Apparatus and method for measuring concentrations of gas components

Abstract: An oxide semiconductor element (8) is arranged in a measuring space (17) in which an atmosphere is introduced. The resistance of the oxide semiconductor element varies corresponding to a concentration of a specific gas component. An oxygen pump, having a solid-electrolyte element (11) with an oxygen ion transmitting property and a pair of electrodes (12,13) arranged on both sides of the solid-electrolyte element is provided. One electrode (12) of the oxygen pump is arranged in the measuring space. The measuring space communicates with the atmosphere through a hole (3) as a gas diffusion means. The resistance of the oxide semiconductor element is measured when the oxide semiconductor element is exposed in the atmosphere so as to obtain a concentration of a specific gas component, while the oxygen partial pressure in the measuring room is controlled by the oxygen pump.

Description: Background of the Invention (1) Field of the Invention The present invention relates to an apparatus and a method for measuring concentrations of gas components, which can measure concentrations of specific gas components even in a gas in which an oxygen partial pressure is varied. (2) Related Art Statement There has been known phenomena such that, if an oxide semiconductor is disposed in an atmosphere or a gas including various specific gas components, a resistance of the oxide semiconductor is varied. Also, there has been known a method for measuring concentrations of specific gas components by utilizing the resistance variation mentioned above. In the method mentioned above, gas components such as CO, CO 2 , NO, NO 2 , H 2 , H 2 O, SO 2 , HCl, H 2 S and so on can be measured. For example, in Japanese Patent Laid-open Publication No. 63-313047 (JP-A-63-313047), SnO 2 is used as the oxide semiconductor, and a concentration of H 2 S is measured. In Japanese Patent Laid-open Publication No. 1-150849 (JP-A-1-150849), TiO 2 is used as the oxide semiconductor, and a concentration of NO is measured. In Japanese Patent Publication No. 45-17038 (JP-B-45-17038), WO 3 is used as the oxide semiconductor, and a concentration of NH 3 is measured. In all the cases mentioned above, a concentration of the gas component is determined by detecting a resistance of the oxide semiconductor in response to a variation of the specific gas concentration, and comparing the resistance thus detected with a predetermined reference curve showing a relation between a resistance and a gas concentration of the oxide semiconductor. Generally, an oxygen concentration is largely varied in an exhaust gas discharged from an internal combustion engine or in an exhaust gas discharged from a dust incinerator. In these exhaust gases, we find that a resistance of the oxide semiconductor is largely varied in response to a variation of an oxygen partial pressure in the exhaust gas as well as the specific gas concentration. Moreover, the detected resistance includes the both influences of the specific gas concentration and the oxygen partial pressure. Therefore, it is not possible to detect a concentration of the specific gas component precisely. For example, as shown in Fig. 4 mentioned below, if the oxygen partial pressure is varied from 0.01 atm to 0.20 atom, resistivities therebetween are largely different from each other while concentrations of NO therebetween are identical with each other. Summary of the Invention An object of the invention is to provide an apparatus and a method for measuring concentrations of specific gas components, in which a precise measurement can be performed even if an oxygen partial pressure is varied. This object is attained with the apparatus of claim 1 and the method of claim 6. Preferred optional features of the apparatus are described in dependent claims 2-5. The features of the preamble to claim 1 and claim 6 respectively are known in combination from the prior art document, EP-A-0517364. In the constitution according to the invention, electrodes are arranged on both sides of a solid-electrolyte element made of zirconia so as to construct an oxygen pump, and one of the electrodes of the oxygen pump is arranged in a measuring room in which an oxide semiconductor is arranged. Therefore, it is possible to control an oxygen partial pressure in the measuring room. Moreover, if a concentration of the specific gas component is measured in an atmosphere in which concentrations of NO or the like and the oxygen partial pressure are largely varied, it is possible to measure the specific gas concentration such as NO or the like precisely with no timing delay i.e. on real time. Therefore, according to the invention, when concentrations of poisonous gases such as NO or the like are measured in an atmosphere such as an exhaust gas or a combustion gas in which the oxygen partial pressure is largely varied, it is possible to realize a precise concentration measurement by utilizing an apparatus for measuring gas concentrations using the oxide semiconductor. Accordingly, the range of the present invention in a field of a measuring apparatus is very large. Brief Description of the Drawings Fig. 1 is a cross sectional view showing one embodiment of an apparatus for measuring a gas concentration according to the invention; Fig. 2 is a cross sectional view illustrating another embodiment of the apparatus for measuring a gas concentration according to the invention; Fig. 3 is a cross sectional view depicting still another embodiment of the apparatus for measuring a gas concentration according to the invention; Fig. 4 is a graph showing one measured result of NO concentration by utilizing a measuring apparatus according to a comparative example in the case of using porous TiO 2 sintered body as an oxide semiconductor; Fig. 5 is a graph illustrating one measured result of NO concentration by utilizing a measuring apparatus according to the present invention in the case of using porous TiO 2 sintered body as the oxide semiconductor; Fig. 6 is a graph depicting another measured result of NO concentration by utilizing a measuring apparatus according to the present invention in the case of using porous TiO 2 sintered body as the oxide semiconductor; Fig. 7 is a graph showing one measured result of CO concentration by utilizing a measuring apparatus according to the comparative example in the case of using porous WO 3 sintered body as the oxide semiconductor; and Fig. 8 is a graph illustrating one measured result of CO concentration by utilizing a measuring apparatus according to the present invention in the case of using porous WO 3 sintered body as the oxide semiconductor. Description of the Preferred Embodiment As an oxide semiconductor element, use is made of an element made of an oxide such as TiO 2 , SnO 2 , ZnO 2 , NiO, Fe-O, WO 2 , In 2 O 3 , Nb 2 O 5 , V 2 O 5 , Mn-O, Cu-O, CoO or the like, or a solid solution or an oxide composite both including respective oxides mentioned above. In the oxide semiconductor element, a ratio between metal ions and oxygen ions and a crystal structure are varied corresponding to a temperature and an oxygen partial pressure during an element producing step or an environmental condition to be used. Moreover, the oxygen semiconductor element may be made of a porous body or a densified sintered body of an oxide semiconductor, or an oxide semiconductor film formed with or on a supporting body. Electrodes used for measuring a resistance are arranged on the oxide semiconductor element and the oxide semiconductor element with the electrodes is accommodated in a measuring room. An apparatus using the oxide semiconductor element can detect all the gas components except for oxygen, but it is preferably used for measuring a specific gas components such as CO, CO 2 , NO, NO 2 , NH 3 , H 2 , H 2 O, SO 2 , HCl or H 2 S. An oxygen pump is formed by a solid-electrolyte element made of zirconia having an oxygen ion conductivity on which a positive electrode and a negative electrode are arranged. In the oxygen pump, an oxygen component becomes an oxygen ion at the positive electrode, the oxygen ion is transmitted through the solid-electrolyte element, and the oxygen ion becomes the oxygen component at the negative electrode and is discharged. Therefore, the oxygen component is moved from a positive electrode side to a negative electrode side. A shape of the solid-electrolyte element is not limited, but a thin plate is preferred. As an electrode, it is preferred to use a porous electrode. The porous electrode can be formed by printing a paste on the element and fired it. Moreover, in order to increase a temperature of the oxide semiconductor to a temperature preferable for measuring a gas concentration, or in order to increase a temperature of the oxygen pump to a temperature at which the oxygen pump is active, it is preferred to embed a heater of a resistance heating type or the like in a wall surface of a container consisting of the measuring room. According to the invention, a voltage applied between a pair of electrodes of the oxygen pump can be adjusted constant. In this case, a ratio of the oxygen partial pressure between one electrode side of the oxygen pump can be adjusted constant. Moreover, the other electrode of a pair of electrodes of the oxygen pump is arranged in an atmosphere out of the measuring room, and a voltage applied to the solid-electrolyte element is adjusted in such a manner that the positive electrode is positioned in the measuring room and the negative electrode is positioned out of the measuring room. In this case, an oxygen partial pressure in the measuring room can be controlled in a small level as compared with an oxygen partial pressure out of the measuring room. Therefore, if an oxygen partial pressure out of the measuring room is largely varied, a variation of an oxygen partial pressure in the measuring room can be made small relatively. Further, the other electrode of a pair of electrodes of the oxygen pump is arranged in an atmosphere out of the measuring room, and a voltage applied to the solid-electrolyte element is adjusted in such a manner that the negative electrode is positioned in the measuring room and the positive electrode is positioned out of the measuring room. In this case, an oxygen partial pressure in the measuring room can be controlled in a large level as compared with an oxygen partial pressure out of the measuring room. Then, if the applied voltage is controlled in a sufficiently large level, it is possible to control an oxygen partial pressure in the measuring room always above a predetermined threshold level C. The construction mentioned above can be adopted if a resistance of the oxygen semiconductor element is not so varied in an oxygen partial pressure range above the threshold level C. Moreover, an oxygen concentration reference room in which an oxygen concentration is maintained in a predetermined range is arranged, and the other electrode of a pair of electrodes of the oxygen pump is arranged in the oxygen concentration reference room. In this case, since an oxygen concentration in the oxygen concentration reference room is maintained at a constant level, it is possible to maintain an oxygen concentration in the measuring room at a constant level, if a voltage applied between a pair of electrodes of the oxygen pump is controlled at a constant level. Therefore, it is possible to control an oxygen concentration precisely in an easy manner. Moreover, an oxygen partial pressure in the measuring room can be controlled in a predetermined range by arranging an oxygen sensor for measuring an oxygen concentration in the measuring room and adding a signal obtained from the oxygen sensor to a voltage applied to the oxygen pump as a feedback room. For example, if a signal obtained from the oxygen sensor is above a predetermined upper limit, a voltage applied to the oxygen pump is controlled in such a manner that the positive electrode is positioned in the measuring room. In this case, the oxygen component in the measuring room is discharged, and thus an oxygen partial pressure in the measuring room can be decreased. Moreover, if a signal obtained from the oxygen sensor is under a predetermined lower limit, a voltage applied to the oxygen pump is controlled in such a manner that the negative electrode is positioned in the measuring room. In this case, the oxygen component is supplied into the measuring room, and thus an oxygen partial pressure in the measuring room can be increased. If the feedback control mentioned above is performed, it is possible to maintain an oxygen partial pressure in the measuring room at a predetermined range in an easy manner even when the oxygen concentration reference room is not arranged and the other electrode of a pair of electrodes of the oxygen pump is exposed in an atmosphere out of the measuring room under the condition such that an oxygen concentration in the atmosphere is largely varied. In the present invention, a gas diffusion resistant means is arranged to the measuring room. This gas diffusion resistant means functions to supply or discharge an atmospheric gas out of the measuring room into or out of the measuring room. The number of the gas diffusion resistant means is not limited to one, but a plurality of the gas diffusion resistant means can be used. Hereinafter, the present invention will be explained in detail with reference to the drawings. Fig. 1 shows an apparatus for measuring a gas concentration with an oxygen concentration reference room. In Fig. 1, a measuring room 17 is formed in a container 2, and a hole 3 is arranged in the container 2 as a gas diffusion resistant means. An atmospheric gas in an outer atmosphere 16 out of the measuring room 17 is supplied or discharged through the hole 3 as shown by arrows B. Respective electrodes 6 and 7 are arranged on both sides of an oxide semiconductor element 8 so as to form a measuring element 5 for measuring a concentration of a specific gas component. The measuring element 5 is arranged in the measuring room 17, and an opening of the container 2 is plugged by a fused member 10. The electrodes 6 and 7 are connected respectively to a controller 18 having a known construction. An oxygen concentration reference room 1 is arranged under the measuring room 17, and a reference gas is introduced into the oxygen concentration reference room 1 through a hole 14. A solid-electrolyte element 11 is arranged in a partition 21, and electrodes 12 and 13 are respectively arranged on both sides of the solid-electrolyte element 11 so as to form an oxygen pump 4. The electrode 12 is faced to the measuring room 17, and the electrode 13 is faced to the oxygen concentration reference room 1. The electrodes 12 and 13 are connected respectively to a controller 19. Moreover, a heater 9 is embedded in the partition 21. In an apparatus for measuring a concentration of a specific gas component shown in Fig. 2, the measuring room 17 is formed in a container 22, and the hole 3 is arranged in a wall 23 of the container 22. The solid-electrolyte element 11 is arranged in the wall 23, and the electrodes 12 and 13 are arranged respectively on both sides of the wall 23 so as to form an oxygen pump 15. The electrode 12 is faced to the measuring room 17 and the electrode 13 is faced to the outer atmosphere 16. Moreover, the heater 9 is embedded in the container 22. In an apparatus for measuring a concentration of a specific gas component shown in Fig. 3, the measuring room 17 is formed in the container 22, and the hole 3 is arranged in the wall 23 of the container 22. The solid-electrolyte element 11 is arranged in the wall 23, and the electrodes 12 and 13 are arranged respectively on both sides of the wall 23 so as to form the oxygen pump 15. The electrode 12 is faced to the measuring room 17 and the electrode 13 is faced to the outer atmosphere 16. Moreover, the heater 9 is embedded in the container 22. In the embodiment shown in Fig. 3, an oxygen sensor 20 is arranged in the measuring room 17, and is connected to the controller 19. Then, if a signal obtained from the oxygen sensor 20 is above a predetermined upper limit, a voltage applied to the oxygen pump 15 is controlled in such a manner that the electrode 12 in the measuring room 17 becomes a positive side. If a signal obtained from the oxygen sensor 20 is under a predetermined lower limit, a voltage applied to the oxygen pump 15 is controlled in such a manner that the electrode 12 in the measuring room 17 becomes a negative level. Hereinafter, actual experimental results will be explained. (Experiment 1) The apparatus for measuring a concentration of a specific gas component having a construction shown in Fig. 1 was prepared. As the container 2, use was made of an air-tight sintered body made of zirconia having the hole 3. Then, a slurry including TiO 2 particles having a particle size of about 0.5 µm was supplied in the container 2, and the slurry was dried and sintered so as to obtain the oxide semiconductor element 8 made of a porous TiO 2 sintered body. Then, the opening portion of the container 2 through which the slurry was introduced was plugged by the fused member 10. As the solid-electrolyte element 11, use was made of zirconia. As a comparative example, an apparatus for measuring a concentration of a specific gas component, having the same construction shown in Fig. 1 except that no oxygen pump 4 was arranged, was prepared. The other construction and the material to be used was the same as those of the apparatus for measuring a concentration of a specific gas according to the invention. At first, a measuring experiment was performed with respect to the apparatus according to the comparative example. A temperature at which the experiment was performed was 600°C, and a concentration of NO in the outer atmosphere was varied in a range of 0~500 ppm as shown in Fig. 4. Moreover, an oxygen partial pressure was also varied at 0.01 atm, 0.10 atm and 0.20 atm as shown in Fig. 4. A total pressure of the outer atmosphere was 1 atm. Measuring results of resistivities of the oxide semiconductor element were also shown in Fig. 4 respectively. As can be understood from Fig. 4, if the oxygen partial pressure is varied, the resistance of the oxide semiconductor element is varied extremely. Under such a condition, if the oxygen partial pressure is varied largely, it is impossible to measure the NO concentration. Moreover, even if a variation of the oxygen partial pressure is small, it is difficult to detect the resistance accurately. Then, a measuring experiment was performed with respect to the apparatus according to the invention. A temperature at which the experiment was performed was 600°C, and a concentration of NO in the outer atmosphere was varied in a range of 0~500 ppm as shown in Fig. 5. Moreover, an oxygen partial pressure was also varied at 0.01 atm, 0.10 atm and 0.20 atm as shown in Fig. 5. A total pressure of the outer atmosphere was 1 atm. Further, the oxygen partial pressure in the oxygen concentration reference room 1 was set at 0.20 atm. Moreover, a voltage of 56 mV was applied between the electrodes 12 and 13 in such a manner that a ratio of the oxygen partial pressure between the electrode 13 side and the electrode 12 side was 20:1, so that the electrode 12 was a positive level. Measuring results of resistivities of the oxide semiconductor element were also shown in Fig. 5 respectively. Then, a voltage of 186 mV was applied between the electrodes 12 and 13 in such a manner that a ratio of the oxygen partial pressure between the electrode 13 side and the electrode 12 side was 20000:1, so that the electrode 12 was a positive level. Measuring results of resistivities of the oxide semiconductor element were also shown in Fig. 6 respectively. As can be understood from the results mentioned above, according to the invention, it is possible to measure the NO concentration accurately even if the oxygen partial pressure in the outer atmosphere is largely varied. (Experiment 2) The apparatuses for measuring a concentration of a specific gas component according to the invention and the comparative example were prepared as is the same as the experiment 1, and a measuring experiment was performed with respect to the apparatus. The different point was that the oxide semiconductor element was made of a porous WO 3 sintered body. At first, a measuring experiment was performed with respect to the apparatus according to the comparative example. A temperature at which the experiment was performed was 400°C, and a concentration of CO in the outer atmosphere was varied in a range of 0~2000 ppm as shown in Fig. 7. Moreover, an oxygen partial pressure was also varied at 0.01 atm, 0.10 atm and 0.20 atm as shown in Fig. 7. A total pressure of the outer atmosphere was 1 atm. Measuring results of resistivities of the oxide semiconductor element were also shown in Fig. 7 respectively. As can be understood from Fig. 7, if the oxygen partial pressure is varied, the resistance of the oxide semiconductor element is varied extremely. Under such a condition, if the oxygen partial pressure is varied largely, it is impossible to measure the CO concentration. Moreover, even if a variation of the oxygen partial pressure is small, it is difficult to detect the resistance accurately. Then, a measuring experiment was performed with respect to the apparatus according to the invention. A temperature at which the experiment was performed was 400°C, and a concentration of CO in the outer atmosphere was varied in a range of 0~2000 ppm as shown in Fig. 8. Moreover, an oxygen partial pressure was also varied at 0.01 atm, 0.10 atm and 0.20 atm as shown in Fig. 8. A total pressure of the outer atmosphere was 1 atm. Further, the oxygen partial pressure in the oxygen concentration reference room 1 was set at 0.20 atm. Moreover, a voltage of 12 mV was applied between the electrodes 12 and 13 in such a manner that a ratio of the oxygen partial pressure between the electrode 13 side and the electrode 12 side was 2:1, so that the electrode 12 was a positive level. Measuring results of resistivities of the oxide semiconductor element were also shown in Fig. 8 respectively. As can be understood from the results mentioned above, according to the invention, it is possible to measure the NO concentration accurately even if the oxygen partial pressure in the outer atmosphere is largely varied. Therefore, according to the invention, it is possible to measure a concentration of a specific gas component even in the atmosphere in which an oxygen concentration is largely varied. Moreover, it is possible to improve a resistance measuring accuracy. As mentioned above, according to the invention, in the apparatus for measuring a concentration of a specific gas component, it is possible to detect the resistance accurately even if an oxygen partial pressure in an atmosphere or a gas flow to be measured. Therefore, in the atmosphere in which the oxygen partial pressure is largely varied such as an exhaust gas or a firing gas, the apparatus for measuring a gas concentration utilizing an oxide semiconductor element can be used for measuring a poisonous gas or a combustion gas in which the oxygen partial pressure is large varied. Moreover, it is possible to perform an accurate measuring operation. Accordingly, the use of the present invention in the field of measuring apparatus is very large.

Claims: An apparatus for measuring a concentration of a specific gas component in an atmosphere which also contains oxygen, comprising an open measuring space (17), a diffusion aperture (3) connecting the atmosphere with said measuring space (17), an oxide semiconductor element (8) which is arranged in said measuring space (17) and whose resistance varies with the concentration of the specific gas component, and an oxygen pump having a solid-electrolyte element (11) which conducts oxygen ions and a pair of electrodes (12,13) arranged on opposite sides of said solid-electrolyte element (11), one said electrode (12) being in said measuring space and communicating with the atmosphere in said measuring space, oxygen partial pressure in said measuring space being controlled by said oxygen pump, and the resistance of said oxide semiconductor element being measured when exposed in said atmosphere, so that the concentration of the specific gas component is obtained from the measured resistance, wherein both said electrode (12) in the measuring space (17) and said oxide semiconductor element (8) are located in a hollow chamber defining the measuring space and characterised in that said hollow chamber has an elongate shape as seen in cross-section with its elongation direction extending from said diffusion aperture (3) to said oxide semiconductor element (8), and said electrode (12) is located in said hollow chamber intermediately between said diffusion aperture (3) and said oxide semiconductor element (8). The apparatus according to claim 1, having a controller (19) for maintaining the voltage applied between said pair of electrodes at a constant level so as to control an oxygen partial pressure in said measuring space. The apparatus according to claim 1 or 2, having an oxygen concentration reference space, in which an oxygen concentration is maintained constant, the other electrode (13) of said pair of electrodes being arranged in said oxygen concentration reference space. The apparatus according to claim 1, 2 or 3, having an oxygen sensor for measuring the oxygen concentration in said measuring space wherein a signal obtained from said oxygen sensor is used as a feedback for control of the voltage applied to the oxygen pump, so as to maintain an oxygen partial pressure in said measuring space in a predetermined range. The apparatus according to one of claims 1 to 4, wherein a heater is arranged in a wall of said chamber. A method for measuring the concentration of a specific gas component in an atmosphere which also contains oxygen, comprising the steps of introducing a gas to be measured into an open measuring space (17) through a gas diffusion aperture (3), controlling an oxygen partial pressure in said measuring space to a predetermined level by means of an oxygen pump utilizing a solid-electrolyte element which conducts oxygen ions and has a pair of electrodes (12,13) on opposite sides with one electrode (12) in said measuring space (17), and measuring resistance of an oxide semiconductor element (8) which is arranged in said measuring space and whose resistance varies in dependence on the concentration of the specific gas component to be measured, and obtaining a measurement of the concentration of the specific gas component from the measured resistance, wherein both said electrode (12) in the measuring space (17) and said oxide semiconductor element (8) are located in a hollow chamber defining the measuring space and characterised in that said hollow chamber has an elongate shape as seen in cross-section with its elongation direction extending from said diffusion aperture (3) to said oxide semiconductor element (8), and said electrode (12) is located in said hollow chamber intermediately between said diffusion aperture (3) and said oxide semiconductor element (8).

Inventors: KATO NOBUHIDE, KATSUDA YUJI, MURASE TAKAO, TAKAHASHI TOMONORI, YOSHIDA TOSHIHITO, KATO, NOBUHIDE, KATSUDA, YUJI, MURASE, TAKAO, TAKAHASHI, TOMONORI, YOSHIDA, TOSHIHITO, Katsuda, Yuji, 207, NGK-Takeda-Minami-Shataku
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1995
Main classification: G01N27
IPC codes: G01N33, G01N27