Silver-Zinc Oxide Electrical Contact Materials by Mechanochemical Synthesis Route

Indian Journal of Pure & Applied Physics Vol. 45, January 2007, pp. 9-15

Silver-zinc oxide electrical contact materials by mechanochemical synthesis route

P B Joshi 1, V J Rao 1, B R Rehani 1 & Arun Pratap 2 1Department of Metallurgical Engineering, M S University of Baroda, Vadodara 390 001 2Department of Applied Physics, M S University of Baroda, Vadodara 390 001 2E-mail: [email protected]

Received 1 March 2006; revised 27 October 2006; accepted 1 November 2006

Mechanochemical synthesis or reactive milling (RM) is a well-established high-energy milling process for production of a wide range of nanocomposite powders using oxides, carbonates, sulphates or hydroxides as the starting precursors. It ensures chemical reactions such as oxidation/reduction, decomposition or phase transformation in solid-state conditions during room temperature milling, which otherwise require high temperatures. The silver-zinc oxide nanocomposite powders by reactive milling of silver oxide and zinc powder particles have been produced. The resultant Ag-ZnO nanocomposite powders are further processed to bulk solid pieces by conventional powder metallurgy route as electrical contact materials for switchgear applications.

Keywords: Mechanochemical synthesis, Reactive milling, Nanocomposite powders, Silver-zinc oxide composites

IPC Code: H01F41/30

1 Introduction high-energy ball milling process comprising repeated Over the years silver-zinc oxide composites have fracturing and rewelding of composite powder emerged as an environment-friendly substitute to particles. The process leads to an intimate dispersion conventional silver-cadmium oxide contact materials of second phase particles within the soft and ductile (causing environmental and health hazards due to metal matrix. The crystallite size of the powder toxic CdO vapours) for switchgear applications such particles gets reduced to nanometric size during MA. as relays, contactors, circuit breakers, switches, etc. 1,2 . Milling or MA process during which a chemical Though Ag-ZnO contacts possess low contact reaction such as metallothermic reduction and/or the resistance, they have unsatisfactory resistance to formation of compounds takes place is termed as welding and greater tendency to contact wear. Mechanochemical synthesis process or Reactive A fundamental approach to improve the Milling 10 (RM). Schaffer and McCormick 11 were the antiwelding behaviour and wear resistance of such first to report reduction of metal oxides by reactive composites resides in uniformly dispersing the second metals using RM route. Later on, the same principle, phase particles of metal oxide in soft silver matrix. In has been utilized by several researchers to produce order to achieve this goal, a variety of techniques metal-metal oxide type nanocomposite powders for have been developed including ball milling, co- electrical as well as magnetic applications 12-14 . Such precipitation, sol-gel process, electroless deposition nanocrystalline composites by virtue of their fine and internal oxidation as alloy powders i.e. IOAP grain size and consequently high density of interfaces process, etc 3-8. have been found to exhibit exotic properties such as increased strength and hardness, enhanced diffusivity, Another technique that has demonstrated improved ductility/toughness, etc 15 . significant potential for synthesis of metal-metal oxide type composite powders with novel structure An attempt has been made in this investigation to and properties is Mechanical Alloying (MA). process and evaluate Ag-ZnO nanocomposite Mechanical alloying was originally developed by J S powders followed by their consolidation to bulk solid Benjamin in late 1960s as a method for production of contact pieces by conventional powder metallurgy oxide dispersion-strengthened superalloys 9. It is a route of press-sinter-hot press. 10 INDIAN J PURE & APPL PHYS, VOL. 45, JANUARY 2007

2 Experimental Details using a PID type temperature programmer/controller The powders used to produce Ag-8 wt.% ZnO system. The density of as-sintered compacts was contact materials were synthesized by using two further improved by hot-pressing at 450 °C at a different processing routes viz., (i) conventional pressure of 1250 MPa. The hot-pressed compacts powder metallurgy route involving mixing or were subjected to evaluation of properties viz. blending of silver and zinc oxide powder particles and density, microhardness, electrical conductivity and (ii) reactive milling or mechanochemical synthesis microstructure. The density of compacts was approach. In conventional powder metallurgy route, measured as per Archimedes’ principle. The the stoichiometric amount of AR grade silver and zinc microhardness was evaluated at 50 g load using the oxide powder particles were milled in a cylindrical microhardness attachment of Neophot-21, Carl Zeiss blender for 30 min at a rotational speed of 130 rpm (Germany) microscope. The electrical conductivity using a roller mill. The blended powder was sieved was measured on 10 mm dia ground and polished through 100 mesh sieve prior to compaction. samples with the help of an electrical conductivity Likewise stoichiometric amount of AR grade meter Type 979 of M/s Technofour, India. silver oxide and zinc powders (corresponding to Ag 2O-6.12 wt.% Zn and equivalent to Ag-8 wt.% 3 Results and Discussion ZnO) after blending were subjected to Figure 1 shows representative XRD traces for as- mechnochemical synthesis in a high-energy attritor to blended and mechanochemically synthesized (i.e. 8 h produce Ag-8 wt.% ZnO composite powders. The milled) Ag 2O-6.12 wt.% Zn powders. The XRD milling was carried out at 450 rpm speed of attrition profile for the as-blended Ag 2O-6.12 wt.% Zn powder and with 15:1 ball to powder ratio. The 6.3 mm shows diffraction peaks corresponding to reactant diameter hardened steel balls (AISI 52100 steel) were phases namely silver oxide and zinc whereas the used as grinding bodies. No process control agent similar profile for reaction-milled powder shows (PCA) was used during milling. The progress of solid peaks corresponding to Ag, Ag 2O and ZnO. The state reaction between silver oxide and zinc powder underlying mechanism for this change in constituent particles during the course of milling was monitored phases may be explained as follows. The oxygen by subjecting them to X-ray diffraction (XRD) on liberated on account of reduction of Ag 2O by Zn Philips X’Pert PRO X-ray diffractometer fitted with during the course of reaction milling reacts with zinc solid state germanium detector. The powder samples powder particles close to the Ag 2O particles in the were scanned within the 2 θ range of 0 o-80 o at a scan attritor vial. In turn, the zinc particles get oxidized to o -1 speed of 0.1269 s using Cu target and Cu-Kα zinc oxide. This is confirmed by the presence of radiation of 0.15406 nm wavelength and 45 kV and diffraction peaks corresponding to ZnO after 8 h 40 mA as power rating. The powder samples were milling and the disappearance of peaks of Zn, drawn for XRD analysis after 2, 4 and 8 h of milling. otherwise present in the diffraction profile for as- The changes in the size and shape morphology of blended Ag 2O-6.12 wt.% Zn powder. Tables 1 and 2 powder particles taking place during the course of give XRD data for different phases present in the as- milling were studied by subjecting them to Scanning blended and reaction-milled powder samples.

Electron Microscopy (SEM). A Jeol JSM-5610 LV Thus, the XRD analysis confirms the make SEM at an accelerating voltage of 15 kV in SE mechnochemically driven oxidation/reduction (secondary electron) mode was used for this purpose. reaction taking place between the Ag 2O and Zn The thermal behaviour of Ag 2O-Zn powder blend was powder particles in the solid state during milling. The assessed by using SHIMADZU DSC-50 Differential diffraction profile for 8 h reaction-milled powder Scanning Calorimeter at a heating rate of 10 °C min -1. sample was used to estimate the crystallite size of the Both conventionally blended powders and matrix phase i.e. silver, using Scherrer method 16 . The mechnochemically synthesized powders were then crystallite size was found to be of the order of 25 nm. consolidated in the form of green compacts of 10 mm A representative DSC scan for Ag 2O-6.12 wt.% Zn dia × 2 mm thickness at 250 MPa pressure in single as-blended powder sample is given in Fig. 2. The action die compaction mode. The green compacts DSC trace shows three endothermic events at 238, were sintered at 930 °C for 60 min in air. The heating 287 and 412 °C corresponding to thermal rate during sintering was controlled at 6-7°C min -1 decomposition of silver oxide to silver and oxygen on JOSHI et al. : SILVER-ZINC OXIDE ELECTRICAL CONTACT MATERIALS 11

Fig. 1 (a) XRD profile for Ag 2O-6.12 wt.% Zn as-blended powder sample; (b) XRD profile for Ag-8 wt.% ZnO 8 h RM powder sample heating. A sharp endotherm corresponding to melting The changes in the shape morphology and size of of zinc is also observed at 391 °C temperature. the powder particles subjected to milling are Likewise one shallow exotherm in the DSC scan at displayed in SEM microphotographs given in Fig. 3. 162 °C appears to be for removal of moisture from the The as-blended Ag 2O-6.12 wt.% Zn powder particles powder sample and the other exotherm at 454 °C are in the form of fine agglomerates. This may be being indicative of oxidation of zinc to zinc oxide. attributed to the fact that major phase in this blend i.e. 12 INDIAN J PURE & APPL PHYS, VOL. 45, JANUARY 2007

Ag O is a powder normally produced by chemical  2 Table 1 XRD data for the diffraction peaks of routes like precipitation and hence such Ag 2O-Zn as-blended powder sample agglomeration tendency. Contrary to this, the coarse Observed value Value as per standard JCPDS plate-like particles are seen in the SEM micrograph for sample Ag 2O phase Zn Phase File no for 8 h reaction-milled sample. These particles are 2θ value d value d value d value expected to be of silver because the attrition milling

33.15 2.73 2.73 - 12-793 of ductile metal like silver usually leads to coarse 33.89 2.70 flake-like particles. The silver particle formation 36.47 2.46 - 2.47 4-831 could be taken as the consequence of reduction of 37.27 2.41 - silver oxide to silver by zinc as a result of 38.35 2.34 - 2.30 4-831 mechanochemical reaction between the constituent 43.42 2.08 - 2.09 4-831 44.55 2.03 - powders during milling. 54.97 1.67 1.67 - 12-793 The properties of bulk-solid hot-pressed compacts 64.65 1.44 1.43 - 12-793 produced from conventionally blended powders and 68.41 1.37 1.37 - 12-793 the mechanochemically synthesized/reaction-milled 70.92 1.32 - 1.33 4-831 77.62 1.22 - 1.23 4-831 powders are given in Table 3. Table 3 also presents

Table 2 XRD data for the diffraction peaks of Ag-ZnO reaction milled powder sample

Observed value Value as per standard JCPDS for sample Ag ZnO Ag 2O File no 2 θ value d value d value d value d value

33.37 2.68 - 2.66 2.73 21-1486, 34.21 2.62 12-793 34.59 2.59 38.55 2.33 2.36 - 2.37 4-783, 12-793 43.74 2.06 2.04 - - 4-783 44.65 2.02 53.12 1.72 - - 1.67 12-793 60.98 1.51 - 1.57 - 21-1486 64.84 1.43 1.44 1.48 1.43 21-1486, 4-783, 12-793 68.84 1.36 - 1.35 1.37 21-1486, Fig. 2 DSC trace for Ag O-6.12 wt.% Zn as-blended 2 12-793 powder sample 77.75 1.22 1.23 - - 4-783

Table 3 Data on properties of Ag-8 wt.% ZnO bulk-solid contact materials

Sr. No. Processing route Designation Property code Hot-pressed density, gcc -1 Microhardness, Electrical conductivity, (Percent Theoretical) kgmm -2 % IACS

1 Ag-8 wt.% ZnO Conventional A 9.4 (96%) 71-81 77 blending route

2 Ag-8 wt% ZnO (equivalent to Ag 2O-6.12 wt% Zn) by Mechanochemical B 9.4 (96%) 84 82 synthesis or Reactive milling route

3 Data on Ag-8 wt% ZnO commercial contact material produced by press-sinter- C 9.82 (100%) 75 83 extrude route* for comparison

*www.Metalor.com, Metalor Inc., USA JOSHI et al. : SILVER-ZINC OXIDE ELECTRICAL CONTACT MATERIALS 13

Fig. 3 SEM micrograph for (a) Ag 2O-6.12 wt.% Zn as-blended and (b) Ag-8 wt.% ZnO 8 h RM powder sample the corresponding data for an equivalent commercial sinter-extrude route (i.e. for material C). It is well- contact material produced by pressing-sintering- known that the extrusion route always gives higher extrusion of silver and zinc oxide powder blend 17 . For density levels (close to theoretical density) compared the sake of convenience, the compacts of three to hot-pressing, in view of higher degree of plastic different routes are designated as A, B and C. deformation associated with the hot extrusion process According to this data, the density of the and the resultant high densification. material A and B is same (equal to 96% of theoretical The microhardness data for the material A given in density) whereas that for a commercial product Table 3 shows a significant variation in the (i. e. material C) is high and equal to 100%. The microhardness value from 71 to 81 kg mm -2. This can process route used to consolidate the powders into be explained on the basis of the microstructure of the bulk-solid pieces in the present investigation (for material A, given in Figure 4(a). The microsection material A and B) has been press-sinter-hot press shows relatively non-uniform dispersion of zinc oxide route whereas that normally used in industry is press- (black areas) in silver matrix along with some 14 INDIAN J PURE & APPL PHYS, VOL. 45, JANUARY 2007

porosity. The hardness value in the oxide dominated mean free path of the electrons as a result of area is higher than that in the rest of the matrix. heterogeneity in the dispersion of oxide phase in Material B offers maximum hardness in view of a silver matrix in such materials. Thus, the material very fine and uniform dispersion of zinc oxide in produced under this investigation by the novel silver matrix. The resistance to contact wear improves mechanochemical synthesis route offers comparable with increase in the microhardness of the contact levels of electrical conductivity as normally observed member. Improved microhardness of material B can in the case of corresponding commercially developed be attributed to greater dispersion hardening of material. otherwise soft silver matrix by the dispersed oxide Figure 4(a) and (b) show the SEM micrographs for phase particles. Ag-8 wt.% ZnO bulk-solid hot-pressed compacts The electrical conductivity of material B matches prepared from conventionally blended powder well with the material C. The lower value of electrical (material A) and mechnochemically synthesized conductivity of material A is on account of reduced powder (material B). The oxide particles in these

Fig. 4 SEM micrograph for Ag-8 wt.% ZnO bulk-solid hot-pressed compacts prepared from (a) Conventionally blended powder (material A) and (b) Reaction milled powder (material B) JOSHI et al. : SILVER-ZINC OXIDE ELECTRICAL CONTACT MATERIALS 15

microstructures appear as black areas whereas the milling route. The bulk solid contact materials silver matrix appears as light/white background. The produced from such powders have properties at least phases seen in the SEM micrographs viz. silver and comparable to those of existing commercial contact zinc oxide were confirmed by EDS (Energy materials and even better in some respects. Dispersive Spectroscopy) as well. The compacts produced from mechanochemically synthesized References powder (i.e. material B) show an improved dispersion 1 Joshi P B & Ramkrishnan P, Materials for electrical and of zinc oxide in silver matrix compared to that in the electronic contacts-processing, properties and applications (Science Publishers, USA), 2004. compacts of blending route. An improvement in the 2 Schoepf T J, Behrens V, Honig T & Kraus A, Trans microhardness in terms of uniform dispersion of oxide IEEE , 25 (2002) 656. phase in silver matrix is responsible for superior 3 Stewart T I, Douglas P & McCarthy J P, Trans IEEE , 13 electrical performance of the contact members in (1977) 12. switchgear device viz. greater resistance to arc 4 Chang H, Pitt C H & Alexander G B, J Mater Engg & Performance , 1 (1992) 255. erosion, better antiwelding behaviour and lower 5 Jost E M, Proc of Holm Conf on Electrical Contacts , contact resistance. Chicago, IL (1983) 177. Finally, it is worth highlighting here that the 6 Pedder D J, Douglas P & McCarthy J P, Proc of Holm Conf mechanochemical synthesis or reactive milling on Electrical Contacts , Chicago, IL (1976) 109. 7 Schreiner, Horst, Rothkegel & Bernhard, United States produces powder particles with their crystallites Patent, 3954459 (1976). having nanometric size (around 25 nm as in this 8 Schreiner, Horst, United States Patent, 4204863 (1980). investigation). Such nanocomposite powders impart 9 Benjamin J S, Scientific American , 234 (1976) 40. advantages to bulk solids produced therefrom namely, 10 Murty B S & Ranganathan S, Int Mat Review , 43 (1998) 101. higher strength and hardness, improved ductility, 11 Schaffer G B & McCormick P G, Appl Phys Lett , 55 (1989) 45. enhanced diffusivity of constituent atoms and hence 12 Takacs L, 125 th TMS Annual Conf , Anahem CA (1996) B84. better sinterability, etc. 13 Zoz H, Ren H & Spath N, Metall , 53 (1999) 423. 14 Joshi P B, Marathe G R, Pratap Arun & Kaushik V K, Int J 4 Conclusion Powder Met , 40 (2004) 67. 15 Suryanarayan C & Koch C C, Hyperfine Interactions , 130 From the present investigation, it can be said that it (2000) 5. is possible to produce Ag-ZnO nanocomposite 16 Scherrer P, Math Phys K , 1 (1918) 98. powders using mechanochemical synthesis or reactive 17 www.Metalor.com, Metalor Inc., USA.