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  • 1995-1999  (5)
  • Chemical Engineering  (5)
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  • 1
    Electronic Resource
    Electronic Resource
    Detailed kinetics of titanium nitride synthesis (1995)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 41 (1995), S. 377-388 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A thermogravimetric analyzer is used to study the synthesis of TiN from Ti powder over a wide range of temperature, conversion and heating rate, and for two Ti precursor powders with different morphologies. Conversions to TiN up to 99% are obtained with negligible oxygen contamination. Nonisothermal initial rate and isothermal data are used in a nonlinear least-squares minimization to determine the most appropriate rate law. The logarithmic rate law offers an excellent agreement between the experimental and calculated conversions to TiN and can predict afterburning, which is an important experimentally observed phenomenon. Due to the form of the logarithmic rate law, the observed activation energy is a function of effective particle size, extent of conversion, and temperature even when the intrinsic activation energy remains constant. This aspect explains discrepancies among activation energies obtained in previous studies. The frequently used sedimentation particle size is a poor measure of the powder reactivity. The BET surface area indicates the powder reactivity much better.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690410219
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Analysis of interface evolution and pattern formation during CVD (1995)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 41 (1995), S. 1926-1943 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Morphological aspects of the evolution of a gas - solid interface during typical CVD processes are presented, as well as a continuum model of CVD growth. A linear stability analysis used determines the effect of reactor conditions on the stability of planar growth. The main focus, however, is numerical solution of governing equations under a wide variety of conditions and with different initial interface shapes as starting point. Simplified solutions under specific deposition conditions and the numerical procedure for solving the complete system of equations are presented. The focuses are on the use of a parametrization that eliminates numerical problems encountered with steep interface gradients and the automatic generation of an adaptive mesh for the domain above the interface. Several examples illustrate the numerical solution procedure. To our knowledge, this is the first attempt to simulate interface evolution during CVD for long deposition times from various initial interface shapes. The simulation revealed several morphological phenomena observed experimentally in previous studies, including the formation of occlusions that contributes to film porosity and was clearly shown by the numerical results. Film uniformity strongly depends on the controlling mechanism of deposition. Severe nonuniformities develop under diffusional limitations, while deposition is very uniform under conditions of kinetics control. Film uniformity could be improved by choosing conditions for which a Damköhler number of deposition, Da, would have the lowest value.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690410810
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Reaction rate modeling in noncatalytic gas-solid systems: Species transport and mechanical stress (1995)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 41 (1995), S. 2614-2624 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: A detailed model to describe the overall reaction rate of the oxidation of titanium is developed. The mathematical model consists of two facets, the first of which involves a detailed description of species transport that accounts for the formation of charged species. This is augmented by a description of the occurrence of mechanical stress due to a Pilling - Bedworth ratio that differs significantly from I as well as differences between precursor and product thermal expansion coefficients. A self imposed electric field is formed across the oxide layer due to different mobilities of the species considered. This field opposes the transport of electrons and enhances the transport of anion vacancies, thus increasing the overall reaction rate compared to a pure diffusion process, while also ensuring that electrical charge is conserved. Large growth stresses result from the unmatched precursor and product volumes, significantly affecting the overall process. These results show that the incorporation of a consistent treatment of mechanical stress forms a necessay part of any accurate description of the overall behavior of a reacting particle.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690411210
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Deflagration and detonation in solid-solid combustion (1997)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 43 (1997), S. 3085-3094 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Solid-solid combustion becomes self-sustaining when the preheating of the fresh mixture is high enough to support a spontaneous chemical reaction. These reactions have high activation energies, requiring significant preheating. Traditionally, conduction has been considered as the main form of preheating, and propagation velocities in the order of a few mm to a few cm per second were found. When acoustic equations are included in the analysis, no significant changes occur for traditional SHS reactions. However, the analysis of a 1-D model propagating at a constant velocity reveals the existence of two other solutions with propagation velocities which are much faster. An SHS deflagration wave is found with combustion temperature lower than the adiabatic value. The propagation velocity is less than the longitudinal sound speed of the medium, but typical Mach numbers vary between 0.6 and 0.95. The third solution is an SHS detonation with temperature above the adiabatic value and supersonic propagation velocity. Since the heat fluxes are extremely high, the hyperbolic conduction model is used.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690431119
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Noncatalytic gas-solid reactions and mechanical stress generation (1995)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 41 (1995), S. 1235-1250 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: The reaction rate in gas-solid systems can be affected by mechanical stresses that arise as the reaction proceeds. Stresses develop due to differences between precursor and product molar volumes and thermal expansion coefficients. Experimental evidence on the interaction of reaction rate and mechanical stress for the Ti/N2 and Ti/O2 systems is provided. A detailed and consistent mathematical model is developed for the reaction taking place at the constrained precursor/product interface. An elastic formulation for the stresses is adopted, and stress generation due to mismatches in linear thermal expansion coefficients and equivalent volume (Pilling-Bedworth ratio) for the precursor and product are considered. The effect of surface energy, which becomes significant for particle sizes below 1 μm, is also included in the model. Both experimentally and theoretically, conditions exist where the mechanical stresses exceed the strength of the material, leading to mechanical breakdown of the product layer, thus causing a discontinuity in the observed reaction rate. The entire processing history, including the reaction, temperature, and pressure profiles, plays an important role in determining the overall reaction kinetics of the powder.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690410519
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    Paper (German National Licenses)
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