Flux growth

Question 1

How is flux growth done?

Answer 1

~ powder of the compound to be grown (plus desired impurities) is mixed with another compound with which it forms a low-temp eutectic liquid

~ these compounds must be fully immiscible in the solid state

Question 2

What are other methods for flux growth?

Answer 2

~ sample powders and their crucibles are in turn sealed under vacuum in a fused silica ampoule

~ in others, the furnace is atmosphere-controlled

Question 3

What is an example of flux growth?

Answer 3

~ Al2O3 is mixed with MoO3 and heated to 1100C. In an unsealed system, the MoO3 is more volatile, so sapphire nucleates and grows from the melt as the composition shifts past the liquidus. Cooling further facilitates the growth of sapphire

Question 4

What is the effect of changing cooling rate?

Answer 4

~ more rapid cooling = fewer, larger crystals

~ slower cooling = more nucleation and a larger number of smaller crystals

Question 5

What is the crystal structure of the crystals that form?

Answer 5

~ these crystals are free of strain and have low levels of imperfection

~ grow with faceting related to the hexagonal crystal structure (hexagonal bipyramid shape)

Question 6

What are the advantages and disadvantages of this method?

Answer 6

~ advantage of lower temp processing
~ disadvantage: to grow larger crystals, growth times can take ten months (Chatman crystals)

Question 7

How else can it be cooled?

Answer 7

~ liquid can be poured off prior to cooling the eutectic temp

~ if cooled to room temp, the eutectic solid must be mechanically or chemically (etchant) separated from the grown crystals