清華材料科學(xué)基礎(chǔ)課件(英文)skja_30.ppt

1、2020/7/23,材料科學(xué)基礎(chǔ),材料科學(xué)基礎(chǔ)Fundamental of Materials,Prof: Tian Min Bo Tel: 62795426 ，62772851 E-mail: Department of Material Science and Engineering Tsinghua University. Beijing 100084,Lesson thirty,2020/7/23,材料科學(xué)基礎(chǔ),6.8 Diffusion Mechanism,.Flicks law in microscopic form One dimensional diffusion.,: pro

2、bability of atom jumping to nearest position,: jump frequency period,2020/7/23,材料科學(xué)基礎(chǔ),2020/7/23,材料科學(xué)基礎(chǔ),simple cubic:,Two dimensional diffusion. Three dimensional diffusion.,FCC( ):,2020/7/23,材料科學(xué)基礎(chǔ),For diffusion of interstitial atoms in BCC,For tetrahedral interstitial For octahedral interstitial,20

3、20/7/23,材料科學(xué)基礎(chǔ),.Diffusion Mechanisms Direct exchange of neighboring atoms.,Atom must acquire additional energy ,called the activation energy, before they jump from normal state to activated state (the saddle point). This mechanism seems to be unrealistic for large,2020/7/23,材料科學(xué)基礎(chǔ),2020/7/23,材料科學(xué)基礎(chǔ),I

4、nterstitial mechanism to be effective for diffusion in interstitial sol.sol. Vacancy mechanism.,Atoms jump directly in to nearby vacant site, being small. Different kind of atoms has different (chemical) affinity for vacancy.,2020/7/23,材料科學(xué)基礎(chǔ),2020/7/23,材料科學(xué)基礎(chǔ),.Calculation of D,P1: probability of ato

5、m acquiring additional,2020/7/23,材料科學(xué)基礎(chǔ),2020/7/23,材料科學(xué)基礎(chǔ),2020/7/23,材料科學(xué)基礎(chǔ),2020/7/23,材料科學(xué)基礎(chǔ),6.9 Empirical rule for diffusion,Factors influencing diffusion Temperature and the diffusion coefficient,D : The activation energy for diffusion of species under consideration. R : The gas constant T : The abs

6、olute temperature D0 : The pre-exponential term,2020/7/23,材料科學(xué)基礎(chǔ),2020/7/23,材料科學(xué)基礎(chǔ),Composition Size,Grain size, Defects Cold wording or irradiation Accelerate diffusion Up-hill diffusion may take place,2020/7/23,材料科學(xué)基礎(chǔ),2020/7/23,材料科學(xué)基礎(chǔ),Structure: D in closed packed structures is smaller than in “open

7、” structures. Diffusion is anisotropy in crystals other than cubic.,2020/7/23,材料科學(xué)基礎(chǔ),Third elements. No effect. Accelerate by forming fine grains or causing lattice distortion. Retard by forming compound. Curious effect.,For self-diffusion,2020/7/23,材料科學(xué)基礎(chǔ),Example,Design an iron membrane that will a

8、llow no more than 1% of the nitrogen to be lost through the membrane each hour, while allowing 90% of the hydrogen to pass through the membrane per hour.,2020/7/23,材料科學(xué)基礎(chǔ),The total number of nitrogen atoms in the container is:,Then:,2020/7/23,材料科學(xué)基礎(chǔ),(minimum thickness of the membrane),(maximum thickness),So iron membrane be