The anisotropy of ac conductivity and dielectric constant of anisotropic conductor–insulator composites

We study the complex ac admittance tensor (ac conductivity and dielectric constant) of anisotropic conductor–insulator composite materials, based on anisotropic two-dimensional RC-networks consisting of randomly placed conductors and capacitors with different conductor existence (bond occupation) probabilities in two directions. We calculate numerically each component of the complex ac admittance tensor by applying a transfer matrix method and reveal the effect of the anisotropy of the bond occupation probability on the frequency characteristics of the ac admittance tensor. It is found that the dual relation holds for each diagonal component of the complex admittance tensor of the anisotropic two-dimensional RC-network. For the effective conductance in the metallic region, the anisotropy depends not only on the anisotropy of the bond occupation probability, but also on the frequency ω. We derive the analytical relation between the anisotropy of the conductance and the anisotropy of the bond occupation probability, at both the dc limit and ω RC = 1. The calculated results on the ac admittance are compared with the effective medium theory and how the accuracy of the theory is related with the microscopic current path is clarified.     

Equivalent network representation for thickness vibration modes in piezoelectric plates with a linearly graded parameter

Equivalent network representation is derived for thickness vibration modes in piezoelectric plates with a linearly graded parameter. The network is composed of a transmission line of a finite length, which is linked to the electric excitation port via ideal transformers connected serially at both ends. The ratios of the two transformers are different from each other and frequency dependent. Two frequency-dependent capacitors of the same value but opposite sign appear at the electric port. The frequency characteristic of the input electric admittance of the resonator, which shows the unique feature of the graded piezoelectric plate, is demonstrated by the equivalent network analysis.     

Frequency Dependence of Electrode Surface Effects in Parallel-Plate Capacitors

Electrode surface effects in parallel-plate capacitors lead to a variation in capacitance and conductance with frequency. A method is presented for the determination of absolute changes in capacitance and absolute values of conductance, as functions of frequency. The method involves the use of two variable parallel-plate capacitors, and requires the measurement of relative capacitance and conductance values over a range of frequencies and at a minimum of two different electrode spacings. A capacitor cell, incorporating two variable capacitor sections is described. The cell is designed for use in the investigation of the frequency characteristics of a range of electrode surfaces. Two current transformer ratio-arm bridges are used to perform the relative admittance measurements. The bridges are compensated for capacitance loading, and together cover the frequency range 11 Hz-52 kHz. Measurement precision is a few parts in 108 for each admittance component. Details of the bridges and measurement techniques are discussed and corrections for mechanical resonance effects and lead impedances are considered. Results are given for rhodium plated and plain brass electrodes under vacuum. Over the full frequency range the fractional capacitance change for rhodium, when referred to a 1-mm electrode spacing, is <4 × 10-7. Tan ? is < 1.5 × 10-7. Oil films and organic contaminants on electrode surfaces are thought to be the most likely sources of frequency dependence.     

Effects of moisture absorption on the electrical parameters of embedded capacitors with epoxy-BaTiO3 nanocomposite dielectric

In this work the effects of moisture absorption on the electrical parameters of embedded capacitors is investigated. Capacitors of two different areas embedded inside a four-layered printed wiring board were selected for this work. The dielectric was a nanocomposite of epoxy and BaTiO₃ which is common dielectric material used in embedded capacitors. These capacitors were exposed to elevated temperature and humidity conditions (85?°C and 85% RH) and two parameters, capacitance and dissipation factor, were measured in situ. The diffusion of moisture in the dielectric was also modeled using the finite element method (FEM), and the changes in electrical parameters were calculated theoretically. The FEM methodology was then verified by applying it on capacitors of different dimensions.     

Properties of thin film capacitors made with reactively sputtered Ta-Al

The dielectric properties of oxides formed on Ta-Al-N films were investigated. The capacitors made from these films exhibit excellent a.c. and d.c. properties and are comparable in performance with TaN capacitors. Owing to the high resistivity of Ta-Al-N films an aluminum underlayer was used for these capacitors to reduce losses at high frequencies. It is concluded that these capacitors are more desirable for applications in hybrid integrated circuits when used together with Ta-Al-N resistors.     

Standard-independent estimation of dielectric permittivity with microdielectric fringe-effect sensors

Microdielectric spectroscopy with planar fringe-effect (FE) interdigital sensors is a useful method for noninvasive characterization of the interfacial properties of the materials. Unfortunately, obtaining an accurate dielectric spectrum is difficult because of the complexity of the probing electrical field created by the FE sensor and the contribution of the sensor substrate and stray elements to the overall measurements. Previously, quantitative microdielectric spectroscopy required the calibration of the FE sensor with standard materials that are known to be dielectrically similar to an unknown sample of interest. This limitation complicates the application of microdielectric spectroscopy, particularly in cases where the monitored sample undergoes a transformation that changes its dielectric permittivity. A standard-independent method for quantitative FE microdielectric measurements is proposed in this paper. The developed method is based on comparison of the theoretically predicted admittance of the FE sensor with the sample of known dielectric properties and the measured sensor admittance. Comparison of the theoretical predictions with the admittance measurements reveals the contribution of the unknown stray elements. The measurements with an unknown sample are then adjusted for the strays. The contribution of the sensor substrate to the sensor measurements is removed using the theoretical model derived from the electroquasistatic approximation of Maxwell equations. The dielectric permittivity of the material being tested is calculated by successively solving the system of complex nonlinear equations for each frequency at which the sensor admittance is measured. The developed method is illustrated by applying it to the dielectric measurements of several dissimilar samples. The results are in excellent agreement with those obtained using the gold standard parallel-plate measurement method over the entire range of frequencies.     

Admittance matrix of asymmetric piezoelectric bimorph with two separate electrical ports under general distributed loads

The dynamic admittance matrix of the asymmetric triple-layer piezoelectric bimorph subjected to the general distributed harmonic loads as well as the flexural moments and the vertical loads at the tip are presented. The top and bottom piezoelectric layers have two separate electrical ports such that each layer can be used as either a sensor or an actuator. The variation principle is used for deriving the motion equations and the conjugate parameters that maintain the symmetry of the admittance matrix. The mechanical displacements and forces at the tip are expressed in a matrix form, which, together with the reciprocal condition, greatly simplify the analysis procedure. The derived admittance matrix under the cantilevered condition is presented by a five-by-five matrix, each row representing the relationships of the displacement and rotation at the tip, the volume averaged displacement, the separate electrical charges with the flexural moment and vertical load at the tip, the magnitude of the distributed load, and the voltages. The matrices, which reduce to simpler forms for several special cases, are then used to determine the two-port electrical admittance. It is shown that the derived admittance matrix covers the various boundary conditions, the electrical parallel and series connections, and the arbitrary lay-up, including the unimorph, used as both sensors and actuators     

A mathematical model of the admittance between tapped windings andtheir shield for ratio instruments

A mathematical model is developed for the equivalent admittance dependent on the measurable admittance distributed between the shield and individual sections of a tapped winding. The model makes possible the calculation of ratio instrument errors, correcting admittances, and the input admittance caused by the admittance to the shield. These parameters strongly depend on both the type of supply and the location of the shield with respect to the winding connection. Experimental results are presented that demonstrate the usefulness of the model for an enhancement of ratio instrument accuracy     

Simplified Noise Evaluation of Microwave Receivers

The noise figure for arbitrary source admittance can be computed from the noise figure at any one source admittance and the output noise when passive one-ports are connected to the receiver input, if input admittance is known. The procedure is demonstrated experimentally at 4000 MHz with offset short circuits as the passive one-ports. The input admittance determined by mismatched noise sources agrees with network analyzer measurements.     

Capacitance-voltage measurements on Schottky diodes with poor ohmiccontacts

The frequency-dependent admittance of Al-GaAs Schottky diodes with nonideal ohmic contacts is tested, using a simple three-component small-signal equivalent circuit model. Both the ohmic and the rectifying contacts of this diode are produced during one process step. This simplifies the formation procedure and saves testing time and costs. However, the nonalloyed ohmic contacts lead to a high series resistance, minority carrier injection, and deep level influence. All of these effects give a frequency-dependent diode admittance. Frequency-dependent admittance analysis in a certain frequency range using the three-component equivalent circuit model leads to the space-charge capacitance of the diode reflecting only the free majority carriers. The method is highly suitable for the automatic routine control of semiconductor material properties, diode, or gate capacitances     

Detection of capacitor electrolyte residues with FTIR in failure analysis

Detecting leakage from liquid aluminum electrolytic capacitors is not easy. Typically there is very little evidence of leakage because the electrolyte is volatile and leaves behind only trace residues. Liquid aluminum electrolytic capacitors are known to cause catastrophic failures where there is complete loss of functionality due to a short or open circuit. In the study presented in this paper, printed circuit board assemblies from a test and measurement system used in a clean room environment failed. Two units failed, causing burning in a particular area on the printed circuit board assembly. The failure area included several surface mount liquid aluminum electrolytic capacitors, and several others were mounted very close to the burnt region. A study was initiated to evaluate the cause of failure. Careful optical inspection revealed some residues on the outer side of the rubber seals of two of the electrolytic capacitors. Through using Fourier transform infrared analysis and a process of experimentation and analysis, it was determined that the residues were produced by liquid electrolyte that leaked out of the capacitor at some point in the field. The leaked electrolyte that came out of the capacitor was believed to be the cause of failure that led to the burning of the printed circuit board assembly.     

Small-signal admittance for Schottky-Richardson emission into an organic layer

The work presents a model of small-signal admittance for Schottky-Richardson emission into an organic layer. The discussion on both parts of complex admittance has been performed. Moreover, the analysis of direct current and small-signal admittance results obtained on a gold/molibdenyl phthalocyanine/gold sandwich system is included.     

Harmonic admittance and dispersion equations--the theorem

The harmonic admittance is known as a powerful tool for analyzing the excitation and propagation of surface acoustic waves (SAWs) in periodic electrode arrays. In particular, the dispersion relationships for open- and short-circuited systems are indicated, respectively, by the zeros and poles of the harmonic admittance. Here, we show that a strict reverse relationship also exists: the harmonic admittance of a periodic system of electrodes may always be expressed as the ratio of two determinants, which have been specifically constructed to describe the eigenmodes of the open- and short-circuited systems. There is no need to solve these equations to find the admittance. The existence of a connection between the excitation and propagation problems was recognized within the coupling-of-modes theory by Chen and Haus (1985) and was recently used to model surface transverse waves by Koskela et al. (1998), but a rigorous mathematical proof was only found later by Biryukov (2000). Here, we reproduce this theorem in detail, give some examples of calculations based on this theorem, and compare the results with measured admittance curves.     

Integrated ferroelectric stacked mim capacitors with 100 nF/mm2and 90 V breakdown as replacement for discretes

This paper shows for the first time integrated thin film ferroelectric metal-insulator-metal capacitors on silicon with a record high capacitance density above 100 nF/mm² combined with a breakdown voltage of 90 V and a lifetime exceeding 10 years at 85degC and 5 V. The high capacitance density was obtained by a combination of material optimizations resulting in a dielectric constant of 1600, and stacking of capacitors. The reliability of these ferroelectric capacitors was studied in detail with accelerated lifetime testing. The high performance of the integrated capacitors in this paper shows great potential for applications demanding high capacitance densities combined with electrostatic discharge protection.     

矩形断面抖振力展向相关性的试验研究

为了研究紊流作用下钝体的非定常气动力(抖振力),以矩形断面为例,通过测压试验研究不同紊流积分尺度下的非定常气动力的分布特性,并结合理论分析方法研究了现有导纳识别方法的缺陷,进而提出基于三维统计理论的两波数三维导纳初步识别方法。结果表明:对于矩形这类钝体断面,其在紊流下的抖振力特性与流线型箱梁(桥梁主梁)接近,即抖振力的相关性高于脉动风速的相关性,且与紊流积分尺度成正比。另外,理论分析结果与实测数据间存在较大的差异,进一步表明传统的二维导纳函数无法反映真实的非定常气动力展向分布情况,而两波数的三维导纳则可揭示展向不同尺度漩涡都对抖振力的贡献。     

基于压电导纳的钢框架螺栓松动检测试验研究

介绍了基于压电导纳的结构健康监测技术基本原理;利用该技术对钢框架结构进行了螺栓松动检测试验研究。即将三个压电陶瓷片粘贴在钢框架节点处的不同构件表面,通过测量框架节点处各压电陶瓷片在螺栓松动前后电导纳的变化来识别损伤.试验结果发现,位于连接板处的压电陶瓷片对该节点处的螺栓松动最敏感,而与螺栓不直接相连的上斜撑上的压电陶瓷电导纳则几乎不受连接板螺栓松动的影响。分别定义了基于电导纳实部和虚部改变的均方根(RMSDR和RMSDI)作为损伤程度识别指标,对框架节点的螺栓松动损伤程度进行了定性识别研究。通过对得到的RMSDR和RMSDI两种损伤指标值对比发现,基于压电导纳实部信息定义的损伤程度识别指标RMSDR能较好地识别框架节点螺栓松动损伤程度,而基于电导纳虚部定义的识别指标RMSDI则不能正确识别。     

用于结构健康诊断的压电阻抗技术

利用自驱动、自传感特性的压电陶瓷片 ,粘贴在外部结构的表面 ,结合动态阻抗的概念 ,提出了一种机电耦合的在线压电阻抗技术用于结构健康诊断。结构健康状况的改变 ,表现为结构动态阻抗的变化 ,则可以通过压电陶瓷片的导率表现出来。以含不同裂纹尺寸的梁结构为例 ,实验分析该种技术的有效性。结果表明 :随着裂纹尺寸的增加 ,压电陶瓷片导率的峰值不仅发生明显偏移 ,而且幅值下降。根据幅值的变化实验给出了能较好地衡量损伤程度的标尺     

薄膜膜系计算的计算机程序设计

根据膜堆的特征矩阵,求出单层膜的光学导纳,在单层膜的基础上,进行层层递推,叠加起来,求出膜块的等交内,计算振幅反射系数,最后求出整个膜系的反射率、透射率。     

Tunable microwave filters with controlled ferroelectric capacitors

Microwave microstrip resonators and filters controlled by ferroelectric capacitors have been simulated and experimentally studied. Control at room temperature and under cryogenic conditions has been ensured by capacitors based on BSTO and STO films, respectively. Cooled devices based on YBa₂Cu₃O_7-δ high-temperature superconducting films have been fabricated. The developed filters are efficiently tuned by applying a control voltage, are small, and have a low level of introduced losses.     

A study on the acoustic boundary admittance. Determination,results and consequences

The acoustic boundary admittance condition represents the stiffness, the mass and the damping behaviour of the surrounding structure. The harmonic analysis of small interior domains (e.g. vehicle cabin) is often carried out only by applying acoustically rigid boundaries since the admittances are unknown or unreliably determined by commonly applied methods. Determination using an impedance tube (or Kundt's tube) does not consider the real sound field; the calculation from the measured reverberation time provides an average admittance (and no phase information) of the whole boundary only. Beginning with the definition of the complex-valued boundary admittance, a brief review of the techniques to determine boundary admittances is followed by a boundary element based method that is suited to calculate the boundary admittances from a known sound pressure field. In addition to known methods, a formulation is found where the nodal admittance is calculated by the quotient of a nodal particle velocity divided by the nodal sound pressure. The nodal particle velocity can be calculated from a known sound pressure field solving a Dirichlet problem. The methods are applied to three simple examples. Finally, other examples are given to demonstrate how the boundary admittance can represent the fluid–structure interaction and phase angle of admittance influences the complex-valued eigenfrequencies.