NANOSTRUCTURED
MATERIALS
What are nanostructured materials??
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a broad class of materials, with microstructures modulated
in zero to three dimensions on length scales less than 100 nm.
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materials with atoms arranged in nanosized clusters, which
become the constituent grains or building blocks of the material
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any material with at least one dimension in the 1-100m range
Conventional materials have grains sizes ranging from microns
to several millimeters and contain several billion atoms each. Nanometer
sized grains contain only about 900 atoms each. As the grain size
decreases, there is a significant increase in the volume fraction of grain
boundaries or interfaces. This characteristic strongly influences
the chemical and physical properties of the material. For example,
nanostructured ceramics are tougher and stronger than the coarser grained
ceramics. Nanophase metals exhibit significant increases in yield
strength and elastic modulus. It has also been shown that other properties(
electrical, optical, magnetic...) are influenced by the fine grained structure
of these materials. Using a variety of synthesis
methods, it is possible to produce nanostructured materials in the
following forms: thin films, coatings, powders and as a bulk material.
There is also considerable interest in the generation of carbon
nanostructures, which are related to the famous Buckyball.
In addition, the use of nano-sized materials as fillers for
composite materials is generating interest. specifically in the case of
polymer nanocomposites.
Classes of nanostructured
materials:
There are several
different types of nanostructured materials. These range from zero
dimensional atom clusters to three dimensional equiaxed grain structure.
Each class has at least one dimension in the nanometer range, as shown
in Figure 1. Atom clusters and filaments are defined as zero
modulation dimensionality and can have any aspect ratio from
1 to ¥. Any
multilayered material with layer thickness in the nanometer range is classified
as one-dimensionally modulated.
Layers in the nanometer thickness range consisting of ultrafine grains
(nanometer range diameter) are two-dimensionally
modulated. This class includes coatings, buried layers
and thin films. The last class is that consisting of three
dimensionally modulated microstructures or nanophase materials.
(from R.W. Siegel, Nanophase
Materials, Encyclopedia of Applied Physics, vol. 11, VCH Publishers
1994, p 173)
Synthesis
The methods employed to produce nanostructured materials
are numerous, with each method having advantages and disadvantages depending
on the desired properties or application. Atom clusters are typically
synthesized via a vapor
condensation route which is essentially evaporation of a
solid metal followed by rapid condensation to form nano-sized clusters.
These resulting powders are essentially agglomerates of the nano-sized
atom clusters. The powders can be then be used as fillers for composite
materials or consolidated into bulk material. The cluster assembly
method can be used to produce ceramic or metal nanostructured powders.
The main advantage of this method is the extremely low contamination levels
and the ability to control the final cluster size by manipulation of the
process parameters such as temperature, gas environment and evaporation
rate.
Several different methods have been developed using the
vapor condensation concept. These include inert
gas condensation, chemical
vapor condensation, laser ablation, electron beam deposition,
arc evaporation....
Chemical Synthesis:
Both metals and ceramics can be produced using a variety of chemical approaches
such as sol-gel or thermal
decomposition. These methods provide large quantities of nano-sized
agglomerates at a low cost. Chemical processes also allow effective
control of the stoichiometry of the final product. However, the precursor
chemicals required may leave residual contamination on the particle surfaces,
which leads to difficulties in compaction and sintering. Additionally,
powders produced through wet chemical techniques often have difficulties
with agglomeration.
Other methods which implement chemical reactions include
Examples:
A common method of producing nanostructured powders is through
mechanical deformation, i.e. milling or shock deformation.
These processes produce nanostructured materials through severe mechanical
deformation of a coarse grained precursor material. The nanometer
sized grains nucleate within the dislocation cell structures located in
the shear bands. The final grain size is a function of the amount
of energy input during the milling as well as time, temperature during
milling and milling atmosphere. The precursor materials can be crystalline
or amorphous materials in the form of powders or tapes. Through mechanical
milling, it is possible to produce nanostructured powders of systems which
are otherwise immicsible. A major disadvantage of this method is
the possibility of contamination from the milling media due to the large
forces and energies involved in the milling process.
Other synthesis methods based on mechanical deformation
: friciton/sliding wear
Three dimensional nanostructured materials are also synthesized
through thermal crystallization of
an amorphous material. By controlling the nucleation and growth during
annealing of an amorphous material, one can produce a bulk material with
average grain sizes of less than 20 nm without the need for consolidation
and sintering steps. However, this process is limited to material
composition which are readily available in the form of a metallic glass,
which has an amorphous microstructure.
Microstructure
The atomic structure of nanostructured materials is unlike
that seen in glasses or crystals because of the large volume fraction (greater
than 50%) of grain boundaries or interfaces. Coarse-grained materials
have less than 3% of all atoms associated with grain boundaries or interfaces.
However, in materials with nano-sized grains between 5% and 50% of all
atoms are associated with grain boundaries or interfaces. Figure
2 shows the percentage of atoms in the grain boundaries as a function of
grain diameter. The grain boundary structure in these nanostructured
materials is similar to that of the conventional coarse grained materials.
Figure 2. Percent of atoms in grain
boundaries/interfaces as a function of grain size.
Properties
Applications
Thin films/Nanolithography
Biomedical/bioengineering:
In this field there is an enormous amount of research dealing with nanostructured
materials and how the biological community can benefit from the unique
properties. Research is currently being done to evaluate the possibility
of using nanostructured ceramics as a implant material. It is hoped
that the nano-sized microstructure will be an advantage in the interaction
of the implant with the bone cells. There is also interest in use
of nanocomposites for the replacement of bone. For more information
on the use of nanostructured materials in the bio field, check out these
sites:
Further information on nanostructured materials and nanotechnology
can be found at the following links:
Research on nanostructured materials
Company websites
Journals
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Last updated 11-24-97 by Paula Crawford
