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Research of combustion of the aluminum nanopowder in the air allowed to determine the character
of the process of burning, the speed of a density variation of the thermal flows and to analyze
the end products of burning.
of the process of burning, the speed of a density variation of the thermal flows and to analyze
the end products of burning.
Aluminum powders are widely applied as additives to rocket fuels and pyrotechnic mixes [1]. An increase of dispersion decreases the powders’ losses and improves the other characteristics of the fuels, but, at that, the content of a metal aluminum is also decreased and the risk of occurrence of pyrophoric properties [2] is higher. The purpose of the work undertaken in the scientific-analytical centre of Tomsk Polytechnic University was research of the specific features of the process of burning of aluminum nanopowder (ANP) in the air.
The examined samples of ANP were obtained with the help of electroblasting experimental installation UDP-4G (fig.1) [3, 4] with an automatic conductor supply and with the frequency of the electric blasts of 0.6–0.8 Hz. The blast conductor was a wire with diameter of 0.35 mm and content of aluminum of 99.8%.
The working gas was argon applied at pressure 0.6 МPа. The conditions for obtaining of nanopowders are presented in table 1. Blasts were done in a quick explosion mode [5] and carried out with an arc stage. Energies – the specific entered energy (W/Ws) and that of an arc stage (Wd/Ws) were regulated by changing the charge pressure (U0) or the length (l) of the conductor (W – energy entered into the conductor at the explosion stage, Wd – energy entered at the stage of the arc discharge, Ws – energy of sublimation of the blasting conductor). Under investigation was a nanopowder passivated in the argon environment with a controllable content of the air.
The phase composition of nanopowders was determined with use of X-ray diffractometer Shimadzu XRD-7000 (CuKα). The size and form of the particles were analyzed by means of raster microscope Jeol JSM-7500FA. The area of the specific surface (Ssp) was measured by a low-temperature adsorption of nitrogen by BET method. The reactionary ability of ANP was determined by the chemical activity [6] with account of the temperature of the beginning of oxidation, the maximal speed of oxidation, the degree of oxidation of ANP and the specific thermal effect of oxidation. For calculation the thermograms obtained by means of Q600 thermoanalyzer were used.
For measurement of the power of the thermal flows during combustion of ANP Termomet-1 (ТМ-1) device [7] was used, preliminary calibrated by means of UTM-1 installation with a thermometrical chamber ensuring a thermal flow with density from 10 up to 2000 W/m2. According to the measurement technique corresponding to GOST 1855-88 (error not more than 1.5%) for determination of the actual density of a thermal flow in the working chamber of the installation EDTP 0924 master transducer was used.
Experiment
Before a sample was placed in TM-1 chamber, a thermogram was recorded and parameters of activity of ANP (fig.2) were calculated. As a result, its stability (pyrophoric ability) was determined at a room temperature.
The major thermal flow from ANP is observed at temperatures below 660°C. It brings the main contribution to the total thermal emission: 68.9% of the metal aluminum is oxidized (table 2) and 5956 J/g of heal are emitted. A sample of ANP was weighed on electronic scales (AND, WTB-200). For ignition of the attachment of ANP a red semi-conductor laser was used with power of 200 mW. During burning of ANP a signal of voltage from the central gauge in the top part of TM-1 chamber was sent to a computer through АDC and continuously recorded in time. For measurement of the thermal flows from the burning samples of ANP attachments from 0.2 up to 0.8 g were used. A typical dependence of variation of the rate of a heat flow on the time during burning of ANP is shown in fig.3.
The maximal speed of growth of the density of a heat flow depends on the size of the attachment (table 3). At that, two stages of burning are typical for ANP: the initial one is rather slow – from the moment of ignition up to the 20th second, and a faster one – from the 20th second up to the 30th second. The first stage with attachment of 0.8 g is characterized by an average speed of the growth of the density of a heat flow of 2.19 W/s∙m2 and the second – 3.79 W/s∙m2.
After burning of ANP in TM-1 chamber the phase composition of the combustion products was analyzed with the help of a roentgenophase analysis (RPA). As fig.4 demonstrates, the basic crystalline phase in the combustion products is aluminum nitride. It is necessary also to point out that the initial ANP is present in those products in quantities comparable with the nitride.
Results
If the mass of an attachment varied from 0.800 up to 0.812 g (table 3, samples 1–7), the speed of growth of the density of the heat flows changed from 1.68 up to 2.65 W/s∙m2. At the first stage a nonstationary burning of ANP was observed [8], and the oxygen, which accumulated during adsorption and dissociation of the molecules of water on the surface of nanoparticles during passivation, burnt out.
After a diffusion of the protons inside of the nanoparticles there happened a restoration of the protons up to the atoms of hydrogen and oxidation of a metal [9]. The mass share of the absorbed hydrogen in ANP reached 1.5%. The enthalpy of aluminum combustion at the first stage depends on the form of an attachment, its density and content of hydrogen, therefore a considerable difference in the density of a heat flow was observed. Similar results were obtained with smaller attachments (table 3, samples 8–14), although, according to the received data, the density of such a flow goes down with the reduction of the weight of the samples.
The second stage of ANP burning goes on more actively and the density of a heat flow increases on average 1.5 times. At that, the spread of the values of density did not decrease – the values of the experimental data differ roughly two times. The average values of the speed of growth of the density of the heat flows for the first and second stages of ANP burning are equal, correspondingly to 2.19 and 3.79 W/s∙m2. The average value of the heat energy during ANP burning in the first series of experiments (attachments of 0.800–0.812 g) equals to
661.16±38.36 J/m2.
Thus, burning of ANP at the first and second stages differs from the burning of combustible materials in a massive state by its nonstationarity. A characteristic property of ANP is dependence of the speed of change of the density of the heat flows on the weight of an attachment. The thermal energy density depends on a correlation of the aluminum nitride and oxide in the combustion end products. ■
The examined samples of ANP were obtained with the help of electroblasting experimental installation UDP-4G (fig.1) [3, 4] with an automatic conductor supply and with the frequency of the electric blasts of 0.6–0.8 Hz. The blast conductor was a wire with diameter of 0.35 mm and content of aluminum of 99.8%.
The working gas was argon applied at pressure 0.6 МPа. The conditions for obtaining of nanopowders are presented in table 1. Blasts were done in a quick explosion mode [5] and carried out with an arc stage. Energies – the specific entered energy (W/Ws) and that of an arc stage (Wd/Ws) were regulated by changing the charge pressure (U0) or the length (l) of the conductor (W – energy entered into the conductor at the explosion stage, Wd – energy entered at the stage of the arc discharge, Ws – energy of sublimation of the blasting conductor). Under investigation was a nanopowder passivated in the argon environment with a controllable content of the air.
The phase composition of nanopowders was determined with use of X-ray diffractometer Shimadzu XRD-7000 (CuKα). The size and form of the particles were analyzed by means of raster microscope Jeol JSM-7500FA. The area of the specific surface (Ssp) was measured by a low-temperature adsorption of nitrogen by BET method. The reactionary ability of ANP was determined by the chemical activity [6] with account of the temperature of the beginning of oxidation, the maximal speed of oxidation, the degree of oxidation of ANP and the specific thermal effect of oxidation. For calculation the thermograms obtained by means of Q600 thermoanalyzer were used.
For measurement of the power of the thermal flows during combustion of ANP Termomet-1 (ТМ-1) device [7] was used, preliminary calibrated by means of UTM-1 installation with a thermometrical chamber ensuring a thermal flow with density from 10 up to 2000 W/m2. According to the measurement technique corresponding to GOST 1855-88 (error not more than 1.5%) for determination of the actual density of a thermal flow in the working chamber of the installation EDTP 0924 master transducer was used.
Experiment
Before a sample was placed in TM-1 chamber, a thermogram was recorded and parameters of activity of ANP (fig.2) were calculated. As a result, its stability (pyrophoric ability) was determined at a room temperature.
The major thermal flow from ANP is observed at temperatures below 660°C. It brings the main contribution to the total thermal emission: 68.9% of the metal aluminum is oxidized (table 2) and 5956 J/g of heal are emitted. A sample of ANP was weighed on electronic scales (AND, WTB-200). For ignition of the attachment of ANP a red semi-conductor laser was used with power of 200 mW. During burning of ANP a signal of voltage from the central gauge in the top part of TM-1 chamber was sent to a computer through АDC and continuously recorded in time. For measurement of the thermal flows from the burning samples of ANP attachments from 0.2 up to 0.8 g were used. A typical dependence of variation of the rate of a heat flow on the time during burning of ANP is shown in fig.3.
The maximal speed of growth of the density of a heat flow depends on the size of the attachment (table 3). At that, two stages of burning are typical for ANP: the initial one is rather slow – from the moment of ignition up to the 20th second, and a faster one – from the 20th second up to the 30th second. The first stage with attachment of 0.8 g is characterized by an average speed of the growth of the density of a heat flow of 2.19 W/s∙m2 and the second – 3.79 W/s∙m2.
After burning of ANP in TM-1 chamber the phase composition of the combustion products was analyzed with the help of a roentgenophase analysis (RPA). As fig.4 demonstrates, the basic crystalline phase in the combustion products is aluminum nitride. It is necessary also to point out that the initial ANP is present in those products in quantities comparable with the nitride.
Results
If the mass of an attachment varied from 0.800 up to 0.812 g (table 3, samples 1–7), the speed of growth of the density of the heat flows changed from 1.68 up to 2.65 W/s∙m2. At the first stage a nonstationary burning of ANP was observed [8], and the oxygen, which accumulated during adsorption and dissociation of the molecules of water on the surface of nanoparticles during passivation, burnt out.
After a diffusion of the protons inside of the nanoparticles there happened a restoration of the protons up to the atoms of hydrogen and oxidation of a metal [9]. The mass share of the absorbed hydrogen in ANP reached 1.5%. The enthalpy of aluminum combustion at the first stage depends on the form of an attachment, its density and content of hydrogen, therefore a considerable difference in the density of a heat flow was observed. Similar results were obtained with smaller attachments (table 3, samples 8–14), although, according to the received data, the density of such a flow goes down with the reduction of the weight of the samples.
The second stage of ANP burning goes on more actively and the density of a heat flow increases on average 1.5 times. At that, the spread of the values of density did not decrease – the values of the experimental data differ roughly two times. The average values of the speed of growth of the density of the heat flows for the first and second stages of ANP burning are equal, correspondingly to 2.19 and 3.79 W/s∙m2. The average value of the heat energy during ANP burning in the first series of experiments (attachments of 0.800–0.812 g) equals to
661.16±38.36 J/m2.
Thus, burning of ANP at the first and second stages differs from the burning of combustible materials in a massive state by its nonstationarity. A characteristic property of ANP is dependence of the speed of change of the density of the heat flows on the weight of an attachment. The thermal energy density depends on a correlation of the aluminum nitride and oxide in the combustion end products. ■
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