Background

Environments of interest are characterised by the nature and the concentration of specific gas-borne species. The measurement and the control of such environments reguires multiple sensors, each designed to target a specific component. For example, indoor air guality usually demands the measurement of 3 gaseous species, H2O, CO2, VOC while exhaust emissions demands the measurement of H2O, CO2, O2, CO. It simplifies matters greatly if the same technology is used to detect all the target gases. Combining different sensor technologies, each for a particular gas, adds complexity, cost and larger module footprints. Of the available gas sensor technologies, metal oxide semiconductor (MOx) technology can detect the largest range of gases of interest. Advantages of MOx are low-cost, simple construction, long-lifetimes, small form factor and amenable to high volume manufacture. The use of a pulse-powered MEMS (MicroElectroChemical Systems) platform ensures the power consumed by having to heat the sensor to 300- 550°C to achieve performance, can be kept to below 10mW. Commercial MOx sensors are available for CO, tVOC, H2O but, apart from FIS Inc. (Japan), not for CO2.

The prevailing technology for CO2 is optical (non-dispersive infra-red). It is unsuitable for direct use in hot humid environments, reguiring cooling and condensation of the incoming gas before measurement can be made, so using it in the exhaust of combustion flues for example would not be viable. Outside of this very specific application, optical CO2 sensors still remain costly despite considerable price reduction over the years, because they are component-rich (a light source, a light detector, an optical tube/path, mirrors, reflectors, optical filters and ancillary circuitry). Some companies offer low-price NDIR sensors but at the expense of accuracy and reliability due to dispensing with a reference detector, being reliant on an ABC (average baseline calculation) algorithm strategy where the lowest point over a defined time period is assumed to be 400ppm CO2. Photo-acoustic are developing fast for non-aggressive environments, exploiting the availability of cheap microphones developed for mobile phones. Solid-state electrochemical sensors are better suited for detecting large shifts in CO2 rather than the more modest changes required. Slow response and recovery times, and a high power consumption due to a high operating temperature are further disadvantages. Colorimetric represents a traditional inexpensive technology where CO2 reacts with a chemically-impregnated paper strip resulting in a change of colour. It comes with a number of performance downsides, including being very qualitative, some subjectivity in interpreting colour change, and single-use only.

Prior Art

Semiconducting metal oxide sensing (MOx) technology offers a compelling fit with the requirements for inexpensive, long life, low-maintenance and miniature CO2 sensors. Unlike the alternative technologies, MOx technology can be miniaturised, reducing power consumption and with the smaller footprint of the MEMS sensors, allowing greater ease of integration with electronic boards. Consequently, there have been a number of attempts, mainly of an academic nature, to develop a semiconductor metal oxide sensor in the past 20 years. Out of these efforts, one semiconductor sensor product has made it to the marketplace, sensor product AB SB-AQ6A-00 from FIS Inc. It is based on lanthanum-doped SnO2. Although SnO2 is the most widely deployed MOx material, its problems with stability and humidity interference are well documented. Other efforts have focused on other gas-sensitive systems such as thick-film Ba-containing oxides (US20120161796, DE4437692) and on rare earth oxy-carbonate systems (Paper Reference 3). In the main, all exhibit high impedance, a positive increase in resistance in the presence of CO2 (pushing the measured resistance values even higher), uncertain discriminatory properties in the dilute concentrations of 350 - 2000ppm for indoor air quality and all with interference to the signal from humidity. An inherent high resistivity in a gas-sensing material poses considerable challenges in designing a workable circuit for signal measurement, adding cost and complexity, thereby discouraging uptake. Interference from humidity is a less desirable feature too. It is difficult to eliminate it as humidity plays a critical role in the reaction mechanism responsible for CO2 detection (Paper reference 4), with the magnitude of the CO2 signal for a given CO2 concentration increasing with increasing humidity levels. Conventional practise is to include in the sensor package/module a humidity sensor - typically a roomtemperature polyimide capacitance sensor - whereby the output is used to correct the measured MOx response signal. This approach can be fraught with difficulties due to the hot MOx sensor ‘seeing’ more dissolved water compared to the humidity sensor, as a result of differences in air flow, ambient temperature and location on the board. Offsets are typically used in an attempt to manage this situation (reference 5) but requires user intervention and as the offset needed is not constant but subject to change with changing conditions, vigilance for this approach to work. The sum total of all of these efforts is inadequate performance to meet the market requirement.

Recent work (W02020012870) on rare earth oxy-carbonates has demonstrated long-term stability with respectable sensitivity once formation of the hexagonal phase is established. This can be achieved by a lengthy transformation from a metastable monoclinic phase, which is itself sensitive to CO2, by aging for 60-80 hours in range 475 - 575°C. However, this transformation is accompanied by an order of magnitude increase in electrical resistance. The disclosure advises that the monoclinic phase is unstable for the family L^ChCOs (where n is one or more rare earth elements selected from the range Scandium - Lutetium) with the exception of samarium oxycarbonate, and will convert to the stable hexagonal phase with continued heating. of the Invention

It is, therefore, an object of aspects of the invention to address at least some of the above-mentioned issues and provide an improved gas sensor that is particularly, although by no means necessarily exclusively, useful for measuring changes in concentration of CO2 in an environment.

In accordance with a first aspect of the present invention, there is provided a gas sensor comprising a gas-sensitive material and means for measuring a change in an electrical property thereof to indicate a change in concentration of a specified gas in the vicinity of the sensor, the gas-sensitive material having a monoclinic crystallographic structure and comprising a chemical compound including a first rare earth oxy-carbonate and a second rare earth element and/or a transition and/or alkaline-earth metal. Rare earth elements are well known in the art as the group of elements made of the lanthanides (i.e. the chemical elements with atomic numbers 57-71 ), scandium and yttrium. The rare earth elements used in the present invention are preferably lanthanides.

The rare earth oxy-carbonate may, beneficially, be a La, Nd or Pr oxy-carbonate. Highly preferred is for the rare earth oxy-carbonate to be a La oxy-carbonate. The use of La as the first rare earth element is believed to encourage the formation of a monoclinic crystalline structure.

The gas-sensitive material may, optionally but preferably, comprise a first rare earth oxy-carbonate and a second rare earth element, wherein the first rare earth oxycarbonate is a La, Nd and/or Pr oxy-carbonate and the second rare earth element is a rare earth element other than La, Nd or Pr. In other words, the second rare earth element is preferably a second rare earth element other that the first rare earth in the oxy-carbonate. Thus, where the first rare earth oxy-carbonate is La, as is preferred, the second rare earth element is a rare earth element other than La.

In an exemplary embodiment, the second rare earth element may be Ho, Ce, Pr, Nd, Er and/or Tb, and preferably is Ho and/or Tb, e.g. Ho. Preferably mixtures of metals are not used as the second rare earth element.

In an embodiment, the gas-sensitive material may comprise a first rare earth oxycarbonate and a transition or alkaline-earth metal. This may be used instead of the second rare earth element or in addition to the second rare earth element.

Transition metals are well known in the art as the group of elements in the d-block of the periodic table. Cu is a preferred transition metal for use in the present invention. This is believed to be due to similarity in ionic size with the first rare earth element.

Alkaline-earth metals are also well known in the art as the group of elements in group 2 of the periodic table. Ca is a preferred transition metal for use in the present invention. As with Cu, this is believed to be due to similarity in ionic size with the first rare earth element. The gas-sensitive material of the present invention has a monoclinic crystallographic structure. Monoclinic crystals are known in the art as a system in which three of the crystal axes all have different lengths, but in which only two of the axes are perpendicular. Crystal structure may be determined using X-ray diffraction techniques.

In an embodiment, the gas-sensitive material may have a monoclinic crystallographic structure in the form of A2-XBXO2CO3, A2-XCXO2CO3, or A2-x- _y B _x C _y O2CO3, wherein A is one or more first rare-earth elements, B is one or more second rare earth elements different to the first rare earth element, C is one or more transition and/or alkaline- earth metals and x and y are positive numbers less than 1 . It will be appreciated that the C in O2CO3 represents carbon and not a transition and/or alkaline-earth metal. Optionally, A may be one or more of La, Nd and Pr and is preferably La, B may be one or more rare earth elements other than A (e.g. other than La, Nd or Pr), C may be one or more transition and/or alkaline-earth metals, x may be less than 1 and y may be less than 0.2; for example, 0 < x < 0.8 and 0 < y < 0.1 .

Preferably, 0.01 < x < 0.7 and more preferably 0.1 < x < 0.6 when applied to second rare earth element B. However, preferably 0.01 < x < 0.1 and more preferably 0.02 < x < 0.05 when applied to transition and/or rare earth metal C. Preferably, 0.01 < y < 0.1 and more preferably 0.02 < y < 0.05.

As mentioned above, the first rare earth oxy-carbonate may, beneficially, comprise or include La.

Examples of monoclinic crystalline gas sensitive materials that may be used in the present invention include:

Lai.9Hoo.i02C03

Lai.8Hoo.202C03 Lai.6Hoo.402C03 Lai.96Cuo.o4 O2CO3 Lai.gCuo.i O2CO3 Lai.96Cao.o4 O2CO3 Lai.58HO0.4CU0.04O ₂ CO3 Lai.9Ceo.i02C03

Lai.6Pro.402C03

Lai.6Ero.402C03

Lai 6Tbo.40 ₂ C03

La-i.sHoo.sC^COs

Lai.4Hoo.602C03

The electrical property is preferably electrical resistance or impedance, and the gas sensor may further include means for measuring changes in said electrical resistance or impedance.

In an embodiment, the gas sensor may be in the form of a CO2 gas sensor further comprising means for measuring changes in said electrical property over time and outputting corresponding values representative of changes in the concentration of CO2 gas in the vicinity of the gas sensor.

The gas sensor may further comprise humidity sensing means for sensing gaseous water in the vicinity of the gas sensor, and correcting means configured to correct an output of the gas sensor using an output of the humidity sensing means. Such a humidity sensing means may, for example, comprise a MO _X humidity sensor having an output representative of a concentration of gaseous water present in the vicinity of the gas sensor. M preferably represents a metal. Preferably, the metal is: a transition metal, preferably a first, second or third order (i.e. period 4, 5 or 6) transition metal such as Ti, Ni or W; a group 13 or 14 metalloid such as Ga, In or Sn; or a lanthanide such as Ce. Mixtures of these metals may also be used, e.g. O2- xTixOs (preferably 0 < x < 2) or La2CuO4. The value taken by x in MO _X is preferably from 1 to 3. For instance, where M is Sn, x is preferably 2.

The humidity sensor may, optionally, be configured to generate an output representative of a percentage of the gas sensor response due to gaseous water present in the vicinity thereof, and the correcting means is configured to correct the output of the gas sensor using said percentage. In an embodiment, the gas sensor may comprise a supporting substrate incorporating electrodes, over which is provided a layer of said gas-sensitive material, and contacts configured to enable an electrical measurement to be taken between said electrodes.

The gas sensor beneficially further comprises an integrated heater element configured to enable the gas-sensitive material to be held at a desired elevated operating temperature.

In accordance with a second aspect of the present invention, there is provided a method of manufacturing a gas sensor, comprising the steps of forming electrodes on a supporting substrate, preparing a gas-sensitive material having a monoclinic crystallographic structure and comprising a chemical compound including a first rare earth oxy-carbonate and a second rare earth element and/or transition and/or alkaline-earth metal, providing a layer of said gas-sensitive material on the supporting substrate such that it is electrically coupled to said electrodes, and providing contacts in or on said electrodes so as to enable an electrical property of said material between said electrodes to be measured.

The method may further comprise providing a supporting substrate patterned with an electrode structure and depositing a layer of said gas-sensitive material over said electrode structure. In an exemplary embodiment, the deposition step may comprise forming said gas-sensitive material in powder form, mixing the powder with a carrier material to form a printing ink, and printing a layer of said ink onto said supporting substrate over said electrode structure.

The method may, beneficially, further comprise providing a heater element in or on said supporting substrate.

In an exemplary embodiment, the method comprises providing a heater element in or on said substrate, forming said gas-sensitive material in precursor form and depositing it on said substrate, and using said heater element to heat said gassensitive material so as to cause it to form a monoclinic crystallographic structure. The method may further comprise providing, on said supporting substrate, a MO _X humidity sensor.

The gas-sensitive material may beneficially comprise a first rare earth oxy-carbonate and a second rare earth element, wherein the first rare earth oxy-carbonate is a La, Nd and/or Pr oxy-carbonate and is preferably a La oxy-carbonate, and the second rare earth element is a rare earth element other than the first rare earth element in the oxy-carbonate.

In an embodiment, the gas-sensitive material may comprise a first rare earth oxycarbonate and a transition or alkaline-earth metal, such as Cu.

In an exemplary embodiment, the gas-sensitive material may have a monoclinic crystallographic structure in the form of A2-XBXO2CO3, A2-XCXO2CO3, or A2-X- yBxCyC^COs, wherein A is beneficially one or more first rare-earth elements, B is optionally one or more second rare earth elements different to the first rare earth element, C is optionally one or more transition and/or alkaline-earth metals and x and y may be positive numbers less than 1 .

The inventors have surprisingly found that the monoclinic phase can be stabilised when more than one rare earth elements or other elements, such as Cu or Ca, are present and, moreover, retain its lower electrical resistance and responsiveness to CO2 gas. Further, the size of the phase domain for the stabilised monoclinic phase is extensive, i.e. has a large solution range. This provides scope for further increases in electrical conductivity through additions of heterovalent cations to alter the number of charge-carrying electronic defects (Book reference 7). Accordingly, the sensing material of a preferred embodiment of the invention belongs to a class of materials with the monoclinic crystallographic structure of the form A2-X BxC^COs, A2-XCXO2CO3, A2-x-yBxCyO2CO3 (where A is (optionally) La, Nd and/or Pr and is preferably La, B is (optionally) one or more rare earth elements other than A, and C is (optionally) a transition metal and/or an alkaline-earth metal, x and y are less than 1 , for example, 0 < x < 0.8 and 0 < y < 0.1 ). In addition to improved conductivity and stability, greater reliability can be achieved by using a second MOx sensor to measure and thereby correct for the effects of gaseous water in the CO2 sensor signal. The improvement over the conventional approach is that the MOx sensor experiences the same micro environment as the CO2 sensor, as well as offering long-term reliability due to its thermal and chemical stability in potentially hot conditions. The MOx materials which can function as humidity sensors compound oxides based on cerium oxide, niobium oxide or titanium oxide, e.g. CaTiOs (Reference papers 8 and 9).

Brief Description of the Drawings

Examples and embodiment of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which:

Figure 1 illustrates a X-ray diffraction analysis of prepared lanthanum oxy-carbonate powders, as described in Example 1 , alongside ‘stick’ patterns of reference phases;

Figure 2 illustrates a X-ray diffraction analysis of prepared mixed element oxycarbonate powders, as described in Examples 2 - 3, alongside ‘stick’ patterns of reference phases;

Figure 3 illustrates graphically a gas response behaviour towards CO2 and H2O for reference lanthanum oxy-carbonate sensors;

Figure 4 illustrates graphically a gas response behaviour towards CO2 and H2O for lanthanum-holmium oxy-carbonate sensors, and reference lanthanum oxy-carbonate sensor.

Figure 5 illustrates graphically a gas response behaviour towards CO2 and H2O for lanthanum-holmium, lanthanum-copper and lanthanum-calcium oxy-carbonate sensors, alongside reference lanthanum oxy-carbonate sensor; Figure 6 illustrates graphically a gas response behaviour towards CO2 and H2O for lanthanum-holmium, lanthanum-copper, lanthanum-holmium-copper oxy-carbonate and cerium oxide sensors, alongside reference lanthanum oxy-carbonate sensor.

Figure 7 is a high level block diagram of a gas sensing system of the invention;

Figures 8, 9, and 10 are perspective, plan, and side views respectively of a discrete gas sensor including a MOS sensor ceramic chip wire bonded to pins in a package base; and

Figure 11 is a schematic flow diagram illustrating steps of a method according to an exemplary embodiment of the invention.

Detailed Description of the Embodiments

A typical electrochemical gas sensor according to an embodiment of the invention would comprise a housing in which is provided a supporting substrate. The supporting substrate which may, for example, be formed of ceramic such as zirconia or alumina, is patterned with a first and a counter-electrode, and an associated track, using any known chip patterning technique, as will be known to a person skilled in the art, such as sputter deposition or screen printing, and the present invention is not necessarily intended to be limited in this regard. Contact pads are typically provided on one side of the substrate, to enable electrical measurements to be made between the first and counter-electrodes. A heater track is also integrated into the supporting substrate.

A gas-sensing material is deposited on top of the electrodes, so as to be electrically coupled thereto, and in thermal communication with the heater track. The gassensing materials that can, optionally, be used are referenced in the Examples described below. However, in general, it is considered to be beneficial to use a chemical compound that has a monoclinic crystallographic structure in the form of A2-XBXO2CO3, A2-XCXO2CO3, or A2-x-yBxCyO2CO3, wherein A is one or more first rare- earth elements and preferably is La, B is one or more second rare earth elements different to the first rare earth element, C is one or more transition and/or alkaline- earth metals and x and y are positive numbers less than 1 . It is to be understood that not all of the gas-sensing material, thus provided, needs to be in the monoclinic phase for the gas sensor to be operable, and some amorphous material and other phases may be present, as will be evident from the Examples described hereinafter.

Fig. 7 is a high level block diagram of a gas sensing system 1 of the invention, having a sense element 2 adjacent a heater element 3. The system 1 comprises a heater controller 4, a circuit 5 for gas sensor conditioning, and a microcontroller 6. As shown in Figs. 8 to 10 the system 1 comprises a discrete transducer incorporating the gas sense element 2 and the heater element 3. The transducer consists of the two-terminal gas sensitive impedance element 2 and the two-terminal heater element 3 which is controlled so as to maintain the sense element 2 at the optimum operating temperature. The sense element 2 is engineered so that its impedance is modulated according to the concentration of the exposed gas. The gas sensor conditioning electronics 4-6 monitor variations in the sensor element 2 impedance. These resistance or impedance variations when combined with calibration algorithms give a measure of value of the target gas concentration. The heater controller 4 monitors the sense element 2 temperature and controls the heater 3 power so as to maintain optimum operating conditions. The micro controller 6 with non volatile memory (NVM) stores calibration coefficients determined at manufacturing and implements a number of data correction algorithms.

Figs. 8 to 10 show a discrete transducer with the MOS sensor element 2 supported from a package base 10 having pins 11 linked to the sense element 2 by wire bonds 12. The sense element 2 has a heated sensor substrate which is thermally isolated from the package base 10 as it is suspended in mid air. Heat loss is primarily by convection from the element 2 surface and by conduction through the bond wires 12. Referring additionally to Figure 11 of the drawings, in an exemplary method of manufacturing a gas sensor according to an aspect of the invention, a supporting substrate (e.g. alumina or zirconia) is provided (at step 800) and patterned with an interdigitated gold electrode structure and a serpentine heater track (at step 802). Next, at step 806, a gas-sensing material is deposited on the supporting substrate (over the electrode structure and heater track), the gas-sensing material (when operable) having a monoclinic crystallographic structure and comprising a chemical compound including a first rare earth oxy-carbonate and a second rare earth element and/or a transition and/or alkaline-earth metal.

In order to form the monoclinic crystal phase in the precursor gas sensing material, it must be heated and, in one embodiment, this may be performed prior to deposition of the material onto the substrate. In this case, the gas-sensing material may be prepared in the form of a particulate or powder state, which can be mixed with a carrier material to enable the material to be deposited onto the substrate by means of, for example, screen printing. In this case, the heater track is provided only to keep the material at a desired elevated operating temperature. However, in an alternative embodiment, the precursor gas-sensing material can be deposited onto the substrate (by any conventional deposition means) and then heated, via the heater, track to form the monoclinic crystal phase. In either case, it will be understood that not all of the precursor material needs to have the monoclinic crystal phase, and some amorphous material, and other phases, may remain or be present in the layer of gas-sensing material deposited on the substrate.

A MOx humidity sensor may also be provided (at step 810) on the substrate, and this may take place after or (more likely) before deposition of the gas-sensing material.

Examples

The following examples support and illustrate aspects of the invention:

Example 1

In accordance with the teaching in W02020012870, a selection of pure lanthanum oxy-carbonate precursor powders, lanthanum oxalate (Alfa Aesar), and lanthanum hydroxide (Absco), were subjected to a heat treatment for 72 hours at 550°C in a box furnace. X-ray diffraction analysis (Table 1 , Figure 1) confirmed the crystallographic structure of the resultant fired powders as primarily hexagonal lanthanum oxycarbonate, La2 O2CO3, (ICDD: 00-070-5540). Example 2

A mixture of lanthanum nitrate hexahydrate powder (Alfa Aesar, 99.9%) and holmium nitrate pentahydrate powders (Alfa Aesar, 99.9%) was weighed out in accordance with the ratio La: Ho equal to 95:5 by atomic weight, and dissolved in deionised water. Each water solution (La and La-Ho) was then added to separate 0.4M oxalic acid solutions to precipitate out the resultant oxalate salt. The precipitate was washed, dried and heated to 550degC for 72 hours in air in a furnace. X-ray diffraction analysis (Table 1 , Figure 1 ) confirmed the fired mixture to be lanthanum oxy-carbonate with the monoclinic phase (ICDD: 00-048-1113), which we denote as La-i.gHoo.i O2COs.

Example 3

Using a similar methodology to Example 1 , the following compounds were prepared: Lai.9Hoo.i02C03 Lai.8Hoo.202C03 Lai.6Hoo.402C03 Lai.96Cuo.o4 O2CO3 Lai.gCuo.i O2CO3 Lai.96Cao.o4 O2CO3 Lai.58HO0.4CU0.04O ₂ CO3 and their crystallographic structures reported in Table 1 , Figure 2. The additional precursor compounds used were calcium nitrate tetrahydrate (Sigma Aldrich, 99%) and copper nitrate hexahydrate (Sigma Aldrich, 99.9%).

Example 4

The powders from the previous examples were sieved through a 63 micron aperture sieve and then made into a screen-printing inks by mixing with an ethyl cellulose - butyl carbitol vehicle. The inks were deposited onto alumina chips patterned with an interdigitated gold electrode structure and a serpentine heater track. The chips were interconnected to a TO-5 4-pin can in such a way to ensure thermal isolation and powered up to 440°C using the integrated heater in air humidified to 50% relative humidity at 21 °C in a dedicated gas test appliance, as described in (US20120161796, Paper Reference 3). The signal measurement circuit, employing a potential divider arrangement, provided an analogue output of the sense element resistance, which was monitored and recorded. The sensors were stabilised for up to 180 minutes in 400ppm CO2-balance humidified air. The measured resistance values recorded as shown in Table 2, demonstrate that all the multiple element oxycarbonates with the stable monoclinic structure have lower values by typically an order of magnitude compared to the counterpart single element hexagonal phases. Table 2 also demonstrates the influence of humidity levels on the measured resistance values. Following the stabilisation period, testing on the sensors was continued by exposing them to different CO2 concentrations spanning the range 400 - 2600ppm and in differing humidity levels. Figures 3-6 demonstrate that the stabilised multiple element formulations are gas-sensitive to CO2, with discrimination between different CO2 concentrations comparable to that for the single element hexagonal counterpart.

Example 5

Following the preparation methodology of Example 1 , the following compounds were prepared:

Lai.9Ceo.i02C03

Lai.6Pro.402C03 Lai.6Ero.402C03 Lai 6Tbo.40 ₂ C03 La-i.sHoo.sC^COs Lai.4Hoo.602C03

The compounds were analysed by X-ray diffraction (Table 3) for the presence of the monoclinic phase (ICDD: 00-048-1113). The additional compounds used were cerium nitrate hexahydrate (99.5%, Alfa Aesar), praseodymium nitrate hydrate (99.9%, Alfa Aesar), erbium nitrate hydrate (99.9%, Alfa Aesar) and terbium nitrate hydrate (99.9%, Alfa Aesar). Example 6

While Example 4 demonstrates lower electrical resistance (higher electrical conductivity) and responsiveness to CO2 gas, it also shows that both of these properties are sensitive to the prevailing humidity levels, as is the case for the hexagonal single element counterpart. To correct for the effect of humidity, it is necessary to measure it in the micro environment of the CO2 sensor. Accordingly, a metal oxide humidity sensor comprising cerium oxide (Sigma Aldrich, 99.9% < 5 microns) and assembled into a sensor in the manner described in Example 4 was tested alongside the CO2 sensors. Figure 6 shows that this metal oxide humidity sensor can function in the same micro-environment as the metal oxide CO2 sensor, thereby enabling a more reliable means of correcting for humidity interference on the response of the CO2 metal oxide sensor. Table 1 : X-Ray Diffraction Results following Annealing at 550C for 72 hours in air Table 2: Baseline Resistance in humidified 400ppm CO2 flowing air Table 3: X-Ray Diffraction Results following Annealing at 550°C for 72 hours in air

References

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5. Sensirion Gas Sensor Module SVM30 Datasheet

6. W02020012870

7. P. Kofstad (1972), Nonstoichiometry, Diffusion and Electrical Conductivity in Binary Metal Oxides, Wiley-lnterscience, New York, 1972

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