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.     

Deformation and Fracture of Polycrystalline Lithium Fluoride

Techniques for the fabrication of polycrystalline LiF test specimens were developed and evaluated using single-crystal LiF as a control. An etch was developed which revealed dislocations on all crystallographic faces of LiF. Large-grained polycrystalline specimens tested in four-point loading underwent 0.076 to 0.798% plastic strain before fracture. In most cases their yield stress was similar to that for single crystals favorably oriented for flow on {110}〈110〉 slip systems. Deformation was inhomogeneous among the grains because of differences in orientation with respect to the applied stress and within individual grains because of interactions at grain boundaries. Grain boundaries were barriers to slip, but stresses resulting from slip in one grain were transmitted to neighboring grains and often caused local deformation near the boundary. In one case, local boundary slip occurred on an (010) plane. Three-grain junctions were areas of high residual stresses, and fractures originated at boundaries at or near three-grain junctions. Fractures were mixed transgranular and intergranular.     

Effect of lamellar thickness on the epitaxial crystallization of PE on oriented iPP films

The effects of lamellar thickness on the epitaxial crystallization of polyethylene on the oriented isotatic polypropylene have been studied by means of transmission electron microscopy. The results obtained from the bright field electron microscopy and electron diffraction show that the epitaxial orientation of the PE crystals on the iPP substrate depends not only on the thickness of the oriented iPP lamellae, but also on the lamellar thickness of PE crystals. No epitaxial orientation relationship between PE crystal and iPP substrate can be found, when the PE crystals are thicker than the lamellar thickness of iPP along the matching direction. This suggests, that the epitaxial nucleation of PE in the PE/iPP epitaxial system is controlled not only by the chain-row matching, but also by a secondary nucleation process. Received: 11 July 1996/Revised version: 30 September 1996/Accepted: 30 September 1996     

Room temperature deposition of functional ceramic films on low-cost metal substrate

In various practical applications, such as high power actuators, high sensitivity sensors, and energy harvesting devices, polycrystalline piezoelectric films of 1–100?µm thickness and sizes ranging from several µm² to several cm² are required. With conventional film deposition processes, such as sol-gel, sputtering, chemical vapor deposition, or pulsed laser deposition, it is difficult to fabricate films with higher thickness due to their low deposition rate and high interfacial stress. The aerosol deposition method (AD), a relatively new deposition technique, can be used to fabricate highly dense thick films at room temperature by the consolidation of submicrometer-sized ceramic particles on various ceramic, metal, glass, and polymer substrates. Ferroelectric BaTiO₃ ceramic films of different thicknesses ranging from 1 to 30?µm were fabricated on a low-cost metallic substrate at room temperature using the AD method. Surface morphology and adhesion of the film were analyzed. Analysis of internal residual stresses revealed an equibiaxial compressive stress state in the as-processed film. Electrical characterization of films annealed at 500?°C shows an enhanced polarization value of ~?14?µC/cm² over that of the as-processed film. This improved property is related to the decreasing internal residual stress. In addition, the BT films prepared in this work were found to withstand electric fields greater than 100?kV/mm, which is possibly related to the inherent relatively defect-free structure of AD films.     

Electromechanical Imaging and Spectroscopy of Ferroelectric and Piezoelectric Materials: State of the Art and Prospects for the Future

Piezoresponse force microscopy (PFM) has emerged as a powerful and versatile tool for probing nanoscale phenomena in ferroelectric materials on the nanometer and micrometer scales. In this review, we summarize the fundamentals and recent advances in PFM, and describe the nanoscale electromechanical properties of several important ferroelectric ceramic materials widely used in memory and microelectromechanical systems applications. Probing static and dynamic polarization behavior of individual grains in PZT films and ceramics is discussed. Switching spectroscopy PFM is introduced as a useful tool for studying defects and interfaces in ceramic materials. The results on local switching and domain pinning behavior, as well as nanoscale fatigue and imprint mapping are presented. Probing domain structures and polarization dynamics in polycrystalline relaxors (PMN-PT, PLZT, doped BaTiO₃) are briefly outlined. Finally, applications of PFM to dimensionally confined ferroelectrics are demonstrated. The potential of PFM for studying local electromechanical phenomena in polycrystalline ferroelectrics where defects and other inhomogeneities are essential for the interpretation of their macroscopic properties is illustrated.     

Thickness‐dependent domain wall reorientation in 70/30 lead magnesium niobate‐ lead titanate thin films

Continued reduction in length scales associated with many ferroelectric film‐based technologies is contingent on retaining the functional properties as the film thickness is reduced. Epitaxial and polycrystalline lead magnesium niobate‐lead titanate (70PMN‐30PT) thin films were studied over the thickness range of 100‐350 nm for the relative contributions to property thickness dependence from interfacial and grain‐boundary low permittivity layers. Epitaxial PMN‐PT films were grown on SrRuO₃/(001)SrTiO₃, while polycrystalline films with {001}‐Lotgering factors >0.96 were grown on Pt/TiO₂/SiO₂/Si substrates via chemical solution deposition. Both film types exhibited similar relative permittivities of ~300 at high fields at all measured thicknesses with highly crystalline electrode/dielectric interfaces. These results, with the DC‐biased and temperature‐dependent dielectric characterization, suggest irreversible domain wall mobility is the major contributor to the overall dielectric response and its thickness dependence. In epitaxial films, the irreversible Rayleigh coefficients reduced 85% upon decreasing thickness from 350 to 100 nm. The temperature at which a peak in the relative permittivity is observed was the only measured small signal quantity which was more thickness‐dependent in polycrystalline than epitaxial films. This is attributed to the relaxor nature present in the films, potentially stabilized by defect concentrations, and/or chemical inhomogeneity. Finally, the effective interfacial layers are found to contribute to the measured thickness dependence in the longitudinal piezoelectric coefficient.     

Segregation and properties at curved vs straight (000) inversion boundaries in piezotronic ZnO bicrystals

TEM and SEM investigations of ZnO bicrystal interfaces were undertaken with an aim to study the correlation of local grain-boundary structure, segregation, and electrical transport perpendicular to the interface. To this end, varistor-like ZnO bicrystals with piezotronic characteristics were chosen with (000)║(000) tail-to-tail orientation with respect to the c-axis. In order to contrast different local grain-boundary structures with different coherency and segregation of bismuth, but identical macroscopic polarization state, two complementary processing techniques were applied. A diffusion-bonded bicrystal with an intermediate thin film containing Zn–Bi–Co–O provided a straight interface as reference. In contrast, a ZnO bicrystal prepared by epitaxial solid-state transformation was manufactured by bonding two ZnO single crystals with a 100 µm thick polycrystalline ZnO varistor material with a typical dopant composition including bismuth and cobalt. This structure was annealed to the point that a bicrystal was formed with the varistor concentration at the boundary, which was strongly curved due to the polycrystalline microstructure still providing a shadow image at the interface. The results highlight a distinct correlation between local interfacial morphology, degree of segregation of bismuth, and degree of nonlinearity of the electrical transport across the interface.     

Effect of Residual Stress on the Luminescence Lifetime of R-Line Emission from Polycrystalline Alumina Formed by Oxidation

In contrast to the simple exponential decay of the luminescence lifetime of the R lines observed from stressed and unstressed single crystals, the luminescence decay from polycrystalline alumina, formed by the oxidation of a ferritic FeCrAlY alloy, is found to fit a stretched exponential. A stretch exponential is found to fit all the luminescence decays in the polycrystalline alumina and the dependence of the stretch parameters under biaxial residual stress and under uniaxial stress is reported. The mean lifetime varies linearly with the residual stress, as monitored by the R2-line shift. Using established piezospectroscopic relations, the stress dependence of the lifetime is found to be one-third of the hydrostatic pressure under uniaxial stress and two thirds under biaxial stresses. In contrast, the stretch exponent doesnt vary with stress.     

Stress Measurement in Single-Crystal and Polycrystalline Ceramics Using Their Optical Fluorescence

A general methodology is developed for determining the state of stress and the numerical value of the stresses from observed shifts and broadening of optical fluorescence lines. The method is based on the piezospectroscopic properties of single crystals. We present general relationships between the measured fluorescence shifts and the stress state for a number of illustrative cases, pertinent to both polycrystalline and single-crystal ceramics under stress. These include measuring the stresses applied to polycrystalline ceramics, the residual stress distribution due to crystallographically anisotropic thermal expansion, and the stresses applied to single crystals. Using the recently implemented technique of performing the fluorescence measurements in an optical microprobe, we also provide experimental tests of the relationships derived.     

Role of morphology,crystal orientation and stoichiometry in the electrical response of perovskite EuTiO3 ceramics

To identify the generation of various contributions in the electrical response of EuTiO₃, a series of pure and doped, morphologically different and crystallographically oriented thin films prepared through RF magnetron sputtering and polycrystalline samples prepared through solid state reaction method have been investigated. Effect of stoichiometry, morphology and crystallographic orientation on the electrical response have been traced. Interestingly, in case of thin films, synthesis in more reducing atmosphere led to an in-plane crystallographic re-orientation from [110] to [100], accompanied by the observation of a prominent intrinsic antiferrodistortive phase transition. At temperatures above the antiferrodistorive phase transition, potential barriers to conduction at grain boundary are found to be directly proportional to the oxygen vacancy concentration, and inversely proportional to the average grain size. The relaxation times obtained from the frequency dependent dielectric response show an anomalous non-monotonic variation with temperature, which arises due to the variation of octahedral tilt angle.     

Do Grain Boundaries Affect Microwave Dielectric Loss in Oxides?

Reducing loss in microwave dielectrics is critical to improving performance in wireless communications systems. Grain boundaries in polycrystalline microwave dielectric ceramics have long been suspected of increasing dielectric loss. They are often cited as the main contributor to the observed difference in dielectric losses between single crystals and polycrystalline ceramics. The exact configuration of grain boundaries is problematic to quantify in practice and their influence on the dielectric loss difficult to distinguish from other defects such as porosity, oxygen vacancies, impurities, and dislocations. Here we measure the sensitivity of a single grain boundary in a magnesium oxide bi-crystal to the polarization of an applied microwave field as a function of temperature.     

The optical spectra characterization of Cr2+:ZnSe polycrystalline synthesized by direct reaction of Zn–Cr alloy and element Se

Cr²⁺:ZnSe materials have attracted much attention as candidates for mid-infrared laser source either in the form of polycrystalline powders, or bulk ceramics, single crystals and nano-materials. In this work, a novel method for synthesizing Cr²⁺:ZnSe polycrystalline by direct reaction of Zn–Cr alloy and element Se (DRAE) was proposed. The zinc alloy containing 0.1 at% Cr was prepared by dissolving Cr in zinc liquid in a closed quartz ampoule. X-ray diffraction (XRD) results showed that the synthesized Cr²⁺:ZnSe polycrystalline was with a Zinc-blend structure. X-ray photoelectron spectroscopy (XPS) spectra showed that there was no un-reacted element of Zn, or Se. Cr²⁺ ions successfully and uniformly doped into ZnSe crystal lattice, which is confirmed by the diffuse reflectance spectrum, Raman spectrum and mid-infrared photoluminescence spectra. Furthermore, the sample showed excellent mid-infrared properties without luminescence quenching in the region 1800–3000 nm, and the decay-time was about 5 μs. The as-synthesized Cr²⁺:ZnSe polycrystalline meets the requirement for the preparation of mid-infrared ceramic or single crystals. These results indicate that the novel strategy of DRAE is valid for the synthesis of other transition metal doped ZnSe materials.     

Measurement of polarization parameters impacting on electrodeposit morphology I: Theory and development of technique

A newly extended theory is presented on the role of polarization characteristics in determining the morphology of thick, polycrystalline metal electrodeposits. The theory is applicable to any system in which a single metal deposits. A simple galvanodynamic scanning procedure is more favourable than cyclic voltammetry, for predicting deposit morphology. The galvanodynamic technique represents an improved way of measuring accurately the nucleation potential and plating potential. According to the extended theory, these potentials can be readily related to the major metallographic structures of polycrystalline electrodeposits.     

Plasticity and Creep in Single Crystals of Zirconium Carbide

High-temperature deformation in ZrC single crystals was studied. Seeded crystals were grown by a direct rf-coupling floating-zone process. Yield stresses were measured from 1080° to 2000°C as a function of stress axis orientation. The Burgers vector was shown to be parallel to the 〈110〉 axes by transmission electron microscopy. Slip was observed on {100}, {110}, or {111} planes, depending on the orientation of the stress axis; it always occurred on the most favorably oriented slip system. The dependence of steady-state creep rate on the applied stress indicated that recovery occurred by a dislocation climb mechanism. Examination of the dislocation structure in deformed crystals by transmission electron microscopy supported this conclusion.     

Residual orientation in injection‐moulded plaques of atactic‐polystyrene I: The effect of processing conditions

Injection moulded polymer articles often have residual macromolecular or crystalline orientation which can have a significant impact on the optical and mechanical properties of the moulded article. Small angle neutron scattering (SANS) was used to measure the molecular shape and orientation of deuterated blends of injection moulded polystyrene. For ～1‐mm‐thick mouldings of uniform rectangular cross‐section, the eccentricity in the SANS pattern gave a direct measure of the residual molecular orientation over the length scale ～100–1,500 Å. The residual orientation was found to vary significantly with injection moulding conditions with comparative residual orientation decreasing with decreasing mould fill‐time, and increasing with mould thickness and moulding temperatures. The orientation was found to be a minimum in the centre of the mould and highest near the surface and the average orientation at a particular position in the mould was found to be strongly correlated with the volume of material deposited as a solid skin layer during injection moulding. POLYM. ENG. SCI., 58:1322–1331, 2018. © 2017 Society of Plastics Engineers     

Electronographic investigation of the degree of preferred orientation of nickel electrodeposits

The influence of a polished platinum substrate upon the degree of orientation of [110]-oriented nickel electrodeposits has been investigated electronographically, in relation to layer thickness and current density. It has been shown that the influence of the substrate is restricted to only very thin layers of electrodeposited nickel. This influence decreases with the increase of current density, or with the decrease of size of the nickel crystallites.

The obtained experimental data are in good agreement with the two-dimensional theory of preferred orientation, when the epitaxial influence of the polycrystalline substrate is taken into consideration.     

Synthesis and Performance of Advanced Ceramic Lasers

This paper reports recent progress in the production of polycrystalline Nd:YAG (Y₃Al₅O₁₂), Nd:YSAG (Y₃Sc_1.0Al_4.0O₁₂), Yb:YSAG ceramics, and a Nd-doped YAG single crystal with an almost perfect pore-free structure by advanced ceramic processing. The laser conversion efficiency of pore-free polycrystalline Nd- and Yb-doped ceramics is extremely high, and their optical qualities are comparable with that of commercial high-quality Nd:YAG single crystals. We have also succeeded in the fabrication of a Nd:YAG single crystal, which can be used for laser oscillation, by the solid-state reaction method. Laser oscillation efficiency was very low when the pores remained inside the single crystal; however, the laser oscillation efficiency of the pore-free Nd:YAG single crystal was slightly higher than that of polycrystalline Nd:YAG ceramics having high optical quality. From this fact, it was recognized that optical scattering occurs mainly in the residual pores inside the Nd:YAG ceramics and the scattering at the grain boundary is very less. In addition, we confirmed that a heavily doped Nd:YAG single crystal can be fabricated by the sintering method. Moreover, we have demonstrated the fabrication of a composite ceramic with complicated structures without the need for precise polishing and diffusion bonding. Advanced ceramic processing, which enables design flexibility of the laser element, presented in this work is important in the development of a high-performance laser (high efficiency, high beam quality, and high output energy, etc.)     

Evolution of Structural and Mechanical Properties of TiN Films on SS 304LN

We investigate the effect of orientation and residual stress on mechanical properties of reactive magnetron‐sputtered TiN thin films on SS 304 LN with a function of substrate temperature. All these films are polycrystalline with a preferred orientation (200). Residual stress of these films were calculated by sin²Ψ technique and found to be in the range of ?2.6 to ?4.5 GPa. The hardness and modulus of these films ranged between 24–29 GPa and 326–388 GPa, respectively. Temperature‐dependent orientation change is clearly observed and this in turn influenced the residual stress. Hardness and modulus of these films exhibited dependence on the orientation and residual stress.     

The mechanics of bondline thickness in balanced sandwich structures

Assuming negligible in-plane stress, non-varying shear stress, and linearly varying transverse stress along the thickness of the adhesive, the closed-form solutions for the interfacial shear and the interfacial peeling stresses in balanced bonded sandwich structures was derived. The nil-shear-stress condition at the free edge of the adhesive was enforced through the use of a decay function. The solutions for the shear and the peel stresses along the interfaces right up to the free edge were validated with the finite element analysis. The solutions were used to investigate (i) the observed increase susceptibility of single lap joints with increase bondline thickness and (ii) the nil report of such susceptibility for electronic assemblies experiencing differential thermal strain. The first investigation was compromised by the inability of the solutions to model the singular stress/strain field at the free-edge of the interfaces. The second investigation revealed a far higher rate of reduction in the magnitudes of the interfacial shear and peeling stresses in structures (electronic assemblies) that experience differential thermal strain than in structures (single lap joints) that experience bending strain.     

Orientation of lamellar crystals and its correlation with switching behavior in ferroelectric P(VDF-TrFE) ultra-thin films

The ferroelectric properties of poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) thin films mainly depend on the crystal orientation and crystallinity. We demonstrate here the thermal history and thickness dependent crystal orientation in P(VDF-TrFE) ultrathin films, and its correlation with local polarization reversal. Upon annealing in the paraelectric phase after spin-cast, the lamellar crystals grow preferentially along the crystallographic a axis, with the c axis (chain axis) parallel to the substrate. In contrast, when crystallized in the paraelectric phase from melt, the lamellar crystals take a flat-on orientation with the crystallographic c-axis normal to the substrate. In addition, local measurement by piezoresponse force microscopy indicates that the flat-on crystals do not display any polarization switching, whereas the edge-on crystals exhibit proper switching upon application of a vertical electric field. Importantly, the coercive field measured from the piezoresponse hysteresis loops does not change with the film thickness in the edge-on oriented lamellar crystals.