1,796,775 research outputs found
Impact of the surface roughness on the electrical capacitance
Full text linkA new hybrid approach consists to use the advantages of both systems namely the high geometric aspects of the electrodes of the ultracapacitor and the high dielectric strength of polymer materials used in dielectric capacitors. The surface roughness of the electrodes of the ultracapacitor is manufactured with nano-porous materials; activated carbon and carbon nanotubes (CNTs). Many compositions of both carbonaceous materials are tested with different insulating materials (liquid and solid) to constitute the hybrid capacitor. It appears that the capacitance increases with the carbonaceous composition: An increasing from 15 to 40% is observed as compared to a plane capacitor, it can be twice with a 100 wt% of CNTs content. But, the impregnation of the insulating material in the surface roughness remains the key point of the realization of the hybrid capacitor. The roughness accessibility is a major property to optimize in order to improve the impregnation of the insulating material to increase the electrical capacitance
Accurate modeling of gate capacitance in deep submicron MOSFETs with high-K gate-dielectrics
No full textGate capacitance of metal-oxide-semiconductor devices with ultra-thin high-K gate-dielectric materials is calculated taking into account the penetration of wave functions into the gate-dielectric. When penetration effects are neglected, the gate capacitance is independent of the dielectric material for a given equivalent oxide thickness (EOT). Our selfconsistent numerical results show that in the presence of wave function penetration, even for the same EOT, gate capacitance depends on the gate-dielectric material. Calculated gate capacitance is higher for materials with lower conduction band offsets with silicon. We have investigated the effects of substrate doping density on the relative error in gate capacitance due to neglecting wave function penetration. It is found that the error decreases with increasing doping density. We also show that accurate calculation of the gate capacitance including wave function penetration is not critically dependent on the value of the electron effective mass in the gate-dielectric region
Influence of carbonaceous electrodes on capacitance and breakdown voltage for hybrid capacitor
Full text linkThis paper presents a new type of capacitor and deals with a hybrid approach where the advantages of two systems, dielectric capacitors and the ultracapacitor are combined. The objective is to increase the capacitance and the energy storage capability, while or at least preserving or decreasing the volume of the passive components. In this aim, the surface area and structural properties of ultracapacitor electrodes and the high dielectric strength of a polymer material are associated. The surface roughness of the carbonbased electrodes, namely (activated carbon—AC, and carbon nanotubes—CNTs), has a good impact on the capacitance. However, the surface roughness also depends on the composition of carbonaceous materials and so does the capacitance. Moreover, the choice of the dielectric material is the key parameter. The better the impregnation of the roughness is, the better is the increase of the capacitance. Since the final objective is to improve the electrical energy stored by the capacitor, the effect of surface roughness on the breakdown voltage is also evaluated
Electrical capacitance tomography for flow imaging: System model for development of image reconstruction algorithms and design of primary sensors
No full textA software tool that facilitates the development of image reconstruction algorithms, and the design of optimal capacitance sensors for a capacitance-based 12-electrode tomographic flow imaging system are described. The core of this software tool is the finite element (FE) model of the sensor, which is implemented in OCCAM-2 language and run on the Inmos T800 transputers. Using the system model, the in-depth study of the capacitance sensing fields and the generation of flow model data are made possible, which assists, in a systematic approach, the design of an improved image-reconstruction algorithm. This algorithm is implemented on a network of transputers to achieve a real-time performance. It is found that the selection of the geometric parameters of a 12-electrode sensor has significant effects on the sensitivity distributions of the capacitance fields and on the linearity of the capacitance data. As a consequence, the fidelity of the reconstructed images are affected. Optimal sensor designs can, therefore, be provided, by accommodating these effect
Design of sensor electronics for electrical capacitance tomography
No full textThe design of the sensor electronics for a tomographic imaging system based on electrical capacitance sensors is described. The performance of the sensor electronics is crucial to the performance of the imaging system. The problems associated with such a measurement process are discussed and solutions to these are described. Test results show that the present design has a resolution of 0.3 femtofarad. (For a 12-electrode system imaging an oil/gas flow, this represents a 2% gas void fraction change at the centre of the pipe) with a low noise level of 0.08 fF (RMS value), a large dynamic range of 76 dB and a data acquisition speed of 6600 measurements per second. This enables sensors with up to 12 electrodes to be used in a system with a maximum imaging rate of 100 frames per second, and thus provides an improved image resolution over the earlier 8-electrode system and an adequate electrode area to give sufficient measurement sensitivit
Gate Capacitance of deep submicron MOSFETS with high-K gate dielectrics
No full textWe study gate capacitance of deep submicron MOSFETs with high-K gate dielectrics. Schrödinger’s equation is solved by applying an open boundary condition at silicon-gate dielectric interface. Self-consistent numerical results reveal that accounting for wave function penetration into the gate dielectric causes the carrier distribution to be shifted closer to the gate dielectric. This effect increases with increasing gate voltage and also increases with the decreasing conduction band offset of the gate dielectric material with silicon. Gate capacitance calculated from conventional modeling is found to be independent of dielectric materials for a given equivalent oxide thickness (EOT). But our study shows that when wave function penetration into the gate dielectric is considered, gate capacitance for a given EOT increases with a decrease in the conduction band offset. Effects of substrate doping density on gate capacitance are found to be negligible when wave function penetration effects are incorporated
Modeling of direct tunneling gate current and gate capacitance in deep submicron MOSFETs with high-K dielectric.
No full textScaling down of MOS device dimensions is accompanied by a decrease in gate-oxide thickness and an increase in substrate doping density. When gate oxide thickness becomes less than 2 nm, a substantial current follows through gate-oxide due to direct tunneling. In order to reduce this current, International Technology Roadmap for Semiconductors (ITRS) has suggested replacement of SiO2 gate insulator layer by high-K dielectrics. For a given equivalent oxide thickness (EOT), high-K dielectrics offer greater physical thickness. The direct tunneling (DT) current and the gate capacitance for an inverted n-MOS device with different dielectrics used as gate insulator is studied. Coupled Schrodinger’s and Poisson’s equations are solved self-consistently. Open boundary conditions, taking account the wavefunction tail inside the gate dielectric within the self-consistent loop are used to solve Schrodinger’s equation. DT current increases exponentially with the decrease of conduction band offset for electrons travelling from silicon substrate to dielectric. As general trend of dielectrics is to decrease of conduction band offset with the increase of dielectric constant, use of high-K material as gate insulator results in prominent influence of direct tunneling of carriers on potential profile. Therefore in DT current calculation effect of wavefunction penetration on potential profile is incorporated within self-consistent loop. Results of this simulation is compared with published experimental results and also with the results of the simulation where penetration effect on potential profile is neglected. Results show that neglect of wavefunction penetration effect on potential profile causes underestimation of DT current. A comprehensive analysis of the effect of wavefunction penetration on the gate capacitance of the MOSFETs with high-K dielectrics is also done. Gate capacitance from conventional modeling is found to be independent of dielectric materials for a given EOT. The study reveals that accounting for wavefunction penetration into the gate dielectric causes gate capacitance to vary from material to material for a given EOT. Consequently wavefunction penetration effects must be considered to determine properties of future generation devices where high-K dielectrics will be employed as gate insulator
Capacitance Tip Timing Techniques in Gas Turbines
Full text linkThe vibration of turbomachinery blades is an important phenomenon to understand, observe and predict
and is the reason for developing a tip timing measurement system. Vibration leads to High Cycle Fatigue
(HCF), which limits blade durability and life. HCF can result in blade failure, having expensive
consequences for the engine involved. The traditional method for monitoring blade vibration under test
conditions is to use blade mounted strain gauges. However, strain gauges are costly and time consuming
to install. They have a limited operating life as they are subjected to the harsh on-engine conditions. Only
a limited number of blades can be monitored with strain gauges as the number that can be used is limited
by the number of channels in the slip ring or telemetry. They can also interfere with the assembly
aerodynamics. Consequently non-intrusive alternative techniques such as tip timing are sought.
Capacitance probe based clearance measurement systems see widespread use in turbomachinery
applications to establish rotor blade tip clearance. This thesis reports investigations into an alternative
and additional use in aero-engine rotor blade tip timing measurement for these commercially available
systems. Tip clearance is of great importance in the gas turbine industry; this is clear from the fact that
gas turbine efficiency has an inverse relationship with tip clearance. Large tip clearance leads to large
leakage flows, hence low efficiency, thus the common use of the capacitance probe clearance
measurement technique in monitoring turbomachinery.
Optical systems have been successfully used to measure rotor blade tip timing on test rigs with several
optical probes mounted equally spaced around the turbomachine casing. However, there are practical
problems associated with mounting such monitoring systems on in-service jet engines. Optical probes
require high maintenance to keep the lenses clean, probably incorporating a purge air system to keep the
lenses from fouling. Such impracticalities and added weight make it unlikely that an optical probe based
tip timing system will be fitted on an in-service engine in the foreseeable future.
In this thesis the scope for a dual use sensor to measure both turbomachinery tip clearance and tip timing
is investigated. Since it is impractical to measure blade tip clearance with an optical probe, then the
obvious choice for such a sensor is a capacitance probe. Therefore, a commercially available FM
capacitance probe based blade tip clearance measurement system is used in a series of tip timing practical
investigations. The equipment and instrumentation designed, assembled and produced to facilitate this
investigation is documented. These include the development of an optical once per revolution sensor and
the design of an independent vibration measurement system based on blade mounted strain gauges.
Through an extensive body of experimental work the practicalities in this alternate use of the tip
clearance measurement equipment have been assessed. System responses pertaining to tip timing
measurement have been investigated, characterised and quantified. The accuracy by which tip timing can
be measured using the system has been reported through the findings of an experimental programme
carried out on a full-sized, low-speed compressor.
Specifically, dual capacitance probe tip timing derived vibration amplitudes have been compared to
those derived from blade mounted strain gauge signals. Sources of error have been identified and
quantified. Amplitudes were found to agree within the calculated error bands. Instantaneous resonant
blade vibrations measured through single capacitance probe tip timing have been correlated with strain
gauge derived vibration levels. This has also been done as the rotor traverses blade resonant speed. In
this case the vibration phase change across resonance expected from theory was successfully detected
through tip timing. Also, the accuracy by which blade time of arrival can be determined by using
capacitance probe tip timing has been assessed using a precision OPR sensor and a non-vibrating
compressor rotor blade. The characteristics of a DC capacitance probe based clearance measurement
system's response to movement in 3D space in proximity to a blade tip have been mapped. Detection of
small vibrations have also been investigated in a series of static impulse tests
Capacitance-Tuning Guides the Electric Antifouling Membrane Design
No full textIn
recent years, conductive membranes have attracted
significant
attention and have been extensively studied for their unique antifouling
mechanisms and regenerative properties. The current strategy primarily
focuses on enhancing the conductivity of membranes. Yet, the resulting
antifouling performance enhancement is limited due to lack of capacitance-tuning.
Herein, we constructed hierarchical-nanostructure membrane carbon
nanotubes–polyaniline–graphene quantum dots (CNT-PANI-GQDs),
first demonstrating a capacitance-tuning strategy for enhancing the
membrane’s antifouling performance. PANI covers the CNTs’
surface and cross-links them, forming a conductive 3D structure polymer
substrate. GQDs assisted with PANI are used for tuning the membrane
capacitance. The CNT-PANI-GQDs had the highest capacitance among all
the membranes and exhibited the highest water flux, the best antifouling
performance, and mechanical stability. Cross-flow filtration experiments
were conducted with the organic model foulants. After treating a 100
ppm bovine serum albumin (BSA) solution with a voltage of −2.5
V and running continuously for one h, the CNTs-PANI-GQDs membrane
maintained a normalized flux of over 97%. Electrochemical measurement
and Derjaguin–Landau–Verwey–Overbeek (DLVO) analysis
revealed that PANI and GQDs simultaneously enhance the pseudocapacitance
and double-layer capacitance, and the hierarchical nanostructure membrane
possesses excellent charge transfer ability and a large electrochemically
active surface area. Increased capacitance leads to greater accumulation
of surface charges and enhances the electrostatic repulsion against
impurities. This work may offer valuable references to guide the design
of electric antifouling membranes to favor water purification applications
GIANT CAPACITANCE OSCILLATIONS RELATED TO THE QUANTUM CAPACITANCE IN GAAS ALAS SUPERLATTICES
No full textWe have observed periodic current and capacitance oscillations with increasing bias on doped GaAs/AlAs superlattices at a temperature of 77 K. The maximum of the observed capacitance is larger than usual geometric capacitances in superlattices, being comparable to the quantum capacitance of the two-dimensional (2D) electron system proposed by Luryi. A model based on well-to-well sequential resonant tunneling due to the movement of the boundary between the electric field domains in superlattice was proposed to explain the origin of the giant capacitance oscillations. It was demonstrated that the capacitance at the peaks of capacitance-voltage (C-V) characteristics reflects the quantum capacitance of the space-charge region at the boundary between the domains (a novel 2D electron system)
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