Transcript 2011-10-21-Belfast-Unit-3-Materials-and-their

Materials and their uses
Structure of Materials
The specification states;
Materials behave as they do because of
their structure; the way their atoms
and molecules fit together
You need to know;
- how the internal structure of a material
influences the way it behaves
- ways in which properties materials can
be modified by altering the structure of
the material
Using Materials
• The first record of the use of salt
dates back to around 6050 BC.
• It was used as part of Egyptian religious
offerings
• In ancient Rome salt was used as a
method of payment
(the origin of the word salary)
• Gold has been highly valued since
prehistoric times.
• It was associated with beauty, power
and wealth.
Around 1300 BC
Egyptian
hieroglyphs from
as early as 2600
BC describe gold
as ‘more plentiful
than dirt’
Around 2000 BC
• The word diamond derives from the
Greek word ‘Adamas’ meaning
unconquerable and indestructible
• Diamonds are thought to have been
mined in India around 800 BC
• Why choose the three materials?
 salt
 gold
 diamond
Materials behave as they do because
of their structure; the way their
atoms and molecules fit together
Properties of Materials
We have known many of the properties of
materials for thousands of years
Metals are shiny, they have a high melting
point, they are malleable, ductile, they are
insoluble and they conduct electricity*
Salt is crystalline, it is soluble in water, it has a
high melting point and it conducts electricity in
solution*
Diamonds are crystalline, they have a high melting
point, they are insoluble and they do not conduct
electricity*
*All later discoveries
Why?
• We know how materials behave – their
properties
•The next question is why?
•An important development in our
scientific knowledge pointed to the
answer
i.e. The understanding that electricity is
a flow of charged particles
The flow of charge is called the current and it is the rate at which
electric charges pass though a conductor. The charged particle can
be either positive or negative.
Conducting electricity
Two types of materials that we know
conduct electricity are
Metals
Salt
The search to find their ‘charged
particles’ eventually led to an
understanding of the structure and
properties of materials
The Atom
Bohr’s Atom
Metals
Metals conduct
electricity
They have charged
particles which are
free to move
Each atom loses control of its outer shell
electron resulting in a lattice of positive
ions surrounded by a ‘sea’ of electrons
Metallic bonding is the result of strong
electrostatic attraction between the
positive core and the negative ‘sea’ of
electrons
The strength of the bond gives metals
their high melting point
The melting point of gold is 1064oC
• Metals objects are formed by casting
• The process is controlled by
temperature and other factors
• As the metal cools small crystals
(grains) appear
• The crystals grow until they form a
solid mass of small crystals
Crystals in metals
• In a crystal the molecules of the
material lock together in a regular and
repeating pattern. If a crystal is
allowed to grow undisturbed, it will form
regular shapes such as cubes, or
hexagonal columns. The type of
substance and how its molecules
interlock determine
the shape of the crystal
Grains
• When the molten metal
solidifies, different
regions crystallise at the
same time
• The crystalline areas are
known as ‘grains’
• Eventually growing grains
meet at grain boundaries
• At these boundaries
there are can be atoms
which do not fit into the
crystal structure
dislocation
Properties of metals
•Hard but malleable and ductile – metals can be
hammered into sheets or drawn into wires because
blocks of atoms or grains can slip over one
another.
the block slip theory – when stress is
applied to the structure, blocks of atoms
become displaced as they slip past one
another
•Conduct electricity because the delocalised
electrons are free to move towards the positive
terminal
•Are shiny because as light shines on the metal the
electrons absorb energy and jump temporarily to a
higher energy. When the electron falls back to its
lower level the extra energy is emitted as light
Flame tests
Lithium
Red
Sodium
Yellow
Potassium
Lilac
Calcium
Brick red
Barium
Green
Flame tests
Aurora Borealis
Cold Working
Metals can be ‘cold –worked’ – forced into new
shapes at a low temperature
Dislocations occur at the grain boundaries as it
is worked.
The more dislocations
a metal has, the more
they get in the way of
each other
The metal becomes
stronger but less
ductile – more brittle
Annealing
• Annealing is a treatment used to
restore softness and ductility to metals
•The metal is put in a furnace to soften the
metal
•It is then allowed to cool slowly so that new
crystals form and there are fewer dislocations
Salt
Salt conducts electricity
when it is dissolved in
water
There must be
charged particles
which are free to
move
Chlorine
35Cl
17
Ions
Sodium atom
Chlorine atom
11 protons + 11 electrons
17 protons + 17 electrons
+11 -11 = 0
Sodium ion
+17 -17 = 0
Chlorine ion
11 protons + 10 electrons
17 protons + 18 electrons
+11 - 10 = +1
+17 -18 = -1
Sodium chloride crystal
Ionic Bonding
As with metals the strong electrostatic
attraction between the positive and negative
ions results in ionic compounds having a high
melting point (salt melts at 808oC)
Ionic compounds conduct electricity when in
solution as the ions (the charged particles)
are free to move to the positive and
negative terminals
Diamond
• Diamond does not conduct electricity
Diamond consists of
atoms of carbon
bonded together to
form a material with
a very high melting
point
It has no charged
particles
An uncut diamond
Bonding in diamond
Carbon atoms are bonded by sharing
electrons in a covalent bond
•
Covalent bonds form
when outer shell
electrons are attracted
to the nuclei of more
than one atom
•Both nuclei attract the electrons
equally so keeping them held
tightly together
Giant Covalent Bonding
Repeating crystal
lattice
High melting point
due to strength
of covalent bonds
(3550oC)
Cannot conduct
electricity as it
has no free
charged particles
Graphite
Like diamond graphite has strong covalent
carbon to carbon bonds and a high melting
point (3720OC)
Graphite conducts
electricity
The bonds between
the covalently
bonded sheets of
carbon are weak
bonds and the
electrons are easily
attracted to a
positive terminal
Fullerenes
C60 Buckyball
Carbon nanotube
Discovered in 1985
Fullerenes are resilient to impact and deformation. This
means, that squeezing a buckyball and then releasing it
would result in its popping back in shape. Or ,if it was
thrown against an object it would bounce back
Buckyballs are also extremely stable in the chemical
sense Their hollow structure allows other atoms to be
carried within them
What decides the type of bond?
Elements on the left of the periodic table
Sodium 2.8.1
(groups 1 & 2) tend to lose electrons
Magnesium 2.8.2
Elements on the right (groups 7 & 8) tend
to gain electrons
Fluorine 2.7
Oxygen 2.6
Electronegativity is a measure of the
tendency of an atom to attract electrons in a
bond – the greater the electronegativity, the
greater the ability to attract the electrons
Electronegativity increases going across a period
and going up a group
losers
gainers
Bond Type
IONIC
Metal  non-metal
Non – metal  non –metal COVALENT
Metal  metal
Low
electronegativity
METALLIC
High
electronegativity
Polymers
The largest group of covalent compounds
are polymers
Polymers are long carbon chains sometimes
with different functional groups added and
all held together by covalent bonds
The bonding in a polymer chain is
strong covalent bonding
The bonding between chains can
create either thermsoftening
plastics or thermosetting plastics
Thermoplastics
• In thermosoftening plastics like poly(ethene) the
bonding is like ethane except there are lots of carbon atoms
linked together to form long chains. They are moderately
strong materials but tend to soften on heating and are not
usually very soluble in solvents.
Can be recycled
A thermosoftening
plastic
Weak bonds
between chains
• Thermosoftening
heat: softens
hard, solid
cool: harden
soft, pliable
• These can be heated enough to be reshaped. This
stretches the cross links. When cooled in the
stretched state they stay stretched and retain the
new shape
• If reheated the
chains are free
to slide back to their
original shape
Thermoset plastics
• Thermosetting plastic structures like melamine have
strong 3D covalent bond network they do not dissolve in any
solvents and do not soften on heating and are much stronger
than thermoplastics
They do not lend themselves
to recycling like
thermosoftening plastics
which can be melted and remoulded.
A thermosetting plastic
Covalent bonds
between chains)
• Thermosetting
Cool: harden
during
manufacture
warm, pliable
permanently
hard
• Both thermoplastics and thermoset plastics
can be strong, tough, rigid and stable towards
chemical attack
Bonds between atoms are strong covalent
bonds so they do not conduct electricity
•Bonds between chains are weak
intermolecular bonds
•When plastics melt or dissolve it is the
intermolecular forces that are broken so
the different parts can slide past one
another
NAME
Uses of thermosets
PROPERTIES
USES
Epoxy resin
Good electrical
insulator, hard,
brittle unless
reinforced, resists
chemicals well
adhesives, bonding of
other materials
Melamine
formaldehyde
Stiff, hard, strong,
resists some
chemicals and stains
Laminates for work
surfaces, electrical
insulation, tableware
Polyester resin
Stiff, hard, brittle
unless laminated,
good electrical
insulator, resists
chemicals well
bonding of other
materials
Urea formaldehyde
Stiff, hard, strong,
brittle, good
electrical insulator
Electrical fittings,
handles and control
knobs, adhesives
NAME
Uses of thermoplastics
PROPERTIES
USES
Polycarbonate
high impact resistance,
temperature resistance and optical
properties
lighting lenses,sunglass/
eyeglass lenses, safety
glasses, compact discs,DVDs
automotive headlamp lenses,
lab equipment and drinks
bottles
Polyamide (Nylon)
Creamy colour, tough, fairly
hard, resists wear, selflubricating, good resistance to
chemicals and machines well
Bearings, gear wheels,
hinges for small cupboards,
curtain rail fittings and
clothing
Polymethyl
methacrylate
(Acrylic)
Stiff, hard but scratches easily,
durable, brittle in small
sections, good electrical
insulator, machines and polishes
well
Signs, covers of storage
boxes, aircraft canopies and
windows, covers for car
lights, wash basins and
baths
Polystyrene:
- conventional
Light, hard, stiff, transparent,
brittle, with good water
resistance
Toys, especially model kits,
packaging, castes for
televisions, 'plastic' boxes
and containers
Cold drawing
• Cold drawing is the process of
stretching out a polymer fibre to line up
the chains
• Cold drawing is used to increase a polymers’
strength.
Crystalline Plastics
• A very strong material can be produced by
arranging the molecules of a plastic to produce a
highly ordered material.
• This material is sometimes called oriented plastic
describing the way the molecules line up
• A recent example of such polymer
engineering is a substance called
‘spectra’ produced by an American
chemical company.
Surgeons
• Spectra fibres have enormous
gloves made of
strength and yet are very flexible. fabric woven
with oriented
plastic
Ceramics
• Ceramics are materials that include
glass, enamel, concrete, cement,
pottery, brick, porcelain, and chinaware.
Ceramics can be defined as inorganic, non metallic
materials. They are typically crystalline in nature and
are compounds formed between metallic and non
metallic elements such as aluminium and oxygen
calcium and oxygen , and silicon and nitrogen
• Ceramics are hard and strong so are used as
structural material such as bricks in houses,
stone blocks in the pyramids
•but not in conditions of tensile stress because
they are brittle (low tensile strength)
•Most ceramics do not conduct electricity
but this depends on the type - chromium
dioxide does, silicon dioxide is a semi–
conductor, aluminium dioxide does not
Bonding in ceramics
• Bonding are usually ionic as in magnesium
oxide or aluminium oxide in which the
ions are arranged in a regular repeating
pattern - a giant lattice
2
Combine with 3
To give
• Bonding can also be covalent e.g. in
silicon carbide or silicon nitride
• The structure is a giant covalent lattice
Semi Conductors
A chip
An LED
A transistor
Semiconductors have had a monumental impact on our
society. You find semiconductors at the heart of
microprocessor chips as well as transistors. Anything
that's computerized or uses radio waves depends on
semiconductors. Silicon is a semi conductor
Silicon bonding
Silicon giant
covalent lattice
•Generally in a giant covalent lattice all the
electrons are tied into the bonds and are
not free to conduct electricity
•In a semiconductor (like silicon) if electrons
get enough energy they escape from the
atom. Heat or light provides this energy
•Given enough energy electrons can escape
to the conduction band (like the delocalised
electrons in metals) and are free to move
and conduct electricity
Conduction in semiconductors
Delocalised
electrons
Electrons
in filled
shells
Doping silicon: diodes and
transistors
You can change the behaviour of silicon and turn
it into a conductor by doping it. In doping, you
mix a small amount of an impurity into the silicon
crystal.
There are two types of impurities
1. phosphorus or arsenic – called N-type
2. boron or gallium – called P-type
N type semiconductor
Si
P
Contaminated
by phosphorus
which has 5
outer electrons
4 of these are
involved in the
covalent bonds
One phosphorus
electron has
nothing to bond
with and is free to
move around. It
needs little energy
to jump to the
conduction band
It takes very little impurity to create enough free
electrons to allow an electric current to flow
P type semiconductor
Si
Contaminated by
boron
which has 3 outer
shell electrons
B
A fourth electron is
taken from a silicon
atom creating a
positive hole
This silicon takes an
electron from another
silicon and so on
….............
The ‘positive hole’ is moving
through the semi conductor
Uses of semi conductors
A diode is the simplest possible semiconductor
device. A diode allows current to flow in one
direction but not the other. You may have seen
turnstiles at a stadium or a subway station that
let people go through in only one direction. A
diode is a one-way turnstile for electrons.
When you put N-type and P-type silicon together
you get a very interesting phenomenon that gives
a diode its unique properties.
Amorphous Materials
An amorphous solid is a solid in which there is
no long-range order of the positions of the
atoms. For instance, common window glass is an
amorphous ceramic, many polymers (such as
polystyrene are amorphous, and even foods such
as candy floss are amorphous solids.
• When glass is broken the
edges of the piece have a
range of shapes and sizes
• If a crystal is broken it cleaves along the grain
of the crystal
• This difference in behaviour is due to the
fact that glass is an amorphous solid - its
particles are jumbled up
•Amorphous solids behave more like liquids than
solids in terms of their structure and are
sometimes referred to as ‘supercooled liquids’ –
liquids cooled below their melting point
This type of solid has no definite melting point
but softens as it is heated (like glass or some
plastics) and can be shaped by heating
Glass
• Glass is an amorphous material usually
produced when the viscous molten
material cools very rapidly to below its
melting point without sufficient time
for a regular crystal lattice to form
Amorphous materials are often prepared by rapidly cooling
molten material, such as glass. The cooling reduces the
mobility of the material's molecules before they can pack
into a more crystalline state. Amorphous materials can also
be produced by additives which interfere with the ability to
crystallize. For example addition of soda to silicon dioxide
results in window glass.
What is glass?
The term glass refers to amorphous oxides, and
especially silicates (compounds based on silicon and
oxygen). Ordinary soda-lime glass, used in windows and
drinking containers, is created by the addition of soda
and lime (calcium oxide) to silicon dioxide. Without
these additives silicon dioxide will (with slow cooling)
form quartz crystals, not glass
Properties of glass
- Solid and hard material
- Disordered and amorphous structure
- Fragile and easily breakable into sharp
pieces
- Transparent to visible light
- Inert and biologically inactive material.
- Glass is 100% recyclable and one of the
safest packaging materials due to its
composition and properties
Tempered/ heat toughened
glass
Tempering: Tempered safety glass is a
single piece of glass that gets
tempered using a process that heats,
then quickly cools the glass to harden
it. The glass is heated in a furnace and
cooled quickly. The outside hardens
but the inside remains fluid and flows
out to the edges compressing the
molecules together. The tempering
process increases the strength of the
glass from five to 10 times that of
untempered glass.
Advantages of toughened glass
• Toughened glass or tempered
glass is a type of safety glass
that has increased strength
and will usually shatter in small,
square pieces when broken. It
is used when strength, thermal
resistance and safety are
important considerations.
Sintering
• In the sintering and pressing process, first
the glass is ground to a fine powder and
mixed with a binder. The mixture is portioned
out into a metal die and pressed. The pressed
article is removed from the die and fired in a
kiln to the sintering temperature, 700900°C.
The result is hard, somewhat porous glass. It
is not transparent or does not otherwise look
similar to molten glass.
Composites
• Composites are combinations
of materials with different
properties
• The parts of the composite
retain their identity and
do not dissolve or
completely merge together
• They act together
Reinforced concrete
fibreglass
Uses of composites
Glass- ceramic composite
• Glass-ceramic is a
mechanically very strong
material and can sustain
repeated and quick
temperature changes up
to 800 – 1000oC.
The Future
•Nanotechnology is the art and science of manipulating
matter at the nanoscale (down to 1/100,000 the width
of a human hair) to create new and unique materials
and products. The opportunities to do things
differently with nanotechnology have enormous
potential to change society.
Dust mite and gears
produced by
nanotechnology

Sunscreen - Many sunscreens contain
nanoparticles of zinc oxide or titanium oxide.
Older sunscreen formulas use larger particles,
which is what gives most sunscreens their whitish
color. Smaller particles are less visible, meaning
that when you rub the sunscreen into your skin, it
doesn't give you a whitish tinge.
Clothing - Scientists are using nanoparticles to enhance your clothing. By
coating fabrics with a thin layer of zinc oxide nanoparticles,
manufacturers can create clothes that give better protection from UV
radiation. Some clothes have nanoparticles in the form of little hairs or
whiskers that help repel water and other materials, making the clothing
stain-resistant.
Antimicrobial bandages - Scientist Robert Burrell
created a process to manufacture antibacterial
bandages using nanoparticles of silver. Silver ions
block microbes' cellular respiration . In other words
silver smothers harmful cells, killing them.
The Future
• Nanotechnology involves using nanoparticles of different
elements or compounds to alter the properties of materials
• e.g.
- developing cheap, disposable solar panels by developing specialist
inks containing silicon nanoparticles
- nanodevices capable of detecting cancer and other diseases at
the earliest stages, pinpointing the location of the disease,
delivering effective drugs only to the site of the disease and
monitoring the progress of the treatment
- nanocatalytic fuel cells capable of powering a laptop with the
equivalent amount of alcohol as 2 or 3 drinks
- implants made of materials that will bond with natural tissues and
not be rejected by the body especially neural and retinal tissue