Alexander E. Gash

New sol–gel synthetic route to transition and main-group metal oxide aerogels using inorganic salt precursors

Abstract We have developed a new sol–gel route to synthesize several different transition and main-group metal oxide aerogels. The approach is straightforward, inexpensive, versatile, and it produces monolithic microporous materials with high surface areas. Specifically, we report the use of epoxides as gelation agents for the sol–gel synthesis of chromia aerogels and xerogels from simple Cr(III) inorganic salts. The dependence of both gel formation and its rate was studied by varying the solvent used, the Cr(III) precursor salt, the epoxide/Cr(III) ratio, as well as the type of epoxide employed. All of these variables were shown to affect the rate of gel formation and provide a convenient control of this parameter. Dried chromia aerogels were characterized by high-resolution transmission electron microscopy (HRTEM) and nitrogen adsorption/desorption analyses, results of which will be presented. The results presented here show that rigid monolithic metal oxide aerogels can be prepared from solutions of their respective metal ion salts (Fe3+, Al3+, In3+, Ga3+, Zr4+, Hf4+, Ta5+, Nb5+, and W6+), provided the formal oxidation state of the metal ion is greater than or equal to +3. Conversely, when di-valent transition metal salts are used precipitated solids are the products.

Nanostructured energetic materials using sol-gel methodologies

Abstract We have utilized a sol–gel synthetic approach in preparing nano-sized transition metal oxide components for new energetic nanocomposites. Nanocomposites of Fe 2 O 3 /Al(s), are readily produced from a solution of Fe(III) salt by adding an organic epoxide and a powder of the fuel metal. These materials can be processed to aerogel or xerogel monolithic composite solids. High resolution transmission electron microscopy (HRTEM) of the dried energetic nanocomposites reveal that the metal oxide component consists of small (3–10 nm) clusters of Fe 2 O 3 that are in intimate contact with ultra fine grain (UFG) ∼25 nm diameter Al metal particles. HRTEM results also indicate that the Al particles have an oxide coating ∼5 nm thick. This value agrees well with analysis of pristine UFG Al powder and indicates that the sol–gel synthetic method and processing does not significantly perturb the fuel metal. Both qualitative and quantitative characterization has shown that these materials are indeed energetic. The materials described here are relatively insensitive to standard impact, spark, and friction tests, results of which will be presented. Qualitatively, it does appear that these energetic nanocomposites burn faster and are more sensitive to thermal ignition than their conventional counterparts and that aerogel materials are more sensitive to ignition than xerogels. We believe that the sol–gel method will at the very least provide processing advantages over conventional methods in the areas of cost, purity, homogeneity, and safety and potentially yield energetic materials with interesting and special properties.

Controlling Material Reactivity Using Architecture.

3D-printing methods are used to generate reactive material architectures. Several geometric parameters are observed to influence the resultant flame propagation velocity, indicating that the architecture can be utilized to control reactivity. Two different architectures, channels and hurdles, are generated, and thin films of thermite are deposited onto the surface. The architecture offers an additional route to control, at will, the energy release rate in reactive composite materials.

Nanostructured Energetic Materials with Sol-Gel Methods

The utilization of sol-gel chemical methodology to prepare nanostructured energetic materials as well as the concepts of nanoenergetics is described. The preparation and characterization of two totally different compositions is detailed. In one example, nanostructured aerogel and xerogel composites of sol-gel iron (III) oxide and ultra fine grained aluminum (UFG Al) are prepared, characterized, and compared to a conventional micron-sized Fe{sub 2}O{sub 3}/Al thermite. The exquisite degree of mixing and intimate nanostructuring of this material is illustrated using transmission and scanning electron microscopies (TEM and SEM). The nanocomposite material has markedly different energy release (burn rate) and thermal properties compared to the conventional composite, results of which will be discussed. Small-scale safety characterization was performed aerogels and xerogels of the nanostructured thermite. The second nanostructured energetic material consists of a nanostructured hydrocarbon resin fuel network with fine ammonium perchlorate (NH{sub 4}ClO{sub 4}) oxidizer present.

The solubility and recrystallization of 1,3,5-triamino-2,4,6-trinitrobenzene in a 3-ethyl-1-methylimidazolium acetate–DMSO co-solvent system

Ionic liquids have previously been shown to dissolve strong inter- and intramolecular hydrogen-bonded solids, including natural fibers. Much of this solubility is attributed to the anions in ionic liquids, which can disrupt hydrogen bonding. We have studied the solubility and recrystallization of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), a very strong inter- and intramolecular hydrogen-bonded solid, in various ionic liquid solvent systems. We discovered that acetate-based ionic liquids were the best solvents for dissolving TATB, while other anions, such as Cl−, HSO4− and NO3− showed moderate improvements in the solubility compared to conventional organic solvents. Ionic liquid–DMSO co-solvent systems were also investigated for dissolving and recrystallizing TATB.

Microreactor flow synthesis of the secondary high explosive 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105)

The secondary high explosive 2,6-diamino-3,5-dinitropyrazine-1-oxide, or LLM-105, has been synthesized using a commercially available flow microreactor system. Investigations focused on optimizing flow nitration conditions of the cost effective 2,6-diaminopyrazine-1-oxide (DAPO) in order to test the feasibility and viability of flow nitration as a means for the continuous synthesis of LLM-105. The typical benefits of microreactor flow synthesis including safety, tight temperature control, decreased reaction time, and improved product purity all appear to be highly relevant in the synthesis of LLM-105. However, the process does not provide any gains in yield, as the typical 50–60% yields are equivalent to the batch process. A key factor in producing pure LLM-105 lies in the ability to eliminate any acid inclusions in the final crystalline material through both a controlled quench and recrystallization. The optimized flow nitration conditions, multigram scale-up results, analyses of sample purity, and quenching conditions for purity and crystal morphology are reported.

Sol-gel preparation of alumina stabilized rare earth areo- and xerogels and their use as oxidation catalysts.

A new sol-gel synthesis route for rare earth (Ce and Pr) alumina hybrid aero- and xerogels is presented which is based on the so-called epoxide addition method. The resulting materials are characterized by TEM, XRD and nitrogen adsorption. The results reveal a different crystallization behavior for the praseodymia/alumina and the ceria/alumina gel. Whereas the first remains amorphous until 875°C, small ceria domains form already after preparation in the second case which grow with increasing calcination temperature. The use of the calcined gels as CO oxidation catalysts was studied in a quartz tube (lab) reactor and in a (slit) microreactor and compared to reference catalysts consisting of the pure rare earth oxides. The Ce/Al hybrid gels exhibit a good catalytic activity and a thermal stability against sintering which was superior to the investigated reference catalyst. In contrast, the Pr/Al hybrid gels show lower CO oxidation activity which, due to the formation of PrAlO3, decreased with increasing calcination temperature.

A Robust Approach to Inorganic Aerogels: The Use of Epoxides in Sol–Gel Synthesis

Over the last decade, the diversity of metal oxide materials prepared using sol–gel techniques has increased significantly. This transformation can be attributed, in part, to the development of the technique known as epoxide-initiated gelation. The process utilizes organic epoxides as initiators for the sol–gel polymerization of simple inorganic metal salts in aqueous or alcoholic media. In this approach, the epoxide acts as an acid scavenger in the sol–gel reaction, driving the hydrolysis and condensation of hydrated metal species. This process is general and applicable to the synthesis of a wide range of metal oxide aerogels, xerogels, and nanocomposites. In addition, modification of synthetic parameters allows for control over the structure and properties of the sol–gel product. This method is particularly amenable to the synthesis of multi-component or composite sol–gel systems with intimately mixed nanostructures. This chapter describes both the reaction mechanisms associated with epoxide-initiated gelation as well as the variety of materials that have been prepared using this technique.

Aerogels and Sol–Gel Composites as Nanostructured Energetic Materials

In the last 10 years there have been a significant number of investigations of the application of aerogels and sol–gel-derived materials and methods to the field of energetic materials (e.g., explosives, propellants, and pyrotechnics) specifically through the synthesis and characterization of nanostructured energetic composites. Aerogels have unique density, composition, porosity, and particle sizes as well as low temperature and benign chemical synthetic methods all of which make them attractive for energetic nanomaterials candidates. The application of these materials and methods to this technology area has resulted in three general types of sol–gel energetic materials (1) sol–gel inorganic oxidizer/metal fuel pyrotechnics (thermite-like composites); (2) sol–gel-derived porous pyrophoric metal powders and films; and (3) sol–gel organic fuel/inorganic oxidizer nanocomposites (propellant and explosive-like composites). This chapter details results from synthesis and characterization research in all three areas. General trends are detailed, analyzed, and discussed. In general, all sol–gel nanostructured energetic material behaviors are highly dependent on several factors including surface area, degree of mixing between phases, the type of mixing (sol–gel or physical mixing), solids loading, and the presence of impurities. Sol–gel methods are attractive to the area of nanostructured energetics because they offer a great deal of many processing options such as monoliths, powders, and films and have broad compositional versatility. These attributes coupled with strong synthetic control of the microstructural properties of the sol–gel matrix enable the preparation of energetic nanocomposites with tunable performance characteristics. Various aspects of the present literature work are reviewed and future challenges for this technological area are presented and discussed.

Safe and Environmentally Acceptable Sol-Gel-Derived Pyrophoric Pyrotechnics

Abstract : It was demonstrated that highly porous sol-gel derived iron (III) oxide materials could be reduced to sub-micron-sized metallic iron by heating the materials to intermediate temperatures in a hydrogen atmosphere. Through a large number of experiments complete reduction of the sol-gel based materials was realized with a variety of hydrogen-based atmospheres (25-100% H2 in Ar, N2, CO2, or CO) at intermediate temperatures (350 C to 700 C). All of the resulting sol-gel-derived metallic iron powders were ignitable by thermal methods, however none were pyrophoric. For comparison several types of commercial micron sized iron oxides Fe2O3, and NANOCATTM were also reduced under identical conditions. All resulting materials were characterized by thermal gravimetric analysis (TGA), differential thermal analysis (DTA), powder X-ray diffraction (PXRD), as well as scanning and transmission electron microscopies (SEM and TEM). In addition, the reduction of the iron oxide materials was monitored by TGA. In general the sol-gel materials were more rapidly reduced to metallic iron and the resulting iron powders had smaller particle sizes and were more easily oxidized than the metallic powders derived from the micron sized materials. The lack of pyrophoricity of the smaller fine metallic powders was unexpected and may in part be due to impurities in the materials that create a passivation layer on the iron. Several recommendations for future study directions on this project are detailed.