Approved for public release; distribution is unlimifed. Title: Author(s). Submitted to. Laser Ignition of Aluminum Nanoparticles in Air Mary M. Sandstrom, David Oschwald, and Steven F. Son 31 st International Pyrotechnics Seminar, Fort Collins, CO, July 11 -1 6, 2004 A '0 LosAlamos NATIONAL LABORATORY Los Alamos National Laboratory, an affirmative action/equalopportunity employer, is operated by the University of Californiafor the U.S. Departmentof Energy under contract W-7405-ENG-36.By acceptance of this article, the publisher recognizes that the U.S. Government retains a nonexclusive,royalty-freelicense to publish or reproducethe publishedform of this contribution, or to allow others to do so, for U.S. Government purposes. Los Alamos National Laboratory requeststhat the publisher identify this article as work performed under the auspices of the US. Department of Energy. Los Alamos National Laboratory strongly supports academic freedom and a researcher's right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness. Form 836 (8100)
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Approved for public release; distribution is unlimifed.
Title:
Author(s).
Submitted to.
Laser Ignition of Aluminum Nanoparticles in Air
Mary M. Sandstrom, David Oschwald, and Steven F. Son
31 st International Pyrotechnics Seminar, Fort Collins, CO, July 11 -1 6, 2004
A ' 0 LosAlamos
N A T I O N A L L A B O R A T O R Y Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by the University of California for the U.S. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for U.S. Government purposes. Los Alamos National Laboratory requests that the publisher identify this article as work performed under the auspices of the US. Department of Energy. Los Alamos National Laboratory strongly supports academic freedom and a researcher's right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness.
Form 836 (8100)
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Laser Ignition of Aluminum Nanoparticles in Air M.M. Sandstrom, D.M. Oschwald, and S.F. Son
Abstract
This paper reports on recent experiments of the ignition of nanoaluminum in air by COz laser heating. Ignition time and temperature were measured as a function of A1 particle size and laser power. The ignition time was determined by high-speed digital images and frrst light as determined by a photodiode. The ignition delay increases with increasing particle size, and the decreasing laser power. Two stage burning is observed. The first reaction takes place on the surface of the powder sample and moves from the center to the edges followed by the second reaction, which takes place within the bulk of the sample. As the particles size increases the material is less likely to burn through out, leaving behind unreacted A1 powder.
Introduction
Micron sized aluminum powders have long been used as an additive in energetic materials, including propellants, explosives and pyrotechnics. When added to rocket propellants, the powders increase the energy and act to raise the flame temperature. When incorporated into explosives, they can enhance late-time effects including air blast, increase reaction temperature and create incendiary effects. Thermites are a pyrotechnic mixture of a metal and a metal-oxide. Aluminum and iron oxide powder mixtures is an example of a thermite. The aluminum reduces the iron oxide, which generates a large amount of heat, slag and iron. These applications have made the ignition and burning of aluminum powders the topic of intensive research over the past four decades [’-I ’I One very important finding from this research is an enhanced reaction performance as particle size decreases.
large enough quantities that they are now commercially available.[’21 This allows for the extensive use of these materials, however, the combustion properties are still not well understood. A comprehensive understanding the combustion properties is critical in determining the performance and safety of energetic materials that utilize Al-nanpowders.
It has been long known that micron sized aluminum is difficult to ignite, and does not typically self propagate in air. However, it has also been observed that nano-aluminum is easily ignited and does self propagate in an oxidizing atmosphere such as air. When the particle size is reduced to tens to hundreds of nanometers, the reactivity of the powder increases significantly. This is most likely due to the increased surface area which decreases the diffusion length scales in the AI nanopo~der.[’~I Another explanation of the combustion behavior of the nano-aluminum may be credited to the “surface effect” of superfine particles.[ 141 This theory suggests that the larger ratio of surface atoms to interior atoms in the nano-powders increases the overall energy of the material since atoms at the surface of the particle are in a higher energy state than the interior atoms. This results in an excess surface energy and surface tension on the particles, which may contribute to a lower melting temperature.
Laser ignition via a C02 laser heating is effective means to apply a controlled amount of thermal energy into an energetic material. This is achieved by controlling the pulse width, power, energy, beam profile, and spot size. For this reason, the laser ignition can be a useful tool for determining the thermal response of energetic materials[ 15- 171
This work reports on the ignition behavior of both nanometer and micron sized aluminum powders by C02 laser heating in air. Ignition as determined by first light was measured for the aluminum powders at varying laser powers.
Recent advances in technology have allowed nano-sized aluminum powders to be produced in
Experimental Aluminum powders
Four different nano-powders and two micron sized powders were used in these experiments. All the nano-sized powders were obtained from Technanogy were synthesized by dynamic gas condensation.[’21 The micron size powders were obtained from Alfa Aesar. The average particle diameter, the oxide layer thickness and/or the percent active aluminum are presented in Table 1. The reported average particle size diameter for the nano-sized powders is calculated from the surface area measured by Brunauer-Emmett-Teller analysis (BET)[ 181. This calculation makes an assumption that the particles are spherical and mono-sized, however, analysis of other powders obtained from Technanogy have shown both irregular particle morphologies and log-normal particle size distributions.[ 19,201 The oxide shell thickness or active aluminum content is measured by thermogravimetric analysis (TGA).[19] [21]
Laser Ignition Experiments
A Coherent 250W C02 (GEM 200PC DEOS) laser was used to ignite the powder samples. The laser was operated at 10.6pm and the beam was TEMoo. The Guassian beam profile was recorded by a laser beam analyzer (Spirocon Model #300pc and Pyrocam), and the laser power was determined by a Coherent power meter(LM-200). The power meter was fitted with a 1 cm diameter with a mask to determine the laser power incident on the sample surface. Approximately 8% of the laser beam is split off and directed onto a Vigo Photovoltaic tri-metal detector (PD-10.6-3) to monitor the laser output. The rise time was determined to be approximately 0.25ms
A lcm diameter quartz cup held the powder sample and was placed on top of a piece of silica aero-gel. The thermal response of the material is measured by an array of diagnostic tools to determine ignition delay and reaction temperatures. A photodiode (Thorlabs DET21O) was used to determine ignition by first light.. Finally, a high-speed video camera is used to monitor entire process. A schematic of the experimental set up is shown in Fig. 1.
Results and Discussion
Figure 2 shows the photodiode and laser output traces for all six samples shot at the same laser power. The irradiance on each sample was determined to be 66W/cm2. The laser pulse width was 0.5 seconds for the nanopowders and 2 seconds for the micron powders. As the average particle increases, the photodiode intensity decreases significantly. Furthermore, the nanometer-sized powders continue to bum after the laser has been shut off while the micron-sized powders cool rapidly and are extinguished. For the 24.9nm, 33.lnm and 52 nm powders, combustion continues until the entire sample has reacted. The time to completion is increased with increasing particle size. The 24.9nm powder is almost completely consumed during the laser pulse, and the remaining sample is consumed quickly after the laser is shut off. The 33.lnm and 53nm powders cool rapidly when the laser is shut off, but do not extinguish. They appear to “reignite,” which can be seen by a local maximum at approximately 4 seconds on the 33. lnm trace, and two seperate maxima at 2.5 and 6 seconds in the 53nm powder. The 133.lnm powder continues to react slowly after the laser has been shut off, but does not completely react throughout the sample since there is unreacted powder left at the bottom of the sample cup. The micron sized powders only burn as long as the laser is on. A hard “disk” is formed at the surface of the sample, and there appears to by no further reaction below this disk (Steve, I need a better way to say this).
Two phase burning has been observed by many researchers for both nano and micron aluminum powders.[3,6, 113 The first phase is characterized by the spread combustion waves over the surface of the sample originating at an ignition source. During the second phase, the temperature of the sample increases rapidly accompanied by bright white radiation that originates from the bulk of the sample. A closer inspection of the photodiode traces show two phase burning in all the samples except the 10-14pm powder. This may be due to the fact that this sample did not actually ignite and burn, but rather just
experienced laser heating. Figure 3 is a comparison of a photodiode trace to the reaction progress as observed by high-speed video on the 38.1 nm sample.
powders. The vigorous (violent?) reaction was unique to the 24.9nm powder, while all other powders exhibited behavior similar to the 38.1 nm material. It is difficult to discern the first stage of the reaction for the 24.9 nm material on the photo diode trace due to an extremely energetic and rapid reaction, however it is evident in the high-speed video. (Steve, do you think reaction behavior of the 24.9 nm powders might support the “surface effect” theory that I talked about in the introduction? I got that reference fiom Michelle Pantoya. I have gone back and forth on commenting on that because I know that the particle size distribution is pretty wide after looking at that sample under SEM. )
expected, the ignition delay is longer with increasing particle size.. The micron sized powders were a “no- go” at the lowest irradiance (39W/cm2). These results indicate that diffusion is the rate-limiting step, therefore the size of the aluminum particles affects the ignition time. The data were fit to a power curve, and the corresponding equations are displayed. All the data have a slope of approximately -1 , except the 10-14pm powder. This result is interesting because it indicates that these materials may behave according to a dual ignition criteria model developed by Pantoflicek and Lebr[22] to explain results from radiant ignition experiments on ammonium perchlorate.[23] Ali and coworkers[24] saw similar behavior in HMX. The model proposed two ignition criteria: minimum surface temperature and minimum energy concentration deposited into the solid. The model predicts at that for lower irradiances, where surface temperature is limiting, the power curve fit will be -2. At higher irradiances, the energy within the condensed phase is limiting, and power curve fit is -1. The data presented here may fit these criteria. Future experiments would include ignition of these materials at lower irradiances to determine if a temperature limiting region.exists.
Figure 4 shows a comparison of the reaction progression between the 24.9nm and the 38.lnm
First light ignition delays as a function of irradiance for all the samples are shown in Fig. 5 . As
Conclusion In this we paper reports on recent experiments of the ignition of nanoaluminum in air by C02
laser heating. Ignition time and temperature were measured as a function of A1 particle size and laser power. The ignition time was determined by high-speed digital images and first light as determined by a photodiode. Two stage burning is observed in nano-aluminum powders and to a lesser extent in the micron sized powders. The first reaction takes place on the surface of the powder sample and moves fiom the center to the edges followed by the second reaction, which takes place within the bulk of the sample. As the particles size increases, the material is less likely to burn through out, leaving behind unreacted A1 powder. The ignition delay increases with increasing particle size, and the decreasing laser power. On a log-log plot, the slope of the data was approximately -1 for all samples except the 10-14pm powder. This indicates that the energy concentration deposited into the samples is the limiting factor for ignition. Moreover, the increase in ignition delay with larger particle sizes indicates that diffusion is the rate limiting step.
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Il'in A.P. and Proskurovskaya, L. T., Two Stage Combustion of an Ultradispersed Aluminum Powder in Air, Combustion Explosion and Shock Waves 26, no. 2, 190-191 (1990). Dreizin, E. L., Experimental Study of Stages in Aluminum Particle Combustion in Air, Combustion and Flame 105,541-556 (1996). Il'in A.P., Y., G.V., and Gromov, A.A., Influence of Additives on Combustion of Ultadisperse Aluminum Powder and Chemical Binding of Air Nitrogen, Combustion Explosion and Shock Waves 32, no. 2,211-213 (1996). Il'in A.P., G., A.A., Vereshchagin, V.I., Popenko, E.M., Surgin, V.A. and Lehn, H., Combustion of Ultrafine Aluminum in Air, Combustion, Explosion, and Shock Wave 37, no. 6,664-668
Politzer, P., Lane, P. and Grice, M. E., Energetics of Aluminum Combustion, J . Phys. Chem A
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Figure 2 Photodiode and laser output responses for all six powders shot at the same laser power. The solid line represents the photodiode and the broken line represents the laser output.
h
W > a, -0 0 U 0 0 L a
.- c
33.lnm 39W/Cm2 1 I I 1 I I
I
0.4
0.2
0.6 L
0.4 -
0.2 -
I o c
Time (s)
0.4
0.2 h
W > a, tJ 0 U 0 0 L a
0 .- c
-0.2
-0.4
33.lnm 39W/Cm2
I I I I
.--- I I I I
I I I I I
0 0.1 0.2 0.3 0.4 0.5
Time (s)
0.1
0.05
- 7
s n
0 5 0 (D h < V
-0.05
-0.1
Figure 3. Photodiode and laser trace. The top graph shows the entire laser pulse and diode trace. The images show the first and stage reactions. The bottom figure shows the initial part of ignition
Figure 4. Comparison of the reaction progress of the 38.lnm powder (top) and the 24.9nm powder (bottom). In both figures, the top row of images show the first phase of combustion, while the bottom row shows the second phase.
100
10
Aluminum Powder Ignition in Air
- c -38.lnm --L-- 133.5nrn - - A - - 3-4urn -- - 10-1 4um
10 100 1000
Irradiance (W/cmA2)
- y = 2488.3 * x"(-1 .102) R= 0.99534
- - y = 5259.3 * x"(-1 .2052) R= 0.99523
- - - y = 1774.8 * x"(-0.99088)
== -= -y = 5860.1 * xA(-1.1769)
1 1 1 1 1 y = 16423 * x"(-1 .1164)
- - - y = 5520.6 * x"(-0.55693)
R= 0.99657
R= 0.97721
R= 0.90577
R= 0.91051
Figure 5 Time to first light as a function of irradiance for all six particle sizes considered
