CA1051514A - Storage battery plate with core of lighter metal - Google Patents

Abstract

BATTERY PLATE AND METHOD OF MAKING SAME

Abstract of the Disclosure A storage battery plate is provided having a core of aluminum, for example, which has its oxide coating replaced by a thin film coating of a metal such as zinc. The zinc in turn is coated with a thin layer of silver. Lead is thereafter de-posited over the silver coating in the usual manner to form a battery plate. Alternately the oxide coating is removed and a thin film of lead is deposited thereon. Such a plate has a higher conductivity and is lighter in weight with superior per-formance characteristics than a conventional lead-antimony plate.

Description

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Ihis invention relates ~o hattery plates, and moro particularly to an improvement in the method of producing said plates. In producing battery plates, it is desirable to produce plates which are capable of carrying high currents for rapid charging and output performance, while at the same time reducing their weight and size. The present invention teaches tlle production of battery plates which have a reduced internal resistance over present lead-antimony plates and which are smaller in ; size and weight.
; It is an object of the present invention to provide an improved battery plate of an aluminum core having a lead coating deposited thereon.
It is a further object of the present invention to provide a method for forming battery plates of higher conductivity and superior charging capability. ~
It is a further object of the present invention according to -a second embodiment thereof to produce an aluminum core battery plate ; having a lead coating deposited thereon without the need for precoatings of materials such as zinc and/or silver.
` The invention may be generally defined as a battery plate for a lead-acid storage battery comprising a substantially oxide-free aluminum core and a coating of substantially pure lead over the core.

A coating of zinc is provided over the substantially pure lead coating and a coating of silver is provided over the zinc coating. Furthermore, a second lead coating is provided over the silver coating.

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'I'he invention includcs a method for making this battery comprising the steps of: (a) placing a substantlally oxide-free aluminum core to be coated and a supply of pure lead in a chamber of reduced atmospheric pressure; (b) heating the lead to its vaporizing point; and (c) condensing lead on said core by maintaining said core at a temperature below the vaporizing point of lead to form a lead film thereon .
In accordance with this invention, in order to obtain as light a battery as possible, the core of the plate of the battery is made of a lightweight conductive metal, preferably aluminum. While other metals such as copper or silver are considered, however, aluminum has the highest conductivity per unit of weight, is most economical and has adequate physical strength, and therefore is preferred.
The aluminum should preferably be of a commercially pure type.
The aluminum is formed into a shape suitable for a battery plate. The plate is then cleaned in a well known manner in a solution of a detergent in which a suitable wetting agent has been placed. It is thereafter washed in water and is provided with a barrier layer to the lead.

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In accordance with this invention, the barrier layer is provided and the internal resistance of the battery cell is minimized by removing the natural aluminum oxide which coats the surface of the aluminum while simultaneously replacing it with a protective conductive coatiny. A protective coating may be applied using metals, such as zinc, nickel, or chrome, but zinc is preferred because it is a better thermal conductor and its thermal expansion and contraction characteristics are more compatible with those of aluminum. The protective coating is applied by dipping the core in a solution consisting of from 13 to 70 ounces of zinc oxide per gallon of sodium hydroxide at room temperature. After the first dipping of from 2 to 5 sec-onds, a second dipping for from 2 to 5 seconds is advis~d. To improve the bonding of the coating additives~ they are added to the solution into which the core is dipped. These may be a `~
solution of 1 to 5 ounces of either ferric chloride or rochelle salts to a gallon of water. These additives insure that a tightly bonded pure zinc filmg without cracks or blisters is le~t on the aluminum surface.
The solution into which the aluminum plate is dipped is at room temperature. A weight of about 0.02 mg. of zinc ~ -per square inch is desirable. The other indicated metals namely, nickelg or chrome, may be deposited using the same method as described for zinc. The aluminum oxide is removed while simultaneously depositing the zinc coating to prevent ~;
reformation of the aluminum oxide, which is an insulator~ and makes poor contact with other materials.
If an inspection reveals~ after the first zinc dip-ping that there are still impurities on the surface, the battery 3 plate is dipped into a solution of 50% nitric acid at room --3~

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temperature ~or a few seconds. Thereafter, the ~atte~y plate is rinsed in pure water and dipped in the original zinc oxide-sodium hydroxide mixture again.
Thereafter, the aluminum plate which is now coated -~ 5 with pure zinc has a second mechanical barrier layer o~ an electrically conductive metal, such as silver, a ~ew angstroms thick, deposited over the zinc coating. This is achieved by placing the battery core for from 2 to 4 seconds in a regular silver plating solution. By way of example, the silver plat-ing solution can be a mixture of 1/2 to 3/4 ounce of silver cyanide to a gallon of water. This solution is mixed with an equal solution of 10 to 12 ounces of potassium cyanide to a gal-- lon of water. Temperature of the solution is maintained be-tween 70 to 85 degrees. The voltage which is applied to the electrolytic solution is from 4 to 6 volts and the current den-sity is from 15 to 25 amperes per square foot.
The two electrically conductive barrier layers which are deposited on the aluminum core prevent any grain migration between the aluminum and the lead and avoid any galvanic action that otherwise acts to deteriorate the cell and tend to increase its resistance.
The final lead coating which is deposited on the cell can be applied by electroplating, spraying, sintering, or any other well-known techniques. With a cell of this construction, purer lead may be employed than otherwise. Antimony is usually present in standard battery plates. It is added to the lead or alloying constituents to increase hardness of the lead used in the manufacture of the plates. Such plates release a large proportion o~ the surface antimony which migrates toward the 3 negative plates~ thereby contaminating the electrolyte and .' .

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adversely affecting battery performance, as by increasing the positive plate corrosion, and increasing the sel~-discharye rate. These problems may be avoided when pure lead is used.
With the battery plate made in accordance with the present in-vention, there is no need for a hard lead plate and therefore, pure lead can be used.
Lead normally has up to 0.5% of antimony, which has little effect in cell operation. Antimony may be removed from the lead which has been deposited on the battery plate by dip-ping the lead covered battery plate into a solution made of a mixture of 2 to 5 volumes of hydrogen peroxide with one volume of sulfuric acid having a specific gravity of 1.2, ~or a time on the order of one hour.
Description of the Second Embodiment In order to o~tain a film of pure lead with good ad-herence qualities directly on an aluminum core, it is necessary to use one or more of the following vacuum techniques: thermal evaporation~ plasma deposition (sputtering) or ion implantation (thermal evaporation with ionization).
TheLmal evaporation of lead onto an aluminum core is accomplished by heating pure lead in a vacuum chamber causing i-t to vaporize and recondense onto the core, to form a lead film thereon. The steps include placing pure lead into a fila-ment boat which is connected to a high current low voltage power supply. The heat from the filament is transferred to the lead by conduction. When the temperature o~ the lead reaches the evapoxation point, it begins to evaporate. The vaporized molecules move from their source to the other solid surfaces in the vacuum chamber such as the core and the chamber walls 3 condensing thereon due to the temperature differential. Con--' ' ~O'~

densation produces the :Lead Eilm ~onnation on the alumin~n core. No removal of the al~ninum oxide is obtained in khis pro-cess. However~ such removal is a desirable objective in mak-ing high performance battery plates, and thus where this tech-nique is used the aluminum oxide is substantially removed bya sodium hydroxide bath or the like prior to placing the alumi-num into the vacuum chamber.
E~AMPI,E I
A pure lead film was deposited on an aluminum core having its oxide coating substantially removed, in a vacuum chamber having a pressure of 2 x 10 5 torr. The rate of depo-sition was 50 angstroms per minute.
EXAMPLE II
In a vacuum chamber having a pressure o~ 2 x 10-7 torr, a film of pure lead was obtained on an aluminum core with a de-position rate of 15l000 angstroms per minute.
In both examples a uniform film having good adhesion properties was obtained.
A second vacuum deposition technique known as ion

2~ sputtering may be employed. In ion sputtering two electrodes are immersed in an inert gas medium at reduced pressure. The material to be coated, ~or example, an aluminum core, is utiliz-ed as the cathode. The coating material to be deposited on the aluminum, for example, lead, is utilized as the anode. A plasma is maintained by either a D.C. or R.F. potential which continual-ly ionizes the inert gas molecules. The electrodes are bom-barded by the gaseous ions in the presence of a high electric field. The kinetic energy of the gaseous ions (on the order of 150 to 600 electron volts) exceeds the bonding energy of the

3~ surface molecules of the electrodes causing them to be dislodged :- . .

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from the sur~ace. In the case of the aluminwn cathode, khis produces a removal of the alumin~n oxide surface layer for ef-fectively cleaning the aluminum plate. In the case of pure lead this bombardment is effective for dislodging molecules from the surface. The lead molecules are accelerated by the electric field thereby to cause a lead film to condense and adhere to the cleaned aluminum cathode.
EX~MPLE III
In a vacuum chamber of 1.0 to 15 x 10-3 torr and utiliz-10ing 50 to 600 watts of R.F. power, a deposition rate of approxi-mately 1,000 angstroms per minute of lead onto the cleaned alumi-num core is obtained.
A third technique combines the first two techniques and produces an increased deposition rate. The third technique is known as ion~plating. As in the sputtering technique, elec-trodes are immersed in an inert gas medium such as argon at re-duced pressure. The alwminum substrate which is to receive the film is connected as the cathode. A plasma is maintained either by a D.C. or R.F. potential. An evaporation source is utilized as the anode and may be a boat or filament together with the evaporant lead. Positive ions in the plasma are accelerated towards the cathode (the aluminum core) by the electrical field gradient thereby bombarding and continuously cleaning the plate's . ~
core prior to the film deposition. ;~
2~ Whila the surface of the aluminum is cleaned by the - plasma bombardment, the pure lead is evaporated by heat. The evaporated lead becomes ionized on passing through the plasma.
The ionized lead atoms are accelarated towards the cathode (the aluminum core) simultaneously with the sputtering action which 3 is cleaning the surface. In this manner the aluminum sur~ace . . .
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is sputtered, removing the al~ninum oxide, both before and during plating. This maintains a clean surface until the first monolayer of the lead film is deposited on the surface, and at the same time roughens the surface to insure strong adhesion of the first monolayer. The higher the velocity with which the lead particles are accelerated, the deeper the penetration of the lead onto the surface of the aluminum.
The plasma bombarding of the core is essential for it is effective to remove the naturally occurring aluminum oxide from the surface of the core. Further, it cleans and etches the surface to reduce surface defects~ The removal of the aluminum oxide and the cleaning of the surface produce greatly improved electrical characteristics. Removal of the oxide also facilitates the proper deposition of lead onto the aluminum, a result which has not heretofore been obtainable without the utilization of intermediate depositions of metals such as zinc between the lead and aluminum surfaces.
It is desirable to provide shutters between the cathode - and anode to collect the removed aluminum oxide and other par-ticles which are freed from the aluminum substrate by the sput-tering process to thereby reduce contamination.

EXAMPLE IV
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Ion plating in a vacuum chamber having a pressure of from 5 x 10~5 to 8 x 10-4 torr/ a film of 1,000 to 15,000 ang-stroms per minuta of high purity lead was deposited on the aluminum coreO The dielectric was kept at about 10 to 13 mm `
at 20 to 25 millitorr with a current density o~ approximately 0.3 to 0.8 m amps per cm2.
Film deposition rates are nearly linear functions of 3 the deposition period. By varying the process parameters of pressure, power and electrode distance the film deposition rate can be changed as desired. The lead may be vapori~ed by resis-tance heating in a dielectric crucible heated by eddy currents induced by an external R.F. diode coil. Use of this technique permits deposition rates up to about 200,000 angstroms per minute.
Although the foregoing description discloses a tech-nique for forming battery plates wherein lead is directly coat-ed onto aluminum as the aluminum oxide layer is removed, it is also possible to utilize the present invention where a mechani- -cal barrier coating is first placed over the aluminum. For ex-ample, as disclosed in the first embodiment of the invention, an aluminum core can have its oxide removed and replaced with -~ pure zinc followed by a thin layer of silver and thereafter a lead film. Additional lead is then deposited over the lead film by the present method. A similar example is an oxide free aluminum plate which has been silver plated and then has a lead film formed thereon.
A third variation is to first deposit a lead film on ~ the aluminum by the present method and to then add a lead coat ing by the electrochemical plating technique of the first em-bodiment. A sufficiently porous coating formed in this manner will eliminate the need for lead paste usually required for ne~ative plates.
A final variation is to deposit directly by the vacu-um techniques of the present embodiment lead oxide on the alumi-num core.
Utilization of plates formed according to this inven-tion will not be subject to the usual hydrogen embrittlement.
3 meir performance is superior in comparison to lead-antimony _ 9_ !

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type plates. Further3 the use o~ such plates will produce a battery cell having higher efficiency and discharge rate at low temperatures. The plates are thinner than the standard plates and thus a larger number of plates can be placed in a given cell. A partial improvement in the performance of a battery having plates according to the present invention is due to the absence of the antimony usually present in lead. The present process by use of pure lead eliminates the electrical couple formed between lead and antimony which tends -to cause self-1~ discharging and create internal resistance in the battery. Anadditional advantage is the improved circulation of electrolyte between the thinner plates which carries away small bubbles usually present in a standard cell further reducing internal re-sistance.
: 15 While I have described embodiments of this invention in some detail, it will be understood that this description and illustrations are offer~d merely by way of example, and that the invention is to be limited in scope only by the appended : claims.
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Claims (3)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A battery plate for a lead-acid storage battery, comprising a substantially oxide-free aluminum core and a coating of substantially pure lead over the core, a coating of zinc over the substantially pure lead coating, a coating of silver over the zinc coating and a second lead coating over the silver coating.

2. The method of preparing a battery plate having a substantially oxide-free aluminum core and a coating of substantially pure lead over the core, comprising:
(a) placing a substantially oxide-free aluminum core to be coated and a supply of pure lead in a chamber of reduced atmospheric pressure;
(b) heating the lead to its vaporizing point; and (c) condensing lead on said core by maintaining said core at a temperature below the vaporizing point of lead to form a lead film thereon.

3. A method as defined in claim 2, wherein, prior to placing the core in said chamber it has layers of zinc, silver and lead deposited thereon.