Effect of PFT substitution on dielectric properties of PFW-PFN perovskite ceramics

Perovskite phase formation and dielectric characteristics of ceramic system with addition of were investigated in order to examine the influence of . The ceramic system powders were synthesized via a B-site precursor route. Peculiar behaviors of frequency dispersion in dielectric constant spectra in the paraelectric region were observed due to increasing conductivity. Lattice parameters, dielectric maximum temperatures, and maximum dielectric constants increased with increasing content.     

Dielectric properties of Sr3CuNb2O9 perovskite ceramics

The dielectric constant ? and loss tangent tanδ of Sr₃CuNb₂O₉ perovskite ceramics prepared by solid-state reactions have been measured at temperatures from 300 to 900 K and frequencies from 25 to 1 × 10⁶ Hz. The results demonstrate that the samples slowly cooled from the temperature of the final, high-temperature firing (1200°C) have relatively low permittivity (? ? 10) and dielectric losses (tanδ ? 0.005 at 1 kHz) at room temperature, with no strong dielectric dispersion and no prominent maxima in the temperature dependences of their permittivity and dielectric loss. The ceramics quenched from 1300°C exhibit a pronounced Debye-type low-frequency relaxation and strong dielectric dispersion in conjunction with high permittivity ? ? 2000 at low frequencies and/or high temperatures. The observed dielectric anomalies in the Sr₃CuNb₂O₉ ceramics can be understood in terms of Maxwell-Wagner relaxation at dielectric inhomogeneities associated with the quenching-induced difference in oxygen-vacancy concentration between the grain bulk and surface layer.     

Effect of Sr substitution on structural,dielectric, magnetic and magnetoelectric properties of rapid liquid sintered BiFe0.8Ti0.2O3 ceramics

Structural, dielectric, magnetic and magnetoelectric properties of polycrystalline Sr doped BiFe_0.8Ti_0.2O₃ ceramics [Bi_1?xSr_x(Fe_0.8Ti_0.2)O₃; x = 0.05, 0.10 and 0.15)] were studied. All the samples were prepared by rapid liquid phase sintering method. Rietveld refinement of X-ray diffraction patterns of all samples confirmed that the samples crystallize in a rhombohedral structure and showed change in Fe–O–Fe bond angle and Fe–O bond length which in turn enhanced magnetization from 0.33 to 0.73 emu/g with the increase in Sr concentration from x = 0.05 to 0.15. The dielectric constant and dielectric loss were observed to increase with the increase in temperature from 30 to 500 °C. An anomalous peak has been observed in dielectric constant versus temperature plot around 300 °C for all the samples, which is close to the magnetic transition temperature of BiFeO₃. The composition-dependent magnetic properties with the expected Fe²⁺/Fe³⁺ ratio fluctuations were correlated by X-ray photoelectron spectroscopy. Magnetic and electric hysteresis loops showed a systematic increase in magnetization and polarization as a result of Sr doping in BiFe_0.8Ti_0.2O₃ ceramics. The value of magnetocapacitance at 10 kHz was observed as 0.95, 1.23 and 1.73 for x = 0.05, 0.10, 0.15 respectively in Bi_1?xSr_xFe_0.80Ti_0.20O₃ ceramics.     

Effect of Zr+4 ion substitution on the structural, dielectric and electrical properties of Sr5LaTi3Nb7O30 ceramics

Polycrystalline Zr-modified Sr₅LaTi₃Nb₇O₃₀ compound was prepared by a high-temperature solid-state reaction technique. X-ray structural analysis confirmed the formation of compound in an orthorhombic system at room temperature. Detailed studies of the dielectric parameters (dielectric constant and tangent loss) as a function of frequency (1–100 kHz) at different temperature (150 to 650 K) were carried out. It was found that as Zr⁺⁴ concentration increases in Sr₅LaTi_{3 – x} ZrxNb₇O₃₀, the value of dielectric constant decreases. These compounds show ferroelectric-paraelectric phase transition of diffuse type at 283, 305 and 320 K for x = 0, 1 and 2 respectively. The transition temperature (T _c) shifts towards higher temperature and maximum dielectric constant value (_max) decreases with increasing Zr⁺⁴ concentration for x = 0 to x = 2. When Ti⁺⁴ cations were completely replaced by Zr⁺⁴ cations (i.e., Sr₅LaZr₃Nb₇O₃₀), the compound does not show any phase transition. Impedance-spectroscopic studies provided an insight into the electrical properties and understanding of relaxation behavior of the material. Measurement of electrical conductivity as a function of temperature suggests that the compounds have a negative temperature coefficient of resistivity (NTCR) with a typical semiconductor behavior.     

Effect of Pr substitution on structural and electrical properties of BiFeO3 ceramics

Investigation on structural, vibrational, dielectric and ferroelectric properties of Bi_1−xPr_xFeO₃ (x = 0.0, 0.15, 0.25) ceramic samples has been carried out. Room temperature Rietveld-refined X-ray diffraction pattern shows the crystal structure of Bi_1−xPr_xFeO₃ is rhombohedral for x = 0 and triclinic for x = 0.15, 0.25. The changes in Raman normal modes with increasing doping concentration infer the structural transformation is due to Pr substitution at A-site in BiFeO₃. Raman spectra also reveal suppression of ferroelectric behavior due to Pr doping. The dielectric parameters, namely, dielectric permittivity (ε′) and loss tangent (tan (δ)) were evaluated as a function of frequency at room temperature. The ferroelectric polarization reduces in Pr doped bulk BFO samples due to structural change.     

Electrical and dielectric properties of nanocrystalline LaCrO3

The X-ray diffraction pattern of nanocrystalline LaCrO₃ synthesized by combustion method reveals the perovskite phase of the material. The synthesized material has an average crystallite size of 24 nm. The characterization techniques such as X-ray diffraction studies, scanning electron microscopy, transmission electron microscopy etc., were employed to study the average particle size, phase and composition of the material. The effects of constituent phase variation on the dielectric, resistivity and impedance properties are examined. The dielectric dispersion with frequency has been explained on the basis of an electron–hole hopping mechanism, this is responsible for conduction and polarization. From the ac conductivity study, it is confirmed that the conduction in the present material is of small polaron type. Such materials may be useful to fabricate the interconnecting materials for the solar cell, gas sensors, gas transducers, optoelectronic devices, catalyst etc.     

Effect of Ce substitution on structural and superconducting properties of Bi-2212 system

In this study, Bi₂Sr₂Ca_1?xCe_xCu₂O_y, where x = 0, 0.01, 0.05, 0.1 and 0.25, superconducting samples were prepared by solid state method and subsequently used as feed in a laser induced directional solidification (LFZ) process. The physical properties of the samples were investigated by powder X-ray diffraction, scanning electron microscopy, dc-electrical resistance, magnetization, and magnetic-hysteresis loops measurements. It has been found that no significant difference has been observed in the critical transition temperatures of samples except for the sample with the highest Ce additions, which shows the lowest T _C compared with the other doped samples. Magnetic hysteresis measurements have shown that the hysteresis loop is greater than the doped samples. In addition, critical current density values obtained from the hysteresis loops measurements by using Bean’s critical state model show a decrease with Ce-addition. All the results indicate that Cerium substitution for Ca produces the deterioration on the superconducting properties, compared with the undoped sample.     

Effects of Ca and Sr substitution on dielectric properties in ceramics

Modification of dielectric characteristics for Ba₃Sm₃Ti₅Ta₅O₃₀ was performed by Ca and Sr substitution for Ba. The temperature coefficient of the dielectric constant () of Ba₃Sm₃Ti₅Ta₅O₃₀ decreased somewhat by Ca substitution, but the dielectric constant () decreased considerably. In the case of Sr substitution, the temperature coefficient could be markedly decreased without considerable decrease of dielectric constant. Ceramics of the two series could obtain dielectric properties of high (> 100) and low loss (10^–3 at 1 MHz).     

Effect of Mn substitution on structural and magnetic properties of ferromagnetic shape memory alloys

The structure and the magnetic transitions have been investigated as a function of Mn in stoichiometric Ni₂MnGa heusler alloys. Particular attention is paid to examine the linear increase of martensite transformation temperature on substituting Mn for Ga. It is observed that the martensite temperature increases and Curie temperature decreases with the effect of Mn content. Room-temperature magnetic measurements show the composition-dependent characteristics with decreasing magnetic saturation values and increasing coercivity values due to decrease in the magnetic exchange interaction strength with increasing Mn in place of Ga. The scanning electron microscopy image confirms that the Mn-rich alloys have the martensitic plates.     

Room temperature ferromagnetism in triple perovskite Sr3CrFeMoO9

Triple perovskite Sr₃CrFeMoO₉ was designed and prepared in reducing atmosphere. The ceramics are single phase with homogeneous, porous-like microstructure. The valences of the interior Cr, Fe, and Mo cations are +3, +2/+3, 0/+6, whereas that of the surface cations are +3, +3, and +6, respectively. The ceramics show well-saturated magnetization-magnetic field hysteresis loop, the saturation magnetizations are 1.0 and 0.6 μ_B per formula unit at 10 K and room temperature, respectively. The ferromagnetic Curie temperature is determined to be 385 K. Furthermore, semiconductor behavior in the measuring temperature range (10–300 K) and large negative magnetoresistance of ?30.7 % at 10 K are observed. Our results may stimulate further works on room temperature ferromagnetism in triple perovskite.     

Effect of La3+ substitution on the structural,electrical and electrochemical properties of strontium ferrite by citrate combustion method

The La³⁺ substituted nanocrystalline strontium ferrite has been prepared by citrate combustion method using metal nitrate salts as cation precursors and citric acid as a fuel. The structural characteristics of the compounds have been evaluated using XRD and FTIR. The existence of the single-phase perovskite structure with nanocrystalline size has been confirmed from the X-ray powder diffraction patterns. The stretching and bending vibrations of the metal cations are confirmed from the FTIR spectra. The electrical conductivity of the materials is found to increase with increasing temperature measured by using a modified four-probe technique. The electrochemical behavior has been studied by using potentiostatic polarization method in KOH solutions at two different concentrations of 1 and 2 M. From the polarization studies it has been found that the material La_0.4Sr_0.6FeO₃ gives the lowest corrosion rate of 0.001 mmpy in 1 M KOH solution.     

非正分钙钛矿锰氧化物(La0.8 Sr0.2)1-xMnO3的电磁特性

报道了A位缺位的钙钛矿锰氧化物(La0.8Sr0.2)1-xMnO3(0≤x≤0.30)多晶样品的相结构、磁性和磁电阻效应.实验表明,当x≥0.20时,化合物主要由磁性钙钛矿相和非磁性Mn3O4相所组成.它们的电阻率随温度的变化曲线均具有双峰特征,高温侧的电阻率峰出现在钙钛矿相的居里温度附近,低温侧的电阻率宽峰则是金属导电性的钙钛矿晶粒和高电阻率的半导体或绝缘体导电性的晶界或相界共同作用的结果.样品的零场电阻率ρ0随着A位缺位量x的增大而增大.适当改变x值,可以改善磁电阻比的温度稳定性.当x=0.30时,化合物磁电阻比MR在一个相对宽的温度范围内(175～328K)基本上保持不变,即MR=(9.1±0.5)%.     

Structural,magnetic and magnetocaloric properties of La0.65Sr0.35V0.1Mn0.9O3 perovskite

A new complex magnetic material La_0.65Sr_0.35V_0.1Mn_0.9O₃, suitable for the magnetic refrigeration, has been investigated. X-ray diffraction result showed that this compound had rhombohedral structure. The substitution of manganese with V leads to a decrease in the Curie temperature, T_C from 378 K to 353 K. Using Arrott plots; it was found that the phase transition for this compound is of the second-order. The magnetocaloric study exposed a quite large value of the magnetic entropy change ～ 1.56 J/kg K and the relative cooling power value of 67 J/kg at magnetic field variation of 1 T.     

Effect of Gd3+ substitution on dielectric properties of nano cobalt ferrite

CoGd_xFe_2 ? xO₄ (x = 0.0, 0.1, 0.3, 0.5) nano magnetic ferrite particles were synthesized by chemical co-precipitation method. The variation of dielectric parameters like dielectric constant, dielectric loss, capacitance and resistance for different Gd³⁺ compositions has been measured at room temperature for frequency dependence in the range of 100 Hz to 10 MHz using impedance analyzer. Results of measurements reveal strong dependence of dielectric parameters on frequency and Gd³⁺ ion content. Dielectric constant, dielectric loss, capacitance and resistance decrease with increasing frequency for all the CoGd_xFe_2 ? xO₄ compositions. Increase in Gd³⁺ ion composition in material, increases the values of dielectric constant, dielectric loss and capacitance while decreases the electrical resistance of nano-particles. A qualitative explanation is given for the composition and frequency dependence of dielectric parameters.     

Nanoindentation of LaCrO3 thin films

Nanoindentation of LaCrO₃ thin films deposited by radio-frequency magnetron sputtering onto stainless steel substrates was performed using an XP Nanoindenter. The “as-deposited” film was amorphous but transformed to an orthorhombic LaCrO₃ perovskite structure after annealing at 1073 K for 1 h. The film thickness in the “as-deposited” state was 800 nm. Single loading/unloadings were performed in the displacement control mode on the crystalline film using different maximum displacements (50, 200, 400, and 800 nm). Therefore, the integral response of the film−substrate system was probed at different distances from the substrate. Nanoindentation experiments on LaCrO₃ perovskite films revealed sharp “pop-in” events at certain loads. Such “pop-ins”, are most likely caused by the orthorhombic-to-rhombohedral phase transition which is known to occur in a LaCrO₃ perovskite structure under pressure. However, such discontinuities have never been observed upon indentation of the amorphous “as-deposited” La-Cr-O thin films, and the pressure found to be typical of this transition in the LaCrO₃ thin films is higher than previous bulk LaCrO₃ sample studies. Mechanical characteristics of the films, such as hardness and Young’s modulus, were also measured.