Double crucible for a glass drawing method and method of making glass fiber or a preform with the double crucible

Abstract

The double crucible for a glass drawing method has a heatable outer crucible ( 1 ) and an inner crucible ( 2 ) surrounded by the outer crucible ( 1 ), which is heatable separately from the outer crucible ( 1 ). Both crucibles ( 1,2 ) have an outlet nozzle ( 1 a, 2 a ) for the glass to be drawn. To make glass fibers from heavy metal oxide glass (HMO-glass) with higher quality and comparatively simple crucible features, the outlet nozzle ( 1 a ) of the outer crucible ( 1 ) extends a certain distance beyond the outlet nozzle ( 2 a ) of the inner crucible ( 2 ). Surfaces of the outlet nozzles coming in contact with the glass melt are polished and are provided on a material, which has a reducing action on heavy metal glass in the melt in all cases. These surfaces also have sufficient mechanical strength for and chemical inertness to heavy metal oxide glass.

1 - 14 . (canceled)  
   
   
       15 . A method of making a glass fiber or preform for said glass fiber with a double crucible, said glass fiber or said preform comprising a core and at least one coating, and 
 wherein said double crucible comprises a heatable outer crucible and an inner crucible surrounded by the outer crucible, said inner crucible being heatable separately from the outer crucible, the outer crucible has an outlet nozzle, the inner crucible has an outlet nozzle, the outlet nozzle of the inner crucible projects or extends beyond the outlet nozzle of the outer crucible; said method comprising the steps of: 
 a) melting a portion of heavy metal oxide glass for the core glass of the glass fiber and another portion of heavy metal oxide glass for the cladding glass in separate gold crucibles from a glass batch in a pre-melting step to form separate glass melts;  
 b) bubbling oxygen through the separate glass melts in the pre-melting step;  
 c) pre-heating the double crucible and pouring the separate glass melts into the pre-heated double crucible, with the glass melt for the core glass in the inner crucible and the glass melt for the cladding glass in the outer crucible;  
 d) holding the glass melts in the double crucible at a predetermined temperature with said oxygen bubbling through the glass melts for a predetermined time interval;  
 e) increasing the temperature and refining the glass melts and optionally passing oxygen over the glass melts;  
 f) tempering the glass melts so that the glass melts reach a glass drawing temperature; and  
 g) after the tempering of step f), drawing the glass fiber or preform from the glass melts.  
   
   
   
       16 . The method as defined in  claim 15 , wherein said heavy metal oxide glass is a glass containing at least one oxide of at least one element and wherein said at least one element is selected from the group consisting of Bi, Sb, Te, Se, As, Cd, Ga and Pb oxide.  
   
   
       17 . The method as defined in  claim 15 , further comprising drawing a coating jacket for the glass fiber from a heavy metal oxide glass melt in a discharge crucible, preparing said heavy metal oxide glass melt in a separate melting step in a gold crucible from a melted glass batch with intensive oxygen bubbling and pouring the heavy metal oxide glass melt into the discharge crucible.  
   
   
       18 . The method as defined in  claim 15 , wherein said glass batch comprises nanoscale raw materials.  
   
   
       19 . The method as defined in  claim 15 , wherein the glass batch comprises raw materials, and further comprising mixing and burning the raw materials in a pre-reaction step prior to the pre-melting step.  
   
   
       20 . The method as defined in  claim 15 , further comprising controlling said drawing according to a diameter of the glass fiber, controlling drawing speed during the drawing of the glass fiber with a first time constant and controlling crucible temperature with a second time constant and wherein said second time constant is longer than said first time constant.  
   
   
       21 . A method of making a glass fiber or preform for said glass fiber with a double crucible, said glass fiber or said preform comprising a core and at least one coating, and wherein said double crucible comprises an outer crucible and an inner crucible surrounded by the outer crucible, said inner crucible being heatable separately from the outer crucible, the outer crucible having an outlet nozzle and the inner crucible having an outlet nozzle, and 
 wherein the outlet nozzle of the inner crucible projects or extends beyond an outlet of the outlet nozzle of the outer crucible, and    wherein at least glass of the at least one coating has a steep viscosity curve and/or a high density.    
   
   
       22 . The method as defined in  claim 15 , wherein the heavy metal oxide glass for the cladding glass is a bismuth oxide glass.