Applied Dc Bias Research Articles

The microwave properties of tin sulfide thin films prepared by RF magnetron sputtering techniques

In this paper we present the microwave properties of tin sulfide (SnS) thin films with the thickness of just 10 nm, grown by RF magnetron sputtering techniques on a 4 inch silicon dioxide/high-resistivity silicon wafer. In this respect, interdigitated capacitors in coplanar waveguide technology were fabricated directly on the SnS film to be used as both phase shifters and detectors, depending on the ferroelectric or semiconductor behaviour of the SnS material. The ferroelectricity of the semiconducting thin layer manifests itself in a strong dependence of the electrical permittivity on the applied DC bias voltage, which induces a phase shift of 30 degrees mm−1 at 1 GHz and of 8 degrees mm−1 at 10 GHz, whereas the transmission losses are less than 2 dB in the frequency range 2–20 GHz. We have also investigated the microwave detection properties of SnS, obtaining at 1 GHz a voltage responsivity of about 30 mV mW−1 in the unbiased case and with an input power level of only 16 μW.

Zero-Bias Power-Detector Circuits based on MoS2 Field-Effect Transistors on Wafer-Scale Flexible Substrates.

The design, fabrication, and characterization of wafer-scale, zero-bias power detectors based on 2D MoS2 field-effect transistors (FETs) are demonstrated. The MoS2 FETs are fabricated using a wafer-scale process on 8μm-thick polyimide film, which, in principle, serves as a flexible substrate. The performances of two chemical vapor deposition MoS2 sheets, grown with different processes and showing different thicknesses, are analyzed and compared from the single device fabrication and characterization steps to the circuit level. The power-detector prototypes exploit the nonlinearity of the transistors above the cut-off frequency of the devices. The proposed detectors are designed employing a transistor model based on measurement results. The fabricated circuits operate in the Ku-band between 12 and 18GHz, with a demonstrated voltage responsivity of 45V W-1 at 18GHz in the case of monolayer MoS2 and 104V W-1 at 16GHz in the case of multilayer MoS2 , both achieved without applied DC bias. They are the best-performing power detectors fabricated on flexible substrate reported to date. The measured dynamic range exceeds 30dB, outperforming other semiconductor technologies like silicon complementary metal-oxide-semiconductor circuits and GaAs Schottky diodes.

Non-enzymatic electrochemical detection of H2O2 using Ni (OH)2 nanoparticles

In this study, we have reported the synthesis of Ni (OH)2 nanoparticles by simple chemical reduction method for non-enzymatic electrochemical sensing of H2O2. The as prepared Ni(OH)2 nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared spectroscopy (FTIR) and energy dispersive X-ray (EDX) spectroscopy techniques as a means of the structural, morphological, and elemental analyses. The electrocatalytic activity of Ni (OH)2 towards reduction of H2O2 in 0.1 M phosphate buffer (PBS) solution was studied through cyclic voltammetry (CV) technique. From the CV responses, it has been realized that the as fabricated Ni (OH)2 based sensor electrode can effectively reduce H2O2 into water and oxygen at an applied dc bias voltage of −1.0 V. Moreover, the electroactive area (A) and rate constant (ks) of the redox process were determined to be 3.36 cm2 and 0.404 s−1; respectively. Our sensor exhibits a low detection limit of 26.4 µM and with a good sensitivity value of 1660 µA.mM−1.cm−2 under a wide linear range of 30–320 µM of analyte concentration. The proposed sensor electrode exhibits excellent redox activity towards sensing H2O2 and offering high stability.

Tunable and Miniaturized Microwave Filters Using Carbon Nanotube-Based Variable Capacitors

In this paper, we introduce a novel concept of tunable and miniaturized filtersthat embed, as voltage-controlled elements, state-of-the-art variable capacitors, based on vertically aligned carbon nanotubes (VACNTs). Starting from a theoretical estimation of the voltage-dependent capacitance between two adjacent CNTs, we extended this physics principle to a large matrix of CNTs, suitably placed on the molybdenum electrodes of an interdigitatedcapacitor (IDC), since molybdenum can withstand the high temperature necessary in the plasma process for the growth of the VACNTs. The IDC is the tunable element of a microwave filter, which must fulfill the need for both reconfigurability (being either a low-pass, a high-pass, or a band-pass filter, at discretion) and low-voltage frequency tuning of reflection&#x002F;transmission coefficients. For all these reasons, a very compact layout made of T-type cells (comprising VACNT-based variable capacitors and distributed inductors) was designed, simulated, fabricated, and tested, targeting the C, X, and K_u bands (4&#x2013;16 GHz) for wireless and radar applications. Taking as a reference the free-space wavelength <i>&#x03BB;</i>₀ at 10 GHz, the band-pass filter has overall dimensions of just 3.19 mm &#x00D7; 3.47 mm (i.e., 0.11<i>&#x03BB;</i>₀ &#x00D7; 0.12<i>&#x03BB;</i>₀), with the minimum of the reflection coefficient shifting of 1.16 GHz (within the X band) for an applied dc bias voltage of just 4 V and spanning between &#x2212;24.81 dB and &#x2212;36.13 dB. Furthermore, the maximum rejection is 31.65 dB, and the 3-dB fractional bandwidth is 12.44&#x0025;. The proposed filters are the proof that nanomaterials can be profitably integrated into microwave components for next-generation transceivers.

(Invited) DC-Bias and Temperature Effect on the Dielectric Properties of a Polyimide Thin Layer

Photo-processable Polyimide is widely used as a thin sensitive layer (less than 4µm) for relative humidity or as a passivation layer (higher than 4µm) for power semiconductor devices (Si, SiC, GaN, etc.). In this work, an interdigitated transducer (IDT) was produced by standard photolithography on glass substrate. Each IDT consists of two interdigitated gold electrodes with filling area of 2×2 mm2 and two wires around the interdigitated electrodes, one to heat the sensor and the other to measure the local temperature. Then the polyimide material was spin coated homogeneously on top of the planar electrodes to obtain different layer thicknesses ranging from 1 to 11 µm.The electrical properties of the sensors (coated IDT with polyimide) were monitored by impedance spectroscopy using a HP4192 4192A LF impedance analysis controlled by a PC, permitting automated data collection. The advantage of our sensor is the possibility to carry out the impedance measurements at different temperatures (from RT to 350°C) by locally heating up the sensor using the integrated heater around the electrodes. The design of the realized sensor is adequate to study the sensitivity of the polyimide layer to relative humidity as well as the DC-bias and temperature effect on its dielectric properties.First, impedance measurements were performed from 1 MHz to 100 Hz to obtain the optimum frequency at which the sensor capacitance changes linearly with relative humidity. Next, capacitive measurements of the sensor were recorded at 100 kHz under different relative humidity levels. The detected signal, the capacitance, follows a linear dependence on the relative humidity once it is varied from 5 to 80%. The coating of the sensor with an optimal thickness of polyimide allows the best performance of the humidity sensor in terms of response and recovery times.Second, we studied the effect of temperature and DC bias on the change in sensor capacitance at a constant frequency of 100 kHz and under a constant flux of 250 sccm N2. The temperature and DC bias were increased from 20 to 350°C and from 0 to 35 V, respectively. Only when the sensor temperature exceeds 280°C does the DC bias begin to have an effect on the measured capacitance. The sharp increase in capacitance is due to the increase in charge carriers. This peak immediately decreases exponentially even before the bias is removed. The decrease can be referred to the loss of energy, due to the transport of charge carriers. Since electromigration is active at temperatures above 200 °C, this could explain why an effect starts to occur when sensors are heated above this threshold. The large increase in capacitance at higher temperature and higher DC current is generally reproducible in terms of shape but not in terms of magnitude. At higher temperature and under higher DC bias, a sharp increase in capacitance usually appears and then followed by an exponential decrease to a constant value even though the sensor is still under the same DC bias.Finally, we performed impedance measurements of our sensors over the frequency range 100 to 1MHz. From these measurements, we deduced the complex electric modulus, which corresponds to the dielectric relaxation processes inside a medium, if an electric field is applied and which is inversely proportional to the complex permittivity. This gives an alternative approach to analyze the electrical response of the Polyimide material and allows to study the relaxation phenomena, to study the long-range conductivities, as well as to identify the localized dielectric relaxation phenomena. The real part of the electrical modulus shows a decrease with the increase of the DC bias (from 0 to 35V) for frequencies below 30 kHz. Its value tends to zero for low frequencies and reaches a plateau for high frequencies. But not affected by the applied bias. This suggests a decrease in relaxation processes by an applied electric field, especially for low frequencies. At frequencies below a few kHz, space charge polarizations are the most dominant mechanism. In the case of a strong electric field obtained by a higher DC polarization, the relaxation process may be prohibited, resulting in a lower value of the electric modulus. It reaches a constant plateau at high frequencies, suggesting that the relaxation mechanism is mainly frequency dependent rather than DC bias dependent, which is in good agreement with the literature. The measurement of the electrical modulus suggests that the relaxation mechanisms are mainly frequency dependent, rather than dependent on the applied DC bias.

Plasma-enhanced electrostatic precipitation of diesel exhaust using high voltage nanosecond pulse discharge

We have demonstrated nearly 80% remediation of diesel particulates from an 18 kW (i.e., 24.4 HP) diesel engine with just 1.6% of the engine power using a transient pulsed plasma in conjunction with an applied DC bias voltage. This result was achieved using a high voltage (~10 kV) DC bias together with nanosecond high voltage pulses (~20 kV) in what we refer to as a plasma-enhanced electrostatic precipitator (PE-ESP). Here, the remediation takes place in a two-step process in which the particles are first charged by the ions in the plasma and then they are swept out to the collecting electrode by the applied DC bias. The fast rise times (i.e., dV/dt≈ 1012V/sec) associated with these nanosecond pulses produce a streamer discharge with ion densities that are more than one order of magnitude higher than conventional DC coronas. These extremely high ion densities produce the enhanced electrostatic precipitation observed here. In fact, little or no remediation was observed with the DC bias only or with the nanosecond pulses only, and it is the combination of these two applied fields (pulsed and DC) that produces the robust enhancement observed here. This general approach of using nanosecond pulse discharge to produce plasma-enhanced electrostatic precipitation opens up new degrees of freedom in the design of new and more compact ESPs.

Enhanced Dielectric Relaxation in Self-Organized Layers of Polypeptides Coupled to Platinum Nanoparticles: Temperature Dependence and Effect of Bias Voltage

Using alternative current impedance spectroscopy, we investigate the dynamical conductivity of hybrid nanomaterials composed of helical polypeptide layers containing platinum nanoparticles (PtNP). The electrical characteristics of the self-organized poly(γ-benzyl-l-glutamate) (PBGL) in bidimensional lamellar assembly in the presence of Pt nanoparticles are well modeled and described by a single equivalent circuit of parallel resistance and capacitance. The latter are determined using a comparison between the measured and calculated Nyquist plots, which allows extracting the characteristic relaxation time and frequency of the dipolar relaxation process. We found that the relaxation frequency in the PBLG–PtNP hybrid materials is enhanced by 4 orders of magnitudes compared to pure PBLG, which indicates a much faster dielectric relaxation in PBLG–PtNP due to dipole orientation and dipole–dipole interactions. The temperature dependence of the relaxation time is analyzed using Arrhenius plots, from which the activation energy of the relaxation process is found to be around 0.1 eV. Such a value close to the peptide vibration energy of the PBLG indicates a vibration-assisted relaxation process and a polaronic charge transport mechanism. An advantage of the PBLG–PtNP nanocomposite material is that the activation energy can be finely tuned by the PBLG degree of polymerization. Finally, an important outcome of this work is the investigation of the dielectric relaxation process in PBLG–PtNP under applied DC bias. We found that the activation energy decreases with increasing bias voltage for all degrees of polymerization of the PBLG molecule. This effect is interpreted in terms of electric field-induced alignment of the dipoles and of increased mobility of the polaronic charge carriers. The presence of piezoelectricity in the hybrid material gives the possibility to use the DC bias as a simple mean of monitoring the dynamical conductivity involving polaronic states.

Investigation of internal fields in organic semiconductors in the presence of traps

In an organic semiconductor optoelectronic device, the built-in field within the active layer is typically determined by the difference in contact potentials of the device. However, the presence of space charges and trap states contribute to the electric field within the thin film. Depending on the maximum applied forward voltage, the trap states can be charged, inducing hysteresis in the optoelectronic response of the system. In this work, we investigate the electric fields inside organic photovoltaic device structures, in the presence of traps, using electroabsorption (EA) spectroscopy. Comparing simulations with our experimental results, we explained the origin of hysteresis in the electroabsorption signal as a function of applied DC bias. We solved Poisson’s equation to estimate the densities of trapped carriers in the active layers. The filled trap densities in poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]thiadiazole)] (PPDT2FBT) were found to be ∼1×1017 and ∼6×1016 cm−3, respectively. From the transient EA measurements, the estimated values of energies of the trap states with respect to the HOMO level were 0.82 and 0.76 eV in P3HT and 0.70 and 0.64 eV in PPDT2FBT, which indicated the presence of midgap traps in these organic semiconductor thin films. Such trap induced changes in the internal fields within the active layers, affect the mobility and carrier transport in the organic optoelectronic devices. The midgap traps lead to exciton quenching and also act as non-radiative recombination centers, resulting in reduction in luminescence efficiency of the active layers.

A Structural and ac Conductivity Study on Li Doped ZnO

In the present work, the incorporation of Li in a wurtzite ZnO host synthesized through a chemical route has been studied experimentally. Starting with Yoshio’s model, [K. Yoshio, A. Onodera, H. Satoh, N. Sakagami and H. Yamashita, Ferroelectrics, 264, 133 (2001)] structural refinement of the powder diffraction data suggests that Li has a mixed occupancy at both the substitutional 2b (≈1.2%) and interstitial 2a (≈12.6%) Wykoff sites. In comparison with a pure ZnO sample, the net result appears to be an overall expansion of the unit cell by ∼0.07% along both a and c -axes. Temperature dependant ac conductivity measurements carried out using a two electrode assembly and the influence of superimposed dc bias suggest that the conduction may be governed by both the presence of interstitial Li and polar defect complexes also. The grains seem to respond more to an applied dc bias (an order of magnitude change in resistivity) than grain boundaries.

Contribution of Triple/Double Phase Boundary Reactions in Mixed Conducting Oxide Cathodes in SOFCs and PCFCs

Improvement of cathodic performance is one of challenges for full-scale commercialization of solid oxide fuel cells (SOFCs) and protonic ceramic fuel cells (PCFCs). At cathodes in SOFCs and PCFCs, oxygen gas is electrochemically reduced. The cathodic reacitons basically takes place only at triple phase boundaries (TPBs) of the electrolyte, the electrode, and the gas phases, because ions (oxide ion or proton), electrons, and oxygen gas molcules exist in the different phases. On the other hand, when a mixed ionic-electronic conductor (MIEC) is used as the electrode material, the reaction site can be expanded from TPBs to double phase boundaries (DPBs) of the electrode and the gas phases, since ions can exist not only in the electrolyte but also in the electrode in this case. Thus, it is generally believed that DPBs are the dominant reaction sites in an MIEC cathode, because the DPB area is much larger than the TPB area. However, as far as we know, no one could experimentally succeed to show how significantly the TPB reaction contributes to the total reaction in an MIEC cathode. In order to properly design a high pefromance cathode, it is important to understand the contribution of TPB/DPB reactions separately and quantitatively.In practical SOFCs and PCFCs, porous electrodes are conventionally used. However, it is not easy to quantitatively evaluate the contributions of TPB/DPB reactions from experiments using porous electrodes because of their complicated microstructures and inhomogeneous electrode reaction distribution [1]. In order to avoid such experimental difficulties with porous electrodes, many researchers applied the model electrodes, such as a dense thin film electrode and a microelectrode. However, these conventional model electrodes tend to overestimate the contribution of the DPB reaction and thus are still not appropriate to investigate the contributions of the TPB/DPB reactions [2]. Instead, our group proposed new and original model electrodes, which is so-called "patterned thin film electrodes", as schematically illustrated in Figs. 1(A) and (B) [2]. These model electrodes are a kind of thin film electrodes, but the area of the electrode/electrolyte contact was limitted by inserting the slitted insulating layer between the electrode and the electrolyte. In the model electrodes of Fig. 1(B), a part of the electrode film was removed to introduce TPBs. By utilizing these model electrodes, the contribution of the DPB reaction can be evaluated from the model electrode (A), whereas the contribution of the TPB reaction from the difference between the model electrodes (A) and (B). These model electrode can be fabricated by helps of photolithography, pulsed laser deposition, and gas spputtering techniques.By applying the aforementioned model electrodes, we first investigated the contributions of TPB/DPB reactions in SOFC MIEC cathodes, for instance an La0.6Sr0.4CoO3-d or La0.6Sr0.4Co0.2Fe0.8O3-d cathode on a Ce0.9Gd0.1O1.95 electrolyte. We consequently found: (i) DPBs are the dominant reaction sites at operation temperatures of conventional SOFCs (i.e. above 973 K), but (ii) contribution of TPB reactions becomes significant as temperature decreases below 873 K, amibient p(O2) increases, and the applied DC bias decrases. These results tell us that the contribution of TPB reactions should be taken into account for desigining the cathode in intermediate tempeature SOFCs, even when MIEC is used as electrode materials. Similar measurements were performed to investigate the cathodic reactions in PCFCs. Here La0.6Sr0.4CoO3-d and BaGd0.3La0.7Co2O6-δ were chosen as cathode materials. As results, it was found that the dominant reaciton site is stongly affected by electrode material, possibly depending on protonic conductivity in the electrode materials. More details of our researches on SOFC and PCFC cathodic reacions by using patterned thin film model electrodes will be reported in the presentation.Acknowledgements : A part of this work was supported by NEDO, Japan. Reference s [1] S. B. Adler, Chem. Rev., 104, 4791 (2004).[2] K. Mizuno, et al., to be submitted. Figure 1

Quantum dynamics of a single fluxon in Josephson-junction parallel arrays with large kinetic inductances

We present a theoretical study of coherent quantum dynamics of a single magnetic fluxon (MF) trapped in Josephson junction parallel arrays (JJPAs) with large kinetic inductances. The MF is the topological excitation carrying one quantum of magnetic flux, $\Phi_0$. The MF is quantitatively described as the $2\pi$-kink in the distribution of Josephson phases, and for JJPAs with high kinetic inductances the characteristic length of such distribution ("the size" of MF) is drastically reduced. Characterizing such MFs by the Josephson phases of three consecutive Josephson junctions we analyse the various coherent macroscopic quantum effects in the MF quantum dynamics. In particular, we obtain the MF energy band originating from the coherent quantum tunnelling of a single MF between adjacent cells of JJPAs. The dependencies of the band width $\Delta$ on the Josephson coupling energy $E_J$, charging energy $E_C$ and the inductive energy of a cell $E_L$, are studied in detail. In long linear JJPAs the coherent quantum dynamics of MF demonstrates decaying quantum oscillations with characteristic frequency $f_{qb}=\Delta/h$. In short annular JJPAs the coherent quantum dynamics of MF displays complex oscillations controlled by the Aharonov-Casher phase $\chi \propto V_g$, where $V_g$ is an externally applied gate voltage. In the presence of externally applied dc bias, $I$, a weakly incoherent dynamics of quantum MF is realized in the form of macroscopic Bloch oscillations leading to a typical "nose" current-voltage characteristics of JJPAs. As ac current with frequency $f$ is applied the current-voltage characteristics displays a set of equidistant current steps at $I_n=2en f$.

Switchable Transducers in GaN MEMS Resonators: Performance Comparison and Analysis.

This work presents a comprehensive comparison of switchable electromechanical transducers in an AlN/GaN heterostructure toward the goal of reconfigurable RF building blocks in next-generation ad hoc radios. The transducers’ inherent switching was achieved by depleting a 2D electron gas (2DEG) channel, allowing an RF signal launched by interdigital transducers (IDTs) to effectively excite the symmetric (So) Lamb mode of vibration in the piezoelectric membrane. Different configurations for applying DC bias to the channel for electromechanical actuation in the piezoelectric are discussed. Complete suppression of the mechanical mode was achieved with the transducers in the OFF state. Equivalent circuit models were developed to extract parameters from measurements by fitting in both ON and OFF states. This is the first time that an extensive comparative study of the performance of different switchable transducers in their ON/OFF state is presented along with frequency scaling of the resonant mode. The switchable transducer with Ohmic IDTs and a Schottky control gate showed superior performance among the designs under consideration.

Experimental analysis of saturated core fault current limiter performance at different fault inception angles with varying DC bias

Economic growth has exponentially increased the power demand resulting in bulk power systems. The distributed generations are also integrating into the grid network to retrieve the benefits of the locally available generating resources. Also, the high-reliability expectations of supply systems lead to the interconnection of power networks. All these modern characteristics escalated the magnitude of short circuit currents in the grids. The security from these currents, therefore, challenged the power network protection systems. The deployment of the scalable and reliable Fault Current Limiter (FCL) technology can enhance the service span of existing components in the power industry. The Saturated Core Fault Current Limiter (SCFCL) can be a commercially viable solution to this problem. This paper reports the test performance of a scaled-down, compact, novel, copper-coils-based SCFCL model. The steady-state and transient-state behavior have been characterized in terms of saturation levels and the fault inception angles. The importance of DC bias selection in the optimum current limiting performance of the device is demonstrated in the results. The results also exhibit that the fault inception at the peak of voltage causes the most asymmetric transient during the current limiting process. Also, the heuristic design methodology based on extensive experimental work has been proposed, taking into account an application of optimum DC bias.

Large and Local Magnetoresistance in a State-of-the-Art Perpendicular Magnetic Medium.

PurposeMagnetotransport properties of granular oxide-segregated CoPtCr films were studied on both macroscopic and microscopic length scales by performing bulk and point-contact magnetoresistance measurements, respectively. Such a perpendicular magnetic medium is used in state-of-the-art hard disc drives and, when combined with magnetotransport phenomena for read/write operations, may lead to a novel concept for magnetic recording with high areal density.Materials and MethodsThe CoPtCr films were deposited by an epitaxy-like sputtering and contained several perpendicularly magnetized granular-media layers with different coercivities; they are very much like the state-of-the-art perpendicular magnetic medium, which can be found in today’s hard disc drives. Magnetoresistive properties of bulk films were assessed by measuring the film resistance in the standard Van der Pauw geometry, while the local transport was probed by the point-contact technique.ResultsThe bulk measurements showed only a negligible magnetoresistance of less than 0.02%. In contrast, the local point-contact measurements revealed giant-magnetoresistance-like changes ΔR in local resistance of the contact R with more than 10,000% ratio ΔR/R.ConclusionThe observed large and local magnetoresistive effect could be tentatively attributed to a tunnel magnetoresistance between oxide-segregated CoPtCr grains with different coercivities. The tunneling picture of electronic transport in our granular medium was confirmed by the observation of tunneling-like current–voltage characteristics of the contacts and bias dependence of the contact magnetoresistance – both the local point-contact resistance and magnetoresistance were found to decrease with the applied dc bias.

Nonlinear electrical transport in Fe3O4-decorated graphene nanoplatelets

This paper investigates the nonlinear properties of graphene nanoplatelets (GNPs), decorated with Fe3O4 magnetic nanoparticles (MaNPs). Nanocomposite MaNP/GNP samples were prepared by a solvothermal method with three different MaNP loading concentrations of 17 wt%, 28 wt% and 40 wt%, and deposited on a metallic interdigitated electrode (IDE). Three different models are proposed to assess measurements, with the objective to explain the electronic transport in the nanocomposites. At first, a thermionic transport model is proposed to fit the DC nonlinear current–voltage characteristics for the three concentrations. It is observed that the barrier height decreases to 0.312, 0.310 and 0.281 eV, following a decrease in the MaNP loading. A second model, dynamic random resistor network (DRRN) further shows that the impedance of IDE increases following the decreasing MaNP loading rate, 40 wt% > 28 wt% > 17 wt%, and that charge transport takes place through a resistor–capacitor (RC) rectifying percolating network. Finally, impedance spectroscopy performed at different applied DC biases shows that a constant phase element (CPE) is necessary in the equivalent circuit in order to fit the Cole–Cole plot AC response of the IDE, instead of the classical parallel RC circuit. The presence of the CPE confirms the hypothesis of random phenomena occurring in the transport according to the DRRN model. CPE is associated with a spatial distribution of different RC circuits, due to disorderness that arises from inhomogeneities in the Fe3O4–GNP samples.

Development of a DC bias structure with reduced RF leakage for suppressing the multipacting effect in the high power coaxial couplers

Coaxial power couplers are being employed for RF power transmission to the low beta superconducting cavities used for proton beam acceleration in the 25 MeV superconducting linear accelerator (linac) demonstration facility of the China Initiative Accelerator Driven System (CiADS) project. However, the operation stability of the coaxial couplers was mainly limited by the multipacting effect, which can be suppressed by applying DC bias voltage between the inner and the outer conductors. In this paper, we found an RF leakage problem that the electromagnetic (EM) field inside the couplers can penetrate the bias capacitor and then propagate along the DC bias cable, thus disturbing the stability of the DC power supply and finally disabling the RF system. Therefore, a simple DC bias “T” transition part has been developed, where the series resistors are introduced to reduce the electromagnetic field propagating along the cable and make the structure intensely stable. The test results indicate that the blocking effect of the resistive solution is better than that of the conventional inductive method. The equivalent circuits of the resistive structure and the inductive structure are established and analyzed. The detailed test and analysis results of the new DC bias structure will be reported.

(Invited) Contribution of Triple/Double Phase Boundary Reactions in Mixed Conducting Oxide Cathodes in SOFCs and PCFCs

Development of high performance cathode is one of largest challenges for the full-scale commercialization of solid oxide fuel cells (SOFCs) using an oxide ion conducting electrolyte and proton ceramics fuel cells (PCFCs) using a proton conducting electrolyte. The cathodic reactions in SOFCs and PCFCs are the electrochemical reduction of oxygen gas. Since ions (oxide ion or proton), electrons, and oxygen gas molcules exist in the electrolyte, the electrode, and the gas phase, respectively, the cathodic reaciton basically takes place only at triple phase boundaries (TPBs). On the other hand, the reaction site can be expanded from TPBs to double phase boundaries (DPBs), when a mixed ionic-electronic conductor (MIEC) is used as the electrode material, because ions can exist not only in the electrolyte but also in the electrode. In such an MIEC cathode, it is generally believed that the dominant reaction sites are DPBs, because the DPB area is much larger than the TPB area. However, as far as we know, there are no clear experimental evidences which quantitatively showed how significantly the TPB reaction contributes to the total reaction in an MIEC fuel cell cathode. It is important to know the contribution of TPB/DPB reactions separately for the material and structure designs of high performance fuel cell cahtodes.In practical SOFCs and PCFCs, porous electrodes are conventionally used. But for evaluating the contributions of TPB/DPB reactions, experiments using porous electrodes are not appropriate because of complicated microstructures and inhomogeneous electrode reaction distribution in porous electrodes [1]. In order to avoid such experimental difficulties with porous electrodes, many researchers applied the model electrodes, such as a dense thin film electrode and a microelectrode. However, these conventional model electrodes tend to overestimate the contribution of the DPB reaction and thus are still not appropriate to investigate the contributions of the TPB/DPB reactions separately [2]. Instead, our group proposed new and original model electrodes, which is so-called « patterned thin film electrodes », as shown in Figs. 1(A) and (B) [2]. These electrodes are sorts of thin film electrodes, but the area of the electrode/electrolyte contact was limitted by inserting the slitted insulating layer between the electrode and the electrolyte. In one type of the model electrodes (Fig. 1(B)), a part of the electrode film was removed to introduce TPBs. By applying these novel model electrodes, the contribution of the DPB reaction can be evaluated from the model electrode (A), while the contribution of the TPB reaction from the difference between the model electrodes (A) and (B). These model electrode can be fabricated by helps of photolithography and pulsed laser deposition techniques.By applying the above-mentioned model electrodes, we first investigate the dominant reaction pathway in SOFC MIEC cathodes, for instance an La0.6Sr0.4CoO3-δ or La0.6Sr0.4Co0.2Fe0.8O3-δ cathode on a Ce0.9Gd0.1O1.95 electrolyte. Consequently, it was found (i) DPBs are the dominant reaction sites at operation temperatures of conventional SOFCs (i.e. above 973 K), and (ii) Contribution of TPB reactions becomes significant with decreasing temperature below 873 K, increasing amibient p(O2), and decrasing the applied DC bias. These results suggested that the contribution of TPB reactions should be taken into account for desigining the cathode in intermediate tempeature SOFCs. Similar measurements were applied to investigated the cathodic reactions in PCFCs. Here La0.6Sr0.4CoO3- δ and BaGd0.3La0.7Co2O6-δ were chosen as cathode materials. It was clearly shown that the dominant reaciton site is stongly affected by electrode material, possibly depending on protonic conductivity in the electrode materials. Moreover, our moedel electrodes can be employed to investigate the degradation mechanism of SOFC or PCFC cathodes. For instance, Cr poisoning phenomena in SOFC cathodes was examined, and, as a results, it was found that several degradation modes exist depending on the position in the electrodes. In the presentation, details of these our research activities on SOFC and PCFC cathodic reacions by using patterned thin film model electrodes will be introduced. Reference s [1] S. B. Adler, Chem. Rev., 104, 4791 (2004). [2] K. Mizuno, et al., to be submitted. Figure 1

Current-induced nonreciprocity and refraction-suppressed propagation in a multilayered graphene-dielectric crystal

Nonreciprocal photonic devices play a significant role in regulating the propagation of electromagnetic waves. Here we theoretically investigate the nonreciprocal properties of transverse magnetic modes in a multilayered graphene-dielectric crystal under an applied DC bias. We find that drifting electrons driven by the external DC electric field can give rise to extremely asymmetric dispersion diagrams. Furthermore, when the drifting electrons travel antiparallel to the normal component of the incident wave vector, negative refraction can be strongly suppressed, causing the energy of light to flow along the direction of the electric current. Our theoretical findings can be used to design nonreciprocal optoelectronic devices and enable light to propagate without refraction.

Bulk photovoltaic effect in organic binary systems consisting of a ferroelectric liquid crystalline semiconductor and fullerene derivatives

The bulk photovoltaic effect of fluorophenylterthiophene derivative 1 bearing a chiral alkyl chain doped with fullerene derivatives 2 and 3 was studied for the construction of organic bulk photovoltaic systems with bulk heterojunction structures. Electrical polarization could be immobilized in the chiral ordered smectic phase of the mixtures of compound 1 with compound 2 or compound 3 by a DC bias application during the phase transition from the chiral smectic C (SmC*) phase to the chiral ordered smectic phases. The spectral sensitivity is expanded to the illumination of visible light in the smectic phases of the FLC 1 doped with the fullerene derivatives while FLC 1 had a spectral sensitivity below 480 nm. Because of segregation of fullerene derivatives in the smectic phases of FLC 1 doped with fullerene derivatives 2 and 3 , electron transport characteristics were enhanced in the smectic phases and the electron mobilities reached 1 × 10 −2 cm 2 V −1 s −1 . As a result, polarity inversion of this bulk photovoltaic effect was observed in FLC 1 doped with the fullerene derivatives. The open circuit voltage was also increased to 0.5 V in the ordered smectic phase of fullerene-doped FLC 1 in spite of the symmetrical structure of the cells. • Binary mixtures consisting of a p- and n-type components for an organic bulk photovoltaic system. • High electron mobilities because of nanosegregation of fullerene derivatives in the LC phases. • Reversible polarity inversion of the bulk photovoltaic effect. • Bulk photovoltaic effect for visible light illumination.

Photovoltaic and impedance spectroscopy characterization of single-junction a-Si:H p–i–n solar cells deposited by simple shadow masking techniques using PECVD

Hydrogenated amorphous silicon p–i–n solar cells with a 1 × 1 cm2 active surface area were fabricated using shadow masks on the 20 × 20 cm2 glass substrate coated with a fluorine-doped tin oxide film. The intrinsic, n-type hydrogenated amorphous silicon (a-Si:H), and p-type a-SiC:H thin films were deposited using plasma-enhanced chemical vapor deposition at 13.56 MHz plasma excitation frequency and on 20 × 20 cm2 and SnO2:F covered glass substrates. Low rf-power densities (less than 0.1 W/cm2) and substrate temperatures (less than 190 °C) were used for this purpose. Raman spectra of the films are dominated by a broad peak around 480 cm−1 that is the characteristic of the amorphous silicon network for all the three types of films. Scanning electron microscopy measurements revealed that the surface of the a-Si:H films deposited on SnO2:F-coated glass substrates (Asahi-VU) replicates the texture of the SnO2:F film. Spectroscopic ellipsometry spectra were analyzed with the Tauc–Lorentz dispersion model, and the results revealed that the optical gap of the intrinsic a-Si:H films is on the order of 1.7 eV, while that of the a-SiC:H is on the order of 1.8 eV. These results were further confirmed by optical transmission measurements. The highest efficiency obtained for solar cells prepared with shadow masking under our condition is on the order of 8.83% with a Voc of 0.856 V, a short circuit current density of 15.6 mA/cm2, and a fill factor of 66.07%. The obtained efficiency is slightly lower than the record efficiency obtained in this family of cells (10.3%) prepared by laser scribing because the low short-circuit current slightly lowers the fill factor. Impedance spectroscopy measurements were performed on the cells in the dark in the frequency range of 1 kHz–100 kHz. The analysis of impedance either in the Nyquist diagram or in the Bode diagram suggests a lumped circuit consisting of resistance Rs in series with a parallel combination of resistance Rp and capacitance Cp that account for the p–i–n structure. The value of Rp changed with the applied DC bias. The value of the series resistance agrees with the value obtained from the current–voltage characteristics of the cell.