Authors:C. Păcurariu, R. Lazău, I. Lazău, and D. Tiţa
The crystallization kinetics of some glass-ceramics obtained from Romanian (Şanoviţa) basalt has been studied in non-isothermal
conditions using DTA technique. The activation energies of the crystallization processes were calculated using the isoconversional
methods Kissinger-Akahira-Sunose and Ozawa-Flynn-Wall. The results obtained show a dependence of the activation energy (Eα) on the crystallized fraction (α) that proves the complex mechanism of the glass-ceramics crystallization process. It has
been proved that the Johnson-Mehl-Avrami model cannot be applied for the studied glass-ceramics crystallization process. The
effect of 2% TiO2 as nucleating agent upon the crystallization kinetics and upon the microstructure of the studied glass-ceramics was analyzed.
Crystallization, morphology and mechanical properties of a spodumene-diopside glass ceramics with adding different amount
of CaO and MgO in Li2O-Al2O3-2SiO2 were investigated. With CaO and MgO addition, the crystallization temperature (Tp) decreased, the value of Avrami constant (n) decreased from 3.2±0.3 to 1.4±0.2, the activation energy (E) increased from 299±3 kJ mol−1 to 537±5 kJ mol−1. The crystalline phases precipitated were h-quartz solid solution, β-spodumene and diopside. The mechanism of crystallization of the glass ceramics changed from bulk
crystallization to surface crystallization. The grain sizes and thermal expansion coefficients increased while flexural strength
and fracture toughness of the glass-ceramics increased first, and then decreased. The mechanical properties were correlated
with crystallization and morphology of glass ceramics.
Authors:M. Goswami, A. Sarkar, B Sharma, V. Shrikhande, and G. Kothiyal
Magnesium aluminum silicate (MAS) glass samples with different concentrations of alumina (7.58 to 14.71 mol%) were prepared
by melt and quench-technique. Total Mg content in the form of MgF2+MgO was kept constant at 25 mol%. MAS glass was converted into glass-ceramics by controlled heat treatment at around 950C.
Crystalline phases present in different samples were identified by powder X-ray diffraction technique. Dilatometry technique
was used to measure the thermal expansion coefficient and glass transition temperature. Scanning electron microscopy (SEM)
was employed to study the microstructure of the glass-ceramic sample. It is seen from X-ray diffraction studies that at low
Al2O3 concentrations (up to 10.5 mol%) both MgSiO3 and fluorophlogopite phases are present and at higher Al2O3 concentrations of 12.3 and 14.7 mol%, fluorophlogopite and magnesium silicate (Mg2SiO4), respectively are found as major crystalline phases. The average thermal expansion co-efficient (aavg) of the glass samples
decreases systematically from 9.8 to 5.510-6C-1 and the glass transition temperature (Tg) increases from 610.1 to 675C with increase in alumina content. However, in glass-ceramic samples the aavg varies in somewhat
complex manner from 6.8 to 7.910-6C-1 with variation of Al2O3 content. This was thought to be due to the presence of different crystalline phases, their relative concentration and microstructure.
Authors:T. Vlase, C. Pǎcurariu, R. Lazǎu, and I. Lazǎn
The crystallization kinetic of the basalt glass ceramic of the oxide composition, (%): SiO2 − 50.82; Al2O3 − 12.05; Fe2O3 − 9.28; CaO − 15.48; MgO − 11.08; Na2O+K2O − 1.14; TiO2 − 0.15, with addition of 10% TiO2 as nucleating agent has been studied using thermal analysis under non-isothermal conditions.
In this order, the non-isothermal DTA curves were obtained at different heating rates between 4 and 20°C min−1 in the temperature range of 25–1000°C using a Derivatograph-C (MOM, Hungary). The kinetic parameters of the crystallization
process were calculated on the basis of Ozawa-Flynn-Wall, Friedman, Budrugeac-Segal and non-parametric kinetic methods.
Authors:C. Păcurariu, R. Lazău, I. Lazău, R. Ianoş, and B. Tiţa
The crystallization mechanism of the glass-ceramics obtained from Romanian (Şanoviţa) basalt in the presence of 3 and 5% CaF2 as nucleation agent has been investigated under non-isothermal conditions using DTA technique. The activation energies of
the crystallization processes were calculated using the Kissinger-Akahira-Sunose, Ozawa-Flynn-Wall, Starink and Tang isoconversional
methods. The monotonous decreases in the activation energy (Ea) with the crystallized fraction (α) confirms the complex mechanism of the glass-ceramics crystallization process. It has
been proved that the Johnson-Mehl-Avrami model cannot be applied for the studied glass-ceramics crystallization process.
Authors:C. Păcurariu, M. Liţă, I. Lazău, D. Tiţa, and G. Kovacs
The paper presents the kinetic study of the crystallization processes which take place at the obtaining of some glass ceramics,
starting from two basalt glasses with different oxide composition. The activation energies have been calculated using Kissinger's
equation and verified with the Ozawa's equation. In this order, the DTA curves have been registered with different heating
rates, between 4 and 20C min-1. By X-ray diffraction it was established that the crystalline phase formed in the crystallization process represent a pyroxenic
solid solution, Ca(Mg,Fe)SiO3.
Authors:Yaohui Li, Kaiming Liang, Bo Xu, and JianWei Cao
Li2O–Al2O3–SiO2 glass-ceramics were prepared with Ta2O5 as nucleating agent, the crystallization mechanism and microstructure evolution were investigated by DTA, XRD, and SEM technologies.
With increasing amount of Ta2O5 from 2 to 6 mol%, the crystallization activation energy decreased from 297.73 to 218.66 kJ mol−1, while the crystallization index increased from 1.76 to 3.39. In addition, the cluster of dendritic crystals and lamellar
structure obtained in T-2 glass-ceramics indicated a typical two-dimensional crystallization mechanism, and the formation
of spherical β-quartz solid solution in T-4 specimens, with average size of 50–70 nm, was mainly due to bulk crystallization
mechanism. It was considered that Ta2O5 promoted the nucleation and crystallization of LAS glass by precipitating the crystalline precursor phase of Ta2O5, which acted as nuclei for the subsequent crystal growth. Eventually, the diffusion and crystallization process, microstructure
morphology, as well as the secondary grain growth were also investigated.