Features of thermal decomposition and ignition of HEMs with metal additives

Transcription

1 Features of thermal decomposition and ignition of HEMs with metal additives A G Korotkikh 1,2, V A Arkhipov 2, I V Sorokin 1 and K V Slyusarskiy 1 1 Institute of Power Engineering, Tomsk Polytechnic University, Tomsk, Russia 2 Research Institute of Applied Mathematics and Mechanics, Tomsk State University, Tomsk, Russia korotkikh@tpu.ru Abstract. The use of metal powder as a fuel in high-energy materials (HEMs) for propulsion is the most energy efficient method allows to the increase of combustion characteristics of composite solid propellants in the combustion chamber, and also specific impulse. Typically, microsized aluminum powder is used in the composition of original HEMs. To improve the ignition characteristics of aluminized HEMs advisable to use the catalyst (nonmetals, metals or their oxides). This paper presents the experimental data of the thermal decomposition and ignition processes of HEMs on based on ammonium perchlorate (AP) and butadiene rubber, containing metal nanopowders (NPs): aluminum, iron and boron. It was found that additive of iron NP in HEM with Alex decreases the ignition time by times under initiation by CO 2 -laser in air at the range of heat flux density W/cm 2 and increases of the recoil force of gasification products outflow from burning surface by 27 % at stationary combustion of propellants due to possible of the catalytic effect, which reduces the beginning temperature of AP high-temperature decomposition by ~20 C, and interaction of thermite mixture of aluminum and iron particles in the reaction layer of propellants. Upon additive of boron NP in HEM with Alex the ignition times are decreased by times, the recoil force of gasification products outflow from burning surface is slightly increased by 9 % while the beginning temperature of high-temperature decomposition of HEM does not change and equals to ~310 C. 1. Introduction According to the study [1] on the aluminum oxidation at combustion of composite solid propellants considerable influence renders the presence on the surface particles of refractory layer of aluminum oxide. Melting point of alumina significantly above the melting point of aluminum. The combustion aluminum particles possible at high temperature gradient near the reaction layer of high-energy materials (HEMs) near the combustion surface with occurrence of cracks and destruction of the oxide layer resulting in oxidation of the active metal. It is well known [1, 2] the destruction of the aluminum oxide layer of particles in possible cooperation with the carbon particles to form aluminum carbide. Analysis of obtained results [3] showed that the main role of iron powder additive is reduced to acceleration of thermal decomposition process of oxidizer and combustible binder occurring in condensed phase of HEM. Furthermore, the iron and titanium powders used for the intermetallic compounds by heating with aluminum powder reacts with high exothermic effect. This indirect to confirming is the reduction of the plate surface temperature at the same value of the ignition time of

2 sample under introduced into the HEMs these metal additives [4]. Additive of boron powder reduces the agglomeration of aluminum nanoparticles on the HEM surface of reaction layer at combustion composite solid propellants [5]. Moreover according to the thermodynamic calculations and experimental data [6] the additives of boron and iron allows to reduce of the mass fraction of condensed combustion products of HEMs. This paper presents the experimental data of thermal decomposition and the ignition characteristics of model HEMs based on ammonium perchlorate and butadiene rubber, containing metal nanopowders: aluminum, iron and amorphous boron. 2. Experimental 2.1. The HEM samples We studied the HEM samples on the basis of bidisperse AP (fraction less than 50 mm and mm in the ratio 40/60), inert combustible-binder (19.7 wt. %) butadiene rubber plasticized by transformer oil, and Alex aluminum NP (15.7 wt. %) obtained in argon using electric explosion of conductors. In the second and third compositions 2 wt. % of the Alex NP was partially replaced by 2 wt. % additives of iron and amorphous boron NPs. According to the measurements on the BET analyzer Nova 2200e in nitrogen the specific surface area of Alex amounts to 7.04 m2/g, iron 1.08 m2/g, amorphous boron 8.63 m2/g. Micrographs of scanning electron microscope Jeol JSM7500F of aluminum, iron and boron nanopowders are shown in Figure 1. According to the manufacturers data the metal content in the tested NP Alex comprised 85 wt. %, iron 92 wt. %, and boron 99.5 wt. %. (a) (c) Figure 1. SEM images of tested aluminum Alex (а), iron (b) and amorphous boron (c) NPs. (b)

3 The studied cylindrical samples of HEMs in diameter 10 mm and height 30 mm produced in the laboratory by extrusion pressing with the subsequent curing. The density of solid samples was in an interval of g/cm 3 depending on the component composition Ignition of HEMs The ignition process study of test CSPs was carried with the use of set up for the radiant heating on the basis of CO 2 -laser with the wavelength of 10.6 mm and power of 200 W (Figure 2). Prior to testing the samples were cut into tablets of 5 mm in height. The CSP samples were placed in the hollow cylinder of 10 mm depth made of ebonite for inhibiting the lateral surface and making one-dimensional flow of gasification products. Figure 2. The scheme of experimental setup based on CO 2 -laser: 1 CO 2 -laser; 2 beam-splitting mirror; 3 thermoelectric sensor of radiation power; 4 shutter; 5 lens; 6 CSP sample; 7 photodiodes; 8 transducer of recoil force; 9 ADCs; 10 PC; 11 thermal imager camera. The test CSP sample (6) was attached to the substrate of recoil force transducer (8) to register the gasification products outflow from the burning surface. When opening the shutter (4) the radiation was focused by the sodium chloride lens (5) to the CSP sample (6). Signals from the recoil force transducer (8) and photodiodes (7) were transmitted to the L-card-E ADC (9) and recorded in the personal computer (10), and then processed with the software application LGraph2. The time delay of start gasification t gas of CSP sample was determined as time interval between the moments of signals change of photodiode near the shutter (7) (or a thermocouple installed in the laser beam behind the shutter), and the recoil force transducer (8). Photodiode (7) registered the moment of opening the shutter, transducer (8) recorded the appearance of recoil force signal of gasification products flowing from the front (irradiated) sample surface. The ignition time t ign of CSPs was determined by difference between the moments of signals from two photodiodes (7), one of which registered the appearance of flame near the end surface of CSP sample. The relative error of delay times measuring of t gas and t ign was equal 5 12 % at the value of confidence probability 0.9. The values of recoil force of gasification products outflow from the end surface of CSP sample during the heating of reaction layer, ignition and combustion of CSPs were determined using recoil force transducer [7]. The radiation power and heat flux density of CO 2 -laser beam was measured by the thermoelectric sensor of radiation power (3). The maximum radiation power was defined in the center of the laser beam. 3. Results and discussion

4 3.1. Thermal decomposition data of HEMs Thermal analysis of HEM samples with metal nanopowders was carried out using of the simultaneous thermal analyzer NETZSCH STA 449 F3 Jupiter under argon at the heating rate of 10 C/min. Thermal decomposition data of test HEMs are presented in Figure 3. (a) (b) (c) Figure 3. Thermal decomposition data of HEMs with Alex (а), Alex/Fe (b) and Alex/B (c). Thermal analysis data shown that at the additive of iron NP in HEM with Alex the beginning temperature of intensive decomposition of HEM sample is reduced by ~20 C due to a possible catalytic effect [8], which decreases the beginning temperature of high-temperature decomposition of ammonium perchlorate. Additive of boron NP in HEM with Alex does not lead to change in the beginning temperature of intensive decomposition and equals ~310 C Ignition parameters of HEMs The values of ignition and gasification times, recoil force of gasification products outflow for the test HEM samples under study in atmospheric conditions were determined with use of laser setup. Processing of signal recording from the recoil force transducer, the time delays of ignition and gasification, the recoil force outflow of gasification products from the burning surface of HEM samples have been got. The ignition time of HEMs dependences vs. the maximum value of the heat flux density (in the center of the laser beam) were determined (Figure 4). Additive of iron NP in HEM with Alex leads to the decrease of the ignition time by times in the range of heat flux density W/cm 2. Additive of boron NP in HEM with Alex leads to the decrease of the ignition time by times in the specified range of q. For HEM sample with Alex, the gasification starts immediately after the shutter opens and the gasification time by 40 times less than the ignition time at the heat radiant flux density of q = 65 W/cm 2. For samples with Alex/Fe and Alex/B, the time moments of ignition start and gasification start coincide. Partial replacement of Alex

5 by iron in HEM at the initiation a CO 2 -laser leads to the increase of the gasification time by 25 times and the increase of the recoil force of gasification products outflow from burning surface by 1.3 times in the period of stationary combustion of HEM. For the partial replacement of Alex by boron in the HEM composition, the gasification time increases by 36 times and the recoil force of gasification products outflow slightly increases by 1.1 times. Figure 4. The ignition time of HEMs with metals nanopowders vs. the heat flux density. The temperature on the reaction layer surface of sample is determined at the moment of visible flame appearance using a thermal imaging camera Jade J 530 SB. The measured temperature values of samples are shown in Table 1. Table 1. The temperature on the reaction layer surface of HEM samples. q, The temperature on the reaction layer surface of samples T, С W/cm 2 with Alex with Alex/Fe with Alex/B ± ± ± ± ± ± 90 The temperature on the reaction layer surface of HEM samples with Alex/Fe and Alex/B at the time of appearance of visible flame is increased by ~130 and 200 C compared with the HEM sample with Alex. With the increase of heat flux density, the difference temperature is increased for sample with Alex/Fe up to 230 C due to a possible thermite reaction of Al+Fe 2 O 3 particles in the reaction layer surface of HEM. For HEMs with Alex/B the temperature on the reaction layer surface does not change and is equal to 770 C. Thus, the additives of iron and boron in HEMs with Alex allows to considerably increase the gasification time and to reduce the ignition time and also to increase the recoil force of gasification products outflow from the burning surface and the temperature on the reaction layer surface of sample. 4. Conclusions 1. The effect of iron and amorphous boron nanoadditives in HEM on the basis of AP, butadiene rubber and 15.7 wt. % of aluminum Alex on ignition characteristics were studied. It was found that additive of iron NP in HEM with Alex decreases the ignition time by times under initiation by CO 2 - laser in air at the range of heat flux density W/cm 2 and increases of the recoil force of gasification products outflow from burning surface by 27 % at stationary combustion of propellants due to possible of the catalytic effect, which reduces the beginning temperature of AP high-

6 temperature decomposition by ~20 C, and interaction of thermite mixture of aluminum and iron particles in condensed phase with the increase of temperature on the reaction layer surface of HEM. 2. Additive of boron NP in HEM with Alex the ignition times are decreased by times, the recoil force of gasification products outflow from burning surface is slightly increased by 9 % while the beginning temperature of high-temperature decomposition of HEM does not change and equals to ~310 C. Acknowledgement The reported study was partially supported by Russian Scientific Foundation Grant References [1] Pokhil P F, Belyaev A F, Frolov Yu V, Logachev V S and Korotkov A I 1972 Combustion of Powdered Metals in Active Media (Moscow: Nauka) [2] 2016 Energetic nanomaterials: Synthesis, Characterization, and Application ed Zarko V E and Gromov A A (Elsevier) [3] Komarova M V, Komarov V F, Vakutin A G and Yashenko A V 2010 Polzunovskiy Vestnik [4] Arkhipov V A, Korotkikh A G, Kuznetsov V T and Sinogina E S 2007 Khimicheskaya Fizika [5] Simonenko V N, Zarko V E, Kiskin A B, Sedoi V S and Birukov Yu A 2001 Energetic Materials: Production, Processing and Characterization of the 32 nd International Annual Conference of ICT [6] Korotkikh A G, Arkhipov V A, Glotov O G and Slyusarskiy K V 2015 IOP Conference Series: Materials Science and Engineering [7] Arkhipov V A, Kiskin A B, Zarko V E and Korotkikh A G 2014 Combustion Explosion and Shock Waves [8] Berner M K, Talawar M B and Zarko V E 2013 Combustion Explosion and Shock Waves