Walter A. Schulze

Review of literature on aging of dielectrics

Abstract This paper is a review of dielectric aging and a discussion of the possible mechanisms that may be responsible for the time dependent decrease in complex dielectric constant. The discussion is divided into three material systems: barium titanate, PZT and relaxors. Aging is considered as it affects the real portion of the dielectric constant, tanδ and P-E hysteresis loops.

A COMBUSTION SYNTHESIS PROCESS FOR SYNTHESIZING NANOCRYSTALLINE ZIRCONIA POWDERS

Materials with nanocrystalline features are expected to have improved or unique properties when compared to those of conventional materials. Methods for the practical and economical production of nanoparticles in large quantities are not presently available. A method based on combustion synthesis for preparing nanocrystalline powders was investigated in this work. Yttria-doped zirconia powders with an average crystalline size of 10 nm were synthesized. The characteristics of the powder (e.g., surface area and phase content) were found to depend strongly on the fuel content in the starting mixture and on the ignition temperature used in the process. The method is expected to be suitable for commercial fabrication of nanocrystalline multicomponent oxide ceramic powders.

Electrical properties of ultrafine-grained yttria-stabilized zirconia ceramics

Nanoparticles of yttria-doped tetragonal zirconia polycrystalline ceramics (Y-TZP) with an average crystallite size of less than 9 nm were prepared by a combustion synthesis process. Dense and fine-grained ({lt}200 nm) Y-TZP ceramics were obtained by fast-firing using temperatures lower than 1400{degree}C and dwell times of less than 2 min. Impedance spectroscopy was employed to measure conductivities of oxygen vacancies in the grain and the grain boundary of the fine-grained Y-TZP. The relationships between the concentration of the oxygen vacancies in the grain boundary and measurable physical parameters were determined semiquantitatively. The oxygen vacancy concentrations and activation energies for the oxygen-ion conduction in the grain and the grain boundary of the fine-grained Y-TZP were found to be independent of the average grain size in the average grain-size range of 90{endash}200 nm. These experimental results suggest that, in order to retain the abnormally high oxygen vacancy concentrations of the Y-TZP nanoparticles and thus enhance the oxygen-ion conductivity, it may be necessary to decrease the average grain size to approximately 10 nm.{copyright} {ital 1997 Materials Research Society.}

Grain-oriented fabrication of bismuth titanate ceramics and its electrical properties

The concept of grain-oriented fabrication in ceramics, which utilizes anisotropy either in morphology or some specific property of the particle, is reviewed. A fabrication method, which maximizes the grain orientation, is described for Bi/sub 4/Ti/sub 3/O/sub 12/. The process utilized plate-like morphology of Bi/sub 4/Ti/sub 3/O/sub 12/ particles and yielded a ceramic with an X-ray density of 95.4% and an unprecedented value of 100% for Lotgerings orientation factor in the direction of orientation. The dielectric and piezoelectric properties of this ceramic are described and compared to single crystal values. Complex impedance analysis of the ceramics was used to explain the dielectric relaxations at elevated temperatures. The use of grain-oriented fabrication as a practical technique for making polycrystalline ceramics with electrical properties close to those of single crystals (in the direction of orientation) is emphasized. The use of grain-oriented Bi/sub 4/Ti/sub 3/O/sub 12/ ceramics as a high-temperature piezoelectric transducer is suggested.<<ETX>>

Fabrication and characterization of ferroelectric PLZT 7/65/35 ceramic thin films and fibers

Integration of ferroelectrics in the current generation of electronic/photonic devices not only enhances the functional capabilities of these devices but also has potential for a variety of new applications. The fabrication of ceramic thin films and fibers is enabled by sol-gel process. This fabrication process is well-controlled, low-cost and compatible with existing fabrication technologies. This study focussed on the fabrication of PLZT 7/65/35 ceramic thin films and fibers from lead acetate hydrate, lanthanum acetate hydrate, zirconium n-propoxide and titanium n-propoxide utilizing acid catalysis. The transformations of the precursors to sol, sol to gel and gel to crystalline phase were characterized using spectroscopy and x-ray diffraction. The films were formed by spin-coating the hydrolyzed precursor sol on various substrates: conductors (Au, Al, Pt), semiconductors (Si [100], Si [111]) and insulators (fused SiO2, Al2O3, MgO). The fibers were drawn from the similar sols. These films and fibers were...

Axially distorted 3–3 piezoelectric composites for hydrophone applications

Abstract Application of reticulated ceramic technology to the fabrication of 3–3 lead zirconate-titanate (PZT)/Spurr epoxy composites has been investigated. Use of this technology provides a substantial improvement in manufacturability over other methods. Directional distortion of the ceramic structure to enhance the composites hydrostatic response is also possible. Composite fabrication is described and the effects of distortion on the piezoelectric properties are presented. The results for K33 and g33 are found to be comparable to other 3–3 PZT/epoxy composites while d33 and hydrostatic gh are reduced with density.

Preparation of nanocrystalline yttria-stabilized zirconia

Nanocrystalline powder with an average crystalline size of 8--12 nm, which was produced by a combustion synthesis process, was used to prepare dense, nanocrystalline articles. Green compacts of high green density were prepared by dry pressing and densified by a fast-firing process. During fast-firing, the dwell temperature significantly affected the final grain size and final density. On the other hand, the ranges of heating rates and dwell times that were used had a much less significant effect on the final density and final grain size. It was determined, however, that a high final density ({gt}99% {rho}{sub th}) and a very fine final average grain size ({lt}200 nm) can be simultaneously achieved under three different firing conditions. The high densification rates are, in part, a result of the minimal coarsening that the particles undergo when the sample is taken rapidly through the temperature regime in which surface diffusion predominates to the temperature regime in which the densification mechanisms of grain boundary and lattice diffusion predominate.

Grain-oriented ferroelectric bismuth titanate thin film prepared from acetate precursor

Abstract Bismuth titanate (Bi 4 Ti 3 O 12 ) thin films were fabricated by a spin coating deposition and rapid thermal processing (RTP) technique. The acetate-derived solution for deposition was synthesized by blending dissolved bismuth acetate in aqueous acetic acid and then adding titanium acetate. A series of electrically insulating, semiconducting and conducting substrates were evaluated for Bi 4 Ti 3 O 12 films deposition. X-ray diffraction indicated that the initial crystallization temperature of the Bi 4 Ti 3 O 12 films was 500 °C or less; a 700 °C crystallization treatment was used to obtain single phase films. The Bi 4 Ti 3 O 12 film crystallographic orientation is shown to depend on three factors: substrate type; number of coating layers; and thermal processing. While preferred c -direction orientation was observed for films deposited on silver foil substrates, preferred a -direction orientation was obtained for films deposited on both Si and Pt coated Si wafers. The films were dense, smooth, crack-free and had grain sizes ranging from 20 to 400 nm. Film thickness and refractive index were determined using a combination of ellipsometry, waveguide refractometry and TEM techniques. The refractive index is close to the value of single crystal BIT.

SYNTHESIS OF YTTRIA-STABILIZED ZIRCONIA NANOPARTICLES BY DECOMPOSITION OF METAL NITRATES COATED ON CARBON POWDER

Weakly agglomerated nanoparticles of yttria-stabilized zirconia (YSZ) were synthesized by a novel process which involved the decomposition of metal nitrates that had been coated on ultrafine carbon black powder, after which the carbon black was gasified. The use of ultrafine, high-surface-area carbon black powder apparently allowed the nanocrystalline oxide particles to form and remain separate from each other, after which the carbon black was gasified at a somewhat higher temperature. As a result, the degree of agglomeration was shown to be relatively low. The average crystallite size and the specific surface area of the as-synthesized YSZ nanoparticles were 5{approximately}6 nm and 130 m{sup 2}/g, respectively, for powder synthesized at 650{degree}C. The as-synthesized YSZ nanoparticles had a light brown color and were translucent, which differs distinctly from conventional YSZ particles which are typically white and opaque. The mechanism of the synthesis process was investigated, and indicated that the gasification temperature had a direct effect on the crystallite size of the as-synthesized YSZ nanoparticles. High-density and ultrafine-grained YSZ ceramic articles were prepared by fast-firing, using a dwell temperature of 1250{degree}C and a dwell time of two minutes or less. {copyright} {ital 1996 Materials Research Society.}

3-3 composite hydrophones from distorted reticulated ceramics

Application of reticulated ceramic technology to the fabrication of 3-3 lead zirconate-titanate (PZT)/epoxy composite hydrophones has been investigated. Directional distortion of the ceramic structure to enhance the composite hydrostatic response has also been studied. Composite fabrication is described and the effects of polymer type, distortion, volume percent PZT and poling field on the hydrophone properties are presented. In general, the results indicate that reticulated ceramic composites (RCCs) have properties which are comparable to other 3-3 PZT/epoxy composites. Composites which contained the stiffer Spurr epoxy significantly out performed those containing a softer Eccogel epoxy. In addition, a distorted structure and higher density improved hydrophone response. The novel use of reticulated ceramic technology provides substantial improvement in manufacturability over other methods and allows fabrication of sensitive, low density hydrophones.