CC BY
38iSHS 2019
Moscow, Russia
KINETIC MEASUREMENTS FOR SOLUTION COMBUSTION SYNTHESIS
PROCESSES
N. Amirkhanyan*", S. L. Kharatyan^, and K. V. Manukyanc
aLaboratory of Kinetics of SHS Processes, Institute of Chemical Physics NAS of Armenia,
Yerevan 0014, Armenia
bDepartment of Chemistry, Yerevan State University, Yerevan 0025, Armenia cNuclear Science Laboratory, Department of Physics, University of Notre Dame, Notre Dame,
Indiana 46556, United States *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10007
Solution combustion synthesis (SCS) is a unique approach for the fabrication of nanoscale materials and thin films for applications ranging from electronics to biomaterials [1]. This method of materials' synthesis involves self-sustained non-catalytic heterogeneous reactions. Because SCS is a self-sustained thermal process where the primary source of heat comes from combustion reactions, it can be considered as a specific type of self-propagating high-temperature synthesis (SHS) or combustion synthesis (CS). In SCS processes, two main features—reaction duration and temperature—define the products' morphology, structure, and composition. Therefore, it is critical to control the reaction conditions and obtain knowledge on the kinetics for the SCS processes to control the properties of the resulting materials. SCS reactions between metal nitrates, such as Ni(NO3)2 and fuel (e.g., glycine, C2H5NO2) can be typically presented as follows:
10m 5 10m 25« 5^ + 9
Ni(NO3)2+ -^-C2HsNO2+-(9-1)O2 = NiO+^-CO2+ N2
where 9 is the fuel-to-oxidizer ratio, 9 =1 implies that all oxygen required for complete combustion of fuel derives from the oxidizer, while 9 > 1 (< 1) implies fuel-rich (or lean) conditions.
The chemical mechanism of these reactions is complex. Therefore, obtaining kinetic data that describes the rate of individual chemical reactions is difficult. Thermal analysis methods (e.g., differential thermal analysis, DTA) were used to extract kinetic parameters of some SCS reactions. For example, for the nickel nitrate-urea system, the obtained apparent activation energy using the Kissinger method was reported to be ~ 180 kJ/mol [2]. DTA analysis of nickel nitrate-glycine system with excessive amounts of glycine fuel revealed a two-step process with different activation energies [3]. It was speculated that the first process with an activation energy of ~ 123 kJ/mol corresponds to the reaction between NH3 and HNO3, which forms during decomposition of glycine and nickel nitrate, respectively. The second reaction may be related to the reduction of NiO by hydrogen. The activation energy of this reaction is lower (~ 111 kJ/mol) and within values for hydrogen reduction (85-110 kJ/mol) reported elsewhere. Thermal analysis methods, however, are limited to low heating rates (0.2-1 K/s). In typical SCS reactions, the heating rates can be as high as 500-1000 K/s. Therefore, the application of realtime combustion diagnostic tools to determine the kinetic parameters is of practical interest.
In this work, we use rapid micro-thermocouple measurements to record time-temperature profiles and determine front propagation velocities in the Ni(NO3)2 + C2H5NO2 system to extract apparent activation energy of the processes. The measured parameters were treated using the theory proposed by Zeldovich and Frank-Kamenetskii [4], according to which the
XV International Symposium on Self-Propagating High-Temperature Synthesis
combustion front propagation velocity (V), maximum temperature (T), and apparent activation energy (Ea) of the processes are related by the following equation:
7 n t Ea
in I — ) = const — —= \TJ 2RT
where R is the gas constant and equals to 8.314 J-K-1-mol-1.
In a typical experiment, reactive solutions were prepared first by dissolving an oxidizer, such as nickel nitrate hydrate (Ni(NO3)2-6H2O and fuels (glycine, methenamine, citric acid) with 9 ratio in deionized water and thoroughly stirred. The obtained solutions were poured into a boat (50 mm in length, 10 mm width, and 10 mm height) and dried at 373 K for the different durations to evaporate the solvent and produce reactive gels. The SCS reaction was initiated by the local preheating (spot of ~ 1 mm3) of gels in the air by a resistively heated tungsten wire. After initiation, the chemical reaction propagates through the gels in the form of a moving combustion wave. The time-temperature profiles (Fig. 1a) of the process is recorded by two 100 p,m K-type thermocouples inserted inside the reactive gels. Multiple runs were used to measure the average combustion temperatures. The front propagation velocities were determined by dividing the distance between thermocouples (D) by temporal distance between thermocouple signals (t). A typical Arrhenius-type plot of ln(V/T) against the reciprocal temperature for Ni(NO3)2 + C2H5NO2 (9 = 0.75) is shown in Fig. 1b. Linear fitting of the data points permits to extract apparent activation energies of 84 ± 8 kJmol-1. We can suggest that such measurements based on real-time combustion diagnostic methods provide more reliable estimation of the apparent activation energies. Other examples of such measurements involving different systems will also be outlined during the presentation. The role of different fuels, solvents and other reaction conditions will be presented and discussed.
Reciprocal Temperature, K"1
Fig. 1. (a) Typical time-temperature profiles for Ni(NO3)2 + C2H5NO2 system to extract maximum combustion temperature, T, and combustion front propagation velocity, V, where D is the distance between thermocouples in cm and t is the temporal distance of thermocouple signals in seconds. (b) Linear fitting of the ln(V/T) against the reciprocal temperature to extract apparent activation energy (Ea) for NiO formation in fuel-lean conditions.
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3. A. Kumar, E.E. Wolf, A.S. Mukasyan, Solution combustion synthesis of metal nanopowders: Nickel-reaction pathways, AIChE J., 2011, vol. 57, pp. 2207-2213.
4. Y.B. Zeldovich, D.A. Frank-Kamenetskii, Theory of thermal flame propagation (in Russian), JPhys Chem, 1938, vol. 12, pp. 100-105.
