4.7. Formation of Thin Film:
Here, we will discuss the four process parameters for the formation of the thin film:
2.7.1 Atomization of precursor solution: Aqueous …show more content…
Electric force: In electric spray deposition the electric force is the main force to drive droplets having magnitude higher than gravitational force (D.Perednis et al., 2004). The electric force (F_e) is given by, F_e = q_dE where E is electric field strength and q_d is dropet size.
c. Stokes force: The stokes force acts on the droplet due to the air resistance. This results due to friction between the droplet and air molecules. Large droplets which move with larger velocity experiences the largest retarding force, F_s = 6πη_ar (v_d-v_a) (1 + 3/8 R_e), here η_a and η_d are the vescosity of air and droplets vescosity, R_e is Rynolds number (Filipovic L. et al., 2013).
d.Thermophoretic force: It is retarding force results in the decrease of droplets velocity as they approach the heated substrate. Thermophoretic forces keep most droplets away from the substrate surface and the film grows from the vapor of droplets passing very close to the heated substrate surface as in of chemical vapor deposition as shown in fig. Droplets after collision with heated substrate changes into powdered form. The droplets closer to the heated substrate but not in actual contact improves the efficiency of film growth (Perednis D. …show more content…
3.2. Schematic of aerosol transport (Chen et al., 1996).
The aerosol droplets experience evaporation of the solvent during the transport to the substrate.
4.7.3 Decomposition of precursor: When the droplet hits the heayed substrate surface many processes takes place simultanaously, evaporation of the solvent, spreading of the droplet, and salt decomposition.
Different steps during the pyrolysis of the aerosol decomposition are shown below:
A) In the first step, an aqueous precursor solution is converted into aerosols through spray nozzle and the solvent evaporation takes place.
B) In this step,the vaporization of the solvents results the formation of precipitate as the liquid droplets approachees the substrate.
C) Pyrolysis of the precipitate occurs in succession before the precipitate reaches the substrate.
D) When the precipitate reaches the substrate, nucleation and the growth of metal oxide thin films on the substrate take place.
E) Finally, the growth of the nuclei leads to the formation of continuous thin layer of metal oxide.
Fig. 2.3. Mechanism of thin film formation by spray pyrolysis