Zhuo Liu

Thermally reduced graphene oxide films as flexible lateral heat spreaders

A thermally reduced graphene oxide film (r-GOF), with tailorable micro-structures and macro-properties, is fabricated by annealing a filtrated graphene oxide film (GOF) in a confined space. The structural evolution of the film at different annealing temperatures is systematically investigated, and further correlated to the thermal conductivity and mechanical performances. With the increase of temperature, more oxygen-containing functional groups are removed from the film by a simultaneous conversion from sp3 to sp2 carbon in the graphitic lattice. As the temperature reached 1200 °C, the r-GOF achieves an ultrahigh thermal conductivity of ca. 1043.5 W m−1 K−1, while 1000 °C is a critical temperature in enhancing the thermal conductivity. Moreover, G1200 exhibits excellent mechanical stiffness and flexibility with a high tensile strength (13.62 MPa) and Youngs modulus (2.31 GPa). The combined conductivity and mechanical performances render the r-GOFs promising materials as flexible lateral heat spreaders for electronics.

Self‐Assembled 3D Graphene‐Based Aerogel with Co3O4 Nanoparticles as High‐Performance Asymmetric Supercapacitor Electrode

Using graphene oxide and a cobalt salt as precursor, a three-dimensional graphene aerogel with embedded Co3 O4 nanoparticles (3D Co3 O4 -RGO aerogel) is prepared by means of a solvothermal approach and subsequent freeze-drying and thermal reduction. The obtained 3D Co3 O4 -RGO aerogel has a high specific capacitance of 660 F g(-1) at 0.5 A g(-1) and a high rate capability of 65.1 % retention at 50 A g(-1) in a three-electrode system. Furthermore, the material is used as cathode to fabricate an asymmetric supercapacitor utilizing a hierarchical porous carbon (HPC) as anode and 6 M KOH aqueous solution as electrolyte. In a voltage range of 0.0 to 1.5 V, the device exhibits a high energy density of 40.65 Wh kg(-1) and a power density of 340 W kg(-1) and shows a high cycling stability (92.92 % capacitance retention after 2000 cycles). After charging for only 30 s, three CR2032 coin-type asymmetric supercapacitors in series can drive a light-emitting-diode (LED) bulb brightly for 30 min, which remains effective even after 1 h.

Biodegradable triboelectric nanogenerator as a life-time designed implantable power source

Mechanical energy in vivo could be harvested by BD-TENG in a designed time frame. Transient electronics built with degradable organic and inorganic materials is an emerging area and has shown great potential for in vivo sensors and therapeutic devices. However, most of these devices require external power sources to function, which may limit their applications for in vivo cases. We report a biodegradable triboelectric nanogenerator (BD-TENG) for in vivo biomechanical energy harvesting, which can be degraded and resorbed in an animal body after completing its work cycle without any adverse long-term effects. Tunable electrical output capabilities and degradation features were achieved by fabricated BD-TENG using different materials. When applying BD-TENG to power two complementary micrograting electrodes, a DC-pulsed electrical field was generated, and the nerve cell growth was successfully orientated, showing its feasibility for neuron-repairing process. Our work demonstrates the potential of BD-TENG as a power source for transient medical devices.

In Vivo Self-Powered Wireless Cardiac Monitoring via Implantable Triboelectric Nanogenerator

Harvesting biomechanical energy in vivo is an important route in obtaining sustainable electric energy for powering implantable medical devices. Here, we demonstrate an innovative implantable triboelectric nanogenerator (iTENG) for in vivo biomechanical energy harvesting. Driven by the heartbeat of adult swine, the output voltage and the corresponding current were improved by factors of 3.5 and 25, respectively, compared with the reported in vivo output performance of biomechanical energy conversion devices. In addition, the in vivo evaluation of the iTENG was demonstrated for over 72 h of implantation, during which the iTENG generated electricity continuously in the active animal. Due to its excellent in vivo performance, a self-powered wireless transmission system was fabricated for real-time wireless cardiac monitoring. Given its outstanding in vivo output and stability, iTENG can be applied not only to power implantable medical devices but also possibly to fabricate a self-powered, wireless healthcare monitoring system.

Self‐Powered Pulse Sensor for Antidiastole of Cardiovascular Disease

Cardiovascular diseases are the leading cause of death globally; fortunately, 90% of cardiovascular diseases are preventable by long-term monitoring of physiological signals. Stable, ultralow power consumption, and high-sensitivity sensors are significant for miniaturized wearable physiological signal monitoring systems. Here, this study proposes a flexible self-powered ultrasensitive pulse sensor (SUPS) based on triboelectric active sensor with excellent output performance (1.52 V), high peak signal-noise ratio (45 dB), long-term performance (107 cycles), and low cost price. Attributed to the crucial features of acquiring easy-processed pulse waveform, which is consistent with second derivative of signal from conventional pulse sensor, SUPS can be integrated with a bluetooth chip to provide accurate, wireless, and real-time monitoring of pulse signals of cardiovascular system on a smart phone/PC. Antidiastole of coronary heart disease, atrial septal defect, and atrial fibrillation are made, and the arrhythmia (atrial fibrillation) is indicative diagnosed from health, by characteristic exponent analysis of pulse signals accessed from volunteer patients. This SUPS is expected to be applied in self-powered, wearable intelligent mobile diagnosis of cardiovascular disease in the future.

Structure and magnetoresistance of the double perovskite Sr2FeMoO6 doped at the Fe site

The structural, magnetic and transport properties of the compounds Sr2(Fe1−xMnx)MoO6 (0 ≤ x ≤ 0.45) have been studied. The saturated magnetization, Curie temperature and low-field magnetoresistance of the compounds decrease with x, while the resistivity increases by several orders of magnitude when x exceeds a critical value. A positive magnetoresistance has been observed for x = 0.45. A possible percolation mechanism is proposed to elucidate the observations; it also suggests a coexistence of (Mn3+, Mo5+) and (Mn2+, Mo6+) valence pairs and a saturated substitution of Mn3+ for Fe3+.

Structural stability and electrical properties of Sr2FeMoO6 under high pressure

The structural stability and electrical properties of Sr2FeMoO6 under high pressure at room temperature have been studied using energy dispersive x-ray diffraction with synchrotron radiation and resistance and capacitance measurements. The x-ray diffraction results show that the structure of Sr2FeMoO6 remains stable up to 40 GPa. The equation of state of Sr2FeMoO6 is obtained from the V/V0–P relationship. The bulk modulus B0 and its first-order derivative B0′ of Sr2FeMoO6 were calculated based on the Birch–Murnaghan equation. The electrical resistance undergoes a metallic transition at about 2.1 GPa. The metallic transition may be caused by a change in the electronic structure induced by high pressure.

Evolution of magnetic behaviour in the graphitization process of glassy carbon

We have carried out DC magnetization measurements on spherical glassy carbon exposed to high temperatures and high pressures. The observed magnetic signals clearly depend on the sintering temperature. On the basis of the graphitization temperature (around 1400oC), the behaviour can be classified into three regions: (1) paramagnetism in the no-graphitization region; (2) ferromagnetism in the near-graphitization region; (3) diamagnetic behaviour after graphitization. The magnetic transitions associated with the process of graphitization are discussed.

Graphene enhanced low-density polyethylene by pretreatment and melt compounding

The rational design and fabrication of structural-functional materials are development trends in materials science. Herein, graphene enhanced low-density polyethylene (LDPE) is prepared by pretreatment and melt compounding, which is a simple and eco-friendly method. The amount of graphene added is controlled from 0 to 0.8 wt% to explore the enhancement effects. The graphene/LDPE nanocomposites (LPGNs) are characterized via SEM, TEM, Raman spectra, XRD, TG-DTA and DSC to research their dispersion morphology, crystal structure and thermal stability. A DMA, Servo Universal Strength Tester and Izod Impact Test Machine were used to study their mechanical properties. The results show that the added graphene is dispersed uniformly in the LDPE matrix, and the crystallinity of the LPGNs increases. The high specific surface area and outstanding properties of graphene improve the thermal stability, storage modulus, and mechanical properties of the LPGNs. Compared with neat LDPE, the Te of the LPGN with 0.8 wt% graphene increased by 58 °C and its tensile strength increased to 138%. A low content of graphene was effective in optimizing the Tm, Te and flame retardant properties of the LPGNs without compromising their mechanical properties.

Robust Multilayered Encapsulation for High-Performance Triboelectric Nanogenerator in Harsh Environment

Harvesting biomechanical energy especially in vivo is of special significance for sustainable powering of wearable/implantable electronics. The triboelectric nanogenerator (TENG) is one of the most promising solutions considering its high efficiency, low cost, light weight, and easy fabrication, but its performance will be greatly affected if there is moisture or liquid leaked into the device when applied in vivo. Here, we demonstrate a multiple encapsulation process of the TENG to maintain its output performance in various harsh environments. Through systematic studies, the encapsulated TENG showed great reliability in humid or even harsh environment over 30 days with a stability index of more than 95%. Given its outstanding reliability, the TENG has the potential to be applied in variety of circumstances to function as a sustainable power source for self-powered biomedical electronics and environmental sensing systems.