Hongliang He

Shock compression of Fe‐Ni‐Si system to 280 GPa: Implications for the composition of the Earth's outer core

The shock Hugoniot of an Fe-9 wt %Ni-10 wt %Si system as a model of the Earths core has been measured up to ~280 GPa using a two-stage light-gas gun. The samples had an average density of 6.853 (±0.036) g/cm3. The relationship between shock velocity (Us) and particle velocity (up) can be described by Us (km/s) = 3.95 (±0.15) + 1.53 (±0.05) up (km/s). The calculated Hugoniot temperatures and the melting curve indicate that the model composition melts above a shock pressure of ~168 GPa, which is significantly lower than the shock-melting pressure of iron (~225 GPa). A comparison of the pressure-density (P-ρ) profiles between the model composition and the preliminary reference Earth model gives a silicon content close to 10 wt %, necessary to compensate the density deficit in the Earths outer core from seismological observations, if silicon is present as a major light element in the Fe-Ni core system.

Time-resolved dynamic tensile spall of pure aluminum under laser irradiation

A diagnostic technique with high time resolution in the velocity-history measurement is presented by coupling an electronic streak camera with a push-pull type velocity interferometer system for any reflector. This technique has been applied to investigate the dynamic tensile spall induced by laser irradiation in pure aluminum by measuring the free surface velocity profiles of samples. Laser irradiation intensities are in the range of 1010–1012W∕cm2. Spall strength in pure aluminum is calculated from the measured free surface velocity profile as a function of the tensile strain rate. The results show a rapid increase with the tensile strain rate more than 106s−1. The damage influence on the tensile spall of laser-irradiated pure aluminum is analyzed by experimental and numerical studies. In the frame of percolation theory, a physical model is proposed to describe the rapid stress release due to void coalescence. A good agreement between the calculation and the experiment is obtained.

Sinoite (Si2N2O) shocked at pressures of 28 to 64 GPa

Abstract Shock recovery experiments of sinoite (Si2N2O) were carried out in a pressure range of 28-64 Gpa. Post-shock samples were investigated by optical microscopy, X-ray diffraction, micro Raman spectroscopy, and transmission electron microscopy coupled with energy dispersive X-ray analysis. Sinoite was stable up to ~28 Gpa, being partially amorphous at 34 Gpa, and almost completely amorphized above 41 Gpa. These results are consistent with the observation that sinoite is found in enstatite chondrites classified into shock stage S2, and imply that sinoite may not be a crystallization product from impact melts and that it may be a metamorphic product of high temperatures and relatively low pressure.

Failure behavior of Pb(Zr0.95Ti0.05)O3 ferroelectric ceramics under shock compression

PZT 95/5 ferroelectric ceramics have been used in shock-driven pulsed-power supplies for many years; their mechanical failure under shock compression plays an important role in dielectric breakdown. Shock experiments have been conducted to understand such failure by measuring the velocity of the free surface or the PZT/sapphire interface. Results confirm that delayed failure exists in PZT 95/5 before dynamic yielding at 2.4 GPa; multipoint measurements indicate that the failure layer in PZT 95/5 was not a plane but a rough front. The delay time and velocity of this layer had been determined by measuring samples of varying thicknesses at fixed pressure; results indicate that this velocity is the same as the shock-wave speed and the delay time decreases with increasing shock stress. At a shock stress of 4.9 GPa, the delay time falls to zero and a ramp wave is observed. This kind of failure is a new phenomenon in electric breakdown of PZT 95/5 under shock compression.

A new interpretation of decomposition products of serpentine under shock compression

Abstract Dense hydrous magnesium silicates (DHMSs) may play an important role in water transport during planetary accretion and as water reservoirs in the Earth’s deep mantle. We show that the dynamic decomposition products of antigorite, Mg3Si2O5(OH)4, can be interpreted as containing the newly discovered, dense hydrous silicate, phase H (MgSiO4H2). The Hugoniot for phase H was calculated based on the Hugoniots for its constituent oxides and equation of state data derived from first-principles calculations. The measured antigorite Hugoniot, previously suggested to decompose into high-pressure phases without generating fluid H2O, was compared with those derived from calculations involving phase H. Sound velocity data were also compared to confirm that the dynamic breakdown product of antigorite at pressures above ~40 GPa is most likely phase H plus MgO without formation of fluid H2O.

Experimental constraints on light elements in the Earth's outer core.

Earth’s outer core is liquid and dominantly composed of iron and nickel (~5–10 wt%). Its density, however, is ~8% lower than that of liquid iron, and requires the presence of a significant amount of light element(s). A good way to specify the light element(s) is a direct comparison of density and sound velocity measurements between seismological data and those of possible candidate compositions at the core conditions. We report the sound velocity measurements of a model core composition in the Fe-Ni-Si system at the outer core conditions by shock-wave experiments. Combining with the previous studies, we found that the best estimate for the outer core’s light elements is ~6 wt% Si, ~2 wt% S, and possible ~1–2.5 wt% O. This composition satisfies the requirements imposed by seismology, geochemistry, and some models of the early core formation. This finding may help us to further constrain the thermal structure of the Earth and the models of Earth’s core formation.

Mesoscopic deformation features of shocked porous ceramic: Polycrystalline modeling and experimental observations

To prevent functional failure, the macroscopic shock response of ceramics needs to be understood. We explored the mesoscopic deformation features of porous ceramics, which are responsible for the measured macroscopic “plastic” wave profiles, using polycrystalline modeling and experiments. A polycrystalline model is established that considers the influence of two major microstructures (multi-voids and grain boundaries) in the porous ceramics. Shock experiments with the recovery of shocked porous lead zirconate titanate ceramics were conducted. The computational results show that shear cracks nucleate around voids under shock because of severe shear stress concentrations. Broken fragments fill the voids and lead to void collapse. Representative long-distance extended cracks and thick crevices are observed in the recovered sample subjected to 3.3 GPa compression. These representative features are reproduced by the polycrystalline model. An initial transgranular crack translates into an intergranular crack af...

Electrical response of Pb(Zr0.95Ti0.05)O3 under shock compressions

PZT ((Pb(Zr0.95Ti0.05)O3)) 95/5 ferroelectric ceramics have been used in shock-driven pulsed-power supplies for many years. Their electrical responses under the high electric field strengths generated by shock compression are important in various applications. Shock experiments were conducted to understand the complicated electromechanical behavior by measuring the depoling current. An equivalent circuit was built to analyze the dielectric parameters of PZT 95/5 ceramics under shock compression. The results indicated that the internal resistivity and breakdown strength decrease with increasing shock stress and abnormal behavior, whereby the internal resistivity increases with increasing electric field strength, which is also observed at 4.3 GPa. Cracks induced by mechanical failure under high stresses were found to be related to the leakage current of the samples and to play an important role in determining the electrical response of PZT 95/5 under shock compression.

Measurement of dynamic tensile strength of nanocrystalline copper by laser irradiation

An approach is developed to investigate the dynamic tensile fracture of nanocrystalline copper by laser irradiation loading. A push-pull type velocity interferometer system for any reflector is used to measure the rear free surface velocity profiles. The dynamic tensile strength of nanocrystalline copper films is determined from these velocity profiles as a function of the tensile strain rate. Results show that the dynamic tensile strength of nanocrystalline copper film is about 3 GPa, which is much higher than that of polycrystalline bulk copper, but lower than that of single crystal copper. This dynamic tensile strength increase may be attributed to constraints on dislocation motion by more grain boundaries in nanocrystalline materials.

Hugoniot and impact-induced phase transition of magnesite

Abstract Hugoniot equation-of-state and release adiabat results are presented for magnesite to a pressure of ~140 GPa. A sharp change in the shock velocity and particle velocity relation suggests that a phase transition to a high-pressure phase occurs at 107±10 GPa. Decomposition of magnesite was observed by abrupt volume expansion during the pressure release from a pressure over the phase transition and by investigating post-shock magnesites recovered from hypervelocity impacts of mini-flyers performed using a laser-driven acceleration. Post-shock magnesites above 95 GPa contained MgO crystallites and the amount of MgO increased with increasing shock pressure.