6.8 Di ffus i on Mechan i sm .. Flick's law in microscop ic form 1. One dimensional diffusion F I: jump frequency period T: probability of atom jumping to nearest position
§6.8 Diffusion Mechanism ◆ Ⅰ.Flick's law in microscopic form 1. One dimensional diffusion. Γ: probability of atom jumping to nearest position 1 = : jump frequency period
21 2 =J12-J21=(n 1·6 J1=(C1-C22=mB3(、c=,dC 2 dx D=-62=-la
1 2 1 2 1 2 2 1 2 1 J = n J = n = 1 1 n C 2 2 2 1 2 1 D = = a ( ) 2 1 1 1 2 2 1 1 2 J = J − J = n − n x C C C J C C d d 2 ( ) 1 2 1 ( ) 2 1 2 1 2 2 1 1 2 = − − = − =
TWo dimensional diffusion D 4 3. Three dimensional diffusion simple cubic D=6 FCC(a-Fe D A[(
simple cubic: 2. Two dimensional diffusion. 3. Three dimensional diffusion. 2 4 1 D = a FCC( − Fe ): 2 2 12 1 ) 2 ( 12 4 a a D = = 2 2 1 6 1 D = a
4. For diffusion of interstitial atoms in bcc D=aTo=aTa For tetrahedral interstitial a For octahedral interstitial a' 40—4 6 D=(=) 6224
4. For diffusion of interstitial atoms in BCC 2 2 D = =a For tetrahedral interstitial For octahedral interstitial 4 1 = 4 0 = 2 2 24 1 ) 2 ( 6 1 6 2 1 a a D a = = = =
I. Di i on mechan i sms 1. Direct exchange of neighboring atoms Atom must acquire additional energy ag called the activation energy, before they jump from normal state to activated state(the saddle point) △G"=△H-TAS=△E+P△V*-TAS* This mechanism seems to be unrealistic for large Ag
◆ Ⅱ.Diffusion Mechanisms 1. 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 G G G = H −TS = E + PV −TS