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Exothermicity of SHS Reactions
Exothermicity of SHS Reactions Alexander G. Merzhanov, Yury M. Maksimov
The main peculiarity of combustion is the exothermicity of the process [1]. High combustion temperatures lead to high reaction rates and complete conversion of initial substances in final products. There are necessary and sufficient conditions for propagation of the combustion wave according to the theory of combustion. The necessary conditions are given as follows β¼
RTad ≪1 E
(1)
γ¼
2 RTad ≪1 cEQ
(2)
where Tad ¼ T0 + Q=c is the adiabatic temperature of the process, Q is the heat of reaction, E is the activation energy, c is the specific heat at constant pressure, T0 is the initial temperature of reagents, and R is the gas constant. The parameters of the combustion reaction Q, E, c and the quantity T0 characterize the initial state of the system. The sufficient condition is determined by the relationship between the heat release in the combustion wave and the heat loss to the environment. In Eqs. (1), (2), small values of the parameters β and γ determine the strong temperature sensitivity and high exothermicity of the reaction. At the same time, in the combustion wave, the exponential temperature dependence of the heat release rate results from the fact that the material is heated along with a sharp, progressive self-acceleration and the very high temperatures with significant ΔT, where ΔT ¼ Tad T0 (Fig. 1). For the combustion wave propagation, the heat loss should be lower than a certain critical value (3) η < ηcr
(3)
The value η is determined by the expression (4) η¼
Tad Tc Tad T0
(4)
where Tc is the combustion temperature in the presence of heat losses. For the case ηcr ¼ γ. For the synthesis of metal borides (IV–VI group), the adiabatic combustion
116
Copyright © 2017 Elsevier Inc. All rights reserved.
Concise Encyclopedia of Self-Propagating High-Temperature Synthesis http://dx.doi.org/10.1016/B978-0-12-804173-4.00051-X
Exothermicity of SHS Reactions
T Tad
DT
T0
x
Fig. 1 Typical thermogram of the SHS process in the combustion wave: T is the temperature and x is the spatial coordinate.
temperatures are in the range of 1400–3500 K. The highest Tad are observed for the reaction with the formation of metal borides of group IV. Adiabatic temperatures during the combustion reaction of metals in nitrogen are high in the most cases (2500–5000 K). For metal nitrides such as Ti, Zr, Hf, and Al, the combustion temperatures exceed the melting temperatures by several hundred degrees. Dissociation of nitrides can take place because of the very high temperatures of Me-N combustion. The degree of dissociation and the combustion temperature depends on the nitrogen pressure. Comparison of the reaction combustion temperatures during the formation of various compound classes shows that the highest Tad are reached during reactions with the formation of nitrides and that the lowest Tad are reached during reactions with the formation of silicides (1500–3000 K). Carbides and borides are in the intermediate position. One of the ways to control SHS is in the change in the combustion temperature, which is achieved by preheating the reaction mixture or by dilution of the reagent mixture with a final reaction product. Thus, for example, dilution of the metal and boron mixture leads to a reduction of the combustion temperature by 200–900°C. The Tad calculations allow us to determine the conditions for the reagents in the reaction zone. There is the liquid-phase and the solid-phase combustion. In a liquid-phase mode (Tc > Tp of reagents and reaction products), the chemical reaction develops only in the liquid phase. In a solid-phase mode (Tc < Tp of reagents and products), the chemical reaction is determined by the process of reaction diffusion. Numerous investigations show that, in most cases, when the IV–VII group metals of the periodic table interact with boron, carbon, nitrogen, silicon, and sulfur, these temperatures are high and reach 2000–4000 K.
REFERENCE [1] Merzhanov AG. Combustion processes in chemical engineering and metallurgy. Chernogolovka: USSR Academy of Sciences; 1976. p. 5–29.
117
The main peculiarity of combustion is the exothermicity of the process [1]. High combustion temperatures lead to high reaction rates and complete conversion of initial substances in final products. There are necessary and sufficient conditions for propagation of the combustion wave according to the theory of combustion. The necessary conditions are given as follows β¼
RTad ≪1 E
(1)
γ¼
2 RTad ≪1 cEQ
(2)
where Tad ¼ T0 + Q=c is the adiabatic temperature of the process, Q is the heat of reaction, E is the activation energy, c is the specific heat at constant pressure, T0 is the initial temperature of reagents, and R is the gas constant. The parameters of the combustion reaction Q, E, c and the quantity T0 characterize the initial state of the system. The sufficient condition is determined by the relationship between the heat release in the combustion wave and the heat loss to the environment. In Eqs. (1), (2), small values of the parameters β and γ determine the strong temperature sensitivity and high exothermicity of the reaction. At the same time, in the combustion wave, the exponential temperature dependence of the heat release rate results from the fact that the material is heated along with a sharp, progressive self-acceleration and the very high temperatures with significant ΔT, where ΔT ¼ Tad T0 (Fig. 1). For the combustion wave propagation, the heat loss should be lower than a certain critical value (3) η < ηcr
(3)
The value η is determined by the expression (4) η¼
Tad Tc Tad T0
(4)
where Tc is the combustion temperature in the presence of heat losses. For the case ηcr ¼ γ. For the synthesis of metal borides (IV–VI group), the adiabatic combustion
116
Copyright © 2017 Elsevier Inc. All rights reserved.
Concise Encyclopedia of Self-Propagating High-Temperature Synthesis http://dx.doi.org/10.1016/B978-0-12-804173-4.00051-X
Exothermicity of SHS Reactions
T Tad
DT
T0
x
Fig. 1 Typical thermogram of the SHS process in the combustion wave: T is the temperature and x is the spatial coordinate.
temperatures are in the range of 1400–3500 K. The highest Tad are observed for the reaction with the formation of metal borides of group IV. Adiabatic temperatures during the combustion reaction of metals in nitrogen are high in the most cases (2500–5000 K). For metal nitrides such as Ti, Zr, Hf, and Al, the combustion temperatures exceed the melting temperatures by several hundred degrees. Dissociation of nitrides can take place because of the very high temperatures of Me-N combustion. The degree of dissociation and the combustion temperature depends on the nitrogen pressure. Comparison of the reaction combustion temperatures during the formation of various compound classes shows that the highest Tad are reached during reactions with the formation of nitrides and that the lowest Tad are reached during reactions with the formation of silicides (1500–3000 K). Carbides and borides are in the intermediate position. One of the ways to control SHS is in the change in the combustion temperature, which is achieved by preheating the reaction mixture or by dilution of the reagent mixture with a final reaction product. Thus, for example, dilution of the metal and boron mixture leads to a reduction of the combustion temperature by 200–900°C. The Tad calculations allow us to determine the conditions for the reagents in the reaction zone. There is the liquid-phase and the solid-phase combustion. In a liquid-phase mode (Tc > Tp of reagents and reaction products), the chemical reaction develops only in the liquid phase. In a solid-phase mode (Tc < Tp of reagents and products), the chemical reaction is determined by the process of reaction diffusion. Numerous investigations show that, in most cases, when the IV–VII group metals of the periodic table interact with boron, carbon, nitrogen, silicon, and sulfur, these temperatures are high and reach 2000–4000 K.
REFERENCE [1] Merzhanov AG. Combustion processes in chemical engineering and metallurgy. Chernogolovka: USSR Academy of Sciences; 1976. p. 5–29.
117