Crystallization Behaviors of CaO-SiO2-Al2O3-Na2O-CaF2-(Li2O-B2O3) Mold Fluxes

Abstract

Both the Matusita equation and the modified Matusita equation for estimating the activation energy associated with non-isothermal crystallization were critically evaluated. The derivation for melts crystallization on cooling indicates that, unlike for the crystallization that occurs on heating, the term 1 À exp (ÀDG/RT) in the basic rate equation of crystal growth and the term R T s 0 exp ÀE=RT ð Þ dT depending on the initial temperature of the cooling process cannot be neglected. It is demonstrated that both the Matusita equation and its modified expression are only valid to estimate the activation energy associated with the crystallization that occurs on heating, but are inapplicable for the melt crystallization that occurs on cooling. It is suggested that the isoconversional methods of Friedman and Vyazovkin should be alternative to determine effective activation energy for melt crystallization that occurs on cooling.Various models have been developed to estimate the activation energy associated with the non-isothermal crystallization from thermal analysis data, including Kissinger equation, [1] Ozawa equation, [2] modified Ozawa-Chen equation, [3] and Matusita equations. [4,5] Among these models, Kissinger equation [1] and Matusita equations [4,5] are the most widely used approaches to determine the activation energy for the crystallization that occurs on heating. [6][7][8][9][10] Moreover, they are applied frequently for the non-isothermal crystallization of polymer melts [11][12][13][14][15] and metallurgical slags [16,17] that occurs on cooling.Matusita and Sakka [4] emphasized that the physical meaning of the crystallization activation energy determined by the Kissinger equation [1] is obscure because the crystallization of glass is advanced by both nucleation and crystal growth, rather than an n-th order reaction. Therefore, they proposed the following generalized expression to determine the activation energy for crystal growth: [4]

Effect of SiO2 on the Crystallization Behaviors and In-Mold Performance of CaF2-CaO-Al2O3 Slags for Drawing-Ingot-Type Electroslag Remelting

“…This result is also in accordance with the findings that the crystallization temperatures of CaO-SiO 2 -CaF 2 -based mold fluxes decrease with the decrease in the mold flux basicity (mass pct CaO/mass pct SiO 2 ). [9,21,22] It is clear from Figure 5(b) that the crystallization temperature of the second crystalline phase CaF 2 in the SiO 2 -free slag is much higher than those in the SiO 2 -containing slags. For CaF 2 -CaOAl 2 O 3 -SiO 2 slags, it appears that increasing SiO 2 addition from 2.8 to 6.8 mass pct has no noticeable effect on the crystallization temperature of the second crystalline phase formed in these slags, as shown in Figure 5(b).…”

“…The results obtained here are consistent with the findings for mold fluxes reported by the present authors. [19,21] Figure 5 shows the CCT diagrams of the first and second crystalline phases precipitated from the slags containing different amounts of SiO 2 , respectively. The crystallization temperatures of the first crystalline phases 11CaOAE7Al 2 O 3 AECaF 2 are observed to decrease with increasing SiO 2 contents in the slag, as shown in Figure 5(a).…”

Control of Crystal Morphology for Mold Flux During High-Aluminum AHSS Continuous Casting Process

“…[13,14] Figure 4 shows the morphology of cuspidine crystal after iso-thermal DSC measurement at different crystallization temperatures for Slag B and D, and Table IV lists their supercooling, Jackson a factor, and corresponding crystal morphology. It should be pointed out that only cuspidine is present at the iso-thermal experimental temperatures since the second phase, i.e., wollastonite, precipitates at very low temperature for slag B according to the XRD identified results as shown in Table II Table IV that cuspidine of melt crystallization in slag B and D have a very similar Jackson a factor, but the undercooling for crystallization of slag B is much greater than that of slag D. Therefore, it can be concluded that the crystal morphology difference between Slag B and Slag D is mainly due to the supercooling difference rather than Jackson a factor.…”

“…But their experimental results also demonstrated that lubrication and flux consumption of lime-alumina-based mold fluxes should be further improved for industrial applicability. In order to understand the causes of lubrication differences between lime-alumina-based mold fluxes and lime-silica-based mold flux further, more careful researches concerning on the crystallization behaviors had been carried out by the present authors, [13][14][15][16] which demonstrated huge differences in crystal morphology between the foregoing two kinds of mold fluxes, as shown in Figure 1. For the CaO-SiO 2 -based mold fluxes, the primary crystalline cuspidine (Ca 4 Si 2 F 2 O 7 ) was typically faceted with moderate size in morphology generally, as shown in Figure 1(a).…”

“…In consideration of the great practical significance, the present investigation was carried out to understand how to control crystal morphology in mold flux as a successive work of our previous findings [13][14][15][16] on both the lime-alumina-based and lime-silica-based mold fluxes. A thermodynamic method was introduced to estimate Jackson a factor of a crystal in multi-component mold fluxes and the parameters were calculated with the help of the thermodynamic databases of FactSage 6.4.…”