Sensor electrode, manufacturing method thereof, and metal paste for electrode formation

Abstract

The present invention relates to a gas sensor electrode including a conductive particle phase made of Pt or Pt alloy and a ceramic particle phase being mixed and dispersed, wherein a rate of content of the ceramic particle phase is 6.0 to 22.0 mass %, and a void ratio is 2.5 to 10.0%, and a dispersion degree of the conductive particle phase per length of 25 μm on the electrode surface is 0.60 to 0.85 μm, and a dispersion degree of the conductive particle phase in the electrode cross section per length of 100 μm in a direction parallel to the electrode surface is 2.0 to 4.0 μm. This electrode can be produced by firing a metal paste made by dispersing, in a solvent, a conductive particle having a core/shell structure in which a core particle such as Pt is covered with a ceramic shell and ceramic powder. The gas sensor electrode according to the present invention has a high electrode activity.

Claims

1. A metal paste for forming a gas sensor electrode, in which (1) a conductive particle having a core/shell structure comprising a core particle made of Pt or Pt alloy and a shell made of ceramic covering at least a portion of the core particle, and (2) ceramic powder, are dispersed in a solvent, wherein the conductive particle is a particle of which average particle diameter is 90 to 500 nm, and the core particle is covered with ceramic of 0.5 to 3.0 mass % with respect to the mass of the conductive particle, a content of the ceramic powder is 5 to 20 mass % with respect to a total mass of the conductive particle and the ceramic powder, a total content of a ceramic component in a metal paste is 6.0 to 22.0 mass % with respect to the total mass of the conductive particle and the ceramic powder, and further, a dispersion degree measured according to a line transect method based on a grind gauge is equal to or less than 15 μm.

2. The metal paste for forming the gas sensor electrode according to claim 1, wherein the particle diameter of the ceramic powder is 100 to 500 nm.

3. The metal paste for forming the gas sensor electrode according to claim 2, wherein the ceramic acting as the shell and the ceramic powder comprise ceramic including ZrO.sub.2.

4. The metal paste for forming the gas sensor electrode according to claim 2, wherein the core particle comprises any of Pt or Pt—Pd alloy including Pd of 30 mass % or less.

5. A manufacturing method for a gas sensor electrode, wherein the metal paste for forming the gas sensor electrode according to claim 2 is applied to a substrate, and is fired at 1300 to 1600° C.

6. The metal paste for forming the gas sensor electrode according to claim 1, wherein the ceramic acting as the shell and the ceramic powder comprise ceramic including ZrO.sub.2.

7. The metal paste for forming the gas sensor electrode according to claim 6, wherein the core particle comprises any of Pt or Pt—Pd alloy including Pd of 30 mass % or less.

8. A manufacturing method for the metal paste for forming the gas sensor electrode according to claim 6, comprising the steps of: making mixed powder by mixing a composite particle having a core/shell structure made of a precious metal particle made of Pt or Pt alloy and a shell made of ceramic covering at least a portion of the core particle and ceramic powder; heating the mixed powder to 700 to 1200° C., adjusting a particle diameter of the composite particle, and forming the conductive particles having the core/shell structure of which average particle diameter is 90 to 500 nm; and dispersing the mixed powder having been subjected to the thermal treatment into a solvent.

9. A manufacturing method for a gas sensor electrode, wherein the metal paste for forming the gas sensor electrode according to claim 6 is applied to a substrate, and is fired at 1300 to 1600° C.

10. The metal paste for forming the gas sensor electrode according to claim 1, wherein the core particle comprises any of Pt or Pt—Pd alloy including Pd of 30 mass % or less.

11. A manufacturing method for the metal paste for forming the gas sensor electrode according to claim 10, comprising the steps of: making mixed powder by mixing a composite particle having a core/shell structure made of a precious metal particle made of Pt or Pt alloy and a shell made of ceramic covering at least a portion of the core particle and ceramic powder; heating the mixed powder to 700 to 1200° C., adjusting a particle diameter of the composite particle, and forming the conductive particles having the core/shell structure of which average particle diameter is 90 to 500 nm; and dispersing the mixed powder having been subjected to the thermal treatment into a solvent.

12. A manufacturing method for a gas sensor electrode, wherein the metal paste for forming the gas sensor electrode according to claim 1 is applied to a substrate, and is fired at 1300 to 1600° C.

13. A manufacturing method for the metal paste for forming the gas sensor electrode according to claim 1, comprising the steps of: making mixed powder by mixing a composite particle having a core/shell structure made of a precious metal particle made of Pt or Pt alloy and a shell made of ceramic covering at least a portion of the core particle and ceramic powder; heating the mixed powder to 700 to 1200° C., adjusting a particle diameter of the composite particle, and forming the conductive particles having the core/shell structure of which average particle diameter is 90 to 500 nm; and dispersing the mixed powder having been subjected to the thermal treatment into a solvent.

14. The manufacturing method for the metal paste according to claim 13, wherein the average particle diameter of the composite particle is 10 to 25 nm.

15. A manufacturing method for the metal paste for forming the gas sensor electrode according to claim 2, comprising the steps of: making mixed powder by mixing a composite particle having a core/shell structure made of a precious metal particle made of Pt or Pt alloy and a shell made of ceramic covering at least a portion of the core particle and ceramic powder; heating the mixed powder to 700 to 1200° C., adjusting a particle diameter of the composite particle, and forming the conductive particles having the core/shell structure of which average particle diameter is 90 to 500 nm; and dispersing the mixed powder having been subjected to the thermal treatment into a solvent.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram for illustrating a structure of a general oxygen sensor.

(2) FIG. 2 is a diagram for illustrating the details of the inside of an electrode of the oxygen sensor (three-phase boundary).

(3) FIG. 3 is cross-sectional pictures and surface pictures of electrode films according to Example 4, Comparative Example 2, and Conventional Example 1.

(4) FIG. 4 is a measurement result of electrode resistances with respect to a platinum weight per unit area of electrode films formed by means of various kinds of metal pastes.

(5) FIG. 5 is a measurement result of electrode activity of electrode films formed using various kinds of metal pastes.

(6) FIG. 6 is a TMA curve illustrating thermal shrinkage of metal pastes according to whether particle diameter adjustment is performed or not (Example 5, Comparative Example 7).

DESCRIPTION OF EMBODIMENTS

First Embodiment

(7) Hereinafter, an embodiment of the present invention will be described. In the present embodiment, Pt is adopted as core particles, composite particles bonded with ZrO.sub.2 (yttria stabilized zirconia) were prepared as ceramic which becomes shells, ZrO.sub.2 (yttria stabilized zirconia) was added thereto as ceramic powder, and thermal treatment was performed to adjust particle diameter. Then, a metal paste was produced, and an electrode made by firing the metal paste was subjected to characteristics evaluation.

(8) To make composite particles having the core/shell structure, Pt fine powder having an average particle diameter of 10 nm and ZrO.sub.2 powder (yttria stabilized zirconia) having an average particle diameter of 10 nm are uniformly mixed by a V-type mixing machine, and this mixed powder was discharged into a plasma atmosphere under an argon atmosphere by means of a high frequency induction heating plasma apparatus. Then, the generated fine powder was collected by a filter. Through this step, composite particle powder having the core/shell structure, in which Pt is adopted as core particles and ZrO.sub.2 is adopted as shells, was obtained. In this case, the composite particle powder was prepared in which the mixing amount of the ceramic with respect to the entire mixed powder was adjusted, and the bonding amount of the ceramic which becomes the shell was changed. In the present embodiment, composite particles having ceramic amounts of 0.5 wt % (Example 1, Comparative Example 1), 1.0 wt % (Examples 2, 3), 1.6 wt % (Examples 4, 5), 3.0 wt % (Examples 6, 7), and 6.1 wt % (Comparative Examples 2, 3) were prepared.

(9) Subsequently, thermal treatment for particle diameter adjustment of the conductive particles was performed. First, ZrO.sub.2 powder having the same composition as the shell was added to the composite particles, and was mixed sufficiently by a planetary mill. Then, after the mixed powder was dried, heating is performed at 800° C. for one hour, and particle diameter adjustment is performed. The amounts of mixing of the ZrO.sub.2 powder with respect to the mixed powder are 5 wt % and 10 wt %.

(10) Then, a metal paste was produced from each mixed powder. In the production of the metal paste, the mixed powder having been subjected to thermal treatment was put into ester alcohol which is an organic solvent, and further, a diamine-based surfactant and ethylcellulose were mixed therewith and, using a triple roll mill, they were mixed and kneaded to be made into paste. The mixing amount of the mixed powder with respect to the entire paste was 80 mass %.

Conventional Example 1

(11) Conductive particles (ceramic amount 1.6 wt % which is a shell) of Examples 4, 5 are prepared as conventional conductive particles having the core/shell structure. Then, thermal treatment is applied to the conductive particles without mixing any ceramic powder, and the thermally treated conductive particles and ceramic powder are mixed (10 wt % with respect to the entire mixed powder). Then, the mixed powder was made into a metal paste.

Conventional Example 2

(12) As a conventional metal paste including conductive particles not having the core shell structure, a metal paste obtained by separately mixing Pt powder and ceramic powder was produced. Pt powder having a particle diameter of 5 μm was used as Pt powder, and ZrO.sub.2 powder (11.4 wt %) was used as ceramic powder, so that powder for a metal paste was made, which was in turn made into a metal paste.

(13) In Examples 1 to 7, Comparative Examples 1 to 3, and Conventional Examples 1 to 2, the particle diameters of the composite particles (conductive particles) in the steps from the production of the composite particles to the particle diameter adjustment in the powder state before made into a paste were converted from specific surface area measurement based on a BET three-points method. In the dispersion property evaluation of the particles, the dispersion degrees are measured according to the line transect method using a grind gauge. In the dispersion degree measurement, a grind gauge (maximum scale 25 μm) manufactured by Tsutsui Scientific Instruments Co., Ltd. was used, a paste was dropped to the maximum groove depth portion of the gauge, line-like marks appearing on a paste coating film obtained by scraping the paste with a scraper were observed, and the depth (μm) of the groove at a point where the third line appears was measured. This was performed three times, and the average thereof was evaluated as the dispersion degree. These values are shown in Table 1.

(14) TABLE-US-00001 TABLE 1 Particle diameter Conductive particle Ceramic powder The amount of ceramic Before After Before After Grind gauge Separately Total thermal thermal thermal thermal dispersion Shell added amount treatment treatment treatment treatment degree Example 1 0.5 wt % 10 wt %  10.5 wt % 18.1 nm 480 nm 169.3 nm 169.8 nm 8.8 μm Example 2 1.0 wt % 5 wt %  6.0 wt % 18.1 nm 481 nm 169.1 nm 169.9 nm 12.5 μm  Example 3 10 wt %  10.9 wt % 18.1 nm 327 nm 169.4 nm 169.7 nm 9.2 μm Example 4 1.6 wt % 5 wt %  6.5 wt % 18.8 nm 140 nm 169.1 nm 169.9 nm 12.7 μm  Example 5 10 wt %  11.4 wt % 18.8 nm 102 nm 169.5 nm 169.3 nm 7.8 μm Example 6 3.0 wt % 5 wt %  7.9 wt % 19.3 nm 155 nm 169.9 nm 169.2 nm 7.2 μm Example 7 10 wt %  12.8 wt % 19.3 nm 101 nm 169.2 nm 169.3 nm 7.5 μm Comparative 0.5 wt % 5 wt %  5.5 wt % 18.1 nm 889 nm 169.6 nm 169.8 nm 13.3 μm  Example 1 Comparative 6.1 wt % 5 wt % 10.8 wt % 27.1 nm  82 nm 169.6 nm 169.2 nm 7.2 μm Example 2 Comparative 10 wt %  15.5 wt % 27.1 nm  77 nm 169.3 nm 169.6 nm 6.5 μm Example 3 Conventional 1.6 wt % 10 wt %* 11.4 wt % 18.1 nm 144 nm 169.5 nm — 17.5 μm  Example 1 Conventional — 11.4 wt %   11.4 wt %  .sup. 5 μm —.sup.  169.5 nm — 11.2 μm  Example 2 *Ceramic powder is added after thermal treatment of conductive particles.

(15) In Table 1, it is understood from the results of Examples, Comparative Examples, and Conventional Example 1 that the conductive particles having the core/shell structure have larger particle diameters due to thermal treatment. However, it is understood that, when the ceramic powder is not mixed before thermal treatment like Conventional Examples, the particle diameter of the conductive particles does not become excessively large, but the dispersion degree increases after the paste is produced. This is because, when the ceramic powder does not exist during thermal treatment, coarse particles are generated locally, and it can be said that the coarse particles exist without collapsing even by the mixing during the production of the paste. These facts indicate that, by mixing the ceramic powder before thermal treatment, the generation of the coarse particles is suppressed, and it is possible to obtain powder having uniform particle diameters.

(16) Subsequently, an electrode was formed from the produced metal paste, and evaluation thereof was performed. The electrode was formed by applying the metal paste onto a 99 mass % zirconia green sheet by screen printing. Thereafter, the electrode was dried at 80° C. for 20 minutes, and firing treatment was performed at 1450° C. for one hour, so that an electrode film was produced. Two types of electrodes were produced, which have φ7.8 mm and thicknesses of 3 μm and 7 μm, respectively.

(17) Structure observation was performed on each electrode film produced, and the structure thereof (the void ratio, the dispersion degree of the conductive particles on the surface and in the cross section) was measured. This measurement is based on image analysis of a structure photograph on the surface and in the cross section of each electrode. To measure the void ratio, the area of a black point in the picture is derived as a void portion, and the ratio of the area is derived based on the area of the observation. To measure the dispersion degree in the electrode cross section, a measurement region of 5 μm×100 μm is extracted from the cross-sectional organization, and in this measurement region, five reference lines (lines of a length 100 μm with an interval of 1 μm) are drawn, the number of dots of the conductive particle phase thereon is measured for each reference line, and the average value thereof was measured. To measure the dispersion degree on the electrode surface, a measurement region of 25 μm×25 μm was extracted from the surface structure, six reference lines (lines of a length 25 μm with an interval of about 4 μm) are drawn, and likewise, the number of dots of the conductive particle phase and the average thereof were measured.

(18) In order to evaluate the electrode activity (electric conductivity) of each electrode, the electrode resistance relative to the platinum weight per unit area is measured according to an alternate current impedance method. As the evaluation condition, under 800° C. atmosphere, the frequency response of an electric current to a voltage of a frequency from 100 kHz to 30 mHz with an amplitude of 20 mV without DC bias was measured. Then, a measurement result of a film thickness of 7 μm according to Conventional Example 2 was adopted as the reference value, those having substantially equivalent characteristics to the reference value were evaluated as “Δ”, those having better characteristics than the reference were evaluated as “◯”, and those having extremely better characteristics than the reference value were evaluated as “⊙”. Those that cannot be measured because of excessively high resistances of the electrode were evaluated as “x”. Table 2 shows results of characteristics evaluations.

(19) TABLE-US-00002 TABLE 2 Electrode structure Dispersion The amount of ceramic degree(μm) Characteristics Separately Total Void Cross evaluation Shell added amount ratio(%) section Surface 3 um 7 um Example 1 0.5 wt % 10 wt %  10.5 wt % 4.49 3.1 0.73 ⊙ ⊙ Example 2 1.0 wt % 5 wt %  6.0 wt % 9.53 3.8 0.84 ◯ ◯ Example 3 10 wt %  10.9 wt % 8.49 3.0 0.60 ⊙ ⊙ Example 4 1.6 wt % 5 wt %  6.5 wt % 6.76 3.9 0.60 ◯ ◯ Example 5 10 wt %  11.4 wt % 4.59 3.3 0.65 ⊙ ⊙ Example 6 3.0 wt % 5 wt %  7.9 wt % 2.75 3.8 0.74 ◯ ◯ Example 7 10 wt %  12.8 wt % 6.49 2.0 0.73 ⊙ ⊙ Comparative 0.5 wt % 5 wt %  5.5 wt % 4.56 5.6 0.86 Δ Δ Example 1 Comparative 6.1 wt % 5 wt % 10.8 wt % 0.11 2.3 0.54 Δ X Example 2 Comparative 10 wt %  15.5 wt % 0.19 2.0 0.80 Δ X Example 3 Conventional 1.6 wt % 10 wt %* 11.4 wt % 5.12 4.7 0.98 Δ Δ Example 1 Conventional — 11.4 wt %   11.4 wt % 14.66 5.0 1.02 X Δ Example 2 *Ceramic powder is added after thermal treatment of conductive particles.

(20) As can be understood from Table 2, the electrode films (Examples 1 to 7) made of metal pastes including the conductive particles having the core/shell structure of which particle diameters are appropriately adjusted and mixed with ceramic powder have a porous structure but still have relatively fine conductive metal dispersed therein. In contrast, the metal paste (Conventional Example 1) obtained by performing thermal treatment with only the conductive particles having the core/shell structure and mixed with ceramic after thermal treatment includes coarse particles, and therefore the dispersion degree of the platinum particles is out of a preferred range, and the metal paste (Conventional Example 1) is inferior in conductivity. The metal paste (Conventional Example 2) made by simply mixing the conductive particles not having the core/shell structure and the ceramic powder can produce an electrode having a porous structure, but the conductive particles are coarse and the resistance value is high, making the metal paste (Conventional Example 2) inferior in conductivity. For this reason, it is difficult for the metal paste to act as an electrode using a thin film.

(21) However, even with the conductive particles having the core/shell structure, when the amount of ceramic which becomes the shells is small, the conductive particles become coarse and, when the ceramic amount is large, it is difficult to obtain a porous electrode (Comparative Examples 1 to 3). As described above, this is considered to be because, when the ceramic is little, the sintering of the conductive particles advances too quickly, and when the ceramic is too much, the timing of the sintering of the conductive particles is out of the expected timing, and it occurs at the same point in time as the sintering of the mixed ceramic powder.

(22) With regard to produced electrodes, structure photographs (cross section, surface) of Example 4, Comparative Example 2, and Conventional Example 1 are shown in FIG. 3. In Conventional Example 1, the coarse powder generated when the conductive particles were thermally treated remained on the electrode. As compared with Example 4, it is understood that the closely packed film with little voids included therein was made according to Comparative Example 2.

Second Embodiment

(23) In this case, the effect of the addition amount of ceramic powder mixed with the conductive particles has been considered. In addition to the electrodes produced according to the first embodiment (Examples 4, 5 and Conventional Example 2), electrodes were produced by means of metal pastes where the amounts of addition of ZrO.sub.2 powder mixed with the conductive particles having the core/shell structure (ceramic amount: 1.6 wt %) are 15 wt % (Example 8), 20 wt % (Example 9), 1 wt % (Comparative Example 4), 3 wt % (Comparative Example 5), and 25 wt % (Comparative Example 6), and the resistance values and the electrode activity thereof were measured.

(24) In the evaluation of the resistance values, the sheet resistance values per platinum weight in the electrode were evaluated. The sheet resistance value was measured by printing and firing an evaluation paste on a zirconia green sheet, making a line of 4 mm×16 mm (film thickness: 3 μm), and measuring the resistance values at both ends thereof by means of a digital multi-meter.

(25) In order to evaluate the electrode activity (electric conductivity) of each electrode, the electrode resistance relative to the platinum weight per unit area was measured according to an alternate impedance method. As the evaluation condition, the evaluation paste was printed and fired on both surfaces of the zirconia green sheet, an electrode which has φ7.8 mm and a thickness of 3 μm was made, and under 800° C. atmosphere, the frequency response of an electric current to a voltage of a frequency from 100 kHz to 30 mHz with an amplitude of 20 mV without DC bias was measured.

(26) These evaluation results are shown in Table 3 and FIGS. 4 and 5. Note that these evaluation results indicate the same examination result obtained in association with the electrode (film thickness: 7 μm) formed by the paste according to Conventional Example 2.

(27) TABLE-US-00003 TABLE 3 The amount of ceramic Evaluation result* Separately Total Sheet resistance Electrode resistance Shell added amount value Ω .Math. mg/□ Ω .Math. mg/cm2 Example 4 1.6 wt % 5 wt % 6.5 wt % 0.0786 24.46 Example 5 10 wt % 11.4 wt % 0.0999 15.75 Example 8 15 wt % 16.4 wt % 0.1612 14.92 Example 9 20 wt % 21.3 wt % 0.4167 18.87 Comparative 1.6 wt % 1 wt % 2.6 wt % 0.0791 611.42  Example 4 Comparative 3 wt % 4.6 wt % 0.0823 150.98  Example 5 Comparative 25 wt % 26.2 wt % Unmeasurable Unmeasurable Example 6 Conventional — 11.4 wt % 11.4 wt % 0.2079 28.46 Example 2 *Examples 4, 5, 8, 9 and Comparative Examples 4 to 6 are measurement values of film thickness 3 μm, and Conventional Example 2 is measurement value of film thickness 7 μm.

(28) It is confirmed from Table 3 and FIG. 4 that the mixed ceramic amount affects the resistance value of the electrode. When the mixing amount of the ceramic is 5 mass % or less, the resistance value of the electrode is substantially constant and this can be said to be a low resistance. When the mixing amount of the ceramic exceeds 10 mass %, an increase in the mixing amount of the ceramic increases the resistance value, and the resistance is too high at 25 mass % (Comparative Example 6), making it impossible to perform the measurement. Even in a case of simply mixing the platinum powder and the ceramic powder according to Conventional Example 2, there would be basically no problem from the perspective of the resistance value.

(29) On the other hand, referring to FIG. 5 which is the measurement result of electric conductivity (the electrode resistance relative to the platinum weight per unit area) in view of the activity as the gas sensor electrode, the resistance per unit area is high and the activity is low when the mixing amount of the ceramics is low (Comparative Examples 4, 5). In contrast, the activity becomes preferable from around a level where the mixing amount of the ceramic exceeds 5 mass %. However, when the mixing amount of the ceramic is 25 mass %, electrical conduction is lost, and therefore, the measurement cannot be performed. The difference in the results between these Example and Comparative Example lies in the difference in the structure between the electrode films, and it is understood that the gas sensor electrode is preferably porous.

(30) Considering both of the measurement results of the resistance value and the electrode activity, it is understood that the mixing amount of the ceramic needs to be 5 to 20 mass % in order to obtain a practical electrode that exhibits the electrode activity and still has a low sheet resistance. The electrode thus produced exhibits better characteristics even with a thinner film than Conventional Example 2. If Conventional Example 2 is made to have a thin film, the electrode activity cannot be measured (see Table 2), and therefore, the measurement result of the thick film is taken into consideration. Because of this fact, the superiority of the paste according to the present invention can be confirmed.

Third Embodiment

(31) In this case, the technical significance of the particle diameter adjustment of the conductive particles having the core/shell structure has been considered. As described above, the particle diameter adjustment of the conductive particles has a meaning of obtaining an electrode having a porous structure by appropriately setting the sintering timing thereof, and more specifically, a meaning of ensuring the electrode activity. In addition, there is also another significant meaning, which is to appropriately set the shrinkage rate during the metal paste firing in order to prevent the substrate from being deformed or broken. Therefore, thermomechanical analysis (TMA) is performed on the metal paste of Example 5. As the measurement condition of this examination, the temperature raising rate was set to 20° C./min while a load of 1 g was applied to a sample of φ5 mm under atmosphere. The shrinkage curve (TMA curve) which is a result thereof is shown in FIG. 6. Note that this measurement is also applied to the metal paste having 10 wt % of ZrO.sub.2 mixed in the conductive particles not having been subjected to the particle diameter adjustment based on the thermal treatment (this will be referred to as Comparative Example 7).

(32) As can be seen from FIG. 6, the metal paste having the ceramic powder mixed in the conductive particles not having been subjected to the particle diameter adjustment in Comparative Example 7 indicates a significant change in the shrinkage rate at a temperature higher than 750° C. which is a shrinkage temperature (a temperature at which 5% shrinkage occurs with respect to 400° C. where the organic component is lost). When the metal paste is applied to the substrate and fired, it is highly possible that the substrate would be broken, for example. In contrast, in the metal paste having been subjected to the particle diameter adjustment, the shrinkage temperature is high (1150° C.), and the shrinkage curve is relatively mild. Therefore, it can be said that by appropriately setting the firing temperature, the electrode film can be formed without deforming or breaking the substrate.

INDUSTRIAL APPLICABILITY

(33) According to the present invention, a porous electrode film can be formed while conductive metal and ceramic particles are dispersed in a fine state. The present invention is preferable for a metal paste for forming a gas sensor electrode of a gas sensor such as an NOx sensor and an oxygen gas sensor electrode, and in addition, the electrode film can be made thin. Therefore, the costs of various kinds of sensor devices can be reduced.