Highly sensitive humidity sensor based on composite film of partially reduced graphene oxide and bacterial cellulose

Breathing is an important physiological activity of human body, which not only reflects the state of human movement, but also is one of the important health indicators. Breathing can change the concentration of water molecules, so monitoring humidity has gradually become a hot topic in modern research. In this study, a humidity sensing composite film with high sensitivity and short response time was made by using the mixture of graphene oxide (GO) and bacterial cellulose (BC) with simple dry film-forming method. L-ascorbic acid was used as reducing agent to reduce GO and improve the conductivity of GO/BC composite film (BG). The influence of different BC contents and the different reduction degree on the resistance change rate of composite film was investigated in details. The maximum resistance change rate of partially reduced BG humidity sensitive composite film reached up to 94%, and the response and recovery time were 13 s and 47 s respectively. Furthermore, the sensor shows obvious resistance change in noncontact sensing test and different breathing states. This kind of humidity sensitive film with fast response and high sensitivity has great potential in human health monitoring and noncontact sensing, and is of great significance in promoting health detection and intelligent life.

Bacterial cellulose/multi-walled carbon nanotube composite films for moist-electric energy harvesting

Generation of renewable and clean electricity energy from ubiquitous moisture for the power supply of portable electronic devices has become one of the most promising energy collection methods. However, the modest electrical output and transient power supply characteristics of existing moist-electric generator (MEG) severely limit its commercial application, leading to an urgent demand of developing a MEG with high electrical output and continuous power generation capacity. In this work, it is demonstrated that a flexible bacterial cellulose (BC)/Multi-walled carbon nanotube (MWCNT) double-layer (BM-dl) film prepared by vacuum filtration can maintain the moisture concentration difference in the film MEG. Unlike previous studies on cellulose based MEG, BM-dl film has a heterogeneous structure, resulting in a maximum output power density of 0.163 μW/cm2, an extreme voltage of 0.84 V, and current of 2.21 μA at RH = 90 %. BM-dl MEG can generate a voltage of 0.55 V continuously for 45 h in a natural environment (RH = 63-77 %, T = 26-27 °C), which is in a leading level among existing reported cellulose-based MEGs. In summary, this study provides new ideas for innovative design of MEG, which is highly competitive in terms of energy supply for the Internet of Things and wearable devices.

Self-healing and anti-freezing graphene-hydrogel-graphene sandwich strain sensor with ultrahigh sensitivity

Hydrogels with specially designed structures and adjustable properties have been considered as smart materials with multi-purpose application prospects, especially in the field of flexible sensors. However, most hydrogel-based sensors have low sensitivity, which inevitably affects their promotion in the market. Herein, a strain sensor comprising a poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA) hybrid hydrogel sandwiched between two graphene layers was successfully constructed in a facile way, and it exhibited many excellent properties including extremely high sensitivity. The incorporation of glycerol ensured the good flexibility and anti-freezing performance of the hydrogel-based sensor even at -15 °C. The dynamic coordination bonds in the hydrogel-based sensor endowed it with excellent self-healing properties. In particular, the sandwich-structured hydrogel sensor showed a very high gauge factor (GF) value of 39 at the strain of 50%, which is much higher than those of most ordinary hydrogel-based strain sensors. A super stable signal value after 5000 strain cycles and a very short response time of 274 ms guaranteed the long-term usability and sensitivity of the hydrogel-based sandwich sensor. More importantly, the hydrogel-based sandwich sensor could detect both large and tiny human motions accurately and instantly in a series of real-time monitoring experiments, showing great potential for intelligent wearable electronic devices.

Synthesis and characterization of poly(phthalazinone ether ketone ketone) copolymers with 4,4′-dihydroxybiphenyls

Meta-poly(phthalazinone ether ketone ketone) copolymers with 4,4′-dihydroxybiphenyls (meta-PPBEKK) was synthesized with 4,4′-dihydroxybiphenyls (BP), 1,3-bis(4-fluorobenzoyl)benzene (meta-DFKK) and 4-(4-hydroxyphenyl)-2,3-diazaphthalene-1-one (DHPZ). The copolymers are high-temperature resistant and have good solubility in many solvents. With increasing molar ratio of BP to meta-DFKK from 0 to 0.2, the addition of BP improves number-average molecular masses, intrinsic viscosity, mechanical properties and fluidity of meta-PPBEKKs. When the molar ratio of BP to meta-DFKK is 0.2, the intrinsic viscosity reaches 1.14 dL g−1, the number-average molecular mass reaches 44700 g mol−1, the maximum thermal elongation ratio reaches 14 with strength of 127.1 MPa and Young’s modulus of 2.4 GPa. With increasing molar ratio of BP to meta-DFKK from 0 to 0.2, meta-PPBEKKs have improved comprehensive properties, and make sense in many fields.

High absorption shielding material of poly(phthalazinone etherketone)/multiwall carbon nanotube composite films with sandwich configurations

Sandwich structure can be induced to achieve excellent electromagnetic interference shielding effectiveness (EMI SE) as is well known. However, how to optimize the structure to achieve performance optimization is still a problem. Herein, a poly(phthalazinone etherketone)/multiwall carbon nanotube (PPEK/MWCNT) composite with homodisperse and high content (15 wt%) of MWCNT was prepared by a water induced phase separation process. A sandwich structure was designed by introducing a wave-transmitting layer between the PPEK/MWCNT composite films. The MWCNT loading of the shielding layer and the wave-transmitting layer thickness (d) were varied to systematically investigate their influence on shielding performance in the X-band (8.2–12.4 GHz). EMI SE shows strong d value dependence. When the shielding layer was fixed, EMI SE showed a trend of decreasing, rising, leveling off and then decreasing again with d value increasing. The d value in the leveling off stage is the best value for performance optimization. The best d value decreased with increasing MWCNT content of the shielding layer. An absorption dominated SE up to 65 dB and a high increment rate of 140% of SE were achieved by optimization of the d value and shielding layer.

Sandwich structure can be induced to achieve excellent electromagnetic interference shielding effectiveness (EMI SE) as is well known.

High performance strain sensor based on buckypaper for full-range detection of human motions

A high-performance strain sensor based on buckypaper has been fabricated and studied. The sensor with an ultrahigh gauge factor of 20 216 can detect a maximum and a minimum strain range of 75% and 0.1%, respectively. During stretching, the strain sensor achieves a high stability and reproducibility of 10 000 cycles, and a fast response time of less than 87 ms. On the other hand, the sensor shows an excellent sensing performance upon pressure. The pressure range, pressure sensitivity and loading-unloading cycles are 0-1.68 MPa, 89.7 kPa-1 and 3000 cycles, respectively. A concept of the optimal value is utilized to evaluate the strain and pressure performances of the sensor. The optimal values of the sensor upon tensile strain and pressure are calculated to be 3.07 × 108 and 1.35 × 107, respectively, which are much higher than those of most strain and pressure sensors reported in the literature. Precise detection of full-range human motions, acoustic vibrations and even pulse waves at a small scale has been successfully demonstrated by the buckypaper-based sensor. Owning to its advantages including ultrahigh sensitivity, wide detection range and good stability, the buckypaper-based sensor suggests a great potential for applications in wearable sensors, electronic skins, micro/nano electromechanical systems, vibration sensing devices and other strain sensing devices.

Education program for controversial defect of recent X-ray instrument termed as a simultaneous small angle X-ray scattering and wide angle X-ray diffraction measuring instrument

Simultaneous rotations of sample and X-ray detected counter are needed to evaluate orientation distribution of crystallites and amorphous chains oriented predominantly parallel to the film surface in addition to exact diffraction peak profiles obtained without the complicated intensity corrections. The rotation mode is known as “θ–2θ scanning” system (θ: film, 2θ: counter). The system has been mainly used in research and development institutes. However, such instruments are not produced at present. Recently, small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD) intensities have been measured by using X-ray beam generated along one direction. The brand name of the instrument is “a simultaneous SAXS and WAXD measuring instrument”. The X-ray beam generated by the instrument has surely high luminance providing high degree resolution of peak profiles by diffraction and/or scattering. The sample stage and detector, however, are fixed, since the intensities for SAXS and WAXD are obtained by the digital display of the number of X-ray photons detected on the imaging plate. Such optical system contains controversial defect on evaluating orientation of crystal planes parallel to the surface of films prepared by T-die and inflation methods as well as the exact profile. The imaging plate cannot detect the diffraction intensity from the crystal planes existing in the angle range between incident beam and Bragg angle associated with the diffraction peak position of the individual crystal planes.

Facile fabrication of polyaniline@γ-MnOOH on a buckypaper ternary composite electrode for free-standing supercapacitors

Ternary composites as electrode materials have attracted extensive attention. Herein, we demonstrated a facile two-step method to construct a new hierarchical nanocomposite by combing buckypaper (BP) with γ-MnOOH nanorods and polyaniline.

Considerable different frequency dependence of dynamic tensile modulus between self-heating (Joule heat) and external heating for polymer--nickel-coated carbon fiber composites

Dynamic tensile moduli of polyethylene--nickel-coated carbon fiber (NiCF) composites with 10 and 4 vol % NiCF contents under electrical field were measured by a homemade instrument in the frequency range of 100--0.01 Hz. The drastic descent of the storage modulus of the composite with 10 vol % was verified in lower frequency range with elevating surface temperature (T(s)) by self-heating (Joule heat). The composite was cut when T(s) was beyond 108 °C. On the other hand, the measurement of the composite with 4 vol % beyond 88 °C was impossible, since T(s) did not elevate because of the disruption of current networks. Incidentally, the dynamic tensile moduli by external heating could be measured up to 130 and 115 °C for 10 and 4 vol %, respectively, but the two composites could be elongated beyond the above temperatures. Such different properties were analyzed in terms of crystal dispersions, electrical treeing, and thermal fluctuation-induced tunneling effect.

Appearance of perfect amorphous linear bulk polyethylene under applied electric field and the analysis by radial distribution function and direct tunneling effect

Without melting flow, linear ultrahigh molecular weight polyethylene (UHMWPE) provided X-ray intensity curve from only amorphous halo at 129.0 °C (surface temperature, Ts arisen by Joule heat) lower than the conventionally known melting point 145.5 °C on applying electric field to UHMWPE-nickel-coated carbon fiber (NiCF) composite. Such surprising phenomenon was analyzed by simultaneous measurements of X-ray intensity, electric current, and Ts as a function of time. The calculated radial distribution function revealed the amorphous structure with disordered chain arrangement. The appearance of such amorphous phase was arisen by the phenomenon that the transferring electrons between overlapped adjacent NiCFs by tunneling effect struck together with X-ray photons and some of the transferring electron flown out from the gap to UHMWPE matrix collided against carbon atoms of UHMWPE. The impact by the collision caused disordering chain arrangement in crystal grains.

Surface characterization for ultrahigh molecular weight polyethylene/hydroxyapatite gradient composites prepared by the gelation/crystallization method

To establish implant longevity of hip prosthesis in orthopedics, a new approach was proposed to improve dramatically the wear resistance and to reduce the surface friction of the acetabular cup as a bearing material in the femoral head. To do so, ultrahigh molecular weight polyethylene (UHMWPE) and hydroxyapatite (HA) composites with four amounts of HA content were prepared by a sol-gel process, and the four composites were hot-molded to make a composite with HA gradient content. When the resultant UHMWPE/HA agglomerates by the sol-gel method were molded in the narrow temperature range of 145-153 °C, the (110) planes with highest density of atoms in the PE crystal unit were oriented predominantly parallel to the resultant film surface. Such an unusual planar orientation contributed excellent wear resistance and low friction on the surface. Polarized light-scattering patterns, SEM images, and FTIR spectra of the specimens with such unusual planar orientation supported that the narrow molding temperature range achieves good dispersion of HA particles and high crystallinity of the UHMWPE matrix on the surface layer. Negative complex Poisson's ratio reduced from complex tensile and shear moduli was attributed to spongy-like tissue formation under crystallization of UHMWPE chains on the HA particle surface. The gradient composite molded maintained the spongy-like structure, which played an important role to avoid the cracking under bending stress.

Orientation of carbon fiber axes in polymer solutions under magnetic field evaluated in terms of orientation distribution of the chain axes of graphite with respect to the carbon fiber axis

Orientation of carbon fiber (CF) axes in poly(vinyl alcohol) (PVA) aqueous solution under magnetic field was evaluated by considering the fact that the c-axes (chain axes of graphite) have orientation distribution with respect to their CF axis. This new approach was proposed to resolve the well-known contradiction that the diffraction image from the (002) plane measured by X-ray shows broad arcs indicating dull orientation of the c-axes with respect to the magnetic field, whereas the corresponding orientation of CF axes observed by SEM reveals high predominant orientation. To pursue the quantitative evaluation, the orientation function of CF axes was obtained from the orientation function of the reciprocal lattice vector of the (002) plane by a somewhat coordinate transformation. The real orientation function of the CF axes with respect to the magnetic field direction showed very sharp distribution profile in comparison to the function of the c-axes and it was in good agreement with the orientation of CF axes observed directly by SEM. Furthermore, the orientation of CF axes up to the equilibrium state was estimated by a common diffusion equation. The calculated results predicted time dependence of the preferential orientation behavior of CFs in PVA solution precisely.

Dielectric studies on the heterogeneity and interfacial property of composites made of polyacene quinone radical polymers and sulfonated polyurethanes

Sulfonated polyurethane (PUI, matrix) is synthesized and composited with polyacene quinone radical polymers (PAQRs, filler). The polarization mechanism of these polymers and composites were investigated in terms of their frequency, temperature, and filler-concentration-dependent dielectric properties. We found that PUI/PAQR composites have a high permittivity, which is attributed to the filler-matrix interfacial polarization and the contact effect. The PAQR-concentration-dependent permittivity of different PUI/PAQR composites reveals a percolation threshold at 20-30 wt % with scaling exponents that indicate the intercluster polarization. The frequency dependence of dielectric response is well-fitted by using the Debye and Cole-Cole functions on the basis of the structural diagrams and equivalent circuit, leading to a detailed evaluation on heterogeneous structures of different PUI/PAQR composites.