Microthermocouples Sensors for Velocity and Temperature Measurements in Gas Flow (original) (raw)
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In this paper, we present a complete thermomechanical study of a micromachined gas sensor substrate. The work has been carried out combining coupled electrothermomechanical three-dimensional finite element modelling simulations with electrical, infrared thermography and interferometric microscopy experimental measurements. The performances predicted by simulations, such as the power consumption (heating efficiency in air of 5.7 • C mW −1), the time response (19 ms), the membrane deflection during operation and the preferential failure sites in the micromachined substrate have been confirmed by experience. Their good agreement validates the model, and allows us to consider the adaptability of this design as a micromachined substrate for integrated gas sensors.
ACCURATE EXTRACTION OF THE TEMPERATURE OF THE HEATING ELEMENT IN MICROMACHINED GAS SENSORS
The sensitivity and selectivity of micromachined gas sensors strongly depend on the temperature of the heating element; therefore an accurate determination of this temperature is required. In this paper, a simple analytical model of the thermal behavior of a heating element placed onto a thermally insulated dielectric membrane is presented. It is demonstrated that simple resistance vs. power measurements are sufficient for a precise determination of the sensor temperature. These measurements performed "once and for all" at the wafer-level on the statistically relevant number of heaters, allowed us to determine a "universal" temperature vs. power curve.
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This paper presents a short response time, all-silica, gas-flow-velocity sensor. The active section of the sensor consists of a 16 µm diameter, highly optically absorbing micro-wire, which is heated remotely by a 980 nm light source. The heated microwire forms a Fabry–Perot interferometer whose temperature is observed at standard telecom wavelengths (1550 nm). The short response time of the sensor allows for different interrogation approaches. Direct measurement of the sensor’s thermal time constant allowed for flow-velocity measurements independent of the absolute heating power delivered to the sensor. This measurement approach also resulted in a simple and cost-efficient interrogation system, which utilized only a few telecom components. The sensor’s short response time, furthermore, allowed for dynamic flow sensing (including turbulence detection). The sensor’s bandwidth was measured experimentally and proved to be in the range of around 22 Hz at low flow velocities. Using time c...
Journal of the Electrochemical Society, 2014
A miniature, ultra-low power, sensitive, microbridge-based thermal conductivity gas sensor has been developed. The batch fabrication of the sensors was realized by CMOS compatible processes and surface micromachining techniques. Doped polysilicon was used as the structural material of the bridge with critical dimension of 500 nm. A model of the microbridge was simulated in COMSOL that couples electrical and thermal physics together and includes minimal simplifications. Modeling results shows that the majority of heat is transferred via conduction through the gas gap under the bridge. Heat loss from constant voltage application was observed to be a function of the thermal conductivity of the gas ambient, resulting in different magnitude of resistance change. Maximum temperature occurs at the center of the bridge and could be as high as 800 K. Modeling results coincide with experimental data in predicting resistance changes. The sensor was tested with nitrogen, carbon dioxide, and helium. The response of the sensor to the mixtures of helium fractions in nitrogen was tested. We demonstrated that the sensitivity for helium in nitrogen was 0.34 m /ppm when operated at 3.6 V supplies (at power level of 4.3 mW). With a Wheatstone bridge with ac excitation and a lock-in amplifier the sensor limit of detection was ∼700 ppm helium in nitrogen. The stability of the sensor was excellent achieving over 30 billion measurements before failure.
Thermal microsensor with a.c. heating for gas-pressure measurements
Sensors and Actuators A: Physical, 1997
A method for gas-pressure measurements by means of a micromachined thennal sensor is described. A 0,15 ~m thick silicon nitride membrane with dynamically changed dissipated power serves as a gas heater. Theoretical and experimental results of the sensor sensitivity in a frequency range of modulated dissipated power from 20 Hz to 1 kHz are presented. A resolution of 0.2 torr is estimated at atmospheric pressure. © 1997 Elsevier Science S.A.
Characterization of an Embedded Micro-Heater for Gas Sensors Applications
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