Mechanical Properties of Microstructural Components of Inorganic Materials

Disertační práce se zabývá studiem strukturních a mechanických vlastností anorganických materiálů. Cílem je nalezení jednotlivých fází ve zkoumaném materiálu a hlavně lokalizace (mechanicky) nejslabšího místa, jeho ovlivnění a následně výroba materiálu o lepších mechanických vlastnostech. Z důvodu velkého množství použitých metod je základní teorie vložena vždy na začátku příslušné kapitoly. Taktéž z důvodu značného množství výsledků jsou na konci kapitol uvedeny dílčí závěry. Práce je rozdělena na tři části, kdy první se zabývá seznámením s možnostmi modelování mikro-mechanických vlastností a provedením experimentů umožňujících posouzení rozsahu platnosti některého modelu. V druhé části je provedeno shrnutí současných možností indentačních zkoušek pro měření mechanických vlastností strukturních složek betonu a praktické zvládnutí metodiky vhodné k užití pro výzkum materiálů zkoumaných domovským pracovištěm. V třetí části je navržena metoda identifikace nejslabších článků struktury anorganických pojiv a její ověření na konkrétním materiálu zkoumaném na domovském pracovišti. V této dizertační práci jsou použity tyto metody: kalorimetrie, ultrazvukové testování, jednoosá pevnost v tlaku, nanoindentace, korelativní mikroskopie a rastrovací elektronová mikroskopie s energiově disperzním spektrometrem. Dílčími výsledky jsou kompletní charakterizace cementových materiálů, upřesnění stávajících poznatků a nalezení optimálního postupu pro charakterizaci. Hlavním výsledkem je inovativní přístup vedoucí k pozitivnímu ovlivnění materiálu.The doctoral thesis deals with study of structural and mechanical properties of inorganic materials. Goal is to find the weakest (mechanically) phases and interfaces of material. By affecting these structures it should be possible consequently produce a material with better mechanical properties. Due to the large amount of used methods the basic theory is discussed always in the beginning of relevant chapter. Similarly, due to the considerable amount of results every chapter includes partial conclusions. The work is divided in three parts. The first deals with the introduction of the possibilities of modeling micro-mechanical properties and performing of experiments that allow assessment of the scope of some model. In second part itis performed an overview of current possibilities of indentation tests for measuring mechanical properties of structural components of concrete and the practical managing of methods suitable for use for materials research examined at our faculty. In third part the method of identifying the weakest points in structure of inorganic binders is proposed and validation on the particular material examined at our faculty is performed. The methods used in this doctoral thesis are: calorimetry, ultrasonic testing, uniaxial compression, nanoindentation, correlative microscopy and scanning electron microscopy with energy dispersive spectrometer. Partial results are a complete characterization of cementitious materials, specification of existing knowledge and finding the optimal procedure for characterization. The main result is an innovative approach that leads to a positive effect on the material.

Micromechanics of oxides - From complex scales to single crystals

Protective oxide scales shield high temperature materials from corrosion, thus ensuring safety and long material life under adverse operating conditions. Cracking and spallation of such scales can lead to fatigue crack initiation and expose the material to further oxidation. It is therefore imperative to measure the fracture properties of oxides so that they can be incorporated in the life estimation models of high temperature materials. Existing models require inputs on oxide properties such as fracture strain and elastic modulus. The established measurement methods are mainly applied for thick (several microns) scales, but for many materials such as superalloys the oxides are thinner (< 1 \ub5m), and the results would be affected by the influence of substrate and residual stresses. Focused ion beam machining (FIB) enables the preparation of micro sized specimens in the size range of these scales. \ua0In this work, a modified microcantilever geometry with partially removed substrate is proposed for testing of oxide scales. Room temperature microcantilever bending of thermally grown superalloy oxide (complex oxide with an upper layer of spinel and lower layer of Cr2O3) revealed the presence of plasticity, which is attributed to the deformation of the upper cubic spinel layer and low defect density of the volume being probed. Due to difficulties in isolating Cr2O3 from the complex oxide layer, dedicated oxidation exposures are performed on pure chromium to generate Cr2O3 which is tested using the same cantilever geometry at room temperature and 600 \ub0C. Results show lower fracture strain at 600 \ub0C in comparison to room temperature and presence of cleavage type of transgranular fracture in both cases, pointing to a need for studying cleavage fracture of Cr2O3. This was analysed using microcantilever bending of single crystal Cr2O3 to identify the preferential cleavage planes. Finally, fracture toughness was also measured through microcantilever bending and micropillar splitting. \ua0Thus, it is shown that micromechanical testing is an effective tool for measuring fracture properties of oxide scales. The fracture study of Cr2O3 scales show that it is a complex process in which the crystallographic texture also plays a role. Surface energy and fracture toughness criterion was unable to explain the fracture behaviour of single crystal Cr2O3 observed from experiments. Such a comprehensive analysis can contribute towards the development of reliable models for oxidation assisted failure

Doctor of Philosophy

dissertationLigaments and tendons are dense, fibrous connective tissue that transmit and bear loads within the musculoskeletal system. They are elastic and viscous, and thus are capable of storing and dissipating energy. Although soft and flexible, they can interface with materials that are orders of magnitude stiffer (e.g., bone) and orders of magnitude more compliant (e.g., muscle). These functions are mediated by a complex network of hierarchically organized fibrillar collagen and accessory proteins and molecules. Tissue constituents form unique structural motifs that span the nanoscale, microscale, mesoscale and macroscale. This multiscale organization enables both a robust mechanical response at the macroscopic joint level and simultaneously provides a microscale environment conducive to cellular proliferation and nutrient transport. The aim of this dissertation was to gain a deeper understanding of how the organization of tissue constituents contribute to mechanical function of tendon and ligament across scale levels. At the nanoscale, the question regarding the role of the proteoglycan decorin was addressed. A novel combination of an in vitro assay, imaging techniques and mechanical testing was used to explore how decorin acts to modify the strength of collagen fibril networks. At the microscale, computational modeling was used to examine how different fibril organizations contribute to the macroscopic volumetric response of tendon and ligament during tensile loading. The volumetric response is believed to drive fluid flux within the tissue, which may play a role in nutrient transport and the apparent viscoelastic response. This flow dependent mechanism was addressed in a study that experimentally measured the volumetric changes in mesocale fascicles during viscoelastic testing. One of the challenges in discerning structure-function relationships in tendon and ligament is the large number of uncontrolled variables, which can be difficult to account for in an experimental setting. To address this challenge, a collagen based tendon surrogate was developed for use as a physical model. The physical model was coupled to a validated micromechanical computational model. This facilitated the testing of hypotheses that would have been difficult to address experimentally. The four studies contained within this dissertation, along with a number of preliminary studies, represent a novel contribution to the field of tendon and ligament mechanics

Small-scale testing of micromechanical response of cemented carbides

Tesi per compendi de publicacions, amb diferents seccions retallades per drets de l'editorCemented carbides are composite materials widely used in different industry fields within applications involving wear, due to their outstanding wear resistance. The most commonly used are WC-Co grades, for Co wettability with the carbide and adhesion characteristics. Emergence of new applications, the existence of advanced characterization techniques, economic and environmental aspects, among others, encourages the development of a new cemented carbides generation containing other binding phases as Ni and Fe or alloys of them. Furthermore, Co powder has been classified as very toxic for the human health and the combination carbide-cobalt hardmetals dust has shown to be even more toxic than both pure cobalt and tungsten. The success of substitution of the main constituents of cemented carbides, have been commonly measured in terms of their final mechanical properties at macroscale such as hardness, toughness and transverse rupture strength; and structural integrity under service-like conditions, such as corrosion resistance, thermal shock and fatigue resistance. In this sense, general framework of microstructural effects – carbide mean grain size, volume fraction and chemical nature of constitutive phases - on the mechanical response of cemented carbides is well established at the macroscale. However, assessment of the individual role of the binder and carbide phases at local scale i.e. microscale, is yet to be studied in depth. Within micromechanical testing, special attention has being paid to the micropillar compression approach because its advantages: the stress-state is nominally uniaxial, allowing a straight conversion of the measured load-displacement data into flow curves; sample preparation by means of Focused Ion Beam (FIB) milling is a relatively easy machining route; it involves the use of a conventional nanoindenter with a flat-end tip; and, it can be performed ex-situ or in-situ by using Scanning Electron Microscopy (SEM) or Transmission Electron Microscopy (TEM) techniques. However, attention have to be paid to sample sizes since it has been well established that intrinsic properties of crystalline materials such as yield stress and strength, can be greatly influenced by extrinsic factors such as volume. For instance, results have evidenced an inverse relation between hardness and the indentation depth at the micro- and nanometric length scales. Regarding cemented carbides, recent studies showed that changes in volume fraction of binder and carbides in samples can lead to wide scatter in results of Young’s modulus measured at the microscale. Following the above ideas, in this PhD thesis uniaxial compression of micropillars and nanoindentation have been selected to evaluate the role of binder and carbides regarding their chemical nature and microstructural dimensions, i.e. carbide mean grain size and binder mean free path, in the mechanical properties and response of cemented carbides at local scales. This thesis is presented by a compendium of scientific publications in which several specific objectives are studied individually. In the first and second publications the sample size and the volume fraction of constitutive phases within the micropillar are studied respectively. Results allowed to overcome the size effect issue – usually found when testing in the micro or nanometer regime – by selecting an appropriate sample size, to accomplish reliability on the mechanical properties evaluated at local length scales. Third and fourth publications are devoted to investigating the mechanical properties of cemented carbides with partial or total substitution of WC or Co as main constitutive phases based on their intrinsic mechanical properties and behavior. Outcomes evidence that small scale testing of complex composite materials such as cemented carbides by means of uniaxial compression of micropillars and nanoindentation, allows to evaluate the role of each constitutive phase on their mechanical behavior.Los carburos cementados son materiales compuestos ampliamente utilizados en diferentes campos de la industria dentro de aplicaciones que implican desgaste, debido a su excelente resistencia al mismo. Los más utilizados son los grados WC-Co, debido a la buena mojabilidad del Co con el carburo. La aparición de nuevas aplicaciones, la existencia de técnicas avanzadas de caracterización, y aspectos económicos y ambientales, fomenta el desarrollo de una nueva generación de carburos cementados que contiene otras fases ligantes como Ni y Fe o sus aleaciones. Además, el polvo de Co ha sido clasificado como muy tóxico para la salud humana y la combinación de polvo de metal duro de carburo y cobalto ha demostrado ser aún más tóxico que el cobalto y el tungsteno puros. El éxito en la sustitución del Co y WC en carburos cementados es medido comúnmente en términos de sus propiedades mecánicas finales a escala macro, como dureza, tenacidad y resistencia; y de su integridad estructural en condiciones de servicio, como resistencia a la corrosión, choque térmico y resistencia a fatiga. En este sentido, los efectos microestructurales (tamaño medio de WC, fracción de volumen y naturaleza química de las fases constitutivas) sobre la respuesta mecánica de estos materiales están bien establecidos a macroescala. Sin embargo, el papel individual cada fase a escala local, es decir, microescala, aún no se ha estudiado en profundidad. Dentro de los ensayos micromecánicos, se ha prestado especial atención a la compresión de micropilares debido a sus ventajas: estado de tensión nominalmente uniaxial, permitiendo la conversión directa de los datos medidos de desplazamiento y carga en curvas de flujo; la preparación de la muestra mediante fresado con haz de iones focalizados (FIB) es una ruta de mecanizado relativamente fácil; implica el uso de un nanoindentador convencional con punta plana; y, puede realizarse ex situ o in situ utilizando técnicas de microscopía electrónica de barrido (SEM) o microscopía electrónica de transmisión (TEM). Sin embargo, se debe prestar atención a los tamaños de muestra, ya que las propiedades intrínsecas de los materiales cristalinos, como el límite elástico y la resistencia, pueden verse muy influidas por factores extrínsecos como el volumen. Por ejemplo, resultados han evidenciado una relación inversa entre la dureza y la profundidad de indentación en las escalas de longitud micro y nanométrica. Con respecto a los carburos cementados, estudios recientes mostraron que cambios en la fracción de volume de ligante y carburos conducen a una amplia dispersión en los resultados del módulo de Young medido a microescala. Siguiendo las ideas anteriores, en esta tesis doctoral se ha seleccionado la compresión uniaxial de micropilares y nanoindentación para evaluar el papel del ligante y los carburos con respecto a su naturaleza química y dimensiones microestructurales, es decir, el tamaño medio del grano de carburo y el camino libre medio del ligante, en las propiedades y respuesta mecánica de carburos cementados a escalas locales. Esta tesis es presentada por un compendio de publicaciones científicas en los que varios objetivos específicos se estudian individualmente. En la primera y segunda publicación se estudia el efecto del diámetro del micropilar y la fracción volumétrica de las fases constitutivas dentro del mismo para superar el problema del efecto del tamaño de la muestra, seleccionando un tamaño apropiado para lograr confiabilidad en las propiedades mecánicas evaluadas localmente. Las publicaciones tercera y cuarta se dedican a investigar las propiedades mecánicas de los carburos cementados con sustitución parcial o total de WC o Co, en función del comportamiento mecánico intrínseco de las fases constitutivas. Los resultados demuestran que las pruebas a pequeña escala de materiales compuestos complejos – como los carburos cementados – mediante compresión uniaxial de micropilares y nanoindentación, permiten evaluar el papel de cada fase constitutiva en su respuesta y propiedades mecánicas. Al hacerlo, se debe seleccionar un tamaño de muestra apropiado para obtener resultados confiables del comportamiento general del material.Els carburs cimentats – també coneguts com a metalls durs – són materials compostos àmpliament usats a diversos camps industrials en aplicacions que comporten desgast, com en eines de tall, mecanitzat o trepat, a causa de la seva excepcional resistència al mateix. Els carburs cimentats més comunament usats són graus de WC-Co, per les característiques d’humectabilitat de cobalt (Co) amb el carbur de tungstè (WC) i la seva adhesió. L’aparició de noves aplicacions, l’existència de tècniques de caracterització avançades, aspectes econòmics i ambientals, entre d’altres, fomenta a el desenvolupament d’una nova generació de carburs cimentats que continguin altres fases d’unió com níquel (Ni) i ferro (Fe) o els seus aliatges. A més, la pols de Co ha estat classificada com a molt tòxica per a la salut humana i la combinació de pols de metall dur carbur-cobalt ha demostrat ser encara més tòxica que el Co o el W purs. L’èxit de la substitució dels constituents principals dels carburs cimentats es mesura habitualment en termes de propietats mecàniques finals, com la duresa, la tenacitat de fractura Palmqvist i la resistència a fractura transversal (TRS) a escala macroscòpica; i en termes d’integritat estructural en condicions similars a servei, com ara la resistència a corrosió, resistència a xocs tèrmics i fatiga, etc. En aquest sentit, el marc general dels efectes de les característiques microestructurals – mida mitjana dels carburs i fracció de volum i naturalesa química de les fases constitutives – en la resposta mecànica dels carburs cimentats està ben establerta en l’escala macroscòpica. No obstant això, encara cal estudiar en profunditat el paper individual de la fase lligant i dels carburs en l’escala local, és a dir, a l’escala micromètrica. Pel que fa als assajos micromecànics, s’ha prestat especial atenció a la compressió de micropilars gràcies als seus avantatges: l’estat de tensions és nominalment uniaxial, permetent una conversió directa de les mesures càrrega-desplaçament a corbes de flux; la preparació de mostres mitjançant un microscopi de feix de ions (FIB) és una tècnica de mecanitzat relativament senzilla; implica l’ús d’un nanoindentador convencional amb una punta plana; i es pot realitzar ex-situ o in-situ mitjançant un microscopi electrònic de rastreig (SEM) o de transmissió (TEM). Tot i això, cal parar atenció a les dimensions de les mostres, ja que està ben establert que les propietats intrínseques dels materials cristal·lins, com ara la tensió i la resistència, poden estar molt influïdes per factors extrínsecs com ara el volum. Per exemple, els resultats han evidenciat una relació inversa entre la duresa i la profunditat d’indentació a les escales micro- i nanomètriques. Respecte als carburs cimentats, estudis recents han demostrat que canvis en la fracció volumètrica de lligant i carburs comporta una àmplia dispersió en els resultats de mòdul de Young mesurat a la microescala. Seguint aquestes idees, en aquesta tesi doctoral s’ha seleccionat la compressió uniaxial de micropilars i nanoindentació per avaluar el paper del lligant i els carburs respecte la seva naturalesa química i dimensions microestructurals, és a dir, grandària mitjana del carbur i camí lliure mig del lligant, en les propietats mecàniques dels carburs cimentats i la seva resposta mecànica a escales locals. Aquesta tesi es presenta com a compendi de publicacions científiques en les quals s’estudien objectius específics individualment. La primera publicació té com a objectiu avaluar l’efecte del diàmetre del micropilar en la resposta micromecànica del WC-Co. A la segona publicació, s’investiguen l’efecte de la mitja mitjana del gra de WC i la fracció de volum de les fases de carbur i lligant. Els resultats han permès superar el problema de l’efecte de mida – habitual quan s’assaja a escales micro- i nanomètrica – seleccionant una mida de mostra adequada per tal d’aconseguir propietats mecàniques fiables avaluades a escales locals. La tercera i quarta publicacions estan dedicades a investigar les propietats mecàniques dels carburs cimentats amb substitució parcial o total de WC o Co com a fase constitutiva principal. En aquest sentit, en la tercera publicació s’usa la tècnica de nanoindentació per avaluar la duresa intrínseca de les fases constitutives i la tensió de flux del lligant constret en un carbur cimentat WC-(W,Ti,Ta,Nb)C-Co. Finalment, en el quart treball s’han estudiat tres materials, un amb Co i dos amb substitució parcial o total de Co com a lligant, respectivament, per tal d’investigar la influència de la naturalesa química del lligant en la resposta mecànica global dels carburs cimentats, segons fenòmens de deformació plàstica i mecanismes de fallada induïts per compressió uniaxial de micropilars. Els resultats derivats de la investigació realitzada durant aquesta tesi doctoral demostren que els assajos a escala petita de materials compostos complexos com ara els carburs cementats mitjançant compressió uniaxial de micropilars i tècniques de nanoindentació permeten avaluar el rol de cada fase constitutiva en les propietats i resposta mecàniques. Per fer-ho, cal seleccionar una mida de mostra adequada per tal d’obtenir resultats fiables del comportament global del material.Postprint (published version

Using Micro-Raman Spectroscopy to Assess MEMS Si/SiO2 Membranes Exhibiting Negative Spring Constant Behavior

We introduce a novel micro-mechanical structure that exhibits two regions of stable linear positive and negative stiffness. Springs, cantilevers, beams and any other geometry that display an increasing return force that is proportional to the displacement can be considered to have a “Hookean” positive spring constant, or stiffness. Less well known is the opposite characteristic of a reducing return force for a given deflection, or negative stiffness. Unfortunately many simple negative stiffness structures exhibit unstable buckling and require additional moving components during deflection to avoid deforming out of its useful shape. In Micro-Electro-Mechanical Systems (MEMS) devices, buckling caused by stress at the interface of silicon and thermally grown SiO2 causes tensile and compressive forces that will warp structures if the silicon layer is thin enough. The 1 mm2 membrane structures presented here utilizes this effect but overcome this limitation and empirically demonstrates linearity in both regions. The Si/SiO2 membranes presented deflect ~17 μm from their pre-released position. The load deflection curves produced exhibit positive linear stiffness with an inflection point holding nearly constant with a slight negative stiffness. Depositing a 0.05 μm titanium and 0.3 μm layer of gold on top of the Si/SiO2 membrane reduces the initial deflection to ~13.5 μm. However, the load deflection curve produced illustrates both a linear positive and negative spring constant with a fairly sharp inflection point. These results are potentially useful to selectively tune the spring constant of mechanical structures used in MEMS. The structures presented are manufactured using typical micromachining techniques and can be fabricated in-situ with other MEMS devices

Automated Quantitative Analyses of Fatigue-Induced Surface Damage by Deep Learning

The digitization of materials is the prerequisite for accelerating product development. However, technologically, this is only beneficial when reliability is maintained. This requires comprehension of the microstructure-driven fatigue damage mechanisms across scales. A substantial fraction of the lifetime for high performance materials is attributed to surface damage accumulation at the microstructural scale (e.g., extrusions and micro crack formation). Although, its modeling is impeded by a lack of comprehensive understanding of the related mechanisms. This makes statistical validation at the same scale by micromechanical experimentation a fundamental requirement. Hence, a large quantity of processed experimental data, which can only be acquired by automated experiments and data analyses, is crucial. Surface damage evolution is often accessed by imaging and subsequent image post-processing. In this work, we evaluated deep learning (DL) methodologies for semantic segmentation and different image processing approaches for quantitative slip trace characterization. Due to limited annotated data, a U-Net architecture was utilized. Three data sets of damage locations observed in scanning electron microscope (SEM) images of ferritic steel, martensitic steel, and copper specimens were prepared. In order to allow the developed models to cope with material-specific damage morphology and imaging-induced variance, a customized augmentation pipeline for the input images was developed. Material domain generalizability of ferritic steel and conjunct material trained models were tested successfully. Multiple image processing routines to detect slip trace orientation (STO) from the DL segmented extrusion areas were implemented and assessed. In conclusion, generalization to multiple materials has been achieved for the DL methodology, suggesting that extending it well beyond fatigue damage is feasible

Mechanics and Mechanisms of Fracture for an Eastern Spruce Subject to Transverse Loading Using Acoustic Emission

Due to its excellent structural qualities and accessibility, wood is among the most often utilized structural materials. Despite its ubiquity, wood poses numerous challenges. It is heterogeneous and anisotropic. It has a complex hierarchical ultrastructure, and the properties can have wide variation within a species, and indeed within an individual tree. This work aims to improve our understanding of the strength and fracture behavior of spruce-pine-fir (south) (SPFs), particularly in cross-grain direction. This study’s primary goal is to examine the relationship between crack propagation and cross grain morphology under the following loading configurations: compact tension, compression, and rolling shear. The broader goal is to be able to use this information to improve our ability to predict the performance of mass timber structures. In order to better characterize micromechanical processes and damage progression, acoustic emission (AE) techniques were applied. In this investigation, fracture in compact tension specimens was characterized by both R-curve and bulk fracture energy approaches. Our results show that the fracture follows a distinct route that deviates from the initial crack direction depending on the end-grain angles. This deviation is driven by a competition between maximum strain energy release rate and minimum crack resistance. For crack propagation in the tangential direction, cracks are confined to an earlywood region, which corresponds to the direction of least resistance. This pattern continues even as the end-grain shifts until an angle of about 40°, when the crack begins to jump across earlywood/latewood rings. At roughly 45°, the crack path shifts to a strictly radial direction, corresponding to a path of least resistance. In order to further quantify different micromechanical mechanisms, acoustic emission monitoring was used to track the propagation of damage. To identify different damage sources, an artificialneural network (ANN) technique was used to detect, classify, and quantify the AE energy sources. Results showed that earlywood cell wall tearing, dominant at 0°, produced higher energy release than cell wall separation, which dominates 90°crack propagation. Fiber bridging was also identified as another damage mechanism that occurs in the later stages of the crack growth, but in cross-grain fracture, it produces minimal AE energy. The same ANN approach was used to identify the damage mechanisms in specimens under rolling shear. Cross-laminated timber’s (CLT) mechanical performance is greatly influenced byrolling shear characteristics. In this work, the impact of end-grain orientation on rolling shear strength and modulus was evaluated. AE signal classification was applied to separate the associated damage modes and to determine the AE energy sources. Macroscopically, damage typically initiates along the glue line, but further crack growth is highly dependent on end grain morphology. Specimens with end-grain parallel to the axis of shear showed tangential propagation along an earlywood line, but as the dominant grain angle changes, cracks jump across growth rings, or if the angle is high enough, shift to a radial direction. AE results showed cell wall tearing to be the dominant energy dissipation mechanism, but cell wall peeling and bridging have significant contributions at higher end-grain angles. Through this research, we are better able to link damage sources to particular micro mechanical energy dissipation. This information is in a suitable form for inclusion in computational models that can be used to simulate structural performance as a function of material morphology

Gallium Nitride Integrated Microsystems for Radio Frequency Applications.

The focus of this work is design, fabrication, and characterization of novel and advanced electro-acoustic devices and integrated micro/nano systems based on Gallium Nitride (GaN). Looking beyond silicon (Si), compound semiconductors, such as GaN have significantly improved the performance of the existing electronic devices, as well as enabled completely novel micro/nano systems. GaN is of particular interest in the “More than Moore” era because it combines the advantages of a wide-band gap semiconductor with strong piezoelectric properties. Popular in optoelectronics, high-power and high-frequency applications, the added piezoelectric feature, extends the research horizons of GaN to diverse scientific and multi-disciplinary fields. In this work, we have incorporated GaN micro-electro-mechanical systems (MEMS) and acoustic resonators to the GaN baseline process and used high electron mobility transistors (HEMTs) to actuate, sense and amplify the acoustic waves based on depletion, piezoelectric, thermal and piezo-resistive mechanisms and achieved resonance frequencies ranging from 100s of MHz up to 10 GHz with frequency×quality factor (f×Q) values as high as 1013. Such high-performance integrated systems can be utilized in radio frequency (RF) and microwave communication and extreme-environment applications.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/135799/1/azadans_1.pd