Effects of barium on the nonlinear electrical characteristics and dielectric properties of SnO2-based varistors

The effects of barium on electrical and dielectric properties of the SnO_2·Co_2O_3·Ta_2O_5 varistor system sintered at 1250℃ for 60min were investigated. It is found that barium significantly improves the nonlinear properties. The breakdown electrical field increases from 378.0 to 2834.5V/mm, relative dielectric constant (at 1kHz) falls from 1206 to 161 and the resistivity (at 1kHz) rises from 60.3 to 1146.5kΩ·cm with an increase of BaCO_3 concentration from 0mol% to 1.00mol%. The sample with 1.00mol% barium has the best nonlinear electrical property and the highest nonlinear coefficient (α=29.2). A modified defect barrier model is introduced to illustrate the grain-boundary barrier formation of barium-doped SnO_{2}-based varistors.     

Influences of annealing on structural and compositional properties of Al_2O_3 thin films grown on 4H–SiC by atomic layer deposition

Annealing effects on structural and compositional performances of Al_2O_3 thin films on 4H–Si C substrates are studied comprehensively. The Al_2O_3 films are grown by atomic layer deposition through using trimethylaluminum and H_2 O as precursors at 300?C, and annealed at various temperatures in ambient N_2 for 1 min. The Al_2O_3 film transits from amorphous phase to crystalline phase as annealing temperature increases from 750?C to 768?C. The refractive index increases with annealing temperature rising, which indicates that densification occurs during annealing. The densification and grain formation of the film upon annealing are due to crystallization which is relative with second-nearest-neighbor coordination variation according to the x-ray photoelectron spectroscopy(XPS). Although the binding energies of Al 2p and O 1s increase together during crystallization, separations between Al 2p and O 1s are identical between as-deposited and annealed sample, which suggests that the nearest-neighbour coordination is similar.     

Electrical and dielectric properties of Na_(1/2)La_(1/2)Cu_3Ti_4O_(12) ceramics prepared by high energy ball-milling and conventional sintering

We report on the measurements of the electrical and dielectric properties of Na_(1/2)La_(1/2)Cu_3Ti_4O_(12)(NLCTO) ceramics prepared by high energy ball-milling and conventional sintering without any calcination steps. The x-ray powder diffraction analysis shows that pure perovskite-like CCTO phase is obtained after sintering at 1025?C–1075?C. Higher sintering temperatures result in multi-phase ceramics due to thermal decomposition. Scanning electron microscope observations reveal that the grain size is in a range of ~ 3 μm–5 μm for these ceramics. Impedance spectroscopy measurements performed in a wide frequency range(1 Hz–10 MHz) and at various temperatures(120 K–470 K) are used to study the dielectric and electrical properties of NLCTO ceramics. A good compromise between high ε(5.7 × 10~3 and 4.1 × 10~3 at 1.1 k Hz and 96 k Hz, respectively) and low tan δ(0.161 and 0.126 at 1.1 k Hz and 96 k Hz, respectively) is obtained for the ceramic sintered at 1050℃. The observed high dielectric constant behavior is explained in terms of the internal barrier layer capacitance effect.     

Nonlinear Electrical Characteristics of Antimony and Copper Doped Tin Oxide Based Varistor Ceramics

The novel CuO-doped dense tin oxide varistor ceramics are investigated. The densification of tin oxide varistor ceramics could be greatly improved by doping copper oxide additives. The introduction of antimony additives into a SnO2. CuO ceramic system would make it possess excellent nonlinearity. The sample doped with 0.05 mol% Sb2 03 possesses the highest nonlinearity coefficient （α = 17.9） and the lowest leakage current density （ JL = 52μA cm^-2） among all the samples. A modified defect barrier model is introduced to explain the formation of the grainboundary barrier. The nonlinear behaviour of （Cu, Sb）-doped SnO2 varistor system could be explained by the barrier model.     

Improvement of sintering,nonlinear electrical,and dielectric properties of ZnO-based varistors doped with TiO_2

The effects of TiO_2 on sintering and nonlinear electrical properties of(98.5-x)ZnO–0.5MnO_2–0.5Co_2O_3-0.5Bi_2O_(3–x)TiO_2(x = 0.3,0.5,0.7,0.9 mol%) ceramic varistors prepared by the ceramic technique are investigated in this work.The optimum sintering temperature of the prepared samples is deduced by determining the firing shrinkage and water absorption percentages.The optimum sintering temperature is found to be 1200℃,at which each of the samples shows a maximum firing shrinkage and minimum water absorption.Also minimum water absorption appears in a sample of x = 0.9 mol%.Higher sintering temperature and longer sintering time give rise to a reduction in bulk density due to the increased amount of porosity between the large grains of ZnO resulting from the rapid grain growth induced by the liquid phase sintering.The crystal size of ZnO decreases with increasing TiO_2 doping.The addition of TiO_2 improves the nonlinear coefficient and attains its maximum value at x = 0.7 mol% of TiO_2,further addition negatively affects it.A decrease in capacitance consequently in the dielectric constant is recorded with increasing the frequency in a range of 30 kHz–200 kHz.The temperature and composition dependences of the dielectric constant and AC conductivity are also studied.The increase of temperature raises the dielectric constant because it increases ionic response to the field at any particular frequency.     

Structural,magnetic, and dielectric properties of Ni–Zn ferrite and Bi_2O_3 nanocomposites prepared by the sol-gel method

Ni–Zn ferrite and Bi_2O_3 composites were developed by the sol-gel method. The structural, magnetic, and dielectric properties were studied for all the prepared samples. X-ray diffraction(XRD) was performed to study the crystal structure.The results of field emission scanning electron microscopy(FE-SEM) showed that the addition of Bi_2O_3 can increase the grain size of the Ni–Zn ferrite. Magnetic properties were analyzed by a hysteresis loop test and it was found that the saturation magnetization and coercivity decreased with the increase of Bi_2O_3 ratio. In addition, the dielectric properties of the Ni–Zn ferrite were also improved with the addition of Bi_2O_3.     

Structural evolution of a-Si:H/SiO2 multilayers upon step by step thermal annealing

a-Si:H/SiO_2 multilayers were prepared by alternatively changing plasma enhanced chemical vapour deposition of a-Si:H layers and in situ plasma oxidation process. Subsequently, as-grown samples were annealed at temperatures from 350℃ to 1100℃ in N_2 ambient with an increment of 100℃. The evolution of bonding configurations and structures with annealing treatments was systematically investigated by Fourier-transform infrared spectroscopy. The peak position of Si－O stretching vibration of SiO_2 layers shift to 1087cm^{-1} after annealing at 1100℃, which demonstrates that the SiO_2 films fabricated by plasma oxidation after high temperature annealing can have similar properties to the thermal grown ones. A Si－O vibration from interfacial SiO_x was identified: the value x was found to increase as increasing the annealing temperature, which is ascribed to the cooperation of hydrogen effusion and reordering of the oxygen bond in SiO_x networks. The H-related bonds were observed in the form of H－Si－O_3 and H－Si－Si_{3-n}O_n (n=1－2) configurations, which are supposed to be present in SiO_2 and interfacial SiO_x layers, respectively. The H atoms bonded in different bonding configurations effuse at different temperatures due to their different desorption energies.     

Spectra and lasing properties of Er~(3+),Yb~(3+):phosphate glasses

The effects of Al_2O_3, Yb_2O_3, Er_2O_3 and OH~- on spectral properties of P_2O_5.Na_2O.SrO.Al_2O_3.Yb_2O_3.Er_2O_3 erbium phosphate glass were studied. 5, 8, and 13 mol% Al_2O_3, 4, 5, 6, 7 and 8 mol% Yb_2O_3 and0.05, 0.2, and 0.4 mol% Er_2O_3 were used. It was found that Al_2O_3 can improve fluorescent lifetime ofEr~(3+)ions, but the integrated absorption cross section of Er~(3+)ions decreases with the increase of Al_2O_3concentration. Evaluating from energy transfer efficiency of Yb~(3+)to Er~(3+)and spectral parameters ofYb~(3+)and Er~(3+),we conclude that 6 mol% Yb_2O_3 and 0.4 mol% Er_2O_3 are needed for LD pumped     

Morphology-controlled preparation of tungsten oxide nanostructures for gas-sensing application

A novel three-dimensional(3D) hierarchical structure and a roughly oriented one-dimensional(1D) nanowire of WO_3are selectively prepared on an alumina substrate by an induced hydrothermal growth method.Each hierarchical structure is constructed hydrothermally through bilateral inductive growth of WO_3 nanowire arrays from a nanosheet preformed on the substrate.Only roughly oriented 1D WO_3 nanowire can be obtained from a spherical induction layer.The analyses show that as-prepared 1D nanowire and 3D hierarchical structures exhibit monoclinic and hexagonal phases of WO_3,respectively.The gas-sensing properties of the nanowires and the hierarchical structure of WO_3,which include the variations of their resistances and response times when exposed to NO_2,are investigated at temperatures ranging from room temperature(20 ℃) to 250 ℃ over 0.015 ppm-5 ppm NO_2.The hierarchical WO_3 behaves as a p-type semiconductor at room temperature,and shows p-to-n response characteristic reversal with the increase of temperature.Meanwhile,unlike the1 D nanowire,the hierarchical WO3 exhibits an excellent response characteristic and very good reversibility and selectivity to NO_2 gas at room temperature due to its unique microstructure.Especially,it is found that the hierarchical VO_3-based sensor is capable of detecting NO_2 at a ppb level with ultrashort response time shorter than 5 s,indicating the potential of this material in developing a highly sensitive gas sensor with a low power consumption.     

Thermal stability of tungsten and tungsten nitride Schottky contacts to AlGaN/GaN

Thin tungsten nitride (WNx) films were produced by reactive DC magnetron sputtering of tungsten in an Ar-N2 gas mixture. The films were used as Schottky contacts on AlGaN/GaN heterostructures. The Schottky behaviours of WNx contact was investigated under various annealing conditions by current-voltage (I-V ) measurements. The results show that the gate leakage current was reduced to 10-6 A/cm2 when the N2 flow is 400 mL/min. The results also show that the WNx contact improved the thermal stability of Schottky contacts. Finally, the current transport mechanism in WNx/AlGaN/GaN Schottky diodes has been investigated by means of I-V characterisation technique at various temperatures between 300 K and 523 K. A TE model with a Gaussian distribution of Schottky barrier heights (SBHs) is thought to be responsible for the electrical behaviour at temperatures lower than 523 K.     

Role of helium in the sliding and mechanical properties of a vanadium grain boundary:A first-principles study

The effects of helium （He） on the sliding and mechanical properties of a vanadium （V） E5（310）/[001] grain boundary （GB） have been investigated using a first-principles method. It has been found that He was energetically favorable sitting at the GB region with a segregation energy of -0.27 eV, which was attributed to the special atomic configurations and charge density distributions of the GB. The maximal sliding energy barrier of the He-doped GB was calculated to be 1.73 J/m^2, 35% larger than that of the clean GB. This suggested that the presence of He would hinder the V GB mobility. Based on the thermodynamic criterion, the total energy calculations indicated that the embrittlement of V GB would be enhanced by He segregation.     

Controllable synthesis of ultrathin vanadium oxide nanobelts via an EDTA-mediated hydrothermal process

Ultrathin VO_2 nanobelts with rough alignment features are prepared on the induction layer-coated substrates by an ethylenediaminetetraacetic acid(EDTA)-mediated hydrothermal process. EDTA acts as a chelating reagent and capping agent to facilitate the one-dimensional(1D) preferential growth of ultrathin VO_2 nanobelts with high crystallinities and good uniformities. The annealed induction layer and concentration of EDTA are found to play crucial roles in the formation of aligned and ultrathin nanobelts. Variation in EDTA concentration can change the VO_2 morphology of ultrathin nanobelts into that of thick nanoplates. Mild annealing of ultrathin VO_2 nanobelts at 350℃ in air results in the formation of V_2O_5 nanobelts with a nearly unchanged ultrathin structure. The nucleation and growth mechanism involved in the formations of nanobelts and nanoplates are proposed. The ethanol gas sensing properties of the V_2O_5 nanobelt networks-based sensor are investigated in a temperature range from 100℃ to 300℃ over ethanol concentrations ranging from 3 ppm to 500 ppm.The results indicate that the V_2O_5 nanobelt network sensor exhibits high sensitivity, good reversibility, and fast responserecovery characteristics with an optimal working temperature of 250℃.     

Effect of Fe concentration on the nonlinear refraction and reverse saturable absorption in ferroelectric X-cut LiNbO_3 crystal

In this Letter, the effects of the iron(Fe) dopant concentration on the nonlinear optical properties of iron-doped ferroelectric X-cut Li Nb O3 crystals plates are studied by using the Z-scan technique with a cw laser at the wavelength of 532 nm. The amount of iron in the compound is varied from 0 to 0.15 mol%. Measurements of nonlinear refractive index n2 and the nonlinear absorption coefficient β are determined. The sign of the nonlinear refractive index is found to be negative and the magnitude is on the order of 10-8cm2∕W. This nonlinear effect increases as the concentration increases from 0 to 0.15 mol%. A good linear relationship is obtained between nonlinear refractive index, nonlinear absorption coefficient, and concentration.     

Structural and dielectric properties of giant dielectric Na_(1/2)Sm_(1/2)Cu_3Ti_4O_(12) ceramics prepared by reactive sintering methods

Na_(0.5)Sm_(0.5)Cu_3Ti_4O_(12)(NSCTO) ceramics have been prepared by reactive sintering of amorphous powder.Spark plasma sintering(SPS) for 10 min at 1025℃ and conventional sintering(CS) for 10 h at 1090℃ have been employed.X-ray diffraction measurements confirmed the pure CCTO-like phase for SPS and CS NSCTO ceramics.The SPS ceramic showed an average grain size of 500 nm, which is much smaller than that of the CS(～ 5 μm) sample.The impedance spectroscopy measurements revealed an electrically inhomogeneous structure in the prepared ceramics.While the resistivities of grains of both ceramic samples were in the same order of magnitude, the resistivity of grain-boundaries of the CS ceramic was three orders of magnitude greater than that of the SPS ceramic.Both of the samples showed giant dielectric constant( 10~3) over wide ranges of temperatures and frequencies.Nevertheless, the room-temperature dielectric loss of the SPS NSCTO(3.2 at 1.1 kHz) ceramic sample was higher than that of the CS NSCTO(0.08 at 1.1 kHz) ceramic sample due to the reduced grain-boundary resistivity of the former.Two dielectric relaxations were detected for each sample and attributed to the relaxations in grains and grain-boundaries.The dielectric behavior of the SPS and CS NSCTO ceramics could be interpreted in terms of the internal barrier layer capacitor(IBLC) model.     

Porosity Evaluation and the Power Spectral Densities Analyses of Carbon-Nickel Composite Films Annealed at Different Temperatures

The densification and the fractal dimensions of carbon-nickel films annealed at different temperatures 300,500,800,and 1000℃with emphasis on porosity evaluation are investigated.For this purpose,the refractive index of films is determined from transmittance spectra.Three different regimes are identified,T500℃,500℃T800℃and T800℃.The Rutherford backscattering spectra show that with increasing the annealing temperature,the concentration of nickel atoms into films decreases.It is shown that the effect of annealing temperatures for increasing films densification at T500℃and T800℃is greater than the effect of nickel concentrations.It is observed that the effect of decreasing nickel atoms into films at 500℃T800℃strongly causes improving porosity and decreasing densification.The fractal dimensions of carbon-nickel films annealed from 300 to 500℃are increased,while from 500 to 1000℃these characteristics are decreased.It can be seen that at 800℃,Elms have maximum values of porosity and roughness.     

Water-based processed and alkoxide-based processed indium oxide thin-film transistors at different annealing temperatures

In this study,indium oxide(In_2O_3) thin-film transistors(TFTs) are fabricated by two kinds of low temperature solution-processed technologies(Ta ≤ 300?C),i.e.,water-based(DIW-based) process and alkoxide-based(2-ME-based)process.The thickness values,crystallization properties,chemical structures,surface roughness values,and optical properties of In_2O_3 thin-films and the electrical characteristics of In_2O_3 TFTs are studied at different annealing temperatures.Thermal annealing at higher temperature leads to an increase in the saturation mobility(μsat) and a negative shift in the threshold voltage(VTH).The DIW-based processed In_2O_3-TFT annealed at 300?C exhibits excellent device performance,and one annealed at 200?C exhibits an acceptable μsat of 0.86 cm~2/V·s comparable to that of a-Si:H TFTs,whereas the 2-ME-based TFT annealed at 300?C exhibits an abundant μsat of 1.65 cm~2/Vs and one annealed at 200?C is inactive.The results are attributed to the fact that the DIW-based process induces a higher degree of oxidation and less defect states than the 2-ME-based process at the same temperature.The DIW-based process for fabricating the In_2O_3 TFT opens the way for the development of nontoxic,low-cost,and low-temperature oxide electronics.     

Room temperature NO_2-sensing properties of hexagonal tungsten oxide nanorods

Hexagonal WO_3 nanorods were synthesized through a facile hydrothermal method. The nanorods properties were investigated by scanning electron microscope(SEM), transmission electron microscope(TEM), energy dispersive spectroscopy(EDS), and x-ray diffraction(XRD). The NO_2-sensing performances in terms of sensor response, response/recovery times and repeatability at room temperature were optimized by varying the heat treatment temperature of WO_3 nanorods. The optimized NO_2sensor(400-℃-annealed WO_3 nanorods) showed an ultra-high sensor response of 3.2 and short response time of 1 s to 5-ppm NO_2. In addition, the 400-℃-annealed sample exhibited more stable repeatability.Furthermore, dynamic responses measurements of annealed samples showed that all the annealed WO_3 nanorods sensors presented p-type behaviors. We suppose the p-type behavior of the WO_3 nanorods sensor to be that an inversion layer is formed in the space charge layer when the sensor is exposed to NO_2 at room temperature.Therefore, the 400-℃-annealed WO_3 nanorods sensor is one of the most energy conservation candidates to detect NO_2 at room temperature.     

Evidence for Grain Boundary Transport from Impedance Spectroscopy of Gabbro at 1－2GPa up to 890℃

Impedance spectra of gabbro were measured at 1-2 GPa and up to 890℃ with applied frequency of 12 to 10^5 Hz. At temperatures below 680℃, only one impedance arc corresponding to the grain interior conduction process occurs. Owing to the grain boundary transport with increasing temperature, the impurities occur at the grain boundaries, resulting in the second arc corresponding to the grain boundary conduction process over the frequency range of 12 to 10^3Hz above 680℃, and the resistivities of the grain interior and the grain boundary conduction mechanisms add in series. The total conductivity of this rock is dominated by the grain interior conductivity and the impurities have no significant effect on the total electrical conductivities.     

Simulation of grain boundary effect on characteristics of ZnO thin film transistor by considering the location and orientation of grain boundary

The grain boundaries (GBs) have a strong effect on the electric properties of ZnO thin film transistors (TFTs). A novel grain boundary model was developed to analyse the effect. The model was characterized with different angles between the orientation of the grain boundary and the channel direction. The potential barriers formed by the grain boundaries increase with the increase of the grain boundary angle, so the degradation of the transistor characteristics increases. When a grain boundary is close to the drain edge, the potential barrier height reduces, so the electric properties were improved.     

Nonlinear optical properties of 75TeO2－20Nb2O5－5ZnO glasses doped with CeO2

Nonlinear optical properties of 75TeO_2－20Nb_2O_5－5ZnO glasses doped with CeO_2 have been investigated with a self-diffracted time-resolved degenerate four-wave mixing (DFWM) technique at different excitation intensities and lattice temperatures. The DFWM signal exhibits three peaks at higher excitation intensities, where a main peak appears at zero delay time and two rather weak side peaks are located symmetrically at the negative and positive time delay. Due to destructive interferences between the fifth- and third-order polarizations, the line-shape of the main peak around the zero time delay evolves from single peak into a double-peak structure with increasing excitation intensity. Two side peaks emerge at the positive and negative time delay and gradually intensify with increasing excitation intensity or lattice temperature, and their positions are independent of the pulse duration, temperature and excitation intensity, which are attributed to the many-body Coulomb interaction.