Electrolytic deposition, definition

Tellurium is a constituent common to several definite compounds having semiconducting properties which can be obtained by electrolytic deposition (e.g. CdTe, ZnTe,...). The low solubility of tellurium oxide in acidic aqueous solutions explains why its kinetics of electrodeposition, in the binary of tertiary alloys involved, is mainly controlled by mass transport. 

In a similar way, electrochemistry may provide an atomic level control over the deposit, using electric potential (rather than temperature) to restrict deposition of elements. A surface electrochemical reaction limited in this manner is merely underpotential deposition (UPD see Sect. 4.3 for a detailed discussion). In ECALE, thin films of chemical compounds are formed, an atomic layer at a time, by using UPD, in a cycle thus, the formation of a binary compound involves the oxidative UPD of one element and the reductive UPD of another. The potential for the former should be negative of that used for the latter in order for the deposit to remain stable while the other component elements are being deposited. Practically, this sequential deposition is implemented by using a dual bath system or a flow cell, so as to alternately expose an electrode surface to different electrolytes. When conditions are well defined, the electrolytic layers are prone to grow two dimensionally rather than three dimensionally. ECALE requires the definition of precise experimental conditions, such as potentials, reactants, concentration, pH, charge-time, which are strictly dependent on the particular compound one wants to form, and the substrate as well. The problems with this technique are that the electrode is required to be rinsed after each UPD deposition, which may result in loss of potential control, deposit reproducibility problems, and waste of time and solution. Automated deposition systems have been developed as an attempt to overcome these problems. 

Table 4 lists selected electrochemical stability data for various lithium salt anions that are commonly used in lithium-based electrolytes, with the measurement approaches indicated. Although it has been known that the reduction of anions does occur, sometimes at high potentials, the corresponding processes are usually sluggish and a definite potential for such reductions is often hard to determine. The reduction of solvents, occurring simultaneously with that of anions on the electrode, further complicates the interpretation efforts. For this reason, only the anodic stability of salt anions is of interest, while the cathodic limit of the salt in most cases is set by the reduction of its cation (i.e., lithium deposition potential). 

The electrodeposition of an alloy requires, by definition, the codeposition of two or more metals. In other words, their ions must be present in an electrolyte that provides a cathode film, where the individual deposition potentials can be made to be close or even the same. Figure ll.l depicts typical polarization curves, that is, deposition... 

One coulomb C or one ampere-second is a unit of quantity of electricity (electric charge) required to deposit 0,001118 grams of silver from a solution of silver nitrate regardless of the time during which the current passed through the electrolytic cell. This definition means that a current of one ampere represents a quantity of electricity equivalent to one coulomb per second. 

According to Faraday s law, a perfectly definite deposition of zinc takes place for a given current running a given time, so that the velocity of reaction is definite too. Thus the nature of the resistance already spoken of (p. 176) which conditions the progress of a chemical reaction is explained, at least in a special case it depends here on the conductivity of an electrolyte, i. e. on the velocity of movement through the medium in question, under the influence of the moving force, which is here electrical. 

According to the definition of Brenner [43], it has become common to classify electrolytic metal deposition from nonaqueous electrolytes according to two groups, i. e., aqueous and nonaqueous . The aqueous group comprises all electrolyte systems from which metals or metallic alloys are deposited that can also be deposited from aqueous solutions. The nonaqueous group includes systems from which metals or metal alloys can be electrodeposited that cannot be plated from aqueous electrolytes. 

Dendritic deposits grow under mass transport-controlled electrodeposition conditions. These conditions involve low concentration of electrolyte and high current density. A dendrite is a skeleton of a monocrystal consisting of stem and branches. The shapes of the dendrites are mainly determined by the directions of preferred growth in the lattice. The simplest dendrites consist of the stem and primary branches. The primary branches may develop secondary and tertiary branches. The angles between the stem and the branches, or between different branches, assume certain definite values in accordance with the space lattice. Thus, dendrites can be two dimensional (2D) or three dimensional (3D). 

At least two of the recognized mechanisms for the formation of electrical double layers (Hunter, elal. 1981 Russel etal., 1989) are relevant to LB film depositions (1) ionization of carboxylic acid group and amphoteric acid groups on solid surfaces, and (2) differences between the affinities of two phases for ions or ionizable species. The latter mechanism includes the uneven distribution of anions and cations between two immiscible phases, the differential adsorption of ions from an electrolyte solution to a solid surface, and the differential solution of one ion over the other from a crystal lattice. Since the solid-liquid and the film-liquid interfaces are flat, large surfaces and since both have a large, solid-like concentration, the analysis that follows applies to both interfaces. For an interface conformed by a thin film of an amphiphilic compound with the hydrophilic end of the molecule in contact with the water subphase, the equilibrium of charges is based on pH and subphase concentration. The effect of pH is highlighted by the definition of the of the carboxylic acid ... 

The stoichiometry of the film depends upon the solute concentration, the pH of the electrolyte, electrolysis current density, and deposition time. A careful analysis of the electrodeposition potential curve appears to be the best way to determine the ideal experimental conditions for the preparation of various definite compounds like CuInSe2 by electrolyhc co-deposition. 

This definition excludes inhibitor films or other corrosion-protecting layers such as paint and so on, which have the same function as passive films but a different origin. Such films are formed from components in the electrolyte (inhibitors) or are deposited in a technical process (e.g. cathodic deposition of paint). 

Faraday s Law Definition. The number of equivalents of any substance liberated or deposited at an electrode is exactly proportional to the quantity of electricity which passes across the metal-electrolyte junction. This is Faraday s law, which is among the most exact in all of nature. It is independent of the shape of the electrode, temperature, the nature of the electrode, and the rate of current passage. 

In the electrolytic cell, the cupric ions and sulfate ions both contribute to the conduction mechanisms. But only cupric ions enter into the electrode reaction and pass through the electrode-solution interface. The electrode therefore acts like a semipermeable membrane which is permeable to the Cu ions but impermeable to the 80 ions. Anions accumulate near the anode and become depleted near the cathode, resulting in concentration gradients in the solution near the electrodes of both ions. This is termed as concentration polarization. Let us determine the current-voltage characteristic of the cell, that is, the concentration polarization. To do this, we must calculate the flux of metal ions (cations) arriving at the cathode and depositing on it. We assume that the overall rate of the electrode reaction is determined by this flux. Once the cation distribution is known, the potential drop can be calculated. Note that anions are effectively motionless and do not produce a current. Let us assume that electrodes of the electrolytic cell are infinite planes at the anode (y = 0) and cathode (y = h) (Figure 6.3). The electrolyte velocity is zero. The definition of the current densities is... 

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