CO2 sensor with low electrical resistance and high insensitivity to humidity, and methods for its manufacture

Abstract

The invention relates to a CO2 sensor with low electrical resistance and high insensitivity to humidity, and to methods for its manufacture. The sensor comprises a gas-sensitive material containing at least one substance from the group consisting of semiconducting metal oxides, rare-earth oxides, or rare-earth oxycarbonates, wherein this substance contains alkali metal atoms and / or alkali metal ions. In particular, this substance is doped with at least one alkali metal and / or loaded with at least one alkali carbonate. In a preferred method for manufacturing the gas-sensitive material, at least one rare-earth oxide or at least one semiconducting metal oxide is heated with at least one alkali carbonate for doping. In an alternative method, the loading of at least one rare-earth oxycarbonate or at least one semiconducting metal oxide is carried out with at least one alkali carbonate.

Description

[0001] The invention relates to a CO2 sensor with low electrical resistance and high insensitivity to humidity, as well as to methods for producing the gas-sensitive material for the sensor, which comprises semiconducting metal oxides, rare earth oxides, rare earth oxycarbonates, or mixtures thereof and additionally contains alkali metal atoms and / or alkali metal ions. In particular, the gas-sensitive material is doped with at least one alkali metal and / or loaded with at least one alkali carbonate.

[0002] CO2 sensors based on rare earth oxycarbonates are known, for example, from WO2022 / 049402A2, EP3751264A3, and US020200400606A. However, the CO2 sensors known from the prior art suffer from the disadvantages of high sensitivity to humidity and relatively high resistance. Both disadvantages mean that complex compensation electronics and a humidity sensor, as well as cost-intensive evaluation electronics, are required to obtain reliable measurement data.

[0003] The object of the present invention is therefore to provide a CO2 sensor with a high insensitivity to humidity and a low electrical resistance, as well as methods for its production.

[0004] To solve this problem, a CO2 sensor is provided in which the gas-sensitive material contains alkali metal atoms and / or alkali metal ions. In particular, the gas-sensitive material comprises alkali metal-doped and / or alkali carbonate-loaded semiconducting metal oxides, and / or rare earth oxides and / or rare earth oxycarbonates, and / or mixtures thereof.

[0005] In a preferred embodiment, the gas-sensitive material of the CO2 sensor according to the invention comprises at least one substance belonging to one of the following substance classes: - Alkali metal-doped and / or alkali carbonate-loaded mixed compound Ln2O2CO3, - Alkali metal-doped and / or alkali carbonate-loaded rare earth oxide Ln2O3, - Alkali metal-doped and / or alkali carbonate-loaded mixed compound Ln2O 2+2x (CO3) 1-x A 2x , - Alkali metal-doped and / or alkali carbonate-loaded mixed compound Ax Ln 2-x OCO3, - Alkali metal-doped and / or alkali carbonate-loaded mixed compound A x Ln2- X (CO3), where Ln stands for lanthanides from lanthanum to lutetium and A for alkali metals Li, Na, K.

[0006] In particular, oxides and oxycarbonates of the rare earth metal gadolinium are well suited as sensor materials, as they exhibit the greatest insensitivity to humidity of all materials tested in the invention.

[0007] In another embodiment, the gas-sensitive material of the CO2 sensor according to the invention comprises alkali metal-doped and / or alkali carbonate-loaded semiconducting metal oxides, e.g. SnO2, WO3, In2O3, ZnO, TiO2, CuO2.

[0008] In order to produce the gas-sensitive material for the sensor according to the invention, in principle all suitable methods known from the prior art can be used with which doping and / or loading of semiconducting metal oxides or metal oxycarbonates with alkali metals or alkali carbonates is possible.

[0009] The simplest method, for example, involves loading semiconducting metal oxides or rare earth oxycarbonates as starting materials with alkali carbonates in a mechanical process, e.g., in a ball mill or a mixing screw. Chemisorption results in the formation of small agglomerates of alkali carbonates on larger particles of the starting material.

[0010] In the alternative process for producing the gas-sensitive material for the CO2 sensor, at least one alkali carbonate of the general formula A2CO3 is used as one of the starting materials, where A stands for alkali metals Li, Na, K.

[0011] According to the invention, two or more different alkali carbonates are preferably used, in particular in a mixing ratio that results in a eutectic mixture. This has the advantage that the melting point of such a mixture is lower than the melting point of the individual substances used.

[0012] In one embodiment of this process, at least one semiconducting metal oxide or a rare earth oxide of the general formula Ln2O3 is used as the second starting material and reacted with at least one alkali carbonate of the general formula A2CO3 in a modified flux process, where Ln stands for lanthanides from lanthanum to lutetium and A for alkali metals Li, Na, K. The alkali carbonate serves as the solvent and the rare earth oxide as the solute.

[0013] In this flux process, which has been specifically modified in the present invention, the starting materials, unlike a conventional flux process, are preferably used in a ratio of approximately 1:1, mixed, and heated to a suitable melting temperature. This produces a product that contains pure, alkali metal-doped and / or alkali carbonate-loaded semiconducting metal oxides or rare earth oxides or rare earth oxycarbonates or mixed compounds. Substances from the substance classes Ln2O3, ALnOCO3, Ln2O are preferably formed. 2+2x (CO3) 1-x A 2x , ALn(CO3), Ln2O2CO3, or mixed compounds. After the reaction, the product is cooled and washed with water as needed.

[0014] The composition of the product and thus of the gas-sensitive material can be varied according to the invention by choosing the reaction conditions such as atmosphere, temperature or reaction time.

[0015] For example, the following reactions take place with rare earth oxides under the CO2 atmosphere: Ln2O3 + A2CO3 + 3CO2 → 2 ALn(CO3) Ln2O3 + A2CO3 + CO2 -> 2 ALnOCO3 Ln2O3 + A2CO3 → Ln2O 2+2x (CO3) 1-x A 2x + (1-x) A2CO3 Ln2O3 + A2CO3 + CO2 -> Ln2O2CO3 + A2CO3

[0016] Under the oxygen atmosphere, the following reaction takes place with rare earth oxides: Ln2O3 + A2CO3 → 2 ALnO2 + CO2

[0017] Depending on the reaction time and temperature, different amounts of residual Ln2O3 remain in the product, which has not reacted completely.

[0018] In addition to a mixture of defined compounds, as described above, compounds are also formed that contain more than one type of alkali metal (example: Li 0.5 N / a 0.5 SMO2).

[0019] The temperature at which the described process takes place is in particular at least 600°C.

[0020] The gas-sensitive material produced by one of the possible methods can then be applied to a sensor substrate, including a MEMS substrate, using conventional techniques.

[0021] The measurement accuracy of the sensor according to the invention in the measuring range up to 1500 ppm is 50 to 150 ppm, which corresponds to 3% to 10% of this measuring range. These values ​​are comparable to the values ​​known from the state of the art for commercially available NDIR CO2 sensors.

[0022] The advantages of the sensor according to the invention compared to the prior art, e.g. according to EP3751264A3, include the high insensitivity to air humidity and the low electrical resistance with comparable accuracy.

[0023] The insensitivity to humidity of the sensor according to the invention has the advantage that it can operate without the otherwise necessary complex compensation electronics and without a humidity sensor. The significantly lower electrical resistance compared to state-of-the-art CO2 sensors also enables a sensor with cost-effective evaluation electronics.

[0024] Both advantages described above achieved by the sensor according to the invention enable an overall very cost-effective CO2 sensor that is also suitable for the consumer device sector.

[0025] Further advantages, features and possible applications of the sensor and the method for its production are described below using the embodiments described below with reference to the figures. Fig. 1. Table of the inventive materials and comparative materials used Fig. 2. Microscope image of the material soc02 (flux-Sm2O2CO3) with size scale displayed 10 µm Fig. 3. Microscope image of the material soc02 (flux-Sm2O2CO3) with size scale displayed 1 µm Fig. 4. Microscope image of the material soc02 (flux-Sm2O2CO3) with size scale displayed at 500 nm Fig. 5. Microscope image of the reference material TSLOC with size scale displayed 10 µm Fig. 6. Microscope image of the reference material TSLOC with size scale displayed 1 µm Fig. 7. Microscope image of the reference material TSLOC with size scale indicated at 500 nm Fig. 8. XRD X-ray diffraction image for the material gocf02 and Gd2O3 Fig. 9. XRD X-ray diffraction image for the material soc03 and Sm2O2CO3 Fig. 10. XRD X-ray diffraction image for the reference material TSLOC and La2O2CO3 Fig. 11. XRD X-ray diffraction image for the material pjloc01 and La2O2CO3 Fig. 12. XRD X-ray diffraction image for the reference material sn01 and SnO2 (cassiterite) Fig. 13. Electrical resistance / CO2 concentration diagram of the material loc02e at different humidity values Fig. 14. Electrical resistance / CO2 concentration diagram of the material goc29b at different humidity values Fig. 15. Electrical resistance / CO2 concentration diagram of the material pjloc01c at different humidity values Fig. 16. Electrical resistance / CO2 concentration diagram of the material soc02b at different humidity values Fig. 17. Electrical resistance / CO2 concentration diagram of the material sn01a at different humidity values Fig. 18. Electrical resistance / rh(%) concentration diagram of the material loc02e at 0 ppm CO2 (zero line) Fig. 19. Electrical resistance / rh(%) concentration diagram of the material goc29a at 0 ppm CO2 (zero line) Fig. 20. Electrical resistance / rh(%) concentration diagram of the material goc29b at 0 ppm CO2 (zero line) Fig. 21. Electrical resistance / rh(%) concentration diagram of the material pjloc01a at 0 ppm CO2 (zero line) Fig. 22. Electrical resistance / rh(%) concentration diagram of the material pjloc01c at 0 ppm CO2 (zero line) Fig. 23. Electrical resistance / rh(%) concentration diagram of the material soc02a at 0 ppm CO2 (zero line) Fig. 24. Electrical resistance / rh(%) concentration diagram of the material soc02b at 0 ppm CO2 (zero line) Fig. 25. Electrical resistance / rh(%) concentration diagram of the material sn01a at 0 ppm CO2 (zero line) Examples of implementation: Production of the gas-sensitive material from rare earth oxides and alkali carbonates according to the modified flux method according to the invention

[0026] For the production of the reference material La2O2CO3 according to the state of the art according to EP3751264A3, lanthanum oxalate is used as the starting material. This material is calcined without prior preparation in a furnace at temperatures of 450-550°C for up to 72 hours. After the heat treatment, the material is ready for use.

[0027] For the production of gas-sensitive materials according to the invention using the modified flux method, a rare earth oxide is used as the starting material, which is mixed with a eutectic mixture of alkali carbonates and heated to temperatures of at least 600°C. By using the eutectic mixing ratio, a lower melting point for the mixture is achieved than the melting points of individual substances. The calcined mixture can be used directly or after purification by repeated washing with water as a gas-sensitive material for the production of a CO2 sensor.

[0028] The manufactured and tested inventive and comparative materials from the state of the art (St.dT) are shown in the table ( Fig. 1) are listed. Preparation of the inventive nano-sensitive material alkali metal-doped or alkali metal-loaded La2O2CO3

[0029] The following eutectic mixing ratio of the alkali carbonates proved to be particularly advantageous in the production of the gas-sensitive material based on La2O2CO3: 0.476 Li2CO3 + 0.270 Na2CO3 + 0.254 K2CO3 -> melting point approx. 600°C

[0030] (In comparison, individual substances have higher melting points: Li2CO3 → Melting point: 723°C Na2CO3 → Melting point: 851°C K2CO3 → melting point: 891°C)

[0031] The complete eutectic mixture is mixed with the rare earth oxide La2O3 in a 1:1 molar ratio and calcined in a furnace at 700°C for 15 hours. This occurs under a CO2 atmosphere or under laboratory air. The mixture is then washed four times with distilled H2O and dried overnight in a drying cabinet at 80°C or in a desiccator. The material is then ready for use.

[0032] Under a CO2 atmosphere, doped or loaded oxycarbonate La2O2CO3 precipitates, or a mixture consisting of doped, loaded, and pure oxycarbonate La2O2CO3, as well as doped, loaded, and pure oxide La2O3. Under laboratory air, only pure, doped, or loaded oxide La2O3 precipitates. Preparation of the inventive gantry-sensitive material alkali metal-doped or alkali metal-loaded Gd2O3

[0033] The complete eutectic mixture of alkali carbonates from the previous example is mixed with the rare earth oxide Gd2O3 in a 1:1 molar ratio and calcined in a furnace at 700°C for 15 hours. This is done under laboratory air. The mixture is then washed four times with distilled water to remove the remaining alkali carbonates and dried overnight at 80°C in a drying cabinet or in a desiccator. The material is then ready for use.

[0034] In this example, only pure, doped or loaded rare earth oxide Gd2O3 precipitates. Preparation of the inventive nano-sensitive material alkali metal-doped or alkali metal-loaded SnO2

[0035] The complete eutectic mixture of alkali carbonates from the previous example is mixed with the semiconducting metal oxide SnO2 in a 1:1 molar ratio and calcined in a furnace at 700°C for 15 hours under a CO2 atmosphere. The material is then ready for use.

[0036] In this example, doped and loaded SnO2 is obtained. Production of the gas-sensitive material according to the invention from rare earth oxycarbonates

[0037] A rare earth oxycarbonate, in this case La2O2CO3, is mixed with alkali carbonates, in this case Li2CO3, in a molar ratio of 1:1. The mixture is then ball milled either dry or with a small amount of water for one hour until homogeneous at 200 rpm. If water is used, care must be taken to achieve an oil-like viscosity of the mixture, and after milling, the mixture must be dried in a drying cabinet.

[0038] The dry mixture can then be applied as a sensitive layer to a sensor substrate, including MEMs substrates, using conventional coating methods. Production of the gas-sensitive material according to the invention from semiconducting metal oxides

[0039] A semiconducting metal oxide, here SnO2, is mixed with alkali carbonates, here Li2CO3, in a molar ratio of 1:1 and heated to 750°C in a CO2 atmosphere for 15 h in an oven. The material is then washed four times with distilled water and dried overnight in a drying oven.

[0040] The dry mixture can then be applied as a sensitive layer to a sensor substrate, including MEMs substrates, using conventional coating methods. Crystal structure investigations of gas-sensitive materials according to the invention

[0041] For the assessment of the crystal structure and size of the prepared materials according to the invention, a microscope image of the material soc02 (flux-Sm2O2CO3) with a size scale of 10 µm ( Fig. 2), 1 µm ( Fig. 3), 500 nm ( Fig. 4) was created.

[0042] For the assessment of the crystal structure and size of the reference materials according to the St.dT, a microscope image of the material TSLOC (La2O2CO3, produced according to EP3751264A3) with a size scale of 10 µm ( Fig. 5), 1 µm ( Fig. 6), 500 nm ( Fig. 7) was created.

[0043] The production of the gas-sensitive material using the modified flux method according to the invention leads to the formation of structures that differ significantly in shape and size from the materials produced according to the prior art according to WO2022 / 049402A2, EP3751264A3, and US020200400606A. The particle sizes of soc02 (~0.5 µm - 1 µm) are approximately 10 times larger than the agglomerates of the comparative material TSLOC according to the St.dT (~50 nm). Compared to TSLOC, which has irregular agglomerate structures, the materials produced using the described method exhibit relatively regular agglomerates in platelet form. These have a predominantly hexagonal crystal structure in the case of the pure, alkali metal-doped and / or alkali carbonate-loaded rare earth oxycarbonates, and a predominantly cubic crystal structure in the case of the pure, alkali metal-doped and / or alkali carbonate-loaded rare earth oxides.

[0044] The XRD X-ray diffractometry material and crystal structure investigations regarding the material composition and crystal size according to Scherrer are carried out in the Fig. 8, Fig. 9, Fig. 10, Fig. 11 and Fig. 12 shown.

[0045] The Scherrer equation is a formula in X-ray diffraction and crystallography that relates the size of submicrometer crystallites in a solid to the broadening of a peak in a diffraction pattern.

[0046] Fig. Figure 8 shows an XRD X-ray diffraction pattern for the materials gocf02 and Gd2O3. The material gocf02 was synthesized or recrystallized from Gd2O3 using the flux method in air, i.e., without CO2. Apart from the reference reflections for Gd2O3, no secondary phase is identifiable.

[0047] Fig. Figure 9 shows an XRD X-ray diffraction pattern for the materials soc03 and Sm2O2CO3. The material soc03 was synthesized or recrystallized from Sm2O3 using the flux method under a CO2 atmosphere at 1020 °C. The synthesis was not fully implemented here. In the range between 45 ° and 55 °, the experimental reflections are shifted to the left and right compared to the reference. This is a clear indication of doping, since the lattice constants of the unit cell are different.

[0048] Fig. Figure 10 shows an XRD image for the reference material TSLOC and La2O2CO3. Apart from the reference reflections for La2O2CO3, no secondary phase is identifiable.

[0049] Fig. Figure 11 shows an XRD X-ray diffraction pattern for the material pjloc01 and La2O2CO3. The material was synthesized or recrystallized from La2O3 using the flux method under a CO2 atmosphere at 700°C. Compared to the material TSLOC, reflections can be seen here, for example, at 38°C, which are attributable to alkali carbonates.

[0050] Fig. Figure 12 shows an XRD X-ray diffraction pattern for the material SnO2 and SnO2 (cassiterite). The material was synthesized and recrystallized from SnO2 using the flux method under a CO2 atmosphere at 700°C. In addition to the reflections for SnO2, the material exhibits many other smaller reflections that can be attributed to alkali carbonates, since the material was not washed.

[0051] The crystallite size was calculated according to Scherrer for the following materials: TSLOC 71 nm pjloc01 272 nm soc02 158 nm soc03 244 nm goc29 286 nm gocf02 380 nm

[0052] It can be seen that the synthesis according to the invention produces crystallites that are two to five times larger than those of the comparison material TSLOC.

[0053] The Gd2O3 used for the synthesis of gocf02 and goc29 comes from Sigma Aldrich and comes with the manufacturer's specified particle sizes <100 nm (measured by BET). Testing the properties of gas-sensitive materials according to the invention

[0054] The material goc29 is a mixture of doped and pure oxide and oxycarbonate. The material gocf02, on the other hand, is a pure or only lightly doped oxide.

[0055] Fig. Figure 13 shows an electrical resistance versus CO2 concentration diagram for the standard dT reference material Ioc02e at various humidity levels. It illustrates a relatively high sensitivity to humidity. In real-world sensor device applications, this requires an additional, relatively accurate humidity sensor for humidity compensation.

[0056] In comparison, the materials according to the invention show Fig. 14 (goc29b), Fig. 15 (pjloc01c), Fig. 16 (soc02b) and Fig. 17 (sn01a) has a low sensitivity to humidity.

[0057] Furthermore, it is evident that the electrical resistance of the materials according to the invention is a factor of 200-1000 lower than that of the material from the standard dT. This allows for significantly more cost-effective sensor signal evaluation electronics, thus enabling a low-cost CO2 air quality measuring device with this CO2 sensor material.

[0058] In Fig. Figure 18 shows the zero line of electrical resistance versus relative humidity (rh(%)) concentration diagram of the standard dT reference material loc02e at 0 ppm CO2. Here, too, a strong sensitivity to relative humidity is evident.

[0059] In Fig. 19 to 25 show the zero line of electrical resistance in relation to relative humidity rh(%) of various inventive sensor materials at 0 ppm CO2.

[0060] In Fig. 18 shows a strong reduction of the electrical resistance for the reference material loc02e known from the St.dT when the humidity increases, which highlights the sensitivity to this.

[0061] In comparison, Fig. 19 to Fig. 25 For the inventive materials goc29, pjloc01, soc02, and sn01, an increase in humidity has only a very slight effect on the electrical resistance. The materials goc29 and soc02 perform best in this regard.

[0062] These results confirm the relatively low sensitivity to humidity and the significantly lower level of electrical resistance of the sensor materials according to the invention compared to state-of-the-art materials. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] WO 2022 / 049402A2 [0002, 0043] EP 3751264A3 [0002, 0022, 0026, 0042, 0043] US 020200400606A [0002, 0043]

Claims

[1] Gas sensor for measuring CO2, comprising a gas-sensitive material containing at least one substance from the group consisting of semiconducting metal oxides, rare earth oxides or rare earth oxycarbonates, characterized by that this substance contains alkali metal atoms and / or alkali metal ions. [2] Gas sensor according to claim 1, characterized by that the gas-sensitive material is doped with at least one alkali metal and / or loaded with at least one alkali carbonate. [3] Gas sensor according to one of claims 1 to 2, characterized by that the gas-sensitive material contains at least one substance from the group of the following substance classes: - Alkali metal-doped and / or alkali carbonate-loaded mixed compound Ln2O2CO3, - Alkali metal-doped and / or alkali carbonate-loaded semiconducting metal oxide, in particular SnO2, WO3, In2O3, ZnO, TiO2, CuO, - Alkali metal-doped and / or alkali carbonate-loaded rare earth oxide Ln2O3, - Alkali metal-doped and / or alkali carbonate-loaded mixed compound Ln2O 2+2x (CO3) 1-x A 2x , - Alkali metal-doped and / or alkali carbonate-loaded mixed compound A x Ln 2-x OCO3, - Alkali metal-doped and / or alkali carbonate-loaded mixed compound A x Ln 2-x (CO3), where Ln stands for lanthanides from lanthanum to lutetium and A for alkali metals Li, Na, K. [4] A method for producing a gas-sensitive material for the CO2 sensor according to any one of claims 1 to 3, characterized by that at least one alkali carbonate of the general formula A2CO3 reacts with at least one rare earth oxide of the general formula Ln2O3 or a semiconducting metal oxide, where Ln stands for lanthanides from lanthanum to lutetium and A stands for alkali metals Li, Na, K. [5] Method according to claim 4, characterized by that at least two alkali carbonates are present in a molar ratio which results in a eutectic mixture. [6] Method according to one of claims 4 to 5, characterized by that the reaction takes place at a temperature of at least 600°C. [7] A method for producing a gas-sensitive material for the CO2 sensor according to any one of claims 1 to 3, characterized by that at least one rare earth oxycarbonate of the general formula Ln2O2CO3 or a semiconducting metal oxide is loaded with at least one alkali carbonate of the general formula A2CO3, where Ln stands for lanthanides from lanthanum to lutetium and A stands for alkali metals Li, Na or K.

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