Magnetoelectrical technique for studying the insulator–metal transition under shock compression

Abstract

The paper describes a magnetoelectrical technique for monitoring the particle velocity in matter acquiring a large electrical conductivity under shock compression. The particle velocity is measured in the electromagnetic skin layer, which travels through matter with a wave velocity. The layer thickness essentially depends on the electrical conductivity of shock-compressed matter. A multielectrode cell also allows the wave velocity to be registered at various spatial bases. Experiments give kinematics parameters of a metallization wave for aluminium powders of varied mass density and particle size. The results unambiguously testify that the conductivity of the aluminium powder arises in the main shock wave carrying full pressure. The results do not confirm the conclusion of Novac et al about metallization of the aluminium powder in an elastic precursor. The thickness of the skin layer for a coarse powder is smaller than the thickness of the shock-compression zone, which offers the opportunity of finding the metallization phase directly in the shock transition zone.

Model of shock-wave magnetic cumulation

“…Third, the conductivity is independent of shock pressure and is uniform throughout the compressed matter. The data for aluminium powders [23,24] demonstrate a nonmonotonic dependence of conductivity σ on shock pressure P . Meanwhile, the dependence σ (P ) for the aluminium powder with density 0.5 g cm −3 [23,24] can be approximated by the constant conductivity ≈ 10 4 cm −1 .…”

“…The data for aluminium powders [23,24] demonstrate a nonmonotonic dependence of conductivity σ on shock pressure P . Meanwhile, the dependence σ (P ) for the aluminium powder with density 0.5 g cm −3 [23,24] can be approximated by the constant conductivity ≈ 10 4 cm −1 . Such a simplification is taken in the initial analysis for finding the effect of conductivity on magnetic cumulation.…”

“…As a result, a satisfactory description of the state of highporosity matter is obtained (in contrast to the Mie-Grüneisen equation of state using extrapolation with respect to pressure). Figure 4 shows the shock-wave data obtained for low-density aluminium powders [22,23] and predictions of the Oh-Persson model. As seen from the figure, the model gives results that are close to the experimental data in this area of the state of matter.…”

“…The present MHD model uses several assumptions on matter conductivity under shock compression. First, the particle size a (responsible for the thickness of the shock-transition zone) is smaller than the thickness of the electromagnetic skin layer in the substance metallized under shock compression a < 1/µ 0 σ (D − u) [23], where σ is the conductivity. Under such a condition, the diffusion time of the magnetic field in the shock-transition zone is smaller than the convection time.…”

“…Under such a condition, the diffusion time of the magnetic field in the shock-transition zone is smaller than the convection time. Therefore, the phase of conductivity origin within the transition zone does not affect the electromagnetic field behind the zone [23]; the magnetic field is continuous through the shock-transition zone. Second, the powder conductivity appears in a stepwise manner in the shock wave and further remains constant.…”