Smokeless, odorless, and delayed response—these are not characteristics typically associated with explosives. But Laboratory researchers have been focusing on a group of compounds—high-nitrogen energetic materials—that are challenging stereotypical notions of how explosives behave. In addition to their use in weaponry, these compounds have applications in fire suppression, entertainment, and automobile safety.

Traditionally, explosives have been substances rich in carbon—for example, the charcoal in gunpowder. Combustion of a carbon-rich explosive in air results in the formation of carbon dioxide and carbon monoxide, together with water vapor and soot (unburned carbon particulates). Energy is rapidly liberated, accompanied by high temperatures (see the combustion primer). When combustion occurs within a gun barrel, the hot gases expand and propel a bullet or other projectile toward its target.

High-nitrogen energetic materials contain more nitrogen than carbon by weight, and their combustion products therefore differ from those that result from carbon-rich compounds. Nitrogen gas (N₂, which makes up 78 percent of Earth's atmosphere) is a main combustion product. Additionally, the high nitrogen content of these materials often leads to high densities; that is, individual molecules of the material pack closely together, so that a small quantity of the compound contains more combustible material than would a less dense compound. This high density is desirable in an explosive, in that large amounts of energy can be liberated from a relatively small amount of material.

The high-nitrogen compounds occupying the attention of a Los Alamos team in DX-2 (Materials Dynamics Group) are derivatives of the 1,2,4,5 tetrazine ring, a benzenelike, or aromatic, chemical structure in which four of the six carbon atoms in the benzene ring are replaced by nitrogen. Many of the tetrazine-derived compounds studied to date share the quality of "insensitivity"; that is, they will detonate only upon reaching a target but not when subjected to high temperature, friction, or accidental impact.

Empirical Chemistry
Because the tetrazine ring does not exist in nature, its similarity to benzene can be misleading. The synthesis of tetrazine compounds does not proceed directly from benzene and can require a long series of reaction steps. As many organic chemists would attest, chemical syntheses are part understanding, part experience, and part trial and error, sometimes with a healthy dollop of luck stirred in.

By modifying the molecular groups bonded to tetrazine rings, the DX-2 team of Mike Hiskey, Darren Naud, David Chavez, and My Hang Huynh has synthesized a host of compounds that differ in the rate and the temperature at which they burn. "You never know exactly quite what you'll get. You set out to make one thing and then find other applications," comments Hiskey, alluding to the fact that it is often difficult to predict a compound's properties before synthesizing it.

One compound the team has synthesized is DHT (dihydrazino-tetrazine). Because it is both a high-nitrogen and a high-hydrogen compound, it tends to burn hotter in air, forming both nitrogen gas and water. These characteristics make it a candidate for so-called thermobaric bombs. First used in Afghanistan during the March 2002 attacks on Al Qaeda caves, these bombs use a primary explosion to propel a solid-fuel explosive into a tunnel, bunker, or cave. The secondary explosive then detonates via a delayed fuse to generate both high heat (therme) and high pressure (baros) within the enclosed space. Finely divided fragments of aluminum metal are often included in the solid explosive, because in the high-energy environment of the explosion, aluminum oxidizes in air. Forming aluminum oxide, this reaction both consumes significant oxygen and generates additional heat (it is highly exothermic).

The army and navy are testing DHT-aluminum as a candidate explosive because, in addition to its high density, its detonation also generates significant volumes of nitrogen gas. When the gas replaces the oxygen consumed by the burning aluminum, a nitrogen-rich atmosphere is created that leads to nitrogen narcosis in anyone exposed to it. This physiological phenomenon was originally described in deep-sea divers. As the total gas pressure increases with increasing dive depth, the partial pressure of nitrogen increases, and more nitrogen becomes dissolved in the blood—and therefore in the brain. Dubbed "rapture of the deep," this misnomer alludes to the fact that nitrogen's intoxicant and soporific effects are equivalent to those of one martini on an empty stomach for each 50 feet beyond a 100-foot dive. Since nitrogen impairs the conduction of nerve impulses, at very high brain concentrations, nitrogen narcosis is fatal.

Cleaner Pyrotechnics
Ironically, the same compound that can be weaponized can also entertain. The hot flame and carbon-free combustion of DHT have made it ideal for a new generation of reduced-toxicity fireworks. Traditional carbon-based fireworks generate large quantities of sooty, sulfurous smoke that interferes with viewing and can also cause respiratory discomfort or asthmatic reactions in audiences. Remaining unseen are constituents that are even more toxic: carbon monoxide and the heavy metals (for example, barium) used as colorants.

The odorless, colorless nitrogen gas released in the combustion of DHT enables brighter, more deeply saturated display colors with only one-tenth the conventional amount of metal-ion colorants. DHT's hot flame also allows the use of safer substitutes for traditional colorants, for example, boric acid instead of barium. Moreover, the absence of carbon monoxide, soot, and sulfurous fumes makes DHT-powered fireworks much safer for indoor use. [figure: DHT-powered fireworks]

Hiskey's interest in fireworks and high-energy chemistry dates back to his teens, and that curiosity provided him with both a practice-based learning environment and a career path. Teaming with Naud and Chavez, Hiskey developed a cost-effective synthesis of DHT, which led to "smokeless" fireworks. Their invention won an R&D 100 Award in 1998 and has drawn the interest of at least one major theme park.

Better Big-Gun Propellants
As most gun owners know, the high gas temperatures and sooty residues produced by the combustion of gun powder mandate frequent cleaning of gun barrels. Less appreciated is the wear and tear on a gun barrel produced by carbon-based propellants. This wear is of particular concern for military guns, whose powerful blast charges propel projectiles to targets miles away. Another tetrazine relative, known as GUZT (guanidinium azotetrazolate), may help reduce such wear when added to gun propellants.

In gun barrels, the propellant is burned behind a projectile, and—in accord with gas laws—the expanding gases from that combustion exert pressure on the projectile, accelerating it toward a target. During this process, two major sources of wear contribute to reducing gun barrel life. First, the combustion's very high temperature deforms metal alloys in the barrel. Second, carbon-based propellants generate significant carbon monoxide, which at elevated temperatures, reacts to form metal carbides that embrittle the barrel's interior, resulting in increased wear. [figure: naval gun propellants]

A high-nitrogen propellant additive like GUZT addresses both issues. Its lower-temperature combustion moderates temperature deformation, while its generation of nitrogen gas helps mitigate reactivity. In sufficient concentration, nitrogen gas can inhibit the carbon monoxide–gun barrel reaction, thereby reducing the extent of metal carbide formation.

For barrels as large and as expensive as those on naval warships, this reduced wear could represent significant cost savings. Barrels are currently replaced after every eight thousand rounds fired, and the navy expects the replacement frequency to rise as more-energetic propellants come into use. But the cost of the barrel is overshadowed by two other factors. Since barrels cannot be replaced at sea, there is the expense of bringing a ship back to port for the work. More important from a strategic standpoint is the impact of this return to port on fleet readiness.

Fighting Fire with Fire
A compound known as BTATz (bistetrazolylaminotetrazine) has shown considerable promise as a fire suppressant. The current suppressant of choice is Halon-1301 (bromotrifluoromethane). Halon works by evaporating within a fire and displacing the oxygen that would sustain the blaze. It has several drawbacks, however. First, it destroys Earth's ozone layer (its production was thus halted in 1994). Second, as a gas, it is stored in pressurized bottles for application as a liquid. Particularly for use in an airplane, these bottles add undesirable weight.

For several years, the U.S. military has been evaluating solid and hybrid (solid/liquid, solid/gas) fire suppressants, and the properties of BTATz rank it among the more promising agents now undergoing evaluation. Almost 80 percent nitrogen by weight, BTATz burns very rapidly to form 0.7 liter of nitrogen gas per gram of solid combusted. Because of its low carbon content, it burns cleanly, without smoke, leaving a minimal residue. And like GUZT, it burns at a temperature hundreds of degrees lower than do carbon-rich compounds of similar molecular weight.

The large volume of inert nitrogen gas rapidly generated by BTATz is precisely what is needed to displace oxygen from the vicinity of a fire. Nitrogen gas is composed of smaller, less-massive molecules than vaporized halon, and nitrogen's more rapid rate of expansion helps cool its surroundings (all gases absorb heat from their environments as they expand). BTATz thus addresses a problem encountered with other fire suppressants, namely, effluent gases that are too hot.

As a solid, BTATz can be formed into a paint that, when applied to a surface (such as the wheel well of a carrier-based aircraft), will remain chemically stable until it ignites. Requiring no heavy storage containers, it is also lighter than halon. BTATz now costs $75 per pound to manufacture, but the navy's Air Warfare Weapons Division at China Lake, California, is seeking to scale up its synthesis, thus further reducing its cost. The navy is also experimenting with combining BTATz with flame inhibitors. Since halon is also the civilian fire suppressant of choice, this research may ultimately produce a widely used, environmentally friendly fire suppressant.

Airbags
Gas-generating solids are used in automobile airbags, and BTATz is also a candidate for this civilian application. Airbags contain sodium azide, a metabolic poison at low doses, potent enough to be used as an agricultural antifungal compound. Readily absorbed through the skin and lungs, it can cause cardiovascular abnormalities, convulsions, and upon prolonged exposure, even death. Although airbags contain very small quantities of the chemical, eliminating it from automobiles is clearly desirable, particularly since it also accumulates in landfills where undeployed airbags are discarded.

In an automobile accident, a crash sensor activates a circuit that sends an electrical discharge into the sodium azide, causing it to ignite and decompose into sodium metal and nitrogen gas, which rapidly expands to inflate the airbag (see sidebar on airbag chemistry). Although currently more expensive than sodium azide, BTATz efficiently generates nitrogen gas, and it may well find an application in this area if its synthesis cost can be lowered. [figure: airbag basics]

Small-Scale Rocket Propellants
From GPS data to live transoceanic newscasts, satellite communications have become central to our way of life. Despite satellites' generally stable orbits, their orientations must occasionally be fine-tuned by tiny rocket motors (microthrusters). Inspection satellites, launched to inspect other orbiting equipment, also use these thrusters.

Yet another tetrazine derivative, dubbed DAAT-N-Ox, shows promise as a solid fuel for such rocket motors. Like BTATz, DAAT-N-Ox burns at a lower temperature than do conventional solid propellants. These lower temperatures allow the use of materials for rocket nozzles that would otherwise melt during combustion. Such nozzles are frequently only about a millimeter wide at their narrowest point, so they would also rapidly become clogged with the residues deposited by hotter-burning carbon-based fuels. However, DAAT-N-Ox burns very cleanly, thus avoiding nozzle clogging. It also burns much more rapidly than any carbon-rich energetic material, perfectly suiting it for the very short propulsive bursts needed to make minor changes in satellite orientation and orbits. With the fastest combustion rate of any solid rocket fuel, DAAT-N-Ox may well be the best solid propellant available. [figure: satellite microthruster]

Variations on a Theme
From microthrusters to naval gun propellants, from thermobaric bombs to smokeless fireworks, from fire suppression to airbag inflation, high-nitrogen compounds promise unique versatility as energetic materials. Their insensitivity, high density, and lower combustion temperatures are attractive characteristics for explosives and rocket motors. Their rapid generation of inert nitrogen in large volumes is ideal for both existing and future applications that require an environmentally friendly gas generator. Although not quite a case of "one compound fits all," these tetrazine derivatives represent variations on a theme, and it appears likely that current and future compounds will find useful niches in both military and civilian life.