The decomposition kinetics of nanocrystalline calcite
Introduction
Calcium carbonate is one of the most widely used fillers in many industrial applications such as plastics, rubber, paper making and medicine. Recently it is found that the nano scale calcium carbonate has much more advantages than the commonly used normal size calcium carbonate and its novel characteristic has attracted wide research interests.
Extensive work has been carried out on the non-isothermal decomposition kinetics of solid system. Multiple techniques were reported in literatures for determining the reaction mechanism and deducing kinetic parameters 1, 2, 3, 4, 5, 6, 7. Contrary to the homogeneous reaction, the mechanism and kinetics of solid decomposition will vary with many factors such as the change of reaction condition, crystal form and particle size. It is mainly owing to the influence of heat transfer and mass transfer on the phase boundaries. Since that, although the dissociation of calcium carbonate has been widely studied, its mechanism and kinetic parameters reported in the literatures are not well consistent 4, 6, 8. Besides, to the nano scale particles, the kinetic behavior of decomposition is particularly rarely noticed. Criado and Ortega [9]has observed that smaller particle would result in the diminution of the activation energy, but their experiment was confined to particles of micro scale. As is known to all, constitutive particle size of nano-material is ranging between molecular cluster and bulk substance. It may result in many special characteristics. In this paper, the thermal decomposition of nanocrystalline calcite is studied to see the effect of particle size on the decomposition parameters and mechanism.
Section snippets
Decomposition kinetics
The commonly used equation in the non-isothermal decomposition kinetics is presented belowin which the kinetic function f(α) is determined by reaction mechanism and speed controlling step. It has an another form asPossible f(α) and expressions are listed in Table 1.
Since the mechanism determination and the kinetic parameter calculation are mutually dependent, non-mechanism equation is firstly introduced to estimate the activation energy as a prerequisite for
Experiments
Purified CO2 gas (about 20–40% concentration, diluted with N2) bubbled though and carbonized 5–10% Ca(OH)2 until the pH value reached 7–8. The precipitate was filtered, dried at 110°C to steady weight, then sifted through 320 mesh. The crystallite is confirmed as calcite by X-ray diffraction pattern and the particle size is estimated at around 40 nm by TEM examining. A reference calcite (99.99%, the particle size is estimated at 5–20 μm by SEM examining) was used for comparison. All the
Conclusions
- 1.
Master plot method and Coats and Redfern equation substantiate that both nanocrystalline calcite and reference calcite dissociate according to contracting volume mechanism R3, which is in accordance with most of the literature reports. Since the particle size is very small, D1 and D4 mechanisms reported in some literatures seem inappropriate in the case of nanocrystalline calcite.
- 2.
The activation energy obtained from non-mechanism equation is consistent with that obtained from mechanism equation.
Acknowledgements
The authors thank Prof. Wu Qingzhou of the Analysis and Testing center of Zhejiang University for his help in the thermal analysis.
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