Heat-exchange method

Question 1

What is the heat exchange method?

Answer 1

~ a sapphire seed crystal (shape of a hockey puck) is placed at the bottom of a single used molybdenum crucible, when is then loaded with pure alumina

~ furnace is evacuated and resistively-heated to melt the alumina, while keeping the seed just below its melting point by passing helium gas through the heat-exchanger beneath the center of the crucible

~ after partial melting of the seed, helium flow is increased to cool the seed and initiate the growth of sapphire onto the seed

~ furnace is held at a constant temp during growth of the crystal, which proceeds out from the seed in three dimensions, following the shape of the crucible

~ after crystallization is finished, the furnace temp and the helium flow dwell, annealing to attenuate internal stresses and the dislocation density

Question 2

What are the sizes of the boules produced?

Answer 2

~ boules of ~34cm diameter weighing 115kg are routinely produced in ~18 days

~ high quality regions can be core-drilled out

Question 3

What is the hardness of sapphire?

Answer 3

~ hard scratch resistance as compared to glass (gorilla glass = 6.17 GPa, sapphire = 18.6 GPa)

Question 4

What are the applications?

Answer 4

~ used for high end watch covers, supermarket scanners, atm touch screen displays

~ using ion implantation, thin lamina can be formed from a rectangular block for smart phone screens; higher cost and required production volume has prohibited it from reaching smart phone market

~ sapphire is used as an electrically insulating substrate for semiconductors (SOS: silicon on sapphire). GaN deposited on sapphire forms a blue LED. This, combined with a phosphor coating yields white light LEDS, which are substantially more efficient than other lighting

Question 5

How are windows made out of these boules?

Answer 5

~ boules are grown and/or cut in a specific orientation, with c-axis perpendicular to the window face for minimum birefringence