Article pubs.acs.org/crystal
Crystallization of Quartz Glass Fibers during the Drawing Process Aleksey Kulesh,*,† Mikhail Eronyan,†,# Igor Meshkovskii,†,# Vladimir Zolotarev,†,# Mikhail Bisyarin,‡,# and Marina Tsibinogina§,# †
Saint Petersburg National Research University of Information Technologies, Mechanics and Optics (ITMO University), 197101 Saint Petersburg, Russia ‡ Saint Petersburg State University, 199034 Saint Petersburg, Russia § Perm National Research Polytechnic University, 614990 Perm, Russia ABSTRACT: An objective of the present paper is to study the surface crystallization of optical fibers made of quartz glass during the drawing process. Calculations based on the rates of fiber cooling and the crystallization process reveal formation of a cristobalite layer on the surface about 6 nm thick. The infrared reflection spectra of the quartz fibers, their pictures obtained with an electron microscope, and the thermal treatment results confirm the occurrence of a cristobalite layer on the fiber surface.
1. INTRODUCTION Fiber light guides (FLG) made of quartz glass have a certain statistical regularity in the distribution of a strength magnitude due to the specific character of the surface defects. The nature of this regularity has not been established conclusively. The strength of the 1 meter standard FLG pieces with a diameter of 125 μm with epoxy acrylate polymer cladding is about 6 GPa. This level of breaking stress of the quartz glass corresponds to a depth of the surface critical crack of about 6 nm, calculated on the basis of the experimental dependence of the quartz fiber strength (σ) on the defect size (r) formed on its surface1 (equation is valid for air at room temperature): σ = D/r 0.5
surface. They may cause the FLG strength to decrease to 0.2 GPa.4 During the drawing process, the fiber gets rapidly cool. Nevertheless, the time of cooling from 1700 to 1550 °C (depending on the fiber diameter) may be sufficient for the formation of the nanoscale surface cristobalite layer. Obviously, this process causes the FLG strength to decrease. The measured strength value of the 124 nm quartz fibers was 26 GPa;5 therefore, the surface crystallization decreased on account of the fiber diameter. The objective of the present paper was to conduct an experimental and computational investigation of the surface crystallization of the FLG made of quartz glass during the drawing process.
(1)
where the constant D is 0.474 × 10−3 GPa·m0.5. The extrapolation of eq 1 to the defect size equal to interatomic spacing in the Si−O bond (0.16 × 10−9 m) gives a strength value of 37.5 GPa corresponding to the theoretical evaluations within the interval 22−44 GPa.2 The preform heating in the fiber drawing process may cause the surface crystallization at temperatures of 1500−1730 °C.3 In the high temperature zone of the preform transformation at temperatures higher than 2000 °C, a certain part of crystals remain unmelted. That leads to appearance of the defect as cristobalite particles with a size of about 5 μm on the fiber © 2015 American Chemical Society
2. MATERIALS AND METHODS Two types of the quartz FLG samples with the same thickness of 125 μm were used in the experiments. The samples of the first type A had a UV cured epoxy acrylate coating, and the second ones B type were uncoated. The FLG pieces of the A type of 150 mm in length were used for the investigation of the strength dependence on the heating within Received: February 20, 2015 Revised: April 30, 2015 Published: May 8, 2015 2831
DOI: 10.1021/acs.cgd.5b00253 Cryst. Growth Des. 2015, 15, 2831−2834
Crystal Growth & Design
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220−310 °C in air, with the purpose to reveal an α−β phase transition in cristobalite.6 The strength (σ) was measured in air at room temperature with a two-point bending method and then approximately evaluated with the formula taken from ref 7
σ = 1.198Eod /D
Table 1. Experimental8 and Calculated Time of Cooling (τ, ms) from 1730 to 200 °C for Different Fibers
(2)
where Eo = 72.2 GPa is a modulus of elasticity of quartz glass; d = the fiber diameter; D = diameter of a neutral axis of the bent fiber leading to the fiber destruction. The fibers of the A type were also used in the strength measurements upon the tensile force applied to fiber pieces of 0.5 m length. The fibers of the B type were investigated with a scanning electron microscope Quanta 200 equipped with an X-ray microanalyzer. The reflection spectra of the dense layer were investigated with an infrared (IR) spectrometer Bruker Tensor 27.
experimental data8
14 17 20 24 34 18
85 100 165 190 375 136
upon calculation in ref 9 (Δτ/τ, %) 125 151 178 213 302 160
upon calculation acc eq 3 (Δτ/τ, %)
(32) (34) (7,6) (11) (−23) (19)
88 (3.5) 117 (14.7) 150 (−10) 198 (4) 333 (−12) 128 (−1.2)
The transformation of eq 3 provides the 125 μm fiber temperature dependence upon its cooling time expressed as follows:
3. EXPERIMENTAL SECTION
t = exp(7.464 − 2.61τ )
The fibers were drawn from the preform with a velocity about 60 m/ min in the atmosphere of the high-purity argon with an admixture of oxygen of 10−4 vol %, and the temperature of a graphite heater was 2100 °C. In order to reveal an impact of the furnace temperature on the A type FLG strength, the fibers were drawn at the heater temperatures of 2070 and 2160 °C. A preform with an outer diameter of 12 mm was fabricated with the MCVD technology, and substrate tubes were made of quartz glass F300. The A type fibers were covered with polymer cladding, and the B type ones of about 2−3 m long were extracted out of the drawing tower between the furnace and cladding deposition block. The A type FLG was heated in the muffle furnace equipped with a wire heater. After 20 min of isothermal treatment fibers were kept in air for at least 2 h at room conditions for stabilization of their strength under conditions of natural humidity. In order to reduce an error of the measured value, we have determined an arithmetic mean value of the 20 measurement results for each treatment temperature. The standard deviation from the arithmetic mean value was about 0.5%. A confidence interval for the strength mean arithmetic value was about 1% at a confidence probability of 97%. For the electron microscopy and IR spectrometry investigation, B type FLG samples of 25−30 mm long were used. The fibers had no contacts with any foreign objects in order to avoid the appearance of defects on its surface. Some fibers had been chemically treated in the 20% HF water solution for 5 min for removing the surface layer.
(4)
A crystallization rate is determined by the activation energy of process (E) as given in ref 6 V (nm/s) = 2.78 × 1019 exp( −E /RT )
(5)
where E = 645 kJ/mol; R = universal gas constant; T = temperature, K. Taking into account dependence 4, eq 5 may be written as V (τ ) = a exp{−b/[exp(c − kτ ) + D]}
where a = 2.78 × 1019, b = 77617, c = 7.464, k = 2.61, and D = 273. The cristobalite layer thickness (δ), having been formed during fiber cooling from 1730 to 1500 °C, was calculated by the integration of V(τ) within the time limits of 0−0.058 s:
∫ V (τ ) d τ = a ∫ exp{−b/[exp(c − kτ ) + D]} dτ
δ(nm) =
Using the transformation of integration variable: z = b/[exp(c − kτ ) + D] − ln a
one can write expression for δ as follows:
4. CALCULATIONS The calculation of the crystalline layer thickness, being formed on the FLG surface during the fiber drawing at 1730−1500 °C, was made on the basis of the published parameters of the cooling dynamics and crystallization rate of the quartz glass. An experimentally found fiber cooling time had revealed its proportionality to the fiber radius to the 3/2 power and independence on the drawing velocity.8 With the use of literature8 results, the time of the fiber cooling process from 1730 °C to the temperature t is well approximated with the following formula: τ = 0.0085r1.5(0.681 − 0.21 lgt )
fiber radii, μm
1 δ= k
∫
z1
z2
e −z
(
(z + ln a) 1 −
D (z b
+ ln a)
)
dz (6)
with integration limits z1 and z2 corresponding to τ = 0 and τ = 0.058 s respectively. This expression enables asymptotic estimates and is more suitable for numerical solution. This expression is more suitable for numerical solution.
5. RESULTS The thickness of the crystalline layer calculated with eq 6 was 6 nm; the same value corresponds to the depth of the critical crack that determines the strength of the standard quartz FLG. Comparison of the IR spectra of the 1 μm cristobalite film and sample surface (B type) confirms crystallization of its surface (Figure 1). The IR spectrum of FLG was compared with the reflection spectrum of 1 μm cristobalite film deposited on pure quartz. The characteristic reflection peak of crisobalite at 620 cm−1 (line 1) disappears after etching the fiber (line 2). This fact may also confirm crystallization of the thin surface layer on FLG.
(3)
where τ = cooling duration, s; r = fiber radius, μm; t = temperature, °C. The analogous calculation of the cooling time using the formula given in ref 9 presents more discrepancy between the experimental data8 and calculations using eq 3, that is, Δτ/τ, % in Table 1 below. That is why, in order to find the cooling time for the 125 μm fiber, we had used eq 3 giving a fiber cooling time of 58 ms at the temperature drop from 1730 to 1500 °C. 2832
DOI: 10.1021/acs.cgd.5b00253 Cryst. Growth Des. 2015, 15, 2831−2834
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Figure 1. Spectral dependence of the reflection coefficient (R) for the B type samples before (1) and after (2) the fiber etching and micron α-cristobalite film (3) on the pure quartz surface.
Jumping strength reduction observed with short FLG pieces (A type sample), having been thermally treated at 270−310 °C (Figure 2), may also indicate formation of the cristobalite Figure 3. Dependence of the destruction probability (F) on the tensile force for the fibers drawn from the initial part (1) and ending part (2) of the preform at temperatures of 2075 °C (dark markers) and 2160 °C (dashed markers). After the high temperature preform collapsing, the flame polishing procedure was not conducted.
analysis of these particles showed the presence of only silicon and oxygen and nothing else.
Figure 2. Impact of thermal treatment on the strength of the A type fiber. The indicated deviations present the confidence interval for 97% confidence probability.
nanoscale layer. The point is that cristobalite demonstrates the structure transformation at 273 °C accompanied by a volume change of 4%.6 A similar jumping change of the fiber strength also has been observed with Corning quartz light guides within the same temperature range. After thermal treating, a cladding continuity on the fiber surface was maintained. Jumping strength reduction observed with short FLG pieces, having been thermally treated at 270−300 °C (Figure 2), was also observed in a recently published paper,10 but heating the silica fiber to 200 °C does not affect its strength.11 The strength change for the FLG 0.5 m pieces (A type sample) demonstrated two levels of braking force −40 and 60 N (Figure 3). The upper level probably is caused by the cristobalite nanolayer, the lower one, by the fritted micron particles. That is why the fibers, having been drawn from an initial part of the preform and still not crystallized, are stronger than the fibers drawn from other parts of the preform. The elevated drawing temperatures enhance the fiber strength probably due to melting of the cristobalite micron particles formed on the preform surface during the fiber drawing. However, the electron microscopic observation has revealed the appearance of separate irregular-shaped 1−10 μm particles on the surface of all samples (Figure 4). The fiber surface occupied by these defects was less than 1%. X-ray microprobe
Figure 4. Particles on the fiber surface composed only of silicon and oxygen.
6. DISCUSSION The calculation of the crystalline layer thickness based on experimental results for the pure quartz glass synthesized from the vapor phase.6 The crystallization rate of other types of quartz glass may significantly differ from that. However, there is practical evidence for independence of the sample strength on the quartz glass type for the short FLG pieces less than 5 mm long. One can explain this fact by the following. During the fiber drawing process, the preform surface is purified due to evaporation of the admixtures. In addition, the quartz glass 2833
DOI: 10.1021/acs.cgd.5b00253 Cryst. Growth Des. 2015, 15, 2831−2834
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(3) Kulesh, A. Yu.; Eronyan, M. A.; Meshkovskii, I. K. In Proceedings of the 3rd International Conference on Chemistry and Chemical Technology, Erevan, 2013. (4) Aulich, H.; Douklias, N.; Graber, K.; et al. Siemens Forsch.Entwicklungsber. 1980, 9, 57. (5) Brambilla, G.; Payne, D. N. Nano Lett. 2009, 9, 831. (6) Leko, V. K., Mazurin, O. V. In Properties of Quartz Glass; Nauka: Leningrad, 1985. (7) France, P. W.; Reeve, M. J.; Paradine, M. H.; Newns, G. R. J. Mater. Sci. 1980, 15, 825. (8) Arridge, R. G. C.; Prior, K. Nature (London), Phys. Sci. 1964, 203, 386. (9) Paek, U. C.; Kurkjian, C. R. J. Am. Ceram. Soc. 1975, 58, 330. (10) Lezzi, P. J.; Xiao, Q. R.; Tomozawa, M.; Blanchet, T. A.; Kurkjian, C. R. J. Non-Cryst. Solids 2013, 379, 95. (11) Proctor, B. A.; Whitney, I.; Johnson, J. W. Proc. R. Soc. A 1967, 297, 534. (12) Ewles; Yowell, R. F. Trans. Faraday Soc. 1951, 47, 1060. (13) Schedrin, V. M.; Telegina, A. A.; Vaskin, V. M. Metals 1977, 6, 57. (14) Maurer, R. D. Appl. Phys. Lett. 1977, 30, 82. (15) Eronyan, M. A.; Zlobin, P. A.; Khokhlov, A. V. Opt. Zh. 2007, 74, 436.
evaporates congruently in the inert medium and has the same composition for any glass type. During the neck down zone preform heating in the course of the drawing process, the quartz glass evaporates, and that process leads to an increase of the oxygen content in the inert argon atmosphere within the furnace volume with reaction 7: SiO2 (l) = SiO(g) +
1 O2 (g) 2
(7)
where (l) and (g) denote the liquid and gaseous state, respectively. The quartz glasses composition differs from cristobalite due to an oxygen deficit of 0.01%.12 The deep-brown color of the condensation products on the preform above the bulb zone show incomplete interaction between SiO and O2, which appears at the glass evaporation. Therefore, the additional oxygen pressure in the inert gas may be higher than the oxygen equilibrium pressure at 1730−1500 °C of about 10−5−10−7 bar.13 In such conditions the oxygen content in the quartz glass will be increased, and the cristobalite layer will be formed on the surface of preform and fiber.
7. CONCLUSIONS As it has been mentioned above, the fibers, having been drawn from an initial part of the preform and still not crystallized, are stronger than the fibers drawn by the end of the process. This fact may indicate the crystallization process on the preform surface event. The crystalline layer thickness may reach several microns. In course of plastic transformation of the bulb, this layer spreads upon the fiber surface, the fiber surface area being larger by a hundred times than the area of the preform. This phenomenon had been reported earlier.3,14 Formation of the micron particles observed with an electron microscope as well as formation of the nanoscale continuous cristobalite layer on the fiber surface may lead to the occurrence of two fiber strength levels.15 Thus, investigation results may show the surface crystallization of both preform and fiber during the drawing process. This fact may be the main cause of the occurrence of two strength levels of the FLG made of quartz glass.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions #
These authors (M.B., M.E., V.Z., I.M., M.T.) contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was done at the ITMO University, and it was supported by the Ministry of Education and Science of the Russian Federation in the framework of Project 02.G25.31.0044.
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REFERENCES
(1) Sakaguchi, S.; Nakahara, M.; Tajima, Y. J. Non-Cryst. Solids 1984, 64, 173. (2) Wiederhorn, S. M. J. Am. Ceram. Soc. 1969, 52, 99. 2834
DOI: 10.1021/acs.cgd.5b00253 Cryst. Growth Des. 2015, 15, 2831−2834
