Ac Properties Research Articles

Application of Nanotechnology in High Frequency and Microwave Devices

The study of the ac properties of nano-electronic systems is one of the most discussed topics recently. To understand the high-frequency electronic properties of carbon nanotubes we should understand passive, ac impedance of a 1D quantum system. In this paper we have tried to use CNT in Substrate Integrated Waveguide (SIW) and Modified SIW (MSIW) to decrease the electrical energy loss. For this, firstly some of the basic concepts about nanoscale electromagnetic properties and the qualitative difference in circuit behavior at nanoscale is presented. Secondly the electrical properties of carbon NanoTube and graphene which are the two most technologically advanced examples have been discussed. Thirdly a comparative study of conductive and substrate loss in SIW and MSIW has been conducted for CNT-graphene system and copper system. Thus we have suggested the use of CNTgraphene instead of copper in this substrate. Simulation results with the CST Microwave Studio show 70% reduction in conductive insertion loss and 40% reduction in total insertion loss in SIW. We have also studied the use of graphene and CNT in MSIW; the result of simulation with HFSS software is presented as well.

Characterization of activated carbon prepared by phosphoric acid activation of olive stones

The effects of activating agent concentration on the pore structure and surface chemistry of activated carbons derived from olive stone with chemical activation method using phosphoric acid as the activating agent were studied. Mass changes associated with the impregnation, carbonization and washing processes were measured. With H3PO4 dilute solutions (60, 70, and 80 wt% H3PO4), the loading of substance on CS increases with concentration. The concentration of the H3PO4 solution seems to control the processes of impregnation, carbonization and washing in the preparation of AC from olive stones by H3PO4 chemical activation. ACs have been characterized from the results obtained by N2 adsorption at 77K. Moreover, the fractal dimension (D) has been calculated in order to determine the AC surface roughness degree. Optimal textural properties of ACs have been obtained by chemical activation with H3PO4 80wt.%. The BET surface areas and total pore volumes of the carbons produced at H3PO4 80wt.% are 1218m2/g and 0.6cm3/g, respectively.

Purification and characterization of the acetyl-CoA synthetase from <italic>Mycobacterium tuberculosis</italic>

Acetyl-CoA (AcCoA) synthetase (Acs) catalyzes the conversion of acetate into AcCoA, which is involved in many catabolic and anabolic pathways. Although this enzyme has been studied for many years in many organisms, the properties of Mycobacterium tuberculosis Acs and the regulation of its activity remain unknown. Here, the putative acs gene of M. tuberculosis H37Rv (Mt-Acs) was expressed as a fusion protein with 6×His-tag on the C-terminus in Escherichia coli. The recombinant Mt-Acs protein was successfully purified and then its enzymatic characteristics were analyzed. The optimal pH and temperature, and the kinetic parameters of Mt-Acs were determined. To investigate whether Mt-Acs is regulated by lysine acetylation as reported for Salmonella enterica Acs, its mutant K617R was also generated. Determination of the enzymatic activity suggests that Lys-617 is critical for its function. We further demonstrated that Mt-Acs underwent auto-acetylation with acetate but not with AcCoA as the acetyl donor, which resulted in the decrease of its activity. CoA, the substrate for AcCoA formation, inhibited the auto-acetylation. Furthermore, the silent information regulator (Sir2) of M. tuberculosis (Mt-Sir2) could catalyze Mt-Acs deacetylation, which resulted in activation of Acs. These results may provide more insights into the physiological roles of Mt-Acs in M. tuberculosis central metabolism.

Application of Dielectric Spectroscopy in ZnO Varistor Ceramics

There are two kinds of charges in dielectric materials, one is bound charge, and the other is free charge. Bound charge will introduce dielectric relaxation under applied AC electric field, which will be detected with dielectric spectroscopy. Free carrier will introduce DC conduction through the sample from one electrode to the other under DC electric field. But what can we do to obtain AC and DC properties of dielectric materials at the same time? In this paper, dielectric characteristics of ZnO varistor ceramics in a wide range of frequency and temperature are reported. DC conductivity is observed at low frequency and high temperature region and grainboundary Schottky barrier is obtained further. Dielectric loss peaks are observed at high frequency and low temperature region and single grainboundary electric breakdown voltage is calculated. At the end of the paper, the advancing trend of dielectric spectroscopy is discussed.

Ferromagnetic resonance investigation of Py/Cr multilayer system

The multilayer thin films consisting of alternating stacks of ferromagnetic Permalloy (Py) and ultra thin non-magnetic (sub-nanometer) spacer (Cr) layers have been investigated by using ferromagnetic resonance (FMR) and dc magnetization measurement techniques. The non-magnetic spacer layer thickness changes from 0.5 Å to 2.5 Å by 0.5 Å steps. Polycrystalline composite metallic films were grown on Si substrate by usual magnetron sputtering techniques at UHV conditions. The magnetic hysteresis curves were recorded by conventional dc magnetization. The ac and dc magnetic properties were investigated by using ferromagnetic resonance. FMR measurements were carried out for different directions of external dc magnetic field in order to search magnetic anisotropy. Two strong and well defined peaks (acoustic and optic mode) were observed in FMR spectrum as the magnetic field direction approaches to the film normal. Their positions and relative intensities helped to characterize coupling species. The magnetic parameters have been deduced by using a theoretical model. It has been found that the exchange coupling parameter between ferromagnetic layers through non-magnetic Cr spacer is ferromagnetic in nature and strongly decreases with increasing spacer layer thickness. The magnetic anisotropy parameters strictly depend on magnetic layer thickness while the dc magnetization is almost constant for a few nanometer thick Permalloy as well. We observed only ferromagnetic coupling between ferromagnetic layers in all samples.

Equivalence of Measurements on Soft Magnetic Materials in the U.K. and Measurements for Operational Conditions

Measurements on soft magnetic materials were made to determine the measurement capabilities available to the partners of a government-funded collaborative research project, with 16 partners from industry and academia. One of the objectives of this project was the development of advanced magnetic materials for novel electrical machines for use in harsh environments such as gas turbines. To establish the measurement confidence necessary to deliver this project objective, an equivalence study was undertaken. A reference core of soft magnetic material that has been measured over a period of many years at the National Physical Laboratory (NPL) was distributed and the dc and ac properties were measured. The measurement systems of the three laboratories are independent and this ensured that no correlations that could skew the results existed. In the analyses, the measurements at NPL were used as the baseline. With the core capabilities established, it was possible to develop the new measurements required to deliver the overall project objectives. At NPL, a standard permeameter method was modified for the measurement of strips. This was incorporated into a stress machine to measure the dc properties of the soft magnetic materials used by the partners, at stress levels of up to 700 MPa. The results for seven materials are presented and the changes in the properties with applied stress compared to establish which materials exhibit favorable properties.

Evidence for Quantum Interference in SAMs of Arylethynylene Thiolates in Tunneling Junctions with Eutectic Ga–In (EGaIn) Top-Contacts

This paper compares the current density (J) versus applied bias (V) of self-assembled monolayers (SAMs) of three different ethynylthiophenol-functionalized anthracene derivatives of approximately the same thickness with linear-conjugation (AC), cross-conjugation (AQ), and broken-conjugation (AH) using liquid eutectic Ga-In (EGaIn) supporting a native skin (~1 nm thick) of Ga(2)O(3) as a nondamaging, conformal top-contact. This skin imparts non-Newtonian rheological properties that distinguish EGaIn from other top-contacts; however, it may also have limited the maximum values of J observed for AC. The measured values of J for AH and AQ are not significantly different (J ≈ 10(-1)A/cm(2) at V = 0.4 V). For AC, however, J is 1 (using log averages) or 2 (using Gaussian fits) orders of magnitude higher than for AH and AQ. These values are in good qualitative agreement with gDFTB calculations on single AC, AQ, and AH molecules chemisorbed between Au contacts that predict currents, I, that are 2 orders of magnitude higher for AC than for AH at 0 < |V| < 0.4 V. The calculations predict a higher value of I for AQ than for AH; however, the magnitude is highly dependent on the position of the Fermi energy, which cannot be calculated precisely. In this sense, the theoretical predictions and experimental conclusions agree that linearly conjugated AC is significantly more conductive than either cross-conjugated AQ or broken conjugate AH and that AQ and AH cannot necessarily be easily differentiated from each other. These observations are ascribed to quantum interference effects. The agreement between the theoretical predictions on single molecules and the measurements on SAMs suggest that molecule-molecule interactions do not play a significant role in the transport properties of AC, AQ, and AH.

Relationship between Thermodynamic Data and Adsorption/Desorption Performance of Acid and Basic Dyes onto Activated Carbons

Adsorption properties of two activated carbons were investigated as a function of several operating parameters such as initial dye concentration, contact time, pH, and temperature. Adsorption/desorption performance was related to the structural and thermodynamic properties of the adsorbent. To evaluate the effect of the adsorbate an acid dye (alizarin red S, ARS) and a basic dye (methylene blue, MB) were used. Results obtained suggest that a low basicity of the adsorbent is a key factor independent of the acid or basic nature of the adsorbate. Langmuir isotherms best represent the adsorption system. Reported maximum adsorption capacities for ARS and MB were 762 mg·g−1 and 742 mg·g−1, respectively. The adsorption process followed the pseudo-first-order model. The thermodynamic parameters such as Gibbs energy (ΔG°), entalphy (ΔH°), and entropy (ΔS°) indicate that the adsorption of the basic dye was significantly dependent on AC properties. Hydrophobic interaction played a dominant role in the adsorption of the basic dye, whereas acid dye was bound through electrostatic interaction. This coincided with the lower activation energy values reported for MB [(9.58 to 18.68) kJ·mol−1] in comparison to ARS [(23.79 to 27.67) kJ·mol−1]. Finally, for adequate desorption it is more important to enhance interaction between dye and surfactant rather than the repulsion between micelles and the adsorbent surface.

Physical properties of Gd 7NiPd 2 single crystal

Investigations of X-ray diffraction, electronic structure, dc-magnetization M(T), ac-magnetic susceptibility χ ac (T) and magnetocaloric properties for the Gd 7NiPd 2 single crystal were performed. A single crystal of Gd 7NiPd 2 was grown by the Czochralski method from a levitating melt. Anomalies in χ ac (T) and M(T)-curves establish that Gd 7NiPd 2 undergoes a long-range ferromagnetic-type ordering at T C = 298 K, followed by a spin-reorientation below 135 K. The magnetization data indicate that there is an excess magnetic moment calculated per Gd 3+ ions. The measured XPS valence band indicates the hybridization effect between Gd 5 d, Ni 3 d and Pd 4 d states. The calculated values of entropy change Δ Sm for the examined compound amount to −6.85 J/K kg for the a-axis and −6.49 for the c-axis at 7 T.

Magnetic Properties of Crystalline NiFe Alloy Prepared by High-Energy Ball Milling and Compacting

The structure and magnetic properties of compacted microcrystalline NiFe (81 wt% of Ni) powder is investigated. The powder of NiFe alloy prepared by ball milling of ribbon (prepared by melt spinning) remains single phase material suitable for compaction in order to prepare soft magnetic material. The bulk samples were consolidated by uniaxial compaction of the powder in vacuum. By measuring of AC and DC magnetic properties it was found out that in bulk samples the displacement of domain walls is the dominant magnetization process, while rotation of magnetization vectors prevails in powder material.

Separation of junction and bundle resistance in single wall carbon nanotube percolation networks by impedance spectroscopy

Single wall carbon nanotube (SWNT) networks of different loadings were measured by impedance spectroscopy. The resistances of the junctions and bundles have been separated by modeling ac impedance spectroscopy data to an equivalent circuit of two parallel resistance-capacitance elements in series. The junction resistance was found to be 3–3.5 times higher than the bundle resistance. The dc and ac properties of the SWNT networks were found to obey a percolation scaling law, with parameters determined by dispersant type and SWNT purity. The values of the critical exponent in all cases were higher than the expected value of 1.3, which is related to widely distributed bundle and junction conductivities.

Graphene RF Transistor Performance

Recent excitement about graphene as a possible material for high-frequency electronics has lead to demonstrations of high-frequency field-effect transistors (FETs) in the last two years. Although graphene FETs operate by a different principle than silicon MOSFETs, and have different dc characteristics, their ac properties are quite similar. Hence the high intrinsic mobility of graphene leads to the expectation of high frequency operation of gated graphene FETs. Demonstrations of frequency response in the GHz regime have been shown using both exfoliated flakes and synthesized graphene. An intrinsic cut-off frequency as high as 100 GHz has been achieved in a 240-nm-gate graphene FET fabricated on a 2" wafer of epitaxially-grown graphene. This value exceeds that of Si MOSFETs at the same gate length, illustrating the potential of graphene for RF applications.

AC and dielectric properties of vacuum evaporated InTe bilayer thin films

III–VI compound semiconductors receive great attention due to its applications in memory devices, switching devices, gas sensors, hybrid solar cells, etc. InTe thin films were prepared by sequential thermal evaporation of In and Te at Ar atmosphere. X-ray diffraction pattern of the films shows that the films posses mixed phase of In2Te3 and In2Te5. Grain size (D) and dislocation density were calculated by using Scherer’s formula. Surface morphology of the film is analyzed by SEM and the surface is found to be agglomeration of well defined grains. EDS analysis reveals that elemental composition is in right ) ) �

Alternating-Current Electrical Conduction Properties of DyCoO3 Ceramics

Abstract The ac , dc conductivity and dielectric properties of DyCoO 3 were reported in the temperature range of 77 - 300Kand in the frequency range of 20 Hz - 100 kHz. It was observed that at low temperature, ac conductivity is much higherthna dc conductivity and the hopping carrier between localized states near the Fermi level was the dominant loss mechanism.A comparison of the measured ac conductivity ( ) was made with some of the models of hopping conductivity of theproposed earlier in the literature. It was observed that in DyCoO 3 the measured ac conductivity, over the entire frequencyand temperature region, can be explained reasonably well by assuming two contributions 1 ( ) and 2 ( ) to the measured ( ). The first, 1 ( ), which dominates at low temperature, may be due to impurity conduction in a small polaron; theecond,s 2 ( ), which dominates at higher temperatures, depending on the frequency of measurements, may be due to thehopping of a small polaron and is reasonable for the dielectric relaxation peak.Key words ac conductivity , dielectric relaxation, small polaron.

Nanocrystalline diamond as an electronic material: An impedance spectroscopic and Hall effect measurement study

Nanocrystalline diamond (NCD) has been grown using a nanodiamond seeding technique, leading to a dense form of this material, with grain sizes around 100 nm. The electrical properties of both intrinsic and lightly boron-doped NCD have been investigated using impedance spectroscopy and Hall effect measurements. For intrinsic material, both grain boundaries and grains themselves initially contribute to the frequency dependant impedance values recorded. However, boundary conduction can be removed and the films become highly resistive. Interestingly, the ac properties of these films are also excellent with a dielectric loss value ∼0.004 for frequencies up to 10 MHz. The dielectric properties of these NCD films are therefore as good as high quality large grain polycrystalline diamond films. In the case of boron-doped material, p-type material with good carrier mobility values (10–50 cm2/V s) can be produced at carrier concentrations around 1017 cm−3.

Numerical Simulations of the Effect of Microstructure on AC Conductivity of MWCNT/Polymer Nanocomposites

AbstractA continuum model is developed to investigate the microstructure‐dependent AC properties of MWCNT/polymer nanocomposites. The AC conductivity of the composite is increased by a higher curl ratio of MWCNTs. At a critical frequency ω0, the AC conductivity switches to a frequency‐dependent region. For high MWCNT content, the curl curliness of MWCNTs has only a weak influence on the AC conductivity. For medium MWCNT content, the AC conductivity became frequency‐dependent for low frequencies with decreasing curl ratio of MWCNTs, which cannot be explained by correlation length theory. An interpretation based on the linear circuit theory is given. With increasing size of MWCNT clusters, the critical frequency ω0 increases. It is also affected significantly by the crystallinity the polymer.magnified image

Calmodulin-Induced Conformational and Hydrodynamic Changes in the Catalytic Domain of Bordetella pertussis Adenylate Cyclase Toxin

Bordetella pertussis, the causative agent of whooping cough, secretes among various toxins an adenylate cyclase (CyaA) that displays a unique mechanism of cell invasion, which involves a direct translocation of its N-terminal catalytic domain (AC, 400 residues) across the plasma membrane of the eukaryotic targeted cells. Once into the cytosol, AC is activated by endogenous calmodulin and produces toxic amounts of cAMP. The structure of AC in complex with the C-terminal part of calmodulin has recently been determined. However, as the structure of the catalytic domain in the absence of calmodulin is still lacking, the molecular basis of AC activation by calmodulin remains largely unknown. To characterize this activation mechanism, we investigated here the biophysical properties of the isolated catalytic domain in solution with or without calmodulin. We found that calmodulin triggered only minor modifications of the protein secondary and tertiary structure but had a pronounced effect on the hydrodynamic properties of AC. Indeed, while the isolated catalytic domain was spherical and hydrated, it underwent a significant elongation as well as compaction and dehydration upon calmodulin interaction. On the basis of these data, we propose a model for the structural transition between the calmodulin-free and calmodulin-bound AC.

Effect of annealing temperature on magnetic properties of cold rolled high silicon steel thin sheet

Abstract 6.5 wt%Si–Fe alloy has excellent soft magnetic properties, but its room temperature brittleness impedes its industrial application. In this paper, we report that 6.5 wt%Si–Fe alloy could be cold rolled to 0.03–0.05 mm in thickness after specific thermomechanical processes including forging, hot rolling, warm rolling and heat treatments. The as rolled sheets have good shape and shining metal luster. Dc and Ac magnetic properties of the 0.05 mm cold rolled 6.5 wt%Si–Fe sheets were measured in dependence of annealing temperatures and times. Higher annealing temperature is helpful for Dc properties, while Ac properties for the high frequency (>10 kHz) reach maximum at 1100 °C.

Influence of Surface Properties of Activated Carbon on Photocatalytic Activity of TiO2 in 4-chlorophenol Degradation

This work shows the influence of specific surface area (BET) and surface pH of different activated carbons on the photocatalytic activity of UV-irradiated TiO2 in the 4-chlorophenol (4CP) photodegradation. Carbons were prepared from sawdust of Tabebuia Pentaphyla wood by physical activation with CO2 and by pyrolysis under N2 flow at tempera- tures from 450 up to 1000oC during 1h Chemical activation was also performed by the impregnation of precursor with ZnCl2, H3PO4 and KOH at different concentrations, followed by activation under N2 flow at 450oC by 1h. Comparison of apparent first-order rate constants between TiO2 against TiO2-AC indicate that TiO2 photoactivity is enhanced or inhibited as a function of AC properties and correlations with surface area and pHPZC were found suggesting that surface pH of car- bons is the most important parameter that influences TiO2 activity in the 4CP photodegradation.

Controlling Losses With Gates and Shields to Perfect a Quantum Hall Impedance Standard

We report new results and insights concerning the effect of side gates, back gates, and ac losses on the ac properties of the quantum Hall resistance (QHR). Three different loss mechanisms have been identified, namely, dissipative losses in the 2-D electron gas (2DEG), relaxation losses in a surface layer above the 2DEG, and dielectric losses in the GaAs substrate. A novel shielding configuration reduces the effect of these loss mechanisms and makes the ac QHR independent of frequency and current to better than 1 part in 108 at frequencies around 1 kHz.