We present two-dimensional measurements of the
laser-induced plasma development and shock wave evolution
in air. The breakdown is induced by a Q-switched
Nd:YAG laser (λ = 1064 nm) with a pulse duration of 4 ns.
To study these fast laser-induced phenomena, we have developed
a high-speed, two frame shadowgraph method. It
enables 2D visualization of a laser-induced event in two time
instances, which are delayed by an arbitrary time interval
in the range from 300 ps to 30 ns. The established method
is based on 30 ps, green (λ = 532 nm), and linearly polarized
laser pulse, which is split into two orthogonally polarized
illumination pulses for direct and delayed illumination
of the breakdown area. Exploiting polarization of the probe
pulses, we capture two temporally and spatially separated
frames with two CCD cameras. Special attention is given to
the subsequent data processing, especially to the minimization
of the systematic error due to alignment of both images,
and to the determination of 2D velocity distribution from the
captured image pairs.
Electrostatic, rotating bell (ESRB) application is one of the most important coating application techniques
for industries with demanding specifications for optical attractiveness of coatings, such as automotive. The
ESRB process involves production of droplets using a high-speed rotating bell, which are subsequently
transported to the substrate being coated via shaping air [1-3]. An electrical potential is applied between the
bell and the substrate which further helps droplet atomization and transport. This research investigates the
effects of inertia, centrifugal force, drag force, and electrostatic force on the atomization mechanism and
particle size distribution using an automotive OEM base coat formulation. Coating flow rate (CFR), shaping
air flow rate (SAFR), bell speed (BS), and electrostatic potential (EP) were used as primary parameters to
create various atomization conditions and particle size distributions. The atomization mechanism, ligament
formation, and particle size distribution were measured using high-speed laser shadowography and image
processing. The effects of governing forces and particle size generated on efficiency of droplet transfer to the
substrate and optical appearance of the coatings were studied to generate operating windows for optimum
process efficiency and appearance.
Various types of capacitively coupled radio frequency (CCRF) discharges are frequently used
for different applications ranging from chip and solar cell manufacturing to the creation of
biocompatible surfaces. In many of these discharges electron heating and electron dynamics
are not fully understood. A powerful diagnostic to study electron dynamics in CCRF
discharges is phase resolved optical emission spectroscopy (PROES). It is non-intrusive and
provides access to the dynamics of highly energetic electrons, which sustain the discharge via
ionization, with high spatial and temporal resolution within the RF period. Based on a time
dependent model of the excitation dynamics of specifically chosen rare gas levels PROES
provides access to plasma parameters such as the electron temperature, electron density and
electron energy distribution function (EEDF). In this work the method of PROES is reviewed
and some examples of its application are discussed. First, the generation of highly energetic
electron beams by the expanding sheath in geometrically symmetric as well as asymmetric
discharges and their effect on the EEDF are investigated. Second, the physical nature of the
frequency coupling in dual frequency discharges operated at substantially different frequencies
is discussed. Third, the generation of electric field reversals during sheath collapse in single
and dual frequency discharges is analysed. Then excitation dynamics in an electrically
asymmetric novel type of dual frequency discharge is studied. Finally, limitations of PROES
are discussed.
Stability of the Si-H bonds on the hydrogen-terminated Si(111) surface has been investigated by sum frequency generation (SFG) spectroscopy in air at room temperature. The SFG observation showed that the Si(111) surface is terminated by a monolayer of monohydride (Si-H) after etching in a concertrated ammonium fluoride (NH4F) solution. The number of Si-H bonds decreased with laser irradiation time and the abstraction rate of hydrogen atoms on Si increased with the increase of input energy of "visible" light. The Si-H bond under irradiation at 1064nm light was more stable than that at 532nm light with a given intensity. A small amount of water in air severely lowered the stability of Si-H bond because of a photoelectrochemical recation under laser irradiation.