Corrosion-resistant zinc alloy powder and method of...

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Reexamination Certificate

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C075S255000, C420S513000

Reexamination Certificate

active

06436539

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to corrosion resistant zinc powders and a method of manufacturing corrosion resistant zinc powders. More particularly, the present invention relates to high performance zinc alloy powders for use in the anodes of primary and secondary cells and batteries belonging to the zinc-alkaline family, such as zinc-manganese dioxide, silver-zinc, nickel-zinc, zinc-air and zinc-oxygen systems.
BACKGROUND OF THE INVENTION
Primary and secondary cells experience a loss of capacity on storing because of a self-discharge, parasitic, nonpower-producing reaction of the active electrodes. In particular, the shelf life of a cell employing a zinc electrode is limited by, among other factors, the open circuit and in-use corrosion of the zinc electrode, which causes discharge of the metallic zinc and evolution of hydrogen gas. Thus, in cells employing a zinc electrode, a central issue with respect to cell longevity is the zinc electrode's resistance to corrosion. Cell longevity is particularly important for zinc electrode batteries that require a long shelf life, for example a year or more, until they are consumed by the electronic application they are intended for.
Zinc anode corrosion is primarily the result of a reaction between the zinc and the aqueous electrolyte, which is commonly an alkaline solution of a Group IA metal hydroxide. One product of this reaction is hydrogen gas, the measurement of which is commonly used to gauge the level of zinc corrosion. The hydrogen gas forms at the cathodic sites of the anode by the decomposition of water. This hydrogen gas is particularly undesirable in sealed batteries where it can lead to bubble formation and excessive pressure build up. Simultaneous to the hydrogen gas generation, active zinc at the anodic sites of the anode oxidizes to zinc hydroxide, zinc oxide and mixtures of zinc hydroxide and zinc oxide. The zinc consumed in this reaction consequently becomes unavailable to produce an electric current, and thereby reduces the electrical capacity of the anode. The self-discharge reactions are as follows:
cathodic sites: 2H
2
O+2
e
→H
2
|+2OH

anodic sites: Zn+2OH

−2
e
→Zn (OH)
2
ZnO+H
2
O
overall: Zn+2H
2
O→H
2
|+Zn(OH)
2
ZnO+H
2
O
To avoid the corrosion of the zinc, a variety of corrosion inhibition techniques have been used. One of the oldest and most effective corrosion inhibition techniques involves the amalgamation of the zinc with mercury. Today, however, environmental policy and laws restrict use of mercury and its compounds as commercial corrosion inhibitors. Other effective techniques of reducing the corrosion reaction include adding corrosion inhibitors to the electrolyte or the zinc and alloying the zinc with an effective amount of a corrosion inhibitor. The challenge with respect to all methods of inhibiting corrosion is to achieve superior corrosion inhibiting performance without significantly sacrificing the zinc anode's discharge performance within the cell.
Several prior patents relate to the technique of using electrolyte additives to reduce corrosion in zinc anodes. In U.S. Pat. No. 4,112,205, a chloride double salt containing both mercuric ion and quaternary ammonium ion is added to the electrolyte to inhibit corrosion. The corrosion inhibitor provides a relatively continuous source of mercuric ion to the zinc as required to minimize corrosion of unamalgamated zinc as zinc oxide goes into solution and new zinc surfaces are exposed. Again for environmental policy reasons, corrosion inhibitors based on mercury and its salts are not practicable.
U.S. Pat. No. 3,945,849 teaches the use of quaternary ammonium salts as inhibitors for zinc anodes in primary battery cells. The patent teaches that the disclosed quaternary ammonium salts are particularly suitable for use as corrosion inhibitors in Leclanche-type cells wherein the electrolyte employed is ammonium chloride. As such, these corrosion inhibitors are not very effective in alkaline based battery cells in which an alkaline electrolyte comprised of an aqueous solution of a Group IA metal hydroxide is used.
U.S. Pat. No. 4,195,120 teaches alkaline cells containing a predominantly zinc anode and, as-a corrosion inhibitor, a surfactant which is an organic phosphate ester of the ethylene oxide-adduct type. The surfactant is added in such a manner that, directly or upon wetting of the anode with the electrolyte, there is adsorption of the surfactant on the surface of the anode material. The patent, however, teaches that the surfactant should be used in combination with a conventional zinc amalgam powder. This is unacceptable for environmental reasons. Moreover, because the inhibitor works as a surfactant, it tends to interfere with the electrochemical reaction during discharge, which can degrade the electrical performance of the cell to an unacceptable level.
Because mercury is hazardous, research has been conducted on additional anode materials that are able to inhibit the generation of hydrogen in its absence. It has been found that zinc and lead, as well as indium, bismuth and/or gallium, combined in predetermined proportions can produce mercury-free zinc powders that provide effective corrosion resistance. A range of techniques have been investigated for treating zinc with these and other corrosion inhibitor metals to produce corrosion resistant zinc powders. Several commonly employed techniques include thermal atomization, cementation, and electrolytic co-deposition.
In the thermal atomization process, zinc alloy powder is produced using a method in which a predetermined amount of lead or other inhibitor metal is added to high-purity zinc, which is typically produced by an electrolytic method, the entire mixture is then melted to form an alloy. The molten alloy is then atomized through an air jet to form a powder comprised of generally spheroidal or dumbbell-shaped particles of a predetermined size. Typically the resulting zinc powders produced by the thermal atomization process have a bulk density of 2.5-3.5 g/cc, a surface area of 0.1-0.4 m
2
/g, and a particle size distribution between 0.0075 and 0.8 mm.
Although the thermal atomization technique allows zinc alloy powders to be produced with a wide variety of compositions, thermal atomization has at least two shortcomings over zinc alloy powders produced using the electrolytic co-deposition technique.
First, the morphology of zinc powder produced by the thermal atomization process is less than optimal. In order to achieve high continuous current drain, a large reservoir of active anode material is needed. Due to space and other considerations, this is generally best achieved by incorporating an active anode element having a highly porous morphology and a large surface area of active anodic material. By contrast, in order to achieve high peak power output, studies show that a tight interparticulate packing structure of the active anodic material is advantageous. This has traditionally come at the expense of porosity in known powdered anodes, which can drastically reduce the current capacity of the battery. Therefore, powder morphologies that have large surface areas and yet allow for a tight interparticulate anode structure are desired for battery and cell applications; such morphologies, however, cannot be obtained by the usual thermal processes.
When thermally prepared unamalgamated zinc is compressed sufficiently to form a self-supporting anode, usually with the help of organic binders, the spheroidal particles become well-packed, resulting in a relatively low zinc surface area overall. Lower surface areas limit the utilization potential of the zinc. In a battery application, this results in poor discharge performance. If thermally prepared zinc is used to form an anode with little or no compaction, such as in the case of a gelled anode, the contact area between the solid spherical particles is quite limited, resulting in batteries with low peak power output.
A second shortcoming o

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