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A self-powered AC magnetic sensor based on piezoelectric nanogenerator
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 455503 (http://iopscience.iop.org/0957-4484/25/45/455503) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 193.140.21.152 This content was downloaded on 28/12/2014 at 12:09
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 455503 (6pp)
doi:10.1088/0957-4484/25/45/455503
A self-powered AC magnetic sensor based on piezoelectric nanogenerator Aifang Yu1, Ming Song1, Yan Zhang1, Jinzong Kou1, Junyi Zhai1 and Zhong Lin Wang1,2 1
Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, People’s Republic of China 2 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, USA E-mail: [email protected] and [email protected] Received 16 July 2014, revised 12 September 2014 Accepted for publication 16 September 2014 Published 21 October 2014 Abstract
An AC magnetic field, which is a carrier of information, is distributed everywhere and is continuous. How to use and detect this field has been an ongoing topic over the past few decades. Conventional magnetic sensors are usually based on the Hall Effect, the fluxgate, a superconductor quantum interface or magnetoelectric or magnetoresistive sensing. Here, a flexible, simple, low-cost and self-powered active piezoelectric nanogenerator (NG) is successfully demonstrated as an AC magnetic field sensor at room temperature. The amplitude and frequency of a magnetic field can both be accurately sensed by the NG. The output voltage of the NG has a good linearity with a measured magnetic field. The detected minute magnetic field is as low as 1.2 × 10−7 tesla, which is 400 times greater than a commercial magnetic sensor that uses the Hall Effect. In comparison to the existing technologies, an NG is a roomtemperature self-powered active sensor that is very simple and very cheap for practical applications. Keywords: magnetic sensor, piezoelectric nanogenerator, self-powered (Some figures may appear in colour only in the online journal) Alternating current (AC) magnetic fields, which can also be carriers of information, occur everywhere and are continuous. How to utilize and detect this energy source has been an ongoing topic over the past few decades. Traditionally, magnetic sensors are based on the Hall Effect, and the sensitivity is 0.5 Oe or 5 × 10−5 tesla. The best commercial magnetic sensor is made up of superconducting quantum interface devices (SQUIDs) [10], and the highest sensitivity is 10−15 tesla. However, it can only be achieved at an ultra-low temperature (less than 4 K). In addition, the instruments and the maintenance costs are expensive. Another important type of magnetic sensor is the giant magnetoresistance (GMR) multilayer sensor [11]. At room temperature, the maximum sensitivity of this sensor is around 4 × 10−10 tesla. Recently, cheap magnetic sensors have been developed using magnetoelectric (ferroelectric/magnetostrictive) composites [12]. These composites demonstrated high sensitivity (1 × 10−12 tesla) at room temperature. However, since ferroelectric
Introduction By using piezoelectric nanowires (NWs) [1], an innovative nanodevice, a piezoelectric nanogenerator (NG) can be utilized as a power source or as a self-powered active sensor. When an NG is used as a power source, it can convert environmental energy produced by small-scale physical action into electric energy. In the past few years, NGs have successfully been used to convert external mechanical energy [2], biomechanical energy [3], thermal energy [4] and magnetic force [5] into electricity. The general characteristics of mechanical actions, such as frequency, magnitude and striking rate, are generally reflected in the output electrical signals of the NG. Therefore, an NG can be used as a self-powered active sensor for monitoring or detecting vortex capture [6], ambient wind velocity [7], low frequency vibrations [8], automobile velocities [9], etc. 0957-4484/14/455503+06$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
A Yu et al
Nanotechnology 25 (2014) 455503
Figure 1. (a) Morphology of the ZnO-textured film. The insert is a picture of the actual device. (b) Schematic diagram of the self-powered AC
magnetic sensor and the corresponding measurement set-up. (c) Proposed mechanism of the self-powered active magnetic sensor.
measurement set-up. The sensor is a cantilever structure with a metallic NdFeB disk at its tip. The NdFeB disk serves as a sensitive unit of the magnetic field. The NG was attached to a rectangle Kapton film, which is used as a cantilever. The Kapton film has excellent mechanical properties. Its tensile strength is about 200 MPa at room temperature. That means it has enough rigidity to function as a reliable cantilever with repeatable bent and unbent cycles. In reference to the material parameters and the geometrical shape of the cantilever: L = 80 mm, W = 40 mm, T = 0.5 mm and E = 3.3 GPa, and the NdFeB proof mass is about 1 g; the resonance frequency is calculated by equation (4) to be 10.1 Hz, which coincides with the experimental result. One end of the cantilever was fixed, and the other end was located in the middle of the solenoid. If the alternating current flows in the solenoid, the NdFeB disk will be attracted by the electromagnetic force. Once the NG is strained, an electric signal will be generated due to the piezoelectric effect. When the direction of the magnetic field is upward, the body of the NG will be bent upward under compressive stress. Because the NWs are densely packed and perpendicular to the substrate, the compressive stress will result in a tensile strain along the NWs’ direction ( the c-axis direction), thus creating a piezopotential drop from the tips of the NWs to the roots, as shown in figure 1(c). By the same token, when the direction of the magnetic field is downward, tensile stress will result in the body of the NG (e.g. a compressive strain along the NWs’
materials have a pyroelectric effect, the best sensitivity had to be reached at a constant environment temperature. Moreover, most of these sensors rely on an externally supplied power source. In this paper, a flexible, simple and low-cost piezoelectric NG is successfully demonstrated as a self-powered active sensor for detecting the frequency and amplitude of an AC magnetic field at room temperature. The output voltage of the NG has a good linearity with the measured magnetic field. The detected minute magnetic field is as low as 1.2 × 10−7 tesla under a resonance frequency of 9.9 Hz. Compared to the existing magnetic sensor technology, NGs offer a new way to lower costs and increase sensitivity. Results and discussion The flexible, small, lightweight NG used in this study was reported previously [8, 13]. Its effective area is about 1.5 cm × 1.2 cm. Its core is comprised of densely packed ZnO NWs. Figure 1(a) shows the morphology of the ZnO NWtextured film grown at the surface of a flexible Kapton (PI) film substrate by a wet chemical method. The top view of the ZnO-textured film is shown in the top-right insert figure of figure 1(a). The intersected hexagonal structure clearly demonstrates that the thin film is constituted of high-density NWs. Figure 1(b) shows a schematic diagram of the selfpowered AC magnetic sensor and the corresponding 2
A Yu et al
Nanotechnology 25 (2014) 455503
Figure 2. Output voltage of the sensor at a forward connection (a) and a reversed connection (b) to the measurement system under a fixed frequency of 11 Hz. (c) Output signals of the sensor when the magnet was removed. (d) Results of the ‘linear superposition’ tests.
NW-textured film and the frequency of the external force. There is a maximum of C at the resonance. K is determined by the NdFeB disk. Z is the impedance of the intrinsic capacitance and resistance within the piezoelectric circuit. It can be seen that output voltage V is the function of magnetic field H(t). So, all of the information of H(t) should be reflected in Voc. Moreover, they have a linear relationship. The characteristics of the corresponding magnetic field can be derived and calculated from the electric signal. Figure 2 gives data of the input magnetic signal and the output voltage obtained from the NG. The converted output voltage from the magnetic field was recorded via an oscilloscope. It can be seen in figures 2(a) and (b) that the obtained electric signal is also alterative, following the frequency and waveform of the magnetic signal with the sinusoidal mode electrical input. For the forward connection in figure 2(a), the measured maximum output voltage is 1.0 mV at the driving frequency of 11 Hz. When the measurement system is reversely connected to the sensor, the output signal is also reversed in figure 2(b). So, the output signals satisfy the switching-polarity test [15]. In addition, when the magnet is removed and the cantilever cannot be bent, the output signal decreases, which rules out the possibility that the signal is coming from the electromagnetic induction. Furthermore, the linear superposition test is carried out to prove the correctness of the output signals in figure 2(d). NG A with 1.12 mV of output voltage is connected to NG B (output voltage of
direction); thus, the roots of the NWs have a higher piezopotential than the top ends. These piezopotential distributions will generate induced charges in the top and bottom electrodes as a result of the electron flow in order to minimize the total energy and will consequently generate an output voltage and current in the external load. The magnetic force (F) between the NdFeB disk and the solenoid can be approximately expressed as F (t ) = ABSH (t)
(1)
where A is a constant, B is the surface magnetic flux density of the NdFeB disk, S is the cross-section area of the NdFeB disk and H is the time dependence of the AC magnetic field to be sensed. In this experiment, because NG was put in the middle of the solenoid, the magnetic force can be approximated to be only the function of time. Assuming a small uniform mechanical strain, the strain of the NG increased when the cantilever amplitude increased by external force for the NG with the cantilever structure [14]. Consequently, the vibration amplitude increases as the amplitude of the AC magnetic field increases linearly. Therefore, the open circuit voltage Voc and current Ioc can be given by Voc ≈ −C.K H (t )
(2)
I oc = Voc Z
(3)
where C is the parameters of the device and is determined by the material parameters of the cantilever as well as the ZnO 3
A Yu et al
Nanotechnology 25 (2014) 455503
Figure 3. (a) Response of the NG to the AC magnetic field with different frequencies. (b) Magnetoelectric coupling versus the frequency.
that at 5 Hz and 13 Hz under the same magnetic field. Because of the magnetic sensor studied in this paper with a cantilever structure, the sensor’s frequency characteristics were demonstrated in figure 3(b). It can be seen that the resonance frequency is about 9.9 Hz. If the sensor works around the resonance frequency, high output voltage will be obtained under the same magnetic field. So, the highest output voltage was observed at 8 Hz in figure 3(a). According to the Bernoulli–Euler theory of a beam [16], the resonance
3.42 mV) in the series. The output voltage (4.47 mV) is enhanced and approximately equal to the sum of the signal from NG A and NG B. Further, the response of the NG to the magnetic field with different frequencies was studied. After changing the frequency of the magnetic field, the corresponding frequency was detected by the NG in figure 3(a). The NG can accurately detect the frequency of the magnetic signals. Notably, the output voltage of the NG at 8 Hz is significantly higher than 4
A Yu et al
Nanotechnology 25 (2014) 455503
frequency of the fundamental bending vibration of the cantilever is expressed by f=
1 2π
EWt 3 4ml 3
(4)
where m is the proof of mass of the cantilever; E is the Young’s modulus of the cantilever material; and L, W and t are the length, width and thickness of the cantilever, respectively. Therefore, changing the length of the cantilever and mass of the NdFeB disk can effectively modulate the resonance frequency of the cantilever. Through changing these two parameters, the NG magnetic sensor can match the frequency in the detecting environment. The response of the NG magnetic sensor to a different magnetic field with sine mode was also investigated by changing the output voltage of the function generator, as shown in figure 4(a). It can be seen that the output voltage of NG increases with the increased amplitude of the magnetic field. To measure the magnetic field dependence of the output voltage of the NG in a wide range, a lock-in amplifier was used to power a Helmholtz coil to generate the AC magnetic field and measure the voltage induced by the NG. The detected voltages in figure 4(b) were measured under the resonance frequency of the NG cantilever. Figure 4(b) exhibits that the output voltage has a good linear relationship with the magnetic field (at the location of the sensor) in a broad region from 1.2 × 10−7 tesla to 2.4 × 10−4 tesla. Since the maximum AC magnetic field generated by our Helmholtz coil is 2.4 × 10−4 tesla, we cannot measure the higher AC magnetic field. Our sensor can be used to measure a higher AC magnetic field than the field in figure 4(b). Such a broad linear region is important and is easy for circuit design and data processing. This result demonstrates the possibility of using an NG as a self-powered sensor to monitor the amplitude of a magnetic field. One can calculate the magnetic field from the obtained output voltage of the NG according to the linear relationship between them. The detected lowest magnetic field is 1.2 × 10−7 tesla at room temperature. So far, the amplitude and frequency of the magnetic field can all be sensed by the NG. Compared to traditional techniques for magnetic field monitoring, the NG-based sensor has the following advantages: (1) it is a self-powered active sensor that does not require an external power source to drive the sensor; (2) its fabrication is very simple and very cheap; (3) it works well at room temperature.
Figure 4. (a) Response of the NG to the AC magnetic field with different amplitudes. (b)Sensitivity of the NG under the resonance frequency.
Experimental Conclusions
The ZnO NW-textured film was first grown at the surface of a flexible Kapton (PI) film substrate by a wet chemical method at 75 °C for 12 h. The nutrient solution we used in the chemical growth process of the ZnO densely packed NW-textured films was 100 mM of Zn (NO3)2.6H2O and 10 mM hexamethylenetetramine (HMTA). The magnetic field to be sensed was produced by a solenoid in which the magnetic field was modulated by a function generator (SR345). The Helmholtz coils that were used consist of two identical
In summary, we demonstrated a flexible, simple and cheap piezoelectric NG as an ultra-high sensitive sensor for detecting an AC magnetic field. The output voltage of the sensor has a good linearity with a measured magnetic field. The detected minute magnetic field is as low as 1.2 × 10−7 tesla. This kind of self-powered magnetic sensor has potential applications for environmental monitoring, diagnostic medicine, magnetic global position systems and defense technology. 5
A Yu et al
Nanotechnology 25 (2014) 455503
circular coils. Each coil has 100 turns. The coil diameter is 7 cm, and the distance between the two coils is 3.5 cm. The obtained magnetic field is uniform in the center of the coils and is well controlled the by input voltage.
[5] Cui N, Wu W, Zhao Y, Bai S, Meng L, Qin Y and Wang Z L 2012 Nano Lett. 12 3701–5 [6] Xu S, Qin Y, Xu C, Wei Y, Yang R and Wang Z L 2010 Nat. Nanotechnology 5 366–73 [7] Zhang R, Lin L, Jing Q, Wu W, Zhang Y, Jiao Z, Yan L, Han R P and Wang Z L 2012 Energy Environ. Sci. 5 8528–33 [8] Yu A, Jiang P and Wang Z L 2012 Nano Energy 1 418–23 [9] Lin L, Hu Y, Xu C, Zhang Y, Zhang R, Wen X and Wang Z L 2012 Nano Energy 2 75–81 [10] Weinstock H 1996 SQUID Sensors: Fundamentals, Fabrication, and Applications (New York: Springer) 329 [11] Baibich M N, Broto J, Fert A, Van Dau F N, Petroff F, Etienne P, Creuzet G, Friederich A and Chazelas J 1988 Phys. Rev. Lett. 61 2472 [12] Zhai J, Xing Z, Dong S, Li J and Viehland D 2006 Appl. Phys. Lett. 88 062510–062510 [13] Hu Y, Zhang Y, Xu C, Lin L, Snyder R L and Wang Z L 2011 Nano Lett. 11 2572–7 [14] Maugin G A 1988 Continuum Mechanics of Electromagnetic Solids (New York: Elsevier Science) p xxii 598 p [15] Yang R S, Qin Y, Li C, Dai L M and Wang Z L 2009 Appl. Phys. Lett. 94 022905 [16] Graff K F 1975 Wave Motion in Elastic Solids Courier (Mineola, NY: Dover) 11–13
Acknowledgement Our research was supported by the ‘thousands talents’ program for pioneer researcher and his innovation team, China and by the Knowledge Innovation Program of the Chinese Academy of Sciences, Grant No. KJCX2-YW-M13. We also thank Weiwei Wu for the fabrication of the ZnO seed layer. References [1] Wang Z L and Song J 2006 Science 312 242–6 [2] Qin Y, Wang X and Wang Z L 2008 Nature 451 809–13 [3] Yang R, Qin Y, Li C, Zhu G and Wang Z L 2009 Nano Lett. 9 1201–5 [4] Yang Y, Wang S, Zhang Y and Wang Z L 2012 Nano Lett. 12 6408–13
6
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My IOPscience
A self-powered AC magnetic sensor based on piezoelectric nanogenerator
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 455503 (http://iopscience.iop.org/0957-4484/25/45/455503) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 193.140.21.152 This content was downloaded on 28/12/2014 at 12:09
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 455503 (6pp)
doi:10.1088/0957-4484/25/45/455503
A self-powered AC magnetic sensor based on piezoelectric nanogenerator Aifang Yu1, Ming Song1, Yan Zhang1, Jinzong Kou1, Junyi Zhai1 and Zhong Lin Wang1,2 1
Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, People’s Republic of China 2 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, USA E-mail: [email protected] and [email protected] Received 16 July 2014, revised 12 September 2014 Accepted for publication 16 September 2014 Published 21 October 2014 Abstract
An AC magnetic field, which is a carrier of information, is distributed everywhere and is continuous. How to use and detect this field has been an ongoing topic over the past few decades. Conventional magnetic sensors are usually based on the Hall Effect, the fluxgate, a superconductor quantum interface or magnetoelectric or magnetoresistive sensing. Here, a flexible, simple, low-cost and self-powered active piezoelectric nanogenerator (NG) is successfully demonstrated as an AC magnetic field sensor at room temperature. The amplitude and frequency of a magnetic field can both be accurately sensed by the NG. The output voltage of the NG has a good linearity with a measured magnetic field. The detected minute magnetic field is as low as 1.2 × 10−7 tesla, which is 400 times greater than a commercial magnetic sensor that uses the Hall Effect. In comparison to the existing technologies, an NG is a roomtemperature self-powered active sensor that is very simple and very cheap for practical applications. Keywords: magnetic sensor, piezoelectric nanogenerator, self-powered (Some figures may appear in colour only in the online journal) Alternating current (AC) magnetic fields, which can also be carriers of information, occur everywhere and are continuous. How to utilize and detect this energy source has been an ongoing topic over the past few decades. Traditionally, magnetic sensors are based on the Hall Effect, and the sensitivity is 0.5 Oe or 5 × 10−5 tesla. The best commercial magnetic sensor is made up of superconducting quantum interface devices (SQUIDs) [10], and the highest sensitivity is 10−15 tesla. However, it can only be achieved at an ultra-low temperature (less than 4 K). In addition, the instruments and the maintenance costs are expensive. Another important type of magnetic sensor is the giant magnetoresistance (GMR) multilayer sensor [11]. At room temperature, the maximum sensitivity of this sensor is around 4 × 10−10 tesla. Recently, cheap magnetic sensors have been developed using magnetoelectric (ferroelectric/magnetostrictive) composites [12]. These composites demonstrated high sensitivity (1 × 10−12 tesla) at room temperature. However, since ferroelectric
Introduction By using piezoelectric nanowires (NWs) [1], an innovative nanodevice, a piezoelectric nanogenerator (NG) can be utilized as a power source or as a self-powered active sensor. When an NG is used as a power source, it can convert environmental energy produced by small-scale physical action into electric energy. In the past few years, NGs have successfully been used to convert external mechanical energy [2], biomechanical energy [3], thermal energy [4] and magnetic force [5] into electricity. The general characteristics of mechanical actions, such as frequency, magnitude and striking rate, are generally reflected in the output electrical signals of the NG. Therefore, an NG can be used as a self-powered active sensor for monitoring or detecting vortex capture [6], ambient wind velocity [7], low frequency vibrations [8], automobile velocities [9], etc. 0957-4484/14/455503+06$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
A Yu et al
Nanotechnology 25 (2014) 455503
Figure 1. (a) Morphology of the ZnO-textured film. The insert is a picture of the actual device. (b) Schematic diagram of the self-powered AC
magnetic sensor and the corresponding measurement set-up. (c) Proposed mechanism of the self-powered active magnetic sensor.
measurement set-up. The sensor is a cantilever structure with a metallic NdFeB disk at its tip. The NdFeB disk serves as a sensitive unit of the magnetic field. The NG was attached to a rectangle Kapton film, which is used as a cantilever. The Kapton film has excellent mechanical properties. Its tensile strength is about 200 MPa at room temperature. That means it has enough rigidity to function as a reliable cantilever with repeatable bent and unbent cycles. In reference to the material parameters and the geometrical shape of the cantilever: L = 80 mm, W = 40 mm, T = 0.5 mm and E = 3.3 GPa, and the NdFeB proof mass is about 1 g; the resonance frequency is calculated by equation (4) to be 10.1 Hz, which coincides with the experimental result. One end of the cantilever was fixed, and the other end was located in the middle of the solenoid. If the alternating current flows in the solenoid, the NdFeB disk will be attracted by the electromagnetic force. Once the NG is strained, an electric signal will be generated due to the piezoelectric effect. When the direction of the magnetic field is upward, the body of the NG will be bent upward under compressive stress. Because the NWs are densely packed and perpendicular to the substrate, the compressive stress will result in a tensile strain along the NWs’ direction ( the c-axis direction), thus creating a piezopotential drop from the tips of the NWs to the roots, as shown in figure 1(c). By the same token, when the direction of the magnetic field is downward, tensile stress will result in the body of the NG (e.g. a compressive strain along the NWs’
materials have a pyroelectric effect, the best sensitivity had to be reached at a constant environment temperature. Moreover, most of these sensors rely on an externally supplied power source. In this paper, a flexible, simple and low-cost piezoelectric NG is successfully demonstrated as a self-powered active sensor for detecting the frequency and amplitude of an AC magnetic field at room temperature. The output voltage of the NG has a good linearity with the measured magnetic field. The detected minute magnetic field is as low as 1.2 × 10−7 tesla under a resonance frequency of 9.9 Hz. Compared to the existing magnetic sensor technology, NGs offer a new way to lower costs and increase sensitivity. Results and discussion The flexible, small, lightweight NG used in this study was reported previously [8, 13]. Its effective area is about 1.5 cm × 1.2 cm. Its core is comprised of densely packed ZnO NWs. Figure 1(a) shows the morphology of the ZnO NWtextured film grown at the surface of a flexible Kapton (PI) film substrate by a wet chemical method. The top view of the ZnO-textured film is shown in the top-right insert figure of figure 1(a). The intersected hexagonal structure clearly demonstrates that the thin film is constituted of high-density NWs. Figure 1(b) shows a schematic diagram of the selfpowered AC magnetic sensor and the corresponding 2
A Yu et al
Nanotechnology 25 (2014) 455503
Figure 2. Output voltage of the sensor at a forward connection (a) and a reversed connection (b) to the measurement system under a fixed frequency of 11 Hz. (c) Output signals of the sensor when the magnet was removed. (d) Results of the ‘linear superposition’ tests.
NW-textured film and the frequency of the external force. There is a maximum of C at the resonance. K is determined by the NdFeB disk. Z is the impedance of the intrinsic capacitance and resistance within the piezoelectric circuit. It can be seen that output voltage V is the function of magnetic field H(t). So, all of the information of H(t) should be reflected in Voc. Moreover, they have a linear relationship. The characteristics of the corresponding magnetic field can be derived and calculated from the electric signal. Figure 2 gives data of the input magnetic signal and the output voltage obtained from the NG. The converted output voltage from the magnetic field was recorded via an oscilloscope. It can be seen in figures 2(a) and (b) that the obtained electric signal is also alterative, following the frequency and waveform of the magnetic signal with the sinusoidal mode electrical input. For the forward connection in figure 2(a), the measured maximum output voltage is 1.0 mV at the driving frequency of 11 Hz. When the measurement system is reversely connected to the sensor, the output signal is also reversed in figure 2(b). So, the output signals satisfy the switching-polarity test [15]. In addition, when the magnet is removed and the cantilever cannot be bent, the output signal decreases, which rules out the possibility that the signal is coming from the electromagnetic induction. Furthermore, the linear superposition test is carried out to prove the correctness of the output signals in figure 2(d). NG A with 1.12 mV of output voltage is connected to NG B (output voltage of
direction); thus, the roots of the NWs have a higher piezopotential than the top ends. These piezopotential distributions will generate induced charges in the top and bottom electrodes as a result of the electron flow in order to minimize the total energy and will consequently generate an output voltage and current in the external load. The magnetic force (F) between the NdFeB disk and the solenoid can be approximately expressed as F (t ) = ABSH (t)
(1)
where A is a constant, B is the surface magnetic flux density of the NdFeB disk, S is the cross-section area of the NdFeB disk and H is the time dependence of the AC magnetic field to be sensed. In this experiment, because NG was put in the middle of the solenoid, the magnetic force can be approximated to be only the function of time. Assuming a small uniform mechanical strain, the strain of the NG increased when the cantilever amplitude increased by external force for the NG with the cantilever structure [14]. Consequently, the vibration amplitude increases as the amplitude of the AC magnetic field increases linearly. Therefore, the open circuit voltage Voc and current Ioc can be given by Voc ≈ −C.K H (t )
(2)
I oc = Voc Z
(3)
where C is the parameters of the device and is determined by the material parameters of the cantilever as well as the ZnO 3
A Yu et al
Nanotechnology 25 (2014) 455503
Figure 3. (a) Response of the NG to the AC magnetic field with different frequencies. (b) Magnetoelectric coupling versus the frequency.
that at 5 Hz and 13 Hz under the same magnetic field. Because of the magnetic sensor studied in this paper with a cantilever structure, the sensor’s frequency characteristics were demonstrated in figure 3(b). It can be seen that the resonance frequency is about 9.9 Hz. If the sensor works around the resonance frequency, high output voltage will be obtained under the same magnetic field. So, the highest output voltage was observed at 8 Hz in figure 3(a). According to the Bernoulli–Euler theory of a beam [16], the resonance
3.42 mV) in the series. The output voltage (4.47 mV) is enhanced and approximately equal to the sum of the signal from NG A and NG B. Further, the response of the NG to the magnetic field with different frequencies was studied. After changing the frequency of the magnetic field, the corresponding frequency was detected by the NG in figure 3(a). The NG can accurately detect the frequency of the magnetic signals. Notably, the output voltage of the NG at 8 Hz is significantly higher than 4
A Yu et al
Nanotechnology 25 (2014) 455503
frequency of the fundamental bending vibration of the cantilever is expressed by f=
1 2π
EWt 3 4ml 3
(4)
where m is the proof of mass of the cantilever; E is the Young’s modulus of the cantilever material; and L, W and t are the length, width and thickness of the cantilever, respectively. Therefore, changing the length of the cantilever and mass of the NdFeB disk can effectively modulate the resonance frequency of the cantilever. Through changing these two parameters, the NG magnetic sensor can match the frequency in the detecting environment. The response of the NG magnetic sensor to a different magnetic field with sine mode was also investigated by changing the output voltage of the function generator, as shown in figure 4(a). It can be seen that the output voltage of NG increases with the increased amplitude of the magnetic field. To measure the magnetic field dependence of the output voltage of the NG in a wide range, a lock-in amplifier was used to power a Helmholtz coil to generate the AC magnetic field and measure the voltage induced by the NG. The detected voltages in figure 4(b) were measured under the resonance frequency of the NG cantilever. Figure 4(b) exhibits that the output voltage has a good linear relationship with the magnetic field (at the location of the sensor) in a broad region from 1.2 × 10−7 tesla to 2.4 × 10−4 tesla. Since the maximum AC magnetic field generated by our Helmholtz coil is 2.4 × 10−4 tesla, we cannot measure the higher AC magnetic field. Our sensor can be used to measure a higher AC magnetic field than the field in figure 4(b). Such a broad linear region is important and is easy for circuit design and data processing. This result demonstrates the possibility of using an NG as a self-powered sensor to monitor the amplitude of a magnetic field. One can calculate the magnetic field from the obtained output voltage of the NG according to the linear relationship between them. The detected lowest magnetic field is 1.2 × 10−7 tesla at room temperature. So far, the amplitude and frequency of the magnetic field can all be sensed by the NG. Compared to traditional techniques for magnetic field monitoring, the NG-based sensor has the following advantages: (1) it is a self-powered active sensor that does not require an external power source to drive the sensor; (2) its fabrication is very simple and very cheap; (3) it works well at room temperature.
Figure 4. (a) Response of the NG to the AC magnetic field with different amplitudes. (b)Sensitivity of the NG under the resonance frequency.
Experimental Conclusions
The ZnO NW-textured film was first grown at the surface of a flexible Kapton (PI) film substrate by a wet chemical method at 75 °C for 12 h. The nutrient solution we used in the chemical growth process of the ZnO densely packed NW-textured films was 100 mM of Zn (NO3)2.6H2O and 10 mM hexamethylenetetramine (HMTA). The magnetic field to be sensed was produced by a solenoid in which the magnetic field was modulated by a function generator (SR345). The Helmholtz coils that were used consist of two identical
In summary, we demonstrated a flexible, simple and cheap piezoelectric NG as an ultra-high sensitive sensor for detecting an AC magnetic field. The output voltage of the sensor has a good linearity with a measured magnetic field. The detected minute magnetic field is as low as 1.2 × 10−7 tesla. This kind of self-powered magnetic sensor has potential applications for environmental monitoring, diagnostic medicine, magnetic global position systems and defense technology. 5
A Yu et al
Nanotechnology 25 (2014) 455503
circular coils. Each coil has 100 turns. The coil diameter is 7 cm, and the distance between the two coils is 3.5 cm. The obtained magnetic field is uniform in the center of the coils and is well controlled the by input voltage.
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Acknowledgement Our research was supported by the ‘thousands talents’ program for pioneer researcher and his innovation team, China and by the Knowledge Innovation Program of the Chinese Academy of Sciences, Grant No. KJCX2-YW-M13. We also thank Weiwei Wu for the fabrication of the ZnO seed layer. References [1] Wang Z L and Song J 2006 Science 312 242–6 [2] Qin Y, Wang X and Wang Z L 2008 Nature 451 809–13 [3] Yang R, Qin Y, Li C, Zhu G and Wang Z L 2009 Nano Lett. 9 1201–5 [4] Yang Y, Wang S, Zhang Y and Wang Z L 2012 Nano Lett. 12 6408–13
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