CO2 Laser Heating Research Articles

Structure and Dynamics in Calcium Aluminate Liquids: High‐Temperature 27Al NMR and Raman Spectroscopy

27Al NMR spectra of CaO–Al2O3 liquids at 2500 K have been obtained using a recently developed technique that involves CO2 laser heating and sample levitation inside the NMR magnet. The spectra consist of single, narrow lines (100 to 200 Hz FWHM), indicating that rapid chemical exchange among molecular species occurs in the liquids. Isotropic chemical shifts vary linearly with composition, becoming more shielded with increasing Al2O3content. This is most likely due to an increase in the average nearest‐neighbor coordination of Al, as the proportion of higher coordinate Al species (AlO5 and AlO6) has been observed to increase with alumina content in glasses along this join. This is also in agreement with ion dynamics simulations of liquids along the CaO–Al2O3 join, which show the average Al coordination to range from 4 at low alumina content to 5 for Al2O3liquid. Correlation times for 27Al spin–lattice relaxation estimated from the NMR line widths are in agreement with shear relaxation times determined from experimental viscosity measurements. This may indicate that oxygen exchange between neighboring aluminate polyhedra is a key component in the mechanism for viscous flow. Viscosities determined from oxygen ion diffusion coefficients via ion dynamics simulation agree well with extrapolation of experimental data, in further support of this. We have also obtained high‐temperature Raman spectra of CaAl2O4 and Ca12Al14O33 glasses and liquids up to 1928 K. The liquid spectra contain structure similar to those of glasses, indicating that the aluminate liquids consist of AlOn polyhedral units on the vibrational time scale, rather than simple ionic species.

Thermometric study of CO2 laser heating and radiative cooling of n-butylbenzene ions

Working in the Fourier-transform ion cyclotron resonance ion trap, the competitive photodissociation of n-butylbenzene ions at 440 nm has been developed as a thermometric probe to monitor the instantaneous internal energy of the ion population. Using this probe, the heating of initially thermal ions by CO2 laser irradiation was followed as a function of time at several laser intensities. The IR multiphoton dissociation of the population was followed simultaneously. The ions were observed to heat up progressively during the induction period, during which no dissociation was observed. Dissociation commenced as the ions leveled off at a steady-state internal energy ∼0.8 eV, and dissociation then proceeded approximately as a first-order decay at constant average internal energy. After heating of the population, the CO2 laser was turned off and IR-radiative cooling of the ions was observed with a cooling rate constant of 0.97 s−1. The kinetics of heating, cooling, and dissociation followed the qualitative expectations from previous work, but quantitative modeling required some modification of the previously used simple harmonic photophysical picture. It seemed most likely that this ion possesses a modest inverse pumping bottleneck with up pumping slowing by a factor of 3 at high internal energies (above 0.8 eV). Enhanced radiative relaxation at internal energies above 1.0 eV can also bring the kinetic modeling into quantitative agreement with experiment.

GROWTH AND CHARACTERIZATION OF Bi2Sr2-xLaxCuOy CRYSTALS RELATED TO SUPERCONDUCTIVITY

Bi-2201 system oxide superconducting crystals with or without doped La are grown by Verneuil method and travelling melt zone method by CO2-laser heating. Single crystals though tiny reveal microstep line structures on their surfaces produced by lateral crystal growth mechanism. The fundamental step height is measured as 12A which is equal to the half unit cell dimension. Bi, Cu and O, respectively, distribute nearly uniformly in the crystal, while Sr and La do not display uniform distribution. As an interesting characterics the concentration of Sr and La is complementary between the two. This suggests the a replacement of Sr in the crystal lattice by La is taking place. Occasionally magnetic susceptibility and electrical resistivity proved the occurrence of superconductivity but the reproducibility is rather inferior.

Mechanism of Material Removal from Silicon Carbide by Carbon Dioxide Laser Heating

The mechanism of material removal from SiC by CO2 laser heating was studied using sintered and single‐crystal α‐SiC. Removal rate and width of the groove showed maxima when plotted as a function of translation speeds. Groove depth decreased as the translation speed of samples increased. Similar results were obtained if argon or air was used as gas assist, which indicated that the material removal mechanism is induced dissociation of SiC. Microstructure of the material deposited in and outside of the groove was studied by SEM. At low scanning speeds, columnar grains 10 to 50 μm long appeared. As the scanning speed increased, columnar grains became smaller and finally only irregular polycrystalline particles were observed. By using Raman spectroscopy, Auger analysis, and X‐ray diffraction, phases inside and outside the groove were identified as Si, β‐SiC, C, and SiO2. Columnar grains were identified as β‐SiC covered with thin layers of C, Si, and SiO2. Slow scanning speeds enhanced the growth of β‐SiC. At slow scanning speed, free silicon was always found in the grooves of lased single crystals but not in the grooves of lased sintered SiC. It can be concluded that the mechanism of material removal from silicon carbide by CO2 laser heating is a vaporization process, and material found in the groove and on the surface near the groove is formed by condensation from the vapor.

Laser micromachining of efficient fiber microlenses

We present results of the use of a CO(2) waveguide laser for the reproducible fabrication of efficient microlenses on single-mode optical fibers. Short intense laser pulses are used both to melt microlens tips to specific radii and to micromachine microlenses by ablative removal of small (~1-mum(2)) areas of glass from the fiber surface. CO(2) laser heating of fiber tips results in more consistent lens curvature than the electric arc commonly used in microlens fabrication. The microlenses formed using a laser microlathe, in which spinning fibers are shaped by simultaneous cutting and heating in the pulsed CO(2) laser beam, show excellent laser-fiber coupling with losses in the 1.5-3-dB range, an improvement of more than 2 dB over coupling with standard microlenses made by the etch and melt technique.

Melt Growth of High‐Critical‐Temperature Superconducting Fibers

To determine the intrinsic properties of the newly discovered high‐critical‐temperature oxide superconductors, their single crystals are necessary. Using the float‐zone technique with CO2 laser heating, coarse‐grained polycrystalline superconducting La‐Sr‐Cu‐O fibers have been grown directly from the melt of a sintered compact. Cu‐O is lost from the liquid and can be compensated by the addition of an excess of CuO in the compact. Superconducting properties (including Tc and Meissner effect), X‐ray diffraction, and EDAX indicate that the La/Sr stoichiometry is not maintained from the sintered compact to the melt‐grown material. Attempts to grow an analogous fiber in the Y‐Ba‐Cu‐O system were unsuccessful because the very high evaporation rates of the constituents precluded the maintenance of a molten zone. The results also indicate that patterning of circuits on thin films of the Y‐Ba‐Cu‐O superconductor can be achieved using laser irradiation.

Experimental determination of laser heated surface temperature distributions

A new scheme has been developed to measure laser-induced temperature distributions on solid surfaces, based on a first-order phase transition of an optically and thermally thin surface film where the transformed zone periphery marks the phase change isotherm of the film material. In order to test the technique, experimental measurements were made on a system for which analytical theoretical calculations and input parameters were available: steady-state 9.27-μm CO2 laser heating of semi-infinite SiC samples. The melting of polycrystalline films of NaCl (m.p. of 801 °C) and the eutectic mixture of NaCl-KCl (m.p. of 660 °C) spray deposited on the SiC surface served as the indicator. For a particular set of irradiation conditions, theoretical calculations were compared with experiment as a function of incident power.

Kinetic evolution during the laser/thermal preparation of Mn3O4from MnCO3

An investigation was performed of the decomposition of MnCO3 by both CW CO2 laser heating and controlled thermal heating in a derivatograph. The process was shown to have different kinetic evolutions in the two cases even though the final product was the same.

The Effect of Laser Heating on Optical Properties of Germania Doped Silica Optical Waveguides

ABSTRACTWe report the use of CO2 laser heating for inducing an increase in the refractive index of germania doped silica films. UV absorption spectroscopy and XPS analysis have confirmed that the index change is a result of GeO2 being reduced to GeO. The technique has been successfully used for the direct writing of integrated optical strip waveguides.

Thermally stimulated scattering in plasmas

A theory for stimulated scattering of a laser beam is formulated where the dominant nonlinearity is the ohmic heating of the plasma. The analysis is carried out with particular reference to experimental investigations of CO2 laser heating of a linear discharge plasma. In the conditions characterizing this experiment local heat conduction is of little importance and the dynamic evolution for the electron temperature is dominated by heating and energy exchange with the ion component. These features are incorporated in the analysis. The resulting set of equations gives a growth rate and characteristic scale size for the filaments, which agrees well with the experimental results.

Nitrification of zirconium by cw CO2 laser irradiation in ambient atmosphere

The preferential nitrification of zirconium can be obtained as a result of cw CO2 laser heating in air in some specific conditions, followed by rapid cooling in a jet of high purity argon.

Clean-Room and Co2-Laser Processing of Ultra High-Purity Al2O3

AbstractClean-room powder processing, furnace firing, and CO2-laser heating techniques to produce high-purity Al2O3 ceramics were applied to a new source of high-purity Al2O3 ( 〉13 ppm total and 〉8 ppm cation detected impurities). The chemical analyses of the material after each stage of processing and firing indicate that the procedures used in this work give no detectable contamination of the material. The microstructure of a CO2-laser ultra high-purity Al2O3 is illustrated. Densification is incomplete for this material. Calcination and deagglomeration were not optimized. Procedures, such as these are required to control the trace impurity contents in a fired ceramic material

Dynamics of CO2 laser heated solenoids

Experimental results are reported for long pulse CO2 laser production and heating of magnetically confined plasma columns. The plasma column is produced by an ionizing and heating wave propagation along the axis of a linear magnetic solenoid when laser radiation is focused into hydrogen gas contained inside the solenoid. The axial behavior is found to be reasonably well described by a "bleaching" wave model which predicts column length as a function of time. Radial behavior, following a transient ionization and expansion phase, is determined by a balance of ion thermal conduction and inverse bremsstrahlung laser heating. A finite ionization time is observed at the gas breakdown front. Energy balance measurements indicate that most of the incident laser energy is effectively coupled to ionization and heating of the plasma. Temperature measurements show good agreement with predictions of simple scaling models from which pressure balance gives a density value in agreement with experiment.

Perturbation of the phosphorescence decay of propynal upon tea CO2 laser irradiation

The phosphorescence decay of propynal in dilute vapour phase (0.7 torr) is greatly accelerated when subjected to CO2 laser heating. The such perturbed decay can be divided into three periods. The first is characterized by the appearance of a short-lived luminescence ( XXX 20 us), the second by a fast non-exponential phosphorescence decay and the third by a single-exponential decay. In relating the absorbed CO2 laser pulse energy to the induced temperature jump it is shown that the latter decay rate increases exponentially with temperature. Since self-quenching and photodecomposition are not significantly changed upon CO2 laser heating, the temperature dependent triplet decay of propynal is attributed to an intramolecular radiationless decay which becomes increasingly more efficient with rising temperature.

Two-dimensional hydrodynamic simulations of a laser heated gas target plasma

The CO2 laser heating and the resulting hydrodynamic expansion of a gas target plasma is examined by means of a two-dimensional computer simulation. These plasmas are found to exhibit characteristic scale lengths of the order of 1 mm. Electron temperatures vary up to about 200 eV whereas ion temperatures vary up to about 50 eV. Electron density is found to vary all the way from above critical in a shock front to well below critical in the laser heated region behind the shock.

Multiple-pass CO2 laser heating of a ϑ-pinch plasma

A 20-cm-long 3.6-cm-diam ϑ-pinch plasma is placed within a coupled optical cavity of a 40-J CO2 TEA laser as an active optical element. Ruby-laser scattering is used to measure the plasma temperature for the non-laser-heated, the single-pass, and the multiple-pass laser-heated plasmas. At times in the pinch cycle when holography shows a favorable density profile, multiple-pass heating equivalent to over five passes of the single-pass energy is observed.

CO2 laser heating of plasma columns in a steady solenoid field

A 250−J CO2 laser has been used to produce plasma columns 12−20 cm in length, 2−4 mm in diameter, from initially uniform neutral hydrogen in a steady solenoidal field up to 100 kG. Interferometric and spectroscopic measurements indicate densities and temperatures on the order of 100 eV and 5×1017 cm−3, respectively. The backward−going laser−driven wave normally present in laser breakdown has been eliminated by pulsed filling of the tube.

CO2 laser heating of a magnetized plasma column

A 280-cm3 volume of a magnetized plasma column has been heated by a 15-J CO2 TEA laser. The 2-eV plasma with peak electron densities of 3×1016 cm−3 was axially irradiated by the collimated unfocused laser beam. A measured temperature increase of approximately 0.5 eV was found to be in excellent agreement with classical absorption by inverse bremsstrahlung. A significant increase in heating was observed using a multipass heating scheme.

Application of the CO2 Laser to Thermal Properties Measurements in Biomaterials

The CO2 laser provides radiation that is strongly absorbed and only weakly back-scattered by high water-content materials, such as biological tissue. Several methods can be employed to obtain radial distributions of laser irradiance that are described by simple mathematical functions. It is therefore possible to use the CO2 laser to noninvasively establish an accurately known surface heat source. An effective thermal conductivity of the material can be determined from the temperature response due to laser heating. Thermal conductivity is reported for agar gel using CO2 laser heating and thermocouple temperature sensing.

CO2 Laser Heating of a Θ-Pinch Plasma by Inverse Bremsstrahlung and Induced Compton Processes

The heating of a Θ-pinch plasma by a pulsed CO2 laser is considered. In addition to the inverse bremsstrahlung the induced Compton effect is taken into account as a heating process. The influence of the pulse duration on the maximum temperature attained is studied. The results show that it is preferable to deliver the laser energy slowly. In addition, if one seeks to transfer the greatest fraction of laser energy to the plasma, the pulse shape becomes important. Tailored pulses providing an increase in the energy delivery rate when the power absorbed by bremsstrahlung diminishes will increase the contribution of the induced Compton effect to the heating.