A solid-state oxygen sensor chip

Abstract

A potentiometric gas sensor 10 comprising a planar electrolytic substrate 12 having a sensing electrode 20, a reference electrode 22, and an embedded heater 28 provided thereon. The embedded heater, t

Description

Field of the invention The present invention relates to a gas sensor, and in particular to a gas sensor for detecting oxygen. 5 Background of the invention Gas and particle sensors are used for monitoring air quality in a wide range of industries. As reported in Gas and Particle Sensors - Technology and Market Trends 2021 (published by Yole Intelligence), oxygen sensors account for 55% of all sensor sales, with zirconia-based electrochemical sensors being the dominant technology. 10 The standard sensor is comprised of a zirconia-based electrolyte element in the form of a disk, tile strip or tube which is coated on opposite sides by electrodes. The opposite sides are exposed to two different atmospheres: one being a measuring atmosphere and the second being a reference atmosphere whose composition is known, and is generally air. Electrodes can be either a metal, or a metal oxide and require electron conductivity. To create separation 15 of the two atmospheres, the ceramic electrolyte has traditionally been fabricated in the form of a closed end tube orthimble with the inner surface exposed to the reference air atmosphere. However, other shapes are possible such as a flat rectangular ‘laminated’ design, assembled by stacking and bonding together multiple individual pre-formed component strips. An internal cavity open to the reference atmosphere or an fully enclosed reference atmosphere features 20 of the laminate design. These types of sensors are known as potentiometric (or Nernstian) sensors. A general description of the technology, and the scientific principles underlying the Nernstian detection mechanism is described in Zhuiykov: Electrochemistry of zirconia gas sensors (2007,1st edition, published by CRC Press: https: / / doi.org / 10.1201 / 9781420047622). Another, very different, design variant is based on a vertical multi-layered planar construction 25 with an inner chamber covered by membrane typically 5-10mm in thickness. The ingress of oxygen into the sensor is controlled by a very fine through-hole (around 50 to 100 microns in diameter) which traverses the membrane. This type of sensor is referred to amperometric or current-limiting as the ingressing oxygen generates a current on reaction with the sensing electrode and, as the rate of ingressing oxygen is controlled by the external oxygen partial 30 pressure and the dimensions of the through-hole, provides a direct link between the current generated and external oxygen. The amperometric variant is complimentary to the potentiometric variant, offering a more restricted detection range but more accurate 02 06 25 measurements. However, blockage of the narrow through-hole and sensitivity to sudden pressure changes can be problematic in some applications. There are a number of ceramic electrolytes that are currently used in potentiometric sensors, e.g. zirconia doped with various oxides (yttrium, calcium scandium, gadolinium oxides), beta-5 alumina and cerium oxide, with yttria-doped zirconia (YSZ) being the cheapest and by far the most widely used. The overriding market for potentiometric sensors is the fossil-fuelled automotive industry where the sensors are used as part of a system which controls the air-fuel mixture in the combustion engine, and to monitor the resultant exhaust emissions. The sensors operate in 10 this environment at high temperatures of around 600 to 900 °C. Although sensor design has evolved in response to the needs of the automotive market, many features have remained largely unchanged, e.g. cost, power consumption, size and interference from volatile organic compounds (VOCs) particularly carbon monoxide. This has greatly limited the suitability of the sensors for other applications, for instance in industries in 15 which the sensors are used at more moderate temperatures such as around room temperature. Examples of these markets include medical, food processing, waste management, nitrogen generation, and semiconductor manufacturing processes. These industries favour technologies which are lower power, lower cost, and have smaller footprints such as galvanic liquid electrolyte sensors, thermo-paramagnetic and tunable laser diode gas 20 sensors. However, each of these sensor technologies has its own distinct merits and limitations. The YSZ technology is unique in operating in hot humid dirty conditions, has no consumable parts and has a detection span from ppb to 100% O2. However, the worldwide transition away from the traditional fossil-fuel related markets requires changes to the design of the YSZ 25 sensor to widen its applicability to these other markets. Surprisingly the design has withstood the radical changes seen in recent times with other sensor technologies in making them small, lightweight, compact, energy efficient with fast boot-up times. The following documents describe efforts that have been made to made to shrink or simplify the design: US 5,827,415 A 30 Shuk: Oxygen Gas Sensing Technologies Application: A Comprehensive Review (2017, DOI: 10.1007 / 978-3-319-47322-2_5) WO 2013 / 160271 A1 02 06 25 Hu et ah Improved Internal Reference Oxygen Sensors with Composite Ceramic Electrodes (2012, J. Electrochem. Soc., 159(10), B811-B817, DOI: 10.1149 / 2.02121 Ojes) Kanekoa et ah Performance of a miniature zirconia oxygen sensor with a Pd-PdO 5 internal reference (2005, Sensors and Actuators B: Chemical, 108 (1-2), 331-334) JP 2004-117099 Dunst et ah. Potentiometric oxygen sensor with solid state reference electrode (2014, Metrol. Meas. Syst., Vol. XXI(2), 205-216) Radhakrishnan et ah. Design, fabrication and characterization of a miniaturized series-10 connected potentiometric oxygen sensor (2005, Sensors and Actuators B 105, 312- 321) Some of the designs disclosed in these documents centre on a variety of planar formats using an enclosed reference in place of the air, multiple layer assemblies with integrated heater, internal chambers for the air reference, via holes to ensure electrical leads are on the same 15 side, or even shifting the design to a silicon substrate and utilising microfabrication techniques patterning and deposition techniques to miniaturise the sensor. The proposed designs have not translated into commercial products, with technical reasons such as oxygen leakage in the reference electrode, warping, delamination and weak signals being mentioned in some cases, and consequently zirconia-based potentiometric gas sensors have remained firmly rooted in 20 the large bulky energy-intensive tubular format. Thus, there remains a need for a low-power, compact, lightweight ceramic- or silicon-based potentiometric sensor. Summary of the invention The present invention provides a potentiometric gas sensor comprising a planar electrolytic 25 substrate having a sensing electrode, a reference electrode, and an embedded heater provided thereon. The embedded heater, the sensing electrode and the reference electrode are provided on the same surface of the substrate. The electrolytic substrate has a surface area, on each planar face, of from 0.1 to 25 mm2. Both the sensing and reference electrodes are in direct contact with the electrolytic substrate, and an insulation layer sits between the 30 electrolytic substrate and the heater with the insulation layer in direct contact with both the electrolytic substrate and the heater. 02 06 25 Also provided is a gas sensing device which comprises a gas sensor of the present invention housed in a casing. The casing may comprise an aperture which is open to the external environment and a carbon filter covering the aperture. Further provided is a method of detecting a gas, such as oxygen, in an environment, said 5 method comprising using a gas sensor of the present invention to measure the amount of the gas in the environment, as well as the use of a gas sensor of the present invention for detecting a gas, such as oxygen, in an environment. A method of preparing a gas sensor of the present invention is also provided. The method comprises: depositing a sensing electrode and a reference electrode directly onto a surface 10 of a planar electrolytic substrate, and providing an embedded heater on the surface of the substrate. An insulation layer sits between the electrolytic substrate and the heater, with the insulation layer in direct contact with both the electrolytic substrate and the heater The embedded heater, the sensing electrode and the reference electrode are provided on the same surface of the substrate, and the substrate has a surface area, on each planar face, of 15 from 0.1 to 25 mm2. Figures Fig. 1 is a schematic depicting an exploded view of a gas sensor (10) of the present invention having a sensing electrode (20), a reference electrode (22) and an embedded heater (28) on the same side of the electrolytic substrate. 20 Fig. 2 is a schematic depicting an exploded view of a gas sensor (10) of the present invention in which a filter coating (30) is present on the sensing electrode (20). Fig. 3 is a schematic depicting an exploded view of a gas sensor (10) of the present invention in which two sensing electrodes (20a, 20b) are present, one of which has a filter coating (30) deposited thereon. 25 Fig. 4 is a schematic depicting an exploded view of a gas sensor (10) of the present invention in which two reference electrodes (22a, 22b) are present, each of which is provided with a different reference couple (24a, 24b). Fig. 5 is a top-down view of the gas sensor (10) depicted in Fig. 1. Fig. 6a is a cross-sectional view of the gas sensor (10) depicted in Fig. 1 from side A. Fig. 6b 30 is a cross-sectional view of the gas sensor from side B. Sides A and B are marked in Fig. 5. 02 06 25 Detailed description of the invention Overall sensor The present invention relates to potentiometric gas sensors. Potentiometric sensors determine the concentration of a gas analyte by comparing the electrical potential of a sensing 5 electrode to that of a reference electrode. The gas sensors of the present invention comprise a planar electrolytic substrate on which a sensing electrode, a reference electrode and an embedded heater are provided. Thus, unlike laminated prior art structures, the gas sensors of the present invention are monolithic chip structures having a single electrolytic substrate. The sensors of the present invention are 10 preferably stand-alone, i.e. they do not comprise a supporting substrate in addition to the electrolytic substrate. The gas sensor is preferably a single-sided sensor. In other words, the sensing and reference electrodes and the heater are all provided on the same surface of the substrate. Any electronics or other associated apparatus is also preferably provided on the same surface of 15 the substrate. Being single-sided dispenses with the need to do a flip-chip during manufacturing, thereby eliminating an assembly step, speeding up throughput and improving yield by avoiding potential damage to the gas sensor during the flipping operation. As will be discussed in greater detail below, the sensing and reference electrodes and the heater are all preferably planar in nature. This allows the gas sensor to be readily 20 manufactured at low cost in a miniaturised form. Furthermore, complex structures of multiple layers of different materials and geometries, with the concomitant costs and slower manufacturing times, can be avoided. The gas sensor of the present invention is preferably a miniaturised gas sensor. The footprint of the gas sensor will generally be determined by the surface area of the planar electrolytic 25 substrate. The footprint of the gas sensor may be at least 0.1 mm2, preferably at least 0.2 mm2, and more preferably at least 0.25 mm2. The footprint of the gas sensor may be up to 5 mm2, preferably up to 4 mm2, and more preferably up to 3 mm2. Thus, the footprint of the gas sensor may be from 0.1 to 5 mm2, preferably from 0.2 to 4 mm2, and more preferably from 0.25 to 3 mm2. 30 The footprint of the gas sensor may be at least 0.5 mm2, preferably at least 1 mm2, and more preferably at least 1.5 mm2. Thus, the footprint of the gas sensor may be from 0.5 to 5 mm2, preferably from 1 to 4 mm2, and more preferably from 1.5 to 3 mm2. 02 06 25 The footprint of the gas sensor is typically in the shape of a rectangle (e.g. a square or nonsquare rectangle). The peripheral edge of the gas sensor will generally be the peripheral edge of the planar electrolytic substrate. The gas sensor may have an overall thickness of at least 100 pm, preferably at least 125 pm, 5 and more preferably at least 150 pm. The gas sensor may have an overall thickness of up to 300 pm, preferably up to 275 pm, and more preferably up to 250 pm. Thus, the gas sensor may have an overall thickness of from 100 to 300 pm, preferably from 125 to 275 pm, and more preferably from 150 to 250 pm. The gas sensor is typically devoid of inner chambers, cavities and via holes. 10 Sensing electrode The gas sensors of the present invention comprise a sensing electrode. A sensing electrode is used to detect gas in an atmosphere. As such, a sensing electrode is open to the atmosphere in which it is used. The sensing electrode may be open to the atmosphere because it is entirely uncovered, or it can be covered by a material which is 15 permeable to gases. The sensing electrode is in electrical contact, and direct physical contact, with the substrate. The sensing electrode is preferably porous, and more preferably has an open-cell porous structure. This facilitates a “triple phase boundary” (TPB) at which gas, electrode and electrolyte can share common points of contact. In some embodiments, the entirety of the 20 sensing electrode is porous. In other embodiments, just an active region (i.e. non-bond pad region - discussed below in greater detail) of the sensing electrode is porous. The sensing electrode is preferably in the form of a planar structure. The planar structure may be in the shape of a rectangle (e.g. a square or non-square rectangle) or a circle. The sensing electrode will typically be a metallic sensing electrode. The sensing electrode 25 may comprise and preferably is a platinum, palladium, gold or silver electrode, with platinum and gold particularly preferred. It is generally preferred forthe sensing electrode to be formed of a single material, even though the material itself may be a composition comprising multiple components. Thus, the sensing electrode is preferably formed as a single, preferably planar, structure of uniform material. 02 06 25 Though less preferred, in some embodiments, different regions of the sensing electrode may be made from different materials. For instance, where the sensing electrode comprises a bond pad (discussed below), the bond pad may be made of a different material from the rest of the sensing electrode. Where different regions are made from different materials, the 5 different regions are all preferably selected from the materials mentioned above. The sensing electrode may comprise a bond pad, i.e. a region of the sensing electrode that is not in direct physical contact with the substrate. Thus, the bond pad is electrically insulated from the substrate other than through non-bond pad area (i.e. the active region) of the sensing electrode. The bond pad may be electrically insulated from the substrate by an insulation 10 layer. The insulation layer preferably sits between the substrate and the bond pad. The insulation layer will typically be in direct contact with both the substrate and the bond pad. In preferred embodiments, the insulation layer covers the entire upper surface of the substrate apart from the areas below the non-bond pad region (i.e. active region) of the sensing and 15 reference electrodes. This allows the electrodes to remain in direct electrical contact with the substrate. The insulation layer may cover at least 50 %, preferably at least 65 %, and more preferably at least 80 % of the surface area of the upper surface of the substrate. The insulation layer may cover up to 95 %, preferably up to 92 %, and more preferably up to 90 % of the surface area 20 of the upper surface of the substrate. Thus, the insulation layer may cover from 50 to 95 %, preferably from 65 to 92 %, and more preferably from 80 to 90 % of the surface area of the upper surface of the substrate. Suitable materials for an insulation layer include glass and are discussed in greater detail below. 25 In alternative embodiments, the sensing electrode may comprise a bond pad which is in direct physical contact with the substrate, provided that it the bond pad is in a densified (i.e. non-porous) form. This ensures that a TPB, as described earlier, is not formed in the bond pad area of the sensing electrode. It will be appreciated that, in these embodiments, the active region of the sensing electrode is preferably still porous (as described above in greater detail). 30 A catalytic coating may be present on the surface of the sensing electrode. It will be appreciated that this is the surface of the sensing electrode that, in the absence of the coating, 02 06 25 would be exposed to atmosphere. The catalytic coating is preferably deposited directly on the sensing electrode. The catalytic coating preferably reduces the interference of contaminant gases. For instance, when the gas sensor is for detecting oxygen gas, contaminant gases include VOCs or oxygen-5 containing gases. The VOCs or oxygen-containing gases can react with oxygen at the sensing electrode thereby reducing the level of oxygen that is detected by the sensor compared to the level that is actually present in the atmosphere. A further advantage of a catalytic coating is that, in fast flowing non-equilibrium environments such as an engine, it can serve to promote the establishment of thermodynamic equilibrium of the gaseous atmosphere in the vicinity of 10 the sensing electrode. In preferred embodiments, the catalytic coating comprises an oxidation catalyst. The oxidation catalyst may comprise a metal oxide or a metal oxide loaded with a precious metal such as platinum, palladium, rhodium or gold, preferably platinum or palladium. For instance, the oxidation catalyst may be selected from Sm2O3, Cr2Os, M2O3 and SnO2, as well as gold, 15 platinum or palladium loaded forms of Sm2O3, Cr2Os, M2O3 and SnO2; these catalysts are particularly suitable for removing VOCs contaminants. It will be appreciated that the catalytic coating allows the gas being detected by the sensor to pass through the coating to the sensing electrode. Thus, the catalytic coating may be seen as a filter coating. 20 The catalytic coating will typically be porous, and most likely microporous or mesoporous. It is known in the art that microporous materials have a pore width not exceeding about 2.0 nm, while mesoporous materials have a pore width which is greater than about 2.0 nm but not exceeding about 50 nm, while microporous materials have a pore width exceeding about 50 nm (IUPAC). 25 The form of the coating will be dependent on the coating method deployed, such as chemical or sputter deposition or the particle-suspension deposition methods such as inkjet, droplet or screen-printing. For the latter, the characteristics of the particles such as size, shape, surface area, the particle loading in the suspension and the viscosity of the suspension will govern the compactness of the coating. The porous nature of the layer allows passage of gases such as 30 oxygen through the coating but the pathway is designed to be tortuous so as to facilitate interaction between the catalytic surfaces and the contaminant gases. 02 06 25 In some embodiments, the gas sensor comprises a single sensing electrode, i.e. no more than one sensing electrode. Where a single sensing electrode is used, it preferably comprises a catalytic coating as described above. However, in other embodiments, the gas sensor comprises multiple, i.e. two or more, such as 5 two or three, sensing electrodes. For instance, the gas sensor may comprise two identical sensing electrodes so that one can act as a backup in case of failure of the first electrode. It will be appreciated that, for the sensing electrodes to be considered identical, they must have the same composition and structure, including the presence or absence of coatings. 10 However, where the gas sensor comprises multiple sensing electrodes, it is generally preferred that the sensing electrodes differ from one another. For instance, the sensing electrodes may differ from one another in composition and / or structure. Thus, the sensing electrodes may differ from one another in their reactivity towards contaminant gases. This means that, when the gas sensor is in use, the level of gas that is detected by each sensing 15 electrode may be compared to provide information about the atmosphere surrounding the gas sensor. In other words, the multi-sensor capability allows for a composition analysis of the atmosphere, for instance by allowing the determination of both target gas levels, e.g. oxygen levels, and contaminant gases, e.g. VOC levels, in an atmosphere. The sensing electrodes may have the same structure but differ in composition, for instance 20 because they comprise different metallurgy. Alternatively, the sensing electrodes may have the same composition but differ in structure, for instance due to the presence of a catalytic coating on just one of the sensing electrodes. The gas sensor may comprise a pair of sensing electrodes which differ only in composition, and an identical backup pair of sensing electrodes. The gas sensor may comprise a pair of 25 sensing electrodes which differ only in structure, and an identical backup pair of sensing electrodes. The gas sensor may comprise a pair of sensing electrodes which differ only in composition, a pair of sensing electrodes which differ only in structure, and optionally backup sensing electrodes for each of the pairs of electrodes. Reference electrode 30 In addition to a sensing electrode, the potentiometric gas sensors of the present invention comprise a reference electrode. 02 06 25 Unlike the sensing electrode, the reference electrode is not open to the environment in which it is used, but instead comprises a hermetically sealed reference environment. Since the electrical potential that is imparted to the reference electrode from the reference environment is known, the amount of gas that is detected by the sensing electrode may be determined by 5 the potential difference between the sensing and reference electrodes. The gas sensor preferably comprises a reference couple which is sealed within the reference environment with the reference electrode. It is this reference couple (by virtue of a generated equilibrium oxygen partial pressure) that imparts the electrical potential to the reference electrode, rather than an air-filled chamber that is seen in the more conventional devices. 10 The reference couple is preferably in direct physical contact with the reference electrode. The reference couple preferably covers the portion of the reference electrode that is in contact with the substrate, but not the bond pads which are open to the environment in which the sensor is used. The reference couple may comprise a mixture of a first metal and a first metal oxide with the 15 first metals being the same, or a mixture of a first metal oxide and a second metal oxide where the first and second metals are the same or, preferably, different. Metal oxides may comprise multiple metals such as two different metals. The reference couple provides the reference electrode with a value of pO2 which is defined thermodynamically by the reaction at equilibrium between the different components of the reference couple and oxygen for a given operating 20 temperature. Exemplary materials for the reference couple are selected from: Pd / PdO, Ni / NiO, Cr / Cr2O3, Ru / RuO2, C0 / C002, CuO / CuO2, Fe3O4 / Fe2O3, CeO2 / ZrO2, and Lai-xSrxMnO3 / YSZ. The reference electrode may be sealed with an insulation layer. Suitable materials for an insulation layer include glass and are discussed in greater detail below. 25 The reference electrode is in electrical contact, and direct physical contact, with the substrate. The reference electrode is provided on the same side of the substrate as the sensing electrode. The reference electrode is preferably provided in the same plane as the sensing electrode. As with the sensing electrode, the reference electrode and the reference couple preferably 30 have a porous, and more preferably an open-cell porous, structure. In some embodiments, the entirety of the reference electrode is porous. In other embodiments, just an active region 02 06 25 (i.e. non-bond pad region - discussed below in greater detail) of the reference electrode is porous. The reference electrode is preferably in close proximity to the sensor electrode. This means that thermal gradients between the sensing and reference electrodes, which can impact the 5 levels of gas detected, are minimised. For instance, the space between the reference electrode and the sensing electrode on the substrate surface is preferably less than 500 pm, preferably less than 300 pm, and more preferably less than 200 pm. Where the gas sensor comprises more than one sensing electrode and / or more than one reference electrode, the space between each of the reference 10 and sensing electrodes on the substrate surface is preferably less than 500 pm, preferably less than 300 pm, and more preferably less than 200 pm. It will be appreciated that the “space” between the electrodes represents the minimum straight-line distance between a peripheral edge of the reference electrode and a peripheral edge of the reference electrode. The reference electrode is preferably in the form of a planar structure. The planar structure 15 may be in the shape of a rectangle (e.g. a square or non-square rectangle) or a circle. The reference electrode will typically be a metallic reference electrode. The reference electrode may comprise and preferably is a platinum, palladium, gold or silver electrode, with platinum and gold particularly preferred. It is desirable to minimise thermal stress within the gas sensor in order to prevent changes in 20 the reference environment as a result of cracks in the sensor. The thermal stress (o) of coatings on substrates are related by coefficient of thermal expansion (a), Young’s modulus of elasticity (E) and the temperature change (AT), covering the ambient temperature upto the firing temperature or the operating temperature, as follows: O — (Cisubstrate — Cicoating) X E X AT 25 provides appropriate guidance on keeping the coefficient of thermal expansion (aCOating) of the reference electrode / reference couple as close to that of the solid-state electrolyte substrate (cisubstrate) and on keeping Young’s modulus (E) as low as possible. It is known that while the coefficient of thermal expansion is a property of the material, the effective Young’s Modulus is both a function of the material and the degree of porosity prevailing in the delivered structure, 30 with increase in porosity accompanied by a commensurate decrease in Young’s modulus. 02 06 25 Thus, the reference electrode and the reference couple preferably have a coefficient of thermal expansion (CTE) that is close to that of the electrolytic substrate on which they are provided. It is generally preferred for the reference electrode to be formed of a single material, even though the material itself may be a composition comprising multiple components. Thus, the 5 reference electrode is preferably formed as a single, preferably planar, structure of uniform material. Though less preferred, in some embodiments, different regions of the reference electrode may be made from different materials. For instance, where the reference electrode comprises a bond pad (discussed below), the bond pad may be made of a different material from the rest 10 of the reference electrode. Where different regions are made from different materials, the different regions are all preferably selected from the materials mentioned above. The reference electrode may comprise a bond pad, i.e. a region of the reference electrode that is not in direct physical contact with the substrate. Thus, the bond pad is electrically insulated from the substrate other than through non-bond pad area (i.e. active region) of the 15 reference electrode. The bond pad may be electrically insulated from the substrate by an insulation layer. The insulation layer preferably sits between the substrate and the bond pad. The insulation layer will typically be in direct contact with both the substrate and the bond pad. Preferably, the bond pad of the reference electrode is electrically insulated from the substrate 20 by the same insulation layer that electrically insulates the bond pad of the sensing electrode from the substrate. Suitable materials for an insulation layer include glass and are discussed in greater detail below. In alternative embodiments, the reference electrode may comprise a bond pad which is in 25 direct physical contact with the substrate, provided that it the bond pad is in a densified (i.e. non-porous) form. This ensures that a TPB, as described earlier, is not formed in the bond pad area of the reference electrode. It will be appreciated that, in these embodiments, the active region of the reference electrode is preferably still porous (as described above in greater detail). 30 In some embodiments, the only difference between the sensing electrode and the reference electrode is that the sensing electrode is open to the atmosphere in which the gas sensor is used, whereas the reference electrode is sealed in a reference environment. 02 06 25 In some embodiments, the gas sensor comprises a single reference electrode, i.e. no more than one reference electrode. However, in other embodiments, the gas sensor comprises multiple, i.e. two or more, such as two or three, reference electrodes. 5 For instance, the gas sensor may comprise two identical reference electrodes, with identical reference environments, so that one reference electrode can act as a backup in case of failure of the other. It will be appreciated that, for the reference electrodes to be considered identical, they must have the same composition and structure, including the presence or absence of coatings. For reference environments to be identical, they must contain the same reference 10 couple. Alternatively, where the gas sensor comprises multiple reference electrodes, the reference electrodes and / or the reference environment may differ from one another. These embodiments are particularly useful where the gas sensor is used in environments in which a range of temperatures are experienced, since higher operating temperatures lead to a small 15 amount of electronic conductivity. In such cases, it is desirable that the difference in pO2 between the test environment and the reference electrode is small (Kanekoa et al ibid.) Having a choice of reference electrodes with different pO2 values is advantageous in ensuring the reference pO2 is similar to the pO2 of the test environment. Thus, in some embodiments where the gas sensor comprises multiple reference electrodes, 20 the reference electrodes differ from one another in their pO2 values. This can be achieved by using reference electrodes that differ from one another in composition or structure, but is preferably achieved by using a different reference environment. A different reference environment can be achieved by using different reference couples in the reference electrodes. Substrate 25 The sensing and reference electrodes are provided on an electrolytic substrate. The substrate is a planar structure, typically in the shape of a rectangle (e.g. a square or non-square rectangle). The substrate may have a thickness of at least 50 pm, preferably at least 75 pm, and more preferably at least 100 pm. The substrate may have a thickness of up to 200 pm, preferably 30 up to 175 pm, and more preferably up to 150 pm. Thus, the substrate may have a thickness of from 50 to 200 pm, preferably from 75 to 175 pm, and more preferably from 100 to 150 pm. 02 06 25 The substrate has a geometric surface area, on each planar face, of at least 0.1 mm2, preferably at least 0.2 mm2, and more preferably at least 0.25 mm2. The substrate has a surface area, on each planar face, of up to 25 mm2, preferably up to 15 mm2, and more preferably up to 5 mm2. Thus, the substrate has a surface area, on each planar face, of from 5 0.1 to 25 mm2, preferably from 0.2 to 15 mm2, and more preferably from 0.25 to 5 mm2. The substrate may have a geometric surface area, on each planar face, of at least 0.5 mm2, preferably at least 1 mm2, and more preferably at least 1.5 mm2. Thus, the substrate may have a surface area, on each planar face, of from 0.5 to 25 mm2, preferably from 1 to 15 mm2, and more preferably from 1.5 to 5 mm2. 10 Preferably each peripheral edge of the substrate is at least 0.25mm, preferably at least 0.4mm, and more preferably 0.5 mm in length. Preferably each peripheral edge of the substrate is up to 5 mm in length. The substrate used in the present invention is a solid-state electrolyte. In preferred embodiments, the substrate is a ceramic material such as a zirconia-based material. Zirconia-15 based materials comprise zirconium dioxide, optionally doped with a stabilising agent. The zirconia-based material may comprise at least 2 %, preferably at least 3 %, and more preferably at least 5 % by weight of a stabilising agent. The zirconia-based material may comprise up to 20 %, preferably up to 15 %, and more preferably up to 10 % by weight of a stabilising agent. Thus, the zirconia-based material may comprise from 2 to 20 %, preferably 20 from 3 to 15 %, and more preferably from 5 to 10 % by weight of a stabilising agent. Suitable stabilising agents include magnesium oxide (MgO), yttrium oxide (Y2O3, yttria), calcium oxide (CaO), hafnium oxide (HfO2) and cerium trioxide (Ce2O3). In particularly preferred embodiments, the substrate material is yttria-stabilised zirconia (YSZ). Heater 25 In addition to sensing and reference electrodes, the gas sensors of the present invention are provided with an embedded heater. This allows the gas sensor to be configured for and used at a particular temperature or temperature range, thereby enhancing the accuracy of the measurements. It also minimises the thermal stresses on the gas sensor that that can occur during repeated thermal cycling, while allowing the sensor to be heated quickly if required. 30 The heater is preferably planar. The heater may be in the form of a track. The track is preferably not straight, but rather windy, often referred to in the art as serpentine, so as to cover a larger area of the gas sensor. 02 06 25 The heater is embedded in the gas sensor, meaning that it is protected from the atmosphere in which the gas sensor is used and is configured to ensure minimal temperature difference between sensing and reference electrodes. The teaching in the prior art in the rare case where a heater is added, e.g. Fig. 10 in WO 2013 / 160271 A1, describes a heater element positioned 5 externally on one side of a two-sided sensor design, clearly resulting in temperature disparities between both electrodes and as a result, inaccuracies in the resulting measurements. The use of an integrated heater allows the gas sensor to be a stand-alone article. Having the integrated heater embedded in the sensor provides more uniform heating of the entire sensor thereby minimising temperature differences between the sensing and reference electrodes 10 e.g. as compared to sensors in which an integrated heater is attached to an outer surface of the device. The embedded heater, the sensing electrode and the reference electrode are provided on the same surface of the substrate. In particularly preferred embodiments, the embedded heater sits in the same plane of the sensor as the sensing and reference electrodes. 15 In some embodiments, the heater is embedded between layers of insulation. The materials used in the insulation layers may be the same or different, but are preferably the same. Similarly, the thickness of the insulation layers may be the same or different, but are preferably the same. An insulation layer sits between the substrate and the heater. This is to avoid electrical 20 interference between substrate and heater. The insulation layer is in direct contact with both the substrate and the heater. This insulation layer is preferably the same insulation layer that is used to electrically insulate the bond pads of the reference and sensing electrodes from the substrate. Preferably an insulation layer sits on the outer surface of the heater. It will be appreciated that 25 the outer surface is the surface that faces away from the substrate. This insulation layer may be the same insulation layer that is used to seal the reference electrode, or there may be a separate insulation layer to seal the reference electrode. The insulation layer that sits on the outer surface of the heater may cover at least 50 %, preferably at least 65 %, and more preferably at least 80 % of the surface area of the upper 30 surface of the substrate. The insulation layer may cover up to 95 %, preferably up to 92 %, and more preferably up to 90 % of the surface area of the upper surface of the substrate. Thus, the insulation layer may cover from 50 to 95 %, preferably from 65 to 92 %, and more preferably from 80 to 90 % of the surface area of the upper surface of the substrate. 02 06 25 Suitable materials for an insulation layer include glass and are discussed in greater detail below. Insulation layers As mentioned above, the gas sensor of the present invention may comprise one or more 5 insulation layers on the upper surface of the substrate (whether directly or indirectly on the upper surface). In some embodiments, the gas sensor may comprise one or more insulation layers on the underside of the substrate. These embodiments are preferred where one or more insulation layers are present on the upper surface of the substrate. The insulation layer(s) on the 10 underside of the substrate typically cover at least 80%, preferably at least 90%, and more preferably the entire surface area of the underside of the substrate. The total thickness and the composition of the insulation layer(s) on the underside of the substrate is preferably the same as the total thickness and composition of the insulation layer(s) on the upper surface of the substrate. This helps to prevent curvature of the chip 15 during use due to relief of thermal stress. It will be appreciated that insulation layers on the underside of the substrate are only preferred where the insulation layer on the upper surface of the substrate covers a significant portion of the substrate surface area. Thus, where an insulation layer covers at least 75 % of the surface area of the upper surface of the substrate, then a corresponding insulation layer on the 20 underside of the substrate is preferred. As mentioned previously, an insulation layer typically sits between the substrate and the heater and this insulation layer preferably also serves as the insulation layer that sits between the substrate and the bond pads of the electrodes. The insulation layer that sits sit between the substrate and the heater may have a thickness of at least 5 pm, preferably at least 8 pm, 25 and more preferably at least 10 pm. The insulation layer may have a thickness of up to 50 pm, preferably up to 40 pm, and more preferably up to 30 pm. Thus, the insulation layer may have a thickness of from 5 to 50 pm, preferably from 8 to 40 pm, and more preferably from 10 to 30 pm. Where an insulation layer sits between the substrate and the heater, an insulation layer is preferably provided on the underside of the substrate preferably having the same 30 composition and thickness as the insulation layer that sits between the substrate and the heater. 02 06 25 An insulation layer may sit on the outer surface of the heater and this insulation layer may also serve as the seal forthe reference electrode. The insulation layerthat sits on the outer surface of the heater may have a thickness of at least 5 pm, preferably at least 8 pm, and more preferably at least 10 pm. The insulation layer may have a thickness of up to 50 pm, preferably 5 up to 40 pm, and more preferably up to 30 pm. Thus, the insulation layer may have a thickness of from 5 to 50 pm, preferably from 8 to 40 pm, and more preferably from 10 to 30 pm. Where an insulation layer sits on the outer surface of the heater, an insulation layer is preferably provided on the underside of the substrate preferably having the same composition and thickness as the insulation layer that sits on the outer surface of the heater. 10 The insulation layers preferably have a composition with a coefficient of thermal expansion (a) that is close to that of the solid-state electrolyte substrate on which it is provided, and preferably lower so that the insulation material, typically glass, is in compression on cooling. For instance, for a YSZ with an a of about 12x106 / oC, a desirable a for the insulation glass should be in the region of 10.5 - 11.5 x 106 / oC for thin YSZ substrates(< 200 microns in 15 thickness). It is appreciated by those skilled in the art that glasses may undergo partial crystallisation during heating and the resultant a of the partially crystallised glass may differ from that of the pure glass. The insulation layers preferably have a composition with a Young’s modulus of less than 80 GPa, and preferably less than 70 GPa. Suitable materials forthe insulation layers include glass, and preferably passivation glass. For 20 instance, the glass may be an aluminosilicate glass. The aluminosilicate glass is preferably an alkaline earth aluminosilicate glass. Each of the insulation layers may have a different composition, but they are preferably the same in composition. In particular, the insulation layers that cover a large cross-sectional area of the substrate (e.g. at least 75 % of the surface area of a side of the substrate) are preferably 25 the same in composition as one another, as this minimises thermal stresses in the gas sensor. Power The gas sensors of the presentation invention advantageous exhibit low power requirements. For instance, the gas sensor may have a power consumption of up to 1000 mW, preferably up to 800 mW, and more preferably up to 600 mW when operating continuously at 550 °C. 30 The gas sensor may have a power consumption of at least 50 mW, typically at least 100 mW, and more typically at least 150 mW when operating continuously at 550 °C. Thus, the gas sensor may have a power consumption of from 50 to 1000 mW, preferably from 100 to 800 mW, and more preferably from 150 to 600 mW. These values are the power consumption 02 06 25 when the sensor is operating continuously at 550 °C. Continuous operation means that the sensor is operated without interruption at the operating temperature. To conserve power, e.g. in order to extend the sensor lifetime or to lengthen the time between changing batteries, the sensor can be operated in a pulse-powered or intermittently powered 5 mode. This means that the sensor alternates between a sleep temperature and the sensing temperature. The sleep temperature may be room temperature - i.e. the sensor is not heated and allowed to reduce to the temperature of the environment in which it used - or, to reduce thermal stress, a temperature which is higher than room temperature but below that of the sensing temperature. When a sensor is operated in an intermittently powered mode, its power 10 consumption will be lower than when it is operated continuously. In preferred embodiments, the gas sensor is battery-powered. This enables the gas sensor to be fully portable. Gas sensing device The gas sensor of the present invention may be used without a casing; however it is generally 15 preferred for the gas sensor to be housed in a casing in order to provide some further protection to the gas sensor. Thus, the present invention provides a gas sensing device which comprises a gas sensor of the present invention housed in a casing. The casing will generally comprise an aperture which is open to the external atmosphere, so that the gas sensor is exposed to the atmosphere in which the gas sensing device is used. 20 A carbon filter, such as an activated carbon filter, may coverthe aperture. This helps to reduce the amount of contaminant gases that come into contact with the sensing electrode. Methods of use The gas sensors of the present invention may be used to measure the concentration of a variety of gases, but they are preferably used for measuring the concentration of oxygen gas 25 in an atmosphere. The sensors of the present invention may be used in high-temperature environments, for instance in an engine such as an internal combustion engine. Thus, the sensors are preferably capable of withstanding temperatures of 850 °C (dependent on the characteristics of the insulation glass selected), though they will typically be operated at a temperature of 500 to 30 600 °C. The sensors may also be used for detecting gases in other environments, such as in a factory or scientific laboratory. Examples of factories in which gas levels should be 02 06 25 controlled include factories producing medical equipment, food processing plants, waste management plants, nitrogen generation plants and semiconductor manufacturing processing plants. Synthesis method 5 A significant advantage of the gas sensors of the present invention is that they are easily manufacturable at relatively low-cost. The present invention provides a method of preparing a gas sensor of the present invention, the method comprising: applying a sensing electrode and a reference electrode to a surface of a planar electrolytic substrate, and providing an embedding heater on the, preferably glass-10 insulated but otherwise blank, surface of the substrate. The electrodes are preferably applied to the substrate by sputter deposition (also known as physical vapour deposition), though other methods may also be used such as chemical vapour deposition, screen printing or inkjet printing may also be used. Where the bond pad regions of the electrodes are in direct contact with the substrate, they may be densified by overprinting the bond pad region e.g. with 15 platinum ink. The method may further comprise depositing a catalytic coating on the sensing electrode. Where insulation layers are present in the gas sensing device, the method may comprise applying these layers by screen printing, for instance screen printing an ink comprised of glass particles suspended in a water-based or organic medium. 20 The construction of the sensor may proceed with, but is not necessarily confined to, the following sequence of steps. The electrode blocks are deposited, followed by the deposition of the glass insulation layers on both sides of the substrate. A heat treatment in air is then carried out whereby the electrodes are both sintered and hardened on to the substrate while the glass layers fuse and then become sufficiently fluid to expel entrapped air and coat the 25 substrate to provide a flat pore-free seal. Alternatively, the glass layers may be deposited firstly and fired, followed by the application and firing of the electrode blocks. Next, the heater track may be deposited and furnace-fired in air to a temperature below that of the preceding heat treatment step, achieve sintering and adhesion to the underlying glass. Finally, the reference couple and the associated sealing glass are deposited, and a final heat treatment 30 step is carried out, either in air or in a controlled atmosphere with reduced oxygen levels. The intention of the heat treatment steps is to burn off all sacrificial organic material used in the deposition formulations and to apply sufficient heat to sinter or seal and functionally activate the heater and electrode systems. 02 06 25 In some embodiments, the method may comprise preparing multiple gas sensors of the present invention by: depositing multiple sensing electrodes and multiple reference electrodes onto the surface of a planar electrolytic substrate (e.g. a tablet, tile or wafer), providing multiple embedded heaters on the surface of the substrate, and cutting the substrate to provide 5 multiple gas sensors of the present invention. Preferably, a single substrate may be cut to provide at least 10 gas sensors, preferably at least 100 gas sensors, and more preferably at least 1,000 gas sensors of the present invention. The substrate that is cut to provide multiple gas sensors may be in the form of a wafer having a width and / or length (in the case of a square or rectangular wafer) and diameter (in the case of a round wafer) of at least 10cm, and 10 preferably up to 15 cm. The substrate and components provided thereon are preferably heat-treated before the substrate is cut into the individual gas sensors. Thus, the substrate and its components are preferably in the form of a fully patterned wafer before the substrate is cut. A water-cooled diamond-tipped saw is particularly suitable for cutting the substrate into multiple gas sensors. 15 The invention will now be described byway of the following non-limiting examples. Examples Example 1 A 120 pm thick YSZ (8%Y2O3-ZrO2) 4 or 6 inch square substrate is patterned, coated with different layers and heat-treated before being diced into individual 1 mm x 2 mm rectangular 20 gas sensors. One of the gas sensors (10) is shown in Figure 1 in exploded view. The same gas sensor (10) is shown from above in Figure 5, and from sides A and B (marked in Figure 5) in Figures 6a and 6b. A screen-printed passivation alkaline-earth aluminosilicate glass layer (14) is present on the upper surface of the YSZ substrate (12) and covers 70 to 90 % of the substrate surface. A 25 glass layer (16) identical in composition and thickness to the passivation layer (14) is also present on the underside of the substrate (12). On the upper surface, there are two rectangular-shaped spaces (18) in the glass (14). Into these spaces (18), a sensing electrode (20) and a reference electrode (22) are deposited, for instance by sputter deposition, as porous platinum blocks. In the embodiment depicted in 30 Figure 1, the electrodes (20 and 22) are deposited within the spaces (18) in the glass insulation layer (14). The platinum block electrodes (20 and 22) are partially overprinted by a platinum ink, e.g. on about 50% of the surface area of the platinum blocks, to provide a densified bond pad region (20” and 22”). The surface area that has not been overprinted functions as the 02 06 25 active region (20’ and 22’) of the electrodes (20 and 22). The active regions (20’ and 22’) and bond pad regions (20” and 22”) can be seen in Figures 5 and 6a. In other embodiments (not shown), the platinum blocks (20 and 22) cover the glass free area (18) and a similar area of adjoining glass coating (14). In these embodiments, the platinum 5 blocks (20 and 22) act jointly as the electrodes in intimate contact with the surface of the YSZ substrate (12) and as bond pads where they have covered the glass surface (14). The platinum block on the lefthand-side as shown in Figure 1 is the sensing electrode (20) as it is open to the surrounding atmosphere. The platinum block on the righthand-side acts as the internal reference electrode (22) where a metal / metal oxide reference couple (24) is 10 deposited over the glass-free area (18), which is then overprinted with a sealing glass (26) to provide a hermetic seal on firing. The area covered by the sealing glass (26) extends beyond the footprint of the both the metal / metal oxide reference couple (24) and the active region (22’) of the reference electrode (22) to ensure the reference environment is sealed fully from the surrounding atmosphere. 15 The heater (28), which is a planartrack, may be coated with an outer passivation glass layer (not shown) for protection purposes, which in turn requires a second glass layer on the underside (not shown) to negate any curvature in the chip due to thermal stresses. The application of the sealing glass (26) may be combined with the application of the outer passivation glass layer (not shown) to protect the heater so as to condense both steps into 20 one step. Example 2 A further gas sensor (10) is shown in Figure 2 in exploded view. This sensor is very similar to that of Example 1, but has a filter coating (30) comprised of an oxidation catalyst such as Sm2O3, Cr2Os or Au / Pt / Pd-loaded Cr2O3 / ln2O3 / SnO2 deposited on the sensing electrode (20) 25 to remove VOC contaminants. In fast flowing non-equilibrium environments, the coating (30) can serve also to promote the establishment of thermodynamic equilibrium of the gaseous atmosphere in the vicinity of the sensing electrode (20). Example 3 A further gas sensor (10) is shown in Figure 3 in exploded view. In this embodiment, the gas 30 sensor (10) functions as a dual sensor with two sensing electrodes (20a, 20b), where a first sensing electrode (20a) is has a filter coating (30) as in Figure 2, and a second sensing electrode (20b) is uncoated as in Figure 1. This allows a pure oxygen measurement and a composite oxygen / VOC measurement to be obtained. By comparing and analysing the information from sensing electrodes (20a, 20b) with and without filter coatings (30), both oxygen and VOC levels can be determined. Example 4 5 A further gas sensor (10) is shown in Figure 4 in exploded view. In this embodiment, the gas sensor is provided with two sealed reference electrodes (22a, 22b), either of identical composition to provide a back-up in case of one being faulty or, as depicted, comprising a different reference couple (24a, 24b) so as to give flexibility in choice in the reference pO2 when different operating temperatures are used. 06 25 02 06 25

Claims

1. A potentiometric gas sensor comprising a planar electrolytic substrate having a sensing electrode, a reference electrode, and an embedded heater provided thereon, wherein the embedded heater, the sensing electrode and the reference electrode are provided on the same surface of the electrolytic substrate, and the electrolytic substrate has a surface area, on each planar face, of from 0.1 to 25 mm2, and wherein both the sensing and reference electrodes are in direct contact with the electrolytic substrate, and an insulation layer sits between the electrolytic substrate and the heater with the insulation layer in direct contact with both the electrolytic substrate and the heater.

2. The gas sensor of Claim 1, wherein the space between the reference electrode and the sensing electrode on the electrolytic substrate surface is less than 500 pm.

3. The gas sensor of Claim 1 or Claim 2, wherein the electrolytic substrate is a ceramic material.

4. The gas sensor of any preceding claim, wherein:the overall thickness of the gas sensor is from 100 to 300 pm; and / orthe overall footprint of the gas sensor is from 0.1 to 5 mm2.

5. The gas sensor of any preceding claim, wherein an insulation layer is provided on the opposite side of the electrolytic substrate from the surface on which the sensing and reference electrodes are provided, wherein the insulation layer has the same composition and thickness as the insulation layer that sits between the electrolytic substrate and heater.

6. The gas sensor of any preceding claim, wherein the sensing and reference electrodes are porous.

7. The gas sensor of any preceding claim, wherein the sensing electrode is a platinum, palladium, gold or silver electrode.

8. The gas sensor of any preceding claim, wherein a catalytic coating comprising an oxidation catalyst is present on the surface of the sensing electrode.02 06 259. The gas sensor of any preceding claim, wherein multiple sensing electrodes are provided on the electrolytic substrate surface, the multiple sensing electrodes differing from one another in composition and / or structure.

10. The gas sensor of any preceding claim, wherein the reference electrode comprises a reference couple which is hermetically sealed within a reference environment.

11. The gas sensor of Claim 10, wherein the reference couple is sealed with a glass insulation layer.

12. The gas sensor of any preceding claim, wherein multiple reference electrodes are provided on the electrolytic substrate surface, each having a reference environment, the multiple reference electrodes:being identical to one another and having identical reference environments;differing from one another in composition and / or structure; orhaving different reference environments from one another.

13. The gas sensor of any preceding claim, wherein an insulation layer sits on the surface of the heater, and an insulation layer is provided on the underside of the electrolytic substrate having the same composition and thickness as the insulation layer that sits on the outer surface of the heater.

14. The gas sensor of any preceding claim, wherein both of the sensing and reference electrodes comprises an active region and a bond pad region.

15. The gas sensor of Claim 14, wherein:the active region is in direct physical with the electrolytic substrate, and the bond pad region is not in direct physical contact with the electrolytic substrate; orthe bond pad is densified and both the active region and the bond pad region are in direct physical contact with the electrolytic substrate.

16. The gas sensor of any preceding claim, wherein the gas sensor has a power consumption of from 50 to 1000 mW when operating continuously at 550 °C.02 06 2517. The gas sensor of any preceding claim, wherein the gas sensor is adapted to withstand temperatures of 850 °C.

18. The gas sensor of any preceding claim, wherein the embedded heater sits in the same plane of the sensor as the sensing and reference electrodes.

19. The gas sensor of any preceding claim, wherein the gas sensor comprises a single electrolytic substrate.

20. The gas sensor of any preceding claim, wherein the gas sensor is devoid of inner chambers, cavities and via holes.

21. A gas sensing device which comprises a gas sensor of any preceding claim housed in a casing, the casing comprising an aperture which is open to the external environment and a carbon filter covering the aperture.

22. A method of detecting a gas in an environment, said method comprising using a gas sensor as defined in any of Claims 1 to 20 or a gas sensing device as defined in Claim 21 to measure the amount of the gas in the environment.

23. Use of a gas sensor as defined in any of Claims 1 to 20 or a gas sensing device as defined in Claim 21 for detecting a gas in an environment.

24. The method of Claim 22 or the use of Claim 23, wherein the environment is an internal combustion engine or a factory or a scientific laboratory.

25. A method of preparing a gas sensor as defined in any of Claims 1 to 20, the method comprising:applying a sensing electrode and a reference electrode directly to a surface of a planar electrolytic substrate, andproviding an embedded heater on the surface of the electrolytic substrate, wherein an insulation layer sits between the electrolytic substrate and the heater, with the insulation layer in direct contact with both the electrolytic substrate and the heater;wherein the embedded heater, the sensing electrode and the reference electrode are provided on the same surface of the electrolytic substrate, and the electrolytic substrate has a surface area, on each planar face, of from 0.1 to 25 mm2.

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