Chapter 5 Flashcards
Why do metals shine?
Free electrons in the metal absorb the light energy and then immediately re-emit it.
How are the grains in a polycrystalline material structured?
They are oriented differently relative to one another
Why do metals resist cracking and why are they tough?
Because they are ductile and so the cracks are blunted and the stress is reduced.
Why are fibre reinforced materials tough?
Cracks can’t propagate through the soft matrix as the matrix shares the stress amongst many strong fibres so if one fibre breaks, the stress is taken up by the other fibres.
Fracture energy
Total energy used to fracture / specimen cross sectional area
Why are alloys less ductile?
As alloy atoms don’t allow dislocations to move and cause slips.
How does rubber stretch?
Chains uncoiling and coiling again between sulphur cross links
Why does plastic deformation occur in polymers?
Chains slip past each other and become more crystalline as they line up.
Why do semi conductors conduct better as the temp increases?
More atoms become ionised and hence there are extra charge carriers.
What is n type doping
Introducing phosphorus impurities to ionise and release free electrons .
What is p type silicon
Adding a boron impurity to create ‘holes’ which act like mobile positive charge carriers.
Semiconductor
Lower charge carrier density and hence intermediate conductivity between conductors and insulators
