Synthesis of Nano Particles of Zirconia

3.7.2. Synthesis of Nano Particles of Zirconia

As said earlier, there are various techniques for the preparation of nano-crystalline zirconia par- ticles. However, the most common and versatile techniques are :

A. Sol-Emulsion-Gel Technique, and

B. Sol-Gel Method.

NANO PARTICLES OF ALUMINA AND ZIRCONIA

137 Although the latter topic is discussed in some details in the chapter - 8, some salient features

will be given here with reference to zirconia along with that of the first technique in the following subsections.

3.7.2.1. Sol-Emulsion-Gel Technique

The use of alkoxides in the preparation of ceramic powders has not always been successful due to its high cost. In any processing technique, the economy is quite an important factor. So, after

a search of so-called inexpensive chemicals, it was found that the ‘zirconyl nitrate’ is relatively cheaper precursor and it can be easily used for the preparation of zirconia nano powder. The chemi- cal reaction in ammonia solution is expressed as :

ZrO(NO 3 ) 2 +H 2 O + 2NH 3 = ZrO 2 + 2NH 4 NO 3

In this process, the nitrogeneous compounds are preferred over chloride compounds due to the fact that they lead to a non-polluting and easy burn-out. This process is basically a combination of both sol-gel and emulsion methods [60].

Here, the Zr ions are entrapped in the water droplets and stabilized in an organic solvent by the addition of ammonia gas. It is through the process of ‘heterogeneous distillation’, water can be removed, thus stabilizing the zirconia gel droplets. The remaining unreacted water is also removed by heterogeneous distillation [61-62].

First of all, the sol is prepared by using zirconyl nitrate and deionised water. Then, the gel formed is heated to around 90°C with continuous stirring until a stable ‘suspension’ of fine colloi- dal particles is obtained. In another beaker, a surfactant like ‘Tween - 80’ (chemical name - poly- oxyethelene sorbitan mono-oleate) is dissolved in xylene. The sol is then added to this ‘xylene solution’, which leads to the formation of small ‘emulsion droplets’ in the reaction mixture, and stabilized due to the presence of the surfactant.

At this stage, suspension is agitated, when the ammonia gas is bubbled through this suspen- sion, and then the ‘emulsion droplets’ transform into the ‘gel’. In the next step, the ultra-sonic agita- tion of the solution is again needed to prevent any ‘agglomeration’ during the gelation process. Finally, the reaction mixture is refluxed and the washing is done by xylene several times before separating the powder by a centrifuging system. The whole process of the ‘sol-emulsion-gel’ is depicted in Figure 3.9,

where all the details of the progression of the process is shown. Finally it should be mentioned that the selection of a ‘surfactant’ for a particular emulsion depends on the hydrophobic-lyophilic balance (HLB) number of the particular surfactant, and the composition of the suspension [63].

NANO MATERIALS

Zirconyl nitrate Deionized water Sol

Surfactant Em ulsion

Ultrasonic Reaction m ixture

Am m onia gas Gel Distilled &Refluxed

Centrifuge Drying

Calcination

Figure 3.9: The flow digram for the sol-emulsion-gel process. [68]

3.7.2.2. The Sol-Gel Technique

This technique of synthesis is based on non-hydraulic sol-gel reaction mechanism between zir- conium iso-propoxide and zirconium chloride at 340°C. By this process, it is possible to synthesize a reasonably large quantity of uniform-sized nano particles without any size selection process. In this

case, ‘zirconium iso-propoxide propanol complex’, Zr[OCH(CH 3 )] 4 .(CH 3 ) 2 COOH, and zirconium chlo- ride are added to extremely pure and degassed tri-octyl phosphine oxide, i.e. TOPO, at 60°C in argon atmosphere. Then, the temperature of the system is increased to 340°C and the whole reaction mixture at

139 this temperature is subjected to vigorous stirring. The reaction mixture is then cooled, dried, and degassed.

NANO PARTICLES OF ALUMINA AND ZIRCONIA

The acetone is now added to assist in the precipitation of the nano-sized zirconia particles. The ‘precipi- tate’ can be taken out by the action of the centrifuge, in order to result in the formation of the nano particles of zirconia. With a proper optimization of the process, it is possible to synthesize a few grams of product, which can be considered as a large production in ‘colloidal chemical’ synthesis of momo- dispersed nano particles.

Zr[OCH(CH ) ](CH ) CHOH 32 3 2 Zirconium chloride

TOPO 60°C, Argon Atm.

340°C stirring

Cooled 60°C

Acetone

Zirconia Nanoparticle Precipitation

Washing

Calcination 800°C

Figure 3.10: The flow digram for the sol-gel process. [68]

NANO MATERIALS

The entire process of sol-gel is shown in Figure 3.10, which clearly shows various steps in- volved. Some points still need to be mentioned here. In this sol-gel process, the use of water forms hard agglomerates, and hence they should be avoided as much as possible, if not completely eliminated. The final product on drying is obtained in the powder form. After a controlled heat-treatment process, the yellow colour of powder changes to white.