Dielectric response and polarization mechanism of alkali silicate glasses in gigahertz to terahertz frequency range

Oxide glasses are dielectric materials with potential applications in high-frequency communications; hence, their dielectric properties in the gigahertz to terahertz frequency range should be investigated. In this study, the dielectric properties of silica glass and five single alkali silicate glasses were measured at 0.5–10 THz using terahertz time-domain spectroscopy and far-infrared spectroscopic ellipsometry. At 0.5–10 THz, the silica glass exhibited low dielectric dispersion with a low dielectric constant and loss. By contrast, the alkali silicate glasses exhibited high dielectric dispersion, and the dielectric constant and loss were higher than those of the silica glass. The shape of the dielectric dispersion profile depended on the alkali-metal ions; it was broader for lighter ions such as Li ions and sharper for heavier ions such as Cs ions. The peak dielectric loss shifted toward a lower frequency as the weight of the alkali-metal ions in the alkali-silicate glass increased. To understand the dielectric dispersion, the complex permittivity was calculated using molecular dynamics simulations. The theoretical results qualitatively agreed with the experimental data. Ion dynamics analysis revealed that alkali-metal ions vibrate and migrate under an applied electric field, which affects the dielectric constant and loss of alkali-silicate glasses at gigahertz to terahertz frequencies. To fabricate filter devices at low temperatures, alkali metals should be added to silicate glass; therefore, a minimum amount of light alkali metals should be used to minimize the dielectric loss of the glass materials while maintaining productivity.     

Microstructural evolution of electrically activated polypropylene/layered silicate nanocomposites investigated by in situ synchrotron wide-angle X-ray scattering and dielectric relaxation analysis

Electric field was found to facilitate the destruction of layer stacking and separation of silicate layers in polypropylene (PP)/layered silicate nanocomposites, resulting in the penetration of polymer chains into silicate galleries. In this study, we describe the real-time microstructural evolution of PP/clay nanocomposites under electric field investigated by in situ synchrotron wide-angle X-ray scattering (WAXS) analysis. We were able to identify two distinctive mechanisms for the formation of nanocomposites depending on the type of electric field. We observed that the exfoliation process prevails in the AC field, while the alignment of silicates parallel to the electric field predominates in the DC field. Dielectric relaxation analysis showed that the different mechanisms originate from different charge distributions of bound ions attached to the clay surfaces due to the applied electric field.     

电场作用下液滴分裂动力学行为的格子Boltzmann模拟

采用格子Boltzmann方法（LBM）伪势模型,耦合流场和电场控制方程,研究了电场作用下油水共存体系中水滴分裂的动力学行为及特性,借助形变率衡量液滴的形变大小,展现了液滴从形变至分裂的动态演变过程,分析了外加电场大小和液滴内外介电常数比对液滴分裂行为的影响。结果表明：外加电场能促使液滴发生振荡形变,且存在临界电毛细数和临界介电常数比决定液滴是否发生分裂：高于临界值,液滴形变率振荡幅度随时间不断增长,最终发生分裂;低于临界值,则液滴形变率振荡幅度不断衰减,并最终趋于一稳定值。在此基础上,综合考虑电场强度与介电常数比的影响,提出了基于现有电毛细数的修正电毛细数唯一地表征电场作用下液滴分裂与否。     

Characterization of dielectric properties of oxide materials in frequency range from GHz to THz

We measured complex dielectric permittivity using THz time-domain spectroscopy (THz-TDS) to clarify the dielectric properties of oxide materials in a frequency range from GHz to THz. Piezoelectric and ferromagnetic oxide single crystals, such as quartz (SiO₂), zinc oxide (ZnO), Bi substituted rear-earth iron garnet (BiRIG), and LiTaO₃ (LT), were used. We obtained the complex dielectric permittivity of these materials in a frequency range from 100 GHz to 2 THz. The ɛ′ and ɛ″ obtained for SiO₂ were in agreement with previous reports. We observed dielectric relaxation in ZnO crystal from 100 GHz to 1 THz, which originated from n-type conductivity. In the BiRIG, the values of the dielectric permittivity increased as the frequency increased, and the values of the dielectric permittivity with the magnetic field were smaller than those without the magnetic field throughout the measured frequency range. In a comparison between congruent LiTaO₃ (CLT) and stoichiometric LiTaO₃ (SLT), the ɛ₃₃ of the CLT was very similar to that of the SLT, but a lot of difference was between the ɛ₁₁ of evident CLT and SLT within the measured frequency region. We determined that the point defects had profound effect on the dielectric performance of the LT.     

Peculiar electric properties of polarized layer in alkaline silicate glasses

Broadband dielectric spectroscopy (BDS) was applied to study polarization phenomena in alkaline silicate glasses, in particular, properties and structure of subsurface (anodic) polarized layers forming in poling with deposited film electrodes of different structures. A model of poled glasses which does not contradict experimental data is proposed. In accordance with the model, a poled glass is presented as two resistor-capacitor circuits in a series connection, one of which is the polarized layer and another is the rest of the sample. It is found that the electric properties of the layers essentially depend on the structure of the anodic electrode used in glass poling. It is also shown that the dielectric response of poled glass samples is mainly determined by the electric properties of the submicron polarized layers and this gives an opportunity to reveal specific properties of the layers rather than ones of the glass sample bulk. Revealed temperature dependence of DC conductivity of the polarized layers obeys Arrhenius's law, and determining activation energy does not depend on the electrode. Finally, it is noted that today above-mentioned information about polarized layers can be obtained only by BDS.     

Time-electric field superposition in electrically activated polypropylene/layered silicate nanocomposites

This study investigated the microstructural evolution of PP/clay nanocomposites under electric field. The storage modulus, which is a kind of mirror of the microstructure, increases while an electric field (both AC and DC) is applied. It was found that time and the electric field strength can be superposed to yield a single mastercurve that is independent of the type and strength of the electric field. In addition, the shift factor scaled differently according to the field type. The SAXS and TEM data revealed that the AC field induces the microstructural evolution of the nanocomposites toward an exfoliated structure, while the DC field induces the alignment of silicate layers. In a DC field, the alignment process occurs as a result of dielectrophoretic motion. However, in an AC field, dielectric relaxation analysis showed that an exfoliation process arises as a result of the breakup of the charge balance.     

Electrical Relaxation in Na2O·3SiO2 Glass

The electrical relaxation associated with alkali diffusion in Na₂O·3SiO₂ glass was studied from 0.2 Hz to 700 kHz at –1° to 163°C. A formalism for analysis of electrical relaxation in conducting dielectrics which associates the nonexponential decay of the electric field to zero and the dispersions in the dielectric constant and the conductivity with a distribution of relaxation times for the electric field was developed and is shown to be in qualitative accord with current molecular theories of electrical relaxation in alkali silicate glasses. A relation between the dc conductivity, the limiting high-frequency dielectric constant, and the average electric field or conductivity relaxation time was derived and is verified experimentally for the Na₂O·3SiO₂ glass. The distribution of electric-field relaxation times for the glass is broad, asymmetric on a logarithmic scale, and weighted in favor of the shorter relaxation times; the distribution narrows with increasing temperature. A reduced electrical relaxation curve which can be used to compare electrical and mechanical relaxations in Na₂O·3SiO₂ glass was generated.     

Theoretical Study of Microwave Enhanced Thermal Decontamination of Oil Contaminated Waste

This paper demonstrates that microwave enhanced thermal decontamination of oil contaminated waste is a potentially important and highly efficiency approach to achieving the required environmental discharge limit. Numerical simulation of microwave thermal decontamination of oil contaminated wastes within an applicator known to support high electric field strengths was used to assess the influence of electric field and power loss density distributions on oil removal from the waste materials. It was found during this study that the water content plays the most important role in oil removal. As the microwave receptors within the material matrix, water molecules absorbed the bulk of the applied microwave energy. This energy was subsequently transferred to the oil in the form of heat, and was shown to cause thermal desorption. The boiling point of oil can also be reduced when it is mixed with water, since free water is beneficial to uniform heating, whilst bound water has a significant effect on the latter stages of oil removal. The experimental and simulation results suggest that the lowest residual oil levels were achieved when the sample material had a high dielectric loss factor and was placed in the position of maximum electric field strength. The cost of using a microwave technique for the remediation of oily waste can be up to 20 times lower than conventional heating.     

The anomalous increase in tunability in Ba0.55Sr0.45TiO3-MgO-Mg2SiO4 composite ceramics

The Ba_0.55Sr_0.45TiO₃–Mg₂SiO₄–MgO composite ceramics were prepared via solid-stated method and the dielectric tunable properties of composite ceramics were investigated. With the increase in dielectrics content, the tunability of composites ceramics increased abnormally first and then decreased, while the anharmonic coefficient increased monotonously. A simple 3D Finite Element model was developed to simulate the dielectric response and an anomalous increase in tunability when increasing the dielectrics content in composites was obtained. The simulation indicated that adding dielectrics into ferroelectrics causes the redistribution of the electric field. Increasing dielectrics content, the average electric field strength in the enhanced area of ferroelectrics increases, which lead to the increase in the tunability. The abnormal increase in tunability originates from the enhancement of the electric field on a fraction of the ferroelectrics around dielectrics.     

Investigation into the field and frequency dependences of the dielectric parameters of α-As<Subscript>2</Subscript>Se<Subscript>3</Subscript> amorphous films

This paper reports on the results of an investigation into the field and frequency dependences of dielectric parameters (such as the permittivity ? and the dielectric loss tangent tanδ) for α-As₂Se₃ amorphous films. It is found that, as the electric field strength increases, the permittivity ? decreases and the maximum of the dielectric loss tangent tanδ shifts toward lower frequencies.     

Basic ideas of microwave processing of polymers

The objective of this effort has been to investigate the relationship between polymer structure and microwave absorptivity. Dielectric loss factor, ε″, loss tangent, tan δ, and oscillator strength, (ε_S ?- ε_∞), were used to evaluate potential material processability under applied microwave radiation. Numerous polymeric materials varying in chemical and physical structures were irradiated in a low power (≤ 100W) electric field at 2.45 GHz. Electromagnetic radiation was applied as either traveling or resonant wave modes in cylindrical and rectangular waveguides. In general, heatability was found to be a direct function of the dielectric loss dispersion dependence on temperature and frequency. The dielectric loss factor obtained at low frequency measurements was found to be directly proportional to the heatability of polymers. A WLF plot was used to predict the shift of dielectric loss maxima into or out of the microwave frequency range.     

Gradient dielectric constant sandwich-structured BaTiO3/PMMA nanocomposites with strengthened energy density and ultralow-energy loss

High energy storage density with low-energy loss polymer films are essential for high-performance electric devices. To avoid the high-energy loss of utilizing nonlinear polymer materials, a sandwich nanostructure comprising a linear polymer poly(methyl methacrylate) (PMMA) matrix embedded with a high dielectric constant BaTiO₃ (BT) interlayer and poly(vinylidene fluoride) (PVDF) binder was constructed using a solution casting strategy. This structural design takes advantage of each component in the composite. The good dispersion of BT particles in the binder, which was incorporated between PMMA, enabled a high dielectric constant and fewer defects. Additionally, the excellent film formation ability of the PVDF binder guarantees the uniform thickness and stable structure of the BT mid-layer, and good miscibility between PVDF and PMMA enhanced the interaction between each layer. Interestingly, since the dielectric constant of PVDF was between BT fillers and PMMA, a dielectric gradient distribution mitigated the local electric field concentration, as proven by the simulation results. Consequently, a low-loss linear PMMA composite film exhibited satisfying breakdown strength and excellent discharged energy density, which were 25% and 460% higher than those of pristine PMMA, respectively.     

Property reinforcement of silicone dielectric elastomers filled with self‐prepared calcium copper titanate particles

In this article, submicron and micron calcium copper titanate (CCTO) crystallites with different morphologies were successfully designed and prepared by directly thermal treatment method and molten salt method, respectively. Then, the silicone elastomer filled with self‐prepared CCTO particles had high dieletric constant, low dielectric loss, and actuated strain which was greatly improved at low electric field. The dieletric constant at 50 Hz obviously increased from 2.15 for pure silicone elastomer to 4.37 and 4.18 for the submicron and micron CCTO/poly (dimethyl siloxane) (PDMS) composites. The dielectric loss of the composites retained at a low value (less than 0.06). Meanwhile, the elastic modulus of CCTO/PDMS composites was increased slightly only with a good flexibility. Compared to pure silicone elastomer (2.25%), the submicron and micron CCTO/PDMS composites with 2 wt % content exhibited a greater strain of 7.69% and 9.83% at a low electric field of 5 V/μm. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42613.     

Electrical properties of flexible graphene reinforced polyimide composites

The electrical properties of polyimide and the composite at different volumes fractions were studied in the frequency range 200–20 kHz and in the temperature range 30–200 °C. Increasing the volume fraction of graphene up to 10%, resulted in an extremely large increase in the dielectric constant, which indicates the composites remarkable ability to store electric potential energy under the effect of alternating electric field. An increase in dielectric constant was also observed with increasing temperature and decreasing frequency. The outstanding dielectric properties of polyimide graphene nanocomposites are attributed to the large volume fraction of interfaces in the bulk of the material. The measured increase in dielectric constant with increasing temperature was attributed to the segmental mobility of the polymer chains. The AC conductivity for polyimide and the composites was calculated from the loss factor and a remarkably high conductivity was obtained for the composites due to the formation of conducting paths in the matrix by the graphene sheets. Also this study showed that the thermal conductivity of the composites increased sharply with increasing graphene concentration. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45372.     

Polarization Response and Thermally Stimulated Depolarization Current of BaTiO3‐based Y5V Ceramic Multilayer Capacitors

Polarization response and thermally stimulated depolarization current (TSDC) of BaTiO₃‐based ceramic multilayer capacitors with Y5V specification were studied. The temperature dependence of dielectric behavior shows that as the dc electric field increases, the polarization response in the whole measurement range (from ?125°C to +350°C) is suppressed. As the temperature rises to about 250°C, dielectric loss significantly increases and has a dependence on dc electric field, due to the leakage behavior at high temperature. According to the hysteresis loops, the calculated electrostatic energy density and energy efficiency are also closely related to polarization‐electric field. Utilizing a fixed measuring polarization condition, two TSDC relaxation peaks are observed and both are associated with oxygen vacancies. It is demonstrated that the weak peak originates from the in‐grain migration of oxygen vacancies and the strong peak with high relaxation temperature is caused by the across grain‐boundary oxygen vacancies. The activation energy estimated for the relaxation of oxygen vacancies across grain boundaries is about 0.78 eV. The main contribution for the leakage behavior is from the across grain‐boundary relaxation of oxygen vacancies. With increasing of temperature and electric field stress, the extrinsic oxygen vacancy defects show more fluent migration, which eventually leads to the resistance degradation and breakdown.     

Calculation of dielectric loss of gamma relaxation process in polyethylene at low temperature

A method has been investigated to obtain the absorption intensity of the dielectric γ relaxation process in polyethylene by calculation. A correlation factor for evaluation of the effective dipole moment of polar groups in solid polyethylene is introduced on the assumption that with expanding distribution of relaxation time and/or increasing temperature, the extent of the canceled dipole moment due to the orientation motions of the polar groups under an alternating electric field will increase. Comparison of the calculated dielectric loss tangent with the observed one supports the assumption.     

THz dielectric properties of AlPO4-BPO4-SiO2 ternaries

0.05AlPO₄-0.05BPO₄-0.90SiO₂ glass-ceramic and glass-free 0.45AlPO₄-0.45BPO₄-0.10SiO₂ ceramic have been prepared by a solid-state reaction process and their terahertz (THz) dielectric responses were characterized by time-domain spectroscopy (TDS) techniques in the frequency range from 0.2 to 1.6 THz. Both samples demonstrate little dielectric dispersions in the (sub) THz frequency range, having dielectric permittivities (ε_r) of ∼3.35(7) and ∼4.06(8), respectively. The dielectric responses are dominated by the phonons and electronic polarizations in the (sub) THz frequency range. The contributions to the relative permittivities from electronic polarization ε_{r ∞} are 1.85(4) and 1.99(4), which are ∼55% and ∼49% of the total ε_r at THz frequencies, respectively_. The dielectric losses of both samples continuously increase with increasing frequency in the THz band, while the loss factor of glass-ceramic is one order of magnitude higher than that of glass-free ceramic. The glass-free sample exhibits extremely high Q×f values of 100,000–190,000 GHz in the THz band, being much higher than that of the glass-ceramic (∼7000–25,000 GHz).     

Bond theory,terahertz spectra,and dielectric studies in donor-acceptor (Nb-Al) substituted ZnTiNb2O8 system

As a donor and acceptor separately, Nb⁵⁺ and Al³⁺ are used to substitute Ti⁴⁺ in the ixiolite ZnTiNb₂O₈ system for the first time. The dielectric responses in the terahertz range of this system are initially studied based on the data from terahertz time-domain transmitted spectroscopy. Combined with ligand field theory, the formation of a secondary phase of ZnAl₂O₄ is reasonably explicated. Then, the origins of the lower dielectric loss and the terahertz wave absorption coefficient are determined to be the high chemical bond covalency and effective phase control. For the composition of ZnTi_0.85(Al_0.5Nb_0.5)_0.15Nb₂O₈ sintered at 1180°C, a low dielectric loss (<5 × 10⁻³ @ 0.5 THz) and absorption coefficient (<10/cm @ 0.5 THz) are obtained, which make this kind of material propitious for developing terahertz dielectric devices.     

Characterization of microwave and terahertz dielectric properties of single crystal La2Ti2O7 along one single direction

New generation wireless communication systems require characterisations of dielectric permittivity and loss tangent at microwave and terahertz bands. La₂Ti₂O₇ is a candidate material for microwave application. However, all the reported microwave dielectric data are average value from different directions of a single crystal, which could not reflect its anisotropic nature due to the layered crystal structure. Its dielectric properties at the microwave and terahertz bands in a single crystallographic direction have rarely been reported. In this work, a single crystal ferroelectric La₂Ti₂O₇ was prepared by floating zone method and its dielectric properties were characterized from 1 kHz to 1 THz along one single direction. The decrease in dielectric permittivity with increasing frequency is related to dielectric relaxation from radio frequency to microwave then to terahertz band. The capability of characterizing anisotropic dielectric properties of a single crystal in this work opens the feasibility for its microwave and terahertz applications.     

Investigation of transport behavior in Ba doped BiFeO3

Bi_1?xBa_xFeO₃ (x = 0.00–0.25) samples were prepared by conventional solid state reaction method. X-ray diffraction revealed the rhombohedrally distorted perovskite structure for undoped BiFeO₃ with a phase transition from rhombohedral to pseudo cubic on Ba substitution. The leakage current density of 10% Ba substituted sample is found to be four orders of magnitude less than that of the pure BiFeO₃. Grain boundary limited conduction and space charge limited conduction mechanisms are involved in low and high electric field regions respectively for all the samples except 10% Ba doped BFeO₃ which obeys grain boundary limited conduction mechanism in whole of the electric field range. Dielectric measurements showed that the dielectric constant and dielectric loss attained their minimum values at 10% Ba substitution. Thus 10% Ba is found to be optimum concentration to have better multiferroic properties. Undoped BiFeO₃ and 5% Ba doped samples have very large values of dielectric constants and leakage current densities which can be attributed to a large number of oxygen vacancies in these samples, indicating an extrinsic response of these compositions.