Role of Nd-Ni on structural,spectral and dielectric properties of strontium-barium based nano-sized X-type ferrites

The influence of neodymium and nickel substitution on structural and dielectric parameters was investigated in strontium-barium X-type hexagonal ferrites having composition SrBaCu_2?xNi_xNd_yFe_28?yO₄₆ (x = 0, 0.2, 0.4, 0.6, 0.8, 1 and y = 0, 0.02, 0.04, 0.06, 0.08, 0.1). Sol-gel method was employed for synthesizing these hexagonal ferrites. The XRD plots of all studied materials which were annealed at 1250 °C show single phase characteristics. Lattice parameter ‘c’ increased as a consequence of larger radius of rare earth ion (Nd³⁺) as compared to (Fe³⁺), while lattice parameter ‘a’ showed very small variation. The cell volume was obtained in the range 2508.32–2523.75 (ų). The inclusion of Nd-Ni also affected X-ray density, bulk density and porosity. The FTIR spectroscopy indicated the particular absorption peaks of hexagonal ferrites and it was performed in the range of 500–700 cm^?1. On account of Nd-Ni doping, the dielectric constant, dielectric loss and AC-conductivity showed decreasing trend. The occupancy of Nd³⁺ ions at octahedral site impedes the valence alternation of Fe³⁺; therefore there was decrease in dielectric permittivity. Ac conductivity has been decreased from 9.14 to 6.49 (Ω cm)^?1 at frequency of 2.7 GHz. The Cole-Cole plots of synthesized materials noticeably revealed grain boundary contribution. The appearance of single semi-circle in impedance Cole-Cole graphs confirms the exceptional role of grain boundaries in the conduction process. The considerably lower dielectric parameters of investigated nano X-type ferrites propose their feasibility for high-frequency applications (phase shifters, dielectric resonators, stealth technology etc).     

Dielectric properties of Sr0.5Ca0.5TiO3: x Pr3+ ceramics

The frequency dependent dielectric and AC conductivity properties of different concentrations of Pr³⁺ doped Sr_0.5Ca_0.5TiO₃ ceramics were investigated for the frequency range 100 Hz to 2 MHz at different temperatures. The morphology of the prepared samples was analyzed by using Field-Emission Scanning Electron Microscope images. The value of dielectric constant and dielectric loss decreases with increase in the frequency of the applied signal in all the samples. Also, the value of dielectric constant and dielectric loss decreased with doping of different concentrations of Pr³⁺ ions. The conductivity of the samples obeys Jonscher's power-law and shows a decrease with increasing doping concentration of Pr³⁺ ions. The higher value of real and imaginary part of impedance at lower frequency indicates the space charge polarization of the material and its absence at higher frequencies was confirmed from the low value of impedance at higher frequency region. The Cole- Cole parameters of the samples were calculated and the semi-circle observed indicates a single relaxation process and can be modeled by an equivalent parallel RC circuit. At lower frequency region, the value of dielectric constant and dissipation factor increases with increase in the temperature. Also the value of conductivity increases with temperature at high frequency region, due to the enhanced mobility of charge carriers.     

弥散相变型铁电体的介电谱

在不同温度下测量了Ba0.74Sr0.26TiO3陶瓷从10^-2～10^6Hz的介电谱。在超低频下，由于空间电荷激发的贡献，样品的介电常数在较高温度下远超过58℃时的居里峰。此时的Cole-Cole图在复ε平面上成为一段圆弧，圆心在实轴以下。     

高温下尼龙11介电谱的研究

研究了尼龙１１在高温下的介电松驰温度谱和频率谱，发现了１个新的超强介电松弛峰－αＨ松弛峰，其松驰强度ΔＥ可达２５３００，松驰活化能ΔＥ为８９．１ｋＧ／ｍｏｌ，αＨ松驰十分符合拜松驰模型，相对介电常数ε′和ε″的柯尔－柯尔图接近于１个半圆，αＨ松驰是由于在高温低频下尼龙１１中不连续氢键形成的缺陷偶极子在电场的作用下产生界面极化而引起的。     

IGBT器件封装用有机硅凝胶宽频介电特性与温度影响

有机硅凝胶材料作为IGBT器件封装用绝缘材料,在器件的运行工况下,器件承受着重复性导通关断电压,对应频谱宽,且器件损耗将引起温度升高。为了能够准确分析器件内部电场特性,运用频域介电谱技术对有机硅凝胶在宽频、宽温度范围内的介电特性进行研究,利用叉指电极,实现对有机硅凝胶在不同温度下的宽频介电谱测试。采用Cole-Cole介电模型对实验数据进行拟合,并分析温度对Cole-Cole模型特征参量的影响规律。研究结果表明：频率和温度对有机硅凝胶的介电特性均有较大影响,在低频高温下,有机硅凝胶材料相对复介电常数的实部、虚部都显著增加。在所提取的Cole-Cole介电模型的特征参量中,直流电导率σ₀及特征参量Δε₁与温度之间的关系都满足Arrhenius方程,热活化能分别为0.233 eV与0.691 eV;弛豫时间τ₁和τ₂随温度的变化规律有所不同,但在高温时都明显增加。对半导体器件封装用有机硅凝胶材料介电特性的认知可以为器件内部电场分析和绝缘设计提供基础数据支撑。     

IGBT器件封装用有机硅凝胶宽频介电特性与温度影响

有机硅凝胶材料作为IGBT器件封装用绝缘材料,在器件的运行工况下,器件承受着重复性导通关断电压,对应频谱宽,且器件损耗将引起温度升高。为了能够准确分析器件内部电场特性,运用频域介电谱技术对有机硅凝胶在宽频、宽温度范围内的介电特性进行研究,利用叉指电极,实现对有机硅凝胶在不同温度下的宽频介电谱测试。采用Cole-Cole介电模型对实验数据进行拟合,并分析温度对Cole-Cole模型特征参量的影响规律。研究结果表明：频率和温度对有机硅凝胶的介电特性均有较大影响,在低频高温下,有机硅凝胶材料相对复介电常数的实部、虚部都显著增加。在所提取的Cole-Cole介电模型的特征参量中,直流电导率σ₀及特征参量Δε₁与温度之间的关系都满足Arrhenius方程,热活化能分别为0.233 eV与0.691 eV;弛豫时间τ₁和τ₂随温度的变化规律有所不同,但在高温时都明显增加。对半导体器件封装用有机硅凝胶材料介电特性的认知可以为器件内部电场分析和绝缘设计提供基础数据支撑。     

考虑激电效应和电磁效应的复电阻率法二维数值模拟研究

复电祖率实测数据是包含激电效应和电磁效应的影响,而传统的复电祖率法二维数值模拟只考虑激电效应会影响后续资料处理结果的精度,因此本文开展同时考虑激电效应和电磁效应的复电祖率法二维正演研究。本文正演是基于二次场满足的麦克斯韦方程组,通过傅里叶变换将其变换到波数域,结合有限单元法,引入Cole-Cole模型,形成正演矩阵方程,采用超松弛迭代双共轭梯度法求解该方程组可得波数域结果,再由反傅里叶变换将其转换至空间域,完成了正演研究。然后通过几个模型算例的计算验证了该算法的正确性,也为复电阻率法二维反演奠定了基础。     

纳米改性变压器油-纸复合绝缘频率响应特性

为寻求提高变压器油纸复合绝缘特性可能途径,开展了纳米改性变压器油-纸复合绝缘频率响应特性研究。利用电介质频率响应法,对比测试了添加纳米改性前后油纸复合绝缘不同微水含量、在不同温度条件下,10^-4～10^-6Hz范围频率响应;建立基于Cole-Cole函数的弛豫模型分析实验结果,并通过最小二乘法拟合得到弛豫模型参数;分析小同微水含量及温度条件下添加纳米粒子对弛豫参数的影响。结果表明：所建立的弛豫模型能较好地反映油纸复合绝缘频率响应特性,弛豫模型参数能反映油纸复合绝缘状态的变化规律;添加纳米粒子引入了新弛豫机制,在低频及高频段,降低了油纸复合绝缘的介质损耗,在0．1～100Hz频率范围内却增大了其介质损耗,且随着温度升高及微水含量增加,影响越显著。     

考虑频变参数的油浸式变压器绕组分数阶传输线模型

建立精确的变压器绕组模型对于准确分析研究绕组各处的电压分布具有重要意义。导线的趋肤效应和油纸介电常数的频变特性分别是造成变压器绕组传输线模型的阻抗和导纳参数频变的主要因素。然而,以往的变压器绕组传输线模型通常将油纸介电常数的频变特性和导线的趋肤效应忽略,影响暂态过电压分布计算的准确性。该文以具有单弛豫过程的Cole-Cole模型为基础,对油浸纸样品的实测介电常数进行拟合,利用拟合模型和平行板电容公式,推导出变压器绕组传输线模型的分数阶导纳参数,进而建立变压器绕组的分数阶传输线模型。通过对分数阶传输线模型进行仿真计算,得到绕组上的过电压波形,将仿真值与实验实测值以及未考虑频变参数时建立的整数阶传输线模型仿真值进行对比,验证了所提分数阶模型的准确性。     

Structural,dielectric, electrical and optical properties of Li/Fe modified barium tungstate double perovskite for electronic devices

In this communication, the synthesis (solid-state reaction route) and characterization (structural and electrical properties) of Li/Fe modified barium tungstate BaWO₄ of chemical formula (BaLi)(FeW)O₆ double perovskite have been reported. Analysis of room temperature X-ray diffraction data shows the formation of a new single-phase (double perovskite) of tetragonal symmetry. The calculated average crystallite size and lattice strains are found to be 42 nm and 0.111% respectively. The scanning electron micrograph shows that grains are distributed uniformly with less porosity in the material. Detailed studies of field (frequency) and thermal (temperature) dependence of capacitive (dielectric) and resistive (impedance and conductivity) characteristics of the prepared material have shown the dielectric dispersion, relaxation, and non-Debye type of conduction mechanism respectively of the material. The existence of a non-Debye type of relaxation process and thermally activated relaxation process in the material has been analyzed by using impedance spectroscopy and conductivity techniques respectively. The presence of the depressed semicircular arcs in both Nyquist and Cole-Cole plots confirms the semiconductor nature of the material, which is well supported from ZSIMPWIN fitted impedance data. Study of room temperature Raman spectra supports the incorporation of Li/Fe of LiFeO₂ in BaWO₄ to produce new double perovskite of a composition BaLiFeWO₆ of tetragonal symmetry. Further, as the temperature dependence of resistance shows a good stability factor, sensitivity factor, and thermistor constant, the material may be useful for thermistor-related devices. The nature of I–V characteristics confirms the presence of the Ohmic type of conductivity of the material. The energy bandgap of 2.85 eV, obtained from the UV–visible spectrum, suggests the material may also be used for optoelectronic devices.