Conductivity and polarisation

Classically, the experimental curve interpretation is often limited to the definition of a distribution function in which the Debye-type relaxation superposition is accounted for by two parameters a and jS. From this description, we learn nothing about the physical processes involved this is often the relaxation frequency zone which permits connection of the observed evolutions to a particular polarisation phenomenon. Moreover, for materials with large free carrier concentration, the conduction and polarisation effects are separated in an arbitrary way, assuming that the contribution to the conduction phenomena is limited to a term e" conduction = (Tdc/cyto over the whole frequency range. 

In a similar way, when results arc expressed in terms of the complex permittivity c e = f, — ie"), the contributions of the conduction and polarisation mechanisms will be separated. For a given value of... 

Electrochemical Techniques Although the linear polarisation resistance technique has moved beyond the infancy status attributed to it in the original material, its inherent limitations remain, i.e. it is a perturbation technique, sensitive to environmental conductivity and insensitive to localised corrosion. Two developments have occurred ... 

At these high frequencies, the retarding effect of the ion-atmosphere on the movement of a central ion is greatly decreased and conductance tends to be increased. The capacitance effect is related to the absorption of energy due to induced polarisation and the continuous re-alignment of electrically unsymmetrical molecules in the oscillating field. With electrolyte solutions of low dielectric constant, it is the conductance which is mainly affected, whilst in solutions of low conductance and high dielectric constant, the effect is mostly in relation to capacitance. 

Another type of polarization arises from a charge build-up in the contact areas or interfaces between different components in heterogeneous systems. This phenomenon is also known as interfacial polarization and is due to the difference in the conductivities and dielectric constants (see below) of the materials at interfaces. The accumulation of space charge is responsible for field distortions and dielectric loss and is commonly termed Maxwell-Wagner polarisation . 

Graphite was known from the 60 s to become metallic under suitable doping with potassium. It was then normal to try and dope with alkali metals the fullerenes Qq molecules discovered in 1985 by H.W. Kroto and R.E. Smalley, and to extend this treatment to balloons or tubes-like C molecules later developed in great quantities. The superconductivities observed here, often at fairly high temperatures, seem of a normal BCS type, with notable electron-phonon coupling and also with electron-electron repulsions weakened by the electrical polarisability of these big and easily excited molecules. Here again, more remains to be done on the detailed characteristics of conduction and superconductivity. 

Core-polarisation and conduction-electron polarisation effects can be studied as can exchange polarisation of diamagnetic atoms in magnetic hosts. The lattice dynamics of the metal lattice are examined via the temperature dependence of the /-factor. Many metals approximate closely to the Debye model, and a Debye temperature has some significance. Impurity doping can... 

This implies that the magnetic field at the iron is produced by the short-range core-polarisation effects, and that the conduction-electron polarisation is negligible because of there being little 4 density at the Fermi surface. This is consistent with the successful interpretations of the hyperfine field values using a 3(/-model only. 

More detailed measurements on the Pt/Fe system have covered compositions in the range 3-50 at. % Pt [81]. The 3 at. % alloy gave Hm = —1260 kG at 4-2 K and (99 keV) = -0-60(15) n.m. The value of the field is almost independent of composition within the range specified. Neutronscattering data have shown that there is no localised magnetic moment on the Pt atoms, so that the field is generated entirely by conduction-electron polarisation, approximately 0-07 unpaired conduction electrons being required per Pt atom. A similar mechanism is believed to act in the cobalt and nickel alloys [84]. 

Magnetic hyperfine splitting is seen at nominally diamagnetic Au in ferromagnetic alloys, presumably as a result of conduction-electron polarisation. The 6i-contribution to the conduction band becomes partially spin-unpaired by interaction with the magnetic atoms. The lines are usually only partly resolved, and the first magnetic data were not analysed successfully... 

A systematic study of the Eu/Yb and Eu/Ba alloys has been made [52, 53]. In the ytterbium system, the Curie temperature falls from 90 to 5 K and the saturation field also falls from 265 to 160 kG as the ytterbium content increases from 0 to 92 at. %. The relationships are linear apart from a discontinuity at 50 at. % where there is a phase change. Similarly for barium the Curie temperature falls from 90 to 40 K and the field from 265 to 206 kG as the barium content rises to 50 at. %. However, the chemical isomer shift is not significantly altered. The sign of the magnetic field is known to be negative from neutron diffraction data. Calculations suggest that a contribution of —340 kG to the field in europium metal arises from core polarisation, that +190 kG comes from conduction-electron polarisation by the atoms own 4/-electrons, and that —115 kG comes from conduction-electron polarisation, overlap, and covalency effects from neighbouring atoms. 

These compounds are sensitive to electrical field. Microwave frequency losses are due to two phenomena polarisation and conduction. The polarisable materials concept will be examined later with some examples based on conducting polymers. Relaxation-inducing dielectric losses can be caused by interfacial polarisa-... 

In heterogeneous systems, an interfacial polarisation is Created due to the space charges. This polarisation corresponds to the electron motion inside conductive charges, dispersed in an insulated matrice (Maxwell-Wagner Model). In fact, this phenomenon will appear as soon as two materials I and 2 are mixed so that c7]/ei C2le.2 with a conductivity and e dielectric constant at zero frequency [ 123]. 

In this technique the sample is subjected to an oscillating sinusoidal electric field. The applied voltage produces a polarisation within the sample and causes a small current to flow which leads the electric field by a phase dilference (5) (Figure 4). Two fundamental electrical characteristics, conductance and capacitance, are determined from measurements of the amplitude of the voltage (V), current (/) and S. These are used to determine the admittance of the sample (T) given by ... 

---