Cathode deposit morphology

Cathode deposit morphology can also be affected by the presence of trace elements in solution. An SEM micrograph of a zinc deposit from a pure or unadulterated electrolyte and time duration of two minutes at 10 mA/cm2 is shown in Figure 3. When U0 parts per billion of antimony was added to this solution, and a similar electrodeposition cycle was performed, an approximate ten-fold increase in crystallite size was obtained (see Figure U). 

Activation-limited growth tends to favor compact columnar or equiaxed deposits, while mass transport-limited growth favors formation of loose dendritic deposits. The deposit morphology is modified by using additives. Additives act as grain refiners and levelers because of their effects on electrode kinetics and the structure of the electrical double layer at the cathode surface. Additives that reduce primarily the nucleation overpotential can be considered to be grain-refining additives because of increased secondary nucleation events. 

Electrorefining can be carried out in acidic or alkaline medium. The acid electrolytes consist of sulfuric acid and stannous sulfate, with additives such as creosulfonic or phenolsulfonic acids and glue to modify deposit formation on the cathodes. The alkaline electrolytes consist of potassium or sodium stannite and free alkali. When compact cathode deposits are required, alkaline electrolytes are inferior to acid sulfate or halogen solutions in terms of electric energy consumption, productivity, cathode morphology, and operating temperature [82, 83]. 

The shape and typical dimensions of this cell are shown in Figure 26.24 [94]. Due to the nonuniform primary current distribution on the inclined cathode (with a local current density that decreases with increasing anode-cathode separation distance), a single electroplating experiment is used to study the effect of current density on deposit morphology (and deposit composition, for the case of alloy plating). 

A wide range of organic molecules are added in relatively low concentration to the electroplating bath to modify the structure, morphology and properties of the cathode deposit. Their development has been almost totally empirical and details of their mode of operation are seldom known. Indeed it is not always clear whether their effect is due to the additive itself or to decomposition products formed in electrode reactions. Several generalizations concerning their operation are, however, possible. 

The deposition of ions at the cathode creates a depletion layer across which the ions must migrate in order to deposit. This layer can vary in thickness according to surface morphology. The depletion layer is more or less defined as the region where the ion concentration differs from that of the bulk solution by >1%. The layer thickness can be decreased by agitation. 

Optimisation of SWCNT production has been attempted within the framework of the arc-discharge method in which anode and cathode were made of graphite rods, a hole in the anode being filled with metal catalysts such as Y (1 at.%) and Ni (4.2 at.%) [7]. A dense collar deposit (ca. 20% of the total mass of graphite rod) formed around the eathode under He (ca. 500 Torr), with 30 V and 100 A de eurrent. It was eonfirmed that this optimal eollar eontained large amounts of SWCNT bundles eonsisting of (10, 10) SWCNTs (diameter 1.4 nm). Such morphology resembles that produced by the laser ablation teehnique [4,5]. 

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