Pressure Wave Propagation Method Research Articles

Polarization of thick polyvinylidene fluoride/trifluoroethylene copolymer films

Thick (800 μm) polyvinylidene fluoride/trifluoroethylene (P(VDF-TrFE)) copolymer films for transducer applications are poled under applied voltage at elevated temperatures. By using different heat treatments, poling temperatures, and poling time, we are able to prepare a uniformly poled film with a single resonance peak at 1.3 MHz, or a nonuniformly poled film with two resonances (1.3 and 2.6 MHz), or a film with bimorph structure with a single resonance at 2.6 MHz. The nonuniform polarization which arises from charge injection from the cathode is checked by the pressure wave propagation method. The polarization mechanisms in these thick films are expected to be similar to those previously reported for thin films. The results obtained in this work may lead to practical applications because they suggest a means for controlling transducer frequency by poling.

Effect of wavelength on pressure wave propagation measurements of thick VF2/VF3 copolymer films

Abstract Using the infrared pulses from a Nd-YAG laser we have recently studied the non-uniform polarization distribution in thick (∼ 1 mm) VF2/VF3 copolymer films by the pressure wave propagation method. In the present pressure wave propagation experiments, shorter wavelength harmonics (532, 355, 266 nm) with narrower pulse widths are generated by a harmonic generator, and the results obtained for a partially poled thick VF2/VF3 copolymer film are reported.

Space charge formation and relaxation in ion-bombarded poly(imide) Kapton films

The space charge formation and relaxation in poly(imide) Kapton thin films, deep-implanted with heavy ions in the MeV amu −1 range, have been studied by on-line measurements, with the pressure wave propagation method. The measured space charge depth profiles correspond to the damage depth profiles due to the nuclear collisions. The space charge buildup in the bulk thus seems to be linked to some end-of-range defects acting as trapping sites. Ionization-induced defects seem to play a negligible role in the formation of the bulk space charge.

Space charge and anomalous discharge currents in crosslinked polyethylene

Transient currents in cross-linked polyethylene (XLPE) have been investigated by using different parameters: temperature T, electrical field E/sub p/ and hydrostatic pressure P. Anomalous discharging currents flowing in the same direction as charging currents were observed from T=30 to 70/spl deg/C, P=0.1 to 30 MPa and E/sub p/=13 to 33 kV/mm. The space charge was measured using the pressure wave propagation method (PWP). Theoretical calculation revealed that this anomalous behavior can result from charge migration under a partial blocking condition at the electrodes. We discuss theoretical results which are in reasonable agreement with our discharge current measurements.

Evidence of strong correlation between space-charge buildup and breakdown in cable insulation

Many processes have been considered over the years to explain the origin of breakdown in cable insulation. Such effects as space charge build-up, tree growth, charge injection, etc. have all been discussed. Various techniques are now available to measure, in a nondestructive way, space charge distributions in insulators. These techniques, for instance the pressure wave propagation (PWP) method, can be used under applied electric stress and thus make it possible to follow the development of space charge in selected regions of the insulators. In this paper we present new evidence linking space charge buildup, tree growth and breakdown in XLPE. We have used the PWP method to monitor the charge distribution as a function of time under dc stress in high insulating thickness cable. We show that for certain insulation systems the space charge buildup can increases the local field to a value which is more than 8/spl times/ the applied electric field, leading to breakdown. Post-mortem analysis followed by optical microscopy shows the presence of electrical trees, the breakdown channel being centered on one of them. The study of space charge evolution in practical insulations permits an understanding of the role of space charge in dc breakdowns. This understanding enables the development of technologies to suppress this effect and hence realize practical dc XLPE transmission cables.

Space charges of different mobilities: A novel data processing method

A new way to process the information obtained in space-charge distribution measurements is presented. It is applied to data given by the pressure wave propagation method. This method can be carried out with or without an applied voltage. It is shown however that it does not always allow, under dc stress, for quantitative evaluation of the charge density, if the material contains space charges of very different mobilities, and particularly of high mobility. The new processing technique described here overcomes this difficulty. It is illustrated by application to cross-linked polyethylene samples. This permits the discrimination of two different kinds of space charge. One is the normal space charge which has been reported many times, the second is a type that displays a high mobility. This charge is not directly visible even at very short times after the suppression of the applied voltage. In these measurements, this very mobile space-charge is always seen as heterocharge near the anode.

Polarization of VDF-TrFE copolymer films at elevated temperature

The buildup of the polarization of VDF-TrFE copolymer films, at room temperature and after various thermal treatments, is studied using the PWP (pressure wave propagation) method. This method allows the nondestructive measurement of the homogeneity of the piezoelectric coefficient. The samples are polarized at 100 degrees C, and the influence of different thermal treatments before polarization, and of the insulator-electrode interfaces, are analyzed. The behavior of the copolymer is compared to that described previously for poling experiments made at room temperature. The effect of electron injection at the cathode by nonblocking electrodes is clearly shown.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A novel method for studying a photorefractive grating

Abstract Photorefractive gratings can be induced by laser illumination. They have been observed so far using optical methods. In this paper we report the direct measurement of the space charge distribution associated with such a grating, using the pressure wave propagation method. The wavefront of a pressure wave travelling through the sample acts as a virtual probe sensitive to electrical charge or field. The photorefractive grating was produced by means of a spatial distribution of linear polarization of laser light.

Application of the pressure wave propagation method to the study of interfacial effects in e-irradiated polymer films

The pressure wave propagation method is applied here to the study of charging and discharging processes of polymer films of spatial interest. The irradiation of the materials is performed by a monoenergetic electron beam in the range 5–50 keV. Since the irradiated surface of the sample is free, we extended the model of the induced signals, previously developed, to take into account the total reflection of the pressure at the polymer-vacuum interface. All the experiments, irradiations and measurements, are operated in a vacuum environment. After the description of the experimental setup, typical signals showing the interfacial effects are discussed. They concern the behavior of the sample-vacuum interface and charge injection at the target-sample interface. These experiments, which were carried out both on metallized and nonmetallized samples bonded to the target electrode, allow a direct observation of charge injection at interfaces.

Observation of charge behavior in organic photoconductor using pressure-wave propagation method

The charge behavior in an organic photoconductor (OPC), which can be used as an electrophotographic receptor, was observed using the pressure wave propagation method. When a pulse pressure wave passes through the sample, a displacement current is generated. The charge distribution is obtained from the measurement of the displacement current. Two kinds of samples were prepared in this study. One was an aggregate OPC which consisted of one layer (sample A), the other was a layered OPC which consisted of the carrier generation layer (CGL) and the carrier transport layer (CTL) (sample B). The motions of charges in the two samples were observed under the application of DC voltage and of light exposure. In the case of aggregate OPC, the carriers were generated with light exposure and both hole and electron carriers drifted under the applied electric field, whereas in the case of the layered OPC, the carriers were generated on the CGL by the electric field only and one polarity of carriers, holes, drifted in the CTL. >

Comparison of polarization distribution measurement by the LIMM and PWP methods

Two different methods of measurement of polarization distributions in piezoelectric materials, the laser intensity modulation method (LIMM) and the pressure wave propagation (PWP) method, are compared. The two techniques are described, and measurements made on different PVDF-based samples, one with a symmetric and the other one with an antisymmetric structure, are reported. Although the LIMM responds to diffusion of heat and the PWP method to the propagation of a pressure wave, the direct observation of the raw data gives similar information in both cases as far as the symmetry of the polarization distribution is concerned. The specific characteristics of each method are analyzed. >

Direct observation of space charge evolution in e-irradiated Kapton films

The pressure wave propagation method was applied to analyze in a vacuum the evolution of the space charge in electron-irradiated Kapton H films. The 125 mu m thick films were irradiated in a vacuum chamber with a monoenergetic electron beam in the 10 to 50 keV energy range. The method allows direct observation of the depth of penetration of electrons and first results on Kapton are presented. The space charge generated in the irradiated and nonirradiated regions was observed during and after the irradiation process. The space charge is influenced by electric field-assisted detrapping of incident electrons and their interaction with the lattice as they drift. Interfacial effects are indicated by the buildup of a negative charge layer at the metal-polymer interface.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurement of Charge Distributions in Organic Photoconductor Using Pulse Ultrasonic Wave

A pressure wave propagation method using pulse ultrasonic wave was used to investigate the charge behavior in an organic photoconductor (OPC) used as an electrophotographic receptor. Pulse ultrasonic waves were generated by applying a high-voltage pulse to a piezoelectric transducer of LiNbO3. The charge distributions in OPC were measured under the conditions of dc voltage application and light exposure. The behavior of the charges in OPC was clearly observed.

Electrical properties of electrode/polyethylene/electrode structures

The pressure-wave propagation (PWP) method allows for the nondestructive measurement of charge distributions in dielectric materials. This method has been used to study the electrical properties of electrode/polyethylene/electrode structures such as those involved in high-voltage cables. The influence of the composition of the insulating resin itself and that of the electrodes are analyzed in polyethylene samples. According to the chosen combination, charge transfer at the interfaces, migration of ionizable impurities, or a strong decrease of both is observed. This application of the PWP method is of particular interest, since it allows for a suitable choice of the materials and structures involved in insulator/conductor interfaces. >

The Pressure wave Propagation Metod for the Analysis of Insulating Materials: Application to LDPE Used in HV Cables

In the field of electrical insulation, polymers are progressively replacing formerly used materials. When such an insulator is subjected to a voltage gradient, an internal space charge and a polarization develop, eventually reducing the apparent electrical strength of the material. We have applied the pressure wave propagation (PWP) method to the study of HV insulators, such as low density polyethylene (LDPE) used for HV cables. The spatial charge distribution is successively measured in samples subjected to various processes. The influence of additives in the development of a space charge under electric stress is directly shown. We have also observed that the nature of the electrode-iinsulator interface strongly affects the charge distribution throughout the insulator, and that charge injection from these interfaces occurs at much lower applied voltage gradients than previously assumed. The evolution in time of the charge distribution in samples submitted to voltage gradients, at 20 and 700C, is studied. These results show that an equilibrium is reached, from which the life expectancy of materials and cables can be evaluated.

New nondestructive method for the investigation of insulator-semiconductor structures

The pressure wave propagation (PWP) method, originally designed to study the electrical properties of polymers, is used to determine the distribution of charges in metal-insulator-silicon and metal-oxide-silicon structures. This method is nondestructive and fast. The electric field distribution in a sample is derived from the signal induced by the propagation of a pressure wave with a short rise time in the structure. As a new application of this method, we demonstrate that it is possible to observe the evolution of the surface states of silicon during ambient oxidation and to reveal the charge distribution in a metal-glass-silicon structure.