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1、1.Kinetics of the nucleationA.Homogeneous nucleation:the probability that crystal nuclei are generated from the melt is the same everywhere.Lecture 2 Crystallization:kineticsexp()exp()VKmIBGRTGRTP DP:controlled by the nucleation barrierD:controlled by the atomic diffusionB.Heterogeneous nucleation:r
2、efers to the process that nuclei are formed at position such as the surfaces,interfaces,particle cracks,vessel walls and catalysts.The nucleation barrier is lower in heterogeneous nucleation than in homogeneous nucleation KKGGf*()f 2()(2cos)(1cos)4Relationships between and GK*cosf()Wetting0901001/2(
3、01/2)GKNonwetting901800(-1)1/21(1/21)GK GK*A.Rate of grain growth in ideal crystals2.Kinetics of grain growthLinear growth rate u uBH TT RTBGRT0.1exp()1exp()Discussion:a.T T0,T 0,G RT,uBH TRT02b.T RT，u Bu T,At higher temperatures,T,uGrain growth at high temperatures is mainly controlled by the trans
4、formation of the liquid to the crystals.It is beneficial to this process by increasing the undercooling,resulting in the increasing u.Grain growth at low temperatures is mainly controlled by the diffusion at the phase interface,which is slower at lower temperatures.A maximum value in the curve of u
5、versus undercoolingB.Grain growth in the real crystals U (T)2 phase phaset=0 V 0t=V=V-VV VVtThe relationship3.The rate of crystallizationInitial stage:VVVI u t1334Modifying:Avrami Christionn:Avrami index VVVI u t3 41exp13 VVktn1exp()Three stage:Induction stage(controlled by IV)Auto catalytic stage(c
6、ontrolled by u)The late stage4.Crystallization processThe relative positions of the curves of the nucleation and grain growth is determined by the characteristics of the system.Crystallization region:The overlapping region of the rate curves of nucleation and grain growth.If there are added nucleati
7、ng agents in the system,crystals can still be formed based on the agents in the metastable region at high temperatures.IvuTRatetimeTemperatureIvuTRateIf there is no overlapping between these two curves,crystallization region does not exist and the melt prefers forming glass to crystallizing.The addition of the nucleating agents is effective on reducing the nucleation barrier by heterogeneous nucleating instead of homogeneous nucleating.As a result,these two curves overlap each other and the crystallization become easier.