Katsuhisa Tanaka

Thermal evolution of gels derived from CH3Si(OC2H5)3 by the sol-gel method

Abstract The structure change of CH3Si(OC2H5)3 (MTES)-derived gels on heating in N2 and NH3 to form C- and N-containing SiO2 glasses, respectively, was examined by IR spectroscopy and thermal analysis. The gel was considered to consist of siloxane polymers containing strained 4-fold siloxane ring structure. The Siue5f8CH3 and 4-fold rings were thermally cleaved around 700°C to form Siue5f8H, Siue5f8OH and carbonous matter in N2, and Siue5f8NH2 as well as Siue5f8H and Siue5f8OH bonds in NH3. The Siue5f8NH2 was condensed at higher temperatures, resulting in N-containing SiO2 glass.

Preparation of multiple oxide BaTiO3 fibres by the sol-gel method

Multiple oxide BaTiO3 gel fibres were prepared by the sol-gel method from Ba(OC2H5)2-Ti(O-isoC3H7)4-H2O-C2H5OH-CH3COOH and Ba(CH3COO)2-Ti(O-isoC3H7)4-H2O-CH3COOH solutions. Relatively long gel fibres of 10cm length were obtained from both solutions in the limited composition region. The latter solution in particular showed a spinnability even when it contained no water. Therefore, the occurrence of spinnability of the solution was considered to be due to the formation of linear polymers composed of bridging acetate groups such as Ti←O-C(CH3)-O-Ti rather than metalloxane bonding as Ti-O-Ti. Addition of water to the solutions seems to break the bridging acetate bonds and replace some of them by bridging oxygen bonds. The as-drawn gel fibres which were X-ray amorphous crystallized into BaTiO3 ceramic fibres of 5mm average length upon heating above 600 ° C. However, the gel fibres drawn from the sols without water became powdery on heating because of the lack of Ti-O-Ti metalloxane bonds. The crystallization behaviour of the BaTiO3 gel fibres is discussed based on the infrared spectroscopy, X-ray diffraction analysis and thermogravimetric-differential thermal analysis.

Transparent tellurite-based glass-ceramics with second harmonic generation

Abstract Transparent glass-ceramics, in which the crystalline phase has basically a face centered cubic structure, have been prepared in the system K 2 Oue5f8Nb 2 O 5 ue5f8TeO 2 , and a second harmonic generation (SHG) is observed. The second harmonic intensity in the glass-ceramics is comparable to that in electrically poled TeO 2 -based glasses. A very small distortion from the cubic structure might be present and an origin of SHG. Transparent glass-ceramics consisting of the cubic crystalline phase have been also fabricated in the system of Li 2 O or Na 2 Oue5f8Nb 2 O 5 ue5f8TeO 2 , but SHG is not observed.

Structure and ionic conductivity of LiClLi2OTeO2 glasses

Abstract The structure and the ionic conductivity of the ternary LiClue5f8Li 2 Oue5f8TeO 2 glasses were investigated. The structure was studied mainly by means of IR spectroscopy. It was observed that the Teue5f8O ax bond, where ax stands axial position of the TeO 4 trigonal bipyramid, became weaker with increasing the LiCl content in binary LiClue5f8TeO 2 glasses, indicating that LiCl works as a network modifier. On the other hand, the gradual increase in the wave number of the peak due to the Teue5f8O ax bond was observed when Li 2 O was replaced with LiCl. This was explained in terms of a difference in bonding characteristics between Teue5f8O eq and Teue5f8Cl eq bonds, where eq denotes the equatorial position. The electrical conductivity of the glasses with a high concentration of Li + ion reached an order of 10 −6 S cm −1 at 25° C. This value was comparable to those of the other fast Li + ion conducting glasses such as LiClue5f8Li 2 Oue5f8B 2 O 3 and LiPO 3 ue5f8LiCl systems. The composition dependence of the electrical conductivity was discussed in relation to the glass structure, especially in terms of bonding characteristics of Teue5f8Cl and Teue5f8O.

STRUCTURE AND HIGH MAGNETIZATION OF RAPIDLY QUENCHED ZINC FERRITE

Abstract Rapidly quenched zinc iron oxides with nominal compositions of x ZnO·(100−x)Fe 2 O 3 where x =50, 55, 60, 65 and 70 (mol%) were prepared by using a twin-roller quenching method. X-ray diffraction analyses indicate that the most predominant phases precipitated in all the as-quenched specimens are zinc ferrites with spinel structure. The lattice parameters of rapidly quenched zinc ferrites are slightly smaller than that of normal spinel ZnFe 2 O 4 . The magnetization of as-quenched 60ZnO·40Fe 2 O 3 is 23.8xa0emuxa0g −1 at room temperature when the external magnetic field is 10xa0kOe. This value is much higher than the magnetization of ZnFe 2 O 4 prepared by the solid state reaction. We suggest that some of the Fe 3+ ions occupy tetrahedral sites as well as octahedral sites. Some of Zn 2+ ions also occupy octahedral sites in the present rapidly quenched zinc ferrites. This leads to the high magnetization caused by strong superexchange interaction between octahedral and tetrahedral Fe 3+ ions, as proposed for ultrafine particles of ZnFe 2 O 4 which show high magnetization even at room temperature. Zn K-edge extended X-ray absorption fine structure (EXAFS) measurements support that there exist six-coordinated Zn 2+ ions in the present rapidly quenched zinc ferrites. In addition, a solid solution of magnetite which is suggested by the lattice parameters of as-quenched specimens contributes to the high magnetization as well.

Induction and relaxation of optical second-order nonlinearity in tellurite glasses

Second harmonic generation has been examined for 30ZnO⋅70TeO2 glass with a two-step poling procedure in order to understand the poling temperature dependence of second harmonic intensity. When the poling temperature increases, the second harmonic intensity increases, manifests a maximum at the temperature which we call an optimum poling temperature, and decreases drastically just below the glass transition temperature. The glass treated with two-step poling, which includes poling at 300u200a°C and subsequent poling at the optimum poling temperature, i.e., 280u200a°C, exhibits much smaller second harmonic intensity and more unambiguous Maker fringe pattern than that poled only at 280u200a°C. This fact suggests that the decrease in second harmonic intensity with an increase in poling temperature cannot be attributed to a reversible process like a thermal fluctuation of dipoles, but is governed by an irreversible one. Based on a linear relation between the optimum poling temperature and glass transition temperature, the...

Growth of fibrous hydroxyapatite in the gel system

Hydroxyapatite (HAP) was prepared in the agar gel system, where Ca2+ ions were incorporated in the gel and PO43− solution was layered over the gel. When the concentration of Ca2+ was lower than 1.0 M and the initial solution Ca/P molar ratio was lower than about unity, fibrous HAP several centimetres to several tens of centimetres in length was grown upwards in the PO43− solution from the gel phase, while at the larger Ca2+ concentration and initial Ca/P molar ratio CaHPO4 · 2H2O (DCPD) in the form of gelatinous precipitate, particulate precipitates or needle-like crystals were preferred to HAP. The fibrous HAP was calcium deficient and composed of small elongated hollow ovals linked in a zigzag row. This was considered to be formed in the following manner. First, Ca2+ ions supplied through pores in the gel reacted with PO43− ions to form a small spherulite at the pore exit, then the spherulite was bloated and elongated by the osmotic pressure or capillary force until a part of the oval was broken for Ca2+ ions to be gushed out into the PO43− solution. The above two processes were repeated to form elongated hollow ovals linked to the preceding ones.

Second Harmonic Generation in Poled Tellurite Glasses

Second harmonic generation has been observed in electrically poled tellurite glasses with 80TeO210Li2O10Nb2O5 and 70TeO215Li2O15Nb2O5 compositions. The poling was carried out at 200 to 300°C for 30 min under applied dc field of 4 to 5 kV. To explain the mechanism generating the second-order nonlinearity in the poled tellurite glasses, it is proposed that strong local polarization is caused by migration of lithium ions and/or periodic orientation of lone pairs of electrons in the tellurium ions.

Do triboluminescence spectra really show a spectral shift relative to photoluminescence spectra

There are many reports in the literature of a spectral difference between the triboluminescence (TL) spectrum and the solid-state photoluminescence (PL) spectrum of the same compound. Numerous reasons have been suggested for this difference including pressure-induced changes to Franck-Condon factors during the lifetime of the TL light emission; self-absorption of the TL emission and fracture-induced symmetry changes perturbing the local field of the TL emitting species. However, in a number of cases, the luminescent spectra were recorded on different equipment with different spectral responses, with the resolution of either luminescent spectrum rarely quoted. To avoid artificial spectral differences, care must be taken to account for the response of each equipment over the wavelength range studied, as well as any resolution difference. We have therefore measured the TL and solid-state PL spectra of sixteen TL compounds on the same spectrometer at an identical resolution. Furthermore, the solid-state photoluminescent lifetime has been determined for all samples and the implication that these values have for observing pressure-induced (Franck-Condon) luminescent spectral changes discussed. Finally, in all cases where a significant difference was observed between TL and PL spectra, either self-absorption of the TL emission or fracture-induced perturbations of the local field have been evoked to explain the difference.

Faraday effect of sodium borate glasses containing divalent europium ions

Faraday effect measurements have been carried out at room temperature for some sodium borate glasses containing Eu2+ ions prepared under a reducing atmosphere. The wavelength dependence of the Verdet constant is analyzed in terms of a theory derived by Van Vleck and Hebb to obtain effective transition wavelength of the 4f7→4f65d transition which causes the Faraday effect of Eu2+ in the visible range. The effective transition wavelength increases with an increase in the optical basicity of glass. This phenomenon is explained in terms of the crystal field theory; because the splitting of 5d levels is larger in the glass with larger optical basicity, the energy required for the 4f7→4f65d transition decreases as the optical basicity of glass increases. The magnitude of Verdet constant increases with an increase in the effective transition wavelength when the concentration of EuO remains constant.