D
J. Chem. Chem. Eng. 6 (2012) 292-298
DAVID
PUBLISHING
Short-Wave Emission and Microdischarges during
Self-Propagating High-Temperature Synthesis
Kirdyashkin Alexandr Ivanovich1, Salamatov Vitaly Georgievich1, Maksimov Yury Mikhailovich1, Sosnin
Eduard Anatolievich2*, Tarasenko Victor Fedotovich2 and Gabbasov Ramil Mahmutovich1
1. Department for Structural Macrokinetics, Tomsk Scientific Center of SB RAS, Tomsk 634021, Russia
2. Institute of High Current Electronics SB RAS, Tomsk 634021, Russia
Received: November 29, 2011 / Accepted: January 12, 2012 / Published: March 25, 2012.
Abstract: Emission in the X-ray and ultraviolet (200-300 nanometers) region of spectrum is found out during combustion of
heterogeneous systems with the formation of condensed products, and pulses from microwave emission with short duration are
recorded as well. Combustion of a Ti-B powder system showed that self-propagating high-temperature synthesis (SHS) is accompanied
by two types of X-ray radiation. Radiation of the first type has the maximum quantum energy ~ 5 keV. It is supposed that this type is
caused by micro-breakdowns due to the charge separation in combustion products. Runaway electrons and soft X-ray radiation are
generated due to the concentration of electric field on microparticles during breakdown. Radiation of the second type has the quantum
energy up to ~ 15 keV. It is supposed that it is caused by exoemission of photons. UV radiation in the region of 200-300 nm is recorded
during SHS in different gases (He, Ar, N2). This radiation is shown to have the highest intensity in helium at the pressure ~ 25 × 103 Pa.
Key words: Self-propagating high-temperature synthesis, spectrophotometry, X-ray.
1. Introduction
Exothermal chemical reactions are widely spread in
natural phenomena and used by people for a long time.
Combustion of heterogeneous systems with the
formation of condensed products is characterized by
the high concentration of chemical energy per unit of
time (more than 1013 W/m3). This energy is higher than
the similar parameter of gas flames by 3-4 orders. The
high concentration of energy allows us to use SHS
systems as an intensive source of optical emission in
the visible region of spectrum [1]. However, emission
properties of combustion in condensed phases are
mostly studied for a visible part of spectrum [1-3]. The
additional information on emission features is
important for a deeper understanding of the chemical
reaction mechanism with the participation of
*
Corresponding author: Sosnin Eduard Anatolievich, Ph.D.,
research fields: lighting engineering, optics, quantum
electronics, photochemistry, photobiology, photomedicine.
E-mail: badik@loi.hcei.tsc.ru.
condensed substances, and for practical use as well.
The aim of this work is to investigate optical and
X-ray radiation during the SHS processes in powder
systems (Ti-B, Zr-B). This work is continuation of
investigations started in the works [4-5], where
recording of emission in the X-ray and ultraviolet UV
(200-300 nm) regions of spectrum was reported during
SHS.
2. Methods of Research for Dynamic and
Emission Spectra of SHS Wave
The experimental arrangement is given in Fig. 1. The
mixtures with a bulk density and weight of 1.0-1.5 g
were placed into a quartz cavity 20 mm in diameter,
which were located on the surface of a molybdenum
foil. The SHS reaction was initiated at a rate of 2-4 K/s
in a forevacuum or atmosphere of inert gases He, Ar,
N2 at the pressure of 70 Pa-100 kPa by heating of the
foil by an electric current. When the critical
temperature of 900-1200 K was reached, the mixture
Short-Wave Emission and Microdischarges during Self-Propagating High-Temperature Synthesis
293
Fig. 1 Experimental arrangement for measurement of dynamic emission spectra: 1—vacuum chamber; 2—green mixture;
3—quartz cavity; 4—molybdenum foil; 5—suspension cloud; 6—metal electrodes (0.2 mm, interelectrode distance 1.5 mm,
Wolfram); 7—ceramic insulator; 8—thermocouple; 9—oscillograph; 10—optical quartz window; 11—high-speed videocamera;
12—spectrometer; 13—computer; 14—quartz optical fiber (d = 0.6 mm); 15—photographic assemblage; 16—dosimeter.
was self-ignited and the reaction proceeded in the
thermal explosion regime. The calculated adiabatic
temperature of the products during SHS (without
considering preliminary heating) is 3190 K for Ti-B
and 3310 K for Zr-B according to Ref. [6].
Optical measurements were conducted in the range
of 200-1100 nm. The schemes of chemical conversions
of reaction systems can be presented in Eqs. (1) and (2).
Ti + 2B TiB2 + 1 eV/atom
(1)
Zr + 2B ZrB2 + 1.1 eV/atom
(2)
The powder mixtures were prepared in a
corresponding stoichiometrical ratio from the
following components: Ti, Zr (PTM and PTsRK
Russian trade marks) and amorphous black boron (of
98 % purity). The current through the reaction products
was recorded by electrodes (Fig. 1) powered by a
constant-voltage supply source. In the absence of the
external power supply, the electrodes recorded the
spontaneous electric polarization of the products
(EMF). The optical emission from the thermal
explosion region was recorded by spectrometers
through an optical quartz window of the reaction
chamber or quartz fiber by the HR4000 (Ocean Optics
B.V., λ = 200-300 nm), EPP2000C-25 (StellarNet Inc.,
λ = 200-850 nm), and “Spectra” (ITM, λ = 300-1100
nm) spectrometers. Spectra were measured in a
dynamic mode with a time resolution up to 5 mс. The
spectrometers were calibrated according to the
continuous spectrum of the SI 10-300u (visible range)
tungsten lamp and the spectrum of the XeCl-BD-P (λ =
307-309 nm) and XeBr-CD-P (λ = 275-285 nm)
excilamps in the near ultraviolet region [7]. The
reactions of mixtures were observed with a high-speed
video recording (Motion Pro X-3 videocamera).
For recording of X-ray radiation it used the acquired
experience in the study of X-ray radiation from
nanosecond gas discharges [8]. The presence and
intensity of X-ray radiation were registered onto a flare
of the RF-3 X-ray photofilm and by an Arrow-Tech,
Inc (Model 138) X-ray dosimeter as well. The
dosimeter had a maximum sensitivity for the quantum
energy more than 16 keV and threshold of sensitivity
for the quantum energy ~ 5 keV. To prevent the action
of optical emission, the photofilm was placed into a
light-impermeable envelope made of dense black paper
~100 m in thickness. In some experiments, the plastic
tape, 120 m in thickness, or metal grid were placed on
the photofilm surface to screen a part of the photofilm
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Short-Wave Emission and Microdischarges during Self-Propagating High-Temperature Synthesis
surface. The photographing assembly (aluminium foil
(10 m), photofilm, absorbing plate, and envelope) and
dosimeter were placed in the top part of the vacuum
chamber at a distance of 20-25 cm above the surface of
the powder mixture (Fig. 1).
The signal from the antennas was registered at
TDS-3034 oscillograph. These antennas differed in the
sizes and time characteristics accordingly.
It should be noted that the effect of reaction product
dispersion during thermal explosion intensifies
released emission at different stages of chemical
conversions, which is connected with the increase in
free emitting surface of condensed phases and
transparency of a reaction media.
3. Results and Discussion
The research showed that the flow of gas emission
(gasification of volatile impurities, evaporation of
components) during the SHS process resulted in the
formation of an emitting suspension cloud. The cloud
had a diameter up to 25 cm and consisted of droplets of
the condensed product 10 - 200 m in size and ionized
gas. The droplets were formed through capillary
coalescence of the melted initial particles of the mixture
with subsequent evolution of a chemical reaction inside
the droplets at the stage of cloud formation.
The characteristic times of the process were as
follows: duration of heating of the droplets to the
maximum temperature tс  1-10 s (data of a
high-speed micro video recording), duration of the
ionized condition of the cloud ti ~ 40-60 s (signal of
the current between the electrodes), and maximum time
of optical emission tо ~ 100-150 s. The quantities tс
and ti reflect the duration of chemical conversions, and
the difference between them is determined by
nonsynchronism of conversion in different droplets of
the suspension cloud. The value of tо characterizes the
integral process duration, including the reaction and
cooling of the products.
The radiative composition of the suspension cloud
for powder systems under study represents a rapidly
changing superposition of continuous and selective
spectra in the optical range of 200-1100 nm (Fig. 2).
The continuous spectrum in the visible and infra-red
region is typical of thermal emission and similar to
emission from a photometric lamp at the corresponding
temperature of a tungsten ribbon. The selective
spectrum reflects the presence of fluorescence from
partially ionized vapors of the (Ti, B, Zr) system
components. The observed spectra are essentially
unbalanced. Their feature is the presence of a
high-energy ultra-violet component (energy of quanta
hν ≥ 4 eV).
The intensities of short-wave spectra depend
essentially on the composition and pressure of the gas
medium in which the SHS process proceeds. The most
intensive emission is observed in the atmosphere of He
at the pressure of 25 kPa (Fig. 2). The time intervals of
the continuous emission spectrum (40-60 s) and
selective lines (25-30 s) differ considerably, that
indicates different mechanisms of their excitation. The
change of emission power versus time in the range of
200-400 nm during combustion of the Ti-B system in
the atmosphere of He at different pressures is shown in
Fig. 3.
According to the performed estimations, the
emission power (integral of intensity in the specified
range of wave lengths) in the ultra-violet interval of
300-400 nm is 35%-75% relating to the emission
power in a long-wave part of 400-1100 nm. The
emission intensities of the suspension cloud in the UV
range are close to the intensity of emission from the
XeBr-CD (280-284 nm, 0.7 kW) and XeCl-BD-S
(interval 307-309 nm, 1.1 kW) excilamps (Fig. 2) and
according to the comparative measurement data,
considerably exceed a level of similar short-wave
emission from gas flames (hydrogen, hydrocarbonic).
The observed selective lines of a spectrum reflect
fluorescence of the Ti, Zr partially ionized vapors.
Fluorescence of the atoms with a degree of ionization
2-3 shows a high power of their excitation (more than
20 eV).
Short-Wave Emission and Microdischarges during Self-Propagating High-Temperature Synthesis
295
Fig. 2 Ultraviolet emission spectra during combustion of the Zr-B, Ti-B systems in different gases at a pressure of 25 kPa and
excilamps: (1) Zr-B (Ar); (2) Ti-B (He); (3) Ti-B (Ar); (4) Ti-B (N2); (5) XeCl-BD-P excilamp; (6) XeBr-CD-P excilamp. Spectra
1-4 correspond to the maximum emission intensity during thermal explosion. Ti(I), Ti(II), Ti(III), Ti(IV), Zr(I), Zr(II), Zr(III)
are the emission lines of the corresponding atoms and ions with different degrees of ionization [9-10].
Fig. 3 Emission power in the range of 200-400 nm during combustion of the Ti-B system in the atmosphere of He at the
pressures of (1) 70 Pa; (2) 25 × 103 Pa; (3) 50 × 103 Pa; (4) 100 × 103 Pa for different time moments. P =
400
 Jd  .
200
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Short-Wave Emission and Microdischarges during Self-Propagating High-Temperature Synthesis
It is considered that emission of a continuous
spectrum is connected with recombination of highly
ionized plasma, similarly to emission during
high-current discharge in xenon [11]. Emission of
selective lines is caused by both recombination of
highly ionized plasma and direct excitation of atoms
(ions) by electrons. It should be noted that the intensity
of UV emission increased after a constant voltage
supplied to the electrodes from an external source. It is
known [3, 12] that SHS is accompanied by a
chemically stimulated plasma flow from the free
surface of condensed phases. The plasma is
characterized by a high concentration of charged
particles (up to 1021-1023 m-3) and the presence of
electrons with an energy of up to 150 eV.
The influence of the composition and pressure of the
gas medium on the characteristics of emission during
SHS is apparently similar to the role of buffer gases
during excitation of gas media by an electron beam (for
example, in laser plasma [13]). It takes place due to the
accumulation of electron energy by gas molecules with
their subsequent transfer to ionized vapors of metals in
the suspension cloud, the decrease in decay of exited
electronic states, and other kinetic effects in plasma.
The investigations have shown that short-wave SHS
emission is not limited only to the ultra-violet region of
a spectrum. The character of a photofilm flare (Figs. 4
and 5) indicates the presence of X-ray radiation of two
types:
As shown in Fig. 4, emission penetrates through a
layer thickness, is the linear absorption factor) and
the dependence of the absorption factor on energy of
photons [14], it follows that the energy of X-ray quanta
causing a flare of a photofilm is 4.5-5.5 keV.
The film flare consisting of separate dots or lines
behind a tantalum foil (Fig. 5) shows that the emission
composition includes the quanta with an estimated
energy more than 15 keV. The total exposition dose of
emission registered by a dosimeter was 10 mR for the
series of 24 explosions. Emission doses for separate
explosions differed considerably from each other after
the level of 2.5 mR. It should be noted that the
registered exposition dose did not change after
wrapping a dosimeter with a film from aluminium 10
m in thickness which excluded the influence on
indications of a microwave emission dosimeter.
Analyzing the obtained data, it is necessary to notice
that the measured exposition dose of X-ray radiation by
a dosimeter should be considered to be an approximate
lower estimate. It is connected with that the maximum
sensitivity of this device is realized for the energy of
quanta more than 16 keV, and in the “soft” X-ray
region the sensitivity of a dosimeter is essentially lower.
It should be also noted the instability of X-ray radiation
for the energy of quanta 15 keV.
barrier (aluminium foil 10 m thick + envelope 80 m
thick) and essentially decreases by a plastic plate, 120
m in thickness, placed into an envelope on the surface
of an X-ray film. The area of film blackening is rather
homogeneous and limited by the contours of a plastic
plate. The photometric measurements for the density of
X-ray film blackening showed that X-ray radiation
decreases by 1.5-2 times after the passage through a
plastic plate. Based on the known relationship: J =
Joed (Jo and J are the emission intensities before and
after the passage through a substance layer, d is the
Fig. 4 Flare of the RF-3 photofilm by X-ray radiation from
thermal explosion of the Ti + 31.1 mass % B system in the
atmosphere of argon at a pressure of 70 Pa: (1) Al foil; (2)
black paper; (3) plastic plate; (4) X-ray film; (5) region
shaded by the absorbing plastic plate.
Short-Wave Emission and Microdischarges during Self-Propagating High-Temperature Synthesis
297
Fig. 5 Flare of the RF-3 (a) and AGFA (b) photofilms by X-ray radiation from thermal explosion of the Ti + 31.1 mass % B
system in the atmosphere of argon at pressure of 70 Pa: (1) Al foil; (2) black paper; (3) X-ray photofilm; (4) tantalum foil (50 m).
Fig. 6 Thermal electromotive force signals measured in a layer of plasma generated during thermal explosion of the Ti +
31.1 B mixture. Po = 100 Pа.
The presence of X-ray radiation can be explained by
two effects. The soft X-ray radiation which provides
rather homogeneous film blackening can be connected
with the emission from microdischarges. Positive and
negative charges are separated in local regions of
plasma cloud during SHS. After that an electrical
breakdown takes place between these regions. The
electrical breakdown is confirmed by the steps of a
discharge current through plasma and voltage with a
duration less than 100 ns, as shown in Fig. 6. The
presence of microdischarges explains the initiation of
UV radiation. Microdischarges result in the formation
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Short-Wave Emission and Microdischarges during Self-Propagating High-Temperature Synthesis
of zones with a high level of ionization.
Recombinational emission is recorded from these
zones, and the increase in density of a current during
microdischarges leads to direct excitation of the
electronic levels of atoms, ions, and molecules.
It is believed that electric field with
microbreakdowns can generate runaway electron
beams with an energy in terms of keV. In turn, the
deceleration of runaway electrons on the particles of
gas and reaction products leads to the formation of
X-ray quanta with energy in terms of keV. The X-ray
film flare consisting of separate dots or lines can be
explained by the influence of emission from a surface
of particles moving along a surface of an envelope.
Exoemission of photons takes place on a surface of
droplets during SHS, as a result, X-ray quanta are
generated with an energy more than 10 keV. The form
of tracks and spots in Fig. 5 confirms this assumption.
[3]
[4]
[5]
[6]
[7]
4. Conclusion
Thus, in this work the properties of optical emission
were studied in the processes of self-propagating
high-temperature synthesis during combustion of the
Ti-B and Zr-B powder systems. Powerful emission was
shown to be recorded not only in the visible and near
UV spectrum, but also in the region up to 200 nm and
X-ray region of a spectrum as well.
Acknowledgments
This work was supported by the Russian Foundation
for Basic Research (No. 11-03-00688). The authors are
sincerely grateful to I. D. Kostyrya, V. A. Panarin, V. I.
Koshelev, Yu. A. Andreev for their help in conducting
experiments.
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