- Email: [email protected]
Optical fibers with composite magnetic coating
October 2002
Materials Letters 56 (2002) 148 – 155 www.elsevier.com/locate/matlet
Optical fibers with composite magnetic coating A. Milutinovic´-Nikolic´ a,*, N. Talijan a, K. Jeremic´ b, R. Aleksic´ b a
Institute of Chemistry, Technology and Metallurgy, Njegosˇeva 12, 11000 Belgrade, Yugoslavia b Faculty of Technology and Metallurgy, Karnegijeva 4, 11000 Belgrade, Yugoslavia
Received 9 April 2001; received in revised form 20 November 2001; accepted 28 November 2001
Abstract The goal of these investigations was to define the process parameters that enable the formation of a composite coating with the most uniform thickness on an optical fiber. The composite coating was obtained by drawing the optical fiber through a reservoir with a die at the bottom. The reservoir was supplied with a dispersion of magnetic powder in a polymer solution. The effect of the dispersion viscosity, powder concentration and drawing rate on the uniformity of the composite coating thickness under laboratory conditions was investigated on a defined system to establish the optimal coating conditions. Dispersions were formed by dispersing different concentrations of Ba-ferrite in a poly(ethylene-co-vinyl acetate) solution in toluene. The thickness of the coatings and its uniformity were determined by microscopic analysis and further evaluated by mathematical – statistical analysis. The smallest standard deviation (S.D.) of the mean value of the diameter of the optical fiber with a composite coating was chosen as the criteria for defining the most uniform composite coating. The results show that the most uniform coatings were obtained with a dispersion viscosity whose value was about 1.2 Pa s. All the process parameters should be established in accordance with this viscosity value, enabling the formation of the most uniform composite coating for the investigated system. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Optical fiber; Composite ferromagnetic coating; Dispersion viscosity
1. Introduction Optical fibers coated with a composite coating containing permanent magnetic material can be used as fiber optic sensors. In these fiber optic sensors, the sensed parameter (magnetic field, electric field, electric current) acts upon the magnetic particles present in the polymer coating layer of the optical fiber in such a way that the action modifies the optical
*
Corresponding author. E-mail address: [email protected] (A. Milutinovic´-Nikolic´).
propagation characteristics of the optical fiber [1]. Thus, the change in the optical path length of the optical fiber with a composite magnetic coating in the presence of a magnetic field provides the fiber with sensitivity to a magnetic field, which can be detected by optical interferometry [2]. Another possible application of using the optical fibers with a composite ferromagnetic coating is for the identification and detection of telecommunication optical fibers, using magnetic methods. Embedding magnetic markers in the cable and/or fiber is one of the possibilities of improving the reliability of networks [3,4]. Standard magnet detector can easily detect optical cables with magnetic markers. Magnetic
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 4 3 1 - 7
A. Milutinovic´-Nikolic´ et al. / Materials Letters 56 (2002) 148–155
markers can be imposed in optical fibers by dispersing them in a composite coating. The composite coating can be made by adapting the existing process of manufacturing optical fibers in stage in which a polymer coating is applied to the drawn fiber. Instead of a solely polymer coating, a coating with particles of magnetic powder dispersed in the polymer melt or solution can be used. An alignment of the magnetic particles should be performed before the hardening of the composite coating. These simple modifications enable the commercial process of producing optical fibers to be used for the production of optical fibers with composite magnetic coating. Appropriate composite coatings should be homogenous and thus enable reliable magnetic detection, while minimizing side effects. Therefore, the primary conditions that a composite coating should satisfy are the coating homogeneity and circularity, as well as the thickness uniformity. Numerous process parameters affect the uniformity of the coating thickness and circularity: composition, concentration of the polymer and magnetic constituents, viscosity of the coating dispersion, drawing rate, etc. [5– 7]. Commercial optical fibers consist of a protective polymer coating made of different acrylates, polyimides, silicones, etc. [8,9]. A copolymer of ethylene and vinyl acetate (EVA) was chosen for the polymer component of the composite coating because of its good adhesive properties. Using EVA, it is possible to produce the coating without the application of a UV or thermal curing process, and hence, the number of process parameters was reduced. The magnetic component of the composite coating can be selected from a variety of permanent ferromagnetic powders (hard ferrite, Sm –Co, Nd – Fe –B, etc.) [3,4]. Ba-ferrite was chosen for the presented investigation, as it is one of the most common and less expensive magnetic materials [10]. The concentration, size (on a micro- or nanometer scale), distribution and orientation (alignment) of the magnetic powder particles also affect the coating homogeneity, and this will be the subject of further investigations. The viscosity of the dispersion of magnetic powder in the polymer solution or melt is one of the parameters that have a great influence on the coating uniformity. If the viscosity is smaller than the optimal, then an inhomogeneous composite coating having a form similar to pearls is formed. If the viscosity is
149
greater than the optimal, then inhomogeneous and too thick coatings are formed [5 – 7]. Therefore, it is necessary to perform the coating with dispersion having the appropriate viscosity. The viscosity of the coating dispersion depends on the composition, temperature, concentration of polymer in solution (in a case of a solution) and the concentration of the magnetic powder in the dispersion [11,12]. The viscosities of EVA solutions in toluene were previously investigated at various temperatures, concentrations and shear rates [13], and it was shown that they exhibit non-Newtonian behavior [11]. For each investigated temperature and concentration of EVA solution in toluene, it was also shown that these solutions are pseudoplastic (viscosity decreases with shear rate and shear stress) and obey a power law [11]: g ¼ bc˙n1
ð1Þ
where g is the viscosity, c˙ = (dv/dx) is the shear rate, and b and n are constants. Dispersing a magnetic powder in a polymer solution also affects the viscosity of the system. Therefore, the viscosity of dispersion varies with the EVA concentration in toluene, the concentration of Baferrite powder, the temperature, the shear rate and, hence, with the drawing rate for a defined die aperture. The influence of the geometry of the die (cylindrical or conical) and especially the diameter of the die on the coating thickness is of great significance [14]. The presented investigations were performed using a laboratory equipment having a defined die geometry at a constant working temperature. The composite coating was formed with dispersions of Ba-ferrite powder in EVA solutions in toluene. The effect of the investigated process parameters (Ba-ferrite concentration, viscosity of dispersion and drawing rate) on the uniformity of the composite coating thickness were investigated to establish the optimal coating conditions.
2. Experimental The following starting materials were chosen for the investigation [15,16]:
Commercial single-mode optical fiber, produced by Alcatel. Attenuation given by the producer [16]
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was 0.34 dB/km at k = 1310 nm and 0.21 dB/km at k = 1550 nm; Polymer: poly(ethylene-co-vinyl acetate)— EVA with 28 mass% of vinyl acetate, produced by DuPont under the commercial name —ELVAX 265, was used in the form of toluene solutions; Permanent magnetic powder: Barium ferrite, BaO6Fe2O3, produced by ‘‘Feriti’’, Belgrade, in the form of single domain particles of 0.6 – 2.5 Am, having an average size of 1.1 Am. The viscosities of the Ba-ferrite dispersions in EVA solutions in toluene were investigated using a rotational viscometer Rheotest, at 38 F 0.5 jC, at shear rates in the interval of 3 – 1390 s1. Experimental procedure is given as a flow chart in Fig. 1. The optical fibers were coated with various concentrations of Ba-ferrite dispersed in 17.5 mass% solution of EVA in toluene. First, the polymer was dissolved in toluene at 60 jC using a magnetic mixer
Fig. 1. Flow chart of the experimental procedure for obtaining optical fibers with a composite coating.
with heater. Complete dissolution was reached after 2.5 h. After the complete dissolution of EVA, different concentrations of Ba-ferrite were dispersed in the solution. Ultrasound (40 kHz) mixing (5 min) followed by mechanical mixing (30 min) were used to homogenize the dispersion. The composite coating was obtained by drawing the optical fiber through a reservoir with the dispersion having a die (F = 0.5 mm) at the bottom. The temperature in the reservoir was controlled and constant (38 F 0.5 jC). Different drawing rates from 0.01 to 0.10 m/s were investigated. The drawing rate was controlled by a PID controller. The microscope technique was used to determine the thickness of the composite coating. The investigation was carried out using a Carl Zeiss JENA microscope with reflected light. The microscope was equipped with a digital (CCD) camera connected to a personal computer through a frame grabber card videoMiro DC 30. For recording the microphotographs, Adobe illustrator 4.2. software was used. The obtained microphotographs were treated with Lira 98 software (developed at the Faculty of Technology and Metallurgy, Belgrade) for quantification of the visual information [17]. The program gives option for line length measurement, which has been used for measuring the projection of the diameters of the optical fibers with a composite coating. For the calibration of the microscope, a calibration glass with a distance between the reference marks of 5 Am was used. The results of the measured diameters were obtained in the form of a table, and further statistical analysis was performed using the program Origin 5.0. For each investigated volume fraction of Ba-ferrite in the dispersion, and therefore, its fraction in the composite coating, the parameters producing the most uniform coating thickness were established. The propagation of coherent light through the developed optical fibers with the most uniform coating was investigated to confirm the coating quality. The influence of an applied external magnetic field on the change of the intensity of the light signal propagated through a light pipe consisting of an optical fiber with composite EVA/Ba-ferrite coating was investigated. A laser operating fiber optic tester FOT 900 produced by EXFO was used for measuring and
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controlling the transmission characteristics of the optical fibers. Investigations were performed at a wavelength of 1310 nm. The measurements were performed in a high-defined DC and low frequency (0– 5 Hz) magnetic field [18].
3. Results and discussion The pseudoplastic behavior was established for Baferrite dispersion in 17.5 mass% EVA solution in toluene for all the investigated volume fractions (u) of dispersed Ba-ferrite powder. The influence of the volume fraction and shear rate on the viscosity of the dispersions of Ba-ferrite is presented in Fig. 2. The obtained dependencies of the logarithm of the viscosity on the logarithm of the shear rate for all the investigated volume fractions of Ba-ferrite are given in Table 1. The optical fibers were coated with various volume fractions of Ba-ferrite dispersed in 17.5 mass% solutions of EVA in toluene. The composition of the composite coating represents the ratio of barium ferrite and EVA since the amount of residual toluene in the formed composite coating is negligible [5– 7]. The composition of the dispersions and the composi-
151
Table 1 Dependence of the viscosity on the shear rate for various dispersions of Ba-ferrite in 17.5 mass% EVA solution in toluene at 38 jC Volume fraction of Baferrite in dispersion, u
Approximated linear dependence
Correlation coefficient
0 (without Ba-ferrite) 0.0123
logg = 0.30863 0.12227logc˙ logg = 0.34276 0.13024logc˙ logg = 0.38361 0.14252logc˙ logg = 0.40785 0.14279logc˙ logg = 0.42850 0.14518logc˙ logg = 0.45117 0.14539logc˙
0.9912
0.0191 0.0283 0.0419 0.0637
0.9916 0.9962 0.9982 0.9962 0.9951
tions of the corresponding composite coating are given in Table 2. Microscopic analysis was performed on all the investigated optical fibers with composite ferromagnetic coatings, obtained with the investigated dispersions and drawing rates. For each sample, photographs were taken from each 10– 20 cm. The diameters of the optical fibers with composite coating were measured. The experimental results were evaluated by a mathe-
Fig. 2. Dependence of the viscosity on the shear rate for various dispersions of Ba-ferrite in 17.5 mass% EVA solution in toluene at 38 jC.
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Table 2 Composition of the investigated dispersions and corresponding composite coatings Number
1 2 3 4 5
Dispersion
Composite coating
EVA
Ba-ferrite
Toluene
Volume
mass%
mass%
mass%
fraction, u
mass%
vol.%
mass%
vol.%
16.3 15.7 14.9 13.9 12.4
7.0 10.4 14.9 20.8 29.0
76.7 73.9 70.2 65.3 58.6
0.0123 0.0191 0.0283 0.0419 0.0637
70.0 60.0 50.0 40.0 30.0
92.8 89.3 84.7 78.7 70.4
30.0 40.0 50.0 60.0 70.0
7.2 10.7 15.3 21.3 29.6
matical – statistical method and the criteria for defining the most uniform composite coating was the standard deviation of the mean value of diameter the optical fiber with composite coating. The smallest standard deviation (S.D.) corresponds to the sample with the most uniform composite coating. First, the uniformity of the thickness of the composite coating, expressed as S.D., was correlated with the drawing rate (v). The dependence of S.D. vs. v for different Ba-ferrite contents in composite coating is shown in Fig. 3. According to the results presented in Fig. 3, the S.D. increases with the increasing of Ba-ferrite content. The dependency of S.D. on the drawing rate shows a minimum for most investigated volume
EVA
Ba-ferrite
fraction of Ba-ferrite. This indicates that for each volume fraction of Ba-ferrite, there is an optimal coating rate, which produces a coating with the most uniform thickness. The next step in the presentation of the results of the statistical analysis of the data obtained from the microscope images of the optical fibers with composite EVA/Ba-ferrite coating was the correlation of the (S.D.) with the viscosity (g) of the dispersion used for the formation of a composite coating. For each investigated volume fraction of Ba-ferrite, the dependence of S.D. on the viscosity of the dispersion was established. The results are shown in Fig. 4. The viscosity was calculated according to the experimentally obtained equations given in Table 1
Fig. 3. Dependence of the standard deviation of the mean value of diameter of optical fiber with composite coating on the drawing rate for various concentrations of Ba-ferrite in the composite coating.
A. Milutinovic´-Nikolic´ et al. / Materials Letters 56 (2002) 148–155
153
Fig. 4. Dependence of the standard deviation of the mean value of diameter of optical fiber with composite coating on the dispersion viscosity for various concentrations of Ba-ferrite in the composite coating.
using the values for the shear rate calculated according to the following simplification: c¼
dv Dv vcoat v0 2vcoat c ¼ ¼ d d dx Dx ddie dof die of 2
ð2Þ
where vcoat is the drawing rate, v0 is the velocity at the die wall (it is assumed that v0 = 0), ddie is the diameter of the die, and dof is the diameter of the original optical fiber. The obtained dependencies of the standard deviation of the mean diameter of the optical fibers with composite coatings on the dispersion viscosity were approximated by a third degree polynomial. These polynomials are given in Table 3.
The dispersion viscosity, where the polynomial dependencies of the standard deviation have their minimum, is the optimal dispersion viscosity providing a composite coating of the most uniform thickness. The results show that the most uniform coatings were obtained with dispersions having a viscosity in a very narrow range (1.20 – 1.24 Pa s) for all the investigated concentrations of Ba-ferrite in the dispersion. This means that all of the process parameters such as Ba-ferrite concentration in composite coating, drawing rate and temperature should be adjusted in such a manner that the coating is performed with dispersion in accordance with the evaluated optimal viscosity. These investigations proved that the dispersion viscosity is the key parameter for the formation of the
Table 3 Polynomial dependence of the standard deviation (S.D.) of the mean value of diameter of optical fiber with composite coating on the viscosity (g) of the coating dispersion for various concentrations of Ba-ferrite in the composite coating Concentration of Ba-ferrite incomposite coating (mass%)
Approximated polynomial dependence
30.0 40.0 50.0 60.0 70.0
S.D. = 20.871 + 637.530g 629.699g2 + 202.148g3 S.D. = 98.4749 + 413.288g 468.303g2 + 162.409g3 S.D. = 1431.91 3214.38g + 2390.35g2 585.373g3 S.D. = 733.805 1214.63g + 505.487g2 0.33875g3 S.D. = 8841.05 20333.5g + 15559.8g2 3957.35g3
Coefficient of correlation
Calculated minimum of functions g (Pa s)
S.D. (Am)
0.9947 0.9949 1 0.9990 0.9903
1.20 1.24 1.21 1.20 1.24
3.71 3.59 5.20 3.96 7.08
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most uniform composite coating in the investigated system. By using the determined values for the optimal dispersion viscosity for each investigated concentration of Ba-ferrite, the optimal drawing rate was determined. These rates were calculated using the equations given in Table 1 and Eq. (2). For 30, 40, 50, 60 and 70 mass% of Ba-ferrite in the composite coating, the following drawing rates should produce the most uniform composite coating: 1.33, 1.42, 1.95, 3.19 and 3.69 102 m/s, respectively. The influence of various applied external highdefined DC and low frequency (0 –5 Hz) magnetic fields on the change of the power intensity of light propagated through a light pipe consisting of an optical fiber with the most uniform composite EVA/ Ba-ferrite coating was investigated. No change in the power of the propagated laser beam through the fiber optic with the composite coating for all the applied magnetic fields was observed. These investigations indicate that the optical fibers with a composite ferromagnetic coating have most probably preserved their transmission characteristics [13], but for unambiguous statement, some further investigation should be performed.
4. Conclusion The process parameters optimized for the investigated system were: single-mode optical fiber; polymer component of the composite coating EVA (17.5 mass% solution in toluene); magnetic component of the composite coating Ba-ferrite powder (with average particle size 1 Am); coating temperature of 38 jC; and a coating die (F = 0.5 mm). The uniformity of the coating thickness was controlled using a microscopic technique and further evaluated by the mathematical – statistical analysis. The standard deviation of the mean value of the diameter of an optical fiber with a composite coating was used as the criteria for defining the uniformity of the coating thickness. The results show that the most uniform coatings were obtained with dispersions having a viscosity in a very narrow range of viscosity (1.20 –1.24 Pa s) for all the investigated concentrations of Ba-ferrite in the dispersion. It was shown that the dispersion viscosity
is the key parameter for obtaining coatings with the most uniform thickness. All the process parameters such as Ba-ferrite concentration in the composite coating, drawing rate and temperature should be established in accordance with the evaluated optimal viscosity.
References [1] P. Extance, R.E. Jones, G.D. Pitt, Fibre optic sensors, EP 131404/25, June 1984. [2] J.E. Lenz, A review of magnetic sensors, Proc. IEEE 78 (6) (1990) 973 – 989. [3] C.J. Arroyo, S. Jin, T. Palstra, Magnetically locatable nonmetallic optical fiber cables, EP 675379/04, October 1995. [4] C.J. Arroyo, S. Jin, T. Palstra, Magnetically locatable optical fiber cables containing integrated magnetic marker materials, EP 675380/04, October 1995. [5] A. Milutinovic´-Nikolic´, N. Talijan, A. Zˇivkovic´, R. Aleksic´, Composites: optical fibers – magnetic powders, Proc. Int. Symposium Light Metals and Composite Materials, Beograd, Yugoslavia, 1999, pp. 49 – 50. [6] A. Milutinovic´-Nikolic´, N. Talijan, R. Aleksic´, Investigation of the conditions of synthesis composite optical fiber-magnetic powder, Sci. Sintering 32 (2) (2000) 73 – 79. [7] A. Milutinovic´-Nikolic´, N. Talijan, M. Srec´kovic´, R. Aleksic´, Investigation of the conditions of synthesis composite optical fiber-magnetic material with preserved transmission characteristics, Proc. Inter. Conf. on Lasers ’99, STS Press, Canada, 2000, pp. 166 – 172. [8] Wagatsuma, F. Yammamoto, T. Kimura, Y. Shuto, S. Yamakawa, Coated optical fiber, fabrication process thereof and fabrication apparatus thereof, US patent 4801186, January 1989. [9] M. Ott, P. Friedberg, Technology validation of optical fiber cables for space flight environments, Optical Devices for Fiber Communication II, SPIE Proc. vol. 4216, Boston MA, in press (http://misspiggy.gsfc.nasa.gov/tva/meldoc/cotsfospie2000. pdf). [10] T. Abraham, Magnets and magnetic materials: a technical economic analysis, Int. J. Powder Metall. 31 (2) (1995) 133 – 136. [11] F. Rodriguez, Principles of Polymer Systems, Second ed., Hemisphere Publishing, Washington, 1982. [12] A. Arsac, C. Carrot, J. Guillet, Rheological characterization of ethylene – vinyl acetate copolymers, J. Appl. Polym. Sci. 74 (11) (1999) 2625 – 2630. [13] A. Milutinovic´-Nikolic´, N. Talijan, M. Srec´kovic´, S. Milic´, S. Pantelic´, R. Aleksic´, Technological procedure and characterization of optical fibers with a composite magnetic coating, Proceedings of International Conference on Lasers ’2000, STS Press, Albuquerque, NM, USA, 2001, pp. 596 – 603. [14] U.C. Paek, Free drawing and polymer coating of silica glass optical fibers, J. Heat Transfer-Trans. ASME 121 (4) (1999) 774 – 788. [15] A. Milutinovic´-Nikolic´, N. Talijan, R. Aleksic´, Mechanical
A. Milutinovic´-Nikolic´ et al. / Materials Letters 56 (2002) 148–155 properties of composites: optical fiber-magnetic powders, Proc. CHISA ’2000, 2 Prague, (2000) P1.100. [16] Alcatel’s Technical Documentation, April 1998. [17] R. Jancˇic´, R. Aleksic´, Influence of formation conditions and precursor viscosity on mean fiber diameter formed used the rotating disk method, Mater. Lett. 42 (2000) 350 – 355.
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[18] A. Milutinovic´-Nikolic´, N. Talijan, M. Srec´kovic´, Lj. Konstantinovic´, S. Milic´, S. Pantelic´, R. Aleksic´, Optical fibers modified with magnetic materials in medicine, Proceedings of International Conference on Lasers ’2000, STS Press, Albuquerque, 2001, pp. 604 – 610.
Materials Letters 56 (2002) 148 – 155 www.elsevier.com/locate/matlet
Optical fibers with composite magnetic coating A. Milutinovic´-Nikolic´ a,*, N. Talijan a, K. Jeremic´ b, R. Aleksic´ b a
Institute of Chemistry, Technology and Metallurgy, Njegosˇeva 12, 11000 Belgrade, Yugoslavia b Faculty of Technology and Metallurgy, Karnegijeva 4, 11000 Belgrade, Yugoslavia
Received 9 April 2001; received in revised form 20 November 2001; accepted 28 November 2001
Abstract The goal of these investigations was to define the process parameters that enable the formation of a composite coating with the most uniform thickness on an optical fiber. The composite coating was obtained by drawing the optical fiber through a reservoir with a die at the bottom. The reservoir was supplied with a dispersion of magnetic powder in a polymer solution. The effect of the dispersion viscosity, powder concentration and drawing rate on the uniformity of the composite coating thickness under laboratory conditions was investigated on a defined system to establish the optimal coating conditions. Dispersions were formed by dispersing different concentrations of Ba-ferrite in a poly(ethylene-co-vinyl acetate) solution in toluene. The thickness of the coatings and its uniformity were determined by microscopic analysis and further evaluated by mathematical – statistical analysis. The smallest standard deviation (S.D.) of the mean value of the diameter of the optical fiber with a composite coating was chosen as the criteria for defining the most uniform composite coating. The results show that the most uniform coatings were obtained with a dispersion viscosity whose value was about 1.2 Pa s. All the process parameters should be established in accordance with this viscosity value, enabling the formation of the most uniform composite coating for the investigated system. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Optical fiber; Composite ferromagnetic coating; Dispersion viscosity
1. Introduction Optical fibers coated with a composite coating containing permanent magnetic material can be used as fiber optic sensors. In these fiber optic sensors, the sensed parameter (magnetic field, electric field, electric current) acts upon the magnetic particles present in the polymer coating layer of the optical fiber in such a way that the action modifies the optical
*
Corresponding author. E-mail address: [email protected] (A. Milutinovic´-Nikolic´).
propagation characteristics of the optical fiber [1]. Thus, the change in the optical path length of the optical fiber with a composite magnetic coating in the presence of a magnetic field provides the fiber with sensitivity to a magnetic field, which can be detected by optical interferometry [2]. Another possible application of using the optical fibers with a composite ferromagnetic coating is for the identification and detection of telecommunication optical fibers, using magnetic methods. Embedding magnetic markers in the cable and/or fiber is one of the possibilities of improving the reliability of networks [3,4]. Standard magnet detector can easily detect optical cables with magnetic markers. Magnetic
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 4 3 1 - 7
A. Milutinovic´-Nikolic´ et al. / Materials Letters 56 (2002) 148–155
markers can be imposed in optical fibers by dispersing them in a composite coating. The composite coating can be made by adapting the existing process of manufacturing optical fibers in stage in which a polymer coating is applied to the drawn fiber. Instead of a solely polymer coating, a coating with particles of magnetic powder dispersed in the polymer melt or solution can be used. An alignment of the magnetic particles should be performed before the hardening of the composite coating. These simple modifications enable the commercial process of producing optical fibers to be used for the production of optical fibers with composite magnetic coating. Appropriate composite coatings should be homogenous and thus enable reliable magnetic detection, while minimizing side effects. Therefore, the primary conditions that a composite coating should satisfy are the coating homogeneity and circularity, as well as the thickness uniformity. Numerous process parameters affect the uniformity of the coating thickness and circularity: composition, concentration of the polymer and magnetic constituents, viscosity of the coating dispersion, drawing rate, etc. [5– 7]. Commercial optical fibers consist of a protective polymer coating made of different acrylates, polyimides, silicones, etc. [8,9]. A copolymer of ethylene and vinyl acetate (EVA) was chosen for the polymer component of the composite coating because of its good adhesive properties. Using EVA, it is possible to produce the coating without the application of a UV or thermal curing process, and hence, the number of process parameters was reduced. The magnetic component of the composite coating can be selected from a variety of permanent ferromagnetic powders (hard ferrite, Sm –Co, Nd – Fe –B, etc.) [3,4]. Ba-ferrite was chosen for the presented investigation, as it is one of the most common and less expensive magnetic materials [10]. The concentration, size (on a micro- or nanometer scale), distribution and orientation (alignment) of the magnetic powder particles also affect the coating homogeneity, and this will be the subject of further investigations. The viscosity of the dispersion of magnetic powder in the polymer solution or melt is one of the parameters that have a great influence on the coating uniformity. If the viscosity is smaller than the optimal, then an inhomogeneous composite coating having a form similar to pearls is formed. If the viscosity is
149
greater than the optimal, then inhomogeneous and too thick coatings are formed [5 – 7]. Therefore, it is necessary to perform the coating with dispersion having the appropriate viscosity. The viscosity of the coating dispersion depends on the composition, temperature, concentration of polymer in solution (in a case of a solution) and the concentration of the magnetic powder in the dispersion [11,12]. The viscosities of EVA solutions in toluene were previously investigated at various temperatures, concentrations and shear rates [13], and it was shown that they exhibit non-Newtonian behavior [11]. For each investigated temperature and concentration of EVA solution in toluene, it was also shown that these solutions are pseudoplastic (viscosity decreases with shear rate and shear stress) and obey a power law [11]: g ¼ bc˙n1
ð1Þ
where g is the viscosity, c˙ = (dv/dx) is the shear rate, and b and n are constants. Dispersing a magnetic powder in a polymer solution also affects the viscosity of the system. Therefore, the viscosity of dispersion varies with the EVA concentration in toluene, the concentration of Baferrite powder, the temperature, the shear rate and, hence, with the drawing rate for a defined die aperture. The influence of the geometry of the die (cylindrical or conical) and especially the diameter of the die on the coating thickness is of great significance [14]. The presented investigations were performed using a laboratory equipment having a defined die geometry at a constant working temperature. The composite coating was formed with dispersions of Ba-ferrite powder in EVA solutions in toluene. The effect of the investigated process parameters (Ba-ferrite concentration, viscosity of dispersion and drawing rate) on the uniformity of the composite coating thickness were investigated to establish the optimal coating conditions.
2. Experimental The following starting materials were chosen for the investigation [15,16]:
Commercial single-mode optical fiber, produced by Alcatel. Attenuation given by the producer [16]
150
A. Milutinovic´-Nikolic´ et al. / Materials Letters 56 (2002) 148–155
was 0.34 dB/km at k = 1310 nm and 0.21 dB/km at k = 1550 nm; Polymer: poly(ethylene-co-vinyl acetate)— EVA with 28 mass% of vinyl acetate, produced by DuPont under the commercial name —ELVAX 265, was used in the form of toluene solutions; Permanent magnetic powder: Barium ferrite, BaO6Fe2O3, produced by ‘‘Feriti’’, Belgrade, in the form of single domain particles of 0.6 – 2.5 Am, having an average size of 1.1 Am. The viscosities of the Ba-ferrite dispersions in EVA solutions in toluene were investigated using a rotational viscometer Rheotest, at 38 F 0.5 jC, at shear rates in the interval of 3 – 1390 s1. Experimental procedure is given as a flow chart in Fig. 1. The optical fibers were coated with various concentrations of Ba-ferrite dispersed in 17.5 mass% solution of EVA in toluene. First, the polymer was dissolved in toluene at 60 jC using a magnetic mixer
Fig. 1. Flow chart of the experimental procedure for obtaining optical fibers with a composite coating.
with heater. Complete dissolution was reached after 2.5 h. After the complete dissolution of EVA, different concentrations of Ba-ferrite were dispersed in the solution. Ultrasound (40 kHz) mixing (5 min) followed by mechanical mixing (30 min) were used to homogenize the dispersion. The composite coating was obtained by drawing the optical fiber through a reservoir with the dispersion having a die (F = 0.5 mm) at the bottom. The temperature in the reservoir was controlled and constant (38 F 0.5 jC). Different drawing rates from 0.01 to 0.10 m/s were investigated. The drawing rate was controlled by a PID controller. The microscope technique was used to determine the thickness of the composite coating. The investigation was carried out using a Carl Zeiss JENA microscope with reflected light. The microscope was equipped with a digital (CCD) camera connected to a personal computer through a frame grabber card videoMiro DC 30. For recording the microphotographs, Adobe illustrator 4.2. software was used. The obtained microphotographs were treated with Lira 98 software (developed at the Faculty of Technology and Metallurgy, Belgrade) for quantification of the visual information [17]. The program gives option for line length measurement, which has been used for measuring the projection of the diameters of the optical fibers with a composite coating. For the calibration of the microscope, a calibration glass with a distance between the reference marks of 5 Am was used. The results of the measured diameters were obtained in the form of a table, and further statistical analysis was performed using the program Origin 5.0. For each investigated volume fraction of Ba-ferrite in the dispersion, and therefore, its fraction in the composite coating, the parameters producing the most uniform coating thickness were established. The propagation of coherent light through the developed optical fibers with the most uniform coating was investigated to confirm the coating quality. The influence of an applied external magnetic field on the change of the intensity of the light signal propagated through a light pipe consisting of an optical fiber with composite EVA/Ba-ferrite coating was investigated. A laser operating fiber optic tester FOT 900 produced by EXFO was used for measuring and
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controlling the transmission characteristics of the optical fibers. Investigations were performed at a wavelength of 1310 nm. The measurements were performed in a high-defined DC and low frequency (0– 5 Hz) magnetic field [18].
3. Results and discussion The pseudoplastic behavior was established for Baferrite dispersion in 17.5 mass% EVA solution in toluene for all the investigated volume fractions (u) of dispersed Ba-ferrite powder. The influence of the volume fraction and shear rate on the viscosity of the dispersions of Ba-ferrite is presented in Fig. 2. The obtained dependencies of the logarithm of the viscosity on the logarithm of the shear rate for all the investigated volume fractions of Ba-ferrite are given in Table 1. The optical fibers were coated with various volume fractions of Ba-ferrite dispersed in 17.5 mass% solutions of EVA in toluene. The composition of the composite coating represents the ratio of barium ferrite and EVA since the amount of residual toluene in the formed composite coating is negligible [5– 7]. The composition of the dispersions and the composi-
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Table 1 Dependence of the viscosity on the shear rate for various dispersions of Ba-ferrite in 17.5 mass% EVA solution in toluene at 38 jC Volume fraction of Baferrite in dispersion, u
Approximated linear dependence
Correlation coefficient
0 (without Ba-ferrite) 0.0123
logg = 0.30863 0.12227logc˙ logg = 0.34276 0.13024logc˙ logg = 0.38361 0.14252logc˙ logg = 0.40785 0.14279logc˙ logg = 0.42850 0.14518logc˙ logg = 0.45117 0.14539logc˙
0.9912
0.0191 0.0283 0.0419 0.0637
0.9916 0.9962 0.9982 0.9962 0.9951
tions of the corresponding composite coating are given in Table 2. Microscopic analysis was performed on all the investigated optical fibers with composite ferromagnetic coatings, obtained with the investigated dispersions and drawing rates. For each sample, photographs were taken from each 10– 20 cm. The diameters of the optical fibers with composite coating were measured. The experimental results were evaluated by a mathe-
Fig. 2. Dependence of the viscosity on the shear rate for various dispersions of Ba-ferrite in 17.5 mass% EVA solution in toluene at 38 jC.
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Table 2 Composition of the investigated dispersions and corresponding composite coatings Number
1 2 3 4 5
Dispersion
Composite coating
EVA
Ba-ferrite
Toluene
Volume
mass%
mass%
mass%
fraction, u
mass%
vol.%
mass%
vol.%
16.3 15.7 14.9 13.9 12.4
7.0 10.4 14.9 20.8 29.0
76.7 73.9 70.2 65.3 58.6
0.0123 0.0191 0.0283 0.0419 0.0637
70.0 60.0 50.0 40.0 30.0
92.8 89.3 84.7 78.7 70.4
30.0 40.0 50.0 60.0 70.0
7.2 10.7 15.3 21.3 29.6
matical – statistical method and the criteria for defining the most uniform composite coating was the standard deviation of the mean value of diameter the optical fiber with composite coating. The smallest standard deviation (S.D.) corresponds to the sample with the most uniform composite coating. First, the uniformity of the thickness of the composite coating, expressed as S.D., was correlated with the drawing rate (v). The dependence of S.D. vs. v for different Ba-ferrite contents in composite coating is shown in Fig. 3. According to the results presented in Fig. 3, the S.D. increases with the increasing of Ba-ferrite content. The dependency of S.D. on the drawing rate shows a minimum for most investigated volume
EVA
Ba-ferrite
fraction of Ba-ferrite. This indicates that for each volume fraction of Ba-ferrite, there is an optimal coating rate, which produces a coating with the most uniform thickness. The next step in the presentation of the results of the statistical analysis of the data obtained from the microscope images of the optical fibers with composite EVA/Ba-ferrite coating was the correlation of the (S.D.) with the viscosity (g) of the dispersion used for the formation of a composite coating. For each investigated volume fraction of Ba-ferrite, the dependence of S.D. on the viscosity of the dispersion was established. The results are shown in Fig. 4. The viscosity was calculated according to the experimentally obtained equations given in Table 1
Fig. 3. Dependence of the standard deviation of the mean value of diameter of optical fiber with composite coating on the drawing rate for various concentrations of Ba-ferrite in the composite coating.
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Fig. 4. Dependence of the standard deviation of the mean value of diameter of optical fiber with composite coating on the dispersion viscosity for various concentrations of Ba-ferrite in the composite coating.
using the values for the shear rate calculated according to the following simplification: c¼
dv Dv vcoat v0 2vcoat c ¼ ¼ d d dx Dx ddie dof die of 2
ð2Þ
where vcoat is the drawing rate, v0 is the velocity at the die wall (it is assumed that v0 = 0), ddie is the diameter of the die, and dof is the diameter of the original optical fiber. The obtained dependencies of the standard deviation of the mean diameter of the optical fibers with composite coatings on the dispersion viscosity were approximated by a third degree polynomial. These polynomials are given in Table 3.
The dispersion viscosity, where the polynomial dependencies of the standard deviation have their minimum, is the optimal dispersion viscosity providing a composite coating of the most uniform thickness. The results show that the most uniform coatings were obtained with dispersions having a viscosity in a very narrow range (1.20 – 1.24 Pa s) for all the investigated concentrations of Ba-ferrite in the dispersion. This means that all of the process parameters such as Ba-ferrite concentration in composite coating, drawing rate and temperature should be adjusted in such a manner that the coating is performed with dispersion in accordance with the evaluated optimal viscosity. These investigations proved that the dispersion viscosity is the key parameter for the formation of the
Table 3 Polynomial dependence of the standard deviation (S.D.) of the mean value of diameter of optical fiber with composite coating on the viscosity (g) of the coating dispersion for various concentrations of Ba-ferrite in the composite coating Concentration of Ba-ferrite incomposite coating (mass%)
Approximated polynomial dependence
30.0 40.0 50.0 60.0 70.0
S.D. = 20.871 + 637.530g 629.699g2 + 202.148g3 S.D. = 98.4749 + 413.288g 468.303g2 + 162.409g3 S.D. = 1431.91 3214.38g + 2390.35g2 585.373g3 S.D. = 733.805 1214.63g + 505.487g2 0.33875g3 S.D. = 8841.05 20333.5g + 15559.8g2 3957.35g3
Coefficient of correlation
Calculated minimum of functions g (Pa s)
S.D. (Am)
0.9947 0.9949 1 0.9990 0.9903
1.20 1.24 1.21 1.20 1.24
3.71 3.59 5.20 3.96 7.08
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most uniform composite coating in the investigated system. By using the determined values for the optimal dispersion viscosity for each investigated concentration of Ba-ferrite, the optimal drawing rate was determined. These rates were calculated using the equations given in Table 1 and Eq. (2). For 30, 40, 50, 60 and 70 mass% of Ba-ferrite in the composite coating, the following drawing rates should produce the most uniform composite coating: 1.33, 1.42, 1.95, 3.19 and 3.69 102 m/s, respectively. The influence of various applied external highdefined DC and low frequency (0 –5 Hz) magnetic fields on the change of the power intensity of light propagated through a light pipe consisting of an optical fiber with the most uniform composite EVA/ Ba-ferrite coating was investigated. No change in the power of the propagated laser beam through the fiber optic with the composite coating for all the applied magnetic fields was observed. These investigations indicate that the optical fibers with a composite ferromagnetic coating have most probably preserved their transmission characteristics [13], but for unambiguous statement, some further investigation should be performed.
4. Conclusion The process parameters optimized for the investigated system were: single-mode optical fiber; polymer component of the composite coating EVA (17.5 mass% solution in toluene); magnetic component of the composite coating Ba-ferrite powder (with average particle size 1 Am); coating temperature of 38 jC; and a coating die (F = 0.5 mm). The uniformity of the coating thickness was controlled using a microscopic technique and further evaluated by the mathematical – statistical analysis. The standard deviation of the mean value of the diameter of an optical fiber with a composite coating was used as the criteria for defining the uniformity of the coating thickness. The results show that the most uniform coatings were obtained with dispersions having a viscosity in a very narrow range of viscosity (1.20 –1.24 Pa s) for all the investigated concentrations of Ba-ferrite in the dispersion. It was shown that the dispersion viscosity
is the key parameter for obtaining coatings with the most uniform thickness. All the process parameters such as Ba-ferrite concentration in the composite coating, drawing rate and temperature should be established in accordance with the evaluated optimal viscosity.
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