Conductivity measurements temperature dependence

Fig. 9.16 Analysis of the conductivity measurements temperature dependence of electronic properties derived for the (Fa)2PFe crystal a of the energy gap 2A at = 2kp, b of the concentration n, c of the mobility [jl, d of the mean scattering time r, and e of the mean free path X of the charge carriers in the...

The concentration of dissolved ionic substances can be roughly estimated by multiplying the specific conductance by an empirical factor of 0.55—0.9, depending on temperature and soluble components. Since specific conductance is temperature dependent, all samples should be measured at the same temperature. Alternatively, an appropriate temperature-correction factor obtained by comparisons with known concentrations of potassium chloride may be used. Instmments are available that automatically correct conductance measurements for different temperatures. 

In parallel with the HEM experiments, studies using a model porous membrane (Nuclepore ) system, with urea and mannitol as the permeants, were conducted. These studies showed that for these permeants, under experimental conditions identical to those of the HEM studies, the measured temperature dependence of permeation was in line with measured activation energies of bulk diffusion (Longsworth, 1953). The measured temperature-dependent ratios P glP2T) were 1.34 0.03 and 1.38 0.02 N = 4, ave. s.d.) for urea and mannitol, respectively (Peck et al., 1995). These ratios were viewed as a reference point to which the permeation temperature-dependence ratios determined for HEM could be compared. 

In 1997, Anderson et al. reported that when more and more extra electrons are introduced into the zeolite hosts,[19] some zeolites indeed show an electrical conductivity increase. The loading of potassium into zeolite L increases the room-temperature conductivity of the latter by 10 000 times. Nevertheless, the measured temperature dependence of conductivity on temperature indicates that the conduction mechanism is characteristic of thermal activation, and therefore that the electrical conduction may involve the redox jump of the K32+/K3+ process (see Figure 9.7). The K/K-A host-guest compound exhibits interesting ferromagnetic behavior,1221 and this magnetism may be related with the formation of a superlattice.[231... 

Experiment 16.9 Conductometric determination cf conversion A conductivity melCT with a double platinum electrode is used for measuring conductivity. (We will go into conductivity and measuring it in more detail in Oiap. 21.) Conductivity is temperature dependent so the use of a thermostat is recommended. To start the reaction, a known amount of tertiary butyl chloride is pipetted into the demineralized water in the measuring cell to start the reaction. 

If the conductivity cell is used in combination with a chromatographic column, the measuring electrodes have the form of capillaries. This makes it possible to keep the cell volume very small, as required the volume is ca. l-2pL. Figure 15 illustrates the principle of an HPLC conductivity detector with two measuring electrodes. The cell must be thermally insulated because the conductivity is temperature dependent. 

The radiation and temperature dependent mechanical properties of viscoelastic materials (modulus and loss) are of great interest throughout the plastics, polymer, and rubber from initial design to routine production. There are a number of laboratory research instruments are available to determine these properties. All these hardness tests conducted on polymeric materials involve the penetration of the sample under consideration by loaded spheres or other geometric shapes [1]. Most of these tests are to some extent arbitrary because the penetration of an indenter into viscoelastic material increases with time. For example, standard durometer test (the "Shore A") is widely used to measure the static "hardness" or resistance to indentation. However, it does not measure basic material properties, and its results depend on the specimen geometry (it is difficult to make available the identity of the initial position of the devices on cylinder or spherical surfaces while measuring) and test conditions, and some arbitrary time must be selected to compare different materials. 

The previous definitions can be interpreted in terms of ionic-species diffusivities and conductivities. The latter are easily measured and depend on temperature and composition. For example, the equivalent conductance A is commonly tabulated in chemistry handbooks as the limiting (infinite dilution) conductance and at standard concentrations, typically at 25°C. A = 1000 K/C = ) + ) = +... 

Temperature The level of the temperature measurement (4 K, 20 K, 77 K, or higher) is the first issue to be considered. The second issue is the range needed (e.g., a few degrees around 90 K or 1 to 400 K). If the temperature level is that of air separation or liquefact-ing of natural gas (LNG), then the favorite choice is the platinum resistance thermometer (PRT). Platinum, as with all pure metals, has an electrical resistance that goes to zero as the absolute temperature decreases to zero. Accordingly, the lower useful limit of platinum is about 20 K, or liquid hydrogen temperatures. Below 20 K, semiconductor thermometers (germanium-, carbon-, or silicon-based) are preferred. Semiconductors have just the opposite resistance-temperature dependence of metals—their resistance increases as the temperature is lowered, as fewer valence electrons can be promoted into the conduction band at lower temperatures. Thus, semiconductors are usually chosen for temperatures from about 1 to 20 K. 

The temperature dependence of the thermal conductivity of CBCF has been examined by several workers [10,13,14]. Typically, models for the thermal conductivity behavior include a density term and two temperaUrre (7) terms, i.e., a T term representing conduction within the fibers, and a term to account for the radiation contribution due to conduction. The thermal conductivity of CBCF (measured perpendicular to the fibers) over the temperature range 600 to 2200 K for four samples is shown in Fig. 6 [14]. The specimen to specimen variability in the insulation, and typical experimental scatter observed in the thermal conductivity data is evident in Fig. 6. The thermal conductivity of CBCF increases with temperature due to the contribution from radiation and thermally induced improvements in fiber structure and conductivity above 1873 K. 

Recently, Dinwiddie et al. [14] reported the effects of short-time, high-temperatme exposures on the temperature dependence of the thermal conductivity of CBCF. Samples were exposed to temperatures ranging from 2673 to 3273 K, for periods of 10, 15, and 20 seconds, to examine the time dependent effects of graphitization on thermal conductivity measured over the temperature range from 673 to 2373 K. Typical experimental data are shown in Figs. 7 and 8 for exposure times of 10 and 20 seconds, respectively. The thermal conductivity was observed to increase with both heat treatment temperature and exposure time. 

Fig. 18. The temperature dependence of the thermal conductivity of hybrid carbon fiber monoliths measured in the to fibers direction at two densities.

In Fig.. I we present the temperature dependence of the conductance for one of the CNTs, measured by means of a three-probe technique, in respectively zero magnetic field, 7 T and 14 T. The zero-field results showed a logarithmic decrease of the conductance at higher temperature, followed by a saturation of the conductance at very low temperature. At zero magnetic field the saturation occurs at a critical temperature, = 0.2 K, which shifts to higher temperatures in the presence of a magnetic field. 

Following the general trend of looldng for a molecular description of the properties of matter, self-diffusion in liquids has become a key quantity for interpretation and modeling of transport in liquids [5]. Self-diffusion coefficients can be combined with other data, such as viscosities, electrical conductivities, densities, etc., in order to evaluate and improve solvodynamic models such as the Stokes-Einstein type [6-9]. From temperature-dependent measurements, activation energies can be calculated by the Arrhenius or the Vogel-Tamman-Fulcher equation (VTF), in order to evaluate models that treat the diffusion process similarly to diffusion in the solid state with jump or hole models [1, 2, 7]. 

If the temperature dependence of conductivity is known in a given solvent, an estimate of an unknown A0 at higher temperatures may be obtained which is much better than that measurable at lower temperatures with the help of the Walden rule ... 

For example, the final heat treatment temperatures In the manufacture will produce different electrochemical properties, even with the same surface treatments (2-4) since the structure and electrical property of glassy carbon depends on the temperature, as Indicated by the single crystal TEM patterns and by measurement of temperature dependent conductivity (5-6). On the other hand. It Is also well established that the electrochemical properties of carbon-based electrodes are markedly affected by surface treatments. 

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