Effects of Ni doping on B-site ordering and magnetic behaviors of double perovskite Sr2FeMoO6

Sr₂FeMoO₆ of double perovskite structure has attracted much attention recently, owing to its substantial low-field magnetoresistance reported at room temperature. In our study, a mechanical activation process is devised for successful formation of Ni-doped double perovskite Sr₂(Fe_{1 − x} Ni _x )MoO₆ (0.00 ≤ x ≤ 0.20). For the first time, a single phase of double perovskite is synthesized in air by using MoO₂ as the starting material. Ni doping in Sr₂FeMoO₆ significantly affects its long-range B-site ordering and magnetic behaviors, where the B-site long-range order parameter is progressively enhanced with increasing Ni doping level. Both magnetization and the Curie temperature in Sr₂FeMoO₆ are enhanced significantly by Ni doping, where they both depend almost linearly on the degree of B-site long-range ordering.     

Determination of polaronic conductivity in disordered double perovskite La2CrMnO6

Journal of Electroceramics - Double perovskite La2CrMnO6 ceramics was sintered using standard high temperature route in ambient air. The orthorhombic Pbnm cell characterized by a total disorder...     

AC and DC conductivity studies in pulsed laser ablated (Ba,Sr)TiO3 thin films

Abstract

A comparison of the ac conductivity of laser ablated (Ba, Sr)TiO₃ thin films was made for films grown at different substrate temperatures. The ac conductivity was studied as a function of frequency and ambient temperature ranging from room temperature to 460°C. Conductivity results from ac and dc measurements were compared in their an respective Arrhenius plots revealing interesting coincidences. The value of the activation energies computed from the Arrhenius plot of à _ac with 1000/T ranged from 0.97 to 1.3 eV in the high temperature region to 0.36 to 0.54 eV in the low temperature region for different samples. The activation energies obtained from pure dc measurement for fields greater than 100 kV/cm across the samples were in the range of 1.06 to 1.32 eV for different samples. The similarity in results suggests a common origin in the de conduction process in the concerned temperature range and was attributed to ionic conduction resulting from oxygen vacancy motion which has been observed to be significant at high temperatures and high fields. For high temperature grown samples the value of activation energy computed from the Arrhenius plots was comparatively less than those grown at lower substrate temperatures. The difference was attributed to the microstructure and the effect of grain boundaries on the motion of oxygen vacancies leading to the conduction process.     

Dielectric and ferroelectric properties of Nb-doped Ba0.8Sr0.2TiO3 ceramics

Ferroelectric and dielectric properties were investigated for Ba_0.8Sr_0.2Ti_(1?5/4x)Nb _x O₃ ceramics with different Nb₂O₅ concentrations. The relations between the ceramic structures and those properties were discussed. The Ba_0.8Sr_0.2TiO₃ doping with 0.01mol% Nb₂O₅ appears to have a strong ferroelectric effect and better dielectric properties. The max permittivity (? _max) is up to 7,521.3 and Ba_0.8Sr_0.2Ti_(1?5/4x)Nb _x O₃ ceramics has higher permittivity even at room temperature. The permittivity presents broadened curves at large temperature ranges, which suggests a non Curie–Weiss behavior near the transition temperature. The diffuse phase transition coefficient (δ) for Ba_0.8Sr_0.2Ti_(1?5/4x)Nb _x O₃ doping with 0.01mol% Nb₂O₅ reaches 0.098, and its P–E loop expresses a diffusing curve. The remanent polarization (2P _r) and coercive field are 31.3 μC/cm² and 10 kV/cm, respectively. The P–E loop presents a diffusing curve, which is relative to the relaxor characteristic.     

交流谐波经MMC的传导机理及叠加特性研究

连接大量交流源、荷是基于模块化多电平换流器(MMC)的中压柔性直流配电网的未来应用场景之一。然而,非理想交流源、荷产生的谐波扰动会使MMC桥臂出现电压波动并激发环流,进而使桥臂电流畸变,导致直流侧出现纹波。为研究交流谐波经MMC的传导机理,通过平均开关函数和桥臂瞬时功率方程,推导了交流负荷谐波电流和交流系统背景谐波电压在MMC内部激发的桥臂电压波动及环流表达式,分析了其传导机理和规律。同时,分析了不同次交流谐波经MMC的传递叠加特性及其对直流侧纹波含量的影响。仿真结果验证了理论分析的正确性。     

尖晶石相LiMg0.2Mn1．8O4的制备与性能研究

以氢氧化锂、柠檬酸以及醋酸锰、醋酸镁为原料,利用低温固相法制备了LiMn2O4及其Mg元素的掺杂产物,采用XRD、FTIR和恒流充放电测试研究了合成产物的性能。XRD测试表明,所有产物均为尖晶石相结构,Mg离子能很好地溶入尖晶石相产物的晶格之中;FTIR结果显示,经过Mg元素掺杂改性后,产物中的Mn（Ⅳ）-O和Mn（Ⅲ）-O键分别存在着蓝移和红移现象;电化学测试则表明,LiMg0.2Mn1．8O4的初始电化学容量较LiMn2O4低,但经过一定的循环次数后,电化学容量超过了LiMn2O4,且在整个循环过程中LiMg0.2Mn1．8O4的容量衰减率较小,循环性能相对于未掺杂前的产物得到了较大的提高。     

Relationship between the dielectric properties and the conductivity of Ba2FeNbO6

In the present work, we performed detailed investigation on the relationship between the dielectric properties and the conductivity of Ba₂FeNbO₆. Our results revealed that (1) the colossal dielectric behavior of the sample can be well understood based on the framework of universal dielectric response, (2) the observed relaxation shows a distinct deviation from the Arrhenius behavior, and (3) the peak intensity of dielectric loss can be well expressed by a relation similar to the Fermi?CDirac distribution function rather than a thermally activation relation. These are the typical features for the hopping motions of polaronic carriers demonstrating that the dielectric properties and the conductivity of Ba₂FeNbO₆ are closely linked.     

Impedance spectroscopy and conduction mechanism of NaYP2O7 ceramic compound

Journal of Electroceramics - In the present study, NaYP2O7 compound was prepared by solid-state method and was characterized by powder X-ray diffraction and Raman spectroscopy. The X-ray...     

Synthesis and electric characterization of rare earth double perovskite Ho2CdZrO6 ceramics

A new double perovskite oxide holmium cadmium zirconate Ho₂CdZrO₆ (HCZ) is synthesized by solid state reaction technique. The crystal structure has been determined by powder X-ray diffraction (XRD) which shows monoclinic phase at room temperature. The frequency dependent electrical data are analyzed in the framework of conductivity and electric modulus formalisms. The scaling behavior of imaginary electric modulus suggests that the relaxation describe the same mechanism at various temperatures. Nyquist plots are drawn to identify an equivalent circuit and to know the bulk and interface contributions. The conduction mechanism is explained by Mott??s theory. At the high temperature range, conductivity data satisfy the variable range hopping (VRH) model. In this regime, the conductivity of sample obeys Mott??s T ^1/4 law indicating 3D charge transport in HCZ compound. High temperature data indicates the formation of thermally activated small polarons.     

Synthesis and sintering properties of (La0.8Ca0.2−x Sr x )CrO3 perovskite materials for SOFC interconnect

Synthesis and sintering properties of the (La_0.8Ca_0.2−x Sr _x )CrO₃ samples doped by two alkaline earth metals in comparison to the doped only by one alkaline earth metal were evaluated by phase analysis, sintering properties, thermal expansion behaviors, and electrical conductivity. The sintered (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, and 0.1) and (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0.2) were found to have orthorhombic and rhombohedral symmetries, respectively. Relative density of the (La_0.8Sr_0.2)CrO₃ sample sintered at 1500^∘C for 5 h was lower than that of the (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, and 0.1) sample. TECs of the (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, 0.1, and 0.2) in air were 11 × 10⁻⁶/^∘C, 11.2 × 10⁻⁶/^∘C, 11.2 × 10⁻⁶/^∘C, and 11.3 × 10⁻⁶/^∘C, respectively. The electric conductivity of the (La_0.8Ca_0.2−x Sr _x )CrO₃ sample was determined.     

Structural characterizations, dielectric properties and impedance spectroscopy analysis of Nd x Sr1-1.5x TiO3 ceramics

The Nd _x Sr_1–1.5x TiO₃ ceramics (x?=?0, 0.008, 0.024, 0.056, 0.104, 0.152 and 0.200) were prepared by solid state reaction method. X-ray diffraction (XRD) results indicated pure perovskite phase for all ceramics and the structure changed from cubic to tetragonal by increasing x value. Scanning electron microscopy (SEM) images showed high dense microstructure for all ceramics, and the grain size was strongly suppressed to ~1 μm for Nd _x Sr_1–1.5x TiO₃ ceramics with x?=?0.008, then gradually increased by further increasing x value and reached ~10 μm when x?=?0.200. The relative dielectric constant ε _r was strongly enhanced to 5,060 for the sample with x?=?0.024, while its dielectric loss tanδ was still lower than 0.03. Together with its relatively high breakdown strength E _b?>?10 kV/mm, this ceramic should be a good candidate for high-voltage capacitor applications with enhanced energy storage densities. Impedance spectroscopy analysis revealed that the Nd _x Sr_1–1.5x TiO₃ ceramics showed electrically heterogeneous, and the space charge polarization at interfaces should contribute to the observed high dielectric constant ε _r.     

Dielectric tunability and microwave properties of Ba0.5Sr0.5TiO3-BaMg6Ti6O19 composite ceramics for tunable microwave device applications

Ba_0.5Sr_0.5TiO₃-BaMg₆Ti₆O₁₉ microwave composite ceramics with low dielectric constant and relatively high tunability were fabricated via the solid-state reaction method. The microstructures and microwave dielectric properties of the composite ceramics have been investigated. BaMg₆Ti₆O₁₉ and Ba_0.5Sr_0.5TiO₃ can be friendly coexistent in the composite material system without obvious chemical reactions. With increasing content of BaMg₆Ti₆O₁₉ from 10 wt.% to 60 wt.%, the dielectric anomalous peaks of ferroelectric-paraelectric phase transition for the composite ceramics are suppressed and broadened. The dielectric constant can be effectively tailored from 2035 to 129. For composite ceramics with 60 wt.% content of BaMg₆Ti₆O₁₉, the dielectric loss still keeps around 0.002 and the tunability is 13.4% measured at a dc-applied electric field of 30 kV/cm. The Q value of composite ceramics with 20 wt.% content BaMg₆Ti₆O₁₉ is 367 and the dielectric constant is cut down to 665 at the microwave band of 1.579 GHz.     

Dielectric and piezoelectric properties of La2O3 doped (Bi0.5Na0.5)0.92(Ba0.8Sr0.2)0.08 TiO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (Bi_0.5Na_0.5)_0.92(Ba_0.8Sr_0.2)_0.08 TiO₃+x mol% La₂O₃(x = 0, 0.1, 0.3, 0.5, 0.8) were synthesized by conventional solid state reaction. The crystal structure of all compositions is mono-perovskite ascertained by XRD. The grain size decreased and diffuse phase transition behavior was more evident with the increasing amount of La₂O₃. The piezoelectric constant d₃₃ and the electromechanical coupling factor k_p showed the maximum value of 165 pC/N and 0.322 at 0.3% and 0.1% La₂O₃ addition, respectively, and rapidly decreased when La₂O₃ addition over 0.5%. The loss tangent tanδ linearly increased and the mechanical quality factor Q_m linearly decreased with the increasing amount of La₂O₃.     

Contribution of dielectric and metallic losses in RF/microwave tunable varactors using (Ba,Sr)TiO3 thin films

Abstract

Recently, there has been significant interest in use of (Ba,Sr)TiO₃ (BST) thin films for tunable high frequency (RF and microwave) components. In a previous work we have shown that BST thin films grown by metalorganic chemical vapor deposition (MOCVD) exhibit films very low losses (as low as 0.003–0.004) and tunabilities over 50% at low operation voltages.

In order to integrate BST thin films in tunable devices, the objectives of this work are : (i) study the effect of bottom and top electrode on the performance of thin film based capacitors, (ii) correlate low frequency (10 kHz) and high frequency (45 MHz - 1 GHz) measurements, (iii) separate the contribution of dielectric losses and metallic losses at both low and high frequency and finally (iv) show the potential usefulness of series capacitors structures.     

Structural study of (1-x)BNKLT-xBZT ceramics using XRD,Raman spectroscopy and XAS

Abstract

The aim of this work is to study the structural phase of (1-x)Bi_0.5(Na_0.74K_0.16Li_0.10)_0.5TiO₃–xBaZr_0.05Ti_0.95O₃ ((1-x)BNKLT-xBZT) ceramics with various x content between 0.025 and 0.150?mol% synthesized via the combustion method. The X-ray diffraction (XRD), Raman spectroscopy and X-ray absorption spectroscopy (XAS), including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), were used to investigate the structural phases of all samples. The XRD pattern and Raman spectra revealed coexisting phases of rhombohedral (R) and tetragonal (T) in all compositions. As the BZT content increased, the phase formation in these samples exhibited higher tetragonality. The quantity of each phase in (1-x)BNKLT-xBZT ceramics can be determined by the XAS technique with Ti K-edge measurements. Increasing the BZT content from 0 to 0.15?mol% caused the R phase to suddenly deceased from 60.9% to 20% while the T phase rapidly increased from 39.1% to 80%. The phase of the structural was determined by several alternative strategies including XRD, Raman and XAS techniques.