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Capacitors and inductors
Published in John Bird, Science and Mathematics for Engineering, 2019
Disadvantages of supercapacitors include: (i) The amount of energy stored per unit weight is generally lower than that of an electrochemical battery(ii) Has the highest dielectric absorption of any type of capacitor(iii) High self-discharge – the rate is considerably higher than that of an electrochemical battery(iv) Low maximum voltage – series connections are needed to obtain higher voltages and voltage balancing may be required(v) Unlike practical batteries, the voltage across any capacitor, including EDLCs, drops significantly as it discharges. Effective storage and recovery of energy requires complex electronic control and switching equipment, with consequent energy loss(vi) Very low internal resistance allows extremely rapid discharge when shorted, resulting in a spark hazard similar to any other capacitor of similar voltage and capacitance (generally much higher than electrochemical cells)
Passive electronic components
Published in Stephen Sangwine, Electronic Components and Technology, 2018
Dielectrics are not perfect insulators, and some current will flow between the plates of a capacitor with a steady voltage applied. Capacitors, therefore, have a d.c. leakage resistance, which for the best capacitor types is as high as 1012 Ω for a capacitance of around 10 nF. Apart from a steady leakage current, capacitors also exhibit dielectric absorption in which charge is absorbed into the dielectric. A capacitor that has been short-circuited and supposedly fully discharged can recover a small voltage as the absorbed charge emerges from the dielectric. This can be a problem in precision analogue-to-digital converters, where the result of an A-to-D conversion can be slightly influenced by the result of the previous conversion because the capacitor has some “memory” of its previous voltage. It can also be a safety problem with large-value, high-voltage capacitors. This point is considered further in Chapter 11.
Capacitance and Capacitance Measurements
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
The dielectric absorption introduces a time lag during the charging and discharging of the capacitor, thus reducing the capacitance values at high frequencies and causing unwanted time delays in circuits. The leakage current, on the other hand, prevents indefinite storage of energy in the capacitor. An associated parameter to leakage currents is the leakage resistance, which is measured in megohms, but usually expressed in megohm–microfarads or ohms–farads. The leakage resistance and capacitance introduces time constants that can vary from a few days for polystyrene to several seconds in some electrolytic capacitors. It is important to mention that the leakage current does not only depend on the properties of the dielectric materials but also depends on the construction and structural integrity of capacitors. This is particularly true for capacitors having values less than 0.1 µF, having very thin dielectric materials between the electrodes.
Enhancement of dielectric and electro-optical parameters of a newly prepared ferroelectric liquid crystal mixture by dispersing nano-sized copper oxide
Published in Liquid Crystals, 2020
Sidra Khan, Shikha Chauhan, Achu Chandran, Michał Czerwiński, Jakub Herman, Ashok M. Biradar, Jai Prakash
In spite of the difficulty in synthesis, there is always a need of new FLC materials with improved material parameters and purity. The present study focuses on the preparation and characterisation of a newly prepared FLC mixture, namely W302. The characterisation of the FLC mixture, W302, has been performed through dielectric spectroscopy, differential scanning calorimetry (DSC), polarising optical microscopy (POM) and other electro-optical methods. A low-frequency dielectric relaxation peak, i.e. p-UHM, is observed in the dielectric absorption spectrum of W302 material. The effect of dispersing copper (II) oxide nanoparticles (nCuO) in the host FLC mixture on p-UHM is also studied. Also, the dielectric permittivity of the nCuO/FLC mixture is enhanced due to the dispersion of nCuO. The effect of CuO NPs on the material parameters of FLC has also been observed.
Dielectric properties of four room temperature ferroelectric and antiferroelectric multi-component liquid crystalline mixtures
Published in Liquid Crystals, 2019
Asim Debnath, Pradip Kumar Mandal
Bias electric field strongly affects the Goldstone mode relaxation process of the SmC* phase. For the mixtures, the frequency dependence of GM dielectric absorption (ε”GM) measured at different bias voltages keeping the temperature fixed is shown in Figure 8. On increasing the bias voltage, the strength of the dielectric absorption maximum (ε”GM) decreases and peaks get broadened. When the bias field reaches a particular value, called critical field (Ec), absorption peaks are completely suppressed, and ε”GM becomes nearly zero. For MIX0 and MIX3, Ec ~ 4V/µm and for MIX1 and MIX2, Ec ~ 2V/µm. Figure 9 shows the variation of ΔεGM and fGM with bias field. From the figure, it is clear that variation of ΔεGM has three parts. In the first part, when the bias voltage is relatively low, ΔεGM is nearly constant. In the second region, ΔεGM decreases linearly and finally reaches a minimum with bias field which may be due to the continuous deformation of helix. After that ΔεGM is independent of the bias field, because for E ≥ Ec all dipoles aligned parallel to the field, and hence the measuring electric field will not cause any deformation in the structure.
Microwave power absorption profile of detergent surfactant agglomerates during microwave heating
Published in Drying Technology, 2018
Muhammad Y. Sandhu, Sharjeel Afridi, Qari Khalid, Ian C. Hunter, Nigel S. Roberts
Separate samples of the above agglomerate were conditioned to different moisture levels by exposure to a range of ambient humidity. After conditioning, the sample had an eRH of 33% which corresponds to a free water level of ∼3 wt% in the agglomerate. Following conditioning, the dielectric properties of the different agglomerate samples were measured and the samples then exposed to microwave heating. The loss factor and dielectric constant of the LAS agglomerate sample were measured using an open circuited microstrip stub partly loaded with the sample as described in Ref.[12] The measurement process consists of measuring the scattering parameters of a loaded and unloaded bandstop microstrip open circuit stub. The complex permittivity of the material is calculated from the shift in resonant frequency of the stub and dielectric absorption.