The Application of Barium Hydroxide in Quartz Crucible Coating

2015/09/24   Bmschem

    High quality quartz crucible manufacturers often use barium coating technology to improve the quality of crucibles in the process of production.

    Barium hydroxide, powdery or granular crystal containing crystal water, is required to reach a purity of 99.99%. It is an ideal chemical raw material for the cold coating of single crystal crucible in the photovoltaic industry. The reason for the application of barium hydroxide is that the equilibrium segregation coefficient of barium in silicon is very small. Thus its concentration in the silicon crystal bar is less than 2.5x10 (-9) /cm_3. Therefore, the quality of crystal bar has not been influenced.

   Quartz crucible itself is amorphous intermediate energy. Phase change will occur and stable cristobalite crystalline will be formed in appropriate conditions. And with the increase of single crystal drawing time, cristobalite crystal may peel off wholly. These peeling cristobalite particles will float in solution. Most of the particles completely dissolve in solution after a certain period of time. However, there are still probabilities that some of the larger particles knock down the surface of crystal bar before their complete dissolution and leads to dislocation. The probability of dislocation increases with the service time and temperature of quartz crucible. 

    Recent studies have found that the quartz crucible wall is coated with barium hydroxide octahydrate (Ba(OH)₂. 8H₂O) (saturated barium hydroxide solution) which reacts with carbon dioxide in the air to form barium carbonate. When the quartz crucible is heated on a single crystal furnace, the barium carbonate will decompose to form barium oxide. Then basi0₃ is formed by the reaction of barium oxide and quartz crucible. Next, a layer of dense and tiny cristobalite crystal is formed on the wall of the quartz crucible due to the basi0₃. These tiny cristobalite crystals are difficult to be infiltrated by solution and desquamate. Even if peeling happens, they will quickly dissolve in solution. So in this way, it greatly improved the service life and crystal grown yield of quartz crucible. Besides, the advantages of forming a layer of cristobalite crystals on the inner wall are that it can increase the strength of the quartz crucible and reduce high temperature softening. 

    In the past, artificial coating is adopted in barium hydroxide coating technique of quartz crucible. The prepared barium hydroxide solution without the protection of nitrogen, generally adopting carrier gas of spray can as compressed air and the influences by personal subjective factors in manual operation, cannot realize the uniformity and stability of coated film. Previous spraying equipments spray barium hydroxide solution directly driven by carbon dioxide. This spraying technology is comparatively simple. Carbon dioxide is easy to react with the solution in barium hydroxide liquid storage tank, which will influence the spraying effect. Besides, the spraying mode does not adopt two fluid pressure atomizing system, so it cannot form uniform atomization and small atomizing particles. Now, we can use nitrogen as protective gas and ensure that the barium hydroxide solution does not react with the carbon dioxide in the air. So impurities do not mix and the barium hydroxide solution is pure. The pollution of other impurities is avoided. When spraying, we adopt two fluid pressure atomizing system and high-purity carbon dioxide. Thus we can get much smaller particles and uniform spraying effects. When quartz crucible is rotary, atomizing nozzle does upper and lower reciprocating motion relative to it. Thus, each parts of the inner wall of quartz crucible contact with the soup fully. Besides, there is a water spray process after spraying soup, which leads to the uniform distribution of soup on the surface of quartz crucible.