Cadmium dendrites

Fig. 7.170. (a) Two-dimensional (left) and three-dimensional (right) silver dendrites (after Wranglen). (b) Three-dimensional cadmium dendrites (after Wranglen). 

In the case of a reentrant groove (Fig. 2.8a), the growth of new layers can be started by one-dimensional nucleation. The growth rates in the cases of one-dimensional and two-dimensional nucleation as rate-determining steps can be compared to each other by COTisidering the growth of two-dimensional flat cadmium dendrites from Fig. 2.8b-d. 

The tip of the twined cadmium dendrite precursor from Fig. 2.8b represents the physical equivalent of the scheme of the growth site from Fig. 2.8a. As shown in Fig. 2.8a, a layer of atoms advance in the direction determined by twining laws, an edge is constantly renewed, in which the new layers can be started by one-dimensional nucleation. Further growth and branching of precursor like that from Fig. 2.8b produces the dendrites shown in Fig. 2.8c, d. The deposition on the lateral flat dendrite surfaces takes place by repeated two-dimensional nucleation, as in deposition on dislocation free surface [33]. This makes the deposition rate in the direction of tip motion many times larger, which results in dendrite shape like that from Fig. 2.8d. 

Cadmium also belongs to the group of normal metals which are characterized by the large exchange current densities. In the case of cadmium, hydrogen evolution is also a slow process and this fact enables the analysis of the formation of cadmium dendrites without the effect of any parallel process. 

Fig. 2.22 Precursors of cadmium dendrites obtained by electrochemical depositions at overpotentials, tj, of (a) = 50 mV deposition time, t = 2 min ...

Precursors of cadmium dendrites [47] obtained by the processes of electrochemical deposition from 0.1 M CdS04 in 0.50 M H2SO4 onto cadmium wire electrodes at different overpotentials are shown in Fig. 2.22. It is obvious that further growth of the dendrite precursors shown in Fig. 2.22 leads to the formation of 2D dendrites (Fig. 2.23). Around the tips of dendrite precursors, as well as around the tips of dendrites, spherical or cylindrical diffusion control can occur, which is in good agreement with the requirements of the mathematical model. 

There is an induction period before initiation of dendritic growth [25, 33,49,50]. During this induction period, dendrite precursors are formed by the growth of suitable nuclei. According to Pangarov and Vitkova [51, 52] the orientation of nuclei is related to the over-potential used. The effect of overpotential of electrodeposition on the shape of cadmium dendrites is illustrated in Fig. 2.23. 

A remedy could be achieved by a decrease in the zinc solubility in the electrolyte or by suppression of dendrite formation cadmium-, lead-, or bismuth oxide,... 

Another problem that must be taken into account with plate electrodes is the formation of metal dendrites which grow from the cathode surface toward the counter electrode, causing eventually a short circuit. If a diaphragm is placed between the electrodes, the dendrites can pass through it and they finally may destroy it. The region of Zn or Cd dendrite (or sponge) formation lies on the polarization curve below the limiting current density for deposition of zinc from zincate solutions and cadmium from CdSO solutions. It seems that the formation... 

The critical overpotential of dendritic growth initiation can be determined by plotting the logarithm of the slopes of the straight lines from Fig. 2.14a as a function of overpotential, and the intersection point of the two straight lines determines //j. The same procedure was followed for the deposition of cadmium from 0.10 M CdS04 in 0.50 M H2SO4 (Fig. 2.14b) [43]. 

The cross sections of the copper and cadmium deposits obtained at //i < // < tjc, and // > //c are shown in Fig. 2.15a, b, respectively. It can be seen that there is no dendrite formation when t] < t, both compact and dendritic deposits are formed when rjidendritic metal is deposited when rj > This is in perfect agreement with findings of Calusaru [44] for the morphology of deposits of the same metals deposited at overpotentials corresponding to full diffusion control. 

Popov KI, Cekerevac MI (1989) Dendritic electrocrystallization of cadmium from acid sulphate solution II the effect of the geometry of dendrite precursors on the shape of dendrites. Surf Coat Technol 37 435-440... 

Popov KI, Maksimovic MD, Totovski DC, Nakic VN (1983) Some aspects of current dcmsily distribution in electrolytic cells I dendritic growth of cadmium at the cathode edge in galvanostatic electrodeposition. Surf Technol 19 173-180... 

For instance, the nickel-iron battery, invented almost at the same time (Edison, 1901) as the nickel-cadmium battery, has a poor charge efficiency, which causes excessive heating and hydrogen release. Another example is nickel-zinc technology, for which further study seems necessary, because it is subject to the formation of dendrites which limit its lifetime. 

Quite recently, Guibal and his research team (Butewicz et al., 2010) have irrrmo-bilized thiourea onto chitosan the new polymer was employed for the sorption and recovery of platinum and palladitrm from acidic solutions (up to 1-2 M HCl concentrations). The kinetics of the sorption process was investigated and the pseudo-second rate equation was used for modehng the uptake kinetics. Similarly, Chanthateyanonth et al. (2010) reported the successful immobilization of vinyl sirlfonic acid sodium salt onto dendritic hyper branched chitosan. The new chitosan derivatives displayed improved water solrrbility as compared to the starting material. In addition, the new material showed better antimicrobial activity and chelating behavior with cadmium(II), copper(II), and nickel(II) than chitosan itself. 

Nickel-Zinc Batteries. The nickel-zinc (zinc/nickel oxide) battery has characteristics midway between those of the nickel-cadmium and the silver-zinc battery systems. Its energy density is about twice that of the nickel-cadmium battery, but the cycle life previously has been limited due to the tendency of the zinc electrode toward shape change which reduces capacity and dendrite formations, which cause internal short-circuiting. 

Pulse plating of zinc, cadmium, nickel, chromium and precious metals in aqueous media and molybdenum, chromium, tungsten, niobium and titanium in fused salts improves the properties of the deposits [1,15]. Dense, not dendritic coatings can be obtained because concentration polarization is minimized by the use of pulse current (PC) [16]. Moreover, other pulse plating effect can improve the surface roughness and morphology of the electroplated coatings [17, 18],... 

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