Mixed Electron Emission from Doped Pb(Zr,Ti)O₃ Ceramics: Microstructural Aspects

A mixed type electron emission, i.e., simultaneous ferroelectric and plasma emission, was observed with a negative driving pulse applied to doped Pb(Zr,Ti)O3 ceramics in the absence of any external potential on the electron collector. During these emission studies, significant microstructural changes on the emission surface were observed, and corresponded to the different emission modes. Erosion craters at the edge of the electrode and small particles near these craters reflected the formation of a dense plasma there. Comparatively, cavities, i.e., grain pullouts, accumulated on the bare ferroelectric surface, the frequency of which depended upon its distance from the grid. This phenomenon is proposed to be a result of fringing fields and the associated strain energy due to 90° domain switching, which could be seen as an evidence that ferroelectric emission occurred in these areas

High Energy Density Dielectrics for Symmetric Blumleins

Multilayer, tape cast ceramics are being developed for use in large area, high voltage devices in order to achieve high specific energy densities (\u3e106 J/m3) and physical size reduction. In particular, symmetric Blumleins are desired with the following properties: High voltage hold off (≥ 300 kV) High, nondispersive permittivity: ≈100 to 900 Ability to be fabricated into various shapes and sizes Surface flashover inhibition at the edgeAbility to be triggered by surface flashover switching The compositions being pursued are based on pure BaTiO3 dielectrics. Our approach is to add glass phase additions which result in not only near theoretical densities, but also allow for fabrication of more complex geometries through high temperature creep. Variations in the volume fraction and connectivity of the glassy phase allow for direct control of the permittivity as well as energy density. Structures up to 5 inches in diameter have been fabricated and pulse-tested at field strengths over 300 kV/cm. A strong dependence of breakdown strength and permittivity has been observed and correlated with microstructure and the glass composition. This paper presents the interactive effects of manipulation of these variables

Processing and Characterization of Samarium and Manganese Modified Lead Titanate Thin Film

Samarium and manganese modified lead titanate thin films were fabricated by spin-coating an amorphous citrate precursor. These films transformed into an oxide film upon heat treatment at 400 degrees Celcius or above. Relatively large area and crack-free thin films could be obtained by this process both easily and inexpensively. The rheological behavior of the precursor solution, as well as its thermal decomposition and phase development were studied by means of DSC/TGA and XRD. The thickness and grain size of the oxide film were examined by TEM and SEM

Surface Switching Characteristics of Variable Permittivity Dielectrics

Flashover voltage, lifetimes, and switch performance of insulators utilizing square thin and thick film electrodes were examined to determine the viability of using thin electrodes for reliable surface discharge switching. Gold, silver, and platinum were sputtered (0.25 µm) and screen printed (15 µm) onto Al2O3, TiO2, and modified BaTiO3 (MBT), then tested in air at 105 Pa, under vacuum (10-3/ torr), and while immersed in an insulating fluid, SF-2 (manufactured by 3M). For the measured range of 0.5 to 3 mm in air, the flashover voltage for all three insulators was found to have a linear dependence on the electrode separation distance with 15 µm thick screen printed electrodes and a square root dependence with 0.25 µm thick sputtered electrodes. Delay times of approximately 20 ns with a corresponding jitter of 6 ns were observed across all three insulators under triggered flashover. Insulators in air with sputtered electrodes had lifetimes of approximately 5 flashovers for dc flashover and 40 for triggered flashover. Screen printed TiO2 and MBT had dc lifetimes of approximately 10 flashovers in air, and 3 flashovers in vacuum and SF-2. Screen printed TiO2 and MBT had triggered lifetimes of greater than 200 flashovers in air, and \u3c3 flashovers in vacuum and SF-2. Screen printed Al2O3 had dc and triggered lifetimes of greater than 200 flashovers in air, vacuum and SF-2. Insulator failure during dc flashover was determined to be due to the formation of a conductive channel between the anode and cathode. Formation of the channel was attributed to insulator thermal and dielectric properties and the presence of vaporized electrode species in the gap region during flashover

Size-Induced Lattice Relaxation in CeO₂ Nanoparticles

Size-induced lattice relaxation was observed for nanoscale CeO2 single crystals with an average size from 4 to 60 nm. Results showed the finest crystallites exhibited no strain-induced line broadening, while high temperature annealing resulted in larger grain sizes and significant strains. The observed shift in the x-ray diffraction lattice parameters was assumed to be due to the formation of defects on the lattice, specifically oxygen vacancies. Modeling revealed that the oxygen vacancy concentration ([V••O]) was found to be ≈4 x 1020/cm3 for the 4 nm crystallites, and decreased two orders of magnitude for larger 60 nm single crystals

Design Considerations for 1-3 Composites Used in Transducers for Medical Ultrasonic Imaging

Ceramic polymer piezoelectric composites with 1-3 connectivity have become an important tool in the design and manufacture of thickness-mode transducers for medical diagnostic ultrasonic imaging. The authors compare annular array transducers made from ceramics alone to those made with these composites to demonstrate the advantages of composites. They examine some of the trade-offs involved in optimizing composite designs for this application. The effects of varying Young\u27s modulus and Poisson\u27s ratio of the polymer phase on the coupling coefficient and high-frequency lateral resonances of the composite are presente

A Study of the Sintering Mechanism of PZT-Based Piezoceramics

The densification behavior a PZT-based composition with additions of BiFeO3 as a flux was studied. Two different PZT powders were studied with varying initial particle sizes. Results clearly showed that BiFeO3 forms a low temperature liquid that aids in the densification behavior, with 3 mole% being optimum. Fine particle sizes substantially improved the density at low temperatures, with the net result that ceramics could be densified in air without the aid of a lead source. The corresponding electromechanical properties were typical of a hard PZT formulation

Room-Temperature Homogeneous Nucleation Synthesis and Thermal Stability of Nanometer Single Crystal CeO₂

Nanometer (about 4~5nm) CeO2 single crystals were first synthesized by room-temperature homogeneous nucleation; the size was determined by electron microscopy and specific surfaced area of the particles. Modeling revealed that the surface energy of as-synthesized nanometer single crystals was in the range of 2.8-3.7J/m2. Crystal growth mechanisms change over the temperature regimes, from boundary diffusion over low-temperature regime (Eα=0.16eV) to bulk diffusion (Eα=0.50eV) over high-temperature region

Method of Manufacture of Multiple-Element Piezoelectric Transducer

An improved method for fabrication of a multiple-element piezoelectric transducer and the transducer produced thereby. A green precursor tape is produced by doctor-blade tape-casting of a slurry containing lead zirconate-titanate (PZT) powder. After drying, individual strips of the tape are stacked between flat plates of previously sintered PZT, and sintered to form PZT strips; Pb from the previously sintered PZT plates makes up any Pb lost from the surfaces of the tape strips during sintering. The PZT strips are stacked interposed by layers of a thermoplastic polymer, and heated to a temperature above the melting point of the polymer, forming a laminate block. This block is then sliced perpendicular to the plane of the layers, forming slabs of alternate PZT and polymer layers; the slabs are then sliced perpendicular to the first slicing planes, forming strips of alternating PZT and polymer material. Electrodes are then added to complete the transducer assembly

Method for Preparation of Nanometer Cerium-Based Oxide Particles

The invention comprises novel undoped and doped nanometer-scale CeO2 particles as well as a novel semi-batch reactor method for directly synthesizing the novel particles at room temperature. The powders exhibited a surface area of approximately 170 m2/g with a particle size of about 3-5 nm, and are formed of single crystal particles that are of uniform size and shape. The particles’ surface area could be decreased down to 5 m2/g, which corresponds to a particle size of 100 nm, by thermal annealing at temperatures up to 1000° C. Control over the particle size, size distribution and state of agglomeration could be achieved through variation of the mixing conditions such as the feeding method, stirrer rate, amount of O2 gas that is bubbled through the reactor, the temperature the reaction is carried out at, as well as heating the final product at temperatures ranging from 150° to 1000° C