ONE thousand years ago, black powder was prepared by grinding saltpeter, charcoal, and sulfur together into a coarse powder using a mortar and pestle. Since then, the equipment for making energetic materials-explosives, propellants, and pyrotechnics-has evolved considerably, but the basic process for making these materials has remained the same. That, however, is changing, thanks to an explosive combination of sol-gel chemistry and modern-day energetic materials research.|
At Livermore Laboratory, sol-gel chemistry-the same process used to make aerogels or "frozen smoke" (see S&TR, November/December 1995)—has been the key to creating energetic materials with improved, exceptional, or entirely new properties. This energetic materials breakthrough was engineered by Randy Simpson, director of the Energetic Materials Center; synthetic chemists Tom Tillotson, Alex Gash, and Joe Satcher; and physicist Lawrence Hrubesh.
These new materials have structures that can be controlled on the nanometer (billionth-of-a-meter) scale. Simpson explains, "In general, the smaller the size of the materials being combined, the better the properties of energetic materials. Since these `nanostructures' are formed with particles on the nanometer scale, the performance can be improved over materials with particles the size of grains of sand or of powdered sugar. In addition, these `nanocomposite' materials can be easier and much safer to make than those made with traditional methods."
Energy Density vs Power, the Traditional Tradeoffs|
Energetic materials are substances that store energy chemically. For instance, oxygen, by itself, is not an energetic material, and neither is fuel such as gasoline. But a combination of oxygen and fuel is.
Energetic materials are made in two ways. The first is by physically mixing solid oxidizers and fuels, a process that, in its basics, has remained virtually unchanged for centuries. Such a process results in a composite energetic material such as black powder. The second process involves creating a monomolecular energetic material, such as TNT, in which each molecule contains an oxidizing component and a fuel component. For the composites, the total energy can be much greater than that of monomolecular materials. However, the rate at which this energy is released is relatively slow when compared to the release rate of monomolecular materials. Monomolecular materials such as TNT work fast and thus have greater power than composites, but they have only moderate energy densities-commonly half those of composites. "Greater energy densities versus greater power—that's been the traditional trade-off," says Simpson. "With our new process, however, we're mixing at molecular scales, using grains the size of tens to hundreds of molecules. That can give us the best of both worlds-higher energy densities and high power as well."
Energetic Nanostructures in a Beaker|
To control the mix of oxidizer and fuel in a given material at the nanometer scale, Livermore researchers turned to sol-gel methodologies. Sol-gel chemistry involves the reactions of chemicals in solution to produce nanometer-size particles called sols. These sols are linked together to form a three-dimensional solid network or skeleton called a gel, with the remaining solution residing in the open pores of the gel. The solution can then be supercritically extracted to produce aerogels (highly porous, lightweight solids) or evaporated to create xerogels (denser porous solids).
"A typical gel structure is extremely uniform because the particles and the pores between them are so small," notes Tillotson. "Such homogeneity means that the material's properties are also uniform. Our main interest in the sol-gel approach is that it will allow us to precisely control the composition and morphology of the solid at the nanometer scale so that the material's properties stay uniform throughout-something that can't be achieved with conventional techniques."
Using these sol-gel-processing methods, the team derived four classes of energetic materials: energetic nanocomposites, energetic nanocrystalline materials, energetic powder-entrained materials, and energetic skeletal materials.
Energetic nanocomposites have a fuel component and an oxidizer component mixed together. One example is a gel made of an oxidizer with a fuel embedded in the pores of the gel. In one such material (termed a thermite pyrotechnic), iron oxide gel reacts with metallic aluminum particles to release an enormous amount of heat. "These reactions typically produce temperatures in excess of 3,500 degrees Celsius," says Simpson. Thermites are used for many applications ranging from igniters in automobile airbags to welding. Such thermites have traditionally been produced by mixing fine powders of metal oxides and metal fuels. "Conventionally, mixing these fine powders can result in an extreme fire hazard. Sol-gel methods can reduce that hazard while dispersing extremely small particles in a uniform way not possible through normal processing methods," adds Simpson. The Livermore team has successfully synthesized metal oxide gels from a myriad elements. At least in the case of metal oxides, sol-gel chemistry can be applied to a majority of elements in the periodic table.
In energetic nanocrystalline composites, the energetic material is grown within the pores of an inert gel rather than mixed into it. One way to initiate the growth is to dissolve the energetic material in the solvent used to control the density of the resulting gel. After the gel is formed, the energetic material in the pore fluid is induced to crystallize within the pores. The Livermore team synthesized nanocrystalline composites in a silica matrix with pores containing the high explosive RDX or PETN. The resulting structures contain crystals so small that they do not scatter visible light and are semitransparent.
In the powder-entraining method, a high concentration of energetic powders (90 percent by weight) is loaded within a support matrix (for example, silica) that takes up a correspondingly small mass. Highly loaded energetic materials are used in a variety of applications, including initiators and detonators. Manufacturing this type of energetic material using current processing technologies is often difficult. Producing detonators with pressed powders is a slow manufacturing process, mixing two or more powders homogeneously is difficult, and precise geometric shapes are not easy to produce. Also, pressing powders is a hazardous process.
Many of these problems may be overcome with the sol-gel process. One result is that the sol-gel explosives formed by adding energetic powders are much less sensitive than those produced by conventional methods. "These results were surprising because conventionally mixed powders generally exhibit increased sensitivity when silica powders are added," says Simpson. "We're still exploring the reasons for this decreased sensitivity, but it appears to be generally true with sol-gel-derived energetic materials."
The final class of energetic material produced by sol-gel methods is energetic skeletal materials. Basically, the sol-gel chemistry is used to create a skeletal matrix, which is itself energetic. Satcher thinks that it might also be possible to form a nanostructure made up of a fuel-oxidizer skeleton with precise stoichiometry (the numerical relationship of elements and compounds as reactants and products in a chemical reaction). "This is something we are still looking into," he adds. In addition to providing materials that have high energy density and are extremely powerful, sol-gel methodologies offer more safe and stable processing. For instance, the materials can be cast to shape or do not require the hazardous machining techniques required by materials that cannot be cast.
Future Looks Bright|
Right now, making energetic materials using the sol-gel technique is in the basic research stage, but results look promising. "Many compositions depend on a simple, inexpensive procedure that we can basically do in an ordinary chemistry beaker," says Tillotson. He notes that the practical advantages of these materials are encouraging. Some of the pluses are less sensitivity, safe mixing, low-temperature synthesis, safe handling, safe processing, and homogeneity leading to better performance.
"We've just begun to explore the possibilities for these new materials and the methodologies that produced them," adds Simpson. "This approach is like a new baby—it has lots of potential. The ramifications are still largely unknown."
Key Words: aerogel, energetic materials, explosives, nanocomposites, PETN, propellants, pyrotechnics, RDX, sol-gel, xerogel.
For more information contact Randy Simpson (925) 423-0379 (firstname.lastname@example.org).