Ceramics and Glasses Flashcards
What are Crystalline Ceramics?
Based on regular arrangements of atoms/ions
What are Amorphous Ceramics?
Atoms/ions are not arranged in a regular long range structure
What are Bonded Ceramics?
Based on crystalline and amorphous structures bonded together
Name some ceramics properties
Low toughness Poor in tension High hardness High melting point Insulators Environmentally stable Typically ionic or covalently bonded
What is ‘Powder Processing’?
Way to bond ceramics with high melting temps. Heat and pressure is applied to ceramic in powder for which fuses into a matrix. Can create unwanted pores
What is meant by a ‘Supercooled Liquid’?
Often refers to amorphous (glass) materials . Amorphous structure is ‘frozen in’ when glass cools.
What is the chemical form of Silica Based ceramics?
SiO2
What is the Silicon solid state structure?
Tetrahedral shape
What are the Crystalline Silica structures?
Quartz
Crystabolite
Tridymite
What are the Amorphous Silica structures?
Fused silica
Vitreous silica
What are the effects of adding oxides to amorphous silica?
Disrupts bonds reducing Tg
What occurs in Crystalline material solidification?
Clear change in volume during solidification
What occurs in Amorphous material solidification?
Does not have a definite temperature where it transforms from liquid to solid
What is the Strain point?
Temperature at which internal stress is relieved within several hours
What is the Annealing point?
Temperature at which internal stress is relieved within a few minutes
What is the Softening point?
Viscosity of approx. 4x106 Pa-s, glass slumps under its own weight
What is the Working point?
Soft enough to be formed into shape
What are some of the Defects of ceramics?
Vacancies
Dislocations
Pores
Grain boundaries - smaller grain size increases strength
What is ‘Thermal shock’?
Uneven rapid heating or cooling can create stresses due to the difference in the expansion pf the interior and the exterior of the ceramic
Describe the effects of Annealing
Heating around the annealing point allows stresses to be relieved
What is ‘Thermal Tempering’?
Glass is heated above the Tg (but below the softening T)
Exterior is cooled rapidly with air or oil
As interior cools it shrinks placing exterior in compression
