Preparations for the SANS Study
5.7.5. Preparations for the SANS Study
This chapter deals with the magnetic properties of a basalt glass containin about 13 wt% iron oxide, which has been heat-treated at different temperatures, i.e. different particles of magnetite with a narrow range of nano-sized grains. On heat-treatment, the basalt glass transforms into a fine-grained glass-ceramic [1]. In order to control the quality of glass-ceramics, it is important to study the nucleation and crystallization behaviour of this system. Initially, X-Ray diffraction (XRD) and transmission elec- tron microscopy (TEM) measurements were carried out on samples heat treated at 600, 650, 700, 800, 900°C for 2, 4 and 8h, and also on the as-annealed glass (i.e. the blank glass). As explained earlier, the XRD showed that there was no crystalline peaks for blank glass, 600 and 650 samples. The peaks due to magnetite appeared for the 700 sample ; the strongest peak being at the d-spacing of 0.250 nm. For the 800 and 900 samples, there were also peaks due to the pyroxene along with those of the other minor phases. However, TEM showed the presence of magnetite in the 650 sample, and also in the 700 sam- ple, while the last two samples showed the presence of a mixture of the phases. The particle sizes of the nano particles of magnetite were estimated to range between 4.5 nm to 7.0 nm between 650 and 900°C for 8 h only.
Since the particles are small, the supermagnetic behaviour of the particles is very interesting as explained above [6-9]. The Mössbauer measurements at 4°K showed that the maximum amount of magnetite was formed at 700°C, and the evolution of the Mössbauer parameters showed that the mag- netite particles have the ‘most improved symmetry’ at this temperature [6, 9].
The study of growth process immediately following nucleation is the most accessible, though indirect, method of inquiry into the nucleation process [3]. In order to determine an optimum nucleation condition for magnetite in the basalt glass-ceramics, small angle neutron scattering (SANS) measure- ments were carried out dynamically at 710°C, as a function of time, on a series of samples (dimensions = 10 mm × 10 mm × 7 mm) heat treated (i.e. nucleated) between 550 and 665°C for different times. The importance of the SANS measurements for such a study has been discussed elsewhere [4]. The longer times upto 32 h were employed for lower temperatures and shorter times upto 8 h for higher tempera-
NANO MATERIALS
tures of heat-treatment. Based on the results mentioned above, the growth of temperature was fixed at 710°C. This was also higher than the Curie temperature of the bulk magnetite (578°C), so that the effect of the ‘magnetic scattering’ on the SANS spectra could be neglected.
The SANS measurements were carried out in the spectrometer D-17 at the Institut Laue-Langevin (Grenoble); the details of this spectrometer are given elsewhere [46]. The detector distance was 2.85 m and λ o = 1.282 nm. Absolute small-angle cross-sections were deduced from the experimental neutron intensity, measuring the incoherent scattering on a thick vanadium sample. The sample was put in a small furnace between the incident beam and the detector, and the temperature was raised to 710°C in about 10 to 120 min, except for one sample (nucleated at 577°C for 19 h) which was extende upto 360 min.