Powder Technology 256 (2014) 251–256

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Powder Technology

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Effects of atmosphere and milling time on the coarsening of copperpowders during mechanical milling

Babu Madavali a, Jin-He Lee a, Jin Kyu Lee a, Kuk Young Cho a, Suryanarayana Challapalli b,⁎, Soon-Jik Hong a,⁎⁎a Division of Advanced Materials Engineering & Institute for Rare Metals, Kongju National University, Cheonan 331-717, Republic of Koreab Department of Mechanical and Aerospace Engineering, University of Central Florida, Orlando, FL 32816-2450, USA

⁎ Corresponding author.⁎⁎ Corresponding author. Tel.: +82 41 521 9387; fax: +

E-mail addresses: Surya@ucf.edu (S. Challapalli), hongs

http://dx.doi.org/10.1016/j.powtec.2014.02.0190032-5910/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 October 2013Received in revised form 4 February 2014Accepted 6 February 2014Available online 15 February 2014

Keywords:Ultra-fine copper powderMechanical millingMorphologyParticle coarsening

The coarsening behavior of copper nanopowders (50–150 nm in size) duringmechanical milling was investigat-ed as a function of milling atmosphere and milling time. Pure copper powders were milled under argon and airatmospheres for up to 80 h and the crystal structure, microstructure, and composition of the milled powderswere investigated. It was shown that on milling the powders for 80 h under air atmosphere, the particle sizewas in the range of 7 to 80 μmdue to sphere-to-sphere and/or sphere-to-flakewelding and from the aggregationof the copper particles duringmilling. On the other hand, theparticle sizewas only 7 to 35 μm,whenmilled underargon atmosphere even after 80 h of milling. There was no difference either in the crystal structure or latticeparameter of the copper powder after milling in the two atmospheres. The powders got contaminated withoxygen; the oxygen content increased with increasing milling time, reaching up to 4.97 wt.% when milled inair for 80 h. The oxygen got incorporated into the copper powders to form oxide layers, which were brokendown duringmilling and thus played an important role in determining the final particle size and microstructureof the powder. A viable explanationwas provided to explain the differences in themicrostructure and sizes of thepowder particles milled under the argon and air atmospheres.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Copper powders have been used for a long time in a wide variety ofapplications such as electronic devices including hybrid integrated cir-cuits and multilayer ceramic capacitors, paints to make a non-metallicmaterial appear to have metallic-like luster, catalysts, coatings toprevent wear and corrosion, and printer electronic technology [1,2].Currently, in the electrical connector technology, there is an increasedinterest in synthesizing pure copper powders owing to their highlydesirable properties such as high electrical conductivity and excellentsolderability, in addition to the high melting point and low materialcost. Due to these reasons, researchers have focused their attention oncopper powders instead of preciousmetals such as silver, gold, platinumor palladium, which are very expensive.

A number of differentmethods have been used to synthesize coppermetal powders. These include atomization [3], electrolysis [4], pyrolysis[5], chemical reduction methods [6], and ball milling (mechanicalmilling) [7–10]. Among these, the mechanical milling (MM) methodhas been very effective in the fabrication of copper powders. In thistechnique, the repeated ball-to-powder collisions produce severe plas-tic deformation of the powder and introduce significant amount of

82 41 568 5776.j@kongju.ac.kr (S.-J. Hong).

strain into the powder. This results in crystallite fracture into smallerparticles down to the nanometer level. Such powders exhibit excellentphysical, chemical and mechanical properties [11,12].

Pure monolithic copper powders and composites based on copperhave been synthesized by milling methods [13–21]. Zuhailawati andMahani [18] synthesized in situ copper-based composites by high ener-gy ball milling and showed that the NbC particles experience crystalliterefinement and lattice strain due tomilling. Reyter et al. [16] studied themicrostructure and electrocatalytic reduction of nitrate on micron cop-per powders throughMMunder air and argon atmospheres. In additionto these, several researchers have successfully used the milling methodto produce nanocomposite materials and bulk nanostructured metals[14,15,21].

Even though themorphology andmicrostructure of copper powderssynthesized by mechanical methods were reported earlier, there havebeen only a few studies on the effect of process parameters such asmill-ing atmosphere and the role of oxygen contamination on the micro-structure of the milled powders. Furthermore, while there have beensome investigations on themilling of coarse copper powders to synthe-size finer and nanocrystalline powders, there have been no studies onthe effect of milling on the structure of nanocrystalline powders.

There have been some studies on synthesizing nanocomposite pow-ders by milling of coarse-grained metal powders and nanocrystallineceramic powders [21]. There have also been reports where continuedmilling of powders for long times have resulted in structural changes

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in addition to morphological changes in powders [8,9]. These includecrystallization of amorphous powders (termed as mechanical crystalli-zation) [22,23], inducing of polymorphic transformations [9,24,25],and cyclic transformations from crystalline to amorphous states[26,27]. Therefore, it will be interesting to observe if any changesoccur during milling of nanocrystalline powders, which is the subjectof the present investigation. As an exploratory preliminary study, wehave employed nanocrystalline pure copper powders.

In this paper, we report on the structure of mechanically millednanocrystalline copper powdersmilled at room temperature and reporton the contamination, particle size, and microstructural features of thecopper powders. The main intention was to analyze the effect ofambient atmosphere (air and argon) and milling time on the structureof the copper powder. Further, the influence of oxygen on the coarsen-ing behavior of the copper nanopowder during MM was also studied.

2. Experimental procedure

Mechanical milling of copper nanopowders (purity better than99.0%) with a particle size in the range of 50 to 150 nmwas conductedin a conventional ball mill for differentmilling times under argon atmo-sphere. 33.3 g of the copper powderwas loaded into a stainless steel vialsealed under argon atmosphere. Stainless steel balls of 5.54 mm indiameter corresponding to a ball to powder weight ratio (BPR) of 15:1were also loaded and the powder was milled at a speed of 400 rpm.The same experimental conditions were also maintained for millingthe copper powder under air atmosphere.

The milled powder was collected at the end of milling and weighedin order to determine the powder yield. More than 90% of yield wasachieved in all the milling experiments under both the air and argonatmospheres.

X-ray diffraction (XRD) patterns were obtained using an X-raydiffractometer (Rigaku, MiniFlex-600) with Cu Kα radiation. The mor-phology of the copper powders was studied using a scanning electronmicroscope (SEM-MIRA LMH II (TESKAN)). The contamination in themilled copper powders was analyzed using two different techniques.One was the energy dispersive spectroscopy (EDS) method in theSEM. The other was by using the PC-controlled ONH-2000 oxygen/hydrogen/nitrogen analyzer (ELTRA-ONH-2000). But, because of thebetter accuracy, only the results of analysis by the oxygen/hydrogen/nitrogen analyzer will be reported here.

3. Results and discussion

3.1. Powder morphology

Fig. 1 presents the SEMmicrographs of the pure copper nanopowderin the as-received (un-milled) condition. The powder is characterizedby spherical morphology and the particle size is in the range of 50 to150 nm, in conformity with the supplier's data.

2 µm

a

Fig. 1. Morphology of the initial copper powder in the as-received (un

Fig. 2 shows the morphology of the copper powder obtained aftermilling for 30 min, 10 h, and 80 h, under both air and argon atmo-spheres. On milling the powder for 30 min under air atmosphere, theinitial spherical powder continued to be almost spherical in shapewith rough surfaces. As is well known [8], during milling the powderparticles get trapped between the grinding media and the containerwalls, and undergo severe plastic deformation. However, it may benoted that due to the relatively ductile nature of the Cu powder, coldwelding has predominated and therefore, even on milling for about30 min, the particle size had increased to about 5 to 25 μm. On millingfor 10 h, the size of the copper powder was found to be coarser andthe morphology had changed to plate-like shape. On milling for 10 h,more than half of the spherical powders got flattened due to plasticdeformation and they got coalesced when the boundaries betweentwo surfaces (flattened and tiny spherical Cu powder) were broughttogether and became a large single particle [8,9]. Thus, the change inshape and coarsening of copper powders could be explained as due tothe coalescence of the smaller particles through coldwelding operations[28]. After the long milling time of 80 h, the copper powder was evencoarser and the morphology of the powder changed to plate-like thinsheets (as shown in Fig. 2(e)) due to the dominant effect of welding. Itmay also be noted that the particles are rather flattened due to thestrong plastic deformation experienced by the particles [8].

To study the influence of the milling atmosphere on the size andmorphology of the copper nanopowders, MM was also carried outunder argon atmosphere and the results are shown in Fig. 2(b), (d),and (f). In conformity with the results of the powder milled under airatmosphere, the powdermilled for 30min under the argon atmospherealso showed that the powder consisted of a monodispersed powderwith a spherical shape and with a particle size of about 5 to 20 μm. Onmilling for 10 h under argon atmosphere, it was observed that thecopper powder showed coarsening due to the aggregation of the softcopper particles. But, it should be realized that the extent of coarseningunder argon atmosphere was less in comparison to the powder milledunder air atmosphere. On increasing themilling time to 80h, the copperpowders were further coarsened. But, an important difference is thatwhile the powder milled under air showed thin sheet-like morphology,the powder milled under argon atmosphere was much less plate-like.This can be attributed to the argon atmosphere preventing contamina-tion of the powder surface. However, under both atmospheres, thesizes of the particles increased onmilling from 30min to 80 h ofmillingtime. Thus, it appears from these results that processing of the copperpowder under the air atmosphere shows larger particles with plate-like morphology. As will be shown later, the oxygen content in thepowders milled under air atmosphere is higher than that milled inargon atmosphere. Therefore, it is expected that the powder particlesmilled in air atmosphere will be covered with a thin oxide layer, makethem brittle and therefore should result in finer particles. In additionto oxygen content making the powder particles brittle, it should alsobe noted that the deformation characteristics of ultrafine particles willbe different from those of coarse powders. It is noted that there is a

500 nm

b

-milled) condition. (a) Low magnification, (b) high magnification.

50 µm50 µm

50 µm 50 µm

50 µm 50 µm

Air atmosphere Argon atmosphere

a

fe

dc

b

Fig. 2.Morphology of the copper powder obtained after high energy ball milling for (a) 30min, (c) 10 h and (e) 80 h under air atmosphere; (b) 30min, (d) 10 h and (f) 80 h under argonatmosphere.

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limit, referred to as the limit of comminution, belowwhich fracturing ofbrittle particles stops [29]. That is, very fine and hard particles deformplastically rather than fracturing. Consequently, the particle size ofpowders milled in the air atmosphere is larger than that milled inargon atmosphere. Even though, a small amount of oxygen may bepresent in the argon atmosphere, it is almost negligible in comparisonto that in the air atmosphere. Thus, the result showed that argon atmo-sphere is more preferable than the air atmosphere for the fabrication ofspherically coarse copper powders.

Fig. 3 presents high magnification micrographs showing the surfacemorphology of copper powdersmilled for different times in the two dif-ferent atmospheres. A comparison of the two sets ofmicrographs clearlyreveals that the powders milled in the argon atmosphere [Fig. 3(b), (d),and (f)] show copper lumps, which are denser than those milled in theair atmosphere [Fig. 3(a), (c), and (e)]. Formation of such large sphericalparticles is mainly due to the sphere-to-sphere and/or sphere-to-flakewelding of smaller particles [11,13,30]. Since the powders milled inthe argon atmosphere experience much less contamination from oxy-gen (and therefore they are more ductile), it is easy for them to getcold welded and therefore they end up as dense particles. On theother hand, due to contamination from oxygen, the particles milledunder the air atmosphere tend to be brittle since they are harder. Itmay be easily appreciated that it is very difficult to pack brittle (andirregular) particles in a dense manner. Consequently, powders milledin air atmosphere end up as porous particles. However, cold welding

also takes place and that is why the particle size increases. Thus, itappears that fabrication of copper powders under argon atmosphereproduces denser and pore-free powder than when milled under theair atmosphere.

Fig. 4(a) and (b) shows the minimum, average, and maximum sizesof the spheroidal copper powder particles as a function of the millingtime when milled under air and argon atmospheres. These size datawere obtained from micrographs similar to those presented in Figs. 2and 3. Considering the size of the particles of powders milled in airatmosphere [Fig. 4(a)], it is clear from Fig. 3(a) that on milling for30 min, the starting powder was spherical in shape and as millingproceeded, the powder particles got welded together to form thickflake-like solids and on milling for 10 h, particles with a size range of8–35 μmwere obtained. After 80 h ofmilling, the powder became coars-er and large plate-like powderswith amaximum diameter of 75–80 μmwere obtained. However, in the case of 10 h milling under air atmo-sphere, the copper powder was smaller in size than that milled for30 min. Nevertheless, the size of the particles was noted to increasefrom 10 h to 80 h.

As may be noted from Fig. 4(b), the sizes of the copper powdersincreased with increasing milling time, because of the repeated coldwelding, fracturing, and re-welding events due to the severe plasticdeformation experienced; even though the cold welding events pre-dominated. This is similar to what was noted when the powders weremilled under air atmosphere. However, the size of the copper powder

Air atmosphere Argon atmosphere

a

fe

dc

b

2 µm

2 µm

2 µm

2 µm

2 µm

2 µm

Fig. 3. Surfacemorphology of the copper powders obtained after high energy ballmilling for (a) 30min, (c) 10 h and (e) 80 h under air atmosphere; (b) 30min, (d) 10h and (f) 80 h underargon atmosphere.

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milled under argon atmosphere was smaller than that milled under airatmosphere. For example, the average size of the copper powder parti-cles milled under argon atmosphere was about half of that milled underair atmosphere. This was ascribed to the low weight percent of oxygenin powders milled under argon atmosphere and ductile deformationbehavior of ultrafine powder particles.

3.2. Crystal structure

Fig. 5(a) represents the XRD patterns of the copper powders milledfor different milling times under both argon and air atmospheres. Itmay be noted that the same trend is observed for the powders milledunder both atmospheres. Note that the intensity of the peaks decreasedand the full width half maximum (FWHM) increased with increasingthe milling time from 10 h to 80 h; attributed to a reduction in theparticle/crystallite size and accumulation of lattice strain, as a result ofmilling [8,9]. The FWHM was calculated with Gaussian curve fittingmethod; these values increased from 0.215 to 0.250 under air atmo-sphere and from 0.245 to 0.256 under argon atmosphere, respectively,when milled for 10 h to 80 h. Fig. 5(b) shows enlarged views of the(111) peak (in the 2θ range of 42–45°) at different milling times inboth atmospheres, and the lattice parameters were calculated fromthese peak positions. There was no difference in the lattice parameterof the powder with milling time and also whether milled in argon orair atmosphere. The presence of the (111), (200) and (220) peaks

confirms that the face-centered cubic structure of the copper powderis also retained after milling [31]. No other XRD peaks correspondingto phases such as CuO and Cu2O were observed, suggesting that thesynthesized copper powder was reasonably pure and that it did notpick up excessive amounts of impurities to form new phases. In spiteof this, it is important to realize that some amount of powder contami-nation occurred during milling.

There have been instances in the literature where the mechanicallyalloyed powders exhibit changes in crystal structure due to phasetransformations [8,9,24,25]. These include pure metals, which mayexhibit polymorphic changes or intermetallics, where either the high-temperature or high-pressure phases are stabilized. This may happendue to an increase in the free energy of the system due to a reductionin particle size and consequent increase in surface energy. However, inthe case of copper metal, there are no polymorphic transformationsand therefore we did not observe any change in the crystal structureof the copper powders after mechanical milling.

3.3. Powder contamination

The contamination in the milled copper powders was analyzedusing the ONH-2000 oxygen/hydrogen/nitrogen analyzer. Table 1presents the oxygen content in the copper powder as a function of themilling time under air and argon atmospheres. It is noted that theamount of oxygen increased with increasing milling time under both

0

10

20

30

40

50

60

70

80

90

Po

wd

er s

ize

(µm

)P

ow

der

siz

e (µ

m)

MiIling Time

minimumAverageMaximum

0

10

20

30

40

50

60

70

80

Milling time

MinimumAverageMaximum

30 min

b

a

10 h 80 h

30 min 10 h 80 h

Fig. 4. Variation of the powder size with milling time (a) under air atmosphere, (b) underargon atmosphere.

a

b

Fig. 5. (a) XRD patterns of the copper powder as a function of milling time under air andargon atmospheres. (b) Enlarged view of the (111) peak in the 2θ range of 42–45°.

Table 1Oxygen content (wt.%) in the milled copper powder, analyzed by the oxygen/nitrogen/hydrogen analyzer, as a function of milling time under air and argon atmospheres.

Milling time (h) Air atmosphere (wt.%) Argon atmosphere (wt.%)

0 (initial powder) 1.71 1.710.5 4.50 3.3210 4.72 3.4580 4.97 4.07

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atmospheres, even though the actual amount of oxygen uptake by themilled powder is different under the argon and air atmospheres. Forexample, the amount of oxygen was gradually increased from 4.50% to4.97% in powders milled for 30 min and 80 h, respectively when milledin the air atmosphere. These values were smaller and increased from3.32% to 4.07% when milled for 30 min to 80 h, respectively, whenmilled in argon atmosphere. A similar situation was also obtainedwhen Ti and Ti alloy powders weremilled in argon and air atmospheres[32,33]. Thus, the oxygen uptake will be much less when the powdersare milled in argon atmosphere.

Fig. 6 shows how the atmosphere will have an effect on the coarsen-ing of copper nanopowders. During the early part ofmilling (first stage),the copper powder is coarsening from the starting nanopowder due tothe predominance of cold welding; this appears to be true under bothatmospheres. During the early period of the second stage, neck growthseems to be occurring and the shape of the powder is irregular (plate-like) and the size is increasing with increasing milling time under theair atmosphere. However, in the case of milling under argon atmo-sphere, individual copper powder particles are present even at 10 h ofmilling and these are combined to form small irregular shapes duringthe later part of the second stage. In addition, during this stage, theoxygen content has increased with increasing milling time as shownin the oxygen analyzer data in Table 1. In the final stage, the contamina-tion of the powder with oxygen increases at long milling times (80 h)and this gets incorporated into the powder forming oxide layers onthe powder surfaces (shown at the bottom in Fig. 6). It may be noted

that oxygen acts as a process control agent [8]. Because of this, theagglomerated coarse powders get cracked and broken down and even-tually form flake-like powders [16]. Even though copper is ductile, itgets work hardened due to the severe plastic deformation experiencedduring ball milling, and the powder becomes brittle and breaks downinto flakes. This strengthening occurs because of dislocationmovementsand dislocation generation within the structure of the material duringmilling. However, the powders milled under argon atmosphere do notturn into flake-shaped particles because of the low oxygen content.

4. Conclusions

The effect of milling atmosphere and milling time on the coarseningbehavior of copper nanopowders during mechanical milling wereexamined in the present work. Coarse and plate-like thin solids with amaximum diameter of about 75–80 μm were produced after 80 h ofmilling under the air atmosphere due to sphere-to-sphere or sphere-to-flake welding during milling. In addition, due to the heavy contami-nation of the powderwith oxygen at longmilling times, the oxygen getsincorporated into the powder to form thick surface oxide layers, whichthen break up the agglomerated powders to flake-like shapes duringmilling under air atmosphere. In contrast to the air atmosphere, thepowder shows the formation of plate-like shape and form irregularthick solid spheres under the argon atmosphere with a maximum

Atmosphere 1st Step

nanoturned into micron powder

copper powderNeckgrouped into single powders

formation and then

Neckgrouped into single powders

formation and thenWeak oxygenlayers

Hugeamount of

dislocations

High oxygenlayers

Crackformation

Hugeamount

ofdislocations

nanoturned into micron powder

copper powder

2nd Step 3rd Step

Plate shape formed

Irregular shape formed

Air Atmos-phere

ArgonAtmos-phere

+

Fig. 6. Schematic diagram showing the stages of coarsening of the copper powder in different atmospheres.

256 B. Madavali et al. / Powder Technology 256 (2014) 251–256

diameter of about 30–35 μm after milling for 80 h. This can be ascribedto the thin oxygen layers due to low levels of contamination whenmilled under argon atmosphere. From the experimental results, it isconcluded that the argon atmosphere is more preferable than the airatmosphere for the processing of copper powders by milling methods.

Acknowledgment

This research was supported by the Basic Science Research Programthrough the National Research Foundation of Korea (NRF) funded bythe Ministry of Education, Science and Technology (NRF-2008-331-D00746).

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