Decrease In Enhancement Factor Research Articles

Universality of hydroxyl radical generation in irradiated nanoenhancer suspensions: Vital role of concentration

Concentration plays an essential role in generating hydroxyl radicals in irradiated nanoenhancer suspensions. In this paper, we used coumarin-3-carboxylic acid as a hydroxyl radical-specific probe to investigate the hydroxyl radical production of different concentration nanodiamonds (NDs) and CeO2 NPs in phosphate-buffered saline under x-ray irradiation. NDs significantly enhanced hydroxyl radical production, and the maximum enhancement of hydroxyl radical production was observed at a concentration of 10 µg/ml, with an enhanced factor of 1.398 ± 0.262. CeO2 NPs can increase and scavenge hydroxyl radicals at different concentration ranges, with the lowest and highest enhanced factors of 0.623 ± 0.069 and 1.738 ± 0.264, respectively. We tested the hydrodynamic diameter at various concentrations to explore the concentration effect further. We found that with increasing concentration, there might be factors, such as hydroxyl radical recombination and nanoparticle agglomeration, that lead to changes in the enhancement factor. Based on the data from previous and present studies, the experimental results indicate that the concentration factor is essential for hydroxyl radical generation in nanoenhancer suspensions under ionizing radiation. We also provide possible mechanisms for enhancing hydroxyl radical production by nanoenhancers in water under ionizing radiation and the decrease in enhancement factor at high concentrations of nanoenhancers.

Local Characterization of Field Emission Properties of Graphene Flowers

AbstractAn experimental study about field emission properties of commercially available graphene flowers cloth is reported. Material characterization by means of X‐ray diffraction, Raman spectroscopy, and X‐ray photoemission spectroscopy confirms the formation of high quality vertical few‐layers graphene nanosheets. A tip‐anode setup is exploited in which nanomanipulated tungsten tip is used as the anode at controlled distance from the emitter in order to reduce the effective emitting area below 1 µm2, giving access to local characterization. A turn‐on field as low as 0.07 V nm−1 and field enhancement factor up to 32 for very small cathode–anode separation distances is demonstrated, in the range 400–700 nm. It is also shown that the turn‐on field increases for increasing distances, while the field enhancement factor decreases. Finally, time stability of the field emission current is reported, evidencing a reduction of the fluctuations for lower current levels.

CO2 chemical absorption into AMP aqueous solution and mass transfer intensification in cascade sudden expansion microchannels

Carbon dioxide chemical absorption into 2-amino-2-methyl-1-propanol (AMP) aqueous solution and mass transfer enhancement in cascade sudden expansion microchannels were investigated by a high-speed digital camera. The influences of the flow rates of gas and liquid phases, the sudden expansion units and the concentration of AMP on the gas-liquid two-phase flow and mass transfer performance are studied systematically. Results showed that the increase of sudden expansion unit is conductive to increase liquid side volumetric mass transfer coefficient and enhancement factor. The maximal enhancement factor could reach to 1.33, and the maximally effective mass transfer efficiency could reach 1.6 under taking the energy consumption into account. The gas phase flow rate has slight effect on mass transfer enhancement factor E, while E decreases with increasing liquid phase flow rate. With the increase of AMP concentration, the volumetric mass transfer coefficient increases, but the enhancement factor decreases. A new correlation with good prediction performance was proposed for the volumetric mass transfer coefficient. Furthermore, the energy consumption and effective mass transfer efficiency were assessed.

Investigation of changes in field electron emission characteristics of industrial fine-grained graphite when operated in an argon atmosphere up to 10–2 Pa

Studying of field electron emission properties of carbon cathodes operating under technical vacuum conditions is a promising scientific field. Massive cathode made of commercial fine-grained graphite of MG (Russian abbreviation) grade is being investigated. Experiments on obtaining current-voltage characteristics and long-term testing are being carried out. The emitter made of fine-grained graphite demonstrates good emission properties under technical vacuum conditions. Carbon cathode is capable of operating at pressures up to 2×10–2 Pa. Increased pressure in the vacuum chamber leads to deterioration of cathode emission properties. Electric field enhancement factors were calculated for all stages of studies. Analysis of experimental data demonstrates decrease in enhancement factor due to ion bombardment of cathode surface during exploitation. This results in higher electric field for operation of investigated graphite cold cathodes.

Performance and Shelf-Life of Electrospun Ag-doped WO3 Nanofiber-Based SERS Substrates

Recent advances in Ag-doped WO3 nanofibers have provided an opportunity to fabricate high quality and low-cost surface-enhanced Raman scattering (SERS) substrates. This work studied the performance and shelf-life of Ag-doped WO3 nanofiber-based substrates. Methylene blue (MB) molecules were used as analytical references. The prepared substrates were stored in ambient air at room temperature for the duration of 12 months. The Raman enhancement factor (EF) of freshly prepared sample was calculated to be 6.2 × 105. In the first 6 months, a small decrease in EF was observed. The EF was found to significantly decrease after samples were stored for 12 months. After 12 months, the relative standard deviations (RSD) of Raman signals changed from 10.35% to 50.29%, indicating non-uniform Raman enhancements at different locations of the sample. These suggested agglomerations of Ag nanoparticles, which deteriorated the Raman enhancement effect, after samples were stored for a prolonged period.

Effect of electric field uniformity on positive streamer and breakdown characteristics of transformer liquids

Streamer development in liquid has different dominant mechanisms under uniform and divergent electric fields, respectively initiation and propagation. This paper presents a study on the influence of electric field uniformity on the pre-breakdown and breakdown characteristics of transformer insulating liquids. Streamers were investigated experimentally in different needle-plane non-uniform field and plane-needle-plane semi-uniform field configurations under standard positive lightning impulses. For the same gap distance, streamers under non-uniform field propagate with a more side-branched structure whereas streamer propagation under semi-uniform field is more constrained to the axial direction. Field enhancement factor was used to indicate the degree of inhomogeneity of the field configuration. Streamers in fields with higher field enhancement factors occupy larger areas. As field enhancement factor reduces, the streamer withstanding probability distribution gets closer to the breakdown probability distribution and hence the breakdown happens at a lower normalized voltage level. Furthermore, a transition phase from a propagation-dominated to an initiation-dominated process of streamer development was evident as the field enhancement factor decreases. The field enhancement factor at which this transition happens was quantitatively derived. It is 54.1 for the investigated mineral oil under positive lightning impulses.

The effect of an inhibitor on the torrefaction of pellets inside a quiescent layer of bentonite clay

Biomass fuel pellets are an important product for the energy sector. The disadvantages of biomass as a sustainable alternative fuel when compared with coal and other fossil fuels are mainly attributed to its low energy density. To improve the biomass properties the torrefaction process can be used. This study aims at investigating the new technique of pellets torrefaction inside a quiescent layer of bentonite under the atmospheric conditions. This process of torrefaction is carried out without the use of inert gas that makes it attractive from the economic and ecological points of view. The main purpose of bentonite in this technique is to limit oxygen access to the biomass from the environment since oxygen excess leads to the decrease in the energy yield. It is also proposed to use the salts of alkali metals as an inhibitor of the oxidation reactions. In this work, the effect of sodium hydrocarbonate addition on the mass and energy yields of biomass is analyzed at varying torrefaction temperature, duration and height of the mineral layer. Pyrolytic behavior of torrefied biomass is analyzed with the use of thermogravimetry and differential scanning calorimetry measurements. It is established that the mass and energy yields occur to be higher in case of torrefaction in the bentonite-soda blend than in case of torrefaction in pure bentonit. Furthermore, the use of bentonite-soda blend favors the decrease in enhancement factor of HHV. It is shown that energyyield≈1.15⋅massyield.

Enhancement of upconversion luminescence of Er and Yb co-doped Y2O3 nanoparticle by Ag half-shell

Upconversion photoluminescence (PL) properties of single Y2O3 nanoparticles doped with Yb and Er (Y2O3:Yb,Er) with a Ag over-layer is studied. We traced the PL and light scattering images of individual nanoparticles by changing the thickness of a Ag over-layer. When the Ag thickness is relatively small and only the upper part of a nanoparticle is covered by Ag (Ag half-shell), the PL is strongly enhanced. On the other hand, when the Ag thickness is increased and a continuous Ag over layer is formed, the enhancement factor decreases. From the correlation between the enhancement factors of the upconversion PL and scattering intensities as well as the change of the PL lifetime, the mechanism of the PL enhancement is discussed.

Effect of outgassing on the field emission property of tetrapod ZnO

AbstractTo study the mechanism of current degradation of a cold cathode, the outgassing measurement of a field emission diode with tetrapod zinc oxide (ZnO) cathode has been taken with a quadrupole mass spectrometer (QMS) in an ultra‐high vacuum (UHV) system. It was found that the H2, CO2, CO, and CH4 were outgassed from the diode during the working process. The partial pressure of outgassing increases with the increased emission current density. The field emission properties of the cold cathode, such as turn‐on field, threshold field and the emission current density, were largely related to the pressure of the outgases. From the outgassing experimental results, the relation between the emission current degradation and the outgassing of field emission devices has been found. Due to the outgassing, the work function of ZnO cold cathode increases, while the field enhancement factor decreases. Therefore, the outgassing plays very important role in the current degradation for the field emission devices (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Metal-enhanced fluorescence based calcium detection: Greater than 100-fold increase in signal/noise using Fluo-3 or Fluo-4 and silver nanostructures

In this paper we describe metal-enhanced fluorescence (MEF) of the Ca 2+ indicators: Fluo-3 and Fluo-4 in close proximity to silver nano-particles, SiFs. When the concentration of Ca 2+ increases, the MEF enhancement factor decreases, which is consistent with our previous findings that the greatest fluorescence enhancements occur for fluorophores with the lowest free-space quantum yield. Over 100-fold enhanced fluorescence intensities (signal/noise) were observed. In addition, the photostability of the fluorophore (Fluo-3) is much more prolonged from the SIFs as compared to glass (a control sample). Our findings strongly suggest the widespread use and the general approach of nanoparticulate SiFs surfaces for the enhanced detection of both intra and extra-cellular calcium and indeed other analytes of interest.

Magnetic Field Dependence of Photo-CIDNP MAS NMR on Photosynthetic Reaction Centers ofRhodobacter sphaeroidesWT

Photochemically induced dynamic nuclear polarization (photo-CIDNP) is observed in frozen and quinone depleted photosynthetic reaction centers of the purple bacteria Rhodobacter sphaeroides wild type (WT) by (13)C solid-state NMR at three different magnetic fields. All light-induced signals appear to be emissive at all three fields. At 4.7 T (200 MHz proton frequency), the strongest enhancement of NMR signals is observed, which is more than 10 000 above the Boltzmann polarization. At higher fields, the enhancement factor decreases. At 17.6 T, the enhancement factor is about 60. The field dependence of the enhancement appears to be the same for all nuclei. The observed field dependence is in line with simulations that assume two competing mechanisms of polarization transfer from electrons to nuclei, three-spin mixing (TSM) and differential decay (DD). These simulations indicate a ratio of the electron spin density on the special pair cofactors is 3:2 in favor of the L-BChl during the radical cation state. The good agreement of simulations with the experiments raises expectations that artificial solid reaction centers can be tuned to show photo-CIDNP in the near future.

On the modifications of oscillator strengths and damping constants of Fe I spectral lines from 500 nm to 510 nm

We modified oscillator strengths and enhancement factors of van der Waals damping constants for 84 moderately strong and weak neutral iron spectral lines between 500 nm and 510 nm, by fitting the solar synthetic spectrum to the observed one. We have found significant difference between the oscillator strengths and damping constants taken from an extensive spectral line list, frequently used for spectral synthesis, and their modified values. Our findings include: (1) the mean value of the distribution of the difference between our and listed oscillator strengths is -0.31dex and its width is 0.42 dex, (2) a decrease of this difference with increasing equivalent width of spectral lines, (3) unusually high values of the obtained enhancement factors in comparison with the results of other authors, (4) a decrease of enhancement factor with increasing excitation potential for lines with equivalent width greater than 7.5 pm, (5) a decrease of enhancement factors with increasing equivalent width and (6) a decrease of enhancement factors with increasing oscillator strengths.

Self-Consistent Second Order Perturbation Theory for the Hubbard Model in Two Dimensions

We apply self-consistent second order perturbation theory (SCSOPT) with respect to the on-site repulsive interaction U to study the Hubbard model in two dimensions. We investigate single particle properties of the model over the entire doping range at zero temperature. It is shown that as doping decreases toward half-filling ω-mass enhancement factor increases, while k -mass enhancement factor decreases. The increase in ω-mass enhancement factor is larger than the decrease in k -mass enhancement factor, so that total-mass is larger than that in the non-interacting case. When particle number density per unit cell n is given by 0.64< n <1.0 interaction enhances anisotropy of the Fermi surface, whereas at lower densities n <0.64 interaction suppresses anisotropy of it. Due to the decrease in k -mass enhancement factor the density of states (DOS) at the Fermi level is suppressed. It is possible to understand the results within the framework of the weak coupling Fermi liquid theory.

Experimental investigation of particulate formation in He-SiH4 modulated RF discharges

Formation processes of particulates in radiofrequency power-modulated SiH4 discharges have been studied using two kinds of laser light scattering, a modified Langmuir probe and absorption methods. The results suggest that particulates are formed by three phases of nucleation, rapid growth and growth saturation. In the nucleation phase, the formation may be caused by short-lifetime radicals such as SiH2, SiH or Si. In the rapid growth phase, SiHn+ (n=0-3) ions and/or SiH3 radicals seem to make an important contribution to particulate growth, if only direct influx of the particles onto a particulate is taken into account. In the growth saturation phase, the saturation may be explained by taking into account the decrease in enhancement factor of the ion-collecting areas of particulates and the increase in their loss due to the reduction of ambipolar field in the periphery of the discharge column. Power modulation in SiH4 radiofrequency discharges is found to be practically very effective for decreasing the sizes of particulates.

An experimental study of the 180° backscattering yield enhancement

A large enhancement in the yields of MeV 4He ions backscattered near 180° from amorphous and polycrystalline materials was reported by Pronko et al. and subsequently interpreted in terms of correlated scattering between the incoming and outgoing trajectories. We have developed a simple magnetic deflection system which detects all ions backscattered within a small (∼0.04°) angular cone centered at exactly 180° to the incident beam direction. Using this system, we have investigated the enhancement in scattering yield from several thick amorphous (or polycrystalline) targets and also from Bi atoms implanted at various energies (i.e. depths) into C and Si targets. The maximum in the observed enhancement factor is almost independent of target material, provided the 180° collision involves a heavy atom for which the kinetic backscattered energy factor is close to unity. In the Bi-implanted targets, the decrease in enhancement factor at larger depths is observed to scale well with x/ E 0 and is essentially independent of the host.