The Influence of Microstructure and Lattice Strain on Tetragonality Factor and Dielectric Properties of Ferroelectric Ceramics BaTiO3BaTiO_3

This work is devoted to direct process of molten salt synthesis and studies on barium titanate ($BaTiO_3$, BT), belonging to ferroelectric crystal group type perovskite $BO_3$. This material thanks to its noncentrosymmetric, fully tetragonal structure possesses at room temperature (up to $T_{c}$ = 135°C) the spontaneous polarization. Due to this fact BT can be applied as piezoelectric material in electromechanical transducers, so as an excellent dielectric in multilayer capacitors and many other devices. With grain size reduction of BT ceramics to nanometric level it leads to permanent transformation into paraelectric state with minimized energy and to lose its specific features as a consequence, even at room temperature. In case of structural agent, means as tetragonality factor, it has a crucial influence on investigated material properties and it is referred in current paper. It is a row of complementary researching methods allows to confirm the presence of desired tetragonal BT phase in nano or micropowders obtained by means of molten salt synthesis route. For different temperature variants of synthesis X-ray diffraction analysis were undertaken and precise unit cells parameters both with tetragonality factor were determined using the Cohen method. Based on structural studies the lattice strains and crystallite sizes were estimated through the Williamson-Hall method. Using scanning electron microscopy the powders morphology and grain size distribution were done. Dielectric measurements of sintered BT ceramics were carried out to determine the Curie temperatures, dielectric permittivities and loss factors in prepared capacitors

The Influence of Microstructure and Lattice Strain on Tetragonality Factor and Dielectric Properties of Ferroelectric Ceramics BaTiO_3

This work is devoted to direct process of molten salt synthesis and studies on barium titanate ($BaTiO_3$, BT), belonging to ferroelectric crystal group type perovskite $BO_3$. This material thanks to its noncentrosymmetric, fully tetragonal structure possesses at room temperature (up to $T_{c}$ = 135°C) the spontaneous polarization. Due to this fact BT can be applied as piezoelectric material in electromechanical transducers, so as an excellent dielectric in multilayer capacitors and many other devices. With grain size reduction of BT ceramics to nanometric level it leads to permanent transformation into paraelectric state with minimized energy and to lose its specific features as a consequence, even at room temperature. In case of structural agent, means as tetragonality factor, it has a crucial influence on investigated material properties and it is referred in current paper. It is a row of complementary researching methods allows to confirm the presence of desired tetragonal BT phase in nano or micropowders obtained by means of molten salt synthesis route. For different temperature variants of synthesis X-ray diffraction analysis were undertaken and precise unit cells parameters both with tetragonality factor were determined using the Cohen method. Based on structural studies the lattice strains and crystallite sizes were estimated through the Williamson-Hall method. Using scanning electron microscopy the powders morphology and grain size distribution were done. Dielectric measurements of sintered BT ceramics were carried out to determine the Curie temperatures, dielectric permittivities and loss factors in prepared capacitors

Dielectric Properties of Compounds Creating Dual-Frequency Nematic Liquid Crystals

In this paper we report how dielectric spectroscopy can help in creating of dual-frequency nematic liquid crystals. Dual-frequency nematic liquid crystals is new class of liquid crystal materials. Such mixture is usually formed by a combination of many components (even more than 10), which can be split into two groups: molecules having large transverse dipole moment and molecules with a large longitudinal dipole moment. The behavior of a base (parent) mixture, functional admixtures and final dual-frequency nematic liquid crystals mixture is investigated by dielectric spectroscopy in wide frequency (100 Hz-10 MHz) and temperature ranges. This allows us to find out why the dual-frequency liquid crystal has an important feature: positive and negative dielectric anisotropy at different frequencies. We present parameters of molecular motions around short (S-mode) and long (L-mode) molecular axes observed in investigated materials and discuss how the creation of final dual-frequency nematic liquid crystals mixture can modify molecular relaxations

Polar nematic phases with enantiotropic ferro- and antiferroelectric behaviour

The recent discovery of a new ferroelectric nematic (NF) liquid crystalline phase has become of utmost interest for the liquid crystal (LC) and the whole soft and condensed matter fields. Contrary to the previously known ferroelectric LC materials, whose ferroelectric characteristics were much weaker, new polar nematics exhibit properties comparable to solid ferroelectrics. This discovery brought about tremendous efforts to further explore compounds showing these phases, and fascinating physical properties have been reported. Herein, we present the first synthesized compounds with the enantiotropic ferro- (NF) and antiferroelectric (NX) nematic phases. The enantiotropic nature and an unprecedentedly broad temperature range of NF and NX phases are confirmed by various experimental techniques: polarized-light optical microscopy (POM) observations, different scanning calorimetry (DSC), dielectric spectroscopy, second harmonic generation (SHG), and molecular modeling. The presented achievements in designing achiral compounds that exhibit enantiotropic polar nematic phases with ferro- and antiferroelectric properties significantly contribute to the development of multicomponent mixtures with a broad temperature range of NF and NX phases down to room temperature. Furthermore, this accomplishment considerably enhances the general understanding of the structural correlations that promote polar nematic liquid crystal phases with high thermodynamic stability. Finally, this work may benefit various applications in photonic devices

Laser speckle reduction using a liquid crystal diffuser enhanced with redox dopants

In this work, a large reduction in the speckle noise is observed using a thin electro-responsive film consisting of a chiral nematic liquid crystal (LC) that has been enhanced with the addition of a redox dopant. Two different redox dopants are investigated over a range of concentrations; one being an electron acceptor and the other being an electron donor redox dopant. Results are presented that show that the incorporation of either of these dopants leads to a greater reduction in the speckle contrast than that observed using just the chiral nematic LC host when subjected to electrohydrodynamic instabilities. Furthermore, it is found that the permanent electrochemical reactions typically observed when ionic dopants, such as CTAB, are used are not observed for these devices resulting in a considerable improvement in terms of the operating lifetime of the speckle reducer technology. To conclude we present results that show that the speckle contrast can be reduced to C = 0.11 ± 0.02 at a temperature of 30ºC and demonstrate the improvement of the quality of an image generated using a modified commercial projector fitted with a monochromatic laser source