Nanocomposites
Nanocomposites are multyphase materials where at least one phase has at least
one dimension of the order 100 nm or less.
- novel properties due to quantum confinement,
- synergistic improvement of properties of the constituents,
- large interface surface area A (alumosilicate-polymer has A about 700 m2/cm3).
d
d
Λ
(b)
(c)
(a)
(a) nanolayered composites with nanoscale bilayer repeat length Λ; (b)
nanofilamentary (nanowire) composites composed of a matrix with embedded
filaments of nanoscale diameter d; (c) nanoparticulate composites composed
of a matrix with embedded particles of nanoscale diameter d.
Engineering applications: functional and structural nanocomposites.
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Functional Nanocomposite - Semiconductor Superlattice
Ec
ψ
Alternating layers.
Quantization of energy of
electrons:
∆E
Egap for
Egap for
En = n2h2/8m∗w2
AlxGa1−xAs
GaAs
w is the well width.
Electronic and
optoelectronic applications
thickness of
GaAs layer
Ev
Energy band diagram of GaAs/GaAlxAs1-x quantum well. An electron
(represented by its wavefunction ψ) is partially confined in the quantum
well of width equal to the GaAs thickness. The barrier height ∆E is equal
to the difference in the energies of the bottom of the conduction band Ec
for the two layer materials. Ev is the energy of the top of the valence band
and Egap is the band gap energy.
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Metal reinforced by ceramic nanoparticles is a structural
composite.
Yield strength is governed by the
motion of dislocations.
hard precipitate particles
Orowan bowing mechanism for
the dislocation motion in
nanocomposites.
⇑
The stress needed to bow around
the particle:
dislocation
σ = Gb/λ,
λ
where G and b are shear modulus
and Burgers vector, λ is the
interparticle spacing.
Precipitate particles of spacing λ acting
as obstacles to dislocation motion.
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Nanolayered Composites
Superlattice is a multylayer structure
of metals, semiconductors, ceramics
and their combinations.
Film deposition techniques are used to
fabricate superlattices:
- Physical Vapor Deposition
(evaporation and sputtering);
- Molecular Beam Epitaxy for precise
deposition of semiconductor
superlattices (GaAs/GaAlxAs1-x);
- Pulsed Laser Deposition especially
effective for multycomponent
materials (high T superconductors,
ferroelectrics);
TEM micrograph showing a
cross-sectional view of an
InAs-GaSb (100) superlattice.
Chemical Vapor Deposition is used
for the various structures.
Langmuir-Blodgett and self-assembly
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is used for multylayered plastics.
Functional Layered Nanocomposites
Ferromagnetic superlattices: Magnetoelectrical effect in Co/Cu structures with
antiparallel alignment (Giant Magnetic Resistance). Is used in hard disk drives.
Bragg reflectors use the layered structures with different electron density.
Reflection strongly depends on the bilayer period. X-ray optics. Polymer
reflecting films.
Electronics and optoelectronics on semiconductor superlattices.
Structural Layered Nanocomposites
Protective coating applications. Hardness enhancement of metallic and
ceramic multylayers for repeat length below 100 nm (dislocation pinning
effect associated with interfaces, dislocation image forces associated with
stress at interfaces impeding the dislocation mobility).
Metallic structures exhibit improved wear resistance and fracture strength.
Coextruded polymer composites composed of brittle (high tensile strength)
and ductile layers have increased fracture toughness (heavy-duty wrapping
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packing materials).
Nanowire Composites
Nanofilamentary composites (Cu99.9%/Nb0.1%) are made from two-phase
ingot by drawing and swaging. Aspect ratio of the Nb filaments is up to 106.
High electrical conductivity and mechanical strength.
Arrays of nanowires. Electrodeposition in nanoporous membranes
(polycarbonates) is the major fabrication method.
Membranes are produced by high energy
ion irradiation (nuclear tracks) and
etching (down to tens of nm).
Electrodeposition is performed in an
electrochemical cell.
Applications for field emitters (flat
displays) and magnetic data storage.
SEM micrograph of electrodeposited Fe/Co nanowires (the polycarbonate
matrix in which the wires were embedded has been completely dissolved).
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Composites with Nanoparticles
Thing film processing methods are used to produce nanoparticulate composites.
Electrodeposition can be used to fabricate Ni/Al2O3 films.
TEM micrographs of Ni/SiO2 granular metal films deposited by physical
deposition. Metal phase shows a percolation threshold at 50% composition.
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Strong structural nanocomposites can be
made of Al2O3 and SiC.
Polymer-based nanocomposites:
thermoplastics reinforced with carbon
nanotubes. The high elastic modulus and
tensile strength makes CNTs the best
filler for nanocomposite polymers.
Self-assembly from hexane dispersion of
FePt and Fe3O4 nanoparticles form
nanparticulate composite.
TEM micrographs of binary
nanoparticle assemblies. (a)
Fe3O4(4nm)-Fe58Pt42 (4nm) assembly;
(b) Fe3O4 (8nm)- Fe58Pt42 (4nm)
assembly; Fe3O4 (12 nm)- Fe58Pt42 (4 nm)
assembly.
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Granular metal/ceramic composites have percolation thershold
for electrical conductance of many orders of magnitude. Novel
electronic applications.
Nanocomposites with ferromagnetic nanoparticles are
promising for magnetic data storage (GMR similar to that
observed in the layered composites).
Control over the composition and size of the
nanoparticles allows a precise tuning physical
properties of particulate nanoconposites.
Technology of particulate nanocomposites is not that
demanding as that of the layered nanocomposites
(selfassembly).
Many particulate nanocomposites are used in industry
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