Application Of Dc Field Research Articles

Innovative NDT Technique Based on Ferrofluids for Detection of Surface Cracks

An innovative NDT technique is proposed for surface inspection of materials not necessarily magnetic or conductive, based on local magnetic field variations due to ferrofluid deposited in the cracks. The feasibility of the technique is assessed preliminarily, based on signal detectability without applied external magnetic field, and under applied DC fields. The signals (local magnetic flux density variations) are quantified analytically, experimentally and numerically. The model agrees well with the tests, showing that the signal increases with the applied field strength, up to the saturation magnetization of the ferrofluid, and decreases with the distance to the crack longitudinal axis, and thus it can provide useful estimations of the signal. The proposed technique, requiring application of external fields to magnetize the ferrofluid to enhance the signal, seems promising: the model suggests that signals associated to cracks significantly smaller than surface cracks in a target application like aircraft skin panel inspection NASA STD-5009 are easily detectable with commercial magnetometers.

Mechanism of intrinsic dipole moment induction in quantum paraelectric SrTiO3

We measure the Ti Kβ resonant X-ray emission spectra of SrTiO3 in the quantum paraelectric phase. We find that charge transfer (CT) excitation is affected by temperature decrease and UV irradiation, which provides direct evidence of the fact that the induced electric dipole moment is derived from the Ti off-center displacement. The UV-assisted dipole moments are aligned parallel to the applied DC electric field, as inferred from the additional change in the CT peak. We conclude that the local polar region formed by UV irradiation evolves under DC field application at low temperature.

철기(Fe Based) 나노결정질 합금 분말코어의 코팅제 및 입도가 연자기적 특성에 미치는 영향

본 연구는 철기(Fe based) <TEX>$Fe_{73.5}Si_{13.5}B_9Nb_3Cu_1$</TEX> 나노결정 합금의 분말코어(powder core)의 연자기적 특성 향상을 위한 기초연구로서, 절연 코팅제의 첨가량 및 분말입도에 따른 투자율, 코어손실 및 DC 바이어스 특성을 주로 조사하였다. 우선 합금조성을 PFC 장치를 이용하여 비정질 합금리본을 제조한 후, 열처리, 미분쇄 및 분급하여 얻어진 합금분말에 절연 코팅제(PEI)의 첨가량을 0.5, 1.0, 2.0, 2.5 wt%로 변화시켜 <TEX>$16ton/cm^2$</TEX>으로 압축성형 및 결정화 열처리하여 제조한 토로이달 나노결정 분말코어(<TEX>$OD12.7mm{\times}ID7.62mm{\times}H4.75mm$</TEX>)는 절연 코팅제 함량이 증가할수록 투자율은 감소하였지만, 코어손실 및 DC 바이어스 특성은 향상됨을 확인하였다. 이러한 이유는 합금분말 절연 코팅제 첨가량이 증가할수록 비정질 합금분말 입자가 적어져 분말코어의 성형밀도가 낮아지기 때문으로 추정되었으며, 절연 코팅제의 함량은 1 wt%가 가장 적합한 것으로 판단되었다. 또한 절연 코팅제 함량을 1 wt%로 고정하고, 합금분말의 입도에 따라 제조한 분말코어의 경우, 실효투자율 및 코어손실은 입도가 클수록 우수하였지만, DC 바이어스 특성은 인가자장이 증가함에 따라 더욱 나빠짐을 확인하였다. 그 이유는 합금분말 표면의 코팅층 두께 차이에 의한 절연효과, 잔류기공 혹은 분말코어의 성형밀도 차이 등에 기인하는 것으로 추정되었다. This is a basic research for improving soft magnetic property of Fe based nano crystalline alloy powder core. The main study is done around characteristics of permeability, core loss, and DC bias depending on amount of insulation coating agent and particle size. First, <TEX>$Fe_{73.5}Si_{13.5}B_9Nb_3Cu_1$</TEX> amorphous alloy ribbon was fabricated by using the planar flow casting (PFC) device. Then, heat treatment and ball milling were done to obtain alloy powder. The amount of polyether imide (PEI) added to it was varied by 0.5, 1.0, 2.0, 2.5 wt% to have compression molding into <TEX>$16ton/cm^2$</TEX>. After going through crystalline heat treatment, the made toroidal nano crystalline powder core (<TEX>$OD12.7mm^*ID7.62mm^*H4.75mm$</TEX>) had smaller permeability as amount of insulation coating agent decreases. However, it was found out that core loss and DC bias characteristics have been improved. The reason for this results were expected to be because green density of power core decreases as amorphous alloy powder particles become smaller as amount of alloy powder insulation coating agent increases, it was determined that 1 wt% of insulation coating agent is appropriate. Also, for powder core made based on alloy powder size with amount of insulation coating agent fixed at 1 wt%, effective permeability and core loss were outstanding as particle size became bigger. However, characteristics of DC bias became worse as applied DC field increases. This is expected to be due to insulation effect, residual pores, or molding density of powder core resulting from thickness of coating on surface of alloy powder.

Strong Direct Magnetic Coupling in a Dinuclear Co(II) Tetrazine Radical Single-Molecule Magnet.

The ligand-centered radical complex [(CoTPMA)2 -μ-bmtz(.-) ](O3 SCF3 )3 ⋅CH3 CN (bmtz=3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine, TPMA=tris-(2-pyridylmethyl)amine) has been synthesized from the neutral bmtz precursor. Single-crystal X-ray diffraction studies have confirmed the presence of the ligand-centered radical. The Co(II) complex exhibits slow paramagnetic relaxation in an applied DC field with a barrier to spin reversal of 39 K. This behavior is a result of strong antiferromagnetic metal-radical coupling combined with positive axial and strong rhombic anisotropic contributions from the Co(II) ions.

Amplification and directional emission of surface acoustic waves by a two-dimensional electron gas

Amplification of surface acoustic waves (SAWs) by electron drift in a two-dimensional electron gas (2DEG) is analyzed analytically and confirmed experimentally. Calculations suggest that peak power gain per SAW radian occurs at a more practical carrier density for a 2DEG than for a bulk material. It is also shown that SAW emission with tunable directionality can be achieved by modulating a 2DEG's carrier density (to effect SAW generation) in the presence of an applied DC field that amplifies SAWs propagating in a particular direction while attenuating those propagating in the opposite direction.

Magnetic-field-induced Heisenberg to XY crossover in a quasi-2D quantum antiferromagnet

The magnetic-field-dependent ordering temperature of the quasi-2D quantum Heisenberg antiferromagnet (QHAF) Cu(pz)2(ClO4)2 was determined by calorimetric measurement in applied dc fields up to 33 tesla. The magnetic phase diagram shows a round maximum at 5.95 K and 17.5 T (at ≈ 1/3 of its saturation field), a 40 percent enhancement of the ordering temperature above the zero field value of 4.25 K. The enhancement and reentrance are consistent with predictions of a field-induced Heisenberg to XY crossover behavior for an ideal 2D QHAF system.

Exchange coupling and magnetic blocking in dilanthanide complexes bridged by the multi-electron redox-active ligand 2,3,5,6-tetra(2-pyridyl)pyrazine

The syntheses and magnetic properties of six new compounds featuring the radical-bridged dilanthanide complexes [(Cp*2Ln)2(μ-tppz˙)]+ (Ln = Gd, 1; Tb, 2; Dy, 3; tppz = 2,3,5,6-tetra(2-pyridyl)pyrazine) and [(Cp*2Ln)2(μ-tppz˙)]− (Ln = Gd, 4; Tb, 5, Dy, 6) are reported. Cyclic voltammograms for compounds 1–3 reveal that the tppz ligand can reversibly undergo multiple redox changes. Hence, in the two sets of compounds isolated, 1–3 and 4–6, the redox-active ligand tppz exists in the monoanionic (tppz˙−) and trianionic (tppz˙3−) forms, respectively. Substantial LnIII–tppz˙− exchange coupling is found for the cationic tppz˙− radical-bridged species of 1–3, as suggested by a rise in χMT at low temperatures. For the Gd compound 1, fits to the data yielded a coupling constant of J = −6.91(4) cm−1, revealing antiferromagnetic coupling to give an S = 13/2 ground state. Both of the TbIII and DyIII-containing compounds 2 and 3 exhibit single-molecule magnet behavior under zero applied dc field. Importantly, the Dy congener shows a divergence of the field-cooled and zero-field-cooled dc susceptibility data at 2.8 K and magnetic hysteresis below 3.25 K. Interestingly, the coupling constant of J = −6.29(3) cm−1 determined for the trianionic tppz˙3− radical-bridged Gd compound 4 is of similar magnitude to that of the tppz˙−-bridged analogue 1. However, the anionic tppz˙3−-bridged species containing TbIII and DyIII centers, compounds 5 and 6, do not exhibit slow magnetization dynamics under zero and applied dc fields. Computational results indicate a doublet ground state for the bridging tppz˙3− unit, with a different distribution for the spin density orientation towards the LnIII centers. These results have important implications for the future design of molecule-based magnets incorporating exchange-coupled lanthanide-radical species.

Large spin-relaxation barriers for the low-symmetry organolanthanide complexes [Cp*2 Ln(BPh4 )] (Cp*=pentamethylcyclopentadienyl; Ln=Tb, Dy).

Single-molecule magnets comprising one spin center represent a fundamental size limit for spin-based information storage. Such an application hinges upon the realization of molecules possessing substantial barriers to spin inversion. Axially symmetric complexes of lanthanides hold the most promise for this due to their inherently high magnetic anisotropies and low tunneling probabilities. Herein, we demonstrate that strikingly large spin reversal barriers of 216 and 331 cm(-1) can also be realized in low-symmetry lanthanide tetraphenylborate complexes of the type [Cp*2 Ln(BPh4 )] (Cp*=pentamethylcyclopentadienyl; Ln=Tb (1) and Dy (2)). The dysprosium congener showed hysteretic magnetization data up to 5.3 K. Further studies of the magnetic relaxation processes of 1 and 2 under applied dc fields and upon dilution within a matrix of [Cp*2 Y(BPh4 )] revealed considerable suppression of the tunneling pathway, emphasizing the strong influence of dipolar interactions on the low-temperature magnetization dynamics in these systems.

Utilization of graphene electrode in transparent microwell arrays for high throughput cell trapping and lysis

Here we present a high-throughput, transparent microfluidic device with embedded microwell arrays sandwiched between transparent electrodes made from graphene (at the bottom) and indium tin oxide (at the top) for dielectrophoretic cell trapping and electrical lysis. Graphene suppresses unwanted faradaic reaction effects on the cells and the medium that is typically observed in ITO based electrodes from application of DC field for electrical lysis. This is because graphene is more electrochemically inert than indium tin oxide (ITO) where ITO undergoes reduction–oxidation (redox) reaction in the presence of electrolyte in most standard cell media. This redox process also compromises ITO's electrical properties and optical transparency over multiple use. The presented microfluidic device shows high efficiency for cell trapping and lysis and an electrochemically stable behavior for long operational life.

Synthesis, crystal structure, and magnetic characterization of the three-dimensional compound [Co2(cbut)(H2O)3]n (H4cbut = 1,2,3,4-cyclobutanetetracarboxylic acid).

A novel cobalt(II) complex of formula [Co2(cbut)(H2O)3]n (1) (H4cbut = 1,2,3,4-cyclobutanetetracarboxylic acid) has been synthesized under hydrothermal conditions and its crystal structure has been determined by means of synchrotron radiation and neutron powder diffraction. The crystal structure of 1 consists of layers of cobalt(II) ions extending in the bc-plane which are pillared along the crystallographic a-axis through the skeleton of the cbut(4-) ligand. Three crystallographically independent cobalt(II) ions [Co(1), Co(2), and Co(3)] occur in 1. They are all six-coordinate with four carboxylate-oxygens [Co(1)-Co(3)] and two cis-[Co(1)] or trans-water molecules [Co(2) and Co(3)] building distorted octahedral surroundings. Regular alternating double oxo(carboxylate) [between Co(1) and Co(1a)] and oxo(carboxylate) plus one aqua and a syn-syn carboxylate bridges [between Co(1) and Co(2)] occur along the crystallographic b-axis, the values of the cobalt-cobalt separation being 3.1259(8) and 3.1555(6) Å, respectively. These chains are connected to the Co(3) atoms through the OCO carboxylate along the [011] direction leading to the organic-inorganic bc-layers with Co(1)-OCO(anti-syn)-Co(3) and Co(2)-OCO(anti-anti)-Co(3) distances of 5.750(2) and 4.872(1) Å. The shortest interlayer cobalt-cobalt separation through the cbut(4-) skeleton along the crystallographic a-axis is 7.028(2) Å. Variable-temperature magnetic susceptibility measurements show the occurrence of antiferromagnetic ordering with a Néel temperature of 5.0 K, followed by a field-induced ferromagnetic transition under applied dc fields larger than 1500 Oe. The magnetic structure of 1 has been elucidated at low temperatures in zero field by neutron powder diffraction measurements and was found to be formed by ferromagnetic chains running along the b-axis which are antiferromagnetically coupled with the Co(3) ions through the c-axis giving rise to noncompensated magnetic moments within each bc-layer (ferrimagnetic plane). The occurrence of an antitranslation operation between these layers produces a weak interlayer antiferromagnetic coupling along the a-axis which is overcome by dc fields greater than 1500 Oe resulting in a phase transition toward a ferromagnetic state (metamagnetic behavior).

Development of Robust HTS-SQUID for Non-destructive Inspection System in Unshielded Environment

Cross-shaped YBa2Cu3O7−x (YBCO) film was overlapped on directly-coupled multi-pickup-coil HTS-dc-SQUID magnetometer in flip-chip configuration as a superconducting shield, and the characterestics of the SQUID were examined in DC and AC magnetic fields. We created slots in the YBCO film and pickup-coil of HTS-SQUID magnetometer for suppression of flux trapping. In low magnetic field environment, we measured the characteristics of the SQUID without and with HTS film. The most of the parameters were same with and without the HTS films, while S1/2ϕ with the HTS film was a bit smaller than that of the bare SQUID. In the DC field, Ic of the HTS-SQUID covered with the HTS film did not change until a DC field over 140 μT was applied to the SQUID, while the bare SQUID's Ic decreased with the increase of the applied DC field. In the AC fields at 10 MHz, the SQUID with the HTS film showed degradation of the parameters such as Vpp and S12ϕ at lower field amplitude than the bare SQUID.

Optimized Braking Torque Generation Capacity of an Eddy Current Brake With the Application of Time-Varying Magnetic Fields

Unlike conventional hydraulic brakes (CHBs), eddy-current brakes (ECBs) are electrically controlled and noncontact-type actuators. Having such advantages makes ECBs a potential alternative to the conventional systems used in vehicles. However, the braking torque generation of ECBs at low speed is insufficient to stop the vehicle. Thus, such brakes are used for assistive braking. Our previous study showed that braking torque increases with an ac field application compared with the torque generated with a dc field application. To increase the braking torque generation to realize a stand-alone ECB, the braking torque generation of the ECB is optimized, using a stochastic search algorithm, with the consideration of comfort and skin effects, as well as geometric and field-dependent factors. The results show that the application of an ac field with varying frequency on a configuration with multiple pole projection areas (PPAs) results in a significant increase in the braking torque compared with the dc field application, and such optimum ECB configuration can generate a sufficient amount of braking torque comparable with those generated by CHBs.

Charge trapping and transport phenomenon in aged and unaged epoxy resin and polyethylene

Summary Polymeric dielectrics are prone to space charge trapping within their volume. This in turn affects many of their dielectric properties including breakdown strength and charge transport. In the current paper, we attempt to characterise charge accumulation and transport in polymeric dielectrics under the application of DC field. Epoxy (EP) resin and polyethylene (PE) are chosen as the dielectric materials to be investigated on account of their prevalence in the high voltage industry. Pulsed electro-acoustic measurements are performed on EP resin and PE specimens. Further, effect of ageing on charge trapping is studied for both EP resin and PE, by thermally ageing the materials under controlled conditions. In addition, the dielectric materials are exposed to a humid environment over long periods and the effect of prolonged exposure to moisture on charge trapping studied. From the experimental results, density of trapped charge is obtained for the different cases. Fourier transform infrared spectroscopy is carried out to identify chemical changes in dielectric materials due to ageing, with an aim to relating physicochemical changes in the material to charge trapping. Copyright © 2014 John Wiley & Sons, Ltd.

The effect of a direct current field on the microparticle charge in the plasma afterglow

Residual charges of individual microparticles forming dense clouds were measured in a RF discharge afterglow. Experiments were performed under microgravity conditions on board the International Space Station, which ensured particle levitation inside the gas volume after the plasma switch-off. The distribution of residual charges as well as the spatial distribution of charged particles across the cloud were analyzed by applying a low-frequency voltage to the electrodes and measuring amplitudes of the resulting particle oscillations. Upon “free decharging” conditions, the charge distribution had a sharp peak at zero and was rather symmetric (with charges concentrated between −10e and +10e), yet positively and negatively charged particles were homogeneously distributed over the cloud. However, when decharging evolved in the presence of an external DC field (applied shortly before the plasma switch-off) practically all residual charges were positive. In this case, the overall charge distribution had a sharp peak at about +15e and was highly asymmetric, while the spatial distribution exhibited a significant charge gradient along the direction of the applied DC field.

Structure and ferroelectric studies of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 piezoelectric ceramics

We have synthesized and studied the structural and ferroelectric properties of lead-free 0.5(Ba0.7Ca0.3)TiO3–0.5Ba(Zr0.2Ti0.8)O3 ceramics in the temperature region of its ferroelectric transition. The synthesized material showed high dielectric constant, low loss and good pyroelectric figure of merit. From the temperature dependent X-ray diffraction measurements, we determined the tricritical point to be in the temperature range of 303–400K. The dielectric measurements indicate a diffuse ferroelectric phase transition (DPT) around 360K in agreement with the X-ray measurements. We studied the evolution of Raman spectra with temperature to understand the nature of phase transition in BaTiO3 (BTO) and the BCTZO. The results indicates that the transition of ferroelectric–paraelectric state is not sharp as in the case of BTO and the polar state persists through the paraelectric state. In general, our study indicates that there are ferroelectric domains of nanometer size beyond the commonly defined transition temperature. The observation of local piezoelectric hysteresis loop indicated the existence of intrinsic ferroelectric property of the ceramic at the nanoscale. The ceramics exhibited electric field tunable dielectric properties with a tunability of 82% at an applied DC field of 40kVcm−1, low dielectric loss of 0.001 and room temperature pyroelectric coefficient of 6×10−8Ccm−2K−1 and the detectivity of 1.9×10−8Ccm−1J−1; larger than those reported for other BaTiO3-based materials. Overall, our results indicate that BCTZO ceramics with coexistence of rhombohedral–tetragonal phases is a promising candidate for lead-free ferroelectric applications.

Giant self-biased magnetoelectric coupling in co-fired textured layered composites

Co-fired magnetostrictive/piezoelectric/magnetostrictive laminate structure with silver inner electrode was synthesized and characterized. We demonstrate integration of textured piezoelectric microstructure with the cost-effective low-temperature co-fired layered structure to achieve strong magnetoelectric coupling. Using the co-fired composite, a strategy was developed based upon the hysteretic response of nickel-copper-zinc ferrite magnetostrictive materials to achieve peak magnetoelectric response at zero DC bias, referred as self-biased magnetoelectric response. Fundamental understanding of self-bias phenomenon in composites with single phase magnetic material was investigated by quantifying the magnetization and piezomagnetic changes with applied DC field. We delineate the contribution arising from the interfacial strain and inherent magnetic hysteretic behavior of copper modified nickel-zinc ferrite towards self-bias response.

Anisotropy of the upper critical fields and the paramagnetic Meissner effect in La1.85Sr0.15CuO4 single crystals

Optimally doped La1.85Sr0.15CuO4 single crystals have been investigated by dc and ac magnetic measurements. These crystals have rectangular needle-like shapes with the long needle axis parallel to the crystallographic c axis (c-crystal) or parallel to the basal planes (a-crystal). In both crystals, the temperature dependence of the upper critical fields (HC2) and the surface critical field (HC3) were measured. The H–T phase diagram is presented. Close to TC = 35 K, for the c-crystal, , whereas for the a-crystal the obtained is much higher than 1.69, predicted by the ideal mathematical model. At low applied dc fields, positive field-cooled branches known as the ‘paramagnetic Meissner effect’ (PME) are observed; their magnitude is inversely proportional to H. The anisotropic PME is observed in both a- and c-crystals, only when the applied field is along the basal planes. It is speculated that the high γa and the PME are connected to each other.

Investigation of Giant Magneto-Impedance Effect in Inorganic Nickel Coated Amorphous Ribbons

Co-based amorphous ribbon Fe5.85Co70.15Mo4B15Si5 (Metglas 2705X) was coated by Nickel complexes in order to increase the giant magneto-impedance response. Influence of inorganic nickel complex coating on giant-magneto impedance (GMI) effect was investigated under varied applied dc fields (–8 kA/m < H dc < 8 kA/m) at a frequency range of 0.1–3.5 MHz. The results showed that the GMI effect and operating frequency on Co-based amorphous ribbons, which were coated by Ni film, have significantly improved as compared to the uncoated samples. The variations on permeability, transverse anisotpy and flux penetration in the coated ribbons cause this improvement in the GMI effect.

Enhancement of COD-Nutrients Removals and Filterability of Secondary Clarifier Municipal Wastewater Influent Using Electrocoagulation Technique

The present work studied experimentally the variations of some characteristics of the mixed liquor solution in wastewater treatment plants (WWTPs) under the effect of electrocoagulation (EC) technique. Unlike conventional methods, electrocoagulation was performed using cylindrical perforated iron electrodes to achieve a good distribution of the applied DC field onto the municipal wastewater. Effects of electrocoagulation time, applied voltage gradient, and aeration on the mixed liquor characteristics were considered. The study figured out the dependency of the local pH and oxidation potential reduction (OPR) values, near anode and cathode on these operational conditions. It was found that electrocoagulation process reached steady state conditions in no more than 60 minutes. At high voltage gradient (6 V/cm), the steady removal efficiencies of COD and nutrients exceeded 89%. While aeration showed moderate influence on removal efficiencies and wastewater sludge filterability, it significantly affected the pH and ORP near the electrodes especially at the low voltage gradient. Results also indicated that the sludge filterability was improved notably by increasing the voltage gradient to have minimal levels of specific-resistance to filtration (SRF).

CYM and RGB colored electronic inks based on silica-coated organic pigments for full-color electrophoretic displays

Chromatic organic pigment–SiO2 composite particles with six different colors (CYM and RGB) were prepared by a modified sol–gel method. With the aid of a cationic polyelectrolyte, poly(diallyldimethylammoniumchloride), amorphous silica was coated onto the surface of organic pigments which was revealed by the variation of the zeta potential and average particle size, together with TEM images and EDX and XPS results. Because of the fine scattering properties of silica to visible light, the intensity of the reflectance peak increased significantly, which makes the composite particles more vivid than the organic pigments. The chromatic electrophoretic suspension was prepared by mixing Span 85 and the composite particles anchored with a self-made block copolymer PLMA-b-PDMAEMA in tetrachloroethylene. All the colored ink particles have similar zeta potentials and mobility of about 30 mV and 4–5 × 10−10 m2 V−1 s−1, respectively. White/color (CYM and RGB) dual-particle electronic ink containing PLMA-b-PDMAEMA treated pigment–SiO2 composite particles and TiO2 grafted with a polymer present excellent performance and quick response under an applied DC field of 0.15 V μm−1.