JPH022534B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention consists of a metal oxide doped with a semiconductor, the conductivity of which changes continuously with the gas composition in the presence of oxygen and oxidizable components. Relating to a semiconductor for a sensor for measuring the content of oxidizable components such as oxygen and/or carbon monoxide in exhaust gas by a change in conductivity, in which the conductivity changes by at least one order of magnitude when the pressure changes by about half an order of magnitude. .

The premise for using a semiconductor in such a sensor is that the conductivity change (Δσ) sensitively responds to a change in the concentration of the gas in the exhaust gas. The exhaust gas mainly contains carbon monoxide (0.4-8% CO) as a harmful component in the air-deficient range, and mainly oxygen (0.5-3% _O2 ) in the air-excess range, with the CO content being
0.03-1%, NOx content 1.01<λ<1.2 and max. 0.4
reach %.

In the air-deficit range, the splitting of CO is of great significance as a prerequisite for warning signals, while in the air-excess range, the analysis of the O ₂ concentration is important, for example, for the control of domestic combustion devices or internal combustion engines that operate with as much air as possible. Therefore, the sensor's responsiveness to CO and O ₂ is an issue.

According to semiconductor theory, the conductivity change Δσ based on the equilibrium between gaseous oxygen and oxygen incorporated in the lattice of the semiconductor oxide is expressed by n4 according to the equation Δσ˜Po ₂ ^±1/n (1). In the case of conductivity change based on adsorption, n may be 2 (−1-valent oxygen ion), and in the case of CO, it is n2. This is evident from curves 1 and 2 in FIG. 1, where measured conductivity (σ) is plotted against oxygen partial pressure (po ₂ ) on a logarithmic scale on both axes. Curve 1 is FeO15 mol%
Relationship at 900℃ for MgO semiconductor containing curve 2
represents the same relationship at 900°C for an MgO semiconductor containing 10 mol% of CoO. For curve 1 we get the ratio d logσ/d logPO ₂ = -1/5.75 = -0.17, and for curve 2 this ratio is 1/3.5 =
It is 0.29. In the case of these pure semiconductor oxides, therefore, when the oxygen partial pressure changes by about two orders of magnitude, the conductivity decreases by about 1/
Changes by 3 digits. This relatively small change in conductivity is too small to be a warning signal within the concentration changes that actually occur in the exhaust gas, and if the temperature dependence of the conductivity is small or if this temperature dependence is detected by a second and identical sensor. If it is compensated automatically, it can only be used with high electrical measurement outlays. This conductivity difference is insufficient for control purposes.

A sensor semiconductor consisting of tin dioxide doped with magnesium oxide or niobium pentoxide is known from DE 26 48 373 A1. Since this semiconductor has a very strong relationship between electrical conductivity and oxygen partial pressure, the signals of such sensors can be further processed in warning or control devices. This is shown in curve 3 of FIG. 1 as an example of a semiconductor of this type consisting of tin dioxide containing 5 mol % MgO. When the oxygen partial pressure changes by about 1/2 order of magnitude, the conductivity changes by about 2 orders of magnitude. This is also expressed by the ratio dlogσ/ _dlogPo2 =-3.5.

This relationship between conductivity and oxygen partial pressure can no longer be explained only by semiconductor theory that introduces equation (1). In this case, an analog amplification of the concentration changes of the gases to be measured, oxygen and oxidizable gases such as carbon monoxide, takes place. This amplification is achieved by deliberately reducing the catalytic activity of the semiconductor surface, which is so weak that the corresponding reaction equilibrium, e.g. according to the equation CO+1/2O ₂ Co ₂ (2), is adjusted very slowly. In any case, this adjustment must take place significantly slower than the adjustment of the adsorption equilibrium of the reaction components, such as CO and O ₂ . This is particularly the case when the adsorption or chemisorption strength of the various gases differs (competitive adsorption), or because of the different rates of the dissociation process of the adsorbate on the solid surface, or because of the reaction mechanism (Angmi-Uahinschel-Ud, Ellie). - redeal) are different. Said analog amplification of the measurement signal therefore involves a chemical conditioning towards reducing the catalytic effect of the semiconductor used as a sensor. The extent to which metal ions are thereby reduced close to the surface cannot yet be determined precisely. The combination of semiconducting behavior and significantly reduced catalytic activity is achieved by a certain chemical composition of the semiconductor, in which the semiconducting behavior is regulated both by chemisorption and by the desorption and incorporation of oxygen into the semiconductor lattice. can do.

The doped tin oxide described in Japanese Patent Application No. 52-128550 can be used very well in the oxygen excess range (λ>1), i.e. small concentration changes of a few percent can be measured, or sensors made from this semiconductor can be Although the combustion process can be controlled with a sufficiently fast response time,
Its measurability does not fully extend to the oxygen-deficient range, and the entire range centered at λ = 1 is
It cannot be detected continuously without sudden changes as occurs with doped Cr ₂ O ₃ semiconductors.

The semiconductor according to the invention is characterized in that it consists of cerium dioxide doped with magnesium oxide, aluminum oxide, yttrium oxide, titanium dioxide, tantalum pentoxide, niobium pentoxide or vanadium pentoxide. The advantage of the semiconductor of the present invention having this feature is that the sensor manufactured from it has λ = 0.9
It has a continuous and uniform course of λ-R characteristics extending over about 4 orders of magnitude in the range of ~1.2.
Although the relationship between electrical resistance and oxygen and carbon monoxide concentrations is not linear, the rise changes continuously so that calibration is easily possible and satisfactory measurement results are obtained over the entire range. The upper detection limit of the measurement range is approximately 500ppm CO.

The semiconductor of the present invention is further advantageously formed by doping cerium dioxide with 0.1 to 10 mol % of said oxide.

As mentioned above, FIG. 1 shows the relationship between conductivity and oxygen partial pressure for various types of semiconductors, plotted on a logarithmic scale on both axes. Figure 2 shows the temperature range 400
The resistance of various semiconductors is shown as a function of oxygen or carbon monoxide content at temperatures up to 500°C, with resistance plotted on a logarithmic scale on the vertical axis. The range shown on the horizontal axis corresponds to a λ value of approximately 0.9 to 1.2 within the gas chamber.

The manufacture of semiconductors is carried out by precipitation of metal hydroxides with ammonia from approximately neutral solutions of metal chlorides or nitrates. The hydroxide is then scorched to form an oxide, which is ground into a fine powder. This powder is compressed into tablets, hardened by sintering, and provided with electrical connection leads. or by mixing the powder with inorganic or organic diluents such as silicates or high-boiling organic admixtures into a spreadable paste and applying this to an inert support such as a ceramic plate made of aluminum oxide using thick or thin film techniques. Place it in The contact is made in this case by means of gold or platinum conductor tracks.

The powder that will later be molded into a semiconductor sensor is manufactured in the following way: Example 1 CeCl ₃ 7H ₂ O 34.8 g and AlCl ₃ 6H ₂ O 0.504
Dissolve 1 g in water and add semi-concentrated ammonia solution (approx.
Precipitate at ~7.5. After precipitation, stir for another 2 hours at 80°C. It is then sedimented, decanted, suspended in water, and centrifuged. The centrifuged material is dried at 150°C for 3 hours and then sintered at 850°C for 8 hours. 800
The heating time to °C is 4-5 hours.

Example 2 CeCl ₃・7H ₂ O37.5g and MgCl ₂・6H ₂ O0.633
g is dissolved in 1 part of water and processed as described in Example 1 below.

10 ⁴ to ^{10 5} N/
Compress the tablet with a pressure of cm ² and store it at 1100°C.
Sinter in air for 12-24 hours. For contacting, the tablet is compressed between two gold contacts, the entirety of which is in a holder, which is connected to an internal combustion chamber similar to known sensors with ionically conductive solid electrolytes such as stabilized zirconium dioxide. Screw into engine exhaust pipe or synthetic exhaust gas test equipment. A direct current of 12 V is applied between the contacts and the electrical resistance is measured as a measure of the conductivity of the semiconductor.

Another way to process the powders obtained in Examples 1 and 2 into sensors is to place the powders on a support plate using thick film technology with customary diluents. For this purpose, the powders obtained in Examples 1 and 2 are milled with toluene and ground in a ball mill to a particle size of 10 to 30 μm. The finely ground oxide is further stirred with common admixtures such as turpentine oil or glycerin in toluene or thick-film technology to form a paste, and then pre-plated with dimensions 0.6 mm x 5 mm x 51 mm made of aluminum oxide. The two conductor tracks are applied using thick-film technology to one end of the arranged carrier plate, the two conductor tracks having a spacing of 0.02 to 2 mm below the applied semiconductor material. The coated semiconductor has a thickness of 20 to 10,000 μm, especially
It has a thickness of 50-100 μm. This material is heated to 1100℃
The parts of the platinum conductor track which are not covered by the semiconductor oxide are covered with borosilicate glass up to the parts that serve as connection leads and baked for 2 hours at 900 DEG C. A heating device consisting of a zigzag-shaped platinum conductor track can be installed on the back side of the plate, and in the case of gas containing a large amount of carbon, this heating conductor track is similarly covered with glass to prevent individual It is advantageous to prevent short circuits between conductor tracks. A direct current of 12 V is applied to the two conductor paths leading to the semiconductor layer, and the electrical resistance is again measured as a measure of the conductivity of the semiconductor.

FIG. 2 shows the relationship between resistance and gas composition for eight semiconductor oxides. In this case the resistance is plotted on a logarithmic scale on the vertical axis. Curve 1 is MgO0.6
Obtained with Cr ₂ O ₃ doped with mol %. This is λ=1
This is a typical example of a semiconductor that shows a large sudden change in resistance in the oxygen excess range, but the resistance no longer changes in the oxygen excess range. This is a characteristic similar to that of a commonly used potentiometer λ sensor. Curve 2 shows the results for a semiconductor sensor consisting of tin dioxide doped with 9% MgO.
This semiconductor is described in the aforementioned Japanese Patent Application No. 52-128550. This curve goes into the oxygen deficient range.Curve 3
and λ does not spread as deeply as in the case of 4, and furthermore λ
It is clear that in the range = 1 the rise of the curve is very steep, it flattens out significantly in the oxygen excess range and then falls very sharply. Such a characteristic is, of course, unsuitable for measurements in the range λ=0.90 to 1.20, since the course of the curve has two inflection points.

Curves 3 and 4 are obtained for doped cerium dioxide according to the invention, the oxide of curve 3 being
0.9 mol % Al ₂ O ₃ , curve 4 doped with MgO 3 mol %. Although the two curves extend more widely into the oxygen-deficient range than curve 2, and the course is not linear, the change in the rise of this curve is 1
direction only (no inflection point) and much more slowly than in curve 2. Therefore, this semiconductor
It can be suitably used for measurements in the λ range of 0.90 to 1.20.

Furthermore, curve 5 is Y ₂ O ₃ 0.3 mol%, curve 6 is
TiO ₂ 2 mol %, curve 7 is Ta ₂ O ₅ 5 mol %, curve 8
shows the properties of cerium dioxide doped with 5 mol % of V ₂ O ₅ . In both cases, the curve rises without an inflection point,
It shows a slope similar to curves 3 and 4 in the range λ=0.90 to 1.20.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the conductivity and oxygen partial pressure of three types of semiconductors, and Figure 2 is a diagram showing the relationship between the resistance and oxygen or carbon monoxide content of various semiconductors, including the semiconductor of the present invention. be.

Claims (1)

[Claims] 1. The semiconductor is made of a doped metal oxide, and its conductivity changes continuously with the gas composition in the presence of oxygen and oxidizable components, such that the oxygen partial pressure in the total amount of exhaust gas is approximately In semiconductors for sensors that measure the content of oxidizable components such as oxygen and/or carbon monoxide in exhaust gases by changes in conductivity, where a change in conductivity changes by at least one order of magnitude, the conductivity changes by at least one order of magnitude. Magnesium oxide, aluminum oxide, yttrium oxide, titanium dioxide,
A semiconductor sensor for measuring the content of oxygen and/or oxidizable components in a gas, characterized in that it consists of cerium dioxide doped with tantalum pentoxide, niobium pentoxide or vanadium pentoxide. 2 Cerium dioxide is 0.1 to 10 mol% of the above oxide
A semiconductor according to claim 1 doped with .