Pr????????s ?? ?????st??? ¦?st?t?t? ? ???????? ???t???

??r??? ????????? ???t???

?????rs?t? ?? ????s?r

??? ?????? ????

Control of Ignition Temperature in Hybrid Thermite-Intermetallic

Reactive Materials

Christian Poupart*, Geoff Maines, Matei Radulescu†

Department of Mechanical Engineering, University of Ottawa

Abstract

Thermite compounds have received a renewed interest due to their ability to store large quantities of energy that is comparable to conventional energetic materials. Such reactive materials can be manipulated to create a nanolaminated structure. It has been shown that an increase in the fraction of nanolaminated particles can reduce the ignition temperature and increase reactivity. In the present study, methods to lower the ignition temperature of aluminium copper-oxide (Al-CuO) are assessed. Arrested reactive milling (ARM) was used on stoichiometric Al-CuO powders to increase the nanolamination and reduce the ignition temperature to

600?C. Milling alone not only reduced the ignition temperature slightly, but for milling times greater than

30 minutes, intermediate phases were produced, which had negative impacts on the reaction characteristics. Another method to reduce the ignition temperature of Al-CuO involved creating a hybrid mixture using a compound with a lower ignition temperature to further decrease the ignition temperature of Al-CuO. ARM was used to lower the ignition temperature of a nickel aluminium (Ni-Al) intermetallic compound down2

to 207?C. Hybrid mixtures were then created with varying concentrations of milled Al-CuO and Ni-Al then

tested in a tubular furnace to determine the ignition temperature dependence on heating rate and concentra- tion of constituents. It has been shown that a 75% concentration of Al-CuO has an ignition temperature of

600?C, corresponding to pure Al-CuO. The ignition temperature of the hybrid mixture has been reduced to

approximately 250-350?C for concentrations of Ni-Al greater than 50%.

1. Introduction

Thermite mixtures involving aluminium have seen increased popularity in recent years due to their high energy content, which is comparable to that of conventional energetic compounds. A study done by Dreizin et al. [1] has shown that volumetric and mass combustion enthalpy is shown to be larger in most metal fuels when compared to popular energetic materials such as TNT, RDX, HMX, CL-20 which are extensively used in civilian and military applications [2, 3]. An example of such thermite mixture is Al-CuO which is largely popular due to its ability to produce large quantities of gaseous products and a reaction front that can propagate at supersonic speeds. This compound can be used in many applications involving propellants, explosives, and pyrotechnics. One of the limitations of this thermite
mixture is the high ignition temperature which has been reported to be that of the melting point of aluminium (660?C)
[4]. This high ignition temperature requires high amounts of energy, and can be problematic for some applications.
One method to lower the ignition temperature involves using nano-scale constituents to create mixtures known as MIC (metastable intermolecular composite). Another method to decrease particle size is the arrested reactive milling(ARM) technique. ARM uses a mechanical mill such as a planetary ball mill to reduce micron-sized parti- cles down to sub-micron level. ARM is the most affordable and effective method to achieve nano-scale constituent interface. In a study done by Dreizin et al. [4] Al-CuO samples were prepared by ARM and yielded slightly lower ignition temperatures. This study will further assess the effects of ARM on the ignition temperature of Al-CuO. Another method to lower the overall ignition temperature of Al-CuO has been explored by Ref. [5] using a hybrid
Al-CuO-Si-Bi2 O3 mixture, where the lowest ignition temperature obtained was 613?C. The objective of this method
is to lower the ignition temperature by introducing a compound that has a lower ignition temperature, to the Al-CuO
mixture. Studies have shown that high energy ball milling has an important effect on the ignition temperature of Ni-Al intermetallic mixture and yielded ignition temperatures as low as 275?C [6, 7]. This study will focus on the effects of
the ignition temperature of Al-CuO when introduced with varying concentrations of Ni-Al. Al-CuO and Ni-Al are to be tested separately to determine the milling parameters to create the hybrid mixture.

2. Experimental Method

The aluminium powder used was 99.9% pure, 325 mesh, sifted to obtain particles of 25µ m. Copper-Oxide powder was also 99.9% pure, 1-5µ m, and nickel powder 99.9% pure, 25µ m. Mixtures of Al-CuO were made at stoichiometry (2Al

*M.A.Sc. Candidate, corresponding author, cpoup046@uottawa.ca

† Associate Professor

+ 3CuO) and (1Ni + 1Al). Mixtures were made using a simple hand mixing method to then proceed with mechanical milling. Powders were mechanically milled using a Pulverisette 7 planetary ball mill. A procedure similar to Ref. [4] was used. The powders were placed in two 80ml hardened steel cups rotating at 450rpm, using a 20:1 powder to ball mass ratio, which yielded 3.7g of powder mixture per cup. Hexane was inserted in each cup (4ml) as a PCA (Process Control Agent) in order to inhibit local reaction of compounds. Milling was performed in an argon environment to prevent oxidation of constituents. Al-CuO and Ni-Al were milled separately in order to determine the dependency of the ignition temperature on milling time and heating rate. Individual mixtures were milled until mechanical activation occurred inside the steel cups, which set the maximum milling time for the respective compound, hence the name arrested reactive milling. Scanning electron microscopy (SEM)and X-ray diffraction (XRD) allow for monitoring the micro-structure and characterizing the phases of milled materials.
Ignition temperatures were determined using a Wat- low 60 Watts tubular furnace. Milled mixtures were compacted into 3mm diameter cylindrical pellets to ap- proximately 65% TMD and heated in the furnace at dif- ferent heating rates until ignition. Figure 1 shows the experimental apparatus to determine the ignition temper- ature of compounds. A micro flow of argon was passed through the furnace in order to prevent oxidation of sam-

Argon f ow

Ceramic tubing

Heating coil

Photo-Diode

ples during heating. Figure 2 shows the temperature evo- lution given by the thermocouple. The spike in tem-

Sample

3mm

Focusing lens

perature coinciding with the large increase of lumines- cence given by the photo-diode characterizes the ignition event.
Figure 1: Experimental setup

Figure 2: Typical temperature evolution plot

3. Experimental Results

3.1 Aluminium Copper-Oxide

Al-CuO was milled using settings decribed in section 2. Stoichiometric mixtures of Al-CuO were milled at differ- ent intervals from 16 minutes to 2 hours, then tested in the apparatus shown in figure 1 to determine the ignition temperature. Results for the ignition temperature dependency on milling time and heating rate are shown in figure 3. Results confirm that the ignition temperature of thermites can be modified using the ARM technique. The 16 minute

700

600

500

Furnace Tests: AlCuO Ignition Temperatures vs Mill Time

16 min

30 min

46 min

1hr

2hr

700

600

500

AlCuO Ignition Temperature vs Heat rate

16 min

30 min

46 min

1hr

2hrs

400

400

300

300

200

200

100

100

0

0 20 40 60 80 100 120 140

Mill Time (min)

0

0 20 40 60 80 100 120

Heat rate (K/min)

Figure 3: Al-CuO ignition temperature plotted versus milling time (left) and heating rate (right)
milled sample showed ignition temperatures varying around 600?C with a certain consistency. For mill times of 30 and 46 minutes, the ignition temperature was either 600?C or approximately 150?C. There seems to be a transition
occurring between 20 and 60 minutes of milling time. XRD and SEM results shown in Ref. [8] show that after 30 minutes of milling, products began forming as a result of mechanical activation. Complete reaction of Al-CuO in the milling cups might have been inhibited by the process control agent hexane. Intermediate phases formed during milling are believed to be responsible for the inconsistency in the ignition temperature. Results have also shown that exothermicity was reduced due to the formation of these products [8].

3.2 Nickel Aluminium

Mixtures of Ni-Al were milled until mechanical activation occurred during the milling process, to yield the maximum milling time. The maximum milling time was established at 42 minutes (shown by Bacciochini et al. [9]). The powders were milled at different intervals (16, 30 and 40 minutes).

Figure 4: SEM micro-structure images 16 minutes (left), 30 minutes (center), 40 minutes (right)
As shown in figure 4, milling of Ni-Al yielded a more refined micro-structure. Nanolamination was increased for longer milling times. A 40 minute milled Ni-Al mixture contains a very large quantity of these nanolaminates, which is believed to reduce the overall ignition temperature of the mixture.
XRD analysis results shown in figure 5 proved that no products were formed during the milling process. This tells us that any of the milled samples can be used for the hybrid mixture.
Figure 6 shows that the ignition temperature of Ni-Al could be reduced to approximately 350?C for a 16 minute milling time. The lowest ignition temperature obtained was for a mill time of 40 minutes. No dependency on the heating rate is found for the Ni-Al mixture.

Figure 5: XRD analysis results

700

600

500

400

300

200

100

0

Nickel Aluminium Ignition Temperature vs Mill Time

0 10 20 30 40 50 60

Mill Time (min)

700

600

500

400

300

200

100

0

Ignition temperature vs heating rate

16 min

30 min

40 min

30 40 50 60 70 80 90 100 110

Heat rate (K/min)

Figure 6: Ni-Al ignition temperature plotted versus milling time (left) and heating rate (right)

3.3 Hybrid Mixture

The procedure for creating a hybrid Al-Cuo-Ni mixture was strongly dependent on the results of individual Al-CuO and Ni-Al mills. The objective of the hybrid mixture is to create a compound that embodies the low ignition of the intermetallic Ni-Al with the high reactivity of Al-CuO without the negative effects of having intermediate phases. The candidates for this mixture were 16 minute milled Al-CuO and 40 minute milled Ni-Al. The goal was to create a mechanically milled mixture with these total milling times, therefore Ni-Al was milled separately for 24 minutes. The newly made 24 minute milled Ni-Al was then mixed with unmilled stoichiometric Al-CuO and milled for an additional
16 minutes. Mixtures were made at varying concentrations of constituents (25, 50 and 75% of Al-CuO by mass).

700

600

500

Ignition Temperature vs AlCuO concentration (mass)

700

600

500

Ignition Temperature vs Heat rate

100% Ni-Al

50% Al-CuO

100% Al-CuO

400

400

300

300

200

100

0

100% AlCuO

100% NiAl

25% AlCuO

50% AlCuO

75% AlCuO

0 0.2 0.4 0.6 0.8 1

AlCuO concentration

200

100

0

30 40 50 60 70 80 90 100

Heat rate (K/min)

Figure 7: Hybrid ignition temperature plotted versus milling time (left) and heating rate (right)
Preliminary results shown in figure 7 show that the ignition temperature of the hybrid mixture is dependent of the concentration of constituents. A 75% Al-CuO concentration had an ignition temperature of 600?C, corresponding to
that of pure 16 minute milled Al-CuO. A transition occurs at 50% concentration, at which point the ignition tempera- ture is approximately 400?C. Further increase in concentration of Ni-Al reduced the ignition temperature of the hybrid mixture down to approximately 250?C. No clear dependency on heating rate was found.

4. Conclusion

The ARM method has proven successful in reducing the ignition temperatures of thermites and intermetallic com- pounds. Products and intermediate phases formed for mixtures of Al-CuO that were milled for more than 16 minutes. Due to the formation of products during milling, the ARM method alone was not a viable option to lower the ignition temperature of Al-CuO. The lowest ignition temperature obtained using ARM was for 16 minutes milled Al-CuO
which had an average ignition temperature of 600?C. Milled nickel-aluminium yielded significantly lower ignition
temperatures averaging 207?C for 40 minute milled samples. The addition of an intermetallic mixture to Al-CuO
greatly reduced the ignition temperature of Al-CuO for concentrations of Ni-Al more than 50%. Hybrid mixtures with
25% concentration of Al-CuO had an ignition temperature corresponding to that of 40 minutes milled pure Ni-Al. More experiments are to be conducted for lower heating rates to expand the data field.

References

[1] E. L. Dreizin, “Metal-based reactive nanomaterials,” Progress in Energy and Combustion Science, vol. 35, no. 2, pp. 141–167, 2009.
[2] J. Agrawal, “Recent trends in high-energy materials,” Progress in energy and combustion science, vol. 24, no. 1, pp. 1–30, 1998.
[3] P. F. Pagoria, G. S. Lee, A. R. Mitchell, and R. D. Schmidt, “A review of energetic materials synthesis,” Ther- mochimica Acta, vol. 384, no. 1, pp. 187–204, 2002.
[4] S. M. Umbrajkar, M. Schoenitz, and E. L. Dreizin, “Exothermic reactions in al–cuo nanocomposites,” Ther- mochimica acta, vol. 451, no. 1, pp. 34–43, 2006.
[5] K. Ilunga, O. Del Fabbro, L. Yapi, and W. W. Focke, “The effect of si–bi< sub> 2</sub> o< sub> 3</sub> on the ignition of the al–cuo thermite,” Powder Technology, vol. 205, no. 1, pp. 97–102, 2011.
[6] K. V. Manukyan, B. A. Mason, L. J. Groven, Y.-C. Lin, M. Cherukara, S. F. Son, A. Strachan, and A. S. Mukasyan, “Tailored reactivity of ni+al nanocomposites: Microstructural correlations,” Journal of Physical Chemistry C, vol. 116, pp. 21027–21038, 2012.
[7] A. Mukasyan, J. White, D. Y. Kovalev, N. Kochetov, V. Ponomarev, and S. Son, “Dynamics of phase transfor- mation during thermal explosion in the al–ni system: Influence of mechanical activation,” Physica B: Condensed Matter, vol. 405, no. 2, pp. 778–784, 2010.
[8] G. Maines, M. I. Radulescu, A. Bacciochini, B. Jodoin, and J. Lee, “Pressure waves generated by metastable intermolecular composites in an aqueous environment,” in Bulletin of the American Physical Society. 18th Biennial Intl. Conference of the APS Topical Group on Shock Compression of Condensed Matter held in conjunction with

the 24th Biennial Inti. Conference of the Inti. Association for the Advancement of High Pressure Science and

Technology (AIRAPT); Seattle, Washington., vol. 58, 7-12 July 2013.

[9] B. A., R. M.I., C.-T. Y., J. Van Dyke, M. Nganbe, Y. M., J. J. Lee, and J. B., "Enhanced reactivity of mechanically­ activated nano-scale gasless reactive materials consolidated by coldspray," Surface and Coatings Technology, vol. 206,pp.4343-4348, 2012.