SOL-GEL METHODThis is a bottom-up approach, where materials and devices are built from molecular atoms which assemble themselves chemically by principles of molecular recognition i.e. this approach starts with atoms and molecules and create larger nanostructures. The sol-gel process is a wet-chemical technique (chemical solution deposition) widely used recently in the fields of materials science and ceramic engineering. Such methods are used primarily for the fabrication of materials (typically a metal oxide) starting from a chemical solution (sol, short for solution) which acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers. Typical precursors are metal alkoxides and metal chlorides, which undergo hydrolysis and polycondensation reactions to form either a network elastic solid or a colloidal suspension (or dispersion) a system composed of discrete (often amorphous) sub-micrometre particles dispersed to various degrees in a host fluid. Formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution. Thus, the sol evolves towards the formation of a gel-like diphasic system containing both a liquid phase and solid phase whose morphologies range from discrete particles to continuous polymer networks. A sol is a dispersion of the solid particles (~0.1-1 mm) in a liquid where only the Brownian motions suspend the particles. A gel is a state where both liquid and solid are dispersed in each other, which presents a solid network containing liquid components.The precursor sol can be either deposited on a substrate to form a film (e.g. by dip-coating or spin-coating), cast into a suitable container with the desired shape (e.g. to obtain monolithic ceramics, glasses, fibers, membranes, aerogels), or used to synthesize powders (e.g. microspheres, nanospheres).

The reagent grade Fe(NO3)3.9H2O has been used as precursor for the preparation of nanocrystalline -Fe2O3 powder by chemical route. Initially, a solution of Fe(NO3)3.9H2O is made with distilled water. A few drops of concentrated nitric acid have been added to keep the pH level of the solution in acidic range. This solution is then stirred for 1 h and then poured into a plastic flat-bottomed container and left for three days in ambient atmosphere for gelation. The gel is then evaporated to obtain as-prepared sample in powder form.

3.2.1.1 Advantages of Sol-gel Technique: Sol-gel process-(a) Can produce thin bond-coating to provide excellent adhesion between the metallic substrate and the top coat.(b) Can produce thick coating to provide corrosion protection performance.(c) Can easily shape material into complex geometries in a gel state.(d) Can produce high purity products because the organo-metallic precursor of the desired ceramic oxides can be mixed, dissolved in a specified solvent and hydrolyzed into a sol, and subsequently a gel, the composition can be highly controllable.(e) Can have low temperature sintering capability, usually 200-600 0 C.(f) Can provide a sample, economic and effective method to produce high quality coatings.