Amartya Mukhopadhyay

Vertically aligned graphene layer arrays from chromonic liquid crystal precursors.

Progress in graphene synthesis has led to increased interest in the assembly of graphene into superstructures, and to the fabrication of novel ordered materials from graphene or engineered graphenic molecular building blocks. [ 1–8 ] Graphene monolayers typically deposit fl at on substrates or associate face-to-face to form horizontal stacked papers or multilayer coatings. [ 8–11 ] The opposite structure, vertically aligned graphene layer arrays on substrates ( Figure 1 A ), are more diffi cult to fabricate, but are expected to show a range of unique properties and behaviors. Their high concentration of edge-sites at the top surface would allow functionalization at high density for superhydrophobic/ philic coatings, high-redox-activity electrode surfaces, or chemically patterned surfaces for cell adhesion and guidance. The vertical layer orientation would allow rapid intercalation and deintercalation of lithium in high-discharge-rate thin fi lm batteries, [ 12 ] and would provide high Z -directional thermal/electrical conductivity. If also ordered in a second dimension, such arrays would be anisotropic in the substrate plane and provide unidirectional in-plane heat spreading or optical polarization. If the heights of arrays can be limited to below 50 nm, they may fi nd application as transparent conductive fi lms, [ 10 , 13 ] or as graphene nanoribbons where the ribbon width is set by the array height. Here we use chromonic liquid crystal precursors to fabricate vertically aligned graphene layer arrays (VAGLAs) on substrates and also demonstrate a method to achieve full two-dimensional order, which we defi ne as further control of graphene layer orientational patterns within the substrate plane using local shear-forces. We also demonstrate one example of a unique property of these arrays – the ability to etch Z directional nanopores by catalytic hydrogenation, in which cobalt nanoparticles tunnel vertically into and through the arrays as they track vertically receding edge-plane surfaces. A promising route to the desired vertical graphene array structure is one based on polyaromatic precursors, [ 2 ] which can adopt edge-on orientation on substrates. [ 5 , 14 , 15 ] Many polyaromatic compounds, however, do not retain supramolecular alignment upon carbonization, and also their solution or vapor deposition typically gives only short-range order in-plane. [ 2 ]

Understanding the Li-storage in few layers graphene with respect to bulk graphite: experimental, analytical and computational study

In order to understand, clarify and provide confirmations in the contexts of the prevalent confusions concerning Li-storage in graphenic carbon (viz. the reduced dimensional scale of graphitic carbon), electrochemical lithiation/delithiation has been performed with CVD-grown fairly pristine well-ordered few-layer graphene films (FLG; ∼7 layers; as a model material). Chronopotentiograms and cyclic voltammograms recorded with the FLG present distinct features corresponding to the transformation between different Li-GICs (i.e., ‘staging’) below 0.3 V against Li/Li+, thus confirming that ‘classical’ Li-intercalation does occur even at such reduced dimensional (nano)scale. Nevertheless, even in this lower potential window (our main focus here), Li-storage in FLG involves contributions from both diffusion- and surface-controlled mechanisms. The Li-capacities recorded with FLG just within this lower potential window, and also upon subtracting any possible contribution from the Cu current collector, were still ∼3–4 times greater than those obtained with similarly grown thicker bulk graphite films (TBG: ∼450 nm; Li-capacity recorded: ∼380 mA h g−1). Contrary to the usual belief, the excess Li-capacity of FLG cannot be explained by the presence of extrinsic/intrinsic defects, which are nearly negligible in the FLG films under consideration. Simulation of Li-storage in graphene via DFT indicated that the excess capacity (after formation of the LiC6 configuration) is associated with additional stable Li-storage on the outer graphene surfaces in the forms of more than one Li-layer (but different from Li-plating) and segregation close to the ‘stepped’ (exposed) edges of the inner graphene layers (but not exactly at the edge sites). Overall, such predicted Li-storage mechanisms are in agreement with the experimentally observed contributions from both ‘classical’ Li-intercalation and surface-controlled processes (even at potentials below 0.3 V), which primarily account for the excess Li-capacities recorded with graphenic carbon.

Does Thermal Conductivity Play a Role in Sliding Wear of Metals in Cryogenic Environment

The thermal conductivity of a metallic test piece is one of the principal parameters that influence the temperature buildup at tribocontacts and this normally plays an important role in the unlubricated dry sliding wear of metallic materials. It is, however, not clear whether thermal conductivity is an equally important parameter in the case of wear of metals at cryogenic temperatures, in particular, at liquid nitrogen temperature (LN 2 ) of ―196°C. In order to assess the influence of such a physical property of selected nonferrous metals on their tribological behavior in the LN 2 environment, we have studied the friction and wear properties of high purity copper (Cu) and titanium (Ti) against the bearing grade steel. These two materials have been processed to produce samples of comparable hardness that have widely different thermal conductivities at room temperature and at test temperature. Wear tests were conducted at three different sliding speeds (0.89 m/s, 1.11 m/s, and 1.34 m/s) under 10 N load, and the friction and wear data were compared. Ti exhibited an order of magnitude higher wear rate (∼10 ―3 mm 3 /N m) as compared with Cu in identical test conditions. While evidences of abrasive wear and adhesive wear, without any oxidative wear, were found in worn Cu surfaces, worn Ti surfaces showed evidences of significant oxidative wear and mechanical damage of tribolayers. Higher wear rate in Ti appeared to be a result of oxidative wear of Ti, which seemed to be driven by the depletion of LN 2 blanket at the tribocontacts under the influence of high flash temperature (14―76°C) as compared with the boiling temperature of LN 2 (―196°C). These results demonstrate that the materials with similar hardness subjected to identical LN 2 wear test conditions can have significantly different wear rates because of the difference in the flash temperatures, which depend on the thermal conductivity of the test pieces.

Optimization of lithium content in LiFePO4 for superior electrochemical performance: the role of impurities

Carbon coated LixFePO4 samples with systematically varying Li-content (x = 1, 1.02, 1.05, 1.10) have been synthesized via a sol–gel route. The Liu2006:u2006Fe ratios for the as-synthesized samples is found to vary from ∼0.96u2006:u20061 to 1.16u2006:u20061 as determined by the proton induced gamma emission (PIGE) technique (for Li) and ICP-OES (for Fe). According to Mossbauer spectroscopy, sample Li1.05FePO4 has the highest content (i.e., ∼91.5%) of the actual electroactive phase (viz., crystalline LiFePO4), followed by samples Li1.02FePO4, Li1.1FePO4 and LiFePO4; with the remaining content being primarily Fe-containing impurities, including a conducting FeP phase in samples Li1.02FePO4 and Li1.05FePO4. Electrodes based on sample Li1.05FePO4 show the best electrochemical performance in all aspects, retaining ∼150 mA h g−1 after 100 charge/discharge cycles at C/2, followed by sample Li1.02FePO4 (∼140 mA h g−1), LiFePO4 (∼120 mA h g−1) and Li1.10FePO4 (∼115 mA h g−1). Furthermore, the electrodes based on sample Li1.05FePO4 retain ∼107 mA h g−1 even at a high current density of 5C. Impedance spectra indicate that electrodes based on sample Li1.05FePO4 possess the least charge transfer resistance, plausibly having influence from the compositional aspects. This low charge transfer resistance is partially responsible for the superior electrochemical behavior of that specific composition.

Crystalline core/amorphous shell structured silicon nanowires offer size and structure dependent reversible Na-storage

One of the major bottlenecks towards the development of the Na-ion battery system is that graphitic carbon (the commonly used anode material for the Li-ion system) is not suitable for use in Na-ion system. Accordingly, in the pursuit to identify and develop a suitable anode material for the upcoming Na-ion battery system, we report here the feasibility of reversible electrochemical Na-alloying in core/shell-structured Si nanowires having crystalline (c-Si) core and amorphous (a-Si) shell. Vapor–liquid–solid mechanism during nanowire growth allowed systematic variations of the a-Si shell thickness around the c-Si core of constant diameter (∼25 nm; as per the size of Sn catalyst-cum-‘nano-template’ particles). This allowed the development of four different sets of nanowires having overall diameters varying between ∼40 (SiNW-40) and ∼460 nm (SiNW-460); thus providing platforms also for investigating the influences of the dimensional scale and structure of Si (viz., amorphous vs. crystalline) towards Na-storage. While negligible reversible Na-capacity could be recorded with the thickest nanowire set, significantly greater Na-capacities could be recorded upon reduction in the overall diameter; leading to a reversible Na-capacity of ∼390 mA h g−1 for the thinnest nanowire set (i.e., SiNW-40), which is also the highest reported to-date for ‘stand-alone’ Si-based electrodes. Shortened Na-transport distance through the a-Si shell and increased influence of the more conductive c-Si core towards the lowering of charge transfer resistance, with reduced nanowire thickness, are the causes for such a remarkable dimensional effect. Experimental evidences and analytical computational studies indicate that Na-capacity gets contributed primarily by the ‘bulk’ of the amorphous Si shell, but (interestingly) not by the crystalline Si core.

Effects of residual stress on overpotentials and mechanical integrity during electrochemical Li-alloying of Al film electrodes

The possibilities of achieving reductions in overpotentials and improvements in mechanical integrity, upon electrochemical Li-alloying/dealloying, by inducing pre-cycling tensile residual stress via facile annealing treatment have been demonstrated in this work. Monitoring of the stress developments in-situ during galvanostatic lithiation/delithiation of Al film electrodes indicated that the mechanical degradations occurred primarily during potential plateaus corresponding to the Al ↔xa0AlLi first-order phase transformations. Such degradations tended to get suppressed, as did the overpotentials needed to drive the electrochemical Li-alloying/dealloying (including initiation of the phase transformation), in the presence of the tensile residual stress. The improved mechanical integrity agreed with features observed in the stress profiles recorded in real-time during galvanostatic cycling. The magnitudes for the reductions in the overpotentials agreed with theoretical estimation based on the incremental residual stress. This exploratory idea related to engineering of residual stress to improve various electrochemical performances may be applicable also to other ‘alloying reaction’ based electrode materials.Graphical Abstract

‘Age-hardened alloy’ based on bulk polycrystalline oxide ceramic

We report here for the first time the development of ‘age-hardened/toughened’ ceramic alloy based on MgO in the bulk polycrystalline form. This route allows for the facile development of a ‘near-ideal’ microstructure characterized by the presence of nanosized and uniformly dispersed second-phase particles (MgFe2O4) within the matrix grains, as well as along the matrix grain boundaries, in a controlled manner. Furthermore, the intragranular second-phase particles are rendered coherent with the matrix (MgO). Development of such microstructural features for two-phase bulk polycrystalline ceramics is extremely challenging following the powder metallurgical route usually adopted for the development of bulk ceramic nanocomposites. Furthermore, unlike for the case of ceramic nanocomposites, the route adopted here does not necessitate the usage of nano-powder, pressure/electric field-assisted sintering techniques and inert/reducing atmosphere. The as-developed bulk polycrystalline MgO–MgFe2O4 alloys possess considerably improved hardness (by ~52%) and indentation toughness (by ~35%), as compared to phase pure MgO.

Li metal battery, heal thyself

Intermittent high-current pulses prevent battery failure In the 1970s, scientists first developed a promising class of rechargeable batteries in which lithium (Li) metal served as the anode and compounds that could reversibly host Li ions inside the lattice formed the cathode (1). Because Li is the lightest and most electropositive metal, this setup allows very high energy and power densities. However, repeated discharge-charge cycles cause growth of Li dendrites from the anode toward the cathode. Such dendrites can eventually penetrate the separator (placed to prevent contact between the electrodes) and touch the cathode, causing short circuiting of the cell and potentially leading to safety hazards (2–5). On page 1513 of this issue, Li et al. (6) show that Li dendrite growth can be suppressed by applying short, intermittent high-current pulses during battery use. These pulses lead to self-healing of the dendrites.

Facile-low temperature route towards development of SiC-based coating on carbon nanotubes for improved oxidation resistance

Coating with silicon carbide is a preferred avenue for improving the oxidation resistance of carbon nanotubes. However, expensive and non-scalable techniques like vapor deposition are commonly used to develop such coatings. Against this backdrop, reported here is a facile, cost-effective and low temperature route, based on wet-chemical synthesis, for obtaining SiC-based coating on multi-walled carbon nanotubes (MWCNTs). The coating was achieved by dispersing surface-modified MWCNTs in silica sol (having optimized viscosity), followed by simultaneous reduction and ‘carburization’ of the silica (wetting MWCNTs) in the presence of Mg at 600xa0°C. X-ray diffraction and RAMAN spectroscopy confirmed the presence of β-SiC, while TEM observations revealed the development of uniform and thin coating (~3xa0nm thick) on undamaged MWCNTs. Thermo-gravimetric and differential thermal analyses up to 1500xa0°C in air confirmed significant improvement in oxidation resistance for the MWCNTs with SiC-based coating (just ~15xa0% mass loss; as against rapid mass loss of ~100xa0% for uncoated MWCNTs due to oxidation). Additionally, undamaged MWCNTs (with SiC-based coating) could be imaged in TEM after exposure in air at 1200xa0°C for longer duration.