Bipolar current problem

Studies of double carrier injection and transport in insulators and semiconductors (the so called bipolar current problem) date all the way back to the 1950s. A solution that relates to the operation of OLEDs was provided recently by Scott et al. [142], who extended the work of Parmenter and Ruppel [143] to include Lange-vin recombination. In order to obtain an analytic solution, diffusion was ignored and the electron and hole mobilities were taken to be electric field-independent. The current-voltage relation was derived and expressed in terms of two independent boundary conditions, the relative electron contributions to the current at the anode, jJfVj, and at the cathode, JKplJ. 

The analytic theory outlined above provides valuable insight into the factors that determine the efficiency of OI.EDs. However, there is no completely analytical solution that includes diffusive transport of carriers, field-dependent mobilities, and specific injection mechanisms. Therefore, numerical simulations have been undertaken in order to provide quantitative solutions to the general case of the bipolar current problem for typical parameters of OLED materials [144—1481. Emphasis was given to the influence of charge injection and transport on OLED performance. 1. Campbell et at. [I47 found that, for Richardson-Dushman thermionic emission from a barrier height lower than 0.4 eV, the contact is able to supply... 

In another variation of the bipolar pulse technique, a bipolar current pulse is applied to a conductance cell, and the voltage is sampled at the end of the second pulse [19]. Analogously, the solution resistance is calculated from Rs = measured applied This technique has also been applied with good success to chemical problems similar to those mentioned above. 

A number of integrated circuit (IC) failure mechanisms are related to the presence of water and impurities at device surfaces. The most catastrophic failures are open or short circuits resulting from electrochemical attack on substrate metallization. Other, more subtle maladies include increased capacitive coupling between conductors (1.), reduced bipolar current gain (2), shifted MOS threshold voltages (3.4), and parasitic MOS devices (5.6). These problems arise from spurious electrical conduction processes in the presence of moisture and ionic contaminants. Polymer encapsulants, such as silicone rubber, provide barriers that prevent the formation of conductive water films on IC surfaces. 

A possible problem of sealing the electrolyte path is found in the Foreman and Veatch cell. This can be avoided by placing the cells in a vessel. The best known example of this is the Beck and Guthke cell shown in Figure 8 (74). The cell consists of a stack of circular bipolar electrodes in which the electrolyte is fed to the center and flows radially out. Synthesis experience using this cell at BASF has been described (76). This cell exhibits problems of current by-pass at the inner and outer edge of the disk cells. Where this has become a serious problem, insulator edges have been fitted. The cell stack has parallel electrolyte flow however, it is not readily adaptable to divided cell operation. 

II. Ease of electrical connection Here the main problem is that of efficient electrical current collection, ideally with only two electrical leads entering the reactor and without an excessive number of interconnects, as in fuel cells. This is because the competitor of an electrochemically promoted chemical reactor is not a fuel cell but a classical chemical reactor. The main breakthrough here is the recent discovery of bipolar or wireless NEMCA,8 11 i.e. electrochemical promotion induced on catalyst films deposited on a solid electrolyte but not directly connected to an electronic conductor (wire). 

A significant step for the commercialization of bipolar electrochemical promotion units has been made recently by Christensen, Larsen and coworkers at Dinex Filter Technology A/S in Denmark.18 20 The goal is the development of an efficient catalyst system for the aftertreatment of Diesel exhausts. This is one of the most challenging problems of current catalytic research. 

Because of the relatively high concentrations of the acid and base as well as the salt solution the limiting current density is in general no problem and a bipolar membrane stack can generally be operated at very high current densities compared to an electrodialysis stack operated in desalination. However, membrane scaling due to precipitation of multivalent ions such as calcium or heavy-metal ions is a severe problem in the base-containing flow stream and must be removed from the feed stream prior to the electrodialysis process with a bipolar membrane. 

Ni-state-of-the-art anodes contain Cr to eliminate the problem of sintering. However, Ni-Cr anodes are susceptible to creep, while Cr can be lithiated by the electrolyte and consumes carbonate, leading to efforts to decrease Cr. State-of-the-art cathodes are made of lithiated-NiO. Dissolution of the cathode is probably the primary life-limiting constraint of MCFCs, particularly under pressurised operation. The present bipolar plate consists of the separator, the current collectors, and the seal. The bipolar plates are usually fabricated from thin sheets of a stainless steel alloy coated on one side by a Ni layer, which is stable in the reducing environment of the anode. On the cathode side, contact electrical resistance increases as an oxide layer builds up (US DOE, 2002 Larminie et al., 2003 Yuh et al., 2002). 

Perchlorate formation in drinking water electrolysis is a serious problem. In experiments using a semitechnical bipolar cell with BDD electrodes and drinking water (40 ppm chloride) even for the lowest current density applied (50 A m-2) perchlorate was measured at 30ppb using a residence time of approximately 1 s. This behaviour does not recommend BDD cells for drinking water treatment without additional measures. 

In spite of this, the practical verification of the idea has been severely hampered to the present-day, mainly due to material problems for the so-called bipolar wall. Even for the lead-acid accumulator, the development is only at the initial stages [479]. Metal-firee batteries suffer somewhat from poor electronic conductivities of the current collectors, as already pointed out in the previous section, and this feature alone would highly justify the introduction of bipolar cell designs. 

CMOS devices can develop a serious problem called latchup, in which junctions in different devices connect and form a forward-biased diode structure, leading to a catastrophic current which destroys the circuit. As illustrated in Fig. 14.5a, the latchnp is caused by the formation of a pnpn device between the terminal of VSS and VDD (see Chap. 9, Sect. 1.3). In a latchup condition, the pnpn device is biased snch that the collector current of the pnp bipolar transistor supplies a base current to the npn bipolar transistor in a positive feedback situation. The latchup can cause device function failure or even self-bumout. Figure 14.5b shows the bipolar components and resistive components of a latchup configuration. The conduction state of a pnp device requires Vq, and the conduction state... 

However, as long as we are inside the symmetrical zone of reciprocal current density, the contribution from the left and right side volumes with respect to the PU electrode position are equal but with opposite signs. The volume of the symmetrical zone is dependent on the reciprocal current path (e.g., whether it is a bipolar or unipolar PU electrode system). The paradox is that the symmetrical very high sensitivity region has zero net sensitivity all contributions cancel. In conclusion, such an electrode configuration will be very sensitive to asymmetry in the tissue near the PU electrode, and the recording may easily be unstable and noisy. Increased PU electrode contact area will reduce the problem, but this implies current disturbance of the J field. 

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