Synthesis of WO3 nanoparticles for superthermites by the template method from silica spheres

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Abstract

Nanosized WO3 tungsten trioxide was prepared by calcination of H3P4W12O40·xH2O phosphotungstic acid, previously dissolved in a silica colloidal solution. The influence of the silica spheres/tungsten precursor weight ratio (x) was investigated. The pristine oxide powders were characterized by XRD, nitrogen adsorption, SEM and TEM techniques. A specific surface area and a pore volume of 64.2 m2 g−1 and 0.33 cm3 g−1, respectively, were obtained for the well-crystallized WO3 powder prepared with x = 2/3 and after the removal of the silica template. The WO3 particles exhibit a sphere-shaped morphology with a particle size of 13 and 320 nm as function of the x ratio.

The performance and the sensitivity levels of the thermites prepared from aluminium nanoparticles mixed with (i) the smallest tungsten (VI) oxide material and (ii) the microscale WO3 were compared. The combustion of these energetic composites was investigated by time resolved cinematography (TRC). This unconventional experimental technique consists to ignite the dried compressed composites by using a CO2 laser beam, in order to determine their ignition delay time (IDT) and their combustion rate. The downsizing WO3 particles improves, without ambiguity, the energetic performances of the WO3/Al thermite. For instance, the ignition delay time was greatly shortened from 54 ± 10 ms to 5.7 ± 0.2 ms and the combustion velocity was increased by a factor 50 to reach a value of 4.1 ± 0.3 m/s. In addition, the use of WO3 nanoparticles sensitizes the mixture to mechanical stimuli but decreases the sensitivity to electrostatic discharge.

Introduction

The tungsten trioxide (WO3) is commonly regarded as a promising material for many applications, in several research fields, due to its very large range of interesting properties including electrochromic [1], [2], [3], photocatalytic [4], gaschromic [5], [6] and optochromic [7] properties. The possibility of using WO3 as an oxidizer in thermites has also been reported in the literature [8], [9], [10]. The thermites are highly energetic materials consisting of fuel (usually aluminium) and oxidizer particles (MoO3, WO3, Fe2O3, CuO, etc.) of microscale dimensions mixed with respect to the stoichiometry given by the general equation:2yAl(s)+3MxOy(s)yAl2O3(s)+3xM(s)+ΔHSuch reactions require high activation energy levels to be primed and their ignition is often performed by using boosting compositions. The combustion of thermites releases a large amount of heat (ΔH) and develops temperatures ranging from 2000 K to 7000 K mainly as a function of the oxide nature [11]. Due to the granular nature of the systems, the kinetic of the reaction is widely controlled by the mass diffusion rate between the reactants. When nanopowders are used, the energetic materials (which are then named nanothermites or superthermites) are more reactive systems due to a better contact between the reactants and consequently a shortened diffusion distance. In this chemical configuration, the combustion behaviour of highly energetic thermites such as Al/MoO3 [10], [12], [13], [14], Al/CuO [15] or Al/Fe2O3 [16], was widely modified exhibiting a decrease in the ignition temperature and delay time and sometimes a modification in the kinetic mechanism [12] in contrast to the microscale oxide. Generally, the nanostructuration seems to be an opportunity to improve the preparation of the energetic material, to control and to optimize their energetic behaviour. With this in mind, we decided to investigate the possibility of synthesizing high surface area WO3 oxide (>40 m2/g [10]) and to evaluate its impact on the energetic behaviour of the Al/WO3 thermites.

During the last few years, a wide variety of processes have been developed to synthesize WO3 powders with very specific properties. The thermal decomposition of a tungsten based inorganic material [17], the precipitation in an acidic solution of tungstite (WO3.H2O) followed by calcination [18], [19], the colloidal process [20], the hydro- or solvo-thermal routes [20], [21], [22], the sol–gel process [2], coupled with an ionic structure-directing agent such as a triblock copolymer [5], [23], [24], [25] or a non-ionic surfactant [26] have been described. More original routes, such as the use of porous materials, e.g. SBA-15 or KIT-6 [27], [28], [29] or alumina membrane [30], have also been mentioned. Whereas the precipitation or calcination methods lead to the preparation of (sub-)micrometric monoclinic tungsten (VI) oxide platelets, the most appropriate method of manufacturing tailored nanosized WO3 material seems to be the sol-gel route and the template process. However, these syntheses sometimes require drastic experimental conditions such as the control of the ambient humidity or the modification of the SBA-15 or Al2O3 template surface properties. In this last example low yields are also obtained.

In the present work, an original method of synthesizing high specific surface area WO3 tungsten trioxide samples is reported. This method involves the calcination in air of H3P4W12O40·xH2O phosphotungstic acid previously dissolved in a silica colloidal solution with nanometric silica spheres used as template agent. The surface properties, morphology and crystallographic structures of the WO3 nanoparticles thus produced were investigated. The combustion behaviour of nano-Al/WO3 composites was also studied in contrast to nanoscale to microscale oxidizers.

Section snippets

Tungsten (VI) oxide synthesis and preparation of Al/WO3 compounds

The synthesis was achieved in atmospheric conditions and with commercially available reagents without further purification. Phosphotungstic acid hydrate (99%), Ludox® HS40 and commercial micrometric WO3 tungsten trioxide (20 μm – SBET = 1.18 m2 g−1, 99.9%) were purchased from Aldrich. 50 nm-sized aluminium particles (SBET = 40 m2 g−1, 99%) and n-hexane (analytical reagent grade) were obtained from Nanotechnologies Inc. and Fisher Scientific, respectively.

Results and discussion

The process used in this study to synthesize ceramic oxide nanoparticles is based on the use of a template which is here a colloidal solution of silica. The facility for silica nanospheres to self-organize into ordered building (simple cubic packing, face-centred cubic or hexagonal close packing) in creating empty spaces or interstitials is well known. The average diameter of the interstitials is as function of the sphere radius following the formula (3):rinterstitial=k×rspherewhere r

Conclusion

Tungsten trioxide nanoparticles were successfully synthesized by using silica nanospheres as porous template. It was shown that the increase in the [silica/tungsten precursor] weight ratio (x) leads to smaller and less aggregated WO3 nanoparticles. The highest specific surface area (64 m2 g−1) and interparticle pore volume (0.33 cm3 g−1) were obtained for an x value of 2/3. The as-prepared nWO3 nanoparticles were used to formulate an aluminium based superthermite, whose sensitivity and

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