Sumio Sakka

Kinetic study of crystallization of glass by differential thermal analysis—criterion on application of Kissinger plot

Abstract For the purpose of analyzing the crystallization process of glass by DTA, the modified Kissinger-type equation was derived on the basis of the nucleation and growth equations, and the validity of the equation was ascertained by applying to the crystallization of Li 2 O·2SiO 2 glass whose kinetic data regarding crystallization are already well-known. The modified Kissinger-type equation is identical with the so-called Kissinger equation only when the crystallization starts at the surface and grows towards the inside of the glass one-dimensionally. The conditions required for applying the Kissinger plot or modified Kissinger-type plot to the crystallization of glass were discussed, and it was concluded that the crystallization mechanism should be known in order to obtain the meaningful activation energy.

The sol-gel transition in the hydrolysis of metal alkoxides in relation to the formation of glass fibers and films

Abstract The sol-gel transition of Si(OC 2 H 5 ) 4 was investigated by measuring the volume, the density, the SiO 2 content, the viscosity and the specific and intrinsic viscosities of the solution. The transition behavior of Si(OC 2 H 5 ) 4 solutions with different amounts of water and different catalysts was compared. The measurements indicated that solutions with a small water content were comprised of linear polymer when catalyzed by HCl. Fibers could be easily drawn from such solutions immediately before gelation. Solutions catalyzed by HCl and containing a large amount of water behaved similarly to solutions catalyzed by NH 4 OH, suggesting that a high water content causes the formation of non-linear or network colloidal polymers in hydrolysis-polycondensation. Such solutions did not exhibit spinnability but gelled into elastic bulk masses which were easily converted to bulk glasses.

Glasses from metal alcoholates

Abstract Developments in the technique of preparing oxide glasses from metal alcoholates through hydrolysis, gelling and heating at low temperatures have been reviewed. This technique makes it possible to produce (1) glasses of very high purity and (2) glasses of compositions which the conventional melting technique could not give because of immiscibility, crystallization and high melting temperatures. This technique can provide glasses having various shapes such as fibers, thin coating films, isolated films and bulk glasses. Discussion was made on the microscopic processes leading to fibers and bulk glasses.

Preparation of glass fibres of the ZrO2-SiO2 and Na2O-ZrO2-SiO2 systems from metal alkoxides and their resistance to alkaline solution

Glass fibres of the ZrO2-SiO2 and Na2O-ZrO2-SiO2 systems containing up to 33 wt% ZrO2 were prepared by a non-melting technique using zirconium n-propoxide, sodium methoxide and silicon tetraethoxide as raw materials. The mixed alkoxide solutions were exposed to moist air for hydrolysis. The fibrous gels were drawn from these solutions in the course of hydrolysis, and converted to the corresponding oxide glass fibres by heating at 500 to 700° C. It was found that chemical durability of the fibres toward alkaline solution increased with ZrO2 content. The weight loss due to the corrosion by 2 N NaOH solution at 96° C for 4 h was around 14 mg dm−2 for the fibres containing 17 to 26 wt% ZrO2, which was comparable to the alkali-resistant glasses so far obtained by the conventional melting technique. The glass fibres containing 33 wt% ZrO2 showed higher resistance.

Determination of the activation energy for crystal growth by differential thermal analysis

A method for determining the activation energy for crystal growth was calculated on the basis of the heat balance in the differential thermal analysis (DTA) measurements and the mechanism of nucleation and growth. The theoretical analysis showed that the term ln[Cpd(δT)/dt+KδT] should be a linear function of l/T, whereCp is the heat capacity of sample and sample holder,K is the heat transfer coefficient,δT is the temperature difference between the sample and reference substance andt is the time. The energy term,ED, obtained by multiplying the slope of the resulting straight line byR is indicative of the activation energy for crystal growth. It was shown thatED should be three times the activation energy for crystal growth when bulk nucleation is dominant, and equal to that for crystal growth only when surface nucleation predominates. The result of the analysis was tested by comparing the experimentally determinedEDs with the activation energy for viscous flow, which was known to represent that for crystal growth. TheED for Li2O·2SiO2 glass with dominant bulk nucleation, approached three times the activation energy for viscous flow, as the heating rate in DTA decreased. TheED for 33.3Li2O·66.7SiO2·3TiO2 glass with dominant surface nucleation approached the activation energy for viscous flow as the heating rate increased, suggesting the validity of the analysis.

Preparation and properties of CaAlSiON oxynitride glasses

Abstract The effect of preparation condition on the melting behavior and the effect of nitrogen content on the properties were investigated for CaAlSiON OXYNITRIDE GLASSES. It was found that melting at lower temperatures produces more homogeneous and clearer glasses than melting at higher temperatures when the glass is melted at temperatures between 1530°C and 1750°C. The density, glass transition and softening temperatures, Vickers hardness and Youngs modulus increased markedly with increasing nitrogen content. The observed values of densities of nitrogen-containing glasses was found to agree well with the additivity calculation based on the assumption that each nitrogen atom in the glass structure is connected with three silicon atoms.

Structure of alkali germanate glasses studied by spectroscopic techniques

Abstract EXAFS spectra and X-ray radial distribution curves were determined for Li2OGeO2, Na2OGeO2 and K2OGeO2 glasses to elucidate the glass structure. The formation of 6-coordinated Ge atoms on the addition of an alkali oxide R2O to GeO2 glass was confirmed. The concentration of 6-coordinated Ge atoms N6 increased with an increase of the R2O content, exhibited a maximum around 20 mol% R2O and decreased again with a further increase of the R2O content. No essential dicrepancies between the techniques employed and between the glass systems investigated were found in the N6 value and the R2O content of glass at the maximum. The small discrepancies were attributed to the different assumptions made for the calculations. It was concluded that the concentration of 6-coordinated Ge atoms N6 follows the formula N6=x/(100-x), where x is mol% R2O. The maximum value of N6 was about 25% for the composition of about 20 mol% R2O.

Aragonite formation through precipitation of calcium carbonate monohydrate

Abstract Formation of aragonite from amorphous, gelatinous CaCO 3 and CaCO 3 · H 2 O precipitated in the solution containing Mg 2+ ion was investigated. Amorphous CaCO 3 was converted to aragonite via CaCO 3 · H 2 O formation on drying in air. The stability of CaCO 3 · H 2 O in air depended on how long it has been immersed in the mother solution. Effect of Mg 2+ ion on the formation of aragonite was discussed on the basis of surface energy of a small particle.

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.

Incorporation of copper into glass by the CuNa ion exchange

Abstract Aluminosilicate glasses containing 20 mol% Na 2 O have been ion-exchanged by heating in CuCl melt mainly at 550°C and the concentration distribution curve of copper has been measured as a function of depth from the surface with an electron microprobe X-ray analyser. It has been found that 30–40% Na 2 O in glass can be replaced by Cu 2 O at the surface, indicating that some Cu + ions change to Cu 2+ ions after entering the glass. The copper concentration profile of glass after ion exchange has been found to be stepwise. It has been shown that the ion-exchange rate increases remarkably with increasing Al 2 O 3 content of glass. In the series of 20Na 2 O·Al 2 O 3 ·(80−x) SiO 2 , the rate of ion exchange markedly increases with increasing Al 2 O 3 content, until it reaches a maximum at the Al 2 O 3 content of 20 mol%. On the other hand, no maximum of the ion-exchange rate has been found at 20–25 mol% for the series of 20Na 2 O·10CaO·xAl 2 O 3 ·(70−x)SiO 2 and the ion-exchange rate continues to increase with Al 2 O 3 content up to 30 mol% Al 2 O 3 . It has also been shown that the CaO component in glass suppresses the ion-exchange rate to some extent.