1,721,078 research outputs found
Annealing and crystallization of Fe40Ni40P14B6 glassy alloy - A DSC study
No full textIn this paper a DSC study is reported of the behavior of Fe40Ni40P14B6 alloy produced by rapid quenching. The experimental results show that relaxation phenomena can be studied directly from the DSC curves. From these experiments, the spread of the E-c values in the literature is attributed to differences in the quenching rates and the presence of variable number of quenched-in nuclei. It is also shown that the microstructure (number and size of crystals) of the non-isothermally devitrified metallic alloy changes with the heating rate; this is a consequence of the shift of crystallization temperatures and, therefore, of the change of the ratio of nucleation and crystal growth rates
Thermal and devitrification behavior of CaO-Ga2O3-SiO2 glasses
No full textA study of the influence of the substitution of Ga2O3 for CaO, at constant O/Si ratio, on thermal properties and non-isothermal devitrification of 2.5CaO.2SiO(2) is reported. Differential thermal analysis (DTA) and X-ray diffraction analysis were used. The Xray diffraction pattern of the crystallized Ga2O3 base glass shows that the alpha CaO SiO2, that should be stable only above 1125 degrees C, forms in the temperature range 900-1000 degrees C. A new ternary crystalline phase, whose reflections are not reported in the JCPDS cards, was found to form during crystallization of the glass. The glass transformation temperature, T-g, and softening, T-s, temperature decrease as Ga2O3 is substituted for CaO. This is the result of the substitution of the network modifying cation Ca2+, of higher coordination number, by a network forming cation Ga3+ in fourfold coordination, in a composition range of relative insensitivity to changes of covalent cross-linking density. The crystal growth activation energy, E-c, decreases with substitution; this is the consequence of the decrease of the structural rigidity and of the shift of crystallization to a higher temperature range. Devitrification involves a mechanism of surface nucleation; surface nuclei behaving as bulk nuclei in samples that soften and sinter before devitryfing. (C) 2000 Elsevier Science Ltd. All rights reserved
A new hypothesis about the mechanism of magnetic-domain formation
No full textThis paper proposes a mechanism of formation of the magnetic domains on cooling a ferromagnetic material from a higher temperature than the Curie temperature. It is derived from the well-known theory of crystallization in an amorphous material through a two-step mechanism of nucleation and crystal growth. Following this approach it is shown that the domain structure should depend on the free-energy change DeltaG per mole of magnetic moment aligning in the same direction. A method to avoid calculation of DeltaG is shown. This approach was successfully applied to explain the differences between the domain structures of the annealed and the as-quenched samples of the amorphous alloy Fe62.5Co6Ni7.5Zr6Cu1Nb2B15. Moreover, it gives an explanation for recent findings on the magnetic microstructure of high-permeability materials
EFFECT OF CHANGING THE SUBSTITUENT OXIDE AND ITS AMOUNT ON THE THERMAL STABILITY TOWARDS CRYSTALLIZATION OF GLASSES OF FORMULA (2.5-X)CaOX/3M2O32SiO2 (M= La, Y, IN, Ga)
No full textCrystallisation behaviour of CaO-M2O3-SiO2 glasses (M=La, Y, In, Ga) is studied using a recently proposed dimensionless parameter, which can be easily calculated from thermoanalytical data. This parameter proves useful in comparing the stability of glass toward crystallisation. The stability to crystallisation increases as M2O3 is substituted for CaO. Moreover, the higher the ionic field strength, Z/R-2, of the substituting cation, the greater the effect. This can be ascribed to: (a) the decrease in density of nonbridging oxygens when a network former oxide (Al2O3, Ga2O3) is introduced (Z/R-2 > 5 Angstrom(-2)); (b) the increase of the transverse bond strength (the greater the higher the ionic field strength of the cation) when a network modifier oxide is introduced (Z/R-2 < 5 Angstrom(-2))
Interpretation of the thermogravimetric curves of ancient Pozzolanic concretes
No full textPozzolanic concretes submitted to thermogravimetric (TG) analysis show a continuous weight loss starting from about 400 C-o. In order to reconcile these observations with those from other analytical methods, it is necessary to attribute this weight loss to CO2 removal. It has been proposed, in the literature, that silicates and CaCO3 react at lower temperatures, producing calcium silicates and CO2. In this paper, the FTIR spectra collected on samples submitted to TG analysis, stopped at conveniently selected temperatures, provide direct evidence that the continuous weight loss recorded with thermogravimetric analysis of the pozzolanic concrete in the temperature range 400-900(o) C is to be attributed to the reaction between the silicates and calcite, with the formation of CO2 and of a silicate that is richer in CaO. Therefore it is justifiable that the whole weight loss, in the temperature range 400-900(o) C, should be taken into account in the calculation of the CaCO3 content of the concrete. Moreover, the described procedure--of recovering small samples (2 mg) from the sample-holder of the TG apparatus at various steps of the heating rate and comparing the FFTIR spectra--can help in identifying the pozzolanic nature of a concrete that, in general, is not easily recognizable from the trend of the thermoanalytical curve
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