Shin-ichi Hyodo

Raman study on silica optical fibers subjected to high tensile stress

For silica (pure and synthetic) optical fibers, on which tensile stresses from 0 to 4 GPa were imposed, Raman studies were carried out. It has become evident by deconvoluting the silica Raman peaks that the main peak intensity at 440 cm−1 decreases with increasing stress while the peaks at 490, 604, and 800 cm−1 remain practically unchanged. The fact that the 490 cm−1 defect line was not affected by the stress applied to fibers is contrary to the conclusion by G. E. Walrafen, P. N. Krishnan, and S. W. Freiman [J. Appl. Phys. 52, 2832 (1981)]. The results obtained in the present study are rather extensively discussed from a microscopic view of silica glass networks.

Storage Effect in Photostimulated Exoelectron Emission from Scratched Aluminum

When photoillumination is interrupted, the exoelectron emission from scratched metal samples decays quickly; when the illumination is resumed, however, the recovered exoelectron emission shoots up to a value significantly higher than before and then decreases gradually–a fact overlooked by previous researchers. This transient phenomenon has been studied in some detail as a function of the interval of the interruption and photon energy. To explain our data we propose a model, according to which there are two excitation processes competing during PSEE; one is the photoexcitation of the electrons at defect-related energy levels above the Fermi level, and the other is the thermal excitation of lower-energy electrons to the empty defect-related levels. Rate equations based on this model were found to be in satisfactory agreement with our observations.

Change of Energy-Level Distribution of Photoemission Sources in Al and Zn Caused by Scratching

Photoemission from Al (99.99% purity) specimens was measured for the photostimulation wavelength ranging from 190 to 700 nm. The specimens were scratched with a steel needle at temperatures from 77 to 290 K and at pressures from 10-5 to 10-7 Torr. Whereas prior to scratching the emission peaks from Al specimens appeared in a range from 250 to 290 nm, they shifted to a 290–350 nm range after scratching. Although this result is nothing but a manifestation of the so-called exoelectron emission, it has not been well recognized that scratching causes a change in the number of photoemission sources at various energy levels. The wavelength at the peak emission was influenced neither by the specimen temperature nor by the degree of vacuum. Similar results have been obtained for Zn specimens, though the yield observed was two orders of magnitude lower than that for Al.

Mean residence time of potassium ions on tungsten as measured with reference to ionization efficiency

Abstract The mean residence time, τ i , of potassium ions on “clean” and oxygenated tungsten has been measured, together with the ionization efficiency, as a function of surface temperature T by using incident K beams of low intensity (10 9 –10 12 atoms cm −2 s −1 ). For T higher than ~900 K the observed τ i followed Frenkels equation τ i = τ i 0 exp ( Q i kT ) as usual and the agreement of the ionic desorption energy Q i and of the pre-exponential factor τ i 0 with the corresponding values of previous experiments was quite satisfactory. Below 830 ~ 910 K, where a steep drop of ionization efficiency began to be noticeable, Arrhenius plots of τ i deviated considerably from linearity. The apparent increment of the desorption energy was shown to be nearly equal to the decrement of thermionic work function of tungsten as obtained from the ionization efficiency and Saha-Langmuir equation. The increase of surface coverage by potassium was accordingly taken as the main cause of the departure of Arrhenius plots from linearity. Under certain conditions of incident beam intensity and surface temperature τ i was observed to make an abrupt change from a higher to a lower value — a difference expressed as 100–140 meV in terms of the difference in ionic desorption energy. This peculiar phenomenon was attributed to the phase change of adsorbed potassium on tungsten.

PMMA fracture surface produced by ultrasonically modulated tensile stress

Ultrasonic shear waves with a large amplitude and a frequency of 3.63 MHz were applied to a notched PMMA plate under uniaxial tension in order to produce artificial Wallner lines by altering the direction of the maximum principal stress during the rapid growth of fracture. The amplitude of the lines generated was found to be of the order of 10–102 nm and has been shown to change proportionally to the input‐wave amplitude. The interference colors observed indicate that the PMMA crack front is very liable to proceed inside the crazed layer ahead of the front; despite the intensive ultrasonic stress modulation very few cracks which reached or crossed the craze‐matrix boundaries were found.

Direct Observation of Twin Formation in Iron-4.4% Silicon Crystals

Single crystals of iron-4.4% silicon and of iron were subjected to impact compression at room temperature. High-speed cine-microscopic pictures were taken at a framing rate of 4–6×105 sec-1 for observation of twin formation in the specimens. The twin formation lasted for a period of an order of 10 µsec, resulting in a fairly uniform distribution of twins with a linear density of 10–20 mm-1. During this period, the twins were apparently formed in successive groups. The velocity of twin propagation was estimated to be higher than 7×102 m/sec-1. The observed twins thickened very rapidly in their early stage of growth.

Tensile Fragmentation of Glass Fibers

When a virgin glass fiber fractures by static tension, it does not break into two or three pieces but shatteres as a cloud of dust, and disappears. The dynamic process of this fragmentation was observed by means of high-speed photography at a rate of 4×105 frames/sec. It was found that the rupture was initially produced at a cross-section of the fiber as in ordinary tensile tests and then fragmentation began to propagate from the section towards the fixed ends of the fiber at a constant speed (177±5 m/sec). Attempts to interpret the results as the effects of nondispersive stress waves propagating in continuous media seemed unsuccessful. It was, however, pointed out that the observed fragmentation velocity was in close agreement with the value of the fission velocity predicted by the wave-mechanical theory of Fitzgerald.

Two-Process Model for a Comprehensive Interpretation of Photostimulated Exoelectron Emission

In order to explain several PSEE time-related phenomena regarding scaratched metals, the authors have proposed a reaction-kinetic model (the Two-Process model), in which particular attention is given to activating inactivated emission sources. On the basis of this model the authors have succeeded in elucidating such PSEE phenomena (mostly observed for scratched aluminum) as a storage effect, the relation between the peak intensity and the ambient pressure, and the dependence of the intensity on the stimulation intensty. In a discussion regarding the parameters involved in the model it is suggested that glow curves produce useful information about the PSEE emission mechanism as well as TSEE measurements.

PSEE-Related Phenomena Indicative of the Meaning of Two-Process Model Parameters for Mechanically Deformed Aluminum

In order to examine the physical meaning of the parameters involved in the two-process model previously proposed by the authors, the following experimental studies were conducted for scratched Al specimens. (1) It was found that the statistical fluctuation in the PSEE yield obeys a Poisson distrubution. This indicates that not only the exo-emission process but also the process of reactivating inactive emission sources should be regarded as a stochastic process. (2) The rate of exo-activation was found to be insensitive to specimen temperatures between 100 and 300 K, and to agree well with the theoretical value obtained from a simple tunneling model. (3) Abrasion did not affect the frequency of the photoluminescence at peaks which were observed at 2.6, 3.0, 3.8 and 4.8 eV. The peak intensity, however, increased after abrasion and then gradually decreased. This is similar to the intensity change in a PSEE yield.

Two-process model of PSEE from scratched metals

Abstract When photoillumination is interrupted, the exoelectron emission from scratched metal samples decays quickly. When the illumination is resumed, however, the recovered exoelectron emission shoots up to a value significantly higher than before and then decreases gradually - a fact overlooked by previous researchers. To explain this “storage effect” in PSEE (photo- stimulated exoelectron emission), the authors have proposed a model, according to which there are two excitation processes competing during PSEE; one is the photoexcitation of the electrons at energy levels within the band gap of surface oxide layers, and the other is the tunneling transition of electrons in bulk metals to occupy the vacant levels of the oxide layers. From the rate equations based on this model and also from the PSEE data obtained for scratched aluminum and zinc, three kinds of quantities, i.e., the number of exo-active sites, the emission rate from the sites and the rate of activating exo-inactive sites, are successfully estimated. Other PSEE phenomena such as the peculiar emission intensity versus time profiles can also be elucidated in view of this model.