In the last 20 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, thermites, 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 thermite-like composites; (2) sol–gel-derived porous pyrophoric metal powders, films, and monoliths; and (3) sol–gel metal or organic fuel/inorganic oxidizer nanocomposites (propellant, explosive, thermite, and pyrotechnic composites). This chapter summarizes 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 the nanomorphology of the network, its surface area, the degree of mixing and contact area between phases, the type of mixing (sol–gel, physical mixing, interpenetration), 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.

Propellants, Explosives, Pyrotechnics has lost one of our own. Professor Tom Brill, Associate Editor of PEP, recently passed away after a long struggle with Parkinson’s disease. Tom impacted the lives of thousands. He was a scholar, a teacher, a scientist, and a technical advisor to national laboratories, where one of us first met him thirty years ago and a long-lasting friendship began. The other one of us first knew Tom as his graduate advisor and became his close colleague, as many of his students did. Tom was a giant in the field of energetic materials, and a scientist of incredible depth and breadth. He received a BS degree at the University of Montana and a Ph.D. degree in Chemistry from the University of Minnesota in 1970. That year he joined the faculty at the University of Delaware, where he remained until retiring in 2006. He was an experimentalist who thrived on creating new methods to investigate chemistry at elevated temperature and pressure related to explosions, rocket propulsion and deep-sea geothermal vents. He lectured throughout the world and worked for many years as a consultant for industry and government laboratories. For 20 years he also taught material properties to art conservation graduate students. One of us gave Tom’s book on art, Light: Its Interaction with Art and Antiquities, as a gift to our own daughter who was a student of art history. Tom was known simply as The Boss by his graduate students while away from the lab, and Dr. Brill while present. He was calm and patient, yet he was also a strict academic advisor, once explaining that in all his decades of teaching he never gave a student a grade that they didn’t deserve. Never arrogant or angry, he taught his students that it wasn’t the time that they spent in the lab that mattered, but rather what they produced. He also taught them how to write a technical paper, distilling a disparate set of data, seemingly too disorganized to make any sense of, into a logical story. At the time that one of us joined his research group in the late 1990s, he was easily the leading academic in the decomposition of energetic materials. It was also at that time that the first signs of Parkinson’s disease began to appear, and shortly afterwards he decided to stop taking on new graduate students in his group, making one of us one of his last PhD students. He always had amazing stories of his adventures to share with his graduate students, whether it was outrunning storms down a high alpine mountain, or eating foods served at a foreign banquet in his honor that, for the average palate, were quite unique to say the least. Tom was a superb contributor to the PEP international community. He had a lasting impact on the energetic materials field, graduating over 45 master and PhD students, publishing over 300 peer-reviewed articles, one book and five co-edited books, and co-founding the Energetic Materials Gordon Research Conference in 1988. He will be greatly missed.

Lanthanide oxide-based aerogels were synthesized employing the so-called epoxide addition sol–gel method already successfully applied for main- and transition metal oxide aerogels. Using chlorides and nitrates as precursors, our aim was to test the transferability of this robust sol–gel methodology to the entire lanthanide series. By adding the proton scavenging organic epoxide, propylene oxide, to hydrated lanthanide trichloride dissolved in ethanol or methanol, uniform monolithic alcogels were obtained. Subsequent processing in supercritical CO2 resulted in monolithic aerogels. No gelling process could be induced by using nitrates, in contrast to previous results with iron oxide or alumina aerogels. All materials were characterized by nitrogen adsorption/desorption analysis, transmission electron microscopy, and powder X-ray diffraction. With the exception of cerium, for which fractions of crystalline CeO2 were found already in the as-prepared material, XRD analysis revealed that the other materials were mainly amorphous. Subsequent heat treatment of the aerogels above 650 °C resulted in nanocrystalline phases for all aerogel materials. However, except for ceria, more detailed TEM and XRD studies provided evidence that crystalline oxychloride phases are formed in addition to fractions of oxide phases. The trends and possible explanations are discussed in this contribution.

Propellants, Explosives, PyrotechnicsVolume 34, Issue 1 p. 5-5 EditorialFree Access Energetics in 2009 Randall L. Simpson, Randall L. Simpson Energetic Materials Center, Lawrence Livermore National Laboratory EditorSearch for more papers by this authorAlexander E. Gash, Alexander E. Gash Energetic Materials Center, Lawrence Livermore National LaboratorySearch for more papers by this author Randall L. Simpson, Randall L. Simpson Energetic Materials Center, Lawrence Livermore National Laboratory EditorSearch for more papers by this authorAlexander E. Gash, Alexander E. Gash Energetic Materials Center, Lawrence Livermore National LaboratorySearch for more papers by this author First published: 04 February 2009 https://doi.org/10.1002/prep.200990001 AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume34, Issue1February 2009Pages 5-5 RelatedInformation

We report the synthesis and characterization for the first example of a low-density nanocrystalline thoria aerogel. The monolithic aerogels were prepared through the sol-gel polymerization of hydrated thorium nitrate in ethanol using ammonium hydroxide and propylene oxide as gelation initiators. The dried ThO2 aerogel was characterized by high-resolution transmission electron microscopy (HRTEM) and nitrogen adsorption/desorption analyses. The aerogel network was determined to be composed of spherical primary particles with features in the 5-20 nm range. These particles were also determined to be highly crystalline as evidenced by the higher magnification TEM examination. The thoria aerogel possesses high surface area (120 m(2)/g) and pore diameters in the micro- and mesoporous range. (C) 2004 Elsevier B.V. All rights reserved.

The general synthesis of metal–silicon mixed-oxide nanocomposite materials, including a variety of both main group and transition metals, in which the metal oxide is the major component is described. In a typical synthesis, the metal-oxide precursor, MClx · yH2O(x = 2–6, y = 0–7), was mixed with the silica precursor, tetramethoxysilane (TMOS), in ethanol and gelled using an organic epoxide. The successful preparation of homogeneous, monolithic materials depended on the oxidation state of the metal as well as the epoxide chosen for gelation. The composition of the resulting materials was varied from M/Si = 1–5 (mol/mol) by adjusting the amount of TMOS added to the initial metal-oxide precursor solution. Supercritical processing of the gels in CO2 resulted in monolithic, porous aerogel nanocomposite materials with surface areas ranging from 100–800 m2 g−1. The bulk materials are composed of metal oxide/silica particles that vary in size from 5–20 nm depending on the epoxide used for gelation. Metal oxide and silica dispersion throughout the bulk material is extremely uniform on the nanoscale. The versatility and control of the synthesis method will be discussed as well as the properties of the resulting metal–silicon mixed oxide nanocomposite materials.

The synthesis and characterization of nickel(II)-based aerogel materials prepared using the epoxide addition method is described. The addition of the organic epoxide propylene oxide to an ethanolic solution of NiCl2·6H2O resulted in the formation of an opaque light green monolithic gel and subsequent drying with supercritical CO2 gave a monolithic aerogel material of the same color. This material has been characterized using powder X-ray diffraction, electron microscopy, elemental analysis, and nitrogen adsorption/desorption analysis. The results indicate that the nickel(II)-based aerogel has very low bulk density (98kgm−3 (∼98% porous)), high surface area (413m2g−1), and has a particulate-type aerogel microstructure made up of very fine spherical particles with an open porous network. By comparison, a precipitate of Ni3(NO3)2(OH)4 is obtained when the same preparation is attempted with the common Ni(NO 3)2·6H2O salt as the precursor. The implications of the difference of reactivity of the two different precursors are discussed in the context of the mechanism of gel formation via the epoxide addition method. The synthesis of nickel(II)-based aerogel, using the epoxide addition method, is especially unique in our experience. It is our first example of the successful preparation of a metal-oxide-based aerogel using a divalent metal ion and may have implications for the application of this method to the preparation of aerogels or nanoparticles of other divalent metal oxides. To our knowledge this is the first report of a monolithic pure nickel(II)-based aerogel materials.

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% H{sub 2} in Ar, N{sub 2}, CO{sub 2}, 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 NANOCAT 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.

The synthesis of Fe–Si mixed oxide nanocomposite materials in which the iron(III) oxide is the major component is described. In a typical synthesis, the iron oxide precursor, FeCl3·6H2O, was mixed with a silica precursor, tetramethyl- or tetraethylorthosilicate, in ethanol and gelled using an organic epoxide. The composition of the resulting materials was varied from Fe/Si (mol/mol)=1–5 by adjusting the amount of silica precursor added to the FeCl3·6H2O solution. Further processing of the gels in supercritical CO2 resulted in monolithic, porous aerogel nanocomposite materials with surface areas ranging from 350–450 m2/g. The bulk materials are composed of iron(III) oxide/silica particles that vary in size from 5–20 nm depending on the epoxide used for gelation. Iron(III) oxide and silica dispersion throughout the bulk material is extremely uniform on the nanoscale. The synthesis method presented is general for the synthesis of several other metal oxide/silicon oxide nanocomposite materials.

The synthesis of iron(III) oxide aerogel monoliths was performed by adding any one of several different 1,2- and 1,3-epoxides to ethanolic Fe(III) salt solutions at room temperature. While all of the epoxides examined resulted in gel formation, robust low-density (similar to30-40 kg/m(3); 99% porous), high-surface-area (similar to250-300 m(2)/g), aerogel monoliths were prepared by the addition of 1,3-epoxide derivatives to solutions of FeCl3.6H(2)O, followed by drying with supercritical CO2. Both types of iron(III) oxide aerogels (those made with 1,2- and 1,3-epoxides respectively) were characterized using elemental analysis, X-ray diffraction, thermal analysis, acoustic measurements, transmission electron microscopy, scanning electron microscopy, and N-2 adsorption desorption analysis. Elemental analyses and powder X-ray diffraction indicated that the strong aerogel monoliths made with the 1,3-epoxides are made up predominately of polycrystalline beta-FeOOH, akaganeite, and those made with the 1,2-epoxides are amorphous. To our knowledge, this is first known report of synthesis and characterization of akaganeite aerogel materials. Transmission electron microscopy analysis indicates that aerogels derived using 1,3-epoxides have a microstructure made up of a highly reticulated network of fibers with diameters from similar to5 to 35 nm and lengths several times that, whereas those resulting from the use of 1,2-epoxides consist of interconnected spherical particles, whose diameters are 5-15 nm. The difference in microstructure results in each type of aerogel displaying very distinct physical and mechanical properties. In particular, the stiffness of the beta-FeOOH aerogels is remarkable for a transition metal oxide aerogel. Monolithic cylinders of beta-FeOOH aerogel can be sintered at 515 degreesC, transforming to alpha-Fe2O3 without shattering.

We report the synthesis and characterization for the first example of a low-density urania aerogel. The monolithic aerogels were prepared through the sol–gel polymerization of hydrated uranyl nitrate in ethanol using propylene oxide as a gelation initiator. The dried UO3 aerogel was characterized by high-resolution transmission electron microscopy and nitrogen adsorption/desorption analyses. The aerogel micro-structure was determined to be composed of primary particles with two distinct morphologies (spherical and fibrous) with features in the 5–20 nm range. These particles were also determined to be highly crystalline as evidenced by the higher-magnification TEM examination. The urania aerogel possesses high surface area (236 m2/g) and pore diameters in the micro- and mesoporous (2–20 nm) range.

In the field of composite energetic materials, properties such as ingredient distribution, particle size, and morphology, affect both sensitivity and performance. Since the reaction kinetics of composite energetic materials are typically controlled by the mass transport rates between reactants, one would anticipate new and potentially exceptional performance from energetic nanocomposites. We have developed a new method of making nanostructured energetic materials, specifically explosives, propellants, and pyrotechnics, using sol-gel chemistry. A novel sol-gel approach has proven successful in preparing metal oxide/silicon oxide nanocomposites in which the metal oxide is the major component. Two of the metal oxides are tungsten trioxide and iron(III) oxide, both of which are of interest in the field of energetic materials. Furthermore, due to the large availability of organically functionalized silanes, the silicon oxide phase can be used as a unique way of introducing organic additives into the bulk metal oxide materials. As a result, the desired organic functionality is well dispersed throughout the composite material on the nanoscale. By introducing a fuel metal into the metal oxide/silicon oxide matrix, energetic materials based on thermite reactions can be fabricated. The resulting nanoscale distribution of all the ingredients displays energetic properties not seen in its microscale counterparts due to the expected increase of mass transport rates between the reactants. The synthesis and characterization of these metal oxide/silicon oxide nanocomposites and their performance as energetic materials will be discussed.

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 Fe2O3/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 per-formed on the nanostructured thermite. The second nanostructured energetic material consists of a nanostructured hydrocarbon resin fuel network with fine ammonium perchlorate (NH4ClO4) oxidizer present.