Displacement deposition mechanisms

The kinetics and mechanisms of the displacement deposition of Cu on a Zn substrate in alkaline media was studied by Massee and Piron (5). They determined that at the beginning of the deposition process, the rate is controlled by activation. The activation control mechanism changes to diffusion control when the copper covers enough of the Zn surface to facilitate further deposition of copper. This double mechanism can explain the kinetic behavior of the deposition process. 

The mechanisms of the crystal-building process of Cu on Fe and A1 substrates were studied employing transmission and scanning electron microscopy (1). These studies showed that a nucleation-coalescence growth mechanism (Section 7.10) holds for the Cu/Fe system and that a displacement deposition of Cu on Fe results in a continuous deposit. A different nucleation-growth model was observed for the Cu/Al system. Displacement deposition of Cu on A1 substrate starts with formation of isolated nuclei and clusters of Cu. This mechanism results in the development of dendritic structures. 

Metal deposition creates a hybrid material of metal and semiconductor and the new material is expected to develop a new function, where microstructuring is cracial. A variety of techniques have been utilized for producing the 2D and 3D stractures. They are controlled physically, mechanically, optically, and electrochemically. Some examples of the 2D or position-selective local deposition are summarized in Table 1. The optical control is only possible on p-type silicon and deposition of less-noble metal. Illumination creates charge carriers and the illuminated spot undergoes deposition. Otherwise, excess free electrons and displacement deposition hinder the selective deposition. 

Differential eleetroehemieal mass speetrometry was used by Vidal-Iglesias et al. to study the HCOOH oxidation on Pd submonolayers deposited on Pt(lOO) and Pt(lll) [170], It was found that the adsorption of S04 inhibits HCOOH oxidation. Competitive adsorption between hydrogen and sulfate was responsible for the voltammetric peak at 0.26 V vs. RHE on Pd, while at 0.3 V vs. RHE, Had was eompletely displaced by the adsorbed sulfate. At E > 0.3 V the S04 adsorption was stronger, further diminishing the HCOOH oxidation current on the Pd modified electrodes [170]. The deposition mechanism and surface analysis of Pd on Pt(l 11) was published by Ball et al. [171]. 

Nickel does not deposit directly on copper without prior catalysis. Palladium or ruthenium metal forms an immersion deposit on copper as the catalysis or activation step of ENIG. Catalysis baths employ the immersion (galvanic displacement) chemical mechanism. 

We have so far assumed that the atoms deposited from the vapor phase or from dilute solution strike randomly and balHstically on the crystal surface. However, the material to be crystallized would normally be transported through another medium. Even if this is achieved by hydrodynamic convection, it must nevertheless overcome the last displacement for incorporation by a random diffusion process. Therefore, diffusion of material (as well as of heat) is the most important transport mechanism during crystal growth. An exception, to some extent, is molecular beam epitaxy (MBE) (see [3,12-14] and [15-19]) where the atoms may arrive non-thermalized at supersonic speeds on the crystal surface. But again, after their deposition, surface diffusion then comes into play. 

Ni3C decomposition is included in this class on the basis of Doremieux s conclusion [669] that the slow step is the combination of carbon atoms on reactant surfaces. The reaction (543—613 K) obeyed first-order [eqn. (15)] kinetics. The rate was not significantly different in nitrogen and, unlike the hydrides and nitrides, the mobile lattice constituent was not volatilized but deposited as amorphous carbon. The mechanism suggested is that carbon diffuses from within the structure to a surface where combination occurs. When carbon concentration within the crystal has been decreased sufficiently, nuclei of nickel metal are formed and thereafter reaction proceeds through boundary displacement. 

Particulate diffusion does not play a significant role in the deposition of pharmaceutical aerosols. However, it is worth noting the mechanism by which diffusion of particles occurs in the lungs. The principle of Brownian motion is responsible for particle deposition under the influence of impaction with gas molecules in the airways. The amplitude of particle displacement is given by the following equation ... 

The displacement mechanism involves placing the iron alloy packed in chromium powder, NH4C1, and 1 i in a sealed retort, which is heated to promote vapor deposition and diffusion processes. The exact chemistry is not known, hut the following steps probably occur ... 

The damage and high concentrations of lattice delects, resulting from atomic displacements produced by the incident alums, can change the chemical reactivity and mechanical hardness of a treated surface, Implantation cun enhance the diffusion of impurities already deposited in a substrate, presumably through the motion of the high concentrations of lattice defects produced by the incident ions. 

There are four types of fundamental subjects involved in the process represented by Eq. (1.1) (1) metal-solution interface as the locus of the deposition process, (2) kinetics and mechanism of the deposition process, (3) nucleation and growth processes of the metal lattice (M a[tice), and (4) structure and properties of the deposits. The material in this book is arranged according to these four fundamental issues. We start by considering the basic components of an electrochemical cell for deposition in the first three chapters. Chapter 2 treats water and ionic solutions Chapter 3, metal and metal surfaces and Chapter 4, the metal-solution interface. In Chapter 5 we discuss the potential difference across an interface. Chapter 6 contains presentation of the kinetics and mechanisms of electrodeposition. Nucleation and growth of thin films and formation of the bulk phase are treated in Chapter 7. Electroless deposition and deposition by displacement are the subject of Chapters 8 and 9, respectively. Chapter 10 contains discussion on the effects of additives in the deposition and nucleation and growth processes. Simultaneous deposition of two or more metals, alloy deposition, is discussed in Chapter 11. The manner in which... 

Deposition by diffusion is the main mechanism for particles smaller than 0.5 pm, and is important in bronchioles, alveoli, and bronchial bifurcations. Aerosol particles are displaced by a random collision of gas molecules this results in particle collision with the airway walls [24]. Deposition by diffusion increases with the decrease in particle size, and breath-holding following inhalation was also found to increase this deposition [25]. 

Immediately upon connecting the cell to a source of direct current, a deposit of gray metallic zinc appears on the surface of the cathode and bubbles of chlorine gas appear at the surface of the anode. A simple chemical test for chlorine may be made by leading this gas into an aqueous sodium iodide solution, whereupon the solution assumes a yellow color caused by displacement of iodine by chlorine. Accordingly, it is concluded that the products of the electrolysis of a zinc chloride solution are elemental zinc and elemental chlorine, and the next problem is that of explaining the mechanism by which these products may be produced. 

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