The preheating of graphite crucible plays an important role in the process of crucible, and the service life of the crucible is greatly enhanced if the preheating method is right, conversely, the service life of crucible is over if the preheating method is incorrect. So what's the right way to warm it?
1).The graphite crucible is a ceramic product, it has a certain number of pores in the crucible wall , they are easy to absorb the moisture, so for the first time using crucible and under the conditions of having no any material in it ,the crucible must be preheated to ensure that no residual moisture in the crucible wall: it is best to put it nearby the stove which has the temperature and baked it more than 24 hours, in this period of time, the crucible need to be turned several times so that the heat evenly and removing the moisture inside of the crucible wall(or we can put some of wood inside the crucible and burn them about 3 hours to achieve the effect of preheating). We can avoid the crucible heating too fast and resulting in crucible thermal expansion,the bottom appeared off and delamination or cracking.
The humidity general is in the fastest evaporation in the 75-100 degrees temperature,so the heating must be slow in this temperature; after 2 hours ,the graphite crucible preheating to 150 to 200 degrees and holding this temperature an hour and then heating 150 degrees for an hour. The crucible don’t stay in this temperature range too long time because in the temperature range 315~650 degree ,the crucible wall will appear rapid oxidation, oxidation will not only shorten the life of the crucible , but also it will impact its use of performance because of its thermal conductivity be reduced.
2).The graphite crucible normally don’t need preheating again after the first time preheating if it is used continuously unless the crucible is exposed in the moist environment again.
3).The operators need make the graphite crucible fast warming up to 850 to 950 degrees in having no material and insulation for half an hour and then dropped to normal temperature and put the material in it after the the graphite crucible preheating. (the life of the preheating crucible will prolong when its temperature is higher than the normal working temperature.) Usually the graphite crucible is heated to a 815~930, it is useful to protect the glaze of the crucible, the high temperature heating can be each a week if it’s possible.