R.D. Roseman

PTCR Effect in BaTiO3: Structural Aspects and Grain Boundary Potentials

Extensive microstructural and structure-property studies on donor doped barium titanate have revealed that the PTCR phenomenon is strongly controlled by the density, number of grain boundaries available to conduction, domain orientation and grain boundary domain coherence. Structural heterogeneities lead to a wide range of grain boundary structures, potential barriers and, therefore, depletion widths. Conduction thus occurs primarily by percolation of electrons through favorably aligned domain pathways and low potential barrier grain boundaries. At the Curie point, the increase in the potential barriers along these pathways is likely to dominate the PTCR effect. To improve theoretical understanding a model needs to take heed of local values of parameters and also incorporate the fact that the bulk of the current flow is only through a certain percentage of grain boundaries. The specific structural factors that have led to an improved qualitative understanding of overall PTCR phenomenon are discussed.

The piezoelectric cochlear implant: Concept, feasibility, challenges, and issues

A better understanding of the fundamental phenomena occurring in both the healthy and the artificially stimulated cochlea will greatly aid in the engineering of more effective cochlear implant devices and will, in general, enhance mankinds knowledge of inner ear function. This study was initiated to probe the feasibility of use of artificial piezoelectric transducer devices, both for the understanding of cochlear phenomena and as a possible cochlear implant. Aspects of feasibility of such an implant, the issues involved, the materials science challenges that need to be overcome to fabricate such a device, and results from initial in vivo experiments are discussed.

Influence of grain boundary, defect and internal stress states on properties of ferroelectric materials

Abstract Performance characteristics in ferroelectric based components, sensors and thin films for memory and storage applications are significantly influenced by aging instability in these perovskite materials. The aging changes can be attributed both to dc electrochemical effects, where migration of ionic charges lead to degradation in permittivity, loss and resistance, and to changes in domain structure and mobility. This latter reflects the presence within the microstructure of defect states, diffuse grain boundary structures and internal stresses. Aging in these materials is also dependent on microstructure, impurities, dopants and stoichiometry, as well as on ambient temperature and applied dc field stresses. Significant improvements in aging and performance characteristics have been achieved with selected dopant additions to BaTiO3, such as ZrO2, which control grain morphology, resulting in structural modifications and decreased domain mobility. Permittivity has been found to be highly dependent on...

Microstructural effects on conductivity in donor doped BaTiO3

The relationship between domain morphology, grain boundary structure, dopant distribution and the effect on conductivity in lightly donor doped barium titanate was characterized using SEM and TEM. The compositional distribution and the structural relationship between adjacent domains were determined using the convergent beam electron diffraction technique. Only 90° a-a type head-to-tail domain structures were found. HREM images of grain boundary regions in annealed samples, in which domains are common to the boundary (25-35% of total boundary), show lattice coherency and depict electronic pathways. Domain configuration near and across grain boundaries contribute to the observed resistivity differences found between samples of varying heat treatment histories. Conduction in the ferroelectric state is modeled as a percolation of conducting electrons along mutually interacting domain pathways. Annealed samples result in an inhomogeneous grain boundary, where only a limited number of grain boundaries and grai...

Microstructural dependence of the voltage sensitivity of the PTCR effect in donor doped barium titanate

Barium titanate positive temperature coefficient of resistivity (PTCR) ceramics are widely used in the electrical device industry. It is known that the PTCR effect shows a voltage sensitivity similar to voltage-dependent resistors. However, the microstructural aspects controlling this phenomenon have not been studied in detail. In this study, the voltage sensitivity of PTCR materials displaying a wide range of microstructures and properties was measured. Results show that though all materials tested showed considerable voltage sensitivity, the extent of the effect is strongly controlled by microstructural features. The presence of a grain boundary insulating second phase has the most significant effect. In these materials the room temperature resistivity is relatively insensitive to the voltage but the high temperature resistivity is significantly decreased. In the absence of such a second phase, resistivities are lowered by similar amounts at all temperatures, including a large decrease in the room temperature value. The difference in behavior between these two types of microstructures has been attributed to the difference in the number of available conducting pathways, and the resultant differences in local electric fields generated in the near-grain boundary regions. Thus, microstructural aspects play a pivotal role in determining the voltage sensitivity of these materials. An understanding of these relationships will enable engineering of better devices.

Structural phase transitions in donor doped BaTiO3 and effects on PTCR behavior

In this study, in situ transmission electron microscopy studies were used to examine and characterize domain formation and phase transformation mechanisms in donor doped, PTCR BaTiO3 materials. A significant and abrupt change in resistivity occurs as a result of a non-uniform boundary structure, the result of variability in strain state and non-uniform potential energy barrier along the grain boundary. In the highly stressed grain boundary regions, the ferroelectric phase transformation is very rapid with martensitic type character. During the phase transformation process, the sudden release of stress and change in structure, in the limited number of conducting regions along the grain boundary, were found to be the dominant factors behind the driving force for a sudden and abrupt change in resistance.

Vibrational and acoustic studies of bending mode piezoelectricity in millimeter-size polyvinylidene fluoride cantilevers

The dynamic bending piezoelectric properties of polyvinylidene fluoride cantilevers in the millimeter size range is reported. These devices are being investigated with the intention of developing a piezoelectric device based inner ear cochlear implant. The size restrictions and fluid environment of the inner ear place special requirements on a piezoelectric device, and it is essential to perform basic studies on sensor materials, deformation modes and device configurations to develop a successful implant. Results from both basic vibration tests and underwater acoustic measurements are presented. Experimental modal analysis reveals that millimeter length cantilevers exhibit three bending resonances under 1 kHz. The modal frequencies are sensitive functions of the length and thickness of the film, and are also affected substantially by the width of the cantilever and the nature of the electrode material. Further, all bending piezoelectric modes display high piezoelectric coupling coefficients in the range 0.2 - 0.35, and damping of < 2%. Experimental results are compared with a theoretical model of unimorph piezoelectric cantilever beams. Underwater acoustic measurements also reveal that single-cantilever devices in the millimeter length display acoustic sensitivities in the -195 to -210 dB range, in the 2 - 10 kHz regime. These sensitivities are comparable to commercial devices of larger size and more complex design. The viability of use of the conducting polymer polypyrrole as the electrode material in polymer piezoelectric sensors is also investigated. Results show that devices with polypyrrole electrodes are at least as sensitive as devices with metal electrodes, and these type all-polymer devices thus have great promise. The results presented in this paper can be used to design an appropriate sensory implant, as well as in other audio frequency applications.

Control and Optimization of Parameters in the ESEM at High Temperatures

High temperature processes play a decisive role in microstructure and property development of engineering materials. A fundamental understanding of the behavior of materials at elevated temperatures must precede control and optimization of these processes in advanced applications. Research in the form of ex-situ studies and theoretical modeling has been unsuccessful in developing a complete understanding of high temperature materials phenomenon. In-situ techniques have been plagued by experimental complexity, lack of resolution and thermal degradation effects. Further, insitu studies are typically performed in sterile environments questioning their ability to simulate real world environments and their interaction with the material. Environmental scanning electron microscopy combines the depth-of-field and high resolution advantages of conventional electron microscopy with novel gas based detection mechanisms, permitting observation of materials in a variety of gaseous atmospheres, at pressures as high as 20 Torr. Environmental scanning electron microscopy could provide researchers with a unique tool in dynamic high temperature investigations of materials and processes.

Temperature and voltage effects on microstructure and electrical behavior of donor modified BaTiO3

Abstract A high temperature NTC resistivity transition (Tc2) near 600°C has been found in donor-modified BaTiO3. Thermal treatment near Tc2 on developed PTCR materials significantly affects the resistivity (lowering the Prt and increasing the resistivity transition) and stabilizes the response with time and cycling. Thermal treatment plus applied voltage (Tc2-Poling) affects both electrical behavior and ferroelectric domain structure in samples which have not been annealed after sintering. Poling results in alignment of the domain structure in direction of the applied voltage, suppressing the resistivity transition, while typical PTCR behavior exists in the anti-poled direction. These properties are dependent on controlling domain structure and grain boundary coherency. It has been found that atmosphere and time during these treatments have little effect on bulk domain alignment and electrical behavior. Other resultant properties include improved aging and voltage-withstand characteristics.

In-situ Development and Study of Conducting Polymer Electrodes on PVDF Substrates for Electro-Acoustic Application in Cochlear Implants

Sensorineural hearing loss (profound deafness) is a result of the inability of the transductory structures in the cochlea (organ of Corti) to convert the mechanical displacement of the basilar membrane to neural signals. A class of devices known as Cochlear Implants can significantly enhance the hearing ability in these patients. Fundamentally different from the existing cochlear implant technology, are the totally implantable piezoelectric based devices that are being developed. The unit is completely self-contained, designed to work without any signal amplifiers or transmission elements, greatly simplifying the stimulation process, and enhancing the cosmetic appearance of the patient. These devices utilize the bending piezoelectric effect. Device design consists of arrays of elements of piezoelectric polymer films with conducting polymer electrodes embedded in a flexible substrate with the whole device coated with an insulating layer. The incoming mechanical energy (pressure waves) into the cochlea generates electrical charge by virtue of the piezoelectric effect of the film. The generated charge is fed to electrical connections evaporated on the substrate and is used to stimulate surviving nerve fibers in the cochlea. In certain environments where acoustic impedance matching is limited by size constraints and conducting liquid medium, the advantage of polymer based devices over ceramics and metal based devices, are their flexibility, low acoustic impedance, and high sensitivity. However, in order to utilize these useful properties, the electrode material is an important issue, since the conventionally used metal electrodes, have high acoustic impedance and also impose mechanical clamping on the soft polymer which can significantly reduce the electromechanical efficiency of the transducer. Due to its flexibility, strong coherent interfaces, and significantly improved acoustic transparency, such an all-polymer electroactive system is compared to a metal-polymer system of similar design and also compared to the current technology.