Nucleation is one of the processes that involves at the beginning of the certain process like freezing, melting, boiling, condensation and crystallization Homogeneous nucleation is known when the nucleation is occurring without any favorable nucleation sites. It requires supercooling or superheating for the medium and it happens in the inner of the uniform substance.
Nucleation and growth phenomena Content Nucleation Growth Nucleation Introduction Homogeneous nucleation Heterogeneous nucleation Primary nucleation Secondary nucleation Introduction Nucleation is one of the processes that involves at the beginning of the certain process like freezing, melting, boiling, condensation and crystallization Nucleation may involve The assembly of proper kinds of atom by diffusion The structural change into one or more unstable intermediate structures The formation of critical sized particle i.e Nuclei of the new solid phase Homogeneous nucleation Homogeneous nucleation is known when the nucleation is occurring without any favorable nucleation sites It requires supercooling or superheating for the medium and it happens in the inner of the uniform substance The total free energy change The critical free energy The critical radius Heterogeneous nucleation γ – surface/interface energy - The formation of nuclei wwithin its own melt with thehelp of foreign substances or substrates is known as heterogeneous nucleation - The phase transformation takes place with the help of impurities - If a metal is solidify on a foreign substrate it is essential that the surface of the substrate should be wet by a liquid metal Once this condition is satisfied, next the liquids solidify easily on the substrate - When angle of contact θ is small, interface between solid and substrate has a low surface energy - Hence, the total free energy for formation of stable nucleus is also decreased and critical radius of the nucleus will be smaller as per Heterogeneous nucleation - Critical nucleus size r* – critical nucleus size; Tm – melting point; ∆Hm – enthalpy of fusion per unit volume - Energy barrier towards nucleation ∆G* – energy barrier towards nucleation Nucleation rate - The undercooling for hetarogeneous nucleation is less than the homogeneous nucleation - Maximum nucleation rate for hetarogenerous nucleation occurs at a higher temperature than homogeneous nucleation Primary nucleation Primary nucleation describes the transition to a new phase that does not rely on the new phase already being present, either because it is the very first nucleus of that phase to form, or because the nucleus forms far from any pre-existing piece of the new phase Primary nucleation occurs in the absence of crystalline material of its own kind and is a stochastic process Secondary nucleation - Secondary nucleation is the birth of new crystals in the presence of parent crystals of the same substance - It is for instance the main source for new crystals in continuous crystallization processes in continuously stirred tanks - Secondary nucleation can occur through several mechanisms, including initial breeding, contact nucleation (also known as collision breeding), and shear breeding Growth phenomena Crystal Growth Mechanisms of growth Growth rate Growth phenomena - The growth step in a phase transformation begins once an embryo has exceeded the critical size, and becomes a stable nucleus - The growth process will cease in any region where particles of the new phase meet because here the transformation will have reached completion Crystal Growth A crystal is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions The action of crystal growth yields a crystalline solid whose atoms or molecules are close packed, with fixed positions in space relative to each other The crystalline state of matter is characterized by a distinct structural rigidity and very high resistance to deformation Mechanisms of growth The interface between a crystal and its vapor can be molecularly sharp at temperatures well below the melting point An ideal crystalline surface grows by the spreading of single layers, or equivalently, by the lateral advance of the growth steps bounding the layers Non-uniform lateral growth - The surface advances by the lateral motion of steps which are one interplanar spacing in height (or some integral multiple thereof) An element of surface undergoes no change and does not advance normal to itself except during the passage of a step, and then it advances by the step height - Non-uniform lateral growth is a geometrical motion of steps — as opposed to motion of the entire surface normal to itself - Alternatively, uniform normal growth is based on the time sequence of an element of surface In this mode, there is no motion or change except when a step passes via a continual change Uniform normal growth - Non-uniform lateral growth is a geometrical motion of steps — as opposed to motion of the entire surface normal to itself Alternatively, uniform normal growth is based on the time sequence of an element of surface Two criteria have been used to make this prediction: - Whether or not the surface is diffuse: a diffuse surface is one in which the change from one phase to another is continuous, occurring over several atomic planes - Whether or not the surface is singular: a singular surface is one in which the surface tension as a function of orientation has a pointed minimum Driving force - Consider next the necessary requirements for the appearance of lateral growth It is evident that the lateral growth mechanism will be found when any area in the surface can reach a metastable equilibrium in the presence of a driving force - Thus, for sufficiently large driving forces, the interface can move uniformly without the benefit of either a heterogeneous nucleation or screw dislocation mechanism What constitutes a sufficiently large driving force depends upon the diffuseness of the interface, so that for extremely diffuse interfaces, this critical driving force will be so small that any measurable driving force will exceed it Growth rate - Q: activation energy (tempt idependent) - C: Temperture idependent constant - The rate of transformation and the time required to complete the transformation to a certain degree are inversely proportional - For example, the time required to complete 50% transformation is represented as t at different temperatures below solidification temperature showsa minim that corresponds to the maximum of the overall transformation rate for listening