Time (min)

0 50 Time (min)

Figure 5.30 : The redissolution of nano particles against the time of growth at 710°C for the samples nucleated at 610°C.

At the growth temperature, if we take the number density at the longest time as N ∞ , then at any time of growth , (N t –N ∞ ) represents the number of nano nuclei which redissolve. The plots of (N t – N ∞ ) as a function of time of growth are shown in Figures 5.30 and 5.31 for all the nucleated samples. It is seen that almost all the curves show a more or less rapid decrease at the initial time of growth. This decrease is slowed down considerably up to certain time for lower temperatures of nucleation. After this stage, there is no more redissolution indicating a stable behaviour. This ‘initial time’ of rapid redissolution,

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i.e. a rapid decrease of (N t –N ∞ ), can possibly be termed as an “incubation period " , which corresponds to a kinetic phenomenon related to bulk atomic diffusion process in the basalt glass.

For different nucleation conditions at relatively lower temperatures, but the same growth tem- perature (i.e. 710°), it is seen that during the ‘incubation period’, the redissolution rate for the sample nucleated at 550°C for 32 h is lower than that of the ‘blank glass’. This is due to higher temperature of nucleation, which seems to stabilize a certain number of nuclei. Since the time of nucleation for this sample is higher at 32 h, its redissolution rate is also lower than that nucleated even at a higher tempera- ture at 577°C for shorter time of 19 h. Hence, it appears that the ‘effect of time’ is more important for the ‘stabilization’ of the nano nuclei - which means a lower redissolution rate - than the ‘temperature effect’ in the lower temperature region of nucleation, i.e. between 525 and 577°C.

30 Nucleated at 634°C

Nucleated at 665°C

0 50 Time (min)

Figure 5.31 : The redissolution of nano particles against the time of growth at 710°C for the samples nucleated at 634°C and 665°C respectively.

For the same nucleation conditions, but different growth temperates, the observations for the redissolution of the nano nuclei are quite interesting. The ‘incubation period’ is longer for lower growth temperature due to a lower rate of atomic diffusion. During this ‘incubation period’, the redissolution

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rate is almost constant at 680°C, then it increases considerably at a higher growth temperature of 710°C, and then it decreases again at still higher growth temperature of 740°C.

The above data can be explained in the following manner : At 680°C, the mobility of the atoms is much lower than that at higher temperatures up to a certain

time, i.e. the ‘incubation period’, so that the redissolution rate is quite negligible, and the process is also quite long. At 710°C, the rate of redissolution increases considerably due to the increase of bulk atomic diffusion with temperature. However, at 740°C, this rate seems to be very much reduced, because many of the nano nuclei had already been redissolved during the increase of temperature, thereby making the attainment of the ‘stable nano nuclei’ much easier. The higher temperature represented the particles for which the Gibbs free energy would decrease with increasing size, favouring immediate growth once the temperature is raised in order to give enough ‘thermal energy’ for bulk atomic diffusion.

It should be noted after the ‘incubation period’, the redissolution rate decreased from 680 to 740°C, indicating this to be a ‘thermally activated’ process. Below this ‘incubation period’, there is some sort of ‘blocking’ in the rearrangement of the diffusing atoms, which not only kept the rate of redissolution constant, but also caused the ‘incubation period’ to be quite long, say at a temperature of 680°C. Therefore, the dynamic study of growth on different nucleated samples shows the ‘advantages’ in revealing some details, which otherwise would have been lost in the conventional technique of study- ing growth in two-stage heat-treatments [48].

For higher temperatures of nucleation, the number of ‘redissolved’ nano nuclei against time of growth (see Figure 5.31) shows a very similar behaviour. There is a rapid decrease of the ‘redissolved’ number density up to around 50 min, and then it reaches a saturation level. This saturation level is lower for higher temperatures of nucleation. At the highest nucleation temperature of 665°C for 8 h, the redissolution rate is almost negligible. This is obviously due to the higher temperature of nucleation. Therefore, in this temperature range of nucleation, i.e. 610 - 665°C, the temperature is an important facor.

By analyzing these curves, if the number density at t → 0 is taken as the maximum number of ‘redissolved’ nano nuclei, then it is noted that this number [N D(max) ] decreases with nucleation time at any of these nucleation temperatures. While this ‘decrease’ is very little for 610°C, it is quite rapid at

634 and 665°C respectively, indicating the beneficial effect of nucleation at or just about T g [45]. All the above data on redissolution clearly show its effect on the nucleation and crystallization beaviour of the nano particles of magnetite, which were ‘innovatively created’ within a basalt glass matrix by a simple procedure of heat-treatments at different temperatures for different time.

In summary, the dynamic ‘on-line’ study of growth of nano particles of magnetite shows a pro- nounced inter-particle interference effect in the SANS spectra. There is a decrease of the number den- sity with the time of growth and thereafter, a saturation occurs due to the growth of the stable nano nuclei. This saturation level is higher for the lower temperatures of nucleation. This is interpreted as due to redissolution of the smaller nano nuclei, as the larger and stable nano nuclei continue to grow. The crystallization behaviour tends to follow an Ostwald ripening mechanism at higher time of growth. For lower nucleation temperature, the time of nucleation is an important factor, whereas for the higher

nucleation temperature near T g , the temperature seems to be an important parameter. The rapid decrease of the number density of the redissolved nano nuclei in the initial time is noted for all the nucleation conditions. At longer time of growth, the redissoluition terminates progressively for lower nucleation temperatures. But for higher nucleation temperatures, there is a saturation effect before the termination of the redissolution process. These data on ‘nano particles’ within such a ‘narrow range of sizes’ is quite revealing and definitely merit further attention for our future endeavour in the search of ‘newer nano materials’.

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