Relative Resistance Change Research Articles

Development and Fabrication of Carbon Nanotube (CNT)/CuO Nanocomposite for Volatile Organic Compounds (VOCs) Gas Sensor Application

AbstractVolatile organic compounds (VOCs) have been recognized as one of the primary trace segments of atmospheric air pollutants. The change in the level of VOCs in the surrounding environment can lead to chronic health issues, respiratory problems, nerve system disorder, and toxicity in kidneys/liver. Thus, monitoring of VOCs concentration in the surrounding environment is significant for avoiding serious health problems. Herein, copper oxide (CuO) nanoparticle and carbon nanotube (CNT) nanocomposite (NC) are presented for the efficient detection of VOCs. The scalable sol‐gel method is adopted for the controlled growth of CNT/CuO NC. The structural, elemental, and morphological analysis is performed by XRD, FTIR spectroscopy, and SEM characterization, respectively. The VOCs sensor was fabricated by drop‐casting the as‐synthesized CNT/CuO NC on interdigitated electrodes (IDEs). The CNT/CuO sensing response is analyzed for six VOCs that include toluene, methanol, acetone, chloroform, xylene, and benzene. The CNT/CuO response towards different VOCs is investigated with respect to change in resistance of the material in the presence of test VOC and in an inert atmosphere. In comparison to other VOCs, the sensor exhibits high sensitivity toward benzene. The estimated change in relative resistance (AR) for benzene is ≈0.62% for 500 ppm concentration. Moreover, the sensor apprehended a detection of benzene with a concentration as low as 5 ppm. The as‐synthesized CNT/CuO NC offers high sensitivity and low detection limit, which benchmark its potential for benzene detection.

Fabrication of Inkjet-Printed Carbon Nanotube for Enhanced Mechanical and Strain-Sensing Performance

This paper presents fabrication of inkjet-printed carbon nanotube film on flexible substrate for wearable electronics applications. The density of CNT films is optimized by droplet spacing (DS) and multiple passes to provide the best strain behavior. It is found that low-density carbon nanotubes have fewer conductive pathways resulting in less change and low GF under applied strain. Conversely, high-density carbon nanotubes have more conductive paths, and they are not easily broken under strain, resulting in poor strain-sensing ability. The inkjet printing process can adjust uniformity and density of CNT film through DS and multiple passes to optimize its strain characteristics. The highest GF of 3.36 was obtained under strain ranging from 71 to 3128 με when CNT printed by DS of 23 μm and 20 passes. The relative change in resistance under various strains, ranging from 71 to 3128 με, had a stable peak value for each 20 strain/release cycle which proved its repeatability and stability. Furthermore, inkjet-printed CNT sensors monitored human movement of various joints and distinguished bending angle demonstrating its potentially practical application in wearable electronics.

Thermoelectric Characteristics of InAs Nanowire Networks Directly Grown on Flexible Plastic Substrates

III-V semiconductor nanowires have shown promise for thermoelectric applications, but their use in practical devices has conventionally been hindered by complex fabrication processes and device integration. Here, we characterize the thermoelectric properties of InAs nanowire networks directly grown on flexible polyimide plastic. The n-type nanowire networks achieve a high room-temperature Seebeck coefficient of −110.8 μV K–1 and electrical conductivity of 41 S cm–1, resulting in a thermoelectric power factor of 50.4 μW m–1 K–2. Moreover, the nanowire networks show remarkable mechanical flexibility with a relative change in resistance below 0.01 at bending radii below 5.2 mm. We further establish the thermoelectric performance of InAs nanowire networks on plastic using a facile proof-of-concept thermoelectric generator producing a maximum power of 0.44 nW at a temperature gradient of 5 K. The findings indicate that direct growth of III-V nanowire networks on plastic substrates shows promise for the development of flexible thermoelectrics applications.

Electrically conductive and piezoresistive polymer nanocomposites using multiwalled carbon nanotubes in a flexible copolyester: Spectroscopic, morphological, mechanical and electrical properties

Nanocomposites of multiwalled carbon nanotubes (MWCNTs) with poly(butylene adipate-co-terephthalate) (PBAT), a flexible aromatic–aliphatic copolyester, were prepared by melt mixing followed by compression moulding to investigate their spectroscopic, morphological, mechanical and electrical properties. A comparison of the Fourier transform infrared (FTIR) spectra of the neat polymer matrix and the composites showed no difference, implying a physical mixing of the matrix and the filler. A morphological investigation revealed the formation of a continuous and interconnected MWCNT network embedded in the polymer matrix with partial agglomeration. Increasing Martens hardness and indentation modulus and decreasing maximum indentation depth with increasing filler concentration demonstrated the reinforcement of the polymer by the MWCNTs. A volume resistivity of 4.6 × 105Ω cm of the materials was achieved by the incorporation of only 1 wt.-% of the MWCNTs, which confirmed a quite low percolation threshold (below 1 wt.-%) of the nanocomposites. The electrical volume resistivity of the flexible nanocomposites was achieved up to 1.6 × 102Ω cm, depending on the filler content. The elongation at the break of the nanocomposites at 374% and the maximum relative resistance changes (ΔR/R0) of 20 and 200 at 0.9 and 7.5% strains, respectively, were recorded in the nanocomposites (3 wt.-% MWCNTs) within the estimated volume resistivity range. A cyclic strain experiment shows the most stable and reproducible ΔR/R0 values in the 2%–5% strain range. The electrical conductivity and piezoresistivity of the investigated nanocomposites in correlation with the mechanical properties and observed morphology make them applicable for low-strain deformation-sensing.

Heat Treatment of Cast and Cold Rolled Al–Yb and Al–Mn–Yb–Zr Alloys

The microstructure, electrical properties and microhardness of as-cast and cold rolled AlYb and AlMnYbZr alloys were investigated. The addition of Mn, Yb and Zr has a positive influence on grain size. A deformed structure of the grains with no changes of their size was observed after cold rolling. The Al3Yb particles coherent with the matrix were observed in the AlYb alloys. The size of the particles was about 20 nm in the initial state; after isochronal treatment up to 540 °C the particles coarsen, and their number density was lower. The deformation has a massive effect on the microhardness behavior until treatment at 390 °C, after which the difference in microhardness changes between as-cast and cold rolled alloys disappeared. Relative resistivity changes show a large decrease in the temperature interval of 330–540 °C which is probably caused by a combination of recovery of dislocations and precipitation of the Al3(Yb,Zr) particles. Precipitation hardening was observed between 100 and 450 °C in the AlYb alloy after ageing at 625 °C/24 h and between 330 and 570 °C in the AlMnYbZr alloy after ageing at 625 °C/24 h.

Dual-network sodium alginate/polyacrylamide/laponite nanocomposite hydrogels with high toughness and cyclic mechano-responsiveness

Ionic conductive hydrogels with good mechanical strength and toughness have arisen a heated debate for their prospect in the fields of human motion sensors, sensory skins, and personal healthcare diagnosis. Nevertheless, the probable candidates rarely manage to meet the mechanical requirements to be used as wearable devices. As possible solutions, the dual network (DN) structure and nanocomposites (NC) have been adopted, respectively. Still, it is highly favorable yet challenging for the single DN or NC to design a hydrogel with toughness, high strength, stretchability, and fatigue resistance for potential applications. Herein, we combined the two commonly used strengthening strategies to prepare the sodium alginate/polyacrylamide/Laponite hydrogels (SPL hydrogels). The designed ionic conductive SPL hydrogels exhibited good mechanical properties, such as tensile strength and stretchability of up to ~206 KPa and ~854%, respectively, compressive strength of up to ~2136 KPa (90% strain), and high toughness (~134.78 kJ/m3). Besides, the designed ionic conductive hydrogels also possessed stable and timely mechano-responsive performances along with relative resistance changes (RRCs). It is envisioned that the designed high strength and toughness hydrogels may be promising for applications as bio-sensors and personal healthcare diagnoses.

Monte Carlo analytical-geometrical simulation of piezoresistivity and electrical conductivity of polymeric nanocomposites filled with hybrid carbon nanotubes/graphene nanoplatelets

Piezoresistivity and electrical conductivity of carbon nanotube (CNT)/graphene nanoplatelet (GNP)-filled polymer nanocomposites are investigated using a 3D Monte Carlo analytical-geometrical model. GNPs and CNTs are considered as randomly distributed solid thin rectangular cube and cylinder, respectively. After establishment of a random CNT/GNP network, electrical conductivity and relative resistance change with strain is calculated. Model considers effect of CNT and GNP deformation on filler separation distance as the dominant factor for percolation tunneling. Comparing model results with experimental data of hybrid nanocomposites showed a good agreement for electrical conductivity and piezoresitivity. Analytical model is developed on the basis of geometrical tunneling percolation theory to consider the effect of several parameters like height of barrier potential, GNP side length, CNT orientation and dimensions on electric behavior of nanocomposites. Results revealed that CNT dispersion state and GNP dimensions have significant effects on the percolation threshold and resistivity change ratio of nanocomposites with strain.

Improved tensile and single-lap-shear mechanical-electrical response of epoxy composites reinforced with gridded nano-carbons

A new carbon nanotube (CNT)/graphene (GN) grid structure is prepared and used to reinforce and health-monitor the epoxy composites. Results reveal that with a 30–50% mesoscopic-porosity of the CNT/GN grid, tensile strength and single-lap-shear (SLS) strength of the CNT/GN/epoxy composites can be improved by 27–41% and 28–49%, respectively. Also, relative resistance change (ΔR/R0%) at damage initiation would sudden change more than 10% for the tensile testing and 6% for the SLS testing. Meanwhile, during the damage evolution stage, the gauge factor obtained from tensile testing reaches 10.2–24.9 improved by 23–200%, and ΔR/R0% from SLS testing achieves 47.4–60.4% increased by 69–116%, respectively, when compared to the baseline composite. Moreover, an equivalent resistance prediction mode for the CNT/GN grid composites is proposed and which can well explain the resistance response mechanism. The result reveals that the gridded CNT/GN sensor is potential in reinforcing and structural health monitoring the epoxy resin-based composites.

Electrically Conductive Kevlar Fibers and Polymer-Matrix Composites Enabled by Atomic Layer Deposition

Multifunctional composites that incorporate nonstructural capabilities such as energy storage, self-healing, and structural health monitoring have the potential to transform load-bearing components in automotive and aerospace vehicles. Imparting electrical conductivity into polymer-matrix composites (PMCs) is an important step in enabling multifunctionality while maintaining mechanical stiffness and strength. In this work, electrically conductive PMCs were fabricated by conformally coating Kevlar 49 woven fabrics with aluminum-doped zinc oxide using atomic layer deposition (ALD). Electrical resistance was measured at the single-fiber, single-tow, and woven fabric levels as a function of coating thickness. The ALD coatings on adjacent fibers merge as their thickness increases, resulting in an interconnected network with improved percolation and lower resistance. After ALD, the fabrics were embedded in an epoxy matrix to manufacture PMCs. The electrical resistance of the composites increased with applied tensile strain, which was attributed to cracking of the conductive coatings. The relative change in resistance as a function of strain varied with coating thickness, which was rationalized by a thin-film fracture mechanics model. This work demonstrates a pathway for scalable and tunable incorporation of electrical conductivity into fiber-reinforced composites without significantly changing their density or load-bearing capabilities.

Relaxation behavior of nonvolatile resistance modulation in Zn:SnO2/PMN-PT heterostructures

Relaxation in resistance switching (RS) has severely hindered the stability of memory devices based on oxide-thin-film/ferroelectric heterostructures. In this work, Zn doped SnO2 (ZTO) thin films were deposited on (111)-0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-0.3PT) substrates to investigate the effect of oxygen vacancies (OVs) on resistance relaxation. The polarization-dependent resistance relaxation is observed in ZTO/PMN-0.3PT heterostructures. The largest relative change in resistance up to 53.6% within 20 min is ascribed to OV diffusion. Local aggregation of positively charged OVs with the application of poling electric field would enhance the Coulomb repulsion and strain in distorted lattice, which could accelerate the OV back-diffusion. Partial positively charged OVs are neutralized in the positive polarization state with the application of +10 kV/cm poling voltage, which weakens the relaxation compared to the negative polarization state. Our work helps to understand the interrelation between OVs and polarization-dependent relaxation of RS properties.

Preparation and application of a stretchable, conductive and temperature-sensitive dual-network nanocomposite hydrogel

Conductive hydrogel has received extensive attention in the design and application of flexible electronic equipment, due to its flexibility, easy processing, good sensitivity and excellent electrochemical performance. In this study, we designed a stretchable conductive and temperature-sensitive dual-network nanocomposite hydrogel in a simple one-pot method. Fourier transform infrared (FTIR), scanning electron microscopy （SEM） and thermogravimetric analysis (TGA) were used to characterize the nanocomposite hydrogel. The formation of the double cross-linked network greatly improves the mechanical properties of the nanocomposite hydrogel, and continuous electron transport path formed by MWCNTs dispersed in a hydrogel network endows the nanocomposite hydrogel great conductive behavior and the hydrogel exhibited a stable relative resistance change during the tensile deformation process. The nanocomposite hydrogel displayed great application potential in wearable devices, smart robots, and flexible strain sensors.

Enhanced Graphene Sensors via Multi-Lasing Fabrication

Resistive strain and bending sensors offer a versatile platform for sensing various physical parameters with relatively little effort and budget. The lightweight, robust and compact sensors are extensively used in manifold low-power applications. Recently, scribed and flexible laser-induced graphene sensors have shown potent capabilities for a variety of measurements, including flow, deflection, and force. Achieving a high sensitivity to various stimuli remains a challenge due to limited change in relative resistance. In this paper, we report a multifunctional LIG sensor with widely tunable properties and significantly enhanced electromechanical performance. A method of repeated laser writing is used to increase the porosity, the uniform carbonization degree and, most importantly, the sensitivity of the LIG sensors. A gauge factor of 91.2 is achieved after three-times laser writing at low power, which is an increase of 750% to one-time laser writing and 720% higher than the ones previously reported for LIG strain sensors. The increase is attributed to a more porous surface morphology that provides more overlapping area and displacement of the graphene layers. A homogeneous bidirectional response was obtained by scribing the electrodes on both faces of the substrate. Parylene C-coating is used to protect the LIG sensors from environmental effects. Coated sensors were packaged to a PCB assembly for easy integration into various applications. An example is a LIG bending sensor customized for velocity profile monitoring of Unmanned Aerial Vehicles in the outdoor environment.

3D‐Printed Flexible Piezoresistive Sensors for Stretching and Out‐of‐Plane Forces

AbstractConductive polymer composites (CPCs) of carbon nanotubes (CNTs) and graphite nanosheet (GNP)‐filled thermoplastic polyurethane (TPU) are 3D‐printed into flexible piezoresistive sensors via fused filament fabrication. The sensor, with a customized lever‐cross structure, allows detection of stretching and out‐of‐plane forces of different magnitudes and frequencies. The out‐of‐plane force direction is obtained by combing the relative electrical resistance change in the cross section of the sensor with a force analysis. The 75‐CNT/25‐GNP sensor (CNT‐to‐GNP mass ratio of 75%‐to‐25%) demonstrates excellent sensing performance at a total nanoparticle loading of 3 wt%. The linearity of the 75‐CNT/25‐GNP sensor is 0.98, while those of the 100‐CNT and 50‐CNT/50‐GNP sensors are 0.93 and 0.86, respectively. The gauge factor of the 75‐CNT/25‐GNP sensor is 52% higher than that of the 100‐CNT sensor, and its sensing strain range is 79% above that of the 50‐CNT/50‐GNP sensor. Excellent sensing stability is demonstrated for the 75‐CNT/25‐GNP sensor after 1500 stretching (out‐of‐plane force) cycles. The synergistic effect of CNTs and GNPs on sensing performance of piezoresistive sensors is clearly shown in this study.

Reduced Graphene Oxide-Wrapped Palladium Nanowires Coated with a Layer of Zeolitic Imidazolate Framework-8 for Hydrogen Sensing

Rapid and sensitive H2 detection is important because of its low threshold for the formation of explosive mixtures in air (∼4%). Palladium’s unique interactions with H2 make it particularly useful in room-temperature H2 sensing, but the formation of water by reaction with O2 at the Pd surface can interfere with sensor response. Here, we report H2 sensors using networks of high aspect ratio and ultrathin reduced graphene oxide (rGO)-coated palladium nanowires (Pd NWs@rGO) with a coating of zeolite imidazole framework (ZIF-8) that serves as a nanofiltration layer. We first produced Pd NWs in high yield by a hydrothermal method, then sonicated them with GO and added a reducing agent to produce Pd NWs@rGO. Thin Pd NWs promote rapid response and high sensitivity, while rGO prevents the formation of additional conductive channels due to expansion of Pd NWs upon H2 exposure, ensuring monotonic sensor response. The coating of ZIF-8 reduces the transport of molecules like oxygen and nitrogen to the Pd NWs while allowing H2 to reach them and H2O to diffuse away from them. The optimized sensors showed a response (relative change in resistance) to 1% H2 in air of up to 2%, with a response time of 5 s and a lower limit of detection of 20 ppm. Relative to previously reported Pd-based H2 sensors with fast response, the Pd NWs@rGO@ZIF-8 nanocomposite-based device provides a low-cost, high-performance sensor solution for fuel-cell vehicles and similar applications that require both rapid and sensitive H2 detection.

Electrical resistance changes of 3D carbon fiber/epoxy woven composites under short beam shear loading along different orientations

Electrical resistance under cyclic loading is important for designing intelligent structures of carbon-fiber reinforced composites. Here we report electrical resistance changes of 3D carbon fiber/epoxy woven composites under short beam shear loading along different orientations. The electric changes were measured in short beam shear tests combined with four-probe method and digital image correlation technology along five orientations, i.e., 0°, 30°, 45°, 60° and 90°. The on-axis (0° and 90°) samples have higher electrical conductivity, shear strength and resistance increase than other off-axis (30°, 45° and 60°) samples. Relative resistance changes become larger during monotonic loading and decrease under cyclic loading. The 45° and 60° samples show a higher relative resistance change compared with other samples under cyclic loading. Relationship between shear strength and relative resistance change was also revealed. This electric-mechanical coupling behavior provides a possible way for characterizing inner damages.

Monitoring of Curing and Cyclic Thermoresistive Response Using Monofilament Carbon Nanotube Yarn Silicone Composites

The curing process and thermoresistive response of a single carbon nanotube yarn (CNTY) embedded in a room temperature vulcanizing (RTV) silicone forming a CNTY monofilament composite were investigated toward potential applications in integrated curing monitoring and temperature sensing. Two RTV silicones of different crosslinking mechanisms, SR1 and SR2 (tin- and platinum-cured, respectively), were used to investigate their curing kinetics using the electrical response of the CNTY. It is shown that the relative electrical resistance change of CNTY/SR1 and CNTY/SR2 monofilament composites increased by 3.8% and 3.3%, respectively, after completion of the curing process. The thermoresistive characterization of the CNTY monofilament composites was conducted during heating–cooling ramps ranging from room temperature (RT~25 °C) to 100 °C. The thermoresistive response was nearly linear with a negative temperature coefficient of resistance (TCR) at heating and cooling sections for both CNTY/SR1 and CNTY/SR2 monofilament composites. The average TCR value was −8.36 × 10−4 °C−1 for CNTY/SR1 and −7.26 × 10−4 °C−1 for CNTY/SR2. Both monofilament composites showed a negligible negative residual relative electrical resistance change with average values of ~−0.11% for CNTY/SR1 and ~−0.16% for CNTY/SR2 after each cycle. The hysteresis amounted to ~21.85% in CNTY/SR1 and ~29.80% in CNTY/SR2 after each cycle. In addition, the effect of heating rate on the thermoresistive sensitivity of CNTY monofilament composites was investigated and it was shown that it reduces as the heating rate increases.

Electric heating behavior of flexible knitted fabrics comprising reduced graphene oxide, with emphasis on resistance temperature-sensitive behavior and decoupling of contact resistance

Electric fabric heaters have demonstrated potential applications in a wide range of fields for medical electrothermal, human healthcare and athletic rehabilitation. Whereas, little attention has been paid to the resistance variations and the interface of electric heaters. Here, this paper focuses on the resistance temperature-sensitive behavior and interfacial electricity of reduced graphene oxide (RGO)/polyester (PET) fabrics, which are obatained through a facile and scalable dip-coating method. When a current of 0.055 ampere (A) is applied, the RGO/PET fabric can achieve an equilibrium temperature about 89 °C in 20 s, with a maximum heating rate of 11.78 °C/s. Besides, the relative resistance changes of RGO/PET fabric are linearly related to the temperature. When the RGO/PET fabric reaches its steady-state temperature of 89 °C, the value of ΔR/R0 drops by ∼30%, showing that the fabric is endowed with temperature sensitivity. These prominent results indicate that the RGO/PET fabric owns great promise in the field of wearable electric heaters. Notably, the contact resistance at the interface of RGO/PET fabric heater is investigated and the mechanism of the temperature in middle part of heater is higher than that of both ends is analyzed. This provides a qualitative decoupling analysis method for the study and analysis of interfacial electricity and electrothermal distribution of carbon materials.

Effective interlaminar reinforcing and delamination monitoring of carbon fibrous composites using a novel nano-carbon woven grid

To date, it is still a challenge to simultaneously reinforce and well monitor the delamination of carbon fiber reinforced plastics (CFRPs) using nanocarbon-based interleaf due to the high electrical conductivity of CFRPs. Herein, this work prepares a novel woven grid composed of carbon nanotubes and graphenes (CG) with various grid densities and suitable resistivity. Results reveal that the CG woven grid-based interleaf could significantly reinforce the interlaminar shear properties of CFRPs, i.e. mode II interlaminar fracture toughness and interlaminar shear strength can be improved by 38–60% and 17–25%, respectively. Meanwhile, the relative resistance change (ΔR/R0%) of CFRPs at the delamination initiation stage can reach ~12% or more, which is greater than that reported by previous literatures. The results suggest that the prepared CG woven grid can not only greatly enhance the delamination resistance, but also can sensitively monitor the delamination of the highly conductive CFRPs.

Modeling electromechanical coupling of liquid metal embedded elastomers while accounting stochasticity in 3D percolation

Liquid metal embedded elastomers (LMEEs) have attracted interest from researchers on account of their combination of low elastic modulus, high flexibility, and tunable electrical and/or thermal conductivity. One interesting feature of LMEEs is that their electrical resistance remains constant when stretched, which is desirable in many real-world applications. This work aims to investigate this unique electromechanical coupling behavior of LMEEs with three-dimensional deterministic and statistical modeling. We first performed finite element analysis (FEA) of the electromechanical coupling response of a percolated liquid metal droplet repeating in the conductive network. Next, we modeled the internal conductive network of LMEE as a simple cubic structure and introduced stochasticity in representing the droplet–droplet connectivity. This combined approach allows us to calculate relative changes in resistance with stretch that are in reasonable agreement with experimental findings. Our analysis provides new theoretical insights that can guide efforts to tailor the electromechanical coupling behavior of LMEEs.

Structural effect of ZnO-Ag chemoresistive sensor on flexible substrate for ethylene gas detection

As a versatile semiconductor material, ZnO has been investigated in many areas. One of the ZnO potential application is its use as the sensing materials, especially for gas sensors. In this study, ethylene gas sensor is successfully fabricated using ZnO-Ag layer on a flexible PET-ITO substrate using a simple electrochemical deposition process. The morphological and structural properties of the layer are characterized and related to the sensors’ performance. From the result, it is shown that the ethylene sensing works best on a smaller crystallite size ZnO-Ag layer, with the most evenly coverage layer. The optimum sensor was made using ZnO and 0.6 mM of Ag. This sensor showed a relative change in resistance or a response value of 5.65 % and 16.01 % under ethylene gas exposure at 30 and 100 ppm, respectively.