Chapter 5 (7 lessons)
5.1 Materials under the microscope
Learning Outcomes
· To understand the properties of a material, we need to know about its structure
· Structures may be classified as crystalline, polycrystalline and amorphous
· The structure of a material may be visible to the naked eye, or we may have to use microscopy or other techniques to reveal it
· Various imaging techniques are available to show the structure of materials at the atomic level; e.g., atomic force and scanning tunnelling microscopes, scanning electron microscope
· The size of atoms/molecules and typical spacings in solids/liquids can be determined
Lesson 1
Objectives: Materials with different properties have different structures. We use a variety of techniques to study structure. How to classify structures.
Starter: Look at the pictures on pg 104. Ask students which images are false colours, which images have artefacts, which show atoms/particles/cells.
Experiments: A10E Looking at bone
A30E Looking at wood and wood products
A40E Looking at textiles
(Students should spend about 10 minutes on each one, listing the properties of the material, drawing its structure and writing a brief “explanation” of why each one has those properties.)
Display: D40O Looking inside glasses, D50O Looking inside wood, D60O Looking inside metals and ceramics, D70O Looking inside polymers. Quickly highlight the different structures and properties of each. Note the scale down the side of each one. Or save till after they have done the oil drop.
Define terms “amorphous”, “crystalline” and “polycrystalline”. Answer questions 1-3 on pg 108. (Don’t give them the book – use whiteboards. Use A-Z Solid materials on CD for definitions.)
Question: Q10C Visible Structures. (This should be finished in class – they will try to spend ages on it.)
Lesson 2
Objectives: Use simple equipment to measure the length of an oil molecule and hence calculate the size of an atom.
Starter: Question 50S Short Answer ‘Ants, atoms and chips’
Experiment: A80E Measuring a molecule. This will take the rest of the lesson to do the experiment, tidy it up and do the calculations. Show them how to set up the equipment. Have a mop ready. Practise first. Tell them that next lesson they learn about the modern way to measure atoms.
Lesson 3
Objectives: Be able to describe how modern microscopes enable us to look at atoms. Build and discuss models of different structures
Starter: D10S Size line for living things
D20S Similar images at different scales
D30S Textile images
D80S Fracture surfaces
Discuss how a SEM (pg 105), an AFM (pg 106) and a STM (pg 11) work.
D150S Making models of metals – introduce ideas of dislocations and grain boundaries.
Experiment: A190E A bubble-raft model of dislocations. Use D140S Bubble raft features to introduce. Note: The room must be well ventilated. It is usually best for staff to light the rafts. The needle ends are easy to break – care must be taken. 15 minutes is long enough if the groups are small enough – make sure the girls have a go!
Experiment: A150E Growth of grains – great experiment. If you can control this growth, you can determine the properties of the material.
Activity 170E Experiment 'Growing grains in a zinc ingot'
A90D Models: Crystal, polycrystal and amorphous' to get vocab of crystalline/polycrystalline/amorphous.
Homework:
Piglet / Pooh / Christopher RobinQ30D The size of an atom
Answer pg 108 questions 4-7.
Be able to describe from memory how either a SEM or an ATM or a STM works. / Q20E Scaling exercises
Make a 3D model of an amorphous or a crystalline or a polycrystalline structure. / Choose and carry out one or more experiments from A50H, A60H, A70H, A100H, A160H. Write brief conclusion for each.
Reading 10T Text to Read 'Biological tissues'
Reading 40T Text to Read 'Introduction to proteins'
5.2 Stiff stuff, strong stuff
Learning Outcomes
· Glasses have a disordered, amorphous structure.
· The brittleness of glass (and other ceramic materials) is a consequence of defects such as fine surface cracks. Cracks propagate easily through glass.
· Metals are generally tough, because cracks do not propagate easily.
· For a crack to propagate through a material, energy is needed to tear apart atoms, create new surface, move atoms around, and send shock waves through the material.
· Fibre-reinforced plastic composites can be tough, though made of brittle fibres. The plastic matrix shares out the extra stress when one fibre breaks
Lesson 4
Objectives: Know the difference between strength and toughness in materials
Know why cracks propagate through some materials, but not others
Starter: range of things with perforations/cracks – chocolate bars, stamps, fizzy drink cans and bottles, Why are stamp perforations round and others rectangular.
Display Q40C Photoelastic stress images and discuss.
Demo A130D Photoelastic stress
Discuss the metal fatigue on the Comet.
Question 40C Comprehension 'Photoelastic stress images'
Most materials should be 10-1000 times stronger than they are. Why? Because of stress around cracks – tough materials blunt cracks.
Display and discuss D80S 'Fracture surfaces', D90O Cracks and stress, D100O Stopping cracks, D110O Strong and tough and D120O Fracture energy and tensile strength.
Make glass fibres and try breaking them – then scratch and try breaking them.
40E Experiment 'More than one spring' – link back to YM.
Homework:
Activity 290H Home Experiment 'Ice and paper composite'
Activity 300H Home Experiment 'A jelly composite'
Activity 310H Home Experiment 'Making and testing composite biscuits' for all – report back
Piglet / Pooh / Christopher RobinAnswer questions 1-3 pg 114
Question 110S Short Answer 'Bone' /
Answer questions 4-6 pg 114
Question 120C Comprehension 'Concrete: A material for all seasons'
/Answer questions 2 , 3 and 6 pg 128
Question 130C Comprehension 'Wire ropes and suspension bridges'
Question 50C Comprehension 'Tendon elasticity'
Question 150S Short Answer 'Springs connected to the Young modulus'5.3: Making more of materials
Learning Outcomes
· Metallic bonds are strong, non-directional bonds; these account for the toughness of metals.
· Because metallic bonds are non-directional, atoms can slide easily past one another; this accounts for the ductility of metals.
· Strong ionic or covalent bonds between the atoms account for the stiffness of ceramic materials and the difficulty of atoms slipping which gives strength and hardness.
· Polymers have long chain molecules of repeating units: in thermosets, cross-linking makes the material rigid; in thermoplastics, molecules are relatively free to rotate and slide past one another.
Lesson 5 – Metals
Objectives: Understand how different bonds account for different properties. Understand the importance of dislocations in metals. Understand that the link between structure and properties has led to the development of designer materials
Starter: Use pg 116 – pg 12- to answer the following questions:
1. Why are alloys less ductile than pure metals? (9ct gold is much harder than 24ct gold.)
2. Why are metals ductile when ceramics aren’t?
3. Why does polythene stretch when polystyrene doesn’t?
4. What is the difference between a plastic material and an elastic material?
Experiment: A190E A bubble-raft model of dislocations. Again! Use D140S Bubble raft features
Discuss metallic bonding
Experiments: A200E Heat treatment of steel
A210E Work hardening of copper
Demo the memory metal springs
Lesson 6 – Polymers, Glasses and Ceramics
Objectives
· Strong ionic or covalent bonds between the atoms account for the stiffness of ceramic materials and the difficulty of atoms slipping which gives strength and hardness.
· Polymers have long chain molecules of repeating units: in thermosets, cross-linking makes the material rigid; in thermoplastics, molecules are relatively free to rotate and slide past one another.
Starter – silly putty!
Activity: Use pg 116 – pg 12- to answer the following questions:
1. Why does polythene stretch when polystyrene doesn’t?
2. What is the difference between a plastic material and an elastic material?
Display Material 70O OHT 'Looking inside polymers'
Discuss bonding in polymers, glasses and ceramics
Some of
Activity 220E Experiment 'Amorphous and crystalline structures'
Activity 230E Experiment 'Sketching a semicrystalline polymer'
Activity 260D Demonstration 'A model for stretching rubber'
Activity 270E Experiment 'Effect of the vulcanisation of rubber on stiffness'
Best as a demo: Activity 280E Experiment 'Design your own rubber'
Homework:
Piglet / Pooh / Christopher RobinAnswer questions 1-6 pg 121 and question 4 pg 128 / Answer question 1 pg 128
Q80S Further questions on metals / Q100S Questions on polymers
Q70X Questions on metals
5.4: Controlling conductivity
Learning Outcomes
· Semiconductors have electrical conductivity intermediate between that of metals and insulators.
· The conductivity of a metal decreases gradually as the temperature increases.
· The conductivity of semiconductors increases rapidly with temperature, as more electrons break free.
· The conductivity of a semiconductor can be increased by doping with other elements.
· In n-type material, the charge-carriers are electrons; in p-type, they are holes
Lesson 7
Objectives: Revise ideas about conductivity covered in Chapter 4.
Starter: revise conductivity. Look at range of devices that use semiconductors.
Explain how metals and semiconductors conduct
Display Material 220O OHT 'Conduction by metals and semiconductors'
Display Material 230O OHT 'Free electron model of metal'
Display Material 250O OHT 'Effect of temperature on conductivity'
Display Material 260O OHT 'Conduction in doped silicon
Activity 320E Experiment 'Calibration of a thermistor' - done better as a competition to measure the temperature of your tea. Bring out ideas of quality of measurement.
Questions: 1-6 on pg 126
Q140D How resistivity changes with temperature
Piglet / Pooh / Christopher RobinAnswer question 5 pg 128.
Check learning outcomes. / Write a summary of Chapter 5 using pg 125 as starting point. / Be able to explain from memory the mechanical, optical and electrical properties of metals, ceramics and polymers. Use pg 127 as starting point.
Question 160C Comprehension 'High-temperature superconductivity'
Question 170X Explanation–Exposition 'Conductivity'
Question 180E Estimates 'Estimating with materials'Homework: They must bring the info on their material for their next lesson.
Lesson 8
Objectives: Revise Chapters 4 and 5 (last time it is discussed in class before mocks.)
Starter: Students write equations and definitions of key words on whiteboards. (Equations: stress, strain, YM, resistivity, conductivity. )
(Key words: tough/brittle, strong/weak, heavy/light, stiff/flexible, hard/soft, resistivity, conductivity, metal, ceramic, polymer, semi-conductor, insulator, superconductor, transparent, translucent, opaque, absorption, transmission, reflection, refraction, bonds, dislocations, work-hardening, grain boundaries.)
Go through any questions/topics which they are unsure about.
Mark each other’s summaries.
Complete the Materials Checklist (should be based on material chosen for presentation and they should be able to do it easily).
Answer review questions.