Synthesis of Nanoparticles from Unstable Precursors

Thermally activated processes of synthesis, as a rule, occur in several stages, the duration of which depend on the composition of the gaseous medium, the reaction regime (kinetic or diffusion), the specific surface area of the initial and intermediate phases, temperature and heating rate [279]. Let us consider in detail some of the processes, especially those which may be carried out in micro/nanoreactor. The thermal decomposition of unstable compound A leads to the formation of solid B and gaseous C phases  

The emergence of a new phase due to local structural fluctuations in a lattice of the original solid phase Ag occurs most often at the boundaries and defect sites of the crystals. Thermodynamically stable nucleus of new phase has often a critical size close to unit cell volume, which differs from the normal one. This gives rise to mechanical stresses in the transformation zone, and even the destruction of the original crystal. Such effect leads to smaller quantity of a desired product. Thermal decomposition reaction, like all topochemical reactions, proceeds more rapidly 

In general terms, the theoretical description of the kinetic competition between the nucleation of new phase and nuclei growth can be represented as a system of equations. It is assumed that the reaction takes place in a continuum where the new phase nuclei have a spherical symmetry. The transformation degree a and its derivative describes the kinetics of chemical reactions. Equation (5.10) presents the parameter a in the integral form in terms of the size distribution function [284]. 

Barium titanate. The continuous miniaturization of integrated electronic devices forces to manufacture active and passive components of smaller size, including multilayer capacitors (MLCCs) [285,286]. This accounts for a significant breakthrough in the synthesis of ultrafine BaTiOs powders in recent years [287]. 

The temperature of calcination is the most sensitive parameter for the particles size. The details of reaction mechanism of BaTiOs formation from barium titanyl oxalate have been studied a lot by thermochemical methods (TGA, DTA), X-ray diffraction, gas chromatography analysis and infrared spectroscopy (FTIR). The decomposition of barium titanyl oxalate proceeds in four stages as it is shown in Fig. 5.16. 

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