635 research outputs found
Self-organized metal nanostructures through laser driven thermocapillary convection
When ultrathin metal films are subjected to multiple cycles of rapid melting
and resolidification by a ns pulsed laser, spatially correlated interfacial
nanostructures can result from a competition among several possible thin film
self-organizing processes. Here we investigate self-organization and the
ensuing length scales when Co films (1-8 nm thick) on SiO_{\text{2}} surfaces
are repeatedly and rapidly melted by non-uniform (interference) laser
irradiation. Pattern evolution produces nanowires, which eventually break-up
into nanoparticles exhibiting spatial order in the nearest neighbor spacing,
\lambda_{NN2}.The scaling behavior is consistent with pattern formation by
thermocapillary flow and a Rayleigh-like instability. For h_{0}\leq2 nm, a
hydrodynamic instability of a spinodally unstable film leads to the formation
of nanoparticles.Comment: 10 pages, 3 figure
Thermodynamic approach to the dewetting instability in ultrathin films
The fluid dynamics of the classical dewetting instability in ultrathin films
is a non-linear process. However, the physical manifestation of the instability
in terms of characteristic length and time scales can be described by a
linearized form of the initial conditions of the films's dynamics. Alternately,
the thermodynamic approach based on equating the rate of free energy decrease
to the viscous dissipation [de Gennes, C. R. Acad. Paris.v298, 1984] can give
similar information. Here we have evaluated dewetting in the presence of
thermocapillary forces arising from a film-thickness (h) dependent temperature.
Such a situation can be found during pulsed laser melting of ultrathin metal
films where nanoscale effects lead to a local h-dependent temperature. The
thermodynamic approach provides an analytical description of this
thermocapillary dewetting. The results of this approach agree with those from
linear theory and experimental observations provided the minimum value of
viscous dissipation is equated to the rate of free energy decrease. The flow
boundary condition that produces this minimum viscous dissipation is when the
film-substrate tangential stress is zero. The physical implication of this
finding is that the spontaneous dewetting instability follows the path of
minimum rate of energy loss.Comment: 8 pages, 3 figures. Under revie
Role of Rare-Earth Oxide Additives on Mechanical Properties and Oxidation Behavior of Si 3 N 4 /BN Fibrous Monolith Ceramics
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65415/1/j.1551-2916.2006.00911.x.pd
Care For Pastors: Learning From Clergy and Their Spouses
Pastors and their spouses face unique challenges because of the nature of pastoral work, and yet most manage these challenges successfully. Five studies are presented which help distinguish between intrapersonal, family, and community forms of care. Pastors rely heavily on intrapersonal forms of coping such as spiritual devotion, hobbies, exercise, and taking time away from work. The marriage relationship is also quite important for most clergy and spouses. Relationships outside the immediate family are not commonly identified as coping resources. Implications are discussed
Robust nanopatterning by laser-induced dewetting of metal nanofilms
We have observed nanopattern formation with robust and controllable spatial
ordering by laser-induced dewetting in nanoscopic metal films. Pattern
evolution in Co film of thickness 1\leq h\leq8 nm on SiO_{2} was achieved under
multiple pulse irradiation using a 9 ns pulse laser. Dewetting leads to the
formation of cellular patterns which evolve into polygons that eventually break
up into nanoparticles with monomodal size distribution and short range ordering
in nearest-neighbour spacing R. Spatial ordering was attributed to a
hydrodynamic thin film instability and resulted in a predictable variation of R
and particle diameter D with h. The length scales R and D were found to be
independent of the laser energy. These results suggest that spatially ordered
metal nanoparticles can be robustly assembled by laser-induced dewetting
Investigation of pulsed laser induced dewetting in nanoscopic metal films
Hydrodynamic pattern formation (PF) and dewetting resulting from pulsed laser
induced melting of nanoscopic metal films have been used to create spatially
ordered metal nanoparticle arrays with monomodal size distribution on
SiO_{\text{2}}/Si substrates. PF was investigated for film thickness h\leq7 nm
< laser absorption depth \sim11 nm and different sets of laser parameters,
including energy density E and the irradiation time, as measured by the number
of pulses n. PF was only observed to occur for E\geq E_{m}, where E_{m} denotes
the h-dependent threshold energy required to melt the film. Even at such small
length scales, theoretical predictions for E_{m} obtained from a
continuum-level lumped parameter heat transfer model for the film temperature,
coupled with the 1-D transient heat equation for the substrate phase, were
consistent with experimental observations provided that the thickness
dependence of the reflectivity of the metal-substrate bilayer was incorporated
into the analysis. The spacing between the nanoparticles and the particle
diameter were found to increase as h^{2} and h^{5/3} respectively, which is
consistent with the predictions of the thin film hydrodynamic (TFH) dewetting
theory. These results suggest that fast thermal processing can lead to novel
pattern formation, including quenching of a wide range of length scales and
morphologies.Comment: 36 pages, 11 figures, 1 tabl
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