David W. Bonnell

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.

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.

Optical characterization of thin film laser deposition processes

Pulsed laser deposition (PLD) has recently been shown to be an effective means of depositing thin films from refractory targets. In this study, Nd/YAG and excimer lasers have been used to deposit thin films from refractory, high Tc superconducting, dielectric, ferroelectric and magnetic targets. Optical emission spectroscopy of the laser induced plume has been used to determine the identity and energy (temperature) of the excited state species present in the laser induced plumes. Temporally and spatially resolved optical emission spectra were obtained using a gated intensified photodiode array detector coupled to a grating spectrometer. The individual emission spectra were analyzed to identify the atomic and ionic species present. The temporal and spatial evolution of individual emission lines were used to determine the velocity of the species in the plume. These results were combined with the results from in situ molecular beam mass spectrometric analysis of the plumes. In addition, studies of the stoichiometry and morphology, as well as the electrical properties, of these PLD thin films were carried out for correlation with the spectroscopic observations.

Application of a new thermochemical measurement method for nuclear materials at temperatures beyond 3000 K

In processing and end-use environments, and particularly nuclear fission reactor excursions, inorganic materials can be subjected to temperatures where liquids and vapors are significant components of the materials system. Classical characterization and thermochemical methods fail at temperatures beyond about 3000 K, due to the reactivity of container materials. Use of a pulsed laser beam as a localized heat source avoids this limitation. Coupling laser heating with molecular beam sampling and mass- and optical-spectroscopy allows us to characterize the thermochemistry of liquid-vapor systems at temperatures of 3000-5000 K, pressures of 0.01-20 bar (1 bar = 10 5 Nm 2 ). and on a nanosecond order-of-magnitude time scale. Materials considered here include C, ZrO 2 , Y 2 O 3 and HfO 2 . New approaches for temperature measurement and for pressure determination, using electron impact mass spectral data coupled with deposition rate measurements, are described.

Imaging and gasdynamic modeling of pulsed laser film deposition plumes

Optical multichannel emission spectroscopy and intensified charge-coupled device (ICCD) imaging have been applied to real-time, in situ gas-phase species identification during the pulsed excimer or Nd:YAG laser deposition of various ceramic thin films. A plume gasdynamic expansion model has been developed and used to predict the outer-edge plume front locations for comparison with those observed in the ICCD images. Good agreement was found between the model and ICCD images, with plume temperatures indicated by the model to be typically between 10,000 and 50,000 K. The systems studied include PbZr0.53Ti0.47O3 (PZT), BaTiO3 , AlN, and BN. When high laser fluences were used, ICCD imaging also revealed strong evidence for interactions between the laser and the near-surface plume. Plume particulates were also noted at long times following the laser pulse.

Species Temporal and Spatial Distributions in Laser Ablation Plumes

The intermediate species present in laser ablation plumes, particularly those formed during pulsed laser deposition (PLD) of thin films, have been identified for a variety of advanced materials systems. Optical multichannel analysis spectroscopy has been used to monitor the atomic and ionic species present, via their spectral emissions. Molecular beam-sampling mass spectrometry has been used to monitor the molecular species present, in addition to atoms and ions. With both monitoring approaches, temporal and spatial species distribution information has been obtained. Velocity distributions, obtained from the time-dependent mass spectral studies, show the effects of isentropic expansion to be predominant when compared with gasdynamic models of the plume evolution process. Also, the plume structure was found to be particularly sensitive to target elemental distribution. Examples of systems studied include high temperature superconductors, refractory compounds, ferroelectrics and nanostructured magnetic films.

Imaging and modeling of pulsed-laser thin-film-deposition plumes

Optical multichannel emission spectroscopy and intensified charge coupled device (ICCD) imaging have been applied to real-time, in situ gas phase species identification during the pulsed excimer or Nd/YAG laser deposition of selected thin films. A plume gasdynamic expansion model has been developed and used to predict the outer edge plume front locations for comparison with those observed in the ICCD images. Good agreement was found between the model and ICCD images, with plume temperatures indicated by the model to be typically between 10,000 K and 30,000 K. Molecular beam mass spectrometry has also been used for real- time, in situ species identification and velocity distribution determinations. The systems studied include PbZr0.53Ti0.47O3 (PZT), BaTiO3, AlN, BN, Al2O3, and Ag. The ICCD imaging of plumes from PZT and BN targets, in particular, revealed that particulate ejecta were present after some regions of the target surface had been modified morphologically by multiple exposures to the laser beam. These ejecta appeared long after the luminous plume had decayed. When relatively high laser fluences were used, ICCD imaging also revealed strong evidence for interactions between the laser and the near-surface plume. This interaction manifests itself as increased emission intensity in the direction of the incoming laser beam. Mass spectrometric studies showing relatively fast velocity components of both neutral and charged plume species support the imaging evidence. The laser-plume interaction results in higher kinetic energies and a much greater effective temperature for a portion of the plume species. The slower component(s) appeared to be more thermal in origin (i.e. controlled by surface vaporization), and apparently reflects the target surface temperature.

Mass Spectrometric Probing of Laser-Induced Materials Vapor Transport: Graphite and Superconducting YBa 2 Cu 3 O x

A very high pressure-sampling mass spectrometer has been used to identify the vapor transport species and determine the thermochemistry and kinetics of laser-induced plumes produced from graphite and superconducting composition YBa 2 Cu 3 O x targets (x = 6.5 to 7). An electron impact ion source was used for the ionization and detection of neutral plume species. The plumes initially contain -1 atm (1 atm = 101.325 kPa) of neutral and charged atomic and molecular species in a vacuum of −7 atm. Time resolved mass spectra were obtained with graphite targets for the neutral plume species C n (n = 1-9) for varying laser fluence, laser-surface interaction geometry, vapor plume-sampling geometry, and target surface morphology. Relatively low abundance charged species C1 + , C 2 + , C 3 + , and impurities Na + and K + were also observed in the laser-induced plume. Mass spectra obtained with superconducting YBa 2 Cu 3 O x targets showed a variety of species in the laser-induced plumes including both neutral and ionic Y, Ba, and Cu. In addition, molecular species such as O 2 , BaO, CuO + , YO and bimetallics (BaCu, YCu) were observed.

Monte Carlo simulations of plume evolution from laser ablation of graphite and barium titanate

Abstract Evolution of laser ablation products from graphite and barium titanate targets has been simulated using a Monte Carlo method, assuming elastic collisions. The calculated species arrival times and intensities, at a hypothetical detection plane, compare favorably with those obtained experimentally by time-resolved mass spectrometric sampling of the expanded vapor plumes. As such experiments are difficult to perform, the simulation method should prove useful for unstudied systems in defining the spatial and temporal distributions of species arriving for example, at a film deposition substrate.