21-11-2012, 01:53 PM
Nucleation and Growth of Crystals
[attachment=40526]
Nucleation and Growth Rates Control Rc
Nucleation, the first step…
First process is for microscopic clusters (nuclei) of atoms or ions to form
Nuclei possess the beginnings of the structure of the crystal
Only limited diffusion is necessary
Thermodynamic driving force for crystallization must be present
NUCLEATION water as example
initiation of freezing
formation of small nuclei -center of crystals
homogeneous or heterogeneous
homogeneous -water -random accumulation of water molecules
heterogeneous -small particles present in the solution act as nuclei
Crystal Growth water as example
can only occur after nuclei are formed
exceed the crystal size
function of :
rate at which the water molecules reacts at the crystal surface
diffusion rate of water molecules from the unfrozen solution to the crystal surface
rate heat is removed
Fluid Processing
Molten glass is processed to maximize clarity or durability.
Pure raw materials are often produced to exacting sizes through precipitation from aqueous solutions.
Large single crystals are grown from pure molten solids for specialty applications.
Critical Cooling Rate
Rapidly cooled liquids skip crystallization and form random amorphous solids.
Calculating this rate involves minimizing both nucleation and grain growth.
Precipitation
The objective of precipitation is to remove salts, metals, or other contaminants present in liquid waste streams. Most often, this deals with the removal of metals at varying pH levels. Generally, the size of a precipitated particle increases if the reaction is allowed to occur with previously precipitated particle.
Explanation for the occurrence of Ostwald ripening
This is a spontaneous process that occurs because larger crystals are more energetically favored than smaller crystals. (This might be hard to believe seeing as how it seems far more common to get many small crystals than a few large ones, but there is a believable explanation.) . While the formation of many small crystals is kinetically favored, (i.e. they nucleate more easily) large crystals are thermodynamically favored. Thus, from a standpoint of kinetics, it is easier to nucleate many small crystals. However, small crystals have a larger surface area to volume ratio than large crystals. Molecules on the surface are energetically less stable than the ones already well ordered and packed in the interior. (Think of packing your vacation clothes in a suitcase. Which ones are more energetic? The ones in the middle or the ones you are packing in on top, trying to get them to fit?) Large crystals, with their greater volume to surface area ratio, represent a lower energy state. Thus, many small crystals will attain a lower energy state if transformed into large crystals and this is what we see in Ostwald ripening.
[attachment=40526]
Nucleation and Growth Rates Control Rc
Nucleation, the first step…
First process is for microscopic clusters (nuclei) of atoms or ions to form
Nuclei possess the beginnings of the structure of the crystal
Only limited diffusion is necessary
Thermodynamic driving force for crystallization must be present
NUCLEATION water as example
initiation of freezing
formation of small nuclei -center of crystals
homogeneous or heterogeneous
homogeneous -water -random accumulation of water molecules
heterogeneous -small particles present in the solution act as nuclei
Crystal Growth water as example
can only occur after nuclei are formed
exceed the crystal size
function of :
rate at which the water molecules reacts at the crystal surface
diffusion rate of water molecules from the unfrozen solution to the crystal surface
rate heat is removed
Fluid Processing
Molten glass is processed to maximize clarity or durability.
Pure raw materials are often produced to exacting sizes through precipitation from aqueous solutions.
Large single crystals are grown from pure molten solids for specialty applications.
Critical Cooling Rate
Rapidly cooled liquids skip crystallization and form random amorphous solids.
Calculating this rate involves minimizing both nucleation and grain growth.
Precipitation
The objective of precipitation is to remove salts, metals, or other contaminants present in liquid waste streams. Most often, this deals with the removal of metals at varying pH levels. Generally, the size of a precipitated particle increases if the reaction is allowed to occur with previously precipitated particle.
Explanation for the occurrence of Ostwald ripening
This is a spontaneous process that occurs because larger crystals are more energetically favored than smaller crystals. (This might be hard to believe seeing as how it seems far more common to get many small crystals than a few large ones, but there is a believable explanation.) . While the formation of many small crystals is kinetically favored, (i.e. they nucleate more easily) large crystals are thermodynamically favored. Thus, from a standpoint of kinetics, it is easier to nucleate many small crystals. However, small crystals have a larger surface area to volume ratio than large crystals. Molecules on the surface are energetically less stable than the ones already well ordered and packed in the interior. (Think of packing your vacation clothes in a suitcase. Which ones are more energetic? The ones in the middle or the ones you are packing in on top, trying to get them to fit?) Large crystals, with their greater volume to surface area ratio, represent a lower energy state. Thus, many small crystals will attain a lower energy state if transformed into large crystals and this is what we see in Ostwald ripening.