08 HOMO HETERO NUCLEATION-1 - Homogeneous Nucleation This...

Info icon This preview shows pages 1–8. Sign up to view the full content.

View Full Document Right Arrow Icon
Homogeneous Nucleation This section deals with the simplest nucleation event, namely the homogeneous nucleation of solid crystals during the freezing of a pure metal. The term homogeneous , as applied to nucleation events is explained. The concept of a critical size or radius, r * and a critical free energy to nucleation D G* , is introduced, Differentiattion between unstable clusters (or embryos) and stable nuclei is made Expressions for r * and D G* in terms of both volume free energy, D G v and undercooling, D T are derived. Also, mention on r * α µ 1/D T and D G * α µ 1/D T 2 made.
Image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
A driving force for solidification, D G v exists below the equilibrium melting temperature, T m and that this is approximately proportional to the degree of undercooling D T : (1) where D H v is the change in enthalpy of solidification. (convention that D G v and D H v are NEGATIVE below T m is used.)
Image of page 2
This driving force is opposed by the increase in energy due to the creation of a new solid- liquid interface.
Image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
By assuming that solid phase nucleates as spherical ‘clusters’ of radius, r , it is shown that the net (excess) free energy change for a single nucleus, D G ( r ) is given by: where gSL is the solid/liquid interfacial energy. (2)
Image of page 4
From this expression, the critical radius r * (defined as the radius at which D G ( r ) is maximum) is given by : (3)
Image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
The associated energy barrier to homogeneous nucleation, D G * is found by substituting r * into equation (2): (4) The temperature-dependence of these terms, i.e. that r * α µ 1/D T and D G * α µ 1/D T 2 is emphasised
Image of page 6
Heterogeneous Nucleation This section continues to look at the liquid-sold transformation, but now introduces the idea that nuclei can form at preferential sites (e.g. mould wall, impurities or catalysts, etc.). By so doing,
Image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 8
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern