Papers in Reaction KineticsCopyright (c) 2018 University of Nebraska - Lincoln All rights reserved.
https://digitalcommons.unl.edu/chemengreaction
Recent documents in Papers in Reaction Kineticsen-usWed, 31 Jan 2018 19:31:05 PST3600PIEZOELECTRIC SENSORS / ACTUATORS FOR USE IN REFRACTORY ENVIRONMENTS
https://digitalcommons.unl.edu/chemengreaction/12
https://digitalcommons.unl.edu/chemengreaction/12Thu, 09 Feb 2006 13:29:37 PST
pierozlectric properties over a large temperature range of from room temperature to approximately 1360 degrees centigrade. The piezarlecttic system is sequentially comprised of a first electrically conductive layer, a layer of (Ta,O,) in other than a rnonoclinic phase, ipreferably onhorhombic demonstrating small x-ray crystalopraphic and peaksi. and a second slecttically conductive layer A preferred method of fabrication involves sputter deposition of both said layer of (Ta,O) md said second electrically conductive layer.
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Hendrik J. Viljoen et al.Convective Regimes in reactive Fluid media due to the interaction with Catalytic Surfaces
https://digitalcommons.unl.edu/chemengreaction/11
https://digitalcommons.unl.edu/chemengreaction/11Thu, 09 Feb 2006 13:01:27 PST
Reactive fluid media enclosed in a cavity with a catalytic surface are analyzed. Nonisothermal chemical reactions on this surface can lead to convective instabilities. A simplified model is developed by using a low-order truncation of a Fourier-type expansion and employing the Galerkin method. A linear stability analysis is presented and it is shown that, under certain conditions, the marginal curve for the onset of oscillatory instabilities can lie below that for monotonic ones. The stability of the convective modes is studied by nonlinear stability analysis and it is shown how they can evolve into periodic and nonperiodic motion patterns. Numerical results are provided to support and confirm analytical predictions.
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Hendrik J. Viljoen et al.Analysis of combustion-driven acoustics
https://digitalcommons.unl.edu/chemengreaction/10
https://digitalcommons.unl.edu/chemengreaction/10Wed, 08 Feb 2006 09:06:31 PST
Abstract-Combustion-driven acoustic oscillations are investigated by performing a onedimensional stability analysis of a burner-stabilized premixed flame. In contrast to other investigators, no initial acoustic wave is assumed in the analysis; the downstream acoustic field results from flame instability. Two models are considered: the thermodiffusive model (uncoupled model) and the fully coupled thermodiffusive-hydrodynamic model. The fully coupled problem exhibits instability at a much lower critical Lewis number than the uncoupled problem.
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L Boshoff-Mostert et al.Reaction Fronts in a Porous Medium. Approximation Techniques versus Numerical Solution
https://digitalcommons.unl.edu/chemengreaction/9
https://digitalcommons.unl.edu/chemengreaction/9Tue, 07 Feb 2006 15:15:31 PST
The flame sheet approximation (FS) and a novel polynomial approximation technique (PA) are compared in terms of their capability to describe reaction fronts of highly exothermic reactions in a porous medium. A one-phase model and a two-phase model of a system with adiabatic walls and a radiant output (to approximate the case of a porous radiant burner) are included in the analysis. By matching the reaction zone solution found by either the FS or PA method with the solutions of the nonreacting zones, the temperature, conversion, and position of the reaction zone were determined. Numerical solutions for catalytic and noncatalytic oxidation reactions were used to compare the predictions of both approaches. It was found that although both techniques yielded good approximations to the solutions, the PA technique proved to be more accurate, producing results with 3.5% of the numerical results. Both methods can find useful application in the analysis of this class of problems.
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Fernando Escobedo et al.Modeling of Solid Phase Detonations
https://digitalcommons.unl.edu/chemengreaction/8
https://digitalcommons.unl.edu/chemengreaction/8Thu, 02 Feb 2006 09:35:09 PST
In contrast to conventional explosives which constitute rapid decomposition of the molecular structure acconlpanied by the release of large volumes of gaseous products, heterogeneous mixtures in the SHS realm react by progression of a thermal wave at velocities far below the speed of sound in such mixtures. Interestingly, ultrafast solid phase reactions can be initiated under the right conditions. A shock wave compresses the solid mixture to densities well beyond the theoretical mean ambient density (TMD) and compression becomes the major form of preheating. In addition, elastic potential energy is pumped into the lattice structure to induce severe distortion and eventually a structural collapse of the lattice on the atomic scale. Mixing and reaction proceed as in a dense gas and condensed products form. A continuum model is presented which addresses the following elements in the process. Compaction of the porous preform is described by an amended equation of state which includes plastic yielding and dilatation. The equation of state of densified material is based on an isobaric modification of the Mie-Gruneisen equation of state to account for anomalous behavior - a phenomenon of density reduction in the shock wave. Pressure is coupled into the kinetics as suggested by Benderskii insofar the activation energy is reduced proportionally to the stored elastic potential energy. Examples are presented of anomalous shock behavior, stable and unstable detonations.
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C Richter et al.Temperature Oscillations of Alternating-Current-Heated Thin Filaments in Ceramic Fiber Production
https://digitalcommons.unl.edu/chemengreaction/7
https://digitalcommons.unl.edu/chemengreaction/7Mon, 19 Dec 2005 12:15:19 PST
Resistively heated filaments are used to manufacture ceramic fibers. When an ac power source is used for heating, the temperature of the filament oscillates. These oscillations influence the deposition rate on the surface of the filament. An analysis of this problem is presented, and it is also shown that these oscillatory effects diminish as the filament gauge increases. The analysis can help to decide whether an ac or a dc power source should be used.
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Hendrik J. Viljoen et al.NATURAL CONVECTION AND SOLID PHASE COMBUSTION
https://digitalcommons.unl.edu/chemengreaction/6
https://digitalcommons.unl.edu/chemengreaction/6Fri, 16 Dec 2005 13:30:17 PST
Self-propagating high-temperature synthesis [SHS] is a combustion process in-volving two or more solid reactants. The typical SHS configuration consists of a cylin-drical preform of mixed powders, placed in an inert gas chamber, and ignited at one end. In past studies, interaction between the solid phase and the ambient gas phase has been limited to heat losses from the solid; the influence of natural con-vection on the solid phase has never been considered. In this study, computational fluid dynamics [CFD] is used, and it is shown that intense convection flow develops in the proximity of the combustion front. Gas flows adjacent to reacted solid mate-rial, heats up, and when it reaches the unreacted solid heat is transferred from the gas to the solid phase, which aids solid phase thermal conduction in preheating the material. The effect is stronger than expected, and it could stabilize the combustion of structured reactants like roll-ups of foils and wires. Combustion parallel and anti-parallel to gravity is investigated for different burning velocities. At low propagation velocities, the natural convection cell forms a torus that is seated above the com-bustion front. At high propagation velocities, the convection flow cannot track the combustion front, and Tollmien-Schlichting waves form. Constant front propagation and planar oscillations of the combustion front lead to increasingly complex flows. Finally, the heat exchange between the gas and solid for constant front propaga-tion is compared to analytical solutions.
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S E. Whitney et al.Effective Thermal Conductivity Estimates Of Particulate Mixtures
https://digitalcommons.unl.edu/chemengreaction/5
https://digitalcommons.unl.edu/chemengreaction/5Thu, 10 Nov 2005 13:07:48 PST
Several theories have been advanced to estimate effective thermal conductivities of particulate mixtures, but most theories have focused on the dilute case. A method is proposed to estimate the effective thermal con-ductivity coefficient of mixtures of arbitrary concentration. Earlier the authors developed a theory to deter-mine the expected contact area between different species. This theory is employed to determine the Kapitza resistance of the heterogeneous mixture and forms part of an overall theory to estimate the effective thermal conductivity. Results are compared with other theoretical estimates and with experiments.
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Christiaan Richter et al.Modeling of a-Si : H deposition in a DC glow discharge reactor
https://digitalcommons.unl.edu/chemengreaction/3
https://digitalcommons.unl.edu/chemengreaction/3Mon, 07 Nov 2005 13:18:49 PST
PECVD reactors are increasingly used for the manufacturing of electronic components. This paper presents a reactor model for the deposition of amorphous hydrogenated silicon in a dc glow discharge of Ar-SiH_{4} The parallel-plate configuration is used in this study. Electron and positive ion densities have been calculated in a self-consistent way. A macroscopic description that is based on the Boltzmann equation with forwardscattering is used to calculate the ionization rate. The dissociation rate constant of SiH_{4} requires knowledge about the electron energy distribution function. Maxwell and Druyvesteyn distributions are compared and the numerical results show that the deposition rate is lower for the Druyvesteyn distribution. The plasma chemistry model includes silane, silyl, silylene, disilane, hydrogen, and atomic hydrogen. The sensitivity of the deposition rate toward the branching ratios SiH_{3} and SiH_{2} as well as H_{2} and H during silyl dissociation is examined. Further parameters that are considered in the sensitivity analysis include anode/cathode temperatures, pressure, applied voltage, gap distance, gap length, molar fraction of SiH_{4}, and flow speed. This work offers insight into the effects of all design and control variables.
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Dariusz Orlicki et al.A Macrokinetic Study of the High-Temperature Solid-Phase Titanium-Carbon Reaction
https://digitalcommons.unl.edu/chemengreaction/2
https://digitalcommons.unl.edu/chemengreaction/2Mon, 07 Nov 2005 12:12:49 PST
An experimental method, electro thermal explosion (ETE), is used to measure the macro kinetic parameters of the high-temperature titanium/carbon reaction. Different stages of the reaction have been identified, but the focus of this study is on the reaction between solid titanium and solid carbon, i.e., prior to the melting of titanium. The reaction has high activation energy, and an electric current is used to heat the cylindrically shaped sample to a specified temperature. The current is shut off at a temperature below the melting point of titanium; any further temperature rise is only due to reaction. The output of the ETE equipment is temperature time data that can be processed to recover the kinetic parameters. The activation energy and preexponential factor of the reaction rate constant are calculated and comprise 214 kJ mol-1 and (6.2 ( 1.5) _ 107 s-1, respectively. An important aspect of solid-phase reactions is the contact area between reactants. The contact area between titanium and carbon particles is calculated, and the reaction constant is corrected for this effect.
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Julia Y. Kostagorva et al.Reaction Kinetics Analyses of Amine-Cured Epoxies: First Shell Substitution Effects
https://digitalcommons.unl.edu/chemengreaction/1
https://digitalcommons.unl.edu/chemengreaction/1Wed, 12 Oct 2005 10:22:41 PDT
Kinetic reaction theory was used to model first shell substitution effects for several amine-cured epoxy resins, subject to the constraint of intermolecular reactions. Moment analysis will allow numerical simulations to be extended beyond the gel point. Functionalities for several chemical moieties were correlated as a function of conversion. Simulations also incorporated population density distribution dynamics. Analyses of several oligomers clearly demonstrated the dependency of oligomeric weight fractions on first shell substitution effects and conversion. The ratio of rate constants for reactions of secondary amino hydrogens relative to reactions of primary amino hydrogens ranged from 0.5 to 1.2 for the resins analyzed.
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Hossein Noureddini et al.