notes 12 (Structure & Properties of Cermamics)
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what are ceramics
Inoganic non-metallic materials. typically compounds of metallic and non-metallic elements
what bonding is found in Ceramics
Ionic or Covalent or a combination of both
List some properties of Ceramics
• Hard & Brittle, aka low toughness and ductility) • Good electrical and thermal insulators • high melting points and good chemical stability
what does the coordination reflect in ceramics
the ratio of the sizes of the ions
what are the 2 factors which determine crystal structure of ceramics
• relative sizes of ions(the more tightly compacted, the more stable) • the maintenance of charge neutrality
what is the coordination number?
the number of adjacent atoms surround a reference atom without overlap of electron orbitals
what can be said about silicates?
• maintain charge neutrality 2. ionically bond (SiO4)4- to one another
glass structure
glass is amorphous(non-crystalline)
describe Carbon
it is Polymorphic: diamond;graphite;fullerness
Diamond characteristics(5)
• strong covalent bonds • hardest known material • very low electrical conductivity • very high thermal conductivity • optically transparent
Graphite properties (5)
• in-plane covalent bonding • gives easy interplanar cleavage • high electrical conductivity i the plane • good strength • low thermal expansion
Types of point defects in ceramics
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
why do interstitial vacancies not exist for anions
because anions are relatively large compared to the interstitial sites
Mechanical Properties of ceramics
• brittle • tensile strengths are very low • compressive strengths are very high • High HARDNESS • the strength is typically dictated by defects/flaws
what can be said about ionic crystals with respect to their slip systems
they have very few slip systems and charge neutrality problems
Do ceramics have plastics deformation?
NO, unless at very high temperatures
what is a flexural test?
measurement of elastics modulus or flexural strength
how doe non crystalline ceramics deform
by viscous flow, like liquids
what is affected by pores in ceramics
the stiffness, the flexural strength
which is usually smaller in size: cations or anions
cations
true or false, the larges the atoms size, the more atoms you can pack around it
true
for like sized atoms, what is the coordintation number
12
list the coordination number for the following cation-anion ratios:
2-linear 3-triangular 4-tetrahedral 6-octahedral 7-cubic
what is particular about the anitfluorite structure:
the positions of cations and anions are reversed