No abstract

A new type of amperometric gas sensor, based on screen printing and lamination strategies, has been fabricated and tested. These sensors use thin film electrodes, printed on plastic substrates to form a small, low power (μW) amperometric sensor package, which uses a very small volume of electrolyte. These sensors have been demonstrated to have improved performance compared to amperometric gas sensors of conventional design, when conventional electrolytes are used. The sensors have also been demonstrated to have promising performance, compared to conventional technology, using room temperature ionic liquid electrolytes (RTILs). The printed sensors have been demonstrated successfully for monitoring of carbon monoxide (CO), ammonia (NH3).

Micro-thermal conductivity detector (µTCD) gas sensors work by detecting changes in the thermal conductivity of the surrounding medium and are used as detectors in many applications such as gas chromatography systems. Conventional TCDs use steady-state resistance (i.e., temperature) measurements of a micro-heater. In this work, we developed a new measurement method and hardware configuration based on the processing of the transient response of a low thermal mass TCD to an electric current step. The method was implemented for a 100-µm-long and 1-µm-thick micro-fabricated bridge that consisted of doped polysilicon conductive film passivated with a 200-nm silicon nitride layer. Transient resistance variations of the µTCD in response to a square current pulse were studied in multiple mixtures of dilute gases in nitrogen. Simulations and experimental results are presented and compared for the time resolved and steady-state regime of the sensor response. Thermal analysis and simulation show that the sensor response is exponential in the transient state, that the time constant of this exponential variation was a linear function of the thermal conductivity of the gas ambient, and that the sensor was able to quantify the mixture composition. The level of detection in nitrogen was estimated to be from 25 ppm for helium to 178 ppm for carbon dioxide. With this novel approach, the sensor requires approximately 3.6 nJ for a single measurement and needs only 300 µs of sampling time. This is less than the energy and time required for steady-state DC measurements.

No abstract

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. There is an increasing demand and variety of applications for miniaturized gas sensors. Applications range from environment safety sensing, transportation systems, medical and health to aid in diagnostics and even food safety and agriculture. Conventional gas detectors including the most widely used, relatively small pellistors 1 require large power consumption (hundreds of milli-Watts to Watts) with slow response time (tens of seconds). With MEMS technology, microhotplates have been built and are capable of reaching operating temperatures of 500• C in 20 ms at a power level of 100 mW. 2,3 Microfabricated thermal resistive bridges used for catalyst heater and resistance thermometer have been reported with 0.2 ms response time at even lower power level. 4 Compared with the calorimetric gas sensing principle used in the previous work, an electrothermal sensing mechanism based on the thermal conductivity of gases does not rely on gas adsorption and reaction with catalyst films. Therefore, the response speed can be more rapid, and the sensor can be operated in a continuous manner and be repeatedly used without memory effect. Miniature thermal conductivity sensors have been developed for gas chromatography systems 5,6 and demonstrated for methane determination in natural gas.7 Summaries of chemical sensor platforms based on thermal conductivity sensors are provided in references. 8,9 Other groups have investigated nanoscale bridge type gas sensors using chemically synthesized carbon nanotubes, 10 nanowires, 11 and nanobelts, 12 and a review by G. Di Francia et al. 13 has been published. Altho...