Structural,magnetic and dielectric behavior of Mg1−xCaxNiyFe2−yO4 nano-ferrites synthesized by the micro-emulsion method

Ca–Ni co-substituted samples of nanocrystalline spinel ferrites with chemical formula Mg_1−xCa_xNi_yFe_2−y O₄ (x=0.0–0.6, y=0.0–1.2) were synthesized by the micro-emulsion method and were annealed at 700 °C for 7 h. The synthesized samples were characterized by x-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, vibrating sample magnetometry (VSM) and dielectric measurements. The XRD and FTIR analysis reveals that single phase samples can be achieved by substituting Ca and Ni ions at Mg and Fe sites respectively in cubic spinel nano-ferrites. The crystallite size of the synthesized samples was found in the range 29–45 nm. The saturation magnetization (M_s) increases from 9.84 to 24.99 emu/g up to x=0.2, y=0.4 and then decreases, while the coercivity (H_c) increases continuously from 94 to 153 Oe with the increase in dopants concentration. The dielectric properties of these nano materials were also studied at room temperature in the frequency range 100 MHz to 3 GHz. The dielectric parameters were found to decrease with the increased Ca–Ni concentration. Further the peaking behavior was observed beyond 1.5 GHz. The frequency dependent dielectric properties of all the samples have been explained qualitatively on the basis of the Maxwell–Wagner two-layer model according to Koop's phenomenological theory. The enhanced magnetic parameters and reduced dielectric properties make the synthesized materials suitable for switching and high frequency applications, respectively.     

Structure,magnetic and electrical transport properties of the perovskites Sm1−xCaxFe1−xMnxO3

The structure of series Sm_1−xCa_xFe_1−xMn_xO₃ (0.0 ≤ x ≤ 1.0) compounds was investigated. The lattice parameters increase with coupled substitution Sm³⁺ by Ca²⁺ and Mn⁴⁺ for Fe³⁺. The variation of parameter, c, is larger than that of a and b, respectivly. The detailed analysis of magnetic properties of series Sm_1−xCa_xFe_1−xMn_xO₃ (0.1 ≤ x ≤ 0.9) shows that local magnetic interaction between Fe³⁺ and Fe³⁺ and Mn⁴⁺ and Mn⁴⁺ at below magnetic transition temperature is antiferromagnetic. Above magnetic transition temperature the presence of large magnetic cluster is proposed and the sizes of magnetic clusters decrease with Mn⁴⁺. The electrical transport behaviors related with small polaron hopping and variable range hopping models.     

Structural and magnetic characterization of MnxZn1 − xFe2O4 (x = 0.2; 0.35; 0.65; 0.8; 1.0) ferrites obtained by the citrate precursor method

In this work the microstructure and magnetic properties of Mn-Zn ferrites powders were investigated. Mn_xZn1 _− xFe₂O₄ powders where x = 0.2; 0.35; 0.5; 0.65; 0.8 and 1.0 were obtained by citrate precursor method. Citrate resin precursor was burned on air atmosphere at 400 °C for 3 h. Mn-Zn powders were calcined at 950 °C during 150 min under inert atmospheres: N₂ and rarefied atmosphere. Thermal analysis of precursor resin, phase evolution and microstructure of Mn-Zn ferrites powders were investigated by TG, DTA, XRD and SEM techniques. The powders calcined under rarefied atmosphere show spinel cubic structure and contamination of α-Fe₂O₃, while powders calcined under N₂ presents only the spinel cubic structure. Particle size was observed by SEM ranging from 80 to 150 nm. The magnetic properties were measured employing a vibrating sample magnetometer (VSM). It was observed that the saturation magnetization M_s increased with the increase of Mn content. The M_s of Mn_0.8Zn_0.2Fe₂O₄ calcined on rarefied atmosphere and Mn_0.8Zn_0.2Fe₂O₄ calcined on N₂ was 23.31 emu g⁻¹ and 56.23 emu g⁻¹, respectively.     

Structural,vibrational, optical,magnetic and dielectric properties of Bi1−xBaxFeO3 nanoparticles

Bi_1−xBa_xFeO₃ (x=0.05, 0.10 and 0.15) nanoparticles were synthesized by the sol–gel method. X-ray diffraction and Raman spectroscopy results showed the presence of distorted rhombohedral structure of Bi_1−xBa_xFeO₃ nanoparticles. Rietveld refinement and Williamson–Hall plot of the x-ray diffraction patterns showed the increase in lattice parameters, unit cell volume and the particle size. Infrared spectroscopy and Raman analysis revealed the shifting of phonon modes towards the higher wavenumber side with increasing Ba concentration. These samples exhibited the optical band gap in the visible region (2.47–2.02 eV) indicating their ability to absorb visible light. Magnetic measurement showed room temperature ferromagnetic behavior, which may be attributed to the antiferromagnetic core and the ferromagnetic surface of the nanoparticles, together with the structural distortion caused by Ba substitution. The magnetoelectric coupling was evidenced by the observation of the dielectric anomaly in the dielectric constant and the dielectric loss near antiferromagnetic Neel temperature in all the samples.     

Synthesis,structural analysis and dielectric properties of Ba8(Mg1−xZnx)Nb6O24 hexagonal perovskites

Ba₈(Mg_1−xZn_x)Nb₆O₂₄ (x=0, 0.2, 0.4, 0.6, 0.8 and 1) ceramics were prepared through the conventional solid-state route. The materials were calcined at 1250 °C and sintered at 1375–1425 °C. The structure of the system was analyzed using X-ray diffraction and vibrational spectroscopic studies. The microstructure of the sintered pellet was analyzed using scanning electron microscopy. The dielectric constant (ε_r), temperature coefficient of resonant frequency (τ_f) and the unloaded quality factor (Q_u) were measured in the microwave frequency region. The τ_f values of the compositions were reduced by varying the value of x from 0 to 1. The dielectric responses to frequency were also studied in the radio frequency region. The compositions have good microwave dielectric properties and hence are suitable for dielectric resonator applications.     

Effect of La substitution on the structural and electrical properties of BaBi4−xLaxTi4O15

Pure and lanthanum doped barium bismuth titanate BaBi_4−xLa_xTi₄O₁₅ (BBLT, x=0, 0.05, 0.15, 0.30) ceramics were prepared utilizing solid state method. The X-ray diffraction (XRD) data confirmed formation of single-phase Aurivillius compounds while SEM micrographs did not show evident grain size change of doped ceramics. Dielectric properties were investigated in 1.21 kHz to 1 MHz frequency range and in the temperature range of 20 to 727 °C. When Bi³⁺ is substituted with La³⁺, a significant disorder was induced and the material exhibited broadening of the phase transition. Impedance analysis confirmed the presence of two semicircular arcs in doped samples suggesting the existence of grain and grain-boundary conduction. The dc-conductivity and activation energies were evaluated for all compositions.     

VUV photoluminescence properties of Y1−xGdxVO4:Eu phosphors prepared by ultrasonic spray pyrolysis

The red-emitting (Y_1−xGd_x)_0.94Eu_0.06VO₄ (0 ≤ x ≤ 1.0) phosphors were synthesized by ultrasonic spray pyrolysis. The (Y_1−xGd_x)_0.94Eu_0.06VO₄ (0 ≤ x ≤ 1.0) phosphors had the tetragonal xenotime structure with a space group of I4₁/amd (1 4 1). The calculated crystallite sizes of the annealed phosphors ranged from 58 to 68 nm. In this study, we discussed the photoluminescence properties of the (Y_1−xGd_x)_0.94Eu_0.06VO₄ phosphors under VUV excitation, depending on Gd content. The emission intensity of the (Y_1−xGd_x)_0.94Eu_0.06VO₄ phosphors increased with increasing Gd content up to x = 0.5, and then decreased with a further increase in Gd content. The purest red color was obtained for the (Y_0.5Gd_0.5)_0.94Eu_0.06VO₄ phosphors.     

Structural and magnetic properties of NiFe2−xBixO4 (x = 0, 0.1, 0.15) nanoparticles prepared via sol-gel method

NiFe_2−xBi_xO₄ (x = 0, 0.1, 0.15) nanopowders were synthesized via sol-gel method. The precursor gels were calcined at 773 K in air for 1 h to obtain the pure nanostructured NiFe_2−xBi_xO₄ spinel phase. The crystal structure and magnetic properties of the substituted spinel series of NiFe_2−xBi_xO₄ have been investigated by means of ⁵⁷Fe Mössbauer spectroscopy, transmission electron microscopy and alternating gradient force magnetometry. Mössbauer spectroscopic measurements revealed that Bi³⁺ cations tend to occupy octahedral positions in the structure of the substituted ferrite, i.e., the crystal-chemical formula of the as-prepared nanoparticles may be written as: (Fe)[NiFe_1−xBi_x]O₄ (x = 0, 0.1, 0.15), where parentheses and square brackets enclose cations on sites of tetrahedral and octahedral coordination, respectively. Selective area electron diffraction studies provided evidence that the samples of the NiFe_2−xBi_xO₄ series, independently of x, exhibit the cubic spinel structure. The values of the saturation magnetization and the coercive field of NiFe_2−xBi_xO₄ nanoparticles were found to decrease with increasing degree of bismuth substitution.     

Magnetic and magnetoresistive properties of La0.65−xEuxSr0.35MnO3

Eu-doped perovskites La_0.65−xEu_xSr_0.35MnO₃ (0.05 ≤ x ≤ 0.30) were synthesized by sol–gel method using citric acid and characterized by X-ray diffraction, magnetization, resistivity and magnetoresistance (MR) experiments. All samples had a single hexagonal perovskite structure. As x increased from 0.05 to 0.30, the Curie temperature T_C for the samples decreased from 352 to 242 K. It was found that two transition points appeared when the resistivity changed with increasing temperature, and upon an application of a magnetic field of 20 kOe the maximum magnetoresistivity of 18% for the La_0.65−xEu_xSr_0.35MnO₃ with x = 0.20 was obtained at room temperature 300 K. The mechanism of the transitions for the samples was explored.     

Raman scattering, electronic structure and microwave dielectric properties of Ba([Mg1−xZnx]1/3Ta2/3)O3 ceramics

Effects of Zn substitution for Mg on the crystal structure, lattice vibrations and microwave dielectric properties of Ba(Mg_1/3,Ta_2/3)O₃ (BMT) ceramics were investigated. Raman scattering spectra for Ba([Mg_1−xZn_x]_1/3Ta_2/3)O₃ (BMZT) ceramics, with x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0, were measured at room temperature. The Raman result shows a dominance of 1:2 ordered structure at all Zn substitution contents. All Raman modes shift to lower frequencies with increasing Zn substitution. Higher Qf value correlates well with narrower width of the breathing Raman mode A_1g(4) and larger relative intensity of 1:2 long-range-ordered mode E_g(2) in BMZT solid solution. First-principle calculation was performed to investigate the electronic structure of 1:2 ordered BMT and Ba(Zn_1/3,Ta_2/3)O₃ (BZT). Covalent bond between Zn and O in BZT is much stronger than that between Mg and O in BMT due to the Zn 3d orbital. Zn substitution for Mg leads to longer and weaker Ta-O bonds, which may be one reason for the variation of Raman spectroscopy and microwave dielectric properties of BMZT system.     

Microstructure and ionic conductivity properties of gadolinia doped ceria (GdxCe1−xO2−x/2) electrolytes for intermediate temperature SOFCs prepared by the polyol method

Gd_0.1Ce_0.9O_1.95 and Gd_0.2Ce_0.8O_1.9 powders were prepared through the polyol process without using any protective agent. Microstructural and physical properties of the samples were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry (TG) and impedance analysis methods. The results of the thermogravimetry/differential thermal analysis (TG/DTA) and XRD indicated that a single-phase fluorite structure formed at the relatively low calcination temperature of 500 °C. The XRD patterns of the samples revealed that the crystallite size of the samples increased as calcination temperatures increased. The sintering behavior and ionic conductivity of pellets prepared from gadolinia doped ceria (GDC) powders, which were calcined at 500 °C, were also investigated. The relative densities of the pellets, which were sintered at temperatures above 1300 °C, were higher than 95%. The results of the impedance spectroscopy revealed that the GDC-20 sample that was sintered at 1400 °C exhibited an ionic conductivity of 3.25×10⁻² S cm⁻¹ at 800 °C in air. This result clearly indicates that GDC powder with adequate ionic conductivity can be prepared through the polyol process at low temperatures.     

Enhanced ferroelectric properties in (Ba1−xCax)(Ti0.94Sn0.06)O3 lead-free ceramics

Lead-free (Ba_1−xCa_x)(Ti_0.94Sn_0.06)O₃ (BCST) (x = 0.01-0.04) ceramics were prepared using a solid-state reaction technique. The effects of Ca content on the phase structure and electrical properties of the BCST ceramics were investigated. High piezoelectric coefficient of d₃₃ = 440 pC/N, planar electromechanical coupling factor of k_p = 45% and dielectric constant ?_r = 6900 were obtained for the samples at x = 0.03. At room temperature, a polymorphic phase transition (PPT) from orthorhombic phase to tetragonal phase was identified in the composition range of 0.02 < x < 0.04.     

Synthesis and NTC properties of YCr1−xMnxO3 ceramics sintered under nitrogen atmosphere

YCr_1−xMn_xO₃ (0 ≤ x ≤ 0.8) negative temperature coefficient (NTC) compositions were synthesized by classical solid state reaction at 1200 °C, and sintered under nitrogen atmosphere at 1500 °C and 1600 °C. XRD patterns analysis has revealed that for x ≤ 0.6, the structure consists of a solid solution of an orthorhombic perovskite YCrO₃ phase with Mn substitute for Cr. For x ≥ 0.8, a second phase with a structure similar to the hexagonal YMnO₃ phase appears. SEM images and calculated open porosity have shown that the substitution of Mn for Cr results in a decrease in porosity. Whatever the sintering temperature, the electrical characterizations (between 25 and 900 °C) have shown that the increase in the manganese content involves the decrease in both resistivity and material constant B (parameter which characterizes the thermal sensitivity of material) when x ≤ 0.6. The magnitude order of the resistivity at 25 °C is of 10⁴-10⁸ Ω cm and activation energies vary from 0.28 to 0.99 eV at low and high temperatures, respectively.     

Moisture sensitivity of YCr(1−x)MnxO3 perovskites

This work deals with the sensitivity to moisture rate in air of perovskite with general formula YCr_(1−x)Mn_xO₃ with x ranging from 0 to 0.8. The combined sensitivity to moisture and temperature let us think the possibility of realizing composite sensors temperature/moisture. This study clearly shows the influence of the Mn content on moisture which affects the working frequency of sensors. Capacitance variation of the most sensitive compositions increases from 100 pF to 1 nF for relative humidity between 20 and 80%. The influence of porosity is also demonstrated.     

Adjustable dielectric properties of Li2CuxZn1−xTi3O8 (x=0 to 1) ceramics with low sintering temperature

Single-phase dielectric ceramics Li₂Cu_xZn_1−xTi₃O₈ (x=0–1) were synthesized by the conventional solid-state ceramic route. All the solid solutions adopted Li₂MTi₃O₈ cubic spinel structure in which Li/M and Ti show 1:3 order in octahedral sites whereas Li and M are distributed randomly in tetrahedral sites with the degree of Li/M cation mixing varying from 0.5 to 0.3. The substitution of Cu for Zn effectively lowered the sintering temperatures of the ceramics from 1050 to 850 °C and significantly affected the dielectric properties. As x increased from 0 to 0.5, τ_f gradually increased while the dielectric constant (ε_r) and quality factor value (Q×f) gradually decreased, and a near-zero τ_f of 1.6 ppm/°C with ε_r of 25.2, Q×f of 32,100 GHz could be achieved for Li₂Cu_0.1Zn_0.9Ti₃O₈ ceramic sintered at 950 °C, which make it become an attractive promising candidate for LTCC application. As x increases from 0.5 to 1, the dielectric loss significantly increases with AC conductivity increasing up to 2.3×10⁻⁴ S/cm (at 1 MHz).     

Synergistic effect in structural and supercapacitor performance of well dispersed CoFe2O4/Co3O4 nano-hetrostructures

Herein, we report a facile homogeneous urea – assisted hydrothermal approach for the design of CoFe₂O₄/Co₃O₄ nano hetrostructure. A variation in Co concentration leads to smartly designed composite material namely CFC-11 and CFC-12 where CFC-12 appreciates the benefits of both CoFe₂O₄ and Co₃O₄ nanoparticles. The physico – chemical properties of as developed materials were investigated by X-ray diffraction (XRD), field emission electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron microscopy (XPS) and Raman spectroscopy. The specific surface area and pore size distribution was determined by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halendo (BJH) respectively. Magnetic measurements via. vibrating sample magnetometer (VSM) demonstrate that saturation magnetization decreases with the incorporation of Co₃O₄ antiferromagnetic nanoparticles. To explore the utility of as designed nano-hetrostructures as supercapacitor electrodes, we employed cyclic voltammetry (CV) and electrochemical impedence spectroscopy (EIS) measurements. A high specific capacitance of 761.1?F?g^?1 at 10?mV?s^?1, admirable cyclic durability of 92.2% and a low resistance value obtained from impedence measurements was observed for CFC-12. The favorable performance demonstrates the synergistic effect of CoFe₂O₄ and Co₃O₄ nanoparticles and thus promise an excellent material for energy storage devices.     

Structural and dielectric properties of Bi1−xSrxMnO3 (0.40≤x≤0.55)

In order to study the effect of Sr substitution on structural and dielectric properties of Bi_1−xSr_xMnO₃ (0.40≤x≤0.55) compounds were synthesized by the solid state reaction method. The as-prepared samples were characterized by X- ray diffraction (XRD) and dielectric measurements to correlate structural changes with dielectric properties. The XRD data were further analyzed by the Rietveld refinement. The highest dielectric constant was observed in Bi_0.55Sr_0.45MnO₃ and Bi_0.5Sr_0.5MnO₃ systems (∼10⁶) mainly because of orientation polarization. The charge ordering temperature decreases with increasing Sr concentration in Bi_1−xSr_xMnO₃ systems.     

A novel synthesis route to Sn1−xRExO2−x/2 nanorods via microwave-induced salt-assisted solution combustion process

Sn_1−xRE_xO_2−x/2 (RE=Y, La, Gd and Nd) nanorods have been prepared by annealing the as-obtained products from microwave-induced KCl-assisted solution combustion reaction. The phase evolution in the synthesis process was investigated by an X-ray diffractometer. Accordingly, the possible growth mechanism of Sn_1−xRE_xO_2−x/2 nanorods was discussed based on oriented attachment by polar forces. The results showed that the Sn_0.8Y_0.2O_1.9 nanorods were rutile-structured single crystals with 8–12 nm diameter and 100–200 nm length. Proper addition of KCl into the redox mixture solution is critical to the formation of Sn_1−xRE_xO_2−x/2 nanorods. The approach is convenient, inexpensive and efficient for the high yield preparation of Sn_1−xRE_xO_2−x/2 nanorods.     

The role of Eu2O3 and CeO2 on the phase formation,infrared emission and magnetic properties of Co0.6Zn0.4Fe2O4 ceramics by a solvothermal method

Nanosized Eu₂O₃ and CeO₂ co-addition CoZn ceramics have been achieved via a hydrothermal method by adjusting the mol ratios of Eu and Ce. The as-prepared samples were characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM), Fourier transform infrared (FTIR), Vibrating sample magnetometer (VSM) and Infrared emission measurement (IRE-2). The particle morphologies of the as-prepared samples evolve from spherical, to self-assembled nanoparticles, and irregular nanoparticles when the mol ratios (x) of Eu and Ce was changed from 0:10 to 10:0. Correspondingly, the main phases of the as-prepared samples change from both cubic spinel CoFe₂O₄ and CeO₂, pure cubic cerianite CeO₂, to amorphous. Meanwhile, the as-prepared samples appear transformed from a ferromagnetic behavior with a saturation magnetization 66.4 emu/g to a paramagnetic behavior with a saturation magnetization of 0.55 emu/g at turning point x=3.5:6.5. While the infrared emissivity is increasing as the x from 0:10 to 3.5:6.5, reach the maximum at 3.6:6.4, and then remain stable when further increasing x till 10:0. Those may be due to the amorphous tendency rising and the particle sizes gradual decreasing with x increasing from 0:10 to 10:0. What is more important is that the solvothermal method is proved to be an efficient way to prepare CoZn nano-ceramics in this study which may open new pathways to magnetic and far infrared therapy.