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This paper illustrates the electrochemical exfoliation of graphite for the synthesis of graphene oxide (GO) with few layers. Innovative design and arrangement of electrodes were adopted, which allowed simultaneous exfoliation and re-exfoliation of graphite anode. Electrolyte having different molar concentrations of an aqueous solution of sulfuric acid (H2SO4) and pH offered successful exfoliation of graphite anode with a varying yield of GO. The addition of sodium hydroxide (NaOH) in the solution affected the exfoliation rate and the morphology of the GO sheets. The transition of GO sheets from clumping–scrolling–flat sheet edge rolling to nanoribbons was observed by the change of composition and pH of the electrolyte. GO sheets showed lateral dimensions ranging from 500 nm to 24 μm, and ribbons and scrolls were found to have dimensions up to 50 μm. With the help of transmission electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy, a mechanism of exfoliation has been proposed. In situ functionalization of graphene helped in imparting stability to the GO in solvents such as ethanol and water for more than 11 months. This study elaborates on the significance of solvated sulfate ions and provides a window of sulfate and hydroxyl ion combination for tuning of structural morphologies of GO.

Delamination and fiber pull-out remain significant problems with carbon fiber-reinforced epoxy composites due to the brittle nature of epoxy and weak fiber–matrix interface. The present study approaches overcome these problems by fabricating a multiscale carbon fiber-epoxy composites laminated composite with graphene nanoplatelets reinforced matrix, using a vacuum-assisted resin infusion method. Incorporation of 0.2 wt.% graphene nanoplatelets in carbon fiber-reinforced epoxy composites enhances fracture toughness by ~ 35% and ultimate tensile strength by ~ 22%, owing to significant adhesion between fiber and matrix. Thorough fiber–matrix interfacial characterization revealed improved modulus gradient across the interface, ~ 38% more energy absorption during delamination, and ~ 42% higher fiber pull-out strength from the matrix.

Silver nanoparticles (Ag NPs) have found wide-spread applications as antibacterial agents, catalysts and in chemical detection via surface-enhanced Raman spectroscopy (SERS). To avoid agglomeration and unwanted release to the environment, Ag NPs are usually loaded onto various substrates. However, simple procedures for large-scale synthesis of this important material are still lacking. Here, we demonstrate a facile, green and scalable synthesis of Ag NPs loaded to ceramic substrates. Environment-friendly biopolymer (gum arabic) and ceramic (silica) nanoparticles were spray-dried to obtain surface-functionalized mesoporous silica substrate. Thereafter, the silver precursor was reduced in situ to obtain Ag NPs attached to the silica substrate. Gum arabic played a crucial role in reducing and attaching Ag NPs to the substrate. The resulting composite material was characterized using X-ray diffraction, high-resolution transmission electron microscopy, field emission scanning electron microscopy and Fourier transform infrared spectroscopy. The antimicrobial efficacy of the synthesized composite was evaluated against a gram-negative bacterium, Escherichia coli and a gram-positive bacterium, Staphylococcus aureus. Complete bactericidal effect was observed for composite harboring silver concentration as low as 19 µg ml−1 and 76 µg ml−1 for Escherichia coli and Staphylococcus aureus, respectively. Furthermore, the composite was used as a catalyst in the reduction reaction of 4-nitrophenol and rhodamine B by sodium borohydride. The rate constant for 4-nitrophenol reduction was found to be 1750 s−1 g−1. The composite demonstrated its applicability as a substrate for surface-enhanced Raman spectroscopy (SERS) wherein very dilute concentration (10−9 M) of crystal violet and rhodamine 6G could be easily detected.

Conversion of diamond polishing powders into graphite nano-crystals and simultaneous densification to high-density graphite were achieved through spark plasma sintering at 2000 °C with 45 MPa pressure. The sintered graphite had a density of 1.84 g/cm3 and hardness of 12.1 ± 0.4 HV. The phase evolution was characterized by X-ray diffraction and Raman spectroscopy. Scanning electron microscopy and transmission electron microscopy revealed that the converted high-density graphite consisted of randomly oriented nano-crystalline grains. The formed graphite was also isotropic in nature.

Herein, sorption of pentavalent neptunium from aqueous acidic solution was carried out onto 1st and 2nd generation of poly(amidoamine) dendrimer functionalized MWCNTs. The Np(V) sorption kinetics followed pseudo second order kinetics with rate constants of 1.4 × 10−4 and 6.84 × 10−4 g mg−1 min−1 for MWCNT–PAMAMG1 and MWCNT–PAMAMG2, respectively. The sorption of Np(V) onto MWCNT–PAMAMGn proceeded via chemisorption and the data fitted well to Langmuir isotherm model. MWCNT–PAMAMGn was found radiolytically stable up to 500 kGy dose of gamma radiation. Sodium carbonate was found to strip Np effectively from sorbent.

The degradation of polymer matrix composite material during carbonization and the processing temperature are some important deciding factors to evaluate the performance of carbon–carbon composite (CC) for the targeted applications. In present study, the carbonization of material at the temperature range of 1200–1500 °C is considered as a critical step for understanding the material performance, as it is essential for the progress in the processing of CCs. The heat, mass and momentum transport equations are used with weight loss kinetics to develop a numerical simulation model for the carbonization process. Afterward, to solve the governing equations, a 2-D finite difference method (FDM) is employed. Using material properties, the developed numerical model predicts the distribution of pressure, porosity, density, specific heat, thermal conductivity as a function of surface temperature for square samples of various thicknesses at different heating rates. The comparisons of the results of the 2-D model are carried out with results of 1-D FDM model published earlier.

Silicon carbide fabric converted from carbon fabric was prepared by a facile halide-activated pack cementation method. The XRD, SEM and EDS analyses confirm the formation of SiC fibres with graded conversion of carbon to SiC from surface to core of individual carbon fibres of carbon fabric. The graded conversion of carbon fibre to SiC was uniform and homogeneous throughout the fabric dimension. The thermokinetics of oxidation in atmospheric environment was studied by TGA analysis. The reaction kinetics and governing mechanism for oxidation of bare carbon fabric and silicon carbide fabric were calculated using Kissinger and Ozawa methods. It was observed that the activation energy for bare carbon fabric and silicon carbide fabric converted from carbon fabric are 62 and $$174 \hbox { kJ mol}^{-1}$$ , respectively. Further, it was observed that the silicon carbide fabric converted from carbon fabric follows the second-order surface reaction oxidation mechanism.

A novel and simple sol–gel route has been used for the fabrication of composite structure composed of carbon fibers and silicon carbide nanowires embedded in dense silicon carbide matrix. The carbonaceous silica sol was impregnated in the carbon fiber preform at atmospheric pressure. The sol impregnated carbon preform was cured and heat treated to convert into silicon carbide. The analysis by X-ray diffraction, scanning electron microscopy, X-ray tomography, and transmission electron microscopy indicates that the impregnated carbonaceous silica gel converts to β-silicon carbide with dense and wire morphology. Different morphological silicon carbide was uniformly distributed inside carbon fiber preform and there was no degradation in thermophysical properties of carbon composite during processing. These results reveal high efficient reinforcement of different morphological silicon carbide in carbon composite, demonstrate a new mechanism of carbon composite reinforcement and suggest a new direction to carbon composite reinforcement.

Photo-stability of laser dyes has been an important consideration for dye lasers. In this work, photo-stability of dye and its effect has been investigated as a function of dye laser wavelengths rather than commonly studied photo-chemical degradation pathways upon excitation with pump laser. Here, we report a significant improvement in photo-stability of the liquid dye laser at peak emission wavelength compared to edge wavelengths of the dye gain curve, which was related with an increase in rate of stimulated emission of dye at peak wavelength. Thus, high laser efficiency harmonizes to high photo-stability of the dye at peak wavelength which is useful for the applications of dye lasers. As a representative case, the photo-stability rates of pyrromethene 567 (PM567) dye at different laser emission wavelengths were studied using a 10-Hz Nd-YAG (532 nm) laser-pumped dye laser set-up under lasing and non-lasing conditions. Deterioration of dye laser efficiency on account of photo-degradation of dye molecules was theoretically simulated using a time-dependent rate equation model, which has been found to be in good agreement with the experimental observations.

The paper deals with the comparative study of nanocrystalline Lanthanum hexaboride (LaB6) thin films grown on various substrates by Pulsed laser deposition and Arc plasma method. Field emission studies were carried out on LaB6 films deposited on various substrates show metallic behavior of the emitters. The high value of field enhancement factors, indicating that the electron emission from LaB6 nanoscale protrusions deposited on emitter surface. The post field emission surface morphology of the emitters showed no significant erosion of the films during continuous operation. The observed behavior indicates that it is linked with the growth of LaB6 films on substrate crystal structure. The LaB6 nanocrystallites/nanowires films were synthesized using arc plasma method shows good emission current stability. The LaB6 micro/nanocrystallites were also obtained by picosecond laser irradiation which gives high enhancement β factor, and good emission current stability along with high current density. The results reveal that nanocrystalline LaB6 films, exhibit high resistance to ion bombardment and excellent structural stability and are more promising emitters for practical applications in field emission based new generation devices.

A combined experimental and theoretical study of the photo-physical, laser properties and molecular structures of three relatively recent Pyrromethene (PM) class dyes, PM597, PM580 and PM567, have been carried out. Laser characteristics of these three PM dyes were compared with three other widely used Rhodamine (RH) class dyes, RH6G, RHB and KRS, using a narrow-band dye laser setup, transversely pumped by the second harmonic (532 nm) of a Q-switched Nd-YAG laser. In addition to generating comparative data of these dyes for optimal use in dye lasers, we observed that unlike the RH dyes, the PM dyes show high efficiencies and wide tunability, despite the low fluorescence yield and high rate of non-radiative decay. Particularly, PM597 dye, in spite of a very low quantum yield of fluorescence (Φ=0.42), high non-radiative decay rate, and a large distortion from planarity in its excited state, when used in a laser cavity it exhibited similar laser efficiency and a beneficially wider tuning curve in comparison to other two PM dyes. Theoretical studies were carried out applying density functional theory and time-dependent density functional theory (DFT/TDDFT) to obtain new information on ground and the first excited state geometrical parameters of the PM dyes. Good correlation between calculated molecular properties and experimental results was observed for the evolution of the longest wavelength absorption maximum.

Many aspects of photophysical, photostability and laser properties of kiton red S dye remain unresolved, particularly for pumping with 578 nm radiation of CVL and 532 nm output of Nd:YAG lasers, and these are studied using different alcohol- and water-based binary solvents. Our results show that methanol is better suited for low and high-repetition-rate KRS dye lasers because of its superior laser efficiency, photostability as well as photothermal properties.

Narrow-band laser performance of alcohol solutions of pyrromethene 567 (PM567) and rhodamine 6G (RH6G) dye was investigated using a home-made GIG-configured dye laser, excited by the second-harmonic radiation (at 532 nm) of a pulsed Nd:YAG laser. Higher laser efficiency was observed with PM567 dye (∼23% peak) in comparison to the commonly used RH6G dye (16.5%), in spite of much lower fluorescence quantum efficiency of the PM567 (0.83) vis-a-vis RH6G (0.98) dye solutions in ethanol. First principle-based electronic structure calculations were performed on PM567 dye in the ground (S0) and excited states (S1) using density functional theory to elucidate the structure and photophysical properties of the dye.

A time-dependent, two-dimensional (in space) rate equation model of a transversely-pumped pulsed dye laser oscillator, which incorporates transverse pump intensity variation in the presence of intracavity dye laser radiation, is proposed to understand and predict its temporal behaviour. The model yields output pulses which agree well with experimental results using rhodamine 6G and kiton red dyes. The shape, amplitude and temporal position of the simulated pulse within the pump pulse vary dramatically across the tuning range of each dye depending on the relative gain and loss values.

Photonic crystals fabricated from the colloidal spheres of polymethyl methacrylate (PMMA) using the inward-growing self-assembly technique were subsequently in-filled with zinc oxide (ZnO) prepared by the sol-gel process. The polymer template was removed by heat treatment and chemical method to get ZnO-inverse photonic crystal. The structural quality of the inverse photonic crystal obtained by the chemical method was found to be superior to that obtained by heat treatment. The ZnO-inverse photonic crystal obtained by the chemical method was further treated at an elevated temperature to ensure the crystalline nature of ZnO. Laser-induced emission studies on ZnO-inverse photonic crystals were carried out at an excitation wavelength of 325 nm. The emission spectra showing UV and visible bands at collection angle of 45° from the direction of excitation helped to establish the role of crystalline ZnO.

Micro/nano structures have been obtained by laser surface treatment on sintered LaB6 pellets employing a picosecond pulsed Nd:YAG laser at a pressure of ∼1×10−3 mbar. The X-ray diffraction pattern of the laser treated pellet shows a set of well defined diffraction peaks, indexed to the cubic phase of LaB6 only. The scanning electron microscope studies reveal formation of micro and nano structures upon laser treatment and the resultant surface morphology is found to be strongly influenced by the laser fluence. Field electron emission studies made on the LaB6 pellet, treated with optimized laser fluence, have been performed in a planar diode configuration under ultra high vacuum conditions. The threshold field required to draw an emission current density of ∼10 μA/cm2 has been found to be ∼2.3 V/μm and a current density of ∼530 μA/cm2 has been drawn at an applied field of 5.2 V/μm. The Fowler-Nordheim plot is found to be linear in accordance with the quantum mechanical tunneling phenomenon, confirming the metallic nature of the emitter. The emission current at the pre-set value ∼10 μA shows very good stability over a period of more than 3 hours. The present results emphasize the effectiveness of a picosecond laser treatment towards fabrication of a nano metric LaB6 emitter for high current density applications.

Small angle neutron scattering (SANS) has been utilized to study the morphology of the multi-walled carbon nanotubes prepared by chemical vapour deposition of acetylene. The effects of various synthesis parameters like temperature, catalyst concentration and catalyst support on the size distribution of the nanotubes are investigated. Distribution of nanotube radii in two length scales has been observed. The number density of the smaller diameter tubes was found more in number compared to the bigger one for all the cases studied. No prominent scaling of the structure factor was observed for the different synthesis conditions.

Narrowband laser performances and photochemical stability of alcoholic solutions of pyrromethene 567 and rhodamine 6G dyes, under high-repetition rate copper vapour laser (at 510 nm), as well as, high-peak intensity Nd:YAG laser (at 532 nm) excitation have been investigated. We have observed that pyrromethene 567 dye solutions offer higher efficiency, wider tuning range, but lower photochemical stability and higher lasing threshold than that of rhodamine 6G dye solutions. An addition of about 100 mM DABCO, as a singlet oxygen quencher, in pyrromethene 567 dye solutions improved its photochemical stability close to that of rhodamine 6G. The observation of higher slope efficiency, in spite of higher threshold pump energy for pyrromethene 567 dye than that of rhodamine 6G dye solutions, was explained by a predictive model on gain characteristics of both dye solutions as a function of pump energy.

The use of graphite as a moderator in a low temperature thermal nuclear reactor is restricted due to accumulation of energy caused by displacement of atoms by neutrons and high energetic particles. Thermal transients may lead to a release of stored energy that may raise the temperature of the fuel clad above the design limit. Disordered carbon is thought to be an alternative choice for this purpose. Two types of disordered carbon composites, namely, CB (made up of 15 wt. % carbon black dispersed in carbonized phenolic resin) and PAN (made up of 20 vol. % chopped polyacrylonitrile carbon fibre dispersed in carbonized phenolic resin matrix) have been irradiated with 145 MeV Ne6+ ions at three fluence levels of 1.0×1013, 5.0×1013 and 1.5×1014 Ne6+/cm2, respectively. The XRD patterns revealed that both the samples remained disordered even after irradiation. The maximum release of stored energy for CB was 212 J/g and that of PAN was 906 J/g. For CB, the release of stored energy was a first order reaction with activation energy of 2.79 eV and a frequency factor of 3.72×1028 per second. 13% of the defects got annealed by heating up to 700 °C. PAN showed a third-order release rate with activation energy of 1.69 eV and a frequency factor of 1.77×1014 per second. 56% of the total defects got annealed by heating it up to 700 °C. CB seems to be the better choice than PAN as it showed less energy release with a slower rate.

Carbon nanotubes have been synthesized by catalytic chemical vapour deposition of acetylene diluted with argon using three different catalysts, namely, nickel formate, cobalt formate and ferrocene. The synthesis was carried out at 700°C in a quartz reactor for 30 minutes. Thermal analysis was carried out in order to determine the yield of the nanotube. It was found that the deposit contains 86% nanotube, with nickel-based catalyst, which was the maximum. The yield of nanotube was 71 times that of the nickel loading. The TEM images reveal helical type of nanotubes with iron catalyst while cobalt and nickel catalysts yielded straight nanotubes. This technique can be explored for the bulk production of carbon nanotube in an economic way.

Water is an inherently safe solvent for laser dyes and posses far better photo thermal characteristics in comparison to the commonly used organic solvents in dye lasers. However, water solution of dye needs a suitable deaggregating additive to suppress non-radiative dimers and to achieve similar laser efficiency. We have carried out systematic studies on comparative performances of dye lasers, pumped by low-repetition-rate Nd-YAG, as well as high-repetition-rate copper vapour lasers using Rhodamine 6G dye solutions in ethanol and various binary solvents of water. Among different additives in water, for the first time, the results of our studies on the application of water solvent in dye lasers could clearly establish that binary solvent comprising of normal or heavy water and about 18% to 25% n-propanol has produced similar efficiency, better photochemical stability and superior thermo-optic properties than ethanol solvent in high-repetition rate oscillator-amplifier operation.

Nonlinear refraction in various polymethyl methacrylate (PMMA) solid hosts when doped with fluorescent organic dyes, commonly used as a laser active medium, has been studied. The observed thermal lensing effect arising from optical nonlinearity, results from non-radiative energy transfer from the dye molecules to the solid matrix and is, therefore, strongly dependent on the thermal properties and material parameters of the polymeric host. The non-linear index of refraction, n2, for each sample of dye doped polymeric host, has been measured employing the standard Z-scan technique. These measured values of n2, have been used to determine the extent of variation in the index of refraction with varying temperature, dn/dT, for the various host materials. Estimation of dn/dT is critical in determining the extent of thermal lensing in the dye doped solid-state medium, that in turn determines the spatial quality and divergence of the generated laser beam delivered by a dye doped solid-state laser system.

Detailed experimental study of laser dye stability and the subsequent effects of photodegradation products on the operation of a Nd:YAG laser pumped Rhodamine-6G pulsed dye oscillator has been carried out. Deterioration of laser output on account of dye photodegradation resulting in the loss of active dye molecules and the generation and accumulation of reaction products that introduce a loss at the laser and pump wavelengths has been theoretically simulated in terms of a time dependent rate equation model. Our experimental observations have been found to be in good agreement with our theoretical estimates. We have also investigated the deterioration in the performance of a copper vapour laser pumped tunable dye laser with a high average power and a high repetition rate operating over an extended period of time. Our theoretical model, when suitably modified, provided useful estimates for the extent of deterioration in laser performance of such a dye laser with a high average power and a high repetition rate on account of photodegradation of the active dye molecules.

Photophysical properties of heavy-water-based rhodamine dye solutions have been investigated, with a view to assess the suitability of heavy water as a solvent for high-power, high-repetition-rate dye lasers and amplifiers. We have measured the quantum yield of fluorescence of the commonly used dyes rhodamine-6G, rhodamine-B and kiton-red, dissolved in heavy water, ethanol and normal water. The performance of a heavy-water-based pulsed rhodamine-6G dye laser has been investigated in broadband, as well as in narrowband wavelength-tunable resonator configurations, yielding laser efficiencies comparable to those achieved with ethanolic solutions of the same dye. We have also studied the thermo-optic properties of normal and heavy water, using the Z-scan technique. Finally, photodegradation rates for laser dyes have been compared in heavy water, normal water and ethanol. Our results establish heavy water as a solvent superior to both ethanol and normal water, on account of the lower thermo-optic effects and the higher photostability of rhodamine dyes when dissolved in heavy water.

The photo-stability of laser dyes, Rhodamine-6G, Rhodamine-B, and Kiton-Red, under high repetition rate (6.3 kHz) Copper-vapour-laser (CVL) irradiation has been investigated. Exhaustive photo-bleaching of these dyes in different solvents has been carried out to study the extent to which the dye photo-degraded products would interfere with the lasing process in high-power CVL-pumped dye lasers. Our results indicate that the photo-degradation of dyes occurs predominantly through excited-state intermolecular reactions, involving the singlet state, rather than the process of nonlinear optical absorption in individual molecules.

For the first time, bamboo-shaped multiwalled carbon nanotubes, having diameter of the order of 50 nm, have been grown on carbon black in a fluidized bed in bulk amount. The activation energy for the synthesis of the product was found out to be around 33 kJ/mol in the temperature range of 700−900 °C. The carbon nanotubes were separated from the carbon black by preferential oxidation of the later, the temperature of which was determined by thermogravimetry. The transmission electron microscopy revealed different features of the nanotubes such as “Y” junction, bend, and catalyst filling inside the nanotubes. Small angle neutron scattering was performed on the nanotubes synthesized at different temperatures. The data were fitted into a suitable model in order to find out the average diameter, which decreases with increase in synthesis temperature. The Monte Carlo simulation predicts the same behavior. Based on the above observations, a possible growth mechanism has been predicted. The oscillation in carbon saturation value inside the catalyst in the fluidized bed has been indicated as the responsible factor for the bamboo-shaped structure.

A Michelson interferometer-based technique has been used to measure the deformation of dielectric-coated mirror, caused by an incident repetitive pulsed laser beam with high average power. Minimum measurable deformation of 17 nm is reported.

A large number of autoionization resonances have been observed in uranium in the energy range 50,590–51,560cm−1 by two-step three-photon ionization technique, using two copper vapor laser pumped dye lasers. A Rydberg series converging to the ionization limit of UII at 1749cm−1 (6 L 13/2) has been identified. Some of these resonances are very narrow with a fwhm of 0.1cm−1. Possible origins of these are discussed.

Using two pulsed tunable dye lasers, a two colour multiphoton ionization process has been utilised to explore the higher energy levels of uranium. One hundred and thirty eight new UI levels have been observed in the 37540–38420 cm−1 region. J assignments of these levels have been suggested on the basis of their excitation from intermediate levels with known and contiguous J values. In eight cases the J assignments are unique.