Field heating effect

Fig. 70. The thermomagnetic curves of polycrystalline LuFe204 showing the field heating effect, (a) in an external field of 4.5 kOe (b) in an external field of 105 Oe. In both cases, the upper curves are for a field-cooled specimen whereas lower...

The field-heating effect has been observed most clearly in LuFe204 but is supposed to exist in other compounds, too, at least in YbFe204. It is crucially dependent on the process of sample preparation, although no difference has been detected in composition, contamination of other elements or phases, lattice constants, etc., between specimens with and without the field-heating effect. The dynamics of the melting of frozen spins in these compounds are problems to be investigated. 

The heating effect is the limiting factor for all electrophoretic separations. When heat is dissipated rapidly, as in capillary electrophoresis, rapid, high resolution separations are possible. For electrophoretic separations the higher the separating driving force, ie, the electric field strength, the better the resolution. This means that if a way to separate faster can be found, it should also be a more effective separation. This is the opposite of most other separation techniques. 

Changes in the composition of a specimen over the analyzed depth can be caused by beam heating or by beam charging of the specimen. Beam heating can lead to selective vaporization of some elements or diffrisional redistribution of the elements. If the surflice charges up to some potential, then electric-field enhanced diffusion can selectively redistribute certain elements. Beam-heating effects usually... 

Micro reactors are seen to have smaller inhomogeneities of the electrical field and less temperature rise in the reaction medium due to the Joule heating effect between the electrodes [70]. Submillimeter interelectrode gaps are expected to reduce the ohmic loss. 

There are a number of papers offering explanations of the breakdown phenomenon. Suggestions have been made that it is due to the presence of macro- and microdefects in oxides (electrolyte-filled fissures, micropores, flaws, etc.).286 Joule heating effects were also considered289 as well as volume increase and the resulting increase of internal stresses during anodization,290 elec-trostriction effects,291 or field-assisted ionic migration.292... 

Fig. 33. Comparison of temperature fields in WS midplane for particles with active outer shell (95% inactive, left diagram) and particles with no reaction heat effects (100% inactive, right diagram). 

The measurement of properties such as the resistivity or dielectric constant of PS requires some kind of contact with the PS layer. Evaporation of a metal onto the PS film-covered silicon sample produces a metal/PS/Si sandwich, which behaves like an MIS structure with an imperfect insulator. Such sandwich structures usually exhibit a rectifying behavior, which has to be taken into account when determining the resistivity [Si3, Bel4]. This can be circumvented by four-terminal measurements of free-standing PS films, but for such contacts the applied electric field has to be limited to rather small values to avoid undesirable heating effects. An electrolytic contact can also be used to probe PS films, but the interpretation of the results is more complicated, because it is difficult to distinguish between ionic and electronic contributions to the measured conductivity. The electrolyte in the porous matrix may short-circuit the silicon filaments, and wetting of PS in-... 

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