F. E. Walker

Bio: F. E. Walker is an academic researcher from Lawrence Livermore National Laboratory. The author has contributed to research in topics: Shock wave & Explosive material. The author has an hindex of 3, co-authored 3 publications receiving 74 citations.

A General Model for the Shock Initiation of Explosives

Abstract: A general model for the shock initiation of explosive reaction in chemical explosives is proposed. The model is based on the concepts of: (1) the kinetics of decomposition in which ions and free radicals produced by the shock wave shear forces initiate chain reactions that contribute to and accelerate the decomposition produced by the thermally activated molecules; (2) the formation of statistically random reaction sites whose number in a specific explosive is a direct function of the shock pressure as the shock transits the explosive; and (3) a critical- energy-fluence requirement for initiation. This model appears to apply to explosive reaction in gases, liquids, and solids.

Quantum Mechanics and Molecular Dynamics Calculations provide new evidence for a free‐radical shock initiation model

Abstract: The contrast between a hydrodynamic model for shock initiation of explosives and a newer microscopic free-radical model is presented. The primary considerations of the free-radical model are that shock energy is very coherent and that it is strong enough to cause mechanical fracture of covalent bonds. The shock front on a microscopic scale is very narrow (∼5 A to 15 A) and thermal equilibrium does not exist in the front. The free atoms and radicals formed by the shear and acceleration forces in the shock front initiate the chemical reaction that leads to hot spots and the eventual decomposition of the explosive materials. Energy-release rates and vibrational velocities of the covalently bound atoms are factors in establishing detonation velocities. A number of explosives phenomena are discussed as the new model provides explanations for them. New information from molecular dynamics and quantum mechanical calculations on shock waves in condensed systems and recent experimental data are shown to support the free-radical model.

Description of a Shock-Wave Velocity Barrier

Abstract: Summary An analogy can be drawn between the sonic barrier in transonic flight, the thermal barrier in supersonic or hypersonic flight, and the limiting of the detonation process in high explosives.

Thinking big (and small) about energetic materials

Abstract: A significant recent development in the field of energetic materials has been the introduction of nanotechnology. Nanoparticle based materials have the potential of releasing more than twice as much energy as the best molecular explosives. The possibility of developing nanoenergetic materials that are optimised for specific applications by controlling the spatial organisation on length scales ranging from nanometres to metres is discussed. To do so a fundamental understanding of the relationships between structure and performance that does not yet exist is required. Experimental measurements using molecular spectroscopy with high time and space resolution are presented that help clarify the fundamental mechanisms of reaction between fuel nanoparticles and surrounding oxidisers, and the relationships between nanoenergetic structure and performance properties such as the energy release rate and the reaction propagation rate.

X‐ray photoelectron spectroscopy and paramagnetic resonance evidence for shock‐induced intramolecular bond breaking in some energetic solids

Abstract: Solid samples of 1,3,5, trinitro 1,3,5, triazacyclohexane (RDX), trinitrotoluene (TNT), and ammonium nitrate were subjected to shock pulses of strength and duration less than the threshold to cause detonation. The recovered shocked samples were studied by x‐ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The results of these measurements indicate that the shock pulse either broke or altered the internal bonds of the molecules of the solid. The results of the shock decomposition are compared with measurements of the uv and slow thermal decomposition of these materials using the same experimental techniques.

Simplified laser-driven flyer plates for shock compression science

Abstract: We describe a simplified system of laser-driven flyer plates for shock compression science and shock spectroscopy. We used commercially available one-box Nd:YAG lasers and beam homogenization solutions to create two launch systems, one based on a smaller (400 mJ) YAG laser and an inexpensive diffusive optic, and one based on a larger (2500 mJ) laser and a diffractive beam homogenizer. The flyer launch, flight, and impact processes were characterized by an 8 GHz fiberoptic photon Doppler velocimeter. We investigated effects of different substrates, adhesives, absorbers, ablative layers, and punching out disks from continuous foils versus fabricating individual foil disks, and found that a simple metal foil epoxied to a glass window was satisfactory in almost all cases. Our simplified system launched flyer plates with velocities up to 4.5 km s−1 and kinetic energies up to 250 mJ that can drive sustained steady shocks for up to 25 ns. The factor that limits these velocities and energies is the laser fluence that can be transmitted through the glass substrate to the flyer surface without optical damage. Methods to increase this transmission are discussed. Reproducible flyer launches were demonstrated with velocity variations of 0.06% and impact time variations of 1 ns. The usefulness of this flyer plate system is demonstrated by Hugoniot equation of state measurements of a polymer film, emission spectroscopy of a dye embedded in the polymer, and impact initiation and emission spectroscopy of a reactive material consisting of nanoscopic fuel and oxidizer particles.

Propagation of shock-induced chemistry in nanoenergetic materials: The first micrometer

Abstract: The propagation of shock-induced chemical reactions over nanometer distances is studied in energetic materials consisting of Al nanoparticles (30, 62, and 110 nm) in the polymer oxidizers nitrocellulose (NC) and Teflon. Picosecond laser flash heating vaporizes the Al particles, which react with surrounding oxidizer and generate a spherical shock wave with a rapidly dropping pressure, that decomposes the NC or Teflon out to a diameter drxn. A methodology is developed to measure drxn as a function of laser energy, that uses the average distance between nanoparticles davg as a length scale and identifies the ablation threshold as occurring when the reaction spheres from multiple particles coalesce. At minimal laser fluences, drxn is slightly larger than the diameter of the polymer sphere needed to just oxidize the nanoparticle. The excess diameter is attributed to the chemical energy of oxidation. At larger laser fluences where chemical energy is unimportant, drxn∝E over the length scale of 50–1500 nm, where E is the energy in the spherical shock. Shock-induced chemical reactions propagate farther with larger nanoparticles and farther in Teflon than in NC. The linear dependence of drxn on E is explained using a hydrodynamic model that assumes chemistry occurs when a pressure P is applied for a given time t, so that Pt=constant.