Chapter I

The New Alchemy And The World of Neglected Dimensions

1. THE NEW ALCHEMY AND THE WORLD OF NEGLECTED DIMENSIONS

1 .1 The Emerging World of Materials Science and Technology

Materials have always influenced the basic fabric of human life. Using

new and ultraprecise technologies, researchers are probing worlds far tinier than

the submicroscopic realm of viruses.' They are peering into the world of atoms

and unlocking the secrets of how they interact. Using technology developed

primarily to deal with the rapidly decreasing dimensions of microelectronics,

modem-day alchemists are rearranging molecules - and even atoms - to assemble

novel substances.

Using modeling software that incorporates a wealth of knowledge from the

exploding field of materials research, one can design entirely new substances,

atom by atom. These include custom-made ceramic superconductors, and

ultrahigh strength plastics and alloys. There are reports1 of a super alloy of five

metals and the resulting extra-tough steel has been tested for bearings in space

shuttle engines and turbo pumps, which now must be rebuilt after every flight.

Researchers are making 'aerogels', often called 'frozen smoke', airy silicon

concoctions that far outperform the best insulators, ceramics pliable enough for

use in car or jet engines and composites embedded with artificial 'nerves' and

'muscles' that let them respond to stress almost as a living thing would, which are

called 'smart materials'.' The most dramatic advances have come in electronics.

To meet the increasing demand for improved performance in storing and

processing data, researchers are working harder than ever on techniques and

equipments to shrink silicon circ~itry.~"' One of the newest tools is the scanning

tunneling microscope (sTM)',~ that uncovers incredibly accurate information

about the positions of individual atoms on surfaces. The new materials are

pushing the speed of chips and computers to the edge of physical limits." These

advances, leading eventually to products such as hand held super computers, will

change the way of life.

The most important outcome of present research in materials science is the

creation of an entirely new class of materials - the nanophase materials often

called in the synonyms nanocrystalline materials or nanostructured materials?22

In nanophase materials, each crystal is less than 100 nm that is smaller than most

viruses. The individual crystals of most metals or ceramics consist of millions or

billions of atoms. But the grains that make up nanophase materials contain just a

few thousand. After these grains are squeezed under pressure, into a solid

material, they exhibit remarkable properties. Such superdense ceramics'37 can be

bend and molded like plastics and extruded or formed into a final shape without

the shrinkage normally typical of ceramics. Nanophase versions of metals such as

copper, palladium, nickel, etc. and inter-metallics are five times stronger as their 23-30 large-grained counterparts. Other extraordinary properties - optical, 31-35

36-40 magnetic and electrical proper tie^^'"^ - have been reported for nanophase

materials.

1.2 History of Nanostructured Materials

Nanostructured materials apparently had its genesis with the Big-Bang.

The structures of the earliest meteorites suggest that matter is formed by the

condensation of atoms into larger masses under the action of gravity.7 Many

examples of the natural nanostructures can be found as well in biological systems

from seashells to the human body.

Fine dispersion of gold14 was synthesized by Michael Faraday as early as

1857. In 1861, British chemist Thomas Graham coined the term colloid as colloid

chemistry was beginning to develop.13 Woltgang Ostwald classified the dispersed

systems of particles with sizes of about 1-100 nm as colloid. John Tyndall,

Hermann Helmholtz, Lord Rayleigh, James Clerk Maxwell and Albert Einstein

were among the physicists who studied the characteristics of colloids around 1900.

In 1915, Wo Ostwald entitled his famous book on colloid chemistry "The World

of Neglected Dimensions", implying that this was a science in its own right."

During 1930s, the new commonly used gas-condensation technique for synthesis

of small particles of materials was pioneered by A. H. Pfund in the United States

and H. G. Burger and P. H. Van Clittert in ~ e r m a n ~ . ' ~ Certain types of fine

particles22 have been produced commercially for more than half a century. During

the same period, Arne Tiselius studied the properties of protein and other

biological colloids and developed a precise electrical migration device.I3 In 1934,

the so-called Langmuir-Blodgett method became a new tool in surface chemistry.

By early 1940s, precipitated and fumed silica fine particles were being

manufactured and sold in the United States and ~ e r m a n ~ . ' ~

As long ago as 1959, Nobel laureate physicist Richard Feynrnann, in a

lecture before the annual meeting of the American Physical Society, predicted

about the creation of materials with new properties by the manipulation of matter

on a small scale:

"I can hardly doubt that when we have some control of the arrangement of things on a small scale we will get an enormously greater range of possible properties that substances can have" and "there is aplenty of room at the bottom".

-Feynman

By these keywords7, he strongly pointed out that there is a lot of scope for

research on materials on a very small scale. By 'small scale', again he meant the

dimensions of particles at the nanometer scale.

The theoretical formulation of quantum confinement of electrons by Royo

Kubo, at Tokyo University in the 1960s did much to launch the interest in the

nanostructuring of condensed matter, although ultrafine particles had long been

synthesized and used for their unique He pointed out that electrons in

metallic ultrafine particles are in a unique situation and do not obey Fermi

statistics because the number of such electrons is small. Kubo also noted that for

particles smaller than 10 nrn, it is difficult either to add or to remove a single

electron and such particles have a stronger tendency to remain electrically neutral.

The way this affects specific heat, magnetization and superconductivity is now r 7,13 called 'Kubo effect . Following Kubo's work, researchers began to understand

that ensembles of matter containing only tens to thousands of atoms could have

structures and properties quite different from those of conventional materials on a

coarser size scale, and work began in earnest around the world to investigate

ultrafine particles and ultrathin films of matter.

Also, in the 1960s, Ryozi Uyeda and his coworkers at Nagoya University

used electron microscope A d electron diffraction to determine the morphologies

and crystal structures of single particles of metals and metallic compounds. They

also made advances in producing relatively clean ultrafine particles through

evaporation and condensation in an inert gas. The 1960s saw the use of arc

furnaces to produce submicron particles of alumina, carbonated tungsten, Si3N4,

and other heat and acid resistant carbonates and nitrides of active metals and rare

earth metals.13

The interest on nanometer-sized particles in the 1970s included both

experimental studies and theoretical calculations of atomic and electronic

structures. Researchers during these years found that ultrafine iron particles that

are carefully covered with an oxide film less than 1 nrn thick are very stable and

resistant to weather corrosion at normal temperatures.43'4 During 1970s magnetic

fluids containing suspended iron oxide nanoparticles, which evolved from

technology developed at NASA in the late 1960s, entered the US market.14 By the

same period researchers in Japan developed 30 nm particles of magnetic alloys for

use in magnetic tapes. They found that an ultrafine particles made form an

iron-nickel-cobalt alloy exhibited excellent recording characteristics. The

Research and Development Corporation of Japan in 1971 instituted a five-year

development programme focused on metallic nanopowder production for magnetic

recording tapes.I4

The synthesis of materials by consolidation of small particles was first

suggested in the early 1980s by Herbert ~ l e i t e r ~ at the University of Saarlandes in

Germany and was applied initially to metals and then to nanophase ceramics. In

1981, a five-year ultrafine particle project known as Exploratory Research for

Advanced Technology (ERATO) was launched in Japan.

The 1990s have seen ever-increasing interest in nanostructured materials.

More than 5000 papers detailing research developments in the field were

published in the scientific literature during this period.

1.3 The Nanophase Regime

The properties of nanoparticles are so strikingly different from those of the

bulk material that some authors are even inclined to classify them as a new (fifth)

aggregate state of matter: liquid phase, solid phase, nanophase, gas phase and

plasma.58 The evolution of the structural, electronic and other properties, as atoms

form progressively larger clusters (nanocrystals) and particles leading to a

macroscopic solid has long been a challenging problem for solid state and

theoretical physicists. Many quantities such as the temperature, surface tension,

surface area and even volume that are used in the description of macroscopic

systems become ill-defmed as the size of the particle decrease^.'^"^ To understand

how molecules evolve into solids, the study of small particles that contain a few to

several thousand atoms are critical. They provide a new arena for probing the

interactions between atoms and offer new hope for the synthesis of new materials

possessing unusual proper tie^.^'

1.4 Unique Properties of Nanoparticles

Some of the unique characteristics of nanoparticles, which identify them from the

bulk material, are:

*:* Large surface-to-volume ratio: The uniqueness of very small particles lies in

their large surface-to-volume ratio.62b3 In the nanophase regime the fraction of

atoms associated with the surface is large and as the size of the particle

decreases the characteristics of the surface atoms become increasingly sharper.

Assuming that grains have the shape of spheres or cubes the volume fraction of

nanocrystalline materials associated with the boundaries can be calculated as

V& = 3d/D where d is the average grain boundary thickness and D is the

average grain diameter.I9 The volume fraction of atoms in the grain

boundaries can be as much as 50 % for 5 nm grains, about 30 % for 10 nm and

3 % for 100 nm grains.

*:* Quantum size effect: The dependence of the specific properties of nanoparticles

on their geometrical dimension is termed quantum size effect. 32-34.64-68

$3 The defect structure : Nanocrystalline materials contain such a high density of

defects (point defects, dislocations, dangling bonds, grain boundaries,

interphase boundaries etc.) that the spacing between neighboring defects

approach interatomic distances?6p12 This increased defect density can play an

important role in the electrical, optical and mechanical properties of

nanocrystalline materials.

*:* Thermodynamic size e f f e ~ t s : ~ ~ . ~ ~ ~ n important class of size effects involves the

dependence of some intensive thermodynamic properties, eg. surface tension,

vapor pressure and characteristics of phase transition, on the sample size and

shape.

Any attempt to generate materials with properties beyond the present

limitations has to focus attention on generating microstructures with new types of

atomic arrangement andlor new chemical compositions.

As far as the atomic arrangement is concerned, all existing materials may

be divided into crystal and glasses, which differ by the presence or absence of

long-range order, whereas the short-range order is s imi~ar?~. '~, '~ As properties

depend primarily on short-range order, the availability of materials without

short-range order appears attractive. By the generation nanocrystalline materials,

a new solid state structure without short-range order (i.e. solids with a completely

disordered 'gas-like' struct~re) '~ and the alloying of components, which are

immiscible in the solid andlor molten state may, however, be achieved.

The basic feature of nanocrystalline materials is the utilization of the atomic

arrangements in the cores of lattice defects (grain or interphase boundaries) to

generate solids with a new type of atomic structure. Because of the constraints

exerted by the two adjacent lattices on the atoms in the core of an incoherent

interphase, the atomic arrangements in the cores of grain or interphase grain

boundaries differ from the unconstrained atomic arrangements of glasses and

ctysta~s.'~

A schematic cross section through a hard sphere model6 of a nanocrystalline

material is shown in figure 1.2. All atoms are assumed chemically identical. As

far as the atomic structure is concerned, two kinds of atoms may be distinguished:

'crystal atoms', the nearest-neighbor configurations of which corresponds to the

lattice configurations; and 'boundary atoms' characterized by the nearest-neighbor

configurations which are different from the lattice configurations. Hence, in terms

Fig.l.2 Schematic representation of a nanocrystalline material distinguishing between the atoms associated with the individual crystallites (full circles) and those constituting the boundary network (open circles)

of nearest neighbor configurations, a nanocrystalline material consists of a

crystalline component (formed by all the crystal atoms) and a boundary

component with a froth-like morphology (formed by all the boundary atoms). The

atomic structure of all the crystallites is identical. However, the atomic structures

of the boundaries are different because their atomic arrangement depends, among

other parameters, on the orientation relationships between the crystallites. If the

crystallites forming the nanocrystalline materials are oriented at random, the same

applies to all other boundaries. As nanocrystalline materials contain typically

about boundaries per cubic centimeters, the interfacial component represents

the sum over 1019 atomic arrangements, all of which are different. If the

interatomic spacing do not occur preferentially, the interfacial component is

expected to exhibit no preferred interatomic spacing apart from excluding

interatomic penetration. In other words, the interfacial component represents a

structure, which exhibits no short-range order. This does not imply that the grain

boundaries are disordered. In fact, every boundary is assumed to have a two-

dimensionally ordered structure, the periodicity and interatomic spacing of which

are different from boundary to boundary. The physical reason for this new type of

structure of a solid material are the crystallographic constraints imposed on the

atoms in the cores of the boundaries by the adjacent crystal lattices of different

orientations. In grain boundaries, the atoms can only relax into structures

compatible with adjacent crystal lattice. The interfacial component has an average

atomic density, which is 10 to 30 % less than the crystal density, depending on the

type of chemical bonding between the atoms. 19,71,72 In glasses and crystals, no

constrains of this type exist. This has led Gleiter to coin the nanocrystals as a

'new form of matter'.19

1.5 Synthesis of Nanocrystalline Materials

Several novel methods have been developed during the past two decades to

prepare nanocrystalline materials. Increased activity on the synthesis of

nanocrystalline materials dates back to the pioneering investigations of Herbert

~ l e i t e r . ~ , ~ He synthesized the ultrafine metal particles in gas condensation

technique. Since then a number of techniques have been developed in which the

starting material can be either in a solid, liquid or gaseous state. These include 71-75 76,77 inert gas condensation, mechanical alloying, rapid ~olidification,7~ spray 79,80 conversion processing, sputtering,8' electrochemica1,8~ arrested (controlled)

precipitation, 30-34.50-51.83-87 88-91 sol-gel process, microemulsion t e ~ h n i ~ u e : ~ - ~ ~ Ball

Milling, 96,97 98.99 hydrothermal method , solvothermal method,Io0 plasma

processing, 101,102 laser ablation, 103-105 . ion exchange rpocess,106 sliding wear,Io7

spark erosion,'08 torsional deformation technique,109 etc. The grain size,

morphology and texture can be varied by suitably modifying the process variables

in these methods.

A central underlying theme in all these methods is to energize the material

to bring it into a highly non-equilibrium (metastable) state (also including a

possible change of state from the solid to liquid or gas) through melting,

evaporation, irradiation, application of pressure, storing of mechanical, energy,

etc. The material is then brought to another lower-energy state by quenching or

related process when it can exist as a supersaturated solid solution, metastable

crystalline or quasicrystalline phase, or even in a glassy state, affording ample

opportunities to modify the crystal structure andlor microstructures. In principle,

any method capable of producing ultrafine polycrystalline materials (by increasing

nucleation rate and reducing the growth rate during formation) can be utilized to

produce nanocrystalline materials.

Even though all of the above methods have been used to produce

nanocrystalline phases, gas condensation, sol-gel process and controlled (arrested)

chemical precipitation techniques have been most commonly employed to produce

nanocrystalline materials in large quantities.

In chemical precipitation technique, the rate of nucleation consists of two

exponential factors.22 Of these, one is an expression of the probability of forming

a nucleus of size greater than the critical size, and the other is concerned with the

rate at which material can be supplied to the nucleus for growth. Once a nucleus

has exceeded the critical size it will continue to grow at a rate controlled by the

provision of fresh material in the correct orientation at the surface of the growing

nucleus and by the ease of formation of new bonds in the required positions. The

requirement for the production of a fine powder having dimensions of few

nanometers is initially for the formation of a large number of nuclei, the

subsequent growth of which is restricted. To produce many nuclei a high degree

of supersaturation is necessary and, as a result, nucleation will tend to be

homogeneous. Under these conditions, the size of the critical 'nucleus is

approximately inversely proportional to the degree of supersaturation. Rates of

nucleation and growth vary widely in reactions in which a precipitate is formed

and, consequently, there are wide differences in the effect of conditions on the

particle size of the final precipitate. After precipitation is complete, there is still

change in the particle size distribution in the precipitate. The agglomeration of the

particles can be eliminated by using a suitable stabilizer or capping agent2* The

capping agents are known to inhibit the growth of particles, since the polymeric

chain is strongly bound to the metal ions near the surface of the nanoparticles. ,3436

When a precursor is used for the production of a nanocrystalline material,

the particle size of the product depends mainly on two factors viz. temperature at

which the decomposition is performed and the rate of decomposition.22 The size

of the particles of the resulting powder can be controlled by suitably selecting the

above two parameters. A t h e r m ~ ~ r a m " ~ can be used to choose these factors.

1.6 Techniques for Characterization of Nanocrystalline Materials

The complete characterization of a nanocrystalline material requires a

variety of techniques and instruments. The nanocrystalline materials should be

characterized on both atomic and nanometerscale. The microstructural factors to

be taken into account during characterization include:

chemical and phase composition

size distribution, morphology and roughness of the primary particles

nature and morphology of the grain boundaries and interphases

surface area of the primary particles

number and size distribution of pores

whether single crystalline, polycrystalline or amorphous

state of surface and interfacial stress

nature and concentration of defects

presence of adsorbed film or adsorbed impurities

state of agglomeration

identification of residual trapped species from processing

Presently, a number of simple as well as sophisticated experimental

techniques are available that can yield structural information on nanocrystalline

materials. These include direct microscopic techniques such as Scanning Electron

Microscopy (SEM), Transmission Electron Microscopy (TEM), High Resolution

Transmission Electron Microscopy (HRTEM), Scanning Tunneling Microscopy

(STM), Field Ion Microscopy (FIM), Atomic Force Microscopy (AFM) and the

less direct x-ray and neutron diffraction techniques. Indirect spectroscopic

techniques such as EXAFS, NMR, Raman and Mossbauer spectroscopies and

Positron Annihilation Spectroscopy (PAS) have also been used. Other useful tools

include DSC, Mass Spectroscopy, X-ray Fluorescence and ESCA (XPS and UPS)

The broadening of x-ray diffraction line is primarily a measure of the

departure from single crystal perfection and regularity in the specimen. IIL,112

Thus, in addition to small size, lattice strain, dislocations, impurities and other

defects lead to line broadening. Nevertheless, as the only method which gives the

size of the primary crystallites irrespective of how they are aggregated or sintered,

x-ray diffraction broadening is of great value for research in nanocrystalline

materials. X-ray line broadening can also be used for analysis of particle shape.

Some of the diffraction lines will be markedly more broader than others. This

broadening is due to the shape and orientation of the particles.

1.7. Improved Properties and Applications

Nanocrystalline materials exhibit a variety of properties that are different

and often considerably improved in comparison with those of the conventional

coarser-grained polycrystalline materials. Several potential technological

applications are suggested based on the special attributes of nanocrystalline

material. 2.4.14

The improved properties, which may be put to use in industrial and technological

applications, include:

Thin films and coatings - the smaller the particle size, the thinner the coating

can be.

Electronic ceramics - reduction in grain size results in reduced dielectric

thickness and improved dielectric properties.

Strength-bearing ceramics - strength increases with decreasing grain size.

Cutting tools - smaller grain size results in a finer cutting edge, which can

enhance the surface finish.

Impact resistance - finer microstructure increases the toughness of high-

temperature steels

Cements - finer grain size yields better homogeneity and density.

Gas sensors - finer grain size gives increased sensitivity.

Adhesives - finer grain size gives thinner adhesive layer and higher bond

strength.

Chromatography - the increase in specific surface area associated with small

particles, allows column lengths to be reduced.

The vast range of technological and industrial applications of fine particles

is largely a direct result of their diversity of structure on microscopic and

mesoscopic scales. Industry has long used fine particles in dyes, pigments,

adhesives and catalysts. Since reducing the grain size to nanometer-dimensions

can provide increased strength and hardness, supertough and superstrong ceramics

can be synthesized.19 Such superceramics may find use in several engineering

applications such as high efficiency gas turbines, and in aerospace and automotive

components.' Nanocrystalline materials can be made into wear-resistant coatings

or pressed and sintered to make rigid objects. Applications of such powders

include fine drill bits and cutting tools.

Since the optical transparency of nanocrystalline ceramics can be controlled

by controlling the grain size and porosity, they may find application in optical

filtration technology. The main demand for optics having surface roughness in the

0.1 nm range arises from the requirements for high energy laser mirrors, to reduce

laser-induced damage,''' and to minimize scatter in laser gyroscopic optics and x-

ray optics.

The increased diffusion rates in nanocrystalline metals and ceramics

considerably reduce the temperature at which sintering can occur.19 The enhanced

diffusivity can be used to make gas sensors and fuel cells capable of operation at

much lower temperature.4

The electrical and magnetic properties of nanocrystalline materials will

probably form the basis of their widespread and industrial applications. The

phenomenon of giant magneto resistance (GMR)'~ is now shown to be present

even in equiaxed nanocrystalline materials. Thus, nanocrystalline materials

exhibiting large GhtR can be promising candidates for the reading heads of the

next generation of information systems. The excellent combination of soft

magnetic properties can be useful in producing tape-wound cores for common

mode chokes, saturable reactors, high frequency transformers and magnetic heads.

Another useful property of nanocomposites is the magnetocaloric effect, which

can be used for magnetic refrigeration replacing compressed gas containing

harmful CFC'~.

Fine particles of latex, which is an organic polymer, are commonly used as

a vehicle for biological and chemical manipulation in the pharmaceutical industry

and other fields." Aerosols, which are fine dispersions of a solid material in a gas,

are used in agriculture, forestry, military technologies, and medicine.

Nanoparticulate thin films possessing desirable electronic qualities would

be of great interest in microelectronics. To meet the increasing demand for

improved performance in storing and processing data, researchers are working

harder than ever on techniques and equipments to shrink silicon circuitry.

Chipmakers are entering a realm where transistors will be too tiny to print on

silicon. Instead, they will be grown in the material as clusters of atoms. 114,115

Nanometer-sized structures perpendicular to the substrate have been produced by

x-ray, electron- and ion-beam t e ~ h n i ~ u e s ~ . " ~ Nano-devices based on GaAs have

captured a significant amount of attention because they offer the highest digital

processing speed coupled with the additional advantages over the silicon of having

a higher temperature tolerance and having greater radiation re~istance.~

A bridge-like Josephson junction having a bridge length of the order of the

coherence length of superconducting electrons, which is about a few nanometers

offers an entirely different approach to the development of high-speed digital

circuits.

1.8 A Look into the Future

Research publications in various journals over the past two decades

indicate that there are tremendous opportunities for synthesizing nanostructured

materials with new architecture. This synthesis can be done from atomic or

molecular precursors via the assembly of atom layers and clusters and by a myriad

of other techniques now becoming available, such as nanoscale lithography and

biological templating.

By extrapolating thin film technology and nanoscale lithography for

miniaturization, it may take approximately one decade for electronic switches to

be reduced to molecular dimensions. The impact of molecular biology and genetic

engineering has thus provided a stimulus to attempt engineer upwards, starting

with the concept that single molecules, each acting as an electronic device in their

own right, might be assembled using nanotechnology, to form molecular

electronic devices (MEDs) or open biochip computers (BCCs). Conducting or

insulating molecular wires would be built up in some areas. This field of

molecular electronic devices is a promising filed.

Nanoelectronics will be the watchword of the coming revolution in

semiconductor devices. Chipmakers are entering a realm where transistors will be

too tiny to print on silicon. Instead, they will be 'grown' as clusters of atoms.

Tinier transistors can be packed more closely, which means more powerful

circuits, which run faster. When these technologies are perfected, they will bring

'quantum leaps' in performance of all things that use chips. A shirt-pocket

supercomputer as well as memory chips that hold a library can be real. A

technique called band-gap-engineering looks like the best way to harness quantum

electronics. Researchers are looking to band-gap engineering to fashion quantum

wires having a cross-section of about 30 x 30 atoms.

It is now becoming apparent that scanning tunneling microscopy (STM)

may provide the basis of a new technology, which can be called scanning

tunneling engineering. An important broadening of STM technology is emerging

through the realization that the tip can be employed for purposes other than

microscopy, and that the tip may be used to modify the surface of the sample in a

number of ways. The STM tip has demonstrated its ability to draw fine lines,

which exhibit nanometer-sized structure, and hence may provide a new tool for

nanometer lithography. "7x"8 The first step in atomic-scale writing and reading

have already been taken, and if the technology could be fully implemented, would

lead to the development of computers unprecedented power and speed.

In scanning probe microscopy, a tiny sharpened tip, such as that in STM,

can move atoms and create atomic-scale images.2.4 If many developmental

hurdles can be overcome, the technology holds the promise of being suitable for

storing bits of information by moving atoms 'on' or 'off a surface.

According to Bernard S. Meyerson, an IBM physicist, "By getting the

dimensional control down to atomic level you can do pretty amazing

things ........." and ".....never bet on what somebody can't do"'. K. Eric Drexler,

the avatar of nanotechnology predicts that, in the course of a few years, by using

nanotechnology one could produce anything form a rocket ship to minute disease-

fighting submarines that roam the blood stream and like biological cells, the robots

that populate in a nanofactory could even make copies of themselves. The

Drexlerian future posits fundamental social changes: nanotechnology could

alleviate world hunger, clean the environment, cure cancer, guarantee biblical life

spans or concoct superweapons of untold horror. For the time being, nanoists can

only wait for these breakthroughs to arrive, while continuing to formulate their

computerized models of molecular machine parts.

The keys to the future of nanostructured materials, however, are our ability

to continue to improve the properties of materials by artificially structuring them

on nanometer length scale and our ability to develop the methods for producing

these materials in commercially viable quantities. Based on what we have learned

to date about mesoscopic physics and on the successes secured by the entry of

nanostructured devices into industry, it appears that the future holds great promise

for nanostructured materials.

1.9 Scope of the present study

Present work consists of the synthesis, characterization and a systematic

investigation of electrical properties of nanophase ZnO and its nanocomposites

with other nanoparticles of A1203, CdO and silver. These materials were selected

for the present study in view of their technological importance. The size and

crystallinity of the nanoparticles were determined using x-ray diffraction and

Transmission Electron Microscopy (TEM). The effect of particle size on the

vibrational properties was investigated by FT-IR spectroscopy. The ac electrical

properties of nano-ZnO and of the nanocomposites Zn0-A1203, ZnO-Ag and

ZnO-CdO were analyzed in detail by using impedance spectroscopy. The

investigation of ac and dc conductivity and dielectric properties of consolidated

nanoparticles of these materials were performed. The change in properties of

these materials compared to their bulk counterparts is attributed to the size effect,

defect structure of the grain boundaries, grain boundary structure modification and

grain boundary interactions.

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