Scale, structure and
behaviour
Lecture 2
MTX9100
Nanomaterjalid
OUTLINE
-How are crystals structured?
-Why and how does nanoworld differ from the
world we live in?
-When does size matter?
What is the smallest particle in matter?
What are things made of?
Everything is made of atoms.
In the Bohr atomic
model, there is a nucleus
consisting of protons
with a positive charge and a
mass of 1.67 × 10−27 kg;
and neutrons
with no charge but with the
same mass as the protons.
The nucleus is surrounded by
electrons with a negative
charge and a mass of 9.11 ×
10−31 kg that revolve around
the nucleus in discrete orbits.
What is particular about electrons?
(1) the number of electrons in an
electrically neutral atom
depends on the number of
protons in the nucleus,
(2) an electron will enter the
orbital possessing the least
possible energy, and
(3) only two electrons can fit into
any one of the energy states.
How do atoms bond together?
Energy of bonding
Ionic bonding
• Occurs between strongly electronegative and
strongly electropositive atoms
• Electron(s) are transferred from electropostive
atom to electronegative atom, thereby forming a
cation (positively charged) and an anion (negatively
charged)
Covalent bonding
Atoms form stable electron structures, i.e. those of inert gases, by
sharing of electrons with other atoms
• F2, Cl2,… – Group VII – diatomic molecules
• Resulting bonds are strongly directional
Metal bonding
Valence electrons ( in outer
shell) leave atoms and form a
“sea” of free electrons
Positively charged ion cores
are shielded from one another
by the free electrons
Free electron acts as the
“glue” that hold positive cores
together
Non-directional
High thermal and electrical conductivity
Van der Waals bonding
Isolated Ar
atom
Due to statistical nature of electron
motion, occasionally the center of
negative charge is spatially different
than the center of positive charge =>
temporary dipole
Secondary
bond
•
Dipole moment
produced by
instantaneousasymme
try of electron
charge distribution
•
Coulombic attraction
occurs between
positive end of one
dipole and negatively
charged end of
another
Hydrogen bonding
Atomic interactions
Interatomic interactions energy
Interatomic forces
Energy – Force
Crystal structure
In solids, atoms are often arranged on a periodic lattice,
forming 3D crystals with many atoms
Crystal structures of metals
Compounds structure
Materials Packing
Energy and packing
Polycrystalline structure
Three-dimensional structures or bulk materials
with a nanometer-sized microstructure are
assembled of nanometer-sized building blocks
or grains that are mostly crystallites.
Engineering materials
There are currently over 50,000 engineering materials!
Schematic classification of nano – materials: (a) three – dimensional structures; (b) two –
dimensional; (c) one – dimensional; and (d) zero – dimensional structures.
Classes of materials
Metallic materials (consist principally of one or more metallic
elements, although in some cases small additions of nonmetallic elements
are present; When a particular metallic element dissolves well in one or
more additional elements, the mixture is called a metallic alloy.
Ceramic materials (are composed of at least two different elements).
Polymeric materials (consist of long molecules composed of
manyorganic molecule units)
Composites (are formed of two or more materials with verydistinctive
properties, which act synergistically to create propertiesthat cannot be
achieved by each single material alone)
Electronic materials
Biomaterials
Nanomaterials
Metals
Characteristics:
– High electrical and thermal conductivity
– Ductile/malleable
– Moderate to high strength
– Atoms arranged on periodic lattice, i.e.
crystalline
Ceramics and polymers
Stoichiometric compounds made of
electropositive
(metallic) and electronegative (nonmetallic) elements
• Examples: Al2O3, SiC, ZrO2 , WC
• Characteristics:
– Low electrical and thermal
conductivity
– High melting point
– Very hard
– Brittle (flaw-sensitive)
Made of long molecules, with very
strong intramolecular bonds but
weak intermolecular bonds.
• Examples: Polyethylene (PE),
polymethylmethacrylate (PMMA, aka
acrylic or plexiglass), polystyrene
(PS), polyvinylchloride (PVC), epoxy,
elastomers
Characteristics:
– Low electrical and thermal
conductivity
– Low melting point
– Relatively weak (compared to
metals and ceramics)
Composites
Composite materials are made of two or more distinct
phases, often from dissimilar material categories, e.g.
polymer + ceramic, metal + ceramic
• Examples: Glass fiber-reinforced polymers (GFRP),
carbon fiber-reinforced polymers (CFRP),
WC/Co (“cermets”), C/C, nanotube reinforced composites
• Characteristics:
– Properties usually
intermediate to those of the
constituents
Scale changes everything
There are enormous
scale differences in
our universe!
At different scales
– Different forces dominate
– Different models better explain
phenomena
Four important ways in which nanoscale
materials may differ from macroscale
materials
– Gravitational forces become negligible and
electromagnetic forces dominate
– Quantum mechanics is the model used to
describe motion and energy instead of the classical
mechanics model
– Greater surface area to volume ratios
– Random molecular motion becomes more
important
Dominance of electromagnetic forces
• Because the mass of nanoscale objects is so small, gravity becomes negligible
– Gravitational force is a function of
mass and distance and is weak
between (low-mass) nanosized particles
– Electromagnetic force is a function of
charge and distance is not affected by
mass, so it can be very strong even
when we have nanosized particles
–The electromagnetic force between
two protons is 1036 times stronger than
the gravitational force!
Sources: http://www.physics.hku.hk/~nature/CD/regular_e/lectures/images/chap04/newtonlaw.jpg
http://www.antonine-education.co.uk/Physics_AS/Module_1/Topic_5/em_force.jpg
Quantum Effects
• Large ZnO particles
– Block UV light
– Scatter visible light
– Appear white
• Nanosized ZnO particles
– Block UV light
– So small compared to the
wavelength of visible light that
they don’t scatter it
– Appear clear
The following are among the most important things that quantum mechanical models can
describe (but classical models cannot):
• Discreteness of energy
• The wave-particle duality of light and matter
• Quantum tunneling
• Uncertainty of measurement
Sources: http://www.apt owders.com/images/zno/im_zinc_oxide_particles.jpg
http://www.abc.net.au/science/news/stories/s1165709.htm ; http://www.4girls.gov/body/sunscreen.jpg
Discreteness of energy
It is the fact that electrons can only exist at discrete energy levels that prevents
them from spiraling into the nucleus, as classical models predict.
This quantization of energy, along with some other atomic properties that are
quantized, give quantum mechanics its name.
In 1901, Max Planck published an analysis that succeeded in reproducing the
observed spectrum of light emitted by a glowing object. To accomplish this, Planck
had to make an ad hoc mathematical assumption of quantized energy of the oscillators
(atoms of the blackbody) that emit radiation.
It was Einstein who later proposed that it is the electromagnetic radiation itself that is
quantized, and not the energy of radiating atoms.
In 1905, Albert Einstein provided an explanation of the photoelectric effect, a
hitherto troubling experiment that the wave theory of light seemed incapable of
explaining. He did so by postulating the existence of photons, quanta of light energy
with particulate qualities.
Extended internal surface
Surface to Volume Ratio Increases
Since reactions occur at the interface of two
substances, when a large percentage of the particles
are located on the surface, we get maximum exposed
surface area, which means maximum reactivity! So
nanosized groups of particles can make great
catalysts.
Random molecular motion is
significant
• Tiny particles (like dust) move
about randomly
– At the macroscale, we barely see
movement, or why it moves
– At the nanoscale, the particle is
moving wildly, batted about by
smaller particles
• Analogy
– Imagine a huge (10 meter) balloon being batted
about by the crowd in a stadium. From an airplane,
you barely see movement or people hitting it; close
up you see the balloon moving wildly.
At the nanoscale, these
motions can be on the same
scale as the
size of the particles and thus
have an important influence
on how particles behave.