EP0537502A1 - Metal- and metal alloy powder comprising microcrystalline, spherical and dense particles and process and installation for preparing same - Google Patents

Abstract

Metal and metal alloy powders comprising microcrystalline, spherical and dense particles, and process and installation for preparing same. Problems in chip production can be overcome by the use of powders according to the invention. The powders according to the invention are characterised by a mean particle diameter of between 0.1 mu m and less than 5 mu m and a grain spectrum within 0.01 mu m and 25 mu m. The process comprises the conversion of a non-spherical metal powder or metal alloy powder into a powder cloud, treatment thereof in an oven at a temperature above the melting point and separation of the powder consisting of spherical particles from the cooled powder cloud. The installation for preparing the powders comprises a device (1) (consisting of (2) to (7)), an oven (8), whose pipe (9) is provided with heating devices (10) and cooling devices (11), and a recovery chamber (13). <IMAGE>

Description

The invention relates to powders of metals and binary and ternary metal alloys in the form of microcrystalline, spherical and dense particles.

For the production of sintered metal blocks and components for the electronics industry, there is an increased demand for microcrystalline metal and metal alloy powders which are approximately spherical, free of cracks and pores, i.e. are dense with a surface that is as smooth as possible.

Metal powders can be produced using chemical and physical processes that have a decisive influence on the properties of the powders. The known processes consist in the production of the powders by atomizing the metal or metal alloy melts in a liquid or gaseous medium (see, for example, DE-OS 33 24 188, DE-OS 33 45 983, GB 952 457, GB 1 123 825 and EP- A 0 120 506). The metal powder particles atomized with water according to DE-OS 33 24 188 have very irregular surfaces, and their specific surface area is considerably larger than that calculated geometrically. When using a gaseous medium, metal powder with spherical particles is obtained, the particle size of which is described in GB 952 457, e.g. is between 10 and 50 µm.

EP-A 0 120 506 describes a special process for the production of even finer metal powder by atomizing a melt stream by means of a gas. The powder particles obtained with this method are dense and non-porous; they are approximately spherical with an average diameter between 5 and 35 microns.

The disadvantage of this known method for the production of powders by atomizing a molten metal stream is the high investment in mechanical engineering, which is particularly important when only small amounts of valuable metal powder, e.g. of precious metals. In addition, this process cannot be used to produce metal powders with an average particle diameter of less than 5 µm.

A completely different process for the production of spherical metal particles according to DE-OS 33 45 985 consists of a certain amount of coarse metal particles, e.g. Put scrap, chips or castings into the melting zone of a fluidized bed furnace, melt the particles, atomize the melting product in droplets in a hot gas stream and solidify the droplets. This method, which is just as expensive as the one described above, is suitable for the production of abrasives, but not for the production of microcrystalline powders.

Very fine metal and metal alloy powders are produced using the classic method by chemical synthesis. For example, metal powders such as the powders of precious metals and their alloys, which are produced by simple chemical reduction of the metal salts, are very fine with a particle diameter between 0.001 and 0.1 µm. They have a large specific surface area of 10 to 40 m² / g, depending on which reducing agent was used. The primary particles of these powders can be agglomerated by thermomechanical treatment. However, these particles are structured so that they usually have a small grain size with a large specific surface. Under these conditions, the powders are very reactive; this is disadvantageous for the production of sintered metals as well as electronic components and microelectronic hybrid circuits.

The great influence exerted by the morphology of the metal or metal alloy powder becomes clear in the case of the ceramic multilayer capacitors with internal electrodes consisting of an alloy of the silver-palladium system.

The two metals form slightly continuous series of single-phase solutions. The temperature of the solid state and that of the liquid is between the melting point of the palladium at 1552 ° C and the melting point of the silver at 961 ° C. In order to produce the capacitors mentioned, the ratio Pd / Ag is chosen so that the melting point is above the sintering temperature of the non-conductor in order to prevent the electrode from melting during the sintering process.

The two metals form oxides when sintered in air. Ag₂O is the most stable oxidation level of silver at room temperature and up to 300 ° C. However, the formation of oxide is impeded by the air and the decrease in the specific surface area of the silver during the first sintering phase.

Palladium, on the other hand, easily forms the oxide PdO in the initial phase of the sintering process. The extent depends on the specific surface of the metal, the heating rate and the partial pressure of atmospheric oxygen. PdO with a tetragonal crystalline structure is thermodynamic up to approx. 800 ° C. stable and returns to the metallic state at higher temperatures.

When the capacitors are sintered, the oxidation of the palladium can be accompanied by a volume expansion of up to 40%. Above 800 ° C the reduction of PdO to Pd is noticeable in a volume reduction. Both reactions cause tension, which is a reason for the flaking.

The oxidation reduction of the palladium is accelerated by adding silver. For example, powders consisting of 70 wt% Ag and 30 wt% Pd reach the oxidation maximum at 520 ° C compared to 790 ° C without silver. The reduction end of PdO is reduced in the same way from 900 ° C to 700 ° C.

During the firing phases, the fine powder can react catalytically with the organic medium to form warm spots, which leads to rapid degassing, bubble formation and the condensers peeling. The shrinkage properties of very fine metal powders are usually very pronounced and cannot compensate for the properties of the surrounding dielectric materials. As a result, the discontinuous metal groups with weaker conductivity form tiny metal "islands".

The invention has for its object to provide inactive metal powders that are fine enough for use in screen printing pastes, but the specific surface area must be small enough so that the catalytic effect is slowed down. A further object of this invention is to develop a method and an apparatus for producing such powders, starting from microcrystalline powders from non-spherical particles, the specific surface area of which is larger than the theoretically calculated value.
The invention therefore relates to powders of metals and binary and ternary metal alloys in the form of micro-crystalline, spherical and dense particles, which is characterized in that the mean particle diameter is between 0.1 μm and less than 5 μm and the grain spectrum within 0.01 μm and 25 µm, preferably within 0.1 µm and 10 µm and in particular within 0.1 µm and 5 µm. The specific surface area of the powders corresponds approximately to the theoretical value calculated on the basis of the average grain size.

The powders according to the invention, but also powders of the same morphology, whose average particle diameter is greater than 5 μm and whose grain spectrum is within 0.01 μm to approximately 50 μm, can be produced by a process, in the form of non-spherical particles in one Oven-introduced metals or their alloys are heated to a temperature above the melting point and the particles obtained are then cooled so that they solidify, which is characterized in that a metal or metal alloy powder or powder from the constituents of the alloy in the form of microcrystalline is not -spherical particles with a larger specific surface than that of the powder to be used as the starting product, this suspended in an inert carrier gas, the suspension (powder cloud) in a tubular furnace with external heating and cools after passing through the heating zones of the furnace and then there s Separates powder from spherical particles from the suspension using known methods.

According to a preferred embodiment of the method, a metal or metal alloy powder is used as the starting product, which was produced by chemical synthesis, preferably by chemical reduction of metal salts with possibly subsequent thermomechanical treatment.

In principle, all powders that can be suspended in the carrier gas can be used as the starting product. The flow rate of the suspension through the furnace and the temperature of the heating zone are controlled so that the molten particles are spherical as they enter the cooling zone. The heating zone is preferably set to a temperature which is 100 to 250 ^o C above the melting temperature. The particles solidify in a cooling device, which can be attached to the outside of the furnace tube or in a part of the tube and / or outside the furnace.

According to the method of the invention, powders of base metals, such as. B. copper, lead, tin, zinc, aluminum and powder of precious metals, preferably silver, gold, palladium and platinum. The method is also applicable to binary and ternary metal alloys.

For example, a powder of an Ag-Pd alloy, produced by chemical reduction of a silver-palladium mixed carbonate using a reducing agent of the aldehyde type in the aqueous phase, has a specific surface area of 10 m² / g (measured by means of N₂ gas adsorption after the BET Method) and a particle diameter of less than 0.1 µm. Thermomechanical treatment of the powder produced gives a powder with a specific surface area of 1 to 2 m² / g. With the method according to the invention, the specific surface area of the latter powder can be reduced to approximately 0.3 to 0.5 m² / g.

The invention also relates to a device for carrying out the method, which is characterized by the features mentioned in connection with the explanation of FIG. 3.

• Fig. 1 zeigt die Morphologie eines Ausgangspulvers einer Ag-Pd-Legierung.
• Fig. 2 zeigt die Morphologie eines Produktes, hergestellt nach dem erfindungsgemäßen Verfahren (Fig. 1 und 2 stellen Aufnahmen mit dem Rasterelektronenmikroskop dar).
• Fig. 3 zeigt im Längsschnitt ein Schema einer Anlage zur Herstellung von Pulvern nach dem erfindungsgemäßen Verfahren.

The invention is explained in more detail with reference to the following examples and the attached figures:

• 1 shows the morphology of a starting powder of an Ag-Pd alloy.
• 2 shows the morphology of a product produced by the method according to the invention (FIGS. 1 and 2 represent images taken with a scanning electron microscope).
• Fig. 3 shows in longitudinal section a diagram of a plant for the production of powders according to the inventive method.

3 contains a device for suspending the powder (1). a tubular furnace (8), one or more devices for cooling the suspension (11) and a device (13) in which the powder of spherical particles is separated from the carrier gas and recovered; the device (1) consists of an airtight housing (2), a filling system the starting powder (3), at least one carrier gas supply (4), a device (5, 6) for intensively mixing the powder with the gas and an outlet (7) connected to the beginning (9a) of the tube (9) of the tubular furnace (8) is connected; the area (9b) of the pipe (9) is surrounded by one or more heating devices (10), the cooling device or devices (11) are inside and / or outside of the pipe (9) between the heating zone (9b) and the pipe outlet ( 9c) and / or inside and / or outside on the chimney (12), which is located between the pipe end (9c) and the recovery chamber (13).

The device (1) and its parts (2) to (6) can be designed in different ways. The powder supply (3) can be carried out, for example, by conventional metering systems which are used for fine powders, such as cellular wheel sluices, metering screws or vibration chutes. Fig. 3 shows a particularly suitable mixing device (5, 6); (5) is a rotor (5) driven by a motor (6). The principle of mixing devices without mobile parts, whereby fixed mixers and powder are introduced on one side, has also proven itself. Instead of via the feed line (4), the carrier gas can also be fed into the mixing chamber (2) via other connections. The tube (9) of the tubular furnace (8) has a heating zone (9c) which is heated by one or more heating registers (10). The heating can be done electrically or with gas; However, an electric heater is preferred because it enables the regulation and setting of the temperature program for the entire heating zone in a simple manner. The pipe (9) is connected at one end (9a) to the mixing chamber (2) and at the other end (9c) to the recovery chamber (13) or any chimney (12) between the recovery chamber and the pipe (9). The cooling system (s) (11) can be of different types and can be arranged at the end of the pipe (9) and / or in the vicinity of the possible chimney (12). The suspension can be cooled in one or more steps respectively. According to the preferred scheme according to Figure 3, the cooling device (11) is at the end of the tube (9);
this cooling system consists of a heat exchanger that is placed around the pipe and works with a refrigerant; (11a) and (11b) represent the supply or discharge of the refrigerant. The optional cooling system (14) with supply and discharge (14a and 14b) of the refrigerant serves to stably close the pipe (9) with the chamber (2) connect. If a pipe element (12) is installed between the pipe (9) and the recovery system (13), which has the shape of a chimney and can be equipped with a cooling device, it will, due to its significantly larger diameter than pipe (9), cause that the flow rate of the suspension decreases. In this way, the powder can be separated from the carrier gas by settling in the simply constructed chamber (13); the powder moves towards the outlet (13b); the carrier gas escapes via the outlet (13a), which is optionally provided with dust filters, or it is completely or partially returned to the mixing chamber. Instead of in the recovery chamber (13) of FIG. 3, the continuous separation of the powder from the cooled suspension can also take place by means of other known devices, for example by means of a dust separator and / or dust filter.

It has surprisingly been found that microcrystalline metal or metal alloy powders which are present as non-spherical particles can be converted according to the invention into powders with spherical particles and a smooth surface in a production device specially developed for this process. This process is particularly advantageous because it is much simpler and can work more economically than the known processes, which also start from solids. In the method according to the invention, the amount of gas required is significantly less than in the named method; the energy and raw material costs are lower. Through targeted selection of the starting product and variation of its residence time in the heating zone by changing the length and temperature of the heating zone and the flow rate of the gas, powders of the desired grain spectra can be produced in areas that have not yet been achieved. The powders produced by the process according to the invention have proven to be particularly suitable for the production of electronic components, such as ceramic multilayer capacitors. Due to its simple construction, the device is particularly suitable for the production of small powder batches of very expensive metals, such as precious metals and their alloys.

In addition, a device for producing the powders according to the invention can also be used to improve the crystal structure of the metal and metal alloy powders, even if they are treated at temperatures which are 100 to 200 ° C. below the melting temperature. In this case, the starting powder can be in the form of spherical or non-spherical particles.

The process is explained in more detail by the following examples:

example 1

The starting powder - see Fig. 1 - is a silver / palladium alloy with 30 wt .-% Pd, which was produced by chemical means and by thermomechanical treatment.

• Morphologie: Agglomerate von 3 bis 4 µm, in Einzelfällen bis zu 17 µm.
• Spezifische Oberfläche: 1,8 m²/g (Gasadsorption noch der BET-Methode))
• Oxydationsgrad: 60 - 80% des enthaltenen Palladiums oxidiert sich bei der maximalen Oxidationstemperatur.

It has the following chemical-physical properties:

• Morphology: agglomerates of 3 to 4 µm, in individual cases up to 17 µm.
• Specific surface area: 1.8 m² / g (gas adsorption still using the BET method))
• Degree of oxidation: 60 - 80% of the palladium contained is oxidized at the maximum oxidation temperature.

Test conditions:

• The method was carried out in a device according to FIG. 3
• Carrier gas; Argon, throughput: 1 1 / min.
• Rotation speed of the rotor: 3000 rpm.
• Oven temperature: 1460 ° C

Results:

The particles are completely spherical - see Fig. 2; the grain size is between 0.2 and 3 µm, the specific surface is 0.43 m² / g (electron microscope). The powder particles are completely homogeneous; the chemical composition has not changed.

Commitment:

The starting powder and the powder obtained were used to produce the internal electrodes of ceramic multilayer capacitors; Manufacture of capacitors using known methods:
The chips based on the starting powder showed some tiny metal islands as well as partial exfoliation of the layers after sintering at 1150 ° C.

The chips based on the spherical powder according to Example 1 showed no flaking after sintering at 1150 ° C.

Example 2:

Starting powder and test conditions are identical to Example 1, the only difference is that the oven temperature is set to 1320 ° C.

Results:

The particles are completely spherical, the grain size is between 0.4 and 4 µm, the specific surface area is 0.34 m² / g. The powder particles are completely homogeneous; the chemical composition has not changed.

The degree of oxidation of the palladium contained is 30% at the maximum oxidation temperature of 575 ° C. The shrinkage measured at 1100 ° C is 12% compared to 40% of the starting powder. The linear coefficient of thermal expansion at 0 and 900 ° C is close to that of the layer connection, i.e. 1.68 x 10⁻⁵ / ° C.

Used for the production of the inner electrodes of ceramic multilayer capacitors, the chips showed no peeling after sintering at 1150 ° C.

Example 3

The starting powder is a silver / palladium alloy with 70% by weight Pd, which was produced chemically and by thermomechanical treatment.

• Morphologie: Agglomerate von 3 bis 5 µm, in Einzelfällen bis zu 12 µm.
• Spezifische Oberfläche: 1,5 m²/g

It has the following chemical-physical properties:

• Morphology: agglomerates of 3 to 5 µm, in individual cases up to 12 µm.
• Specific surface: 1.5 m² / g

Test conditions:

• 3,
• Carrier gas: argon, flow rate: 5 1 / min.
• Rotation speed of the rotor: 3000 rpm.
• Oven temperature: 1500 ° C

Results:

The particles are perfectly spherical; the grain size is between 0.2 and 3 µm, the specific surface is 0.5 m² / g. The powder particles are completely homogeneous; the chemical composition has not changed.

Example 4

• Morphologie: Agglomerate von 5 bis 7 µm, in Einzelfällen bis zu 25 µm.
• Spezifische Oberfläche: 1 m²/g

The starting powder consists of silver particles that have been obtained by chemical reduction. It has the following chemical-physical properties:

• Morphology: agglomerates of 5 to 7 µm, in individual cases up to 25 µm.
• Specific surface: 1 m² / g

Test conditions:

• 3,
• Carrier gas: argon, flow rate: 5 1 / min.
• Rotation speed of the rotor: 3000 rpm.
• Oven temperature: 1200 ° C

Results:

The particles are perfectly spherical; the grain size is between 2 and 5 µm.

Example 5

• Morphologie: Agglomerate bis zu 50 µm.
• Spezifische Oberfläche: 0,8 m²/g

The starting powder consists of copper particles obtained from a powdered copper formate by thermal dissociation under nitrogen at 300 ° C.

• Morphology: agglomerates up to 50 µm.
• Specific surface: 0.8 m² / g

Test conditions:

• 3,
• Carrier gas: argon.
• Rotation speed of the rotor: 3000 rpm.
• Oven temperature: 1300 ° C

Results:

The particles are perfectly spherical; the average grain size is 8 µm, the specific surface 0.2 m² / g.

Claims (6)

Powder of metals and binary and ternary metal alloys in the form of microcrystalline, spherical and dense particles,
characterized,
that the average particle diameter is between 0.1 µm and less than 5 µm and the particle size range is between 0.01 µm and 25 µm, preferably within 0.1 µm and 10 µm. Powder according to claim 1,
characterized,
that they consist of precious metals or of binary and ternary precious metal alloys, preferably silver, gold, palladium or platinum or their alloys. Process for the production of metal powders as well as binary and ternary metal alloy powders in the form of microcrystalline, spherical and dense particles with a grain spectrum within 0.01 µm and 50 µm, in the form of non-spherical particles brought into a furnace or their alloys to a temperature heated above the melting point and the particles obtained are then cooled so that they solidify, characterized in that that a metal or metal alloy powder or powder from the constituents of the alloy in the form of microcrystalline, non-spherical particles with a larger specific surface than that of the powder to be used is used as the starting product, this is suspended in an inert carrier gas, the suspension (powder cloud) in conducts a tubular furnace with external heating and cools after passing through the heating zones of the furnace and then separates the powder from spherical particles from the suspension using known methods. Method according to claim 3,
characterized,
that the temperature of the heating zone of the furnace is set 100 to 250 ^o C higher than the melting temperature. Process for the production of noble metal powders and binary and ternary noble metal alloy powders in the form of microcrystalline, spherical and dense particles, the mean particle diameter of which is between 0.1 µm and less than 5 µm and the particle size range within 0.01 µm and 25 µm, preferably within 0.1 µm and 5 µm, is according to one of claims 3 or 4,
characterized,
that a metal or metal alloy powder is used as the starting product, which was obtained by chemical synthesis, preferably by chemical reduction of metal salts with possibly subsequent thermomechanical treatment. Device for carrying out the method of claims 3 to 5, characterized by a device (1) for suspending the powder, a tubular furnace (8), one or more devices (11) for cooling the suspension and a device (13) in which the powder is separated from the carrier gas and recovered; the device (1) comprises an airtight housing (2), a device (3) for filling the powder, at least one carrier gas supply (4), a device (5, 6) for intensive mixing of the powder with the gas and an outlet (7 ) connected to the beginning (9a) of the tube (9) of the tubular furnace (8); the area (9b) of the tube (9) is surrounded by one or more heating devices (10); the one or more cooling devices (11) are inside and / or outside of the pipe (9) between the heating zone (9b) and the pipe outlet (9c) and / or inside and / or outside of the chimney (12) which is between the pipe end (9c) and the recovery chamber (13) is attached.

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