Composite electrical contact material comprising Ag and intermetallic compounds
United States Patent [1 1 Shibata Apr. 29, 1975 [75] Inventor: Akira Shibata, Yokohama, Japan [73] Assignee: Chugai Denki Kogyo Kabushiki-Kaisha, Tokyo, Japan [22] Filed: Jan. 4, I973 {21] Appl. No.: 320,888  
 [30] Foreign Application Priority Data Feb. 15. 1972 Japan 4. 47-15453 [52] U S CI 252/514; 252/512; 252/519- 252/521 [51] Int. Cl. H0lb H02 [58] Field of Search 252/519. 521 514. 512  
 [56] References Cited FOREIGN PATENTS OR APPLICATIONS 1.257.654 12/1971 United Kingdom 252/514 OTHER PUBLICATIONS Hara et 211.. Silver and Base Metal Alloy Contact Materials, In Proc. Holm Sem. on Elec. Contact. Phenomena. (1969).  
 Primary E.\&#39;aminerBenjamin R. Padgett Assistant E.\&#39;amin(rR. E. Schafer Attorney, Agent, or Firm-Shlesinger. Fitzsimmons &amp; Shlesinger [57] ABSTRACT Process for the preparation of a composite electrical contact material consisting of an alloy composed of 84 to 90 weight percent of silver, 1.0 to 4.0 weight percent of the components consisting of at least two of zinc, tin or antimony, and less than 0.5 weight percent of the intermetallic compound of one of the group 2A elements of the Periodic Table with nickel or cobalt. the balance being cadmium, said alloy being formed into a strip or contact form and subjected to an internal oxidation at a temperature of 700 to 750C.  
 3 Claims, 5 Drawing Figures PATENTEDAPRZSIQYS 3,880,777  
 SHEET 10F 2 (PRIOR ART) FIG. 3  
 PATENTEDAPRZSIHYS 3,880,777  
 SHEET 2 [IF 2 COMPOSITE ELECTRICAL CONTACT MATERIAL COMPRISING AG AND INTERMETALLIC COMPOUNDS This invention relates to a composite electric contact material and method of preparing the same.  
  Electrical contact material of the silver/cadmium oxide alloy has a superior physical property in point of anti-weld characteristic. low contact resistance and long service life. Especially. the compound alloy material obtained by internal oxidation is advantageous economically and has an excellent property compared to that obtained by powder metallurgical methods.  
  It is noted that the property of the contact material obtained by internal oxidation depends considerably on the kind and property of the oxides precipitated by internal oxidation, and the shape and capacity ofprecipitation particles. The nonfusibility and current breaking performance of the contact material are governed markedly by the volume and refractory characteristics of the oxides. It was recently realized that the refractoriness and anti-weld characteristic of the silver alloy. mixed and melted with tin, zinc or antimony to provide non-fusible oxide. could be improved through oxidation. Addition of nickel and cobalt in a trace amount to the silver alloy is effective to prevent coarse particles of precipitated oxides from forming in the course of internal oxidation. But, certain anisotropy is encountered in the diffusion property of these addition constituents in the oxidation mechanism, and nuclei of oxidized precipitates may not be formed or those of acicular structures may be formed. giving thus rise to abnormal consumption of contact elements. Even the particles of these refractory oxides dislocate within the silver grains. coalesce and turn into coarse particles, on account of the high thermal energy generated at the time of interruption of larger current. resulting in a de creased volume ratio and increased fusibility of such oxides with increased consumption of the contact and silver loss.  
  This invention aims, therefore, at the provision of electrical contact material capable of retaining its structure and high strength up to a higher temperature and withstanding larger electrical impulse through the combination of the strengthened grain boundaries of polycrystalline structure and local strains produced in the matrix. I  
  It is often experienced that the conventional internal oxidation often results in the increased oxide concentration in the surface zone and decreased oxide concentration in the inner section. The material in the inner section becomes ductile and, therefore, the uniform contact performance can hardly be expected.  
  In view of the above. this invention provides a process for the preparation of a composite electrical contact material consisting of an alloy composed of 84 to 90 weight percent of silver. 1.0 to 4.0 weight percent of the components consisting of at least two of zinc. tin, or antimony. and less than 0.5 weight percent of the intermetallic compound of one of the group 2A elements of the Periodic Table with nickel or cobalt. the balance being cadmium, said alloy being formed into a desired configuration and subjected to an internal oxidation at a temperature of 700 to 750C.  
 In the accompanying drawing:  
  FIG. 1 is a photomicrograph at a magnification of 200 X of a cross section of a contact of the cadmium oxide-silver type in accordance with the present invention.  
  FIG. 2 is a photomicrograph at a magnification of 200 X of a cross section of a contact of the cadmium oxide-silver type made in accordance with a conventional internal oxidation,  
  FIG. 3 is an enlarged explanatory view of the contact obtained in accordance with the present invention, in which the letter P indicates minute particles and the letter 1 the width of crystalline silver grain,  
  FIG. 4 is a photomicrograph similar to FIG. 1, in which the internal oxidation of the alloy was made at a temperature below 700C, and  
  FIG. 5 is a photomicrograph similar to FIG. 4, in which its internal oxidation was conducted at a temperature above 750C.  
  Working principles of this invention as summarized above are as follows.  
  Elements belonging to group 2A of the Periodic Table have the property of comminuting the crystals of the metallic alloy. while nickel and cobalt have the property of retarding the growth of precipitated particles and hence lowering the velocity of diffusion thus contributing to the formation of minute precipitated particles. It has been found out that addition of less than 0.5 percent in weight of the intermetallic compound of Be, Mg or Ca with Ni or C0 leads to the retarded growth of the silver crystalline grains and to the precipitation of the oxides in the grain boundaries, thus preventing the diffusion of the internal solute metal into the grain boundaries and the macroscopic difference in concentration of the oxides in the outer and inner zones. It was also found that addition of such compound in higher than 0.5 weight percent lead to embrittled alloy material and segregation in its structure. Minute particles (P in FIG. 3) of the convenient size of l to 3 ,u are precipitated in the grains (of the size I in FIG. 3). Precipitation of too fine particles leads to marked decrease in the silver volume and aggravated conductivity and generation of excessive heat. thus leading to worsened current breaking characteristics. If the particles are coarse on the contrary, silver can easily diffuse and move about and is very liable to fuse and agglomerate at the time of current interruption, thus leading to the worsened characteristics of the compound contact material. The temperature of internal oxidation in the range of 700 to 750C is convenient for the reason that the precipitated oxide particles otherwise will become so fine that they can not be observed under the microscope, causing temperature increase and aggravated nonfusibility and resistance to consumption of the contact material, at a temperature below 700C (See FIG. 4), while the growth of silver crystal grains will be enhanced at a temperature higher than 750C thus promoting the diffusion of solute metal into the grain boundaries and self-diffusion of silver. but will result in enhanced silver concentration in the silver crystal grain boundaries, but extinction of oxide walls at boundaries and larger difference of oxides at the surface and inner zones (See FIG. 5).  
  Mixture of a constituent material of at least two of Sn, Sb and Zn with Cd is added to the alloy metal for the following reason. Evaporation and fusion energies of the oxides CdO, ZnO, SnO and SbO as well as refractoriness of such oxides differ from each other and CdO has the lowest value among these oxides. It is therefore assumed that sublimation of CdO occurs at the lowest temperature. Therefore, it is desirable to oxidize and precipitate them as eutectic or intermetallic compounds thereof. This results in change in the surface concentration ratios of the various oxides at the time of interruption of larger current is eliminated. Sublimation of CdO occurs at 760C, while the sublimation temperatures or melting points of ZnO, SnO and SbO are varied. It is therefore evident that contact material consisting of silver/cadmium oxide and only one of ZnO, SnO&#39; and SbO has its nonfusibility lowered for a certain intermediate current range. On the other hand. the contact material of the present invention has an improved resistivity to such temperature range and has an excellent characteristic as contact material for a current breaker which has to be frequently opened or closed. The composition of the present invention compound material is selected so as to correspond to the required refractoriness and also to be convenient for formation of the intermetallic compound. The difference of diffusion of sn, Sb and Zn in the silver solid solution inevitably produces acicular or anistropic structures in the alloy of silver/cadmium oxide to which only one of Zn, Sb and Sn is added. Such undesirable structures can be eliminated by adding thereto a plurality of the above constituents. The ratio of 1 to 4 percent is convenient for the reason that the addition of such constituents in less than 1 percent is rather ineffective and the addition in higher than 4 percent retards the process of internal oxidation. For example, ZnO is used as a main constituent for the preparation of the contact material for electromagnetic switches which are required to be durable. And, in addition to said ZnO, SnO or SbO is added in view of the evaporation characteristics of such oxides in the higher temperature range and resultant nonfusibility and refractoriness of such oxides at higher temperatures. Where current interruption characteristics is required exclusively. SnO and SbO are used as these constitutents. For high-voltage conductors, SbO is employed in addition to ZnO and SnO, because SbO readily forms an intermetallic compound with SnO and ZnO. Other applications can also be considered within the scope of the present invention.  
 EXAMPLE Non-oxidized silver alloys (Samples 1 to 4) each having the composition in weight ratio as listed below were made.  
  The above samples were subjected to an internal oxidation at a temperature of 730C i C and under the pressure of 3 atm of oxygen.  
  Comparative test samples 5, 6, 7, and 8 were prepared as shown below.  
 Composition Oxidation temp.  
 Sample 5 Ag C dO( 15%) 700C Sample 6 Ag C dO( 15%) 800C Sample 7 Ag CdO( 137:) ZnO(Z7r) 700C Ni0(0.2/1) Sample 8 Ag CdO( 13%) SnO(27r) 700C CoO(0.2&#39;71  
  These samples, 6 mm in diameter and 2 mm in thickness, were tested for their physical and electric properties. Results were as follows.  
 The samples were further tested for refractoriness by measuring the following values.  
 Item A The decrease in the outside diameters as observed when the samples were maintained at 1,000C for 10 minutes in the atmosphere.  
 Item B The weight change as observed when the samples were used as contacts in a circuit (200V, 5,000A), the circuit being closed and opened 10 times.  
 ltem C:- The weight decrease as observed when the samples were used as contacts in a circuit (200V, 50A). Circuit was opened and closed 100,000 times.  
 ltem D:- The times of 200V-300A, 10 times.  
 ltem E:- The times of 200V-50OA, 10 times.  
 ltem F:- The times of 200V-1000A, 10 times.  
 ltem G:- The times of 200V-2000A, 10 times.  
 ltem l-l:- The times of 200V-500A, 10 times.  
 Item l:- The times of 200V-10000A, 10 times.  
 The measured values on the above items are listed in the following table.  
 welding when applied welding when applied welding when applied welding when applied welding when applied welding when applied -Continued ltems A B C D E F G H 1 (mm! (mg) (mg) (times) Samples No. ll 43 224 2 5 8 9 4 9 No. 6 ll 56 I5. 4 (i 8 8 5 7 No. 7 0.4 32 7.5 l 5 (i 7 3 5 No. 8 0.1 I5 0 3 2 Z l 2 ing an alloy oxidized at a temperature of 700 to 750C and composed of 84 to by weight of silver, 1.0 to 4.0% by weight of at least two metals selected from the group consisting of zinc, tin and antimony. and at least 0.1% but less than 0.5% by weight of an intermetallic compound consisting of one of the group 2A elements of the Periodic Table with nickel or cobalt, the balance being cadmium.  
 2. A composite electrical contact material as claimed in claim 1, which has a polycrystalline structure comprising crystalline silver grains provided with grain boundaries of highly concentrated hard oxides and contained by refractory compound oxide particles precipitated in the grains.  
  3. A composite electrical contact material as claimed in claim 2, in which the dimension of each grain is less than 500 ,u. and that of precipitated particles is l to 3