Ba Vapor Research Articles

Thermodynamic analyses of thermal evaporation of BaSi2

Thermal evaporation is a useful method for the deposition of high-quality photovoltaic BaSi2 films. The elemental processes of BaSi2 film formation are, however, complicated due to vapor composition change during BaSi2 evaporation. In this study, we thermodynamically calculate the vapor composition for quantitative explanation of the evaporation process. The Ba and Si vapor fluxes are calculated from relative partial molar enthalpies of Ba and Si in Ba–Si melt. Calculations show a tendency of decreasing Ba vapor flux and increasing Si flux with decreasing mole fraction of Ba in Ba–Si melt, by which the vapor composition change from being Ba-rich to being Si-rich during thermal evaporation of BaSi2 is successfully explained. The calculation results are discussed by comparing with the depth profiles of evaporated films deposited at room temperature and deposition rate. The present results would be useful as a theoretical basis of thermal evaporation of BaSi2.

On availability of the optimal regimes in a Knudsen Cs-Ba thermionic converter at high emitter temperatures

A thermionic energy converter (TIC) functioning in collisionless regime at high emitter temperatures demonstrates very high output performance. In practice, such a regime has been realized at moderate temperatures (up to 2300 K) when its inter-electrode gap is filled with Cs and Ba vapors. Changing the Ba vapor pressure, one can vary the emitter work function and obtain optimum output performances, moreover maximum output power being available with any emitter material. The higher the emitter temperature, the more pronounced a shift of this maximum is toward higher Ba pressure, and at high temperature a TIC happens to be working in the collision regime before the maximum power point. We have analysed some experimental data in the literature when studying the emission parameters of refractory metals in the presence of the Ba vapor as well as in the Cs-Ba TIC experiments. Through our analysis, as well as results on the TIC optimization, the Ba optimum pressure is evaluated to obtain maximum power at the high emitter temperature for a set of refractory metals. We conclude that the maximum output performance of the Knudsen TIC with W and Re emitters can be reached up to the emitter temperature as high as 2600 K.

Synthesis and characterization of barium silicide (BaSi2) nanowire arrays for potential solar applications.

In order to utilize nanostructured materials for potential solar and other energy-harvesting applications, scalable synthetic techniques for these materials must be developed. Herein we use a vapor phase conversion approach to synthesize nanowire (NW) arrays of semiconducting barium silicide (BaSi2) in high yield for the first time for potential solar applications. Dense arrays of silicon NWs obtained by metal-assisted chemical etching were converted to single-crystalline BaSi2 NW arrays by reacting with Ba vapor at about 930 °C. Structural characterization by X-ray diffraction and high-resolution transmission electron microscopy confirm that the converted NWs are single-crystalline BaSi2. The optimal conversion reaction conditions allow the phase-pure synthesis of BaSi2 NWs that maintain the original NW morphology, and tuning the reaction parameters led to a controllable synthesis of BaSi2 films on silicon substrates. The optical bandgap and electrochemical measurements of these BaSi2 NWs reveal a bandgap and carrier concentrations comparable to previously reported values for BaSi2 thin films.

Collision induced phenomena in the autoionizing spectrum of Ba and Sr

Broadening and shift of spectral lines in dense Ba and Sr vapor have been studied below and above their first ionization limits. The latter are well-known autoionizing spectral lines that exhibit many different spectral features. However, at larger pressures of noble gases some previously unobserved or very weak spectral phenomena become visible and their studies might be also linked to energy conversion purposes. Originally, we used nanosecond laser which was scanned in a certain spectral region where the two-photon excitation of autoionization lines occurred. This pulsed laser was overlapped by a mode-locked femtosecond laser and a very sensitive thermionic detection was used to observe autoionizing spectral lines. We also presented our preliminary results with hot strontium vapor excited to the resonance level at 460.7 nm.

Linewidth oscillations in the 5d3/2nd autoionizing series of barium

The first autoionization region of barium was studied using a single-color two-photon excitation scheme. The resonance energies and linewidths of barium 5d3/2nd 1D0, 2 series members were obtained by fitting each spectral line with a Beutler-Fano profile function. The quantum defects of the series were calculated using the obtained resonance energies. The behavior of the linewidth and quantum defect of the series members for n = 8 to 30 was then studied to find out about different interactions involving this autoionizing series such as inter-series coupling. Atomic Ba vapor was produced in a hot-pipe oven, and a tunable dye laser pumped by an excimer laser was used to excite autoionizing Rydberg states. We observed spectral linewidth oscillations in the range of the main quantum numbers 8 to 24, and explained this phenomenon based on interconfiguration interaction among autoionizing series of Ba .

Collision induced modification of spectral lines in the first autoionization region of barium

The spectrum of barium above the first ionization potential is significantly modified by collisions with rare gas atoms. Atomic Ba vapor was produced in a heat-pipe oven. The two ground state electrons of Ba were excited simultaneously using non-resonant two-photon absorption to populate the 5d 3/2 n d 3/2, J = 0, 2 autoionizing states. The spectral region corresponding to this series was studied when Ba atoms were unperturbed, and when they were perturbed by the noble gases Ar, Kr, and Xe. Besides the expected shift and broadening of autoionizing series due to collisions with rare gases, we observed new spectral bands and significant changes to the shape of the existing lines.

Nonlinear magneto-optical effects in Ba vapor

We report experimental measurements of linear and nonlinear magneto-optical polarization rotation on an intercombination transition of Ba vapor (λ = 791.1 nm). We observed a maximum polarization rotation angle in Faraday configuration of 15 mrad, accompanied by reduction of absorption.

Radiation trapping effects in Cu, Mn, Ca, Sr, and Ba vapor lasers

The radiation trapping effect was investigated in Cu, Mn, Ca, Sr, and Ba vapor lasers with a cylinder discharge tube. Compared with the negligible impact induced by the gas temperature, the lower level population density of radiation trapping was found to dominate the effective spontaneous emission probability. Meanwhile a tube radius scanning from 1 mm to 20 mm revealed that the threshold parameters of radiation trapping effect were affected little by the tube radius when the radius changed from 20 mm to 5 mm, but they varied visibly when the tube radius decreased from 5 mm to 1 mm. Furthermore, the threshold temperature and the lower level population density of the radiation trapping effect in six resonant lines in Cu, Mn, Ca, Sr and Ba vapor lasers were calculated, and the threshold temperature was found to be close to the initial lasing temperature of the corresponding resonance level to metastable level transition.

Radiometric efficiency of low pressure barium discharges

We have studied, both experimentally and numerically, the radiometric efficiency of low-pressure barium–argon positive column discharges. Using a 14 mm inner diameter alumina discharge tube to reduce chemical reactivity, we have operated discharges with Ba vapor pressures corresponding to a range of oven temperatures from 600 to 800 °C, Ar buffer gas pressures of 0.5 and 5 Torr, and currents of 100–800 mA. The spectral power distribution has been determined by examining the visible radiation emitted perpendicular to the discharge axis from a small aperture in the side of the alumina discharge tube. Absolute emitted powers were determined by comparison to a calibrated tungsten–quartz–halogen lamp. The measured efficiencies for production of visible radiation from the positive column ranged from 2.5% to 52%. These efficiencies correspond to luminous efficacies ranging from 6 to 97 lm/W. Strong emission on Ba ion lines at 455.4, 493.4, 614.2, and 649.7 nm contributed significantly to the best efficiency, 52%, and yielded a white light. A numerical model, incorporating a substantial number of excited levels and atomic processes, has been used to examine the same conditions. The model reproduces most of the major experimental trends.

A qualitative comparison of barium behaviour in the PHEBUS FPT0 test and analytical tests

Thermochemical considerations have been used to understand the difference between the barium releases observed in the PHEBUS FPT0 in-reactor severe accident test (<1%) and those observed in the ORNL and VERCORS analytical experiments (>40%). BaO is slightly soluble in UO 2, and BaUO 3 may also be formed as a separate phase, as may BaZrO 3 and more complex solid-solution phases. Barium vapour pressure is generally a monotonic function of the mole fraction of BaO in the fuel; however, the Ba vapour pressure decreases in the region between the solidus and the liquidus, because of a simultaneous decrease of Ba concentration in both liquid and solid phases. Zirconium and iron can also reduce the Ba volatility. These phenomena may be used to rationalize the low release of Ba observed in the PHEBUS FPT0 experiment.

SPECTROSCOPY AND DYNAMICS OF THE LASER INDUCED INTRACLUSTER (Ba··FCH3)* -&gt; BaF* + CH3 AND Ba* + FCH3 REACTION

An overview of the spectroscopy and dynamics of the intracluster Ba··FCH3 + hv → BaF* (Ba*) + CH3 (CH3F) bimolecular reaction is presented. The Ba··FCH3 weakly bound complex is produced by laser vaporisation of a solid Ba sample followed by supersonic expansion of the Ba vapour with the solvent gas. Typical mass spectra obtained by the laser ionisation technique and time-of-flight mass spectrometry are presented. The Ba··FCH3+ ionisation potential was found to be 4·55 ± 0·03 eV. Laser induced photodepletion of the Ba··FCH3 complex was observed at distinct wavelengths of the excitation laser, together with two open channels for complex photofragmentation: a reactive channel, giving BaF as a product, and a non-reactive channel where Ba is formed. The photodepletion spectrum was measured in the 545–630 nm region, displaying two distinct regions that were interpreted as two different electronic states of the complex. The lifetime of the excited A-state was estimated by applying the inverse Fourier transform to the measured spectrum, giving a lifetime of about 250 fs. This result was confirmed in a pump and probe femtosecond experiment, where a lifetime of about 270 ± 30 fs was measured. In addition to the photodepletion spectrum, product action spectra were measured. This allowed the direct determination of the reaction probability as a function of excitation energy. In the A-state, the BaF action spectrum has been measured over the 16065–16340 cm-1 energy range. The reaction probability, PR (E), displays a peak structure with an energy spacing of 10·9 cm-1 that may be related to the internal motion of the transition state of the reaction. For the B-state, product action spectra for both the reactive and non-reactive channel are presented for the 17795–18250 cm-1 energy range. Here, the reaction probabilities show an oscillatory behaviour with opposite phase, which could be due to quantal interferences between the two photofragmentation channels. In addition, the energy spacing of the oscillations can be related to the internal motions of the transition state. All these features are discussed taking into account the interplay between the spectroscopy and dynamics associated with each electronically excited state of the complex that governs the photoinitiated harpooning reaction.

Laser preparation of bunched ion beams

Laser ionization at ISOLDE is performed with high repetition laser beams in the ion source unit and for daughter elements with low repetition lasers synchronized with laser desorption from a secondary target. We are preparing an implementation of thermal adsorption followed by laser desorption and laser ionization in the target ion source unit of ISOLDE. The combination of resonance ionization with thermal bunching allows the preparation of bunched and chemically pure ion beams. Bunched ion beams of Mg or Ca are prepared for post-acceleration at REX-ISOLDE. In order to investigate the process of adsorption and pulsed laser desorption, experiments are performed with Ba vapour. In a test setup Ba is effusing from an oven and adsorbed on a cooled surface, which can be heated by a pulsed Nd: YAG laser beam. The adsorption and bunched release of Ba atoms is detected via resonance absorption of the 6s 2 1S 0 → 6s6p 1P 1 atomic line at λ = 553 nm with a single mode dye laser.

ACUTE INHALATION TOXICITY STUDIES OF n -BUTYL ACETATE

Aerosolized n -butyl acetate (n -BA) was reported to have an LC50 value of approximately 160 ppm in a study conducted by 3M at NOTOX. Six follow-up studies at three laboratories (BASF, BRRC, NOTOX) were conducted in an attempt to reproduce this value. Two other LC50 values for aerosolized n -BA were generated (390 and 1096 ppm). However, 4 studies did not result in any lethalities from aerosolized n -BA at concentrations of 415.5, 798, 1033.5, 1411, 1442, 1517, 1575, 4429.2, 5082, and 9312 ppm. Thus, no consistent LC50 value for aerosolized n -BA was determined. Statically saturated and dynamically evaporated n -BA studies were conducted to investigate whether the aerosolization process of n -BA had an impact on the toxicity. No deaths were observed from animals exposed to statically generated n -BA vapor at a concentration of 6867 ppm and dynamically evaporated n -BA vapor at concentrations of 2027, 3013, 4429.2, and 6335 ppm. Exposures of aerosolized n-BA from old and current productions of n -BA were monitored for small particles (minimum size 0.01 m) to determine any effect the presence of these particles might have on toxicity. No differences in the particle concentrations between the old and current productions of n -BA were noted, and particle size/concentration appeared unrelated to toxicity. Two other parameters were investigated: Relative humidity was elevated (100%), and different atomizers were used to investigate the particle effect on the toxicity of aerosolized n -BA. No lethalities were observed using the different productions of n -BA, elevated relative humidity, different atomizers, or different air sources for the aerosolization of n -BA. Findings in the 3M/NOTOX study not observed in any other studies were (1) low chamber relative humidity, (2) brief times to death of the animals, and (3) histopathological finding of vesicular emphysema. Occupational exposure to n BA does not result from the usage of aerosolized pure n -BA, but rather mixtures con- taining n -BA in which n -BA is released as a vapor. Aerosolized n -BA was not found at the workers' breathing zone in a furniture paint spray study. The animal vapor data suggest only low to moderate toxicity at high concentrations. Thus, this anomaly of low and variable LC50 values of aerosolized pure n -BA does not appear to be of great concern in occupational settings. Consequently, the American Conference of Governmental Industrial Hygienists (ACGIH) in 1996 maintained a threshold limit value (TLV) value of 150 ppm.

Electron temperature measurement of Ba photoplasma by using an electrostatic probe

We study experimentally and theoretically the time evolution of the electron temperature Te of a photoionized barium vapor which expands freely into a vacuum. Using the gas dynamics fundamental equations and assuming the plasma expansion to be adiabatic, a model is built and analytical expressions are derived for the electron and ion temperatures, velocity, and density time evolutions. The experimental apparatus consists essentially of a vacuum chamber, a Joule effect furnace which produces the Ba vapor. A cylindrical plasma created between the two vertical plates is produced by two-step ionization of the vapor. The electron temperature is measured with a cylindrical electrostatic probe biased by a slowly variable voltage ramp. The current-voltage curve is built step by step with a boxcar averager. The results involve different parameter variations like vapor density or sampling time of the probe current for studying the time evolution of electron temperature. Finally, it was found that the adiabatic cooling model agrees well with the experimental electron temperature evolution, but was limited below 0.025 eV by the Langmuir probe accuracy.

Multiphoton ionization of barium with a tunable pulsed laser in the range 570-608 nm

Multiphoton ionization (MPI) of barium atom using an excimer-pumped pulsed dye laser has been studied in the wavelength range 570-608 nm at a laser intensity approximately 10.0 GW cm-2. Resonances in MPI were recorded by monitoring either the ions produced in the multiphoton process using a thermionic detector or fluorescence from the ions formed in their various excited states of Ba+. The observation of photoion and fluorescence spectra were made on Ba vapour in the heat-pipe oven in the moderate-density regime ( approximately 1013 atoms/cm3) so that the collisional effects are negligible. Fine-structure resolution of the final ionic state (6p 2P1/2,3/2), produced after four-photon autoionizing decay was obtained. The decay of the Ba atom after four-photon absorption preferentially populates the 6p 2P3/2 state relative to the 6p 2P1/2 state.

A study of the reactions between Ba(1S) and N2O, O2, and CO2

A kinetic study is presented of the reactions between Ba(1S) atoms and N2O, O2, and CO2 over an extended temperature range. Ba atoms were produced in an excess of each oxidant and He bath gas by the pulsed 2-photon (193.3 nm) dissociation of BaO, produced in situ by the reaction between Ba vapor and the oxidant, and then monitored by time-resolved laser induced fluorescence spectroscopy at λ=553.55 nm (Ba(61P−61S)). The following results were obtained (at the 2σ level of uncertainty): k(Ba+N2O, 242&amp;lt;T&amp;lt;850 K) =(2.99±0.14)×10−10 exp[−(5.7±0.17) kJmol−1/RT]; k(Ba+O2, 254&amp;lt;T&amp;lt;566 K) =(4.89±0.38)×10−10 exp[−(7.06±0.23) kJmol−1/RT]; k(Ba+CO2, 257&amp;lt;T&amp;lt;849 K)=(3.97±0.21)×10−10 exp[−(7.66±0.18) kJmol−1/RT] cm3 molecule−1 s−1. These results are in good agreement with recent molecular beam studies. In the case of Ba+N2O, two line-of-centers collision models are employed to demonstrate that the effect of vibrational excitation of N2O is to enhance the chemiluminescence quantum yield as well as the reaction cross section. Comparison with the chemiluminescence cross section for the reaction Ba+N2O →BaO(A1Σ+ and A′1Π ), obtained in a beam-scattering experiment, indicates a branching ratio of (18.3±6.7)% into these states.

Observation of pair absorption and self-broadening in Ba vapor

Reduced absorption coefficients γBaBa were measured for the far-wing absorption of the self-broadened Ba6s2 1S0–5d6p 1P1 line, Ba(6s2 1S0) +Ba(6s2 1S0)+ℏω → Ba*(5d6p 1P1)+Ba(6s21 S0)and the pair absorption Ba(6s2 1S0)+Ba(6s2 1 S0) +ℏω→Ba*(6s6p 1P1 +Ba*(6s5d 1D2). The self-broadening shows the red-shaded asymmetric profile: γBaBa on the red wing is proportional to Δν−3/2, where Δν is the detuning from the line center. The pair absorption shows a broad asymmetric peak having a blue tail and a blue satellite. The estimated γBaBa, which is based on the quasistatic line-broadening theory and the long-range dipole–dipole interaction between the relevant atomic excited levels, shows reasonable agreement for the red wing of the self-absorption whereas it gives larger values by a factor of 3 for the blue tail of the main peak for the pair absorption.

Phase conjugation by degenerate four-wave mixing in barium vapor

We report what is to our knowledge the first observation of phase-conjugate waves by degenerate four-wave mixing in Ba vapor near its 6s(2)(1)S(0) ? 6s6p(1)P(1) resonance transition. A maximum reflectivity of 36% at 10 mbars of Ar buffer gas pressure is observed together with a temporal pulse shortening of 50%. Conical emission in the two pump waves after their passage through the nonlinear medium is also observed. The dependence of the phase-conjugate signal on the pressure of the Ar buffer gas shows a small state-changing collisional decay rate.

Effects of Material Transfer by Liquid and Vapor on the Microstructure of Sintered Y‐Ba‐Cu‐O Superconductors

Effects of a slight compositional change on the microstructure of sintered Y‐Ba‐Cu‐O superconducting ceramics were investigated. The compositional change was provided either by the liquid which has been formed at the hot end of the rectangular bar specimen sintered under gradient temperature or by the vapor from Y‐Ba‐Cu‐O melt. The results indicated that the Cu and Ba vapor pressure of the sintering atmosphere must be closely controlled to obtain an enhanced densification and a desired single‐phase microstructure.

Opacity effects on near-resonance-rayleigh scattering in a barium plasma

Near-resonance-Rayleigh scattering (NRRS) was used to measure the effects of opacity on the resonance transition of Ba +. The plasma is generated by illuminating a barium heat-pipe-oven (Ba vapor density ≳ 10 14 cm −3) with a flash-lamp dye-laser pulse (10 5W/cm 2) tuned to the barium resonance transition at 553.5 nm. The Ba + ion is probed with 90° NRRS from an Nd: YAG-pumped dye laser tuned over the line profile of the Ba + resonance transition at 493.4 nm. The influence of the oven temperature, pumping laser-beam size, and probe-laser detuning on the opacity of this transition was investigated. The experimental results indicate a minimum detuning from the Ba + resonance line center to avoid opacity problems in NRRS.