Crystal structure and microwave dielectric properties of NaPb2B2V3O12(B=Mg,Zn) ceramics

Two low temperature sintered NaPb₂B₂V₃O₁₂ (B?=?Mg, Zn) ceramics with garnet structure were synthesized through conventional solid state reaction route and their crystal structure and microwave dielectric properties were investigated for the first time. Rietveld refinements of XRD patterns show both the compounds belong to cubic symmetry with space group Ia-3d. Observed number of Raman bands and group theoretical predictions also confirm cubic symmetry with space group Ia-3d for both NPMVO and NPZVO. At the optimum sintering temperature of 725?°C NPMVO has a relative permittivity of 20.6?±?0.2, unloaded quality factor (Q_uxf) of 22,800?±?1500?GHz (f?=?7.7?GHz) and temperature coefficient of resonant frequency +25.1?±?1?ppm/°C while NPZVO has relative permittivity of 22.4?±?0.2, Q_uxf of 7900?±?1500?GHz (f?=?7.4?GHz) and near zero temperature coefficient of resonant frequency of -6?±?1?ppm/°C at 650?°C. The relative permittivity of the compounds is inversely related to the corresponding Raman shifts.     

Dielectric relaxation and thermodynamic properties of polyvinylpyrrolidone using time domain reflectometry

Temperature‐dependent values of dielectric permittivity ε′ and dielectric loss ε″ of polyvinylpyrrolidone (PVP, commercialized as PVP K‐60) solution of average molecular weight 160 000 g mol^?1 were measured. The measurements were carried out in the frequency range 10 MHz to 20 GHz using time domain reflectometry at temperatures from 25 to 0 °C. The dielectric spectra can be described by the Davidson‐Cole model. Dielectric parameters such as the static dielectric constant ε₀, the high frequency limiting dielectric constant ε_∞, the relaxation time τ₀ and the distribution parameter β and thermodynamic parameters such as the free energy of activation ΔF_τ, the enthalpy of activation ΔH_τ and the entropy of activation ΔS_τ were determined. The average free energy of activation was found to be in the range 12.55–14.65 KJ mol^?1 and the enthalpy of activation was found to be 6.86 KJ mol^?1. Entropies of activation were found to be positive at all the measured temperature values and these large positive values of entropies reveal a less ordered structure of the PVP solution. The Kirkwood correlation factor g and the dipole moment µ were also determined for PVP solution. The results were compared with the results of the PVP‐water system studied previously. Copyright © 2011 Society of Chemical Industry     

Enhanced Piezoelectric and Dielectric Responses in 92.5%(Bi0.5Na0.5) TiO3 ‐7.5%BaTiO3 Ceramics

Structure and piezoelectric coefficient (d₃₃) of lead‐free 7.5% mole BaTiO₃‐doped (Bi_0.5Na_0.5) TiO₃ (BNT‐7.5%BT) polycrystalline piezoceramics have been characterized systematically as a function of poling electric (E) field. Dielectric permittivity and loss were also measured as functions of frequency and temperature. The piezoelectric coefficient d₃₃ after poling at E = 35 kV/cm can reach d₃₃~186 pC/N, which is the highest value reported among (1?x) BNT–xBT compositions. A prior poling E field can reduce rhomobherdal lattice distortion, and enhance tetragonal phase and polarization ordering, that contribute significantly to the rapid raise of d₃₃ and lower depolarizing temperature (T_d). The reduced dielectric permittivity for the poled sample is attributed to ordered state and the pinning of field‐induced nanodomain walls by the presence of oxygen vacancies.     

Artificial Neural Networks Modeling of Ozone Bubble Columns: Mass Transfer Coefficient,Gas Hold-Up,and Bubble Size

This study aims at applying artificial neural network (ANN) modeling approach in designing ozone bubble columns. Three multi-layer perceptron (MLP) ANN models were developed to predict the overall mass transfer coefficient (k_La, s^?1), the gas hold-up (? _G , dimensionless), and the Sauter mean bubble diameter (d_S , m) in different ozone bubble columns using simple inputs such as bubble column's geometry and operating conditions. The obtained results showed excellent prediction of k_La, ? _G , and d_S values as the coefficient of multiple determination (R² ) values for all ANN models exceeded 0.98. The ANN models were then used to determine the local mass transfer coefficient (k_L , m.s^?1). A very good agreement between the modeled and the measured k_L values was observed (R² ?=?0.85).     

Use of different size grids to control the surface chemistry of plasma‐polymerized acrylic acid films in a hybrid discharge

The surface chemistry of plasma‐polymerized acrylic acid (ppAc) films were controlled in a two‐stage (primary and processing) hybrid radio frequency (RF) discharge by changing the grid wire spacing (d_s). Two regions were defined in terms of d_s with respect to Debye length (λ_d) in the primary chamber at the grid to control the electron temperature (T_e) and surface chemistry of the ppAc films deposited in the processing chamber. A higher T_e (>3 eV) in the processing plasma was possible for d_s > λ_d, whereas decreasing d_s relative to λ_d reduced T_e. X‐ray photoelectron spectroscopy was used to characterize the ppAc films deposited on a glass substrate. The ppAc films surface characterization showed the maximum proportion of carbon atoms as carboxylic/ester [C(?O)OX] functionalities in C1s at the surface of films for the grid with d_s ≈ λ_d. The proportion of carbon atoms as ? [C(?O)OX] and COX in C1s at the surface decreased when d_s decreased relative to λ_d. The proportion of carbon atoms as carbonyl (C?O) at the film surface showed very good stability for all of the d_s values explored in this study. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2219–2224, 2007     

Defect chemistry and dielectric behavior of Sr0.99Ce0.01Ti1−xO3 ceramics with high permittivity

Sr_0.99Ce_0.01Ti_1-xO₃ (SCT, x?=?0, ±?0.0025, ±?0.0050, 0.0075) ceramics were prepared by solid state reaction methods and sintered in air atmosphere at different temperatures, with a soaking time of 2?h. The dielectric properties of all samples presented excellent temperature independence over a broad temperature range from 25 to 330?℃ and frequency independence between 10?kHz and 1?MHz. Sr_0.99Ce_0.01Ti_0.9925O₃ (SCT0.9925) ceramics sintered in air atmosphere exhibited a high permittivity (~5400) and a low dielectric loss (~0.01) measured at room temperature and 1?kHz. XPS and complex impedance spectroscopy analysis confirmed that the high permittivity and low dielectric loss were attributed to the fully ionized oxygen vacancies and giant defect-dipoles in Ti-deficient samples. However, a higher dielectric loss of Ti-rich samples is owing to the destruction of giant defect dipoles, in which highly localized electrons were transformed into hopping electrons.     

Structures and Properties of Pb(Zr0.5Ti0.5)O3−Pb(Zn1/3Nb2/3)O3− Pb(Ni1/3Nb2/3)O3 Ceramics for Energy Harvesting Devices

Piezoelectric ceramics with large energy density coefficient d₃₃·g₃₃ value have been found suitable for piezoelectric energy harvesting applications. In this study, the phase structures and piezoelectric properties of xPb(Zr_0.5Ti_0.5)O₃?yPb(Zn_1/3Nb_2/3)O₃?(1?x?y)Pb(Ni_1/3Nb_2/3)O₃ (xPZT?yPZN?(1?x?y)PNN) ceramic were investigated with systematically varying PZN and PNN components. The ternary phase diagram of PZT?PZN?PNN system was illustrated and the composition region of morphotropic phase boundary (MPB) was determined. Piezoelectric and dielectric measurements verify that the materials in MPB region all present large d₃₃ and d₃₃·g₃₃ values. In particular, very high d₃₃·g₃₃ coefficients of 20162.2 × 10^?15 m²/N and 21026.3 × 10^?15 m²/N are observed from samples 0.75PZT?0.15PZN?0.1PNN and 0.8PZT?0.05PZN?0.15PNN with compositions located on the rhombohedral phase side near MPB because the dielectric coefficient ε₃₃^T/ε₀ decreases faster than the d₃₃ coefficient at this side.     

Influence of the stoichiometry,microstructure, and metallization method on the dielectric properties of 0.94 Na0.5Bi0.5TiO3-0.06 BaTiO3

The morphotropic composition of the lead-free solid solution between Na_0.5Bi_0.5TiO₃ and BaTiO₃ (0.94 Na_0.5Bi_0.5TiO₃-0.06 BaTiO₃ or NBT-6BT) is of particular interest for the next generation of high-temperature capacitors but remains plagued by the diversity of dielectric properties reported in the literature. In order to explain the apparent inconsistencies among the reported dielectric properties of NBT-6BT, we examine the influence of stoichiometry, phase separation, and metallization method. We show that the nominal stoichiometry has a crucial effect, since increasing the nominal Na/Bi ratio increases conductivity and dielectric losses (tan δ). It also increases the real part of the permittivity (ε’) and the frequency dispersion of both ε’ and tan δ, thereby altering the shape of the evolution with temperature of the dielectric properties. Moreover it increases the depolarization temperature (T_d) and decreases the temperature of maximum permittivity (T_m). Phase separation also occurs during the synthesis of NBT-6BT as Na evaporation leads to the formation of secondary Ba-containing phases. We report that these phases can have a positive impact on the dielectric properties: a moderate volume fraction (2.5 to 3.0%) and average grain surface (0.9 to 3.0 µm²) of these secondary Ba-containing phases increase the relative permittivity, decrease the dielectric losses, and increase the insulation resistance. We also show that the metallization method impacts the dielectric properties and therefore may contribute to the differences between various reports. The dielectric properties of NBT-6BT samples are measured during successive heating/cooling cycles and reveal that the permittivity value is lower during the first heating when silver paste, even cured, is used. These three components contribute to explaining the diversity of the reported dielectric properties of NBT-6BT.     

Processing and Properties of Porous PZT Ceramics from Particle‐Stabilized Foams via Gel Casting

In the present work, porous lead zirconate titanate (PZT) ceramics with porosity ranging from 27.8% to 72.4% were fabricated by the gel‐casting process of particle‐stabilized wet foams with the initial solid loading of 10–30 vol%. The phase, the microstructure, the dielectric property, and the piezoelectric property were characterized. The relative permittivity (ε_r) and longitudinal piezoelectric strain coefficient (d₃₃) of the investigated samples decreased with increasing porosity. Both the values of hydrostatic strain coefficient (d_h) and hydrostatic voltage coefficient (g_h) increased moderately with the increase in porosity, which was beneficial for enhancing the value of hydrostatic figure of merit (HFOM). As a result, the prepared sample possessed a maximal HFOM value of 15236 × 10^?15 Pa^?1 with the porosity of 72.4%. The acoustic impedance (Z) of specimens decreased linearly with increasing porosity, and had the lowest value of 1.95 MRayls, making them promising candidates for application of medical ultrasonic imaging or underwater sonar detectors.     

Dielectric relaxation studies on thermotropic side‐chain liquid crystalline polymers

The dielectric activity of a thermotropic side‐chain liquid crystalline polymer is analysed. The sub‐glass relaxations are deconvoluted by two different empirically based equations (Fuoss–Kirkwood and Havriliak–Negami). The conductivity contributions are studied following a methodology proposed by Macdonald and Coelho. The effects of the orientation of the mesogenic group under electrical fields are also analysed. We propose a general method to estimate the curve of loss permittivity with a determined value of S_d based on the knowledge of two experimental curves, each corresponding to one well‐known orientation. © 2001 Society of Chemical Industry     

Technology and Properties of PBZTS Ceramics

This paper presents the results of preparing and investigating the solid solution of lead barium zirconate titanate stannate (Pb_1?xBa_x)[(Zr_1?yTi_y)_1?zSn_z]O₃ with x = 0.25, y = 0.35 and z = 0.00, 0.02, 0.04, 0.08, 0.10. Ceramic samples were obtained from oxides and carbonates using conventional ceramic technology and pressureless sintering. The results of Energy‐dispersive X‐ray spectroscopy (EDS) investigations, XRD studies, as well as dielectric measurements and electromechanical investigations are presented. It was stated that at the room temperature, the structure of the investigated samples is pseudo‐cubic, typical for relaxors. Maximal value of elementary cell parameter is observed for z = 0.02, and for higher values of z, we observe almost linear decrease with increasing z. It was found that with increasing content of the Sn, the temperature T_m at which dielectric permittivity reaches its maximum decreases. Analyzing P‐E hysteresis loops it was stated that the phase transition in the investigated samples takes place at temperatures approximately 100°C lower than the temperature of the dielectric permittivity maximum. The temperature of phase transition was calculated also from hysteresis loops and compared with that obtained from measurements of dielectric permittivity.     

Phase structure and electrical properties of lead free Na0·5Bi0·5TiO3–CaTiO3 ceramics

Abstract

Abstract

(1?x)Na_0·5Bi_0·5TiO_3?xCaTiO₃ ceramics with x?=?0–0·2 were prepared by solid state sintering method. Structural and morphology studies carried out by X-ray diffraction and scanning electron microscopy indicate the change in crystal structure from rhombohedral to orthorhombic symmetry (R3C to Pnma). The morphotropic phase boundary of this system was found to lie around x?=?0·08–0·14, where the orthorhombic and rhombohedral symmetries coexist. The orthorhombic phase is stabilised for x>0·14, indicating that the rhombohedral phase of Na_0·5Bi_0·5TiO₃ is susceptible to orthorhombic distortion brought about by Ca substitution. Calcium substitution in Na_0·5Bi_0·5TiO₃ caused an obvious decrease in peak temperature and a decrease in relative permittivity. The compositional variation of the fundamental dielectric behaviour is discussed in relation to the crystal chemistry of the system. The highest piezoelectric constant d₃₃ of 85 pC N^?1 is achieved for x?=?0·1, with the coercive field of 18 kV cm^?1 and the dielectric maximum temperature of 148°C.     

Shielding and dielectric properties of sulfonic acid‐doped π‐conjugated polymer in 8.2–12.4 GHz frequency range

This article deals with dielectric and electromagnetic interference shielding properties of the polyaniline doped with dodecyl benzene sulfonic acid (DBSA) synthesized by microemulsion polymerization of aniline in aqueous solution of DBSA. Dielectric constant and shielding effectiveness due to absorption (SE_A) were calculated using S‐parameter obtained from the vector network analyzer in 8.2–12.4 GHz frequency range. Maximum SE_A of 26 dB (>99%) was achieved for polymer sample. The real part ε′ of complex permittivity shows small variation, whereas the imaginary part ε″ is found to decrease with the increase in frequency. Different formulations have been performed to see the effect of monomer to dopant ratio on intrinsic properties of polyaniline. Further characterization of polymer was carried out by UV–visible and thermal gravimetric analysis, whereas the conductivity measurements were carried out by the four‐probe method.© 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Structure,Infrared Reflectivity and Microwave Dielectric Properties of (Na0.5La0.5)MoO4–(Na0.5Bi0.5)MoO4 Ceramics

A series of temperature‐stable microwave dielectric ceramics, (1?x)(Na_0.5La_0.5)MoO₄–x(Na_0.5Bi_0.5)MoO₄ (0.0 ≤ x ≤ 1.0) were prepared by using solid‐state reaction. All specimens can be well sintered at temperature of 580°C–680°C. Sintering behavior, phase composition, microstructures, and microwave dielectric properties of the ceramics were investigated. X‐ray diffraction results indicated that tetragonal scheelite solid solution was formed. Microwave dielectric properties showed that permittivity (ε_r) and temperature coefficient of resonant frequency (τ_f) were increased gradually, while quality factor (Q × f) values were decreased, at the x value was increased. The 0.45(Na_0.5La_0.5)MoO₄–0.55(Na_0.5Bi_0.5)MoO₄ ceramic sintered at 640°C with a relative permittivity of 23.1, a Q × f values of 17 500 GHz (at 9 GHz) and a near zero τ_f value of 0.28 ppm/°C. Far‐infrared spectra (50–1000 cm^?1) study showed that complex dielectric spectra were in good agreement with the measured microwave permittivity and dielectric losses.     

Thermal-stability of electric field-induced strain and energy storage density in Nb-doped BNKT-ST piezoceramics

In this work, the relationship between the structural mechanisms and macroscopic electrical properties of the Nb-modified 0.96(Bi_0.5Na_0.84K_0.16TiO₃)–0.04SrTiO₃ (BNKT–ST) system were elucidated by using temperature dependent and in situ synchrotron X-ray diffraction (XRD) techniques. For the composition x?=?0.0175, a large-signal piezoelectric coefficient (S_max/E_max?=?d₃₃*) of 735 pm?V^?1 at 6?kV mm^?1 was observed at room temperature. Interestingly, at a higher temperature of 110?°C, the sample still showed a large d₃₃* of 570 pm V^?1. Furthermore, the temperature-invariant electrostrictive coefficient for this sample was found to be 0.0285?m⁴?C^?2 over the temperature range of 25–170?°C. Moreover, the energy density for x?=?0.030 sample was ～1.0?J?cm^?3 with an energy storage efficiency of ?70% in the temperature range of 25–135?°C. These results suggest that the synthesized Nb-modified BNKT–ST system is promising for the design of ceramic actuators as well as capacitor applications.     

Investigation of ferroelectric,piezoelectric and mechanically coupled properties of lead-free (Ba0.85Ca0.15) (Zr0.1Ti0.9)O3 ceramics

Lead-free piezoelectric ceramic (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ (BCZT) has been synthesised by solid-state sintering method and the effect of grain size on ferroelectric, piezoelectric, dielectric, mechanical and piezo-electro-mechanical properties was systematically studied. The compacted powders were sintered at three temperatures i.e. 1450°°C, 1500°C and 1550°C for an optimised duration of 5?h and they have exhibited well resolved morphotropic phase boundary with an average grain size ranging from 10 to 25?µm. Enhanced piezoelectric charge, d₃₃ ～ 560 pC/N and voltage, g₃₃ ～ 14.3?mV?m/N coefficients were obtained for the 1450°C sintered BCZT sample. A maximum strain of around ～0.14% was obtained which is comparable to that of lead-based piezoelectrics. Variation of relative permittivity with temperature revealed that the curves are independent of frequency, indicating the typical relaxor ferroelectric nature of the samples. A systematic study on cyclic loading was performed to evaluate piezo-electro-mechanical coefficients at different loads which showed hysteresis behaviour. High value of elastic modulus (E) and hardness (H) i.e. 262.7?±?38.1?GPa and 13.7?±?1.7?GPa were exhibited by samples sintered at 1450°C, which is higher than that of BCZT synthesised by wet-chemical methods. The results are discussed.     

The Direct and the Converse Magnetoelectric Effect in Multiferroic Cobalt Ferrite–Barium Titanate Ceramic Composites

We report on a systematic study of the magnetoelectric effect in cobalt ferrite (CoFe₂O₄)—barium titanate (BaTiO₃) ceramic composites with (0‐3) connectivity. Both the converse magnetoelectric coefficient, α_C, and the direct voltage magnetoelectric coefficient, α^E, were measured in dependence on composition and electric and magnetic bias fields. The strongest ME effect was observed in the composition (1?x) CoFe₂O₄–xBaTiO₃ with x = 0.5 yielding values α_C = 25 psm^?1 and α^E = 3.2 mV/(cm·Oe). We show that the proper conversion between these two coefficients demands knowledge about the dielectric permittivity of the sample. For low BaTiO₃ content the dielectric coefficient of the composite yields a better correspondence, whereas for high BaTiO₃ content the sample's average dielectric coefficient yields a better match. The influence of mutual orientation of polarization and magnetization on the ME effect is addressed. We found that for measurements performed parallel to the polarization direction (longitudinal effect), the ME coefficient is approximately twice as large and of opposite sign in comparison to the measurements perpendicular to the polarization direction (transverse effect). This difference has been rationalized in terms of the different contributions of the material coefficient tensor components to the ME effect, the demagnetizing factor, and losses. The obtained results provide a better understanding of peculiarities of the ME effect in bulk ceramic composites.     

Crosslink density and solvent swelling of filled and unfilled stocks

The network segment density (γ_g) by solvent-swollen compression modulus of gum stocks and of stocks containing nonreinforcing fillers was calculated to be 6.6093 × 10^?9h₀Sf/?_Bd² in mmole/m³ of gel at 25°C, where h₀ is the height of the unswollen sample in cm, S is the slope of the height versus applied weight curve in g per mil (0.001 in.), ?_B is the volume fraction of binder, d is the diameter of the initial sample in cm, and f is a factor equal to (1? ?_s/?_B)^1/3/(1? ?_S)^2/3, where ?_S is the volume fraction of extract. The volume fraction of crosslinked polymer containing nonreinforcing fillers at equilibrium in a solvent (V₂) was determined for the cases where the filler is insoluble and partially adheres to the binder, where the filler is partially solube and there is no binder–filler adhesion, and where the filler is completely soluble in the swelling solvent. The relations were tested and found to hold for polyester–polyurethane stocks containing plasticizer and soluble and insoluble fillers. Log V₂ fell on single straight-line curves with respect to log γ_g for gum stocks and filled stocks both when V₂ was measured in solvents which dissolved none or dissolved part of the fillers.     

Experimental study and DFT calculations of improved giant dielectric properties of Ni2+/Ta5+ co-doped TiO2 by engineering defects and internal interfaces

High-performance ceramics with chemical formula (Ni_1/3Ta_2/3)_xTi_1?xO₂ with excellent dielectric properties are demonstrated. The dopants of Ni²⁺ and Ta⁵⁺ in TiO₂ caused the formation of oxygen vacancies and free electrons. The (Ni_1/3Ta_2/3)_xTi_1?xO₂ exhibited low loss tangent of 0.046 and a high dielectric permittivity of 3.5–4.5 × 10⁴ with a very weak dependence on temperature (?60 to 200 °C). Broadband dielectric spectroscopy shows at least four dominant sources in the dielectric relaxation response in the temperature range of ? 253–210 °C. DFT calculations indicate the formation of defect clusters, which are the largest contributors to the dielectric response, and these are found to be dominant even at temperatures down to ? 253 °C. Both grain boundary and surface layer mechanisms in the ceramics contribute to the dielectric response at the relatively high temperatures. The sample–electrode contact effect associated with oxygen vacancy diffusion is dominant at high temperatures above 150 °C.     

Temperature induced dielectric polarization evolution in polycrystalline Pb(Cd1/3Nb2/3)O3

The results of X-ray diffraction (XRD), scanning electron microscopy (SEM), dilatometric and dielectric measurements performed for the same polycrystalline Pb(Cd_1/3Nb_2/3)O₃ (PCN) sample are presented. These results reveal the complex character of polycrystalline PCN dielectric properties; in particular, no structural phase transition (PT) was observed. The thermal expansion coefficient indicates glass-like behaviour of PCN. The frequency and temperature dependencies of complex dielectric permittivity were measured and analyzed in terms of diffused/relaxor transition of the ferroelectric polarization. Temperature induced evolution of the relative dielectric permittivity was found to involve two processes. Dielectric permittivity changes, originating from thermally induced evolution of thermal equilibrium of interactions between main structural lattice and two sub-lattices present in the sample were interpreted in terms of thermal evolution of polar cluster sizes in the region of relaxor/glass-like transition.