JPH03118459A - Exhaust gas sensor - Google Patents

Abstract

PURPOSE:To broaden the range of usable temperature by providing a gas sensitive layer part comprising a metallic oxide indicating gas sensitivity and a metallic oxide at least a part of which undergoes the formation of solid solution and which has the valence larger than that of the above described metallic oxide. CONSTITUTION:A part of a metal oxide whose valence is larger than a metal oxide indicating gas sensitivity undergoes the formation of solid solution. Then, the metal elements of the metallic oxide having the larger valence readily substitutes for the elements in the crystal structure of the metallic oxide indicating the gas sensitivity and readily undergoes the formation of the solid solution. The metal elements having the larger valence act as electron doners for the metallic oxide indicating the gas sensitivity in a gas sensitive layer. Thus, the variation in resistance value in lean and rich atmospheres can be sufficiently secured, and the temperature changing rate of the resistance value in both atmospheres can be decreased. Therefore, the air-fuel ratio can be detected over a broad temperature range without using a heater, a switching circuit for a reference resistor and the like.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to an exhaust gas sensor for detecting oxygen concentration in exhaust gas exhausted from internal combustion engines of automobiles, etc. The present invention relates to an exhaust gas sensor having a gas-sensitive layer portion consisting of a gas-sensitive layer portion, which can be used over a wide temperature range.

[Prior art and its problems]

Conventionally, in order to purify exhaust gas emitted from internal combustion engines such as automobiles, the oxygen concentration in the exhaust gas is measured and the composition of the exhaust gas is maintained within a desired range based on the measurement results. Such exhaust gas purification systems have been developed and put into practical use.

As an oxygen concentration detection sensor used in this exhaust gas purification system, an exhaust gas sensor using titanium oxide whose resistance value changes depending on the oxygen concentration has been put into practical use. This exhaust gas sensor has a constant voltage power supply 11 as shown in FIG.
On the other hand, it consists of a configuration in which a reference resistor 12 and the titanium oxide 13 are connected in series, and the voltage drop on the circuit is mainly The air-fuel ratio is detected by reading from the output terminal 14 whether it is occurring due to the reference resistor 12 or the titanium oxide 13. FIG. 4 shows the relationship between the resistance value and temperature of titanium oxide in this exhaust gas sensor in a lean atmosphere and a rich atmosphere. As shown in the figure, the resistance value 15 in a lean atmosphere and the resistance value 16 in a rich atmosphere both decrease as the temperature increases. As described above, the air-fuel ratio is detected depending on whether the voltage drop occurs at the reference resistor 12 or the titanium oxide 13. Therefore, the resistance value 17 of the reference resistor 12 must be between the resistance value 15 in a lean atmosphere and the resistance value 16 in a rich atmosphere, and the operating temperature range as an exhaust gas sensor is narrow, 400 to 700 degrees Celsius. It is.

In order to solve the problem that the conventional exhaust gas sensor has a narrow operating temperature range, technical development is actively being carried out, and several methods have been proposed to solve these problems. One of them is a "gas detection device" (Japanese Unexamined Patent Application Publication No. 5-118), which uses an electric circuit to switch the reference resistance value from the resistance value or output of titanium oxide, or to correct the comparison voltage.
4-85097), ``Signal processing circuit for titania type 02 sensor'' (Japanese Patent Laid-open No. 56-22944), and ``Air-fuel ratio detection device'' (Japanese Patent Laid-Open No. 56-42132). However, these types of exhaust gas sensors that are corrected using electric circuits have a drawback that the detection circuit is complicated and is prone to malfunctions.

In addition, as another method, a method for compensating for the temperature dependence of titanium oxide using an NTC thermistor as a reference resistance, ``Oxygen Detector in Gas'' (Japanese Unexamined Patent Publication No. 53-61396)
There is. However, this method has problems in that it is difficult to obtain an NTC thermistor with a desired resistance value, and the NTC thermistor must be placed near titanium oxide for temperature compensation. Furthermore, we have developed an ``air-fuel ratio sensor'' (Japanese Unexamined Patent Publication No. 53-84797) that maintains titanium oxide at a predetermined temperature by heating it with a heater.
No.).

However, this exhaust gas sensor whose temperature is controlled by a heater has the problem that a new power source for the heater is required and the manufacturing process for the exhaust gas sensor is increased.

Therefore, the present inventors conducted intensive research to solve the problems of the prior art as described above, and as a result of conducting various systematic experiments, they came up with the present invention.

[Purpose of the invention]

An object of the present invention is to provide an exhaust gas sensor that can be used over a wide temperature range.

The present inventors have focused on the following regarding the problems of the prior art described above. That is, by reducing the rate of temperature change in the resistance value of the gas-sensitive layer between a lean atmosphere and a rich atmosphere without impairing the change in the resistance value of the gas-sensitive layer that ranges over 3 to 4 orders of magnitude between lean and rich atmospheres. We have focused on the fact that even a conventional type of exhaust gas sensor as shown in FIG. 3 can be provided as an exhaust gas sensor that can detect lean atmospheres and rich atmospheres in a sufficiently wide temperature range.

[Description of the first invention] Configuration of the first invention The exhaust gas sensor of the first invention includes a pair of electrodes.

In an exhaust gas sensor comprising a gas-sensitive layer disposed between the electrodes, the gas-sensitive layer includes a metal oxide exhibiting gas-sensitivity and the metal oxide at least partially dissolved in the metal oxide. It is characterized by being composed of an oxide of a metal with a higher valence than that of the metal oxide.

! The mechanism by which the exhaust gas sensor of the first invention exerts its effects is not yet clear, but it is thought to be as follows.

That is, the gas-sensitive layer of the exhaust gas sensor of the first invention comprises a metal oxide exhibiting gas-sensitivity and an oxide of a metal having a higher valence than the metal oxide, which is at least partially dissolved in the metal oxide. It consists of. A metal element having a higher valence than the metal oxide is likely to be substituted into a solid solution in the crystal lattice of the gas-sensitive metal oxide during the firing process during the manufacture of the exhaust gas sensor. These elements act as electron donors to the gas-sensitive metal oxide in the gas-sensitive layer. Therefore, the activation energy for generating thermally excited electrons in the conduction band of the gas-sensitive metal oxide in the gas-sensitive layer is lowered, and as a result, the rate of temperature change in the resistance value of the gas-sensitive layer can be reduced. can.

Effects of the First Invention The exhaust gas sensor of the first invention is an exhaust gas sensor that can be used over a wide temperature range.

That is, by dissolving a metal element having a higher valence than the metal oxide that acts as an electron donor into the metal oxide forming the gas-sensitive layer, a sufficient change in resistance value between lean and rich atmospheres is ensured. Furthermore, since the temperature change rate of the resistance value in both atmospheres can be reduced, it can operate as an exhaust gas sensor that detects the air-fuel ratio over a wider temperature range than conventional methods without using a switching circuit such as a heater or reference resistance. .

[Description of other inventions of the first invention] Other inventions of the first invention will be described below.

In the exhaust gas sensor of the present invention, the electrode is not particularly limited as long as it is a heat-resistant conductor, and usually one whose main component is gold or a platinum group metal is used.

Further, the gas-sensitive layer portion is composed of a metal oxide exhibiting gas-sensitivity and an oxide of a metal having a higher valence than the metal oxide, which is at least partially dissolved in the metal oxide.

The metal oxide is made of a gas-sensitive substance, and an oxide semiconductor is used that responds to oxygen concentration as a change in resistance. Specifically, titanium oxide (Ti0z), tin oxide (
S n Ot ), zinc oxide (ZnO), niobium oxide (
Nb205), tantalum oxide (Ta2nt), cerium oxide (CeOz), and the like. These oxide semiconductors have non-stoichiometric compositions; for example, titanium oxide has a non-stoichiometric composition.
A chemical equilibrium ← 2 is established with oxygen. Because the number of electrons in the conduction band increases or decreases with respect to the oxygen concentration, the resistance value changes rapidly when the stoichiometric air-fuel ratio is reached.

When the metal oxide is titanium oxide, examples of metal oxides with a higher valence than metal oxides include oxides of metals with a pentavalent valence, such as oxides of niobium, tantalum, etc. I can do it. In addition, in the case of tin oxide, oxides such as antimony, in the case of zinc oxide, oxides such as aluminum and chromium, and in the case of niobium oxide, oxides such as tungsten,
When the oxide such as molybdenum is tantalum oxide, examples include oxides such as tungsten and molybdenum, and when cerium oxide, examples include oxides such as tantalum.

The oxide of a metal with a higher valence than this metal oxide is
At least a portion thereof is dissolved in the metal oxide, which is the gas-sensitive substance. The solid solution of the metal oxide is preferably uniformly dispersed at least in the current passing portion of the gas-sensitive layer. This is because in order to obtain the effect of adding an oxide of a metal with a higher valence than the metal oxide, it is necessary to form a conductive path made of a solid solution of the oxide of the metal in the current passing section. .

The gas-sensitive layer portion of the present invention comprises the metal oxide and an oxide of the metal having a higher valence than the metal oxide. Note that this gas-sensitive layer portion is preferably a porous sintered body. This is to suppress a decrease in responsiveness to oxygen concentration due to the inherent characteristic of porous sintered bodies that the electrical conductivity does not necessarily match the number of electrons in the conduction band.

Further, it is preferable that the proportion of the oxide of a metal having a higher valence than the metal oxide in the gas-sensitive layer portion is 0.02 to 0.5 mol% with respect to the metal oxide exhibiting gas-sensitivity. In order to obtain the effect of addition of a metal oxide having a higher valence than the metal oxide in the gas-sensitive layer on the resistance characteristics of the metal oxide, it is necessary to This is because it is necessary to add 0.02 mol % or more in order to reduce the oxygen concentration and maintain responsiveness to oxygen concentration. Furthermore, if the amount added exceeds 0.5 mol%, there will be too many electrons in the conduction band, and the change in resistance of the metal oxide between lean and rich atmospheres will become small, resulting in poor performance as an exhaust gas sensor. At the same time, the response speed of the sensor output to changes in the air-fuel ratio also decreases.

Here, the solid solution effect of the oxide of a metal having a higher valence than the metal oxide of the gas-sensitive layer will be explained using titanium oxide as an example of the metal oxide.

In other words, N with a pentavalent valence relative to titanium oxide
The solid solution effects of b 20 s and Ta 20 s are as follows.

N b 20 S and Ta 20 s form a solid solution in titanium oxide by replacing the lattice position of Ti'+ with Nb'' and Ta5+.In order to maintain the electrical neutrality as a whole, they form a solid solution by substitution. Ti4+ at a lattice position with the same amount as Nb5'' or Ta'+ becomes Ti3+. This Ti3+
acts as a donor that provides electrons to the conduction band of titanium oxide through the equilibrium reaction of Ti"Ti"+e, so it is possible to lower the activation energy of thermally excited electron generation in the conduction band of titanium oxide, As a result, the temperature change rate of the resistance value of titanium oxide can be reduced.

In addition, when the gas-sensitive layer part is titanium oxide and the oxide of a metal with a higher valence than the metal oxide is niobium oxide or tantalum oxide, Nb" (ion radius: 0.69 people), T
Since the ionic radius of a5+ (ion radius: 0.68 people) is almost equal to that of Ti'' (ion radius = 0°68 people) at the lattice position of titanium oxide, niobium oxide or tantalum oxide is a substituted solid solution for titanium oxide. It is easy to add niobium oxide and tantalum oxide to titanium oxide, and the above-mentioned addition effect can be achieved even more, and the above-mentioned addition effect can be sufficiently obtained with a relatively small amount of niobium oxide and tantalum oxide compared to titanium oxide. The solid solution limit is also large, and the solid solution state is thermodynamically stable, so segregation does not occur even when used for a long time under high temperature conditions, and the added effect can be maintained. It has excellent durability as a sensor.

Next, a specific example of the method for producing a gas-sensitive layer of the present invention will be briefly described below.

Here, an example will be described in which titanium oxide is used as a metal oxide and niobium oxide or tantalum oxide is used as an oxide of a metal having a higher valence than the metal oxide.

First, a method for preparing a gas-sensitive layer made of titanium oxide and niobium oxide or tantalum oxide will be described.

The first method uses titanium oxide powder and a predetermined amount of niobium oxide (N b 20 s ) or tantalum oxide (
The powder of Ta20s) is mechanically pulverized and mixed using a ball mill. Since this is a mechanical mixing method, in order to achieve more uniform mixing, the particle size of the starting materials must be small, preferably 5 μm or less. Also,
The purity of the starting material is also preferably 3N or higher in order to obtain the desired addition effect. The ball mill used is preferably made of plastic such as nylon in order to prevent contamination with impurities as much as possible, and the mixing time is preferably 6 hours or more.

The second method is to add titanium oxide powder to an alcohol solution in which a predetermined amount of niobium or tantalum alkoxide is dissolved, and to mix and dry the mixture. In this method, niobium or tantalum alkoxide is hydrolyzed by moisture in the air during the drying process, and the surface of the titanium oxide powder is uniformly coated as niobium oxide or tantalum oxide. Therefore, the state of dispersion of niobium and tantalum in the titanium oxide powder is more uniform than in the mechanical mixing method. Further, the particle size of the titanium oxide powder used needs to be small, preferably 5 μm or less, and its purity is preferably 3N or more in order to obtain the desired addition effect.

The third method is to add a predetermined amount of niobium or tantalum alkoxide to an alcohol solution in which titanium alkoxide is dissolved, and to mix and dry the mixture. In this method, since all starting materials are solutions, the state of dispersion of niobium or tantalum in the titanium oxide produced after hydrolysis is the most uniform. Furthermore, it is easy to use 5N or more niobium, tantalum, or titanium alkoxide as a starting material, and is a preferred method for adding 0.1 mol % or less.

Next, the titanium oxide powder to which niobium or tantalum has been added as described above is molded to give it mechanical strength as an exhaust gas sensor, and then heated at a temperature of 1000 to 1400°C.
C. to obtain the exhaust gas sensor of the present invention. Note that the firing may be performed by heating in an electric furnace or by microwave heating. At this time, the firing time can be shortened as the firing temperature is higher, and in the case of heating in a furnace at a firing temperature of 1200° C., the firing time is about 4 hours. During this firing process, the added niobium and tantalum are substituted and dissolved in the titanium oxide.

The exhaust gas sensor of the present invention may have a titanium oxide gas sensing portion formed into a disk shape and sandwiched between a pair of platinum electrodes.

Alternatively, it may be formed on a ceramic substrate using a hybrid technology such as a thick film technology.

〔Example〕

The present invention will be explained in detail below.

First Example An exhaust gas sensor was prepared using titanium oxide as a metal oxide and niobium and tantalum as oxides of metals having a higher valence than the metal oxide, and a performance evaluation test of the sensor was conducted.

First, powders of titanium oxide and niobium oxide (NbzOs) with a purity of 3N and a particle size of 2 μm are prepared, and niobium oxide is 0.1 mol% and 0.2 mol% as an oxide for 2 g of titanium oxide.
, 0.5 mol%, and 1.0 mol% using a nylon and polypropylene ball mill (inner capacity 250 m1) to which 100 mA' of reagent grade ethanol was added.
After mechanically mixing for 24 hours, the mixture was dried to prepare a titanium oxide powder to which a predetermined amount of niobium oxide was added.

In addition, titanium oxide with a purity of 3N and a particle size of 2 μm and penta-1-propoxy tantalum [Ta(Oi
Prepare Pr)s) and add penta-oxide to 2g of titanium oxide.
1-propoxytantalum as an oxide after hydrolysis 0
．． 1 mol%, 0.2 mol%, 0.5 mol%, and 1
．． Weighed 0 mol%, reagent grade ethanol 100ml17
After thorough stirring and mixing in a Pyrex beaker with a capacity of 500 ml, penta-1-propoxytantalum was hydrolyzed and solidified as a gel on the surface of the titanium oxide powder by being left at room temperature for one week. Thereafter, the mixture was maintained at 500° C. for 6 hours in an electric furnace in an air atmosphere to completely burn off residual alcohol and alkoxide, thereby preparing titanium oxide powder to which a predetermined amount of tantalum oxide was added.

Next, 5N purity tetra-1-propoxy titanium I:T
i (-0-i -Pr ) 4 ],] penta-1-propoxyniobium Nb(Oi Pr)s) and penta-1-propoxytantalum [Ta(-0-i
-Pr) 5] was prepared, and 0.02 mol% of penta-1-propoxyniobium and penta-1-propoxytantalum were added as oxides after hydrolysis to 70 g of tetra-i-propoxytitanium. Weigh the mole%,
Capacity 500m including reagent grade ethanol 10ml
After thorough stirring and mixing in a Pyrex beaker (No. 1), the mixture was left at room temperature for one week to hydrolyze each alkoxide as a starting material and solidify as a gel. Thereafter, the mixture was held at 500° C. for 6 hours in an electric furnace in an air atmosphere to completely burn off the remaining alcohol and alkoxide groups. Then, a ball mill made of nylon and polypropylene (inner capacity 250ml) was added with 100ml of reagent grade ethanol.
ml) for 24 hours, and then dried using a nylon net with a mesh width of 5 μm to prepare titanium oxide powder with a particle size of 5 μm or less to which predetermined amounts of niobium oxide and tantalum oxide were added.

The titanium oxide powder prepared as described above and the titanium oxide powder with a purity of 3N and a particle size of 2 μm as starting materials were molded into a tablet with a diameter of 13 mm and a thickness of about 2 mm at a pressure of about 350 kg/cJ under reduced pressure using a tablet molding machine. After molding into tablets, the mixture was calcined in an electric furnace at 1200° C. for 4 hours in an air atmosphere. The porosity of the sintered body containing only titanium oxide is 17.7%, and the porosity of the sintered titanium oxide body containing niobium oxide and tantalum oxide is 0.02 to 0.0% as shown in Table 1.
In the range of 5 mol%, it exceeded 20%.

The disk-shaped sintered body 2 obtained above was ion-coated with platinum 3a on both sides to a thickness of 300 mm, as shown in FIGS. 1 and 2, and then mechanically pressed to a diameter of 0.1 m.
[Pt(
Ammonia plating solution containing NH3)4E ci2 (
-1,OV for 20 min against platinum counter electrode in 200 ml)
The platinum electrode 3 was formed by plating platinum on the ion-bladed platinum layer by applying a voltage of . At this time, the platinum electrode 3 with the platinum wire 3b connected to the titanium oxide sintered body 2 was formed because plating was performed so as to wrap around the pressed platinum wire. As a result, an exhaust gas sensor 1 according to this embodiment of the present invention was obtained.

Next, a performance evaluation test of this exhaust gas sensor was conducted. First, the exhaust gas temperature can be changed from 300 to 900 degrees Celsius, and the air-fuel ratio (λ) can vary between excess fuel (rich atmosphere: λ=0.9) and fuel shortage (lean atmosphere: λ=1.
The exhaust gas sensor sample produced in the above process was held in a propane gas burner tester that could be controlled between 1) and the resistance value in a steady state was measured. The results are shown in FIGS. 5 to 10. In addition, the amount of niobium oxide added is 0.
．． For 02 mol%, 0.5 mol%, and 1.0 mol%, the resistance values in a steady state were measured at 600°C in a rich atmosphere or a 1.0 mol% atmosphere, and then switched to the opposite atmosphere, and the logarithmic resistance values were measured. As a change, the time required for 50% response was measured and evaluated as response speed. Table 2 shows the response time when switching from a lean atmosphere to a rich atmosphere as TI, and the time when switching from a rich atmosphere to a lean atmosphere as T2.

In Table 2, Figure 5, the lean atmosphere resistance value of the sample containing only titanium oxide is 15, and the resistance value of the rich atmosphere resistance value is 16, while the lean atmosphere resistance value of the sample with 0.02 mol% niobium oxide added is 5A and 16. For both resistance values 5B in the rich atmosphere, the rate of temperature change is reduced. This decrease in temperature change rate is observed when the amount of niobium oxide added is 0.02 mol% or more.

In FIGS. 6 to 9, compared to the resistance value 15 in a lean atmosphere and the resistance value 16 in a rich atmosphere for samples containing only titanium oxide, the resistance values are 0.05 mol%, 0.1 mol%, 0.2 mol%, 0. For the lean atmosphere resistance values 6A, 7A, 8A, and 9A and the rich atmosphere resistance values 6B, 7B, 8B, and 9B of the samples to which 0.5 mol % of niobium oxide was added, the rate of temperature change is reduced.

In Fig. 1θ, the resistance value 15 in the lean atmosphere of the sample containing only titanium oxide is 3 compared to the resistance value 16 in the rich atmosphere.
Although it is ~4 orders of magnitude larger, the temperature change rate of resistance value in the sample with 1 mol% niobium oxide is lower, but the resistance value of 10A in a lean atmosphere is higher than the resistance value of 10A in a rich atmosphere.
It increases by only 1 to 2 orders of magnitude compared to B. This means that
This indicates that the sample's responsiveness to oxygen concentration has decreased, meaning that its characteristics as an exhaust gas sensor have been impaired. This phenomenon occurs when the amount of niobium oxide added is 0.5
Permitted if it exceeds mol%.

From the above results, it can be seen that 0.02 to 0.5 mol % of niobium oxide is effective in reducing the rate of change in the resistance value of titanium oxide.

From Table 2, when the amount of niobium oxide added is 0.5 mol% or less, the response time satisfies the responsiveness as a sensor, but when it exceeds 0.5 mol%, the response time is 1. It can be seen that the response time as a sensor is reduced beyond seconds.

From the above results of this example, it can be seen that an effective amount of niobium oxide to be added to titanium oxide in order to improve the characteristics as an exhaust gas sensor is 0.02 to 0.5 mol %.

Further, the resistance characteristics of the sample obtained by adding tantalum oxide to the titanium oxide of this example were also measured using a propane gas burner tester. 0.02 mol% of tantalum oxide, 0
．． The temperature characteristics of the resistance value of titanium oxide added at 5 mol % and 1.0 mol % are shown in FIGS. 11 to 13. From this, it was confirmed that 0.02 to 0.5 mol% of tantalum oxide is effective for reducing the rate of change in the resistance value of titanium oxide.

[Brief explanation of drawings]

1 and 2 show an exhaust gas sensor according to a first embodiment of the present invention, FIG. 1 is a schematic perspective view thereof, FIG. 2 is a sectional view taken along line AA' in FIG. 1, and FIGS. 4 shows the prior art, FIG. 3 is a circuit diagram of the air-fuel ratio detection device, and FIG. 4 is a diagram showing the relationship between the resistance and temperature of the oxygen sensor element of the device.
5 to 13 are diagrams showing the results of performance evaluation tests conducted using the exhaust gas sensor of the first embodiment of the present invention (diagrams showing changes in the resistance-temperature characteristics of the gas-sensitive layer). . l...Exhaust gas sensor, 2...Gas sensitive layer portion, 3...Electrode, 5A, 6A, 7A, 8A, 9A, 10A. 1 lA, l 2A, 13A... Change in resistance-temperature characteristics in lean atmosphere 5B, 6B
, 7B, 8B, 9B, l0B11B, 12B, 13B

Claims (1)

[Claims] An exhaust gas sensor comprising a pair of electrodes and a gas-sensitive layer disposed between the electrodes, wherein the gas-sensitive layer includes a metal oxide exhibiting gas-sensitivity, and a gas-sensitive layer disposed between the electrodes. and an oxide of a metal having a higher valence than the metal oxide, which is at least partially solid-dissolved in the exhaust gas sensor.