Otto J. Gregory

Experimental Investigation of Gas Flow in Microchannels

We present an experimental investigation of laminar gas flow through microchannels. The independent variables: relative surface roughness, Knudsen number and Mach number were systematically varied to determine their influence on the friction factor The microchannels were etched into silicon wafers, capped with glass, and have hydraulic diameters between 5 and 96 μm. The pressure was measured at seven locations along the channel length to determine local values of Knudsen number, Mach number and friction factor. All measurements were made in the laminar flow regime with Reynolds numbers ranging from 0.1 to 1000

High temperature stability of indium tin oxide thin films

A robust high temperature strain gage based on indium-tin-oxide (ITO) has been used to measure static and dynamic strains at temperatures up to 1400 °C. These thin film, ceramic strain gages have several advantages over metal strain gages including a large gage factor and increased chemical and electrical stability at very high temperatures. Electron spectroscopy for chemical analysis (ESCA) of ITO films deposited onto high purity alumina substrates and subjected to temperatures up to 1400 °C indicated that the ITO films had undergone an interfacial reaction with the substrate. In addition, the interfacial reaction appears to have been responsible for high temperature stabilization through the formation of an ITO/Al2O3 solid solution. When similar ITO films were deposited onto alumina with a platinum diffusion barrier, there was no evidence of an interfacial reaction. Thermodynamic calculations indicated that bulk ITO may not be stable in air ambients at temperatures above 1300 °C but the alumina substrates stabilized the ITO to temperatures well beyond this value.

Preparation and piezoresistive properties of reactively sputtered indium tin oxide thin films

Oxygen deficient thin films of indium tin oxide (ITO) were prepared by r.f. reactive sputtering from a high-density ITO target (90 wt.% In2O3 and 10 wt.% SnO2 in various Ar:O2 mixtures for the purpose of investigating their use in a variety of strain gage applications. The resulting thin films were transparent in the visible spectrum (optical bandgap of 3.3-3.4 eV), tested n-type by hot probe and exhibited room-temperature resistivities in the range 0.01 to 0.10 ω cm after annealing. Room-temperature gage factors (G = ΔR/Ro1/e) as large as −77.71 were measured on patterned ITO films. These gage factors are considerably larger than those reported for refractory metal alloys (G = 2). A large, negative piezoresistive response (negative gage factor) was observed for all ITO films similar to the responses observed for n-type silicon. The piezoresistive response was reproducible and linear, with little or no hysteresis observed with strains up to 700μm m− . Additionally, optical gage factors based on changes in the ITO bandgap due to strain were established for these films using UV-Vis spectroscopy. Temperature coefficients of resistance (TCRs) of ITO films as low as +230 ppm °C−1 were realized in nitrogen ambients at temperatures up to 500 °C. In oxygen-bearing ambients, two distinct regions were observed; one having a TCR as low as −429 ppm °C− and another, at temperatures up to 1100 °C, having a TCR of −1560 ppm °C−1. Large gage factors combined with relatively low TCRs make these ITO films excellent candidates for use as high-temperature strain sensors. The relationship between processing parameteers and piezoresistive properties of these ITO films is reviewed and prospects using these films as high-temperature strain sensors is discussed.

A self-compensated ceramic strain gage for use at elevated temperatures

Abstract A ceramic thin film strain gage has been developed for static strain measurements at temperatures up to 1400°C. These thin film sensors are ideally suited for in situ strain measurement in harsh environments, such as those encountered in the hot sections of gas turbine engines. However, the wide bandgap semiconductor used as the active strain elements in these devices also exhibited a large temperature coefficient of resistance (TCR). Thus, to reduce the apparent strain or thermally induced strain in these static strain sensors to an acceptable level, a novel self-compensation scheme was demonstrated using thin film platinum resistors placed in series with the active strain element. A mathematical model was developed and design rules were established for the self-compensated circuitry using this approach. Close agreement between the model and actual static strain results have been achieved using this approach, and reliable static strain measurements have been made over an extended temperature range.

Metallic and Ceramic Thin Film Thermocouples for Gas Turbine Engines

Temperatures of hot section components in todays gas turbine engines reach as high as 1,500 °C, making in situ monitoring of the severe temperature gradients within the engine rather difficult. Therefore, there is a need to develop instrumentation (i.e., thermocouples and strain gauges) for these turbine engines that can survive these harsh environments. Refractory metal and ceramic thin film thermocouples are well suited for this task since they have excellent chemical and electrical stability at high temperatures in oxidizing atmospheres, they are compatible with thermal barrier coatings commonly employed in todays engines, they have greater sensitivity than conventional wire thermocouples, and they are non-invasive to combustion aerodynamics in the engine. Thin film thermocouples based on platinum:palladium and indium oxynitride:indium tin oxynitride as well as their oxide counterparts have been developed for this purpose and have proven to be more stable than conventional type-S and type-K thin film thermocouples. The metallic and ceramic thin film thermocouples described within this paper exhibited remarkable stability and drift rates similar to bulk (wire) thermocouples.

An apparent n to p transition in reactively sputtered indium–tin–oxide high temperature strain gages

Abstract A robust strain gage based on alloys of indium–tin–oxide (ITO) has been developed, which is capable of measuring strain at temperatures up to 1450 °C. These thin film sensors are ideally suited for in-situ strain measurement in harsh environments since they are non-intrusive, have minimal impact on vibration patterns due to their negligible mass and are robust enough to withstand the high ‘g’ loading associated with rotating components. Thus, this ITO strain gage is well suited to meet instrumentation requirements in advanced propulsion systems. Static strain tests performed at temperatures as high as 1400 °C have resulted in a relatively large and repeatable piezoresistive response. However, in the vicinity of 950 °C, a change in sign of the piezoresistive response from −G to +G was observed, suggesting that the active ITO strain element had been converted from a net ‘n-carrier’ to a net ‘p-carrier’ semiconductor. The ‘n’ to ‘p’ transition has been shown to be reversible over many temperature cycles from room temperature to 1400 °C. This repeatability implies that the carrier species is the predominate factor controlling the observed changes in the resistance and gage factor. Consistent with this change in sign of the gage factor was a change in the sign of the slope of emf vs. temperature (dV/dT) for hot probe measurements made on the same ITO films that were thermally cycled over the temperature range (600 °C to 1300 °C). This finding supports the premise that change in sign of the gage factor from −G to +G occurred within the same temperature range (∼950 °C) and that a change in the charge carrier type was responsible for observed transition in both cases.

Ceramic temperature sensors for harsh environments

A ceramic thermocouple based on indium-tin-oxide (ITO) thin films is being developed to measure the surface temperature of gas turbine engine components employed in power and propulsion systems that operate at temperatures in excess of 1500/spl deg/C. By fabricating ITO elements with substantially different charge carrier concentrations, it was possible to construct a robust ceramic thermocouple. A thermoelectric power of 6.0 /spl mu/V//spl deg/C, over the temperature range 25-1250/spl deg/C, was realized for an unoptimized ITO ceramic thermocouple. The charge carrier concentration difference in the legs of the ITO thermocouple was established by r.f. sputtering in oxygen-rich and nitrogen-rich plasmas. SEM micrographs revealed that after high-temperature exposure, the surfaces of the nitrogen prepared ITO films exhibited a partially sintered microstructure with a contiguous network of ITO nanoparticles. Thermal cycling of ITO films in various oxygen partial pressures showed that the temperature coefficient of resistance was nearly independent of oxygen partial pressure at temperatures above 800/spl deg/C and eventually became independent of oxygen partial pressure after repeated thermal cycling below 800/spl deg/C. Based on these results, a versatile ceramic sensor system has been envisioned where a ceramic thermocouple and strain sensor can be combined to yield a multifunctional ceramic sensor array.

High temperature strain gages based on reactively sputtered AlNx thin films

Abstract Thin film strain sensors based on reactively sputtered aluminum nitride are being developed for a variety of advanced aerospace applications, where the measurement of both static and dynamic strain is required at elevated temperatures. The non-stoichiometric AlNx thin films are particularly attractive for strain sensor applications at elevated temperatures since they exhibit a relatively large gage factor G, and a relatively low temperature coefficient of resistance (TCR), and they are electrically stable at high temperature, “c” axis oriented (0002) AlNx thin films were prepared by reactive r.f. sputtering from high purity aluminum targets. By varying the nitrogen content in the plasma, AINx films useful for strain gage applications were produced. The resulting films exhibited room temperature resistivities in the range 1 × 10−3 Ω cm to 5 × 102 Ω cm, were semi-transparent in the visible spectrum (optical band gaps in the range 4.6–5.8 eV) and tested “p” type by hot probe. TCRs ranged from +825 to −1200 ppn °C−1 after repeated thermal cycling to 1100 °C, depending on the nitrogen content in the film and the room temperature resistivity. Sputtering parameters were adjusted to yield a minimum value of +109 ppm°C−1. Large positive gage factors were measured at room temperature for all semiconducting AlNx films and the films exhibited a nearly linear piezoresistive response with little or no hysteresis when cycled from tension to compression. Gage factors on the order of 15 were realized for the AlNx films (gage factors of 2 are normally observed for refractory metal alloys). Annealing the AlNx films in argon at temperatures up to 1100 °C had minimal effect on the gage factor. This suggests that the piezoresistive response was more dependent on the defect structure of the as-deposited films than on the resistivity or subsequent thermal processing. Both the TCR and gage factor were correlated with the as-deposited resistivity and the nitrogen content in the films. The relationship between processing parameters and properties of these A1Nx films is reviewed here, and prospects of using such films as high temperature strain sensors are discussed.

Thin film thermocouples for advanced ceramic gas turbine engines

Abstract Thin film temperature sensors were developed for both the rotating and non-rotating components of ceramic-based gas turbine engines. These type-S thin film thermocouples were fabricated on hot isostatically pressed Si 3 N 4 substrates (Norton-TRW NT154), whose surfaces were altered to maximize adhesion. Annealing cycles and sputtered oxide interlayers were utilized in the fabrication to prevent blistering of the metallic thin films and enhance adhesion. Completed sensors were tested at temperatures up to 1300°C, with a variety of temperature gradients applied along the length of the test bars. Both oxidizing and inert gas atmospheres were used to investigate environmental effects on sensor performance. Sensor tests performed in oxidizing atmospheres resulted in the formation of volatile platinum and rhodium oxides, which caused significant drift and ultimate failure. Sensor tests performed in inert gas environments had drift rates 5–10 times lower than those observed in oxidizing atmospheres. Therefore, sputtered oxide overcoats were employed to minimize adverse reactions between the sensor elements and the atmosphere, the results of which will be discussed in terms of sensor performance.

Simulation of Thermal Conductivity of Nanofluids Using Dissipative Particle Dynamics

The effective thermal conductivity of Al2O3/water and CuO/water nanofluids were modeled by numerically solving steady heat flow in one-dimensional microchannels. This was accomplished by using energy conserving dissipative particle dynamics (DPDe).The effects of the interfacial thermal resistance and the Brownian motion of nanoparticles were incorporated in the model by modifying the conductive interaction parameter in the energy equation. The results were presented in the form of the thermal conductivity of nanofluids as functions of particle volume fraction and temperature, and were compared with the available experimental and analytical results. The present model agreed well with the experimental results for Al2O3/water nanofluid, whilethere were discrepancies between the model and the results for CuO/water nanofluid.