of wurtzite ZnO:Ga grows geometrically and settles in tangentially, providing favorable conduction path for stacked layer devices. Nano-sheet like structures produced at the surface are interconnected and provide conducting path across the surface; however, those accommodate a lot of pores in between that help better light trapping and reduce the reflection loss. The optimized ZnO:Ga thin film prepared at RF power of 200W has 〈002〉 oriented grains of average size ∼10nm and exhibits a very high conductivity ∼200Scm−1 and elevated transmission (∼93% at 500nm) in the visible range. The optimized ZnO:Ga film has been used as the transparent conducting oxide (TCO) window layer of RF-PECVD grown silicon thin film solar cells in glass/TCO/p-i-n-Si/Al configuration. The characteristics of identically prepared p-i-n-Si solar cells are compared by replacing presently developed ZnO:Ga TCO with the best quality U-type SnO2 coated Asahi glass substrates. The ZnO:Ga coated glass substrate offers a higher open circuit voltage (VOC) and the higher fill factor (FF). The ZnO:Ga film being more stable in hydrogen plasma than its SnO2 counterpart, maintains a high transparency to the solar radiation and improves the VOC, while reduced diffusion of Zn across the p-layer creates less defects at the p-i interface in Si:H cells and thereby, increases the FF. Nearly identical conversion efficiency is preserved for both TCO substrates. Excellent c-axis orientation even at low growth temperature promises improved device performance by extended parametric optimization.

Quantitative and simultaneous analysis of the polarity of polycrystalline ZnO seed layers and related nanowires grown by wet chemical deposition

The polarity in ZnO nanowires is an important issue since it strongly affects surface configuration and reactivity, nucleation and growth, electro-optical properties, and nanoscale-engineering device performances. However, measuring statistically the polarity of ZnO nanowire arrays grown by chemical bath deposition and elucidating its correlation with the polarity of the underneath polycrystalline ZnO seed layer grown by the sol–gel process represents a major difficulty. To address that issue, we combine resonant x-ray diffraction (XRD) at Zn K-edge using synchrotron radiation with piezoelectric force microscopy and polarity-sensitive chemical etching to statistically investigate the polarity of more than 107 nano-objects both on the macroscopic and local microscopic scales, respectively. By using high temperature annealing under an argon atmosphere, it is shown that the compact, highly c-axis oriented ZnO seed layer is more than 92% Zn-polar and that only a few small O-polar ZnO grains with an amount less than 8% are formed. Correlatively, the resulting ZnO nanowires are also found to be Zn-polar, indicating that their polarity is transferred from the c-axis oriented ZnO grains acting as nucleation sites in the seed layer. These findings pave the way for the development of new strategies to form unipolar ZnO nanowire arrays as a requirement for a number of nanoscale-engineering devices like piezoelectric nanogenerators. They also highlight the great advantage of resonant XRD as a macroscopic, non-destructive method to simultaneously and statistically measure the polarity of ZnO nanowire arrays and of the underneath ZnO seed layer.

アルミニウム微量添加が酸化亜鉛（ZnO）バリスタ素子特性に与える影響

It is well known that aluminum doping is very effective for improving the Voltage-Current (V-I) characteristics of ZnO varistor elements comprising Bismuth oxide and Antimony oxide. Although it is established that the presence of aluminum lowers the resistance of ZnO grains by valence control, in this study, it is proposed that the homogeneity of micro structure of ZnO elements is more effective for improvement of V-I characteristics. The relationship between electrical characteristics and microstructure at different aluminum doping levels and sintering temperatures is shown with V-I characteristics, SEM observation and capacitance measurement of ZnO elements. As the effect of Al doping, it was found that moderation of ZnO grain growth causes the homogeneity of microstructure of ZnO varistors. In detail, reduction of the volume ratio of high resistive parts like thicker grain boundaries and multiple point between ZnO grains leads to excellent non-ohmic V-I characteristics. In addition, it is suggested that the tunnel effect by thinner grain boundaries is important as the mechanisms of improvement of V-I characteristics.

The effect of SiO2on electrical properties of low‐temperature‐sintered ZnO–Bi2O3–TiO2–Co2O3–MnO2‐based ceramics

AbstractZnO–Bi2O3–TiO2–Co2O3–MnO2‐based (ZBTCM) varistors were fabricated via the conventional solid‐state method, and the effect of SiO2content on the phase transformation, microstructure, and electrical properties of theZBTCMhad been investigated. Results showed that this varistor can be sintered at a low temperature of 880°C with a high sintering density above 0.95 of the ZnO theoretical density. In these components, SiO2acts as a controller in ZnO grain growth, decreasing the grain size of ZnO from 3.67 to 1.92 μm, which in turn results in an increase in breakdown voltageE1mAfrom 358.11 to 1080 V/mm. On the other hand, SiO2has a significant influence on the defect structure and component distribution at grain‐boundary regions. When SiO2content increases from 0 to 4 wt%, the value of the interface state density (Ns) increases sharply. At the same time, the electrical properties are improved gradually, and reach an optimized value with the nonlinear coefficient (α) up to 54.18, the barrier height (ϕb) up to 2.90 eV, and the leakage current (IL) down to 0.193 μA/cm2.

Nanoflower structures of ZnO–SnO2 arranged on nanoporous Al2O3 for sensing ethanol and methanol gases

A compound sensing structure based on nanoflower-like ZnO---SnO2 coated on the top of nanoporous anodic aluminum oxide (AAO) layer to form ZnO---SnO2---AAO composite as a gas sensing structure is developed for detecting ethanol and methanol simultaneously. The ZnO---SnO2 powders were synthesized by hydrothermal synthesis method. The SnO2 was grown densely and uniformly that has unique loose and porous nano structures to increase the accessible surface area for improving gas diffusion and mass transport. A solenoid-type heater was designed to surround the sensing column for controlling operation temperature to promote gas sensitivity. The ethanol and methanol responses of ZnO---SnO2---AAO are high because the porous AAO greatly improves the specific adsorption surface area, and the ZnO produces more electron donor states or oxygen vacancies to enhance oxygen adsorption as well as the heterojunction of SnO2 and ZnO grains offers access to facile electronic interaction to enhance the surface reaction between adsorbed oxygen and current-carrying electrons. By analyzing a series of temperature-varied cyclic tests, the optimal operation temperatures of 180 °C for sensing ethanol and 260 °C for sensing methanol were obtained to have the strongest output responses. The developed ZnO---SnO2---AAO gas sensing composites include the advantages: simple fabrication process, low cost, high specific surface area and porosity, rapid detection, high sensitivity, stable stability, and good repeatability.

Growth process optimization of ZnO thin film using atomic layer deposition

The work reports experimental studies of ZnO thin films grown on Si(100) wafers using a customized thermal atomic layer deposition. The impact of growth parameters including H2O/DiethylZinc (DEZn) dose ratio, background pressure, and temperature are investigated. The imaging results of scanning electron microscopy and atomic force microscopy reveal that the dose ratio is critical to the surface morphology. To achieve high uniformity, the H2O dose amount needs to be at least twice that of DEZn per each cycle. If the background pressure drops below 400 mTorr, a large amount of nanoflower-like ZnO grains would emerge and increase surface roughness significantly. In addition, the growth temperature range between 200 °C and 250 °C is found to be the optimal growth window. And the crystal structures and orientations are also strongly correlated to the temperature as proved by electron back-scattering diffraction and x-ray diffraction results.

Structural, morphological and magnetic properties of Zn1−xCoxO prepared by sol—gel and hydrothermal method combined**Project supported by the 46th Institute of China Electronics Technology Group Corporation (No. 0231182346).

Cobalt-doped ZnO powder samples were prepared by sol—gel and hydrothermal method combined. The prepared powder samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), electron paramagnetic resonance (EPR) and vibrating sample magnetometry (VSM). XRD patterns show that all the samples have a single pure phase with wurtzite structure suggesting that Co2+ occupied Zn2+ sites in the ZnO crystal lattice. SEM images show that crystal grains of cobalt-doped ZnO are hexagonal cone or spheroidal. EPR pattern shows that all of the samples possess oxygen vacancy. Measurement of magnetism indicates that all the samples exhibit room temperature ferromagnetism (RTFM) embedded in a dominant diamagnetic or paramagnetic signal. The dominant signal turns from diamagnetic to paramagnetic with the increase of Co concentration. The magnetic behavior can attribute to defects and Co doping.

Microstructures and electrical properties of ZnO–V2O5–MnO2 varistors with low-temperature sintering

The microstructures and electrical properties of 95 at.%ZnO + xV2O5 + (5−x)MnO2 varistors (x = 1, 2, 3, 4, 5 at.%) sintering at 900 °C were investigated. The microstructure of all the samples consisted of ZnO grain with β-Zn3(VO4)2 and γ-Zn3(VO4)2 as minority secondary phases. The average grain size decreased from 10.4 to 4.5 μm with the decrease of V2O5 mol fraction and it is attributed that ZnO grains could probably grow rapidly in the presence of a rich liquid phases related to V2O5 concentrations. The breakdown voltage increases from 6444 to 1446 V/cm with increase of V2O5 mol fraction due to the decrease of the average grain size. The samples containing 2 at.% V2O5 and 3 at.% MnO2 varistors exhibited the best nonlinear properties, with present nonlinear coefficient (20.8).

Sb2O3첨가제가 ZnO 배리스터의 전기적 특성에 미치는 영향

The leakage conduction and critical voltage characteristic of ZnO ceramic were investigated as a function of Sb₂O₃ concentration. Leakage conduction in the ohmic region increased with increasing Sb₂O₃ concentration and was attributed to the potential barrier height. The nonlinear coefficient increased with an increasing amount of Sb₂O₃. It was found that increases in the apparent critical voltages were associated with the lowered donor concentration in the grain boundary of between two ZnO grains. And the decrease of donor concentration on doping with Sb₂O₃ additive was attributed to the lowered capacitance in the grain boundary layer.

High-frequency capacitance-voltage characteristics of the heterogeneous structure based on the model of spherical semiconductor particles in a dielectric

The dependence of the parameters of the capacitance effect in heterogeneous dispersed two-component structures based on semiconductors from the bulk fraction of the semiconductor component is modeled. The used method for determining the changes of the energy bands bending on the surface of the spherical semiconductor particle by applying dc electric field allowed to calculate the changes of the dipole moment and effective (taking into account the polarization of the free charge) dielectric constant of this semiconductor particle. This result allowed to use the known models of the dielectric constant of two-component structures for the description of the capacitance field effect in the heterogeneous structures. The relations allowing to estimate the value of the bulk donor concentration in the semiconductor component of the matrix of the heterogeneous system and the statistical mixture have been obtained. The approbation of the obtained calculation relations to evaluate the donor concentration in the ZnO grains of zinc oxide varistor ceramics leads to the correct values that are consistent with estimates of other methods and models. It is established that the sensitivity of the relative dielectric constant to the applied dc electric field is dependent on the bulk fraction of the semiconductor particles in the heterogeneous structures. The bulk fraction of the semiconductor particles significantly affects on the dielectric constant beginning with the values from [Formula: see text] for matrix systems and [Formula: see text] for statistical mixtures.