F. Branda

Surface and bulk crystallization in non-isothermal devitrification of glasses

Abstract The surface and bulk crystallization of Li 2 O · 2 SiO 2 glass has been studied by differential thermal analysis and the influence of the specific surface area of the sample and the nucleation heat treatment on the crystallization kinetics is pointed out. The kinetic parameters were also evaluated from the DTA curves and related to the crystallization mechanism. The results agree well with the isothermal data reported in the literature.

Evaluation of glass stability from non-isothermal kinetic data

Abstract Non-isothermal devitrification of 1.1Li 2 O·1.9SiO 2 glass has been studied and is compared with that of Li 2 O·2SiO 2 glass reported in the literature. In both cases the devitrification mechanism appears to be controlled by the crystal-glass interface reaction. The maximum nucleation rate temperature is, also, the same. With respect to the Li 2 O·2SiO 2 glass, the crystal growth activation energy, E c , is slightly greater and the DTA devitrification peaks are shifted towards lower temperatures. If T p is the devitrification peak temperature on a DTA curve recorded under conveniently selected experimental conditions, a dimensionless parameter, E c / RT p , can be defined which can be used to compare the devitrification tendency of different glass compositions. The glass studied has a greater value of E c / RT p with respect to the parent glass and in the light of the present work thus shows a lower thermal stability.

Nucleation and crystal growth in Na2O · 2 CaO · 3 SiO2 glass: a DTA study

Abstract The non-isothermal devitrification of Na 2 O · 2 CaO · 3 SiO 2 glass has been studied by differential thermal analysis in order to evaluate, from DTA curves, the temperature of maximum nucleation rate, T m , and the activation energy values, E c , for crystal growth. The temperature, T m =580°C, is very close to the glass transition temperature, T g =570°C, and the value of E c =78 Kcal mole −1 for the surface crystal growth is nearly the same as the value E c =89 kcal mole −1 for the bulk crystal growth; both are consistent with the activation energy for viscous flow. It is also pointed out that the nucleation rate—temperature curve and the crystallization rate—temperature curve are partially overlapped.

Structure and devitrification behaviour of sodium, lithium and barium borophosphate glasses

Thermal properties and devitrification behaviour of sodium, lithium and barium borophosphate glasses have been studied by differential thermal analysis. The results are in good agreement with the hypothesis that BO 4 groups form when a network modifier oxide (Na 2 O or Li 2 O or BaO) is added to B 2 O 3 . The excess of negative charge is compensated by metallic cations or positively charged PO 4 groups. Activation energy for crystal growth and devitrification mechanism under non isotermal condition in sodium, lithium and barium metaphosphate glasses were also evaluated.

Sol–gel synthesis and crystallisation of 3CaO·2SiO2 glassy powders

Abstract A gel of composition 3CaO·2SiO 2 was synthesised by means of sol–gel route involving hydrolysis of tetramethyl orthosilicate (TMOS) with calcium nitrate in alcoholic medium, and polycondensation reaction. The gel was submitted to DTA and TGA analysis and the resulted material was examined by FTIR spectroscopy and X-ray powder diffractometry. The thermal treatments required for the gel into glass conversion led to a partially devitrified product. The XRD analysis showed that the structure obtained is highly depolymerised.

Kinetics of crystal growth in Na2O · 2 SiO2 glass. A DTA study

Abstract The non-isothermal devitrification of Na 2 O·2 SiO 2 glass is investigated by differential thermal analysis (DTA). The kinetic parameters for the viscous flow of the glass and the temperature of maximum crystallization rate are evaluated from DTA curves. The limits of the Arrhenius-type relationship between the rate constant of crystal growth and the absolute temperature are also discussed.

Tg and FTIR of (2.5 − x)CaO·x/3M2O3·2SiO2 (M=Y, La, In, Al, Ga) glasses

A comparative study of the effect of substituting oxides of (M=Y, La, In, Al, Ga) to CaO in 2.5CaO2SiO2 glass in the narrow compositional range of the binary CaO–SiO2 system in which homogeneous glasses can be obtained (0.4 < CaO/(CaO + SiO2) < 0.55). A plot of glass transformation temperature, Tg, vs the ionic field strength, Z/r2, where Z and r are the charge and the radius of the cation, is useful in discussing the role of oxides in the glassy structure. It is hypothesized that, in the composition range, studied Al2O3 and Ga2O3 act as network forming oxides, while La2O3, Y2O3 and In2O3 act as network modifying oxides. FTIR spectra agree with this hypothesis and with the expectations based on the criteria reported in the literature, in particular with the one proposed by McMillan that the network modifier cations have an ionic field strength Z/r2 < 5 A−2.

Hydroxyapatite coating of titanium by biomimetic method

The biomimetic method was used in order to deposit, on titanium substrates, an hydroxyapatite (HA) coating. The bioactive HA layer was obtained by using, in the first stage of the process, a glass having the composition 2.5CaOċ2SiO2 different from the one proposed for the application of the biomimetic method. This glass can be obtained via sol–gel, a method that allows one to obtain, easily, very pure products. The growth of HA crystals was confirmed by Fourier transform infrared, SEM, EDS and X-ray photoelectron spectroscopy (XPS) results. The experimental results suggest that, as reported in the literature for other supports, the silicate ions released from the glass in the first stage bind themselves to the titanium support. In particular, from XPS analysis it is evident that the titanium substrate is well covered by a calcium phosphate layer of the type of HA.

Non-isothermal devitrification behaviour of diopside glass

Abstract The non-isothermal devitrification of diopside CaO · MgO · 2SiO 2 glass has been studied. The crystal growth activation energy has been evaluated as E c ± 137 ± 10 kcal mol −1 . The devitrification occurs in bulk samples through a surface nucleation mechanism. Surface nuclei behave as bulk nuclei in very finely powdered samples sintered before devitrifying. Optical and scanning electron microscopy support these conclusions.

Nucleation in glass-forming systems. A DTA study.

Abstract A simple method recently proposed (ref. 1) for evaluating the temperature of maximum nucleation rate is applied to several different glass-forming systems. The results well agree with those obtained by means of traditional methods.