O2 Mixture Research Articles

Burnup analysis for HTR-10 reactor core loaded with uranium and thorium oxide

Abstract We used MCNP6 computer code to model HTR-10 core reactor. We used two types of fuel; UO2 and (Th+Pu)O2 mixture. We determined the critical height at which the reactor approached criticality in both two cases. The neutronic and burnup parameters were investigated. The results indicated that the core fueled with mixed (Th+Pu)O2, achieved about 24% higher fuel cycle length than the UO2 case. It also enhanced safeguard security by burning Pu isotopes. The results were compared with previously published papers and good agreements were found.

Localized oxidation of Zr-2.5Nb alloy in high temperature carbon dioxide environment containing hydrochloric acid

Zr-2.5Nb pressure tube alloy with uniform oxide layer was shown to undergo extensive nodular corrosion when exposed to a gas mixture of CO2 and O2 at high temperature in presence of hydrochloric acid. Substantial hydrogen pick-up with radial hydrides at the nodule base was observed. A bare metal surface, however, did not undergo nodular corrosion. A thicker initial oxide was less prone to nodular corrosion. A global mechanism was proposed which postulates that a site with localized damage (chemical/mechanical) of the uniform oxide layer, yet surrounded by the remaining oxide, tends to oxidize at an enhanced rate resulting in nodules.

Synthesis and in situ nitrogen doping of ZnO nanomaterials using a microwave plasma system at atmospheric pressure

A microwave plasma system operated at atmospheric pressure was utilized to not only synthesize ZnO nanostructures from a micro-sized Zn powder, but also dope with nitrogen simultaneously by changing the ratio of O2 and N2 gas mixture. When pure N2 gas was used, ZnO was not produced due to oxygen deficiency. When the ratio of O2 to N2 was 10:90, 20:80, and 50:50 (vol.%), tetrapod-shaped ZnO rods with a diameter of 29.8 and a length of 256.5 nm were synthesized. When only oxygen was used, ZnO nanorods with a diameter of 626.5 and a length of 852.6 nm were primarily synthesized. Furthermore, it was confirmed from Raman and X-ray photoelectron spectroscope (XPS) analyses that the doping concentration of nitrogen increased with the increase of N2 gas proportion to 50, 80, and 90 vol.%. The increase of the absorbance and the luminescence in the visible light region were confirmed by the change of the energy level due to the N-doping on ZnO. The most effective gas mixture ratio of O2 to N2 for synthesis of ZnO nanomaterials and in situ N-doping was confirmed to be 20:80 (vol.%). This work demonstrates the effectiveness of a microwave plasma system at atmospheric pressure which is available for synthesis of ZnO nanomaterials and simultaneous functional doping on it.

Inelastic Collisions of O2 with He at Low Temperatures.

Rotationally inelastic collisions of O2 with He in the 10-34 K thermal range are investigated by means of an experimental procedure based on supersonic gas jets probed by Raman spectroscopy. The procedure employs a kinetic master equation (MEQ) that describes the time evolution of the rotational populations of O2 along supersonic jets of O2 and He mixtures. The MEQ is expressed in terms of experimental quantities (number density and rotational populations) measured here, and state-to-state rate coefficients for the O2:He inelastic collisions calculated here, plus those for O2:O2 collisions from the literature. An agreement with the experiments is accomplished for temperatures between 10 and 34 K. Within this thermal range, the role of the fine structure due to electron spin in the collision dynamics of O2 is discussed.

Characterization of Gaseous Plasma Sustained in Mixtures of HMDSO and O2 in an Industrial-Scale Reactor

Optical emission spectroscopy and mass spectrometry was used to characterize gaseous plasma in an industrial reactor of volume 5 m3 during deposition of protective coatings. Plasma was created in mixtures of hexamethyldisiloxane (HMDSO) and oxygen at the powers between 1 and 8 kW. The plasma density was somehow below 1014 m−3. The flows of both gases were varied up to 200 sccm while the effective pumping speed was adjusted by changing the roots pump rotation speed between 250 and 4000 rpm. At such conditions the HMDSO was only partially dissociated to fragments. The behaviour of optical emission lines and mass ion currents was well correlated indicating that even one single technique was sufficient to monitor the behaviour of plasma at various discharge conditions. The optical emission spectroscopy as a simple and economic method is therefore suitable for controlling key processing parameters in such a plasma reactor.

Analysis of Mass Transport Phenomena in PEMFC Cathode Electrode: Effects of Operating Conditions

Electrochemical conversion systems that use air as the oxidant, like proton exchange membrane fuel cells (PEMFCs), direct liquid fuel cells and metal/air batteries, strongly depend on mass transport of reagents for power production. To meet the power density, reliability and cost requirements that promote further commercialization of fuel cell vehicles, a sufficient understanding of properties-performance correlations is strictly desired. In particular, the dependence of fuel cell performance and mass transport parameters on operating conditions and structural properties of the membrane electrode assembly (MEA) should be better understood. Previously we developed a method for determining the oxygen mass transfer coefficient which is based on measurements of limiting current distributions using O2 mixtures with different diluents (from He to C3H8) [1]. The method allows us to separate gas phase molecular diffusion (km, N2 ) and a combination of Knudsen diffusion and ionomer/water films (kK+film ). In this work we studied effects of operating conditions and gas diffusion layer (GDL) properties on oxygen mass transport and PEMFC performance to gain a better understanding of the transport phenomena in fuel cell cathodes and to further develop the aforementioned method. A segmented cell and data acquisition system were operated with commercially available 100 cm2 MEAs with anode and cathode loadings of 0.1 and 0.2 mgPt cm-2 and thicknesses of 3-4 and 6-7 μm, respectively. Cathode GDLs were also varied to elucidate the impacts of the microporous layer (MPL) and catalyst layer. 25BA (no MPL) and 25BC (with MPL) diffusion medias from Sigracet SGL were used for the cathode electrode. Thus, operation of MEA-25BA provided us with results on mass transport predominantly in the catalyst layer, since the GDL does not include MPL. The experiments were performed at two operating temperatures: 60 and 80°C, varied relative gas humidities and back pressures. Details of experimental procedures and the determination of the mass transfer coefficient are presented in [1]. Fig. 1 presents results illustrating the impact of reagent humidification on the mass transfer coefficients and PEMFC performance. The oxygen mass transfer coefficients (kK+film and km, N2 ) determined for MEA-25BA are higher than for MEA-25BC at all studied humidification values and temperatures, most likely due to the effect of MPL. kK+film increases with RH and temperature, however, temperature does not have a significant influence. The oxygen mass transfer coefficient in the gas phase (N2) was found to grow when RH increased from 32 to 50% at 60°C, but further increases in gas humidification did not affect its value (Fig. 1 a). At the same time, operation at 80°C demonstrated an increase in km, N2 with humidification (Fig. 1 b). Behavior of mass transfer coefficient arising from the MPL (kK, MPL ) showed a similar development at both temperatures: a decrease in oxygen transport with RH. The observed trends in mass transfer coefficients with RH and temperature can be attributed to the gas permeability of Nafion and its dependence on humidification, which is more critical at low RH operation [2]. Effects of high gas humidification are associated with liquid water formation and its removal through the evaporation mechanism as well as though liquid-phase motion due to capillary effects of the MPL [3]. Apparently, the effects of water seem to be more pronounced at 60°C, where mass transfer coefficients in the gas phase and in the MPL decrease with RH. Higher operating temperature appears to be beneficial for oxygen transport in GDE and results in greater values of the mass transfer coefficients in the gas phase. At the same time variation of back pressure allowed us to determine the pressure-independent component of oxygen transport and further separate contributions to the oxygen transport in the cathode electrode structure [4]. Detailed analysis of the PEMFC performance, voltage losses and oxygen mass transfer coefficients at varied operating conditions will be presented and discussed. ACKNOWLEDGEMENTS We gratefully acknowledge funding from ONR (N00014-18-1-2127) and ARO (W911NF-15-1-0188). The authors are thankful to Hawaiian Electric Company, G. Randolf and J. Huizingh for support of the operation as well as T. Carvalho for assisting with SEM. REFERENCES T. V. Reshetenko, J. St-Pierre, J. Electrochem. Soc., 161, F1089-F1100 (2014).K. Broka, P. Ekdunge, J. Appl. Electrochem., 27, 117-123 (1997).J.H. Nam, M. Kaviany, Int. J. Heat Mass Transfer, 46, 4595-4611 (2003).D.R. Baker, D.A. Caulk, K.C. Neyerlin, M.W. Murphy, J. Electrochem. Soc., 156, B991-B1003 (2009). Figure 1

P‐13.12: The Lifetime and Yield Improvement by Plasma Treatment to Anode Surface in Active Matrix Organic Light Emitting Diode Display

The active matrix organic light emitting diode (AMOLED) display industry has been undergoing rapid development and increasing competition as heavy investment enters the industry continuously. The performance and yield of one company’s products, closely related to OLED device fabrication process, are the critical factors to its core competence [1]. Here, the lifetime and Mura defects could be improved simultaneously by introducing plasma treatment to anode surface of array driver circuit were proved. The lifetime was increased at least 50% and the Mura defects caused by residue on anode surface were decreased at least 75%. Also, the effects of different kinds of plasma gas, pure O2 and mixture of SF6 and O2 were compared and the mechanism was discussed. Our work demonstrated that the plasma treatment to anode surface before organic materials evaporated was beneficial and necessary to enhance the product performance of AMOLED display.

Investigating the Promise of Incorporating a Redox Mediator in Li–O2/CO2 batteries

In recent years, continuous studies have highlighted the languished research state of Li–O2, and the impelling development of energy storage devices with analogously high theoretical energy density [1]. In preliminary studies, we established that fed O2 and CO2 mixtures (Li–O2/CO­2) swiftly shift the reaction mechanism toward an exclusive formation of Li2CO3 in a TEGDME-based electrolyte. The proposed O2-assisted pathway is initiated by an electron transfer from the cathode, with subsequent CO4 – formation and complete Li2O2 suppression. Compared with O2-driven counterparts, the CO2-based Li–air cell suffers, however, from higher overcharges during the decomposition of Li2CO3 at ~4.5 V, rendering impracticable a commercialization of these technologies [2]. Here we propose for the first time the incorporation of a redox mediator (RM) toward facile Li2CO3 oxidation in Li–O2/CO­2 devices. Lithium bromide (LiBr) possessing two redox couples (Br−/Br3 − and Br3 −/Br2) has gathered increasing attention in conventional Li–air cells and was considered a suitable candidate in this study. All galvanostatic measurements were carried out under 30% CO2. At optimized concentrations, hindered recharge potentials (~0.5 V) at similarly sustained cyclability were demonstrated. Further dissection of the significance of involved Br−/Br3 −/Br2 species inferred, however, an additional role of the RM in the stabilization of intermediate discharge species and in the undesirable formation of side-products. In this context, drawn conclusions reinforced the need to consider a deeper evaluation of the effects of a RM inclusion beyond an overcharge decrease and the urge to stabilize RM/RM+ species participating in deleterious side-reactions with neighboring species and cell components.

Effects of Working Medium Gases on Emission Spectral and Temperature Characteristics of a Plasma Igniter

Ar, N2, O2, and N2-O2 mixture under different O2 components were separately used as the working medium gas of a plasma igniter to study the influence of working-medium-gas type and O2 concentration on emission spectral and temperature characteristics of a direct-current arc plasma igniter. The emission spectra of the plasma jet were also analyzed using the optical emission spectroscopy method, and the influence rule of working medium gas and O2 concentration on electron temperature and vibrational temperature of the plasma jet was calculated and analyzed. The experimental results show that the emission spectra of the plasma jet had significant differences when diverse working gases were used as the working medium of the igniter. With increasing O2 concentration in the working medium gas, the spectral line intensity of oxygen-containing particles in emission spectra of the plasma jet was significantly enhanced, and the spectral intensities of nitrogen-containing particles, NO molecular bands, the second positive system of N2C3∏u−B3∏g, and the first negative system of N2+B2∑u+−X2∑u+ were also enhanced obviously. Both electron and vibrational temperature of the plasma jet increased gradually with increasing O2 concentration in the working medium gas, and first increased and then decreased with increasing axial distance from the plasma jet.

Homologous substrate-temperature dependence of structure and properties of TiO2, ZrO2, and HfO2 thin films deposited by reactive sputtering

Group 4 metal oxides TiO2, ZrO2, and HfO2 were deposited on glass substrates at substrate temperatures ranging from 100 to 800 °C by reactive direct current magnetron sputtering using an Ar and O2 mixture as discharge gas. On the basis of the obtained cross-sectional and surface morphologies, crystallographic structures, and film properties, the homologous substrate-temperature dependence of the film structure and properties for the sputter-deposited metal oxide thin films are discussed. The x-ray diffraction measurements show the diffraction patterns characteristic of the anatase (tetragonal) structure at substrate temperatures below 400 °C and those of the rutile (tetragonal) structure at substrate temperatures above 600 °C for TiO2 thin films and the patterns attributable to the monoclinic structure with 〈−111〉 orientation for ZrO2 and HfO2 thin films. Scanning electron microscopy (SEM) observations show voided columnar structures with rough surfaces for TiO2 thin films and fine columnar structures with smooth surfaces for ZrO2 and HfO2 thin films, which is supported by the results obtained from atomic force microscopy (AFM) analysis. The quantitative data on the lattice strain, crystallite size, surface roughness, and refractive index are plotted against the homologous substrate temperature, Tsub/Tm (Tsub: substrate temperature and Tm: melting point of thin film materials). The lattice strain reaches approximately zero at Tsub/Tm = 0.3–0.35, suggesting that the residual stress is relaxed at this point, and the crystallite size reaches a plateau at the same range of Tsub/Tm. The surface roughness increases sharply after passing Tsub/Tm = 0.25–0.30, and the refractive index shows a rise at Tsub/Tm = 0.30–0.35. The images obtained by SEM and AFM are categorized based on the property transition observed in the quantitative property changes. The data obtained in this work are utilized to systematically study the effectiveness and appropriateness of the homologous (normalized) substrate temperature to explain the changes in structure, morphology, and properties of oxide thin films. Furthermore, it is suggested that the structure zone model, which is normally applied to sputtered metal films, is also applicable to sputtered oxide thin films.

Experimental study on the mechanism of enhanced oil recovery by air-assisted cyclic steam stimulation process in extra-heavy oil reservoirs

ABSTRACTWith the increasing demand of energy, extra-heavy oil has attracted more and more attention at present. Cyclic steam stimulation (CSS) process is one of the main methods to produce extra-heavy oil, however with the increase of stimulation cycle, the periodic oil production of well decreases obviously and the water cut increases significantly. As non-condensable gas, surfactant, high-temperature foam, horizontal well technology, and other assisted CSS methods have been applied in heavy oil reservoirs, it indicates that CSS process has been developing toward diversification. Air-assisted cyclic steam stimulation (AACSS) process has become a more and more popular technique for developing extra-heavy oil due to its abundant sources and low cost. In this paper, static oxidative experiments were carried out to study the mechanism of low-temperature oxidation (LTO) reaction of extra-heavy oil and the effect of temperature, pressure, and catalyst on the LTO reaction. In addition, four groups of CSS experiments were conducted to compare the development performance of different stimulation methods, such as conventional CSS, first-injection AACSS, simultaneous-injection AACSS, and later-injection AACSS. The results show that the system pressure and O2 content decreased, but carbon oxides increased after LTO reaction. The residue O2 content decreased from 21% to below 12% under the safety risk of explosion caused by the mixture of O2 and hydrocarbon gases. During LTO process, both cracking reaction and polymerization reaction of molecule occurred in the extra-heavy oil, which increased the content of saturates and aromatics, while decreased the content of heavy components. LTO reaction also increased the oil viscosity because of the increase of asphaltene content. But pyrolysis reaction would upgrade the extra-heavy oil and decrease the asphaltene content of extra-heavy oil under high-temperature condition. Besides, the higher the reaction temperature and pressure, the faster the reaction and the lower the O2 content. Moreover, the catalyst could enhance the LTO reaction and O2 consumption. Based on the larger scale sand pack experiments, the results indicate that AACSS could effectively improve the oil recovery compared with conventional CSS, and air injection after injecting steam during each cycle could achieve the highest recovery.

Filamentary nanosecond surface dielectric barrier discharge. Experimental comparison of the streamer-to-filament transition for positive and negative polarities

Streamer-to-filament transition is a general feature of high pressure high voltage nanosecond surface dielectric barrier discharges (nSDBDs) for mixtures containing molecular gases. The transition is observed at high pressures and voltages in a single-shot experiment a few nanoseconds after the start of the discharge. A set of experimental results comparing streamer-to-filament transition and properties of plasma in the filaments for the identical high voltage pulses of negative and positive polarity is presented. The transition curves in voltage–pressure coordinates are obtained for N2:O2 mixtures with different content of molecular oxygen, from 0% to 20%, at the pressure range 1–12 bar. Continuous optical spectra are compared for both polarities in 6 bar synthetic air. Electron density is calculated from Stark broadening of Hα line at λ = 656.3 nm in the discharge and in early afterglow, 40 nanoseconds after the end of the high voltage pulse. Hydrodynamic perturbations are measured using schlieren imaging in 1–6 bar air for streamer and filamentary mode for both polarities. The review of common and distinctive features of the filamentary single-shot nSDBD for two polarities of the applied pulse is provided.

Detonation limits in methane-hydrogen-oxygen mixtures: Dominant effect of induction length

New-concept detonation combustors, for example, rotating detonation engines (RDEs), motivate investigations of the phenomena and theory relevant to detonation limits. Because CH4–H2 binary fuel mixtures have excellent combustion performance in engines, it is interesting to investigate the application possibilities of such mixtures in advanced detonation engines. Therefore, the detonation characteristics (e.g., initiation, propagation and failure) and their mechanisms for methane-hydrogen mixtures under different thermodynamic conditions need further study. In this work, the physical connection between detonation limits and induction length (ΔI) of detonation structures for CH4–H2–O2 mixtures is investigated; the dominant effect of induction length on the detonation limits is examined. The results show that a single-headed spinning structure is a unique feature of detonation limits; hence, the detonation limits can be qualitatively estimated by this phenomenon. The relation between λ and ΔI is proportional, and the proportionality factor is 34.62; this relation is applicable in methane/hydrogen and oxygen mixtures with various fuel contents. By scaling the critical pressure pc with ΔI near the detonation limits, the relation between them is shown to be an exponential function: ΔI = 201.2·(1+pc)−2.1.

Kinetics of the topochemical reaction of the solid solutions of magnesia spinels: Mg(Cr0.5Fe0.5)2O4, Mg(Al0.5Cr0.5)2O4, Mg(Al0.5Fe0.5)2O4 with sulphur oxides

Spinel-containing materials belong to an important group of refractories used as high-temperature unit linings. A crucial element of the conditions in which they are used is gaseous corrosion caused by sulphur oxides: SO2 and SO3. In previous investigations into reactions of magnesia spinels with sulphur oxides it was found that spinels’ reactivity could be considerably influenced by a phase transition (order – disorder) in the cation sublattice, resulting in a change of their reactivity in relation to SO2/SO3.The aim of the study was to investigate the kinetics of topochemical reactions between equimolar solid solutions of magnesia spinels and sulphur oxides before and after the order – disorder phase transformation in the structure begins. Research into the reaction of equimolar solid solutions Mg(Cr0.5Fe0.5)2O4, Mg(Al0.5Cr0.5)2O4, Mg(Al0.5Fe0.5)2O4 with SO3 was undertaken due to the fact that spinels form solid solutions in basic refractories. To conduct kinetic measurements, a semi-flow reactor was designed and constructed, in which investigations were carried out at the temperatures of 773 and 973 K and time range: 0–7 h. A mixture of air and SO2 (13%) was used.The obtained results have been compared with the kinetic results from the previous work obtained on two-cation spinels: MgAl2O4, MgFe2O4, MgCr2O4. The influence of the degree of inversion in spinel structure on the kinetics of the process was discussed.

Steady flame streets in a non-premixed microburner

Three-dimensional simulations of a non-premixed microburner (channel height Lz=0.75 mm) are used to study flame structure and stability, for both H2–O2 and CH4–O2 mixtures, matching conditions of previously reported experiments. Thermal quenching and slow diffusive mixing lead to incomplete combustion and, for some flow rates, steady flame streets form in the channel for CH4–O2, matching experimental observations. Still smaller-scale burners, with channel heights Lz=0.375 mm and Lz=0.25 mm, are also simulated, and flame streets are seen even for H2–O2 cases due to the strong thermal quenching. A wall-chemistry model is used to assess the importance of wall quenching of H and O radicals. Wall recombination kinetics weaken the flames, reduce temperature by over 100 K, significantly reduce the length of the flame diffusion tails, and reduce overall combustion completeness. The basic mechanisms observed to be important in the microburner channel are included in an analogous one-dimensional diffusion-flame model, which includes a heat-loss factor motivated by the full burner. For similar conditions to the microburner and high heat loss, the solution oscillates sufficiently strongly to extinguish the flame, as observed in some microburner cases. For more modest thermal quenching, the oscillations persist and are analogous to the stable flame streets seen in the microburner.

Impact of OH Radical Generator Involvement in the Gas‐Phase Radical Reaction Network on the Oxidative Coupling of Methane—A Simulation Study

The impact of OH• generation during the oxidative coupling of methane (OCM) is simulated using state‐of‐the‐art gas‐phase chemistry and a comprehensive chemical kinetic model. The inclusion of the quasi‐equilibrated formation of OH• from a H2O–O2 mixture into the combustion chemistry network enhances the CH4 conversion rate and C2 selectivity, consistent with the previously proposed mechanism involving catalytically generated OH•. The OH‐pathway increases the concentration resulting in an enhanced transformation rate from to C2H6 (second order in ) more than CO (first order in ). Relative to other H‐abstracting radical species, the OH• weakens the sensitivity of the H abstraction rate constant to C—H bond energy, or lowers , which comparatively slows the C2H6 conversion rate relative to CH4, thus enhancing C2 selectivity. Concurrent dehydrogenation of C2H6 to C2H4 maximizes the C2H4 selectivity even after O2 depletion. With the involvement of the OH•‐mediated pathway, this study addresses the effects of temperature and CH4/O2 ratio on the achievable C2 selectivity and C2H4 yield. The maximum C2H4 yield reaches 32% at a CH4/O2 ratio of 3, temperature of 1100–1200 °C, and total pressure of 1 atm.

Effect of reaction conditions on the evolution of surface functional groups in O2/H2O combustion process of demineralized coal char

O2/H2O combustion is now regarded as a novel and promising technology of coal utilization for next-generation oxy-fuel combustion. The combustion reaction mainly occurs in the mixture of O2 and H2O instead of recycled flue gas (mainly CO2). The purpose of this research is to clarify the evolution characteristics of O/N-containing complexes (C(O)/C(N)) during O2/H2O combustion. Fourier transform infrared spectroscopy (FT-IR), temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) were employed in this study. The addition of H2O in the atmosphere promoted the generation of NO and HCN in char combustion. There was a good linear relationship between the C(O) amount and char reactivity, as well as the NO conversion ratio, during the O2/H2O combustion process. Moreover, trace amounts of the precursors of CO2 with low thermal stability remained on the particle surface in the O2/H2O combustion. The enhancement of the reaction temperature revealed positive effects on inhibiting NO emissions in the O2/H2O combustion. The relative amount of pyrrole (N-5), which was the precursor of HCN, increased apparently at the expense of pyridine (N-6) and quaternary nitrogen (N-Q) in the early reaction stage. When the char conversion degree exceeded 50%, N-Q and N-6 became the dominant forms of C(N).

Spatial structure of radio-frequency capacitive discharge plasma with ring-shaped hollow electrode using Ar and O2 mixture gases

The spatial structures of plasma parameters in a radio-frequency capacitive discharge plasma with a ring-shaped hollow electrode operated in argon were measured at various oxygen gas concentrations for plasma processing by moving a cylindrical Langmuir probe. Their distinctive profiles are obtained around the ring-shaped hollow trench. The electron temperature Te at the hollow trench of radial position r = 37.5 mm has a peak value at an O2 gas concentration of 0%, whereas at O2 gas concentrations of 5 and 10% Te decreases everywhere except for positions close to the hollow trench. The plasma potential Vs at an O2 gas concentration of 0% is higher than that at O2 gas concentrations of 5% and 10%. The electron density ne has a peak value at the hollow trench at all O2 gas concentrations and decreases with increasing O2 gas concentration. Their axial profiles indicate unique distributions around the hollow inlet at a radial position of r = 37.5 mm, while at a radial position of r = 0 the profiles are almost uniform. It is found that a double layer is formed from the axial profiles of the plasma potential at r = 37.5 mm for all oxygen gas concentrations.

Effect of Oxygen Mixture Ratio on the Properties of ZnO Thin-Films and n-ZnO/p-Si Heterojunction Diode Prepared by RF Sputtering

ZnO thin-films are grown on a p-Si(111) substrate by RF sputtering. The effects of growth temperature and O2 mixture ratio on the ZnO films are investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and roomtemperature photoluminescence (PL) measurements. All the grown ZnO thin films show a strong preferred orientation along the c-axis, with an intense ultraviolet emission centered at 377 nm. However, when O2 is mixed with the sputtering gas, the half width at half maximum (FWHM) of the XRD peak increases and the deep-level defect-related emission PL band becomes pronounced. In addition, an n-ZnO/p-Si heterojunction diode is fabricated by photolithographic processes and characterized using its current-voltage (I-V) characteristic curve and photoresponsivity. The fabricated n-ZnO/p-Si heterojunction diode exhibits typical rectifying I-V characteristics, with turn-on voltage of about 1.1 V and ideality factor of 1.7. The ratio of current density at ± 3 V of the reverse and forward bias voltage is about 5.8 × 103, which demonstrates the switching performance of the fabricated diode. The photoresponse of the diode under illumination of chopped with 40 Hz white light source shows fast response time and recovery time of 0.5 msec and 0.4 msec, respectively.

Streamer propagation in the atmosphere of Titan and other N2:CH4 mixtures compared to N2:O2 mixtures

Streamers, thin, ionized plasma channels, form the early stages of lightning discharges. Here we approach the study of extraterrestrial lightning by studying the formation and propagation of streamer discharges in various nitrogen-methane and nitrogen-oxygen mixtures with levels of nitrogen from 20% to 98.4%. We present the friction force and breakdown fields Ek in various N2:O2 (Earth-like) and N2:CH4 (Titan-like) mixtures. The strength of the friction force is larger in N2:CH4 mixtures whereas the breakdown field in mixtures with methane is half as large as in mixtures with oxygen. We use a 2.5 dimensional Monte Carlo particle-in-cell code with cylindrical symmetry to simulate the development of electron avalanches from an initial electron-ion patch in ambient electric fields between 1.5Ek and 3Ek. We compare the electron density, the electric field, the front velocities as well as the occurrence of avalanche-to-streamer transition between mixtures with methane and with oxygen. Whereas we observe the formation of streamers in oxygen in all considered cases, we observe streamer inceptions in methane for small percentages of nitrogen or for large electric fields only. For large percentages of nitrogen or for small fields, ionization is not efficient enough to form a streamer channel within the length of the simulation domain. In oxygen, positive and negative streamers move faster for small percentages of nitrogen. In mixtures with methane, electron or streamer fronts move 10–100 times slower than in mixtures with oxygen; the higher the percentage of methane, the faster the fronts move. On Titan with methane percentages between 1.4% and 5%, a successful streamer inception would require a large electric field of 4.2 MV m−1 (3Ek). Such large fields might not be present and explain the non-detection of Titan lightning by the Cassini/Huygens mission.