Homogeneous Nucleation
As liquid cools to temperatures below the equilibrium freezing
temperature, two factors combine to favor nucleation. First, since atoms
are losing their thermal energy, the probability of forming clusters to form
larger embryos increases. Second, the larger volume free energy difference
between the liquid and the solid reduces the critical size (r*) of the nucleus.
Homogeneous nucleation occurs when the undercooling becomes large
enough to cause the formation of a stable nucleus.
The size of the critical radius r* for homogeneous nucleation is given by
where _Hf is the latent heat of fusion per unit volume, Tm is the equilibrium
solidification temperature in kelvin, and ?T = (Tm - T) is the undercooling
when the liquid temperature is T. The latent heat of fusion represents the
heat given up during the liquid-to-solid transformation. As the
undercooling increases, the critical radius required for nucleation
decreases. Table 9-1 presents values for ?sl, ?Hf, and typical
undercooling s observed experimentally for homogeneous nucleation. The
following example shows how we can calculate the critical radius of the
nucleus for the solidification of coppe
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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