size effect of nanomaterials
TRANSCRIPT
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Size Effect Of Nanomaterials
(Solid state Phase Transformation and Bandgap Variation)
Presented By_ To_
Sanjeeb Limbu(14305019) Dr.A.Subramania Sir
M.Tech.-Nanoscience & Technology Associate Professor
(FIRST YEAR) (CNST)
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Solid state Phase Transformation:
In the Universe matter exists in three states; solid, liquid and gaseous
state.
The phase of a solid substance is stable when the thermodynamic
variables like volume, pressure, temperature and energy are minimum.
If any of the thermodynamic variables is varied, the Gibb's free energy
of the system also changes continuously. If the variation in free energy
leads to change in structural details of a phase, a “Phase transformation
or phase transition" is said to occur.
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Phase diagram:
Phase diagrams are maps that give the relationship between
phases in equilibrium in a system as function of temperature
pressure and composition.
We can study the phase changes keeping one of the variable
as constant and varying the other two variables.
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One component system:
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Two component system:
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The Lead-Tin phase diagram:
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Size dependence of phase transition:
There are many examples phase transition occurring when
the particle dimension is reduces below a critical value.
CERAMICS: Monoclinic-tetragonal transition of Zirconia.
normally zirconia crystallises in monoclinic form.
but when size is reduced to 10 nm range it crystallises in
tetragonal structure.
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Size dependence of phase transition:
METALS: The bulk nickel crystallises in fcc structure.
when the grain size reduced beyond 4nm , the nickel
crystallises in hcp structure.
SEMICONDUCTORS: In materials such as CdS nano
particles shows wurtzite structure up to 5 nm, less than that it
shows zinc blende structure.
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Band gap variation:
Bandgap: The energy gap between valence and conduction
band is of fundamental importance for the properties of a solid.
When the size of a solid is reduced to the nanometer length
scale any change of the gap may significantly alter the
material’s physics and chemistry.
Therefore, the science and the technology of nanomaterials
needs to take into account a bandgap, which is different from
that of the bulk.
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Band gap variation:
The relation between particle size and bandgap is well
explained in quantum physics by the relation
E is energy Eigen value (bandgap)
A is particle size
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The figure below represents the variation of band gap in
bulk and different quantum dot structures
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Example: Calculated optical bandgap energies for silicon crystallites
dependent on their diameter
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The bandgap also differs for different nano morphologies
such as nanowires, dots and sheets.
The figure below represents the variation of bandgap of
different nano morphologies as a function of size.
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References:
Material Science and Engineering- V Raghavan
Materials Science- MS Vijaya and G Rangarajan
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QUERIES !!!!!!!!
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