United States Patent [19] [11] 4,294,616 Kim et a1. [45] Oct. 13, 1981

[54] ELECTRICAL CONTACTS 4,138,251 2/1979 Hirsbrunner et a1...... 75/234 _ 4,141,727 2/1979 Shida et a1...... 75/234 X [75] Inventorsr Han J- Klm, Greensburg, P34 4,204,863 5/1980 Schreiner ...... 75/234 Thomas E. Peters, Chelmsford; John Gustafson, Harvard, both of Mass. FOREIGN PATENT DOCUMENTS {73} Assigneez GTE Products Corporation’ 5421284 7/1979 Japan ...... 75/206 Stamford, Conn Primary Examiner—Richard E. Schafer [21] Appl‘ NO: 628 Attorney, Agent, or Firm-Robert E. Walter [22] Filed: Jan. 2, 1979 [571 _ ABSTRACT [51] 1111.01.3 ...... 110111 1/02 An electrical Contact of the type having a Working [52] US. Cl...... ‘15/234; 75/206; Surface and Comprising a ductile metal and Weld inhibit- - 200/266; 428/565; 423/929 ing material includes a wetting agent present in an [58] Field Of Search ...... 75/206, 234; 428/929; amount Suf?cient to reduce the surface energy between 423/565; 200/266 said contact surface and the liquid phase of the ductile 56 R f C_ d metal. Improved wettability of the surface results in [ 1 e erences "e reduced arc erosion due to the spattering loss of ductile U.S. PATENT DOCUMENTS metal. Also, described is a method for selecting and 3,954,459 5/1976 Schreiner et al...... 75/206 ‘8511118 Proposed addltlves 4,011,052 3/1977 Davies ...... 200/266 X 4,115,325 9/ 1978 Santala et a1...... 75/234 5 Claims, 1 Drawing Figure

STANDARD EROSIONRATE—NORMAL1ZEDTOSTANDARDCONTACT

1 I 3O 40 50 PEAK CURRENT —AMPS U.S. Patent Oct. 13, 1981 4,294,616

I I I I I I I I

STANDARD 809O

‘F//. so30 CURRENT-AMPSPEAK

.LQVlNOO CIHVCINVLS OJ. GEIZI‘IVWHON-EILVH NOISOHEI 4,294,616 1 2 increased with the attendant result that the bulk proper ELECTRICAL CONTACTS ties, i.e. electrical conductivity, of the ?nal material are unfavorably effected. Prior art studies relating to an This invention relates to electrical contacts which are additive of another material to the metal and weld in typically used on contact pieces in medium to high hibiting material have attributed observed differences to voltage switching apparatus, more particularly to microstructual changes induced by the third material or contact materials of the silver-cadmium oxide type. to low work function and arc dispersive effects of the To be suitable for the above applications, the contact third material itself. As distinguished from the works of material should have high thermal conductivity, high prior researchers, the present invention describes a electrical conductivity, high resistance to corrosion, method of selecting a third material which operates to high mechanical strength, low contact resistance, good control a speci?c mechanism causing erosion of the arc-interrupting cababilities, good formability and fabri contact. cability, and minimal tendency for interfacial welding or sticking and maximum resistance to arc erosion. SUMMARY OF THE INVENTION It is known to make electrical contacts from a con 5 Electrical contact materials comprising a ductile ductive material and an additive material that tend to metal and a weld inhibiting material are increasingly inhibit welding by weld embrittlement. The conductive being used as contact materials for a variety of switch- ‘ material imparts high electrical and thermal conductiv ing applications. Silver~cadmium oxide is extensively ity to the contact while the additive, which is usually in used because of its excellent arc erosion resistance and the form of a metal oxide, contributes to the desirable 20 antiwelding properties. Although silver-cadmium oxide properties of weld resistance, arc extinguishing, arc erodes at a slower rate than most other contact materi erosion resistance and increased strength properties of als, there is, nevertheless, a loss of material during arc the overall contact material. ing cycles. Generally, two modes of material loss can be In practice, silver is preferable for the conductive characterized: (1) the vaporization and/or decomposi material and cadmium oxide is preferable as a weld 25 tion of the contact material, and (2) the blow-off or inhibiting material. However, there is substantiation, spattering loss of droplets of the ductile metal used in theoretical and empirical, that other conductive mate the contact material. The present invention is particu rial could be used, particularly copper and possibly larly directed to the suppresion of the blow-off or spat other highly conductive materials such as gold and tering mode of material loss. platinum. The oxides of zinc, tin, indium, antimony, In accordance with the principles of the present in bismuth and lead may be used to impart the weld inhib vention, there is provided an electrical contact material iting characteristics to the conductive material. comprising a ductile metal or metal alloy having a rela It is known in the prior art to improve the desired tively high electrical and thermal conductivity, a suf? properties of the electrical contact material by a third cient amount of material to impart weld inhibiting quali— material added to the mixture of conductive material 35 ties to said contact, and a wetting agent present in an and embrittling material. The following discussion is amount suf?cient for reducing surface energy between relevant to additives of a third material. said contact surface and the liquid phase of said metal or US. Pat. No. 3,969,112 to Kim and Reid describes the alloy. addition of an alkali metal to a mixture of silver and cadmium oxide prior to the sintering step to improve DRAWINGS the assintered densities of the resultant alloy material. FIG. 1 illustratesa comparision of erosion rates in US. Pat. No. 3,694,197 to Zdanuk et al. discloses that graph form. reduced arc erosion is obtained in silver-cadmium oxide alloys by adding beryllium, cerium, scandium, anti DETAILED DESCRIPTION mony, gallium, indium, strontium, yttrium and thallium 45 The electrical contact material comprises a ductile metals. metal or metal alloy having relatively high electrical US Pat. No. 3,893,821 to Davies discloses the addi and thermal conductivity for imparting these desirable tion of lanthanum strontium chromite to a silver cad characteristics to the contact. Silver is ideally suited for mium oxide contact material. use as the conductive metal due to its chemical nonreac US. Pat. No. 4,011,053 to Davies discloses the addi tivity and commercial availability. It is contemplated tion of an oxide of a metal selected from Group IA and that other metals such as copper, gold or platinum to IIA of the periodic table to an electrical contact mate gether with alloys thereof may also be used. rial. A suf?cient amount of a weld inhibiting material is U.S. Pat. No. 3,785,810 to Durrwachter et al. dis included with the ductile metal alloy to impart embrit closes a silver cadmium oxide contact material having 55 tlement qualities to the ?nal electrical contact. The at least two additive other alloy components selected weld embrittlement material which preferably does not from the group consisting of calcium, antimony, magne‘ alloy with the ductile metal contributes to the breaking sium, beryllium, aluminum, tin, manganese and zirco of the welds which form when contact is established nium. , between electrical contacts. This property can be mea In two separate articles, Proceedings of the 17th sured according to a standard ASTM weld test wherein Holm Conference on Electrical Contacts, pp. 16-25, the weld strength of contacts which are closed and 1971 and Proceedings of the 18th Holm Conference on subjected to a speci?c current at a speci?c voltage are Electrical Contacts, pp. 333-348, 1972, Shen, Zdanuk separated and the force due to welding of the contacts and Krock studied the effects of numerous single ele is measured. Typical weld inhibiting materials include ment additions on the physical and electrical properties 65 oxides, especially the oxides of antimony, indium, tin, of silver cadmium oxide contact materials. zinc, bismuth and cadmium. . Heretofore, to obtain improved weld inhibiting prop Since the weld inhibiting material is generally a poo erties, the amount of weld inhibiting material has been electrical and thermal conductor and has poor mechani~ 4,294,616 3 4 ca] properties, it is generally preferable that the electri propensity of the ductile metal to form microscopic cal contact material comprise a major portion by weight droplets on the contact surface. Preferably the wetting ductile metal and a minor portion by weight of the agent possesses the ability to substantially lower the embrittlement material. The addition of an excess surface tension of the liquid metal on the contact surface amount of weld inhibiting material undesirably reduces without otherwise signi?cantly diminishing the physi the conductivity of the electrical contact. Preferably cal, mechanical and electrical properties of the contact the electrical conductivity of the ?nal contact is greater or its performance. Wetting agents that form alloys or than about 40 percent I.A.C.S. and even more prefera intermetallic compounds with the ductile metal can bly greater than about 50 percent I.A.C.S. The afore reduce the electrical conductivity of the contact and mentioned value of electrical conductivity is in refer should be avoided. Unless the wetting agent itself con ence to those standards known in the art as the Interna tributes to the ?nal desired properties of the contact tional Annealed Copper Standards (I.A.C.S.) wherein material, it is preferable to use only relatively small pure copper is rated as possessing 100 percent conduc amounts of the wetting agent in proportion to the over tivity. More speci?cally pure copper in accordance all composition of the contact. Preferably small with the above standards is rated at 10.371 ohms circu 5 amounts of a wetting agent which imparts a high degree lar mils/ft. of wetting to the surface of the contact is homoge To obtain the desirable weld embrittlement proper neously distributed over the working surface and dis ties of the contact, the embrittlement material should be persed in the metal matrix so that the entire working at least uniformly distributed on the surface of the surface will have desirable wetting properties. contact in the metal matrix. According to one method 20 The wetting agent or material capable of forming a of preparing contacts, the ductile metal and metal oxide wetting agent should be selected to permit its incorpo embrittlement material are intimately mixed by blend ration into the contact under conditions of manufacture ing the powders of the materials. The powder mixture is or use or permit the formation of the wetting agent pressed into a compact and the compact is sintered by during such conditions. More preferably the wetting heating and pressing. 25 agent is an oxide of a metal other than the ductile metal In accordance with another process for preparing and has a substantially greater wettability with the liq contact materials, the ductile metal is alloyed with an uid phase of the ductile metal than the wettability of the other metal or second metal which can be oxidized at a embrittlement material with the liquid phase of the subsequent time to form the embrittlement material. It is ductile metal. By' incorporating the wetting agent into required that the second metal be oxidized more readily the contact, the favorable properties of low surface than the ?rst metal so internal oxidation results in a energy are imparted to the contact to give increased structure having the weld inhibiting characteristics of surface wettability. the .contact. In practice, the preferred metals are silver As hereinbefore mentioned, the in situ formation of for the ?rst metal'and cadmium for the second metal. the wetting agent is contemplated. When the wetting Preferably the second metal is present in an amount up 35 agent is a compound of the metal, the metal itself or to its limit of solubility in the ?rst metal or ductile metal. other compound of the metal maybe incorporated into Typical other metals that may be used are copper for the contact provided the wetting agent is subsequently the ?rst metal and zinc or tin for the second metal. formed. For example, when the wetting agent is a metal According to the process, a powder mixture of the ?rst oxide, the metal in elemental form or other metal com metal and oxide of the second metal is heated, prefera 40 pound may be incorporated into the contact provided bly in a reducing atmosphere at a temperature to avoid the metal oxide wetting agent is subsequently formed. It excessive sintering to form an alloy powder. The alloy is contemplated that the metal or metal compound nec powder is then heated in a suitable oxidizing atmo essary for the formation of the wetting agent or the sphere to internally oxidize the second metal. Electrical wetting agent itself may be present as a separate phase contacts prepared from powders made according to the 45 or alloyed with the ductile metal. above process have a uniform distribution of the oxide The wettability and relative surface energy of a wet of the second metal in a matrix of the ?rst metal. Due to ting agent can be determined by measuring the contact the fact that the materials are substantially preoxidized, angle between the liquid phase of the ductile metal and the compositions remain relatively stable during the the wetting agent. A solid/liquid interface is. present conditions of operation. 50 between the solid contact and liquid ductile metal. The According to another method of making electrical contact angle which is equal to or greater than zero, is contacts, the alloy of the ?rst metal and second metal measured between the liquid and the solid wetting agent may be formed directly into the proper contact shape and is the angle at the line of contact between the solid which may be in the form of a thin strip or wire. Subse and liquid as measured in the liquid phase. Although quent oxidation of the material at suitable conditions to many methods may be used for measuring surface en oxidize the second metal and not the ?rst metal results ergy, it has been found that the following method is in a metal oxide of the second metal being formed satisfactory for obtaining a relative comparison of the within a matrix of the ?rst metal. The oxide of the sec surface energies of candidate materials for wetting ond metal is decreasingly present as the surface of the agents. ~ contact is penetrated. The contact angle between a molten metal and a In accordance with the principles of the present in metal oxide is measured by ?rst pressing ?ne powder of vention, the electrical contact includes a wetting agent the wetting agent into a compact. Next the compact is present as an additive in the contact material in an sintered by heating. The resulting product should have amount suf?cient for reducing the surface energy be a ?at surface free from surface contamination so that tween the working surface of the contact and the liquid 65 subsequent contact angle measurements are due to ma phase of the ductile metal or alloy. The wetting agent terial differences alone rather than the nature of the minimizes the spattering mode of erosion by lowering surface. A short length of a small diameter wire of the the surface energy of the contact thereby reducing the ductile metal is placed on the surface of the compact. 4,294,616 5 6 The compact and ductile metal are heated to a tempera With the silver-cadmium oxide contacts, the contact ture suf?cient to liquify the metal and a photograph is angle that the ?nal electrical contact exhibits with the taken. The contact angle is then approximated from the liquid du'ctile metal is substantially the same as the photograph. By comparing the contact angles of vari contact angle that cadmium oxide in substantially pure ous proposed wetting agents with the contact angle form exhibits with the liquid metal. With silver-cad formed between the ductile metal and weld inhibiting mium oxide contacts containing a major portion of material, a wetting agent may be selected. The wetting silver, it has been found'that small amounts of well agent should have a substantially smaller contact angle dispursed, wetting agent can signi?cantly reduce the with the liquid phase of the ductile metal than the droplet formation by reducing the surface energy be contact angle that the weld inhibiting material has with 10 tween the ductile metal droplet and the ?nal contact the liquid phase of the ductile metal. material. This results in reduced arc erosion by way of In general, the electrical contact material comprises a reduction of spattering. major portion by weight ductile metal and a minor Although the above work was done with silver-cad portion embrittlement material with the wetting agent mium oxide materials, it is theorized that materials hav comprising an amount replacing the embrittlement ma ing a lower surface energy than the weld inhibiting terial and present in an amount from an effective material may be incorporated in electrical contact mate amount which reduces the surface energy to an amount rials formed from other ductile materials and embrittle- ' less than an amount which undesirably effects the desir ment material. The value of the material as a wetting able ?nal properties of the electrical contact material. agent can be measured by comparing its tendency to be Typically, the contact comprises about 50 to about 95 wetted by the molten metal with the tendency of the percent by weight ductile metal, from about 5 to about weld inhibiting material to be wetted by the molten 50 percent by weight weld inhibiting material and metal. greater than about 0.001 percent by weight wetting The following examples are given to enable those agent. skilled in the art to more clearly understand and prac The contact angle of silver with oxides was measured 25 tice the present invention. They should not be consid in accordance with the above procedure. The following ered as a limitation upon the scope of the invention, but oxides in the order set forth were found to exhibit in merely as being illustrative and representative thereof. creasing contact angles with the oxide of copper exhib-, Example l-sAbout 6 grams of cadmium oxide parti iting the smallest contact angle and the oxide of alumi cles on the order of less than two microns is pressed num exhibiting the largest contact angle. The oxides in using a pressure of 560 kg per cm2 to obtain compacts order of decreasing wettability are copper oxide, ger 0.2 cm in thickness. The resulting compact is heated to maniurn oxide, tantalum oxide, niobium oxide, yttrium a temperature of 915° C. at 15° C. per minute and held oxide, cadmium oxide, cerium oxide, chromium oxide, at the ?nal temperature for about 1.5 hours to sinter the cobalt oxide, iron oxide, ruthenium oxide, zirconium compact. After cooling, a short length of silver wire oxide, zinc oxide, and aluminum oxide. 35 having a small diameter is placed on the surface of the To obtain an improved wetting angle in the ?nal compact. The compact and silver wire is heated to a contact surface using a relatively small amount of wet temperature of about 1050° C. whereby the silver is in ting agent, it is desirable to employ a wetting agent the liquid phase. A photograph is taken of the liquid having a substantially smaller contact angle with the silver on the surface of the contact at the above temper liquid phase than the contact angle that the weld inhibit ature. From the photograph the contact angle of the ing material has with said liquid phase. Cadmium oxide silver with the cadmium oxide compact is determined to has a contact angle of approximately 82°, yttrium oxide be about 82°. about 80° and niobium oxide about 73". When added in Example 2—In a manner similar to Example 1, the relatively small amounts to the ?nal contact material, oxides of copper, germanium, tantalum, niobium and niobium and yttrium oxides will not have a substantially 45 yttrium in powder form are pressed into a compact smaller contact angle with the liquid silver phase so that which is sintered. The contact angles are measured to a relatively small amount of these materials incorpo give contact angles of 22, 35, 64, 73 and SO'degrees rated into the contact would not have a signi?cant ef respectively. fect in reducing arc erosion. The oxides of copper, Example 3—In a manner similar to Example 2, the germanium and tantalum exhibited contact angles of contact angles of the oxides of cerium, cobalt, iron, 22°, 35° and 64° respectively. These materials have a ruthenium, zirconium, zinc and aluminum were mea contact angle substantially less than cadmium oxide and sured. All of these oxides exhibited contact angles when properly incorporated into the ?nal contact mate greater than the about 82° contact angle exhibited by rial in small amounts will result in a decrease in arc cadmium oxide. The oxide of cerium exhibited contact erosion due to spattering. 55 angle of 94”. It was found that germanium and tantalum be present Example 4—Submicr0n size silver powder is dry in the form of single oxide or double oxide compounds, blended with cadmium oxide powder and a metal oxide are the most preferred materials for use with cadmium wetting agent. The cadmium oxide and metal oxide oxide. Their presence in small amounts did not seem to wetting agent each have particle sizes of less than 2 impart undesirable characteristics to the ?nal contact. microns as measured by a scanning electron micro Although the silver cadmium oxide contact prepared scope. The metal oxide is added in a quantity suf?cient with copper oxide displayed the most favorable wetting to occupy about one volume percent of the contact properties of those contacts tested, the contact exhib material. Lithium nitrate dissolved in distilled water is ited undesirable cracking in the are effected layer. Typi added to the powdered blend to form a slurry. The cal double oxides are formed as an oxide of the wetting 65 amount of lithium nitrate added is suf?cient so as to agent and the weld inhibiting material. For germanium result in a dry powder mixture having about 0.1 weight and tantalum the compounds have the chemical formula percent lithium nitrate. It is added for purposes of densi Cd3GeO4 and Cd2Ta2O7. ?cation as set forth in US. Pat. No. 3,969,112. After 4,294,616 7 8 drying and sieving, about 3 grams of the powdered increase at low current. This effect masked the true blend containing the wetting agent and densification aid erosion rate of the material. Scanning electron micro is pressed using a pressure of 560 kg per cm2 to obtain scope examination of the anode surfaces showed mor compacts 0.2 cm in thickness having approximately 50 phological differences similar to those noted for the percent porosity. For comparison purposes, a compact cathodes. Well formed droplets of silver were on the not containing a wetting agent but including the densi? surface of the standard contact while the surface con cation aid is prepared. The compacts are heated for taining the wetting agent additives were wetted by the ?red sintering to 915° C. at 15° C. per minute and held the silver. Some cracking occurred in the anode surface at that temperature for 1.5 hours for ?nal sintering. employing copper oxide. Contacts are prepared according to the above proce 0 Example 7—The contacts prepared in accordance dure using the following blends of powder: (1) 21.3 with the procedure set forth in Example 5 were tested grams of Ag, 3.55 grams of CdO and 0.15 grams of and erosion rate lower than the standard contact mate CuzO, (2) 21.3 grams of Ag, 3.54 grams of CdO and 0.16 rial was noted. The absence of droplet formation on the grams of Cd2GeO4, (3) 21.3 grams of Ag, 3.45 grams of contact surfaces indicates an improvement in the wet CdO and 0.25 grams of Cd2Ta2O7, and (4) 21.3 grams of 15 ting behavior of the contacts containing wetting agent. Ag, 3.70 grams of CdO. Example 8—Submicron size silver powder is dry Example 5—The procedure of Example 4 was re blended with cadmium oxide powder having a particle peated using powder blends containing germanium size less than two microns. The resulting blend is about oxide and tantalum oxide represented by the formulae 85.2 percent silver by weight with the remainder cad G602 and T3205. 20 mium oxide. Sufficient germanium dioxide is added in a Example 6—The erosion rates of the contacts pre quantity to occupy about one volume percent of the pared in Example 4 were measured in a static gap arc contact material. The germanium dioxide is added as an chamber. The electrode spacing was 0.5 mm. An arc aqueous solution containing 3.85 grams per liter of ger was initiated by the application of a high voltage break manium oxide per liter. The resulting slurry is mixed, down pulse of 15 kv and 2 microseconds duration 25 dried and sieved. Next the powder is placed in an alu which developed a conducting channel through which mine boat and heated at about 10° C./min to a tempera a capacitor bank was discharged. The parameters of the ture of 350° C. in a nitrogen atmosphere containing capacitor bank were chosen to give a half-sinusoid arc about 10% hydrogen to effect reduction of the cad wave form of 8.3 millisecond duration and peak cur mium oxide and result in the formation of an alloy pow rents adjustable to 100 amps. The tests were carried out 30 der of cadmium and silver. The germanium oxide re in an atmosphere of 80 percent nitrogen, 20 percent mains essentially unchanged. The resulting alloy pow oxygen of 99.9 purity. The samples were subjected to der is then oxidized at 350° C. to precipitate a dispersion 6X103 current pulses at a repition rate of one pulse of cadmium oxide within the grains of silver. After every ten secondsf The low duty cycle eliminated any cooling to room temperature the powder is again sieved long term temperature rise in the contact assembly. The 35 and is mixed with sufficient lithium nitrate in a methyl erosion of the contacts was determined by weighing the alcohol solution to result in a powder when dried hav sample before and after arcing. The results of the test ing about 0.1 weight percent lithium nitrate. About 2 are illustrated in the drawing. The cathodic erosion rate grams of the powder is pressed using a pressure of 2800 for the materials containing the wetting agent, blends kg/cm2 to obtain a compact about 0.25 cm in thickness 1-3 of Example 4, are plotted in FIG. 1 relative to the having approximately 40 percent porosity. The com erosion rate for the silver-cadmium oxide material with pact is then sintered by heating according to the sched out the wetting agent, blend 4 of Example 4. The error ule set forth in Example 4. The above procedure is bars equal plus or minus one standard deviation and followed except for the germanium dioxide addition to represent the amount of scatter of the measurements produce an electrical contact not containing a wetting about the smoothing functions used to represent the 45 agent and hereinafter referred to as the standard data. The erosion rate for materials containing the addi contact. tives is signi?cantly less than that of the silver~cadmium Example 9—The contacts prepared as in Example 8 oxide material without wetting agent, hereinafter called are tested according to the procedure set forth in Exam the standard contact. The difference is particularly ple 6. At an arc current of 200 amps, GeO was 84 per pronounced at the higher currents where appreciable LII cent of the erosion exhibited by the standard material. contact melting occurs and silver droplet loss is most The present invention has been described with refer pronounced. The ranking of the wetting agents ob ence to the speci?c embodiments thereof, it should be tained from the contact angle measurements is pre understood by those skilled in the art that various served in erosion rate measurements. That is copper changes may be made and equivalence may be substi oxide, the material with the lowest interfacial energy, tuted without departing from the spirit and scope of the (contact angle equal to 22°) is also the most effective of invention as set forth in the appended claims. the wetting agents in lowering the erosion rate. Scan What is claimed is: ning electron microscope examination of the eroded 1. An electrical contact having a working surface cathodes reveal well formed droplets of silver, approxi consisting essentially of from about 50 to about 95 per mately one micron in diameter, on the surface of the 60 cent by weight silver or silver alloy, from about 5 to standard contact. The well de?ned droplet is indicative about 50 percent vby weight of cadmium oxide, and an , of poor wettability between the droplet and the contact additional oxide selected from the oxides of germanium, surface. The contact containing wetting agent have tantalum, and mixtures thereof, said additional oxide surfaces exhibiting a smooth morphology indicative of being present in an effective amount sufficient to reduce good wetting between the silver droplets and the cath the surface energy between said working surface and ode surface. The anode was the upper most electrode in the liquid phase of said silver and being substantially the test. Material vaporized from the cathode con homogeneously distributed on said surface in a silver densed on the cooler anode causing an apparent weight matrix. 4,294,616 9 10 2. An electrical contact according to claim 1 wherein additional oxide are substantially homogeniously dis said wetting agent is a cadmium germanium oxide. tributed throughout said contact. 3. An electrical contact according to claim 1 wherein 5. An electrical contact according to claim 1 wherein said wetting agent is a cadmium tantalum oxide. said cadmium oxide is formed by the internal oxidation 4. An electrical contact according to claim 1 wherein 5 of cadmium within a silver matrix. said contact is sintered and said cadmium oxide and said * “‘ * * *

1O

15

2O

25

30

35

40

45

50

55

6O

65