Design, fabrication and performance of flexible pressure sensors based on microstructures
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摘要: 随着科技的快速发展，电子皮肤和柔性可穿戴设备由于在人体运动、健康监测、智能机器人等领域具有重要应用而引起了人们广泛的关注。传统的基于贵金属或金属氧化物半导体的压力传感器成本高、柔韧性差，而新型的基于微结构的柔性压力传感器具有灵敏度高、应变范围宽、低成本、低功耗、响应速度快等优势，在电子皮肤和柔性可穿戴设备等方面发挥重要作用，成为当前柔性电子材料与器件主要研究热点之一。本文系统总结了近年来颇受关注的基于金字塔形、微球形、微柱形、仿生结构、褶皱等不同柔性基底微结构和多孔导电聚合物材料的柔性压力传感器在材料选择、结构设计、制备方法、传感性能等方面取得的重要进展，并对柔性压力传感器的未来发展进行了展望。Abstract: With the rapid development of science and technology, electronic skin and flexible wearable devices have attracted wide attention because of their important applications in human motion, health monitoring, intelligent robots and other fields. The traditional pressure sensors based on noble metal or metal oxide semiconductor have high cost or poor flexibility, while the flexible pressure sensors based on microstructures have the advantages of high sensitivity, wide strain range, low cost, low power consumption and fast response, which play an important role in electronic skin and flexible wearable devices and have become one of the main research hotspots of materials and devices in flexible electronics. This review systematically summarizes the important progress made in the material selection, structural design, preparation methods and sensing performance of flexible pressure sensors based on different flexible substrate microstructures such as pyramid, microsphere, micro-column, bionic structure and fold and porous conductive polymer materials. Finally, the future development of flexible pressure sensors is prospected.
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果蔬、食品、乳酸菌、血液、疫苗等对环境温度敏感的物质，采用冷链物流耗能较大，而利用材料在相变过程中储存或释放能量调节周围环境的温度，可以改进并解决其保质运输与储存的问题。相变储能材料(PCMs)储能密度大、相变温度稳定、装置简单、设备灵活，能快速解决能源在时间及空间上不匹配的矛盾[1-2]，在军工、航天、纺织、制冷设备、食品及农产品等领域有广泛的工程应用价值[3-9]。在冷链物流包装中，大多数果品贮藏温度为0~8℃，且具有相变温度稳定性好、相变潜热高等优点。相变储能材料按照材料类别可以分为无机相变储能材料、有机相变储能材料及共晶相变储能材料[10-11]。无机相变材料具有较高导热性和相变潜热，主要应用于中高温材料系统[12]，但是普遍存在严重的过冷和相分离现象[13]。有机相变储能材料基本没有过冷度及相分离现象、化学性能稳定且成本低，但是储能密度相对较低，主要应用于中低温材料系统[14-15]。共晶相变储能材料是由两种或两种以上的成分组成的低共熔物，在一定程度上克服了有机相变储能材料与无机相储能材料的局限性，但在导热性能、循环稳定性和储热性能方面仍需改善[10]。
目前常用的有机相变储能材料主要包括石蜡、酯、脂肪酸、醇和烷烃等，固体成型性好、不易燃、不易发生过冷和相分离现象[16-18]。脂肪酸的过冷度小，有可逆的融化和凝固性能，是性能良好的有机相变储能材料。月桂酸、正癸酸、棕榈酸、肉豆蔻酸及它们的共混物是应用比较多的相变材料[19-22]。石蜡是一种固-液相变材料，具有相变潜热高、无过冷和相分离、熔点低、化学性质稳定、价格低廉等优点，通常用来改善正十二烷、正十四烷等烷烃的导热性能或熔点[23-24]。在0℃左右的有机相变储能材料中，正癸醇结构简单、相变潜热较高、性质稳定，是一种适合的相变材料，通常将其与其他材料复配得到可调节相变温度的材料[17]。
由于单一有机相变储能材料存在相变温度不可调、相变潜热较低等缺点，无法满足冷链物流用相变储能材料熔点为0~8℃、相变潜热高的要求，且部分有机相变储能材料价格昂贵，无法在生产生活中大量应用。因此，本文选用正癸酸、月桂酸甲酯、正癸醇、月桂酸及十四烷，通过物理共混法制备正癸酸-月桂酸甲酯、正癸酸-正癸醇、月桂酸-十四烷三种二元有机复配物，并针对二元有机复配物在相变过程中的泄漏问题利用凝胶对其进行吸附以期获得适用于果品保质包装与物流技术的有机相变储能材料。
1. 实验部分
1.1 原材料
正癸酸，99%，分析纯；正癸醇，98%；月桂酸，99%，优级纯；十四烷，98%；月桂酸甲酯，99%；N-异丙基丙烯酰胺，98%；N,N'-亚甲基双丙烯酰胺(MBA)，99%；过硫酸铵，≥98%，分析纯；四甲基乙二胺，99%；聚乙二醇1000；聚乙二醇4000，化学纯；聚乙二醇8000，分析纯；聚乙二醇10000，上海阿拉丁生化科技股份有限公司。聚乙二醇200，化学纯，北京国药集团化学试剂有限公司。
1.2 样品制备
1.2.1 凝胶的制备方法
利用在氧化还原体系下引发单体进行原位自由基聚合的方法制备凝胶。首先，选择1 mol/L的N-异丙基丙烯酰胺(NIPAM)作为单体，MBA为交联剂，为单体总质量的0.5%，聚乙二醇1000(PEG1000)为致孔剂，为单体总质量的40%，将NIPAM、MBA、PEG1000共同溶解于去离子水中。其次，在过硫酸铵(APS)和四甲基乙二胺(TEMED)氧化还原体系中保持20℃引发聚合反应24 h，形成以PEG1000作致孔剂的聚N-异丙基丙烯酰胺(PNIPAM)凝胶，凝胶的命名规则为PNIPAM-y%PEGx，x代表聚乙二醇的分子量，y代表聚乙二醇占NIPAM质量的百分比(表1)。例如，PNIPAM-40%PEG1000为以PEG1000作为致孔剂的PNIPAM凝胶，且致孔剂含量为NIPAM质量的40%。制备过程如图1所示。APS广泛应用于水相自由基聚合，TEMED作为促进剂用于加快聚合反应的进行。第三步，将制备好的凝胶在去离子水中浸泡三天，每6 h换一次去离子水，以去除多余的单体及不参与反应的PEG1000，将浸泡处理之后的PNIPAM取出并放置于−25℃的冰箱中冷冻24 h，随后将冷冻好的PNIPAM凝胶放进冷冻干燥机(LGJ-10C 型，江苏金坛市环宇科学仪器厂)冷冻干燥72 h，冷肼温度为−55~−50℃，真空度为0.5~50 Pa。
表 1 聚N-异丙基丙烯酰胺(PNIPAM)-聚乙二醇(PEG)凝胶的命名Table 1. Naming of poly(N-isopropylacrylamide) (PNIPAM)-polyethylene glycol (PEG) gelSample Mass ratio of PEG∶NIPAM/% Molecular weight of PEG PNIPAM-y%PEGx y x Note: NIPAM—N-isopropylacrylamide. ![]()
图 1 PNIPAM-40%PEG1000凝胶制备过程Figure 1. PNIPAM-40%PEG1000 gel preparation processPEG1000—Polyethylene glycol 1000; APS—Ammonium peroxodisulphate; TEMED—Tetramethylethylenediamine1.2.2 二元有机复配物的制备方法
依据混合物凝固点下降原理，利用两种单一物质的物理共混加热法制备二元有机复配物，均匀的二元有机复配物体系的FTIR图谱如图2所示。首先，称取一定摩尔比的正癸酸、月桂酸甲酯于透明玻璃瓶中，将其置于60℃的恒温水浴锅中融化10 min，正癸酸融化完全。其次，用旋涡震荡仪(XW-80A型，佛山予华仪器科技有限公司)混合10 min，再用超声波震荡仪将混合物震荡2 min，正癸酸与月桂酸甲酯充分混合，得到正癸酸-月桂酸甲酯二元有机复配物体系，二元有机复配物体系没有分离现象，正癸酸-正癸醇及月桂酸-十四烷二元有机复配物体系采用相同的方法制备。
1.2.3 相变材料的制备方法
NIPAM的分子链上同时具有亲水性的酰胺基团和亲油性的异丙基团。当NIPAM通过自由基聚合形成PNIPAM-40%PEG1000凝胶，具有大分子网状结构，凝胶在液体中可显著溶胀。将冷冻干燥后的PNIPAM-40%PEG1000凝胶浸泡于二元有机复配物中进行溶胀。PNIPAM-40%PEG1000凝胶在二元有机复配物中的溶胀过程实际上是两种相反趋势的平衡过程，溶剂分子试图渗透到网络内部，使得凝胶体积溶胀从而导致三维分子网络的伸展平衡，交联点之间的分子链的伸展降低了它的构象熵值，分子网络的弹性收缩力使网络收缩。当两种相反的倾向互相抵消时，达到溶胀平衡，凝胶的溶胀程度用溶胀率(Swelling ratio，SR)表示。PNIPAM-40%PEG1000凝胶在三种二元有机复配物中溶胀(水浴60℃)，得到PNIPAM-40%PEG1000/正癸酸-月桂酸甲酯、PNIPAM-40%PEG1000/正癸酸-正癸醇和PNIPAM-40%PEG1000/月桂酸-十四烷三种相变储能材料。
1.3 性能测试与结构表征
1.3.1 傅里叶红外测试
参考GB/T21186—2007[25]“傅里叶变换红外光谱仪”的实验方法，利用傅里叶红外扫描仪(Nicolet 6700型，赛默飞世尔Thermo-fisher科技(中国)有限公司)对正癸酸、月桂酸甲酯、正癸醇、月桂酸、十四烷、正癸酸-月桂酸甲酯、正癸酸-正癸醇及月桂酸-十四烷进行红外测试，将单一有机物的图谱与二元有机复配物的图谱进行对照，考察二元有机复配物是否有新的吸收峰生成，判断两种单一有机物的化学相容性。
1.3.2 差示扫描量热分析测试
参考GB/T 19466.3—2004[26]“塑料 差示扫描量热法(DSC)”的实验方法，利用差示扫描量热仪(TGA/DSC1型，梅特勒-托利多国际贸易(上海)有限公司(METTLER TOLEDO))对正癸酸、月桂酸甲酯、正癸醇、月桂酸、十四烷、正癸酸-月桂酸甲酯、正癸酸-正癸醇及月桂酸-十四烷进行DSC测试，测定其相变初始温度、相变终止温度及相变潜热。称取10 mg左右的样品放置于铝坩埚中，精确度为0.01 mg，用压样机进行压制，实验参比侧放置了一个标准的空铝坩埚。将温度从30℃降温至−30℃(速度：20℃/min)，再升温至30℃(速度：20℃/min)，这个升降温过程重复进行两次，去除材料的热历史，将物质在30℃恒温保持5 min，再降温至−30℃得到冷冻放热曲线(速度：5℃/min)，然后再升温至30℃得到融化吸热曲线(速度：5℃/min)。月桂酸、正癸酸这两种物质的温度设置为0~80℃。DSC升降温速率是由N2作用控制的，其中N2作为反应气的速率为20 mL/min，N2作为保护气的速率为150 mL/min。月桂酸甲酯、正癸醇、十四烷、正癸酸-月桂酸甲酯及正癸酸-正癸醇的测试温度为−30~30℃，而正癸酸、月桂酸及月桂酸-十四烷的测试温度为−30~50℃。
参考GB/T 14797.3—2008[27]“浓缩天然胶乳 硫化胶乳 溶胀度的测定”的实验方法，对凝胶在二元有机复配物的溶胀性能进行测试。将冷冻干燥好的凝胶称重(万分之一天平，AR224CN型，奥豪斯仪器有限公司)W1
，将其浸入二元有机复配物中，在60℃的水浴环境下充分溶胀24 h，过程中每6 h测量一次凝胶的重量W2 ，并计算凝胶在二元有机复配物中的溶胀率Q： Q=W2−W1W1×100% (1) 其中：Q为凝胶在二元有机复配物中的溶胀率；W1、W2为凝胶溶胀前、后的质量。
2. 二元有机复配物热物性能估算
基于表2所示的正癸酸、月桂酸甲酯、正癸醇、月桂酸和十四烷的热物性数据，利用凝固点下降定律、热力学第二定律和相平衡关系，评估两种或多种物质共混时的相变温度。
表 2 有机物的热物性数据Table 2. Thermophysical data of organic matterSample Phase transition
temperature/℃Latent heats of
phase transition/
(J·g−1)Decanoic acid 31.39 153.72 Methyl laurate 4.74 179.25 1 decanol 6.13 200.31 Lauric acid 44.54 181.14 Tetradecane 5.68 215.85 利用Schroder Van Laar公式从理论上估算二元有机复配物体系的相变温度，即
{T=11TA−lnXARΔHAT=11TB−lnXBRΔHBXA+XB=1 (2) 其中：T为二元有机复配物的最低共熔点温度(K)；XA、XB为组分A、组分B是摩尔分数(%)；TA、TB为组分A、组分B的理论熔点(K)；R为气体常数且取值为8.315 (J·mol−1·K−1))；ΔHA、ΔHB为组分A、组分B的相变焓(J·mol−1)。
对于表2所列的单一有机物的热物性数据，由式(2)分别得到五种物质的Schroder Van Laar公式：
正癸酸-月桂酸甲酯：
{lnXA=10.46−3184.58/TlnXB=16.63−4620.83/TXA+XB=1 (3) 正癸酸-正癸醇：
{lnXA=10.46−3184.58/TlnXB=13.64−3809.57/TXA+XB=1 (4) 月桂酸-十四烷：
{lnXA=13.47−4369.92/TlnXB=18.47−5150.03/TXA+XB=1 (5) 通过MATLAB软件分别计算上述方程组(3)~(5)，获得正癸酸-月桂酸甲酯、正癸酸-正癸醇和月桂酸-十四烷二元复合物体系的理论共熔点及对应的摩尔比，如表3所示。显然，这三种二元有机复配物的理论最低共熔点在−1~4℃的范围内，接近于目标相变温度0~8℃，故可进一步通过实验分析来探究它们的热物性能。
表 3 二元有机复配物最低共熔点Table 3. The lowest common melting point of binary organic compoundSample The lowest common
melting point/℃Molar
ratioDecanoic acid-methyl laurate −0.86 29∶71 Decanoic acid-1 decanol −0.74 29∶71 Lauric acid-tetradecane 3.58 13∶87 3. 实验结果与讨论
3.1 二元有机复配物的热物性能
图2为正癸酸、月桂酸甲酯、正癸醇、月桂酸、十四烷及二元有机复配物(正癸酸-月桂酸甲酯、正癸酸-正癸醇、月桂酸-十四烷)的FTIR图谱，由单一有机材料物理共混加热得到二元有机复配物，其FTIR图谱中并没有新的官能团生成，这说明二元有机复配物各组分之间能够共存，即共混过程不发生化学反应。
图3和图4分别描述了不同二元有机复配物的差示扫描量热曲线及热物性数据，“↑exo”表示放热方向，
T0 、T1 、ΔH分别表示起始温度、终止温度和相变焓。通过对比分析这两幅图，可获得二元有机复配物的热物性能。![]()
图 3 二元有机复配物的差示扫描量热曲线Figure 3. Differential scanning calorimetry curves of binary organic compound![]()
图 4 二元有机复配物的热物性数据Figure 4. Thermophysical data of binary organic compoundT0—Initial temperature; T1—Final temperature; ΔH—Enthalpy of phase change对于正癸酸-月桂酸甲酯二元有机复配物，当组分摩尔比在3∶97~30∶70范围内，月桂酸甲酯能够溶解正癸酸，差示扫描量热曲线光滑且只有一个吸热峰。当组分摩尔比在33∶67~66∶34范围内，月桂酸甲酯饱和，无法溶解过量的正癸酸，在室温下有不溶物质，正癸酸-月桂酸甲酯的差示扫描量热曲线出现第二个吸热峰，如图3(a)、图3(b)和图3(c)所示。当组分摩尔比在3∶97~66∶34范围内，二元有机复配物相变初始温度在1.62~3.48℃的范围内变化，波动比较小，相变潜热处于165.35~193.40 J/g之间。当正癸酸与月桂酸甲酯的摩尔比为30∶70，相变初始温度为1.62℃、相变终止温度为8.15℃、相变焓为193.40 J/g，如图4(a)所示，此时相变潜热最高，性价比最高，相变初始温度和相变终止温度合适，适用于果品保质包装与物流技术。
对于正癸酸-正癸醇二元有机复配物，当组分摩尔比在3∶97~36∶64范围内，正癸醇能够溶解正癸酸，差示扫描量热曲线光滑且只有一个吸热峰。当组分摩尔比在39∶61~66∶64范围内，正癸醇饱和，无法溶解过量的正癸酸，在室温下有不溶物质，正癸酸-正癸醇的差示扫描量热曲线出现第二个吸热峰，如图3(d)和图3(e)所示。当组分摩尔比在3∶97~66∶34范围内，二元有机复配物的相变初始温度在−2~8℃范围内变化，相变潜热处于154.97~198.27 J/g之间。相变初始温度先从4.6℃降低到−2℃，当正癸酸摩尔比达到24%时，相变初始温度开始上升到0℃以上，最后稳定到4℃左右，波动较大。正癸酸-正癸醇二元复配物的相变初始温度范围广、相变潜热较高、规律性较明显。当正癸酸与正癸醇的摩尔比为36∶64，相变初始温度为3.80℃、相变终止温度为11.72℃、相变焓为180.94 J/g，如图4(b)所示，此时相变潜热较高，性价比最高，相变初始温度和相变终止温度合适，亦适用于果品包装。
对于月桂酸-十四烷二元有机复配物，当组分摩尔比在3∶97~21∶79范围内，十四烷能够溶解月桂酸，差示扫描量热曲线光滑且只有一个吸热峰。当组分摩尔比在24∶76~30∶70，十四烷饱和，无法溶解过量的月桂酸，在室温下有不溶物质，月桂酸-十四烷的差示扫描量热曲线出现第二个吸热峰，如图3(f)所示。二元有机复配物的相变初始温度为5~6℃且波动最小，相变潜热为205.80~217.94 J/g。十四烷成本较高，它与月桂酸的复配有利于大幅度降低成本。当月桂酸与十四烷的摩尔比为21∶79，相变初始温度为5.51℃、相变终止温度为26.74℃、相变焓为216.46 J/g，如图4(c)所示，此时相变潜热较高，性价比最高，也适用于果品包装。
3.2 相变储能材料的热物性能
3.2.1 MBA交联剂含量对溶胀程度的影响
PNIPAM凝胶在二元有机复配物的溶胀程度取决于pH值、温度以及凝胶的交联强度等，选用单体浓度分别为0.5 mol/L、1 mol/L、1.5 mol/L和2 mol/L的PNIPAM凝胶进行溶胀，以探究单体浓度对溶胀程度的影响。由于NIPAM浓度为0.5 mol/L的凝胶成胶性弱，无法得到成型性好的凝胶，故无法进行溶胀实验。如图5(a)所示，单体浓度为1 mol/L的PNIPAM凝胶具有最高的溶胀度，在其他条件不变时，单体浓度越小，越有利于凝胶对二元有机复配物的吸附。选用交联剂含量分别为NIPAM质量的0.25%、0.5%、0.75%和1%的PNIPAM凝胶进行溶胀，进一步探究MBA交联剂含量对溶胀程度的影响。图5(b)所示的实验结果表明，交联剂含量为NIPAM质量的0.25%的PNIPAM凝胶具有最高的溶胀度，在其他条件不变时，交联剂越少，越有利于凝胶对二元有机复配物的吸附，但是交联剂含量为NIPAM质量的0.25%的PNIPAM凝胶在溶胀之后过于柔软，形状稳定性不好。因此制备PNIPAM凝胶的单体浓度为1 mol/L，交联剂含量为NIPAM质量的0.5%。
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图 5 PNIPAM凝胶在二元有机复配物中的溶胀度Figure 5. Swelling ratios of PNIPAM gels in binary organic compoundsC—Concentration3.2.2 PEG作致孔剂对PNIPAM-y%PEGx凝胶物化性能的影响
在PNIPAM凝胶的制备过程中加入致孔剂PEG，得到PNIPAM-y%PEGx凝胶，可以有效增加凝胶中胞元的尺寸。致孔剂聚乙二醇在PNIPAM-y%PEGx凝胶形成的过程中并不参与反应，只是占据一定的空间，在凝胶形成之后，可以通过去离子水浸泡法去除致孔剂聚乙二醇。图6为PNIPAM、PNIPAM-40%PEG1000两种凝胶的FTIR图谱，显然两种凝胶的FTIR图谱相同。其中3300 cm−1为NH的伸缩振动，2945 cm−1为聚合物主链上饱和CH3的振动吸收，1650 cm−1属于C=O的双键伸缩振动(酰胺I带)，1540 cm−1属于CNH的摇摆振动(酰胺II带)，1387 cm−1和1459 cm−1为CH3的吸收峰，在PN100040凝胶的FTIR图谱中1150~1050 cm−1处没有观察到典型的COC吸收，说明PNIPAM-40%PEG1000凝胶中没有PEG1000，PEG1000分子仅在凝胶聚合过程中充当致孔剂，没有参与PNIPAM-40%PEG1000凝胶的形成反应，在反应完成后通过在去离子水中浸泡除去。
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图 6 PNIPAM和PNIPAM-40%PEG1000凝胶的FTIR图谱Figure 6. FTIR spectra of the gel PNIPAM and PNIPAM-40%PEG10003.2.3 PEG分子量及含量对溶胀程度的影响
凝胶PNIPAM与凝胶PNIPAM-y%PEGx内部含有丰富的微胞结构，如图7所示，从凝胶PNIPAM及PNIPAM-y%PEGx的SEM图像(S4800场发射扫描电镜冷场，S4800型，日立Hitachi公司)可以看出，PEG分子量越大，形成胞元的尺寸越大，但是溶胀度不会随着其一直增大，溶胀结果见图8所示。实验结果表明，PNIPAM-40%PEG1000凝胶在二元有机复配物中具有最高的溶胀率：在正癸酸-月桂酸甲酯中，PNIPAM-40%PEG1000凝胶的溶胀度可以达到53.31%；在正癸酸-正癸醇中，PNIPAM-40%PEG1000凝胶的溶胀度可以达到53.70%；在月桂酸-十四烷中，PNIPAM-40%PEG1000凝胶的溶胀度可以达到52.47%。这主要是由于当PEG的分子量为4000、8000和10000时，由于微胞尺寸较大且毛细管力较弱，造成微胞之间导热性能较低且凝胶吸附能力差；当不含PEG或PEG分子量为200时，凝胶为微胞单元较小，吸附二元有机复配物的能力较差，相应所得的有机相变储能材料的相变潜热也较小。凝胶完全浸没在二元有机复配物中成为一个整体，整个微胞呈现出有些透明凝胶状态，这表明此时二元有机复配物的质量超过了凝胶的饱和吸附量。用自封袋分别装入100 g的PNIPAM-40%PEG1000/正癸酸-月桂酸甲酯、PNIPAM-40%PEG1000/正癸酸-正癸醇及PNIPAM-40%PEG1000/月桂酸-十四烷，并进行冷冻处理，随后将其放置于30℃的恒温恒湿箱中8 h(模拟夏季高温情况)，最后从自封袋中取出相变储能材料并进行称重，质量损失率分别为2.8%、5.7%和5.1%，说明PNIPAM-40%PEG1000凝胶能够有效减少二元有机复配物的泄露量。
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图 8 PNIPAM-y%PEGx凝胶在二元有机复配物中的溶胀性能Figure 8. Swelling performance of PNIPAM-y%PEGx gel in binary organic compounds3.2.4 相变温度和相变潜热
相变温度和相变潜热是决定相变储能材料能否应适用于冷链物流的重要条件。通过PNIPAM-y%PEGx凝胶对二元有机复配物进行吸附得到相变储能材料，添加致孔剂PEG可以提高凝胶内部微胞的尺寸，提高凝胶的溶胀率，但是胞元越大，凝胶的毛细管作用力就越小，因此凝胶的溶胀率不会随着致孔剂分子量及含量的增加而一直增加，如图8所示。PNIPAM-40%PEG1000凝胶在三种二元有机复配物中的溶胀效果最好，相变潜热最高，如图9所示。这三种相变储能材料的热物性能为：(1) 正癸酸-月桂酸甲酯(摩尔比30∶70)对应的相变储能材料为PNIPAM-40%PEG1000/正癸酸-月桂酸甲酯，其相变初始温度为3.2℃、相变焓为188.10 J/g；(2) 正癸酸-正癸醇(摩尔比36∶64)对应的相变储能材料为PNIPAM-40%PEG1000/正癸酸-正癸醇相变储能材料，其相变初始温度为1.2℃、相变焓为177.74 J/g；(3) 月桂酸-十四烷(摩尔比21∶79)对应的相变储能材料为PNIPAM-40%PEG1000/月桂酸-十四烷相变储能材料，其相变初始温度为4.2℃、相变焓为206.17 J/g。
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图 9 PNIPAM-y%PEGx/二元有机复配物的相变焓Figure 9. Enthalpy of phase transition of PNIPAM-y%PEGx/binary organic compounds4. 结 论
(1) 通过物理共混法得到三种相变温度适当、相变潜热较高的二元有机复配物：正癸酸-月桂酸甲酯、正癸酸-正癸醇和月桂酸-十四烷，这三种二元有机复配物无过冷和相分离现象、相变温度范围是0~5℃，且具有可逆的熔化和凝固性能，满足果品保鲜包装的要求。
(2) 由于二元有机复配物在相变过程中会有液相生成，不可避免地会发生泄漏，利用聚N-异丙基丙烯酰胺(PNIPAM)凝胶的吸油性可以对三种二元有机复配物进行吸附，可以减少其在应用过程中的泄漏量。
(3) 当在PNIPAM凝胶的制备过程中加入致孔剂聚乙二醇1000(PEG1000)，凝胶在二元有机复配物的溶胀度提高，制备出三种适用于果品保质包装与物流技术的有机相变储能材料：PNIPAM-40%PEG1000/正癸酸-月桂酸甲酯、PNIPAM-40%PEG1000/正癸酸-正癸醇及PNIPAM-40%PEG1000/月桂酸-十四烷相变储能材料。
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图 2 压力传感器转导方式示意图((a)压阻式；(b)电容式；(c)压电式)
Figure 2. Schematic diagram of pressure sensor transduction methods ((a) Piezoresistive; (b) Capacitive; (c) Piezoelectric)
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图 3 微图案化Si模具和柔性微结构化AgNWs/PDMS复合介电膜的制备流程图 (a)、柔性电容式压力传感器的结构 (b)、在施加压力下具有不同类型介电层的压力传感器的灵敏度曲线 (c) 以及传感器对放置和取出小纸片的瞬态响应 (d)[22]
Figure 3. Schematic illustration for the fabrication of micropatterned Si mould and flexible microstructured AgNWs/PDMS composite dielectric film (a), architecture of the flexible capacitive pressure sensor (b), sensitivity curves of the pressure sensor with different types of dielectric layer under applied pressure (c), and transient response of sensor to placing and taking out a small sheet of paper (d)[22]
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图 4 连锁微柱阵列柔性压力传感器的制作过程示意图 (a)、传感器在100 Pa外部压力下的实时响应曲线 (b)、具有不同微柱阵列HAR值的传感器的灵敏度曲线 (c) 以及在不同压力下传感器的相对电阻变化响应曲线 (d)[80]
Figure 4. Schematic diagram of fabrication process of interlocking micro-column array flexible pressure sensor (a), real-time response curve of sensor under 100 Pa external pressure (b), sensitivity curves of sensors with different micro-column array HAR values (c), andresponse curve of relative resistance change of the sensor under different pressures (d)[80]
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图 5 利用PS微球结构制备可调压阻式压力传感器的制作示意图 (a)；三种不同传感器结构的压力敏感度对比 (b)；不同施加压力下压力传感器的电流-电压(I-V)曲线 (c)[81]
Figure 5. Schematic diagram of pressure sensor with adjustable pressure resistance made of polystyrene microsphere structure (a)；Comparasion of the pressure sensitivity for three different sensor structures (b); Current-voltage (I-V) curve of pressure sensor under different applied pressures (c)[81]
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图 8 通过浸涂工艺制备PEDOT:PSS涂覆三聚氰胺海绵的示意图 (a)、压缩条件下PEDOT:PSS@MS的结构变化示意图 (b) 和PEDOT:PSS@MS和压缩下的PEDOT:PSS@MS的SEM图像 (c)[87]
Figure 8. Schematic diagram of PEDOT:PSS coated melamine sponge prepared by dip coating process (a), schematic diagram of structural change of PEDOT:PSS@MS under compression (b), and SEM images of PEDOT:PSS@MS and PEDOT:PSS@MS under compression (c)[87]
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图 10 基于泡沫石墨烯制备的柔性压力传感器的原理示意图 (a)、施加压力时传感器相对电阻的变化 (b)、施加相同压力时，传感器的相对电阻变化 (c) 以及人在行走和跳跃时传感器的相对电阻变化 (d)[90]
Figure 10. Schematic diagram of flexible pressure sensor based on foam graphene (a), change of relative resistance of sensor when pressure is applied (b), change of relative resistance of sensor when the same pressure is applied (c), and change of relative resistance of sensor when people walk and jump (d)[90]
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图 12 基于网络架构的宽量程压力传感器(NWPS)的制作过程 (a)、蚁巢的照片 (b)、NWPS的分解图 (c)、NWPS用手指弯曲的插图 (d)和利用NWPS的高灵敏度和宽检测范围的各种应用 (e)[7]
Figure 12. Fabrication process of wide range pressure sensor (NWPS) based on network architecture (a), photograph of ant nests (b), exploded diagram of the NWPS (c), illustration of the NWPS bending with a finger (d), and various applications exploiting the high sensitivity and wide detection range of the NWPS (e)[7]
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表 1 压阻式、电容式和压电式柔性压力传感器比较
Table 1 Comparison of piezoresistive, capacitive and piezoelectric flexible pressure sensors
Type Working principle Sensitivity Advantages Disadvantages Piezoresistive sensor Resistive change S=δ(ΔR/R0)δP Low cost, simple design and operation, easily detectable signals and high sensitivity Nonlinearity and hysteresis Capacitive sensor Capacitive change S=δ(ΔC/C0)δP Fast response, high accuracy, high sensitivity and low hysteresis High cost, complicated detection circuit, nonlinear output and low load capacity Piezoelectricity sensor Electric current change Piezoelectric effect Wide frequency band, fast response and reliable operation High cost, complicated operation Notes: S—Sensitivity; ΔR—Change in resistance; R0—Initial resistance value of the sensor when it is not under pressure; P—Pressure per unit area of the sensor; ΔC—Change in capacitance; C0—Initial capacitance value in the unstressed state. 
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