Laser ablation of nanoscale particles with 193 nm light
Laser interaction with nanoscale particles is distinct and different from laser-bulk material interaction, where a hot plasma is normally created. Here, we review our studies on 193 nm laser ablation of various nanoscale particles including NaCl, soot, polystyrene, and gold. The 20 ns laser beam with fluences up to 0.3 J/cm 2 irradiates nanoparticles in a gas stream at laser repetition rates from 10 to 100 Hz. The particle size distributions before and after irradiation are measured with a scanning mobility particle sizer (SMPS), and particle morphology is examined with electron microscopy. All the nanomaterials studied exhibit a similar disintegration pattern and similar particle formation characteristics. No broadband emission associated with particle heating or optical breakdown is observed. The nanoparticles formed after irradiation have a smaller mean diameter and an order of magnitude higher number concentration with a more spherical shape compared to the original particles. We use the photon-atom ratio (PAR) to interpret the laser-particle interaction energetics.
Spontaneous emission from C2(d 3Πg) and C3(A 1Πu) during laser irradiation of soot particles
In order to investigate the contribution of non-thermal processes to laser-induced vaporization of soot, we have recorded temporally and spectrally resolved emission from C2(d 3Πg–a 3Πu; Swan system) and C3(A 1Πu–X ; Swings system) following laser irradiation of soot at 532 and 1064 nm over a wide range of laser fluences. We compared the measured spectra with simulated spectra from C2-Swan and C3-Swings emission to gain new insight into the formation mechanism of the excited species. This comparison shows that the vibrational and rotational energy distributions of the nascent C2(d 3Πg) and C3(A 1Πu) depend strongly on laser wavelength and are not in local thermodynamic equilibrium with the soot. These results suggest non-thermal ejection of highly excited C2(d 3Πg) and C3(A 1Πu) from the particle surface. Multi-photon laser-induced fluorescence from thermally sublimed C2(a 3Πu) and C3(X ) is unlikely to be the source of the observed emission. This work provides fluence thresholds at 532 and 1064 nm for the appearance of spontaneous emission from C2(d 3Πg) and C3(A 1Πu) with sufficient intensity to be observable in the presence of laser-induced incandescence from soot and emphasizes the importance of non-thermal vaporization processes during laser irradiation of soot.
EFFECT OF SAMPLE PREPARATION ON WATER TREE RESISTANCE OF XLPE/SEBS BLENDS
The effects of an aromatic polymer, such as poly[styrene-b-(ethylene-co-butylene)b-styrene] (SEBS), and its preparation processes on water tree resistance, morphology, crystallinity, dielectric constant and dielectric loss tangent of crosslinked polyethylene (XLPE)/SEBS and XLPE/SEBS/ethylene vinyl acetate (EVA) blends were investigated. The XLPE/SEBS samples were characterized by means of differential scanning calorimetry, scanning electron microscopy, dielectric spectral analysis and the accelerated water treeing experiment. The results clearly showed that although SEBS was partially compatible with LDPE and it was dispersed evenly in XLPE matrix, the blends demonstrated excellent dielectric performances, such as dielectric constant and dissipation factor. The crystallinity of XLPE/SEBS blends was decreased with increase in SEBS content and it was further decreased with the addition of EVA. SEBS could effectively improve the water-tree resistance of XLPE and both SEBS and EVA were found to improve the water tree resistance of XLPE synergistically as expected. The differences in sample preparation processes could change the phase morphology and dielectric performance of the blends, and could also influence the dispersion of SEBS in XLPE matrix as well as the crystallinity of the blends. Different preparation processes led to different water tree resistance behaviours of the samples. The improvement of SEBS dispersion in XLPE matrix helped to improve the water tree resistance of XLPE.