Takao Kawano

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.

Positronium formation in SiO2 films grown on Si substrates studied by monoenergetic positron beams

The annihilation characteristics of positrons in SiO2 films grown on Si substrates were studied by using monoenergetic positron beams. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured as a function of incident positron energy for SiO2 (166 nm)/Si specimens fabricated by thermal oxidation. From the measurements, it was found that about 90% of positrons implanted into the SiO2 film annihilate from positronium (Ps) states. This fact was due to the trapping of positrons by open‐space defects and a resultant enhanced formation of Ps in such regions. For the SiO2 film grown at 650 °C, the lifetime of ortho‐Ps was found to be shorter than that in the film grown at 1000 °C. This result suggests that the volume of open‐space defects in the SiO2 film decreased with decreasing the growth rate of the SiO2 film.

Annealing Properties of Defects in B^+- and F^+-Implanted Si Studied Using Monoenergetic Positron Beams

Annealing properties of defects in F+- and B+-implanted Si were studied using monoenergetic positron beams. For F+-implanted specimen with a dose of 2×1013 F/cm2, before annealing treatment, the mean size of the open volume of defects was estimated to be close to the size of divacancies. After rapid thermal annealing (RTA) at 700° C, vacancy-fluorine complexes and vacancy clusters were formed. The mean size of the open volume for the vacancy-fluorine complexes was estimated to be close to the size of monovacancies, and their annealing temperature was determined to be 800° C. For F+-implanted specimen with a dose of 4×1015 F/cm2, complexes between vacancy clusters and fluorine atoms were introduced during solid-phase epitaxial growth of the amorphous region, and they were observed even after RTA at 1100° C. Effects of additional B+ implantation on annealing properties of defects are also discussed.

Effects of Recoil-Implanted Oxygen on Depth Profiles of Defects and Annealing Processes in P+-Implanted Si Studied Using Monoenergetic Positron Beams

Effects of oxygen atoms recoiled from SiO2 films on depth profiles of defects and annealing processes in P+-implanted Si were studied using monoenergetic positron beams. For an epitaxial Si specimen, the depth profile of defects was found to be shifted toward the surface by recoil implantation of oxygen atoms. This was attributed to the formation of vacancy-oxygen complexes and a resultant decrease in the diffusion length of vacancy-type defects. The recoiled oxygen atoms stabilized amorphous regions introduced by P+-implantation, and the annealing of these regions was observed after rapid thermal annealing (RTA) at 700° C. For a Czochralski-grown Si specimen fabricated by through-oxide implantation, the recoiled oxygen atoms introduced interstitial-type defects upon RTA below the SiO2/Si interface, and such defects were dissociated by annealing at 1000° C.

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.

Defects in electron irradiated vitreous SiO2 probed by positron annihilation

Defects in 3 MeV electron irradiated vitreous SiO2 (v-SiO2) were probed by the positron annihilation technique. For unirradiated v-SiO2 specimens, almost all positrons were found to annihilate from positronium (Ps) states. The high formation probability of Ps was attributed to the trapping of positrons by open-space defects. The formation probability of Ps was decreased by the electron irradiation. The observed inhibition of the Ps formation was attributed to the trapping of positrons by point defects introduced and/or activated by the irradiation. From measurements of the lifetime distribution of Ps, it was found that, by the electron irradiation, the mean size of open-space defects was decreased and the size distribution of such defects was broadened.

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 .

Application of Proton-conducting Ceramics and Polymer Permeable Membranes for Gaseous Tritium Recovery

In order to carry out deuterium plasma experiments on the Large Helical Device (LHD), the National Institute for Fusion Science (NIFS) is planning to install a system for the recovery of tritium from exhaust gas and effluent liquid. As well as adopting proven conventional tritium recovery systems, NIFS is planning to apply the latest technologies such as proton-conducting ceramics and membrane-type dehumidifiers in an overall strategy to ensure minimal risk in the tritium recovery process. Application of these new technologies to the tritium recovery system for the LHD deuterium plasma experiment is evaluated quantitatively using recent experimental data.

Vacancy-type defects in ion-implanted diamonds probed by monoenergetic positron beams

Vacancy-type defects introduced by 180-keV B+-, C+- and N+-ion implantation in synthesized diamonds (type Ib) were probed by the positron annihilation technique. For an unimplanted specimen, the diffusion length of positrons was shorter and the lifetime of positrons was longer than those for a type IIa specimen. These facts were attributed to the trapping or the scattering of positrons by nitrogen-related defects. For ion-implanted specimens, the depth distributions of vacancy-type defects were determined from measurements of Doppler broadening profiles of the annihilation radiation as a function of incident positron energy. The obtained profiles of defects were in agreement with those of an energy loss of ions calculated by the Monte Carlo method. From measurements of lifetime spectra of positrons, the dominant defect species introduced by the B+- or N+-ion implantation were identified as complexes of vacancy clusters and these impurities.

Defects in Ion-Implanted 3C-SiC Probed by a Monoenergetic Positron Beam

Defects introduced by 200-keV N2+- or Al+-implantation into 3C–SiC were probed by a monoenergetic positron beam. Depth profiles of the defects were determined from measurements of Doppler broadening profiles of the annihilation radiation as a function of incident positron energy. For ion implanted specimens at high substrate temperature (≥800° C), the major species of defects was identified to be vacancy clusters. The depth profile of vacancy-type defects was found to be shifted towards the surface of the specimen by implantation at high temperatures. Upon furnace annealing after the implantation, an agglomeration of vacancy-type defects was observed, and interstitial clusters were introduced below the vacancy-rich region.