Kunihito Nagayama

Elastoplastic properties under shock compression of Al2O3 single crystal and polycrystal

Elastoplastic properties of Al2O3 single crystal (14–38 GPa) and polycrystal (6–33 GPa) under shock compression were studied by the measurements of Hugoniot compression curves and stress histories. The Hugoniot‐elastic limit (HEL) stresses were determined to be 14.4–17.3 GPa for single crystals and 6.7–9.6 GPa for polycrystals with 2.7%–3.2% porosity. In the plastic region, the single‐crystal Hugoniot stresses, at a given density, exceed the isothermal x‐ray static data and its extrapolation by 6.5–8 GPa. And, the polycrystalline Hugoniot data exceeds, at a given density, the single‐crystal Hugoniot data by about 1 GPa. These Hugoniot offsets are not caused by temperature increase, but rather by the shear strength and the remaining porosity. The stress histories of a single crystal show very similar shapes to that of a perfect elastoplastic material. The apparent yield stresses of single crystal at the HEL and in the plastic region are estimated to be 9.5–12 GPa and 4–6 GPa, from the HEL data and the quas...

Keyed‐powder gun for the oblique‐impact shock study of solids in several 10 s of GPa region

Experimental equipment for the precise oblique‐impact shock study of solids in several 10 s of GPa region was developed. A powder gun with a keyed launcher (keyed‐powder gun), which is capable of accelerating projectiles of 20–40 g in weight without rotation to velocities of even greater than 2 km/s, was produced for the first time. The gun has a double‐wall structure launcher with a key whose muzzle shape resembles an old moon (basic bore diameter is 27 mm). Measurement of projectile velocities in the launcher by the reflected‐light method, including a logic trigger circuit for shock‐wave measurements, is combined with this gun. It was confirmed that the parallel‐plate impact experiment (normal type or oblique type), in which the tilt angle of parallel plates is less than 0.18° could be performed, allowing precise measurements of impact velocity and shock‐wave data by electrical and optical methods. Further, the keyed‐powder gun should find other widespread applications, e.g., for hypervelocity‐aerodynam...

Shock compression of zirconia ZrO2 and zircon ZrSiO4 in the pressure range up to 150 GPa

The shock compression state of zirconia ZrO2 and zircon ZrSiO4 in the pressure range up to 150 GPa (1.5 Mbar) are studied on the basis of the measurements of shock velocities, particle-velocity histories, free surface motions, and electrical conductivities. Zircon transforms, and zirconia probably does, to high pressure phases up to 90 GPa. The shock velocity (Us) — particle velocity (Up) Hugoniots can be described as Us=4.38+1.37 Upkm/s above 90 GPa for ZrO2, and Us=6.50+0.49 Upkm/s (mixed phase region), and Us=1.54+2.30 Upkm/s (high pressure phase region) for ZrSiO4. The corrected isothermal densities of the high pressure phase ZrSiO4 are roughly consistent with the isothermal ones of mixtures of ZrO2 and SiO2. Bulk sound velocities in the high-pressure phase region of these oxides are discussed in comparison with other dioxides. Electrical conductivities of these oxides increase from lower than 10−12 S/m to greater than 100 S/m in the shock-stress range up to 70 GPa, and remain as constant values up to higher than 100 GPa.

New method of magnetic flux compression by means of the propagation of shock‐induced metallic transition in semiconductors

A new procedure of generating ultrahigh pulsed magnetic field was described. The method is based on the fact that semiconducting materials undergo shock‐induced metallic transition at very high pressures. An approximate numerical analysis using the magnetohydrodynamics equations was carried out for silicon with normal and low initial densities. It is shown that flux concentration may become efficient for low‐impedance materaials.

Anisotropic elastic limits and phase transitions of rutile phase TiO2 under shock compression

Anisotropic elastic limits and phase transitions of the rutile phase TiO2 under shock compression are studied, on the basis of the measurements of particle‐velocity histories and free‐surface motions along the [100], [110], and [001] axis directions in the stress range up to 38.6 GPa. The stresses of Hugoniot elastic limit (HEL) along the [100] and [001] axis directions are determined to be 7.8±0.5 and 4.1±0.3 GPa, respectively. These different HEL stresses are reasonably analyzed by an inherent yield stress of 3.0±0.3 GPa. A Hugoniot kink at 12.2±0.6 or 17.0±0.7 GPa along the [100] or [110] axis direction, respectively, and two kinks at 10.6±0.5 and 33.7±1.3 GPa along the [001] axis direction are observed. Considering the volume changes and the recovery experiments of α‐PbO2 phase, two former ones and the last one are suggested to correspond to the beginning of phase transitions to a α‐PbO2 phase and a hexagonal phase, respectively. These anisotropic behaviors clearly show the typical evidences of the un...

Experimental study of a new mechanism of magnetic flux cumulation by the propagation of shock‐compressed conductive region in silicon

A new mechanism of magnetic flux compression proposed by one of the authors has been experimentally proved promising for the generation of ultrahigh pulsed fields. A cylindrical implosion type device is used here, and the thin metal liner normally used is replaced by a solid cylinder of crystalline or powdery silicon. As predicted by the theory, the field compression factor depends strongly on the initial density of the silicon, on the explosive, and on the scale. A typical value of the field multiplication factor is 17.8 for a 45‐mm‐diam silicon cylinder of density 1.1 g/cm3.

Optical study of cylindrically converging blast waves

Cylindrically converging blast waves have been produced by detonating cylindrical explosive shell. Formation of shock waves has been observed adopting the laser shadowgraphy. Spectroscopic measurements have also been made with a monochromator combined with a photomultiplier. A temperature of 12 000±2000 K has been observed, which is 10 000 K lower than that estimated from the propagation velocity of the shock wave.

Electrical response of BaTiO3 ceramics to the shock-induced ferroelectric-paraelectric transition

The electrical response of BaTiO3 ceramics to the strong shock compression well above the Hugoniot‐elastic limit or ferroelectric‐paraelectric transition pressure is studied to separate and identify the causes of shock‐induced electrical response of ferroelectric materials. Measurements of voltage and current histories under shock compression are performed by using poled and nonpoled specimens, assemblies of both parallel and normal modes, and explosive systems. In order to consider the effects due to elastic‐plastic deformation and shock‐induced conduction, the Hugoniot compression curve and electrical conductivities are also measured by using nonpoled specimens. The Hugoniot‐elastic limit stress is determined to be 4.8–6.4 GPa for porous specimens with 1.7–4.4% porosity, and the shock velocity (Us) and particle velocity (Up) Hugoniot can be closely described as Us =4.29+1.66Up km/s in the plastic region. The electrical conductivity values at 12‐ and 29‐GPa peak stresses are measured to be 2.1×10−3 and 4...

Flash Radiographic Observation Of Shock Wave Propagation In Solids

Flash radiographic method has been used to observe the propagation of plane shock waves in solids, particularly in laminated composites. Preliminary experiments on PMMA sample shows that the method is useful for determining the shock wave parameters in condensed media. Then the method has been applied to composite samples consisting of alternate layers of PMMA and aluminium. A plane shock wave was transferred to these bi-laminar system parallel to the laminations. The shock front in each PMMA plate was observed to be slightly curved because of the impedance mismatch between constituents. Across the shock front, an abrupt change in area ratio of constituents was also observed. The average Hugoniot data obtained for laminated composites have been compared with previous theoretical models.