Combustion and Flame 173 (2016) 288–295

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Combustion and Flame

journal homepage: www.elsevier.com/locate/combustflame

Oxidation kinetics and combustion of boron particles with modified

surface

Kerri-Lee Chintersingh, Mirko Schoenitz, Edward L. Dreizin

∗

New Jersey Institute of Technology, Newark, NJ 07102, United States

a r t i c l e i n f o

Article history:

Received 3 June 2016

Revised 31 August 2016

Accepted 31 August 2016

Keywords:

Ignition delays

Powder processing

Particle-seeded flame

Oxide layer

Metal combustion

a b s t r a c t

This work is aimed to modify a commercial 95% pure boron powder to improve its ignition. The powder

is processed using acetonitrile to dissolve the oxidized and hydrated surface layers. The washed powder

re-oxidizes and agglomerates readily, which can be prevented by additionally washing it in toluene and

other liquid hydrocarbons, prior to exposing it to an oxidizing atmosphere. The presence and removal

of the hydrated oxide layer and oxidation of boron powders are studied using thermo-gravimetry. The

processed powders are shown to retain the active boron present in the starting material and maintain

their reactivity after extended exposure to an oxidizing environment. The powders are also injected in

a pre-mixed air-acetylene flame and their ignition and combustion are studied based on their optical

emission. Flame temperature is measured using optical emission spectroscopy. Powder particles washed

in acetonitrile, toluene and hexane exhibit substantially shorter ignition delays compared to the starting

commercial boron particles. Full-fledged combustion is unaffected by this processing: both, burn times

and flame temperatures are nearly identical for commercial and modified powder particles.

© 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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1. Introduction

Boron is an attractive material for metal combustion applica-

tions, such as explosives [1] and solid fuel additives for rocket pro-

pellants [2] , because of its high gravimetric and volumetric heats of

oxidation [3] . However, its long ignition delays [4,5] and extended

combustion times [6,7] present major challenges for its practical

use.

There have been considerable experimental and theoretical ef-

forts [6,8–14] to understand and characterize the oxidation kinetics

and combustion mechanism of boron. Specific focus was on long

ignition delays [7,15] . It has been generally agreed that a natural

surface layer of oxide (B 2 O 3 ) or hydroxide (approximately 0.5 nm

[16] ) inhibits boron oxidation and therefore delays ignition [2] ,

which leads to reduced bulk burn rates.

An accelerated removal of the boron oxide layer is expected

to lead to shorter ignition delays, motivating multiple relevant ef-

forts, e.g., see review [17] . Different studies considered addition

of coating agents [18] , metals [19] , metal anhydrides or oxides

[8,20–24] , rare-earth metal catalysts [21] , polymers and fluorinated

compounds [2,25,26] .

Experiments [21] showed that for boron nanoparticles ball

milled with ceria (20 wt%) ignition delays were reduced by almost

∗ Corresponding author.

E-mail address: dreizin@njit.edu (E.L. Dreizin).

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http://dx.doi.org/10.1016/j.combustflame.2016.08.027

0010-2180/© 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved

factor of 6. This was based on chemiluminescence measurements

t a wavelength of 546 nm for particles fed in the combustion

roducts of an ethanol flame in a customized combustor. Unfortu-

ately, including 20 wt% of an oxide substantially reduces the en-

rgy density of an energetic composition.

The effects of metal hydrides (Ca, Li, Ti and Zr) on the ignition

nd combustion of 99% boron particles in a CO 2 laser were stud-

ed in Ref. [24] . Results indicated that the lithium hydride blended

ith a mass ratio of 10:1 in favor of boron, reduced ignition delays

rom 140 ms to as low as 90 ms. However, lithium hydride is diffi-

ult to handle and make compatible with processes involved with

reparation of energetic formulations.

Adding fluorinated compounds or working with fluorine-

ontaining gaseous oxidizers was shown to improve boron com-

ustion and reduce ignition delay times, which was associated

ith alternate reaction pathways [2] . Practical introduction of flu-

rine in the reactive material may be difficult. Ignition of mixed

olytetrafluoroethylene (PTFE)/boron systems was considered in

ef. [26] . It was concluded that for thermodynamically attractive,

oron-rich compositions, adding PTFE was ineffective for promot-

ng ignition.

Ignition of mixed boron and magnesium powders was analyzed

n Refs. [19,27] . Selective ignition of magnesium occurred indepen-

ently of the composition; it remained unclear whether ignition of

agnesium was effective in igniting boron. Another approach to

odifying ignition behavior of boron was proposed in Ref. [28] ,

here the boron surface was coated with a carbide layer. The

.

K.-L. Chintersingh et al. / Combustion and Flame 173 (2016) 288–295 289

Table 1

List of materials prepared for investigation.

Sample ID Starting material Washing cycles

In acetonitrile, 3 In toluene, 5 In hexane, 3

A 95% pure boron No No No

B 95% pure boron Yes No No

C 95% pure boron Yes Yes No

D 95% pure boron Yes Yes Yes

E 99% pure boron No No No

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oating was formed by prolonged exposure of boron particles to

ydrocarbons at elevated temperatures. However, X-ray photoelec-

ron spectroscopy showed that the surface layer was substantially

xidized, and that the carbide surface layer was unfavorable for

apid ignition and heat release of the sample.

Generally, a challenge with most boron additives or coatings is

hat they reduce the overall material energy density. Significant ki-

etic effects accelerating removal of the boron oxide are observed

ith substantial amounts of such added components, and thus,

aterials with significantly reduced heats of combustion. The ob-

ective of this effort is to reduce boron particle ignition delays

ithout imposing an energetic penalty associated with most ad-

itives. The approach here is to dissolve the natural boron oxide

r boric acid layer from the particle surface while preventing the

owder from rapid re-oxidation.

. Experimental section

.1. Materials

Commonly, boron powders are coated with a surface layer of

oron oxide, B 2 O 3 , which readily converts into boric acid, B(OH) 3 ,

n humid environments [29] . Reported data on solubility of boron

xide are scarce, while boric acid can be dissolved in many sol-

ents, most easily in water and ethanol [30] . However, most ef-

ective solvents contain oxygen and thus may oxidize the exposed

oron surface. To avoid such oxidation, an oxygen-free polar sol-

ent, acetonitrile, was considered in this work. No literature data

n solubility of boron oxide or boric acid in acetonitrile were

ound. Therefore, solubility of boron oxide in acetonitrile was in-

estigated preliminarily. Alfa Aesar acetonitrile 99.5% was used. A

.5-g batch of 99.98% commercial boron oxide by Alfa Aesar was

laced in a beaker with 30 mL of acetonitrile at room tempera-

ure and agitated by a magnetic stirrer for 10 min. The suspension

as allowed to settle for 1 h. The clear liquid was siphoned from

he top and centrifuged. The weight of the solid precipitated from

he obtained clear solution was then determined. The solubility of

oron oxide in acetonitrile was found to be 30 g/L. The precipitate

btained in room air with about 70% humidity had characteristic

rystal structure of boric acid. Similar experiments with Alfa Aesar

5% n-hexanes and ChemPUR® commercial grade toluene showed

hat less than 0.1 g/L of solid could be recovered. These prelimi-

ary experiments guided preparation of modified boron samples,

s discussed below.

All boron samples used in experiments are described in

able 1 . Sample A, an amorphous boron powder, 95 wt% pure from

B Boron was used as both the reference and starting material.

ample E, a crystalline boron powder with 99 wt% purity, by Alfa

esar was used as an additional reference material. Samples B, C,

nd D were obtained starting with sample A and using washing

ycles aimed to remove or minimize the thickness of the natu-

al boron oxide or boric acid layer. 10 grams of the starting pow-

er were loaded into a 50 mL steel vial of a SPEX Certiprep 80 0 0

eries shaker mill with 30 mL of acetonitrile. The mixture was

haken for 30 min with no milling media. The sample was allowed

o settle, and the acetonitrile was siphoned off without exposing

he washed boron to air. To rinse the washed boron, fresh ace-

onitrile was added, and the sample was vigorously stirred using

cientific Industries Inc. Vortex Genie X4674 shaker. Using an LW

cientific Ultra 8F-1 centrifuge, the acetonitrile solution was then

eparated from the boron powder and siphoned off. This cycle was

epeated three times. After the third cycle, toluene was added,

nd the material was rinsed with toluene five more times. Some

amples were additionally rinsed three times with hexane. After

reparation, samples remained stored in the solvent used in their

espective last washing cycle. Table 1 lists different materials pre-

ared and compared in this study.

The prepared samples were characterized using laser scattering,

ptical and scanning electron microscopic (SEM) techniques. Im-

ges of the prepared samples A, B, C, and D obtained using a LEO

530 field emission SEM are shown in Fig. 1 . Sample B washed in

cetonitrile only consists of strongly agglomerated particles. There

re large crystalline formations consistent with precipitates of ox-

des remaining after rinsing with acetonitrile only. This is indica-

ive of incomplete removal of the solvent, and this material was,

herefore, not used in further experiments. Particle shapes and ag-

lomerate morphologies observed for as received boron, sample A,

nd both samples C and D, finally processed using toluene and

exane, respectively are essentially the same. Note, however, that

he sizes of agglomerates in samples C and D are somewhat greater

han those in sample A.

Particle sizes were analyzed for samples A, C, and D, using SEM

mages of particles fed into the burner used in the combustion ex-

eriments (see below). The powder exiting from the burner was

ollected for fifteen seconds on a 1-kV charged aluminum surface

ositioned 4 cm above the burner to enhance collection of fine

articles electrophoretically. SEM image processing enabled us to

ccount for agglomeration and fractal dimension [31] of the ac-

ual particles fed into the flame. Images were collected at differ-

nt magnifications. For each magnification, the images were pro-

essed to identify particles and to measure their sizes using Im-

geJ, a freely available software package by the National Institute

f Health. The size distributions were then constructed using all

ecorded images. The equivalent particle diameters were measured

nd corrected by the agglomerate fractal dimension (FD), deter-

ined using the box counting method [32] . The FD value was ap-

roximately 1.8 for all materials. For additional details describing

ow particle size distributions are obtained from SEM image pro-

essing see Ref. [31] . The resulting particle size distributions are

hown in Fig. 2 . Preparing the samples to account for agglomera-

ion shifts the distributions towards larger sizes. It is also observed

hat washed samples C and D are somewhat more agglomerated

ompared to the commercial powder A. This is consistent with

ualitative observation made while examining the obtained SEM

mages shown in Fig. 1 . These size distributions were correlated

ith the measured burn times and ignition delays.

.2. Particle combustion experiments

Aerosolized particles were injected in an air-acetylene flame.

article emission streaks were photographed. The locations where

treaks were observed to originate were treated as locations where

articles ignited; thus, ignition delays were obtained accounting

or the injected particle velocities calculated for the experimen-

al flow conditions. Particle burn times and temperatures were ob-

ained from the optical emission measurements. Both, burn times

nd ignition delays were correlated with the measured particle size

istributions.

The experimental setup is shown schematically in Fig. 3 ; it has

een described in detail elsewhere [9,33,34] . The particles entered

he flame through a brass tube with 2.39 mm internal diameter,

laced axially at the center of the burner’s nozzle. Pre-mixed air

290 K.-L. Chintersingh et al. / Combustion and Flame 173 (2016) 288–295

Fig. 1. SEM Images of as-received and processed boron powder samples.

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and acetylene gases were fed through a 50-mm long, 19.13-mm

inner diameter tube that was tapered to form a 5.16-mm nozzle

of the burner. The acetylene and air flow rates were 0.64 L/min

4.72 L/min, respectively, creating a flame with an equivalence ra-

tio of 0.34. Approximately 0.1 g of powder was loaded onto a cus-

tom built powder screw feeder and fed into the burner using nitro-

gen as a carrier gas. The nitrogen flow rate was 0.94 L/min and the

powder mass feed rate was ca. 0.27 mg/min. All flow rates were

measured using individual rotameters.

Particle emission traces were recorded using two Hamamatsu

R3896-03 Photomultiplier Tubes (PMTs) equipped with 700 and

800-nm interference filters. The light to the PMTs was fed using a

bifurcated fiber optics bundle, with its optical inlet positioned ap-

proximately 5 cm above the top of the burner and 17 cm away from

the burner. The height of the field of view was 25 cm, while the

streaks were typically originating at the height of 2 cm and ended

no higher than approximately 13 cm above the burner.

Recorded particle emission signals were processed using a cus-

tom MATLAB code [33] to determine particle burn times and the

combustion temperatures. Data were collected at a rate of 10 0,0 0 0

samples per second using a 16 bit PCI-6123 National Instruments

data acquisition board and LabView software for a total of 8 sec-

onds for each run. A total of 60 runs were processed to obtain and

analyze more than 300 well-separated individual particle emission

peaks for each powder sample. The 700 nm wavelength signal was

selected for burn time analysis because it had the greatest signal

to noise ratio. For initial processing, the baseline was identified as

an average signal value without any particle peaks recorded for

10 ms (or 10 0 0 data points.) The durations of the individual emis-

sion pulses were interpreted as particle burn times. The pulse du-

ations were obtained when the signal amplitude for each pulse

xceeded the noise level by more than 10% of its maximum value.

Particle combustion temperatures were determined from the

ime-integrated optical emission spectra from multiple particle

treaks. A StellarNet BW16 spectrometer was positioned 5 cm

bove and 23 cm away from the burner tip. Emission spectra were

ecorded with an integration time of 1 s. For each sample, 1500

pectra were collected. The data were processed to filter out peaks

f the boron combustion products (4 80–6 80 nm) and emission

rom contaminations by sodium (589 nm) and potassium (740–

10 nm). Effect of wavelength on emissivity was neglected, so that

he remaining spectra were used to obtain the temperature by fit-

ing them with Planck’s formula, where the temperature served as

n adjustable parameter.

.3. Thermogravimetric experiments

Thermogravimetry was used to investigate oxidation kinetics,

hich is expected to govern particle ignition. Samples were heated

p to 1100 °C in oxygen using a TA Instruments model Q50 0 0IR

hermo-gravimetric analyzer. Sample masses ranging from 2 to

mg were loaded into the instrument in an alumina crucible. The

amples were prepared in different solvents. To prevent uncon-

rolled oxidation of samples removed from the solvent, samples

ere loaded as suspensions. For consistency, as received commer-

ial boron (sample A) was also loaded under hexane.

Prior to each experiment, both the balance and furnace were

urged with 20 mL/min and 50 mL/min of argon respectively at

0 °C for at least 60 min, during which the solvent evaporated and

K.-L. Chintersingh et al. / Combustion and Flame 173 (2016) 288–295 291

Fig. 2. Particle size distributions of commercial and washed boron powders.

Fig. 3. Schematic diagram of the combustion experimental setup.

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Temperature, °C200 400 600 800 1000

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3.51%

2.64%

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0.55%

3.47%E

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B E

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C

Fig. 4. Thermogravimetry curves of fresh and washed boron samples. Heating rate:

10 K/min.

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stable mass reading was recorded. That reading corresponded to

he mass of the initial dry sample.

During the oxidation measurements, the furnace gas was

witched to oxygen at 20 mL/min. The temperature was first in-

reased and held at 40 °C for 60 min before heating at 10 K/min for

ll powders. The weight changes for the boron samples heated in

n oxidizing environment are shown in Fig. 4.

To explore the effect of aging, dried powders were placed in an

ven at 30 °C. The powders were kept in an air-filled sealed glass

essel together with an open container with a saturated potassium

arbonate (K 2 CO 3 ) solution in order to keep the relative humidity

onstant at 44%. Aged samples were analyzed using TG measure-

ents after 7, 14 and 120 days.

. Results

Thermogravimetric (TG) measurements show two major mass

hanges occurring during heating: (1) mass loss occurring in the

emperature range of approximately 10 0–40 0 °C and (2) mass gain

t higher temperatures. TG traces for all samples are shown in

ig. 4 . The traces are offset vertically for clarity. The inset

hows magnified portions of the TG traces focusing on the low-

emperature mass loss.

The low-temperature mass loss is most significant for the com-

ercial 95% pure boron, sample A. It exceeds 5%, which is the

anufacturer-specified level of impurities in this material. The

ass occurs in two apparent steps: the first, sharper step occurs

n near 100 °C and the second step occurs in a broader tempera-

ure range up to 400 °C. This correlates well with reported temper-

ture ranges for decomposition of boric acid [35] and magnesium

ydroxide [36] , respectively. Thus, the mass loss represents dehy-

ration of the natural hydroxide surface layer. The magnitude of

he mass loss implies that the oxide layer is rather thick for a 95%

ure boron. This also means that a large fraction of the powder is

navailable for oxidation, which could lead to long ignition delays.

he mass loss is the smallest for the high-purity, 99% crystalline

oron, sample E. Interestingly, the mass gain signifying oxidation

f boron begins for this material at a lower temperature than for

ll other samples. However, using such powders in practical en-

rgetic formulations is impractical because of their high cost and

nstability for storage in oxidizing environments.

The low-temperature mass loss is reduced substantially for

ll washed powders, samples B, C, and D. The difference be-

ween washed powders in terms of the magnitude of the low-

emperature mass loss is insignificant; repeated measurements

how the reproducibility of the low-temperature mass loss step

ithin 2.6–3.6% of the total mass change. The mass gain signifying

xidation of boron occurs similarly for all samples A, B, C, and D.

mall differences in the magnitude of the mass gain step are likely

aused by irregularities of sintering oxidizing samples at elevated

emperatures and thus are not important.

Effect of aging on the magnitude of both low-temperature mass

oss and mass gain measured in the TG tests is illustrated in

able 2 . The experiments were performed with sample D rinsed

n toluene and hexane. Exposure of this material to air at 30 °Cnd controlled humidity did not result in appreciable changes in

292 K.-L. Chintersingh et al. / Combustion and Flame 173 (2016) 288–295

Table 2

Summary for temperature-programmed oxidation (TPO- TG) of washed boron sam-

ples aged up to 120 days in air at 30 °C and 44% relative humidity.

Samples Low-temperature Final mass

mass loss (%) gain (%)

D: rinsed in toluene and hexane 3.51 127.3

Aged sample 7 days 1.86 129.6

Aged sample 14 days 1.99 130.3

Aged sample 120 days 0.78 135.1

Fig. 5. Points of ignition on boron particle streaks.

0

20406080

100

Freq

uenc

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)

050

100150200250

Average distance35 mm

21 mm

Distance from burner (mm)0 50 100 150 200

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100

150

200

28 mm

A: 95% pure boron

C: rinsed in toluene

D: rinsed in toluene and hexane

Fig. 6. Frequency distributions of ignition distances for processed and unprocessed

boron powders. (A) is commercial boron, (C) is washed boron modified with toluene

and (D) is washed boron modified with toluene and hexane.

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its oxidation behavior. In fact, the low-temperature mass loss is

somewhat reduced and following mass gain are slightly increased

for the samples aged for 7, 14, and 120 days. Therefore, no signs

of oxidation during aging were detected. The negligible change ob-

served may have been a result of the relatively low (44%) humidity

used in the aging experiments. In the hindsight, this exposure may

have further dehydrated the boron oxide layer.

Images of luminous streaks of burning boron particle were pro-

cessed to identify the locations where particles ignited. The images

were captured using a SONY DSC-H50 digital camera with expo-

sure time of 1/125 s (8 ms), aperture of 2.875 and focal length of

5 mm. A characteristic photograph with ignition points labeled for

clarity is shown in Fig. 5 . The ignition was assumed to occur at the

nset of each single particle streak; the distance from that point to

he burner was measured. These measurements are summarized

or three powders in Fig. 6 . For commercial boron, multiple parti-

les were observed to ignite relatively far from the burner. Fewer

uch particles were observed for both washed boron samples. The

verage distances for the samples A, C and D are 35, 28, and 21 mm

espectively. Thus, despite their larger agglomerate sizes ( Fig. 2 ),

ashed and modified boron particles ignite closer to the burner

han as received, commercial boron. The ignition distances are the

hortest for the powder D, rinsed in hexane.

The distances at which ignition occurs were recast into particle

gnition delays. The flow field above the air-acetylene flame was

escribed using a numerical model, taking into account the flame

emperature and composition of the reaction products [31] . Parti-

les were assumed to be carried by the gas; the average gas ve-

ocity was used to estimate the time delays corresponding to the

istances at which the ignition was observed. Preliminary calcu-

ations showed that the slip was negligible for all particle sizes

sed in experiments. Finally, it was assumed that larger particles

gnited further away from the burner, and thus the particle size

istributions obtained by the SEM image processing and shown in

ig. 2 were correlated with the distribution of ignition distances

hown in Fig. 6 . For each particle size an ignition distance was

dentified, which was recast into the ignition delay specific for this

article size. Ignition delays for different size particles for all three

owders are shown in Fig. 7 . These results may be affected by

rregularities in the measured statistical distributions, present for

oth particle sizes and ignition locations. Thus, only the general

rends implied by the data shown in Fig. 7 may be significant. Ig-

ition delays are substantially reduced for both modified powders

ompared to the commercial 95% pure boron. The effect is partic-

larly strong for finer particles, less than 6 μm. The shortest igni-

ion delays are observed for the powder washed in hexane, sample

. For larger agglomerates, the difference in ignition delays is di-

inished. Even longer ignition delays are observed for the coarsest

K.-L. Chintersingh et al. / Combustion and Flame 173 (2016) 288–295 293

Fig. 7. Ignition distance and ignition delay correlation to particle size for processed

and un-processed boron powders. (A) is unprocessed boron, (C) is processed boron

rinsed in toluene and (D) is processed boron rinsed in toluene and hexane.

PM

T O

utpu

t (V

) C: rinsed in toluene

A: 95% pure boron

Time (ms)0 10 20 30 40 50

D: rinsed in toluene and hexane

Fig. 8. PMT Output signals filtered at 700 nm for different boron powders burning

in the products of an air-acetylene flame.

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200 A: as received

Num

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100

150 C: washed in toluene

Amplitude, V10-4 10-3 10-2 10-1 1000

10

20

30

40 D: washed in hexane

Base

line

Base

line

Base

line

Fig. 9. Histograms for emission pulse amplitudes for particles of boron samples

burned in hydrocarbon flame. The baseline band shows typical ranges of signals

produced by the hydrocarbon flame emission.

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articles rinsed in toluene as compared to commercial boron. Look-

ng back at the data shown in Fig. 6 suggests that these longer de-

ays are due to single streaks detected for sample C far away from

he burner. Thus, the longer ignition delays shown for large parti-

les of sample C in Fig. 7 were not obtained based on the statisti-

ally significant experimental data.

Combustion of different powders was also characterized based

n optical emission measurements. Typical emission traces pro-

uced by multiple burning particles of different powders injected

n the air-acetylene flame are shown in Fig. 8 . Pulses produced

y burning particles appear on top of a somewhat noisy back-

round signal produced by the flame emission. For sample A, most

article pulses had characteristic two-peak shapes, as shown in the

nset. For samples C and D, most pulses had only one well-resolved

eak, however. There were more overlapping pulses for samples C

nd, especially, D, suggesting more agglomerated particles fed into

he burner. Overlapping pulses were excluded from the analysis fo-

used on identifying the burn times of different boron particles.

The pulse durations were determined from the emission peaks

nd correlated with particle sizes. For the correlation of the re-

pective statistical distributions of pulse durations and particle

izes to be meaningful, it is important to establish that emission

ulses were recorded for particles of all sizes. The amplitudes of

he recorded emission pulses are also expected to scale with the

article sizes. Thus, if the smallest emission pulses are noticeably

reater than the detection limit, defined by the emission baseline

roduced by the air-acetylene flame, it is likely that pulses pro-

uced by all particles are reliably detected. The frequency distri-

utions of the emission pulse amplitudes measured in the present

xperiments are shown in Fig. 9 for samples A, C, and D. For each

ase, the baseline level of the emission signal is shown, with a

and indicating noise in the flame emission. In all cases, the small-

st pulses are reliably greater than the baseline emission, suggest-

ng that pulses from all particles were detected.

The burn times (or emission durations) obtained from individ-

al particle emission pulses were sorted into logarithmic bins. The

umber frequency distributions were plotted as histograms shown

n Fig. 10 . The burn times are very similar to one another for dif-

erent boron powders.

Correlation of the boron particle burn times with their size dis-

ributions yielded the trends, which describe the effect of parti-

le size on its burn time shown in Fig. 11 . The plots for processed

owders are shifted to larger sized particles, reflecting the differ-

nces in their respective particle size distributions. The trends for

he processed boron powders overlap with that for the starting, as

eceived commercial boron. This suggests that the effect of wash-

ng and surface modification on full-fledged boron combustion is

egligible. The burn time, t b , in ms, as a function of the particle

iameter, d , in μm, is reasonably well described as t b ≈ 4.68 ·d

0.74

or all powders.

Examples of typical time-integrated emission spectra produced

y different burning powders are shown in Fig. 12 . The molecular

eaks of BO are clearly visible for all powders. In addition, com-

on impurity peaks by K and Na are observed. These peaks were

294 K.-L. Chintersingh et al. / Combustion and Flame 173 (2016) 288–295

50

100

150

200 A: 95% pure boron

1 10Burn times, ms

05

101520

2530

D: rinsed in toluene and hexane

Freq

uenc

y, n

0

20

40

60

80

100 C: rinsed in toluene

Fig. 10. Burn time distributions of processed and unprocessed amorphous boron

samples in a hydrocarbon premixed air- acetylene flame.

Fig. 11. Burn times as a function of diameter for processed and unprocessed boron

powder burnt in the combustion products of pre-mixed air- acetylene flame.

.u.a,ytisnetninoissi

mE

C: washed, functionalizedin toluene

Tavg=2750 K

A: 95% pure boron

Tavg=2630 K

Wavelength (nm)100 200 300 400 500 600 700 800 900

D: washed, functionalized in hexane

Tavg=2960K

Fig. 12. Emission spectra for different boron samples burning in the combustion

products of an air- acetylene flame.

3

9

h

t

m

u

t

F

f

a

n

i

S

m

g

b

i

c

a

m

a

a

p

h

T

i

s

t

a

a

s

removed manually to fit the remaining spectra with the emis-

sion given by Planck’s Law while treating temperature as an ad-

justable parameter. The fits are also shown in Fig. 12 . These inte-

grated spectra yielded average temperature values of 2630 ± 370 K,

2750 ± 170 K and 2960 ± 460 K for samples A, C and D, respec-

tively. The differences between average temperatures are within

the experimental error bars for all materials. Individual, time-

dependent particle temperatures could not be recovered from the

recorded emission traces filtered at different wavelengths (700 and

800 nm) because of a non-gray background emission produced by

the air-acetylene flame. Although the temperature could not be ob-

tained, the time-dependent intensity ratios for pulses with differ-

ent durations (and thus produced by particles of different sizes)

were compared to one another. No effect of particle size on the

intensity ratio was observed for any material. Thus, the same ma-

terial particles of all sizes burned at approximately the same tem-

peratures.

. Discussion

The low-temperature mass loss exceeding 5% observed for the

5% pure boron powder suggests that commercial boron contains

ydroxides as products of aging in humid environments. It is likely

hat the composition of the surface layer also includes impurities,

ost likely magnesium, which can also produce hydroxide that is

nstable at elevated temperatures. Washing the powder in acetoni-

rile reduces the mass loss by approximately 50%, as shown in

ig. 4 . The mass loss is not eliminated, suggesting that only a

raction of the surface layer is dissolved. The mass loss occurs at

bout the same temperature for all powders, suggesting that the

ature of the reaction causing it has not changed. It is likely that

n all cases the mass loss is due to decomposing hydrated oxides.

ome such layers are captured between agglomerated particles and

aybe difficult to dissolve. Using ultrasonication or other deag-

lomeration methods may thus be beneficial to produce a cleaner

oron powder.

Removal of the acetonitrile-oxide solution by decanting

s inefficient, leading to remaining boron oxide after drying

ementing powder particles together. This is evident by the char-

cteristic crystallites ( Fig. 1 B). The solution can be effectively re-

oved by replacing the solvent with toluene, and ultimately hex-

ne. These solvents are less polar than acetonitrile and do not bind

s strongly to the boron particle surface. Consequently, particle–

article contacts become energetically more favorable in toluene or

exane, leading to the formation of larger but loose agglomerates.

hese agglomerates appear to be slightly smaller in hexane than

n toluene. Overall, however, the morphology of the powder is pre-

erved ( Fig. 1 ) as well as its oxidation behavior ( Fig. 4 ). Therefore,

he washed and surface-modified powders, which do not include

ny additional material components, are as energetically attractive

s commercial boron. It is also important that such materials are

table in the oxidizing environment near the room temperature, as

K.-L. Chintersingh et al. / Combustion and Flame 173 (2016) 288–295 295

s

e

(

d

b

g

s

f

c

e

c

s

u

c

n

i

t

c

b

a

t

a

a

w

e

t

d

o

4

b

o

e

c

p

a

s

m

t

a

p

A

A

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

hown in Table 2 . Thus, such modified powders are expected to be

asy to adopt as components of energetic formulations.

Shorter distances at which the washed powder particles ignite

Fig. 6 ) and respectively shorter ignition delays ( Fig. 7 ) clearly in-

icate that the partial removal of the hydrated oxide layer from

oron surface accelerates its ignition. This is consistent with the

enerally accepted understanding of the mechanism of the first

tage of boron combustion, involving removal of the oxide layer

rom the particle surface [14,37] . The reaction that follows, so-

alled full-fledged combustion, which is detected here by optical

mission of the burning particles, is not affected by the initial pro-

essing. Both reaction rates and flame temperatures are nearly the

ame for “as received” and washed powders. This suggests that the

noxidized or “active” boron is not changed by the present pro-

essing. Washing in acetonitrile and rinsing with toluene or hexane

either impede nor accelerate the full-fledged combustion.

Note finally that the temperatures of the particles prior to their

gnition were estimated considering their heating in a gas flow of

he air-acetylene flame products. Calculations performed for parti-

les of different sizes assumed particles to remain chemically inert

efore they ignited. It was predicted that most particles reached

nd followed the gas temperature before they were flown to loca-

ions, where ignition was observed experimentally. Thus, no char-

cteristic ignition temperature could be determined. This, once

gain, supports the idea that the full-fledged combustion begins

hen the oxide layer becomes sufficiently thin, and when accel-

rated rate of oxidation leads to a sharp increase in the particle

emperature and its respective emission signal. This can occur at

ifferent tem peratures, depending on the thickness of the initial

xide layer and on the rate of particle heating.

. Conclusions

Hydrated oxide layers can be readily removed from surface of

oron powders by washing them in acetonitrile. Effective removal

f the solvent can be achieved by rinsing with toluene. Additional

xposure to other hydrocarbon liquids, such as hexane, does not

hange powders chemically, but may affect agglomeration of boron

articles.

Powder particles washed in acetonitrile and rinsed with toluene

nd hexane and injected in an air-acetylene flame exhibit sub-

tantially shorter ignition delays compared to the starting com-

ercial boron particles. Full-fledged combustion is unaffected by

he present processing: both burn times and flame temperatures

re nearly identical for the commercial and washed/functionalized

owder particles.

cknowledgment

This work was supported by the US Defense Threat Reduction

gency (Grant no. HDTRA1-11-1-0060).

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