Nanoparticle–electrode collisions as a dynamic seeding route for the growth of metallic nanostructures

The collisions between colloidal metal nanoparticles and a carbon electrode were explored as a dynamic method for the electrodeposition of a diverse range of electrocatalytically active Ag and Au nanostructures whose morphology is dominated by the electrostatic interaction between the charge of the nanoparticle and metal salt.

Charge mediated semiconducting-to-metallic phase transition in molybdenum disulphide monolayer and hydrogen evolution reaction in New 1T’ phase

The phase transition of single layer molybdenum disulphide (MoS2) from semi-conducting 2H to metallic 1T and then to 1T' phases, and the effect of the phase transition on hydrogen evolution reaction (HER) are investigated within this work by density functional theory. Experimentally, 2H-MoS2 has been widely used as an excellent electrode for HER and can get charged easily. Here we find that the negative charge has a significant impact on the structural phase transition in a MoS2 monolayer. The thermodynamic stability of 1T-MoS2 increases with the negative charge state, comparing with the 2H-MoS2 structure before phase transition and the kinetic energy barrier for a phase transition from 2H to 1T decreases from 1.59 eV to 0.27 eV when 4 e- are injected per MoS2 unit. Additionally, 1T phase is found to transform into the distorted structure (1T' phase) spontaneously. On their activity toward hydrogen evolution reaction, 1T'-MoS2 structure hydrogen coverage shows comparable hydrogen evolution reaction activity to the 2H-MoS2 structure. If the charge transfer kinetics is taken into account, the catalytic activity of 1T'-MoS2 is superior to that of 2H-MoS2. Our finding provides a possible novel method for phase transition of MoS2, and enriches understanding of the catalytic properties of MoS2 for HER.

Charge mediated semiconducting-to-metallic phase transition in molybdenum disulfide monolayer and hydrogen evolution reaction in new 1T′ phase

The phase transition of single layer molybdenum disulfide (MoS2) from semiconducting 2H to metallic 1T and then to 1T′ phases, and the effect of the phase transition on hydrogen evolution reaction (HER) are investigated within this work by density functional theory. Experimentally, 2H-MoS2 has been widely used as an excellent electrode for HER and can get charged easily. Here we find that the negative charge has a significant impact on the structural phase transition in a MoS2 monolayer. The thermodynamic stability of 1T-MoS2 increases with the negative charge state, comparing with the 2H-MoS2 structure before phase transition and the kinetic energy barrier for a phase transition from 2H to 1T decreases from 1.59 to 0.27 eV when 4e– are injected per MoS2 unit. Additionally, 1T phase is found to transform into the distorted structure (1T′ phase) spontaneously. On their activity toward hydrogen evolution reaction, 1T′-MoS2 structure shows comparable hydrogen evolution reaction activity to the 2H-MoS2 structure. If the charge transfer kinetics is taken into account, the catalytic activity of 1T′-MoS2 is superior to that of 2H-MoS2. Our finding provides a possible novel method for phase transition of MoS2 and enriches understanding of the catalytic properties of MoS2 for HER.

The role of tunnel junction resistances and defects on electron transport mechanism in networks of two-dimensional disordered conductors

The effect of tunnel junction resistances on the electronic property and the magneto-resistance of few-layer graphene sheet networks is investigated. By decreasing the tunnel junction resistances, transition from strong localization to weak localization occurs and magneto-resistance changes from positive to negative. It is shown that the positive magneto-resistance is due to Zeeman splitting of the electronic states at the Fermi level as it changes with the bias voltage. As the tunnel junction resistances decrease, the network resistance is well described by 2D weak localization model. Sensitivity of the magneto-resistance to the bias voltage becomes negligible and diminishes with increasing temperature. It is shown 2D weak localization effect mainly occurs inside of the few-layer graphene sheets and the minimum temperature of 5 K in our experiments is not sufficiently low to allow us to observe 2D weak localization effect of the networks as it occurs in 2D disordered metal films. Furthermore, defects inside the few-layer graphene sheets have negligible effect on the resistance of the networks which have small tunnel junction resistances between few-layer graphene sheets

Socio-cultural influences on vehicle defects in the Omani heavy vehicle industry

With recent economic growth in Oman there is increased use of heavy vehicles, presenting an increase in heavy vehicle crashes, associated fatalities and injuries. Vehicle defects cause a significant number of heavy vehicle crashes in Oman and increase the likelihood of fatalities. The aim of this study is to explore factors contributing to driving with vehicle defects in the Omani heavy vehicle industry. A series of qualitative participants observations were conducted in Oman with 49 drivers. These observations also involved discussion and interviews with drivers. The observations occurred at two road-side locations where heavy vehicle drivers gather for eating, resting, vehicle check-up, etc. Data collection was conducted over a three week period. The data was analysed using thematic analysis. A broad number of factors were identified as contributing to the driving of vehicles with defects. Participants indicated that tyres and vehicle mechanical faults were a common issue in the heavy vehicle industry. Participants regularly reported that their companies use cheap, poor quality standards parts and conducted minimal maintenance. Drivers also indicated that they felt powerless to resist company pressure to drive vehicles with known faults. In addition, drivers reported that traffic police were generally in effective and lacked skill to appropriately conduct roadside inspection on trucks. Further, participants stated that it was possible for companies to avoid being fined during annual or roadside vehicle inspections if members of the company knew the traffic police officer conducting the inspection. Moreover, fines issued by police are generally directed to the individual driver rather than being applied to the company, thus providing no incentive for companies to address vehicle faults. The implications of the findings are discussed.

Virtues of marginally metallic oxides

Transition-metal oxides at the metal-insulator boundary, especially those belonging to the perovskite family, exhibit fascinating phenomena such as insulator-metal transitions controlled by composition, high-temperature superconductivity and giant magnetoresistance (GMR), Interestingly, many of these marginally metallic oxides obey the established criteria for metallicity and have a finite density of states at the Fermi;level. The perovskite manganates exhibiting GMR, on the other hand, are unusual in that they possess very high resistivities in the 'metallic' state and show no significant density of states at the Fermi level, Marginal metallicity in oxide systems is a problem of great complexity and contemporary interest and its understanding is of crucial significance to the diverse phenomena exhibited by these materials.

Micropillar compression studies on a bulk metallic glass in different structural states

Uniaxial compression experiments on 0.3, 1 and 3 mu m diameter micropillars of a Zr-based bulk metallic glass in as-cast, shot-peened and structurally relaxed conditions were conducted. Shear band formation and stable propagation is observed to be the plastic deformation mode in all cases, with no detectable difference in yield strength according to either size or condition. The limitations of uniaxial compression tests in assessing the influence of various material conditions on plasticity, when it is inhomogeneous in nature, are illustrated.

An Investigation of Friction During Friction Stir Welding of Metallic Materials

The technique of friction stir welding (FSW) puts effective use frictional heat for the purpose of joining metallic materials. In this research article, we present and discuss an experimental method to determine the coefficient of friction during FSW. The experiments were conducted to study the interaction between the FSW tool (a die steel) and the base metal (a high strength aluminum alloy) at various contact pressures (13MPa, 26MPa, and 39MPa) and rotation speeds (200rpm, 600rpm, 1000rpm, and 1400rpm). The experimental results, the microstructure, and the process temperature reveal the experimental setup to be capable of simulating the conditions during FSW. The coefficient of friction was found to vary from 0.15 to 1.4, and the temperature increased to as high as 450C. The coefficient of friction was found to increase with temperature. There exists a critical temperature at which point a steep increase in the coefficient of friction was observed. The critical temperature decreases from 250C at a contact pressure of 26MPa to 200C at contact pressure of 34MPa. Below the critical temperature at a specific contact pressure the maximum coefficient of friction is 0.6, and above the critical temperature it reaches a value as high as 1.4. The steep increase in the coefficient of friction is found to be due to the seizure phenomenon and the contact condition during FSW between the tool and the workpiece (base metal) is found to be sticking.

Influence of non-stoichiometric defects on nonlinear absorption and refraction in Nd:Zn co-doped lithium niobate

Nonlinear absorption and refraction phenomena in stoichiometric lithium niobate (SLN) pure and co-doped with Zn and Nd, and congruent lithium niobate (CLN) were investigated using Z-scan technique. Femtosecond laser pulses from Ti:Sapphire laser (800 nm, 110 fs pulse width and 1 kHz repetition rate) were utilized for the experiment. The process responsible for nonlinear behavior of the samples was identified to be three photon absorption (3PA). This is in agreement with the band gap energies of the samples obtained from the linear absorption cut off and the slope of the plot of Ln(1 − TOA) vs. Ln(I0) using Sutherland’s theory (s = 2.1, for 3PA). The nonlinear refractive index (n2) of Zn doped samples was found to be lower than that of pure samples. Our experiments show that there exists a correlation between the nonlinear properties and the stoichiometry of the samples. The values of n2 fall into the same range as those obtained for the materials of similar band gap.

Composition-controlled metal-insulator transitions and minimum metallic conductivity in the oxide systems LaNi1-xMxO3 (M = Cr, Mn, Fe, or Co)

The composition-controlled metal-insulator transition in the perovskite systems LaNi1-xMxO3 (M = Cr, Mn, Fe, and Co) has been investigated by transport measurements over the temperature range 12-300 K. These systems, which have critical electron densities (nc) in the range (1-2) -1020 electrons cm-3, exhibit sharp metal-insulator transitions at the base temperature. The corresponding minimum metallic conductivity (Ï-min), separating the localized and itinerant electronic regimes, is of the order of 102 ohm-1 cm-1. Particular attention is paid to the idea of Ï-min scaling with nc, and our present results are compared with earlier studies of the metal-insulator transition in low (e.g., Ge:Sb) and high (e.g., metal-ammonia, supercritical Hg) electron-density systems. A link is established between the transport and magnetic properties of the title systems at the metal-insulator transition.

Assessment of quality defects in macadamia kernels using NIR spectroscopy

Spectral data were collected of intact and ground kernels using 3 instruments (using Si-PbS, Si, and InGaAs detectors), operating over different areas of the spectrum (between 400 and 2500 nm) and employing transmittance, interactance, and reflectance sample presentation strategies. Kernels were assessed on the basis of oil and water content, and with respect to the defect categories of insect damage, rancidity, discoloration, mould growth, germination, and decomposition. Predictive model performance statistics for oil content models were acceptable on all instruments (R2 > 0.98; RMSECV < 2.5%, which is similar to reference analysis error), although that for the instrument employing reflectance optics was inferior to models developed for the instruments employing transmission optics. The spectral positions for calibration coefficients were consistent with absorbance due to the third overtones of CH2 stretching. Calibration models for moisture content in ground samples were acceptable on all instruments (R2 > 0.97; RMSECV < 0.2%), whereas calibration models for intact kernels were relatively poor. Calibration coefficients were more highly weighted around 1360, 740 and 840 nm, consistent with absorbance due to overtones of O-H stretching and combination. Intact kernels with brown centres or rancidity could be discriminated from each other and from sound kernels using principal component analysis. Part kernels affected by insect damage, discoloration, mould growth, germination, and decomposition could be discriminated from sound kernels. However, discrimination among these defect categories was not distinct and could not be validated on an independent set. It is concluded that there is good potential for a low cost Si photodiode array instrument to be employed to identify some quality defects of intact macadamia kernels and to quantify oil and moisture content of kernels in the process laboratory and for oil content in-line. Further work is required to examine the robustness of predictive models across different populations, including growing districts, cultivars and times of harvest.