KNO3 Diode-Laser Induced

Article pubs.acs.org/JPCA

Spectroscopic Characterization of B/KNO3 Diode-Laser Induced Combustion J. Sivan and Y. Haas* Chemistry Institute, Hebrew University of Jerusalem, Givat Ram, Jerusalem, 91904 Israel ABSTRACT: The combustion of a B/KNO3 pyrotechnic mixture was characterized by its chemiluminescence for the first time. The reaction was initiated by a continuous wave (cw) diode laser inside a novel multipurpose reaction cell, whose design and construction are described. As in the case of the extensively studied oxidation of boron by O2, the most intense luminescence, recorded in the 400−600 nm range, is assigned to BO2. Its appearance delay time (10−2 to 10−1 s) equals that measured for the pressure increase and is shortened as the laser power is increased. A band observed at 355 nm appears at longer delay times than the BO2 bands. The band, and some weaker ones, may be assigned to BO, although some bands expected for BO (based on reaction between B atoms and O2) are absent from the spectra. This observation is discussed in the text, and possible emission from BN is discussed. If the band is assigned to BO, the absence of known bands may be due to specific E−V resonance energy transfer. Possible oxidation mechanisms consistent with the different delay ignition times are discussed.

1. INTRODUCTION Pyrotechnic mixtures have a variety of uses in military and civilian applications.1 Of special interest are B/KNO3 mixtures that are high energy pyrotechnic mixtures used extensively as propellant igniters and gas generators. On the one hand, these mixtures have a high onset temperature of the combustion process (∼800 K).2,3 On the other hand, they possess a high specific caloric value (∼1700 kcal/g). This makes them suitable candidates as insensitive components in ignition chains. Although the mechanism of boron combustion in oxygen has been extensively studied4−8 to the best of our knowledge the combustion mechanism of B/KNO3 has not been the focus of any study. This is probably due to the fact that it was assumed the combustion in this system proceeds in a piecewise process, beginning with thermal decomposition of KNO3 (∼700 K) and followed by oxidation of boron particles by oxygen. However, to utilize the inherent insensitivity of these mixtures, and to allow safer more robust usage in a potentially dangerous system, a better understanding of the reaction mechanisms of this system needs to be gained. In this work a new experimental setup, described in the following section, was used to measure the time dependent pressure buildup as well as visible emission of the combustion products. The mixtures were ignited by a high power continuous wave (cw) laser diode and the emission from the combustion products was recorded by a high speed spectrometer and a photodiode. The pressure−time history during the combustion was also recorded. The visible emission spectrum of B/KNO3 combustion products is reported for the first time. The most intense bands are assigned to BO2 (A−X band). Other bands that appear near 355 and 385 nm may be tentatively assigned to the BO A−X © XXXX American Chemical Society

transitions, although known bands of similar intensities of this electronic transition are not observed. This assignment, its implications, and other possible alternatives are discussed.

2. EXPERIMENTAL SETUP A schematic of the experimental setup is presented in Figure 1. The setup consists of a high power laser diode (Axcel Photonics HF-808-010W-25C) operating at 808 nm and delivering up to 10W optical power through a 200 μm core fiber, a lens system, and a closed pressure vessel to which several view ports are attached. Pyrotechnic pellets were prepared by pressing a mixture of 29% (weight) commercial grade boron powder (1 μm mean grain diameter), 66% potassium nitrate, and 5% polymeric binder (Laminac 4116) into a cylindrical sleeve (10 mm inner diameter). The pressing pressure was 520 kg/cm2 and the total weight of the pellet was maintained (150 mg). All the samples reported here had a density of 1.65 ± 0.05 g/cc, calculated by dividing the powder weight by the physical volume of the pellet. The sample is introduced into the pressure vessel using a sample holder, which attaches to the backward flange of the vessel. The vessel is made of stainless steel and is roughly cylindrical. The inner diameter of the tube is 36 mm, and its length is about 150 mm. When locked into place, the sample holder positions the sample between two concentric view ports that are perpendicular to the vessel’s axis. The viewports have Special Issue: Curt Wittig Festschrift Received: March 22, 2013 Revised: May 1, 2013

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dx.doi.org/10.1021/jp402820f | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Figure 1. Experimental setup (not to scale): 1, fiber from the laser diode coupled to a collimating lens on a kinematic stage; 2, fixed focusing lens; 3, focusing lens on the translational stage; 4, stainless steel pressure vessel; 5, laser input window (BK glass); 6, diagonal view port with a sapphire window; 7, perpendicular view port with a sapphire window; 8, quick release sample holder; 9, pressed pyrotechnic pellet; 10, lens holder with focusing lens; 11, fiber to spectrometer; 12, fast response photodiode with band-pass filter; 13, CMOS camera; 14, bore for pressure transducer.

The laser is controlled by a homemade current modulator allowing quick current modulation (up to 1 MHz) with little or no overshooting (∼5% peak to peak). The laser power is controlled by changing the current amplitude. A calibration curve of power at target plane vs driving current was constructed using a power meter (OphirOptronics 10A-P). In all the experiments the laser power at the target was maintained between 1 and 5 W. To prevent condensation of solid particles on the view ports, each view port was fitted with interconnecting gas inlet tubes (about 2 mm OD), upstream from the window. The purpose of these tubes is to allow flushing of the system with a desired gas, and thus to decrease solid combustion product adherence to the windows, reducing the sensitivity to window opacity. In our experiments, no gas, pure helium, pure O2, or pure N2 was used. A schematic of the working principle of this system is presented in Figure 2. The use of these gases allowed longer operation times between successive cleanings of the windows.

an inner diameter of 9 mm, defining the minimum optical clearance. Through these view ports, which are fitted with sapphire windows, light emission of the combustion products was observed and recorded. The origin of this emission is the hot gas plume that is ejected from the surface of the burning sample and propagates perpendicular to the aforementioned view-ports. Emission is detected by focusing the light through biconvex lenses (Thorlabs LB1014) into two separate detectors. One is a spectrometer (OceanOptics HR2000+), coupled to the viewport by a 600 μm core fiber. The spectrometer is able to record full spectra (190−650 nm) at a maximum rate of 500 Hz. Before combustion product emission spectra were recorded, a radiant intensity calibration of the spectrometer was performed by recording the spectra of a low pressure Ar/Hg lamp. This calibration allows correcting for instrument spectral response but is not enough to determine absolute values of radiant intensity. The other detector is a biased silicon photodiode (Thorlabs DET100A), fitted with a bandpass filter (Thorlabs FB550-10, CW 550 nm ± 10 nm), allowing a much higher acquisition frequency (up to 1 MHz). In some experiments the spectrometer was replaced with another photodiode fitted with a bandpass filter for 350 nm (Thorlabs FB350-10, CW 350 nm ± 10 nm). This allowed the high acquisition frequency of two specific wavelengths. Three similar viewports, with a diagonal viewing angle of the sample were not used in this set of experiments but are present for future work and observation of the combustion flash. A dynamic pressure transducer (Omega DPX-101-250 amplified by Omega ACC-PS1) with a 1 MHz bandwidth was attached to the pressure vessel via a bore located roughly 10 mm downstream from the pyrotechnic sample. The laser beam is introduced into the pressure vessel via an antireflection (AR)-coated glass window (Thorlabs WG11050B) using a 200 μm fiber (N.A. 0.22). It was focused onto the sample by a set of three AR-coated lenses. The first is an aspheric lens mounted on a kinematic stage for beam alignment (Thorlabs F220SMA-B, f = 9.2 mm). The second is a fixed biconvex lens (Thorlabs LB1811-B, f = 35 mm), and the third is another biconvex lens (Thorlabs LB1901-B, f = 75 mm) mounted on a transitional stage. By moving the lens using the stage, we can adjust the beam diameter at the target plane from 1.0 mm to about 3.0 mm. The beam width (1/e of maximum intensity) was measured beforehand using a beam profiler (Edmund Optics 69-099), positioned at the target location. The beam had a nearly Gaussian profile.

Figure 2. Schematic representation of the inlet gas flow system operation: 1, sample holder; 2, view ports; 3, gas inlets; 4, combustion plume; 5, gas outlet; 6, laser input window.

The general protocol of the experiments performed is as follows: (1) Laser power is set by setting the driving current. The laser pulse duration is set between 200 and 2000 ms. (2) The sample is placed inside the sample holder and the pressure vessel is sealed. (3) All acquisition systems are synchronized with the laser driver signal. (4) The laser is activated by a applying a gated current and the pyrotechnic pellet is ignited. B



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3. RESULTS Initial experiments were performed using a constant laser-beam width (D = 1.0 mm). Figure 3 presents the pressure buildup in

Figure 5. Time dependent emission, recorded simultaneously by a photodiode with a 550 nm bandpass filter and a photodiode with a 350 nm bandpass filter. It is evident that emission at 550 nm appears at a shorter interval from application of current to the laser than emission at 350 nm.

Figure 3. Pressure history curves for different laser intensities

the closed vessel as a function of time from laser initiation for these experiments. Figure 4 shows light emission (at 550 nm)

Figure 6. Time to first light (measured using photodiode at 550 or 350 nm) and time of initial pressure increase versus absorbed laser intensity. The first light at 550 nm coincides approximately with initial pressure increase. The time to first light at 350 nm lags by 20−60 ms relative to the time of first light at 550 nm.

Figure 4. Light emission at 550 nm for different laser intensities

as a function of time for different laser intensities. The dependence of ignition delay time on emission wavelength can be seen in Figure 5. Ignition delay times were determined as the first time when the first derivative of the measured quantities (either pressure transducer signal or photodiode signal) is higher than twice the standard deviation of the derivative before laser activation. In this sense the ignition delay time can be determined either by the appearance of first light or by the initial increase of pressure. As expected,9 ignition delay times (measured by the appearance of first light or the initial increase in pressure) decrease as the laser intensity increases. A logarithmic scale plot of the time at which first light is observed at 550 or 350 nm (measured from the moment current is applied to the laser diode) or the time when the pressure begins to increase versus absorbed laser intensity can be fitted to a linear curve as can be seen in Figure 6. To estimate actual delivered heat flux, it was assumed that 80% of the laser intensity was absorbed by the solid sample.10 Apart from measuring the time to first light using a photodiode, high speed acquisition (1 sample /2 ms) spectra of the emission gases were recorded. The spectra were taken

with oxygen, nitrogen, or helium rinsing the system (at 2 bar) The emission spectra at different times from laser activation can be seen in Figure 7 for the case of nitrogen flushing of the system. The assignment of the peaks in the 400−600 nm range to BO2 emission is based on the similarity of the spectra to previous work11−13 and appears to be well founded. In contrast, the assignment of the peaks in the 300−400 nm range to BO is more ambiguous and is further discussed in the next section. Lines due to K and Na atoms (present as an impurity of potassium nitrate) are also seen. The apparent decrease of intensity at 640 nm is due to a sharp decrease of the spectrometer’s grating efficiency. The main difference between spectra taken with oxygen flushing the system and nitrogen or helium flushing the system is the ratio between peaks of the band centered at 500 nm and the peaks of the band centered at 355 nm. There is almost no difference in relative peak intensities between nitrogen and helium flushing the system. Figure 8 shows a comparison of these spectra, after normalizing the intensity to the peak at 547 nm. The spectra presented in this figure were obtained by C



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combustion mechanism of pure boron in O2.5−8,14,15 It was probably assumed that combustion of these mixtures is similar to combustion of boron in an oxygen environment, with potassium nitrate serving essentially as a source of oxygen for the combustion. The experimental setup described in this work makes the study of this assumption possible by observing emission from specific combustion products. The system also enables the study of ignition delay times, not only by measuring time to first light (as was studied in previous works for B/ KNO3) but also by measuring the onset of pressure increase. Time dependent spectra are also essential to the study of B/ KNO3 combustion mechanism, because they allow discriminating between time dependent behaviors of different electronically excited combustion products. 4.2. Combustion Mechanism. To discuss our results, a viable combustion mechanism for B/KNO3 is proposed. Because potassium nitrate decomposes at around 700 K, one can assume that potassium nitrate will decompose prior to the beginning of boron particle combustion. This decomposition creates an oxygen rich environment in which subsequent oxidation of the boron particle may follow. To test this hypothesis, a thermochemical equilibrium calculation was performed with NASA’s Chemical Equilibrium Algorithm (CEA).16 The mole fractions of KNO3 decomposition products at an assigned pressure of 2 atm (conditions before ignition) and temperatures ranging from 700−2000 K are listed in Table 1.

Figure 7. Luminescence spectra of the gaseous products of B/KNO3 as a function of time after laser initiation. The laser intensity was 405 W/cm2. The pressure chamber was rinsed with N2. The total pressure at the end of burning was about 3.25 atm.

Table 1. Mole Fractions of KNO3 Decomposition Products at Different Assigned Temperaturesa species K KNO3 KNO2(l) KNO2 KO KO2(l) K2O N2 O2

Figure 8. Spectra of combustion products rinsed with oxygen, nitrogen, and helium. The intensity of each spectrum was normalized to the peak at 547 nm (000−000 BO2 transition). Spectra are displaced for clarity.

700 K

1500 K

2000 K 0.23

0.02 0.65 0.04

0.02 0.07

0.47

0.33

0.24 0.25

0.03 0.17 0.48

a

Species in the liquid phase are denoted by (l), all other species are in the gas phase.

summing over the entire duration of the combustion (for each experiment). The resulting spectra (after normalizing the intensity) were averaged for each gas (2 sets for each gas). It can be seen that all the intensities of the peaks attributed to the BO2 A−X band are similar when normalized to the BO2 000−000 transition, for all flushing gases. In contrast, the intensity of the three peaks at 347, 355, and 385 nm, relative to the BO2 peaks, clearly depend on the flushing gas. Upon flushing with O2, the intensity of these peaks decreases relative to that of the BO2 emission, whereas when helium or nitrogen are used, the intensities are not changed compared to those of nonflushed systems. However, the relative intensity between these peaks remains constant and they all decrease by an equal amount relative to the BO2 peaks. Peaks at wavelengths higher than 547 nm are also better resolved when the system is flushed with O2. This is probably due to increased emission of BO2 relative to blackbody radiation from burning particles.

Table 1 shows that a very prominent decomposition product is molecular oxygen. This supports the assumption that potassium nitrate serves essentially as a high density oxygen source to the system. The results at 700 K predict that the major decomposition products of potassium nitrate are oxygen and potassium nitrite (KNO2), similar to experimental results reported by Freeman17 for KNO3 decomposition. This reaction is endothermic (+24.5 kcal/mol), requiring an external heat input (supplied in our system by the laser) to proceed. Freeman also mentions that above 700 K further decomposition of the alkali nitrite occurs, which is supported by our calculations. Once the potassium nitrate decomposes, the condensed matrix which fastens the solid boron particles collapses, leaving the boron particles (which have a mean diameter of 1 μm) to drift in an oxygen rich environment. Oxidation of the boron particles can then follow well studied oxidation patterns. For example, the model proposed by Williams and Li6 and later adopted by Yeh, Kuo,7 and Hussmann,15 suggests that boron

4. DISCUSSION 4.1. General Comments. Little work has been conducted on the combustion mechanism of B/KNO3 as opposed to the D



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particles burn in three stages. The first stage is a preheating stage in which the surface temperature of the boron particle increases. In the second stage the particle’s oxide layer is removed by a complex mechanism, which involves solid boron dissolving into the molten oxide layer or gaseous oxygen dissolving into the layer. In this process a polymeric (BO)n chain is formed inside the molten layer. This polymer may then undergo reaction with oxygen at the surface of the particle: B(s) + B2O3(l) →

3 (BO)n (l) n

4.3. Assignment of the 355 nm Band. The spectra of the combustion products resemble spectra taken when boron particles are ignited in oxygen.11,13,14,18 The same BO2 A−X bands reported independently for instance by Gole, 11 Spalding,13 and Yuasa14 are clearly seen and assigned in this work. Apart from BO2 bands, a relatively strong band is observed at 355 nm, with a shoulder at 347 nm and a much weaker band at 385 nm. One of the first researchers to assign these peaks to BO A−X emission was Mulliken.19 Gole11 and also Hinchen20 have also observed these peaks when reacting boron with O2, NO2, and O3. They have also assigned these peaks to BO A−X transitions. The peaks at 355 and 385 nm were assigned to v′ = 4 → v″ = 0 and v′ = 2 → v″ = 0, respectively. However, both researchers have observed stronger emissions from other transitions as well, specifically v′ = 3 → v″ = 0 at 366 nm, v′ = 5 → v″ = 0 at 335 nm, v′ = 0 → v″ = 0 at 422 nm, and many others. These are not observed in our spectra. If the peaks at 355 and 385 nm are indeed ascribed to BO emission, this would suggest in the B/KNO3 case a very selective population of some specific vibrational levels of the excited state. Thus, only v′ = 4 and v′ = 2 (to a lower extent) are seen to emit. The emission from these states is also very selective. Transitions from v′ = 4 to v″ = 1 and v″ = 3 have very low Franck−Condon factors so their absence is reasonable; the much stronger transition from v′ = 4 to v″ = 4 (475 nm) should be observable but may be concealed by the stronger 140 → 000 BO2 transition. The v′ = 4 to v″ = 3 band (405 nm) may be masked by emission of potassium atoms. Of the bands originating at v′ = 2 only the v′ = 2 → v″ = 0 at 385 nm is intense enough to be discerned above the noise. On the one hand, researchers, such as Gole, have been able to observe emission from the BO A−X band. On the other hand, these emissions are not present in the papers by Yuasa, by Spalding, by Maligne, and in this work. A possible explanation is that in this work, as well as the other works mentioned, the boron is present in solid form whereas in the studies by Gole and Hinchen gaseous boron is reacting. BO produced from liquid boron and molecular oxygen, via reaction R6, is most likely formed in the ground state; some may be formed in the excited A state, via reaction R7, with a low vibrational occupation, up to v′ = 2. This is in contrast to the work done by Gole and Hinchen, in which boron atoms in the gas phase were reacted with oxidizers. The reaction B(g) + 1 /2O2(g) → BO(g), occurring in the gas phase has a high enthalpy of reaction (ΔH = −130 kcal/mol), allowing BO(g) to be formed directly in the excited state, with a high vibrational occupation. To explain the appearance of emission from specific vibrational levels of the electronically excited BO molecules, a suitable excitation mechanism must be proposed. One possible mechanism is excitation of ground state BO molecules by collision with electronically excited BO2(g) molecules:

(R1)

1 (BO)n (l) + O2 (g) → BO2 (a) + O (a) n

(R2)

1 (BO)n (l) + BO2 (a) → B2O3(l) n

(R3)

1 (BO)n (l) + O(a) → BO2 (g) n

(R4)

The overall global reaction of this mechanism is ΔH = −68 kcal/mol

B(s) + O2 (g) → BO2 (g)

(R5)

The (a) denotes gas phase species adsorbed on the particle surface. The energy released by this reaction is enough to excite the A electronic state of BO2 (υ00 ∼ 52 kcal/mol), meaning that some of the BO2 can be formed in the excited state. This explains the observation of strong BO2 bands at 430 nm and higher wavelengths, occurring simultaneously with initial increase of pressure. The second stage of combustion occurs after the oxide layer is removed; oxygen is now free to react directly with the “clean” boron surface:8 B(l) +

1 O2 (g) → BO(g) 2

ΔH = −7 kcal/mol (R6)

B(l) + O(g) → BO(g)

ΔH = − 71 kcal/mol

(R7)

1 O2 (g) → BO2 (g) 2

(R8)

BO(g) + BO2 (g) → B2O3(g)

(R9)

BO(g) + BO(g) → B2O2 (g)

(R10)

BO(g) +

Reaction R6 is one possible reaction that leads to the production of BO in the gas phase. The low reaction enthalpy suggests that unlike reaction R5, it is not very likely that BO is formed in the excited A state (υ00 ∼ 67 kcal/mol) in this reaction. On the other hand, liquid boron particles may react directly with atomic oxygen (an initial product of KNO3 decomposition), by reaction R7. The enthalpy of reaction of this reaction is high enough for BO to be formed in an electronically excited state, however not at high lying vibrational levels (v′ = 1 at the most). When BO is formed, in either the ground or the excited state, it occurs at a delay, equal to the time that is required for the removal of the oxide layer, relative to the initial formation of BO2. This may explain the delay between time of first light of the bands at 430 nm and higher (BO2 bands) and the time of first light of the band at 355 nm. Possible assignment of this band to BO emission is discussed in the following section.

BO(g)(vinitial″) + BO2 *(g)(vinitial′) → BO*(g)(vfinal′) + BO2 (g)(vfinal″)

(R11)

A similar E−V energy transfer mechanism was proposed by Gole.11 For this mechanism to be feasible the energy difference between products and reactants must be negative and small, because there is no efficient mechanism to remove excess energy in a single collision. From the energy gap law, the E



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efficiency of such a mechanism will depend exponentially on the aforementioned energy difference. In Table 2 the energy difference for several such collisions (yielding the maximal possible negative energy difference) is presented.

mixtures, although he did not have the means to positively identify this species in his system. Flushing the cell with O2 during combustion leads to reduction of the intensities of the bands at 355 nm and at 385 nm relative to those of the BO2 emission bands. However, the relative intensity of these bands compared to one another remains constant. This behavior is consistent with the assumptions that the bands originate from a single emitting species. This decrease supports the assignment of the peaks to BO or BN because the decrease of emission upon flushing with O2 can arise from a decrease in BO or BN concentration due to oxidation to BO2. A similar oxidative decrease in emission of K and Na atoms emission (at 405 and 588 nm, respectively) can be explained by the same mechanism. The excited atoms react with oxygen when the system is flushed with O2 lowering their concentration and emission intensity. The role of alkali metals as oxygen scavengers is a well-documented fact.22

Table 2. Few Possible Collision Induced V−E transfer between BO and BO2* reactant v level

product v level

BO2*

BO

BO2

BO*

ΔEa,b

2 2 2 1 1 1 1

3 4 4 4 5 3 4

0 1 2 0 1 0 1

4 4 4 3 3 2 2

0.0 −1.5 −3.1 −2.0 −3.4 −0.3 −1.8

Energy in kcal/mol. bEnergy calculated according to E = Te + ωe(v + 0.5) − ωexe(v + 0.5)2. a

5. SUMMARY A system to record emission spectra and ignition delay times of pyrotechnic mixtures ignited by a laser diode is presented. Ignition of B/KNO3 pellets is studied experimentally and high speed acquisition emission spectra of combustion products are reported for the first time. Preliminary results indicate that combustion products contain BO2 similar to the case of combustion of boron lumps or particles in oxygen rich environments. The ignition delay times measured by the pressure rise are equal to those observed by the rise in BO2 luminescence. This indicates that BO2* is a primary product of the reaction. In contrast, the 355 nm emission appears at a longer delay and must be due to a secondary product. The species involved in this emission as well as the weaker one at 385 nm is discussed. A likely candidate is BO; however, this would require very selective population of the excited BO A state, specifically the v′ = 4 and v′ = 2 vibrational levels. A highly specific emission pattern (v ′ = 4 → v″ = 0 and v′ = 2 → v″ = 0) may be explained by an E−V collisional energy transfer: electronically and vibrationally excited BO is formed by collisions with excited BO2*. The efficiency of energy transfer from excited BO2* to BO depends on the vibrational level of the molecules before and after collision. Only collisions between molecules in low lying vibrational levels, which have a minimal difference between the energy of the molecules before and after collision, will be efficient. The intensity of the 355 nm peaks relative to the intensity of the BO2 bands depends on the nature of the environment. In an oxygen rich environment these peaks decrease relative to the BO2 bands. This trend is compatible with assigning the bands to emission from either BO or BN. However, this assignment requires an explanation as to why other peaks of similar magnitudes are absent from the spectra. This point warrants further investigation. The delayed first light at the 355 nm band relative to first emission of BO2 bands is consistent with the band being attributed to BO emission also from a different point of view. According to previously proposed models:6,7,15 BO is a product of the second stage of combustion that occurs only when all of the oxide layer is removed from the burning boron particle.

From Table 2 it can be seen that only two vibrationally selective collisions of BO2* and BO will result in a minimum of an energy difference; one resulting in formation of BO*(v′=4) and another resulting in occupation of BO*(v′=2). Other possible collisional energy transfers either require high vibrational occupation before the collision or result in a large energy difference between reactants and products, making them unlikely. This is one possible explanation for the very specific emission observed at 355 nm (v′ = 4 → v″ = 0) and 385 nm (v′ = 2 → v″ = 0). Because one of the collision partners (BO2) is in an electronically excited state, the total yield of these reactions is expected to be small. An alternative to this assignment is assignment of the peaks at 355 and 385 nm to emission from BN. Mulliken19 has shown that emission of BO can be mistaken for BN. Expected peak positions in order of decreasing relative amplitudes (noted in parenthesis) are 360 nm (1), 381 nm (0.27), 363 nm (0.18), 383 nm (0.17), and 344 nm (0.11). The peak locations were taken from Reddy et al.21 The peak intensity was estimated by multiplying the reported Franck−Condon factor by the estimated vibrational level Boltzmann occupation, assuming a temperature of 2000 K. The temperature was estimated by running the thermochemical code CEA for the B/KNO3 mixture for a closed cell combustion problem (assigned enthalpy and pressure). Table 3 summarizes the possible assignments of the peaks at short wavelengths. Formation of solid BN as an equilibrium product is predicted by the CEA calculation. Gillard10 has postulated on the possible existence of BN in the combustion products of B/KNO3 Table 3. Observed and Possible Assignments of Bands below 410 nm obsda wavelength (nm) 347 355 360 385 405b

BO transition

BN transition Δvc = +1

v′ = 4 → v″ = 0 v′ = 4 → v″ = 1 v′ = 4 → v″ = 2

Δv = 0 Δv = −1 Δv = −2

Wavelength of emission peak. bObscured by atomic K emission. cΔv = v′ − v″.

a

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(20) Hinchen, J. J. Kinetics for the Quenching and Relaxation of Boron Oxide. J. Chem. Phys. 1993, 99, 4403−4410. (21) Reddy, R. R.; Ahammed, N. Y.; Gopal, R. K.; Azeem, P. A.; Anjaneyulu, S. RKRV Potential Energy Curves, Dissociation Energies, γ-Centroids and Franck-Condon Factors of YO, CrO, BN, ScO, SiO and AlO Molecules. Astrophys. Space Sci. 1999, 262, 223−240. (22) Birchall, J. D. On the Mechanism of Flame Inhibition by Alkali Metal Salts. Combust. Flame 1970, 14, 85−96.

AUTHOR INFORMATION

Corresponding Author

*Tel: +97226585067. Fax: +97225618033. E-mail: Yehuda. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank IMI-RSD pyrotechnics laboratory, for the preparation and thermal characterization of B/KNO3 pellets. REFERENCES

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KNO3 Diode-Laser Induced

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