ARTICLE pubs.acs.org/JPCC
Development of Highly Active Titania-Based Nanoparticles for Energetic Materials David L. Reid,*,† Kevin R. Kreitz,‡ Matthew A. Stephens,‡ Jessica E. S. King,† Ponnusamy Nachimuthu,§ Eric L. Petersen,‡ and Sudipta Seal*,† †
Department of Mechanical, Materials, & Aerospace Engineering, Advanced Materials Processing & Analysis Center, Nanoscience & Technology Center, University of Central Florida, Orlando, Florida 32816, United States ‡ Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843, United States § WR Wiley Environmental Molecular Sciences Laboratory, Interfacial & Nanoscale Science Facility, Pacific Northwest National Laboratory, Richland, Washington 99352, United States
bS Supporting Information ABSTRACT:
Recent advances in nanostructured fuels and oxidizers may lead to high-performance energetic materials for propulsion, but these nanoparticulates present serious challenges due to their inherent instability and safety hazards and difficulty of manufacture. In this paper, we develop an alternate route, the use of nanoscale metal-oxides to catalyze reactions between micrometer-scale energetic constituents. Methods to synthesize TiO2-based nanoparticles that are highly active toward energetic reactions and effectively incorporate them into energetic composites are reported. Activity was maximized by tuning the physical and chemical properties of the nano-TiO2 dispersion in the composite. An 81% increase in combustion rate was achieved with a nanoparticle loading of 1 wt %, making energetically active nano-TiO2 a viable material for advanced propulsion, without the hazards and difficulties of competing technologies.
’ INTRODUCTION In the development of advanced materials for propulsion, there is great promise in the use of nanotechnology to increase energy densities, energy release rates, safety, and environmental compatibility. Nanostructured solid oxidizers have been found to dramatically increase the reactivity of composite energetic materials.1 Nanoaluminum is emerging as an attractive fuel for rocket propulsion and hydrogen production.25 Encouraging results have been reported in the use of catalytic nanostructured additives in liquid hydrocarbon fuels.6 The use of catalytic nanoscale additives, primarily metal-oxides, in solid-phase energetic materials has the potential to greatly increase energy release rates without the extreme hazards of nanoparticulate fuels and oxidizers. Recently, the authors found that TiO2, a material longthought to be inactive79 or detrimental10 toward energetic reactions, can become highly active when the material dimensions are reduced to the nanoscale. In particular, we found anatase nanoparticles to be more active than rutile or amorphous TiO2.11 However, we have also found that the incorporation of r 2011 American Chemical Society
nanoscale metal-oxides in solid energetic materials does not necessarily produce enhanced reaction rates relative to equivalent micrometer-scale powders.12 This unexpected result indicates that the activity of metal-oxides toward energetic reactions at composite interfaces is not a clear function of particle size. A determination of the physical and chemical parameters that control the activity of nanoscale metal-oxides is therefore an important step in the advancement of energetic materials for propulsion. With this understanding, it is our goal to develop new energetically active nanomaterials for improved performance, stability, and safety in propulsion. Given the potential of nanoscale anatase TiO2 as an active component in energetic reactions, we selected this material as the basis for the current study. Energetic materials have high densities of stored chemical energy that can be rapidly released. This class of materials includes Received: January 30, 2011 Revised: April 28, 2011 Published: May 11, 2011 10412
dx.doi.org/10.1021/jp200993s | J. Phys. Chem. C 2011, 115, 10412–10418
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Highly active TiO2 nanoparticles containing substitutional Fe3þ synthesized by the hybrid impregnationcoprecipitation method are isolated and incorporated without agglomeration into a solid propellant.
fuels, propellants, pyrotechnics, and explosives. As a model energetic material, we use a composite solid propellant containing the crystalline oxidizer ammonium perchlorate (AP) and catalytic powder additives dispersed in a matrix of the combustible polymer hydroxyl-terminated polybutadiene (HTPB). It is well established that certain metal-oxides, most notably Fe2O3, catalyze the thermal decomposition of AP and therefore increase reaction rates during combustion of the composite.8 TiO2 is one of the most widely studied materials in heterogeneous catalysis, but its activity toward combustion and other energetic reactions remains controversial and relatively unexplored. Pure TiO2 is generally a poor combustion catalyst,13 but its activity has been increased by doping substitutionally with transition metal ions such as Fe3þ. Pecchi and co-workers14 studied the combustion activity of Fe-doped TiO2 and found it to increase with Fe loadings up to 15%. In a separate study,15 Pecchi and co-workers found catalytic activity toward methane combustion was maximized when Fe3þ ions occupied substitutional positions in the titania lattice rather than being present on the surface as clusters of Fe2O3. Although the mechanism by which substitutional Fe3þ increases activity toward combustion is unclear, Fe doping is known to have a strong impact on TiO2 photocatalytic activity. Small amounts (1000 rpm) at 60 °C the nanoparticles became physically embedded in HTPB due to the much higher viscosity of the polymer. As the nanoparticle concentration in the aqueous phase became depleted, the water was removed by evaporation. After mixing was complete, isophorone diisocyanate was added, and the propellants were extruded via syringe into 6.4 mm OD Teflon tubing, cured for one week at 63 °C, and then stripped of Teflon and cut into 25 mm strands for testing. The propellant strands were mounted horizontally in a high-pressure bomb reactor, and the burn inhibitor was applied to the sides so that the strands burned in a linear fashion. The strands were ignited via a nichrome wire stretched across one end. As the propellants burned, the pressure change and emission spectra were recorded and used to calculate the combustion rate in centimeters per second. A schematic of the strand burner facility is shown in Figure S1 in the Supporting Information. Detailed descriptions of the strand burner and data analysis methodology are given in Carro et al.23
’ RESULTS AND DISCUSSION Materials Characterization. A list of the prepared nanoparticles, the abbreviations used in the figures, and selected material and performance properties are shown in Table 1. Spray-dried powders were characterized and used with and without calcination, while unagglomerated particles were used as-is from the synthesized suspensions. Unagglomerated nanoparticles were imaged by TEM, which revealed faceted
