Microwave absorption properties of multiferroic BiFeO3 nanoparticles

Multiferroic BiFeO₃ (BFO) nanoparticles ranging from 60 nm to 120 nm were synthesized successfully by a sol-gel method, and the microwave absorption properties of BFO nanoparticles were investigated in the range of 12.4 GHz to 18 GHz. The reflection loss of BFO nanoparticles is more than 10 dB (or more than 90%) in the 13.1 GHz-18 GHz range and reaches to 26 dB at 16.3 GHz, which indicated that the BFO is a good candidate for microwave absorption application. The excellent microwave absorption properties of BFO nanoparticles could be attributed to the good electromagnetic match as a consequence of the coexistence of ferroelectric and weak ferromagnetic order in BFO nanoparticles, which has been confirmed by electric and magnetic measurement. Moreover, the nanosize-confinement effect may also have contribution to the high reflection loss of BFO nanoparticles.     

Dielectric properties and heating effect of multiferroic BiFeO3 suspension

Bismuth ferrite (BiFeO₃) ceramic particles of micro-meter size were prepared by a solid-state route. This study revealed an appropriate method to measure the dielectric properties of BiFeO₃ particle suspension using a system designed by ourselves. The heating effect of the BiFeO₃ suspension with applied AC voltage was confirmed. Application of this material in hyperthermia treatment of biological tissues with the goal of tumor therapy may be possible.     

Hydrogen-induced degradation in multiferroic BiFeO3 ceramics

Chemical stability of multiferroic BiFeO₃ (BFO) ceramics has been studied through electrochemical hydrogen charging, in which BFO ceramic pellets were placed in 0.01 M NaOH solution with hydrogen deposited on their electrodes from the electrolysis of water. The properties of the samples were greatly degraded after the treatment: The leakage current was increased by orders of magnitude, the capacitance and the dielectric loss were dramatically increased in low frequency region. It was proposed that hydrogen entered into BFO lattice and existed at interstitial sites and the degradation was explained by hydrogen's reduction of some Fe³⁺ to Fe²⁺ in BFO. Hydrogen-induced degradation occurs relatively easily in BFO and much attention should be paid to it for BFO-based devices.     

The synthesis of lead-free ferroelectric Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3 thin films by sol-gel method

(1 − x)Bi_0.5Na_0.5TiO₃-xBi_0.5K_0.5TiO₃ [BNT-BKT-100x] thin films have been successfully deposited on Pt/Ti/SiO₂/Si substrates by a sol-gel process together with rapid thermal annealing. A morphotropic phase boundary (MPB) between Bi_0.5Na_0.5TiO₃ and Bi_0.5K_0.5TiO₃ was determined around x ∼ 0.15. Near the MPB, the film exhibits the largest grain size, the highest ε value (360) and the largest P_r value (13.8 μC/cm²). The BNT-BKT thin film system is expected to be a new and promising candidate for lead-free piezoelectric applications.     

Structural, dielectric, magnetic, magnetodielectric and impedance spectroscopic studies of multiferroic BiFeO3-BaTiO3 ceramics

Polycrystalline (1−x)BiFeO₃-xBaTiO₃ (x = 0.00, 0.10, 0.20 and 0.30) ceramics have been prepared via mixed oxide route. The effect of BaTiO₃ substitution on the dielectric, ferroelectric and magnetic properties of the BiFeO₃ multiferroic perovskite was studied. From XRD analysis it revealed that BaTiO₃ substitution does not affect the crystal structure of the (1−x)BiFeO₃-xBaTiO₃ system up to x = 0.30. Improved dielectric properties were observed in the prepared system. An anomaly in the dielectric constant (?) was observed in the vicinity of the antiferromagnetic transition temperature. Experimental results suggest that in the (1−x)BiFeO₃-xBaTiO₃ system, the increase of BaTiO₃ concentration leads to the effective suppression of the spiral spin structure of BiFeO₃, resulting in the appearance of net magnetization. The dependence of dielectric constant and loss tangent on the magnetic field is a evidence of magnetoelectric coupling in (1−x)BiFeO₃-xBaTiO₃ system. The impedance analysis suggests the presence of a temperature dependent electrical relaxation process in the material, which is almost similar for all the concentrations in the present studies. The electrical conductivity has been observed to increase with rise in temperature showing a typical negative temperature coefficient of the resistance (NTCR) behaviors analogous to a semiconductor and suggests a non-Debye type of electrical relaxation.     

Electrical and luminescence properties of Er-doped Bi0.5Na0.5TiO3 ceramics

The influences of Er content on the dielectric and photoluminescence performances of Bi_0.5Na_0.5TiO₃-xEr (x = 0, 0.005, 0.01, 0.015, 0.02, 0.03) ceramics have been investigated. The results show that Bi_0.5Na_0.5TiO₃-xEr ceramics with x = 0.01 Er have maximum values of photoluminescence and piezoelectric properties. A bright green emission at 550 nm and enhanced piezoelectric response are achieved in the ceramic Bi_0.5Na_0.5TiO₃-0.01Er at room temperature. Furthermore, the photoluminescence performance of the ceramics is significantly enhanced by electric poling.     

Giant strain in Nb-doped Bi0.5(Na0.82K0.18)0.5TiO3 lead-free electromechanical ceramics

The effect of Nb substitution on the crystal structure, ferroelectric, and electric field induced strain properties of Bi_0.5(Na₈₂K_0.18)_0.5TiO₃ (BNKT) ceramics has been investigated. The coexistence of rhombohedral and tetragonal phases was found in undoped BNKT ceramics, however, Nb doping induced a phase transition to a pseudocubic phase with high electrostriction coefficients. When 3 mol% Nb was substituted on Ti ions, the electric field induced strain was markedly enhanced up to S_max/E_max = 641 pm/V, which is higher than those previously reported on non-textured lead-free electromechanical ceramics.     

Sol-gel synthesis of lead-free (K0.5Bi0.5)0.4Ba0.6TiO3 nanorods

A kind of lead-free ferroelectric nanorods, (K_0.5Bi_0.5)_0.4Ba_0.6TiO₃, has been prepared by the sol-gel process. The phase formation, structure and morphological analyses of (K_0.5Bi_0.5)_0.4Ba_0.6TiO₃ were investigated by XRD, FTIR, Raman and TEM. The results revealed that single-crystalline (K_0.5Bi_0.5)_0.4Ba_0.6TiO₃ nanorods with width around 80 to 120 nm, and length around 200-300 nm were obtained by calcining dried gels at 800 °C for 2 h. Raman analysis of (K_0.5Bi_0.5)_0.4Ba_0.6TiO₃ nanorods indicated that A₁(TO₂) and E(TO) mode incorporated into one broad peak at around 285 cm^− 1, which can be attributed to the cation disorder (Bi, K, Ba) on the 12-fold coordinated A site of ABO₃ structure.     

Dielectric and ferroelectric properties of A-site non-stoichiometric Na0.5Bi0.5TiO3-based thin films

The A-site non-stoichiometry of [(Na_0.7K_0.2Li_0.1)_0.45Bi_0.55]TiO_3 + x (NKLBT) films were epitaxially deposited on LaNiO₃(100)/Si substrates using metal organic decomposition. The structural evolution of NKLBT films annealed at different temperatures is studied and a single perovskite phase can be found at the low temperature of 600 °C. Ferroelectric hysteresis measurement shows a higher remanent polarization value of 15.6 μC/cm² with a lower coercive field of 89 kV/cm at 450 kV/cm due to the lower concentration of oxygen vacancies by the donor doping effect. The frequency dependence of the capacitance-voltage behavior and the correlation between the capacitance-voltage and polarization-electric field are analyzed. The dielectric constant and dissipation factor are measured to be 690 and 0.04, respectively, at a frequency of 1 MHz.     

Photoconductivity in BiFeO3/La0.7Sr0.3MnO3 heterostructure

In this letter, an oxide heterostructure has been fabricated by successively growing La_0.7Sr_0.3MnO₃ (LSMO) and BiFeO₃ (BFO) layers on LaAlO₃ (100) by pulsed laser deposition. Analysis of the leakage current at different temperature demonstrated that the Poole-Frenkel dominated the leakage current mechanism. Additionally, the BiFeO₃/La_0.7Sr_0.3MnO₃ heterostructure exhibits a positive colossal magnetoresistance (MR) effect over a temperature range of 50-320 K. The maximum MR values are determined to be about 45.32% at H = 0.5 T and 28.34% at H = 0.3 T. At last, we report photoconductivity in BiFeO₃/La_0.7Sr_0.3MnO₃ film under illumination from 160 mW/cm² and 200 mW/cm² green-light source, and photoconductivities increase with the intensity of light enhanced.     

Optimal growth windows of multiferroic BiFeO3 films and characteristics of ferroelectric domain structures

Multiferroic BiFeO₃ films of smooth surface and fully-saturated ferroelectric hysteresis loops have been grown by RF magnetron sputtering. The (001)-oriented epitaxial films showed a large remanent polarisation of 61 µC/cm². A strategy to grow BiFeO₃ films of good ferroelectric property was demonstrated, that was using fast growth rate to achieve accurate stoichiometry for the BiFeO₃ phase and at the same time to avoid the formation of impurity phases associated with the fast growth by accurate control of thermodynamic parameters such as oxygen partial pressure and temperature, as well as proper selection of substrates. Piezoresponse force microscopy revealed fine spontaneous domains for highly resistive epitaxial films, which were switchable under DC biases. For the polycrystalline films of increased density of free carriers, single-domain grains of about 200 nm in diameter were observed due to effective compensation of depolarisation field by free carriers and therefore allowing larger domains.     

Dielectric properties of Ba0.6Sr0.4TiO3-Sr(Ga0.5Ta0.5)O3 solid solutions

Ba_0.6Sr_0.4TiO₃-Sr(Ga_0.5Ta,sub>0.5)O₃ solid solutions are prepared by a solid-state reaction method, and their dielectric and tunable characteristics are investigated. The solid solutions with cubic perovskite structures are obtained for compositions of 10-50 mol% Sr(Ga_0.5Ta_0.5)O₃. It is observed that the addition of Sr(Ga_0.5Ta_0.5)O₃ into Ba_0.6Sr,sub>0.4TiO₃ causes a shift in the phase transition peak to a lower temperature. Ba_0.6Sr_0.4TiO₃-Sr(Ga_0.5Ta,sub>0.5)O₃ solid solutions exhibit depressed and broadened phase transition peaks, resulting in decreased dielectric constants and dielectric losses at room temperature. With the increase of Sr(Ga_0.5Ta_0.5)O₃ content, the dielectric constant, loss tangent, and tunability are decreased. 0.9Ba_0.6Sr_0.4TiO₃-0.1Sr(Ga_0.5Ta,sub>0.5)O₃ has a dielectric constant ϵ = 534 and a tunability of 16% at 100 kHz under 2.63 kV/mm. The dielectric characteristics of Ba_0.6Sr_0.4TiO₃-Sr(Ga_0.5Ta,sub>0.5)O₃ ceramics at microwave frequencies are also evaluated.     

Effect of MnO addition on structure and electrical properties of (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics prepared by citrate method

(Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ ceramics added with 0–0.8 wt.% MnO were prepared by a citrate method, and the influence of the MnO addition on the structure and electrical properties was investigated. The results indicate that the addition of small amounts of MnO did not cause a remarkable change in crystal structure, but resulted in an evident evolution in microstructure. The dielectric constant (_r) and piezoelectric constant (d₃₃) significantly decrease with increasing MnO content, while the electromechanical coupling factor (k_p) presents a slight variation in the range of 0.25–0.28. The dissipation factor (tan δ) and mechanical quality factor (Q_m) attain a minimum value of 1.5% and a maximum value of 304 when adding 0.4 and 0.5 wt.% MnO, respectively. This research demonstrates that doping effect and microstructural evolution contribute cooperatively to the electrical properties of the ceramics.     

Highly enhanced mechanical quality factor in lead-free (K0.5Na0.5)NbO3 piezoelectric ceramics by co-doping with K5.4Cu1.3Ta10O29 and CuO

Effects of Cu doping on the ferroelectric and piezoelectric properties of 0.0038 mol K_5.4Cu_1.3Ta₁₀O₂₉ modified (K_0.5Na_0.5)NbO₃ ceramics have been investigated. On the basis of analyses on crystal structure and polarization hysteresis, it is suggested that Cu ions reveal amphoteric doping behavior in KNN ceramics. At doping levels up to 1 mol%, the Cu ions substitute pentavalent B-site cations, acting as acceptors that generate O-vacancies to resultantly harden the ceramics. At doping levels above 1.5 mol%, however, Cu ions play a role as donors by replacing monovalent A-site cations. A specimen doped with 0.5 mol% CuO shows an extremely high mechanical quality factor of 3053, which is higher than those of any other reports on KNN-based ceramics.     

Dielectric and piezoelectric properties of LiSbO3 doped 0.995 K0.5Na0.5NbO3-0.005BiFeO3 piezoelectric ceramics

LiSbO₃ doped and undoped 0.995 K_0.5Na_0.5NbO₃-0.005BiFeO₃ piezoelectric ceramics with high properties have been fabricated in air by the conventional ceramic processing. By adding LiSbO₃ to K_0.5Na_0.5NbO₃-BiFeO₃ ceramics, the dielectric and piezoelectric properties evidently increase. The doped ceramics exhibit good electrical properties. The enhanced piezoelectric properties of the ceramics should be attributed to optimum LiSbO₃ substitution and better microstructure with high density. Results show that LiSbO₃ doped K_0.5Na_0.5NbO₃-BiFeO₃ lead-free piezoelectric ceramics are a promising lead-free piezoelectric material for applications in different devices.     

Enhanced reduction properties of mesostructured Ce0.5Zr0.5O2 solid solutions

Mesostructured Ce_0.5Zr_0.5O₂ solid solutions (M-CZ) were hydrothermally synthesized using Gemini surfactant as the template. X-ray diffraction (XRD), small-angle X-ray diffraction (SAXRD), N₂ adsorption–desorption isotherms and high-resolution transmission electronic microscopy (HRTEM) were adopted to characterize the samples. The product had a surface area of 123.5 m² g⁻¹ with maximum oxygen storage capacity (OSC) of 0.58 mol O₂/mol Ce. Oxygen anions in the M-CZ can be repeatedly released and resumed during the redox recycles. Reduction of Ce⁴⁺ to Ce³⁺ or lower valence and Zr⁴⁺ to Zr³⁺ were fulfilled with an obvious color change during the temperature programmed reduction (TPR) process, while the crystal structure of the product remained unchanged even after severe reduction. The mesostructure of the product can improve the reductive ability of Ce⁴⁺ and Zr⁴⁺ cations, which was beneficial to the enhancement of OSC.     

Dependence of magnetic properties on the Fe ion in Ba-doped BiFeO3 multiferroic films

Ba-doped BiFeO₃ thin films were prepared on Pt/TiO₂/SiO₂/Si (100) substrate at various oxygen pressures by pulsed laser deposition. The influences of oxygen pressure on the crystalline structures, magnetic and electrical properties of these films were studied. Studies showed that the oxygen vacancy content and accordingly the leakage current density of the films decreased with the increase of the oxygen pressure, and the concentration of the Fe²⁺ increased with the decrease of the oxygen pressure. Strong ferromagnetism was observed in these films. The magnetization of the films increased with the decrease of the oxygen pressure and the largest remnant magnetization of 2.46 × 10⁴ A/m was obtained at an oxygen pressure of 0.7 Pa. This enhancement of the magnetization can be attributed to the increase of the Fe²⁺ concentration which may be related closely to the increase of the oxygen vacancy in the films.     

Enhancing the magnetic characteristics of BiFeO3 nanoparticles by Ca,Ba co-doping

Multiferroic nanoparticles (NPs) of pristine and Ca, Ba co-doped BiFeO₃ were synthesized by a facile sol–gel route. Co-doping was done by fixing the total dopant concentration at 5 mol% and then the relative concentrations of Ca and Ba was varied. Structural, optical and magnetic properties of the NPs were investigated using different techniques. UV–Vis absorption spectra of BiFeO₃ NPs showed a substantial blue shift of ∼100 nm (530 nm → 430 nm) on Ca, Ba co-doping which corresponds to increase in band gap by 0.5 eV. ⁵⁷Fe Mössbauer spectroscopy confirmed that iron is present only in 3⁺ valence state in all co-doped samples. The coercive field increased by 18 times for Bi_0.95Ca_0.01Ba_0.04FeO₃ samples, which is the maximum enhancement, observed amongst all the 5 mol% doped samples. At the equimolar (2.5 mol% each) concentration of co-dopants, the coercive field shows a significant enhancement of about 9 times (220 Oe → 2014 Oe) with concomitant increase in saturation magnetization by 7 times. Thus, equimolar co-doping causes simultaneous enhancement of the twin aspects of magnetic properties thereby making them better suited for device applications.     

Morphotropic phase boundary and piezoelectric properties of (Bi1/2Na1/2)1 − x(Bi1/2K1/2)xTiO3-0.03(Na0.5K0.5)NbO3 ferroelectric ceramics

Lead-free piezoelectric ceramics, (Bi_1/2Na_1/2)_1 − x(Bi_1/2K_1/2)_xTiO₃-0.03(Na_0.5K_0.5)NbO₃ (x = 0.10-0.40) were synthesized by conventional solid-state sintering. A morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases was confirmed. Two dielectric anomalies can be observed, showing diffused phase transition behavior. There is no shift of the dielectric maximum temperature with frequency due to the contribution of space charge at high temperatures, similar to pure (Bi_1/2Na_1/2)TiO₃. The materials near MPB show a strong compositional dependence with the optimal properties of a d₃₃ of 167 pC/N, a k_p of 35.5%, a P_r of 27.6 μC/cm² and a E_c of 27.9 kV/cm, suitable for future application.     

Microwave-hydrothermal synthesis of the multiferroic BiFeO3

In this work, three different methods for preparing BiFeO₃ polycrystals are compared: hydrothermal synthesis, microwave heating in the solid state and the combination of both, that is a hydrothermal method using microwave heating. The best materials, without high purity reactants, are obtained in few minutes by the last procedure, a new, very fast, reproducible and environment-friendly method of synthesis, which is described and discussed here.