Oxide crystal growth using gas lasers

The use of CO₂-N₂-He gas lasers as a heat source in a floating-zone recrystallisation technique of crystal growth is described. So far, single crystals of Y₂O₃, CaZrO₃, MgAl₂O₄ and Al₂O₃ have been grown in this manner and factors controlling their quality are discussed. The problems associated with this technique of crystal growth are reviewed.     

Further developments in oxide crystal growth using gas lasers

It is shown that a DC energised CO₂-N₂-He laser system leads to a lower loss of material by evaporation than a comparable AC system when applied to oxide crystal growth. The reasons for this behaviour are discussed and a method for minimising evaporation using an AC system is described.     

Paint removal using lasers

Experiments to investigate the potential for practical laser graffiti-removal systems are reported. A universal engineering curve for the time needed for removal of paint from nonconductive substrates that was valid over a range of 10(7) in intensity was measured with a variety of lasers. Comparable times were measured for conductive substrates, when pulses shorter than the thermal conduction times were used. Analysis suggests that Q-switched Nd:YAG lasers may be the most efficient means for removing graffiti and other unwanted paint. An 1-m(2) area of paint 14 μm thick can be removed in approximately 10 min with a 50-Hz laser system of 15-W average power.     

Flow investigation in pumping loops of gas lasers

A simple method of flow visualization is shown to be effective for optimization of pumping loops of gas lasers. The results of an experimental study of flows in the closed-pumping loops models are reported.Research Institute of Nuclear Problems, Belorussian State University, Minsk, Belarus. Translated from Inzhenerno-Fizcheskii Zhurnal, Vol. 63, No. 1, pp. 38–43, July, 1992.     

Photocatalytic degradation of carbofuran using semiconductor oxides

The photocatalytic degradation of carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate) was investigated in an aqueous solution using Degussa P-25 TiO2 and ZnO as photocatalysts. The progress of degradation was monitored using TOC analyzer, HPLC, GC-MS and UV-vis spectrophotometer. The effects of various experimental parameters such as initial concentration of carbofuran, pH of the solution, catalyst loading and light intensity were systematically studied in order to achieve maximum degradation efficiency. The complete mineralization of carbofuran was confirmed by TOC analyzer. The degradation with ZnO showed less efficiency than TiO2. The formation of NO(3)(-) was identified and quantified using HPLC. In addition, four different intermediates formed during the degradation process were also identified and characterized by GC-MS. The mineralization rate was compared with lamps of wavelength 254 and 365 nm under similar conditions. The rate with 254 nm was observed to be very close to that of 365 nm.     

Analysis of dioxins by gas chromatography/resonance-enhanced multiphoton ionization/mass spectrometry using nanosecond and picosecond lasers

Dioxins in a soil sample were measured using gas chromatography/resonance-enhanced multiphoton ionization/time-of-flight mass spectrometry coupled with different types of laser sources. The fourth-harmonic emission (266 nm) of a nanosecond Nd:YAG laser (1 ns) provided low ionization efficiency, especially for highly chlorinated dioxins/dibenzofurans (CDDs/CDFs). The ionization efficiency was improved using the fourth-harmonic emission (266 nm) of a picosecond Nd:YAG laser (4 ps), due to shorter singlet excited-state lifetimes. It was, however, difficult to efficiently ionize hepta-CDD and octa-CDD/CDF, because of their shorter lifetimes, which were induced by stronger spin-orbit coupling that led to efficient relaxation of the excited molecule to triplet levels. The ionization efficiency was substantially improved using the fifth-harmonic emission (213 nm) of the picosecond Nd:YAG laser (4 ps), in which the analyte molecule that was relaxed to triplet levels was efficiently ionized using a photon with sufficient energy for ionization, although the pulse energy obtained at 213 nm was only one-third of the pulse energy obtained at 266 nm. The limits of detection achieved for 17 toxic polychlorinated dibenzo-p-dioxins/polychlorinated dibenzofurans (PCDDs/PCDFs) were 0.41-45 pg. The analytical instrument developed in the present study performed sufficiently well for the practical trace analysis of dioxins in soil samples.     

High-bandwidth laser-pulse generator using continuous-wave lasers

A laser-pulse generator for the transfer of continous-wave (cw) absolute radiometric calibrations to high-speed pulse photodetectors is described. The cw input lasers are modulated by a spinning polygon mirror in a ring cavity to produce high-speed, constant-amplitude, constant-shape laser pulses. Constantamplitude outputs range temporally from cw to 15-ns full-width at half-maximum (FWHM) Gaussian pulses. An all-reflective optical design permits operation of the device from the visible to the far-infrared spectral regions. Design and performance estimates for the pulse generator as well as experimental verification are presented.     

High-speed machining of martensitic stainless steel using PcBN

The performance of PcBN cutting tool during its application in the mass production of components made from AISI 440B stainless steel has been considered. The experimental tests have been performed at cutting speed ranging between 350–500 m/min at dry cutting conditions. The machining operations that have been explored included facing, turning, grooving and boring and the 3D topography of the machined surface are presented. The results show that good surface finish similar to grinding and dimensional accuracy can be achieved with PcBN tools.     

Modern combustion analysis techniques using lasers

Principall laser techniques for the analysis of elementary combustion processes are outlined, including the laser Doppler velocimeter for measuring flow velocities, the correlation spectro scopy method for measuring soot diameters and the coherent antiStokes Raman scattering method for temperature measurement. Research activities in these fields are describ ed, based on recent reports in the literature, and practical applications centring on engine combustion are tabulated.     

Synthesis of iron-copper alloy using electrical discharge machining

The present work investigates the novel route for the synthesis of Fe-Cu alloy using electric discharge machining (EDM). The Synthesis of Fe-Cu alloy is difficult by equilibrium processes because of their immiscible nature. An attempt was made to investigate the synthesis of Fe-Cu alloy by EDM process where the discharge can lead to a very high temperature and subsequent quenching to result in alloy formation. The electrode was made up of copper and die steel was used as workpiece. The characterization of generated debris was carried out by X-ray diffraction analysis, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM). The nano-phase granular particles of Fe-Cu alloy were confirmed by TEM and selected area diffraction pattern analysis. SEM morphology results reveal that the generated particles were both, spherical and non-spherical shape and size ranging between 50?nm and 30?µm. The EDS analysis indicates that the spherical particles were Fe-rich and non-spherical particles were Cu-rich.     

The role of morphology and crystallographic structure of metal oxides in response of conductometric-type gas sensors

This review paper discusses the influence of morphology and crystallographic structure on gas-sensing characteristics of metal oxide conductometric-type sensors. The effects of parameters such as film thickness, grain size, agglomeration, porosity, faceting, grain network, surface geometry, and film texture on the main analytical characteristics (absolute magnitude and selectivity of sensor response (S), response time (τ_res), recovery time (τ_rec), and temporal stability) of the gas sensor have been analyzed. A comparison of standard polycrystalline sensors and sensors based on one-dimension structures was conducted. It was concluded that the structural parameters of metal oxides are important factors for controlling response parameters of resistive type gas sensors. For example, it was shown that the decrease of thickness, grain size and degree of texture is the best way to decrease time constants of metal oxide sensors. However, it was concluded that there is not universal decision for simultaneous optimization all gas-sensing characteristics. We have to search for a compromise between various engineering approaches because adjusting one design feature may improve one performance metric but considerably degrade another.