Metal paste for gas sensor electrode formation

Abstract

To be provided is a metal paste from which an electrode having high electrode activity as a sensor electrode of various gas sensors can be produced. The present invention is a metal paste for forming a gas sensor electrode obtained by dispersing a conductive particle including Pt or a Pt alloy and a ceramic powder including zirconia or stabilized zirconia, or any of zirconia and stabilized zirconia and one or more oxides of La, Ce, Pr, Nd, Sm, and Hf in a solvent, the metal paste further including an inorganic oxide particle containing alumina and an insoluble particle that is insoluble in the solvent, in which 0.5 or more to 3.0 mass % or less of the inorganic oxide particle and 1.0 to 5.0 mass % of the insoluble particle are dispersed based on the mass of the solid content of the conductive particle, the ceramic powder, the inorganic oxide particle, and the insoluble particle.

Claims

1. A metal paste for forming a gas sensor electrode, the metal paste consisting of a dispersion of: a) a conductive particle consisting of Pt or a Pt alloy, b) a ceramic powder consisting of zirconia or stabilized zirconia, or any of zirconia and stabilized zirconia and one or more oxides of La, Ce, Pr, Nd, Sm, and Hf in a solvent, c) an inorganic oxide particle containing alumina, and d) an insoluble particle that is insoluble in the solvent, wherein the insoluble particle is one or more selected from the group consisting of acryl, polyethylene, polyethylene terephthalate, polycarbonate, fluorine resin, and theobromine, wherein 0.5 to 3.0 mass % of the inorganic oxide particle and 1.0 to 5.0 mass % of the insoluble particle are dispersed based on mass of solid content of the conductive particle, the ceramic powder, the inorganic oxide particle, and the insoluble particle to form the gas sensor electrode.

2. The metal paste for forming a gas sensor electrode according to claim 1, wherein a particle diameter of the inorganic oxide particle is 5 to 500 nm.

3. The metal paste for forming a gas sensor electrode according to claim 1, wherein a particle diameter of the insoluble particle is 0.5 to 3 μm.

4. The metal paste for forming a gas sensor electrode according to claim 1, wherein the conductive particle comprises any of Pt and a Pt—Pd alloy containing 30 mass % or less of Pd.

5. The metal paste for forming a gas sensor electrode according to claim 1, wherein a particle diameter of the conductive particle is 5 nm to 2 μm.

6. The metal paste for forming a gas sensor electrode according to claim 1, wherein a dispersed amount of the ceramic powder is 10 to 20 mass % based on the mass of the solid content.

7. The metal paste for forming a gas sensor electrode according to claim 1, wherein the solvent is one or more kinds of ethylene glycol, propylene glycol, ethylene glycol monophenyl ether, benzyl alcohol, kerosene, paraffin, γ-butyrolactone, N-methyl pyrrolidone, butyl carbitol, turpentine oil, α-terpineol, and terpineol.

8. The metal paste for forming a gas sensor electrode according to claim 2, wherein a particle diameter of the insoluble particle is 0.5 to 3 μm.

9. The metal paste for forming a gas sensor electrode according to claim 2, wherein the conductive particle comprises any of Pt and a Pt—Pd alloy containing 30 mass % or less of Pd.

10. The metal paste for forming a gas sensor electrode according to claim 3, wherein the conductive particle comprises any of Pt and a Pt—Pd alloy containing 30 mass % or less of Pd.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram for describing the configuration of a general oxygen sensor.

(2) FIG. 2 is a diagram for describing the inside (three-phase interface) of an electrode of an oxygen sensor.

(3) FIG. 3 is a cross sectional photograph and a surface photograph of an electrode produced in an embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

(4) Hereinafter, an embodiment of the present invention will be described. In this embodiment, metal pastes were produced in 63 wt % of Pt (a particle diameter of 0.7 μm) used as a conductive particle, 15 wt % (8 mol) of YSZ (yttria-stabilized zirconia: a particle diameter of 0.2 μm) was used as a ceramic powder, and further, various inorganic oxide particles and insoluble particles each having a different particle diameter were mixed. Incidentally, diamond used as an insoluble particle was MD800 produced by Tomei Diamond Co., Ltd. and graphite was SGP-3 produced by SEC Carbon Ltd. Then, these metal pastes were applied to substrates and then calcined to form electrodes. Electrical characteristics of these electrodes were evaluated.

(5) The metal paste was produced as follows. Each powder was mixed and then placed in terpineol as a solvent. Further, a diamine-based surfactant and ethyl cellulose were added thereto, followed by being mixed and kneaded in a three-roll mill to form a paste. The mixing amount of the mixed powder was 80 mass % with respect to the whole paste.

(6) After the metal paste was produced, an electrode was formed and evaluated. The electrode was formed by applying the metal paste on a 99 mass % YSZ green sheet (a thickness of 0.3 mm) by screen printing. Thereafter, the resultant was calcined at 1450° C. for 1 hour to form an electrode. The electrode was produced such that the dimension thereof after calcination would be 2 mm×4 mm and 10±3 μm thick.

(7) Since the evaluation on the formed electrodes was intended for conductivity (resistance), the evaluation was performed based on a current value in a state where a DC voltage (300 mV) was applied in air at 700° C. As for the evaluation, based on the measured current values, a case where the current value was less than 5 mA was designated as “x,” a case where the current value was 5 mA or more but less than 5.5 mA was designated as “Δ,” a case where the current value was 5.5 mA or more but less than 6 mA was designated as “◯,” and a case where the current value was 6 mA or more was designated as “⊙.”

(8) Further, in order to evaluate the electrode activity of each electrode, electrode resistance with respect to platinum weight per unit area was measured by an AC impedance method. The evaluation condition was as follows. A paste was printed on both side of a zirconia green sheet under the same condition as described above to prepare a processed and calcined electrode, and a current frequency response with respect to voltages with a frequency from 100 kHz to 100 mHz at an amplitude of 20 mV without DC bias in atmospheric atmosphere at 700° C. was measured. Then, the case of more than 20Ω was evaluated as “x,” the case of more than 15Ω but 20Ω or less was evaluated as “Δ,” the case of more than 10Ω but less than 15Ω was evaluated as “◯,” and the case of 10Ω or less was evaluated as “⊙.” The evaluation results of these electrode characteristics are shown in Table 1.

(9) TABLE-US-00001 TABLE 1 Added Electrode amount of Inorganic oxide particle Insoluble particle characteristics Test Conductive ceramic Particle Added Particle Added Electrode No. particle powder Type diameter amount Type diameter amount Conductivity activity 1 Pt (0.7 μm) 15 wt % Al.sub.2O.sub.3  10 nm 0.5 wt % Acryl 0.8 μm 3.0 wt % ◯ ◯ 2 1.1 wt % ⊙ ⊙ 3 2.3 wt % ⊙ ⊙ 4 3.0 wt % ◯ ◯ 5 2.3 wt % 0.1 μm Δ ◯ 6 1.5 μm ⊙ ⊙ 7 0.8 μm 1.0 wt % ⊙ ⊙ 8 2.0 wt % ⊙ ⊙ 9 4.0 wt % ⊙ ⊙ 10 — — 0 0.1 μm 3.0 wt % X X 11 0.8 μm X Δ 12 1.5 μm X X 13 Al.sub.2O.sub.3  10 nm 2.3 wt % — — 0 X X 14 — — 0 — — 0 X X 15 10 wt % Al.sub.2O.sub.3  10 nm 2.3 wt % Acryl 0.8 μm 3.0 wt % ⊙ ⊙ 16 15 wt % 200 nm ⊙ ⊙ 17 500 nm ◯ ⊙ 18  10 nm 2.0 μm ⊙ ⊙ 19 3.0 μm ⊙ ⊙ 20 5.0 wt % ◯ ◯ 21 MgO  10 nm 2.0 wt % 0.8 μm 3.0 wt % ⊙ ⊙ 22 Al.sub.2O.sub.3  10 nm 2.3 wt % PE 1.0 μm ⊙ ⊙ 23 PET 1.2 μm ⊙ ⊙ 24 Al.sub.2O.sub.3 200 nm 0.5 wt % Diamond 0.8 μm  10 wt % ◯ ◯ 25 1.1 wt % ⊙ ⊙ 26 3.0 wt % ◯ ◯ 27 Al.sub.2O.sub.3 200 nm 2.3 wt % Diamond 0.8 μm .sup. 5 wt % ◯ ◯ 28 .sup. 8 wt % ⊙ ⊙ 29  10 wt % ⊙ ⊙ 30 13.5 wt %  ⊙ ⊙ 31  15 wt % ◯ ⊙ 32 17.5 wt %  X X 33 Al.sub.2O.sub.3 200 nm 2.3 wt % Diamond 0.2 μm  10 wt % ◯ Δ 34 0.5 μm ⊙ ⊙ 35 1.4 μm ⊙ ⊙ 36 Al.sub.2O.sub.3 200 nm 2.3 wt % Carbon 3.0 μm .sup. 5 wt % ⊙ ◯

(10) First, from a study on the addition effect of the inorganic oxide particle and the insoluble particle, it can be confirmed from the comparison of Test No. 3 with Test Nos. 10 to 14 that favorable electrode characteristics are exhibited by adding both of the components to the metal paste. The electrode characteristics are not sufficient not only in a case where both of the inorganic oxide particle and the insoluble particle are not added, but also in a case where only one of them is added. Further, even when the particle diameter of the insoluble particle is adjusted as in Test Nos. 10 to 12, it is not possible to improve the electrode characteristics if the inorganic oxide particle is not added.

(11) Regarding the added amount of the inorganic oxide particle in a range (0.5 to 3.0 mass %) studied in this embodiment, the electrode acts as an effective electrode (Test Nos. 1 to 4 and Nos. 24 to 26). It can be said that a preferable added amount is 1.1 to 2.3 mass %. Further, regarding the particle diameter of the inorganic oxide particle, with a range of 10 nm to 500 nm, a favorable result can be achieved (Test Nos. 3, 16, and 17). In addition, it is inferred from the result of Test No. 17 that, when the particle diameter exceeds 500 nm, the electrode characteristics may be adversely affected. Incidentally, as the inorganic oxide particle, in addition to alumina, magnesia is also applicable (Test No. 21).

(12) Further, regarding the types of the insoluble particle, in addition to organic substances such as acryl, PE, and PET (Test Nos. 1 to 23), diamond and carbon were applicable (Test Nos. 24 to 36). In the case of using the organic substance, when the added amount thereof was 1.0 to 5.0 mass %, favorable electrode polarity was confirmed (Test Nos. 3, 7 to 9, and 20). Furthermore, in the case of using diamond or carbon, when the added amount was 1.0 to 15.0 mass %, favorable electrode characteristics were confirmed (Test Nos. 27 to 32, and 36).

(13) Regarding the particle diameter of the insoluble particle, the insoluble particles having a particle diameter of 0.8 to 3.0 μm exhibited favorable characteristics (Test Nos. 3, 6, 18, 19, 34, and 35). However, in the case of the insoluble particles having a fine particle diameter of 0.2 μm or less, conductivity is slightly insufficient (Test Nos. 5 and 33). For this reason, the particle diameter is preferably larger than 0.2 μm.

(14) Incidentally, regarding the ceramic powder, study on the added amounts of 10 mass % and 15 mass % was conducted in this embodiment, and both of the added amounts exhibited favorable results (Test Nos. 3 and 15).

(15) FIG. 3 shows results obtained when the cross section of the electrode produced in Test No. 3 and the cross section of the electrode produced in Test No. 14 are observed. As clearly seen from FIG. 3, the electrode produced in Test No. 3 has a porous structure having a large number of pores. Further, a state where a platinum particle (a white portion in the photograph) is not excessively sintered but is appropriately dispersed is shown. The electrode produced in Test No. 14 was dense with very few pores.

INDUSTRIAL APPLICABILITY

(16) According to the present invention, a porous electrode film can be formed while the conductive metal and the ceramic powder are dispersed in a fine state. The present invention is preferably used as a metal paste for forming an oxygen sensor electrode or a sensor electrode of a gas sensor such as NOx sensor and can make the film thickness of an electrode film thin. Therefore, the present invention can lower costs of various sensor devices.