Grain refiners deposits

Fibrous stmctures represent a grain refinement of columnar stmcture. Stress-reHeving additives, eg, saccharin or coumarin, promote such refinement, as do high deposition rates. These may be considered intermediate in properties between columnar and fine-grained stmctures. 

The production of such tailor-made nanomaterials by electrochemical procedures is advantageous because the two crudal steps in nanocrystal formation -nudeation and growth of nudei - can be controlled by physical (current, voltage, time, temperature) and chemical (grain refiners, complex formers) parameters during the deposition process [42, 43]. 

Fig. 8.3 The activity of different grain refiners (butanediamine, ammonium ethylenediamine tetra-acetic acid, benzosulfimide) on the nanostructure of gold deposits [29].

The grain refinement is achieved by an increase in the nucleation rate. The first four deposit types are seen with two-dimensional nucleation, but UD type requires three-dimensional or secondary nucleation. The nucleation intensity increases with increasing inhibition and deposition current density. The FI type of crystals is usually seen at low inhibition. With increasing current density, the deposit form changes from whiskers to prismatic crystals, dendrites, and finally powder deposits. The BR type is seen at moderate inhibition level and the deposit follows the structure of the base material. 

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. 

Grain refinement is realized in a deposit by having a high nucleation rate or by suppressing crystal growth or both. Several tricks are possible ... 

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