Preparation of Composite Film from Non-Suspended Solution By Electrochemical Technique
Authors: Koiwa Ichiro , Chokki Kenta , Watanabe Nobuaki , Yabe Kazuhiro 
Published: February 2020

A composite plating method is very useful to add new function in plated films. However, the composite plating method has serious problems due to particles in the bath, deterioration of bath stability and difficulty of filtering to maintain quality of plated films. Oue et. al.1) reported a composite films, Zn-Al2O3, plated from non-suspended solution bath without particles by using quaternary ammonium salts. In this study, two type composite films, Zn-Al2O3 and Zn-TiO2, have been prepared from non-suspended solution by electrochemical technique.
First, results of Zn-Al2O3 composite films were reported. The zinc matrix with aluminum oxide particles composite films from non-suspended solution containing quaternary ammonium salts were investigated. The solution contains 520 mol/m3 ZnSO4·7H2O and 79 mol/m3 Al2(SO4)3·14-18H2O as metal source. A quaternary ammonium salts of Benzyldimethyltetradecylammonium chloride dehydrate (BDTAC, Wako Co.,Ltd.) was used in this study. Transmission electron microscopy (TEM) was used to evaluate the composite films. Bath pH at 2.5 is the best of all and high aluminum content ratio values, Al/(Al+Zn), more than 30at% are obtained. The aluminum content ratio values increase with increasing current density under the condition of bath pH at 2.5 and 3.0. However, the films plated bath whose bath pH adjusted at 2.0 showed almost no aluminum content. Since the codeposition mechanism is pH jump at the cathode electrodes, pH value of 2.0 is too low. From transmission electron microscopy (TEM) measurement, two different deposits were observed as shown in Fig.1, one is needle-shaped deposit and the other is closely packed fine particle area. Magnified photograph shows closely packed fine particle area and fine particles are clearly observed. From results of composition analyses, the D area, needle-shaped deposit, is composed of 82.6 at% zinc, 17.0 at% aluminum and 15.7at% oxygen. As the same matter, the E area, closely packed fine particle area, is composed of 45.6 at% zinc, 29.2 at% aluminum, 20.6 at% oxygen and 4.5 at% sulfur. The zinc content of area D, needle-shaped deposit, is much higher than that of area E, closely paced fine particle area. On the other hand, the aluminum content of area E is much higher than that of area D. It is concluded based on above results that the needle-shaped deposit is mainly composed of Zn or ZnO, and the fine particle is mainly composed of Al2O3 from electron diffraction measurement. From results of transmission high energy electron diffraction (THEED) measurement, the fine particles were determined as θ-Al2O3. Therefore, first, aluminum ion formed Al(OH)3 near the cathode electrode by increasing pH value. Then it was dehydrated and it changed to θ-Al2O3. From X-ray diffraction (XRD) measurement, no peaks due to θ-Al2O3 have been observed. The size of θ-Al2O3 particles, whose diameter is about 5nm, is too small to detect by XRD measurement. However, transmission high energy electron diffraction (THEED) measurement shows higher sensitivity and the crystal structure is determined as θ-Al2O3. The origin of sulfur is not clear.
Second, the results of Zn-TiO2 composite films were reported. The solution contains 520 mol/m3 ZnSO4·7H2O and 79 mol/m3 TiSO4·1-2H2O as metal source. A quaternary ammonium salts of Benzyldimethyltetradecylammonium chloride dehydrate (BDTAC, Wako Co.,Ltd.) was also used in this study. The plating bath temperature was controlled at 40 oC. The titanium was difficult to plate because it is easy to precipitate as a hydroxide. A complexing agent was very important to prevent from precipitating. In this study, six kinds of complexing agents ethylenediamintetraacetic acid (EDTA), glycine, formic acid, trisodium citrate, ascorbic acid and gluconic acid, have been investigated. A critical pH value to precipitate was evaluated by changing complexing agent. The critical pH values by different complexing agents, ethylenediamintetraacetic acid (EDTA), glycine , formic acid, trisodium citrate, ascorbic acid and gluconic acid, were 1.65, 1.85, 2.06, 2.65, 2.67 and 5.50, respectively. The gluconic acid is one of the most promising candidates of theses 6 complexing reagent. The 0.5g or 1.0g gluconic acid added to the plating baths and bath pH was controlled at 0.5 or 1.0. A current density was controlled from 50 to 500 A/m2. The highest TiO2 content value, 1.53 at%, was obtained at following condition, gluconic acid concentration was 1.0 g/l, bath pH was 0.5, and current density was 100 A/m2. Therefore, the two types of composite films, Zn-Al2O3 and Zn-TiO2, have been prepared by using quaternary ammonium salts, BDTAC, from non-suspended solution.
Acknowledgement: This work was partially aided by MEXT-supported Program for the Strategic Research Foundation at Private Universities.
[1] S. Oue, H. Nakano, S. Kobayashi, T. Akiyama, and K. Okumura, J. Surface Finishing Soc. Jpn.,53 920 (2002) .

Figure 1