Investigation of phase transformations in iron and cerium using a polyvinylidene fluoride pressure gauge

Abstract

This paper presents results of studies of shock-induced phase transformations in iron (polymorphic α-ε transition) and cerium (isomorphic γ-α transition) using a polyvinylidene fluoride pressure gauge.Key words: shock wave, phase transformation, iron, cerium, polyvinylidene fluoride pressure gauge.Shock wave propagation in some solids leads to phase transformations that change their crystal structure. The formation of new crystal modifications over short time intervals (≈10 −7 sec) is one of the most interesting topics in shock wave physics. To obtain a realistic picture of the phenomenon, it is necessary to know the transformation kinetics [1].Polymorphic transformation under shock-wave compression was first detected by a kink in the shock adiabat of iron at a pressure p ≈ 13 GPa (α-ε transition) [2]. The existence of this transition was then supported by an electric resistance jump under static compression at p = 13.3 GPa and at room temperature [3]. The emergence of methods for continuous recording of shock profiles in material samples has simplified the detection of phase transitions in both shock waves and rarefaction waves. This is done, as a rule, using Manganin gauges (pressure measurements) and laser interferometry (mass velocity measurements) [4]. The use of a dynamic pressure gauge based on the ferroelectric polymer polyvinylidene fluoride (PVDF) 10-30 μm thick with a time resolution of ≈10 −9 sec [5-7] will probably allow one to record not only phase transformations but also information on their kinetics. This assumption is based on the fact that the gauge output current is proportional to the time derivative of pressure [5][6][7]. This parameter can be more sensitive to the content of the new phase in the material than the pressure or free-surface velocity.In the present paper, we report results of a study of two metals -iron and cerium. The first of them has been studied fairly well [4,8] and is used here as the reference material. The second metal (cerium) possesses unusual thermodynamic properties in the vicinity of the isomorphic γ-α phase transition under statistic conditions [9]. The volume jump due to this transition is anomalously large -16.5% [9]. We are aware of only one paper on the phase transformation in shock compressed cerium [10].The experiments with iron and cerium were performed similarly. A sample of the material tested was placed between the plate of an explosive plane shock wave loading device and a fluoroplastic disk 10 mm thick. A PVDF gauge 20 μm thick with a working area of 4 mm 2 was placed at the sample-fluoroplastic interface. A shock wave in the plate was produced by TNT explosion products (loading through an air gap of 5 mm). The plate consisted of several layers of materials with different acoustic rigidities. The required characteristics of the shock wave were obtained by choosing the number and materials of the layers and the thicknesses and heights of the explosive charge. The gauge signal was recorded by a TDS 5052 digital oscillograph. The measuring lines (RK-50-9-...

Study of phase transitions in cerium in shock-wave experiments

“…The data of time dependence on pressure profiles with the use of x(t) diagrams and the D(u) relation for cerium [5] allow the definition of the Lagrangian velocity of the unloading wave C Lagr and the Eulerian velocity C Eul by taking into account the compressibility σ . Obtained experimental data are presented in Table 1 and Table 2.…”

Application of polyvinylidene fluoride for pressure measurements in an underwater explosion of aluminized explosives

“…Polyvinylidene fluoride (PVDF) is a piezoelectric polymer, which has been widely used in shock/impact pressure measurements [5,6]. It could be processed into many complicated shapes to make thin pressure transducers with a nanosecond time response.…”