trunk/chuckDissertation/conclusion.tex

\section{Conclusions}
\label{sec:conclusion}

Our heat-transfer calculations have utilized the best current
estimates of the interfacial heat transfer coefficient (G) from recent
experiments.  Using reasonable values for thermal conductivity in both
the metallic particle and the surrounding solvent, we have obtained
cooling rates for laser-heated nanoparticles that are in excess of
10$^{13}$ K / s.  To test whether or not this cooling rate can form
glassy nanoparticles, we have performed a mixed molecular dynamics
simulation in which the atoms in contact with the solvent or capping
agent are evolved under Langevin dynamics while the remaining atoms
are evolved under Newtonian dynamics.  The effective solvent viscosity
($\eta$) is a free parameter which we have tuned so that the particles
in the simulation follow the same cooling curve as their experimental
counterparts.  From the local icosahedral ordering around the atoms in
the nanoparticles (particularly Copper atoms), we deduce that it is
likely that glassy nanobeads are created via laser heating of
bimetallic nanoparticles, particularly when the initial temperature of
the particles approaches the melting temperature of the bulk metal
alloy.

Improvements to our calculations would require: 1) explicit treatment
of the capping agent and solvent, 2) another radial region to handle
the heat transfer to the solvent vapor layer that is likely to form
immediately surrounding the hot
particle,\cite{Hu:2004lr,kotaidis:184702} and 3) larger particles in
the size range most easily studied via laser heating experiments.

The local icosahedral ordering we observed in these bimetallic
particles is centered almost completely around the copper atoms, and
this is likely due to the size mismatch leading to a more efficient
packing of 5-membered rings of silver around a central copper atom.
This size mismatch should be reflected in bulk calculations, and work
is ongoing in our lab to confirm this observation in bulk
glass-formers.

The physical properties (bulk modulus, frequency of the breathing
mode, and density) of glassy nanobeads should be somewhat different
from their crystalline counterparts.  However, observation of these
differences may require single-particle resolution of the ultrafast
vibrational spectrum of one particle both before and after the
crystallite has been converted into a glassy bead.

Revision:	3483
Committed:	Tue Jan 13 14:39:50 2009 UTC (15 years, 8 months ago) by chuckv
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#	User	Rev	Content
1	chuckv	3483	\section{Conclusions}
2			\label{sec:conclusion}
3
4			Our heat-transfer calculations have utilized the best current
5			estimates of the interfacial heat transfer coefficient (G) from recent
6			experiments. Using reasonable values for thermal conductivity in both
7			the metallic particle and the surrounding solvent, we have obtained
8			cooling rates for laser-heated nanoparticles that are in excess of
9			10$^{13}$ K / s. To test whether or not this cooling rate can form
10			glassy nanoparticles, we have performed a mixed molecular dynamics
11			simulation in which the atoms in contact with the solvent or capping
12			agent are evolved under Langevin dynamics while the remaining atoms
13			are evolved under Newtonian dynamics. The effective solvent viscosity
14			($\eta$) is a free parameter which we have tuned so that the particles
15			in the simulation follow the same cooling curve as their experimental
16			counterparts. From the local icosahedral ordering around the atoms in
17			the nanoparticles (particularly Copper atoms), we deduce that it is
18			likely that glassy nanobeads are created via laser heating of
19			bimetallic nanoparticles, particularly when the initial temperature of
20			the particles approaches the melting temperature of the bulk metal
21			alloy.
22
23			Improvements to our calculations would require: 1) explicit treatment
24			of the capping agent and solvent, 2) another radial region to handle
25			the heat transfer to the solvent vapor layer that is likely to form
26			immediately surrounding the hot
27			particle,\cite{Hu:2004lr,kotaidis:184702} and 3) larger particles in
28			the size range most easily studied via laser heating experiments.
29
30			The local icosahedral ordering we observed in these bimetallic
31			particles is centered almost completely around the copper atoms, and
32			this is likely due to the size mismatch leading to a more efficient
33			packing of 5-membered rings of silver around a central copper atom.
34			This size mismatch should be reflected in bulk calculations, and work
35			is ongoing in our lab to confirm this observation in bulk
36			glass-formers.
37
38			The physical properties (bulk modulus, frequency of the breathing
39			mode, and density) of glassy nanobeads should be somewhat different
40			from their crystalline counterparts. However, observation of these
41			differences may require single-particle resolution of the ultrafast
42			vibrational spectrum of one particle both before and after the
43			crystallite has been converted into a glassy bead.
44