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(11) | EP 0 205 679 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Production of molded silver-graphite articles |
| (57) The present invention provides a process for producing a molded graphite-silver article
by blending graphite and a silver containing compound and heating to reduce the compound
to metallic silver, mixing the product with binder/ solvent, shaping the resultant
mix and heating the same. |
[0002] Graphite brushes and contacts are sometimes made with silver powder or are sometimes impregnated with silver to decrease their contact resistance or to decrease their specific resistance. The present invention relates, in one embodiment, to a process for the manufacture of such brushes or contacts which improves the uniformity and predictability of the silver level in the product by introducing the silver as a silver compound with the graphite filler and reducing the silver compound to elemental silver prior - to mixing the filler with the binder.
[0003] The patent art evidences activity in this area and this discussion will be limited to the most relevant patents. Thus U.S. Patent No.3,146,130 teaches the impregnation of porous electrodes with a solution of an ammonium complex silver prepared by dissolving a silver compound in a nitrogen-containing solvent and heating to evaporate the solvent. U.S. Patent No.2,934,460 to Ramadanoff concerns a method for impregnating carbonaceous brushes with silver and silver sulfide in a precipitate such as acetone or ethylene diamine; heating to precipitate silver nitride in situ rapidly and then further heating the brush to reduce the silver nitrate to silver. U.S. Patent No.4,220,884 describes a method of making a carbon contact brush by melting a metal such as tin or an alloy thereof with lead, zinc and silver; immersing a porous carbon body therein to impregnate it with molten metal and removing the body from the molten metal to solidify the metal therewithin.
[0004] None of these patents suggest or hint at the incorporation of the metal with graphitic particles and pitch prior to shaping and cannot produce the same articles.
[0005] According to the present invention there is provided a process for producing a molded graphite-silver article containing silver particles uniformly distributed throughout said article, characterized by the steps of:
(a) mixing graphite particles with a silver containing compound selected from silver'nitrate, silver acetate, silver carbonate and silver protein complexes;
(b) heating said mixture to reduce the silver compund to metallic silver;
(c) milling the resulting product and mixing it with a pitch binder and a solvent to produce a slurry;
(d) removing volatiles from the slurry and milling the residue;
(e) molding the residue to step (d) to produce a dense body of a desired shape; and
(f) heating the dense body in an inert atmosphere to carbonize the pitch binder and convert the body into the molded graphite-silver article.
[0008] In step (d) the milled residue is preferably heated from 1600C to 1700C whereupon it is then cooled, further milled and reheated from 150°C to 250°C and then further milled.
[0010] The molded graphite-silver article preferably contains from 28 to 55 percent by weight of silver.
[0011] A simplified flow diagram of the present invention, shown for a particular embodiment, is as follows:
[0012] In accordance with one embodiment of the present invention, graphite and silver nitrate are blended together and heated to an elevated temperature of 600°C to 800°C or to where the silver nitrate is converted to metallic silver. The mixture of silver-graphite is milled and combined with a binder and a solvent in a mixer. This blend is milled and molded into desired shapes and baked. The raw materials can be mixed in any proportions to obtain the desired percent of silver in the product.
[0013] A particular use of the present process is for making silver-graphite contacts. The silver-graphite contacts universally used in the field today containa a 50-55% silver level. However, other silver levels ranging from 1% to 99% can be achieved by altering the formulations used in this process.
Thus if one combines 153 grams of silver nitrate with 50 grams of graphite, follows the processing steps, and adds 28 grams of pitch binder per 70 grams of silver-graphite, then the process will yield a brush with approximately 50% silver by weight. Proceeding similarly with adjusted levels of the components yielded silver levels of 28%, 37%, 44%, 52%, and 55%.
[0014] Various solvents such as trichlorethylene, methylene chloride, toluene, mixtures thereof and other aromatic hydrocarbons can be used to dissolve the pitch. Silver nitrate is the preferred silver-containing compound but others may be used as well such as silver acetate, silver carbonate, and silver-protein complexes. Any graphite material natural or artificial can be used. Similarly a wide range of pitch binders can be used.
[0015] All reactions described in this process are time-temperature related and can be altered to suit a particular need.
[0016] The screening process following a final 3-hour, 160°C - 170°C temperature treatment is particularly important to the uniformity of the final material as it reduces the silver particle agglomeration. However, other screen sizes can be used to obtain any product desired.
[0017] This material can be baked at temperatures ranging from 600°C to 1400°C to obtain the desired physical properties.
EXAMPLE 1
[0020] Intimately mix 153 grams of silver nitrate (crystals, powder, or in aqueous solution) and 50 grams of graphite in a container. Hill the mixture to a fine powder. Place the powder in a sagger lined with Grafoil flexible graphite, place a lid on the sagger, fire the contents in an inert atmosphere in a Lindberg furnace at 700°C for 3 hours. Grafoil is a trademark of Union Carbide Corporation. Hill the resulting silver-graphite mixture and weigh this powder to determine the amount of "available silver-graphite." Place one gram of pitch per 2.5 grams available silver-graphite into a beaker. Add 500 milliliters of trichlorethylene to the pitch and stir the slurry until the pitch has dissolved. Add the silver-graphite to the pitch solution and stir until a smooth slurry is formed. Continue mixing for 45 minutes.
[0021] Pour this slurry into evaporating pans. Keep the slurry stirred while pouring into the pans. Place the pans into a 90°C oven until all volatiles have been evaporated-(approximately 1 to 2 hours). Let the mix cool. Crush the mix into small pieces (1/2" diameter maximum). Place the crushed mix into drying pans and place in a 160°C to 170°C oven for 3 hours. Let the mix cool. Hill the mix in a swing hammer mill to a fine powder of around 200 mesh and place in the drying pans. Place the pans in a 160°C to 170°C oven for 3 hours. Hill the mix in a swing hammer mill to a fine powder and screen through a 200 mesh screen. The portion of mix not going through 200 mesh may be remilled and rescreened. The screened mix is heat treated at 150°C to 250°C for 15 minutes and granulated in a Stokes Granulator through a 50 mesh screen. Then the granulates are molded in a press such as a Dorst Press. Pressures developed during the pressing operation may be in the range of 5 to 15 tons per square inch, or even higher. The molded shape then is baked in a Lindberg furnace at 1000°C to 1200°C under an inert atmosphere before machining to finished size and X-ray inspection.
[0022] In other examples of the invention the following contacts were made as in Example 1 and had the indicated characteristics.
EXAMPLE 2
[0023] The finished contacts contained 28%. silver and had an average compressive strength of 5500 psi with an average density of 1.77 g/cm3 and a specific resistance of 0.0016 ohm inches.
EXAMPLE 3
[0024] The contacts contained 37% silver, had an average compressive strength of 12,000 psi with an average density of 2.29 g/cm 3 and a specific resistance of 0.0011 ohm inches.
EXAMPLE 4
[0025] The contacts contained 44% silver, had an average compressive strength of 15,500 psi with an average density of 2.72 g/cm3 and a specific resistance of 0.0012 ohm.inches.
EXAMPLE 5
[0026] The contacts contained 52% silver, had an average compressive strength of 14,580 psi with an average density of 3.00 g/cm3 and a specific resistance of 0.00095 ohm inches.
[0028] While there have been described herein what are at present considered preferred embodiments of the invention, it will be obvious to those skilled in the art that minor modifications and changes may be made without departing from the essence of the invention. It is therefore to be understood that the exemplary embodiments are illustrative and not restrictive to the invention, the scope of which is defined in the appended claims and that all modifications that come with the meaning and ranges-of equivalency of the claims are intended to be included therein.
1. A process for producing a molded graphite-silver article containing silver particles
uniformly distributed throughout said article characterized by the steps of:
(a) mixing graphite particles with a silver containing compound selected from silver nitrate, silver acetate, silver carbonate and silver protein complexes;
(b) heating said mixture to reduce the silver compound to metallic silver;
(c) milling the resulting product and mixing it with a pitch binder and a solvent to produce a slurry;
(d) removing yolatiles from the slurry and milling the residue;
(e) molding the residue of step (d) to produce a dense body of a desired shape; and
(f) heating the dense body in an inert atmosphere to carbonize the pitch binder and convert the body into the molded graphite-silver article.
2. A process according to claim 1 wherein the silver-containing compound in step (a)
is silver nitrate.
3. A process according to claim 1 or 2, wherein the silver-containing compound is
used in crystal, powder or solution form.
4. A process according to any of claims 1 to 3, wherein the solvent in step (c) is
selected from trichlorethylene, methylene chloride, toluene and mixtures thereof.
5. A process according to any of claims 1 to 4, wherein the following step is added:
(g) machining the molded graphite-silver article of step (f) to a desired size.
6. A process according to any of claims 1 to 5, wherein in step (d) the milled residue
is heated from 160° to 170°C whereupon it is then cooled, further milled and reheated
from 150°C to 250°C and then further milled.
7. A process according to any of claims 1 to 6, wherein in step (f) the molded graphite-silver
article is heated under an inert atmosphere at a temperature from 600°C to 1400°C.
8. A process according to any of claims 1 to 7, wherein the molded graphite-silver
article contains from 28 to 55 percent by weight of silver.
9. A process according to any of claims 1 to 8, wherein in step (c) about one gram
of pitch per 2.5 grams of silver-graphite is employed.
10. A process according to any of claims 1 to 9, wherein the mixture in step (b) is
heated at a temperature of 600°C to 800°C to reduce the silver compound to metallic
silver.