Isotropic Temperature Research Articles

Random laser radiation behavior of liquid crystal in photonic crystal fiber carrier

A hollow-core photonic-crystal fiber filled with a mixture of nematic liquid crystal TEB30A, chiral agent S-811 and laser dye PM597 was pumped by a frequency-doubled Nd:YAG laser with a wavelength of 532 nm. The laser emission spectra was measured and the random laser radiation behavior in the photonic-crystal fiber carrier was investigated. When side-pumping was applied to the fiber, the emitted random laser with a wider radiation direction from the side face had a wavelength range of 590−605 nm and an FWMH of 0.3 nm. When end-pumping was employed to the fiber, the emitted random laser from the end face had a wavelength range of 580−605 nm and an FWMH of 0.3 nm. After the sample was heated to the isotropic temperature, the laser emission with both pumping methods was shut down. The experimental results demonstrate that the dye doped liquid crystal mixture in the micropore induce the random laser emission in the photonic-crystal. The change in the mean free path of photon transport and the fluctuation of the dielectric tensor of chiral nematic liquid crystals with temperature are the main factors affecting the laser intensity.

Stochastic and Quasi-adiabatic Electron Heating in Quasi-parallel Shocks

Abstract Using Magnetospheric Multiscale observations at the Earth’s quasi-parallel bow shock we demonstrate that electrons are heated by two different mechanisms: a quasi-adiabatic heating process during magnetic field compression, characterized by the isotropic temperature relation T / B = ( T 0 / B 0 ) ( B 0 / B ) α with α = 2/3 when the electron heating function ∣ χ e ∣ < 1 , and a stochastic heating process when ∣ χ e ∣ > 1 . Both processes are controlled by the value of the stochastic heating function χ j = m j q j − 1 B − 2 div ( E ⊥ ) for particles with mass m j and charge q j in the electric and magnetic fields E and B . Test-particle simulations are used to show that the stochastic electron heating and acceleration in the studied shock are accomplished by waves at frequencies (0.4–5) f ce (electron gyrofrequency) for bulk heating, and waves f > 5 f ce for acceleration of the tail of the distribution function. Stochastic heating can give rise to flat-top electron distribution functions, frequently observed near shocks. It is also shown that obliquely polarized electric fields of electron cyclotron drift and ion acoustic instabilities scatter the electrons into the parallel direction and keep the isotropy of the electron distribution. The results reported in this paper may be relevant to electron heating and acceleration at interplanetary shocks and other astrophysical shocks.

Microwave-Assisted Synthesis of Mushrooms Like MWCNT/SiO2@ZnO Nanocomposite: Influence on Nematic Liquid Crystal E7 and Highly Effective Photocatalytic Activity in Degradation of Methyl Blue

Mostly-uniform spherical mushrooms like MWCNT/SiO2@ZnO nanocomposite was synthesized under microwave power and characterized using TEM, XRD, EDX, TG/DTA and FT-IR techniques. The modified chemical process was used in three steps for the synthesis of the nanocomposite. Having a nano-capillary structure of MWCNT as nano-core, chemically and physically inert silica cover are a vital backbone for ZnO, which is a semiconductor. Because of the excellent chemical and physical properties of MWCNT/SiO2@ZnO nanocomposite, it has shown unique catalytic properties in the photocatalytic degradation of methyl blue. Approximately all of the methyl blue was degraded in water in 10 min. under sun-light and after 60 min., rarely left from dye in the water. The thermal and electrical behavior of mushrooms like nanocomposite doped E7 was investigated. According to the obtained results, the nematic to isotropic phase transition temperature of sample enhanced with nanocomposite dispersion and threshold voltage of mushroom like nanocomposite doped Nematic Liquid Crystal E7 decreased was founded.

Quasi-adiabatic and Stochastic Heating and Particle Acceleration at Quasi-perpendicular Shocks

Abstract Based on Magnetospheric Multiscale observations from the Earth’s bow shock, we have identified two plasma heating processes that operate at quasi-perpendicular shocks. Ions are subject to stochastic heating in a process controlled by the heating function for particles with mass m j and charge q j in the electric and magnetic fields and . Test-particle simulations are employed to identify the parameter ranges for bulk heating and stochastic acceleration of particles in the tail of the distribution function. The simulation results are used to show that ion heating and acceleration in the studied bow shock crossings is accomplished by waves at frequencies (2–10)f cp (proton gyrofrequency) for the bulk heating, and for the tail acceleration. When electrons are not in the stochastic heating regime, , they undergo a quasi-adiabatic heating process characterized by the isotropic temperature relation . This is obtained when the energy gain from the conservation of the magnetic moment is redistributed to the parallel energy component through the scattering by waves. The results reported in this paper may be applicable also to particle heating and acceleration at astrophysical shocks.

Electrical properties of the octyl cyanobiphenyl nematic liquid crystal dispersed with graphene oxide

Graphene oxide (GO) with different concentrations was dispersed to pristine octyl cyanobiphenyl nematic liquid crystal. The characteristics of the used GO were investigated with X-Ray diffraction (XRD), scanning electron microscopy (SEM) and UV–Vis spectroscopy. Also the bandgap values of GO were calculated. The impact of doping concentration on the thermal and electrical properties of prepared samples was investigated. The phase transition temperatures of samples were studied by differential scanning calorimeter. The isotropic phase transition temperature has decreased by the addition of graphene oxide ratio in samples. The electrical properties of the samples have been examined by measuring dielectric permittivity, capacitance and impedance of samples with an impedance analyzer. Also, electrical behaviors of pure and GO-dispersed samples under bias voltage were studied. The relaxation frequencies of specimens were obtained. The dielectric anisotropy and relaxation frequency also increased due to the presence of graphene oxide flakes. The dielectric permittivity $${\varepsilon }^{^{\prime}}$$ of GO-dispersed samples enhanced under the bias voltage was revealed.

Tunable and Cooperative Thermomechanical Properties of Protein-Metal-Organic Frameworks.

We recently introduced protein-metal-organic frameworks (protein-MOFs) as chemically designed protein crystals, composed of ferritin nodes that predictably assemble into 3D lattices upon coordination of various metal ions and ditopic, hydroxamate-based linkers. Owing to their unique tripartite construction, protein-MOFs possess extremely sparse lattice connectivity, suggesting that they might display unusual thermomechanical properties. Leveraging the synthetic modularity of ferritin-MOFs, we investigated the temperature-dependent structural dynamics of six distinct frameworks. Our results show that the thermostabilities of ferritin-MOFs can be tuned through the metal component or the presence of crowding agents. Our studies also reveal a framework that undergoes a reversible and isotropic first-order phase transition near-room temperature, corresponding to a 4% volumetric change within 1 °C and a hysteresis window of ∼10 °C. This highly cooperative crystal-to-crystal transformation, which stems from the soft crystallinity of ferritin-MOFs, illustrates the advantage of modular construction strategies in discovering tunable-and unpredictable-material properties.

Effect of iron oxide (γ-Fe2O3) nanoparticles on the morphological, electro-optical and dielectric properties of a nematic liquid crystalline material

The effect of Iron Oxide Nanoparticles (γ-Fe2O3 NPs) on a room temperature nematic liquid crystalline material namely 4-(4′-hexylcyclohexyl) isothiocyanatobenzene (6CHBT) has been studied. The ferronematic composites have been characterized using differential scanning calorimetry, polarizing optical microscopy, dielectric spectroscopy and electro-optical measurements. It is found that the ferronematic with the low concentration of γ-Fe2O3 NPs enhanced the orientational ordering of nematic matrix due to the strong orientational coupling between magnetic dipole moments associated to nanoparticles and nematic liquid crystal director, which is the essential feature of these systems. It resulted the improvement of various parameters of liquid crystals important for the application. In the ferronematics, the nematic to isotropic transition temperature increased as compared to that of the pure material. Dielectric spectroscopy has shown an increase of dielectric anisotropy in the composite systems. Electro-optical results reveal a decrement in both the splay elastic constant and the electric threshold voltage, in line with a noteworthy improvement of the steepness of transmission- voltage curves.

Quasistatic mechanical behavior of HMX- and TATB-based plastic-bonded explosives

Data on the macroscopic quasistatic mechanical behavior of pressed HMX- and TATB-based plastic-bonded explosives (PBXs) are listed in this paper. This review shows that (1) few characterizations are available for TATB-based PBXs. This gap is filled in this paper. An extensive database is detailed for the CEA M2 explosive composition. The HMX and TATB database then enables selection of the deformation mechanisms to be considered: viscoelasticity, damage-induced anisotropy and its effectivity (i.e.: whether or not the damage is influenced by the loading direction), plasticity with kinematic and isotropic hardenings, pressure and temperature dependencies and asymmetric failure threshold. The review also shows that (2) HMX- and TATB-based materials share close elastic and ultimate properties when the compositions (binders, solid volume fractions) and the mechanical behavior of the two crystals differ. The constitutive laws proposed in the literature are reiterated. In our opinion, a universal law could be proposed in the near future, each material being considered by its own set of parameters. The objective of this paper is to draw up the guidelines for model improvement.

Incorporation of magnesium oxide nanoparticles in to nematic liquid crystalline matrix

Incorporation of magnesium oxide nano particles (MgONPs) in a nematic liquid crystal display material namely 4-pentyl-4'-cyanobiphenyl (5CB) have been analyzed by differential scanning calorimeter, polarizing light microscopy and dielectric spectroscopy. MgONPs have been dispersed in two different concentrations in pure 5CB viz. 0.2 and 0.6 wt%. It has been experimentally noticed that with the increase of MgONPs concentrations in 5CB, the nematic to isotropic transition temperatures decreases and dielectric anisotropy increases. Relaxation frequency of an observed relaxation mode corresponding to molecular rotation about the short axis is almost unchanged after dispersion of MgONPs. The ionic conductivity has increased by approximately one order of magnitude for 0.6 wt% MgONPs and decreased for 0.2 wt%.

Influence of carbon quantum dots on electro–optical performance of nematic liquid crystal

A simple synthesis of carbon quantum dots (CQDs) from green sources has attracted increasing attention nowadays. Here we report the synthesis of water-soluble fluorescent carbon quantum dots from mandarin juice by hydrothermal method. When excited at 364 nm, CQDs give maximum fluorescence at 462 nm. As verified from Transmission Electron Microscopy, obtained CQDs were very small, with an average diameter of 2.37 ± 0.42 nm. These very small-sized nanoparticles were dispersed to nematic liquid crystal mixture E7. As is known, particle size is a significant parameter for nanoparticle dispersed liquid crystal applications. The threshold voltage of pure and CQDs doped liquid crystals was derived from dielectric spectroscopy measurements, and it was observed that the threshold voltage increased with the increasing CQDs doping concentration. The nematic to the isotropic phase transition temperature of samples was investigated with DSC thermograms and POM. The phase transition temperatures gradually decreasing with increasing CQDs doping concentration were revealed.

Liquid-crystalline dimers containing cholesteryl groups and azobenzene moieties with branched allyl chain: synthesis and properties

ABSTRACT A series of liquid-crystalline dimers containing a cholesteryl group and a azobenzene moiety with branched allyl chain were synthesised, all exhibiting chiral nematic phase (N*). The chemical structure and liquid crystal properties of the dimers were characterised by FTIR, 1 H-NMR, DSC, POM and XRD. The isotropization temperatures of the dimers were in a decreasing trend with increasing the length of the flexible spacer while anomalies occurred in the melting temperatures and crystallinities of the dimers. Nevertheless, it is in evidence that increasing the flexible spacer length is feasible to reduce the crystallinity of the dimer and then subcool the dimer to glass state to maintain the selective reflection. Besides, it was shown that the dimer with increased flexible spacer length tended to attain selective reflection at longer wavelengths.

Syntheses and Properties of New Smectic Reactive Mesogens and Their Application in Guest-Host Polarizer

We designed and synthesized new smectic liquid crystalline monomers containing photopolymerizable acrylate groups and a mesogenic core unit composed of two benzene rings, one pyrimidine-based heteroaromatic ring and an acetylene linking group. The structures of monomers were characterized by 1H nuclear magnetic resonance spectroscopy, and their mesomorphic behaviors were investigated by differential scanning calorimetry and polarized optical microscopy. Both of monomers showed an enantiotropic phase transition behavior with exhibiting both nematic phase and highly ordered smectic phase. Both of monomers showed smectic phases during the cooling process in the temperature range of 144.2–94.0 °C and 154.0–85.8 °C, respectively. Neither of these compounds showed a homogeneous alignment even at various temperatures, and therefore, their 50:50 mixture was chosen for the preparation of guest-host mixture because of its lowest crystallization point, wide smectic phase temperature range and superior aligning property. Polymerizable “host-guest” mixture formulated from the host liquid crystalline monomer mixture, dichroic dye and additives was injected into the sandwiched glass substrates having rubbed alignment layers at the isotropic temperature. Subsequent in-situ photopolymerization by UV irradiation at room temperature successfully resulted in a thin film polarizer with good polarizing properties. The fabricated guest-host polarizer showed a dichroic ratio (DR) of 13.1 and a degree of polarization (DOP) of 95.5% with the thickness of 5 µm.

Reconfigurable Reflective Colors in Holographically Patterned Liquid Crystal Gels

Thin film holographic gratings have attracted significant attention due the tailorability of their photonic properties. The ability of holography to create materials with unique photonic properties is particularly promising when combined with stimuli-responsive liquid crystals. Holographic polymer-dispersed liquid crystals (H-PDLCs) are an example of such a system where nonuniform irradiation is used to template alternating polymer-rich and polymer-poor regions with a periodicity related to the illumination wavelength. These systems typically use nonmesogenic monomers at a concentration range where a significant volume fraction of the final system is polymer. Here, we present a reconfigurable material that was generated by holographically polymer stabilizing liquid crystals (H-PSLCs) using liquid crystalline monomers, thereby enabling both alternating regions to be liquid crystalline. The “stabilizing effect” of the liquid crystalline polymer leads to a material with regions of periodic transition temperatures. As written, the polymer-rich regions have a higher isotropic transition temperature than the polymer-poor regions. Upon heating these samples beyond the isotropic temperature of the polymer-poor regions, a vivid wavelength specific reflection appears due to the refractive index mismatch between the ordered liquid crystalline polymer-rich regions and the disordered isotropic polymer-poor regions. When these materials are further heated to the isotropic temperature of the polymer-rich regions, the samples again return to a transparent state because the refractive index modulation between the alternating isotropic regions is negligible. By tailoring the liquid crystalline isotropic transition temperature to be near-room temperature, modest AC-fields can be used as an Ohmic heat source to drive the thermal color change.

Dielectrical, electro-optical and textural studies of 5CB nematic liquid crystal doped with TiO2 and Cu-TiO2 nanoparticle

ABSTRACT We investigated dielectric and viscoelastic properties of the 4-pentyl-4-biphenylcarbonitrile nematic liquid crystal also known as 5CB doped by 0.5%wt of pure and copper-incorporated TiO2 nanoparticles. Optical microscopy observations showed uniform copper-incorporated TiO2 nanoparticles dispersion into the nematic 5CB while we have seen nanoparticles aggregations for pure TiO2. Further, except the pure TiO2 nanoparticles, the nematic isotropic transition temperature of all doped samples slightly increased by about 1.4–2.4°C as compared to the pure 5CB. This indicates that the order parameter was increased. Significant decrease in the dielectric anisotropy, response time, splay elastic constant and threshold voltage was observed for all samples. The rotational viscosity has been deduced from response time data, it was found that rotational viscosity shows a slight increase with respect to pure 5CB for copper-incorporated TiO2 and a clear decreasing for pure TiO2 nanoparticles.

Orientational order induced by a polymer network in the isotropic phase of liquid crystal.

We studied the paranematic ordering induced by a polymer network in the isotropic phase of a liquid crystal (LC) that occurs in polymer-stabilized cells with bend configuration of the LC director (π cells) fabricated via photopolymerization of photoreactive monomer RM 82 added in small concentrations (3-5 wt %) to a nematic LC [4-cyano-4'-pentylbiphenyl (5CB)] when low voltage was applied across the cell. The polymer network formed in the nematic phase of the LC consists of fine fibrils that are aligned along the LC director and thus mirror the bend deformation of the LC at the time of polymerization. When heated to temperatures above the nematic-to-isotropic (N-I) phase transition such highly ordered polymer network anchors LC molecules providing ordering of the LC around the fibrils which results in unusually high optical retardation of the cell, R_{cell}. We present a theoretical model that relates R_{cell} to the degree of order of the fibrils, the anchoring energy of the LC molecules on the surface of the polymer fibrils, and the fibril radius r_{0}. Fitting of the experimental R_{cell}(T) curves with the developed model reveals correlation of r_{0} with the nematic correlation length ξ_{0} which characterizes penetration of the nematic order in the isotropic phase of the LC. Accepting ξ_{0} as a material constant of about 1 nm leads to a very small radius of the fibrils, r_{0}∼1nm, which is also supported by other reported experimental data. High optical retardation and fast electro-optical response of the cells at the temperatures deep into the isotropic phase point toward the enhancement of the polymer-induced paranematic order by a well-oriented layer of LC molecules that are absorbed on the surface of fibrils. Application of high voltage at the isotropic phase temperatures results in high variations of the optical retardation of the cells. Characteristic on and off response times were about 10-100 μs, independent of the cell gap. Combination of large voltage-driven changes of the optical retardation occurring in the low-viscosity isotropic state with switching times that are at least two orders of magnitude shorter than the typical relaxation times of the cells operating in the nematic phase make such polymer-stabilized π cells very promising for application in fast electro-optical switches and light modulators.

Textural and electro-optical study of a room temperature nematic liquid crystal 4̍-pentyl-4-biphenylcarbonitrile doped with metal oxide nanowires in planar and in-plane switching cell configurations

ABSTRACT This work shows the doping effect of metal oxide nanowires (alumina nanowires) on the textural and electro-optical characteristics of nematic liquid crystal (5CB) in planar and in-plane switching (IPS) cell configurations. Results indicate the increase in nematic to isotropic phase transition temperature after doping in both planar and IPS cells. A decrease in threshold voltage in addition to increase in contrast ratio, birefringence, and band gap energy was observed after doping with alumina nanowires in both types of cells. Overall analysis shows that IPS cell configuration has more improved threshold voltage and contrast ratio in addition to decrease in birefringence and transmission intensity, as compared to planar cell configuration. No effect of cell configuration was observed on energy band gap and found same value for both planar and IPS cell configurations, as energy band gap is a material specific property.

Entropy modes in multi-component plasmas confined by a dipole field

In this paper, the collisionless electrostatic instability of the entropy mode is investigated linearly in multi-component plasmas confined by a dipole magnetic field, which commonly exists in space environments, as well as laboratory plasmas, such as Collisionless Terrella eXperiment, Levitated Dipole eXperiment, and Dipole Research EXperiment. We focus on characteristics of the entropy mode driven by the plasma density or/and the temperature gradient at low plasma beta (=8πP0/B02). The theoretical analysis of this work agrees with the calculated results qualitatively. It is indicated that the peak growth rate of the instability is in the regime of k⊥ρi ∼ 1, and the entropy mode tends to be more stable as the percentage of the heavy ion increases. For multi-component plasmas, each component has the entropy mode feature of its own, instead of simply averaging all the components. While for the electron with an isotropic temperature, the use of weighted harmonic average can be a good approximation for simplification.

An Optical Steam Sterilization Sensor Based On a Dual-Responsive Supramolecular Cross-Linked Photonic Polymer.

An optical time–temperature steam sensor is presented based on the loss of structural color in a supramolecularly cross-linked cholesteric liquid crystal photonic coating. A gradual decrease in the selective reflection band is observed upon exposure to temperatures above 105 °C related to the cholesteric to isotropic transition temperature. The linear polymers with carboxylic acid side chains provide physical cross-linking through hydrogen bonding that allows a time–temperature-dependent order loss through the dynamic equilibrium between supramolecular dimer and free monomer states. Steam is accelerating the color loss, and autoclave experiments show that the photonic supramolecular polymer is applicable as a steam sterilization sensor for medical applications.

An Azoester-containing Photoresponsive Linear Liquid Crystal Polymer with Good Mesophase Stability

Photoresponsive linear liquid crystal polymers (LLCPs) are attractive because of the excellent stimuli-responsibility and the good processability. In this study, a new photoresponsive LLCP containing azoester (PC11AE6) with good mesophase stability was synthesized by ring-opening metathesis polymerization. By introducing photoresponsive azoester mesogenic unit, which has high rigidity and a large length-diameter ratio, the resultant polymer possesses a broad mesophase temperature interval (isotropic temperature = 180 °C). A study on mesophase by 2D-wide angle X-ray diffraction indicated that the mesogens were orientated spontaneously into smectic A phase after annealing. The orientated films and fibres exhibited macroscopic, rapid and reversible deformations under light irradiation as a result of the photoisomerization of azoester as confirmed by UV-Vis absorption spectrophotometry. We anticipate that this work provides a strategy for preparing LLCP with a broad mesophase temperature range, which is positive for potential applications.

Magnetic field tuning of spin resonance in TmFeO3 single crystal probed with THz transient

TmFeO3, a canted antiferromagnet, has two intrinsic spin resonance modes in the terahertz (THz) frequency regime: quasi-ferromagnetic (q-FM) mode and quasi-antiferromagnetic (q-AFM) mode. Both the q-FM and q-AFM modes show strong magnetic field and temperature dependence. Hereby, by employing THz time-domain spectroscopy combined with external magnetic field and low temperature system, we systematically investigated the magnetic field induced frequency shift of q-FM and q-AFM modes as well as the temperature driven spin reorientation phase transition in TmFeO3 single crystal. In contrast to the isotropic temperature dependent two-mode, the magnetic field dependence of two-mode is strongly anisotropic: the magnetic field applied along c-axis (a-axis) can harden (soften) the spin resonance frequency of q-FM mode for Γ4 phase of TmFeO3, and the field applied along b-axis shows negligible frequency shift for the q-FM mode, with the q-AFM mode relatively stable. The present study provides solid evidence that the magnetic anisotropy in rare earth orthoferrite plays a dominant role in the q-FM mode and the occurrence of spin reorientation phase transition. With the magnetic anisotropic energy obtained from the temperature dependent q-FM and q-AFM mode frequencies, we can predict both magnetic field and temperature dependence of spin resonance in TmFeO3 single crystal via phenomenological analysis.