Method and apparatus for manufacturing fused silica crucible, and the fused silica crucible
A method of manufacturing a fused silica crucible by heating and melting a vitreous silica powder compact shaped into a mold using arc discharge of electrodes arranged around a rotation shaft of the mold, includes the steps of: arranging the electrodes in a ring shape, and setting a ratio W/R of a horizontal distance W between the electrode front end and the surface of the vitreous silica powder compact to a vitreous silica powder compact opening radius R, for at least a predetermined time during arc heating, to be in the range of 0.002 to 0.98.
1. A method of manufacturing a fused silica crucible by heating and melting a vitreous silica powder compact shaped into a mold using arc discharge of electrodes arranged around a rotation shaft of the mold, comprising the steps of:
arranging the electrodes in a ring shape;
heating and melting the vitreous silica powder by not generating a sustainable arc between the electrodes opposed to each other through a center portion of the ring but forming a stable ring-shaped arc that connects the neighboring electrodes; and
setting a ratio W/R of a horizontal distance W between the electrode front end and the surface of the vitreous silica powder compact to a vitreous silica powder compact opening radius R, for at least a predetermined time during arc heating, to be in the range of 0.002 to 0.98, wherein the bubble content of a transparent layer of the bottom and a transparent layer of the wall of the manufactured crucible is equal to or less than 0.03%.
2. The method according to claim 1 , wherein a ratio H/R of a vertical distance H between the electrode front end and the surface of the vitreous silica powder compact to the vitreous silica powder compact opening radius R, for at least a predetermined time during arc heating, is set to be in the range of 0.02 to 9.2.
3. The method according to claim 1 , wherein a ratio H/H 2 of the vertical distance H between the electrode front end and the surface of the vitreous silica powder compact to the vitreous silica powder compact height H 2 , for at least a predetermined time during arc heating, is set to be in the range of 0.0007 to 4.
4. The method according to claim 1 , wherein a ratio D/R of the horizontal distance D between the neighboring electrodes to the vitreous silica powder compact opening radius R, for at least a predetermined time during arc heating, is set to be in the range of 0.04 to 1.1.
5. The method according to claim 1 , wherein the ring-shaped arc is formed by using an electrode configuration in which the electrodes are arranged in a ring-shape at even intervals such that the absolute value of an alternating current phase difference θ between the neighboring electrodes is in the range of 90°≦θ≦180°.
6. The method according to claim 1 , wherein the ring-shaped arc is formed by using an electrode configuration in which a ratio of a radius r of a circumference around the mold rotation shaft of the ring to the vitreous silica powder compact opening radius R is in the range of 1 to ¼, for at least a predetermined time during arc heating.
7. An apparatus for manufacturing a fused silica crucible by rotational molding, comprising:
electrodes arranged in a ring shape around a rotation shaft of a mold; and
an electrode position setting device capable of setting:
a ratio W/R of a horizontal distance W between the electrode front end and the surface of the vitreous silica powder compact charged into the mold to a vitreous silica powder compact opening radius R to be in the range of 0.002 to 0.98,
a ratio H/R of a vertical distance H between the electrode front end and the surface of the vitreous silica powder compact to the vitreous silica powder compact opening radius R to be in the range of 0.02 to 9.2,
a ratio H/H 2 of the vertical distance H between the electrode front end and the surface of the vitreous silica powder compact to the vitreous silica powder compact height H 2 to be in the range of 0.0007 to 4, and
a ratio D/R of the horizontal distance D between the neighboring electrodes to the vitreous silica powder compact opening radius R to be in the range of 0.04 to 1.1,
wherein:
a sustainable arc is not generated between the electrodes opposed to each other through a center portion of the ring, but a stable ring-shaped arc that connects the neighboring electrodes is formed, and
the bubble content of a transparent layer of the bottom and a transparent layer of the wall of the manufactured crucible is equal to or less than 0.03%.
8. The apparatus according to claim 7 , wherein the apparatus includes an electrode configuration in which the electrodes are arranged in a ring-shape such that the absolute value of an alternating current phase difference θ between the neighboring electrodes is in the range of 90°≦θ≦180°.
9. The apparatus according to claim 7 , wherein the electrode position setting device sets a ratio of a radius r of a circumference around the mold rotation shaft of the ring to the vitreous silica powder compact opening radius R to be in the range of 1 to ¼.
10. The apparatus according to claim 7 , wherein one electrode configuration of four two-phase AC electrodes, six two-phase AC electrodes, eight two-phase AC electrodes, ten two-phase AC electrodes, three three-phase AC electrodes, six three-phase AC electrodes, nine three-phase AC electrodes, twelve three-phase AC electrodes, fifteen three-phase AC electrodes, four four-phase AC electrodes, eight four-phase AC electrodes, twelve four-phase AC electrodes, and sixteen four-phase AC electrodes, is included.