Four-probe configuration

However, biofilms and pili increased in conductivity when these materials were cooled from the room temperature, until a crossover temperature after which disorder dictates the response (Fig, 7,13) [4, 68], It was suggested that potential contact effects negated the relevance of these measurements [78], but the studies were performed with a four-probe configuration that avoids such artifacts. Furthermore, the increase in conductivities was observed as the temperature was dropped through physiologically relevant temperatures of... 

Sone et at. used a four-probe configuration to study the impact of temperature and RH on the ionic conductivity of a Nafion 117 membrane [50]. They quantified the conductivity change with RH and showed that the ionic conductivity of the membrane decreased with the heat-treatment temperature ranging from 80 to 120 °C. Heat treatment led to the microstructural change of the membrane and caused it to lose some of its ability to take up water. 

Contact resistances are very small in comparison with the resistance of the sample, so sandwich-type configuration of electrodes with guard ring can be used (no need for four-probe configuration). 

Conventional electrochemical experiments at the ITIES are based on a four-electrode configuration with a reference and an auxiliary electrodes positioned in each liquid phase (16,17). A four-electrode potentiostat is used to apply a voltage between the reference electrodes and to measure the current flowing between auxiliary electrodes. The interfacial charge transfer is assumed to be rate limiting, and the whole potential drop occurs mostly within a thin interfacial layer. However, the ITIES in such an experiment is not microscopically probed directly, and the nature of CT reaction (i.e., ET vs. IT) typically remains uncertain. 

Dopant profiles in the films were obtained by secondary-ion mass spectrometry (SIMS). The concentration of electrically active impurities was evaluated using four-probe resistivity measurements and calibration plots of resistivity versus dopant concentration. In the resistivity measurements, we used a coplanar contact configuration with a contact separation of 1 mm. The electric field was no higher than 102 V/cm. 

Figure 2.22 z-velocity in the xy plane at the outlet of the reference channel and four different configurations of the probe. Figure taken from Hettel et al. (2013). 

Various methods have been proposed for reducing or eliminating the effects of alternating current polarization impedances (Ferris, 1959, 1963 Schwan and Ferris, 1968 Schwan, 1951, 1963). Since the impedance is a function of current density at the electrode surface, an obvious solution is the elimination of current through the probes. Unfortunately, it is impossible to do this in a two-electrode system if one is attempting to measure impedance however, it can be accomplished with a four-electrode configuration (Ferris, 1959, 1963). 

Through-plane measurements of total GDL conductivity with varying compressive loads have been measured with four-probe arrangements in axially symmetric configurations (see Section 5.3.3). Several efforts with multiple GDL samples in a stack have been reported. However, the separation of interfacial and effective bulk conductivities (oj in thin, highly conductive samples can be challenging. 

To effectively eliminate interfacial impedance from conductivity measurements, a four-point probe method has been widely used [7,49,50,60-62,64]. Compared with the two-point probe configuration, two additional Pt probes... 

Figure 5 illustrates the doping and compensation process and the usual configuration for conductivity measurement. The precursor polymer (an electrical insulator) is mounted with four probes typically in a van der Pauw arrangement(60). The voltage V is measured between two adjacent probes while a current i is passed between the other two. The conductivity a is determined as (ln2)i/itdV, where d is the sample thickness. In the insulating state o is very low and strongly temperature activated. 

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

Note that the PSpice results are shown in a slightly different configuration because of the PROBE waveform display program. Three of the four Q output waveforms are shown in the lower section of the plot (analog waveform), while the clock (a digital signal) is shown at the top. 

Sheet resistance can be measured with a four-point probe. The probes may be in line or in a square pattern, as shown in Figure 9. In either configuration, a constant current I is passed through two of the probes, and the voltage difference between the other two is read. Provided the conducting layer is thin (t < 0.60 d), the sheet resistance can be calculated from... 

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