Journal of Materials Science & Technology  2020 , 44 (0): 42-47 https://doi.org/10.1016/j.jmst.2019.10.019

Research Article

A flexible and high temperature tolerant strain sensor of La0.7Sr0.3MnO3/Mica

Min Guoa, Cheng Yanga, Dong Gaoa, Qiang Lia, Aihua Zhanga, Jiajun Fengc, Hui Yanga, Ruiqiang Taoa, Zhen Fana, Min Zenga, Guofu Zhoubc, Xubing Lua*, J.- M. Liuad

a Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Optical Information Materials and Technology, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China
b Guangdong Provincial Key Laboratory of Optical Information Materials and Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China
c National Center for International Research on Green Optoelectronics, South China Normal University, Guangzhou 510006, China
d Laboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

Corresponding authors:   * E-mail address: luxubing@m.scnu.edu.cn (X. Lu).

Received: 2019-08-14

Revised:  2019-09-13

Accepted:  2019-10-8

Online:  2020-05-01

Copyright:  2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

Flexible sensors have been widely investigated due to their broad application prospects in various flexible electronics. However, most of the presently studied flexible sensors are only suitable for working at room temperature, and their applications at high or low temperatures are still a big challenge. In this work, we present a multimodal flexible sensor based on functional oxide La0.7Sr0.3MnO3 (LSMO) thin film deposited on mica substrate. As a strain sensor, it shows excellent sensitivity to mechanical bending and high bending durability (up to 3600 cycles). Moreover, the LSMO/Mica sensor also shows a sensitive response to the magnetic field, implying its multimodal sensing ability. Most importantly, it can work in a wide temperature range from extreme low temperature down to 20 K to high temperature up to 773 K. The flexible sensor based on the flexible LSMO/mica hetero-structure shows great potential applications for flexible electronics using at extreme temperature environment in the future.

Keywords: Flexible sensor ; Mica ; La0.7Sr0.3MnO3 ; High temperature ; Multimodal sensing

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Min Guo, Cheng Yang, Dong Gao, Qiang Li, Aihua Zhang, Jiajun Feng, Hui Yang, Ruiqiang Tao, Zhen Fan, Min Zeng, Guofu Zhou, Xubing Lu, J.- M. Liu. A flexible and high temperature tolerant strain sensor of La0.7Sr0.3MnO3/Mica[J]. Journal of Materials Science & Technology, 2020, 44(0): 42-47 https://doi.org/10.1016/j.jmst.2019.10.019

1. Introduction

Flexible and wearable electronic products have attracted enormous attentions due to their potential applications in artificial intelligence robot, wearable health care technologies and portable personal electronics compared with conventional rigid silicon based electrons. The core components of building flexible and wearable electronic devices are various kinds of flexible sensors, such as stress sensors [[1], [2], [3]], photoelectric sensors [4,5], magnetic field sensors [6,7] and temperature sensors [8,9]. Flexible strain sensor has been the most widely studied flexible sensor, which can convert various mechanical signals into electrical signals in the surrounding environment, indicating potential application in motion detection [10,11], life activity monitoring [12,13], and artificial intelligence [14,15]. Zhong et al. reported a multifunctional electronic skins prepared by aligned carbon nanotubes in flexible polymer composites [16]. This strain sensor exhibits high accuracy in the measurement of bending scale, low energy consumption (<10 μW) and excellent bending stability, demonstrating a capability for portable motion detectors. Park et al. developed highly flexible, scalable and sensitive strain sensors based on silver nanowire network composites and polydimethylsiloxane (PDMS) elastomer sandwich structures [17]. They demonstrated a practical application of the strain sensors by making gloves fixed with five strain sensors for finger movement detection and virtual character control in virtual environments. Wang et al. presented a flexible vessel-like sensor consisting of braided cotton hose substrate, carbon nanotubes (SWCNTs)/ZnO@polyvinylidene fluoride (PVDF) arrays and flexible PVDF fibrous membrane [18]. It can detect physical signals such as temperature, strain and frequency, implying potential applications in wearable or portable physical sensing.

The above mentioned flexible strain sensors are mainly composed of two parts. One is the active layer and the other is the flexible substrate. The active layer mainly consists of carbon nanotubes [19,20], conductive polymers [21,22], nanomaterials [23,24], etc. The flexible substrate mainly consists of PDMS [25,26], polyethylene terephthalate [13,27], PVDF [2,28] and other polymer materials [20,24]. Among them, flexible substrates are mainly polymers with low melting point (generally less than 300 °C) which can not withstand high temperature and become brittle at low temperature. The flexible electronics reported at present are not suitable for high temperature environment, which limits its application in aerospace, metallurgical industry, underground energy exploration and other extreme high temperature environments.

In order to fabricate flexible strain sensor for high temperature applications, our idea is to find a flexible substrate which can withstand high temperature. Mica (KAl2(Si3Al)O10(OH)2) is an inorganic material with layered structure. A thin mica sheet (about 100 μm) exhibits excellent flexibility and high transmittance in the ultraviolet-visible-infrared light range [29]. Its high melting point (1150-1300 K) enables it to withstand 700 °C high temperature environment. Therefore it is believed to be an excellent flexible substrate material for high temperature environment.

In this work, we adopt functional oxide LSMO thin film and high temperature resistant flexible substrate mica to construct a flexible strain sensor which can work at high temperature. Although several function oxides have been deposited on Mica substrate with high quality, such as CoFe2O4 [30], La0.67Sr0.33MnO3 [31], VO2 [32] and MoO2 [33]. No work has been done for strain sensor application especially working at high temperature. In our work, we fabricated LSMO/Mica hetero-structure by pulsed laser deposition for flexible strain sensor application. This strain sensor exhibits good flexible performance with excellent bending properties, outstanding resistance endurance at room temperature. Moreover, it can work well in a wide temperature range between 20 K and 773 K. The response of the sensor to the magnetic field has also been studied, with a relatively large magnetoresistance change of 32% at 20 K under 2 T magnetic field, implying its multimodal sensing ability.

2. Experimental

2.1. Preparation

Fig. 1(a) shows the detailed preparation processes of La0.7Sr0.3MnO3/Mica hetero-structure. The polycrystalline LSMO films were deposited on mica (001) substrate through pulsed laser deposition using a commercial La0.7Sr0.3MnO3 ceramic target. A KrF excimer laser ( = 248 nm), operated at 2 Hz repetition rate and a fluence of ~2.91 J/cm2 was used to ablate the target. A growth temperature of 650 °C and an oxygen partial pressure of 10 Pa were adopted during the growth of the LSMO thin film. After deposition, the fabricated samples were subsequently cooled down with a temperature decreasing rate of 1 °C/min in 1 mbar of pure O2 ambient. Pt electrodes with 100 nm thickness were grown at 10-4 Pa oxygen pressure on 4 corner of the LSMO film by a shadow mask for electrical measurement.

Fig. 1.   (a) Schematic diagram of preparation processes of La0.7Sr0.3MnO3/Mica hetero-structure; (b) cross-sectional SEM image of the mica sheet; (c) AFM surface morphology of the La0.7Sr0.3MnO3 film on mica; (d) XRD pattern of La0.7Sr0.3MnO3/mica hetero-structure.

2.2. Characterization

Crystal structures of the LSMO thin films were characterized by X-ray diffraction (XRD) using a PANalytical X’Pert Pro diffractometer with Cu Kα radiation. Surface morphologies were measured by atomic force microscope (AFM) (Cypher Asylum Research Ltd.). Cross-sectional image of Mica were characterized by field emission scanning electron microscopy (FE-SEM, ZEISS-Ultra55). The electrical transport and magnetoresistance property were investigated by Van der Pauw method at a temperature range from 20 K to 300 K using a physical property measurement system (PPMS9, Quantum Design). The resistance value measured by Van der Pauw method was defined by the sensed voltage divided by the sensing current.

The mechanical bending properties were characterized by using a self-made test system composed of a Keysight B2902A source meter, a stepping-motor and control software. The sample resistance was measured by monitoring its voltage by applying a constant sensing current when bending the sensor by stepping-motor.

3. Result and discussion

It is well known that mica has a layered structure and the adjacent layers are attracted by Van der Waals forces instead of chemical bonds, so it is convenient to gain a thin and flat mica sheet. Fig. 1(b) shows a typical SEM image for a cleaved mica sheet, indicating a layered structure and a 4.47 μm thickness. The surface morphology of LSMO thin film on mica was characterized by AFM. As shown in Fig. 1(c), the LSMO thin film shows a smooth surface, and the root of mean square is around 1.62 nm over a 5 μm × 5 μm scanning area. Fig. 1(d) shows the XRD θ-2θ scanning patterns of the LSMO/Mica hetero-structure, which reveals a polycrystalline structure of the LSMO thin film.

The resistance changes under different bending state are investigated by Van der Pauw four-point method. During the test, one end of the device is fixed and the other end is driven to move by a stepping motor. Consequently, different bending states were realized. Resistances of the device in bending and unbending states are defined as Rb and R0, respectively. The real-time resistance change is defined as ΔR=Rb-R0, then the real-time resistance change rate can be defined as ΔR/R0=(Rb-R0)/R0. Fig. 2(a) shows the resistance change rate of the device as a function of bending radius. The inset of Fig. 2(a) shows a schematic diagram of the resistance measurement setup during bending state, the sensing current direction is same to the bending direction. The ΔR/R0 shows an increase monotonically with the decrease of the bending radius. At the bending radius of 3 mm, a ΔR/R0 of 5.0% can be observed. Fig. 2(b) shows the instantaneous resistance change rate with continuous bending/unbending cycles. The inset of Fig. 2(b) shows a single bending/unbending cycle, in which the bending radius changes with the sequence of flat→10 mm→8 mm→5 mm→8 mm→10 mm→flat. After 6 bending/unbending cycles, the resistance state at each flat state and different bending states nearly remain constant, indicating good repeatability and stability of the device during the various bending states. Fig. 2(c) depicts a long time resistance fatigue test of the device under various bending/unbending cycles. The time from bending to unbending for one cycle is 8 s. It was clearly shown that there was no performance deterioration after 3600 bending-unbending cycles. After 8 h of long-term bending/unbending, the change in ΔR/R0 of the device only shows a reduction by less than 0.5%. The ΔR/R0 values in the beginning, intermediate, and ending stages are very consistent and repeatable, as shown in the inset of Fig. 2(c), implying good stability and reliability of the present strain sensor.

Fig. 2.   (a) Resistance changes (ΔR/R0 (%)) upon different bending radius (the inset shows a schematic diagram of the resistance measurement setup during bending state); (b) the instantaneous change of the resistance under three different bending radius; (c) repetitive measurement of the resistance changes over 8 h bending time under bending radius of 8 mm. the three top figures show some typical cycles at the initial, intermediate, and ending stages of the testing process, respectively.

Fig. 3(a) shows the temperature dependence of resistance for the LSMO thin films at different bending radius. As the bending radius decreased, the resistance of LSMO thin film increased. Moreover, the strain sense of the device can be observed within a wide temperature range from 100 K to 300 K. Fig. 3(b) depicts the time dependent resistance of the LSMO thin film at 20 K at different bending states. For the resistances observed at different bending radius, all of them nearly remain constant after 10 h measurement at 20 K, suggesting a good stability of the flexible strain sensor even at extreme low temperature. The high temperature tolerance of the flexible strain sensor has also been studied. Fig. 3(c) shows the bending radius dependent resistance measured at different elevated temperatures. For the device under the same bending state, the resistance decreases with the increase of the ambient temperature, showing typical semiconductor conducting characteristic. Nevertheless, for the strain sensor at the same elevated temperature, the resistance increases with the decrease of curvature radius, showing the same change tendency as that at room temperature. The present results prove that the LSMO/Mica strain sensor can work not only at room temperature but also at high temperatures up to at least 773 K. Fig. 3(d) shows the quantitative values of resistance changing rate (ΔR/R0) as a function of ambient temperatures at different curvature radius. Compared with room temperature, the magnitude of resistance change is more obvious at high temperature, which indicates that the device is more sensitive to bending action at high temperature. The present LSMO/Mica strain sensors show working temperatures of 773 K and 100 K, which are much higher/lower than that of previously reported flexible strain sensors [9,34], indicating their great application potentials in the field of harsh electronics in the future.

Fig. 3.   (a) Temperature dependent resistances under different bending radius of curvature; (b) time-dependent resistances under different bending states at 20 K; (c) resistance as a function of bending radius of curvature at different high temperatures; (d) temperature-dependent resistance changes under different bending states.

Multimodal sensing with a single sensor was highly demanded in future flexible electronics. Considering that LSMO is a well known material with magnetoresistance (MR) effect, we also carried out the study on the magnetic resistance and magnetic sensing ability of LSMO thin films in our work. Fig. 4(a) shows the temperature dependent resistance for the film in 0 T and 1 T magnetic fields. Noticeable MR effect can be clearly observed. The resistance measured with 1 T magnetic field is clearly smaller than that measured without magnetic field, especially at low temperature. Fig. 4(b) shows the magnetic field dependent MR% for the film at different temperatures. The MR changes are generally defined as MR%={[R(H,T)-R(0,T)]}/R(0,T)×100%, where R(H,T) and R(0,T) are the temperature dependent resistance values under applied field and zero field, respectively. The MR effect increased obviously as the temperature reduces from room temperature to low temperature, which is consistent with what has been reported results [35,36]. The maximum value of MR is about 32% under the applied field of 2 T measured at 20 K. Fig. 4(c) shows the magnetic resistance of LSMO thin film with different bending radius at 20 K, which indicates that clear MR effect can be observed even at bending states. Furthermore, when the temperature increases to 300 K, the MR effect still remains for both flat and bending states. The present results shown in Fig. 4 imply that the LSMO/Mica hetero-structure can work for a multimodal sensor to measure strain and magnetic signal with a single device.

Fig. 4.   (a) Temperature dependence of the resistance at 0 T and 1 T magnetic field; (b) magnetic field dependent MR% at different temperatures for the LSMO thin film; resistance-magnetic field characteristics of LSMO thin films under different bending radius of curvature at 20 K (c) and 300 K (d).

Based on the above shown results, we carried out experiments to measure the strain and magnetic signal by using the same device structure of LSMO/Mica. The LSMO/Mica strain sensors were first pasted on gloves to monitor the movement of finger joint. Fig. 5(a) shows the time dependent resistance change (ΔR/R0) of the strain sensor. In each bending motion, the fingers are bent to the same position and released after holding for 2 s. It can be seen that the sensor is very sensitive to the bending action of the finger, and the resistance can be quickly restored to its original value after releasing during the repeated bending/releasing cycles. For the magnetic signal measurement, we adopted a commercial Nd2Fe14B permanent magnet as magnetic field source. The measurement setup was schematically shown in the inset of Fig. 5(b). The sensor was arranged to move toward the magnet for some distance, and then move back to the original position. During the movement, the resistance was in-situ monitored. The same process was repeated for several times, and the distance moving forward decreases with the increase of the cycle number. The time dependent resistance change during the moving forward/moving backward cycles was shown in Fig. 5(b). It can be seen that the smaller the distance between the sensor and the magnet, the higher the magnetic field intensity, and the more obvious the resistance changes. When the distance between the sensor and the magnet is 1 mm, the resistance change rate can reach 2.5%, indicating a clear magnetic sensing ability.

Fig. 5.   (a) Resistance change of the sensor during the finger bending/unbending cycles (the inset shows the actual photograph of the LSMO/Mica sensor fixed on fingers during the bending and unbending states); (b) time dependent MR change upon different magnetic field intensity or distance between magnet and sensor (the inset is a schematic diagram of the measurement setup for the magnetic signal).

4. Conclusion

In summary, we have fabricated a flexible sensor composed of functional oxide LSMO deposited on flexible mica substrate. A single LSMO/Mica sensor shows multimodal sensing ability to detect strain as well as magnetic signals. As a strain sensor, at the bending radius of 3 mm, a ΔR/R0 of 5.0% can be observed, indicating a good sensitivity. After 3600 bending/unbending cycles, the change in ΔR/R0 of the device only shows a reduction by less than 0.5%, indicating excellent endurability. As a magnetic sensor, a maximum MR change of 32% can be observed under the applied field of 2 T measured at 20 K. Most importantly, the LSMO/Mica flexible sensor can work in a wide temperature range between 20 K and 773 K, demonstrating its great application potential in extreme harsh environments.

Acknowledgment

This work was supported financially by the National Natural Science Foundation of China (No. 51872099), the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2016), the Guangdong Innovative Research Team Program (No. 2013C102), the Guangdong Provincial Key Laboratory of Optical Information Materials and Technology (No. 2017B030301007) and Science and Technology Program of Guangzhou (No. 2019050001).


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A versatile metal nanowiring platform enables the fabrication of Ag nanowires (AgNW) at a desired position and orientation in an individually controlled manner. A printed, flexible AgNW has a diameter of 695 nm, a resistivity of 5.7 μΩ cm, and good thermal stability in air. Based on an Ag nanowiring platform, an all-NW transistors array, as well as various optoelectronic applications, are successfully demonstrated.
[24] Y.Y. Yu, C.H. Chen, C.C. Chueh, C.Y. Chiang, J.H. Hsieh, C.P. Chen, W.C. Chen, ACS Appl. Mater. Interfaces 9 (2017) 27853-27862.

DOI      URL      PMID      [Cited within: 2]      Abstract

-1) was successfully exploited. These superior properties were revealed to originate from the reversible phase separation endowed by the nanogranular-like morphology formed in Ag. Owing to such discrete nanomorphology, the free volume within this Ag electrode is susceptible to the applied tensile strain and the ensuing change in conductivity enables the realization of an efficient strain sensor. Besides, a representative PTB7-th:PC71BM organic photovoltaic (OPV) using this electrode (with the assistance of a wrinkled scaffold to reinforce the stretchability of the active layer) can exhibit a power-conversion efficiency (PCE) of 6% along with high deformability, for which 75% of its original PCE is retained after 50 cycles of stretching under a 20% strain. Meanwhile, a representative all-polymer OPV consisting of a PTB7-th:N2200 blend, in which the N2200 has a better mechanical stretchability than that of PC71BM, can maintain over 96% of its original PCE after 50 cycles of stretching (under a 20% strain) without employing the wrinkled scaffold. Such promising performance in stretchable OPVs is among the state-of-the-art results reported to date.]]>
[25] B. Wang, T. Shi, Y. Zhang, C. Chen, Q. Li, Y. Fan, J. Mater. Chem. C 6 (2018) 6423-6428.

[Cited within: 1]     

[26] X. Gao, M. Zheng, X. Yan, J. Fu, M. Zhu, Y. Hou, J. Mater. Chem. C 7 (2019) 961-967.

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[27] X. Ren, K. Pei, B. Peng, Z. Zhang, Z. Wang, X. Wang, P.K. Chan, Adv. Mater. 28(2016) 4832-4838.

DOI      URL      PMID      [Cited within: 1]      Abstract

An organic flexible temperature-sensor array exhibits great potential in health monitoring and other biomedical applications. The actively addressed 16 × 16 temperature sensor array reaches 100% yield rate and provides 2D temperature information of the objects placed in contact, even if the object has an irregular shape. The current device allows defect predictions of electronic devices, remote sensing of harsh environments, and e-skin applications.
[28] W. Zhang, C.Y. Hou, Y.H. Zhang, H.Z. Wang, Nanascale 9 (2017) 17821-17828.

[Cited within: 1]     

[29] Y.H. Chu, NPJ Quantum Mater. 2(2017) 67.

[Cited within: 1]     

[30] H.J. Liu, C.K. Wang, D. Su, T. Amrillah, Y.H. Hsieh, K.H. Wu, Y.C. Chen, J.Y. Juang, L.M. Eng, S.U. Jen, Y.H. Chu, ACS Appl. Mater. Interfaces 9 (2017) 7297-7304.

DOI      URL      PMID      [Cited within: 1]      Abstract

2O4, CFO) and flexible muscovite was fabricated via van der Waals epitaxy. The combination of X-ray diffraction and transmission electron microscopy was conducted to reveal the heteroepitaxy of the CFO/muscovite system. The robust magnetic behaviors against mechanical bending were characterized by hysteresis measurements and magnetic force microscopy, which maintain a saturation magnetization (Ms) of ∼120-150 emu/cm3 under different bending states. The large magnetostrictive response of the CFO film was then determined by digital holographic microscopy, where the difference of magnetostrction coefficient (Δλ) is -104 ppm. The superior performance of this bimorph is attributed to the nature of weak interaction between film and substrate. Such a flexible CFO/muscovite bimorph provides a new platform to develop next-generation flexible magnetic devices.]]>
[31] J. Huang, H. Wang, X. Sun, X. Zhang, H. Wang, ACS Appl. Mater. Interfaces 10 (2018) 42698-42705.

DOI      URL      PMID      [Cited within: 1]      Abstract

0.67Sr0.33MnO3 (LSMO) thin films have been deposited on flexible mica substrates. The crystallinity and microstructure of the films have been characterized to show the good epitaxial quality of the films. The LSMO thin films on mica present excellent ferromagnetic and magnetoresistance properties (such as saturation magnetization Ms of 125-400 emu/cm3 at 10 K and a high MR value of ∼45% at 5 K under 1 T for the 50 mTorr deposited sample), which is even better than the ones on conventional rigid single-crystal oxide substrates. More interestingly, no deterioration of the properties is observed under mechanically bending condition, which demonstrates the good mechanical stretchability and property stability of the LSMO thin films on mica. The demonstration of functional oxides integrated on flexible mica substrates paves a route toward future flexible spintronics and electronics.]]>
[32] C.I. Li, J.C. Lin, H.J. Liu, M.W. Chu, H.W. Chen, C.H. Ma, C.Y. Tsai, H.W. Huang, H.J. Lin, H.L. Liu, P.W. Chiu, Y.H. Chu, Chem. Mater. 28(2016) 3914-3919.

DOI      URL      [Cited within: 1]     

[33] C.H. Ma, J.C. Lin, H.J. Liu, T.H. Do, Y.M. Zhu, T.D. Ha, Q. Zhan, J.Y. Juang, Q. He, E. Arenholz, P.W. Chiu, Y.H. Chu,Appl. Phys. Lett. 108(2016), 253104.

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[34] N.T. Tien, S. Jeon, D.I. Kim, T.Q. Trung, M. Jang, B.U. Hwang, K.E. Byun, J. Bae, E. Lee, J.B. Tok, Z. Bao, N.E. Lee, J.J. Park, Adv. Mater. 26(2014) 796-804.

DOI      URL      PMID      [Cited within: 1]      Abstract

Diverse signals generated from the sensing elements embedded in flexible electronic skins (e-skins) are typically interfered by strain energy generated through processes such as touching, bending, stretching or twisting. Herein, we demonstrate a flexible bimodal sensor that can separate a target signal from the signal by mechanical strain through the integration of a multi-stimuli responsive gate dielectric and semiconductor channel into the single field-effect transistor (FET) platform.
[35] G. Ren, S. Yuan, Z. Tian, Thin Solid Films 517 (2009) 3748-3751.

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