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.     

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.     

Synthesis, structural and microwave dielectric properties of Al2W3−xMoxO12 (x = 0-3) ceramics

Low dielectric ceramics in the Al₂W_3−xMo_xO₁₂ (x = 0-3) system have been prepared through solid state ceramic route. The phase purity of the ceramic compositions has been studied using powder X-ray diffraction (XRD) studies. The microstructure of the sintered ceramics was evaluated by Scanning Electron Microscopy (SEM). The crystal structure of the ceramic compositions as a result of Mo substitution has been studied using Laser Raman spectroscopy. The microwave dielectric properties of the ceramics were studied by Hakki and Coleman post resonator and cavity perturbation techniques. Al₂Mo_xW_3−xO₁₂ (x = 0-3) ceramics exhibited low dielectric constant and relatively high unloaded quality factor. The temperature coefficient of resonant frequency of the compositions is found to be in the range −41 to −72 ppm/°C.     

Coexistence of room temperature ferroelectricity and ferrimagnetism in multiferroic BiFeO3-Bi0.5Na0.5TiO3 solid solution

The structure, ferroelectric and magnetic properties of (1 − x)BiFeO₃-xBi_0.5Na_0.5TiO₃ (x = 0.37) solid solution fabricated by a sol-gel method have been investigated. X-ray diffraction and Raman spectroscopy measurements show a single-phase perovskite structure with no impurities identified. Compared with pure BiFeO₃, the coexistence of ferroelectricity and ferrimagnetism have been observed at room temperature for the solution with remnant polarization P_r = 1.41 μC/cm² and remnant magnetization M_r = 0.054 emu/g. Importantly, a magnetic transition from ferrimagnetic (FM) ordering to paramagnetic (PM) state is observed, with Curie temperature T_C ∼ 330 K, being explained in terms of the suppression of cycloid spin configuration by the structural distortion.     

Preparation and characterization of La1−xKxFeO3 (x = 0-1) by self-propagating high-temperature synthesis for use as soot combustion catalyst

This paper proposes La_1−xK_xFeO₃ prepared by self-propagating high-temperature synthesis (SHS) as an alternative to platinum catalysts for promoting diesel soot combustion. The catalytic property of eleven products SHSed with different substitution ratios of potassium (x = 0-1) was experimentally evaluated using a thermobalance. In the mass loss curves of the product, T₅₀ was defined as the temperature at which the weight of the reference soot decreases to half its initial weight. The BET specific surface area of SHSed La_1−xK_xFeO₃ depended on x strongly. All the products showed good oxidation catalytic activity. Despite having the smallest surface area (0.11 m²/g) among the obtained products, La_0.9K_0.1FeO₃ (x = 0.1) was found to be the best catalyst with the lowest T₅₀ (442 °C). T₅₀ of La_1−xK_xFeO₃ decreased with increasing x for x > 0.2. The products with x = 0.6 and 0.8 were the second-best catalysts in terms of their T₅₀. Moreover, average apparent activation energy of La_0.9K_0.1FeO₃ (x = 0.1) calculated by Friedman method using TG was as much as 61 kJ/mol lower than that of Pt/Al₂O₃ catalyst. In conclusion, potassium-substituted SHSed La_1−xK_xFeO₃ can be used as an alternative to Pt/Al₂O₃ for soot combustion.     

Effect of A site and B site doping on structural, thermal, and dielectric properties of BiFeO3 ceramics

Polycrystalline Bi_1−xBa_xFe_1−yM_yO₃ (M = Co and Mn; x = 0.1, y = 0.1) were synthesized by solid-state route method to study the compositional driven structural transformations in multiferroics. Room temperature X-ray diffraction patterns confirmed the formation of perovskite structure. Rietveld-refined crystal structure parameters revealed the existence of rhombohedral R3c symmetry for both the samples. The samples were found to be nearly free from any other secondary phases. Raman analysis reveals that Ba atom substitutes Bi site and Mn and Co atom substitutes Fe site into the BiFeO₃ with the shifting of phonon modes. The red shift is attributed to Co or Mn doping where as blue shift occurs from Ba doping. The differential scanning calorimetry reveals the corresponding Neel temperature 370 °C and 326 °C for Co and Mn doped samples. Ba and Co substitution with x = 0.1 and y = 0.1 has not affected the Neel temperature of the parent BiFeO₃ as well of Ba and Mn substitution. The variation of frequency dispersion in permittivity and loss pattern due to A-site and B-site substitution in BiFeO₃ was observed in the dielectric response curve.     

Microstructure, ferroelectric, and piezoelectric properties of (1 − x − y)Bi0.5Na0.5TiO3-xBaTiO3-yBi0.5Ag0.5TiO3 lead-free ceramics

Lead-free (1 − x − y)Bi_0.5Na_0.5TiO₃-xBaTiO₃-yBi_0.5Ag_0.5TiO₃ (BNT-BT-BAT-x/y, x = 0-0.10, y = 0-0.075) piezoelectric ceramics were synthesized by conventional oxide-mixed method. The microstructure, ferroelectric, and piezoelectric properties of the ceramics were investigated. Results show that a morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases of BNT-BT-BAT-x/0.04 ceramics is formed at x = 0.06-0.08. The addition of BAT has no obvious change on the crystal structure of BNT-BT ceramics while it causes the grain size of the ceramics to become more homogenous. Near the MPB, the ceramics with x = 0.06 and y = 0.05-0.06 possess optimum electrical properties: P_r ∼ 42.5 μC/cm², E_c ∼ 32.0 kV/cm, d₃₃ ∼ 172 pC/N, k_p ∼ 32.6%, and k_t ∼ 52.6%. The temperature dependences of k_p and polarization versus electric hysteresis loops reveal that the depolarization temperature (T_d) of BNT-BT-BAT-0.06/y ceramics decreases with increasing y. In addition, the polar and non-polar phases may coexist in the BNT-BT-BAT-x/y ceramics above T_d.     

Thermoelectric properties of Bi2SexTe3−x prepared by Bridgman method

Bi₂Se_xTe_3−x crystals with various x values were grown by Bridgman method. The electrical conductivity, σ, was found to decrease with increasing Se content. The highest σ of 1.6 × 10⁵ S m⁻¹ at room temperature was reached at x = 0.12 with a growth rate of 0.8 mm h⁻¹. The Seebeck coefficient, S, was less dependent on Se content, all with positive values showing p-type characteristics, and the highest S was measured to be 240 μV K⁻¹ at x = 0.24. The lowest thermal conductivity, κ, was 0.7 W m⁻¹ K⁻¹ at x = 0.36. The electronic part of κ, κ_el, showed a decrease with increasing Se content, which implies that the hole concentration as the main carriers was reduced by the addition of Se. The highest dimensionless figure of merit, ZT, at room temperature was 1.2 at x = 0.36, which is attributed to the combination of a rather high electrical conductivity and Seebeck coefficient and low thermal conductivity.     

Hydrothermal synthesis of superconductors Ba1−xKxBiO3 and double perovskites Ba1−xKxBi1−yNayO3

Superconductors Ba_1−xK_xBiO₃ and body-centered double perovskites Ba_1−xK_xBi_1−yNa_yO₃ have been selectively synthesized by a facile hydrothermal route. The appropriate ratio and adding sequence of initial reagents, alkalinity, reaction temperature and time are the critical factors that influence the crystal growth of the compounds. The purity and homogeneity of the crystals were detected by the ICP, SEM, EDX and TEM studies. Magnetic measurements show that the superconducting transition temperatures T_C of Ba_1−xK_xBiO₃ decrease from 22 K (for x = 0.35) to 8 K (for x = 0.55) with increasing the K doping level.     

CuIn1−xAlxS2 thin films prepared by sulfurization of metallic precursors

CuIn_1−xAl_xS₂ thin films (x = 0, 0.09, 0.27, 0.46, 0.64, 0.82 and 1) with thicknesses of approximately 1 μm were formed by the sulfurization of DC sputtered Cu-In-Al precursors. All samples were sulfurized in a graphite container for 90 min at 650 °C in a 150 kPa Ar + S atmosphere. Final films were studied via X-ray diffraction (XRD), scanning electron microscopy (SEM) and micro-Raman spectroscopy. It was found that all samples were polycrystalline in nature and their lattice parameters varied slightly nonlinearly from {a = 5.49 Å, c = 11.02 Å} for CuInS₂ to {a = 5.30 Å, c = 10.36 Å} for CuAlS₂. No unwanted phases such as Cu_2−xS or others were observed. Raman were recorded at a room temperature and the most intensive and dominant A₁ phonon frequency varied nonlinearly from 294 cm⁻¹ (CuInS₂) to 314 cm⁻¹ (CuAlS₂).     

Electrochemical hydrogen storage properties of non-stoichiometric La0.7Mg0.3−xCaxNi2.8Co0.5 (x = 0-0.10) electrode alloys

The microstructure and electrochemical hydrogen storage characteristics of La_0.7Mg_0.3−xCa_xNi_2.8Co_0.5 (x = 0, 0.05 and 0.10) alloys prepared by arc-melting and subsequent powder sintering method are investigated. The electrochemical measurement results show that the cycle stability after 100 charge/discharge cycles first increases from 46.4% (x = 0) to 54.3% (x = 0.05), then decreases to 43.2% (x = 0.10), and the high rate dischargeability increases from 64.5% (x = 0) to 68.5% (x = 0.10) at the discharge current density of 1200 mA/g. The electrochemical impedance spectroscopy analysis indicates that the electrochemical kinetics of the alloy electrodes is improved by increasing Ca. The entire results exhibit that a suitable content of Ca (x = 0.05) can improve the overall electrochemical hydrogen storage characteristics of the alloys.     

Relationships between Zr substitution for Ti and microwave dielectric properties in Mg(ZrxTi1−x)O3 ceramics

The influence of Zr substitution for Ti on the microwave dielectric properties and microstructures of the Mg(Zr_xTi_1−x)O₃(MZ_xT) (0.01 ≤ x ≤ 0.3) ceramics was investigated. The quality factors of Mg(Zr_xTi_1−x)O₃ ceramics with x = 0.01-0.05 were improved because the solid solution of a small amount of Zr⁴⁺ substitution in the B-site could increase density and grain size. An excess of Zr⁴⁺ resulted in the formation of a great deal of secondary phase that declined the microwave dielectric properties of MZ_xT ceramics. The temperature coefficient of resonant frequency (τ_f) of Mg(Zr_xTi_1−x)O₃ ceramics slightly increased with increasing Zr content, and the variation in τ_f was attributed to the formation of secondary phases.     

Preparation and electrical properties of Bi0.5Na0.5TiO3-BaTiO3-KNbO3 lead-free piezoelectric ceramics

Lead-free (1 − x)Bi_0.47Na_0.47Ba_0.06TiO₃-xKNbO₃ (BNBT-xKN, x = 0-0.08) ceramics were prepared by ordinary ceramic sintering technique. The piezoelectric, dielectric and ferroelectric properties of the ceramics are investigated and discussed. The results of X-ray diffraction (XRD) indicate that KNbO₃ (KN) has diffused into Bi_0.47Na_0.47Ba_0.06TiO₃ (BNBT) lattices to form a solid solution with a pure perovskite structure. Moderate additive of KN (x ≤ 0.02) in BNBT-xKN ceramics enhance their piezoelectric and ferroelectric properties. Three dielectric anomaly peaks are observed in BNBT-0.00KN, BNBT-0.01KN and BNBT-0.02KN ceramics. With the increment of KN in BNBT-xKN ceramics, the dielectric anomaly peaks shift to lower temperature. BNBT-0.01KN ceramic exhibits excellent piezoelectric properties and strong ferroelectricity: piezoelectric coefficient, d₃₃ = 195 pC/N; electromechanical coupling factor, k_t = 58.9 and k_p = 29.3%; mechanical quality factor, Q_m = 113; remnant polarization, P_r = 41.8 μC/cm²; coercive field, E_c = 19.5 kV/cm.     

Microwave dielectric properties of temperature stable Li2ZnxCo1−xTi3O8 ceramics

The Li₂Zn_xCo_1−xTi₃O₈ (x = 0.2-0.8) solid solution system has been synthesized by the conventional solid-state ceramic route and the effect of Zn substitution for Co on microwave dielectric properties of Li₂CoTi₃O₈ ceramics has also been investigated. The microwave dielectric properties of these ceramics show a linear variation between the end members for all compositions. The optimized sintering temperatures of Li₂Zn_xCo_1−xTi₃O₈ ceramics increase with increasing content of Zn. The specimen with x = 0.4 sintered at 1050 °C/2 h exhibits an excellent combination of microwave dielectric properties with ?_r = 27.7, Q_u × f = 57,100 GHz and τ_f = −1.0 ppm/°C.     

Preparation, structure and electrical conductivity of the pyrochlore-type phase Sm2Zr2O7 codoped with bivalent magnesium and trivalent dysprosium cations

The pyrochlore-type phases with the compositions of SmDy_1−xMg_xZr₂O_7−x/2 (0 ≤ x ≤ 0.20) have been prepared by pressureless-sintering method for the first time as possible solid electrolytes. The structure and electrical conductivity of SmDy_1−xMg_xZr₂O_7−x/2 ceramics have been studied by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance spectroscopy measurements. SmDy_1−xMg_xZr₂O_7−x/2 (x = 0, 0.05, 0.10) ceramics exhibit a single phase of pyrochlore-type structure, and SmDy_1−xMg_xZr₂O_7−x/2 (x = 0.15, 0.20) ceramics consist of pyrochlore phase and a small amount of the second phase magnesia. The total conductivity of SmDy_1−xMg_xZr₂O_7−x/2 ceramics obeys the Arrhenius relation, and the total conductivity of each composition increases with increasing temperature from 673 to 1173 K. SmDy_1−xMg_xZr₂O_7−x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest total conductivity value is about 8 × 10⁻³ S cm⁻¹ at 1173 K for SmDy_1−xMg_xZr₂O_7−x/2 ceramics.     

Thermoelectric properties of Cu1−xPtxFeO2 (0.0 ≤ x ≤ 0.05) delafossite-type transition oxide

The samples of Cu_1−xPt_xFeO₂ (0 ≤ x ≤ 0.05) delafossite were synthesized by solid state reaction method for studying thermoelectric properties. The properties of Seebeck coefficient, electrical conductivity and thermal conductivity were measured in the high temperature ranging from 300 to 960 K. The results of Seebeck coefficient, electrical conductivity and power factor were increased with increasing Pt substitution and temperature. The thermal conductivity was decreased from 5.8 to 3.5 W/mK with increasing the temperature from 300 to 960 K. An important results, the highest value of power factor and ZT is 2.0 × 10⁻⁴ W/mK² and 0.05, respectively, for x = 0.05 at 960 K.     

Properties of praseodymium-doped bismuth potassium titanate (Bi0.5K0.5TiO3) synthesised using the soft combustion technique

Bismuth potassium titanate (Bi_0.5K_0.5TiO₃; BKT) and praseodymium-doped BKT (Bi_0.5(1−x)Pr_xK_0.5TiO₃; BPKT) powders were synthesised using the soft combustion technique. Fine particles of 10-100 nm of BKT and BPKT were produced. A single phase BKT was obtained with a minimum of 0.5 mol of glycine. Various compounds of Bi_0.5(1−x)Pr_xK_0.5TiO₃ where x = 0.01, 0.03, 0.05, 0.10, 0.15 and 0.20 were prepared. Pure BKT and BPKT powders were obtained after calcination at 800 °C for 3 h. After sintering at 1050 °C for 5 h, pure BKT and BPKT pellets were obtained for x = 0 and 0.01. However, for BPKT with x = 0.03, 0.05, 0.10, 0.15 and 0.20, a minor amount of Bi₄Ti₃O₁₂ (BIT) secondary phase was present after sintering at 1050 °C for 5 h. The crystallite size and grain size of all the samples followed similar trends, first increasing from x = 0 (undoped BKT) to x = 0.05 and then decreasing above x = 0.05. Among the undoped and doped samples, BPKT with x = 0.05 had the highest dielectric properties (?_r = 713.87) due to its large crystallite size (68.66 nm), large grain size (∼435 nm) and high relative density (93.39%).     

Electrical properties of binder-free thick film BaYxBi1−xO3 NTC thermistors

The microstructure and electrical properties of BaY_xBi_1−xO₃ thick film negative temperature coefficient thermistors, fabricated by screen printing, were investigated. The sintered thick films were the single-phase solid solutions of the BaY_xBi_1−xO₃ compounds with a monoclinic structure. The added Y₂O₃ led to a significant decrease in the grain size of the thermistors. The resistivity and coefficient of temperature sensitivity for the BaY_xBi_1−xO₃ (0 ≤ x ≤ 0.15) thick film NTC thermistors decreased first with increasing x in the range of x < 0.04 and then increased with further increase in x.     

Structure, dielectric and electric properties of (Ba0.68−xSr0.308Bi0.006Na0.006Mgx)(Ti0.99Sn0.01)O3 ceramics

(Ba_0.68−xSr_0.308Bi_0.006Na_0.006Mg_x)(Ti_0.99Sn_0.01)O₃ ceramics were synthesized by solid-state reaction process. The samples (X ≤ 0.010) are a mixture of cubic (major) and rhombohedral (minor) phases. The rhombohedral phase causes a large dielectric loss in low temperature regions and plays an important role in diffuse phase transition of ceramics. While X > 0.010, the rhombohedral phase decreases and gradually disappears. The dielectric loss of ceramics in the low temperature regions decreases, and the samples change from the diffuse phase transition to the phase transition of second order, and then to of first order. In the temperature range of 270-370 °C, intrinsic conduction dominates conductivity of ceramics.     

High permittivity and low dielectric loss of the Ca1−xSrxCu3Ti4O12 ceramics

The Ca_1−xSr_xCu₃Ti₄O₁₂ (CSCTO) giant dielectric ceramics were prepared by conventional solid-state method. X-ray diffraction patterns revealed that a small amount of Sr²⁺ (x < 0.2) had no obvious effect on the phase structure of the CSCTO ceramics, while with increasing the Sr²⁺ content, a second phase of SrTiO₃ appeared. Electrical properties of CSCTO ceramics greatly depended on the Sr²⁺ content. The Ca_0.9Sr_0.1Cu₃Ti₄O₁₂ ceramics exhibited a higher permittivity (71,153) and lower dielectric loss (0.022) when measured at 1 kHz at room temperature. The ceramics also performed good temperature stability in the temperature range from −50 °C to 100 °C at 1 kHz. By impedance spectroscopy analysis, all compounds were found to be electrically heterogeneous, showing semiconducting grains and insulating grain boundaries. The grain resistance was 1.28 Ω and the grain boundary resistance was 1.31 × 10⁵ Ω. All the results indicated that the Ca_0.9Sr_0.1Cu₃Ti₄O₁₂ ceramics were very promising materials with higher permittivity for practical applications.