Minoru Hanaya

Discovery of homogeneous-nucleation-based crystallization in simple glass-forming liquid of toluene below its glass-transition temperature

Abstract Crystallization process of toluene was investigated at low temperatures in the glass-transition region to examine the universality of the existence of homogeneous-nucleation-based (HNB) crystallization in fragile liquids, and to check the dependence of the crystallization process on the entropy of fusion, Δ fus S . The HNB crystallization was actually observed in the supercooled liquid of toluene as in the cases of o -terphenyl, salol, triphenylethylene and benzophenone, while toluene has much smaller Δ fus S than the latter substances. The magnitude of the crystallization rate was essentially reproduced by the numerical calculation with taking the nucleation-enhancement effect in the vicinity of the liquid–crystal interface and the high molecular-rearrangement rate due to a β-process. These results suggest that the HNB crystallization and thus homogeneous-nucleation process are not affected so much by the magnitude of Δ fus S , and that the HNB crystallization is a universal phenomenon, independent of the magnitude of Δ fus S , in fragile liquids.

Anomalous AgI composition dependence of the thermal and dielectric properties of AgI-Ag2O-P2O5 glasses : evidence for the formation of amorphous AgI aggregate regions as dominating the fast Ag+ ion conduction

Heat capacities of (AgI)x(Ag3PO4)1−x glasses with x = 0.70 and 0.80 were measured to estimate the jump, ΔCp, in the β-glass transition due to freezing of the Ag+ ion rearrangement. Dielectric relaxation properties of (AgI)x(Ag2PO3.5)1−x with x = 0, 0.20, and 0.40 and (AgI)x(Ag3PO4)1−x glasses with x = 0.30 and 0.40 were examined to estimate the activation energies, Δea, along the conduction path. ΔCp was found to appear only at larger x than a critical value xc = 0.60 for (AgI)x(Ag3PO4)1−x glasses. xc, depending on the structure of the glass network former, is larger than the values of 0.35 and 0.54 for (AgI)x(AgPO3)1−x and (AgI)x(Ag2PO3.5)1−x, respectively, glasses. Δea was found to show its inflection at around the respective xc values for the (AgI)x(Ag3PO4)1−x and (AgI)x(Ag2PO3.5)1−x glasses. These observations are discussed to support the amorphous AgI aggregate model for the fast Ag+ ion conduction path.

Activation properties of Ag+-ion conduction in bulk amorphous AgI: estimation from extrapolation of the AgI composition dependence in AgI?Ag2O?P2O5 glasses

AgI-based fast-ion conducting glasses with very high AgI compositions from the (AgI)x(AgPO3)1?x, (AgI)x(Ag2PO3.5)1?x, and (AgI)x(Ag3PO4)1?x systems were prepared successfully by using a rapid-press quenching and a twin-roller quenching method. The ac dielectric measurements showed common relaxation properties of Ag+-ion conduction in the glasses independently of the species of the glass network formers of AgPO3, Ag2PO3.5, and Ag3PO4, and the activation energies, ??a, for Ag+-ion conduction were observed to converge upon the same magnitude of ~26 kJ mol?1 at the AgI composition limit of x = 1. This indicates the formation of amorphous AgI regions in the glasses, and the value of ??a = 26 ? 1 kJ mol?1 estimated at x = 1 was concluded to correspond to that for bulk amorphous AgI which has never been obtained experimentally.

A Novel Alkoxysilyl Azobenzene Dye Photosensitizer with Alkylamino Group for Dye-Sensitized Solar Cells

A newly designed alkoxysilyl azobenzene dye with alkylamino group, 4-diethoxyphenylsilyl-4′-dimethylaminoazobenzene, was synthesized and examined as the photosensitizer for a dye-sensitized solar cell (DSSC). The novel azobenzene dye exhibited a strong absorption band in the visible region, and the solar cell using the dye as the sensitizer showed an incident monochromatic photon-to-current conversion efficiency (IPCE) value of 66 % at 440 nm and a light-to-electric energy conversion efficiency of 2.2 % under simulated sunlight irradiation of AM-1.5G one sun condition. The energy conversion efficiency obtained here is the highest value among those reported so far for azobenzene-sensitized solar cells, indicating the potential of alkoxysilyl dyes as photosensitizers for DSSCs.

Adsorption and Sensitizing Properties of Azobenzenes Having Different Numbers of Silyl-Anchor Groups in Dye-Sensitized Solar Cells

The effects of the silyl-anchor group number on the adsorption and the sensitizing properties were examined in sensitizing dyes for dye-sensitized solar cells by using 4-alkoxysilyl and 2,4-dialkoxysilyl azobenzenes. The TiO2 electrode with the 2,4-disilyl dye exhibited higher durability to water and better photovoltaic performance than that with the 4-monosilyl dye, although the amount of 2,4-disilyl dye molecules adsorbed on the TiO2 electrode was slightly smaller than in the case of 4-monosilyl dye.

Improvement of Photovoltage in Dye-Sensitized Solar Cells with Azobenzene and Azulene Sensitizing Dyes by Applying Br3-/Br- Redox Mediator

In dye-sensitized solar cells (DSSCs) fabricated with azobenzene and azulene derivatives as the photosensitizing dyes, the replacement of the conventional I3-/I- redox mediator with Br3-/Br- redox mediator possessing more positive redox potential has been examined to improve open-circuit photovoltages (Voc) of the cells. The cells with the Br3-/Br- redox mediator exhibited high Voc of ~0.8 V, resulting in higher light-to-electric energy conversion efficiencies (η) than the cells with the I3-/I- redox mediator by a factor of 1.7 under the simulated sunlight irradiation (AM-1.5G, 100 mW cm-2). The results indicate a high potential of the Br3-/Br- redox mediator in DSSCs.

Photoluminescence and Electroluminescence Properties of (Ca,Sr)TiO3:Er

Powder samples of (Ca,Sr)TiO3:Er were prepared by a solid-state reaction method. Photoluminescence due to f-f transitions of Er3+ was not induced by band-gap excitation of (Ca,Sr)TiO3 but by f-f transitions of Er3+. An electroluminescent device in which thin films of Ca0.6Sr0.4TiO3:Er and SnO2:Sb are stacked alternately was prepared by sol-gel and spin-coating methods. Very weak electroluminescence due to f-f transition of Er3+ was observed in the device.

Electroluminescence of a Multilayer in which Thin Films of NaNbO3:Pr Phosphor and SnO2:Sb Transparent Conductor Are Alternately Stacked

Oxide inorganic electroluminescent device in which thin films of Pr-doped NaNbO3 phosphor and Sb-doped SnO2 transparent conductor are alternately stacked was prepared by sol-gel and spin-coating methods. Red electroluminescence was observed due to f-f transitions of Pr3+ ions by applying 5-kHz ac voltages to the device. The luminance was 1.0 cd m−2 at 25 V ac and5.0 cd m−2 at 34 V ac.

Effect of B2O3 or SiO2 Fluxes on Morphology and Size of Pr-Doped CaTiO3 Phosphor Particles and on their Photoluminescence Properties

Powder of Pr-doped CaTiO3 red phosphor was prepared at 1473 K by a conventional solid-state reaction method with addition of B2O3 or SiO2 as a flux. Primary particle sizes of the prepared samples were increased by using B2O3 flux but decreased by using SiO2 flux. Clear planes, edges, or steps were observed on surfaces of the primary particles. The intensity of photoluminescence induced by irradiation of light with a wavelength longer than ∼350 nm was enhanced about three times by using B2O3 flux. The intensity of photoluminescence induced by irradiation of light with a wavelength shorter than ∼350 nm was enhanced about twice by using either B2O3 or SiO2 fluxes. The origin for the flux effects is discussed.

Effect of the Introduction of a CF3 Group to a Silyl-Anchor Azobenzene Dye on Sensitization Property in Dye-Sensitized Solar Cells

The effect of the introduction of a CF3 group to a silyl-anchor moiety of the sensitizer on the sensitizing property for the dye-sensitized solar cell was examined by using alkoxysilyl-anchor azobenzene dyes. The cell sensitized by the silyl-anchor dye with the electron-withdrawing CF3 group in the silyl-anchor moiety exhibited higher photovoltaic performance than that by the silyl-anchor dye without the CF3 group.