No abstract

No abstract

Lattice cells containing point defects in ferroelectric single crystals can behave as a second phase. In addition to the role these defects have in the variation of several chemical and physical properties of the ferroelectric, they modify those physical properties of the ferroelectric that are manifested in the hysteresis curve. These effects are: the shearing of the loop by all kinds of defects, the shifting of the hysteresis curve along the field axis (internal bias) by oriented polar defects, and the changing (mostly the increasing) of the coercive field strength. An electrostatic model is used to calculate the strength of these effects.

The static relative permittivity and density of 1-heptanol, 3-heptanol, 3-methyl-2-hexanol, 2-methyl-2-hexanol have been measured as a function of pressure up to 3.5 kbar and from -30 degrees to 100 degrees C. The static relative permittivity has been observed to be a sensitive function of pressure, temperature and the isomeric structure of the alcohol. The Kirkwood dipole correlation factor g has been calculated from the experimental results and its pressure and temperature dependence has been discussed in terms of isomeric structure. At a temperature of -30 degrees and a pressure of 3 kbar, an increase in g by a factor of 8 above the ambient pressure value for 2-methyl-2-hexanol compares with a similar increase by a factor of 1.4 for 3-methyl-2-hexanol. The results are compared with other studies of the effects of pressure on alkanols and their isomers. Reference is made to the possible importance of the existence of different optically active isomers in the samples studied.

There are significant differences in the field dependence of conductivity in low-density polyethylene as reported by various investigators. These differences are attributable partly to the presence of different kinds of impurities in variable concentrations in polyethylene samples of various origin, but are mainly due to the different measurement procedures adopted. The significance of the time which elapses after the application of voltage to the electrodes, as well as the effect of conditioning, e.g. thermal and voltage stressing of the specimen before and during the measurement, is discussed. Whether it is always necessary to obtain a steady-state current in order to determine the true conductivity is somewhat debatable.

A general description is given of the behavior of polymeric dielectrics in low fields, including polarisation and depolarisation, and transient and steady state conduction, in isothermal and non-isothermal conditions. An attempt is made to present a unified interpretation applicable to all of these phenomena.

A model for the conduction mechanism in polymer insulating materials based on a classical concept is described; the application may possibly be extended beyond polymers. By means of this model, a formula expressing the field dependence of conductivity is derived. The values given by this formula have been checked against published experimental data from several sources, and good agreement was obtained.

The complex relative permittivity of 1-heptanol, 3-heptanol, 3-methyl-2-hexanol and 2-methyl-2-hexanol has been measured from -30 degrees C to 20 degrees C at pressures up to 350 MPa in the range of frequencies between 50 Hz and 6 MHz. The dispersion in 1-heptanol and 3-heptanol is well characterised by the Debye equation, whereas the other two materials exhibit more than one relaxation time. The activation parameters are calculated and are found to be strongly dependent on the position of the hydroxyl group, and in the case of the branched isomers, of the methyl group. These results are compared with those in the literature for similar octanol isomers, and are discussed in terms of their dependence on liquid structure. The existence of closely ordered and loosely ordered chains is proposed to explain the results.