Method of producing vitreous silica crucible
The method of producing a vitreous silica crucible, comprises deposition step of depositing silica powder on an inner wall surface of a cylindrical rotation mold having a bottom while rotating the mold; and a fusion step to obtain a vitreous silica crucible by heating and fusing the silica powder deposited on the inner wall surface of the mold and thereby vitrifying the silica powder. In the deposition step, a density of the deposited silica powder layer on the inner wall surface of the mold is controlled within a predetermined range by controlling the electrostatic charging voltage of the silica powder to be in a range of 1.0 kV or less in absolute value, and supplying the thus controlled silica powder to the inner wall surface of the mold. Thus, a wall thickness of the vitreous silica crucible is controlled.
1. A method of producing a vitreous silica crucible, comprising:
performing deposition of a silica powder by depositing silica powder as a single layer of unmelted silica powder on an inner wall surface of a cylindrical mold that has a bottom while rotating the mold;
shaving the wall using a device to control the thickness of the single layer of unmelted silica powder in the target thickness; and
performing fusion of the single layer of unmelted silica powder to obtain a vitreous silica crucible comprising a single layer of vitreous silica by heating, fusing through a one-step arc heat fusing process after the step of shaving, and thereby vitrifying the silica powder deposited on the inner surface of the mold,
wherein:
during the deposition of the silica powder, electrostatic charging voltage of the unmelted silica powder is controlled to be in a range of 0.0 kV to −1.0 kV in absolute value by controlling a relative humidity of an atmospheric gas being in contact with the unmelted silica powder to be not lower than 55%, by adding humidified gas, water vapor, or droplets of pure water to the atmospheric gas being in contact with the unmelted silica powder without performing an arc heat fusing process at the same time,
the thus controlled unmelted silica powder is supplied to the inner wall surface of the mold to maintain the density of the deposited unmelted silica powder layer on the inner wall surface of the mold within a predetermined range, wherein the thickness of the unmelted silica powder layer deposited on the bottom portion of the mold is controlled to be within ±1% compared with the target thickness, and
the one-step arc heat fusing process performed after the step of shaving is the only arc heat fusing process performed.
2. A method of producing a vitreous silica crucible according to claim 1 , wherein the relative humidity is controlled to be in a range of 60% to 90%.
3. A method of producing a vitreous silica crucible according to claim 1 , wherein the electrostatic charging voltage is controlled to be in a range of 0.0 kV to −0.9 kV in absolute value.