Chemische Charakterisierung und Spurenanalytik
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The equation of state for plasmas containing negative and positive ions of elements and molecules formed by these elements is modeled under the assumption that all ionization processes and chemical reactions are at local thermal equilibrium and the Coulomb interaction in the plasma is described by the Debye–Hückel theory. The hierarchy problem for constants of molecular reactions is resolved by using three different algorithms for high, medium, and low temperatures: the contraction principle, the Newton–Raphson method, and a scaled Newton–Raphson method, respectively. These algorithms are shown to have overlapping temperature ranges in which they are stable. The latter allows one to use the developed method for calculating the equation of state in combination with numerical solvers of Navier–Stokes equations to simulate laser-induced Plasmas initiated in an atmosphere and to study formation of molecules and their ions in such plasmas. The method is applicable to a general chemical network. It is illustrated with examples of Ca–Cl and C–Si–N laser-induced plasmas.
based on the use of the signal from hydrogen-containing polyatomic ions formed in the inductively coupled plasma. Prior to analytical experiments, a theoretical study was performed to assess the concentration of polyatomic species present in an equilibrium Ar-O-D-H plasma, as a function of temperature and stoichiometric composition. It was established that the highest sensitivity and linearity measurement of D concentration in a wide range can be achieved by monitoring the ions of D2 and ArD, at masses 4 and 42, respectively. Results of the calculations are in good agreement with the experiments.
Signal stability, spectral interferences, as well as the effect of plasma parameters were also assessed. Under optimized conditions, the limit of detection (LOD) was found to be 3 ppm atom fraction for deuterium when measured as ArD (in calcium and potassium free water), or 78 ppm when measured as D2. The achieved LOD values and the 4 to 5 orders of magnitude dynamic range easily allow the measurement of deuterium concentrations at around or above the natural level, up to nearly 100% (or 1 Mio ppm) in a standard quadrupole ICP-MS instrument. An even better performance is expected from the method in high resolution ICP-MS instruments equipped with low dead volume sample introduction systems
Laser induced plasma (LIP) is a highly dynamic, short living event which presents significant difficulty for both diagnostics and modeling. The former requires precise spatially- and time-resolved measurements on a micron-nanosecond scale while the latter needs numerous descriptive parameters; many of them can only be obtained from experiment. Diagnostics and modeling should always complement each other for obtaining a truthful picture of LIP.
In this presentation, a newly developed collisional-dominated model will be presented. The model is based on the coupled Navier-Stokes, state, radiative transfer, material transport, and chemical equations. The model incorporates plasma chemistry through the equilibrium approach that relies on atomic and molecular partition functions. Several chemical systems are modeled including Si-C-Cl-N and B-H-Cl systems.
The model is used to study the equilibrium states of the systems as functions of the concentrations of plasma species and plasma temperature. The model also predicts the evolution of number densities of atomic and molecular species in the expanding plasma plume.
High purity halides of III-VI group elements, especially chloride and fluorides, are used in gas phase technologies for obtaining high purity materials and coatings. The reduction of halides in hydrogen-halide mixtures can be achieved in various discharge plasmas, e.g. inductively coupled, ark, and even laser-induced plasmas. Existing models of such plasmas are not sufficiently accurate to predict a yield of the targeted compounds and to describe the plasma processes involved in formation of these compounds. Besides, a construction of costly plasma-chemical reactors can be alleviated by the prior modeling of plasma processes that may occur in such reactors.
A goal of this work is to extend the model, which was initially developed for laser induced Plasmas, to plasmas used in chemical reactors, in particular, the inductively-coupled-RF discharge Plasma. The model predicts equilibrium chemical compositions of reaction mixtures as functions of plasma temperature and stoichiometry of reactants. The mixtures investigated are BCl3/H2/Ar and BF3/H2/Ar where Ar serves as the plasma-forming gas and H2 as a binding agent which binds the active species Cl and F and Cl- and F-containing intermediates to produce gaseous B and its condensate. An additional goal is to obtain information about intermediate reaction products for different ratios of BCl3/H2 and BF3/H2 and at different temperatures and different Ar flow rates.
It is found that the desired components B and B2 appear at appreciable concentrations of >0.1% and ~0.01% respectively only at temperatures above 3000 K. It is also established that the effect of charged species on the reaction products is miniscule for temperatures below 5000 K. The expected yield of boron as a function of the original mole fraction H2/BCl3 and H2/BF3 is calculated. The mole fractions are varied in the range 0.1-1000 and the temperature in the range 1000-10000 K. It is shown that the yield of boron increases with increasing the molar ratio H2/BCl3 and H2/BF3 up to ~100 in the temperature range 2000-5000 K. At higher temperatures, T>5000 K, the boron concentration reaches its maximum and does not depend on the concentration of hydrogen; all molecules dissociate and chemical reactions proceed only between charged particles (mostly elemental ions) and electrons. The calculated plasma parameters and composition are compared with experimental data obtained by optical emission spectroscopy. The calculated plasma temperature and electron density are shown to be in good agreement with the measured ones.
An improved algorithm for calibration-free laser induced breakdown spectroscopy (CF LIBS) will be presented which includes several novel features in comparison with previously proposed similar algorithms. In particular, it allows using spectral lines with arbitrary optical thickness for the construction of Saha-Boltzmann plots, retrieves the absorption path length (plasma diameter) directly from a spectrum, replaces the Lorentzian line profile function by the Voigt function, and allows for self-absorption correction using pre-calculated and tabulated data rather than approximating functions. The tabulated data embody the solutions of the radiative transfer equation for numerous combinations of optical thicknesses and line widths. The algorithm is thoroughly verified using synthetic spectra.
Laser-induced plasmas are widely used in many areas of science and technology; examples include spectrochemical analysis, thin film deposition, material processing, and even jet propulsion. Several topics will be addressed. First, general phenomenology of laser-induced plasmas will be discussed. Then, a chemical model will be presented based on a coupled solution of Navier-Stokes, state, radiative transfer, material transport, and chemical (Guldberg-Waage) equations. Results of computer simulations for several chemical systems will be shown and compared to experimental observations obtained by optical imaging, spectroscopy, and tomography. The latter diagnostic tools will also be briefly discussed.
This technical note highlights the fact that CF-LIBS algorithms work in mole fractions, while results of spectrochemical analysis are usually reported in mass fractions or mass percent. Ignoring this difference and not converting mole fractions to mass fractions can lead to errors in reported concentrations determined by the CF-LIBS method and inadequate comparison of these concentrations with certified concentrations. Here, the key points of the CF-LIBS algorithm are reproduced and the formulae for converting a mole fraction to a mass fraction and vice versa are given. Several numerical examples are also given, which show that the greater the difference between the molar mass of an individual element in a sample and the average molar mass, the greater the discrepancy between the mole and mass fractions.
A goal of this work is to extend the model, which was initially developed for laser induced plasmas, to plasmas used in chemical reactors, in particular, the inductively-coupled-RF discharge plasma. The model predicts equilibrium chemical compositions of reaction mixtures as functions of plasma temperature and stoichiometry of reactants. The mixtures investigated are BCl3/H2/Ar and BF3/H2/Ar where Ar serves as the plasma-forming gas and H2 as a binding agent which binds the active species Cl and F and Cl- and F-containing intermediates to produce gaseous B and its condensate. An additional goal is to obtain information about intermediate reaction products for different ratios of BCl3/H2 and BF3/H2 and at different temperatures and different Ar flow rates.
It is found that the desired components B and B2 appear at appreciable concentrations of >0.1% and ~0.01% respectively only at temperatures above 3000 K. It is also established that the effect of charged species on the reaction products is miniscule for temperatures below 5000 K. The expected yield of boron as a function of the original mole fraction H2/BCl3 and H2/BF3 is calculated. The mole fractions are varied in the range 0.1-1000 and the temperature in the range 1000-10000 K. It is shown that the yield of boron increases with increasing the molar ratio H2/BCl3 and H2/BF3 up to ~100 in the temperature range 2000-5000 K. At higher temperatures, T>5000 K, the boron concentration reaches its maximum and does not depend on the concentration of hydrogen; all molecules dissociate and chemical reactions proceed only between charged particles (mostly elemental ions) and electrons. The calculated plasma parameters and composition are compared with experimental data obtained by optical emission spectroscopy. The calculated plasma temperature and electron density are shown to be in good agreement with the measured ones.
Laser metal deposition is a rapidly evolving method for additive manufacturing that combines high performance and simplified production routine. Quality of production depends on instrumental design and operational parameters that require constant control during the process. In this work, feasibility of using optical spectroscopy as a control method is studied via modeling and experimentally. A simplified thermal model is developed based on the time-dependent diffusion-conduction heat equation and geometrical light collection into detection optics. Intense light emitted by a laser-heated spot moving across a sample surface is collected and processed to yield the temperature and other temperature-related parameters. In a presence of surface defects the temperature field is distorted in a specific manner that depends on a shape and size of the defect. Optical signals produced by such the distorted temperature fields are simulated and verified experimentally using a 3D metal printer and a sample with artificially carved defects. Three quantities are tested as possible metrics for process monitoring: temperature, integral intensity, and correlation coefficient. The shapes of the simulated signals qualitatively agree with the experimental signals; this allows a cautious inference that optical spectroscopy is capable of detecting a defect and, possibly, predicting its character, e.g. inner or protruding.
The spatial heterodyne detection principle has a great potential in spectroscopy. It has an optical setup similar to that of a Michelson interferometer, with the mirrors replaced by diffraction gratings positioned at fixed, equal distances from the beamsplitter and are slightly tilted. The resulting interference pattern is recorded by a digital camera and the spectrum is recovered by using Fourier transformation. Although SHS was initially developed for astronomical and satellite-based atmospheric measurements, but in recent years it has been started to be applied in other branches of spectroscopy too. Recently the area of laser-induced breakdown spectroscopy (LIBS) has also discovered the potential of SHS. The main appeal of SHS detection in LIBS includes the compactness and robustness of the setup (in view of field applications) and the flexibility to optimize the setup for either high sensitivity or for high resolution, which can be benficially exploited in applications like stand-off measurements, quantitative analysis with isotope resolution, etc.
In the present work, we have improved and further optimized our initial LIBS-SHS setup described in a previous conference. By using optical simulations, we have modelled the light transmission efficiency, instrumental function and imaging properties of the system. We significantly improved and automated the spectral and image data processing sequence. The optimizations carried out resulted in an improved spectral resolution and repeatability, a lower spectral background and the elimination of the central line artifact originating from the Fourier transformation procedure. A detailed characterization of the LIBS spectroscopy performance (e.g. resolution, spectral coverage, tuning range, linearity, etc.), including a comparison with that of a LIBS setup based on a conventional dispersion CCD spectrometer was also performed.