Preparation of Glasses with Nano Particles

6.1.2. Preparation of Glasses with Nano Particles

For bismuth-containing glasses, most of the work was done on a composition : 10 Na 2 O, 18

B 2 O 3 , 64 SiO 2 , 8 Bi 2 O 3 (mole%). These glasses were identified as showing ‘memory switching’ [1, 3].

NANO MATERIALS

Some work was also carried out on samples of composition : 25 Na 2 O, 10 CaO, 55 SiO 2 , 10 Bi 2 O 3 (mole%) to show that the electrical behaviour due to nano metallic bismuth is independent of the chemi- cal composition of the glass matrix. Finally, in order to show the effect of nano-bismuth particles more

clearly, some work was also done on the ‘base glass’ compositions : 10 Na 2 O, 26 B 2 O 3 , 64 SiO 2 and 25 Na 2 O, 20 CaO, 55 SiO 2 (mole%) respectively. The compositions of different glasses, which were used to study the effect of nano-bismuth particles on their electrical properties are summarized in Table 6.1, and which are numbered as 1 to 4. The compositions of the glasses, which were used to elucidate the effect of the dispersion of nano-crystalline selenium particles on the resulting composite matrix, are summarized in Table 6.2, and which are numbered as 5 to 8.

Table 6.1 : Compositions of Glasses containing Bismuth.

Glass No.

Mole% SiO 2 Mole% B 2 O 3 Mole% CaO

Mole% Na 2 O Mole% Bi 2 O 3

Table 6.2 : Compositions of Glasses containing Selenium.

Glass No.

Mole% SiO 2 Mole% B 2 O 3 Mole% Na 2 O

Wt.% Se 5 53.3 32.0 14.7 0.0 6 53.5 26.6 19.7 0.2 7 53.8 21.1 24.8 0.3 8 48.0 19.2 28.8 4.0

The glasses were prepared by melting reagent-grade chemicals in alumina crucibles in an electri- cally heated furnace at temperatures ranging from 1200 to 1400°C under normal atmospheric condi- tions. The selenium purity was 99.9%, which was introduced as powder in the mixture. The glasses were quenched by pouring the melts onto the aluminium moulds. These were then annealed at 500°C for an hour and then cooled slowly to room temperature within the furnace after being switched off.

A substantial amount of selenium was lost during melting, because of the low melting tempera- ture of these selenium containing glasses (i.e. nos. 5 to 8) [4]. Therefore, the final compositions of these samples were determined after estimating their selenium contents by standard chemical methods [5]. For preparing glass 8, which contained the largest amount of selenium, the oxide glass was first made with adequate molar percentages of different components. This glass was then powdered and mixed with the appropriate amount of selenium powder in order to get the required composition. The mixture was taken in a quartz tube, which was evacuated and then sealed. The sealed quartz ampoule was heated to a temperature to 1000°C for 24 hours with gentle rocking for homogenization purposes. The quartz ampoule was taken out and allowed to cool at room temperature. The optical absorption spectra of the glasses 5 to 8 confirmed that selenium was present in its elemental form and not as an oxide [4].

ELECTRICAL PROPERTIES

231 The micro-structure of the glasses was studied with a Philips EM-301 Transmission Electron

Microscope (TEM) as per the procedure described in chapter 2. The TEM micrographs show darker regions, which are ascribed to the presence of metalic bismuth grains, which are of non-spherical geom- etry and of different nano-sizes varying between 5 nm and 100 nm. In the TEM micrographs of the selenium containing glasses also, there are darker regions, which are ascribed to the presence of metallic nano particles of metallic selenium with size varying from 5 nm to 200 nm. The electron diffraction pattern of these glasses show that the metallic granules of selenium are crystalline [1, 2].

For electrical measurements, circular metallic electrodes (both aluminium or gold giving the same results) of diameters varying between 1 and 2 cm were evaporated onto two faces of the samples, which were ground and polished to a thikness in the range 1 to 2 mm. Fo glasses 5 to 8, silver paint was used as electrodes. The DC resistivity was measured by plotting their voltage-current characteristics over a decade of voltage by using an ‘Electrometer’ (Type 1230A, General Radio, U. K.). The linearity of the I-V curves was obtained for all the samples in the temperature range – 160°C to +200°C.

For AC resistivity measurements, a guard-ring was also deposited on one of the sample faces, and three-terminal measurements were carried out over a frequency range 100 Hz – 100 KHz with a ‘Capacitor Bridge’ (Type 716C General Radio, U.K.). For measurements at higher frequency, i.e. between 100 KHz and 30 MHz , a ‘Boonton Q-Meter’ (Type 260A) was used.

The possibilities of switching phenomena in selenium containing glasses were explored by sub- jecting them to a Na + ⇔ Ag + ion exchange, which was followed by a reduction treatment in hydrogen. The electrical measurements on these samples were carried out by the method described elsewhere [2]. The I-V characteristics of these samples were carried out by a ‘Transistor Curve Tracer’ (Type 575, Tektronix, USA). For all the electrical measurements on selenium containing glasses, the samples were kept in a dark chamber so that ‘photo-conductive’ effects could be avoided.