JP2021134367A - Noble metal paste - Google Patents

Abstract

To provide a noble metal paste that improves electrode activity.SOLUTION: Provided is a noble metal paste that comprises a metal powder made of platinum or platinum alloy, a solid electrolyte powder made of zirconia or yttria-stabilized zirconia, an organometallic compound containing at least one of organoaluminum compound, organomagnesium compound and organocalcium compound, and a vehicle.SELECTED DRAWING: None

Description

The present invention relates to a noble metal paste. More specifically, it relates to a noble metal paste for producing an electrode of a gas sensor such as oxygen or NOx.

In order to obtain high electrode activity in the electrodes of gas sensors such as oxygen and NOx, platinum or a platinum alloy as an electrode catalyst, zirconia or yttria-stabilized zirconia as a solid electrolyte, and a gas such as oxygen are used in the electrode film. It is important to form a large number of surfaces (three-phase interfaces) where the three components of the above are in contact.

In Patent Document 1, platinum powder, which is an electrode material, is used as the electrode forming method for the oxygen sensor. A method is proposed in which yttria-stabilized zirconia powder is mixed at a predetermined ratio, a solvent and a resin are further added to obtain a paste, and the paste is applied, dried, and fired on an yttria-stabilized zirconia substrate to form an electrode. Has been done.

Japanese Unexamined Patent Publication No. 57-165758

It is important to form a large number of three-phase interfaces in order to obtain high electrode activity at the electrode (electrode catalyst layer) of the gas sensor.

Patent Document 1 discloses that in a paste containing platinum powder, yttria-stabilized zirconia powder, solvent and resin, the particle size of platinum powder and yttria-stabilized zirconia powder is reduced as a means for increasing the three-phase interface. Has been done. However, if the particle size of platinum powder or yttria-stabilized zirconia is reduced, the same particles agglomerate in the pasting step, resulting in insufficient dispersion in the paste. Therefore, there is a limit to the particle miniaturization method. The problem is that the three-phase interfaces are not sufficiently obtained in the above-mentioned prior art, and it is a problem to form a large number of three-phase interfaces.

An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a noble metal paste having improved electrode activity.

The platinum or platinum alloy in the electrode film aggregates and grows under the influence of heat in the process of firing after the paste is applied and printed on the substrate. As a result, a large number of three-phase interfaces are not formed, and the electrode activity is lowered. The step of firing the paste is indispensable for the formation of electrodes, and it is important how to suppress the aggregation or grain growth of this platinum or platinum alloy in this firing step.

As a result of diligent studies to achieve the purpose of obtaining a higher electrode activity, the present inventors have added a metal powder made of platinum or a platinum alloy and a solid electrolyte powder made of zirconia or ittria-stabilized zirconia, and a firing process. When an organometallic compound which becomes a metal oxide in the above, for example, an organometallic compound containing at least one of an organic aluminum compound, an organic magnesium compound, and an organic calcium compound is selected to be included in the components of the paste, a three-phase interface is selected. It was found that a large number of compounds were formed and the electrode activity was improved.

According to the present invention, it is possible to provide a noble metal paste having improved electrode activity.

It is a figure which shows the aluminum distribution in the cross section of the electrode film of Example 1. FIG. It is a figure which shows the aluminum distribution in the cross section of the electrode film of the comparative example 1. FIG. It is a figure which shows the SEM image of the electrode film cross section of Example 1. FIG. It is a figure which shows the SEM image of the cross section of the electrode film of the comparative example 1. FIG.

Hereinafter, the platinum or platinum paste of the present invention will be described in more detail.

The platinum or platinum paste of the present invention contains at least one of a metal powder made of platinum or a platinum alloy, a solid electrolyte powder made of zirconia or yttria-stabilized zirconia, and an organoaluminum compound, an organomagnesium compound, and an organocalcium compound. It is a paste characterized by containing an organometallic compound containing and a vehicle.

Hereinafter, platinum or platinum paste will be described.

The metal powder of the present invention is made of platinum or a platinum alloy. For example, Pt powder, PtRh powder, and PtAu powder can be used. The particle size of the metal powder is preferably 0.1 μm or more and 10 μm or less. The ratio of platinum or platinum alloy powder is preferably 40 to 80 mass% in the total paste.

Zirconia or yttria-stabilized zirconia is used as the solid electrolyte powder contained in the present invention. The particle size of the solid electrolyte powder is preferably 0.01 μm or more and 1 μm or less. The ratio of the solid electrolyte powder is preferably 2 to 25 mass% in the total paste.

The organometallic compound can be, for example, at least one or more of an organoaluminum compound, an organomagnesium compound, and an organocalcium compound. It is the metal oxide that inhibits the aggregation or grain growth of the metals, but unlike the case where it is added to the paste as metal oxide particles, the organic metal compound is dissolved in the solvent that forms the vehicle, and the paste is applied and printed. Can be evenly distributed over the entire film. Further, the metal oxide produced by oxidizing the organometallic compound in the firing process is fine, and the effect of suppressing the aggregation of platinum or the platinum alloy and / or the effect of suppressing the grain growth can be obtained.

Organoaluminium compounds, organomagnesium compounds, and organocalcium compounds are metal compounds having a structure represented by M (OR) n (OCOR') (3-n), and M in the structural formula represents the corresponding metal element. In this structural formula, R and R'may be the same or different, and R and R'may contain a phenyl group and are composed of an alkane and an olefin. The alkane and olefin can include a hydroxy group, a carbonyl group, an amino group, an amide group, an ether group, an ester group and an oxime group.

Examples of the organic aluminum compound include aluminum isopropylate, aluminum ethylate, aluminum ethyl acetoacetate diisopropylate, aluminum trisethyl acetoacetate, aluminum trisacetyl acetonate, aluminum bisethyl acetoacetate, monoacetyl acetonate, and acetoalkoxyaluminum diisopropi. Examples thereof include rate, aluminum stearate, trimethylaluminum, triphenylaluminum, acetylacetonatoaluminum, and (3-ethoxy-1,2-dimethylpropan-1-en-3-onyl) bisbutoxyaluminum.

Examples of the organic magnesium compound include magnesium isopropylate, magnesium ethylate, magnesium ethyl acetoacetate diisopropylate, magnesium trisethyl acetoacetate, magnesium tris acetyl acetonate, magnesium bisethyl acetoacetate, monoacetyl acetonate, and acetoalkoxymagnesium diisopropi. Examples thereof include rate, magnesium stearate, trimethylmagnesium, triphenylmagnesium, acetylacetonatomagnesium, (3-ethoxy-1,2-dimethylpropan-1-en-3-onyl) bisbutoxymagnesium, hydroxylaurylmagnesium and the like.

Examples of organic calcium compounds include calcium isopropylate, calcium ethylate, calcium ethyl acetoacetate diisopropylate, calcium trisethyl acetoacetate, calcium tris acetyl acetonate, calcium bisethyl acetoacetate, monoacetyl acetonate, and acetoalkoxy calcium diisopropi. Rate, calcium stearate, trimethyl calcium, triphenyl calcium, acetylacetonato calcium, (3-ethoxy-1,2-dimethylpropan-1-en-3-onyl) bisbutoxycalcium, calcium glutarate, calcium glycoside, etc. Be done.

The organometallic compound is dissolved in a solvent that forms a vehicle, and when the paste is fired, it is oxidized to become an oxide of the metal. When the organic metal compound is dissolved in a vehicle-forming solvent, the metal of the organic metal compound is different from the metal of the metal powder or the solid electrolyte powder when fired, so that the metals of platinum or the platinum alloy are aggregated and / or Physically inhibits grain growth. Along with this, a large number of three-phase interfaces are formed, and the electrode activity is improved.

The proportion of the organometallic compound is preferably 2 mass% to 20 mass% in the total paste. In particular, 4 mass% to 12 mass% is more preferable. If it is below this range, the platinum or platinum alloy aggregation inhibitory effect cannot be sufficiently obtained, and if it exceeds this range, the proportion of the organic metal compound in the entire paste becomes too large, and paste kneading and paste viscosity adjustment become physically difficult. .. In addition, the firing shrinkage rate becomes too large, making film formation difficult.

Further, a metal oxide powder which is at least one of alumina, titania, and yttria may be further contained. These metal oxide particles are added for the purpose of adjusting the shrinkage characteristics of the entire paste during firing and for the purpose of imparting adhesion to the ceramic substrate. The particle size of the metal oxide powder is preferably 0.1 μm or more and 5 μm or less, and more preferably 0.4 μm or more and 1.2 μm or less. The proportion of the metal oxide powder is preferably 0 mass% to 5 mass% in the total paste. If it exceeds this, the metal oxide inhibits the formation of an electric circuit in the obtained film, and the electric resistance becomes high.

Regarding the ratio of the organometallic compound to the metal oxide powder, the ratio of the metal derived from the inorganic metal compound to the total metal is preferably 0 to 60 mass% in terms of metal. For example, when an organoaluminum compound and alumina are used in combination, the amount of aluminum derived from inorganic aluminum is preferably 0 to 60 mass% in terms of the amount of aluminum.

Further, a powder consisting of at least one of carbon powder and acrylic powder may be contained. These powder particles need to be particles that are insoluble in the solvent that forms the vehicle. These particles disappear in the process of firing, form pores in the firing film, form a large number of three-phase interfaces, and contribute to increasing the density of reaction points where gas molecules undergo an electrochemical reaction. The particle size of these powders is preferably 0.5 μm or more and 3 μm or less. The ratio of these powders is preferably 0.5 mass% to 15 mass% in the total paste. If it exceeds this range, excessive pores are formed in the membrane, the electrical conductivity of the membrane is lowered more than necessary, and the strength of the membrane is lowered. The amount of carbon powder and acrylic powder should be kept to a minimum depending on the reactivity required for the film to be formed. As the carbon powder, diamond, carbon black or the like can be used.

The vehicle contains a solvent and a resin.

As the solvent, butyl carbitol, butyl carbitol acetate, tarpineol, dihydroterpineol, N-methylpyrrolidone, mesitylene, 2-ethylhexanol, tridecane, texanol, and the like can be used.

As the resin, a cellulose resin such as ethyl cellulose, an acrylic resin, a rosin resin, a terpene resin, a butyral resin and the like can be used.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Examples 1 to 5)
A vehicle consisting of PtRh alloy powder (Pt: Rh = 1: 1 (weight ratio)), zirconia powder, carbon powder, alumina powder, organoaluminum, resin / solvent is weighed so as to have the component composition shown in Table 1, and the primary mixture is used. After smelting, finish kneading was performed with three rolls to obtain a paste. Here, as the organoaluminum compound, (3-ethoxy-1,2-dimethylpropan-1-en-3-onil) bisbutoxyaluminum was used.

(Comparative Example 1)
Alumina powder having the same amount of aluminum as in Example 1 was added instead of organic aluminum, weighed so as to have the composition shown in Table 1, primary kneaded, and then finish kneaded with three rolls for comparison. I got the paste for.

(Example 6)
A vehicle composed of Pt powder, zirconia powder, carbon powder, organoaluminum, resin / solvent was weighed so as to have the composition shown in Table 2, and after primary kneading, finish kneading was performed with three rolls to obtain a paste. .. Here, as the organoaluminum compound, (3-ethoxy-1,2-dimethylpropan-1-en-3-onil) bisbutoxyaluminum was used.

(Comparative Example 2)
Alumina powder was used instead of organoaluminum, weighed so as to have the component composition shown in Table 2, primary kneaded, and then finish kneaded with three rolls to obtain a paste of Comparative Example 2.

In Examples and Comparative Examples, the volume ratio (%) of the metal oxide (alumina) to (precious metal + zirconia) was made to be substantially the same. The calculated values of the volume ratio are shown in Tables 1 and 2.

The pastes of Examples and Comparative Examples were screen-printed on both sides of a green sheet made of a zirconia solid electrolyte in a predetermined pattern, and fired under predetermined conditions (1500 ° C. for 10 minutes) to prepare an electrode film to obtain a sample element.

The impedance of the above sample element was measured at 600 ° C. under a nitrogen atmosphere in which nitric oxide was controlled. The measurement was performed using an LCR meter at a frequency of 100 kHz to 10 mHz. The result is Nyquist plotted, a profile consisting of multiple semicircles is obtained, a curve fit is performed assuming an equivalent circuit consisting of multiple capacitors and resistors, and the diffusion resistance derived from the diffusion of oxygen gas and the oxidation of oxygen molecules are performed. The components were separated into reaction resistance derived from the reduction reaction.

From Table 1, it can be seen that when the metal powder is PtRh powder, Example 1 using the organoaluminum compound has lower reaction resistance and diffusion resistance than Comparative Example 1 using the alumina powder. The smaller the reaction resistance and diffusion resistance, the higher the performance as an electrochemical electrode. Further, it can be seen that Examples 3 to 5 in which carbon powder was used in addition to the organoaluminum compound had lower reaction resistance and diffusion resistance than Example 1 in which the organoaluminum compound was added. Further, it can be seen that Example 2 in which the alumina powder was added in addition to the organoaluminum compound had lower reaction resistance and diffusion resistance than in Comparative Example 1 in which the alumina powder was used. Here, the ratio of aluminum derived from the organoaluminum compound to aluminum derived from the alumina powder in Example 2 is 40 mass% of the total aluminum in terms of the amount of aluminum.

From Table 2, it can be seen that when the metal powder is Pt powder, Example 6 using the organoaluminum compound has lower reaction resistance and diffusion resistance than Comparative Example 2 using alumina powder. The smaller the reaction resistance and diffusion resistance, the higher the performance as an electrochemical electrode.

EDS mapping measurements were performed on the cross sections of the sample elements of Example 1 and Comparative Example 1 with an electron microscope. FIG. 1 is a diagram showing an aluminum distribution in the cross section of the electrode film of Example 1. FIG. 2 is a diagram showing an aluminum distribution in the cross section of the electrode film of Comparative Example 1. From FIGS. 1 and 2, it can be seen that in Example 1, aluminum is more uniformly present than in Comparative Example 1.

SEM images of the cross sections of the sample elements of Example 1 and Comparative Example 1 were taken with an electron microscope. FIG. 3 is a diagram showing an SEM image of a cross section of the electrode film of Example 1. FIG. 4 is a diagram showing an SEM image of a cross section of the electrode film of Comparative Example 1. From FIGS. 3 and 4, it can be seen that the aggregation and grain growth of the platinum alloy are suppressed in Example 1 as compared with Comparative Example 1.

Claims (4)

With metal powder made of platinum or platinum alloy,
Solid electrolyte powder consisting of zirconia or yttria-stabilized zirconia,
Organometallic compounds containing at least one of organoaluminum compounds, organomagnesium compounds, and organocalcium compounds,
Including vehicle,
A precious metal paste characterized by that. The noble metal paste according to claim 1, wherein the organic metal compound is contained in an amount of 2 mass% to 20 mass% based on the total paste. The noble metal paste according to claim 1 or 2, further comprising a metal oxide powder which is at least one of alumina, titania, and yttria. The noble metal paste according to any one of claims 1 to 3, further comprising a powder which is at least one of a carbon powder and an acrylic powder.

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