Aluminum displacement deposition

Activation by Thermal Decomposition of Metallic Oxides. The surface of alumina, AI2O3, may be activated by employing laser or ultraviolet irradiation to decompose AI2O3 (68). Decomposition of AI2O3 results in the generation of aluminum particles that are catalytic for electroless deposition of Cu (the first reaction probably is displacement deposition). 

Even not recognized as such, the galvanic displacement deposition of noble metals such as Au or Ag onto Fe, Zn, Cu, or similar substrates is known since the times of early Mediterranean cultures and, possibly, before. In the sixteenth century, the recovery of copper from copper mine waters by contacting dilute process streams with iron scrap was successfully achieved [2]. Since that time, many different galvanic displacement deposition processes have been developed. Examples used on industrial scale include application of aluminum, iron, or zinc powders for the removal of copper, silver, gold, or other noble metals from waste solutions. Similar approaches are used for the solution purification in hydrometallurgical plants, electronics, electrochemical experiments, etc. 

Considering that during the galvanic displacement deposition of copper the aluminum substrate is dissolved, alternatively, the rate of this process can be studied in the following way. Due to dissolution of aluminum, the concentration of Al(III) ions in the solution should increase with time. Eventually, complete aluminum substrate could be dissolved under proper conditions, during the deposition of copper. By applying Faraday s law to Eq. (9.8), the concentration of Al(III)... 

The metallic impurities present in an impure metal can be broadly divided into two groups those nobler (less electronegative) and those less noble or baser (more electronegative) as compared to the metal to be purified. Purification with respect to these two classes of impurities occurs due to the chemical and the electrochemical reactions that take place at the anode and at the cathode. At the anode, the impurities which are baser than the metal to be purified would go into solution by chemical displacement and by electrochemical reactions whereas the nobler impurities would remain behind as sludges. At the cathode, the baser impurities would not get electrolytically deposited because of the unfavorable electrode potential and the concentration of these impurities would build up in the electrolyte. If, however, the baser impurities enter the cell via the electrolyte or from the construction materials of the cell, there would be no accumulation or build up because these would readily co-deposit at the cathode and contaminate the metal. It is for this reason that it is extremely important to select the electrolyte and the construction materials of the cell carefully. In actual practice, some of the baser impurities do get transferred to the cathode due to chemical reactions. As an example, let the case of the electrorefining of vanadium in a molten electrolyte composed of sodium chloride-potassium chloride-vanadium dichloride be considered. Aluminum and iron are typically considered as baser and nobler impurities in the metal. When the impure metal is brought into contact with the molten electrolyte, the following reaction occurs... 

The extensive tissue damage associated with hemochromatosis is usually ascribed to the formation of free radicals that damage subcel-lular membranes, causing the organelles to become leaky (105, 148). However, comparison with aluminum suggests other mechanisms may also be operative. Thus iron, like aluminum (Section III), may cause damage because it displaces magnesium and calcium from key biochemical interaction sites. Also, insoluble iron deposits may stimulate the formation of free radicals, as well as produce them directly, and may activate other defense mechanisms in the body that attempt to remove or sequester particulate matter, as may happen in certain cases of aluminum overload (Section III). 

It is obvious from Eq. (9.13) that the dependence of log [Cu(II)]t/[Cu(II)]o on time should be linear. Indeed, the experimental results on the deposition of copper from an alkaline solution onto aluminum powder via the galvanic displacement reaction show that there is a linear dependence between log [Cu(II)]t/[Cu(II)]o and time as illustrated in Fig. 9.1 [3]. As can be seen from Fig. 9.1, the linear relationship was confirmed for all the investigated surface areas of the aluminum substrate. Furthermore, these results clearly show that an increase in the surface area of the substrate leads to the increase in the rate of copper deposition, which is a consequence of Eq. (9.13). 

The rate of the galvanic displacement reactions depends also on pH and temperature. In general terms, when dealing with the amphoteric metals, e.g., aluminum, zinc, or similar, that act as substrates on which the deposition takes place in the alkaline solutions (above pH 8 or so), an increase in pH leads to an increase of deposition of the more noble metal. This is due to increase in the rate of oxidation or dissolution of the substrate. 

Based on the results from Karavasteva [7], it seems that the kinetics and consequently the surface morphology of the deposited copper via the galvanic displacement reaction onto zinc, iron, and aluminum are strongly influenced by... 

Fig. 9.11 SEM images of copper deposited onto aluminum substrates upon immersion in the alkaline Cu(II) solutions via the galvanic displacement reaction (Reproduced from Ref. [3] with permission from The Electrochemical Society)...

A related phenomenon is the common occurrence of secondary deposition in molten salts. Thus, a metal may be deposited as a result of a chemical reaction between a metallic ion in the melt and another preferentially-deposited, reactive metal corresponding to the solvent cation(s). (Notably, there is still discussion as to whether sodium or aluminum is the primarily deposited metal during operation of the Hall-Heroult cell. " ) Such occurrences can be rationalized in theoretical terms if the primary deposition of the solute metal is precluded by an observed limiting current exceeding that predicted, or if its deposition potential is displaced in a cathodic direction by activation overpotential. Some authors have preferred an explanation which involves underpotential deposition (vide infra). 

Synthetic routes derived from molecular and non-molecular precursors have expedited the development of technologically important 2- and 3- dimensional materials. Such approaches have often proved superior to conventional ceramic techniques in that high purity bulk samples or thin films can be prepared at lower temperatures much more rapidly. Predominant among the precursor methods are those based on decomposition reactions. These either involve gaseous species, such as those used in chemical vapor deposition (CVD), or solids. Examples include the pyrolysis of the gas-phase precursor [(CH3)2A1(NH2)]3 to produce aluminum nitride (i) and the thermal decomposition of solid state carbonate precursors of calcium and manganese (Cai j,Mn C03, 0 < x < 1) to produce several of the known ternary compounds in the Ca-Mn-O system (2). Single-displacement reactions are also common as precursor methods. These approaches usually involve gas-phase reactions and are also used in CVD techniques. Examples here include the formation oi... 

As pointed out on page 1007, a very important halide complex of aluminum is cryolite, NasAlFg. Natural deposits of cryolite were discovered in Greenland in 1794 and occur almost nowhere else. For aluminum production, natural cryolite has been largely displaced by cryolite synthesized in a lead-clad vessel by using the following reaction ... 

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