A visualization of the solvent/TATB mix. The red, white, blue and gray moleculese are the explosive, TATB, the green balls are the fluoride anions and the blue/gray regions are the organic cation portion of the solvent. The new solvent promises less toxic processing and safer nuclear weapons, among other potential benefits.  (Source: LLNL)

Commercial grade TATB is shown on the left. Performance is directly correlated to the crystal quality. The new organic solvent-produced TATB is on the right, and offers visibly superior crystals and the vastly superior performance.  (Source: LLNL)
Researchers are replacing toxic inorganic compounds, with "greener" organic compounds during processing

Explosives and green tech aren't two things you might always think of as going hand in hand.  However, new breakthrough research from the Lawrence Livermore National Laboratory (LLNL) not only promises to improve conventional explosives, but also make processing less toxic through the use of organic compounds.

The research focuses on a class of compounds known as molecular crystals.  Molecular crystals are extremely powerful compounds that are also surprisingly stable.  They are used in drugs, pigments, agrochemicals, dyes and optoelectronics, as well as in explosives.  Since many of these compounds in unusable disordered form are bound together by strong hydrogen bonds, processing via typical organic solvents in nearly impossible due to lack of solubility.

As a result, toxic inorganic solvents are typically used to partially dissolve and reform the material into useful crystals.  The process is inefficient and produces toxic byproducts, such as ammonium chloride.

Researchers at the LLNL, under the Transformational Materials Initiative (TMI) Laboratory Research and Development project, discovered a class of organic solvents that could be used and would process the material more efficiently.  They turned to ionic liquids – a special type of molten salt that becomes liquid under the boiling point of water (100 degrees Celsius).  This type of solvent is considered attractive due to its lack of vapor pressure and tendency not to evaporate even under high temperature (evaporation of solvents can yield toxic fumes and also limits the usable liquid).  Also, scientists can custom tailor the types of positive and negative ions they want to use.

Lead author Amitesh Maiti used quantum simulations to narrow the selection of ions to a class of solvents with fluoride ions, which is very good at dissolving hydrogen bonds.  He describes, "The design of custom solvents through first principles modeling opens up new possibilities for the dissolution of materials that are hard to dissolve."

After the theoretical work was complete, researchers led by Phil Pagoria tested dissolving and reforming molecular crystals with the solvent.  The result was large defect-free (97 percent pure) crystals, which were vastly superior to current commercial offerings.

The key crystal compound tested was a special type of explosive known as TATB (1,3,5-triamino-2,4,6-trinitrobenzene).  This explosive is widely used for its exceptional stability by the Department of Energy, the Department of Defense and the mining industry.  The refined crystalline version is even more stable.  Larry Fried, the project’s principal investigator and a co-author of the paper states, "Improving crystal quality and purity leads to explosive materials that are safer (less likely to react violently) when subjected to mechanical impact or heat."

This is of particular interest to national security, as TATB is thought to be used in all of Britain's nuclear warheads, and the majority of the U.S.'s warheads as a detonator, due to its already favorable stability.  The purer crystals could provide detonators capable of surviving even more severe punishment without an accidental detonation.

Aside from TATB the technique could be used to process a broad array of other commercial substances containing molecular crystals, including polymers (plastics) and molecular solids (pharmaceuticals, paints, propellants, explosives).  For example, the same solvents were shown to break down cellulose into basic sugars.  This could prove a boon to efforts to create ethanol and other biofuels from cellulose, a typically energetically intensive process.

In the short term it means the ability to not only process higher quality, safer TATB, but to minimize the environmental impact from processing.

The research is reported in the Sept. 1 issue of the journal Physical Chemistry Chemical Physics.

Other researchers on the project included Alex Gash, Yong Han, Christine Orme and Richard Gee.  

"There is a single light of science, and to brighten it anywhere is to brighten it everywhere." -- Isaac Asimov
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