Applications: silicon nitride Flashcards
When was silicon nitride developed?
~ developed in the 1960-70s for use in reciprocating (automobile) and turbine (jet) engines, allowing them to operate at higher temp and so more efficiently
What is the structure of silicon nitride?
~ Si3N4 bonding is 70% covalent
~ two polymorphs: α and β Si3N4
–> for α: stacking sequence is ABCDABCD
–> for β: stacking sequence is ABABAB
~ both α and β are hexagonal
Where is the α –> β transformation?
~ α is the low temp polymorph, α –> β transformation is at 1420C
~ with some impurity solid solution, β is stable at room temp
How is powder synthesis conducted?
~ commercially available Si3N4 powder is most typically made via reacting Si powder with nitrogen gas (3Si + 2N2 –> Si3Ni4) at 1250-1400C
Why is silicon mixed with seeds of Si3N4 powder?
~ to hasten the reaction
~ serves as a diluent to control the exothermic rxn, which would otherwise facilitate excessive temp rise, producing more β phase
~ this yields a 90:10 mixture of α and β polymorphs
Where does the Si powder come from?
~ comes from carbothermal reduction SiO2 in an electric arc furnace (~2350C)
What is another route of formation of Si3N4?
~ direct carbothermal reduction of silica
~ residual carbon may remain
How else are high purity powders also made?
~ via vapor-phase rxns
~ powder made from this rxn is amorphous, but after heating to 1400C, it becomes mostly α-Si3N4
What happens upon milling?
~ Si3N4 particle surfaces oxidize upon milling; thin (nanometer scale) SiO2 coatings are inevitable
How is silicon nitride fabricated? Why?
~ the predominately covalent bonding in Si3N4 hinders atomic migration so that at the temps required for significant diffusivity, Si3N4 beings to decompose by volatilization of nitrogen, making solid state sintering difficult
~ thus, many parts are hot-pressed, hot isostatic pressed, or rxn bonded
