Solid state sintered alumina

Question 1

What are the applications?

Answer 1

~ high temp applications (crucibles, furnace tubes, thermocouple sheaths)

~ hip prostheses

Question 2

What is high purity alumina?

Answer 2

~ >99.7 wt%
~ mixed with ~0.25wt% MgO, fired in H2 at 1600-1700C

Question 3

Why is MgO added to high purity alumina?

Answer 3

~ MgO is a grain growth inhibitor, eliminates grain growth anisotropy

~ fracture strength goes up with lower grain size

Question 4

Why is H2 used during firing high purity alumina?

Answer 4

~ hydrogen is interstitially soluble in Al2O3 and can diffuse through the lattice from shrinking pores

~ by contrast, the N2 in the air is not soluble: the pressure buildup in pores leaves ~5% porosity when firing in air

Question 5

Why is alumina opaque?

Answer 5

~ because of porosity and second phase light scattering as well as differentially-oriented grains meeting at GB (with index of refraction varying with orientation in the hexagonal lattice) scattering light

Question 6

What is lucalox?

Answer 6

~ more dense, translucent (90-95% transmittance)

~ developed to encapsulate sodium vapor lamps (street lighting)

~ sintered at 1700-1800C, high temps to facilitate grain growth –> fewer grain boundaries, less light scattering