EP0205679B1 - Production of molded silver-graphite articles - Google Patents
Production of molded silver-graphite articles Download PDFInfo
- Publication number
- EP0205679B1 EP0205679B1 EP19850302890 EP85302890A EP0205679B1 EP 0205679 B1 EP0205679 B1 EP 0205679B1 EP 19850302890 EP19850302890 EP 19850302890 EP 85302890 A EP85302890 A EP 85302890A EP 0205679 B1 EP0205679 B1 EP 0205679B1
- Authority
- EP
- European Patent Office
- Prior art keywords
- silver
- process according
- graphite
- article
- molded
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
- H01B1/02—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of metals or alloys
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
- H01B1/04—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of carbon-silicon compounds, carbon or silicon
Definitions
- the present invention relates to the production of molded graphite-silver articles.
- 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.
- 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.
- 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.
- the silver-containing compound is desirably used in crystal, powder or solution form.
- step (c) about one gram of pitch per 2.5 grams of silver-graphite is preferably employed.
- step (d) the milled residue is preferably heated from 160°C to 170°C whereupon it is then cooled, further milled and reheated from 150°C to 250°C and then further milled.
- the molded graphite-silver article of step (f) may then be machined to a desired size.
- the molded graphite-silver article preferably contains from 28 to 55 percent by weight of silver.
- 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.
- a particular use of the present process is for making silver-graphite contacts.
- the silver-graphite contacts universally used in the field today contain 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.
- 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.
- 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.
- other screen sizes can be used to obtain any product desired.
- This material can be baked at temperatures ranging from 600°Cto 1400°C to obtain the desired physical properties.
- 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.
- 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.
- the finished contacts contained 28% silver and had an average compressive strength of 5500 psi with an average density of 1.77 g/cm 3 and a specific resistance of 0.0016 ohm inches.
- 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.
- the contacts contained 44% silver, had an average compressive strength of 15,500 psi with an average density of 2.72 g/cm 3 and a specific resistance of 0.0012 ohm inches.
- the contacts contained 52% silver, had an average compressive strength of 14,580 psi with an average density of 3.00 g/cm 3 and a specific resistance of 0.00095 ohm inches.
Description
- The present invention relates to the production of molded graphite-silver articles.
- 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.
- 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.
- 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.
- 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 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 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.
- The silver-containing compound is desirably used in crystal, powder or solution form.
- In step (c) about one gram of pitch per 2.5 grams of silver-graphite is preferably employed.
- In step (d) the milled residue is preferably heated from 160°C to 170°C whereupon it is then cooled, further milled and reheated from 150°C to 250°C and then further milled.
- The molded graphite-silver article of step (f) may then be machined to a desired size.
- The molded graphite-silver article preferably contains from 28 to 55 percent by weight of silver.
-
- 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.
- A particular use of the present process is for making silver-graphite contacts. The silver-graphite contacts universally used in the field today contain 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%.
- 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.
- All reactions described in this process .are time-temperature related and can be altered to suit a particular need.
- 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.
- This material can be baked at temperatures ranging from 600°Cto 1400°C to obtain the desired physical properties.
- The invention is further illustrated in a non limiting fashion by the following examples.
- The following procedure produces a 50-55% by weight silver contact or brush.
- Intimately mix 153 grams of silver nitrate (crystals, powder, or in aqueous solution) and 50 grams of graphite in a container. Mill 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. Mill 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.
- 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 (21" 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. Mill 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. Mill 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.
- In other examples of the invention the following contacts were made as in Example 1 and had the indicated characteristics. -
- 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.
- The contacts contained 37% silver, had an average compressive strength of 12,000 psi with an average density of 2.29 g/cm3 and a specific resistance of 0.0011 ohm inches.
- 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.
- 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.
- X-ray examination showed these pieces had excellent silver distribution.
- 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.
Claims (10)
Priority Applications (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US06/516,233 US4534887A (en) | 1983-07-22 | 1983-07-22 | Molded-to-size silver-graphite articles and process for making same |
EP19850302890 EP0205679B1 (en) | 1985-04-25 | 1985-04-25 | Production of molded silver-graphite articles |
DE8585302890T DE3576251D1 (en) | 1985-04-25 | 1985-04-25 | PRODUCTION OF SHAPED BODIES FROM SILVER GRAPHITE. |
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
EP19850302890 EP0205679B1 (en) | 1985-04-25 | 1985-04-25 | Production of molded silver-graphite articles |
Publications (2)
Publication Number | Publication Date |
---|---|
EP0205679A1 EP0205679A1 (en) | 1986-12-30 |
EP0205679B1 true EP0205679B1 (en) | 1990-02-28 |
Family
ID=8194205
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP19850302890 Expired EP0205679B1 (en) | 1983-07-22 | 1985-04-25 | Production of molded silver-graphite articles |
Country Status (2)
Country | Link |
---|---|
EP (1) | EP0205679B1 (en) |
DE (1) | DE3576251D1 (en) |
Family Cites Families (9)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US2448685A (en) * | 1944-04-29 | 1948-09-07 | Nat Carbon Co Inc | Electrical contact brush |
US2640944A (en) * | 1949-06-04 | 1953-06-02 | Union Carbide & Carbon Corp | Electrical contact brush |
US3165480A (en) * | 1957-12-18 | 1965-01-12 | Union Carbide Corp | Electrical contact brush |
US2934460A (en) * | 1958-03-31 | 1960-04-26 | Union Carbide Corp | Method for impregnating a carbonaceous brush with silver |
DE1148286B (en) * | 1959-08-28 | 1963-05-09 | Varta Ag | Process for the impregnation of the porous support structures of electrodes, especially sintered electrodes, for electrical accumulators with active mass |
US3455023A (en) * | 1967-07-31 | 1969-07-15 | Rowe Ind Inc | Silver-carbon brush block and method of making same |
JPS543682B2 (en) * | 1974-08-29 | 1979-02-26 | ||
US4188279A (en) * | 1976-10-26 | 1980-02-12 | Mobil Oil Corporation | Shaped carbon articles |
US4534887A (en) * | 1983-07-22 | 1985-08-13 | Union Carbide Corporation | Molded-to-size silver-graphite articles and process for making same |
-
1985
- 1985-04-25 EP EP19850302890 patent/EP0205679B1/en not_active Expired
- 1985-04-25 DE DE8585302890T patent/DE3576251D1/en not_active Expired - Lifetime
Also Published As
Publication number | Publication date |
---|---|
EP0205679A1 (en) | 1986-12-30 |
DE3576251D1 (en) | 1990-04-05 |
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