Fluorine incorporation into porous silica by gas phase doping with C2F6 in N2

Frank Froehlich; Claudia Aichele; Stephan Grimm; Kay Schuster

doi:10.1364/OME.3.001839

1. Introduction

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 [4] and improves optical transmittance in the UV and VUV range. Defect centers in the silica were reduced [5], 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 [6]. 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) [12] and outside vapor deposition (OVD) [13] 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. Experiments

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. [18] were used. The species SiO_1.5F was identified as solid fluorinated species by Dumas et al. [19], 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.

Fig. 1 Reactor setup for the gas phase fluorination of porous VAD pellets.

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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.

Fig. 2 FTIR spectra of (a) carbon-containing gases and (b) carbon-free gases during gas phase fluorination with C₂F₆ (qualitative comparison of characteristic bands; staggered spectra with different concentrations of the gases in nitrogen) (Media 1).

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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 [21]. 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.

2 C_{2} F_{6} \to 3 {CF}_{4} + C (s)

Nevertheless, C₂F₆ practically exists at room temperature because its decomposition is inhibited. But this metastability also means that decomposition starts when the kinetic restrictions disappear at higher temperatures.

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), SiO_1.5F and C (graphite), and the four gases SiF₄, CO₂, CO, and hexafluorodisiloxane Si₂F₆O. Figure 3 shows their equilibrium amounts.

Fig. 3 (a) Silicon-containing species and (b) carbon-containing species in the equilibrium of the C₂F₆-SiO₂-N₂ system.

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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):
$3 S {iO}_{2} (s) + 2 C_{2} F_{6} \to 3 {SiF}_{4} + 3 {CO}_{2} + C (s)$ $2 {SiO}_{2} (s) + C_{2} F_{6} \to {Si}_{2} F_{6} O + 1 .5 {CO}_{2} + 0 .5 C (s)$
Both equations can be connected to the equilibrium:
$2 {Si}_{2} F_{6} O ⇄ 3 {SiF}_{4} + {SiO}_{2} (s)$
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
${CO}_{2} + C (s) ⇄ 2 CO$
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):
$3 {SiO}_{2} (s) + 2 C_{2} F_{6} \to 3 {SiF}_{4} + 2 {CO}_{2} + 2 CO$ $2 {SiO}_{2} (s) + C_{2} F_{6} \to {Si}_{2} F_{6} O + {CO}_{2} + CO$
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 SiO_1.5F is only a side reaction:

3 {SiO}_{2} (s) + {SiF}_{4} \to 4 {SiO}_{1 .5} F (s)

Table 1 presents the main and trace products of fluorination at a typical process temperature of 1000 °C.

Table 1. Equilibrium concentrations of species in the two phases of the SiO₂-C₂F₆-N₂ system at 1000 °C

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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.

Fig. 4 Influence of the C₂F₆ starting amount on (a) the silicon-containing species and (b) the carbon-containing species in the C₂F₆-SiO₂ system (equilibrium composition at 800 °C).

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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 SiO_1.5F, 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):
$2 C_{2} F_{6} + 3 {CO}_{2} \to 4 {COF}_{2} + {CF}_{4} + 2 CO$
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% SiO_1.5F (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.

Fig. 5 Influence of the C₂F₆ starting amount on the equilibrium concentration of SiO_1.5F in the solid phase at 800 °C (C₂F₆-SiO₂ system).

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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.

Fig. 6 Equilibrium composition in the C₂F₆-SiO₂-N₂-O₂ system after the removal of solid carbon by adding oxygen.

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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 SiO_1.5F, 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.

Fig. 7 Hydrogen-containing species in the C₂F₆-SiO₂-N₂-OH system.

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Fig. 8 Influence of OH on SiF₄ and SiO₂ in the equilibrium of the C₂F₆-SiO₂-N₂-OH system.

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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).

{SiF}_{4} + 2 H_{2} O ⇄ {SiO}_{2} (s) + 4 HF

This equilibrium shifts with the rise in temperature from SiF₄ to HF. At room temperature the calculation predicts the coexistence of SiF₄ and H₂O. Above 1000 °C, HF is the dominating hydrogen species. The comparison of the HF curve in 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 SiO_1.5F 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 SiO_1.5F 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 SiO_1.5F.
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%.

Fig. 9 Quartz carrier tube with an etched silica pellet and carbon depositions after fluorination with 5 vol% C₂F₆ in N₂ up to 1300 °C.

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The following products were detected by FTIR (except carbon):

Main products: SiF₄, CO₂, CO, CF₄, C(s)
Trace products: H₂O, HF, Si₂F₆O, COF₂

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.

Fig. 10 IR active products during fluorination of porous silica with 5 vol% C₂F₆ in N₂: (a) fluorine-free and (b) fluorine-containing gases (heating phase; the numbers at the bars refer to the kinetic phases in Table 2).

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Fig. 11 Deactivation of fluorination with C₂F₆ by carbon soot, demonstrated at the selected species CO₂ and CF₄ (Overview of heating, holding, and cooling phase; the arrow marks the unique sorption phase).

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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).

Table 2. Kinetic phases during the first heating of an untreated silica pellet in 5 vol% C₂F₆ and N₂ up to 1300 °C (FTIR indicator molecules bold)

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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:

2 \equiv Si-OH \to \equiv Si-O-Si \equiv + H_{2} O

In the presence of C₂F₆, the release of H₂O traces is synchronized with CO₂ traces and can be described by the reaction:

6 \equiv Si-OH + C_{2} F_{6} \to 6 \equiv Si-F + 3 H_{2} O + {CO}_{2} + CO

An estimation shows that a reaction of all OH groups of the pellet (bulk and surface OH) within a temperature interval of 50 K according to reaction (10) would consume ≈2.7% of the C₂F₆. However, the absence of any detectable C₂F₆ turnover at this phase proves that only the small fraction of the surface OH groups was fluorinated by 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): $2 {SiF}_{4} + H_{2} O \to {Si}_{2} F_{6} O + 2 HF$ $2 {SiF}_{4} + 2 {SiO}_{2} (s) \to {Si}_{2} F_{6} O + 2 {SiO}_{1 .5} F (s)$
Both routes require SiF₄ as a source; the IR results cannot emphasize a single route.
5. Traces of COF₂: If C₂F₆ and CO₂ coexist in the gas phase, the fluorination of CO₂ can take place according to Eq. (7). Marshall et al. [24] also discussed COF₂ as an intermediate, which can again be consumed by SiO₂.

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. Discussion

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:

6 {SiO}_{2} (s) + C_{2} F_{6} \to 6 {SiO}_{1 .5} F (s) + {CO}_{2} + CO

This pure fluorination causes an increase in sample mass and contradicts both the experiment and the calculation.

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.

Fig. 12 Kinetic framework of the fluorination of porous silica with C₂F₆ in N₂ (without sorption phase).

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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).

6. Conclusions

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 SiO_1.5F. 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.

Acknowledgments

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 SiO_1.5F, the kinetics, and the diffusion of fluorine in silica.

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Phase	Species	mol%
Solid	SiO₂	96.356
	SiO_1.5F *	3,613
	C	0.030
Gas	SiF₄	6.600
	CO₂	4.447
	CO	4.445
	Si₂F₆O	0.032
	N₂	84.476

	Kinetic phase	Temperature	Typical products
1	Thermodesorption of adsorbed H₂O	100...250 °C	H₂O
2	Sorption phase	850...1050 °C	H₂O, CO₂, SiO_1.5F (s) ?
3	Fluorination and etching	1050...1250 °C	SiF₄, CO₂, CO, SiO_1.5F(s) Traces: HF, Si₂F₆O, COF₂
4	Thermal decomposition of C₂F₆	1100...1300 °C, above 1250 °C dominating	CF₄, C(s)

Phase	Species	mol%
Solid	SiO₂	96.356
	SiO_1.5F *	3,613
	C	0.030
Gas	SiF₄	6.600
	CO₂	4.447
	CO	4.445
	Si₂F₆O	0.032
	N₂	84.476

	Kinetic phase	Temperature	Typical products
1	Thermodesorption of adsorbed H₂O	100...250 °C	H₂O
2	Sorption phase	850...1050 °C	H₂O, CO₂, SiO_1.5F (s) ?
3	Fluorination and etching	1050...1250 °C	SiF₄, CO₂, CO, SiO_1.5F(s) Traces: HF, Si₂F₆O, COF₂
4	Thermal decomposition of C₂F₆	1100...1300 °C, above 1250 °C dominating	CF₄, C(s)

Fluorine incorporation into porous silica by gas phase doping with C₂F₆ in N₂

Abstract

1. Introduction