Titania-silica glass with uniform hydroxyl concentration
A process of forming a titania-silica glass body, the process including exposing a titania-doped silica soot body to a first thermal treatment by heating the body to a first temperature T1 between about 800° C. and about 1100° C. for a first time duration t1 calculated using the equation: t 1 > L c 2 4 α , wherein L c is the characteristic length (cm) of the body and α is the thermal diffusivity (cm 2 /sec) of the body. The process further including exposing the body to a second thermal treatment by heating the body to a second temperature T2 between about 1050° C. and about 1250° C. wherein, after the second thermal treatment, a peak-to-valley difference of hydroxyl concentration amongst a plurality of segments of the body is about 70 ppm or less.
1 . A process of forming a titania-silica glass body, the process comprising:
exposing a titania-silica soot body to a first thermal treatment by heating the body to a first temperature T1 between about 800° C. and about 1100° C. for a first time duration t1 calculated using the equation:
t
1
>
L
c
2
4
α
wherein L c is a length (cm) of the body and a is the thermal diffusivity (cm 2 /sec) of the body;
increasing the temperature of the body from the first temperature T1 to a second temperature T2 at a rate of about 20° C./hour or greater; and
exposing the body to a second thermal treatment by heating the body at the second temperature T2 between about 1050° C. and about 1250° C. for a second time duration t2 such that the second temperature T2 is greater than the first temperature T1,
wherein, after the second thermal treatment, a peak-to-valley difference of hydroxyl concentration amongst a plurality of segments of the body is about 70 ppm or less, the hydroxyl concentration being measured using a Fourier transform infrared spectroscopy in transmission, and the plurality of segments includes every adjacent segment across a specific length and a specific width of the body, the specific length being about 25 mm or more and the specific width being about 25 mm or more.
2 . The process of claim 1 , wherein the peak-to-valley difference of hydroxyl concentration is about 60 ppm or less.
3 . The process of claim 1 , wherein the peak-to-valley difference of hydroxyl concentration is about 50 ppm or less.
4 . The process of claim 1 , wherein the thermal diffusivity α is 0.003 cm 2 /sec.
5 . The process of claim 1 , wherein, after the second thermal treatment step, the body has a silica concentration of about 80 wt. % or more.
6 . The process of claim 1 , wherein after the second thermal treatment, the body has a titania concentration from about 6.0 wt. % to about 12.0 wt. %.
7 . The process of claim 1 , wherein the rate is about 30° C./hour or greater.
8 . The process of claim 7 , wherein the rate is about 45° C./hour or greater.
9 . The process of claim 1 , wherein the rate is from about 30° C./hour to about 100° C./hour.
10 . The process of claim 1 , wherein the specific length is about 50 mm or more and the specific width is about 50 mm or more.
11 . The process of claim 10 , wherein the specific length is about 100 mm or more and the specific width is about 100 mm or more.
12 . The process of claim 1 , further comprising, during at least one of the first thermal treatment and the second thermal treatment, exposing the body to an atmosphere having a partial pressure of about 0.50 atm or less.
13 . The process of claim 12 , wherein the partial pressure is about 0.20 atm or less.
14 . The process of claim 13 , wherein the partial pressure is less than about 0.02 atm.
15 . The process of claim 14 , wherein the atmosphere is substantially free of water and steam.
16 . The process of claim 1 , further comprising, during at least one of the first thermal treatment and the second thermal treatment, exposing the body to an atmosphere comprising steam and having a partial steam pressure from about 0.10 atm to about 1.00 atm.
17 . The process of claim 1 , further comprising, during the second thermal treatment, exposing the body to an atmosphere comprising steam and having partial steam pressure of about 0.10 atm or less.
18 . The process of claim 17 , wherein the body is not exposed to steam during the first thermal treatment and during a hold-time t hold during the second thermal treatment, and the body is exposed to an atmosphere comprising steam during the second thermal treatment for a time t rem , the time t rem being the remainder of the second thermal treatment after the hold-time t hold , and the hold-time t hold being greater than or equal to the first time duration tl of the first thermal treatment.
19 . The process of claim 1 , wherein the average hydroxyl concentration of the plurality of segments is about 1500 ppm or less.