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Grain Boundary Diffusion (grain_boundary + diffusion)

Distribution by Scientific Domains

Polymers and Materials Science	75%

Selected Abstracts

Plasticity and Grain Boundary Diffusion at Small Grain Sizes,

ADVANCED ENGINEERING MATERIALS, Issue 8 2010
Gerhard Wilde
Bulk nanostructured,or ultrafine-grained materials are often fabricated by severe plastic deformation to break down the grain size by dislocation accumulation. Underlying the often spectacular property enhancement that forms the basis for a wide range of potential applications is a modification of the volume fraction of the grain boundaries. Yet, along with the property enhancements, several important questions arise concerning the accommodation of external stresses if dislocation-based processes are not longer dominant at small grain sizes. One question concerns so-called "non-equilibrium" grain boundaries that have been postulated to form during severe deformation and that might be of importance not only for the property enhancement known already today, but also for spectacular applications in the context of, e.g., gas permeation or fast matter transport for self-repairing structures. This contribution addresses the underlying issues by combining quantitative microstructure analysis at high resolution with grain boundary diffusion measurements. [source]

Analysis of Nanocrystalline and Microcrystalline ZnO Sintering Using Master Sintering Curves

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2006
Kevin G. Ewsuk
Master sintering curves were constructed for dry-pressed compacts composed of either a nanocrystalline or a microcrystalline ZnO powder using constant heating rate dilatometry data and an experimentally determined apparent activation energy for densification of 268±25 and 296±21 kJ/mol, respectively. The calculated activation energies for densification are consistent with one another, and with values reported in the literature for ZnO densification by grain boundary diffusion. Grain boundary diffusion appears to be the single dominant mechanism controlling intermediate-stage densification in both the nanocrystalline and the microcrystalline ZnO during sintering from 65% to 90% of the theoretical density (TD). Based on both the consistency of the calculated activation energy as a function of density and the narrow dispersion of the sintering data about the master sintering curve (MSC) for the nanocrystalline ZnO, there is no evidence of either significantly enhanced surface diffusion or grain growth during sintering relative to the microcrystalline ZnO. The MSC constructed for the nanocrystalline ZnO was used to design time,temperature profiles to successfully achieve four different target sintered densities on the MSC, demonstrating the applicability of the MSC theory to nanocrystalline ceramic sintering. The most significant difference in sintering behavior between the two ZnO powders is the enhanced densification in the nanocrystalline ZnO powder at shorter times and lower temperatures. This difference is attributed to a scaling (i.e., particle size) effect. [source]

Diffusion-controlled growth of wollastonite rims between quartz and calcite: comparison between nature and experiment

JOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2002
R. Milke
Abstract Growth rates of wollastonite reaction rims between quartz and calcite were experimentally determined at 0.1 and 1 GPa and temperatures from 850 to 1200 °C. Rim growth follows a parabolic rate law indicating that this reaction is diffusion-controlled. From the rate constants, the D,,-values of the rate-limiting species were derived, i.e. the product of grain boundary diffusion coefficient D, and the effective grain boundary width, ,. In dry runs at 0.1 GPa, wollastonite grew exclusively on quartz surfaces. From volume considerations it is inferred that (D,CaO,)/(D,SiO2,),1.33, and that SiO2 diffusion controls rim growth. D,SiO2, increases from about 10,25 to 10,23 m3 s,1 as temperature increases from 850 to 1000 °C, yielding an apparent activation energy of 330±36 kJ mol,1. In runs at 1 GPa, performed in a piston-cylinder apparatus, there were always small amounts of water present. Here, wollastonite rims always overgrew calcite. Rims around calcite grains in quartz matrix are porous and their growth rates are controlled by a complex diffusion-advection mechanism. Rim growth on matrix calcite around quartz grains is controlled by grain boundary diffusion, but it is not clear whether CaO or SiO2 diffusion is rate-limiting. D,, increases from about 10,21 to 10,20 m3 s,1 as temperature increases from 1100 to 1200 °C. D,SiO2, or D,CaO, in rims on calcite is c. 10 times larger than D,SiO2, in dry rims at the same temperature. Growth structures of the experimentally produced rims are very similar to contact-metamorphic wollastonite rims between metachert bands and limestone in the Bufa del Diente aureole, Mexico, whereby noninfiltrated metacherts correspond to dry and brine-infiltrated metacherts to water-bearing experiments. However, the observed diffusivities were 4 to 5 orders of magnitude larger during contact-metamorphism as compared to our experimental results. [source]