MANIFESTATION OF INTRAMOLECULAR VIBRATIONS IN THE FLUORESCENCE SPECTRA OF MOLECULES IN POLAR MEDIA

The effect of a low frequency intramolecular vibration on the nonstationary fluorescence spectrum of a molecule and the dynamics of the Stokes shift is analyzed in the framework of the nonstationary perturbation theory based on the operator of the interaction energy of the electric field of the exciting pump pulse with a dissolved molecule. Using a well-tested spin-boson model, which takes into account not only the reorganization of the relaxation polarization modes of the medium, but also intramolecular vibrations, an analytical expression for the time-resolved fluorescence signal is written, and the conditions under which the oscillations of the maximum of nonstationary fluorescence spectra can manifest themselves are analyzed. It is shown that the reorganization of the medium does not suppress the oscillations of the spectrum maximum.

Simulation of BNNSs Dielectrophoretic Motion under a Nanosecond Pulsed Electric Field

Using a nanosecond pulsed electric field to induce orientation and arrangement of insulating flake particles is a novel efficient strategy, but the specific mechanism remains unclear. In this study, the dielectrophoretic motion of boron nitride nanosheets (BNNSs) in ultrapure water under a nanosecond pulsed electric field is simulated for the first time. First, the simulation theory is proposed. When the relaxation polarization time of the dielectric is much shorter than the pulse voltage width, the pulse voltage high level can be considered a short-term DC voltage. On this basis, the Arbitrary Lagrangian–Euler (ALE) method is used in the model, considering the mutual ultrapure water–BNNS particles-nanosecond pulsed electric field dielectrophoretic interaction, to study the influence of different BNNSs self-angle α and relative angle β on local orientation and global arrangement. The particles are moved by the dielectrophoretic force during the pulse voltage high level and move with the ultrapure water flow at the zero level, without their movement direction changing during this period, so the orientation angle and distance changes show step-like and wave-like curves, respectively. The model explains the basic mechanism of dielectrophoretic motion of BNNSs under a pulsed electric field and summarizes the motion law of BNNSs, providing a reference for subsequent research.

Relaxation Polarization Dependence of Circular Vector Gratings in Azobenzene Glassy Molecular Films

Holographic grating recording and relaxation is studied in different azobenzene molecular glassy films by circularly orthogonally polarized 532 nm laser beams L and R. The readout was made by circularly polarized (R or L) 632.8 nm laser beam. Sandwich-type samples (glass-film-glass) were also studied. Maximum diffraction efficiency of 81% has been achieved in sandwich-type AR-173 film. The following relaxation features have been found: after reaching diffraction efficiency (DE) maximum no DE decay took place; DE read out by R-polarized beam was always higher than that by L-polarized beam; in sandwich-type samples DE decayed until zero when read out by R-polarization whereas DE was zero when read out by L-polarization. 50% relaxation times varied from 4 to 44 minutes, and they mainly decreased when grating period was increased. The observed relaxation peculiarities can be understood if one assumes that volume birefringence grating (VBG) is recorded followed by volume density grating (VDG) and surface relief grating (SRG) recording. R-polarization "feels" all gratings whereas L-polarization only VDG and SRG. At large exposures VDG and SRG dominate. These results confirm the conclusion made by J.Mikelsone in her 2018 PhD thesis that birefringence grating recording in azobenzene materials is a neccessary condition for SRG appearance.

Effect of sintering temperature on magnetoelectric coupling in 0.2Ni0.9Zn0.1Fe2O4-0.8Ba0.9Sr0.1TiO3 composite ceramics

Magnetoelectric composites have attracted much attention due to their intriguing physical properties and potential applications. They might have strong magnetoelectric coupling effect above room temperature, but it mainly depends on the sintering schedule. In this paper, 0.2Ni0.9Zn0.1Fe2O4-0.8Ba0.9Sr0.1TiO3 (NZFO-BSTO) composite ceramics were prepared by conventional solid sintering method. Effects of the sintering temperature (1050, 1100, 1150 and 1200?C) on the microstructure, dielectric and multiferroic properties were investigated in detail. XRD results confirm that the prepared ceramics show bi-phase structure, which can be indexed as NZFO and BSTO. No obvious impurity phase was observed when the sintering temperature is less than 1200?C, indicating that there is no apparent chemical reaction occuring at the magnetic and ferroelectric interface. All of the samples show relatively dense and uniform structure. The mean grain size of the composites increases from 220 to 650 nm when the sintering temperature increases from 1050?C to 1200?C. The sample ST-1100 has the best frequency stability of dielectric constant, while it presents the smallest dielectric loss. All specimens present two dielectric peaks, the first one is attributed to the diffuse phase transition of BSTO, while another one generated at higher temperature corresponds to the relaxation polarization. The sample ST-1100 shows excellent ferroelectric properties, the value of remnant polarization is about 5.1 ?C/cm2 and the coercive electric field value is ~20 kV/cm. The ceramics ST-1050 and ST-1200 show larger leakage current. All samples show paramagnetic behaviour with small remnant magnetization (~0.3 emu/g) and coercive magnetic field (~30Oe). The sample ST-1100 has maximum magnetoelectric coupling coefficient of 9.6mV/cm?Oe when the magnetic field is near 1100Oe.

Поляризационные процессы в тонких слоях аморфного MoS-=SUB=-2-=/SUB=-, полученных методом высокочастотного магнетронного распыления

The polarization processes in thin layers of amorphous molybdenum disulfide MoS_2 are studied by dielectric spectroscopy techniques. The process of dipole-relaxation polarization is observed. The microscopic parameters of the system are calculated, and the relaxation time of the dipole-polarization process, as well as the activation energies E _ a and E _σ of the relaxation process and conductivity, respectively, are determined. The fact that the two activation energies are close to each other suggests that the processes of relaxation and charge transport are driven by the same mechanism.

Дипольное упорядочение и ионная проводимость в NASICON-подобных структурах типа Na-=SUB=-3-=/SUB=-Fe-=SUB=-2-=/SUB=-(PO-=SUB=-4-=/SUB=-)-=SUB=-3-=/SUB=-

The problems of the structural features, the dielectric and conducting properties of Na_3Fe_2(PO_4)_3 polycrystals prepared by hot pressing have been studied. The Na_3Fe_2(PO_4)_3 polycrystals in the α and β phases are shown to have higher conducting properties than the α and β phases of the single crystals grown by the solution–melt method. The regularities of the appearance of the dipole ordering, the ionic and superionic conductivities related to phase transitions and structural changes in the {[Fe_2(PO_4)_3]^3–}_3∞ rhombohedral crystalline frame during polymorphic transformations are refined. The problems of the relaxation polarization in the α and β phases of Na_3Fe_2(PO_4)_3 are discussed. A model for explaining the dipole ordering and the ionic conductivity in Na_3Fe_2(PO_4)_3 is proposed.

Calcium titanate ceramics obtained by combustion synthesis and two-step sintering

Calcium titanate powder was successfully prepared from duck eggshell and anatase titanium dioxide with a magnesium inductant via combustion synthesis in argon. As-combusted products were leached with diluted HCl. In XRD analysis, as-leached powders exhibited a major phase of CaTiO3 with a perovskite structure. The particle size, observed by SEM, was approximately 240 nm. As-leached powders were densified by single-step (SST) and two-step sintering (TSS) to produce calcium titanate ceramics. The first step of all TSS conditions was fixed at 1350C and holding times (t1) at this temperature were varied up to 120 min. Calcium titanate ceramic obtained from holding for 120 min had a grain size of 2.18 ?m, relative density of 86.68% and a dielectric constant of 92. Two-step sintered ceramic had the highest density (95.73%) and best dielectric properties (dielectric constant = 110, dielectric loss = 0.02) when the holding temperature (T2) was 1250?C. Calcium titanate ceramics processed by two-step sintering had denser microstructures and higher dielectric constants than single-step sintered ceramic due to grain boundary diffusion and the simplicity of relaxation polarization.

Effects of molar ratio on dielectric, ferroelectric and magnetic properties of Ni0.5Zn0.5Fe2O4-BaTiO3 composite ceramics

Ni0.5Zn0.5Fe2O4-BaTiO3 (NZFO-BTO) magnetoelectric composite ceramics with different molar ratios (mNZFO:mBTO = 1:1.5, 1.5:1 and 2:1, defined as N1B1.5, N1.5B1 and N2B1, respectively) were prepared successfully by using a joint hydrothermal method and sol-gel technique and sintering at 1000 ?C. Meanwhile, the dielectric, ferroelectric and magnetic properties of the composites were investigated. The presence of bi-phase structure in the composites was verified with X-ray diffraction analyses. The scanning electron microscopy images and energy dispersion spectrum results confirmed that the bulk-like grains (2 to 5 ?m) and spherelike grains (_0.5 ?m) could be attributed to NZFO and BTO, respectively. The dielectric constant and loss increased with increasing NZFO/BTO molar ratio because the carrier concentration of NZFO is higher than that of BTO. Thus, the dielectric constant of the N2B1 ceramics is more than 7800 at low frequency of 100Hz and room temperature, while only less than 2000 for the N1B1.5 composite. Two peaks can be observed in the temperature dependence of the dielectric constant curves. One is near 120 ?C, which corresponds to the Curie temperature of BTO, while the other peak occurs at about 320 ?C, corresponding to the relaxation polarization. The remanent polarization increases with increasing the content of ferroelectric BTO. The maximum value at 1 kHz was observed for the N1B1.5 sample and it is larger than 4.5 ?C/cm2, while the minimal value was obtained for the N2B1 composite and is only 1.2 ?C/cm2. Magnetic properties were also measured and it was observed that magnetization increases with increasing the molar ratio. The largest saturation magnetization has the N2B1 composite (_51.74 emu/g) due to the larger concentration of NZFO phase. However, the sample N1B1.5 shows the largest coercive field due to the highest interface interaction. This study provides guidelines for the fabrication of NZFO-BTO magnetoelectric composite ceramics.