Gregory C. Stangle

Preparation of fine multicomponent oxide ceramic powder by a combustion synthesis process

An important, rather novel procedure for the synthesis of submicron crystalline multicomponent oxide ceramic powders has been studied. The synthesis of CuFe 2 O 4 powder, a ferrite material, has been used as a model system for understanding the synthesis process. The effect of the fuel content, powder packing, and surface heat loss has been investigated in terms of the maximum reaction temperature and reaction period, phase formation, and particle size and morphology. It has been shown that the maximum temperature and reaction period can be tailored to produce different phases. The submicron features of the synthesized powders are indicated by the large surface area values obtained from BET measurement.

Simultaneous momentum, heat and mass transfer with chemical reaction in a disordered porous medium: application to binder removal from a ceramic green body

A theoretical model has been developed to describe simultaneous momentum, heat, and mass transfer phenomena in disordered porous materials. The model can be applied to a wide variety of engineering-related fields, e.g., the drying and/or burnout of processing aids in the colloidal processing of advanced ceramic materials. Simulations based on the model predict the local temperature and mass distribution of the porous body as a function of time and position. This information can then be coupled with known mechanical properties of the body to predict internal stresses generated during removal of liquid from the body. The theoretical model has potential application to many engineering problems, e.g., the optimization of processing conditions in the design of an improved binder removal process. The model is evaluated using experimental data on binder removal from a ceramic green compact consisting of submicron a-Al,O, powder dispersed in a paraffin wax; the agreement between the simulated and experimental re&lti & good. -

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.}

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.

Sedimentation in flocculating colloidal suspensions

A combined experimental and theoretical investigation of the sedimentation of unstable colloidal ceramic suspensions has been performed. Suspensions containing submicron-sized α-Al 2 O 3 particles were prepared at various pH values in order to modify suspension stability. Particle volume fraction during sedimentation was determined as a function of position and time by gamma-ray densitometry. A population balance model was developed to account for various coagulation and decoagulation mechanisms that affect sedimentation behavior in flocculating suspensions. Model predictions were then compared with experimental measurements, in order to establish the validity of the theoretical model.

Ignition criteria for self-propagating combustion synthesis

Ignition criteria for gasless self-propagating combustion synthesis have been investigated through an ignition temperature analysis. The calculations were based on the dimensionless energy and mass continuity equations where the dimensionless parameters associated with the rate of local heat generation (β), activation energy (γ), the rate of surface heat loss by convection (ω), the rate of surface heat loss by radiation (δ), and the rate of reaction (λ) were incorporated. The relative significance of each of these parameters on the ignition of the self-propagating combustion reaction was evaluated to be γ > β > δ > ω. The ignition region, transition region, and nonignition region were identified for selected conditions. The correlations between ignition behavior and the material properties, the thermodynamic and kinetic properties, as well as the experimental conditions were discussed. The calculations indicated that only those systems with δH/C p > 1.5 × 10 3 (K) will give rise to a self-propagating combustion reaction without external energy input. Thus, this value can be used as an approximate guide for the existence of self-sustaining combustion. The calculations provide a sound basis toward interpreting experimental observations and developing a fundamental understanding of the process.

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.}

The mechanism and kinetics of the niobium-carbon reaction under self-propagating high-temperature synthesis-like conditions

The mechanism and kinetics of the chemical reaction between Nb(s) and C(s) under self-propagating high-temperature synthesis (SHS)-like (or combustion synthesis-like) conditions have been studied. Experiments were designed and conducted in order to produce a transport-resistance-free reaction between Nb and C under time-temperature conditions that are characteristic of the combustion synthesis process. To do so, a reaction couple, consisting of carbon and either a thin niobium foil or a fine niobium wire, was used. The effects of the temperature history and the formation of a liquid phase on the reaction were studied. In addition, theoretical experiments of the reaction were also conducted. The results showed that at high temperatures, layered niobium carbide phases formed in a direction that was parallel to the original carbon-niobium interface. As might be expected, local melting played a very significant role in the reactions. The mechanism and kinetics of these reactions provide a fundamental understanding of the manner and rate by which a powder-based Nb/C SHS process takes place, and, by extension, to a large, general class of solid-solid material synthesis processes that are based on the SHS (or combustion synthesis) process.