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Stress sensing with Co based ferrite composites
Journal of Magnetism and Magnetic Materials 242–245 (2002) 1460–1463
Stress sensing with Co based ferrite composites M. Pasqualea,*, C.P. Sassob, M. Vellutoa, S.H. Limc a
IEN Galileo Ferraris, Strada delle Cacce 91, 10135 Torino, Italy Politecnico di Torino, Dipartimento di Elettronica, Torino, Italy c Korea Institute of Science and Technology, Seoul, South Korea
b
Abstract We measure the magnetic and magneto-mechanical properties of polymer-bonded Co ferrites to analyze their behavior as stress sensors in a wide temperature and compressive stress range. The composites, possessing a magnetostriction of 30 10 6 and a Young’s modulus of 50 GPa, are good candidates for a range of sensing applications, with good chemical stability and good high frequency characteristics. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetostriction; Ferrites; Polymer-bonded composites; Magneto-elastic sensors
1. Introduction Co based ferrites are a promising magnetostrictive material for industrial use, since they possess intrinsic robustness in harsh environments, the ability to work at high frequency, and a price advantage over other magnetostrictive materials such as rare earth based alloys. Recently the class of Co based ferrites, a magnetic material with a high negative magnetostriction [1], has been applied to torsional stress sensing, an interesting field due to many possible applications in the automotive industry. This paper presents the magnetic and magneto-mechanical characteristics of a set of Co ferrite composites, which are bonded with a polymer binder. In order to test the magnetic and magnetomechanical properties, the ferrite powders formed with a phenol-type binder (3.5–3.8 wt%) are subjected to quasistatic magnetic fields using a conventional electromagnet positioned within a climate controlled chamber. Magnetic tests were performed at temperatures ranging from 201C and 1101C. During a set of magnetic measurements a compressive stress is also applied to verify the sensing characteristics of these composites. The high frequency magnetic characteristics up to 10 kHz are also investigated, but no macroscopic eddy currents can be *Corresponding author. E-mail address: [email protected] (M. Pasquale).
observed given the low conductivity of both the ferrite powder and the phenol based binder. It is shown that the magnetic and magneto-mechanical properties of the composites are sensitive to measurement temperature in the studied range but they can still be used successfully as sensors if temperature effects are properly taken into account. The composites posses a magnetostriction of the order of lE 10 5 and are particularly suitable for stress sensing in the 0–30 MPa range, given a 50 Gpa Young’s modulus.
2. Experimental Rectangular ferrite samples were obtained from Co ferrite powders of composition CoO–Fe2O3 bonded by a polymer. The samples were made by mixing the powders (under 45 mm) with a phenol-type binder (3.5–3.8 wt%) and by cold pressing with a typical compaction pressure of 0.5 GPa. Quasi-static magnetic measurements were made using an electromagnet with adjustable poles of 7 cm diameter, which is capable of producing a maximum field of 1.5 T. Dynamic (AC) measurements up to 10 kHz were made with a specially designed yoke of thin amorphous metal connected to a 5 kW power supply. The applied magnetic field was measured using a set of custom built tensiometer coils, while
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 1 0 3 - 9
M. Pasquale et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1460–1463
magnetization was measured using air-flux compensated pick-up coils. The rectangular samples with typical dimensions 10 mm in length and 3.5 mm in width and thickness were fitted with a resistive strain gauge on one long side, thus allowing for the easy application of a longitudinal compression. The samples can be mechanically loaded with a calibrated non-magnetic spring made of phosphorous bronze, where the load was measured by another resistive strain gauge. Strain measurements were made with resistive 120 O strain gauges connected in half bridge configuration to a digital signal processor to increase the signal to noise ratio. Temperature control was achieved within a 1 m3 volume controlled climate chamber. All signals (magnetic field, magnetization, strain and temperature) were fed into a four channel digital storage oscilloscope (with up to 14 bits vertical resolution) and then post processed and analyzed with a PC.
3. Analysis of results Typical experimental results are depicted in Fig. 1 for quasi-static magnetization and strain vs. applied field obtained on a composite sample of pure Co ferrite+ binder at room temperature. A typical coercive field of 18–20 kA/m is observed and a magnetization (J) of about 0.425 T is obtained at a maximum field of 440 kA/ m. The observed magnitude of J is slightly smaller than a saturation value of 0.458 T, which can be calculated from saturation magnetization (Js ) of Co ferrite (0.53 T) [2] and its volume fraction (86.5%). A peak strain of 38 10 6 is achieved at the same field. A substantial portion of the strain ( 20 10 6) can be achieved in the applied field range between 50 and 130 kA/m. With these characteristics the Co ferrite composites can be successfully used for the design of stress sensitive inductive
Fig. 1. Quasi-static (0.1 Hz) magnetization and strain vs. internal field in a pure Co ferrite composite bonded with a polymer binder. Coercive field is of the order of 20 kA/m and magnetization at 440 kA/m reaches 0.425 T.
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elements in AC circuits. For this purpose we have measured magnetic loops at room temperature under a constant unidirectional compressive stress up to 30 MPa. Upon application of stress, changes are observed in the coercive field and maximum permeability. In Fig. 2 it is shown that the maximum permeability increases dramatically when an increasing compressive stress is applied. The increase is between 80% and 120% with respect to the zero applied stress case. The behavior can be easily exploited for stress sensing, since changes in maximum permeability can be directly translated into inductance and voltage changes in a coupled electric circuit. To investigate the effect of temperature on the stress sensing properties of the Co ferrite composites we have measured quasi-static (0.1 Hz) hysteresis loops at different temperatures, under no applied stress or a 30 MPa compressive stress. The temperature range 201C up to 1101C was chosen to cover typical operating temperatures in the automotive environment. In Fig. 3 it is shown that temperature effects play a significant role in determining the permeability behavior of the Co ferrites. An 80% increase of the maximum relative permeability has been detected from 201C to 1101C at zero applied stress, probably due to an increase in domain wall mobility with increasing temperature. A reduced, but still notable increase in permeability can be observed when temperature is varied with a 30 MPa compressive stress (about 15%). Magnetic loops, instead, show a small typical decrease of Js with temperature, since the Curie point for this type of ferrites is 5201C [2]. Dynamic measurements were also performed; they show that the ferrite composites can be used at audio frequencies without the detrimental effect
Fig. 2. Permeability variation vs. applied stress (percentage increase normalized at zero stress). Measurements are performed at room temperature. The lower induction level can be achieved with fields of the order of 50 kA/m. The higher induction level requires fields of about 130 kA/m. The figure shows that the Co ferrite can be used as a high efficiency stress sensor.
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M. Pasquale et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1460–1463
dependence is connected to loss processes at the magnetic domain level. This behavior can be compared to the case of thin laminations of amorphous magnetic materials, where the low thickness does not permit the onset of classical losses [4]. The dynamic characteristics are mostly due to the high intrinsic resistivity of the ferrite (>107 mO cm) and the polymer binder, while the lower slope above 5 kHz cannot be easily explained. The loss behavior above 5 kHz can be related to the changes in conductivity due to the grain boundary capacitance, which may cause variations in the conductivity. This similar behavior was also observed in other ferrite media [5,6].
Fig. 3. Permeability variation vs. temperature at 0.35 T magnetization (percentage increase normalized at room temperature and zero stress) for applied compressive stresses of 0 and 30 MPa. This figure shows that the efficiency of stress sensing decreases with increasing temperature, but remains sufficient for detection.
Fig. 4. Total loss per cycle Ptot =f vs. Of obtained from high frequency hysteresis loop measurements. The loss follows a Of law up to 5 kHz in accordance with the predictions of the statistical theory of losses [3,4]. The figure shows that the polymer bonded Co ferrite powders are not affected by macroscopic eddy current losses up to about 5 kHz, since the presence of macroscopic eddy currents indicates a linear dependence of Ptot =f on f : The nonlinearity above 5 kHz may be due to capacitive coupling of the ferrite particles.
of eddy current shielding or the onset of macroscopic eddy currents. It is shown in Fig. 4 that the total loss per cycle Ptot =f increases with Of law up to about 5 kHz, without any macroscopic eddy current effect. According to the statistical theory of loss [3], eddy currents flowing in the whole sample cross-section lead to a linear dependence of loss on frequency, while the square root
4. Summary We have characterized polymer bonded Co ferrite composites for use as stress sensors. The results of the analysis show that maximum permeability can be used as a sensing parameter. Typical excitation fields should be in the range of 20–130 kA/m (above the coercive point). A very large change of about 100% in the maximum permeability is observed in the 0–30 MPa compressive stress interval (at room temperature and 0.35 T magnetization). Temperature also plays a role in determining the magnetization properties of bonded Co ferrites: the permeability increases about 60% at zero applied stress, when temperature increases form 20 to 1101C (Fig. 3) while this variation is reduced to about 15% when the sample is heated under a 30 MPa compressive stress. This implies that stress changes at high temperatures can be detected with a decreasing sensitivity, since both temperature and compressive stress tend to increase the squareness of the hysteresis loops. The polymer content may also be a part of the temperature related sensitivity since the mechanical coupling is strongly affected by the nature and characteristics of the binder. Also the magnetostriction constant is reduced by an order of magnitude due to the presence of the polymer binder, even though this is a desirable effect to allow for a wider stress sensing range. Another important issue is related to the relative softness of these Co ferrites: results found in the literature [1] report that a lower coercivity can be achieved by appropriate treatments, a matter to be discussed in future work, where the optimization of magnetic properties should be sought through proper bonding or sintering procedures.
Acknowledgements Financial support from the Italy-Korea cooperation research program is gratefully acknowledged. SHL also thanks Research Center for Advanced Magnetic
M. Pasquale et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1460–1463
Materials (an ERC at Chungnam National University) for a financial support.
References [1] Y. Chen, J. Snyder, K.W. Dennis, R.W. McCallum, D.C. Jiles, J. Appl. Phys. 87 (2000) 5798.
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[2] R.A. McCurrie, Ferromagnetic MaterialsFStructure and Properties, Academic Press, London, 1994, p. 124. [3] G. Bertotti, IEEE Trans. Magn. 28 (1992) 2599. [4] G. Herzer, H.R. Hilzinger, Phys. Scripta T24 (1988) 22. [5] M.J. Tung, W.C. Chang, C.S. Liu, T.Y. Liu, C.J. Chen, T.Y. Tseng, IEEE Trans. Magn. 29 (1993) 3526. [6] D. Stoppels, J. Magn. Magn. Mater. 160 (1996) 323.
Stress sensing with Co based ferrite composites M. Pasqualea,*, C.P. Sassob, M. Vellutoa, S.H. Limc a
IEN Galileo Ferraris, Strada delle Cacce 91, 10135 Torino, Italy Politecnico di Torino, Dipartimento di Elettronica, Torino, Italy c Korea Institute of Science and Technology, Seoul, South Korea
b
Abstract We measure the magnetic and magneto-mechanical properties of polymer-bonded Co ferrites to analyze their behavior as stress sensors in a wide temperature and compressive stress range. The composites, possessing a magnetostriction of 30 10 6 and a Young’s modulus of 50 GPa, are good candidates for a range of sensing applications, with good chemical stability and good high frequency characteristics. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Magnetostriction; Ferrites; Polymer-bonded composites; Magneto-elastic sensors
1. Introduction Co based ferrites are a promising magnetostrictive material for industrial use, since they possess intrinsic robustness in harsh environments, the ability to work at high frequency, and a price advantage over other magnetostrictive materials such as rare earth based alloys. Recently the class of Co based ferrites, a magnetic material with a high negative magnetostriction [1], has been applied to torsional stress sensing, an interesting field due to many possible applications in the automotive industry. This paper presents the magnetic and magneto-mechanical characteristics of a set of Co ferrite composites, which are bonded with a polymer binder. In order to test the magnetic and magnetomechanical properties, the ferrite powders formed with a phenol-type binder (3.5–3.8 wt%) are subjected to quasistatic magnetic fields using a conventional electromagnet positioned within a climate controlled chamber. Magnetic tests were performed at temperatures ranging from 201C and 1101C. During a set of magnetic measurements a compressive stress is also applied to verify the sensing characteristics of these composites. The high frequency magnetic characteristics up to 10 kHz are also investigated, but no macroscopic eddy currents can be *Corresponding author. E-mail address: [email protected] (M. Pasquale).
observed given the low conductivity of both the ferrite powder and the phenol based binder. It is shown that the magnetic and magneto-mechanical properties of the composites are sensitive to measurement temperature in the studied range but they can still be used successfully as sensors if temperature effects are properly taken into account. The composites posses a magnetostriction of the order of lE 10 5 and are particularly suitable for stress sensing in the 0–30 MPa range, given a 50 Gpa Young’s modulus.
2. Experimental Rectangular ferrite samples were obtained from Co ferrite powders of composition CoO–Fe2O3 bonded by a polymer. The samples were made by mixing the powders (under 45 mm) with a phenol-type binder (3.5–3.8 wt%) and by cold pressing with a typical compaction pressure of 0.5 GPa. Quasi-static magnetic measurements were made using an electromagnet with adjustable poles of 7 cm diameter, which is capable of producing a maximum field of 1.5 T. Dynamic (AC) measurements up to 10 kHz were made with a specially designed yoke of thin amorphous metal connected to a 5 kW power supply. The applied magnetic field was measured using a set of custom built tensiometer coils, while
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 1 0 3 - 9
M. Pasquale et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1460–1463
magnetization was measured using air-flux compensated pick-up coils. The rectangular samples with typical dimensions 10 mm in length and 3.5 mm in width and thickness were fitted with a resistive strain gauge on one long side, thus allowing for the easy application of a longitudinal compression. The samples can be mechanically loaded with a calibrated non-magnetic spring made of phosphorous bronze, where the load was measured by another resistive strain gauge. Strain measurements were made with resistive 120 O strain gauges connected in half bridge configuration to a digital signal processor to increase the signal to noise ratio. Temperature control was achieved within a 1 m3 volume controlled climate chamber. All signals (magnetic field, magnetization, strain and temperature) were fed into a four channel digital storage oscilloscope (with up to 14 bits vertical resolution) and then post processed and analyzed with a PC.
3. Analysis of results Typical experimental results are depicted in Fig. 1 for quasi-static magnetization and strain vs. applied field obtained on a composite sample of pure Co ferrite+ binder at room temperature. A typical coercive field of 18–20 kA/m is observed and a magnetization (J) of about 0.425 T is obtained at a maximum field of 440 kA/ m. The observed magnitude of J is slightly smaller than a saturation value of 0.458 T, which can be calculated from saturation magnetization (Js ) of Co ferrite (0.53 T) [2] and its volume fraction (86.5%). A peak strain of 38 10 6 is achieved at the same field. A substantial portion of the strain ( 20 10 6) can be achieved in the applied field range between 50 and 130 kA/m. With these characteristics the Co ferrite composites can be successfully used for the design of stress sensitive inductive
Fig. 1. Quasi-static (0.1 Hz) magnetization and strain vs. internal field in a pure Co ferrite composite bonded with a polymer binder. Coercive field is of the order of 20 kA/m and magnetization at 440 kA/m reaches 0.425 T.
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elements in AC circuits. For this purpose we have measured magnetic loops at room temperature under a constant unidirectional compressive stress up to 30 MPa. Upon application of stress, changes are observed in the coercive field and maximum permeability. In Fig. 2 it is shown that the maximum permeability increases dramatically when an increasing compressive stress is applied. The increase is between 80% and 120% with respect to the zero applied stress case. The behavior can be easily exploited for stress sensing, since changes in maximum permeability can be directly translated into inductance and voltage changes in a coupled electric circuit. To investigate the effect of temperature on the stress sensing properties of the Co ferrite composites we have measured quasi-static (0.1 Hz) hysteresis loops at different temperatures, under no applied stress or a 30 MPa compressive stress. The temperature range 201C up to 1101C was chosen to cover typical operating temperatures in the automotive environment. In Fig. 3 it is shown that temperature effects play a significant role in determining the permeability behavior of the Co ferrites. An 80% increase of the maximum relative permeability has been detected from 201C to 1101C at zero applied stress, probably due to an increase in domain wall mobility with increasing temperature. A reduced, but still notable increase in permeability can be observed when temperature is varied with a 30 MPa compressive stress (about 15%). Magnetic loops, instead, show a small typical decrease of Js with temperature, since the Curie point for this type of ferrites is 5201C [2]. Dynamic measurements were also performed; they show that the ferrite composites can be used at audio frequencies without the detrimental effect
Fig. 2. Permeability variation vs. applied stress (percentage increase normalized at zero stress). Measurements are performed at room temperature. The lower induction level can be achieved with fields of the order of 50 kA/m. The higher induction level requires fields of about 130 kA/m. The figure shows that the Co ferrite can be used as a high efficiency stress sensor.
1462
M. Pasquale et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1460–1463
dependence is connected to loss processes at the magnetic domain level. This behavior can be compared to the case of thin laminations of amorphous magnetic materials, where the low thickness does not permit the onset of classical losses [4]. The dynamic characteristics are mostly due to the high intrinsic resistivity of the ferrite (>107 mO cm) and the polymer binder, while the lower slope above 5 kHz cannot be easily explained. The loss behavior above 5 kHz can be related to the changes in conductivity due to the grain boundary capacitance, which may cause variations in the conductivity. This similar behavior was also observed in other ferrite media [5,6].
Fig. 3. Permeability variation vs. temperature at 0.35 T magnetization (percentage increase normalized at room temperature and zero stress) for applied compressive stresses of 0 and 30 MPa. This figure shows that the efficiency of stress sensing decreases with increasing temperature, but remains sufficient for detection.
Fig. 4. Total loss per cycle Ptot =f vs. Of obtained from high frequency hysteresis loop measurements. The loss follows a Of law up to 5 kHz in accordance with the predictions of the statistical theory of losses [3,4]. The figure shows that the polymer bonded Co ferrite powders are not affected by macroscopic eddy current losses up to about 5 kHz, since the presence of macroscopic eddy currents indicates a linear dependence of Ptot =f on f : The nonlinearity above 5 kHz may be due to capacitive coupling of the ferrite particles.
of eddy current shielding or the onset of macroscopic eddy currents. It is shown in Fig. 4 that the total loss per cycle Ptot =f increases with Of law up to about 5 kHz, without any macroscopic eddy current effect. According to the statistical theory of loss [3], eddy currents flowing in the whole sample cross-section lead to a linear dependence of loss on frequency, while the square root
4. Summary We have characterized polymer bonded Co ferrite composites for use as stress sensors. The results of the analysis show that maximum permeability can be used as a sensing parameter. Typical excitation fields should be in the range of 20–130 kA/m (above the coercive point). A very large change of about 100% in the maximum permeability is observed in the 0–30 MPa compressive stress interval (at room temperature and 0.35 T magnetization). Temperature also plays a role in determining the magnetization properties of bonded Co ferrites: the permeability increases about 60% at zero applied stress, when temperature increases form 20 to 1101C (Fig. 3) while this variation is reduced to about 15% when the sample is heated under a 30 MPa compressive stress. This implies that stress changes at high temperatures can be detected with a decreasing sensitivity, since both temperature and compressive stress tend to increase the squareness of the hysteresis loops. The polymer content may also be a part of the temperature related sensitivity since the mechanical coupling is strongly affected by the nature and characteristics of the binder. Also the magnetostriction constant is reduced by an order of magnitude due to the presence of the polymer binder, even though this is a desirable effect to allow for a wider stress sensing range. Another important issue is related to the relative softness of these Co ferrites: results found in the literature [1] report that a lower coercivity can be achieved by appropriate treatments, a matter to be discussed in future work, where the optimization of magnetic properties should be sought through proper bonding or sintering procedures.
Acknowledgements Financial support from the Italy-Korea cooperation research program is gratefully acknowledged. SHL also thanks Research Center for Advanced Magnetic
M. Pasquale et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 1460–1463
Materials (an ERC at Chungnam National University) for a financial support.
References [1] Y. Chen, J. Snyder, K.W. Dennis, R.W. McCallum, D.C. Jiles, J. Appl. Phys. 87 (2000) 5798.
1463
[2] R.A. McCurrie, Ferromagnetic MaterialsFStructure and Properties, Academic Press, London, 1994, p. 124. [3] G. Bertotti, IEEE Trans. Magn. 28 (1992) 2599. [4] G. Herzer, H.R. Hilzinger, Phys. Scripta T24 (1988) 22. [5] M.J. Tung, W.C. Chang, C.S. Liu, T.Y. Liu, C.J. Chen, T.Y. Tseng, IEEE Trans. Magn. 29 (1993) 3526. [6] D. Stoppels, J. Magn. Magn. Mater. 160 (1996) 323.