Nano Crystalline

8/13/2019 Nano Crystalline

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Nanocrystalline Materials

Introduction

Nanocrystalline materials are single- or multi-phase polycrystalline solids with a grain

size of a few nanometers (1 nm = 109m = 10 ) typically less than 100 nm! "ince the

grain sizes are so small a significant #olume of the microstructure in nanocrystalline

materials is composed of interfaces mainly grain $oundaries i!e! a large #olume

fraction of the atoms resides in grain $oundaries! %onse&uently nanocrystalline

materials e'hi$it properties that are significantly different from and often impro#ed

o#er their con#entional coarse-grained polycrystalline counterparts! aterials with

microstructural features of nanometric dimensions are referred to in the literature as

nanocrystalline materials (a #ery generic term) nanocrystals nanostructured materials

nanophase materials nanometer-sized crystalline solids or solids with nanometer-sized

microstructural features! Nanostructured solids is perhaps the most accurate

description e#en though nanocrystalline materials will $e the appropriate term if one is

dealing with solids with grains made up of crystals!

Nanocrystalline structures are not really #ery new! Nanocrystalline phases were

detected in samples of lunar soils! any con#entional catalytic materials are $ased on

#ery fine microstructures! Nanostructures formed chemically under am$ient conditions

can also $e found in natural $iological systems from seashells to $one and teeth in the

human $ody! hese materials are nota$le in that they are simultaneously hard strong

and tough! herefore a num$er of in#estigations ha#e $een conducted to mimic nature

($iomimetics) and also artificially synthesize nanostructured materials and study their

properties and $eha#ior! hese in#estigations ha#e clearly shown that one could

engineer (tailor) the properties of nanocrystalline materials through control of

microstructural features more specifically the grain size!

he nanocrystalline materials pioneered $y *leiter were preceded $y studies of

nanoparticles $y researchers such as +yeda! ,t present the #ery $road field of

nanostructured materials includes (i) nanoparticles (ii) nanocrystalline materials and

(iii) nanode#ices! he potential applications for the #arious inds of nanoscale materials

include dispersions and coatings high surface area materials functional nanostructures

(e!g! optoelectronic de#ices $iosensors nanomachines) and $ul nanostructured

materials for structural or magnetic applications!

Nanocrystalline materials can $e classified into different categories depending on the

num$er of dimensions in which the material has nanometer modulations! hus they can

$e classified into (a) layered or lamellar structures ($) filamentary structures and (c)

e&uia'ed nanostructured materials! , layered or lamellar structure is a one-dimensional


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(1.) nanostructure in which the magnitudes of length and width are much greater than

the thicness that is only a few nanometers in size! /ne can also #isualize a two-

dimensional (.) rod-shaped nanostructure that can $e termed filamentary and in this

the length is su$stantially larger than width or diameter which are of nanometer

dimensions! he most common of the nanostructures howe#er is $asically e&uia'ed

(all the three dimensions are of nanometer size) and are termed nanostructured

crystallites (three-dimensional 2.3 nanostructures)!

he nanostructured materials may contain crystalline &uasicrystalline or amorphous

phases and can $e metals ceramics polymers or composites! 4f the grains are made

up of crystals the material is called nanocrystalline! /n the other hand if they are

made up of &uasicrystalline or amorphous (glassy) phases they are termed

nano&uasicrystals and nanoglasses respecti#ely! *leiter has further classified the

nanostructured materials according to the composition morphology and distri$ution of

the nanocrystalline component! a$le shows this classification of the three types of

nanostructures! ,mongst the a$o#e ma'imum research wor is conducted on the

synthesis consolidation and characterization of the 2.-nanostructured crystallites

followed $y the 1.-layered nanostructures! 5hile the former are e'pected to find

applications $ased on their high strength impro#ed forma$ility and a good com$ination

of soft magnetic properties the latter are targeted for electronic applications! 6elati#ely

few in#estigations ha#e $een carried out on the .-filamentary nanostructures!

Dimensionalit

yDesignation

Typical Synthesis

methods

/ne

.imensional

7ayered

(lamellar)

8apor .epositionlectrodeposition

wo

.imensional:ilamentary %hemical 8apor .eposition

hree

.imensional%rystallites

*as %ondensationechanical ,lloying ;

illing

Synthesis

Nanocrystalline materials can $e synthesized either $y consolidating small clusters or$reaing down the $ul material into smaller and smaller dimensions! *leiter used the

inert gas condensation techni&ue to produce nanocrystalline powder particles and

consolidated them in situ into small diss under ultra-high #acuum (+


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dimensionality of the product o$tained! Nanostructured materials ha#e $een

synthesized in recent years $y methods including inert gas condensation mechanical

alloying spray con#ersion processing se#ere plastic deformation electrodeposition

rapid solidification from the melt physical #apor deposition chemical #apor processing

co-precipitation sol-gel processing sliding wear spar erosion plasma processing

auto-ignition laser a$lation hydrothermal pyrolysis thermophoretic forced flu' system

&uenching the melt under high pressure $iological templating sonochemical synthesis

and de#itrification of amorphous phases! ,ctually in practice any method capa$le of

producing #ery fine grain-sized materials can $e used to synthesize nanocrystalline

materials! he grain size morphology and te'ture can $e #aried $y suita$ly

modifying;controlling the process #aria$les in these methods! ach of these methods

has ad#antages and disad#antages and one should choose the appropriate method

depending upon the re&uirements! 4f a phase transformation is in#ol#ed e!g! li&uid to

solid or #apor to solid then steps need to $e taen to increase the nucleation rate and

decrease the growth rate during formation of the product phase! 4n fact it is this

strategy that is used during de#itrification of metallic glasses to produce nanocrystalline

materials! he choice of the method depends upon the a$ility to control the most

important feature of the nanocrystalline materials #iz! the microstructural features

(grain size layer spacing etc!)! /ther aspects of importance are the chemical

composition and surface chemistry or cleanliness of the interfaces! 'tremely clean

interfaces can $e produced and retained during processing and su$se&uent

consolidation $y conducting the e'periments under +


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echanical alloying produces nanostructured materials $y the structural disintegration

of coarser-grained structure as a result of se#ere plastic deformation! echanical

alloying consists of repeated welding fracturing and rewelding of powder particles in a

dry high-energy $all mill until the composition of the resultant powder corresponds to

the percentages of the respecti#e constituents in the initial charge! 4n this process

mi'tures of elemental or prealloyed powders are su$>ected to grinding under a

protecti#e atmosphere in e&uipment capa$le of high-energy compressi#e impact forces

such as attrition mills #i$rating $all mills and shaer mills! , ma>ority of the wor on

nanocrystalline materials has $een carried out in highly energetic small shaer mills!

he process is referred to as mechanical alloying when one starts with a $lended

mi'ture of elemental powders and as mechanical milling when one starts with single

component powders such as elements or intermetallic compounds! 5hile material

transfer is in#ol#ed in mechanical alloying no material transfer is in#ol#ed in

mechanical milling! hese processes ha#e produced nanocrystalline structures in pure

metals intermetallic compounds and immisci$le alloy systems! 4t has $een shown that

nanometer-sized grains can $e o$tained in almost any material after sufficient milling

time! he grain sizes were found to decrease with milling time down to a minimum

#alue that appeared to scale in#ersely with the melting temperature! ?och and

"uryanarayana ha#e recently summarized the process of mechanical alloying;milling

and the characteristics and properties of the nanocrystalline materials thus o$tained!

@owder contamination (from the milling tools and;or the atmosphere) is usually a matter

of concern with this process especially when reacti#e metals and;or long milling times

are in#ol#ed some remedial measures ha#e $een suggested in recent years! 4n recent

years the process of se#ere plastic deformation of $ul solids ($y the e&ual-channel-

angular pressing torsion straining and accumulati#e roll $onding techni&ues) has $een

shown to produce ultrafine-grained structures! #en though the grain size is strictly not

in the nanometer range (it is usually a$out 0!2A0!B m) there has $een considera$le

amount of wor on the structure and properties of materials produced $y these

methods essentially due to the possi$ility of producing $ul materials possessing

su$micron grain sizes!

2. lectrodeposition (for !iquid starting phase)

his is a simple and well-esta$lished process and can $e easily adapted to produce

nanocrystalline materials! lectrodeposition of multilayered (1.) metals can $e

achie#ed using either two separate electrolytes or much more con#eniently from one

electrolyte $y appropriate control of agitation and the electrical conditions (particularly

#oltage)! ,lso 2. nanostructure crystallites can $e prepared using this method $y


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utilizing the interference of one ion with the deposition of the other! r$ and his

colla$orators ha#e e'tensi#ely used this process to study the synthesis and properties

of 2. nanocrystalline materials! 4t has $een shown that electrodeposition yields grain

sizes in the nanometer range when the electrodeposition #aria$les (e!g! $ath

composition p


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inert gas condensation method produces e&uia'ed (2.) crystallites! he crystal size of

the powder is typically a few nanometers and the size distri$ution is narrow! he crystal

size is dependent upon the inert gas pressure the e#aporation rate and the gas

composition! 'tremely fine particles can $e produced $y decreasing either the gas

pressure in the cham$er or the e#aporation rate and $y using light (such as

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Nano Crystalline