Medium Frequencies

Medium frequency (up to 100MHz) The features described above (with the exception usually of the chopped-signal variant) are available but the time base components and amplifiers are designed for much wider frequency bands. 

Combustion of a propellant in a rocket motor accompanied by high-frequency pressure oscillation is one of the most harmful phenomena in rocket motor operation. There have been numerous theoretical and experimental studies on the acoustic mode of oscillation, concerning both the medium-frequency range of 100 Hz-1 kHz and the high-frequency range of 1 kHz-30 kHz. The nature of oscillatory combustion instability is dependent on various physicochemical parameters, such... 

The use of glass and quartz capillaries is, in principle, not restricted to what nowadays are medium frequencies (400-600 MHz). This type of high pressure NMR equipment will certainly be further improved, mainly to study biological samples by high-field H NMR. To overcome the serious sensitivity problem inher-... 

This type of control system is operated at medium frequency and is currently limited to a maximum power of about 5 kW. ... 

High-frequency or diagnostic ultrasound in clinical imaging (3-10 MHz) Medium-frequency or therapeutic ultrasound in physical therapy (0.7-3.0 MHz) Low-frequency or power ultrasound for lithotripsy, cataract emulsification, liposuction, tissue ablation, cancer therapy, dental descaling, and ultrasonic scalpels (18-100 kHz)... 

X 10-3 V-750 V Varies, often protected General use, medium frequency... 

An estimate of the T, lifetime can be made from the measured line width of the Ti < So absorption. Figure 2.18 presents the SitO0) T O0) S0(Oo) OODR spectrum measured under medium-frequency resolution ( 0.05 cm-1) [29]. The rotational structure, depicted in Figure 2.18 consists of a prominent band with two satellites (50 times weaker) at 6 cm 1 to lower energies. From... 

The transistors of the output-side inverter are pulse-width modulated in such a way that in every phase, a near-sinusoidal current is created after smoothing by motor inductivity. The frequency of this current can be varied to adjust the speed of the motor. The switching frequency of small to medium frequency converters is on the order of 2 to 8 kHz and adjustable. In frequency converter operation, this frequency is quite audible. The larger the motor, the smaller the switching frequency selected. In large drives, switching frequency drops to 400 Hz. 

The impedance spectroscopy is most promising for electrochemical in situ characterization. Many papers have been devoted to the AB5 type MH electrode impedance analysis [15-17]. Prepared pellets with different additives were used for electrochemical measurements and comparing. Experimental data are typically represented by one to three semicircles with a tail at low frequencies. These could be described to the complex structure of the MH electrode, both a chemical structure and porosity [18, 19] and it is also related to the contact between a binder and alloy particles [20]. The author thinks that it is independent from the used electrolyte, the mass of the electrode powder and the preparing procedure of electrode. However, in our case the data accuracy at high frequencies is lower in comparison with the medium frequency region. In the case, the dependence on investigated parameters is small. In Figures 3-5, the electrochemical impedance data are shown as a function of applied potential (1 = -0.35V, 2 = -0.50V and 3 = -0.75V). 

The complex-plane impedance diagram is given in Figure 4.126. At both high and medium frequencies the complex-plane impedance is characterized by a well pronounced semicircle, while at the low-frequency range a tail appears. This tail s shape is strongly dependent on the value of the CPE exponent, as can be seen in AppendixD (Model Dll). 

In a H2/02 (air) fuel cell, in general, the spectra, i.e., the Nyquist plots obtained by EIS measurements, very often have three features, which are denoted as high-frequency, medium-frequency, and low-frequency. The high-frequency region of an impedance spectrum is associated with the internal ohmic resistance and the contact capacitance in the granular electrode structure of the membrane electrode assemblies, whereas the medium- and low-frequency regions represent the charge-... 

It is very often observed that the Nyquist plot has only a single impedance arc. Here we define this as a medium-frequency feature. Figure 5.20 shows some typical single-arc Nyquist spectra, which were calculated by Springer et al. [18],... 

The diffusion layer inside the MEA can cause the low-frequency end of the impedance to bend over the real axis, giving rise to a double semicircle. These two arcs for the fuel cell cathode process are defined as a medium-frequency feature and a low-frequency feature. Typical two-arc Nyquist plots from simulation (calculated by Springer et al. [18]) and from experiments are presented in Figure 5.23 and Figure 5.24, respectively. 

Two impedance arcs, which correspond to two relaxation times (i.e., charge transfer plus mass transfer) often occur when the cell is operated at high current densities or overpotentials. The medium-frequency feature (kinetic arc) reflects the combination of an effective charge-transfer resistance associated with the ORR and a double-layer capacitance within the catalyst layer, and the low-fiequency arc (mass transfer arc), which mainly reflects the mass-transport limitations in the gas phase within the backing and the catalyst layer. Due to its appearance at low frequencies, it is often attributed to a hindrance by finite diffusion. However, other effects, such as constant dispersion due to inhomogeneities in the electrode surface and the adsorption, can also contribute to this second arc, complicating the analysis. Normally, the lower-frequency loop can be eliminated if the fuel cell cathode is operated on pure oxygen, as stated above [18],... 

The medium-frequency arc, the size of which is strongly dependent on the anode potential, corresponds to methanol electrooxidation kinetics. Note that different anode catalyst compositions and operating conditions of the DMFC can be evaluated by assessing their effects on anode performance. 

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