OPTIMIZATION OF GROWTH RATE OF THIN FILMS OF CADMIUM SULFIDE AT ELECTROCHEMICAL SEDIMENTATION

the results of a study of the process of deposition of cadmium sulfide from an aqueous electrolyte were shown in this work. As part of the full factorial experiment (FFE), a mathematical model of dependence between the average speed growth cathode CdS films, and the deposition conditions was built. A relationship between current efficiency and deposition factors such as pH, temperature and electrolyte concentration was demonstrated.

The composition of the films produced electrochemically deposition depends on the potential in which the synthesis is carried out.Therefore for the purpose of standardization of structure of the studied films, sedimentation was carried out in the potentsiostatic mode.The factors that affect the process of deposition films, such parameters were selected: the pH value; electrolyte temperature (T); electrolyte concentration (C).

One of the major factors of defining composition of the films of CdS
Processing of experimental data was carried out by method, described in [2].
Within FFE technique, k parallel series of experiments were conducted (where k = 1, 2, 3) at g locations factorial space (where g = 1, 2, 3…8).Table 1 shows the combination of minimum and maximum values of the factors investigational factor space.In communication that calculations for a full factorial experiment uses normalized values, through x 1 , x 2 , x 3 denote pH, electrolyte temperature and concentration, respectively.
Thickness of the samples received during the experiment has been measured on a microscope of «МИИ-4».Based on the thickness of these films the average speed of growth was calculated for each test, and calculated the average speed of three parallel series of experiments, each of the 8 points of the test factor space (see table 2).
Table 2 Average speed film growth The experimental results are tested for reproducibility by means of Cochran's criterion [2], which is based on the law of the distribution ratio of the maximum empirical variance to the sum of all variances.Then normalized values of the model in (1) was obtained: where V' mod is the mean rate of growth of films obtained by modeling using normalized Applying the criterion of Fisher [2], it was confirmed by the adequacy of the model.That allowed us to obtain the equation ( 2) interprets the results of experiments in terms of named: -0.63X 1 X 3 0.119X 2 X 3 +0.026X 1 X 2 X 3 -0.217,where V « mod is the mean rate of growth of films obtained in the simulation with the use of the named variables; X 1 , X 2 , X 3 -named values of pH, temperature and electrolyte concentration, respectively.
The dependence described by the received equation, the average growth rate of a film from sedimentation conditions, is graphically presented in figure 2. As can be seen from this figure, in an acidic medium at pH=1 the film growth is nearly three times more active than the electrolyte with pH=6.
This results from the fact that in acidic environment there is a sodium thiosulphate decomposition on reaction [3]: This reaction is a sulfur source for formation reaction of CdS.For the films obtained in the medium with pH = 1, the maximum growth rate is locate at high temperature and a low electrolyte concentration.Increasing the CdS film growth rate with increasing temperature of the electrolyte it can be explained by: first, an increase in the internal energy reserve of cadmium and sulfur ions, second diffusion growth on the cathode surface, and third acceleration of thiosulfate decomposition.In When using an electrolyte with pH = 6, a significant increase was observed only at elevated temperatures and high electrolyte concentration, in other cases only traces of CdS found on the cathode surface.

Fig. 1 .
Fig. 1.Cyclic voltammogram of water solution 0,347M CdSO4 and 0,347M Nа2S2O3, pH=2, at a Pt electrode.Potential ramp rate 5mV•s-1: a -potential of reaction's beginning of cathodic sedimentation of CdS; b-potential site's beginning of limit current of sedimentation's reaction of CdS; cpotential of reaction's beginning of metal cadmium's sedimentation; d-the peak corresponding to process of joint etching of CdS and Cd; e-peak corresponding to CdS etching process

values; x 1 , x 2 , x 3 .
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Fig. 2 .
Fig. 2. Dependence of average growth rate of films on temperature and concentration of electrolyte: a-pH = 1; b-pH = 6