Pressure Wave Propagation Method Research Articles

Space charge measurement at solid/liquid interface by PWP method

The presence of electric charges within liquid is relatively unexplored despite its role in electrochemical energy storage and industrial safety. Quantifying charge distribution in liquids and at solid/liquid interfaces is therefore crucial to the efficiency and longevity of devices. Over time, metrologies using stimulus to measure space charges in solids have been transposed to liquids. The stimulus disturbs electrostatic equilibrium to induces electrical response. This study builds upon measuring electric charges in the Electrical Double Layer (EDL) using the Pressure-Wave-Propagation (PWP) method. It presents how the pressure wave amplitude, the liquid conductivity and the liquid nature impact the electrical response.

Migration behavior of space charge packet researched by using electron beam irradiation and real-time space charge distribution measurement in piezo-pressure wave propagation (PWP) method

The space charge packet is a special sort of space charge phenomenon that is characterized by the migration of aggregated space charge in the form of a packet in the sample. Currently, most of the experimental and simulation studies on the generation and migration of space charge packets focus on the space charge packets with positive polarity in polyethylene and their cross-linking products, while the characteristics of the space charge packets with negative polarity still need studying. This paper presents an integrated experimental system for studying space charge packet with negative polarity, which enables experimental studies combining electron beam irradiation technique and real-time space charge distribution measurement. The beam flux for electron beam irradiation is controlled by a metal grid with an optical fiber-electric relay, and the space charge distribution measurement is performed by the piezo- pressure wave propagation method. The system achieves a withstand voltage value of 17 kV and a measuring resolution of 25 ns for space charge distribution measurement and is suitable for experimental studies of various material samples with different thickness ranges under different electric fields. In this work, the migration behaviors of space charge packets with negative polarity in polypropylene (PP) and polymethyl methacrylate (PMMA) samples under different applied electric fields (15, 20, 25, 30 kV/mm) are studied by using the experimental system. The relationship between the migration properties of carriers with negative polarity (electrons) and the electric field can be extracted from the experimental results. The “negative differential mobility” phenomenon is found for both materials, i.e. the migration rate decreases with the increase of the electric field. The threshold electric field for the “negative differential mobility” phenomenon of electrons in PP sample is about 26.0 kV/mm while the threshold electric field for the “negative differential mobility” phenomenon of electrons in PMMA sample is 19.5 kV/mm, and the phenomenon disappears at an electric field of 27.5 kV/mm. The electric field where the "negative differential mobility" phenomenon of electrons appears and disappears in different materials can be extracted by using the experimental system proposed in this paper.

Space Charge Accumulation in Linear Low-Density Polyethylene with Surface Fluorination of the Semiconductive Electrode

Several fluorinated polyethylene surface studies have proved that the fluorination reaction can effectively prevent charge from being injected into polyethylene. Therefore, this paper explores the influence of fluorinated electrode on space charge behavior in polyethylene. In this study, the space charge distribution inside polyethylene is measured by laser-PWP (Pressure Wave Propagation) method while altering electrode fluorination duration and fluorination temperature. The experimental results show that measured homocharge injection is increased in polyethylene to which a fluorinated semiconductive electrode is heat pressed. Attenuated Total Reflectance-Infrared (ATR-IR) spectroscopy and Scanning Electron Microscope (SEM) are also performed, revealing that the fluorinated layer of semiconductive electrode on the interface between polyethylene and electrode block impurities from entering polyethylene. The measured space charge distribution, being the planar average amount, also reveals impurity generation since heterocharge formation is observed. As such impurities are blocked, in the meantime the electrodes' homocharge injection remains un-affected, thereby the measured apparent charge is greatly increased.

Space charge effect and application in solid insulation dielectrics

<p indent="0mm">Space charge effect and its application in solid insulation dielectrics are long-term issues with continuous development. The study of space charge effect is of great significance to high-voltage direct current (HVDC) transmission. This paper summarizes the development history and research progress of space charges in solid dielectrics, from the aspects of measurement technologies, damage, suppression, application, and simulation, etc. Currently, the most widely used space charge distribution measurement technologies can be roughly divided into two categories, acoustic effect technique and thermal effect technique, whose spatial resolution and application range are different. For the acoustic effect technique, pressure wave propagation method and pulsed electro-acoustic method are most commonly used with proven technique. The spatial resolutions of these two are similar, both can reach an average of <sc>10 μm.</sc> With extremely narrow pulses, the highest spatial resolution can reach about <sc>1 μm.</sc> The thermal effect techniques, such as thermal pulse method and laser intensity modulation method, have more advantages in spatial resolution and practicability, especially for micron-thick film detection. Future research may focus on the more widely application of space charge distribution measurement technologies to electrical equipment in operation. For the trap parameters and charge storage properties of insulation dielectrics, there is still a lack of reliable and practical measurement technologies. The results of the isothermal decay method are more reliable and credible, but the required measurement time is too long. Therefore, it has been rarely used in practice in recent years. The most commonly used methods, thermal stimulated discharge current method and photo stimulated discharge method, each has its own disadvantages. And their results for the same sample are quite different for mutual corroboration. In recent years, some researchers are more focused on piezo-stimulated current method, but this method is still in a preliminary stage of experiment and application. In general, the space charge effect has pros and cons. On one hand, space charge accumulation in solid insulation dielectrics may cause electric field distortion and discharge damage, eventually lead to breakdown and destruction of insulation. In order to prevent these damages, many effective methods have been proposed, such as surface modification of insulation dielectrics and material modification by additives. However, these methods have not yet been practically used in industrial applications. On the other hand, space charge effect also has many efficient applications. One of the most important applications is electret, a dielectric material that quasi-permanently stores real charges in surface or volume traps. A large number of significant contributions have been published on preparation process, charge characteristics and applications of electrets. The simulation research of space charges can mainly be divided into physical model research of space charge behavior and data processing of experimental results. The development of space charge simulation is relatively backward compared to other research fields as the physical model of space charge behavior has not yet been built completely. The issues of space charge effect and its application are involved with multiple factors. Although some technical and theoretical developments have been achieved in recent years, there are still some uncertain questions remained to be explored.

An analytical amplitude model for negative pressure waves in gaseous media

The negative pressure wave propagation method is an established Structural Health Monitoring (SHM) technique to locate leakages and defects in pipelines. The amplitude of negative pressure waves can be exploited to obtain information about the severity of the damage (e.g. leak flow rate and leak area) and are useful to determine the smallest detectable leakage flow rate of the negative pressure wave method. The current manuscript proposes an analytical amplitude model of negative pressure waves in gaseous media. The isentropic fluid equations that represent the leak phenomena are combined with acoustical equations expressing the generation of the negative pressure waves. The presented model has been experimentally validated over a broad range of pressurization cases, gaseous media, leak sizes and pipe diameter. The derivation of the model provides insight in the physics behind the occurring phenomenon while the model reveals all parameters affecting the negative pressure wave amplitude. It is derived that, depending on the pressurization type, a different formulation has to be used to predict the amplitude of the negative pressure waves. The amplitude model is, in view of its applications, both presented in function of the leakage flow rate as well as in function geometrical size of the leak.

Erratum: Pressure wave propagation method for space charge measurement in coaxial geometry [Dec 18 2139-2146

Erratum: Pressure wave propagation method for space charge measurement in coaxial geometry [Dec 18 2139-2146

Pressure wave propagation method for space charge measurement in coaxial geometry

The existence and accumulation of space charge in the insulation of polyethylene cable has a significant influence on the conductivity, aging and breakdown of the cable. Therefore, it is of great importance to study the space charge distribution in high voltage polyethylene cable. This paper proposes a simplified approximate analysis of pressure wave propagation measurement method in coaxial geometry. The experimental results show that, as in the case of planar sample, the peak of current in coaxial geometry obeys the simple model proposed.

Physical model for space charge distribution measured by pressure wave propagation method in coaxial geometry

With the rapid development of the science and technology, the application of the high voltage power cable has become more and more extensive. Now, it is generally accepted that space charge has an important effect on the electrical properties of insulating material in a high voltage cable. The measurement of space charge is the research base for the behaviors and properties of space charge in the polymer dielectric. Actually, the pressure wave propagation (PWP) method and pulsed electroacoustic (PEA) method are two sophisticated methods of measuring the space charge. However, these two methods are based on a planar sample. For measuring the space charge in a real cable, it is necessary to need the correct and precise mathematical expressions for the PWP method and PEA method. According to the theoretical analysis of the space charge distributions in the plate samples, measured by the pressure wave propagation method, we propose a physical model and its mathematical method of treating space charge distribution data measured in a coaxial geometry. In terms of Poisson equation, the influences of pressure waves on coaxial samples can be divided into two parts, namely, sample deformation and particle displacement. These two parts take into consideration the variations of the sample electric field, dielectric constant and density of space charge disturbed by pressure waves. Therefore, the voltage and current equations about the space charge distribution in the coaxial structure are found. The mathematical expression for the current measured indicates that compared with the current measured in the planar structure, which is proportional to the space charge distribution, the current signal measured in the coaxial structure should be further corrected. This paper also shows the experimental results which are the induced current signals picked from the planar sample and coaxial sample respectively. The results indicate that the current measured in the planar sample is proportional to the space charge distribution. However, the current measured in the planar sample is related to the inner and outer diameter of the dielectric, which verifies the correctness of the mathematical expression. Due to the influence of the coaxial structure of the high voltage cable, the pressure wave focusing effect is obvious as the pressure wave propagates along the axis, which causes the measurement signal to increase gradually with the propagation of sound wave. As a consequence, the electric field and the space charge density will change apparently. Due to the influence of the pressure wave focusing effect, the current and voltage signal will be amplified more obviously in cable, and the current measured by the PWP method shows the distribution of space charge density in cable.

Thermally assisted charge injection at electrode/polyethylene interfaces evidenced by direct observation of space charge

A layered sample consisting of a polyethylene (PE) film and a both surface fluorinated PE plate was used to investigate charge injection at the semiconductive, Al, or Au electrode/PE interface by space charge observation based on the pressure wave propagation method, to determine whether the charge injection is a thermal excitation process. Space charge observation in the layered sample subjected to a dc field of positive or negative 5 kV/mm at 20 or 60 °C shows that charge injection at the electrode/PE film interface and charge accumulation at the film/surface fluorinated plate interface do not stop so long as the electrode field is not reduced to zero, independent of electrode material. The charge injection and accumulation rates strongly depend on the charging temperature. At 60 °C, it only needs 15 min for the electrode field to decrease to zero or the charge accumulation at the film/surface fluorinated plate interface to reach a maximum value, while the electrode field is still about 47 % of its initial value even after 240 min charging at 20 °C. The very likely existence of electrode-induced gap states at the electrode/PE interfaces and the thermal excitation of charge carriers from the electrode-induced gap states near the charge neutrality level to the bulk states of PE can account for the observations of the charge injection behavior.

Comparative Study on Space Charge Distribution Measurements using PEA and PWP Methods on High Voltage Insulation

Major drawbacks to high voltage insulator especially on electrical aging process have been an issue for decades. The internal electric field distribution due to the presence of local space charge will distort, which in turn may cause the insulating material to degrade and finally can lead to accelerated aging and electrical breakdown. Comprehension of space charge buildup in solid insulator is crucial to determine how aging occurs, thus enabling the prediction of insulator lifetime. Space charge measurement has been extensively researched in recent years and has become a common method of investigating solid insulator properties. Pressure Wave Propagation (PWP) and Pulsed Electro-Acoustic (PEA) are non-destructive dielectric testing methods used to determine the space charge distribution within the insulation. This paper presents the basic principles, typical measurement setup and comparison of results from several published papers on application of PWP and PEA methods to investigate space charge accumulation. The space charge distribution in the sample is measured as current and voltage signals obtained from PWP and PEA methods respectively. Judging from extracted results of other research, both methods show the capability to detect premature aging activities in the insulator, which is necessary for condition monitoring of high voltage insulators.

Reduction of space charge breakdown in e-beam irradiated nano/polymethyl methacrylate composites

Fast discharge of numerous space charges in dielectric materials can cause space charge breakdown. This letter reports the role of nanoparticles in affecting space charge breakdown of nano/polymethyl methacrylate composites. Space charge distributions in the composites, implanted by electron beam irradiation, were measured by pressure wave propagation method. The results show that the nanoparticles have significant effects on the isothermal charge decay and space charge breakdown in the nanocomposites. The resistance to space charge breakdown in the nanocomposites is attributed to the combined action of the introduction of deep trapping states and the scattering effect by the added nanoparticles.

Influence of surface fluorination temperature on space charge accumulation in polyethylene

Polyethylene (PE) discs prepared by hot-pressing, each with a thickness of 0.8 mm, are surface fluorinated in a laboratory vessel by an F2/N2 mixture containing 12.5% F2 by volume at different temperatures of 35, 55, and 70 ℃ for the same time of 2 h. The influence of fluorination temperature on space charge accumulation in PE is investigated by the pressure wave propagation method. The results show an obvious decrease in space charge accumulation in the fluorinated sample subject to a direct current high voltage with the increase of fluorination temperature, and there exists almost no charge accumulation in the sample fluorinated at 70 ℃. Attenuated total reflection infrared analyses indicate a substantial change in chemical composition of the sample surface layers by the fluorinations and an obvious increase in degree of fluorination with fluorination temperature increasing. Contact angle measurements and surface energy calculations indirectly indicate a significant increase in permittivity of the fluorinated layers by the fluorinations. Open-circuit thermally stimulated discharge current measurements further reveal different charge trapping properties of the fluorinated layers and the improved barrier properties of the fluorinated layer to the diffusion of the chemical species from the semi-conductive electrode to the PE with increase of fluorination temperature, thus indicating a corresponding decrease in free volume of the surface layer. The decrease in the free volume plays a more important role in suppressing the space charge accumulation, compared with the increase in permittivity and the change in charge trap.

Influence of oxyfluorination time on space charge behavior in polyethylene

Linear low density polyethylene (LLDPE) samples were surface oxyfluorinated for different times to investigate the influences of oxyfluorination time on charge injection from carbon black loaded poly(ethylene-co-vinyl acetate) (EVA) electrode and charge accumulation in bulk. Oxyfluorination led to the substantial variations in chemical composition, forming large numbers of various polar groups in the surface layers, depending on oxyfluorination time, as indicated by attenuated total reflection infrared (ATR-IR) analyses. Space charge measurements based on the pressure wave propagation method revealed obvious dependence of the charge injection and accumulation on oxyfluorination time when the samples were submitted to an average direct current field of about 50 kV/mm at 40°C. Suppression of the charge injection and accumulation by the oxyfluorinated surface layer was enhanced with extending oxyfluorination time. The total amounts of the positive and negative charges within the samples oxyfluorinated for 1, 6, and 13 h were calculated to be around 67, 50, and 29% of the total amount in the original sample, respectively. Diffusion of the volatile byproducts within the EVA electrode into LLDPE was evidenced by the ATR-IR analyses or indirectly by open-circuit thermally stimulated discharge current measurements. The diffusion influenced the charge injection and accumulation by changing charge traps in the surface layer and the field assisted ionization of the diffused volatile byproducts, and was influenced by the surface oxyfluorination depending on the treatment time. The influences of oxyfluorination on charge injection and accumulation were attributed to the remarkable increase in surface layer permittivity and the changes in its charge trap and barrier property to the diffusion of volatile by-products. The change in surface layer permittivity was indirectly evaluated by surface energy calculations.

Charge Accumulation in Various Electron‐Beam‐Irradiated Polymers

AbstractCharge accumulations in electron‐beam (e‐beam)‐irradiated polymers are investigated by observing them using the pressure wave propagation method. The purpose of our research work is to measure the charge distribution in e‐beam‐irradiated dielectric materials for spacecrafts. We have been developing a system for measuring such a charge distribution in dielectric materials. Using this system, we tried to measure the charge distribution in various typical polymers (poly(ethylene terephthalate), polystyrene, low‐density polyethylene) under e‐beam irradiation. As a result, it is found that charging characteristics of insulating material is strongly influenced by the existence of benzene ring in the molecular architecture. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley &amp; Sons, Inc.

Electric Charge Accumulation in Polar and Non-Polar Polymers under Electron Beam Irradiation

The electric charge accumulation under an electron beam irradiation (40 keV and 60 keV) was measured by using the pressure wave propagation (PWP) method in the dielectric insulation materials, such as polar polymeric films (polycarbonate (PC), polyethylene-naphthalate (PEN), polyimide (PI), and polyethylene-terephthalate (PET)) and non-polar polymeric films (polystyrene (PS), polypropylene (PP), polyethylene (PE) and polytetrafluoroethylene (PTFE)). The PE and PTFE (non-polar polymers) showed the properties of large amount of electric charge accumulation over 50 C/m3 and long saturation time over 80 minutes. The PP and PS (non-polar polymer) showed the properties of middle amount of charge accumulation about 20 C/m3 and middle saturation time about 1 to 20 minutes. The PC, PEN, PI and PET (polar polymers) showed the properties of small amount of charge accumulation about 5 to 20 C/m3 and within short saturation time about 1.0 minutes. This paper summarizes the relationship between the properties of charge accumulation and chemical structural formula, and compares between the electro static potential distribution with negative charged polymer and its chemical structural formula.

Significant suppression of space charge injection into linear low density polyethylene by surface oxyfluorination

Surface oxyfluorination of linear low density polyethylene (LLDPE) was performed to suppress space charge injection and accumulation under direct current high voltage. Significant suppression effect that there is almost no space charge inside LLDPE was obtained, as observed by space charge measurements based on the pressure wave propagation method. Oxyfluorination led to the substantial variation in chemical composition, forming the various polar groups in surface layer, as indicated by attenuated total reflection infrared analyses. The suppression mechanisms are mainly attributed to the charge trap change and the remarkable increase in permittivity of the oxyfluorinated surface layer, as investigated by open-circuit thermally stimulated discharge current measure-ments and surface energy calculations, respectively.

From the editors' desk

In this issue we have four articles. The first is "Recent Developments in the Pressure Wave Propagation Method," by Stephane Hole from ESPCI, Paris. The pressure wave propagation (PWP) method, also known as the laser-inducedpressure- pulse (LIPP) method, was one of the earliest nondestructive methods of measuring space charge and electric field profiles in solid dielectrics, and dates from 1977.

Recent developments in the pressure wave propagation method

Performing measurements is an essential part of understanding physical phenomena. Hence, numerous measuring techniques have been developed to study space charge build-up in dielectrics. This macroscopic accumulation of charge modifies the electric field and can lead to aging and premature failure of electrical insulation. On the other hand, charge accumulation is essential to the operation of devices such as electret- or piezoelectric-based sensors. It is therefore essential to study charge build-up in order to design insulating materials in which excessive charge does not accumulate, or electrets in which a sufficiently large amount of charge accumulates to ensure adequate device operation, and the stability of that charge in time is sufficient to ensure an acceptable device life-time. This paper focuses on the measurement method using a pressure wave to perturb the charges. That method is commonly called the pressure-wave-propagation (PWP) method. A pressure wave enters the sample through one of its interfaces. As it travels through the material at the velocity of sound, the charges encountered are mechanically displaced.

Suppression effect of surface fluorination on charge injection into linear low density polyethylene

To suppress charge injection from electrodes, direct fluorination using fluorine gas was used for linear low density polyethylene (LLDPE) since it is one of the most effective methods of the polymer surface modification. Surface fluorination of the LLDPE plates was obtained as indicated by attenuated total reflection infrared spectroscopy. Remarkable suppression of charge injection by the surface fluorination was observed by space charge distribution measurements using the pressure wave propagation method. Comparing with the remarkable bipolar charge distribution in bulk of the original LLDPE, there is less space charge in bulk and it mostly exists in the fluorinated surface layers. The possible mechanisms of the charge injection suppression are discussed, one of which, the effect of fluorination on the charge traps in surface layer was investigated by the thermally stimulated discharge technique. The results indicate that fluorination has charge traps in the surface layer remarkably deepened and charges captured in the deep traps can block or shield the further charge injection.

Morphology‐related packetlike space‐charge behavior in linear low‐density polyethylene doped with Al2O3 nanoparticles

AbstractPacketlike space‐charge behavior and the isothermal decay processes of the injected charge in neat linear low‐density polyethylene (LLDPE) and LLDPE doped with Al2O3 nanoparticles were investigated by the pressure wave propagation method. The 1‐mm sheet samples, sandwiched by semiconductive electrodes, were submitted to 40 kV/mm of direct current field at various temperatures for 3 h. The charge‐injecting rate and the apparent mobility of packetlike space charge under direct‐current stress were compared among the samples subjected to different blending processes with or without nanoparticles. The slight doping concentration showed a significant influence on the space‐charge dynamics, with a lower injecting rate and apparent mobility for higher doped samples. The isothermal decay processes of the injected charge indicated trap‐modulated features. The phenomena were considered to be related to the changing morphology of the matrix material. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009