John W. Hastie

High-temperature mass spectrometry: Instrumental techniques, ionization cross-sections, pressure measurements, and thermodynamic data (IUPAC Technical Report)

An assessment of high-temperature mass spectrometry and of sources of inaccuracy is made. Experimental, calculated, and estimated cross-sections for ionization of atoms and inorganic molecules typically present in high-temperature vapors are summarized. Experimental cross-sections determined for some 56 atoms are generally close to theoretically calculated values, especially when excitation–autoionization is taken into account. Absolute or relative cross-sections for formation of parent ions were measured for ca. 100 molecules. These include homonuclear diatomic and polyatomic molecules, oxides, chalcogenides, halides, and hydroxides. Additivity of atomic cross-sections supplemented by empirical corrections provides fair estimates of molecular cross-sections. Causes of uncertainty are differences in interatomic distances and in shapes of potential energy curves (surfaces) of neutral molecules and of molecular ions and tendency toward dissociative ionization in certain types of molecules. Various mass spectrometric procedures are described that render the accuracy of measured thermodynamic properties of materials largely independent of ionization cross-sections. This accuracy is comparable with that of other techniques applicable under the conditions of interest, but often only the mass spectrometric procedure is appropriate at high temperatures.

Geometries and entropies of metal trifluorides from infrared spectra: ScF3, YF3, LaF3, CeF3, NdF3, EuF3 and GdF3☆

Abstract The metal trifluorides of Sc, Y, La, Ce, Nd, Eu and Gd have been isolated in solid rare-gas and N 2 matrices and infrared absorption spectra obtained over the range 33–4000 cm −1 . All possible fundamental vibration frequencies were observed for the trifluorides of La, Ce, Nd, Eu and Gd, indicating a non-planar C 3 ν structure. The symmetric stretch frequency, ν 1 , indicative of C 3 ν symmetry, was also observed for YF 3 , though less obviously due to matrix effects. For ScF 3 no distinct ν 1 frequency was observed under normal isolation conditions, which suggests either a D 3 h or nearly planar C 3 ν geometry for this species. For CeF 3 and NdF 3 , absorptions not attributable to vibrations of these species were assigned as possible electronic spectra. From the observed frequencies, entropies were calculated to an uncertainty of 1 cal/deg mol, which allowed discrepancies in existing thermodynamic data to be resolved. For ScF 3 , where the symmetry is indefinite, the uncertainty is 2.5 cal/deg mol. Matrix diffusion experiments revealed the formation of dimer species which appear to be isostructural with those of the Group IIIA halides. Unusual matrix effects were interpreted in terms of molecular orientation effects and a possible weak bonding of N 2 to ScF 3 .

Definitions of terms relating to phase transitions of the solid state

Definitions of phase transitions that relate specifically to the solid state are presented. Various types of transition have been defined; they include: (i) structural transitions of the solid state, (ii) transitions related to the change in the state of matter, i.e., from a solid phase to either a liq. or gaseous phase, or vice-versa, (iii) transitions related to changes in compn., (i.v.) metallurgical transitions, (v) transitions related to changes in the electronic structures of crystals, (vi) transitions that change thermodn. properties or the disorder of a system, and (vii) liq.-crystal transitions. [on SciFinder (R)]

Mass spectrometric analysis of 1 atm flames: Apparatus and the CH4O2 system

A mass spectrometric system is described for the measurement of reactive intermediates in 1 atm flames. The system has been tested on CH 4 O 2 and CH 4 O 2 N 2 flames and provides for the first time a complete analysis of such flames for species in excess of 10 −5 mole fraction concentration.

Mass spectrometric studies of flame inhibition: Analysis of antimony trihalides in flames

Abstract The chemistry of SbBr3 and SbCl3 in 1 atm premixed fuel rich CH4 O2 and CH4 O2 N2 flames has been studied. Using line-of-sight mass spectrometric techniques, concentrations profiles were obtained for the major species SbX3, HX, CH3X, X, Sb, and SbO, where X = Br or Cl. Reaction mechanisms are indicated and their relation to flame inhibition discussed. Evidence for a negligible perturbation of the flame kinetics by the sampling procedure is given.

Composition and gas dynamics of laser ablated AlN plumes

Abstract The formation, composition and propagation of pulsed laser produced plasma plumes from an AlN target have been studied in real time by spatially and temporally resolved optical spectroscopy. The mean front velocity has been measured and appears to be slightly slower in the presence of added gas than in vacuum. In the initial stage of expansion, this velocity is almost the same for all neutral particles in the plume in the initial stage of expansion. Velocities of neutral and ionic species have been measured by their time of arrival from the target to a quadrupole mass spectrometer and their expansion beam Maxwell-Boltzmann (MB) temperature determined. The plume electronic temperature has been determined by assuming a local thermodynamic equilibrium of the emitting species.

Vaporization of simulated nuclear waste glass

Abstract Industrial development of glass-forming processes for nuclear waste disposal, particularly borosilicate glasses, requires basic data on glass vaporization thermodynamics. Using special high-temperature mass-spectrometric methods, species partial pressure data have been obtained for a non-radioactive borosilicate process glass containing simulated nuclear waste isotopes. Alkali metaborates were observed to be dominant vapor species and their partial pressures indicate significant transport under likely process conditions. The relative order of significanc of vapor transport of radionuclides was found to be Cs ≳ Re (∼ Tc) > Ru ⪢ Sr. Significant losses by vaporization can also occur during the initial glass-forming process. Decomposition and hydrolysis of the cesium formate starting material appear to be the significant reactions. Detailed thermal decomposition studies indicate that the degradation of cesium formate to yield cesium carbonate involves a complex stepwise pathway. The resultant carbonate releases Cs(g) at a rate up to an order of magnitude higher than for the pure carbonate due to reduction interactions with the carbon produced by formate decomposition.

Gas-dynamic effects in the laser-pulse sputtering of AlN: is there evidence for phase explosion?

Abstract The overall light intensity (fluorescence) of the sputtered atoms, ions, and molecules has been measured for polycrystalline AlN which was bombarded with 248 nm laser pulses in the presence of a background pressure of N 2 . AlN is unusual in that, in spite of a ∼6 eV band gap, it is easily rendered highly absorptive of 248, 308, or 694 nm laser pulses. In fact, since it is well established in other work that ∼1.5 J/cm 2 (308 nm) brings AlN to the melting temperature, ∼3050 K, we will assume that the fluence used here (∼20 J/cm 2 ) was more than enough to bring the target surface first to a temperature sufficient for normal vaporization but finally to the vicinity of the thermodynamic critical temperature, T tc . As a result a significant quantity of particles can be assumed to have been expelled by phase explosion. The tentativeness in the argument rests in the problem that some part of the incident fluence beyond ∼1.5 J/cm 2 will have been consumed in laser–plume interaction. Nevertheless there is evidence in work by Pedraza et al. [A.J. Pedraza, J-Y. Zhang, H. Esrom, Mater. Res. Soc. Symp. Proc. 285 (1993) 209] that both AlN and Al respond linearly to the fluence up to at least 6 J/cm 2 . It was found that the assumed phase-exploded particles decelerated rapidly, possibly due to their encounter with the normally vaporized particles, or possibly due to an electric field arising from positive charging of the target surface. The fluorescence maximum (which can be safely assumed to be also a density maximum) was then nearly stationary, a situation which characterized the lowest background pressures of N 2 (≤3.5 Pa). At higher pressures (≥3.5 Pa) a second fluorescence maximum appeared nearer the contact front and was found to move. Following the suggestion of Horwitz 1 we take this feature as being an artifact of electrons near the contact front diffusing (or scattering) backwards and causing fluorescence which is unrelated to the particle density. From the velocity of the contact front one obtains explicit information on the mean kinetic energies ( E 4 ) of the particles in the plume (1.5–2 eV). Another estimate of E 4 follows from the initial expansion observed from 0–200 ns (1.5–3 eV). Such energies suggest, independently of the fact that the fluence was high (∼20 J/cm 2 ), that a temperature near T tc was reached and that phase explosion may have occurred. We finally note that, however tentative is the claim for phase explosion, it is certain that a close relative of phase explosion, due to subsurface heating, was not involved. This is because the numerical demonstrations of subsurface heating have been flawed.

Development and Application of Very High Temperature Mass Spectrometry: Vapor Pressure Determinations Over Liquid Refractories

Existing thermodynamic and vaporization data for liquid refractories are based either on estimates or on data extrapolated from studies on the solids obtained at much lower temperatures. Previously, we have shown that pulsed laser heating, coupled with time-dependent mass spectrometry of the free-expansion vapor plume, can be used for semi-quantitative measurements of vaporization thermochemistry. The present work extends this approach with the development of (a) more direct, and more accurate, methods for determining the system temperature and pressure; (b) improved experimental and theoretical determinations of key parameters such as ionization cross sections; and (c) improved characterization of the gas dynamic expansion and thermal equilibration processes. Example material systems considered include C, SiC, Al2O3, ZrO2—7%Y2O3, and Y2O3 at temperatures and total pressures typically in the range of 3000 to 5000 K and 0.01 to 10 bar, respectively (1 bar = 105 Nm-2).

Particulate reduction in the pulsed laser deposition of barium titanate thin films

Abstract A particulate-reduction approach has been developed, based on a pulsed supersonic gas-jet that selectively deflects particulates from the deposition stream during pulsed laser deposition of BaTiO 3 thin films. In situ imaging, using an intensified CCD camera, has shown that the desirable atomic, ionic, and molecular species move toward the substrate with velocities on the order of 10 6 cm s −1 , while the undesirable particulates move at velocities on the order of 10 4 cm s −1 and slower. This separation of velocities is sufficient that the pulsed gas-jet can be timed to impact the particulates after the vapor species reach the substrate, but while the particulates are still near the target, allowing for near-maximum deflection. Key parameters with this approach are the sharpness and the timing of the valve opening and closing. Initial results show that at least an order of magnitude reduction in the number of film particulates is achieved.