JPS605548B2 - Zirconia sintered body for oxygen sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a zirconia sintered body suitable for use in an oxygen sensor for detecting oxygen concentration in exhaust gas of an automobile internal combustion engine.

Conventionally, as this type of zirconia mirror body, one in which zirconium oxide and yttrium oxide are solidified is known.

The zirconia sintered body having this composition has good oxygen ion conductivity and has the property of conducting oxygen ions even at a relatively low temperature (approximately 400 oo). By the way, according to confirmation experiments conducted by the present inventors, when the composition range with the best oxygen ion conductivity was selected, zirconium oxide was 97 to 89 mol%, and yttrium oxide was 3 to 11 mol%.

Generally, a high firing temperature of approximately 18,000C or higher is required to cause a solid solution reaction between zirconium oxide and yttrium oxide, and at such a high firing temperature, crystal growth becomes difficult and the bulk specific gravity decreases. becomes weaker in strength. Therefore, in order to lower the firing temperature, it is considered to add oxidized frog, which is known as a dawn accelerator in the general ceramic field. When the present inventor actually added silicon oxide, the firing temperature dropped to about 160 mm, but since a minute current was passed through the zirconia Akatsuki silk body to stabilize the terminal voltage, etc. We found that the terminal voltage tends to increase under relatively low temperature conditions (approximately 400 pm or less). We need to investigate the cause of this. When we crushed the zirconia striations and examined them using an electron microscope, we found that a film of silicon oxide was present at the grain boundaries of the crystal structure where zirconium oxide and yttrium oxide were dissolved. This is because the diffusion of oxygen ions is hindered and the electrical resistance of the zirconia pseudo-consolidation increases. As mentioned above, when the terminal voltage of the zirconia silk body increases, the following problems occur.

That is, when using an oxygen sensor as a measure against exhaust gas from an automobile internal combustion engine, the terminal voltage that the oxygen sensor shows depending on the difference in oxygen concentration between the exhaust gas and the atmosphere is compared with a set voltage, and based on this comparison, Feedback control is performed to bring the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine close to the stoichiometric air-fuel ratio, and under such operating conditions, as mentioned above, if the terminal voltage of the zirconia competitive charge is large in the low-temperature castle, the output voltage will also increase. As a result, the terminal voltage characteristics of the oxygen sensor as a whole deviate, and the above set voltage becomes relatively low.
Therefore, incorrect air-fuel ratio control is performed. According to confirmation experiments conducted by the present inventors, it has been confirmed that when the increase in the terminal voltage of the zirconia crystal body exceeds 0.1V, the above-described deliberate air-fuel ratio control is performed. Furthermore, as the ambient temperature around the zirconia sintered body rises, the electrical resistance of the zirconia sintered body decreases because the zirconia sintered body itself has negative resistance-temperature characteristics, so the terminal voltage decreases. .

Therefore, the above-mentioned problems do not occur in high-temperature castles. In view of the above points, the present invention uses a small amount of boron oxide that does not hinder the diffusion of oxygen ions, and uses aluminum oxide to suppress the decrease in the sintering promotion effect caused by reducing the amount of silicon oxide. By doing so, it is an object of the present invention to provide a zirconia sintered body which can fully satisfy the braning accelerating effects of silicon oxide and aluminum oxide, and which has almost no adverse effects from these sintering accelerators. In the present invention, since the zirconia aged compact exhibits a terminal voltage depending on the oxygen concentration difference between the reference gas side and the detection gas side, it must be made delicate, and therefore the fired high specific gravity is at least 5.35.
(evening/earth) or more is necessary. In other words, a high firing latitude is required. In addition, oxygen ions must be diffused well. As mentioned above, if silicon oxide or aluminum oxide is used, the firing temperature can be lowered to promote crystallization, but as mentioned above, if the amount of silicon oxide is large, the diffusion of oxygen ions will be hindered. Therefore, if these are taken care of, silicon oxide is 0. The amount of aluminum oxide is preferably 0.1% to 1% by weight. Note that 99.5 to 89.4% by weight consists of zirconium oxide and yttrium oxide. below,
An example will be shown in which the effect on the appearance voltage and bulk specific gravity is shown when the mixing ratio of each Z2, Y203, Si02, and AI203 is changed.

First, the implementation procedure will be explained. Commercially available Zr02, Y203, AI2Q, and Si02 are weighed in predetermined mol %, mixed and pulverized in a pot mill for 1 feeding time.

The mixture, pulverized product, water and binder (e.g. PVM, C
MC, etc.) and granulate using a known spray dryer. The granulated material is molded into a mold with a diameter of 2mm and a thickness of 2mm. The molded product is sintered at 1600 to 1750 qo.
The calcined bulk specific gravity of the zirconia calcined mackerel body thus obtained is measured by a mercury displacement method. Thereafter, as shown in FIG. 1, Pt electrodes 2 and 3 are provided on both sides of this zirconia aggregate 1 using an existing method to prepare a sample. Next, connect Pt wire lead wires to the Pt electrodes 2 and 3 on both sides and put them in a furnace with air atmosphere for 400 minutes.
Heat to. 400 o'clock When it reaches 0, the constant current power supply E turns 0.
．． A current of tsubo A is applied to the sample, and after a predetermined period of time, the terminal voltage generated at both electrodes 2 and 3 of the competitive body in the sample is read using a high internal impedance voltmeter.

We measured how the terminal voltage changes over time. As a result, a relationship as shown in FIG. 2 is obtained, and for each example, the terminal voltage was measured after 5 minutes. The results for each example are shown in Table 1. The terminal voltage is shown as the value after 5 minutes. (Note) Zr02 and Y
When AI203 and Si02 are added to the oxygen ion conductor material with 203, it becomes 100% by weight,
For example, Example No. 1 is AI2030 and SI02.
=1.

Since it is 7% by weight, the remainder is 98%.

3% by weight is Zr02+Y203.

Note that mol% and weight% are used differently for Zr02 and Y.
Since 203 has a solid solution composition, it is better to express it in mol%, and since SI02 and AI203 are accelerators and are added to Zr020 and Y203, it is better to express it in weight%. Good. As understood from Table 1 above, Example No. 1.No. In Nos. 11 and 18, the terminal voltage exceeded 0.1 V after 5 minutes, and the terminal voltage tended to increase with time.

This increase over time is considered to be due to the polarization effect. This is Example No. 18 and Example No.
A comparison with 3 is shown in FIG. These example skin o.
In all cases, the amount of Si02 exceeds 0.6% by weight. Also, Example No. 7.No. 16 has a bulk specific gravity of 5
．． 35th evening/day, indicating that sufficient sintering has not been carried out. These Example No. 7.No. 16
has a large amount of M203 and is influenced by AI203. Furthermore, Example No. In No. 20, the composition range of Z2 and Y203 was outside the range of the present invention, and the oxygen ion conductivity was somewhat poor. Along with this, the oxygen ion diffusion was also poor, and the terminal voltage exceeded 0.1 V. considered to be a thing. Next, one structure of an oxygen sensor using the zirconia crystals of the present invention will be shown and explained.

In Fig. 3, reference numeral 1 denotes an oxygen concentration detection element made of zirconia crystals, which has a cup-like shape with one end closed and the other end solidified with 93 mol% zirconia and 7 mol% yttoria. It has the formation of Porous first and second electrodes 2 and 3 made of platinum having a catalytic action are formed on the outer peripheral surface of the oxygen concentration detection element 1 by methods such as chemical plating, vacuum deposition, and paste phosphorization. There is.
Note that the second electrode 3 on the outer peripheral surface of the oxygen concentration detection element 1
is formed over the outer circumferential surface of the oxygen concentration detection element 1 up to the area on the open □ side that is not exposed to exhaust gas. Further, a heat-resistant and porous protective coating 4 made of a metal oxide such as magnesia or alumina vinyl (MgAl204) is formed on the surface of the second electrode 3 that is exposed to exhaust gas. Reference numeral 5 denotes a cylindrical housing made of heat-resistant metal, and is arranged on the outer peripheral surface of the oxygen concentration detection element 1.

Between the housing 5 and the oxygen concentration detection element 1, conductive graphite powder 6, ring-shaped asbestos 7, and a ring body 8 made of conductive metal are arranged. One end of a protective tube 9 made of steel is placed. Then, a conductive metal smoothing ring 10 is arranged on the flange 9a at the end of the protective tube 9, and the housing 5 is
The oxygen concentration detection element 1 can be detected by caulking the upper end of the
At the same time as the housing 5 is fixed, one end of the protective tube 9 is fixed. Note that a flange 11 is fixed to the housing 5 for fixing it to an exhaust pipe (not shown). The area of the outer peripheral surface of the oxygen concentration detection element 1 that is not exposed to exhaust gas is covered by one end of the protection tube 9 as described above. Reference numeral 12 denotes a stem made of conductive metal (for example, stainless steel), which has a through hole 12a in the center, and is fixed to the inner peripheral side of the oxygen concentration detection element 1 via conductive graphite powder 13.

An insulator 14 made of alumina or the like is caulked and fixed to the other end of the protective tube 9, and a hollow pipe 15 made of conductive metal (stainless steel) is located at the center of the insulator 14.
is fitted with iron, and the flange 15a of the pipe 15 and the stem 12
A spring 16 is interposed between the step portion 12b and the step portion 12b.
The set load of the spring 16 presses the stem 12 against the oxygen concentration detection element 1 and firmly fixes it. 17 is a stainless steel wire, one end of which is welded and fixed to the through hole 12a of the stem 12, and the other end of which is welded and fixed to the upper interior of the hollow pipe 15.

A lead wire 19 of the connector 8 is crimped through a terminal 20 on the opposite side of the hollow pipe 15 to the flange 15a. 2
Reference numeral 1 denotes a double protection tube having a large number of small holes 21a. It is attached to the bottom edge.

The first electrode 2 of the oxygen concentration detection element 1 is connected to the graphite powder 1
3. Stem 12, stainless steel wire 17, lead wire 19 via pipe 15 or via spring 17 midway
The second electrode 3 is electrically connected to the housing 5 via the conductive graphite powder 6 and the conductive ring 8. In addition, 22 is a heat-resistant rubber tube,
It is attached to the protective tube 9 via a collar 23. As described in detail above, according to the present invention, the competitive bonding of zirconium oxide and yttrium oxide in oxygen ion conductor materials can be sufficiently promoted by silicon oxide and aluminum oxide, and moreover, the oxygen ion This has an excellent effect in that diffusion is hardly hindered.

[Brief explanation of the drawing]

FIG. 1 is an electrical wiring diagram for explaining the implementation procedure of the present invention;
FIG. 2 is a characteristic diagram for explaining the effects of the present invention, and FIG. 3 is an overall configuration diagram showing an example of an oxygen sensor using the ant body of the present invention. 1... Zirconia Akatsuki solid, oxygen concentration detection element. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1 Oxygen ion conductor material consisting of 97 to 89 mol% of zirconium oxide and 3 to 11 mol% of yttrium oxide, 99.5 to 89.4% by weight, 0 aluminum oxide
．． A zirconia sintered body for an oxygen sensor, comprising a composition of 5 to 10% by weight of silicon oxide and 0.6% by weight or less of silicon oxide.