Measurement of the Mobility

The conductivity of a solid is given by Eq. (1). If we assume that, as in conventional extrinsic semiconductors, the concentration of free carrier is simply given by the density of dope, the mobility would be proportional to the conductivity, and the measurement of the conductivity would be a very simple way to have access to the mobility of an organic material. Unfortunately, this is rarely the case, and an independent measurement of the free carrier density would be most often required. As rehable determinations of the latter is generally not available, a direct measurement of the mobility is generally the necessary counterpart to conductivity measurements. 

1 eV) has been recognized to give good ohmic contacts to most oligothiophenes, whereas aluminum (work function 4.2 eV) generally forms a blocking contact. 

Although conductivity cannot be used by itself to estimate the charge mobility, it may be useful to study the transport anisotropy of organic materials. Its temperature dependence may also be of interest, insofar as the free carrier density remains constant, which is indeed the case in extrinsic semiconductors in certain temperature ranges. 

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Hoppel, W.A., Measurement of the Mobility Distribution of Tropospheric Ions, Pure and App. Geophvs. 81 230-245 (1970). 

Dynamic probe methods Another indirect strategy for emalysis of gel structure is the measurement of the mobility of dynsumic probes whose sizes are well characterized. For example, dynsumic light scattering or any other method for diffusivity determination (for examples, see 37) can be used to measure the motions, through a gel matrix, of a series of spherically shaped particles with varying sizes. To oversimplify greatly, if, as probe size is raised, a dramatic decrease in diffusivity is found, then the "mesh" size of the polymer gel may be estimated. 

Thermionic and photoelectric methods have been successfully employed in the measurement of the mobility of alkali metals on W surfaces. The migration of adsorbed gases like H2, O2, and CO over a metal surface may be followed in the F.E.M., and data concerning the mobility of these adsorbates are of particular interest, since they are frequently involved in surface reactions and other chemisorption phenomena. 

In the preparation and stablization of small, supported-catalyst particles, the consideration of surface mobility is essential. If the active component is in a high state of dispersion, conditions under which high mobility is attained must be avoided, since these conditions lead to particle size growth. On the other hand, a poorly dispersed component may be partially redispersed by treatment in a more highly mobile state. In supported catalyst systems, the interaction between the dispersed species (the active component) and the support is always of important concern, and a measure of the mobility of the active component is an indirect measure of this important interaction. 

There is a direct proportionality between the NMR correlation time - as measure of the mobility — and the IR frequency shift of the OH-bond of water - as measure of the H-bond interaction and order effects — in organic solvents100). 

The specific conductivity (y) is a measure of the mobility of ions in an electrolyte or electrons in a metallic conductor. Thus, y is about 1 or 107 S/m for a 0.1 kmol/m3 aqueous salt solution or for a metal such as iron, respectively. Such a difference in charge mobility makes the temperature dependence of % [i-e.,(l/x)3x/97k] positive for ions of about 2.5% per K, but negative for metals and alloys of approximately an order of magnitude lower (Prentice, 1991). 

When chain segments can move relatively freely in cured polymers, it is most likely due to low crosslink density or the mobility of the molecular chain structure. The glass transition temperature is a measure of the mobility of the molecular chains in the polymer network as a function of temperature. The glass transition is the reversible change in a polymer from (or to) a rubbery condition to (or from) a hard and relatively glassy state condition (Fig. 3.14). This transition occurs at a temperature called the glass transition temperature or Tg. It is... 

In a search for the defining structural parameter of a composite, the free volume of disperse system proved to be the most sound one from the physical standpoint Presumably, for disperse systems the free volume is a measure of the mobility of filler particles, just as for liquids it is a measure of the mobility of molecules. But as applied to highly-loaded coarse systems of the type solid particles — liquid — gas this notion requires a certain correction. In characterizing the structure of such specific systems as highly-loaded coarse composites, it should be noted that to prevent their settling and separation into layers under the action of vibration, the concentration of the finest filler fraction with the largest specific surface in dispersion medium should be the maximum possible. Because of this and also because of the small size of particles (20-40 pm), the fine fraction suspended in the dispersion medium practically does not participate in the formation of the composite skeleton, which consists of coarser particles. Therefore... 

Returning to the expressions for the mobility given by Eqs. (3.18) and (3.20), it must be stressed that the thickness and field dependences are artifacts of the time-of-flight experiment and are not intrinsic properties of the mobility, which is only time-dependent. The artifact arises because the transit time changes with the sample thickness or applied field and the measurement of the mobility Hu(0 then applies to a different time. It is also of note that the measured mobility in Fig. 3.13... 

Although there has been considerable effort to calculate the mobility of foam in porous media from first principles, utilizing usually measured rock properties ( 6), a different approach is used here. In this research, major emphasis has been on measurement of the mobility of C02 foam in rock core samples. The... 

Several studies have shown that clustering also occurs around negative ions in supercritical fluids. The mobility data for C F in xenon [see Fig. 3(a)] indicate that the clusters around C F are slightly smaller [see Fig. 3(b)] than those around the positive ion but the pressure dependence is similar. Measurements of the mobility of in argon at a temperature just above the critical temperature indicated the O2 ion also moves with a large solvation shell. 

Previous studies about dynamics of water near interfaces by quasi-elastic neutron scattering involved measurements of the mobility of water on the surface of Nafion membranes [63,64], the diffusive motions and the density of states of water in silica gels [65], and the interfacial melting of ice in graphite and talc pow-... 

Some measure of the mobility of the radicals within the glass may be obtained, since elevation of the temperature from 77° K to 90° K led to the recombination of the radicals. The variation in viscosity of substances which are glass-like at room temperature is less easily controlled, although some experiments have been de-scribed in which radicals are trapped in plastic films. 

Diffusion coefficient (polarographic, D, chromatographic, D ) A measure of the mobility of a species in units of cm /s. 

In this section, the reader will be confronted with and introduced to some comparatively elemental facts on the theory underlying interpretation of the shielding parameter accessible for normal molecules in isotropic solutions, where normal refers to molecules which are not oversized (such as vanadium bound to proteins), and were we therefore are in the so-called extreme narrowing limit , characterised by the condition 2TruQT << 1, where vq is the measuring frequency and the molecular correlation time, a measure of the mobility of a solute molecule in a solvent. Extreme narrowing simply means that the molecule is freely mobile and the frequency applied to obtain NMR information does not influence the respective parameters. Although the term contains the component extreme , we are well in the domain of normal conditions. 

The number of electrons in the system is dEJdfi n/2, while a measure of the mobile bond order is ( ln)cEJc VC/2. This value is close to the accurate one, 0.79, for the graphene sheet with a coordination number 3. 

Measurements of the mobility of membrane-associated components, particularly those associating with the cytoplasmic side of the lipid bilayer and positioned to interact with the intracellular domain of the receptor, are few (Bruckert et al., 1992 Kwon and Neubig, 1992 Kwon, 1992). Measurements of the diffusivity of membrane-associated effectors (see Fig. 2), for example, will be important in determining the role of diffusion in receptor/effector coupling and signal transduction. 

Exner and Simon163 have proposed that the transmission of the inductive effect through an aromatic ring may serve as a measure of the mobility of the -electrons and thus also as an indirect measure of the aromatic character of the skeleton. From an examination of the two series 20 and 21 the effects of substituents on the dissociation constants... 

Integrated electrosurface investigations, which include measurements of the mobile charge ct , by means of low-frequency dielectric dispersion (Lyklema et al. 1983) and electrokinetic charge by electrophoresis were applied by many groups in the world for particle surface... 

MORB- MORB-normalized spidergrams are most appropriate for evolved basalts, andesites normalized and crustal rocks — rocks to which MORB rather than primitive mantle could be spider diagrams parental. This form of spider diagram was proposed by Pearce (1983) and is based upon two parameters (Figure 4.25). Firstly, ionic potential (ionic charge for the element in its normal oxidation state, divided by ionic radius) is used as a measure of the mobility of an element in aqueous fluids. Elements with low (<3) and high (>12) ionic potentials are mobile and those with intermediate values are generally... 

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