Process for manufacturing lanthanum oxycarbonate powder and its use as an active layer in CO2 sensors

Abstract

A process for manufacturing a lanthanum oxycarbonate powder comprising the following steps: a step of grinding an initial powder comprising lanthanum hydroxide La(OH)3 in the presence of a control agent, said control agent comprising ethylene glycol and said grinding resulting in a paste and a subsequent step wherein said paste is subjected to a heating step allowing the oxidation of the La(OH)3 powder into a La2O2CO3 powder. Figure for abstract: no figure

Description

Title of the invention: Method for manufacturing a lanthanum oxycarbonate powder and its use as an active layer in CO sensors 2 Field of invention

[0001] The invention relates to a method for manufacturing a lanthanum oxycarbonate powder; its use for manufacturing a CO2 sensor or detector as well as the design of a CO2 sensor using this powder. Prior art

[0002] Air pollution in cities as well as the indoor air quality of public and private places constitute a major challenge of the 21st century. Human beings spend 85% of their time in enclosed spaces in which they are exposed to numerous pollutants that can have harmful effects on health, cognitive abilities as well as their general well-being. The nature of these pollutants is mainly anthropogenic and depends, among other things, on the characteristics of the building, the activities and human behavior within it. In addition, the 2020 health crisis has raised new concerns about the hygiene of private spheres and has motivated the need to implement monitoring of this environment.

[0003] Air quality is defined as the concentration of these polluting gases in the air, indexed against hazard thresholds whose values ​​are set by the European Union and the French government, in accordance with the directives of the World Health Organization (WHO). Among these gases, CO2 is subject to particular monitoring, since it is a relatively common gas released in quantity by many human activities, but also by our own bodies. Under normal conditions, its concentration in the air is around 420 ppm. As soon as it exceeds 1,000 ppm, harmful effects are felt in humans, such as reduced concentration, increased breathing rate or even headaches. These effects worsen when its concentration exceeds 10,000 ppm, and can even cause irreversible damage to the health of the subject.The COVID crisis has also proven the causal link between increased CO2 concentration and the increased risk of virus transmission in an enclosed space. To prevent and protect individuals from the risks caused by prolonged exposure to a CO2-rich environment, suitable sensors have been developed.

[0004] There are 3 families of CO2 sensors, based on different physical detection principles. We thus distinguish photoacoustic sensors [1], sensors non-dispersive infrared (NDIR) sensors [2], which are very dominant in this market, but expensive and bulky, and chemical sensors [3-4]. This last category includes Metal Oxide Semiconductor (MOS) sensors, operating on the principle of molecular adsorption. They generally consist of an insulating substrate (3) carrying a heating element (5) on which a sensitive layer (7) is deposited, the seat of adsorption, housed between two electrodes (9), like the sensor schematically represented in [Fig.l] [4].

[0005] The adsorption of gas on the sensitive layer causes the variation of the conductivity thereof which is detectable by the external load connected to the measuring electrodes. Concerning the development of the materials of the sensitive layer, adsorption being a surface phenomenon, increasing the surface of the materials concerned will have a direct impact on the detection properties of the sensor using such materials. There are 2 ways to increase the surface of a material:

[0006] Division, which consists of reducing the size of the powder grains.

[0007] Porosity, that is to say the introduction of cavities deeper than wide called "pores" inside the powder grains. According to IUPAC, pores are classified into 3 categories depending on their size, namely: - micropores, with a size less than 2 nm; - mesopores, with a size between 2 and 50 nm; - macropores, which have a size greater than 50 nm.

[0008] Increasing the division state and / or introducing pores into a material generally makes it possible to increase the surface area that can be accessible to the gas.

[0009] Nanoscale oxide powders used as active layer for gas sensors are prepared using various methods including sol-gel method, chemical coprecipitation, metal-organic decomposition (MOD), plasma vapor deposition (PECVD), chemical or physical vapor deposition, low pressure flame deposition (LPFD), laser ablation and mechanical milling [5-6]. The latter method has never been used to prepare lanthanum oxycarbonates.

[0010] Lanthanum oxycarbonate of formula La2O2CO3, has properties of sen interesting and better sibility, selectivity and stability than some oxides [5]. Lanthanum oxycarbonate exists in 2 crystalline forms: monoclinic and hexagonal [7].

[0011] There are CO2 sensors using lanthanum oxycarbonate [8-9].

[0012] Lanthanum oxycarbonate-based microsensors have also been made by grinding in a mortar commercial nanopowders of lanthanum oxide La2O3 which carbonated in an uncontrolled manner during storage

[10] . These powders were then suspended in solution, and then deposited by drop-coating or screen- printing on pre-machined substrates, before being subjected to annealing to remove the solvent. The resulting sensors do not meet industrial requirements in terms of reproducibility and / or cost, as well as in terms of reducing the size and / or power consumption of these sensors, and maintaining their stability, sensitivity and / or selectivity.

[0013] The invention aims in particular to remedy these shortcomings. The method according to the invention thus makes it possible to obtain a lanthanum oxycarbonate powder having a larger specific surface area than known powders, which improves the performance of the sensors and this according to a simple, inexpensive and environmentally friendly method. In addition, the powder obtained also makes it possible to carry out homogeneous deposits of sensitive layers on the surface of a sensor by simple drop coating, which greatly simplifies the manufacture of CO2 sensors. Description of the invention

[0014] It has been surprisingly determined that grinding lanthanum hydroxide powders under particular conditions combined with a specific heat treatment makes it possible to obtain lanthanum oxycarbonate (nano)powders having a mesoporous texture particularly suited to its use in CO2 sensors. This method is, moreover, simple and economical to implement and reproducible and can be easily transposed to the industrial level.

[0015] To this end, the invention relates to a process for the manufacture of a lanthanum oxycarbonate powder comprising the following steps: - a step of grinding an initial powder comprising lanthanum hydroxide La(OH)3 in the presence of a control agent, said control agent comprising ethylene glycol, and said grinding resulting in a paste; - a subsequent step where said paste is subjected to a heating step allowing the oxidation of the La(OH)3 powder into a La2O2CO3 powder.

[0016] Surprisingly, the combination of a grinding step in the presence of ethylene glycol and a heating step of the paste thus obtained makes it possible to obtain a La2O2CO3 powder having mesoporosity. This result is not obtained with other control agents. The mesoporous lanthanum carbonate powder thus obtained can have numerous applications, in particular in the field of CO2 sensors and / or detectors.

[0017] The grinding step is preferably carried out by high energy grinding.

[0018] High energy grinding is a method of producing divided materials (powders), which consists of agitating more or less violently a powder and balls contained in a sealed enclosure called a bowl. The desired effect is the increase in the specific surface area of ​​the ground material by reducing the grain size: the more The greater the size reduction, the more grains there are, and therefore the greater the total surface area of ​​the material. High-energy grinding involves a high-speed deformation mechanism (103 to 104 per second), resulting in repeated fracture and cold welding processes

[11] . These steps are repeated until the steady state is reached, i.e. until the fracture and cold welding phenomena of the powder grains are balanced.

[0019] A mill that can be advantageously used to carry out high-energy grinding is a "planetary" or "vario-planetary" type mill. In this type of mill, one or more bowls containing the powder to be ground and balls are each fixed on a rotating support. The rotating support(s) are arranged on a plate that rotates in the opposite direction to the supports. These movements allow grinding by impact and friction of the balls on the powder and of the powder on the walls of the bowls. The term "vario" indicates that it is possible to modify the rotation speeds of the bowls and the plate, the ratio of these speeds becoming an adjustable parameter of the grinding. Obviously, a planetary mill whose speed ratio parameters are fixed in advance according to the method of the invention will be just as effective.Other mills (e.g. vibration or attrition mills) can also be considered as long as they are capable of developing a grinding similar to the high energy (vario)planetary mill.

[0020] The material of the bowls and the balls is generally, but not necessarily, identical and is chosen according to the mechanical properties of the material to be ground (hardness, abrasion resistance, etc.). According to a particularly preferred variant of the invention, the material of the bowls and / or the balls comprises zirconia oxide or agate (SiO2). The diameter of the balls, which is generally the same for all the balls used, but not necessarily, can affect the efficiency of the grinding (i.e. the speed at which the stationary state is reached), the nature and the particle size of the final material. Balls with a diameter ranging from 3 to 10 mm are preferably used. In particular, balls with a diameter of 5 mm have shown good results.

[0021] The mass ratio of balls to powder mass represents the proportions of the loading of the bowl and affects the time between two impacts. Generally, the higher it is, the more the grinding efficiency is increased, until an efficiency plateau is reached where According to the method of the invention, the mass ratio of said balls relative to said powder comprising lanthanum hydroxide varies from 10:1 to 30:1 and, preferably, is around 20:1.

[0022] The higher the speeds of the bowls and the plate, the more energy is involved, and the more efficient the grinding can be. However, an excessive increase in this parameter intensifies the risks of amorphization and can also cause a rise in temperature. In addition, there is a "critical speed", which constitutes a threshold from which the centrifugal force is such that the balls remain stuck to the walls of the bowl, which makes the grinding ineffective. In the context of planetary grinding, the speed of the plate is preferably chosen in the range from 1200 rpm to 1500 rpm, advantageously from 135 rpm to 1450 rpm and more particularly around 1400 rpm. The speed of the bowl(s) is preferably chosen in the range from 150 rpm to 300 rpm, advantageously from 200 rpm to 280 rpm and more particularly around 240 rpm.

[0023] The presence of a control agent makes it possible to condition the powder to be ground and the grinding properties, such as limiting the rise in temperature, increasing the grinding efficiency or protecting the powder against contamination. The most frequently used control agents are paraffin oil or glycerol. Preferably, the control agent used comprises more than 80% by mass of ethylene glycol. Even more preferably, the control agent consists essentially of ethylene glycol.

[0024] According to the method of the invention, the mass ratio between the control agent and the mass of powder comprising lanthanum hydroxide varies from 0.25 to 0.75 and preferably from 0.45 to 0.55. The control agent is in liquid form and forms, with the initial powder, a “paste”. The term paste is used in its most generic sense to identify a mixture of liquid and solid particles. However, the mass ratios which are preferred make it possible to obtain a viscous mixture.

[0025] The atmosphere injected inside the bowls can alter the chemical composition of the powders. It can be selected to synthesize a new compound, or be neutral if the ground material is reactive to certain gases or sensitive to contamination. However, according to the method according to the invention, the grinding can take place in air and, preferably, under normal pressure conditions.

[0026] The grinding temperature influences the resulting nature of the material by promoting diffusion, decomposition and amorphization phenomena, and may depend on the nature of the material of the balls and bowls. It is therefore advantageous to grind in cycles, taking breaks between each cycle, to compensate for an increase in temperature which may be undesirable.

[0027] Also, during grinding, cycles of 5 to 30 minutes (e.g. approximately 10 min.) can take place interspersed with pause times to carry out cooling. These pause times can be from 1 to 20 min. (e.g. approximately 5 min.).

[0028] The grinding step is advantageously carried out for a duration of more than 2 hours, preferably a period of more than 3.5 hours, in particular a period of 3.5 to 7 hours, for example around 4 hours. This duration is the grinding time and excludes any possible break period.

[0029] A powder is a discrete collection of small pieces of a solid called grains, in generally smaller than one tenth of a millimeter (100 pm). The size of the powder grains can be characterized by various techniques such as dry sieving, analytical centrifugation, laser diffraction, microscopy, zeta potential. The initial powder comprising lanthanum hydroxide La(OH)3 may comprise more than 60%, preferably more than 80% by mass of lanthanum hydroxide. Since lanthanum hydroxide can be obtained from lanthanum oxide La2O3 by simple contact with ambient air, the initial powder may also comprise a certain proportion of this oxide. However, the initial powder is advantageously made up and / or consists essentially of lanthanum hydroxide.

[0030] The step of heating the paste obtained by grinding the initial powder and the control agent can be carried out at a temperature above 400°C, for example ranging from 400°C to 475°C, preferably ranging from 430°C to 460°C, and advantageously around 450°C for a duration which can be between 30 min. and 1 h.

[0031] At the end of the heating step, a La2O2CO3 powder is obtained. Advantageously, a simple grinding step is carried out to obtain a fine powder; simple grinding by mortar may be sufficient. This powder is of reduced dimensions compared to the dimensions of the original powder. It may comprise or consist of nanoparticles (1-100 nm), microparticles (1-1000 pm), fine particles (100-2500 nm) or coarser particles (2500 - 10,000 nm).

[0032] Another object of the invention is the use of a powder according to the invention for the manufacture of a CO2 sensor.

[0033] For sensor applications, the important characteristic is the accessible surface of the powders to gases. For this purpose, the texture, i.e. the state of division of the powders and / or their specific surface area of ​​the powders produced is characterized by adsorption of nitrogen at 77K. This method makes it possible to obtain adsorption-desorption isotherms from which it is possible to calculate this surface

[12] .

[0034] Thus another aspect of the invention is a La2O2CO3 powder obtained, or obtainable, by the process according to the invention and described above. The powder according to the invention is a mesoporous powder which has a specific surface area greater than or equal to 16 m2 / g, said surface area being measured by the BET N2 at 77°K method, preferably greater than or equal to 17 m2 / g and more preferably greater than or equal to 18 m2 / g.

[0035] Another aspect of the invention relates to the use of the powder according to the invention for the manufacture of a CO2 sensor and / or detector. This aspect of the invention comprises a manufacturing method where a layer of a liquid-solid mixture comprising a powder according to the invention, mixed, preferably homogeneously, with a liquid (called carrier liquid), is deposited on a substrate.

[0036] The carrier liquid and its quantity are advantageously chosen so as to be adapted to the technique of depositing the mixture on the substrate. The drop coating technique is preferred for its simplicity of implementation.

[0037] To carry out such a drop deposition, a mixture comprising or consisting essentially of ethanol and glycerol, for example in respective proportions 9 / 1 v / v, makes it possible to obtain particularly effective results because this mixture has sufficient viscosity and adhesion. Ethanol could be replaced by other alcohols.

[0038] The concentration of solid powder and carrier liquid may range from 10 mg / mL to 50 mg / mL, for example from 20 mg / mL to 40 mg / mL. With regard to a carrier liquid comprising an ethanol / glycerol mixture, a concentration of 30 ± 5 mg / mL is particularly suitable.

[0039] Since it is advantageous for the solid / liquid carrier mixture to be homogeneous, the mixture is stirred and homogenized by known means: magnetic bar, sonication, etc.

[0040] Advantageously, the powder obtained according to the method according to the invention is mixed quickly with the carrier liquid after the heat treatment (or heating) described above, which makes it possible to avoid or reduce contamination. For example, the powder is used immediately, within hours (e.g. 1 to 24 hours) or within days (e.g. 1 to 3 days) following its production.

[0041] Once the powder / carrier liquid mixture has been deposited on the substrate, the carrier liquid is then advantageously removed by heating, to leave on the surface of the support a layer comprising, consisting of and / or consisting essentially of a La2O2 CO3 powder. Such a layer constitutes a sensitive layer making it possible to capture ambient CO2. This deposition step can be repeated more than once on the same support.

[0042] The substrate comprises an insulating material. For a micro-sensor a material such as silicon is used because it allows fine cuts. The silicon surface may have been oxidized (for example by applying an oxygen plasma) so as to make it hydrophilic.

[0043] Metal electrodes, for example in the form of combs, are advantageously arranged on the surface of the support and leaving a space to receive the La2O2CO3 powder.

[0044] Drop coating can be carried out by taking and depositing at least one drop of 10 qL on the support (generally 1 to 3 drops). After this deposition, the substrate is advantageously heated to remove the carrier liquid.

[0045] When the carrier liquid comprises a mixture of compounds, successive heating steps may be applied, for example at 80°C (advantageously for 5 minutes), then at 180°C (advantageously for 5 minutes) in the case of ethanol and glycerol respectively.

[0046] The manufacturing method according to the invention is particularly suitable for the manufacture of CO2 microsensors. By "microsensor" is meant a sensor whose largest dimension does not exceed 5 cm, and preferably 1 cm, even more advantageously less than 4 mm.

[0047] Another object of the invention is therefore a sensor, and more particularly a CO2 microsensor. In particular, the sensor can be obtained by the method described above. The sensor comprises an insulating substrate supporting a sensitive layer or element, the seat of the adsorption, positioned between two electrodes where said sensitive layer comprises a powder of La2O2CO3 according to the invention. Advantageously, said insulating substrate of said sensor carries a heating element on which said sensitive layer is deposited. The electrodes may comprise at least one metal and / or a metal alloy. For example, the electrodes may comprise a superposition of a layer of titanium and platinum. The electrodes may be arranged in an interdigitated configuration, consisting of fingers, for example 30, 0.5 to 10 mm long (for example about 4 mm), 5 to 100 qm wide (for example 50 qm) and spaced 5 to 100 qm apart from each other (for example 50 qm).This configuration makes it possible to increase their resistance by increasing their length and to increase their contact surface with the sensitive element. Brief description of the figures

[0048] The invention will be better understood on reading the following description given solely by way of example and with reference to the appended figures in which:

[0049] [Fig.l] is a schematic representation of a standard MOS gas sensor;

[0050] [Fig.2] represents the adsorption-desorption isotherm of a powder obtained according to the method of Example 1 as well as the isotherms of the comparative examples also described in Example 1;

[0051] [Fig.3] is an enlargement of a part of the graph of [Fig.2];

[0052] [Fig.4] is a set of a top view and a sectional view of a sensor of CO2 according to a variant of the invention described in example 3;

[0053] [Fig.5] represents the variation of the resistivity of a sensor as a function of time according to example 3 in relation to the CO2 concentration for 3min of CO2 exposure at 1000, 5000 and 10,000 ppm, Sensor B polarized at 3V - Operating temperature 350 °C - Flow rate 250 ml / min - 50% RH.

[0054] [Fig.6] represents the response of the sensor of example 3 as a function of time for 3min CO2 exposure at 1,000, 5,000 and 10,000 ppm, Sensor B polarized at 3V - Operating temperature 350 °C - Flow rate 250 ml / min - 50% RH. Examples

[0055] Example 1: Preparation of lanthanum oxycarbonate powders according to the invention and comparative examples

[0056] A commercial powder of lanthanum oxide La2O3 (Sigma Aldrich, Lanthanum (III) oxide, purity S 99.9%; reference L4000-100G) was left in the open air for 7 days in order to obtain a lanthanum hydroxide La(OH)3. The nature of the phase was verified by X-ray diffraction. This initial powder of La(OH)3 has a BET surface area of ​​3 m2 / g. It was then ground using the Fritsch Pulverisette 4 vario-planetary mill.

[0057] All the parameters of this grinding are reported in Table 1 below:

[0058] [Tables 1] Bowl and ball material ZrO2 Bowl volume 45 mL Ball diameter 5 mm Ball mass / powder mass ratio 20:1 Control agent mass / powder mass ratio 2:1 Grinding atmosphere Ambient air Table top speed 1400 rpm. Bowl speed 240 rpm. Grinding cycle 10 min. grinding - 5 min. break Grinding time 4h Control agent A: Ethylene glycol

[0059] After grinding, the preparation obtained consists of a paste of La(OH)3 and ethylene glycol introduced. This then undergoes annealing in a Na-bertherm muffle furnace. This heat treatment makes it possible to transform La(OH)3 into La2O2CO3 and to create the porosity of the powders. The heat treatment is as follows: - Increase in ambient temperature to 80°C in 30 min.; - Isothermal for 30 min. at 80°C; - Rise from 80°C to 450°C in 3 hours; - Isothermal (maintaining the temperature at 450°C) for 30 min.; - Rapid cooling to room temperature. Cooling is limited by the inertia of the furnace.

[0060] Following this heat treatment, the sample consists of concave chips a few millimeters long, generally white or gray. These shavings are collected and then reduced to powder using an agate mortar. Comparative examples

[0061] In order to establish the particular texture of the powder according to the invention and the specificity of the claimed method, comparative examples were carried out: lanthanum oxycarbonate powders were produced with two other control agents: - Control agent B: Paraffin oil; - Control agent C: Glycerol.

[0062] The preparation of these powders was carried out strictly according to the same conditions as those described above, only the control agent was modified. The nature of the powder obtained after the thermal step was analyzed by infrared spectrometry (FTIR Spectrum 2 Mettler). In all three cases, the bands observed are characteristic of La2O2CO3 regardless of the control agent. Analysis of the porosity of the powders obtained

[0063] The shape of the nitrogen adsorption-desorption isotherms at 77K allows us to conclude on the type of porosity of a material. The specific areas determined by the Brunauer Emmett and Teller (BET) method

[12] allow us to quantify the area accessible to nitrogen, also called specific area (or even BET area) of the samples. The adsorption-desorption isotherms were measured using a surface area analyzer (Gemini VII - Micromeritics). The results are presented in Table 2. The initial La(OH)3 powder used has a BET area of ​​3 m2 / g. Thus, the specific area of ​​the powder obtained was multiplied by 6 compared to the area of ​​the initial powder.

[0064] [Tables2] Porosity Type and BET Areas Recorded According to the Control Agent Used Control Agent Porosity Type (Porous Radius) BET Area (m2 / g) Ethylene Glycol Mesoporous (Rp~ 100 nm) 18 Paraffin Oil Macroporous (Rp > 200 nm) 13 Glycerol Macroporous (Rp > 200 nm) 8

[0065] Example 2: 6 hour grinding duration

[0066] The process of Example 1 according to the invention was repeated identically with the exception of the grinding time which was set at 6 hours. The results measured as previously are as follows:

[0067] [Tables3] Porosity type and BET areas recorded according to grinding time Grinding time Porosity type BET area (m2 / g) 4h Mesoporous (Rp~ 100 nm ) 18 6h Mesoporous (Rp~ 100 nm ) 20 X-ray diffraction analysis indicates that the samples consist of a mixture of the hexagonal and monoclinic phases of La2O2CO3. Both phases appear to be produced in equal proportions for a 4-hour grinding session. The monoclinic phase is slightly dominant for the 6-hour grinding session.

[0068] Example 3: preparation of a CO2 microsensor by deposition of sensitive layers of lanthanum oxycarbonate

[0069] The deposits of the sensitive layers are carried out by drop deposition (or "drop coating"). Firstly, the La2O2CO3 powder obtained in Example 2 (6 hours) recovered at the end of the heat treatment described above is diluted in an ethanol-glycerol mixture (0.9 mL ethanol for 0.1 mL glycerol) to create an apparently homogeneous mixture of solid powder and liquid at 30 mg / mL, slightly viscous and more adherent thanks to the glycerol. Then, this mixture is stirred for 72 hours until completely homogenized. However, it is considered entirely possible to reduce the time of this stirring, the condition being to visually ensure that there are no more powder clumps.

[0070] In parallel, the substrate used was made to order in the French Renater network in the Femto-ST clean room on an oxidized silicon wafer cut from a mask designed by the research team. Each substrate consists of an inter-digitated metal comb (Ti / Pt) shown in [Fig.4]. The electrodes are deposited by cathode sputtering. The silicon wafer is pre-cut in the clean room and the substrate is removed with tweezers. The latter is protected by a resin which is dissolved by successive cleanings with acetone and ethanol. This cleaning is done in an ultrasonic bath for 5 minutes for each solvent. The substrate is then rinsed with deionized water and dried in dry air. To continue and complete the cleaning, the substrate is subjected to an oxygen plasma flow for 3 minutes to effectively remove all organic contamination from the surfaces and make the surface more hydrophilic.The deposition was carried out within the following 30 minutes, during which time the substrate surface is considered “clean”.

[0071] Drop coating is carried out using a micropipette, by taking and depositing a 10 qL drop. After this deposition, the substrate is heated for 5 minutes at 80°C, then at 180°C for 5 minutes again, to remove the ethanol and glycerol respectively. Finally, the electrical contact pads of the substrate are cleaned with acetone to allow the deposition of the electrical measurement electrodes on the metal electrodes without the deposit interfering.

[0072] The substrate used is shown in [Fig.4]. The dimensions are as follows: A: 2750 pm, B: 4100 pm, C and D: 50 pm. The maximum height of the electrodes is 200 nm: 190 nm for platinum and 10 nm for titanium.

[0073] These specific substrates do not have a heating element, which means that they will have to be heated if the operating temperature needs to be adjusted. In our example, the substrate is deposited on a heating ceramic that can go up to 420°C and is controlled by a regulator. As explained in [Fig.4], the electrodes can be arranged in an interdigitated configuration, composed of 30 fingers approximately 4 mm long, 50 pm wide and spaced 50 pm apart. This configuration makes it possible to increase their resistance by increasing their length and to increase their contact surface with the sensitive element. The electrodes consist of a 190 nm platinum layer deposited on a 10 nm titanium bonding substrate, for a total height of 200 nm. They emanate from 2 large rectangular platforms visible at the bottom of [Fig.4], which will be used to establish electrical contact between the sensor and the test bench using contact tips.

[0074] This type of substrate is robust, compact, simple to produce and suitable for several deposition methods. Thus, it meets the criteria of low cost and replaceability.

[0075] [Fig.6] represents the response of the sensor to the detection of CO2 at 355°C. Sensor B was exposed on a test bench to different temperatures (OMICRON test bench), reached thanks to the heating ceramic under the substrate. The sensor was biased at 3V and exposed under 50% relative humidity for 3 min. to 3 different CO2 concentrations (1000, 5000 and 10000 ppm), with breaks of 20 min. between each exposure. During the whole operation, the resistance of the sensor is recorded using the multimeter (Keithley) of the test bench, its variation acting as response as shown in [Fig.5]. For all the measurements that were undertaken, the carrier air flow rate was set at 250 mL / min., the humidity set at 50% RH and the sensor was placed in darkness.Thanks to the 3 concentration measurements, if the evolution of the response is linear, the sensitivity of the sensor can then be evaluated over this concentration range, or the variation of response per unit of concentration of gas introduced. We then classically obtain a variation of the response similar to that represented in [Fig.6].

[0076] When the sensor is not exposed to CO2, a basic resistance Ro of the sensor of several tens of megaohms is recorded. As soon as the gas is introduced, The signal amplitude increases sharply to reach resistances of several hundred megaohms, then a slowdown in the growth is noted, characteristic of the approach to the steady state. These high resistance values ​​are linked to the high intrinsic resistivity of the material which induces low currents involved, of the order of ten nanoamperes. This low current is also attributed to the noise of the measurement which tends to gain in amplitude when the resistance of the sensor increases, therefore in particular when it is exposed to gas. As soon as exposure to gas ceases, an immediate drop in the signal amplitude is observed, with this same slowdown in the decrease as the Ro plateau approaches, an indicator of the sensor's response time.

[0077] To estimate the sensitivity of the sensor over this concentration range, the sensor response was obtained by dividing the recorded resistance by Ro. The value of this resistance normalized for each concentration when the CO2 supply is cut off, therefore close to its maximum value in this case, was obtained and the variation of the resistance normalized as a function of the concentration. The increase in the normalized response slows down as the concentration increases, due to the increase in the number of occupied adsorption sites. By extension, it can be assumed that this curve reaches a plateau for concentrations above 10,000 ppm, where all the adsorption sites are occupied.Using a trend curve, we can then extract the sensitivity of the sensor over the range 1000 ppm to 10000 ppm: we note a sensitivity of 0.07% RO / ppm, which corresponds, if we multiply this value by the value of Ro (approximately 39.25 MQ here), to an absolute sensitivity of 27.47 kQ / ppm over this range. By repeating the experiment for several temperatures, we can then plot the sensitivity as a function of the operating temperature. This makes it possible to establish a clear maximum sensitivity of approximately 0.12%R0 / ppm, or 94kQ / ppm for a temperature of 350°C. The humidity level above 10% had little influence on this sensitivity, but makes it possible to limit the measurement noise caused by the low currents involved and the high resistivity of the deposit.

[0078] Method for measuring the texture of powders after grinding and heat treatment: adsorption-desorption of nitrogen at 77K

[0079] The state of division and the possible porosity of the powders produced was measured using the standard BET method using N2 as the adsorbed gas at 77°K. This method makes it possible to obtain the area per unit mass, or mass area, or specific area. The principle of the BET method consists of the linearization of the adsorption isotherm previously obtained in a range of partial pressures P / PO lower than 0.35 by the BET transform equation.

[0080] The adsorption isotherm is measured using a known surface probe molecule called “adsorbable”, here dinitrogen N2, which is introduced in the vicinity of the sample to coat the surface of the latter. By measuring the pressure of the medium, and obtaining isothermal adsorption curves, it is possible, using a theoretical model of particles (BET model) to go back to the quantity of adsorbed nitrogen material (i.e. the number of adsorbed molecules), and therefore to the area of ​​the material as a function of the pressure knowing the surface occupied by an adsorbable molecule.

[0081] This technique is well known and is part of the general knowledge of those skilled in the art. In summary, the measurement begins with a pre-treatment under vacuum at temperature of the sample which has been previously introduced into a glass tube and weighed. Then, the purified sample in its tube is weighed again in order to know the exact mass of adsorbent. This is a crucial step because all the measured quantities will be reduced to the mass of powder. The tube containing the sample is then installed on a surface area analyzer.

[0082] The measurement of the adsorbed quantity is carried out at a constant temperature maintained by a liquid nitrogen bath at 77K at a given pressure. This measurement then provides the value of the adsorbed quantity for an incoming pressure of nitrogen. The measurement is repeated for a discrete set of pressures which increases up to the saturated vapor pressure of the PO adsorbable, then again by decreasing the pressure down to low pressures in order to characterize adsorption and desorption. All of these points trace a curve of the adsorbed quantity as a function of the relative equilibrium pressure P / PO at a given temperature, called the adsorption-desorption isotherm, the shape of which provides information on the type of porosity by comparison with the isotherms of the IUP AC classification (for International Union of Pure and Applied Chemistry)

[10] ,

[11] . List of references

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Claims

Claims

1. A process for manufacturing a lanthanum oxycarbonate powder comprising the following steps: - a step of grinding an initial powder comprising lanthanum hydroxide La(OH)3 in the presence of a control agent, said control agent comprising ethylene glycol and said grinding resulting in a paste; - a subsequent step where said paste is subjected to a heating step allowing the oxidation of the La(OH)3 powder into a La2O2CO3 powder.

2. The method of claim 1, wherein said grinding step is carried out by planetary mill where said initial powder is placed in a bowl comprising balls.

3. The method according to claim 2, wherein said grinding step is carried out for a period of more than 2 hours, preferably a period of more than 3.5 hours, in particular a period of 3.5 to 7 hours.

4. The method according to any one of claims 1 to 3, wherein said heating step is carried out at a temperature ranging from 400°C to 475°C, preferably ranging from 430°C to 460°C, in particular 450°C.

5. The method according to any one of claims 2 to 4, wherein the mass ratio between the control agent and the mass of powder comprising lanthanum hydroxide varies from 0.25 to 0.75 and preferably from 0.45 to 0.

55.

6. The method according to any one of claims 1 to 3 wherein the mass ratio of said beads to said powder comprising lanthanum hydroxide varies from 10:1 to 30:1 and preferably is around 20:

1.

7. A La2O2CO3 powder obtained, or obtainable, by the process according to claim 1 to 6, said powder being a mesoporous powder and having a specific surface area greater than or equal to 16 m2 / g, said surface area being measured by the BET N2 at 77°K method, preferably greater than or equal to 17 m2 / g and more preferably greater than or equal to 18 m2 / g, preferably greater than or equal to 15 m2 /

8. ë- A CO2 sensor comprising an insulating substrate supporting a sensitive layer, seat of adsorption, positioned between two electrodes where said sensitive layer comprises a La2O2CO3 powder according to claim 7.

9. The sensor according to claim 8, wherein said insulating substrate carries a heating element (5) on which said sensitive layer is deposited.

10. A method of manufacturing a CO2 microsensor wherein a layer of a liquid-solid mixture comprising a powder as described in claim 7 is mixed with a liquid and deposited on a substrate, said liquid being a mixture comprising ethanol and glycerol, for example in respective proportions 9 / 1 v / v.

11. Use of a powder according to claim 7 for the manufacture of a CO2 sensor.

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