ΔE-Effect Magnetic Field Sensors

Many conceivable biomedical and diagnostic applications require the detection of small-amplitude and low-frequency magnetic fields. Against this background, a magnetometer concept is investigated in this work based on the magnetoelastic ΔE effect. The ΔE effect causes the resonance frequency of a magnetoelastic resonator to detune in the presence of a magnetic field, which can be read-out electrically with an additional piezoelectric phase. Various microelectromechanical resonators are experimentally analyzed in terms of the ΔE effect and signal-and-noise response. This response is highly complex because of the anisotropic and nonlinear coupled magnetic, mechanical, and electrical properties. Models are developed and extended where necessary to gain insights into the potentials and limits accompanying sensor design and operating parameters. Beyond the material and geometry parameters, we analyze the effect of different resonance modes, spatial property variations, and operating frequencies on sensitivity. Although a large ΔE effect is confirmed in the shear modulus, the sensitivity of classical cantilever resonators does not benefit from this effect. An approach utilizing surface acoustic shear-waves provides a solution and can detect small signals over a large bandwidth. Comprehensive analyses of the quality factor and piezoelectric material parameters indicate methods to increase sensitivity and signal-to-noise ratio significantly. First exchange-biased ΔE-effect sensors pave the way for compact setups and arrays with a large number of sensor elements. With an extended signal-and-noise model, specific requirements are identified that could improve the signal-to-noise ratio. The insights gained lead to a new concept that can circumvent previous limitations. With the results and models, important contributions are made to the understanding and development of ΔE-effect sensors with prospects for improvements in the future

Modeling of Thermally Aware Carbon Nanotube and Graphene Based Post CMOS VLSI Interconnect

This work studies various emerging reduced dimensional materials for very large-scale integration (VLSI) interconnects. The prime motivation of this work is to find an alternative to the existing Cu-based interconnect for post-CMOS technology nodes with an emphasis on thermal stability. Starting from the material modeling, this work includes material characterization, exploration of electronic properties, vibrational properties and to analyze performance as a VLSI interconnect. Using state of the art density functional theories (DFT) one-dimensional and two-dimensional materials were designed for exploring their electronic structures, transport properties and their circuit behaviors. Primarily carbon nanotube (CNT), graphene and graphene/copper based interconnects were studied in this work. Being reduced dimensional materials the charge carriers in CNT(1-D) and in graphene (2-D) are quantum mechanically confined as a result of this free electron approximation fails to explain their electronic properties. For same reason Drude theory of metals fails to explain electronic transport phenomena. In this work Landauer transport theories using non-equilibrium Green function (NEGF) formalism was used for carrier transport calculation. For phonon transport studies, phenomenological Fourier’s heat diffusion equation was used for longer interconnects. Semi-classical BTE and Landauer transport for phonons were used in cases of ballistic phonon transport regime. After obtaining self-consistent electronic and thermal transport coefficients, an equivalent circuit model is proposed to analyze interconnects’ electrical performances. For material studies, CNTs of different variants were analyzed and compared with existing copper based interconnects and were found to be auspicious contenders with integrational challenges. Although, Cu based interconnect is still outperforming other emerging materials in terms of the energy-delay product (1.72 fJ-ps), considering the electromigration resistance graphene Cu hybrid interconnect proposed in this dissertation performs better. Ten times more electromigration resistance is achievable with the cost of only 30% increase in energy-delay product. This unique property of this proposed interconnect also outperforms other studied alternative materials such as multiwalled CNT, single walled CNT and their bundles

Modeling and Simulation in Engineering

The general aim of this book is to present selected chapters of the following types: chapters with more focus on modeling with some necessary simulation details and chapters with less focus on modeling but with more simulation details. This book contains eleven chapters divided into two sections: Modeling in Continuum Mechanics and Modeling in Electronics and Engineering. We hope our book entitled "Modeling and Simulation in Engineering - Selected Problems" will serve as a useful reference to students, scientists, and engineers

Conductive behaviour of carbon nanotube based composites

This project was basically exploratory in the electrical properties of carbon nanotube (CNT) based materials. The direct current (DC) conductivity of CNT/polymer composites was computed by using equivalent circuit method and a three dimensional (3-D) numerical continuum model with the consideration of tunneling conduction. The effects of the potential barrier of polymer and the tortousity of CNTs on the conductivity were analyzed. It was found that both of percolation threshold and DC conductivity can be strongly affected by the potential barrier and the tortousity. The influence of contact resistance on DC conductivity was also computed, and the results revealed that contact resistance and tunneling resistance had significant influences on the conductivity, but did not affect the percolation threshold. The microstructure-dependent alternating current (AC) properties of CNT/polymer composites were investigated using the 3-D numerical continuum model. It was found that AC conductivity and critical frequency of CNT/polymer composites can be enhanced by increasing the curl ratio of CNTs. In the mid-range CNT mass fraction, with increasing curl ratio of CNTs, AC conductivity, interestingly, became frequency-dependent in low frequency range, which cannot be explained by reference to the percolation theory. A proper interpretation was given based on the linear circuit theory. It was also found that the critical frequency can also be affected by the size of CNT cluster. Series numerical formulas were derived by using a numerical capacitively and resistively junction model. In particular, this work introduced an equivalent resistor-capacitor (RC) circuit with simple definitions of the values of contact resistance and average mutual capacitance for CNT/polymer nanocomposites. Theoretical results were in good agreement with experimental data, and successfully predicted the effect of morphology on the AC properties of CNT/polymer composites. DC and AC conductivities of multi-walled carbon nanotube (MWCNT)/graphene oxide (GO) hybrid films were measured for selected MWCNT mass fractions of 10%, 33.3%, 50%, 66.7%, and 83.3% using four-probe method. The experimental results were fitted using scaling law, and relatively high percolation threshold was found. This high percolation threshold was understood in terms of the potential energy and intrinsic ripples and warping in the freestanding graphene sheets. The capacitance of these hybrid films were measured using the voltmeter-ammeter-wattmeter test circuit with different voltages and heat treatments. The MWCNT/GO film showed relatively high specific capacitance (0.192F/cm3 for the mass fraction of 83.3%) and power factor compared to conventional dielectric capacitors. Both of measured capacitance and power factor can be enhanced by increasing testing voltages. The capacitance of MWCNT/GO films rapidly decreased after heat treatments above 160 ℃. This decrease was caused by redox reaction in the GO sheets. The capacitive behaviour of MWCNT/GO hybrid films was also interpreted by using the equivalent circuit model. Single-walled carbon nanotube (SWCNT) and SWCNT/Poly(vinyl alcohol) (PVA) films were used to form a piezoresistive strain sensor. Both of static and dynamic strain sensing behaviours of SWCNT and SWCNT/PVA films were measured. It was found that the sensitivities of these films decreased with increasing their thicknesses. The SWCNT film with a thickness of 1900 nm and SWCNT/PVA film exhibited viscoelastic sensing behaviour, because van der Waals attraction force allowed axial slippages of the smooth surface of nanotubes. A numerical model was derived based on the dynamic strain sensing behaviour. This model could be useful for designing CNT strain sensors. Finally, thermoelectric power (TEP) of deformed SWCNT films with various thicknesses was measured. It was observed that positive TEP of SWCNT films increased with increasing stain above the critical point. The experimental results were fitted by using a numerical model in terms of a variation of Nordheim-Gorter relation and fluctuation induced tunneling (FIT) model. From the numerical model, it was found that the increase of TEP above the critical strain resulted from the positive term of the contribution from the barrier region, and the effect of barrier regions decreases with increasing the thickness of the film

Spintronic terahertz emitters based on ferro- and ferrimagnetic thin film systems

Elektromagnetische Strahlung im Terahertzfrequenzbereich von 0.1 bis 30 THz bietet zahlreiche Anwendungsmöglichkeiten, beispielsweise in Spektroskopie- und Bildgebungsverfahren, sowohl im Bereich der Grundlagenforschung, als auch für industrielle Prozesse. Da der Terahertzspektralbereich jedoch zwischen den mit etablierten elektronischen und optischen Emittern gut zugänglichen Mikrowellen- und Infrarotspektralbereichen liegt, sind Emittersysteme immer noch teuer und limitiert in Bezug auf Leistung und Bandbreite. Ein neues vielversprechendes Konzept für die Terahertzerzeugung stellen die sogenannten spintronischen Emittersysteme dar. Diese bestehen aus Multilagensystemen ferromagnetischer (FM) und nichtmagnetischer (NM) Metallschichten mit Schichtdicken im Bereich weniger Nanometer. Die Anregung einer FM/NM-Bilage mit einem femtosekundenlangen optischen Laserpuls führt zur Ausbildung eines spinpolarisierten Ladungsstromes Js von der FM in die NM Schicht, der auf der Anregung spinpolarisierter Elektronen der FM Schicht über das Ferminiveau beruht. In der NM Schicht wird Js auf Grund des inversen Spin-Hall-Effekts in einen transversalen Ladungsstrompuls Jc konvertiert, der zur Emission elektromagnetischer Strahlung im Terahertzfrequenzbereich führt. Die Abstrahlcharakteristik der Emitter kann durch Variation der verwendeten Materialien, Schichtdicken, oder durch die Verwendung komplexerer Multilagensysteme optimiert werden. Die vorliegende Arbeit präsentiert Studien zu spintronischen Terahertzemittersystemen basierend auf verschiedenen magnetischen dünnen Filmen kombiniert mit Pt und W Schichten. Die Abhandlung kann in zwei Teilbereiche untergliedert werden. Das Ziel der ersten drei Studien war die Untersuchung des Einflusses der magnetischen Eigenschaften unterschiedlicher FM und insbesondere auch ferrimagnetischer (FI) Legierungsschichten auf die Terahertzemission. Hierfür wurden Bilagensysteme bestehend aus NM Pt Schichten in Kombination mit FM CoFe, sowie FI TbFe und GdFe Legierungsschichten mit variierendem Eisengehalt (0 ≤ x ≤ 1) hergestellt. Die laserangeregte Terahertzemission wurde in Abhängigkeit des angelegten Magnetfelds, der Fluenz des Anregungslasers und der Temperatur gemessen. Die Ergebnisse wurden unter Einbeziehung detaillierter Untersuchungen der strukturellen, magnetischen, elektrischen und optischen Eigenschaften der Proben erklärt. Der zweite Teilbereich der Arbeit befasst sich mit der Entwicklung komplexerer Multilagenterahertzemittersysteme, welche das Schalten der Amplitude zwischen Zuständen hoher und niedriger Terahertzemission ermöglichen und zudem Potential für eine Steigerung der Terahertzamplitude bieten. Hierfür wurde, basierend auf den Terahertzemissioncharakteristiken des zuvor untersuchten FI Pt/GdFe Systems, ein Emitter entwickelt, der das Schalten der Terahertzamplitude durch Veränderung der Temperatur ermöglicht. In einer weiteren Studie wurde zudem die Anwendung eines Spin-Valve-Systems als magnetisch schaltbares Emittersystem demonstriert. Dieses ermöglicht ein reversibles Schalten der Emissionsamplitude mit kleinen angelegten magnetischen Feldern in der Größenordnung weniger Millitesla.Electromagnetic radiation in the terahertz (THz) frequency range from 0.1 to 30 THz can be highly useful for spectroscopy and imaging experiments in fundamental scientific research as well as for industrial applications. However, as THz regime bridges the gap between electronic and optical frequencies, emitter systems are still expensive and limited in power and bandwidth. A novel approach to overcome these challenges is given by the so-called spintronic terahertz emitters, which are based on ferromagnetic (FM) and non-magnetic metal (NM) layers with thicknesses of a few nanometers. Excitation of a FM/NM bilayer with a femtosecond optical laser pump pulse leads to the formation of an ultrafast spin current Js from the FM toward the NM layer, which is caused by the excitation of spin-polarized electrons of the FM layer above the Fermi level. In the NM layer, Js is converted into a transverse charge current pulse Jc due to the inverse spin Hall effect, which leads to the emission of electromagnetic radiation in the THz frequency regime. The emission properties of the emitters can be optimized by utilizing different materials, layer thicknesses, or more complex multilayer structures. The present work shows studies of spintronic THz emitter systems that are based on different magnetic thin films combined with Pt and W layers. The experimental studies can be divided into two parts. The main goal of the first part was to investigate how the magnetic properties of different FM and in particular also ferrimagnetic (FI) materials are reflected in the THz emission properties of a spintronic emitter system. Therefore, thin bilayers consisting of FM CoFe, or FI TbFe or GdFe alloy thin films with varying Fe content (0 ≤ x ≤ 1), combined with Pt layers have been prepared. The laser-excited spintronic THz emission has been investigated in dependence on the applied magnetic field, the temperature, and the pump fluence of the excitation laser. The results have been explained with regard to detailed characterizations of the structural, magnetic, electrical, and optical properties of the samples. The second goal of this work was set on the development of more functional multilayer emitter systems that allow for the control of the THz emission amplitude between a high- and a low-amplitude state and also might open the way for higher THz emission amplitudes. Based on the results of the previously investigated FI Pt/GdFe bilayer emitter system, a new concept of a THz emitter that can be switched by a temperature change from a high- to a low-amplitude state has been developed. Additionally, the use of a spin-valve system as a spintronic emitter system that allows for the switching of the emission amplitude by small applied magnetic fields in the range of a few millitesla has been demonstrated

Design and Modeling of Fiber Optical Current Sensor Based on Magnetostriction

A novel fiber optical current sensor (FOCS) which is based on a giant magnetostrictive material, Terfenol-D (T-D) is modeled and prototyped. Several experiments have been conducted to validate the expected results. Magnetostriction is defined as the change in dimensions of a material under the influence of an external magnetic field. The cause of the change in length is due to the rotation and re-orientation of the small magnetic domains in the magnetostrictive material. The magnetostriction of Terfenol-D is modeled and investigated using several software packages. Here, a magnetostriction-based FOCS using a Terfenol-D/epoxy composite is investigated. Particularly, the FOCS is based on applying magnetostrictive composite material to transform an external magnetic field into a corresponding mechanical strain caused by the magnetostriction of the composite. The composite is incorporated in the FOCS for increased durability, flexibility in shape, extended frequency response, and tensile strength compared to monolithic materials. Coupling Terfenol-D with a fiber Bragg grating (FBG) is an excellent method of magnetic field sensing. It consists of an FBG embedded in the composite that converts magnetostrictive strain into frequency chirp of the optical signal in proportion to a magnetic field. This will form a sensor that is compact, lightweight, and immune from electromagnetic interference. For electromagnetic interference mitigation and optimal signal condition, an FBG, which can be easily integrated with an optical fiber network and reflect a narrow band of wavelengths based on grating periods, is used to encode strain information onto an optical signal. This FOCS has potential in detecting power systems faults due to its advantages over the conventional current transformers. Experiments have been performed to investigate the effect of direct current (DC) and alternate current (AC) on the response of the FOCS. Consistent results that indicate its reliability have been obtained. The experiment results matched the predicted response. The effect of the temperature on the response of the FOCS also has been investigated. Finally, future research directions are presented for the enhancement of the FOCS technology