Four-probe conductivity test

A four-probe conductivity apparatus (Keithley 6517, USA) was used for the electrical conductivity test. The tests were repeated four times on the film with a thickness of 100 pm. 

A simple way to measure the resistivity of soil is a four-pin method in which four probes are drilled into the ground along a straight line at equal distances a and depth b. Then a voltage V is applied to the two inner probes and a current, T, is measured in the two outer probes (Figure 22.16). This test can also be conducted with the help of a ground icster as discussed in Section 22.3, which normally also has a provision for ihis test. 

Fig. 1.67 Conductivity measurement for PSSA-PVDF membrane using four-probe apparatus and a.c. complex impedance technique. Membrane was tested at room temperature and was fully hydrated.

Many polymers and most PMC show no or only comparatively low electrical conductivity, which limits application of electromagnetic test methods. One of the main exceptions is CFRP. The continuous network of carbon fibers in CFRP allows for electrical resistance measurements, eg, based on the four-probe method. Electrical potential (101) or resistance methods are used in fracture mechanics to detect delaminations (102) and to monitor damage in CFRP (103). Whether the application of electrodes is nondestructive depends on the intended use. 

Figure 8.5 Instrumentation for monitoring and corrosion experiments. (A) "geochemistry-corrosion" skid near the well head of CPK3 on the reinjection side of the Soultz geothermal cycle A Four probes used for continuous fluid monitoring (temperature, conductivity, pH, and Eh) and chambers for corrosion research and material testing. B Laboratory autoclave system and coupon mounting system.

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] ... 

The electrical conductivity of low-resistance polymers and polymer composites can be determined by two four-electrode techniques the standard method for testing bar-shape specimens of semiconductors and conductors and the four-point probe method developed for nondestructive testing of sheets and thin films. In both methods two external probes (electrodes) are used as the cunent electrodes and the two inner probes are used to measiue the voltage drop. Using four probes eliminates possible erroneous results due to the probe resistance and the contact resistance between the electrode and the tested material. Hiis method also allows the elimination of the influence of the barriers at the electrode/material contact on the temperature dependence of the conductivity. 

Four-Cartridge Test (Ref 16a, p 52). The test, designed in Germany under the name "Detonationsfahigkeit Probe , was conducted as follows ... 

Quantitative conductivity of a CP sample is typically measured by a four-point probe technique. CPs for biomedical applications have typically yielded conductivities of between 1 and 200 S cm [44—46,49,50,138]. However, four-point probe measurements can be problematic, as the CP film must be removed from the substrate prior to testing. There have been a number of different methods used for film removal, including doublesided tape, razor blades, and backing the film with polydimethylsiloxane (PDMS) [61]. Fonner et al. found the double-sided tape technique to be most effective, as the CP film was removed firom the substrate undamaged and the presence of the tape did not affect the conductivity measurement [61]. 

PEM for DMFC is mainly characterized with ionic conductivity and methanol crossover. Ionic conductivity is tested with the four-point probe method. Through plane and in plane ionic conductivity is characterized with temperature and humidity [22]. Methanol crossover can be measured with diffusion cell method, pervapora-tion of methanol solution, dictating of CO amount from cathode out stream [23]. An oxidant-impermeable property is also one of the important factors in PEM for DMFC. The properties of PEM in DMFC are also characterized with an interfacial resistance, an electro-osmotic drag, methanol crossover through MEA, and so on. In this chapter, development of PEM for DMFC are discussed. Approaches for PEM in DMFC are classified as fluorinated polymer, hydrocarbon polymer, modification of polymer materials, and the technical approaches are described as well. 

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