Hong-ng Li

Donor Compensation and Carrier-Transport Mechanisms in Tin-doped In2O3 Films Studied by Means of Conversion Electron 119Sn Mössbauer Spectroscopy and Hall Effect Measurements

Donor compensation and carrier-transport mechanisms in tin-doped indium oxide (ITO) thin films were investigated, from the viewpoint of chemical state of tin atoms, by conversion electron Mossbauer (CEM) spectroscopy and Hall effect measurement. Carrier density (ne) and Hall mobility (µH), changed with the post-deposition anneals. The carrier density decreased with air-anneal and increased with reducing atmosphere. Analyses of the CEM spectra indicated that the decrease in ne with air-anneal was attributed to the compensation of tin donors by excess interstitial oxygen (tin-oxygen complex). Also, we could explain the increase in ne with reducing atmosphere in terms of the dissociation of the tin-oxygen complex. From the analyses of µH, we arrived at the possibility that the tin-oxygen complex behaved as the screened dipole scattering center.

Estimation of free volume in polystyrene-polyphenylene ether blend probed by the positron annihilation lifetime technique

Abstract Variations of size, content and size distribution of free volumes in linear polystyrenes (PS) synthesized by radical processes, with the addition of miscible polyphenylene ether (PPE) in PS, were observed by positron annihilation lifetime measurements. The average free volume radius of polystyrene was proved to increase from 0.30 nm (PPE: 0%) to 0.34 tun (PPE: 100%) and the average intensity, relevant to free volume content, decreased with the increase of PPE. The size distributions of free volume of various PS to PPE ratios also reveal that average free volume size increases and that smaller free volumes decrease by the increase of PPE ratio. The evidence that the size distribution curve of PS and PPE mixture (1:1) is more similar to that of PS suggests that free volumes produced in PS are stable with the addition of PPE and that more positroniums annihilate in smaller free volumes generated in PS than in bigger ones formed by PPE, since numbers of free volume overwhelm those of PPE.

Free volumes and their distribution in crosslinked polysiloxanes probed by positron annihilation lifetime technique

In order to correlate the size, numerical concentration, and size distribution of free volumes with the nanostructure of crosslinked polysiloxane (cured MQ resin), free volumes of polysiloxanes of different degree of crosslinkage polysiloxanes were observed by positron annihilation lifetime measurements in the temperature range from 30 K to room temperature. The free-volume diameters of the highly crosslinked polysiloxane were found to be larger in the temperature range studied and their distributions become broader above Tg (150 K) than those for the lowly crosslinked polysiloxane, whereas free volume contents (number density) were lower for the highly crosslinked polysiloxane. The results suggest the heterogeneous and distorted nanostructure of prepolymers in the highly crosslinked polysiloxane.

Variation of free volume size and content of shape memory polymer — Polyurethane — Upon temperature studied by positron annihilation lifetime techniques and infrared spectroscopy

Positron annihilation lifetime measurement and Fourier transform infrared spectrometry were applied to the study of temperature dependencies of free volume parameters and hydrogen bonds in segmented polyurethane, specially fabricated as a shape memory polymer. The variation of free volumes in amorphous region were correlated to that of hydrogen bonding and the shape memory mechanism of polyurethane is elucidated from a microscopic point of view. The relationship between free volume contents and the formation of hydrogen bonds significantly suggests that the shape memory occurs due to the configurational recovery of the microscopic regulation around urethane bundles of hard segment and consequent restoration of soft-segment in polyurethane.

Study of free volume parameters of polystyrenes of different molecular architecture by positron annihilation lifetime measurement

Differences of size, content and size distribution of free volumes in linear branched and three-armed polystyrenes, synthesized by radical and anionic processes, were observed by positron annihilation lifetime measurements. The temperature dependence of an average free volume radius was quite similar among polystyrenes of different architectures and molecular weight distributions. The free volume radius increased with temperature, from 0.27 nm (T:∼60 K) to 0.29 nm (T:∼260 K) and 0.30 nm (Tg:363 K), then to 0.35 nm (423 K), showing turning at γ and α transition temperature. The free volume content decreased from 60 K to ∼220 K to ∼300 K showing peculiar minimum at 220 K to 300 K depending on the molecular shape, increased above ∼320 K, upto 340 to ∼360 K. The free volume contents decreased with an increase of molecular weight and by an addition of oligomer or plasticiser, suggesting differences in relaxation time or molecular motion between the edge and middle portions of molecular chain and filling effect of smaller molecules in free volumes, respectively. The apparent free volume fraction showed clear variations atTβ andTg. Size distribution of free volumes suggested more complicated behavior of free volume upon the molecular relaxations and filling effect.

Estimation of Chemical States and Carrier Density of Sn-doped In2O3 (ITO) by Mössbauer Spectrometry

Chemical states of Sn in Sn-doped In2O3 (ITO) powders and films were studied by Mössbauer spectrometry. From the deconvolution analysis of the spectra it was found that there were 7- and 8-fold coordinated, electrically inactive Sn4+ species in the host lattice in addition to two electrically active Sn4+ species. The concentration of the electrically inactive dopants and the carrier densities could be estimated by Mössbauer spectra. The estimated carrier densities for the powders and films were in good agreement with the measured values.

Variation of size, numerical concentration and size distribution of free volume of heat shrink polyethylene prepared by electron irradiation cross-linkage with temperature probed by positron annihilation technique

Variations of free volumes of cross-linked heat shrink low density polyethylene, prepared by the electron beam irradiation, in terms of temperature are studied through the analysis of positron annihilation parameters, such as S parameter, mean lifetime, lifetime and intensity of pick-off reaction of ortho-positronium (o-Ps), and a size distribution of free volume. Recognizable differences are observed for all parameters between the unirradiated and irradiation cross-linked polyethylene. Differences of parameters are also observed for the expanded polyethylene before and after the thermal treatments for shrinkage. Recognizable differences of free volumes as a function of temperature are not observed in all parameters between the calcium carbonate unfilled and filled heat shrink polyethylenes.

VARIATION OF FREE VOLUME PARAMETERS OF POLYALKYLMETHACRYLATE DERIVATIVES IN THE RANGE FROM 15 TO 300 K PROBED BY POSITRON ANNIHILATION LIFETIME MEASUREMENT

Positron annihilation lifetime measurements of PMMA, PEMA, PiPMA, PnPMA and PnBMA were performed in the temperature range between 15 and 300 K, where δ, γ and β relaxational transitions occur for these polyalkylmethacrylates. The variations of free volume size and content calculated from the longest lifetime component against temperature are correlated to the results obtained by dielectric, viscoelastic and dynamic mechanical relaxation data of the polymers. The variations of free volume sizes and contents, apparent free volume fraction and size distribution of the polyalkylmethacrylates are well correlated with the rotational transitions of side chains. The location of free volumes, in which positronium annihilates can be estimated near the vicinity of alkyl groups bound to oxygen atom of side ester chains.

Crystallinity dependence of free-volume size and its distribution of ethylene-vinyl alcohol copolymer evaluated by positronium lifetime measurement

The size distribution of free-volume (<~0.1 nm3) of ethylene-vinyl alcohol copolymer with various crystalline degrees was estimated by positronium lifetime measurement. With increasing degree of crystallinity, the size distribution significantly narrowed and the intensity of positronium decreased. This indicates that the inhomogeneity reduces with the increase of the degree of crystallinity. It is found that the free-volume fraction estimated is reflected by the fraction of the amorphous region.

Mössbauer spectrometric study of Sm2Fe17Nx(x=0-8)

Ternary rare-earth-iron-nitrogen compounds of Sm2Fe17Nx with x ranging from 0 up to 8, which have a rhombohedral structure of Th2Zn17 and a high Curie temperature up to 475–480°C depending on the degree of nitrogenation, were prepared by the nitrogenation of arc-melted Sm2Fe17 in mixed ammonia and hydrogen gas atmospheres of various ratios, and Mössbauer spectra of these compounds were observed at room temperature in order to analyse the physicochemical state of the nitrogenated products. The obtained data were analysed by focusing main attention on the effect of degree of nitrogenation on the variation of spectral profiles. Paramagnetic peaks due to an amorphous compound became prevalent as x became higher than ~ 2.3.