Interfaces resistance

The effect may be reduced by the introduction of surfactants which tend to concentrate at the interface where they exert a stabilising influence, although they may introduce an interface resistance and substantially reduce the mass transfer rate. Thus, for instance, hexadecanol when added to open ponds of water will collect at the interface and substantially reduce the rate of evaporation. 

The possible existence of an interface resistance in mass transfer has been examined by Raimondi and Toor(12) who absorbed carbon dioxide into a laminar jet of water with a flat velocity profile, using contact times down to 1 ms. They found that the rate of absorption was not more than 4 per cent less than that predicted on the assumption of instantaneous saturation of the surface layers of liquid. Thus, the effects of interfacial resistance could not have been significant. When the jet was formed at the outlet of a long capillary tube so that a parabolic velocity profile was established, absorption rates were lower than predicted because of the reduced surface velocity. The presence of surface-active agents appeared to cause an interfacial resistance, although this effect is probably attributable to a modification of the hydrodynamic pattern. 

However, under working conditions, with a current densityj, the cell voltage E(j) becomes smaller than the equilibrium cell voltage eq, as the result of three limiting factors (i) the overvoltages Tia and T],- at both electrodes due to a rather low reaction rate of the electrochemical reactions involved (T] is deflned as the difference between the working electrode potential and the equilibrium potential , so that i = ( + T]), (ii) the ohmic drop J both in the electrolyte and interface resistances e and (iii) mass transfer limitations for reactants and products (Figure 1.2). ... 

Zone I the E vsj linear curve corresponds to ohmiclosses j in the electrolyte and interface resistances a decrease of the specific electric resistance from 0.3 to 0.15 Dcm gives an increase in the current density j (at 0.7 V) from 0.25 to 0.4 A cm, that is, an increase in the energy efficiency and in the power density of 1.6-fold. 

The realization of the MEA is a crucial point for constructing a good fuel cell stack. The method currently used consists in hot-pressing (at 130 °C and 35 kg cm ) the electrode structures on the polymer membrane (Nafion). This gives non-reproducible results (in terms of interface resistance) and this is difficult to industrialize. New concepts must be elaborated, such as the continuous assembly of the three elements in a rolling tape process (as in the magnetic tape industry) or successive deposition of the component layers (microelectronic process) and so on. 

What makes an interface polarizable In other words, what makes an interface resist or accept potential changes This question can be answered, but the answer has to be in terms of the rates at which charges transfer across the interface, i.e., in electrodic terms (see Section 7.2). 

When material is transferred from one phase to another across a separating interface, resistance to mass transfer causes a concentration gradient to develop in each phase (Figure 3-1). 

Let us now construct an atomic model for the interface reactions and particle transfer across boundaries in order to interpret such kinetic parameters introduced before as the exchange current or the interface resistance. Tb this end, we replot Figure 10-7 as shown in Figure 10-9 a. This scheme allows us to quantify the processes occurring at a stationary interface in an electric field under load. Let us further simplify the model and consider crystals with immobile anions and the interface AY/AX as shown in Figure 10-9 b. AY merely serves as a source for the injection of atomic particles (SE s) into the sublattices of AX, or as a sink for SE s arriving from... 

AC/ is known as the overpotential in the electrode kinetics of electrochemistry. Let us summarize the essence of this modeling. If we know the applied driving forces, the mobilities of the SE s in the various sublattices, and the defect relaxation times, we can derive the fluxes of the building elements across the interfaces. We see that the interface resistivity Rb - AC//(F-y0) stems, in essence, from the relaxation processes of the SE s (point defects). Rb depends on the relaxation time rR of the (chemical) processes that occur when building elements are driven across the boundary. In accordance with Eqn. (10.33), the flux j0 can be understood as the integral of the relaxation (recombination, production) rate /)(/)), taken over the width fR. 

Fig. 5 dotted lines). The overall spectra shapes are qualitatively reproduced, except for the peak of the 50 nm spectra which is noticeably different from the experimental data. At large Ln, the predicted mini-gap is clearly smaller than in the experiment. Changing the s values or introducing a small interface resistance in the calculation did not improve the fit. We tried to take into account the dependence of the elastic mean free path with the normal metal thickness Ln, and hence the related variation of the characteristic length n... 

It is generally considered that there exist three resistances in series in transfer processes of gas-solid, gas-liquid, liquid-liquid, and liquid-solid systems gas or liquid side resistance, the so-called external resistance, interface resistance, and internal resistance of particle/droplet. The interface resistance possibly results from the accumulation of impurities on the interface. Reduction of any one of these three types of resistance can enhance transfer processes. 

Minkowycz, W.J. and Sparrow, E.M., Condensation Heat Transfer in the Presence of Non-condensables, Interface Resistance, Superheating, Variable Properties and Diffusion, Int. J. Heat Mass Transfer, Vol. 9, p. 1125,1966. 

The strategy of hybrid and gel electrolytes is very promising for lithium-ion batteries, but it seems less viable for lithium-metal batteries due to the reactivity of lithium metal with the encapsulated solvent. In fact, high conductivity is not the only parameter in selecting a successful polymer electrolyte for the development of lithium batteries a low interface resistance and a high interface stability over time are also required to assure good cyclability and long life. 

Polymer electrolytes have been shown to stabilize the lithium/electrolyte interface, yielding stable and low interface resistance, especially when ceramic additives such as y-LiA102 are used. Furthermore, the 7-LiA102 ceramic additive has been shown to stabilize the polymer amorphous phase and to slow down the recrystallization process [99-103]. Thus, the unique electrochemical performance of lithium metal can be applied in practical devices by substitution of the liquid electrolyte with a solid one whose conductivity and stability can be enhanced with ceramic additives. 

Fig. 1 shows the potential profile near the joint boundary in the FGM. The abrupt potential change was found within a width of about 0.5 mm, which is also reported for the SiGe-FGM[2]. Since the potential gradient corresponds to an increase of resistivity at the joint interface, the interface resistivity must be minimized by optimizing the sintering in the further studies in order to attain high efficiency of energy conversion. 

Fig. 2 shows the temperature dependence of <7 for the FGM and the components. The 7 value of all the samples monotonously decreased with an increase of temperature, indicating that the samples are the typical degenerated semiconductors. The 7 value for the FGM was almost intermediate between those for the components and never become lower than those of the components in spite of the existing interface resistivity. At Ti>500 K, the O value for FGM was larger than that for the components. 

Although the origin of the interface resistivity is not clear so far, an inspection into the low resistive interface of the p-n junction will give us a solution for preparing high performance of interface in stepwise carrier concentration FGMs. 

Note If we want to know how much thermal resistance is typically attributable to thermal grease, we must remember that without this grease we would have air in the spaces between the device and heatsink, and that is a very poor thermal conductor. Thermal grease lowers this interface resistance significantly by filling the... 

Another governing relationship, however, is Ohm s law, which leads to a dependency of the corrosion current on both the polarization characteristics of the anodic and cathodic reactions and on the total electrical resistance of the system, Rtotal. Rtotal includes the resistance in the metal between anodic and cathodic areas, RM a metal junction resistance if different metals are associated with the two areas, Rac any anode- or cathode-solution interface resistance, Rai and Rci and the solution resistance, Rs. The latter depends on the specific resistivity or conductivity of the solution and the geometry of the anode-solution-cathode system. 

Figure 183 (a) The admittance spectrum of a typical film in the low-frequency range the darkened squares are the measured data and the open squares are derived from (fc) the circuit model for low-frequency characteristics containing the interface admittance, Ri, Cj, and the film admittance, Rf, Cj. For the curve fit, the interface capacitance, C,, is 0.25 /xF and the interface resistance, R, is 1 kQ the film capacitance, C/, is 0.1 /xF and the film resistance, Rf, is 2.5 M 2. 

Estimates of the order of magnitude for each of these resistances indicate that several simplifications can be made [33], that is, the axial resistance of both the pipe wall and the liquid-wick combination may be treated as open circuits and neglected and the liquid-vapor interface resistances and the axial vapor resistance can typically be assumed to be negligible, leaving only the pipe wall radial resistances and the liquid-wick resistances at both the evaporator and condenser. 

Figure 12.5 depicts schematically the gas- and aqueous-phase concentrations of A in and around a droplet. The aqueous-phase concentrations have been scaled by HART, to remove the difference in the units of the two concentrations. This scaling implies that the two concentration profiles should meet at the interface if the system satisfies at that point Henry s law. In the ideal case, described by (12.45), the concentration profile after the scaling should be constant for any r. However, in the general case the gas-phase mass transfer resistance results in a drop of the concentration from cA(oo) to cA(Rp) at the air-droplet interface. The interface resistance to mass transfer may also cause deviations from Henry s law equilibrium indicated in Figure 12.5 by a discontinuity. Finally, aqueous-phase transport limitations may result in a profile of the concentration of A in the aqueous phase from [A(/ ,)J at the droplet surface to [A(0)] at the center. All these mass transfer limitations, even if the system can reach a pseudo-steady state, result in reductions of the concentration of A inside the droplet, and slow down the aqueous-phase chemical reactions. 

---