Kichinosuke Yahagi

Radiation effects on pure silica core optical fibers by γ-rays: relation between 2 eV band and non-bridging oxygen hole centers

The defect center causing the 2 eV absorption band induced in pure silica core fibers by 60Co γ-rays is discussed. The 2 eV band and non-bridging oxygen hole center (NBOHC, SI–O) show almost the same thermal annealing characteristics. The peak wavelength of the 2 eV band in low-OH (6 ppm) fibers is 630 nm, but shifts to shorter wavelengths according to the amount of OH group the sample contains, and appears at 600 nm in high-OH (700 ppm) fibers. The peak shape is broader in high-OH fibers than in low-OH fibers. This shift and broadening is attributable to a hydrogen bond between NBOHC and a hydrogen atom. Based on the above results, the defect center responsible for the 2 eV band is considered to be NBOHC.

Dielectric Properties and Morphology in Polyethylene

Polyethylene consists of crystalline parts and amorphous parts. Trap sites for electrons or ions exist on a boundary between them. Mechanical drawing or isothermal crystallization of a bulk from its melt provides increased trap sites due to change of morphology in crystalline lamellae. The trap sites for electrons might be physical cavities, where the cavity probably is best pictured as an irregular space bounded by a particular local arrangement of molecular chains which would be changed by drawing or annealing. The TSC peak is enhanced by drawing, and also we observe particular SCL current characteristics modified by the increase of trap sites. 90% of the vinyl groups in polyethylene are reported to be located at the surface of the crystallite and are able to become trap sites. However, no increase of vinyl groups (910 cm-1 and 990 cm-1) in IR spectra could be detected in a film drawn twice. The morphology of crystalline lamellae is changed by drawing. Breakdown strength is controlled by the tilting of slip planes in crystalline lamellae due to an applied force. Crystalline blocks aligned along the tensile axis might be resistive for electrons attempting to accelerate across aligned chains. It is well known that homogeneous deformation of spherulites due to straining provides a direction for development of an electrical tree. In our experiment, the breakdown strength is higher in the case of a small difference in density between crystalline lamellae and amorphous parts.

Gamma-Ray-Induced Absorption Bands in Pure-Silica-Core Fibers

Radiation-induced optical absorption bands in pure synthetic silica-core fibers were studied. The effect of Cl contamination on the absorption change of the irradiated fibers in relation to the radiation resistance was examined, and it was deduced that low-Cl fibers have good radiation resistance even when they have a low OH content. The effect of the OH content on the radiation response in low-Cl fibers was thus investigated. The radiation-induced 630 nm band is enhanced in higher-OH-content silica-core fibers. The 660 nm and 760 nm bands were also observed in certain irradiated fibers. The former increases with dose up to 0.1~2 kGy but decreases at higher doses. The mechanisms of the radiation-induced bands are discussed.

Temperature dependence of surface flashover voltage of polyethylene in vacuum

The surface flashover voltage over cylindrical spacers made of polyethylene in vacuum under impulse voltage application decreases monotonically with increasing temperature of the sample surface. This temperature dependence may be explained well by the mechanism that electrons injected from a cathode‐insulator‐vacuum junction bombard the sample surface to cause desorption of adsorbed gases and vaporization of sample material, thereby triggering surface flashover.

Effect of Carrier Traps in Polyethylene under Gamma‐Ray Irradiation

Stimulated currents are detected with increasing temperature in polyethylene films following the irradiation with high‐energy electron beams at 77°K. A peak current occurs at about 200°K in low‐density specimens and 250°K in high‐density ones.Induced currents I under gamma‐ray irradiation of polyethylene at room temperature depend on dose rate R via I ∝ RΔ. In high‐density polyethylene preirradiation by electron beams of about 5×107 rads at room temperature Δ increases from 0.5 to 0.84 and is accompanied by a decrease of induced current. In the low‐density specimens Δ decreases from 0.8 to 0.6 and the induced current increases with the same preirradiation. After boiling the preirradiated specimen for 5 min, Δ becomes 0.5 in both cases, and in the high‐density specimens the induced current I returns to that of the nonirradiated sample. This behavior suggests that the increase and decrease of carrier traps by these treatments influence the induced current characteristics. Another factor influencing the indu...

Transient Electric Current in p-Methoxybenzylidene-p-n-Butylaniline

A peak current was observed in the transient electric current in an n-type nematic liquid crystal, p-methoxybenzylidene-p-n-butylaniline (MBBA) by the application of a rectangular pulse voltage. It was conjectured that the peak current was obtained due to the carrier transit from one electrode to the other. Experiments on the dependences of the applied voltage and sample thick ness on the transient current were carried out. The carrier mobility was calculated ca. 1.6×10-5 cm2/Vsec at 30°C from these experiments. The activation energy was derived as ca. 0.20 eV from the temperature dependence of the current. Modified Waldens rule seems applicable to the transient electric current in MBBA.

Electrical Properties of Rutile (TiO2) Thin Film

Rutile thin films of Au(Ti, Al)–TiO2–Pt sandwich system were prepared on a substrate glass by evaporation of Ti metal and oxidation of it by heat treatment. Through AC and DC measurements, the capacitance obtained was about 2000 pF/mm2, the dielectric constant was estimated as 60~110, tan δ was 1~10%, the conductivity was about 10-13Ω-1cm-1 and the electrical strength was 5~10 V (0.3~0.5 MV/cm), respectively. In the micro-sec- and micro-sec-pulse conduction measurement, the relation of current and time became I∝t-n, in which the value of n was 0.7~0.8. The polarity effect on the current was considered to be due to the work functions of electrodes. The current carrier was estimated to be electronic from the measurement of photoconduction and the polarity effect in the pulse conduction.

Effect of dielectric constant on surface discharge of polymer insulators in vacuum

The effect of dielectric constant on surface discharge of polymer insulators (polyethylene, polycarbonate, polytetrafluoroethylene, and polyamide) with right cylindrical shape was investigated. The experimental results showed that dc flashover voltage was decreased with increase of dielectric constant in polymers.

Effect of Thermal Aging on Breakdown Strength of Polyethylene

Effect of thermal aging on breakdown strength of low-density polyethylene has been investigated with fixed electrodes. Thermal aging of polyethylene induces oxidation and rearrangement of molecular chains which cause an increase of its crystallinity and a decrease in the long period. In the initial stages of thermal aging, the amorphous parts grow strained with an increase in the crystallinity, and oxidation products introduced in the crystalline surface regions may act as deep traps for electronic carriers which in turn cause an increase in the breakdown strength at room temperature. In the advanced stages of thermal aging and in the high temperature range where crystalline melts, carbonyl groups in amorphous parts due to oxidation may become active and act as hopping sites for accelerated electrons. Therefore, the electronic conductivity increases and the breakdown strength decreases.

Gamma-Ray Induced Absorption Band at 770 nm in Pure Silica Core Optical Fibers

The effects of the drawing conditions, OH content and irradiation temperature on radiation-induced optical absorption bands in step-index fibers with pure silica cores clad with silicone or F-doped silica have been investigated. A loss peak was induced at 770 nm in silicone-clad fibers with both low and high OH-content silica cores, but the loss peak occurred at 630 nm in F-doped silica-clad fibers under γ-ray irradiation. The drawing conditions affected the growth of the loss peak in both silicone-clad and F-doped silica-clad fibers. The 770 nm band decreased under γ-ray irradiation exceeding 100 Gy at room temperature but did not decrease at 77 K. Radiation hardening was produced in the 770 nm band by γ-ray irradiation at room temperature or by room temperature annealing after irradiation at 77 K. The loss peaks at 770 nm and 630 nm in the two kinds of fiber are discussed in relation to the differences in the cladding materials.