Akihiko Nukui

Oxyfluoride tellurite glasses doped by erbium: thermal analysis, structural organization and spectral properties

Abstract The effects of fluorine substitution in zinc tellurite glass system doped with rare earths on the spectral properties of the Er 3+ ions are investigated. Differential thermal analysis, vibrational spectroscopies studies and in situ high-temperature X-ray diffraction measurements have been considered in term of fluorine influence. As a function of composition, we have principally measured optical absorption, spontaneous emission, and lifetime measurements. Judd–Ofelt intensity parameters of Er 3+ in these host glasses were determined and used to calculate radiative transition rates and lifetimes. The bulk composition variation and addition of fluoride compounds in tellurite glasses result among others, in broad emission spectra, improved emission lifetime and difference in relative band intensities compared to pure oxide glass.

In situ high-temperature X-ray observation of structural changes of tellurite glasses with p-block oxides; ZnO-TeO2 glasses

Abstract The thermal behavior of each of the three x ZnO·(100− x )TeO 2 (where x =10, 20 and 30 mol%) glasses was studied by an in situ high-temperature X-ray diffractometer, using a rapid measurement system, and differential thermal analysis (DTA). DTA results compared with X-ray diffraction data resulted in phase relations, at high temperature, of the three different glasses. The characteristic temperatures (glass transition and crystallization temperatures) determined for the three glasses showed a dependency on the ZnO content. In the three glasses, the α-TeO 2 and ZnTeO 3 were found to crystallize first, followed by the crystallization of Zn 2 Te 3 O 8 . The phases observed at high temperatures in the three glasses were the same, except for an unknown phase found in the 30ZnO·70TeO 2 glass. Existence-temperature ranges of the phases observed in the three glasses were found to be different. The relation between the first crystallization phase and the local structures of the ZnO–TeO 2 glass systems is discussed.

EXAFS AND RDF STUDIES OF TEO2-LI2O GLASSES

Local arrangements around Te atoms in TeO 2 -Li 2 O glasses with four different Li 2 O contents (15, 20, 25, and 30 mol %) are observed by Te K EXAFS spectroscopy and x-ray diffraction by making use of synchrotron radiation. EXAFS results based on the two-shell fitting method indicate that interatomic distances of Te-O in axial (ax) sites decrease from 0.208 to 0.197 nm with increasing Li 2 O contents, while distances between tellurium atom and oxygen atoms in equatorial (eq) sites change slightly from 0.190 to 0.188 nm. Total coordination numbers seem to decrease slightly with increasing Li 2 O contents. These results suggest the coordination states of tellurium atoms are changed from TeO 4 trigonal bipyramids to TeO 3+1 polyhedra and TeO 3 trigonal pyramids. RDF results also suggest the change of coordination states of tellurium atoms. TeO 3+1 polyhedra in glasses are considered to be connected at the vertices with Te- eq O ax -Te or Te- ax O ax -Te linkage as seen in crystalline α-Li 2 Te 2 O 5 .

Thermochromic property of tellurite glasses containing transition metal oxides

Abstract Thermochromic properties of the tellurite glasses containing transition metal oxides have been measured. The transmission spectra were measured in the range from the UV to the visible region at various temperatures below the glass transition temperatures. The cut-off wavelength in the optical spectra was shifted toward ‘red’ wavelengths at rates of approximately 8 × 10 −4 eV/K with increasing temperature. The shift of the transmission edge with temperature took place reversibly. The shift rates were compared with those of lead-silicate and lead-borate glasses containing transition metal oxides.

The role of precursors in the structure of SiO2-Al2O3 sols and gels by the sol-gel process

Binary sols and gels of SiO2-Al2O3 were prepared using tetraethyl orthosilicate and each of four aluminum compounds; aluminum di (sec-butoxide) ethylacetoacetic ester chelate (AC), aluminum nitrate nonahydrate (AN), aluminum formoacetate (AF), and boehmite sol (BS) made from aluminum i-propoxide. The structure and the evolution of the Si-O-Al bonds in SiO2-Al2O3 sols and gels were investigated by 27Al nuclear magnetic resonance (NMR), Infrared absorption spectra, DTA, and X-ray diffraction. The formation of Si-O-Al bonds differs depending on the aluminum compounds used as raw materials. The ratio of Al(IV) to {Al(IV) + Al(VI)} is related to the microstructural homogeneity of the gels. When AC is used as a raw material, the Si-O-Al bonds are formed in the sol state and resultant gel shows good microstructural homogeneity. In case of AN, the Si-O-Al bonds are not formed either in the sol or the wet gel state. The bonds are formed by drying the gel before heat-treatment temperature reaches 300 ∼ 400°C, resulting in good microstructural homogeneous gel. When AF is used, the Si-O-Al bonds are formed in the sol state but the ratio of Al(IV) to {Al(IV) + Al(VI)} is lower than when using AC. Microstructural homogeneity of the gel is ranked between AC or AN and BS. Using BS, the Si-O-Al bonds are not formed in the sol solution, and the change in the coordination number of the gel is similar to that of boehmite gel. The Microstructural homogeneity of the gel is the worst among the BS gels, which were prepared by using the four aluminum raw materials.

EXAFS study on the local environment of Cu+ ions in glasses of the Cu2O–Na2O–Al2O3–SiO2 system prepared by Cu+/Na+ ion exchange

Abstract The local environment around Cu + in Cu + /Na + ion-exchanged sodium aluminosilicate glasses was investigated as a function of the ratio x (x= Cu + /[ Cu + + Na + ]) by extended X-ray absorption fine structure spectroscopy (EXAFS). Glass specimens with various ratios were prepared by ion exchange in 20Na 2 O–10Al 2 O 3 –70SiO 2 (mol%) glass at 400°C followed by concentration equalizing treatment. The coordination number N of Cu + in the glasses of x 20Cu 2 O–(1− x )20Na 2 O–10Al 2 O 3 –70SiO 2 (mol%) increased accompanying the increase of the interatomic distance r Cu–O with increasing x: N=2.2, 2.8 and 3.7, and r Cu – O =0.184, 0.188 and 0.191 nm for the samples of x=0.2, 0.4 and 0.63, respectively. The change in the coordination of Cu + was discussed in relation to the structural evolution caused by ion exchange. These changes seem to induce a change in the environment around Cu + from twofold to fourfold coordination structure. The fourfold site for Cu + offered a favorable situation for ionic conduction, because these highly ion-exchanged glass samples showed high electrical conductivity.

Study of thermal behaviour of 30Li2O·70TeO2 glass by X-ray dynamic observation

The thermal behaviour of a 30Li2O·70TeO2 glass was studied by use of an in situ high-temperature X-ray diffractometer equipped with a position-sensitive detector (PSD). The dynamical structural changes were also observed by differential thermal analysis (DTA) and differential scanning calorimetry (DSC) measurements. Differences and contradictions exist in previous studies related to the phase relation at high temperature, including the process of phase change. It is found that in the exothermic process after a glass transition between 553 and 623 K, an unknown phase of lithium tellurite (LT-X) crystallizes at first. At almost the same temperature, a minor component of vitreous TeO2 appears during decomposition of vitreous 30Li2O·70TeO2 of the initial phase. After that, the thermal changes of the lithium tellurite and the TeO2 components proceed independently. The LT-X phase converts to α-Li2Te2O5, and the vitreous TeO2 phase is followed by crystallization of an α-TeO2 phase. In the endothermic process between 673 and 793 K, the α-Li2Te2O5 phase transforms into a β-Li2Te2O5 phase and the minor crystalline α-TeO2 phase melts. Then, β-Li2Te2O5 incongruently melts to form Li2TeO3 and a molten TeO2 phase. Li2TeO3 continuously melts at around 763 K. The results show a possibility of reversible change between the α-Li2Te2O5 and the β-Li2Te2O5 phases.

Correlation between specific heat and change of refractive index formed by laser spot heating of tellurite glass surfaces

Abstract Correlation between the specific heat and the refractive index change formed by laser spot heating on tellurite glass surfaces has been investigated. The ternary tellurite glasses of TeO 2 –Na 2 O–Al 2 O 3 , TeO 2 –Na 2 O–GeO 2 and TeO 2 –Na 2 O–TiO 2 doped with 2 mol% of CoO were irradiated by a green light beam spot (532 nm) from a second harmonic generator of a Q switch pulse YAG laser. The refractive index map of the glass surface was measured with a He–Ne laser beam using a scanning ellipsometric technique, showing the formation of the refractive index dot patterns lower than the matrix by 0.05–0.22. The specific heat of the glasses was measured by CO 2 laser flashing method. The glasses containing Al 2 O 3 possessed the lowest specific heat among the three glass systems, giving the largest refractive index change on the glass surfaces. A good linear correlation between the reciprocal of the specific heat and the refractive index change was found in cases where the refractive index change was less than 0.1, suggesting that the refractive index change derived from the change of the fictive temperature of the glasses.

Refractive index patterning of tellurite glass surfaces by ultra short pulse laser spot heating

Dot patterns of refractive indices were formed by the laser pulse irradiation on the tellurite glasses. The ternary tellurite glasses of TeO2-Na2O-Al2O3, TeO2-Na2O-GeO2 and TeO2-Na2O-TiO2 doped with 2 mol% of CoO were irradiated by a femtosecond pulse laser beam (800 nm) or by a green light beam (532 nm) from a second harmonic generator of a Q switch pulse YAG laser. The refractive index map of the glass was composed with an He-Ne laser beam by an scanning ellipsometric technique at a resolution of 100 μm × 50 μm, indicating that the spots possessing refractive index lower by about 0.05–0.38 than the surroundings were formed at the region irradiated by the laser beam. The irradiation of the femtosecond laser beam generated the dot patterns roughly equivalent to the beam size. The change of refractive index could be tunable by adjusting laser power, suggesting that the process could be applied to optical recording.

Increased crystal porosity and enhanced gas adsorption by intracolumnar gliding for broadband gas detection

A new three-level structural approach based on anion-induced intracolumnar gliding of stacked coordinatively unsaturated units is described for design of metal-organic crystals which exhibit concentration- and gas-specific adsorption of a variety of small molecules and could be employed for multiple and rapid qualitative and quantitative broadband detection of gases or gaseous mixtures.