Manufacture of synthetic silica glass

1. A method of manufacturing a silica-based glass, comprising the steps:

forming an emulsion of an aqueous phase in a non-aqueous liquid silica precursor material;

supplying said emulsion as a spray of droplets into a synthesis flame, whereby said precursor material is converted in the flame into a silica-containing soot; and

collecting said soot on a substrate either as a porous soot body for subsequent consolidation to glass or directly as a substantially pore-free glass,

wherein the aqueous phase comprises a solution of at least one salt of at least one metallic dopant species.

2. A method according to claim 1 , wherein said synthesis flame is provided by a synthesis burner.

3. A method according to claim 1 , wherein said synthesis flame is provided by a plasma heating device.

4. A method according to claim 1 , wherein said silica precursor material is a non-aqueous chlorine-free silicon compound.

5. A method according to claim 4 , wherein the non-aqueous chlorine-free silicon compound is selected from the group consisting of a siloxane compound and a mixture of siloxane compounds.

6. A method according to claim 5 , wherein said siloxane compound is selected from the group consisting of hexamethyldisiloxane, octamethylcyclotetrasiloxane, decamethylcyclopentasiloxane, dodecamethylcyclohexasiloxane and mixtures of at least two thereof.

7. A method according to claim 1 , wherein the non-aqueous phase of the emulsion further includes an emulsifier.

8. A method according to claim 7 , wherein the emulsifier is a silicone-based surfactant.

9. A method according to claim 1 , wherein said metallic dopant species includes at least one compound capable of conversion in said synthesis flame to the oxide of at least one metal selected from Groups 1 to 16 and the lanthanide and actinide series of the Periodic Table of the Elements.

10. A method according to claim 1 , wherein at least one non-metallic dopant species is incorporated into the glass.

11. A method according to claim 10 , wherein such non-metallic species includes an oxide of an element selected from boron and phosphorus.

12. A method according to claim 1 , wherein each droplet in the spray contains all of the ingredients required for conversion by oxidation or hydrolysis in the flame to said silica-based glass.

13. A method according to claim 12 , wherein each droplet in the spray contains an emulsion of said aqueous phase in said non-aqueous liquid silica precursor material.

14. A method according to claim 13 , wherein the diameter of the majority of micro-droplets of said aqueous phase in said emulsion is less than 2 μm.

15. A method according to claim 1 , wherein each droplet in the spray contains the at least one dopant species.

16. A method according to claim 2 , wherein the emulsion is atomised within the synthesis burner by pneumatic means.

17. A method according to claim 16 , wherein the pneumatic means comprises a flow of atomising gas.

18. A method according to claim 2 , wherein the emulsion is atomised within the synthesis burner by ultrasonic means.

19. A method according to claim 18 , wherein the ultrasonic means comprises an ultrasonic horn.

20. A method according to claim 2 , wherein emulsification takes place within or proximate to the burner, and is facilitated by an ultrasonic horn adapted for the simultaneous feeding and emulsification of two liquid phases.

21. A method according to claim 1 , wherein said silica-based glass is subjected to a subsequent homogenisation process.

22. A method of reducing the NOx emissions from an atomisation burner used in the manufacture of silica-based glass, wherein said atomisation burner is fed with an emulsion of an aqueous phase in a non-aqueous chlorine-free liquid silica precursor, in a method according to claim 1 .

23. A method according to claim 1 , wherein said synthesis flame is augmented by a plasma heating device.