Direct Examinations of PTC Action of Single Grain Boundaries in Semiconducting BaTiO3 Ceramics

R-T and I-V characteristics of single grains and grain boundaries in large-grained BaTiO₃ PTC ceramics were studied with a two-probe technique using a micromanipulator and fine Al wire. The PTC originates in the grain boundary only and behaves differently in each boundary. Even below T_c , the ceramic resistance depends almost entirely on the boundary. I-V characteristics in the boundary follow Ohm's law and conduction by a space-charge-limited current with a trap, using different applied voltages. The PTC anomaly relates to activation of the trap in the boundary, not to barrier height. A band model in the intergranular layer, with dielectric BaTiO₃ and the trap, is proposed.     

Electrical Nonuniformity of Grain Boundaries within ZnO Varistors

ZnO varistors with and without ZnO crystalline seeds have been prepared through conventional ceramic processing. Their electrical nonuniformity has been carefully examined using microcontact measurements of single grain boundaries, current-voltage ( I-V ) characteristics, and dielectric temperature spectra of bulk samples. Three types of grain boundaries with barrier heights of 0.2, 0.5, and 0.6 eV are identified in a ZnO varistor with the seeds, while only one with 0.4 eV barrier height has been found in a varistor without the seeds. Using a computerized electric circuit simulation, the influence of boundary thickness and grain size on I-V characteristics of varistor is investigated extensively. The simulated results show that currents passing across various grain boundaries are quite different. For a model varistor (1 × 1 × 1 mm³) with different grain size, a good agreement between measured data and simulated curves can be achieved.     

Polarization of High-Permittivity Dielectric NiO-Based Ceramics

Polarization and dielectric behavior of Li and Ti co-doped NiO (LTNO) ceramics was investigated. Analysis of the ceramic microstructure and composition indicate that an obvious grain boundary was formed and surrounded the grains, and Ti ions distributed in grains and grain boundaries. The concentration of Ti had a remarkable effect on the dielectric properties of the LTNO ceramics. Positron annihilation spectra measurements implied that a large amount of defects exist in the LTNO ceramic samples. Anomaly polarization–electric field hysteresis loops were observed in the high-permittivity dielectric LTNO ceramics, which are totally different from those for ferroelectric materials and are due to defect-dipoles induced, and electrical conduction effects of samples could also have an influence on the hysteresis loops. The high dielectric constant response of the LTNO ceramics is enhanced partially by the boundary layer capacitor mechanism, and partially by the huge polarization of a large amount of defect dipoles.     

Nonlinear Current-Voltage Characteristics with Negative Resistance Observed at ZnO-ZnO Single-Contacts

A couple of zinc oxide (ZnO) single crystals with single boundaries (ZnO-ZnO single-contacts) are fabricated by the traditional vapor reaction method and their electrical properties are characterized. The ZnO-ZnO single-contacts obtained show nonlinear current-voltage ( I-V ) characteristics without varistor-forming constituents. Some of the ZnO-ZnO single-contacts show pronounced nonlinear I-V characteristics with negative resistance. The I-V characteristics of the ZnO-ZnO single-contacts are apparently similar to those of ZnO varistors; however, there are marked differences in the electric structure of the boundaries between the ZnO-ZnO single-contacts and ZnO varistors. The capacitance-voltage ( C-V ) relations of the ZnO-ZnO single-contacts are quite different from that of ZnO varistors and no evidence for the formation of double Schottky barriers at the boundary region are found. A very slow response to current stress is a feature of ZnO-ZnO single-contacts and it is suggested that any thermal processes including Joule heat would modify the carrier transport efficiency through the boundaries.     

Multiferroic properties of La/Er/Mn/Co multi-doped BiFeO3 thin films

Bi_0.9-xLa_xEr_0.1Fe_0.96Co_0.02Mn_0.02O₃ (BLa_xEFMCO) thin films were prepared by sol-gel method. The grain size, grain boundary resistance, oxygen vacancies and the amount of Fe²⁺ of the films were reduced by multi-ion doping to reduce the built-in electric field of the films. An applied voltage was adopted to regulate the effects of the directional alignment of the oxygen vacancies, defects, and defect pairs on the ferroelectric domains at the grain boundaries to control the ferroelectric polarization of the films. Meanwhile, the capacitance peak also reveals the effects of the ferroelectric domains switching, the migration of oxygen vacancies, and the directional alignment of defect pairs on the ferroelectric properties. In addition, the remnant polarization value of the BLa_0.01EFMCO thin film reaches 152?μC/cm², the squareness of the hysteresis loop (R_sq) is calculated to be 1.03, and the maximum switching current is 1.50?mA. The typical butterfly curves under positive and negative electric fields indicate the films with the enhanced ferroelectric properties. Moreover, the BLa_0.01EFMCO thin film exhibits the enhanced ferromagnetic properties, and its saturation magnetization (M_s) is 2.32 emu/cm³. Therefore, the ferroelectric properties of the BFO film can be enhanced by the multi-ion doped BFO film to reduce the grain boundary resistance (Rgb), the interface Schottky barrier formed by the asymmetric electrode material at the top and bottom of the film, and the built-in electric field formed by the film internal defect or defect pairs.     

Single-Grain Boundaries in PTC Resistors

Thin semiconducting barium titanate ceramic bars consisting of single grains joined together in series have been prepared, and the positive temperature coefficient of resistivity (PTCR) characteristics of strictly single-grain boundaries in the materials were investigated. The resistivity ( R )-temperature ( T ) characteristics obtained for the present samples can be classified into typically three categories: (1) normal type PTCR characteristics, similar to those observed in usual ceramic samples, (2) saw-tooth type PTCR characteristics, characterized by an abrupt increase in resistivity by more than three orders of magnitude at the Curie point, immediately follwoed by a monotonous decrease in it, and (3) flat type R–T characteristics, with substantially little or no resistivity jump. Of these R–T characteristics, normal type PTCR characteristics were the most frequently observed (about 60%; a total of 65 samples were examined). Flat type R–T characteristics were least frequently (about 10%) observed. Single boundaries with these three types of PTCR characteristics exhibited essentially the same ferroelectric capacitance–temperature characteristic; this demonstrates that the temperature dependence of the dielectric constant above the Curie point was not responsible for the PTCR anomalies. Single boundaries with normal and saw-tooth type PTCR characteristics showed significantly nonlinear current-voltage characteristics above the Curie point, which may be interpreted to be caused by a current strongly affected by traps (or surface acceptor states) present at the grain boundaries.     

The Linear Thermal Expansion of Bulk Nanocrystalline Ingot Iron from Liquid Nitrogen to 300 K

The linear thermal expansions (LTE) of bulk nanocrystalline ingot iron (BNII) at six directions on rolling plane and conventional polycrystalline ingot iron (CPII) at one direction were measured from liquid nitrogen temperature to 300 K. Although the volume fraction of grain boundary and residual strain of BNII are larger than those of CPII, LTE of BNII at the six measurement directions were less than those of CPII. This phenomenon could be explained with Morse potential function and the crystalline structure of metals. Our LTE results ruled out that the grain boundary and residual strain of BNII did much contribution to its thermal expansion. The higher interaction potential energy of atoms, the less partial derivative of interaction potential energy with respect to temperature T and the porosity free at the grain boundary of BNII resulted in less LTE in comparison with CPII from liquid nitrogen temperature to 300 K. The higher LTE of many bulk nanocrystalline materials resulted from the porosity at their grain boundaries. However, many authors attributed the higher LTE of many nanocrystalline metal materials to their higher volume fraction of grain boundaries.     

Investigation of electrical conduction mechanism in double‐layered polymeric system

The electrical conduction in solution‐grown polymethylmethacrylate (PMMA), polyvinylidenefluoride (PVDF) and PMMA‐PVDF double‐layered samples in the sandwich configuration (metal‐polymer‐metal) was investigated at different fields in the range 100–120 kV/cm as a function of temperature in the range 293–423 K for samples of constant thickness of about 50 μm. Certain effects which lead to a large burst of current immediately after the application of field were observed in double‐layered samples. An attempt was made to identify the nature of the current by comparing the observed dependence on electric field, electrode material and temperature with the respective characteristic features of the existing theories on electrical conduction. The observed linear I‐V characteristics show that the electrical conduction follows Pool‐Frenkel mechanism in PMMA and PVDF samples. Whereas, the non‐linear behavior of current‐voltage measurements in PMMA‐PVDF double‐layered samples have been interpreted on the basis of space charge limited conduction (SCLC) mechanism. The conductivity of the polymer films increased on formation of their double‐layer laminates. The polymer‐polymer interface act as charge carrier trapping centres and provides links between the polymer molecules in the amorphous region. The interfacial phenomenon in polymer‐polymer heterogeneous system has been interpreted in terms of Maxwell‐Wagner model. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Impedance Spectroscopy of Undoped BaTiO3 Ceramics

The electrical properties of ceramic BaTiO₃ were investigated by ac impedance spectroscopy over the ranges 25°-330°C and 0.03 Hz-1 MHz. Results are compared with those obtained from fixed-frequency measurements, at 1 kHz and 100 kHz. Fixed-frequency Curie-Weiss plots show deviations from linearity at temperatures well above t _c. The ac measurements show that grain boundary impedances influence Curie-Weiss plots in two ways: at high temperatures, they increasingly dominate the fixed-frequency permittivities; at lower temperatures, closer to T ^c, the high-frequency permittivity contains a contribution from grain boundary effects. Methods for extraction of bulk and grain boundary capacitances from permittivity and electric modulus complex plane plots are discussed. The importance of selecting the appropriate equivalent circuit to model the impedance response is stressed. A constriction impedance model for the grain boundary in BaTiO₃ ceramics is proposed: the grain boundary capacitance is neither temperature-independent, nor shows Curie-Weiss behavior. The grain boundary is ferroelectric, similar to the grains, but its impedance is modified by either air gaps or high-impedance electrical inhomogeneity in the region of the necks between grains; the activation energy of the constriction grain boundary impedance differs from that of the bulk, suggesting differences in defect states or impurity levels.     

Suitability of mullite for high temperature applications

High temperature mechanical behaviour of mullite has been studied. Our study include tensile, flexural and compressive creep behaviour and fracture up to 1400 °C. The results obtained in creep are analysed and compared with previous work in the literature. Two regions with different behaviour can be distinguished. The creep rates in bending, tension and compression are very similar in the first region at low stresses and temperatures. It is shown that in this region creep takes place by accommodated grain boundary sliding assisted by diffusion. At higher stresses slow crack growth from defects present in the sample occurs. The stress at which this transition in the deformation mechanism happens is dependent on several factors, the loading system during testing, the grain size, the amount and distribution of glassy phase and the environment. It is claimed the existence of a network of mullite–mullite grain boundaries free of glassy phase associated to the low surface energy of [001] planes. The diffusion rate through these boundaries controls the creep rate, and explains the high creep resistance of mullite. The results presented in this work lead to the conclusion that the mechanism controlling high temperature deformation resistance of mullite materials in a wide range of stress–temperature working conditions is independent of the glassy phase content. Slow crack growth limit the use of mullite at high stresses and temperatures.     

Electromechanical Response of Polycrystalline Barium Titanate Resolved at the Grain Scale

Ferroic materials are critical components in many modern devices. Polycrystalline states of these materials dominate the market due to their cost effectiveness and ease of production. Studying the coupling of ferroic properties across grain boundaries and within clusters of grains is therefore critical for understanding bulk polycrystalline ferroic behavior. Here, three‐dimensional X‐ray diffraction is used to reconstruct a 3D grain map (grain orientations and neighborhoods) of a polycrystalline barium titanate sample and track the grain‐scale non‐180° ferroelectric domain switching strains of 139 individual grains in situ under an applied electric field. The map shows that each grain is located in a very unique local environment in terms of intergranular misorientations, leading to local strain heterogeneity in the as‐processed state of the sample. While primarily dependent on the crystallographic orientation relative to the field directions, the response of individual grains is also heterogeneous. These unique experimental results are of critical importance both when building the starting conditions and considering the validity of grain‐scale modeling efforts, and provide additional considerations in the design of novel ferroic materials.     

Weak-Beam Dark-Field Microscopy of Complex Stress States in X7R-Type BaTiO3 Dielectric Core–Shell Structures

X7R-type BaTiO₃ materials were analyzed using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Powder XRD indicated that the materials had pseudocubic lattices, but core–shell grain structures predominated in bright-field (BF) TEM images. Electron diffraction patterns across the core–shell boundaries and convergent beam electron diffraction patterns of cores and shells indicated that coherent grain boundaries existed between cores and shells. The flat dielectric constant–temperature curves obtained from these materials can be interpreted in terms of the internal stress states in individual grains. The stress states were observed using weak-beam dark-field (WBDF) microscopy, and strain contours formed by distorted crystal planes were visible in the WBDF images. The contours observed were dependent on the stress state of the crystal instead of crystal symmetry and the stress distribution in individual grains was determined by both the thickness ratio of shell and core, and the geometrical relationship of the core and the shell. Twins observed in this material were determined to be growth rather than mechanical twins, through observation of the strain contour distribution.     

Direct Characterization of Grain-Boundary Electrical Activity in Doped (Ba0.6Sr0.4)TiO3 by Combined Imaging of Electron-Beam-Induced Current and Electron-Backscattered Diffraction

Simultaneous measurements of remote electron beam induced current (REBIC) and orientation imaging microscopy (OIM) in a scanning electron microscope (SEM) have been applied to a polycrystalline (Ba_0.6Sr_0.4)TiO₃ with a positive temperature coefficient of resistivity (PTCR) to elucidate a grain-boundary character dependence of the potential barrier formation. The absence of electrical activity in a coherent Σ3 twin boundary is clearly imaged. The resistivity of individual grain boundaries estimated from a resistive contrast image is interpreted in terms of geometrical coherency, which is defined by the degree of coincidence in the reciprocal lattice points.     

Optical and electrical properties of thin films of WO3 electrochemically coloured

Strong blue colouration has been induced in rf sputtered thin films of WO₃ by electrochemical injection of H⁺, Li⁺, and Ag⁺ ions from various solid and liquid electrolytes. Electrical conductivity and optical properties of the coloured films are reported. Comparison of these properties with those of single crystal tungsten bronzes of equivalent composition is made. Evidence, electrical and optical, for a non-uniform distribution of injected ions produced by relatively fast diffusion down grain boundaries in these polycrystalline WO₃ films is presented. A model for the optical absorption consisting of two components, due to (i) conduction electron intra-band transitions (in states close to crystalline surfaces) and (ii) transitions from unionized donor states to the conduction band (in the grain boundary phase), is tentatively proposed.     

Single Crystals of Pb(Mg1/3Nb2/3)O3—35 mol% PbTiO3 from Polycrystalline Precursors

A potentially more cost-efficient method of growing single-crystal relaxor-based ferroelectric materials has been investigated. Seed single crystals of Pb(Mg_1/3Nb_2/3)O₃(PMN)—;35 mol% PbTiO₃(PT) were embedded within polycrystalline powders and annealed at temperatures from 900° to 1200°C. The boundary of the single crystal migrated through the polycrystal matrix under the influence of grain boundary curvature; growth distances of several millimeters were observed, verifying the feasibility of the approach. The grown single crystals exhibited macroscopic cubic growth morphologies with (100) faces. Strain levels as high as 0.68% under an electric field of 30 kV/cm were observed in initial measurements.     

Comparison study of macro and micro scale AC and DC conductivity measurements with Impedance Spectroscopy and Atomic Force Microscopy techniques applied in PBZT ceramics

Ferroelectric materials systematically enter into the structure of microelectronic devices. The ability to increase the packing density of the ferroelectric structures, and thus the piezoelectric coefficients of the final device, is primarily limited by the fact that such tiny ferroelectric structures may not preserve their microscopic properties at macroscopic scale. A problem of current interest in ferroelectric research is to get to know how to modify the domain structure and the piezoelectric properties of the material, if the polycrystalline material consists of grains and grain boundaries, in which electrical properties differ significantly. In this paper, we have combined the Impedance Spectroscopy (IS), as a method for detecting such inequality in the form of separated impedances, and Atomic Force Microscopy (AFM), as techniques for direct local engineering and investigation of grain and grain boundaries conductivity. We would like to present hitherto unreported connection between values of electrical parameters obtained by both methods.     

A Review of the Influence of Grain Boundary Geometry on the Electromagnetic Properties of Polycrystalline YBa2Cu3O7−x Films

Shortly after the discovery of high-temperature superconducting (HTS) materials in the late 1980s, it was revealed that grain boundaries in these complex oxides are strong barriers to current flow. This fact has remained one of the most significant challenges to a viable HTS conductor, and necessitated the development of technologies capable of producing biaxially textured substrates in long lengths. Multiple studies have reported that the critical current density ( J _c) across grain boundaries in the perovskite-like superconductor YBa₂Cu₃O_{7− x} (YBCO) falls off exponentially below the intragrain J _c beyond a critical misorientation angle θ_c of only ≈2°–3°. Here we review our recent work demonstrating that certain grain boundary geometries permit significant enhancements of J _c well beyond the conventional J _c(θ) limit, and also that the grain boundary structure in YBCO films is tied closely to the films' deposition technique. Pulsed laser deposition, a physical vapor deposition technique, results in a columnar grain structure and planar grain boundaries that exhibit the typical J _c(θ) dependence. Ex situ growth processes, where the YBCO film is converted from a previously deposited precursor, can result in laminar grain growth with highly meandered grain boundaries. These latter grain boundary structures are directly correlated to greatly improved J _c values over a wide range of applied magnetic fields. Consequently, very high J _c values are possible in polycrystalline HTS wire even when significant misorientations between grains are present.     

Zinc Oxide Varistor Gas Sensors: I, Effect of Bi2O3 Content on the H2-Sensing Properties

Current ( I )-voltage ( V ) characteristics of porous ZnO varistors with different Bi₂O₃ content have been investigated in air as well as in H₂-air mixtures in the temperature range room temperature (RT)-600°C. The I-V characteristics measured at RT remained unchanged in the presence of H₂, but the breakdown voltage clearly shifted to a lower electric field in the temperature range 400–600°C. The breakdown voltage decreased with increasing H₂ concentration in air. The optimum amount of Bi₂O₃ for the largest decrease was found to be 1.0 mol%. Thus, ZnO varistors can be used as a new type of H₂ sensor. The results presented in this study also suggest the important role of excess oxygen ions existing at the ZnO-ZnO grain boundaries in developing the Schottky barrier as well as in the H₂-sensing mechanism of the varistors.     

Grain‐size–dependent dielectric properties in nanograin ferroelectrics

A series of ferroelectric ceramic models with grain and grain‐boundary structures of different sizes are established via Voronoi tessellations. A phase‐field model is introduced to study the dielectric breakdown strength of these ferroelectric ceramics. Afterward, the relation between the electric displacement and electric field and the hysteresis loop are calculated using a finite element method based on a classical and modified hyperbolic tangent model. The results indicate that as the grain size decreases, the dielectric strength is enhanced, but the dielectric permittivity is reduced. The discharge energy density and energy storage efficiency of these ferroelectric ceramics extracted from the as‐calculated hysteresis both increase along with a decrease in their grain size at their breakdown points. However, under the same applied electric field, the ferroelectric ceramic with a smaller grain size possesses a lower discharge energy density but a higher energy storage efficiency. The results suggest that ferroelectric ceramics with smaller grain sizes possess advantages for applications in energy storage devices.     

Preparation and Electric Properties of Dense Nanocrystalline Zinc Oxide Ceramics

This communication reports on the preparation and electric properties of dense nanocrystalline ZnO ceramics. By spark plasma sintering, nanocrystalline (∼100 nm) ZnO ceramics with a high density of 98.5% were obtained at a very low temperature of 550°C. Electric property measurement revealed a novel conduction nonlinearity in the sample sintered at 500°C. This phenomenon is due to the nanometerization of ZnO crystal and the grain boundary layer with an amorphous interfacial layer.