Melt-derived Glass Research Articles

Effect of Preparation Conditions on the Properties of CdSe Microcrystal-Doped SiO2 Glass Thin Films Prepared by RF-Sputtering

CdSe microcrystal-doped SiO2 glass thin films were prepared by an RF-sputtering technique under various conditions. The mean size and the size distribution of microcrystals in the films were found to be controlled by the sputtering conditions. A marked narrowing of size distribution of CdSe microcrystals was achieved in the films, compared to the melt-derived CdSe microcrystal-doped glasses. The relationship of the blue shift of the absorption edge to the reciprocal square of the mean particle size deviated from those theoretically deduced from the exciton confinement and the electron-hole confinement, and this can be deduced from the significant contribution of the Coulomb force between electrons and holes.

The formation of unusual glasses by sol-gel processing

It has been claimed that one of the main advantages of the sol-gel technique relates to its potential to produce new glasses from compositions which would normally crystallize if processed by quenching a melt. This article shows that, due to the intrinsic nature of gels (high OH-content, residual carbon, high fictive state), sol-gel glasses crystallize faster than melt-derived glasses at any temperature. Additionally, gels must be (in general) slowly heated from room temperature to approximately half the liquidus temperature to densify to a glass. That is a riskier path than quenching from the liquid state because the nucleation region is crossed on heating, allowing crystal growth from a plethora of nuclei. Finally, there has been no report on a dense gel-derived glass made from systems prone to crystallize. It is concluded that the gel route is not as good as conventional processing to reach the vitreous state and is unlikely to lead to dense glasses of reluctant vitrifying compositions.

Raman study of the structure of sodium aluminosilicate gels

The structure of sodium aluminosilicate gels of various composition, prepared by sol-gel methods using TEOS and nitrates, was studied with Raman spectroscopy. Some of these gels, 12.5Na 2O12.5Al 2O 375SiO 2 (albite) and 7.5Na 2O7.5Al 2O 385SiO 2, can be easily densified into glasses by heat treating them at 1100°C. The similarity between Raman spectra of the gel-derived and melt-derived glasses, in particular the absence of the non-bridging-oxygen peak at 1095 cm −1, shows that for the albite composition the structure consists of a 3D-network of interconnected AlO 4/SiO 4 tetrahedra with the negative charge of the AlO 4 groups compensated by the sodium ions. The network structure of the glass phase is already present in the gel phase at temperatures as low as 500°C. For other compositions, such as 15.5Na 2O15.5Al 2O 369SiO 2 , densification of the gel into a glass is not possible because of crystallization during the heat treatment.

Preparation of lithium aluminosilicate glass-ceramic monolith from metal alkoxide solution

Glass-ceramic monoliths with a composition of Li2O·Al2O3·4SiO2 have been synthesized by the sol-gel technique using metal alkoxides as starting materials. Heating dried gel monoliths of about 36 mm diameter and height to appropriate temperatures gave glass-ceramic monoliths of about 20 mm diameter and height without the occurrence of cracks, while a melt-derived glass body of the same composition was cracked or softened on heating. The glass-ceramic monoliths obtained by heating gel monoliths at 1000 ‡C for 40 h were a porous body, whose bulk density and porosity were 1.62 g cm−3 and 34%, respectively. On heating gel monoliths, Β-eucryptite crystals were first precipitated around 750 ‡C, followed by precipitation of Β-spodumene crystals at 830 ‡C. At higher temperatures the latter grew at the expense of the former phase. The crystallized specimens exhibited very low thermal expansion coefficient ranging from −13-12×10−7 ‡C−1 at temperatures from room temperature to 700 ‡C depending on the heat-treatment temperature of gels, indicating that a porous glass-ceramic monolith with ultra-low thermal expansion can be prepared using the present sol-gel method.

Crystallization behavior of sol-gel derived glasses

The synthesis of cordierite and anorthite by an all-alkoxide route has been demonstrated. Amorphous gel-derived anorthite powders are shown to exhibit crystallization behavior similar to that of melt-derived glasses. Crystallization of amorphous gel-derived cordierite powders takes place by the formation of a transient, metastable stuffed β-quartz phase, which converts to cordierite at higher temperatures - similar to the sequence of phase development in melt-derived glasses which are close to cordierite in composition.

Electronic spectra of transition metal ions in gel-derived and melt-derived glasses

The absorption spectra of 3d transition elements Cr, Mn, Fe, Co, Ni and Cu were studied in SiO 2R m O n compositions (R: a transition element), which were prepared at 500°C as coating films through the sol-gel process from a solution consisting of Si(OC 2H 5) 4, H 2O, HCl and the nitrate of a transition metal. Generally speaking, the characteristic features of the peaks corresponding to a particular valence and coordination state of an element were similar to those reported for silica glass prepared at a high temperature of 1450°C, although the concentration of the particular valence and coordination state was not equal. This indicates that the SiO 2 network similar to that prepared at high temperatures is already constructed at low temperatures around 500°C. The oxidation state of copper in glasses prepared or treated in air atmosphere was studied with melt-derived silicate and gel-derived silica glasses by measuring the optical absorption due to Cu 2+ ions and making chemical analysis. It has been shown that the redox equilibrium of copper in glass shifts towards the higher valence state as the temperature decreases in the wide temperature range from about 1550°C to 500°C and that at least a portion of the copper is present as Cu 2+ ions in SiO 2 glass prepared at low temperatures up to about 500°C.

Preparation of 33 mol% Na 2)-67 mol% SiO 2 glass by gel-glass transformation

It is difficult to prepare gels of the binary Na 2OSiO 2 system because the presence of OH − ions promotes quick and controlled gellation. A reproducible method for obtaining monolithic gel samples of 33 mol% Na 2O-67 mol% SiO 2 (33N) is described based upon hydration and polycondensation reactions of SiO(CH 3) 4 and NaOCH 3. Conversion of the dried gels to glasses and glass-ceramics is followed by the evolution of physicochemical properties and by comparing infrared reflection structural features of the gel-derived glasses with conventional melt-derived glasses. Densification in gel-glass transformation in the range from 490–520°C produces non-porous glass monoliths with density and structure equivalent to melt-derived 33N glasses. At higher temperatures (520°C) uncontrolled crystallization occurs.