Showing papers by "Sumio Sakka published in 1986"

Formation of a high-strength bioactive glass-ceramic in the system MgO―CaO―SiO2―P2O5

Abstract: Formation of a high-strength bioactive glass-ceramic in the system MgO-CaO-SiO2-P2O5 was investigated by observing the microstructure of the crystallized products. Crystallization of the parent glass in a bulk form led to the occurrence of large cracks in the crystallized product. This was attributed to the precipitation of fibrousβ-wollastonite crystals growing perpendicular to the outer surfaces of the glass after uniform precipitation of fine oxyapatite/fluoroapatite crystals. On the other hand, crystallization of the same glass in a powder compact led to the formation of a crack-free dense crystallized product due to uniform precipitation of both apatite and wollastonite fine crystals throughout the glass article. The uniform precipitation of the wollastonite crystals was attributed to the simultaneous formation of fine crystals in the individual glass particles.

Hydrolysis and polycondensation of dimethyldiethoxysilane and methyltriethoxysilane as materials for the sol-gel process

Abstract: In order to understand better the hydrolysis-polycondensation of Si(OC 2 H 5 ) 4 having four functional groups, the corresponding reaction of (CH 3 )Si(OC 2 H 5 ) 3 and (CH 3 ) 2 Si(OC 2 H 5 ) 2 having three and two functional groups, respectively, has been investigated. The tri-functional silane exhibited a similar polycondensation behavior to that of tetra-functional silane; the uniform solution increased in viscosity with time, finally gelling. Fibers could be drawn from the solution before gelation. On the other hand, the bi-functional silane was phase-separated at a certain stage of polycondensation, and no gelation was found over a reasonable time period, probably due to ring molecule formation. On the basis of these data, discussion is presented on the nature of polymers found in the spinnable solution.

X-ray diffraction study of Na2OGeO2 melts

Abstract: The structure of Na2OGeO2 melts in the temperature range from 1100 to 1150°C has been investigated with the high temperature X-ray diffraction technique. Comparing the radial distribution functions obtained for the melts with those for the corresponding glasses, the first peak due to the GeO interatomic distance is invariant upon melting, although it becomes broader due to thermal vibration. The second peak for the GeGe interatomic distance for melts shifts toward the large distance, which is explained by broadening of the GeOGe bond angle, not by the thermal expansion of the GeOGe bond. The composition dependences of GeO distances and coordination numbers of the Ge4+ ion of the melts are found to be almost the same as the corresponding glasses, indicating that even in melts at such high temperatures 6-fold coordinated Ge4+ ions are present and their content changes with the Na2O content as in the case of the corresponding glasses.

Glass formation and optical properties of glasses in the systems (R2O or R'O)-Ta2O5Ga2O3

Abstract: Gallate glasses containing no conventional glass formers were obtained in the systems (Na 2 O, K 2 O or Cs 2 O)-Ta 2 O 5 Ga 2 O 3 and (Sr or BaO)-Ta 2 O 5 Ga 2 O 3 by an ordinary crucible-melting technique. The glasses showed high optical transmission in the infrared as well as in the visible region. Infrared spectroscopic analysis suggested that the Ga 3+ ions are tetrahedrally coordinated in the glasses. The glass-forming tendency of the melt and the infrared transmission of the glasses are discussed in terms of the glass structure.

Structures of alkali niobium gallate glasses

Abstract: Raman spectroscopy was carried out on glasses in the system Cs2ONb2O5Ga2O3 and K2ONb2O5Ga2O3, in order to elucidate the structure of the glasses. The Raman bands assigned to the GaO vibration have indicated that Ga3+ ions are four-fold coordinated by oxygens in all the glasses used in this study, indicating that Ga2O3 plays a role of network former in gallate glasses. The structural state of Nb5+ ions has been also discussed.

Migration of Cu+ ions in Cu2OAl2O3SiO2 glasses on heating in air

Abstract: The behavior of copper ions in the Cu 2 O·Al 2 O 3 ·4SiO 2 (in moles) glass on heating in air at temperatures up to 500°C was studied. When the glass, in which about 90% of Cu was present as Cu + ions, was heated in air above 300°C, a CuO layer was formed on the surface. The amount of CuO was increased with heating temperature and time, corresponding to the decrease in weight of the glass. Furthermore, the fraction of Cu 2+ ions in the glass increased. These observations suggest that oxygens do not diffuse into the glass, but Cu + ions migrate to the surface from the interior to balance the surplus positive charge produced by the oxidation of Cu + to Cu 2+ ions inside the glass. The following reaction scheme for the formation of the CuO layer was proposed; 2Cu + (interior) + 2 1 O 2 (surface) → Cu 2+ (interior) + CuO(surface).

Formation of alumina fibers by unidirectional freezing of gel

Abstract: Alumina fibers have been prepared by the unidirectional freezing of gel, which consists of three steps: gelation of the sol by dialysis in water, unidirectional freezing of the gel, and thawing of the frozen gel. It was found that alumina fibers were formed when the following conditions were satisfied: (1) a high Al2O3 concentration of the sol, (2) the presence of feather-like aggregations of fibrous particles in the sol, (3) the appropriate dialysis time and (4) the rate of freezing appropriate for the cellular growth of ice fiber crystals in unidirectional freezing.

Structural changes of glass-ceramics of the Cu2O-Al2O3-SiO2 system on heating in air

Abstract: The behaviour of copper ions in low thermal expansion glass-ceramics prepared from Cu2O · Al2O3 ·nSiO2 glasses, or inβ-spodumene type Cu2O · Ae2O3 ·nSiO2 crystals included in the glass-ceramics on heating in air was investigated. On hewing at 300 to 500° C, the copper ions behaved as in the corresponding glasses. Cuprous ions in the glass-ceramics orβ-spondumene type crystals were oxidized into the cupric state, and at the same time an equal amount of cuprous ions to those oxidized were expelled out of the specimen for the requirement of electrical charge neutrality and then reacted with oxygen to form CuO on the surface. The oxidation of cuprous ions and the decrease of the copper content of theβ-spondumene type crystals brought about a considerable decrease in the lattice spooings.

Hydration of phosphate and borate glasses in an autoclave

Abstract: Kinetics of hydration of CaOAl 2 O 3 P 2 O 5 and Na 2 OCaOB 2 O 3 Al 2 O 3 glasses in an autoclave at high temperatures and high pressures has been investigated. The hydration of the phosphate glasses may occur as a result of hydrolysis of glass constituents to form orthophosphate crystals. Cabal glasses which do not contain any alkali oxides have shown a quite high resistance to water. Substitution of sodium for calcium deteriorates the chemical durability of Cabal glasses.