JP2002357580A - Humidity sensor and method for manufacturing sensor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a humidity sensor for effectively preventing poisoning due to lead or the like of an electrode and a humidity sensitive layer, even an exhaust gas of an automobile ort the like and superior in durability or the like having stable humidity sensitive characteristic over a long period, and to provide a method for manufacturing a sensor element. SOLUTION: The humidity sensor S is provided with a poisoning prevention layer 6, where the circumference of coarse particle powder comprises composite powder covered by particulate powder an voids without filling a gap between the composite powders with the particulate powder disperse and exist. In addition, the coarse particle powder and the particulate powder comprise ceramic powder, and are particularly preferably titania powder, having a peak at 1 μm or shorter and the powder of the composite oxide containing aluminum atoms, such as spinel having a peak at 10 μm or longer. The poisoning preventing layer can be formed by applying slurry containing a solvent, such as ceramic powder different in a specific surface, an organic binder and methanol on the surface of a protective layer 5 to be dried.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a humidity sensor for detecting the content of water vapor in an atmosphere to be measured by a change in electric resistance of a moisture-sensitive layer, and a method of manufacturing a sensor element.

[0002]

2. Description of the Related Art When a humidity sensor having a sensor element is exposed to exhaust gas as a humidity sensor for measuring moisture in the exhaust gas, the electrodes and the moisture-sensitive layer are exposed to poisonous substances such as lead, phosphorus and silicon. It poisons and deteriorates with time, and the electrical resistance of the moisture-sensitive layer changes significantly. As a result of studying to solve such problems, we used a humidity sensor with a protective layer made of a complex oxide such as spinel to use it as a humidity sensor to measure the moisture in the exhaust gas of automobiles. In this case, the initial characteristics were good. However, when these sensors are exposed to exhaust gas from automobiles or the like for a long time, there is a problem that the influence of poisoning substances cannot be sufficiently prevented.

[0003]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a sensor element having excellent durability, in which poisoning of an electrode and a moisture-sensitive layer by a poisoning substance is prevented for a long time. An object of the present invention is to provide a method for manufacturing a humidity sensor and a sensor element provided.

[0004]

The humidity sensor of the present invention comprises:
A humidity sensor comprising: a moisture-sensitive layer, a pair of electrodes formed in contact with the moisture-sensitive layer, a protective layer, and a sensor element having a poisoning-preventive layer formed on the surface of the protective layer. Is composed of a composite powder in which a ceramic powder having a large particle diameter (hereinafter, also referred to as a coarse powder) is covered with a small ceramic powder (hereinafter, also referred to as a fine powder). Are arranged so that pores not filled with are dispersed.

By forming the poisoning prevention layer as described above, the poisoning substance is trapped by the fine powder and does not reach the humidity sensor electrode and the moisture-sensitive layer. can do. On the other hand, since the fine powder is carried by the coarse powder, the poisoning prevention layer is hardened by continuous use at a high temperature and peels off from the sensor element surface as in the case of the poisoning prevention layer composed of only the fine powder. Problem can be prevented. Further, the fine powder is supported so as to cover the surface of the coarse powder, but pores having a size approximately equal to that of the coarse powder are formed between the composite powders. Since the gap is not completely filled, the poisoning prevention layer does not become clogged even if the poisoning substance is deposited, and a decrease in the response of the sensor can be prevented.

[0006] The particle size distribution of the primary particles of the ceramic powder constituting the poisoning prevention layer as described above has at least two peaks, the peak with the smallest particle size is 10 µm or less, and the particle size is the smallest. When the peak on the larger side is 0.1 μm or more, a highly effective poisoning prevention effect can be obtained as a desirable poisoning prevention layer. Here, the peak having the smallest particle size is preferably 1 μm or less, and 0.2 μm or less.
μm or less, particularly 0.05 μm or less.
The peak having the largest particle size is preferably 1 μm or more, particularly preferably 10 μm or more. When the protective layer, which is the base of the poisoning prevention layer, is formed by screen printing or thermal spraying, it is desirable that the coarse particles bite well.

The "ceramic powder" contained in the "poisoning prevention layer" includes high-temperature exhaust gases such as titania, alumina, silica, and complex oxides containing aluminum atoms such as spinel and mullite. It is preferred to choose from oxide powders that are chemically stable. However,
Powders other than oxides can be used as long as they are chemically stable. In this case, two or more types of ceramic powders having different compositions may be mixed. When the ceramic powder of one composition is a fine powder and the ceramic powder of the other composition is a coarse powder, the degree of freedom in selecting the powder is widened, and it becomes easy to prepare a powder having a desired particle size distribution. This is convenient because ceramic powder having a high poisoning prevention effect can be used as fine powder, and ceramic powder having high durability at high temperatures can be used as coarse powder.

The two or more types of ceramic powders having different compositions preferably contain a titania powder having a particle size distribution peak at 1 μm or less and a ceramic powder other than titania having a particle size distribution peak at 10 μm or more. . Titania is considered to have excellent ability to adsorb poisonous substances. In particular, anatase-type titania is easy to obtain a powder having a small particle size, and has a high poisoning prevention effect. As the ceramic powder other than titania, a ceramic powder that is not easily thermally shrunk, such as a composite oxide containing an aluminum atom such as spinel or mullite, is particularly preferable.

The titania powder is 0.003 to 0.5.
It is particularly preferable that the ceramic powder other than titania has a peak at 15 μm and is combined so as to have a peak at 15 to 50 μm, since an appropriate gap is formed in the poisoning prevention layer. When such a powder is contained, the poisoning substance is sufficiently adsorbed, and the poisoning prevention layer does not peel off from the protective layer due to heat shrinkage, and has better durability with less deterioration of poisoning resistance. It can be a poisoning prevention layer having.

That is, 1 μm or less, preferably 0.003
When a powder having a small particle size having a peak of particle size distribution at ~ 0.5 μm and a powder having a large particle size having a peak of particle size distribution of 10 μm or more, preferably 15 to 50 μm, the poisoning prevention layer is As shown in FIGS. 1 (a) and 1 (b), a powder composed of composite particles having a large number of particles having a small particle diameter adhered to the particle surface of a powder having a large particle diameter has a size of a moderately coarse powder. Since the shape is formed in a state in which the pores are formed, the air permeability is sufficiently maintained, and the poisoning substance is surely adsorbed, so that a highly durable poisoning prevention layer can be obtained.

As the coarse powder and the fine powder, powders having the same composition but different crystal phases can be selected. In particular, it is preferable to use an anatase titania powder as the fine powder and to use a rutile titania powder as the coarse powder. All of these powders are titania powders but have different crystal phases, and are provided as fine powder or coarse powder having a narrow particle size distribution, so that the poisoning prevention layer having good air permeability is provided. Suitable for forming

The particle size of the anatase type titania powder is preferably such that the peak of the particle size distribution is 0.5 μm or less,
It is more preferably in the range of 0.003 to 0.5 μm from the viewpoint of the poisoning prevention effect. The particle size of the rutile titania powder is preferably such that the peak of the particle size distribution is 1 μm or more, and more preferably in the range of 3 to 8 μm in view of the effect of preventing poisoning. Anatase titania powder having an extremely small particle size of about 0.003 to 0.5 μm,
By combining with rutile-type titania powder having a larger particle diameter than this, a poisoning prevention layer having an excellent action of trapping poisonous substances can be obtained. Further, by using ceramic powders having the same composition, formation of composite particles becomes easy, and a poisoning prevention layer having a high poisoning prevention effect can be formed.

When evaluating the particle size distribution of the anti-poisoning layer of a product, one of the factors is to determine the particle size in the field of view of an electron microscope.
Alternatively, it can be read from a photograph taken. When the particle diameter is read from the visual field of the electron microscope, the circumscribed circle diameter of each of the primary particles that can be visually confirmed is measured to determine the particle diameter. Many measurements of the above particle size (100
(About 0) of the primary particles, and the particle size distribution is calculated. When oxide powders having different compositions are used, for the oxide powders of the respective compositions, the particle diameter can be measured, and the particle size distribution can be measured. When measuring the particle size in a state where the powders are mixed, without measuring the particle size distribution for each ceramic powder having a different composition, if the particle size distribution is measured using the particle size randomly sampled from the poisoning prevention layer, Good. As a result, the peak having the smallest particle diameter should be 10 μm or less, and the peak having the largest particle diameter should be 0.1 μm or more.

On the other hand, the particle size distribution of the fine powder may be difficult to measure with a general scanning electron microscope or the like. In this case, the measurement can be performed in the same manner as described above by using a high-resolution electron microscope. It can also be calculated from the Scheller equation using the small-angle X-ray scattering method for measuring the particle size distribution of the powder. The particle size distribution can also be measured by a commonly used method such as a laser beam diffraction method or a centrifugal sedimentation method. However, it is often difficult to measure the particle size distribution of the same sample from the fine region to the coarse region by the same measurement method. In that case, the particle size distribution of the poisoning prevention layer may be identified from the respective particle size distributions by measuring the particle size distribution of the fine region and the coarse region by another measuring method.

The method for manufacturing the sensor element of the humidity sensor according to the present invention includes: (1) one or more types of first ceramic powder; and (2) primary particles of the first ceramic powder having a particle size distribution peak. The maximum particle size (hereinafter, referred to as 10% particle size or d10) of 10% of the particles having a smaller particle size than the peak of the particle size distribution and 90% of the particles having a smaller particle size is smaller than the peak of the particle size distribution. The difference in the maximum particle size (hereinafter, referred to as 90% particle size or d90) of the particles of the one or more second particles having a particle size distribution of not more than twice the particle size of the peak value of the particle size distribution.
A ceramic powder, (3) an organic binder, and (4) a solvent or water are kneaded to prepare a paste for forming a poisoning prevention layer, and the paste for forming a poisoning prevention layer is used as a protective layer for a humidity sensor element. A method can be used in which a coating film is formed by coating the surface, and then the coating film is heated and dried to form the poisoning prevention layer.

According to this manufacturing method, a powder having a uniform particle size near the peak of the particle size distribution is used as the second ceramic powder which is a coarse particle powder in the poisoning prevention layer. It is easy to form a poisoning prevention layer in which pores having the size of a granular powder are dispersed. In addition, by mixing an inorganic binder into the paste for forming a poisoning prevention layer as appropriate, the fine powder adheres to the surface of the coarse powder, so that a good poisoning prevention layer can be formed. Further, it is desirable that both the first ceramic powder to be fine powder and the second ceramic powder to be coarse powder are oxides having high heat resistance.

[0017] In particular, the specific surface area as the first ceramic powder is used titania powder or the like is 2~500m ² / g, a specific surface area as the second ceramic powder 0.1 to 100 m ²
/ G of a composite oxide containing an aluminum atom. Furthermore, an anatase type titania powder having a specific surface area of ² to 500 m ² / g as the first ceramic powder, and a specific surface area of 0.2 as the second ceramic powder.
A poisoning prevention layer can be formed in the same manner by using rutile type titania powder of 1 to 10 m ² / g.

The first ceramic powder has a specific surface area of 2 to 50.
A 0 m ² / g, it is particularly preferably 5 to 300 m ² / g. When the specific surface area is less than 2 m ² / g, both the physical capture and reaction of the poisoning substance decrease, and
If it exceeds m ² / g, the powder is liable to agglomerate, the reaction activity with the poisoning substance becomes too high, and the obtained humidity sensor gradually deteriorates the poisoning resistance under a high temperature environment, which is not preferable. .

Meanwhile, the specific surface area of the second ceramic powder is 0.1 to 100 m ² / g, especially 0.3～10M ² /
g is preferable. This specific surface area is 0.1 m ² /
If it is less than g, a uniform poisoning preventing layer having a smooth surface cannot be formed, and if it exceeds 100 m ² / g, aggregation of the poisoning preventing layer cannot be sufficiently suppressed.
Further, when the specific surface area of the second ceramic powder is in the above range, the pores are dispersedly formed in the gaps of the composite powder, so that the poisoning prevention layer having good air permeability can be obtained. The specific surface area can be measured by the BET method. Also,
When the specific surface area of the powder is particularly large, a fully automatic surface area measuring device manufactured by Yuasa Ionics, Inc.
2 ".

The first ceramic powder and the second ceramic powder are each 15 parts by weight when the poisoning prevention layer forming paste is 100 parts by mass (hereinafter simply referred to as “parts”).
Parts or more. Either one, especially the first
If the amount of the ceramic powder is less than 15 parts, the poisoning substance cannot be sufficiently captured. On the other hand, if the amount of the second ceramic powder is less than 15 parts, pores are not appropriately formed between the composite powders in the poisoning prevention layer, and air permeability cannot be maintained. In order to form appropriate pores in the poisoning prevention layer, 20 to 50
More preferably, it is contained. It should be noted that the ceramic powder may be mixed with other ceramic powders other than the respective powders which are the main constituent components of the present invention, but the particle size distribution of the entire ceramic powder deviates from the gist of the present invention. Mixing is undesirable.

The mixing ratio of the first ceramic powder and the second ceramic powder is not particularly limited, but when one of them is 100 parts, the other is 40 to 250 parts, especially 80 to 130 parts. It is preferable that the amount be about the same. If there is no large difference in the amount ratio of these powders, the effect of capturing the poisoning substance is excellent, and the poisoning prevention layer in which the holes are appropriately dispersed can be formed more efficiently.

The above "paste for forming a poisoning prevention layer" can be obtained by mixing ceramic powder, an organic binder, a solvent such as methanol and xylene or water, and an inorganic binder as appropriate. The coating film formed on the surface of the protective layer is dried at 50 to 150 ° C. for about 5 to 60 minutes,
After solidification and hardening and drying, the sensor element is
In an asy furnace or the like whose temperature has been adjusted to about 00 to 700 ° C, particularly about 400 to 600 ° C, in a reducing atmosphere or in the air, 2
By heating for about 0 to 60 minutes, a sensor element having a poisoning prevention layer having a predetermined poisoning prevention action and thickness can be obtained.

The thickness of the anti-poisoning layer is preferably 50 to 500 μm, particularly preferably about 100 to 300 μm. If the thickness is too small, the poisoning substance may not be sufficiently captured. On the other hand, if it exceeds 500 μm, the responsiveness of the obtained humidity sensor decreases, and furthermore, the poisoning prevention layer tends to be easily peeled off from the protective layer, which is not preferable.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described specifically with reference to examples. (1) Production of Humidity Sensor A slurry containing alumina powder was prepared, and an alumina green sheet having a thickness of 400 μm was formed by a doctor blade method. Thereafter, an output extraction lead pattern, a heater pattern, and a resistance temperature detector pattern were formed on each of the alumina green sheets by screen printing. Next, a predetermined surface of each of these alumina green sheets is brought into contact with each other and pressed to form a laminate.
For 2 hours, followed by firing to produce an insulating substrate 1.

Then, a platinum paste is printed on the surface of the insulating substrate, dried at 120 ° C. for 15 minutes, and then dried at 1200 ° C.
For 10 minutes, and baked to form a lower electrode 2 having a thickness of 15 μm.
Was formed. Next, a paste containing a mixed powder in which Al ₂ O ₃ powder, TiO ₂ powder and SnO ₂ powder are blended at a predetermined mass ratio is printed on the surface of the lower electrode 2, dried at 60 ° C. for 1 hour, and dried at 1200 ° C. Hold for 2 hours and bake to a thickness of 20
A moisture-sensitive layer 3 having a thickness of ３０30 μm was formed. Thereafter, a platinum paste is printed on the surface of the moisture-sensitive layer 3, dried at 120 ° C. for 15 minutes, held at 1200 ° C. for 10 minutes, and baked to a thickness of 15 μm.
m of upper electrodes 4 were formed. Next, a paste made of spinel (MgAl ₂ O ₄ ) is printed on the surface of the upper electrode,
After drying at 60 ° C. for 1 hour, it was kept at 1200 ° C. for 2 hours and fired to form a protective layer 5 having a thickness of 30 μm.

Thereafter, a powder and a powder, and a predetermined amount of a solvent and an alumina sol having the types and ratios shown in Tables 1 and 2 were mixed by a pot mill using a nylon ball to prepare a slurry. In addition, 100% by mass of the slurry
In Experimental Examples 1 to 9, the total amount with powder was 70% by mass, methanol containing an organic binder was 23% by mass, and alumina sol was 7% by mass.
Then, the total amount with the powder was 50% by mass, water was 40% by mass, and alumina sol was 10% by mass.
Then, the total amount with the powder was 56% by mass, water was 35% by mass, and alumina sol was 9% by mass. Thereafter, the substrate on which the protective layer is formed is immersed in a slurry, a coating film is formed on the surface of the protective layer, and dried at 120 ° C. to a thickness of 50 to 300 μm.
m (preferably 150 to 250 μm) of the poisoning prevention layer 6 was formed to produce a sensor element. Next, after assembling the sensor element to a protective tube socket, the sensor was heated at 500 ° C. to obtain a humidity sensor S.

[0027]

[Table 1]

[0028]

[Table 2]

(2) Performance Evaluation of Humidity Sensor Appearance of Poisoning Prevention Layer The appearance of the poisoning prevention layer of the humidity sensor obtained in (1) was visually observed. The evaluation criteria are as follows: ；: no cracks or the like are observed, △: some cracks are generated, x: cracks are generated in all of them. Poisoning resistance (durability) An 1800 cc engine was used, and the durability pattern was based on a life cycle pattern. As the fuel, leaded gasoline containing 0.4 g of lead per liter was used.
The sensor mounting position is closer to the engine, 500-80
A position through which a high-temperature exhaust gas of 0 ° C passes and a position through which a low-temperature exhaust gas of 350 to 700 ° C pass away from the engine. After performing the durability test for 100 hours in this manner, the lead durability performance of each humidity sensor was evaluated by the flow splitting method using the apparatus shown in the schematic diagram of FIG. Table 1 shows the results
And 2 together. The evaluation criteria were as follows: ；: moisture sensitivity was hardly degraded, Δ: moisture sensitivity was slightly degraded (difference in resistance at 40% humidity before and after the endurance test was 10% or less), x: humidity sensitivity The characteristics are deteriorated (the difference between the resistance values before and after the durability test at a humidity of 40% exceeds 10%).

According to the results in Table 1, Experimental Examples 3 to 8 and Experimental Example 1 in which the powder to be the fine powder and the powder to be the coarse powder are within the preferred range of the invention described in claim 14.
In Nos. 1 to 14, cracks and the like were hardly observed on the surface of the poisoning prevention layer, and the poisoning resistance was excellent. Further, it was observed that pores having a size of the order of coarse powder were scattered inside the poisoning prevention layer. In Experimental Example 1 in which the peak values of the powders were all less than 0.1 μm, the viscosity of the slurry was too high, and a coating film serving as a poisoning prevention layer could not be formed.

Furthermore, in Experimental Example 2 where no powder was contained, the durability was greatly deteriorated, and in Experimental Examples 9 and 15 where no powder was contained, cracks were generated in all of them, making them impractical. . In Experimental Examples 3 and 11 in which the amount ratio of the powder was low, although the appearance was good, the poisoning prevention layer was formed in a state where the surface of the coarse powder was not sufficiently covered with the fine powder, and the durability was high. Tend to deteriorate. In Experimental Example 8 in which the amount ratio of the powder was high, cracks were observed on the surface of the poisoning prevention layer in some products. However, good performance was shown even after durability for the one without cracks.

According to the results shown in Table 2, although the peak value of the particle size distribution of the powder, which is a fine powder, is larger than that in Table 1, the results are within the preferred range of the present invention.
22 and Experimental Examples 25 to 29, a sensor element in which no crack or the like was observed on the surface of the poisoning prevention layer could be manufactured. Experimental examples 20 to 22 and 27 to 29 in which the ratio of the powder to the powder was within a desirable range also exhibited excellent poisoning resistance. The peak values of the powders were all 10
In Experimental Example 16 exceeding μm, the particles were too large, and the durability was greatly deteriorated. Furthermore, even in Experimental Examples 17 and 24 containing no powder, the durability was greatly deteriorated, and in 23 and 30 containing no powder, cracks occurred in all of them, making them impractical. In Experimental Examples 18, 19, 25, and 26 in which the amount ratio of the powder was low, the poisoning prevention layer was formed in a state where the surface of the coarse powder was not sufficiently covered with the fine powder, although the appearance was good. And the durability tended to deteriorate.

Further, Experimental Example 3 in which the powder has a relatively wide particle size distribution outside the desirable range of the present invention.
In Nos. 1 to 32, the pores are not appropriately formed in the poisoning prevention layer, so that cracks do not occur at the time of manufacture, but the poisoning resistance is poor, and the poisoning substance tends to deteriorate the humidity sensitivity of the sensor. Was.

Example 1 As a raw material, an anatase type titania powder having a specific surface area of 10 m ² / g and a particle size distribution peak at 0.2 μm was used.
g, specific surface area 0.5 m ² / g, peak of particle size distribution is 34
20 g of spinel powder, 28 g of water, and 3 g of alumina sol having a diameter of μm were stirred by a pot mill using nylon balls for 2 hours and mixed to prepare a paste. Thereafter, the sensor element having the protective layer prepared in (1) is immersed in this paste, about 20 mg of the paste is applied to the surface of the protective layer, and dried at 120 ° C. for 10 minutes to obtain a thickness of 150 to 150 μm. A 250 μm anti-poisoning layer was formed to produce a sensor element. Next, after assembling to a protective tube socket, heating was performed at 500 ° C. for 30 minutes to obtain a humidity sensor.

The surface of the anti-poisoning layer thus formed was smooth and no cracks were observed.
The anti-poisoning layer is formed in a state where the fine-grained powder sufficiently covers the surface of the coarse-grained powder, and pores of the same size as the coarse-grained powder are dispersed inside the poisoning-preventing layer. Was observed to be present. Further, as a result of evaluation in the same manner as in (2), it was confirmed that the poisoning resistance was also extremely excellent.

Further, a humidity sensor (embodiment) incorporating the sensor element having the poisoning prevention layer and an oxide powder containing no fine powder (Experimental Example 17 in Table 2) were used to form the poisoning prevention layer. The humidity sensor (comparative product) in which the formed sensor element was incorporated was exposed to an exhaust gas generated from a fuel to which a predetermined amount of silicon was added for 12 hours, and then the moisture sensitivity was evaluated by a split flow method. As a result, as shown in FIG. 2, it was found that the moisture sensitivity was considerably deteriorated with time in the comparative product, whereas the moisture sensitivity was less deteriorated in the actual product. Also,
According to the X-ray powder diffraction pattern of the anti-poisoning layer, crystal phases of anatase-type titania, spinel and alumina were recognized.

Example 2 As raw materials, 22.5 g of anatase type titania powder having a specific surface area of 500 m ² / g and a particle size distribution peak of 0.007 μm, a specific surface area of 0.7 m ² / g and a particle size distribution peak of 7 μm 22.5 g of rutile type titania powder in
After preparing a paste in the same manner as in Example 1 except that 35 ml of methanol and 2.8 g of alumina sol were used, the sensor element having the protective layer prepared in (1) was immersed in this paste, 20 mg of paste was applied on the surface of the protective layer, and dried at 120 ° C. for 10 minutes to form a poisoning prevention layer having a thickness of 150 to 250 μm, thereby producing a sensor element. Next, after assembling to a protective tube socket, heating was performed at 500 ° C. for 30 minutes to obtain a humidity sensor.

The surface of the anti-poisoning layer thus formed was smooth and no cracks or the like were observed.
The anti-poisoning layer is formed in a state where the fine-grained powder sufficiently covers the surface of the coarse-grained powder, and pores of the same size as the coarse-grained powder are dispersed inside the poisoning-preventing layer. Was observed to be present. In addition, as a result of evaluation in the same manner as in (2), it was confirmed that although some cracks were generated due to thermal shrinkage, they had durability that could be put to practical use.

Further, a poisoning prevention layer is formed using a humidity sensor (embodiment) incorporating the sensor element having the poisoning prevention layer and a titania powder containing no fine powder (Experimental example 2 in Table 1). The humidity sensor (comparative product) incorporating the sensor element thus obtained was exposed to an exhaust gas generated from a fuel to which a predetermined amount of silicon was added for 12 hours, and then the humidity sensitivity was evaluated by a split flow method. As a result, as shown in FIG. 2, the resistance of the comparative product increases as the time of exposure to the exhaust gas increases,
It was found that while the moisture-sensitive properties were considerably deteriorated, the resistance of the actual product was small and the moisture-sensitive properties were little deteriorated.
Further, according to the X-ray powder diffraction pattern of the poisoning prevention layer, crystal phases of anatase titania, rutile titania and alumina were recognized.

FIG. 3 is a front view showing the appearance of the humidity sensors S of the first and second embodiments. FIG. 4 is a cross-sectional view showing a state in which a lower electrode, a moisture-sensitive layer, an upper electrode, a protective layer, and a poisoning prevention layer are laminated and formed on an insulating substrate. As shown in FIG. 4, the humidity sensor S includes an insulating substrate 1,
The lower electrode 2, the moisture-sensitive layer 3, the upper electrode 4, the protective layer 5 and the poisoning prevention layer 6 are sequentially formed on the surface. The poisoning prevention layer covers all of the insulating substrate, the lower electrode, the moisture-sensitive layer, the upper electrode, and the protective layer.

It should be noted that the present invention is not limited to the specific embodiments described above, but can be variously modified within the scope of the present invention in accordance with the purpose and application. That is, the specific surface area of the ceramic powder such as titania, the thickness of the poisoning prevention layer, the air permeability (porosity), and the like can be appropriately adjusted. Further, a protective layer having the same composition as the protective layer can be further formed on the surface of the poisoning prevention layer.

[0042]

According to the present invention, an exhaust poisoning prevention layer containing a fine ceramic powder having a specific particle size and a ceramic powder having a relatively large particle size and a narrow particle size distribution can reduce exhaust gas. Even if it comes in contact with a poisoning substance such as lead contained, poisoning is efficiently prevented, and this poisoning prevention layer is hardly peeled off from the protective layer, and excellent performance with less deterioration of moisture sensitivity is achieved. Humidity sensor can be obtained. Further, by using a paste for forming a poisoning prevention layer containing ceramic powders having different specific surface areas, claims 1 to 13 are provided.
Can easily be manufactured.

[Brief description of the drawings]

FIG. 1A is an explanatory view showing a poisoning prevention layer formed on a sensor element of Experimental Example 20 by a scanning electron microscope photograph at a magnification of 200 ×. (B) is an explanatory view in which the central part of (a) is enlarged and shown by a scanning electron microscope photograph at a magnification of 2000 times.

FIG. 2 is a graph showing that the humidity sensor of Example 2 has excellent durability.

FIG. 3 is a front view showing the appearance of the sensor element after a poisoning prevention layer is formed.

FIG. 4 is a sectional view taken along line A-A 'of FIG.

FIG. 5 is a schematic diagram showing an outline of an apparatus used for evaluating the humidity sensitivity of a humidity sensor by a split flow method.

[Explanation of symbols]

S; humidity sensor, 1; insulating substrate, 2; lower electrode, 3; moisture-sensitive layer, 4; upper electrode, 21, 41; output lead wire, 5; protective layer, 6; Cylinders,
201; mass flow (wet), 202; mass flow (dry), 30; constant temperature water bath, 40a; first saturation bath, 4
0b: second saturation tank, 50; evaluation tank, 60; temperature / humidity tester.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Ryuji Inoue 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi Inside Japan Specialty Ceramics Co., Ltd. (72) Inventor Takafumi Oshima 14-18 Takatsuji-cho, Mizuho-ku, Nagoya F-term (reference) 2G046 AA09 BA01 BB02 BF01 EA02 EA08 EA09 EA12 FB02 FB03 FE03 FE20 FE38 FE39 FE44

Claims (19)

[Claims] 1. A humidity sensor comprising: a humidity sensitive layer, a pair of electrodes formed in contact with the moisture sensitive layer, a protective layer, and a sensor element having a poisoning prevention layer formed on a surface of the protective layer. The poisoning prevention layer is composed of a composite powder in which a ceramic powder having a large particle diameter (hereinafter, also referred to as coarse powder) is covered with a ceramic powder having a small particle diameter (hereinafter, also referred to as fine powder). A humidity sensor characterized in that voids not filled with fine powder are dispersed in gaps between composite powders. 2. The particle size distribution of the primary particles of the ceramic powder constituting the poisoning prevention layer has at least two peaks, the peak with the smallest particle size is 10 μm or less, and the peak with the largest particle size is 2. The humidity sensor according to claim 1, wherein a peak of the humidity is 0.1 μm or more. 3. The humidity sensor according to claim 1, wherein the protective layer is formed by screen printing or thermal spraying. 4. The humidity sensor according to claim 1, wherein at least a part of the ceramic powder is an oxide powder. 5. The ceramic powder according to claim 2, wherein the ceramic powders have different compositions.
The humidity sensor according to any one of claims 1 to 4, further comprising at least one type of ceramic powder. 6. A humidity sensor comprising: a humidity sensitive layer, a pair of electrodes formed in contact with the moisture sensitive layer, a protective layer, and a sensor element having a poisoning prevention layer formed on the surface of the protective layer. The poisoning prevention layer is composed of titania powder and ceramic powder other than titania, the particle size distribution of the primary particles of the titania powder has a peak at 1 μm or less, and the particle size distribution of the primary particles of the ceramic powder other than the titania is A humidity sensor having a peak at 10 μm or more. 7. The humidity sensor according to claim 6, wherein the titania powder is an anatase-type titania powder or a rutile-type titania powder. 8. The ceramic powder other than titania is a powder of a composite oxide containing aluminum atoms.
Or the humidity sensor according to 7. 9. The particle size distribution of primary particles of the titania powder has a peak at 0.003 to 0.5 μm, and the particle size distribution of primary particles of the ceramic powder other than titania is 15 to
9. The method according to claim 6, which has a peak at 50 μm.
The humidity sensor according to the item. 10. A humidity sensor comprising: a humidity sensitive layer, a pair of electrodes formed in contact with the moisture sensitive layer, a protective layer, and a sensor element having a poisoning prevention layer formed on the surface of the protective layer. The poisoning prevention layer is composed of two or more types of ceramic powders having the same composition and different crystal phases, wherein one of the ceramic particles is a coarse powder having a large particle size, and the other is a ceramic particle of the other crystal phase. Is a fine powder having a small particle diameter. 11. The ceramic powder according to claim 10, wherein said powder is anatase titania powder and rutile titania powder.
The humidity sensor as described. 12. The method according to claim 11, wherein the particle size distribution of primary particles of the anatase-type titania powder has a peak at 0.5 μm or less, and the particle size distribution of primary particles of the rutile-type titania powder has a peak at 1 μm or more. Humidity sensor. 13. The particle size distribution of primary particles of said anatase type titania powder has a peak at 0.003 to 0.5 μm, and the particle size distribution of primary particles of said rutile type titania powder has a peak at 3 to 8 μm. Item 13. The humidity sensor according to item 11 or 12. 14. A method for manufacturing a sensor element having a moisture-sensitive layer, a pair of electrodes formed in contact with the moisture-sensitive layer, a protective layer, and a poisoning prevention layer formed on the surface of the protective layer. 1) one or more first ceramic powders having a particle size distribution peak of primary particles of 10 μm or less; (2) a first particle size distribution peak of primary particles of 0.1 μm or more and the first
The maximum particle size (hereinafter, referred to as 10% particle size) of 10% of the particles having the larger particle size than the peak of the particle size distribution of the primary particles of the ceramic powder, and the smaller particle size side. One or more second ceramic powders having a particle size distribution in which the difference in the maximum particle size (hereinafter referred to as 90% particle size) of 90% of the particles is not more than twice the particle size of the peak value of the particle size distribution;
(3) An organic binder and (4) a solvent or water are kneaded to prepare a paste for forming a poisoning prevention layer, and the paste for forming a poisoning prevention layer is applied to the surface of the protective layer to form a coating film. ,
Thereafter, the coating film is heated and dried to form the anti-poisoning layer. 15. The first ceramic powder and the second ceramic powder.
The method according to claim 14, wherein at least a part of the ceramic powder is an oxide powder. 16. The first ceramic powder has a specific surface area of 2
A titania powder to 500m ² / g, the sensor element according to claim 14 or 15 is a powder of a composite oxide containing aluminum atoms of the second ceramic powder is a specific surface area of 0.1 to 100 m ² / g Production method. 17. The method according to claim 17, wherein the first ceramic powder has a specific surface area of 2
~ 500 m ² / g anatase titania powder,
The second ceramic powder has a specific surface area of 0.1 to 10 m ² /
The method for producing a sensor element according to claim 14, wherein the rutile type titania powder is g. 18. The paste for forming a poisoning prevention layer is
The method according to any one of claims 14 to 17, wherein the first ceramic powder and the second ceramic powder each include 15 to 50 parts by mass when the amount is 0 parts by mass. 19. The method according to claim 14, wherein the mass ratio of the first ceramic powder to the second ceramic powder is 1: 2 to 2: 1.