We present evidence for breathing modes in magnetron sputtering plasmas: periodic axial variations of plasma parameters with characteristic frequencies between 10 and 100 kHz. A set of azimuthally distributed probes shows synchronous oscillations of the floating potential. They appear most clearly when considering the intermediate current regime in which the direction of azimuthal spoke motion changes. Breathing oscillations were found to be superimposed on azimuthal spoke motion. Depending on pressure and current, one can also find a regime of chaotic fluctuations and one of stable discharges, the latter at high current. A pressure-current phase diagram for the different situations is proposed.
High speed imaging of high power impulse magnetron sputtering discharges has revealed that ionization is localized in moving ionization zones but localization disappears at high currents for high yield targets. This offers an opportunity to study the effect ionization zones have on ion energies. We measure that ions have generally higher energies when ionization zones are present, supporting the concept that these zones are associated with moving potential humps. We propose that the disappearance of ionization zones is caused by an increased supply of atoms from the target which cools electrons and reduces depletion of atoms to be ionized.
Polymer colloids
arise in a variety of contexts ranging from synthetic
to natural systems. The structure of polymeric colloids is crucial
to their function and application. Hence, understanding the mechanism
of structure formation in polymer colloids is important to enabling
advances in their production and subsequent use as enabling materials
in new technologies. Here, we demonstrate how the specific pathway
from precipitation to vitrification dictates the resulting morphology
of colloids fabricated from polymer blends. Through continuum simulations,
free energy calculations, and experiments, we reveal how colloid structure
changes with the trajectory taken through the phase diagram. We demonstrate
that during solvent exchange, polymer–solvent phase separation
of a homogeneous condensate can precede polymer–polymer phase
separation for blends of polymers that possess some degree of miscibility.
For less-miscible, higher-molecular-weight blends, phase separation
and kinetic arrest compete to determine the final morphology. Such
an understanding of the pathways from precipitation to vitrification
is critical to designing functional structured polymer colloids.
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