Dilatometric and dielectric studies of [(CH3)2NH2]5Cd3Cl11 crystal

Thermal expansion of the DMACC crystal along three crystallographic axes was measured in the temperature range 300-160 K. An anomaly of thermal expansion was observed along the b-axis and detailed dielectric measurement along the c-axis showed a kink of dielectric permittivity at 179 K. These results evidence the phase transition at this temperature in DMACC crystal.     

Dielectric and dilatometric studies of (CH3)2NH2Ga(SO4)2 · 6H2O (DMAGaS)

The results of dielectric, pyroelectric, spontaneous polarization and dilatometric investigations in the temparature interval 110 K-295 K are presented for DMAGaS crystal. At both phase transition temperature, 136 K and 116 K, discontinuous changes of dielectric permittivity, spontaneous polarization and length of bars are observed. Temperature dependences of alongation along the ferroelectric direction and of spontaneous polarization are quite similar.     

Electrical Properties of a Bismuth Layer-Structured Ba2Bi4Ti5O18 Single Crystal

Ba₂Bi₄Ti₅O₁₈ single crystals were grown, and their dielectric permittivity, conductivity, and ferroelectricity were investigated along the a -(or b -)axis and the c -axis separately. The dielectric permittivity at 1 MHz along the a -(or b -)axis was 2000 at the Curie temperature (360°C); this value was 8 times greater than that along the c -axis. The dc conductivity was greater along the a -(or b -)axis than that along the c -axis, by one order of magnitude. In regard to the ferroelectricity, the saturated remanent polarization was 120 mC/m² and the saturated coercive field was 3 MV/m along the a -(or b -)axis; values of 8.5 mC/m² and 0.81 MV/m, respectively, were observed along the c -axis. The Ba₂Bi₄Ti₅O₁₈ single crystals had large electrical anisotropies, which were due to the layered structure.     

Dielectric relaxation and viscoelastic behavior of polyurethane–titania composites: dielectric mixing models to explain experimental results

Polyurethane–titania nanocomposites with varying composition are prepared through two different mixing methods. The effect of titania on dielectric, mechanical, and thermal properties is investigated for different composites. A variety of electrical tests (like impedance and dielectric constant) are performed on the resultant composites and it was found that the dielectric constant of composites increased significantly, whereas impedance (|Z|) decreased with the increase in titania concentration. The effect of temperature on dielectric properties was also studied and it was found that the dielectric constant increased up to a certain temperature and beyond that it decreased. From mechanical testing, it is observed that the properties depend on both the composition and mixing methods. The glass transition temperature (T _g) of soft and hard segments along with vulcanization temperature (T _v) observed from the differential scanning calorimetry (DSC) are found to shift with the incorporation of titania into the PU matrix. The viscoelastic behavior of the nanocomposites was studied by dynamic mechanical analysis (DMA), and increase in storage modulus (E′) was achieved through addition of titania to the PU matrix, especially in the low strain region, whereas some decrease was observed in the higher strain region. Finally, different dielectric models were compared with the experimental data and the best match was achieved by the Lichtenecker model, especially at 1 kHz, which can be used as a predictive rule for different volume contents of titania filler in the PU matrix.     

Effect of fillers on electrical properties of epoxy composites

The effect of fillers such as glass, ferrite, and carbon on the dielectric properties of epoxide resin along with conductivity has been studied as a function of frequency (1–100 kHz) and temperature (30–200°C). The glass transition temperature is not much affected by the presence of the fillers. The observed higher values of dielectric permittivity and loss in the case of filled polymers are attributed to Maxwell–Wagner–Sillars polarization.     

Dielectric and Ferroelectric Properties of [(CH3)2NH2]3CuCl5 Single Crystal

The detailed studies of dielectric properties of [(CH₃)₂NH₂]₃CuCl₅ single crystal were performed. Distinct anomalies of permittivity along three crystallographic direction were observed at 277.5 K and 252.7 K on cooling. The observed permittivity behaviour evidenced the first order character of both phase transitions. The hysteresis loops characteristic of a ferroelectric phase were observed in the range of temperature 277.5-252.7 K. An appearance and disappearance of spontaneous polarization confirms the first order phase transitions in this crystal. The permittivity measured along the ferroelectric a-axis under influence of external electric field in paraelectric phase showed clearly a nonlinear increase with increase of electric field intensity applied to the crystalline sample.     

Abnormal Grain Growth and New Core–Shell Structure in (K,Na)NbO3-Based Lead-Free Piezoelectric Ceramics

A unique core–shell structure was observed in coarse grains in (K,Na)NbO₃ (KNN)-based lead-free piezoelectric ceramics. It is morphologically different from the chemical inhomogeneity-induced core–shell grain structure reported previously in BaTiO₃-based ceramics. The core region is composed of highly parallel nanosized subgrains, whereas the shell region consists of larger-sized but similar self-assembled subgrains. The electron-backscattered diffraction analysis and selected area electron diffraction pattern confirmed that coarse grains with a core–shell structure were single-crystalline-like grains. The formation process of such coarse grains was then discussed based on mesocrystal growth along with the classical theory of grain growth. The two studied KNN-based systems showed a similar grain growth transformation: from self-assembled aggregation clusters with nanosized subgrains to a typical core–shell grain structure when the sintering temperature was increased only by a range of 10°–20°C. The volatilized alkali metal oxides and liquid phase were supposed to accelerate such grain growth transformation. When abnormal grown grains with a core–shell structure occurred, both systems showed the highest densities and dielectric constants along with the lowest dielectric losses, while their piezoelectric properties tended to decline.     

Low‐Temperature Magnetic and Dielectric Anomalies in Rare‐Earth‐Substituted BiFeO3 Ceramics

In this work, we report on the magnetic and dielectric anomalies observed in dense Bi_1–xRE_xFeO₃ ceramics (RE = Dy, Tb; 0 ≤ x ≤ 0.3) at cryogenic temperatures. For compositions with a high content of rare‐earth ions, thermomagnetic experiments revealed a distinct anomaly in the magnetization curves at temperatures below 200 K. The temperature of the magnetic anomaly along with a thermal hysteresis was found to be dependent on the rare‐earth concentration and magnetic field strength. Low‐temperature dielectric measurements showed an anomalous relaxor‐like behavior of the relative permittivity and dielectric loss in highly doped ceramic samples. The anomalies in low‐temperature magnetization and dielectric response are suggested to result from the presence of GdFeO₃‐like orthoferrite phase and/or bismuth rare‐earth‐mixed iron garnet impurities.     

Fracture phenomena in micro‐ and nano‐layered polycarbonate/poly(vinylidene fluoride‐co‐hexafluoropropylene) films under electric field for high energy density capacitors

The long‐term dielectric lifetime properties of multilayered polycarbonate/poly(vinylidene fluoride‐co‐hexafluoropropylene) [PC/P(VDF‐HFP)] films were measured as a function of the layer thickness. An optimum layer thickness of 160 nm was determined with the longest dielectric lifetime. The morphology of the damaged sites after dielectric breakdown was examined using scanning electron microscope. Acoustic emission detection system was coupled with the dielectric setup to correlate fracture events and dielectric breakdown to thereby elucidate the mechanisms of the enhancements in dielectric lifetime properties. Two types of acoustic signals were always observed during the breakdown process for multilayered films. The high‐amplitude signals were attributed to the formation of breakdown pinholes caused by the primary discharge from top to bottom electrode. The subsequent low‐amplitude signals were attributed to internal discharges that could further damage the film. The total number of acoustic hits, in particular, low‐amplitude hits, increased with decreasing layer thickness, indicating more internal discharges occurred along the layered interface. It was concluded that the breakdown event initiated at a defect initiated “hotspot” formed because of internal pressure buildup. The film was punctured when the pressure buildup inside the film overcame the mechanical strength of the film. More number of PC layers and layer interfaces were desirable to slow down and divert the damage propagation through the film thickness direction. The crazes in P(VDF‐HFP) can, however, easily propagate across PC layers with less than 160 nm layer thickness. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39877.     

Theoretical estimation of dielectric loss of oxide glasses using nonequilibrium molecular dynamics simulations

To theoretically explore amorphous materials with a sufficiently low dielectric loss, which are essential for next-generation communication devices, the applicability of a nonequilibrium molecular dynamics simulation employing an external alternating electric field was examined using alkaline silicate glass models. In this method, the dielectric loss is directly evaluated as the phase shift of the dipole moment from the applied electric field. This method enabled us to evaluate the dielectric loss in a wide frequency range from 1 GHz to 10 THz. It was observed that the dielectric loss reaches its maximum at a few THz. The simulation method was found to qualitatively reproduce the effects of alkaline content and alkaline type on the dielectric loss. Furthermore, it reasonably reproduced the effect of mixed alkalines on the dielectric loss, which was observed in our experiments on sodium and/or potassium silicate glasses. Alkaline mixing was thus found to reduce the dielectric loss.     

A transport model considering charge adsorption inside pores to describe salts rejection by nanofiltration membranes

In the present work, rejection of mineral salts by nanofiltration membranes is modeled by a new knowledge model considering a charge distribution along the membrane pores. The model is classically based upon the coupling between the extended Nernst–Planck equation of transport within the pores with the Donnan, steric and dielectric interfacial exclusion. In literature, this kind of model is currently being used to describe salt rejections and it requires assessment of the membrane charge density and the dielectric constant of the solution inside the pores or the dielectric constant of the membrane material. Experimental estimation of these parameters, especially the membrane charge density, is difficult. For this reason, these parameters are usually adjusted so as to fit experimental rejections. The novelty of the proposed approach lies with the membrane charge density, which is not considered constant along the pores. Indeed, in this work, the membrane charge density was introduced in the model by means of adsorption isotherms, which were determined beforehand from the streaming potential measurements. In this case, only the dielectric constant inside pores had to be adjusted. The good agreement found between experimental and simulated rejection curves with coherent dielectric constants proves that the model theory is consistent. It has also proved that the estimated membrane charge density was, clearly, not overestimated as it was previously reported and made it possible to foresee a fully predictive model.     

Microstructure Evolution of Pulsed Laser-Deposited (Ba, Sr)TiO3 Films on MgO for Microwave Applications

To develop low-loss tunable microwave circuits, based on the field dependence of dielectric permittivity, phase pure (Ba_0.5, Sr_0.5)TiO₃ doped with 1% W (BST) thin films 0.3-μm thick were deposited on single crystal MgO wafers by pulsed laser deposition. The BST films were characterized by X-ray θ–2θ scans and pole figure analysis, field emission scanning electron microscopy (FESEM), and cross-sectional transmission electron microscopy (TEM), coupled with selected-area electron diffraction (SAED). Although, the X-ray θ–2θ scan indicated an epitaxial nature of BST with an out-of-plane orientation of (100), the pole figure analysis confirmed the presence (4–6%) of (111)-oriented grains in a matrix of (100) textured grains. The columnar grains exhibited an in-plane (i.e., along the plane perpendicular to the growth direction) grain size that was thickness-dependent. The cross-sectional TEM, coupled with SAED in the thickness direction, corroborated the pole figure analysis. Additionally, from X-ray analysis, it was observed that the textured films were under in-plane tension. The deposited film was characterized at microwave frequencies (1–20 GHz) using interdigitated electrodes deposited on top of the film. The film was characterized by a relatively low dielectric Q of 5–7. A 17% change in the capacitance was observed when applying a 40 V bias. From the observed microstructure, a preliminary understanding of its evolution and its relationship with the microwave dielectric properties is discussed, and some ideas to obtain truly epitaxial BST films are presented.     

Vibrational and dielectric properties and ideal strength of Si2N2O ceramic from first principles

Silicon oxynitride (Si₂N₂O) is a functional ceramic having many possible applications. In this work, we studied the vibrational and dielectric properties and ideal strength as well as the related deformation mechanism of Si₂N₂O using first principles calculations. Factor group analysis was performed and vibrational frequencies were calculated to assign the observed vibrational modes. Infrared dielectric and absorption spectra were simulated and their origins were revealed. Tensile, compressive and shear strengths along the representative directions were investigated, which gave Si₂N₂O an ideal strength agreeing well with its experimental hardness. The calculated minimum compressive stress also coincides well with the starting amorphization pressure observed in shock experiments, providing a theoretical method to study mechanical effects of shock wave. Analyses on the deformation mechanisms at the critical strains imply that Si-O bond is stronger than other bonds, which is opposite to the previous speculation.     

A Numerical Investigation of Stacked Oxide Junctionless High K with Vaccum Metal Oxide Semiconductor Field Effect Transistor

Silicon - The amalgamation of high dielectric permittivity material along with vaccum based dual material junctionless transistor with surrounding gate (DUM-HIK-VAC-JLSG-MOSFET) was investigated in...     

Green synthetic route of carbon quantum dot-reinforced graphene oxide-poly(vinylidene fluoride-co-hexa fluoropropylene) nanocomposites: Toward high dielectric constant and suppressed loss

A simple and green method was developed to fabricate carbon quantum dot@ graphene oxide filled poly (vinylidene fluoride-co-hexa-fluoropropylene) [CQD@GO-P(VDF-HFP)] nanocomposite films via solution casting technique. The synthetic approach was to bring CQD from bilva leafs, a renewable and sustainable resource. The effect of CQD on dielectric properties, dielectric constant, and dielectric loss in the presence of GO along with P (VDF-HFP) matrix were investigated. The result showed that the nanocomposites having 1.5 wt % of CQD@GO-P(VDF-HFP) with higher dielectric constant (≈144) at 100 Hz and suppressed loss (<1) at 1000 Hz, which is well supported by field emission scanning electron microscopy (FESEM) study. The FESEM study shows a river iceland morphology with a channel-like structure along with voids and pores that may provide a conducting network, which tends to have Maxwell Wagner-Sillars or interfacial polarization results in a high-end properties outcome. Furthermore, the suppressed loss enhanced the possibility of end use performance of CQD@GO-P(VDF-HFP) matrix with a referral memorandum of percolation theory. Thus, the present work demonstrated a new approach to develop high dielectric constant and negligible loss materials in the field of embedded devices for electronic industries through green synthetic approach. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47850.     

Dielectric and thermal expansion properties of LHPG grown potassium lithium niobate single crystals

Compositions of potassium lithium niobate K₆Li_4+xNb₁₀O₃₀ (x for excess mole % of Li₂O) with tetragonal tungsten bronze structure were prepared as crystal fibers by the Laser Heated Pedestal Growth technique. The lattice constants of KLN were measured by using X-ray diffraction and typical values for a = 12.5703 ± 0.0018 and c = 3.9485 ± 0.0012 were measured. This paper reports the dielectric permittivity measurements on single crystal along the a- and c- axis in the temperature range of 90-450K and frequency range of 10²-10⁵ Hz. The dielectric permittivity at room temperature along a- and c- axis was 430 and 330 respectively. Mechanism for the low temperature dielectric relaxation will be discussed. Thermal expansion behavier in the a- and the c-direction will also be reported.     

Piezoelectric resonance in KAP single crystals

The frequency response of the dielectric constant (ε_r), dielectric loss (tanδ) and impedance (Z) of Potassium acid pthalate (KAP) single crystals, monitored along the polar axis, is characterised by the presence of strong mechanical resonances, in the 50-200 kHz region. The observed resonance behaviour, which is dependent on the geometric size of the sample, is attributed to the piezoelectric nature of the sample. Studies on the effect of variation of temperature and pressure, on the stiffness coefficient (C), indicate that C increases with increasing pressure and decreasing temperature. The electro-mechanical coupling coefficient (k) has been evaluated based on the resonance-antiresonance method and its dependence on temperature and pressure has been studied.     

Dieletric and relaxation behaviors of (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 single crystal

Dielectric permittivity along the [111] direction has been measured as a function of temperature for a relaxor ferroelectric single crystal (PbMg_1/3Nb_2/3O₃)_0.67(PbTiO₃)_0.33 (PMN-33%PT). A sharp ferroelectric phase transition was observed near 425 K and 429 K for cooling and heating processes, respectively. As temperature decreases, a diffuse phase transition (which begins near 330 K upon cooling) was detected. In addition, the nature of the thermal hysteresis for the dielectric permittivity confirms that these transitions (near 330 and 425 K upon cooling) are diffuse first-order and first-order, respectively. The frequency-dependent dielectric data ε'₁₁₁ (ƒ, T) prove the existence of an electric dipolar relaxation process between 350 and 400 K. The activation energy, the Vogel-Fulcher temperature and attempt frequency corresponding to this relaxation process are also calculated.     

Dielectric properties of amorphous cellulose acetate-butyrate polymer films

The dielectric constant, loss tangent and AC conductivity of solution grown cellulose acetate-butyrate films of 5μm thickness were studied as a function of temperature (300-450 K) in the frequency range 10 Hz-10 MHz. The decrease in dielectric constant was observed with the increase in frequency as well as with temperature. A loss peak was observed in the dielectric loss spectra and was identified as the β-relaxation peak. The frequency-dependent conductivity was also studied. The results were interpreted in terms of electronic conduction via hopping processes.     

Thermal and morphological studies on γ‐Fe2O3 polystyrene composites and the affect of additives

γ‐Fe₂O₃polystyrene (PS) composite films were prepared by a gel‐casting technique to obtain monodisperse composite films. To understand the effect of additives on the prepared composite films, additives such as rice husk ash and thiourea were made to disperse into the PS matrix. The as‐prepared γ‐Fe₂O₃ PS composite films, along with their additives, were subjected to characterization and study by X‐ray diffraction, scanning electron microscopy, thermal, IR, and dielectric measurement techniques. These studies showed monodisperse and chemically homogenous composite films with an increase in thermal behavior. An interesting self‐assembly of rod‐like nanoparticles of γ‐Fe₂O₃ particles into the polymer matrix, which formed spherical packets, was observed for the γ‐Fe₂O₃PS composite film. The electrical behavior of these films was interesting, as some showed conduction whereas others showed an increase in dielectric behavior. This nature was explained by the dielectric measurements. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 778–788, 2004