Passive device for detecting hazardous gas, having an active material based on transition metal oxide

Abstract

Structure for passive detection of gas, in particular of reducing gas such as hydrogen, comprising: - at least one porous element (5) comprising an "active" material (20) provided for reacting with the reducing gas, the active material (20) being formed of a metal oxide MyOx and of a catalyst, with M being a transition metal with y ϵ [1; 4] and 1 < x < 5, the metal oxide MyOx being capable of undergoing a reduction in the presence of said gas so as to produce a release of heat, - one or more heat-sensitive visual indicators (30; 330A,..., 330F; 301,..., 304) arranged on the porous element (5), comprising one or more thermochromic and / or thermoluminescent regions, configured to change color or hue, respectively, or to emit light radiation following said release of heat resulting from said reduction.

Description

[0001] PASSIVE HAZARDOUS GAS DETECTION DEVICE WITH ACTIVE MATERIAL BASED ON TRANSITION METAL OXIDE

[0002] DESCRIPTION

[0003] TECHNICAL FIELD AND PREVIOUS ART

[0004] The present invention relates to the field of hazardous gas detection and more specifically concerns passive devices for detecting such gases.

[0005] Reducing gases play an important role in various industrial processes. Some of these gases pose safety and risk management challenges. Besides the toxicity of some, 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, particularly 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 chemicals to create dangerous reactions.

[0006] These dangers require constant monitoring and appropriate detection systems.

[0007] 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 and the risk of sparks. Such gas detection devices can operate even in the event of an accident involving high levels of explosive gases such as hydrogen or methane.

[0008] Passive gas detection systems exist. Paints exist that react with certain toxic gases and change color in their presence. Glazing that operates on a similar principle has also been developed.

[0009] However, such detection structures typically have disadvantages such as: 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.

[0010] Furthermore, 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.

[0011] The problem arises of finding a new passive gas detection structure that is improved with respect to at least the disadvantages mentioned above.

[0012] DESCRIPTION OF THE INVENTION

[0013] It is therefore an object of the present invention to provide a passive gas detection structure comprising:

[0014] - at least one porous element comprising a so-called "active" material designed to react with the gas, the active material being formed of at least one metal oxide M y O x and a catalyst, with M a transition metal and l < y < 4 and l < x < 5, the metal oxide MyOx being capable of reacting in the presence of said gas so as to produce a release of heat,

[0015] - one or more thermosensitive visual indicators arranged on the porous element, comprising one or more thermochromic and / or thermoluminescent regions, configured respectively to change color or tint, or emit light radiation following said heat release resulting from said reduction.

[0016] Advantageously, the passive detection structure comprises: at least a first thermochromic or thermoluminescent region having a first activation temperature and at least a second thermochromic or thermoluminescent region, of different composition from the first thermochromic region and having a second activation temperature different from the first activation temperature and in particular higher than the first activation temperature.

[0017] In the case of thermochromic regions, the first thermochromic region is configured so that following an exceedance of the first activation temperature, the first thermochromic region changes from a so-called "starting" hue or color to a first "detection" hue or color, while the second thermochromic region is configured so that following an exceedance of the second activation temperature, the second thermochromic region changes hue or color and changes to a second detection hue or color, different from the first detection hue or color of the first thermochromic region.

[0018] In the case of thermoluminescent regions, the first thermoluminescent region is configured so that following an exceedance of the first activation temperature, the first thermoluminescent region emits light radiation while the second thermoluminescent region does not emit light radiation, the second thermoluminescent region being configured so that following an exceedance of the second activation temperature, the second thermoluminescent region emits light radiation.

[0019] This allows for more precise detection at different concentration levels and enables semi-quantitative measurement.

[0020] Alternatively, or in combination, among these regions, the detection structure may include a first thermochromic region having a first activation temperature and a second activation temperature higher than the first activation temperature. The first thermochromic region is configured such that, upon exceeding the first activation temperature, the first region changes from a so-called "starting" hue or color to a first "detection" hue or color, and upon exceeding the second activation temperature, it changes from the first detection hue or color to a second detection hue or color, different from the first detection hue or color. "Porous element" means that said element has an open porosity through which the gas can flow.

[0021] Passive detection structures are particularly applicable to the detection of hazardous gases, i.e. toxic and / or explosive and / or flammable.

[0022] The structure is particularly applicable to the detection of reducing gases. In this case, the metal oxide M y O x is capable of undergoing reduction in the presence of said gas so as to produce a release of heat.

[0023] The detection structure can be applied particularly to one of the following reducing gases: H2, CO, H2S, SO2, NH3.

[0024] Advantageously, the catalyst can be a metal from group VIII or group I, preferably chosen from the following metals: Pt, Pd, Ni, Cu, Ag, Au.

[0025] According to one possible embodiment, the metal oxide M y O x can be chosen from the following metal oxides: WO3, MOO3, CuO, Fe2Û3, TiÛ2, ZnO, ZrO2, SnÛ2, NiO, CeÛ2, V2O5.

[0026] 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.

[0027] Alternatively, when the gas to be detected is H2S or NH3, the metal oxide can be WOs and the catalyst Au.

[0028] Preferably, in a case where the gas to be detected is SO2, the metal oxide can be WOs-based and the catalyst Ag-based.

[0029] The porous element can take various forms.

[0030] According to a first possible implementation, the porous element can be formed of porous supports joined together and between which the active material is integrated.

[0031] Alternatively, the porous element may comprise a porous substrate containing 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.

[0032] 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.

[0033] Advantageously, the "active" material is integrated in powder form into the porous element. This improves detection sensitivity.

[0034] According to an embodiment of the detection structure in which the porous element comprises a porous support with open porosity, the thermosensitive visual indicator(s) can be fixed to this support by means of an adhesive.

[0035] One particular embodiment provides for the porous element made of woven glass or SiC or a polymer material.

[0036] Depending on a specific implementation of the structure, the porous element can be formed from at least one porous substrate based on a fusible material, particularly a polymer. This fusible material must have a melting point lower than the ignition point of the gas to be detected. In the case of hydrogen, a fusible material with a melting point lower than hydrogen's ignition point is chosen. Melting the substrate can help limit the risk of the exothermic reaction running away and igniting the gas as a result.

[0037] Advantageously, the passive detection structure can include:

[0038] - at least one thermochromic region with reversible color or hue change, and

[0039] - at least one thermochromic region with an irreversible change in color or hue.

[0040] The thermosensitive visual indicator(s) of the detection structure are typically arranged on a so-called "detection" face of the porous element.

[0041] In one design, the porous element may include a so-called "buffer" zone in contact with the sensing face, in which the content of sensitive material decreases as one approaches the sensing face. Alternatively, this buffer zone contains no sensitive material. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The present invention will be better understood on the basis of the following description and the accompanying drawings, in which:

[0043] Figure 1 serves to illustrate an example of a passive gas detection structure, in particular for hazardous gases such as certain reducing gases, equipped with a so-called "active" metal oxide-based material, designed to react with the gas by producing a release of heat associated with a heat-sensitive indicator configured to change its appearance, in particular its color or hue, following said release of heat resulting from this reaction";

[0044] Figure 2 serves to illustrate an alternative embodiment with several distinct temperature-sensitive indicators having different respective activation temperatures.

[0045] Figure 3 serves to illustrate a heat-sensitive label that can be used as a heat-sensitive indicator of the detection structure.

[0046] Figures 4A and 4B serve to illustrate another type of thermosensitive label that can be used as a thermosensitive indicator of the detection structure and that includes several distinct thermochromic regions.

[0047] Figures 5A, 5B and 5C serve to illustrate a manufacturing process for an example of a passive gas detection structure, in particular for reducing gas.

[0048] Figures 6A, 6B and 6C serve to illustrate another manufacturing process for another example of a passive gas detection structure.

[0049] Figure 7 illustrates a step in a passive gas detection structure manufacturing process consisting of spreading the active material in powder form between two porous supports.

[0050] Figure 8 serves to illustrate a particular realization of a passive structure 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.

[0051] Identical, similar, or equivalent parts of the different figures carry the same numerical references to facilitate transitions between figures. The different parts represented in the figures are not necessarily shown on a uniform scale, to improve readability.

[0052] DETAILED DESCRIPTION OF SPECIFIC METHODS OF IMPLEMENTATION

[0053] An example of a passive gas detection structure implemented according to the invention is shown in Figure 1.

[0054] By "passive," we mean that this structure operates without a power supply. The detection structure can also function without an electronic circuit, component, or means of transduction that produces an electronic signal.

[0055] The gas detection system is specifically designed for detecting hazardous gases, including flammable, explosive, and / or toxic gases. It is particularly suited to reducing gases such as hydrogen (H2) or any of the following: carbon monoxide (CO), hydrogen sulfide (H2S), sulfur dioxide (SO2), and ammonia (NH3).

[0056] The gas detection structure has a porous element 5 represented schematically in Figure 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 a so-called "active" material 20.

[0057] In the particular embodiment shown, the porous substrate 11 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.

[0058] However, other geometries can be adopted.

[0059] The active material 20 is designed to react with the gas to be detected, such that this chemical reaction produces heat release. 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 M y O x (with M a transition metal such as, for example: W, Mo, Ti, Ce, Ni, Sn, Cu and with y typically greater than or equal to 1 and less than 5) chosen so that it can be reduced in the presence of the gas to be detected. For example, the metal oxide M yO x can be one of the following metal oxides: WO3, MOO3, CuO, Fe2U3, TiCh, ZnO, SnCh, NiO, CeCh, V2O5.

[0060] Preferably, the active material 20 also includes or is associated with a catalyst to enable the reaction to start at room temperature.

[0061] In a specific 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:

[0062] Reaction 2 with M the metal used, H(ad) the hydrogen dissociated by the catalyst, A r H° and A r H°' the enthalpies of chemical reactions, that is to say the heat released (in kJ / mol).

[0063] In this particular example of hydrogen detection, we choose a metal M, such that the couple M y 0 x / IVI y 0 x-i exhibits a standard electrochemical potential E°(M y 0 x / M y 0 x -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.

[0064] Advantageously, the metal M is chosen such that the standard electrochemical potential E°(M y 0 x / M y 0 x -i) of the couple M y 0 x / IVI y 0 x -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.

[0065] In general, apart from the electrochemical potential, other physicochemical characteristics can be taken into account to select the metal oxide M y O x namely the activation energy (E a ) of the reaction between the metal oxide (M y O x) and the gas to be detected and the exothermicity of the reduction (these reactions corresponding to reactions 1 and 2 given above). Typically, the higher the E value a The lower the concentration, the higher the reaction kinetics. Generally, the greater the exothermic nature of the reduction of the metal couple, the more effective the detection principle. 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:

[0066] 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.

[0067] Another advantage of such an active material 20 is that it allows reaction with gas at low concentrations of that gas.

[0068] 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.

[0069] According to a preferred embodiment, the active material 20 is a compound based on M y O x 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 chosen from the following metals: Ru, Rh, Ir, Pd, Pt, Ni, Cu, Ag, Au.

[0070] The catalyst is typically chosen based on the metal oxide of the active material 20 and the gas to be detected.

[0071] For example, to detect H2 or CO, the active material 20 can be a mixture or compound of WOs / Pt or WOs / Pd.

[0072] A WO3 / AU mixture can be advantageously used when the gas to be detected is H2S or NH3.

[0073] To detect SO2, a mixture or compound of WCh / Ag can be advantageously used.

[0074] Advantageously, the active material 20 can be in powder form, distributed throughout the pores 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. An adhesive with good thermal conductivity, for example, 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] As an 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 the "front face", with at least one thermosensitive visual indicator whose appearance is likely to be 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 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.

[0079] The thermochromic region 30 is in this case based on a material or substance designed to change color or hue 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 temperature known as the "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 so-called "initial" hue to a so-called "detection" hue different from the initial hue, or from a so-called "initial" color to a so-called "detection" color different from the initial color. This 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—that is, the minimum temperature at which it is likely to change color or hue—higher than the maximum ambient temperature in which the porous element 5 is likely to be placed, in order to prevent 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 capable of changing its hue or color from an initial hue or color to a second color or color. 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 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 example, a support consisting 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 WO1 / Pt, CuO / Pd, CuO / Pt, or MoC1 / 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 drops 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-in-time (i.e., limited-duration) gas leak detection by retaining a trace of a leak that occurred in the past but has since been resolved or stopped.

[0086] Advantageously, the thermochromic region 30 can be designed with a composition that allows it to adopt more than two distinct hues or colors, in order to identify more than two distinct temperature ranges and thus detect more than two ranges of gas concentration. In this case, the thermochromic region 30 has several activation temperatures, each corresponding to a threshold beyond which the region changes hue or color.

[0087] Such a thermochromic region 30 can be made, for example, using liquid crystals.

[0088] 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).

[0089] The thermochromic region 30 can also be in the form of a layer or label or patch or ribbon containing at least one thermochromic dye.

[0090] Thermochromic dyes can be based on compounds such as spirolactones, fluorans, spiropyrans, or fulgides. These dyes typically alternate between a colored and a colorless state depending on the pH of the environment. In acidic environments, protonated forms predominate, while in basic environments, 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 are available in several basic colors. When several dyes are combined, the resulting color is a mixture of the individual hues. A thermochromic dye can also be combined with a conventional dye or pigments. Thus, when the thermochromic dye decolorizes, it reveals the color of the underlying conventional dye or pigment.

[0091] 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.

[0092] An adhesive with good thermal resistance, for example such as an epoxy adhesive, can be used here.

[0093] In the specific embodiment shown in Figure 1, the thermochromic region 30 is not directly in contact with the porous substrate 11 or the active material 20 on or within that substrate. The thermochromic region 30 is instead bonded, and in particular glued, to another support 22, preferably also porous, which is itself positioned against the porous substrate 11 and may itself be 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.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 Figure 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 Figure 8, the porous substrate 11 is arranged between two porous supports 12, 12', with the porous substrate 11 and the supports 12, 12' stacked. The substrate 11 in the center of the stack has the largest quantity of active material 20. A thermochromic region is located 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 decreases. Preferably, a buffer zone without active material is provided in the support near the front face 5A.

[0096] Such an arrangement, with a decreasing quantity of active material as one approaches the thermochromic region 30, allows for the gradual consumption of the detected gas and, for example, in the case of hydrogen or another flammable gas, limits any potential runaway reaction. In support 12, where the quantity of active material is lower, the detected gas is consumed slowly, and as diffusion progresses through the stack, the efficiency increases but the concentration of detected gas decreases.

[0097] An alternative embodiment illustrated in Figure 2 provides for a passive gas detection structure, this time comprising a plurality of distinct thermochromic regions 30i, 3Ü2, 30s, 3Ü4, for example in the form of several thermosensitive labels or dye layers.

[0098] Regions 30i, 3C>2, 3C>3, and 3C can advantageously have distinct compositions, giving them distinct activation temperatures. Thus, a first thermochromic 30i region with a first activation temperature can be designed to change hue or color when subjected to a temperature above this first activation temperature, while a second thermochromic 3C>2 region with a second activation temperature can be designed to change hue or color when subjected to a temperature above a second activation temperature, distinct from the first and, for example, higher than the first activation temperature. For example, the difference between the respective activation temperatures can be several degrees or several tens of degrees Celsius.When the second activation temperature is higher than the first activation temperature, the second thermochromic region 3Û2 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.

[0099] Regions 30i, 3Û2, 3Û3, 304 can all be irreversible hue or color change or all reversible hue or color change.

[0100] Alternatively, it is also possible to associate both one or more thermochromic regions with the irreversible change of color or hue, and one or more thermochromic regions with the reversible change of color or hue.

[0101] This allows for both the preservation of a record of past hazardous gas detections and the ability to indicate that the presence of that 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.

[0102] According to a particular embodiment example, one or more thermochromic regions with irreversible color or tint change can be provided based on an inorganic thermochromic dye such as, for example, iron oxyhydroxide: FeO(OH) capable of changing from yellow to dark red or copper(I) oxide: CU2O capable of changing irreversibly from red to black or mercury(II) sulfide: HgS.

[0103] The thermochromic region(s) may be provided or associated with a temperature threshold indication or a temperature range to allow, in association with a particular color or hue in which the thermochromic region 30 is located, the indication of a temperature threshold or temperature range reached during the exothermic reaction between the sensitive material and the gas reacting with that material.

[0104] A specific embodiment of a thermochromic region is shown in Figure 3 and takes the form of a label 300 with a heat-sensitive dye zone 302, similar to a label marketed by TH-industrie under the name "traffic light | THERMAX". In a first, so-called "normal" state, in the absence of gas to be detected or when the gas concentration is below a detection threshold, the label 300 adopts an initial hue 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 hue or color with a first temperature range, for example, below 50°C.The label includes another indicator 312, which associates a first detection tint 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 second detection tint 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.

[0105] In this example, we have a thermochromic region with two activation temperatures. When the thermochromic region is exposed to a temperature above the first activation temperature of 50°C, it changes from green to orange. When it is exposed to a temperature above the second activation temperature of 70°C, it changes from orange to red. These color changes are reversible.

[0106] According to another example of a particular implementation, 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 the company 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 Figure 4A, regions 330A, 330B, 330C, 330D, 330E, and 330F share the same hue or color, for example, beige or orange, indicating that several temperature thresholds, tA, ts, te, to, tE, and tr, have been exceeded. These thresholds correspond to different gas concentration levels. The temperatures tA, ts, te, to, tE, and tr can range, for example, from 30 °C to 300 °C.

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

[0110] As an alternative or in combination with a thermochromic region 30 whose hue or color changes with temperature, an indicator made of a thermoluminescent material or substance can also be used; that is, one whose luminescence (light emission) varies in response to a temperature change. This thermoluminescent material can 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 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] As a first step, a solution containing a metal oxide is acidified. This acidification can be achieved, for example, using a cation exchange resin that is brought into contact with the solution. For instance, a 13 mL 0.5 M sodium tungstate (ISWC) solution can be acidified to obtain a solution containing a precursor, in this case, H₂WO₄aq.

[0114] The solution containing the precursor is collected and to which another solution containing a catalyst is added. In one particular example, this other solution could be a solution of FUPtCle. A volume of 4 mL at 0.125 M could be used, resulting in an atomic ratio of 1:13 of catalyst to the metal oxide WO3. This ratio is optimal for a good reaction while minimizing the amount of catalyst required. A solvent, typically organic, is then added. This solvent could be ethanol, for example, in a volume of approximately 8 mL for the aforementioned volumes of precursor and catalyst solutions.

[0115] The active coating can be obtained in powder form by evaporating 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, a 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 multifaceted. First, the tungsten trioxide undergoes crystalline transformation, creating vacancies. The presence of these vacancies enhances the material's reactivity. Finally, if the catalyst is introduced as FbPtCle or any other form in solution, calcination is performed to break the bonds between the chlorine and platinum groups and to reduce the platinum to its metallic, catalytically active form. This heat treatment is typically carried out at a temperature of several hundred degrees Celsius, for example, typically between 300°C and 650°C, for a duration 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 30 to 60 minutes.

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

[0119] Another example of active material preparation, in a specific 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) solution (4 g 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, 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 Büchner funnel.

[0121] Alternatively, before filtration, the resulting Cu(OH)2 precipitate can be suspended and then ultrasonically heated to a temperature of, for example, 80°C. The ultrasonic treatment can be carried out for several tens of minutes, for example, 1 hour and 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 duration of, for example, one hour.

[0122] As an alternative or complement to ultrasound, the precipitate can be calcined at a temperature of, for example, 400°C for a period of, for example, one hour. The catalyst can then be added either in liquid or solid powder form. For solid powder addition, a mixture in a given proportion is prepared and then stirred using a dedicated powder stirring system. When adding the catalyst in liquid form, a precursor, for example, FhPtCl, is dissolved in water or pure ethanol. The liquid is then brought into contact with the powder, and the mixture is allowed to evaporate. 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 in which a primary vacuum is applied.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 via a solid-state method, 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 FbPtCk.

[0123] Another method for creating 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 of carbon or metal, such as several nanometers thick, using 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 application of the active coating to the substrate can be carried out electrochemically, in particular using an electroplating device.

[0125] A bilayer coating, or a coating based on a transition metal such as tungsten or copper with a noble metal such as platinum or palladium, can be created to produce an active coating. A bilayer coating, consisting of a first layer of transition metal such as Cu or W with a thickness ranging from 1 to 200 µm, and a second layer of noble metal such as Pt or Pd with a thickness ranging from several nanometers to several hundred nanometers, can be produced. Such a bilayer coating can be created, for example, by successive immersions in two baths and the use of electrochemical techniques such as 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] As an example, a two-layer or mixed coating can be applied using a protocol such as the following:

[0128] An electrically conductive or conductive substrate is attached to a cathode with an electrical contact and introduced into an aqueous electrochemical solution composed, for example, of 150 to 250 g / L of CuSO4·5H2O and 15 to 100 g / L of H2SO4. The anode is formed, for example, of thick copper wire placed around the substrate to obtain a homogeneous deposit. For the deposition of a noble metal such as platinum, the substrate remains in contact with the cathode and is introduced into an aqueous electrolytic solution based, for example, on 23 to 58 g / L of PtClNa2 and 10 to 390 g / L of HCl. The anode is replaced by a platinum wire placed 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 substrate composition, and its thermal resistance. Heat treatment can be carried out at a temperature, for example, between 150°C and 400°C for a duration of, for example, between 10 and 90 minutes.

[0130] Steps in an example 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 (figure 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 a treatment can be carried out for example in an oven 503 (figure 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 secured by means of an adhesive applied to a peripheral area of ​​the first support 12 and / or the second support 12'. The bonding 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 urethane methacrylate adhesive, such as LOCTITE® 402 (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 affixed (Figure 5C) to one face of one of the 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 join supports 12 and 12'.

[0136] According to another possible implementation 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, particularly flexible supports. The active material can then be placed 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' (figure 6A) is preferably carried out in such a way as to preserve an access zone 607 between the supports 120, 120'. For this, the glue can be applied at several points while keeping at least one part unglued.

[0139] The active material is then applied (Figure 6B), maintaining a space between the respective edges of the supports 120, 120'. The active material can be, for example, a viscous liquid paste 602 injected into this space. The paste can be made by mixing an active powder, for example, a WO3 and Pt powder (the preparation of which has been described previously), with a solvent, particularly an organic one, such as pure acetone. A thermochromic region 30, for example, in the form of a heat-sensitive dye label, is then affixed (Figure 6C) to one face of one of the porous supports 120, 120'.

[0140] According to a variation (Figure 7) of the example described above, rather than dispensing the active material in liquid form, it can be introduced directly as a powder. The supports 120, 120' are then re-joined to reduce or eliminate the space between them, and the stack of supports 120, 120' is agitated to ensure the powder is evenly distributed.

[0141] 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.

[0142] As another example, supports 120, 120', 12, 12' and / or the porous SiC-based substrate 11 can also be used. Such a material has the advantage of good thermal conductivity and exhibits good thermal resistance.

[0143] According to another implementation option, supports 120, 120' made of cellulose acetate, or based on polyimide or a porous polymer, particularly a biopolymer such as cellulose with relatively high temperature resistance, typically between 150 and 380 °C, can also be used. The use of a polymer with such high temperature resistance is advantageous because it allows for passive safety measures, especially when the detected gas is flammable, such as hydrogen.

[0144] 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 polymer's melting point, the substrate and / or supports made from 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.

[0145] As previously mentioned, one particular embodiment involves creating the thermochromic region(s) of the passive detection structure using liquid crystals. These liquid crystals may optionally be in microencapsulated form, thus enclosed within capsules.

[0146] Typically, a thermochromic region with multiple activation thresholds and configured to adopt at least three distinct hues or colors can be formed from a liquid crystal-based ink or a mixture of thermotropic liquid crystals or, with controlled organization, for example aligned nematic or chiral / cholesteric nematic.

[0147] The document "Design of chiral dimesogens containing cholesteryl groups; formation of new molecular organizations and their application to molecular photonics", by V. Ajav Mallia and N. Tamaoki, 2003, presents, for example, such types of liquid crystals.

[0148] A thermotropic mixture of cholesteryl esters, exhibiting a cholesteric phase (chiral nematic), can be specifically provided. Such a mixture can be formed, for example, based on COC (Oleyl cholesteryl carbonate; CAS #: 17110-51-9; SigmaAldrich; ref #: 151157), CP (Cholesteryl nonanoate, also called cholesteryl pelargonate; CAS #: 1182-66-7; SigmaAldrich; ref #: C78801), and CBz (Cholesteryl benzoate; CAS #: 604-32-0; SigmaAldrich; ref #: C75802).

[0149] According to a specific embodiment, such a mixture can be prepared in proportions of 30 / 60 / 10 by mass percentage of COC, CP, and CBz, respectively. For example, to prepare 4 g of such a mixture, 1.2 g of COC, 2.40 g of CP, and 0.4 g of CBz are used. The different components are weighed in a single container, which is then heated, for example, using a hot plate, to a temperature of approximately 60 °C, preferably with stirring at a speed of 200 to 300 rpm, until a homogeneous liquid without visible crystals is obtained. The mixture is then allowed to cool while maintaining it above its melting point. Such a liquid crystal mixture can then be applied to a substrate. Advantageously, a polymer substrate, preferably black, can be used.

[0150] In one particular example, such a support can be made by mixing, for example, PDMS (polydimethylsiloxane) base and a crosslinking agent, then adding carbon black, for example, at a mass percentage of approximately 1% relative to the polymer. After homogenization and degassing under vacuum, the polymer mixture can be deposited onto a plate, for example, a glass plate, using a spin-coating method. It can then be baked at a temperature of, for example, 100 °C for 30 minutes. After cooling, a black polymer support is obtained, with a thickness that can be several hundred micrometers, for example, 200 µm.

[0151] If it is desired to increase the hydrophilicity of the surface of this polymer support, an optional O2 plasma treatment step can then be carried out. This can improve wetting and facilitate the deposition and adhesion of the liquid crystal mixture.

[0152] The deposition of this mixture onto the polymer support can be achieved by preheating it, for example to a temperature between 50 and 60 °C, so that it becomes fluid.

[0153] A specific application method involves placing the mixture as at least one drop at the edge of the polymer substrate, then spreading this drop, for example with a blade or scraper (the so-called "doctor blade" method), preferably over the entire surface of the substrate to form a uniform thickness of, for example, several tens of micrometers. The mixture can then be allowed to cool to room temperature so that it solidifies and forms a film.

[0154] Preferably, this film is covered with a transparent protective layer, for example a transparent PDMS (polydimethylsiloxane) polymer layer. Such a protective layer can be formed, for example, by centrifugal coating.

[0155] Another type of liquid crystal that can be used to form a thermochromic region at several activation temperatures is nematic liquid crystals, particularly chirally doped cyanobiphenyls. Such a type of liquid crystal is discussed, for example, in the book "Thermochromy and Thermotropic Materials" by Seeboth, A.; Lotzch, D. 1st ed.; Jenny Stanford Publishing: New York, NY, USA, 2013; ISBN 9788578110796, especially page 8, chapter 1.2.1.2.

Claims

DEMANDS 1. Passive gas detection structure comprising: - at least one porous element (5) comprising an "active" material (20) intended to react with the gas, the active material (20) being formed of a metal oxide M y O x and a catalyst, with M a transition metal of which ye[i ; 4] and 1 < x < 5 , |' O metallic oxide M y O x 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, ..., 3O4) disposed on the porous element (5) and comprising thermochromic and / or thermoluminescent regions, configured respectively to change color or hue, or emit light radiation following said heat release resulting from the reaction between the active material (20) and said gas, - at least one first region (30i) among said regions having a first activation temperature and at least one second region (3O2) among said regions, having a composition different from the first region and having a second activation temperature different from the first activation temperature, and / or - at least one first region (30i) among said regions being thermochromic and having a first activation temperature and a second activation temperature higher than the first activation temperature, the first thermochromic region being configured so that following an exceedance of the first activation temperature, the first region changes from a so-called "starting" hue or color to a first "detection" hue or color and following an exceedance of the second activation temperature, changes from the first detection hue or color to a second detection hue or color, different from the first detection hue or color.

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

3. Structure according to claim 1 or 2, wherein the gas is a reducing gas, the metal oxide M y O xbeing 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, 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 M y O x is chosen from: WO3, MOO3, CuO, Fe2Û3, TiÛ2, ZnO, SnÛ2, NiO, CeÛ2, ZrO2, V2O5.

7. Structure according to any one of claims 1 to 6, - in which the gas is H2 or CO, the metal oxide is WO3 and the catalyst is Pd or Pt, or - in which the gas is H2S or NH3, the metal oxide being WO3 and the catalyst being Au, or - in which the gas is SO2, the metal oxide is WO3 and the catalyst is 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, said one or more thermosensitive visual indicators being fixed to at least of said porous supports (12), or - a porous substrate (11) comprising the active material (20), the porous substrate (11) being attached 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) of glass, or of 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, wherein among said regions is: - at least one thermochromic region (30i) with reversible color or hue change, and - at least one thermochromic region (3O2) with irreversible change of color or tint.

14. Detection structure according to any one of claims 1 to 13, 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).

15. Detection structure according to any one of claims 1 to 14, wherein the first region (30i) is a thermochromic region configured to, following an exceedance of the first activation temperature, adopt a first hue or a first detection color and wherein the second region (3O2) is a thermochromic region configured to, following an exceedance of the second activation temperature, adopt a second detection hue different from the first detection hue or a second detection color different from the first detection color.

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