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 2000 BC

Around 1300 BC

Egyptian hieroglyphs from as early as 2600 BC describe gold as ‘more plentiful than dirt’

• 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 diamondMaterials behave as they do because of their structure; the way their atoms and molecules fit together

Properties of MaterialsWe have known many of the properties

of materials for thousands of years

Diamonds are crystalline, they have a high melting point, they are insoluble and they do not conduct electricity*

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*

*All later discoveries

Why?

• We know how materials behave – their properties

i.e. The understanding that electricity is a flow of charged particles

•An important development in our scientific knowledge pointed to the answer

•The next question is why?

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 determinethe 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 testsFlame tests

LithiumRed

SodiumYellow

Potassium

Lilac

CalciumBrick red

BariumGreen

Flame tests

Aurora Borealis

Cold Working

Dislocations occur at the grain boundaries as it is worked.

The metal becomes stronger but less ductile – more brittle

Metals can be ‘cold –worked’ – forced into new shapes at a low temperature

The more dislocations a metal has, the more they get in the way of each other

Annealing

• Annealing is a treatment used to restore softness and ductility to metals

•It is then allowed to cool slowly so that new crystals form and there are fewer dislocations

•The metal is put in a furnace to soften the metal

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

11 protons + 11 electrons

+11 -11 = 0Sodium ion

11 protons + 10 electrons

+11 - 10 = +1

Chlorine atom

17 protons + 17 electrons

+17 -17 = 0Chlorine ion

17 protons + 18 electrons

+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

An uncut diamond

It has no charged particles

Diamond consists of atoms of carbonbonded together to form a material witha very high melting

point

Bonding in diamond

• Covalent bonds form when outer shell electrons are attracted to the nuclei of more than one atom

Carbon atoms are bonded by sharing electrons in a covalent bond

•Both nuclei attract the electrons equally so keeping them held tightly together

Giant Covalent BondingRepeating crystal lattice

Cannot conduct electricity as it has no free charged particles

High melting point due to strength of covalent bonds (3550oC)

GraphiteLike diamond graphite has strong covalent

carbon to carbon bonds and a high melting point (3720OC)

Graphite conducts electricityThe bonds between the covalently bonded sheets of carbon are weak bonds and the electrons are easily attracted to a positive terminal

Fullerenes

C60 BuckyballCarbon 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 (groups 1 & 2) tend to lose electronsElements on the right (groups 7 & 8) tend to gain electrons

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

Sodium 2.8.1

Magnesium 2.8.2

Fluorine 2.7

Oxygen 2.6

Electronegativity increases going across a period and going up a group

losers gainers

Bond Type

Non – metal non –metalMetal metal

Metal non-metal IONICCOVALENT

METALLICLow electronegativity

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 bondingThe 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.

A thermosoftening plastic

Weak bonds between chains

Can be recycled

• Thermosoftening heat: softens

hard, solid soft, pliable

cool: harden

• 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

A thermosetting plastic

Covalent bonds between chains)

They do not lend themselves to recycling like thermosoftening plastics which can be melted and re-moulded.

• Thermosetting Cool: harden

duringpermanently

manufacture hardwarm, pliable

• Both thermoplastics and thermoset plastics can be strong, tough, rigid and stable towards chemical attack

•When plastics melt or dissolve it is the intermolecular forces that are broken so the different parts can slide past one another

•Bonds between chains are weak intermolecular bonds

Bonds between atoms are strong covalent bonds so they do not conduct electricity

Uses of thermosetsNAME 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

Uses of thermoplasticsNAME 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, self-lubricating, 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.

• Spectra fibres have enormous strength and yet are very flexible.

Surgeons gloves made of 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

•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

•but not in conditions of tensile stress because they are brittle (low tensile strength)

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 lattice2 To

give

Combine with 3

• Bonding can also be covalent e.g. in silicon carbide or silicon nitride

• The structure is a giant covalent lattice

Semi Conductors

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

A chip

A transistor

An LED

Silicon bonding

Silicon giant covalent lattice

•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

•Generally in a giant covalent lattice all the electrons are tied into the bonds and are not free to conduct electricity

Conduction in semiconductors

Electrons in filled shells

Delocalised electrons

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

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

Si

P

It takes very little impurity to create enough free electrons to allow an electric current to flow

P type semiconductor

Contaminated by boron

which has 3 outer shell electrons

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

B

Si

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

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

•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

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, 700 900°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 anddo 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•  

Dust mite and gears produced by nanotechnology

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.

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.

- implants made of materials that will bond with natural tissues and not be rejected by the body especially neural and retinal tissue

- nanocatalytic fuel cells capable of powering a laptop with the equivalent amount of alcohol as 2 or 3 drinks

- 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

- developing cheap, disposable solar panels by developing specialist inks containing silicon nanoparticles