Michihisa Kyoto

Characterization of fluorine'doped silica glasses

Fluorine-doped silica glasses containing up to 2 wt% were prepared by the vapour-phaseaxial-deposition (VAD) sintering process. The characteristics of these glasses were investigated by Raman spectroscopy, vacuum-ultraviolet-ultraviolet (v.u.v-u.v.) spectroscopy, and viscosity measurements. From the Raman spectroscopic investigation, it has been shown that the Si-F bond structure in the fibres is the same as that in bulk glass and is not affected by codoped additives such as B2O3. From the u.v.-v.u.v, spectroscopic investigation, it has been shown that the absorption band at 7.6 eV in high-purity silica glass is removed with the addition of fluorine. This addition produces a silica glass with most excellent transparency in u.v. and v.u.v. regions. Also it was observed that the optimum addition is around 1 wt%. This fact proves that fluorine incorporation in glass prevents the generation of defects related to optical loss in fibres. From the viscosity study, it was found that the viscosity decreases with an increase of the fluorine content in glass and the activation energy also decreases with increasing fluorine content.

PROPERTIES OF BI-PB-SR-CA-CU-O GLASS-CERAMIC FIBERS FORMED BY GLASS-DRAWING METHOD

Flexible Bi-Pb-Sr-Ca-Cu-O glass fibers were prepared by the glass-drawing method. They showed superconducting properties after crystallization by heat treatment. The Tc and Jc values of the fibers were very sensitive to starting compositions and heat treatment conditions. The fiber with the starting composition of Bi2.1Pb0.9Sr2Ca2Cu3Oy showed superconductivity with a Jc=1480 A/cm2 (77 K, zero magnetic field).

Transition and relaxation processes of polyethylene, polypropylene, and polystyrene studied by positron annihilation

Transition and relaxation processes of polyethylene (PE), polypropylene (PP), and polystyrene (PS) were studied by the positron annihilation technique. From measurements of lifetime spectra of positrons as a function of temperature, the lifetime of ortho-positronium, τ3, and its intensity, I3, were found to increase above 260 K for PP. This fact was attributed to a cooperative motion of large segments of molecules above the glass transition temperature, Tg. For PE, above Tg (140 K), the value of τ3 increased, but the temperature coefficient of I3 was negative below 230 K. From this fact, for PE, the molecular motions that cause the glass transition were associated with a rearrangement of molecules by local motions such as kink motions. The discrepancy between the results for PE and PP was attributed to the presence of methyl groups in PP and the resultant suppression of the local motions. For PS (Tg = 340 K), the molecular motions were found to start above 260 K, but those were suppressed by an interphenyl correlation. Detailed annihilation characteristics of positrons in polymers were also discussed.

Fluorescence of pyrene in sol-gel silica derived from triethoxysilane

Abstract The fluorescence of pyrene probe molecules was measured in a sol-gel reaction system of triethoxysilane (HTEOS)/tetraethoxysilane (TEOS). There was minimal change in the ratio of the intensity of the third peak to the first peak (I3/I1) for the fluorescence spectra in the starting sol-gel solutions for various mixtures of HTEOS and TEOS. On the contrary, I3/I1 in silica xerogels increased linearly as HTEOS increased. These results indicate that the solvent composition mainly determines the environmental polarity of pyrene probes in initial solutions, while the HTEOS composition determines the polarity in xerogels, and that those silica xerogels derived from HTEOS are hydropholic. The infrared spectra of these xerogels suggest that the hydrophobic character of the xerogels derived from HTEOS is largely ascribed to the reduction of SiOH groups and adsorbed water molecules on these groups.

Some considerations of the SERS effect of L‐phenylalanine in the near‐infrared region using silver colloid solution

The surface-enhanced Raman scattering (SERS) spectra of L-phenylalanine were examined in the near-infrared region using silver colloid solutions. It was shown that near-infrared SERS (NIR-SERS) was not detected for L-phenylalanine in a normal silver colloid solution, whereas in the visible region a distinct SERS intensity enhancement was realized for such sample systems. In the present study, this situation was considered from the viewpoint that the degree of aggregation of colloid particles necessary to cause the SERS effect is different between visible and near-infrared radiation. In this work, silver colloid solution containing dissolved L-phenylalanine was dropped on to a filter-paper and this filter-paper sample was then dried. It was confirmed by SEM (scanning electron microscopy) that as a result of this sample preparation, the degree of aggregation of colloid particles increased considerably. It was shown that the near-infrared SERS was detected with high sensitivity for L-phenylalanine adsorbed on silver particles condensed on filter-paper. From consideration of the magnitudes of the SERS intensity enhancement in this case, it is suggested that the mechanism of NIR-SERS for the present sample system is the electromagnetic effect, related to the excitation of surface plasmons (the surface plasmon polariton mechanism).

Open spaces in the subsurface region of polyethylene probed by monoenergetic positron beams

Open spaces in the subsurface region (<10 μm) of very low density polyethylene were probed by a monoenergetic positron beam. From measurements of Doppler broadening spectra measurements of the annihilation radiation and the lifetime spectra of positrons as a function of incident positron energy, the size of the open spaces in the region of 0–3 μm was found to be larger than that in the deeper region. This was attributed to the cooperative motion of large segments of molecules which is activated near the surface. After the freezing in of such motions (below 230 K), although the lineshape parameter S in the bulk was almost constant, S in the region of 0–3 μm decreased with decreasing temperature. This discrepancy was associated with the presence of the open spaces with an excess content and the resultant contraction of amorphous structure near the surface.

Study of relaxation processes in polyethylene and polystyrene by positron annihilation

Relaxation processes in polyethylene (PE) and polystyrene (PS) were studied by positron annihilation technique. For PE, above the glass transition temperature, T g , the size of free volumes and its concentration were increased by the micro-Brownian motion of molecules. For PS, local motions of molecules in backbone chains were found to start above 260 K. However, these local motions were suppressed by an interphenyl correlation. For both PE and PS, below 250-260 K, the formation probability of positronium atoms increased with decreasing temperature. This fact was assigned to the freezing in of the local motions of molecules. For PS, an onset of the local motions of molecules was observed above 100 K. These motions were expected to be associated with liberation of phenyl groups.

Free volumes in polystyrene probed by positron annihilation

Free volume characteristics in three samples of monodisperse polystyrene were investigated by positron annihilation technique over a temperature range from 300 to 380 K. The number-average molecular weight of the samples ranged from 5730 to 1,524,000. The observed lifetime spectra were resolved into three components, where the longest lifetime, τ 3 , was associated with the pick-off annihilation of ortho-positronium (o-Ps) trapped by free volumes. The change of the temperature coefficient of τ 3 was observed at around 350 K, at which the value of τ 3 was a constant value of 2.3 ns for all specimens with different molecular weights. There was no discrete change of τ 3 in intensity, which is corresponding to the number of free volumes. The size of free volume at glass transition was evaluated to be 0.1 nm 3 .

Bi−Pb−Sr−Ca−Cu−O superconducting fibers drawn from melt-quenched glass preforms

Bi-Pb-Sr-Ca-Cu-O glass fibers were drawn from melt-quenched glass preforms by the method that is ordinarily applied to optical fiber fabrication. The drawn fibers were very flexible, and their surfaces were smooth. The glass fibers were crystallized by annealing and had superconducting properties. The annealed (1133 K, 240 hour) Bi1.6Pb0.4Sr2Ca2Cu3Ox fiber exhibited superconductivity with a Tc(zero) of 68 K and a critical current density (30 K, zero magnetic field) of 22 A/cm2.

Preparation of Bi1.5SrCaCu2Ox Glass Fibers Using Melt-Quenched Glasses

Bi1.5SrCaCu2Ox glass preforms were prepared by the melt-quenching method, and their thermal characteristics and viscosity were investigated to determine the possibility of drawing them into glass fibers. Furthermore, Bi1.5SrCaCu2Ox glass fibers were drawn from glass preforms by the method that is ordinarily applied to optical fiber fabrication. The drawn fibers were very flexible, and the maximum length was 1200 mm.