CC BY
15PLASMA-CHEMICAL ACTIVATION OF SHS IN EXTERNAL
ELECTRIC FIELDS
A. I. Kirdyashkin*" and R. M. Gabbasov"
aTomsk Scientific Center SB RAS, Tomsk, 634055 Russia
*e-mail: kirdyashkin_a@mail.ru
DOI: 10.24411/9999-0014A-2019-10061
Application of external electric field refers to known methods for controlling the combustion dynamics of gas and condensed systems. Field effects are provided by its (i) thermal and (ii) nonthermal factors: (i) the temperature acceleration due to Joule heating and (ii) direct effect on kinetics of heat and mass transfer and chemical reactions [1]. Nonthermal factors in gas systems are realizing during electric discharge in flame through an ionic wind and the generation of active chemical centers (radicals, ions, etc.).
For the combustion synthesis of inorganic materials, the thermal fields effects are caused by the electric current through bound particles skeleton of the green components and reaction products [2, 3]. In order to realize this effect in practice, it is required the Joule heading up to 100-300 W/cm3. Last condition is technically difficult to perform in a large volume of a mixture. It is important to implement less energy-consuming nonthermal field factors which are not fully studied for combustion synthesis.
The purpose of this work is to investigate nonthermal field effects on the ignition and combustion of metal-containing powder systems by the examples Ni-Al and Mo-Ti-B mixtures.
Research methods and materials
Powders Ni, Al, Mo, B, and Ti presented in Table 1 were used as initial reagents. Composition of the mixtures Ni + (13.3-31.5) wt % Al and Mo + 12.1 wt % B + 6.5 wt % Ti was chosen in such a way to conduct the exothermic synthesis of condensed nickel aluminides, molybdenum and titanium borides, according to the brutto-reaction schemes: Ni + aAl ^ 0.5(3a-1)NiAl + 0.5(1-a)Ni3Al; Mo + PB + yTi ^ MoB + yTiB2, where a = 0.33-1.00, p = 1.32, y = 0.16 are the mole stoichiometric coefficients.
The mixtures were pressed into cylindrical briquettes (d = 20 mm, h = 10 mm) or placed inside the quartz tube (d = 11 mm, h = 40 mm). The relative density of the mixtures was Pr = 0.20-0.55. Field effects were studied in the processes described below.
A. Propagation of combustion in mixtures inside a quartz tube (Fig. 1a). From various sources, constant, alternating and unipolar pulse voltages were applied to the ends of the sample. Constant voltage: 0.1-4.5 kV. Alternating voltage 0.5-4.5 kV, frequency of 1-200 kHz. Pulse voltage: pulse amplitude 1-25 kV, pulse duration 0.005-3 p,s and pulse frequency 1 kHz. The constant and alternating voltage were applied through the circuit: «sample - ballast resistance - reference resistance». Pulse voltage was applied directly to the mixture. The dynamics of the combustion was controlled with a video camera. Gas medium: Ar, 100 kPa.
B. Ignition of the pressed powder compact with an electro-spiral through a gap of 3 mm (Fig. 1b). The direct voltage of ±650 V was applied to the electrical circuit «sample - spiral -ballast resistance - reference resistance» with the realization of self-sustained glow discharge between the spiral and the sample. The temperature dynamics was controlled by the C-type thermocouple. The latter was pressed into the near-surface layer of the sample. Gas medium: Ar, 100 Pa.
C. Ignition of one briquette of a mixture on another - translation of burning through a gap of
2.5 mm (Fig. 1c). The direct voltage of ± 100^500 V was applied to the electrical circuit «sample 1 - sample 2 - ballast resistance - reference resistance». Gas medium: Ar, 100 Pa.
Voltage (V), current (i) in electrical circuits was measured by the voltage across the reference resistances. In all cases, except for using pulse voltage, current was specially limited within 20 mA by selecting appropriate ballast resistance. For the case A, the average rate of temperature rise did not exceed 2.5 K/s. Taking into account the heat capacity of the substances and the volume of the samples, this fact means that Joule heating is within 10 W/cm3.
Fig. 1. Experimental setup for the study of non-thermal effects of electric fields on combustion (a) and ignition (b, c) of powder systems Ni-Al, Ti-Mo-B. DC, AC, PC are sources of constant, alternating and pulsed voltage, respectively; Ri , Rv are reference resistances. Rb = 200^230 kQ is the ballast resistance.
Table 1. Mixture components.
Powders Trademark C, wt.% d, ^m
Ni PNK-L5 99.9 < 20
Al ASD-4 99.7 < 10
Al ALEX 96.5 0.1
Mo MP 99.6 < 5
B 99A 99.5 < 0.5
Ti PTM1 99,0 < 45
C is the component concentration; d is the particle diameter.
Results and Discussion
Processes A. According to the data obtained, direct electric field has no effect on the dynamics of combustion. In alternating and pulse field, an average combustion rate may increase by a factor of two, with the effect amplified with increasing amplitude and frequency of oscillations and decreasing the duration of pulses of applied voltage. The effect of the field decreases with increasing a relative density and electrical conductivity of the mixture, increases together with the dispersity of initial particles, and depends nonlinearly on the chemical composition of samples. During combustion, the effect of the field has a cumulative nature. Electric field, starting from the ignition of the mixture, accelerates gradually the movement of reaction wave. Switching off the voltage during combustion leads to the monotonous deceleration of wave to its initial value. Preliminary treatment of mixtures by applying voltage for 10^15 s (maximum combustion duration for the samples used), followed by switching off voltage before ignition does not have a significant effect on combustion (Fig. 2). The effect of the field increases the degree of chemical conversions of condensed combustion products (Table 2).
Processes B. When a potential of the sample is positive, the ignition temperature decreases by 20%, and the ignition delay time decreases by 30% in comparison with ignition without voltage. For a negative potential, an increase in these parameters is observed (Table 3).
Processes C. The voltage in the gas layer between the samples of Ni + 31.5 wt % Al leads to the discharge induced by emissive combustion plasma of the sample 1. This is confirmed by the occurrence of current in the electrical circuit in the time interval between the completion of
combustion in the sample 1 and the ignition of the sample 2. The supply of a positive potential to sample 2 makes it possible to reduce the delay time of its ignition by more than an order of magnitude. Negative potential provides less effect (Table 4).
Fig. 2. Changes in the current combustion rate (uc) when alternating voltage is switch on and off at various times. V = 4kV, 125kHz. S is the voltage switching-on; O is the voltage outage; I is the ignition.
Table 2. Phase composition of combustion products.
Green mixtures
Normal conditions
Alternating voltage: 4 kV, 125 kHz
Ni+14wt.%Al Mo+12.1wt%B+6.5wt.%Ti
Ni, NiAl Mo, MoB, TÎ3B4
Ni3Al, Ni|, NiAl| Mo, MoB t, Ti3B4 I
Table 3. The effect of a constant electric field on the ignition of a Ni + 31.5 wt % Al mixture from an electrospiral. Al-ASD4, pr = 0.52._
Voltage, V
Power of electric spiral, W
1000
2200
7i, K ti, s Ti, K ti, s Ti, K ti, s
0 - - 1033 6.4 1183 2.1
650 448 44 873 4.5 923 1.5
-650 no ignition no ignition 1343 9.3 1433 3.0
ti is the ignition delay time; Ti is the ignition temperature
Table 4. The effect of a constant electric field on the ignition of one sample of a mixture of Ni + 31.5 wt % Al from another. Al-ASD4, pr =0,52_
Voltage, V
ti, s
0
1.7
+120 1.0
+140 0.6
+200 0.1
-400 1.0
0
The field effect on the ignition of the mixture with a positive potential is explained by the acceleration of reactions on the surface of Ni and Al powders exposed to negatively charged plasma particles of gas discharge (electrons and negative ions). This effect is known in a wide range of topochemical reactions on the solid surface of various metals that form volatile products [4]. The structure of the Ni + 31.5 wt % Al mixture which formed after a short treatment of the sample with the radiant heat of electric spiral was studied. It was found that for the sample with a positive potential, thin Ni coatings were formed on the surface of Al particles. Probable cause of Ni deposition is topochemical reactions induced by discharge: - surface of Ni particles: Ni (solid) + 4CO (gas) ^ Ni(CO)4 (gas),
- surface of Al particles:
Ni(CO)4 (gas) ^ Ni (solid) + 4CO (gas).
The source of CO is impurities of the initial Ni powder. The described processes provide the gas-phase transport of Ni and, as a consequence, the acceleration of chemical interaction between Ni and Al powders.
A similar plasma-chemical effect, apparently, accelerates combustion in alternating and pulse fields. This effect is caused by gas discharge in the porous space of the mixture, which induces the topochemical reactions before the combustion wave. The condition for discharge is met for highly porous mixtures in the case of comparable values of the specific electrical conductivity for the bound skeleton from mixture particles and plasma inside pores. Electro-transport through the porous space of the mixture confirms the presence of asymmetric electric current in the sample during combustion. Such current is typical for gas plasma. The increase in the combustion rate together with the frequency of the voltage is explained by improvement in the uniformity of discharge in the porous space. Diffuse discharge in gases takes place at a pressure of 100 kPa and, as a rule, at sufficiently high frequency and the short duration of pulsations of the electric fields [1]. Concluding Remarks.
- Gas-discharge plasma in direct electric field with the Joule heading up to 5 W/cm2 decreases the ignition temperature by 20^24% and significantly reduces the ignition delay time of the Ni-Al mixture.
- Alternating and pulse electric fields with Joule heating of less than 10 W/cm3 accelerate the combustion of highly porous Ni-Al, Mo-B-Ti powders mixture up to 250% and increase the degree of chemical conversions in combustion products.
- Changes in the characteristics of combustion are explained by the plasma-chemical effect of gas discharge in the peripheral porous space of powder mixtures. The effect provides the acceleration of gas-phase transport of the mixture components due to activation of the reactions on the particles surface.
This work was carried out within the state task for TSC SB RAS (no. 0365-2019-0004) and supported by RFBR (project no. 18-48-700037).
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2. I.A. Filimonov, N. Kidin, Thermal Effect of an External Electric Field, Int. J. Self-Propag. High-Temp. Synth., 2004, vol. 13, no. 4, pp. 263-284.
3. Z.A. Munir, Field effects in Self-propagating solid-state synthesis reactions, Solid State Ionics, 1997, vol. 101, pp. 991-1001.
4. A.N. Magunov. Heat transfer of a nonequilibrium plasma with a surface - in Russian, 2005, p. 312.
