Combustion and Flame 212 (2020) 245–251 Contents lists available at ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame Titanium enhanced ignition and combustion of Al/I 2 O 5 mesoparticle composites Wanjun Zhao 1, Xizheng Wang, Haiyang Wang, Tao Wu, Dylan J. Kline, Miles Rehwoldt, ∗ Hui Ren 1, Michael R. Zachariah Department of Environmental and Chemical Engineering, University of California Riverside, Riverside, CA 92521, USA a r t i c l e i n f o a b s t r a c t Article history: Aluminum (Al) is widely used in thermites, and iodine pentoxide (I 2 O 5 ) is a strong oxidizer that can re- Received 11 October 2018 lease gas phase iodine. The result is a potentially very powerful and effective energetic composite with Revised 25 April 2019 biocidal properties. However, the alumina coating on the Al presents a diffusion barrier that increases Accepted 28 April 2019 the ignition temperature. In this paper, nano titanium powders (nTi) were added at various mass ratios to Al/I 2 O 5 and a composite mesoparticle was assembled by electrospray. Combustion studies showed that Keywords: nano Ti can enhance the reactivity of Al/I 2 O 5 thermites significantly with increases in pressure, pres- Titanium surization rate and dramatic decreases in burn time. Part of this behavior can likely be attributed to Aluminum less sintering enhancing the completeness of reaction in the Ti added cases. Addition of titanium to alu- Thermite minum was found to have a significant impact on ignition and when Al is replaced with Ti, the ignition Iodine temperature was ∼300 °C below that of the corresponding aluminum thermite. These results show that titanium can be used as a fuel to tune energetic behavior and iodine release temperature and in some ways superior to aluminum. ©2019 Published by Elsevier Inc. on behalf of The Combustion Institute. 1. Introduction show significant decrease in ignition temperature compared to Al- based ones [4] . Combustion behavior can also be moderated with Thermites are a class of energetic materials composed of metal the additive of other fuels. For example, a 20 mol% addition of and metal oxide with greater stored potential energy than tradi- boron to Al/CuO increases the pressurization rate ∼3 X [5] . Adding tional CHNO based systems. The most common fuel used in ther- 2 wt% nano Ag into Al/CuO has been shown to increase combus- mites is aluminum due to its high reaction enthalpy and low cost. tion speed by over 60% [6] . However, the native oxide coated on aluminum can increase the For Al NPs-based thermites, traditional oxidizers include CuO, ignition delay [1]. For some applications, it may be desirable to Fe2 O3 , Bi2 O3 , WO3 , etc. [7–11]. Other oxidizers including KIO4 [12], lower the ignition temperature. Wang et al. found that ammonium K2 S2 O8 [13], Ag2 O [14,15], AgIO3 [16], I2 O5 [15,17,18], B2 O3 [19–21], perchlorate (AP) can lower the ignition temperature of aluminum some metal iodates [22,23] , etc. have also been explored for their because the oxide shell can be corroded by the acid released from high reactivity. Among them, Ag2 O, Ca(IO3 )2 , Cu(IO3 )2 and I2 O5 are the decomposition of AP [1] . example oxidizers that have been considered as possible biocidal Others have added metals to Al-based thermites, such as tita- oxidizers because they leave a biocide (silver or iodine) as a reac- nium, nickel, zinc, boron, magnesium to tune either the ignition or tion product [13–17,22–24]. I2 O5 has attracted considerable interest combustion behavior [2–6] . Shoshin and Dreizin investigated the because of its high iodine content, ∼76% iodine mass fraction. Wu ignition temperature of Al–Ti alloys and found that the ignition et al. showed that Al/I2 O5 is extremely effective as oxidizer and io- temperature decreased from 1877 °C (for neat Al) to 969 °C (for a dine generator, and thus as biocidal energetic material [25] . And Al0.75 Ti0.25 alloy) [2]. Aly et al. showed that Al–Ni and Al–Zn com- in fact Martirosyan et al. found that the maximum peak pressure posites made by short term milling ignited at a lower tempera- produced by Al/I2 O5 is comparable to Al/Bi2 O3 system which is a ture than unmilled aluminum [3] . Similarly, AlMg-based thermites particularly violent thermite mixture [18] . In this paper, we explore the use of titanium as an additive to tune the ignition and combustion performance of Al/I2 O5 . Based ∗ Corresponding author: on our previous work, we incorporated small amounts of nitrocel- E-mail address: [email protected] (M.R. Zachariah). lulose (NC) into the system by electrospray considering that fuel 1 Present address: Beijing Institute of Technology, Beijing 10 0 081, China. https://doi.org/10.1016/j.combustflame.2019.04.049 0010-2180/© 2019 Published by Elsevier Inc. on behalf of The Combustion Institute. 246 W. Zhao, X. Wang and H. Wang et al. / Combustion and Flame 212 (2020) 245–251 and oxidizer can be mixed more uniformly and the sintering of Al Table 1 = Thermite formulations (fuel/oxidizer equivalence ratio 1.0). can be inhibited by the gas release from NC [26]. I2 O5 particles were synthesized via an aerosol spray pyrolysis (ASP) technique Thermites Al (wt%) Ti (wt%) I 2 O 5 (wt%) NC (wt%) [17] and the mesoparticles of Al/Ti/I O with varying amounts of 2 5 Al–I 2 O 5 23.8 0 71.2 5 Al and Ti content were prepared by electrospray. The peak pres- 90%Al–10%Ti–I 2 O 5 20.7 4.1 70.2 5 sure, pressurization and burn time during the combustion were 70%Al–30%Ti–I2 O5 15.2 11.6 68.2 5 measured by a constant volume combustion cell. The ignition tem- 50%Al–50%Ti–I2 O5 10.3 18.3 66.4 5 30%Al–70%Ti–I O 5.9 24.3 64.8 5 perature and species released from the thermites were measured 2 5 Ti–I 2 O 5 0 32.3 62.7 5 by time-resolved temperature-jump time-of-flight mass spectrom- eter (T-Jump/TOFMS). We found that adding Ti to Al/I2 O5 system can increase the pressurization rate, i.e. reactivity, by a factor of ray diffraction (XRD, Bruker D8 Advance using Cu K α radiation). X- 10 0 0, and the ignition temperature can be reduced from ∼610 °C ray photoemission spectrometry (XPS, Kratos AXIS 165 spectrom- to ∼300 °C. eter) and XRD were used to determine the nature of the oxide layer of nTi particles. Morphology and elemental analysis of elec- 2. Experimental section trosprayed Al/Ti/I2 O5 thermite composites were measured by SEM and energy dispersive spectrometer (EDS) conducted on a Hitachi 2.1. Chemicals SU-70 instrument. Thermogravimetry/mass spectra (TG/MS) results of I2 O5 , Al/I2 O5 , Ti/I2 O5 ,TiO2 /I2 O5 and Al2 O3 /I2 O5 were obtained Iodic acid (HIO3 ) (99.5 wt%) was purchased from Sigma- with a TA Instruments Q600 at a heating rate of 10 °C/min up to Aldrich and the nano-aluminum powders (nAl) (Alex, ∼80 nm), 700 °C in argon connected by a heated micro-capillary ( ∼300 °C) with ∼81 wt% active aluminum, confirmed by thermogravime- to the MS to obtain the simultaneous MS of gaseous product. try analysis, were purchased from Novacentrix. The nano-titanium (nTi) (50–80 nm) powders, with ∼70 wt% active titanium, were purchased from US Research Nanomaterials Inc. Anatase tita- 2.5. Combustion cell test < nium dioxide (TiO2 ) nanoparticles ( 50 nm) and aluminum ox- < A constant-volume combustion cell was used to measure the ide (Al2 O3 ) nanoparticles ( 50 nm) were purchased from Sigma Aldrich. Collodion solution (4–8 wt% in ethanol/diethyl ether) was combustion behavior of thermite samples [27] . In a typical experi- purchased from Fluka Corp and a diethyl ether (99.8%)/ethanol ment, 25 mg of loose thermite powders was loaded into the com- ∼ 3 (99.8%) mixture (volume ratio: 1:3) was used to dilute the collo- bustion cell with a constant volume of 20 cm and ignited with a dion solution. resistively heated nichrome coil. The pressure and optical emission from the thermite reaction were recorded by the attached piezo- 2.2. Preparation of iodine pentoxide via aerosol spray pyrolysis electric pressure sensor and a photodetector, respectively. The peak width at half height of the optical emission was defined as the burn time. Submicron particles of I2 O5 were obtained via ASP [17]. Typ- ically, 1 g HIO3 dissolved in 50 mL deionized water (20 mg/mL) was sprayed into aerosol droplets with ∼1 μm size by an atomizer 2.6. T-jump ignition and time-resolved mass spectrometry ( ∼35 psi pressure air). The atomized droplets passed through sil- measurement icon diffusion dryer, where most of the water was absorbed and then passed into a tube furnace at 400 °C where the chemical de- The ignition of thermites and oxygen release of the oxidizer composition was complete and I2 O5 particles were obtained. were investigated by temperature-jump/time-of-flight mass spec- trometry (T-jump/TOF-MS) coupled with a high-speed camera (Vi- 2.3. Preparation of the Al/Ti/I 2 O 5 meso-particles sion Research Phantom v12.0, 67,056 frames/s) [28]. Typically, a thin layer of the thermite sample was coated onto a platinum wire To explore the influence of the titanium on Al/I2 O5 thermites, (76 μm in diameter, 10 mm in length).