Elastic modulus measurements of vertically aligned multi walled carbon nanotubes carpets by using the nanoindentation technique

Electrical, thermal and mechanical properties of Vertically Aligned Multi Walled Carbon NanoTubes (VA-MWCNT) make them an ideal candidate to replace some of conventional materials in micro and nano-electronic components. Integrating this material in micro components requires a good knowledge of their properties. As the electrical and thermal properties, the MWCNT mechanical properties are difficult to assess. Several techniques have been developed to estimate the CNT Young's modulus and the obtained results cover a large range of scale. In this study, we propose an indirect technique for MWCNT carpet Young's modulus measurements by using the nanoindentation technique. Nanoindentation tests are performed on a metallic film deposited on MWCNT. The measured equivalent reduced modulus takes into account the elastic properties of the metallic thin film and those of the MWCNT substrate. Bec et al. model, introduced in 2006, is used to separate elastic properties, and thus determine the MWCNT reduced Young’s modulus which is estimated between 329 and 352 GPa. Knowing the indenter mechanical properties, we estimate the Young’s modulus in the 461 to507 GPa range.     

Preparation of nanosized hollow silica spheres from Na2SiO3 using Fe3O4 nanoparticles as templates

Nanosized hollow silica spheres with average diameters from 43 to 70 nm were prepared by removal of Fe₃O₄ templates with hydrochloric acid from silica-coated Fe₃O₄ core–shell composites. The shells of the hollow silica spheres had nanopores with average diameters of 0.92–1.25 nm. When the silica-coated Fe₃O₄ core–shell composites were prepared at a high pH value or with a low mole ratio of Na₂SiO₃ to Fe₃O₄, the resulting hollow silica spheres consisted of highly porous shells. When the silica-coated Fe₃O₄ core–shell composites were prepared with a high mole ratio of Na₂SiO₃ to Fe₃O₄, the resulting hollow silica spheres had large diameters and thick shells. The release rate of herbicide, ammonium glyphosate, could be tuned by using hollow silica spheres with different shell thicknesses.     

Facile synthesis and electrochemical properties of MnO2/carbon nanotubes

The poor electrical conductivity of MnO2 limits its use as an electrode material. To overcome this limitation, we report an easy and rapid approach to deposit nanosized MnO2 onto multi-wall carbon nanotubes functionalized with hydroxyl groups （MWCNTs-OH） by chemical reduction of KMnO4 with MnSO4 in aqueous solution under ambient conditions. Characterization with XRD and TEM reveals that the obtained MnO2/MWCNTs-OH composite is nanocrystalline and partially covered by Mn02 nanosheets with a thick- ness of 1-3 nm at a MnO2 loading of 20 wt%. Cyclic voltammetry （CV） and galvanostatic charge-discharge measurements reveal that the MnO2/MWCNTs-OH composite with a MnO2 loading of 20 wt% has a relatively high specific capacitance of 234 F/g at a scan rate of 2 mV/s and exhibits good cycling stability. Furthermore, the oxygen reduction reaction （ORR） shows that MnO2/MWCNTs-OH composite may have potential applications as a non-noble metal electrocatalyst in fuel cells and metal-air batteries.     

Effects of temperature change on the rheological property of modified multiwall carbon nanotubes

Applied Mathematics and Mechanics - Solvent-free nanofluids hold promise for many technologically significant applications. The liquid-like behavior, a typical rheological property of solvent-free...     

Predicting the elastic properties of single-walled carbon nanotubes

Advances in the prediction of the mechanical properties of single-walled carbon nanotubes (SWNTs) are reviewed in this paper. Based on the classical Cauchy-Born rule, a new computational method for the prediction of Young's modulus of SWNTs is investigated. Compared with the existing approaches, the developed method circumvents the difficulties of high computational efforts by taking into consideration of the microstructure of nanotube and the atomic potential of hydrocarbons. Numerical results of Young's modulus and its variation with respect to the deformation gradient tensor are given and discussed. The results obtained are in good agreement with those obtained by laboratory experiments and other numerical methods.     

Enhanced wettability and thermal stability of nano-SiO2/poly(vinyl alcohol)-coated polypropylene composite separators for lithium-ion batteries

To improve the electrolyte wettability and thermal stability of polypropylene (PP) separators, nano-SiO₂/poly(vinyl alcohol)-coated PP composite separators were prepared using a simple but efficient sol–gel and dip-coating method. The effects of the tetraethoxysilane (TEOS) dosage on the morphology, wettability, and thermal stability of the composite separators were investigated using Fourier-transform infrared spectroscopy, scanning electron microscopy, and contact-angle measurements. All the composite separators gave a smaller contact angle, higher electrolyte uptake, and lower thermal shrinkage compared with the PP separator, indicating enhanced wettability and thermal stability. Unlike the case for a traditional physical mixture, SiOC covalent bonds were formed in the coating layer. The composite separator with a TEOS dosage of 7.5 wt% had a unique porous structure combining hierarchical pores with interstitial voids, and gave the best wettability and thermal stability. The ionic conductivity of the composite separator containing 7.5 wt% TEOS was 1.26 mS/cm, which is much higher than that of the PP separator (0.74 mS/cm). The C-rate and cycling performances of batteries assembled with the composite separator containing 7.5 wt% TEOS were better than those of batteries containing PP separators.     

On models in the theory of stability of multiwalled carbon nanotubes in matrix

Models in the theory of stability of multiwalled carbon nanotubes in a polymer matrix are justified. Some results on the fracture mechanics of nanocomposites are presented. New areas of research in mechanics suggested by a group of well-known scientists are discussed __________ Translated from Prikladnaya Mekhanika, Vol. 42, No. 6, pp. 3–21, June 2006.     

单壁碳纳米管的扭转力学特性研究

利用基于高阶Cauchy-Born准则所建立的单壁碳纳米管本构模型,针对不同手性的单壁碳纳米管的扭转力学特性进行了研究.研究发现结构呈现对称性的锯齿型和扶手型单壁碳纳米管具有完全对称的扭转特性,而结构不对称的手性型单壁碳纳米管具有正反相异的扭转特性.同时,针对一系列手性不同的单壁碳纳米管的扭转力学特性展开了详细的研究.研究的部分结果与采用其他方法得到的结果进行了对比,证实了所提出方法以及预测结果的有效性和可行性.     

Atomistic-continuum modeling for mechanical properties of single-walled carbon nanotubes

The study attempts to explore the influences of the surface effect resulting in an initial relaxed unstrained deformation and the in-layer non-bonded van der Waals (vdW) atomistic interactions on the mechanical properties of single-walled carbon nanotubes (SWCNTs) using a proposed atomistic-continuum modeling (ACM) approach. The modeling approach incorporates atomistic modeling, by virtue of molecular dynamics (MD) simulation, for simulating the initial unstrained equilibrium state, and equivalent-continuum modeling (ECM), by way of finite element approximations (FEA), for modeling the subsequent static/dynamic behaviors.SWCNTs with various radius and two different chiralities, including zigzag and armchair type, are presented. To validate the proposed technique, the present results are compared with the literature data, including numerical and experimental values. Results show that the derived elastic moduli (1.2–1.4 TPa) when considering these two nanoeffects tend to be more consistent with the published experimental data. In specific, they can increase up to 17–23% Young’s modulus, 5–15% shear modulus, 6–11% natural frequencies and 10–30% critical buckling load of the SWCNTs, implying that without considering these two effects, the material behaviors of SWCNTs would be potentially underestimated.     

Microwave-assisted polyol synthesis of LiMnPO4/C and its use as a cathode material in lithium-ion batteries

We synthesized LiMnPO₄/C with an ordered olivine structure by using a microwave-assisted polyol process in 2:15 (v/v) water–diethylene glycol mixed solvents at 130 °C for 30 min. We also studied how three surfactants—hexadecyltrimethylammonium bromide, polyvinylpyrrolidone k30 (PVPk30), and polyvinylpyrrolidone k90 (PVPk90)—affected the structure, morphology, and performance of the prepared samples, characterizing them by using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, charge/discharge tests, and electrochemical impedance spectroscopy. All the samples prepared with or without surfactant had orthorhombic structures with the Pnmb space group. Surfactant molecules may have acted as crystal-face inhibitors to adjust the oriented growth, morphology, and particle size of LiMnPO₄. The microwave effects could accelerate the reaction and nucleation rates of LiMnPO₄ at a lower reaction temperature. The LiMnPO₄/C sample prepared with PVPk30 exhibited a flaky structure coated with a carbon layer (∼2 nm thick), and it delivered a discharge capacity of 126 mAh/g with a capacity retention ratio of ∼99.9% after 50 cycles at 1C. Even at 5C, this sample still had a high discharge capacity of 110 mAh/g, demonstrating good rate performance and cycle performance. The improved performance of LiMnPO₄ likely came from its nanoflake structure and the thin carbon layer coating its LiMnPO₄ particles. Compared with the conventional polyol method, the microwave-assisted polyol method had a much lower reaction time.     

Controllable microfluidic strategies for fabricating microparticles using emulsions as templates

Microfluidic techniques provide flexible strategies for fabrication of uniform advanced microparticles with well-tailored sizes, shapes, structures, and functions from controllable emulsion templates. This review highlights recent progress on controllable synthesis of microparticles using versatile microfluidic emulsions as templates. First, highly controllable and scalable microfluidic techniques for the generation of defined emulsions are introduced. Versatile microfluidic strategies for fabricating microparticles from diverse controllable emulsion templates are then summarized, including solid microparticles with spherical, non-spherical, and Janus configurations, porous microparticles with flexible pore structures, and compartmental microparticles with controlled internals. Finally, the future development of microfluidic techniques for microparticle fabrication is briefly discussed.     

Mesoporous composite of LiFePO4 and carbon microspheres as positive-electrode materials for lithium-ion batteries

Mesoporous LiFePO₄/C microspheres consisting of LiFePO₄ nanoparticles are successfully fabricated by an eco-friendly hydrothermal approach combined with high-temperature calcinations using cost-effective LiOH and Fe³⁺ salts as raw materials. In this strategy, pure mesoporous LiFePO₄ microspheres, which are composed of LiFePO₄ nanoparticles, were uniformly coated with carbon (∼1.5 nm). Benefiting from this unique architecture, these mesoporous LiFePO₄/C microspheres can be closely packed, having high tap density. The initial discharge capacity of LiFePO₄/C microspheres as positive-electrode materials for lithium-ion batteries could reach 165.3 mAh/g at 0.1 C rate, which is notably close to the theoretical capacity of LiFePO₄ due to the large BET surface area, which provides for a large electrochemically available surface for the active material and electrolyte. The material also exhibits high rate capability (∼100 mAh/g at 8 C) and good cycling stability (capacity retention of 92.2% after 400 cycles at 8 C rate).     

Solid-state synthesis of Li[Li0.2Mn0.56Ni0.16Co0.08]O2 cathode materials for lithium-ion batteries

Layered Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ cathode materials were synthesized via a solid-state reaction for Li-ion batteries, in which lithium hydroxide monohydrate, manganese dioxide, nickel monoxide, and cobalt monoxide were employed as metal precursors. To uncover the relationship between the structure and electrochemical properties of the materials, synthesis conditions such as calcination temperature and time as well as quenching methods were investigated. For the synthesized Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ materials, the metal components were found to be in the form of Mn⁴⁺, Ni²⁺, and Co³⁺, and their molar ratio was in good agreement with stoichiometric ratio of 0.56:0.16:0.08. Among them, the one synthesized at 800 °C for 12 h and subsequently quenched in air showed the best electrochemical performances, which had an initial discharge specific capacity and coulombic efficiency of 265.6 mAh/g and 84.0%, respectively, and when cycled at 0.5, 1, and 2 C, the corresponding discharge specific capacities were 237.3, 212.6, and 178.6 mAh/g, respectively. After recovered to 0.1 C rate, the discharge specific capacity became 259.5 mAh/g and the capacity loss was only 2.3% of the initial value at 0.1 C. This work suggests that the solid-state synthesis route is easy for preparing high performance Li[Li_0.2Mn_0.56Ni_0.16Co_0.08]O₂ cathode materials for Li-ion batteries.     

Revealing the correlation between structural evolution and long-term cyclability of the LiNi0.5Co0.2Mn0.3O2/artificial graphite pouch cells at various rates

In recent years, researches on improving high-voltage performance of lithium-ion batteries incorporating LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) and artificial graphite (AG) have been widely reported. However, limited attentions have been paid to understand the effects and influence mechanisms of charge and discharge rates and charge limit currents on cyclability of NCM523/AG cells. Herein, a ∼1.9 Ah NCM523/AG pouch cell is employed, whose electrochemical and structural evolutions after 800 cycles at various rates are comprehensively investigated. We find that cycling performances are strongly influenced by charge rate, followed by limit current and discharge rate. The cell charged at a high rate and cell charged until reaching a low limit current both exhibit low capacity retentions compared to the cell discharged at a high rate. Possible failure reasons are analyzed by advanced characterizations. Results reveal that NCM523 cathodes of the cells deteriorated early experience severe transition metal dissolution, lattice distortion, and partial phase transformation. Meanwhile, the deposited transition metals on AG anodes catalyze the electrolyte consumption, lithium plating and active area loss. Finally, these side reactions notably increase cell impedance and electrochemical polarization. Undoubtedly, these findings clearly outline the challenges and optimization direction for high-rate NCM523/AG cells.     

The coupled effects of mechanical deformation and electronic properties in carbon nanotubes

Coupled effects of mechanical and electronic behavior in single walled carbon nanotu besare investigated by using quantum mechanics and quantum molecular dynamics. It is found that external applied electric fields can cause charge polarization and significant geometric deformation in metallic and semi-metallic carbon nanotubes. The electric induced axial tension ratio can be up to 10% in the armchair tube and 8.5% in the zigzag tube. Pure external applied load has little effect on charge distribution，but indeed influences the energy gap. Tensile load leads to a narrower energy gap and compressive load increases the gap. When the CNT is tensioned under an external electric field, the effect of mechanical load on the electronic structures of the CNT becomes significant, and the applied electric field may reduce the critical mechanical tension load remarkably. Size effects are also discussed.     

Rheological properties and percolation in suspensions of multiwalled carbon nanotubes in polycarbonate

This paper is concerned with several issues related to the rheological behavior of polycarbonate/multiwalled carbon nanotube nanocomposites. The composites were prepared by diluting a masterbatch of 15 wt.% nanotubes using melt-mixing method, and the dispersion was analyzed by SEM, TEM, and AFM techniques. To understand the percolated structure, the nanocomposites were characterized via a set of rheological, electrical, and thermal conductivity measurements. The rheological measurements revealed that the structure and properties were temperature dependent; the percolation threshold was significantly lower at higher temperature suggesting stronger nanotube interactions. The nanotube networks were also sensitive to the steady shear deformation particularly at high temperature. Following preshearing, the elastic modulus decreased markedly suggesting that the nanotubes became more rigid. These results were analyzed using simple models for suspensions of rod-like particles. Finally, the rheological, electrical, and thermal conductivity percolation thresholds were compared. As expected, the rheological threshold was smaller than the thermal and electrical threshold.     

Mesoporous composite of LiFePO4 and carbon microspheres as positive-electrode materials for lithium-ion batteries

Mesoporous LiFePO4/C microspheres consisting of LiFePO4 nanoparticles are successfully fabricated by an eco-friendly hydrothermal approach combined with high-temperature calcinations using cost-effective LiOH and Fe3＋ salts as raw materials.In this strategy,pure mesoporous LiFePO4 microspheres,which are composed of LiFePO4 nanoparticles,were uniformly coated with carbon（1.5nm）.Benefiting from this unique architecture,these mesoporous LiFePO4/C microspheres can be closely packed,having high tap density.The initial discharge capacity of LiFePO4/C microspheres as positive-electrode materials for lithium-ion batteries could reach 165.3 mAh/g at 0.1 C rate,which is notably close to the theoretical capacity of LiFePO4 due to the large BET surface area,which provides for a large electrochemically available surface for the active material and electrolyte.The material also exhibits high rate capability（100 mAh/g at 8 C） and good cycling stability（capacity retention of 92.2%after 400 cycles at 8 C rate）.     

On the van der Waals interaction of carbon nanotubes as electromechanical nanothermometers

Electromechanical carbon nanothermometers are devices that work based on the interactions and relative motions of double-walled carbon nanotubes (DWCNTs). In this paper, the mechanics of carbon nanotubes (CNTs) constituting two well-known configurations for nanothermometer, namely shuttle configuration and telescope configuration are fully investigated. Lennard-Jones (LJ) potential function along with the continuum approximation is employed to investigate van der Waals (vdW) interactions between the interacting entities. Accordingly, semi-analytical expressions in terms of single integrals are obtained for vdW interactions. Acceptance condition and suction energy are studied for the shuttle configuration. In addition, a universal potential energy is presented for the shuttle configuration consisting of two finite CNTs. Also, for the telescope configuration, extensive studies are performed on the distributions of potential energy and interaction force for various radii and lengths of CNTs. It is found that these geometrical parameters have a considerable effect on the potential energy.     

The influence of mechanical deformation on the electrical properties of single wall carbon nanotubes

Recent experimental studies and atomistic simulations have shown that carbon nanotubes (CNTs) display strong interplay between the mechanical deformation and electrical properties. We have developed a simple and accurate method to determine atom positions in a uniformly deformed CNT via a continuum analysis based on the interatomic potential. A shift vector is introduced to ensure the equilibrium of atoms. Such an approach, involving only three variables for the entire CNT, agrees very well with the molecular mechanics calculations. We then study the effect of mechanical deformation on the band gap change of single wall CNTs under tension, torsion, and combined tension/torsion via the k-space tight-binding method. Prior studies without this shift vector lead to significant overestimation of the band gap change. It is established that the conducting CNTs may easily become semi-conducting ones subject to mechanical deformation, but the semi-conducting CNTs never become conducting ones upon deformation.