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This paper proposes a new interface constitutive model for fully grouted rock-bolts and cable-bolts based on pull-out test results. A database was created combining published experimental data with in-house tests. By means of a comprehensive framework, a Coulomb-type failure criterion accounting for friction mobilization was defined. During the elastic phase, in which the interface joint is not yet created, the proposed model provides zero radial displacement, and once the interface joint is created, interface dilatancy is modeled using a non-associated plastic potential inspired from the behavior of rock joints. The results predicted by the proposed model are in good agreement with experimental results. The model has been implemented in a finite element method (FEM) code and numerical simulations have been performed at the elementary and the structural scales. The results obtained provide confidence in the ability of the new model to assist in the design and optimization of bolting patterns.

This research work investigates the phenomenon of interbar currents in induction motors equipped with die-cast aluminum cages. Flowing in the lamination and distorting the bar current distributions, the interbar currents cause additional stray losses as well as an increase of the joule loss in the cage bars. The contact resistance between the bars and the rotor laminations is the key modeling element for a correct prediction of the interbar currents and the related extra losses. The study presents a new noninvasive method to obtain a reliable estimation of the contact resistance. The proposed hybrid approach is based on a mix of experimental data and Finite Element method (FEM)-based simulation results. A reliable three-dimensional FEM model of a four-pole 15-kW induction motor with closed rotor slots has been used to investigate the impact of variable contact resistance on the interbar currents, rotor joule losses, and the voltage drops along the bars, opening a new perspective for this complex phenomenon and its impact on the stray losses. Finally, the proposed methodologies are critically discussed and the obtained results compared with other relevant research works.

Investigating and predicting the magnetization of bulk superconducting materials and developing practical magnetizing techniques is crucial to using them as trapped field magnets (TFMs) in engineering applications. The pulsed field magnetization (PFM) technique is considered to be a compact, mobile and relative inexpensive way to magnetize bulk samples, requiring shorter magnetization times (on the order of milliseconds) and a smaller and less complicated magnetization fixture; however, the trapped field produced by PFM is generally much smaller than that of slower zero field cooling (ZFC) or field cooling (FC) techniques, particularly at lower operating temperatures. In this paper, the PFM of two, standard Ag-containing Gd-Ba-Cu-O samples is carried out using two types of magnetizing coils: 1) a solenoid coil, and 2) a split coil, both of which make use of an iron yoke to enhance the trapped magnetic field. It is shown that a significantly higher trapped field can be achieved using a split coil with an iron yoke, and in order to explain these how this arrangement works in detail, numerical simulations using a 2D axisymmetric finite element method based on the H-formulation are carried to qualitatively reproduce and analyse the magnetization process from both electromagnetic and thermal points of view. It is observed that after the pulse peak significantly less flux exits the bulk when the iron core is present, resulting in a higher peak trapped field, as well as more overall trapped flux, after the magnetization process is complete. The results have important implications for practical applications of bulk superconductors as such a split coil arrangement with an iron yoke could be incorporated into the design of a portable, high magnetic field source/magnet to enhance the available magnetic field or in an axial gap-type bulk superconducting electric machine, where iron can be incorporated into the stator windings to 1) improve the trapped field from the magnetization process, and 2) increase the effective air-gap magnetic field.

Ankara : The Department of Electrical and Electronics Engineering and The Institute of Engineering and Science of Bilkent University, 2010. Thesis (Ph.D.) -- Bilkent University, 2010. Includes bibliographical references leaves 40-49. Capacitive micromachined ultrasonic transducers (cMUTs) are used to transmit and receive ultrasonic signals. The device is constructed from circular membranes fabricated with surface micromachining technology. They have wider bandwidth with lower transmit power and lower receive sensitivity compared to the piezoelectric transducers, which dominate the ultrasonic transducer market. In order to be commercialized, they must overcome these drawbacks or find new application areas, where piezoelectric transducers perform poorly or cannot work. In this thesis, the latter approach, finding a new application area, is followed to design wide band and highly efficient airborne transducers with high output power by maximizing the radiation resistance of the transducer. The radiation impedance describes the interaction of the transducer with the surrounding medium. The real part, radiation resistance, is a measure of the amount of the power radiated to the medium; whereas the imaginary part, radiation reactance, shows the wobbled medium near the transducer surface. The radiation impedance of cMUTs are currently not well-known. As a first step, the radiation impedance of a cMUT with a circular membrane is calculated analytically using its velocity profile up to its parallel resonance frequency for both the immersion and the airborne applications. The results are verified by finite element simulations. The work is extended to calculate the radiation impedance of an array of cMUT cells positioned in a hexagonal pattern. The radiation impedance is determined to be a strong function of the cell spacing. It is shown that excitation of nonsymmetric modes is possible in immersion applications. A higher radiation resistance improves the bandwidth as well as the efficiency and the transmit power of the cMUT. It is shown that a center-to-center cell spacing of 1.25 wavelength maximizes the radiation resistance for the most compact arrangement, if the membranes are not too thin. For the airborne applications, the bandwidth can be further increased by using smaller device dimensions, which decreases the impedance mismatch between the cMUT and the air. On the other hand, this choice leads to degradation in both efficiency and transmit power due to lowered radiation resistance. It is shown that by properly choosing the arrangement of the thin membranes within an array, it is possible to optimize the radiation resistance. To make a fair analysis, same size arrays are compared. The operating frequency and the collapse voltage of the devices are kept constant. The improvement in the bandwidth and the transmit power can be as high as three and one and a half times, respectively. This method may also improve the noise figure when cMUTs are used as receivers. A further improvement in the noise figure is possible when the cells are clustered and connected to separate receivers. The results are presented as normalized graphs to be used for arbitrary device dimensions and material properties. Şenlik, Muhammed N Ph.D.

ÖZET KAPASİTİF MİKRO-İŞLENMİŞ ULTRASONİK ÇEVİRİCİLERİN MODELLENMESİ VE KARAKTERİZASYONU Ayhan Bozkurt Elektrik ve Elektronik Mühendisliği Doktora Tez Yöneticisi: Prof. Dr. Abdullah Atalar 5 Ocak 2000 Kapasitif Mikro-işlenmiş Ultrasonik Çevirici (kMUÇ), ultrasonik ses dalgalarının üre tilmesi ve algılanması için kullanılan bir cihazdır. Cihaz, silikon bir taban üzerinde mikro-fabrikasyon yöntemiyle üretilir. Cihazı oluşturan hücreler, boyutları on mikro nlar mertebesinde olan ve silikonun mekanik olarak sağlam bir bileşiğinden yapılan zarlardır. Çeviricinin kendisi santimetre cinsinden ölçülebilen boyutlara sahip olduğu için bir çeviriciyi oluşturan toplam hücre sayısı binlerle ifade edilir. Akustik dalgaların uyarılması/algılanması, zar ile taban arasındaki kapasitansa dayanır: Akustik dalgaların varlığı, cihaz üzerindeki DC öngeriliminde algılama için kullanılabilecek küçük bir AC dalgalanmaya yol açarken, sürücü devre tarafından DC öngerilime eklenen küçük bir AC bileşen zar üzerindeki elektrostatik çekim kuvvetini değiştirerek zarı titreştirip akustik dalgalar üretir. Dizilim (array) yapılarının kolay biçimlendirilmesi, mekanik yapıların sürücü/algılama devreleriyle entegrasyonu ve düşük maliyet kMUÇ cihazlarının temel avantajları arasında yer almaktadır. Bu çalışmada, kMUÇ cihazlarını karakterize etmekte kullanılan temel teoriler gelişti rilmiştir. Analitik formulasyon, bir Sonlu Eleman Metodu (SEM) modelinin doğruluğunusınamak için kullanımıştır. SEM modeli, daha sonra analitik modeli bulunmayan yapıların analizinde kullanılmıştır. Elektrotları belli amaçlar için biçimlendirilmiş kMUÇ'larm analizi, SEM modeli kullanılarak çözülen özel problemler arasında yer almaktadır. Daha özel bir çalışma, sualtı uygulamaları için kullanılan 25 //m yarıçaplı bir cihazın elek trot yarıçapının 10 /im yapılmasıyla cihazın bant genişliğinin % 100 arttırılabileceğini göstermiştir. SEM analizi, daha sonra taban kayıplarını da içerecek şeklide genişletilmiştir. Bu, SEM modeline Emici Sınır Koşulları'nın (ESK) eklenmesini gerektirmiştir. Ayrıca bir Normal Mod Teorisi analiziyle problemin fiziksel temelinin daha iyi anlaşılabilmesi sağlanmıştır. Bu analizle, merkez frekansı 2.5 MHz ve taban kalınlığı 500 //m olan bir çevirici için hakim dalga modunun en düşük sıralı anti-simetrik Lamb dalgası modu olduğu tespit edilmiştir. Bu çalışmada ayrıca kMUÇ cihazlarının üretimi için bir mikro- fabrikasyon yöntemi geliştirilmiştir. Bu yöntemle, zar malzemesi olarak amorf silikon, kalıp malzemesi olarak da silikon nitrat kullanılarak 40 /im çaplı altıgen biçimli çeviriciler üretilmiştir. Zar ve hava boşluğu kalınlıklarının her ikisi de 0.5 /im olarak seçilmiştir. Zarlar, elektrot olarak kullanılmak üzere 8, 16 veya 24 fim çaplı altın plakalarla kaplan mışlardır. 0.7x0.7 cm2'lik bir çevirici alanı üzerinde 24 bin aktif hücre inşaa edilmiştir. Merkez frekansı 2 MHz olarak ölçülen çeviricilerden analitik kMUÇ modelini destekleyen ölçüm sonuçlan alınmıştır. Anahtar Sözcükler: Kapasitif Mikro-işlenmiş Ultrasonik Çevirici (kMUÇ), Sonlu Eleman Metodu (SEM), Emici Sınır Koşullan (ESK), Normal Mod Teorisi, Mikro- fabrikasyon Yöntemi. ABSTRACT MODELING AND CHARACTERIZATION OF CAPACITIVE MICROMACHINED ULTRASONIC TRANSDUCERS Ayhan Bozkurt Ph. D. in Electrical and Electronics Engineering Supervisor: Prof. Abdullah Atalar January 5, 2000 The Capacitive Micromachined Ultrasonic Transducer (cMUT) is a device used for the generation and detection of ultrasonic sound waves. The device is constructed on a sil icon substrate using a microfabrication process. Individual cells constituting the device are membranes which have dimensions in the order of tens of microns, and are made up of a mechanically strong compound of silicon. The transducer itself has dimensions mea sured in centimeters, thus the total number of cells that make up a transducer is in the order of thousands. The excitation/detection of acoustic waves relies on the capacitance between the substrate and membrane: The presence of acoustic waves induces a small AC variation on the DC bias on the device, which can be used for detection, while a small AC component added to the DC bias by the drive circuit changes the electro-static attraction force on the membrane causing it to vibrate, producing acoustic waves. Basic advantages of cMUT devices include easy patterning of array structures, integration of drive/detection electronics with mechanical structures, and low cost. In this study, basic theory describing the characteristics of cMUT devices were de veloped. The analytic formulation was used to test the validity of a Finite ElementMethod (FEM) model. The FEM model, then, was employed in the analysis of struc tures for which no analytical models are present. Specific problems solved using the FEM model included the characterization of cMUT devices with judiciously patterned electrodes. A more specific study showed that the bandwidth of an immersion device with an active area of radius 25 //m can be increased by 100% by simply setting the electrode radius to 10 /im. The FEM analysis was, then, extended to handle the effects of substrate loss, which required the incorporation of an Absorbing Boundary Condition (ABC) into the model. A Normal Mode Theory analysis was conducted to give better insight to the physical nature of the effect of substrate loss to device characteristics. The dominant wavemode for a transducer of central frequency 2.5 MHz was found to be the lowest order anti-symmetric lamb wave mode (A0), for a silicon substrate of thickness 500 /xm. A microfabrication process was developed for the production of cMUT devices. Hexagonally shaped transducers of radius 40 /xm were fabrictated on a conducting sili con substrated with silicon nitride as the sacrificial layer and amorphous silicon as the membrane material. Both the gap and membrane thicknesses are set to 0.5 /im. 8, 16, and 24 /xm gold plates were deposited as top eletrodes. The total number of active cells were 24 thousand for a substrate size of 0.7x0.7 cm2. Some experimental results were obtained from the fabricated transducers to support the analytical cMUT model. The device is found to have a central frequency of 2 MHz. Keywords : Capacitive Micromachined Ultrasonic Transducer (cMUT), Finite Ele ment Method (FEM) Modeling, Absorbing Boundary Condition (ABC), Normal Mode Theory, Microfabrication Process. 83