New high-energy luminescence bands from Co2+ in ZnSe

Three sharp absorption features in the energy range 2.36–2.55 eV have been detected in the transmission spectrum of Co-diffused ZnSe, and a number of luminescence transitions originating from the lowest of these states at 2.361 eV have been observed. Photoluminescence excitation spectra prove that these are high energy excited states of the Co²⁺_Zn impurity, a conclusion confirmed by comparison of measured and predicted luminescence energies. This represents the first identification of luminescence branching from a higher excited state of a transition metal ion in any semiconductor. The sharp, weakly phonon-coupled transitions involve either intra-impurity excitation or transitions from the impurity to localised states split off from a minimum in the conduction band. The implications of these observations for the mechanism of host-impurity energy transfer and for the nature of the excited state wavefunctions are discussed.     

Test-based thermal explosion model for HMX

We present a thermal explosion (cookoff) model for an HMX-based plastic bonded explosive (LX-10). The thermal–chemical–mechanical response of LX-10 is modeled based on the measurements from the scaled thermal explosion experiment (STEX) at the Lawrence Livermore National Laboratory. Confined LX-10 is heated at a rate of 1 °C/h until an explosion is observed. The modeled cookoff problem is simulated by the Arbitrarily Lagrangian–Eulerian hydrocode (ALE3D) that can handle a wide spectrum of time scales that vary from a structural to a high speed shock physics time scale. In addition to a comprehensive model for energetic material, the confinement material namely an AerMet 100 steel is modeled as a Steinberg–Guinan material with a Johnson–Cook failure model with a statistical failure distribution. By using the size distribution data from the fragmentation experiment, the metal fracture and fragmentation due to an explosion are modeled. The explosion temperature is predicted to within 1°. Calculated wall strain provides violence associated with the thermal explosion process and agrees favorably with the measured STEX data.     

Direct sample fraction deposition using electrospray in narrow-bore size-exclusion chromatography/matrix-assisted laser desorption/ionization time-of-flight mass spectrometry for polymer characterization

A direct sample fraction deposition method was developed for off-line size-exclusion chromatography (SEC)/matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. By using electrospray, the SEC eluent, together with a suitable matrix solution added coaxially, was directly deposited on the MALDI plate. Owing to the formation of very small droplets in electrospray, solvent evaporation is much faster. The fractionation volume in narrow-bore SEC, which can directly be collected in one MALDI spot, can easily be optimized in the range of a few microlitres. In addition, fairly homogeneous sample spots were obtained. The possible influence of composition variation of the SEC effluent on the analytical results using direct fraction deposition was investigated; no substantial effects were observed. The applicability of the method was demonstrated by characterizing a broad poly(methyl methacrylate) sample. Copyright 2000 John Wiley & Sons, Ltd.     

Characterization of thermally-damaged LX-17

Thermal damage was applied to LX-17 at 190°C for several hours. The damaged LX-17 samples, after cooled down to room temperature, were characterized for their material properties, safety and performance. Mass losses upon thermal exposure were insignificant (<0.1 mass%). The damaged LX-17 samples expanded, resulting in a bulk density reduction of 4.3%. Subsequent detonation measurements (cylinder tests) were conducted on the thermally-damaged LX-17 samples. The results showed that the fractions of damaged LX-17 reacted were slightly lower than those of pristine LX-17. The thermally damaged LX-17 samples had average detonation velocity of 7.341 mm μs⁻¹, lower than that (7.638 mm μs⁻¹) of pristine LX-17. The average detonation energy density for the damaged LX-17 was 5.18 kJ cm⁻³, about 6.0% lower than the detonation energy density of 5.51 kJ cm⁻³ for the pristine LX-17. The break-out curves showed reaction zone lengths for pristine LX-17 and damaged LX-17 were similar but the damaged samples had ragged detonation fronts. DSC curves showed no significant difference between pristine LX-17 and damaged LX-17 with a peak temperature of 381°C observed.