ISSN:0010-2202    

DOI:10.1080/00102209308947232

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A mathematical model which describes the process when a spherical metallic particle is exposed to an oxidizing atmosphere, has been formulated. We refer to the oxidation process in a generic sense, and the model can be used for the formation of oxides, nitrides and sulphides in general. The oxidation process is modeled for oxides which form n-type semiconductors with the defects in the form of interstitial cations and electrons (e.g., Al2O3) in terms of mass and energy transport phenomena.It accounts for the release of metal cations and electrons at the metal-oxide interface, the transport of these species through the oxide layer due to concentration and eleclric field gradients, and the formation of the metal oxide at the outer interface. As a result a hollow oxide sphere develops as the metal is converted 10 oxide. If the energy released by the oxidation reaction exceeds the heat losses, self-heating will occur. Such a rise in temperature accelerates the release of cations and electrons from the metal into the oxide as well as the transport of cations. This can eventually lead to the total combustion of the particle. The model is solved numerically by the Galerkin Finite Element Method. The effects of several process variables are investigated for the aluminum-oxygen case study: initial passivating oxide layer thickness, average particle size, storage temperature and oxidant partial pressure, and heat losses. For certain limiting conditions, a simplified model is developed, and an approximate analytical treatment is presented.

ISSN:0010-2202    

DOI:10.1080/00102209308947233

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The combustion synthesis of the YBa2Cu3Oz–xceramic superconductor was achieved from CuBaO2and Y2O3mixtures, using both self-propagating high-temperature synthesis (SHS) and thermal explosion modes. Particular attention was paid to establishing the mechanism of formation of the YBa2Cu3O3–2product. Previous studies included effects of BaO2and Cu particle sizes, along with quenching of reacting pellets. It was shown that the mixture of reactants undergoes a complex sequence of reactions before eventually forming the final product. In the present work, the effect of the Y2O3particle size has been investigated. The initiation of YBa2Cu3O7–xformation has also been studied by providing a portion of the copper as either Cu2O or CuO. It is shown that the oxidation of Cu to form Cu2O is a necessary step for the reaction to become self-sustaining, but that excessive oxidation to Cu2O inhibits the reaction, leading to partially reacted pellets.

ISSN:0010-2202    

DOI:10.1080/00102209308947234

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Abstract–The synthesis of multicomponent ceramic systems by combustion reactions is investigated. These systems can be either composites or solid solutions depending on the miscibility of the individual phases. Representative systems studied include the TiR2–TiC and Si2N4–SiC system for the first case and the Ti(C, N) and SIALONS for the second. Various self-propagating reaction schemes have been considered for the synthesis ofeach particular system. The effects of parameters like particle size of the reactant powders, dilution of the solid phase with a final product (lowering of a combustion temperature) and gas pressure, on the combustion characteristics and especially on the phase composition of the synthesized product arc studied in detail. in order to determine the optimum synthesis conditions for the systems under study.

ISSN:0010-2202    

DOI:10.1080/00102209308947235

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Abstract–Theoretical analyses are provided for the establishment of a self-propagating high-temperature synthesis (SHS) diagram for TiC. Such diagrams include boundaries between SHS and non-SHS regimes and between stable and unstable wave propagation regions, Experimental results obtained on this carbide were in agreement with the predicted SHS/non-SHS boundary calculated for an activation energy of 397 kJ mol–1(95 kcal mol–1). consistent with a reaction controlled by the diffusion of carbon in titanium carbide.

ISSN:0010-2202    

DOI:10.1080/00102209308947236

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Abstract–Combustion synthesis of the Ni,AI intcrmetallic was studied using the self-propagating hightemperature synthesis (SHS) mode. Since the reaction wave propagates rather quickly through the sample in SHS, a successful application of this process requires an understanding of the influence of processing variables on its dynamics, and on the phase composition and microstructure of the product. Thus, the effects of green density, ignition power, preheating temperature, and the particle sizes of aluminum and nickel were investigated. In addition, nickel particles with different morphology were used. It was found that higher green density, lower ignition power and higher preheating temperature lead to fully reacted product with a well-developed microstructure. The reactant particle sizes and their morphology also significantly influence the product properties. The obtained results arc helpful in understanding the mechanism of for mation of Ni3Al.

ISSN:0010-2202    

DOI:10.1080/00102209308947237

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

It has been observed that the gasless combustion of thermites can proceed in a variety of nonsteady propagation modes that range form periodic to chaotic in character. While the nature of the transition from steady to nonsteady, but periodic, combustion is now weJl understood, the various mechanisms that lead to more complex te.g., chaotic) modes of burning are is not. However, it has been shown that mode interactions which arise after the neutral stability boundary is crossed do lead to combustion waves that exhibit more complicated spatial and temporal behavior. In the present work, we foucus on the case of temporally resonant mode interactions and show how such interactions can provide a new route to chaos in gaslcss systems. The resulting character of the propagating combustion wave is that or chaotic. multiple-point combustion, in which one may observe the random movement of hot spots that appear, disappear and reappear on the sample surface, as reported in recent experiments. Various quantitative measures of this attractor are then presented.

ISSN:0010-2202    

DOI:10.1080/00102209308947238

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Abstract-We consider a model of filtration combustion in a thin porous layer immersed in a bath of gaseous oxidizer. which provides for exchange of the oxidizer between the pores and the bath. We employ approximate analytical methods to derive explicit analytical expressions for various quantities associated with uniformly propagating solutions, including the combustion temperature, the propagation velocity, and the final depth of conversion, as well as results about the structure of the solution. We also employ numerical computation to find profiles for the reaction rate, temperature. pressure, density and depth of conversion, as well as to determine the stability of the solutions as a function of various parameters. We identify a new pulsating instability associated with gas mass transfer between the pores and the bath, and describe the mechanism of pulsations.

ISSN:0010-2202    

DOI:10.1080/00102209308947239

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Abstract-We propose a mathematical model for the combustion of porous deformable condensed materials, which we use to describe the deformation of the high temperature products, induced by the pressure difference of the gas outside and inside the sample, in the absence of any external forces. The deformation occurs as a result of pore compaction (expansion), resulting in a more (less) dense product material. To describe the evolution of porosity we derive an equation which allows us to define a characteristic time ofdeformation fJ. If td is sufficiently smaller than the characteristic time of combustion t, the deformation process is sufficiently fast to compensate for pressure gradients. so that pressure is equalized almost instantaneously, and filtration is suppressed. If td> trdeformation occurs solely in the product, and does not affect the propagation velocity. We determine various characteristics of a uniformly propagating combustion wave, and the materials produced by it, such as the propagation velocity. combustion temperature, final depth of conversion and final porosity of the product, as a function of the thermophysical parameters of the system. We also identify a regime of pulsating propagation, in which case the final porosity of the products is periodic in space. We show that both the uniformly propagating wave and the pulsating propagating wave solutions are stable, each corresponding to its own parameter regime. We also find that the deformation process can affect stability. In particular, the effect of viscosity is found to be stabilizing.

ISSN:0010-2202    

DOI:10.1080/00102209308947240

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor