High-pressure Shock Loading Research Articles

Hybrid mechanism of electrical breakdown in ferroelectric materials under high-pressure shock loading

The unique ability of ferroelectrics to generate high voltage under shock loading is limited by electrical breakdown within the shock-compressed ferroelectric material. Breakdown is a hybrid process of initiation and growth. The possible mechanisms of electrical breakdown in ferroelectric films and bulk ceramics subjected to high-pressure shock loading are discussed and experiments designed to elucidate which mechanisms govern breakdown. Gigapascal shock loading experiments were performed on poled Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 ferroelectric film specimens in the range of 32–156 μm thickness to determine the dependence of the breakdown field on thickness and on film specimens in the range of 4–16 mm length to determine the dependence of the breakdown field on the duration of shock compression. The resulting breakdown-field vs thickness and breakdown-field vs shock transit time dependencies are consistent with a hybrid electron emission initiation and Joule heating microchannel growth mechanism. Further analysis of data previously obtained on shock-compressed 0.27Pb(In1/2Nb1/2)O3–0.47Pb(Mg1/3Nb2/3)O3–0.26PbTiO3 ferrvoelectric single crystals and Pb(Zr0.65Ti0.35)O3, Pb0.99(Zr0.52Ti0.48)0.99Nb0.01O3, Pb0.99(Zr0.95Ti0.05)0.98Nb0.02O3 bulk ceramics is consistent with this dual mechanism. It appears that neither chemical composition nor microstructure (single crystal vs polycrystalline) of the ferroelectric material has a significant effect on the breakdown mechanism in shocked ferroelectrics.

Quasi-static compression of shock loaded, single crystal tantalum micropillars

This paper presents the mechanical behaviour of single crystal Ta (6.5 GPa–22 GPa) 1-D shocked along the three principal crystallographic directions [100], [110] and [111]. Room temperature mechanical testing was carried out on micropillars to make possible precise load and displacement measurements. The findings exhibit a marked variation in the flow curve and strain hardening between the three orientations, and relative changes are noticed with shock loading. The yield strength (YS) increases linearly as a function of shock load. The YS was highest along [111] (282 MPa) compared with the [100] and [110] orientations (194 and 206 MPa). The strain hardening rates without pre-shock loading are found to be in the order [111] > [100] > [110] and this is not altered by pre-shock loading. Deformation was accomplished by slip on {112} planes. Further, assessments are considered to identify effective plastic strain accumulation during shock loading. The accumulation of effective plastic strain was found to be similar for all directions for shock loading up to 6.5 GPa, however, above 15 GPa the effective strain was lower for [110] compared to [100] and [111] directions. An attempt is made here to assess the flow curve hardening behaviour and variation in yield strength based on a different combination of pair orientation to throw some light into materials response at high-pressure shock loading.

Shock-induced plasticity and phase transformation in single crystal magnesium: an interatomic potential and non-equilibrium molecular dynamics simulations

An effective and reliable Finnis–Sinclair (FS) type potential is developed for large-scale molecular dynamics (MD) simulations of plasticity and phase transition of magnesium (Mg) single crystals under high-pressure shock loading. The shock-wave profiles exhibit a split elastic–inelastic wave in the [0001]HCP shock orientation and a three-wave structure in the [10-10]HCP and [-12-10]HCP directions, namely, an elastic precursor, a followed plastic front, and a phase-transition front. The shock Hugoniot of the particle velocity (U p) vs the shock velocity (U s) of Mg single crystals in three shock directions under low shock strength reveals apparent anisotropy, which vanishes with increasing shock strength. For the [0001]HCP shock direction, the amorphization caused by strong atomic strain plays an important role in the phase transition and allows for the phase transition from an isotropic stressed state to the product phase. The reorientation in the shock directions [10-10]HCP and [-12-10]HCP, as the primary plasticity deformation, leads to the compressed hexagonal close-packed (HCP) phase and reduces the phase-transition threshold pressure. The phase-transition pathway in the shock direction [0001]HCP includes a preferential contraction strain along the [0001]HCP direction, a tension along [-12-10]HCP direction, an effective contraction and shear along the [10-10]HCP direction. For the [10-10]HCP and [-12-10]HCP shock directions, the phase-transition pathway consists of two steps: a reorientation and the subsequent transition from the reorientation hexagonal close-packed phase (RHCP) to the body-centered cubic (BCC). The orientation relationships between HCP and BCC are (0001)HCP ⟨-12-10⟩HCP // {110}BCC ⟨001⟩BCC. Due to different slipping directions during the phase transition, three variants of the product phase are observed in the shocked samples, accompanied by three kinds of typical coherent twin-grain boundaries between the variants. The results indicate that the highly concentrated shear stress leads to the crystal lattice instability in the elastic precursor, and the plasticity or the phase transition relaxed the shear stress.

Observations of shock-loaded tin and zirconium surfaces with single-pulse X-ray diffraction

A single-pulse X-ray diffraction (XRD) diagnostic has been developed for the investigation of shocked material properties on a very short time scale. The diagnostic, which consists of a 37-stage Marx bank high-voltage pulse generator coupled to a needle-and-washer electron beam diode via coaxial cable, produces line-and-bremsstrahlung X-ray emission in a 40 ns pulse. The molybdenum anode produces 0.71 Å characteristic Kα lines used for diffraction. The X-ray beam passes through a pinhole collimator and is incident on the sample with an approximately 2 mm×5 mm spot and 1° full width at half maximum angular divergence. Coherent scattering from the sample produces a Debye-Scherrer diffraction pattern on an image plate located at 75 mm from the polycrystalline sample surface. An experimental study of the polycrystalline structures of zirconium and tin under high-pressure shock loading has been conducted with single-pulse XRD. The experimental targets were 0.1-mm-thick foils of zirconium and tin using 0.4-mm-thick vitreous carbon back windows for shock loading, and the shocks were produced by either Detasheet or PBX-9501 high explosives buffered by 1-mm-thick 6061-T6 aluminum. The diffraction patterns from both shocked zirconium and tin indicated a phase transition into a polymorphic mix of amorphous and new solid phases.

D-25 Observations of Shocked Metallic Surfaces with Single-Pulse X-ray Diffraction

A single-pulse X-ray diffraction (XRD) diagnostic has been developed for the investigation of shocked material properties on a very short time scale. The diagnostic, which consists of a 37-stage Marx bank high-voltage pulse generator coupled to a needle-and-washer electron beam diode via coaxial cable, produces line and bremsstrahlung X-ray emission in a 40-ns pulse. A selected anode of either silver or molybdenum produces characteristic K lines used for diffraction. The X-ray beam passes through a pinhole collimator and is incident on the sample with an approximately 2-mm by 5-mm spot and 1-width-half-maximum angular divergence. Coherent scattering from the sample produces a Debye-Scherrer diffraction pattern on an image plate 75 mm from the polycrystalline sample surface. An experimental study of the polycrystalline structures of zirconium and tin under high-pressure shock loading has been conducted with single-pulse XRD. The experimental targets were 0.1-mm-thick foils of zirconium and tin using 0.4-mm-thick vitreous carbon back windows for shock-loading, and the shocks were produced by either Detasheet or PBX-9501 high explosives buffered by 1-mm-thick 6061-T6 aluminum. The diffraction patterns from both shocked zirconium and tin indicated a phase transition into a polymorphic mix of amorphous and new solid phases.

Rare twin linked to high-pressure phase transition in iron

At approximately 13 GPa, body-centered cubic alpha-iron undergoes a fully reversible, pressure-induced phase transition into hexagonal close-packed epsilon-iron. Microstructural evidence of this phase transition has been identified in the fully reverted alpha-iron as a large number of {3 3 2} 〈 1 1 3 〉 twins found primarily as secondary twins within {1 1 2} 〈 1 1 1 〉 primary twins. The {3 3 2} 〈 1 1 3 〉 twins were produced during high-pressure shock-loading of 1018 steel at a peak pressure above the alpha-epsilon phase transition pressure. The twins were identified using electron backscattered diffraction and transmission electron microscopy.

Deformation of nanocrystalline materials at ultrahigh strain rates – microstructure perspective in nanocrystalline nickel

Nanocrystalline materials with grain sizes smaller than 100 nm have attracted extensive research in the past decade. Due to their high strength, these materials are good candidates for high pressure shock loading experiments. In this paper, we investigated the microstructural evolutions of nanocrystalline nickel with grain sizes of 10-50 nm, shock-loaded in a range of pressures (20-70 GPa). A laser-driven isentropic compression process was applied to achieve high shock-pressures in a timescale of nanoseconds and thus the high-strain-rate deformation of nanocrystalline nickel. Postmortem transmission electron microscopy (TEM) examinations reveal that the nanocrystalline structures survive the shock deformation and that dislocation activity is the prevalent deformation mechanism when the grain sizes are larger than 30 nm, without any twinning activity at twice the stress threshold for twin formation in micrometer-sized polycrystals. However, deformation twinning becomes an important deformation mode for 10-20 nm grain-sized samples.

PVDF shock sensors: applications to polar materials and high explosives

Ferroelectric polymers (PVDF) with well-defined and precisely known electrical properties are now routinely available from commercial sources. Electrical processing with the Bauer cyclic poling method can produce individual films with well-defined remanent polarization up to 9 /spl mu/C/cm/sup 2/. These polymers provide an unusual opportunity to study the structure and physical properties of materials subjected to shock loading. The behavior of PVDF has been studied over a wide range of pressures using high-pressure shock loading and has yielded well-behaved, reproducible data up to 25 GPa in inert materials. The application of PVDF gauges for recording shock waves induced in polar materials such as Kel-F, PMMA, or in reactive materials is hampered by observations of anomalous responses due to shock-induced polarization or an electrical charge released inside a shock-compressed explosive. A solution using an appropriate electrical shielding has been identified and applied to PVDF for shock measurement studies of Kel-F, and for Hugoniot measurements of high explosives (PH). Furthermore, shock pressure profiles obtained with in situ PVDF gauges in porous HE (Formex) in a detonation regime have been achieved. Typical results of shock pressure profile versus time show a fast superpressure of a few nanoseconds followed by a pressure release down to a plateau level and then by a pressure decay. More accurate measurements are reported with electrically improved PVDF gauges as well as with 0.25 mm/sup 2/ active area PVDF gauges.

Shock-induced chemical reactions in titanium–silicon powder mixtures of different morphologies: Time-resolved pressure measurements and materials analysis

The response of porous titanium (Ti) and silicon (Si) powder mixtures with small, medium, and coarse particle morphologies is studied under high-pressure shock loading, employing postshock materials analysis as well as nanosecond, time-resolved pressure measurements. The objective of the work was to provide an experimental basis for development of models describing shock-induced solid-state chemistry. The time-resolved measurements of stress pulses obtained with piezoelectric polymer (poly-vinyl-di-flouride) pressure gauges provided extraordinary sensitivity for determination of rate-dependent shock processes. Both techniques showed clear evidence for shock-induced chemical reactions in medium-morphology powders, while fine and coarse powders showed no evidence for reaction. It was observed that the medium-morphology mixtures experience simultaneous plastic deformation of both Ti and Si particles. Fine morphology powders show particle agglomeration, while coarse Si powders undergo extensive fracture and entrapment within the plastically deformed Ti; such processes decrease the propensity for initiation of shock-induced reactions. The change of deformation mode between fracture and plastic deformation in Si powders of different morphologies is a particularly critical observation. Such a behavior reveals the overriding influence of the shock-induced, viscoplastic deformation and fracture response, which controls the mechanochemical nature of shock-induced solid-state chemistry. The present work in conjunction with our prior studies, demonstrates that the initiation of chemical reactions in shock compression of powders is controlled by solid-state mechanochemical processes, and cannot be qualitatively or quantitatively described by thermochemical models.

X-Ray Diffraction Study of Shock-Modified Tetragonal Phases of SnO2 and MnO2

AbstractX-ray diffraction line-profile analysis on tetragonal forms of SnO2 (cassiterite), MnO2 (pyrolusite), and previously studied TiO2 (rutile), which were subjected to high pressure shock loading, show that residual lattice strain and coherent “crystal” size are a function of shock parameters. An interesting observation on a sample of MnO2 concerns the recovery of cubic Mn2O3 (bixbyite) in the material subjected to 22 GPa, indicating a shock-induced chemical synthesis.

Annealing and Strain Effects on Tl-Ba-Ca-Cu-O Crystals and Ceramics

ABSTRACTSuperconducting transition temperatures (Tc'S) in the Tl-Ba-Ca-Cu-O system are shown to be a function of processing for both single crystals and bulk ceramics. Crystals with the identical structural phase and nominal composition but slightly different lattice constants can have significant differences in Tc. We report the effects of high temperature annealing in vacuum or flowing oxygen on Tc for single crystal plates of two common phases with nominal stochiometries Tl2Ba2Ca1Cu2O8 and Tl2Ba2Ca2Cu3O10. Contradictory results on nominally similar crystals suggest that strain, Tl content and TI/Ca site disorder are as important as the oxygen stoichiometry in determining Tc This interpretation is supported by complementary annealing studies in bulk ceramic, including a powder where Tc increased by 5 K following high-pressure shock loading which introduced substantial residual strain.

X-ray Line Broadening Study on Shock-modified Zirconia

Zirconia (Zr02) powder compacts have been subjected to controlled, quantitative high pressure shock loading at peak pressures from 5-27 GPa and preserved for post-shock analysis. The overlapping broadened X-ray diffraction peak profiles have been separated by least-squares fitting procedures. The separate lines have been analysed in the usual manner to determine the residual lattice strain and the coherent crystallite sizes. Maximum modification effects are observed near 20 GPa with strain values near 3 x 10-3 and size values near 200 A.

X-Ray Line Broadening Study on Shock-Modified Hematite

AbstractHematite (α-Fe2O3) powder compacts have been subjected to controlled, quantitative high pressure shock loading at peak pressures from 8-27 GPa and preserved for post shook analysis. The broadened x-ray diffraction peak profiles have been analyzed to determine the residual lattice strain and the coherent crystallite sizes. Maximum modification effects are observed near 17 GPa with strain values near 3 x 10-3 and size values near 200 Å suggesting annealing at higher shock pressure, resulting from the higher shock temperature.

Morin transition of shock-modified hematite.

Hematite (\ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$) powder compacts have been subjected to controlled, quantitative high-pressure shock loading at peak pressures from 8 to 27 GPa and preserved for postshock analysis. Analysis of broadened x-ray line profiles shows a reduction of average crystallite size to about 60 nm and retained lattice strains near 0.1% in the recovered samples. M\"ossbauer and magnetization measurements show drastically altered behavior of the Morin transition: large fractions of the Fe sites do not exhibit the transition, and those that do have a significantly reduced transition temperature and an increased thermal hysteresis. The M\"ossbauer quadrupole splitting is sensitive to the shock compression conditions and provides evidence for a less than 90\ifmmode^\circ\else\textdegree\fi{} spin flip at the Morin transition for the higher-pressure shock loading. A small fraction of the hematite is converted to magnetite (${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$) under the most extreme shock condition examined. In some cases the observed effects can be attributed to either the reduced crystallite size or the large defect density associated with the residual strain.

Submicrosecond shock-induced chemical reactions in solids: First real-time observations

We report observations of a chemical reaction on a 100 ns time scale in a nickel-aluminum powder mixture while under high-pressure shock loading. Optical pyrometer measurements show values of spectral radiance that can only be explained by high temperatures due to an exothermic reaction. Streak photography shows spatial evolution of the reaction zone in regions of the sample predicted to experience shock pressures and temperatures known to initiate formation of nickel-aluminum alloys.

A model for time dependence in shock-induced thermal radiation of light

High-speed optical pyrometry has seen increasing application in the measurement of shock temperatures in initially transparent solids and liquids; however, the information contained in the time-dependent intensity of the emitted light has frequently been overlooked. A model has been developed for this time dependence in the observed intensity of light emitted from materials undergoing high-pressure shock loading. Most experimental observations of this time dependence can be explained on the basis of geometric effects only, without having to invoke intrinsic time dependences of the source intensity (due to changes in temperature, emissivity or shock-wave structure). By taking advantage of this fact, observed time dependences can be used to determine the absorption coefficient of shocked materials and their effective emissivities, thereby providing more precise temperature measurements. The model is invoked under various limiting conditions to explain time dependences previously observed in NaCl, CaO, Mg2SiO4 (forsterite), SiO2 (quartz), MgO, and CaAl2Si2O8 (anorthite) glass. As an example, the linear absorption coefficient at 650 nm of NaCl shocked to 75 GPa is found to be 13 cm−1, close to previously published values based on a similar but less general model.

Synthesis of zinc ferrite under high-pressure shock loading

Mechanically blended stoichiometric mixtures of zinc oxide and ferric oxide powders have been subjected to high-pressure shock-wave loading and preserved for post-shock analysis. Partial conversion to a non-stoichtometric spinel structure zinc ferrite was found by X-ray diffraction. The ferrite is strongly magnetic as shown by static magnetization measurements.

Synthesis of nickel aluminides under high-pressure shock loading

We report the first shock synthesis of nickel aluminides by explosive loading of the blended mixture of elemental nickel and aluminum powders. Insofar as we are aware, this is the first synthesis of these compounds from fully dense powder compacts. The major end product was ordered Ni 3Al which was identified by X-ray diffraction and electron-probe microanalysis. Transitional reaction zones or heat-affected zones contained NiAl, Ni 2Al 3, and NiAl 3. Microhardness (DPH) of Ni 3Al was 438 ± 8. This is the same hardness obtained for cold-rolled or rapidly solidified Ni 3Al containing boron additions.

X-ray diffraction line broadening of shock-modified titanium carbide

Titanium carbide powder compact samples have been subjected to controlled high-pressure shock loading with well-characterized peak pressures between 17 and 27 GPa and preserved for post-shock study. The broadened X-ray diffraction peak profiles have been analyzed with the Warren—Averbach technique to determine the residual lattice strain and the coherent crystallite sizes. Although residual strain values in the powders shock-modified to 27 GPa are similar to those reported for wet-milled powders, the shocked powders show less anisotropy in strain, more anisotropy in crystallite shape and larger strain gradients near dislocation cores than milled powders.

Line Broadening Studies on Highly Defective TiO2 Produced by High Pressure Shock Loading

Enhanced solid state reactivity of materials both during and after shock compression has been attributed to the introduction of large numbers of defects into the crystalline lattices and to reduction in the particle and crystallite size of powders [1]. In particular, orders of magnitude increases in the catalytic activity has been observed In shock-modified TiO2 [2]. Line broadening of x-ray diffraction profiles provides a means to determine the coherent crystallite size and the residual lattice strain resulting from defect concentrations. The present study on shock-loaded rutile is a detailed Investigation of the influence of shock loading on residual lattice strain and coherent crystallite size. Annealing of shock-modified rutile powders is also studied.