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Rheology Study of Silica-Zirconia Sols for Elaboration of Silica-Zirconia Nanostructured Optical Fibers by Inverse Dip Coating G. Brasse,*,† C. Restoin,† D. Soule´,‡ and J-M. Blondy† XLIM Research Institute-UMR CNRS, 123 AVenue Albert Thomas, 87000 Limoges, France and UniVersity Institute of Technology, Alle´e A Maurois, 87000 Limoges, France ReceiVed: June 2, 2010; ReVised Manuscript ReceiVed: NoVember 16, 2010
This article exposes a study of the rheological properties of silica-zirconia sols in acidic conditions, which are used for the development of nanostructured optical fibers by an original “inverse dip-coating” method. This new generation of optical fibers, which contains zirconia nanocrystals dispersed inside an amorphous pure silica core, must present a homogeneous core to allow good waveguiding properties. Thus, good control of the rheological properties of the silica-zirconia sols at its origin is needed to optimize the development process. After having described the chemical synthesis of the sols, the principle of the rheological measurement is then explained and a short reminder of a few and essential rheological notions is provided. The evolution of the rheological behavior of the sols is then studied as a function of the time and thus as a function of the polymerization advancement. Finally, in order to extend and stabilize the development process of these fibers, an estimation of the sol viscosity is done by taking into account the different viscosity designation notions that are inherent to the flow regime of the fluid. Introduction Optical fibers have been considerably developed and studied during this past decade, especially the microstructured optical fibers, the so-called photonic crystal fiber (PCF).1-3 Recent advances in technology related to this field of applications have demonstrated the possibility of designing a new generation of optical fibers, which present a nanostructured core associated with good waveguiding properties.4,5 The core of such a fiber is composed of zirconia nanocrystals dispersed in an amorphous silica matrix and seems to be an interesting choice for reaching unconventional emission wavelengths.6-8 The sol-gel process,9 which is based on conversion of a liquid sol into a solid gel phase by a series of hydrolysis and condensation reactions of the precursors, has been used to synthesize the nanostructured material that constitutes the core of the fiber. Indeed, this chemical process has been extensively developed during the past few years, especially for the synthesis of silica glass doped with rare earth ions or transition metals10-12 as well as for the synthesis of materials which have original microstructural properties. In addition, a specificity of this original method leads to the possibility of developing such a material at low temperatures, by comparison to the traditionnal melting processes used in glass technology. Development of such a nanostructured optical fiber has been already explained in previous articles4,5 and is based on a method called “inverse dip coating” that consists of dip coating the inner wall of a preform with a liquid sol of silica and zirconia precursors, which becomes a nanostructured glass after a series of various heat treatments.13 The geometry, purity, and homogeneity of the core are essential parameters for allowing guidance of the light along the fiber. Thus, it is absolutely * To whom correspondence should be addressed. Phone:+33 (0)5 55 50 77 32. Fax: +33 (0)5 55 50 72 53. E-mail:
[email protected]. † XLIM Research Institute-UMR CNRS. ‡ University Institute of Technology.
necessary to understand the rheological behavior of the initial silica-zirconia sols to control the protocol and especially coating steps. Previous studies found in the literature refer to the rheological behavior of pure colloidal silica sols for dip-coating or spincoating applications on plan samples, but to our knowledge, no study has been conducted on the rheological behavior of silica-zirconia sols, especially in acidic conditions that lead to formation of polymeric silica-zirconia gels. This article describes a study concerning the evolution of the rheological properties of such sols during their polymerization, in order to determine the time interval when it is preferable to realize the coating of the preform as well as to determine the sol viscosity when the sol layers are deposited. A presentation of the chemical preparation and the principle of the rheological measurements are first explained. Then, the experimental results are described and discussed, after providing a few essential basics about the rheological behaviors. Experimental Section 1. Preparation of the Sols. The silica and zirconia sols were, respectively, prepared in various concentrations from tetraethylorthosilicate (Fisher) and zirconium n-propoxyde (Alfa Aesar) as metal precursor. Hydrochloric acid (Alfa Aesar) has been chosen as a catalyst to promote formation of a polymeric gel, while deionized water and propanol-1 (Fisher) were selected as solvents. Due to the high reactivity of the zirconium precursor with humidity, the sols were prepared in a glovebox under an argon atmosphere and a chelating agent is added to the sol to control the hydrolysis rate of zirconium n-propoxyde. In this way and according to previous studies concerning the microstructural properties of the material that is finally desired to develop the core of the fiber, the sol composition (30 mol % ZrO2:70 mol % SiO2) has been achieved by mixing the ZrO2 sol into the SiO2 sol without any caution. Rheological studies were realized with a rotational rheometer Rheomat RM 180 at 20 °C on three sol compositions: S1, S2,
10.1021/jp105048s 2011 American Chemical Society Published on Web 12/14/2010
Rheology Study of Silica-Zirconia Sols
Figure 1. Descriptive representation of the rotational rheometer Rheomat 180.
and S3, which have, respectively, metallic precursor concentrations equal to 0.5, 0.8, and 1 mol · L-1. 2. Principle of the Rheological Measurements. The Rheomat RM 180 is a classical rotation-type viscometer, which uses a motor-driven bob rotating in a fixed cup. The studied fluid is sheared in the gap between the bob and the cup, as described in Figure 1, and the measured shear stress is used to calculate the rheological parameters and especially the viscosity. The measuring bob is 24 mm wide in diameter and 36 mm long, while the stationary measuring tube is 32.54 mm wide in diameter. Because of the versality of the RM 180 viscometer, a viscosity measurement can be made by immersing the measurement bob in an open vessel or in a closed measurement tube where temperature can be accurately controlled. In this study, the temperature of the measurements has been set at 20 °C. The inner cylinder rotates with an angular rate equal to ω0, while the external cylinder is kept stationary. As a consequence, the fluid is submitted to a laminar shear movement. The speed difference between both cylinders induces a rate gradient in the fluid that can be decomposed in a multitude of coaxial cylindrical layers. The angular rates of these layers vary continuously from 0 for the one in contact with the external cylinder to ω0 for the layer in contact with the mobile cylinder. Following the relative motion of each layer, there is always a shear rate γ˙ ) (dγ)/(dt) and a shear stress τ everywhere in the fluid. For symmetric reasons, the shear stress and shear rate are constants over the surface of a same layer but depend on the radial position; so, γ˙ and τ are functions of the radius r from the centerline. It is therefore possible to determine the evolution of the dynamic viscosity with the time and rheological behavior of the fluid from the rheograms representing the function τ ) f(γ˙ ). Thus, the viscosity of the fluid sample is characterized by a flow curve, which is generated automatically by the RM 180 viscometer in the measurement programs. A measurement is decomposed in three steps: during the first step (0-60 s), the angular rate of the inner cylinder and thus the shear rate applied on the fluid increase for 60 s; then during step 2 (60-80 s), the rate is kept constant for 20 s; finally, during the third step (80-140 s), there is a decrease of the shear rate applied on the fluid for 60 s. The total duration of a measurement is 140 s, and each measurement is realized on the sols at a regular time interval until formation of the gel. Finally, the angular rate of the inner mobile can be correlated
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Figure 2. Graphic representation of the angular rate of the inner mobile as a function of time.
to the time on the whole duration of the measurement from the graph of Figure 2. Results and Discussion 1. Rheological Study Interpretation. 1.1. Rheology Background. a. Bibliography. The rheology of sol-gel materials has been extensively investigated by Brinker et al.9 Most of the rheological measurements were performed to determine the gel point of the sol rather than the evolution of the rheological behavior and the effect of sol rheology on the deposited film uniformity and thickness. Sacks et al. also performed some rheological measurements of both acid- and base-catalyzed silica sols,14 especially to determine the dynamic viscosity of the liquid at the gelation point and to try to relate the dynamic viscosity of the fluid to the thickness of a coating. For low and moderate shear rates, the studied silica-zirconia sols can be described according to two simple rheological models: the Newtonian model and the power law model. b. Newtonian Fluids. This model is generally applicable when the sol has not yet polymerized to any great degree. The equation that relates the shear rate to the shear stress in the Newtonian regime is given by the relation
τ ) -µγ˙ where τ is the shear stress, µ is the dynamic viscosity of the fluid, and γ˙ is the shear rate.15,16 c. Power Law Model. This model is generally applicable once polymerization has begun and is quite advanced; the relation that characterizes this model is given by
τ ) -Kγ˙ n where K is the consistence factor of the fluid and n the gap between the real behavior and the Newtonian behavior.15,16 When n ) 1, this model is reduced to the Newtonian model, whereas the fluid has a shear-thinning regime for n < 1. It is also possible that n > 1, and in this case the fluid is called shear thickened. For fluids, which obey a power law model as mentioned above, it is necessary to define an effective viscosity ηeff to describe their viscosity such as
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ηeff ) K
| | dγ dt
Brasse et al.
n-1
d. Notion of Apparent Viscosity. The appelation “apparent viscosity” is used to denominate the resistance of the fluid toward its flow. This physical quantity corresponds to the dynamic viscosity when the flow regime is Newtonian, but as soon as the fluid behavior tends to become shear thinning, this quantity is called effective viscosity and is variable as a function of the shear rate applied on the studied fluid, which is characterized only by the constants K and n. 1.2. Rheology Study Interpretation. A superposition of the measurements performed on the sol S1 every 15 min from t ) 30 to t ) 195 min is shown in Figure 3. These curves represent the evolution of the apparent viscosity with time along the acquisition. In this way, we can correlate each moment of these measurements with the rotational rate of the inner mobile and especially with the shear rate of the fluid. The measurements are realized every 15 min for sol S1, every 10 min for sol S2, and every 5 min for sol S3. This choice concerning the time intervals between each acquisition is explained by the sol polymerization rate, which is even faster as the alkoxide concentration is high. It is indeed not envisageable to realize an acquisition on a sol after its gelation, because the structure of a gelled sol becomes extremely brittle: a rotation of the inner mobile would destroy irreversibly the structure of the gel. In addition, we can consider in this study that the time necessary to reach the maximum rate of the mobile is negligible compared to the time of all other experimental durations and as compared to the times relative to evolution of the studied structure for constant stress or deformation. The modifications observed through this first set of experiments highlight very well the evolution of the rheological behavior of the studied fluid. Furthermore, similar observations are made in the case of sols S2 and S3, except that their polymerization rates are faster and thus their gelation time. To better understand the rheology of silica-zirconia sols in acidic conditions, it is needed to analyze their rheogram, which represents the shear stress τ applied on the fluid as a function of the mobile rotational rate gradient γ˙ . Figure 4 displays the rheogram of this fluid measured at 105 min after its preparation: a straight line can be observed whether the mobile rate gradient increases or decreases. This behavior is noticeable until 120 min after the sol synthesis, which means there is a linear relation between τ and γ˙ and that the fluid leads in a Newtonian regime during this period. Over this time, the rheograms reveal that the apparent viscosity decreases progressively when the rate gradient of the mobile increases: the sol tends to become more and more shear thinning as polymerization progresses. Moreover, the progressive apparition of a thixotrop character, which is revealed by formation of a hysteresis cycle on the rheogram, seems to follow the modifications that occur at a molecular scale in the fluid. This phenomenom relates the fact that the behavior of such a sol is not the same after having been sheared by the mobile: it means that the flow regime characteristic of the fluid depends on the previous treatments applied. We can explain the origin of such evolution by the fact that the polymeric chains, which form the fluid, are lining up under the constraint applied to the rotational mobile, the effect of which is to fluidize its flow. Then, the constant strength that is applied on the precursor sol progressively breaks the weak chemical boundaries between the molecular chains and lead to a thixotrop regime: it means that after having been destructured, the fluid needs a time delay to recover its initial organization at the molecular scale.
Figure 3. Superposition of the measurements presenting the evolution of the apparent viscosity as a function of the shear rate gradient applied by the rotational cylinder on sol S1; acquisition every 15 min.
Figure 4. Rheogram of the silica-zirconia sol realized at t ) 105 min after the sol preparation.
When the degree of polymerization is very advanced, a stress yield must be applied on the fluid to allow the rotation of the mobile, as it is the case at 195 min for the sol S1: it is the first sign that the gelification will occur very soon. After having studied the evolution of the rheological behavior of such a fluid as a function of polymerization advancement, it is now judicious to determine the evolution of the dynamic viscosity η with the time. Thus, its tendency is given by the graph shown in Figure 5, and thanks to these data, it would be possible to extract the value of the dynamic viscosity of the so-studied silica-zirconia sols, while they lie in a Newtonian regime. Nevertheless, even if it is preferable to dip coat the inner wall of the preform while the fluid has a Newtonian character it has an apparent viscosity η less than 0.013 Pa · s; it is also interesting to deepen the study when the fluid has a shearthinning character. As mentioned previously, these kinds of fluids, which obey to a power-law model, are characterized by two parameters: the consistence factor K and the factor n. The last one expresses the difference between the Newtonian regime and the real regime of the fluid. Evolution of the factor K is given by the graph shown in Figure 6 for measurements done t > 135 min after sol preparation. A hysteresis is noticeable and confirms the thixotropy of the silica-zirconia sols for the period mentioned above. Furthermore, analogous observations can be done concerning the evolution of the factor n that is pictured in Figure 7.
Rheology Study of Silica-Zirconia Sols
Figure 5. Evolution of the dynamic viscosity of the silica-zirconia sol S1 in the Newtonian regime for t < 120 min after sol preparation.
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Figure 8. Framing of the effective viscosity of the silica-zirconia sol S1 in the shear-thinning regime for various shear rates gradient (100-3600 s-1) applied by the moving cylinder.
TABLE 1: Time Intervals Corresponding to the Newtonian Regime and to the Shear-Thinning Regime as well as the Gelation Point for Sols S1, S2, and S3 C ) 0.5 mol/L C ) 0.8 mol/L Newtonian regime shear thinning regime ge´lification point
Figure 6. Evolution of the consistence factor of the silica-zirconia sol S1 in the shear-thinning regime realized for t > 135 min after sol preparation.
Figure 7. Evolution of the factor n of the silica-zirconia sol S1 in the shear-thinning regime realized for t > 135 min after sol preparation.
When the so-studied fluid lies in the shear-thinning regime, it is thus possible to estimate its effective viscosity as a function of the shear rate, which is applied on it from
µeff ) K
| | dγ dt
n-1
0-135 min 135-205 min 205 min
0-50 min 50-100 min 100 min
C)1 mol/L 0-25 min 25-50 min 50 min
In this way it is easy to frame the effective viscosity evolution between a maximum for small shear rates and a minimum at low shear rates as shown in Figure 8. For sol S1, whose behavior has been related previously, it is noted that gelling occurs 205 min after sol preparation for a metal precursor concentration fixed at 0.5 mol · L-1. Thanks to this study, it is possible to approach the gelling time and thus the viscosity of the sol just before it gels by framing this instant. Indeed, when the sol has gelled, it becomes a solid and any viscosity measurement can be done in this state, considering our equipment which is adapted for studying large kinds of fluids: The sample studied does not flow, and rotation of the inner mobile destroys irreversibly the gel. As a consequence of these observations, we are able to pinpoint the transformation of sol S1 in a solid gel phase between 210, which is the last measurement done on the fluid, and 215 min, the time for which a measurement could not be done because of irreversible destruction of the gel. These considerations can be extended to all of the sols, S1, S2, and S3: only the gelling time and polymerization rate vary. Table 1 shows the time intervals corresponding to the Newtonian regime, shear-thinning regime, and gelling time for each sol. Regarding these experimental measurements, it is clear that the higher the concentration in a metal precursor is the sooner gelling occurs. Conclusion To summarize, this study is a part of a larger project that aims to create original optical fibers by using the sol gel process and optimize a development method based on the coating of the inner wall of pure silica preforms. To obtain optical fibers with good waveguiding and optical properties, the purity and homogeneity of the core are essential criteria, which depend especially on the quality of the deposited sol layers. In this way, a study of the rheological behavior of silica-zirconia sols of various concentrations has been realized in order to control the coating step to optimize the
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development process of the fibers and allow a robust reproductibility of the results as well as better understand the evolution of such a fluid. Hence, to deposit a homogeneous and uniform sol layer, it is more advisible to use a fluid that lies in a Newtonian regime to avoid a dependence of the sol viscosity with the shear rate along the inner wall of the silica preform when it flows during the coating step. In addition, special care has been accorded to determine the viscosity of the fluid as a function of the polymerization time. The viscosity is indeed one of the most important parameters as it influences the thickness of the deposited layers and thus the core diameter of the final fiber. Complementary experiments are currently underway in order to relate the sol layer thickness with the sol viscosity and to compare these results with them predicted by the various existing theoretical relations, such as the Landau-Levich equation. Finally, the influence of the angular velocity of the inner mobile on the flow of the studied fluid is still not very well known, especially the role of the critical velocity that demarcates the transition between a Newtonian flow and the turbulent regime. It could be judicious to study in the future the influence of this parameter on the rheological transitions that occur in such a fluid as its polymerization degree progresses.
Brasse et al. References and Notes (1) Knight, J. C.; Birks, T. A.; Russell, P.; Atkin, D. M. Opt. Lett. 1996, 21, 19. (2) Russell, P. Science 2003, 299, 358. (3) Knight, J. C.; Broeng, J.; Birks, T. A.; Russell, P. Science 1998, 282, 1476. (4) Brasse, G.; Restoin, C.; Auguste, J. L.; Hautreux, S.; Blondy, J. M. Appl. Phys. Lett. 2007, 91, 121920. (5) Brasse, G.; Restoin, C.; Auguste, J. L.; Hautreux, S.; Blondy, J. M. Opt. Mater. 2009, 31, 765–768. (6) Sargent, E. H. AdV. Mater. 2005, 17, 5. (7) Klimov, V. I.; Ivanov, S. A.; Nanda, J.; Acherman, M.; Bezel, I.; McGuire, J. A.; Piryatinski, A. Nature 2007, 447, 441. (8) Riello, P. S.; Bucella, B. D.; Fossa, F.; Benedetti, A.; Trave, E.; Mazzoldi, P. Opt. Mater. 2006, 28, 1261. (9) Brinker, C. J. the Physics and Chemistry of Sol-Gel Processing; Academic: New York, 1990; Chapter 14. (10) Nogami, M. J. Non-Cryst. Solids 1985, 69, 415–423. (11) Bang, J.; Yang, H.; Holloway, P. H. J. Chem. Phys. 2005, 123, 084709. (12) Costa, V. C.; Lochhead, M. J.; Bray, K. L. Chem. Mater. 1996, 8, 783. (13) Gaudon, A.; Lallet, F.; Boulle, A.; Lecomte, A.; Soulestin, B.; Guinebretie`re, R.; Dauger, A. J. Non-Cryst. Solids 2006, 352, 2152. (14) Sacks, M. J. Non Cryst. Solids 1987, 92, 383. (15) Couarraze, G.; Grossiord, J. L. Initiation a` la rhe´ologie, 3rd ed.; Tec & Doc, 2000. (16) Boch, P. Mate´riaux et processus ce´ramiques; Hermes Science Europe Ltd., 2001; Vol. 36.
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