DIFFUSION (material transport by atoms motion, movement of atoms from site to site, requires sufficient vibration energy, can change structure in solids)– interfusion-atoms of one molecule diffuse into another, concentration changes w/ time. Self-diffusion– all atoms exchanging positions are the same type, no change in concentration, ex. metals. Steady-state diffusion– diffusion flux does not change with time. Fick’s 1stLaw– J=-D(ΔC/Δx), D-const of proportionality [m2/s}, C-concentration gradient [kg/m3], x-distance [m], J-diffusion flux [kg/m2-s]. Fick’s 2ndLaw– (dC/dt)=D(d2C/dx2)-> (Cx-Co)/(Cs-Co)=1-erf(x/2√(Dt)), Co=initial concentration, Cs=surface concentration, Cx=concentration at depth ‘x’, erf()=Gaussian error function. Erf(0.5)=0.52, x=sqrt(Dt), C~50%. Erf(1)=0.85, x=2sqrt(Dt), C~15%. Factors Affecting Diffusion– species & temperature. Diffusion coef increases when T rises, atomic size, structure (activation energy), phase change, cold work. D(t)=Doe^(Qd/(RT)), Do=temp-independent preexponential [m2/s], Qd=activation energy [J/mol], R=gas const., T=abs temp. metals– movement of atoms; ceramics– ion pairs move to preserve electroneutality (long-term), single point defects may not exist, cations diffuse quicker than anions; polymers–
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