Large Capacitance Values Research Articles

TUNABLE NMR RECEIVER FRONT-END USING VARIABLE MEMS CAPACITOR

In this paper, a tunableMicroElectro Mechanical Systems Nuclear Magnetic Resonance (MEMS NMR) receiver front-end, which consists of three main parts, is presented. In the first part, a proper microcoil for detecting [Formula: see text] and [Formula: see text] with complete specifications regarding the sample is optimized to get the better Signal to Noise Ratio (SNR) and quality factor (Q). In the second part, the passive network is discussed and the amounts of the desired capacitor and the desired tunability range are calculated and based on the calculations and the fabrication process, a new MEMS tunable capacitor is presented. Due to the large capacitance value of the required capacitor, the proposed capacitor has a fixed part with a capacitance value of 28[Formula: see text]pf and a variable part with a tunability range of 193% (2.25[Formula: see text]pf–6.6[Formula: see text]pf) with 1.5 A applied to a thermal actuator. A buckling effect due to the weight of the fingers of the capacitor and thermal stress after the release is analyzed and a fabrication process based on the routine processes is proposed. In the third part, a low noise amplifier (LNA) is presented for the proposed receiver with a gain of 47.6[Formula: see text]dB at the bandwidth of 384 MHz and noise figure of 0.5[Formula: see text][Formula: see text]. In this work, modeling and investigation of surface microcoils and also the capacitor are conducted using finite element simulation in COMSOL Multiphysics and post-processing. The method for obtaining signal sensitivity and SNR of the microcoils are based on the MR signal generation by employing the principle of reciprocity.

Simple programmable voltage reference for low frequency noise measurements

The paper presents a circuit design of a low-noise voltage reference based on an electric double-layer capacitor, a microcontroller and a general purpose DAC. A large capacitance value (1F and more) makes it possible to create low-pass filter with a large time constant, effectively reducing low-frequency noise beyond its bandwidth. Choosing the optimum value of the resistor in the RC filter, one can achieve the best ratio between the transient time, the deviation of the output voltage from the set point and the minimum noise cut-off frequency. As experiments have shown, the spectral density of the voltage at a frequency of 1 kHz does not exceed 1.2 nV/√Hz; the maximum deviation of the output voltage from the predetermined does not exceed 1.4 % and depends on the holding time of the previous value. Subsequently, this error is reduced to a constant value and can be compensated.

A droplet-based passive force sensor for remote tactile sensing applications

A droplet-based flexible wireless force sensor has been developed for remote tactile-sensing applications. By integration of a droplet-based capacitive sensing unit and two circular planar coils, this inductor-capacitor (LC) passive sensor offers a platform for the mechanical force detection in a wireless transmitting mode. Under external loads, the membrane surface of the sensor deforms the underlying elastic droplet uniformly, introducing a capacitance response in tens of picofarads. The LC circuit transduces the applied force into corresponding variations of its resonance frequency, which is detected by an external electromagnetic coupling coil. Specifically, the liquid droplet features a mechanosensitive plasticity, which results in an increased device sensitivity as high as 2.72 MHz N−1. The high dielectric property of the droplet endows our sensor with high tolerance for noise and large capacitance values (20–40 pF), the highest value in the literature for the LC passive devices in comparable dimensions. It achieves excellent reproducibility under periodical loads ranging from 0 to 1.56 N and temperature fluctuations ranging from 10 °C to 55 °C. As an interesting conceptual demonstration, the flexible device has been configured into a fingertip-amounted setting in a highly compact package (of 11 mm × 11 mm × 0.25 mm) for remote contact force sensing in the table tennis game.

Needle-like Co[sbnd]Mo[sbnd]O with multi-modal porosity for pseudocapacitors

The needle-like CoMoO with multi-modal porosity was synthesized via simple strategy of chemical coprecipitation in hydrothermal condition. The resulted products were characterized with SEM, EDXS, XPS, XRD, Raman, BET, FTIR and electrochemical techniques. The needle-like CoMoO exhibited excellent electrochemical performance as supercapacitor. A large capacitance value of 1180 F/g was obtained with good cycle life from the CV plots at 5 mV/s or 302 F/g was calculated from the discharge curve at 1 A/g. The good electrochemical performance may be attributed to the multi-modal microstructure of the needle-like CoMoO with the combination of microporosity, mesoporosity and macroporosity which leads to the large surface area and the excellent contact probability, diffusion efficiency and storage ability of OH− ions. It’s observed for the first time that MoO3 in the needle-like CoMoO gradually flows away with the charge-discharge. The presence of Mo may not directly contribute to the electrochemcal capacitance of the materials but it helps to form the structure of the obtained needle-like CoMoO and thus indirectly affect the electrochemical performance of the needle-like CoMoO.

Impact of Sr-doping on structural and electrical properties of Pb(Zr0.52Ti0.48)O3 thin films on RuO2 electrodes

Abstract In this paper, we investigated the impact of Sr-doping on the structural properties and electrical characteristics of lead zirconate titanate [Pb(Zr 0.52 Ti 0.48 )O 3 , PZT] thin films deposited on RuO 2 electrodes by a sol-gel process and spin-coating technique. We used X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, and field-emission transmission electron microscopy to explore the structural, morphological, chemical, and microstructural features, respectively, of these films as a function of the growth condition (strontium doping concentrations varied from 1, 3, and 5 mol%). The PZT thin film processed at the 3 mol% Sr exhibited the best electrical characteristics, including a low leakage current of 2.27×10 −7 A/cm 2 at an electric field of 50 kV/cm, a large capacitance value of 2.74 μF/cm 2 at a frequency of 10 kHz, and a high remanent polarization of 37.95 μC/cm 2 at a frequency of 5 kHz. We attribute this behavior to the optimal amount of strontium in the PZT film forming a perovskite structure and a thicker interfacial layer at the PSZT film-RuO 2 electrode interface.

Complete passive vibration suppression using multi-layered piezoelectric element, inductor, and resistor

This paper describes passive technique for suppressing vibration in flexible structures using a multi-layered piezoelectric element, an inductor, and a resistor. The objective of using a multi-layered piezoelectric element is to increase its capacitance. A piezoelectric element with a large capacitance value does not require an active electrical circuit to simulate an inductor with a large inductance value. The effect of multi-layering of piezoelectric elements was theoretically analyzed through an equivalent transformation of a multi-layered piezoelectric element into a single-layered piezoelectric element. The governing equations were derived using this equivalent transformation. The effect of the resistances of the inductor and piezoelectric elements were considered because the sum of these resistances may exceed the optimum resistance. The performance of the passive vibration suppression using an LR circuit was compared to that of the method where a resistive circuit is used assuming that the sum of the resistances of the inductor and piezoelectric elements exceeds the optimum resistance. The effectiveness of the proposed method and theoretical analysis was verified through simulations and experiments.

Techniques to Reduce Driving Energy of 1-Pixel Displays

We propose a pair of techniques to lower the energy consumed by driving 1-pixel liquid-crystal displays (LCDs). The first one employs multiple capacitors while the other divides the LCD into two to lower the drive voltage. Simulations show that large capacitance values and many capacitors reduce the energy consumed, and stacking two thin low voltage LCDs maximizes the effect by decreasing the overhead consumed by the microcontroller. Actual polymer-dispersed LCDs (PDLCDs) are tested to confirm that the techniques work as effectively as the simulations imply. The results show that the overall energy consumption of large PDLCDs is reduced more than 70% using multiple capacitors, and the combination of the two techniques successfully reduces the driving energy even for small LCDs. While the polarity reversal of the proposed technique does incur some delay, it was confirmed by a flicker test that the technique does not degrade image quality.

Direct fabrication of 3D graphene on nanoporous anodic alumina by plasma-enhanced chemical vapor deposition.

High surface area electrode materials are of interest for a wide range of potential applications such as super-capacitors and electrochemical cells. This paper describes a fabrication method of three-dimensional (3D) graphene conformally coated on nanoporous insulating substrate with uniform nanopore size. 3D graphene films were formed by controlled graphitization of diamond-like amorphous carbon precursor films, deposited by plasma-enhanced chemical vapour deposition (PECVD). Plasma-assisted graphitization was found to produce better quality graphene than a simple thermal graphitization process. The resulting 3D graphene/amorphous carbon/alumina structure has a very high surface area, good electrical conductivity and exhibits excellent chemically stability, providing a good material platform for electrochemical applications. Consequently very large electrochemical capacitance values, as high as 2.1 mF for a sample of 10 mm3, were achieved. The electrochemical capacitance of the material exhibits a dependence on bias voltage, a phenomenon observed by other groups when studying graphene quantum capacitance. The plasma-assisted graphitization, which dominates the graphitization process, is analyzed and discussed in detail.

Tunneling Current Induced Frequency Dispersion in the C-V Behavior of Ultra-Thin Oxide MOS Capacitors

Huge frequency dispersion appears in the capacitance-voltage (C-V) characteristic of metal-oxide-semiconductor capacitors (MOSCAPs) in depletion bias region especially when the oxide thickness is extremely small. The low frequency C-V curves have large capacitance value and behave as a peak at depletion bias. By using TCAD simulation, it was proven that the tunneling effect which is significant in thin oxide MOSCAPs plays an important role for the contribution of the frequency dispersion. The conventional theory of interface traps density is not the only cause of frequency dispersion. Consequently, extraction of interface traps density by the conventional high-low frequency method will introduce misleading error of interface property in ultra-thin oxide MOS devices.

Analysis of Resistive Switching Characteristics in Surface Anchored Metal Organic Framework (SURMOF) Films

In the quest for new nonvolatile memory devices many new device concepts and materials systems have emerged in recent years. Notably MRAMs, FeRAMs, STTRAMs and RRAMs have attracted considerable attention and research efforts. Particularly in RRAM, the majority of these new developments are focusing on inorganic materials systems and very little work has been reported on organic or hybrid organic-inorganic memory related materials systems. Here we report on the resistive switching characteristics of a class of hybrid designer solids HKUST-1(Hong Kong University of Technology1) (or [Cu3(BTC)2]) SURMOF porous conductors which were loaded with ferrocene. SURMOF films are also referred to as porous coordination polymers (PCPs). The electrical conductivity of SURMOF films can be modulated by loading the pores with appropriate organic semiconductors. Loading with ferrocene or TCNQ can enable ion transport in addition to electron transport thus change the electrical conductivity. The SURMOF samples for this study have been grown on surface-functionalized Au-substrates by employing a layer-by-layer, liquid-phase epitaxial method. Device fabrication was carried out by depositing top electrodes from either Cu or Au using a commercial e-beam deposition system. Two different device stacks were fabricated and tested. The first device consisted of an Au/SURMOF/Cu/Au stack with SURMOF film thickness of around 75nm. The Cu/Au top electrode was deposited by e-beam evaporation with a thicknesses of 200nm and 25nm respectively and patterned with a shadow mask to produce disks of 200µm in diameter. The second device consisted of an Au/SURMOF/Al/Au stack with a SURMOF thickness of 10nm, 20nm and 50nm. The Al/Au top electrode was deposited by e-beam evaporation with a thicknesses of 700nm and 100nm respectively, and patterned with a shadow mask to yield disks of 150µm in diameter. A slow voltage ramp typically between -1.5V to +1.5V was used. Measurements have been performed on pristine samples directly after synthesis. Initially pristine SURMOF switching devices are in a high resistance state. In our device configuration the bottom electrode is grounded and the voltage is applied to the top electrode. In devices with Cu/Au as top electrode, ON was observed on negative voltage sweeps and OFF was observed on positive voltage sweeps. In devices with Al/Au top electrodes, ON was observed on positive voltage sweeps and OFF was observed on negative voltage sweeps. After increasing the voltage from 0 V to max. 0.75 V, a transition to a low resistance state is observed. The voltage was swept in the reverse polarity to switch from low to high resistance state. Current as high as 50mA was required to switch the device from low to high resistance state. Measurements were also performed on the unloaded SURMOF devices. Compared to the loaded device, the switching characteristics are quite poor. The observed phenomena are crucially dependent on ferrocene loading. Ferrocene loading increased the conductivity within the MOF layer and therefore lowers the resistance. The devices could be switched using pulse width longer than 20 ns. Although the device switched within tens of nanoseconds, the time of switching could not be established due to large device capacitance values. The polarity of the SET and RESET in the devices with the Cu as the top electrode rules out the possibility of Cu ion migration being the reason for formation and dissolution of the low resistance path during switching. Additionally, devices with Al top electrode were also measured. Similar to the Cu top electrode, the Al top electrode devices also showed resistive switching.

Influence of the charge double layer on solid oxide fuel cell stack behavior

While the charge double layer effect has traditionally been characterized as a millisecond phenomenon, longer timescales may be possible under certain operating conditions. This study simulates the dynamic response of a previously developed solid oxide fuel cell (SOFC) stack model that incorporates the charge double layer via an equivalent circuit. The model is simulated under step load changes. Baseline conditions are first defined, followed by consideration of minor and major deviations from the baseline case. This study also investigates the behavior of the SOFC stack with a relatively large double layer capacitance value, as well as operation of the SOFC stack under proportional-integral (PI) control. Results indicate that the presence of the charge double layer influences the SOFC stack's settling time significantly under the following conditions: (i) activation and concentration polarizations are significantly increased, or (ii) a large value of the double layer capacitance is assumed. Under normal (baseline) operation, on the other hand, the charge double layer effect diminishes within milliseconds, as expected. It seems reasonable, then, to neglect the charge double layer under normal operation. However, careful consideration should be given to potential variations in operation or material properties that may give rise to longer electrochemical settling times.

Determination of the dissipation in superconducting Josephson junctions

The results relative to macroscopic quantum tunneling rate, out of the metastable state of Josephson junctions, are examined in view of determining the effect of dissipation. We adopt a simple criterion in accordance to which the effect of dissipation can be evaluated by analyzing the shortening of the semiclassical traversal time of the barrier. In almost all the considered cases, especially those with relatively large capacitance values, the relative time shortening turns out to be about 20% and with a corresponding quality factor Q ≃ 5.5. However, beyond the specific cases here considered, still in the regime of moderate dissipation, the method is applicable also to different situations with different values of the quality factor. The method allows, within the error limits, for a reliable determination of the load resistance RL, the less accessible quantity in the framework of the resistively and capacitively shunted junction model, provided that the characteristics of the junction (intrinsic capacitance, critical current, and the ratio of the bias current to the critical one) are known with sufficient accuracy.

N-doped monolithic carbon aerogel electrodes with optimized features for the electrosorption of ions

N-doped carbon aerogels were synthesized by the polycondensation of resorcinol–formaldehyde–melamine mixtures. The prepolymerization of the precursors was determinant for the preparation of meso/macroporous aerogels with varied nitrogen content, regardless the solution pH and M/R molar ratio. The effect was more pronounced at pH 6, as the hydrogels prepared by the one-step route displayed essentially a microporous character, as opposed to the mesoporous network after prepolymerization. The prepolymerized carbon aerogels presented large capacitance values due to the optimum balance between pore structure, improved wettability and low polarization resistance for the highly doped materials. Salt deionization capacity revealed the importance of the macropore structure in the monolithic configuration of the aerogels for a fast electrosorption of ions.

A binder-free multi-walled carbon nanotube electrode containing oxygen functionalities for electrochemical capacitors

A method for the incorporation of immobile oxygen-containing functional groups onto multi-walled carbon nanotubes (MWCNTs) rooted in a metallic substrate for supercapacitor (SC) application is presented. Direct growth on the current collector (a 316 stainless steel (SS) mesh) offers the benefit of minimal MWCNT agglomeration, avoidance of using binder, and maintaining the 3-dimensional nanostructure. MWCNTs are grown by thermal-chemical vapour deposition (t-CVD) using SS as substrate and functionalized using an Ar/C2H6/O2 glow discharge. When used as a binder-free SC electrode, these functionalized MWCNTs (f-MWCNTs) show excellent surface stability and large specific capacitance values. At low charging/discharging current densities (0.2mAcm−2 or 3A g−1), a specific capacitance of 288±10F g−1 is achieved. The material shows good rate capability up to charging/discharging current density of 10mAcm−2 (37A g−1) and displays excellent reversibility. Consequently, these new binder-free f-MWCNT SC electrodes are excellent candidates for future investigations of MWCNT-based SC electrodes.

Spherical activated carbon derived from spherical cellulose and its performance as EDLC electrode

ABSTRACTSpherical activated carbons (ca. 30–100 μm in diameter) were synthesized from commercial spherical cellulose beads. The addition of guanidine phosphate was performed as the pretreatment, and CO2 activation was applied for enhancing the specific surface area. The addition of guanidine phosphate drastically improved the yield of carbon during the activation process. The specific surface area reached 1545 m2 g−1 for the sample heat‐treated at 850°C for 1 h in flowing N2 and continuously for 3 h in flowing CO2 activation. The calculated capacitance values per area (F m−2) for the pore size of less than 1 nm was large in the case of small current density. The distortion of the solvation shell could be the reason for the large capacitance value for the small pore size of less than 1 nm. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40950.

Harmonic-Suppressed Miniaturized-Element Frequency Selective Surfaces With Higher Order Bandpass Responses

We introduce a new technique for designing miniaturized-element frequency selective surfaces (MEFSSs) with bandpass responses and no spurious transmission windows over extremely large bandwidths. The proposed harmonic-suppressed MEFSSs consist of multiple metallic and dielectric layers. Each metallic layer is in the form of a two-dimensional arrangement of capacitive patches or an inductive wire grid with extremely sub-wavelength periods. Harmonic-free operation in these structures is achieved by using multiple, closely spaced capacitive layers with overlapping unit cells to synthesize a single, effective capacitive layer with a larger capacitance value. This allows for reducing the unit cell size of a conventional MEFSS considerably and moving the natural resonant frequencies of its constituting elements to considerably higher frequencies. Consequently, the spurious transmission windows of such MEFSSs, which are caused by these higher order harmonics, can be shifted to very high frequencies and an extremely broad frequency band free of any spurious transmission windows can be obtained. Using this technique, an MEFSS with a second-order bandpass response is designed to operate at 3.0 GHz with 20% fractional bandwidth and be free of spurious transmission bands up to 27.0 GHz. A prototype of this harmonic-free MEFSS is fabricated and experimentally characterized in the lab. Measurement results confirm harmonic-free operation of the proposed FSS for incidence angles in the $\pm {\hbox {60}}^{\circ} $ range for both the ${\hbox {TE}}$ and ${\hbox {TM}}$ polarizations of incidence.

Electrochemical impedance spectroscopy analysis of porous silicon prepared by photo-electrochemical etching: current density effect

Electrical impedance characteristics of porous silicon nanostructures (PSiNs) in frequency function were studied. PSiNs were prepared through photo-electrochemical etching method at various current densities (15–40 mA/cm2) and constant etching time. The atomic force microscope images of PSiNs show that pore diameter and roughness increase when current density increases to 35 mA/cm2. The surface roughness subsequently decreases because of continuous etching of pillars, and a second etching process occurs. Photoluminescence spectra show blue and red shift with increasing applied current density that is attributed to PSiNs size. Variations of electrical resistance and capacitance values of PSiNs were measured using electrochemical impedance spectroscopy analysis. These results indicate that PSiNs prepared at 20 mA/cm2 current density have uniform porous structures with a large number of pillars. Furthermore, this PSiNs structure influences large values of charge transfer resistance and double layer capacitance, indicating potential application in sensors.

A 4 V-electrochemical capacitor using electrode and electrolyte materials free of metals

A 4 V asymmetric capacitor has been assembled using non-porous activated mesophase carbon microbeads (AMCMB) as the negative electrode, a graphitized carbon (GMCMB) as the positive electrode and quaternary alkyl ammonium based organic electrolyte. AMCMB has been heated under the reduction atmosphere of hydrogen/nitrogen after a chemical activation. It has small specific surface area (24.48 m2 g−1), but delivers a large capacitance value due to the irreversible “electrochemical activation” in the initial galvanostatic charge–discharge process. The storage charge behavior of AMCMB has also been investigated by ex situ XRD, Raman spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy. In terms of specific capacity, energy density, power density and cycle ability, the AMCMB/GMCMB capacitor is superior the other capacitors like AC/graphite, AC/AC.

Capacitance of carbon-based electrical double-layer capacitors

Experimental electrical double-layer capacitances of porous carbon electrodes fall below ideal values, thus limiting the practical energy densities of carbon-based electrical double-layer capacitors. Here we investigate the origin of this behaviour by measuring the electrical double-layer capacitance in one to five-layer graphene. We find that the capacitances are suppressed near neutrality, and are anomalously enhanced for thicknesses below a few layers. We attribute the first effect to quantum capacitance effects near the point of zero charge, and the second to correlations between electrons in the graphene sheet and ions in the electrolyte. The large capacitance values imply gravimetric energy storage densities in the single-layer graphene limit that are comparable to those of batteries. We anticipate that these results shed light on developing new theoretical models in understanding the electrical double-layer capacitance of carbon electrodes, and on opening up new strategies for improving the energy density of carbon-based capacitors.

Effect of nano-filler on the performance of multiwalled carbon nanotubes based electrochemical double layer capacitors

This article presents the main features of electrochemical double layer capacitor (EDLC), made up of multiwalled carbon nanotubes as electrode materials and electrolyte based on nano silica dispersed in poly(vinylidene fluoride-co-hexafluoropropylene)-based magnesium ion conducting nano composite gel polymer electrolyte. The capacitance value was stable up to 5000 voltammetric cycles and even more. The performance characteristics of EDLC have been tested using cyclic-voltammetry, impedance spectroscopy, and charge-discharge techniques. Analysis shows that the nano-filler based EDLC has relatively larger capacitance value as compared to filler-free based electrolyte. The maximum capacitance of 92 mF cm−2, equivalent to single electrode specific capacitance of 92 F g−1 was achieved. It corresponds to energy-density of 10.5 W h kg−1 and power-density of 3.25 kW kg−1.