COMPARISON OF PROPERTIES OF SILVER-METAL OXIDEELECTRICAL CONTACT MATERIALS

V. Ćosović*, N. Talijan*, #, D. Živković**, D. Minić***, Ž. Živković**

*Institute of Chemistry, Technology and Metallurgy, University of Belgrade, Njegoševa 12,11000 Belgrade, Serbia

**University of Belgrade, Technical Faculty Bor, VJ 12, 19210 Bor, Serbia***University of Priština, Faculty of Technical Sciences, 38220 Kosovska Mitrovica, Serbia

(Received 01 November 2011; accepted 10 December 2011)

Abstract

Changes in physical properties such as density, porosity, hardness and electrical conductivity ofthe Ag-SnO2 and Ag-SnO2In2O3 electrical contact materials induced by introduction of metal oxidenanoparticles were investigated. Properties of the obtained silver- metal oxide nanoparticlecomposites are discussed and presented in comparison to their counterparts with the micro metaloxide particles as well as comparable Ag-SnO2WO3 and Ag-ZnO contact materials. Studied silver-metal oxide composites were produced by powder metallurgy method from very fine pure silver andmicro- and nanoparticle metal oxide powders. Very uniform microstructures were obtained for allinvestigated composites and they exhibited physical properties that are comparable with relevantproperties of equivalent commercial silver based electrical contact materials. Both Ag-SnO2 and Ag-SnO2In2O3 composites with metal oxide nanoparticles were found to have lower porosity, higherdensity and hardness than their respective counterparts which can be attributed to better dispersionhardening i.e. higher degree of dispersion of metal oxide in silver matrix.

Keywords: silver based electrical contacts, metal oxide nanoparticles, powder metallurgy,physical properties

# Corresponding author: ntalijan@tmf.bg.ac.rs

J o u r n a l o f

M i n i n g a n d

M e t a l l u r g y

J. Min. Metall. Sect. B-Metall.48 (1) B (2012) 131 - 141

1. Introduction

Electrical contact materials based on

silver-metal oxide composites are veryimportant functional materials which arecommonly used in different low voltage

DOI:10.2298/JMMB111101013C

switchgear devices such as contactors,relays, circuit breakers and switches [1-4].Since electrical contact materials are used indiverse exploatation conditions, differenttypes of silver based contact materials havebeen developed in order to meet therequirements for different applications. Theusefulness of an electrical contact material ishence determined by a variety of electricaland mechanical properties, exploatation andload conditions it can withstand andeconomical reasons [4-6]. The essentialproperties for these materials from theperformance point of view are high electricaland thermal conductivity, good mechanicalproperties as well as anti-welding behaviorand oxidation resistance which would ensurepreventing of erosion and welding of thecontact by electric arc generated duringswitching operations.

Ever since Ag-SnO2 and Ag-ZnO contactmaterials have emerged as anenvironmentally-friendly substitute for Ag-CdO they have been continually improvedregarding their performance andapplicability. Commercial production andapplication of the AgSnO2 contact materialsis limited by their rather poor over-temperature behavior due to high contactresistance caused by the coalescence of SnO2particles in surface layers and poorworkability resulting from high hardness andlarger particle size of SnO2 owing to thetraditional manufacturing process [7].Nowadays, it is known that the functionalproperties of the Ag-SnO2 electrical contactmaterials can be improved by addition of asecond metal oxide component of such asIn2O3, Bi2O3, CuO or WO3 which increasedispersion of main oxides (SnO2) in silver

matrix and contribute to the activation ofsintering process [8, 9]. It was found thatsmall addition of In2O3 to Ag-SnO2 causesforemost improvement of mechanicalproperties while keeping the good values ofelectrical conductivity. Alternatively,addition of WO3 improves sinterability andwettability of SnO2 in molten Ag thusimproving the anti-welding and over-temperature behavior [9]. On the other hand,Ag-ZnO materials utilize the high meltingpoint of ZnO resulting in that it’sdecomposing is more difficult than that ofSnO2 under the same exploitationconditions, thus preventing the appearanceof holes, as well as the fact that the ZnOparticles effectively protect against silversputtering [10]. However, apart frompossessing low contact resistance Ag-ZnOmaterials have unsatisfactory resistance towelding and greater tendency to contact wear[11].

In addition to the chemical composition,microstructure has the most important role indetermining properties of electrical contactmaterials. Hence, a general course of actionto improve the anti-welding behavior andwear resistance of these materials is to obtainuniform dispersion of metal oxide particlesin a soft silver matrix. Generally speaking,when additives are used to alter theinteraction of a silver matrix and base oxidesthus altering the materials properties, finermicrostructures are more favorable.Although, the influence of the oxide particlesize on the switching behavior is still notwell defined it is generally accepted thatsmaller metal oxide particles promoteformation of anti-welding characteristics inthe contact surface exposed to the arc upon

V. Ćosović / JMM 48 (1) B (2012) 131 - 141132

the solidification of the molten Ag.Moreover, under certain conditions erosionrate can be decreased by a decrease of themetal oxide particle size. When usingsmaller oxide particles, if there is sufficientwetting by the molten Ag to preventformation of oxide layers the contactresistance can be considered not critical [12].However, there is a limit to reduction of theparticle size given that if the particle spacingis on the order of the mean free electron pathof the conduction electrons a significantreduction in bulk conductivity can beexpected. It was found that the optimalmicrostructure can be achieved by ahomogenous distribution of metal oxideparticles with diameters in range of 50-200nm [12].

The influence of metal oxidenanoparticles on the microstructure andphysical properties such as density, porosity,hardness and electrical conductivity of Ag-SnO2 and Ag-SnO2In2O3 electrical contactmaterials was investigated. Properties of theobtained silver-nanoparticle metal oxidecomposites are discussed and presented incomparison to their microparticle metaloxide counterparts as well as equivalent Ag-SnO2WO3 and Ag-ZnO contact materials.

2. Experimental

2.1 Starting powders

Studied silver based electrical contactmaterials were produced by powdermetallurgy (PM) method from pure silverpowder obtained by chemical synthesis routeand very fine commercial powders (SnO2 -99.9%, In2O3 - 99.99%, ZnO - 99.0%, WO3 -98.5%) as well as commercial SnO2 andIn2O3 nano powders produced by Sigma-Aldrich.

Silver powder of high purity was made viaaqueous precipitation of pure silver fromsilver nitrate by mixing of solutions of silvernitrate and sodium hydroxide with addition offormaldehyde. As particle size of precipitateis affected by reaction kinetics, processingconditions were carefully selected.

Particle size distribution of the producedsilver powder was analyzed using MalvernInstruments laser diffractometer Mastersizer2000 with the Scirocco 2000 module, whilemorphology was investigated by means ofscanning electron microscopy (SEM).Corresponding SEM image and particle sizedistribution curve are presented in Fig. 1a andFig. 1b, respectively.

V. Ćosović / JMM 48 (1) B (2012) 131 - 141 133

a) b)Figure 1. Obtained silver powder: a) SEM image and b) particle size distribution curve

The particles show rather narrow sizedistribution with the mean particle size of thesilver powder determined by laserdiffractometry having following values:d(0.1) = 1.126 μm, d(0.5) = 1.521 μm andd(0.9) = 4.237 μm.

Particle size and shape of the usedcommercial SnO2 and In2O3 nano powderswas analyzed by means of transmissionelectron microscopy (TEM). The obtainedTEM image of used SnO2 nano powder(Fig.2a) reveals the particle size in range 20-50 nm, while TEM image In2O3 nanopowder (Fig. 2b) demonstrates the particlesize in range 50-100 nm.

2.2 Sample preparation and processingconditions

Powder metallurgy method (PM) wasselected for preparation of the investigatedsamples since it provides uniformmicrostructure, high electrical conductivityand gives possibility for quick and easyproduction of samples with a variety of

chemical compositions i.e. possibility ofproduction of silver based compositescontaining two or more metal oxides.Compositions of the prepared samples aregiven in Table 1.

The technological procedure included dryand wet homogenization of powdermixtures, pressing, sintering, additionalmechanical treatment (forging) andcharacterization. Since the starting powderswere in the form of agglomerates consistingof very fine particles, homogenization bothwet and dry was done in several steps.Uniformity of the obtained mixtures wascontrolled using scanning electronmicroscopy. The samples were pressed intotablets Ø16 × 3 mm by hydraulic press underthe pressure of 100 MPa in a steel dye. Thesamples were sintered for 2h at 800oC inelectro-resistive oven with programmabledigital temperature controller with theaccuracy ±1oC in the air atmosphere. Aftersintering, the samples were forged (at 800oC)with the low degree of reduction (~15%).Subsequently the samples were annealed at750oC for 30 min and then quenched inwater.

V. Ćosović / JMM 48 (1) B (2012) 131 - 141134

a) b)Figure 2. – TEM images of used commercialnano powders: a) SnO2 and b) In2O3

Table 1 – Composition of the investigatedsilver-metal oxide electrical contact materials

No SampleComposition

Ag SnO2 In2O3 WO3 ZnO

1 Ag-SnO2 92 8 - - -

2 Ag-SnO2 nano 92 8 - - -

3 Ag-SnO2In2O3 89.1 8 2.9 - -

4 Ag-SnO2In2O3 nano 89.1 8 2.9 - -

5 Ag-SnO2WO3 90 9.5 - 0.5 -

6 Ag-ZnO 92 - - - 8

2.3 Characterization

Microstructure of the samples aftersintering and mechanical treatment andchemical composition of the observed phaseswere studied on polished cross-sectionsurfaces using JEOL JSM-6610LV scanningelectron microscope equipped with OxfordInstruments energy dispersive spectrometer(EDS). Density of the obtained samples wasdetermined by standard methods. Applyingthe procedure given in more detail in [13]theoretical density of samples was calculatedand by comparison between experimentaland theoretical densities the porosity ofsamples was determined.

Theoretical density of composites, ρt, wascalculated according to relation [13-15]:

(1)

where V is volume fraction, ρ – densitywhile f and m indexes correspond to filler(metal oxide powder) and matrix (silverpowder), respectively.

Porosity of the investigated composites, τ,was determined by comparison ofexperimental and theoretical densities of thesamples according to relation [13-15]:

(2)

where ρe – is experimentally obtainedvalue of composite density.

Hardness measurements were carried outafter sintering and mechanical treatment onpolished samples at room temperature usinga Vickers hardness tester applying load of 5kp. The reported hardness values are an

average of three readings. Electricalconductivity of the investigated materialswas measured using Foerster SIGMATEST2.069 eddy current instrument formeasurements of electrical conductivity ofnon-ferromagnetic metals based on thecomplex impedance of the measuring probe,with the 8 mm probe.

3. Results and discussion

Microstructures of the investigated silver-metal oxide electrical contact materials areillustrated by SEM images (Fig.3) of thepolished cross-sections of the producedsamples after sintering, mechanicaltreatment and subsequent annealing. Thepresented images demonstrate thathomogeneous distribution of the metal oxideparticles in the silver matrix i.e. very uniformmicrostructures with reasonably smallporosity were obtained for all investigatedmaterials. This is important from the point ofview that uniformly dispersed oxide particlescan strengthen the otherwise soft silvermatrix resulting in higher hardness,improved anti-welding behavior and wearresistance.

Combined SEM-EDS analysis providedbetter insight into the microstructure of theinvestigated silver based electrical contactmaterials. Considering that secondaryelectron SEM images provide moreinformation regarding the topography andmorphology i.e. almost 3d images of theobserved surfaces, they were used toillustrate presence of pores in the obtainedmicrostructures. The EDS analysis was usedfor determination of the composition of theobserved phases and the identified phases are

V. Ćosović / JMM 48 (1) B (2012) 131 - 141 135

t f m f f

t

V V 1

t e

t

100

marked on presented figures.Fig.4 shows microstructures of the Ag-

SnO2 composite material with micro (sample1) and nano metal oxide particles (sample 2).As suggested by Fig. 3a and Fig. 3b, themicrostructures of the sample 1 and sample 2are quite similar. Only at highermagnifications differences in microstructure

become apparent, as it is illustrated by anenlarged detail of the obtainedmicrostructure of the sample 2 given in Fig4b which reveals better dispersion of SnO2nanoparticles in silver matrix and lowerporosity of the composite.

More noticeable differences between themicrostructures can be observed on the

V. Ćosović / JMM 48 (1) B (2012) 131 - 141136

Fig. 3– SEM metallographic images of the polished cross-sections of the investigated silver-metal oxide composites

images of the Ag-SnO2In2O3 samples withmicro and nano metal oxide powderspresented in Fig.5a and Fig.5b, respectively.Besides the obvious absence of the coarseIn2O3 particles, the microstructure of thesample with nano metal oxide powders givenin Fig. 5b appears to be much finer and lessporous. Better connectivity of the silvergrains can be observed as well.

Microstructures of the investigated Ag-SnO2WO3 (sample 5) and Ag-ZnO (sample

6) materials are illustrated by secondaryelectron SEM images presented on Fig. 6with marked chemical compositions. Fig. 6ademonstrates more or less the same structureof the sample as of the other analyzedsamples, with SnO2 and WO3 particlessituated predominantly on silver grainboundaries.

However, in contrast to other investigatedmaterials, within microstructure of thesample 6 (Fig. 6b) zinc oxide particles that

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a) b)Fig. 4. SEM images with results of EDS analysis of AgSnO2 samples with

a) micro (sample 1) and b) nano metal oxide particles (sample 2)

a) b)Fig. 5. SEM images with results of EDS analysis of Ag-SnO2In2O3 samples with

a) micro (sample 3) and b) nano metal oxide particles (sample 4)

are present in the silver matrix can be foundboth on grain boundaries and dispersedinside the silver grains. Given that prior tosintering, during mixing and pressing stages,the contact surfaces between powderparticles are plentiful of pores and defectssome of the finer ZnO particles could bewedged between larger Ag particles. Some ofthese contact surfaces will turn into grainboundaries and some will disappeargradually during sintering [16], henceenclosing ZnO particles inside the grain.Consequently, this provides an increase ofthe degree of dispersion which could lead toan improvement in material’s performance.

The physical properties of theinvestigated contact materials after sintering,mechanical treatment and subsequentannealing are compared in Table 2.

It can be noticed that the relatively highvalues of electrical conductivity wereobtained despite the existence of porosity inthe materials (Table 2.). Particularly sincepores are potential scattering centers for theelectrons and consequently reduceconductivity [17]. Furthermore, good valuesof hardness can be attributed to the highdensity and very uniform distribution of themetal oxides in silver matrix.

Both materials containing metal oxide

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a) b)Fig. 6. SEM images with results of EDS analysis of a) Ag-SnO2WO3 (sample 5) and

b) Ag-ZnO (sample 6)

No SampleDensity [g/cm3]

Porosity [%]

Hardness [HV] 5 kP

Electricalconductivity [% IACS]

1 Ag-SnO2 9.37 7.08 68 68.61

2 Ag-SnO2 nano 9.89 1.95 81 56.11

3 Ag-SnO2In2O3 9.4 6.14 73 53.02

4 Ag-SnO2In2O3 nano 9.54 4.74 75 56.98

5 Ag-SnO2WO3 9.32 7.33 71 52.63

6 Ag-ZnO (92-8) 9.61 2.08 82 66.63

Table 2. Physical properties of the investigated contact materials

nanoparticles (samples 2 and 4) exhibithigher values of hardness than theircounterparts (samples 1 and 3). The observedenhancement of hardness (Table 2.) can beattributed to a greater dispersion hardeningof soft silver matrix by the dispersed oxidephase nanoparticles. This assumption issupported by the SEM images of thecorresponding microstructures presented onFig. 4 and Fig. 5 which also demonstratebetter dispersion of metal oxidenanoparticles.

Higher value of electrical conductivity ofthe sample 1 containing micro SnO2 particles(Table 2.) can be explained by lowerdispersion of metal oxide particles andexistence of connected pure silver grains(oxide free zones) which would attribute tohigher electrical conductivity on one side butalso derogate mechanical properties on theother, as it is illustrated by lower values ofhardness.

Favorable physical properties and verygood electrical performance exhibited by theinvestigated Ag-ZnO contact material(sample 6 in Table 2.) can be ascribed togreater dispersion of very fine ZnO particlesin silver matrix as it is illustrated by thecorresponding SEM image of the sample 6microstructure presented on Fig. 6b.

It should be pointed out that besideschemical composition and particle sizeprocessing conditions play a very importantrole in determining final properties of thePM produced materials [18]. The significantinfluence of the sintering regime andmechanical treatment on the electricalconductivity of the investigated silver metaloxide composites is demonstrated on Fig. 7,illustrating difference in electrical

conductivity of green compacts and samplesafter applied sintering regime andsubsequent forging.

Considering that it well known that apartfrom sintering regime applied pressureduring consolidation stage determinesphysical properties of materials (porosity,bulk density and hardness) furtherenhancement of materials properties ispossible by application of higher pressure forsample processing. Many published studiessuggest that pressure even up to 900 MPa[10] can be applied.

The presented experimentally obtainedresults of microstructural analysis anddetermined mean values of density, hardnessand electrical conductivity of theinvestigated electrical contact materials arecomparable to each other and at the sametime comparable to the same characteristicsof the equivalent commercially availableelectrical contact materials, particularly Ag-CdO. This is significant, considering that the

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Fig. 7. Electrical conductivity of theinvestigated silver metal oxide compositesbefore and after sintering and subsequentmechanical treatment

existing EU directives (RoHS, WEEE)restrict the use of potentially hazardoussubstances among which Cd and necessitateintroduction of more environmentallyfriendly electrical contact materials such assilver-metal oxide composites investigated inthis study.

4. Conclusion

The influence of metal oxidenanoparticles on microstructure and physicalproperties such as density, porosity, hardnessand electrical conductivity of the Ag-SnO2and Ag-SnO2In2O3 electrical contactmaterials was investigated. Properties of theobtained silver- nanoparticle metal oxidecomposites are discussed and presented incomparison to their counterparts with themicro metal oxide particles as well ascomparable Ag-SnO2WO3 and Ag-ZnOcontact materials. The microstructuralanalysis revealed that very uniformmicrostructures of the all investigatedcomposites were obtained. This resulted indensity, hardness and electrical conductivityof the all investigated composites beingcomparable with relevant properties ofequivalent commercial silver based electricalcontact materials. Another important point isthat the analyzed properties of theinvestigated materials are in the same rangeof values as those of Ag-CdO, particularlysince they are considered as its moreenvironmentally friendly substitute. Incomparison to materials with micro oxideparticles, both Ag-SnO2 and Ag-SnO2In2O3composites with metal oxide nanoparticlesare less porous and exhibit higher degree ofdispersion of metal oxide in silver matrix.

Moreover, the both materials containingmetal oxide nanoparticles possess highervalues of hardness and density than theirrespective counterparts. The observedenhancement of hardness can be attributed toa greater dispersion hardening of otherwisesoft silver matrix by the dispersed oxidephase nanoparticles. High hardness and verygood electrical performance exhibited by theinvestigated Ag-ZnO contact material can beascribed as well to the greater dispersion ofvery fine metal oxide particles in silvermatrix. This is very significant especiallyfrom an application point of viewconsidering that better dispersion of oxidesand higher hardness can improve anti-welding behavior and wear resistance.

It is expected that the differences inproperties and performance of investigatedelectrical contact materials with micro- andnano- metal oxide particles will becomemore apparent after further tests on surfaceerosion and welding resistance which wouldillustrate the benefits of introduction of metaloxide nanoparticles in silver matrix to thefull extent.

Acknowledgement

This work has been supported by theMinistry of Education and Science of theRepublic of Serbia under project ON172037.

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