Nanoparticles Review.pdf

8/22/2019 Nanoparticles Review.pdf

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CHAPTER 1

Nanoparticles from Mechanical Attrition

Claudio L. De Castro, Brian S. Mitchell

Department of Chemical Engineering, Tulane University,New Orleans, Louisiana, USA

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. Historical Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3. Principles of Milling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

4. Attrition Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44.1. SPEX Shaker Mills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44.2. Planetary Ball Mills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54.3. Attritor Mills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54.4. Comparison of Mills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

5. Milled Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65.1. Solid-State Amorphization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.2. Metals, Alloys, and Intermetallics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.3. Ceramics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

5.4. Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105.5. Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6. Contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

7. Characterization of Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

8. Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1. INTRODUCTION

Unlike nanoparticles produced from bottom-up pro-cesses such as self-assembly and templated synthesis,nanoparticles from mechanical attrition are produced by

a top-down process. Such nanoparticles are formed ina mechanical device, generically referred to as a mill,in which energy is imparted to a course-grained mate-rial to effect a reduction in particle size. Under cer-tain conditions, the resulting particulate powders canexhibit nanostructural characteristics on at least two lev-els. First, the particles themselves, which normally pos-sess a distribution of sizes, can be nanoparticles if theiraverage characteristic dimension (diameter for spheri-cal particles) is less than 100 nm [1]. Second, many of

the materials milled in mechanical attrition devices arehighly crystalline, such that the crystallite (grain) sizeafter milling is often between 1 and 10 nm in diame-ter. Such materials are termed nanocrystalline [2]. The

sizes of the nanocrystals and the nanoparticles may ormay not be the same. In some of the nanostructuredmaterials literature, particularly that involving bottom-upprocesses, the term nanocrystal is reserved for crys-talline particles with low concentrations of defects, suchas are found in single crystals, whereas nanoparticlesare those nanoscale particles that contain gross internalgrain boundaries, fractures, or internal disorder, whetherthe crystals they contain are nanocrystalline or not [3].However, we will see that because of the large amount

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Synthesis, Functionalization and Surface Treatment of Nanoparticles

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2 De Castro and Mitchell

of strain imparted to particles during the milling pro-cess, it is virtually impossible to obtain defect-free crys-tals via mechanical attrition. As a result, we will adhereto the more general definitions; that is, nanocrystals are110 nm in diameter, and nanoparticles are less than100 nm in diameter. Thus, it is possible to have both

nanocrystalline nanoparticles and coarser particles thatcontain nanocrystals. Both types of materials producedby mechanical attrition will be addressed in this chapter.

The importance of nanoparticles lies in their inher-ently large surface-to-volume ratio relative to that oflarger particles. These high surface areas can potentiallyimprove catalytic processes and interfacially driven phe-nomena such as wetting and adhesion. Nanoparticleshave the potential for use in structural and device appli-cations in which enhanced mechanical and physical char-acteristics are required. As for the internal structure ofthe nanoparticles, it has been found that nanocrystallinematerials have comparative advantages over their micro-crystalline counterparts in hardness, fracture toughness,and low temperature ductility [4, 5]. As new methodsfor surface modification and postattrition processing ofnanoparticles are developed, the potential applicationsfor them continue to grow.

The early work on the production of nanostructuredmaterials by mechanical attrition has been reviewed pre-viously [615]. However, significant information contin-ues to become available in this rapidly evolving field,particularly with regard to the range of material classesto which it is applied. We attempt here to provide anupdated review of the subject, as mechanical attrition is

used to form nanoparticles in new materials such as poly-mers [16] and FCC metals [17], and as improvementsin the milling process that optimize the formation ofnanoparticles continue to be made.

2. HISTORICAL PERSPECTIVE

The attrition, or milling, of materials has been a majorcomponent of the ceramic processing and powder metal-lurgy industries for many years. The objectives of millinginclude particle size reduction (comminution or grind-ing); amorphization; particle size growth; shape changing(flaking); agglomeration; solid-state blending (incompletealloying); modifying, changing, or altering properties ofa material (density, flowability, or work hardening); andmixing or blending of two or more materials or mixedphases. However, the primary objective of milling is oftenpurely particle size reduction.

Mechanical attrition began as a way to simultaneouslyblend and commute (decrease in size) metal powders ina process called mechanical alloying. Mechanical alloy-ing (MA) is a powder technique that allows productionof homogeneous materials from blended elemental pow-der mixtures. John Benjamin and his colleagues at thePaul D. Merica Research Laboratory of the International

Nickel Company (INCO) developed the process around1966. The technique was the result of an intense researcheffort to produce nickel-based superalloys for gas turbineapplications. Benjamin has summarized the historical ori-gins of the process and the background work that led tothe development of MA [1820]. To summarize, they pro-

duced a nickel-chromium-aluminum-titanium alloy firstproduced in a small high-speed shaker mill and later in aone-gallon stirred ball mill, starting the birth of MA as amethod to produce oxide dispersion-strengthened (ODS)alloys on an industrial scale. The process developed byBenjamin was initially referred to as milling/mixing andlater termed mechanical alloying by Ewan C. Mac-Queen, a patent attorney for INCO. The formation ofan amorphous phase by mechanical grinding of a Y-Cointermetallic compound in 1981 [21] and in the Ni-Nbsystem by ball milling of blended elemental mixturesin 1983 [22] established MA as a potential nonequilib-rium processing technique. Beginning in the mid-1990s, anumber of investigations were carried out to synthesize avariety of stable and meta-stable phases, including super-saturated solid solutions, crystalline and quasi-crystallineintermediate phases, and amorphous alloys [2327]. Sincethen, mechanical alloying has been applied to virtually allmaterial classes, including metals, ceramics, and polymers.

3. PRINCIPLES OF MILLING

The fundamental principle of size reduction in mechan-ical attrition devices lies in the energy imparted tothe sample during impacts between the milling media.

The impact process is shown in Figure 1. This model

Figure 1. Model of impact event at a time of maximum impacting force,showing the formation of a microcompact. Reprinted with permissionfrom [28], E. Kuhn, ASM Handbook, Vol. 7, Materials Park, OH, 1984. 1984, ASM.


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Nanoparticles from Mechanical Attrition 3

Figure 2. Process of trapping an incremental volume of powderbetween two balls in a randomly agitated charge of balls and pow-der. (ac) Trapping and compaction of particles. (d) Agglomeration. (e)Release of agglomerate by elastic energy. Reprinted with permissionfrom [28], E. Kuhn, ASM Handbook, Vol. 7, Materials Park, OH, 1984. 1984, ASM.

represents the moment of collision, during which par-ticles are trapped between two colliding balls within aspace occupied by a dense cloud, dispersion, or massof powder particles [28]. Figure 2 shows the process oftrapping an incremental volume between two balls. Com-paction begins with a powder mass that is characterizedby large spaces between particles compared with the par-ticle size. The first stage of compaction starts with therearrangement and restacking of particles. Particles slidepast one another with a minimum of deformation andfracture, producing some fine, irregularly shaped parti-cles. The second stage of compaction involves elastic andplastic deformation of particles [29]. Cold welding mayoccur between particles in metallic systems during thisstage. The third stage of compaction, involving particlefracture, results in further deformation and/or fragmen-tation of the particles.

For brittle materials, particle fracture is well describedby Griffith theory. According to the theory, the stress, F,at which crack propagation leading to catastrophic failure(fracture) occurs in the particle is approximated by

F

E

c(1)

Table 1. Selected Youngs moduli for isotropic solids at roomtemperature.

Material E (106 psi) E (1010 N/m2)

Aluminum 10 7-Iron 30 21Copper 16 11

Silicon 16 11Tungsten 59 41Titanium carbide (TiC) 45 31Alumina (Al2O3) 58 40Magnesia 45 31Hard rubber (ebonite) 06 04Polystyrene (atactic) 04 03Polyethylene (branched) 003 002

where c is the length of the crack, E is the modulusof elasticity (Table 1), and is the surface energy ofthe milled substance (Table 2). When stress at the cracktip equals the strength of cohesion between atoms, thecrack becomes unstable and propagates, leading to frac-ture [28]. As fragments decrease in size, the tendencyto aggregate increases, and fracture resistance increases.Particle fineness approaches a limit as milling continuesand maximum energy is expended [30]. According to Har-ris [31], the major factors contributing to grind limit are:

Increasing resistance to fracture. Increasing cohesion between particles, with decreas-

ing particle size causing agglomeration. Excessive clearance between impacting surfaces.

Coating of the grinding medium by fine particles thatcushion the microbed of particles from impact. Surface roughness of the grinding medium. Bridging of large particles to protect smaller parti-

cles in the microbed. Increasing apparent viscosity as particle size

decreases.

Table 2. Selected surface energies of solid materials.

Material Surface energy, (J/m2)

Au 140Cu 1.431.70

Ag 1.141.20Ni 190NaCl 030Al2O3 0905MgO 1.001.20TiC 119LiF 034CaF2 045BaF2 028CaCO3 023Si 124Poly(tetrafluoroethylene) 00185Poly(ethylene) 0033Poly(styrene) 0034


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Figure 3. Change of average particle size as a function of milling timefor MA Fe-20at.%Co powders by (a) fixed r.p.m. operation and (b)cyclic operation. Reprinted with permission from [30], Y. D. Kim et al.,

Mater. Sci. Eng. A 291, 17 (2000). 2000, Elsevier Science.

Decreasing internal friction of slurry as particle sizedecreases.

The grinding limit is clearly demonstrated in the studiesof Kim et al. [30]; after milling of Fe-Co powders for 30 h,the MA process reached a steady state where the parti-cles have become homogenized in size and shape (Fig. 3).

This work also clearly distinguishes between particle size,which approaches a limit of 10 m with a planetary ballmill, and crystallite size, which approaches 10 nm (Fig. 4).Figure 4 also illustrates the strain buildup in the particlesas milling time increases and grain sizes decrease.

4. ATTRITION DEVICES

Different types of milling equipment are availablefor mechanical alloying and nanoparticle formation.

Figure 4. Changes of average grain size and strain for MA Fe-20at.%Co powders with milling time by cyclic operation. Reprinted with per-mission from [30], Y. D. Kim et al., Mater. Sci. Eng. A 291, 17 (2000). 2000, Elsevier Science.

Figure 5. SPEX 8000D dual mixer/mill.

They differ in their capacity, efficiency of milling, andadditional arrangements for heat transfer and particleremoval. A brief description of the different mills avail-able for MA can be found below.

4.1. SPEX Shaker Mills

Shaker mills such as SPEX (Fig. 5), which mill about1020 g of powder at a time, are most commonly usedfor laboratory investigations and for alloy screening pur-poses. The common variety of the mill has one vial (seeFig. 6), containing the sample and the grinding media,which is secured in the clamp and swung energetically

back and forth several thousands times a minute. Theback-and-forth shaking motion is combined with lateralmovements of the ends of the vial. With each swing ofthe vial the milling media, typically hard, spherical objectscalled milling balls, impact against each other and theend of the vial, both milling and mixing at the same time.Because of the amplitude (about 5 cm) and speed (about1200 rpm) of the clamp motion, the ball velocities arehigh (on the order of 5 m/s), and consequently the forceof the balls impact is usually great. Therefore, these millscan be considered a high-energy variety.

The most recent design of this mill has provisionfor simultaneously milling the powders in two vials

to increase the throughput. This machine incorporates

Figure 6. SPEX stainless steel vial set for SPEX 8000D mill.


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forced cooling to permit extended milling times. A vari-ety of vial materials are available for the SPEX mills,including hardened steel, alumina, tungsten carbide, zir-conia, stainless steel, silicon nitride, agate, plastic, andpolymethylmethacrylate. A majority of the research onthe fundamental aspects of MA has been carried out with

some version of these SPEX mills.

4.2. Planetary Ball Mills

Another popular mill for conducting MA experiments isthe planetary ball mill (referred to as Pulverisette) inwhich a few hundred grams of the powder can be milledat a time. The planetary ball mill owes its name to theplanet-like movement of its vials. These are arranged ona rotating support disk, and a special drive mechanismcauses them to rotate around their own axes. The cen-trifugal force produced by the vials rotating around theirown axes and that produced by the rotating support disk

both act on the vial contents, consisting of material to beground and the grinding balls. Since the vials and the sup-porting disk rotate in opposite directions, the centrifugalforces alternately act in like and opposite directions. Thiscauses the grinding balls to run down the inside of thevialthe friction effectfollowed by the material beingground. Grinding balls lift off and travel freely throughthe inner chamber of the vial and collide against theopposing inside wallthe impact effect.

Even though the disk and the vial rotation speeds couldnot be independently controlled in the earlier versions ofthis device, it is possible to do so in modern versions. In asingle mill there can be either two (Pulverisette 5 or 7) orfour (Pulverisette 5) milling stations. Recently, a single-version mill was also developed (Pulverisette 6). Grindingvials and balls are available in a variety of different mate-rials, including agate, silicon nitride, sintered corundum,zirconia, chrome steel, Cr-Ni steel, tungsten carbide, andpolyamide. An example of the particles that result fromattrition in a planetary mill is shown in Figure 7.

4.3. Attritor Mills

A conventional ball mill consists of a rotating horizon-tal drum half-filled with steel balls that range from 0.318

to 0.635 cm in diameter. As the drum rotates the ballsdrop on the metal powder that is being ground; the rateof grinding increases with the speed of rotation. At highspeeds, however, the centrifugal force acting on the steelballs exceeds the force of gravity, and the balls are pinnedto the wall of the drum. At this point the grinding actionstops. An attritor (a ball mill capable of generating higherenergies) consists of a vertical drum with a series ofimpellers inside it. Set progressively at right angles toeach other, the impellers energize the ball charge, causingpowder size reduction due to the impact between balls;between the balls and the container wall; and betweenthe balls, the agitator shaft, and the impellers. Some size

Figure 7. SEM of Bi4Ti3O12 milled for different times: (a) 3, (b) 9, (c)

15, and (d) 20 h. Reprinted with permission from [43], L. B. Kong et al.,Mater. Lett. 51, 108 (2001). 2001, Elsevier Science.

reduction appears to take place by interparticle collisionsand ball sliding. A motor rotates the impellers, which inturn agitate the steel balls in the drum.

Attritors (Fig. 8) are the mills in which large quanti-ties of powder (from about 0.5 to 40 kg) can be milledat a time. Attritors of different sizes and capacities areavailable. The grinding tanks or containers are avail-able in stainless steel or stainless steel coated with alu-mina, silicon carbide, silicon nitride, zirconia, rubber,or polyurethane. A variety of grinding media are also

available: glass, flint stones, stealite ceramic, mullite, sil-icon carbide, silicon nitride, Sialon, alumina, zirconium

Figure 8. Attrition ball mill. Reprinted with permission from [28],E. Kuhn, ASM Handbook, Vol. 7, Materials Park, OH, 1984. 1984,ASM.


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silicate, zirconia, stainless steel, carbon steel, chromesteel, and tungsten carbide.

The operation of an attritor is simple. The powder tobe milled is placed in a stationary tank with the grindingmedia. The mixture is then agitated by a shaft with arms,rotating at a high speed of about 250 rpm. This causes

the media to exert both shearing and impact forces onthe material. The laboratory attritor works up to 10 timesfaster than conventional ball mills.

4.4. Comparison of Mills

The final product size from mechanical attrition is deter-mined by the energy input during milling, the ball-to-powder weight ratio, and the overall temperature duringmilling. Borner and Eckert [32] investigated the effectof energy input by milling iron powders with the use ofa SPEX milling machine and a Pulverisette 5, amongother lower energy mills. They determined that the SPEX

shaker mill provides the largest input and therefore leadsto a fast decrease of grain size to less than 20 nm. Thesteady-state grain size was achieved after 4 h of milling.The Pulverisette mill provides a smaller energy impactduring the collision. After 32 h of milling the grain sizeachieved was 40 nm at 90 rpm, 31 nm at 180 rpm, and20 nm at 360 rpm (Fig. 9).

The different classifications of mills and the typicalrange of particle sizes they produce are summarized inFigure 10. Notice that vibratory mills are the typical classof mills used to produce nanoparticles. Figure 11 showshow the different mill types can be used to attrit materialsof all types, ranging from the more commonly used brittlematerials, to the newer, plastic and viscoelastic materials.In the next section, we describe recent research resultson nanoparticle formation and nanocrystallinity in thesedifferent materials classes.

Figure 9. Average grain size for Fe powders vs. milling time with differ-ent mills. Reprinted with permission from [32], I. Borner and J. Eckert,


Figure 10. Typical size capabilities of common classes of size reduc-tion equipment. Reprinted with permission from [28], E. Kuhn, ASMHandbook, Vol. 7, Materials Park, OH, 1984. 1984, ASM.

5. MILLED MATERIALS

Though traditionally limited to metallic materials, in par-ticular FCC metals, mechanical alloying is now being uti-lized to form nanoparticles and nanocrystalline particlesfrom a variety of materials. We report here on recentresearch results that employ MA as the principal pro-cessing method and the primary benefits it provides fordifferent types of materials.

Figure 11. Applicability of various classifications of comminutionmachines to different material types.


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5.1. Solid-State Amorphization

A solid alloy with a noncrystalline atomic arrangementis called an amorphous (metallic) alloy. The synthesis ofan amorphous phase in the Ni-Nb system by MA startingfrom blended elemental powders of Ni and Nb in 1983[22] has given rise to increased research activity in this

area. Amorphous phases have been synthesized by MAfrom blended elemental powder mixtures, pre-alloyedpowders and/or intermetallics, mixtures of intermetallics,or mixtures of intermetallics and elemental powders.

The effect of process variables on amorphizationbehavior has been studied in several alloy systems. Themost important variables that have been studied aremilling energy and milling temperature. Increased millingenergy, which is achieved by higher ball-to-powder weightratio and increased speed of rotation, is expected to intro-duce more strain and increase the defect concentrationin the powder, thereby leading more readily to amor-

phization. On the other hand, higher milling energies canalso produce more heat, resulting in crystallization of theamorphous phase. A balance between these two effectswill determine the nature of the final product phase.

Eckert et al. [33] reported that in Ni-Zr system milled ina planetary ball mill at a low intensity did not produce anyamorphous phase, because of the lack of energy. How-ever, when the intensity was increased, amorphous phaseformation was observed in a wide composition range of3083% Ni. At even higher intensities, amorphous phaseformation was observed between 66 and 75% Ni. Theseobservations suggest that with increasing milling energy,

the heat generated is also high, which crystallizes theamorphous phases. It appears that the maximum amor-phization range is observed at intermediate values ofmilling intensity; too low an intensity does not provideenough energy to amorphize, while at very high intensi-ties, the amorphous phase formed would crystallize.

There have been conflicting results on the effect ofmilling temperature on the nature of the phase formed.Koch et al. [34] summarized the results of varying thetemperature of the mill on the kinetics of amorphizationin intermetallics. They concluded that generally a lowermilling temperature accelerated the amorphization pro-cess. Since a nanostructured material can easily be pro-duced at lower milling temperatures, the increased grainboundary area drives the crystal-to-amorphous transfor-mation. For example, the time required for amorphiza-tion in NiTi was 2 h at 190 C, 13 h at 60 C, and 18 hat 220 C [34].

Mechanical alloying introduces contamination into themilled powder and thus alters the constitution and stabil-ity of powder products. Generally, the presence of addi-tional elements favors amorphization. Contamination hasalso been found to affect the stability of the amorphousphases formed. Formation of a crystalline phase has beenreported on milling of Ti-Al powders after an amorphous

phase had been formed [35]. Lattice measurements sug-gested that this FCC crystalline phase is due to theincreased nitrogen contamination of the milled powder,and the crystalline phase has been identified as TiN.

5.2. Metals, Alloys, and Intermetallics

Metals are routinely milled to the nanoparticle size range,with typical crystallite sizes in the 110-nm range. Theaverage grain size of a NiAl intermetallic compound wasas low as 5 nm after 100 h of milling [36] and as low as4.7 nm for Fe-C alloys milled in a horizontal ball millingmachine [37].

Kim et al. [30] studied the effect of fixed and cyclicoperation on the formation of nanocrystalline Fe-Coalloy powders produced by mechanical alloying in a plan-etary ball mill. The first method was conventional millingwith fixed velocity (1300 rpm). The second methodinvolved an operation cycle characterized by a time inter-

val of 4 min at 1300 rpm followed by 1 min of oper-ation at 900 rpm. In both cases, the milling time wasvaried from 1 to 100 h. It is generally known that theMA process reaches a steady state where the particleshave homogeneous size and shape. The steady state wasachieved after 30 h of milling for fixed operation and15 h of milling for a cyclic operation. The advantage ofthe cyclic operation is due to the fact that the periodicalchanges of the milling velocity in cyclic operation breakthe balance of deformation, fracture, and welding in theprocess and maximize the effect of fracture. The steady-state crystallite grain size obtained was in the range of

1015 nm. Interestingly, during the first hour of millingthe grain size of Fe-Co increased and then decreased tothe final 1015 nm after achieving steady state.

Raghu et al. [38] studied the differences betweennanocrystalline copper-tungsten alloys milled in argonand air. They determined that the particle size initiallydecreases and then increases in powders milled in air(Fig. 12). The crystallite sizes of copper and tungstendecrease continuously with milling time in all millingatmospheres (Fig. 13). Crystallite size levels off at a valueof 10 nm for copper and 15 nm for tungsten after 8 h ofmilling. It was concluded that whereas fracture is dom-inant in milling in argon, adequate welding for the MAprocess appears to be available during milling in air.

Eckert and Borner [36] studied nanostructure forma-tion of ball-milled NiAl intermetallic compounds in aplanetary ball mill with hardened steel balls and vial at arotation speed of 180 rpm. Figure 14 illustrates that theaverage grain size and the rms strain scale with composi-tion of the material. Whereas the grain size is reduced toonly 12 nm for Ni46Al54, it decreased to about 5 nm forNi-rich compounds. Blending Ni46Al54 with elemental Nito give an overall composition of Ni50Al50 and subsequentmechanical alloying reduced the grain size from 12 nmafter 100 h to about 9 nm after 300 h of total milling time


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Figure 12. Particle size of Cu-5vol%W powders milled in differentatmospheres. Reprinted with permission from [38], T. Raghu et al.,


(Fig. 15). On the other hand, the grain size of 100-h ball-milled Ni60Al40 increased upon blending with elementalAl and further mechanical alloying. This demonstratesthat the ultimate grain size is coupled with the com-position of the material. Table 3 summarizes grain sizeresults obtained by milling different metals, alloys, andintermetallics.

Figure 13. Average grain size of (a) copper and (b) tungsten powdersmilled in argon and air. Reprinted with permission from [38], T. Raghuet al., Mater. Sci. Eng. A 304306, 438 (2001). 2001, Eslevier Science.

Figure 14. Average grain size (filled symbols) and atomic-level strain(open symbols) for NixAl100x powders after 100 h of milling as a func-tion of composition. Reprinted with permission from [36], J. Eckert andI. Borner, Mater. Sci. Eng. A 239240, 619 (1997). 1997, ElsevierScience.

Figure 15. Average grain size obtained for Ni56Al54 and Ni60Al40 pow-ders after 100 h of ball milling and after addition of elemental Ni orAl and subsequent mechanical alloying as a function of total millingtime. For comparison the average grain size for Ni 50Al50 obtained after100 h of continuous ball milling is also shown. Reprinted with permis-sion from [36], J. Eckert and I. Borner, Mater. Sci. Eng. A 239240, 619(1997). 1997, Elsevier Science.

Table 3. Typical grain sizes for metals, alloys, and intermetallics forMA.

Milling MillingComposition technique Grain size (nm) time (h) Reference

Fe-Co powders Rotary ball mill 1015 30 [30]Fe Vibratory mill 20 4 [32]NiAl Vibratory mill 12 100 [36]Ni silicides Vibratory mill 1017 30 [1]Fe-C Horizontal ball 4.7 500 [37]

millFe3Al Vibratory mill 12.6 100 [76]


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5.3. Ceramics

Recent studies show that highly exothermic reactions, forexample, the formation of TiC [39], can be initiated byhigh-energy ball milling. This process is referred to asa mechanically induced self-propagating reaction (MSR).In the studies of Xinkun et al. [39], raw powders of Ti

and C were mixed in a mole ratio of 1:1 and milled.TiC particles with 20-nm crystallites were fabricated justafter reaction at 120 min. It took 10 to 20 h for the Tiand C powders to react completely. The reaction of Tiand C powders was investigated by measuring the tem-perature of the vial outer wall. Figure 16 shows the rela-tion between the temperature of the outer wall and themilling time and indicates that the temperature of the vialincreased slowly as the milling time was extended. Whenthe milling time reached 115 min, the temperature of thevial increased abruptly and reached its maximum (from319 to 332 K). The temperature peak was associated with

the exothermic reaction of Ti and C. After the peak, thevial temperature decreased gradually during subsequentmilling.

Traditionally, the raw materials used in mechanicalalloying include at least one ductile material used as ahost or binder for the other ingredients. However, Davisand Koch [40] mechanically alloyed a brittle Si-Ge sys-tem to obtain a solid solution Si(Ge) and observed a lat-tice change with the increased duration of MA. Recently,Zhang and Tam [41] studied the mechanical alloying ofan all-ceramic-phase component, TiC + TiN. Two com-positions were explored, 70% TiC + 30% TiN and 50%TiC + 50% TiN (by weight). MA was conducted for upto 80 h in a Fritsch planetary ball mill at a ball-to-powderratio of 20:1. Figure 17 shows a plot of the lattice param-eter as a function of milling time for 70% TiC + 30%TiN milled with steel balls. A similar plot was obtainedfor 50% TiC + 50% TiN milled with WC balls. The twocompositions show a similar trend in lattice parameterchange, irrespective of milling media, and therefore it isindicated that solid solutions occur during the MA pro-cess. It was also determined that the rate of change of the

Figure 16. Temperature of outer vial wall as a function of milling timefor milling of Ti and C powders. The sharp increase in wall temperatureat 115 min is attributed to formation of TiC. Reprinted with permissionfrom [39], Z. Xinkun et al., Mater. Sci. Eng. C 16, 103 (2001). 2001,Elsevier Science.

Figure 17. Lattice parameter as a function of MA time for 70wt%TiC + 30wt% TiN as determined from XRD patterns. Reprinted withpermission from [41], S. Zhang and S. C. Tam, J. Mater. Processing 67,12, (1997). 1997, Elsevier Science.

lattice parameter with MA time is almost the same forthe two compositions; this shows that in MA, the reac-tion rate is independent of the concentration of thereactant as long as the latter is not depleted, whichis different from a chemical reaction, where the reac-tion rate is proportional to the concentration of reac-tants. The reduction of crystallite size was very rapid inthe beginning, and after 40 h it was in the range of 2030 nm (Fig. 18). In the studies of Indris et al. [42], thesystem 1 xLi2O:xB2O3 reacts either at high temper-atures or because of mechanochemical treatment. Indris

determined that the reaction product is not determinedby the overall composition of the mixture, but by the con-ditions at the particle interfaces.

Indris et al. [42] also studied the oxide ceramics Li 2O,LiNbO3, LiBO2, B2O3, and TiO3 as monophase materi-als. Nanostructures of these oxides were prepared in a

Figure 18. Reduction in average grain size as a function of milling timefor 50wt% TiC + 50wt% TiN. Reprinted with permission from [41],S. Zhang and S. C. Tam, J. Mater. Processing 67, 112, (1997). 1997,Elsevier Science.


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Figure 19. Average grain size of Li2O, LiNbO3, and B2O3 powders forvarious milling times. Reprinted with permission from [42], S. Indriset al., J. Mater. Synth. Processing 8, 245 (2000). 2000, Plenum.

high-energy ball mill Spex 8000 with alumina milling vialswith a ball-to-powder ratio of 2:1 to produce at least 2 gof powder. Figure 19 shows the average grain size forLiNbO3, Li2O, and B2O3. Although the kinetics of grainsize reduction is different, all samples show a similar sat-uration value for a final grain size of about 23 nm.

Kong et al. [43] prepared Bi4Ti3O12 ceramics by com-bining Bi2O3 and TiO2 powders. Although the particlesize was reduced to 100200 nm after 3 h, no reactionbetween the powders took place. After 9 h of milling,peaks for Bi4Ti3O12 appeared in the XRD pattern, indi-cating reaction between the starting powders. After 15 h,

the reaction was completed, with a further reduction inparticle size. Kong et al. also studied lead zirconiumtitanate (PZT) ceramics [44, 45] because of its excel-lent dielectric, piezoelectric, and electro-optic proper-ties. PZT powders were formed in a planetary ball millfrom PbO, ZrO2, and TiO2. The grain size from XRDranged between 35 nm (4 h of milling) and 26 nm (24 hof milling). The particle size, as determined with SEM,ranged from 120 nm to 65 nm.

5.4. Composites

Mechanical alloying (MA) has been successfully used tosynthesize a number of commercially important alloysand composites [4651]. The mechanically alloyed pow-ders have attributes such as fine powder size and flexi-bility of alloying choices. In particular, MA has uniqueadvantages in producing metal matrix composite pow-ders. The low-temperature solid-state process eliminatesreactions between the reinforcement particles and thematrix. It is commonly known that for particulate rein-forcement of metal matrix composites, the mechanicalproperties are influenced significantly by the quantity andthe distribution mode of the reinforcements and by thenature of the interfaces between the reinforcements and

the matrix. Finer size reinforcements produce higher spe-cific mechanical properties. However, they also encountersignificant agglomeration problems for that size range,resulting in poor dispersion and subsequently less thanoptimal mechanical properties.

The main problem with the fine-grained structure is

its instability at high temperatures. Because of the largeexcess of free energy, significant grain growth has beenobserved in several nanocrystalline materials [52, 53].Senkov et al. [53] studied grain growth of TiAl 3/Ti5Si3composites produced by mechanical alloying and hotisostatic pressing (HIP). Blends of elemental and pre-alloyed powders were used for the MA. Intimate mixingof the elements and powder amorphization occurred dur-ing the MA. The average grain size increased from 140to 540 nm when the HIP temperature was increased from850 C to 1100 C.

Hwang and Nishimura [54] demonstrated that mag-nesium matrix nanocomposites could be mechanochem-

ically synthesized by milling elemental precursors, ofwhich two of the elemental powders (normally, ametal and an element such as C) form the secondaryparticles. Nanocomposites have also been successfullymechanochemically synthesized by Fan et al. [55] withNb-Si-Ti-C mixtures, Zhou et al. [56] with Ni-Al-Ti-C mix-tures, and Takahashi and Hashimoto [57] in the Cu-M-Csystem (where M = Zr, Hf, Nb, Ta, or Ti). Hwang andNishimura [54] used a mixture of Mg, Ti, and C powderto synthesize nanometer-sized TiC particles embeddedin a nanocrystalline Mg matrix by mechanochemicalreaction. Powder compositions corresponding to Mg-

xTiC (where x = 515 volume %) were milled. TheXRD pattern shows the development of the Mg-15%TiCnanocomposite during milling (Fig. 20). After 3 h ofmilling, the sample still contained the three forms ofelemental powders, but with further milling (6 h), Tipeaks were reduced and TiC peaks appeared. After 24 hof milling, the powder showed only the Mg and TiC

Figure 20. Evolution of Mg-Ti-C nanocomposite during milling andafter heat treatment. Reprinted with permission from [54], S. Hwangand C. Nishimura, Scripta Mater. 44, 2457 (2001). 2001 ElsevierScience.


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Figure 21. Average grain size of MmM5phase in MmM5-Mg compositeprepared by 20 h of milling as a function of Mg content. Reprintedwith permission from [58], M. Zhu et al., Mater. Sci. Eng. A 286, 130(2000). 2000, Elsevier Science.

peaks, indicating that Ti and C reacted during millingto form TiC. The grain sizes of TiC particles and Mggrains of as-milled samples were approximately 5.6 and43.1 nm, respectively, and those of heat-treated sampleswere approximately 7.1 and 62.3 nm, respectively. Mg-Ti-C nanocomposites exhibited remarkably high ductilitywhile retaining compressive strength similar to that ofMg-TiC nanocomposite.

Zhu et al. [58] studied the effect of Mg contenton the microstructure of mechanical alloyedMmNi35(CoAlMn)15-Mg (abbreviated as MmM5-Mg).Figure 21 shows that the grain size of the MmM 5 phaseincreases with Mg content in the composite. The authorsexplained this observation by the consideration that Mg

absorbed part of the energy imparted to the MmM5phase. MmM5 particles were gradually embedded inMg as milling proceeded. The larger the amount ofMg in the composite, the less energy was consumed inrefining the grain size of MmM5 and thus the grain sizeof MmM5 increased with increasing Mg content.

Lee and Munir [59] studied the synthesis of denseTiB2-TiN nanocrystalline composites through mechanicaland field activation. Powder mixtures were mechanicallyactivated through ball milling. The powders were blendedin a stoichiometric ratio according to the reaction

Ti+

05B+

05BN

05TiB2+

05TiN

They were mixed in a Shaker mixer (Glenmill) for 24 hand ball milled in a planetary mill. Figure 22 shows therefinement of the grain size and the increase in strain asa function of milling times for Ti. The small grain size ofunmilled Ti powder (71111 nm) may be related to theparticular preparation method of this powder, namely,that it was made from sponge Ti. As shown in Figure 22,the strain value changed very little in early stages ofmilling, but after 2 h of milling, the strain increasedmarkedly. Within the first 6 h of milling, the decreasein crystallite size for Ti was relatively linear with milling

Figure 22. Average grain size and strain for Ti as a function of millingtime, for the WilliamsonHall (WH) and HalderWagner (HW) meth-ods. Reprinted with permission from [59], J. W. Lee and Z. A. Munir,

J. Am. Ceram. Soc. 84, 1209 (2001). 2001, The American CeramicSociety.

time. Figure 22 also shows that the minimum crystal-lite size of Ti that can be obtained before the onset ofa reaction in the mill is in the range of 16.733.3 nm.Figure 23a shows the results of crystallite size analyseson milled powders (10 h) that were subsequently reactedin the spark plasma synthesis (SPS) apparatus for 1 or12 min, and Figure 23b shows the results on powdersthat had reacted during 12 h of milling and subsequently

reacted in the SPS for 5 to 12 min. It is interesting tonote that the crystallite size of the product phases after 1min of SPS treatment is roughly the same whether theyare formed in the SPS or previously during milling. How-ever, when the treatment is longer (12 min), the size ofthose formed in the SPS is about one-half that of thoseformed during milling.

5.5. Polymers

Unlike the materials previously described here, thereis little work on nanoparticle polymers by mechanicalalloying. We include here recent progress toward theformation of polymer nanoparticles and nanocrystallinepolymer particles.

High-energy milling of polymeric materials subjectsthe blend components to a complex deformation fieldin which shear, multiaxial extension, fracture, and coldwelding proceed concurrently. This technique has beensuccessfully used to prepare blends of thermoplastics[6065] and to cause allotropism in semicrystalline poly-mers [66, 67]. Smith et al. have demonstrated thatnanostructured polymer blends composed of poly(methylmethacrylate) (PMMA) and either poly(ethylene-alt-propylene) (PEP) or polyisopropene (PI) can be


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Figure 23. Average grain size of TiN and TiB2 formed in spark plasmasintering (SPS). (a) Reactant powders milled for 10 h (no product for-mation). (b) Reactant powders milled for 12 h (with powder formation).

Reprinted with permission from [59], J. W. Lee and Z. A. Munir, J. Am.Ceram. Soc. 84, 1209 (2001). 2001, The American Ceramic Society.

produced by cryogenic mechanical alloying [16]. In cryo-genic mechanic alloying the vial is sealed under argon(


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that in the case of semicrystalline PEEK, a polymer witha higher degree of amorphization was obtained as a resultof this mechanical treatment.

6. CONTAMINATION

A major concern in the processing of nanoparticles byMA is the nature and amount of impurities that contam-inate the milled powder. Contamination can arise fromseveral sources, including

impurities in starting powders, vials and grinding media, milling atmosphere, and control agents added to the powders.

During MA the powder particles become trappedbetween the grinding medium and undergo severe plasticdeformation; fresh surfaces are created because of the

fracture of the powder particles. Collisions occur betweenthe grinding medium and the vial and among the grindingballs. All of these effects can cause deterioration of thegrinding medium and vial, resulting in the incorporationof these impurities into the powder. The extent of con-tamination increases with increasing milling energy andmilling time.

Various attempts have been made to minimize powdercontamination during MA. One way of minimizing thecontamination from the grinding medium and the con-tainer is to use a material for the container and grindingmedium that is the same as the powder being milled.

For example, one could use copper milling balls and acopper vial for milling copper and copper alloy powders.However, milling effectiveness may be limited in suchcases. In general, to minimize contamination from thecontainer and grinding medium, the container and grind-ing medium should be harder/stronger than the powderbeing milled. Nonetheless, contamination is difficult tocompletely avoid in MA.

The problem of milling atmosphere is serious and hasbeen found to be a major cause of contamination. It hasbeen observed that if the container is not properly sealed,the atmosphere surrounding the container, usually air,leaks into the container and contaminates the powder.This is particularly problematic for oxidation-sensitivematerials, such as pure metals. De Castro and Mitchellstudied contamination caused by high-energy milling witha SPEX 8000 mixer/mill [73]. Aluminum with a purity of99.9% or better and an initial particle size in the rangeof 50100 m was milled in stainless steel and tungstencarbide vials with milling media of the same compositionas the vial. A ball-to-powder ratio of 10:1 was used in allcases. Ethanol was added as surfactant during milling andsubsequently removed. Figures 25 and 26 show contami-nation as a function of milling time in stainless steel andtungsten carbide, respectively. For the stainless steel vials,

Figure 25. Contamination of aluminum powders milled in stainlesssteel vials.

contamination is reported as the sum of the atomic per-centages of Fe, V, and Cr, as determined from X-ray flu-orescence (XRF). Vanadium and chromium are commonalloying elements in stainless steel. For milling in WC,contamination is reported as percentage WC as deter-mined by XRF. As can be seen, contamination increasesdramatically with milling time, especially in the WC vial.The objective of current studies is to reduce the contam-ination caused by the milling media/vials and at the sametime retain, or further reduce in size, the desired nanos-tructure of the powders.

7. CHARACTERIZATION OF NANOPARTICLES

The powders obtained after MA must be characterizedfor their size, shape, surface area, phase constitution,and microstructural features. Additionally, one could also

characterize the transformation behavior of the mechan-ically alloyed powders in annealing and other treatments.The measurement of crystallite size and lattice strain inmechanically alloyed powders is very important, since thephase constitution and transformation appear to be crit-ically dependent upon them. We attempt here to give avery brief overview of the common techniques used todetermine both particle and crystallite size, in an attemptto help differentiate between nanocrystals and nanopar-ticles, both of which can result from mechanical attrition.

Figure 26. Contamination of aluminum powders milled in tungsten car-bide vials.


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Those wishing to obtain more detailed information onthese well-established analytical techniques are directedto their respective literature sources.

The crystalline size or grain size in nanoparticles canbe determined with several techniques that rely upon thepeak width in X-ray diffraction patterns. In the Hall

Williamson method [74, 75], crystalline size, D, is givenby

cos = 2 sin +094

D(3)

where is the peak full width at half-maximum(FWHM), is the diffraction angle, is the wavelengthof the X-ray, and is the effective strain associated withmechanical alloying and ultrafine grain size. The crys-talline size D can be obtained by extrapolating sin tozero to eliminate the strain term.

The most widely used equation to determine grain size

from XRD data is the Scherrer equation,

Lo =K

cos (4)

where K is a constant of order 1 dependent on parti-cle shape, is the wavelength of the X-ray radiation, is the diffraction angle, and is the FWHM, after cor-rections concerning instrument broadening. It should benoted that for both the HallWilliamson and Scherrermethods of determining grain size, one or several peaksfrom the XRD pattern can be selected for analysis, andthe grain size reported for each, or as an average of mul-

tiple peak determinations. In either case, the grain sizedetermination from these methods gives a bulk-averagevalue.

The size and shape of powder particles may be deter-mined accurately with direct methods of either scanningelectron microscopy (SEM) for relatively coarse powdersor transmission electron microscopy (TEM) for fine pow-ders. In contrast to XRD, which gives a bulk-averagevalue for grain size, observations of particle size viaSEM/TEM give only values of an isolated sample areaand, as such, may not be representative of the entire sam-ple, vis--vis particle size distribution. As outlined previ-

ously, the values of particle size should not be confusedwith crystalline size, as determined from XRD data.

8. SUMMARY AND CONCLUSIONS

Mechanical alloying is a simple and useful processingtechnique that is now being employed in the productionof nanocrystals and/or nanoparticles from all materialclasses. Although a variety of mechanical alloying devicesexist, the high-energy ball mill is typically used to pro-duce particles in the nanoscale size range. Particle sizereduction is effected over time in the high-energy ballmill, as is a reduction in crystallite grain size, both of

which reach minimum values at extended milling times.The distinction between nanoparticles and nanocrystalsin the current MA literature is not clear. Grain sizes, asdetermined with standard X-ray diffraction techniques,are typically reported as a function of milling time; how-ever, the actual particle sizes that result from milling,

while assumed to be larger than the reported grain sizesfor polycrystalline materials, often go unreported. More-over, what are often reported as nanocrystals may not besuch in the technical definition of the term (


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