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 Joshi1, 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: 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, co-

precipitation, sol-gel process, electroless deposition

and internal oxidation as alloy powders i.e. IOAP

process, etc3-8

.

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 superalloys9. 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

Milling10

(RM). Schaffer and McCormick11

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 applications12-14

. 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, etc15

.

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.

INDIAN J PURE & APPL PHYS, VOL. 45, JANUARY 2007

10

2 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

Ag2O-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 52100 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 0o-80

o at a scan

speed of 0.1269o s

-1 using Cu target and Cu-Kα

radiation of 0.15406 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 Ag2O-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 as-

blended and mechanochemically synthesized (i.e. 8 h

milled) Ag2O-6.12 wt.% Zn powders. The XRD

profile for the as-blended Ag2O-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, Ag2O and ZnO. The

underlying mechanism for this change in constituent

phases may be explained as follows. The oxygen

liberated on account of reduction of Ag2O by Zn

during the course of reaction milling reacts with zinc

powder particles close to the Ag2O 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 as-

blended Ag2O-6.12 wt.% Zn powder. Tables 1 and 2

give XRD data for different phases present in the as-

blended and reaction-milled powder samples.

Thus, the XRD analysis confirms the

mechnochemically driven oxidation/reduction

reaction taking place between the Ag2O 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 method16

. The

crystallite size was found to be of the order of 25 nm.

A representative DSC scan for Ag2O-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

JOSHI et al.: SILVER-ZINC OXIDE ELECTRICAL CONTACT MATERIALS

11

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 Ag2O-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.

Fig. 1(a) XRD profile for Ag2O-6.12 wt.% Zn as-blended powder sample; (b) XRD profile for Ag-8 wt.% ZnO 8 h RM powder sample

INDIAN J PURE & APPL PHYS, VOL. 45, JANUARY 2007

12

Ag2O 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 3Data on properties of Ag-8 wt.% ZnO bulk-solid contact materials

Property Sr. No. Processing route Designation

code Hot-pressed density, gcc-1

(Percent Theoretical)

Microhardness,

kgmm-2

Electrical conductivity,

% IACS

1 Ag-8 wt.% ZnO Conventional

blending route A 9.4 (96%) 71-81 77

2 Ag-8 wt% ZnO (equivalent to

Ag2O-6.12 wt% Zn) by Mechanochemical

synthesis or Reactive milling route

B 9.4 (96%) 84 82

3 Data on Ag-8 wt% ZnO commercial

contact material produced by press-sinter-

extrude route* for comparison

C 9.82 (100%) 75 83

*www.Metalor.com, Metalor Inc., USA

Table 1XRD data for the diffraction peaks of

Ag2O-Zn as-blended powder sample

Value as per standard Observed value

for sample Ag2O phase Zn Phase

JCPDS

File no

2θ value d value d value d value

33.15

33.89

2.73

2.70

2.73 - 12-793

36.47

37.27

2.46

2.41

-

-

2.47 4-831

38.35 2.34 - 2.30 4-831

43.42

44.55

2.08

2.03

-

-

2.09 4-831

54.97 1.67 1.67 - 12-793

64.65 1.44 1.43 - 12-793

68.41 1.37 1.37 - 12-793

70.92 1.32 - 1.33 4-831

77.62 1.22 - 1.23 4-831

Fig. 2DSC trace for Ag2O-6.12 wt.% Zn as-blended

powder sample

Table 2XRD data for the diffraction peaks of Ag-ZnO reaction

milled powder sample

Value as per standard Observed value

for sample Ag ZnO Ag2O

JCPDS

File no

2 θ value d value d value d value d value

33.37

34.21

34.59

2.68

2.62

2.59

- 2.66 2.73 21-1486,

12-793

38.55 2.33 2.36 - 2.37 4-783,

12-793

43.74

44.65

2.06

2.02

2.04 - - 4-783

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,

12-793

77.75 1.22 1.23 - - 4-783

JOSHI et al.: SILVER-ZINC OXIDE ELECTRICAL CONTACT MATERIALS

13

the corresponding data for an equivalent commercial

contact material produced by pressing-sintering-

extrusion of silver and zinc oxide powder blend17

. 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 press-

sinter-extrude route (i.e. for material C). It is well-

known 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

Fig. 3SEM micrograph for (a) Ag2O-6.12 wt.% Zn as-blended and (b) Ag-8 wt.% ZnO 8 h RM powder sample

INDIAN J PURE & APPL PHYS, VOL. 45, JANUARY 2007

14

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. 4SEM 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

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.

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17 www.Metalor.com, Metalor Inc., USA.