S. D. Gilev

Detonation Properties and Electrical Conductivity of Explosive–Metal Additive Mixtures

Detonation properties of mixtures of condensed high explosives with metal additives are studied. A scheme of measurement of high electrical conductivity of detonation products (σ > 10 Ω−1 · cm−1) with a time resolution of ∼ 10 nsec is developed. It is shown that the properties of detonation products depend significantly on the content of the additive in the HE and on dispersion and density of the mixture. The electrical conductivity of detonation products of the compositions examined reaches ∼ 5 · 103 Ω−1 · cm−1, which is more than three orders higher than the electrical conductivity of the HE without the additive. Significant variation of electrical conductivity of detonation products over the conducting region thickness has been found. The main conductivity corresponds to a sector ∼ 1 mm long near the detonation front. The overdriven state of the detonation wave has a strong effect on electrical conductivity and conducting region thickness. It is assumed that the behavior of electrical conductivity with time is caused by successive processes of shock compression of the HE, excitation of the chemical reaction (including the reaction of the additive with detonation products), and expansion of detonation products. The measurement technique used is highly informative due to the possibility of studying detonation in various regimes.

Electrical conductivity of copper powders under shock compression

The electrical conductivity of some compositions based on the copper powder under shock compression is measured. The commercial copper powder, mixtures of copper with glass microspheres, and copper-aluminum mixtures are studied. The electrical conductivity is measured by the author’s electrocontact technique, which allows the insulator-metal transition to be measured. Dependences of the electrical conductivity of powders on the shock wave pressure are obtained. As for aluminum powders examined previously, this dependence for the copper powder has a nonmonotonic character. The maximum electrical conductivity is ≈ 9·104 Ω−1 · cm−1. With a further increase in the shock pressure, the electrical conductivity decreases approximately by an order of magnitude, which is explained by intense temperature heating. The results for the electrical conductivity at high shock pressures is qualitatively consistent with known broad-range models of conductivity proposed by Garanin and Bakulin.

High electrical conductivity of trotyl detonation products

A new measurement scheme makes it possible to study the conductivity of detonation products of condensed explosives with a time resolution of about 10 ns. Experiments with cast trotyl show that conduction under detonation is a complex phenomenon associated with the chemical reaction zone and the expansion of the reaction products. The time variation of the electrical conductivity has a sharp peak (≈250 Ω−1 cm−1) and a plateau (≈35 Ω−1 cm−1). The peak corresponds to the highest conductivity value that has been ever observed for the products of chemical explosive detonation. The results support the validity of the contact method for measuring the detonation conductivity of trotyl.

Measurement of electrical conductivity of condensed substances in shock waves (Review)

Available experimental techniques of electrical conductivity measurements under strong shock compression are analyzed. Dielectric-semiconductor, dielectric (semiconductor)-metal, and metal-metal (semiconductor) transitions are considered. Methods and schemes of contact and contactless measurements in inert and electrically active media, implemented by various authors, are discussed. In-depth analysis of measurement circuits, two-dimensional and three-dimensional modeling of currents, fields, and hydrodynamic flows, passing from the electric engineering model to the field electromagnetic model, and allowance for transitional electrodynamic processes have contributed to the significant recent improvement of the time resolution and to extending the range of conductivity registration under shock compression. A typical feature of new techniques is solving a differential equation for the electrical circuit or finding electrical conductivity by solving an inverse boundary-value problem for the magnetic diffusion equation. In particular, the problem of electrical conductivity registration on dielectric (semiconductor) — metal transitions, which has been known since the 1950s, is solved in this manner. Difficulties, constraints, and unsolved problems of experimental techniques are discussed.

Electromagnetic Field and Current Waves in a Conductor Compressed by a Shock Wave in a Magnetic Field

A physically correct and mathematically rigorous solution of the problem on the structure of an electromagnetic field formed when a shock wave enters a conducting half–space in a transverse magnetic field is obtained. It is shown that only physically grounded boundary conditions lead to a noncontrovercial pattern of the electromagnetic field and a system of currents in a conductor. The main parameters and characteristic times are found, which determine the structure of current waves in a metal. The solution in the uncompressed region is determined by the parameter R1 = µ0σ1D2t and that in the compressed region by the parameter R2 = µ0σ2(D—U)2t (σ1 and σ2 are the electric conductivities of the uncompressed and compressed substance, respectively, µ0 is the magnetic permeability of vacuum, D is the wave–front velocity, U is the mass velocity, and t is the time). The parameter for the compressed substance R2 coincides with the parameter obtained previously for the shock–wave dielectric—metal transition; the governing parameter for the uncompressed substance R1 is obtained for the first time. The asymptotic solutions of the problem for small and large times and the special case R1 = R2 considered help in understanding the physical meaning of the solution found.

Semiconductor-metal transition in selenium under shock compression

Phase transitions in selenium are studied by time-resolved measurements of the electrical conductivity under shock compression at a pressure of up to 32 GPa. The pressure dependence of the electrical conductivity (σ(P)) has two portions: a sharp increase at P < 21 GPa and a plateau at P > 21 GPa. The experimental data and the temperature estimates indicate that, at P < 21 GPa, selenium is in the semiconductor state. The energy gap of semiconducting selenium decreases substantially under compression. At P > 21 GPa, the electrical conductivity saturates at ∼104 Ω−1 cm−1. Such a high value of the electrical conductivity shows the effective semiconductor-metal transition taking place in shock-compressed selenium. Experiments with samples having different initial densities demonstrate the effect of temperature on the phase transition. For example, powdered selenium experiences the transition at a lower shock pressure than solid selenium. Comparison of the temperature estimates with the phase diagram of selenium shows that powdered selenium metallizes in a shock wave as a result of melting. The most plausible mechanism behind the shock-induced semiconductor-metal transition in solid selenium is melting or the transition in the solid phase. Under shock compression, the metallic phase arises without a noticeable time delay. After relief, the metallic phase persists for a time, delaying the reverse transition.

Application of the Electromagnetic Model for Diagnosing Shock–Wave Processes in Metals

To verify the electromagnetic model of shock compression of a conductor in an electromagnetic field, shock–wave experiments with Constantan are performed. The test results show that the electromagnetic model gives a qualitatively correct description of the phenomenon. Some disagreement between the numerical and experimental data may be caused by factors ignored in the model (a finite thickness of the shock–wave front and the nonuniformity of the shock wave and electromagnetic field in the measurement cell). Electric conductivity of Constantan is determined experimentally under conditions of single shock compression. These studies justify the electromagnetic model of shock compression of metal in a magnetic field and form the basis for development of new techniques for dynamic experiments.

Generation of magnetic field by detonation waves

The generation of a magnetic field by a system of detonation waves in a condensed explosive is reported. The convergence of the detonation waves, which exhibit a high conductivity in the chemical reaction zone, increases the magnetic field at the axis of the system. The fact of magnetic field generation is demonstrated experimentally. Features of the new method of magnetic cumulation are discussed. A simple compression model that qualitatively agrees with experimental data is proposed.

Current Waves Generated by Detonation of an Explosive in a Magnetic Field

The structure of the electromagnetic field in detonation of a condensed explosive in a magnetic field is analyzed qualitatively. Propagation of a detonation wave in a magnetic field leads to generation of an electric current in explosion products. The physical reason for current generation is the “freezing” of the magnetic field into the conducting substance at the detonation front and subsequent extension of the substance and the field in the unloading wave. The structure of the current layer depends on the character of the boundary magnetic fields and conditions on the surface of initiation of the explosive. Detonation of the explosive in an external magnetic field B0 generates a system of two currents identical in magnitude but opposite in direction. The structure of the arising current and its absolute value are determined by the parameter R1 = μ0 σ0D2t (μ0 is the magnetic permeability of vacuum, σ0 is the electrical conductivity of detonation products, D is the detonation‐front velocity, and t is the time). The value of the current increases with the detonation‐wave motion, and the linear current density is limited from above by 2B0/μ0. For R1 ≫ 1, the electric field in the conducting layer is significantly nonuniform; for detonation products with a polytropic equation of state, a region of a constant‐density current is adjacent to thedetonation front. The results of this analysis are important for interpretation of experiments performed and development of new methods for studying the state of the substance in the detonation wave.

Electrical resistance of copper under shock compression: Experimental data

The electrical resistance of copper foil under shock compression is measured. The electrical resistance and electrical conductivity are plotted as functions of the shock pressure in the interval up to 20 GPa. These dependences are monotonic and have no visible inflections or singularities. A qualitative dependence of the electrical resistance of the metal on the shock impedance of the material of the block containing the sample is found. A comparison of the data obtained in this study with results of other authors shows that it is important to take into account the block material, the shape and thickness of the sample, and the procedure of determining the state of the sample.