gas sensor

Abstract

This suppresses deterioration of the gas detection unit due to red-hot heating and reduces external impact. [Solution] The gas sensor 1 includes a sensor element 30 that detects the concentration of a gas by a change in resistance. The sensor element 30 includes a substrate 31, a pair of electrodes 32 provided on one surface 31a of the substrate 31, a gas detection element 33 placed on the pair of electrodes 32, and a heater 35 provided on one surface 31a or the other surface 31b of the substrate 31, which controls the temperature of the gas detection element 33 to a predetermined operating temperature lower than the temperature of a hot spot generated by the self-heating of the sensor element 30.

Description

[Technical Field]

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

[0002] Patent Document 1 discloses an oxygen sensor that has power supplies at both ends of a wire and an ammeter for measuring the current flowing through the wire. This oxygen sensor detects the oxygen concentration by utilizing the hot spot phenomenon, in which a part of the wire becomes red-hot due to the self-heating of the wire, which is the gas detection part. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2007-85816 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] In gas sensors that detect the concentration of gases such as oxygen as described above, the temperature of the hot spot rises to around 1000°C due to the self-heating of the gas detection unit, raising concerns about deterioration of the gas detection element and external impacts due to the red-hot state of the gas detection unit.

[0005] Therefore, the present invention has been made in view of the above-mentioned problems, and aims to suppress deterioration of the gas detection unit due to red-hot heating and its impact on the outside. [Means for solving the problem]

[0006] According to an aspect of the present invention, a gas sensor includes a sensor element that detects the concentration of a gas based on a change in resistance value. The sensor element includes a substrate, a pair of electrodes provided on one surface of the substrate, a gas detection element placed on the pair of electrodes, and a heater provided on one surface or the other surface of the substrate that controls the temperature of the gas detection element to a predetermined operating temperature lower than the temperature of a hot spot generated by self-heating of the gas detection element.

Advantages of the Invention

[0007] According to an aspect of the present invention, the temperature of the gas detection element is adjusted by the heater to be lower than the temperature of a hot spot generated by self-heating of the gas detection element and to a predetermined operating temperature at which the concentration of the gas can be detected. Thereby, deterioration due to red heat of the gas detection element and influence on the outside can be suppressed.

Brief Description of the Drawings

[0008] [Figure 1] FIG. 1 is a perspective view showing the appearance of the gas sensor according to the present embodiment. [Figure 2] FIG. 2 is an exploded view illustrating the structure of the gas sensor. [Figure 3] FIG. 3 is a perspective view showing the structure of the sensor element constituting the gas sensor. [Figure 4] FIG. 4 is a top view showing the upper surface of the sensor element. [Figure 5] FIG. 5 is a side view showing the longitudinal side surface of the sensor element. [Figure 6] FIG. 6 is a bottom view showing the lower surface of the sensor element. [Figure 7] FIG. 7 is an explanatory view explaining the relationship between the upper surface and the lower surface of the sensor element. [Figure 8] FIG. 8 is a side view showing a modified example of the sensor element. [Figure 9] FIG. 9 is a top view showing a modified example of the electrode of the sensor element. [Figure 10] FIG. 10 is a perspective view showing a first modified example of the terminal of the gas detection element. [Figure 11] Figure 11 is a perspective view showing a second modified example of the terminals of the gas detection element. [Modes for carrying out the invention]

[0009] Embodiments of the present invention will be described below with reference to the drawings. Throughout this specification, the same or equivalent elements will be denoted by the same reference numerals.

[0010] Figure 1 is a perspective view showing the external appearance of the gas sensor 1 according to this embodiment.

[0011] The gas sensor 1 is a detector that detects the concentration of a gas by a change in resistance. The gas sensor 1 according to this embodiment is an oxygen gas sensor that detects the concentration of oxygen as the target of detection, and is used, for example, in an environment with a low oxygen concentration of less than 0.1%.

[0012] Figure 1 shows the components that make up the appearance of the gas sensor 1, including a cap 40 that protects the sensor element for detecting oxygen concentration, a filter 41 to which the oxygen to be detected is supplied from the outside, and a number of pins 11 that are connected to an external circuit such as a power supply.

[0013] In this embodiment, oxygen is used as an example of the target of detection for the gas sensor 1, but the configuration of this embodiment may also be applied to a gas sensor that detects the concentration of other gases such as carbon dioxide or flammable gases.

[0014] Next, the overall structure of gas sensor 1 will be briefly explained with reference to Figure 2.

[0015] Figure 2 is an exploded view illustrating the structure of the gas sensor 1.

[0016] The gas sensor 1 comprises a plurality of pins 11, two bases 12a and 12b, a base board 20, a sensor element 30, a cap 40, and a filter 41.

[0017] The pin 11 is a rod-shaped conductive metal. The five pins 11 consist of two pins 11a for detecting the resistance value of the sensor element 30, two pins 11b for supplying power for temperature control of the sensor element 30, and one pin 11c for identifying that the two pins 11b are temperature control pins. The two pins 11b can also be used for flushing, for example.

[0018] The two bases 12a and 12b are stands for mounting the base board 20 and are made of, for example, plastic. Base 12a is fixed with two pins 11a inserted through it, and base 12b is fixed with two pins 11b and one pin 11c inserted through it.

[0019] The base substrate 20 is a circuit board that holds the pins 11. The base substrate 20 is an insulator and is formed from, for example, a glass epoxy substrate or an insulating ceramic substrate. In this embodiment, a through hole 20a is formed in the center of the base substrate 20. In addition, wiring patterns 21a to 21d for electrically connecting the pins 11 and the sensor element 30 are provided on the surface of the base substrate 20.

[0020] The wiring patterns 21a and 21b are formed from a conductive metal such as copper or aluminum. Pinholes are formed at one end of each of the wiring patterns 21a and 21b, and one end of each pair of wiring patterns 21a and 21b is electrically connected to a pair of pins 11a via the pinholes.

[0021] A pinhole is formed at one end of each pair of wiring patterns 21c and 21d, and each end of the pair of wiring patterns 21c and 21d is electrically connected to the pins 11b at both ends via the pinhole.

[0022] The other ends of the wiring patterns 21a to 21d are connected to the sensor element 30 via wires 22a to 22d, respectively. The wires 22a to 22d are made of, for example, gold, copper, or the like.

[0023] The sensor element 30 is suspended by four wires 22a to 22d. The structure of the sensor element 30 will be described later in Figures 3 to 8.

[0024] The cap 40 is a cylindrical case that houses the sensor element 30 and is attached to the base substrate 20. The cap 40 is made of a metal such as aluminum. A vent hole 40a is formed in the center of the upper surface of the cap 40 to allow the oxygen to be detected to pass through. The vent hole 40a is covered by a filter 41 to prevent the entry of foreign matter other than the target to be detected.

[0025] Next, the structure of the sensor element 30 will be described in detail with reference to Figures 3 to 7. In the following, the surface of the sensor element 30 viewed from the filter 41 side (above) will be referred to as the top surface, and the surface of the sensor element 30 viewed from the opposite side (below) will be referred to as the bottom surface. Also, the surfaces of the sensor element 30 viewed from the left and right directions, which are perpendicular to the vertical direction, will be referred to as the side surfaces.

[0026] Figure 3 is a perspective view showing the structure of the sensor element 30. Figure 4 is a top view showing the top surface of the sensor element 30. Figure 5 is a side view showing the longitudinal side of the sensor element 30. Figure 6 is a bottom view showing the bottom surface of the sensor element 30. Figure 7 is an explanatory diagram illustrating the relationship between the top and bottom surfaces of the sensor element 30.

[0027] The sensor element 30 includes a substrate 31, a pair of electrodes 32, a gas detection element 33, a bonding material 34, and a heater 35. The bonding material 34 is shown with dashed lines or omitted to facilitate understanding of the structure of the sensor element 30.

[0028] The substrate 31 is a plate-shaped insulator with high heat resistance. The substrate 31 is formed from, for example, an insulating ceramic substrate. Examples of insulating ceramic substrates include alumina substrates.

[0029] The substrate 31 in this embodiment is formed in a rectangular shape. For example, the length of the substrate 31 in the longitudinal direction is approximately 1 mm to approximately 5 mm, and the length in the short direction is designed to be approximately 0.5 mm to approximately 2.5 mm. The shape of the substrate 31 may be circular, elliptical, square, or polygonal instead of rectangular.

[0030] As shown in Figures 3 to 5, a pair of electrodes 32 are provided on the upper surface 31a, which is one side of the substrate 31.

[0031] Each pair of electrodes 32 is an electrode to which a pair of wires 22a and 22b (see Figure 2) are connected by a bonding material 34 or solder (not shown). Each pair of wires 22a and 22b is connected to a pair of pins 11a mounted on a base substrate 20 via a pair of wiring patterns 21a and 21b. The bonding material 34 is a paste mainly composed of metals such as gold, silver, or platinum.

[0032] A predetermined voltage is applied between a pair of electrodes 32 through a pair of wires 22a and 22b, and in this state, the current flowing through the gas detection element 33 between the pair of electrodes 32 is detected. The gas detection method is not limited to current detection; detection by voltage is also possible.

[0033] The electrode 32 is a conductive film mainly composed of metal, and for example, gold, platinum, or silver can be used as the metal. As shown in Figure 4, the electrode 32 in this embodiment is formed in a rectangular shape. The shape of the electrode 32 may be circular, elliptical, or polygonal instead of rectangular.

[0034] A gas detection element 33 is mounted on a pair of electrodes 32. The gas detection element 33 is a hot spot type gas detection element that detects the concentration of a gas by utilizing the hot spot phenomenon, in which a part of it glows red due to self-heating.

[0035] In this embodiment, the gas detection element 33 changes its resistance value according to the oxygen concentration present around it. A voltage is applied to the gas detection element 33 through a pair of electrodes 32, and the current flowing through the gas detection element 33 in this state is detected.

[0036] As shown in Figure 3, the gas detection element 33 is, for example, a rectangular prism-shaped bulk body with a longitudinal length of approximately 0.5 mm to approximately 2 mm and a transverse length and height of approximately 0.2 mm to approximately 0.5 mm. The shape of the gas detection element 33 may be circular, elliptical, polygonal, or other columnar shape. A columnar gas detection element 33 has a certain thickness compared to a typical linear or thin-film gas detection element.

[0037] In this embodiment, as shown in Figure 5, the gas detection element 33 is manufactured such that its thickness W2 is greater than the thickness W1 of the substrate 31. This increases the volume of the gas detection element 33, thereby suppressing its degradation. It also allows for more efficient transfer of heat from the heater 35 to the gas detection element 33.

[0038] The gas detection element 33 of this embodiment comprises a gas detection unit 331 and a pair of terminals 332.

[0039] The gas detection unit 331 is formed from a material used in hot-spot type gas detection elements. For example, the gas detection unit 331 is formed from a semiconductor material whose resistance increases with oxygen deficiency.

[0040] In this embodiment, the gas detection unit 331 is a ceramic having a porous structure and is formed from a ceramic sintered body. The gas detection unit 331 is composed of, for example, a laminate formed by stacking a plurality of ceramic sheets.

[0041] Next, the composition constituting the gas detection unit 331 will be described.

[0042] For example, the composition of the gas detection unit 331 has the composition formula RE1+x Ba 2―x Cu3O δ (However, RE is a rare earth element, δ represents an oxygen non-stoichiometric amount, and the substitution amount x satisfies 0 ≦ x ≦ 2), a part of the composition represented by is replaced by at least one of any element selected from the elements of Group 2 of the periodic table and any element selected from the lanthanoid series elements. It is possible to use a specific composition.

[0043] The gas detection unit 331 of this embodiment is, for example, formed of a semiconductor material obtained by adding y volume% (however, 0 < y ≦ 7.5) of copper oxide (CuO) to the above specific composition.

[0044] In the above composition formula, RE is at least one element selected from rare earth elements (Sc (scandium), Y (yttrium), La (lanthanum), Nd (neodymium), Sm (samarium), Eu (europium), Gd (gadolinium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), Lu (lutetium)).

[0045] The rare earth element RE can be used alone with any one of the above elements, or a plurality of them can be mixed and used. In addition, since it becomes easy to control the composition of the semiconductor material and manage during production, it is preferable to use the same element for the two REs present in the composition formula.

[0046] In this embodiment, the composition formula: RE 1+x Ba 2―x Cu3O δ A part of the composition represented by is a composition in which a part of the composition is replaced by at least one of any element selected from the elements of Group 2 of the periodic table and any element selected from the lanthanoid series elements.

[0047] Here, the Group 2 element of the periodic table is any one element selected from beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).

[0048] In addition, the lanthanoid element is any one element selected from lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu).

[0049] Composition formula: RE 1+x Ba 2―x Cu3O δ In the composition represented by, the substitution amount x satisfies 0 ≤ x ≤ 2. The reason for this will be explained below.

[0050] Under the same temperature conditions, as the value of x approaches 0, the resistivity of the semiconductor material constituting the gas detection unit 331 decreases, and the difference in the resistivity of the semiconductor material according to temperature tends to increase.

[0051] Also, generally, the dependence of the electrical conductivity σ [S / m] of the oxygen gas detection element at a predetermined temperature on the oxygen partial pressure P O2 [atmospheric pressure] is known to be expressed as follows using the factor m.

[0052]

Equation

[0053] In Equation (1), the value of the factor m varies depending on the type of defects, impurity concentration, measurement temperature, etc. of the semiconductor oxide (semiconductor material). According to Equation (1), at a predetermined temperature, the smaller the absolute value of the factor m (>0), the better the responsiveness (sensitivity) of the oxygen gas sensor.

[0054] Therefore, when the temperature of the measurement environment changes, if the change amount of the absolute value of factor m is small, it can be said that the temperature dependence is small. From this, it can be said that a semiconductor-type oxygen gas detection element has good sensitivity and high stability when the absolute value of factor m in formula (1) is small and the change amount of the value of factor m due to temperature change is small.

[0055] Note that factor m can be calculated as follows using the oxygen partial pressure P1 in the ambient gas surrounding the semiconductor material, the resistance value R1 of the semiconductor material at the time of oxygen partial pressure P1, the oxygen partial pressure P2, and the resistance value R2 of the semiconductor material at the time of oxygen partial pressure P2.

[0056]

Equation

[0057] As described above, from the viewpoint of obtaining a semiconductor material with good sensitivity and high stability by reducing the resistivity of the semiconductor material and reducing the change amount of factor m due to temperature change, the value of x shall satisfy 0 ≦ x ≦ 2.

[0058] From the viewpoint of keeping the resistivity of the semiconductor material small and maintaining good detection sensitivity for oxygen gas, the value of x preferably satisfies 0 < x ≦ 1.2.

[0059] On the other hand, from the viewpoint of suppressing the change amount of factor m and securing a temperature range in which it can function with good sensitivity, in the composition represented by the composition formula: RE 1+x Ba 2―x Cu3O δ it is preferable to apply a composition in which the substitution amount x is x = 2, that is, the composition formula: RE 1+x Ba 2―x Cu3O δ is represented by the composition formula: RE3Cu3O δ

[0060] Composition formula: RE 1+x Ba 2―x Cu3O δIn the semiconductor material represented by [formula], the rare earth element RE is preferably lanthanum (La), neodymium (Nd), or samarium (Sm) which has a large solid solubility limit of barium (Ba) from the viewpoint of easily adjusting the value of x within the above range, that is, suppressing the resistivity of the semiconductor material to a low level and obtaining a predetermined temperature range with good detection sensitivity and high stability. Among them, it is particularly preferable to use neodymium (Nd).

[0061] The gas detection unit 331 may further contain a composition represented by the composition formula RE2BaCuO5 (where RE is a rare earth element). In the semiconductor material constituting the gas detection unit 331, the composition formula: RE 1+x Ba 2―x Cu3O δ By including both the composition represented by [formula] and the composition represented by the composition formula RE2BaCuO5, the resistivity of the semiconductor material can be lowered, the change amount of the factor m due to temperature change can be reduced to improve the sensitivity, and good detection sensitivity can be obtained in a wide temperature range.

[0062] In the present embodiment, copper oxide (CuO) can be added to the above specific composition constituting the gas detection unit 331 at y volume% (here, 0 < y ≤ 30) with respect to the total volume of the composition. The reason for this will be explained below.

[0063] Even if the addition amount of CuO to the above specific composition is a very small amount near 0 volume%, the effect of enhancing the sinterability of the semiconductor material can be obtained. However, if the addition amount of CuO to the above specific composition becomes excessive, it will also inhibit the contact opportunity between the basic structure contributing to the detection reaction of oxygen gas and oxygen gas, leading to a decrease in the detection sensitivity of oxygen gas in the gas detection unit 331.

[0064] Therefore, the composition formula: RE 1+x Ba 2―x Cu3O δFrom the perspective of enhancing the sinterability of the semiconductor material represented by , the addition amount y volume % of CuO to the above specific composition must be more than 0 volume %, and from the perspective of preventing a decrease in the detection sensitivity of oxygen gas in the gas detection unit 331, the addition amount y volume % of CuO must be 30 volume % or less. On the other hand, if the upper limit of CuO addition exceeds 30 volume %, CuO that exhibits conductivity at high temperatures forms a conductive path, and the detection sensitivity of oxygen gas decreases.

[0065] From the perspective of enhancing the sinterability of the semiconductor material and preventing a decrease in the detection sensitivity of oxygen gas, the addition amount y volume % of CuO to the composition constituting the gas detection unit 331 is preferably 0 < y ≦ 7.5. More preferably, 5.0 ≦ y ≦ 7.5.

[0066] Also, the composition formula: RE 1+x Ba 2―x Cu3O δ The composition represented by has a complex perovskite structure.

[0067] The complex perovskite structure has a cubic unit cell, with a metal element R located at each vertex of the cube, another metal element M located at the body center, and further, with oxygen O arranged at the face center of each cube face centered on the metal element M. The octahedron (MO6) composed of oxygen O and the metal element M can be easily distorted by the interaction with the metal element R. As a result, it has a structure that can undergo a phase transition to a more asymmetric orthorhombic (rhombic) or tetragonal crystal. Therefore, since the semiconductor material constituting the oxygen gas detection part in the oxygen gas detection element has a complex perovskite structure, when it comes into contact with oxygen gas, the crystal structure is likely to change and a resistivity change is likely to occur.

[0068] Among the above-described semiconductor materials, in particular, a semiconductor material containing both a composition represented by the composition formula: Nd 1.6 Ba 1.4 Cu3O δ and a composition represented by the composition formula: Nd2BaCuO5 and containing 5.0 volume % or more and 7.5 volume % or less of CuO is preferably used.

[0069] Next, a brief explanation will be given regarding the manufacturing method of the composition constituting the gas detection unit 331.

[0070] First, a binder resin or the like is mixed with the raw materials containing the above composition, and the mixture is thrilled to form a sheet. Then, a laminate is obtained by stacking the sheet-like mixture to a predetermined thickness. After that, the laminate is cut to a predetermined size and shape, and the cut laminate is fired to produce the bulk gas detection unit 331.

[0071] As described above, the gas detection unit 331 of this embodiment is a ceramic sintered body and is composed of a laminate formed by stacking multiple ceramic sheets. In the laminate after firing, the binder resin and other components are burned away, so a porous structure that is not dense is formed. For this reason, a laminate with a porous structure has a larger specific surface area than a dense composition, and the oxygen to be detected can easily penetrate the laminate, thus improving the detection sensitivity of the gas detection unit 331.

[0072] Alternatively, as another manufacturing method, a powder of raw materials containing the above composition may be molded to a predetermined size and shape, and the molded powder may be calcined to create a bulk gas detection unit 331. Or, a binder resin or the like may be mixed with the raw materials containing the above composition, and the bulk gas detection unit 331 may be produced by extrusion molding or the like.

[0073] Terminals 332 are provided at both ends of the gas detection unit 331. The pair of terminals 332 are films formed by baking a paste containing gold, silver, etc., onto them. Examples of methods for forming the terminals 332 include dipping or coating.

[0074] In this embodiment, the terminals 332 are formed so as to cover the end of the gas detection unit 331. Each of the terminals 332 is connected to the electrode 32 by a bonding material 34.

[0075] When the terminals 332 at both ends of the gas detection element 33 are connected to the pair of electrodes 32 by a bonding material 34, the gas detection element 33 is placed on the pair of electrodes 32 such that a space is formed between the gas detection element 33 and the upper surface 31a of the substrate 31, as shown in Figure 5.

[0076] By forming a space between the gas detection element 33 and the upper surface 31a of the substrate 31, it is possible to release the strain on both caused by the difference in thermal expansion coefficients between the gas detection unit 331 and the substrate 31. Therefore, compared to the case where the gas detection unit 331 is in direct contact with the upper surface 31a of the substrate 31, the occurrence of cracks in the electrode 32, gas detection unit 331, and bonding material 34 can be suppressed.

[0077] For example, even if the substrate 31 is an alumina substrate and the gas detection unit 331 is a ceramic having a porous structure, the occurrence of cracks can be suppressed.

[0078] Next, the heater 35 provided on the other surface of the substrate 31, the lower surface 31b, will be described with reference to Figures 6 and 7. In Figure 7, the thickness of the heater 35 is omitted to make it easier to understand the positional relationship between the heater 35 located on the lower surface 31b and the pair of electrodes 32 located on the upper surface 31a, which is opposite to the lower surface 31b.

[0079] The heater 35 controls the temperature of the gas detection element 33 to a predetermined operating temperature lower than the temperature of the hot spot generated by the self-heating of the gas detection element 33.

[0080] The hotspot temperature referred to here is the temperature necessary to stably detect the gas concentration solely through self-heating. The predetermined operating temperature is the temperature at which the gas detection element 33 can detect the gas concentration, and is, for example, the temperature range in which the factor m shown in equation (1) above is 100 or less. Specific examples of the predetermined operating temperature include the temperature range in which optimal detection sensitivity is obtained, and do not exclude temperatures necessary for adjustments such as flushing.

[0081] More specifically, the heater 35 controls the temperature of the gas detection element 33 to a predetermined operating temperature while a voltage is applied between the pair of electrodes 32. The gas detection element 33 may be adjusted to a predetermined operating temperature by both heating by the heater 35 and the heat generated in the gas detection element 33 by the voltage applied between the pair of electrodes 32, or it may be adjusted to a predetermined operating temperature by heating by the heater 35 alone.

[0082] In this embodiment, the temperature range in which the optimal detection sensitivity of the gas detection element 33 can be obtained is defined as a predetermined operating temperature, which is 450°C to 650°C. Therefore, the heater 35 controls the temperature of the gas detection element 33 to within the temperature range of 450°C to 650°C defined as the predetermined operating temperature.

[0083] The lower limit of the temperature range in which the gas concentration can be detected is approximately 400°C. From the viewpoint of reducing the power consumption of the gas sensor 1, for example, the predetermined operating temperature may be set to a temperature range of 400°C to 650°C. In this temperature range, it is possible to detect the oxygen concentration even in a low-concentration environment of 1 ppm of oxygen. Furthermore, the same conditions can be applied to the above-mentioned specific composition that does not contain copper oxide (CuO).

[0084] The heater 35 is a metal layer of the heating element, and the metal layer is made of, for example, platinum. As shown in Figure 6, the heater 35 has a heater section 351 and a pair of heater electrode sections 352, and in this embodiment, the heater section 351 and the pair of heater electrode sections 352 are integrally formed.

[0085] The heater section 351 is formed in a meandering shape. The shape of the heater section 351 is not limited to a meandering shape; for example, it may be a shape in which alternating trimmings are made to a strip-shaped metal layer, or a spiral shape. Heater electrode sections 352 are arranged at one end and the other end of the heater section 351, respectively.

[0086] Each pair of heater electrode sections 352 is an electrode to which a pair of wires 22c and 22d (see Figure 2) are connected by solder, gold paste, silver paste, or the like (not shown). A pair of pins 11b attached to the base substrate 20 are connected to the pair of wires 22c and 22d via a pair of wiring patterns 21c and 21c, respectively.

[0087] A predetermined current is supplied to the heater section 351 between the pair of heater electrode sections 352 through the pair of wires 22c and 22d, causing the heater section 351 to heat up. In this embodiment, as shown in Figure 6, the connecting portions 353 of the heater section 351 and the pair of heater electrode sections 352 are arranged point-symmetrically.

[0088] As a result, the heat point of the heater section 351 that generates the most heat is more likely to occur at the center of the point symmetry, making it possible to accurately heat the central part of the gas detection element 33 that you want to heat.

[0089] Furthermore, in this embodiment, as shown in Figure 7, in the heater 35 provided on the lower surface 31b of the substrate 31, the connecting portion 353 between the heater portion 351 and the heater electrode portion 352 overlaps with a pair of electrodes 32 provided on the upper surface 31a of the substrate 31 via the substrate 31.

[0090] As a result, the gas detection element 33 and the heater 35 are in approximately the same position on the upper surface 31a and lower surface 31b of the substrate 31. Therefore, the center of the heater 35 coincides with the center of the gas detection element 33, and the connecting portion 353 of the heater 35 coincides with the electrode 32.

[0091] Therefore, the heat generated at the heat point of the heater section 351 is accurately transferred to the center of the gas detection element 33 via the air, and the heat generated at the connecting section 353 is more easily transferred to the gas detection section 331 via the electrode 32.

[0092] Therefore, the gas detection element 33 can be heated efficiently and uniformly. Thus, the power consumption of the heater 35 can be reduced. Further, by heating the gas detection element 33 uniformly, deterioration of the gas detection element 33 can be suppressed.

[0093] As described above, by providing the sensor element 30 with the heater 35 that controls the temperature of the hot spot type gas detection element 33, the temperature of the gas detection element 33 can be easily controlled to a predetermined operating temperature suitable for detecting the concentration of a gas, as compared with a conventional sensor element that does not have the heater 35.

[0094] The gas detection unit 331 of the present embodiment has a composition formula of RE 1+x Ba 2―x Cu3O δ , (where RE is a rare earth element, δ represents an oxygen non-stoichiometric amount, and the substitution amount x satisfies 0 ≦ x ≦ 2), and a part of the composition is replaced with at least one of an element selected from the elements of Group 2 of the periodic table and an element selected from lanthanoid series elements, and copper oxide (CuO) is added in a volume percentage of y% (where 0 < y ≦ 7.5).

[0095] The temperature range in which the sensitivity is best in this composition is from 450°C to 650°C. On the other hand, in order to stably operate the gas detection element 33 so that the oxygen concentration can be detected only by self-heating, it is necessary to raise the temperature of the hot spot generated in the gas detection element 33 to around 950°C, preferably 1000°C.

[0096] The inventors attempted to adjust the voltage applied to the gas detection element 33 through the pin 11b to lower the temperature of the hot spot of the gas detection element 33 to a predetermined operating temperature with good detection sensitivity, but found that this temperature adjustment is difficult.

[0097] As a countermeasure, the sensor element 30 in this embodiment is equipped with a heater 35 on the surface of the substrate 31. This makes it possible to control the temperature of the hot spot type gas detection element 33 to a predetermined operating temperature, thereby suppressing deterioration of the gas detection element 33 due to excessive temperature rise and external impacts caused by overheating of the gas detection element 33.

[0098] Furthermore, the inventors discovered that in low-oxygen environments with an oxygen concentration of less than 0.1%, hot spots due to self-heating of the gas detection element 33 do not occur, making it difficult to detect the oxygen concentration.

[0099] Even in such cases, the heater 35 can be driven to maintain a predetermined operating temperature that provides good detection sensitivity. Therefore, the concentration of the gas to be detected can be accurately detected even in low-concentration environments where the target gas is less than 0.1%.

[0100] In this embodiment, the heater 35 is provided on the lower surface 31b of the substrate 31, but it may also be provided on the upper surface 31a of the substrate 31. In this case, the heater 35 is positioned around the gas detection element 33 on the upper surface 31a of the substrate 31. If there are no design problems with the sensor element 30, the heater 35 may also be placed below the gas detection element 33.

[0101] Next, a modified example of the gas sensor 1 will be briefly described with reference to Figures 8 to 11.

[0102] Figure 8 is a longitudinal side view showing a modified example of the sensor element 30 in this embodiment.

[0103] The modified sensor element 30A includes a buffer layer 36 in addition to the configuration of the sensor element 30 shown in Figure 5. Since the other components of the sensor element 30A are the same as those shown in Figure 5, the same reference numerals are used for identical components, and their descriptions are omitted.

[0104] The buffer layer 36 is provided between the pair of electrodes 32 on the upper surface 31a of the substrate 31, that is, below the gas detection unit 331. Parts of the pair of electrodes 32 are provided so as to overlap on the buffer layer 36.

[0105] The buffer layer 36 plays a role in suppressing thermal distortion of the gas detection element 33, and the thermal expansion coefficient of the buffer layer 36 is closer to the thermal expansion coefficient of the gas detection element 33 than that of the substrate 31. For example, the buffer layer 36 is made of a composition similar to that of the gas detection element 33.

[0106] By providing such a buffer layer 36, the difference in thermal expansion coefficients between the buffer layer 36 and the gas detection element 33 becomes smaller than the difference in thermal expansion coefficients between the substrate 31 and the gas detection element 33. Therefore, crack occurrence can be suppressed compared to the configuration shown in Figure 5.

[0107] Figure 9 is a top view showing a modified example of the pair of electrodes 32 in this embodiment.

[0108] In the modified version, the pair of electrodes 32A are formed so that the portion on which the gas detection element 33 is mounted protrudes. This reduces the amount of paste formed on the upper surface 31a of the substrate 31 while also reducing the excess current flowing through the electrodes 32A.

[0109] Figure 10 is a perspective view showing a first modified example of the terminal 332 of the gas detection element 33 in this embodiment, and Figure 11 is a perspective view showing a second modified example of the terminal 332 of the gas detection element 33 in this embodiment.

[0110] As shown in Figure 10, the pair of terminals 332A in the first modified example are formed on the four surfaces of the gas detection unit 331: the longitudinal end face, the bottom face, and both sides. Also, as shown in Figure 11, the pair of terminals 332B in the second modified example are formed only on the two surfaces of the gas detection unit 331: the end face 3321 and the bottom face 3322.

[0111] Thus, with respect to terminals 332A and 332B, the number of surfaces covering the end of the gas detection unit 331 is smaller, and the area is smaller, compared to terminal 332 shown in Figure 3. As a result, the force pulling the gas detection element 33 in one direction and the other in the longitudinal direction due to the thermal expansion of the bonding material 34, which is a metal paste such as gold, is weakened, and the occurrence of cracks in the gas detection element 33 and the bonding material 34 can be suppressed.

[0112] Next, the effects and advantages of this embodiment will be described.

[0113] In this embodiment, the gas sensor 1 includes a sensor element 30 that detects the concentration of a gas by a change in resistance. The sensor element 30 includes a substrate 31, a pair of electrodes 32 provided on the upper surface 31a of the substrate 31, a gas detection element 33 mounted on the pair of electrodes 32 and capable of detecting the concentration of a gas by self-heating, and a heater 35 provided on the upper surface 31a or the lower surface 31b of the substrate 31, which controls the temperature of the gas detection element 33 to a predetermined operating temperature lower than the temperature of a hot spot generated by the self-heating of the sensor element 30.

[0114] In this configuration, the heater 35 adjusts the temperature of the gas detection element 33 to a predetermined operating temperature that is lower than the temperature of the hot spot of the gas detection element 33 and capable of detecting the gas concentration. This suppresses deterioration of the gas detection element 33 due to red-hot heating and external influences.

[0115] Furthermore, in this embodiment, the heater 35 maintains the temperature of the gas detection element 33 at a predetermined operating temperature, and the gas detection element 33 detects the gas concentration in a low-concentration environment where the gas is at least 0.1% or less.

[0116] With this configuration, even in low-concentration environments of less than 0.1%, where no hot spots occur on the gas detection element 33, the heater 35 maintains the temperature of the gas detection element 33 at a predetermined operating temperature, thus enabling the detection of gas concentration.

[0117] Furthermore, in this embodiment, the gas detection element 33 is placed on a pair of electrodes 32 such that a predetermined space is formed between the upper surface 31a of the substrate 31 and the gas detection element 33.

[0118] With this configuration, compared to the case where the substrate 31 and the gas detection element 33 are in direct contact, it is easier to release the strain caused by the difference in thermal expansion coefficients between the substrate 31 and the gas detection element 33, thereby suppressing the occurrence of cracks in the gas detection element 33 and the bonding material 34.

[0119] Furthermore, in this embodiment, the gas sensor 1 is provided between a pair of electrodes 32 on the upper surface 31a of the substrate 31 and further includes a buffer layer 36 that suppresses thermal strain of the gas detection element 33.

[0120] With this configuration, the difference in thermal expansion coefficients between the gas detection element 33 and the buffer layer 36 is smaller than the difference in thermal expansion coefficients between the substrate 31 and the gas detection element 33. Therefore, the difference in expansion between the gas detection element 33 and the buffer layer 36 when heated by the heater 35 is reduced. As a result, the stress on the gas detection element 33 and the buffer layer 36 is relieved, which suppresses the occurrence of cracks and fractures in the gas detection element 33 and the bonding material 34. Therefore, the output of the gas sensor 1 can be stabilized.

[0121] Furthermore, in this embodiment, the gas detection element 33 consists of a gas detection unit 331 and terminals 332 provided at both ends of the gas detection unit 331, and the gas detection unit 331 is a laminate formed by stacking multiple ceramic sheets.

[0122] With this configuration, the laminate has a porous structure, which allows the gas to be detected to easily penetrate the laminate. Furthermore, the specific surface area is larger compared to a dense composition, thus improving the detection sensitivity of the gas detection unit 331.

[0123] Furthermore, in this embodiment, the terminals 332 of the gas detection element 33 are provided on at least the lower surface and end surface of the gas detection unit 331.

[0124] This configuration ensures the conductivity necessary for detecting changes in resistance while avoiding unnecessarily increasing the size of terminal 332, thereby suppressing the occurrence of cracks in the gas detection unit 331 and other components caused by thermal expansion of terminal 332.

[0125] Furthermore, in this embodiment, the thickness W2 of the gas detection element 33 is greater than the thickness W1 of the substrate 31.

[0126] This configuration increases the volume of the gas detection element 33, thereby suppressing its degradation. Furthermore, it allows for more efficient transfer of heat from the heater 35 to the gas detection element 33. In this way, the degradation of the gas detection element 33 is suppressed while simultaneously improving the heat transfer performance of the heater 35.

[0127] Furthermore, in this embodiment, the heater 35 is provided on the lower surface 31b of the substrate 31 and has a heater portion 351 and a pair of heater electrode portions 352 provided at both ends of the heater portion 351. The connecting portion 353 between the heater portion 351 and the heater electrode portions 352 overlaps with a pair of electrodes 32 provided on the upper surface 31a of the substrate 31 via the substrate 31.

[0128] With this configuration, the heat generated at the most heat-generating heat point in the heater section 351 is accurately transferred to the center of the gas detection element 33 via the air, and the heat generated at the connecting section 353 is more easily transferred to the gas detection section 331 via the electrode 32.

[0129] Therefore, the gas detection element 33 can be heated efficiently and uniformly. Consequently, the power consumption of the heater 35 can be reduced. Furthermore, by heating the gas detection element 33 uniformly, the deterioration of the gas detection element 33 can be suppressed.

[0130] Furthermore, in this embodiment, the connecting portions 353 between both ends of the heater portion 351 and the pair of heater electrode portions 352 are arranged point-symmetrically.

[0131] According to this configuration, since the heat points of the heater unit 351 are likely to occur at the center of point symmetry, it becomes possible to accurately heat the central portion of the gas detection element 33 to be heated.

[0132] Also, in the present embodiment, the gas detection element 33 is a ceramic sintered body. The ceramic sintered body has a composition formula of RE 1+x Ba 2―x Cu3O δ , (where RE is a rare earth element, δ represents the oxygen non-stoichiometric amount, and the substitution amount x satisfies 0≦x≦2), and a part of the composition represented by this is substituted by at least one of any element selected from the elements of Group 2 of the periodic table and any element selected from the lanthanoid series elements, or a material obtained by adding copper oxide (CuO) to the composition in a volume percentage of y% (where 0 < y≦7.5).

[0133] According to this configuration, it is possible to provide an oxygen detection element having excellent moisture resistance, high fusing resistance, and good sensor characteristics for oxygen concentration measurement, and an oxygen sensor using the same. Further, according to the material obtained by adding copper oxide (CuO) to the above composition, since CuO acts as a sintering aid, the sintering temperature of the semiconductor material formed from the composition in which a part of the composition represented by the composition formula RE 1+x Ba 2―x Cu3O δ is substituted by at least one of any element selected from the elements of Group 2 of the periodic table and any element selected from the lanthanoid series elements can be lowered.

[0134] Therefore, for example, by adding CuO to the composition constituting the gas detection element 33, it is possible to realize a high-density sinterability comparable to that when fired at the normal firing temperature at a firing temperature lower than the normal firing temperature. Also, when the firing temperature is increased to the normal firing temperature, the shrinkage amount increases, and a more highly dense sintered body can be obtained.

[0135] Furthermore, in this embodiment, the predetermined operating temperature for the gas detection element 33 made of the above material is 450°C to 650°C.

[0136] With this configuration, the temperature range of 450°C to 650°C is the temperature range in which the gas detection element 33 can obtain optimal detection sensitivity. Therefore, the oxygen concentration can be detected with greater accuracy compared to when the temperature of the gas detection element 33 is adjusted to other temperature ranges.

[0137] Furthermore, in this embodiment, as shown in Figure 2, the gas sensor 1 includes a base substrate 20 that holds a plurality of pins 11, and a cap 40 that is attached to the base substrate 20 to protect the sensor element 30. The electrodes 32 of the sensor element 30 are each connected to the pins 11a.

[0138] This configuration allows the sensor element 30 to be protected by the cap 40, while a voltage can be applied between the pair of electrodes 32 of the sensor element 30 from the pair of pins 11a extending opposite the cap 40.

[0139] Furthermore, in this embodiment, as shown in Figure 2, the pair of electrodes 32 are connected to the pin 11a via wires 22a and 22b, respectively, and the pair of heater electrode portions 352 are connected to the pin 11b via wires 22c and 22d, respectively. In this state, the sensor element 30 is suspended by wires 22a to 22c.

[0140] In this configuration, the sensor element 30, which has a gas detection element 33 mounted on a substrate 31, is suspended by wires 22a to 22c. This allows for easy replacement of only the gas detection element 33 if any abnormality occurs in the gas detection element 33, compared to the case where a thin film gas detection element 33 is formed on the substrate 31 by printing. Therefore, the gas sensor 1 can be used for a long period of time.

[0141] Although this embodiment has been described above, the above embodiment is merely one example of how the present invention can be applied, and it is not intended to limit the technical scope of the present invention to the specific configuration of the above embodiment.

[0142] For example, in this embodiment, a through hole 20a is provided in the base substrate 20, but it is not necessary to provide a through hole 20a. [Explanation of Symbols]

[0143] 1. Gas sensor 11 (11a, 11b) pins 20 base boards 22a~22d Wire 30, 30A sensor element 31 circuit boards 32, 32A electrodes (a pair of electrodes) 33 Gas detection element 331 Gas detection unit 332, 332A, 332B (3321, 3322) terminal (end surface, bottom surface) 35 Heater 351 Heater section 352 Heater electrode section 353 Connecting part 36 Buffer Layers 40 caps W1 board thickness W2 Thickness of the gas detection element

Claims

1. It is equipped with a sensor element that detects the concentration of a gas by changing its resistance value. The aforementioned sensor element is circuit board and A pair of electrodes provided on one side of the substrate, A gas detection element placed on the pair of electrodes, The substrate includes a heater provided on one or the other surface of the substrate, which controls the temperature of the gas detection element to a predetermined operating temperature lower than the temperature of a hot spot generated by the self-heating of the gas detection element, Gas sensor.

2. A gas sensor according to claim 1, The heater maintains the temperature of the gas detection element at the predetermined operating temperature. The gas detection element detects the concentration of the gas in a low-concentration environment where the gas is at least 0.1% or less. Gas sensor.

3. A gas sensor according to claim 1 or claim 2, The gas detection element is placed on the pair of electrodes such that a predetermined space is formed between one surface of the substrate and the gas detection element. Gas sensor.

4. A gas sensor according to claim 1, A gas sensor further comprising a buffer layer provided between the pair of electrodes on one side of the substrate to suppress thermal strain of the gas detection element.

5. A gas sensor according to claim 1, The gas detection element consists of a gas detection unit and terminals provided at both ends of the gas detection unit. The gas detection unit is a laminate formed by stacking multiple ceramic sheets. Gas sensor.

6. A gas sensor according to claim 5, The terminals of the gas detection element are provided on at least the lower surface and end surface of the gas detection unit. Gas sensor.

7. A gas sensor according to claim 1, The thickness of the gas detection element is greater than the thickness of the substrate. Gas sensor.

8. A gas sensor according to claim 1, The heater comprises a heater section and a pair of heater electrode sections provided at both ends of the heater section. The heater is provided on the other side of the substrate, The connection portion between the heater portion and the heater electrode portion overlaps with the pair of electrodes provided on one side of the substrate via the substrate. Gas sensor.

9. A gas sensor according to claim 1, The heater comprises a heater section and a pair of heater electrode sections provided at both ends of the heater section. The connection portion between the heater section and the heater electrode section is arranged point-symmetrically. Gas sensor.

10. A gas sensor according to claim 1, The gas detection element is a ceramic sintered body, The aforementioned ceramic sintered body has a compositional formula RE １+ｘ Ba ２―ｘ Cu ３ O δ A composition in which a portion of the composition represented by (where RE is a rare earth element, δ represents the unstoichiometric amount of oxygen, and the substitution amount x satisfies 0 ≤ x ≤ 2) is substituted with at least one of the elements selected from Group 2 of the periodic table and the elements selected from the lanthanide system. Gas sensor.

11. A gas sensor according to claim 10, The predetermined operating temperature is between 450°C and 650°C. Gas sensor.

12. A gas sensor according to claim 1, A base board that holds multiple pins, The system includes a cap that is attached to the base substrate to protect the sensor element, The pair of electrodes of the sensor element are each connected to the pins. Gas sensor.

13. A gas sensor according to claim 12, The pair of electrodes are each connected to the pin via a wire, The pair of heater electrode sections are each connected to the pin via a wire, The sensor element is suspended by the wire. Gas sensor.

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