Narrow Results

Triangular voltage waveform was employed to distinguish the contributions of dielectric permittivity, electric conductivity and domain switching in current-electric field curves. At the same time, it is shown how those contributions can affect the shape of the electric displacement — electric field loops (D–E loops). The effects of frequency, temperature and microstructure (point defects, grain size and texture) on the ferroelectric properties of several ferroelectric compositions is reported, including: BaTiO₃; lead zirconate titanate (PZT); lead-free Na_0.5K_0.5NbO₃; perovskite-like layer structured A₂B₂O₇ with super high Curie point (T_c); Aurivillius phase ferroelectric Bi_3.15Nd_0.5Ti₃O₁₂; and multiferroic Bi_0.89La_0.05Tb_0.06FeO₃. This systematic study provides an instructive outline in the measurement of ferroelectric properties and the analysis and interpretation of experimental data.

Ba(Mg${}_{1∕3}$Nb${}_{2∕3})$O₃ (BMN) doped and undoped Ba${}_{0.45}$Sr${}_{0.55}$TiO₃ (BST) thin films were deposited via radio frequency magnetron sputtering on Pt/TiO₂/SiO₂/Al₂O₃ substrates. The surface morphology and chemical state analyses of the films have shown that the BMN doped BST film has a smoother surface with reduced oxygen vacancy, resulting in an improved insulating properties of the BST film. Dielectric tunability, loss, and leakage current (LC) of the undoped and BMN doped BST thin films were studied. The BMN dopant has remarkably reduced the dielectric loss ($\sim$38%) with no significant effect on the tunability of the BST film, leading to an increase in figure of merit (FOM). This is attributed to the opposing behavior of large Mg${}^{2+}$ whose detrimental effect on tunability is partially compensated by small Nb${}^{5+}$ as the two substitute Ti${}^{4+}$ in the BST. The coupling between ${\text{Mg}}_{\mathrm{\text{Ti}}}^{″}$ and V${}_{\mathrm{\text{O}}}^{••}$ charged defects suppresses the dielectric loss in the film by cutting electrons from hopping between Ti ions. The LC of the films was investigated in the temperature range of 300–450K. A reduced LC measured for the BMN doped BST film was correlated to the formation of defect dipoles from ${\text{Mg}}_{\mathrm{\text{Ti}}}^{″}$, V${}_{\mathrm{\text{O}}}^{••}$ and Nb${}_{\mathrm{\text{Ti}}}^{•}$ charged defects. The carrier transport properties of the films were analyzed in light of Schottky thermionic emission (SE) and Poole–Frenkel (PF) emission mechanisms. The result indicated that while the carrier transport mechanism in the undoped film is interface limited (SE), the conduction in the BMN doped film was dominated by bulk processes (PF). The change of the conduction mechanism from SE to PF as a result of BMN doping is attributed to the presence of uncoupled Nb${}_{\mathrm{\text{Ti}}}^{•}$ sitting as a positive trap center at the shallow donor level of the BST.

In the work nano silica has been irradiated by 2×10¹³ cm^-2s^-1 neutron flux at different times up to 20 h. The temperature and frequency dependencies of real and imaginary parts of dielectric constant of the nanomaterial exposed to neutron flux influence and initial state has been comparatively analyzed. From analysis results it has been revealed that the permittivity of nano SiO₂ increases in general tendency with influence of neutron flux. The mutual dependence of the real and imaginary parts of dielectric constant of nano SiO₂ particles has been reviewed. From the cases similar to Cole–Cole diagrams existing in the dependencies it has been revealed that the value of the relaxation period is compatible with polarization of the nano particles. It has been observed an increase in polarization with increase of influence period of neutron flux. Mechanisms of all effects observed in the experiments have been given.

Owing to the special planar structure and very high resistivity, $Z$-type hexaferrites possess the high initial permeability and cut-off frequency, indicating great application potential in high-frequency devices. Strontium Co₂Z hexaferrites (Sr₃Co₂Fe${}_{24}$O${}_{41}$) were successfully prepared by the sol–gel method and calcined at the different temperatures from $1175^{\circ }$C to $1250^{\circ }$C. The influence of the calcined temperatures on the microstructure, phase structures, magnetic properties and microwave absorption behavior of all samples was investigated in detail. All results indicated that strontium hexaferrites go through the $M$-, $Y$-, $Z$-, and $W$-phase transformation process from $1175^{\circ }$C to $1250^{\circ }$C, and S-1200 (calcined at $1200^{\circ }$C) is confirmed to be Co₂Z-type hexaferrites. S-1200 showed the highest imaginary permeability while the imaginary permittivity of all samples is close to zero, indicating the excellent microwave adsorption behavior of strontium Co₂Z hexaferrites. The reflection loss results indicate that S-1200 exhibits the best microwave absorption performance with the RL value of −32.4 dB for 4.0 mm thickness at 4.72 GHz, according to the impedance matching and quarter-wavelength theory.

Al³⁺-doped Ni–Zn ferrites with composition of Ni_0.5Zn_0.5(Fe_1-xAl_x)₂ O₄ (where x = 0, 0.012, 0.023, 0.035 and 0.045) were prepared by a method named "one-step synthesis." The magnetic and dielectric properties of the as-prepared Ni–Zn ferrites were investigated. X-ray diffraction data indicated that all the ferrite samples had a single-phase spinel structure. The addition of Al³⁺ resulted in a reduction of the grain size, lattice constant, density, shrinkage and saturation magnetization of the as-prepared samples. The Curie temperatures, however, raised first and reduced later with increasing contents of Al³⁺ in the samples, but still kept high values. Both the real and imaginary parts of permeability of ferrites decreased with increasing amount of Al³⁺ doped before they reached peak value. As the applied frequency increased to higher than about 10 MHz, the real part of permeability of non-doped ferrite had already become lower than those of ferrites doped with Al³⁺. Moreover, the doping of Al³⁺ made their utility of magnetic permeability move to much higher frequency. The observed decreases in dielectric constants and dielectric loss tangent could be attributed to the decreased electron conductivity by substituting Fe³⁺ with Al³⁺ in the samples. The as-prepared high-performance soft magnetic materials prepared using such a simple and low-cost method will be much promising in high-frequency applications and industrialization.

Nanocrystalline 3C-SiC was irradiated by neutron flux ($2\times 10{}^{13}$ n$\cdot$cm${}^{-2}$s${}^{-1})$ up to 20h in the TRIGA Mark II type research reactor. The experiments have been conducted in the 0.1Hz–2.5MHz frequency and 100–400K temperature ranges. The frequency dependencies of real and imaginary parts of the permittivity of nanomaterial were analyzed comparatively before and after neutron irradiation. After neutron irradiation, there was increase in dopant element concentration in the nanocrystalline 3C-SiC particles. Concentration of new dopant elements in the 3C-SiC nanomaterial directly affects dielectric polarization and leads to increased permittivity. Simultaneously, after neutron irradiation, agglomeration and amorphous transformation influence on the polarization of nanocrystalline 3C-SiC. Moreover, 3C-SiC nanoparticle interface polarization gives rise to dispersion. It was found that ionic polarization was dominant in the nanocrystalline 3C-SiC particles.

Polyvinyl chloride (PVC) is widely used as insulator in electrical engineering especially as cable insulation sheaths. In order to improve the dielectric properties, polymers are mixed with ceramics. In this paper, PVC composites with different weight percentages 2 wt.%, 5 wt.%, 8 wt.% and 10 wt.% were prepared and investigated. Loss index (${𝜀}^{″}$) and dielectric constant (${𝜀}^{\prime }$) have been measured using an impedance analyzer RLC. Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) have been used as characterization techniques. The incorporation of BaTiO₃ does not modify the crystallinity and the morphology of the PVC but reduces the space charges, therefore the dielectric losses. The frequency response analysis has been followed in the frequency ranges (20–140 Hz and 115–1 MHz). Relaxation frequencies have been evaluated in each frequency range. Experimental measurements have been validated using Cole–Cole’s model. Experimental results show well that BaTiO₃ as a filler improves the dielectric properties of PVC.

SF₆–N₂ mixtures are becoming the best alternative for SF₆ in electrical systems. Plasmachemical modeling and the results of a positive surface streamer are reported in this paper. The model exhibits excellent streamer transition from the gas gap to the insulator surface. The surface charge accumulations on the dielectric insulators are compared and discussed. Results showed that the density of negative ions is much larger than that of electrons in the surface streamer channel. The dominant and less pronounced constitute the main negative ions in the electronegative surface streamer because of the strong electron attachments to SF₆ molecules.

Nanocrystalline 3C-SiC is irradiated by neutron flux ($2\times 10^{13}$n/cm²s) up to 20h in the TRIGA Mark II type research reactor. At the first stage, silicon carbide nanoparticles were analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM) devices before and after neutron irradiation. Amorphous transformation and agglomeration effects on the permittivity of nanocrystalline silicon carbide (3C-SiC) were comparisons investigated before and after neutron irradiation. Dielectric spectroscopies of nanocrystalline 3C-SiC have been conducted at the frequency ranges of 0.1Hz–2.5MHz and temperature ranges of 100–400K. Real and imaginary parts of the permittivity of the nanomaterial were analyzed for comparison before and after neutron irradiation. Neutron irradiation increased dopant elements concentration in the nanocrystalline 3C-SiC particles, and that directly affects dielectric polarization and increased permittivity. All the mechanisms of the observed effects are given in the work.

In this paper, we report the effect of sulfur doping on the electrical and dielectric properties of semiconducting Sb₂Se₂S over wide ranges of temperatures (298–473K) and frequencies (42–10⁶Hz). Sb₂Se₂S system has been prepared by the direct fusion and cooling cycle of a mixture of the constituent elements, in stoichiometric ratio and purity 99.999%, in vacuum-sealed silica tubes. X-ray analysis showed a decrease in the cell parameters $a$, $b$ and $c$ upon doping with sulfur. However, the pure and doped Sb₂Se₃ showed the single orthorhombic phase structure. The permittivity of Sb₂Se₂S showed a decrease with increasing frequency due to a decrease in the average bond strength. While, ac conductivity increased with the frequency increase, obeying the Jonscher’s universal dynamic law. The conductivity temperature dependence is well described by the correlated barrier hopping model. The activation energy calculated from DC conductivity is found at higher value (0.79eV) as compared to that reported in the literature for other antimony selenide compounds. Accordingly, a new Sb₂Se₂S compound is suggested which may be useful for electronic devices.

The solid solutions of Pb_1-xSm_x(Zr_0.50Ti_0.50)_1-x/4O₃ (PSZT), x = 0.00, 0.03, 0.06 and 0.09 were prepared by a mixed oxide method at high temperature (sintering temperature 1200°C). Preliminary X-ray structural analysis suggests the formation of single-phase compounds at room temperature in tetragonal crystal system. The microstructural SEM analysis shows that the grains of the materials are almost spherical and also uniformly distributed over the surface. Detailed studies of dielectric properties (ε, tanδ and σ) of the materials in a wide temperature range (30–500°C) at different frequencies (10³–10⁶ Hz) reveal that the compounds have transition temperatures well above the room temperature. The analysis of the diffusivity of the dielectric peaks in these compounds provides values between 1 and 2, where the higher values indicate great disorder in the system. The temperature dependence of ac conductivity and the value of activation energy in different regions suggest that the conduction process is of mixed type (i.e. singly ionized in ferroelectric region and doubly ionized in paraelectric phase).

The film structures on the base of soft and crystalline ferroelectrics (poly(vinylidene fluoride - trifluoroethylene) copolymer and deuterated triglycine sulfate) were produced. The volume content of the DTGS microcrystal particles embedded in the polymer matrix of the composite films was up to 10%. It was found crystalline phase changes in the polymer matrix at the DTGS inclusions presence. The DTGS crystal particles increased dielectric permittivity and electrical conductivity of the composite films. The temperature dependences of the dielectric permittivity indicated increasing the phase transition temperature of polymer matrix at the DTGS inclusions growth.

In this report, the processes of texture formation in grain-oriented ferroelectric ceramics based on layer-structured ferroelectric Bi₄Ti₃O₂ (LSBT) prepared by hot forging method are considered. The microstructural and X-ray methods revealed the axial textured formation in ferroelectric ceramic that are used to estimate the orientation factor of ceramics. For the first time, the domain structure changes when poling the anisotropic ferroelectric ceramics are investigated. The anisotropy of electromechanical, piezoelectric and ferroelectric properties of ferroelectric ceramics due to the crystal texture existence in it is studied. The aim of this study is to study the processes of crystalline texture formation in polycrystalline BLSF and to establish the dependence of the electrophysical properties of ceramics on the degree of texturing. Ceramics were textured using the hot stamping (HS) method developed at the Research Institute of Physics. The mechanism of the method is that the workpiece is subjected to uniaxial pressure and free radial deformation occurs due to the plastic flow of the material until the workpiece fills the free volume of the mold, which is created by placing the workpiece in the mold with a gap. The study of the microstructure of ceramics showed that an increase in the firing temperature in the range 950–1050${}^{\circ }$C causes a sharp decrease in porosity and increases the density to 7.95 g/${\mathrm{\text{cm}}}^3$, which is 98% of theoretical. An X-ray analysis was performed and microstructural studies were carried out, which revealed the formation of an axial texture in ceramics. The features of the switching processes of textured ceramics are revealed. The characteristics of the polarization switching of ceramics in the directions parallel and perpendicular ($\perp$) of the pressure axis during hot processing were obtained from the dielectric hysteresis $P$($E$) loops, i.e., axis axial texture. The $\perp$-cut ceramics are characterized by a more complete polarization switching, which is associated with the additional orientation of the (001) crystallographic planes in the textured material, as well as the presence of a threshold switching field. In the temperature range from -196 to + 600${}^{\circ }$C, the anisotropy of the electro physical properties of ceramics due to the presence of a crystalline texture in it was studied. The dielectric constant, electrical conductivity, piezoelectric and elastic coefficients were measured for sections of ceramics of different orientations relative to the axis of the texture. The anisotropy of the dielectric constant and electrical conductivity manifests itself weakly at room temperature and increases sharply when approaching the Curie temperature. In the temperature range +20–400${}^{\circ }$C, the high thermal stability of the piezoelectric module $d_{33}$, measured by the quasistatic method, was established.