The incorporation of fluorine into porous silica by gas phase doping with hexafluoroethane C2F6 in N2 was studied using thermodynamic calculations and complementary fluorination experiments up to 1300 °C. With online FTIR analysis of the gas phase, the main products SiF4, CO2, CO, and CF4, and the traces H2O, HF, Si2F6O, and COF2 were detected and three consecutive kinetic phases deduced: the unique sorption phase, a combined etching and fluorination phase, and the thermal decomposition of C2F6 accompanied by a deposition of solid carbon. The chemical species were connected to a framework of chemical reactions. Carbon deposition and mass loss by etching are principle problems of this fluorine precursor, that cannot be completely avoided. According to the origin, three types of carbon can be distinguished. Because of its high thermodynamic stability, SiF4 is a key product of etching.
© 2013 Optical Society of America
The demand for optical fibers is driven by application fields such as telecommunication, laser welding, imaging, and fiber sensors. All these applications require low losses, which can be met by materials with a high transmittance and by an adjustment of the refractive index in sophisticated fiber structures. The combination of glasses with higher and lower refractive indexes, which can be achieved by modifying the silica glass with different dopants, is common to all fiber designs. Germanium increases and boron and fluorine decrease the refractive index.
Since the first reports on fluorine doping of silica glass [1–3], fluorine has become an important dopant for optical fibers because of its several additional benefits. The insertion of fluorine also influences material dispersion  and improves optical transmittance in the UV and VUV range. Defect centers in the silica were reduced , leading to a better resistance against UV and x-ray radiation as well as against hydrolysis. Further advantages include a lower expansion coefficient compared to pure silica glass, a decrease in the vitrification temperature and viscosity, and a low hydroxyl content.
A survey of the preparation methods to fluorine doped silica is presented in . Most fluorine doped silica is prepared by gas phase synthesis. Modified chemical vapor deposition (MCVD) [1,7–9] and plasma chemical vapor deposition (PCVD) [10,11] are two medium scale methods which enable the flexible preparation of preforms for multi-cladding fibers by the stacking of layers but which are too expensive for mass production. Vapor phase axial deposition (VAD)  and outside vapor deposition (OVD)  are two large scale methods which produce large porous silica bodies for further processing to large rods and preforms and even optical components (e.g. lenses).
Fluorine incorporation in thermal gas phase synthesis (MCVD, VAD, OVD) can be divided into an one-step process with the deposition of fluorine-doped silica and a two-step process with the deposition of porous silica and the subsequent fluorine incorporation by a gas phase process [14,15]. Our investigations compare different fluorine precursor gases as sources for fluorine incorporation into porous VAD material according to the latter process. The results of our systematic fluorination experiments will be presented in a series of articles where every article stands for one fluorine precursor. This article starts the series with hexafluoroethane.
2.1 Thermodynamic calculations
The object of thermodynamic calculations was the prediction of the ideal temperature window and redox conditions for C2F6 fluorination, of the upper limit of the achievable fluorine concentration and the existence ranges of critical byproducts.
The calculations were performed using the software HSC Chemistry for Windows (Outokumpu Research, Finland, version 6) and provided the equilibrium composition of the chemical systems at a constant pressure of 1 bar in the temperature range 0-1500 °C with the assumptions of ideal gases and an ideal mixture of the condensed phases.
The basic calculations were performed for pure C2F6 and for the system C2F6-SiO2-N2. The extended calculations introduced oxygen and hydrogen into the latter system to investigate the stability of graphite under different redox conditions and to consider the hydroxyl groups of the flame-deposited silica.
For most species, the thermodynamic data of [16,17] have been applied. For hexafluorodisiloxane the data of Shinmei et al.  were used. The species SiO1.5F was identified as solid fluorinated species by Dumas et al. , its thermodynamic data were published by Kirchhof et al. [15,20].
The program is able to handle large sets of species, but only a small set of species has a real practical significance. Thus, figures only present selected species with a mole fraction >0.1 mol% without the carrier gas nitrogen. The chemical equations derived are only formal stoichiometric relationships between the chemical species, which can strongly differ from real kinetics. Because most species are gases, only the condensed species were labeled by phase in the equations.
2.2 Fluorination experiments
For the gas phase fluorination of porous silica, pellets were prepared from VAD green bodies (mass: ≈1 g, thickness: 10 mm, diameter: 16 mm, BET surface area: 13.8 m2/g, wide pore size distribution from meso to macro pores, relative density compared to silica glass: 20%, content of OH groups ≈1500 wt%).
These samples were treated in a quartz tube reactor with 5 vol% C2F6 in nitrogen gas (Fig. 1).The total gas flow was set to 50 sccm. During fluorination, the temperature was increased to 1300 °C at a heating rate of 10 K/min. The temperature was held at 1300 °C for 60 min and finally cooled down at a cooling rate of 10 K/min. The influence of the reactor was considered by reference fluorination of the empty quartz tube.
The gas mixture after the sample passed a heated gas cuvette (10 cm optical path length, 150 °C) and the inline analysis of the composition was performed by a nitrogen-purged FTIR spectrometer iS 10 (Thermo Fisher Scientific; resolution of 2 cm−1).
The complex infrared spectra are a superposition of the spectra of the species in Fig. 2.
The FTIR analysis of the gas mixture has some limitations:
- 1. The gases represent the final state at 150 °C in the gas cell after the hot reactor. Transient species in the hot reaction zone with a short lifetime cannot be seen.
- 2. The combined detection of trace and main components is challenging.
- 3. The absorption bands of similar gases can overlap.
- 4. Infrared-inactive gases are undetectable.
- 5. A full-quantitative analysis of the gas mixture was not made, because some species are not available as pure calibration gases (COF2, Si2F6O, HF).
Taking these limitations into account, a semi-quantitative analysis of the spectra was performed by assigning the infrared vibrations to the different infrared active gases and the numerical integration of the undisturbed ranges of selected absorption bands . The detailed integration and baseline parameters were provided as supplementary material (see Media 1).
The fluorine content of the fluorinated pellet was determined by wet chemical analysis (DIN 51084).
3. Results of thermodynamic calculations
3.1 Thermodynamic stability of pure C2F6
The calculation started with 10 kmol C2F6 and considered a total of 20 species. However, up to 1500 °C only gaseous CF4 and solid C (graphite) are crucial.
In the thermodynamic equilibrium, pure C2F6 should be completely disproportionated into CF4 and graphite according to Eq. (1) in the whole temperature range of the calculation.
The theoretical calculation predicts two problems for the real fluorination of silica with C2F6:
- 1. The disproportionation of C2F6 should compete against the reaction of C2F6 with SiO2. It reduces the fluorination yield because C2F6 is converted into the almost inert CF4.
- 2. Graphite deposition must be expected. A sooted reactor vessel as well as soot particles in the porous silica sample could be a serious consequence of this decomposition.
3.2 The thermodynamic equilibrium in the SiO2-C2F6 system
3.2.1 Equilibrium composition in the system SiO2-C2F6-N2 system
The starting composition of this calculation was chosen to correspond to the experiment with much more SiO2 than the fluorine-containing precursor. C2F6 was diluted with N2 at a concentration of 5 vol% and did not completely consume the SiO2. The calculation started with 10 kmol of SiO2 and 5 kmol of C2F6 in 95 kmol of N2 and considered 83 species (7 solid species and 76 gaseous species). The most crucial up to 1500 °C are the solid species of SiO2 (glass), SiO1.5F and C (graphite), and the four gases SiF4, CO2, CO, and hexafluorodisiloxane Si2F6O. Figure 3 shows their equilibrium amounts.
Again, C2F6 completely disappears, but in the presence of SiO2 the thermal decomposition of C2F6 is replaced by the reaction of C2F6 with silica. The composition at the thermodynamic equilibrium is determined by two interacting chemical equilibria:
- 1. The SiF4-disiloxane equilibrium (Fig. 3(a)):
The direct reaction of C2F6 with silica can lead to SiF4 as well as to Si2F6O (hexafluorodisiloxane F3Si-O-SiF3):
Both equations can be connected to the equilibrium:
The balance of this equilibrium shifts with the rising temperature from Si2F6O to SiF4 in the temperature range 0...400 °C with a turning point at ≈120 °C. However, traces of disiloxane must also be expected above 400 °C.
- 2. The Boudouard equilibrium (Fig. 3(b)):
Depending on the temperature, the Boudouard equilibrium
determines the distribution of the carbon of the consumed C2F6 to the three carbon species. Below ≈400 °C, the SiF4-Si2F6O equilibrium favors CO2 and C(s) according to the Eqs. (2a) and (3a), and above ≈800 °C, the equilibrium favors CO2 and CO according to the Eqs. (2b) and (3b):
With nitrogen as the carrier gas, CO and CO2 are generated in an equimolar ratio above 800 °C; solid carbon is not critical.
The heterogeneous reaction of C2F6 with SiO2 according to the Eqs. (2a), (2b), (3a) and (3b) is the dominating etching reaction, which converts solid silica to the gaseous silicon species SiF4 and Si2F6O. This etching causes an undesired loss of material (74.8 wt% loss at 1000 °C). The desired fluorination reaction to the species SiO1.5F is only a side reaction:
Table 1 presents the main and trace products of fluorination at a typical process temperature of 1000 °C.
3.2.2 Variation of the C2F6 starting amount in the binary SiO2-C2F6-system
To answer the question of the theoretical upper limit of fluorine concentration, the following calculations were started in a binary SiO2-C2F6 system with 10 kmol of SiO2 and increased step-by-step by 0...10 kmol of pure C2F6 at constant temperatures of 800, 1000 and 1200 °C. Compositions with a C2F6 excess correspond to experiments with a sooted silica surface. Figure 4-5 show selected results of the calculation at 800 °C.
Again, C2F6 is extinct in the equilibrium. The reaction of SiO2 with an increasing starting amount of C2F6 can be divided into two phases with a turning point at 6.66 kmol of C2F6, where SiO2 completely disappears:
- 1. Etching of silica (Fig. 4(a)):
Below a molar C2F6/SiO2 ratio of 2:3, C2F6 converts SiO2 to SiF4 and almost equimolar amounts of CO and CO2 according to the Eqs. (2a) and (2b). This etching process consumes the silica matrix together with the fluorinated solid species SiO1.5F, produced by Eq. (6).
Traces of Si2F6O develop proportional to SiF4 according to the Eqs. (3a) and (3b) but completely disappear together with SiO2, revealing that disiloxane can only exist in presence of silica. Traces of graphite are only produced at 800 °C according to the Eqs. (2a) and (3a).
- 2. Fluorination of CO2 in absence of silica (Fig. 4(b)):
If all of the SiO2 is consumed, the amount of SiF4 produced cannot be further increased. The excess C2F6 reacts with CO2 instead of SiO2 according to Eq. (7):
The new species carbonylfluoride is an indicator of SiO2 deficiency conditions. A further increase in the temperature up to 1200 °C promotes COF2 and diminishes CF4.
Fluorination with pure C2F6 yields a maximum concentration of 5.4 mol% SiO1.5F (1.72 wt% F) in the solid phase (Fig. 5), compared to 3.6 mol% with diluted C2F6 (Table 1). The theoretical upper limit must be higher because the reaction products CO and CO2 dilute the gas mixture, and graphite traces dilute the solid phase.
In summary, a higher starting amount of C2F6 cannot improve the fluorine concentration in the solid phase, because additional C2F6 only etches additional SiO2.
3.2.3 Addition of oxygen to the SiO2-C2F6-N2 system
Because carbon deposition is a major problem in fluorination with C2F6, the thermodynamic calculations also examined the possibilities of preventing soot. Figure 6 presents a calculation in which a part of the N2 was replaced by O2. It started with 10 kmol of SiO2, 5 kmol of C2F6, and 3 kmol of O2 in 92 kmol of N2. Figure 6 can be compared to Fig. 3 of the oxygen-free calculation.
The addition of oxygen cannot shift the temperature at which the etching products of the Boudouard equilibrium switch from CO2/C to CO2/CO. The main effect of the successive addition of oxygen is an increase in the CO2 ratio of both pairs. Thus, below ≈400 °C, carbon is oxidized to CO2, and above ≈800 °C, CO is oxidized to CO2 (see Fig. 3(b)). Figure 6 shows the ideal case of the complete removal of solid carbon by the addition of a great deal of oxygen. All C and CO was oxidized to CO2, and the excess O2 remained in the equilibrium gas phase.
Whereas the impact of oxygen addition on the carbon species is strong, its impact on the silicon species can be neglected. Below 800 °C, the additional oxygen shifts the SiF4-Si2F6O-balance weakly to disiloxane by favoring Eq. (3a) and constraining Eq. (2a). Above 800 °C, the thermodynamic calculation does not predict a significant influence of the added oxygen on SiO1.5F, SiF4 and SiO2.
Finally, adding oxygen should avoid carbon deposition without affecting the fluorine concentration in the silica.
3.2.4 Addition of OH to the SiO2-C2F6-N2 system
Silica material produced by flame hydrolysis processes always contains hydroxyl groups. Thus, in this calculation we added 1 kmol of OH to a starting set of 10 kmol of SiO2 and 5 kmol of C2F6 in 94 kmol of N2, which corresponds to silica with 9 mol% OH (2.75 wt%). The OH content is more than ten times larger than in typical VAD material, but it enables the combined analysis of the most important hydrogen species and the influence of OH on the silicon species. Whereas 104 species were considered, in the equilibrium only 12 species achieved an amount >0.1 kmol. Figure 7 shows the hydrogen-containing species, and Fig. 8 shows selected silicon-containing species.
The OH decomposes consecutively with the rise in temperature to H2O(l), H2O(g), H2 and HF (Fig. 7). H2O and HF are connected with SiF4 and SiO2 by the important hydrolysis equilibrium (8).Fig. 7 with the curves of SiF4 and SiO2 in Fig. 8 proves that HF is generated at the cost of SiF4. Otherwise the sample weight loss produced by the etching of SiO2 is reduced. The concentration of SiO1.5F in silica is almost independent of the starting amount of OH.
In summary, fluorination of silica material with a high OH concentration could reduce sample losses by etching. This effect is weak, but in practice kinetic and diffusion benefits of the small polar HF molecule can be important.
3.3 Summary of the thermodynamic calculations
Four fundamental chemical reactions determine the gas phase fluorination of SiO2 by C2F6 in the temperature range 0-1500 °C:
- 1. The thermal decomposition of C2F6
- 2. The conversion of solid SiO2 into gaseous SiF4 by reaction with C2F6 (etching)
- 3. The fluorination of silica by incorporating SiO1.5F into the network of the silica matrix
- 4. The Boudouard equilibrium
Under special conditions the following reactions must also be considered:
- 5. Under SiO2 deficiency conditions, excess C2F6 can fluorinate CO2 to COF2.
- 6. Below 400 °C the disiloxane Si2F6O competes against SiF4 and SiO1.5F.
- 7. Silanol groups of the silica cause the hydrolysis of SiF4 and can generate HF.
Graphite formation and etching of the silica sample are principle problems of fluorination with C2F6. The first problem can be completely prevented by adding oxygen, and the second one can only be reduced by an additional hydrogen species.
4. Results of the fluorination experiments
4.1 Reaction products and concentrations
The theoretical thermodynamic calculations were accompanied by adequate fluorination experiments on VAD pellets according to the conditions of section 2.2. The fluorinated pellet in Fig. 9 demonstrates the material loss by etching (8.4 wt%) and the deposition of carbon on the top side of the pellet. Also, the reaction tube was polluted by carbon and the turnover of C2F6 reached a maximum of 100%.
The following products were detected by FTIR (except carbon):
Figure 10 compares their relative IR absorbance (the band area of every IR species was normalized to their maximum band area of the whole experiment). The influence of growing carbon soot during fluorination with C2F6 is demonstrated in Fig. 11.
4.2 Kinetic phases
Four kinetic phases can be distinguished during the first heating of a porous silica pellet in a flowing C2F6-N2-mixture (Table 2).
4.2.1 Thermodesorption of adsorbed H2O
This “drying” of the sample (first H2O peak in Fig. 10(a)) is not a typical fluorination step and would also take place in pure nitrogen. This step can be avoided by predrying.
4.2.2 Sorption phase
The sorption phase does not have any detectable C2F6 turnover or any visible changes like etching and sintering and can be identified in Fig. 10(a) by the simultaneous peaks of H2O and CO2 at ≈930 °C. It takes place only during the first heating in C2F6 (Fig. 11). At the end of this phase, all surface OH groups of the porous silica sample are fluorinated. Bulk OH groups in the silica matrix are unaffected because hydroxyl and fluorine diffusion are uncritical at this temperature [22,23].
The condensation of adjacent surface OH groups would also take place without C2F6:
Because CO2 has the highest extinction coefficient of the three gaseous products, it can be used as the indicator molecule for this phase.
4.2.3 Fluorination and etching
This phase is separated from the sorption phase, overlaps with the next phase, and is the real fluorination step of the overall reaction. Within 150 K the C2F6 turnover reaches 100%, and six IR active products appear simultaneously (Fig. 10), which can be assigned to five reactions:
- 1. Conversion and etching according to Eq. (2b): The high thermodynamic stability of SiF4 is the reason for the conversion of the C2F6 precursor to SiF4 and for the mass loss of the silica sample. SiF4 is the typical indicator molecule of this phase, whereas CO and CO2 are byproducts.
- 2. Fluorination according to Eq. (6): Because of the low SiF4 consumption, this step cannot be tracked using the FTIR of the gases. However, the fluorine incorporated in the solid sample was proved through wet chemical analysis.
- 3. Conversion of bulk OH into HF: Etching uncovers fresh bulk material and turns bulk OH into new surface OH. In contrast to the sorption phase, the OH groups react to HF and not to H2O. This can be explained by the hydrolysis of SiF4 according to Eq. (8).
- 4. Disiloxane traces: At least two reaction routes to Si2F6O can be constructed: the partial hydrolysis of SiF4 according to Eq. (11) and a fluorination according to Eq. (12):
Both routes require SiF4 as a source; the IR results cannot emphasize a single route.
All reactions of this phase take place both at the heating and at the cooling step (for example, see the peaks of CO2 in Fig. 11). Reactions 2-5 depend on the first reaction, which dominates this kinetic phase.
4.2.4 Thermal decomposition of C2F6
The thermal decomposition of C2F6 according to Eq. (1) quenches the fluorination above 1230 °C (Fig. 10(b)). The typical fluorination products decrease or disappear, whereas the silica-free zone above the silica pellet becomes an effective carbon generator. The disproportionation into CF4 and carbon has three consequences:
- 1. Competition: C2F6 decomposed above the sample is no longer available for reaction with silica (decreasing selectivity of fluorination)
- 2. Carbon as diffusion barrier: Carbon masks the silica surface and stimulates the decomposition of C2F6 (positive feedback). This autocatalytic effect of the growing soot layer demonstrates the dwell step in the middle of Fig. 11. The decomposition product CF4 grows together with carbon, whereas the etching indicator CO2 decreases.
- 3. Inertness of CF4: Reactive C2F6 was converted into inert CF4. Even at 1300 °C the CF4 turnover reaches only several percent.
4.3 Comparison calculation / experiment
- 1. The flow reactor works far from the thermodynamic equilibrium. After 1 hour at 1300 °C, the mass loss in the real experiment achieves 8.4 wt% (predicted equilibrium mass loss 74.5 wt%). Also, the detected fluorine concentration achieves only 0.77 wt% F (prediction 1.13 wt%). Endlessly proceeding fluorination in flowing C2F6 would consume the whole sample.
- 2. The calculation predicts carbon soot below 800 °C according to Eq. (2b); however the actual experiment did not prove a significant indication of soot up to this temperature.
- 3. As predicted, etching by conversion into gaseous SiF4 plays an important role and conforms to Eq. (2b) (parallel formation of CO and CO2).
- 4. Decomposition into carbon and CF4 according to Eq. (1), predicted for pure C2F6, also occurs in the presence of SiO2 in the experiment above 1100 °C and can even terminate fluorination.
- 5. The predicted products Si2F6O and COF2 can be detected as traces during etching.
5.1 Competition between etching and fluorination
CO and CO2 are evidence of the reaction of C2F6 with SiO2 because these carbon oxides can only receive oxygen from silica.
In principle direct fluorination by C2F6 without SiF4 can be constructed:
Direct fluorination by C2F6 producing SiF4 according to Eq. (2b) is more likely because of the high thermodynamic stability of the symmetric SiF4 molecule. Most of the SiF4 leaves the reaction zone and is lost (etching), but a part of SiF4 can fluorinate silica in a second reaction according to Eq. (6) and (12) (indirect fluorination by SiF4). The ratio of etching and indirect fluorination can be determined from mass loss and the fluorine content of the pellet: from 16 C2F6 molecules, reacted with silica, only one actually fluorinates the sample.
In conclusion, etching and indirect fluorination are two competing processes that belong together. Fluorination of silica by C2F6 is not possible without mass loss by etching.
5.2 Consequences of carbon formation
In porous silica samples with a high internal surface even carbon traces must be avoided. Three potential sources can become the origin of carbon during fluorination of porous silica with C2F6:
- 1. Etching carbon according to Eq. (2a): The thermodynamic calculation predicts this reaction only below 800 °C. This carbon is a byproduct of the conversion of SiO2 to SiF4 and must be expected in the cooler parts of the sample and quartz reactor. Its synthesis has not been proved definitely.
- 2. Disproportionation carbon according Eq. (1): The carbon formation from this reaction starts above 1000 °C, increases with the temperature, and is the most important limitation of fluorination with C2F6 in N2. The carbon grows as layers on the sample and the reactor and is also carried out as soot by the flowing gas.
- 3. Carbon from trapped carbon-containing gases: Because high fluorine content lowers the sintering temperature, sintering can start during fluorination, trapping carbon-containing gases like C2F6, CF4, CO2 or CO in the pores. The subsequent vitrification step of the porous green body at temperatures >1500 °C converts these trapped carbon compounds into carbon, which is visible as a gray veil in the vitrified silica glass and which disappears closer to the body surface. This is a hidden fault, because the body seems to be carbon-free before vitrification.
Possibly between 800 and 1000 °C in C2F6/N2, a soot-free fluorination window exists which can only be met with small samples and a precise temperature control. Detection of SiF4, CO2, and CO by FTIR and the analysis of their stoichiometric relationship should enable an inline control of the carbon deposition.
Addition of remarkable amounts of oxygen should completely inhibit the deposition of carbon, but it cannot prevent trapping of CO2 in the pores of the sample, which can be later reduced to carbon by sintering.
5.3 Kinetic framework
The formal kinetic relationships derived from the FTIR investigations of the gas mixture during fluorination experiments were connected to a kinetic framework in Fig. 12, which provides orientation for further investigations of fluorination with C2F6. The framework contains all trace and main species which were verified by FTIR. The formal equations recommend stoichiometric relationships between the species and do not claim completeness.
The real kinetic relationship should be a more complex reaction, and the elementary steps of the heterogeneous reaction between the silica surface and C2F6 are still unknown. The detection of solid species during fluorination would require an in situ technique (at 1000 °C).
The chemistry of the fluorine incorporation into porous silica by gas phase treatment with C2F6 in N2 was investigated using thermodynamic calculations and complementary fluorination experiments up to 1300 °C on pellets of VAD material. SiF4, CO2, CO, CF4, and carbon soot are main products of the gas phase process. H2O, HF, Si2F6O, and COF2 can be detected as traces. The fluorination of the solid silica was described with SiO1.5F. Every gas species, predicted by the theory, can be detected in the experiment. From the detected species, a framework of chemical reactions was created as a “road map” of fluorination. Three kinetic phases can be distinguished during the heating of a dried silica pellet in C2F6/N2: an initial sorption phase during the first heating, a combined etching and fluorination step, and the thermal decomposition of C2F6 into CF4 and carbon.
Mass loss of the sample by etching and carbon deposition are principle problems of the C2F6 fluorine precursor. The etching product SiF4 cannot be avoided because of its high thermodynamic stability. The carbon development can be diminished by optimized temperature and redox conditions but remains a risk.
This research was supported by the Federal Ministry of Economics and Technology of Germany (Central Innovation Program Small Firms, Project Flexfaser, designation KF 2206904DF9).
We thank Sonja Unger for her comparison with fluorine doping in MCVD and Johannes Kirchhof for the discussions about the thermodynamics of SiO1.5F, the kinetics, and the diffusion of fluorine in silica.
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