Deposition electroless

The basic components of an electrolytic cell for the electrodeposition of metals 

The overall reactions of electrodeposition and electroless deposition may be used to compare these two processes. The process of electrodeposition of metal M is represented by 

In this process, z electrons are supplied by an external power supply. The overall reaction of electroless metal deposition is 

In this section, we discuss the electrochemical model of electroless deposition, 

Fundamentals of Electrochemical Deposition, Second Edition. By Milan Paunovic and Mordechay Schlesinger Copyright 2006 John Wiley Sons, Inc. 

In electroless deposition the two electrochemical reactions, reduction of oxidation of Redsoiution occur at the same electrode, at the same electrode-electrolyte interface [Eq. (8.2) and Fig. 8.1]. Thus, in electroless deposition there is a statistical division of the catalytic sites on the substrate into anodic and cathodic sites. Since these catalytic sites are part of the same piece of metal (substrate), there is a flow of electrons between these sites. 

In this chapter we discuss the electrochemical model of electroless deposition (Sections 8.2 and 8.3), kinetics and mechanism of partial reactions (Sections 8.4 and 8.5), activation of noncatalytic surfaces (Section 8.6), kinetics of electroless deposition (Section 8.7), the mechanism of electroless crystallization (Section 8.8), and unique properties of some deposits (Section 8.9). 

An electrochemical model for the process of electroless metal deposition was suggested by Paunovic (10) and Saito (18) on the basis of the Wagner-Traud (1) mixed-potential theory of corrosion processes. According to the mixed-potential theory of electroless deposition, the overall reaction given by Eq. (8.2) can be decomposed into one reduction reaction, the cathodic partial reaction 

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Electrogalvanizing Electrography Electrogravimetry Electrohydrodynamics Electro-Katadyn process Electrokinetics Electroless deposition Electroless nickel Electroless plating... 

In electroless deposition, the substrate, prepared in the same manner as in electroplating (qv), is immersed in a solution containing the desired film components (see Electroless plating). The solutions generally used contain soluble nickel salts, hypophosphite, and organic compounds, and plating occurs by a spontaneous reduction of the metal ions by the hypophosphite at the substrate surface, which is presumed to catalyze the oxidation—reduction reaction. 

Electroless Electrolytic Plating. In electroless or autocatalytic plating, no external voltage/current source is required (21). The voltage/current is suppHed by the chemical reduction of an agent at the deposit surface. The reduction reaction must be catalyzed, and often boron or phosphoms is used as the catalyst. Materials that are commonly deposited by electroless plating (qv) are Ni, Cu, Au, Pd, Pt, Ag, Co, and Ni—Fe (permalloy). In order to initiate the electroless deposition process, a catalyst must be present on the surface. A common catalyst for electroless nickel is tin. Often an accelerator is needed to remove the protective coat on the catalysis and start the reaction. 

In Ni—P electroless deposits, there can be as much as 10% by weight of phosphoms. The amount depends on the added complexing agents and the pH. The Ni—P deposits are fine-grained supersaturated soHd solutions, which may be precipitation hardened by heat treatment to form dispersed Ni P particles in a nickel matrix. 

The growth of electroless plating is directiy traceable to (/) the discovery that some alloys produced by electroless deposition, notably nickel phosphoms, have unique properties (2) the growth of the electronics industry, especially the development of printed circuits (see Electronic coatings Integrated circuits) and (i) the large-scale introduction of plastics into everyday life. 

The important beneficial effects that substrate roughness can bring were firmly established in the late sixties and early seventies, principally as a result of work in two areas. The first was associated with the electroless deposition of metals onto plastics such as ABS and polypropylene. In the process the plastics must be etched in a way which produces pits on a micrometre scale. Such a topography had been shown to be a necessary, but not sufficient condition for adequate adhesion [40]. 

The most extensively used reducing agent for the electroless deposition of nickel is hypophosphite", and the reaction is as follows ... 

Resistance to corrosion Most authors who compare resistance to corrosion of electroless nickel with that of electrodeposited nickel conclude that the electroless deposit is the superior material when assessed by salt spray testing, seaside exposure or subjection to nitric acid. Also, resistance to corrosion of electroless nickel is said to increase with increasing phosphorus level. However, unpublished results from International Nickel s Birmingham research laboratory showed that electroless nickel-phosphorus and electrolytic nickel deposits were not significantly different on roof exposure or when compared by polarisation data. 

Hardness The hardness of electroless deposits is higher after heating to intermediate temperatures, the final value depending upon temperature and time of heating. Values of maximum hardness of nickel-phosphorus after heating to various temperatures are plotted in Fig. 13.11 the variation of... 

Autothermal reaction of ethanol through Pd-Ag membrane reactor prepared by sequential electroless deposition... 

Porter LA, Choi HC, Ribbe AE, Buriak JM (2002) Controlled electroless deposition of noble metal nanoparticle films on Germanium surfaces. Nano Lett 2 1067-1071... 

Bhattacharya RN, Rajeshwar K, Noufi RN (1984) Electroless deposition of CdTe thin films. J Electrochem Soc 131 939-942... 

Sharma NC Kainthla RC, Pandya DK, Chopra KL (1979) Electroless deposition of semiconductor films. Thin Solid Films 60 55-59... 

Photovoltaic response parameters for electrodeposited (polycrystalline) CdTe thin film electrodes in sulfide-polysulfide or alkaline sodium telluride PEC have been reported, primarily with no reference to the stability of the cells [100], In view of the instability of CdTe in aqueous solutions, Bhattacharya and Rajeshwar [101] employed two methods for the characterization of their electrodeposited CdTe-based PEC. In the first one, a coating of Pb02 (-100 nm thick) was deposited on the CdTe film surface by electroless deposition, and the coated films... 

Saliba, R., Mingotaud, C., Argoul, F. and Ravaine, S. (2001) Electroless deposition of gold films under organized monolayers. 

According to Ref. [12], template for synthesis of nanomaterials is defined as a central structure within which a network forms in such a way that removal of this template creates a filled cavity with morphological or stereochemical features related to those of the template. The template synthesis was applied for preparation of various nanostructures inside different three-dimensional nanoporous structures. Chemically, these materials are presented by polymers, metals, oxides, carbides and other substances. Synthetic methods include electrochemical deposition, electroless deposition, chemical polymerization, sol-gel deposition and chemical vapor deposition. These works were reviewed in Refs. [12,20]. An essential feature of this... 

Electroless Deposition Processes for Thin-Film Media. 253... 

Structure-Magnetics Interrelationships for Electrolessly Deposited Media. 258... 

The electrochemical mechanism was rejected by Salvago and Cavallotti [26] on the basis that it does not explain several features of electroless deposition of ferrous metals it does not account for the isotopic composition of the H2 gas evolved it does not explain the effect of the various solution components on reaction rate and it does not account for the homogeneous decomposition of very active solutions or the fact that they can give deposition on insulating surfaces. These authors put forward a chemical mechanism, involving various hydrolyzed nickel species, which they claim explains the observed behavior of the system ... 

Mital et al. [40] studied the electroless deposition of Ni from DMAB and hypophosphite electrolytes, employing a variety of electrochemical techniques. They concluded that an electrochemical mechanism predominated in the case of the DMAB reductant, whereas reduction by hypophosphite was chemically controlled. The conclusion was based on mixed-potential theory the electrochemical oxidation rate of hypophosphite was found, in the absence of Ni2 + ions, to be significantly less than its oxidation rate at an equivalent potential during the electroless process. These authors do not take into account the possible implication of Ni2+ (or Co2+) ions to the mechanism of electrochemical reactions of hypophosphite. 

The substrate was also found to influence the properties of the electrolessly deposited vertical media CoNiMnP, CoNiReMnP, and CoNiReP. The c-axis orientation had a larger degree of perpendicular orientation for films deposited on electroless NiP than for those deposited on Cu foil, presumably because of the smaller roughness of the former substrate [43]. The double-layer (magnetically soft interface, magnetically hard bulk) properties of CoNiReP deposited on a NiMoP underlayer [57] have already been discussed. 

Electrodeposition is more flexible than electroless deposition, in that it is not limited by the requirement of having a catalytically active surface. Electrodeposition allows a wider variation in the alloy composition and in the deposit properties than does electroless deposition. This flexibility has not been widely exploited, however, and most of the electrodeposited alloys have had compositions similar to those obtained by electroless deposition (i.e. CoP or CoNiP). 

The structures of electroplated hard alloys have been less extensively studied than those of similar electrolessly deposited materials. Sallo and co-workers [118-120] have investigated the relationship between the structure and the magnetic properties of CoP and CoNiP electrodeposits. The structures and domain patterns were different for deposits with different ranges of coercivity. The lower-f/c materials formed lamellar structures with the easy axis of magnetization in the plane of the film. The high-Hc deposits, on the other hand, had a rod-like structure, and shape anisotropy may have contributed to the high coercivity. The platelets and rods are presumed to be isolated by a thin layer of a nonmagnetic material. 

Films of CoB have been prepared by electroless deposition. Chang et al. [25] deposited magnetically soft amorphous films, which could be annealed to give materials with an Hc of 250 Oe. Depending on the annealing temperature, the films crystallized as the hep or fee modifications of Co. Matsui and co-workers [22] obtained crystalline materials in the as-deposited state, the crystalline characteristics being determined by processing conditions. A maximum HQ of 300 Oe was observed for films with 10.0 preferred orientation. 

Alloys of CoSnP [17] and CoZnP [16] have been prepared by electroless deposition. Both Zn and Sn increase the Hc, and Sn also improves the corrosion resistance and the appearance. For Sn alloys, X-ray diffraction measurements suggest that the Sn addition orients the c axis parallel to the plane of the film and decreases the grain size. 

Electrolessly deposited vertical media are generally CoNiP alloys containing Mn or Re or both. The Ni is added to lower the Ms and hence to lower the demagnetizing... 

M.H. Pournaghi-Azar and H. Razmi-Nerbin, Voltammetric behaviour and electrocatalytic activity of the aluminum electrode modified with nickel and nickel hexacyanoferrate films, prepared by electroless deposition. J. Electroanal. Chem. 456, 83-90 (1998). 

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