Title of Invention

AN IMPROVED PROCESS FOR THE PREPARATION OF SYNERGISTIC SILVER-METAL OXIDE COMPOSITE POWDERS

Abstract

An improved process for the preparation of synergistic silver-metal oxide composite materials comprising the steps of - providing at least one metal oxide powder selected from cadmium oxide (5 - 20% by wt.), zinc oxide (5 - 15% by wt.), tin oxide (5 - 15% by wt.), nickel oxide (5 - 20% by wt.), or a mixture of one or more of said metal oxides and indium oxide (5 - 15% by wt.); blending one or more of said metal oxide(s) with 80% - 95% silver powder; milling the blend in a high energy attrition mill; annealing the milled product, at temperatures between 200°C and 300°C for a period between 3 and 5 hrs., deagglomerating the milled and annealed product to provide the composite powder.

Full Text

This invention relates to silver-metal oxide composite powders and to powder metallurgical process for preparing such composite powders. The invention further relates to electrical contacts made from such silver-metal oxide composite powders. BACKGROUND & PRIOR ART Currently the electrical-contact grade composite materials are prepared on commercial scale by powder metallurgical processes involving mixing or blending of respective constituents of the composite powder system; followed by their pressing, sintering, repressing/ hot pressing/ extrusion / rolling etc., into final compact. The final compact processed in this manner quite often does not have uniform dispersion amongst the constituent members of the composite system and hence leads to inferior physical, mechanical and electrical contact properties. In US Patent 4023961, to Plassey Inc., silver-10 % cadmium oxide composite material in the powder form has been prepared using coprecipitation technique by intimately mixing silver nitrate and cadmium nitrate solutions and spraying them into potassium car-bonate solution with nitrogen carrier gas. The precipitates of the mixed carbonates are washed, dried in air oven, and decom-posed by heating in air at about 500" c for about 1 to 1.5 hrs. time to give silver- cadmium oxide composite powder particles. The same cycle of washing,drying and decomposition is repeated to produce Ag-MeO powder of controlled levels of sodium and potassi-um impurities. In order to make contact material this is further processed by conventional powder metallurgy processing steps of pressing, sintering and post-sintering operations. British patent 1,397,319 teaches the combination of internal oxidation and powder metallurgical methods of compaction and densification. The process involves formation of a silver-cadmium alloy powder of fine particle size by the low temperature reduction and alloying of a silver and cadmium oxide powder mixture. The internal oxidation of fine Ag-Cd alloy powder is then carried out at relatively low temperature in times typically 103 times shorter than are required in conventional internal oxidation (IO) process. US Patent 3969112, GTE Laboratories Inc., describes the addition of alkali metal, generally in the form of a salt thereof to a mixture of silver and cadmium oxide, prior to the sintering step, to improve the as-sintered density of the resultant composite material with improved electrical contact properties. DRAWBACKS While the aforesaid methods lead to composite powders suitable for producing electrical contact properties but it is felt that by providing improved composite powders the density of the sintered mixture may be considerably improved and thereby consequently improving the mechanical, electrical and contact properties of the electric contact materials made therefrom. The invention herein have found that by using an improved powder metallurgical process, as hereinafter described, improved silver-metal oxide composite powders can be provided and electrical contact materials made from such powders with improved dispersion of metal oxide phase in silver matrix, ultimately offering bene-fits in terms of improved physical mechanical, electrical and contact properties when made into electrical contacts. SUMMARY OF INVENTION According to one embodiment of the invention there is provided an improved process for the preparation of silver-metal oxide composite powder comprising the steps of i) providing at least one metal oxide powder selected from a group of metal oxides selected from cadmium oxide (5-20 % by wt.)/ zinc oxide (5 - 15 % by wt.), tin oxide (5 - 15 % by wt.)/ nickel oxide (5 - 20 % by wt.), or a mixture of one or more of the aforesaid metal oxide(s) with indium oxide (5-5 % by wt.) ; ii) blending one or more of said metal oxide(s) with 80%-95% silver powder ; iii) milling the said blend thereby obtained in a high energy attrition mill ; iv) annealing the milled product, at temperatures between 200"C and 300" C for a period between 3 and 5 hrs. ; v) deaggloraerating the milled and annealed product. According to another aspect of the invention the composite powder obtained by the aforesaid steps is converted into desired articles by the steps of vi) compacting the deaggloraerated powder under pressure to form green compacts ; vii) sintering of the green compacts ; viii) repressing of the sintered compact, and/or hot pressing or extrusion or rolling. Preferably the post-sintering step (viii) above is carried out by repressing the sintered compacts at pressures between 800 and 900 MPa, preferably at 585 MPa pressure, at ambient temperature followed by the step of hot pressing at pressures between 500 and 600 MPa, preferably at 585 MPa and temperature between 350"C and 500" C, preferably at 400" C to the desired size and shape. According to further aspect of the invention there is provided a 4iKt*t&fi*' silver-metal oxide composite powder consisting of a blend com-prising : i) one or more of 5-20% by wt. cadmium oxide ; 5-15% by wt. zinc oxide ; 5-15% by wt. tin oxide ; 5-15% by wt. nickel oxide ; 5-15% of a mixture of one or more of the aforesaid metal oxide(s) and indium oxide ; and ii) 80-95% by wt. silver oxide said composite powder having been obtained by subjecting the starting compounds to the steps of blending, milling, annealing and deagglomeration in the manner described hereinabove. According to a still further aspect the present invention relates to shaped articles, especially electrical contact materials having been formed by subjecting the composite powder described above to the steps of compaction, sintering and hot pressing by the method of the invention described above. DETAILED DESCRIPTION According to one embodiment of the invention, the metal oxide powder is zinc oxide powder used in amounts of between 10-15 % by wt. and the balance silver powder. Typically, the zinc oxide powder is used in an amount of 10.8 % by wt. and the balance is silver powder (89.2 % by wt.). Silver in powder form and one or more of the metal oxides in powder form are blended in the de-sired proportions in a powder blender. It has been found that tin oxide tends to give brittle composites as its proportion in the composite increases above 15 % by wt. When the mixture of metal oxides chosen is tin oxide and indium oxide, for blending with silver powder, indium oxide used is generally from trace quanti-ties upto 2 wt.% max. by weight of the total blend. The combined metal oxides i.e. tin oxide and indium oxide from 5-15 % by wt., while the balance is silver. The blend of the constituent members silver-metal oxides of the composite contact system, thus prepared, is subjected to high energy milling, in an attrit or mill - one fora of which is de-scribed in Example 1 and shown in the accompanying drawings. In this attritor mill, grinding balls and powder charges are held in a stationary, vertical water cooled vessel and agitated by impel-lers radiating from a rotating shaft. The particles of the con-stituent materials are repeatedly flattened, fractured and re-welded during the course of high energy milling. With these starting raw materials milling operation is carried out in normal atmosphere and no particular inert gas blanket of nitrogen or argon is necessary for preparing silver-metal oxide composite system, although, its use is not outside the scope of the present invention. Also preferably, the step of milling in the high energy attrition mill is carried out in air and with circulation of water at ambient temperature. The deagglomeration of the milled and annealed powder is done by sieving it through 100 mesh sieve. The compacting of the deagglomerated powder is made under pressures between 300 and 500 MPa, preferably at 400 MPa pressure. The step of annealing of the milled/homogenised powder is carried out in air. Annealing reduces internal stresses / strains in the material. The annealed composite powder is then sieved to get de-agglomerated powder particles. Alternately, light pulverising or ball milling may be used for deagglomerating. The sieved mass is then compacted in conventional manner using hydraulic presses. The green compact so formed is then sintered in air atmosphere. The sintering is effected at between 850" C and 930" C, preferably at 930" C in air for a period between 30 and 120 min. preferably for 60 min. The sintered compact is again pressed or hot pressed or extruded or rolled to an extent sufficient to achieve near to theoretical final density and the desired size and shape, without affecting the excellent homogeneity of dispersion in the composite material attained before this step, Suitable combination(s) of post-sintering operations, such as repressing the sintered mass followed by hot pressing, may be adopted to get high density compact. The composite so obtained may be extruded to the desired size and shape. The high energy milling step is believed to have surprisingly significantly improved the physical, mechanical and electrical contact properties of these composites over those of composites prepared from silver powder and metal oxide(s) powder by metallurgical processes without this step. The milling step provides excellent homogeneity of dispersion between the constituent members of the final composite electrical contact material and offers high degree of finer dispersion between the constituent members. This has been confirmed by x-ray diffraction studies and photomicrographs such as shown in accompanying drawings in respect of samples of Example 1. The composites as well as the process used in the present invention are environmentally friendly. The improved process of the invention will now be illustrated with the help of non-limiting examples. Three Examples are given below. The first and the third carry a control experiment and results of evaluation of the products of all the examples and the control experiments are given in Table 1 at the end of the three examples. Discussion on all the three are given thereafter. For better understanding of the invention the same is further illustrated with accompanying drawings in which Fig.1 is an attritor used in the step of milling in the method according to the present invention ; and Figs. 2a and 2b are photomicrographs showing morphology of the conventional sintered and repressed material and the sintered and repressed material of the present invention respectively. The following Examples are by the way of illustration only and in no way restrict the scope of the invention. EXAMPLE 1 Preparation of Ag-ZnO(10.8% by wt.) composite Raw Materials: Silver powder: -44 microns AR grade Zinc oxide powder : -5 microns, AR grade Equipaent used: Blender : Laboratory blender fitted with baffle plates. High-energy attrjtor mill : S.S. vertical stationary vessel 1 of 130 cc capacity with rotating shaft 2 and impellers 3 as per the accompanying drawing (Fig 1) along with AISI 52100 grade hardened steel balls 4 as grinding bodies. The vessel 1 has a water inlet 5 and water outlet 6 and inlet 7 and outlet 8 for controlling the conditions in the vessel. Annealing air oven : Laboratory oven of 250 c (Max.) Single action die compaction press : Hydraulic press of 100 ton capacity Sintering furnace : Resistance heating type tube furnace with PID programmer controller Hot pressing die assembly : Developed in laboratory by wrapping heating pad around the die body. Process: Both silver powder and zinc oxide powder were put in a cylindrical blender fitted with baffle plates and mixing/blending operation was carried out for 20 minutes. The blended powder was then charged into the high-energy attrition mill. Ball to charge ratio was 17:1. Attritor speed : 300 rpm. Running time : 4 hrs. Water at ambient temp was circulated in the outer jacket of the attritor during the course of milling until the charge was taken out. The extent of milling was monitored by subjecting the powder samples to x-ray diffraction after 30 min, 60 min., 120 min., 180 min, and 240 min attrition time. The milled powder was annealed in an air oven at 110 C for 4 hrs. The powder was then sieved through 100 mesh sieve to ensure deagglomeration of the powder particles. The sieved powder was then pressed in single action die compaction mode at ~400 MPa pressure. The green compacts were sintered in a sintering furnace at 930 C for 1 hour in air. And then they were repressed at 830 MPa pressure and finally hot pressed -400 C at 585 MPa pressure. Resultant Product: The resultant product was examined for its properties by standard methods (MPIF/ASTM) and the results are given in Table 1. Density was measured by Archimedes' method; Miqrohardness was measured by microhardness attachment of Neoport -2 optical microscope; Electrical conductivity was measured by electrical conductivity meter using type 757 of Technofom Ltd., (Pune make) Dispersion of zinc oxide in silver matrix was observed by optical microscopy at 100x magnification. Control Experiment Similar process runs were carried out on Ag-10.8 wt % ZnO compos-ite system using all the above process steps except the step of high energy milling in an attritor. The resultant product was examined for its properties. The re-sults are given in Table 1. The optical photomicrographs for the Ag-ZnO final hot-pressed compacts for above two conditions are given in Fig.2 showing photomicrographs of silver zinc-oxide composite contacts prepared from powders synthesized by (a) as per control expt. in Example 1 (b) as per Example 1 with high energy attrition milling step. EXAMPLE 2 Preparation of Ag-ZnO(l0.8% by wt.) Composite The procedure of Example 1 was repeated with following variations at the milling stage : ball to charge ratio: 11:1 attrition speed : 180 rpm attrition period : 12 hrs. Resultant product The resultant product was examined for its properties. The results are given in Table 1. EXAMPLE 3 Preparation of Ag-cdo (15.0 % by wt.) Composite The procedure of Example 1 was repeated with following variations: Raw Material : Zinc oxide was replaced by cadmium oxide. Attrition period : 3 hrs 30 min. Resultant product The resultant product was examined for its properties as in Example 1 The results are given in Table 1. Control Expt. similar process runs were carried out on Ag-15.0 wt % CdO composite system using all the above process steps except the step of high energy milling in an attritor . Resultant product The resultant product was examined for its properties. The results are given in Table 1. SUMMARY OF RESULTS OF EVALUATION OF PRODUCTS OF EXAMPLES 1,2 AND 3 (Table Removed) * Control Experiment The photomicrographs in Fig. 2 in the accompanying drawings clearly shows vast difference in the morphology of the dispersion of the zinc oxide in silver matrix. Uniform dispersion of the oxide phase is obtained in process route involving high energy attrition milling as in Example 1. The data reported in Table 1 for Ag-ZnO as well as Ag- CdO composite systems prepared by the process of the present invention clearly show improvement over those obtained by the conventional process route of only blending (without attrition). The above values are comparable to those reported in literature for Ag-MeO contacts made by other processes such as IOAP process, co-precipitation etc. The process of present invention for production of silver-metal oxide contact materials is cleaner and simpler. The silver metal oxide composite powder as described herein is not obtained by mere admixture resulting only in the aggregation of the properties of the components thereof and is neither a product of chemical reaction We claim: 1. An improved process for the preparation of synergistic silver-metal oxide composite materials comprising the steps of i) providing at lease one metal oxide powder selected from cadium oxide (5-20% by wt), zinc oxide (5-15% by wt.), tin oxide (5-15% by wt), nickel oxide (5-20% by wt.), or a mixture of one or more of said metal oxides and indium oxide (5-15% by wt.); ii) blending one or more of said metal oxide(s) with 80%-95% silver powder; iii) milling the blend in a high energy attrition mill; iv) annealing the milled product, at temperatures between 200°C and 300°C for a period between 3 and 5 hrs., v) deagglomerating the milled and annealed product to provide the composite powder. 2. The process as claimed in claim 1 wherein the composite powder thus formed is converted into an article, said process comprising the steps of; i) compacting the deagglomerated powder under pressure to form green compacts; ii) sintering of the green compacts; iii) repressing of the sintered compact and / or hot pressing or extrusion or rolling. 3. The process as claimed in claim 2 wherein the step (viii)of the process comprises repressing the sintered compacts at pressure between 800 and 900 MPa, preferably at 585 MPa pressure, at ambient temperature and hot pressing at pressures between 500 and 600 MPa, preferably at 585 MPa and temperature between 350° C and 500°C, preferably at 400°C to the desired article. 4. The process as claimed in claims 1, 2 or 3 wherein the blend comprises 10-15% by wt. zinc oxide powder and 85-90% by wt. silver powder. 5. The process as claimed in claim 4 wherein the blend comprises 10.8% by wt. zinc oxide powder and 89.2% by wt. silver powder. 6. The process as claimed in any preceding claims wherein the milling is carried out in a high energy attrition mill, in air and with circulation of water at ambient temperature of 25°C - 35°C. 7. The process as claimed in any preceding claims wherein the step of deagglomeration of the milled and annealed powder is done by sieving it through a 100 mesh sieve. 8. The process as claimed in any preceding claim wherein compacting of the deagglomerated powder is done under pressure between 300 and 500 MPa, preferably at 400 MPa pressure. 9. The process as claimed in any preceding claim wherein sintering is done at between 850°C and 930°C, preferably at 930° C in air for a period between 30 and 120 min. preferably for 60 min. 10. A silver-metal oxide composite powder consisting of a blend comprising: i) one or more of 5-20% by wt. cadmium oxide ; 5-15% by wt. zinc oxide; 5-15% by wt. tin oxide; 5-15% by wt. nickel oxide; 5-15% by wt. of a mixture of one or more of the aforesaid metal oxide(s) and indium oxide ; and ii) 80-95% by wt. silver oxide said composite powder having been obtained by subjecting the starting compounds to the steps of blending, milling, annealing and deagglomeration in the manner hereinbefore described. 11. An improved process for the preparation of synergistic silver-metal oxide composite powder and repressed compact as well as shaped articles made thereby substantially as herein described and illustrated with reference to the examples and the accompanying drawings.

Documents:

598-del-1997-abstract.pdf

598-del-1997-claims.pdf

598-del-1997-correspondence-others.pdf

598-del-1997-correspondence-po.pdf

598-del-1997-description (complete).pdf

598-del-1997-drawings.pdf

598-del-1997-form-1.pdf

598-del-1997-form-19.pdf

598-del-1997-form-2.pdf

598-del-1997-form-4.pdf

598-del-1997-gpa.pdf

598-del-1997-petition-138.pdf