Nanoscale

PAPER

Cite this: DOI: 10.1039/c7nr09129j

Received 8th December 2017,Accepted 26th January 2018

DOI: 10.1039/c7nr09129j

rsc.li/nanoscale

Magnetorheological elastomers enabled high-sensitive self-powered tribo-sensor for magneticfield detection†

Song Qi,‡a Hengyu Guo,‡b,c Jie Chen,b Jie Fu,a Chenguo Hu, *b Miao Yu *a andZhong Lin Wang *c

The monitoring of the magnetic field is the most significant process for academic or industrial appli-

cations. In this study, we design a self-powered magnetic-field sensor based on the magnetorheological

elastomer (MRE) and triboelectric nanogenerator (TENG) that can be used for both time-varying and

uniform magnetic field (UMF) sensing. This TENG-based magnetic-field sensor (TMFS) relies on contact

electrification and electrostatic induction of TENG to generate an electrical signal in response to the mag-

netic-induced deformation of MRE without using an external power supply. Enabled by the unique

sensing mechanism and excellent magnetic-induced deformation of MRE, the TMFS exhibits a fast

response (20 ms) and good magnetic-field sensing performance. The TMFS with 60 wt%-MRE shows a

maximum sensitivity of 16 mV mT−1 of the magnetic field ranging from 40 to 100 mT experimentally, and

the sensitivity and detection range of TMFS can be adjusted by several parameters of the device. Besides

the contribution to the effective detection of UMF, this novel sensor provides a new idea for the mag-

netic-field measurements in self-powered mode.

1. Introduction

Magnetic field is a vector quantity, which means that it hasboth magnitude and direction. As the magnetic field has anessential role in environmental surveillance, mineral explor-ing, and safety monitoring, the investigation on the detectionof magnetic field is of the greatest importance. Currently, avariety of technologies are used for the measurement of mag-netic field, such as the Hall effect, fluxgate, superconductingquantum interference device (SQUID), magneto-resistance,magneto-diode or other semiconductor effects.1,2 Some ofthem are utilized for sensing the presence or variation of thefield, whereas the others are used for detecting the magneticfield’s precise value and vector property. Each of the tech-niques possesses unique advantages that make it more suit-

able for particular situations. However, the signal generationof above detection techniques has to be supplied by externalpower.

Recently, self-powered technologies have caught the atten-tion of a large number of researchers.3–5 Among them, tribo-electric nanogenerator (TENG) is a burgeoning technology thatcan directly collect the small-scale mechanical energy andconvert it into electrical energy.6–10 Based on the couplingeffect of the electrostatic induction and contact electrifica-tion,7,11,12 TENG can convert almost all forms of mechanicalenergies, such as vibrations,13,14 rotation,15–17 wind,18,19

flowing water20,21 and even acoustic energy,22–24 into electricalenergy. Owing to the highly sensitive responses to mechanicaltriggering in TENGs, various self-powered devices and sensorsbased on TENG have also been designed for sensing measure-ments.25,26 Yang et al. have reported a TENG-based self-powered magnetic sensor for detecting the variation of thetime-dependent magnetic field.27 The results indicate that thechange in magnetic field can be measured accurately by theoutput voltage of the sensor, and show a high detection sensi-tivity. However, this sensor is designed simply for detectingthe changes in magnetic field, but it cannot be used to detectthe intensity and direction of the uniform magnetic field(UMF).

As with TENG, magnetorheological elastomer (MRE) hasalso received significant attention. The difference is that MRE

†Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nr09129j‡These authors contributed equally to this study.

aKey Lab for Optoelectronic Technology and Systems, Ministry of Education,

College of Optoelectronic Engineering, Chongqing University, Chongqing, 400044,

P. R. China. E-mail: yumiao@cqu.edu.cnbDepartment of Applied Physics, Chongqing University, Chongqing, 400044,

P. R. China. E-mail: hucg@cqu.edu.cncBeijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences;

National Center for Nanoscience and Technology (NCNST), Beijing, 100083,

P. R. China. E-mail: zhong.wang@mse.gatech.edu

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is an intelligent material, which consists of micronized mag-netic particles suspended in a nonmagnetic elastic matrix.28–31

With the excellent magnetic-control properties, MRE exhibitsgreat potential for applications in the fields of noise reduction,vibration attenuation, smart sensing, electromagnetic shield-ing, etc.32–36 When exposed to UMF, the magnetic particles ofMRE have a tendency to form a chain-like structure parallel tothe direction of UMF. The movement of the magnetized par-ticles results in the deformation of MRE. The deformation ofMRE in response to an external magnetic field has beenstudied by many researchers,37–39 and several papers havereported smart devices based on the magnetic-induced defor-mation of MRE.40–42 It has been proved that the magnitude ofdeformation is strongly dependent on the magnetic field.Therefore, by using the excellent magnetic-induced defor-mation of MRE together with the displacement sensitivity ofTENG, this study can provide a useful measuring method forthe detection of UMF.

Herein, to measure the intensity and direction of UMF,this paper reports the development of a self-powered sensorbased on TENG to detect the magnetic-induced deformationof MRE. The structure and mechanism of TMFS have beenillustrated in detail. The material properties of MRE andoutput signals of the sensor have also been systematicallystudied. Besides, the influences of MRE, UMF and the struc-tural parameters of TMFS on the output voltage of the sensorare discussed, and the relevant physical mechanism is ana-lyzed. These results reveal that TMFS has considerable poten-tial for the application of magnetic field measurements inself-powered mode.

2. Experimental section2.1 MRE film fabrication

A schematic diagram of the fabrication process of the MREfilm is shown in Fig. 1a. MRE was fabricated by mixing carbo-nyl iron particles (CIPs, type: CN; size distribution: 1–8 μm;provided by BASF) with the silicone rubber (Ecoflex 0020; pro-vided by Smooth-On, Inc.) at different mass fractions (20 wt%,30 wt%, 40 wt%, 50 wt%, 60 wt%). The CIPs were first mixedwith the uncured liquid silicone rubber by mechanical stirringand ultrasonication for 1 h to ensure particles’ uniform distri-bution. Subsequently, the cross-linker of the silicone rubberwas added into the aforementioned suspension whilemechanically stirring for 30 minutes. It deserves to be men-tioned that the resulting mixture was degassed for 20 minutesin a vacuum oven at 30 °C to remove the bubbles. Finally, themixture was placed in an acrylic mould and formed into filmswith different thicknesses.

2.2 Device fabrication

Fig. 1c illustrates the structure of TMFS, which mainly con-sists of upper and lower parts. The lower part is the defor-mation component and comprises of MRE and latex film overits surface. To ensure that the central portion of MRE was

free to deform, MRE was cut into a disc form with a 5 mmdiameter and attached to the acrylic ring plate. In addition, alatex film was attached to the MRE film to act as the positivetribo-material. The upper part was the TENG componentwhich consisted of fluorinated ethylene propylene (FEP), polyethylene terephthalate (PET) and copper. It is worth mention-ing that the TENG fabrication process started from the prepa-ration of several circular film components. The FEP, PET andcopper films were stacked together in turn, and these multi-layered structures were attached to the acrylic plate. Toenlarge and ensure the deformation direction of MRE,another MRE film was completely fixed onto the acrylic plateof the TENG component as shown in Fig. 1d. Finally, the as-prepared TENG layers were assembled in parallel with thedeformation element, and the TENG layers received thesupport of four springs and bolts. The gap between the upperpart and lower part was 1.5 mm, and it could be adjusted bythe screw cap.

2.3 Materials characterization and output measurements

The morphologies of MRE and FEP were characterized viascanning electron microscopy (SEM; MIRA3TESCAN). Themechanical properties of MRE were measured by aCTM2100 materials test machine (CTM, Shanghai, People’sRepublic of China). The output signals of the TENG unit weremeasured by a voltage preamplifier (Keithley 6514 SystemElectrometer). The software platform was constructed basedon LabVIEW, which was capable of conducting real-time dataacquisition and analysis.

Fig. 1 Structure design of TMFS. (a) Preparation process of the MREfilm. (b) Photograph of MRE film and designed TMFS; the scale bar is5 mm. (c) Schematic illustration of the structure of TMFS, which consistsof upper and lower parts. The lower part is the deformation elementcontaining MRE and latex films, and the upper part consists of FEP, PETand copper. Insets 1 and 2 show the SEM image of MRE and FEP surfaceand the scale bar is 50 µm and 500 nm, respectively. (d) Schematicdiagram of the deformation mechanism of TMFS under UMF.

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3. Results and discussion3.1 Mechanism of the sensor

Fig. 1 shows the structure design of TMFS. The measurementsof UMF can be achieved by the electric signal of TENGinduced from the magnetic-induced deformation of MRE.Fig. 1b shows the photograph of MRE film and designedTMFS. The flexible MRE film has the sensitive magnetic-induced deformation property, and the deformation takes thepositively charged latex film closer to the negatively chargedFEP as shown in Fig. 1d. The electrostatic induction causes thechange in the open-circuit voltage across the two electrodes,and the magnitude of the electric output signal is indicative ofthe local magnetic field. The mechanisms of the magnetic-induced deformation of MRE and TENG are discussed indetail in the following sections.

3.1.1 Mechanism of the magnetic-induced deformation ofMRE under UMF. The CIPs were distributed randomly in thematrix of isotropic MRE as shown in inset 1 of Fig. 1c.Fig. S1† shows the magnetization curves of CIP and MRE,which indicated that both of them have excellent soft mag-netic properties, ensuring the excellent monitoring repeatabil-ity of TMFS. Fig. S2† shows the magnetic simulation of therepresentative volume element (RVE) of MRE. The per-meability of CIPs was much greater than that of the matrix,resulting in a larger magnetic flux density in CIPs as shown inFig. S2a.† It could be seen that the magnetic flux density alsodepends on the particle size and spacing. Fig. S2b† shows themagnetic stress tensor generated on the particle surface, andthe magnitude of the tensor is related to the direction of themagnetic field. These directional stress tensors made the uni-formly dispersed CIPs to form a chain structure parallel to thedirection of the magnetic field, which caused the magnetic-induced deformation of MRE. To ensure that the deformationonly occurred along the vertical direction, the edge of thecircular MRE film was bonded to the acrylic ring plate.Besides, to enlarge the deformation of MRE, another MREfilm was completely fixed onto the acrylic plate of the TENGunit. This completely fixed MRE could provide an additionalmagnetic attraction for the deformable MRE film, whichcould improve the sensitivity of the sensor. To explain themagnetic-induced deformation response of the MRE filmfrom physical mechanisms, a theoretical model was proposedbased on the linear elastic theory of the MRE film. To sim-plify, all CIPs were assumed to be homogeneous spheres thatcould be treated as identical dipoles, and the distancesbetween particles were equal. Besides, we assumed magne-tized fixed MRE as a uniform plate magnet and ignored themutual magnetic induction between the two MREs as shownin Fig. 2a. In an external magnetic field H0, CIPs of deform-able MRE became magnetized and were additionally attractedby magnetized fixed MRE. The magnetic moment mm of themagnetized fixed MRE under the magnetic field H0 is as perthe following equation:

mm ¼ M � Við1Þ ð1Þ

Here, M and Vi are the magnetization intensity and volumeof fixed MRE, respectively. With a magnetic moment mm, fixedMRE can induce an inhomogeneous stray field Hi in the sur-rounding space, and the Hi mainly depends on the per-meability and volume of fixed MRE, external magnetic field H0

and the distances between the space point and fixed MRE. Tosimplify, we assume that Hi is a uniform magnetic field paral-lel to H0 for a single magnetic dipole. Hence, Hi can beexpressed as follows:

Hi ¼ αf ðλÞH 0 ð2ÞHere, α is a constant which depends on the permeability,

susceptibility, shape of MRE etc. f (λ) is the function of the dis-tance λ between the space point and fixed MRE. For thedeformable MRE, due to the large demagnetizing factor ofCIPs, it can be assumed that the magnetic moment mp of theparticles in deformable MRE depends linearly on the magneticfield strength H,43 where H = H0 + Hi, as per the followingequation:

mp ¼ 4π � μ0 � μ1 � χ � r33

H ð3Þ

Here, r is the particle radius, μ0 and μ1 are the vacuum per-meability and permeability of MRE, respectively and χ is thesusceptibility of CIPs. Under a magnetic field, mp of dipoleswere oriented to form a structure that is ordered according tothe shape shown in Fig. 2a. The orientation of mp was parallelto the orientation of H. With increasing magnetic field, boththe percentage of rotated particles and the rotation angle of

Fig. 2 Schematic diagrams of working mechanisms of the TMFS. (a)Schematic illustration of the structure with magnetic dipoles in theelastic matrix of MRE film under UMF: m! is magnetic dipole and H

!is

magnetic field vector in the Oxyz coordinates axis system. Insets 1, 2and 3 show the photographs of the magneto-deformation of MRE under0 mT, 100 mT and 200 mT, respectively; the scale bar is 1 mm. (b)Schematic illustrations of the charge distributions of TENG underdifferent magnetic field strengths, indicating the relationship betweenthe magneto-deformation of MRE and the charge distribution of TENG.The second half of the illustration shows the numerical calculations ofthe potential distribution across the electrodes of TENG under differentmagnetic field strengths as evaluated by COMSOL.

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each particle increased, which resulted in the elongation ofMRE.44 The magnetized particles tended to attract to eachother, which led to the formation of an ordered structure withthe particles aligning along the magnetic-field direction. Then,under magnetic field H, the force (attraction) between twomagnetic dipoles can be expressed as45 follows:

F ′ ¼ 2πμ0r6H2

d4ð4Þ

Here, μ0 is the vacuum permeability, H is the quantity ofmagnetic field H, and d is the distance between the centers ofthe magnetic dipoles. Under the action of F′, according to theHooke’s law, the elastic medium reacts with the elastic force asper the following equation:

F′el ¼ kΔd ð5ÞHere, k and Δd are the coupling constant and strain

between two neighboring particles, assuming that the dipolemoment is kept constant when MRE is deformed. When thetwo forces are balanced, the equality occurs as per the follow-ing equation:

F ′ ¼ 2πμ0r6H2

d4¼ F′el ¼ kΔd ð6Þ

From eqn (2) and (6), we can deduce the expression for theexternal magnetic field strength H0 as follows:

H0 ¼ d2

r3ðαf ðλÞ þ 1Þ

ffiffiffiffiffiffiffiffiffiffikΔd2πμ0

sð7Þ

Assuming f (λ) is a constant, eqn (7) can be simplified asfollows:

H0 � ρffiffiffiffiffiffiΔd

pð8Þ

Here, ρ ¼ d2

r3ðαf ðλÞ þ 1Þ

ffiffiffiffiffiffiffiffiffiffik

2πμ0

sis a constant. From eqn (8)

we could deduce that the magnetic field H0 was correlatedwith the deformation between the neighboring particles Δd,which indicated that the relationship between the deformationand magnetic field strength is an increasing function. Insets 1,2 and 3 of Fig. 2a show the photographs of the magnetic-induced deformation of MRE under 0 mT, 100 mT and200 mT, respectively. As expected, the deformation of MREfilm increased with increasing magnetic field strength. A quali-tative fitting analysis was carried out as shown in the sup-plementary content.

3.1.2 Mechanism of TENG. The electric signal generationmechanism of TENG is based on the coupling of contact tri-boelectrification and electrostatic induction as illustrated inFig. 2b. Because of the large composition percentage of fluo-rine that has the highest electronegativity among all elements,FEP is one of the most triboelectric negative materials.46 FEPalways has a tendency to gain negative charges when incontact with almost any other material. In addition, the latexfilm over the surface of the MRE film generally contains posi-

tive charges. In the absence of a magnetic field, no defor-mation occurs on the MRE film, and the gap between FEP andlatex film is stable. The triboelectric charges on the surfaces ofFEP and latex are balanced by their opposite counterparts,which do not induce changes in the open-circuit voltage acrossthe two electrodes. When an external magnetic field is appliedto TMFS, the magnetic-induced deformation of MRE takes thelatex film closer to the FEP film. The electrostatic inductioncauses the change in the open-circuit voltage across the twoelectrodes. The deformation and the deformation rate of theMRE film result in an electric output signal by TENG. Themagnitude of the electric output signal is indicative of thelocal magnetic field. On the other hand, the magnitude ofmagnetic-induced deformation of MRE is strongly dependenton the angle between the MRE film and magnetic field. As aconsequence, different angles cause different electric outputsignals. This is the mechanism we will use to measure theintensity and the direction of UMF by detecting the electricoutput signal of TENG.

3.2 Magnetic-induced deformation character of the MRE film

The magnitude of the deformation has been evaluated by thedisplacement of the center point in the circular MRE film,which is detected by the laser range finder. The schematicdiagram of the measurement system is shown in Fig. 3a. UMFis generated with an electromagnet, and the strength can becontrolled by the current I in the coil. The generated magneticflux density B in the coil has been tested by a teslameter, andthe relationship between the continuously changing currentand B is calculated by using the ANSYS software. Fig. 3b showsthat the deformation initially increases slowly with the increas-ing magnetic field strength, which can be attributed to theelastic interaction to resist deformation caused by the mag-netic field. As the magnetic field continuously increases, thedeformation is found to exponentially increase with theincreasing magnetic field. It is also observed that the unloadcurve and the loading curve are almost consistent. It can beseen from Fig. 3c that the deformation has been enhanced bythe increment of CIP content. It is primarily because thespacing of particles becomes smaller as the particle contentincreases. The enhanced magnetic interaction forces betweenCIPs cause larger deformations. Fig. S4a† shows the curve ofthe magnetic-induced deformation of MRE with different CIPcontents under increasing and decreasing magnetic fieldstrengths. Noticeably, the sample with the 60 wt% CIP has thehighest sensitivity for the magnetic-induced deformationamong the samples. Fig. S6a† illustrates the influence of theMRE film thickness on the deformation performance, and thedeformation is increased with the decreasing thickness.Another phenomenon that can be seen is that the unloadingcurves are above the loading curves; this hysteretic effect canbe attributed to the Mullins effect. Under tensile strain, themolecular chains of silicone rubber are stretched anduntwisted. The strain of molecular chains does not immedi-ately recover in the unloading stage, which causes a largerdeformation. The elastic restoring force has been reduced in

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the thin sample, which causes the obvious hysteretic effect. Toachieve better stability, the MRE film with 1 mm thickness hasbeen chosen as the material components of the sensor devicein the following section.

Deformation stability of MRE film has been tested by ameasurement system based on the magnets and linear motor;the schematic diagram of the measurement system is shownin Fig. 3d. Through the cycle test, Fig. 3e shows an excellentdeformation stability of MRE, which can be attributed to theexcellent mechanical properties of MRE. Fig. S6b† shows themechanical properties of the MRE samples. The 60 wt%sample has the tensile strength of 1.09 MPa and breakingelongation of 580%. Fig. 3b shows that the maximal magnetic-induced deformation of the MRE sample with 60 wt% CIPs is1 mm and the corresponding stretch strain is below 10%. Thisshows that the elastic strain of deformation is in the linearviscoelastic region, which ensures the stability and repeatabil-ity of the magnetic-induced deformation of MRE film in thefollowing tests. Fig. 3f shows the response time of the mag-netic-induced deformation. Owing to the slider movementtime of 50 ms, we can deduce the response and reset times ofthe magnetic-induced deformation to be about 20 and 30 ms,respectively.

3.3 Output performances of the TMFS

To investigate the performance of TMFS systematically, severalparameters were discussed in both experimental and theore-tical studies. Fig. 4a and b show the output current of TMFS

under different types (rectangle and sine) of time-varying mag-netic fields. It could be seen that the current outputs were gen-erated when the magnetic field was changed. The outputcurrent was positive when the magnetic field increased, andthe output current was negative when the magnetic fielddecreased. The influences of the magnitude and the frequencyof the time-varying magnetic field on the output current ofTENG are shown in the Fig. S7.† These results were in goodaccordance with the output performance of the sensordesigned by Yang et al.27 It showed that TMFS can be used forthe measurement of time-varying magnetic field.

Fig. 4c shows the voltage curves of TMFS under transientUMF with different strengths. It can be observed that theresponse and reset of output voltage is almost instantaneousunder transient UMF, and the magnitude of output voltageincreases with the increasing magnetic field strength. This canbe attributed to the electrostatic induction between the FEPfilm and latex film as discussed before. Larger deformationleads to smaller spacing between the FEP film and the latexfilm, which causes a greater output voltage. Fig. 4d shows therelationship between the deformation of MRE and the outputvoltage of TENG, and the response shape of deformation isalmost the same as that of output voltage. This reveals thatTMFS has an excellent response and recovery property, with a20 ms response time and a 30 ms reset time.

Fig. 4e shows the voltage curves of TMFS under a uniformlyincreased and decreased UMF, and the loading curve and theunloading curve are almost symmetrical. It indicates that

Fig. 3 Deformation performances of the MRE film in TMFS. (a) Schematic diagram of the measurement system for detecting the magneto-defor-mation. The UMF was generated with an electromagnet, and magnetic field strength could be controlled by the current I in the coil. The displace-ment of the center point in the circular MRE film was detected by the laser rangefinder. (b) The curve of the magneto-deformation of MRE with the60 wt% CIPs under increasing and decreasing magnetic field strengths. (c) The magneto-deformation of MRE with different contents of CIPs under160 mT. (d) The schematic diagram of the measurement system for the deformation stability. The transient UMF was generated with two parallelmagnets, which were mounted on the slider of the guide rail. The displacement between the magnets was controlled by the linear motor, and themovement time of the slider was 50 ms. (e) The cycle tests of the magneto-deformation. The time for each cycle was 1s, and it included the transientincrease and decrease of the magnetic field. (f ) A single-cycle curve of magneto-deformation, the response time has been marked.

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TMFS has excellent stability under different time-dependentUMF, which can be attributed to the excellent soft magneticproperties of CIPs. In addition, Fig. 4e also reveals that theMRE film with higher CIP content in TMFS has the higheroutput voltage and detection sensitivity. These curves showsome nonlinear characteristics, which can be attributed to thenonlinear relationship between magnetic-induced deformationand magnetic field strength. The output voltage initiallyincreases slowly with the increasing magnetic field strength,same as that for the deformation of MRE film. As the magneticfield strength reaches a threshold level, the output voltage ofTMFS is found to exponentially increase with the increasingmagnetic field. Eventually, saturation phenomena are observedin the voltage curves under the high magnetic fields.

As mentioned before, the curve of TMFS with 60 wt%-MREcan be divided into three stages. Fig. 4f shows the differentsensitivities of TMFS with 60 wt%-MRE in these three stages,with sensitivities of 3.1 mV mT−1, 16 mV mT−1, and 1.4 mVmT−1, respectively. By comparing the curves in Fig. 4e, it canbe observed that the saturation strength decreases with theincreasing CIP content, which provides guidance for thematerial selection for different measurement demands.

Among the results obtained, TMFS with the 60 wt%-MRE isfound to have a high sensitivity at magnetic field ranging from40 to 100 mT, and TMFS with the 20 wt%-MRE has a high sat-uration strength. This is mainly because the magnetic inter-action of CIPs decreases with the decreasing CIP content. Thesmaller deformation result in the curve of TMFS with the20 wt%-MRE only shows the first stage in the test range.Among them, TMFS with the 60 wt%-MRE has a high sensi-tivity at the magnetic field ranging from 40 to 100 mT, andanother sensor with the 20 wt%-MRE has a high saturationstrength and better linearity. Fig. S8a† reveals the influence ofthe gap between the upper part and the lower part on theoutput voltage of TENG; the results indicate that the sensitivityand saturation strength have been reduced and enhanced,respectively with the increase in the gap.

The inset of Fig. 4g shows the schematic diagram of thedirection measurement of UMF, and the change of the anglebetween the MRE film and magnetic field is obtained by rotat-ing the TMFS with the magnetic field normal to the centeraxis. The output performance has been tested for the 90°, 60°,30° and 0°. Fig. 4g shows the different voltage responses ofTMFS under the increasing magnetic field with different direc-

Fig. 4 Output performances of TMFS for measuring strength and direction of magnetic field. (a) Cyclic changes of the magnetic field and thecorresponding output current of TMFS. Inset shows the schematic diagram of the measurement of time-varying magnetic field. The change and thechanging rate of magnetic field is controlled by the movement of the magnet. (b) Cyclic changes of the sinusoidal magnetic field and the corres-ponding output current of TMFS. (c) Output voltage of TENG under transient UMF with different strengths. (d) The relationship between the defor-mation of MRE and the output voltage of TENG. (e) Output voltage of TENG under a uniformly increased and decreased UMF; different curvescorresponding to MRE with different CIP contents. (f ) The different sensitivities of TMFS with 60 wt%-MRE in different stages, with sensitivities of3.1 mV mT−1, 16 mV mT−1 and 1.4 mV mT−1. (g) Output voltage of TENG under a uniformly increased UMF; different curves corresponding todifferent angles between the MRE film and magnetic field. Inset shows the schematic diagram of the direction measurement of UMF; the sensor isrotated with the magnetic field normal to the center axis and the angle between the MRE film and magnetic field is reduced from 90° to 0°. (h) Thecycle test of output performances of TMFS under transient UMF. The time for each cycle is 1.5 s, and it includes the transient increase and decreaseof the magnetic field.

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tions from which we can observe that the voltage is increasedwith the increasing angle. Fig. S8b† shows the output voltageof TENG under 150 mT transient UMF with differentangles. Also, it can be observed that the voltage is decreasedwith the decreasing angle. It is mainly because CIPs tending toform a chain structure parallel to the direction of the magneticfield, and the movement trends decrease when the angledecreases. These angle-dependent output performancesprovide a viable method to detect the direction of UMF.Finally, the continuous test has been carried out to investigatethe stability of the output performance of TMFS, and noobvious output change can be observed in the voltage curve asshown in Fig. 4h.

4. Conclusions

In summary, we demonstrated a self-powered magnetic-fieldsensor for the sensing of a time-varying and uniform magneticfield. In this design, TENG was used for detecting the defor-mation of MRE film, and the magnitude of electric outputsignal in TENG based on the electrostatic induction indicatedthe strength of the local magnetic field. Based on the excellentmagnetic-induced deformation of MRE and the irreplaceableand unbeatable characteristics of TENG for perceiving the dis-placement changes, the designed TMFS showed high sensi-tivity and stability in the measurement of magnetic field. Thehighest average sensitivity could be reached at 16 mV mT−1,and the response time and reset time were about 20 and30 ms, respectively. The results also showed that the sensitivityof the sensor could be adjusted by MRE and device para-meters. Finally, the direction measurement of UMF could beachieved based on the unique layer structure of TMFS. Ourstudy suggests that the TMFS can be used for magnetic-fieldmeasurements under no power conditions with advantages ofself-power, low cost, simple fabrication, rapid response andhigh sensitivity.

Author contributions

The manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.

Conflicts of interest

The authors declare no competing financial interests.

Acknowledgements

Research supported by the Hightower Chair foundation andthe “thousands talents” program for pioneer researcher andhis innovation team, China. This research is funded by theEquipment Research Joint Fund Project of Chinese Ministry of

Education (grant no. 6141A02022108) and NSFC (51572040and 51775064). The authors are grateful for the support.

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