Impedance drift

Some branched paraffinic hydrocarbons solidify as glasses. Their electronic properties are affected by the structural disorder. Irradiation of glassy samples by y-rays leads to a buildup of trapped charge carriers in the bulk. Their optical properties have been investigated extensively (Hamill, 1968). The presence of these traps may impede drift mobility measurements. Some data on the drift mobility of electrons in 3-methylpentane at temperatures between 4.2 K and 77 K are due to Maruyama and Funabashi (1972). The electrons were photo-injected from a thin layer of IMPD (see Chapter 2) which was deposited on the cathode. Only a fraction of the injected electrons reached the anode. The mobility of these electrons as a function of temperature is shown in Figure 9. Between 4.2 and 33 K the mobility remained virtually constant at 0.03 cm V s" while between 33 and 77 K the mobility was thermally activated with an activation energy of 10 meV. Included is the electron mobility between 250 and 350 K which is thermally activated with an energy of 0.2 eV (Kalinowski et al., 1975). 

It appears that both the Hitachi and DuPont polyimide films, when cured, significantly impede the drift of sodium ions at normal device operating temperatures. There is, however, evidence that underlying device oxides can be contaminated by sodium and/or moisture or other polar molecules during the application and curing of the polyimide films. Qeaner resins and adequate device stabilization may control this problem. Further work will be required to characterize contamination levels associated with specific aspects of the processing, such as the adhesion promoter and the polyimide resins themselves. 

Unfortunately, due to technical difficulties, reference electrodes have not been widely employed in fuel cell measurements. The primary cause is the geometric restriction imposed by the thin solid electrolyte (for example, a typical electrolyte thickness is about 50, um for a Nation 112 membrane). Additional factors such as the shape and position of the reference electrode must be taken into account in order to obtain reliable and meaningful results. In reality, the requirements for a reference electrode are less stringent than expected, since the main problem is the drift of the reference potential during measurement, which can result in large error due to the strong potential dependence of the impedance. 

While the line-shapes parameters may not be unequivocally associated with a set of deterministic or theoretical parameters for a given system, the measurement model approach has been shown to adequately represent the impedance spectra obtained for a large variety of electrochemical systems. The line-shape models represent the low-frequency stationary components of the impedance spectra (in a Fourier sense). Regardless of their interpretation, the measurement model representation can be used to filter and thus identify the nonstationary (drift) and high-frequency (noise) components contained in the same impedance spectrum. 

The amplifier gain can be reduced by using a PMT load resistance or amplifier input impedance of more than 50 The increased load resistor in conjunction with the cable capacitance results in a pulse broadening without loss in amplitude. Load resistors of the order of 1 kf2 can be used. To avoid baseline drift in the amplifier, AC coupling is used. The coupling constant is selected as to minimise the baseline walk at higher pulse rates. Nevertheless, pile-up in the amplifier limits the useful pulse rate to typically a few 10 kHz. The general measurement setup is shown in Fig. 6.22. 

The effect is normally one of degree. The measurement of a corrosion potential does not influence the surface condition. Electrochemical noise and impedance measurements carried out at the corrosion potential also have little effect as does a polarization resistance measurement if the perturbation is small, although rest potential drift may be a problem if potential control techniques are used. Techruques involving large potential differences will in general modify a surface significantly. 

While the crystal oscillator amplification ensures there is no freqnency drift, the system still requires the use of the matching network, consisting of an inductor and capacitor that antomatically and continuously maintain the impedance of 50 O seen by the generator. 

In the Prefaces to Volumes 4 and 5 of Practical Aspects of Trapped Ion Mass Spectrometry the Editors have explained that, in defining the scope of these publications, it is considered that an ion is trapped when its residence time within a defined spatial region exceeds that had the motion of the ion not been impeded in some way. Ion mobility spectrometry ( IMS ) operated at atmospheric pressure, which falls clearly within this definition, involves the determination of the time taken for the components of a packet of ions to move through a defined distance under the influence of a specified electric field gradient and against the connter current flow of a drift gas at ambient pressure. 

The open-circuit potential was measured in real time by using an ultra-low input bias current instrumentation amplifier, providing an accurate differential measurement of voltage. The very high input impedance and very low input bias current minimize the effect of the measurement on the OCP. The gain of the amplifier was set to 1 in order to eliminate instability effects, temperature drift, etc., of the external resistor needed to set a higher amplifier gain. 

In two-phase pipe flows, the slip velocity or, equivalently, the drift flux is related to the void fraction. This leads to the occurrence of kinematic waves which convey void fraction signals. Kinematic waves have been investigated experimentally in air-water two-phase flows by inducing small void fraction disturbances at the inlet of vertical ducts, the average void fraction varying from 0.01 (bubbly flows) to 0.41 (slug flows). The temporal fluctuations of the void fraction are detected in regularly spaced cross sections by non intrusive impedance probes. The statistical... 

---