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Chemically prepared zinc oxide powders are fabricated for the production of high aspect ratio varistor components. Colloidal processing was performed to reduce agglomerates to primary particles, form a high solids loading slurry, and prevent dopant migration. The milled and dispersed powder exhibited a viscoelastic to elastic behavioral transition at a volume loading of 43–46%. The origin of this transition was studied using acoustic spectroscopy, zeta potential measurements, and oscillatory rheology. The phenomenon occurs due to a volume fraction solids dependent reduction in the zeta potential of the solid phase. It is postulated to result from divalent ion binding within the polyelectrolyte dispersant chain and was mitigated using a polyethylene glycol plasticizing additive. This allowed for increased solids loading in the slurry and a green body fabrication study to be presented in our companion paper.
Chemically prepared zinc oxide powders were processed for the production of high aspect ratio varistor components (length/diameter >5). Near-net-shape casting methods including slip casting and agarose gelcasting were evaluated for effectiveness in achieving a uniform green microstructure that densifies to near theoretical values during sintering. The structure of the green parts was examined by mercury porisimetry. Agarose gelcasting produced green parts having low solids loading values and did not achieve high fired density. Isopressing the agarose cast parts after drying raised the fired density to greater than 95%, but the parts exhibited catastrophic shorting during electrical testing. Slip casting produced high green density parts, which exhibit high fired density values. The electrical characteristics of slip-cast parts are comparable with dry-pressed powder compacts.
Silver nanoparticles were incorporated in a dispersion of micron-sized silver spheres for testing as a low-temperature reactive component to form conductive particle networks. The development of conductivity depended on the arrangement of the micron-sized particle network, the amount of material reacted to form necks at the points of contact of micron-sized particles, and sintering of the particle network. Nanoparticles reacted to bond the micron-sized particles, but the stress issues involved in nanoparticle sintering can cause macroscopic cracking. Critical processing variables include the state of particle dispersion, the heating rate, and the fraction of nano-sized material.
Calculations of the Hamaker constants representing the van der Waals interactions between conductor, resistor and dielectric materials are performed using Lifshitz theory. The calculation of the parameters for the Ninham-Parsegian relationship for several non-aqueous liquids has been derived based on literature dielectric data. Discussion of the role of van der Waals forces in the dispersion of particles is given for understanding paste formulation. Experimental measurements of viscosity are presented to show the role of dispersant truncation of attractive van der Waals forces
Calculations of the Hamaker constants representing the van der Waals interactions between conductor, resistor and dielectric materials are performed using Lifshitz theory. The calculation of the parameters for the Ninham-Parsegian relationship for several non-aqueous liquids has been derived based on literature dielectric data. Discussion of the role of van der Waals forces in the dispersion of particles is given for understanding paste formulation. Experimental measurements of viscosity are presented to show the role of dispersant truncation of attractive van der Waals forces.
The “aging” characteristics of an acetic acid/methanol solvent-based lead zirconate titanate (PZT) precursor solution, prepared by the Inverted Mixing Order (IMO) process, have been studied for an extended period of time. The changes in film properties were characterized using optical microscopy, optical scattering, and ferroelectric testing. Films generated from the IMO process exhibit an increase in thickness as a function of solution age due to chemical “aging” (esterification) of the precursor solution. This increased thickness results in a decrease in the microstructural uniformity, which affects the electrical and optical properties. In order to understand and eventually control this phenomenon, we have quantified the “aging” of this solution using a variety of analytical methods, including 1H NMR spectroscopy, pH measurements, and Fourier transform infrared (FTIR) spectroscopy. It is of note that we have discovered a method that circumvents this “aging” problem by removal of the volatile material, forming an IMO powder which can be redissolved to produce high quality PZT thin films whenever desired.
Due to the importance of ferroelectric and high-permittivity perovskite thin films for a wide range of applications, there has been extensive research devoted to understanding the mechanisms responsible for the degradation observed with time, temperature, and/or external field stress. The three most important degradation phenomena for ferroelectric materials such as Pb(Zr, Ti)O3 (PZT) and BaTiO3 are ferroelectric fatigue, ferro-electric aging, and resistance degradation. Ferroelectric fatigue is the loss of switchable polarization by repeated polarization reversals. Ferroelectric aging is characterized by a spontaneous change with time in the polarization-voltage (P-V) response. Resistance degradation is a deterioration of the insulating properties of a dielectric under direct-current (dc) bias and elevated temperature.
These degradation processes ultimately limit the lifetime and reliability of devices that use ferroelectric and high-permittivity perovskite dielectrics. Fatigue and aging lead to reliability concerns for electronic (nonvolatile memories), piezo-electric, electro-optic, and pyroelectric applications. Likewise resistance degradation typically limits the lifetime of ceramic capacitors and high-dielectric constant thin films such as (Ba, Sr)TiO3, which is the principal candidate material for very high-density dynamic random-access memories (DRAMs).
Because of the importance of these degradation processes, it is critical to understand them and to develop methods of eliminating or mitigating their effects. By combining results from studies on thin films with ones on ceramics and single crystals, a consistent picture of the mechanisms involved in these degradation processes is emerging. In this article, we discuss these degradation mechanisms with particular emphasis on the interaction between ferroelectric domains and charge trapping and the role of oxygen vacancies and associated defect dipoles.
Pb(Zr,Ti)O3 (PZT) thin films are being developed for use in optical and electronic memory devices. To study ferroelectric switching behavior, we have produced relatively untextured PZT thin films on Si substrates. We have developed a method for using x-ray diffraction to observe domain switching in situ. Our study involved the use of a micro-diffractometer to monitor the switching behavior in relatively small (0.7mm diameter) electroded areas. Diffraction analyses were done while DC voltages were applied and removed, representing several places in the hysteresis loop. In particular, we were looking for relative intensity changes in the [h 00],[00l] diffraction peaks as a function of position in the hysteresis loop. Our study indicates that the 90° domain switching exhibited by bulk ferroelectrics, is very limited in films on Si when grain sizes are less than about 1 μm.
Switchable polarization can be significantly suppressed in ferroelectric (FE) materials by optical, thermal, and electrical processes. The thermal process can occur by either annealing the FE in a reducing environment or by heating it in air to 100°C while impressing a bias near the switching threshold. The optical process occurs while biasing the FE near the switching threshold and illuminating with bandgap light. And the electrical suppression effect occurs by subjecting the FE to repeated polarization reversals. Using electron paramagnetic resonance, polarization-vol tage measurements, and charge injection scenarios, we have been able to elucidate both electronic and ionic trapping effects that lead to a suppression in the amount of switchable polarization in FE materials. The relative roles of electronic and ionic effects in the same material can depend on the stress condition. For instance, in oxidized BaTiO3 crystals, optical and thermal suppressions occur by electronic domain pinning; electrical fatigue in the BaTiO3 crystals also appears to involve electronic charge trapping, however, it is suggested that these electronic traps are further stabilized by nearby ionic defects. In sol-gel PZT thin films with either Pt, RuO2, or La-Sr-Co-O electrodes it appears that the polarization suppression induced by electrical fatigue, a temperature/bias combination, or a light/bias combination are all primarily due to the trapping of electronic charge carriers to first order.
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