Chuntai Liu

Electrically conductive thermoplastic elastomer nanocomposites at ultralow graphene loading levels for strain sensor applications

An electrically conductive ultralow percolation threshold of 0.1 wt% graphene was observed in the thermoplastic polyurethane (TPU) nanocomposites. The homogeneously dispersed graphene effectively enhanced the mechanical properties of TPU significantly at a low graphene loading of 0.2 wt%. These nanocomposites were subjected to cyclic loading to investigate the influences of graphene loading, strain amplitude and strain rate on the strain sensing performances. The two dimensional graphene and the flexible TPU matrix were found to endow these nanocomposites with a wide range of strain sensitivity (gauge factor ranging from 0.78 for TPU with 0.6 wt% graphene at the strain rate of 0.1 min−1 to 17.7 for TPU with 0.2 wt% graphene at the strain rate of 0.3 min−1) and good sensing stability for different strain patterns. In addition, these nanocomposites demonstrated good recoverability and reproducibility after stabilization by cyclic loading. An analytical model based on tunneling theory was used to simulate the resistance response to strain under different strain rates. The change in the number of conductive pathways and tunneling distance under strain was responsible for the observed resistance-strain behaviors. This study provides guidelines for the fabrication of graphene based polymer strain sensors.

Lightweight conductive graphene/thermoplastic polyurethane foams with ultrahigh compressibility for piezoresistive sensing

Lightweight conductive porous graphene/thermoplastic polyurethane (TPU) foams with ultrahigh compressibility were successfully fabricated by using the thermal induced phase separation (TISP) technique. The density and porosity of the foams were calculated to be about 0.11 g cm−3 and 90% owing to the porous structure. Compared with pure TPU foams, the addition of graphene could effectively increase the thickness of the cell wall and hinder the formation of small holes, leading to a robust porous structure with excellent compression property. Meanwhile, the cell walls with small holes and a dendritic structure were observed due to the flexibility of graphene, endowing the foam with special positive piezoresistive behaviors and peculiar response patterns with a deflection point during the cyclic compression. This could effectively enhance the identifiability of external compression strain when used as piezoresistive sensors. In addition, larger compression sensitivity was achieved at a higher compression rate. Due to high porosity and good elasticity of TPU, the conductive foams demonstrated good compressibility and stable piezoresistive sensing signals at a strain of up to 90%. During the cyclic piezoresistive sensing test under different compression strains, the conductive foam exhibited good recoverability and reproducibility after the stabilization of cyclic loading. All these suggest that the fabricated conductive foam possesses great potential to be used as lightweight, flexible, highly sensitive, and stable piezoresistive sensors.

Continuously prepared highly conductive and stretchable SWNT/MWNT synergistically composited electrospun thermoplastic polyurethane yarns for wearable sensing

Highly conductive and stretchable yarns have attracted increasing attention due to their potential applications in wearable electronics. The integration of conductive yarns with large stretching capability renders the composite yarns with new intriguing functions, such as monitoring human body motion and health. However, simultaneously endowing the yarns with high conductivity and stretchability using an easily scalable approach is still a challenge. Here, highly conductive and stretchable yarns based on electrospun thermoplastic polyurethane (TPU) fiber yarns successively decorated with multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) were prepared by a combined electrospinning, ultrasonication adsorbing, and bobbin winder technique. The improved thermal stability of the SWNT/MWNT/TPU yarn (SMTY) indicated strong interfacial interactions between the CNTs and electrospun TPU fibers. The synergism between the successively decorated SWNTs and MWNTs significantly enhanced the conductivity of the TPU yarns (up to 13 S cm−1). The as-fabricated yarns can be easily integrated into strain sensors and exhibit high stretchability with large workable strain range (100%) and good cyclic stability (2000 cycles). Moreover, such yarn can be attached to the human body or knitted into textiles to monitor joint motion, showing promising potential for wearable electronics, such as wearable strain sensors.

Organic vapor sensing behaviors of conductive thermoplastic polyurethane–graphene nanocomposites

Conductive thermoplastic polyurethane (TPU) nanocomposites filled with graphene were fabricated and tested for organic vapor sensing. The observed finely dispersed graphene in the TPU matrix benefited from the formation of efficient conductive paths and the generation of stable electrical signals. Organic vapor sensing behaviors of the conductive polymer composites (CPCs) were evaluated using four kinds of organic vapors possessing different polarities (p), including cyclohexane (p = 0.1), tetrachloromethane (CCl4, p = 1.6), ethylacetate (p = 4.3) and acetone (p = 5.4). Unlike conventional CPCs that only respond to certain specific groups of organic vapors, the current CPCs showed a novel negative vapor coefficient (NVC) effect for all tested vapors. This observed NVC was due to both the inherent microphase segregation structure of TPU containing soft and hard segments and the wrinkled structure of graphene. In successive immersion-drying runs (IDRs) at 30 °C, fast response, good reversibility and reproducibility were observed for the non- and low- polar vapors (cyclohexane and CCl4), but residual resistance was observed for polar organic vapors (ethylacetate and acetone) after their desorption. The temperature dependent vapor sensing behaviors indicated that the vapor sensing responsivity increased with increasing the temperature due to higher absorption activation energy at higher temperature. This study provides guidelines for the fabrication of organic vapor sensors using CPCs possessing fast response, good discrimination ability and reproducibility.

Photodegradation of polycyclic aromatic hydrocarbon pyrene by iron oxide in solid phase

To better understand the photodegradation of polycyclic aromatic hydrocarbons (PAH) in solid phase in natural environment, laboratory experiments were conducted to study the influencing factors, kinetics and intermediate compound of pyrene photodegradation by iron oxides. The results showed that the pyrene photodegradation rate followed the order of alpha-FeOOH>alpha-Fe(2)O(3)>gamma-Fe(2)O(3)>gamma-FeOOH at the same reaction conditions. Lower dosage of alpha-FeOOH and higher light intensity increased the photodegradation rate of pyrene. Iron oxides and oxalic acid can set up a photo-Fenton-like system without additional H(2)O(2) in solid phase to enhance the photodegradation of pyrene under UV irradiation. All reaction followed the first-order reaction kinetics. The half-life (t(1/2)) of pyrene in the system showed the higher efficiencies of using iron oxide as photocatalyst to degrade pyrene. Intermediate compound pyreno was found during photodegradation reactions by gas chromatography-mass spectrometry (GC-MS). The photodegradation efficiency for PAHs in this photo-Fenton-like system was also confirmed by using the contaminated soil samples. This work provides some useful information to understand the remediation of PAHs contaminated soils by photochemical techniques under practical condition.

Comparative assessment of the strain-sensing behaviors of polylactic acid nanocomposites: reduced graphene oxide or carbon nanotubes

Reduced graphene oxide (RGO)/polylactic acid (PLA) and carbon nanotubes (CNTs)/PLA nanocomposites were prepared by ultrasound-assisted dispersion and a hot-pressing method for comparative studies. The RGO and CNT nanofillers, which had a hydrogen-bonding interaction with the PLA matrix, were uniformly dispersed in the PLA matrix, and low percolation thresholds (0.11 wt% for RGO/PLA and 0.80 wt% for the CNTs/PLA nanocomposites) were achieved in the PLA nanocomposites. The addition of RGO resulted in weak crystallization ability and significant enhancement in thermal stability of the PLA matrix owing to the two-dimensional characteristic of RGO, while opposite results were obtained for the CNTs nanocomposites (the degree of crystallinity Xc is 49.82% for 0.8 wt% CNTs and 9.21% for 0.8 wt% RGO, respectively). During ten extension–retraction cycles, the values of the max and min ΔR/R0 for the RGO/PLA nanocomposites shift upwards gradually with the increase of the cycle number, resulting from the breakdown of conductive networks caused by the slippage of overlapped RGO layers; while the values of the max and min ΔR/R0 for CNTs/PLA nanocomposites decrease gradually owing to the formation of a better conductive network caused by the rearrangement of CNTs. This study is meaningful for the application of conductive polymer composite based strain sensors in many fields, such as structural health monitoring, wearable electronic devices, soft robotics, etc.

Reinforced carbon fiber laminates with oriented carbon nanotube epoxy nanocomposites: Magnetic field assisted alignment and cryogenic temperature mechanical properties

The epoxy nanocomposites with ordered multi-walled carbon nanotubes (MWCNTs) were used to influence the micro-cracks resistance of carbon fiber reinforced epoxy (CF/EP) laminate at 77 K, Oxidized MWCNTs functionalized with Fe3O4 (Fe3O4/O-MWCNTs) with good magnetic properties were prepared by co-precipitation method and used to modify epoxy (EP) for cryogenic applications. Fe3O4/O-MWCNTs reinforced carbon fiber epoxy composites were also prepared through vacuum-assisted resin transfer molding (VARTM). The ordered Fe3O4/O-MWCNTs were observed to have effectively improved the mechanical properties of epoxy (EP) matrix at 77 K and reduce the coefficient of thermal expansion (CTE) of EP matrix. The ordered Fe3O4/O-MWCNTs also obviously improved the micro-cracks resistance of CF/EP composites at 77 K. Compared to neat EP, the CTE of ordered Fe3O4/O-MWCNTs modified CF/EP composites was decreased 37.6%. Compared to CF/EP composites, the micro-cracks density of ordered Fe3O4/O-MWCNTs modified CF/EP composites at 77 K was decreased 37.2%.

An overview of metamaterials and their achievements in wireless power transfer

Metamaterials have been deployed for a wide range of fields including invisible cloak, superlens, electromagnetic wave absorption and magnetic resonance imaging, owing to their peculiar electromagnetic properties. However, few investigations on metamaterials were focused on wireless power transfer (WPT). WPT is the transmission of electrical energy from a power source to an electrical load without conductors like wires or cables. Metamaterials can enhance the transfer efficiency and enlarge the transfer distance due to their ability of focusing magnetic flux, which opens up a novel approach to promoting the development and application of WPT. This review paper aims to provide an overview of the fabrications, exotic properties, and their applications especially in the WPT field. Meanwhile, the perspective and future challenges of metamaterials and WPT are proposed.

Piezoresistive behavior of porous carbon nanotube-thermoplastic polyurethane conductive nanocomposites with ultrahigh compressibility

Ultrahigh compressibility has been observed in the lightweight porous carbon nanotube (CNT)-thermoplastic polyurethane (TPU) nanocomposites prepared by the thermally induced phase separation (TIPS) technique. The porous structure has significantly reduced the density to approximately 0.1 g·cm−3. The nanocomposites prepared with a sonication time of 16 min and a filler content of 0.51 vol. % possess uniform CNT distribution and show the highest saturated electrical conductivity. Furthermore, the observed CNT-dependent cell structure changes indicate that the added CNTs favor the formation of thicker and stronger cell structure to enhance its reproductivity as a piezoresistive sensor. Piezoresistive behaviors were then conducted under stepwise and cyclic compression. The porous nanocomposites possess fast sensing capacity over a wide strain range (up to 90%). In addition, good piezoresistive recoverability and reproducibility were observed in the nanocomposites after stabilization by cyclic compression. This study provides a guideline for fabricating porous electrically conductivenanocomposites as promising candidates for the flexible, high sensitive, and stable piezoresistance sensors.

Synergistic effect induced ultrafine SnO2/graphene nanocomposite as an advanced lithium/sodium-ion batteries anode

SnO2/graphene materials have received extensive attention in broad applications owning to their excellent performances. However, multi-step and harsh synthetic methods with high temperatures and high pressures are major obstacles that need to be overcome. Herein a simple, low-cost, and scalable approach is proposed to construct ultrafine SnO2/graphene nanomaterials effectively under constant pressure and at the low temperature of 80 °C for 4 h, in which ultrafine SnO2 nanoparticles grow on graphene sheets uniformly and firmly via Sn–O–C bonding. This result depends on the synergetic effect of two reactions, the reduction of graphene oxide and formation of SnO2 nanoparticles, which are achieved successfully. More importantly, the constructed SnO2/graphene material exhibits excellent electrochemical properties in both lithium-ion batteries and sodium-ion batteries. As an anode material for lithium-ion batteries, it displays a high reversible capacity (1420 mA h g−1 at 0.1 A g−1 after 90 cycles) and good cycling life (97% at 1 A g−1 after 230 cycles), whereas in sodium-ion batteries, it maintains a capacity of 1280 mA h g−1 at 0.05 A g−1 and 650 mA h g−1 at 0.2 A g−1 after 90 cycles. The proposed synthetic methodology paves the way for the effective and large scale preparation of graphene-based composites for broad applications such as energy storage, optoelectronic devices, and catalysis.