Ostwald Ripening for Nano Particles and the Growth

5.7.6.3. Ostwald Ripening for Nano Particles and the Growth

In order to examine the growth process, a sample nucleated at 577°C for 19 h was crystallized at three different temperatures, i.e. 680 C, 710°C and 740°C respectively. The plots of the cube of the diam- eter (D 3 = 8R 3 S ) of the nano particles are shown against the crystallization time in Figure 5.29. It should be

3 nm

Crystallized at 0 740°C

1 3– (× D

710°C

680°C 0 Time (min)

A plot of the cube of the diameter of the nano crystallites against the time of

crystallization at three different growth temperatures.

MAGNETIC PROPERTIES

223 pointed out that a similar plot of V [= (4 π/3)R 3 S ] against t C could also be shown to indicate the ripening

process. It is seen from the figure that at 680°C, the growth rate is very slow, and the number of nuclei remains constant up to 60 min. This curve also shows that the redissolution of the smaller nano particles is almost negligible, because of the lower growth temperature [3]. At 710°C, the particle diameter increases rapidly up to 80 min, and then it increases ‘slowly but linearly’ with time, which indicates an ‘Ostwald Ripening’ process.

At 740°C, the rapid increase of diameter of the nano particles is up to 40 min only, and then it shows a linear behaviour towards higher time. It also shows that the redissolution process terminates earlier due to a higher growth temperature. In the linear region, the growth rate shows an increase between 710 and 740°C, which indicates that the crystallization of nano particles of magnetite in a basalt glass is a ‘thermally activated’ process.

It should be noted that in the basalt glass, depending on the temperature, there is an ‘eqilibrium volume fraction’ of magnetite of less than 10%. Actually, the Ostwald ripening only becomes predomi- nant when the voume fraction is close to this limiting value. However, in some glass systems, e.g. the simpler system of lithium disilicate, the Ostwald ripening would occur for crystallinities approaching 100%, and hence would be negligible for the low crystal volume fraction applicable in most of the nucleation studies of that system [48]. In actual practice, the “impingement” of the crystals could occur at much lower crystallinities, rather than Ostwald ripening. Therefore, it should be kept in mind that Ostwald ripening is only important in certain circumstances [49, 50]. It may be construed that there might be a ‘hidden’ ripening mechanism operative at certain growth temperatures with different nucle- ated samples [2].

As mentioned, the ‘blank glass’ when studied at 710°C showed sufficient scattered intensity with a maximum even at 11 min. The particle radius (R S ) varied between 3.77 nm and 6.13 nm between

11 and 120 min. This shows that the ‘blank glass’ itself was already nucleated with a high density of nuclei (N = 19.0 × 10 16 at 11 min), which could have occurred either during quenching or during annealing. No measurement could be performed on this glass either at room temperature or at the an- nealing temperature, i.e. 525°C, to determine the number and size of the nuclei in the original sample. This could not be done because of the contribution of the ‘magnetic scattering’ together with that of nucleation scattering (i.e. due to density and concentratiuon fluctuations). However, the Mössbauer measurements on this glass at 4°K showed that there is a short-range ‘magnetic ordering’ of Fe atoms of dimension 1.2 nm (see the section 5.5) [6]. It can be thought that this might have affected the overall nucleation and crystallization behviour of the small nano particles of magnetite in a basalt glass matrix.