Combustion synthesis of Sr0.5Ba0.5Nb2O6 and effect of fuel on its microstructure and dielectric properties

Nano-sized Sr_0.5Ba_0.5Nb₂O₆ (SBN50) powder has been synthesized, at very short reaction time, for the first time by a novel combustion method. Ba(NO₃)₂ and Sr(NO₃)₂ were used as source of Sr and Ba, respectively, while Nb-oxalate was used as the source of niobium. Urea, hexamethyltetramine (HMT) and glycine were used as fuel. The crystallite sizes in the powder ranged between 14-125 nm. X-ray diffraction analysis showed complete SBN50 phase formation at 700 °C, when urea/HMT was used as fuel, and at 800 °C when glycine was used as fuel. Ferroelectric-paraelectric phase transition temperature (T_c) close to 40 °C was observed when urea and HMT were used and the T_c was −49 °C when glycine was used. When urea was used as fuel highest dielectric constant was observed for the pellets sintered at 1250 °C for 4 h. Low dielectric loss was observed when HMT was used as fuel. Larger grain sizes in the sintered pellets were observed when glycine was used as fuel.     

Preparation and electrical properties of Ca5Si3 and Sr5Si3 powders

Single phase Ca₅Si₃ and Sr₅Si₃ powders were prepared, and their electric and thermoelectric properties were investigated. The Ca₅Si₃ and Sr₅Si₃ powders are synthesized by exposure of the Si powders to Ca and Sr fluxes, respectively. It is found that both silicides show a p-type conduction and semiconductor-like behavior. The electronic band structures of the silicides are calculated using the first-principles total-energy calculation program in pseudopotential schemes with plane-wave basis functions. The calculated result predicts the possibility of a semiconductor-like property with a sharp pseudogap at the Fermi level for Ca₅Si₃, as experimentally obtained. The silicide would be expected to be a new semiconductor-like conducting material.     

Polaronic relaxation in La0.8Bi0.2Fe0.7Mn0.3O3

In this work, the dielectric properties of La_0.8Bi_0.2Fe_0.7Mn_0.3O₃ ceramics have been investigated in a temperature range of 76–320 K and a frequency range of 300 Hz–10 MHz. Two thermally activated dielectric relaxations were observed with the activation energy around 0.283 ± 0.007 eV for the low-temperature relaxation and 0.268 ± 0.007 eV for the high-temperature relaxation. Annealing in N₂ and O₂ can destroy and create the high-temperature relaxation, respectively. But the treatments have no significant influence on the low-temperature relaxation. The low-temperature relaxation was found to be bulk effect related to the dipolar effect due to the hopping polarons, and the high-temperature relaxation was associated with the Maxwell–Wagner relaxation due to surface-layer effect caused by hopping polarons blocked and trapped at the surfaces of the sample.     

Structural and dielectric properties of ferroelectric Sr1−xBaxBi2(Nb0.5Ta0.5)2O9 and Sr0.5Ba0.5Bi2(Nb1−yTay)2O9 ceramics

Ferroelectric Sr_1−xBa_xBi₂(Nb_0.5Ta_0.5)₂O₉ and Sr_0.5Ba_0.5Bi₂(Nb_1−yTa_y)₂O₉ were synthesized by solid state reaction route. X-ray diffraction studies confirm the formation of single phase layered perovskite solid solutions over a wide range of compositions (x=y=0.0, 0.25, 0.50, 0.75 and 1). The lattice parameters and the Curie temperature (T_c) have been found to have linear dependence on x and y. Transmission electron microscopy (TEM) suggest the lowering of orthorhombic distortion with increasing Ba²⁺ substitution. Variations in microstructural features as a function of x and y were monitored by scanning electron microscopy (SEM). The dielectric constant at room temperature increases with increase in both x and y. Interestingly Ba²⁺ substitution on Sr²⁺ site induces diffused phase transition and diffuseness increases with increasing Ba²⁺ concentration.     

Effect of dysprosium substitution on electrical properties of SrBi4Ti4O15

SrBi_4−xDy_xTi₄O₁₅ (with x = 0.02, 0.04, 0.06 and 0.08) powders have been synthesized using the stoichiometric amounts of nitrates and oxides of the constituent materials through sol–gel method. The compound so formed is characterized using X-ray diffraction. The density and lattice parameters are calculated. The impedance and electrical conductivity properties are investigated. The imaginary part of impedance as a function of frequency shows Debye like relaxation. Impedance data presented in the Nyquist plot which is used to identify an equivalent circuit and the fundamental circuit parameters are determined at different temperatures. The results of bulk a.c. conductivity as a function of frequency at different temperatures are presented. The dielectric behavior was investigated. Permittivity was calculated based on the relaxation frequency using an alternative approach based on the variation of the imaginary impedance component as a function of reciprocal angular frequency. The frequency dependence of real and imaginary permittivities was also investigated.     

Structural study of Sr0.3Ba0.7Nb2O6 and La0.030Sr0.255Ba0.700Nb2O6 ceramic systems

Sr_0.3Ba_0.7Nb₂O₆ (SBN) and La_0.030Sr_0.255Ba_0.700Nb₂O₆ (LSBN) ceramic compounds have been prepared using the traditional ceramic method at two different calcination temperatures (900 and 1000 °C) and later sintered both at 1400 °C. A study of the effects of the calcination temperatures and La substitution on the morphological, compositional, and structural properties of SBN and LSBN is presented using scanning electronic microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analysis. From Rietveld refinement processes, the XRD patterns were interpreted to evaluate such effects in the structural parameters and the site occupation factors of the heavy metals and oxygen atoms. The effect of the incorporation of La resulted in a 0.25% cell contraction and turned out to be higher than the 0.08% dilation effect produced by the increase of calcination temperature. The La ion with similar effective ionic radius and higher electronegativity is incorporated into the structure occupying the A₁ site just like the Sr ions in the SBN compound. Differences in the site occupation factors between the SBN and LSBN samples lead to substantial changes in the physical properties such as temperature of relative dielectric constant maximum, relative dielectric constant, and dielectric loss, correlated with the distortion and the relative orientation of the oxygen octahedra.     

Enhancement of remnant polarization in multilayered Bi4Ti3O12/(Bi3.25La0.75)Ti3O12 films obtained by chemical solution deposition

Enhancement of remnant polarization was observed in artificially multilayered Bi₄Ti₃O₁₂ (BT)/(Bi_3.25La_0.75)Ti₃O₁₂ (BLT) films_. The multilayer were prepared on platinum coated silicon substrate by chemical solution deposition and compared with the single-phase BT, BLT and (Bi_3.5La_0.5)Ti₃O₁₂ films_. The multilayered film with a stacking periodicity of 60 nm BLT/30 nm BT shows a remnant polarization (2Pr) of about 61 μC/cm², which is much higher than those of the single-phase films. In addition, the multilayered films show a good fatigue-endurance character. After post-annealing the multilayered films at 700 °C for a long time (20 h), its remnant polarization decreased to a value close to the corresponding uniform film. Some possible mechanisms behind the polarization enhancement were proposed.     

Effects of lanthanum modification on electrical and dielectric properties of Pb(Zr0.70,Ti0.30)O3 ceramics

Lanthanum modified lead zirconate titanate ceramics with lanthanum content changing from 7.6 to 9.0 at.% La and a Zr/Ti ratio of 70/30 (PLZT 100x/70/30) have been prepared by conventional high temperature solid-state reaction technique. The studies of the ferroelectric, electromechanical and dielectric properties of the ceramics were carried out. The results showed the enhanced antiferroelectricity stability when the lanthanum content increases. The polarization and strain decreased with the increasing La content. The dielectric spectra of all the PLZT samples show a dispersive behavior. With increasing La concentration, the maximum dielectric constants ε_m and the transition temperatures T_m were reduced.     

Preparation, characterization and microwave absorption properties of polyaniline/Co0.5Zn0.5Fe2O4 nanocomposite

The polyaniline (PAni)/Co_0.5Zn_0.5Fe₂O₄ nanocomposite was prepared by an in situ polymerization in an aqueous solution. The products were characterized by Fourier transform infrared (FT-IR) spectrometer, ultraviolet-visible (UV-vis) spectrometer, X-ray diffraction (XRD) and transmission electron microscope (TEM). The average particle size of the PAni/Co_0.5Zn_0.5Fe₂O₄ was estimated to be about 70 nm by TEM. The reflection loss (dB) of the nanocomposite was measured at different microwave frequencies in X-band (8.2-12.4 GHz), U-band (12.4-18 GHz) and K-band (18-26.5 GHz) by radar cross-section (RCS) method according to the national standard GJB-2038-94. The results showed the reflection loss of the PAni/Co_0.5Zn_0.5Fe₂O₄ nanocomposite was higher than that of the PAni. The maximum reflection loss of the PAni/Co_0.5Zn_0.5Fe₂O₄ nanocomposite was about −39.9 dB at 22.4 GHz with a bandwidth of 5 GHz (full frequency width at about a half of the peak response). In conclusion, this sample is a good microwave shielding and absorbing materials at higher frequency.     

Synthesis and electrical properties of sodium bismuth niobate Na0.5Bi2.5Nb2O9

The crystal structure of Na_0.5Bi_2.5Nb₂O₉ was refined from its powder X-ray diffraction (XRD) data. Sodium bismuth niobate at 23 °C is orthorhombic, A2₁am, with a=5.4998(3) Å, b=5.4602(2) Å, c=24.952(1) Å, and Z=4. The piezoelectric properties were investigated using the dense bulk ceramics. The electromechanical coupling coefficients (k_ij) and electrical quality factors (Q_m) are k₃₁=3.2%, k_t=10.0%, and Q_m=3800. Single crystals were also grown from stoichiometric melts using a slow cooling technique. The dielectric and ferroelectric properties of the crystal indicates that a spontaneous polarization clearly occurs in the crystallographic a-b plane.     

Effect of postdeposition annealing on the structural and electrical properties of thin Dy2TiO5 dielectrics

This paper describes the effect of postdeposition annealing on the structural and electrical characteristics of high-k Dy₂TiO₅ dielectric films deposited on Si (100) through reactive cosputtering. We used X-ray diffraction, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and atomic force microscopy to investigate the structural and morphological features of these films after they had been subjected to annealing at different temperatures. The Dy₂TiO₅ dielectrics annealed at 800 °C exhibited excellent electrical properties such as high capacitance value, small density of interface state, almost no hysteresis voltage, and low leakage current. This phenomenon is attributed to a rather well-crystallized Dy₂TiO₅ structure and the reduction of the interfacial layer at oxide/Si interface. This film also shows almost negligible charge trapping under high constant voltage stress.     

Structure and dielectric performance of K-doped (Pb0.5Ba0.5)ZrO3 thin films

In the present investigation (Pb_0.5Ba_0.5)ZrO₃ (PBZ) thin films doped by K (KPBZ) from 0 to 5 mol% were successfully deposited on Pt-buffered silicon substrates by a sol-gel method. The K content dependence of microstructure and electrical properties of KPBZ thin films were studied in detail. It was found that, although all the films displayed a pure perovskite structure without obvious difference, the surface roughness of KPBZ films was decreased with increasing K content. Dielectric measurements showed that the figure of merit (FOM) values of KPBZ thin films were greatly increased by K-doping, and at the same time that the temperature-dependent stability was also improved. Thus, K doping is a promising way to optimize the overall electrical properties of PBZ thin films for potential application in tunable devices.     

Oxygen ionic conduction in brownmillerite CaAl0.5Fe0.5O2.5+δ

The oxygen permeability of CaAl_0.5Fe_0.5O_2.5+δ brownmillerite membranes at 1123-1273 K was found to be limited by the bulk ionic conduction, with an activation energy of 170 kJ/mol. The ion transference numbers in air are in the range 2×10⁻³ to 5×10⁻³. The analysis of structural parameters showed that the ionic transport in the CaAl_0.5Fe_0.5O_2.5+δ lattice is essentially along the c axis. The largest ion-migration channels are found in the perovskite-type layers formed by iron-oxygen octahedra, though diffusion in tetrahedral layers of the brownmillerite structure is also possible. Heating up to 700-800 K in air leads to losses of hyperstoichiometric oxygen, accompanied with a drastic expansion and, probably, partial disordering of the CaAl_0.5Fe_0.5O_2.5+δ lattice. The average thermal expansion coefficients of CaAl_0.5Fe_0.5O_2.5+δ ceramics in air are 16.7×10⁻⁶ and 12.6×10⁻⁶ K⁻¹ at 370-850 and 930-1300 K, respectively.     

Dielectric and piezoelectric properties of Y2O3 doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free piezoelectric ceramics

Y₂O₃ doped lead-free piezoelectric ceramics (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (0-0.7 wt%) were synthesized by the conventional solid state reaction method, and the effect of Y₂O₃ addition on the structure and electrical properties was investigated. X-ray diffraction shows that Y₂O₃ diffuses into the lattice of (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ to form a solid solution with a pure perovskite structure. The temperature dependence of dielectric constant of Y₂O₃ doped samples under various frequencies indicates obvious relaxor characteristics different from typical relaxor ferroelectric and the mechanism of the relaxor behavior was discussed. The optimum piezoelectric properties of piezoelectric constant d₃₃ = 137 pC/N and the electromechanical coupling factor k_p = 0.30 are obtained at 0.5% and 0.1% Y₂O₃ addition, respectively.     

Enhancement of Sinter Densification of SrO-BaO-Nb2O5-SiO2 Tungsten-Bronze Glass-Ceramics by Doping with P2O5

Densification behaviors of SrO—BaO—Nb₂0₅—SiO₂ based glass—ceramics prepared by conventional sintering were investigated with an emphasis on the influence of P₂O₅ content.Although P₂0₅ dopant did not modify the surface crystallization mechanism,it resulted in a decrease of the glass transition temperature,which facilitates the viscous glass flow necessary for sintering.However,premature crystallization of（Sr,Ba）Nb₂O₆ induced by addition of excess amount of P₂0₅ essentially retarded sintering due to the formation of closed pores in the matrix.The SrO-BaO-Nb₂0₅-Si0₂ glass with 1.0 mol%P₂0₅（SBN-1P） showed the best sinter densification,which was accomplished at about 850℃.     

Synthesis and field effect characteristics of Na2Ti3O7 nanowires

Na₂Ti₃O₇ nanowires with diameters of about 80-130 nm and lengths up to several tens of micrometers are synthesized via a simple hydrothermal method and characterized by the field-emission scanning electron microscopy and X-ray diffraction. Back-gate field-effect transistors based on these nanowires are fabricated on indium tin oxide glass substrates with polymethyl-methacrylate-co-glyciclyl-methacrylate as the gate insulator layers. Typical p-type semiconductor material properties are observed in our investigations. The field-effect mobility is about 0.1 cm²/Vs. The capacitance per unit area of the dielectric is 3.43 nF/cm² (dielectric constant, k = 3.9). The on/off ratio is around 10³ at the conduction of 10 V.     

Microstructure and electrical conductivity of LaNi0.6Fe0.4O3 prepared by combustion synthesis routes

The continuing development of new materials suitable for solid oxide fuel cells operating at about 650-800 °C is of great interest in recent days. The present investigation deals with the development of a perovskite composition-LaNi_0.6Fe_0.4O₃ (LNF)-prepared following two combustion synthesis routes: citrate-gel (LNC) and urea (LNU). The powders were sintered over a wide temperature range (900-1400 °C) and sintering behavior for LNC and LNU was compared. The thermal expansion coefficient (TEC), electrical and microstructural characteristics of LNF was thoroughly investigated. Electrical conductivities were found to be one and a half times higher than that of most commonly used cathode material, La(Sr)MnO₃. Moreover, the TEC value of LNF was found to be ≈11.4×10⁻⁶ K⁻¹ at 800 °C. The study opens up a possibility of using LNF as a promising cell component for SOFC.     

Dielectric properties of the spin-1/2 dimer compounds Ba3Cr2O8 and Sr3Cr2O8

We have investigated the dielectric and magnetic properties of the spin-1/2 compounds Ba₃Cr₂O₈ and Sr₃Cr₂O₈. For Ba₃Cr₂O₈, the real part dielectric constant ε′(T) exhibit a frequency-independent peak near the Jahn-Teller transition temperature T_JT = 70 K. However, no anomaly in the ε′(T) curve is observed near T_JT = 275 K for the isostructural compound Sr₃Cr₂O₈. The difference in the ε′(T) behaviors may be attributed to the different onset temperatures of the phonon mode splitting in the samples. Dielectric relaxation analysis revealed that extrinsic contribution due to Maxwell–Wagner effect dominated at high temperatures for both compounds. The magnetization data reveal the occurrence of antiferromagnetic interactions between the Cr⁵⁺ ions and the χ(T) curves can be described by the Bleaney–Bowers equation for both compounds.     

Synthesis and characterizations of Nd doped SrFe12O19 nanoparticles

This is the first report ever on Nd³⁺ doped M-type hexaferrite nanoparticles: SrNd_xFe_12−xO₁₉ (0 ≤ x ≤ 1) prepared by citrate precursor using the sol–gel technique followed by gel to crystallization. The influence of the Nd³⁺ substitution, Fe³⁺/Sr²⁺ molar ratio and the calcination temperature on the crystallization of ferrite phase have been examined using powder X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), inductance capacitance resistance meter bridge (LCR) and vibrating sample magnetometer (VSM). The structural analysis reveals that the Nd³⁺ ions rearrange themselves in the host lattice without disturbing the parent lattice and Fe³⁺/Sr²⁺ molar ratio less than 12 is more favorable to achieve single phase hexaferrite at calcination temperature 900 °C for 4 h. Mid-IR analysis confirms that Nd³⁺ occupies the octahedral site. Detailed studies of electrical properties of prepared materials have been investigated in the frequency range 100–1000 Hz at room temperature by LCR meter and two probe technique. The result shows that the electrical properties strongly depend upon the frequency of applied field and dopant concentration. The magnetic measurements showing a considerable improvement in coercivity with the substitution of Nd³⁺ on iron sites, while the unsubstituted hexaferrites have highest value of specific saturation magnetization.     

Enhancement on effective piezoelectric coefficient of Bi3.25Eu0.75Ti3O12 ferroelectric thin films under moderate annealing temperature

Bi_3.25Eu_0.75Ti₃O₁₂ (BET) thin films were deposited on Pt/Ti/SiO₂/Si(111) substrates by a metal-organic decomposition method. The effects of annealing temperatures 600-800 °C on microstructure, ferroelectric, dielectric and piezoelectric properties of BET thin films were studied in detail. The spontaneous polarization (87.4 × 10^− 6 C/cm² under 300 kV/cm), remnant polarization (65.7 × 10^− 6 C/cm² under 300 kV/cm), the dielectric constant (992.9 at 100 kHz) and the effective piezoelectric coefficient d₃₃ (67.3 pm/V under 260 kV/cm) of BET thin film annealed at 700 °C are better than those of the others. The mechanisms concerning the dependence of the enhancement d₃₃ are discussed according to the phenomenological equation, and the improved piezoelectric performance could make the BET thin film a promising candidate for piezoelectric thin film devices.