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Introduction
Published in Peter E. J. Flewitt, Robert K. Wild, Physical Methods for Materials Characterisation, 2017
Peter E. J. Flewitt, Robert K. Wild
If ceramic crystals were of perfectly organised structures and uniform microstructure, these materials would have mechanical properties that exceed those achieved. Indeed, failure of a ceramic is generally a consequence of a microstructural defect, or a combination of defects, such as inclusions, pores, voids and distributions of irregular-size grains. Mechanical failure occurs from pre-existing flaws: high mechanical stresses which exceed the local tensile strength can effect crack propagation from flaws followed by rupture (Hodgkins et al. 2010). Apart from their known high-temperature applications, some polycrystalline electronic ceramics are used extensively by communications, electronic and appliance industries. Among the best-known ceramics for these applications are the ZnO varistors, boundary layer capacitors, ferrites and positive temperature coefficient devices. These owe their unusual electrical properties to the presence and character of their grain boundaries since the single crystals of these materials do not exhibit the same phenomena as the polycrystals. A single defect in a ceramic capacitor can cause electrical breakdown and short circuiting and similarly for piezoceramics, where during the ensuing polarisation the electrical breakdown can be followed by mechanical failure. As in other materials, crystal lattice imperfections, in particular lattice vacancies and dislocations, influence thermal conductivity, electrical and magnetic properties.
Solid State RF, EO, and Millimeter Devices Incorporating Rare Earth Materials
Published in A. R. Jha, Deployment of Rare Earth Materials in Microware Devices, RF Transmitters, and Laser Systems, 2019
As mentioned previously, HEMT devices with T-gate structures and field plates will offer higher gain, improved power-added efficiency, and significantly higher CW power output. Appropriate flange material with higher thermal conductivity and optimal flange thickness, advance interfacing materials, heat-spreading elements, advanced epoxies with improved thermal conductivity, and efficient thermal management technology will aid in the rapid removal of heat from the transistor’s junctions. To maintain high operational reliability, GaN transistors must be biased with source grounded at the drain voltage around 65 V, while the gate voltage is dynamically adjusted to maintain the drain current close to 150 mA. Thermal conductivity and coefficient of thermal expansion parameters for advanced packaging materials for GaN transistors (HEMTs) are summarized. The materials suggested will help the transistors to retain their power output and reliability even when the junction temperature approaches 300°C. The SPS concept is best suited to the design and development of sandwiched (BNT/BIT/BNT) composite ceramic capacitors for high-power electronic systems, where the surface temperatures can exceed 300°C in a matter of a few seconds. Note the BNT/BIN/BNT composite ceramic capacitors use rare earth elements and their oxides such as barium, N d, Ti oxide for BNT ceramic capacitors, and bismuth, titanate, titanium oxide for the BIN ceramic capacitor. The combination of these three ceramic capacitors will lead to the formation of a laminated ceramic capacitor with outstanding parameters, which will be best suited for power electronic systems, where operating temperatures can approach 300°C. These composite ceramic capacitors will play a vital role in the design and development of high-power electronic systems.
Development of a low-cost electrochemical sensor for monitoring components in wastewater treatment processes
Published in Environmental Technology, 2022
Rafael Frederico Fonseca, Marcelo Zaiat
Cyclic voltammetry was the electrochemical methodology applied in this study [21]. The signal was generated by a digital analogue converter and was conditioned (offset and amplitude) by a proper electronic circuit. A precision resistor, or proof resistor, was added between the signal condition electronics and the electrode to calculate the current flow in the circuit by measuring the voltage over it. This potential was amplified ten times using an instrumentation amplifier from Texas Instruments to a suitable level to be acquired by the data acquisition board USB-6361 from National Instruments. Four acquisition channels were implemented to allow parallel measurements. The characterisation data were saved in a block matrix, with a header in each one informing the supplementation data followed by the electrochemical data lines. A ceramic capacitor was placed in parallel with the resistor to filter high-frequency noises. To ensure the beginning and end of the electrochemical reactions, an electromagnetic relay was added to the circuit. The electrode schematic is shown in Figure 1(a) and it was based on the ALS Co. interdigitated array design. The input signal was applied between wires A and C, according to Figure 1(b). Its size was chosen to fit the test tube used in the host laboratory and the other two wires were set to be used as reference electrodes.
True Maximum Power Extraction in Photovoltaic Systems using High Gain Energy Efficient DC-DC Converter
Published in Electric Power Components and Systems, 2022
The switching frequency of HGDC is set to 50 kHz. The HGDC is fed from two series connected Shell make “SM55” PV modules (Table 1). A 1 µF ceramic capacitor is connected at the output of PV modules to decouple the dominant component of the current ripple. A 1kΩ resistor is used as the load to extract the PMPP from the PV modules at an output voltage of around 325 V when the irradiance level is 1000 W/m.2 During the simulation, the duty cycle of the HGDC and the converters considered for comparison is manually adjusted so that they draw an average input current of IMPP. This approach does not affect the evaluation of HGDC and other converters as the primary goal is to measure the ripple current drawn from the PV module/s at MPP under steady state condition.
New DC Hybrid Filter for Attenuating Low-Frequency Ripple of AC-DC Power Converter
Published in Electric Power Components and Systems, 2019
Jinn-Chang Wu, Hurng-Liahng Jou, Tse-Yu Lin
The DC hybrid power filter is a parallel type active decoupling circuit. For alleviating the problems of voltage stress and power loss of the conventional parallel type active decoupling circuit, the concept of hybrid power filter, used in the AC distribution power system, is used in the proposed DC hybrid power filter. The DC hybrid power filter is configured by a passive power filter, a single-arm power converter and a buck power converter. Similar to the conventional AC hybrid power filter, the passive power filter can reduce the voltage rating of single-arm power converter, and the operation of single-arm power converter is used to improve the filter performance of passive power filter in the DC hybrid power filter. The DC hybrid power filter can suppress the twice-utility-frequency current flowing from the AC-DC power converter to the DC load, and the ripple of output DC voltage can be attenuated so as to avoid the use of large electrolytic capacitors. The passive power filter contains of a capacitor which can block DC current. Accordingly, the single-arm power converter can be used to generate an AC current with no DC bias. Hence, the circuit of power converter can be simplified as compared with the conventional AC hybrid power filter. However, the capacitor of passive power filter can also block the DC real power from the DC bus of AC-DC power converter. Hence, a buck power converter is used to supply a real power to the DC bus of single-arm power converter to neutralize the power loss of DC hybrid power filter. The use of the buck power converter in the proposed DC hybrid power filter will complicate the power circuit. Fortunately, the power rating of buck power converter is very small. A filter capacitor Cf is connected to the DC load in parallel to filter out the switching harmonic from the AC-DC power converter and the DC hybrid power filter. The capacitance of filter capacitor Cfis small, and a ceramic capacitor can be used.