Metrics for Explosivity, Inerting & Compatibility
Overview and Significance
Determining if a material or formulation is detonable and determining if an adulterant has inerted a detonable material are extremely difficult problems that cannot be properly addressed unless better metrics are developed. That development is the goal of this project. Because the potential matrix of threatening combinations of fuels and oxidizers is large, we seek to determine the characteristics required for detonability; bounding the problem in terms of oxidizer and its ratio with each fuel. In the laboratory, we probe characteristics such as heat and gas release, and a full suite of chemical, thermal and sensitivity analyses to correlate to larger scale detonation performance tests. A method which can successfully determine what formulations are potentially detonable would also reveal if “inerting” of an explosive had successfully made it non-detonable or just “safer”. Either question, what is potentially detonable and if adulteration has achieved non-detonability, currently requires very large-scale testing or a reliable small-scale test. The goal of the R1-B projects is development of the latter-a reliable small-scale test which screens large scale threat combinations quickly and inexpensively. We have taken here a number of approaches to this problem. They are discussed below:
Approach 1: How well an explosive functions is highly dependent on bulk properties, e.g. density, lattice structure, but whether a chemical can detonate at all, requires that the molecule have certain molecular features. To be an explosive, the molecule must be able to react with chemistry that produces heat and gas; and this must happen rapidly enough that the detonation front is supported by the energy release. Examination of the atoms making up the molecule allows prediction of whether heat and gas can be produced. This aspect of the molecule is being investigated under Approach 1 with full details as referenced in paper and in R1-A.1. The thermal and burn behaviors of 11 solid oxidizers and combinations of 13 fuels were determined; burn rate was found to roughly correlate with standard reduction potentials. The thermal studies highlighted the importance of a melt or phase change of one component of the formulation in triggering the reaction. These studies also indicated that the choice in oxidizer, outweighed the choice in fuel, in determining the total energy released. These exciting observations are the first steps in finding behaviors observed on the milligram scale that may correlate with detonability measured on the kilogram scale. Figure 1 is a plot of temperature of decomposition vs heat of decomposition. The fact that explosives clearly group in a different region than non-explosives suggests we can use thermal analysis of small samples as one metric to rate detonability. The critical question of whether the reaction can happen fast enough to support detonation is usually found experimentally. Other approaches in this project are examining the reaction that may or may not support steady detonation.
Approach 2 is looking at one of the fundamental molecular properties–dissociation energies during gas phase ion impact with an inert gas. By examining a variety of explosive and non-explosive compounds in an ion-trap or a triple-quadrupole mass spectrometer, a correlation may be observed between ease of fragmentation from the energy input required and the rank order of detonability.
In the past researchers developed a method for mass spectrometeric (MS) applications which was termed survival yield. The basic idea behind it was to supply enough energy to the molecule to see when only 50% of it was left over and the rest was gone due to fragmentation. Recently, with the advent of new technologies and the progress in MS field resulted in reviving of this application as a proof of a concept for established ionization methods such as desorption electrospray ionization (DESI), electrospray ionization (ESI), and the newly developed accessories (mainly ionization probes) such as laser electrospray MS (LEMS). However, most of these applications are being performed on very simple molecules termed thermometer ions, which in the process produce only one or two fragments that can be easily identified and analyzed.
Unfortunately, for most compounds (including virtually all energetic materials), this is not the case. Usually an array of fragments is formed; some cannot even be accounted for because of the constraints of the instrument, itself. Therefore, a new method must be developed which still has similar basis of the survival yield approach, but accounts for its limitations.
The concept is to produce and isolate ions of individual molecules within either an ion trap or a collision cell of a triple quadrupole mass spectrometer (MS). Once the ion has been isolated from the matrix (liquid phase, impurities, fragments or other unwanted ions), the collision energy provided by the MS can be gradually increased to observe several unique molecular properties: 1) the minimum energy eliciting initial fragmentation; 2) 50 % dissociation; 3) 100 % dissociation; 4) the window of energy associated with fragmentation; and 5) a product ion spectrum. Each analysis requires less than 500 micrograms and presently takes 5 to 40 minutes to produce up to 6 full scans from 0 to 50 normalized collision energy units (eV). Initially, the ion trap MS is being used for method development and proof of principle. A quick-look experiment shows that innocuous compounds can be differentiated from energetic ones, the latter being fragmented with much less energy. Once this method is optimized, we will attempt to establish consistent parameter sets across all compounds, e.g., solvent selection, tune conditions, percent of ion trap fill, etc. At what concentration a compound ills the ion trap may provide additional information about the ionization efficiencies. Because the ion-trap is attached to an exact mass detector (Orbitrap), molecular fragments can be assigned molecular formulas within 10 ppm accuracy.
Approach 3: Materials characterized as “explosives” release sufficient energy to “support” or “propagate” a detonation. Military explosives have been classified as such using detonation tests of prescribed size and initiating charge. Homemade explosives (HMEs) often fail these tests because they release too little energy to support detonation in the prescribed tests; therefore, they are not recognized as real explosive threats. However, these HMEs will perform as explosive materials if the charge size is increased beyond a material- specific size, the critical diameter (Dcr). At sizes less than Dcr, an explosive will not propagate detonation; any conventional explosivity or detonability test performed under the critical diameter of the material will indicate that the material is not an explosive. The critical charge size of many potential threat materials is so large that they are frequently not perceived as threats, when in reality they were simply tested below Dcr. For example, as dictated by shipping regulations, ammonium nitrate (AN) is not classed as an explosive, rather as DOT 5.1, because it does not propagate detonation at a diameter of 3.65 cm. However, with sufficient AN (e.g. when the diameter exceeds 100 cm) it becomes detonable, as was accidentally demonstrated by the explosion in West Texas in April, 2013. Field testing at large scales is hazardous, expensive and slow. Thus, the goal of the R1-B projects is to determine whether a material is detonable at any scale by performing experiments with less than a few pounds of the material in question. A further complication exists in screening a material for explosivity. To confirm that a material is an explosive, traditional testing must be done well above critical diameter and with a sufficient initiating charge. Thus, detonation failure can occur for several reasons including: (1) The material is too small in size; (2) It is insufficiently initiated; or (3) It is not an explosive. Traditional detonability tests do not differentiate.
For non-ideal explosives, a term which describes most HMEs, small-scale testing necessarily means studying these materials well below their Dcr. When steady detonation is not possible, conventional metrics, such as detonation velocity, yield little information. New diagnostics must be devised. Several approaches to this problem have been considered. Our initial approach was over-compensating for edge losses.
Approach 4 was actively soliciting other groups to join us in this effort. As a result, a group at Los Alamos National Lab (LANL) successfully probed evidence of detonable characteristics using 25 mL samples of hydrogen peroxide aqueous solutions of varying concentrations. While they were successful at that scale, they used instrumentation unique to that lab. It has also been demonstrated by LANL researchers that the reaction zone of detonating nitromethane (NM) can be observed using photon Doppler velocimetry (PDV). We believe that a similar approach used to characterize a failing detonation can yield useful information about the material’s capacity to detonate, i.e. confirming or denying the existence of a critical diameter.
Theoretically, PDV can be adapted as an embedded probe inside the explosive material; thus, it would act as a time-resolved velocimeter similar to MI but without the size constraints of microwave waveguides and with very high spacial and temporal resolution.Phase 2 Year 2 Annual Report
Students Currently Involved in Project
- Ryan Rettinger
- Matt Porter
- Devon Swanson
- Tailor Busbee
- Maria Donnelly
- Jon Canino