JP2008224637A - Gas sensor and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas sensor having high measuring sensitivity, excellent stability with time, and high responsiveness. <P>SOLUTION: A reference electrode 4 is formed on a part of the surface of a base material 1, and a solid electrolyte thin film 2 is formed on a surface part including a part of the base material 1 surface where the reference electrode 4 is not formed between the part of the base material 1 surface where the reference electrode 4 is not formed and the reference electrode 4 surface, and a detection electrode 3 is formed at least on a part of the solid electrolyte thin film 2 surface, and a distance between the detection electrode 3 and the reference electrode 4 in a parallel direction to the base material 1 surface is set to 0-500 μm. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a gas sensor in which carbon dioxide gas, nitrogen oxide gas, sulfur oxide gas and the like are detected with high accuracy and deterioration with time is suppressed.

Carbon dioxide sensors using solid electrolytes have a configuration of sensing electrode (working electrode) / solid electrolyte / reference electrode (reference electrode), and an equilibrium potential type that measures the electromotive force generated between both electrodes has been proposed. Various materials for the construction have been studied. Examples of solid electrolytes include sodium ion conductor NASICON (Na ₃ Zr ₂ Si ₂ PO ₁₂ ) and Na-β-Al ₂ O ₃ , oxygen ion conductor YSZ (Y ₂ O ₃ -added ZrO ₂ ), and fluorine ions. The use of the conductors PbSnO ₄ , LaF ₃ is disclosed. As these solid electrolytes, a dense sintered body or a single crystal is usually used (see Non-Patent Document 1).
As for the reference electrode, generally, a noble metal mesh such as gold or platinum, or a paste is used.

  In addition, attempts have been made to make the solid electrolyte layer a thin film. As this thin film type sensor, a reference electrode layer, a solid electrolyte thin film layer, a detection electrode layer, a detection material are formed on a dense gas non-permeable ceramic substrate. A carbon dioxide sensor having a structure in which layers are stacked in this order is disclosed. Here, the reference electrode is provided with a film of a noble metal such as platinum, palladium, rhodium, ruthenium, iridium, gold or silver. Further, an oxygen ion conductor is used for the solid electrolyte, and a mixture of alkali metal carbonate and alkaline earth metal carbonate is used for the detection material (see Patent Document 1).

  Furthermore, studies are also made on the configuration of the working electrode (detection electrode). That is, a technique is disclosed in which two layers are provided depending on the mixing ratio of the electron conductive material, the mixing ratio of the metal salt having a solid electrolyte dissociation equilibrium and the electron conductive material is 0.5 to 14% by volume, and the thickness is 20 μm or less. (See Patent Document 2).

Japanese Patent Publication No. 7-85071 Japanese Patent No. 3256618 Crystal lattice defects and their application in material development (editors: Hiroshi Yamamura, Hironobu Iwahara, publisher; IPC, published in 2002), pp. 314-328

  However, in each of the above sensors, the detection electrode (working electrode, working electrode, detection electrode layer) and the reference electrode (reference electrode, reference electrode layer) are stacked via a layer made of a solid electrolyte. For this reason, if a pinhole is generated in the solid electrolyte, the detection electrode and the reference electrode are slightly short-circuited, resulting in a problem that accurate measurement becomes difficult.

  Accordingly, an object of the present invention is to obtain a gas sensor having high measurement sensitivity, excellent stability over time, and high responsiveness.

  In the present invention, a reference electrode is formed on a part of the surface of the substrate, and the portion of the substrate surface that does not form the reference electrode and the portion of the substrate surface that does not form the reference electrode among the reference electrode surface A solid electrolyte thin film is formed on the surface portion including, a detection electrode is formed on at least a part of the surface of the solid electrolyte thin film, and a distance between the detection electrode and the reference electrode in a direction parallel to the surface of the substrate By setting the thickness to 0 μm or more and 500 μm or less, the above-described problem has been solved.

  As the solid electrolyte, an oxygen ion conductor mainly composed of cerium oxide or an oxygen ion conductor mainly composed of zirconium oxide can be used. Further, as the detection electrode, a detection substance mainly composed of a lithium compound generated by a reaction between lithium and a gas species compound to be detected, and an electron conductive metal can be used. Furthermore, as the reference electrode, a metal selected from the group consisting of platinum, palladium, rhodium, ruthenium, and iridium can be used.

  In the sensor according to the present invention, the distance between the reference electrode and the detection electrode in the direction parallel to the substrate surface is 0 μm or more, that is, the reference electrode and the detection electrode are not opposed to each other through the solid electrolyte thin film, and The linear distance between the two can be set shorter. For this reason, the pinhole which can be made into a solid electrolyte can prevent a micro short circuit with a reference pole and a detection pole, and can raise the conductivity between both. Thereby, the obtained gas sensor has high measurement sensitivity, is excellent in stability with time, and has high responsiveness.

Hereinafter, the present invention will be described in more detail.
The gas sensor according to the present invention is a sensor having a substrate 1, a solid electrolyte thin film 2, a detection electrode 3 and a reference electrode 4, as shown in FIG.

[Configuration of gas sensor]
In this gas sensor, first, a reference electrode 4 is formed on a part of the surface of the substrate 1 as shown in FIG. The solid electrolyte thin film 2 is formed on the surface portion of the base material 1 that does not form the reference electrode 4 and the surface portion of the surface of the reference electrode 4 that includes the surface portion of the base material 1 that does not form the reference electrode 4. Is formed. That is, the fixed electrolyte thin film 2 is directly formed on the base material 1 and is also formed on the reference electrode 4 formed on the base material 1 as necessary. As will be described later, the distance between the detection electrode 3 and the reference electrode 4 in the direction parallel to the surface of the substrate 1 (hereinafter, may be abbreviated as “D” in some cases, including FIGS. 1A and 1B). Since it is 0 μm or more (D ≧ 0), the detection electrode 3 is not formed on the surface of the solid electrolyte thin film 2 formed on the surface of the reference electrode 4. However, the distance between the detection electrode 3 and the reference electrode 4 in the direction parallel to the surface of the substrate 1 is 0 μm or more, as shown in FIG. 1B, one end surface of the detection electrode 3 and one of the reference electrodes. Including the case where the end face of each other is formed on the same plane (D = 0). In the case of D = 0, it is preferable to form the solid electrolyte thin film 2 on the surface of the reference electrode 4 including the one end face because the detection electrode 3 is easily formed.

  A detection electrode 3 is formed on at least a part of the surface of the solid electrolyte thin film 2. The detection electrode 3 is provided such that the distance (D) between the detection electrode 3 and the reference electrode 4 in the direction parallel to the surface of the substrate 1 is 0 μm or more and 500 μm or less.

  Since the distance (D) is 0 μm or more, the detection electrode 3 is not formed on the surface of the solid electrolyte thin film 2 formed on the surface of the reference electrode 4. For this reason, even if a pinhole exists in the solid electrolyte thin film 2, it is possible to prevent the detection electrode 3 and the reference electrode 4 from being short-circuited. On the other hand, the upper limit of the distance is 500 μm, preferably 100 μm or less. If it is larger than 500 μm, the distance between the detection electrode 3 and the reference electrode 4 becomes long and the decrease in conductivity becomes large, which is not preferable.

  Since this gas sensor is an equilibrium potential detection type gas sensor based on a solid electrolyte, generally, it has high sensitivity and high gas selectivity.

[Base material]
The base material 1 is a material for providing the solid electrolyte thin film 2, the detection electrode 3, the reference electrode 4, and the heater 5, and includes quartz glass, sapphire, strontium titanate, aluminum nitride, silicon nitride, magnesium oxide, and the like. Can be used. If the solid electrolyte thin film 2, the detection electrode 3, the reference electrode 4, and the like can be placed on the base material, the base material can be as small as about 5 mm square × 0.5 mm thickness. The amount of heat and time required can be greatly reduced.

[Solid electrolyte thin film]
The solid electrolyte thin film 2 refers to a thin film made of a solid electrolyte. The solid electrolyte is an oxygen ion conductor mainly composed of zirconium oxide, cerium oxide, or the like. Examples of the solid electrolyte mainly composed of zirconium oxide include stabilized zirconia in which an alkaline earth metal oxide such as calcium oxide or yttrium oxide is added to zirconium oxide. Examples of the solid electrolyte mainly composed of cerium oxide include those added with gadolinium oxide or calcium oxide.

The solid electrolyte thin film 2 is formed in a thin film shape, and the thickness of the solid electrolyte thin film controls the thickness of an ion exchange layer formed by a reaction between a lithium compound as a detection substance described later and the solid electrolyte thin film. For example, in the case of a carbon dioxide gas sensor in which lithium carbonate as a lithium compound is baked on a solid electrolyte mainly composed of a zirconium oxide sintered body to form a detection electrode, Li ₂ ZrO ₃ is formed at the interface between zirconium oxide and lithium carbonate. It is estimated that an ion bridge layer (ion exchange layer) is formed. In this ion exchange layer, both Li ⁺ ions supplied from lithium carbonate and O ²⁻ ions supplied from zirconium oxide are conducted, and charge transfer occurs, whereby carbon dioxide gas can be detected. Therefore, optimizing and stabilizing the thickness of the ion exchange layer is important for the temporal stability of the sensor output.

  Since the thickness of the ion exchange layer does not increase over the thickness of the solid electrolyte thin film over time, changes in sensor characteristics over time can be suppressed. And since the diffusion of lithium is suppressed by being a thin film, the ion exchange layer hardly grows in the film surface direction. Therefore, by reducing the thickness of the solid electrolyte thin film 2, it is possible to suppress changes in sensor characteristics over time, particularly deterioration over time.

  For this reason, the film thickness of the solid electrolyte thin film 2 is preferably 0.1 μm or more and 10 μm or less. Since the ionic conduction is in the direction of the film surface of the solid electrolyte thin film, if it is less than 0.1 μm, the ionic conduction characteristics deteriorate and the responsiveness deteriorates. On the other hand, when the thickness exceeds 10 μm, the thickness of the ion exchange layer formed by the reaction between the lithium compound as the detection substance and the solid electrolyte thin film becomes too thick, so that the detection sensitivity decreases.

[Reference electrode]
As the reference electrode 4, a metal selected from platinum, palladium, rhodium, ruthenium, iridium and the like is used. The formation state of the reference electrode 4 may be a film shape, but is not particularly limited.

[Detection pole]
The detection electrode 3 is an electrode made of a detection substance and an electron conductive metal. This detection substance contains a lithium compound containing lithium as a main component. This lithium compound varies depending on the type of gas to be detected. In the case of carbon dioxide, lithium carbonate, in the case of nitrogen oxide gas, lithium nitrate or lithium nitrite, in the case of sulfur oxide gas, lithium sulfate or lithium sulfite Become. Therefore, the lithium compound is a compound produced by reacting metallic lithium with a compound of the gas species to be detected.

  As the detection substance, the lithium compound may be used alone, but it is an alkaline earth metal such as strontium, barium, calcium, etc. and a gas type compound to be detected (in the case of carbon dioxide, carbonate or nitrogen oxide gas). Moisture resistance is increased by mixing compounds produced by reaction with nitrate compounds such as nitrate or nitrite in the case, and sulfate compounds such as sulfate or sulfite in the case of sulfur oxide gas. Can do.

Further, the detection substances include lithium phosphate (Li ₃ PO ₄ ), phosphoryl lithium nitride (Li _{3 + x} PO _4−y N _z , 0 <x, y, z <1), and lithium titanate (Li ₄ Ti One or more compounds containing phosphorus such as ₅ O ₁₂ ) may be mixed. By mixing these compounds, lithium ion conductivity can be improved and the response speed can be further increased.

  Each of the above carbonates, nitric compounds, sulfuric compounds, phosphorus-containing compounds, etc. may be a single compound or a compound mixture of a plurality of compounds selected from these.

  In addition to the detection substance, the electron conductive metal is mixed and used for the detection electrode. This is because the reaction between lithium ions, detection gas, oxygen and electrons occurs at the interface between the electron conductive metal and the detection substance in the detection electrode. For this reason, when these lithium ions, detection gas, oxygen, and electrons are efficiently supplied and the number of contact interfaces is increased, the above reaction can be more easily generated and the responsiveness can be improved. For this reason, it is important that the thin film made of an electron conductive metal forms a fine network.

  Examples of the electron conductive metal include metals having catalytic effects such as gold, platinum, palladium, rhodium, ruthenium, iridium, aluminum, copper, nickel, iron, and stainless steel. Among these, a metal element that reacts with metallic lithium and does not react with the detection gas is preferable. Specifically, among the metals described above, gold or platinum is preferable.

  When the detection electrode 3 having a thin film made of the detection substance and a thin film made of an electron conductive metal is used, the detection substance thin film 3a is formed on the surface of the detection electrode 3. A laminate in which a thin film 3b made of an electron conductive metal is formed (see FIG. 1A), a laminate in which a thin film made of the electron conductive metal is formed, and the detection substance thin film is formed on the surface thereof ( Or a mixed thin film (not shown) in which the thin film made of the electron conductive metal and the detection substance thin film are formed at the same time, and both are formed integrally.

  When the thickness of the detection electrode 3 is preferably 1 μm or less, and more preferably 0.5 μm or less, high responsiveness on the order of seconds can be achieved. When the thickness is 1 μm or more, the rate is limited by the low ionic conductivity of the lithium compound, and the responsiveness decreases. On the other hand, the lower limit of the thickness of the detection electrode 3 is preferably 0.01 μm, and preferably 0.05 μm. When the thickness is less than 0.01 μm, a portion where the electron conductive metal is isolated in an island shape is generated, and a portion that cannot be electrically connected to the detection electrode is generated, and the sensitivity tends to be lowered.

[Relationship between detection electrode and reference electrode]
As described above, the reference electrode 4 is formed on a part of the surface of the substrate 1. The solid electrolyte thin film 2 is formed on the surface portion of the base material 1 that does not form the reference electrode 4 and the surface portion of the surface of the reference electrode 4 that includes the surface portion of the base material 1 that does not form the reference electrode 4. Is formed. Furthermore, the detection electrode 3 is provided at a location where the distance between the detection electrode 3 and the reference electrode 4 in a direction parallel to the surface of the substrate 1 is at least 0 μm, at least part of the surface of the solid electrolyte thin film 2. . For this reason, the detection electrode 3 and the reference electrode are stacked through the solid electrolyte thin film 2, that is, the detection electrode 3 is formed on the surface of the solid electrolyte thin film 2 formed on the surface of the reference electrode 4. There is no. Therefore, the detection electrode 3 and the reference electrode 4 are not short-circuited by a pinhole or the like generated in the solid electrolyte thin film. In order to eliminate pinholes, manufacturing in a high-level clean room from which dust is removed is necessary, which increases manufacturing costs. Can be manufactured at a high yield.

  Furthermore, since the distance between the detection electrode 3 and the reference electrode 4 in the direction parallel to the surface of the substrate 1 is 0 μm or more, the distance between the two is the shortest and only the thickness of the solid electrolyte thin film 2 between them. Thus, the conductivity between the two can be further increased, and high measurement sensitivity can be obtained.

[heater]
If necessary, the base material 1 of the gas sensor according to the present invention is heated on the back surface opposite to the surface on which the sensor element including the solid electrolyte thin film 2, the detection electrode 3, and the reference electrode 4 is formed. A heater 5 is formed. Thereby, heating of this gas sensor is attained. This heater is preferably in the form of a film, and examples of the material include metals such as platinum and oxides such as ruthenium oxide.

  When a film made of metal is used as the heater, the metal film is formed by screen-printing a metal paste such as platinum paste on the base material 1 before forming the sensor element and baking it at about 1000 ° C. be able to.

[Mechanism of gas sensor according to the present invention]
In the equilibrium potential detection type gas sensor according to the present invention, the voltage is measured with a high input impedance of about 1 GΩ (gigaohm) using a voltmeter, but it is inevitably about several tens of pA (picoampere). Therefore, the ion exchange reaction continuously occurs in the ion exchange layer.

  For example, when the carbon dioxide gas is detected in detail, the following chemical reaction (1) proceeds, and the electromotive force (E) changes due to the carbon dioxide gas concentration change according to the corresponding Nernst equation (2).

・ Chemical reaction formula (1)
Detection electrode: Li ₂ CO ₃ → 2Li ⁺ + CO ₂ + 1 / 2O ₂ + 2e ⁻
Reference electrode: 1 / 2O ₂ + 2e ⁻ → O ²⁻

・ Nernst formula (2)
E = E ₀ + (RT / 2F) In (P _CO2 )
Here, E ₀ : constant, R: gas constant, T: absolute temperature (K) of sensor element, F: Faraday constant, P _CO2 ; carbon dioxide gas concentration. The oxygen concentration (P _O2 ) in the air can be regarded as constant.

  At the detection electrode, lithium ions are supplied from the detection substance and electrons are supplied from the electron conductive metal, and reacts with carbon dioxide gas and oxygen in the atmosphere to generate lithium carbonate. On the other hand, at the reference electrode, oxygen ions generated from oxygen in the atmosphere pass through the solid electrolyte thin film and exchange electric charges with lithium ions in the ion exchange layer to form an electric circuit. By improving and stabilizing the conductivity of lithium ions and oxygen ions in each interface and in the solid electrolyte thin film, the sensor output is highly accurate, highly responsive, and effective over time.

  When detecting NOx gas, the detection electrode is a compound mainly composed of lithium nitrate, and when detecting SOx gas, it is a compound mainly composed of lithium sulfate.

[Manufacturing method of gas sensor]
Next, a method for manufacturing a gas sensor according to the present invention will be briefly described with reference to FIG.

  First, a reference electrode manufacturing process for forming a reference electrode 4 on a part of the surface of the substrate 1 is performed. Examples of the film forming method include a vapor phase method, a sol-gel method, and a baking method. Examples of the vapor phase method include a vapor deposition method, an ion plating method, a sputtering method, and a laser ablation method, but are not limited thereto.

  Next, the solid electrolyte thin film in which the solid electrolyte thin film 2 is laminated on the surface portion including the portion of the surface of the base material 1 that does not form the reference electrode 4 and the surface of the base material 1 that does not form the reference electrode 4 among the surface of the reference electrode 4. A lamination process is performed. Specifically, in the case of FIG. 1A, a solid on which the solid electrolyte thin film 2 is formed on the surface of the base material 1 other than where the reference electrode 4 is formed and on the surface of the reference electrode 4. An electrolyte thin film lamination process is performed. That is, in the case of FIG. 1A, a solid electrolyte thin film laminating step for forming the solid electrolyte thin film 2 is performed on the surface of the reference electrode 4 as well as on the surface of the base material 1 where the reference electrode 4 is not formed. .

  Examples of a method for forming the solid electrolyte thin film 2 include a vapor phase method, a sol-gel method, and a firing method. Examples of the vapor phase method include a vapor deposition method, an ion plating method, a sputtering method, and a laser ablation method, but are not limited thereto. Examples of the sol-gel method include a method using a metal organic compound as a raw material. Furthermore, examples of the firing method include a method in which a colloidal powder is applied and then fired.

  After the solid electrolyte thin film laminating step, a detection electrode manufacturing step is performed in which a film of the detection electrode 3 made of a detection substance and an electron conductive metal is formed on the surface of the solid electrolyte thin film 2. Examples of the film forming method include a vapor phase method, a gas deposition method, a sol-gel method, and the like, but are not limited thereto.

  The gas deposition method is a method of forming a film by spraying a raw material powder in a solid state on a substrate by a gas flow. The sol-gel method uses a metal organic compound as a raw material and utilizes a sol-gel change.

  In the gas deposition method, the forming area ratio of both of the detection substance and the electron conductive metal raw material powder can be controlled by the mixing ratio of the raw material powders or the gas flow rate control of both raw material powder nozzles. In addition, after the electron conductive metal thin film is initially formed in an island shape with a thickness of several nm to several tens of nm, both may be formed simultaneously as described above.

  Examples of the vapor phase method include a vapor deposition method, an ion plating method, a sputtering method, and a laser ablation method.

  As a raw material handling method in this vapor phase method, there are a method using a target compound as a raw material and a method using a metal element as a raw material. In the case of adopting a method using this metal element as a raw material, metal lithium is used as the metal element, and when using the alkaline earth metal, both metal lithium and alkaline earth metal are used. And it forms into a film-forming atmosphere of the compound of the gas seed | species of detection object, and a detection substance thin film is formed. For example, when forming a lithium carbonate thin film, a vapor phase method is performed using metallic lithium as a raw material and a film forming atmosphere as a carbon dioxide gas atmosphere.

  By the way, the detection electrode 3 has both a thin film made of the detection substance and a thin film made of an electron conductive metal. The following three methods can be adopted as a method for producing both the thin films. First, as shown in FIG. 1 (a), the detection substance thin film 3a is formed in a place where the detection electrode is formed, and then the surface of the detection substance thin film is made of the electron conductive metal. In this method, the thin film 3b is formed. The second method is a method of forming a thin film made of the electron conductive metal, and then forming the detection substance thin film on the surface of the electron conductive metal thin film. Although not shown, the third method is a method of simultaneously forming a thin film made of the electron conductive metal and a thin film of the detection substance thin film at a place where the detection electrode is formed.

  In the method of sequentially forming the detection substance thin film and the electron conductive metal thin film at the place where the detection electrode 3 is formed, two patterns are formed by using a mask. Thin films can be stacked.

  In the method of simultaneously forming the thin film made of the electron conductive metal and the thin film of the detection substance thin film at the place where the detection electrode 3 is formed, By controlling the generation amount of the gaseous raw material at the generation source, the formation area ratio of both can be controlled.

  By the way, when using metal lithium or metal lithium and alkaline earth metal as a raw material for the vapor phase method, argon gas is used in addition to the method of forming a film in the film formation atmosphere of the compound of the gas species to be detected. A method of forming a metal lithium-containing thin film under an inert gas film formation atmosphere such as heat treatment at a predetermined temperature and alloying in a dry atmosphere and then reacting with water and a detection gas species may be used. . Also by this, the target detection substance thin film can be formed.

  The dry atmosphere is preferably an atmosphere having a dew point of −40 ° C. or lower, and the heat treatment temperature is preferably 100 ° C. or higher and 500 ° C. or lower. If the dew point is higher than −40 ° C., the reaction between water and metallic lithium may occur in part. On the other hand, when the heat treatment temperature is lower than the above temperature, it is difficult to alloy them, and as a result, sufficient performance may not be obtained. On the other hand, although the heat treatment temperature may be higher than the above temperature, the above temperature range is sufficient because there is no great difference in the performance of the obtained thin film.

  By the way, when both the metallic lithium and the alkaline earth metal are used when forming the detection substance thin film, the metallic lithium and the alkaline earth metal are separately in the vapor phase method or in the metal state under the above drying conditions. Then, both metal lithium and alkaline earth metal are efficiently obtained as a compound of the detection gas species by reacting with a detection gas species such as carbon dioxide gas humidified to the above temperature range. Can be preferred.

Further, when producing the detection electrode 3 for detecting sulfur oxide gas, after forming a solid electrolyte thin film made of a compound or compound mixture containing metallic lithium, phosphorus, and sulfur as described above, The detection electrode 3 can be manufactured by reacting the detection gas species. The obtained sensing electrode consists of Li ₂ SO ₄ , Li ₂ S ₂ O ₆ , Li ₆ P ₆ O ₁₈ , Li ₃ PO ₄ , LiPO _3, etc., and a mixture of these hydrates, and Li ion conduction It is a thin film sensing electrode with a high degree of sulfur oxide detection characteristics.

  Among the methods for producing the detection electrode 3, a laser ablation method, a metal lithium film, or an alloy film of a metal element and metal lithium constituting an electron conductive metal is formed by the above method, and after heat treatment, A method of forming the detection electrode 3 by reacting water and detection gas species is more preferable.

  By employing the above manufacturing method, the gas sensor according to the present invention can be manufactured.

Although embodiment is shown below, it is not limited to these. In the following examples, a carbon dioxide sensor will be described.
(Example 1)
[Manufacture of carbon dioxide sensor]
The heater formation process which forms the heater 5 with said method on the back surface of the base material 1 is performed. That is, a metal paste such as platinum paste or ruthenium oxide was screen-printed on the back surface of the base material 1 before forming the sensor element and baked at about 1000 ° C.
A mask is formed on the surface of a 5 mm square and 0.5 mm thick quartz glass plate substrate, and only a part of the mask is opened. Then, a DC sputtering method is used to form platinum to a thickness of 0.1 μm. Four membranes were produced.
Next, a mask is formed so that a part of the base material where the reference electrode 4 is not provided and a part of the surface of the reference electrode 4 are opened, and a cerium oxide film (GDC) to which gadolinium oxide is added has a thickness of 1.0 μm. The solid electrolyte thin film 2 was formed in a thick thickness. This solid electrolyte thin film 2 is prepared by placing a substrate in an excimer laser ablation film forming apparatus, setting the substrate temperature to 650 ° C., and pulsed excimer laser light on a sintered target of cerium oxide powder to which gadolinium oxide is added. The film was formed by a method of irradiation and treatment in an oxygen gas atmosphere.
Next, the detection electrode 3 is a surface of the solid electrolyte thin film 2 that does not overlap with the reference electrode 4 in the vertical direction and is located 1 μm away from the end surface of the reference electrode 4. Formed. In the formation of this detection electrode, a binary evaporation method was used, and gold and metallic lithium were simultaneously evaporated from separate crucibles to form a film. After film formation, the mixture was heated to about 500 ° C. in the atmosphere to form a mixed thin film of lithium carbonate and gold.
Then, a gold lead wire was connected to the detection electrode 3 and the reference electrode 4 with a gold paste to obtain a carbon dioxide gas sensor, and the gas sensor shown in FIG. 1A was obtained (note that the detection electrode 3 is a mixed thin film, This is the only difference from FIG.
The following measurements and evaluations were performed using the obtained carbon dioxide sensor. The results are shown in Table 1.

[Measurement of sensor output]
A voltage was applied to the heater 5 so that the sensor driving temperature was 370 ° C.
Carbon dioxide is added to a mixed gas of 22% oxygen, 78% nitrogen, and 60% relative humidity at room temperature, and the carbon dioxide concentration is increased to 350 ppm, the same level as the atmosphere, and the standard gas is increased to 1000 ppm. Were made and switched to flow into the measurement container.
The produced carbon dioxide sensor was placed in a measurement container and connected to an electrometer having an input impedance of 1 GΩ (gigaohm). The temperature in the measurement vessel was kept constant at 50 ° C., and a carbon dioxide concentration 350 ppm mixed gas and a 1000 ppm mixed gas were alternately flowed, and the output was measured.
In addition, the influence of the temperature fluctuation of the sensor part due to the gas flow rate change at the time of gas switching was corrected according to the Nernst equation based on the temperature detected by the thermocouple attached to the sensor.
(Theoretical value of ΔE at 370 ° C. ({theoretical value of sensor output at CO ₂ (1000 ppm) −sensor output at CO ₂ (350 ppm))} is 29 mV.)

[Measurement stability]
When the carbon dioxide concentration was switched from 350 ppm to 1000 ppm, the stability of whether or not it could be measured immediately was evaluated visually.

[Measurement time]
The time when the output was stabilized when the carbon dioxide concentration was increased from 350 ppm to 1000 ppm and the time when the output was stabilized when the carbon dioxide concentration was decreased from 1000 ppm to 350 ppm were measured.

[Stability over time]
The carbon dioxide concentration was kept constant at 350 ppm, the measurement temperature was kept at 50 ° C., and the change in the output was measured over 350 days. As a result, when the value was within a variation of ± 0.5 mV, “◯” was indicated, and when there was a variation exceeding the range, “X” was assigned.

(Example 2)
A carbon dioxide sensor was prepared in the same manner as in Example 1 except that the solid electrolyte thin film was replaced with yttrium oxide-added zirconium oxide, and the output was measured. The results are shown in Table 1.

(Examples 3 to 6)
A sensor element was produced in the same manner as in Example 1 while changing the reference electrode, and the sensor output was measured under the above conditions. The results are shown in Table 1.

(Examples 7 to 15)
A sensor element was produced by the same method as in Example 1 except that lithium carbonate and other compounds shown in Table 1 were used as the detection substance, and the sensor output was measured under the same conditions as in Example 2. The results are shown in Table 1.

  However, the detection electrode was prepared by excimer laser ablation. As the target, a disk-shaped target having a diameter of 20 mm and a thickness of 5 mm was used. The disk-shaped target was vertically divided into three equal parts, one third of which consisted of other compounds, and another 3 minutes. 1 was Li metal, and the remaining third was a gold metal plate. The target was rotated to irradiate the target with laser light, and other compound layers and metal layers were alternately formed. After the film formation, the detection electrode was obtained through the same process as in Example 1.

The chemical formulas of the lithium compounds used in Table 1 and the compounds used for other substances are as follows.
That is, lithium carbonate is Li ₂ CO ₃ , strontium carbonate is SrCO ₃ , barium carbonate is BaCO ₃ , calcium carbonate is CaCO ₃ , lithium phosphate is Li ₃ PO ₄ , and phosphoryllithium nitride is Li _3.3 PO _{3. 7} N _0.3 and lithium titanate are Li ₄ Ti ₅ O ₁₂ .

(Other sensors)
As for nitrogen oxide gas and sulfur oxide gas, when sensors were similarly produced and the sensor detection test was conducted in the same manner, the same output and stability over time could be obtained.

(A) (b) Sectional drawing which shows the example of the gas sensor concerning this invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Base material 2 Solid electrolyte thin film 3 Detection electrode 3a Detection substance thin film 3b Thin film 4 made of an electron conductive metal Reference electrode 5 Heater D Distance between the detection electrode and the reference electrode in a direction parallel to the surface of the substrate

Claims (11)

Forming a reference electrode on a part of the surface of the substrate;
A solid electrolyte thin film is formed on the surface portion including the portion of the base material surface that does not form the reference electrode, of the portion of the base material surface that does not form the reference electrode and the surface of the reference electrode.
Forming a sensing electrode on at least a part of the surface of the solid electrolyte thin film;
A gas sensor, wherein a distance between the detection electrode and the reference electrode in a direction parallel to the surface of the substrate is 0 μm or more and 500 μm or less. The solid electrolyte is an oxygen ion conductor mainly composed of cerium oxide or an oxygen ion conductor mainly composed of zirconium oxide,
The detection electrode is composed of a detection substance mainly composed of a lithium compound produced by a reaction between metallic lithium and a gas species compound to be detected, and an electron conductive metal,
The gas sensor according to claim 1, wherein the reference electrode is a metal selected from the group consisting of platinum, palladium, rhodium, ruthenium, and iridium.   3. The gas sensor according to claim 2, wherein the detection substance contains, in addition to the lithium compound, one or more elements selected from alkaline earth metals and a compound produced by reacting with a compound of a gas species to be detected.   The gas sensor according to claim 2 or 3, wherein the detection substance contains one or more compounds selected from the group consisting of lithium phosphate, phosphoryl lithium nitride, and lithium titanate in addition to the lithium compound.   The gas sensor according to claim 2, wherein the electron conductive metal is either gold or platinum. A reference electrode manufacturing process for manufacturing a reference electrode on a part of the surface of the substrate is performed.
Next, a solid electrolyte thin film stack in which a solid electrolyte thin film is stacked on a surface portion including the portion of the base material surface that does not form the reference electrode and the surface of the base material that does not form the reference electrode among the surface of the reference electrode Perform the process,
Next, a detection electrode manufacturing method in which a detection electrode film made of a mixture of a detection substance and an electron conductive metal is formed on at least a part of the surface of the solid electrolyte thin film by a gas phase method, a gas deposition method, or a sol-gel method. Perform the process,
A method of manufacturing a gas sensor, wherein the detection electrode manufacturing process is performed such that a distance between the detection electrode and the reference electrode in a direction parallel to the surface of the substrate is 0 μm or more and 500 μm or less.   The gas sensor manufacturing method according to claim 6, wherein the film forming method in the detection electrode manufacturing process is any one of a vapor phase method selected from a vapor deposition method, an ion plating method, a sputtering method, and a laser ablation method.   The gas sensor according to claim 7, wherein metal thin film or a metal lithium and an alkaline earth metal is used as a raw material for the gas phase method, and a detection substance thin film is formed in a film formation atmosphere of a compound of a gas species to be detected. Manufacturing method.   A thin film made of the electron conductive metal is formed in a place where the detection electrode is formed, and then the detection substance thin film is formed on the surface of the electron conductive metal thin film, or the detection electrode is The method for manufacturing a gas sensor according to claim 6, 7 or 8, wherein the film forming of the thin film made of the electron conductive metal and the film forming of the detection substance thin film are simultaneously performed at a place where the film is formed. As a raw material of the vapor phase method, using metal lithium, or metal lithium and alkaline earth metal, forming a metal lithium-containing thin film in an inert gas film formation atmosphere,
A thin film made of the electron conductive metal is formed in a place where the detection electrode is formed, and then the detection substance thin film is formed on the surface of the electron conductive metal thin film, or the detection electrode is In the place where the film is to be formed, the thin film made of the electron conductive metal and the thin film of the sensing substance are simultaneously formed
Next, the method for producing a gas sensor according to claim 7, wherein heat treatment is performed at a temperature of 100 ° C. or more and 500 ° C. or less in a dry atmosphere having a dew point of −40 ° C. or less, and then reacted with water and a detection gas species.   The method for producing a gas sensor according to claim 8, 9 or 10, wherein a compound or compound mixture containing phosphorus and sulfur is used together with the metallic lithium.

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