Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-12-26
2003-11-18
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S233000, C420S566000
Reexamination Certificate
active
06649306
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to lead-calcium-tin-silver alloys for use in the positive grids for lead acid batteries. The alloy may be used to form thin grids by any method, including both expanded metal processing and book mold casting. Grids formed with the alloy harden rapidly, can be cured without resort to extraordinary measures and are stable and easily recyclable.
BACKGROUND OF THE INVENTION
Modern automobile starting batteries require large numbers of thin grids. Proposed 42-volt battery systems will require even more and thinner grids. Sealed VRLA batteries for electric vehicle or hybrid electric service also require thin grids for rapid recharge. Thin positive grids also have utility in stationery batteries for uninterruptible power service or telecommunications service.
Production of thin grids whether conventional book mold cast, continuously cast, concast strip followed by expansion or direct continuous cast followed by rolling, results in a handling of the grid or the strip at high temperatures. The thinner the grid, the more difficult is the grid to handle at high temperatures. Production processes try to rapidly decrease the grid temperature with air, water, or water-cooled trim dies and platens depending on the process. The reduction in temperature is important for lead-calcium alloy grids because these are generally very weak at elevated temperatures and must be cooled to lower temperatures to prevent deformation or thickness change due to inadequate hardness. Despite rapid cooling to room temperature, many grid materials produced from low calcium alloys are extremely difficult to handle due to inadequate hardness at room temperature.
Thicker grids such as those of 0.060″ and above generally have more mass and are better able to be handled despite the low mechanical properties. Thus, thick grids can be cooled to room temperature more slowly than thinner grids. They may be able to be handled in pasting with lower hardness than thinner grids.
The mechanical properties of lead-calcium grid alloys are dependent not only on the temperature but also on the rate of aging after cooling to room temperature. The rate of aging is much more important in thin grids than thick grids.
During the past ten years, lead-calcium-based alloys have replaced lead-antimony alloys as the materials of choice for positive grids of both automobile and stationary lead-acid batteries. Lead-antimony alloys corrode more rapidly than lead-calcium alloys, antimony is released by grids during corrosion, and during the recharge process antimony is transferred to the negative plate where it causes unacceptable loss of water from the electrolyte, particularly in areas of high heat. Lead-calcium alloys do not suffer the water loss during service and, thus, can be processed into grids for maintenance or sealed lead-acid batteries.
Lead-calcium alloys have a very low freezing range and are capable of being processed into positive and negative grids by a variety of grid manufacturing processes, such as conventional book mold casting, rolling and expanding, continuously casting followed by expansion or punching, continuous grid casting, and continuous grid casting followed by rolling. The continuous grid manufacturing processes decrease battery grid and plate production costs.
About ten years ago, the automobile manufacturers modified the exterior of the vehicles to make them more aerodynamic. This design change caused considerably less air to flow through the engine compartment, considerably increasing the underhood temperature.
At that time, lead-calcium alloys were used that generally contained relatively high calcium content (0.08% or higher) and relatively low tin content (0.35-0.5%). Positive grids produced from these alloys hardened rapidly and could be handled and pasted into plates easily. The addition of aluminum to the lead calcium alloys and the method of manufacturing these alloys dramatically reduced calcium oxide generation during processing and permitted production of grids with much better control of the calcium content.
These alloys contained Pb
3
Ca. The higher underhood heat environment leads to increased corrosion of the positive grids in these alloys due to the presence of this Pb
3
Ca in the alloy and failure of the batteries due to corrosion and growth of the positive grids. New lead-calcium alloys were developed to address these problems. They are described in U.S. Pat. Nos. 5,298,350, 5,434,025, 5,691,087, 5,834,141, 5,874,186, as well as DE 2,758,940. These alloys contain much lower calcium than previous alloys because lower calcium produces lower corrosion rates.
Silver has been added to lead and lead alloys for many years to reduce the corrosion of the lead alloy when used as an anode or positive grid of a battery. Rao et al. in U.S. Pat. No. 4,456,579, Nijhawan in U.S. Pat. No. 3,990,893, and Geiss in U.S. Pat. No. 4,092,462 describe lead-antimony alloys for battery grids containing silver as an additive to reduce grid corrosion. The lead-calcium alloys referred to above also contain silver, which further reduces the rate of corrosion, and contain sufficient tin to react with virtually all the calcium to form stable Sn
3
Ca. The grids produced from the lead-calcium-tin-silver alloys have very high resistance to corrosion and growth of the positive grids during testing and in vehicle use, particularly at elevated temperatures.
Rao describes a lead-calcium-tin-silver alloy for positive automobile battery grids in U.S. Pat. No. 5,298,350 which contains 0.025-0.06% calcium, 0.3-0.7% tin, 0.015-0.045% silver, and may contain 0.008-0.012% aluminum. Further refinements of the alloy for direct cast strip are taught in Rao et al. in U.S. Pat. No. 5,434,025 where the calcium range is reduced to 0.02-0.05%, the tin content reduced to 0.3-0.5%, and the silver range increased to 0.02-0.05%. This patent also teaches utilizing strontium or mixed calcium/strontium as a replacement for the calcium. Rao et al. also teach in U.S. Pat. No. 5,691,087 the use of lead-calcium-tin-silver alloys for positive plates of sealed batteries with a composition of 0.025-0.06% calcium, 0.3-0.9% tin, and 0.015-0.045% silver. Rao et al. further refine the lead-calcium-tin-silver alloys for positive grids using the same calcium content ranges described above, but with higher tin contents and a lower level for the silver content based on the methods of grid production. In U.S. Pat. No. 5,874,186, Rao et al. teach an alloy having 0.03-0.05% calcium, 0.65-1.25% tin and 0.018-0.030% silver.
Anderson et al. in U.S. Pat. No. 5,834,141 describe a wider calcium range 0.035-0.085%, higher tin content 1.2-1.55%, and lower silver content 0.002-0.035% range than the patents of Rao and Rao et al. According to Anderson et al., the composition must be varied depending on the method of grid manufacture. If the alloy is to be book mold cast, the alloy must include aluminum and have 0.035-0.055% calcium, 1.2-1.55% tin, 0.025-0.035% silver and 0.005% aluminum. In contrast, a grid formed by the expanded metal process must contain 0.045-0.085% calcium, 1.2-1.55% tin and 0.002-0.0049% silver.
Larsen describes a method of producing directly cast strip of at least 0.060″ thickness from lead-calcium-tin-silver alloys in U.S. Pat. No. 5,948,566. Larsen's alloy contains 0.01-0.06% calcium, 0.03-1.0% tin, 0.01-0.06% silver and optionally 0.003-0.01% aluminum. Assmann describes similar alloys in German patent DE 2758940 with a calcium content of 0.02-0.1%, a tin content of 0.44-1.90%, and a silver content of 0.02-0.1%. Yasuda et al in U.S. Pat. No. 4,939,051 describes the use of a foil of lead-silver-tin pressure bonded to a rolled sheet for a grid production process by expansion. Reif et al. in U.S. Pat. No. 4,725,404 describes the use of copper and/or sulfur to modify the grain structure of lead-calcium (tin) alloys. Finally, Knauer in U.S. Pat. No. 6,114,067 describes a lead alloy containing about 0.06-0.08% calcium, 0.3-0.6% tin, 0.01-0.04% silver and 0.01-0.04% copper which strengthens relatively
Alejandro R
Gillis Theresa M.
Kalafut Stephen
RSR Technologies, Inc.
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