Abstract
The kinetics, the mechanism and the thermodynamics of activated state of formation of primary strontian feldspar via sinter-crystallization of non-equilibrium melt during the thermal treatment of ceramic body was investigated in this work via differential thermal analysis using isoconversional Kissinger kinetic equation. The process of formation of non-equilibrium melt and subsequent crystallization of primary strontian feldspar requires the activation energy of 631±3 and 664±2kJmol−1, respectively. The investigation of mechanism of formation of primary strontian feldspar reveals that the process is driven by the surface nucleation and diffusion controlled growth of the new phase. The nucleation rate decreases with the time of process and non-equilibrium melt can be formed only in metastable equilibrium with activated state of strontian feldspar. Deep consideration of kinetic data leads to the deduction of new kinetic approach that enables single calculation of activation energy and frequency factor of heterogeneous processes as well as the dependence of thermodynamic parameters of activated state on temperature. Further consideration of kinetic data reveals that the activation energy is directly proportional to the function of csch (z)+1. For z=e, this term enables to derive the value for the parameter B(x) in empirical equation for Arrhenius temperature integral p(x) proposed by Doyle to be 1.0642.
Highlights
Feldspars are intensively studied due to their importance in petrography [1,2], understanding the Earth's geology [2,3], soil sciences, where some clay minerals are produced by weathering of feldspars directly or through intermediate phases [4,5], as well as in the production of ceramics, glass and glass-ceramics [6,7,8,9]
The kinetics, mechanism and thermodynamics of activated state of the process of sinter-crystallization of primary strontian feldspar from non-equilibrium melt was evaluated from the results of differential thermal analysis
The change of the peak temperature and other peak parameters (Fig. 1 (b)) during the processes of endothermic formation of non-equilibrium melt (Tm1 in detail in Fig. 1(a)) and exothermic sintercrystallization of strontian feldspar (Tm2) with the heating rate was utilized for the determination of the kinetic constants of sintercrystallization process of primary SrAl2Si2O8
Summary
Feldspars are intensively studied due to their importance in petrography [1,2], understanding the Earth's geology [2,3], soil sciences, where some clay minerals are produced by weathering of feldspars directly or through intermediate phases [4,5], as well as in the production of ceramics, glass and glass-ceramics [6,7,8,9]. Synthetic strontian feldspar (Sr-celsian, strontium aluminosilicate feldspar, SAS, SrAl2Si2O8) is of great technological interest due to low thermal expansion coefficient [16,17]), low and thermally stable dielectric constant, low dielectric losses, high mechanical strength, chemical inertness, and high melting temperature They are known as the materials for the matrix of fiberreinforced ceramic composites, protective coatings, electro-ceramics and refractories [18,19,20,21,22,23,24,25] as well as for the preparation of glass ceramics [26,27,28] and luminescent pigments of MAl2Si2O8:Xn þ ; where M1⁄4 Ca, Sr, Ba and Xnþ denotes ions of doped element [29,30,31,32,33,34,35]. The other aspects of the process of strontian feldspar synthesis are described in previous work [36]
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