PASSIVE HAZARDOUS GAS DETECTION DEVICE WITH ACTIVE MATERIAL BASED ON TRANSITION METAL OXIDE

Abstract

Passive gas detection structure, in particular for reducing gases such as hydrogen, comprising: - at least one porous element (5) having an "active" material (20) designed to react with the reducing gas, the active material (20) being formed of a metal oxide MyOx and a catalyst, with M a transition metal with the metal oxide MyOx being capable of undergoing reduction in the presence of said gas so as to produce heat release, - one or more thermosensitive visual indicators (30; 330A,…, 330F; 301, …, 304) disposed on the porous element (5), comprising one or more thermochromic and / or thermoluminescent regions, configured respectively to change color or hue, or to emit light following said heat release resulting from said reduction. Figure for the abstract: 1.

Description

Title of the invention: PASSIVE HAZARDOUS GAS DETECTION DEVICE WITH ACTIVE MATERIAL BASED ON TRANSITION METAL OXIDE. TECHNICAL FIELD AND PRIOR ART

[0001] The present invention relates to the field of hazardous gas detection and more specifically concerns passive devices for detecting such gases.

[0002] Reducing gases play an important role, particularly in various industrial processes. Some of these gases pose problems in terms of safety and risk management. In addition to the toxicity of some of them, such as carbon monoxide (CO) and hydrogen sulfide (H2S), reducing gases also exhibit significant flammability and / or explosive potential. For example, hydrogen (H2) and carbon monoxide (CO) can cause explosions if exposed to an ignition source. Leaks of certain gases, even in small quantities, can lead to fires or explosions, especially in confined environments with inadequate ventilation. Another significant hazard is the formation of explosive mixtures with air. Some gases are also capable of reacting with other chemical substances and creating dangerous reactions.

[0003] These dangers require constant monitoring and appropriate detection systems.

[0004] There is a need to develop new so-called "passive" detection devices, that is, devices without a power supply and even without electronic circuits or components, in order to limit maintenance operations and the risk of sparking. Such gas devices can allow operation even in the event of an accident with high levels of explosive gases such as hydrogen or methane.

[0005] Passive gas detection structures exist. Indeed, there are paints that react with certain toxic gases and change color in their presence. Glazing operating on a similar principle has also been developed.

[0006] However, such detection structures typically have the following disadvantages: a detection threshold, in terms of minimum detectable concentration, often too high, typically greater than 1%, and a response time that is too long, generally on the order of at least several hundred seconds.

[0007] In addition, some of these structures do not function as well, depending on whether one is in an inert / anoxic atmosphere, i.e. without the presence of oxygen, or whether one is in the presence of oxygen.

[0008] The problem arises of finding a new passive gas detection structure improved with respect to at least the disadvantages mentioned above. Description of the invention

[0009] It is therefore an object of the present invention to provide a passive gas detection structure comprising:

[0010] - at least one porous element comprising a so-called "active" material intended to react with the gas, the active material being formed of at least one metal oxide MyOxet and a catalyst, with M a transition metal and <y<4etl<x<5, l’oxyde métallique MyOx étant apte à réagir en présence dudit gaz de sorte à produire un dégagement de chaleur,

[0011] - one or more heat-sensitive visual indicators arranged on the porous element, comprising one or more thermochromic and / or thermoluminescent regions, configured respectively to change color or hue, or emit light radiation following said heat release resulting from said reduction.

[0012] By "porous element", it is understood that said element has an open porosity through which the gas can flow.

[0013] The passive detection structure applies in particular to the detection of dangerous gases, i.e. toxic(s) and / or explosive(s) and / or flammable(s).

[0014] The structure is particularly applicable to the detection of reducing gases. In this case, the metal oxide MOX is capable of undergoing reduction in the presence of said gas so as to produce a release of heat.

[0015] The detection structure can be applied particularly to one of the following reducing gases: H2,CO, H2S, SO2, NH3.

[0016] It can also be applied to the detection of CH4.

[0017] Advantageously, the catalyst can be a metal from group VIII or group I, of preference chosen from the following metals: Pt, Pd, Ni, Cu, Ag, Au.

[0018] According to one embodiment, the metal oxide MOX can be chosen from the following metal oxides: WO3, MoO3, CuO, Fe2O3, TiO2, ZnO, ZrO2, SnO2, NiO, CeO2.

[0019] Advantageously, when the gas to be detected is H2 or CO, the metal oxide can be based on WO3 and Pd or Pt as a catalyst.

[0020] Alternatively, when the gas to be detected is H2S or NH3, the metal oxide can be WO3 and the catalyst Au.

[0021] Preferably, in a case where the gas to be detected is SO2, the metal oxide can be based on WO3 and the catalyst on Ag.

[0022] The porous element can take various forms.

[0023] According to a first possible embodiment, it can be formed of porous supports joined together and between which the active material is integrated.

[0024] Alternatively, the porous element may comprise a porous substrate comprising the active material, the porous substrate being attached to at least one porous support on which said one or more thermosensitive visual indicators are fixed.

[0025] According to another variant, the porous element may comprise a porous support on which said one or more thermosensitive visual indicators are fixed, a porous substrate on which or in which the active material is integrated, the porous substrate being intercalated between the porous support and another porous support, said porous support, said porous substrate and said other porous support being stacked.

[0026] Advantageously, the "active" material is integrated in powder form into the porous element. This improves detection sensitivity.

[0027] According to an embodiment of the detection structure in which the porous element comprises an open-porosity porous support, the thermosensitive visual indicator(s) can be fixed to this support by means of an adhesive.

[0028] A particular embodiment provides for the porous element based on braided glass or SiC or a polymer material.

[0029] According to a particular embodiment of the structure, the porous element can be formed of at least one porous substrate based on a fusible material, in particular a polymer, the fusible material having a melting point lower than that of the flammability point of the gas to be detected. In the case where the gas is hydrogen, a fusible material is chosen having a melting point lower than that of the hydrogen flammability point. Melting of the substrate can help limit the risks of runaway exothermic reaction and ignition of the gas due to this runaway reaction.

[0030] Advantageously, the passive detection structure may include:

[0031] - at least one thermochromic region with a change of color or hue, reversible and,

[0032] - at least one thermochromic region with a change of color or hue, irreversible.

[0033] Advantageously, the passive detection structure may also include:

[0034] - at least a first thermochromic region having a first temperature activation,

[0035] - at least a second thermochromic region, of a different composition than the first thermochromic region and having a second activation temperature different from the first activation temperature.

[0036] The thermosensitive visual indicator(s) of the detection structure are typically arranged on a so-called "detection" face of the porous element.

[0037] According to one aspect, the porous element may include a so-called "buffer" zone in contact with the detection face, in which the content of sensitive material decreases as one approaches the detection face. Alternatively, this buffer zone does not contain any sensitive material. Brief description of the drawings

[0038] The present invention will be better understood on the basis of the following description and the accompanying drawings in which:

[0039] [Fig. 1] serves to illustrate an example of a passive gas detection structure, in particular for dangerous gases such as certain reducing gases, having an "active" material based on a metal oxide, intended to react with the gas by producing a release of heat associated with a thermosensitive indicator configured to change its appearance, in particular its color or hue, following said release of heat resulting from this reaction0;

[0040] [Fig.2] serves to illustrate an alternative embodiment with several indicators distinct thermosensitives having different respective activation temperatures.

[0041] [Fig.3] serves to illustrate a heat-sensitive label that can be used as thermosensitive indicator of the detection structure.

[0042] [Fig.4A] [Fig.4B] serve to illustrate another type of thermosensitive label that can be used as a thermosensitive indicator of the detection structure and comprising several distinct thermochromic regions.

[0043] [Fig.5A] [Fig.5B] [Fig.5C] serve to illustrate a manufacturing process for an example of a passive gas detection structure, in particular for reducing gas.

[0044] [Fig.6A] [Fig.6B] [Fig.6C] serve to illustrate another manufacturing process of another example of a passive gas detection structure.

[0045] [Fig.7] serves to illustrate a step in a process for manufacturing a passive gas detection structure consisting of spreading the active material in powder form between two porous supports.

[0046] [Fig.8] serves to illustrate a particular realization of a passive structure in in which the porous element is formed from a stack of porous supports, the active material being integrated into a porous substrate intercalated between these supports.

[0047] Identical, similar, or equivalent parts of the different figures bear the same numerical references so as to facilitate the transition from one figure to another. different parts represented in the figures are not necessarily shown on a uniform scale, in order to make the figures more legible.

[0048] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0049] An example of a passive gas detection structure implemented according to the invention is shown in [Fig.1].

[0050] By "passive," it is understood that this structure operates without a power supply. The detection structure can also operate without an electronic circuit, electronic component, or transduction means producing an electronic signal.

[0051] The gas detection structure is particularly applicable to the detection of hazardous gases, and is specifically designed for flammable, explosive, and / or toxic gases. The gas detection structure is particularly relevant for reducing gases such as, for example, H2 or one of the following reducing gases: CO, H2S, SO2, NH3. The structure can also be applied, for example, to the detection of CH4.

[0052] The gas detection structure has a porous element 5 represented schematically in [Fig.1] and which includes at least one porous substrate 11 in the form of at least one porous layer or a porous block coated or incorporating an "active" material 20.

[0053] In the particular embodiment illustrated, the porous substrate has a rectangular shape and a small thickness, for example between 10 µm and 1 cm. In a more advantageous range, the thickness of the porous substrate 11 is between 50 µm and 5 mm.

[0054] However, other geometries may be adopted.

[0055] The active material 20 is designed to react with the gas to be detected, such that this chemical reaction produces a release of heat. The chemical reaction exploited here is a redox reaction with heat release. The active material 20 typically comprises a metal oxide, and in particular a transition metal oxide MxOy (with M a transition metal such as, for example: W, Mo, Ti, Ce, Ni, Sn, Cu, and with x typically greater than or equal to 1 and less than 5), chosen so as to be able to be reduced in the presence of the gas to be detected.

[0056] For example, the metal oxide MyOx can be one of the following metal oxides: WO3, MoO3, CuO, Fe2O3, TiO2, ZnO, SnO2, NiO, CeO2.

[0057] Preferably, the active material 20 also comprises or is associated with a catalyst to enable the reaction to start at room temperature.

[0058] In a particular application example where the gas to be detected is dihydrogen, the following reactions can be implemented by the active material 20 of the detection structure: (with catalyst) Reaction 1 02^ 2My0xM + Reaction 2

[0059] with M the metal used, H(ad) the hydrogen dissociated by the catalyst, ArH° and ArH°' the enthalpies of the chemical reactions, i.e. the heat released (in kJ / mol).

[0060] In this particular example of hydrogen detection, a metal M is chosen such that the couple MyOx / MyOx i has a standard electrochemical potential E°(MyOx / MyOx i) between the standard potential of the H2O / H2 couple and that of the O2 / H2O couple, which are between -0.8 and +1.22 V / ESH respectively.

[0061] Advantageously, the metal M is chosen such that the standard electrochemical potential E°(My0x / My0x i) of the couple My0x / My0x i is in a range between -0.6 V / ESH and +1.0 V / ESH and even more advantageously in a range between -0.5 V / ESH and +0.5 V / ESH.

[0062] In general, apart from the electrochemical potential, other physicochemical characteristics can be taken into account when selecting the metal oxide MyOx: namely, the activation energy (Ea) of the reaction between the metal oxide (MyOx) and the gas to be detected, and the exothermicity of the reduction and oxidation (these reactions corresponding to reactions 1 and 2 given above). Typically, the lower the Ea value, the higher the reaction kinetics. Generally, the higher the exothermic nature of the reduction and oxidation of the metal couple, the more effective the detection principle.

[0063] In a particular embodiment where the gas to be detected is, for example, H2 and the active material 20 is, for example, tungsten trioxide (WO3), the following reaction can be implemented: [00641 WO W +H2^ Reaction 3

[0065] Such an active material 20 has the particularity and advantage of being able to react with dihydrogen even when the oxygen concentration is low, typically for dihydrogen concentrations between 1.81 and 0.03 % v / v, and which can thus be less than 0.5 % v / v.

[0066] Another advantage of such an active material 20 is that it allows reaction with the gas for low concentrations of that gas.

[0067] In the case of operation under an atmosphere containing oxygen, for example in ambient air, such an active material 20 has the advantage of being able to re-oxidize, which allows for passive reconditioning of the detector and thus for the measurement to be carried out several times.

[0068] According to a preferred embodiment, the active material 20 is a compound based on MyOx and a catalyst or is associated with a catalyst. This catalyst is typically metallic and based on at least one metal from group VIII or I, in particular selected from the following metals: Ru, Rh, Ir, Pd, Pt, Ni, Cu, Ag, Au.

[0069] The catalyst is typically chosen according to the metal oxide of the active material 20 and the gas to be detected.

[0070] For example, to detect H2 or CO, the active material 20 can be a mixture or compound of WO3 / Pt or WO3 / Pd.

[0071] A W03 / Au mixture can advantageously be provided when the gas to be detected is H2S or NH3.

[0072] To detect SO2, a mixture or compound of WO3 / Ag can be advantageously used.

[0073] Advantageously, the active material 20 can be in the form of a powder distributed throughout the porosity of the porous substrate 11. The open porosity then allows the passage of a gas while sequestering the powder. Alternatively, the active material 20 can be in the form of a coating covering the porous substrate.

[0074] An adhesive having good thermal conductivity properties, for example of at least 1 W / m / K, can be used to bond the active material to the porous substrate 11 while allowing optimal heat transfer. For example, a super glue doped with silver particles, such as RS PRO adhesive; ref #: 238-4458, with a thermal conductivity of 2 W / m / K, or a thermal adhesive such as a two-component epoxy adhesive loaded with metal oxide, such as RS Pro adhesive; ref #: 155-8320, with a thermal conductivity of 1.1 W / m / K, can be used.

[0075] By way of example, the porous substrate 11 may be in the form of one or more porous layers, of braided glass fiber, or based on porous SiC, or paper (cellulose), or at least one polymer, in particular a polymer foam.

[0076] The porous element 5 comprising an active material 20 is further coated, at the level of a first face 5A called "front face" with at least one thermosensitive visual indicator whose appearance is capable of being modified, reversibly or irreversibly, following the release of heat caused by the exothermic reaction between the reducing gas and the sensitive material 20.

[0077] Preferably, the porous element 5 has porosity and is permeable to gases, in particular to the gas to be detected and to oxygen, both at the level of its front face 5A of detection, and at its rear face 5B opposite to the front face 5A.

[0078] According to a particular embodiment, the thermosensitive indicator may comprise or be in the form of at least one thermochromic region 30.

[0079] The thermochromic region 30 is in this case based on a material or substance designed to change color or shade when subjected to a change in temperature and in particular a release of heat resulting from a reaction between the detected reducing gas and the active material 20. For example, the thermochromic region 30 may be based on at least one of the following types of compounds: spirolactones, fluorans, spiropyrans, or fulgides.

[0080] To enable the change of color or hue, the heat release is such that a so-called "activation temperature" of the thermochromic region 30, specific to the material or substance from which this region 30 is formed, is reached or exceeded. "Change of color or hue" here refers to a transition from a first hue to a second hue different from the first hue, or from a first color to a second color different from the first color, and also includes a change between a colored form and a colorless form, and vice versa.

[0081] The thermochromic region 30 is designed with a minimum activation temperature, i.e., the minimum temperature at which it is likely to change color, that is higher than a maximum ambient temperature at which the porous element 5 is likely to be placed in order to avoid any false detection. For example, the minimum activation temperature could be between 40 and 60 °C. Alternatively, the thermochromic region 30 can be chosen with an activation temperature low enough not to damage the detection structure.

[0082] The composition of the thermochromic region 30 can be such that it allows for a reversible change of hue or color. When subjected to sufficient heating resulting from a reaction between a gas and the active material 20, causing it to exceed its activation temperature, the region 30 is then capable of changing its hue or color from a first, initial hue or color to a second color or hue. Then, when the temperature of the thermochromic region 30 drops below its activation temperature, it will change its hue or color again, returning to the first, initial hue or color. In the case of a thermochromic region 30 with reversible color change, the detection structure has the particular advantage of being reusable.

[0083] The quantity of active material 20 is preferably adapted to reach the activation temperature of the thermochromic region 30 at a given predetermined gas concentration.

[0084] As a specific embodiment, a support formed of two glass fiber filters with a diameter of approximately 4 cm, between which 40 to 50 mg of active material based on a metal oxide doped with a catalyst, such as WO3 / Pt, CuO / Pd, CuO / Pt, or MoO3 / Pt, is introduced, can be heated to a temperature, for example, of approximately 140 °C for 1.8% v / v H2. If we consider a linear variation that passes through 20 °C at 0% H2, a temperature of 80 °C for 0.9% v / v H2 can be reached.

[0085] In the case of a thermochromic region 30 with irreversible or permanent color change, once activated, the color or hue remains the same, even when the temperature falls below the activation temperature and returns, for example, to ambient temperature. The detection structure in this case has the advantage of being able to memorize a point leak detection (i.e., of limited duration) of gas by retaining a trace of leak detection that occurred in the past but has since been resolved or stopped.

[0086] Advantageously, the thermochromic region 30 can be provided with a composition enabling it to adopt a number of distinct tints or colors greater than 2, in order to identify more than 2 distinct temperature ranges and thus to detect more than two ranges of gas concentration to be detected.

[0087] According to a particular embodiment, the thermochromic region 30 can be in the form of a thermosensitive paint which can be based, for example, on spirolactone(s), and / or fluoran(s), and / or spiropyran(s), and / or fulgide(s).

[0088] The thermochromic region 30 can also be in the form of a layer or a label or a patch or a ribbon comprising at least one thermochromic dye.

[0089] The thermochromic dye can be based on compounds such as spirolactones, fluorans, spiropyrans, or fulgides. Such dyes generally alternate between a colored and a colorless state depending on the pH of the medium. In an acidic environment, the protonated forms predominate, while in a basic environment, the reduced forms become predominant. This equilibrium can shift with temperature, particularly in the presence of other components. These dyes define the hue of the product in its colored state and exist in several basic colors. When several dyes are combined, the resulting color is a mixture of the hues of each. A thermochromic dye can also be combined with another dye or other colorants. conventional pigments. Thus, when the thermochromic dye fades, it reveals the color of the underlying conventional dye or pigment.

[0090] The label or patch or ribbon containing a thermochromic dye is arranged on the porous element 5 and preferably fixed to this porous element 5 for example by gluing.

[0091] An adhesive having good thermal resistance, for example such as an epoxy adhesive, can be used here.

[0092] In the particular embodiment shown in [Fig. 1], the thermochromic region 30 is not directly in contact with the porous substrate 11 or the active material 20 on or in that substrate. The thermochromic region 30 is instead bonded, and in particular glued, to another support 22, preferably also porous, itself placed against the porous substrate 11 and possibly glued to the porous substrate 11. Like the porous substrate 11, the support 12 can form a filter, and / or can be, for example, in the form of at least a thin layer of braided fiberglass, or a fiberglass filter, or a paper filter, or a foam. The support 12 may optionally be identical to the porous substrate 11 incorporating the active material 20.

[0093] The support 12 may include at least one area without active material which serves as an intermediate buffer region between the active material 20 and the thermochromic region 30 in order to limit heating of the latter and not to deteriorate it.

[0094] In the particular embodiment illustrated in [Fig.1] where the porous substrate 11 and the porous support 12 have a flat appearance, the stacking of these elements 11,12 forms a closed, flat, gas-permeable assembly.

[0095] In an alternative embodiment illustrated in [Fig.8], the porous substrate 11 is arranged between two porous supports 12, 12', the porous substrate 11 and the supports 12, 12' being stacked.

[0096] The substrate 11 at the center of the stack has the largest quantity of active material 20. A thermochromic region is disposed on one face of a porous support 12, this face forming the front face 5A, also called the "detection face" of the structure. As one moves away from the substrate 11 and towards this front face 5A of the detection structure, the quantity of active material is designed to decrease. Preferably, a buffer zone without active material is provided in the support near the front face 5A.

[0097] Such an arrangement, with a decreasing quantity of active material as one approaches the thermochromic region 30, allows for a gradual consumption of the detected gas and, for example, in the case where this gas is hydrogen or another flammable gas, limits a possible runaway reaction. In the support 12, where the quantity of active material is The weaker the gas detected, the more slowly it is consumed, and as diffusion progresses through the stack, the efficiency increases but the concentration of gas detected decreases.

[0098] An alternative embodiment illustrated in [Fig.2] provides for a passive gas detection structure, this time comprising a plurality of distinct thermochromic regions 30i, 302, 303, 304, for example in the form of several thermosensitive labels or dye layers.

[0099] Regions 30i, 302, 303, 304 may advantageously have distinct compositions, giving them distinct activation temperatures. Thus, a first thermochromic region 30i, having a first activation temperature, may be designed to change hue or color when subjected to a given temperature above this first activation temperature, while a second thermochromic region 302, having a second activation temperature, may be designed to change hue or color when subjected to a temperature above a second activation temperature, distinct from the first activation temperature and, for example, higher than the first activation temperature. For example, the difference between the respective activation temperatures may be several degrees or several tens of degrees Celsius.

[0100] When the second activation temperature is higher than the first activation temperature, the second thermochromic region 302 can retain its hue or color if the given temperature to which it is subjected remains between the first activation temperature and the second activation temperature.

[0101] Regions 30i, 302, 303, 304 may all be irreversibly hue or color change or all be reversibly hue or color change.

[0102] Alternatively, it is also possible to associate both one or more thermochromic regions with the irreversible change of color or tint, and one or more thermochromic regions with the reversible change of color or tint.

[0103] This allows both the preservation of a record of past hazardous gas detections and the ability to indicate that the presence of this hazardous gas has disappeared, while also being ready to perform a new hazardous gas detection. It thus combines the ability to maintain a leak history with the ability to detect a leak in real time.

[0104] The thermochromic region(s) may be provided or associated with a temperature threshold indication or a temperature range(s) to allow, in association with a particular color or a particular shade in which the thermochromic region 30 is located, the indication of a temperature threshold or a temperature range reached during the exothermic reaction between the sensitive material and the gas reacting with that material.

[0105] A particular embodiment of a thermochromic region is shown in [Fig. 3] and takes the form of a label 300 with a thermosensitive dye zone 302, similar to a label marketed by TH-industrie under the name "traffic light I THERMAX". In a first "normal" state, in the absence of gas to be detected or when the gas concentration is below a detection threshold, the label 300 adopts a first tint or color, for example, green. The label 300 may be provided with a first indicator 311, either explanatory or serving as a legend, associating the first tint or color with a first temperature range, for example, below 50°C.The label includes another indicator 312, associating a second shade or color, for example yellow or orange, with a second temperature range, for example between 50°C and 70°C, corresponding to a temperature likely to be reached when the hazardous gas is detected in small quantities. An indicator 313 associates a third shade or color, for example red, with a third temperature range, for example above 70°C, which corresponds to the detection of hazardous gas above a certain threshold.

[0106] According to another particular embodiment example, it is possible to use a heat-sensitive tape with k temperature thresholds (with k > 1) and of the THERMAX type as developed by the company TH-industrie or a thermal tape as developed by the company RS Pro.

[0107] In figures 4A and 4B, a 300' thermosensitive label similar to a 6 temperature indicator from the THERMAX range developed by TH-industrie is this time equipped with six distinct thermochromic regions 330A, 330B, 330C, 330D, 330E, 330F each associated with a temperature threshold indicator.

[0108] In [Fig. 4A], regions 330A, 330B, 330C, 330D, 330E, and 330F have the same hue or color, for example, a beige or orange color, indicating that several temperature thresholds, tA, tB, te, tD, tE, and tF, have been exceeded. These thresholds correspond respectively to different gas concentration thresholds. The temperatures, tA, tB, tC, tD, tE, and tE, can be, for example, temperatures between 30 °C and 300 °C.

[0109] In [Fig. 4B], only two regions, 330A and 330B, have the given hue, indicating that the temperature thresholds tA and tB have been exceeded. Regions 330C, 330D, 330E, and 330F, with a different hue or color than the given color, indicate that a temperature threshold tc has not been exceeded or has not yet been exceeded. Thus, a gas has also been detected here, but with a gas concentration lower than that indicated by the label in [Fig. 4A].

[0110] As an alternative or in combination with a thermochromic region 30 whose hue or color changes according to temperature, an indicator made of a thermoluminescent material or substance may also be provided, that is to say, one whose luminescence (light emission) varies in response to a temperature change. This thermoluminescent material may be based on barite, calcite, celestine, cryolite, danburite, fluorite, or sphalerite.

[0111] The active material 20 with metal oxide and catalyst intended to react with the gas can be produced for example using a sol-gel type method.

[0112] An example of a process for manufacturing active material by the Sol-Gel method will now be given, in a particular case where the metal oxide is WO3 and the catalyst is Pt, the active material being obtained here in powder form.

[0113] According to a first step, a solution containing a metal oxide is acidified. This acidification can be carried out using, for example, a cation exchange resin which is brought into contact with the solution. Acidification of a 13 mL 0.5 M sodium tungstate (Na2WO4) solution can be performed to obtain a solution containing a precursor, in this case H2WO4 aq.

[0114] The solution containing this precursor is recovered, to which another solution containing a catalyst is added. In a particular example, this other solution could be a solution of H2PtCl6. A volume of 4 mL at 0.125 M could, for example, be used, resulting in an atomic proportion of 1:13 of catalyst relative to the metal oxide WO3. Such a proportion corresponds to an optimal proportion for a good reaction while minimizing the amount of catalyst to be used. A solvent, typically organic, is then added. This solvent could, for example, be ethanol, in a volume that could be, for example, on the order of 8 mL for the aforementioned volumes of precursor and catalyst solutions, respectively.

[0115] Obtaining the active coating in powder form can be achieved by evaporation of the solvent. To accelerate the evaporation process, the solution can be heated to a temperature, for example, between 50°C and 60°C and / or placed under vacuum using an evaporator, for example, of the rotary type.

[0116] Such a process typically includes a coating drying step. This drying can be carried out in ambient air or at a temperature, for example, between 50°C and 60°C in order to accelerate the process, for example for a period of several hours, for example 2 hours.

[0117] A calcination step is then typically carried out. The purpose of this step is multiple. First, the tungsten trioxide is converted into crystalline form and vacancies are created. The presence of such vacancies is favorable to reactivity of the material. Finally, if the catalyst is introduced as H2PtCl6 or any other form in solution, calcination is carried out to break the bonds between the chlorine and platinum and to reduce the platinum to its metallic, or catalytically active, form. Such heat treatment is typically performed at a temperature of several hundred degrees Celsius, for example, typically between 300°C and 650°C, for a duration typically of at least several tens of minutes, for example, typically between 10 and 90 minutes. Advantageously, such treatment is carried out between 450°C and 550°C for a duration of between 30 and 60 minutes.

[0118] The substance obtained at the end of these steps can then be ground to form a powder.

[0119] Another example of an active material embodiment in a particular case where the active material is composed of CuO for the metal oxide and Pt involves starting with a CuCl2 powder (CAS#: 7447-39-4) which is dissolved. For example, 6.8 g of CuCl2 can be dissolved in 50 mL of deionized water to obtain a concentration of 0.1 M.

[0120] This solution is then made basic. To do this, another basic solution is prepared, for example, sodium hydroxide (NaOH IM) with 4 g of NaOH in 100 mL of deionized water. The basic solution can then be added dropwise to the precursor solution until a very basic pH is reached, for example, of 14. A blue precipitate of Cu(OH)2 is obtained, which is added to the water until a pH close to neutral or neutral (pH = 7) is reached. The Cu(OH)2 precipitate is then filtered, for example, using a vacuum filtration device, such as a Buchner filter.

[0121] Alternatively, before filtration, the Cu(OH)2 precipitate obtained can be suspended and then passed through ultrasound while being heated to a temperature, for example, 80°C. The ultrasound treatment can be carried out for a period of several tens of minutes, for example, 11 to 30 minutes. Next, filtration is performed using a Büchner funnel. Finally, drying is carried out at a higher temperature, for example, 200°C, for a period of, for example, one hour.

[0122] As an alternative or complement to ultrasound, the precipitate can be calcined at a temperature, for example, T=400°C for a duration of, for example, one hour. The catalyst can then be added either in liquid or solid form as a powder. When added in solid form as a powder, a mixture in a given proportion is prepared and then stirred using a dedicated powder stirring system. When added in liquid form, a precursor of the catalyst can be, for example, H2PtCl6. Dissolved in water or pure ethanol, the liquid is brought into contact with the powder, and the mixture is then evaporated. Evaporation can be accelerated by heating to a temperature of approximately 50 to 75 °C, depending on the solvent, using a rotary evaporator or a desiccator with a primary vacuum. A further calcination step can be carried out at a temperature typically between 300 and 650 °C for a duration of, for example, several tens of minutes. When the addition is made in solid form, a calcination step is more optional and can be included depending on the catalyst. In the case of a metallic catalyst, calcination is optional. Calcination is performed when the catalyst is, for example, in the form of H2PtCl6.

[0123] Another method for producing the active coating can be implemented electrochemically on an electrically conductive or conductive substrate. A non-conductive substrate can be made conductive by depositing, for example, a thin layer, for example on the order of several nanometers, of carbon or metal by physical vapor deposition (PVD). A graphite spray or a conductive varnish can also be used to form a thin conductive layer on the substrate.

[0124] The active coating on the substrate can be produced by electrochemical means, in particular using an electroplating device.

[0125] A bilayer coating or a layer based on a transition metal such as tungsten or copper with a noble metal such as platinum or palladium can be produced to create the active coating. A bilayer coating, consisting of a first layer of transition metal such as, for example, Cu or W with a thickness of, for example, between 1 and 200 µm, and a second layer of noble metal such as, for example, Pt or Pd with a thickness, for example, between several nanometers and several hundred nanometers, can be produced. Such a bilayer coating can be produced, for example, by successive immersions in two baths and the use of an electrochemical technique such as, for example, electroplating.

[0126] Alternatively, a so-called "mixed" coating can be produced by electrochemical method using a mixture of suitable electrolyte solutions, or by multiple alternating dips in a first electrochemical bath and in a second electrochemical bath.

[0127] By way of example, a two-layer or mixed coating can be produced using a protocol such as the following:

[0128] An electrically conductive or made conductive substrate is fixed with an electrical contact on a cathode and introduced into an electrochemical aqueous solution formed for example of CuSO4,5H2O from 150 to 250 g / L and H2SO4 from 15 to 100 g / L. The anode is formed, for example, from thick copper wire placed around the substrate to obtain a homogeneous deposit. For the deposition of a noble metal such as platinum, the substrate is kept in contact with the cathode and introduced into an aqueous electrolytic solution based, for example, on PtCl6Na2 at a concentration of 23 to 58 g / L and HCl at a concentration of 10 to 390 g / L. The anode is replaced by a platinum wire and positioned around the substrate to achieve a homogeneous deposit.

[0129] Once the bilayer or mixed coating is obtained, the presence of oxygen in the air can lead to oxidation of the transition metal. The presence of a noble metal such as platinum allows for a rapid oxidation reaction at room temperature. Optionally, the coating can be heat-treated at a temperature that depends on the transition metal, the thickness of the active coating, the composition of the substrate, and its thermal resistance. The heat treatment can be carried out at a temperature, for example, between 150°C and 400°C for a duration, for example, between 10 and 90 minutes.

[0130] Steps of an example of a manufacturing process for a passive detection structure are schematically illustrated in Figures 5A to 5C.

[0131] The active material is first deposited on a porous substrate 11, in this example by dipping ("Dip-Coating" according to Anglo-Saxon terminology) in a solution 501 ([Fig.5A]).

[0132] The porous substrate 11, once functionalized, is then subjected to a heat treatment of its coating, in particular calcination at a temperature, for example, between 300 and 600 °C, for a duration, for example, between

[0133] 15 min and 1 h 30. Advantageously, this calcination is carried out at a temperature between 450 and 550 °C and for a duration of between 30 min and 1 h. Such treatment can be carried out for example in a 503 oven ([Fig.5B]).

[0134] Next, the functionalized and calcined porous substrate 11 is placed between a first support 12 and a second support 12', both also porous and advantageously having a larger surface area than the porous substrate 11. The first porous support 12, the porous substrate 11, and the second porous support 12' are stacked.

[0135] This stack can be joined by means of an adhesive applied to a peripheral area of ​​the first support 12 and / or the second support 12'. The attachment can be achieved using an adhesive with good heat resistance, such as an epoxy adhesive. For example, DURALCO™ 4538 adhesive (FINAL Advanced Materials; # ref: 1ADH001812), which withstands temperatures up to 235°C, can be used. A methane methacrylate adhesive, such as LOCTITE® 402 adhesive (RS PRO; # ref: 243-2663), can be used for applications up to 150°C. A thermochromic region 30, for example in the form of a heat-sensitive dye label, is then attached ([Fig. 5C]) to one face of a porous supports 12, 12' between which the porous substrate 11 is arranged. This fixing can be achieved using an adhesive, for example such as that used to bond supports 12 and 12'.

[0136] According to another possible embodiment of the detection structure, the porous substrate 110 onto which the active material is applied can itself result from the assembly of several elements or layers, for example, two porous supports, in particular flexible supports. The active material can then be arranged between these supports.

[0137] Thus, in the particular embodiment illustrated in Figures 6A to 6C, the supports 120, 120' porous, for example, are first placed one on top of the other and fixed to one another by applying glue in a peripheral or perimeter area of ​​at least one face of at least one of the supports 120, 120'.

[0138] The assembly of the porous supports 120, 120' ([Fig.6A]) is preferably carried out in such a way as to preserve an access zone 607 between the supports 120, 120'. For this purpose, the glue can be applied at several points while keeping at least one part unglued.

[0139] The active material is then applied ([Fig. 6B]) while maintaining a space between the respective edges of the supports 120, 120'. The active material can be, for example, in the form of a viscous liquid paste 602 which is injected into this space. The paste can result from mixing an active powder, for example, a WO3 and Pt powder, the preparation of which has been described previously, with a solvent, in particular an organic one, such as, for example, pure acetone.

[0140] A thermochromic region 30 for example in the form of a thermosensitive dye label is then fixed ([Fig.6C]) on one face of one of the porous supports 120, 120'.

[0141] According to a variant ([Fig. 7]) of the example described above, rather than dispensing the active material in liquid form, it can be introduced directly in powder form. The supports 120, 120' are then re-joined to reduce or eliminate the space between them, and the stack of supports 120, 120' is then agitated to ensure that the powder is evenly distributed.

[0142] The support(s) 12, 12', 120, 120' and / or the porous substrate 11 used in either of the examples described above may be, for example, made of fiberglass to allow resistance to high temperature, while having high porosity and low thermal conductivity.

[0143] According to another example, supports 120, 120', 12, 12' and / or the porous SiC-based substrate 11 can also be provided. Such a material has the advantage of having good thermal conductivity and exhibits good thermal resistance.

[0144] According to another possible embodiment, supports 120, 120' made of cellulose acetate, or based on polyimide or a porous polymer, in particular a biopolymer such as cellulose with relatively high temperature resistance, typically between 150 and 380 °C, can also be provided. The use of a polymer with temperature resistance in the aforementioned range is advantageous because it allows for the implementation of passive safety, particularly when the detected gas is a flammable gas such as hydrogen.

[0145] In this case, when the concentration of hazardous gas is too high, the active material begins to heat up. If the temperature reached exceeds the melting point of the polymer, the substrate and / or supports based on such a polymer may liquefy, which can limit or even stop the exothermic reaction. For example, in the case of a sensor whose substrate is a polymer with a melting point between 150 and 380 °C, such as cellulose or polyimide, this melting point remains well below the auto-ignition temperature of hydrogen, typically around 572 °C.

Claims

Demands

1. Passive gas detection structure comprising: - at least one porous element (5) having an "active" material (20) intended to react with the gas, the active material (20) being formed of a metal oxide MyOx and a catalyst, with M a transition metal of which ye[l; 4[ and 1 < x < 5, the metal oxide MyOx being capable of reacting in the presence of said gas so as to produce a release of heat, - one or more thermosensitive visual indicators (30; 330A,..., 330F; 30i, • 304) disposed on the porous element (5), comprising one or more thermochromic and / or thermoluminescent regions, configured respectively, to change color or tint, or emit light radiation following said release of heat resulting from the reaction between the active material (20) and said gas.

2. Structure according to claim 1, wherein the gas is a toxic and / or explosive and / or flammable gas.

3. Structure according to any one of claims 1 or 2, wherein the gas is a reducing gas, the metal oxide MyOx being capable of undergoing reduction in the presence of said gas.

4. Structure according to any one of claims 1 to 3, wherein the gas is one of the following gases: H2,CO, H2S, CH4, SO2, NH3.

5. Structure according to any one of the preceding claims, wherein the catalyst is a metal of group VIII or group I, preferably selected from the following metals: Pt, Pd, Ni, Cu, Ag, Au.

6. Structure according to any one of claims 1 to 5, wherein the metal oxide MyOx is selected from: WO3, MoO3, CuO, Fe2O3, TiO2, ZnO, SnO2, NiO, CeO2, ZrO2.

7. Structure according to any one of claims 1 to 6, - wherein the gas is H2 or CO, the metal oxide being WO3 and the catalyst being Pd or Pt, or - wherein the gas is H2S or NH3, the metal oxide being WO3 and the catalyst being Au, or - wherein the gas is SO2, the metal oxide being WO3 and the catalyst being Ag.

8. Structure according to any one of claims 1 to 7, wherein the porous element (5) comprises: - porous supports (120, 120'; 12, 12') joined together between which the active material (20) is integrated, or - a porous substrate (11) comprising the active material (20), the porous substrate (11) being joined to at least one porous support (12) on which said one or more thermosensitive visual indicators are fixed, or - a porous support (120) on which said one or more thermosensitive visual indicators are fixed, a porous substrate (110) on which or in which the active material (20) is integrated, the porous substrate being intercalated between which the porous support and another porous support (120, 120'), said porous support, said porous substrate and said other porous support being stacked.

9. Structure according to any one of claims 1 to 8, wherein active material (20) is integrated in powder form into the porous element (5).

10. Structure according to any one of claims 1 to 9, wherein said porous element (5) comprises a porous support (12, 110, 120) with open porosity on which said one or more thermosensitive visual indicators are fixed, in particular by means of an adhesive.

11. Structure according to any one of the preceding claims, the porous element (5) being formed of at least one porous substrate (11) made of glass, or SiC, or based on polymer material.

12. Structure according to any one of the preceding claims, wherein the porous element (5) is formed of at least one porous substrate (11) based on a fusible material, in particular a polymer, having a melting temperature below a gas ignition point.

13. Structure according to any one of the preceding claims, comprising: - at least one reversible thermochromic change region (30i) of color or tint, and - at least one irreversible thermochromic change region (302).

14. Detection structure according to any one of claims 1 to 13, - at least a first thermochromic region (30i) having a first activation temperature, - at least a second thermochromic region (302), of different composition from the first thermochromic region and having a second activation temperature different from the first activation temperature.

15. Detection structure according to any one of claims 1 to 12, wherein the thermosensitive visual indicator(s) are disposed on a so-called "detection" face (5A) of the porous element (5), the porous element comprising a buffer zone in contact with the detection face (5A) in which the content of sensitive material (20) decreases as one approaches the detection face or which does not comprise sensitive material (20).