Yani Zhang

The applications of carbon nanotubes and graphene in advanced rechargeable lithium batteries

Advanced rechargeable lithium batteries are desired energy storage devices for electric vehicles. These batteries require their electrodes to have high electrical and thermal conductivity, an appropriate high specific surface area, an outstanding hierarchical architecture, high thermal and chemical stability and to be relatively low cost and environmentally benign. Carbon nanotubes (CNTs) and graphene are two candidate materials that could meet these requirements, and thus have been widely studied. The present paper reviews the applications of CNTs and graphene in batteries, with an emphasis on the particular roles (such as conductive, active, flexible and supporting roles) they play in advanced lithium batteries. We will summarize the unique advantages of CNTs and graphene in battery applications, update the most recent progress, and compare the prospects and challenges of CNTs and graphene for future full utilization in energy storage applications. The effects and mechanisms of heteroatoms doping, the distribution of pore sizes, different architectures (anchored, sandwich-like and wrapped hybrid architecture) are discussed in detail.

A modified Weibull model for tensile strength distribution of carbon nanotube fibers with strain rate and size effects

Fundamental studies on the effects of strain rate and size on the distribution of tensile strength of carbon nanotube (CNT) fibers are reported in this paper. Experimental data show that the mechanical strength of CNT fibers increases from 0.2 to 0.8 GPa as the strain rate increases from 0.00001 to 0.1 (1/s). In addition, the influence of fiber diameter at low and high strain rate conditions was investigated further with statistical analysis. A modified Weibull distribution model for characterizing the tensile strength distribution of CNT fibers taking into account the effect of strain rate and fiber diameter is proposed.

Ultra-sensitive and wide-dynamic-range sensors based on dense arrays of carbon nanotube tips

Electrochemical electrodes based on dense and vertically aligned arrays of multi-walled carbon nanotubes (MWCNTs) were produced. The open tips of individual hollow nanotubes are exposed as active sites while the entangled nanotube stems encapsulated in epoxy collectively provide multiplexed and highly conductive pathways for charge transport. This unique structure together with the extraordinary electrical and electrochemical properties of MWCNTs offers a high signal-to-noise ratio (thus high sensitivity) and a large detection range, compared with other carbon-based electrodes. Our electrodes can detect K(3)FeCN(6) and dopamine at concentrations as low as 5 nM and 10 nM, respectively, and are responsive in a large dynamic range that spans almost 5 orders of magnitude.

Flexible SiC/Si3N4 Composite Nanofibers with in Situ Embedded Graphite for Highly Efficient Electromagnetic Wave Absorption

SiC/Si3N4 composite nanofibers with in situ embedded graphite, which show highly efficient electromagnetic (EM) wave absorption performance in gigahertz frequency, were prepared by electrospinning with subsequent polymer pyrolysis and annealing. By means of incorporating graphite and Si3N4 into SiC, the EM wave absorption properties of the nanofibers were improved. The relationship among processing, fiber microstructure, and their superior EM wave absorption performance was systematically investigated. The EM wave absorption capability and effective absorption bandwidth (EAB) of nanofibers can be simply controlled by adjusting annealing atmosphere and temperature. The nanofibers after annealing at 1300 °C in Ar present a minimum reflection loss (RL) of -57.8 dB at 14.6 with 5.5 GHz EAB. The nanofibers annealed in N2 at 1300 °C exhibit a minimum RL value of -32.3 dB at a thickness of 2.5 mm, and the EAB reaches 6.4 GHz over the range of 11.3-17.7 GHz. The highly efficient EM wave absorption performance of nanofibers are closely related to dielectric loss, which originated from interfacial polarization and dipole polarization. The excellent absorbing performance together with wider EAB endows the composite nanofibers potential to be used as reinforcements in polymers and ceramics (SiC, Si3N4, SiO2, Al2O3, etc.) to improve their EM wave absorption performance.

Solid state dendrite formation in an amorphous magnetic Fe77.5Si13.5B9 alloy observed by in situ hot stage transmission electron microscopy

Crystallization of an initially amorphous Fe77.5Si13.5B9 melt spun alloy was studied by in situ hot stage transmission electron microscopy. The morphology of the crystal was initially equiaxed, followed by a morphological transition to an unusual solid state dendrite morphology. This morphological transition is generally observed after the crystal size reaches a critical value of about 45nm. The experimental results of dendrite growth and the morphological transition are compared to theoretical predictions.Crystallization of an initially amorphous Fe77.5Si13.5B9 melt spun alloy was studied by in situ hot stage transmission electron microscopy. The morphology of the crystal was initially equiaxed, followed by a morphological transition to an unusual solid state dendrite morphology. This morphological transition is generally observed after the crystal size reaches a critical value of about 45nm. The experimental results of dendrite growth and the morphological transition are compared to theoretical predictions.

Effect of channel length on the electrical response of carbon nanotube field-effect transistors to deoxyribonucleic acid hybridization

Summary A single-walled carbon nanotube (SWCNT) in a field-effect transistor (FET) configuration provides an ideal electronic path for label-free detection of nucleic acid hybridization. The simultaneous influence of more than one response mechanism in hybridization detection causes a variation in electrical parameters such as conductance, transconductance, threshold voltage and hysteresis gap. The channel length (L) dependence of each of these parameters necessitates the need to include them when interpreting the effect of L on the response to hybridization. Using the definitions of intrinsic effective mobility (µe) and device field-effect mobility (µf), two new parameters were defined to interpret the effect of L on the FET response to hybridization. Our results indicate that FETs with ≈300 µm long SWCNT exhibited the most appreciable response to hybridization, which complied with the variation trend in response to the newly defined parameters.

Fabrication of microscale carbon nanotube fibers

Carbon nanotubes (CNTs) have excellent mechanical, chemical, and electronic properties, but realizing these excellences in practical applications needs to assemble individual CNTs into larger-scale products. Recently, CNT fibers demonstrate the potential of retaining CNTs superior properties at macroscale level. High-performance CNT fibers have been widely obtained by several fabrication approaches. Here in this paper, we review several key spinning techniques including surfactant-based coagulation spinning, liquid-crystal-based solution spinning, spinning from vertical-aligned CNT arrays, and spinning from CNT aerogel. The method, principle, limitations, and recent progress of each technique have been addressed, and the fiber properties and their dependences on spinning parameters are also discussed.

A highly torsionable fiber-shaped supercapacitor

To bestow intelligent functions upon clothing textiles, wearable electronics must be able to accommodate complex deformations. Both stretchable and bendable building blocks have previously been demonstrated, but torsionable components are still lacking. Here we report a fiber-shaped flexible supercapacitor that is highly stable under torsional deformations, with a very small capacitance variation around its average value (<±2%) even under severe twisting. The torsionable supercapacitor is fabricated from shearable carbon nanotube films. When such a film is integrated with an elastic fiber as an electrode, it is capable of withstanding torsion levels up to 20 000 rad m−1 with relatively stable conductivity. As a result, fiber-shaped supercapacitors fabricated from such torsionable electrodes can maintain an almost unchanged capacitance during twisting.

SiC Nanofiber Mat: A Broad-Band Microwave Absorber, and the Alignment Effect

Fiber alignment is a key factor that determines the physical properties of nanofiber mats. In this work, SiC nanofiber mats with or without fiber alignment are fabricated via electrospinning and the microwave electromagnetic properties of their silicone resin composites (5 wt %) are investigated in 2-18 GHz. By comparing with the composite containing SiC whisker, it is found that the nanofiber mats show superior dielectric loss and a minimal reflection loss (RL) of around -49 dB at 8.6 GHz and 4.3 mm thickness, associated with a broad effective absorption (<-10 dB) bandwidth (EAB) of about 7.2 GHz at 2.8 mm thickness. Moreover, the performance can be further enhanced (RL = -53 dB at 17.6 GHz and 2.3 mm thickness) by aligning the nanofiber in the plane of mat, accompanied by the shift of absorption peak to higher-frequency direction and broader EAB up to 8.6 GHz at 3 mm. In addition, the stacking ways of aligned SiC nanofiber mats (either parallel or perpendicular) are proved to have a negligible effect on their microwave properties. Compared with parallel stacking of the aligned mats, cross-stacking (perpendicular) only leads to a slight drop of the attenuation ability. It confirms that alignment of nanofiber in the mats offers a more effective approach to improve the microwave absorption properties than changing the ways of stacking. Furthermore, it is worth mentioning that the low loading fraction (5 wt %) is a great advantage to reduce the weight as well as the cost for large-scale production. All of these facts indicate that the aligned SiC nanofiber mats can serve as a great lightweight and broad-band microwave absorber.

Flexible Fe3Si/SiC ultrathin hybrid fiber mats with designable microwave absorption performance

Flexible Fe3Si/SiC ultrathin fiber mats have been fabricated by electrospinning and high temperature treatment (1400 °C) using polycarbosilane (PCS) and ferric acetylacetonate (Fe(acac)3) as precursors. The crystallization degree, flexibility, electrical conductivity, dielectric loss and microwave absorption properties of the hybrid fibers have been dramatically enhanced by the introduction of Fe. Fe3Si nanoparticles with a diameter around 500 nm are embedded in SiC fibers. As the Fe3Si content increases from 0 to 6.5 wt%, the related saturation magnetization (Ms) values increase from 0 to 8.4 emu g−1, and the electrical conductivity rises from 7.9 × 10−8 to 3.1 × 10−3 S cm−1. Moreover, the flexibility of Fe3Si/SiC hybrid fiber mats is greatly improved and remains intact after 500 times 180°-bending testing. Compared with pure SiC fibers, the Fe3Si/SiC hybrid fibers process higher dielectric and magnetic loss, which would be further advanced as more Fe3Si phase is introduced. At the optimal Fe3Si content of 3.8 wt%, the Fe3Si/SiC fibers/silicon resin composite (5 wt%) exhibits minimal reflection loss (RL) of −22.5 dB at 16.5 GHz and 2.5 mm thickness with a wide effective absorption bandwidth (EAB, RL < −10 dB) of 8.5 GHz. The microwave absorption performance can be further promoted by multi component stacking fiber mat composites which contain both low and high Fe3Si content layers. Furthermore, the position of the microwave absorption bands can also be simply manipulated by designing the stacking components and structure.