Light transmission at the uniaxial-biaxial lyotropic nematics phase transitions

Results are reported on light transmission versus the temperature near the phase transitions between lyotropic nematics. Strong pretransitional effects are observed when the phase transitions occur between biaxial nematic and uniaxial phases. The light is strongly depolarized both at the calamitic-biaxial and at biaxial-discotic phase transitions. The results are qualitatively explained.     

Effect of fluorine on the electrical conductivity of amorphous Si

We report the effect of fluorine on the dc electrical conductivity of amorphous Si films prepared by cathodic sputtering.     

Effect of quenching on the electrical conductivity of doped KCl crystals

Electrical conductivity and optical absorption of divalent cation and anion doped KCl crystals have been measured before and after quenching from 750 °C. The conductivity after quenching is found to decrease in undoped and cation doped crystals while it increases in the anion doped crystals. This is due to higher coagulation rate of cation impurities as compared to anion impurities. Since the divalent impurities diffuse in the form of impurity-vacancy complexs, the higher mobility of cation vacancies (which form dipoles with divalent cation impurities) accounts for the higher aggregation rate of the cation impurities. The aggregation rate, during annealing after quenching, has also been found to depend on concentration.The author is indebted to Prof. H. N.Bose for helpful discussions. Thanks are also due to Dr. M. L.Mukherjee for providing the crystals.     

Effect of hydrogenation on the electrical conductivity of amorphous silicon carbide

The d.c. conductivity of RF sputtered hydrogenated amorphous SiC films have been studied in the temperature range of 350–130 K. High temperature data yield activation energy gap which increases significantly with hydrogen content, while the low temperature conductivity is explained by hopping between localized states.     

Effect of cylindrical filler aggregation on the electrical conductivity of composites

This work reports on the effect of carbon nanotube aggregation on the electrical conductivity and other network properties of polymer/carbon nanotube composites by modeling the carbon nanotubes as hard-core cylinders. It is shown that the conductivity decreases for increasing filler aggregation, and that this effect is more significant for higher cylinder volume fractions. It is also demonstrated, for volume fractions at which the giant component is present, that increasing the fraction of cylinders within clusters leads to a break of the giant component and the formation of a set of finite clusters. The decrease of the giant component with the increase of the fraction of cylinders within the cluster can be related to a decrease of the spanning probability due to a decrease of the number of cylinders between the clusters. Finally, it is demonstrated that the effect of aggregation can be understood by employing the network theory.     

Effect of uniaxial stress on electrical conductivity of sulphur nitride polymer

The effect of uniaxial stress on the room temperature electrical conductivity of polymeric sulfur-nitride is reported. The results provide additional evidence of the importance of interchain coupling on the electrical properties of (SN)_x and demonstrate that the perpendicular transport is limited by interfiber contacts.     

Non-ideal change of electrical conductivity on solidification and remelting of salt eutectics: 1. Oxyanion-based systems

A number of salt eutectics have shown non-ideal electrical conductivity on melting and solidification, indicating some overlap between regions of ‘solid-like’ and ‘melt-like’ conductivities. Solidified eutectics showed much higher conductivities than expected from the values of the component salts, which is attributed to higher interfacial areas between the salt phases formed on solidification, with ionic transport being facilitated in these regions, which would have relatively high disorder.     

Effect of plastic deformation on the electrical conductivity of NH4Cl

Excess electrical conductivity is induced by plastic deformation in NH₄Cl crystals. Study of the isothermal annealing of the excess conductivity at different temperatures in the range 80–150°C gives an activation energy of 0.08 eV for the carrier responsible for excess conduction, which is suggested to be a proton.     

Effect of destabilizing fields on hydrodynamic instabilities in nematics

The effect of destabilizing fields on the roll instability (RI) threshold for shear flow and on the homogeneous instability (HI) threshold for plane Poiseuille flow of nematic HBAB (μ _s>0) is studied on the basis of the continuum theory of nematics for flow cells of infinite lateral width. It turns out that the critical shear rate and wave vector at RI threshold decrease with increasing destabilizing field but do not approach zero at the Freedericksz transition. However calculations show that beyond the Freedericksz threshold HI may be favourable over a range of destabilizing field with shear in the stabilizing role. For plane Poiseuille flow a similar analysis points to the existence of a HI threshold in the presence of destabilizing field beyond the Freedericksz threshold again with shear acting as a stabilizing field. These results are compared with theoretical results obtained previously for MBBA.     

Effect of external field on coherent backscattering in nematics

For a one-elastic-constant model of nematic liquid crystal the optical theorem is shown to produce an explicit relationship between the scattering length of extraordinary wave mode and magnetic coherence length. The Monte Carlo simulation of coherent backscattering is performed accounting for the long-range orientational fluctuations and scattering length anisotropy; the coherent backscattering peak is shown to change quite weakly while the magnetic field varies several orders.     

Thermal conductivity of Rochelle salt

The temperature dependence of the anisotropic thermal conductivity of a Rochelle salt single crystal was investigated in different crystallographic directions in the range from −35 to +40°C. No anomalous behaviour was found near the Curie points.     

The influence of anchoring energy on the prolate shape of tactoids in lyotropic inorganic liquid crystals

A model was constructed to describe the prolate shape of anisotropic regions, tactoids, coexisting with the isotropic phase in lyotropic inorganic liquid crystals. The elastic energy of the tactoid, the surface energy, and the interaction energy between the director field and the boundary of the tactoid were taken into account. Large-sized tactoids were shown to be prolate because of the competition between the elastic energy of the nematic phase of the tactoid and the surface energy. Small-sized tactoids were prolate because of the competition of the surface energy with the anchoring energy between the director and the boundary of the tactoid. The suggested model was applied to experimental data to determine the ratio of the elastic constants K₃/K₁ and the ratio between the anchoring energy W and the surface tension σ depending on the “time of aging” of vanadium pentoxide sols in water.     

Effect of structural misalignments on the temperature variation of basal plane electrical conductivity of graphite

The amount and extent of structural misaligments in natural graphite crystals have been determined, and the temperature variation of the basal plane electrical conductivity (σ_⊥) of naturally occurring graphite has also been studied from 300 to 90 K. The conductivity (σ_⊥) has been found to obey a law σ_⊥α($\text{1}\text{T}$) down to a certain temperature θ (θ varying from sample to sample), below which the variation deviates from linearity towards lower values of σ_⊥. This behaviour, which was earlier thought to be a characteristic of graphite and whose origin could not be traced, has been shown to be an effect of the structural misalignments usually present in natural samples of graphite.