AC Surface Research Articles

Fischer−Tropsch Synthesis over Activated-Carbon-Supported Cobalt Catalysts: Effect of Co Loading and Promoters on Catalyst Performance

The effects of Co loading and K, Ce, and Zr promoters on initial activities and selectivities of activated-carbon-supported cobalt (Co/AC) Fischer−Tropsch synthesis (FTS) catalysts, were studied under conditions of 240 °C, 2.4 MPa, 650 h-1, and H2/CO = 2:1 in a fixed-bed reactor. The reduction and metal dispersion properties of the catalysts were investigated through temperature-programmed reduction (TPR) and CO chemisorption. Detectable hydrocarbons up to C20 were formed on the Co/AC catalysts. Co loading affected initial syngas conversion and CH4 selectivity in the following order: 20%Co > 15%Co > 10%Co > 7%Co, whereas it had an opposite effect on content of C12−C20 in the liquid organic product. K, Ce, and Zr promoters remarkably changed the initial catalytic performance of the Co/AC catalysts. Addition of K to the Co/AC catalyst significantly decreased the FTS activity and CH4 selectivity, but increased the water gas shift (WGS) activity. Addition of Ce improved the Co/AC catalyst activity, accompanied by high CH4 selectivity. Neither CO2 nor CH4 selectivity changed greatly on the Zr-promoted Co/AC catalyst even though the Zr promoter increased the FTS activity. The TPR and CO adsorption studies showed that K, Ce, and Zr promoters improved Co dispersion and interaction between Co oxide and the AC surface, which might be the reasons for the subsequent catalyst performance, especially for the Ce- and Zr-promoted catalysts. The TPR study also showed that the reduction steps of the reductively decomposed Co/AC catalysts by pure H2 are Co3O4 → CoO → Co.

Investigation of shear stiffness and rutting in asphalt concrete mixes

Field and laboratory testing programs were set up to evaluate the in-situ shear properties of asphalt concrete mixes using the newly developed in-situ shear stiffness testing (InSiSSTTM) facility versus the laboratory evaluation using the resilient modulus and torsion testing. The LTPP SPS-9A 870900 test site, which has six similar pavement sections with different AC surface mix properties, was tested in the field using the InSiSSTTM and core samples were extracted from the site and tested in the laboratory. The results of the testing program were correlated with the rutting of the test sections over a 4-year period. In this paper, the InSiSSTTM facility is briefly introduced and the interpretation of the data collected is presented. The experimental program and analysis procedures are then outlined. The analysis of variance was used to test the significance of the results, and a bivariate analysis was performed for correlating rutting (as a criterion variable) and the different laboratory and field measured material properties (as predictor variables). Finally, a regression analysis between the in-situ shear stiffness and pavement rutting is presented. The results of the study showed that the in-situ shear stiffness had the highest correlation coefficient with rutting rate, and this might be a suitable measure to characterize the asphalt mixes and evaluate the rutting potential of asphalt pavements. This important result should be useful to the pavement engineers interested in the evaluation of rutting using a simple field measure.Key words: in-situ testing, laboratory testing, shear stiffness, shear properties, asphalt concrete, pavements, rutting, long-term performance.

Electrocatalytic activity of surface adsorbed ruthenium–alizarin complexone toward the oxidation of benzyl alcohol

The surface electrochemical behavior of an adsorbed alizarin complexone (abbreviated as AC) and its surface coordination with Ru(II) were studied in aqueous solution at a pH range of 0–6. The surface complex of ruthenium with AC displays strong electrocatalytic activities toward benzyl alcohol. Based on the rotating disk electrode measurement, it is believed that the electrocatalytic oxidation of benzyl alcohol is a two-electron and two-proton process with benzaldehyde as a major product. On the other hand, ruthenium–AC surface complex has also shown catalytic activities toward electro-oxidation of several small organic molecules such as methanol, formic acid, formaldehyde, ethanol, and acetaldehyde.

DC reactor type 고온호전도 한류기의 턴간 절연 특성

Fault current limiters (FCL) are extensively needed to suppress fault currents, especially for trunk power systems heavily connected to high voltage/large current transmission lines. Due to its ideal electrical behavior, high-temperature superconductor fault current limiter (HTSFCL) becomes one of the most important developing trends of limiters in power system. This paper describes the result of an investigation of the dielectric characteristics of turn-to-turn insulation for pancake and solenoid type reactor coil in liquid nitrogen. The influence of thickness in a variety length, on AC, DC and impulse surface flashover has been investigated. Also, the relationships between the number of turn and breakdown characteristics were clarified. The information gathered in this test series should be helpful in the design of liquid nitrogen filled DC reactor type HTSFCL.

Raman phonons and ageing-related surface disorder in Na xCoO 2

The polarized Raman spectra from ab and ac surfaces of single crystal Na x CoO 2 ( x≈0.7), parent compound of recently discovered superconductor Na x CoO 2 · yH 2O, are reported and discussed. The crystals were hexagon platelets of typical size 3 × 3 × 0.1 mm. Three of the five (A 1g+E 1g+3E 2g) Raman active phonons were unambiguously identified at 458 (E 1g), 494 (E 2g) and 574 (A 1g) cm −1. The spectra from ab and ac surfaces differ significantly and provide evidence that within hours after preparation the ac surface, unlike the ab one, is strongly disordered. Within several days the disorder extends over the ab surface too.

Atomic Bridging and Barrier-Type AC Surface Photovoltage Measurements on Iron- and Copper-Contaminated Silicon Surfaces

The rise time of the alternating current (ac) surface photovoltage (SPV) of n-type silicon (Si) contaminated by Fe and/or Cu aqueous solutions was investigated. After a rinse time of about 1000 min in Fe contaminated water, three monolayers (0.59 nm) of oxide were formed, with a resulting incorporation of Fe. The variation of ac SPV signals indicated the appearance of a negative charge corresponding to the increasing concentration of Fe on surface of the Si wafer. This negative charge can be accounted for by postulating a bridging mechanism between trivalent Fe ions, and quadrivalent Si ions, forming an network in the native oxide. In contrast to Fe, Cu was deposited at the beginning of the rinsing on partially hydrophobic n-type Si surfaces as an Cu atomic layer or Cu particles (maximum Cu concentration: around resulting in a Cu-Si contact. Because the reduction potential of is larger than that of Si, the process was accomplished through electron capture. It is postulated that the Cu-Si contact may have formed a Schottky barrier between the Cu and n-type Si, resulting in the occurrence a so-called barrier-type ac SPV. This may prove to be a promising method for fabricating atomic layer Schottky devices. © 2003 The Electrochemical Society. All rights reserved.

AC surface flashover strength and barrier effect of LN 2 for HTS transformer with simulated electrode

In the response to an increasing demand for electrical energy, much effort aimed to develop and commercialise HTS power equipments is going on around the world. For the development, it is necessary to establish the dielectric technology in LN 2. Hence many types of dielectric tests should be carried out to understand the dielectric phenomena at cryogenic temperature and to gather various dielectric data. Among the many types dielectric tests, the characteristic of surface flashover and the barrier effect were conducted with the simulated electrode after analysing the insulating configuration of the pancake-coil-type HTS transformer. The influence of a barrier on the dielectric strength was measured according to the size of the barrier, the position of the barrier and the effect of the back-electrode. It was shown that the effectiveness, namely the ratio of the breakdown voltage in presence of barrier to the voltage without barrier, is highest when the barrier is placed at the needle electrode side. The effect increased up to 1.8 times when collar length is 10 mm. The flashover characteristic with back-electrode was remarkably lower than the characteristic without one in the case the electrodes located at the same surface of dielectric plate. On the contrary, in the case the barrier was placed between the electrodes, the characteristic was even improved slightly.

Field dependent permittivity of composite materials containing ferromagnetic wires

A type of a composite material is proposed, the microwave permittivity of which changes under the effect of a dc magnetic field applied to the whole composite sample. The composite consists of short ferromagnetic wires embedded into a dielectric matrix. A strong field dependence of the permittivity is seen in the vicinity of the antenna resonance, where the dispersion behavior can experience a transformation from a resonant spectrum to a relaxation one under the effect of the field. This permittivity behavior is due to a high sensitivity of the ac surface impedance of a ferromagnetic wire to a magnetic field, known as the magnetoimpedance (MI) effect. If the resonance-like dispersion behavior is realized, the real part of the effective permittivity can be made negative past the resonance for wire inclusion concentrations well below the percolation threshold. Applying a magnetic field, the negative peak continuously decreases as the dispersion tends to become of a relaxation type. The effective permittivity is analyzed within a one-particle approximation, by considering a wire piece as an independent scatterer and solving the scattering problem with the impedance boundary condition. A magnetic field is assumed to be applied in parallel to the wire. A new integrodifferential equation for the current distribution in a wire is obtained, which is valid for the surface impedance matrix of a general form. This work demonstrates the possibility of using the MI effect to design field-controlled composites and band-gap structures.

Surface discharge measurements of polymeric materials

The measurement of surface charge is an important parameter for evaluating the electrical performance of high voltage insulation materials. The method proposed allows on line measurement of charge and can generate two-dimensional images that represent the charge behaviour on the surface of the material under test. The measurement system utilises Pockels effect by using a wafer of bismuth silicon oxide (BSO) as the sensing element. This method allows non-destructive measurements since no direct electric signal or component interferes with the surface charge. Unlike conventional surface charge measurement systems, this approach can dynamically observe and simultaneously quantify surface charge. This allows the surface charge distribution under the influence of a time varying electric field to be studied. The system's accuracy and sensitivity have been improved through the use of an optical phase modulator. The system uses a high speed CCD camera and a purpose built synchronisation control circuit to obtain a high temporal resolution. This has allowed the study of charge behaviour at mains power frequencies. Surface charge distributions have been observed by applying AC voltages to a needle electrode in direct contact with the BSO. AC surface discharge behaviour of polymeric materials bonded to the back-face of the BSO has also been investigated.

Narrow dip around zero magnetic field in magnetization hysteresis loops of thin YBCO superconducting films

A narrow dip is observed around zero magnetic field in magnetization curves M(B) of superconducting YBCO films with about 10 nm thickness. This anomaly occurs in the same field range with an anomaly of ac surface impedance Z(B) found recently in thin YBCO films. Because the thickness of our films is considerably less than the London penetration depth, two-dimensional limit of the critical state model is applied. In the framework of this model the magnetic field dependence of the critical current density jc(B) is found. The obtained jc(B) function agrees well with that found in the ac surface impedance investigation.

Pore Structure Effects on Ca-Based Sorbent Sulfation Capacity at Medium Temperatures: Activated Carbon as Sorbent/Catalyst Support

The reaction between three different Ca-based sorbents and SO2 were studied in a medium temperature range (473–773 K). The largest SO2 capture was found with Ca(OH)2 at 773 K, 126.31 mg SO2•g Ca(OH)2 −1, and the influence of SO2 concentration on the sorbent utilization was observed. Investigations of the internal porous structure of Ca-based sorbents showed that the initial reaction rate was controlled by the surface area, and once the sul-fated products were produced, pore structure dominated. To increase the surface area of Ca-based sorbents available to interact with and retain SO2, one kind of CaO/activated carbon (AC) sorbent/catalyst was prepared to study the effect of AC on the dispersion of Ca-based materials. The results indicated that the Ca-based material dispersed on high-surface-area AC had more capacities for SO2 than unsupported Ca-based sorbents. The initial reaction rates of the reaction between SO2 and Ca-based sorbents and the prepared CaO/AC sorbents/cata-lysts were measured. Results showed that the reaction rate apparently increased with the presence of AC. It was concluded that CaO/AC was the active material in the des-ulfurization reaction. AC acting as the support can play a role to supply O2 to increase the affinity to SO2. Moreover, when AC is acting as a support, the surface oxygen functional group formed on the surface of AC can serve as a new site for SO2 adsorption.

Barrier-Type AC Surface Photovoltage in Silicon with a Copper-Contaminated Surface

An AC surface photovoltage (SPV) was induced in n-type silicon (Si) wafers after the deposition of undesirable copper (Cu) atoms on the surface. Chemical analysis reveals that Cu ions (Cu2+) are deposited on Si surfaces, through electron capture because the oxidation reduction potential of Cu2+ is higher than that of Si and, as a result, a Cu–Si contact is formed. This Cu–Si contact may create a Schottky barrier between the Cu and n-type Si, resulting in the appearance of an AC SPV in n-type Si. In this note, a barrier-type SPV is proposed.

The relationship among AC surface spark-over voltage, specific capacitance of solid insulator and bubbles in LN/sub 2/ and LHe

The characteristics of AC surface electrical discharge on solid insulators in liquid nitrogen (LN/sub 2/) or liquid helium (LHe) are studied. The insulation structure used is a model of a power cable end. Bubbles are generated due to discharge at a lower voltage than that of spark-over. However, it was experimentally and computationally confirmed that the bubbles do not influence the spark-over voltage. In the structure, static capacitance is formed on the insulator surface. The surface spark-over voltage depends on the static capacitance. The relation between surface spark-over voltage and the static capacitance is experimentally considered for several insulators. The results show that the spark-over voltage is lower for larger static capacitance. The relation between the spark-over voltage and the capacitance can be expressed by an equation V=A/spl middot/C/sub sp//sup -B/.

Interpretation of AC surface current density by deconvolution of potential differences in solution

In localized electrochemical impedance techniques, the measurable AC potential difference in solution is related to the AC surface current distribution simply by using Ohm’s law. However, this relationship is not true for the cases in which current distribution is non-uniform. In order to correctly obtain the surface current density, deconvolutions of the solution potential difference using the base line stripping procedure and the Fast Fourier Transform method have been simulated and discussed. A more accurate spatial estimate of the surface current density distribution can be obtained by deconvoluting the measured potential difference distribution in the solution than by simply using Ohm’s law. The accuracy of the deconvolution using the base line stripping procedure is greatly dependent on the errors of measurement, and experimental data with a very low noise level are therefore required. Deconvolution by a Fast Fourier Transform is also very sensitive to noise and discontinuity in the potential difference distribution, but the sensitivity can be dramatically reduced to an acceptable level by using a low pass filter.

Characteristic of AC surface spark voltage in LN/sub 2/ and LHe

The AC surface spark-over voltages on solid insulators with a back electrode in liquid N/sub 2/ (LN/sub 2/) and liquid He (LHe) were measured and discussed. The larger gap length did not give a higher spark-over voltage. The spark-over voltage in LHe was about two-thirds of that in LN/sub 2/. Experimental studies were carried out on how bubbles in liquid influence surface spark-over voltage. The result shows that it does not influence the voltage.

Fixed oxide charge in n-type silicon wafers studied by ac surface photovoltage technique

A fixed oxide charge (Qf) in n-type silicon (Si) wafers is investigated in thermally oxidized Si wafers intentionally contaminated with aluminium (Al), where Al-induced negative charge clarified by an ac surface photovoltage plays the role of a charge marker. Qf, which is defined to be positive, in n-type Si wafers is found to be almost completely neutralized by abundant majority carriers (electrons in n-type Si) during thermal oxidation at high temperature. Consequently, the positive oxide charge might disappear in n-type Si wafers. Instead, the resultant oxide charge in thermally oxidized n-type Si wafers is often negative because of possible interface-trapped charge.

Decrease in ac surface partial discharges on an insulator with backing electrode by use of a hollow electrode

Alternating-current surface partial discharges (PDs) in air from the triple junction (TJ) of a cylindrical insulator with a backing electrode were decreased by use of a hollow electrode. The voltage applied was varied over a range from 1 to 1.5 times the PD initiation voltage (Vpd). The thickness of the insulator was varied over a range from 2 mm to 7.5 mm. For applied voltages of 1.2 × (Vpd) and 7.5 mm thickness, the combined magnitude of the PD pulses for the hollow electrode was about 80% smaller than that for the nonhollow electrode; the total number of pulse events was 40% to 50% smaller than for the nonhollow electrode; and PD pulses stronger than approximately 7 × 10−11 C were suppressed significantly. Calculation of the electric field around the TJ suggested that the generation and propagation of the PD pulse resulted from an increase in potential and strength of the vertical field component on the surface of the insulator near the TJ. It was observed from powder figures that the propagation of PDs on the hollow insulator kept their length about 50% shorter than on the nonhollow insulator. © 1998 Scripta Technica, Inc. Electr Eng Jpn, 121(3): 1–8, 1997

Monitoring of Ultra-Trace Contaminants on Silicon Wafers for ULSI by a Novel Impurity Extraction and AC Surface Photovoltage Methods

Monitoring of detrimental micro-contamination coming from various sources has been strived to upgrade process cleanliness in silicon (Si) ultra-large-scale-integration (ULSI), and to improve the device performance and chip yield. Processed wafers were investigated by means of chemical analytical and nondestructive ac surface photovoltage (SPV) methods. In this investigation, the pack extraction method (PEM), where sample wafers are enclosed in cleaned Teflon bags with aqueous acid solutions, were expanded to extract impurities. The PEM collect anions (SO 4 2- , NO 3 - , Cl - , F - , etc.), cations (NH 4 + , Na + , K + , P 5+ , B 3+ , etc.) and detrimental metal impurities in Si wafers of both front and back surfaces, in contrast to the previously proposed technique (1) . The PEM combined with ion chromatography and/or high resolution inductively coupled plasma mass spectroscopy facilitated to detect surface impurities to the level of 10 12 -10 13 atoms/m 2 which is equivalent to ppt in actual sample volume. Monitoring harmful impurities was successful in upgrading the cleanliness in Si wafers and device manufacturing processes. Based on the chemical analysis of SO 4 2- at wafer surfaces having inverted layers, ac SPV in n-type Si was found to decrease in proportion to increase in SO 4 2- concentration. SO 4 2- causes interface traps in native oxide, thus reducing the ac SPV. Hence, this nondestructive method is a useful technique for characterizing SO 4 2- as well as other metal impurities as proposed in the previous report (2) .

背後電極を持つ絶縁物表面で発生する交流沿面部分放電のくぼみ電極による低減

背後電極を持つ絶縁物表面で発生する交流沿面部分放電のくぼみ電極による低減

Dynamic observation of needle-plane surface-discharge using the electro-optical Pockels effect

Dynamic observation of surface charge distribution is the main advantage of the electro-optical Pockels effect technique over the dust figure technique, the photographic Lichtenberg figure technique, and the static potential probe scanning method. This technique is demonstrated here to observe the surface charge distribution deposited by partial discharge during application of one period of an 8 kV sinusoidal voltage to a needle-dielectric plane electrode system with zero gap spacing. Polarity effect and backdischarge are prominent in ac surface discharges. The observed polarity effect in the charge pattern shows that if the dielectric surface is initially free of surface charge, positive 'streamer' channels burst out intermittently and radially from the needle tip resulting in a spoke-like surface charge distribution, while negative streamers expand almost uniformly in all radial directions resulting in a nearly circular surface charge distribution. This behavior results in quasi-permanent positive surface charges. Residual negative surface charge from prior discharges has a considerable influence on the trajectory of subsequent positive streamer discharges, i.e. causing deviation of positive streamer channels from the radial direction; whereas residual positive surface charge has little influence on the radial development of surface charge from subsequent negative discharges. This measurement technique has a potential for widespread application in investigating the dynamics of surface charging phenomena.