final 2 Flashcards
(13 cards)
bonding in ceramics
ceramics have ionic/covalent bonding. these are strong and directional with high bond energy. electrons are not free to move which means poor electrical/thermal conductivity. strong directional bonds resist deformation but fracture more easily, so harder but brittle.
bonding in metals
metals use metallic bonding which are non-directional. electrons are delocalized and free to move, allowing atoms to slide more easily. this makes metals ductile and deformable rather than brittle and hard.
peierls-nabarro stresss
minimum stress needed to move a dislocation through a crystal lattice. when bonds are strong it needs more stress to move. when the dislocation is narrow is needs more stress to move.
dislocations in ceramics vs metals
metals have wide dislocations because of the non-directional metallic bonding, which makes dislocations easier to move making the material more ductile. ceramics have narrow dislocations (ionic/covalent bonds) so they are harder to move which makes them brittle.
what do narrow deformations mean in ceramics
high minimum stress which means there is little plastic deformation before permanent deformation.
von mises criterion
predicts when a material will start to plastically deform. ceramics cant plastically deform because there is no easy dislocation motion, so instead of yielding they fracture suddenly.
why are ceramics hard
their atoms resist being moved because of strong bonds and high minimum stress needed to move a dislocation, which makes ceramics hard.
why are ceramics brittle
when stress exceeds the limit, they crack instead of deforming due to the restricted dislocation motion and not being able to plastically deform
preparation, composition, microstructure of PSZ
mix zirconia with yttria then heat the powder to remove moisture. shape by pressing or extrusion then sinter to densify the ceramic and control the microstructure. this mix is partially stabilized high-temp crystal phase. only part of the high-temperature tetragonal or cubic phase is stable at room temp.
toughening mechanisms which make PSZ tough
the metastable tetragonal/cubic phase is key for transformation toughening. under mechanical stress, tetragonal particles transform into monoclinic phase. results in 4% volume expansion which creates compressive strength at the crack tip. makes it harder for the crack to grow. slows/deflects crack propagation. crack growth is resisted, and more energy is required to propagate the crack. now PSZ has a fracture toughness up to 10 times higher than normal ceramics
how is high purity silica produced using flame hydrolysis
silicon tetrachloride reacts with water vapor in a hydrogen-oxygen flame. the silica particles form as white soot (fine amorphous powder). then HCl gas is removed by ventilation. silica soot is collected, then heated in a furnace to fuse it into a clear, dense glass. this becomes the core of the optical fiber.
why is flame hydrolysis silica ideal for optical fibers
very high purity (avoids impurities which scatter or absorb light), amorphous structure (no crystals means no light scattering), very fine particles (ideal for fusion into smooth clean glass), uniform composition (ensures constant refractive index for fiber optics)
why is flame hydrolysis silica not ideal for sintering
very fine particles - high surface area which leads to agglomeration and uneven shrinkage.
amorphous structure - no grain boundaries which means it is hard to drive sintering by diffusion
porous and fluffy - traps air and causes defects during sintering
low packing density - requires high sintering temp and hard to uniformly densify
