Ashok Kumar Tyagi

The role of SnO2 quantum dots in improved CH4 sensing at low temperature

The role of quantum dots (QDs) of SnO2 in detecting low concentrations of methane (CH4) at a relatively low temperature of ∼150 °C with high response (S ∼ 3.5%) and response time below 1 min is reported. A simple room temperature single step chemical process was adopted for the growth of SnO2 nanoparticles of a size around 2.4 nm. These nanoparticles were subsequently annealed at 800 °C to increase the grain size to 25 nm. The as-prepared SnO2 nanoparticles, being smaller than the corresponding Bohr radius (2.7 nm), showed a strong quantum confinement effect with a blue shift in the band gap energy from 3.6 eV for the bulk SnO2 to 4.37 eV for the QDs. These QDs exhibited a strong sensing response to CH4 in comparison to the annealed sample. A low activation energy of 90 meV, as estimated from the temperature dependent S plot for SnO2 QDs, was found to be the driving force for such unusual high sensitivity at a low operating temperature. X-ray diffraction, transmission electron microscopy, along with Raman spectroscopy measurements are used for the detailed structural studies. The critical role of the chemisorbed oxygen species present at different operating temperatures on the surface of the off-stoichiometric quantum sized SnO2 and bulk-like annealed samples are discussed in light of the adsorption kinetics.

Improvement in Tribological Properties by Modification of Grain Boundary and Microstructure of Ultrananocrystalline Diamond Films

Grain boundaries and microstructures of ultrananocrystalline diamond (UNCD) films are engineered at nanoscale by controlling the substrate temperature (TS) and/or by introducing H2 in the commonly used Ar/CH4 deposition plasma in a microwave plasma enhanced chemical vapor deposition system. A model for the grain growth is proposed. The films deposited at low TS consist of random/spherical shaped UNCD grains with well-defined grain boundaries. On increasing TS, the adhering efficiency of CH radical onto diamond lattice drops and trans-polyacetylene (t-PA) encapsulating the nanosize diamond clusters break due to hydrogen abstraction activated, rendering the diamond phase less passivated. This leads to the C2 radical further attaching to the diamond lattice, resulting in the modification of grain boundaries and promoting larger sized clustered grains with a complicated defect structure. Introduction of H2 in the plasma at low TS gives rise to elongated clustered grains that is attributed to the presence of atomic hydrogen in the plasma, preferentially etching out the t-PA attached to nanosized diamond clusters. On the basis of this model a technologically important functional property, namely tribology of UNCD films, is studied. A low friction of 0.015 is measured for the film when ultranano grains are formed, which consist of large fractions of grain boundary components of sp(2)/a-C and t-PA phases. The grain boundary component consists of large amounts of hydroxylic and carboxylic functional groups which passivates the covalent carbon dangling bonds, hence low friction coefficient. The improved tribological properties of films can make it a promising candidate for various applications, mainly in micro/nanoelectro mechanical system (M/NEMS), where low friction is required for high efficiency operation of devices.

Chemically grafted graphite nanosheets dispersed in poly(ethylene-glycol) by γ-radiolysis for enhanced lubrication

Graphite nanosheets (Gr-NS) dispersed in poly(ethylene-glycol) (PEG200) medium were subjected to various doses of γ-irradiation. Hydroxyl functional groups present in PEG are chemically grafted through hydrogen bonding with hydroxyl, carbonyl and carboxylic groups of Gr-NS. The grafting process is driven by the generation of active radicals from solvent radiolysis. Chemical grafting was investigated using X-ray photoelectron spectroscopy (XPS) and Fourier transform infra-red (FTIR) spectroscopy. The results of spectroscopic studies revealed reduction in oxygen functionality of PEG-Gr-NS at higher doses of γ-irradiation. The γ-irradiation not only bridges the functionalities between PEG and PEG-Gr-NS but edge and basal plane defects in Gr-NS are further reduced as is evident from Raman analysis. The inter-planar sheet distance in Gr-NS is increased due to intercalated chemical grafting with PEG molecules. The chemical grafting between PEG and Gr-NS and reduction in defects enhance the tribological properties with a decrease of 26% and 32% for the friction coefficient and wear, respectively as compared to PEG alone. The lubrication mechanism is mediated through inter-planar weak forces when PEG is chemically grafted with Gr-NS. The electrostatic interaction of PEG with Gr-NS acts as a molecular bridge thus enhancing the sustainability of tribo-stress. Additionally, in the presence of functionalized PEG-Gr-NS tribo-contact conditions, evidence of deposited graphitic tribo-film was observed from micro-Raman spectroscopy inside the steel wear track. This film further enhanced lubrication mediated through low shear strength interlayer graphite sheets and therefore, antiwear properties were synergistically improved.

CNT-ZnO Nanocomposite Thin Films: O2 and NO2 Sensing

The CNT-ZnO nanocomposites were synthesized by addition of commercially available MWCNT during growth of ZnO nanoparticles employing a wet chemical route. These nanocomposites were then spin coated and characterized using X-ray diffraction, scanning electron microscopy, current-voltage characteristics and O2 (5-20%) / NO2 (2-20 ppm) gas sensing at 250°C operating temperature in N2 atmosphere (0.4±0.03 mbar). The addition of CNT in ZnO is found to increase the sensitivity for both O2 and NO2 gas sensing. The 0.1 wt % CNT addition in ZnO is observed to appreciably enhance the NO2 gas sensitivity while 1.0 wt % CNT addition in ZnO showed highest sensitivity for O2 gas detection.

Surface functionalization-enhanced magnetism in SnO2 nanoparticles and its correlation to photoluminescence properties

A high value of magnetic moment of 0.08 emu g−1 at room temperature for SnO2 nanoparticles (NPs) was observed. Surface functionalization with octadecyltrichlorosilane (OTS) enhanced the saturation magnetic moment of NPs to an anomalously high value of 0.187 emu g−1 by altering the electronic configuration on the NPs surface. Surface functionalization also suppressed photoluminescence (PL) peaks arising from oxygen defects around 2 eV and caused an increase in the intensities of two peaks near the violet region (2.6–3 eV). PL studies under a uniform external magnetic field enriched the understanding of the role of OTS. Both OTS and external magnetic fields significantly modulated the luminescence spectra by altering the surface electronic structure of NPs. Extra spins on the surface of SnO2 NPs created by the surface functionalization process and their influence on resultant magnetic moment and luminescence properties are discussed in detail.

Deformation of SS 304 LN during Scratch Test and Influence on Evolution of Coefficient of Friction

An AISI 304 LN nuclear grade forged, metallographically polished specimen was subjected to progressive load scratch tests using a spheroconical indenter at three different scratch speeds 1, 3, and 27 mm/min. The present study attempts to address the evolution of coefficient of friction with scratch speed invoking its correlation with scratch induced deformation in the specimen. At higher scratch speeds, plastic deformation rates were higher which caused friction coefficient to be of higher magnitude. This was correlated with dynamically obtained high resolution optical images that revealed deformation driven microstructural alterations. These alterations significantly influenced the evolution of friction coefficient which was intimately related to plasticity of the surface.

Selective Capacitive Sensor for Ammonia Hydroxide at Room Temperature

Nanostructured yttria stabilized ZrO2 (YSZ) films are prepared by electron beam deposition and subsequently characterized by using X-ray diffraction, field emission scanning electron microscopy, and UV-Vis spectroscopy. A capacitive device with Al/YSZ/Al structure acts as a sensor for analytes like ammonia solution, acetone, and alcohols. Low powered read-out system based on an astable mode frequency generator is employed for monitoring the response continuously. The YSZ capacitive sensor operates at room temperature with a decrease in frequency on exposure to ammonia solution and the response varies linearly with concentrations (1%-30%). In contrast, highly volatile analytes like acetone, ethanol, and dichloromethane exhibit an increase in frequency after the exposures. Water exposure produces no significant change in the sensor response. The unique selectivity of YSZ towards ammonia solution is attributed to the modified dielectric constant of the YSZ films due to preferential binding of water molecules to the analyte.

The role of SnO 2 quantum dots in improved CH 4 sensing at low temperature

The role of quantum dots (QDs) of SnO2 in detecting low concentrations of methane (CH4) at a relatively low temperature of ∼150 °C with high response (S ∼ 3.5%) and response time below 1 min is reported. A simple room temperature single step chemical process was adopted for the growth of SnO2 nanoparticles of a size around 2.4 nm. These nanoparticles were subsequently annealed at 800 °C to increase the grain size to 25 nm. The as-prepared SnO2 nanoparticles, being smaller than the corresponding Bohr radius (2.7 nm), showed a strong quantum confinement effect with a blue shift in the band gap energy from 3.6 eV for the bulk SnO2 to 4.37 eV for the QDs. These QDs exhibited a strong sensing response to CH4 in comparison to the annealed sample. A low activation energy of 90 meV, as estimated from the temperature dependent S plot for SnO2 QDs, was found to be the driving force for such unusual high sensitivity at a low operating temperature. X-ray diffraction, transmission electron microscopy, along with Raman spectroscopy measurements are used for the detailed structural studies. The critical role of the chemisorbed oxygen species present at different operating temperatures on the surface of the off-stoichiometric quantum sized SnO2 and bulk-like annealed samples are discussed in light of the adsorption kinetics.

Enhanced ammonia sensing properties using Au decorated ZnO nanorods

Nanostructured Au-ZnO sensor systems have recently attracted attention for improving sensing behavior towards ethanol, acetone, hydrogen, CO, but rarely towards ammonia. In this report, an enhanced sensing response towards ammonia at near room temperatures using Au decorated ZnO nanorods (NR) is reported in comparison to pristine ZnO NR of hexagonal wurtzite structure. A facile two step synthesis method is adopted involving hydrothermal synthesis for the preparation of high aspect ratio ZnO NR of 40 nm diameter and 100 nm length followed by chemical growth methods for nanoparticle Au decoration of 5 nm average particle size. UV-Visible Diffuse Reflectance Spectroscopy confirms presence of Au nanoparticles along with ZnO NR with characteristic surface plasmon resonance and ZnO exciton peak. Gas sensing studies revealed that the Au-decorated ZnO NR can detect ammonia even at 50°C and shows highest response at 100°C than pristine ZnO NR which shows a response maximum at 300°C.

Air oxidation of the bulk amorphous alloy Zr46.75Ti8.25Cu7.5Ni10Be27.5 studied by using a TGA

The oxidation in air of the bulk amorphous alloy Zr46.75Ti8.25Cu7.5Ni10Be27.5 in its amorphous state and the supercooled liquid state was studied in the temperature range of 588?633?K by using a thermogravimetric analyser (TGA). The oxidation kinetics of the alloy obeys the parabolic rate law during oxidation in its amorphous state as well as in the supercooled liquid state. The results were compared with those obtained in our previous study of the air oxidation of another well-known bulk alloy, Zr65Cu17.5Ni10Al7.5, in the temperature range of 591?684?K. The value of the parabolic rate constant (K) for the alloy Zr46.75Ti8.25Cu7.5Ni10Be27.5 (e.g. K=5.9? 10?9?g?cm?2?s?1/2 at 588?K) is about two orders of magnitude less than its value for the other bulk alloy Zr65Cu17.5Ni10Al7.5 (K=6.8? 10?7?g?cm?2?s?1/2 at 588?K), thus suggesting that the bulk amorphous alloy Zr46.75Ti8.25Cu7.5Ni10Be7.5 displays a better oxidation resistance in air than Zr65Cu17.5Ni10Al7.5.