Silver-zinc oxide electrical contact materials by mechanochemical synthesis route

Transcription

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

2 10 INDIAN J PURE & APPL PHYS, VOL. 45, JANUARY Experimental Details The powders used to produce Ag-8 wt.% ZnO contact materials were synthesized by using two different processing routes viz., (i) conventional powder metallurgy route involving mixing or blending of silver and zinc oxide powder particles and (ii) reactive milling or mechanochemical synthesis approach. In conventional powder metallurgy route, the stoichiometric amount of AR grade silver and zinc oxide powder particles were milled in a cylindrical blender for 30 min at a rotational speed of 130 rpm using a roller mill. The blended powder was sieved through 100 mesh sieve prior to compaction. Likewise stoichiometric amount of AR grade silver oxide and zinc powders (corresponding to Ag 2 O-6.12 wt.% Zn and equivalent to Ag-8 wt.% ZnO) after blending were subjected to mechnochemical synthesis in a high-energy attritor to produce Ag-8 wt.% ZnO composite powders. The milling was carried out at 450 rpm speed of attrition and with 15:1 ball to powder ratio. The 6.3 mm diameter hardened steel balls (AISI steel) were used as grinding bodies. No process control agent (PCA) was used during milling. The progress of solid state reaction between silver oxide and zinc powder particles during the course of milling was monitored by subjecting them to X-ray diffraction (XRD) on Philips X Pert PRO X-ray diffractometer fitted with solid state germanium detector. The powder samples were scanned within the 2θ range of 0 o -80 o at a scan speed of o s -1 using Cu target and Cu-K α radiation of nm wavelength and 45 kv and 40 ma as power rating. The powder samples were drawn for XRD analysis after 2, 4 and 8 h of milling. The changes in the size and shape morphology of powder particles taking place during the course of milling were studied by subjecting them to Scanning Electron Microscopy (SEM). A Jeol JSM-5610 LV make SEM at an accelerating voltage of 15 kv in SE (secondary electron) mode was used for this purpose. The thermal behaviour of Ag 2 O-Zn powder blend was assessed by using SHIMADZU DSC-50 Differential Scanning Calorimeter at a heating rate of 10 C min -1. Both conventionally blended powders and mechnochemically synthesized powders were then consolidated in the form of green compacts of 10 mm dia 2 mm thickness at 250 MPa pressure in single action die compaction mode. The green compacts were sintered at 930 C for 60 min in air. The heating rate during sintering was controlled at 6-7 C min -1 using a PID type temperature programmer/controller system. The density of as-sintered compacts was further improved by hot-pressing at 450 C at a pressure of 1250 MPa. The hot-pressed compacts were subjected to evaluation of properties viz. density, microhardness, electrical conductivity and microstructure. The density of compacts was measured as per Archimedes principle. The microhardness was evaluated at 50 g load using the microhardness attachment of Neophot-21, Carl Zeiss (Germany) microscope. The electrical conductivity was measured on 10 mm dia ground and polished samples with the help of an electrical conductivity meter Type 979 of M/s Technofour, India. 3 Results and Discussion Figure 1 shows representative XRD traces for asblended and mechanochemically synthesized (i.e. 8 h milled) Ag 2 O-6.12 wt.% Zn powders. The XRD profile for the as-blended Ag 2 O-6.12 wt.% Zn powder shows diffraction peaks corresponding to reactant phases namely silver oxide and zinc whereas the similar profile for reaction-milled powder shows peaks corresponding to Ag, Ag 2 O and ZnO. The underlying mechanism for this change in constituent phases may be explained as follows. The oxygen liberated on account of reduction of Ag 2 O by Zn during the course of reaction milling reacts with zinc powder particles close to the Ag 2 O particles in the attritor vial. In turn, the zinc particles get oxidized to zinc oxide. This is confirmed by the presence of diffraction peaks corresponding to ZnO after 8 h milling and the disappearance of peaks of Zn, otherwise present in the diffraction profile for asblended Ag 2 O-6.12 wt.% Zn powder. Tables 1 and 2 give XRD data for different phases present in the asblended and reaction-milled powder samples. Thus, the XRD analysis confirms the mechnochemically driven oxidation/reduction reaction taking place between the Ag 2 O and Zn powder particles in the solid state during milling. The diffraction profile for 8 h reaction-milled powder sample was used to estimate the crystallite size of the matrix phase i.e. silver, using Scherrer method 16. The crystallite size was found to be of the order of 25 nm. A representative DSC scan for Ag 2 O-6.12 wt.% Zn as-blended powder sample is given in Fig. 2. The DSC trace shows three endothermic events at 238, 287 and 412 C corresponding to thermal decomposition of silver oxide to silver and oxygen on

3 JOSHI et al.: SILVER-ZINC OXIDE ELECTRICAL CONTACT MATERIALS 11 Fig. 1 (a) XRD profile for Ag 2 O-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 of zinc is also observed at 391 C temperature. Likewise one shallow exotherm in the DSC scan at 162 C appears to be for removal of moisture from the powder sample and the other exotherm at 454 C being indicative of oxidation of zinc to zinc oxide. The changes in the shape morphology and size of the powder particles subjected to milling are displayed in SEM microphotographs given in Fig. 3. The as-blended Ag 2 O-6.12 wt.% Zn powder particles are in the form of fine agglomerates. This may be attributed to the fact that major phase in this blend i.e.

4 12 INDIAN J PURE & APPL PHYS, VOL. 45, JANUARY 2007 Table 1 XRD data for the diffraction peaks of Ag 2 O-Zn as-blended powder sample Observed value Value as per standard for sample Ag 2 O phase Zn Phase 2θ value d value d value d value JCPDS File no Ag 2 O is a powder normally produced by chemical routes like precipitation and hence such agglomeration tendency. Contrary to this, the coarse plate-like particles are seen in the SEM micrograph for 8 h reaction-milled sample. These particles are expected to be of silver because the attrition milling of ductile metal like silver usually leads to coarse flake-like particles. The silver particle formation could be taken as the consequence of reduction of silver oxide to silver by zinc as a result of mechanochemical reaction between the constituent powders during milling. The properties of bulk-solid hot-pressed compacts produced from conventionally blended powders and the mechanochemically synthesized/reaction-milled 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 for sample Ag ZnO Ag 2 O 2 θ value d value d value d value d value JCPDS File no Fig. 2 DSC trace for Ag 2 O-6.12 wt.% Zn as-blended powder sample , , , 4-783, , Table 3 Data on properties of Ag-8 wt.% ZnO bulk-solid contact materials Sr. No. Processing route Designation code Hot-pressed density, gcc -1 (Percent Theoretical) Property Microhardness, kgmm -2 Electrical conductivity, % IACS 1 Ag-8 wt.% ZnO Conventional blending route 2 Ag-8 wt% ZnO (equivalent to Ag 2 O-6.12 wt% Zn) by Mechanochemical synthesis or Reactive milling route 3 Data on Ag-8 wt% ZnO commercial contact material produced by press-sinterextrude route* for comparison * Metalor Inc., USA A 9.4 (96%) B 9.4 (96%) C 9.82 (100%) 75 83

5 JOSHI et al.: SILVER-ZINC OXIDE ELECTRICAL CONTACT MATERIALS 13 Fig. 3 SEM micrograph for (a) Ag 2 O-6.12 wt.% Zn as-blended and (b) Ag-8 wt.% ZnO 8 h RM powder sample the corresponding data for an equivalent commercial contact material produced by pressing-sinteringextrusion of silver and zinc oxide powder blend 17. For the sake of convenience, the compacts of three different routes are designated as A, B and C. According to this data, the density of the material A and B is same (equal to 96% of theoretical density) whereas that for a commercial product (i. e. material C) is high and equal to 100%. The process route used to consolidate the powders into bulk-solid pieces in the present investigation (for material A and B) has been press-sinter-hot press route whereas that normally used in industry is presssinter-extrude route (i.e. for material C). It is wellknown that the extrusion route always gives higher density levels (close to theoretical density) compared to hot-pressing, in view of higher degree of plastic deformation associated with the hot extrusion process and the resultant high densification. The microhardness data for the material A given in Table 3 shows a significant variation in the microhardness value from 71 to 81 kg mm -2. This can be explained on the basis of the microstructure of the material A, given in Figure 4(a). The microsection shows relatively non-uniform dispersion of zinc oxide (black areas) in silver matrix along with some

6 14 INDIAN J PURE & APPL PHYS, VOL. 45, JANUARY 2007 porosity. The hardness value in the oxide dominated area is higher than that in the rest of the matrix. Material B offers maximum hardness in view of a very fine and uniform dispersion of zinc oxide in silver matrix. The resistance to contact wear improves with increase in the microhardness of the contact member. Improved microhardness of material B can be attributed to greater dispersion hardening of otherwise soft silver matrix by the dispersed oxide phase particles. The electrical conductivity of material B matches well with the material C. The lower value of electrical conductivity of material A is on account of reduced mean free path of the electrons as a result of heterogeneity in the dispersion of oxide phase in silver matrix in such materials. Thus, the material produced under this investigation by the novel mechanochemical synthesis route offers comparable levels of electrical conductivity as normally observed in the case of corresponding commercially developed material. Figure 4(a) and (b) show the SEM micrographs for Ag-8 wt.% ZnO bulk-solid hot-pressed compacts prepared from conventionally blended powder (material A) and mechnochemically synthesized 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)

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