Improved dielectric and non-ohmic properties of Ca2Cu2Ti4O12 ceramics prepared by a polymer pyrolysis method

Non-ohmic and dielectric properties of Ca₂Cu₂Ti₄O₁₂ (CaCu₃Ti₄O₁₂/CaTiO₃ composite) ceramics prepared by a polymer pyrolysis method (PP-ceramic samples) are investigated. The PP-ceramics show a highly dense structure and improved non-ohmic and dielectric properties compared to the ceramics obtained by a solid state reaction method (SSR-ceramic samples). ?′ (tan δ) of the PP-ceramic samples is found to be higher (lower) than that of the SSR-ceramic samples. Interestingly, the PP-ceramic sintered at 1050 °C for 10 h exhibits the high ?′ of 2530 with weak frequency dependence below 1 MHz, the low tan δ less than 0.05 in the frequency range of 160 Hz-177 kHz, and the little temperature coefficient, i.e., |Δ?′| ≤ 15 % in the temperature range from −55 to 85 °C. These results indicate that the CaCu₃Ti₄O₁₂/CaTiO₃ composite system prepared by PP method is a promising high-?′ material for practical capacitor application.     

Piezoelectric and dielectric properties of Dy2O3-doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free ceramics

(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ + x wt% Dy₂O₃ with x = 0-0.3 ceramics were synthesized by conventional solid-state processes. The effects of Dy₂O₃ on the microstructure, the piezoelectric and dielectric properties were investigated. X-ray diffraction pattern confirmed that the coexistence of tetragonal and rhombohedral phases in the (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ composition was not changed by adding 0.05-0.3 wt% Dy₂O₃. SEM images indicate that all the ceramics have pore-free microstructures with high density, and that doping of Dy₂O₃ inhibits the grain growth of the ceramics. The addition of Dy₂O₃ shows the double effects on decreasing the piezoelectric and dielectric properties for 0 < x < 0.15 when Dy³⁺ ions substitute B-site Ti⁴⁺ ions, and increasing the properties for 0.15 < x < 0.3 when Dy³⁺ ions enters into A-site of the perovskite structure. The optimum electric properties of piezoelectric constant d₃₃ = 170 pC/N and the dielectric constant ?_r = 1900 (at a frequency of 1 kHz) are obtained at x = 0.3.     

Study of structural, dielectric and electrical behavior of (1 − x)Ba(Fe0.5Nb0.5)O3-xSrTiO3 ceramics

Polycrystalline samples of BaFe_0.5Nb_0.5O₃ and (1 − x)Ba(Fe_0.5Nb_0.5)O₃-xSrTiO₃ [referred as BFN and BFN-ST respectively] (x = 0.00, 0.15 and 0.20) have been synthesized by a high-temperature solid-state reaction technique. The XRD patterns of the BFN and BFN-ST at room temperature show a monoclinic phase. The microstructure of the ceramics was examined by the scanning electron microscopy (SEM) and shows the polycrystalline nature of the samples with different grain sizes, which are inhomogeneously distributed through the sample surface. Detailed studies of dielectric and impedance properties of the materials in a wide range of frequency (100 Hz-5 MHz) and temperatures (30-270 °C) showed that properties are strongly temperature and frequency dependent. Complex Argand plane plot of ?″ against ?′, usually called Cole-Cole plots is used to check the polydispersive nature of relaxation phenomena in above mentioned compounds. Relaxation phenomena of non-Debye type have been observed in the BFN and BFN-ST ceramics, as confirmed by the Cole-Cole plots.     

AC conductivity and dielectric properties of GeSexTl0.3 amorphous thin films

AC conductivity and dielectric properties have been studied for amorphous thin films with different thicknesses of glassy system GeSe_xTl_0.3 with X = 3, 4 that prepared with thermal evaporation technique. The measurements are taken at temperature range (303-403 K) and frequency range (10²-10⁵ Hz).AC conductivity σ_ac(ω) is found to be proportional to ω^s where s < 1. The temperature dependence of the ac conductivity and the parameter s can be discussed with the aim of the correlated barrier-hopping (CBH) model.The dielectric constant ?′and the dielectric loss ?″ showed frequency and temperature dependence. The maximum barrier height W_M calculated from the dielectric measurements according to Giuntini equation are in good agreement with that proposed by the theory of hopping of charge carriers over a potential barrier as suggested by Elliott in the case of chalcogenide glasses.     

Multiferroic and magnetoelectric properties of single-phase Bi0.85La0.1Ho0.05FeO3 ceramics

Single-phase Bi_0.85La_0.1Ho_0.05FeO₃ multiferroic ceramics were prepared by a rapid liquid sintering method. The ceramics exhibited an obvious ferroelectric loop with a remnant polarization of 11.2 μC/cm² and also showed weak ferromagnetism with the remnant magnetization of 0.179 emu/g at room temperature. A considerable enhancement of the polarization on magnetic poling and a dielectric anomaly in the vicinity of the antiferromagnetic transition temperature due to the intrinsic magnetoelectric coupling effect were observed in Bi_0.85La_0.1Ho_0.05FeO₃ ceramics. The dielectric constant for the Bi_0.8La_0.1Ho_0.05FeO₃ samples at room temperature decreases with increasing applied magnetic fields, and the coupling coefficient (?′(H) − ?′(0))/?′(0) reaches −1.04% at H = 10 kOe.     

Dielectric properties of polycrystalline Cu-Zn ferrites at microwave frequencies

The real dielectric constant ?′ and complex dielectric constant ?″ of Cu_1−xZn_xFe₂O₄ have been measured at room temperature in the high frequency range 1 MHz to 1.8 GHz. At low frequencies the dielectric loss is found to be constant up to 1.4 GHz and there is a sudden rise at 1.5 GHz. A qualitative explanation is given for the composition, frequency dependence of the dielectric constant and dielectric loss of Cu_1−xZn_x Fe₂O₄. These are correlated with the W-H plot which gives the information about change in the average crystal size and strain of the samples. The micro-morphological features of the samples were obtained by Scanning Electron Microscopy (SEM). The micrograph shows that the increase of the Zn content in Cu ferrite increases the grain size.     

Ferroelectric relaxor behavior and dielectric spectroscopic study of 0.99(Bi0.5Na0.5TiO3)-0.01(SrNb2O6) solid solution

0.99(Bi_0.5Na_0.5TiO₃)-0.01(SrNb₂O₆) was prepared by simple solid state reaction route. Material stabilized in rhombohedral perovskite phase with lattice constants a = 3.9060 Å, α = 89.86° and a_h = 5.4852 Å, c_h = 6.7335 Å for hexagonal unit cells. Density of material was found 5.52 g_m/cm³ (92.9% of theoretical one) in the sample sintered at 950 °C. The temperature dependent dielectric constant exhibits a broad peak at 538 K (?_m = 2270) at 1 kHz that shows frequency dependent shifts toward higher temperature - typical relaxor behavior. Modified Curie-Weiss law was used to fit the dielectric data that exhibits almost complete diffuse phase transition characteristics. The dielectric relaxation obeys the Vogel-Fulcher relationship with the freezing temperature 412.4 K. Significant dielectric dispersion is observed in low frequency regime in both components of dielectric response and a small dielectric relaxation peak is observed. Cole-Cole plots indicate polydispersive nature of the dielectric relaxation; the relaxation distribution increases with increase in temperature.     

Low-firable high-K dielectric in the Zrx(Zn1/3Nb2/3)1−xTiO4 ceramic system

Composite ceramics in the solid solution of Zr_x(Zn_1/3Nb_2/3)_1−xTiO₄ (x = 0.1-0.4) have been prepared by the mixed oxide route. Formation of solid solution was confirmed by the X-ray diffraction patterns. The microwave dielectric properties, such as dielectric constant (?_r), Q × f value and temperature coefficient of resonant frequency (τ_f) have been investigated as a function of composition and sintering temperature. With x increasing from 0.1 to 0.4, the dielectric constant decreases from 70.9 to 43.2, and the τ_f decreases from 105 to 55 ppm/°C. The Q × f value, however, increases with increasing x value to a maximum 26,600 GHz (at 6 GHz) at x = 0.3, and then decreases thereafter. For low-loss microwave applications, a new microwave dielectric material Zr_0.3(Zn_1/3Nb_2/3)_0.7TiO₄, possessing a fine combination of microwave dielectric properties with a high ?_r of 51, a high Q × f of 26,600 GHz (at 6 GHz) and a τ_f of 70 ppm/°C, is suggested.     

Structural impact of tungsten ions on ZnO-Sb2O3-As2O3 glass system

Five glasses in the quaternary system 5 ZnO-(50 − x) As₂O₃-45 Sb₂O₃: x WO₃ with the values of x ranging from 0 to 20 mol% (in steps of 5 mol%) are prepared. The samples are characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy (EDS) and differential thermal analysis (DTA) techniques. The DTA studies have indicated that the glass forming ability decreases with the increasing content of WO₃. A number of studies, like, spectroscopic (optical absorption, IR, Raman, ESR spectra) and dielectric studies (dielectric constant ?, loss tan δ, a.c. conductivity σ_a.c.) over a wide range of frequency and temperature and dielectric break down strength at room temperature, have been carried out and are analysed in the light of different oxidation states and environment of tungsten ions in these glasses. These glasses have potential photonic applications.     

X-ray diffraction and dielectric studies across morphotropic phase boundary in (1 − x) [Pb(Mg0.5W0.5)O3]-xPbTiO3 ceramics

We present here the results of comprehensive X-ray diffraction and dielectric studies on several compositions of (1 − x)[Pb(Mg_0.5W_0.5)O₃]-xPbTiO₃ (PMW-xPT) solid solution across the morphotropic phase boundary. Rietveld analysis of the powder X-ray diffraction data reveals cubic (space group Fm3m) structure of PMW-xPT ceramics for the compositions with x ≤ 0.42, tetragonal (space group P4mm) structure for the compositions with x ≥ 0.72 and coexistence of the tetragonal and cubic phases for the intermediate compositions (0.46 ≤ x ≤ 0.68). Temperature dependence of the dielectric permittivity above room temperature exhibits diffuse nature of phase transitions for the compositions in the cubic and two phase region while the compositions with tetragonal structure at room temperature exhibit sharp ferroelectric to paraelectric phase transition. The PMW-xPT compositions with coexistence of tetragonal and cubic phases at room temperature exhibit two anomalies in the temperature dependence of the dielectric permittivity above room temperature. Using results of structural and dielectric studies a partial phase diagram of PMW-xPT ceramics is also presented.     

High-dielectric-constant and low-loss microwave dielectric in the (1 − x)La(Mg0.5Ti0.5)O3-x(Ca0.8Sr0.2)TiO3 solid solution system

The microwave dielectric properties and microstructures of (1 − x)La(Mg_0.5Ti_0.5)O₃-x(Ca_0.8Sr_0.2)TiO₃ ceramics, prepared by a mixed oxide route, have been investigated. The forming of solid solutions was confirmed by the XRD patterns and the measured lattice parameters for all compositions. A near zero τ_f was achieved for samples with x = 0.5, although the dielectric properties varied with sintering temperature. The Q × f value of 0.5La(Mg_0.5Ti_0.5)O₃-0.5(Ca_0.8Sr_0.2)TiO₃ increased up to 1475 °C, after which it decreased. The decrease in dielectric properties was coincident with the onset of rapid grain growth. The optimum combination of microwave dielectric properties was achieved at 1475 °C for samples where x = 0.5 with a dielectric constant ?_r of 47.12, a Q × f value of 35,000 GHz (measured at 6.2 GHz) and a τ_f value of −4.7 ppm/°C.     

Characterization of sol-gel synthesised lead-free (1 − x)Na0.5Bi0.5TiO3-xBaTiO3-based ceramics

The crystal structure, microstructure, dielectric and ferroelectric properties of (1 − x)Na_0.5Bi_0.5TiO₃-xBaTiO₃ ceramics with x = 0, 0.03, 0.05, 0.07 and 0.1 are investigated. A structural variation according to the system composition was investigated by X-ray diffraction (XRD) analyses. The results revealed that the synthesis temperature for pure perovskite phase powder prepared by the present sol-gel process is much lower (800 °C), and a rhombohedral-tetragonal morphotropic phase boundary (MPB) is found for x = 0.07 composition which showing the highest remanent polarization value and the smallest coercive field. The optimum dielectric and piezoelectric properties were found with the 0.93Na_0.5Bi_0.5TiO₃-0.07BaTiO₃ composition. The piezoelectric constant d₃₃ is 120 pC/N and good polarization behaviour was observed with remanent polarization (P_r) of 12.18 pC/cm², coercive field (E_c) of 2.11 kV/mm, and enhanced dielectric properties ?_r > 1500 at room temperature. The 0.93Na_0.5Bi_0.5TiO₃-0.07BaTiO₃-based ceramic is a promising lead-free piezoelectric candidate for applications in different devices.     

Microwave dielectric properties of (Zn1−xMgx)TiO3 (ZMT) ceramics for dielectric resonator antenna application

Present investigation provides experimental studies on cylindrical dielectric resonator antennas (CDRAs) fabricated from (Zn_1−xMg_x)TiO₃ (ZMT) ceramic material with different substitution of Mg in place of Zn (x = 0.1, 0.2, 0.3, 0.4 and 0.5) along with measurement of material permittivity in C-band of microwave frequencies. The dielectric properties of the ZMT ceramic materials with different x values (x = 0.1-0.5) have been measured at microwave frequencies using Hakki-Coleman method modified by Courtney. The value of dielectric constant and loss tangent decreases with the increase in Mg content. The dielectric constant is minimum for x = 0.3 and loss tangent is minimum for x = 0.5. The variations of return loss and input impedance versus frequency and radiation patterns of CDRAs at their respective resonant frequencies are studied experimentally. The measured results for resonant frequency and return loss bandwidth of the CDRAs are also compared with theoretical ones. The measured resonant frequencies of all the four CDRAs are nearly in agreement with respective theoretical values and the values of input resistance at respective resonant frequencies of the CDRAs match well with the coaxial feed impedance of 50 Ω. The designed CDRAs have broad beam, low side lobe and cross-polarized lobe levels.     

Large magnetic entropy change near room temperature in La0.7(Ca0.27Ag0.03)MnO3 perovskite

In this paper, the magnetic properties and magnetocaloric effect (MCE) of La_0.7(Ca_1−xAg_x)_0.3MnO₃ (x = 0, 0.1, 0.2, 0.7, and 1) powder samples are reported. Our polycrystalline compounds were synthesized using the solid state reaction method at high temperature. Magnetization measurements versus temperature showed that all our samples exhibited a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature, T_C, has been found to increase from ∼250 K for x = 0-270 K for x = 1. Ag doping weakens the first order phase transition, and at higher Ag doping, the phase transition is of second order. For the La_0.7(Ca_0.27Ag_0.03)MnO₃ composition, the maxima of the magnetic entropy changes from the applied magnetic field (ΔS_M) at 2 and 5 T are about 4.5 and 7.75 J/kg K, respectively, at the Curie temperature of ∼263 K. The relative cooling power (RCP) values without hysteresis loss are about 102 and 271 J/kg for the applied fields of 2 and 5 T, respectively. Due to the large ΔS_M, large RCP, and high Curie temperature, La_0.7(Ca_0.27Ag_0.03)MnO₃ is promising for application in potential magnetic refrigeration near room temperature.     

Structural, magnetic and dielectric properties of La2−xCaxNiO4+δ (x = 0, 0.1, 0.2, 0.3)

The synthesis, structural, magnetic and dielectric properties of a new type of high permittivity materials La_2−xCa_xNiO_4+δ (x = 0, 0.1, 0.2, 0.3) (abbreviated as LCNs) were reported. The samples were prepared through conventional solid state reaction route. Detailed structural information was retrieved by Rietveld refinement; normalized bond length and bond valence was calculated to investigate the compression/dilation effect of bonds and atoms in unit cell. It can be found all samples belong to K₂NiF₄ structure with space group I4/mmm. Doping of Ca in La₂NiO_4+δ shrinks the unit cell and makes the structure tend to become instable. Three types of (La, Ca)-O bonds, and two kinds of Ni-O bonds exist in LCNs. Along c axis there are alternately compressed (La,Ca)O₉ dodecahedra and lengthened NiO₆ octahedra. Room temperature magnetic measurements show that the materials are paramagnetic and Ca doping can improve the spontaneous magnetization. Furthermore, all samples have colossal values of the dielectric constant (?) at frequencies lower than 1 kHz. Interestingly, La_1.8Ca_0.2NiO_4+δ maintains its high permittivity at frequencies up to 1 MHz while La_1.7Ca_0.3NiO_4+δ has the lowest dielectric loss (tan δ). Calcium doping can effectively enhance ? and inhibit tan δ. The distortion of (La,Ca)O₉ dodecahedra can well explain their dielectric properties.     

Enhanced magnetoelectric properties in Bi0.95Ho0.05FeO3 polycrystalline ceramics

Polycrystalline samples of Ho doped BiFeO₃ were prepared by solid state reaction method and effect of partial substitution of Ho on dielectric, magnetic and ferroelectric properties was studied. High temperature dielectric results show two dielectric anomalies both in ? and tan δ, out of which, anomaly at higher temperature (∼400 °C) could be ascribed to antiferromagnetic Néel temperature which, is a signature of magnetoelectric coupling. The magnetic moment is greatly improved and the maximum magnetization was found to be 0.736 emu/g. Saturated ferroelectric hysteresis loops were observed for Bi_0.95Ho_0.05FeO₃ with remnant polarization (P_r) = 1.59 μC/cm², maximum polarization (P_max) = 2.56 μC/cm² and coercivity (E_c) = 5.45 kV/cm. We have conducted comprehensive magnetoelectric and magnetodielectric properties at room temperature. Magnetic field induced ferroelectric hysteresis loop observed in Bi_0.95Ho_0.05FeO₃ is of prime importance.     

Dielectric and magnetic behavior of BaCd2−xSrxFe16O27 W-type hexagonal ferrites

We have prepared BaCd_2−xSr_xFe₁₆O₂₇ (x = 0, 0.5, 1, 1.5 and 2.0) W-type hexagonal ferrites by standard ceramic method. In this work, the structural, dielectric and magnetic properties have been studied of the prepared samples. The XRD analysis of the samples reveals single phase behavior sintered at 1400 °C for 6 h. The saturation magnetization (M_s) shows increasing behavior with the increasing concentration of Sr²⁺. While the coercivity (H_c) decreases rapidly up to 409 G for x = 1.5 and then increases for (x > 1.5) due to the preference of Cd²⁺ for tetrahedral sites and the number of spin-down magnetic ions. The real and imaginary parts of the dielectric constant (?′,?″) and dielectric loss tangent (tan δ) are determined in the frequency range 0.1-10⁷ Hz. It is observed that both the real and imaginary parts of the dielectric constant and tan δ decrease with the increasing concentration of Sr²⁺, which is due to the contribution of Cd²⁺ ions in addition to Fe³⁺ and Fe²⁺ ions to interfacial polarization.     

Synthesis and study of nanocrystalline Ni-Cu-Zn ferrites prepared by oxalate based precursor method

Nanocrystalline ferrites of compositions Ni_0.5+1.5xCu_0.3Zn_0.2Fe_2−xO₄ (0 ≤ x ≤ 0.5) have been synthesized by using oxalate based precursor method at very low temperature. The Ni-Cu-Zn ferrite powder particles were obtained at 450 °C and they exhibit a crystallite size of 16-24 nm. The lattice constants were found nearly equal in all these samples due to minute difference in the ionic radius between Ni²⁺ and Fe³⁺ ions. The thermal analysis has showed the ferrite phase formation at very low temperature 377 °C. The two main spectroscopic bands corresponding to lattice vibrations were observed in the wavelength range from 300 to 1000 cm⁻¹. The IR bands at 570 cm⁻¹ (v₁) and 390 cm⁻¹ (v₂) were assigned to tetrahedral (A) and octahedral [B] groups. The spectroscopic bands shift with the increase of doping concentration. The magnetization was found to decrease with increasing doping concentration. The dielectric constant (?′) and dielectric loss tangent (tan δ) decreased with increase of frequency. The dielectric constant and dielectric loss obtained for the nanocrystalline ferrite samples appeared to be lower than that of the ferrites prepared by other synthesis techniques.     

Structure and dielectric characterization of xNd(Mg1/2Ti1/2)O3-(1 − x)Ca0.6La0.8/3TiO3 in the microwave frequency range

The crystal structures, phase compositions and the microwave dielectric properties of the xLa(Mg_1/2Ti_1/2)O₃-(1 − x)Ca_0.8Sr_0.2TiO₃ composites prepared by the conventional solid state route have been investigated. The formation of solid solution is confirmed by the XRD patterns. Doping with B₂O₃ (0.5 wt.%) can effectively promote the densification and the dielectric properties of xNd(Mg_1/2Ti_1/2)O₃-(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics. It is found that xNd(Mg_1/2Ti_1/2)O₃-(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics can be sintered at 1375 °C, due to the liquid phase effect of B₂O₃ addition observed by Scanning Electronic Microscopy. At 1375 °C, 0.4Nd(Mg_1/2Ti_1/2)O₃-0.6Ca_0.6La_0.8/3TiO₃ ceramics with 1 wt.% B₂O₃ addition possesses a dielectric constant (?_r) of 49, a Q × f value of 13,000 (at 8 GHz) and a temperature coefficients of resonant frequency (τ_f) of 1 ppm/°C. As the content of Nd(Mg_1/2Ti_1/2)O₃ increases, the highest Q × f value of 20,000 GHz for x = 0.9 is achieved at the sintering temperature 1400 °C.     

Synergistic microwave and structural studies of C-type Ho2Si2O7

Ho₂Si₂O₇ material exists in four polymorphs, a triclinic low temperature phase (type-B), a monoclinic modification (type-C), high temperature monoclinic (type-D), and high temperature orthorhombic modification (type-E). The structural properties are measured by XRD and the morphology is noted through scanning electron microscopy (SEM). The dc electrical resistivity (ρ) as a function of temperature and dielectric properties of C-type Ho₂Si₂O₇ in the microwave region is measured. The activation energy is calculated from ln ρ versus 1/k_BT plot. The activation energy is 0.119 ± 0.001 eV. Both the real (?′) and imaginary parts of permittivity (?″) decrease slightly as the frequency increases up to 1.5 GHz, after that ?′ increases while ?″ decreases as the frequency increases. At around 2.45 GHz, resonance is observed.