|
(11) | EP 0 205 679 B1 |
| (12) | EUROPEAN PATENT SPECIFICATION |
|
|
| (54) |
Production of molded silver-graphite articles Herstellung von Formkörpern aus Silbergraphit Production d'objets moulés en argent-graphite |
|
|
||||||||||||||||||||||||||||
| Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). |
[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 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.
[0008] 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.
[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 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.
[0014] 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%.
[0015] 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.
[0016] All reactions described in this process .are time-temperature related and can be altered to suit a particular need.
[0017] 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.
[0018] This material can be baked at temperatures ranging from 600°Cto 1400°C to obtain the desired physical properties.
Example 1
[0021] 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.
[0022] 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.
[0023] In other examples of the invention the following contacts were made as in Example 1 and had the indicated characteristics. -
Example 2
[0024] 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
[0025] 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.
Example 4
[0026] 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
[0027] 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.
[0029] 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 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.
2. A process according to claim 1 wherein the silver-containing compound in step (a)
is silver nitrate.
3. A process according to 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.
1. Verfahren zum Herstellen eines geformten Graphit-Silber-Erzeugnisses, das gleichmäßig
durch das Erzeugnis hindurch verteilte Silberteilchen aufweist, gekennzeichnet durch
die Schritte:
(a) Mischen der Graphitteilchen mit einer Silber enthaltenden Verbindung, die aus Silbernitrat, Silberacetat, Silbercarbonat und Silbereiweiß-Komplexen ausgewählt ist;
(b) Erhitzen der Mischung, um die Silberverbindung zu metallischem Silber zu reduzieren;
(c) Vermahlen des Endproduktes und Vermischen dessen mit einem Harzbindemittel und einem Lösungsmittel, um eine Aufschlämmung zu erzeugen;
(d) Entfernen flüchigter Bestandteile aus der Aufschlämmung und Vermahlen des Rückstandes;
(e) Formen des Rückstandes aus Schritt (d), um einen kompakten Körper beliebiger Gestalt zu erzeugen; und
(f) Erhitzen des kompakten Körpers in einer inerten Atmosphäre, um das Harzbindemittel zu verkohlen und den Körper in das geformte Graphit-Silber-Erzeugnis zu überführen.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß die Silber enthaltende Verbindung
in Schritt (a) Silbernitrat ist.
3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß die Silber enthaltende
Verbindung in Kristall-, Pulver- oder Lösungsform verwendet wird.
4. Verfahren nach irgendeinem der Ansprüche 1-3, dadurch gekennzeichnet, daß das Lösungsmittel
in Schritt (c) aus Trichlorethylen, Methylenchlorid, Toluol und einer Mischung von
diesen ausgewählt wird.
5. Verfahren nach irgendeinem der Ansprüche 1-4, dadurch gekennzeichnet, daß folgender
Schritt hinzugefügt wird:
(g) Bearbeiten des geformten Silber-Graphit-Erzeugnisses gemäß Schritt (f) zu einer gewünschten Gestalt.
6. Verfahren nach irgendeinem der Ansprüche 1-5, dadurch gekennzeichnet, daß in Schritt
(d) der vermahlene Rückstand auf 160° bis 170°C erhitzt wird, woraufhin dieser dann
abgekühlt, weitervermahlen und auf 150°C bis 250°C wiedererwärmt und dann weitervermahlen
wird.
7. Verfahren nach irgendeinem der Ansprüche 1-6. dadurch gekennzeichnet, daß in Schritt
(f) das geformte Graphit-Silber-Erzeugnis unter einer inerten Atmosphäre bei einer
Temperatur von 600°C bis 1400°C erhitzt wird.
8. Verfahren nach irgendeinem der Ansprüche 1-7, dadurch gekennzeichnet, daß das geformte
Graphit-Silber-Erzeugnis 28 bis 55 Gew.-% Silber enthält.
9. Verfahren nach irgendeinem der Ansprüche 1-8, dadurch gekennzeichnet, daß in Schritt
(e) etwa 1 g Harz pro 2,5 g Silbergraphit verwendet wird.
10. Verfahren nach irgendeinem der Ansprüche 1-9, dadurch gekennzeichnet, daß die
Mischung in Schritt (b) einer Temperatur von 600°C bis 800°C erhitzt wird, um die
Silberverbindung zu metallischem Silber zu reduzieren.
1. Procédé pour produire un article moulé en graphite-argent contenant des particules
d'argent réparties unformément dans tout ledit article, caractérisé par les étapes
qui consistent:
(a) à mélanger les particules de graphite-à un composé contenant de l'argent, choisi parmi du nitrate d'argent, de l'acétate d'argent, du carbonate d'argent et des complexes d'argent et de protéines;
(b) à chauffer ledit mélange pour réduire le composé d'argent en argent métallique;
(c) à broyer le produit résultant et à le mélanger à un liant formé de poix et un solvant pour produire une suspension;
(d) à éliminer les substances volatiles de la suspension et à broyer le résidu;
(e) à mouler le résidu de l'étape (d) pour produire un corps dense d'une forme souhaitée; et
(f) à chauffer le corps dense dans une atmosphère inerte pour carboniser le liant formé de poix et convertir le corps en l'article moulé en graphite-argent.
2. Procédé selon la revendication 1, dans lequel le composé contenant de l'argent
dans l'étape (a) est du nitrate d'argent.
3. Procédé selon la revendication 1 ou 2, dans lequel le composé contenant de l'argent
est utilisé sous la forme d'un cristal, d'une poudre ou d'une solution.
4. Procédé selon l'une quelconque des revendications 1 à 3, dans lequel le solvant
de l'étape (c) est choisi parmi du trichloréthylène, du chlorure de méthylène, du
toluène et des mélanges de ces substances.
5. Procédé selon l'une quelconque des revendications 1 à 4, dans lequel l'étape suivante
est ajoutée: (g) l'usinage de l'article moulé en graphite-argent de l'étape (f) à
une dimension souhaitée.
6. Procédé selon l'une quelconque des revendications 1 à 5, dans lequel, à l'étape
(d), le résidu broyé est chauffé de 160° à 170°C, à la suite de quoi il est alors
refroidi, de nouveau broyé et réchauffé de 150°C à 250°C, puis encore broyé.
7. Procédé selon l'une quelconque des revendications 1 à 6, dans lequel, à l'étape
(f), l'article moulé en graphite-argent est chauffé dans une atmosphère inerte à une
température de 600°C à 1400°C.
8. Procédé selon l'une quelconque des revendications 1 à 7, dans lequel l'article
moulé en graphite-argent contient 28 à 55% en poids d'argent.
9. Procédé selon l'une quelconque des revendications 1 à 8, dans lequel, à l'étape
(c), environ un gramme de poix pour 2,5 g d'argent-graphite est utilisé.
10. Procédé selon l'une quelconque des revendications 1 à 9, dans lequel le mélange
de l'étape (b) est chauffé à une température de 600°C à 800°C pour réduire le composé
d'argent en argent métallique.