Field cooling effect

The behavior of a polar dielectric in an electric field is of the same kind. If the dielectric, is exposed to an external electric field of intensity X, and this field is reduced in intensify by an amount SX, the temperature of the dielectric will not remain constant, unless a certain amount of heat enters the substance from outside, to compensate for the cooling which would otherwise occur. Alternatively, when the field is increased in intensity by an amount SX, we have the converse effect. In ionic solutions these effects are vciy important in any process which involves a change in the intensity of the ionic fields to which the solvent is exposed—that is to say, in almost all ionic processes. When, for example, ions are removed from a dilute solution, the portion of the solvent which was adjacent to each ion becomes free and no longer subject to the intense electric field of the ion. In the solution there is, therefore, for each ion removed, a cooling effect of the kind mentioned above. If the tempera-... 

Fig. 17. Field cooled magnetization measured for increasing temperature at hqH = 2.5 mT on single crystalline YNi2B2C with the two isotopes l0B (solid lines) and 11B (dotted lines), clearly indicating a boron isotope effect...

Fig. 5 Magnetocapacitance and magnetoelectric effects in Tb MnCri, Magnetic field-induced change in the dielectric constant (a) and (b), electric polarization along the c and a axes, respectively (c) and (d). Magnetic fields are applied along the b axis. The data for (d) were collected after magnetic field cooling. The numbers in (d) denote the order of measurements at 9 K (from ref. 14).

There are two aspects to perfect diamagnetism in superconductors. The first is magnetic field exclnsion if a material in the normal state is zero field cooled (ZFC), that is, cooled below Tc to the superconducting state withont any magnetic field present, and then it is placed in an external magnetic field, the field will be excluded from the superconductor. The second aspect is magnetic field expulsion. If the same material in its normal state is placed in a magnetic field, the field will penetrate and have almost the same value inside and outside because the permeability fx is so close to the free space value fXo. If this material is then field cooled (FC), that is, cooled below E in the presence of this applied field, the field will be expelled from the material this is the Meissner effect that was mentioned earlier. 

Fig. 1. Cartoon depicting the spin-cooling effect of optical polarization on an ensemble of nuclear spins (assuming /= 1/2 and positive gyromagnetic ratio). Normally (at thermal equihbrium), the numbers of spins aligned parallel and antiparallel to the magnetic field (Bq) are nearly equal, yielding a low net spin polarization - and consequently, a tiny detectable magnetization, Mq. However, optical polarization can provide the means to drive the population distribution far away from equilibrium, thereby increasing M by several orders of magnitude. In the spin-temperature model, such polarization can often result in nuclear spin ensembles with miUi-Kelvin effective temperatures. (After Ref [110].)...

The thermal structure reveals the existence of a barrier layer in the SCS. The barrier layer usually weakens the cooling effect entrained at the bottom of the mixed layer. There are barrier layers in both the NSCS and SSCS, but they are thinner than that in the western equatorial Pacific. A barrier layer in the SSCS has a seasonal variation, and its depth has a positive correlation with temperature in the mixed layer. In addition, the barrier layer often exists in summer and autumn. The structure of the barrier layer in the SSCS is significantly modulated by the wind field, as well as by development of the mixed layer. In summer, relatively fresh water in the upper layer in the SSCS piles up in the southeast SCS because of the combined action of southeastward Ekman transport and downwelling in the eastern SCS. The high temperature water at the bottom of the mixed layer remains in a thermally uniform layer after separating from the mixed layer. The deepest barrier layer lies in the southeastern SCS, at about 30 m depth. The location of the thickest barrier layer almost overlaps the SCS Warm Water, which suggests that the heat barrier effect may stimulate the development of the SCS Warm Water. 

A ferroelectric crystal does not normally show any observable polarisation, because the domain structure leads to overall cancellation of the effect. PolycrystaUine ceramics would be expected to be similar. In order to form a material with an observable polarisation the crystals are poled. This process involves heating the crystals above the Curie point, Tc, and then cooling them in a strong electric field. The effect of this is to favourably orient dipoles so that the crystal or polycrystaUine ceramic shows a strong ferroelectric effect The majority of ferroelectric materials used are, in fact, polycrystal-Une. 

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