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.     

Low-voltage ZnO varistor fabricated by the solution-coating method

A novel solution nano-coating technique, by coating ZnO powder with a mixed solution of dopants, has been developed to produce high performance low-voltage ZnO varistors. The sintering temperature in the present route is about 50 °C lower than that in the conventional oxide mixing route. The microstructure and electrical characteristics were examined by XRD, SEM and dc power supply and the results showed that the specimens prepared by the solution-coating route have bigger grain sizes, more evenly distributed intergranular phases, higher densities and nonlinearity coefficients, lower breakdown fields and leakage currents than those from the conventional oxide mixing route. The improved current–voltage properties are attributed to the excellent performance of the nano-composite ZnO powder and the advantages of the solution nano-coating technique.     

Combustion synthesis of doped nanocrystalline ZnO powders for varistors applications

Doped nanocrystalline ZnO powders in the size range between 15 and 250 nm were synthesized by chemical combustion method. The powders were characterized for their physical, structural and chemical properties by BET, X-ray diffraction, FESEM, TEM and XPS. These powders were consolidated into dense varistors discs by compaction, sintering and evaluated for their I-V characteristics. Post-calcinations of these powders were found to have great influence on the green density and sinterability. The formations of phases after sintering were confirmed by XRD analysis and EDX. The varistor properties have been studied for different calcination temperatures and compositions. Breakdown voltage as high as 9.5 kV/cm and coefficient of nonlinearity 134 were obtained. Leakage current density was found to be ∼1.29 μA/cm² for a specific composition and condition. These studies demonstrate the feasibility of one step synthesis of doped ZnO nanopowder and their consolidation into ZnO fine grain varistor exhibiting improved performance.     

High-current measurement of the grain resistivity in zinc oxide varistor ceramics

A new pulse technique for grain resistivity measurement in varistor ceramics is suggested. Such technique allows obtaining more precise value of the grain resistivity due to the use of the concept of differential electrical resistance. This technique can be used in the current density range where the overheating of varistor sample is insignificant. The technique was verified using commercial ZnO varistors. Grain resistivities of 0.60±0.02 Ω cm at 293 K and of 3.40±0.13 Ω cm at 77 K were obtained. This result indicates the negative temperature coefficient of grain resistance in ZnO varistor in the range (77–293) K. The contribution of the grain boundaries to the current–voltage characteristic of ZnO varistor is estimated on the basis of the measured grain resistivity and the current–voltage data. It is shown that the electrical conduction in ZnO varistor is controlled by grains if the current density exceeds approximately 1000 А сm⁻².     

Microstructure development in (Co,Ta)-doped SnO2-based ceramics with promising varistor and dielectric properties

Following the $\mathit{3}Sn{\mathit{(}IV\mathit{)}}_{Sn\mathit{(}IV\mathit{)}}^{\times }\leftrightharpoons Co{\mathit{(}II\mathit{)}}_{Sn\mathit{(}IV\mathit{)}}^{\mathrm{″}}+2Ta{\mathit{\left(}V\mathit{\right)}}_{Sn\mathit{(}IV\mathit{)}}^{\cdot }$ charge compensation mechanism we optimized densification and electrical properties of Ta₂O₅-doped SnO₂–CoO ceramics. We show that incorporation of acceptor dopant Co²⁺ in SnO₂ is promoted after the addition of donor dopant like Ta⁵⁺, whereas any surplus of Co would form secondary Co₂SnO₄ phase. A balanced addition of both dopants is needed to promote densification, and any surplus of donor dopants that remain present at the grain boundaries retard the grain growth and deteriorate electrical properties. Varistor and dielectric properties are then strongly influenced by donor doping. Optimum varistor properties (α = 40, U_T = 272 V/mm, I_L = 1.2 μA) were measured for the sample with 1 mol% Ta₂O₅ and the best dielectric properties (ε = 6525; tan(δ) = 0.057@1kHz) were measured for the sample with 0.10 mol% Ta₂O₅ with the largest SnO₂ grain sizes.     

Preparation and electrical properties of multilayer ZnO varistors with water-based tape casting

Multilayer ZnO varistors were prepared by water-based tape casting with water-soluble acrylic as binders. Zeta potentials of the doped ZnO suspensions as a function of pH with and without dispersant were measured. Viscosity measurements were used to find the optimum dispersant concentration needed to prepare a stable slurry. Viscosity properties of the tape casting slurry were investigated. The results showed that aqueous acrylic binders have shear thinning properties suitable for tape casting of ceramic powders. Scanning electron microscopy (SEM) studies revealed that the green sheets have a smooth defect-free surface and that the multilayer varistor (MLV) ceramics prepared by water-based tape casting have a fine grain microstructure with a uniform grain size and dopant distribution. The multilayer ZnO varistors prepared by water-based tape casting display comparable good electrical properties to those prepared by solvent-based tape casting. This is believed to be attributed to the well dispersed water-based slurry, which makes more uniform dopant distribution throughout the multilayer ZnO varistors. Therefore, water-based tape casting is suitable for the manufacture of high performance multilayer ZnO varistors.     

Cold sintering ZnO based varistor ceramics with controlled grain growth to realize superior breakdown electric field

Controlling the grain growth and grain boundary morphology is of great importance in the manipulation of electrical properties of electro-ceramics. However, it has been a challenge to achieve dense varistor ceramics with grain sizes in submicrons and nanometers using conventional thermal sintering at high temperatures. Here we present a strategy to fabricate dense ZnO based ceramics with controlled grain growth and thin grain boundaries using cold sintering process (CSP). With CSP, the sintering temperature of ZnO based ceramics dramatically drops from 1100 °C to 300 °C. The Bi₂O₃, Mn₂O₃, and CoO dopants suppress the grain growth of ZnO under CSP conditions, and Bi-rich intergranular films (2?5 nm) can be observed along grain boundaries. The cold sintered ZnO-Bi₂O₃-Mn₂O₃-CoO ceramic shows a non-linear coefficient of 33.5, and a superior breakdown electric field of 3550 V/mm. This work thus demonstrates that CSP is a promising technique for designing new submicron-/nano-ceramics with superior performances.     

Interface states and MWS polarization contributions to the dielectric response of low voltage ZnO varistor

The main purpose of this work is to study the dielectric response of commercial low voltage ZnO varistors by means of dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization currents (TSDC) in a wide temperature range. Four relaxation processes have been studied here and all of them demonstrate Cole-Davidson behaviour. The first two faster relaxation mechanisms are known processes in ZnO varistors and those are related to the ZnO bulk traps. The next one faster relaxation mechanism attributed to the MWS polarization which should be related to the intergranular Bi-rich microregions. The remaining slower relaxation mechanism is associated to the grain boundaries interfaces. Based on the block model for the ZnO varistor a gradual reduction in the total depletion width is observed at a temperature about 330 K, which can be considered that is due to the gradual decrease of the interface states density at this temperature region.     

Inversion boundary induced grain growth in TiO2 or Sb2O3 doped ZnO-based varistor ceramics

In low-voltage varistor ceramics, the phase equilibrium and the temperature of liquid-phase formation are defined by the TiO₂/Bi₂O₃ ratio. The selection of a composition with an appropriate TiO₂/Bi₂O₃ ratio and the correct heating rate is important for the processing of low-voltage varistor ceramics. The total amount of added Bi₂O₃ is important as the grain growth is slowed down by a larger amount of Bi₂O₃-rich liquid phase at the grain boundaries. Exaggerated grain growth in low-voltage varistor ceramics is related to the occurrence of the liquid phase and the presence of TiO₂ which triggers the formation of inversion boundaries (IBs) in only a limited number of grains, and as a result the final microstructure is coarse grained. The Zn₂TiO₄ spinel phase only affects grain growth in compositions with a TiO₂/Bi₂O₃ ratio higher than 1.5. In high-voltage varistor ceramics, just a small amounts of Sb₂O₃ trigger the formation of IBs in practically every ZnO grain, and in compositions with a Sb₂O₃/Bi₂O₃ ratio lower than 1, grain growth that is controlled entirely by an IBs-induced grain growth mechanism results in a fine-grained microstructure. The spinel phase interferes with the grain growth only at higher Sb₂O₃/Bi₂O₃ ratios.     

Tb-doped ZnO:PDMS based flexible nanogenerator with enhanced piezoelectric output performance by optimizing nanofiller concentration

Zinc oxide (ZnO) is one of the most prospective material for optoelectronic, piezoelectric, spintronic and gas sensing applications. Doping rare-earth elements in ZnO leads to significant enhancement in its electrical properties. Herein, pure and Tb-doped ZnO nanoparticles were synthesized via chemical co-precipitation method and their structural, morphological and electrical properties were systematically studied. As a result of Tb-doping, hexagonal shaped ZnO nanorods converted to taper-like nanoparticles. Tb–ZnO nanoparticles exhibited high Curie temperature (T_c ~ 225 °C) and enhanced dielectric properties as compared to pure ZnO nanoparticles. Electrical conduction studies revealed a significant reduction in the leakage current as a result of Tb-doping. Piezoelectric nanogenerators based on synthesized nanoparticles were fabricated and piezoelectric output voltage of the Tb-doped ZnO based nanogenerator was measured to be remarkably enhanced (by up to ~ 4 times) compared to pure ZnO based nanogenerator. These results indicate Tb–ZnO:PDMS composite films on flexible substrates could be promising candidate for energy harvesting application.     

Nondestructive characterization of grain boundary Z-directional lateral surface in ZnO using nanorobot combined with SEM

Different from focusing on grain boundary upper surface in plane X–Y, a unique approach of nanorobot-based nondestructive characterization of grain boundary Z-directional lateral surface within bulk ZnO ceramic can be creatively developed under scanning electron microscope (SEM). By rolling-over bulk ZnO, two-dimensional profiles and grain boundaries in Z-directional lateral surfaces have been imaged in plane Y–Z and individually electrically characterized nondestructively. Experiments demonstrate that it is feasible to realize nondestructive characterization of grain boundary Z-directional lateral surface structures and electrical properties using nanorobot combined with SEM. Relative height differences between grain boundaries within Z-directional lateral surface can characterize the relative position relationships. Z-directional lateral surface structures can further extend irregular grain boundary lengths in plane Y–Z to interpret surface effects of nonlinear electrical properties. Relative minor electrical reactive effects in grain indicate grain boundary dominate in nonlinear macroscopic electrical properties. Furtherly, it can be advanced to promote a nondestructive characterization of grain boundary.     

High performance multi-layer varistor (MLV) from doped ZnO nanopowders by water based tape casting: Rheology,sintering, microstructure and properties

In this work, we report the fabrication of a high performance multi-layer varistor (MLV) via water based tape casting method using novel compositions of nanomaterials. Bi₂O₃, CaO and Co₃O₄ doped ZnO nanopowders were prepared by solution combustion synthesis (SCS) route, calcined at different temperatures (550, 650, 750 and 850?°C) and characterized by TEM, XRD, SEM and AFM. The nanopowder (crystallite size ~30?nm) calcined at 650?°C for 1?h was used as the starting material for MLV fabrication. Compositions of the slurry containing doped ZnO nanopowders, binder and plasticizer in water solvent were optimized for the fabrication of thick film. The rheological properties of the slurries having different solid loadings were analysed and thick films of various thicknesses (50–500?µm) were prepared by varying the feeding rate of tape casting. The film roughness of 38.3?nm for the thick film made from 40?wt% solid slurry was found to be superior compared to other samples due to the presence of reduced crack and shrinkage. MLV fired at 950?°C for 1.5?h exhibited a coefficient of nonlinearity of 18 and breakdown voltage of 291.5?V that yields superior properties compared to commercial MLVs.     

A study of photoluminescence properties and performance improvement of Cd-doped ZnO quantum dots prepared by the sol-gel method

ABSTRACT: In the present work, ZnO quantum dots (QDs) have been prepared by the sol-gel method, and the performance of the QDs have been improved. The effect of Cd concentration on the structural and luminescent properties of the QDs, as well as the effect of the mass ratio of trioctylphosphine oxide (TOPO)/octadecylamine (ODA), has been investigated. The ZnO and Cd-doped ZnO QDs have hexagonal wurtzite structures and are 3~6 nm in diameter. When the Cd content was increased, the QD particle size was reduced; this effect was confirmed in the corresponding ultraviolet-visible (UV) spectra. The fluorescence intensity was simultaneously enhanced significantly. Both the UV and fluorescence spectra were blue-shifted. The luminous intensity was further enhanced when the QDs were modified with TOPO/ODA. FTIR and XRD techniques proved that the polymer successfully coated the surfaces of the QDs. A TOPO/ODA mass ratio of 1:2 was determined to result in the best optical performance among the different ratios examined. The results showed that the described synthetic method is appropriate for the preparation of doped QDs with a high fluorescence quantum efficiency.     

A novel photocatalyst AgBr/ZnO/RGO with high visible light photocatalytic activity

A novel ternary photocatalyst AgBr/ZnO/RGO, where AgBr/ZnO is supported on reduced graphene oxide, is synthesized via a facile hydrothermal–impregnation method. The resultant composite presents a lamellar structure with AgBr nanoparticles homogeneously dispersing on the surface. The photocatalytic experiment for methyl orange dye degradation under visible light irradiation shows that ternary composite AgBr/ZnO/RGO has an activity 12.8 times and 2.3 times higher than binary photocatalysts ZnO/RGO and AgBr/ZnO respectively. More importantly, the ternary composite also demonstrates a good photostability. Metallic Ag is produced during the photocatalytic process, which may serve as the electron transfer mediator in the vectorial Z-scheme transfer of photogenerated charge carriers at the interface of AgBr/ZnO/RGO. The effective separation of photogenerated electrons and holes was proposed to be responsible for the enhancement of visible light photoactivity.     

Significantly enhanced breakdown field with high grain boundary resistance and dielectric response in 0.1Na0.5Bi0.5TiO3-0.9BaTiO3 doped CaCu3Ti4O12 ceramics

Electrical performances are strongly associated with the electrical heterogeneity of grains and grain boundaries for CaCu₃Ti₄O₁₂ (CCTO) ceramics. In this work, the dielectric ceramics of 0.1Na_0.5Bi_0.5TiO₃-0.9BaTiO₃ (NBT-BT) doped CCTO were fabricated by a conventional solid-state reaction method, and the ceramics were sintered at 1100 °C for 6 h. Relatively homogeneous microstructures are obtained, and the average grain sizes are characterized about 0.9∼1.5 μm. Impressively, a significantly enhanced breakdown field of 13.7 kV/cm and a noteworthy nonlinear coefficient of 19.4 as well as a lower dielectric loss of 0.04 at 1 kHz are achieved in the 0.94CCTO-0.06(NBT-BT) ceramics. It is found that the improved electrical properties are attributed to the increased grain boundary resistance of 3.7 × 10⁹ Ω and the Schottky barrier height of 0.7 eV. This is originated from the NBT-BT compound doping effect. This work demonstrates an effective approach to improve electrical properties of CCTO ceramics by NBT-BT doping.     

Li2AGeO4 (A = Zn,Mg): Two novel low-permittivity microwave dielectric ceramics with olivine structure

Two low-permittivity dielectric materials Li₂AGeO₄ (A?=?Zn, Mg) were prepared via the solid-state reaction method. X-ray diffraction analysis and Rietveld refinement indicated that both ceramics crystallize in an orthorhombic olivine structure with a space group Pmn2₁. Dense ceramics with high relative density and homogeneous microstructure were obtained. Li₂ZnGeO₄ densified at 1200?°C possessed a relative permittivity ε_r?=?6.5, a quality factor Q?×?f?=?35,400?GHz, and a temperature coefficient of resonant frequency. Li₂MgGeO₄ exhibited ε_r?=?6.1, Q?×?f?=?28,500?GHz, and τ_f?=?–74.7?ppm/°C when sintered at 1220?°C. Additionally, the large negative τ_f values of Li₂AGeO₄ (A?=?Zn, Mg) ceramics were successfully adjusted compensated by forming composite ceramics with CaTiO₃ and near-zero τ_f values of +2.9?ppm/°C and +5.8?ppm/°C were achieved in 0.92Li₂ZnGeO₄-0.08CaTiO₃ and 0.90Li₂MgGeO₄-0.10CaTiO₃, respectively.     

A novel nano-sized MoS2 decorated Bi2O3 heterojunction with enhanced photocatalytic performance for methylene blue and tetracycline degradation

In this paper, MoS₂ was used as a band-suitable semiconductor to construct the Bi₂O₃/MoS₂ heterostructured photocatalysts for the first time via a deposition-hydrothermal method. The XRD, SEM and HRTEM analysis indicated that the surface of Bi₂O₃ was decorated with MoS₂ nanoparticles and Bi₂O₃/MoS₂ heterojunctions were formed. The performances on photocatalytic degradation of methylene blue (MB) and tetracycline (TC) were evaluated under visible light irradiation. The results demonstrated that the Bi₂O₃/MoS₂ heterojunctions displayed remarkably improved photocatalytic activity for both MB and TC degradation, compared to the base material (Bi₂O₃). Specifically, as the molar ratio of MoS₂ was 23.81%, the obtained Bi₂O₃/MoS₂-23.81 heterojunctions exhibited promising photocatalytic activities, and approximately 100% MB and 97% TC were degraded within 100 min, respectively. The superior photocatalytic activity was mainly attributed to its large surface area, high visible-light harvesting and the efficient separation of photogenerated electrons and holes caused by the unique heterojunction architecture. Notably, the Bi₂O₃/MoS₂ heterojunctions showed remarkable stability in recycling photocatatlytic experiments. The active species trapping and terephthalic acid (TA) fluorescence experiments indicated that the •OH was the major reactive oxidizing species for MB degradation. Furthermore, the intermediates were detected by UPLC-MS spectrometry and the possible degradation pathways for MB and TC were proposed. Finally, a possible reaction mechanism of Bi₂O₃/MoS₂ heterojunctions for the photodegradation MB was also proposed. This interesting interfacial architecture strategy will provide useful insights for designing and fabricating new class of binary heterojunctions with high-efficient photocatalytic activity towards practical application.     

A novel strategy to achieve NaGdF4:Eu3+ nanofibers with color‐tailorable luminescence and paramagnetic performance

Luminescent‐magnetic bifunctional NaGdF₄:Eu³⁺ nanofibers were fabricated through the bond of electrospinning followed by calcination with fluorination technology for the first time. The structure, morphologies, luminescence, and magnetism of nanofibers have been characterized using various techniques. X‐ray diffraction measurement indicates that NaGdF₄:Eu³⁺ nanofibers are hexagonal phase. Scanning electron microscope measurement shows that the mean diameters of electrospinning‐made polyvinyl pyrrolidone/[NaNO₃+Gd(NO₃)₃+Eu(NO₃)₃] composite nanofibers and NaGdF₄:Eu³⁺ nanofibers are, respectively, 428±4 and 231±4 nm under the confidence level of 95%. Under 274‐nm ultraviolet light excitation, NaGdF₄:Eu³⁺ nanofibers exhibit characteristic ⁵D_3,2,1,0→⁷F_J emissions of Eu³⁺ and the tendency of color tones of samples varies from blue, cold white, warm white to red via varying Eu³⁺ content. In addition, samples exhibit paramagnetic features and the magnetic properties of NaGdF₄:Eu³⁺ nanofibers are tailorable by modulating the doping concentration of Eu³⁺. More importantly, the color‐tailorable luminescence and paramagnetic properties are simultaneously realized in single‐phase NaGdF₄:Eu³⁺ nanofibers, which ideally suit to apply in many fields such as lighting and color displays, bioimaging, and magnetic resonance imaging. This design conception and construction strategy may provide some new guidance for synthesizing other rare‐earth fluorides nanomaterials of multifarious morphologies.