JP7290215B2 - oxygen sensor element - Google Patents

Description

The present invention relates to a material composition of a gas (oxygen) sensor element using a ceramic sintered body.

There is a demand for oxygen concentration detection in various gases, such as detection of oxygen concentration in exhaust gas of internal combustion engines, detection of oxygen concentration for combustion control of boilers, etc., and various materials are used as oxygen concentration detection elements. Oxygen sensors are known. For example, as a material composition of an oxygen sensor using a ceramic sintered body, an oxygen sensor using composite ceramics in which LnBa ₂ Cu ₃ O _7-δ and Ln ₂ BaCuO ₅ (Ln is a rare earth element) are mixed is known. (Patent document 1).

The oxygen sensor using the ceramic sintered body wire as described above is a hot spot type oxygen sensor using a hot spot phenomenon in which a part of the wire becomes red-hot when a voltage is applied. Such an oxygen sensor can be reduced in size, weight, cost, and power consumption, and is expected to be put to practical use in the future.

Japanese Patent Application Laid-Open No. 2007-85816 (Patent No. 4714867)

The above-described conventional oxygen sensor has a problem of durability because the wires are likely to melt due to hot spots generated when the sensor is driven. Such fusing of the wire is considered to be caused by the formation of a liquid phase in a localized portion (especially grain boundary) inside the hot spot.

In addition, since the materials that make up the conventional oxygen sensor element are prone to hydroxylation and carbonation, the sensor element deteriorates due to surrounding gas components such as water vapor and carbon dioxide when the oxygen concentration in the gas is detected. There is a problem that the durability is poor. Therefore, it has been difficult to put sensor elements with improved durability into practical use with conventional material compositions.

The present invention has been made in view of the above-mentioned problems, and its particular object is to provide a highly durable (mechanical strength) and highly reliable sensor that has high moisture resistance and does not impair sensor characteristics at the same time. An object of the present invention is to provide an oxygen sensor element.

The following configuration is provided as one means for achieving the above objects and solving the above problems. That is, the present invention is an oxygen sensor element that is made of a ceramic sintered body and detects oxygen concentration based on a current value when a voltage is applied, wherein the ceramic sintered body has a composition formula of LnBa ₂ Cu ₃ O Part of Ba in _7-δ (Ln is a rare earth element and δ represents a nonstoichiometric amount of oxygen) is the same rare earth element Ln as the rare earth element Ln in the composition formula, or the rare earth element Ln in the composition formula It is characterized by having a composition represented by the composition formula Ln _1+x Ba _2-x Cu ₃ O _7-δ (the amount of substitution x is 0<x≦1.2) substituted with a different rare earth element Ln.

For example , the replacement amount x is characterized by 0.4≦x≦0.8. For example, neodymium (Nd) is selected from the rare earth element Ln. For example, the composition represented by the composition formula Nd _1+x Ba _2-x Cu ₃ O _7-δ is mixed with the composition represented by the composition formula Ln ₂ BaCuO ₅ (Ln is a rare earth element). do. Further, for example, the composition represented by the composition formula Ln _1+x Ba _2-x Cu ₃ O _7-δ is characterized by having a composite perovskite structure. Further, for example, the ceramic sintered body is characterized by being a linear sensor element.

An oxygen sensor according to the present invention is characterized by using any one of the above oxygen sensor elements as an oxygen concentration detection element. For example, in the oxygen sensor, the oxygen sensor element is housed in a protective tube having vent holes at both ends.

According to the present invention, it is possible to provide an oxygen sensor element having excellent moisture resistance, high fusing resistance, and good sensor characteristics for oxygen concentration measurement, and an oxygen sensor using the same.

Fig _{. 1} (a) is an external appearance photograph showing the result of a moisture resistance test of an oxygen sensor element according to a conventional example having a composition _{YBa2Cu3O7- [delta]} , Fig. 1(a) is the external appearance before the test, and Fig. 1(b) is the external appearance after the test. is. 2A and 2B are appearance photographs showing results of a moisture resistance test of the oxygen sensor element according to the first embodiment of the present invention, FIG. 2A being an appearance before the test and FIG. 2B being an appearance after the test. FIG. 10 is an appearance photograph of a sample showing the results of a humidity resistance test when changing the replacement amount of Ca and La and the standing time of the sample for some of the conventional compositions of the oxygen sensor element according to the first embodiment; FIG. FIG. 4 is a diagram showing XRD measurement results of a test sample having a conventional composition (conventional example) and a test sample (example) according to the first embodiment; FIG. 5 is a diagram showing results of evaluating oxygen responsiveness as an oxygen sensor for a test sample according to a conventional example and a test sample according to an example of the first embodiment; 4 is a flow chart showing in chronological order an oxygen sensor element according to the first embodiment and a manufacturing process of an oxygen sensor using the oxygen sensor element. 1 is an external perspective view of an oxygen sensor using the oxygen sensor element according to the first embodiment; FIG. FIG. 10 is a sample appearance photograph showing the results of a humidity resistance test when changing the amount of Nd replacement and the standing time of the sample for each sample of the oxygen sensor element according to the second embodiment of the present invention; FIG. FIG. 4 is a diagram showing X-ray diffraction (XRD) measurement results of oxygen sensor element samples with different Nd substitution amounts; FIG. 5 is a diagram showing the fusion resistance characteristics of each oxygen sensor element sample; FIG. 10 is an enlarged view of a portion A (near 2θ=46°) of FIG. 9; FIG. 5 is a graph showing measurement results of oxygen responsiveness of oxygen sensor elements with different amounts of Nd substitution; FIG. 4 is a diagram showing the results of differential thermal analysis (DTA) measurement of each oxygen sensor element sample; FIG. 10 is an SEM photograph showing the results of SEM observation of fracture surfaces of sintered bodies of oxygen sensor elements with different amounts of Nd substitution; FIG.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings and the like. The oxygen sensor element according to the embodiment of the present invention is made of a ceramic sintered body, and when it is connected to a power supply and an electric current flows, the central part of the sintered body generates heat at a high temperature, and the heat generating part (called a hot spot). is used as an oxygen concentration detector. Further, an oxygen sensor using the oxygen sensor element according to the present embodiment as a sensor element detects the oxygen concentration based on the current value flowing through the sintered body as the sensor element.

[First embodiment]
The oxygen sensor element as an oxygen concentration detector according to the first embodiment of the present invention has a composition of LnBa ₂ Cu ₃ O _7-δ (hereinafter also referred to as a conventional composition). , any one element selected from elements of Group 2 of the periodic table, that is, beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra); The elements of the lanthanide series, namely 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).

In the above composition, Ln is a rare earth element (for example, 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), etc.).

In the following description, as an oxygen sensor element according to the first embodiment, a composition YBa ₂ Cu ₃ O _7-δ in which Ln is Y (yttrium) in the conventional composition LnBa ₂ Cu ₃ O _7-δ is partially replaced with calcium (Ca) and lanthanum (La) are substituted, and the composition is Y _1-z Ca _z Ba _2-z La _z Cu ₃ O _7-δ (0.25≦z≦1) A sintered body will be described as an example.

First, the results of comparative verification between a sample made using the oxygen sensor element material according to the first embodiment and a sample made of a conventional sensor element material will be described. Here, a disc-shaped oxygen sensor element (hereinafter also referred to as a test sample) having a diameter of about 16 mm and a thickness of about 2 mm was prepared by sintering a powder compact having a composition described later, and subjected to a moisture resistance test. etc. These samples are lumps (bulk bodies) of the respective composition materials themselves, and have a shape and size that facilitate observation of changes in appearance before and after the test.

<Results of moisture resistance test>
Moisture resistance test results of the oxygen sensor element having the conventional composition and the oxygen sensor element according to the first embodiment will be described. Here, a test was conducted in which the sample was left for a predetermined period of time in an environment of 40° C. and 93% RH.

FIG. 1 is a sample appearance photograph showing the results of a moisture resistance test of a conventional composition test sample (z=0, also referred to as conventional example) having the composition YBa ₂ Cu ₃ O _7-δ . FIG. 1(a) shows the appearance of the sample before the test, and FIG. 1(b) shows the appearance of the test sample left in an environment of 40° C. and 93% RH for 50 hours.

On _{the other hand , FIG .} 10 is an appearance photograph showing the result of a moisture resistance test of the oxygen sensor element (also referred to as an example) according to the first embodiment in which =1. FIG. 2(a) shows the appearance of the oxygen sensor element before the test, and FIG. 2(b) shows the appearance after the test sample was left in an environment of 40° C. and 93% RH for 500 hours. .

FIG. 3 shows the composition Y _1-z Caz Ba of the oxygen sensor element according to the first embodiment, in which part of the conventional composition YBa ₂ Cu ₃ O _7-δ is replaced with calcium (Ca) and lanthanum ₍ La). In _2-z La _z Cu ₃ O _7-δ , the substitution amount of Ca and La was changed (z = 0 to 1), and the standing time in an environment of 40°C and 93% RH was changed (0 hour 500 hours) is an appearance photograph showing the result of observing the appearance of each sample.

Table 1 shows the result of determination of element deterioration by X-ray diffraction (XRD) measurement for each oxygen sensor element shown in FIG.

In Table 1, the x mark is a sample in which a barium carbonate peak occurs and the existing peak is reduced (with deterioration of the element), and the ○ mark is a sample in which another peak such as barium carbonate is not observed and the existing peak is reduced. This is a sample in which there was no change and it was determined that there was no deterioration in the element.

As a result of the appearance observation shown in FIGS. 1(b) and 3, it can be seen that after the moisture resistance test, barium carbonate or the like was generated on the surface of the oxygen sensor element of the conventional composition (z=0), causing a phenomenon of white discoloration. . From this, it was found that the oxygen sensor element stopped reacting with oxygen and deteriorated. Specifically, as shown by the XRD measurement results in Table 1 and the appearance observation results in FIG. 3, the element of the conventional composition sample deteriorated after 50 hours in an environment of 40° C. and 93% RH.

On the other hand, in the oxygen sensor element according to the example, when the substitution amount z of Ca and La is 0.25, the sample surface turns white when left in an environment of 40° C. and 93% RH for 200 hours. When the amount of substitution was z=0.5, the sample surface partially turned white when left in an environment of 40° C. and 93% RH for 500 hours. In the XRD measurement results of Table 1, deterioration of the elements was recognized for these example samples.

In addition, for the samples of each example in which the amount of Ca and La substitution was z = 0.75 and z = 1, the results of appearance observation shown in Figs. As a result, no deterioration of the element was observed even after standing for 500 hours in an environment of 40° C. and 93% RH.

It was found that the oxygen sensor element according to the example, which has a composition in which a part of the conventional composition is replaced with Ca and La, improves the humidity resistance of the sensor element by performing such replacement. As for the amount of Ca and La substituted, it can be seen that an improvement in moisture resistance is observed even when z=0.25, but z=0.75 or more is desirable. From this, it was found that the oxygen sensor element according to the first embodiment was excellent in moisture resistance.

In order to consider the mechanism by which the moisture resistance of the oxygen sensor element according to the first embodiment is improved, the X-ray diffraction (XRD) measurement results of the test samples described above will be described in more detail.

FIG. 4 shows XRD measurement results of a test sample of a conventional composition (conventional example, z = 0) and a test sample according to the first embodiment (example, 0.25 ≤ z ≤ 1) is. Specifically, when a part of the conventional composition is replaced with calcium (Ca) and lanthanum (La), the substitution amounts are z = 0, z = 0.25, z = 0.5, z = 0.75, Samples were prepared with z=1, and XRD measurement was performed on each sample. Note that FIG. 4 shows an enlarged view of the vicinity of 2θ=23°.

The oxygen sensor element of the conventional example has a composition formula of LnBa ₂ Cu ₃ O _7-δ , and when the oxygen deficiency in the crystal structure is reduced, the tetragonal crystal (a=b≠c) changes to the orthorhombic crystal (a≠b≠c). ). FIG. 4 shows diffraction patterns in each state (both (100) and (010) planes are present in the orthorhombic crystal because a≠b). In the orthorhombic state, defects are likely to occur inside the crystal, and it is assumed that the gap between lattices is large.

As shown in FIG. 4, as the amount of substitution z, in which part of the conventional composition is substituted with calcium (Ca) and lanthanum (La), increases, the peak of the orthorhombic (010) plane decreases and the tetragonal It can be seen that the peak of the (100) plane increases. As the results of the appearance observation and XRD measurement described above show, the reason why the Ca and La substitution improves the moisture resistance is that the introduction of Ca and La with different ionic radii into the crystal structure causes the orthorhombic phase to change. It is presumed that this is due to the effect of suppressing metastasis.

FIG. 4 shows that the XRD measurement at room temperature confirmed the tetragonal diffraction pattern of the LnBa ₂ Cu ₃ O _7-δ composite perovskite structure. In addition, regarding the rare earth elements (Ln) used in the test samples according to the examples, it was confirmed that the humidity resistance of the sensor element was excellent in the order of yttrium (Y) > gadolinium (Gd) > Nd (neodymium).

<Evaluation results of sensor characteristics>
FIG. 5 shows the results of evaluating the oxygen responsiveness as an oxygen sensor for a test sample (z=0) having a conventional composition and a test sample (0.25≦z≦1) according to an example. there is Here, for each test sample, an environment of standard air (oxygen concentration 21%) is set in the period T1 of FIG. (oxygen concentration 21%) environment.

As shown in FIG. 5, the amount of change (response) in the sensor output of the test sample (z=0) of the conventional composition is 32%, and the test sample ( 23%, 22%, 19%, and 14% of change in sensor output (response) were obtained for each substitution amount (z = 0.25, z = 0.5, z = 0.75, z = 1). was taken.

In addition, since the rise and fall of the current change at each change point of the oxygen concentration of T1 → T2 → T3 are also steep, the oxygen responsiveness was evaluated between the test sample having the conventional composition and the test sample according to the example. It can be seen that there is no difference in

Thus, from the oxygen responsive evaluation results of the test samples according to the examples in which part of the conventional composition was replaced with Ca and La, the substitution amount z of Ca and La was in the entire range of 0 ≤ z ≤ 1, It became clear that good sensor characteristics (sensor output, response speed) can be obtained. It was also found that the amount of substitution had little effect on the response speed. From the evaluation results in FIG. 5, it can be seen that the amount of change in current tends to decrease as the amount of substitution z increases.

In the oxygen sensor element according to the first embodiment represented by the _composition formula Y1 _{- zCazBa2 - zLazCu3O7 -δ} described above, barium (Ba) is further replaced with strontium (Sr). Verification of the composition formed by substitution was performed. As a result, it was found that moisture resistance and sensor characteristics could be ensured even in the composition subjected to such substitution.

Next, an oxygen sensor element according to the first embodiment and a method for manufacturing an oxygen sensor using the same will be described. FIG. 6 is a flow chart showing in chronological order the manufacturing process of the oxygen sensor element according to the first embodiment and the oxygen sensor using the oxygen sensor element.

In step S1 of FIG. 6, raw materials for the oxygen sensor element are weighed and mixed. Here, Y ₂ O ₃ , La ₂ O ₃ , BaCO ₃ , CaCO ₃ and CuO, for example, as materials for the oxygen sensor element are weighed and mixed using an electronic balance or the like so as to obtain a predetermined composition.

As Ln (rare earth element) of the oxygen sensor element material, yttrium (Y) is exemplified here, but other single rare earth element or a mixture of multiple rare earth elements may be used. , any rare earth element can be used. Also, Ln ₂ BaCuO ₅ may be added to this mixture.

In step S2, the oxygen sensor element raw materials weighed and mixed in step S1 are pulverized by a ball mill. For pulverization, a solid-phase method such as a bead mill using beads as a pulverization medium, or a liquid-phase method can be used.

In subsequent step S3, the pulverized material (raw material powder) is heat-treated (calcined) at 900° C. for 5 hours in the air. Reactivity and particle size are adjusted by calcination. The calcination temperature may be 880 to 970°C, but is preferably 900 to 935°C.

Next, it shifts to a granulation step. Specifically, granulated powder is produced in step S4. Here, granulated powder is produced by adding an aqueous solution of a binder resin (for example, polyvinyl alcohol (PVA)) to the calcined mixture.

In the following step S5, a pressing pressure is applied to the granulated powder by, for example, a uniaxial pressing method to form a plate-like member (press-formed body) having a thickness of, for example, 300 μm. Forming can also be performed by hydrostatic pressing, hot pressing, doctor blade, printing, or thin film methods.

Dicing is performed in step S6. In dicing, the formed plate-like member is cut according to a predetermined product size and shape (for example, linear body shape of 0.3×0.3×7 mm). Since the smaller the diameter of the oxygen sensor element, the better the power saving, the product size may be any size other than the above.

In step S7, the oxygen sensor element after dicing is subjected to binder removal, and the oxygen sensor element is sintered in the atmosphere at, for example, 920° C. for 10 hours. The sintering temperature can be 900 to 1000° C., but since the optimum temperature differs depending on the composition, the sintering temperature may be changed depending on the composition. Annealing treatment may be performed after this.

In step S8, both ends of the oxygen sensor element are dip-coated with silver (Ag) and dried at 150° C. for 10 minutes to form electrodes. In step S9, for example, silver (Ag) wires with a diameter of 0.1 mm are attached to the electrodes formed in step S8 by a bonding method such as wire bonding, and dried at 150° C. for 10 minutes. The terminal electrodes thus formed are baked, for example, at 670° C. for 20 minutes in step S10.

Materials other than silver (Ag), such as gold (Au), platinum (Pt), nickel (Ni), tin (Sn), copper (Cu), resin electrodes, etc. good. For the dipping of the electrode, a film deposition method such as a printing method or sputtering may be used. Furthermore, as the final step in FIG. 6, the electrical characteristics of the oxygen sensor element manufactured through the above steps may be evaluated, for example, by a four-probe method.

<About the oxygen sensor>
In the oxygen sensor using the oxygen sensor element according to the first embodiment, the heat generating portion (hot spot) in the central portion of the oxygen sensor element serves as the oxygen concentration detection portion. For example, an oxygen sensor 1 shown in FIG. 7 has a structure in which an oxygen sensor element 5 is housed inside a cylindrical glass tube 4 made of heat-resistant glass that functions as a protective member for the oxygen sensor element. Metal conductive caps (bases) 2a and 2b made of copper (Cu), for example, are fitted to both ends of the glass tube 4 in order to electrically connect the oxygen sensor 1 to the outside.

The silver (Ag) wires attached to both ends of the oxygen sensor element 5 are electrically connected to the conductive caps 2a and 2b by lead-free solder, and the oxygen sensor element 5 is connected to the glass tube 4 so that the oxygen sensor element 5 does not come into contact with the glass tube 4. is arranged so that the longitudinal direction of the glass tube 4 is aligned with the axial direction of the glass tube 4 . In addition, the gas (oxygen) to be measured smoothly flows into the glass tube 4 through the ventilation holes 3a and 3b provided on the end faces of the conductive caps 2a and 2b, respectively, and the oxygen sensor element 5 is exposed to the gas. and can accurately measure the oxygen concentration in the atmosphere.

The outer dimensions (size) of the oxygen sensor 1 are, for example, the diameter of the glass tube is 5.2 mm, the length is 20 mm, and the diameter of the air hole is 2.5 mm. ) is replaceable through a vent hole in the glass tube.

The protective member for the oxygen sensor element 5 may be, for example, a ceramic case, a resin case, or the like, other than the glass tube described above. Also, the silver (Ag) wire attached to the oxygen sensor element 5 and the conductive caps 2a and 2b may be connected by a joining method such as leaded solder, welding, or caulking.

Although not shown, in the oxygen sensor using the oxygen sensor element according to the first embodiment, when a predetermined voltage is applied to the oxygen sensor from a power source, the oxygen sensor element generates a current corresponding to the ambient oxygen concentration. flow, the oxygen concentration of the atmosphere to be measured is measured based on the value of the current measured by an ammeter.

As described above, in the oxygen sensor element according to the first embodiment, a part of the conventional composition represented by the composition formula LnBa ₂ Cu ₃ O _7-δ is selected from the elements of Group 2 of the periodic table. A composition formula Ln _1-z Ca _z Ba _2-z La _z Cu ₃ O in which an element, such as calcium (Ca), is substituted with any one element selected from lanthanoid elements, such as lanthanum (La) It has a composition represented by _7-δ (Ln is a rare earth element, and the amount of substitution z is 0.25≦z≦1).

With such a composition, Ca and La with different ionic radii are introduced into the crystal structure to suppress the phase transition to the orthorhombic system, and the Ca and La substitution improves the moisture resistance of the sensor element. As a result, a highly durable and highly reliable oxygen sensor element can be provided without impairing sensor characteristics.

Further, in the first embodiment, an example was given in which a part of the conventional composition was replaced with Ca and La, but beryllium (Be) other than Ca, magnesium (Mg), calcium (Ca), barium (Ba), radium Even if any element selected from other elements of Group 2 of the periodic table such as (Ra) is substituted with any element selected from other elements of the lanthanide system other than La, Ca, La substitution It can be assumed that the same effect as in the case of

[Second embodiment]
The oxygen sensor element according to the second embodiment of the present invention has a composition obtained by substituting the Ba site of the conventional composition LnBa ₂ Cu ₃ O _7-δ described above. Specifically, _{a composition Ln 1+x Ba 2-x Cu 3 O 7 - δ ( 0} <x≤1.2).

Therefore, results of comparative verification between a sample made of the oxygen sensor element material according to the second embodiment and a sample made of a conventional sensor element material will be described. In the following, Nd _1+x Ba _2-x Cu ₃ O _7-δ in which Ln is Nd will be described as an example _. It may be Gd _0.6 Cu ₃ O _7-δ .

<Results of moisture resistance test>
A disk-shaped oxygen sensor element having a diameter of about 16 mm and a thickness of about 2 mm was produced, and a moisture resistance test was performed by leaving the sample in an atmosphere of 40° C. and 93% RH for a predetermined period of time.

FIG. 8 shows the substitution amount x of Nd in the composition Nd _1+x Ba _2-x Cu ₃ O _7-δ of the oxygen sensor element according to the second embodiment at x=0, 0.2, 0.3, As 0.4, 0.6, 0.8, 1.0, 1.2, the appearance observation results of each sample when changing the standing time (0 hours to 500 hours) in the above environment (appearance over time It is a photograph showing a change).

As shown in FIG. 8, in the conventional sample without Nd substitution (x=0), a white precipitate formed on the surface of the oxygen sensor element within 50 hours. In addition, in the samples with x = 0.4 or more, the appearance did not change even after 500 hours, indicating that the moisture resistance of the oxygen sensor element samples with the above composition improved as the Nd substitution amount x increased. I understand. In particular, it was found that when 0.4 ≤ x ≤ 1.2, there is moisture resistance for 500 hours or longer.

FIG. 9 shows the results of X-ray diffraction (XRD) measurement of oxygen sensor elements with different amounts of Nd substituted as described above. Also, Table 2 shows the determination results of element deterioration by XRD measurement.

In Table 2, the x mark indicates that the sensor element sample has deteriorated, and the o mark indicates that the element sample has almost no deterioration. Further, as a result of the XRD measurement shown in FIG. 9, the crystal structure of the NdBa ₂ Cu ₃ O _7-δ (Nd-123) phase was confirmed as the main phase in all the samples. In other words, it was found that each sample had almost the same crystal structure as the conventional sample even when the substitution amount x was increased.

Moreover, precipitation of Nd ₂ CuO ₄ was confirmed in the samples with x=0.8 or more (indicated by ▼ in the figure). Therefore, the solid solubility limit of Nd in Nd _1+x Ba _2-x Cu ₃ O _7-δ is considered to be x=0.6 to 0.8. In the sample, the difference in the number of peaks from the conventional example suggests that the crystal structure changed from an orthorhombic system to a tetragonal system due to Nd substitution.

Although not shown, peaks of BaCO ₃ and CuO were confirmed in the samples with x=0 to 0.2 subjected to the moisture resistance test for 50 hours. On the other hand, in the samples with x=0.4 or more, the peak did not change even after 500 hours.

<Evaluation results of fusing resistance>
FIG. 10 shows the fusing resistance characteristics summarizing the power until fusing of each sample, where the horizontal axis represents the replacement amount x and the vertical axis represents the fusing power. As shown in FIG. 10, it can be seen that Nd replacement improves the fusing power in the range of 0.2≦x≦1.0.

<Change in crystal structure>
NdBa ₂ Cu ₃ O _7-δ (Nd-123), which is the basic composition, and NdBaSrCu ₃ O ₇ with Sr substitution show a change in crystal structure between 400°C and 950°C, with a peak around 2θ = 47°. splits, or the intensity ratio of split peaks changes.

On the other hand, in the oxygen sensor element according to the second embodiment, as a result of high-temperature XRD measurement of the sintered body with the substitution amount x = 0.6, from room temperature (25 ° C.) to 950 ° C., a peak near 2θ = 47 ° was always constant, and no change was observed in the crystal structure. The fact that the Nd-substituted sample has a stable crystal structure up to high temperatures is considered to be one of the reasons for the improvement in the above-described fusion resistance.

FIG. 11 shows the portion A ((006), (020), (200) plane peaks (around 2θ=46°)) surrounded by a dotted line in FIG. In the range of 2≦x≦0.8, the lattice constant decreased as the amount of substitution x increased. It is considered that this is because Nd has a smaller ionic radius than Ba, so that Nd is in a substitution solid solution.

From FIG. 11, when x=0, the peaks of the (020) plane and the (200) plane are split, and the crystal structure is orthorhombic (a≠b). When 0.2≦x≦0.8, the peaks of the (020) plane and the (200) plane overlap, and it is considered that the crystal structure has changed to a tetragonal system (a=b). Since this tendency also appears in Sr substitution, it is considered that the crystal structure is changed by substituting Nd ions having a small ionic radius for the Ba sites. For this reason, it is considered that the same effect can be obtained even with rare earth elements other than Nd having an ionic radius smaller than that of Ba.

Moreover, in the range of 0.8≦x≦1.2, the peaks of the (020) plane and the (200) plane, which overlapped in the range of 0.2≦x≦0.8 as described above, split again. This suggests the possibility that the addition of Nd in excess of the solid solubility limit causes Nd to form a solid solution at another site.

<Sensor characteristics>
FIG. 12 shows the measurement results of the oxygen responsiveness of the oxygen sensor element having the composition formula of Nd _1+x Ba _2-x Cu ₃ O _7-δ when the amount of Nd substitution is changed as described above. Here, as in the first embodiment, for each test sample, the period T1 is switched to the environment of standard air (oxygen concentration 21%), the subsequent period T2 is switched to the environment of oxygen concentration 1%, and the next period T3 was again switched to the environment of standard air (oxygen concentration 21%) and the oxygen response was measured.

As shown in FIG. 12, the current change of about 30% occurred in all the samples subjected to Nd substitution.

In addition, since the rise and fall of the current change at each change point of the oxygen concentration of T1 → T2 → T3 are steep, there is no difference in oxygen responsiveness between the sample with the conventional composition and the sample according to the example. I understand.

In addition, it was found that the sample (x=0.6) according to the example can reduce the current value by about 30% compared to the conventional example. This is probably because the hole concentration decreased and the resistance increased by substituting Nd ³⁺ for Ba ²⁺ . From this, the sample according to the second embodiment can suppress an increase in power consumption (current) even if the wire diameter is made larger than that of the conventional example to improve the mechanical strength. As a result, the increased resistance value can increase the degree of freedom in designing the wire diameter and power consumption.

<Firing temperature>
Next, the firing temperature of the oxygen sensor element according to the second embodiment will be explained. In the samples with substitution amounts of x=0, 0.2, 1.0, and 1.2, pellet shrinkage was large as shown in FIG. 8, and sintering was easy. FIG. 13 shows the result of differential thermal analysis (DTA) measurement to investigate the cause.

In FIG. 13, in the samples (calcined powder) with x=0, 1.0, and 1.2, endothermic peaks are observed at locations indicated by downward arrows. The peak near 820° C. at x=0 is considered to be due to decarboxylation of BaCO ₃ remaining during calcination (synthesis of Gd-123 phase). Similarly, the peak near 970° C. at x=0 is considered to be the melting peak of the phase containing BaO—CuO as the main component.

The endothermic peaks at x=1.0 and 1.2 are considered to be the melting peaks of the phase caused by Nd--Cu--O from the results of FIG. Therefore, the reason why the samples with x=0, 0.2, 1.0 and 1.2 progressed in sintering is considered to be the formation of a liquid phase during firing.

14(a) to (c) show the results of SEM observation of the fracture surface of each sample fired at 980° C. for 10 hours with the Nd substitution amount x=0.4, 0.6, 0.8. It is a photograph. All SEM photographs are backscattered electron images at a magnification of 1000. In addition, in FIG. 13, since no endothermic peak is observed in the samples with x=0.4, 0.6, and 0.8, it is possible to obtain a stable porous structure by firing at a higher temperature than before. rice field.

Since Gd-123, which has a conventional composition, is easily sintered, it needs to be fired at a relatively low temperature such as 920° C. for 10 hours. However, since the temperature of the hot spot generating portion in the oxygen sensor element is considered to be around 950° C., the progress of sintering during operation of the oxygen sensor becomes a problem. By controlling the amount of Nd substitution, it is possible to suppress the formation of liquid phase, which enables firing at high temperatures.

Therefore, it is possible to improve the output stability of the sensor element when the oxygen sensor is in operation (when a hot spot occurs). In addition, since the liquid phase is less generated, deformation during sintering is suppressed, and variation in shape during sintering can be reduced.

Note that the oxygen sensor element according to the second embodiment and the method of manufacturing the oxygen sensor using the same are the same as the manufacturing process according to the first embodiment shown in FIG. 6, so description thereof will be omitted. However, in the second embodiment, in step S1 of FIG. 6, materials for the oxygen sensor element, such as Nd ₂ O ₃ , BaCO ₃ , and CuO, are weighed using an electronic balance or the like so as to have a predetermined composition. and mix.

The configuration of the oxygen sensor using the oxygen sensor element according to the second embodiment is also the same as the configuration shown in FIG. 7, so illustration and description are omitted here.

As described above, the oxygen sensor element according to the second embodiment has a composition Ln _1+x in which the Ln/Ba ratio is controlled in the conventional composition represented by LnBa ₂ Cu ₃ O _7-δ (Ln is a rare earth element). _Nd 1+x Ba _2-x Cu ₃ O having Ba 2-x Cu 3 O 7 _{- δ} (0<x≦1.2) and substituted with neodymium ₍ Nd) selected from rare earth elements, for example It has a composition represented by _7-δ .

In this way, by replacing Ba sites with Nd ions having a small ionic radius, not only moisture resistance and durability, but also heat resistance (fusing resistance) and sinterability (deformation resistance) can be improved. In addition, since the composition is a three-component system, it is possible to suppress variations in manufacturing of the sensor element, thereby providing an oxygen sensor that is easy to manufacture.

Furthermore, since the resistivity of the element can be controlled according to the substitution amount x of Nd, it becomes possible to control the current value during the operation of the sensor, adjust the element wire diameter, and the like.

Since the current density in the sensor element is inversely proportional to the square root of the wire diameter of the element, the higher the current density, the greater the load on the sensor element. Therefore, it is considered that the thicker the wire diameter, the lower the current density, leading to a longer life of the device. In the element of the conventional composition, if the element diameter is increased, the current value (power consumption) during operation increases, so the wire diameter had to be reduced.

On the other hand, in the oxygen sensor element (composition: Nd _1+x Ba _2-x Cu ₃ O _7-δ ) according to the second embodiment, even if the wire diameter is increased by controlling the resistivity of the element, The current value can be suppressed to some extent. In addition, it is considered that heat insulation can be ensured by increasing the element diameter (reducing the specific surface area). Furthermore, an increase in mechanical strength due to an increase in wire diameter and an improvement in handling in mass production processes can be expected.

In addition, in the second embodiment, an example in which part of the conventional composition is substituted with Nd was given, but even if it is substituted with other rare earth elements other than Nd, or if it is substituted with a combination of different rare earth elements, , and Nd substitution.

1 Oxygen sensors 2a, 2b Conductive caps 3a, 3b Vent hole 4 Glass tube 5 Oxygen sensor element

Claims (8)

An oxygen sensor element made of a ceramic sintered body that detects oxygen concentration based on a current value when a voltage is applied,
In the ceramic sintered body, part of Ba in a composition formula LnBa ₂ Cu ₃ O _7-δ (Ln is a rare earth element and δ represents a non-stoichiometric amount of oxygen) is replaced with the same rare earth element as the rare earth element Ln in the composition formula. represented by the composition formula Ln _1+x Ba _2-x Cu ₃ O _7-δ (the amount of substitution x is 0<x≦1.2) substituted with the element Ln or a rare earth element Ln different from the rare earth element Ln in the composition formula An oxygen sensor element characterized by having a composition of 2. The oxygen sensor element according to claim 1 , wherein the substitution amount x satisfies 0.4≤x≤0.8. 3. The oxygen sensor element according to claim 1 , wherein neodymium (Nd) is selected from said rare earth element Ln. A composition represented by the composition formula Ln ₂ BaCuO ₅ (Ln is a rare earth element) is mixed with the composition represented by the composition formula Ln _1+x Ba _2-x Cu ₃ O _7-δ . Item 3. The oxygen sensor element according to Item 1 or 2 . 5. The oxygen sensor element according to claim 1, wherein the composition represented by the compositional formula Ln _1+x Ba _2-x Cu ₃ O _7-δ has a composite perovskite structure. . The oxygen sensor element according to any one of claims 1 to 5 , wherein the ceramic sintered body is a linear sensor element. An oxygen sensor comprising the oxygen sensor element according to any one of claims 1 to 6 as an oxygen concentration detection element. 8. The oxygen sensor according to claim 7 , wherein said oxygen sensor element is housed in a protective tube having vent holes at both ends.

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