Stress and Magnetic Field Bimode Detection Sensors Based on

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A Stress and Magnetic Field Bi-mode Detection Sensor Based on Flexible CI/CNTs-PDMS Sponge Li Ding, Shouhu Xuan, Lei Pei, Sheng Wang, Tao Hu, Shuaishuai Zhang, and Xinglong Gong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11333 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 22, 2018

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ACS Applied Materials & Interfaces

A Stress and Magnetic Field Bi-mode Detection Sensor Based on Flexible CI/CNTs-PDMS Sponge Li Ding, Shouhu Xuan*, Lei Pei, Sheng Wang, Tao Hu, Shuaishuai Zhang, Xinglong Gong* CAS Key Laboratory of Mechanical Behavior and Design of Materials, Department of Modern Mechanics, University of Science and Technology of China (USTC), Hefei 230027, P. R. China KEYWORDS: smart material, sponge, sensor, bi-mode, magneto-sensitivity

ABSTRACT: This work reports a porous carbonyl iron particles/multi-walled carbon nanotubes-polydimethylsiloxane composite (PCMC) with high flexibility and low density. In comparison to the solid product, the porous PCMC possesses a larger elongation and deformation. Due to the excellent magnetic-mechanic-electric coupling performance, the flexible composite exhibits bi-mode sensitivity to both of the external stresses and magnetic field. Typically, the normalized resistance variation (∆R/R) of PCMC reaches 82.8% and 52.2% when the compression strain and tension strain are 60% and 50%, respectively. Moreover, ∆R/R induced by bending, twisting and magneto-stress also changes remarkably. When a 144 mT magnetic field is applied, the ∆R/R of PCMC increases with 3.6%. To further understand the magnetic-mechanic-electric coupling mechanism, a conductive-network sensing model is proposed and analyzed. Finally, based on the bi-mode PCMC sensor array, a smart chessboard which can precisely discriminate special chesses with different masses and magnets is

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developed. This study provides a new fabrication method for next-generation 3D smart sensors toward artificial electronics and soft robotics.

INTRODUCTION Electrically conductive nanocomposites with high flexibility and stretchability are essential to wearable electronics with wide application in electronic skins, health monitors, and robots.1-7 In comparison to the traditional rigid metallic sensors, the piezoresistive polymer nanocomposites, which can efficiently transduce mechanical signals into electrical signals, have attracted increasing research interests in flexible electronic sensors due to their excellent deformation capability. The piezoresistive polymer nanocomposites are conventionally fabricated by doping the polymer matrix with conductive micro/nano-fillers, such as graphene, carbon nanotube, and silver nanowires. During the past decades, various nanocompoaites with 1D fibrous structure,8 2D thin/bulk film, multilayer structure,5 and 3D geometry structure,4 have been successfully developed as the sensing materials because of the simple structure and easy collection. Porous conductive sponges (PCS) are considered as ideal materials for piezoresistive sensors attributed to their more flexible skeleton, higher compressibility, and lower density than the conventional bulk sensors.9-11 The porous structures made by gas foaming, template leaching, phase separation, or other techniques provide the materials with an extensive range of applications for oil/water separation, adsorption, proton conduction, energy storage and conversion, sensing, biomedical scaffolds, etc.12-17 Moreover, the porous polymeric sensors have a large compression strain-resistance region and display a high sensitivity in the low-pressure region owing to the reduced stiffness.18-19 Among them, the polydimethylsiloxane (PDMS) based


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PCSs have been proven to be a wonderful sensor because of their excellent deformability and failure strain. It was found that the PCSs could be obtained by dip-coating conductive nanomaterials into porous PDMS or by using the nanomaterials-coated water-soluble particles as the porous template.20-22 However, because the conductive nanomaterials cannot be completely transferred and embedded in the pore walls, the detachment of the nanomaterials may occur when the samples contact and rub against other objects. Moreover, a reduction in the long-term durability may be caused by the reduced stiffness and recovery ability of the porous-structure polymer. During the past decade, several PDMS based PCSs have been developed and they exhibited broad potential in piezoresistive sensors.22-24 Here, the single functionality, expensive precursor, and complicated fabrication confined their further processing. To better understand the intrinsic mechanical-conductive coupling nature and enlarge its practical application, exploring facile approaches to fabricate high performance PDMS based PCSs is in great demand. Additionally, the sensors are usually worked in a complex multi-field circumstance, thus a multi-functional sensor which can distinguish different stimuli becomes an urgent requirement. Magnetoresistivity is a typical magnetic-electric coupling behavior, of which the conductivity changes with an external magnetic field, finding high potential in magnetic field sensors.25-30 Usually, the magnetic particles together with conductive materials fill in the polymer matrix to form pseudo-chains. Both the isotropic and anisotropic magnetic/conductive nanocomposites have been developed and they can be used to implement magnetic field sensors.31-38 Because of the low magnetic force and large polymer obstacle, the low-field magnetoresistance of the magnetoresistive polymer sensor become a critical problem. In consideration of the high flexibility and large deformation of the porous polymer, the magnetoresistive PDMS sponge will be favorable for high performance magnetic field sensors. More importantly, the


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magnetoresistivity constitutes an interesting improvement over the commonly preferential percolation of conductive nanomaterials. As a result, designing a magnetic/conductive PDMS sponge seems to be a considerable attempt for the magnetoresistive and piezoresistive bi-mode sensor. Here, a novel stress/magnetic field bi-sensitive composite sponge was constructed by embedding multi-walled carbon nanotubes (CNTs) and carbonyl iron (CI) particles into porous PDMS matrix through a facile sacrificing template method. The porous carbonyl iron particles/multi-walled carbon nanotubes-polydimethylsiloxane composite (PCMC), namely porous conductive/magnetic composite, possesses the advantages of simple manufacturing, low precursor cost, no pollution, large-scale production, and multi-functionalities. It exhibits preeminent deformability, good strain-dependent electrical properties, and outstanding magnetic controllability. PCMC can not only quantify the multiple mechanical strain induced by compression, tension, flexion, and torsion but also monitor the changes of magnetic field. Their magnetic-mechanic-electric coupling performance is systematically investigated. In addition, a conductive-network sensing model is proposed and analyzed for a detailed insight into the mechanic-electric and magneto-electric conducting behavior. Furthermore, an electronic chessboard based on the real-time tactile sensing PCMC arrays is constructed and it can efficiently locate external complex loadings through sensing the magnetic field and pressure. RESULTS AND DISCUSSION Design and structural characterization of CI/CNTs-PDMS sponge. The fabrication processes of porous conductive/magnetic composite are schematically illustrated in Figure 1a. Briefly, CNTs (Figure 1b), CI particles (Figure 1c) and the curing agent were mixed with PDMS


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precursor. Then the CI/CNT/PDMS mixture was poured into the prepared water-soluble particulates (carbamide, sugar, and salt) filled mold (more details please see the Experimental Section). Finally, it was vulcanized at 90 °C under 14 MPa for 20 min and then dissolved in distilled water to form the sponge structure. Taking into consideration of various requirements, such as deformability, elasticity, magnetic-sensitivity, stiffness, porosity, and morphological characteristics, the hybrid polymer with curing ratio of 1:30, CI mass ratio of 50%, CNT mass ratio of 1.5% and the salt filled mold were ultimately selected for PCMC. More experimental details and discussion are described in Figure S1,2. The microstructure of as-prepared PCMC (porosity of 65%) and cross-sectional views of their morphologies are shown in Figure 1d,e. The distribution of cavities was uniform and few structural defects were found. CI particles and CNTs were well dispersed in the PDMS matrix (Figure S3a) and the particles were completely wrapped so as to be effectively protected from oxidation (the SEM image of the interior wall of a cavity, Figure S3b). The structural schematic illustration and the product are shown in Figure 1f. The prepared PCMC (diameter: 10 mm; height: 6.5 mm) can be placed on a dandelion without causing damage, showing a lightweight advantage (Movie S1). Moreover, the incorporation of CI/CNTs in the backbone of PDMS sponge strengthened PCMC. The PDMS sponge has the disadvantage of poor dimensional stability and poor recovery ability, greatly reducing its structural stability and reliability, thus limiting its application in the industrial field. However, after doping the CI and CNTs (Figure S3c), the tensile modulus (Figure 1g) and reversibility (Figure S3d) of PCMC were highly improved compared with pure PDMS sponge. According to the creep curves with shear stress of 200 Pa, 99.9% recovery of PCMC was reached within 5 s, demonstrating the significant improving creep recovery (compared to the 97.7% recovery of PDMS sponge) (Figure S3e).


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Under a wide range of the applied shear stress, the CI/CNTs-PDMS sponge exhibited exceptional elasticity, which were almost as good as nonporous PDMS polymer (Figure S3f). Mechanic-electric coupling properties. To study the stress/strain responsive mechanical properties of PCMC, a series of experiments were conducted to test the structural deformation/recovery and dynamic response to external force. After removing the applied strain of 60%, the product backed to 99.9% of the original height, indicating a significant high recovery rate (Figure 2a). Importantly, PCMC was instantly deformed and recovered within tens of milliseconds after being hit by a river stone (4.46 g, at a height of 10 cm) (Figure 2b, Movie S2). The result demonstrated that PCMC has the rapid response, great deformation and excellent recovery capabilities. The cyclic compression loading-unloading test was conducted to investigate the stability. Figure 2c depicts the schematic of the compressive experimental setup. The upper and lower faces of PCMC cylinder were mounted with copper electrodes by using silver adhesive, connecting with the Modulab MTS which could supply a direct voltage excitation and measure the responsive current. The upper slice was used to supply the compressive displacement and record the force signal simultaneously. When the compression strain changed linearly by 60%, the resistance variation increased together with the increase of normal force due to the gradual closure of the cavities upon loading. When decompressed, normal force rapidly decreased but the resistance change was relatively sluggish owing to the viscoelasticity of polymer and then eventually recovered in seconds. The gauge factor (GF), the ratio of relative change in electrical resistance ∆R/R (R is the initial resistance and ∆R is the resistance variation) to the mechanical strain ε, was 1.38 under 60% strain (Figure 2d).


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Then the influence of compressive rates and strain on the tendency and peak value of electrical resistance variation were investigated. The compressive rate was set as 50 mm/min, 40 mm/min, 20 mm/min, and 10 mm/min, respectively and the compressive strain varied from 0 to 50% circularly (Figure 2e). The results showed the test compressive rates had no significant effect in the resistance change in a certain compressive rate range. Figure 2f exhibited the influence of compression strain on the electrical properties. When the strain was set at 10%, 20%, 30%, 40%, 50%, and 60%, ∆R/R increased from 0 to 3.4%, 7.1%, 12.4%, 19.3%, 40.1%, and 82.8%, respectively, indicating that not only mechanical properties but also electrical properties of PCMC exhibited excellent reversible deformation ability, high sensitivity and stability performance. Moreover, two responsive piezoresistivity varied at two pressure regions (Figure 2f- ). Although the GF was smaller at the low strain region than at the high strain region (Figure 2f- ), a higher sensitivity was shown in the lower pressure region because of the lower Young’s modulus. As the PCMC continued to be compressed, Young’s modulus increased causing the reduction of piezosensitivity. Furthermore, the mechanical and electrical performance of PCMC under stretching loading were evaluated (Figure 3a). The ∆R/R gradually increased with increasing elongation from 0 to 50%. It reached 52.2% under 50% tensile strain (Figure 3b) and the corresponding tensile force reached 240 mN (Figure 3c). Both the electrical and mechanical signals returned to the original state in each stretching-releasing cycle. PCMC displayed fast response, high sensitivity in the low-pressure region, and excellent repeatability properties. The triadic relation curves of compressive/tensile strain, peak values of normalized resistance variations and normal forces are further described in Figure S4. The varying tendency of strain dependent normalized resistance was basically consistent with the normal force’, pointing out the great potential of PCMC as


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tensile-stress/strain detection sensor. Furthermore, electrical properties with different tensile rates were also investigated (Figure 3d,e). The results show that the test tension rates exhibited few effect on the electrical response, as does the compression rate. The super-soft and flexibility naturally endows PCMC with bending and torsion properties. In order to investigate the electrical sensing capability at various angles, the PCMC sample was fixed on a plastic film substrate to emulate the e-skin and then the assembled sample was clamped by the home-made holders (Figure S5). As the falling height (∆H) of the holder increased from 3 to 8 mm with 1 mm step (Figure 4a), the bending angle increased and the peak value of normalized resistance variation increased correspondingly, indicating that the variation in electric performance is distinctly dependent on the curving angle. As the falling heights were 3 mm, 4 mm, 6 mm, 8 mm, 10 mm, and 12 mm, the matching angles were 82, 95, 113, 131, 146, and 158 degrees and the average normalized resistance variations were 2.0, 2.4, 3.0, 4.3, 5.1, and 6.0%, respectively (Figure 4b). Additionally, varied torsion angles were set to measure the electrical dynamic response and recovery of PCMC (Figure S5). Normalized resistance variations as a function of different twisting angles are shown in Figure 4c. The stability of electrical signal changes was further evaluated by periodic tension/flexion and release cycles. As exhibited in Figure 4d, after 80 cycles of deformations at an elongation of 15% or 500 cycles at a bending angle of 105 degree, PCMC essentially recovered their resistance after releasing, indicating the excellent durability. In summary, PCMC is a kind of ultra-flexible porous material and it is highly sensitive to the external applied stresses. Table S1 shows a comparison of the sensitivities between PCMC and flexible strain sensors reported previously. The remarkable and stable electrical variation of


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PCMC to external stimuli, demonstrating PCMC has the great potential as an enticing candidate for multifunctional sensor including tension, compression, flexure, and torsion sensing system.

Magnetic-electric coupling performance and magneto-elastic model. Owing to the incorporated CI particles, PCMC also exhibited a remote contactless actuation and magnetism sensing characteristics in addition to quantifying various mechanical stimuli. The deformation behavior of PCMC cylinder affected by magnetic field was shown in Figure S6. Here, a laminar sample was chosen in the magnetic responsive experiments due to the restricted measuring setup. The magnetic responsive performance was systematically investigated using the experimental setup shown in Figure 5a, which consists of four parts: a rheometer (Physica MCR 301) equipped with an electro-magnetic accessory, a DC power supply, an electrical property test system Modulab MTS, and a pre-programed control and data storage-analyzing system (software). The magnetic field was generated by adjusting the electromagnetic coil current which was afforded with the DC power supply machine. The detailed magnetic field is shown in Figure S7 and the relationship between the current and magnetic flux density at the center is shown in Figure S8, which has the linearity of = 0.9995 for the fitting curve. With increasing of the magnetic flux density (86 mT, 101 mT, 115 mT, 130 mT, and 144 mT, respectively), the electrical response ∆R/R was increased (the corresponding values were 0.5%, 1.1%, 1.7%, 2.6%, and 3.6%) in the periodic loading and unloading experiments (Figure 5b). Here, analogous to the strain factor of a strain gauge GF, GF is defined as the ratio of relative resistance variation to magnetic flux density change. Two sensitivity ranges can be observed (Figure 5c). The GF are 0.07-0.14 and 0.14-0.25 in the ranges of 0.086-0.115 T and 0.115-0.15 T, respectively. The


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stability of the magnetic field dependency was further investigated. Under applying 100 mT for more than 100 cycles, the electrical response to magnetic excitation maintained good stability and reliability (Figure 5d). In addition, the responsive performance of PCMC in other magnetic conditions is also investigated (Figure S9). As shown in Table S1, compared to other reported flexible magnetic field sensors, PCMC exhibits a good gauge factor. It is worth mentioning that PCMC was prepared by the reused mold and the commercial CNT and salt, demonstrating that it has the advantage of cheaper raw material and simpler fabrication method comparing with other magnetic field sensors.32-33 To this end, the porous conductive/magnetic composite has the potential as a low-cost portable magnetic field sensor. To better understand the magnetic-electric principle, the magneto-elastic behavior is first analyzed. By applying the external magnetic field, CI particles become magnetized. According to a Langevin function, a magnetization per unit volume is = coth + 1⁄

(1)

where is the saturation magnetization, H is the magnetic field strength, and = ⁄ . represents the mean of the volume of magnetic particles. T is the temperature. is the Boltzmann constant. The fitting magnetization curve of CI particles is shown in Figure 5e. The magnetization generates the interaction among CI particles. ! is defined as the magnetic force of particle i exerted by particle j. According to the point dipole approximation, ! can be described as:39 ! = "#−% ∙ %! + 5% ∙ ' ! %! ∙ ' ! (' ! − % ∙ ' ! %! − %! ∙ ' ! % )

(2)

*

% = +

(3)


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where % means the magnetic moment vector of particle i, ' ! is the normalized relative positional vector from particle i to particle j, k is a geometrical-morphological constant. For convenience, the CI particles are regarded as monodisperse microspheres with a diameter (7 µm)40 equal to the mean diameter measured from experiments and they are considered to be uniformly dispersed in the backbone wall. For a pair of particles positioned along the magnetic field, the magnetic force ,∥ = 2/ (leading to the attraction); for two particles in orthogonal direction to the magnetic field, the magnetic force ,0 = −/ (leading to the repulsion). The interaction relationship among regularly arranged CI particles with an applied external magnetic field is qualitatively illustrated in Figure 5f. Furthermore, a simple schematic diagram of the magneto-induced deformation process for PCMC’s backbone walls is shown in Figure 5g (detailed in the next paragraph). For PCMC, one backbone wall can be thought as a directional pseudo-chain formed by the aggregation of CI particles. By applying the external field, magnetic interactions generate a magneto-induced stress tensor as: 2

1 = ∑ ∑!6 5 ! ! 3

(4)

where 5 ! is the distance between particles, is the volume of the backbone wall. As described above, pseudo-chains parallel to the magnetic field induce the tension while those perpendicular to the magnetic field generate compression. By using the simulation, the magneto-induced normal stress Pz of a backbone wall (assumed as a cylinder: the diameter is 70 µm and the height is 400 µm) when applied parallel and perpendicular magnetic field are obtained (Figure 6a). According to Eq. 1-3, P exhibited a quadratic dependence on the magnetic field strength (7 = 8 ) under small fields (Figure 6b,c). The relations of magnetic flux density, normalized resistance variation and magnetic stress are shown in Figure S4. The magneto-induced stress acts


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like a mechanical force field on the PCMC. Through magneto-elastic coupling, CI particles tend to align along or perpendicular to the field direction under magnetic field, which leads to the complex deformation of the porous structure thereby giving rising to the deformation and movement of CNTs. It will bring on a change in the number of effective conducting paths and thus the change of electrical signals. The deformation under the effect of P is similar to that induced by mechanical stress (detailed in the next paragraph). When the magnetic field is removed, CI particles restore their location due to the elasticity of polymer, resulting in the recovery of mechanical and electrical properties. Figure 5g displays the structure deformation process from an initial random dispersion of CI particles without magnetic field (the approximate orientation follows the backbone walls) to a stable columnar pattern with the applied external magnetic field (inducing the deformation of porous structure).

Bi-mode sensing mechanism. The electrical property of PCMC is highly relied on the CI/CNT interconnected networks within the porous structure. The electrical performance of PCMC is mainly controlled by the electrical properties of network components (carbon nanotubes and iron particles) and a combination of conductive network morphology.41 The intratube/particle resistance of CNT/CI and the inter-tube/particle resistance (that is the resistance of the effective bridge crossing between adjacent conductive components) are defined as , 9 , respectively. A simple conductive network, assuming that there are :; parallel paths and :

Stress and Magnetic Field Bimode Detection Sensors Based on

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