Chapter 5- Looking inside materials

Question 1

define crystaline

Answer 1

a solid material whoes atoms/molecules/ions are arranged in a ordered lattice structure that can extend in an direction

Question 2

define amorphous

Answer 2

a material whoes atoms are not ordered in a defined lattice shape

Question 3

Why are ceramics brittle?

Answer 3

they are brittle because dislocations are very uncommon and therefore slip and plastic deformation cant happen, also the impurity s make it easy for cracks to propagate through the material. The covelant bonds are directions and cannot slip. Cracks act as stress concentrators.

Question 4

ceramics are strong becuase

Answer 4

they have lots of strong covelant bonds holding it together in a rigid lattice, which means the whole material is strong.

Question 5

glass is a

Answer 5

amorphous solid , made from strong directional covelant bonds.

Question 6

general properties of a ceramic

Answer 6

brittle, hard, high compressive strength, prone to thermal shock, resistant to heat, insulators (exempt some are superconductors when very cold)

Question 7

what is toughened glass?

Answer 7

glass that has been annealed so that the edge is under compressive stress and the inside is under tensile stress. when broken it turns into a fine powder.

Question 8

what makes rubbers very elastic

Answer 8

cross links in the polymer chains that allow the chains the stretch from their tangled state and be returned.

Question 9

What increases tensile strength in polymers?

Answer 9

longer polymer chains, and crosslinks

Question 10

explain what happens when you stretch a polymer past its elastic limit.

Answer 10

Once its past its elastic limit the cross links cant pull it back together.
the chain starts off as a amorphous jumbled chain, and stress is applied these chains unravel and straighten to become more crystalline, which allows the plastic to extend drastically, once they have been straightened fully the C-C bonds will start to break and form a neck in the plastic, which then breaks.

Question 11

What are plasticizes?

Answer 11

small molecules added to plastics to improve their flexibility.

Question 12

what is a matrix?

Answer 12

the material that binds the reinforcing fibers together in a composite eg plastic in glass fibre.

Question 13

what is a reinforcement?

Answer 13

a composite building material eg glass in fibre glass.

Question 14

what is a fibre-reinforced composite?

Answer 14

the fibre is the primary load bearing component(eg glass)

Question 15

What is a dispersion strengthened composite?

Answer 15

The matrix is the major load bearing component.

Question 16

What is a particle reinforced composite?

Answer 16

the load is shared by the matrix and the particles (eq MDF)

Question 17

What is a hole?

Answer 17

A hole is the absence of an electron in a particular place in an atom, its a positive electrical charge carrier.

Question 18

What is doping?

Answer 18

the addition of foreign atoms to a pure semi-conductor crystal in order to improve its conductivity.

Question 19

what is a semi conductor

Answer 19

A material which is an insulator at 0K but conducts better with increasing temperature.
intrinsic semiconductors havent been doped.
Covelantly bonded.

Question 20

N-type semiconductor?

Answer 20

have impurities added with add extra ELECTRONS which improves conductivity.

Question 21

P-type semiconductor?

Answer 21

have impurities added with add extra HOLES which improves conductivity.

Question 22

define polycrystline

Answer 22

have structures composed of many smaller crystallites. These grains(crystallites) have a crystalline structure. the grains form in different sizes and directions to create a overall polycrystaline structure.

Question 23

Grain boundarys are…

Answer 23

where two or more crystallites(grains) meet in a polycrystaline structure.

Question 24

2/3 types of metalic lattice structures

Answer 24

Body centered cubic packing- each particle touches 8 others.
Hexagonal (and cubic closest packing) closest packing- each particle touches 12 others. made from 3 layers that are stacked so the particles sit in the depressions of the lower layer. (more ductile due to small congregations in layers that allow slip to happen more easily)

Question 25

The two processes that happen when a metal cools?

Answer 25

whole process is called Crystallization, which is split into;
nucleation-formation of stable nuclei
crystal growth-branching tendrils that happen as the liquid becomes a solid.

Question 26

What makes metals very ductile and malleable?

Answer 26

the non-directional metallic bonds allow slip to occur very easily, also lattice defects make it more ductile.

Question 27

What are the kinds of lattice defects in a metal?

Answer 27

Dislocations - plane defect
Grain Boundary’s -lattice defects
Stacking defects
vacancy - point defect

Question 28

What is a dislocation?

Answer 28

An extra hale a plane of atoms. The dislocation means that the rows of atoms can slips one at a time which reduces the stress required to deform it.

Question 29

What is a stacking defect?

Answer 29

a defect in how a plane rests on the lower plane eq being directly on top and not sitting in the depressions.

Question 30

What is a vacancy?

Answer 30

where a atom is missing from the ideal crystal structure, fixed with a solute atom being substituted in.

Question 31

Why are metals much weaker then in theory.

Answer 31

Metals have stone non-directional metallic bonds and are polycrystlline in structure. However they contain dislocations which can move throughout the structure requiring only a few bonds to break at once , which can reduces stress required by 1000x .

Question 32

Why are alloys usually more resilient to stress?

Answer 32

the bigger atoms in the lattice pin the dislocation and stop them from moving which reduces slip .

Question 33

What is annealing?

Answer 33

Heat metal then allow to cool slowly

The structure disperses- which increases toughness but lowers yield strength.

Question 34

What is Quenching

Answer 34

Heat metal then cool rapidly
Freezes the structure in the high temp form
which makes it very brittle but very hard and strong

Question 35

What is tempering

Answer 35

Quench (heat and cool rapidly) then reheat slowly
This controls the dispersion between the structures
so it regains toughness
and regains some yield strength

Question 36

Three ways to changing structure to a metal (in order to change properties)

Answer 36

Healing +cooling to get the right combination of strength and toughness.
Alloying = harder +stronger but reduce toughness
Work hardening- increases the number of dislocations and reduces grain size which means they entangle this improves yield strength and reduces toughness.

Question 37

Things to include in a questions relating structure to properties.

Answer 37

Amorphous, crystalline or polycrystalline structure
type of bonding and theire strenght etc
then other relevant stuff eq imperfections in the lattice,
MAKE SURE TO USE SCIENCY WORDS
then relate them to the properties.