1,721,057 research outputs found
Non-isothermal devitrification of 0.9Li2O0.1BaO 2SiO2 glass
No full textThe effect of the substitution of a small amount of Li2O by BaO on devitrification of Li2O.2SiO2 has been studied. Thermoanalutical techniques were used to get information about the devitrification mechanism and to evaluate the crystal growth activation energy and the maximum nucleation rate temperature. Nucleation rates were found to be hardly lowered by the substitution of 0.1 mol of Li2O by Ba
Evaluation of Glass Stability From Nonisothermal Kinetic Data
No full textNon-isothermal devitrification of 1.1Li2O.1.9SiO2 glass has been studied and is compared with that of Li2O.2SiO2 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 Li2O.2SiO2 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
Devitrification Behavior of Glasses In the System (1-x)Na2O, (x/3)Al2O3, 1.2WO3 (0-less-than-or-equal-to-x-less-than-or-equal-to-0.60)
No full textThe materials obtained from the glasses were subjected to differential thermal analysis and x-ray diffraction. Trends in the glass formation and transformation temperatures are explained by the formation of AlO4 tetrahedral units and the consequent increase in the degree of polymerisation. From a kinetic point of view it is suggested that the Al2O3 affects the relative positions of the nucleation and crystal growth temperature ranges until a concentration is reached at which AlO4 units are themselves able to leave the glass network to form crystalline phase during cooling
A New Parameter To Evaluate Nucleation In Glasses From Dta Curves
No full textNon-isothermal kinetic data relative to five silicate glasses are compared. The analysis of data suggests the use of a new parameter to obtain a rough estimate of the ability of different glass compositions to form nuclei and to select glass compositions which can, easily, give fine-grained glass ceramics
Nucleation in glass and differential thermal analysis
No full textThe crystal nucleation of Li2O-2SiO2 glass is studied by DTA. A simple method based on the shift of the DTA crystallization peak temperature, for evaluating the influence of temperature and time of heay treatments on bulk nucleation, is described. The experimental results are iun good agreement with results obtained by other workers
Kinetics of nonisothermal devitrification
No full textThe nonisothermal devitrification of GeO2 and 3BaO.5SiO2 glasses was studied to show that, from DTA curves, the activation energy of crystal growth can be calcd., surface and bulk crystn. can be distinguished, and usually no data concerning nucleation rate can be obtained. To describe the glass-crystal transformation a W.A. Johnson and R.F. Mehl (1939) and M. Avrami (1941) rate equation was assumed. The results obtained agree with those obtained under isothermal conditions
The Effect of the Substitution of B2O3 To P2O5 On T(g) and Electrical-properties of 0.65((1+x)Ag2OP2O5)0.35AgI Glasses
No full textGlasses of the series 0.65((1+x)Ag2OYB2O3 (1-Y)P2O5)0.35AgI (0 less-than-or-equal-to x less-than-or-equal-to 0.6 and 0 less-than-or-equal-to y less-than-or-equal-to 0.2) have been prepared. Their glass transformation temperature, T(g), electrical conductivity, sigma, and conduction activation energy, E(sigma) and preexponential factor, sigma0, have been evaluated. The substitution of B2O3 to P2O5 has been found to be useful in making both T(g) and sigma to increase. The results have been interpreted on the basis of a structure composed of AgI microdomains in a phosphate or borophosphate matrix and on the basis of the diffusion path model for ionic conduction
Nonisothermal Devitrification Behavior of Diopside Glass
No full textThe non-isothermal devitrification of diopside CaO . MgO . 2SiO2 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
Thermal-properties and Devitrification Behavior of (2-x)cao-center-dot-x/3y(2)o(3)center-dot-2sio(2) Glasses
No full textThe thermal properties (glass transformation, T-g, softening, T-s, and exo-peak, T-p, temperatures), and the devitrification behaviour of glasses in the system CaO-Y2O3-SiO2 are reported The T-g, T-p and E(c) results can be explained on the basis of the increased structural rigidity when Ca+2 ions are substituted by Y+3 ions, with the formation of stronger bonds to the oxygen. For the molar ratio of Ca/Si = 0.7 or more, wollastonite is no longer formed; at the same time a new and as yet unidentified phase appears. Surface nucleation occurs preferentially, but, in finely powdered samples, which soften and sinter before devitrifying, surface nuclei behave virtually as bulk nuclei
Nucleation Kinetic Data From Dta Curves
No full textNon-isothermal kinetic data relative to five silicate glasses are compared. A new parameter is proposed, whose usefulness in roughly estimating the ability of different glass compositions to form nuclei and, so, give fine grained glass ceramics, is discussed
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