Thermal effects, lasers

In this paper, the performanees of laser-ultrasound are estimated in order to identify lacks of weld penetration. The laser-ultrasonic technique is applied to cylindrical metallic strucmres (few mm thick) in a single-sided control. The results obtained for different materials (gold-nickel alloy and tantalum) are presented by B-sean views for which the control configuration is discussed with regard to the thermal effects at the laser impact. This testing is performed for different lacks of weld penetration (up to 0.5 mm for a thickness of 2 mm) even in the presence of the weld bead, which corresponds to an actual industrial problem. 

It should be noted that the high repetition rate ( 80MHz) laser may give a large deviation of nonlinear parameters from the expected values since the thermal effect may be superimposed on the Z-scan signals [16]. 

In the present work, a somewhat different approach was chosen in order to produce conducting polymers the basic idea was to start with a cheap material, like PVC, and try to remove all the hydrogen and chlorine atoms from the polymer chain. The purely carbon material thus obtained was expected to exhibit the electrical conductivity of a semimetal, while being insensitive to the atmospheric oxygen. In this paper, we report for the first time how PVC can be completely dehydrochlorinated by simple exposure to a powerful laser beam that combines both the photochemical and the thermal effects. 

The total optical path difference between the two arms of the interferometer, for a sample length of about 50 mm, is of the order of 10 mm or less, minimizing the systematic error due to laser frequency fluctuations. To reduce the thermal effects on the interferometer assembly, the interferometer support plate is stabilized to a temperature slightly higher than room temperature and insulated from air currents by a polystyrene foam shield. The temperature variation of the interferometer support is kept below 0.1 K. 

CIR-FTIR spectroscopy provides a direct technique for studying in situ hydrous metal oxide surfaces and molecules adsorbed on these surfaces (37). By itself, FTIR spectrometry is a well established technique which offers numerous advantages over dispersive (grating) IR spectrometry (1) improved accuracy in frequency measurements through the use of a HeNe laser (2) simultaneous frequency viewing (3) rapid, repetitive scanning which allows many spectra to be collected in a small time interval (4) miriimal thermal effects from IR beam and (5) no detection of sample IR emissions (38). 

Upon absorption of an intense laser pulse by a solid sample, temperature is known to rise [112] however, the cooling time should be a microsecond-order event. In the present study, the authors eliminated the possibility of thermal effects contributing to the photo-coloration by simply splitting the laser pulse and delaying the two pulses by up to a few nanoseconds. In this case, the overall two-... 

Thermal effects can be overcome by vapourizing samples with ultraviolet (UV) KrF and ArF lasers, thus making possible in situ oxygen isotope analysis of silicates (Wiechert and Hoefs 1995 Fiebig et al. 1999 Wiechert et al. 2002). 

Juodkazis S, Misawa H, Vanagas E, Li M (2006) Thermal effects and breakdown in laser microfabrication. In Online Proc LAMP2006 4th int congress on laser advanced materials processing, Kyoto, 16-19 May, 2006. JLPS, Osaka, pp 06-60... 

Surface diffusion can be studied with a wide variety of methods using both macroscopic and microscopic techniques of great diversity.98 Basically three methods can be used. One measures the time dependence of the concentration profile of diffusing atoms, one the time correlation of the concentration fluctuations, or the fluctuations of the number of diffusion atoms within a specified area, and one the mean square displacement, or the second moment, of a diffusing atom. When macroscopic techniques are used to study surface diffusion, diffusion parameters are usually derived from the rate of change of the shape of a sharply structured microscopic object, or from the rate of advancement of a sharply defined boundary of an adsorption layer, produced either by using a shadowed deposition method or by fast pulsed-laser thermal desorption of an area covered with an adsorbed species. The derived diffusion parameters really describe the overall effect of many different atomic steps, such as the formation of adatoms from kink sites, ledge sites... 

The editor has encountered numerous studies of laser ignition, usually of explosives. These demonstrate that the ignition is normally a thermal effect, caused by heating solid particles, not photochemical, and thus the MIE is not lower than other methods. This may not obtain if the light be of a frequency (visible or uv) sufficient to excite the early steps of the explosion reaction, as, for example, photodissociation of halogens to the atomic radicals which are the start of their explosive reactions with fuels. 

At higher frequencies, the laser-Raman effect affords, in principle, the possibility of detecting non-thermal excitation of vibrations. These would be found from a higher than thermal ratio of anti-Stokes to Stokes lines. The Raman effect in biological systems has recently been reviewed by Webb (21). Unfortunately only two relevant measurements have been carried out, so far, but both demonstrate non-thermal excitation. A difficulty affecting reproducibility arises here from the effect of a laser beam on a biological system as discussed in (21), in the case of individual cells. The best way to avoid this appears to be the use of a flow instrumentation so that each cell is subjected to the laser beam for a very short period only (22). 

Following this work, we investigated the possibility of polarization state control with a simple quarter waveplate (QWP). We were interested to determine whether a QWP could be employed in the scattered light beam to measure the SCP form of ROA. In this position there would be no thermal effects due to laser heating. Further, the QWP is a relatively thin object which probably would suffer only small thermal effects even if it were placed in the laser beam. We found that if the QWP were zeroth order, a 0+ A / 4 plate rather than an n+A / 4 plate where n is some integer, typically 30 or 40, representing the number of full wave retardations, the SCP form of ROA could be successfully measured [32]. 

During the last decade, there has been considerable interest in studying the interaction between ultraviolet radiation and polymers by the use of pulsed excimer laser (1-41. In fact, some attractive applications in microelectronics and surgery have been successfully implemented (5.), and further informations about the different mechanisms (photochemistry, thermal effect...) involved at the polymer surface have been invoked in order to elucidate their relative contributions. More recently, the attention has been focused on this type of polymer surface modifications to improve some surface properties like the adhesion in metallized polymer structures. 

J/cm laser fluence using 1064 nm excitation. The observation that the measured fluorescence lifetime using 1064 nm excitation was independent of laser fluence therefore provides evidence that thermal effects are unimportant in this work. 

Shebanova ON, Lazor P (2003) Raman study PF magnetite (Fe304), laser-induced thermal effects and oxidation. J Raman Spectrosc 34 845-852... 

SERS spectra in Ag hydrosols were recorded using a Jobin-Yvon HG2S monochromator equipped with a cooled RCA-C31034A photomultiplier and a data acquisition facility. To reduce the thermal effects due to the laser light, a defocused beam with low power (20 mW) was used. Raman data were obtained with exciting lines supplied by Ar"- and Kr"- lasers (406.7, 413.1, 457.9, 488.0, 514.5, 520.8, 568.2, 647.1,676.4 nm) or by He-Ne laser (632.8 nm). AU spectra were corrected to account for monochromator and photomultiplier efficiency. Power density measurements were performed with a power meter instrument (model 362 Scientech, Boulder, CO, USA) giving 5% accuracy in the 300-1,000 run spectral range. 

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