Effect Of Surface Oxidation Research Articles

The role of the oxygen functional groups in adsorption of copper (II) on carbon surface

The effect of carbon surface oxidation on the adsorption of Cu(II) ions from aqueous solution was studied in order to explain the role of the oxygen functional groups in the binding of copper ions. Pristine carbonaceous adsorbent was oxidized to a various extent of oxygen uptake (Fenton-like oxidation < persulphate in H2SO4 < H2O2 in HNO3). Equilibrium adsorption tests were performed in acetate buffer at pH ≈ 5. The results show that the adsorption capacity of pristine adsorbent is expectable low (~0.1 mmol g−1). The oxidized samples adsorb Cu(II) at a considerably higher level of ~1.4 mmol g−1 despite the degree of surface oxidation. Analysis of the surface groups (FTIR, TPD) and surface charge (zeta potential) of used adsorbents and their Cu(II) saturated counterpart lead to the finding that Cu(II) ions are mostly bonded by complexation with the dissociated carboxylic groups (partly formed by anhydrides hydrolysis) probably in the form of Cu(Ac)+ formed in the acetate buffer. The extent of dissociation is given by equilibrium pH during the adsorption and does not depend on the total amount of the surface groups. Thus, the content of active sites and consequently adsorption capacity is independent on the degree of oxidation when pH is kept constant. The results indicate that even moderate oxidation treatment of carbonaceous materials can produce highly effective adsorbents for Cu(II) immobilization.

The effect of plasma treatment on the surface chemistry and structure of ZnMgAl coatings

A principal investigation on the effect of atmospheric pressure plasma on Zn1.5Mg2.5Al coating was performed by means of X-ray photoelectron- and Auger electron spectroscopy as well as by transmission electron microscopy. The type of the medium used to generate the plasma influenced significantly the impact of the proceeded treatment. Although a mild surface cleaning and oxidation effect was observed for air, N2 or H2O plasma, mixed air + H2O and N2 + H2O plasma types already caused a remarkable altering of the chemical composition, structure and thickness of the coating surface layer, a similar effect to an initial corrosion attack.

The effect of surface oxides and grain sizes on the deuterium permeation behavior of niobium membranes

Group 5 transition metal niobium is considered as the promising hydrogen separation membrane material because of its theoretical highest atomic hydrogen permeability, good mechanical strength, low price and easy shaping. However, the permeability of hydrogen in the dense metal membrane is affected by surface oxides, gain sizes, defects and so on. In this paper, the effect of surface oxides and grain sizes on the permeation behavior of deuterium through niobium membranes is studied. The niobium membranes with micro- and nanocrystalline were first treated by mechanically or electro-polished to remove the surface oxides, and then the deuterium permeability tests were performed in the temperature range of 600–800 ℃ at the driving pressures of 50 kPa by gas-driven permeation method. Results showed that the niobium membrane with nanocrystalline and “clean” surface had the highest deuterium permeation than that with microcrystalline or unclean surface. More importantly, grain size and grain boundary had a greater influence on the deuterium permeation behavior of niobium membranes than the thin surface oxides. These results shed new light on the enhancement of hydrogen and its isotopes permeation in the niobium-based hydrogen separation membranes. Further work is to study the plasma-driven permeation of deuterium through nanocrystalline niobium membrane in order to obtain the super-permeation rate.

Combined effects of nanosecond laser-induced surface oxidation and nanostructure formation for selective colorization of nickel surfaces

Metal surfaces can be nanostructured by laser irradiation making use of different effects ranging from thin film generation to nanostructures formation. Here, the colorization of pure nickel surfaces by nanosecond laser irradiation in air is demonstrated and studied in detail. Correlations between chromaticity coordinates and laser-processing parameter show that the accumulated fluence is the dominating factor in creation of a specific colored surface. The color of the laser-irradiated surfaces shows both angle-dependent and angle-independent in reliance on the processing conditions. The examination of the laser-colored surface demonstrates that lateral and vertical organized, laser-induced structures with nano-, micro-, and mesoscopic scales can be found simultaneously which contribute to the colorization in a particular manner. A comprehensive analysis of the processes involved in the color formation at nickel was performed by examining the surfaces by SEM, reflectometry, XPS, and XRD to verify the multi-process mechanisms of color formation. The most saturated colors result from interference effects within the redeposited layers. It was found that controlling the hatching distance applying optimized laser fluence enables a wider color range and allows a very precise setting of the color. Based on the extracted laser-processing parameters, the surface coloration of arbitrary pattern with desired optical properties becomes practicable, and nanosecond laser color marking can, therefore, be expanded to potentially new applications.

Electrical Properties of Monocrystalline Thin Film Si for Solar Cells Fabricated By Rapid Vapor Deposition with Nano-Surface Controlling Double Layer Porous Si in H2

Introduction To reduce the Si thickness with maintaining the high quality is a promising approach to reduce the cost of monocrystalline Si solar cell. A major method to fabricate monocrystalline thin Si is epitaxy by Chemical Vapor Deposition (CVD) and Layer Transfer Process (LTP) as shown in Fig. 1. A seed layer and a sacrificial layer such as double layer porous Si (DLPS) which consist of a Low Porous Layer (LPL) and a High Porous Layer (HPL) are fabricated on the surface of a monocrystalline Si wafer, and then Si is epitaxially deposited on the seed layer. This wafer can then be reused in LTP, thus further reducing the material cost of these Si cells. There remain two challenging issues: (ⅰ) crystal defect introduced during epitaxy caused by the roughness of the seed layer1) and (ⅱ) low deposition rate and yield of epitaxy by CVD. To solve problem (ⅰ), we proposed a Zone Heating Recrystallization (ZHR) method2) to smoothen the DLPS surface as shown in Fig.2. The structure of the DLPS surface can be modified by using an upper lamp heater to scan the surface in one direction and a bottom heater to pre-heat Si substrate. To solve problem (ⅱ), we proposed a Rapid Vapor Deposition (RVD) method3) as shown in Fig.3. By depositing Si under a high vapor pressure by heating the source Si to over 2000℃, the deposition rate of over 10 μm/min with a higher yield is achieved. By applying both the ZHR and RVD methods, we successfully reduced the roughness of a DLPS surface and obtained monocrystalline Si with Si wafer level. The critical effect of lowering the roughness of a DLPS surface to Rms < 0.3 nm was confirmed1)4). In this work, to fabricate high quality solar cells, we investigated the key factors in improving the roughness of a DLPS surface and evaluated the semiconductor properties of monocrystalline thin film Si. Experiment DLPS was prepared by 2-step anodization of p-type monocrystalline Si (100) wafers (≦0.005 Ω・cm) with 2 mA/cm2 and 415 s for LPL and 200 mA/cm2 and 5 s for HPL, respectively. The prepared DLPS were thermally treated by ZHR with adjusting the heat levels of both the upper and bottom heater and the scan rate of the upper heater under H2(0～5%)/N2 atmosphere to investigate the effect of surface oxidation. RVD were carried out with using p-type monocrystalline Si (100) (1-10 Ω・cm) wafers as the source. The thickness of Si was required to be >40 μm to obtain the sufficient light absorbance, but in our RVD apparatus, only <10 µm-thick Si can be fabricated in one deposition due to the structure of the source boats, so Si was epitaxially deposited on DLPS by RVD in 5 time depositions and the film (48 μm-thick) (Sample-1) was obtained. To increase the deposition rate, the tungsten boat to put Si was modified and the film with 35 μm-thick was obtained in one time deposition (Sample-2). The roughness of the DLPS surface were analyzed using atomic force microscopy. To measure the mobility and the carrier concentration, Si was epitaxially deposited directly on non-doped Si (100) (≧1000 Ω・cm) wafer and the hall effect measurement was carried out. The lifetime of obtained Si films were measured after passivated with Al2O3 film on both sides by atomic layer deposition. Results and discussion The minimum roughness of DLPS surface and the heating amount which minimize the roughness of DLPS surface was decreased by changing the atmosphere from N2 to H2/N2. It is supposed that the structural change occurs easier by inhibiting the effect of oxidation, possibly due to the presence of H2. The life time of Sample-2 was much higher than that of Sample-1. In the multiple RVD process, a native oxide film seemed to be formed on the surface of a Si film during re-setting sources in air atmosphere. Thus, a thin Si with almost wafer-level quality was obtained by increase the thickness by RVD in one deposition. Conclusion The importance to inhibit the effect of oxidation during ZHR was clarified. Nearly wafer-level, 35 µm-thick Si was obtained by RVD in one time deposition by increasing deposition rate. Acknowledgements The part of this research was supported by KAKENHI (KIBAN A) and Center for Low Carbon Society Strategy (LCS), JST. 1)K.Hasegawa,C.Takazawa,M.Fujita,S.Noda and M.Ihara,CrystEngComm,20(13),1774-1778(2018). 2)A.Lukianov,K.Murakami,C.Takazawa and M.Ihara,Appl.Phys.Lett.,108(213904),1-4(2016). 3)Y.Yamasaki,K.Hasegawa,T.Osawa and S.Noda,CrystEngComm,18,3404-3410(2016). 4)C.Takazawa,M.Fujita,K.Hasegawa,A.Lukianov,X.Zhang,S.Noda and M.Ihara,ECS Transaction,75(31),11-23(2016). Figure 1

Energy Coupling of Laser Radiation on AISI 304 Stainless Steel: Effect of High Temperatures and Surface Oxidation.

The industrial application of laser materials processing methods is still far ahead of research into the physical phenomena occurring during these processes. In particular, the effect of high temperatures on the energy coupling of laser irradiation of metals is poorly understood. However, most processes in laser materials treatment involve temperatures above the melting point or even cause evaporation. This study therefore evaluates the effect of high temperatures on the energy coupling efficiency of stainless steel experimentally for three typical laser wavelengths (515 nm, 1.07 µm, 10.6 µm). As a result, it is shown that the effect of temperature on the energy coupling efficiency depends on the wavelength. In this context the relevance of the X-point phenomenon known from the emissivity theory could be demonstrated for laser material processing. Further, the effect of a process-induced surface oxidation is analyzed. At temperatures above 650 °C the energy coupling efficiency dramatically increases to around 65% at melting point and stays at this high level even in the liquid phase.

PLASMA-ETCHING AND MODIFICATION OF POLYETHYLENE FOR IMPROVED SURFACE STRUCTURE, WETTABILITY AND OPTICAL BEHAVIOR

In the present paper, the surface modification of low-density polyethylene (LDPE) polymer is done by plasma-etching to tune its surface structure, wettability and optical behavior to make it useful for technical applications. For this purpose, two gasses (N[Formula: see text] and (O[Formula: see text] are used as the discharge precursors in a home-built plasma reactor. The plasma-treated LDPE surface etch-rate (control other surface properties) is high at the beginning and slows down as the treatment time increases due to surface restructuring. The etched surfaces are analyzed by scanning electron microscopy (SEM), which indicate greater surface changes due to O2plasma compared to that of N2. Also, the surface hardness is slightly low at the first treatment time and increases rapidly at higher exposure durations. Besides, the friction coefficient is significantly changed by plasma treatment, suggesting the formation of cohesive surface skin. The obtained X-ray diffraction (XRD) patterns show that the plasma-treated LDPE samples suffer disordering and structural changes which increase with raising the treatment duration. Surface restructuring is attributed to the combined effects of active species (from plasma) bombardments and surface oxidation. Also, the surface chemistry changes are evaluated using attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy which reveals chain scission after N2plasma treatment. Whereas, the O2plasma-treated samples suffer surface oxidation and formation of polar groups which offer some surface oxidation coatings. Furthermore, the surface wettability has been determined by the sessile drop method and shows enhancement upon plasma treatment due to the combined influence of surface chemistry and morphology. Also, the surface free energy (SFE) and adhesion are found to increase with the plasma exposure time due to surface activation. The optical behavior of LDPE is studied using ultraviolet–visible (UV–Vis) spectrophotometer which indicates that the optical bandgap performance depends on the amorphous or crystalline nature of the polymer. Also, the conjugated carbon atoms were examined and correlated to the reduced bandgap. In conclusion, the studied home-built glow discharge plasma reactor could be utilized efficiently to tune polymer surface properties to be used in high technology applications.

Removing Non-Size-Dependent Electron Spin Decoherence of Nanodiamond Quantum Sensors by Aerobic Oxidation

Fluorescent nanodiamonds (NDs) are crystal defect-based light-emitting nanoparticles that can be applied to quantum information science and quantum sensing. Of particular interest are nitrogen vacancy (NV) centers that allow optical access to their coherently controllable electron spin systems, leading to spin-photon quantum devices and nanoscale sensing of various physical parameters. However, the NV spin coherence time (T2) in NDs has been limited to one or two orders of magnitude shorter than those in bulk diamonds owing to the complicated surface effect that decoheres the NV spin systems. Here, we study the relation between the surface properties and T2 of single NV centers in NDs by systematically analyzing the effect of surface oxidation. We apply aerobic and acidic oxidation methods with various heating temperatures and processing times and find that aerobic oxidation most effectively oxidizes the surface and extends T2 by a factor of 1.6 to the original NDs. The ND-size dependence of T2 clearly shows that the surface oxidation removes a constant decoherence contribution irrespective of the ND size and that a new surface derived decoherence source emerges when the ND size reduces below 50 nm. The present results provide quantitative information on the decoherence sources of NV spin systems of NDs and will enable a strategic surface modification for better spin manipulations of NV centers in the context of quantum information science and nanoscale quantum sensing.

Effect of Ni/NiO Heterostructure on Hydrogen Evolution Performance in AEM Water Electrolysis: In-Operando Raman Study

The effect of the presence of NiO in Ni-based catalysts (Ni/NiO heterostructures) on the kinetics of the hydrogen evolution reaction (HER) has been studied ex-situ and simulated by kinetic models provided by Oshchepkov et al.1 , 2 A tenfold increase in the activity for the HER has been observed. The kinetic model also shows a shift from the Heyrovsky–Volmer mechanism to the Tafel–Volmer mechanism due to the presence of NiO.1 , 2 Danilovic et al. provided a four times increase of the HER activity of Ni/ Ni(OH)2 in an alkaline solution.3 Another result by Bates et al. 4 showed enhanced HER at adjacent Ni/NiOx sites. In these catalysts metallic Ni behaves as a basin for the Hads intermediate of the HER, while the oxide behaves as a basin for OHads, thus increasing reaction rates of the HER via water reduction.4 NiO or β-Ni(OH)2 is expected to partially cover the Ni surface. The oxidation state for Ni cannot be determined without additional in-situ studies. The precise identification of the catalytic active sites and elucidation of the reaction mechanisms in these systems remain a grand challenge. Our results confirm that Ni/NiO heterostructure has a higher HER activity compared to bare Ni or NiO. Our Ni/NiO heterostructure achieves -10 mA/cm2 at low overpotential (226 mV) compared to bare Ni (354 mV) or NiO (372 mV) in 1 mol dm-3 KOH. In-situ confocal Raman spectroscopy with cyclic voltammetry (CV) can provide useful information about which surface species are being produced and reduced during cycling and the form in which they exist in-situ, which will help explaining the reasons for the superior performance of Ni/NiO heterostructure. In this work, we have performed in-situ Raman studies of the electrochemical activity of Ni, NiO and Ni/NiO heterostructures for the HER under alkaline conditions (0.1 and 1 mol dm-3 KOH) and deionized water only. Intermediate formation and structural changes have been evaluated during linear sweep voltammetry and cyclic voltammetry and their relation to the kinetic enhancement for the HER in Ni-NiO heterostructures. In-situ Raman was used to analyze the degradation of Ni/NiO heterostructures under chronoamperometric measurements at different applied overpotentials. References Oshchepkov, A. G., Bonnefont, A., Parmon, V. N. & Savinova, E. R. On the effect of temperature and surface oxidation on the kinetics of hydrogen electrode reactions on nickel in alkaline media. Electrochim. Acta (2018). doi:10.1016/j.electacta.2018.02.106Oshchepkov, A. G. et al. Exploring the Influence of the Nickel Oxide Species on the Kinetics of Hydrogen Electrode Reactions in Alkaline Media. Top. Catal. 59, 1319–1331 (2016).Danilovic, N. et al. Enhancing the Alkaline Hydrogen Evolution Reaction Activity through the Bifunctionality of Ni(OH) 2 /Metal Catalysts. Angew. Chemie 124, 12663–12666 (2012).Bates, M. K., Jia, Q., Ramaswamy, N., Allen, R. J. & Mukerjee, S. Composite Ni/NiO-Cr2O3 catalyst for alkaline hydrogen evolution reaction. J. Phys. Chem. C 119, 5467–5477 (2015).

Lithiation of Oxidised Silicon

The electrochemical energy storage market is growing rapidly, and is currently dominated by lithium ion batteries. Replacing the standard graphite anode with silicon, whose theoretical gravimetric capacity is roughly ten times higher, promises a step-change improvement in capacity. Unfortunately, the use of silicon in lithium ion battery anodes frequently leads to poor cycle life, related at least in part to the challenges in creating a stable solid-electrolyte interphase (SEI). This issue is made more complex by the fact that the silicon used in composite anodes is typically nanostructured, with a poorly defined and often oxidized surface, especially when the silicon was prepared at elevated temperature. SiOx has been proposed as an alternative anode material to Si, whereas SiO2 is an electronic insulator. It is therefore valuable to understand the effect of surface oxides in more detail. In this work, we take a fundamental approach to understanding how an oxide on a silicon surface affects SEI formation and lithiation. Silicon wafers with precisely controlled thermal SiO2 thicknesses were prepared by thinning thicker oxides down to 1-30nm SiO2 using buffered hydrofluoric acid solution. These wafers were then cycled against a Li counter and reference electrode in Gen2 electrolyte (1.2M LiPF6 in EC:EMC (3:7 by wt.)) using custom three-electrode cells, which use an O-ring seal on the silicon to prevent contact between the electrolyte and the edges or rear of the wafers. Galvanostatic cycling at 20µAcm-2 indicates that while Si with 1nm SiO2 will lithiate above 0V vs Li, Si with 3nm SiO2 will only lithiate after a potential spike to -0.4V vs Li. Thereafter, it actually cycles with higher Coulombic efficiency (CE) than Si with 1nm SiO2. However, Si with 5-30nm SiO2 initially requires voltages below -1.0V vs Li to sustain 20µAcm-2, resulting in localized Li plating at pinholes in the SiO2 rather than lithiation. The voltages required to induce this pinhole plating will be compared to the dielectric breakdown strength of the SiO2 layers. To understand these phenomena in more detail, cyclic voltammetry (CV) was performed on Si with 1-5nm SiO2 (1mV/s, see abstract figure). Lithiation only occurred in the 1.0nm sample in the first cycle, but by the fourth cycle, lithiation occurred in samples with up to 2.9nm SiO2. Samples with 3.9nm and 4.8nm SiO2 on the other hand exhibit no lithiation or reversible electrochemistry during ten CV cycles. Two distinct delithiation peaks are observed for 1.0 and 1.4nm SiO2, but only the higher-voltage peak is observed for the 2.9nm SiO2, suggesting a unique delithiation chemistry that may be correlated to the high CE during galvanostatic cycling, and which will be studied by XPS. The peak currents (0.05V for lithiation, 0.5V for delithiation) decrease exponentially with SiO2 thickness, indicating that tunneling electron transfer across the SiO2 may play a key role in the reaction kinetics. Overall, we find that dense thermal oxides of 5nm or more severely impede lithiation, whereas 1nm SiO2 allows lithiation to proceed unimpeded. Intermediate thicknesses of ~3nm alter the delithiation chemistry while yielding improved CE during galvanostatic cycling. These effects will be discussed in relation to the bonding of the SiO2 network in thermal oxides, as compared to native or sputtered oxides. Figure 1

Quenching experiment on Cr-alloy-coated cladding for accident-tolerant fuel in water pool under low and high subcooling conditions

A transient pool boiling experiment was conducted to investigate the quenching performance of Cr-alloy-coated cladding in the simulated conditions of a loss-of-coolant accident (LOCA). Cr-alloy-coated cladding is a potential candidate for accident-tolerant fuel (ATF), which is developed to suppress hydrogen generation at high temperature in the event of an LOCA or beyond design basis accident. Cr-alloy-coated cladding and Zircaloy-4 cladding were used in the quenching experiment under low and high subcooling conditions, i.e., 0 °C and 50 °C. The thicknesses of the Cr-alloy-coated layer were 20 and 50 μm. Each cladding specimen was heated up to 600 °C in a furnace and quenched in a water pool at constant temperature. The quenching experiment showed the formation of film boiling and transition to nuclear boiling at the low subcooling temperature of 0 °C. No film boiling was observed at high subcooling at 50 °C. The effect of surface roughness was found to be insignificant for quenching performance. Rather, cladding oxidation, particularly for the Zircaloy-4 specimens, had a remarkable influence on the minimum film boiling temperature (MFBT) and quenching time. The Cr-alloy-coated cladding did not show the effect of surface oxidation as expected because of very low cladding oxidation. There is no significant difference in the quenching performance of the Zircaloy-4 cladding and the Cr-alloy-coated cladding at the high subcooling temperature of 50 °C. Furthermore, the Cr-alloy-coated cladding tends to decrease the quenching time with higher MFBT as the coating thickness decreases.

Impact of surface oxidation on the structural, electronic transport, and optical properties of two-dimensional titanium nitride (Ti3N2) MXene

In this study, the effects of surface oxidation on the structural, electronic transport, and optical properties of two-dimensional titanium nitride (Ti3N2) MXene are examined using density functional theory calculations. Our computations demonstrate that surface-oxidized titanium nitride (Ti3N2O2) MXene is thirty percent more stable than the bare Ti3N2. Adsorption energy calculations show that oxygen terminal groups strongly adhere to the Ti atoms on the surface of the bare Ti3N2. The covalent bonding between outer Ti and N atoms in Ti3N2 MXene is partially transferred to the ionic bonding owing to the surface oxidation and resulting in the formation of covalent bonding between outer Ti and O atoms in the Ti3N2O2 MXene. This surface oxidation has a negative impact on the metallic conductivity of the bare Ti3N2 due to the reduction of density of the states at the Fermi level. The current of Ti3N2 decreases after the termination of the surface atoms with oxygen atoms. Surface oxidation reduces both absorption and the reflectivity of the Ti3N2 in the visible spectrum. However, Ti3N2O2 displays considerably higher absorption and reflectivity compared to the Ti3N2 at higher photon energies. Our results provide a reference for theoretical and experimental studies on the optoelectronics applications of titanium nitride MXenes.

Effects of carbon steel surface oxidation on critical heat flux in downward-face pool boiling

In this study, a downward-face pool boiling critical heat flux (CHF) experiment was performed using a carbon steel plate soldered to a copper base. The results of the experiment were analyzed and compared with those of a carbon steel and copper block experiment previously conducted at the same test facility. After polishing the surface using sandpaper, heat was applied to the surface by cartridge heating until CHF was attained. Due to the oxidation of the carbon steel, the surface changed gradually during boiling. The boiling cycle was repeated several times, and the CHF variations were observed. Detailed images of the heating surface behavior during boiling acquired by two synchronized high-speed cameras with different heat fluxes were analyzed. It was found that the more oxidized the surface became, the fewer bubbles were generated and the higher the CHF became; however, the bubble film departure frequency remained the same in all conditions. The contact angle and microscope surface images of the oxidation material were also obtained, and it was determined that the combined effect of the increase in wettability and decrease in nucleation site density was likely the reason that the CHF increased. This gradual oxidation process could be beneficial in actual situations since it can increase the upper limit of in-vessel retention external reactor vessel cooling.

Investigation of Coatings for Langmuir Probes: Effect of Surface Oxidation on Photoemission Characteristics

AbstractSurface oxidation of Langmuir probes is an important issue for probe measurements in space environments. Followed by our previous work about the oxidation effect on the collection of ambient plasma electrons and ions, here we present its effect on photoemission from the probe surface of various materials. Photoemission is either a contamination for traditional Langmuir probes or a necessity for electric field probes in low‐density plasma. Our results show that all materials after oxidation have a varying degree of reduction in photoemission. The photoemission of copper, gold, and niobium drops most significantly followed by DAG213 (a resin‐based graphite coating), TiN (titanium nitride), and rhenium. Iridium, DAG213, and AquaDAG (graphite coating) have the largest photoemission after oxidation, making them appropriate coating candidates for electric field probes. Both DAG materials show a large photoemission enhancement after the oxidation products are cleaned off the surface. A long exposure test shows that the photoemission from iridium slowly degrades. Due to the high surface conductivity of oxidized iridium shown in previous work, it is suggested that iridium can be oxidized before flight to minimize the photoemission when being used as a coating for Langmuir probes. Overall, iridium is found to be a coating material appropriate for both electric field probes and Langmuir probes.

Influence of carbon nanostructure and oxygen moieties on dopamine adsorption and charge transfer kinetics at glassy carbon surfaces

Abnormal levels of the neurotransmitter dopamine have been linked to a variety of neurochemical disorders including depression and Parkinson's disease. Dopamine concentrations are often quantified electrochemically using biosensors prepared from carbon electrode materials such as carbon paste or glassy carbon. The charge transfer kinetics of dopamine is highly sensitive to carbon surface termination, including the presence of certain oxygen functional groups and adsorption sites. However, the nature of the binding sites and the effects of surface oxidation on the voltammetry of dopamine are both poorly understood. In this work the electrochemical response of dopamine at glassy carbon model surfaces was investigated to understand the effects of altering both the carbon nanostructure and oxygen surface chemistry on dopamine charge transfer kinetics and adsorption. Glassy carbon electrodes with low oxygen content and a high degree of surface graphitisation were prepared via thermal annealing at 900 °C, whilst highly oxidised glassy carbon electrodes were obtained through electrochemical anodisation at 1.8 V vs Ag/AgCl. The carbon surface structure and composition in each case was studied via X-Ray Photoelectron Spectroscopy. Voltammetry in solutions of dopamine at acidic pH confirmed that both annealing and anodisation treatments result in carbon surfaces with rapid charge transfer kinetics. However, dopamine adsorption occurs only at the low-oxygen, highly-graphitized carbon surface. Density functional theory studies on graphene model surfaces reveal that this behaviour is due to non-covalent interactions between the π-system of dopamine and the basal sites in the annealed surface. Simulations also show that the introduction of oxygen moieties disrupt these interactions and inhibit dopamine adsorption, in agreement with experiments. The results clarify the role of oxygen moieties and basal plane sites in facilitating both the adsorption of and charge transfer to DA at carbon electrodes.

Effects of anode oxidation on dynamic behavior of a propagating arc magnetohydrodynamic actuator

The RailPAc is a magnetohydrodynamic flow actuator which employs a high speed gliding arc to alter flow parameters over an aerodynamic surface. Experiments were performed to determine the effect of anode surface oxidation on the dynamics of the RailPAc flow actuator with electrodes spaced 2 mm and 5 mm apart. The arc was characterized with electrical measurements and high-speed imaging. The RailPAc power supply provided currents of ∼1 kA and voltages of ∼100 V. Oxide layers were created on the surface of the anode by heating at 200 °C for 1 to 120 minutes. Fully reduced anodes were used as control cases. Anode oxidation was found to increase the speed and consistency of the arc’s movement. Greater oxidation layer thickness was found to increase the efficiency of the arc by reducing the circuit resistance. Electrode damage was significantly reduced for strongly oxidized anodes. Effects of oxidation on arc motion were stronger for narrower electrode spacing. Computational studies suggest the primary mechanism of action by the oxidation layer on the arc is through increasing surface resistivity which leads to field enhancement at the arc root edges.

Experimental investigation of transition process from nucleate boiling to microbubble emission boiling under transient heating modes

AbstractThis study investigated the transition from nucleate boiling to microbubble emission boiling (MEB) on a small heating surface under transient heating modes. During the short unstable boiling process after critical heat flux with a rapid increase in wall temperature, capillary waves were observed over the vapor film at subcooling of 30–40 K. The visualization results in this process demonstrated that the partial collapse of vapor film induced strong jet toward the heating surface and therefore avoided complete dryout thereon. This mechanism certainly plays an important role in the transition to stable MEB. Further comparison between the wall temperature and boiling states indicates that the wall temperature at onset point of stable MEB is determined by the predominance of MEB mode over film boiling in the transition process. Effects of liquid subcooling and surface oxidation on the transition process were also investigated at the same time.

EMISSIVITY MODEL OF STEEL 316L AT 800–1100 K DURING OXIDE LAYER GROWTH ON THE SPECIMEN SURFACE

This work strived to study the variation in the spectral emissivity of steel 316L specimens with wavelength at a certain temperature. The spectral emissivity and radiances from the specimen surface were measured by multispectral radiation thermometry at wavelengths of 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, and 2.1 μm and at temperatures from 800 to 1100 K in increments of 20 K. The specimen was heated and kept at a certain temperature in air for 6 h. The temperature of the specimen surface was measured by two thermocouples, which were symmetrically welded onto the front surface of the specimens near the measuring area for accurate monitoring of the surface temperature. The average of their readings was regarded as the real temperature. Variation in the spectral emissivity with wavelength was studied at different temperatures and different heating times. Effect of surface oxidation on the fitting accuracy of the emissivity models was evaluated. Two favorite models were determined. With the radiant values measured by thermometry and in combination with the least-squares fitting technique, two favorite models were examined. On the whole, both the five-parameter LLWE and LWE models could yield the best overall temperature prediction and could farthest relieve the effect of surface oxidation on the accuracy of temperature prediction over the present wavelength and temperature ranges.

N/Rh共掺杂金红石TiO2表面对CO气体光学气敏传感特性的影响

N/Rh共掺杂金红石TiO2表面对CO气体光学气敏传感特性的影响

Effect of Carbon Corrosion on Wettability of PEM Fuel Cell Electrodes

Large transport resistances at high current densities hinder the proton-exchange-membrane fuel cells from reaching performance-cost-durability targets. Extensive carbon corrosion in the cathode catalyst layers (CCL) has been shown to correlate to pore closures and increase in surface oxides of carbon support, which has been associated with the increase in wettability of CCLs. This study investigated effects of microstructure and surface oxides on the change of wetting properties of CCLs after different accelerated stress tests. The apparent contact angles were measured over a broad range of carbon mass losses at 0.3 mgPt/cm2. The apparent contact angles fell sharply after small extent of carbon loss followed by a plateau, and decreased again after 123 μg/cm2 (35% carbon support loss). The microstructural changes were characterized by surface roughness (atomic force microscopy) and porosity (focus-ion-beam scanning electron microscopy and thermogravimetric analysis). The surface roughness and porosity were used to parameterize Wenzel's, Cassie-Baxter's, and combined models to elucidate the changes in Young's contact angle. The fittings of the models revealed that the CCLs sustained their wettability after up to 35% of carbon support loss due to the increases in the proportion of hydrophobic fluorocarbon chains on the surface of the CCLs measured from XPS.