<p>This thesis tackles the behavior of two planar shaped opposing air jets. The word opposing means that the two air jets are positioned against each other to have a common center-line so that they impinge on each other. Opposing planar jets have several applications in industry which include: galvanization, chemical mixing processes, and combustion. Opposing planar jets are found to produce high level low frequency acoustic tones. In this study, the acoustic response of the opposing planar jets is investigated for different operating conditions. Acoustic tone analysis is performed for a wide range of jet exit velocities and separation distances between the two jets. Results show that the jets produce strong acoustic tones over the whole investigated range of test conditions. The acoustic response depends on the operating conditions of the jets and the acoustic tone is found to be generated by a self-sustained flow oscillation of the two jets.</p> <p>In order to further understand the reported oscillation phenomenon, particle image velocity (PIV) is utilized. Images of the oscillating flow field are acquired and image analysis is performed to obtain various flow dynamic properties. A computational fluid dynamics simulation is performed to help in highlighting the oscillation behavior of the jets. Results reveal that the jets exhibit anti-symmetric ‘’flapping’’ oscillation behavior. The jets are found to initially deflect away from their common centerline due to high stagnation pressure in the impingement region, while they deflect back to their common centerline due to cross stream oscillating velocity in the entrainment regions around the jets.</p> / Master of Applied Science (MASc)

Yes / In the equivalent frequency-based model, the antenna array gain is utilised to characterise the frequency response of the beam squint effect generated by the antenna array. This impact is considered for a wide range of uniform linear array (ULA) and uniform planar array (UPA) designs, including those with and without tapering configurations. For a closer look at how the frequency response of the array adapts to the variations in the incidence angle of the signal, the bandwidth of the spectrum is varied and investigated. To study this effect, we have considered using the gain array response as an equivalent channel model in our approach. Beam squinting caused by distortion in the frequency response gain can be verified by one of two equalisers: a zero-forcing (ZF) equaliser or a minimum mean square error (MMSE) equaliser. Different cases with their analysis and results are studied and compared in terms of coded and uncoded modulations. / This work was supported in part by the Satellite Network of Experts V under Contract 4000130962/20/NL/NL/FE, and in part by the Innovation Program under Grant H2020-MSCA-ITN-2016 SECRET-722424.

The work herein details an optical droplet measurement system based on planar multi-angle Mie scattering. Sizing information consists of a mean droplet diameter and droplet distribution estimates for every individual point within a planar (2D) area of interest. The planar method makes possible the fast acquisition of data within a large field of interest, and uses relatively inexpensive instrumentation. As presented, the method demonstrated the ability to measure water droplets from a typical simplex spray nozzle, across the range of 5-50 micrometers within +/-10% of known values, and in addition return an estimate of the shape and width of the size distribution at each location within the planar region of interest. Measurements demonstrating the agreement between results from this current method and known PDA data were successfully completed for a 1-gallon-per-hour spray nozzle, and repeatability was demonstrated in 2.5-gallon-per-hour and 4.5-gallon-per-hour nozzles. Additionally the limits of the technique are explored with simulated data. Conclusions from these exercises show that the multi-angle planar Mie scattering method is capable of measuring droplet distribution characteristics and means within a nominal range of 0.3 micrometers up to 150 micrometers. / Ph. D.

In recent years, the SiC power semiconductor has emerged as an attractive alternative that pushes the limitations of junction temperature, power rating, and switching frequency of Si devices. These advanced properties will lead converters to higher power density. However, the reliability of the SiC semiconductor is still under investigation, and at the same time, the standard Si device packages do not meet the requirement of high temperature operation. In order to take full advantage of SiC semiconductor devices, high density, high temperature device packaging needs to be developed. In this dissertation, a high temperature wirebond package for multi-chip phase-leg power module using SiC devices was designed, developed, fabricated and tested. The details of the material selection and thermo-mechanical reliability evaluation are described. High temperature power test shows that the presented package can perform well at the high junction temperature. In order to increase the power density, reduce the parasitic parameters, and enhance the electrical, thermo-mechanical performance over wirebond packages, planar package is utilized to better take advantages of SiC device. This dissertation proposed a novel package, in which the interconnections can be formed on small dimensional pads and enclosed pads that may baffle the regular solder based connection in other planar packages. Electrical and thermo-mechanical tests of the prototype module demonstrate the functionality and reliability of the presented planar packaging methodology. In this dissertation, together with the design example, the manual module layout design and automatic module layout design process are also presented. Furthermore, a systematic optimal design process and parametric study of the heatsink-fan cooling system by applying the analytical model is described. This dissertation also established a systematic testing procedure which can rapidly detect defects and reduce the risk in high temperature packaging testing. Finally, a wirebond module and a planar module are designed for 175 ºC junction temperature and 250 ºC junction temperatures. All the key concepts and ideas developed in this work are implemented in the prototype module development and then verified by the experimental results. / Ph. D.

Additive manufacturing (AM) has received much attention from researchers over the past decades because of its diverse applications in various industries. AM is an advanced manufacturing process that facilitates the fabrication of complex geometries represented by computer-aided design (CAD) models. Traditionally, designed parts are fabricated by extruding material layer-by-layer using a tool path planning obtained from slicing programs by using CAD models as an input. Recently, there has been a growing interest in non-planar AM technologies, which offer the ability to fabricate multilayer constructs conforming to freeform surfaces. Non-planar AM processes have been utilized in various applications and involved objects of varying material properties and geometric characteristics. Although the current state of the art suggests AM can provide novel opportunities in conformal manufacturing, several challenges remain to be addressed. The identified challenges in non-planar AM fall into three categories: 1) conformal 3D printing on substrates with complex topography of which CAD model representation is not readily available, 2) understanding the relationship between the tool path planning and the quality of the 3D-printed construct, and 3) conformal 3D printing in the presence of mechanical disturbances. An open-loop non-planar tool path planning algorithm based on point cloud representations of object geometry and a closed-loop non-planar tool path planning algorithm based on position sensing were proposed to address these limitations and enable conformal 3D printing and spatiotemporal 3D sensing on objects of near-arbitrary organic shape. Three complementary studies have been completed towards the goal of improving the conformal tool path planning capabilities in various applications including fabrication of conformal electronics, in situ bioprinting, and spatiotemporal biosensing: i. A non-planar tool path planning algorithm for conformal microextrusion 3D printing based on point cloud data representations of object geometry was presented. Also, new insights into the origin of common conformal 3D printing defects, including tool-surface contact, were provided. The impact and utility of the proposed conformal microextrusion 3D printing process was demonstrated by the fabrication of 3D spiral and Hilbert-curve loop antennas on various non-planar substrates, including wrinkled and folded Kapton films and origami. ii. A new method for closed-loop controlled 3D printing on moving substrates, objects, and unconstrained human anatomy via real-time object position sensing was proposed. Monitoring of the tool position via real-time sensing of nozzle-surface offset using 1D laser displacement sensors enabled conformal 3D printing on moving substrates and objects. The proposed control strategy was demonstrated by microextrusion 3D printing on oscillating substrates and in situ bioprinting on an unconstrained human hand. iii. A reverse engineering-driven collision-free path planning program for automated inspection of macroscale biological specimens, such as tissue-based products and organs, was proposed. The path planning program for impedance-based spatiotemporal biosensing was demonstrated by the characterization of meat and fruit tissues using two impedimetric sensors: a cantilever sensor and a multifunctional fiber sensor. / Doctor of Philosophy / Additive Manufacturing (AM), commonly referred to as 3D printing, is a computer-aided manufacturing process that facilitates the fabrication of personalized and customized models, tissues, devices, and wearables. AM has several advantages over traditional manufacturing processes. For example, directing computer-driven robotics enables control over spatial structure and composition of parts. While 3D printing is typically performed using layer-by-layer planar tool paths generated by slicing programs, non-planar 3D printing is an emerging area that has recently been examined for various post-processing applications. Processes that enable material deposition conforming to complex geometric and freeform objects (e.g., anatomical structures), are central to various industries, including additive manufacturing, electronics manufacturing, and biomanufacturing. In this dissertation, tool path planning methods and real-time control strategies for non-planar 3D printing onto stationary and moving arbitrary surfaces, and various conformal electronics and in situ bioprinting applications will be presented. In addition to the tool path planning methods for 3D printing, a collision-free path planning program will be proposed for the inspection of large tissues and organs. The utility of the proposed method will be demonstrated through electrical impedance-based biosensing of meat and fruit to characterize their compositional and physiochemical properties which are used for quality assessment.

The trend in isolated DC/DC bus converters is to increase the output power in the same brick form factors that have been used in the past. Traditional intermediate bus converters (IBCs) use silicon power metal oxide semiconductor field effect transistors (MOSFETs), which recently have reached the limit in terms of turn on resistance (RDSON) and switching frequency. In order to make the IBCs smaller, the switching frequency needs to be pushed higher, which will in turn shrink the magnetics, lowering the converter size, but increase the switching related losses, lowering the overall efficiency of the converter. Wide-bandgap semiconductor devices are becoming more popular in commercial products and gallium nitride (GaN) devices are able to push the switching frequency higher without sacrificing efficiency. GaN devices can shrink the size of the converter and provide better efficiency than its silicon counterpart provides. A survey of current IBCs was conducted in order to find a design point for efficiency and power density. A two-stage converter topology was explored, with a multiphase buck converter as the front end, followed by an LLC resonant converter. The multiphase buck converter provides regulation, while the LLC provides isolation. With the buck converter providing regulation, the switching frequency of the entire converter will be constant. A constant switching frequency allows for better electromagnetic interference (EMI) mitigation. This work includes the details to design and implement a hard-switched multiphase buck converter with planar magnetics using GaN devices. The efficiency includes both the buck efficiency and the overall efficiency of the two-stage converter including the LLC. The buck converter operates with 40V - 60V input, nominally 48V, and outputs 36V at 1 kW, which is the input to the LLC regulating 36V – 12V. Both open and closed loop was measured for the buck and the full converter. EMI performance was not measured or addressed in this work. / Master of Science / Traditional silicon devices are widely used in all power electronics applications today, however they have reached their limit in terms of size and performance. With the introduction of gallium nitride (GaN) field effect transistors (FETs), the limits of silicon can now be passed with GaN providing better performance. GaN devices can be switched at higher switching frequencies than silicon, which allows for the magnetics of power converters to be smaller. GaN devices can also achieve higher efficiency than silicon, so increasing the switching frequency will not hurt the overall efficiency of the power converter. GaN devices can handle higher switching frequencies and larger currents while maintaining the same or better efficiencies over their silicon counterparts. This work illustrates the design and implementation of GaN devices into a multiphase buck converter. This converter is the front end of a two-stage converter, where the buck will provide regulation and the second stage will provide isolation. With the use of higher switching frequencies, the magnetics can be decreased in size, meaning planar magnetics can be used in the power converter. Planar magnetics can be placed directly inside of the printing circuit board (PCB), which allows for higher power densities and easy manufacturing of the magnetics and overall converter. Finally, the open and closed loop were verified and compared to the current converters that are on the market in the 48V – 12V area of intermediate bus converters (IBCs).

This thesis examines the packaging of high-temperature SiC power electronic devices. Current-voltage measurements were conducted on as-received and packaged SiC power devices. The planar structure was introduced and developed as a substitution for traditional wire-bonding vertical structure. The planar structure was applied to a high temperature (>250oC) SiC power device. Based on the current-voltage (I-V) measurements, the packaging structures were improved, materials were selected, and processes were tightly controlled. This study applies two types of planar structures, the direct bond and the bump bond, to the high-temperature packaging of high-temperature SiC diode. A drop in the reverse breakdown voltage was discovered in the packaging using a direct bond. The root cause for the drop in the breakdown voltage was identified and corrective solutions were evaluated. A few effective methods were suggested for solving the breakdown issue. The forward I-V curve of the planar packaging using direct bond showed excellent results due to the excellent electrical and thermal properties of sintered nanosilver. The packaging using a bump bond as an improved structure was processed and proved to possess desirable forward and reverse I-V behavior. The cross-sections of both planar structures were inspected. High-temperature packaging materials, including nano-silver paste, high-lead solder ball and paste, adhesive epoxy, and encapsulant, were introduced and evaluated. The processes such as stencil printing, low-temperature sintering, solder reflowing, epoxy curing, sputtering deposition, electroplating, and patterning of direct-bond copper (DBC) were tightly controlled to ensure high-quality packaging with improved performance. Finally, the planar packaging of the high temperature power device was evaluated and summarized, and the future work was recommended. / Master of Science

yes / A novel miniaturized planar double inverted-F antenna is presented. The antenna design is based on the electromagnetic coupling of two air dielectric PIFA antennas, combined with a broadband rectangular plate feed structure to achieve ultra-wideband characteristics. The computed and experimental impedance bandwidths show good agreement over an UWB frequency band from 3.1 GHz to 10.6 GHz for |S11| < -10dB. The antenna is electrically small, with size 0.31 x 0.16 x 0.09 wavelengths at 3.1 GHz and 1.06 x 0.55 x 0.31 wavelengths at 10.6 GHz. The simulated and measured gain and radiation patterns show acceptable agreement and confirm that the antenna has appropriate characteristics for short range wireless applications. / MSCRC

Conventional wire bond power modules have limited application for high-current operation, mainly because of their poor thermal management capability. Planar power modules have excellent thermal management capability and lower parasitic inductance, which means that the planar packaging method is desirable for high-power applications. For these reasons, a planar power module for an automotive motor drive system was developed, and a gate-driver circuit with an over-current protection was planned to integrate into the module. This thesis discusses a current-sensing method for the planar module, and the integrated gate driver circuit with an over-current protection. After reviewing several current-sensing methods, it becomes clear that most popular current-sensing methods, such as the Hall-Effect sensor, the current transformer, the Shunt resistor, and Rogowski coils, exhibit limitations for the planar module integration. For these reasons, a giant magneto resistive (GMR) magnetic-field detector was chosen as a current-sensing method. The GMR sensor utilizes the characteristics of the giant magneto resistive (GMR) effect in that it changes its resistance when it is exposed to the magnetic-flux. Because the GMR resistor can be fabricated at the wafer level, a packaged GMR sensor is very compact when compared with conventional current sensors. In addition, the sensor detects magnetic-fields, which does not require direct contact to the current-carrying conductor, and the bandwidth of the sensor can be up to 1 MHz, which is wide enough for the switching frequencies of most of motor drive applications. However, there are some limiting factors that need to be considered for accurate current measurement: • Operating temperature • Magnetic-flux density seen by a GMR resistor • Measurement noise If the GMR sensor is integrated into the power module, the ambient temperature of the sensor will be highly influenced by the junction temperature of the power devices. Having a consistent measurement for varying temperature is important for module-integrated current sensors. An experiment was performed to see the temperature characteristics of a GMR sensor. The measurement error caused by temperature variation was quantified by measurement conditions. This thesis also proposes an active temperature error compensation method for the best use of the GMR sensor. The wide current trace of the planar power module helps to reduce the electrical/thermal resistance, but it hinders having a strong and constant magnetic-field-density seen by the GMR sensor. In addition, the eddy-current effect will change the distribution of the current density and the magnetic-flux-density. These changes directly influence the accurate measurement of the GMR sensor. Therefore, analyzing the magnetic-flux distribution in the planar power module is critical for integrating the GMR sensor. A GMR sensor is very sensitive to noise, especially when it is sensing current flowing in a wide trace and exposed to external fields, neither of which can be avoided for the operation of power modules. Post-signal processing is required, and the signal-conditioning circuit was designed to attenuate noise. The signal-conditioning circuit was designed using an instrumentation amplifier, and the circuit attenuated most of the noise that hindered accurate measurement. The over-current protection circuit along with the gate driver circuit was designed, and the concept was verified by experiments. The main achievements of this study can be summarized as: • Characterization of conventional current-sensing methods • Temperature characterization of the GMR resistor • Magnetic-flux distribution of the planar power module • Design of the signal-conditioning circuit and over-current protection circuit / Master of Science

Point-of-load (POL) converters have been used extensively in IT products. Today, almost every microprocessor is powered by a multi-phase POL converter with high output current, which is also known as voltage regulator (VR). In the state-of-the-art VRs, the circuits are mostly constructed with discrete components and situated on the motherboard, where it can occupy more than 1/3 of the footprint of the motherboard. A compact POL is desirable to save precious space on motherboards to be used for some other critical functionalities. Recently, industry has released many modularized POL converters, in which the bulky inductor is integrated with the active components to increase the power density. This concept has been demonstrated at current levels less than 5A and power density around 600-1000W/in³. This might address the needs of small hand-held equipment such as smart phones, but it is far from meeting the needs for the applications such as laptops, desktops and servers, where tens and hundreds of amperes are needed. A 3D integrated POL module with an output current of tens of ampere has been successfully demonstrated at the Center for Power Electronic Systems (CPES), Virginia Tech. In this structure, the inductor is elaborated with low temperature co-fire ceramic (LTCC) ferrite, as a substrate where the active components are placed. The lateral flux inductor is proposed to achieve both a low profile and high power density. Generally, the size of the inductor can be continuously shrunk by raising the switching frequency. The emerging gallium-nitride (GaN) power devices enable the creation and use of a multi-MHz, high efficiency POL converter. This dissertation firstly explores the LTCC inductor substrate design in the multi-MHz range for a high-current POL module with GaN devices. The impacts of different frequencies and different LTCC ferrite materials on the inductor are also discussed. Thanks to the DC flux cancellation effect, the inverse coupled inductor further improves the power density of a 20A, 5MHz two-phase POL module to more than 1kW/in³. An FEA simulation model is developed to study the core loss of the lateral flux coupled inductor, which shows the inverse coupling is also beneficial for core loss reduction. The ceramic-based 3D integrated POL module, however, is not widely adopted in industrial products because of the relatively high cost of the LTCC ferrite material and complicated manufacturing process. To solve that problem, a printed circuit board (PCB) inductor substrate with embedded alloy flake composite core is proposed. The layerwise magnetic core is laminated into a multi-layer PCB, and the winding of the inductor then is formed by the copper layers and conventional PCB vias. As a demonstration of system integration, a 20A, 1.5MHz integrated POL module is designed and fabricated based on a 4-layer PCB with embedded flake core, which realizes more than 85% efficiency and 600W/in³ power density. The application of standardized PCB processes reduces the cost for manufacturing the integrated modules due to the easy automation and the low temperature manufacturing process. Combining the PCB-embedded coupled inductor substrate and advanced control strategy, the two-phase 40A POL modules are elaborated as a complete integrated laptop VR solution. The coupled inductor structure is slightly modified to improve its transient performance. The nonlinearity of the inductance is controlled by adding either air slots or low permeability magnetic slots into the leakage flux path of the coupled inductor. Then the leakage flux, which determines the transient response of the coupled inductor, can be well controlled. If we directly replace the discrete VR solution with the proposed integrated modules, more than 50% of the footprint on the motherboard can be saved. Although the benefits of the lateral flux inductor have been validated in terms of its high power density and low profile, the planar core is excited under very non-uniform flux. Some parts of the core are even pushed into the saturation region, which totally goes against the conventional sense of magnetic design. The final part of this dissertation focuses on evaluating the performance of the planar core with variable flux. The counterbalance between DC flux and AC flux is revealed, with which the AC flux and the core loss density are automatically limited in the saturated core. The saturation is essentially no longer detrimental in this special structure. Compared with the conventional uniform flux design, the variable flux structure extends the operating point into the saturation region, which gives better utilization of the core. In addition, the planar core with variable flux also provides better thermal management and more core loss reduction under light load. As conclusions, this research first challenges the conventional magnetic design rules, which always assumes uniform flux. The unique characteristics and benefits of the variable flux core are proved. As an example of taking advantages of the lateral flux inductor, the PCB integrated POL modules are proposed and demonstrated as a high-density VR solution. The integrated modules are cost-effective and ready to be commercialized, which could enable the next technological innovation for the whole computing and telecom industry. / Ph. D.