Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-07-06
2004-03-16
Barr, Michael (Department: 1762)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S670000, C427S437000, C427S443100, C106S001240, C106S001280
Reexamination Certificate
active
06706420
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electroless plating of a platinum-rhodium alloy onto a substrate. More particularly, this invention pertains to an aqueous platinum and rhodium plating bath, a process for plating a uniform coating of a platinum-rhodium alloy onto various substrates using an electroless plating composition, and a platinum-rhodium plated article formed therefrom.
2. Description of the Related Art
Plating of metals is a well known process employed to alter the existing surface properties or dimensions of a substrate. For example, a substrate may be plated for decorative purposes, to improve resistance to corrosion or abrasion, or to impart desirable electrical or magnetic properties to a substrate. Plating is a common practice in many industries, including the manufacture of a variety of electronic packaging substrates, such as printed circuit boards.
There are various methods of plating known in the art, including electroplating and electroless plating. Electroplating involves the formation of an electrolytic cell wherein a plating metal represents an anode and a substrate represents a cathode, and an external electrical charge is supplied to the cell to facilitate the coating the substrate.
Electroless plating involves the deposition of a metallic coating from an aqueous bath onto a substrate by a controlled chemical reduction reaction which is catalyzed by the metal or alloy being deposited or reduced. This process differs from electroplating in that it requires no external electrical charge. One attractive benefit of electroless plating over electroplating is the ability to plate a substantially uniform metallic coating onto a substrate having an irregular shape. Frequently, electroplating an irregularly shaped substrate produces a coating having a non-uniform deposit thickness because of varying distances between the cathode and anode of the electrolytic cell. Electroless plating obviates this problem by excluding the electrolytic cell. Another benefit of electroless plating over electroplating is that electroless plating is autocatalytic and continuous once the process is initiated, requiring only occasional replenishment of the aqueous bath. Electroplating requires an electrically conductive cathode and continues only while an electric current is supplied to the cell. Also, electroless coatings are virtually nonporous, which allows for greater corrosion resistance than electroplated substrates.
In general, an electroless plating bath includes water, a water soluble compound containing the metal to be deposited onto a substrate, a complexing agent that prevents chemical reduction of the metal ions in solution while permitting selective chemical reduction on a surface of the substrate, and a chemical reducing agent for the metal ions. Additionally, the plating bath may include a buffer for controlling pH and various optional additives, such as bath stabilizers and surfactants. The composition of a plating bath typically varies based on the particular goals of the plating process. For example, U.S. Pat. No. 6,042,889, teaches an electroless plating bath and having a hypophosphite reducing agent and employing one of several different “mediator ions”, including rhodium and platinum ions, for the purpose of converting a non-autocatalytic metal-reduction reaction into an autocatalytic reaction to plate a substrate with copper.
Rhodium and platinum metals have been widely employed in the art of plating for their excellent physical and chemical properties. Platinum-rhodium alloys are employed as a thin coatings on substrates, in electronics, optics, fuel cells, electrical contacts, automotive catalysts, gas sensors, corrosion protection, insoluble electrodes, gas turbine engines, supported catalysts, jewelry, medical implants and many other applications. Metallic platinum-rhodium alloys provide the unique combination of excellent physical and chemical properties such as hardness, wear resistance, corrosion resistance, electrical and thermal conductivity, optical reflectance, catalytic activity and others. However, in, electroless deposition of platinum-rhodium alloy coatings is difficult because of high stability of platinum and rhodium complex ions to chemical reduction, and substantially higher stability of rhodium ions comparing to platinum ions.
U.S. Pat. No. 4,285,784 teaches a process for plating a platinum-rhodium alloy on a substrate by electroplating the metals from a bath comprising platinum and rhodium cyanide complexes in a molten cyanide bath containing a cyanide salt, such as sodium cyanide or a mixture of potassium cyanide and sodium cyanide. This plating bath is disadvantageous because of the extreme toxicity of cyanides, high cost of equipment, poor uniformity of the coating thickness, and because only thermally stable conductive substrates are sufficient for the process. U.S. Pat. No. 3,671,408 provides a rhodium-platinum plating bath and process comprising forming an aqueous mixture of rhodium sulfate, platinum P salt (Pt(NH
3
)
2
(NO
2
)
2
) and sulfamic acid. In plating a rhodium-platinum alloy, a high rhodium containing bath is used for a predominantly rhodium containing alloy, or a high platinum containing bath for an alloy having more platinum than rhodium. Also, concentrated sulfuric acid is added to the mixture to improve the density of the plated alloy. This process is also disadvantageous because of the high cost of electroplating equipment and poor uniformity of coating thicknesses.
Japanese patent JP58204168, teaches a process for electroless plating of platinum-rhodium alloy onto a substrate using an aqueous plating bath comprising a platinum and rhodium as ammine chlorides, hydroxylamine salt as a stabilizer and hydrazine as a reducing agent. This process is also disadvantageous because the accumulation of non-volatile chloride byproducts and nonconsumable elements from the plating bath lead to an impure rhodium plating and inhibit replenishment of the plating bath, leading to termination of the plating process.
It has also been found that purity of a deposited platinum-rhodium alloy is important for a number of applications like corrosion protection and catalysis. For example, U.S. Pat. No. 5,788,823 describes that the presence of 0.01 to 0.001% sulfur and phosphorus and chlorine in a noble metal corrosion protective coating reduces its protective action and the lifetime of gas turbine components by 25%. Further, chlorine impurity substantially reduces efficiency and lifetime of Pt—Rh catalysts. (See N. Cant, Catal. Today (1998), 44(14), 93-99; D. Martin, Stud. Surf. Sci Catal. (1995), 96, 801-11; H. Abderrahm, Proc.-Int. Congr. Catal., 9
th
(1988), 3, 1246-52).
The present invention solves the problems of the prior art by employing a process for electroless plating of a platinum-rhodium alloy using a composition comprising an aqueous solution comprising a platinum nitrite and/or ammine-nitrite salt, a rhodium nitrite and/or ammine-nitrite salt, ammonium hydroxide as a complexing agent and hydrazine hydrate as a reducing agent. The composition of this aqueous solution is essentially free of sulfur, phosphorus, chlorine and non-volatile components that cause impure plating, allowing for improved appearance and properties of the plated alloy. Further, the process generates essentially no hazardous substances and the absence of non-volatile components avoids the accumulation of byproducts that degrade the plating bath, allowing for virtually unlimited replenishment of the bath. The aqueous solution is also capable of plating non-conductive and thermally unstable substrates, and the plating equipment is simple and inexpensive. Moreover, the unique composition of the plating bath allows metallic rhodium and platinum to be precipitated from the plating bath by boiling without undesirable contaminants.
This invention provides a simple low-cost method of a deposition of high purity platinum-rhodium alloy coatings on virtually any material of any geometrical shape, including fibers and
Kozlov Alexander S.
Narasimhan Dave
Palanisamy Thirumalai
Barr Michael
Fredrick Kris T.
Honeywell International , Inc.
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