Preparation and microwave absorption properties of BiFeO<Subscript>3</Subscript> and BiFeO<Subscript>3</Subscript>/PANI composites

BiFeO₃(BFO) particle was successfully synthesized by normal citric acid sol–gel method and the size of BiFeO₃ particle is about 200 nm. BiFeO₃/polyaniline (PANI) composites with the different weight ratio were synthesized by in situ emulsion polymerization. The citric acid doped PANI is fibrous and form a loose structure outside the BFO particle. With the increasing of PANI, the conductivity value of composites are increasing to 9.34?×?10^?2 S/cm. Moreover, the permittivity also enhance with the increasing of conductivity, which contribute to the improvement of dielectric loss. Microwave absorbing properties were investigated with a vector network analyzer in 1–18 GHz. The minimum reflection loss (RL) value is about ?40.2 dB at 8.3 GHz when the thickness is 3.5 mm, and the maximum bandwidth less than ?10 dB is 3.5 GHz (from 13.5 to 18 GHz) at the thickness of 2 mm. 3 mm millimeter-wave-attenuation properties were also tested, and the maximum attenuation value of BFO/PANI composites reach 15.71 dB. The composites can dissipate microwave energy into heat effectively by dielectric relaxation because of the suitable conductivity. The interface scattering and multiple reflections also play a important role because of the increasing of a loose structure. The BFO/PANI composite can be taken as a promising lightweight and multiband microwave absorber.     

Synthesis and dielectric properties of BiFeO<Subscript>3</Subscript> derived from molten salt method

BiFeO₃ powder was synthesized in NaCl media at temperature range from 700 to 800 °C, using Bi₂O₃ and Fe₂O₃ as raw materials. Effects of calcining temperature and salt ratios on the synthesis of BiFeO₃ powder had been investigated. It was found that NaCl effectively promoted the formation of BiFeO₃. Almost pure BiFeO₃ phase with a very small amount of Bi₂Fe₄O₉ phase was synthesized at 750 °C with salt weight ratios of 1:1. A large amount of BiFeO₃ phase decomposed to Bi₂Fe₄O₉ and Bi₂₅FeO₃₉ phase when the temperature was up to 800 °C. In the present method, the calcining temperature played an important role in the formation of BiFeO₃ phase. BiFeO₃ ceramics derived from molten salt method were prepared and exhibited the higher dielectric constant.     

Effects of addition of BiFeO<Subscript>3</Subscript> on phase transition and dielectric properties of BaTiO<Subscript>3</Subscript> ceramics

Samples of xBiFeO₃–(1 − x)BaTiO₃ (x = 0, 0.02, 0.04, 0.06, 0.07 and 0.08) were synthesized by solid state reaction technique and sintered in air in the temperature range 1,220–1,280 °C for 4 h. X-ray diffraction data showed that 2–8 mol% BiFeO₃ can dissolve into the lattice of BaTiO₃ and form single perovskite phase. The crystal structure changes from tetragonal to cubic phase at room temperature when 8 mol% of BiFeO₃ was added into BaTiO₃. Scanning electron microscope images indicated that the ceramics have compact and uniform microstructures, and the grain size of the ceramics decreases with the increase of BiFeO₃ content. Dielectric constants were measured as functions of temperatures (25–200 °C). With rising addition of BiFeO₃, the Curie temperature decreases. For the sample with x = 0.08, the phase transition occurred below room temperature. The boundary between tetragonal and cubic phase of the BiFeO₃–BaTiO₃ system at room temperature locates at a composition between 7 and 8 mol% of BiFeO₃. The diffusivity parameter γ for compositions x = 0.02 and x = 0.07 is 1.21 and 1.29, respectively. The relaxor-like behaviour is enhanced by the BiFeO₃ addition.     

Structural,Optical and Multiferroic Properties of (Nd,Zn)-Co-doped BiFeO<Subscript>3</Subscript> Nanoparticles

The pure BiFeO₃ (BFO) and (Nd, Zn)-co-doped BFO nanoparticles were prepared by a sol-gel method. The crystal structure and optical and multiferroic properties of the samples were systematically investigated. Rietveld refinement showed that the samples crystallized in rhombohedral R3c structure. In UV-visible diffuse absorption spectra, an apparent blue shift can be observed after co-doping, which indicates a possible application in photocatalyst and photoconductive devices. Compared with a pure BFO sample, the leakage current density of the x = 0.05 sample decreases about 3 orders of magnitude. The remanent magnetization (M _r) of the x = 0.10 sample reaches 0.105 emu/g while the coercive field (H _c) is as high as 7.0 kOe. The (Nd, Zn) co-doping into BFO nanoparticles has been proved to be an effective way to improve the optical and multiferroic properties.     

Structural,dielectric and electrical properties of Ca modified BaSn<Subscript>0.15</Subscript>Ti<Subscript>0.85</Subscript>O<Subscript>3</Subscript> Ceramics

Polycrystalline samples of (Ba_1-xCa_x)(Sn_0.15Ti_0.85)O₃ (x = 0, 0.03, 0.06 and 0.09) of perovskite structural family have been prepared by a high-temperature solid-state reaction technique. Preliminary room temperature X-ray study confirmed the formation of single-phase compound with tetragonal structure. The Scanning electron microscope shows the uniform distribution of the grains throughout the surface of the samples. Dielectric studies as a function of frequency at different temperature suggests that the compounds undergo diffuse phase transition (DPT). The d.c. and a.c. conductivities have been investigated over a wide range of temperature and the activation energy was calculated.     

Structural,dielectric and electrical properties of CaBa<Subscript>4</Subscript>SmTi<Subscript>3</Subscript>Nb<Subscript>7</Subscript>O<Subscript>30</Subscript> ferroelectric ceramic

The polycrystalline sample of CaBa₄SmTi₃Nb₇O₃₀, a member of tungsten bronze family, was prepared by solid-state reaction method. X-ray diffraction analysis shows the formation of single-phase compound with an orthorhombic structure at room temperature. Scanning electron micrograph of the material shows uniform distribution of grains. Detailed studies of dielectric properties of the compound as a function of temperature at different frequencies suggest that the compound has a dielectric anomaly of ferroelectric to paraelectric type at 198°C, and exhibits non-relaxor kind of diffuse phase transition. The ferroelectric nature of the compound has been confirmed by recording polarization-electric field hysteresis loop. Piezoelectric and pyroelectric studies of the compound have been discussed in this paper. Electrical properties of the material have been analyzed using complex impedance technique. The Nyquist plots manifest the contribution of grain boundaries (at higher temperature), in addition to granular contribution (at all temperatures) to the overall impedance. The temperature dependence of dc conductivity suggests that the compound has negative temperature coefficient of resistance (NTCR) behaviour. The frequency dependence of ac conductivity is found to obey Jonscher’s universal power law. The observed properties have been compared with calcium free Ba₅SmTi₃Nb₇O₃₀ compound.     

Phase,microstructural characterization and dielectric properties of Ca-substituted Sr<Subscript>5</Subscript>Nb<Subscript>4</Subscript>TiO<Subscript>17</Subscript> ceramics

The effect of Ca substitution for Sr on the phase, microstructure and microwave dielectric properties of the Sr_5−x Ca _x Nb₄TiO₁₇ composition series was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), an LCR meter, and vector network analyzer. Below 1450 °C, Sr_5−x Ca _x Nb₄TiO₁₇ (x = 1, 2, 3, or 4) compositions formed single-phase Sr₄CaNb₄TiO₁₇, Sr₃Ca₂Nb₄TiO₁₇, Sr₂Ca₃Nb₄TiO₁₇, and SrCa₄Nb₄TiO₁₇ ceramics, respectively. At x = 0 and 5, Sr₅Nb₄TiO₁₇ and Ca₅Nb₄TiO₁₇ formed, but along with Sr₂Nb₂O₇ (at x = 0) and CaNbO₃ and CaNb₂O₆ (at x = 5) secondary phases. Above 1450 °C, all the compositions formed two-phase ceramics. At low frequencies, a phase transition was observed in the composition Sr₅Nb₄TiO₁₇. The substitution of Ca for Sr enabled processing of highly dense Sr₂Ca₃Nb₄TiO₁₇, with ε_r ~ 53.4, τ_f ~ −6.5 ppm/°C and Q _u  × f _o  ~ 1166 GHz. Further investigations are required to improve the quality factor of these ceramics for possible microwave applications.     

Structural and dielectric properties of Bi doped Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>TiO<Subscript>3</Subscript> ceramics

The dielectric properties of 0.1–15% mol bismuth doped Ba_0.6Sr_0.4TiO₃ (BST) ceramics have been investigated systematically. The solubility limit of bismuth is determined as about 10 mol% by means of both X-ray diffraction and scanning electron microscopy, which is further verified by the fact that the lattice constant of the samples above 10 mol% is almost invariable. The temperature dependence of the dielectric permittivity suggest that the ferroelectric behavior transit to relaxor ferroelectric type when impurity concentration reaches 5 mol%, and further to relaxor behavior for samples above 10 mol% Bi content, which is verified by the absence of a hysteresis loop. Thermal expansion results show differences between 5 and 10 mol% doped samples. Dielectric tunability at room temperature decreases with bismuth content increasing. The variation of properties was attributed to the impurity induced polar regions and former long-order structure.     

The synergetic electromagnetic properties and enhanced microwave absorption of BiFeO<Subscript>3</Subscript>/BaFe<Subscript>7</Subscript>(MnTi)<Subscript>2.5</Subscript>O<Subscript>19</Subscript> composite

The effective microwave absorption materials could contribute to alleviating the electromagnetic wave pollution. However, conventional microwave absorption materials usually suffer from insufficient absorption intensity and the narrow effective absorption bandwidth. Herein, BiFeO₃/BaFe₇(MnTi)_2.5O₁₉ composites are proposed to address these issues through offering synergetic electromagnetic properties and proper electromagnetic properties. BiFeO₃ combined with BaFe₇(MnTi)_2.5O₁₉ exhibits dielectric multiple relaxation behaviors, strong ferromagnetic resonance and electromagnetic matching, ensuring increased multi-band microwave absorption. Accordingly, the minimum reflection loss (RL) of the composite with volume ratio of 1.5:1 reaches ??48 dB, and the bandwidth less than ??10 dB covers multi-frequencies at C, X and Ku band. These results suggest that BiFeO₃/BaFe₇(MnTi)_2.5O₁₉ composite could be a promising microwave absorption material in imaging, healthcare, information safety and military fields.     

Magneto-electric characteristics in (Mn,Cu) co-doped BiFeO<Subscript>3</Subscript> multiferroic nanoparticles

Multiferroic nanoparticles having general formula BiFe_0.99-xMn_xCu_0.01O₃ (x = 0, 0.01, 0.02, 0.03 & 0.04) were prepared by a chemically derived method to explore the magneto-electric characteristics of this new class of materials. X-ray diffraction confirmed that all the samples had rhombohedraly distorted cubic perovskite 3D lattice. Lattice constant was increased with increasing concentration of Mn. Micrographs obtained from a field emission scanning electron microscope revealed a fine distribution of well-shaped particles while the particle size was increased with increased contents of Mn. Enhanced hopping mechanism induced by substitution of Mn at the lattice sites of Fe resulted in an increased AC conductivity. Ferroelectricity was observed to increase with increased Mn, attributed mainly to the leakage current due to free charge carriers instigated by multiple oxidation states of Fe and Mn. It has been observed that antiferromagnetic bismuth ferrite begins to show ferromagnetic behavior due to the collapse of antiferromagnetic spin structure with increased Mn contents.     

Structural,optical spectroscopy,optical conductivity and dielectric properties of BaTi<Subscript>0.5</Subscript>(Fe<Subscript>0.33</Subscript>W<Subscript>0.17</Subscript>)O<Subscript>3</Subscript> perovskite ceramic

Fe and W co-substituted BaTiO₃ perovskite ceramics, compositional formula BaTi_0.5(Fe_0.33W_0.17)O₃, were synthesized by the standard solid-state reaction method and studied by X-ray diffraction, scanning electron microscopy and spectroscopy ellipsometry. The prepared sample remains as double phases with the perovskite structure. The structure refinement of BaTi_0.5(Fe_0.33W_0.17)O ₃ sample was performed in the cubic double and hexagonal setting of the \(\text {Fm}\boldsymbol {\bar {{3}}\mathrm {m}}\) and P6 ₃ /mmc space groups. Spectral dependence of optical parameters; real and imaginary parts of the dielectric function, refractive index, extinction coefficient and absorption coefficient were carried out in the range between 1.4 and 4.96 eV by using the ellipsometry experiments. Direct bandgap energy of 4.36 eV was found from the analysis of absorption coefficient vs. photon energy. In addition, the oscillator energy, dispersion energy and zero-frequency refractive index values were found from the analysis of the experimental data using Wemple–DiDomenico single-effective-oscillator model.     

Structural and dielectric properties of PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>-PbZrO<Subscript>3</Subscript> solid solutions synthesized at high pressures and temperatures

(1 ? x)PbMg_1/3Nb_2/3O₃ · xPbZrO₃ (1 ? x)PMN · xPZ) solid solutions have been synthesized at a pressure of 5 GPa and temperatures from 1300 to 1700 K, and their structural and dielectric properties have been studied. The composition dependences of the average unit-cell parameter and dielectric permittivity for the solid solutions indicate that the PMN-PZ system has a morphotropic phase boundary near x = 0.65. The solid solutions have a cubic structure for x < 0.65, a rhombohedral structure in the range 0.65 < x < 0.9, and an orthorhombic structure (similar to that of PbZrO₃) for x > 0.9. The temperature and frequency dependences of dielectric permittivity suggest that the (1 ? x)PMN · xPZ samples with x < 0.65 consist of two ferroelectric phases: a relaxor with antipolar dipole order and a normal ferroelectric with a diffuse phase transition. The effect of annealing temperature on the ferroelectric state of the samples with x < 0.65 is examined. In the composition range 0.65 < x < 0.9, the samples have normal ferroelectric properties, independent of annealing temperature.     

Enhancement of dielectric and magnetic properties in phase pure,dense BiFeO<Subscript>3</Subscript> nanoceramics synthesized by spark plasma sintering techniques

Phase pure, dense BiFeO₃ (BFO) ceramics with average grain sizes of ~?110 nm, ~?450 nm, and ~?1.15 µm were fabricated by spark plasma sintering method. BFO ceramics exhibited grain-size-dependent magnetic properties, which ascribed to the antiferromagnetism–ferromagnetism (AFM–FM) transition. For BFO nanoceramics (~?110 nm), such transition was much significant, and contributed to a large exchange bias field of H_EB?=?500 Oe at 5 K. In addition, BFO nanoceramics (~?110 nm) exhibited lower leakage current and higher resistivity compared to the larger-grained BFO ceramics (~?450 nm and ~?1.15 µm). The calculated activation energies (E_a) and X-ray photoelectron spectroscopy analyses revealed the existence of different types of defects in BFO ceramics with different grain sizes.     

Structure,dielectric and ferroelectric properties of lead free (K,Na)(Nb)O<Subscript>3</Subscript>-xBiErO<Subscript>3</Subscript> piezoelectric ceramics

The effect of BiErO₃ (BE) as a doping material on the structural, dielectric and ferroelectric properties of (KNa)NbO₃ ceramics was explored in this research. Co-existence of two phase regions was confirmed in the composition range at x?=?0.5% and x?=?1.0%. The addition of BE content led to a decrease of the grain size and the ceramics became denser. Bulk P–E hysteresis loops were obtained with a maximum polarization of P _max = 30.56 µC/cm² and a remnant polarization of P _r = 25.10 µC/cm², along with a coercive field of E _c  ~ 11.26 kV/cm. The results revealed that a field strain value of ~?0.26 for x?=?0.5% of BE substitution was attained. This presents outstanding piezoelectric and dielectric properties.     

Skyrmion lattices in the BiFeO<Subscript>3</Subscript> multiferroic

This work examines conditions for skyrmion lattice stability in BiFeO₃ multiferroic films possessing record high ferroelectric and antiferromagnetic transition temperatures, giant polarization, and a giant magnetoelectric effect. Using analytical and numerical calculations, we demonstrate stability of solitary spin vortices (skyrmions) and skyrmion lattices in BiFeO₃ films owing to the Dzyaloshinskii-Moriya interaction. Our results confirm that BiFeO₃ can be used as a matrix for chips with ultrahigh data density, up to 10 Tb/cm².     

Dynamical Magnetoelectric Coupling in Multiferroic BiFeO<Subscript>3</Subscript>

In this study, the magnon excitations in multiferroic BiFeO₃ (BFO) have been discussed. The studies are based on the spin wave theory and Katsura’s model. The influence of the spin wave excitations on the terahertz absorption is discussed. The antiferromagnetic and ferroelectric interactions in multiferroic BFO were included using an effective fermion Hamiltonian. This Hamiltonian is bosonized and diagonalized, using Holstein–Primakoff and Bogoliubov transformations, respectively. An effective boson Hamiltonian is diagonalized to determine the excitation energy of the spin wave. The results obtained in this study are in qualitative agreement with the experimental data.     

Effect of Pb(Ti,Sn)O<Subscript>3</Subscript> on the dielectric properties of high dielectric constant X8R BaTiO<Subscript>3</Subscript>-based ceramics

The high dielectric constant X8R dielectric materials could be sintered at 1,240 °C by doping 2.5 mol% Pb(Ti,Sn)O₃ additives into the BaTiO₃ ceramics, with a dielectric constant greater than 3,400 at 25 °C, dielectric loss lower than 2.0% and temperature coefficient of capacitance (TCC) less than ±15% from −55 to 150 °C, which satisfied X8R specification. The effects of Pb(Ti,Sn)O₃ on the microstructure and dielectric properties of BaTiO₃-based ceramics were investigated. Doped with Pb(Ti,Sn)O₃ additives, the partial solid solution was formed between Pb(Ti,Sn)O₃ and BaTiO₃. Due to the high Curie point of Pb(Ti,Sn)O₃, the Curie point of the ceramics was markedly shifted to higher temperature about 150 °C, and the temperature coefficient of capacitance curves was flattened. The increase of the tetragonality (c/a ratio) and the fine microstructure were resulted in the increase of dielectric constant. With Pb(Ti, Sn)O₃ content up to 3 mol%, the depression of Ti⁴⁺’s polarization and the decrease of the tetragonality (c/a ratio) were resulted in the decrease of dielectric constant.     

Nanostructure of CaMgSi<Subscript>2</Subscript>O<Subscript>6</Subscript> (Diopside) and CaTiO<Subscript>3</Subscript> (Perovskite) mechanically activated in carbon dioxide

The micro- and nanostructure of materials resulting from mechanochemical interactions of natural diopside (CaMgSi₂O₆) and synthetic perovskite (CaTiO₃) with CO₂ have been studied by transmission electron microscopy (TEM) and high-resolution TEM. The results indicate that CO₂ absorption is accompanied by CO₂ “dissolution” in the form of CO ₃ ^2? ions in the structurally disordered silicate or titanate matrix. The diopside activation product is a quasi-homogeneous amorphous carbonate-silicate phase. The mechanically activated perovskite is a nanocomposite consisting of CaTiO₃ nanocrystals embedded in a carbonated amorphous titanate matrix.