Chapter 2: Experimental Techniques
2.1 Sample Preparation
Bulk samples of ceramics and oxides are generally prepared by solid state route. Solids do not usually react together at ambient temperature over laboratory time scales due to the lack of diffusion and mixing of constituent elements at the atomic scale. So, it is necessary to heat the reactants at high temperatures to overcome the kinetic barriers. The powders of stoichiometric ratio of starting compounds are often pressed into pellets before heating to high temperature in order to increase the contact between the particles. Reaction times are usually several days and it is best to repeat the process to ensure homogeneous samples. The starting materials are usually single cation oxides, carbonates, nitrates or hydroxides which decompose to form oxides when
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heated. Samples used in the present thesis work were prepared by using different techniques such as solid state route, mechanical milling and by co-precipitation.
2.1.1 Solid state reaction
The stoichiometric ratios of starting compounds were weighed using an electronic balance supplied by Mettler Toledo model no. AG135 with an accuracy of ±0.01 mg. The weighed compounds were grinded under the medium of acetone (99%) using an agate mortar and pestle. The homogeneous mixture of starting compounds was transferred into an alumina crucible and was presintered at 400oC for over 24 h followed by furnace cooling to room temperature. The presintered powder was grinded again to get a homogeneous mixture. The presintered powder was pressed into cylindrical shape pellets by using a 13 mm die and a hydraulic press supplied by Techno Search instruments, Thane, India with a maximum load capacity of 6 Ton/cm2. The sintering in pellet form was carried out in a step by step process in air at different temperatures with several intermediate grindings and repelletizing. The final sintering temperature was 900oC.
2.1.2 Mechanical Alloying Technique
Mechanical alloying (MA) of stoichiometric ratio of starting compounds was carried out by high-energy ball milling using a planetary ball mill (Insmart, India). The photographic view of a planetary ball mill, comprising of a horizontal support disc on which vials are mounted is shown in Fig. 2.1(a). The schematic view of MA process is shown in Fig.2.1(b). The vial rotates in a direction opposite to that of the disc, thereby simulating a planetary motion. This planetary motion result in a large centrifugal force acting on the balls kept inside the vial. This causes the balls collide with themselves and with the wall of the vial with high impact. The mixture of starting compounds along with balls in the vial are subjected to repeated cold welding and fracture at the surfaces of the balls and the vial. This process leads to the disintegration of the powders, resulting first in the refinement of crystallite size and ultimately in atomic level mixing of the elements and alloy formation. Hence, size refinement is a natural consequence of a MA process. The refinement and alloying processes are determined by the milling parameters such as the powder to ball weight ratio, ball size, rotation speed, milling time, etc. The nature of the milling vial, the balls and the milling media play an important role in the process of mechanical alloying.
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In the present work, dry milling of high purity oxides such as SnO2 and transition element oxides has been carried. The hardened steel vial and hardened steel balls were used for milling all the powder compositions. The ball to powder ratio of 10:1 and the rotating speed of 500 rpm were maintained during the milling process. In order to avoid excessive heating, it was programmed to stop for 10 minutes after every 15 minutes of continuous milling. For a comparison, we have also carried out the ball milling of selected samples by using tungsten carbide vial and balls with the help of a commercial ball mill.
Fig. 2.1 (a) Photographic view of planetary ball mill and (b) schematic view of mechanical milling process
2.1.3 Co-precipitation Technique
Coprecipitation technique is a convenient method for preparing the highly homogeneous and stoichiometric composition. It is also relatively easy to prepare the materials in nanocrystalline form. Here the size of the nanoparticle depends upon various parameters such as, the type of salt used, reaction temperature, pH value, ionic strength of the system, etc. In a typical procedure, 1.48 mmol of Co(NO3)2·6H2O and 82.5 mmol of SnCl2 were dissolved in 150 mL of deionized water. To this pink solution, concentrated aqueous NH3 was added drop wise with stirring, over
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about 1 hr. The addition of NH3 was stopped when the pH of the solution reached 12 and during this time the precipitation has ceased. Stirring was continued for another 2 hour and then the contents were left undisturbed. After 3 days, the solid Co doped SnO2 was separated from the colorless supernatant layer using a centrifuge and washed with deionized water. Finally the crystalline Co doped SnO2 powder was dried in oven at 80ºC for several hours. The same procedure was followed for obtaining pure/undoped SnO2 from SnCl4. The above product was heated at 200oC for 12 hrs followed by grinding. The precursor powders thus obtained were divided into different batches and were annealed at 400oC, 600oC and 800oC in air and N2 gas atmosphere respectively.