Gas Sensor

Abstract

To provide a gas sensor having improved sensitivity.SOLUTION: A gas sensor includes: a base material; first and second electrodes provided on the base material; and a gas detector connected to the first and second electrodes. The gas detector is composed of calcium ferrite and zirconium as essential components and a semiconductor oxide containing at least one metallic element selected from among aluminum, silver, and tin.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a gas sensor. [Background technology]

[0002] In recent years, carbon monoxide, carbon dioxide, and NO have become increasingly prevalent in living environments such as offices and homes, as well as in agricultural and bio-related fields. x Demand for monitoring specific gases such as HCl, HCl, and HCl is increasing. Accordingly, gas sensors for detecting these specific gases have been proposed (see Patent Document 1).

[0003] In gas sensors such as that described in Patent Document 1, a semiconductor containing tin oxide particles as a main component is often used as a gas detection section for detecting gas. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2017-106857 Summary of the Invention [Problem to be solved by the invention]

[0005] Gas sensors using tin oxide alone have a limit to their gas detection sensitivity. For this reason, attempts have been made to increase the detection sensitivity for the desired gas to be detected by coating the surface of tin oxide particles with lanthanum (La) oxide (La2O3), which is known to be highly reactive with CO2 gas, for example.

[0006] However, it is difficult to obtain a gas sensor with good detection sensitivity that can be adapted to various required use environments (measurement temperature environments), and there is still room for improvement.

[0007] An object of the present invention is to provide a gas sensor using a semiconductor oxide that has good detection sensitivity in accordance with various required measurement temperature environments. [Means for solving the problem]

[0008] As a result of extensive research, the inventors have noticed that the adsorption characteristics of the test gas on the surface of the semiconductor oxide and the space charge layer formed on the surface of the semiconductor oxide contribute to the detection performance of the test gas, and have completed the present invention.

[0009] According to one aspect of the present invention, there is provided a gas sensor comprising a substrate, a first electrode and a second electrode disposed on the substrate, and a gas detection unit connected to the first electrode and the second electrode, wherein the gas detection unit is made of a semiconductor oxide containing calcium ferrite and zirconium as essential elements and at least one metal element selected from the group consisting of aluminum, silver, and tin. [Effects of the Invention]

[0010] According to the above aspect, by using calcium ferrite and zirconium as essential elements and co-doping at least one metal element selected from aluminum, silver, and tin, it is possible to obtain a gas sensor with excellent detection sensitivity in accordance with various required measurement temperature environments. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a plan view illustrating a gas sensor according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram illustrating the mechanism of CO2 gas detection in a conventional tin oxide semiconductor. [Figure 3] FIG. 3 is a schematic diagram illustrating a space charge layer formed on a conventional tin oxide semiconductor in air. [Figure 4] FIG. 4 is a schematic diagram illustrating a space charge layer formed on a conventional tin oxide semiconductor in CO2 gas as a test gas. [Figure 5] FIG. 5 is a schematic diagram illustrating a space charge layer formed in the gas detection portion of the gas sensor according to this embodiment in air. [Figure 6] FIG. 6 is a schematic diagram illustrating a space charge layer formed in the gas detection portion of the gas sensor according to this embodiment in CO2 gas as the test gas. [Figure 7] FIG. 7 is a schematic diagram illustrating the mechanism of CO2 gas detection on a semiconductor surface. [Figure 8] FIG. 8 is a schematic diagram illustrating the detection mechanism when O 2 gas is negatively adsorbed as O − on a semiconductor surface. [Figure 9] FIG. 9 is a schematic diagram illustrating the detection mechanism when O2 gas is negatively adsorbed as O2- on the semiconductor surface. [Figure 10] FIG. 10 is a schematic diagram illustrating a state in which oxygen is negatively charged and adsorbed in the O- state onto CaFe2O4 into which an oxide layer has been introduced. [Figure 11] FIG. 11 is a diagram illustrating a method for producing a semiconductor oxide according to this embodiment. [Figure 12] FIG. 12 is a schematic diagram illustrating an evaluation test device for evaluating a gas sensor. [Figure 13] FIG. 13 is a circuit diagram for explaining a circuit configuration for evaluating the detection performance of a gas sensor. [Figure 14] FIG. 14 is a diagram showing the relationship between the detection sensitivity of CO2 gas and the measurement temperature. [Figure 15] FIG. 15 is a diagram showing the relationship between the O2 concentration (PO2) and the resistance value of the gas sensor using the semiconductor oxide (CaFe2O4) of the specimen 1. [Figure 16] FIG. 16 is a diagram showing the relationship between the O2 concentration (PO2) and the resistance value of the gas sensor using the semiconductor oxide (5 mol % Zr-CaFe2O4) of the specimen 2. [Figure 17] FIG. 17 is a diagram showing the relationship between the O2 concentration (PO2) and the resistance value of a gas sensor using a semiconductor oxide (1 mol % Al, 5 mol % Zr-CaFe2O4) of specimen 3. [Figure 18] FIG. 18 is a diagram showing the relationship between the O2 concentration (PO2) and the resistance value of the gas sensor using the semiconductor oxide (1 mol % Ag, 5 mol % Zr-CaFe2O4) of the specimen 4. [Figure 19] FIG. 19 is a diagram showing the relationship between the O2 concentration (PO2) and the resistance value of the gas sensor using the semiconductor oxide (1 mol % Sn, 5 mol % Zr—CaFe2O4) of the specimen 5. [Figure 20] FIG. 20 is a diagram showing the relationship between the CO2 gas detection sensitivity of each gas sensor under test and the gradient of the approximation line derived from the O2 concentration dependency of the resistance value. [Figure 21] FIG. 21 is a diagram showing the relationship between the slope of the approximation line derived from the O2 concentration dependency of the resistance value in each gas sensor test piece and the measurement temperature. [Figure 22] FIG. 22 shows the relationship between the resistance ratio (RO2 / RN2) of the electrical resistance (RN2) of the gas sensor when it is in N2 gas and the electrical resistance (RO2) of the gas sensor when it is exposed to an atmosphere containing O2 gas, and the measurement temperature. [Figure 23] FIG. 23 is a graph showing the relationship between the resistance ratio (RO2 / RN2) of the electrical resistance (RN2) of the gas sensor when it is in N2 gas and the electrical resistance (RO2) of the gas sensor when it is exposed to an atmosphere containing O2 gas, and the detection sensitivity. [Figure 24] FIG. 24 is a graph showing a comparison of the change in the thickness of the depletion layer as a function of the resistance ratio (RO2 / RN2) between the electrical resistance (RN2) of the gas sensor when it is in N2 gas and the electrical resistance (RO2) of the gas sensor when it is exposed to an atmosphere containing O2 gas. DETAILED DESCRIPTION OF THE INVENTION

[0012] [Gas sensor] <Gas sensor configuration> An example of a gas sensor 10 according to an embodiment of the present invention will be described in detail with reference to the drawings.

[0013] FIG. 1 is a partially cutaway plan view of a gas detection section 14 of a gas sensor 10 according to an embodiment of the present invention.

[0014] The gas sensor 10 shown in FIG. 1 is a thick-film sensor having a substrate 11, a first electrode 12, a second electrode 13, and a gas detection portion 14, all of which are formed on the substrate 11 in the form of a flat plate.

[0015] An insulating material or a semi-insulating material can be used for the substrate 11. Examples of insulating materials that can be used include structural ceramics such as alumina, silicon dioxide, mullite, magnesium oxide, and forsterite, as well as glass and sapphire. Examples of semi-insulating materials that can be used include silicon carbide. In addition, any material that is typically used as a substrate for a gas sensor can be used for the substrate 11.

[0016] When a flat substrate 11 is used, the thickness of the substrate 11 can be 0.05 mm or more and 1.0 mm or less. From the viewpoint of the strength of the substrate 11, the thickness of the substrate 11 is preferably 0.09 mm or more. Furthermore, from the viewpoint of heat dissipation, the thickness of the substrate 11 is preferably 1.0 mm or less.

[0017] The first electrode 12 and the second electrode 13 can be made of any material that is typically used as an electrode. Suitable conductive materials include Cu, Al, Ag, Au, Pt, Ni, Cr, and Sn. Lead wires (not shown) are attached to the first electrode 12 and the second electrode 13, respectively, for electrical connection to an external electrical circuit or the like. The lead wires can be selected from materials such as Cu, Al, Ag, Au, Pt, Ni, Cr, and Sn.

[0018] In this embodiment, the gas sensor 10 is formed, for example, by thick-film printing, as shown in Fig. 1. The first electrode 12 and the second electrode 13 can each be formed in a comb-like shape. The first electrode 12 and the second electrode 13 are arranged on the surface of the substrate 11 such that the comb teeth 12a constituting the first electrode 12 and the comb teeth 13a constituting the second electrode 13 are alternately interdigitated with each other. The gas sensor 10 may be formed in a film shape by coating, dropping, or the like, instead of by thick-film printing.

[0019] The first electrode 12 and the second electrode 13 can be formed on the surface of the base material 11 by pattern film formation using a sputtering method, an ion plating method, a vacuum deposition method, or a laser ablation method depending on the metal element used. The first electrode 12 and the second electrode 13 can also be formed by printing an electrode material on the surface of the base material 11. Other bonding methods such as wire bonding can also be used.

[0020] The thickness of the first electrode 12 and the second electrode 13 can be 0.05 μm or more and 20 μm or less. From the viewpoint of detection performance for the detectable gas, it is preferably 1 μm or more, and from the viewpoint of cost, it is preferably 10 μm or less.

[0021] The inter-electrode distance between the teeth 12a of the first electrode 12 and the teeth 13a of the second electrode 13 is 80 μm or more and 200 μm or less. The ratio (S / L0) of the electrode line width L0 of the teeth 12a of the first electrode 12 and the teeth 13a of the second electrode 13 to the spacing S between the teeth 12a of the first electrode 12 and the teeth 13a of the second electrode 13 can be 0.27 or more and 4.00 or less. From the viewpoint of sensor characteristics, it is preferably 0.80 or more and 2.00 or less.

[0022] Gas detection unit 14 is disposed so as to overlap comb teeth 12a of first electrode 12 and comb teeth 13a of second electrode 13 so as to electrically connect first electrode 12 and second electrode 13. Gas detection unit 14 is made of a material that can electrically adsorb gas molecules to be detected, and detects the presence of gas molecules by utilizing a change in resistance value that accompanies the adsorption of gas molecules.

[0023] In this embodiment, the semiconductor oxide powder constituting the gas detection unit 14 is made into a paste using a binder, and the paste is formed on the substrate 11 by screen printing or the like. Note that the paste may contain an insulating material such as glass.

[0024] In the gas sensor 10 shown in FIG. 1, the gas detection portion 14 can be formed by coating in an area of ​​a predetermined length L1 and width W1 so as to cover the spaces between the comb teeth of the first electrode 12 and the second electrode 13.

[0025] In the thick-film gas sensor 10, the thickness of the gas detection portion 14 can be set to 0.05 μm or more and 10 μm or less.

[0026] 1, the gas detection unit 14 may be applied to a predetermined region across the first electrode 12 and the second electrode 13, or may be applied only between the first electrode 12 and the second electrode 13. Although not shown in FIG. 1, a protective layer may be disposed on the surface of the substrate 11, and the first electrode 12 and the second electrode 13 may be disposed on the surface of the protective layer.

[0027] The overall dimensions of the gas sensor 10, that is, the length L2 and the width W2, can be set appropriately according to the environment in which the gas sensor 10 is used.

[0028] 1, the gas sensor 10 is used together with a heater that heats the gas sensor 10 to the detection temperature of the gas to be detected. The heater may be formed integrally with the substrate 11 of the gas sensor 10.

[0029] <Gas detection section> In this embodiment, the gas detection unit 14 detects oxygen (O2) as O - The semiconductor oxide is capable of forming an oxide layer that adsorbs negative charges in a negative state.

[0030] The gas detection unit 14 is made of a semiconductor oxide that essentially contains calcium ferrite (CaFe2O4) particles and zirconium (Zr), and also contains at least one metal element selected from aluminum (Al), silver (Ag), and tin (Sn). This promotes negative charge adsorption of oxygen onto the surface of the semiconductor oxide, and also converts oxygen into O - In this case, an oxide layer (Ag2O, Al2O3 or SnO2) can be formed that can be adsorbed in the above state.

[0031] The content of Zr is more than 0 mol % and 10 mol % or less in terms of a molar fraction relative to the iron (Fe) contained in CaFe2O4.

[0032] Even a small amount of Zr, even near 0 mol%, can contribute to the formation of a three-dimensional porous structure in CaFe2O4. As the Zr content increases, the state of the micropores in the three-dimensional porous structure can be improved. However, if the Zr content exceeds 10 mol%, it becomes difficult to form micropores in the three-dimensional porous structure of Zr and CaFe2O4, and the adsorption performance of the test gas decreases, resulting in a decrease in the detection performance of the test gas.

[0033] As described above, in order to form a good three-dimensional porous structure in CaFe2O4 and to improve the detection performance of the target gas, the Zr content is preferably 3 mol % or more and 7 mol % or less in terms of the molar fraction relative to Fe in CaFe2O4.

[0034] Furthermore, the content of at least one metal element selected from Al, Ag, and Sn, either singly or in total, is preferably more than 0 mol % and 10 mol % or less relative to the total amount of CaFe2O4.

[0035] If the content of at least one of the metal elements Al, Ag, and Sn exceeds 10 mol%, the formation of a three-dimensional porous structure by Zr and CaFe2O4 is hindered, and the surface area of ​​the semiconductor oxide particles decreases, resulting in a decrease in the detection performance of the target gas.

[0036] In this embodiment, the semiconductor oxide is preferably CaFe2O4 containing Zr and any one of Al, Ag, and Sn alone, and the content of any one of Al, Ag, and Sn is 1 mol% with respect to the total amount of CaFe2O4.

[0037] <Explanation of the gas detection mechanism in the gas detection unit> (tin oxide) Here, a conventional gas sensor using a semiconductor oxide containing tin oxide as a main component in the gas detection section will be described as a comparison with the gas sensor according to the present invention.

[0038] In gas sensors that use a semiconductor oxide primarily composed of tin oxide in the gas detection section, attempts have been made to improve detection performance by adding basic oxides such as CaO or La2O3 to the tin oxide to strengthen the interaction with CO2. However, gas sensors using tin oxide have had the problem of being unable to efficiently detect changes in resistance. This is due to the gas detection mechanism of tin oxide.

[0039] That is, since tin oxide is an n-type semiconductor oxide, when the CO2 adsorption reaction shown in the following reaction formula (1) occurs on the surface of the tin oxide, the electron concentration on the surface of the tin oxide decreases and the electrical resistance increases. Here, (ad) is the symbol indicating that a negative charge is adsorbed on the surface of the semiconductor oxide.

[0040] O - (ad)+CO2+e - → CO3 2- (ad) (1)

[0041] Fig. 2 is a schematic diagram illustrating the CO2 gas detection mechanism in a conventional tin oxide semiconductor. Fig. 3 is a schematic diagram illustrating the space charge layer formed in a conventional tin oxide semiconductor in air. Fig. 4 is a schematic diagram illustrating the space charge layer formed in a conventional tin oxide semiconductor in CO2 gas as the detected gas.

[0042] A space charge layer (hereinafter referred to as a depletion layer) is formed in the tin oxide semiconductor due to the negative charge adsorption of oxygen. The thickness of the depletion layer increases as the negatively adsorbed oxygen reacts with the CO2 gas being detected.

[0043] However, when a basic oxide such as CaO is added to a tin oxide semiconductor, the desorption reaction of O2 takes precedence over the adsorption of CO2 gas above a certain temperature, which limits the measurable temperature range of the gas sensor below that certain temperature (for example, 200°C to 400°C).

[0044] Furthermore, in this measurable temperature range, as mentioned above, the thickness of the depletion layer increases due to the reaction between the oxygen negatively charged and the test gas in the tin oxide semiconductor. This increases the resistance of the tin oxide semiconductor itself. Therefore, even if CO2 gas is adsorbed to the tin oxide semiconductor and the resistance value changes, it becomes difficult to detect the change in resistance due to the adsorption of CO2 gas.

[0045] (Calcium ferrite) The inventors of the present invention have noticed that, among p-type semiconductors containing alkaline earth metals, oxides primarily composed of calcium ferrite (CaFe2O4) can be used as the gas detection section of gas sensors to improve CO2 gas detection sensitivity compared to tin oxide semiconductors. Among alkaline earth metals, calcium (Ca) has the advantage of being easier to synthesize as a pure substance due to its smaller ionic radius than strontium (Sr) or barium (Ba). Furthermore, CO2, being an oxidizing gas, can strongly interact with the surface of basic oxides such as CaFe2O4.

[0046] In addition, CaFe2O4 is a p-type semiconducting oxide. Therefore, as the CO2 adsorption reaction shown in the above-mentioned reaction formula (1) progresses, the hole concentration in CaFe2O4 increases and the electrical resistance decreases.

[0047] Fig. 5 is a schematic diagram illustrating a space charge layer (depletion layer) formed in the gas detection unit 14 of the gas sensor 10 according to this embodiment in air. Fig. 6 is a schematic diagram illustrating a space charge layer (depletion layer) formed in the gas detection unit 14 of the gas sensor 10 according to this embodiment in CO gas as the detection gas.

[0048] When the CO2 adsorption reaction occurs in CaFe2O4 in the gas detection unit 14, the hole (positive hole) concentration in CaFe2O4 increases. That is, due to the negative charge adsorption of CO2 onto the CaFe2O4 surface, the thickness of the depletion layer formed in CaFe2O4 decreases from the state of the depletion layer shown in Figure 5 to the state of the depletion layer shown in Figure 6.

[0049] Therefore, in the adsorption reaction of CO2 gas onto the surface of CaFe2O4, the change in resistance value due to the adsorption of CO2 gas can be efficiently detected even in a specific temperature range (for example, from 200°C to 400°C).

[0050] Furthermore, by adding 1 to 10 mol% of Zr to the Fe in CaFe2O4, a three-dimensional porous structure can be formed in the CaFe2O4 particles, which increases the specific surface area and improves the detection sensitivity of the target gas.

[0051] However, even when CaFe2O4 is used in the gas detection section, there is still room for improvement in order to improve the detection performance of the gas to be detected to a level that meets various requirements in actual application situations.

[0052] As a result of extensive research, the inventors have discovered that the detection sensitivity of the gas to be detected can be further improved based on the following considerations, and have thus achieved the semiconductor oxide according to this embodiment.

[0053] In a gas sensor that detects a test gas based on a change in the resistance value of a gas detection element made of a semiconductor oxide, the test gas is detected based on a change in the conductivity of the semiconductor that occurs due to a reaction between the test gas and oxygen that has been negatively charged and adsorbed on the surface of the gas detection element.

[0054] FIG. 7 is a schematic diagram illustrating the mechanism of CO2 gas detection on the surface of a semiconductor serving as a gas detector.

[0055] When the gas to be detected is carbon dioxide (CO2), as shown in Figure 7, the CO2 gas and the negatively charged oxygen (O - In other words, the reaction shown in the above reaction formula (1) is promoted, making it possible to detect the test gas.

[0056] When the test gas is oxygen (O2), reaction formulas (2) and (3) have been reported as adsorption reactions accompanied by electrically detectable changes in conductivity (N. Yamazoe, K. Suematsu, K. Shimanoe, Sensors and Actuators B, 163, 128-135 (2012)).

[0057] O2+2e - ⇔ 2O - (ad) (2) O2+4e - ⇔ 2O 2- (ad) (3)

[0058] Figure 8 shows the relationship between oxygen and O - FIG. 9 is a schematic diagram illustrating the detection mechanism (corresponding to reaction formula (2)) when O2 gas is adsorbed as a negative charge on the semiconductor surface. 2- FIG. 1 is a schematic diagram illustrating the detection mechanism (corresponding to reaction formula (3)) when negatively charged oxygen (O - (ad),O 2- (ad)) depending on the context, simply O - Or O 2-The symbol (ad) may be omitted, as in the following.

[0059] O2 gas can be detected by the negative charge adsorption reactions occurring on the semiconductor surface according to the reaction formulas (2) and (3) shown in FIGS. 8 and 9.

[0060] Furthermore, when the gas to be detected is CO2, not only the above-mentioned reaction formula (1) but also the following reaction formula (4) may exist.

[0061] O 2- (ad)+CO2→ CO3 2- (ad) (4)

[0062] The adsorption of CO2 onto the surface of a semiconductor oxide, as shown in reaction (4), does not involve a change in the conductivity of the semiconductor, and therefore cannot be detected electrically. 2- (ad) is O - (ad) is not stable, so oxygen is O 2- When adsorbed in this state, the reaction on the oxide surface is O - It is thought to exist in this state.

[0063] O 2- (ad) → O - (ad)+e - ···(5)

[0064] The surface state of the gas sensor 10 using calcium ferrite for the gas detection part 14 is such that the reactions according to the above reaction formulas (1) and (4) can proceed immediately after the gas detection part 14 is formed by thick film printing or the like. After a certain period of time has passed, oxygen is absorbed onto the calcium ferrite surface, and the reaction proceeds as O - It is thought that the state transitions to a state that exhibits the reaction of reaction equation (1).

[0065] Usually, performance tests are conducted on products before shipping on production lines. Therefore, in product manufacturing, it is required to quickly obtain a state (desired surface state) that is ready for shipping without waiting for the performance test for a certain period of time until the surface state where the reaction of reaction formula (1) can proceed is reached. To achieve this, it is necessary to first determine whether oxygen is adsorbed onto the negative charge of O - (ad) and O 2- If reactions (2) and (3) can be promoted to reach the state of (ad), the test gas will be O - This makes it easier to detect the change in conductivity when the gas is adsorbed to the site, thereby improving the detection performance of the gas to be detected.

[0066] Therefore, the inventors attempted to introduce an oxide layer into CaFe2O4 used as the gas detection unit 14, which would easily adsorb oxygen (O2) as a negative charge and cause reactions (2) and (3) accompanied by a change in conductivity.

[0067] Figure 10 shows the structure of CaFe2O4 with an oxide layer. - The negative charge adsorption state (hereinafter referred to as O - ,O 2- means negatively charged adsorbed species).

[0068] The present inventors have added Zr to CaFeO as an essential component, as a metal element capable of promoting the reactions (1) and (4) in CO gas. - As a metal element capable of forming an oxide layer that promotes the adsorption of O, at least one metal element selected from aluminum (Al), silver (Ag), and tin (Sn) is present in the semiconductor oxide. - It was thought that this could form an oxide layer that promotes the adsorption of

[0069] As shown in Figure 10, the oxide layer of aluminum (Al), silver (Ag), tin (Sn), etc. introduced into CaFe2O4 containing Zr as an essential component is -It is thought that this forms new adsorption sites that promote the adsorption of the oxide. As shown in Figure 10, the oxide layer does not need to completely cover the surface of the semiconductor oxide particles, but only needs to be scattered on the particle surface.

[0070] In addition, in the semiconductor oxide applied to the gas sensor 10 according to this embodiment, the electrical resistance (R N2 ) and a predetermined oxygen concentration (P O2 The electrical resistance (R O2 ) and R O2 and P O2 The slope of the approximate line based on the log-log plot of the adsorption state of oxygen (O - Or O 2- ) can be used as an indicator.

[0071] Adsorption state of oxygen (O - Or O 2- The index representing the negative charge adsorption amount of oxygen will be explained later.

[0072] [Calcium ferrite manufacturing method] Next, a method for producing a semiconductor oxide according to this embodiment, which contains calcium ferrite and zirconium as essential elements and at least one metal element selected from aluminum, silver, and tin, will be described.

[0073] The method for producing a semiconductor oxide according to this embodiment includes the steps of preparing a mixed solution of multiple metal ions as starting materials, adding an organic acid to the resulting mixed solution to prepare a precursor solution containing a metal-organic acid complex, evaporating the precursor solution to dryness to obtain a precursor, subjecting the precursor to a first calcination treatment, and, after the first calcination treatment, subjecting the precursor to a second calcination treatment.

[0074] In this embodiment, the first calcination treatment is a treatment in which the mixture is calcined at a temperature of 300°C to 600°C for 10 to 120 minutes, and the second calcination treatment is a treatment in which the mixture is calcined at a temperature of 600°C to 1400°C for 1 to 24 hours. In the second calcination treatment, it is preferable to set the treatment time toward the upper limit of the effective temperature range at the lower end of the temperature range, and it is preferable to set the treatment time toward the lower limit of the effective temperature range at the upper end of the temperature range. For example, at 600°C, the treatment time is preferably 20 hours or more.

[0075] In the production of CaFe2O4, any starting material that can dissolve Ca and Fe as metal ions can be used, such as nitrates such as calcium (II) nitrate and iron (III) nitrate. Carbonates can also be used. Metal oxides are also acceptable.

[0076] For Zr, metal salts such as chlorides, nitrates, acetates, etc., or alkoxides can be used. From the viewpoint of high solubility in alcohols that can be used as solvents, it is preferable to use alkoxides.

[0077] For Al, Ag, and Sn, metal salts such as chlorides, nitrates, and acetates can be used. For Al and Ag, metal nitrates are preferably used from the viewpoint of high solubility in alcohol, water, and the like that can be used as solvents. On the other hand, for Sn, which cannot form a chemically stable nitrate, metal chlorides or metal acetates are preferably used.

[0078] The solvent for the mixed solution may be deionized water or an organic solvent such as methanol, ethanol, acetylacetone, or ethylene glycol. In this embodiment, it is preferable to use ethanol in order to dissolve the metal salt or metal oxide well.

[0079] To the mixed solution, an organic acid, such as malic acid, tartaric acid, citric acid, or malonic acid, is added in an amount equal to the total number of moles of the metal ions.

[0080] The addition of an organic acid can form stable chelate complexes with metal ions such as Ca, Fe, Al, Ag, and Sn. Furthermore, the addition of an organic acid to the mixed solution enables low-temperature firing, allowing the preparation of fine particles of semiconductor oxide. This allows the production of semiconductor oxide powder with a high specific surface area.

[0081] It is preferable to use malic acid from the viewpoint of making the CaFe2O4 particles obtained in the subsequent steps flaky and coarse.

[0082] In this embodiment, the amount of Zr added is preferably 10 mol % or less, and more preferably 3 mol % or more and 7 mol % or less, in terms of the molar fraction relative to Fe contained in CaFe2O4.

[0083] In this embodiment, the amount of each of Al, Ag, and Sn added is preferably 10 mol % or less, more preferably 1 mol %, of the metal element added in terms of molar fraction relative to the total amount of CaFe2O4.

[0084] FIG. 11 is a diagram illustrating a method for producing a semiconductor oxide according to this embodiment.

[0085] In this embodiment, as an example, as shown in FIG. 11 , starting materials including a nitrate of Ca 1-a, a nitrate of Fe 1-b, and an alkoxide of Zr 1-c, and further including a metal salt or a metal chloride 1-d selected as necessary from a metal nitrate of Al, a metal nitrate of Ag, and a metal chloride of Sn, are dissolved in ethanol in a stoichiometric ratio to form a mixed solution 2.

[0086] Malic acid 3 is added as an organic acid to the mixed solution 2 to prepare a metal-malic acid complex solution 4. In this embodiment, the metal-malic acid complex solution 4 is maintained at a temperature of 80°C to 120°C for 1 hour to remove water or ethanol. Next, the solution is maintained at a temperature of 180°C to 220°C for 3 hours, which promotes the thermal decomposition of the nitrate.

[0087] Subsequently, the metal-malic acid complex solution 4 is evaporated to dryness to produce a precursor solid 5. The precursor solid 5 is kept at a temperature of 300° C. to 500° C. for 30 minutes in the atmosphere to remove any remaining organic matter.

[0088] The obtained precursor solid 5 is subjected to a first calcination treatment. This first calcination treatment is a treatment for removing residual organic substances such as carbonic acid, nitric acid, malic acid, and ethanol in the reaction system. The temperature of the first calcination treatment can be set to a temperature higher than the thermal decomposition temperature of the organic acid. From this point of view, in this embodiment, the temperature of the first calcination treatment is set to 300°C or higher and 600°C or lower, and the time is set to 10 minutes or higher and 120 minutes or lower.

[0089] Following the first calcination treatment, a second calcination treatment is carried out to obtain a high-purity semiconductor oxide, and the calcination treatment is carried out at a temperature of 600°C to 1400°C for 1 hour to 24 hours.

[0090] After the calcination process is completed, the obtained precursor solid 5 is pulverized into powder to obtain powder 6 of semiconductor oxide.

[0091] By the above operations, a semiconductor oxide powder 6 containing CaFe2O4 and Zr as essential elements and at least one metal element selected from Al, Ag, and Sn is obtained.

[0092] [Other embodiments] Although the embodiments of the present invention have been described above, the above embodiments merely illustrate some of the application examples of the present invention, and are not intended to limit the technical scope of the present invention to the specific configurations of the above embodiments.

[0093] For example, the shape and dimensions of the substrate 11 of the gas sensor 10 are not limited to those shown in Fig. 1. Furthermore, the shapes of the comb teeth 12a and 13a and the shapes of the electrode portions connected to the comb teeth are not limited to those shown in Fig. 1. [Example]

[0094] A semiconductor oxide according to an embodiment of the present invention was fabricated, and a gas sensor for evaluation test was fabricated using the obtained semiconductor oxide, and the gas detection performance of the semiconductor oxide was measured. A method for fabricating a test specimen and a method for evaluating the same will be described below.

[0095] [Preparation of specimen] <Preparation of semiconductor oxide> Specimen 1: Preparation of CaFe2O4 CaFe2O4 was produced according to the process shown in Figure 13. Calcium nitrate (II) and iron nitrate (III) were used as starting materials. These nitrates were dissolved in deionized water, and an amount of malic acid equal to the total number of moles of metal ions was added to prepare a metal-organic acid complex solution. This solution was evaporated to dryness to obtain a precursor powder. The obtained precursor powder was subjected to a first calcination treatment in air at 400°C for 2 hours. This was followed by a second calcination treatment at 700°C for 12 hours. This yielded CaFe2O4 for specimen 1.

[0096] Specimen 2: Preparation of Zr-added CaFe2O4 The starting materials used were 99% pure calcium nitrate(II) tetrahydrate (Ca(NO3)2·4H2O), 99% pure iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O), and Zr butoxide (zirconium(IV) butoxide (85% 1-butanol solution)).

[0097] For these, the starting materials were prepared so that the molar ratio of calcium nitrate, iron nitrate, Zr alkoxide, and organic acid (malic acid) was 1:1.9:0.1:3, and these were mixed to obtain a metal-organic acid complex solution. Subsequently, this solution was evaporated to dryness to obtain a precursor solid. The obtained precursor powder was subjected to a first calcination treatment in air at 400°C for 2 hours, followed by a second calcination treatment at 700°C for 12 hours. As a result, 5 mol% Zr-CaFe2O4 (CaFe 1.9 Zr 0.1 O4) was obtained.

[0098] Specimen 3: Preparation of Zr and Al co-doped CaFe2O4 Zirconium (Zr) and aluminum (Al) were used as the different metals to be added. As with specimen 2, Zr alkoxide was prepared so that the amount of Zr added was 5 mol% relative to the Fe component of CaFe2O4, and aluminum nitrate (III) was prepared so that the amount of Al added was 1 mol% relative to CaFe2O4, and mixed with calcium nitrate (II) and iron nitrate (III). The conditions for the first and second calcination treatments were the same as those for specimen 1. As a result, a 1 mol% Al, 5 mol% Zr-CaFe2O4 (chemical composition: 0.5 mol% Al2O3-CaFe 1.9 Zr 0.1 O4) was obtained.

[0099] Specimen 4: Preparation of Zr and Ag co-doped CaFe2O4 Zirconium (Zr) and silver (Ag) were used as the different metals to be added. Zr alkoxide was prepared so that the amount of Zr added was 5 mol% relative to the Fe component of CaFe2O4, and silver nitrate (I) was prepared so that the amount of Ag added was 1 mol% relative to CaFe2O4, and mixed with calcium nitrate (II) and iron nitrate (III). The conditions for the first and second calcination treatments were the same as those for specimen 1. As a result, a 1 mol% Ag, 5 mol% Zr-CaFe2O4 (chemical composition: 0.5 mol% Ag2O-CaFe 1.9 Zr 0.1 O4) was obtained.

[0100] Specimen 5: Preparation of Zr and Sn co-doped CaFe2O4 Zirconium (Zr) and tin (Sn) were used as the different metals to be added. Zr alkoxide was prepared so that the amount of Zr added was 5 mol% relative to the Fe component of CaFe2O4, and tin (II) chloride was prepared so that the amount of Sn added was 1 mol% relative to CaFe2O4, and mixed with calcium nitrate (II) and iron nitrate (III). The conditions for the first and second calcination treatments were the same as those for specimen 1. As a result, a 1 mol% Sn, 5 mol% Zr-CaFe2O4 (chemical composition: 1 mol% SnO2-CaFe 1.9 Zr 0.1 O4) was obtained.

[0101] <Fabrication of gas sensors> An alumina substrate was used as the base material, gold (Au) was used as the electrode material, and the gas detection unit 14 was fabricated using each of the above specimens 1-5, thereby fabricating the thick-film gas sensor shown in FIG.

[0102] The spacing S between the teeth 12a of the first electrode 12 and the teeth 13a of the second electrode 13 was set to 80 μm, and the electrode line width L0 between the teeth 12a of the first electrode 12 and the teeth 13a of the second electrode 13 was set to 200 μm. The number of teeth on both the first electrode 12 and the second electrode 13 was 17. That is, the total number of teeth was 34, and the number of spaces between the teeth 12a and the teeth 13a was 33.

[0103] After coating, the substrate was dried at 150°C for 10 minutes, and then heat-treated at 700°C for 2 hours to obtain a gas sensor as a test specimen.

[0104] <X-ray diffraction measurement of semiconductor oxide powder> Powder X-ray diffraction (XRD) measurements were performed to confirm the crystal structure of the obtained semiconductor oxide powders of specimens 1 to 5. The XRD patterns confirmed that each of the obtained specimens 1 to 5 was CaFe2O4 containing Zr as the main component, and each specimen was a semiconductor oxide powder containing any of Al, Ag, and Sn as a heteroelement.

[0105] [Gas sensor evaluation] (Evaluation circuit) 12 is a schematic diagram illustrating an evaluation test apparatus 20 for evaluating a gas sensor. The evaluation test apparatus 20 includes a test tube 21 that seals the gas sensor 30, platinum lead wires 22 and 23 connected to the gas sensor 30, a quartz tube 24 that covers the lead wires 22 and 23 connected to the gas sensor 30, and a heater 25 for heating the test tube 21. The lead wires 22 and 23 are connected to the first electrode 12 and the second electrode 13 (see FIG. 1), respectively.

[0106] Test gases are supplied to the test tube 21 from test gas cylinders 26a, 26b, 26c, and 26d via mass flow meters 27a, 27b, 27c, and 27d for adjusting the gas flow rates.

[0107] The detection results from the gas sensor 30 are input to the PC via an electrometer. The PC analyzes the detection results from the gas sensor 30, manages the temperature of the heater 25, and controls the gas flow rates in the mass flow meters 27a to 27d.

[0108] The gas sensor 30 was manufactured in accordance with the gas sensor 10 shown in FIG.

[0109] FIG. 13 is a circuit diagram illustrating a circuit configuration applied to the evaluation test device 20 for evaluating the detection performance of the gas sensor 30 (gas sensor 10).

[0110] The gas sensors fabricated using each of specimens 1-5 were connected in series with an external resistor to form a circuit. The change in resistance was measured when a predetermined voltage was applied to various test gases and temperature conditions, including oxygen (O2), nitrogen (N2), and CO2 diluted with synthetic dry air (gas concentration: 5000 ppm).

[0111] (CO2 detection sensitivity) In the test using CO2 as the test gas, the gas sensors made using each of specimens 1-5 were subjected to a test in which 0.10 dm2 of CO2 with a concentration of 5000 ppm was added to synthetic dry air at the measurement temperature. 3 The change in resistance value of the gas sensor was measured by increasing the CO2 concentration by flowing the gas at a flow rate of 1 / min. The measurement temperature was set to 250°C to 500°C.

[0112] The electrical resistance (R air ) and the electrical resistance (R CO2 ) and the resistance change rate ΔR CO2 In this embodiment, the resistance change rate ΔR CO2 (%) means gas detection sensitivity.

[0113] Resistance change rate ΔR CO2 (%)=(R CO2 -R air ) / R air

[0114] The relationship between the measurement temperature (horizontal axis) and the rate of resistance change (vertical axis) is shown in FIG.

[0115] (Evaluation of oxygen adsorption state on semiconductor oxide surfaces) It has been reported that the state of oxygen adsorption on the surface of a semiconductor oxide can be evaluated based on the relationship between the resistance value of the semiconductor oxide and the oxygen partial pressure in the detection atmosphere (N. Yamazoe, K. Suematsu, K. Shimanoe, Sensors and Actuators B, 163, 128-135 (2012)).

[0116] According to this report, the resistance value of the semiconductor oxide is P O2 1 / 2 When the electrochemical reaction (7) is proceeding, O - is in a negatively charged state.

[0117] R ∝ P O2 1 / 2 +C1(C n : constant) (6) O2+2e - → 2O - (ad) (7)

[0118] Furthermore, as shown in the following formula (8), the resistance value of the semiconductor oxide is P O2 1 / 4 When the electrochemical reaction (9) is proceeding, O 2- is in a negatively charged state.

[0119] R∝(P O2 1 / 2 +C2) 1 / 2 +C3=P O2 1 / 4 +C4···(8) O2+4e - → 2O 2- (ad) ···(9)

[0120] As described above, the state of oxygen adsorption on the surface of the semiconductor oxide can be predicted from the relationship between the oxygen partial pressure and the resistance value.

[0121] Therefore, the state of oxygen adsorption on the surface of semiconductor oxide was evaluated based on the above findings using oxygen and nitrogen as test gases.

[0122] N2 gas, O2 gas, and a mixture of N2 and O2 were injected into the test gas sensor at a concentration of 0.10 dm 3 The electrical resistance (R N2 ) and a predetermined oxygen concentration (P O2 The electrical resistance (R O2 The measurement temperature was set to 300°C to 450°C.

[0123] The result obtained is R N2 About R N2 and PO2 The slope of the approximate line based on the plot was calculated. The slope obtained in this way indicates the adsorption state of oxygen (O - Or O 2- ) can be used as an indicator.

[0124] The relationship between the O2 gas concentration in the detection atmosphere and the change in resistance value for the gas sensors using each test piece is shown in Figs. 15 to 19.

[0125] FIG. 20 shows the relationship between the CO2 gas detection sensitivity of each test gas sensor and the slope of the approximation line derived from the O2 concentration dependency of the resistance value.

[0126] On the other hand, the rate of negative charge adsorption of O2 gas was evaluated using O2 gas or N2 gas, and the ratio of the resistance value of the gas sensor in N2 gas to the resistance value of the gas sensor in O2 gas was used.

[0127] The test gas sensor was filled with 0.10 dm 3 The change in the resistance value of the gas sensor was measured while O2 gas was flowing at a flow rate of 0.10 dm 3 The electrical resistance (R N2 ) and the electrical resistance (R O2 The measurement temperature was set to 300°C to 450°C. N2 and R O2 Therefore, the resistance ratio of the gas sensor (R O2 / R N2 ) was taken as the amount of negatively charged O2 adsorption.

[0128] The relationship between the slope of the approximation line derived from the O2 concentration dependence of the resistance value of each gas sensor specimen and the measurement temperature is shown in Figure 21. In addition, the electrical resistance (R N2 ) and the electrical resistance (R O2 ) and resistance ratio (R O2 / R N2 ) and the measurement temperature are shown in Figure 22. O2 / R N2 The relationship between the resistance ratio (R O2 / R N2 ) is shown in FIG. 24.

[0129] [Evaluation results] (CO2 detection sensitivity results) FIG. 14 is a diagram showing the relationship between the detection sensitivity of CO2 gas and the measurement temperature.

[0130] Figure 14 shows the temperature dependence of CO2 detection sensitivity in air for the gas sensors using specimens 1-5. When CO2 gas with a concentration ranging from 0 ppm to 5000 ppm was passed through synthetic dry air at a specified flow rate, the resistance of the gas sensor decreased compared to the value in synthetic dry air.

[0131] This is thought to be due to the increase in hole concentration on the CaFe2O4 surface and the decrease in electrical resistance due to reaction formula (1).It was found that all test specimens tended to show the highest detection sensitivity for CO2 gas at a measurement temperature of around 350°C.

[0132] Furthermore, at a measurement temperature of around 350°C, a significant improvement in CO2 detection sensitivity was confirmed by co-doping Zr with Al, Ag, or Sn (specimen 3-5). There is a measurement temperature range in which the semiconductor oxides co-doped with Zr with Al, Ag, or Sn (specimen 3-5) exhibit better detection sensitivity than calcium ferrite alone (specimen 1) or calcium ferrite with only Zr added (specimen 2).

[0133] Therefore, by selecting Al, Ag, or Sn as the metal element to be added to calcium ferrite together with Zr, the gas sensor can be adjusted to have good detection sensitivity depending on the set measurement temperature range.

[0134] In particular, when Al is added to calcium ferrite together with Zr, it exhibits good detection sensitivity over a wide temperature range (300-400°C) compared to the other two metal elements. This eliminates the need to strictly control the heating environment to obtain good detection sensitivity in the gas sensor, and allows the gas sensor to operate with good detection sensitivity using a simple device.

[0135] Furthermore, when Ag is added to calcium ferrite together with Zr, it shows good detection sensitivity from a low temperature range around 250°C compared to the other two metal elements. Therefore, it can contribute to power saving compared to gas sensors that show good detection sensitivity at 300-400°C. In addition, the applicable temperature range is lowered, so it can be used in a wider range of environments.

[0136] In this way, by appropriately selecting the metal element to be added together with Zr, it is possible to provide a gas sensor having desirable detection sensitivity according to the environment in which it is used.

[0137] (Evaluation of oxygen adsorption state on semiconductor oxide surfaces) Figure 15 shows the O2 concentration (P O2 ) and the resistance value.

[0138] In the gas sensor of specimen 1, the O2 concentration (P O2 ) increased, the resistance value decreased. This indicates that the electrical conductivity increased. According to the slope of the approximation line based on the double logarithmic plot shown in Figure 15, the semiconductor oxide of specimen 1 exhibited a P O2 1 / 4 This means that the oxygen adsorbed on the surface of the semiconductor oxide of specimen 1 is proportional to O 2- This suggests that the ions are adsorbed in this state.

[0139] Fig. 16 shows the O2 concentration (P O2 ) and the resistance value.

[0140] In the gas sensor of specimen 2, the O2 concentration (P O2 ) increased, the resistance value decreased. This indicates that the electrical conductivity increased. From the slope of the approximation line based on the double logarithmic plot shown in Figure 16, it can be seen that the semiconducting oxide of specimen 2 exhibited a high P O2 1 / 4 and P O2 1 / 2 It is considered that there is a proportional relationship for a given number between

[0141] At 350°C, P O2 1 / 2 This indicates that in the range of 350 to 450°C, the oxygen adsorbed on the surface of the semiconductor oxide of specimen 2 is proportional to O 2- and O - This suggests that both conditions exist.

[0142] From this, it can be seen that the semiconductor oxide of specimen 2 has a higher O content than the semiconductor oxide of specimen 1 due to the addition of Zr. - Therefore, by adding Zr to CaFe2O4, not only does it increase the specific surface area, but it also increases the O2 content, which becomes the adsorption reaction site for CO2 gas. - It can accelerate the creation of the site.

[0143] Fig. 17 shows the O2 concentration (P O2 ) and the resistance value.

[0144] In the gas sensor of specimen 3, the O2 concentration (P O2 ) increased, the resistance decreased. This indicates that the electrical conductivity increased. From the slope of the approximation line based on the double logarithmic plot shown in Figure 17, it can be seen that the semiconducting oxide of specimen 3 exhibited a high P O2 1 / 4 and P O2 1 / 2 It is considered that there is a proportional relationship for a given number between

[0145] In addition, at around 300°C, P O2 1 / 2 This shows that the semiconductor oxide of specimen 3 is mainly proportional to O in the range of 350 to 450°C. 2- At around 300°C, the adsorption is mainly in the form of O - It was suggested that the adsorption was in the state

[0146] Therefore, according to the semiconductor oxide of specimen 3, by adding Al together with Zr to CaFe2O4, O, which becomes an adsorption reaction site for CO2 gas, is obtained at 300 to 400°C, preferably 300 to 350°C. - It can accelerate the creation of the site.

[0147] Fig. 18 shows the O2 concentration (P O2 ) and the resistance value.

[0148] In the gas sensor of specimen 4, the O2 concentration (P O2 ) increased, the resistance value decreased. This indicates that the electrical conductivity increased. From the slope of the approximation line based on the double logarithmic plot shown in FIG. 18, it can be seen that the semiconductor oxide of specimen 4 exhibited a high P O2 1 / 4 and P O2 1 / 2 It is considered that there is a proportional relationship for a given number between

[0149] In addition, at around 300°C, P O2 1 / 2 This shows that the semiconducting oxide of specimen 4 is mainly proportional to O in the range of 350 to 450°C. 2- At around 300°C, the adsorption is mainly in the form of O - It was suggested that the adsorption occurred in this state.

[0150] Therefore, according to the semiconductor oxide of specimen 4, by adding Al together with Zr to CaFe2O4, O, which becomes an adsorption reaction site for CO2 gas at 300℃ to 350℃, - It can accelerate the creation of the site.

[0151] FIG. 19 shows the O2 concentration (P O2 ) and the resistance value.

[0152] In the gas sensor of specimen 5, the O2 concentration (P O2 ) increased, the resistance value decreased. This indicates that the electrical conductivity increased. From the slope of the approximation line based on the double logarithmic plot shown in FIG. 19, it can be seen that the semiconductor oxide of specimen 5 exhibited a O2 1 / 2 tends to be proportional to

[0153] From this, it can be seen that the semiconductor oxide of specimen 5 is mostly oxygen at 400°C. - It was suggested that the adsorption was in the state

[0154] Therefore, according to the semiconductor oxide of specimen 5, by adding Sn together with Zr to CaFe2O4, O, which becomes an adsorption reaction site for CO2 gas, is formed at 350℃ to 400℃. - It can accelerate the creation of the site.

[0155] Next, the adsorption state of oxygen (O - Or O 2- 20 is a graph showing the relationship between the CO2 gas detection sensitivity of each gas sensor under test and the gradient of the approximation line derived from the O2 concentration dependency of the resistance value.

[0156] Figure 20 shows the region where the slope is -0.5, i.e., where oxygen is O - This shows that the detection sensitivity of CO2 gas is maximum in the region where CO2 gas is adsorbed. - is stable against O, but2- O - is not stable against O 2- Even if adsorbed in this state, on the surface of the semiconductor oxide, O - A reaction occurs in which the state changes to

[0157] According to Figure 20, if the slope can be made closer to -0.5, most of the oxygen adsorbed on the semiconductor oxide can be converted to O - Therefore, the gas sensor 10 according to this embodiment can be configured to have an adsorbed state of oxygen (O - Or O 2- If the slope, which is an index representing the degree of sensitivity, is in the range of -0.75 to -0.25, good detection performance can be demonstrated.

[0158] FIG. 21 is a diagram showing the relationship between the slope of the approximation line derived from the O2 concentration dependency of the resistance value in each gas sensor test piece and the measurement temperature.

[0159] According to FIG. 21, when the measurement temperature is 400°C to 450°C, the value of the slope, which is an index representing the state of adsorbed oxygen, is -0.25 in most of the specimens except for the semiconductor oxide of specimen 1. That is, oxygen is mainly O 2- This suggests that the adsorption occurs in this state.

[0160] At 350°C to 400°C, the slope is -0.5 for the semiconductor oxides of specimens 2 and 5. That is, most of the oxygen is O - This suggests that the adsorption occurs in this state.

[0161] At 300°C to 350°C, the slope is -0.5 for the semiconductor oxides of specimens 3 and 4. That is, oxygen is mainly O - This suggests that the adsorption occurs in this state.

[0162] According to the above, the resistance value R of the gas sensor in nitrogen gas at 300°C to 400°C N2 and oxygen gas concentration P O2If the semiconductor oxide has a slope of an approximated line based on a double logarithmic plot of the above in the range of −0.3 to −0.7, a gas sensor that exhibits high detection performance can be obtained by operating it in a measurement temperature range corresponding to this slope.

[0163] Therefore, by appropriately selecting the metal element to be added to CaFe2O4 in combination with Zr from Al, Ag, and Sn, the adsorption state of oxygen (O - Or O 2- ) can be adjusted to around -0.5 (for example, -0.3 to -0.7). - It is possible to prepare a semiconductor oxide capable of adsorbing in this state according to the required measurement temperature, thereby improving the detection sensitivity of a gas sensor constructed using the semiconductor oxide.

[0164] Next, we will explain the index that represents the amount of negative charge adsorption of oxygen. Figure 22 shows the electrical resistance (R N2 ) and the electrical resistance (R O2 ) and resistance ratio (R O2 / R N2 ) and the measurement temperature.

[0165] The inventors have used the resistance value R of the semiconductor oxide in nitrogen gas as an index representing the amount of negative charge adsorption of O2 in the semiconductor oxide used in the gas sensor according to this embodiment. N2 and the resistance value R in oxygen gas O2 Resistance ratio (R O2 / R N2 In this embodiment, the resistance ratio (R O2 / R N2 ) can be used as an index representing the thickness of the space charge layer (hereinafter sometimes referred to as the depletion layer) formed in a semiconductor oxide due to the adsorption of oxygen as a negative charge.

[0166] According to FIG. 22, in the semiconductor oxide of specimen 2-5 in which Zr is co-doped with either Al, Ag or Sn, the resistance ratio (R O2 / R N2 ) is the resistivity ratio (R O2 / R N2 ), which suggests that the amount of negative charge adsorption of oxygen increases in CaFe2O4 co-doped with Zr and either Al, Ag, or Sn.

[0167] Furthermore, by appropriately selecting the metal element to be added to CaFe2O4 and combined with Zr from Al, Ag, and Sn, it is possible to prepare a semiconductor oxide with good detection sensitivity at the required measurement temperature.

[0168] Figure 23 shows the electrical resistance (R N2 ) and the electrical resistance (R O2 ) and resistance ratio (R O2 / R N2 ) and the detection sensitivity. Also, FIG. 24 shows the relationship between the electrical resistance (R N2 ) and the electrical resistance (R O2 ) and resistance ratio (R O2 / R N2 ) and the change in the thickness of the depletion layer.

[0169] Figure 23 suggests that the semiconducting oxide of specimen 2-5, in which Zr is co-added with either Al, Ag, or Sn, can obtain CO detection sensitivity comparable to that of the semiconducting oxide of CaFe2O4 with Zr addition (specimen 2).

[0170] For example, in specimen 2, the resistance ratio (R O2 / R N2) is in the range of 0.1 to 0.2, the change in the resistance value of the semiconductor oxide due to the depletion layer is suppressed, and it is thought that the change in the resistance value due to the adsorption of the test gas such as CO2 gas can be suppressed to a level that does not affect it.

[0171] In sensors that detect gases to be detected based on changes in the resistance of a gas detection section made of a semiconductor oxide, changes in the thickness of the depletion layer due to CO2 gas adsorption are detected as changes in electrical resistance. Therefore, if the amount of negative charge adsorption is large and the depletion layer is too thick, the O2 gas, which becomes an adsorption reaction site for CO2 gas, is detected as a change in electrical resistance. - Even if the adsorbed oxygen in this state reacts with CO2, it is expected that the change in resistance value will be small.

[0172] According to the results in Figure 23, 0.1 ≤ R O2 / R N2 ≦0.2, it is considered that the change in the thickness of the depletion layer relative to the change in CO2 gas concentration becomes large. Therefore, the electrical resistance (R N2 ) and the electrical resistance (R O2 ) and resistance ratio (R O2 / R N2 ) is 0.1≦R O2 / R N2 By preparing a semiconductor oxide with a value of ≦0.2, the thickness of the depletion layer is prevented from becoming too large, making it easier to capture changes in resistance due to the adsorption of test gases such as CO2 gas onto the surface of the semiconductor oxide.

[0173] The semiconductor oxide (specimen 2) made of CaFe2O4 with added Zr forms a good three-dimensional porous structure. This is thought to be due to the increased specific surface area caused by the increased porosity and the suppression of changes in the depletion layer thickness due to CO2 gas adsorption, resulting in good detection sensitivity. If a metal element other than Zr were added to the semiconductor oxide of specimen 2, it would be expected that the detection sensitivity would decrease because it would be difficult to form or maintain a good three-dimensional porous structure. However, Figure 23 shows that the semiconductor oxides of specimens 3 to 5 exhibit similar changes in the depletion layer as specimen 2 with only Zr added, maintaining good detection sensitivity. This is because when Al, Ag, or Sn are selected as metal elements to be co-doped with Zr, it is likely that the porosity of the semiconductor oxide can be maintained well without significantly disrupting the three-dimensional porous structure. [Explanation of symbols]

[0174] 10 Gas Sensor 11 Base material 12 1st electrode 12a comb teeth 13 Second electrode 13a comb teeth 14 Gas detection unit 20 Evaluation test equipment 21 test tube 22,23 Lead wire 24 Quartz tube 25 Heater 26a, 26b, 26c, 26d Test gas cylinders 27a, 27b, 27c, 27d Mass flow meter 30 Gas Sensor

Claims

1. A substrate; a first electrode and a second electrode disposed on the substrate; a gas detection unit connected to the first electrode and the second electrode, The gas detection unit A semiconductor oxide essentially containing calcium ferrite and zirconium and containing at least one metal element selected from the group consisting of aluminum, silver, and tin. Gas sensor.

2. 2. The gas sensor according to claim 1, the content of the zirconium is 10 mol % or less in terms of molar fraction relative to iron contained in the semiconductor oxide; Gas sensor.

3. 3. The gas sensor according to claim 1, the content of the zirconium is 3 mol % or more and 7 mol % or less in terms of a molar fraction relative to the iron contained in the semiconductor oxide; Gas sensor.

4. 4. The gas sensor according to claim 1, the content of at least one of the metal elements selected from aluminum, silver, and tin relative to the total amount of the semiconductor oxide is 1 mol %; Gas sensor.

5. 5. The gas sensor according to claim 1, the slope of a log-log plot of the resistance of the semiconductor oxide versus oxygen concentration is −0.7 or more and −0.3 or less; Gas sensor.

6. 5. The gas sensor according to claim 1, a ratio of the resistance value of the semiconductor oxide in nitrogen gas to the resistance value in oxygen gas is 0.1 or more and 0.2 or less; Gas sensor.

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