CAD oriented frequency,temperature, and DC bias dependent small‐signal,scalable circuit model of parallel‐plate paraelectric varactors

Simple, CAD compatible small‐signal scalable circuit model for the thin film parallel‐plate ferroelectric varactors is reported. It is based on the measured permittivity and loss tangent of the ferroelectric films along with the fundamental theoretical relationships of ferroelectrics in paraelectric phase. The model makes use of the measured DC field (voltage) at the inflection point of the C‐V curve where the nonlinearity (dC/dV) and the tunability are maximum. Both the capacitance and the equivalent loss resistance (dielectric losses) in the ferroelectric film take into account the low permittivity layers at the boundaries of the nano‐columns of the film. The frequency dependent losses in the top and bottom plates of the varactors and the parasitic inductance of the plates are also taken into account. The model is valid for ferroelectric films having columnar structure. It is useful in wide temperature, DC field, and frequency ranges. © 2008 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2009.     

Plant fibre reinforced cement components for roofing

Composites of blast furnace slag (BFS) based cement mortar reinforced with vegetable fibres are presented. Roofing components are produced with these composites through a simple and low-energy consuming method, including ordinary vibration and curing in a wet chamber. Composites reinforced with eucalyptus pulp, coir fibres and with a mixture of sisal fibre and eucalyptus pulp gave a suitable performance, with compressive strength higher than 20 MPa and modulus of rupture (MOR) higher than 3 MPa. The performance of tiles made with these composites is in accordance with international requirements, with maximum load higher than 450 N, in wet conditions.     

Modeling of conductor losses in capacitors with rectangular and circular plates

Analytic models are developed for the losses in the plates of MIM capacitors. The formulas are derived for the equivalent loss tangent and resistance of the arbitrary thick rectangular electrodes. An equivalent surface resistance is introduced for an arbitrary thick conductor. The accuracy of the model is demonstrated by comparison with rigorous Sonnet simulation and available alternative approach. The formula for the resistance of electrically thick circular electrodes is obtained. The derived expressions are verified via comparison with rigorous HFSS (high frequency structure simulator) simulations using eigenmode solver. © 2008 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2009.     

Combustion synthesis of SOFC materials in a continuous reactor

La_0.8Sr_0.2CrO₃ and LaMg_0.2Cr_0.8O₃ materials for use in solid oxide fuel cells (SOFCs) were synthesized in a continuous SHS reactor from powder mixtures of La₂O₃, MgO, SrCO₃, Cr₂O₃, and Cr. Based on XRD data, the degree of conversion was found to grow with increasing combustion temperature and burning velocity. SHS-produced powders were reground in a vibratory mill, pressed into cylindrical pellets (3–4 mm in height and 10 mm in diameter), and sintered. The required level of properties was attained for sintering temperatures above 1600°C. The article is published in the original.     

Do we really need ferroelectrics in paraelectric phase only inelectrically controlled microwave devices?

Typical paraelectric materials (e.g., SrTiO₃, KTaO₃, Ba_xSr_1-xTiO₃, x<0.5) and electrically tunable microwave devices based on these materials are briefly reviewed. The analysis shows that in spite of the recent year's extensive efforts, no considerable improvement in the microwave losses in thin paraelectric films has been achieved. Thin films, regardless of fabrication method and substrate type, have much lower dielectric permittivity than bulk single crystals, and the loss tangent at microwave frequencies (f>10 GHz) is of the order of 0.01 (at zero dc-bias field) at room temperature. Nevertheless, quite promising component and subsystem level devices are successfully demonstrated. Use of ceramic (bulk and thick film) ferroelectrics in tunable microwave devices, currently considered for industrial applications, offer cost reduction. In this paper, explicitly for the first time, we consider possibilities and benefits of using ferroelectrics in polar phase in electrically controllable microwave devices. Examples of using ferroelectrics in polar state (e.g., Na_0.5K_0.5NbO₃, SrTiO₃ in antiferroelectric phase) in electrically tunable devices are reported     

Modeling gap discontinuity in coplanar waveguide using quasistatic spectral domain method

An efficient computer technique based on the spectral domain method is presented to solve the problem of modeling coplanar gap discontinuity. Comparison with available published data is done to confirm the accuracy of the method. © 2000 John Wiley & Sons, Inc. Int J RF and Microwave CAE 10: 150–158, 2000.     

Electric-Field-Controlled Permittivity Ferroelectric Composition for Microwave LTCC Modules

A low-temperature cofired ferroelectric ceramic composition for electrically tunable radio frequency devices is introduced. A sintering temperature of 950°C for BaSrTiO₃–MgO was obtained following the combined addition of B₂O₃ and Li₂CO₃. The effects of these sintering aids on densification, microstructure, and ferroelectric properties were investigated. The permittivity and dissipation factor values were 234 and 0.0010, respectively, measured at 286.3 K, 1 kHz. The electrically tunable ferroelectric structure for microwave measurement was fabricated throughout the tape-casting process. Measured permittivity was 130 at 26 GHz and tunability >15% (4 V/μm).     

Effect of enhanced current crowding in a CPW with a thinferroelectric film

Enhanced current crowding is predicted at the edges of the conducting strips in thin ferroelectric film coplanar waveguides (CPWs). A minimum current crowding depth is expected, which depends on the CPW geometry and dielectric constant of the ferroelectric film. In practical CPW devices the enhanced current crowding can increase the conductor losses by up to four times. The current crowding depth may be smaller than the skin depth or London penetration depth (for high temperature superconducting CPW)     

On the Fixed-Parameter Tractability of the Maximum Connectivity Improvement Problem

Theory of Computing Systems - In the Maximum Connectivity Improvement (MCI) problem, we are given a directed graph G = (V,E) and an integer B and we are asked to find B new edges to be added to G...     

Application notes - basic parameters of coplanar-strip waveguides on multilayer dielectric/semiconductor substrates, Part 1: high permittivity superstrates

A simple method and closed-form analytic formulas for symmetric CPS based on single layer substrate (infinite and finite thickness) and multilayer lossy dielectric (semiconductor) substrate, where the permittivity of the superstrate layers decrease away from the strips, are proposed. Since the line models are given analytically, the dependencies of the line parameters may be analyzed and optimized in wide ranges of sizes, permittivities, and losses (substrate resistivity). In general, the formulas are reversible. The measured line parameters may be used to calculate the permittivity and loss tangent (resistivity) of one of the substrate layers if the parameters of the other layers and strips are known. The method may be easily extended to include larger number of substrate layers.