notes 12 (Structure & Properties of Cermamics)

Question 1

what are ceramics

Answer 1

Inoganic non-metallic materials. typically compounds of metallic and non-metallic elements

Question 2

what bonding is found in Ceramics

Answer 2

Ionic or Covalent or a combination of both

Question 3

List some properties of Ceramics

Answer 3

1. Hard & Brittle, aka low toughness and ductility)
2. Good electrical and thermal insulators
3. high melting points and good chemical stability

Question 4

what does the coordination reflect in ceramics

Answer 4

the ratio of the sizes of the ions

Question 5

what are the 2 factors which determine crystal structure of ceramics

Answer 5

1. relative sizes of ions(the more tightly compacted, the more stable)
2. the maintenance of charge neutrality

Question 6

what is the coordination number?

Answer 6

the number of adjacent atoms surround a reference atom without overlap of electron orbitals

Question 7

what can be said about silicates?

Answer 7

1. maintain charge neutrality

2. ionically bond (SiO4)4- to one another

Question 8

glass structure

Answer 8

glass is amorphous(non-crystalline)

Question 9

describe Carbon

Answer 9

it is Polymorphic: diamond;graphite;fullerness

Question 10

Diamond characteristics(5)

Answer 10

• strong covalent bonds
• hardest known material
• very low electrical conductivity
• very high thermal conductivity
• optically transparent

Question 11

Graphite properties (5)

Answer 11

• in-plane covalent bonding
• gives easy interplanar cleavage
• high electrical conductivity i the plane
• good strength
• low thermal expansion

Question 12

Types of point defects in ceramics

Answer 12

vacancies: can exist for both cations and anions
Interstitials: can exist for cations
Frenkel defect: a cation vacancy-cation interstitial pair
shottky defect: a paired set of cation and anion vacancies

Question 13

why do interstitial vacancies not exist for anions

Answer 13

because anions are relatively large compared to the interstitial sites

Question 14

Mechanical Properties of ceramics

Answer 14

1. brittle
2. tensile strengths are very low
3. compressive strengths are very high
4. High HARDNESS
5. the strength is typically dictated by defects/flaws

Question 15

what can be said about ionic crystals with respect to their slip systems

Answer 15

they have very few slip systems and charge neutrality problems

Question 16

Do ceramics have plastics deformation?

Answer 16

NO, unless at very high temperatures

Question 17

what is a flexural test?

Answer 17

measurement of elastics modulus or flexural strength

Question 18

how doe non crystalline ceramics deform

Answer 18

by viscous flow, like liquids

Question 19

what is affected by pores in ceramics

Answer 19

the stiffness, the flexural strength

Question 20

which is usually smaller in size: cations or anions

Answer 20

cations

Question 21

true or false, the larges the atoms size, the more atoms you can pack around it

Answer 21

true

Question 22

for like sized atoms, what is the coordintation number

Answer 22

12

Question 23

list the coordination number for the following cation-anion ratios:

Answer 23

2-linear
3-triangular
4-tetrahedral
6-octahedral
7-cubic

Question 24

what is particular about the anitfluorite structure:

Answer 24

the positions of cations and anions are reversed

Question 25

silica is polymorphic; which materials can it form

Answer 25

quartz, crystobalite, tridymite

Question 26

In layered silicates, how are adjacent layers bonded together

Answer 26

van der waals forces

Question 27

what is the structure of Fullerness

Answer 27

Hollow spherical cluster of 60 carbon atoms

Question 28

what is the elastic behaviour like (graphically) for ceramics

Answer 28

linear

Question 29

if a ceramic contains pres, what property would this affect?

Answer 29

Stiffness and Flexural strength