Method for manufacturing PtAu alloy powder

Abstract

To provide a production method of a PtAu alloy powder having a high alloying degree, and a narrow particle size distribution.SOLUTION: A production method of a PtAu alloy powder includes: a reduction step of adding an aqueous solution containing a Pt compound and an Au compound to a reductant-containing aqueous solution, and reducing the Pt compound and the Au compound to obtain a PtAu reduction powder; and a heat treatment step of mixing the PtAu reduction powder with an oxide powder (sinter inhibitor) followed by heat treatment at 350-1,050°C. Au in the PtAu alloy powder is 0.5-40 wt%.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing PtAu alloy powder.

Background Art

[0002] As a component of the sensor electrodes of various gas sensors such as oxygen sensors, CO sensors, NOx sensors, etc., a PtAu paste is screen-printed and fired to form a conductive film. The composition of the PtAu paste consists of PtAu alloy powder, ceramic powder for adhering the PtAu alloy powder to the substrate, an organic vehicle, etc. The PtAu alloy powder is desired to have a narrow particle size distribution and high alloying degree particles without segregation from the viewpoints of sensor performance and durability.

[0003] For example, Patent Document 1 describes a production method for obtaining alloy nanoparticles of nano-order with high alloying degree used for the electrode catalyst of a fuel cell by adjusting the reduction potential of a noble metal compound solution.

[0004] However, in the production method of Patent Document 1, it is necessary to adjust the oxidation-reduction potential difference between two kinds of metal ions to 110 mV or less by changing the noble metal compound as the starting material, the reduction temperature, and the reducing agent concentration. Further, when obtaining micro-order particles available for a noble metal paste by wet reduction, there is a problem that the particles aggregate and the alloying degree becomes low.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] Conventional manufacturing techniques for PtAu alloy powder, which involve only a reduction process, have problems such as a broadening of the particle size distribution due to particle aggregation and a decrease in the degree of alloying. Therefore, a new manufacturing method is needed.

[0007] The object of the present invention is to provide a method for producing PtAu alloy powder with a high degree of alloying and a narrow particle size distribution. [Means for solving the problem]

[0008] The inventors have found that by mixing oxide powder as a sintering inhibitor with powder obtained by wet reduction to prevent aggregation due to heat treatment, and then applying heat, grain growth of alloy particles is suppressed, thereby solving the above problem and enabling the production of PtAu alloy powder with a narrow particle size distribution and a high degree of alloying.

[0009] In other words, the present invention provides a reduction step of adding an aqueous solution containing a Pt compound and an Au compound to an aqueous solution containing a reducing agent, thereby reducing the Pt compound and the Au compound to obtain a PtAu reduction powder. The process includes a heat treatment step in which the PtAu reduction powder is mixed with an oxide powder (sintering inhibitor) that suppresses the progression of sintering and grain growth of PtAu particles during heat treatment, and the mixture is heat-treated at 350 to 1050°C. A method for producing PtAu alloy powder, characterized in that the Au content in the PtAu alloy powder is 0.5 to 40 wt%.

[0010] Furthermore, in the above manufacturing method, the oxide powder (sintering inhibitor) may be a powder containing one or more of zinc oxide, copper oxide, and calcium oxide.

[0011] Furthermore, the PtAu alloy powder produced by the above manufacturing method may have a degree of alloying of 85% or more, an average particle size of 0.5 to 10.0 μm, and a span value of 1.3 to 1.8. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a PtAu alloy powder with a high degree of alloying and a narrow particle size distribution. [Brief explanation of the drawing]

[0013] [Figure 1] This is a scanning electron microscope (SEM) image of Example 11. [Figure 2] This is a scanning electron microscope (SEM) image of Comparative Example 1. [Modes for carrying out the invention]

[0014] The method for producing the PtAu alloy powder of the present invention will be described in detail below.

[0015] The present invention includes a reduction step in which an aqueous solution containing a Pt compound and an Au compound is added to an aqueous solution containing a reducing agent, and the Pt compound and Au compound are reduced to obtain a PtAu reduced powder.

[0016] In this invention, an aqueous solution containing a Pt compound and an Au compound is used.

[0017] Examples of Pt compounds include hexachloroplatin(IV) acid (H2[PtCl6]), tetrachloroplatin(II) acid (H2(PtCl4)), and tetraammineplatin(II) dichloride (Pt(NH3)4Cl2).

[0018] Examples of Au compounds include tetrachloroauric acid (H[AuCl4]), sodium gold(I) sulfite (Na3[Au(SO3)2]), and dichloro(1,10-phenanthroline) gold(III) chloride (AuCl2(C)). 12 Examples include H8N2(Cl) and others.

[0019] Specifically, a mixture of noble metal compounds is obtained by mixing an aqueous solution containing a Pt compound with an aqueous solution containing an Au compound in a predetermined ratio. The ratio of Pt to Au in the PtAu alloy powder can be adjusted by changing the mixing ratio of the Pt compound and the Au compound.

[0020] In the present invention, an aqueous solution containing a pH-adjusted reducing agent is used. As the type of the reducing agent, hydrazine derivatives such as hydrazine sulfate and hydrazine hydrochloride can be used.

[0021] As the pH adjuster, basically aqueous ammonia is preferable, but when the Au ratio is 20% or more, an aqueous sodium hydroxide solution is desirable to prevent the formation of fulminating gold.

[0022] For the preparation of the reducing solution, the reducing agent is dissolved in pure water, the pH is adjusted, and it is heated and maintained. The concentrations of the reducing agent and the pH adjuster in the reaction solution are not particularly limited, but when the amount of the pH adjuster is reduced, the primary particle size tends to increase. Therefore, the addition amount can be adjusted so as to achieve an appropriate pH according to the target primary particle size. The pH of the solution containing the reducing agent is preferably 7.5 or more and 14.0 or less. Preferably, it is 8.0 or more and 10.0 or less.

[0023] An aqueous solution containing a Pt compound and an Au compound (noble metal compound mixture) is added to the aqueous solution containing the reducing agent and stirred. In the reduction step, by adding the noble metal compound mixture to the heated and maintained reducing solution, the Pt compound and the Au compound are reduced, and PtAu reduced powder is obtained.

[0024] The obtained slurry is washed and dried to take out the PtAu reduced powder.

[0025] The present invention includes a heat treatment step of mixing the PtAu reduced powder and an oxide powder (sintering inhibitor) and performing heat treatment at 350 to 1050 °C. The heat treatment atmosphere can be, for example, the atmosphere.

[0026] By mixing the oxide powder (sintering inhibitor), sintering can proceed during heat treatment, and grain growth of PtAu particles can be suppressed. When heat treatment is performed at a temperature exceeding 1050 °C, Au may scatter, so heat treatment at 1050 °C or lower is desirable. When heat treatment is performed at a temperature below 350 °C, the degree of alloying of the powder decreases. The heat treatment time is preferably about 1 to 5 hours.

[0027] In this invention, it is preferable that the Au content in the PtAu alloy powder is 0.5 to 40 wt%. The ratio is adjusted by the mixing ratio of the Pt compound and the Au compound. If the Au ratio is higher than this, the degree of alloying decreases.

[0028] The oxide powder (sintering inhibitor) should not dissolve in solid solution with PtAu and should be easily removed by chemical treatment such as acid after heat treatment. For example, the oxide powder (sintering inhibitor) can be a powder containing one or more of zinc oxide, copper oxide, or calcium oxide. That is, the oxide powder (sintering inhibitor) may contain one of zinc oxide, copper oxide, or calcium oxide, or it may contain two or more of zinc oxide, copper oxide, or calcium oxide. It is preferable to use an oxide powder with an average particle size of 0.3 to 10 μm so that it mixes well with the PtAu reducing powder.

[0029] The amount of oxide powder added can be, for example, 1 to 15 times the weight of the PtAu reduced powder. Preferably, 1.5 to 10 times the weight is desirable. Increasing the amount of oxide powder added tends to decrease the average particle size of the PtAu alloy powder.

[0030] Nitric acid, hydrochloric acid, or sulfuric acid can be used as the acid to remove the sintering inhibitor. For example, by adding the heat-treated mixed powder while stirring dilute nitric acid and stirring to dissolve the sintering inhibitor, PtAu alloy powder (gray-black powder) can be obtained.

[0031] The average particle size of PtAu alloy powder can be controlled by the heat treatment temperature and the oxide powder / PtAu reduced powder ratio. If the heat treatment temperature is lower than 350°C, the diffusion of metal atoms within the particles does not proceed, resulting in a lower degree of alloying. If the heat treatment temperature exceeds 1050°C, grain growth is promoted by interparticle sintering, resulting in a larger average particle size and span value. According to the present invention, a PtAu alloy powder can be obtained with an alloying degree of 85% or more, an average particle size of 0.5 to 10.0 μm, and a span value of 1.3 to 1.8, which represents the narrowness of the particle size distribution. From the viewpoint of forming conductive circuits, the upper limit of the average particle size is preferably 8.0 μm or less, and more preferably 5.0 μm or less. The lower limit is preferably 1.0 μm or more.

[0032] In the present invention, a PtAu powder with a high degree of alloying can be obtained regardless of the valency of the compound. However, it is preferable that the Pt compound is a tetravalent Pt compound and the Au compound is a trivalent Au compound. [Examples]

[0033] The following describes embodiments of the present invention, but the present invention is not limited to these embodiments.

[0034] (Example 1) A mixture of noble metal compounds was prepared by mixing an aqueous solution of hexachloroplatinic acid (H2[PtCl6]) with a Pt concentration of 200 g / L containing 28.5 g of Pt, and an aqueous solution of tetrachloroplatinic acid (H[AuCl4]) with an Au concentration of 400 g / L containing 1.5 g of Au.

[0035] At room temperature, 200 g of hydrazine sulfate and 150 mL of 28% aqueous ammonia were added to 5 L of pure water to adjust the pH to 8.5. The mixture was then heated to 60°C while stirring to prepare the reducing solution.

[0036] When a mixture of noble metal compounds was added to the above reducing solution and stirred, a black powder was obtained.

[0037] The above black powder was washed with pure water, filtered, and dried. The dried black powder was then crushed in a mixer to obtain 29.4 g of reduced PtAu powder.

[0038] Next, zinc oxide powder with an average particle size of 0.5 μm was added in an amount twice the weight of the PtAu reduced powder, and the mixture was further mixed. In this process, the PtAu reduced powder and zinc oxide powder were mixed to obtain a mixed powder.

[0039] The mixed powder was placed in an alumina boat and heat-treated at 1000°C for 1 hour in air.

[0040] PtAu alloy powder (grayish-black powder) was obtained by adding the heat-treated mixed powder to 3 L of a 20% nitric acid aqueous solution and stirring for 2 hours to dissolve the zinc oxide.

[0041] The PtAu alloy powder (grayish-black powder) was washed with pure water, filtered, and dried to obtain PtAu alloy powder with an average particle size of 2.0 μm and an alloying rate of 99.1%. The yield was 93%.

[0042] The particle size of the obtained PtAu alloy powder was measured using the laser diffraction scattering particle size distribution method. D10, D50, and D90, corresponding to the cumulative particle size distribution values ​​of 10%, 50%, and 90%, were determined, and D50 was taken as the average particle size. The span value, which is an indicator of particle size variation, was calculated using the following formula. A smaller span value indicates a narrower particle size distribution and more uniform particle size. Span value = { (D90 - D10) / D50}

[0043] The degree of alloying of the obtained PtAu alloy powder was determined by X-ray diffraction (XRD) measurement. The XRD spectrum was measured, and the diffraction peak of the PtAu alloy powder (220) plane was fitted to three Lorentz functions: the Pt(220) plane, the Au(220) plane, and the PtAu alloy(220) plane. The peak areas x1 for the Pt(220) plane, x2 for the Au(220) plane, and X for the PtAu alloy(220) plane were obtained, and the degree of alloying was calculated using the following formula. Alloying degree={ X / (x1 + x2 + X)} ×100

[0044] (Examples 2-15) PtAu alloy powder was obtained in the same manner as in Example 1, except that the Pt compound, Au compound, Au content of the alloy powder, oxide species used as a sintering inhibitor, oxide powder / PtAu powder weight ratio, and heat treatment temperature were changed according to the information in Table 1 below.

[0045] (Comparative Example 1) Comparative Example 1 is an example in which no heat treatment process is performed. In the same manner as in Example 1, a mixture of noble metal compounds was added to the reducing solution and reduced, then washed, dried, and crushed to obtain the PtAu reduced powder of Comparative Example 1.

[0046] (Comparative Example 2) Comparative Example 2 is an example of heat treatment without adding oxide powder (sintering inhibitor). The PtAu alloy powder of Comparative Example 2 was obtained by manufacturing in the same manner as in Example 12, except that the heat treatment was performed without adding zinc oxide powder (sintering inhibitor).

[0047] (Comparative Example 3) Comparative Example 3 is an example where the Au content is 60%. Except for the mixing ratio of the Au compound being 60%, it was manufactured in the same manner as in Example 1 to obtain the PtAu alloy powder of Comparative Example 3.

[0048] The average particle size, span value, and degree of alloying of the PtAu alloy powders of Examples 1-15 and Comparative Examples 1-3 were measured, and the results are shown in Table 1. Scanning electron microscope (SEM) images of Example 11 and Comparative Example 1 are shown in Figures 1 and 2.

[0049] As is clear from the results in Table 1 and Figures 1 and 2 below, Examples 1 to 15 according to the present invention yielded particles with uniform particle size and a high degree of alloying. On the other hand, Comparative Example 1 had particles with a low degree of alloying because it was not heat-treated, Comparative Example 2 showed a wide particle size distribution due to the aggregation of particles, and Comparative Example 3 had particles with a wide particle size distribution and a low degree of alloying.

[0050] Comparing the narrowness of the particle size distribution, Table 1 shows that Comparative Example 2, which underwent heat treatment without mixing with oxide powder, has a span value of 2.0, while Example 1 has a span value of 1.5, indicating that particles with a narrow particle size distribution were obtained.

[0051] [Table 1]

Claims

1. A reduction step involves adding an aqueous solution containing a Pt compound and an Au compound to an aqueous solution containing a reducing agent, and reducing the Pt compound and the Au compound to obtain a PtAu reduced powder. The process includes a heat treatment step of mixing the PtAu reduction powder and oxide powder (sintering inhibitor) and heat-treating them at 350 to 1050°C. The Au content in the PtAu alloy powder is 0.5 to 40 wt%. A method for producing PtAu alloy powder, characterized by the following features.

2. The method for producing PtAu alloy powder according to claim 1, characterized in that the oxide powder (sintering inhibitor) is a powder containing one or more of zinc oxide, copper oxide, or calcium oxide.

3. A method for producing PtAu alloy powder according to claim 1 or 2, characterized in that the degree of alloying of the PtAu alloy powder is 85% or more, the average particle size is 0.5 to 10.0 μm, and the span value representing the narrowness of the particle size distribution is 1.3 to 1.8.

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