JP4504775B2 - Conductive paste - Google Patents

Description

  The present invention relates to platinum powder and a method for producing the same, and in particular, platinum powder used as a raw material for a thick film conductive platinum paste for forming a conductive circuit and a heating element circuit on ceramics, and used for automobiles. The present invention relates to a platinum powder having excellent antibacterial properties that can be used as a raw material for a gas sensor, a medical device, particularly a dental medical material.

  A conductive paste is used as a material for forming a conductive film (wiring, electrode, etc.) having a predetermined pattern on a ceramic wiring board or other electronic components used in hybrid integrated circuits, multichip modules, and the like. In order to reduce CO, NOx, and HC in exhaust gas from automobiles, etc., a gas sensor that measures the oxygen concentration is placed in the exhaust system, and the air-fuel ratio of the combustion mixture supplied to the engine is determined based on the measured value. I have control. A conductive paste material is used as the tip electrode material of such a gas sensor.

  In these conductive pastes, platinum (Pt), palladium (Pd), gold (Au), silver (Ag), copper (Cu), nickel (Ni), etc. are mainly used as the metal powder that is the main component forming the conductive portion. Although used, it has been clarified that platinum powder is particularly suitable as a metal powder for conductive paste in the process of practical development.

Conventional methods for producing platinum-based powders can be broadly classified into the following three types.
(1) Melt spraying method (atomizing method): This method is a method of heating a metal constituting the powder to a molten state, spraying the molten metal into droplets, and cooling.
(2) Liquid phase reduction method: This method is a method of reducing a solution (raw material salt solution) containing a metal salt constituting the powder.
(3) Spray pyrolysis method: In this method, a solution containing a metal salt constituting the powder is sprayed into droplets, and the droplets are heated at a temperature higher than the metal salt decomposition temperature (near the melting point of the metal). Then, the metal salt is thermally decomposed to deposit metal powder.

  With the downsizing, high density, high performance, etc. of electronic parts, there is a demand for platinum powder for conductive pastes with excellent conductive performance. In order to satisfy this demand, it is desirable that the platinum powder for conductive paste has a small particle size and is nearly spherical. Furthermore, high temperature resistance is also required.

Of the three manufacturing methods described above, in the melt spraying method (1), it is necessary to melt platinum in a crucible. Since the melting point of platinum is 1772 ° C., the platinum in the tundish must be heated and maintained at 1800 ° C. or higher, but in order to ensure fluidity from the nozzle, a higher temperature of 2000 ° C. or higher. It is necessary to heat and hold the area. However, carbon, which is a conventional crucible material, has a problem that the crucible itself oxidizes and wears when heated for a long time in the above-mentioned high temperature range, and a problem that carbon reacts with platinum and impurities are mixed. Occurs. As another crucible material, silicon oxide (silica; SiO ₂ ) is conceivable, but since this refractory temperature is about 1750 ° C., a platinum melting crucible material that requires heating and holding in the above temperature range. Use as is difficult. Therefore, in reality, it was impossible to melt and spray platinum powder using a conventional crucible material.

  In the liquid phase reduction method of (2), since the obtained platinum powder has a polyhedron, the characteristics required as a conductive paste cannot be fully expressed.

  In the spray pyrolysis method of (3), spherical platinum powder can be produced in the range of about 0.1 to 1 μm. This is described, for example, in JP-A-6-172802 (Patent Document 1), JP-A-6-235007 (Patent Document 2), JP-A-6-279816 (Patent Document 3), and the like. .

  However, with the miniaturization of electronic parts, high-temperature fired ceramics have developed, and firing at a temperature higher than the firing temperature range of conventional conductive pastes has become necessary. Therefore, when a conventional conductive paste is used as it is and high-temperature baking is performed, a large number of pores (bubbles) are generated in the film, resulting in defects such as peeling from the circuit board, damage, and abnormal shrinkage.

  Also, with the recent increase in combustion temperature in the engine compartment, the environment to which the gas sensor is exposed becomes even hotter. For example, when conventional platinum powder is used, problems such as abnormal shrinkage of the sensor and thermal damage Occurs. Considering the use of the conductive paste using the platinum powder produced by the spray pyrolysis method of (3) under such a high temperature environment, for example, the particle diameter is as fine as 0.1 to 1 μm, and the crystal Since the particle size is as small as about 1 to 20 nm, the low temperature sintering property is excellent. However, on the other hand, when used in a high temperature range exceeding the sintering temperature, since the sintering proceeds again, there are problems such as deterioration of electrical characteristics and shrinkage / deformation of the sensor due to the above defects. Arise.

In response to the above problems, Japanese Patent Laid-Open No. 8-7644 (Patent Document 4) proposes “noble metal powder and conductive paste for high-temperature firing”. Here, the surface of a noble metal powder such as gold, silver or platinum is coated with a compound of a metal other than the noble metal and an organic acid, and a noble metal powder that can be fired at a high temperature by heat treatment in an inert gas atmosphere is provided. . However, this method complicates the process and further increases the cost. At the same time, it becomes difficult to control the thickness of the compound film, so that instability of quality becomes a problem.
JP-A-6-172802 JP-A-6-235007 JP-A-6-279816 JP-A-8-7644

The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a conductive paste suitable for use in an electronic component or a gas sensor used in a high temperature range.

  Still another object of the present invention is a spherical platinum powder having a fine grain size but having an appropriate crystal grain size and good sphericity, and initiation of sintering between the powders. An object of the present invention is to provide a conductive paste having excellent heat resistance and thermal stability because the temperature is sufficiently higher than the use environment temperature.

The conductive paste according to the present invention contains platinum powder in an amount of 40 wt% to 95 wt% with respect to the total weight of the paste. Of this platinum powder, 5% by weight or more of the powder with respect to the total weight of the paste is a water atomized platinum powder produced by the water atomization method, and the remaining powder is a liquid phase reduced platinum powder. The water atomized platinum powder has a crystal grain size of 50 to 500 nanometers (nm), a powder average particle size of 1 to 10 μm, and a sphericity of 0.7 to 0.9.

Platinum powder is preferably sintered initiation temperature in an inert gas atmosphere at 600 ° C. or higher, more preferably 1000 ° C. or higher. A more desirable sintering start temperature is 1200 ° C. or higher.

The water atomized platinum powder is, for example, a powder obtained after solidification by spraying water onto fine droplets from a molten platinum.

  One application of the conductive paste is a gas sensor, and the other application is an electronic component circuit.

  The method for producing platinum powder according to the present invention includes a step of dropping a molten platinum in a tundish from a nozzle outlet having a diameter of 8 mm or less, and a high-pressure gas flow that flows together with the molten metal flow while surrounding the molten platinum flow falling from the nozzle outlet. A step of dispersing the molten metal flow into a large number of fine droplets by reducing the pressure of the high-pressure gas surrounding the molten metal flow, and solidifying the fine droplets by spraying water on the fine droplets And a step of causing.

The diameter of the nozzle outlet is more preferably 5 mm or less. Further, the melting temperature of the molten platinum in the tundish is preferably 1800 ° C. or higher and 2500 ° C. or lower, more preferably 2000 ° C. or higher and 2300 ° C. or lower. The material used for the tundish crucible and nozzle is preferably selected from the group consisting of aluminum oxide (Al ₂ O ₃ ), magnesium oxide (MgO) and zirconium oxide (ZrO ₂ ).

  The technical significance, action, effect and the like of each configuration of the present invention will be described in the following items.

1. Platinum powder (1) Crystal grain size of the powder The crystal grain size of the platinum powder is an important factor governing the sinterability and electrical conductivity between the powders in the case of a conductive paste. The crystal grain size of the platinum powder according to the present invention is 50 to 500 nanometers (nm).

  When the crystal grain size is less than 50 nm, the sintering start temperature is lower than 600 ° C. Therefore, when the conductive paste is used in a high temperature environment, re-sintering proceeds as described above, and pores ( A large number of bubbles) are generated, and defects such as peeling or damage from the sensor tip, abnormal shrinkage, etc. occur. As a result, problems such as deterioration of electrical characteristics and contraction / deformation of the sensor occur. In addition, when the crystal grain size is reduced, the rate of current decrease due to phonon scattering at the grain boundary increases, and there is a problem in that the characteristics of the conductive paste are degraded.

  On the other hand, even when the crystal grain size increases beyond 500 nm, the above sinterability (sintering start temperature) does not change greatly, and in the range of the crystal grain size of 500 nm or more, the current decrease rate with respect to the crystal grain size Will not change significantly.

  In addition, since the platinum powder produced by the conventional liquid phase reduction method has a crystal grain size as small as about 5 to 30 nm, sintering between the powders starts at around 300 to 400 ° C. Therefore, it is difficult to apply to a conductive paste for a gas sensor used in an environment exceeding 600 ° C.

(2) Average particle diameter of powder The particle diameter of platinum powder is one of the factors governing the sinterability between powders. In the present invention, the average particle diameter of the platinum powder is desirably 1 to 10 μm. When the average particle size is less than 1 μm, the surface diffusion between the powders is promoted, so that the sintering start temperature is below 600 ° C., and the above-described defects are generated. In addition, problems such as powder agglomeration and a decrease in tap density occur, making it unsuitable for a conductive paste. On the other hand, when the thickness exceeds 10 μm, it becomes difficult to form a fine electronic circuit pattern, which is a required characteristic as a paste. In addition, in the platinum powder produced by the conventional spray pyrolysis method, although the crystal grain size is equivalent to the platinum powder of the present invention, since the average grain size of the powder is as fine as 0.5 to 1 μm or less, the powder surface Since diffusion is easy to proceed, sintering between powders starts around 400 ° C. Therefore, it is difficult to apply to a conductive paste for a gas sensor used in an environment exceeding 600 ° C.

(3) Sintering start temperature of powder The sintering start temperature in the inert gas atmosphere of platinum powder is 600 degreeC or more, More preferably, it is 1000 degreeC or more. The sintering start temperature can be measured using a thermomechanical analyzer (TMA).

Thermomechanical analysis is an analytical technique that measures the deformation of a material as a function of temperature in addition to non-vibrating loads such as compression, tension, torsion and bending while changing the temperature of a sample according to a certain program. Specifically, a 5 mm square and 2 mm thick platinum sample pressed at 250 kg / cm ² in a heating furnace is placed, and a probe connected to the position detection unit and the load generation unit is applied to the platinum sample. A thermocouple for temperature measurement is installed near the sample. The sample temperature is changed in a heating furnace (10 ° C./min) while applying a load to the sample through the probe from the load generation unit. Corresponding to the temperature change, a shape change due to transition such as the softening point, sintering start point, and melting point of the sample is measured by the position detection unit as the position change amount of the probe.

  When the sintering start temperature is lower than 600 ° C., for example, when a conductive paste is used for the gas sensor, the temperature of the exhaust gas measurement environment exceeds 600 ° C., and thus defects due to re-sintering of the platinum powder occur.

  If the sintering start temperature of the platinum powder is 600 ° C. or higher, the above-mentioned defects will not occur, and the sintering start temperature is 1000 ° C. or higher by adjusting the crystal grain size and the average particle size of the powder. As a result, it can be used as a conductive paste for a gas sensor in a high temperature environment.

(4) The sphericity of the powder The sphericity of the platinum powder has a strong correlation with the filling property, that is, the tap density. The sphericity of the platinum powder according to the present invention is preferably 0.7 to 0.9. If it is less than 0.7, the tap density becomes low and it is not suitable for the conductive paste. On the other hand, according to the method of the present invention, it is difficult to obtain platinum powder having a sphericity exceeding 0.9.

  The sphericity is a value defined as follows.

A: Specific surface area calculated from particle diameter (spherical surface area)
B: Actual measurement specific surface area (BET method)
Sphericality = A / B
If the value of sphericity A / B is 1, it is a true sphere, and the closer the value is to 1, the better the sphericity.

2. Method for Producing Platinum Powder FIGS. 1 and 2 are illustrative views showing the main part of the method according to the present invention. The method of the present invention will be described with reference to these drawings.

In the tundish 1, a molten platinum is stored. High-pressure gas is introduced into the passage of the molten platinum stream 2 falling from the nozzle outlet of the tundish 1. The material used for the tundish crucible and nozzle is aluminum oxide (Al ₂ O ₃ ), magnesium oxide (MgO), or zirconium oxide (ZrO ₂ ). The temperature of the molten platinum in the tundish is 1800 ° C. or higher and 2500 ° C. or lower, more preferably 2000 ° C. or higher and 2300 ° C. or lower. The nozzle diameter is 8 mm or less, more preferably 5 mm or less. For example, air is used as the high-pressure gas. The high pressure gas flows along with the molten metal stream while surrounding the molten metal stream 2. As shown in FIG. 2, this area becomes a pressure area.

  By rapidly expanding the high-pressure gas flowing together with the molten metal stream 2 to the surroundings, the pressure surrounding the molten metal stream 2 is rapidly decreased. As shown in FIG. 2, this region becomes a decompression / dispersion region. In this region, the molten metal stream 2 is dispersed to the surroundings by abrupt pressure reduction to form a large number of fine platinum droplets.

  Next, the fine droplets are rapidly solidified by spraying water onto the dispersed fine platinum droplets. As shown in FIG. 2, this region becomes a solidified region.

  According to the present invention described above, since the molten metal flow 2 is dispersed in a large number of fine platinum droplets without applying external impact, the fine droplets are likely to become spherical due to their surface tension. . Moreover, since the molten metal flow is spread to the surroundings by abrupt pressure reduction and a large number of droplets are produced, the particle size of the droplets can be reduced. Since the dispersed fine and spherical droplets are rapidly solidified by spraying with water, the finally obtained platinum powder has a crystal grain size in an appropriate range, a small particle size, and good sphericity. Have

  According to the above method, a platinum powder having a crystal grain size of 50 to 500 nm, an average grain size of 1 to 10 μm, and a sphericity of 0.7 to 0.9 can be obtained. If the pressure difference from the high pressure region to the reduced pressure region is increased, platinum powder having a finer particle size can be obtained.

  As a technique for increasing the pressure difference, for example, the following can be considered.

  (A) To reduce the cross-sectional area of the nozzle passage through which the molten metal flow passes.

  (B) A large amount of high-pressure gas is introduced into the nozzle passage.

  (C) A means for causing a sudden pressure reduction is provided at the outlet of the nozzle passage.

  (D) Improving the shape of the nozzle passage.

  When the nozzle diameter is increased beyond 8 mm, the flow rate of the molten metal coming out of the nozzle increases, and the volume of platinum droplets obtained in the reduced pressure / dispersion region increases beyond the appropriate amount. The particle diameter of the increases. As a result, the average particle size of the platinum powder exceeds 10 μm. In order to make the average particle size of the platinum powder smaller, the nozzle diameter is desirably 5 mm or less.

Conventionally, carbon or silicon oxide (silica) is used as a material for the tundish crucible and nozzle. When carbon is used, since the molten metal temperature is 1800 ° C. or higher, the carbon is oxidized under such a high temperature environment, and the crucible is worn and damaged. Moreover, the problem that the impurity containing carbon exists in platinum powder arises by reaction with platinum molten metal. Silica has a refractory temperature of 1750 ° C. and cannot be used at a melting temperature exceeding 1800 ° C. Therefore, in the present invention, aluminum oxide (Al ₂ O ₃ ) or magnesium oxide (MgO) is used because it is an oxide ceramic having a fireproof temperature of 1800 ° C. or higher and does not react with platinum at such a high temperature range. Alternatively, it has been found that zirconium oxide (ZrO ₂ ) is a material for a tundish crucible and nozzle suitable for dissolving platinum.

  The temperature of the molten platinum in the tundish is 1800 ° C. or higher and 2500 ° C. or lower, more preferably 2000 ° C. or higher and 2300 ° C. or lower. In order to stably flow out the molten platinum from the nozzle, it is necessary to heat and hold the platinum in the crucible at a melting temperature of platinum of 1772 ° C. or higher. However, when heated to 2500 ° C. or higher, even the above oxide ceramics are thermally damaged, so the temperature of the platinum molten metal is set to 1800 to 2500 ° C. However, when the nozzle diameter is 5 mm, it is desirable to heat to 2000 ° C. or higher in order to reduce the viscosity of the molten platinum and ensure efficient outflow conditions. On the other hand, from the viewpoint of the durability of the crucible, by making it 2300 ° C. or less, there is an economic effect that the crucible is not worn or damaged even when used for a long time. Therefore, as a more preferable platinum molten metal temperature, it is 2000-2300 degreeC.

  As a method for producing a platinum powder having a larger particle diameter, the above-described method of the present invention may be used, or a conventional gas atomization method or water atomization method may be used.

3. Conductive paste using platinum powder The conductive paste containing the platinum powder of the present invention having the characteristics described above has excellent high-temperature use environment performance. In this case, not only the platinum powder of the present invention but also a platinum powder produced by a conventional production method can be mixed and used. At that time, platinum powder in the range of 40% to 95% on the basis of weight with respect to the total weight of the paste is included, and at the same time, 5% or more of the platinum powder of the present invention is included on the basis of weight with respect to the total weight of the paste. If the platinum powder is less than 40% by weight based on the total weight of the paste, sufficient conductivity cannot be obtained. The upper limit of the platinum powder content with respect to the total weight of the paste is 95% on a weight basis. When the content of the platinum powder exceeds 95%, the viscosity as a paste is lowered and the conductive paste cannot be used.

  When the content of the platinum powder of the present invention is less than 5% with respect to the total weight of the paste, the sintering start temperature is lower than 600 ° C. The platinum powder content should be 5% by weight or more.

  The conductive paste as described above is suitable for an electronic component circuit or a gas sensor used in a high temperature range.

  Regarding the platinum powder (sample number 11 and sample number 12) by the water atomization method of the present invention and the platinum powder (sample number 13) by the liquid phase reduction method as a comparative example, the results of investigating the sintering start temperature The heat shrinkage data is shown in FIG. The sintering start temperature was measured using the above-described thermomechanical analyzer.

  The platinum powder of sample number 11 is an example of the present invention, the crystal grain size is 223 nm, and the average particle size of the powder is 9 μm.

  The platinum powder of sample number 12 is an example of the present invention, the crystal grain size is 175 nm, and the average particle size of the powder is 5 μm.

  The platinum powder of sample number 13 is a comparative example, and has a crystal grain size of 5 nm and an average particle size of 0.6 μm.

  The crystal grain size was measured using a high-resolution Rietveld analysis method using PAnalytical high-speed detectors X'Celeratore and X'Pert-Highscore. The Rietveld analysis method is a method for directly refining the structural parameters and the lattice constant for the entire powder X-ray diffraction pattern. The diffraction pattern includes a lot of information such as lattice constant, crystal structure parameters (occupancy, atomic coordinates), lattice strain, crystallite size, mass fraction, and the like. According to this Rietveld analysis method, crystal data of a substance can be grasped.

  As shown in FIG. 3, in the platinum powder of the sample number 11 and the platinum powder of the sample number 12 which are examples of the present invention, a mild heat shrink phenomenon is confirmed from about 1240 ° C. over 1200 ° C. That is, since the sintering start temperature is 1200 ° C. or higher, it can be sufficiently used as a platinum powder for conductive paste even in a high temperature environment exceeding 1000 ° C.

  On the other hand, in the platinum powder obtained by the liquid phase reduction method of Sample No. 13, which is a comparative example, a rapid heat shrink phenomenon is confirmed at about 400 ° C. From this, in the platinum powder by the conventional manufacturing method, the crystal grain size is a fine particle less than 50 nm, and since the average particle size is a fine powder less than 1 μm, the sintering start temperature between the powders is less than 600 ° C. became. Therefore, it is judged that it is difficult to use as a platinum powder for conductive paste in a high temperature environment exceeding 600 ° C.

  FIG. 4 and FIG. 5 show the observation results of the platinum powder by the water atomization method of the present invention and the appearance of the platinum powder by the liquid phase reduction method as a comparative example, with a scanning electron microscope. FIG. 4 is a photomicrograph showing the platinum powder of the present invention obtained by the method of the present invention (both a and b have a magnification of 2000), and FIG. 5 is a platinum by a liquid phase reduction method as a comparative example. It is the microscope picture (a is 5000 times magnification, b is 10,000 times magnification) which shows powder.

  As is clear from FIG. 4, the platinum powder of the present invention is a true spherical powder. As a result of particle size distribution analysis and shape analysis, the average particle size is 8.6 μm and the sphericity is 0.82. there were.

  On the other hand, as is clear from FIG. 5, it is clear that the platinum powder obtained by the liquid phase reduction method has a polyhedron and is not spherical. The average particle size was 0.6 μm.

  The X-ray diffraction results of the platinum powder shown in Example 2 are shown in FIGS. FIG. 6 shows a platinum powder by a water atomization method as an example of the present invention, and FIG. 7 shows a platinum powder by a liquid phase reduction method as a comparative example.

  In the platinum powder by the water atomization method of the present invention shown in FIG. 6, a sharp platinum diffraction peak is detected. On the other hand, in the platinum powder obtained by the liquid phase reduction method as a comparative example shown in FIG. 7, a broad and broad diffraction peak is detected. This result also shows that the crystal particle diameter of the platinum powder by the water atomization method of the present invention is larger than that of the platinum powder by the liquid phase reduction method.

  A conductive paste was prepared at a blending ratio as shown in Table 1. Here, platinum powder (crystal particle size: 175 nm, average particle size: 5 μm) by the water atomization method of the present invention and platinum powder (crystal particle size: 5 nm, average particle size) produced by the liquid phase reduction method as a comparative material. : 1.6 μm) were used. A powder obtained by mixing these at a ratio shown in Table 1 was added to a mixed slurry of low molecular weight acrylic resin and tapinol, and platinum powder was dispersed by a ball mill to prepare a conductive paste.

  The conductive paste was printed to a thickness of 300 μm, dried and cured at 150 ° C. for 10 minutes, and 10 mm diameter test pieces were prepared. After each test piece was heated to 600 ° C., the shrinkage change of the test piece was evaluated. The results are shown in Table 1.

In the conductive pastes of Sample Nos. 21, 22, 23 and 25 , which are examples of the present invention, the content of platinum powder in the paste is 40% by weight or more and 95% or less, and the platinum powder of the present invention is 5% by weight or more. By containing, abnormal shrinkage does not occur even in holding in a high temperature environment of 600 ° C. or higher. In particular, as shown in Sample Nos. 23 and 25, when the content of the platinum powder of the present invention was 25% by weight or more, abnormal shrinkage was not confirmed even after heating at 800 ° C. From these results, the conductive platinum paste defined by the present invention can be used in a high temperature environment. Sample No. 24 is not an example of the present invention because it does not contain liquid-phase reduced platinum powder, but it is recognized that it exhibits an excellent shrinkage suppression effect.

  On the other hand, the following problems were confirmed in the conductive pastes of sample numbers 26 to 29, which are comparative examples.

  Sample number 26: Since the total content of platinum powder is as small as 12%, the conductivity as a paste is lowered. Since the platinum powder content of the present invention was as low as 2%, there was no effect of improving the sintering start temperature, and abnormal shrinkage occurred.

  Sample No. 27: Since the platinum powder content of the present invention was as low as 3%, there was no effect of improving the sintering start temperature, and abnormal shrinkage occurred.

  Sample number 28: Since the total content of platinum powder is as low as 20%, the conductivity as a paste is lowered.

  Sample number 29: Since the total content of platinum powder was as high as 98%, sufficient viscosity as a paste was not obtained, and a good paste was not obtained.

  As mentioned above, although embodiment of this invention was described with reference to drawings, this invention is not limited to the thing of embodiment shown in figure. Various modifications and variations can be made to the illustrated embodiment within the same range or equivalent range as the present invention.

  The present invention can be advantageously applied to platinum powder used for conductive pastes, gas sensors, electronic component circuits, and the like.

Fig. 2 schematically shows a method according to the invention. It is a figure which divides and shows the field in the method of the present invention. It is a figure which shows the heat contraction data in the temperature rising process of the platinum powder of the example of this invention, and the platinum powder of a comparative example. It is a microscope picture of the appearance of platinum powder by the water atomization method which is an example of the present invention. It is a microscope picture of the appearance of platinum powder by the liquid phase reduction method which is a comparative example. It is a figure which shows the X-ray-diffraction result of the platinum powder by the water atomization method which is an example of this invention. It is a figure which shows the X-ray-diffraction result of the platinum powder by the liquid phase reduction method which is a comparative example.

Explanation of symbols

1 tundish, 2 molten metal flow.

Claims (3)

40 wt% or more and 95 wt% or less of platinum powder with respect to the total paste weight,
Of the platinum powder, 5 wt% or more of the powder based on the total weight of the water is a water atomized platinum powder prepared by a water atomization method, the remaining powder is a liquid phase reduced platinum powder,
The water atomized platinum powder has a crystal grain size of 50 to 500 nanometers (nm), a powder average particle size of 1 to 10 μm, and a sphericity of 0.7 to 0.9. . The conductive paste according to claim 1, wherein the water atomized platinum powder has a sintering start temperature in an inert gas atmosphere of 600 ° C. or higher. The electrically conductive paste of Claim 2 whose sintering start temperature of the said water atomized platinum powder is 1000 degreeC or more.