Philippe Breuil

Thin films for gas sensors

Abstract Thin tin dioxide films are used as sensitive elements for gas sensors. The physical properties of such materials and their electrical performance largely depend on their methods of production. This work presents two methods used to produce thin dioxide films, i.e., chemical vapour deposition and reactive evaporation. Optimization of the deposition parameters enables us to process highly sensitive materials and even materials able to discriminate between certain gases, such as alcohol, benzene or hydrogen sulphide.

Tin dioxide gas sensor as a tool for atmospheric pollution monitoring: Problems and possibilities for improvements

The monitoring of atmospheric pollution needs network of gas sensors in order to increase the number of measuring points. For this goal, tin dioxide sensors have been evaluated. Their potentials and limitations (selectivity and stability), are presented via on-site experiments of urban pollution control. The study shows that some global information indicating mainly the traffic pollution can be obtained. The general problem of long-term stability and selectivity of SnO2 sensors is discussed. Some specific solutions are proposed, for example, a chemical treatment in the case of instability due to SO2. The problem of the dual response to oxidising and reducing gases is discussed, especially in regard to CO/NO2 detection. To solve this problem, the use of active filters is proposed. For example, a thin film of rhodium deposited above the sensing material allows to filter NO2. The use of a platinum filter has also been applied to improve the selectivity for the reducing gases. The results point out the difficulty to control this type of device in thin film technology. On the contrary, with thick films obtained by screen-printing, the possibility to separate HC from VOC (CH4 from CO and C2H5OH) is demonstrated. Some preliminary results concerning the use of a MnO2 filter are also presented in order to control the ozone reaction. The possibilities offered by signal processing are finally discussed using a multi-variable approach with 90 days aged sensors. AC measurements are exploited to improve the selectivity for the oxidizing gases NO2 and O3. The results obtained with a model built with atmospheric air as carrier gas appear promising.

Advances in the development of micropattern gaseous detectors with resistive electrodes

The aim of this paper is to highlight very promising developments of a new family of micropattern gaseous detectors characterized by the presence in their layout of at least one resistive electrode. These novel detectors combine in one design the best features of RPCs (i.e. the protection against sparks) and conventional micropattern gaseous detectors (i.e. the high granularity and space resolution).

Progress in the development of a S-RETGEM-based detector for an early forest fire warning system

We present a prototype of a Strip Resistive Thick GEM (S-RETGEM) photosensitive gaseous detector filled with Ne and ethylferrocene (EF) vapours at a total pressure of 1 atm for an early forest fire detection system. Measurements show that it is one hundred times more sensitive than the best commercial ultraviolet (UV) flame detectors; and therefore, it is able to reliably detect a flame of ~ 1.5 × 1.5 × 1.5 m3 at a distance of about 1 km. An additional and unique feature of this detector is its imaging capability, which in combination with other techniques, may significantly reduce false fire alarms rate when operating in an automatic mode. Preliminary results conducted with air-filled photosensitive gaseous detectors are also presented. The main advantages of this approach include both the simplicity of manufacturing and affordability of construction materials such as plastics and glues specifically reducing detector production cost. The sensitivity of these air-filled detectors at certain conditions may be as high as those filled with Ne and EF. Long-term tests of such sealed detectors indicate a significant progress in this direction. We believe that our detectors utilized in addition to other flame and smoke sensors will exceptionally increase the capability to detect forest fire at a very early stage of development. Our future efforts will be focused on attempts to commercialize such detectors utilizing our aforementioned findings.

Detectors for alpha particles and X-rays operating in ambient air in pulse counting mode or/and with gas amplification

Ionization chambers working in ambient air in current detection mode are attractive due to their simplicity and low cost and are widely used in several applications such as smoke detection, dosimetry, therapeutic beam monitoring and so on. The aim of this work was to investigate if gaseous detectors can operate in ambient air in pulse counting mode as well as with gas amplification which potentially offers the highest possible sensitivity in applications like alpha particle detection or high energy X-ray photon or electron detection. To investigate the feasibility of this method two types of open- end gaseous detectors were build and successfully tested. The first one was a single wire or multiwire cylindrical geometry detector operating in pulse mode at a gas gain of one (pulse ionization chamber). This detector was readout by a custom made wide -band charge sensitive amplifier able to deal with slow induced signals generated by slow motion of negative and positive ions. The multiwire detector was able to detect alpha particles with an efficiency close to 22%. The second type of an alpha detector was an innovative GEM-like detector with resistive electrodes operating in air in avalanche mode at high gas gains (up to 104). This detector can also operate in a cascaded mode or being combined with other detectors, for example with MICROMEGAS. This detector was readout by a conventional charge -sensitive amplifier and was able to detect alpha particles with 100% efficiency. This detector could also detect X-ray photons or fast electrons. A detailed comparison between these two detectors is given as well as a comparison with commercially available alpha detectors. The main advantages of gaseous detectors operating in air in a pulse detection mode are their simplicity, low cost and high sensitivity. One of the possible applications of these new detectors is alpha particle background monitors which, due to their low cost can find wide application not only in houses, but in public areas: airports, railway station and so on.

Hydrogen detection on a cryogenic motor with a SnO2 sensors network

In this work, SnO2 sensors have been used in order to detect leaks of hydrogen on a cryogenic motor. In a first part, the SnO2 sensors performances have been evaluated on a laboratory testing bench under different hydrogen and oxygen concentrations in the nitrogen carrier gas. A particular attention has been focused on the influence of oxygen on the hydrogen response of the sensor. In a second part, a multi-sensors device has been developed for a network application. Before its use, this device has been valued with eight sensors on the laboratory testing bench. The sensors have been then located at different places on the cryogenic motor in order to monitor the distribution of the hydrogen concentrations around the motor and to establish a real cartography of this latter. The experimental results obtained with the network of sensors prove their ability to control the good functionality of the different devices of the motor.

An electrical percolation model for tin dioxide polycrystalline thin films

The physical properties of tin dioxide and its electrical performance largely depend on its morphological properties. As mentioned by several authors, the size of crystallites relative to the Debye length appears to be a determining factor for the sensitivity to gas. In a previous paper concerning thin films elaborated by reactive evaporation, it has been established that the electrical conductivity value depends on the relative density of the film.

Preconcentration Modeling for the Optimization of a Micro Gas Preconcentrator Applied to Environmental Monitoring

This paper presents the optimization of a micro gas preconcentrator (μ-GP) system applied to atmospheric pollution monitoring, with the help of a complete modeling of the preconcentration cycle. Two different approaches based on kinetic equations are used to illustrate the behavior of the micro gas preconcentrator for given experimental conditions. The need for high adsorption flow and heating rate and for low desorption flow and detection volume is demonstrated in this paper. Preliminary to this optimization, the preconcentration factor is discussed and a definition is proposed.

Development of new hole-type avalanche detectors and the first results of their applications

We have developed a new detector of photons and charged particles-a hole-type structure with electrodes made of a double layered resistive material: a thin low resistive layer coated with a layer having much higher resistivity. One of the unique features of this detector is its capability to operate at high gas gains (up to ) in air or in gas mixtures with air. They can also operate in a cascaded mode or be combined with other detectors, for example with GEM. This opens new avenues in their applications. Several prototypes based on this new detector and oriented to practical applications were developed and successfully tested: a detector of soft X-rays and alpha particles, a flame sensor, a detector of dangerous gases. All of these devices could operate stably even in humid air and/or in dusty conditions. The main advantages of these devices are their simplicity, low cost, and high sensitivity. For example, due to the avalanche multiplication, the detectors of flames and dangerous gases have a sensitivity of 10-100 times higher than commercial devices. We therefore believe that new detectors will have a great future.

A micro gas preconcentrator with improved performances for environmental monitoring

This paper presents the optimization of a micro gas preconcentrator applied to environmental monitoring. Different types of adsorbent materials introduced in silicon microchannels with various designs have been investigated and a special focus has been dedicated to the modes of operation of the preconcentrator. The optimization of the device and its operation were driven by its future application in outdoor environments. Parameters such as the preconcentration factor, cycle time and the influence of the humidity were considered along the optimization process. As a result of this study, a preconcentrator with a total cycle time of 10 minutes and the use of single wall carbon nanotubes (SWCNTs) as adsorbent exhibits a good preconcentration factor for VOCs with a limited influence of the humidity.