Transient current temperature

The transient current, derivable from equation 1, is given in equations 2 and 3 where T is the transit time and I is the absorbed photon flux. The parameter a can be further derived as equation 4 (4), where Tis the absolute temperature and is the distribution width (in units of kT) of a series of exponential traps. In this context, the carrier mobdity is governed by trapping and detrapping processes at these sites. 

Transient currents in biased samples at room temperature. 

In [119], the hydrogen adsorption and desorption reactions in thin palladium electrodes were studied using the potential step method in order to analyze the mechanism of phase transformation. Transient current responses were recorded at the onset of the potential step for 47 pm thick Pd electrodes in 1 mol dm H2SO4 at ambient temperature. A model based on a moving boundary mechanism was proposed to account for the experimental i-t curves. It was found that the hydrogen adsorption reaction shows interfacial kinetic limitations and only numerical solutions can be obtained. Such kinetic limitations were not found for the desorption reaction and a semianalytical solution that satisfactorily fits the experimental data was proposed. 

Figure 92 Typical transient current pulses for holes in amorphous selenium (see Ref. 422a), illustrating the effect of temperature on the degree of the dispersion in carrier transport. Left linear current (i) and time (t) axes right normalized values in logarithmic axes log(i/io) vs. log(t/t0). The arrows indicate the position of the knee dividing the two regimes of logarithmic dependence. Similar behavior can be observed in organic solids (see e.g. Ref. 422b).

Fig. 3.12. Examples of the (a) electron and (b) hole transient current pulses at different temperatures, showing the increasingly dispersive behavior at low temperature.

Hole Transport in PMPS. In the experiments with layered structures (20) and visible excitation (to which PMPS is transparent), transient currents were observed only when the top electrode was negatively biased with respect to the substrate. The substrate was composed of a visible photoconductor (charge generation layer) overcoated aluminum ground plane. When the polymer top surface was directly (intrinsically) photoexcited with pulsed 337-nm excitation, current transit pulses were observed only when the top electrode was positively biased. Therefore, under the experimental conditions described, only hole transient transport could be directly observed. Transit pulses were nondispersive over a wide range of temperature. Figure 14 illustrates the relative increase in dispersion with decreasing temperature. In addition, no evidence for anomalous thickness dependence at the transit time was obtained, even at the lowest temperature. 

Figure 1. Transient currents in A1-PI-A1 structure at various temperatures. The 3.3 micron film is a BTDA - MPDA/ODA PI, and is subjected to 100V. A1-PI-A1 structure is used. Between each transient, the sample is discharged at 370°C for 10,000 sec. (Reproduced with permission from Ref. 10. Copyright 1989 M.I.T.)...

Fig. 12.7 The transient current, i.e. the current as a function of the time following a light pulse in crystals of (Me-Me-DCNQI)2Cu as a mixture of/ig/cfe (see Fig. 12.5), following optical excitation with a ps light pulse. The measurement was carried out at three different temperatures in the insulating region OPl and OPS near the phase transition at higher and lower temperatures (reentry, compare...

In most cases, the measurements are carried out isothermally in the frequency domain and the terms dielectric spectroscopy (DS) and dielectric relaxation spectroscopy (DRS) are then used. Other terms frequently used for DRS are impedance spectroscopy and admittance spectroscopy. Impedance spectroscopy is usually used in connection with electrolytes and electrochemical studies, whereas admittance spectroscopy often refers to semiconductors and devices. Isothermal measurements in the time domain are often used, either as a convenient tool for extending the range of measurements to low frequencies (slow time-domain spectroscopy, dc transient current method, isothermal charging-discharging current measurements) or for fast measurements corresponding to the frequency range of about 10 MHz - 10 GHz (time-domain spectroscopy or time-domain reflectometry). Finally, TSDC is a special dielectric technique in the temperature domain, which will be discussed in Section 2.2. 

Additionally, we seek diagnostic tools to understand how the fuel cell performance varies with the location in an individual fuel cell and between fuel cells in a stack. Spatial and cell-to-cell variations in current, temperature, reactant concenhation, and other parameters occur, especially at moderate to high currents and during load transients, and tools are needed which can measure or directly observe these effects. In an operating stack, the number of sensors are Umited due to various cost and size constraints, but laboratory diagnostics are very sophisticated. To understand distributed effects such as flooding in PEFCs or temperature distribution in SOFCs, direct visualization tools and sensors are... 

Figure 12-23 shows simulation of time-of-flighl photocurrenl transients at variable temperature for a system containing 0.25 eV traps at a concentration c=3xl0. This translates into orjj= 0.080 eV. The current decays monotonously by several orders of magnitude to finally merge into a plateau followed by a rapid fall-off that reflects discharge of the carriers at the exit contact. This is in accord with experiment [74],... 

Figure 8.27. Transient effect of current on the rate of CO oxidation on Pt (solid lines) and on catalyst potential (broken lines) inlet compositions and temperatures (a) pco=0.47 kPa, po2-10 kPa, T=412°C (b) pco=2.9 kPa, po2=0.40 kPa, T=555°C.33 Reprinted with permission from Academic Press.

When 2M methanol solution is fed to the stack at a flow rate of 2 ml/min and the stack is operated at a constant voltage output of 3.8V, the transient response of the stack current density is shown in Fig. 3 varying the flow rate of air to the cathode. The stack was maintained at a temperature of 50°C throughout the experiment. As shown in the figure, while the stack current is maintained at the air flow rates higher than 2 L/min, the stack current begins unstable at the slower flow rates. A similar result is shown in Fig. 4 for varying methanol flow rate at an air flow rate of 2 lymin. At a methanol flow rate of 8 ml/min, the current density reaches initially a current density value of about 130 mA/cm and then starts to decrease probably due to medianol crossover. As the methanol flow rate decreases, the stack current density increases slowly until the methanol flow rate reaches 3 ml/min because of the reduced methanol crossover. The current density drops rapidly from the methanol flow rate of 2 ml/min. 

Figure 13.5 Potential-step electro-oxidation of formic acid on a Pt/Vulcan thin-film electrode (7 p,gptcm, geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M HCOOH upon stepping the potential from 0.16 to 0.6 V (electrol)Te flow rate 5 p,L s at room temperature). (a) Solid line, faradaic current transients dashed line, partial current for HCOOH oxidation to CO2. (b) Solid line, m/z = 44 ion current transients gray line, potential-step oxidation of pre-adsorbed CO derived upon HCOOH adsorption at 0.16 V, in HCOOH-ftee H2SO4 solution.

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