JP6703404B2 - Gas sensor element and gas sensor - Google Patents

Description

The present invention relates to a conductive oxide sintered body, a conductive member using the same, a gas sensor element, and a gas sensor.

A ceramic product as an electronic component includes a ceramic base material and an electrode provided on the base material, and the electrode is generally made of metal. Examples of such a product include a multilayer ceramic capacitor having a Ni (nickel) electrode, a Pd (palladium) electrode, or a Pt (platinum) electrode, an Ag (silver) electrode, a Cu (copper) electrode, or an Ag-Pd electrode. There are LTCC components (low-temperature co-fired ceramics) having Pd, piezo actuators having Pd electrodes, semiconductor packages having W (tungsten) electrodes, and spark plugs having Ir (iridium) electrodes or Pt electrodes.

However, when firing a metal such as Ni, Cu, or W together with the ceramic base material, it is difficult to exhibit the original performance of the ceramic base material because it is necessary to control the atmosphere according to the metal used. There is a problem of high cost. On the other hand, since Ag has a low melting point (962° C.), it needs to be fired at a low temperature. As a result, the material of the ceramic base material is limited, and further, the characteristics of the ceramic base material may deteriorate due to firing at a low temperature. Further, since noble metal materials such as Pd, Ir and Pt are expensive, it is difficult to apply them to electrodes that require a large area.

Since the above-mentioned various problems occur when the electrodes of the ceramic product are made of metal, a configuration in which the electrodes are replaced with a conductive oxide (ceramics) is also conceivable. As an oxide exhibiting conductivity, Patent Document 1 discloses a lanthanum cobalt-based oxide having a negative resistance-temperature characteristic in which the resistance value decreases with an increase in temperature. Further, Patent Document 2 discloses a lanthanum cobalt-based oxide having a characteristic that the absolute value of the B constant is large at high temperatures. However, the conductive oxides described in Patent Documents 1 and 2 have high resistivity at room temperature and insufficient conductivity.

As described above, conventional oxides have extremely low electric conductivity and large absolute value of B constant (temperature coefficient) as compared with metals, and thus it is difficult to substitute for metals. Note that ruthenium-based oxides (RuO ₂ , SrRuO _3, etc.) are known as oxides having high conductivity, but there is a problem that Ru is expensive. Therefore, the applicant of the present application has disclosed in Patent Document 3 an oxide sintered body having a high conductivity and a small B constant (temperature coefficient), which is suitable as a conductive material. Further, Patent Documents 4 to 6 also disclose various perovskite type oxides.

Japanese Patent No. 3286906 JP, 2002-87882, A International Publication No. 2013/150779 JP-A-02-269949 JP, 03-165253, A Japanese Patent Laid-Open No. 2002-084006

When a member made of a conductive oxide is provided on a ceramic substrate, in addition to the high conductivity of the conductive oxide, for example, from the viewpoint of suppressing warpage and cracks of the ceramic product, the conductive oxide It is preferable to make the thermal expansion coefficients of the sintered body and the base material close to each other. For example, the coefficient of thermal expansion of alumina and zirconia (YSZ), which are generally used as the base material of ceramic products, are about 8×10 ⁻⁶ K ⁻¹ and about 10×10 ⁻⁶ K ⁻¹ , respectively, and therefore, the conductivity It is preferable that the thermal expansion coefficient of the porous oxide sintered body be as close as possible to these. Therefore, a conductive oxide sintered body capable of appropriately controlling the thermal expansion coefficient while achieving high conductivity has been desired.

The present invention has been made to solve the above problems, and can be implemented as the following modes.
One aspect of the present invention is a gas sensor element including an integral sintered body obtained by integrally firing an electrode formed of a conductive oxide sintered body and a base material made of ceramics. The aggregate is a perovskite type oxide crystal represented by the composition formula: RE _a Cr _b Fe _c Ni _d O _x (where RE represents a rare earth element, a+b+c+d=1, 1.3≦x≦1.7). A crystalline phase having a structure, wherein a, b, c, d are 0.459≦a≦0.524, 0.025≦ b≦0.100, 0.125≦c≦0.250, 0. 200≦d≦0.300 is satisfied.
With such a configuration, the thermal expansion coefficient is controlled in the range of 11.6×10 ⁻⁶ K ⁻¹ to 12.4×10 ⁻⁶ K ⁻¹ while achieving room temperature conductivity of 100 S/cm or more. It will be possible.
In addition, the present invention can also be realized in the following forms.

(1) According to one aspect of the present invention, a conductive oxide sintered body is provided. This conductive oxide sintered body is represented by a composition formula: RE _a Cr _b Fe _c Ni _d O _x (where RE represents a rare earth element, a+b+c+d=1, 1.3≦x≦1.7). Including a crystal phase having a perovskite type oxide crystal structure, wherein a, b, c and d are
0.459≦a≦0.524,
0<b≦0.100,
0.125 ≤ c ≤ 0.250,
0.200≦d≦0.300,
Meet
According to this conductive oxide sintered body, the thermal expansion coefficient is from 11.6×10 ⁻⁶ K ⁻¹ to 12.4×10 ⁻⁶ K ⁻¹ while achieving room temperature conductivity of 100 S/cm or more. It becomes controllable in the range of.

(2) In the conductive oxide sintered body of the above-mentioned form, b may satisfy 0.025≦b≦0.100.
According to the conductive oxide sintered body of this aspect, it becomes easier to suppress the coefficient of thermal expansion within a desired range.

(3) In the conductive oxide sintered body of the above aspect, the rare earth element RE may be La.
According to the conductive oxide sintered body of this aspect, the room temperature conductivity of the conductive oxide sintered body can be further increased, and the absolute value of the B constant of the conductive oxide sintered body can be further increased. Can be made smaller.

(4) In the conductive oxide sintered body of the above-mentioned form, the a may satisfy 0.473≦a≦0.512.
According to the conductive oxide sintered body of this aspect, the sinterability and conductivity of the conductive oxide sintered body can be further enhanced.

(5) The conductive oxide sintered body of the above-mentioned form may be substantially free of alkaline earth metal elements.
According to the conductive oxide sintered body of this embodiment, the weight change of the conductive oxide sintered body is suppressed in a wide temperature range from room temperature to around 900° C., and it is suitable as a conductive material used in a high temperature environment. It can be an oxide sintered body.

The present invention can be realized in various forms, for example, a conductive oxide sintered body, various electrodes using the same, electric wiring, a conductive member, a gas sensor element, a gas sensor (specifically, Can be realized in the form of an oxygen sensor, a NOx sensor, etc.), a thermoelectric material, a heater material, a temperature detecting element, and a manufacturing method thereof.

3 is a flowchart showing a method for producing a conductive oxide sintered body according to an embodiment. Explanatory drawing which shows an example of a gas sensor element. The flowchart which shows the manufacturing method of a gas sensor. Sectional drawing which shows the structure of an oxygen sensor. The figure which shows the composition and characteristic of several samples. The figure which shows B constant and thermoelectromotive force of a typical sample. The graph which shows the relationship between the value of b of a typical sample, and conductivity, or a thermal expansion coefficient. The figure which shows the baking temperature of another typical sample, and the relationship of specific gravity.

A. Composition of Conductive Oxide Sintered Body A conductive oxide sintered body as one embodiment of the present invention includes a crystal phase having a perovskite type oxide crystal structure that satisfies the following composition formula (1). It is an oxide sintered body.

RE _a Cr _b Fe _c Ni _d O _x ... (1)
Here, RE represents a rare earth element, a+b+c+d=1, and 1.3≦x≦1.7. Further, the coefficients a, b, c and d satisfy the following relationship.
0.459≦a≦0.524 (2a)
0<b≦0.100 (2b)
0.125≦c≦0.250 (2c)
0.200≦d≦0.300 (2d)

The rare earth element RE may include one or more of various rare earth elements such as La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm (promethium), and Sm (samarium). It is possible, and it is preferable to contain one or more of La, Pr, and Nd. In particular, it is preferable that La is contained as the rare earth element RE, because a conductive oxide sintered body having higher room temperature conductivity can be obtained. Further, it is preferable that the rare earth element RE contains La only, because the absolute value of the B constant becomes smaller.

If the relations of the above formulas (2a) to (2d) are satisfied, the coefficient of thermal expansion is 11.6×10 ⁻⁶ K ⁻¹ to 12.4× while achieving room temperature conductivity of 100 S/cm or more. It becomes possible to control in the range of 10 ⁻⁶ K ⁻¹ . Here, "room temperature conductivity" means the conductivity at 25°C. When the coefficient a of the rare earth element RE is less than 0.459 or exceeds 0.524, room temperature conductivity of 100 S/cm or more may not be obtained. If the coefficient a exceeds 0.524, the sinterability may be poor. When the coefficient b of Cr is 0, the coefficient of thermal expansion may become too large. Further, when the coefficient b exceeds 0.100, room temperature conductivity of 100 S/cm or more may not be obtained. When the coefficient c of Fe is less than 0.125 or exceeds 0.250, room temperature conductivity of 100 S/cm or more may not be obtained. When the coefficient d of Ni is less than 0.200 or exceeds 0.300, room temperature conductivity of 100 S/cm or more may not be obtained.

When the composition of the conductive oxide satisfies the relationships of the above formulas (2a) to (2d), sintering should be performed within the firing temperature range of oxides such as alumina and zirconia generally used as a base material. Will be possible. Specifically, the sinterable temperature to be described later can be controlled at 1525°C to 1600°C.

Furthermore, when the composition of the conductive oxide satisfies the above formulas (2a) to (2d) and the rare earth element RE is La, while achieving room temperature conductivity of 118 S/cm or more, , The coefficient of thermal expansion can be controlled in the range of 11.6×10 ⁻⁶ K ⁻¹ to 12.4×10 ⁻⁶ K ⁻¹ . In this case, the sinterable temperature described later can be controlled at 1550°C to 1600°C.

In the conductive oxide sintered body in which the coefficients a, b, c and d satisfy the relationships of the above expressions (2a) to (2d), the coefficient a preferably further satisfies the following expression (3a). Thereby, the sinterability and conductivity of the conductive oxide sintered body can be further enhanced.

0.473≦a≦0.512 (3a)

Further, in the conductive oxide sintered body in which the coefficients a, b, c, d satisfy the relationships of the above expressions (2a) to (2d), the coefficient b may further satisfy the relationship of the following expression (3b). preferable. This makes it easier to keep the coefficient of thermal expansion within a desired range.

0.025≦b≦0.100 (3b)

The coefficient x of O (oxygen) is theoretically x=1.5 when all the oxide sintered bodies having the above composition are made of a perovskite phase. However, the amount of oxygen atoms may deviate from the stoichiometric composition depending on the ratio of each metal element contained in the perovskite type oxide, the environmental temperature or the atmosphere. Therefore, in the formula (1), the range of x is defined as 1.3≦x≦1.7 as a typical example.

The conductive oxide sintered body according to the embodiment of the present invention only needs to contain the perovskite phase having the above composition, and may contain other oxides. For example, in the case where a powder X-ray diffraction (XRD) measurement of a conductive oxide sintered body detects an oxide peak of RE.MO ₃ (where M is Cr, Fe, or Ni), It can be determined that the conductive oxide sintered body contains the perovskite phase. However, the conductive oxide sintered body preferably contains 50% by mass or more of the perovskite phase having the above composition. Further, the conductive oxide sintered body is allowed to contain an extremely small amount of alkaline earth metal element within a range that does not affect the conductivity, but it is substantially free of alkaline earth metal element. Preferably. By doing so, even when the conductive oxide sintered body is exposed to a wide range of temperatures from room temperature to around 900° C., it becomes difficult for oxygen to be absorbed or released, so that the weight change of the sintered body is suppressed. Get smaller. Thereby, an oxide sintered body suitable as a conductive material used in a high temperature environment can be obtained. In the present specification, "substantially free of alkaline earth metal element" means that the content ratio of the alkaline earth metal element is 0 when the content ratio of the constituent elements is evaluated by ICP emission spectroscopy. It means 3% or less. This ICP emission analysis is performed based on JIS K0116, and the nitric acid dissolution method is applied to the pretreatment of the sintered body.

B. Method for Manufacturing Conductive Oxide Sintered Body FIG. 1 is a flowchart showing a method for manufacturing a conductive oxide sintered body as one embodiment of the present invention. In this embodiment, the conductive oxide sintered body is formed by the solid-phase reaction method. The solid-phase reaction method is a raw material powder that is a compound containing constituent metal elements such as oxides, carbonates, or nitrates, and the metal element in the raw material powder has a predetermined content depending on the composition of the oxide to be produced. This is a well-known method of synthesizing a desired oxide by weighing and mixing so as to have a ratio and then heat-treating (baking).

When manufacturing a conductive oxide sintered body, first, raw material powders are weighed and mixed (step T110). In the present embodiment, the raw material powder is weighed, and then wet-mixed and dried to prepare the raw material powder mixture. As the raw material powder, for example, La(OH) ₃ , Cr ₂ O ₃ , Fe ₂ O ₃ and NiO can be used. It is preferable to use powders having a purity of 99% or more as the raw material powders. As the La raw material, it is also possible to use _La 2 _{O 3} in place of La _{(OH) 3,} it is preferable to use La _{(OH) 3,} it is preferable not to use _La 2 _{O 3} .. The reason for this is that La ₂ O ₃ has water absorption property, so that it is difficult to prepare it accurately, which may lead to a decrease in conductivity and a decrease in reproducibility.

Then, the obtained raw material powder mixture is calcined to obtain a calcined powder (step T120). The calcination can be performed, for example, in the air atmosphere at 800 to 1400° C. for 1 to 5 hours.

After the calcination, an appropriate amount of organic binder is added to the calcination powder to granulate (step T130). Specifically, in step T130, for example, an appropriate amount of an organic binder is added to the calcined powder, the mixture is put into a resin pot together with a dispersion solvent (for example, ethanol), and wet mixed and pulverized using zirconia cobblestone to obtain a slurry. Just do it. Then, the obtained slurry is dried at 80° C. for about 2 hours, and further granulated through a 250 μm mesh sieve to obtain a granulated powder.

Then, the obtained granulated powder is molded (step T140). The molding can be performed using, for example, a pressing machine (molding pressure; 98 MPa). Then, the obtained molded body is fired at a firing temperature higher than the calcination temperature to obtain a conductive oxide sintered body (step T150). The firing can be performed, for example, at 1300 to 1700° C. for 1 to 5 hours in the air atmosphere. After firing, the flat surface of the conductive oxide sintered body may be polished if necessary.

In the present embodiment, since the metal elements in the raw material powder are hardly lost during the manufacturing process, the ratio of each metal element contained in the obtained conductive oxide sintered body is the raw material mixed in the step T110. It substantially corresponds to the ratio of each metal element in the powder.

Whether or not the conductive oxide sintered body satisfies the composition defined by the equation (1) can be confirmed by ICP analysis of the conductive oxide sintered body. At the time of analysis, the sample may be pulverized in a mortar or the like and dissolved in a 50 vol% nitric acid aqueous solution. If the element to be measured in the sample is 1000 ppm or more, the ICP-AES method (ICP emission spectroscopy) may be used, and if it is less than 1000 ppm, the ICP-MS method (ICP mass spectrometry) may be used. .. ICAP6000 (manufactured by Thermo scientific) can be used for the ICP-AES method, and iCAPQ (manufactured by Thermo scientific) can be used for the ICP-MS method. The analysis by the ICP-MS method may be performed in the collision mode. The composition ratio of the oxide sintered body can be obtained by creating a calibration curve using the standard solution and correcting the analytical value.

C. Sensor Element FIG. 2(A) is a gas sensor element as an embodiment of the present invention, and is a front view showing an example of a gas sensor element using the conductive oxide sintered body of the above-described embodiment. 2(B) is a sectional view thereof. The gas sensor element 100 of the present embodiment is an oxygen sensor element that constitutes an oxygen sensor. The gas sensor element 100 extends in the longitudinal direction, which is the axial direction, and has a bottomed cylindrical base material 110 made of ceramic (solid electrolyte), and a noble metal external electrode 120 formed on the outer surface of the base material 110. It has a standard electrode 130 (reference electrode) formed on the inner surface of the base material 110.

The reference electrode 130 is a conductor layer formed of the conductive oxide sintered body of the above-described embodiment. In the gas sensor element 100 of this embodiment, the reference electrode 130 is formed over almost the entire inner surface of the base material 110. The reference electrode 130 is a reaction prevention layer (for example, Gd and Ce) formed on the inner surface of the base material 110 for preventing a reaction between the reference electrode 130 (conductive oxide sintered body) and the base material 110. May be formed on the reaction preventing layer. On the other hand, the external electrode 120 is in contact with a measured gas such as exhaust gas, while the reference electrode 130 is in contact with a reference gas (for example, the atmosphere) containing an oxygen concentration serving as a reference for detecting the oxygen concentration. Although the length of the reference electrode 130 in the axial direction depends on the design of the gas sensor element 100, a length of 1 to 10 cm is typical.

The solid electrolyte forming the substrate 110 can be formed of, for example, zirconium oxide (ZrO ₂ ) to which yttrium oxide (Y ₂ O ₃ ) is added, that is, yttria-stabilized zirconia (YSZ). Alternatively, by another solid electrolyte such as stabilized zirconia to which an oxide selected from calcium oxide (CaO), magnesium oxide (MgO), cerium oxide (CeO ₂ ) and aluminum oxide (Al ₂ O ₃ ) is added. The gas sensor element 100 may be configured. Here, the stabilized zirconia is not limited to fully stabilized zirconia, but includes partially stabilized zirconia. In addition, in the middle of the axial direction of the gas sensor element 100 (base material 110), a flange portion 140 protruding in the radial direction is formed over the entire circumference.

FIG. 3 is a flowchart showing a method for manufacturing the gas sensor element 100. When manufacturing the gas sensor element 100, first, the base material 110 is molded (step T210). Specifically, the material of the base material 110 (for example, yttria-stabilized zirconia powder) is press-molded and cut into the shape (cylindrical shape) shown in FIG. 2 to obtain a green body (unsintered body). ..

Then, an external electrode 120 is formed on the outer surface of the green body by printing or dipping using Pt or Au paste (step T220).

Then, in order to form the reference electrode 130, a calcined powder paste of a conductive oxide is applied to the inner surface of the green body (step T230). Specifically, the calcined powder of the electrode material manufactured according to steps T110 and T120 of FIG. 1 is dissolved in a solvent such as terpineol or butyl carbitol together with a binder such as ethyl cellulose to prepare a paste, and a green body Apply to the inside of.

The gas processing element 100 is completed by drying and firing the green body applied with the calcined powder paste as described above (step T240). The firing in step T240 may be performed in the air atmosphere at 1250 to 1600° C. for 1 to 5 hours. It should be noted that the various manufacturing conditions in the manufacturing methods of FIGS. 1 and 3 described above are examples, and can be changed as appropriate according to the application of the product.

D. Gas Sensor FIG. 4 is a sectional view showing an example of a gas sensor as an embodiment of the present invention. The gas sensor 300 of the present embodiment is, for example, an oxygen sensor used to detect the oxygen concentration in the exhaust gas of an internal combustion engine. The gas sensor 300 has an elongated shape extending along the axis O. In the following description, the lower side of FIG. 4 is referred to as the front end side, and the upper side is referred to as the rear end side. Further, a direction that is perpendicular to the axis O and goes outward from the axis O is referred to as a “radial direction”. The gas sensor 300 includes the gas sensor element 100 of the above-described embodiment, the metal shell 20, the protector 62, the outer cylinder 40, the protective outer cylinder 38, and the lead wire 60 drawn from the reference electrode 130 of the gas sensor element 100. Is equipped with.

The metal shell 20 is a member made of metal (for example, stainless steel) surrounding the gas sensor element 100, and the tip of the gas sensor element 100 projects from the tip of the metal shell 20. The inner surface of the metallic shell 20 is provided with a step portion 20b whose inner diameter is reduced toward the front end direction. Further, in the vicinity of the center of the metal shell 20, a polygonal flange portion 20c protruding outward in the radial direction is provided for engaging a mounting tool such as a hexagon wrench. Further, a male screw portion 20d is formed on the outer surface on the tip side of the collar portion 20c. By attaching the male screw portion 20d of the metal shell 20 to, for example, a screw hole of an exhaust pipe of an internal combustion engine and disposing the tip of the gas sensor element 100 in the exhaust pipe, it is possible to detect the oxygen concentration in the gas to be detected (exhaust gas). It will be possible. A gasket 29 for preventing gas leakage when the gas sensor 300 is attached to the exhaust pipe is further fitted and inserted into a step portion between the front surface of the flange portion 20c and the rear end of the male screw portion 20d.

The protector 62 is a tubular member made of metal (for example, stainless steel), and covers the tip of the gas sensor element 100 protruding from the tip of the metal shell 20. The rear end of the protector 62 is bent outward in the radial direction. The protector 62 is fixed by sandwiching the rear end portion between the front surface of the collar portion 140 of the gas sensor element 100 and the step portion 20b of the metal shell 20. When assembling the metal shell 20 and the gas sensor element 100, first, the protector 62 is inserted into the metal shell 20 from the rear end side of the metal shell 20, and the rear end of the protector 62 is connected to the step portion of the metal shell 20. Abut on 20b. Then, the gas sensor element 100 is further inserted from the rear end side of the metal shell 20, and the surface on the front end side of the collar portion 140 is brought into contact with the rear end portion of the protector 62. The external electrode 120 formed on the outer surface of the gas sensor element 100 comes into contact with the protector 62 at the collar portion 140 and is electrically connected to the metal shell 20 via the protector 62. It should be noted that the protector 62 is formed with a plurality of holes for taking the exhaust gas into the protector 62. The exhaust gas flowing into the protector 62 from the plurality of holes is supplied to the external electrode 120 as a gas to be detected.

In the space between the rear end side of the collar portion 140 of the gas sensor element 100 and the metal shell 20, a powder filling portion 31 in which a powder material containing talc powder is compressed and filled is arranged. The gap between the metal shell 20 and the metal shell 20 is sealed. A cylindrical insulating member (ceramic sleeve) 32 is arranged on the rear end side of the powder filling portion 31.

The outer cylinder 40 is a member formed of a metal material such as stainless steel, and is joined to the rear end of the metal shell 20 so as to cover the rear end of the gas sensor element 100. A metal ring 33 made of a metal material such as stainless steel is arranged between the inner surface of the rear end of the metal shell 20 and the outer surface of the tip of the outer cylinder 40. Then, the metal shell 20 and the outer cylinder 40 are fixed by crimping the front end portion of the outer cylinder 40 at the rear end portion of the metal shell 20. By performing this caulking, the bent portion 20a is formed on the rear end side of the collar portion 20c. By forming the bent portion 20a at the rear end portion of the metal shell 20, the insulating member 32 is pressed toward the front end side to crush the powder filling portion 31, and the insulating member 32 and the powder filling portion 31 are caulked and fixed. At the same time, the gap between the gas sensor element 100 and the metal shell 20 is sealed.

Inside the outer cylinder 40, a substantially cylindrical insulating separator 34 is arranged. The separator 34 is formed with an insertion hole 35 which penetrates the separator 34 in the direction of the axis O and into which the lead wire 60 is inserted. The lead wire 60 is electrically connected to the connection terminal 70. The connection terminal 70 is a member for taking out the sensor output to the outside, and is arranged so as to be in contact with the reference electrode 130. Inside the outer cylinder 40, a substantially columnar grommet 36 is arranged in contact with the rear end of the separator 34. An insertion hole through which the lead wire 60 is inserted is formed in the grommet 36 along the axis O. The grommet 36 can be formed of, for example, a rubber material such as silicon rubber or fluororubber.

On the side surface of the outer cylinder 40, a plurality of first vent holes 41 are opened side by side in the circumferential direction at a position closer to the tip side than the position where the grommet 36 is arranged. An annular breathable filter 37 is covered on the radially outer side of the rear end portion of the outer cylinder 40 so as to cover the first ventilation hole 41. The protective outer cylinder 38 is enclosed. The protective outer cylinder 38 can be formed of, for example, stainless steel. On the side surface of the protective outer cylinder 38, a plurality of second vent holes 39 are opened side by side in the circumferential direction. As a result, through the second ventilation hole 39 of the protective outer cylinder 38, the filter 37, and the first ventilation hole 41 of the outer cylinder 40, to the inside of the outer cylinder 40 and further to the reference electrode 130 of the gas sensor element 100, The outside air can be introduced. The filter 37 is held between the outer cylinder 40 and the protective outer cylinder 38 by swaging the outer cylinder 40 and the protective outer cylinder 38 on the front end side and the rear end side of the second ventilation hole 39. The filter 37 can be formed of, for example, a porous structure made of a water-repellent resin such as a fluorine-based resin. Since the filter 37 has water repellency, the reference gas (in the internal space of the gas sensor element 100 does not pass through external water). Atmosphere) can be introduced.

According to the conductive oxide sintered body of the embodiment configured as described above, the coefficient of thermal expansion is 11.6×10 ⁻⁶ K ⁻¹ to 12 while achieving room temperature conductivity of 100 S/cm or more. It becomes possible to control in the range of 4×10 ⁻⁶ K ⁻¹ . That is, it can be controlled so as to have a thermal expansion coefficient close to that of ceramics (for example, YSZ) generally used as a base material of a gas sensor element while achieving high room temperature conductivity. Therefore, in the gas sensor element 100 in which the conductive oxide sintered body and the base material are combined, it is possible to suppress the occurrence of warpage, cracks and the like. By thus suppressing the occurrence of warpage, cracks, and the like of the gas sensor element 100, the detection accuracy of the gas sensor 300 including the gas sensor element 100 can be increased and the durability can be improved.

Furthermore, according to the conductive oxide sintered body of the embodiment, the sinterable temperature described later can be controlled from 1525° C. to 1600° C., and firing can be performed in such a high temperature range. Therefore, even when it is used in combination with a ceramic that should be fired at a relatively high temperature, such as stabilized zirconia such as YSZ that constitutes the base material 110 of the gas sensor element 100, it is fired at the same time as the ceramic that constitutes the base material. Thus, the manufacturing process can be simplified. Further, since the ceramics constituting the base material and the conductive oxide sintered body are close in firing temperature (sinterable temperature described later) as described above, in a ceramic product such as a gas sensor element 100 obtained by combining the two, It is possible to suppress peeling and cracking of the conductive oxide sintered body and improve the adhesion of the conductive oxide sintered body to the base material.

E. Modification /Modification 1
In the gas sensor element 100 of the above-described embodiment, the reference electrode 130 is made of a conductive oxide sintered body, but may have a different structure. For example, instead of the reference electrode 130 or in addition to the reference electrode 130, the external electrode 120 may be made of a conductive oxide sintered body.

-Modification 2
In the above embodiment, the gas sensor element 100 is the element for the oxygen concentration sensor, but the electrode of the element for the gas sensor (for example, the NOx sensor) for measuring the concentration of another type of gas has the same conductivity as that of the embodiment. An oxide sintered body may be applied.

-Modification 3
The oxide sintered body according to the embodiment of the present invention includes, for example, various electrodes other than those for the gas sensor described above, electric wiring, conductive members, thermoelectric materials, heater materials, and temperature detecting elements, and metal. Can be used as a substitute for For example, the conductive member can be realized by having a conductor layer formed of a conductive oxide sintered body on the surface of a ceramic base material.

FIG. 5 and FIG. 6 are explanatory diagrams collectively showing evaluation results obtained by producing 20 kinds of oxide sintered bodies of Sample S1 to Sample S20 and examining the composition and performance of each sample. Here, samples S1 to S14 are examples, and samples S15 to S20 are comparative examples. FIG. 5 shows the electrical conductivity, the coefficient of thermal expansion, and the temperature at which sintering is possible for all the samples S1 to S20. Further, FIG. 6 shows B constants and thermoelectromotive forces for representative samples S2, S3, S5, S15, and S16 among the samples shown in FIG. The method for producing each sample is as follows.

The oxide sintered body of each sample was manufactured according to the manufacturing method described with reference to FIG. 1, and finally subjected to planar polishing to obtain a rectangular parallelepiped sample of 3.0 mm×3.0 mm×15 mm. In step T110, the raw material powders were weighed and mixed according to the composition shown in FIG. The rare earth element RE was La in samples S1 to S13 and S15 to S20, and Pr in sample S14.

At the time of evaluation, the composition of each sample was fired at a plurality of firing temperatures, and the water absorption rate of the obtained sintered body was examined to obtain the sinterable temperature and the specific gravity. It was Of the evaluation results shown in FIGS. 5 and 6, the items other than the sinterable temperature are the evaluation results of the sample fired at the sinterable temperature obtained as described above. The evaluation method is as follows.

<Measurement of conductivity>
The conductivity of each sample was measured by the DC 4-terminal method. Pt was used for the electrodes and electrode wires used for the measurement. In addition, a voltage/current generator (monitor 6242 manufactured by ADC Corp.) was used for the conductivity measurement.

<Measurement of thermal expansion coefficient>
For each sample, the coefficient of thermal expansion (linear expansion coefficient α) when the temperature was changed from room temperature to 1000° C. was measured. The measurement conditions are as follows.
Measuring device: Rigaku TMA8310
Standard sample: Al ₂ O ₃
Measurement atmosphere: Air atmosphere Temperature rising rate: 10.0°C/min

<Evaluation of sinterable temperature and specific gravity>
Firing was performed using a plurality of firing temperatures for the composition of each sample, the sinterability (water absorption) was evaluated based on JIS-R-1634, and the firing temperature was derived. Specifically, first, the dry weight W1, the underwater weight W2, and the saturated water weight W3 of each sample after firing at various firing temperatures are measured, and these values and the following equations (4) and (5) are measured. Was used to calculate the water absorption rate and the specific gravity. The lowest firing temperature at which the water absorption rate was 0.10 wt% or less was defined as the sinterable temperature for the composition of each sample. Here, ρ1 in the equation (5) is the density of the liquid medium (water) at the time of the test. The good sinterability of the sintered body means that the water absorption of the sintered body is low, and it can be said that a sintered body having a lower water absorption generally has a larger specific gravity.

Water absorption rate (%)=(W3−W1)/W1×100 (4)
Specific gravity (g/cm ³ )=W1/(W3-W2)×ρ1 (5)

<Measurement of B constant>
The B constant was calculated according to the following equation (6) from the electric conductivity at 75° C. and 870° C. measured by the method described in <Measurement of electric conductivity>.

B constant=ln(ρ1/ρ2)/(1/T1-1/T2) (6)
ρ1=1/σ1
ρ2=1/σ2
ρ1: Resistivity (Ωcm) at absolute temperature T1 (K)
ρ2: Resistivity (Ωcm) at absolute temperature T2 (K)
σ1: conductivity (S/cm) at absolute temperature T1 (K)
σ2: conductivity (S/cm) at absolute temperature T2 (K)
T1=348.15 (K)
T2 = 1143.15 (K)

<Measurement of thermoelectromotive force>
The thermoelectromotive force was measured by the steady-state DC method. Each sample (3.0 mm×3.0 mm×15 mm) was wound at two positions slightly closer to the center than both ends in the longitudinal direction and separated from each other by a predetermined distance, and was used as an electrode for measuring the electric potential difference of conductivity. Further, Au was vapor-deposited on both ends of the sample by sputtering, and both ends of the sample were sandwiched by quartz tubes with the outer platinum electrodes formed of a Pt plate or Pt net being provided on both ends of the sample to fix the sample. During the measurement, a constant current was applied between the two outer platinum electrodes, and high temperature air was sent into the quartz tube on one side to generate a temperature difference between the outer platinum electrodes. Furthermore, an R thermocouple (Pt-Pt13Rh) was attached to the outer platinum electrode, and the temperature difference was read. The temperature difference was generated stepwise by changing the air flow rate, the potential difference-temperature difference correlation was determined, and the thermoelectromotive force at 770° C. was calculated by the least square method. In addition, RZ2001k by Ozawa Scientific Co., Ltd. was used for the measurement. Moreover, the measurement was performed in an air atmosphere.

The samples S1 to S14 in FIGS. 5 and 6 all satisfy the compositions given by the expressions (1) and (2a) to (2d). These samples S1 to S14 had sufficiently high room-temperature conductivity, and had sufficiently good characteristics in terms of coefficient of thermal expansion and sinterable temperature. When the oxide sintered body is used as a conductor, it preferably has as high a room temperature conductivity as possible. The samples S1 to S14 of the examples all have room temperature conductivity of 108 S/cm or more, which is preferable in that they have a high room temperature conductivity of more than 100 S/cm. On the other hand, the samples S16 to S18 and S20 of the comparative examples have low room temperature conductivity of less than 100 S/cm. Samples S15 and S19 had high room temperature conductivity but low sinterable temperature.

Of the samples S1 to S14 of the example, the samples S2 and S14 have the same coefficients a, b, c, d, but differ from each other in that the rare earth elements RE are La and Pr, respectively. Of these two samples S2 and S14, the sample S2 in which the rare earth element RE is La is preferable because it has a higher room temperature conductivity than the sample S14 in which the rare earth element RE is Pr.

Further, among the samples S1 to S14 of the examples, the sample S10 in which the value of a was a=0.459, which was the smallest, had a relatively low sinterable temperature as compared with the samples of the other examples. .. Therefore, by setting a to 0.473 or more (see the sample S11 in which the value of a is the second smallest value of a=0.473 among the samples S1 to S14 in the example), the conductive oxide sintered body can be obtained. It can be said that the sinterability can be further improved. Further, among the samples S1 to S14 of the examples, the sample S13 in which the value of a was a=0.524, which had the largest value of a, had a relatively low conductivity as compared with the samples of the other examples. Therefore, by setting a to 0.512 or less (see sample S12 in which the value of a is the second largest value of a=0.512 among samples S1 to S14 in the example), the conductive oxide sintered body can be obtained. It can be said that the conductivity of can be further improved.

In the samples S2, S3, and S5 of the representative example shown in FIG. 6, the absolute value of the B constant is 110 or less, which is sufficiently small, and sufficiently high conductivity can be obtained even when the temperature changes. confirmed. Although not shown in the drawings for the other samples of the example, it was confirmed that almost the same tendency was exhibited. That is, the samples S1 to S14 of the example have a B constant suitable for use as a conductor layer.

Further, in the samples S2, S3, and S5 of the representative examples shown in FIG. 6, the absolute value of the thermoelectromotive force at 770° C. was 15 μV/K or less, which was sufficiently small. Therefore, in the sensor using such a conductive oxide sintered body, it was confirmed that high accuracy can be maintained even if the length of the sensor element in the axial direction is made longer. Although not shown in the drawings for the other samples of the example, it was confirmed that almost the same tendency was exhibited. That is, the samples S1 to S14 of the examples show thermoelectromotive force suitable for use as the conductor layer.

FIG. 7A shows the proportion of Cr atoms in the composition formula (value of b) based on the measurement results of a representative sample, specifically, the sample of d=0.275 in the sample of FIG. ) And room temperature conductivity. Similarly, FIG. 7B is a graph showing the relationship between the ratio of Cr atoms (value of b) in the composition formula and the thermal expansion coefficient based on the measurement result of the sample with d=0.275.

From FIG. 7(A), the smaller the value of b (the smaller the content of Cr), the higher the room temperature conductivity, and by setting b≦0.100, the high room temperature conductivity exceeding 100 S/cm can be obtained. I understand. Further, from FIG. 7(B), when the value of b is 0 (does not contain Cr), the coefficient of thermal expansion is higher than that of oxides such as alumina and stabilized zirconia generally used as a base material. However, it is understood that the coefficient of thermal expansion can be lowered and brought close to the coefficient of thermal expansion of the above oxides by making the value of b positive (containing Cr). It can be seen that the thermal expansion coefficient decreases as the value of b increases. As described above, by satisfying the above-described formula (2b), a thermal expansion coefficient close to that of ceramics generally used as a base material of a conductive oxide sintered body while achieving high room temperature conductivity. Can be said to be controllable.

Further, from FIG. 7(B), by setting the value of b to 0.025 or more, the coefficient of thermal expansion is lowered, and the thermal expansion of oxides such as alumina and stabilized zirconia generally used as a base material is reduced. It can be seen that the effect of approaching the coefficient increases. Therefore, by satisfying the above-mentioned formula (3b), while achieving a high room temperature conductivity, a thermal expansion coefficient close to that of ceramics generally used as a base material of a conductive oxide sintered body is obtained. The controllable effect can be further enhanced as shown.

FIG. 8 is a graph showing the relationship between firing temperature and specific gravity for a representative sample. Since each sample shown in FIG. 8 has a different Cr atom ratio (value of b) in the composition formula, the value of b is also shown. As shown in FIG. 8, in any of the samples, the higher the firing temperature, the higher the specific gravity (the higher the denseness). Further, as shown in FIG. 8, the graph tends to shift to the lower side as the value of b increases, so that the smaller the value of b (the smaller the ratio of Cr atoms), the higher the specific gravity at the lower sintering temperature. Can be realized (increased precision). The higher the specific gravity, the better, but it is preferably 5.0 g/cm ⁻³ or more. From the above, it is possible to adjust the firing temperature of the conductive oxide sintered body by controlling the value of b in an appropriate range. Therefore, firing of oxides such as alumina and stabilized zirconia generally used as a base material is performed. By selecting a conductive oxide sintered body that is suitable for the temperature, it is possible to achieve sufficiently high density in the conductive oxide sintered body even if it is fired at the same temperature as the base material (simultaneous firing). It will be possible.

The present invention is not limited to the above-described embodiments, examples, and modified examples, and can be realized with various configurations without departing from the spirit of the invention. For example, the technical features in the embodiments, examples, and modifications corresponding to the technical features in each mode described in the section of the outline of the invention are to solve some or all of the above problems, or In order to achieve some or all of the above-mentioned effects, it is possible to appropriately replace or combine them. If the technical features are not described as essential in this specification, they can be deleted as appropriate.

20... Metal shell 20a... Bent part 20b... Step part 20c... Collar part 20d... Part 29... Gasket 31... Powder filling part 32... Insulation member 33... Metal ring 34... Separator 35... Insertion hole 36... Grommet 37... Filter 38 Protective outer cylinder 39... Second ventilation hole 40... Outer cylinder 41... First ventilation hole 60... Lead wire 62... Protector 70... Connection terminal 100... Gas sensor element 110... Base material 120... External electrode 130... Reference electrode 140... Tsuba Part 300... Gas sensor O... Axis

Claims (8)

A gas sensor element comprising an integral sintered body obtained by integrally firing an electrode formed of a conductive oxide sintered body and a base material made of ceramics ,
The conductive oxide sintered body,
Compositional formula: crystal having a perovskite type oxide crystal structure represented by RE _a Cr _b Fe _c Ni _d O _x (where RE represents a rare earth element, a+b+c+d=1, 1.3≦x≦1.7) Phase, wherein a, b, c, d are
0.459≦a≦0.524,
0.025≦ b≦0.100,
0.125≦c≦0.250,
0.200≦d≦0.300,
A gas sensor element characterized by satisfying: The gas sensor element according to claim 1 , wherein
The gas sensor element, wherein the rare earth element RE is La. The gas sensor element according to claim 2 , wherein
Where a is
0.473≦a≦0.512
A gas sensor element characterized by satisfying: The gas sensor element according to any one of claims 1 to 3 ,
A gas sensor element, which is substantially free of alkaline earth metal elements. The gas sensor element according to any one of claims 1 to 4 ,
The gas sensor element is an oxygen sensor element. The gas sensor element according to claim 5 , wherein
A gas sensor element, wherein the electrode is a reference electrode. A gas sensor comprising the gas sensor element according to any one of claims 1 to 4 . Oxygen sensor, characterized in that it comprises an oxygen sensor element according to claim 5 or 6.

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