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3.14/3.40J/22.71J Recap

Diffusion Process & Surface Evolution

Ting-Yun Huang

2012 Nov. 15th

Outline• Diffusion of 1-2-v system: Interdiffusivity

• Kirkendall Effect

• Different diffusion mechanisms

• Fast diffusion path and lattice diffusion

• Capillary force/Surface energy

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Diffusion of 1-2-v system: Interdiffusivity

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• Crystal frame

• From crystal frame to lab frame (observed)

• Interdiffusivity:

– Note that D is usually concentration dependent, thus the here only

applies to small region

Kirkendall Effect

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Direct exchange mechanism

Ring mechanism Kirkendall’s experimental setup. Use Mo as fictitious marker.

JOM.49 (6) (1997), pp. 15-19.

Kirkendall Effect

• Vacancy Mechanism

• Criticism from Mehl

• The measured Zn concentration varies, and the diffusivity is different in Zn and Cu.

• Nanoparticle system.

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JOM.49 (6) (1997), pp. 15-19.

Kirkendall Effect- Nanoscale

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• Hollow nanocrystals

• Kirkendall effect forming the

nano porous materials

• Applications: due to large surface

area, can be applied to nano-

reactor and catalysts, etc.

Yin et al., 304 (5671): 711-714, Science

Diffusion of different mechanisms

•

– Di: diffusivity, controlled by migration energy.

– xi: molar fraction of sites

– i: denotes the species, can be vacancy, interstitials.

– : the migration energy of the species

– : the formation energy of the species

• Vacancy mechanism/Interstitial mechanism

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Fast diffusion path and lattice diffusion

• Dsurface(T)>>DGBs(T)~Dcores(T)>>Dlattice(T)

– for 0<T<Tmelt

• Mixed diffusion mechanism

• Lattice + Grain boundaries

• Lattice + Dislocation core

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Surface energy/Young-Laplace Pressure

• : n dependent. (n: unit normal)

• Wulff construction/Faceting of crystals

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THANK YOU!Questions? Comments?

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