Improvement in microwave dielectric properties of Sr<Subscript>2</Subscript>TiO<Subscript>4</Subscript> ceramics through post-annealing treatment

Sr₂TiO₄ ceramics were synthesized via the conventional solid-state reaction process, and the effects of post-annealing treatment in air on the microwave dielectric properties and defect behavior of title compound were investigated systematically. The Q?×?f values could be effectively improved from 107,000 GHz to 120,300 GHz for the specimens treated at 1450 °C for 16 h. The thermally stimulated depolarization currents (TSDC) revealed two kinds of defect dipoles [\( \left({\mathrm{Ti}}_{\mathrm{Ti}}^{\hbox{'}}-{V}_{\mathrm{O}}^{\bullet \bullet}\right) \) and \( \left({V}_{\mathrm{Sr}}^{"}-{V}_{\mathrm{O}}^{\bullet \bullet}\right) \)] and oxygen vacancies \( \left({V}_{\mathrm{O}}^{\bullet \bullet}\right) \) were considered the main defects in Sr₂TiO₄. Under a post-annealing treatment in air, the concentrations of such defects in the ceramics decreased. Meanwhile, the impedance spectrum revealed the activation energy of the grain boundaries increased. These evidences could account for the improvement of Q?×?f values. Accompanied with a high ε_r of 40.4 and a large τ_f of 126 ppm/°C, the enhanced high-Q Sr₂TiO₄ ceramics can be good candidates for applications in wireless passive temperature sensing.     

Decentralized robust tracking control for uncertain robots

To realize decentralized robust tracking control for robots with uncertain parameters, a new design method is proposed. A robust tracking controller designed by this method consists of two parts: a feedforward controller and a feedback robust controller. A feedforward control is first applied and error dynamics are introduced. For each joint error subsystem a robust controller is designed in two steps: first, a nominal controller is designed for the nominal plant to achieve desired tracking performance, then a robust compensator is added to restrain the influence of the perturbation, that is the difference of the real plant from the nominal plant. The controller designed by the proposed method is a linear time-invariant one. It is shown that robust stability and robust tracking property can be achieved by applying the controller with a sufficiently wide frequency bandwidth. An important feature of the method is that the controller parameters can be tuned on-line easily.List of symbols _max(B), _min(B) the maximum and minimum eigenvalues, respectively, of a symmetric positive definite matrix B - -      

A new approximation method with a rational fraction for the parameters of a power transmission line by a genetic algorithm

In this paper a new automatic approximation method with low-order rational functions for the frequency-dependent characteristics of a high-voltage single-phase transmission line, i.e. characteristic impedance Z_C(jP) and transfer function A( jw ) = e ^{- g( jw )L} A\left( {j\omega } \right) = {\rm e}^{ - \gamma \left( {j\omega } \right)L} , by a genetic algorithm is presented. The results prove that this method is very likely to find the global optimum, that it is easy to use and has a high efficiency, especially for simulating power systems.     

Analytical evaluation of the charge carrier density of organic materials with a Gaussian density of states revisited

An analytical solution for the calculation of the charge carrier density of organic materials with a Gaussian distribution for the density of states is presented and builds upon the ideas presented by Mehmeto?lu (J Comput Electron 13:960–964, 2014) and Paasch et al. (J Appl Phys 107:104501-1–104501-4, 2010). The integral of interest is called the Gauss–Fermi integral and can be viewed as a particular type of integral in a family of the more general Fermi–Dirac-type integrals. The form of the Gauss–Fermi integral will be defined as

$$\begin{aligned} G\left( \alpha ,\beta ,\xi \right) =\mathop {\displaystyle \int }\limits _{-\infty }^{\infty }\frac{ e^{-\alpha \left( x-\beta \right) ^{2}}}{1+e^{x-\xi }}\hbox {d}x\text {,} \end{aligned}$$

where \(G\left( \alpha ,\beta ,\xi \right) \) is a dimensionless function. This article illustrates a technique developed by Selvaggi et al. [3] to derive a mathematical formula for a complete range of parameters \(\alpha \), \(\beta \), and \(\xi \) valid \(\forall \) \(\alpha \) \( \varepsilon \) \( {\mathbb {R}} \ge 0\), \(\forall \) \(\beta \) \(\varepsilon \) \( {\mathbb {R}} \), and \(\forall \) \(\xi \) \(\varepsilon \) \( {\mathbb {R}} \).

     

Effects of ZnO on the piezoelectric properties of Pb(Mn1/3Sb2/3)O3-Pb(Zr,Ti)O3 ceramics

Perovskite-type 0.05 Pb(Mn_1/3Sb_2/3)O₃-0.95 Pb(Zr_0.5Ti_0.5)O₃ (PMS-PZT) was synthesized by conventional bulk ceramic processing technique. ZnO as a dopant up to 0.5 mol% was incorporated into the PMS-PZT system, and the effects on piezoelectric properties were investigated. Pyrochlore phase was not detected to form during the synthesis of the PMS-PZT system with 0∼0.5 mol% ZnO addition. The highest density of 7.92 g/cm³ was obtained when sintered at 1200°C for 2 hrs. Piezoelectric properties as a function of ZnO content were evaluated using a gain phase analyzer. Piezoelectric charge constant (d₃₁) and piezoelectric voltage output coefficient (g₃₁) increased up to −130 pC/N and −24.9 × 10³Vm/N, respectively, with increasing ZnO content. Mechanical quality factor (Q _m) was shown to reduce considerably with increasing ZnO content. When 0.3 mol% of ZnO was added into the system, electromechanical coupling factor (k _p) and relative dielectric constant () reached to the maximum of 56% and 1,727, respectively.     

Delta-doped tunnel FET (D-TFET) to improve current ratio ($$I_\mathrm{ON}/I_\mathrm{OFF}$$) and ON-current performance

A two dimensional (2D) analytical drain current model has been developed for a delta-doped tunnel field-effect transistor (D-TFET) that can address the ON-current issues of the conventional TFET. Insertion of a highly doped delta layer in the source region paves the way for improved tunneling volume and thus provides high drain current as compared with TFETs. The present model takes into account the effects of the distance between the delta-doping region and the source–channel interface on the subthreshold swing (SS), current ratio, and ON-current performance. The D-TFET is predicted to have a higher current ratio \(\left( {\frac{I_\mathrm{ON} }{I_\mathrm{OFF} }\cong 10^{11}} \right) \) compared with TFETs \(\left( {\frac{I_\mathrm{ON} }{I_\mathrm{OFF} }\cong 10^{10}} \right) \) with a reasonable SS \(\left( {{\sim }52\,\mathrm{mV/dec}} \right) \) and \(V_\mathrm{th}\) performance at an optimal position of 2 nm from the channel. The surface potential, electric field, and minimum tunneling distance have been derived using the solution of the 2D Poisson equation. The accuracy of the D-TFET model is validated using the technology computer aided design (TCAD) device simulator from Synopsys.     

Die Berechnung von Übergangsvorgängen bei Induktionsmaschinen mit Stromverdrängungsläufern

Übersicht Zur rechnerischen Untersuchung nichtstationärer Vorgänge bei Asynchronmaschinen mit Käfigläufern wird die Stromverdrängung mit Hilfe der Doppelkäfignäherung in einem auf der Grundlage der Zweiachsentheorie aufgebauten elektromechanischen Gleichungssystem berücksichtigt. Die Einflüsse der Eisensättigung in der Leerlaufkennlinie und in der Kurzschlußkennlinie können in erweiterten Gleichungen mit beachtet werden. Die Auswertung des Systems erfolgt in der Nachbildung am Analogrechner. Die Rechenergebnisse von Hochlauf-, Einschalt- und Netzumschaltungsuntersuchungen für große Motoren werden angegeben.

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Contents The behaviour of squirrel-cage induction machines is described by a system of differential equations based on the cross-field theory. With respect to the skin effect the rotor is represented by a double-cage approximation. Saturation of the main flux and leakage flux as well can be taken into account by an expanded form of equations. An analogue computer was used for numerical computation; results are given for the transient performance of large motors in cases of starting, starting with pony-motor and reclosing on an auxiliary power supply line.

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Übersicht der wichtigsten Formelzeichen

Indizes S Ständer - L Läufer allgemein - o Oberkäfig - u Unterkäfig - h Hauptfeld - a, b, c Dreiphasensystem - , , o Komponentensystem - N Nennwert - K Kurzschlußwert; Kippwert - auf Streuung bezogen - R Regulierläufer (Schleifringläufer) - G Gegenwirkung (Last) - A Anlauf - * konj. komplexer Wert Veränderliche Unabhängig =2f·t Zeitwinkel Abhängig u Spannungen - i Ströme - verkettete Flüsse - m Drehmoment - s Schlupf - _s Korrekturfaktor für Ständerwiderstand - Korrekturfaktor für Streuwegsättigung - Komplexe Zusammenfassung der ,-Komponenten u=u +ju ; +j i=i +ji Konstanten Kurzschlußreaktanzen - Kurzschlußdämpfungen - Kurzschlußströme - Nennflüsse Weitere Koeffizienten T _A mech. Anlaufzeitkonstante - Gewichtsfaktoren bei der Hauptflußbildung Im allgemeinen werden Veränderliche mit kleinen, konstante Wert mit großen Buchstaben bezeichnet.     

Evaluation of material constants in NdCa4O(BO3)3 piezoelectric single crystal

A neodymium calcium oxoborate NdCa₄O(BO₃)₃ piezoelectric single crystal that belongs to the monoclinic system with point group m was grown by the Czochralski technique. A practical evaluation method was developed to determine the 27 independent material constants for acoustic wave device applications. A longitudinal effect face–shear vibration was analyzed and used in the resonance–antiresonance measurement. This method avoided measuring d ₁₁ and d ₃₃ directly by use of X-bar and Z-bar, in which leak of electric field would cause large errors because of the very small dielectric constants. At room temperature, dielectric constants were ${{\varepsilon _{11}^{\text{T}} } \mathord{\left/{\vphantom {{\varepsilon _{11}^{\text{T}} } {\varepsilon _0 }}} \right.\kern-0em} {\varepsilon _0 }} = 9.9$ , ${{\varepsilon _{22}^{\text{T}} } \mathord{\left/{\vphantom {{\varepsilon _{22}^{\text{T}} } {\varepsilon _0 }}} \right.\kern-0em} {\varepsilon _0 }} = 15$ , ${{\varepsilon _{33}^{\text{T}} } \mathord{\left/{\vphantom {{\varepsilon _{33}^{\text{T}} } {\varepsilon _0 }}} \right.\kern-0em} {\varepsilon _0 }} = 10$ and ${{\varepsilon _{13}^{\text{T}} } \mathord{\left/{\vphantom {{\varepsilon _{13}^{\text{T}} } {\varepsilon _0 }}} \right.\kern-0em} {\varepsilon _0 }} = - 0.8$ , respectively. All the independent dielectric and elastic constants were determined in this work. The simulation of surface acoustic wave velocity showed a good agreement with the measured value.     

A Novel Oxygen Sensor Based on a Metal–Metal Oxide Scale System

A novel oxygen sensor was proposed. The sensor, an electrochemical cell, was composed of a metal as reference electrode, its oxide scale as electrolyte and Pt or other adequate materials as sample electrode. It is expected that the electrolyte is self-restorative because it can be restored by high temperature oxidation. The emf measurements were carried out at 873 K in cells using zirconium. At atm, the emf vs. plot lies on a straight line and its gradient is 2.303RT/4F, suggesting t _ion=1 at the surface of the scale. The emf steeply decreases with decreasing at atm which can not be explained by the increase in the electronic conductivity and is explained by a gas laminar film phase at the surface of the sample electrode.     

Electrical properties of PZT piezoelectric ceramic at high temperatures

Advanced aeronautic and space structures need active components that can function at high frequencies and temperatures. Piezoelectric materials can provide frequency response but their use at elevated temperatures is limited. The reason for the lack of insertion of piezoelectric for high temperature active component and sensors are two fold. First, the database of piezoelectric properties that describe the piezoelectric constants is lacking for high temperatures engineering applications. Most studies measure the dielectric constants to determine the Curie temperature but do not provide piezoelectric coefficients as a function of temperature. Second, piezoelectric materials with Curie temperature (T _C) exceeding 500 °C are sought for aeronautics and aerospace applications. This investigation describes a measurement system that captures the impedance dependence upon temperature for piezoelectric materials. Commercially available lead zirconate titanate (PZT) was studied as to determine the piezoelectric activity to define the operating envelope with respect to temperature. The elastic properties , piezoelectric coefficients , dielectric properties , and electro-mechanical coupling factors were determined as a function of temperature. The coupling factor k ₃₃ was found to be relatively constant to 200 °C and exhibit slight temperature dependence above 200 °C. The temperature sensitivity for both piezoelectric coefficient and electromechanical coupling factor were very small; the slopes and Δk ₃₃/k ₃₃ were found to be 0.01 and (−0.07) respectively in the range of 120 to 200 °C. This measurement technique will populate databases that describe the piezoelectric properties of commercially available PZT piezoelectric materials. It can also facilitate the assessment of new piezoelectric materials that are currently being developed for higher temperature applications. This work is supported by US Air Force grant F49620-03-1-0128.     

Transport Properties of the Mixed-Conducting SrFe1.2Co0.3Ox

Single phase SrFe_1.2Co_0.3O_x sample with layered crystal structure was prepared using a solid state reaction method. Electrical conductivity and apparent oxygen diffusion coefficients of the SrFe_1.2Co_0.3O_x sample were measured as functions of temperature in atmospheres of various oxygen partial pressures . Total and ionic conductivities were determined by using the conventional four-probe and electron blocking four-probe methods, respectively. The apparent oxygen diffusion coefficient was derived from the time-dependent conductivity relaxation data of the reequilibrium process after abruptly changing the in the surrounding atmosphere. Several atmospheres of different were established by the use of premixed gas cylinders. The conductivity of SrFe_1.2Co_0.3O_x increases with increasing temperature and . At 900°C in air, the total conductivity and ionic conductivity are 10 and 8S · cm^-2, respectively. The ionic transference number ( 0.8 in air) does not have strong temperature dependence. The activation energy increases with decreasing . In air, the activation energy has a low value of 0.37 eV. The apparent oxygen diffusion coefficient was at 950°C over a wide range of .     

Theorie der Raumladungs-Sperrschicht einer Niederdruckentladung zur Verwendung für eine Theorie der Rückzündung

Zusammenfassung Im ersten Teil der Arbeit wird die Theorie über die Raumladungs-Sperrschicht an der Grenze zwischen einem Niederdruck-Plasma und einer negativen Elektrode ausführlich dargestellt. Auf Grund des Boydschen Schichtkriteriums wird über die Ergebnisse von Bohm hinausgehend eine neue Randbedingung allgemein für die positive Säule im Bereich der ambipolaren Diffusion aufgestellt. Die Gültigkeit dieser Randbedingung wird an früheren Sondenmessungen von Klarfeld geprüft und bestätigt gefunden. Mittels der neuen Randbedingung wird in Verbindung mit früheren Messungen die Beweglichkeit der Quecksilber-Ionen bestimmt, die sich, bezogen aufp=1, als allein von der Elektronentemperatur abhängig erweist. Diese Ergebnisse werden bei der Betrachtung der Entionisierung des Gitter-Anodenraumes und des Ionenrückstromes zur Anode verwendet. Der Entionisierungsvorgang wird bei vereinfachter Anodensystemgeometrie sowohl statisch (mit Vernachlässigung der Sperrschicht vor der Anode) als auch dynamisch mit Berücksichtigung der Sperrschicht betrachtet. Es werden Meßergebnisse wiedergegeben und Übereinstimmung der Theorie mit der Erfahrung gefunden. Der von Koch angegebene Ansatz für den dynamischen Ionenrückstrom wird erweitert, womit die von Zipfel gemessenen hohen dynamischen Rückströme erklärt werden können. Es ergibt sich sowohl theoretisch als auch auf Grund einer dargestellten Sondenmeßmethode, daß die gemessenen Anodenrückströme bis zu sehr hohen Steilheiten der anlaufenden Sprungspannung (du /dt) praktisch reine Ionenströme sind.Übersicht der verwendeten Symbole A Konstante der Feldemissionsgleichung definiert durch Gl. (64) - a Korrekturgröße definiert durch Gl. (75) - B Konstante der Feldemissionsgleichung definiert durch Gl. (64) - b _i Beweglichkeit der Hg-Ionen;b _i1=b_ip - C wird mehrmals als Konstante verwendet - C ₁ - C ₂ - D Dimensionslose Größe, DurchschlagsbedingungD1 - D _a Ambipolare Diffusionskonstante für Hg,D _a1=D_a·p - d Abstand, Gitter-Anode - E Elektrische Feldstärke, an der AnodeE ₀, im Plasma an der Grenze zur SperrschichtE - e Elementarladung,e=1,602·10^–19 As - F _eff effektive Anodenoberfläche - J ₀,J ₁ Besselfunktion o. Ordnung und 1. Ordnung - i Anodenstrom - i _R Anodenrückstrom, ^: Spitzenwert, Index stat: statisch, Index dyn: dynamisch - j Stromdichte, ^: Spitzenwert, IndexV: Verschiebungsstromdichte - j ⁺ Ionenstromdichte zur Anode, ^: Spitzenwert. Bedeutung der Indizes:stat, statisch;dyn, dynamisch;K, Konvektion;P, vom Plasma;S, in der Sperrschicht erzeugt - j ⁺ Ionenwandstromdichte - j ^– Elektronenstromdichte, IndexF, in einem Feldemissionszentrum der Anode - k Boltzmann-Konstantek=1,38·10^–23 VAs/grad - l wird mehrmals als Längenmaß verwendet - M Masse eines Hg-IonsM=3,33·10^–25 kg - m Masse eines Elektronsm=9,106·10^–31 kg - m _F Feldverstärkungsfaktor Gl. (61) - n Neutralgasdichte; Index o, vor der Anode beit=0; Index 1, fürp=1,n ₁=3,54×10²² Atome je m³ und Torr - n _i, n_e Ionen- und Elektronendichte;n _e0, in der Symmetrie-Mitte des Plasmas;n _e, an der Plasmagrenze - p auf 0 °C reduzierter Druck,p=n/n ₁ - mittlerer Ionisierungsquerschnitt innerhalb der Schicht Gl. (71) - mittlerer Umladequerschnitt innerhalb der Schicht Gl. (79) - R Gefäßradius; Gitter- bzw. Blendenlochradius - r Zylinderkoordinate - S definiert durch Gl. (3) - T Zeitkonstante des gemessenen Rückstromes - T ₀ Zeitbereich für die Wahrscheinlichkeit Gl. (85) - T _A Temperatur der Anode - t Zeit - t Zeitdauer bis die Sperrschicht eine Sonde vor der Anode erreicht - U _e Elektronentemperatur, 11600 °K entsprechen 1 V - u Potential;u , Potentialdifferenz an der Sperrschichtu ^* , beim Spitzenwert des Rückstromesu _spr, Sprungspannung (Anode-Kathode) - u Anfangsgeschwindigkeit der Ionen an der Sperrschicht in Volt - Koordinate - - Korrekturgröße definiert durch Gl. (22) - Koeffizient der Elektronenemission durch auftreffende Ionen - Sperrschichtdicke, ₀ definiert durch Gl. (22) - ₀= 8.86 · 10^-12 As/Vm - - _i , definiert in Gl. (13) und Spreizfaktor siehe Gl. (73) - ₁ 1. Nullstelle der FunktionJ ₀, ₁=2,405 - Faktor für die dynamische Rückstromdichte Gl. (39) - Wandkorrekturfaktor für die Ladungsträgerdichte Gl. (1) - _i Raumladungsdichte in der Sperrschicht - definiert in Gl. (73) Die Ergebnisse der vorliegenden Arbeit werden in der im Arch. f. Elektrotechnik 49 (1964), H. 4, erscheinenden Theorie der Rückzündung bei Quecksilberdampf-Stromrichtergefäßen angewendet.     

Die verlustbehaftete Fernleitung ohne und mit Längskompensation

Übersicht Häufig werden Fernleitungen unter der Annahme berechnet, daß die längs und quer zur Leitung auftretenden Verluste vernachlässigbar seien. Die Anwendung dieses Rechenverfahrens setzt jedoch voraus, daß die Abhängigkeit des Betriebsverhaltens der Leitung von den Leitungsverlusten bekannt ist. Zweck der vorliegenden Arbeit soll es sein, die dafür geltenden Gesetzmäßigkeiten abzuleiten. In einem ersten Teil wird die Fernleitung ohne Längskompensation behandelt. Dazu werden zwei Lösungsverfahren angegeben und die abgeleiteten Formelsysteme mit Hilfe eines Digitalrechners für Fernleitungen von 400 bis 800 kV Betriebsspannung sowie von 400 bis 1200 km Länge ausgewertet. In einem zweiten Teil wird die längskompensierte Fernleitung näher behandelt und für die einstufige Kompensation dargelegt, daß das in einer früheren Arbeit vorgeschlagene Rechenverfahren in modifizierter Form auch bei Mitberücksichtigung der Leitungsverluste anwendbar ist.Übersicht der benutzten Formelzeichen l Länge - R ₀ Widerstandsbelag - L ₀ Reaktanzbelag - C ₀ Suszeptanzbelag - A ₀ Ableitungsbelag - Ersatzkonstanten in derII-Schaltung - Absolutbetrag der Längsimpedanz - Z Wellenwiderstand der verlustfreien Leitung - Phasenkonstante der verlustfreien Leitung - l Leitungswinkel der verlustfreien Leitung bei Entnahme ihrer natürlichen Leistung - N _nat natürliche Leistung der verlustfreien Leitung - V Leitungswinkel der verlustbehafteten Leitung - V _k Leitungswinkel der verlustbehafteten Leitung mit Längskompensation - N _w1,N _w2 Wirkleistungen - N _b1,N _b2 Blindleistungen - N _vk verkehrte Kurzschlußleistung - J ₁,J ₂ Leiterströme - U ₁,U ₂ Leiterspannungen - Wirkungsgrad - 1, 2 Zustände am Anfang und Ende der Leitung - x, y Koordinaten des Ossanna-Diagramms - a, b, c, Hilfsgrößen - q, r, s Hilfsgrößen in Gl. (27) - I, II... Nummer des Vierpols - Leistungsverhältnis - k Kompensationsgrad - (N _nat)k natürliche Leistung der kompensierten Leitung - X Kompensationsreaktanz - u Realteil von - v Imaginärteil von - * Mit Stern versehene Formelzeichen gelten für das Spannungsverhältnis 1 - · komplexe Größen mit Punkt über den Formelzeichen Mit 20 TextabbildungenForschungsassistent am vorgenannten Institut.     

Influence of quadrature‐axis reactance of large turbine generator on operation

Two‐pole large generators are extensively used in power utilities. Growing demand for electric power in recent years has led to substantial increases in the capacity of generators. In line with this trend, our company has manufactured two‐pole turbine generators in the 900,000‐kVA to 1,200,000‐kVA classes. As the capacity of generators becomes larger, equivalent circuits are increasingly expected to improve their precision in estimating operation characteristics. The Park model has been widely used as an equivalent circuit model. The Park model with quadrature axis transient reactance $x_{q}^{\prime}$ is claimed to be useful for accurately estimating the load rejection characteristics of turbine generators. However, it is difficult to measure $x_{q}^{\prime}$, few studies based on measured data have appeared. As one of the few examples, we have conducted a measured data on a 500,000‐kVA‐class generator. Recently, the authors obtained test data on $x_{q}^{\prime}$ of a 900,000‐kVA‐class generator. In this study, based on these data, we studied the effect of $x_{q}^{\prime}$ using the finite element method and a transient analysis program (EMTP‐ATP). © 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 177(3): 17–27, 2011; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/eej.21099     

20-nm T-gate composite channel enhancement-mode metamorphic HEMT on GaAs substrates for future THz applications

In this paper, the RF and DC behaviours of a SiN-passivated 20-nm gate length metamorphic high electron mobility transistor (MHEMT) on GaAs substrate with \({\updelta }\)-doped sheets on either side of the composite channel are studied using the Synopsys TCAD tool. The 20-nm enhancement-mode MHEMT with \({\updelta }\)-doped sheets on either side of the \(\hbox {In}_{0.75}\hbox {Ga}_{0.25}\hbox {As}\)/InAs/ \(\hbox {In}_{0.75}\hbox {Ga}_{0.25}\hbox {As}\) multilayer channel shows a transconductance of 3000 mS/mm, cut-off frequency (\({f}_{\mathrm{T}}\)) of 760 GHz and a maximum-oscillation frequency (\({f}_{\mathrm{max}}\)) of 1270 GHz. The threshold voltage of the device is found to be 0.07 V. The room-temperature Hall mobilities of the two-dimensional sheet charge density (2DEG) are measured to be over \(12800\,\hbox {cm}^{2}\)/Vs with a sheet charge density larger than 4 \(\times \) \(10^{12}\,\hbox {cm}^{-2}\). These high-performance enhancement-mode MHEMTs are attractive candidates for future terahertz applications such as high-resolution radars for space research and also for low-noise wide-bandwidth amplifier for future communication systems.     

Transient skin effect in a rectangular conductor placed in a semi-closed slot

Contents It is demonstrated that the Bubnow-Galrekin method can be applied for the analysis of the non-steady state in conductors. The Joule power losses in a rectangular conductor placed in a semi-closed slot and the electrodynamic force acting on this conductor are calculated. The equivalent circuit of the impedance of the conductor is also considered. The investigations are made by using the Bubnow-Galerkin method for the parabolic equation.

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Übersicht Es wird gezeigt, daß man die Methode von Bubnow-Galerkin zur Analyse der nichtstationären Vorgänge in den Leitern anwenden kann. Es werden die Stromwärmeverluste für den rechteckigen Leiter in der halbgeschlossenen Nut einer elektrischen Maschine und die auf den Leiter wirkende Kraft berechnet. Die Ersatzschaltungen für die Impedanz einer Maschinennut werden weiter betrachtet. Zur Analyse der parabolischen Differentialgleichung wird die Methode von Bubnow-Galerkin angewandt.

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Symbols B the magnetic induction - B _x ,B _y Components of the magnetic induction - C Operator in a Hilbert space - E z-component of the electric field - F Electrodynamic force - F _x ,F _y Components of the electrodynamic force - H Hilbert space - H _C Energetic space of a positive definite operatorC - I Current - Imaginary unit - L Inductance - Laplace transform - P Power - R Resistance - Z Impedance - z ^* Dimension coupled with complex dimensionz - Rez, Imz, |z| Real part, imaginary part and modulus of complex numberz - Magnetic permeability - Conductance - Pulsation - ² Scalar Laplacian - 1 _x , 1 _y Unit vectors - (x/y) Scalar product of elementsx,y of Hilbert spaceH - Norm of elementx of Hilbert spaceH -      

Structure and microwave dielectric properties of Sm(2−x)/3Li x TiO3

The structural evolution, and microwave dielectric properties of ceramics (x = 0.0 ≤ x ≤ 0.5) were investigated in this work. X-ray diffraction (XRD) results show that samples with x > 0.3 exhibit a single perovskite phase. Impurity phases of Sm₂Ti₂O₇ and TiO₂ appear and their amount increases with the decrease of x when x ≤ 0.3. TEM observation indicates that the A-site is ordered in x = 0.5, but not in x = 0.3). The dielectric constant decreases with the increase of x for 0.1 ≤ x ≤ 0.4 and then increases with further increase in x up to x = 0.5. The Q×f value decreases with the decrease of x due to the increased occurrence of Sm₂T₂O₇ secondary phase, defects and twinning boundaries. The temperature coefficient of resonant frequency is negative and its absolute value decreases greatly with the decrease of x value.     

First-principle calculations of electronic and ferromagnetic properties of $$\hbox {Al}_{1-x}\hbox {V}_{x}\hbox {Sb}$$

We have used the first-principle calculations of density functional theory within full-potential linearized augmented plane-wave method to investigate the electronic and ferromagnetic properties of \(\hbox {Al}_{1-x}\hbox {V}_{x}\hbox {Sb}\) alloys. The electronic structures of \(\hbox {Al}_{0.25}\hbox {V}_{0.75}\hbox {Sb}, \hbox {Al}_{0.5}\hbox {V}_{0.5}\hbox {Sb}\) and \(\hbox {Al}_{0.75}\hbox {V}_{0.25}\hbox {Sb}\) exhibit a half-metallic ferromagnetic character with spin polarization of 100 %. The total magnetic moment per V atom for each compound is integral Bohr magneton of 2 \(\mu _{\mathrm{B}}\), confirming the half-metallic feature of \(\hbox {Al}_{1-x}\hbox {V}_{x}\hbox {Sb}\). Therefore, these materials are half-metallic ferromagnets useful for possible spintronics applications.