Laura Smilowitz

The β-δ phase transition in the energetic nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine: Thermodynamics

In this paper we present second harmonic generation (SHG) experiments designed to confirm the mechanism and quantify the transformation kinetics of the β–δ solid state phase transition in the organic nitramine molecule octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The β phase adopts a centrosymmetric crystallographic configuration (P21/c) while the δ phase adopts a noncentrosymmetric one (P61(P65)). As expected, this results in a very poor generation of SHG intensity from the β phase, while the δ phase is very efficient, rivaling KH2PO4 in absolute efficiency. SHG thus provides a very high sensitivity zero background probe of the δ phase. We discuss the use of this signal as a quantitative measure of the δ phase mole fraction in ensembles of free HMX crystals and crystals embedded in a visco–elastic polymer matrix. We report imaging experiments where the spatial characteristics of the transformation are shown to be consistent with nucleation from a low density of initial sites, followed by rapid...

Enhanced optical limiting in derivatized fullerenes

We have observed enhanced optical limiting behavior in solutions of a derivatized fullerene (phenyl-C(61)-butyric acid cholesteryl ester) from 532 to 700 nm. Transient absorption measurements determined the spectral and temporal regions of interest for optical limiting in C(60) and in C(60) derivatives that are due to a reverse saturable absorption mechanism and predicted enhanced limiting at longer wavelengths. Intensity-dependent transmission measurements made at several wavelengths confirmed these results. The increased solubility and the broadened ground-state absorption of the functionalized C(60) make it suitable for use as an optical limiter in the red and the near infrared.

On the nucleation mechanism of the β-δ phase transition in the energetic nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine

In our previous work on the mechanism for the β-δ solid-solid phase transition in octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), we used an empirical mechanism for the nucleation step and hypothesized a defect mechanism that was greatly affected by the presence of a nitroplasticizer/estane binder in the HMX formulation. Since then, we have acquired further evidence for this and have separated out the components of the binder to confirm that it is the nitroplasticizer that controls the nucleation energy in HMX formulations containing a nitroplasticizer/estane binder. While the exact distribution of nucleation energies as a function of synthesis route/defect type has not been worked out, it is likely that the solubility of the HMX in the nitroplasticizer is responsible for lowering the nucleation energy at the crystal surface, and therefore determines the nucleation rate for the formulation.

On the Difference in Impact Sensitivity of Beta and Delta HMX

The beta-to-delta phase transition in HMX has been implicated as the primary reason behind the increased sensitivity of the explosive as it is heated. Both physical and chemical changes accompany the transition, but no study has conclusively shown which specific change, or set of changes, is responsible. We present evidence that shows that the mechanical differences, in and of themselves, do not result in increased sensitivity to shock compression of HMX.

Absorption spectroscopy of nonlinear excitations in polyaniline

We present near steady state photoinduced absorption spectra of two forms of polyaniline. Both the emeraldine base (nondegenerate ground state) and pernigraniline (degenerate ground state) exhibit long lived photoinduced nonlinear excitations with transient absorptions within the π→π* gap. In the case of the emeraldine base, two well‐resolved photoinduced absorption bands are observed with maxima at 0.9 and 1.47 eV, accompanied by a bleaching of the 2 eV absorption edge. The different relaxation rates of the two bands indicate that they arise from two independent photoinduced nonlinear excitations. The results are discussed in terms of intrachain and interchain excitations. The possibility of deep traps on defect sites in pernigraniline is suggested.

Femtosecond to nanosecond dynamics in fullerenes: Implications for excitedstate optical nonlinearities

We compared detailed dynamics of the excited-state absorption for C60 in solution, thin films, and entrapped in an inorganic sol-gel glass matrix. Our results demonstrate that the microscopic morphology of the C60 molecules plays a crucial role in determining the relaxation dynamics. This is a key factor for applications in optical limiting for nanosecond pulses using reverse saturable absorption. We find that the dynamics of our C60-glass composites occur on long (ns) timescales, comparable to those in solution; thin film samples, by contrast, show rapid decay (<20 picoseconds). These results demonstrate that C60-sol-gel glass composites contain C60 in a molecular dispersion, and are suitable candidates for solid-state optical limiting. Multispectral analysis of the decay dynamics in solution allows accurate determination of both the intersystem crossing time (600±100ps) and the relative strengths of the singlet and triplet excited-state cross sections as a function of wavelength from 450–950 nm. The triplet excited-state cross section is greater than that for the singlet excited-state over the range from 620–810 nm.

Ignition Chemistry in HMX from Thermal Explosion to Detonation

We present a global chemical decomposition model for HMX based materials. The model contains three component processes, the initial beta to delta phase transition, solid to gas decomposition and gas phase ignition, for which all kinetic and thermodynamic parameters are fixed by independent measurement. We present an isothermal ignition calculation over the range of temperatures from thermal explosion to detonation. The calculation is performed for a sphere of material and the critical diameter and time for ignition are determined. The sample diameter, and thus the balance of heat generation and dissipation, is the only degree of freedom in the calculation. The results of the calculation are in good agreement with data with respect to both the ignition times and length scales over the full temperature range of energetic response in HMX.

Imaging nanometer-thick patterned self-assembled monolayers via second-harmonic generation microscopy

We have used the inherent surface sensitivity of second-harmonic generation to develop an instrument for nonlinear optical microscopy of surfaces and interfaces. This optical technique is ideal for imaging nanometer-thick, chromophoric self-assembled monolayers (SAMs), which have been patterned using photolithographic techniques. In this paper, we demonstrate the application of second-harmonic generation microscopy to patterned SAMs of the noncentrosymmetric molecule calixarene and discuss the resolution and sensitivity limits of the technique.

Coherent pulse interrogation system for fiber Bragg grating sensing of strain and pressure in dynamic extremes of materials

A 100 MHz fiber Bragg grating (FBG) interrogation system is described and applied to strain and pressure sensing. The approach relies on coherent pulse illumination of the FBG sensor with a broadband short pulse from a femtosecond modelocked erbium fiber laser. After interrogation of the FBG sensor, a long multi-kilometer run of single mode fiber is used for chromatic dispersion to temporally stretch the spectral components of the reflected pulse from the FBG sensor. Dynamic strain or pressure induced spectral shifts in the FBG sensor are detected as a pulsed time domain waveform shift after encoding by the chromatic dispersive line. Signals are recorded using a single 35 GHz photodetector and a 50 G Samples per second, 25 GHz bandwidth, digitizing oscilloscope. Application of this approach to high-speed strain sensing in magnetic materials in pulsed magnetic fields to ~150 T is demonstrated. The FBG wavelength shifts are used to study magnetic field driven magnetostriction effects in LaCoO3. A sub-microsecond temporal shift in the FBG sensor wavelength attached to the sample under first order phase change appears as a fractional length change (strain: ΔL/L<10-4) in the material. A second application used FBG sensing of pressure dynamics to nearly 2 GPa in the thermal ignition of the high explosive PBX-9501 is also demonstrated. Both applications demonstrate the use of this FBG interrogation system in dynamical extreme conditions that would otherwise not be possible using traditional FBG interrogation approaches that are deemed too slow to resolve such events.

MODELING THERMAL IGNITION AND THE INITIAL CONDITIONS FOR INTERNAL BURNING IN PBX 9501

Work has been ongoing in our group for several years to produce a global chemistry model of thermal ignition for octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX) based plastic bonded explosives valid over the entire temperature range of energetic response. We have made considerable progress recently, resulting in both the first broadly accurate model of this type and the possible identification of a crucial component of the chemical mechanism governing thermal decomposition and ignition. The model is similar in kind, but very different in detail from previous models produced by us and others. The model is based on independently measured parameters for known processes in the chemistry of HMX decomposition. We have applied the model in simple calculations of ignition time over the full temperature range of energetic response for HMX, including directly observed ignition induced by fast shear and compression. In this paper we present the model and new calculations relevant to the parameterization of the model to the formulation PBX 9501.