- Email: [email protected]
thermoplastic polymer composites
Composites Science and Technology 68 (2008) 3230–3233
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Magnetic levitation force measurement on HTSC ceramic/thermoplastic polymer composites S. Bhadrakumari a,*, P. Predeep b, A. Sheela c a
Department of Physics, M.S.M. College, Kayamkulam, Kerala 690 502, India Laboratory for Unconventional Electronics and Photonics, NIT, Kozhikode, Kerala 673 601, India c Department of Chemistry, M.S.M. College, Kayamkulam, Kerala 690 502, India b
a r t i c l e
i n f o
Article history: Received 18 April 2008 Received in revised form 5 August 2008 Accepted 7 August 2008 Available online 22 August 2008 Keywords: A. Composites A. Ceramics A. Polymer B. Magnetic levitation
a b s t r a c t Composite samples of YBa2Cu3O7 x (Y-123) and linear low density polyethylene (LLDPE) are prepared using Y-123 as filler. An experimental study of magnetic levitation force using a simple, inexpensive electronic balance measurement technique is presented. The repulsive force versus distance or magnetic field is also measured and this measurements show strong hysteretic behavior. This behavior is similar to the sintered superconductor ceramics and is consistent with the hysteresis in magnetization of superconductor. The volume fraction dependence and sample thickness dependence of the levitation force at fixed distance or field is also studied. Results suggest that these composite materials are most suitable for levitation applications. They are very strong which is needed to withstand high rate spinning. They are also very flexible in processing techniques and can be easily reshaped. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The possible applications of superconductor/polymer composites are in the areas of magnetic shielding and levitation where contact from one superconducting grain to another is not essential to the function. It is the property of diamagnetic flux exclusion and not the zero resistivity of these composites [1] makes it a potential candidate for these applications. Levitation of a magnet above a high Tc superconductor (HTSC) is a universally observed phenomenon and it is one of the most attractive properties for superconducting applications. It could be used to construct a levitated vehicle or bearings in rotating machinery. The latter may be a more practical and promising near-term application. There have been intensive studies on the magnetic levitation forces of high Tc superconductors for bearing applications and some simple prototype bearing devices have been made [2–4]. They use the levitation capability and lateral stiffness of ceramic superconductors to spin magnetic bearings at high speeds with a stable system configuration. Moon et al. [5,6] and some other workers [7,8] have quantitatively measured force versus the magnet/superconductor separation, mechanical stiffness and lateral force. Hysteretic behavior has been generally observed [9] and is explained in terms of the flux penetration and pinning mechanism [10]. In fact the flux pinning [10] can sustain a magnetic field gradient which is related to a macroscopic current. The strong diamagnetic behavior leads to remarkable phenomenon such as * Corresponding author. Tel.: +91 4762847776. E-mail address: [email protected] (S. Bhadrakumari). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.08.005
magnetic levitation. To achieve a large interaction force between a superconductor and a field source, it is important to obtain superconducting materials allowing for high current density extended to electrical paths as large as possible. Owing to the hysteretic magnetic behavior of HTSC, the interaction magnetic force can be either repulsive or attractive [11] leading respectively to magnetic levitation and suspension. High temperature superconducting materials, like many other ceramics, are inherently brittle in nature and therefore pose problems when they are to be moulded and die-cast in to pieces of even the simple shapes. It is rather difficult to fabricate them in to intricate shapes and bearing applications also demand high mechanical strength. Further HTSCs are plagued with compositional instability and they are susceptible to environmental degradation. This difficulty has been overcome to some extent by an ingenious use of high temperature superconductor/polymer composites with proper choice of polymeric materials to achieve the desired physical properties [12,13]. The polymer matrices provide more flexibility in mechanical properties and great machinability and protect the superconductor from environmental degeneration. The polymer processing techniques, such as compression, extrusion and injection moulding, could be used to form superconducting composites in to complex forms. Therefore it is very important to evaluate magnetic levitation properties for composite materials. Weinberger et al. [13,14] have studied the levitation force for YBaCuO/paraffin wax composite and suggested a prototype bearing using this type of composite [13]. Results show that composites are promising material for
S. Bhadrakumari et al. / Composites Science and Technology 68 (2008) 3230–3233
levitation bearing applications, but they did not report the volume fraction dependence (only one composition – 50 vol %). Besides, paraffin wax is not an ideal matrix material for bearing applications. This study suggests a versatile thermoplastic linear low density polyethylene (LLDPE) to be used as a matrix for the superconducting ceramic material YBa2Cu3O7 x (Y-123). LLDPE is a semicrystalline material. Semicrystalline polymers have a range of properties depending [15] on the amount of crystallinity. These properties can be further enhanced through the use of fillers. Also LLDPE has versatile properties like flexibility, elongation at break and puncture resistance compared to other branches of the polyethylene. Y-123 is selected [16] for the superconductor part since this ceramic material shows a more abrupt field penetration and they crystallize in orthorhombic oxygen deficient perovskite structure with a transition temperature of around 95 K. Superconductor/polymer composites offer better mechanical properties, greater machinability and easier shaping of the material in to useful devices. We have studied the levitation force against distance and magnetic field on these composites with a wide range of different compositions. We have also studied the sample thickness dependence of force for fixed composition material which gives information both on flux penetration and about practical considerations in applications. We have recently tested the ability of these composites to levitate a small magnet at liquid nitrogen temperature [17]. 2. Experimental The YBa2Cu3O7 x powder used in this study was prepared by solid state sintering reaction. The Y2O3, CuO and BaCO3 (in a 1:4:6 mass ratio) were thoroughly mixed by using a mortar and pestle. The mixed powder was kept inside a furnace for calcination at 900 °C for 36 h. Then the powder was reground and similar processes were repeated thrice. Again the sample masses were pulverized in to fine powders and pressed in to pellets of 2 mm thickness and 13 mm diameter by using a hydraulic press. These pellets were sintered at 950 °C in air for 36 h. After the sintering process, the furnace was cooled slowly and was kept at a temperature of 500 °C under O2 for about 24 h. Then the furnace was slowly cooled to room temperature at the rate of 25 °C/h. Powder X-ray diffraction pattern of the powder revealed no peaks other than those of YBa2Cu3O7 x. The superconductor (Y-123)/polymer (LLDPE) composites were prepared by melt-mixing of the components at various temperatures around the melting point of LLDPE and at varying mixing rates. This technique was adopted because it allows solvent-free mixing [18] for the Y-123 filler. Volume percentage of Y-123 ranged from 0% to 80% and all samples were 13 mm diameter and 2 mm thick. For measuring force dependence on sample thickness, nine samples with 13 mm diameter and different thickness were made from the same material – 50 vol % composite.
Fig. 1. The schematic diagram of the force versus distance measurement set-up.
3231
The schematic diagram of the set-up for force versus distance measurement is shown in Fig. 1. A very sensitive low compliance sensor element incorporated in a Wheatstone bridge was mounted on a microscope travel cross head. A small neodymium iron boron permanent magnet was glued on a small aluminum bar which was then attached at the end of sensor beam. The magnet in this dipole configuration was arranged to be normal to the sample surface. A sample was placed in a glass dewar and mounted against the thin front wall, as shown in the diagram. The magnet was positioned before the centre of the sample. The minimum approach distance of magnet and sample was limited by the thickness of the glass wall. By moving the travel cross head, the magnet could be moved toward or away from the sample and the distance could be precisely measured to a resolution of 1 lm. The force between the magnet and sample strained the sensor beam which gave a voltage output signal over the bridge and was detected by a digital meter. The system was calibrated in terms of voltage output (mV) against force (mN) and a linear relationship was found. The sample was cooled down in liquid nitrogen with the magnet 2 cm away. This was necessary to minimize the amount of magnetic flux trapped inside the superconducting sample. The magnet then was moved continuously towards the sample and the repulsive force was measured as a function of the magnet-sample separation. After taking the magnet close enough to just touch the glass window, it was then moved away from the sample. During this movement cycle, the hysteresis loop was plotted. The magnetic field strength along the permanent magnet axis versus distance was also measured using this set-up together with a Hall Gaussmeter probe. The measured force versus distance relationship could be converted to the relationship of the force versus magnetic field strength. For measuring the force against the volume percentage of the superconducting powder in the composites and sample thickness dependence at fixed distance or field, we used the simple electronic balance technique [17] (the apparatus is shown in Fig. 2). We have already measured the force versus volume percentage of the superconducting powder in these composites [17]. A small neodymium iron boron magnet was placed on the balance pan and the sample in liquid nitrogen container was located above the magnet as shown in the drawing. The weight change of the magnet was due to the repulsive force and the value of the force is given by F = M (kg) 9.8 (m s 2) (N), where M is the apparent mass change of the magnet. All samples were cooled down away from the magnet (zero field-cooled) and then located at the same
Fig. 2. The schematic drawing of the electronic balance measurement technique for measuring the forces at fixed distance.
3232
S. Bhadrakumari et al. / Composites Science and Technology 68 (2008) 3230–3233
position. The force was measured at the same distance and field strength. 3. Results and discussion The repulsive force versus distance between sample and magnet for the composites with 20, 40, 50 and 80 vol % of Y-123 is shown in Fig. 3. The measurement was made when moving the magnet towards the sample. By measuring magnetic field on the axis of the permanent magnet against distance from the pole face of the magnet, the data for force versus distance were converted to the relationship between force and magnetic field strength. Although the magnetic field at the surface of the disc sample may vary considerably due to the small magnet size, it is still appropriate to use the axial measurements to plot useful qualitative graphs as shown in Fig. 4. All samples have the same surface areas and the same field gradient along the radius. From the curves, all samples have similar force versus distance or force versus magnetic field relationship, which indicates the same levitation mechanism. The force increases with magnetic field because of increasing magnetization of superconductor. Fig. 5 gives the results of the force versus vol % of Y-123 at a fixed distance measured with the set-up shown in Fig. 2. From the graph, the values of force are approximately proportional to the volume percentage of Y-123 in composites. The drop at 80 vol % composition is due to the increasing air porosity in the
Fig. 3. Magnetic repulsive force versus the sample-magnet distance for the composites with 20, 40, 50, and 80 vol % of Y-123 powder.
Fig. 4. Repulsive force versus the magnetic field strength for the composites.
composite. The scanning electron microscope (SEM) photographs (unpublished material) showed that the 80 vol % composite contained some big voids which were due to the scarce polymer binder and the measured composite density actually decreased [17]. Counting the air voids as the third phase, the actual Y-123 superconductor phase decreases for the same dimension sample. The 100 vol % data was for a pure sintered Y-123 disc with about the same dimension of the composite sample. This result suggests that for achieving large levitation forces, higher load composites are desirable (but not exceeding 70% in this case). However, at very high loadings (typically, 70% and above), the porosity and the brittleness of materials also increase greatly. So in practice, a compromise has to make between high mechanical strength and high repulsive force. It should be noted that in these composites, even though the superconducting particles are basically separated from each other and there are no dc current paths, the levitation force is not diluted and the actual values are approximately proportional to the volume fraction of Y-123 filler. This indicates that the shielding current loops responsible for levitation is localized in individual grains and not between grains and the weak link currents at grain boundaries have a very small effect. It also suggests that this composite material can be used to replace the hard and brittle superconductor ceramics in levitation bearing applications. The hysteresis behaviour of force versus magnetic field for 50% Y-123 composite is shown in Fig. 6. This composite shows a large
Fig. 5. Force versus vol % of Y-123 for the composites. The 100% data was for a pure sintered Y-123 disc.
Fig. 6. The hysteresis loop of force versus magnetic field for 50% Y-123 composite.
S. Bhadrakumari et al. / Composites Science and Technology 68 (2008) 3230–3233
3233
Force increases linearly with thickness at beginning and begins to saturate at around 4 mm. This is qualitatively in agreement with the conclusion by Hellman, et al. [21], who observed the levitation height dependence on sample thickness and theoretically estimated the dependence based on the energy cost of flux penetration for Y-123 superconductor. Wang, et al. [22] measured the influence of sample thickness on levitation force for the sintered Y-123 sample and obtained very similar results to that on our composites. This proves again that this composite has the same levitation mechanism with that of the sintered sample and the intra-granular contribution of shielding currents plays an important role. For gaining maximum levitation force, it is necessary to increase sample thickness to the optimum. For this particular composite around 4 mm is sufficient. Thicker samples would increase the weight of composite required but gain very little increased levitation force. 4. Conclusions Fig. 7. Force versus magnetic field for 50% Y-123/LLDPE composites with different sample thickness.
Fig. 8. Force versus sample thickness at fixed field strength for 50% Y-123/LLDPE composite material.
Levitation forces for Y-123/LLDPE composites have been quantitatively measured against magnet – sample separation and magnetic field strength. They showed similar levitation properties and hysteretic behaviour as pure sintered superconductors and the intra-granular contribution of shielding currents plays an important role. This information is very useful for levitation applications. It is found that these composites have an appreciable levitation force even though the superconducting particles are basically separated from each other and there are no dc current paths. This proves that the shielding current loops which contribute to the levitation are localized in individual grains and not between grains. The grain boundaries in sintered sample act as weak links. So the associated magnetic forces are essentially not diluted in these composites. The levitation forces are also dependent on the material thickness in accordance with a flux penetration mechanism. A certain thickness is necessary to maximize the levitation force. In comparison with the pure high Tc superconducting ceramic, the composites offer a better chance to construct material in to complex shapes with better mechanical strength and low weight. Therefore this superconducting composite can be used to replace the hard and brittle superconductor ceramics in levitation bearing applications. References
hysteresis loop which is also found in sintered superconductors by other workers [5–8]. It corresponds to the magnetization hysteresis and is due to the trapped magnetic flux [19,20]. When the magnetic field increases, flux partially penetrates in to the superconductor and gives an increasing magnetization and therefore results in a repulsive force component. When the field decreases, some flux is trapped inside the superconductor and this gives a positive component of magnetization which results in an attractive force component. Therefore the total repulsive force will drop. The interaction between flux and superconductor is mainly intra-granular because this composite give similar hysteretic behavior. The results suggest that the levitation force and the height of levitation are dependent on thermal and magnetic history. It is found that levitation forces are dependent on sample thickness and it may be related to flux penetration. Nine disc samples with the same diameter but different thickness were fabricated from same composite material – 50% Y-123/LLDPE composite for this measurement. The force versus magnetic field for the different samples is shown in Fig. 7. The thicker sample gives higher force and stronger field dependence of the force. The force versus sample thickness at fixed distance and field strength is shown in Fig. 8.
[1] Tjukanov E, Cline RW, Krahn R, Hayden M, Reynolds MW, Hardy WN, et al. Phy Rev B 1987;36:7241. [2] Takahata R, Yotsuya T. IEEE Trans Magn 1991;27(2):2423. [3] Moon FC, Chang PZ. Appl Phys Lett 1990;56(4):397. [4] Weeks DE. Rev Sci Instrum 1990;61(1):195. [5] Moon FC, Yanoviak MM, Ware R. Appl Phys Lett 1988;52(18):1534. [6] Chang PZ, Moon FC, Hull JR, Mulcahy TM. J Appl Phys 1990;67(9):4358. [7] Davis LC. J Appl Phys 1990;67(5):2631. [8] Weeks DE. Rev Sci Instrum 1990;61(1):197. [9] Navau C et al. Phys Rev B 2001;64:214507. [10] Brandt EH. Appl Phys Lett 1988;53:1554. [11] Sarkar AK, Salyer LO, Kumar B. J Am Ceram Soc 1989;72:1247. [12] Nies CW, Smith BE, Nickel JM, Cao WW, Srinivasan TT, Bhalla AS, et al. Mater Res Bull 1988;23:623. [13] Unsworth J, Du J, Crosby BJ, Bryant P. Mater Res Bull 1991;26:1041. [14] Weinberger BR, Lynds L, Hull JR. Supercond Sci Technol 1990;3:381. [15] Barone JR. Composites A 2005;36:1518. [16] Ravi-Chandar K, Vipulanandan C, Dharmarajan N, Reddy KP. Intersoc Energy Covers Eng Cont 1988;23(2):525. [17] Bhadrakumari S, Predeep P. Supercond Sci Technol 2006;19:808–12. [18] Bruneel E, Persyn F, Hoste S. Supercond Sci Technol 1998;11:88–93. [19] Matsushita T. Cryogenics 1990;30:314. [20] Jin S, Sherwood RC, Gyorgy EM, Tiefel TH, van Dover RB, Nakahara S, et al. Appl Phys Lett 1989;54(6):584. [21] Hellman F, Gyorgy EM, Johnson Jr DW, O’Bryan HM, Sherwood RC. J Appl Phys 1988;63(2):447. [22] Wang J, Yanoviak MM, Raj R. J Am Ceram Soc 1989;72(5):846.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Magnetic levitation force measurement on HTSC ceramic/thermoplastic polymer composites S. Bhadrakumari a,*, P. Predeep b, A. Sheela c a
Department of Physics, M.S.M. College, Kayamkulam, Kerala 690 502, India Laboratory for Unconventional Electronics and Photonics, NIT, Kozhikode, Kerala 673 601, India c Department of Chemistry, M.S.M. College, Kayamkulam, Kerala 690 502, India b
a r t i c l e
i n f o
Article history: Received 18 April 2008 Received in revised form 5 August 2008 Accepted 7 August 2008 Available online 22 August 2008 Keywords: A. Composites A. Ceramics A. Polymer B. Magnetic levitation
a b s t r a c t Composite samples of YBa2Cu3O7 x (Y-123) and linear low density polyethylene (LLDPE) are prepared using Y-123 as filler. An experimental study of magnetic levitation force using a simple, inexpensive electronic balance measurement technique is presented. The repulsive force versus distance or magnetic field is also measured and this measurements show strong hysteretic behavior. This behavior is similar to the sintered superconductor ceramics and is consistent with the hysteresis in magnetization of superconductor. The volume fraction dependence and sample thickness dependence of the levitation force at fixed distance or field is also studied. Results suggest that these composite materials are most suitable for levitation applications. They are very strong which is needed to withstand high rate spinning. They are also very flexible in processing techniques and can be easily reshaped. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The possible applications of superconductor/polymer composites are in the areas of magnetic shielding and levitation where contact from one superconducting grain to another is not essential to the function. It is the property of diamagnetic flux exclusion and not the zero resistivity of these composites [1] makes it a potential candidate for these applications. Levitation of a magnet above a high Tc superconductor (HTSC) is a universally observed phenomenon and it is one of the most attractive properties for superconducting applications. It could be used to construct a levitated vehicle or bearings in rotating machinery. The latter may be a more practical and promising near-term application. There have been intensive studies on the magnetic levitation forces of high Tc superconductors for bearing applications and some simple prototype bearing devices have been made [2–4]. They use the levitation capability and lateral stiffness of ceramic superconductors to spin magnetic bearings at high speeds with a stable system configuration. Moon et al. [5,6] and some other workers [7,8] have quantitatively measured force versus the magnet/superconductor separation, mechanical stiffness and lateral force. Hysteretic behavior has been generally observed [9] and is explained in terms of the flux penetration and pinning mechanism [10]. In fact the flux pinning [10] can sustain a magnetic field gradient which is related to a macroscopic current. The strong diamagnetic behavior leads to remarkable phenomenon such as * Corresponding author. Tel.: +91 4762847776. E-mail address: [email protected] (S. Bhadrakumari). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.08.005
magnetic levitation. To achieve a large interaction force between a superconductor and a field source, it is important to obtain superconducting materials allowing for high current density extended to electrical paths as large as possible. Owing to the hysteretic magnetic behavior of HTSC, the interaction magnetic force can be either repulsive or attractive [11] leading respectively to magnetic levitation and suspension. High temperature superconducting materials, like many other ceramics, are inherently brittle in nature and therefore pose problems when they are to be moulded and die-cast in to pieces of even the simple shapes. It is rather difficult to fabricate them in to intricate shapes and bearing applications also demand high mechanical strength. Further HTSCs are plagued with compositional instability and they are susceptible to environmental degradation. This difficulty has been overcome to some extent by an ingenious use of high temperature superconductor/polymer composites with proper choice of polymeric materials to achieve the desired physical properties [12,13]. The polymer matrices provide more flexibility in mechanical properties and great machinability and protect the superconductor from environmental degeneration. The polymer processing techniques, such as compression, extrusion and injection moulding, could be used to form superconducting composites in to complex forms. Therefore it is very important to evaluate magnetic levitation properties for composite materials. Weinberger et al. [13,14] have studied the levitation force for YBaCuO/paraffin wax composite and suggested a prototype bearing using this type of composite [13]. Results show that composites are promising material for
S. Bhadrakumari et al. / Composites Science and Technology 68 (2008) 3230–3233
levitation bearing applications, but they did not report the volume fraction dependence (only one composition – 50 vol %). Besides, paraffin wax is not an ideal matrix material for bearing applications. This study suggests a versatile thermoplastic linear low density polyethylene (LLDPE) to be used as a matrix for the superconducting ceramic material YBa2Cu3O7 x (Y-123). LLDPE is a semicrystalline material. Semicrystalline polymers have a range of properties depending [15] on the amount of crystallinity. These properties can be further enhanced through the use of fillers. Also LLDPE has versatile properties like flexibility, elongation at break and puncture resistance compared to other branches of the polyethylene. Y-123 is selected [16] for the superconductor part since this ceramic material shows a more abrupt field penetration and they crystallize in orthorhombic oxygen deficient perovskite structure with a transition temperature of around 95 K. Superconductor/polymer composites offer better mechanical properties, greater machinability and easier shaping of the material in to useful devices. We have studied the levitation force against distance and magnetic field on these composites with a wide range of different compositions. We have also studied the sample thickness dependence of force for fixed composition material which gives information both on flux penetration and about practical considerations in applications. We have recently tested the ability of these composites to levitate a small magnet at liquid nitrogen temperature [17]. 2. Experimental The YBa2Cu3O7 x powder used in this study was prepared by solid state sintering reaction. The Y2O3, CuO and BaCO3 (in a 1:4:6 mass ratio) were thoroughly mixed by using a mortar and pestle. The mixed powder was kept inside a furnace for calcination at 900 °C for 36 h. Then the powder was reground and similar processes were repeated thrice. Again the sample masses were pulverized in to fine powders and pressed in to pellets of 2 mm thickness and 13 mm diameter by using a hydraulic press. These pellets were sintered at 950 °C in air for 36 h. After the sintering process, the furnace was cooled slowly and was kept at a temperature of 500 °C under O2 for about 24 h. Then the furnace was slowly cooled to room temperature at the rate of 25 °C/h. Powder X-ray diffraction pattern of the powder revealed no peaks other than those of YBa2Cu3O7 x. The superconductor (Y-123)/polymer (LLDPE) composites were prepared by melt-mixing of the components at various temperatures around the melting point of LLDPE and at varying mixing rates. This technique was adopted because it allows solvent-free mixing [18] for the Y-123 filler. Volume percentage of Y-123 ranged from 0% to 80% and all samples were 13 mm diameter and 2 mm thick. For measuring force dependence on sample thickness, nine samples with 13 mm diameter and different thickness were made from the same material – 50 vol % composite.
Fig. 1. The schematic diagram of the force versus distance measurement set-up.
3231
The schematic diagram of the set-up for force versus distance measurement is shown in Fig. 1. A very sensitive low compliance sensor element incorporated in a Wheatstone bridge was mounted on a microscope travel cross head. A small neodymium iron boron permanent magnet was glued on a small aluminum bar which was then attached at the end of sensor beam. The magnet in this dipole configuration was arranged to be normal to the sample surface. A sample was placed in a glass dewar and mounted against the thin front wall, as shown in the diagram. The magnet was positioned before the centre of the sample. The minimum approach distance of magnet and sample was limited by the thickness of the glass wall. By moving the travel cross head, the magnet could be moved toward or away from the sample and the distance could be precisely measured to a resolution of 1 lm. The force between the magnet and sample strained the sensor beam which gave a voltage output signal over the bridge and was detected by a digital meter. The system was calibrated in terms of voltage output (mV) against force (mN) and a linear relationship was found. The sample was cooled down in liquid nitrogen with the magnet 2 cm away. This was necessary to minimize the amount of magnetic flux trapped inside the superconducting sample. The magnet then was moved continuously towards the sample and the repulsive force was measured as a function of the magnet-sample separation. After taking the magnet close enough to just touch the glass window, it was then moved away from the sample. During this movement cycle, the hysteresis loop was plotted. The magnetic field strength along the permanent magnet axis versus distance was also measured using this set-up together with a Hall Gaussmeter probe. The measured force versus distance relationship could be converted to the relationship of the force versus magnetic field strength. For measuring the force against the volume percentage of the superconducting powder in the composites and sample thickness dependence at fixed distance or field, we used the simple electronic balance technique [17] (the apparatus is shown in Fig. 2). We have already measured the force versus volume percentage of the superconducting powder in these composites [17]. A small neodymium iron boron magnet was placed on the balance pan and the sample in liquid nitrogen container was located above the magnet as shown in the drawing. The weight change of the magnet was due to the repulsive force and the value of the force is given by F = M (kg) 9.8 (m s 2) (N), where M is the apparent mass change of the magnet. All samples were cooled down away from the magnet (zero field-cooled) and then located at the same
Fig. 2. The schematic drawing of the electronic balance measurement technique for measuring the forces at fixed distance.
3232
S. Bhadrakumari et al. / Composites Science and Technology 68 (2008) 3230–3233
position. The force was measured at the same distance and field strength. 3. Results and discussion The repulsive force versus distance between sample and magnet for the composites with 20, 40, 50 and 80 vol % of Y-123 is shown in Fig. 3. The measurement was made when moving the magnet towards the sample. By measuring magnetic field on the axis of the permanent magnet against distance from the pole face of the magnet, the data for force versus distance were converted to the relationship between force and magnetic field strength. Although the magnetic field at the surface of the disc sample may vary considerably due to the small magnet size, it is still appropriate to use the axial measurements to plot useful qualitative graphs as shown in Fig. 4. All samples have the same surface areas and the same field gradient along the radius. From the curves, all samples have similar force versus distance or force versus magnetic field relationship, which indicates the same levitation mechanism. The force increases with magnetic field because of increasing magnetization of superconductor. Fig. 5 gives the results of the force versus vol % of Y-123 at a fixed distance measured with the set-up shown in Fig. 2. From the graph, the values of force are approximately proportional to the volume percentage of Y-123 in composites. The drop at 80 vol % composition is due to the increasing air porosity in the
Fig. 3. Magnetic repulsive force versus the sample-magnet distance for the composites with 20, 40, 50, and 80 vol % of Y-123 powder.
Fig. 4. Repulsive force versus the magnetic field strength for the composites.
composite. The scanning electron microscope (SEM) photographs (unpublished material) showed that the 80 vol % composite contained some big voids which were due to the scarce polymer binder and the measured composite density actually decreased [17]. Counting the air voids as the third phase, the actual Y-123 superconductor phase decreases for the same dimension sample. The 100 vol % data was for a pure sintered Y-123 disc with about the same dimension of the composite sample. This result suggests that for achieving large levitation forces, higher load composites are desirable (but not exceeding 70% in this case). However, at very high loadings (typically, 70% and above), the porosity and the brittleness of materials also increase greatly. So in practice, a compromise has to make between high mechanical strength and high repulsive force. It should be noted that in these composites, even though the superconducting particles are basically separated from each other and there are no dc current paths, the levitation force is not diluted and the actual values are approximately proportional to the volume fraction of Y-123 filler. This indicates that the shielding current loops responsible for levitation is localized in individual grains and not between grains and the weak link currents at grain boundaries have a very small effect. It also suggests that this composite material can be used to replace the hard and brittle superconductor ceramics in levitation bearing applications. The hysteresis behaviour of force versus magnetic field for 50% Y-123 composite is shown in Fig. 6. This composite shows a large
Fig. 5. Force versus vol % of Y-123 for the composites. The 100% data was for a pure sintered Y-123 disc.
Fig. 6. The hysteresis loop of force versus magnetic field for 50% Y-123 composite.
S. Bhadrakumari et al. / Composites Science and Technology 68 (2008) 3230–3233
3233
Force increases linearly with thickness at beginning and begins to saturate at around 4 mm. This is qualitatively in agreement with the conclusion by Hellman, et al. [21], who observed the levitation height dependence on sample thickness and theoretically estimated the dependence based on the energy cost of flux penetration for Y-123 superconductor. Wang, et al. [22] measured the influence of sample thickness on levitation force for the sintered Y-123 sample and obtained very similar results to that on our composites. This proves again that this composite has the same levitation mechanism with that of the sintered sample and the intra-granular contribution of shielding currents plays an important role. For gaining maximum levitation force, it is necessary to increase sample thickness to the optimum. For this particular composite around 4 mm is sufficient. Thicker samples would increase the weight of composite required but gain very little increased levitation force. 4. Conclusions Fig. 7. Force versus magnetic field for 50% Y-123/LLDPE composites with different sample thickness.
Fig. 8. Force versus sample thickness at fixed field strength for 50% Y-123/LLDPE composite material.
Levitation forces for Y-123/LLDPE composites have been quantitatively measured against magnet – sample separation and magnetic field strength. They showed similar levitation properties and hysteretic behaviour as pure sintered superconductors and the intra-granular contribution of shielding currents plays an important role. This information is very useful for levitation applications. It is found that these composites have an appreciable levitation force even though the superconducting particles are basically separated from each other and there are no dc current paths. This proves that the shielding current loops which contribute to the levitation are localized in individual grains and not between grains. The grain boundaries in sintered sample act as weak links. So the associated magnetic forces are essentially not diluted in these composites. The levitation forces are also dependent on the material thickness in accordance with a flux penetration mechanism. A certain thickness is necessary to maximize the levitation force. In comparison with the pure high Tc superconducting ceramic, the composites offer a better chance to construct material in to complex shapes with better mechanical strength and low weight. Therefore this superconducting composite can be used to replace the hard and brittle superconductor ceramics in levitation bearing applications. References
hysteresis loop which is also found in sintered superconductors by other workers [5–8]. It corresponds to the magnetization hysteresis and is due to the trapped magnetic flux [19,20]. When the magnetic field increases, flux partially penetrates in to the superconductor and gives an increasing magnetization and therefore results in a repulsive force component. When the field decreases, some flux is trapped inside the superconductor and this gives a positive component of magnetization which results in an attractive force component. Therefore the total repulsive force will drop. The interaction between flux and superconductor is mainly intra-granular because this composite give similar hysteretic behavior. The results suggest that the levitation force and the height of levitation are dependent on thermal and magnetic history. It is found that levitation forces are dependent on sample thickness and it may be related to flux penetration. Nine disc samples with the same diameter but different thickness were fabricated from same composite material – 50% Y-123/LLDPE composite for this measurement. The force versus magnetic field for the different samples is shown in Fig. 7. The thicker sample gives higher force and stronger field dependence of the force. The force versus sample thickness at fixed distance and field strength is shown in Fig. 8.
[1] Tjukanov E, Cline RW, Krahn R, Hayden M, Reynolds MW, Hardy WN, et al. Phy Rev B 1987;36:7241. [2] Takahata R, Yotsuya T. IEEE Trans Magn 1991;27(2):2423. [3] Moon FC, Chang PZ. Appl Phys Lett 1990;56(4):397. [4] Weeks DE. Rev Sci Instrum 1990;61(1):195. [5] Moon FC, Yanoviak MM, Ware R. Appl Phys Lett 1988;52(18):1534. [6] Chang PZ, Moon FC, Hull JR, Mulcahy TM. J Appl Phys 1990;67(9):4358. [7] Davis LC. J Appl Phys 1990;67(5):2631. [8] Weeks DE. Rev Sci Instrum 1990;61(1):197. [9] Navau C et al. Phys Rev B 2001;64:214507. [10] Brandt EH. Appl Phys Lett 1988;53:1554. [11] Sarkar AK, Salyer LO, Kumar B. J Am Ceram Soc 1989;72:1247. [12] Nies CW, Smith BE, Nickel JM, Cao WW, Srinivasan TT, Bhalla AS, et al. Mater Res Bull 1988;23:623. [13] Unsworth J, Du J, Crosby BJ, Bryant P. Mater Res Bull 1991;26:1041. [14] Weinberger BR, Lynds L, Hull JR. Supercond Sci Technol 1990;3:381. [15] Barone JR. Composites A 2005;36:1518. [16] Ravi-Chandar K, Vipulanandan C, Dharmarajan N, Reddy KP. Intersoc Energy Covers Eng Cont 1988;23(2):525. [17] Bhadrakumari S, Predeep P. Supercond Sci Technol 2006;19:808–12. [18] Bruneel E, Persyn F, Hoste S. Supercond Sci Technol 1998;11:88–93. [19] Matsushita T. Cryogenics 1990;30:314. [20] Jin S, Sherwood RC, Gyorgy EM, Tiefel TH, van Dover RB, Nakahara S, et al. Appl Phys Lett 1989;54(6):584. [21] Hellman F, Gyorgy EM, Johnson Jr DW, O’Bryan HM, Sherwood RC. J Appl Phys 1988;63(2):447. [22] Wang J, Yanoviak MM, Raj R. J Am Ceram Soc 1989;72(5):846.