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Laser surface treatment of metals is a process in which heating or melting of the surface of the metal by laser irradiation is followed by a rapid cooling via self-quenching. This process can produce metallurgical structures with improved wear and corrosion resistance. For zirconium alloys, laser melting (glazing) has been used to modify the microstructure of the surface layers of Zircaloy-2 and -4 tubing, and Zr-2.5Nb pressure tube alloy (1-4) without affecting the bulk properties. A supersaturated [alpha][prime]-martensitic structure was produced by the laser-glazing process. In addition, laser glazing produced a rough surface finish, which was attributed to rippling effects during the rapid solidification. In the present work the authors have investigated laser surface heating because such a treatment is expected to produce a similar microstructure as laser glazing, but with a better surface finish. Laser irradiation could raise the specimen's surface temperature into the [beta]-phase region and the subsequent rapid cooling should be sufficient to achieve the martensitic phase transformation, without requiring surface melting. The microstructure produced by laser surface heating was characterized by transmission electron microscopy. The depth profiles of the laser surface heated layers were obtained using microindentation. This paper reports the results obtained in this investigationmore » of microstructural changes produced in the surface layers of Zr-2.5Nb alloy by laser surface heating.« less

A three dimensional numerical model has been developed to calculate the transient temperature field during the laser transformation hardening of metal surfaces. The model takes into account the finite dimensions of the substrate, the temperature dependence of the physical properties, the laser scanning velocity, and the surface heat loss through convection and radiation. The model is used to predict the temperature distribution in the substrate and the phase transformation depth resulting from the laser surface treatment. Good agreement is demonstrated with experimental results for the laser hardening of a zirconium alloy.MST/1867

A prototype on-line rotor temperature monitor based on phosphor decay-time measurements using optical fibers was tested on two large electrical generators.

A non-destructive remote sensing technique for obtaining quantitative thermal diffusivity depth profiles of near-surface inhomogeneities in opaque solids is reported. In this technique, the differece in the photothermal signal frequency response of a sample with an inhomogeneous surface layer compared with that of a homogeneous sample of the same material has been utilized. The signal was monitored by the radiometric emission from the sample surface. Applications to the inspection of laser-processed Zr-2.5 Nb alloy samples are presented. Profiles obtained from this method are consistent with the profiles obtained from destructive methods such as microhardness testing.

The influence of (NH4)2S treatment on the surface properties of GaP is presented for the first time. Changes in the chemical composition and Fermi level position are characterized using X-ray photoelectron spectroscopy. These measurements show that after surface treatment, the GaP surface is free of native oxides. A strong enhancement (~ 50 times) in the cathodoluminescent efficiency was observed for sulfur passivated samples. Schottky diode characteristics were used to reveal changes in the barrier height and interface state density due to surface treatment. Changes in carrier concentration were also found to be reflected in measured Raman spectra. The passivation mechanism is also briefly discussed.

A photoluminescence study was made of the green emission band in GaP:Zn,O grown by liquid-phase epitaxy. The spectra are shown to originate from donor-acceptor-pair transitions involving sulfur and zinc. This conclusion is supported by an analysis of the peak energy, peak shape, excitation-intensity dependence of the spectrum, and secondary-ion-mass-spectrometry data. A series of three LO(\ensuremath{\Gamma}) phonon replicas are identified. Temperature-dependence measurements of the no-phonon donor-acceptor-pair peak and the Zn-O bound-exciton-complex peak intensities reveal the important role that green transitions play in reducing the recombination rate of the primary red recombination photoluminescence.

Experimental data have been generated for the bead‐on‐plate keyhole welding of mild steel using the TEM 00 beam from a 1.5 kW CO2 laser and a range of material thickness from 0.64 to 2.6 mm. For each thickness, the maximum welding speed was measured for laser powers from 600 to 1500 W, using the criterion of full penetration along a 5 cm length. Analysis of the data has yielded a simple correlation between laser power (P,kW), penetration depth (d,cm) and welding speed (v,cm/s). When the specific welding energy per unit thickness (Pd−1v−1) is plotted against welding speed (v), the data are observed to lie on a single curve. An analytical expression of this relationship has been derived for speeds above 1 cm/s: P=3.83d + 0.69dv (1≤v

We report on radioluminescence (RL) studies of the wide band gap semiconductors GaP and GaInAsP exposed to tritium gas. All samples were prepared by liquid phase epitaxy. RL tests were performed as a function of tritium pressure, sample temperature, and time, enabling the limiting factors for developing high efficiency visible RL sources to be identified. The studies were complemented by photoluminescence (PL) and cathodoluminescence (CL) measurements. Reduction of surface recombination velocity was shown to be essential for obtaining low threshold CL response and improved PL efficiency. This factor resulted in tritium-activated RL visible to the naked eye. With appropriate materials optimization, these structures should be suitable for developing high efficiency RL devices.

We report on studies of the surface properties of InGaAsP epitaxial layers. A strong correlation was found between the near-surface band structure and the luminescent efficiencies. Sulfur treatment strongly influenced carrier recombination dynamics by altering the near-surface field. This profoundly effected electrical and optical properties of the layers.

Tritium is a radioisotope that emits beta radiation on its decay with a half life exceeding twelve years. The average emitted beta particle energy is ~6 keV. Typically for a semiconductor with bandgap on the order of ~1-2 eV for example, one incident beta particle could result in the production of ~1000 electron-hole pairs in the semiconductor. Beta decay thus represents an interesting generation source for the design of semiconductor-based light sources requiring no external energy supply. In this work we study the luminescence mechanisms resulting from such excitation in materials from the GaInAsP alloy system.

A low-temperature spectrally resolved study was made of the decay characteristics of green emission in liquid-phase epitaxially grown GaP:Zn,O. An analysis of the steady-state and transient characteristics of this emission is shown to be consistent with a proposed model. The dependence of the pair transition rate on the donor-acceptor pair separation is discussed with reference to a theoretical model based upon the overlap of donor and acceptor wave functions within the effective-mass approximation. A previously reported value of 12.2 A for the zinc acceptor (shallow impurity) effective Bohr radius in GaP is confirmed

Experimental data have been generated for the bead-on-plate keyhole welding of mild steel using the TEM∞ beam from a 1.5 kW CO2 laser and a range of material thicknesses from 0.64 to 2.6 mm. For each thickness, the maximum welding speed was measured for laser powers from 600 to 1500 W, using the criterion of full penetration along a 5 cm length. Analysis of the data has yielded a simple correlation between laser power (P,kW), penetration depth (d,cm) and welding speed (v,cm/s). An analytical expression of this relationship has been derived for speeds from 1 to 16 cm/s. Stainless steel samples yield similar results. This correlation is interpreted in terms of the minimum energy envelope or range for laser welding as proposed by other workers and also in terms of the Swift-Hook and Gick theory of laser welding. Data on the effect of polarization are also presented and discussed in terms of the present correlation. Above a critical welding speed the welding efficiency is enhanced when the polarization of the laser beam is aligned with the direction of welding.Experimental data have been generated for the bead-on-plate keyhole welding of mild steel using the TEM∞ beam from a 1.5 kW CO2 laser and a range of material thicknesses from 0.64 to 2.6 mm. For each thickness, the maximum welding speed was measured for laser powers from 600 to 1500 W, using the criterion of full penetration along a 5 cm length. Analysis of the data has yielded a simple correlation between laser power (P,kW), penetration depth (d,cm) and welding speed (v,cm/s). An analytical expression of this relationship has been derived for speeds from 1 to 16 cm/s. Stainless steel samples yield similar results. This correlation is interpreted in terms of the minimum energy envelope or range for laser welding as proposed by other workers and also in terms of the Swift-Hook and Gick theory of laser welding. Data on the effect of polarization are also presented and discussed in terms of the present correlation. Above a critical welding speed the welding efficiency is enhanced when the polarization of the...

Tunable infrared output has been produced by stimulated Raman scattering of visible dye laser radiation in hydrogen gas using a capillary waveguide to enhance conversion to the third Stokes ( S 3 ). With 568 nm pumping an output energy of 2 mJ is observed near 2 μ m, representing a 4% energy conversion efficiency into S 3 ). Higher wavelengths yield much lower energies (0.5 mJ at 3 μ m and 0.02 mJ at 4.55 μ m), reflecting the observed dependence of third Stokes output on the inverse of the fifth power of the generated wavelength.

Irradiation of natural isotopic abundance formaldehyde with a narrow linewidth ultraviolet laser has yielded high deuterium enrichments (>600 at 5 Torr, 303 K) at five wavelengths in the range 340 to 355 nm. The temperature and pressure dependence have been measured for both the enrichment and the selectivity, the latter being determined from high resolution spectra of deuterated and natural formaldehyde. In general, for the conditions investigated (293–343 K; 2–26 Torr), both selectivity and enrichment are decreasing functions of temperature and pressure, but in certain cases the selectivity is independent of pressure over a small range. The highest enrichment (1110) was measured at 2 Torr, 293 K, 352.061 nm. The results can be explained in terms of hot bands as well as Doppler broadening and pressure broadening of formaldehyde absorption bands. For all five high enrichment wavelengths, the enrichment is larger than the selectivity at low pressure (

The deuterium enrichment in the laser dissociation of formaldehyde at 345.6 nm has been measured as a function of pressure, temperature and laser linewidth. The highest enrichment (809) was obtained at 4 Torr, 20° C and 0.02 cm −1 . The data are analyzed in terms of a simple model.

An experimental study has been performed of the carbon-14 enrichment achievable in the ultraviolet laser dissociation of mixtures of14CH2O and12CH2O. The14C:12C enrichment factor in the CO product has been measured at 352.2 nm as a function of laser linewidth, formaldehyde temperature and pressure, and degree of carbon-14 depletion. The enrichment decreases with an increase in each of these parameters; the highest enrichment, 1190, was measured at 0.02 cm−1, 303 K, 1 Torr and a14C depletion of 10%. The results are assessed in terms of their application to radiocarbon dating, in particular for groundwater dating required for nuclear waste disposal, and to the enrichment of14C operating wastes extracted from nuclear reactors to a level suitable for commercial sale.

The infrared absorption spectrum of thick samples of solid CO2 (~4 mm path length) has been investigated in the frequency range 1100–1600 cm−1 at temperatures from 88 to 160 K. The (ν1, 2ν2) Fermi doublet is inactive because of the crystal symmetry but absorption is observed with maxima at the frequencies ν1 ± ai, 2ν2 ± ai(i = 1, 2, 3, 4). At 88 K the values of ai are 60, 75, 95, and 108 cm−1; at lower temperatures the maxima are sharper and the ai greater. These four frequencies correspond to maxima in the phonon frequency distribution. The agreement with existing theoretical calculations is not good; there is better agreement with the zone boundary frequencies determined for two symmetry directions by inelastic neutron scattering experiments. The present results are consistent with previously observed combinations with the allowed ν2 and ν3 internal modes.

The anomaly in the density variation of the integrated absorption of the pressure-induced v1 band of carbon dioxide is shown experimentally to be represented by a linear term in the density expansion of the intensity, which is important only in the lower density region (0–20 amagat). A calculation indicates that the linear term arises from the infrared-active absorption of the v1 band of C16O18O.

The ν1 band of gaseous carbon dioxide has been studied in pressure-induced absorption at temperatures of ~ 190, ~ 300, and ~ 470 K, over a density range from 0.5 to 300 amagat, and with path lengths from 0.007 to 56 m. The observed temperature variation of the binary absorption coefficient can be satisfactorily accounted for only by adding a quadrupole–quadrupole interaction term to the usual Lennard–Jones model for the inter-molecular potential. The band profile is in agreement with the theory of quadrupole-induced absorption. There is some increase in the intensity of the band near the critical point due to the divergence of the correlation length. A very marked increase in the intensity is possibly prevented by the "cancellation effect".

A tunable ultraviolet laser has been used to investigate the deuterium enrichment resulting from the photopredissociation of formaldehyde. An enrichment factor of 254 ± 30 has been measured at 345.6 nm starting from a 4 Torr sample of natural formaldehyde.

Tunable infrared output has been produced by the difference-frequency mixing of 532 nm and dye laser radiation from a Nd: YAG-pumped dye laser in a crystal of LiIO 3 . More than 100 μJ can be generated in a diffraction-limited beam from 3.5 to 4.2 μm and from 4.4 to 5.4 μm with a maximum energy of greater than 800 μJ at 3.8 μm.

A realistic modified valence force field for the bending modes of monochlorodiacetylene is presented and involves important changes to two previously published force fields.

The two components of the (ν1, 2ν2) Fermi doublet of gaseous CO2 in an absorption path length of 56 m at 192 °K show a complex structure. When the allowed C16O18O absorption and the pressure-induced CO2 absorption are removed by computational procedures, the residual spectrum consists of two similar symmetric patterns of five maxima. These are interpreted in terms of the rotation and vibration of (CO2)2 dimers held by quadrupole–quadrupole interaction in the locked T position at an intermolecular distance of 4.1 Å.

The use of phosphor thermometry in generator rotor temperature monitoring over the 60-150 degrees C range has been investigated, using measurements of the decay time of the visible emission stimulated in europiumdoped yttrium oxysulfide by UV laser excitation. A fiber-optic system for light delivery and collection has been designed and tested in laboratory measurements on a rotating disk. Measurements of the effect of oil and magnetic field on sensor performance are reported.