Thin Layer Electrodeposition Growth Of Pb-Sn Alloy

Thin layer electrodeposition is an ideal system for investigation of non-equilibrium aggregation. In this paper the morphological evolution in the binary thin layer electrodeposition was studied for the first time by Pb-Sn alloy system. The results showed that the microstructures of the deposits have an important impact on the morphologies of the deposits. Additionally, thin layer electrodeposition are widely employed in the modern industry and technology, so we also investigated many aspects on the thin layer electrodeposition of Pb-Sn alloy, such as microstructure, growth kinetics and conjugate dissolution/deposition behavior. The main results are as follows:1. Morphological evolution of Pb-Sn binary electrodeposits In order to understand the formation mechanisms of the different patterns in the non-equilibrium growth, thin layer electrodeposition was widely used for the investigation of the different patterns such as dendrite, fractal and so on. However, all these studies only involved of unitary (pure metal) electrodeposition system, so it was impossible to investigate the impact of the solution's composition and the deposit's microstructure on the formation and transition of the morphology. In this paper the binary electrodeposition was used to study the morphological transition for the first time. As the proportion of the two kinds of metallic ions varied, the morphologies of the deposits evaluated from the dendritic pattern to ramification, dense branch and finally to the fractal structure.Microstructure analysis showed that the morphological variations resulted from the influence of the composition on the crystallographic texture. The deposit of pure Pb (or Sn) was regular dendrite with strong anisotropy because of preferred orientation of crystal growth. When a few Sn (or Pb) was added, Sn (or Pb) atoms were embedded in the crystal lattice of Pb (or Sn) and form a Pb-based (or Sn-based) solid solution. Once the content of Sn (or Pb) exceeded the solubility of Sn (or Pb) in Pb (or Sn), the superfluous Sn (or Pb) atoms precipitated out and formed small Sn (or Pb) crystal grains in the deposit. Mixed growth of the Pb and Sn crystal grains weakened the preferred orientation growth and led to tip splitting of the branches. With the increase of the Sn content the texture of the deposit changed into the pseudo-eutectic phase where the Pb crystal grains were mixed with more and more Sn grains and the sizes ofthe grains became smaller and smaller. Thus the mixture with large dispersity broke the intrinsic anisotropy of crystal growth and allowed the randomness to become the dominant factor in the growth of the deposits.Morphological evolution of Pb-Sn codeposits in the thicker electrochemical cell was also studied in this paper. When the electrolyte layer was thicker, the deposit appeared more patterns in the macroscopic scale such as dendrite, ramification, compact, fractal and filamentous. However, this macroscopic morphology transition came down to two kinds of transiting processes in the microscopic scale. One was the transition of branch formation from anisotropic dendrites to ramified branch. The other was the evolution of the tip of branches from polyhedron side-branch to smooth globule. Microstructure analysis showed that these two kinds of transitions were caused by the change in the texture of the deposit. The dendritic side-branch of the pure lead (or pure tin) was consists of crystal grains of pure lead (or pure tin), so the side-branch developed into the polyhedrons due to the preferred orientation of crystal growth. When the Sn (or Pb) was added, the pseudo-eutectic phase formed. The pseudo-eutectic, which was formed by the mixture growth of the crystal grains of Pb and Sn, allowed the electrodeposition to appear isotropic growth, and the side-branch with the shape of regular polyhedron transited into elliptic deposit. With the increase of Sn (or Pb) content, the region of pseudo-eutectic became large, so the deposit exhibited more and more characteristics of the isotropic growth, Le. globular growth, and the tip of branches transited from regular polyhedron into globular deposit gradually.2. Non-equilibrium microstructure of Pb-Sn binary electrodeposit Generally, the microstructure of the alloy obtained from electrodeposition was different from that prepared by solidification method. Especially in thin layer electrodeposition, the deposited alloy often processed unique non-equilibrium microstructure due to the variation of physical and chemical environment in front of the interface. In this paper, we observed these microstructures and discussed their formation mechanisms.The experiments showed that the deposits of pure Pb and Sn were dendrites, and that the side-branches of the dendrite consisted of several large crystal grains. Because of preferred orientation of crystal growth, the side-branch developed into regular polyhedron. For the deposit with the lower Sn content (Pbo.9Sno.i), its morphologywas irregular dendrite and its microstructure possessed solid solution-eutectic-solid solution three layers. The mechanism for this phenomenon was interpreted as follows: In the initial stage the Pb content in the center part of the dendrite deposit was higher than that in the solution due to preferred deposition of Pb, so the texture of the deposit was Pb-based solid solution. Excessive deposition of Pb would lead to the increase of Sn2+ content around the interface, so in the later stage of the growth the Sn content in the deposit was higher than that in the solution, and the pseudo-eutectic structure with 16.8 at. % Sn was formed.In the later stage of the electrodeposition growth, the Pb content in the deposits would increase gradually, which led to the formation of Pb-based solid solution. Our studies indicated that the increase of Pb content in the later stage of the growth was caused by the decrease of the current density on the interface. During the electrodeposition growth, the current density in the interface will change due to irregular shape of the deposit and the screen factor, i.e. the current density in the interface that situates at the root of the whole deposit will decrease gradually as the electrodeposition proceeds. According to Tafel relation, the decrease of the current density led to the reduction of overpotential. Under the lower overpotential, the Pb with more positive equilibrium potential is preferred to deposit. Thus the content of Pb in the deposit increased as the electrodeposition proceeds.Using the principle that the current density influenced the texture of the deposit, we fabricated a deposit with multi-layered microstructure. This method was rather simple, easy-controlled and cheap in comparison to the previous controlled measures. It can be expected that the method will find application in material technology and industry. 3. Growth kinetics of Pb-Sn binary thin layer electrodeposition Both the morphology and the microstructure of the deposit varied as the growth proceeds, this variation means that the growth process of the Pb-Sn thin layer electrodeposition was very complex. In this paper growth kinetics of PbcsSnoj thin layer electrodeposition was investigated.The experiments showed that as the electrodeposition proceeded, the growth mode of the deposit changed from the cake-like structure to massive branches and finally to compact growth. We suggested that this morphological transition aroused from the change in the growth kinetics. In the initial stage of the growth, the electrodeposition process was controlled by the charge transfer. In this case, thegrowth velocity was very high, so the deposit grew into the thin branches. Moreover, in the initial stage the cation around the interface was enough and the area of the interface was small, so the macroscopic morphology appeared to be a cake-like structure. Then the growth changed into the process controlled by ion diffusion. In this stage the cathodic current kept constant and the area of the interface became more and larger, so the current density on the interface decreased gradually. In the case of little current density, the thin branch became thicker, the tip of the branch developed globule, and the deposit appeared the morphology of massive branches. When the current density decreased to a critical value, the deposit could not grow into the branch morphology but a compact structure.The experiments also indicated that in the later growth stage the deposit possessed a layered structure with the periodic composition distribution. We suggested that the reason for this unique phenomenon was the generation of hydrogen came from the hydrolysis of water in the solution. Because of the generation of H2, the cathodic current began to oscillate, which caused the cunent density in the growth interface to be unstable and the formation of the layer structure with different composition.4. Conjugate dissolution/deposition behavior in the thin layer electrodepositionAccording to the electrochemical principle, after the deposit separates from the deposit and becomes an isolated segment, the segment will stop growing. However, we found a novel phenomenon in the thin layer electrodeposition of Pb: generation of the bubble disturbed the growth of the Pb dendrites and made the deposit break down. The deposit segment moved toward the anode in the thin cell after it broke away from the dendrite deposit linked to the cathode. When the experimental condition varied, disengaged deposit segment appeared the different behaviors. If the concentration of the solution was lower, the deposit segment did not move but shortened gradually. If the size of the deposit segment was very short, the disengaged depsosit norther move nor shorten but keep constant. Further investigation showed that these unique behaviors were aroused from the conjugated dissolution/deposition reaction taking place on the two ends of the deposit segment. When the amount of dissolving lead was equal to the deposited lead, the segment appeared to move; when the amount of dissolving lead was larger than thedeposited lead, the segment appeared to shorten gradually; when the dissolution and deposition reactions did not occur, the disengaged deposit remained stationary.The phenomenons that the disengaged deposit segment moved or shorten in the electrochemical cell had not reported in the previous 3D elecreodeposition studies. We suggested that the mechanism resulting in these phenomenons was related to the characteristics of thin layer electrochemical cell. Our studies showed that due to the lower conductivity and the thinner thickness of the solution layer, the resistance of the solution became very large, which led to the higher overpotential at the ends of the deposit segment and quicker rate of the electrode reaction. The quicker rate of the electrode reaction was the reason for the novel behavior of disengaged deposit in the thin layer electrochemical cell. According to equivalent resistance model, we developed a new model for calculate the overpotential at the ends of the deposit segment. Additionally, the microprobe was used to measure the overpotential. The calculative value agreed with the one measured by experiment. The result indicated that as the length of the deposit segment increased, the polarization overpotential reduced. Once the overpotential reduced to a critical potential, the electrode reaction could not take place and the disengaged deposit keep constant.Additionally, we investigated the conditions and the differences that led to the different behaviors of deposit segment, such as movement, shortness or keeping stationary. The results showed that in the case of low concentrations, the anodic reaction was only the dissolution of lead and the cathodic reactions were both the deposition of lead and the generation of hydrogen gas. Conjugate dissolution -deposition reactions require that the charges in the anodic reactions should be equal to those in the cathodic reactions. Thus, the consumption of lead due to the dissolution was more than that for the deposition of lead, and the disengaged deposit segment would shorten gradually. When the concentration of the solution was high, cathodic reaction was dominated by lead deposition and the anodic reaction was the dissolution of lead, so the disengaged deposit appeared movement. Furthermore, it could be seen from the polarization plots that no matter what condition it was, the deposited lead could not be larger than the amount of dissolving lead. Thus, the behavior that the disengaged deposit grew up could not happen.