Nicholas Tiliakos

Temperature-Regulated Nonlinear Microvalves for Self-Adaptive MEMS Cooling

In this paper, thermal buckling of doubly clamped microfabricated nickel beams is implemented as a passive actuation mechanism to drive temperature-regulated nonlinear micro-valves for adaptive microcooling applications. The nonlinear buckling phenomenon is combined with the nonlinear change in flow rate through parallel plates with a variable spacing. The thermal buckling mechanism and parallel plate flow are modeled analytically, and nondimensional characteristic design curves have been generated. Passive flow-control microvalves were fabricated using deep reactive ion etching and a through-mold nickel electroplating process over a thin sacrificial layer. The model is validated with experimental results from the microfabricated temperature-regulated microvalves. Experimental characterization using an integrated micromachined heat exchanger with air as the working fluid shows the desired nonlinear valving behavior with mass flow rates of up to 5 mg/s for a temperature increase of 50degC, corresponding to 0.25 W of heat removal. It is shown that temperature-induced elastic instabilities in microfabricated structures can be modeled and manipulated to create a nonlinear adaptive valving mechanism. The modeling approach, microfabrication process, and full characterization of the microvalves are presented.

A MEMS sensor for mean shear stress measurements in high-speed turbulent flows with backside interconnects

A floating-element capacitive MEMS shear stress flow sensor was fabricated and shown to successfully measure mean-turbulent skin friction in high-speed compressible duct flow. The sensor was designed for harsh environments (e.g. high-temperature, high-shear stress) with novel through-substrate interconnects for robust packaging and remote circuitry for capacitance measurement. This paper extends our previous work on the MEMS shear stress sensor design [1] and backside interconnect process development [2] to provide complete device fabrication and testing in a high-speed flow.

A MEMS-Based Shear Stress Sensor for High Temperature Applications

Micro-electro-mechanical systems (MEMS) are an enabling technology that has lead to various miniature sensor concepts. Utilizing recent advances in silicon carbide (SiC) MEMS fabrication techniques allows for the development of a new series of sensors that leverages the high temperature capabilities of SiC. One such sensor concept is a shear stress sensor that can operate over a high dynamic range, and at very high temperatures, with an application emphasis on ground and flight testing in supersonic and hypersonic flow. The application of this fundamental sensor element and capacitance sensing design to very high temperature and very high shear environment, however, brings another set of challenges that involve the associated packaging and electrical control scheme. While this project is still a work in progress, we present an overview of our efforts to design, develop, fabricate and test a MEMS shear stress sensor for hypersonic aeropropulsion test and evaluation applications.

Preliminary Testing of a MEMS-based Shear Stress Sensor for High Speed Flow Applications

Micro-electro-mechanical systems (MEMS) are an enabling technology that has led to various miniature sensor concepts. One such sensor concept is a shear stress sensor that can operate over a wide dynamic range, and at very high temperatures, with an application emphasis on ground and flight testing in supersonic and hypersonic flow. In this paper we present our research and development efforts for a MEMS-based shear stress sensor that can be used to directly measure the shear stress on ground based articles tested in high speed tunnels, and eventually on flight-test articles. Preliminary data showing sensor operation in high subsonic flow is presented and discussed.

Characterization and Modeling of Thermal Buckling in Eccentrically Loaded Microfabricated Nickel Beams for Adaptive Cooling

The design, fabrication and testing of micromachined nickel beams buckling under thermal loading will be presented in this paper. The focus will be on characterizing key design parameters important to the implementation of electroplated nickel beams as the actuation mechanism in a thermally adaptive microvalve. An analytical model of the thermal buckling phenomena has been developed and validated with test results from electroplated nickel beams with slight eccentricities. Highly nonlinear deflection versus temperature curves were predicted by the closed form model and match well with experimental measurements. Buckling deflections of more than 50μm were achieved at actuation temperatures under 100°C. The nickel beam fabrication process will be presented, as well as various fabrication related issues impacting the actuation capabilities of the beams.Copyright

Non-Intrusive Sensor for in-situ Measurement of Recession Rate of Heat Shield Ablatives

The development of a new sensor for in-situ non-intrusive measurements of recession rate of heat shield ablative materials is described. The sensor utilizes a focused ultrasound approach to non-intrusively detect the ablative materials surface loss while simultaneously correcting for acoustic velocity dependencies on temperature. The latter correction is done via a closed loop feedback approach that yields the average acoustic velocity through the ablative material. The multi-source focusing approach is atypical of current ultrasound based sensors used for ablation recession rate measurement, which require a-priori knowledge of temperature distribution within the ablative to yield accurate data on recession rate. The paper describes the early development of the sensor system resulting in a proof-of-concept breadboard system that demonstrates its unique operational aspects and possibilities as a heat shield health monitoring system for future spacecraft.

Real-Time Ablation Recession Rate Sensor System for Advanced Reentry Vehicles

The development of a new sensor system for in-situ, real-time measurements of recession rate of heat shield ablative materials is described. The sensor utilizes a focused ultrasound approach to non-intrusively detect the ablative materials surface loss while simultaneously correcting for acoustic velocity dependencies on temperature. The latter correction is done via a closed loop feedback approach. The multi-source focusing approach is atypical of current ultrasound based sensors used for ablation recession rate measurement, which require a-priori knowledge of temperature distribution within the ablative to yield accurate data on recession rate. The paper describes the operating principles of the sensor and reports on results obtained in testing the breadboard sensor system in a relevant environment at ATK GASL’s POET facility.

Performance Characteristics of a Variable Density Pin Array Micro-Heat Exchanger

The design, fabrication and preliminary performance of a variable density pin array micro heat exchanger for micro-cooling applications is reported. Various pin diameters and density configurations were analyzed for their ability to provide maximum uniform heat transfer over the active area of the micro-heat exchanger using air as the working fluid for maximum mass flow rates through the micro-heat exchanger in the range of 40 mg/sec to 60 mg/sec. Fabrication of the micro-heat exchanger was performed using deep reactive ion etching (DRIE) technique on a 0.5 mm thick silicon wafer with nominal feature sizes in the range of 5 microns to 20 microns. Performance data is presented based on analysis and comparison to a baseline configuration with no pins.Copyright