General Conditions of Kinetics of Vitrification

5.7.2.2. General Conditions of Kinetics of Vitrification

The number I of nucleus produced per unit volume per unit time, i.e. the rate of nucleation, and the rate of crystal growth U both depend on the temperature, as shown in Figure 5.20. This figure needs an explanation.

For a liquid to form a glass, it must be cooled rapidly to avoid crystallization. Below the tempera- ture of fusion (T f ), the liquid consists of a stable phase. As the liquid enters a stage of supercooling below (T f ), the growth can theoretically take place between T f and T 3 . However, the formation of the initial nucleus necessary before the growth can take place, occurs between T 2 and T 4 .

I, U

ΔT

Figure 5.20 : The rate of nucleation and crystallization curves for a glass.

Thus, the critical region consists of a temperature between T 2 and T 3 , and the possibility of crystallization will depend on the manner that one curve is imposed on the other (see Figure 5.20) as well as on the absolute values of the respective rates in the overlap region

If on the common interval T 2 –T 3 , either I or U (or, I and U together) are too small, the crystal- lization is not detectable - and the system passes to a vitreous state. On the other hand, if I and U are quite important, i.e. strong overlap of the two curves, the total crystallization could not be avoided.

NANO MATERIALS

If in this interval T 2 –T 3 , I is weak but U is strong, the crystallization could give rise to a small number crystals distributed in a glassy phase, whereas for the opposite case (i.e. I is strong, but U is weak), it will give rise to a partially crystallized material with finer grains.

Based on the above description, it can be said that for a solid glass like a ‘basalt glass’, both nucleation rate (I) and crystal growth rate (U) are dependent on the temperature, and hence this idea can

be used to devise a strategy for ‘heat-treatment schedule’. The maximum in I appears at a lower tempera- ture than the maximum U. The most effective heat-treatment schedule firstly for nucleation is obviously attained at the temperature that gives the maximum nucleation rate.

Hence, a glass that has already cooled down (after forming) to room temperature is generally re- heated to this temperature, which is followed by a heat-treatmernt at a higher temperature for crystal growth. In a typical ‘temperature-vs-time’ curve, the temperature is first raised quickly to the nucleation zone and kept there for some time, which is enough for enough number of nuclei to be formed in the glass. Then, the temperature is raised again to the crystal growth zone quickly and kept at this tempera- ture for sufficient time for the nuclei to form stable crystal, and then the temperature is dropped rapidly to room temperature. This schedule depends on many factors including the composition of the glass. This is called 'two-stage' heat treatment schedule (see ref. [48, 50] later), which has also been used in case of basalt glass to precipitate nano particles of magnetite [2, 3].