APPLICATION OF MAGNETIC POWDER FOR MAGNETIC FIELD

Download PDF
0 downloads 0 Views 144KB Size Report
Comment
the action modifies the optical propagation characteristics of the optical fiber. The influence of .... 1 – glass tube; 2 – optical fiber with composite magnetic coating ...
APPLICATION OF MAGNETIC POWDER FOR MAGNETIC FIELD SENSING ELEMENTS D. NEDELJKOVIC1, V. RADOJEVIĆ2, LJ. BRAJOVIĆ3, J. STAJIĆ-TROŠIĆ1, A. GRUJIĆ1, N. TALIJAN1, R. ALEKSIĆ2 1

Institute of Chemistry, Technology and Metallurgy, Belgrade, Serbia and Montenegro [email protected] 2 Faculty of Technology and Metallurgy, Belgrade, Serbia and Montenegro 3 Faculty of Civil Engineering, Belgrade, Serbia and Montenegro Received December 21, 2004

Multi-mode optical fiber with magnetic composite coating was investigated as an optical fiber sensor for magnetic field sensing. The optical fiber magnet sensor element presented in this paper was constructed on the base of intensity-based optical fiber vibration sensors. The commercial multi-mode optical fiber was coated with magnetic composite. The composite coating was formed with dispersions of permanent magnet powder of Nd-Fe-B in poly (ethylene-co-vinyl acetate)-Eva solutions in toluene. The composite coating can be made by adapting the existing process of manufacturing optical fibers in stage in which polymer coating is applied to the drawn fiber. The sensed parameter – magnetic field 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 propagation characteristics of the optical fiber. The influence of the applied external magnetic field on the change of intensity of the light signal propagate through developed optical fibers were investigated. In this paper the influence of the magnetic powder and particle size on the optical propagation characteristics of optical fiber were particularly investigated. Sensor element that was constructed was very sensitive for external magnetic field, and completely reversible. Key words: Composite materials; optical fiber sensor; propagation properties.

1. INTRODUCTION

The measurement of magnetic field has been a critical part in various technical areas. High sensitivity magnetic field measurement has been utilized to detect the presence of large ferromagnetic objects which change the magnetic field distribution. The introduction of optical fiber has changed the way of telecommunications and related fields. The optical fiber provides a large band width, low cost in mass production, and low transmission loss in communication channels. Optical fiber coated with a composite polymer-magnetic coating can 

Paper presented at the 5th International Balkan Workshop on Applied Physics, 5–7 July 2004, Constanþa, Romania. Rom. Journ. Phys., Vol. 50, Nos. 9– 1 0 , P. 971–976, Bucharest, 2005

972

D. Nedeljkovic et al.

2

be used as an optical fiber magnetic sensor [1–3]. Also, these new fiber optic sensors can be used to the areas that are too harsh to measure with conventional systems since the optical fiber is usually made by dielectric materials which have the properties of high resistance to vibration, electromagnetic interference, thermal shock and corrosion. The optical fiber has a shape of long cylindrical wave guide with different types of core/cladding size and shape. The measurements using optical fiber is described as the light propagates in the cylindrical optical wave guide with core and cladding modes, the external excitation disturbs the modes to change their propagation properties which can be detected and measured. Optical fiber magnetic sensor in this study consists of two optical fibers held in close proximity to each other, and are based on the principle of intensity modulation [4]. One of those optical fibers (the one coated with magnetic composite) is cantilevered in a capillary tube. The cantilevered section moves in the opposite direction to the rest of the sensor in response to an applied external magnetic field, and amount of light coupled between the two fibers is modulated. The composite coating can be made by adapting the existing process of manufacturing optical fibers in stage in which polymer coating is applied to the drawn fiber [5–7]. Instead of a solely polymer coating, a coating with particles of magnetic powder can be used. Appropriate composite coatings should be homogenous and thus enable reliable magnetic detection, while minimizing the side effects. After serial of experiments of dependence of coating velocity and concentration of magnetic powder in polymer on uniformity of coating, optimal conditions for coating of optical fiber with magnetic coating were reached. 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 coating without the application of 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 magnetic powders (hard ferrite, Sm-Co). SmCo5 was successfully used [5, 6] and now melt – spun Nd-Fe-B [8, 9] was chosen for investigation. 2. EXPERIMENTAL

For this investigation, the following starting materials were chosen: • Commercial multi-mode optical fiber. • Polymer: Poly (ethylene-co-vinyl-acetate) – EVA produced by DuPont under the commercial name ELVAX 265. Polymer was used in a form of toluene solutions with 20 mass % of polymer. • Permanent magnetic powder of Nd-Fe-B. After many experiments with monomode optical fiber and SmCo5 [5–7] it

3

Magnetic field sensing elements

973

was supposed that the best results for uniform coating and optimal coating speed would be got with coating with 50% Nd-Fe-B and 50% EVA. Experimental procedure was: • Dissolving EVA in toluene at 60°C, mixing 2.5 h. • Dispersing magnetic powder in EVA solution in toluene with 50% mass fraction. • Dispersion homogenization, ultrasound and mechanical mixing. • Removing of basic polymer coating of optical fiber in length of 2–3 cm • Coating of the same optical fiber with composite coating. • Positioning two fibers in glass tube, and cantilevering • Fixing the ends of glass tube. • Measurements of the changes in the intensity of transmitted light under the magnetic field. Scheme of optical fiber magnetic field sensing element (OFMSE) is presented on Fig. 1.

Fig. 1. – Optical fiber with composite magnetic coating for magnetic field sensing. 1 – glass tube; 2 – optical fiber with composite magnetic coating; 3 – optical fiber with basic polymer coating; 4 – glue.

3. RESULTS AND DISCUSSION

The influence of the applied external magnetic field on the change of intensity of the light signal propagated trough optical fiber sensor element was investigated. The light from the light emitting diode (λ = 849 nm) is launched to an optical fiber with deposited composite coating which had particles of permanent magnet. The optical signal is received by the optical fiber that is fixed at the other end of glass tube. The intensity of the light from this fiber is detected by a photo detector. The amplified output signal from the photo detector is connected to the data acquisition system based on the A/D card, personal computer and the specially developed software in Pascal. If the OFMSE is not in a magnetic field the ends of optical fibers 2, 3 (Fig. 1.) in a tube and collinear and the intensity of the propagated light is maximum. If the permanent magnet is approached to the sensing element, the light intensity decreases. Equipment for

974

D. Nedeljkovic et al.

4

measurements of change in intensity of light signal in external magnetic field is presented on Fig. 2.

Fig. 2. – Equipment for measurements of change in intensity of light signal in external magnetic field.

The strength of magnetic field and response of signal intensity of the sensing element are presented on Fig. 3. as dependence of attenuation of signal in the external magnetic field. It is obvious that the OFMSE shows sensibility on the magnetic field and reversibility of signal. Histeresis of OFMSE is presented on Fig. 4. as a dependence of photo detector voltage of strength of magnetic field.

Fig. 3. – Change in intensity of light signal in external magnetic field.

5

Magnetic field sensing elements

975

Fig. 4. – Histeresis of optical fiber magnetic sensor.

The presented results are from the preliminary examination. Composite coating which was made had good reversibility and sensitivity of OFMSE in external magnetic field. Goal of further experiments is improving of constructive characteristics of OFMSE by changing of length of optical fiber with composite magnetic coating. 4. CONCLUSION

In this paper possibility of new use of composite material optical fiber – magnetic powder/polymer was investigated and because of that, preliminary experiments have been done. The process of coating optical fiber with composite magnetic coating polymer/magnetic powder was developed. The polymer component of composite magnetic coating consisted of 50 mass % EVA dissolved in toluene and magnetic component was powder of Nd-Fe-B. It was shown that this configuration of OFMSE is sensitive and reversible in external magnetic field. In order to improve efficiency of OFMSE constructed in this way, it is necessary to test dependence of sensitivity and reversibility on type and concentration of magnetic powder in further experiments. Acknowledgement. This work has been supported by the Ministry of Science and Environmental Protection of the Republic of Serbia. REFERENCES 1. E. Udd, Fiber Optic Smart Structures Proc. IEEE, Vol 84, Iss 6, 1996, pp 884–894. 2. F. Bucholtz, D. M. Dagenais, K. P. Koo, S. Vohra, Recent developments in fiber optic magnetostrictive sensors, Proc. SPIE Vol 1367, 1991, pp. 226–235.

976

D. Nedeljkovic et al.

6

3. P. Extance, R. E. Jones, G. D. Pitt, Fibre optic sensors EP 131404/ 25, Jun 1984. 4. Y. C. Yang, K. S. Han, Smart. Mater. Struct. 11(2002) 337–345. 5. V. Radojevic, D. Nedeljkovic, N. Talijan, D. Trifunovic, R. Aleksic, Jour. of Magnetism and Magnetic Materials, (2004); 272–276 (e1755–e1756). 6. A. Milutinovic-Nikolic, N. Talijan, K. Jeremic, R. Aleksic, Mat. Lett. 56(3), (2002) 148–155. 7. A. Milutinovic-Nikolic, N. Talijan, R. Aleksic, Model of coating optical fibers with composite coating polymer-magnetic powder, Sci. Sinter. 32(2), (2000) 73–79. 8. Talijan Nadežda, Tomas Žak, Stajić-Trošić Jasna, Vladimir Menušenkov, Journal of Magnetism and Magnetic Materials; 258–259 (2003) 577–579. 9. N. Talijan, V. Ćosović, J. Stajić-Trošić, T. Zak, Journal of Magnetism and Magnetic Materials; (2004); 272–276 (e1911–e1912).