Solid Electrolyte for Use in Oxygen Concentration Sensor

This invention relates to an oxygen sensor used in detecting the oxygen concentration in a gas such as the exhaust gas from an internal combustion engine of automotive vehicles. More particularly, it relates to a solid electrolyte consisting essentially of sintered zirconia, which has a good stability of crystal structure and a good oxygen ion conductivity.

Among the solid electrolytes of this sort for use in an oxygen sensor, there has been known a sintered body of stabilized zirconia which is prepared by mixing zirconia with a stabilizer such as yttria, calcia, or ytterbia followed by sintering at high temperatures, as disclosed in U.S. Pat. No. 4,219,359 (Miwa et al.).

However, it has been found that the solid electrolyte composed of said stabilized zirconia, although expected to be of a crystal structure constituted by a cubic phase alone, sometimes exhibits a poor oxygen ion conductivity, an insufficient stability of crystal structure leading to destruction of the structure, and, moreover, a very poor thermal shock resistance.

The inventors examined a sintered body of stabilized zirconia, which had presented the above problems, by finely pulverizing said sintered body as a sample and analyzing the pulverized sample by the powder X-ray diffraction method. A monoclinic phase was detected beside the cubic phase. This suggests that while only the cubic phase is present initially, partial phase transition from cubic to monoclinic takes place under the practical use conditions, e.g. when a temperature change in the atmosphere was made or continuous operation in a low temperature range (e.g. about 200.degree. to 300.degree. C.) carried out. The partial transition of a cubic phase into a monoclinic is accompanied with a volume expansion which tends to give rise to the destruction of the structure of the sintered body. The oxygen ion conductivity seems to be reduced due to such a destruction as well as the presence of the monoclinic phase having an intrinsically low oxygen ion conductivity. Thus, some of the sintered zirconia forming a solid electrolyte undergo the partial transition from a cubic phase to monoclinic phase during practical use, even though they initially had a crystal structure of the cubic phase alone. Therefore, this presents an important problem of how to reduce the proportion of the monoclinic phase.

Thus, the inventors have found the fact that when the cubic-phase sintered zirconia was finely ground, there may be detected a monoclinic phase.

A primary object of this invention is to provide a solid electrolyte for use in an oxygen sensor exhibiting an excellent thermal shock resistance as well as a good oxygen ion conductivity by maintaining the monoclinic phase content of the sintered zirconia in a pulverized state at up to about 5% by weight based on the total of both the phases and the surface-average grain size before grinding, at about 5 microns or less, thereby preventing the sintered body from destruction of the structure.

Other objects and features of this invention will become apparent from the following description.

In order to maintain the surface grain size at or below about 5 microns and the proportion of monoclinic phase in the sintered material at or below about 5% by weight, as specified in this invention, the concentration of a stabilizer such as yttria and the sintering temperature should be properly selected; namely, the concentration of a stabilizer such as yttria is selected, for example, within the range of from about 6.5 to 8% by mol (the remainder being zirconia) and the sintering temperature of the material within the range of from about 1420.degree. to 1570.degree. C., preferably from about 1470.degree. to 1520.degree. C.

According to this invention, there is provided also a method for manufacturing a solid electrolyte for use in an oxygen concentration sensor composed of a sintered body of zirconia having a crystal structure entirely of a cubic phase, which sintered body has a surface-average grain size of about 5 microns or less and in which the crystal structure changes, upon finely grinding, into a structure of intermingled cubic and monoclinic phases, the proportion of said monoclinic phase being about 5% or less based on the total of both the phases, which process comprises mixing zirconia with about 6.5 to 8% by mol of a stabilizer such as yttria, grinding the resulting mixture to a powder having a BET specific surface area of about 9 to 20 m.sup.2 /g, adding a molding aid to the ground mixture to prepare a slurry containing about 40 to 45% by weight of water, spraydrying the slurry, molding the dried material into a predetermined configuration, and firing the molded material at a temperature of about 1420.degree. to 1570.degree. C. under an oxidizing atmosphere at normal pressure.

The technical-grade zirconia being used as a raw material in manufacturing the solid electrolyte of this invention contains, as a rule, certain amounts of impurities such as hafnium oxide, titanium oxide, iron oxide, and so on. Therefore, the constituents of the solid electrolyte of this invention are permitted to contain the above-noted impurities.

Furthermore, in preparing the sintered zirconia according to this invention, a sintering promoter such as aluminum oxide and/or silicon oxide can be used. The suitable amount added of a promoter ranges from about 1 to 5% by weight for aluminum oxide and from about 0.2 to 2% by weight for silicon oxide, said amounts being based on the total weight of zirconia and the stabilizer such as yttria.

An example of the structure of an oxygen sensor employing the soid electrolyte of this invention is described below by referring to the accompanying drawings.

FIG. 1 is a schematic drawing to illustrate the method employed in this invention for measuring the surface-average particle size.

FIG. 2 is an electric connecting diagram to illustrate the method employed in this invention for measuring the resistance.

FIGS. 3 and 4 are microphotographs of a solid electrolyte to illustrate the invention.

FIG. 5 is a partial sectional view illustrating one example of the construction of an oxygen sensor employing the solid electrolyte of this invention.

In FIG. 5, 1 is an oxygen concentration detecting element formed from the solid electrolyte comprising about 92.5% by mol of zirconia and about 7.5% by mol of yttria in the form of solid solution. The element 1, in a cup-like form open at one end and closed at the other, is coated on inner and outer surfaces with porous layers of the first electrode 2 and the second electrode 3, which are composed of platinum and are applied by chemical plating, vacuum deposition or paste baking. The second electrode is covered with a heat-resistant porous protective coating of metal oxides such as magnesia-alumina spinel (MgAl.sub.2 O.sub.4) and the like, in the area where it is exposed to the exhaust gas. Tubular housing 5 made of a heat-resistant metal is provided around the outer surface of the oxygen concentration detecting element 1. In the annular space between the housing 5 and the oxygen concentration detecting element 1, there are disposed conductive graphite powder 6, asbestos ring 7, and conductive metallic ring 8. One end of protective tube 9 made of a conductive heat-resistant metal, e.g. stainless steel, is inserted into said annular space. This end of the protective tube 9 carries spacer ring 10 of a conductive metal supported on flange 9a. The upper edge of housing 5 is fixed to the protective tube 5 by calking to hold in place both the housing 5 and the protective tube 9 against the oxygen concentration detecting element 1. The housing 5 is provided with flange 11 to fix the device to the exhaust line (not shown). The area of the outer surface of oxygen concentration detecting element 1, where it is not exposed to the exhaust gas, is covered with a portion of the protective tube 9, as mentioned above. Stem 12, made of a conductive metal, e.g. stainless steel, and having through-hole 12a along the center line, is fixed, through conductive graphite powder 13, to the inner wall of the oxygen concentration detecting element 1. Insulator 14 of alumina or the like is disposed in the upper end portion of the protective tube 9 and held in place by calking the upper edge of the protective tube. Hollow tube 15 made of a conductive metal, e.g. stainless steel, is inserted through the insulator 14 and spring 16 is mounted between the flange 15a of tube 15 and the shoulder 12a formed in the stem 12. The stem 12 is tightly fixed in place by being pressed against the oxygen concentration detecting element 1 by the pressure exerted from the set load of the spring 16. A piece of stainless steel wire 17 is fixed by welding at one end thereof to the wall of through-hole 12a of the stem 12. A leading wire 19 from connector 18 is connected to the end of hollow tube 15 opposite to the flange 15a by calking the terminal piece 20. A duplex protective tube 21 having a large number of small holes 21a is fixed to the lower end of housing 5 so as to cover the area of outer surface of the oxygen concentration detecting element 1, where it is exposed to the exhaust gas. The first electrode 2 of the oxygen concentration detecting element 1 is electrically connected to the leading wire 19 through the graphite powder 13, stem 12, stainless steel wire 17, and hollow tube 15 or intervening spring 17. The second electrode 3 is electrically connected to the housing 5 through the conductive graphite powder 6 and conductive ring 8. Heat-resistant rubber tubing 22 is fitted around the protective tube 9 through the intervening collar 23.

The invention is illustrated below in detail with reference to Example in which yttrium oxide was used as the stabilizer, though other metallic oxides such as, for example, calcium oxide, magnesium oxide, scandium oxide and ytterbium oxide may be used.

Commercially available zirconium oxide of about 99.5% in purity, containing a small amount of hafnium oxide, (HfO.sub.2) and having a BET specific surface area of 2.5 m.sup.2 /g was blended with commercially available yttrium oxide of about 99.9% in purity, having a BET specific surface area of about 3.8 m.sup.2 /g, in predetermined molar ratios as shown in Table 1. The resulting mixture was ground so as to obtain a powder having a specific surface area of about 12 m.sup.2 /g. The powdered mixture was mixed with a molding aid such as polyvinyl alcohol or the like to form a slurry containing about 40 to 45% by weight of water. The slurry was spray-dried and molded into a rectangular, disc or tubular specimen by means of a hydrostatic press at a pressure of about 1 ton/cm.sup.2. Each specimen was fired at a predetermined temperature, as shown in Table 1, for 3 hours under an oxidizing atmosphere at normal pressure. The fired rectangular specimen was tested for the flexural strength, stability of structure, and surface-average grain size by the methods described later. The specimen in a disc form was polished to obtain a mirror-finished surface and the monoclinic phase content of the surface was determined; the monoclinic phase content of the pulverized specimen was also determined. Another disc specimen was ground to a thickness of 2.5 microns, then coated on both sides with a silver paste in a circular area of 200 mm in diameter, and baked. The disc thus treated was tested for the resistance at a temperature of 400.degree. C. and a content current of 10 .mu.A (measurement was made after the lapse of 10 minutes from the time when the temperature reached 400.degree. C. and the electric current was turned on). The tubular specimen was evaluated for the thermal shock resistance according to the methods and test conditions as described below.

The procedures and conditions of each test were as described below.

(a) Flexural strength: A sintered body was cut and ground to a rectangular rod of 5.times.8.times.60 mm in size and tested for the flexural strength by the known three-point bending test method.

(b) Stability of structure: After heating a rectangular sintered body at 200.degree..+-.15.degree. C. for 1000 hours, the fracture was examined by a scanning electron microscope (SEM) at 10,000 magnifications to determine the presence or absence of intergranular rupture caused by volume expansion originated from the phase transition.

(c) Monoclinic component content: A sintered body was finely pulverized to a particle size below 10 microns whereby, if a tetragonal phase was present in the sintered body, the tetragonal phase was completely transformed into a monoclinic phase and the monoclinic component content was calculated by the equation:

When a tetragonal phase is detected at the surface or lapped surface of the sintered body, there arises a question whether or not the tetragonal phase is completely eliminated on pulverizing. With this respect, it was confirmed that after pulverization no X-ray diffraction peak of the tetragonal phase was found, under the following conditions, in the sample containing initially a tetragonal phase, indicating that the phase transition had been completed. In the above equation M and C represent monoclinic and cubic phases, respectively. The X-ray diffraction intensity I is a value integrated by peak-height x half-width.

Conditions for X-ray diffraction:

Target: (Rotary type) Cu

Tube Voltage: 40 KV

Tube current: 80 mA

Divergency slit: 1.degree.

Receiving slit: 1 mm

Goniometer speed: 2.theta.-1.degree./minute

Chart speed: 20 mm/minute

Time constant: 1 second

Full scale: 4,000 cps-400 cps

Provided with graphite monochrometer Limit of detection for monoclinic phase: >0.5 weight %

(d) Surface-average grain size: The surface-average grain size was determined on the SEM photograph of the sintered body surface taken at 1,000-10,000 magnifications. A straight line was drawn at random across the photograph, as shown schematically in FIG. 1. The diameter of each of the grains lying across the line was determined in terms of the maximum width (d in FIG. 1) of the grain measured in the direction vertical to said straight line. Another straight line was drawn parallel to the above line with a sufficient space therebetween so as not to cross over the grains which had already been measured and the diameter determination was repeated on all the grains lying across the straight line. The determination was further repeated until the number of measured grains had amounted to 20-50. The average value of the determined diameters was taken as a surface-average grain size.

(e) Resistance: The disc specimen 1 provided with silver electrodes 2 and 3, as shown above, was connected to a circuit shown in FIG. 2 and the resistance R was determined under a constant current load. In FIG. 2, A is an ammeter, V a voltmeter, R a rheostat, and E an electric source.

(f) Thermal shock resistance: The aforesaid tubular specimen of the sintered body was a tube closed at one end having an inner diameter of about 4 mm and an outer diameter of about 8 mm, which was prepared by molding a granulated material by means of a common hydrostatic press at a pressure of about 1 ton/cm.sup.2, followed by baking. Inside the closed end of the tubular sintered body, was placed the heat sensing junction of a CA thermocouple which was connected to a recorder to record the temperature with the lapse of time, whereby the rate of increase or decrease of temperature (.degree.C./second) could be calculated. The relative position of a burner to the zirconia sintered body was adjusted so that the flame of the burner supplied with a mixed fuel gas of compressed air and a city gas could touch the zirconia sintered body at the closed end and over a length of about 1.5 mm measured from the closed end. The recorder operated simultaneously with the lighting of the fuel gas to determine the rate of temperature change. After this thermal shock test, the cooled zirconia sintered body was dyed with fuchsine to examine the cracking and the rate of temperature increase which caused the cracking was determined for each of the zirzonia sintered bodies prepared under varied conditions. The higher the rate of temperature change which caused cracking, the higher the thermal shock resistance was rated.

The results obtained in the example were as shown in Table 1.

As is seen from Table 1, in test Nos. 1 to 3, a monoclinic phase is detected at the surface of the sintered body as well as in the powder formed by pulverizing; the amount of monoclinic phase is large in every case and the occurrence of intergranular rupture is observed; and the resistances are comparatively large. On the other hand, it is seen that in test Nos. 5, 6, 7, 9 and 11, monoclinic phase is not detected at the surface of sintered body but detected in the powder prepared by pulverizing; no intergranular rupture is detected and the resistances are comparatively small. As for the test No. 6, however, it is seen that the surface-average grain size is as large as 17.5 microns and both the flexural strength and the thermal shock resistance are somewhat inferior. In the case of test No. 4 where the monoclinic phase is not detectable at the surface of sintered body but becomes detectable when it is pulverized, it is seen that the monoclinic phase content (i.e. 5.5%) of the powder is larger than those in the cases of test Nos. 5, 7, 9 and 11 so that the intergranular rupture is observed and the resistance is comparatively large in test No. 6.

In the cases of test Nos. 8, 10, 12, 14, 15 and 16 where the monoclinic phase is not detected either at the surface of sintered body or in the powdered state, it is seen that the surface-average grain sizes are large, while the flexural strengths are considerably low and the thermal shock resistances are also low.

In FIGS. 3 and 4, there are shown SEM photographs of the surfaces of sintered bodies of zirconia used in test Nos. 9 and 13, respectively. The magnifications are 2,000 and hence a length of 20 mm in an actual scale on the photographs corresponds to 10 microns. It is seen that with the decrease in surface-average grain size of sintered body, both the thermal shock resistance and the flexural strength become inferior.

On the basis of the results shown in Table 1, the present inventors examined conditions necessary for the sintered body of zirconia to exhibit such desirable characteristics as excellent thermal shock resistance, sufficient stability of structure, low resistance and excellent oxygen ion conductivity. As a result, it has been found that the monoclinic phase content of the sintered body in a pulverized state should be about 5% or less and the surface-average grain size of the sintered body prior to the pulverization should be about 5 microns or less.

As fully described above, according to this invention a solid electrolyte can be effectively stabilized in structure and kept from failure of the structure; an excellent thermal shock resistance and a high oxygen ion conductivity are also resulted. Thus, this invention may provide an oxygen sensing solid electrolyte suitable for practical use.