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Wireless communication technology plays a crucial role in the development of aviation and aerospace owing to the increasing demand for high performance, lightweight, environmentally adaptability, and structural stability. With these advantages, three-dimensional woven spacer structure becomes a potential platform for microstrip antennas, which is a significant candidate in wireless technology. In this study, we proposed an ultra-light-weight Kevlar/polyimide 3D woven spacer microstrip antenna (3DWS-MA-T) by embedding the conductive copper threads into the fabric to form an integrated structure. The fabricated 3D woven spacer composites (3DWSCs) overcome the conflict between lightweight and strong mechanical properties, exhibiting ultra-low volume density of 0.34 g/cm3 and compressive strength of 3.8 MPa. The resulting all-textile multifunctional 3DWS-MA-T achieved superb electromagnetic performance (gain value > 5 dB) with a good impedance matching (S11 value 5 dB) for aerospace applications

Flexible strain sensors with high sensitivity are indeed critical for smart wearable devices, they are used for detecting the tiny deformation of human skin induced by pulse, heartbeat or throat vibration. Herein, to design a highly sensitive strain yarn sensor, we produced a carbon nanotube (CNT) composite yarn by partially composing CNT yarn with epoxy (EP)/acetone polymer solution through a quick coating process. By this process, core/sheath structured CNT composite yarn (C/S-CY) with a thin sheath layer of less than 2.50 μm was obtained. The composite yarn exhibited 145.9% improved tensile strength (308.7 MPa) compared with pristine CNT yarn. In addition, the C/S-CY showed high strain sensitivity (Gauge factor ~ 8.65) with a linear piezoresistive (~ 98% linearity) response during the tensile strain of 2%. The core/sheath structured CNT composite yarn maintained proper sensing and tensile performance after multiple stretching cycles (2% strain). The as produced C/S-CY could find its potential applications in detecting movement of the finger, elbow joint and facial micro-expression, which indicated the potential applications of this sensor yarn in wearable electronics, flexible smart devices, or soft multifunctional systems.

Carbon nanotube film (CNTF) assembled by floating catalyst chemical vapor deposition (FFCVD) is a promising material for smart textile, multifunctional composites and soft robotics due to their lightweight, flexibility and electromechanical properties. However, bulk CNTF exhibit unsatisfactory properties, due to the loose and porous packing structures. In this study, an innovative and effective strategy was proposed to densify CNTF by a compression in acetone (wet compression). Due to low surface tension, acetone is easy to penetrate the CNTF and act as a lubricant thus decrease the interactions between the neighboring carbon nanotubes (CNT) under load. Therefore, the CNTF was condensed in thickness from 11.3 to 8.4 µm (34.5% decrease) with a CNT volume fraction increment (~ 11% to ~ 14.7%) under wet-compression treatment. Consequently, the wet-compressed CNTF tensile strength, electrical conductivity and gauge factor enhanced by 97%, 34% and 43% along the direction of CNT alignment and by 213%, 42% and 101% towards the CNT transverse direction. As a potential application, modified CNTF/epoxy composites were fabricated as strong de-icing parts of an airplane. The tensile strength increased to 742 MPa (by 21%) and effectively melted ice by 250 times weight ratio within 1 min; performing as strengthened lightweight multifunctional materials. To summarize, our work provides a universal approach to enhance the overall mechanical properties of the CNTF for several applications.

Carrier velocity modulation by asymmetrical concave quantum barriers to improve the performance of AlGaN-based deep-ultraviolet light emitting diodes has been investigated. The concave structure is realized by inserting a low Al composition AlGaN layer in the barrier, and thus forming a concave region on energy band. The measured device performance shows a significant improvement under 0–250 mA operation current, with the maximum external quantum efficiency and light output power of 1.02% and 8.0 mW, which are more than double (2.32 and 2.35 times) of that for the conventional one, respectively. Simulation confirms that the concave quantum barriers make it possible for electrons to be scattered, which reduce the electron energy before being injected into quantum wells, and thus the capability of quantum wells to capture electrons is enhanced. Meanwhile, the vertical transport of holes within the active region is also enhanced because the asymmetrical setting of the concave layer in the quantum barriers helps holes accelerate under the polarization field, and then get more energy to overcome the barrier.

The composite structures with lightweight, excellent mechanical and thermal properties are crucial to apply in the fields of industrial, civil as well as aerospace engineering. In this study, three-dimensional glass woven spacer fabric-reinforced epoxy resin composites (3DWSCs) were designed and manufactured. By increasing the interlacement of the pile yarn (IPY) from 2 to 6 ends/cm, the 3DWSCs presented similar light densities (0.3–0.42 g/cm3) but significant improvement in specific strength was from 3.9 to 24.6 MPa g−1 cm3, which is much higher than that of other reported 3DWSCs. This was because the pile yarns in the 3DWSC with dense IPY structure (6 ends/cm) were connected and supported with each other, and thus, their strength was fully utilized during compressive process. Contrary to the compressive strength, with the IPY increasing from 2 to 6 ends/cm, the thermal insulation of the 3DWSCs exhibited limited variation (~ 12.5%) and showed comparable performance with wood due to its high percentage of hollow structure (~ 75%). Therefore, the optimal 3DWSC structure (IPY is 6 ends/cm) with connected pile yarn structure exhibited excellent compressive strength as well as good thermal insulation, which showed great potentials in practical applications.

A highly tough and strain sensitive composite was manufactured by symmetric plasma functionalized CNT film and epoxy, considering the limited plasma permeability in a compact CNT assembly. Due to the homogeneously active chemical groups generated on CNTs, the composites demonstrated good resin infiltration and robust CNT-epoxy bonding, resulting in 220.1% improved peeling strength (331.2 N/m). Their tensile toughness reached 134.9 MJ/m3, which is around 5 times higher than other reported carbon based (CNT, Graphene, Carbon Fiber) composites. In addition, it showed excellent mechanical properties with tensile strength of 510.4 MPa and strain sensitivity with gage factor of ∼4.0 under a tensile strain above 40%. Our study for the first time revealed the improved interface interaction in a fiber-matrix system by uniform functionalized CNT film and created the tough and strain sensitive composites with good ductility. It can be applied as advanced composites in ballistic structure, aerospace field and some smart systems.

Carbon nanotubes demonstrate excellent electrical, mechanical and thermal properties due to their unique structure. To bring this astonish properties into macroscale, CNTs were assembled into large scale film via FCCVD method. However, the obtained CNT film mechanical properties remained unsatisfactory and leaves room for further improvement. Herein, different mixtures of Epoxy and PVA polymer matrix were incorporated via vacuum assisted resin transfer molding (VARTM) technique into CNT film and successfully enhanced its mechanical properties. The obtained CNT film/Epoxy and CNT film/PVA tensile strength increased by 3.5 and 4.7 times compared to pristine CNT film tensile strength. In addition, polymer matrix compatibility with CNT film was investigated by interlaminar strength method. CNT films/epoxy composite exhibited significantly higher peeling strength (633.7 N/m) compared to those with PVA (70.8 N/m) demonstrating the good affinity of epoxy with CNT film.

3-D orthogonal woven composites (3DOWC) have attracted great attention in the industrial and civil fields, due to their excellent mechanical properties. However, due to the mutual perpendicular yarns in three directions, its mechanical properties especially for fatigue behavior are very unique but critical for structural design in the practical applications. An experimental study was carried out for the quasi-static three-point bending and fatigue behavior of 3-D orthogonal woven composite (3DOWC). The 3-D orthogonal woven fabric (3DOWF) was reinforced with epoxy resin with the technique of Vacuum Assisted Resin Transfer Molding (VARTM). The damage process of specimens in quasi-static three-point bending tests was detected by acoustic emission (AE) detector. Simultaneously, the critical load levels i.e. the stress levels used in the bending fatigue experiments were also obtained according to AE results. The bending fatigue tests were performed at different stress levels, and the S N curves, stress-strain curves, stiffness degradation curves and residual strength of the composite specimens were also obtained. For the three-point bending fatigue loading condition, the characterizations of the three-stage failure process and crack propagation behavior were observed and illustrated. This study is the first time to systemically illustrate the fatigue stress level of 3-D orthogonal glass woven composites by acoustic emission (AE) detector and the detailed failure process are very important to guide the parameter and structural of the 3DOWC in real applications.

Enhancement of light extraction efficiency (LEE) of AlGaN-based deep-ultraviolet (DUV) light emitting diodes (LEDs) has been attempted by adopting Ag-nanodots/Al reflective electrodes on a highly transparent complex p-type layer. By thinning the p-GaN to several nm, highly DUV transparent p-type layer is achieved, making it meaningful for the application of reflective electrodes composed of Ag-nanodots and Al film to allow most light emitted upward to be reflected back to the sapphire side. By this approach, the maximum light output power and external quantum efficiency of the DUV-LEDs with optimized Ag nanodots/Al electrodes are severally increased by 52% and 58%, respectively, compared to those with traditional Ni/Au electrodes when the current is below 200 mA.

Hall data for a p-Al0.4Ga0.6N/Al0.67Ga0.33N superlattice layer. The hole concentration in this p-AlGaN superlattice layer is 3.7×1018 cm-3 at room temperature.

Lightweight, high strength, and flame-retardant properties are highly required for practical applications of composites, such as in areas of aerospace, aircraft, and automobiles. However, it is challenging to achieve the above properties simultaneously due to the intrinsic conflict between lightweight and excellent mechanical performance. Herein, we overcome this conflict in a 3D Kevlar/polyimide composite (K/PIC) innovatively by mimicking the structural features of the beetle's elytra. As a result, the K/ PIC has a void content of 84% and a low volume density of 0.23 g/cm3, but it can support more than 16 000 times its own weight without any damage. It exhibits a specific strength of 23.5 MPa/g·cm−3, which is superior to that of many other reported lightweight composites. Additionally, the as-prepared K/PIC could not be ignited and could retain around 93.2% of its original compressive strength even after being subjected to the flame of an alcohol burner for 300 s. The results demonstrate that similar lightweight, high strength, and flame-retardant composites can be fabricated by this underlying principle in future and be applied in extreme environments, such as aeronautics and aerospace.

Carbon nanotubes thin films (CNTf) in polymers should demonstrate excellent mechanical, thermal and electrical properties. However, such anticipated properties are hardly possible due to a number of factors. For example, the weak bonding of CNTf with polymers at the interface which results in easy delamination and deteriorated properties of the CNTf/polymer composites. More still, the declined conductivity of composites highly contributed by the insulating polymers. Herein, an approach to solve the mentioned problems is reported which briefly involved first modifying the polymer (epoxy resin) with the CNT in controlled percentages, and then introducing the CNTf to the modified epoxy solution which synergistically created a new CNT-to-CNT interphase and interlocked interactions, hence, presenting enhanced peeling (74.38%) and tensile (164.76%) strength in modified composites as compared to the unmodified CNTf/epoxy composites. Moreover, the strain sensing, conductivity, and temperature responses of the composites were also significantly improved as required in various high-performance composites.

Flexible strain sensors with good linear sensitivity, mechanical compatibility with systems and robustness over repeated usages are very desirable in wearable electronics, smart textiles, and other multifunctional structures. To this end, carbon nanotube/polyvinyl alcohol (CNT/PVA) coated yarn with a core-sheath structure (inner pure CNT core and outer CNT-PVA sheath) was fabricated by dipping the aerogel-spun CNT yarn in various PVA solutions for a short period of time. The as-produced CNT/PVA coated yarn has an electrical conductivity of 447.1 S/cm and exhibits a linear piezoresistive response, with a high gauge factor (the ratio of electrical resistance change to strain change) of 2.36. Compared with the pure CNT yarn, the PVA coated yarn has an improved tensile strength by 71.8%, Young’s modulus by 157.3%, abrasion-resistance by 100%, and enhanced stability after cyclic loading. This demonstrates a promising strain sensor for system integration into flexible intelligent devices or other advanced composites.

With the highly ordered structure and thus probably fast carrier transport, one-dimensional (1D) conducting polymers are very promising as organic thermoelectric (TE) materials. Many efforts have been made toward the elucidation of TE transport in polymer nanowires assembly; however, the mechanism for the improvement in thermoelectric property is still far from clear. Here, we systematically investigate the TE transport in 12-nanometer-wide poly(3,4-ethylenedioxythiophene) nanowires (PEDOT NWs) assembly. The iron (III) chloride oxidized PEDOT NWs shows high electrical conductivity σ (∼540 S/cm), an enhanced Seebeck coefficient S that is 2.6 times of that of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate and tuned power factor (PF∼35.8 μW/m·K2). To the best of our knowledge, it outperforms the TE performance of all reported 1D conducing polymers based films. More importantly, we carefully interpret the origin of σ and S enhancements in PEDOT NWs-based films. The high σ mainly results from the significant increment of carrier mobility μ, which is mainly due to preferred edge-on orientation and high crystallinity of PEDOT chains. The large S in PEDOT NWs-based films stem from the sharp feature of the density of states at Fermi level.

As a promising structural material, three dimensional woven fabric reinforced composites are subjected to machining or drilling during manufacturing processes for components assembly. In-plane tensile and flexural properties of a three dimensional orthogonal woven E-glass/epoxy composites (3DOWC) with 4 and 6 mm drilled holes (DH) and moulded-in holes (MH) are investigated. 2D woven E-glass/epoxy laminated composites (2DWC) with the same drilled hole sizes are also tested for comparison. Two different calculation methods based on gross and net sectional area are adopted to evaluate the effects of the holes on mechanical performance of the composites. The 3DOWC-MH possesses the highest tensile and flexural strength, followed by 3DOWC-DH and 2DWC-DH. As the hole diameter increases, the mechanical properties of all the composites degrade, while those of the 3DOWC-MH increases when the net sectional areas are used for calculation. X-ray 3D microscopy and scanning electron microscopy are employed to analyze damages at submicron level resulted from the tests. Various failure modes such as fiber fracture, matrix cracking and tow debonding are observed, while serious delamination occurs in 2DWC but not in 3DOWC.

The electrical performance of delicate electro-devices is subjected to the external environment, suffering significant performance decay in conditions with extremely low temperature. In this study, a highly stretchable and high effective carbon nanotube (CNT) heater was developed for multiple applications. The CNT-based heater has good stretchability with the strain of 105%, and a high thermal response that reached 206 °C with a limited applied voltage (2 V) and heating up time (~30 s). Our results indicated that the conductivity of the heater could remain unchanged (only 0.2% variation) at 100 cycles repeating deformation. We demonstrated a potential application of using CNT heater as a “battery starter” to provide an external heating source for batteries at low temperatures. Furthermore, our work provided a universal approach to design flexible and wearable heater to offer promising opportunities for thermal management in wearable applications and electro-devices.

The effects of cryogenic treatment (−196 °C) on properties of the carbon nanotube (CNT) fiber/epoxy composite are investigated using a temperature controlled cooling process with low cooling rate. The CNT fiber/epoxy single fiber composite fragmentation test is performed in conjunction with electrical resistance measurements to compare the interfacial properties at ambient and cryogenic temperatures. Weibull distribution for CNT fiber strengths shows a decreased variation (the shape parameter increases from 4.98 to 6.93) for cryo-treated fibers although their mechanical properties are not significantly influenced by the treatment. The electrical resistance measurement shows an increased electrical resistance changing rate with strain for the cryogenic treated sample, indicating better fiber/matrix interaction. In addition, the cryogenic treatment also improves interfacial bonding strength by 31% owing to the effect of differential thermal shrinkage for the CNT fiber and epoxy. An asymmetrical interfacial stress distribution is observed corresponding to the asymmetrical birefringence patterns of the fragmented fiber in the single fiber composite, which could be due to the twisted structure of the CNT fiber.

The interfacial bonding between carbon nanotube (CNT) films and epoxy is usually fairly weak for high quality composites and difficult to be measured experimentally. In this study, the interfacial bonding strength between a CNT film and epoxy is measured using a peeling test. The CNT/epoxy interfacial bonding strength is altered by surface functionalization of the CNT film using atmospheric pressure helium/oxygen plasma. Furthermore, composites with the control and the modified CNT film impregnated in epoxy are manufactured, and their mechanical properties including peeling strengths and tensile strengths are improved remarkably due to enhanced CNT/epoxy bonding after plasma functionalization. The peeling strength between the CNT film and epoxy is increased by 156.6% and so is the fracture energy. The tensile strength of the functionalized CNT film/epoxy composites is 74.4% higher than that with original CNT film. The effect of the treatment time (0.1, 0.2 and 0.3s) is assessed by surface chemical and physical analyses, showing an improvement in the amount of oxygen-containing functional groups on CNT surface, and better dispersion of the CNTs in ethanol with increased treatment time. This benefits the wetting and infiltration into CNTs by epoxy to obtain a stronger interfacial bonding.

The weak wear and interfacial properties of the aramid fiber were the main obstacles to limit its practical applications. In this study, a simple and environmental friendly method, cryogenic treatment was adopted to treat the Kevlar fiber by different cooling rates. After cryogenic conditions, the tensile, wear and interfacial shear strength (IFSS) properties of Kevlar fiber were all enhanced. The tensile strength of the aramid fiber conditioned in sharp cooling rate (QM process) increased by 24.9%, while that of the fiber conditioned in low cooling rate (TPCM process) slightly increased. Moreover, the abrasion duration of the fiber increased 51.6% and 51.1% under QM and TPCM treatments. The scanning electron microscope (SEM) and atomic force microscope (AFM) results showed that the surface roughness of the aramid fibers increased 32% and 73% after conditioned in QM and TPCM processes, respectively. Consequently, the IFSS of the fiber/epoxy resin increased 5.7% and 19% once treated by QM and TPCM processes. The cryogenic treatment was demonstrated as an effective method to enhance tensile strength, abrasion resistance and interfacial properties of Kevlar fiber for practical applications.

In this paper, a simple and innovative method is proposed to monitor the effects of seawater ageing in glass fibre reinforced polymer (GFRP) composites using quantum dots (QDs). The cadmium sulfide (CdS) QDs modified with 3-(trimethoxysilyl)-1-propanethiol (MPS) were firstly synthesized, and then the modified CdS QDs were labeled on the glass fibre by chemical bond. GFRP composites with QDs at the interface between the fibre and matrix were obtained. The sigle-fibre-fragment tests of specimens were done to verify the modified CdS QDs did not affect the interfacial bonding of the matrix and fiber. The effects of seawater ageing on the adhesion behaviour between the fibre and the matrix of QD/GFRP hybrids were investigated based on confocal fluorescence microscopy images. After 6 months of immersion in seawater, the glass transition temperature ( T g ), tensile strength, and flexural strength of specimens decreased 2.5%, 13.8%, and 9.8%, respectively. Furthermore, the thermal and mechanical properties of specimens can be calculated by the fluorescence intensity quantificationally.

The role of low-resistivity substrate on vertical leakage current (VLC) of AlGaN/GaN-on-Si epitaxial layers has been investigated. AlGaN/GaN high-electron-mobility transistors (HEMTs) grown on both p-type and n-type Si substrates with low resistivity are applied to analyze the vertical leakage mechanisms. The activation energy (Ea) for p-type case is higher than that for n-type at 0&ndash, 600 V obtained by temperature-dependent current-voltage measurements. An additional depletion region in the region of 0&ndash, 400 V forms at the AlN/p-Si interface but not for AlN/n-Si. That depletion region leads to a decrease of electron injection and hence effectively reduces the VLC. While in the region of 400&ndash, 600 V, the electron injection from p-Si substrate increases quickly compared to n-Si substrate, due to the occurrence of impact ionization in the p-Si substrate depletion region. The comparative results indicate that the doping type of low-resistivity substrate plays a key role for VLC.

The anisotropic extraction dependence of polarized light on propagation path in AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) is investigated by simulations and photoluminescence (PL) measurements. Theoretical calculations based on k⋅p approximation and Monte Carol ray tracing indicate that there are two kinds of polarized sources with different angular distributions in ~280 nm AlGaN-based LEDs, s-polarized (spherical-shaped) and p-polarized (dumbbell-shaped) sources, which have different extraction behaviors. It is found that the total light extraction intensities are improved with decreasing the propagation path, and the lateral surface extraction gradually becomes dominant. Moreover, the extraction intensity of s-polarized light improves more than that of p-polarized light when the propagation path decreases, leading to a greater polarization degree. Polarization-resolved PL measurements show that the polarization degree of extracted light from lateral facet of the AlGaN multiple quantum well sample can be enhanced from 1% to 17% as the average propagation path reduces by 0.6 mm, which is consistent with the simulation results of the anisotropic dependence of light extraction on propagation path. Our results are significant for understanding and modulating the anisotropic extraction behavior of polarized light to realize high efficiency AlGaN-based DUV LEDs.

Research on lightweight advanced materials that combine heat insulation and sound isolation becomes more attractive day by day. In this study, a 3D spacer structural Kevlar/polyimide composite (3DSSK/PIC) with light weight, heat insulation and sound absorbing properties was prepared by a combination of structural design, hand paste resin method and hot-pressing process. The fabricated 3DSSK/PIC possesses more than 80% of void as well as the ultra-light density (0.2 g/cm3). Consequently, the thermal insulation performance of 3DSSK/PIC (34%) is superior to that of polystyrene foam (adiabatic ratio ΔT ~20%) and similar to that of wood block (ΔT ~35%). Furthermore, the sound absorption coefficient of the 3DSSK/PIC is higher than 0.2 in the range of 600 ~ 2500 Hz, demonstrating its super sound absorption performance. In addition, compressive tests and SEM images were conducted to characterize the mechanical and structural performance. Therefore, the ultra-light 3D fabric reinforced composite with ultra-high thermal insulation and sound-absorbing properties justify a great potential in a wide variety of applications in the industry engineering, automobile, or aerospace.

The tensile properties of an epoxy-based adhesive subject to high strain rate loadings at low temperatures were studied. Tensile split Hopkinson bar incorporating with an environmental chamber was utilized to conduct tensile property tests. Mechanical Parameters, like strength, Young's Modulus, strain to failure and fracture energy at low temperatures (−20°C and −40 °C) with different strain rates were investigated. The results show that at room temperature (RT), the strength and stiffness both increase significantly with the increase of strain rate. However, the increment of strength at low temperatures is insignificant. Interestingly, at RT and −20 °C, strain to failure increase with the increase of strain rates. From the stress-strain curves, strain softening phenomenon at high strain rates are observed at RT and −20 °C.This could be explained by the localized temperature rise in the specimens when the strain rate reaches high. At −40 °C, strain to failure decreases with the increase of strain rate and strain softening disappears. The larger temperature difference between the environment and the localized temperature rise in the specimens may reduce the strain softening phenomenon. Effects of high strain rate on tensile properties of epoxy adhesive at low temperatures are different from that on polyurethane-based adhesive in the literature.