Bath instability

Process Control. Some hot nickel and flash electroless copper solutions are plated to the point of exhaustion and then discarded. Most baths are formulated to give bath fives of >6 turnovers of the bath constituents some reach steady-state buildup of the by-products and can be used indefinitely. AU. regenerable solutions should be filtered to remove particulates that can cause deposit roughness and bath instability. 

The deposition at the surface usually occurs as a continuous film with a uniform surface morphology. On the other hand with an increase in the concentration of the reducing agent or temperature, the deposition of powders occurs, a phenomenon which is usually described as bath instability. 1... 

From the present discussion, it is obvious that at lower concentrations of both metal ion and reducing agent, or at lower temperamres, the deposition will proceed smoothly. When the concentration of reducing agent is increased and at significantly higher temperatures, the rate of deposition or, more precisely, the rate of reduction of metal ions rapidly increases and provokes the bath instability. In order to produce a continuous metal film/coating, the deposition in the bulk solution must be avoided. 

The bath instability is a term that is frequently used in the published literature to describe the conditions where the deposition of the desired coating is significantly diminished. Under these conditions, significant roughening of the metal deposit, and more frequently, deposition of powders, not only at the surface but, as well, in the bulk solution, is observed. Schematically, this is presented in Fig. 9.22 as a dependence of the rate of deposition on temperature. It is to be noted that dependences of the rate of deposition as function of Cr exhibit a similar trend. 

Although, the general rate of the reduction of metal ions should increase with an increase in Cr , and T, even at the point of the so-caUed bath instability is reached, the rate of deposition of metal at the desired surfaces significantly... 

For the review of all these mechanisms, the reader is referred to the Ref. [1]. In summary of the proposed listed mechanisms, it is, based on the experimental observations and the present knowledge, quite unlikely that the atomic hydrogen and hydride ion mechanisms are applicable in the description of the autocatalytic deposition. The universal mechanism is not applicable, since every single autocatalytic deposition (e.g., Ni, Co, Pd, Pt, Ag, An, Cu, Bi, etc.) must have a specific mechanism, and a generalization is quite difficult to achieve. It seems that the metal hydroxide and up to some extent the electrochemical mechanisms are the only mechanisms that can explain most of the characteristics of the autocatalytic deposition of metals and alloys. The discussions of the mechanistic aspects of autocatalytic deposition in details is out of the scope of the present book however, the metal hydroxide mechanism [1, 16-18] seems as the most acceptable way to explain the properties including the surface morphology of the deposits produced via the autocatalytic deposition. The metal hydroxide mechanism is based on the fact that under the conditions of autocatalytic deposition there is an unavoidable pH rise at the surface where the reaction in question takes place. Due to hydrolysis, hydrolyzed species can form and further be absorbed and/or reduced at the surface. This mechanism explains quite well the bath instability and formation of powders within the bulk electrolyte. When other parameters are constant, in general terms, an increase in pH of the solution leads to an increase in the rate of deposition, as schematically presented in Fig. 9.24. 

The production of metallic powders through hydrometallurgical or electrodeposition routes has extensively been studied over the last century. The formation of metallic powders during electroless deposition of films of metals or alloys is observed when the so-called bath instability takes place [1, 16, 17]. As such, the electroless deposition was far less investigated than its electrolytic counterpart. The bath instability is usually seen in the experimental conditions at elevated temperatures or when the concentration of the reducing agent is too high. 

Similarly, other metallic powders can be produced by the electroless deposition when bath instability takes place [1,16,17]. A few examples showing Ni powder produced with hydrazine, Co powder produced with hydrazine or hypophosphite, and Ag powder produced with formaldehyde are presented in Fig. 7.5. Hypophosphite, hydrazine, and formaldehyde were used as reducing agents of the respective ions. 

The rate of plating may be lowered drastically by the addition of complexants to the formulation. Additives which increase the rate to an acceptable level without causing bath instability are termed exaltants . These are generally anions which are thought to function by catalysing oxidation of the anodic process. Common examples include sodium succinate, glycine and fluoride ions. 

The small dispersed particles may become active nuclei for autocatalytic deposition of metal, leading to bath instability. 

Most electrodeposition baths operate at very low solids with 10-15% being typical. This can cause bath instability problems if the neutralising agent is too volatile and evaporates whilst in the bath. Being relatively high capital cost equipment means that quality performance is required from the coating. 

Successful molecular dynamics simulations should have a fairly stable trajectory. Instability and lack of ec uilibratioii can result from a large time step, treatment of long-range cutoffs, or unrealistic coiiplin g to a temperature bath. ... 

Assuming that the pj (t) and Qj (t) can be interpreted as a TS trajectory, which is discussed later, we can conclude as before that ci = ci = 0 if the exponential instability of the reactive mode is to be suppressed. Coordinate and momentum of the TS trajectory in the reactive mode, if they exist, are therefore unique. For the bath modes, however, difficulties arise. The exponentials in Eq. (35b) remain bounded for all times, so that their coefficients q and q cannot be determined from the condition that we impose on the TS trajectory. Consequently, the TS trajectory cannot be unique. The physical cause of the nonuniqueness is the presence of undamped oscillations, which cannot be avoided in a Hamiltonian setting. In a dissipative system, by contrast, all oscillations are typically damped, and the TS trajectory will be unique. 

The checkers found that considerable decomposition occurred when this procedure was employed, perhaps because it happened to be carried out on a warm humid day they found it preferable to remove the chloroform by distillation under reduced pressure from a water bath at this step and the succeeding ones. Because of the instability of the dibromide, it is advisable to convert it to coumarilic acid on the day it is made. 

Because of modest thermal Instability of the material, one should distill at a bath temperature below 150°C. When the bath temperature exceeds 150°C, considerable decomposition of the allylic tributyltin occurs and a poorer yield is realized. The checkers measured a bp of 100°C at 0.1 mm. 

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