Looking Inside Materials

Looking Inside Materials

1. Estimating size of atoms and molecules

   • Oil experiment
     • Oil drop dropped into still water and let it spread - eventually it will reach one atom thick
     • h = thickness of layer
     • r = radius of oil drop
     • R = radius of patch
     • Uncertainty
       • Largest source is measurement of oil drop as it is the smallest value to measure and is cubed
     • Size of atom by density
       • If we know the mass of a gold atom and its density of gold we can estimate the diameter of 1 atom
       • density = mass/volume

2. Metal structure and ductility

   • Metals are crystalline - which means particles are arranged in a regular pattern over distances many times the spacing of the particles
   • Some pure metals are malleable and ductile
   • Disslocations - defect or irregularity within a crystalline structure
   • It is the movement of dislocations that make metals ductile
   • Dislocations reduce the amount of energy needed to deform the metal
   • Alloys
     • Metal alloys tend to be less ductile than pure metals
     • Alloys are formed by the addition of other metallic elements that usually have different sized atoms - which pins dislocations in the metal making slippages more difficult to occure

3. Amorphous and crystalline materials

   • Solids form when liquids cool
   • Amorphous - disordered structure
   • Solids are often amorphous when cooled rapidly which traps the particles in the amorphous state
   • Slow controlled cooling results in a crystalline structure

4. Polycrystalline materials

   • Many solids are a mixture of both crystalline and amorphous - this is called polycrystalline
   • A polycrystalline structure consists of a number of grains all oriented differently to eachother but with an ordered structure within the grain
   • As liquid cools crystals form in different points within it, each crystal grows out into the remaining liquid until it runs into neighbouring crystals - where the grains meet is called the grain boundary

5. Stress concentration and crack propogation

   • The strength of a material is affected by tiny cracks and flaws in the structure of the material
   • The stress concentration around cracks can be hundred or thousands of times the applied stress - this can lead to cracks working through the specimen until it cracks
   • Bending a glass rod example (DIAGRAM BELOW)
     • Glass becomes strained elastically
     • At the tip of the crack two neighbouring atoms are pulled apart
     • The next two atoms are pulled apart and so on
     • The crack moves through the materials like a zip, propogating through the material

6. Toughness in metals

   • Toughness is a measure of the energy needed to extend cracks through a material - more energy needed in a tough metal
   • When a stress is applied to a crack the metal deforms plastically in the region of the crack which makes the crack broader, reducing the stress around the crack
   • Strength is the measure of breaking stress
   • Tough is the opposite of brittle

7. Ceramics

   • Ceramics undergo brittle fracture
   • The bonds in ceramics are directional - meaning the atoms are locked in place and cannot slip making the material hard and brittle
   • Ceramics have rigid structures - bonded covalently and directionally locking the atoms in place
   • The covalent bonds are strong

8. Metals

   • Metal ion crystalline with a sea of electrons with non directional bonds between them
   • The bonds are strong but the ions can easily change places
   • The bonds are strong so metal is stiff
   • The ions can slip so metals are ductile and tough
   • Stiffness
     • Metals behave elastically for small strains - up to about 0.1%
     • Up to this point the metal extends because the spacing between ions increases and when the force is removed it returns to it original length

9. Polymers

   • Long chain molecules that can extend elastically up to 1% strains
   • Polyethene is floppy because it is free to rotate around its bonds
   • The bonds are strong so they are difficult to break
   • All this make polyethene strong and flexible
   • But not all polymers are flexible
   • Cross links are bonds that tie polymers togeter at regular intervals along the chain to make the material more stiff
   • Rubber is a runny white liquid until sulphur is added to form cross links with the polymer chains - called vulcanistation
   • In entangled regions of polymers (amorphous) the chains fold randomly
   • In crystalline parts of the polymer the chains line up
   • When stretched plastically the chains slip past eachother and more will become lined up in chains (crystalline)

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