Deck 15 - Looking inside materials 3 Flashcards
What sort of structures do ceramics have?
Ceramics are crystal structures built from ions of two or more elements.
What is the second flaw about ceramics(apart from that they are brittle)?
Ceramics objects are often made by shaping the raw materials and then heating them.
Explain how a process called sintering can be used to turn silicon nitrade into a turbine blade.
Silicon nitride powder is placed in a mould and heat under high pressure. The surfaces of the particles melt and flow so they become glued together.
What is a cermet?
A “Cermet” is a combination of ceramic particles in a metal matrix. Such a material can be hard tough and strong.
Explain how silicons atomic structure relates to its properties.
Atoms share electrons with neighbouring atoms to form covalent bonds. - These bonds are directional: they lock atoms in place. - The atoms cant slip due to the covalent bonds so silicon is hard and brittle. - The atoms are linked in a rigid structure.
Explain the structure of NaCl and explain how its structure relates to its properties.
-Some atoms give electrons to other atoms to form an ionic bond. - Because oppositely charged ions attract ions hold each other in place. - The bonds are strong: salt crystals are stiff - The ions cannot slip: salt crystals are hard and brittle
Explain the structure of metal and explain it links to its properties.
-Atoms in metals are ionised. The free electrons move between the ions. - The negative charge on the electrons “glues” the ions together, but the ions can easily change places. - The bonds are strong: metals are stiff - The ions can slip: Metals are ductile and tough.
What happens when you stretch a metal?
Stretching a metal stretches bonds- but not by much.
Explain how polyethene’s bonds react when stretched.
- Polythene is a long flexible chain molecule that folds up. - Stretching polythene rotates bonds, making it longer.
Name and explain how two different polymers make chain rotations difficult.
-Polystyrene has benzene rings sticking out sideways which make chain rotations difficult. - Bakelite- Has many cross-linked chains. These cross-links stop the chains from unfolding.
What is polythene flexible but strong?
Its free to rotate its bonds by strong covalent bonds.
Show how the structure of rubber changes when stretched.
Explain what happens to polythene’s structure when stretched.
Polythene is semicrystalline. When stretched plastically, the chains slip past each other. More of the material becomes crystalline.
What is plastic deformation in polymers due to?
Plastic deformation in polymers is due to chains slipping past one another.
What is the vulcanisation of rubber?
Rubber is heated with sulfur. Cross-links are formed between the polymer chains by the sulfur atoms.
Name to different semi-conducting materials.p
Silicon and germanium
Give the uses of semiconductors for electronic devices.
Transistors are the devices made from semi-conductors. Transistors are used to control and amplify electric signals.
Draw a conductivity vs temperature/k graph for metals and explain what you would see.
- The conductivity of metals decreases a little as temperature increases.
- No more electrons become free to move. Moving electrons scatter from the wibrating lattice.
- Metals do not conduct well when they are hot because charge carriers become less mobile, but the number stays the same.
Draw the conductivity vs temperature graph for silicon and explain the observations one would see.
- The conductivity of a pure semiconductor increases dramatically as temperature increases.
- At higher temperatures, more atoms become ionised. The conductivity increases increase because there are more charge carriers free to move. Effects of extra lattice vibrations are much smaller.
Explain silicons conductivity.
- Silicon has a poor conductivity because it is made from silicon which forms 4 covalent bonds, all the electrons are used in these bonds so there are no delocalised electrons.
- However, less than one a million million silicon atoms are ionised giving a very small fraction of electrons that are free to move.
Explain how silicon is doped with phosphorous to get more electrons.
- The phosphorous atoms ionise, giving electrons freedom to move throughout the material.
- Phosphorous has five electrons in its outer shell. Four are shared with silicon atoms. One becomes free to move and conduct, leaving positive phosphorus ions.
- Silicon combined with phosphorus is known as n-type silicon.
Explain how silicon is doped with boron to make p-type silicon.
-Boron has three electrons in its outer shell. One more is stolen to give four to share with silicon atoms. The “electron hole” left behaves like a mobile charge positive charge.
