Thermodynamics combustion synthesis

Modem computational programs [4] and thermodynamic tables [5] now make it possible to explicitly calculate metal-oxygen flame temperatures, thereby opening up a unique aspect of combustion thermodynamics that could be important in the consideration of metal as fuels and as reactants in combustion synthesis. 

A theoretical analysis of combustion synthesis of refractory nitrides was presented by Munir and Holt in 1987.37 They predicted the existence of an activation energy due to chemical reaction or mass-transport. Glassman et al. in 1987,38 in their thermodynamic analysis of TiN formation, examined the possibility of creating TiN by a self-sustained reaction of the metal particles and nitrogen gas in a rocket motor. They reported that for the stoichiometric ratio of 0.5 mole N2/mole titanium, the reaction has... 

Raffaella Ocone and Gianni Astarita, Kinetics and Thermodynamics in Multicomponent Mixtures Arvind Varma, Alexander S. Rogachev, Alexandra S. Mukasyan, and Stephen Hwang, Combustion Synthesis of Advanced Materials Principles and Applications J. A. M. Kuipers and W. P. Mo, van Swaaij, Computional Fluid Dynamics Applied to Chemical Reaction Engineering... 

Through thermodynamic calculation for adjusting the combustion temperatures of different layer of TiBj-Cu FGM, SHS/HIP was successfully used both for combustion-synthesis of the fundamental FGM constitute and for densification of FGM in one step, and TiBj-Cu FGM samples without macro-defects has been obtained. 

The foundation for the combustion synthesis technique lies in the thermodynamic concepts used in the field of propellants and explosives, and its extrapolation to the combustion synthesis of ceramic oxides and its thermodynamic interpretation are discussed extensively by several researchers. The success of this process is closely linked to the mix of constituents of a suitable fuel or complex-ing agent (e.g., citric acid, urea, and glycine) in water and an exothermic redox reaction between the fuel and the oxidant (e.g., nitrates). 

Combustion synthesis also termed self-propagating high-temperature synthesis (SHS) is a strong exothermic chemical reaction that leads to the formation of new thermodynamically stable materials with structures often not obtained under different reaction conditions. In some way or the other, both Goldschmidt and Matignon (Chapter 2) can be considered to be the first to have apphed this concept successfully to synthesize materials. However, the term SHS in today s perception is linked mainly to the work of Russian Chemist Alexander G. Merzhanov ( 1931) who has been working on this concept since the 1960s [1, 2]. 

None of the programs can predict kinetics, that is, the rate of reaction, the activation energy, or the order of the reaction. These parameters can only be determined experimentally. Except for CHETAH, the primary use of the programs is to compute the enthalpies of decomposition and combustion. In fact, acid-base neutralization, exothermic dilution, partial oxidation, nitration, halogenation, and other synthesis reactions are not included in the programs except for CHETAH, which can be used to calculate the thermodynamics of essentially any reaction. 

As discussed above, the pyrolysis of biomass at high temperature (>1000 °C) results in the formation of synthesis gas, a valuable mixture of CO and H2. The decomposition of carbohydrate to synthesis gas is an endothermic reaction since the heating value of product is —125% of that of the feedstock (Reaction 1). The reaction becomes nearly thermo-neutral upon burning about 1/4 of the products. Since the thermodynamics favors the combustion of H2 over CO, the gasification reaction resemble the theoretical Reaction (2). Indeed numerous gasification processes feed 02 or air to drive the gasification reaction. 

The ODH of ethylbenzene to styrene is a highly promising alternative to the industrial process of non-oxidative dehydrogenation (DH). The main advantages are lower reaction temperatures of only 300 500 °C and the absence of a thermodynamic equilibrium. Coke formation is effectively reduced by working in an oxidative atmosphere, thus the presence of excess steam, which is the most expensive factor in industrial styrene synthesis, can be avoided. However, this process is still not commercialized so far due to insufficient styrene yields on the cost of unwanted hydrocarbon combustion to CO and C02, as well as the formation of styrene oxide, which is difficult to remove from the raw product. 

The main characteristics of the green mixture used to control the CS process include mean reactant particle sizes, size distribution of the reactant particles reactant stoichiometry, j, initial density, po size of the sample, D initial temperature, Tq dilution, b, that is, fraction of the inert diluent in the initial mixture and reactant or inert gas pressure, p. In general, the combustion front propagation velocity, U, and the temperature-time profile of the synthesis process, T(t), depend on all of these parameters. The most commonly used characteristic of the temperature history is the maximum combustion temperature, T -In the case of negligible heat losses and complete conversion of reactants, this temperature equals the thermodynamically determined adiabatic temperature (see also Section V,A). However, heat losses can be significant and the reaction may be incomplete. In these cases, the maximum combustion temperature also depends on the experimental parameters noted earlier. 

Thermodynamic calculations have also been used to determine the equilibrium products (Mamyan and Vershinnikov, 1992 Shiryaev et ai, 1993), and to illustrate new possibilities for controlling the synthesis process, even for complex multicomponent systems. Correlating these calculations with the equilibrium phase diagram for each system provides a basis for predicting possible chemical interactions and even limits of combustion during CS of materials. Some examples are discussed in the following subsections. 

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