Ceramics- Properties of Products Flashcards
Theoretical density
ρt. Density based on molecular mass and crystal lattice. Maximum density theoretically achievable.
Measured density
ρm. Density based on real ceramic material. Bulk density including pores
Relative density
Measured density over theoretical density
Porosity
1-relative density
1-ρm/ρt
How does thermal conductivity vary with porosity
It decreases roughly linearly as porosity increases
Mechanical properties as function of porosity
E=E0(1-aP) Where E is the mechanical property E0 is property of 100% density material P is porosity a is constant Used for vickers hardness and YM
Two components of thermal conductivity
Conductivity due to electrons and due to phonons. Effect from phonons dominant in ceramics (electrons for metals)
Phonon
Elastic vibration of the crystal lattice which carries the heat through the material
How does average grain size affect thermal conductivity?
As average grain size increases the thermal conductivity increases (fewer grain boundaries to disturb propagation of phonons)
Grain boundary area in a unit volume
Surface area of sphere over volume of sphere. Equals 3/r
Working out how thermal resistivity depends on grain size
Equal to 1/K. Proportional to grain boundary area in a unit volume. Rearrange to find K proportional to r
Effect of grain size on mechanical properties
By hall-petch relation.
E=E0+a/rt(g)
a is constant
g is grain size
Works for creep, yield strength, fracture toughness
Smaller grains increases value of property
How do grain boundaries affect plastic deformation in ceramics?
Ceramic plastic deformation related to glide (slip) of dislocations through the crystal structure. Grain boundaries act as barriers to this glide of dislocations
