Treatment of silica based soot or an article made of silica based soot

Abstract

One embodiment of the disclosure relates to a method of cleaning silica-based soot or an article made of silica-based soot, the method comprising the step of treating silica-based soot or the article made of silica-based soot with at least one of the following compounds: (i) a mixture of CO and Cl.sub.2 in a carrier gas such that the total concentration of CO and Cl.sub.2 in the mixture is greater than 10% (by volume, in carrier gas) and the ratio of CO:Cl.sub.2 is between 0.25 and 5; (ii) CCl.sub.4 in a carrier gas, such that concentration CCl.sub.4 is greater than 1% (by volume, in carrier gas). Preferably, the treatment by CCl.sub.4 is performed at temperatures between 600 C., and 850 C. Preferably, the treatment with the CO and Cl mixture is performed at temperatures between 900 C. and 1200 C. The carrier gas may be, for example, He, Ar, N.sub.2, or the combination thereof.

Claims

1. A method of treating silica-based soot or an article comprising a silica-based soot layer, said method comprising; treating said silica-based soot or said article comprising a silica-based soot layer, said silica-based soot or said silica-based soot layer comprising ZrO.sub.2, with CCl.sub.4 in a carrier gas at a treatment temperature between 600 C. and 850 C. such that a concentration of CCl.sub.4 is greater than 1%, by volume, and a treatment time, t.sub.treatment, ZrO2, is:
t.sub.treatment,ZrO2(in min)>t.sub.diffusion,ZrO2+t.sub.reaction,ZrO2, a diffusion time, t.sub.diffusion, ZrO2, and a reaction time, t.sub.reaction, ZrO2, have the following values: $t_{\mathrm{diffusion},\mathrm{ZrO}2}\left(\mathrm{in}\min \right)=\frac{L^2}{60D_{\mathrm{eff},{\mathrm{CCl}}_4}};$ and $t_{\mathrm{reaction},\mathrm{ZrO}2}\left(\mathrm{in}\min \right)=\frac{5.75{10}^{-6}{d}_p\left(\mathrm{in}m\right)\mathrm{Exp}\left[12000/T\right]}{y_{\mathrm{CCl}4}\left(\mathrm{in}\mathrm{atm}\right)}.$ and L>0 and is a thickness of said silica-based soot or said silica-based soot layer, D.sub.eff, CCl4 (in cm.sup.2/sec) is a diffusion rate of CCl.sub.4 through said silica-based soot or said article comprising a silica-based soot layer, T is the treatment temperature, y.sub.CCl4 is a partial pressure of CCl.sub.4, and d.sub.p>0 and is an initial particle size of said ZrO.sub.2.

2. The method of treating silica-based soot or an article comprising a silica-based soot layer according to claim 1, wherein the treatment temperature T is between 750 C. and 850 C.

3. The method of treating silica-based soot or an article comprising a silica-based soot layer according to claim 1, wherein said treatment time, t.sub.treatment,ZrO2, is at least 20 min.

4. The method of treating silica-based soot or an article comprising a silica-based soot layer according to claim 3, wherein said treatment time, t.sub.treatment, ZrO2, is at least 50 min.

5. The method of treating silica-based soot or an article comprising a silica-based soot layer according to claim 3, wherein said treatment time, t.sub.treatment, ZrO2, is at least 90 min.

6. A method of treating a porous silica-based soot preform with a soot layer having a thickness L, said method comprising: treating said silica-based soot preform, said silica-based soot preform comprising ZrO.sub.2, with CCl.sub.4 in a carrier gas, such that a concentration of CCl.sub.4 is greater than 1%, by volume, and a treatment time, t.sub.treatment, ZrO2, with CCl.sub.4 is:
t.sub.treatment,ZrO2(in min)>t.sub.diffusion,ZrO2+t.sub.reaction,ZrO2, a diffusion time, t.sub.diffusion, ZrO2, and a reaction time, t.sub.reaction, ZrO2, have the following values: $\begin{array}{c}{t}_{\mathrm{diffusion},\mathrm{ZrO}2}\left(\mathrm{in}\min \right)=\frac{L^2}{60D_{\mathrm{eff},{\mathrm{CCl}}_4}},\\ t_{\mathrm{reaction},\mathrm{ZrO}2}\left(\mathrm{in}\min \right)=\frac{5.75{10}^{-6}{d}_p\left(\mathrm{in}\mathrm{.Math.m}\right)\mathrm{Exp}\left[12000/T\right]}{y_{{\mathrm{CCl}}_4}\left(\mathrm{in}\mathrm{atm}\right)},\end{array}$ and L>0 and is said soot layer thickness, D.sub.eff, CCl4 (in cm.sup.2/sec) is a diffusion rate of the CCl.sub.4 through said porous silica-based soot preform, T is a treatment temperature for treating said porous soot preform in said CCl.sub.4 in a carrier gas, y.sub.CCl4 is a partial pressure of CCl.sub.4, and d.sub.p>0 and is an initial particle size of said ZrO.sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates the decrease in Cr.sub.2O.sub.3 particle size (initial size=0.25 m) at 1100 C. at different concentrations of Cl.sub.2;

(2) FIG. 2 illustrates the decrease in Cr.sub.2O.sub.3 particle size (initial size=0.25 m) at 1100 C., at different concentrations of Cl.sub.2 and CO, according to one embodiment;

(3) FIG. 3 illustrates the decrease in Cr.sub.2O.sub.3 particle size (initial size=0.25 m) with time, when soot is treated, according to at least one embodiment, at three different temperatures with a mixture of CO and Cl.sub.2;

(4) FIG. 4A illustrates the decrease in Cr.sub.2O.sub.3 particle size versus time, for different initial particle sizes when soot is treated, according to one embodiment, at 1100 C. with a mixture of CO and Cl.sub.2;

(5) FIG. 4B illustrates the decrease in Cr.sub.2O.sub.3 particle size versus time, for different initial particle sizes when soot is treated, according to one embodiment, at 1175 C. with a mixture of CO and Cl.sub.2;

(6) FIG. 4C illustrates the decrease in Cr.sub.2O.sub.3 particle size versus time, for different initial particle sizes when soot is treated according to one embodiment, at 1175 C. with a mixture of CO and Cl.sub.2, but at higher concentrations compared to FIG. 4B.

(7) FIG. 5 illustrates the decrease in ZrO.sub.2 particle size (initial size=1 m) after treatment according to at least one embodiment, with different concentrations of CCl.sub.4, at 527 C.;

(8) FIG. 6 illustrates the decrease in ZrO.sub.2 particle size (initial size=1 m) after treatment according to at least one embodiment, with different concentrations of CCl.sub.4, at 627 C.;

(9) FIG. 7 illustrates the decrease in ZrO.sub.2 particle size (initial size=1 m) after treatment according to at least one embodiment, with CCl.sub.4 at three different temperatures; and

(10) FIG. 8 illustrates the decrease in ZrO.sub.2 particle size versus time, for different initial particle sizes, when soot is treated according to one embodiment at 727 C. with CCl.sub.4.

DETAILED DESCRIPTION

(11) Various embodiments will be further clarified by the following examples.

(12) One embodiment of the disclosure relates to a method of cleaning of silica-based soot or an article made of silica-based soot. According to this embodiment the method comprises the step of:

(13) treating silica-based soot or the article made of silica-based soot with at least one of the following compounds: (i) a mixture of CO and Cl in a carrier gas such that the total concentration of CO and Cl.sub.2 in said mixture is greater than 10%, by volume and the ratio of CO:Cl.sub.2 is between 0.25 and 5; (ii) CCl.sub.4 in a carrier gas, such that concentration CCl.sub.4 is greater than 1% by volume, in carrier gas. For example, CCl.sub.4 concentration in carrier gas may be 1.5%, 2 vol %, 3 vol %, 5 vol %, 10 vol %, 15 vol %, 20 vol %, 25 vol %, 30 vol %, 35 vol %, or 40 vol %.) Preferably, the treatment by CCl.sub.4 is performed at temperatures between 600 C., and 850 C. Preferably, the treatment with the CO and Cl mixture is performed at temperatures between 900 C. and 1200 C., more preferably, between 1000 C. and 1200 C., and even more preferably between 1100 C. and 1200 C. (e.g., 1050 C., 1075 C., 1100 C., 1125 C., 1150 C., 1175 C., or 1190 C.). The carrier gas may be, for example, He, Ar, N.sub.2, or the combination thereof.

(14) Preferably, the treatment by CCl.sub.4 is performed for at least 2 min, more preferably for at lest 5 min, more preferably at lest 10 or 20 min, for example for at least 50 min. Preferably, the treatment with the CO and Cl.sub.2 mixture is performed for at least 5 min, preferably at least 20 min or 30 min, more preferably for at least 100 min. Preferably, the ratio of CO to Cl.sub.2 is between 0.5 and 2, more preferably between 0.75 and 1.5.

(15) According to one embodiment the method of cleaning silica-based soot or an article made of silica-based soot comprises the following steps:

(16) treating said silica-based soot or said article made of silica-based soot with (i) CCl.sub.4 in a carrier gas, such that concentration CCl.sub.4 is greater than 1%, by volume; and (ii) a mixture of CO and Cl.sub.2 in a carrier gas such that the total concentration of CO and Cl.sub.2 in said mixture is greater than 10%, by volume and the ratio of CO:Cl.sub.2 is between 0.25 and 5; wherein the treatment with CCl.sub.4 is performed either before, or after the treatment with the mixture of CO and Cl.sub.2. Preferably, the treatment by CCl.sub.4 is performed at temperatures between 600 C., and 850 C., and the treatment with the mixture of CO and Cl.sub.2 is performed at temperatures between 900 C. and 1200 C. (e.g., 950 C., 975 C., 1000 C., 1025 C., 1050 C., 1075 C., 1100 C., 1125 C., 1150 C., 1175 C., 1190 C.). Preferably the treatment with CCl.sub.4 is performed for at least 2 min, and the treatment with a mixture of CO and Cl.sub.2 is performed for at least 5 min.

(17) FIG. 1 illustrates the decrease in Cr.sub.2O.sub.3 particle size at three different concentrations of chlorine (Cl.sub.2 was utilized alone, without CO), at the treatment temperature of 1100 C. The initial size of the Cr.sub.2O.sub.3 particles was 0.25 m. In these three examples the Cl.sub.2 concentration in the carrier gas was 5%, 10%, and 25% by volume respectively, which is higher than the typically used concentrations of Cl.sub.2 when treating optical preforms.

(18) FIG. 1 illustrates that even when the initial Cr.sub.2O.sub.3 particle sizes are relatively small (about 0.25 m) the amount of time needed to eliminate the Cr.sub.2O.sub.3 particles is relatively long even when high concentration of chlorine (about 10%) is used at high temperatures (about 1100 C.). For example, FIG. 1 shows that at a Cl.sub.2 concentration of 10%, the required treatment time to eliminate 0.25 m Cr.sub.2O.sub.3 particles is approximately 5 hrs (300 min). Even when the Cl.sub.2 concentration is 25%, the required treatment time to eliminate 0.25 m Cr.sub.2O.sub.3 particles is approximately 2 hrs.

(19) As shown in FIG. 2, the soot treatment time is dramatically decreased when a mixture of carbon monoxide and chlorine is utilized. For example, FIG. 2 illustrates that the addition of 10% CO to 10% of Cl.sub.2 in a carrier gas results in a ten-fold decrease in treatment time (from about 300 min, as shown in FIG. 1, to about 30 min). The initial particle sizes of Cr.sub.2O.sub.3 particles of FIG. 2 is identical to those of FIG. 1 (about 0.25 m), and the treatment temperature is also the same (about 1100 C.). FIG. 2 illustrates that the shortest treatment time is achieved when the ratio of chlorine to carbon monoxide is 1:1. It is noted that when the concentration of Cl.sub.2 is increased (e.g., to about 15% (0.15 atm), 20% (0.2 atm), 25% (0.25 atm)), with the corresponding increase n CO concentration, the treatment time for soot containing Cr.sub.2O.sub.3 particles may be decreased even further.

(20) The particle size decrease rates at different temperatures, when silica soot is treated with a CO and Cl.sub.2 mixture at 10% concentrations of each CO and Cl.sub.2 is shown in FIG. 3. For example, when the treatment (particle processing) temperatures is 900 C., the treatment time for the elimination of metal oxide particles such as Cr.sub.2O.sub.3, when the initial particle sizes are about 0.25 m, is about 140 min-150 min. When the treatment temperature is 1100 C., the treatment time for elimination of metal oxide particles, such as Cr.sub.2O.sub.3, is about 30-35 min (when the initial particle sizes are 0.25 m). Thus, in order to decrease the silica soot process treatment time when using the CO/Cl.sub.2 mixture, it is advantageous to have the reaction(s) at temperatures greater than 900 C., more preferably greater than 1000 C. and even more preferably greater than 1100 C. (e.g., 1125 C., 1150 C., 1175 C., 1190 C.). However, at temperatures above 1200 C., there can be significant etching of the silica soot by the CO/Cl.sub.2 mixture and some sintering of the soot preform. Consequently, the maximum temperature is preferably not higher than 1200 C.

(21) The rate of particle size decrease for different initial particle size is illustrated in FIG. 4A. We discovered that when Cr.sub.2O.sub.3 particles are large enough such that their presence in the soot preform can later result in fiber breaks, the treatment time needs to be chosen appropriately, based on the maximum particle size. For example, if one knows that silica soot contains Cr.sub.2O.sub.3 particles that are up to 2 m in diameter, (if the treatment temperature is 1100 C.) the processing time with the CO/Cl.sub.2 mixture (10%/10% vol % in carrier gas) should be about 4 to 4.5 hrs. If the Cr.sub.2O.sub.3 particles have a maximum diameter of 5 m, the processing time with the CO/Cl.sub.2 mixture (same concentration) should be about 10-12 hrs (if the treatment temperature is 1100 C.). However, these processing times can be decreased if the temperature during CO/Cl.sub.2 is increased to 1150 C.-1175 C. It is noted for that when the concentration of Cl.sub.2 is increased (e.g., to about 15%, 20%, or 25%) with the corresponding increase in CO concentration, such that the ratio of CO:Cl.sub.2 is between 0.5 and 2 (preferably, between 0.75, and more preferably 1) the treatment time for soot containing larger Cr.sub.2O.sub.3 particles (e.g., 1-10 m diameter, or larger) may be decreased even further. FIG. 4B illustrates the rate of particle size decrease for different initial particle size when the treatment temperature is 1175 C. (same concentrations (10 vol % of Cl.sub.2 and 10 vol % of CO, in carrier gas). FIG. 4C illustrates the rate of particle size decrease for different initial particle size when the treatment temperature is 1175 C., but the concentrations Cl.sub.2 and CO (in carrier gas) were each increased to 30 vol % (0.3 atm).

(22) It is noted that the four plots in FIGS. 4A-4C are parallel to one another. Thus, to determine the appropriate treatment time for the soot containing maximum particle sizes other than those depicted in FIGS. 4A-4C, one needs to simply draw line parallel to those shown in FIG. 4A-4C, but starting at the appropriate particle size level. Similarly, if other metal oxides are present in the soot, or if using uses different concentrations of Cl.sub.2 and CO, one can create a graph similar to that of FIG. 4A-4C, to determine the appropriate treatment time for the metal oxide particles of the specific compositions that are present in the soot and that are present in sizes that are large enough that requires either their elimination, or the decrease in size of these particles below a certain level.

(23) We discovered that for Cr.sub.2O.sub.3 particles of size d.sub.p (cross sectional length, or diameter, in m) treated at temperature T (in K) with mixture of chlorine and carbon monoxide gases having chlorine and carbon monoxide concentrations of y.sub.C12 and y.sub.CO (in atm), the treatment time should be greater than the diffusion time and reaction time, as shown below, i.e.:
t.sub.treatment,Cr203(in min)>t.sub.diffusion+t.sub.reaction,Cr203

(24) where the diffusion reaction time is a function of soot layer thickness L (in cm) and diffusion rate of the CO/Cl.sub.2 mixture D.sub.eff (in cm.sup.2/sec) through the porous soot preform, or loose silica soot and is given as:

(25) $\begin{array}{cc}{t}_{\mathrm{diffusion}}\left(\mathrm{in}\min \right)=\frac{L^2}{60D_{\mathrm{eff}}}& \left[2\right]\end{array}$

(26) and the reaction time is given as:

(27) $\begin{array}{cc}{t}_{\mathrm{reaction},\mathrm{Cr}2O3}\left(\mathrm{in}\min \right)=\frac{4.3{10}^{-4}\left(d_p\left(\mathrm{in}m\right)\right)\mathrm{Exp}\left[12000/T\left(\mathrm{in}K\right)\right]}{y_{\mathrm{Cl}2}{x}_{\mathrm{Cl}2}\left(1-1.35x_{\mathrm{Cl}2}+0.372x_{\mathrm{Cl}2}^2\right)}& \left[3\right]\end{array}$

(28) Methods to find the diffusion rate (D.sub.eff) of a gas in porous soot preform are known in the art. In Eq. [3], parameter x.sub.C12 is given as x.sub.C12: (y.sub.C12)/(y.sub.C12+y.sub.CO), where y.sub.C12 and y.sub.CO are the partial pressure of chlorine and carbon monoxide respectively. According to some embodiments the treatment time with the CO/Cl.sub.2 mixture is greater than 5 min, for example 10 min to 100 hrs. Preferably the treatment time with the CO/Cl.sub.2 mixture is 10 min to 15 or 20 hrs, for example 20 min, 30 min, 50 min, 1 hr, 1.5 hrs, 2 hrs, 3 hrs, 4 hrs, 6 hrs, 8 hrs, 10 hrs, 12 hrs., or some period of time there between.

(29) If ZrO.sub.2 particles are present in the soot, chemical treatment of soot with chlorine and CO mixture is not very effective. That is, although some ZrO.sub.2 particles will be eliminated or minimized in size to the appropriate level during the treatment with the CO/Cl.sub.2 mixture as described above, not all of the ZrO.sub.2 particles will be decreased in size to the appropriate levels (0.1 m, or smaller). However we discovered that the treatment of ZrO.sub.2 particles with CCl.sub.4 is very effective. We have found that CCl.sub.4 treatment of silica soot, for example of optical preforms containing porous silica soot is most effective in temperature range between 600 C.-850 C. Below 600 C., the reaction of CCl.sub.4 is slow and above 850 C., CCl.sub.4 can decompose and react with silica muffle and silica soot particles, as well as form elemental carbon.

(30) The rate of ZrO.sub.2 particle size decrease at different CCl.sub.4 concentrations, temperatures and for the specified initial ZrO.sub.2 particle sizes is shown in FIGS. 5-8. More specifically, FIGS. 5 and 6 illustrate that the decrease in ZrO.sub.2 particle size occurs faster with higher concentrations of CCl.sub.4 and at higher temperatures. For example, as shown in FIG. 5, at a treatment temperature of 527 C., when the concentration CCl.sub.4 was increased from 10% to 30% (0.1 atm to 0.3 atm) the complete elimination of ZrO.sub.2 particles up to 1 m in cross-section occurred in 62 min instead of 185 min. As shown in FIG. 6, at a treatment temperature of 627 C., the treatment proceeded much faster than at 527 C., and when the concentration of CCl.sub.4 was increased from 10% to 30% (0.1 atm to 0.3 atm) the complete elimination of ZrO.sub.2 particles up to 1 m in cross-section occurs in less than 15 min, instead of 35 min. FIG. 7 illustrates the decrease in ZrO.sub.2 particle size (initial size=1 m) at different temperatures, for CCl.sub.4 concentrations of 10 vol %, in carrier gas (0.1 atm). As shown in FIG. 7, when the temperature was changed from 527 C. to 727 C., the treatment time was decreased from about 185 min to about 10 min. FIG. 8 illustrates the decrease in ZrO.sub.2 particle size (initial size=1 m) for different initial particle sizes at 727 C. for CCl.sub.4 concentrations of 0.1 atm (i.e., 10%, per volume, in carrier gas). This figure indicates that the treatment time needs to be chosen appropriately, given the particle sizes present in the soot. For example, if one knows that silica soot contains ZrO.sub.2 particles that are up to 2 m in diameter if the treatment temperature is 727 C., the processing time with the CCl.sub.4 (assuming a concentration of 10% (or 0.1 atm)) is less than 20 min. If the ZrO.sub.2 particles have a maximum diameter of Sum, the processing time with the CCl.sub.4 (assuming concentration of 0.1 atm) is about 45-50 min. If the ZrO.sub.2 particles have a maximum diameter of 10 m, the processing time with CCl.sub.4 (assuming a concentration of 10%) is about 90-95 min. However, these processing times can be decreased if the concentration of CCl.sub.4 is increased.

(31) Experimental Data:

(32) Two grams of silica soot was placed inside a tube furnace equipped with a fused silica muffle. A flow of 2.36% CCl.sub.4 in He was established and the furnace was ramped 10 C./min to 950 C. An infrared spectrum of the gas products downstream of the silica soot sample was acquired every 60 seconds. The results show that:

(33) Below 400 C.: no reaction, only desorption of water from soot.

(34) 400 C.-600 C.: drying/doping of soot:
H.sub.2O+CCl.sub.4.fwdarw.2HCl+COCl.sub.2 (drying)
2H.sub.2O+CCl.sub.4.fwdarw.4HCl+CO.sub.2 (drying)
SiO.sub.2+CCl.sub.4.fwdarw.COCl.sub.2+SiOxCly (doping)

(35) 600 C.-850 C.: radical decomposition of CCl.sub.4:
CCl.sub.4.fwdarw.:CCl.sub.2+2.Math.Cl.fwdarw.C.sub.2Cl.sub.4+Cl.sub.2 (decomposition and recombination)

(36) Above 850 C.: etching of SiO.sub.2, carbonization:
SiO.sub.2+CCl.sub.4.fwdarw.SiCl.sub.4+CO.sub.2 (etching)
CCl4.fwdarw.C+4.Math.Cl (carbonization)

(37) This data indicates that in order to (i) maximize etching and volatilization of Cr.sub.2O.sub.3 and ZrO.sub.2 and (ii) minimize etching of silica (muffle and preform) and carbonization, it is preferable to carry out the CCl.sub.4 treatment of soot at about 800 C. Carbonization would not be a serious issue if the carbon stayed within the muffle, but the experimental data indicate that the carbon particles formed are small enough to be entrained outside of the muffle and deposit inside the piping downstream of the furnace, which could cause maintenance issues later on.

(38) In one experiment, 8.7 g of silica soot doped with 1 wt % Cr.sub.2O.sub.3 (average particle size of 2 m), 1 wt % of Fe.sub.2O.sub.3 (average particle size of 3 m), and 1 wt % of ZrO.sub.2 (average particle size of 5 m) was pressed into a silica soot pellet with a density of 0.90 g/cm.sup.3 and treated with a mixture of 5% CCl.sub.4 in He (this embodiment utilized He as carrier gas) at 800 C. for 45 minutes. The chemical analysis of the resulting pellet showed 0.53 wt % of Cr.sub.2O.sub.3, 0.13 wt % of Fe.sub.2O.sub.3, and 0.062 wt % or ZrO.sub.2. This experimental data further demonstrates the effectiveness of CCl.sub.4 at removing ZrO.sub.2, and in reducing the amount and size of Fe.sub.2O.sub.3 and Cr.sub.2O.sub.3 particles in silica soot.

(39) We discovered that for ZrO.sub.2 particles of size d.sub.p (cross sectional length, or diameter, in m) treated at temperature T (in K) with mixture of carbon tetrachloride gas having concentration of y.sub.CCl4 (in atm), the treatment time should be greater than the diffusion time and reaction time, as shown below, i.e.:
t.sub.treatment,ZrO2(in min)>t.sub.diffusion+t.sub.reaction,ZrO2[4]
where the diffusion reaction time is a function of soot layer thickness L (in cm) and diffusion rate of the CCl.sub.4 D.sub.eff,CCl4 (in cm.sup.2/sec) through the porous soot preform and is given as:

(40) $\begin{array}{cc}{t}_{\mathrm{diffusion}}\left(\mathrm{in}\min \right)=\frac{L^2}{60D_{\mathrm{eff},\mathrm{CCl}4}}& \left[5\right]\end{array}$
and the reaction time is given as:

(41) $\begin{array}{cc}{t}_{\mathrm{reaction},\mathrm{ZrO}2}\left(\mathrm{in}\min \right)=\frac{5.75{10}^{-6}{d}_p\left(\mathrm{in}m\right)\mathrm{Exp}\left[12000/T\right]}{y_{\mathrm{CCl}4}\left(\mathrm{in}\mathrm{atm}\right)}.& \left[6\right]\end{array}$

(42) According to some embodiments the treatment time with the CCl.sub.4 is greater than 2 min, for example 5 minutes to 100 hours. Preferably the treatment time with the CO/Cl.sub.2 mixture is 10 min to 15, for example 20 min, 30 min, 50 min, 1 hr, 1.5 hrs, 2 hrs, 3 hrs, 4 hrs, 6 hrs, 8 hrs, 10 hrs, 12 hrs, or some period of time there between.

(43) The invention is further illustrated by the following examples depicted in Tables 1 and 2. Examples 1-54 of Table 1 show the total treatment times for the 6 cm thick silica based soot layers with maximum initial Cr.sub.2O.sub.3 particles of different sizes, when treated with different combinations of chlorine and carbon monoxide mixtures at various treatment temperatures, such that the size of the Cr.sub.2O.sub.3 particles was reduced to zero (i.e., no Cr.sub.2O.sub.3). As illustrated in these examples, the diffusion times are much smaller than the reaction times, and the total treatment time is almost equal to the reaction time. Examples 55-90 of Table 2 show the total treatment times (resulted in reduction of ZrO.sub.2 particles sizes to zero) for the 6 cm thick soot layers with initial ZrO.sub.2 particles of different sizes, when treated with different concentrations of carbon tetrachloride at various treatment temperatures. Many soot bodies articles include both of these impurities and therefore would need treatment with CCl.sub.4 (between 600 C. and 850 C.) and CO+Cl.sub.2 mixture (between 900 C. and 1200 C.). It is also noted that the in some applications total elimination of Cr.sub.2O.sub.3 and ZrO.sub.2 particles may not be necessary, as long as their diameter is decreased below a critical size. For example, in fiber preforms, one may decide to simply decrease the particle sizes to a maximum size of about 0.2 m or less, or to 0.1 m or less, so that they are unlikely to become a cause of fiber breaks.

(44) TABLE-US-00001 TABLE 1 Treatment of Cr.sub.2O.sub.3 containing soot with CO/Cl.sub.2mixture Soot Soot Initial Total Treatment Layer Layer Cr.sub.2O.sub.3 Chlorine Carbon Diffusion Reaction Treatment Example Temperature Thickness Density Diffusivity, Particle concent. monoxide Time Time Time # [C.] (cm) (g/cm.sup.3) cm.sup.2/sec Size (m) (atm) (atm) xCl.sub.2 (min) (min) (min) 1 900 6.00 0.60 1.01 0.25 0.1 0 1.00 0.59 1354.87 13 55 2 900 6.00 0.60 1.01 0.25 0.1 0.05 0.67 0.59 168.51 169 3 900 6.00 0.60 1.01 0.25 0.1 0.1 0.50 0.59 142.62 143 4 900 6.00 0.60 1.01 0.25 0.1 0.2 0.33 0.59 151.22 152 5 900 6.00 0.80 0.64 0.25 0.1 0 1 0.94 1354.87 1356 6 900 6.00 0.60 1.01 0.25 0.2 0 1 0.59 677.43 678 7 900 6.00 0.60 1.01 0.25 0.2 0.1 0.67 0.59 84.25 85 8 900 6.00 0.60 1.01 0.25 0.2 0.2 0.5 0.59 71.31 72 9 900 6.00 0.60 1.01 0.25 0.2 0.4 0.33 0.59 75.61 76 10 900 6.00 0.60 1.01 1 0.1 0.1 0.50 0.59 570.47 571 11 900 6.00 0.60 1.01 1 0.2 0.2 0.5 0.59 285.23 286 12 900 6.00 0.60 1.01 1 0.4 0.4 0.5 0.59 142.62 143 13 900 6.00 0.60 1.01 5 0.1 0.1 0.5 0.59 2852.35 2853 14 900 6.00 0.60 1.01 5 0.2 0.2 0.5 0.59 1426.17 1427 15 900 6.00 0.60 1.01 5 0.4 0.4 0.5 0.59 713.09 714 16 900 6.00 0.60 1.01 10 0.1 0.1 0.5 0.59 5704.70 5705 17 900 6.00 0.60 1.01 10 0.2 0.2 0.5 0.59 2852.35 2853 18 900 6.00 0.60 1.01 10 0.4 0.4 0.5 0.59 1426.17 1427 19 1000 6.00 0.60 1.09 0.25 0.1 0 1.00 0.55 606.58 607 20 1000 6.00 0.60 1.09 0.25 0.1 0.05 0.67 0.55 75.44 76 21 1000 6.00 0.60 1.09 0.25 0.1 0.1 0.50 0.55 63.85 64 22 1000 6.00 0.60 1.09 0.25 0.1 0.2 0.33 0.55 67.70 68 23 1000 6.00 0.80 0.68 0.25 0.1 0 1 0.88 606.58 607 24 1000 6.00 0.60 1.09 0.25 0.2 0 1 0.55 303.29 304 25 1000 6.00 0.60 1.09 0.25 0.2 0.1 0.67 0.55 37.72 38 26 1000 6.00 0.60 1.09 0.25 0.2 0.2 0.5 0.55 31.93 32 27 1000 6.00 0.60 1.09 0.25 0.2 0.4 0.33 0.55 33.85 34 28 1000 6.00 0.60 1.09 1 0.1 0.1 0.50 0.55 255.40 256 29 1000 6.00 0.60 1.09 1 0.2 0.2 0.5 0.55 127.70 128 30 1000 6.00 0.60 1.09 1 0.4 0.4 0.5 0.55 63.85 64 31 1000 6.00 0.60 1.09 5 0.1 0.1 0.5 0.55 1277.00 1278 32 1000 6.00 0.60 1.09 5 0.2 0.2 0.5 0.55 638.50 639 33 1000 6.00 0.60 1.09 5 0.4 0.4 0.5 0.55 319.25 320 34 1000 6.00 0.60 1.09 10 0.1 0.1 0.5 0.55 2554.00 2555 35 1000 6.00 0.60 1.09 10 0.2 0.2 0.5 0.55 1277.00 1278 36 1000 6.00 0.60 1.09 10 0.4 0.4 0.5 0.55 638.50 639 37 1150 6.00 0.60 1.19 0.25 0.1 0 1.00 0.50 224.57 225 38 1150 6.00 0.60 1.19 0.25 0.1 0.05 0.67 0.50 27.93 28 39 1150 6.00 0.60 1.19 0.25 0.1 0.1 0.50 0.50 23.64 24 40 1150 6.00 0.60 1.19 0.25 0.1 0.2 0.33 0.50 25.06 26 41 1150 6.00 0.80 0.75 0.25 0.1 0 1 0.80 224.57 225 42 1150 6.00 0.60 1.19 0.25 0.2 0 1 0.50 112.28 113 43 1150 6.00 0.60 1.19 0.25 0.2 0.1 0.67 0.50 13.96 14 44 1150 6.00 0.60 1.19 0.25 0.2 0.2 0.5 0.50 11.82 12 45 1150 6.00 0.60 1.19 0.25 0.2 0.4 0.33 0.50 12.53 13 46 1150 6.00 0.60 1.19 1 0.1 0.1 0.50 0.50 94.55 95 47 1150 6.00 0.60 1.19 1 0.2 0.2 0.5 0.50 47.28 48 48 1150 6.00 0.60 1.19 1 0.4 0.4 0.5 0.50 23.64 24 49 1150 6.00 0.60 1.19 5 0.1 0.1 0.5 0.50 472.77 473 50 1150 6.00 0.60 1.19 5 0.2 0.2 0.5 0.50 236.38 237 51 1150 6.00 0.60 1.19 5 0.4 0.4 0.5 0.50 118.19 119 52 1150 6.00 0.60 1.19 10 0.1 0.1 0.5 0.50 945.54 946 53 1150 6.00 0.60 1.19 10 0.2 0.2 0.5 0.50 472.77 473 54 1150 6.00 0.60 1.19 10 0.4 0.4 0.5 0.50 236.38 237

(45) TABLE-US-00002 TABLE 2 Treatment of ZrO.sub.2 containing soot with CCl.sub.4/Reaction of CCl.sub.4 with ZrO.sub.2 Soot Soot Initial Total Treatment Layer Layer ZrO.sub.2 CCl.sub.4 Diffusion Reaction Treatment Example Temperature Thickness Density Diffus.sup.ivity, Particle conc. Time Time Time # [C.] (cm) (g/cm.sup.3) .sup.cm2/sec Size (um) (atm) (min) (min) (min) 55 600 6.00 0.60 0.61 1 0.1 0.98 53.72 54.70 56 600 6.00 0.80 0.42 1 0.1 1.43 53.72 55.14 57 600 6.00 0.60 0.61 1 0.2 0.98 26.86 27.84 58 600 6.00 0.60 0.61 2 0.1 0.98 107.43 108.42 59 600 6.00 0.60 0.61 2 0.2 0.98 53.72 54.70 60 600 6.00 0.60 0.61 2 0.4 0.98 26.86 27.84 61 600 6.00 0.60 0.61 5 0.1 0.98 268.58 269.57 62 600 6.00 0.60 0.61 5 0.2 0.98 134.29 135.27 63 600 6.00 0.60 0.61 5 0.4 0.98 67.15 68.13 64 600 6.00 0.60 0.61 10 0.1 0.98 537.16 538.15 65 600 6.00 0.60 0.61 10 0.2 0.98 268.58 269.57 66 600 6.00 0.60 0.61 10 0.4 0.98 134.29 135.27 67 700 6.00 0.60 0.69 1 0.1 0.87 13.08 13.95 68 700 6.00 0.80 0.469 1 0.1 1.28 13.08 14.36 69 700 6.00 0.60 0.69 1 0.2 0.87 6.54 7.41 70 700 6.00 0.60 0.69 2 0.1 0.87 26.16 27.03 71 700 6.00 0.60 0.69 2 0.2 0.87 13.08 13.95 72 700 6.00 0.60 0.69 2 0.4 0.87 6.54 7.41 73 700 6.00 0.60 0.69 5 0.1 0.87 65.39 66.26 74 700 6.00 0.60 0.69 5 0.2 0.87 32.70 33.57 75 700 6.00 0.60 0.69 5 0.4 0.87 16.35 17.22 76 700 6.00 0.60 0.69 10 0.1 0.87 130.79 131.66 77 700 6.00 0.60 0.69 10 0.2 0.87 65.39 66.26 78 700 6.00 0.60 0.69 10 0.4 0.87 32.70 33.57 79 850 6.00 0.60 0.805 1 0.1 0.75 2.52 3.26 80 850 6.00 0.80 0.53 1 0.1 1.13 2.52 3.65 81 850 6.00 0.60 0.805 1 0.2 0.75 1.26 2.00 82 850 6.00 0.60 0.805 2 0.1 0.75 5.04 5.78 83 850 6.00 0.60 0.805 2 0.2 0.75 2.52 3.26 84 850 6.00 0.60 0.805 2 0.4 0.75 1.26 2.00 85 850 6.00 0.60 0.805 5 0.1 0.75 12.59 13.34 86 850 6.00 0.60 0.805 5 0.2 0.75 6.30 7.04 87 850 6.00 0.60 0.805 5 0.4 0.75 3.15 3.89 88 850 6.00 0.60 0.805 10 0.1 0.75 25.19 25.93 89 850 6.00 0.60 0.805 10 0.2 0.75 12.59 13.34 90 850 6.00 0.60 0.805 10 0.4 0.75 6.30 7.04

(46) Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that any particular order be inferred.

(47) It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the invention. Since modifications combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and their equivalents.