Search

To get insight into the thermal management of twisted bilayer graphene based devices, the interpretation of temperature variation on twisting properties becomes crucial. In this aspect, we report the temperature dependent Raman scattering of twisted bilayer graphene (tBLG) on the Si substrate having SiO2 thickness 300 nm. The tBLG samples have been directly grown on the copper foil substrate using a home-built low pressure chemical vapor deposition (LPCVD) setup. The temperature dependent Raman scattering has been studied on two samples of tBLG named as tB1 and tB2. The twisting nature of bilayer graphene film has been analyzed using R, G, and 2D peak characteristics in Raman spectra. The Raman mapping with respect to different peak parameters of G and 2D peak (intensity, width, and position), intensity ratio (I2D/IG) has been employed to probe the uniformity of tBLG film. In tB1, the R peak has been found at 1395.82 cm−1 and not observed in sample tB2 at room temperature. The temperature dependence of G and 2D peak frequency in Raman spectra has been investigated in the temperature range from 80 to 450 K. In tB1 and tB2, the temperature coefficient of G peak has been found to be - (0.97 ± 0.34) × 10−2 cm−1/K, and - (1.08 ± 0.15) × 10−2 cm−1/K, respectively. Further, the extracted value of temperature coefficient of 2D peak in tB1, and tB2 are - (2.92 ± 0.80) × 10−2 cm−1/K, and - (3.01 ± 0.29) × 10−2 cm−1/K respectively. Additionally, the twist angle has been estimated as ∼23°-27° for T ≤ 300 K and 3°-8° for T > 300 K in tB1. In tB2, it has been found to be ∼13°-16° for the temperature range of 80–200 K and 5°-9° for 250–450 K. These findings shed light on the twisting behavior of bilayer graphene film and its anharmonic properties.

Methylammonium lead bromide (CH3NH3PbBr3) colloidal quantum dots (QDs) exhibit strong green photoluminescence (PL) with high photoluminescence quantum yield (PLQY) making it valuable for various optoelectronic applications. Under the influence of polar gaseous molecules, hybrid halide perovskites show changes in its structural and electrical properties. We, for the first time, have investigated the influence of NH3 gas molecules on the optical properties of CH3NH3PbBr3 colloidal QDs. The investigations carried out under a controlled environment reveal that even the presence of 37 ppm of ammonia (NH3) gas molecules causes a significant reduction in the PL intensity of CH3NH3PbBr3 colloidal QDs. The reduction rate of PL intensity can be tuned with the concentration of NH3 gas molecules. We propose that the decrease in PL intensity is because of the formation of a non-luminescent NH4PbBr3 phase under the presence of NH3 gas molecules. Further, the non-luminescent NH4PbBr3 retransformed into luminescent CH3NH3PbBr3 on the introduction of methylamine (CH3NH2) gas molecules. This reversible alternation in PL properties enables us to demonstrate its application for (NH3) gas sensing. The advantage of using CH3NH3PbBr3 colloidal QDs for luminescence-based sensing is that its green emission is visible with the naked eye even under daylight, which is easy to detect.

Strontium tellurite, lead tellurite and bismuth niobium tellurite glasses containing 1 to 2 mol% of Eu3+ and Er3+ were prepared by melt-quenching and found to contain anti-glass inclusions (droplet phase) of size of several micrometers. Anti-glasses are disordered crystals that have long-range order of the cations such as Sr2+, Pb2+, Bi3+, Nb5+ and Te4+ but have a disordered arrangement of the anions (O2−), therefore these materials exhibit sharp peaks in the X-ray diffraction patterns due to the long range cationic order but the Raman spectra show broad bands due to the vibrational disorder. The glass matrix and anti-glass inclusions in the samples were characterized by synchrotron micro-X-ray Fluorescence (XRF), Raman and photoluminescence mapping studies. The XRF line analysis confirmed that the anti-glass and glass matrix phases have equal concentration of metal ions. The samples containing Eu3+ show intense red photoluminescence at an excitation wavelength of 375 nm, while Er3+ doped samples exhibit strong green emission by the up-conversion of infrared photons (980 nm). The spatially resolved XRF, Raman and photoluminescence spectroscopy found that the inclusions and the glass matrix have the same metal ion concentration, short-range order, Te O speciation and the light emission properties.

The present study aims to demonstrate the application of sugarcane bagasse as an excellent field emitter. Field emission property of as-pyrolyzed sugarcane bagasse (p-SBg) before and after the plasma treatment has been investigated. It has been observed that electronic nature of p-SBg transformed from semiconducting to metallic after plasma treatment. Maximum current and turn-on field defined at 10 μA/cm2 was found to be 800 μA/cm2 and 2.2 V/μm for as-pyrolyzed sugarcane bagasse (p-SBg) and 25 μA/cm2 and 8.4 V/μm for H2-plasma treated p-SBg. These values are found to be better than the reported values for graphene and activated carbon. In this report, pyrolysis of bagasse has been carried in a thermal chemical vapor deposition (Th-CVD) system in inert argon atmosphere. Scanning electron microscopy (SEM), X-ray Diffraction (XRD), High-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) have been used to study the structure of both pre and post plasma-treated p-SBg bagasse’s sample. HRTEM study reveals that carbonaceous structures such as 3D-nanographene oxide (3D-NGO), graphite nanodots (GNDs), carbon nanotubes (CNTs), and carbon onions are present in both pre-treated and plasma-treated p-SBg. Hence, we envision that the performed study will be a forwarding step to facilitate the application of p-SBg in display devices.

Herein, we demonstrate the synthesis of dual-mode Gd2O3:Eu3+, Er3+, Yb3+ phosphor by a facile high-temperature solid-state method as a spectral convertor for the enhancement of conversion efficiency of solar cell. The dual-mode Gd2O3:Eu3+, Er3+, Yb3+ phosphor has cubic crystal structure with space group Ia 3 . The surface morphology of dual-mode Gd2O3:Eu3+, Er3+, Yb3+ phosphor has been investigated by scanning electron microscopy. The spectroscopic features of dual-mode Gd2O3:Eu3+, Er3+, Yb3+ phosphor have been examined by photoluminescence (PL) and time-resolved photoluminescence (TRPL). The dual-mode Gd2O3:Eu3+, Er3+, Yb3+ phosphor gives strong red emissions at 611 nm and 663 upon excitation wavelengths of 200–500 nm and 980 nm, respectively. The dual-mode Gd2O3:Eu3+, Er3+, Yb3+ phosphor has average lifetime ~ 598.7 µs. Further, PL intensity distribution in dual-mode Gd2O3:Eu3+, Er3+, Yb3+ phosphor have been examined by PL confocal microscopy. The spatially resolved PL mapping results reveal that dual-mode Gd2O3:Eu3+, Er3+, Yb3+ phosphor has PL emission throughout the sample at excitation wavelengths of 375 nm and 980 nm, respectively. Hence, the obtained results suggest that dual-mode Gd2O3:Eu3+, Er3+, Yb3+ phosphor could be a promising luminescent material for spectral convertor to improve the efficiency of next generation silicon solar cell.

We report the optical response and temperature-dependent excited-state carrier dynamics in the flake of Bi2Se3, which is cleaved from its single crystal. The optical properties are explored using the visible-near infrared (VIS-NIR), infrared (IR), and photoluminescence (PL) spectroscopies. The VIS-NIR spectra are then used to find the Ɛ1 and Ɛ2 using Kramer’s Koning relations, confirming OBT of 2 eV and 1.4 eV. The DFT (density functional theory) calculations are also carried out to confirm the experimental optical transitions (OBT). To probe these OBT in the bismuth selenide, we have studied the temperature, fluence, and excitation dependent ultrafast transient reflectance over a wide spectral range from 2.58–0.77 eV (VIS-NIR) with different excitation energies 3.02 eV, 2.61 eV, 1.9 eV, and 1.4 eV to explore several previously unseen transitions that do not appear in PL spectroscopy and room temperature carrier analysis. The temperature-dependent excited-state dynamics are investigated at 5–300 K. This study clearly indicates the existence of Moss-Burstein shift in the visible region and Pauli blocking effect in the NIR region in the Bi2Se3 topological insulator. Moreover, the low-temperature TRUS confirms the transition to the second surface state present in bismuth selenide.

A bright novel red emission was obtained from Eu3+: CaLa2ZnO5 (CLZO) Phosphors under visible excitation. We have successfully synthesized Eu3+: CLZO Phosphors by the solid state reaction method. The phase formation and surface morphological studies were analyzed by X-ray diffraction and Field emission scanning electron microscopy respectively. Thermal analysis was done by TG/DTA. Photoluminescence spectral profiles reveal that the strong red emission has been noticed at 626 nm (5D0→7F2) from the Eu3+: CLZO Phosphors under the excitation of 467 nm. The emission and excitation spectral profiles were systematically analyzed and their bands were assigned with corresponding electronic transitions. Upon increasing the Eu3+ concentration, the photoluminescence performance has also been increased remarkably. The optimized concentration of the Eu3+ was noticed at 14 mol%. Upon co-doping with Dy3+ ions to Eu3+: CLZO Phosphors, the red emission pertaining to Eu3+ was appreciably enhanced through energy transfer form Dy3+ to Eu3+ ions. The energy transfer mechanism was substantiated by several fluorescent methods. The prominent red emission of the Dy3+/Eu3+: CLZO Phosphors under blue excitation which suggests that these phosphor materials could be promising candidates for red luminescent optical devices.

A prominent sensitization effect of In 3+ ions is observed in In 3+ +Eu 3+ : PVA polymer composites under UV excitation. Consequently, it enhances the red emission performance of Eu 3+ ions in PVA system. We have successfully synthesized Eu 3+ : PVA, In 3+ : PVA and In 3+ +Eu 3+ : PVA polymer films by traditional solution casting method. The structural and ion-polymer interaction studies have been analyzed from XRD and FTIR spectral profiles. Eu 3+ doped PVA polymer composites are exhibited a red emission at 619 nm ( 5 D 0 → 7 F 2 ) under 396 nm ( 7 F 0 → 5 L 6 ) of excitation. Upon co-doping with In 3+ ions in different concentrations to the Eu 3+ : PVA polymer film, it exhibits predominant red emission than singly doped Eu 3+ : PVA under 396 nm of excitation due to energy migration from In 3+ to Eu 3+ . Successful emission photons of In 3+ ions are collectively absorbed by the Eu 3+ ions which lead the improvement of red emission. Optimized sensitization concentration of the In 3+ ions has been found to be 0.01 wt%. Possible energy migration phenomenon is elucidated by several fluorescent dynamics. The energy transfer process is substantiated by lifetime decay analysis and overlapped spectral studies. The Commission International de I-Eclairage chromaticity coordinates were calculated. The quantum efficiencies of the Eu 3+ ions and In 3+ ions in singly doped and co-doped polymer systems have been evaluated. From these results, these co-doped In 3+ +Eu 3+ : PVA composite polymer films might be proposed as encouraging candidates for bright red fluorescent materials for several photonic applications.

Transition metal oxides and sulfides have great potential for energy storage devices due to their large theoretical energy storage capacities. A facile technique was used for the synthesis of nanostructured and phase pure cobalt oxide (Co 3 O 4 ) and subsequently converting it to cobalt sulfide (Co 9 S 8 ). The effect of sulfurization on energy storage capacity of the cobalt oxide was explored. Microstructural characterizations using X-ray diffraction and scanning electron microscopic reveal formation of phase pure and nanostructured Co 3 O 4 and Co 9 S 8 . It was observed that the areal capacitance of Co 3 O 4 (983 mF/cm 2 ) improved significantly after converting to Co 9 S 8 (7358 mF/cm 2 ). The CV curves of the Co 9 S 8 electrode on bending showed outstanding stability with no change in energy storage properties. New insights into the better performance of Co 9 S 8 over Co 3 O 4 based on electrochemical investigations are presented. The performance of the Co 9 S 8 as an electrode material for energy storage applications was further investigated by fabricating a supercapacitor device. The supercapacitor device showed outstanding stability up to 5000 cycles of charge-discharge study. The performance of the supercapacitor was observed to be improving with temperature. The supercapacitor displayed ~100% enhancement in energy storage property on increasing temperature from 10 to 70 °C. Our results suggest that hydrothermally grown Co 9 S 8 on nickel foam can be utilized for high capacity, flexible and binder free electrode for energy storage applications.

Nanofibers of cobalt manganese oxide (CoMn2O4) were grown using an electrospun technique. Structural and microstructural characterizations confirm the formation of phase pure CoMn2O4 with high porosity. The potential application of CoMn2O4 nanofibers as an electrode material for energy storage device was studied using cyclic voltammetry and galvanostatic charge-discharge measurements. A specific capacitance of 121 F/g was observed with enhanced cyclic stability. Furthermore, an energy storage device was fabricated by sandwiching two electrodes separated by an ion transporting layer. The device showed a specific capacitance of 241 mF/cm2 in 3 M NaOH electrolyte. The effect of temperature on the charge storage properties of the device was also investigated for high temperature applications. The device showed about 75% improvement in the charge storage capacity when the temperature was increased from 10 to 70 °C. This research suggests that nanofibers of CoMn2O4 could be used for fabrication of energy storage devices which could operate in a wide temperature range with improved efficiency.

Growth characteristics of high Indium content InGaN/GaN heterostructure on a-plane ( 11 2 ¯ 0 ) sapphire substrate by plasma assisted molecular beam epitaxy have been investigated in detail. The structural analysis is carried out by high-resolution X-ray diffraction measurements. A reduction in the lattice constant by 0.15% confirms the highly crystalline nature of the grown GaN film. Field Emission Scanning Electron Microscopy measurements divulged smooth morphology with less hexagonally pitted GaN surface. This high quality MBE grown GaN on ( 11 2 ¯ 0 ) sapphire is utilized as a template to grow InGaN film with high indium composition (41%). The elemental distribution and thickness of the grown film is ascertained by Secondary Ion Mass Spectroscopy which demonstrates a sharp interface between the grown structure. The photoluminescence emission spectrum shows a sharp near band edge emission of GaN at 3.42 eV along with a broad band emission related to defects and near band edge emission of InGaN (1.9 ± 0.05 eV) at room temperature. This study provides a new insight into the structural quality of In-rich InGaN/GaN based heterostructures on a-plane sapphire, which could be useful to develop highly-efficient nitride solar cell devices.

To explore the practical feasibility of exotic carbon, various types of geometries of CNT configurations have been investigated such as CNT film, vertically aligned nanotubes, 3D pillared graphene-CNT network etc. High-performance and applicability of CNT derived electrodes in electrochemical energy storage depend on the structural design and high aspect ratio geometry. Here, we have strategically designed electrode derived from a special type of 3D geometry known as carbon hollow cylinders (CHCs) made up of CNTs arranged in the radial direction. The exceptional geometry provides a high areal capacity of 513.92 C/cm2 at an applied current density of 16 mA/cm2. Also, high specific energy of 41.13 mWh/cm2 at the specific power of 5694.92 mW/cm2 originated from supercapacitors and battery response of the electrode material was attained, which idealized the fundamental of theory of composite type electrode material. Hence, proposed geometry sets a stepping stone for a paradigm shift in light-weight electrode which is not only binder-free but also designed with a special geometry that provides an exceptional higher areal capacity with stable network.

To attain the expected outcomes of any measurement, probing the quality of graphene is a prerequisite among the various graphene samples. Herein, we investigate the identification of the number of layers in mechanically exfoliated graphene using Raman spectroscopy. The intensity ratio (I2D/IG) and peak width of the 2D peak have been used to distinguish the number of layers. The 2D peak consists of a single Lorentzian fit having peak width 24 cm−1 in single layer graphene (SLG) and splits into four, six and five components for bilayer, trilayer, and five layers respectively. Further, the quality of graphene has been analyzed by I2D/IG, Raman mapping and the charge carrier mobility. In addition, the high quality of graphene has been encapsulated in terms of two phonon scattering processes and the variation in I2D/IG of SLG as a result of crossover from electron-electron to electron-hole scattering. The maximum mobility of SLG has been obtained ∼11,931 cm2V−1s−1 when the peak width and I2D/IG are 23.88 cm−1 and 5.36 respectively. Hence, the present study sets a platform to use Raman scattering as a probe for quantification of graphene layers and Raman mapping with spatial resolution provides the solution for specific area selection to fabricate the next generation graphene based quantum devices.

Designing of electrode with electrochemically rich structure, outstanding mechanical robustness and high electrical conductivity remains a challenge. We report a design of entangled Sr–Co Layered double hydroxides (LDHs) two-dimensional structure for such electrode. The hierarchical electrode with entangled nanosheet exhibit high specific capacitance of 1415 F g-1 at 5 A g-1 current density, excellent rate capability, high energy (12.28Whkg−1) and power density (567.788 W kg-1) and outstanding stability (80% capacity retention over 5000 cycles). The asymmetric device cell of Sr–Co LDH/ACC gives high energy density (27.78 Whkg-1) and power density (499.96Wkg-1) with cyclic performance of 1000 cycles with 95.24% capacitance retention. Further, we provide a detailed analysis of the various electrolyte environment experienced by the electrode and their effect on electrochemical properties. We pick out three different electrolytes; LiOH, NaOH, KOH and show that both selected electrolyte and metal combination for LDHs paves the way for outstanding electrochemical performance.

Herein, we developed a strategy for fabrication of iron oxide infiltrated vertically aligned multiwalled carbon nanotubes (MWCNT forest) sandwiched with reduced graphene oxide (rGO) sheets network for high performance electromagnetic interference (EMI) shielding application which offers a new avenue in this area. Such engineered sandwiched network exhibits enhanced shielding effectiveness compared to conventional EMI shielding materials. This network of exotic carbons demonstrates the shielding effectiveness value more than 37 dB (>99.98% attenuation) in Ku-band (12.4–18 GHz), which is greater than the recommended limit (∼30 dB) for techno-commercial applications.

Herein, we report a facile synthesis of iron oxide nanoparticles by a hydrothermal route. The X-ray diffraction analysis confirms that these nanoparticles are of pure magnetite (Fe3O4) phase. Further, the morphology and average particle size were investigated using scanning electron microscopy. The average particle size of these nanoparticles was observed to be ~ 65 nm. The magnetic measurement reveals the ferromagnetic nature of the synthesized Fe3O4 nanoparticles at room temperature. The coercivity and remanence magnetization were observed to be 98 Oe and 0.51 μB/molecule, respectively. Fe3O4 nanoparticles showed a sharp transition (Verwey transition) around 120 K in M vs. T measurements. The observation of the Verwey transition indicates the high quality and phase purity of the synthesized Fe3O4. Moreover, the Fe3O4 nanoparticles were electrochemically characterized for their potential application as an electrode for ultra-capacitors. The specific capacitance of 97 F/g at the current of 1 mA was observed with excellent cyclic stability. The present facile synthesis method could be a potential approach for fabrication of ultra-capacitors using cheap and environment friendly ferromagnetic iron oxide nanoparticles for high performance energy materials.

Multilayer films of polyaniline (PANI) and graphene oxide (GO) were deposited on indium tin oxide (ITO) electrode for supercapacitor application. The graphene oxide was synthesized using chemical method. The X-ray diffraction study confirms the formation of graphene oxide. The supercapacitive behavior of the PANI and PANI–GO electrodes were studied using cyclic voltammetry (CV) and galvanostatic charge–discharge techniques in 1 M H 2 SO 4 solution. The electrochemical analysis confirms that the multilayer film has improved specific capacitance compared to pure polyaniline film. The specific capacitance of 201 F/g and 429 F/g was achieved for PANI and PANI–GO multilayer electrode. High electrochemical performance of the PANI–GO electrode could be due to increasing active sites for the deposition of polyaniline provided by large surface areas of graphene oxide sheets and the synergistic effect between polyaniline and graphene oxide. These results demonstrated the importance and great potential of graphene oxide in the development of high-performance energy-storage system based on polyaniline.

Herein, a novel synthesis process for large scale in situ growth of three dimensional carbon nanotube (CNT) pillared graphene hybrid nanostructure network (GCNT) is demonstrated where simultaneous reduction of graphene oxide (GO) and formation of CNT pillared on the reduced GO (RGO) sheet is carried out in a single-step in a chemical vapor deposition setup. Furthermore, this GCNT (graphene-carbon nanotube lattice) network exhibits optical bi-functionality where GCNT quenches the luminescence of Rhodamine organic dye while displays its own defect-induced strong photoluminescence (PL) which is merely reported in literature. Additionally, the quenching effects are examined using Rhodamine solution with variant carbon nanostructures. Moreover, time-resolved spectroscopic studies also show the effect of quenching in lifetime experiments. The quenching lifetime follows the sequence GCNT < RGO < CNT which is in good correlation with observed PL quenching results.

Present research is focussed on the development of a novel composite material for highly-efficient absorption of electromagnetic interference (EMI) pollution. Herein, we report synthesis and characterization of polypyrrole (PPY) composite containing multi-layered graphene (MLG) anchored with titanium dioxide (TiO 2 ) via in-situ oxidative polymerization of pyrrole. The tuning of the microwave signals has achieved via composites with different weight ratios of MLG and TiO 2 . The maximum Total shielding effectiveness (SE T ), 53 dB was observed for PPY/MLG/TiO 2 (5% composite) in the frequency range of 12.4–18 GHz. The observed results suggest that the composite material could be a new alternative for building block of electromagnetic shielding applications.

Multi-walled carbon nanotube (MWCNT)/portland cement(PC) composites have been fabricated to evaluate their electromagnetic interference (EMI) shielding effectiveness (SE). The results show that they can be used for the shielding of EMI in the microwave range. The incorporation of 15 wt.% MWCNTs in the PC matrix produces a SE more than 27 dB in X-band (8.2–12.4 GHz), and this SE is found to be dominated by absorption. Furthermore, the structural analysis, surface morphology and surface interaction of MWCNTs with PC matrix have been explored using XRD, SEM and X-ray photoelectron spectroscopy technique.

Highly efficient green light emission was observed from Mg doped ZnO nanoparticles synthesized via facile wet chemical route with an average particle size ∼15 nm . The XRD analysis confirmed the growth of wurtzite phase of ZnO nanoparticles. Moreover, the optical properties of these nanoparticles were investigated by different spectroscopic techniques. The resulted nanoparticles exhibit intense green emission peaking at 530 nm (2.34 eV) upon 325 nm (3.81 eV) excitation. The photoluminescence (PL) intensity of visible emission depends upon the doping concentration of Mg. The PL intensity was found maximum up to 4% doping of Mg, and beyond it exhibits a decrees in emission. Furthermore, by varying the band gap from 3.50 to 3.61 eV, the PL spectra showed a near band edge (NBE) emission at wavelength around 370 nm (3.35 eV) and a broad deep level emission in the visible region. The obtained highly luminescent green emission of ZnO nanoparticle would be an ultimate choice for next generation portable optoelectronics device materials.

Designing of bio-compatible nanomagnets with multiple functionalities receives immense scientific attention due to their potential applications in bio-labeling, medical diagnosis and treatment. Here we report the synthesis of Nickel (Ni) incorporated single-walled carbon nanotube (SWCNT) hybrid and bio-compatible bundles having interesting magnetic and photoluminescence (PL) properties. The SWCNT exhibits a high-crystallinity and it has an average diameter of ∼1.7 nm. Ni particles of 10-20 nm were incorporated within the SWCNT bundles. These hybrid bundles exhibit PL and it is attributed to the presence of delocalized π electrons and their recombination at the defective sites of SWCNT. Magnetic characterization revealed that the SWCNT/Ni hybrid bundle possesses a high (50 Oe) coercivity compared to bulk Ni and a long range ferromagnetic ordering at room temperature. MTT-assay has been conducted to study the cytotoxicity of these hybrid nanostructures.

The primary aim of the present paper is focused on the gram-scale (>1 g) synthesis and characterization of graphitic/carbon nanofibers (GNF). The GNFs were grown by spray pyrolysis of benzene. The optimum growth conditions for the high-yield GNFs are as follows: spray nozzle diameter (inner diameter) ∼0.52 mm, flow rate of benzene solution ∼5 ml/min with reaction temperature ∼750 °C and flow rate of hydrogen as carrier gas ∼1500 cc/min. The structural and microstructural characterizations have been evaluated by X-ray diffraction and transmission as well as by Scanning electron microscopy. The typical length and diameter are ∼60 μm and ∼250 nm, respectively. Most of the GNF samples exhibit coiled/helical-like fibers. The yield of GNF in the spray pyrolysis growth mode has been found to be of the order of gram-scale (1.45 g) per run. The details of the experimental parameters for optimum growth conditions have been studied and will be described.

This paper reports mischmetal (Mm) as an effective catalyst for fast desorption kinetics (3.7 wt% in 60 min in which ∼3.3 wt% observed in 30 min and ∼5 wt% in 180 min at the desorption temperature of 150 °C for the 2 mol% Mm-doped material) and rehydrogenation (up to 35 cycles) of the light weight hydrogen storage material NaAlH 4 . In fact this catalyst Mm has been shown to be better than the presently known catalyst Ti. For Mm, its presence when admixed in NaAlH 4 is discernible. Also the higher reversible hydrogen storage capacity (4.77 wt% which is 86% of the total reversible storage capacity) of the total is achievable in the 12th cycle. The possible modes of catalyst Mm for fast desorption kinetics has been outlined and the most feasible mechanism in terms of weakening of Na and AlH 4 bonding has been put forward.

In this paper, we have carried out investigations on Ti doping to increase the desorption kinetics of sodium aluminum hydride (NaAlH4) materials. Ti powder can be directly used for doping in the preparation of Ti doped NaAlH4 materials (Ti:NaAlH4). Out of various materials corresponding to NaAlH4 - x mol % Ti ( x = 1 , 2 , 3 , 4 , 5 , 6 ) , we have found that the material with x = 3 mol % is the optimum material. It shows the maximum storage capacity ∼ 5.23 wt % for the first two reactions at 150 and 210 °C, respectively. The first decomposition reaction of Ti doped NaAlH4 material was found to depend upon milling time. The optimum milling time for Ti doping was 10 min under argon atmosphere at 1500 rpm. The desorption kinetics improved nearly three times for Ti doped materials as compared to the procured materials. Rehydrogenation occurred at 110 °C under ∼ 10 MPa pressure in hydrogen environment within 8 h. The hydrogen storage capacity of this material was found to gradually decrease to 2.93 wt% after 15th cycles. The structural and microstructural characterizations have been evaluated by X-ray diffraction and scanning electron microscopy. The results of the present investigations will be described and discussed.

In this paper, we have carried out investigations on admixing carbon nanotubes (CNT) to increase the desorption rate of hydrogen in NaAlH 4 . So far only transition metal Ti has been attempted as the suitable dopant for making NaAlH 4 (purified version) a viable hydrogen storage material by increasing the hydrogen desorption rate. Out of the various materials corresponding to NaAlH 4 – x mol% CNT ( x = 2, 4, 6, 8 and 12), we have found that the material with x = 8 mol% is the optimum material. It shows highest desorption rate leading to 3.3 wt.% of H 2 at ∼160 °C within 2 h for the first dissociation reaction. The CNT admixed NaAlH 4 has also been found to exhibit good rehydrogenation characteristics (reversibility on hydrogenation up to ∼4.2 wt.%). A feasible mechanism for the improvement of hydrogen desorption characteristics has been put forward.

The nano-variants of carbon including graphitic nanofibres (GNF) have recently been considered to be exotic (light weight, high storage capacity) hydrogen storage materials. In the present paper, we report growth of aligned bundles of GNF. The length and width of cross-section of the bundles is ∼50 and ∼25 μm, respectively. The length of individual GNF is ∼50 μm and diameter ∼0.25 μm. The GNFs have been synthesized through thermal decomposition of acetylene using palladium (Pd) sheets as catalyst. This represents a new form of catalyst. The GNFs bundles grown by the present method are easier to hydrogenate. They adsorb hydrogen at a lower pressure of ∼80 atm as against ∼120 atm for the GNF grown in the earlier studies. The storage capacity obtained in the present investigation is ∼17 wt.%. Electron microscopic investigations reveal that as against the as grown GNF, the hydrogenated version embodies microstructures exhibiting fragmentation of graphitic layer bundles. The reasons for the growth of GNF in the form of aligned bundles, the ease of hydrogenation and relevance of GNF fragmentation after hydrogenation have been outlined.

The present paper deals with the synthesis of MmNi 5− x Fe x alloys ( x =0.05, 0.1, 0.3, 0.5). It has been found that Fe substitution, because of the higher electron attractive power of the Fe when substituted, leads to enhancement of the hydrogen storage capacity. Thus, the material MmNi 4.9 Fe 0.1 shows a storage capacity of ∼1.66 wt.%, which is one of the highest capacities for any as synthesised AB 5 type alloy. The P – C isotherms at various temperatures have been simulated by a suitable mathematical model. For this, the randomised variables Δ H and Δ S have been evaluated experimentally. These have been used as input parameters in the modelling. The simulated and experimental curves have been found to match closely. It is suggested that mathematical modelling is useful for predicting P – C – T behaviours when the hydride is used at different temperatures.

The present study is aimed at microstructural characterization and hydrogenation behaviour of graphitic nanofibres (GNF) prepared by ethylene gas. The GNF were prepared by thermal cracking of ethylene gas at a temperature of ∼600°C for time durations of 2 h . The structural characterizations have been evaluated by X-ray diffraction and transmission electron microscopy. It was found that the typical length of GNF was in the range of ∼1 to ∼6 μm and most of GNF sample exhibits coil like configuration. The microstructural characteristics as studied by transmission electron microscopy (TEM) techniques after hydrogenation/dehydrogenation were found to consist of curved voids and were significantly different as compared to as grown GNF. The graphitic nanofibres were hydrogenated at 120 atm for 24 h and then dehydrogenated upto 1 atm . The P-C-T curves drawn based on several dehydrogenation runs have revealed the hydrogen storage capacity of present GNF as ∼15 wt % . In addition to hydrogen storage capacity, we have also investigated the desorption kinetics of GNF samples, the hydrogen was desorbed at the rate of ∼57 ml 3 / min .

The present paper deals with the synthesis and hydrogenation behaviour of graphitic nanofibres. The GNF were grown by thermal cracking of hydrocarbon gases (ethylene, acetylene) at a temperature of 600°C for a time duration of 2 h. The structural characterizations have been evaluated by X-ray diffraction and transmission as well as scanning electron microscopy. It was found that the as grown graphitic nanofibres, unlike graphite, have preferred orientations corresponding to (101). The hydrogenation behaviour was studied through charging at a pressure of 120 atm. and then discharging at a pressure up to 1 atm. The P–C–T curve drawn based on several dehydrogenation runs has revealed the GNF to have hydrogen storage capacity of ∼10 wt%.

The magnetic/mechanical spacing between the transducer and the disk significantly decreases due to thermal expansion of pole tips at stressed high temperature and high humidity tests. The protruded pole tips and alumina overcoat result increased thermal asperities and can interfere with the higher disc asperities and be damaged due to high contact. The damage at the head-disk interface due to protruded pole tips and alumina overcoat may become a major roadblock in the drive mechanical performance when flying height is below 10 nm. In this study, the thermal PTR defined as change in PTR with temperature is measured using an optical profiler and an AFM for heads having stack design with single and dual layers of writer coil. The pole tips protrude above the ABS surface by 3–4 nm when the temperature of the head is raised by 50°C. Heads with single layer of writer coil exhibit significantly lower thermal PTR than those with dual layers of coils.

The exponential increase in areal density of recording of magnetic storage devices has reduced the recorded bit size to nano length scale. This has profound implications on all aspects of the storage system including recording device modeling, materials, fabrication, metrology, characterization, and tribology of the head-disk interface. In this paper, the impact of nanotechnology in extending the data storage device storage systems is explored with an emphasis on the fabrication and characterization of nanolayers and structures.

The stability of a system consisting of a flexible shaft with a single central rotor supported by two finite elliptical journal bearings was considered. The Reynolds equation was solved numerically for several values of the eccentricity ratio, the L D ratio and the dimensionless velocity of the journal centre. The resulting pressure profiles are used to determine the load capacity and the spring and damping coefficients. Limiting speeds of stable operation are obtained from the roots of characteristic equations for the corresponding bearing-rotor system. The operating load, ellipticity, L D ratio and shaft flexibility significantly affect the limit of stable operation. Elliptical bearings are suitable for stiff and moderately flexible rotors.

Journal bearings operating at high speeds often exhibit vibrational instability. Circular bearings are known to be prone to vibrations and, in general, a non-circular bearing geometry enhances shaft stability. Among the several possible configurations of non-circular bearings, elliptical, offset, three-lobe and tilted three-lobe bearings have attracted attention. The present analysis is a theoretical prediction of stability for a hybrid two-lobe bearing obtained by displacing the lobe centres of an elliptical bearing. It has been found that an orthogonally displaced bearing is more stable than the hitherto known bearings and at the same time it is easier to manufacture. Numerical results have been presented for nine combinations of horizontal and vertical displacements of lobes and three L D ratios so that a suitable profile and L D ratio can be selected by a designer in a specific application.