F. Bauer

Influence of composition on relaxor ferroelectric and electromechanical properties of poly(vinylidene fluoride-trifluoroethylene- chlorofluoroethylene)

We investigate the influence of the composition, especially the chlorofluoroethylene content, on the ferroelectric and electromechanical properties of the poly(vinylidene fluoride-trifluoroethylene- chlorofluoroethylene) (PVDF-TrFE-CFE). It was found that increasing the CFE from 0to9mol% gradually converts the normal ferroelectric of the copolymer (in the compositions range of VDF/TrFE mole ratio between 64∕36 to 75∕25) to a relaxor ferroelectric, resulting in a nearly hysteresis free polarization loop and high electrostrictive response. On the other hand, increasing CFE content causes reduction in crystallinity, which will affect the elastic modulus and the induced polarization level of the polymer. These competing effects determine the desired terpolymer compositions for given applications. The electromechanical strain as a function of induced polarization and macropolar phase fraction was modeled by a modified electrostrictive relation, which closely matches experimental data. It is found that the elec...

PVDF shock sensors: applications to polar materials and high explosives

Ferroelectric polymers (PVDF) with well-defined and precisely known electrical properties are now routinely available from commercial sources. Electrical processing with the Bauer cyclic poling method can produce individual films with well-defined remanent polarization up to 9 /spl mu/C/cm/sup 2/. These polymers provide an unusual opportunity to study the structure and physical properties of materials subjected to shock loading. The behavior of PVDF has been studied over a wide range of pressures using high-pressure shock loading and has yielded well-behaved, reproducible data up to 25 GPa in inert materials. The application of PVDF gauges for recording shock waves induced in polar materials such as Kel-F, PMMA, or in reactive materials is hampered by observations of anomalous responses due to shock-induced polarization or an electrical charge released inside a shock-compressed explosive. A solution using an appropriate electrical shielding has been identified and applied to PVDF for shock measurement studies of Kel-F, and for Hugoniot measurements of high explosives (PH). Furthermore, shock pressure profiles obtained with in situ PVDF gauges in porous HE (Formex) in a detonation regime have been achieved. Typical results of shock pressure profile versus time show a fast superpressure of a few nanoseconds followed by a pressure release down to a plateau level and then by a pressure decay. More accurate measurements are reported with electrically improved PVDF gauges as well as with 0.25 mm/sup 2/ active area PVDF gauges.

Shock profile induced by short laser pulses

Standard 25‐μm‐thick polyvinilydene fluoride (PVDF) piezoelectric gauges and new 450‐μm‐thick P(VDF 70%, TrFE 30%) piezoelectric copolymer have been used to record shock profiles at the back face of metallic targets irradiated by laser pulses of 2.5 and 0.6 ns duration at a 1.06 μm wavelength. The records are fully explained with simplified space–time diagram analysis. The pressure profile applied at the front face of the target has been determined from these records combined with numerical simulations of wave propagation through the target. A numerical code describing the interaction of laser with matter (FILM) has also been used for computing the applied pressure. Both methods lead to very close results. The peak pressure dependence on incident laser intensity is determined up to 30 GPa at 1012 W/cm2.

Properties of ferroelectric polymers under high pressure and shock loading

Abstract Ferroelectric polymers are the most recent class piezoelectric and pyroelectric materials developed. The most common piezoelectric polymers are PVDF, based on the monomer CH 2 CF 2 and copolymers PVDF with C 2 F 3 H. The effects of frequency, temperature and hydrostatic pressure on the dielectric properties, molecular relaxations, and phase transitions of PVDF and a copolymer with 30% trifluoroethylene will be discussed. Pressure causes large slowing down of the β molecular relaxations as well as large increases in the ferroelectric transition temperatures and melting points, but the magnitudes of the effects are different for the different “transitions”. A unique application of these polymers as time-resolved dynamic stress gauges based on PVDF studies under very high pressure shock compression is discussed. In particular piezoelectric response of shock compressed PVDF film prepared with attention to mechanical and electrical processing exhibits precise, well defined reproducible behavior to 35 GPa. P(VDF-TrFE) copolymers exhibit unique piezoelectric properties over a wide range of temperature depending on the compositions. Under high shock pressure loading, unique piezoelectric response is also observed. The first record of detonation profile is presented. Results of PVDF shock sensors subjected to X-ray deposition and to neutron fluence above 10 13 n/cm 2 while stressed at a peak level of 2 GPa will be also discussed.

Relaxor fluorinated polymers: Novel applications and recent developments

It has been found that by introducing defects into the P(VDF-TrFE) copolymers, it is possible to convert the polymer from a normal ferroelectric to a relaxor ferroelectric. A new class of ferroelectric polymers, i.e., the terpolymers of P(VDF-TrFE-CFE) or of P(VDF-TrFE- CTFE), was developed from the normal ferroelectric PVDF-TrFE polymer by employing proper defect modifications which eliminate detrimental effects associated with a normal first order F-P transition while maintaining high material responses. Relevant studies show that this class of electroactive polymers offers unique properties in comparison with other ferroelectric polymers. The syntheses of these relaxor ferroelectric polymers have been done by a combination of the suspension polymerization process and an oxygen-activated initiator at a temperature of 40 °C. Films from cast solution can be made in different length and thicknesses. Stretching of these films increases the performance as well as the mechanical properties. These relaxor-ferroelectric terpolymers P(VDF-TrFE-CFE), P(VDF-TrFE-CTFE) are multifunctional i.e. electrostrictive material, dielectric for electric energy storage. The terpolymer exhibits high electrostrictive strain (>7%) with relatively high modulus (>0.4GPa) and high electrocalorific effect. Dielectric polymers with high dipole density have the potential to achieve very high energy density. Examples of devices applications using unimorphe systems are presented. Micropump and Optical device concerning a liquid-filled varifocal lens on a chip are described.

PVDF Shock Compression Sensors in Shock Wave Physics

Early works have shown that highly reproducible piezoelectric film PVDF (Poly(vinylidene fluoride)) can be reliably used in a wide range of precise stress and stress‐rate measurements. The direct stress‐derivative or stress‐rate PVDF signals have nanosecond resolution and higher operating stress limits than any other technique. PVDF stress gauges have been used in many fields of shock wave physics. The present paper summarizes some of original applications of the PVDF gauges. Blast and shock in air measurements will be presented. Pressure responses of inert materials and polymer‐materials will be recalled. Furthermore, example of pressure and particle velocity histories using PVDF and laser interferometry (VISAR) will be presented. Simultaneous measurements using VISAR and PVDF gauge will be discussed. The question of the validity of shock pressure profiles obtained with “in situ” PVDF gauges in one High Explosive in a detonation regime will be discussed.

Characterization of laser‐driven shocks of high intensity using piezoelectric polymers

Piezoelectric elements made of polyvinylidene‐fluoride‐trifluoroethylene copolymer P(VDF/TrFE) have been used to measure the pressure induced on the rear surface of metallic foils irradiated by infra‐red laser pulses of 1.5 ns duration, with intensities up to 3 TW/cm2. The application of such piezoelectric materials under those conditions is new, and a special effort has been made for their improvement and characterization under plate impact loading. Then, the laser experiments have been carried out. First, peak pressures of 0.5–7.5 GPa transmitted in the copolymer have been derived from the peak voltage measured at each shot, using simple assumptions. Next, a more accurate analysis of the data, involving computer simulations, has provided the pressure profiles characterizing the laser shocks driven on the front surface of the targets, over a wide range of laser intensities. Peak pressures of 7–60 GPa on the irradiated surface have been inferred from that analysis. The results have been compared to values...

PVDF Gauge Piezoelectric Response under Two‐Stage Light Gas Gun Impact Loading

Stress gauges based on ferroelectric polymer (PVDF) studies under very high pressure shock compression have shown that the piezoelectric response exhibits a precise reproducible behavior up to 25 GPa. Shock pressure profiles obtained with “in situ” PVDF gauges in porous H.E. (Formex) in a detonation regime have been achieved. Observations of a fast superpressure of a few nanoseconds followed by a pressure release have raised the question of the loading path dependence of the piezoelectric response of PVDF at high shock pressure levels. Consequently, studies of the piezoelectric behavior of PVDF gauges under impact loading using a two‐stage light gas gun have been conducted recently. Symmetric impact as well as non symmetric impact and reverse impact techniques have been achieved. Strong viscoplastic behavior of some materials is observed. In typical experiments, the piezoelectric response of PVDF at shock equilibrium could be determined. These results show that the PVDF response appears independent of the...

Piezoelectric P(VDF-TrFE) thick copolymers: Response in current mode under shock loading

The most common piezoelectric polymers are PVDF, and copolymers PVDF with C2F3H. A unique application of these polymers are time-resolved dynamic stress gauges based on PVDF studies under shock loading. P(VDF-TrFE) copolymers exhibit unique piezoelectric properties over a wide range of temperature depending on the compositions. The materials may be formed directly from a melt, but their piezoelectric properties are strongly dependent on the annealing technique as well as on the poling process. Copolymer plates of 240, 500 μm in thickness have been fabricated. On the plates, small areas have been poled using the ISL cyclic process. Under shock loading up to 14 GPa, unique piezoelectric response is observed. Experimental results obtained in current mode are presented and discussed.

Ferroelectric polarization of PVF 2 and VF 2 /C 2 F 3 H copolymers: Piezoelectric properties under dynamic pressure and shock loading

Polyvinylidene fluoride PVF<inf>2</inf> and VF<inf>2</inf>/C<inf>2</inf>F<inf>3</inf>H copolytners present strong piezoelectric activities under specific conditions of crystallization and polarization. The piezoelectric activity of PVF<inf>2</inf> depends on the polarization process as it is presented in the hysteresis loops P(E) (P polarization, E electrical field). The remanent polarization attains 10 p.C/cm² for biaxially stretched PVF<inf>2</inf> films. Results obtained on VF<inf>2</inf>/C<inf>2</inf>F<inf>3</inf>H copolymers are also reported. Properties of these well clean and uniform poled ferroelectric materials, under static, dynamic pressure and shock wave action are presented and discussed. These organic piezoelectric materials are strong candidates for shock gauges and new transducers.