ENGR2000_diffusion - Introduction to Engineering Materials...

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Unformatted text preview: Introduction to Engineering Materials ENGR2000 Chapter 5: Diffusion Dr. Priya Thamburaj Goeser Practical Examples of Diffusion • The controlled diffusion of P or other dopants into Si wafers. • The controlled diffusion of oxygen and carbon dioxide through a membrane in a heart-lung machine permits surgeons to operate on the heart. • Carburization of steels. 5.1 Introduction • Many reactions & processes that are important in the treatment of materials rely on the transfer of mass either within a specific solid or from a liquid, gas or another solid phase. • Diffusion – a process of mass transport – involves the movement of one atomic species into another. Types of Diffusion • Inter-diffusion or Impurity diffusion – Atoms of one metal diffuse into another • Self-diffusion – Atoms in pure metals exchanging positions Measurement of the self-diffusion • Diffusion of radioactive isotopes in a matrix of the pure material (same atomic species). • With time the radioactive isotopes diffuse into the surrounding material. 5.2 Diffusion Mechanisms • Diffusion is the stepwise migration of atoms from lattice site to lattice site. • Conditions required for an atom to move as above: 1. An empty adjacent site 2. The atom must have sufficient energy to break bonds with its neighbor atoms and then cause some lattice distortion during the displacement. Vacancy Diffusion • Interchange of an atom from a normal lattice site to an adjacent vacant lattice site. Vacancy Diffusion • Diffusing atoms & vacancies exchange positions. • Diffusion of atoms in one direction corresponds to the motion of vacancies in the opposite direction. • Self-diffusion & inter-diffusion occur by this mechanism. Interstitial Diffusion • Atoms migrate from an interstitial position to a neighboring one that is empty. Interstitial Diffusion • Atoms must be small enough to fit into the interstitial positions. • Inter-diffusion occurs by this mechanism. Diffusion mechanisms in metals & metal alloys • Interstitial diffusion occurs more rapidly – Interstitial atoms are smaller & more mobile. – There are more empty interstitial positions than vacancies. 5.3 Steady-State Diffusion • Consider two adjacent atomic planes of a solid solution of A and B atoms. • The diffusion flux (# of A atoms moving from plane 1 to plane 2 per unit area per unit time) depends on… 1. concentration difference between the planes 2. jump distance 3. frequency at which atoms attempt to jump Fick’s First Law ( ) ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ Δ − = + RT Q D D dx dC D J x C C D...
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This note was uploaded on 10/24/2009 for the course ENGR 2000 taught by Professor Goeser during the Fall '08 term at Armstrong State University.

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ENGR2000_diffusion - Introduction to Engineering Materials...

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