Temperature sensing in harsh environment is desired in many areas, such as coal gasification, aerospace, etc. Single crystal sapphire is an excellent candidate for construction of harsh environment sensors due to its superior mechanical and optical properties even at temperature beyond 1600°C. The temperature inside a coal gasifier can be as high as 1200°C. And there is dramatic temperature gradient between the inner and outer layers of the gasifier refractory. Previous work has been done at Virginia Tech's Center for Photonics Technology to design and fabricate a sapphire wafer based Fabry-Perot interferometer (FPI) sensor for temperature sensing in coal gasifiers. The sensor head is based on the use of sapphire wafer which is attached to a lead-in sapphire fiber to be applied in the ultrahigh temperature region; and the sapphire fiber is spliced to a multi-mode fused silica fiber for quality signal transmission in lower temperature areas. One of the challenges encountered by this approach is the shear force to the sapphire fiber, which is caused by the differential thermal expansion between the inner and outer layers of the gasifier refractory. This shear force may be so significant to break the sensor probe. This thesis proposed a free space based interrogation sensing system to address that problem. In this free space based interrogation sensing system, only the sensor head is placed in the inner refractory wall, while all the other parts of the system are placed in the outer refractory or outside the gasifier at the ambient room temperature. An optical frequency domain reflectometry (OFDR) based multiplexed technique is applied in the sensor design to realize temperature measurement at multiple locations along the optical path. In this work, three sapphire wafers based multiplexed temperature sensor is fabricated and calibrated in laboratory. This multiplexed high temperature sensor shows linear response in the range of 20°C ~ 1000°C, with a sensitivity of 1.602?10??/°C and a resolution of 1.3°C. / Master of Science

The thermocouple systems used for the measurement of surface temperature in high temperature applications such as advanced aerospace propulsion systems and diesel engine systems are expected to perform in rapidly fluctuating and extremely high heat fluxes corresponding to high temperatures (in excess of 1400 K) and high speed flows. Traditionally, Pt/Pt-Rh based thin film thermocouples have been used for surface temperature measurements. However, recent studies indicated several problems associated with these thermocouples at temperatures exceeding 1000 K, some of which include poor adhesion to the substrate, rhodium oxidation and reaction with the substrate at high temperatures. Therefore, there is an impending demand for thermoelectric materials that can withstand severe environments in terms of temperature and heat fluxes. In this study, thin films of titanium carbide and tantalum carbide as well as two families of conducting perovskite oxides viz., cobaltites and manganates (La(1-x)SrxCoO3, M(1-x)Cax MnO3 where, M=La,Y) were investigated for high temperature thin film thermocouple applications as alternate candidate materials. Thin films of the carbides were deposited by r.f. sputtering while the oxide thin films were deposited using pulsed laser ablation. Sapphire (1102) was used as substrate for all the thin film depositions. All the thin films were characterized for high temperature stability in terms of phase, microstructure and chemical composition using x-ray diffraction, atomic force microscopy and electron spectroscopy for chemical analysis respectively. Electrical conductivity and seebeck coefficients were measured in-situ using a custom made device. It was observed that TiC/TaC thin film thermocouples were stable up to 1373 K in vacuum and yield high and fairly stable thermocouple output. The conducting oxides were tested in air and were found to be stable up to at least 1273 K. The manganates were stable up to 1373 K. It was observed that all the oxides studied crystallize in a single phase perovskite structure. This phase is stable up to annealing temperatures of 1373 K. The predominant electrical conduction mechanism was found to be small polaron hopping. Stable and fairly high electrical conductivities as well as seebeck coefficients accompanied with phase, structure, composition and microstructure stability indicate that these materials hold excellent promise for high temperature thin film thermocouple applications. / Master of Science

Infrared sensor satellites are used to monitor the conditions in the earth's upper atmosphere. In these systems, the electronic links connecting the cryogenically cooled infrared detectors to the significantly warmer amplification electronics act as thermal bridges and, consequently, the mission lifetimes of the satellites are limited due to cryogenic evaporation. High-temperature superconductor (HTS) materials have been proposed by researchers at the National Aeronautics and Space Administration Langley's Research Center (NASA-LaRC) as an alternative to the currently used manganin wires for electrical connection. The potential for using HTS films as thermal bridges has provided the motivation for the design and the analysis of a spaceflight experiment to evaluate the performance of this superconductive technology in the space environment The initial efforts were focused on the preliminary design of the experimental system which allows for the quantitative comparison of superconductive leads with manganin leads, and on the thermal conduction modeling of the proposed system (Lee, 1994). Most of the HTS materials were indicated to be potential replacements for the manganin wires. In the continuation of this multi-year research, the objectives of this study were to evaluate the sources of heat transfer on the thermal bridges that have been neglected in the preliminary conductive model and then to develop a methodology for the estimation of the thermal conductivities of the HTS thennal bridges in space. The Joule heating created by the electrical current through the manganin wires was incorporated as a volumetric heat source into the manganin conductive model. The radiative heat source on the HTS thermal bridges was determined by performing a separate radiant interchange analysis within a high-Tc superconductor housing area. Both heat sources indicated no significant contribution on the cryogenic heat load, which validates the results obtained in the preliminary conduction model. A methodology was presented for the estimation of the thermal conductivities of the individual HTS thermal bridge materials and the effective thermal conductivities of the composite HTS thermal bridges as functions of temperature. This methodology included a sensitivity analysis and the demonstration of the estimation procedure using simulated data with added random errors. The thermal conductivities could not be estimated as functions of temperature; thus the effective thermal conductivities of the HTS thermal bridges were analyzed as constants. / Master of Science

Infrared sensor satellites, which consist of cryogenic infrared sensor detectors, electrical instrumentation, and data acquisition systems, are used to monitor the conditions of the Earth's upper atmosphere in order to evaluate its present and future changes. Currently, the electrical instrumentation (connections), which act as thermal bridges between the cryogenic infrared sensor and the significantly warmer data acquisition unit of the sensor satellite system, constitute a significant portion of the heat load on the cryogen. As a part of extending the mission life of the sensor satellite system, the researchers at the National Aeronautics and Space Administration's Langley Research Center (NASA-LaRC) are evaluating the effectiveness of replacing the currently used manganin wires with high-temperature superconductive (HTS) materials as the electrical connections (thermal bridges). In conjunction with the study being conducted at NASALaRC, the proposed research is to design a space experiment to determine the thermal savings on a cryogenic subsystem when manganin leads are replaced by HTS leads -printed onto a substrate with a low thermal conductivity, and to determine the thermal conductivities of HTS materials. / Master of Science

With the increasing use of aluminum in naval vessels and the ever-present danger of fires, it is important to have a good understanding of the behavior of aluminum at elevated temperatures. The aluminum samples 5083-H116 and 6061-T651 were examined under a variety of loading conditions and temperatures. Tensile testing was completed on both materials to measure strength properties of elastic modulus, yield strength, and ultimate strength as well as reduction of area from room temperature to 500 deg C taking measurements every 50 deg C. These tests showed how much the material weakened as temperature increases. Low temperatures had a minimal effect on strength while exposure to temperatures between 200 and 300 deg C had the most significant impact. Creep testing was also completed for these materials. These tests were completed at temperatures between 200 and 400 deg C in 50 deg C increments. Stresses for these tests were in the range of 13 to 160MPa for 5083 aluminum and between 13 to 220MPa for 6061 aluminum. These tests showed a significant relationship between stress and temperature and how changes to one can cause a very different resulting behavior. In addition to the creep testing, three creep models were examined as a means of predicting creep behavior. These models included a power law, exponential, and hyperbolic-sine versions and were able to predict creep results with decent accuracy depending on the stress used in the model. / Master of Science

Particle ingestion and deposition is an issue of concern for gas turbine engines operating in harsh environments. The ingested particles accelerate the deterioration of engine components and thus reduce its service life. This effect is observed to a greater extent in aircrafts/helicopters operating in particle laden environment. Understanding the effects of particle ingestion at engine representative condition leads to improved designs for turbomachinery. Experiments have been in an Aerothermal Rig facility at Virginia Tech to study particle deposition at engine representative temperatures. The Aerothermal Rig was upgraded to achieve air temperatures of up to 1100°C at the test section. The experiments are performed using Arizona Road Dust (ARD) of 20-40 μm size range. The temperature of air and particles are around 1100°C at a constant velocity of 70 m/s. The target coupon is made of Hastelloy X, a nickel-based alloy and the angle at which the particles impact the coupon varies from 30° to 80°. The experiments were performed with different amounts of total particle injected, concentration, and coupon angle to understand their effects on deposition. Similar research was carried out in the past at the same facility to study particle deposition at temperatures up to 1050°C and 70 m/s flow velocity. However, this previous research only studied how the coupon angle affects particle deposition; other parameters such as total particle input and particle concentration were not studied. It was found that particle deposition increases significantly at higher temperatures beyond 1050°C for higher coupon angle and amount of sand injected. Results from current study also show that deposition increases with increase in total sand injected, concentration, and coupon angle for a given temperature and velocity. / Master of Science

A study is presented to explore the suitability of CFD as a tool in the design and analysis of total temperature probes. Simulations were completed using 2D axisymmetric and 3D geometry of stagnation total temperature probes using ANSYS Fluent. The geometric effects explored include comparisons of shielded and unshielded probes, the effect of leading edge curvature on near-field flow, and the influence of freestream Mach number and pressure on probe performance. Data were compared to experimental results from the literature, with freestream conditions of M=0.3-0.9, p_t=0.2-1 atm, T_t=300-1111.1 K. It is shown that 2D axisymmetric geometry is ill-suited for analyses of unshielded probes with bare-wire thermocouples due to their dependence upon accurate geometric characterization of bare-wire thermocouples. It is also shown that shielded probes face additional challenges when modeled using 2D axisymmetric geometry, including vent area sizing inconsistencies. Analysis of shielded probes using both 2D axisymmetric and 3D geometry were able to produce aerodynamic recovery correction values similar to the experimental results from the literature. 2D axisymmetric geometry is shown to be sensitive to changes in freestream Mach number and pressure based upon the sizing of vent geometry, described in this report. Aerodynamic recovery correction values generated by 3D geometry do not show this sensitivity and very nearly match the results from the literature. A second study was completed of a cooled, shielded total temperature probe which was designed, manufactured, and tested at Virginia Tech to characterize conduction error. The probe was designed utilizing conventional total temperature design guidelines and modified with feedback from CFD analysis. This test case was used to validate the role of CFD in the design of total temperature probes and the fidelity of the solutions generated when compared to experimental results. A high level of agreement between CFD predictions and experimental results is shown, while simplified, low-order model results under predicted probe recovery. / Master of Science

An experimental investigation was undertaken to analyze the behavior of sandwich composite structures exposed to one-sided heating where a debond exists between the unexposed facesheet and core material. Sandwich composites of plain weave E-glass/epoxy facesheets and an end-grain balsa wood core manufactured using the Vacuum Assisted Resin Transfer Molding (VARTM) technique were the only materials analyzed. These were selected due to their current use in naval vessels and the heightened interest in the fire response properties of balsa wood and its utility as a core material. In order to better understand the interfacial behavior, Mode I Double Cantilever Beam (DCB) fracture tests were performed at ambient, 60 C, and 80 C to determine the influence of the decreased Mode I fracture toughness. While ambient testing showed that stable crack growth could be obtained, high temperature tests resulted in considerable damage occurring to the core at the crack-front preventing stable crack growth. This can be attributed to the significant decrease in the balsa core strength and material properties even for small increases in temperature. Additionally, Mode II Cracked Split Beam (CSB) tests were performed at ambient temperature to examine the sliding dominant crack-growth. Again, the occurrence of balsa core damage prevented stable crack-growth and an accurate measurement of Mode II fracture toughness was not obtained. Intermediate-scale compression testing with one-sided heating at two heat flux levels was performed with a custom designed load frame on sandwich composite columns. This enabled the influence of the debond to be measured using a 3D-Digital Image Correlation (DIC) technique spatially linked with a thermographic camera. The DIC allowed for a detailed observation of debond growth and buckling prior to global failure of the test article. A behavior similar to that observed in the Mode I DCB fracture tests occurred: as the interfacial temperature increased, the amount of crack growth decreased. This crack growth was followed by a core failure at the crack-front, triggering a global failure of the test article. This global failure for test articles containing a debond manifested itself primarily as an anti-symmetric post-buckling shape. Test articles with no debond exhibited the typical progression of the out-of-plane displacement profile for a fixed-fixed column. As the out-of-plane displacement increased, core failure ultimately occurred near the gripped region where the zero-slope condition is required, triggering global failure of the no debond test article. These tests highlight that the reduction in strength and material properties of the end-grain balsa wood core significantly outweigh the reduction in interfacial fracture toughness due to the increased temperatures. / Master of Science

Flow temperature sensing remains important for many hypersonic aerodynamics and propulsion applications. Flight test applications, in particular, demand robust and accurate sensing, making thermocouple sensors attractive. Even for these extremely well-developed sensors, the prediction of stresses (hoop, radial, and axial) within thermocouple sheaths for custom-configured probes remains a topic of great concern for ensuring adequate lifetime of sensors. In contemporary practice, high-fidelity simulations must be run to prove if a new design will work at all, albeit at significant time and expense. Given the time and money it takes to run high-fidelity simulations, rapid optimization of sensor configurations is often impossible, or at a minimum, impractical. The developments presented in this Thesis address the need for hypersonic flow temperature sensor structural predictions which are compatible with rapid design iteration. The derivation and implementation of a new analytical, low-order model to predict stresses (hoop, radial, and axial) within the sheath of a thermocouple are provided. The analytical model is compared to high-fidelity ANSYS mechanical simulations as well as simplified experimental data. The predictions using the newly developed structural low-order model are in excellent agreement with the numerically simulated results and experimental results with an absolute maximum percent error of approximately 4% and 9.5%, respectively, thus validating the model. A MATLAB GUI composed of the combination of a thermal low-order model outlined in additional references [1] through [6] and the new structural low-order model for thermocouples was developed. This code is capable of solving a highly generalized version of the 1-D pin fin equation, allowing for the solution of the temperature distribution in a sensor taking into account conduction, convection, and radiation heat transfer which is not possible with other existing analytical solutions. This temperature distribution is then used in the analytical structural low-order model. This combination allows for the thermal and structural performance of a thermocouple to be found analytically and compared quickly with other designs. / M.S. / Thermocouples are a device for measuring temperature, consisting of two wires of different metals connected at two different points. This configuration produces a temperature-dependent voltage as a result of the thermoelectric effect. Preexisting curves are used to relate the voltage to temperature. Thermocouples are extensively used in high-temperature high-stress environments such as in rockets, jet engines, or any high-corrosive environment. Accurately predicting the stresses within the sheath of a metal-clad thermocouple in extreme conditions is required for many research areas including hypersonic aerodynamics and various propulsion applications. Even for these extremely well-developed and widely used sensors, the accurate prediction of stresses within the metal sheath remains a topic of great concern for ensuring the sensor’s survivability in these extreme conditions. Current engineering practice is to use high-fidelity numerical simulations (Finite Element Analysis) to predict the stresses within the sheath. Perhaps the biggest drawback to this approach is the time it takes to model, mesh, and set-up these simulations. Comparative studies between different designs using numerical simulations are almost impossible due to the time requirement. This Thesis will present an analytically derived quasi-3D solution to find the stresses within the sheath. These equations were implemented into a low-order model that can handle varying temperature, geometry, and material inputs. This model was validated against both high-fidelity numerical simulations (ANSYS Mechanical) and a simplified experiment. The predictions using this newly developed structural low-order model are in excellent agreement with the numerically simulated results and experimental results.

The decline of easily accessible reserves pushes the oil and gas industry to explore deeper wells, where the ambient temperature often exceeds 210 °C. The need for high temperature operation, combined with the need for real-time data logging has created a growing demand for robust, high temperature RF electronics. This thesis presents the design of an intermediate frequency (IF) variable gain amplifier (VGA) for downhole communications, which can operate up to an ambient temperature of 230 °C. The proposed VGA is designed using 0.25 μm GaN on SiC high electron mobility transistor (HEMT) technology. Measured results at 230 °C show that the VGA has a peak gain of 27dB at center frequency of 97.5 MHz, and a gain control range of 29.4 dB. At maximum gain, the input P1dB is -11.57 dBm at 230 °C (-3.63 dBm at 25 °C). Input return loss is below 19 dB, and output return loss is below 12 dB across the entire gain control range from 25 °C to 230 °C. The variation with temperature (25 °C to 230 °C) is 1 dB for maximum gain, and 4.7 dB for gain control range. The total power dissipation is 176 mW for maximum gain at 230 °C. / Master of Science / The oil and gas industry uses downhole communication systems to collect important information concerning the reservoirs such as rock properties, temperature, and pressure, and relay it back to the surface. The electronics in these communication systems have to withstand temperatures exceeding 210 °C. Current high temperature electronics are not able to handle such temperatures for extended periods of time. Advancements in semiconductor technologies allow for fabrication of semiconductors that withstand high temperatures, such as GaN (Gallium Nitride). This thesis describes the design of a VGA (variable gain amplifier), which is an essential part of a downhole communication system. The VGA is designed using GaN semiconductor technology. Measured results show that the VGA shows good performance from 25 °C to 230 °C without the use of any cooling techniques, and can thus be used to design nextgeneration robust, high speed downhole communication systems.