JPS6222421B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing an oxygen concentration detection element made of a transition metal oxide and used in an exhaust gas purification system for detecting the oxygen concentration in gas. The air/fuel ratio (A/F) in the gas mixture typically supplied to internal combustion engines or combustion equipment.
When is near the stoichiometric point, the oxygen partial pressure in the gas in the equilibrium state after combustion changes suddenly. In automotive internal combustion engines, in order to minimize the emissions of harmful components such as NOx, CO, and HC in the exhaust gas, the air-fuel ratio (A/
F) needs to be rapidly controlled within a range very close to the stoichiometric point. To this end, a sensing element is required that exhibits rapid response characteristics to changes in oxygen concentration in the gas. Further, this sensing element is required to have excellent durability so that it can be used in high-temperature exhaust gas for a long time. As a sensing element used for the aforementioned purposes
There are oxygen concentration battery elements using ZrO ₂ ceramics and electrical resistor elements using transition metal oxide ceramics such as titanium oxide. Regarding the latter, it basically uses transition metal oxide ceramics such as titanium oxide, which exhibits an electrical resistance value depending on the oxygen concentration in the detection gas, as the gas-sensitive body, and further increases the response speed. Therefore, it is practically useful to support a noble metal catalyst thereon.
FIG. 1 is an example of a graph showing the relationship between the electrical resistance value of a titanium oxide type sensing element and the air-fuel ratio (A/F). This transition metal oxide ceramic is sintered into a porous state in order to obtain high-speed gas responsiveness, and controlling the porosity of the pores is important in order to satisfy gas responsiveness and longevity. . Conventionally, the manufacturing method for this element was to calcinate transition metal oxide powder as a raw material at a high temperature (for example, 1000°C), wet mill it in a ball mill, and then turn it into a slurry consisting of a binder such as polyvinyl butyral, a plasticizer, a solvent, etc. is formed, dried into a tape shape using a wet molding method, then punched out, and the ends of the precious metal wires that will become the electrode wires are buried and joined together, which is then sintered at high temperature.
Get the element. Control of porosity is greatly influenced by powder calcination conditions, pulverization conditions, slurry composition, and calcination conditions. If highly active powder is to be sintered to a suitable porosity, it must be fired at a low temperature, and such devices have poor life characteristics when operated at high temperatures. The present invention provides a method for manufacturing a sensing element that provides optimum porosity without being significantly influenced by powder manufacturing conditions, element manufacturing conditions, etc. That is, butyryl group 35~ as a binder
40wt%, acetyl group 12-15wt%, hydroxyl group 0-3wt
% of cellulose acetate butyrate resin in combination with n-butyl acetate as a solvent. Mix this binder system with ceramic powder,
When wet-molded into a sheet, the solvent evaporates during the drying process, and the binder resin exceeds its solubility in the solvent, causing gelation. At this time, the ceramic powder is aggregated into secondary particles of 5 to 20 microns, and a compact with a coarse density is finally completed. By processing this into a sensor shape using a predetermined method and firing it, a sensor with optimal porosity can be obtained. Even if highly active raw materials are used, grain growth only reaches the size of secondary particles and is difficult to progress beyond that.
Therefore, the particle size and porosity can always be controlled to be the same without being greatly influenced by the particle size and activity of the raw materials. Hereinafter, the present invention will be described with reference to examples. The titanium oxide raw material is calcined at 1090℃ for 4 hours, wet-pulverized in a ball mill for 20 hours, and then dried. 40 g of the obtained titanium oxide powder, 9 g of cellulose acetate butyrate resin (trade name CAB-381-05, manufactured by Eastman Chemical Company), 11 g of dibutyl phthalate as a plasticizer, and 55 g of n-butyl acetate as a solvent.
Knead in a ball mill for 10 hours to make slurry,
Create a sheet with a thickness of 200μ using the doctor blade method. Punch out this sheet into a disk with a diameter of 3.5 mm. A pair of platinum wires with a wire diameter of 250μ are placed parallel to each other at a spacing of 1.5 mm between the two disks obtained in this way, and the disks punched out in the same way are stacked on top of each other and pasted together under pressure. match. An organic binder was used as a binding agent during lamination. The chip thus produced was sintered at 1250°C for 1 hour to obtain a sensing element. The table below lists the manufacturing conditions and characteristics of devices made using a similar method. A for binder system in the table
Those marked with are those made by this method, and those marked B are those made by the conventional method, and both porosity and responsiveness are less affected by process conditions than in the conventional method. The above embodiment corresponds to No.5.

[Table] The details of the evaluation method are as follows. The response time of the sensing element was measured by using a propane gas burner and connecting the sensing element as shown in FIG. A propane gas burner is a combustion device in which the mixture ratio of air and propane gas is automatically switched at high speed alternately from λ = 0.9 to λ = 1.1, and the detection element is located in the high temperature exhaust gas of this burner (about 700°C). installed on. In Figure 2, 1 is a detection element, 2 is a 20KΩ fixed resistor, E ₀ is a constant voltage power supply,
E is the output voltage. Corresponding to the change in λ mentioned above, the resistance value of the sensing element 1 changes greatly, and therefore E/
E ₀ also changes significantly. The response speed is the voltage ratio E/E ₀
The time required for λ to change from 0.9 to 0.2 (λ=0.9
It is the sum of the time required for the change (when changing from λ = 1.1 to λ = 1.1) and vice versa (when changing from λ = 1.1 to λ = 0.9). The durability test used a gasoline combustion device, and the sensing element of the test sample was left in combustion exhaust gas at a temperature of 750 to 800 degrees Celsius for 100 hours in a combustion state with excess fuel. As described above, the present invention provides a method for manufacturing a stable oxygen concentration sensing element that is not affected by process conditions.

[Brief explanation of the drawing]

Figure 1 shows the air-fuel ratio (A/
A graph showing an example of the relationship between F) and electrical resistance value,
FIG. 2 is a circuit diagram of a measuring circuit used to measure the response speed of the sensing element.

Claims (1)

[Claims] 1 Cellulose acetate butyrate resin consisting of 35 to 40 wt% butyryl groups, 12 to 15 wt% acetyl groups, and 0 to 3 wt% hydroxyl groups as a binder, n-butyl acetate as an organic solvent, and a plasticizer and a slurry consisting of transition metal oxide powder. A method for manufacturing an oxygen concentration sensing element, characterized in that it is manufactured from a sheet formed by a wet molding method.