Analysis of ZnO varistors prepared from high-energy mechanical activation

ZnO varistors were prepared by high-energy mechanical activation to study its effect on the microstructure and electrical properties. Comparison of different permutation and combination of ZnO powders and additives subjected to activation for 0, 6 and 12 h were given in I–V, C–V, complex resistance study. The variation of Schottky barrier height Φ _b, donor concentration N _d, interface state density N _t and depletion layer width L were calculated in different samples. Finally, individual grain boundaries were investigated by SSPM (Scanning Surface Potential Microscopy) under in situ applied fields, allowing determination of the voltage dependence of grain boundary electronic properties.     

Nonuniformity of electrical Characteristics in microstructures of ZnO surge varistors

The nonuniformity of electrical characteristics of grain boundaries in ZnO varistors was systematically analyzed. The high nonuniformity exists in barrier voltages and nonlinearity coefficients in different grain boundaries. The barrier voltages have normal distributions, only a few grain boundaries were electrically active, and the grain boundaries can be simply classified into good, bad, and ohmic ones according to the electrical characteristics of grain boundaries. The average barrier voltage is equal to 3.3 V by the direct method, but it is only 2.3 V by the indirect method. There is a high difference between the barrier voltages by direct and indirect measurement methods. These few good grain boundaries are responsible for the good varistor effect, and control the leakage current of ZnO varistor at low values of applied voltage. The Al/sub 2/O/sub 3/ dopants affect the electrical characteristics of grain boundaries by changing the electron status in grain boundary and intragrain.     

Structural Origin of Dimensional Effect in ZnO Varistors

The relations among breakdown field E _1mA/cm ² (electric field at 1 mA/cm²), nonlinear coefficient α (measured via current-voltage, I-V, characteristics), and change in breakdown field (ΔE _1mA/cm ²/E _{1 mA/cm} ²) (obtained via current impulse measurement), and thickness d of ZnO varistors were investigated. The dimensional effect refers to the variation in E _{1 mA/cm} ², α, and ΔE _{1 mA/cm} ²/E _{1 mA/cm} ² with thickness of the samples. The dispersion of the ZnO grain size and the aspect ratio of the ZnO grains are used in characterizing the microstructure to represent the degree of heterogeneity of the grain size distribution and irregularity of the shape of the ZnO grains. The distribution of the ZnO grain size is statistically analyzed and found that the critical thickness d _c increases linearly with the dispersion of the ZnO grain size. The breakdown electric field can be empirically termed as E _{1 mA/cm} ² ∝ exp(bd), where d is the thickness and b represents the transitional behavior for the curve of the E _{1 mA/cm} ² versus d plot. Based on the inflecting response of this curve, b ₁ represents the small thickness domain prior to the inflection and b ₂ represents the large thickness domain in the post-inflection domain. It is observed that b ₂ is directly proportional to aspect ratio of the ZnO grains. Also it is revealed from the analysis that b ₁ increases with the increase in the critical electric field while b ₂ decreases with the increase in the critical electric field. Based on these observations it is suggested that the dimensional effect of ZnO varistors originates from the distribution of the grain size and proven via experiments involving the measurements of E _{1 mA/cm} ², α, and ΔE _{1 mA/cm} ²/E _{1 mA/cm} ². The dimensional effect is the macroscopic expression of population behavior of varistors that is consisting of the ZnO grains as well as the grain boundaries.     

Evaluation by numerical analysis of magnetic shielding characteristics of bulk high‐Tc superconductor containing weak links

High‐T_c superconductors (HTS), which have the characteristics of critical current density over 3 × 10⁴ A/cm² at liquid nitrogen temperature (77 K) and 1 T, can be produced. Thus, they are promising for many practical applications such as a magnetic bearing, magnetic levitation, flywheel, and magnetic shielding. Since the HTS characteristics are not homogeneous in some specimens due to grain boundaries and cracks, the distribution of magnetic characteristics should be assessed. Thus, we have measured the distribution of the magnetic flux density on the surface of the HTS using a Hall element, and have evaluated its magnetic characteristics. The measurement of magnetic characteristics using a Hall element is difficult regarding the distribution of the magnetic flux density on the actual surface and inside of the HTS sample. In this research, we conducted a quantitative evaluation of the magnetic shielding characteristics of the HTS including weak links under a static magnetic field with the three‐dimensional finite element method analysis. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 146(1): 9–17, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10209     

Microvaristors: Functional Fillers for Novel Electroceramic Composites

Microvaristors are tiny electroceramic particles, which have highly nonlinear, voltage controlled electrical transport properties and can be used as active fillers in a variety of insulating matrix materials for functional composites. Due to the internal grain boundary structure, each individual microvaristor particle shows an IV-characteristics similar to the one known from bulk ceramics, except for the scaled down switching voltage. By controlling the material formulation, the particle morphology and the sintering conditions the switching characteristics of microvaristors can be tailored for specific applications.In the present paper the basic properties of ZnO microvaristors are described and it is shown how they impart their nonlinearity to the composite. A single microvaristor can withstand surprisingly high current loadings, without major changes in their electrical properties. Combined with the high manufacturing flexibility known from polymer processing, the varistor composites can be used for new solutions in overvoltage protection or control of electrical fields.     

A measurement method for atmospheric ion mobilities based on cylindrical electrodes in direct current corona discharge

Ion mobility is one of fundamental parameters to describe the motion of ions in an electric field and a significant quantity to calculate the ionized field and ion current density of ultrahigh‐voltage direct current (UHVDC) power lines. This paper presents a measurement method for atmospheric ion mobilities in a DC corona discharge. An apparatus with coaxial cylindrical electrodes is designed and utilized. A vibrating electric field meter is employed to measure the ionized field of the electrode. Experimental results show that under atmospheric conditions, the average positive and negative ion mobilities are 1.76 × 10⁻⁴ and 1.88 × 10⁻⁴m²·V⁻¹·s⁻¹, respectively. The proposed method is compared with previous works of others. The impact of fluctuations of the measured ionized field and ion current density on the results of ion mobilities is also analyzed. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

The annealing effect on properties of ZnO thin film transistors with Ti/Pt source-drain contact

ZnO-based thin film transistors (TFTs) with Ti/Pt contacts were fabricated on SiO₂/Si substrates. The as-deposited ZnO TFT did not work well as a TFT device but the annealed ZnO TFT showed acceptable characteristics with a mobility (μ_sat), threshold voltage (V_th), on/off ratio and subthreshold swing (SS) of 0.8 cm²/V.s, 2.5 V, over 10⁶ and 0.84 V/dec, respectively. Complete oxygen loss was observed in ZnO after annealing at 300°C under a N₂ atmosphere. The annealing process altered the crystallinity, density and composition of the ZnO active layers due to the formation of oxygen vacancies as shallow donors. This process is expected to play an important role in controlling the TFT performance of ZnO. In addition, it is expected to form the basis of the future electronic devices applications, such as transparent displays and active matrix organic lighting emitted displays (AMOLED).     

Electrical and optical properties of epitaxial and polycrystalline undoped and Al-doped ZnO thin films grown by pulsed laser deposition

Undoped and Al-doped (1.6%) ZnO films were prepared on (0001) sapphire and fused silica substrates using a pulsed laser deposition technique. The ZnO films on sapphire substrates were epitaxially grown, while the ZnO films on fused silica substrates were texturally grown. The films on sapphire substrates were ordered along the in-plane direction and had grains in which the c-axis was well aligned normal to the substrate. However, the films on fused silica were randomly oriented along the in-plane direction and had poor c-axial aligned grains. The structure analyses showed that the epitaxial ZnO films had low-angle grain boundaries, while the textured polycrystalline ZnO films had high-angle tilt and twist grain boundaries. The nature of the grain boundaries influenced the electrical and optical properties of the undoped and Al-doped ZnO films. Resistivity, Hall mobility, carrier concentration, and near band edge emission of the films were measured at room temperature and discussed in connection with the nature of grain boundaries.     

Development of new hydrogen storage material FeTi(Ni) for improved hydrogenation characteristics

The present study deals with the hydrogen storage characteristics of a transition metal composition Fe_0.5Ni_0.5Ti_1.3 synthesized by melting the metal powders in a previously outgassed graphite crucible using radio frequency (r.f.) induction furnace (12 kW) in an argon atmosphere. X‐ray diffraction (XRD) studies revealed that the as‐synthesized sample is multiphasic in nature and embodies FeTi, FeO and Ti phases. Microstructural features (SEM) signify the presence of secondary phases and cracks. The hydrogen storage capacity and kinetics determined at different temperatures were found to be 1.26 wt% at 350°C. Fe_0.5Ni_0.5Ti_1.3 exhibits faster hydrogen desorption kinetic (5 × 10⁻⁴ m³ s⁻¹ kg⁻¹), which is about two times higher than that of the parent FeTi_1.3. Thermodynamic parameters ΔH (change in enthalpy) and ΔS (change in entropy) were found to be −35.07 kJmol⁻¹ and 72.07 Jmol⁻¹ K⁻¹ respectively. © 2006 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

High Temperature Characterization of Electrical Barriers in ZnO Varistors*

Two main models have been proposed to describe the potential barriers in ZnO varistors: the surface oxidation and the surface states. It has been difficult to decide which of them better corresponds to the experimental observations. High temperature electrical characterization of these materials is an important tool to understand the formation of the electrical barriers. In this work, using literature data describing ZnO varistor characteristics at high temperature, up to 1153°C, we calculate the energy position of the equilibrium Fermi level at the grain boundary interface, and found that this parameter decreases with the increase of temperature, and for temperatures higher than ～700°C it stays close to the ZnO band gap without crossing it. This behavior shows that the interface never presents a p-type character, a starting point to develop the surface states model. On the other hand, 700°C is a temperature too low for the surface oxidation mechanism to be operative. It is then proposed that, during cooling down to ～700°C, the interface Fermi level stays close to the middle of the band gap due to the adsorption and subsequent reaction of oxygen with ZnO surfaces/grain boundaries. For lower temperatures, when the interface Fermi level separates from the middle of the band gap, it is proposed that it follows the variation of the bulk Fermi level, which in turn is caused by shallow donors in ZnO. A calculation assuming a reduced electroneutrality condition, gave a donor density of ～ 3 × 10¹⁷ cm^-3, which corresponds approximately to the density of carriers in the material for temperatures down to room temperature. This value is in a good agreement with those available in the literature. Knowing both the bulk and the interface Fermi levels, it is then possible to calculate the barrier height at any temperature, and it is observed that it is almost constant from room temperature up to ～400°C, with a value of 0.8 eV, and than decreases monotonously up to 1153°C. Taking these values, it is possible to calculate the variation of the low voltage conductivity with temperature, and it is found that, apart from the variation between room temperature and 400°C, with no special significance, the decrease of the barrier height from 400°–1153°C induces an extra change of the conductivity from which a fictitious activation energy of 1.5 eV is obtained. Therefore, these two energies are not related to shallow and deep donors in ZnO grains.     

Structural and electrical properties of Sb-doped p-type ZnO thin films fabricated by RF magnetron sputtering

We investigated the Sb-doping effects on ZnO thin film using RF (radio frequency) magnetron sputtering and RTA (rapid thermal annealing). The structural and electrical properties of the thin films were measured by X-ray diffraction, SEM (scanning electron microscope), and Hall effect measurement. Thin films were deposited at a high temperature of 800°C in order to improve the crystal quality and were annealed for a short time of only 3 min. The structural properties of undoped and Sb-doped films were considerably improved by increasing oxygen content in the Ar-O₂ gas mixture. Sb-doping also significantly decreased the electron concentration, making the films p-type. However, the crystallinity and surface roughness of the films degraded and the mobility decreased while increasing Sb-doping content, likely as a result of the formation of smaller grain size. From this study, we observed the transition to the p-type behavior at 1.5 at.% of Sb. The thin film deposited with this doping level showed a hole concentration of 4.412?×?10¹⁷ cm^?3 and thus is considered applicable to p-type ZnO thin film.     

Effects of doping Y2O3 on the microstructure and electrical properties of ZnO-Bi2O3−based varistor ceramics

ZnO-based varistor ceramics doped with different amount of Y₂O₃ have been made by two-step solid-state reaction route including the pre-calcination and subsequent sintering procedures, using nanosized ZnO powder and corresponding additives as the raw material. The phase composition, microstructure and electrical properties were studied by means of X-ray diffractometry (XRD), scanning electron microscopy (SEM) and direct current electrical measurement. It was found that the electrical properties of the varistor ceramics sintered at 950 °C from the powder pre-calcined at 800 °C were enhanced by doped appropriate amount of Y₂O_3. Particularly, ZnO varistors doped with 1.2 mol% Y₂O₃ possessed the best comprehensive electrical properties with the breakdown field of 2113 V/mm, the nonlinear coefficient of 184.6 and the leakage current of 0.4 μA. Y₂O₃ phase, Y-rich phase and the other secondary phase particles were confirmed to distribute along the grain boundaries of predominant ZnO grains from XRD and SEM analyses. The results illustrated that doping Y₂O₃ should be a promising route to obtain varistor ceramics with excellent electrical properties.

     

Measurement method of plasma current and density in atmospheric pressure plasma jet

We present a measurement method of the plasma current and density in an atmospheric‐pressure plasma generated using a quartz tube, helium gas, and copper foil electrode by applying a high RF voltage. The plasma in the form of a bullet is released as a plume or jet into the atmosphere. To study the characteristics of the atmospheric‐pressure plasma, the plasma current is measured using a current probe, and the drift velocity of plasma plume is measured using a photodetector. The current of the plasma plume is estimated by subtracting the ground line current from the power line current in the circuit. The density of plasma plume n is estimated from the plasma plume current I and the drift velocity v as I = envS, where S is the cross section of plasma plume. The density of the released plasma into the atmosphere is estimated as ∼10¹⁸ m⁻³ by the method. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Varistor behavior of Mn doped ZnO ceramics prepared from nanosized precursors

Polycrystalline ZnO doped with MnO, from 2 to 15?mol%, was prepared from nanosized precursors. The effect of Mn doping and sintering temperature on phase evolution, microstructure and V-I characteristics were investigated. SEM images showed that the great merit of using nanoparticles is that the samples with high microstructural uniformity and lower grain size can be achieved. Varistor behavior was observed in all specimens, even in the undoped ceramics due to the oxidation process of zinc interstitial defects at grain boundaries. The electric field versus current density (E-J) curves indicated that the breakdown field E_b increased and the nonlinear coefficient ?? decreased with the increase in doping level. 2?mol% Mn doped ceramic sintered at 1100?°C exhibited the highest nonlinear coefficient, ???=?40. The stability test under DC stress was performed for the undoped ZnO ceramics. ZnO varistor sintered at 1300?°C showed not only high nonlinearity, but also high stability under DC stress.     

Preparation of organic semiconductive thin films by laser ablation: Control of structure and electric properties by selecting wavelength,fluence, and substrate temperature

Amorphous organic semiconductive thin films with electric conductivities ranging between 10⁻⁵ and 10¹ Scm⁻¹ are prepared on several temperature-controlled substrates by excimer laser ablation (ELA) of 3, 4, 9, 10-perylenetetracarboxylic dianhydride (PTCDA) with 193 nm (ArF), 248 nm (KrF) and 308 nm (XeCl) beams. The structure, electric conductivity, and carrier species of the prepared films depend strongly on the ablation wavelength, fluence, and substrate temperature. Thermoelectromotive force measurements demonstrate conversion of carrier species from n-type to p-type with increasing fluence of a 308-nm beam from 0.2 to 4.0 Jcm⁻²pulse⁻¹. A film prepared on a substrate at 300 °C by ELA with a 308-nm beam partially contains a polyperinaphthalene (PPN) structure with electric conductivity of 10⁻² to 10⁻¹ Scm⁻¹. © 1998 Scripta Technica, Electr Eng Jpn, 125(2): 19–26, 1998     

New current–voltage transferring device with different metals

A new concept of an electrical shunt with different materials (aluminum and copper) has been developed to be used as an alternative current measurement device. The device provides better current measurement characteristics compared with the conventional current measurement devices such as a shunt resistor with an ammeter or the multiple shunts consisting of molybdenum having a low temperature coefficient and a Rogowski coil with an integrated circuit. The currents in several electrical circuits have been measured using the developed current–voltage transferring device (CVTD) as voltages between the aluminum and copper elements. The measured voltages (V_m) are proportional to measuring currents (I_m), which is shown as the following the experimental equation V_m [mV] =kI_m [A], in which k is a coefficient depending on the configuration of the CVTD. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

TEM,XPS and SIMS Analyzes on Grain Boundary of Lanthanum Chromites

The morphological characteristics, chromium valence state, and cation transport in the vicinity of grain boundary in La_1-xCa_xCrO₃ were investigated by using TEM/EDS, XPS and SIMS techniques. The width of grain boundary was around 1 nm where anomalous enrichment of calcium was observed. Higher valence state of chromium such as Cr⁶⁺(d⁰) was detected in the grain boundaries whereas Cr³⁺(d³) and Cr⁴⁺(d²) were dominant in the bulk. Very fast interdiffusion of alkaline earths was observed in the Sr²⁺-La_0.75Ca_0.25CrO₃ system. All observed phenomena were correlated by assuming the A-site vacancy which may be induced by the formation of Cr⁶⁺ at grain boundaries.     

Magnetization and Electric Properties of Pr-doped ZnO

Magnetic susceptibility of ZnO-varistors doped with Pr-ion was measured to elucidate the valence state of the Pr-ion and the effect of thermal treatment. Magnetic susceptibility _χ(T) of the Pr-ion obeyed Curie-Weiss law, from which its valence state was estimated. The valence state of the Pr-ion varied with annealing condition of specimens, and it was indicated that the Pr-ion in ZnO ceramics was more reducible in comparison with pure \hboxPrO_x ceramics. Nonlinear current-voltage characteristics was diminished with the reduction of the valence state of Pr ions. According to Auger electron spectra, the Pr-ions segregated at grain boundaries in ZnO ceramics.     

Giant Piezoresistive Effects in Single Grain Boundaries of Semiconducting Barium Titanate Ceramics*

Extremely large piezoresistive effects with a gage factor (elastoresistance) of > 1 × 10⁶ in single grain boundaries of thin ceramic bars of semiconducting barium titanate have been observed at room temperature. Thin barium titanate ceramic bars with a diameter in the range of 10 to 20 μm were prepared to consist of single grains joined together in series. Large piezoresistive effects were observed for some of the single grain boundaries in the present samples under compressive stresses, but no distinct piezoresistance was observed in the grain bulk. A giant piezoresistive effect with a gage factor of 3 × 10⁷ was observed for a single grain boundary which exhibited a saw-tooth type PTCR (positive temperature coefficient of resistivity) characteristic with a significantly large bias dependence of it. This demonstrates that the piezoresistive phenomenon may be interpreted in terms of the change of the potential barrier height due to the change of ferroelectric domain morphologies in the vicinity of grain boundaries under mechanical and electric stresses.     

Effect of heat treatment on electrical properties of zinc oxides

The electrical properties of ZnO ceramics were studied from their voltage-current characteristics, thermally stimulated current (TSC), and heat-treatment effects. ZnO ceramic samples with a few mol% of insulating oxide components were prepared. After sintering at 1200°C in air, some of the sample were heat treated at various temperatures between 450 and 900°C in air. On the voltage-current curves, the current varies as a function of applied dc voltage according to the relation I = CV^α, where α is the nonlinear exponent. The nonlinear exponent α decreased with the heat-treatment temperature in the low-temperature region, became the lowest around 600–750°C, and almost recovered at 900°C. Three TSC peaks (designated by P₁, P₂ and P₃ from the low-temperature side) were observed in the nonheat-treated samples. The samples heat treated at 600 and 750°C showed only the P₁ and P₂ peaks; the P₃ peak increased in magnitude with the increase of biasing temperature t_b, and it shifted toward the high-temperature side. These results suggest that the electrical properties of ZnO ceramics will be associated with the phase transition of the Bi₂O₃- rich intergranular layers by heat treatment.