Task 241-High Temperature, Optical Sapphire Pressure Sensors for Hypersonic Vehicles (037391)

Project Status: Current




The study of hypersonic boundary layers is critical to the efficient design of hypersonic vehicles for rapid global and space access. The harsh environment makes conventional instrumentation unsuitable for time-accurate, continuous, direct measurements. The development of a high temperature sensor for direct measurement of pressure is vital to the understanding of shock-wave/boundary layer interactions which directly influence critical vehicle characteristics such as lift, drag, and propulsion efficiency. The University of Florida and Florida State University, in conjunction with the Interdisciplinary Consulting Corporation, are developing harsh environment MEMS sensors using high temperature materials such as sapphire. The proposed effort is focused on a continued effort towards the design and characterization of a fiber optic based pressure sensor with a remote photo-diode optical readout. The microphone is composed of a compliant, platinum coated, sapphire diaphragm bonded over a cavity containing a single optical fiber. The diaphragm deflection is detected via intensity modulation due to the motion of the reflective platinum coated sapphire diaphragm. The optical signal is routed via the high temperature sapphire fiber to a remote photo-diode allowing for isolation of the electronics from the harsh environment. High temperature materials such as sapphire and platinum present unique challenges in terms of microfabrication. Conventional silicon microfabrication techniques are largely inapplicable. The current research is focused on sensor performance with respect to the underlying thermocompression bonding and ultrafast laser micromachining manufacturing processes. Thermocompression bonding is intended to mate an optically smooth, platinum coated sapphire diaphragm to a sapphire substrate via a platinum foil. Ceramic-metal thermocompression bonding through a combination of extremely high temperatures and the application of force is a proven fabrication technology for sapphire-platinum-sapphire bonds; however, the formation of moving parts for MEMS devices via this fabrication technology is still an active research topic. In addition to thermocompression bonding, ultrafast laser micromachining is used for sapphire material removal. A unique, picosecond, laser micromachining station is being utilized at the University of Florida. The use of a picosecond laser for micromachining allows the ablation of material with minimal time for heat transfer to the surrounding material. This is critical, as the absorption of heat can cause microcracking, material dislocations, and localized residual stress leading to material property shifts such as fracture toughness that greatly affect performance and can lead to device failure. This facility is the first U.S.-based, Oxford Lasers Ltd. picosecond laser micromachining station contained within an academic setting. Prior and on-going solid mechanics investigations will be leveraged to extend this research to high temperature sensor characterization. Currently, the proposed pressure sensor has been fabricated using the proposed manufacturing methods and calibrated under static and dynamic pressure loads at room temperature. During the next phase of this research, we propose to test the sensor in a high temperature environment in the hot jet facility at FSU. This facility is capable of generating supersonic heated jets up to 2000°F. The research and development will leverage on-going activities in predictions of light-matter material evolution with respect to mechanical performance. It will also provide us with key information to identify potential challenges in integrating such sensors on a field deployable hypersonic vehicle.