Ceramic composites for high temperature applications must be designed with crack arrest capability to improve the resistance to flaws produced in service, such as by thermal shock. Laminated composites containing Al2O3 layers in 3mol%Y2O3-ZrO2 (TZ3Y) were fabricated by electrophoretic deposition (EPD) and pressureless sintering. The layering design (Al2O3 layer thickness and volume fraction) was varied to determine the influence on fracture behaviour. The residual stress in Al2O3 layers was measured using a fluorescence spectroscopy technique. The fracture strength of 15 different laminates, and monolithic Al2O3 and TZ3Y, was tested in 4-point bending at room temperature. Vickers indentation (10 kg load) was used to simulate natural flaws at the sample surface before testing as a measure of flaw tolerance. Fracture ranged from catastrophic failure, to multi-stage failure and complete delamination (in processing). Transitions in behaviour were found related to a geometrical parameter derived from the strain energy release rate for edge cracks. The strength of three Al2O/TZ3Y composites was compared with monolithic Al2O3 and TZ3Y for a range of indentation loads (up to 20 kg). The strength of the composites was similar to monolithic TZ3Y but the flaw tolerance was improved due to multi-stage fracture. The strength and flaw tolerance (using 10 kg indentation) of two Al2O3/TZ3Y composites and monolithic TZ3Y was measured < 1300°C. The multi-stage fracture behaviour disappeared > 25 °C, and there was no beneficial effect of the Al2O3 layers on the strength. Superplastic deformation of the TZ3Y layers at 1300°C was prevented by the constraint of the Al2O3 layers. Recommendations are made about the design of flaw tolerant ceramic laminates for high temperature use. / Thesis / Master of Engineering (ME)

The design of efficient and stable solar selective coatings for Concentrating Solar Power (CSP) central receivers requires a comprehensive knowledge about the incorporated materials. In this work solar selective coatings were grown by filtered cathodic vacuum arc (FCVA) deposition. The complete stacks consist of an infrared reflection layer, an absorber layer of C:ZrC nanocomposites and an antireflection layer. The Carbon-transition metal nanocomposites were studied as absorber materials because they show appropriate optical properties, i.e. high absorption in the solar region and low thermal emittance. Furthermore metal carbides are thermally and mechanically stabile in air at high temperatures. In order to optimize the absorber layer, the metal content was controlled by adjusting the pulse ratio between the two arc sources. The elemental composition of the absorber layers was determined by Ion Beam Analysis. X-Ray diffraction (XRD) measurements show the formation of metal carbides when the metal content is high enough. The optical properties of the deposited coatings were characterized by spectroscopic ellipsometry (SE). The reflectance spectra of the complete selective coating were simulated with the optical software CODE. Bruggeman effective medium approximation (EMA) was employed to average the dielectric functions of the two components which compose the nanocomposite in the absorber layer. Good agreement was found between simulated and measured reflectance spectra of the solar selective multilayer.

Electronics are normally designed to operate at temperatures less than 125 oC. For high temperature applications, the use of those normal electronics becomes challenging and sometimes impractical. Conventionally, many industries tried to push the maximum operating temperature of electronics by either using passive/active cooling systems or tolerating degraded performance. Recently, there has been a demand for more robust electronics that can operate at higher temperature without sacrificing the performance or the use of any weighty, power hungry, complex cooling systems. One of the major industries that need electronics operating at high temperature is the oil and gas industry. Electronics have been used within the field in many areas, such as well logging downhole telemetry systems, power networks, sensors, and actuators. In the past, the industry has managed to use the existing electronics at temperatures up to 150 oC. However, declining reserves of easily accessible natural resources have motivated the oil and gas industry to drill deeper. The main challenge at deep wells for downhole electronics is the high temperatures as the pressures are handled mechanically. The temperature in deep basins can exceed 210 oC. In addition, existing well logging telemetry systems achieve low data transmission rates of less than 2.0 Mbps at depth of 7.0 Km which do not meet the growing demand for higher data rates due to higher resolution sensors, faster logging speeds, and additional tools available for a single wireline cable. The main issues limiting the speed of the systems are the bandwidth of multi-conductor copper cable and the low speed communication system connecting the tools with the telemetry modem. The next generation of the well logging telemetry system replaces the multi-conductor wireline between the surface and the downhole with an optical fiber cable and uses a coaxial cable to connect tools with the optical node in downhole to meet the growing needs for higher data rates. However, the downhole communication system between the tools and the optical modulator remains the bottleneck for the system. The downhole system is required to provide full duplex and simultaneous communications between multiple downhole tools and the surface with high data rates and able to operate reliably at temperatures up to 230 oC. In this dissertation, a downhole communication system based on radio frequency (RF) transmission is investigated. The major contributions of our research lie in five areas. First, we proposed and designed a downhole communication system that employs RF systems to provide high speed communications between the downhole tools and the surface. The system supports up to six tools and utilizes frequency division multiple access to provide full duplex and simultaneous communications between downhole tools and the surface data acquisition system. The system achieves 20 Mbps per tool for uplink and 6 Mbps per tool for downlink with bit error rate (BER) less than 10-6. Second, a RF front-end of transceiver operating at ambient temperatures up to 230 oC is designed and prototyped using Gallium Nitride (GaN) high electron mobility transistor (HEMT) devices. Measurement results of the transceiver's front end are reported in this dissertation. To our knowledge, this is the first RF transceiver that operates at this high temperature. Third, current-voltage and S-parameters characterizations of the GaN HEMT at ambient temperatures of 250 oC are conducted. An analytic model that accurately predicts the behavior of the drain-source resistor (RDS) of the GaN transistor at temperature up to 250 oC is developed based on these characterizations. The model is verified by the analysis and the performance of the resistive mixer. Fourth, a passive upconversion mixer operating at temperatures of 250 oC is designed and prototyped. The designed mixer has conversion loss (CL) of 6.5 dB at 25 oC under local oscillator (LO) power of 2.5 dBm and less than 0.75 dB CL variation at 250 oC under the optimum biasing condition. Fifth, an active downconversion mixer operating at temperatures up to 250 oC is designed and prototyped. The proposed mixer adopts a common source topology for a reliable thermal connection to the transistor source plate. The designed active mixer has conversion gain (CG) of 12 dB at 25 oC under LO power of 2.5 dBm and less than 3.0 dB CG variation at 250 oC. Finally, a novel high temperature negative adaptive bias voltage circuit for a GaN based RF block is proposed. The proposed design comprises an oscillator, voltage doubler, and temperature dependent bias controller. The voltage offset and temperature coefficient of the generated bias voltage can be adjusted by the bias controller to match the optimum biasing voltage required by a RF building block. The bias controller is designed using a Silicon Carbide (SiC) bipolar junction transistor. / PHD

"The next generation of aluminum alloys will have to operate at temperatures approaching 300°C. Traditional aluminum alloys cannot perform at these temperatures, but aluminum alloys reinforced with fine ceramic particles can. The objective of this research is to develop a process to synthesize Al-TiC composites by the Rotating Impeller Gas-Liquid In-situ method. This method relies on injecting methane into molten aluminum that has been pre-alloyed with titanium. The gas is introduced by means of a rotating impeller into the molten alloy, and under the correct conditions of temperature, gas flow, and rotation speed, it reacts preferentially with titanium to form titanium carbide particles. The design of the apparatus, the multi-physics phenomena underlying the mechanism responsible for particle formation and size control, and the operation window for the process are first elucidated. Then a parametric study that leads to the synthesis of aluminum reinforced with TiC microparticles and nanoparticles is described. Finally, potential technical obstacles that may stand in the way of commercializing the process are discussed and ways to overcome them are proposed. "

Die vorliegende Arbeit befasst sich mit dem Betriebsverhalten und den Auslegungsrichtlinien für ein hochtemperaturfähiges Übertragungselement aus Stahl für elastischen Klauenkupplungen. Im Grundlagenkapitel wurden hierfür zunächst aus den Eigenschaften und Auslegungsvorschriften handelsüblicher Klauenkupplungen wichtige Kenngrößen und Kennwerte bestimmt und relevante Aspekte der Tribologie und der Federtechnologie beleuchtet. Mittels FEM wird die Spannungsverteilung und das Verformungsverhalten des Übertragungselements bei Drehmomentbelastung untersucht und anhand einer Parameterstudie der Einfluss verschiedener Konstruktionsparameter analysiert. Zusätzlich werden Berechnungen und Simulationen zur Dämpfung und zur Betriebsfestigkeit durchgeführt. Die gewonnenen Erkenntnisse wurden anschließend durch experimentelle Untersuchungen mit statischer und dynamischer Belastung sowie durch Lebensdauerversuche und Betriebslastenversuche verifiziert und ergänzt. Hierbei wurden zu Vergleichszwecken auch handelsübliche Übertragungselemente aus TPE untersucht. Auf Basis der Untersuchungsergebnisse wurden anschließend Richtlinien für die Auslegung und den Betrieb des Übertragungselements abgeleitet. / The present work deals with the operating behaviour and the selection guidelines for a new high-temperature steel transmission element for elastic jaw couplings. In the state oft the art chapter, important parameters and characteristic values were first determined from the properties and selection specifications of commercially available claw couplings and relevant aspects of tribology and spring technology were highlighted. FEM is used to investigate the stress distribution and the deformation behaviour of the transmission element under torque loading and to analyse the influence of various design parameters. In addition, calculations and simulations for damping and fatigue strength are carried out. The knowledge gained was then verified and supplemented by experimental investigations with static and dynamic loads as well as by service life tests and operating load tests. For comparison purposes, commercially available transmission elements made of TPE were also investigated. Based on the test results, guidelines were then derived for the selection and operation of the new transmission element.