Four-probe technique

Accordingly, the ionic conductivity in an electrolyte with negligible electronic conduction (/jon jtolal) may be determined by Ohm s law, provided that unpolarizable electrodes are employed. To overcome this limitation, separate voltage probes in the shape of identical electronic leads connected to the electrolyte at positions separated by a distance L may be employed (four-probe technique [38]). Under these... 

In agreement with this analysis, single-crystal conductivity measurements using the four-probe technique reveals semiconducting behaviour for /r -(TMTTF)2Re04, as shown in Fig. 6.30. In this case aRj — 0.011 cm and Ea — 0.17 eV. 

Figure 6.30. Resistivity measurements using the four-probe technique performed on a /u. -(TMTTF)2Re04 thin single crystal. Reprinted with permission from Perruchas et al, 2005.

Resistivity measurements are also routinely made with an ac four probe technique. The wiring would follow according to Figure 1 and the measuring currents used would be in the range 0.1 - 10 mA with frequencies of 100 Hz (9). For flux creep now known to modify susceptibility and critical current measurements care must taken with ac measurements of resistivity although for the low current densities involved the effect will not likely be observed except very close to Tc (10) or in a magnetic field. 

The magnetoresistance measurements were carried out at 1.5 K and in magnetic fields up to 10 tesla (T). The sample was a flux-grown crystal with a dimension of 2.5 X 0.70 X 0.45 mm3, and the current direction were T//[ 1120]. The dc measurements were made using a standard four-probe technique. 

Upon doping with iodine vapor. Determined on thin films of polymers cast on glass substrate by the use of the four-probe technique. 

Electrical resistivity was measured by a d.c. method using four-probe techniques to avoid problems arising from contact resistance. Pressure contacts were used for both current and potential probes. At low temperatures, the current contacts could be improved by ultrasonically tinning the ends of the samples. 

Conductivity of (BN) iSO F and comparison with Cg SO F. In our early studies (12), a four-probe technique was employed, in which four platinum wires were used for electrical contact, and the samples were prepared by pressing powdered polycrystalline material into pellets. Because the platinum wires and the pellet surface are not ideally flat, a uniform intimate contact could not be assured between the wires and the pellet. The boundary effects due to the polycrystalline nature of the pellet sample also render such conductivity measurements unreliable. Attempts to use a contactless radio frequency inductive technique described by Zeller et al. (22) failed because this technique is not sensitive to low conductivities. A four-point probe measurement (21) on an intercalated highly oriented boron nitride sample was used in the present set of conductivity measurements. The <7295k 1.5Scm . The specific conductivity increased with decreasing temperature (see Fig. 1), it having nearly twice the room temperature value at 77 K. This indicates metallic behavior. 

To measure the Seebeck coefficient a, heat was applied to the sample which was placed between the two Cu discs. The thermoelectric electromotive force (E) was measured upon applying small temperature difference (JT <2 E) between the both ends of the sample. The Seebeck coefficient a of the compound was determined from the E/JT. The electrical resistivity p of the compound was measured by the four-probe technique. The repeat measurement was made rapidly with a duration smaller than one second to prevent errors due to the Peltier effect [3]. The thermal conductivity k was measured by the static comparative method [3] using a transparent Si02 ( k =1.36 W/Km at room temperature) as a standard sample in 5x10 torr. 

The cesium [hydrogen bis(sulfate)] tetracyanoplatinate complex exhibits a room-temperature electrical conductivity (four probe technique) of approximately 113 ohms 1 cm 1, while that of the Rb analog (Pt—Pt - 2.83 A) is 1500-2000 ohms"1 cm 1. 

Tenn. Electronic spectroscopy was performed on a Varian Cary 17 recording spectrophotometer and a Nicolet Fourier Transform Infrared instrument was used to record spectra between 4000 and 200 cm l of samples in Csl. Electrical measurements were performed at room temperature on compacted samples by a four probe technique. TGA was performed on a DuPont 990 Analyzer using a DuPont 951 TGA module. TGA for the GC/MS analysis was performed on a DuPont 950 instrument. Volatile products were collected in a Tenax containing tube which was attached to the GC inlet port, GC/MS was performed on a Hewlett Packard 5982A coupled to a 5934A data system. DSC was performed on a DuPont 1090 analyzer using a DuPont 910 DSC module. 

Composites. The (CH)X/LDPE composites were prepared using the Ti(0Bu)4./Et3Al Ziegler-Natta catalyst system as previously described (10). The amount of (CH)X incorporated was determined by monitoring the acetylene uptake during the polymerization. Electrically conductive derivatives were prepared by immersion of the composites in a saturated 12/pentane solution for 24-48 hours. Electrical conductivities were measured by standard four-probe techniques. 

Experimentally, the excess noise is the difference between the mean-square voltage fluctuations < Sv > measured in the presence and absence of a quiet d.c. voltage VQ = IqR. We determine it using a four-probe technique designed to eliminate the relatively small excess contact noise. 

Measurements on d.c. conductivity were carried out by four-probe technique in the temperature range 77 to 350 K. 

FIGURE 2.1.18 Sheet conductivity o of the vacuum-gap rubrene single-crystal OFET, measured as a function of Vg at different T using the four-probe technique. (From Podzorov, V. et al., Phys. Rev. Lett., 93, 086602, 2004.)... 

The previously described four-probe technique [43,44] allows a separate determination of the source and drain contact resistances. If contacts would behave as Schottky barriers, one would expect the voltage drop at source to be substantially higher than that at drain. This is what is indeed observed with bad contacts. However, good contacts show comparable drops at both electrodes. A possible origin of this behavior has been recently put forward [46]. The model assumes that the regions immediately adjacent to the electrodes are made of organic material of quality different from that of the rest of the conducting channel, with very low mobility. 

As depicted in Figure 2.4.10(b), contact resistance at the source and drain electrodes results in a smaller than expected slope of the potential versus channel position profile. The profile is estimated by linear extrapolation between Vi and V2. Individual source and drain contact resistances are calculated by dividing the voltage drops AVs and AV by the source-drain current, respectively. By isolating the source and drain contact contributions to the total contact resistance, the gated four-probe technique provides more information than the transmission line technique, and it is possible to determine in one device (vs. several). An important caveat for the gated four-probe technique is that the extrapolated channel potential profile wiU only be valid for strict linear regime OFET operation (Vq V, ), where the channel potential profile can be expected to be linear and uniform. 

The resistivity was then measured for PSSA-PVDF membranes with the four probe technique at room temperature under fully hydrated conditions to more accurately determine the conductivity of the samples, and compare these values to the those reported in the literature. The complex impedance plot of a representative sample of PSSA-PVDF is shown in Fig. 1.67, which corresponds to a conductivity value of 54 mS cm . The conductivity of Nafion 117... 

Fig. 1.115 Schematic of apparatus used in the "four probe technique of measuring the conductivity of membrane samples.

The Tc and Hc2 were resistively measured using a standard dc four probe technique. Tc was taken at 50 % of the transition curves. The measured values of the low temperature Nb resistivity of a sample 60 nm thick is PNb=10 pflcm, while its critical temperature is Tc=8.3 K. The low temperature resistivity of the Cuo.42Nio.58 film 60 nm thick is pCUNi=50 p 2cm. 

Conductivity measurements were carried out at room temperature using a four-probe technique in the helium filled drybox. Four platinum pressure contacts were made to the free-standing polyacetylene films. Electrical feedthroughs allowed equipment including a digital multimeter (Hewlett Packard 3468A) to be connected externally to the drybox. All measurements were made at least three times on several independent samples. 

In this part, the composition of T-CN nanocomposites was determined by FT-IR. Morphology of the nanocomposites was characterized by using SEM and transmission scanning electron microscope [TEM, JEOL JEM-2000EX). The conductivities of the nanocomposite films were measured by the four-probe technique (GZFTL RST-8) at room temperature using a semiconductor device analyzer. 

Compressed pellet electrical conductivity of DMTTTI was measured at room temperature using a four-probe technique. Electrical contacts were made with conductive silver paste (GC Electronics) using 1 micron gold wires. The room temperature conductivity is observed to be 1 x lo"" S/cm. This compares very well with that reported for TTTI. ... 

To evaluate the sensing characteristics of the conjugated polymer or its nanocomposite using four-probe technique of resistivity measurement, four electrical contacts are made on the layer by copper wire and silver paste. Subsequently the change in voltage between two middle probes on the sample layer is measured in analyte environment and the results are reported in terms of the change in resistivity of the sample at constant current applied trough terminal two probe. The resistivity of the sample layer can be calculated by the equation [12] ... 

Electrical conductivities were calculated from the mean resistance values using the Van der Panw Equation (8.1) for the samples measured by the four-probe technique [1]. 

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