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Horst Hahn
Technische Universität Darmstadt, Institute of Materials Science, Thin Films Division, Darmstadt, Germany
1Abstract
In this introductory paper an attempt is made to give an overview of the area of nanostructured
„materials irrespective of the synthesis process. The various microstructural features such as
clusters or isolated nanoparticles, agglomerated nanopowders, consolidated nanomaterials and
nanocomposite materials as well as all materials classes are considered. As an important com-
ponent of modern research on nanomaterials a section describes the various characterization
tools available. Based on these remarks some properties of nanostructured materials will be
summarized emphasizing the property-microstructure relationships. Finally, a brief outlook on
applications and initial industrial use of nanomaterials is presented.
2 Introduction
Nanostructures are plentiful in nature. In the universe nanoparticles are distributed widely and
are considered to be the building blocks in planet formation processes. Biological systems have
built up inorganic-organic nanocomposite structures to improve the mechanical properties or to
improve the optical, magnetic and chemical sensing in living species. As an example, nacre
(mother-of-pearl) from the mollusc shell is a biologically formed lamellar ceramic, which „ex-
hibits structural robustness despite the brittle nature of its constituents. [1] Figure 1 shows an
SEM imge of a fracture surface of an abalone shell exhibiting the CaCO
3
-platelets which are se-
Figure 1:
SEM image of a fracture surface of a Korean abalone shell showing the individual calcium-carbonate
platelets separated by organic compounds.
Unique Features and Properties of Nanostructured Materials
Nanomaterials by Severe Plastic Deformation. Edited by M. J. Zehetbauer and R. Z. Valiev
Copyright © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 3-527-30659-5
4
parated by organic compounds which exhibit nanometer dimensions. These systems have evol-
ved and been optimized by evolution over millions of years into sophisticated and complex
structures. In natural systems the bottom-up approach starting from molecules and involving
self organization concepts has been highly successful in building larger structural and functio-
nal components. Functional systems are characterized by complex sensing, self repair, informa-
tion transmission and storage and other functions all based on molecular building blocks.
Examples of these complex structures for structural purposes are teeth, such as shark teeth,
which consist of a composite of biomineralized fluorapatite and organic compounds. These
structures result in the unique combination of hardness, fracture toughness and sharpness, see
Figure 2.
Another example for a biological nanostructure is opal which exhibits unique optical proper-
ties. The self cleaning effects of the surfaces of the lotus flower have been attributed to the com-
bined micro- and nanostructure which in combination with hydrophobic groups give the surface
a water and dirt repellent behavior. [2] In the past few years, numerous companies have realized
products resembling the surface morphology and chemistry of the lotus flower such as paint,
glass surface and ceramic tiles with dirt repellent properties. The realization that nature can pro-
vide the model for improved engineering has created a research field called „bio-„mimicking or
bio-inspired materials science. It has been possible to process these ceramic-organic nanocom-
posite structures which provide new technological opportunities and potential for applications.
[3] Other exciting results have been published such as the biomimetic growth of synthetic fluo-
rapatite [4] in the laboratory and promising new technical applications of these nanomaterials
are envisioned. [5] Other man-made nanostructures were manufactured for their attractive opti-
cal properties, such as the colloidal gold particles in glass as seen in medieval church windows.
While plentiful man made materials with nanostructures have been in use for a long time
(partially without knowing it) a change of the scientific and technological approach can be iden-
tified over the past two decades. This change can be related to a few key ideas and discoveries:
the idea of assembling nanostructures from atomic, molecular or nanometer sized building
blocks, [6] the discovery of new forms of carbon, i.e., fullerenes [7] and carbon nanotubes, and
the development of scanning probe microscopy, [8] such as scanning tunneling microscopy
(STM) and atomic force microscopy (AFM). With the visionary goals many researchers world-
Figure 2: Example of a nanostructure found in nature: shark tooth with unique mechanical properties. The over-
all dimension of the tooth can reach several cm.
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wide have worked intensively on the development of novel or improved synthesis methods, new
and better characterization techniques and the measurement and the design of the properties of
nanostructured materials. In this paper some aspects of the immensely wide field will be de-
scribed. However, as the field of nanostructured materials is very broad including all classes of
materials as well as composites it is not possible and not attempted to consider all developments
and all research groups and industries working in this area.
3 Synthesis
The microstructure and properties of nanostructured materials depend in an extreme manner on
the synthesis method as well as on the processing route. Therefore, it is of utmost importance to
select the most appropriate technique for preparation of nanomaterials with desired properties
and property combinations. Synthesis techniques can be divided into bottom-up and top-down
approaches. The top-down approach starts with materials with conventional crystalline microst-
ructures, typically metals and alloys, and defects such as dislocations and point defects are in-
troduced by severe plastic deformation such as in equal channel pressing. The recrystallization
of the material leads to finer and finer grain sizes and under certain processing conditions to na-
nostructured materials. The advantage of these approaches is the fact that bulk nanostructured
materials with theoretical density can be prepared. An alternative to obtain theoretical dense
materials is the pulsed electrodeposition method developed by Erb and El-Sherik which yields
nanocrystalline strips, however, only with thicknesses of several hundred microns. [9] The bot-
tom-up approach includes many different techniques which are based on liquid or gas phase
processes. Classically, wet chemical processes such as precipitation and sol-gel have been em-
ployed to obtain nanoparticles, however, with the disadvantage of severe agglomeration. In the
gas phase metallic and ceramic nanoparticles have been synthesized by using Inert Gas Conden-
sation, Flame Pyrolysis (Aerosol process by Degussa) and chemical vapor based processes. The
major microstructural features in preparing nanoparticles for subsequent use are: nanometer si-
zed primary particles with narrow size distributions, minimum amount of agglomeration, good
crystallinity, etc.
Two techniques, chemical vapor synthesis (CVS) in the gas phase [10] and electrodeposition
under oxidizing conditions (EDOC) in the liquid phase, [11] together with the resulting micro-
structures will be presented in more detail and the advantages and disadvantages be discussed.
CVS is based on chemical vapor deposition (CVD) for the synthesis of thin films and coatings
by the decomposition of metalorganic precursors. Whether thin films are deposited by heteroge-
neous „nucleation or nanoparticles are formed in the gas phase by homogeneous nucleation is
determined by the residence time of the precursor in the hot zone of the reactor. The most im-
portant parameters determining the growth regime and the particle size are the total pressure,
the precursor partial pressure and the temperature of the reaction zone. A typical reactor set-up
is shown schematically in Figure 3 with one precursor source, the hot wall reactor, the thermo-
phoretic collector, the pumping unit and the control devices for pressure and temperature. The
hot wall concept operating at reduced pressures has been successfully scaled up in a cooperation
project with a large German corporation involved in the synthesis of nanopowders such as car-
bon black, titania and silica. [12]
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When two precursors are used, the precursor delivery can be modified in the following way:
(1) two precursors are introduced simultaneously into the reaction zone yielding doped nano-
particles (i.e. alumina doped zirconia); [13]
(2) two precursors are introduced into two concentric reaction tubes, reacted to form nanoparti-
cles and then mixed in the gas phase to yield a nanocomposite structure (i.e., alumina
mixed with zirconia) and
(3) in the first reaction zone the first precursor is decomposed to form nanoparticles by homo-
geneous nucleation which are subsequently coated in a second reaction zone by introducing
the second precursor under conditions which favor CVD deposition (i.e., alumina surface
coated zirconia). [14]
The experimental set-up of case 3) can be further modified by using a plasma reaction zone
with pulse option which allows the controlled functionalization with organic molecules and
polymeric shells. [15,16] Figure 4 shows a high resolution electron image of polymer coated ti-
tania nanoparticles where the crystalline titania core can be clearly distinguished from the amor-
phous organic shell on several grains.
Further evidence of the complete coating can be obtained by surface analysis, FTIR studies
and by dispersion „experiments in different organic liquids and water. The modification of the
surfaces of nanoparticles allows the improvement of dispersibility in various aqueous and or-
ganic solvents which is important for many ceramic processing steps (dip- or spin-coating, slur-
ries for ceramic processing, etc.) and for technical applications of dispersions. Additionally, the
inorganic core/polymer shell structure allows the preparation of polymer nanocomposites with
excellent separation between the inorganic nanoparticles.
A further variation by exact control of all synthesis parameters allows the growth of thick
nanocrystalline coatings on dense and porous substrates. Depending on the substrate tempera-
ture the porosity of the coating can be changed over a wide range up to theoretical density. This
intermediate stage, called CVD/CVS, has been successfully used to deposit a nanocrystalline
coating of yttrium stabilized zirconia on porous anode substrates for high temperature solid ox-
ide fuel cell applications. Figure 5 shows a high resolution scanning electron image of a coated
anode substrate. [17]
The processes leading to particle formation have been modeled and simulated by many au-
thors. The detailed description of these efforts is beyond the scope of this paper. A comprehen-
Figure 3: Schematic diagram of the major components of a CVS hot wall reactor: precursor source (liquid precur-
sor delivery system, LPDS), hot wall reaction zone, thermophoretic particle collector, pumping system, and pres-
sure and temperature control.
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sive overview of the modeling is given by Winterer. [18] Recently, with the availability of large
scale supercomputers and parallel PC clusters, the simulation of atomic processes involving
millions of atoms has become available. Several authors have employed the molecular dynam-
ics (MD) simulation technique to obtain details of the processes during particle formation, ag-
glomeration, and sintering. As the nanoparticles contain only a limited number of atoms, it is
possible to study diffusion and rearrangement processes leading to aggregation and particle
growth. Atomistic simulations are extremely useful in describing the initial stages of sintering
and equilibrium particle morphologies which determine the final structure and properties of
Figure 4: High resolution TEM of titania nanoparticles (crystalline core) coated with an amorphous polymeric
shell.
Figure 5: High resolution SEM of a nanocrystalline coating of yttrium stabilized zirconia on a porous anode sub-
strate.
