Two-point probe

One technique is referred to as the "spreading resistance" method. In this procedure, a wafer is fractured and the edge containing the film is beveled, as shown in Figure 15. Then, a two-point probe is used to measure resistivity at a sequence of points traversing the interface between the substrate and the epi film. By relating the local resistivity to carrier concentration, one is able to deduce the concentration of dopant atoms over the epi layer. This technique is effective for even highly-doped layers. 

The sample was placed in an iodine chamber for doping. The sample, which still remained off-white in the dry state, appeared to be conductive as judged by the two-point probe method (instead of the four-point method for measuring resistance). Values as small as 500 ohms were measured when touching the sample with probes approximately 1 cm apart. 

A custom-made two-point probe with a linear geometric array was used to measure the specific... 

Regarding the conductivity cell geometry, two configurations have been developed to measure proton conductivity the two-point probe method [52-60] and the four-point probe method [7,49,50,56,60-65]. These will be discussed in the following sections. 

In the two-point probe configuration, two electrodes serve as both current and voltage sensing probes, as shown in Fig. 5.11 [60] in the diagram, open window A allows the membrane to be exposed to the environment. For this method, the current and voltage are measured from two identical probes. Because the electrode/membrane interfacial impedance is always included in... 

FIGURE 5.11 Schematic diagram of a conductivity cell for proton conductivity measurement with the two-point probe method. TTie membrane size is about 2.4 cm x 1.0 cm the distance between the two Pt strips is 0.4 cm [60]. 

Figure 5.12 shows the impedance spectra of the Nation 115 membrane, obtained by the two-point probe method with different distances between voltage sensing probes (Pt strips), at room temperature and under fully hydrated conditions [60]. Each impedance spectmm shows a semicircle in the high-frequency domain and a straight line with an angle of 45° in the lower-frequency domain. The membrane resistance is extracted from the lower-frequency intercept of the semicircle at the Zreai axis. It can be seen from Fig. 5.12 that at low frequency, the behavior of the Pt/Nafion interface is blocked by the 45° line, indicating that interface impedance has very little impact on the results of membrane conductivity. 

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 5.12 AC impedance spectra of Nation 115 membrane obtained by the two-point probe method with different distances between voltage sensing probes (Pt strips), at room temperature and under fuUy hydrated conditions. 2P = two-probe S = strip 0.4, 1.6, and 2.7 represent the distance in centimeters between the two probes [60]. 

In principle, the results measured by the four-point probe method should be more accurate and reliable than those measured by the two-point probe method. However, the latter method is easier. In addition, it has been reported [56] that different measurement frequency ranges should be applied for different... 

PEMs are key materials in PEM fuel cells, and the proton conductivity of a PEM can largely determine the performance of a fuel cell. Thus, an understanding of proton conduction and a knowledge of how to measure the proton conductivity of PEMs are essential prerequisites for developing novel membrane materials for PEM fuel cells. This chapter first addressed the various proton conduction mechanisms, then discussed in detail the methods for measuring proton conductivity, including current interruption, EIS, two-point probe and four-point probe AC impedance methods, and EC-AFM. The influences of both temperature and RH/water content on the proton conductivity of PEMs were also discussed. Generally, an increase in the temperature and/or RH can improve the proton conductivity of a PEM. 

The conductivity of PEDOTPSS layers is usually determined by depositing uniform thin films onto a nonconductive substrate. The sheet resistance, is measured via four-point or two-point probes. The resistivity, p, or its inverse, the conductivity, o, are calculated by multiplying times the layer thickness, d, according to... 

Without absorbing water, these PEMs tend to be rather rigid and are poor ionic conductors. The ionic conductivity would dramatically increase with water content [78]. Therefore, ionic conductivity tests are mostly conducted in water or in water vapor with adequate related humidity. Two types of ionic conductivity for PEMs were used in-plane and through-plane conductivities. The former represents the conductivity along the membrane surface direction, and the latter refers to the conductivity across the membrane thickness direction. In addition, there are two methods for conductivity measurement two-point probe electrode and four-point probe electrode. The latter method is more accurate but the former uses a simpler device. Therefore, comparison of ionic conductivities between membranes must be of the same type and measured through the same method. The aforementioned conductivity measurements are suitable for both proton conductivity and anion conductivity. Proton conductivity (o) is calculated by the following equation [79-82] ... 

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