Avrami model experimental results

The kinetic evolution is usually represented by a sigmoid-type curve. Such a typical curve is given in Figure 9 for the case of displacement reaction, Ni + CuO Cu -F NiO realized in planetary ball mill [58]. Author of this overview analyzed these experimental results by one of the most frequently used kinetic model applied to various solid-state reactions, namely Johnson-Mehl-Avrami equation ... 

A theoretical isotherm for the two-stage series model, as calculated from Eq. (9.66), is illustrated in Fig. 9.22.(55) The contribution to the total crystallinity of each of the individual steps is also shown. In this example n was taken equal to 1 and 3. For the parameters adopted in this particular example the apparent value of the observed n is about 2. Such two-stage processes can lead to fractional values of the Avrami exponent as well as values that vary with the extent of the transformation. The results depend on the choice of parameters. Considering the number of arbitrary parameters involved, and the selection of the time scale, it is not surprising that agreement can be obtained with different sets of experimental data. The physical significance of this approach is of concern since the model taken is arbitrary. The same kind of result can be obtained for a variety of heterogeneous nucleation mechanisms. 

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