Gerhard Wilde, in Physical Metallurgy (Fifth Edition), 2014 26.5.2.2 Effect of Grain-Boundary Migration on Diffusion Typically surface diffusion operates to redistribute the mass over the neck surface in a cooperative process. Without surface diffusion, a hillock forms, similar to an ant hill. The mass flows along the grain boundary to deposit on the neck surface. Then the interparticle grain boundaries, and internal grain boundaries in the particles, act as vacancy annihilation sites. In sintering by grain boundary diffusion, mass is removed from the interparticle grain boundary and deposited on the neck surface. Subsequent models applied refined concepts to shrinkage and densification. Since powder compacts are composed of large numbers of grain boundaries, it is reasonable to ignore differences in diffusion rates with orientation and assume average behavior.Ī theoretical description of grain boundary diffusion controlled sintering came from Coble. Indeed the grain boundary area peaks during intermediate stage sintering. As surface area is consumed and surface diffusion declines in importance, the simultaneous emergence of new grain boundaries increases the role of grain boundary diffusion. Grain boundary diffusion depends on grain boundary area per unit volume. Once the various processes are isolated, heating cycles are manipulated to maximize one event over another, such as by two-stage heating to minimized surface diffusion while enhancing grain boundary diffusion. Usually grain boundary diffusion acts in cooperation with other transport processes, and this often confuses studies seeking to find a single dominant process. Indeed, it is commonly considered to be the dominant process for metals. Įarly sintering studies learned that sintering was enabled by grain boundaries. Although the grain boundary is rather narrow, probably only five atoms wide, it is an active transport path that promotes densification. The defective character of the grain boundary allows mass flow along this interface with an activation energy that is usually intermediate between that for surface diffusion and volume diffusion. A grain boundary is essentially a collection of repeated misorientation steps.
Grain boundaries form within the neck between individual particles as a consequence of random grain contacts leading to misaligned crystals. It dominates the sintering of most metals and many compounds. Grain boundary diffusion is important for sintering densification. German, in Sintering: from Empirical Observations to Scientific Principles, 2014 Grain Boundary Diffusion Our results unravel the processing-induced GB behaviors in LWO-Mo, which pave the way towards further optimized processing for various types of functional membranes.Randall M. To minimize any Zr contamination, either a competent alternative for ZrO 2 or a careful introduction of certain secondary phases (SPs) was proposed. The segregated Zr took more than half of the W sites of the LWO-Mo, resulting in a strained LWO structure and locally concentrated oxygen vacancies. At atomic scale, structural and chemical analyses on these GB features were carried out quantitatively. Employing comprehensive transmission electron microscopy (TEM) techniques, the residual Zr was found to segregate to the grain boundaries (GBs) of the LWO-Mo, either as thin layers or as individual nanograins. However, how these milling balls affect the final LWO-Mo membranes, is still largely unknown. During several essential steps of the membrane processing, an intensive employment of ZrO 2 milling balls is usually inevitable. Molybdenum substituted lanthanum tungstate membranes (LWO-Mo) offer a good alternative for the separation of hydrogen from gas mixtures.
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