EP2078190A1

EP2078190A1 - Combustion gas sensor

Info

Publication number: EP2078190A1
Application number: EP07821611A
Authority: EP
Inventors: André De Haan; Marc Debliquy
Original assignee: Chimie Inorganique et Organique en abrege "Sochinor" Ste
Current assignee: Chimie Inorganique et Organique en abrege "Sochinor" Ste
Priority date: 2006-10-19
Filing date: 2007-10-19
Publication date: 2009-07-15
Also published as: US8234906B2; CA2666560A1; WO2008046926A1; JP2010507088A; US20110100087A1; CN101617221A

Abstract

The invention relates to a combustion gas sensor, said sensor comprising one or more metallic oxides forming an adsorption semi-conductor having an electric resistance that varies according to the adsorbed gas, and said semi-conductor being a direct gas adsorption semi-conductor without catalyzed chemical reaction, wherein said sensor is designed for detecting nitrogen oxides in the case of a strong fire and/or said sensor is designed for detecting, in case of smoldering fire, the partially unburnt gases, in particular alcohols, aldehydes, ketones, carboxylic acids or amines.

Description

Gas sensor emitted by a combustion.

The present invention relates to a gas sensor emitted by a combustion, which sensor comprises one or more metal oxides forming a gas adsorption semiconductor whose resistance changes as a function of the adsorbed gas.

Such a sensor is known from the patent application WO 98/08084. This document describes the possibility of making sensors based on organic semiconductors such as phthalocyanines used alone or mixed with inorganic semiconductors or the use of inorganic semiconductors only. These sensors detect both smoldering fires, that is to say, flameless fires, and live fires, that is to say with flames. These sensors formed by organic and inorganic semiconductors have the function of detecting the presence of one or more gases emitted during combustion.

Fire detection is also known smoke detectors, whose ionic or optical sensors detect solid particles present in the fumes. However, these types of detectors have a certain number of problems because their reactivity depends in particular on the size of the particles emitted. In addition, some fires, such as alcohol, do not emit solid particles and can not be detected. As a result, these smoke detectors do not detect all types of fires.

The detection of gas by semiconductors is well known, but it is important in fire detection to detect only the gases emitted during a combustion to avoid any risk of interference and therefore false alarms.

The interest of fire detection by means of sensors detecting the gases emitted during a combustion essentially lies in the following facts. They allow a much earlier detection of the focus because the speed of propagation of gases is much higher than those of the solid particles detected by the detectors of ionic or optical smoke. In addition, their detection spectrum can be widely extended to all types of fires provided they detect the gases emitted during any type of combustion. This criterion is obviously not satisfied in the case of CO carbon monoxide detectors which normally detect only this single flue gas. Indeed, certain types of hearths emit only very little CO, such as for example a quick fire of alcohol.

Combustion gas detectors are also particularly interesting in that they are insensitive to dust and can therefore be used in dusty, industrial or other environments.

Sensors based on metal oxides that can be used in fire detection, as for example described in. US 6,046,054 or UK 2267968 A, generally use either tin oxide SnO 2, doped with noble metals, or double oxides CrTiO _x , CrRbO _x , SrTiO ₃ , as for example described in. EP 0609316 B1, EP 0656111 B1, US 5,767,388, or US 5,635,628. Their essential function is to detect the emission of combustible gases and, in particular, for the detection of fire, the emission of carbon monoxide CO while attempting to avoid the interfering gases. Other types of gas sensors are based on electrochemical cells that specifically detect carbon monoxide CO. Unfortunately, since they only detect the presence of a specific gas (CO), they do not detect all types of fires. Sensors based on metal oxides such as tin oxide SnO 2, gallium Ga2θ3, ..., working at high temperatures above 400 ⁰ C. In the ambient atmosphere, their semi-conductivity is conditioned by the oxygen of the air which adsorbs itself on the surface. In the presence of a combustible gas, such as carbon monoxide, hydrocarbons and partially oxidized hydrocarbons, the latter undergo on the surface of the sensor a catalytic combustion with oxygen previously adsorbed. This results in a decreasing the amount of oxygen adsorbed on the surface and therefore an evolution of the electrical resistance of the sensor. These sensors are to be classified as "catalytic" sensors based on metal oxides.

These types of detectors are however only usable in the context of the detection of smoldering fires for which the carbon monoxide CO emission is sufficiently high. They do not detect fires with flames that emit very little unburnt and carbon monoxide.

Since they detect only smoldering fires, the field of application of this type of sensor used alone is relatively small and they must generally be coupled to other types of sensors (optical, thermal, ...).

"Catalytic" sensors based on doped tin oxide SnO ₂ or Ga ₂ O gallium oxide have many interferences and very poorly resist standardized corrosion tests in the presence of SO ₂ sulfur oxide. In addition, their sensitivity is influenced by ambient humidity.

These sensors work at ordinary temperature on a principle of adsorption and desorption of the gases emitted during a combustion. It is no longer a question of "catalytic" sensors, but of "adsorption" sensors for combustion gases on the surface of the semiconductor.

A priori, working at room temperature may be of interest in terms of energy consumption because the sensor should not be heated. The energy consumption of a sensor is, indeed, a very important factor because on a control line that can contain several hundred, it is necessary to provide a battery supply that in case of power failure by the sector must be able to maintain surveillance for 72 hours. If the consumption of each detector is too high, the quantity and the cost of the required batteries become prohibitive. However, these sensors usable at ordinary temperature exhibit very large fluctuations of the basic signal according to the revolution of the composition of the atmosphere and of the ambient temperature. The detection of a focus can in this case be performed only by sophisticated electronic means that evaluate in particular the evolution of the response of a sensor for predetermined periods of time.

Indeed, phthalocyanine is a semi-conductor whose semi-conductivity is conditioned in particular by oxygen in the air, but especially by natural atmospheric pollutants such as ozone and nitrogen oxides (the resistivity p fluctuating between 10 ⁷ and 10 ⁹ Ω ^"1 cm ^{" 1} ). For against, in a vacuum or under an inert atmosphere, it is completely insulating (p> October ¹⁵ Ω ^"1 ^{cm" 1).}

These sensors used at room temperature are therefore very sensitive to the ozone O3 content, NO _x nitrogen oxides and atmospheric humidity. The dependence of these sensors on ozone and nitrogen oxides strongly limits their use because the sensitivity of the detectors fluctuates as the content of these atmospheric pollutants depending on the season. It is therefore important to make sensors with a much greater sensitivity to gaseous agents to be detected, such as flue gases, than they have to natural doping elements and interfering gases.

When the resistance changes in the direction of an increase, the sensitivity S of the sensors is defined as the ratio of the resistance R _ag in the presence of the gaseous agent to be detected on the resistance R _am b that it presents to the atmosphere at this instant S ₁ - = R _ag / Ramb; if the resistance decreases, the sensitivity is expressed by the inverse ratio Sj _,

Since phthalocyanine is a semiconductor conditioned by the ambient atmosphere, it is impossible to reduce its sensitivity to ozone by eliminating it by a filter, because in this case the phthalocyanine becomes an insulator. Finally, over time, sensors made from phthalocyanine in powder form as described in EP 0 918 985 B1, undergo sintering which progressively decreases their sensitivity.

The influence of humidity on the response of these sensors also hinders their use. Indeed, as these sensors generally work on the basis of the evolution of their resistance as a function of time, sudden changes in the humidity level in the air can cause false alarms (opening of bathroom doors, showers However, in addition to sensitivity to fluctuations in the ambient atmosphere including humidity, the fact of using sensors working at room temperature opens another problem related to the recovery time of the sensors after the action of an atmospheric agent of any kind: presence of cigarette smoke, alcohol fire for fondues, candles, various household solvents, perfumes, any fires ... The recovery time is the duration of the return to the usual basic line by desorption of the adsorbed gases, becomes relatively long which induces during a strong intoxication a memory effect during a few hours to 24 hours and the detector remains not operational during this period.

All of these problems make their use very problematic in fire detection since in the field of security, the response must imperatively be constant, reproducible over time and at all times. The object of the present invention is to provide a combustion-emitted gas sensor which is capable of detecting either a fire with flames, or a flameless fire, or both a fire with a flameless fire and being relatively insensitive to fire. fluctuations in the ambient atmosphere. For this purpose, a sensor according to the invention is characterized in that said semiconductor is a direct gas adsorption semiconductor without catalyzed chemical reaction, which sensor is arranged to detect oxides of nitrogen in the case of a high-fire, and / or to detect, in the case of a smoldering fire, partially unburned gases, in particular alcohols, aldehydes, ketones, carboxylic acids, or the amines. The conductimetric or resistive sensors based on metal oxides semiconductors with direct adsorption of gas without catalyzed chemical reaction thus carried out are not conditioned by the composition of the ambient atmosphere (O2, O3, NO _x , H ₂ O,. ..). It has also been found, surprisingly, that by detecting nitrogen oxides using a sensor comprising metal oxides semiconductors direct adsorption of gas without catalyzed chemical reaction, it was possible to detect a bright fire. In addition, it has been found, also surprisingly, that by detecting partially unburned gases using a sensor comprising metal oxides semiconductors direct adsorption of gas without catalyzed chemical reaction, it was possible to detect a smoldering fire. Finally these sensors can be used in temperature ranges higher than the atmosphere but relatively low, namely between 150 and 350 ° C °.

A first preferred embodiment of a sensor according to the invention is characterized in that it comprises a heating element, in particular an electrical resistance, for heating the metal oxide at a temperature of between 150 and 350 ^° C. This makes it possible to heat the sensor and thus bring it into a plurality of ranges of use as a function of the chosen temperature. This plurality of ranges makes it possible to adjust the sensitivity of the sensor according to its end use, for example a detector for fire, tobacco smoke, etc.

A second preferred embodiment of a sensor according to the invention is characterized in that the metal oxide is chosen from tungsten oxide WO ₃ , chromium oxide Cr ₂ O ₃ , copper oxide CuO , lanthanum oxide La ₂ O ₃ , or certain oxides doubles such as Cr _x Ti _y θ3, or a mixture thereof. These metal oxides are widely distributed commercially.

A third preferred embodiment of a sensor according to the invention is characterized in that it is housed in a housing provided with a metal grid. The presence of the metal grid makes it possible to avoid the fluctuations due to the evolutions of atmosphere and thus ensures a better functioning of the sensor.

The invention also relates to a use of one or more metal oxides forming a direct adsorption gas-free semiconductor without catalyzed chemical reaction and whose electrical resistance changes as a function of the adsorbed gas to detect oxides of nitrogen in the case of a bright fire and / or to detect, in the case of a smoldering fire, partially unburned gases, in particular alcohols, aldehydes, ketones, carboxylic acids or even amines. The invention also relates to a method for manufacturing and calibrating such a sensor.

Finally, the invention relates to a method of operating a sensor according to the invention wherein the heating element is supplied with electric current either in continuous mode or in pulsed mode. These embodiments of these sensors thus make it possible to greatly limit energy consumption.

The invention will now be described in greater detail with the aid of the drawings in which: Figure 1a illustrates the adsorption and catalytic reaction by a metal oxide for oxygen and non-carbon monoxide; FIG. 1b illustrates the phenomenon of direct adsorption without catalytic reaction on a metal oxide for nitrogen oxide and formaldehyde; FIG. 2 illustrates the evolution of the resistance of a sensor according to the invention during a fire with a flame; FIG. 3 illustrates the evolution of the resistance of a sensor according to the invention during a flame-free fire; FIG. 4 illustrates the evolution of the resistance of a sensor according to the invention in the presence of a reducing gas; FIG. 5 illustrates the evolution of the resistance of a sensor according to the invention as a function of the temperature of the sensor; and Figures 6, 7 and 8 illustrate embodiments of sensors according to the invention.

In the drawings the same reference has been assigned to the same element or a similar element.

The sensors based on metal oxides, such as tin oxide SnO 2 or gallium Ga 2 O 3, generally work at temperatures above 400 ^° C. In the ambient atmosphere, their semi-conductivity is conditioned by the oxygen of the air that adsorbs on the surface. The figure illustrates that in the presence of a combustible gas, such as carbon monoxide (CO), hydrocarbons and partially oxidized hydrocarbons undergo on the surface of the sensor a catalytic combustion with oxygen previously adsorbed. In fact, when oxygen, present in the ambient air, comes into contact with the free electrons of the n-type metal oxide present on a substrate forming part of the sensor, this oxygen will be adsorbed and captured. free electrons

02 + 2 ^e - → 20 -

This results in a decrease in the number of free electrons and therefore in the amount of oxygen adsorbed on the surface. On the other hand, the electrical resistance of the sensor will increase. These sensors are to be classified among the so-called "catalytic" sensors based on metal oxides. In addition, when CO is present in the atmosphere, there will be a catalytic reaction with the oxygen that has been adsorbed CO + O ^" → CO2 + e ^~

This will again increase the number of free electrons and decrease the resistance of the sensor. The sensor according to the invention uses one or more metal oxides said to direct adsorption of the gas emitted by combustion without catalyzed chemical reaction, as described in the previous paragraph. The peculiarity of these metal oxides forming a "direct adsorption without catalyzed chemical reaction" semiconductor stems, on the one hand, from the fact that they are semiconductors not conditioned in the sense that the semiconductivity is due between others to stoichiometric and crystalline defects of the sensitive layer itself. The natural semi-conductivity is therefore not due to the presence of oxygen or atmospheric pollutants on the sensitive layer. As a result, in the case of direct adsorption semiconductors, the detection is due to a direct and unambiguous action of the gas to be detected on the semiconductor without prior catalyzed chemical reaction with the pre-adsorbed species. Figure 1b illustrates this phenomenon. When, for example, a nitrogen oxide (NO 2) comes into contact with the metal oxide of the sensor according to the invention, there will be capture of free electrons NO 2 + e → NO 2

The number of free electrons will decrease and the resistance of the sensor will increase. In case of presence of aldehyde (H2C = O) there will be a release of free electrons and a decrease in the resistance of the sensor.

Thus, for example, during a high heat and therefore a combustion with flames, the temperature is relatively high and by reaction of oxygen and atmospheric nitrogen, oxides of nitrogen are formed and especially NO2 . It has now been found, surprisingly, that once generated, this gas acts directly on the sensitive layer of the sensor without an intermediate catalyzed chemical reaction (FIG. 1b). Since nitrogen oxide NO2 is a strong oxidant, it has a free electron-sensing effect present in the semiconductor and in fact decreases the number of negative charge carriers in the case of a semiconductor of the type not. This results in a significant increase in the electrical resistance of the sensor. Conversely, in the case of a smoldering fire and therefore of a combustion without flames, the gases emitted by the fire are partially unburned. These gases are partially oxidized and include in particular alcohols, ketones, aldehydes, carboxylic acids, amines, etc. These molecules are electronically structured electron donors, which therefore directly increase the number of of negative charge carriers of a n-type semiconductor. The electrical resistance of the sensor therefore decreases in significant proportions. The direction of these evolutions of resistance is obviously reversed in the presence of a p-type semiconductor.

This direct action of the gases to be detected generally makes the unconditioned metal oxide sensors much more selective than the conditioned catalytic sensors. The invention is therefore based on the use of one or more metal oxides forming a direct adsorption gas semiconductor without catalyzed chemical reaction and whose electrical resistance changes as a function of the adsorbed gas to detect oxides of nitrogen in the case a bright fire and / or to detect, in the case of a smoldering fire, partially unburned gases, in particular alcohols, aldehydes, ketones, carboxylic acids or even amines.

The fact that the detection is effected by direct action of the gas without using catalytic combustion with oxygen also has very important consequences on the behavior of the sensors. Indeed, they can in particular be used at much lower temperatures, in particular in a range between 150 and 350 ^° C. This range is much lower than that used by the "catalytic" sensors with conventional metal oxides and which is between 400 and 900 ⁰ C. Among the sensors based on unconditioned metal oxides with direct adsorption of the gas to be detected, there may be mentioned, for example, tungsten oxide WO 3, chromium oxide Cr ₂ θ 3, oxide of copper CuO, or lanthanum oxide La2θ3, or even some double oxides such as Cr _x Ti _y θ3. These are semi-conductors that are not conditioned by the atmosphere and are therefore much less sensitive to natural atmospheric fluctuations. These oxides can be used alone, as a mixture or in superposed layers. In addition, given the lower working temperature than the usual sensors based on "catalytic" oxides, the response of the sensors is solely due to the adsorption and desorption equilibria of the combustion gases which modify their electrical resistance. The sensors thus produced require relatively simple electronics. It suffices, depending on the configuration of the sensor, to measure, for example, the environment with an electrical resistance of the order of 10 ⁵ Ω and to set two alarm thresholds, one at 10 ⁷ Ω for smoldering fires and the other at 10 ³ Ω for live fires. The lights used correspond to those described by European (EN54-7) and American (UL 268) standards. The electrical resistance therefore evolves by a factor of 100 (S ≈ 100) in one direction or the other as illustrated in FIGS. 2 and 3. FIGS. 2 and 3 illustrate the evolution of the resistance of a sensor according to FIG. invention during a fire with flame respectively a fire without flame. With a fire with flame the resistance will increase whereas with a fire without flame it will decrease, of course if the metal oxide used is of the type n. However over time the resistance will evolve very little in the usual atmospheres (S _ma χ ≈ 6). These changes in the atmosphere are still greatly diminished when the sensor is placed in a housing whose wall is provided with a metal grille (S _ma χ ≈ 2). Indeed, these fluctuations in the atmosphere are mainly due to a low sensitivity to ozone. As a result, when the sensor is placed in its housing, the ozone is largely destroyed on the walls and thus hardly affects the resistance of the sensor. The presence of this metal grid significantly reduces the fluctuations due to changes in atmosphere and thus ensures better operation of the sensor. It should also be noted that the sensors according to the invention are largely insensitive to fluctuations in ambient humidity. These sensors can therefore not only be used in usual conditions with significant fluctuations in humidity, but also in more specific applications: marine environment, dryers, saunas,

The fact of having sensors whose temperature is regulated also makes it possible to use them in places where the temperature conditions are between -50 and 300 ^° C., such as, for example, cold stores or the steel, cement industry. .. In these environments, the environment is often dusty and the ambient temperatures very diverse (low or high).

During the onset of a bright fire with flame (Figure 2), the sensors detect the appearance of nitrogen oxide NO2 which is always emitted under these conditions because the flame temperature is very high and in this case Nitrogen oxide is formed by the reaction of nitrogen and oxygen in the air. In the case of an n-type semiconductor, the presence of nitrogen oxide NO ₂ , which can reach levels of the order of a few ppm, reduces the number of surface charge carriers. semiconductor and the electrical resistance of the sensor therefore increases in very large proportions. For example, in the case of a normalized heptane fire, the NO ₂ content of nitrogen oxide measured by chemiluminescence reaches ~ 1 ppm. Figure 2 (dashed line) shows that the response of the sensor submitted to air at a NO2 content of 1 ppm is exactly similar. This gas can therefore be used during a sensor calibration procedure or during the sensitivity tests provided by the standards. It should be noted that in normal atmospheres, nitrogen oxide NO2 levels rarely exceed 50 ppb. At the onset of a smoldering fire (Figure 3), the temperatures reached by the outbreak are much lower and do not allow the formation of nitrogen oxide. In this case, combustions are incomplete and give rise in particular to the appearance of partially unburned gases including in particular ROH alcohols, RHCO aldehydes, RiR ₂ CO ketones or more or less oxidized R1R2R3N amines which are susceptible, by adsorbing on the sensor, to increase the number of negative charge carriers on the surface, which has the effect of greatly reducing the resistance of a sensor made with an n-type semiconductor. FIG. 4 shows that these different types of gas respectively presenting the alcohol, aldehyde, ketone and even amine functions have an effect that goes in the direction of a decrease in the electrical resistance of the sensors. Figure 4 indeed shows the response of the sensor to 1000 ppm injections of formaldehyde, acetone and ethanol. Smoldering fires emit all these types of gases at various levels depending on the nature of the fuel and the temperature of the furnace. All these gases cause an evolution of the resistance of the sensor in the same direction and their effects are cumulative. The detection for smoldering fires can not therefore be attributed to a single gas but to all of those having these types of function. These same partially unburned gases can also be used to calibrate the sensor. It is obvious that the directions of evolution of the resistance as a function of the type of focus are reversed in the presence of a p-type semiconductor.

Note also that the sensors concerned by the present invention are very specific vis-à-vis the gases emitted during a combustion (nitrogen oxides, aldehydes, ketones, ...). In fact, unlike the "catalytic" metal oxides used at higher temperatures (> 400 ° C.), such as, for example, tin oxide, they in no way react with combustible gases such as hydrogen and carbon monoxide. alkanes (methane, propane, ...). This behavior is due to the fact that the evolution of the resistance of the sensors is solely attributable to the adsorption of the combustion gases on the surface of the metal oxides and not to a catalytic reaction between them. combustion gas and oxygen previously adsorbed on the surface as is the case for "catalytic" oxides.

The choice of the nature of the semiconductor and the operating temperature also makes it possible to increase or decrease the sensitivity of the sensor to different types of fire (with or without flames) as shown in FIG. To enable this temperature adjustment of the sensor, the latter is equipped with a heating element, in particular an electrical resistance. The heating element is arranged to heat the semiconductor to a temperature of between 150 and 350 ^° C. Thus the sensors according to this preferred form of the invention can therefore be adapted to the particular circumstances of use of the sensor depending on the types of risks encountered and of the place (tunnels, car parks, industries, dusty environments, overheating of electric cables, ...). Figure 6 shows a general configuration of the sensors. They comprise an insulating support 1 consisting for example of alumina, of surface-oxidized silicon, of silicon provided with interlayer layers such as, for example, silicon nitride, or even other oxides or nitrides which are completely insulating with respect to resistances of the sensitive layers to be measured (R _SU p ort _P> "Rcouche sensitive) - When this condition is fulfilled, the nature of the support has little effect on performance, it determines that the embodiments and therefore consumption 'electric energy.

This support is conventionally provided with two interdigitated electrodes 2 made of a noble metal such as, for example, gold, platinum, etc., or even ruthenium oxide RuO 2. The realization of this circuit depends on the type of support used. For example, on alumina supports it can be produced by screen printing of inks containing the metals to be deposited or by deposition of the metal powder (dispersed in a solvent and spread out in a thin layer) followed by laser sintering. On silicon-based substrates, the electrodes can be made by the usual photolithography techniques used in the field of microelectronics.

The support is preferably also provided with the heating element formed by a heating resistor 3 consisting of either poly-silicon (also called polysilicon) or ruthenium oxide or a noble metal coil such as, for example, the platinum whose electrical resistance has an excellent coefficient of temperature (0.3% / ° K) that will accurately set the working temperature of the sensor. The deposition of this resistance can be achieved by the same techniques as those used for electrode deposition. This heating resistor can be located according to the configuration on one or the other face of the support. This heating element preferably comprises temperature control means arranged to, depending on the temperature of use, adjust the sensitivity of the sensor either to detect all types of fires, or to detect said smoldering fires, or to detect said fires . These adjustment means can be formed by using a variable resistance as electrical resistance or by varying the electric current supplying the electrical resistance. The surface comprising the electrodes is covered by the sensitive layer consisting of the adsorption semiconducting metal oxide (s) not conditioned by the ambient atmosphere. The deposits can be made by different techniques depending on their field of application. The nature of the metal oxides, the techniques used for the deposition of the sensitive layer and the conditions of use such as the temperature of the sensor can also be differentiated according to the field of application.

Indeed, as described in the exemplary embodiments, the fire detection is not reduced to fire risks only in the domestic sector or in the tertiary sector (building, administration, hospitals, hotels, etc.). It is also important to have sensors that can be used for example in dusty industrial environments, in covered car parks, tunnels, cold stores, etc.

The sensitive layer deposits can be made on the supports by screen printing of particular inks. These are obtained by the dispersion of the powder of direct-adsorption semiconducting metal oxides in an organic solvent which contains the appropriate additives (surfactants, thickeners, etc.). These additives in particular have the effect of keeping the solid particles in suspension and to prevent their coagulation. It is therefore important to fix their surface charge and electrical surface potential ζ so that they repel and do not agglomerate. The size of the oxide particles dispersed in these inks is between 0.005 and a few μ.

In a first embodiment according to the invention, the support consists of an Al ₂ θ 3 wafer 3 × 3 mm and 0.5 mm thick, the electrodes arranged in interdigital double combs for example having a width of 150 μm and a spacing of 200 μm. The heating resistor (heating) located on the same side is platinum and has a resistance at 25 ° C for example 17 Ω. Depending on the field of use, the temperature will be set between 150 and 350 ⁰ C which will correspond in the example chosen to resistances fixed by the electronic control respectively 25.6 and 33.8 Ω.

The sensitive layer is deposited either by a sol-gel process when the particle size is very small (<10 μm) or by screen printing of the appropriate ink containing in particular the direct adsorption semiconductor metal oxide (s). gas to be detected. The layer is then subjected to drying and thermal removal of the adjuvants of the ink. The sol-gel or screen-printing process can be repeated a number of times (one or more identical or different layers) depending on the sensitivity that it is desired to confer on it. When the temperature is set to an average value of

250 ⁰ C (heating = 11.7 Ω), the results obtained in this configuration are those described in Figures 2 and 3 which corresponds to sensitivities S of about 100 for smoldering and fires with flames.

When the temperature is set at a lower value, of the order of 200 ^° C., the sensitivity S is around 50 for smoldering fires and 200 for fires with flames.

Conversely, when the temperature is set at a higher value, of the order of 300 ° C, the sensitivity S is around 250 for smoldering fires and 30 for fires with flames.

Figure 5 illustrates the evolution of the sensitivity S as a function of the temperature T for these two types of fires. Depending on the type of risk to be covered and the location of use, the temperature can be adjusted to the most appropriate value.

In this first configuration, depending on the temperature to be achieved, the electrical energy consumption is around 300 mW.

In conventional fire detection, where no interference is desired and perfectly meet all types of standardized fires as described in Figures 2 and 3, up to six "adsorption" oxide layers can be deposited which determines the layer thickness which thus varies between, for example, 5 to 50 μ. The thickness is, in fact, a determining factor of the sensitivity of the sensor. For example, if one wishes to realize not a fire detector but rather a smoke detector, the sensitivity is increased by reducing, for example, to 1 the number of layers and maintaining the temperature in the area that increases sensitivity to smoldering fires. (high temperature). Under these conditions, the presence of a smoker is easily detected, since the sensitivity S reaches the value 30 in a few minutes for a cigarette half-consumed in a room of 6 x 4 x 4 m. Similarly, for sensors with low number of layers, the overheating (without visible combustion or visible smoke emission) of electrical cables can be easily detected. Indeed, although the actual combustion has not yet begun, the occluded gases in the cable sheath are degassed. Thus, for example, in an electrical cabinet of 0.5 x 1, 2 x 2 m, the simple overheating of an electric cable 10 cm long and 1.5 mm ² at 80 ⁰ C already causes a change in the resistance of the sensor by a factor of 100 (S = 100) in about 2 minutes. The results obtained are obviously identical if it is the overheating of a printed circuit rather than a cable or more generally the overheating of any electrical component comprising in its structure organic polymers.

In a second configuration (FIG. 5), the sensor is made on oxidized or nitrided silicon supports (4 = substrate, 1 = oxide or nitride). These 2 x 2 x 1.5 mm supports are hollowed out of the bulk of the silicon substrate on their underside so as to greatly reduce their thickness at the location where the electrodes and the heating resistor (3) will be arranged. These electrodes (2) are deposited by conventional photolithography techniques of microelectronics. In this case, either the upper surface of the sensor is roughened so that the oxide layers can be deposited as previously by screen printing, by a sol-gel process or by sputtering, ie the surface is smooth and only the two last techniques can be implemented. Sputtering is, for example, used when producing a tungsten oxide WO3 layer at a tungsten cathode and a low oxygen partial pressure.

For heating, another configuration is to encircle, on the upper face, the electrodes by a platinum coil or polysilicon. The sensors thus produced have the same performance as those made on AI2O3 alumina but the power consumption is reduced by a factor of fifteen (ie, for example, 20 mW).

Another way to further reduce energy consumption is to operate the sensor in a pulsed and non-continuous mode. In this case, it is a question of feeding the heating resistance of the sensor during, for example, 2 seconds every 10 or 20 seconds. The heating time and consequently the power consumption is greatly reduced and it is thus possible, depending on the heating and pause times chosen, to reduce the power consumption again by a factor of five to one-half.

In a third configuration shown in Figure 6 (not to scale), the silicon support (4) has a rectangular shape whose dimensions are, for example 2 x 2 mm. On this support, different layers of either oxide or silicon nitride are formed or deposited on the upper face 3. This support is also hollowed out on its underside to reduce its thickness to, for example, 5 microns. Above the excavated surface and in a rectangle of, for example, 50 x 100 microns, the electrodes 2, the heating resistor 1 and the sensitive layer are deposited as in the previous example. Finally a lamella corresponding to the rectangle where is located the sensitive layer is cut on 3 sides (Figure 7). The fourth side to establish the electrical contacts to the electrodes and the heating resistor. Under the effect of the mechanical tensions of expansion and compression of the layers of silicon, oxide and / or silicon nitride and under the effect of the temperature this lamella is raised.

As a result, when feeding the heating resistor in continuous or pulsed mode, only this extremely thin lamella is heated and the energy consumption is again strongly reduced (factor two to ten). The behavior of these sensors vis-à-vis different types of homes is obviously the same, because the sensitivity does not depend on the extent of the surface of the sensor.

All of these actions make it possible, depending on the risk to be covered and the field of application, to adapt the detector to the needs actually encountered.

Claims

claims

A gas sensor emitted by combustion, which sensor comprises one or more metal oxides forming an adsorption semiconductor whose electrical resistance changes as a function of the adsorbed gas, characterized in that said semiconductor is a direct adsorption semiconductor. of gas without catalyzed chemical reaction, which sensor is arranged to detect oxides of nitrogen in the case of a live fire.

2. A gas sensor emitted by a combustion, which sensor comprises one or more metal oxides forming an adsorption semiconductor whose electrical resistance changes as a function of the adsorbed gas, characterized in that said semiconductor is a semiconductor with direct adsorption of gas without catalyzed chemical reaction, which sensor is arranged to detect in the case of a smoldering fires partially unburned gases, in particular alcohols, aldehydes, ketones, carboxylic acids, or amines.

3. Gas sensor according to claim 1, characterized in that said semiconductor is a direct adsorption gas semiconductor without catalyzed chemical reaction, which sensor is arranged to detect in the case of a smoldering gas fires partially unburned, in particular alcohols, aldehydes, ketones, carboxylic acids or amines.

4. Sensor according to one of claims 1 to 3, characterized in that it comprises a heating element, in particular an electrical resistance, for heating the semiconductor to a temperature between 150 and 350 ⁰ C.

5. Gas sensor according to claim 4, characterized in that the heating element comprises temperature control means arranged to, depending on the temperature of use, adjust the sensitivity of the sensor is to detect all types of fires, either to detect said smoldering fires or to detect said fires.

6. Sensor according to claim 4 or 5, characterized in that the metal oxide and the heating element are chosen so as to allow the detection of overheating of cables or electrical components comprising organic polymers.

7. Sensor according to one of claims 1 to 6, characterized in that the metal oxide is selected from WO3 tungsten oxide, chromium oxide Cr ₂ θ 3, copper oxide CuO, oxide ₂ Os lanthanum, or certain double oxides such as Cr _x Ti _y θ3, or a mixture thereof.

8. Sensor according to one of claims 1 to 7, characterized in that the metal oxides are applied in superposed layers.

9. Sensor according to one of claims 1 to 8, characterized in that it is housed in a housing provided with a metal gate.

10. Sensor according to claims 1 to 9, characterized in that it is arranged to detect tobacco smoke.

11. Use of one or more metallic oxides forming a direct gas adsorption semiconductor without catalyzed chemical reaction and whose electrical resistance changes as a function of the adsorbed gas to detect oxides of nitrogen in the case of a live fire .

12. Use of one or more metal oxides forming a direct gas adsorption semiconductor without catalyzed chemical reaction and whose electrical resistance changes as a function of the adsorbed gas to detect, in the case of smoldering fires, partially unburned gases, in particular alcohols, aldehydes, ketones, carboxylic acids or amines.

13. Use according to claim 11 or 12, wherein the operating temperature of the semiconductor is adjustable at a temperature between 150 and 350 ⁰ C.

The method of manufacturing a combustion gas sensor according to one of claims 1 to 10, wherein at least one or more metal oxides forming an adsorption semiconductor whose electrical resistance changes according to the adsorbed gas are applied to a support, characterized in that, as an adsorption semiconductor, a direct adsorption semiconductor gases without catalyzed chemical reaction is applied.

15. The method of claim 14, characterized in that the metal oxide is deposited in one or more times by one of the ink-screen printing techniques, sol-gel process or by sputtering on an insulating support made of alumina or silicon. covered by layers of oxides or nitrides of silicon, which support is then provided with electrodes and a heating resistor for maintaining the temperature of the oxide layer at a temperature chosen between 150 and 350 ^° C.

16. A method of operating a sensor according to one of claims 4, 5 or 6, characterized in that the heating element is supplied with electric current either in continuous mode or in pulsed mode.

17. A method of calibrating a sensor according to one of claims 1 to 10, characterized in that the sensitivity of the sensor is calibrated by nitrogen oxides in the case of fires and by alcohols, aldehydes, ketones, caboxylic acids and amines in the case of smoldering fires.