Flexible capacitive pressure sensor with a wide detection range based on porous carbon nanotube、carboxyl iron powder/silicone composite
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摘要: 柔性压容传感器由于其结构简单、响应速度快、灵敏度高、成本低等优点，在健康监测、机器人、可穿戴设备等领域有非常广泛的应用需求。然而，传统的柔性压容传感器难以兼顾有效应力检测下限和有效应力检测上限，这限制其在更宽领域范围的应用。本文以糖颗粒(SPs)为造孔剂，羰基铁粉(CIPs)为磁响应填料，碳纳米管(CNTs)为导电填料，通过与硅橡胶复合制得柔性多孔CNT、CIP/硅橡胶复合材料，并以此为介电层材料制备得到柔性压容传感器，其在0-5 Hz频率范围内的有效应力检测范围为0.07-180 kPa，优于大部分文献报道的柔性压容传感器。由于具有宽的有效应力测试范围，长周期服役稳定性和快速响应，该传感器可用于监测人体呼吸、手臂运动、机械臂运动和语音识别等方面，在健康监测、可穿戴电子设备以及智能机器人等领域具有很好的应用前景。Abstract: Featured by simple structure, fast response, high sensitivity, and low cost, etc., flexible capacitive pressure sensor has been widely used in the fields of health care, robotics, wearable devices and so on. However, the trade-off between the effective upper and lower detection limits greatly restricts the applications of the flexible capacitive pressure sensor. In this work, a flexible and porous carbon nanotube (CNT)/carbonyl iron particle (CIP)/silicone composite was produced by using sugar particles (SPs) as the as pore-forming agents, CIPs as the magneto-responsive fillers, CNTs as the conductive fillers and silicone rubber as the flexible matrix. After serving as the dielectric layer, the porous CNT、CIP/silicone composite endows the capacitive pressure sensor produced a wide effective detection range of 0.07-180 kPa (at the frequency range of 0-5 Hz), much wider than most capacitive pressure sensors reported. In virtue of the wide detection range, long-term stability and fast response, the sensor produced is capable of monitoring human breath, arm movement, talking, and robotic movement, thus showing great promise in health monitoring, wearable electronic devices, and intelligent robotics, etc.
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图 1 柔性多孔碳纳米管(CNTs)、羰基铁粉(CIPs)/硅橡胶复合材料的制备流程图
Figure 1. Schematic diagram of the preparation of flexible porous carbon nanotube (CNT)、carbonyl iron particle (CIP)/silicone composite
图 2 扫描电子显微镜照片：糖颗粒(SPs) (a)，CIPs (b)，CNTs (c)和多孔CNT、CIP/硅橡胶复合材料 (d-f)
Figure 2. SEM images of sugar particles (SPs) (a), CIPs (b), CNTs (c) and porous CNT、CIP/silicone composite (d-f)
图 3 (a) 不同CIPs含量的CIP/硅橡胶复合材料的压缩应力-应变曲线；(b) 不同SPs添加量的多孔CIP/硅橡胶复合材料的压缩应力-应变曲线
Figure 3. (a) Compressive stress-strain curves of the CIP/silicone composite with various CIP content; (b) compressive stress-strain curves of the porous CIP/silicone composite with various SP content
图 4 (a) 不同CNTs含量的多孔CNT、CIP/硅橡胶复合材料的电导率；(b) 不同CNTs含量的多孔CNT、CIP/硅橡胶复合材料的相对介电常数-压缩应变曲线。
Figure 4. (a) Electrical conductivity of the porous CNT、CIP/silicone composites with various CIP content; (b) dielectric constant - compressive strain curves of the porous CNT、CIP/silicone composites with various CIP content.
图 5 (a) 多孔CNT、CIP/硅橡胶压容传感器在0 mT和300 mT下的电容响应-压缩应力曲线；(b) 0.7 g的塑料块放在传感器表面时的电容响应。
Figure 5. (a) Capacitance response-compressive stress curves of the porous CNT、CIP/silicone capacitive pressure sensor at 0 and 300 mT; (b) capacitance response of the sensor when placing a 0.7 g plastic block on its top surface.
ΔC/C0—Relative change rate of the capacitance of the sensor compared to the initial capacitance during the compression process.
图 6 无磁场时，柔性多孔CNT、CIP/硅橡胶压容传感器的动态响应曲线。
Figure 6. Capacitive response of the flexible porous CNT、CIP/silicone capacitive pressure sensor at 0 mT.
图 7 施加300 mT的磁场后，柔性多孔CNT、CIP/硅橡胶压容传感器的动态响应。
Figure 7. Capacitive response of the flexible porous CNT、CIP/silicone capacitive pressure sensor at 300 mT.
图 8 柔性多孔CNT、CIP/硅橡胶压容传感器的长周期服役稳定性测试。(a-b) 0 mT时，传感器对4000次循环载荷：1.5 kPa (a) 和50 kPa (b)的电容响应。(c-d) 300 mT时，传感器对4000次循环载荷：20 kPa (c) 和180 kPa (d) 的电容响应。
Figure 8. Long-term stability of the flexible porous CNT、CIP/silicone capacitive pressure sensor. (a-b) Capacitive response of the sensor to 4000 cycles of compressive stresses of 1.5 (a) and 50 kPa (b) under 0 mT. (c-d) Capacitive response of the sensor to 4000 cycles of compressive stresses of 20 (c) and 180 kPa (d) under 300 mT.
图 9 柔性多孔CNT、CIP/硅橡胶复合材料传感器在0 mT (a) 和300 mT (b) 下的响应时间。
Figure 9. Response time of the flexible porous CNT、CIP/silicone capacitive pressure sensor at 0 (a) and 300 mT (b).
图 10 (a) 柔性多孔CNT、CIP/硅橡胶压容传感器的工作机制；(b) 柔性多孔CNT、CIP/硅橡胶复合材料在施加磁场前后的横截面变化
Figure 10. (a) Working principle of the flexible porous CNT、CIP/silicone capacitive pressure sensor; (b) cross-section of the flexible porous CNT、CIP/silicone composite before and after applying a magnetic field
图 11 柔性多孔CNT、CIP/硅橡胶压容传感器的应用展示。(a) 实时监测鼻息；(b) 实时监测腹部呼吸；(c) 实时监测人说话时的面部肌肉运动；(d) 实时监测手臂抓取哑铃；(e) 在有无磁场下实时检测机械手臂夹持不同质量的重物
Figure 11. Applications of the flexible porous CNT、CIP/silicone capacitive pressure sensor. (a) Real-time monitoring of the nasal breathing; (b) real-time monitoring of the abdominal respiration; (c) real-time monitoring of the facial muscle movements when a testee speaking; (d) monitoring of the arm movement; (e) monitoring of a robotic arm holding objects of various weights before and after applying a magnetic field
表 1 含有0.5% CNTs，60% CIPs和150% SPs的CNT、CIP、SP/硅橡胶复合材料在超声波洗涤前后的质量变化
Table 1. Mass changes of the CNT、CIP、SP/silicone composite with 0.5% CNTs，60% CIPs and 150% SPs before and after ultrasonic washing
Sample Initial mass/g Remaining mass/g Percentage of mass remaining 1 5.9669 3.0848 51.70% 2 5.9715 3.0742 51.48% 3 6.0228 3.1354 52.06% 4 6.0303 3.1216 51.77% Notes: The theoretical residual mass fraction of CNT, CIP, SP/silicone rubber composite after completely removing SPs is 51.7%, and the contents of CNTs, CIPs and SPs in this paper are relative values, relative to the relative mass ratio of the base silicone rubber. For example, CNT, CIP, SP/silicone rubber composite containing 0.5% CNTs, 60% CIPs and 150% SPs refers to the composite prepared by mixing 0.5% CNTs, 60% CIPs and 150% SPs with silicone rubber as 100% according to the mass ratio. 
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表 2 不同CIPs含量的CIP/硅橡胶复合材料在压缩应变为70%时的压缩模量
Table 2. Compressive modulus of the bulk CIP/silicone composites with various CIP content at the strain of 70%
Magnetic field 50% CIP 60% CIP 70% CIP 0 mT 386 kPa 471 kPa 566 kPa 300 mT 487 kPa 664 kPa 720 kPa
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表 3 不同SPs添加量的多孔CIP/硅橡胶复合材料在压缩应变为70%时的压缩模量
Table 3. Compressive modulus of the porous CIP/silicone composites with various SP content at the strain of 70%
Magnetic field 100%r SP 150% SP 200% SP 0 mT 201 kPa 150 kPa 103 kPa 300 mT 377 kPa 326 kPa 199 kPa
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表 4 不同柔性传感器的有效应力检测范围
Table 4. Pressure detection ranges of the flexible pressure sensors reported.
Sensor Production Method Detection Range/kPa Refs. Bi2Te3 and Sb2Te3 film sensor Deposition, sputtering, and laser ablation 0.2-1.5 [25] Graphene/polyurethane foam sensor Freeze-drying, dip-coating, and chemical reduction 30-500 [26] Graphene/polydimethylsiloxane sensor Templating and spray-coating 0.2-25 [27] Graphene paper sensor Templating, dip-coating and thermal reduction 0.3-25 [28] Binary spiky/spherical nanoparticle film sensor Spray-coating and polymerization 0.05-120 [29] Polyaniline foam sensor Electro-deposition, chemical etching and pre-cracking 0.004-5 [30] Porous CNT、CIP/silicone composite sensor Mixing, curing and washing 0.07-180 This work
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