Takehiko Mizuno

Electroluminescence in insulating polymers in ac electric fields

EL (electroluminescence) in several polymeric materials was investigated in the electric field range <90 kV/mm. We used polymeric films with semi-transparent gold electrodes: low density polyethylene, polypropylene, polytetrafluoroethylene and polyvinylchloride. The EL characteristics were very low EL inception fields (2 to 20 kV/mm) which strongly depend on the material and very low energy photon emission (mainly in the red region of 1.6 to 1.8 eV). These results reveal that EL emission occurs predominantly in the metal-polymer interface region rather than in the bulk of the polymer at the applied field. We present a model of EL emission in the metal-polymer interface region to explain our results qualitatively. In this model, the mechanism of EL emission is the radiative recombination of the electrons and holes trapped at the surface states of the polymer, which causes photon emission in the red region. The EL inception field and EL intensity depend on the density of the surface states of the polymers.

Investigation of Charge Injection in Gas-Impregnated Polyethylene by Measurement of Electroluminescence under AC Voltage

Electroluminescence (EL) in polyethylene (PE) impregnated with N2, O2 or SF6 gas has been measured under ac voltages to investigate the effect of impregnation gas on charge injection. EL is strongly dependent on the processes of electron injection and transport in the surface region of PE. The results of our measurements of EL indicate that the process of electron injection is greatly affected by the type of impregnation gas in PE. In particular, O2 gas which is electronegative and chemically reactive markedly affects the charge injection process, causing a reduction in EL inception voltage and barrier height at the electrode-polymer interface. The surface states induced by the presence of O2 gas or by oxidation of PE enhance the electron injection. However, electrons injected into such surface states likely contribute little to EL. N2 gas also causes a reduction in the EL intensity but does not induce a change in the EL inception voltage and the barrier height in comparison with those in the degassed case. N2 gas probably affects an electron transport process such as intermolecular conduction in the surface region of PE rather than the electron injection process.

LIGHT EMISSION BEFORE AND DURING PREBREAKDOWN ON POLYTETRAFLUOROETHYLENE SURFACE WITH METALLIZED ELECTRODES UNDER AC VOLTAGE APPLICATION IN VACUUM

Light emission before and during electrical breakdown from the surface of polytetrafluoroethylene (PTFE) with metallized electrodes has been investigated with ac electric field application along the polymer surface, using a photon counting method, in order to understand the initiation mechanism of the prebreakdown. Two distinct stages of light emission were observed depending on the applied voltage: a low-level stable light emission, electroluminescence (EL), before prebreakdown and an irregular intense light emission during prebreakdown. Before prebreakdown, charge injection from the electrode directly into the polymer surface layer results in EL emission and the formation of long-term electron space charges away from the electrode. The crucial factor of prebreakdown initiation is the strong modification of the local electric field near the electrode because of space charge formation in the surface layer before prebreakdown. The prebreakdown with intense light emission is initiated by detrapping the long-term trapped electrons toward the electrode via the vacuum and/or the surface layer in the positive half-cycle of ac voltage.

Electroluminescence from surface layer of insulating polymer under ac voltage application

Electroluminescence from a surface layer of polytetrafluoroethylene (PTFE) has been investigated using a photomultiplier tube and an intensified charge-coupled device (ICCD) camera under ac or half rectified ac (HRAC) electric fields tangential to the polymer surface. The electroluminescence (EL) characteristics from the polymer surface layer showed a similarity to those from PTFE and other polymer films subjected to ac or HRAC electric fields in the normal direction to the surface. ICCD imaging of EL indicated a large area of EL activity where the EL intensity was maximum at the electrode edge and EL was still observable 2 mm from the electrode. The injection and transport of electrons and holes via surface states in the surface layer and the subsequent electron-hole recombination give rise to EL, strongly depending on the spatial distribution of the surface states. The injection length of charges can be estimated to be at least 2 mm from the electrode in the tangential direction to the surface.

Light emission in PE with a needle-like microvoid at the metal-polymer interface under impulse voltages

Light emission from polyethylene with a needle-like microvoid has been investigated under the application of a negative impulse voltage in order to examine the temporal behavior of discharge in the microvoid at the metal-polymer interface. The light emission was observed distinctly during the voltage rise and fall phases with a period of no light emission between the two light emitting phases. The emitted light consisted of sequential light pulses which appeared regularly, according to the temporal variation rate of the voltage. The sequential light pulses corresponded to the sequential discharge in the microvoid which occurred to keep the voltage across the microvoid below the discharge inception voltage. The characteristics of light emission revealed the occurrence of intermittent discharges in the microvoid during the voltage variation phase of the impulse voltage, caused by charge deposition on the polymer surface of the microvoid.

Electroluminescence from polymeric halides subjected to an AC voltage

Electroluminescence (EL) from polymeric halides and hydrocarbons has been investigated under voltages to clarify the effect of halogens in the polymers on the EL. We used polymer films with semi-transparent gold electrodes, namely, polytetrafluoroethylene (PTFE), polyvinylchloride (PVC), low-density polyethylene (LDPE), and polypropylene (PP). The light emission from PVC and PTFE started from lower fields and had a larger light intensity than that from LDPE and PP. However, the basic natures of the physical mechanisms of light emission involved seemed to be similar in all samples. The characteristics of light emission can be explained by considering the energy band structure of electron at the surface of polymer. The necessary condition of light emission will be the formation of long-term electron space-charges near the metal-polymer interface sufficiently enough to generate the reverse field for the hole injection. Halogens in the polymers act as strong trapping centers for injected electrons and enhance the formation of long-term electron space-charges. The light may be emitted dominantly by radiative recombination of electrons and holes.

Electroluminescence in polyethylene subjected to impulse voltages

Electroluminescence (EL) from HDPE (high-density polyethylene) films subjected to repetitive rectangular-shaped impulse voltages has been investigated at electric fields <80 kV/mm in order to clarify the temporal behavior of carriers injected into the polymer. EL emission was distinctly observed during the voltage rise and fall phases. During the voltage plateau and the zero voltage phases, the EL intensity did not instantly decrease to zero but a very weak EL emission lasted for at least 3 ms. The number of EL pulses was determined by the amplitude of the impulse voltage, whereas the EL intensity strongly depended on the temporal variation rate of the voltage. The total number of EL pulses during both phases of the voltage rise and fall was almost equal. The EL spectra did not depend on the phase and the amplitude of the impulse voltage. The above behavior of EL emission can be explained qualitatively by the electron-hole recombination in the metal-polymer interface region and showed the picture of temporal behavior of carriers, carrier injection and transport, in the interface region for the field relaxation at the interface.