Patent Application
20200393431
This Application claims priority from Japanese Patent Application No. 2019-111312 filed on Jul. 14, 2019, which is incorporated herein by reference in their entirety.
The present invention relates to a carbon dioxide gas sensor, to a method for producing a gas sensor, and to a method for producing a rare earth oxide which can be used for a sensing layer of a gas sensor.
Detection of carbon dioxide gas (hereinafter, also referred to as CO2 gas) is attracting attention not only in the field of environmental safety, such as management of buildings and parking places, but also in the fields of agriculture and food-related industries. In a current standard technology, detection of CO2 gas is conducted using a Non Dispersive Infrared (NDIR) CO2 gas sensor. However, NDIR is expensive and bulky, and thus, there are problems that it is difficult to install. Therefore, a high performance chemoresistive CO2 gas sensor achieving low cost and having a simple structure has been desired.
As a promising chemoresistive material used for a CO2 gas sensor, rare earth metal oxycarbonate (rare earth oxycarbonate) has been proposed (for example, see Non-Patent Literatures 1 to 4). Although there are families of rare earth metal oxycarbonates having different rare earth metals and different crystal polymorphism, it is reported that a monoclinic lanthanum dioxycarbonate (La2O2CO3) is the most suitable material for a CO2 gas sensor (Non-Patent Literature 2).
In addition, semiconductor materials consisting of tin oxide (SnO2) particles coated with lanthanum oxide (La2O3) or gadolinium oxide (Gd2O3) are known to be able to serve as a CO2 gas sensor (for example, see Patent Literature 1).
A chemoresistive gas sensing layer material of higher performance, a method of production thereof, and a gas sensor therewith are desired for the purpose of practical application in a thin film gas sensor.
The inventors investigated chemoresistivity of rare earth metal compounds, and as a result, the inventors discovered oxides suitable for a gas sensing layer of a CO2 gas sensor and a method of production thereof, and finally completed the present invention.
According to one embodiment, the present invention relates to a carbon dioxide gas sensor comprising an insulating substrate and a gas sensing layer formed on one major surface of the insulating substrate via electrodes, wherein the gas sensing layer comprises one or more compounds selected from rare earth oxides represented by Ln2O3, Ln being at least one rare earth metal element selected from Sc, Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Pr, Yb and Lu.
In the above gas sensor, the rare earth oxide represented by Ln2O3 is preferably at least one of Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Er2O3 or Yb2O3.
In the above gas sensor, the rare earth oxide preferably comprises a rare earth oxide having a cubic crystal structure as a main component.
According to another embodiment, the present invention relates to a method for producing a carbon dioxide gas sensor comprising a step of forming an insulating substrate and a gas sensing layer formed on one major surface of the insulating substrate via electrodes, wherein the gas sensing layer comprises at least one compound selected from rare earth oxides represented by Ln2O3, Ln being at least one rare earth metal element selected from Sc, Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Pr, Yb and Lu.
According to yet another embodiment, the present invention relates to a method for producing a rare earth oxide represented by Ln2O3, Ln being a rare earth metal element selected from Sc, Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Pr, Yb and Lu, comprising a step of heating the rare earth metal carboxylate or the rare earth metal carbonate, or the hydrate thereof at 425 to 575° C. for 2 to 80 hours.
According to yet another embodiment, the present invention relates to a cubic crystal structure of the rare earth oxide represented by Ln2O3 produced by the above method of the rare earth oxide represented by Ln2O3.
The above crystal structure is preferably for use in a sensing layer of a carbon dioxide gas sensor.
According to the present invention, a compact, high-performance, chemoresistive CO2 gas sensor comprising a gas sensing layer containing a rare earth oxide can be obtained. Also, according to the present invention, a rare earth oxide having chemoresistivity which can be used for a gas sensing layer can be produced.
Hereinafter, the embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the embodiments described below.
According to the first embodiment, the present invention relates to a CO2 gas sensor.
The insulating substrate 3 may be any substrate as long as it can ensure electrical insulation between the heating layer 4 and the electrodes 2. For example, a silicon substrate with the oxide film and an alumina substrate can be used, but the insulating substrate 3 is not limited thereto. The heating layer 4 is provided on one major surface of the insulating substrate 3. The heating layer 4 may be any layer as long as it can heat gas sensing layer 1 to a predetermined operating temperature through the insulating substrate 3. Pt film etc. can be used as the heating layer, but the heating layer 4 is not limited thereto. In the illustrated aspect, a gas sensor provided with a heating layer is exemplified; however, a heating layer may not be an essential constituent of the gas sensor of the present invention. A heating layer or an alternative heating device will be described below.
The electrodes 2 are provided on the major surface of the insulating substrate 3 opposite to the heating layer 4. The electrodes 2 are preferably a platinum (Pt) film or a gold (Au) film, and usually, comb teeth-shaped electrodes can be used.
The gas sensing layer 1 is provided on a major surface of the insulating substrate 3 so as to cover the electrodes 2. The gas sensing layer 1 comprises a chemoresistive material, and may optionally comprises an inorganic binder, aggregate, and conductive material etc. In the present invention, the chemoresistive material is a rare earth oxide. The rare earth oxide is preferably one or more compounds selected from rare earth oxides represented by Ln2O3. In the chemical formula, Ln is selected form Sc (scandium), Y (yttrium), Nd (neodymium), Sm (samarium), Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Pr (praseodymium), Yb (ytterbium) and Lu (lutetium). The rare earth oxide may be a composite metal oxide, which may comprise two or more metals selected from the above in any proportion. Among these, Sm2O3, Eu2O3, Gd2O3, Dy2O3, Er2O3 and Yb2O3 are particularly preferable in terms of sensor signal and stability.
In the present invention, a main component of the rare earth oxide as a chemoresistive material is preferably rare earth oxide having a cubic crystal structure. “A main component of the rare earth oxide is rare earth oxide having a cubic crystal structure” means that at least 80%, preferably 90% of the rare earth oxide has cubic crystal structure, preferably the rare earth oxide substantially consists of rare earth oxide having a cubic crystal structure, and further preferably 100% of the rare earth oxide is rare earth oxide having a cubic crystal structure. The content (%) of the rare earth oxide having a cubic crystal structure in the rare earth oxide can be calculated by measuring the ratio of peaks using an X-ray diffractometer.
In the present invention, gas sensing layer 1 may not necessarily comprise a semiconductor such as SnO2 as a chemoresistive material, and preferably, the rare earth oxide can be used alone as a chemoresistive material. When the SnO2 is not comprised, an advantage of being able to enhance selectivity against interfering components such as H2, CO and ethanol can be obtained.
Examples of an optional component of the gas sensing layer 1 include a binder and an aggregate for maintaining mechanical strength of the gas sensing layer 1. As a binder and an aggregate, those which are usually used can be used within the range not inhibiting chemoresistivity of the rare earth oxide, and for example, inorganic binders such as alumina sol can be exemplified but they are not limited to a specific material. Examples of other optional components include a conductive material for adjusting the resistivity of the gas sensing layer 1. These optional components may be included in an amount of 20 mass % or less, preferably 15 mass % or less, relative to the total mass of the gas sensing layer 1.
The heating layer 4 of the gas sensor is electrically connected to a driving processor, which is not shown, and the driving processor drives the heating layer 4. The gas sensing layer 1 is electrically connected to a driving processor, which is also not shown, via the electrodes 2 of the gas sensor, and the driving processor can read an electrical resistance value (referred to a sensor resistance value) of gas sensing layer 1. In the present embodiment, as a heating device for heating the gas sensing layer to a predetermined temperature, the heating layer provided on the side of the insulating substrate opposite to the gas sensing layer is illustrated. However, in the present invention, the shape of the heating device is not limited to a heating layer, and the arrangement of the heating device is also not limited to the aspect shown in the figure. In one embodiment, the heating device may be provided on the same surface of the insulating substrate as the gas sensing layer with the heating device being separated from the gas sensing layer. In another embodiment, the heating device may be provided on the major surface of the insulating substrate opposite to the surface on which the gas sensing layer is provided, and the heating device may be provided so as to be partially or completely embedded. In another embodiment, the heating device may be provided according to an aspect in which the heating device does not come into contact with the stack of the insulating substrate and the gas sensing layer, and for example, the heating device may be provided in a housing which contains the insulating substrate and the gas sensing layer. In any case, the heating device may be a heating layer or a heater which is not in the form of a layer, and may include one or more heating device, as long as the heating device can heat the gas sensing layer to a predetermined temperature.
Next, the gas sensor according to the present embodiment will be described with reference to a method of production. The method of production of the gas sensor according to the present embodiment comprises a step of forming the gas sensing layer 1 comprising the rare earth oxide illustrated above.
In producing the gas sensor, the heating layer 4 is formed on one major surface of the insulating substrate 3, and the electrodes 2 are formed on the other major surface. The heating layer 4 and the electrodes 2 on the insulating substrate 3 can be formed by a commonly used method. The heating layer 4 and the electrodes 2 can be respectively connected to a driving processor, which is not shown, by a commonly used method. As for a sensor provided with a heating device other than a heating layer, the heating device can be attached to a suitable place by a commonly used method and connected to a driving power source etc.
Forming the gas sensing layer 1 comprises a step of preparing solid powder of one or more rare earth oxides selected from rare earth oxides represented by Ln2O3 (Ln is same as defined above) which are main components of the gas sensing layer 1, and a step of mixing one or more rare earth oxides and a solvent and, if necessary, an optional component such as a binder to form a film on the insulating substrate 3 on which the electrodes 2 are formed.
The rare earth oxide prepared before film formation may comprise a rare earth oxide having a cubic crystal structure as a main component, and optionally, may comprise a rare earth oxide having a hexagonal crystal structure, and preferably may comprise 1000% of rare earth oxide having a cubic crystal structure.
In the step of mixing one or more rare earth oxides and a solvent, solvents which have high boiling point and lower volatility such as propane-1,2-diol, ethyl carbitol, diethylene glycol monoethyl ether, and ethylene glycol can be used. The rare earth oxide and the solvent are mixed thoroughly to obtain a paste, then a film is formed by a screen printing method, drop coating method, spray coating method etc. at a desired thickness on the insulating substrate 3 on which the electrodes 2 are formed. Then, the obtained film is dried at 60 to 80° C. for 10 to 15 hours. After drying, the film is preferably heat-treated for 10 to 15 minutes under the heat treatment conditions identical to those for producing the rare earth oxide. Thus, the gas sensor can be obtained in which the heating layer 4 can be driven and electrical resistance values of the gas sensing layer can be read by electrifying the sensor.
As another aspect of the gas sensor according to the present embodiment, a diaphragm-type thin film gas sensor can be mentioned.
The Si substrate 16 is formed of silicon (Si), and through holes are formed on the Si substrate at the locations directly over which the gas sensing layer 11 is positioned. The thermally insulating support layer 15 covers the openings of the through holes to form a diaphragm, and is provided on the Si substrate 16. Specifically, the thermally insulating support layer 15 has a three-layer structure comprising thermally oxidized SiO2 layer 15a, CVD-Si3N4 layer 15b and CVD-SiO2 layer 15c. The thermally oxidized SiO2 layer 15a is formed as a heat insulation layer, and has a function of reducing heat capacity by preventing heat generated in the heating layer 14 from being conducted to the side of the Si substrate 16. Furthermore, this thermally oxidized SiO2 layer 15a has high resistance to plasma etching, which facilitates formation of through holes on the Si substrate 16 by plasma etching. The CVD-Si3N layer 15b is formed on upper side of the thermally oxidized SiO2 layer 15a. The CVD-SiO2 layer 15c enhances adhesion to the heating layer 14, and in addition, ensures electrical insulation. SiO2 layer formed by CVD (chemical vapor deposition method) has a low internal stress.
The heating layer 14 may be a Pt—W film in the form of thin film, and is provided on the upper side of approximately the center of the thermally insulating support layer 15. Furthermore, the heating layer 14 is connected to a driving processor, which is not shown, and is configured to be subjected to power feeding. The insulating substrate 13 may be a sputtered SiO2 layer for ensuring electrical insulation, and is provided so as to cover the thermally insulating support layer 15 and the heating layer 14. The insulating substrate 13 can ensure electrical insulation between the heating layer 14 and the electrodes 12a. Furthermore, the insulating substrate 13 can enhance adhesion to the gas sensing layer 11.
The bonding layer 12b is, for example. Ta film (tantalum film) or Ti film (titanium film), and a left-and-right pair of the bonding layers 12b is provided on the insulating substrate 13. These bonding layers 12b are interposed between the electrodes 12a and the insulating substrate 13 to enhance bonding strength. The electrodes 12a are, for example, Pt film (platinum film) or Au film (gold film), and a left-and-right pair of the electrodes 12a is provided so as to serve as sensing electrodes of the gas sensing layer 11. The gas sensing layer 11 is formed astride a pair of the electrodes 12a on the insulating substrate 13 across. In particular, the composition of the gas sensing layer 11 is the same as described in the embodiment with reference to
Similarly to the sensor according to the first aspect, the heating layer 14 of the gas sensor is electrically connected to a driving processor, which is not shown in the figures, and the driving processor drives the heating layer 14. Furthermore, the gas sensing layer 11 is electrically connected to a driving processor, which is also not shown, via the electrodes 12a of the gas sensor so that the driving processor can read electrical resistance values of the gas sensing layer 11.
A diaphragm-type gas sensor can be also obtained by forming a sensing layer using a specific rare earth oxide by the method described above to produce the gas sensor having the structure shown in
Such gas sensors having a diaphragm structure may provide high thermal insulation and low heat capacity. Furthermore, in the gas sensor, heat capacity of each constituent of electrodes 12a, gas sensing layer 11 and heating layer 14 can be reduced by techniques such as MEMS (micro-electrical-mechanical system). Therefore, temperature change over time is greater during driving of the heater, and thus, thermodesorption can be achieved in an extremely short time.
In the present embodiment, the gas sensor is described by showing specific examples of structures of the sensors in
According to the second embodiment, the present invention relates to a method of producing a rare earth oxide. The method of producing a rare earth oxide (Ln is the same as defined above) represented by Ln2O3 comprises a step of heating the rare earth metal carboxylate or the rare earth metal carbonate, or the hydrate thereof at 425 to 575° C. for 2 to 80 hours in a gas atmosphere.
In the production method of the present embodiment, the rare earth metal carboxylate or the rare earth metal carbonate, or the hydrate thereof can be used as a starting material. As a rare earth metal constituting a rare earth metal carboxylate, those corresponding to Ln in the target rare earth oxide represented by Ln2O2 can be used, and the rare earth metal can be selected from Ln defined above. Specific examples of rare earth metal carboxylates include, but are not limited to, oxalates represented by Ln2[C2O4]3 or oxalate hydrates represented by Ln2[C2O4]3.nH2O, carbonates represented by Ln2[CO3]3 or hydrate thereof, acetates represented by Ln[CH3COO]3 or hydrates thereof.
In a step of heating, a rare earth metal carboxylate or a rare earth metal carbonate, or a hydrate thereof which is in the form of solid powder at room temperature can be preferably placed in a heat resistant open-type alumina container and the like, and heated in a heating furnace. The heating temperature is preferably 425 to 575° C., and this is preferably maintained at a constant temperature during heating. The heating time may be 2 to 80 hours. The atmosphere during heating is not particularly limited, but it may be air, a closed system, or an atmosphere to which gas such as air can be continuously supplied. As one example of an atmosphere to which gas such as air can be continuously supplied, the atmosphere may be used in which gas comprising 350 to 500 ppm of carbon dioxide and moisture of 20 to 80% relative humidity at 20° C. can be supplied. However, supplying a gas including carbon dioxide and moisture is not essential.
Among rare earth oxides, the heating conditions in producing Nd2O3 is preferably at 525 to 575° C. for about 2 to 80 hours, or at 475 to 525° C. for about 50 to 80 hours. The heating conditions in producing Sm2O3 is preferably at 525 to 575° C. for about 2 to 80 hours, or at 475 to 525° C. for about 15 to 80 hours, or at 425 to 475° C. for about 60 to 80 hours. For all rare earth oxides, producing under the conditions of heating at high temperature for a long time is preferable in terms of heat stability, for example, heating at 525 to 575° C. for 50 to 80 hours is preferable.
According to the production method of the rare earth oxide in accordance with the second embodiment, a rare earth oxide comprising rare earth oxide having a cubic crystal structure as a main component can be produced. Such a rare earth oxide can be used for a chemoresistive material for a gas sensor, especially for a constituent of a gas sensing layer. The gas sensor comprising the rare earth oxide produced according to the present embodiment as a constituent of the gas sensing layer is also useful as a detection sensor of carbon dioxide gas. In particular, carbon dioxide gas can be selectively detected while being distinguished from various gases such as hydrogen gas, carbon monoxide gas ethanol.
Hereinafter, the present invention will be described in more detail with reference to the Examples of the present invention. However, the present invention is not limited to the scope of the following Examples.
(1) Producing and Evaluation of Rare Earth Oxide Commercially available solid powder of Ln2[C2O4]3.nH2O (Ln is Ce, Gd, Er, all produced by Sigma-Aldrich Co. LLC) or Ln(C2H3O2)3.nH2O (Ln is Nd. Sm, Eu, Dy, Yb, all produced by Sigma-Aldrich Co. LLC) was used as a starting material. The powder of the starting material was placed in an alumina container and heated using a heating furnace. Air was continuously supplied to the heating furnace by a pump during heating. After the specified heating treatment, the obtained product was subjected to crystal structure analysis using an X-ray diffractometer. The starting materials, heat treatment conditions and XRD results of the products after heat treatment are shown in Table 1.
In table 1, “ox” means an oxalate, “ac” means an acetate. An“in” means that a monoclinic oxycarbonate was produced. A“−” means that a heating experiment was not conducted in the corresponding conditions.
The crystal structures of the rare earth oxides produced in the respective conditions shown in Table 1 were investigated by X-ray crystallographic diffraction, and thus, the results shown in Table 2 below were obtained for crystal structures of the rare earth oxides which had been obtained by heat treatment at 550° C. for 72 hours. In Table 2, “cubic” means a cubic crystal structure, “hexagonal” means a hexagonal crystal structure, and “cubic+hexagonal” means a state in which a cubic crystal structure and a hexagonal crystal structure were mixed. The crystal structure of the rare earth oxides used as a component of a sensing layer for producing the gas sensor in Example (2) described below was investigated by X-ray crystallographic diffraction after evaluation of the gas sensor properties of Example (3). As a result, no changes in crystal structures were observed for any of the oxides.
(2) Production of Gas Sensor
Among the oxides produced in above (1), eight rare earth oxides obtained by heat treatment at 550° C. for 72 hours were used to produce the gas sensors. Specifically, the gas sensor shown in
The Pt heater was connected to a DC power source, which is not shown, and thus the sensor was enabled to be heated to a temperature of 250° C., 300° C. or 350° C. The gas sensing layer 1 was connected to an electrical resistance measurement apparatus which is not shown via the electrodes 2 to provide a configuration which enabled measurement of DC resistance of the gas sensing layer at 10 second intervals.
(3) Evaluation Results
(3-1) CO2 Sensor Signal, Sensitivity and Durability
CO2 sensor signal and sensitivity of the eight rare earth oxides shown in Table 1 were compared.
(3-2) Selectivity and CO2 Sensitivity Over Range Up to High Concentration
(3-3) Comparison with Commercially Available Product with Respect to CO2 Sensor Signal
Next, the CO2 sensor signal of Gd2O3 and Dy2O3 each having a cubic crystal structure produced in Example (1) of the present invention and obtained by heat treatment at 50° C. and 72 hours were compared to the CO2 sensor signal of commercially available Gd2O3 and Dy2O3 (Sigma-Aldrich Co. LLC, product number 637335, 637289, specification of particle size <100 nm). Measurements of CO2 gas sensor signal (Rg/R0) were conducted at CO2 concentration of 1000 ppm, 20° C., 50% RH and a sensor operating temperature of 300° C. similarly to the above (3-1). The comparison results of CO2 sensor signal is shown in
The gas sensor according to the present invention is useful as a MEMS solid-state gas sensor with low power consumption, which is intended to be battery-driven.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019-111312 | Jun 2019 | JP | national |
