System to control indoor air quality in energy efficient buildings

This work looks at monitoring air quality in indoor environments through the integration of several sensing technologies into a single robust, reliable and cheap detection platform, which shares air pre-conditioning and electronics. Target gases and detection limits have been set according to recommendations of different agencies in Europe and the US. The system has reached detection limits stated by the OSHA (Occupational Safety and Health Administration) for benzene. The pre-conditioning fluidic platform has also been designed, simulated, fabricated and tested with sensors so the gas flow has been optimized. Field tests in real buildings are being carried out to contrast current measurement procedures and results with the obtained using the device under development. The main aim of the system is to control HVAC (Heat Ventilation and Air Conditioning) in energy-efficient way while keeping a high air quality standard inside the building

Laser-induced periodic surface structures on ZnO thin film for high response NO2 detection

Femtosecond laser-induced periodic structures (LIPSS) have been processed on ZnO thin film gas sensor devices for nitrogen dioxide (NO2) detection. From the morphology point of view, the nanostructures have been identified as high spatial frequency LIPSS (HSFL) with an average period of 145 nm. Through Raman analysis, a decrease of the typical wurtzite ZnO structure is shown, with a possible increase of defects such as Zn interstitials. The response under NO2 is enhanced if compared with the only-annealed ZnO thin film for concentrations as low as 1 ppm, reaching 1 ppb of detection limit (LOD) for the sensors with LIPSS. The Zn interstitials defects could be the source of the adsorbed NO2 species increasing the sensitivity. Reproducible results have been measured during 11 weeks in a row

Comparison of the Characteristics of Small Commercial NDIR CO2 Sensor Models and Development of a Portable CO2 Measurement Device

Many sensors have to be used simultaneously for multipoint carbon dioxide (CO2) observation. All the sensors should be calibrated in advance, but this is a time-consuming process. To seek a simplified calibration method, we used four commercial CO2 sensor models and characterized their output tendencies against ambient temperature and length of use, in addition to offset characteristics. We used four samples of standard gas with different CO2 concentrations (0, 407, 1,110, and 1,810 ppm). The outputs of K30 and AN100 models showed linear relationships with temperature and length of use. Calibration coefficients for sensor models were determined using the data from three individual sensors of the same model to minimize the relative RMS error. When the correction was applied to the sensors, the accuracy of measurements improved significantly in the case of the K30 and AN100 units. In particular, in the case of K30 the relative RMS error decreased from 24% to 4%. Hence, we have chosen K30 for developing a portable CO2 measurement device (10 × 10 × 15 cm, 900 g). Data of CO2 concentration, measurement time and location, temperature, humidity, and atmospheric pressure can be recorded onto a Secure Digital (SD) memory card. The CO2 concentration in a high-school lecture room was monitored with this device. The CO2 data, when corrected for simultaneously measured temperature, water vapor partial pressure, and atmospheric pressure, showed a good agreement with the data measured by a highly accurate CO2 analyzer, LI-6262. This indicates that acceptable accuracy can be realized using the calibration method developed in this study

Formaldehyde sensing mechanism of SnO2 nanowires grown on-chip by sputtering techniques

Tin dioxide nanowires have been grown by thermal oxidation of sputtered thin films by means of a VLS method. A tin sputtered layer catalyzed by gold nanoparticles, acts as material seed for the localized growth of NWs directly on gas sensor devices, avoiding the manipulation and transport of the nanowires to the electrodes. XRD and HRTEM analysis show that the nanowires crystallize in a rutile structure with a [100] preferential growth direction and are single-crystalline with diameters lower than 50 nm. The response of nanowires to formaldehyde has been compared to thin film based sensors. A sensitivity of 0.10 ppm-1 is reported, twofold the sensitivity of the thin film and short response and recovery times are measured (6 times shorter than thin films). The sensing mechanism proposed for the SnO2 NWs under formaldehyde exposure is explained by means of conduction measurements and FT-IR analysis. Oxygen species chemisorbed on the surface of each SnO2 nanowire produce a band bending, which generates a potential barrier (of 0.74± 0.02 eV at 300 ºC) between the point contact of different nanowires. As evidenced by IR spectroscopy at 300 ºC, electrons in the conduction band and in mono-ionized oxygen vacancies (at 0.33 eV below the bottom of the conduction band) are responsible for gas detection

Investigation of the Carbon Monoxide Gas Sensing Characteristics of Tin Oxide Mixed Cerium Oxide Thin Films

Thin films of tin oxide mixed cerium oxide were grown on unheated substrates by physical vapor deposition. The films were annealed in air at 500 °C for two hours, and were characterized using X-ray photoelectron spectroscopy, atomic force microscopy and optical spectrophotometry. X-ray photoelectron spectroscopy and atomic force microscopy results reveal that the films were highly porous and porosity of our films was found to be in the range of 11.6–21.7%. The films were investigated for the detection of carbon monoxide, and were found to be highly sensitive. We found that 430 °C was the optimum operating temperature for sensing CO gas at concentrations as low as 5 ppm. Our sensors exhibited fast response and recovery times of 26 s and 30 s, respectively

WO3 processed by direct laser interference patterning for NO2 detection

In this paper two kind of sensors based on WO3 sputtered by magnetron sputtering and annealed at 600 °C have been studied. The first kind was processed by two-dimensional direct laser interfering patterning (DLIP) and the second one without any additional treatment. Morphological and structural characterization have shown a hole structure in a periodic line-pattern for the DLIP-processed sensors and a flat surface for the only-annealed sensors, both with a tetragonal WO3 phase. TOF-SIMS analysis has revealed that the first WO3 layers are reduced for both samples, which could improve sensing performance. Promising response enhancement of DLIP-processed sensors has been observed for low concentrations of NO2 (from 0.5 ppm to 5 ppm) at 200 °C, lowering the limit of detection (LOD) to 10 ppb, half of the LOD of the only-annealed sensors (20 ppb). Cross sensitivity to CO and HCHO have been investigated and the sensing mechanisms discussed