Th. Becker

GAS-KINETIC INTERACTIONS OF NITROUS OXIDES WITH SNO2 SURFACES

The kinetics of interaction of nitrous oxides (NOx) with SnO2 sensor surfaces were investigated. It is shown that NO, similar to CO, undergoes a simple reducing interaction with SnO2 sensor surfaces producing a conductivity increase. In comparison, NO2 was found to exhibit a highly complex behaviour. At surface temperatures lower than 200°C, NO2 was found to reversibly oxidise SnO2 surfaces independently of the NO2 concentrations applied. In this latter case, reduced conductivities were observed. Approaching surface temperatures of the order of 400°C, a reducing interaction took over which gave way to the normally observed oxidising behaviour as NO2 concentrations above 1 ppm were applied. In this latter high-temperature/high-concentration regime, poisoning effects were observed which caused the sensor resistance baseline to drift away. These latter results indicated that the poisoning effect of NO2 on SnO2 surfaces is associated with the formation of higher molecular species containing two nitrogen atoms at least.

Air pollution monitoring using tin-oxide-based microreactor systems

Abstract Recent research on metal-oxide gas sensors has shown that such devices respond to environmentally relevant concentrations of O 3 , NO 2 , NO and CO. Such sensors, therefore, hold the potential of becoming inexpensive monitors of air pollution. In our work, tin-oxide sensors were embedded into tiny microchambers with internal volumes of 270 μl to form microreaction chambers. Enclosing samples of polluted air within such chambers, gas depletion reactions can be observed, which provide analytical information about the pollutant species enclosed. We have extended this approach to a microanalysis system containing two sensor chambers, one equipped with a thin-film and the other with a thick-film tin-oxide sensor element. Performing experiments on such a system, we were able to obtain analytical information about air pollutants in relevant concentrations for indoor as well as outdoor applications. We further show that in addition to the very gas-analysis functions, our air-monitoring system can also be equipped with calibration and self-test features.

MICROMACHINED THIN FILM SNO2 GAS SENSORS IN TEMPERATURE-PULSED OPERATION MODE

Abstract Gas detection measurements based on a micromachined SnO 2 gas sensor with periodically pulsed heater voltage are presented. Additionally, the field-effect-induced changes in resistivity of the sensitive layer caused by the heater voltage were investigated. The combination of both results leads to an improved design for low power SnO 2 gas sensors. In temperature-pulsed mode, the sensor resistances were measured at constant delays after the pulse edges. The measurements were carried out with the common test gases carbon monoxide and nitric dioxide in synthetic air with 50% humidity. In the cold pulse phase, the CO sensor response is higher and shows only a slow decrease with increasing pulse duration. The sensor sensitivity is related to the pulsed heated mode, on the one hand, and the continuously heated, on the other. The comparison of the measurement results reveals that the temperature-pulsed operation mode (TPOM) caused a significant reduction of power consumption and higher sensitivity.

Ozone detection using low-power-consumption metal–oxide gas sensors

Abstract Thin films of RGTO–SnO 2 deposited onto micromachined heater elements were characterised for their sensitivity towards prominent air pollutants such as O 3 , NO 2 , NO and CO. We find that, unlike their thick-film counterparts, thin-film SnO 2 sensors exhibit a very high sensitivity towards oxidising gases such as NO 2 , and in particular O 3 . Sensitivities towards reducing species such as CO and NO are less pronounced than in thick-film sensors and in the nature of small cross-sensitivities on the main sensitivity to ozone. The differences between thin and thick-film devices are discussed, and it is shown that thin-film devices on micromachined heater elements hold promise for fabricating low-power-consumption gas sensors capable of detecting environmentally relevant concentrations of ozone.

Design and Fabrication of Heat Storage Thermoelectric Harvesting Devices

Thermoelectric energy harvesting requires a substantial temperature difference ΔT to be available within the device structure. This has restricted its use to particular applications such as heat engine structural monitoring, where a hot metal surface is available. An alternative approach is possible in cases where ambient temperature undergoes regular variation. This involves using a heat storage unit, which is filled with a phase-change material (PCM), to create an internal spatial temperature difference from temperature variation in time. In this paper, key design parameters and a characterization methodology for such devices are defined. The maximum electrical energy density expected for a given temperature range is calculated. The fabrication, characterization, and analysis of a heat storage harvesting prototype device are presented for temperature variations of a few tens of degrees around 0 °C, corresponding to aircraft flight conditions. Output energy of 105 J into a 10- Ω matched resistive load, from a temperature sweep from +20 °C to -21 °C, then to +25 °C is demonstrated, using 23 g of water as the PCM. The proposed device offers a unique powering solution for wireless sensor applications involving locations with temperature variation, such as structural monitoring in aircraft, industrial, and vehicle facilities.

Microfluidic system for the integration and cyclic operation of gas sensors

This article describes a microfluidic system for the detection of gases. Different flow channels, each to be controlled by a microvalve, lead to chambers where thin film gas sensors have been integrated. This system is intended for alleviating the problems of microstructured thin film gas sensors such as cross-sensitivity, slow saturation of the sensor signal and baseline drift. With the microfluidic system presented, these problems may be overcome by a cyclic operation of the gas sensors with each cycle leading through subsequent processes of purging, calibration and measurement. All system components were fabricated using silicon micromachining technologies including the gas sensors integrated into the system. Preliminary measurements on such microchamber gas sensors are presented.

Investigation of the Performance of Thermoelectric Energy Harvesters Under Real Flight Conditions

Energy-autonomous wireless sensor nodes (WSNs) in aircraft, acting as health monitoring systems (HMS), have the potential to reduce aircraft maintenance costs. Thermoelectric energy harvesting is a solution for self-powered systems, since it captures enough energy to power up a WSN. The energy harvesting device used in this work consists of a thermoelectric generator (TEG) attached to the inner part of the fuselage and to a thermal storage device, in order to artificially enhance the temperature difference between the bottom and the top surface of the TEG during take-off and landing. In this study, the results of 28 flight tests during a 6-month flight campaign of two identical energy harvesting devices are presented. The results are clustered into two different classes, each having its own characteristics. The two classes comprise typical, similar to standard European short/mid-range flights, as well as atypical flight profiles, where specific flight tests have been performed. In addition, for each class, different parameters such as flight altitudes, flight duration, and temperature profiles are investigated. Moreover, a detailed comparison between a typical and an atypical flight profile is given. In general, for a typical flight profile, the experimental results are in good agreement with simulations predicting the energy output. The average energy output is sufficient to power up a wireless sensor.

Gas mixture analysis using silicon micro-reactor systems

A novel way of operating metal-oxide gas sensors is demonstrated that extends the gas-analyzing facilities of conventional thin-film gas sensors. In our approach, thin-film metal-oxide gas sensors on micro-machined heater substrates are embedded into tiny silicon micro-chambers to form micro-reactor devices. Analyzing samples of polluted air, such micro-reactors can be operated either in constant-flow or no-flow modes. Whereas in the first mode, essentially normal sensor behavior is observed, gas depletion reactions are observed in the latter. Such depletion reactions are shown to provide, in a straightforward way, analytical information about gas mixtures which is difficult to obtain under normal sensor operating conditions. As an application example, we demonstrate how a micro-reactor device can be used to analyze samples of polluted air for their O/sub 3/ and NO/sub 2/ contents.

Flight Test Results of a Thermoelectric Energy Harvesterfor Aircraft

The idea of thermoelectric energy harvesting for low-power wireless sensor systems in aircraft and its practical implementation was recently published. The concept of using a thermoelectric generator (TEG) attached to the aircraft inner hull and a thermal storage device to create an artificial temperature gradient at the TEG during take-off and landing from the temperature changes of the fuselage has passed initial tests and is now subject to flight testing. This work presents preflight test results, e.g., vibration and temperature testing of the harvesters, the practical installation of two harvesting devices inside a test plane, and the first test flight results. Several flight cycles with different flight profiles, flight lengths, and outside temperatures have been performed. Although the influence of different flight profiles on the energy output of the harvester can be clearly observed, the results are in good agreement with expectations from numerical simulations with boundary conditions evaluated from initial climate chamber experiments. In addition, the flight test demonstrates that reliable operation of thermoelectric energy harvesting in harsh aircraft environments seems to be feasible, therefore paving the way for realization of energy-autonomous, wireless sensor networks.

Efficient Power Management for Energy-Autonomous Wireless Sensor Nodes for Aeronautical Applications

Efficient power management is a key component for energy-autonomous wireless sensor nodes. Thermoelectric energy harvesting is a possible solution for powering such sensor nodes. In this paper, we present a power management circuit that has significant improvements compared with earlier versions. Advancements were made to the rectifier part and the storage controller. The improved rectification circuit is able to control the p-metal–oxide–semiconductor (p-MOS) transistors with a negative instead of zero voltage at the gate. Furthermore, modifications to enhance the total efficiency of the storage controller are applied. The storage controller is a direct current (DC)–DC converter and is controlled by a pulse frequency modulation signal, provided by a microcontroller. In the modified version, the microcontroller is always in low-power mode, and an external circuit is used to control the storage controller. Changes in the software are applied so that the microcontroller is always set in low-power mode. The runtime with and without a load are compared, and the overall self-discharge time is evaluated.