Co 2 sensor, and processes for the manufacture thereof

Abstract

The invention relates to a CO2 sensor having low electrical resistance and high resistance to air humidity. The invention also relates to processes for manufacturing same. 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, said substance containing at least two different alkali metals. In particular, this substance is doped with at least two alkali metals and / or loaded with at least two alkali carbonates. In a preferred process for manufacturing the gas-sensitive material, at least one rare earth oxide or at least one semiconducting metal oxide is heated with at least two different alkali carbonates for doping, alkali carbonates being present at a molar ratio which results in a eutectic mixture. In an alternative process, at least one rare earth oxycarbonate or at least one semiconducting metal oxide is loaded with at least two different alkali carbonates.

Description

[0001] CCh sensor with low electrical resistance and high insensitivity to humidity and method for its manufacture

[0002] Description

[0003] The invention relates to a CC 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 at least two different alkali metals. In particular, alkali metals are present in the form of doped atoms and / or in the form of ions from alkali carbonates.

[0004] CO2 sensors based on rare earth oxycarbonates are known, for example, from W02022 / 049402A2, EP3751264A3, and US020200400606A. However, the disadvantages of state-of-the-art CO2 sensors include high sensitivity to humidity and relatively high resistance. Both of these disadvantages require complex compensation electronics, a humidity sensor, and cost-intensive evaluation electronics to obtain reliable measurement data.

[0005] A prior art publication (Yamauchi et al. J. Eur. Cerarn. Soc. 28 (2008) 27-34) addresses the problem of the stability of materials made from a rare earth oxide and a lithium carbonate under different reaction conditions. Furthermore, patent application JP2004239 831A addresses the task of providing a CO2 sensor that is less sensitive to humidity. The proposed solution involves a material containing semiconducting metal oxides or rare earth oxides and an alkali carbonate.

[0006] 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.

[0007] To solve this problem, a CCh sensor is provided in which the gas-sensitive material contains at least two different alkali metals. 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.

[0008] In a preferred embodiment, the gas-sensitive material of the CCh sensor according to the invention comprises at least one substance belonging to one of the following substance classes:

[0009] - Alkali metal-doped and / or alkali carbonate-loaded mixed compound Ln2O2CO3,

[0010] - Alkali metal-doped and / or alkali carbonate-loaded rare earth oxide Ln2Ü3,

[0011] - Alkali metal-doped and / or alkali carbonate-loaded mixed compound Ln2O2 + 2x(CO3)l-xA2x,

[0012] - Alkali metal-doped and / or alkali carbonate-loaded mixed compound AxLn2-xOCO3,

[0013] - 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] In the alternative process for producing the gas-sensitive material for the CO2 sensor, at least two different alkali carbonates of the general formula A2CO3 are used as starting materials, where A stands for alkali metals Li, Na, K.

[0019] According to the invention, two or more different alkali metal 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. 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 two different alkali metal carbonates of the general formula A2CO3 in a modified flux process, where Ln stands for lanthanides from lanthanum to lutetium and A stands for alkali metals Li, Na, K. Alkali metal carbonates serve as the solvent and the rare earth oxide as the dissolved substance.

[0020] 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 results in 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 Ln2Os, AlnOCOs, Ln2O2 are preferably formed. + 2x(CO3)i-xA2x, ALn(CO3), Ln2O2CO3, or mixed compounds. After the reaction, the product is cooled and washed with water as needed.

[0021] According to the invention, the composition of the product and thus of the gas-sensitive material can be varied by choosing the reaction conditions such as atmosphere, temperature or reaction time.

[0022] For example, the following reactions take place with rare earth oxides under the CC atmosphere:

[0023] Ln2Ü3 + A2CO3 + 3CO2 -> 2 ALn(CO3)

[0024] Ln2Ü3 + A2CO3 + CO2 -> 2 ALnOCOs

[0025] Ln2O3 + A2CO3 -> Ln2O 2+ 2x(CO3)l-xA2x + (1-x) A2CO3

[0026] Ln2Ü3 + A2CO3 + CO2 -> Ln2O2CO3 + A2CO3 Under the oxygen atmosphere, the following reaction takes place with rare earth oxides:

[0027] Ln2Ü3 + A2CO3 -> 2 ALnCh + CO2

[0028] Depending on the reaction time and temperature, different amounts of residual Ln2Ü3 remain in the product, which has not reacted completely.

[0029] 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: Lio.sNao.sSmC ).

[0030] The temperature at which the described process takes place is in particular at least 600°C.

[0031] 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.

[0032] 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 NDI-CCh sensors.

[0033] 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.

[0034] The presence of different alkali metals in the gas-sensitive material according to the invention appears to play an important role in the improved stability and the associated higher insensitivity to air humidity as well as in the lower electrical resistance compared to prior art materials.

[0035] 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 the prior art CCh sensors also enables a sensor with cost-effective evaluation electronics.

[0036] Both advantages described above achieved by the sensor according to the invention enable an overall very cost-effective CCh sensor that is also suitable for the consumer device sector.

[0037] 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.

[0038] Fig. 1. Table of the materials used according to the invention and comparison materials

[0039] Fig. 2. Microscope image of the material soc02 (flux-Sr OzC ) with size scale indicated 10 pm

[0040] Fig. 3. Microscope image of the material soc02 (flux-Sm2O2CO3) with size scale indicated 1 pm

[0041] Fig. 4. Microscope image of the material soc02 (flux-Snri2O2CO3) with size scale indicated at 500 nm

[0042] Fig. 5. Microscope image of the reference material TSLOC with size scale indicated 10 pm Fig. 6. Microscope image of the reference material TSLOC with size scale indicated 1 pm

[0043] Fig. 7. Microscope image of the reference material TSLOC with size scale indicated at 500 nm

[0044] Fig. 8. XRD X-ray diffraction pattern for the material gocf02 and Gd2Ü3

[0045] Fig. 9. XRD X-ray diffraction pattern for the material SOC03 and Sm2O2CO3

[0046] Fig. 10. XRD X-ray diffraction pattern for the reference material TSLOC and La2O2COs

[0047] Fig. 11. XRD X-ray diffraction pattern for the material pjlocOl and La2O2CO3

[0048] Fig. 12. XRD X-ray diffraction pattern for the reference material sn01 and SnO2 (cassiterite)

[0049] Fig. 13. Electrical resistance / CO2 concentration diagram of the material Ioc02e at different humidity values

[0050] Fig. 14. Electrical resistance / CO2 concentration diagram of the material goc29b at different humidity values

[0051] Fig. 15. Electrical resistance / CO2 concentration diagram of the material pjlocOl c at different humidity values

[0052] Fig. 16. Electrical resistance / CO2 concentration diagram of the material soc02b at different humidity values

[0053] Fig. 17. Electrical resistance / CO2 concentration diagram of the material sn01a at different humidity values. Fig. 18. Electrical resistance I rh(%) -concentration diagram of the material Ioc02e at 0 ppm CO2 (zero line).

[0054] Fig. 19. Electrical resistance / rh(%) concentration diagram of the material goc29a at 0 ppm CO2 (zero line)

[0055] Fig. 20. Electrical resistance I rh(%) -concentration diagram of the material goc29b at 0 ppm CO2 (zero line)

[0056] Fig. 21. Electrical resistance I rh(%) -concentration diagram of the material pjlocOI a at 0 ppm CO2 (zero line)

[0057] Fig. 22. Electrical resistance I rh(%) -concentration diagram of the material pjlocOlc at 0 ppm CO2 (zero line)

[0058] Fig. 23. Electrical resistance I rh(%) -concentration diagram of the material soc02a at 0 ppm CO2 (zero line)

[0059] Fig. 24. Electrical resistance I rh(%) -concentration diagram of the material soc02b at 0 ppm CO2 (zero line)

[0060] Fig. 25. Electrical resistance / rh(%) concentration diagram of the material sn01a at 0 ppm CO2 (zero line)

[0061] Implementation examples

[0062] Production of the gas-sensitive material from rare earth oxides and alkali carbonates according to the modified flux method according to the invention

[0063] For the production of the prior art reference material La2O2CO3 according to EP3751264A3, lanthanum oxalate is used as the starting material. This is calcined in a furnace at temperatures of 450–550°C for up to 72 hours without prior preparation. After the heat treatment, the material is ready for use. 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 as a gas-sensitive material for the production of a CO2 sensor, either directly or after purification by repeated washing with water.

[0064] The manufactured and tested inventive and comparative materials from the state of the art (St.dT) are listed in the table (Fig.1).

[0065] Preparation of the inventive qanssensitive material alkali metal-doped or alkali metal-loaded

[0066] 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:

[0067] 0.476 L2CO3 + 0.270 Na2CO3 + 0.254 K2CO3 -> melting point approx. 600°C

[0068] (In comparison, individual substances have higher melting points:

[0069] Li2CO3 -> Melting point: 723°C

[0070] Na2CO3 -> Melting point: 851 °C

[0071] K2CO3 -> melting point: 891 °C)

[0072] The complete eutectic mixture is mixed with the rare earth oxide La2Os 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. Under a CCH atmosphere, doped or loaded oxycarbonate L32O2CO3 precipitates, or a mixture consisting of doped, loaded, and pure oxycarbonate La2O2CO3, and doped, loaded, and pure oxide La2Os. Under laboratory air, only pure, doped, or loaded oxide La2O3 precipitates.

[0073] Preparation of the inventive quantifiable material alkali metal-doped or alkali metal-loaded Gd2O3

[0074] The complete eutectic mixture of alkali carbonates from the previous example is mixed with the rare earth oxide Gd2Os 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.

[0075] In this example, only pure, doped or loaded rare earth oxide Gd2Ü3 precipitates.

[0076] Preparation of the inventive quantifiable material alkali metal-doped or alkali metal-loaded SnO2

[0077] 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.

[0078] In this example, doped and loaded SnO2 is obtained.

[0079] Production of the gas-sensitive material according to the invention from rare earth oxycarbonates

[0080] A rare earth oxycarbonate, here La2O2CO3, is mixed with alkali carbonates, here U2CO3, in a molar ratio of 1:1. The mixture is then mixed homogeneously in a ball mill at 200 rpm for one hour, either dry or with a small amount of water. 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.

[0081] The dry mixture can then be applied as a sensitive layer to a sensor substrate, including MEMs substrates, using conventional coating methods.

[0082] Production of the gas-sensitive material according to the invention from semiconducting metal oxides

[0083] 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 a furnace. The material is then washed four times with distilled water and dried overnight in a drying oven.

[0084] The dry mixture can then be applied as a sensitive layer to a sensor substrate, including MEMs substrates, using conventional coating methods.

[0085] Crystal structure investigations of inventive gas-sensitive materials

[0086] To assess the crystal structure and size of the prepared materials according to the invention, a microscope image of the material soc02 (flux-Srri2O2CO3) was created with the size scale displayed at 10 pm (Fig. 2), 1 pm (Fig. 3), and 500 nm (Fig. 4).

[0087] To assess the crystal structure and size of the reference materials according to the standard dT, a microscope image of each of the material TSLOC (La2O2CO3, produced according to EP3751264A3) was taken, with the size scale displayed at 10 pm (Fig. 5), 1 pm (Fig. 6), and 500 nm (Fig. 7). 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 state of the art according to WO2022 / 049402A2, EP3751264A3, and US020200400606A. The particle sizes of SocO2 (~0.5 pm - 1 pm) are approximately 10 times larger than the agglomerates of the reference material TSLOC according to the standard dT (~50 nm). Compared to TSLOC, which has irregular agglomerate structures, the materials produced by the described process 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.

[0088] The XRD X-ray diffraction material and crystal structure investigations regarding the material composition and crystal size according to Scherrer are shown in Figures 8, 9, 10, 11 and 12.

[0089] 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.

[0090] Fig. 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.

[0091] Fig. 9 shows an XRD X-ray diffraction pattern for the materials soc03 and S1T12O2CO3. 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 0 and 55 0The experimental reflections are shifted to the left and right compared to the reference. This is a clear indication of doping, as the lattice constants of the unit cell are different. Fig. 10 shows an XRD image for the reference material TSLOC and La2O2CO3. Apart from the reference reflections for La2O2CO3, no secondary phase is identifiable.

[0092] Fig. 11 shows an XRD X-ray diffraction pattern for the material pjlocOl and l_a2O2CO3. The material was synthesized or recrystallized from La2Ü3 using the flux method under a CO2 atmosphere at 700°C. In comparison to the material TSLOC, reflections can be seen here, for example, at 38°C, which are attributable to alkali carbonates.

[0093] Fig. 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.

[0094] A calculation of the crystallite size according to Scherrer was carried out for the following materials:

[0095] TSLOC 71nm pjlocOl 272nm soc02 158nm soc03 244nm goc29 286nm gocf02 380nm

[0096] 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.

[0097] The Gd2O3 used for the synthesis of gocf02 and goc29 was supplied by Sigma Aldrich and has a particle size of <100 nm (measured by BET). Testing the properties of gas-sensitive materials according to the invention

[0098] 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.

[0099] Fig. 13 shows an electrical resistance / 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.

[0100] In comparison, the materials according to the invention in Fig. 14 (goc29b), Fig. 15 (pjlocOlc), Fig. 16 (soc02b) and Fig. 17 (sn01a) show a low sensitivity to air humidity.

[0101] 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.

[0102] Fig. 18 shows the zero line of electrical resistance versus relative humidity (rh(%)) concentration diagram of the standard dT reference material Ioc02e at 0 ppm CO2. Here, too, a strong sensitivity to relative humidity is evident.

[0103] Fig. 19 to 25 show the zero line of the electrical resistance in relation to the relative humidity rh(%) of various inventive sensor materials at 0 ppm CO2. Fig. 18 shows a strong reduction in electrical resistance as the humidity increases for the comparison material IocO2e known from the state of the art, highlighting its sensitivity to this. In comparison, Fig. 19 to Fig. 25 show only a very slight influence on the electrical resistance for the inventive materials goc29, pjlocOl, socO2 and snO1 as the humidity increases. The materials goc29 and socO2 perform best in this regard. These results confirm the relatively low sensitivity to humidity and the significantly lower level of electrical resistance of the inventive sensor materials compared to prior art materials.

Claims

Patent claims 1 . A CO2 sensor with low electrical resistance and high insensitivity to humidity, 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 and at least two different alkali metals.

2. Gas sensor according to claim 1, characterized in that alkali metals are present in the form of doped atoms and / or in the form of ions from alkali carbonates.

3. Gas sensor according to one of claims 1 to 2, characterized in 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, ln2O3, ZnO, TiC, CuO, - Alkali metal-doped and / or alkali carbonate-loaded rare earth oxide Ln2C>3, - Alkali metal-doped and / or alkali carbonate-loaded mixed compound Ln2O 2+ 2x(CO3)i-xA2x, - Alkali metal-doped and / or alkali carbonate-loaded mixed compound AxLn2-xOCO3, - 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.

4. A method for producing a CCk sensor, which comprises preparing the gas-sensitive material by the following steps: - Providing at least two different alkali carbonates of the general formula A2CO3 in a molar ratio which results in a eutectic mixture, and at least one rare earth oxide of the general formula l_n2O3 or a semiconducting metal oxide, - Reaction of the alkali carbonates with the rare earth oxides or semiconducting metal oxides, where Ln stands for lanthanides from lanthanum to lutetium and A for alkali metals Li, Na, K.

5. A process for producing a CO2 sensor according to claim 4, characterized in that the temperature is increased to at least 600°C for the reaction of the alkali carbonates with at least one rare earth oxide or a semiconducting metal oxide.

6. A method for producing a CO2 sensor, which comprises preparing the gas-sensitive material by the following steps: - Providing at least one rare earth oxycarbonate of the general formula Ln2Ü2CO3 or a semiconducting metal oxide and at least two different alkali carbonates of the general formula A2CO3, - Loading the rare earth oxycarbonate or the semiconducting metal oxide with alkali carbonates, where Ln stands for lanthanides from lanthanum to lutetium and A for alkali metals Li, Na or K.

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