ABSTRACT
The physical properties of a liquid crystal-ionic liquid system were investigated. Low-frequency dielectric spectroscopy for 4-cyano-4'-pentylbiphenyl (5CB) doped with 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF 4) for the nematic and isotropic phase of host substances was performed. We obtained electrical conductivity values in the range from 298.2 K to 313.2 K and the conductivity anisotropy was confirmed. Further study of the relaxation process for bmim + allowed us to extract the relaxation frequencies and amplitudes from experimental data and confirm the temperature scaling; the thickness of the interfacial layers was estimated for the homogeneous and homeotropic alignments of the prepared composite. An attempt to unfold the ion contribution on the charge transport was made in order to better understand the electrode polarization process. In this work, the influence of the alignment layer and phase state on the interfacial layer formation in liquid crystal media will be explained better.
ABSTRACT
We present the depolarization field effects (Maxwell-Wagner-Sillars effect) for the thermal transport properties of polymer dispersed liquid crystal composites under a frequency-dependent electric field. The experiments were conducted on polystyrene/4-Cyano-4'-pentylbiphenyl (PS/5CB) PDLCs of 73 vol.% and 85 vol.% liquid crystal (LC) concentrations. A self-consistent field approximation model is used to deduce the electrical properties of polymer and LC materials as well as the threshold electric field. Electric field-varying (at constant frequency) experiments were also conducted to calculate the interfacial thermal resistance between the LC droplets and polymer matrix as well as to find the elastic constant of LCs in droplet form.
ABSTRACT
In this work, a new method to characterize vertical cracks by lock-in thermography is presented. The heat transfer process induced by a modulated thermal excitation located in the vicinity of the crack is simulated using a finite element method computer package. The propagation of heat flow along the solid surface is disturbed when crossing an inhomogeneity. The disturbance of the thermal-wave allows a quantitative analysis of the crack. The main idea consists of exploiting the second derivative of the amplitude image in order to highlight the useful signal. In addition, an image analysis procedure based on the use of Laplacian calculations is proposed. To support this approach, experimental tests were performed and compared with mathematical simulations. The results demonstrate the potential of active lock-in thermography as a contactless tool for crack-depth estimation.
ABSTRACT
Starting from the Debye model for frequency-dependent specific heat and the Vogel-Fulcher-Tammann (VFT) model for its relaxation time, an analytic expression is presented for the heat capacity versus temperature trace for differential scanning calorimetry (DSC) of glass transitions, suggesting a novel definition of the glass transition temperature based on a dimensionless criterion. An explicit expression is presented for the transition temperature as a function of the VFT parameters and the cooling rate, and for the slope as a function of fragility. Also a generalization of the results to non-VFT and non-Debye relaxation is given. Two unique ways are proposed to tackle the inverse problem, i.e., to extract the fragility from an experimental DSC trace. Good agreement is found between theoretically predicted DSC traces and experimental DSC traces for glycerol for different cooling rates.
ABSTRACT
An enhanced accurate method of measuring the thermal diffusivity of liquids by the sample's thickness scan of the phase of the photopyroelectric signal is presented. The method, making use of the absolute values of the phase and sample thickness, leads to very accurate results for the room temperature values of thermal diffusivity (about +/-0.3%). The high accuracy of the method is due to a very precise control of the sample's thickness variation (0.1 microm step), to a proper localization of the thickness scan range, and to a new procedure of data analysis. The high accuracy of the method recommends it for the study of processes associated with small changes of the thermal parameters.
