Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-07-06
2002-09-24
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C427S098300, C427S126500, C427S443100, C106S001240, C106S001210
Reexamination Certificate
active
06455175
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electroless plating of rhodium onto a substrate. More particularly, this invention pertains to an aqueous rhodium plating bath, a process for plating a uniform coating of rhodium onto various substrates using an electroless plating composition, and a 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.
Rhodium is employed as a thin coating on substrates, in electronics, optics, fuel cells, electrical contacts, automotive catalysts, gas sensors, corrosion protection, insoluble electrodes, gas turbine engines, X-ray mirrors, supported catalysts, jewelry, medical implants and many other applications. Metallic rhodium (Rh) provides 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.
Rhodium coatings are typically deposited by electroplating, vacuum deposition (sputtering), or chemical vapor deposition (CVD) techniques. Vacuum deposition of rhodium coating is expensive, requires complicated equipment and cannot be done uniformly on geometrically complicated substrates. Deposition of rhodium coating by the CVD process is expensive, requires complicated equipment and cannot be done on temperature sensitive substrates like electronic components or polymer membranes for fuel cells because of the CVD method requires thermal decomposition of vapors of rhodium chemical compounds on substrate surface at 200+° C.). CVD rhodium coatings are contaminated with 2-35% carbon. (See A. Etspuler, et al., “Deposition Of Thin Rhodium Films By Plasma-Enhanced Chemical Vapor Deposition”, Appl. Phys., A, 1989, v. 48, pp. 373-375). Also, deposition of CVD coatings on powders is not easy.
Rhodium electroplating is carried out from highly acidic sulfuric or phosphoric electrolytes (See R. Morrissey, “Rhodium Plating”, Plating and Surface Finishing 1999, v. 86, No. 11, p. 62; and Modem Electroplating, Edited by F. Lowenheim, John Wiley & Sons, NY, 1974, pp. 350-352). Electrically non-conductive substrates like polymer membranes for fuel cells and ceramic monolithic supports for automotive catalysts cannot be plated. Non-precious metals can be properly plated only after pre-plating with nickel or precious metals because of high acidity of rhodium electrolyte. Geometrically complicated substrates cannot be uniformly plated because of poor distribution of current density in the rhodium electrolyte. Plating on powders is practically not available. Also, sulfur and phosphorus from the electrolyte contaminate an electroplated rhodium coating. Rhodium purity is critical for a number of applications like corrosion protection and catalysis. It has been shown that the presence of 0.01-0.001% 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% (See U.S. Pat. No. 5,788,823).
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 ions, for the purpose of converting a non-autocatalytic metal-reduction reaction into an autocatalytic reaction to plate a substrate with copper.
Rhodium exhibits good hardness, catalytic, friction and wear properties, but present application techniques are very expensive. Known efforts to plate rhodium onto substrates have been difficult because of problems in producing a pure metallic rhodium coating on a substrate and undesirable degradation of the aqueous bath.
U.S. Pat. No. 4,361,602, teaches a process for electroless plating of rhodium onto a substrate using a aqueous composition, or plating bath, comprising a rhodium salt, a nickel salt as an activating agent and an alkylamine borane wherein the molar ratio of nickel salt to rhodium salt ranges from 0.01 to 2.0.
The disclosed process uses rhodium as a chloride, bromide, perchlorate or a sulfate. This process is disadvantageous in that the presence of nickel, boron, carbon or chloride in the plating bath are non-volatile compounds that lead to impure rhodium plating. Also, the accumulation of non-volatile byproducts and non-consumable elements from the plating bath inhibit replenishment of the plating bath, leading to undesirable termination of the plating process.
Japanese patent JP58204168, teaches a process for electroless plating of rhodium onto a substrate using an aqueous plating bath comprising a rhodium as a chloride, hydroxyl amine salt as a stabilizer and hydrazine as a reducing agent. This process is also disadvantageous because the accumulation of non-volatile chloride byproducts and non-consumable 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. Additionally, Swiss patent CH 606475 and German patent DE 2607988 teach a process for electroless plating of rhodium with similar problems resulting from non-volatile chloride and sodium byproducts.
The present invention solves the problems of the prior art by employing a process for electroless plating of rhodium using a composition comprising an aqueous solution comprising a rhodium ammine nitrite salt, ammonium hydroxide as a complexing agent and hydrazine hydrate as a reduc
Kozlov Alexander S.
Narasimhan Dave
Palanisamy Thirumalai
Fredrick Kris T.
Honeywell International , Inc.
Jones Deborah
Savage Jason
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