FR2682625A1 - POWDERS OF METALS AND METAL ALLOYS IN THE FORM OF SPHERICAL AND COMPACT MICROCRYSTALLINE GRAINS, AND PROCESS AND DEVICE FOR THE MANUFACTURE OF POWDERS. - Google Patents

Abstract

(a) Powder of metals and metal alloys in the form of spherical and compact microcrystalline grains, and method and apparatus for their manufacture. b) Powders characterized in that the average grain diameter is between 0.1 x 10-6 m and less than 5 x 10-6 m and whose particle size is between 0.01 x 10-6 m to 25 x 10-6 m, preferably 0.1 x 10-6 m to less than 10 x 10-6 m, a powder of the metal or alloy obtained by chemical synthesis, preferably by reduction, is used as starting material chemical metal salts; this device (1) is composed of a sealed enclosure (2), a system for introducing the powder (3), at least one inlet (4) for the carrier gas, a device (5, 6) allowing the powder to be intimately mixed with the gas and an outlet (7) which is joined with the inlet (9a) of the tube (9).

Description

Powdered metals and metal alloys in the form of grains

  The present invention relates to powders of metals and binary and ternary alloys of metals in the form of microcrystalline grains.

spherical and compact.

  For the manufacture of bodies from sintered metals as well as elements of the electronics industry, there is an increase in the demand for microcrystalline powders of metals and alloys whose particles are almost spherical, free from cracks and pores, so

  compact, with the smoothest surface possible.

  It is possible to manufacture metal powders by chemical and physical processes which have a determining influence on the characteristics of the powders. Known processes are the production of powders by spraying molten metal or alloy baths with 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 particles of metal powders sprayed with water, for example according to DE-OS 33 24 188, have very irregular surfaces and their surface

  is much larger than that calculated geometrically-

  In addition, spherical particles of metal powder with a spectrum of GB 952 457 are obtained by means of a gaseous medium.

  example is between 10 and 50 x 10-6 m.

  EP-A-0 120 506 discloses the special embodiment of a process for obtaining even finer metal powders by atomizing a jet of molten material with the aid of a gas. The powder particles obtained with this method are compact and pore-free; their shape is almost spherical and their average diameter varies between 5

and 35 x 10-6 m.

  The disadvantage of these known processes for obtaining powders by spraying a jet of molten material is the important technical investment which weighs particularly heavily when small quantities of high-value metal powders such as In addition, it is not possible to manufacture, with these processes, metal powders whose average

  particles is less than 5 x 10-6 m.

  A completely different method of manufacturing spherical metal particles according to German Offenlegungsschrift No. 33 45 983 is to charge a defined amount of coarse metal particles, such as scrap, chips or moldings, into the melting zone of a furnace. fluidized, to melt the particles, to spray the melted product into droplets in a stream of hot gas and to solidify these droplets This process, as expensive as the previous ones, is suitable for the manufacture of particles used as abrasive, but obviously not in the manufacture of microcrystalline powders. Very fine powders of metals and alloys are obtained conventionally, by chemical synthesis. For example, metal powders, such as powders of precious metals and alloys of precious metals, prepared by simple chemical reduction of metal salts, are extremely thin, with particle diameters from 0.001 X 10-6 m to 0.1 x 10-6 m They have a high specific surface area of 10 to 40 m 2 / g, depending on the reducer used The primary grains of this powder can However, the grains have a morphology such that a small granulometry is

  usually associated with a high specific surface area.

  Under these conditions, the powders are very reactive and it is a disadvantage to produce sintered metals and electronic components and

  hybrid microelectronic circuits.

  The important influence of the morphology of metal or alloy powders is evidenced in the case of multilayer ceramic capacitors with internal electrodes based on aluminum alloys.

silver-palladium system.

  The two metals easily form continuous series of single-phase solutions The temperatures of solidus and liquidus vary continuously from the melting point of palladium at 15520 C to the melting point of silver at 9610 C. To manufacture the mentioned capacitors, the ratios Pd / Ag are chosen such that the temperature of the solidus is above the sintering temperature of the dielectric to avoid melting of

the electrode during sintering.

  The two metals form oxides when sintered in air Ag 2 O is the oxidized phase of the most stable silver at temperatures around room temperature and up to 3000 C. However, the formation of The oxide is hindered by the atmospheres and by the decrease in the silver surface area during the first stages of sintering. Palladium, for its part, easily forms

  Pd O oxide during the initial stages of sintering.

  The importance of the latter depends on the specific surface of the metal, the heating rate and the atmospheric oxygen partial pressure Pd O, of tetragonal crystalline structure, is thermodynamically stable phase up to approximately 8000 C, returning to the state of metal at higher temperatures. When sintering capacitors, the oxidation of palladium may be accompanied by a volume expansion up to 40% Above 8000 C, the reduction of Pd O in Pd is manifested by a decrease in volume These two reactions cause

  constraints that are a cause of delamination.

  The oxidation-reduction of palladium is accelerated by the introduction of silver. For example, the powders containing 70% by weight Ag, 30% by weight Pd, result in the maximum of the oxidation of palladium at 5200 C against 7900 C in lack of silver The end of the reduction of Pd O is also modified with a decrease of 9000 C to 7000 C. During the cooking stages, the fine powder can interact catalytically with the organic medium of the ink, creating hot spots and leading to rapid release of flue gas,

  blistering and delamination of the capacitors.

  The shrinkage characteristics of very fine metal powders are usually excessive and can not match those of the (surrounding) dielectric material. As a result, tiny "islands" of metal are formed resulting from discontinuous metallic groups of low conductivity. The object of the present invention is to develop non-active metal powders which are fine enough for use in screen-printed pastes, but which have a specific surface area sufficiently small to inhibit catalytic activity. Another object of the present invention is to find a method and a device for manufacturing such powders from microcrystalline powders consisting of non-spherical grains whose specific surface area is greater than the

calculated theoretical value.

  For this purpose, the invention relates to a process of the type defined above remedying the disadvantages mentioned and characterized in that the average diameter is

  between 0.1 and less than 5 x 10-6 and whose

  Zero-size is spread within 0.01 to 25 x 10-6 m, preferably 0.1 to 10 x 10-6 m and in particular

  from 0.1 x 10-6 m to less than 5 x 10-6 m The special surface

  of these powders is close to the theoretical value

  than calculated from the average particle size.

  The powders according to the invention, but also the powders of the same morphology, whose average diameter is greater than 5 × 10 -6 m and whose particle size is between 0.01 × 10 -6 m and about 50 × 10 -6 m. may be produced by a process comprising introducing the metals or their alloys in the form of non-spherical grains into an oven to heat them to a temperature above the melting temperature and cooling the spherical grains obtained to solidify them, which is characterized in that the starting material used is a metal or alloy powder, or powders of the constituents of the alloy, in the form of non-spherical microcrystalline grains, with a specific surface area greater than that of the powders to be manufactured, this starting material is suspended (cloud of powder) in an inert carrier gas, this suspension is introduced into a tubular furnace with external heating, the suspension is cooled after passing through the oven heating zone and then recovering the powder from the suspension

using known methods.

  According to another characteristic of the process of the invention: the temperature of the heating zone of the oven is set to a temperature of 100 to 2500 ° C. above the melting temperature, a powder of the metal or aluminum is used as starting material. alloy obtained by chemical synthesis, preferably by chemical reduction of metal salts and optionally followed by a thermomechanical treatment. The invention also relates to a device for carrying out the process, characterized in that it comprises a tubular furnace, one or more cooling devices for the suspension, a powder recovery device allowing the separation of the powder from the carrier gas, this device is composed of a sealed enclosure, a system for introducing the powder, at least an inlet of the carrier gas, a device for intimately mixing the powder with the gas and an outlet which is joined with the inlet of the tube of the tubular furnace or a zone of the tube is surrounded by one or more heating devices, the one or more Ir cooling devices are arranged in or around a part of the tube situated between the heating zone and the outlet of the tube and / or optionally in or around a chimney located between the outlet and the recovery device. As a starting material (base powder) all the powders which are suitable for being suspended in the carrier gas can be used in principle. The speed of the suspension current through the furnace and the temperature of the heating zone are adjusted in such a way that spherical fused grains are obtained at the inlet of the cooling zone. Preferably, the heating zone is set at a temperature of 100 to 250 ° C. higher than the melting temperature In a cooling zone which can be arranged around or in a portion of the oven tube and / or positioned at

  outside the oven tube the grains solidify.

  The process for producing the desired powders is characterized in that the starting material used is a powder of the metal or alloy obtained by chemical synthesis, preferably by

  chemical reduction of metal salts and possible

  followed by thermomechanical treatment.

  According to the process of the invention, it is possible to manufacture powders of base metals, such as, for example, copper, lead, tin, zinc, aluminum and powders of precious metals, preferably silver, gold or palladium. and Platinum Beyond, the process is applicable to binary and ternary alloys of metals. The invention will be better understood by means of

  the following description of two examples and

  FIGS. Figure 1 shows the morphology of a starting powder of an Ag-Pd alloy; Figure 2 that of the product obtained according to the method of the invention (Figures 1 and 2 correspond to scanning electron microscope photos); Figure 3 shows a diagram of a powder manufacturing plant according to the invention. By way of example, a powder of an Ag-Pd alloy prepared by chemical redox reaction using an aldehyde-type reducing agent in an aqueous medium from a mixed carbonate of silver and palladium, is characterized by a specific surface of the order of 10 m 2 / g (measured by the adsorption of a gas according to the BET method) and a grain diameter of less than 0.1 x 10-6 By a thermomechanical treatment of the powder obtained, it is possible to obtain a powder whose specific surface is between 1 and 2 m 2 / g. By the process of the invention from the powder mentioned above, the specific surface is reduced to

about 0.3 0.5 m 2 / g.

  FIG. 3 shows a schematic device in longitudinal section of a functional installation for the manufacture of powders according to the method described and comprises a device for suspending the powder 1, a tubular furnace 8, one or more cooling devices of the suspension 11 and a powder recovery device 13 for separating the powder from the carrier gas, said device 1 is composed of a sealed enclosure 2, a powder introduction system 3, at least one arrival of the carrier gas, a device 6 for intimately mixing the powder with the gas and an outlet 7 which is joined with the inlet 9a of the tube 9 of the tubular furnace 8 o a zone 9b of the tube 9 is surrounded by one or more heating devices 10 or the cooling devices 11 are arranged in or around a portion of the tube 9 located between the heating zone 9b and the outlet 9c of the tube and / or optionally in or aut of a chimney 12 located between the outlet 9c and the device

recovery 13.

  The device and the elements that make it up 2 to 6 can be organized in different ways. The powder feed 3 can be done using conventional dosing systems used for fine powders, such as, for example, cellular wheelhouses. metering screw or vibrating chute Figure 3 shows a particularly advantageous mixing device 5, 6; the reference 5 represents a propeller driven by a motor 6 The principle of mixing devices 5, 6 without moving parts, fixed mixers, in which one introduces at one end the powder and the carrier gas, is also suitable Instead of being introduced by a feed 4, the carrier gas can also be introduced into the mixing chamber 2 by different pipes The tube 9 of the tubular furnace 8 contains a heating zone 9 b heated by means of one or more pulling registers 10 heating can be electric or gas; however, the electric heating will be preferred because it allows the adjustment and the easy adjustment of the temperature program over the entire length of the heating zone. The tube 9 is connected on one side 9 a to the mixing chamber 2 and the another side 9 c to the recovery device or possibly to a stack 12 placed between the

  recovery device and the end of the tube 9.

  The cooling system or systems 11 may be in various ways and may be placed towards the end of the tube 9 and / or near a possible chimney 12. The cooling of the suspension may be carried out in one or more steps. recommended, the cooling device 11 shown in Figure 3 is located near the end of the tube 9; this cooling system consists of a heat exchanger placed around it

  of the tube, operating with a cooling medium

  Such as the water 11a and 11b represent the arrival and departure of the cooling medium. The optional cooling system 14 with supply and exhaust ducts 14a and 14b of the cooling medium serves to connect the tube 9 to the chamber 2 If placed between the tube 9 and the recovery system 13 a tubular element 12 having the shape of a chimney and can be equipped with a cooling device, it will serve, thanks to its diameter much more large than that of the tube 9, to reduce the flow rate of the suspension It is possible, in this way, to separate in the rooms of simple design 13, the powder of the carrier gas by sedimentation; the powder is directed towards the outlet 13b, the carrier gas escapes through the outlet 13 possibly connected to dust filters, or is recycled completely or partially into the suspension chamber 2 Instead of the recovery device 13 shown in FIG. 3, the separation of the powder contained in the cooled suspension can also be carried out by means of other known devices, with the aid of

  cyclone separators and / or dust filters.

  It could not be expected that microcrystalline powders of metals or metal alloys in the form of non-spherical particles would be converted into spherical particle and smooth surface powders in accordance with the invention patent and in a This process is particularly advantageous in that it is substantially simpler and it can operate in a more cost-effective manner than the aforementioned process whose

  starting point is also solid particles.

  In the process of the invention, the amount of gas is much lower than in the above process; energy and raw material costs are therefore lower By selecting the starting powders and by acting on their residence time in the heating zone by the length and temperature of the heating zone and the flow velocity of the heating zone. gas, it is possible to manufacture powders with the spectrum

  desired particle size in inaccessible areas.

  The powders obtainable by the process of the invention have proved particularly advantageous for the manufacture of electronic constituents such as multilayer ceramic capacitors. Because of its simple structure, the device is particularly suitable for the manufacture of small loads of very expensive metal powders, for example those of precious metals

and their alloys.

  It should be noted that a device for producing powders of the invention can also be used to improve the crystallinity of the powders of metals and metal alloys, even if they are treated at a temperature of less than 1000 ° C. to 2000 ° C. the melting temperature In this case, the starting powder

  can be in the form of spherical or non-spherical grains

  spherical. is explained using the method

following examples.

Example 1

  The starting powder (see FIG. 1) is a palladium silver alloy containing 30% by weight of Pd obtained chemically and thermomechanically. The physico-chemical characteristics are as follows: Morphology: aggregates of 3 to 4 x 10-6 m average diameter, up to 17 x 10-6 m in one direction. Specific surface 1.8 m 2 / g (gas adsorption according to BET). Oxidation rate: 60 to 80% of the palladium content

  oxidizes at the maximum oxidation temperature.

  Experimental conditions: The process is carried out in a device according to FIG.

  Carrier gas: argon, flow rate: 1 1 / min.

  Rotation speed of the propeller 3000 rpm. Oven temperature: 14600 C. Results: The particles see Figure 2 are perfectly spherical, the particle size is 0.2 to 3 x 10-6 m, the specific surface is 0.43 m 2 / g (electron microscope) The powder has perfect solid homogeneity; the chemical composition

remains unchanged.

  Use The starting powder and the powder obtained were used to produce the internal electrodes of multilayer ceramic capacitors; Manufacture of capacitors in known manner: The chips based on the starting powder had a few tiny islands of metal and a partial delamination of the layers after sintering.

11500 C.

  The chips based on the powder of spherical grains according to Example 1 showed no delamination after sintering at 11500 C.

Example 2

  The starting powder and the experimental conditions are identical to Example 1 with the only difference that the temperature of the oven is regulated at 13200 C. Results The particles are perfectly spherical, the particle size ranges from 0.4 to 4 × 10 -6 m, the specific surface is 0.34 m 2 / g The powder has a perfect solid homogeneity and the

  chemical composition is unchanged.

  The oxidation rate of the palladium content is 30% at the maximum oxidation temperature at 5750 C. The shrinkage measured at 11000 C is of the order of 12% against 40% for the starting powder The coefficient of linear expansion , between O at 900 'C, is close to

  that of the massive material, 1.68 x 10-5 / OC.

  Used to produce the internal electrodes of multilayer ceramic capacitors, the chips showed no delamination after sintering at 11500 C.

Example 3

  The starting powder is an alloy of palladium silver with 70% by weight of palladium obtained by

  chemical and thermomechanical treatment.

  The physicochemical characteristics are as follows: Morphology: aggregates of 3 to 5 x 10-6 m average diameter, up to 12 x 10-6 m in one direction.

Specific surface 1.5 m 2 / g.

Experimental conditions

Device according to Figure 3.

  Carrier gas: argon, flow rate: 5 l / min.

  Rotation speed of the propeller: 3000 rpm.

  Oven temperature: 15000 C. Results: The particles are perfectly spherical, the particle size is 0.2 to 3 x 10-6 m, the specific surface is 0.5 m 2 / g The powder has a perfect solid homogeneity and the composition

chemical remains unchanged.

Example 4

  The starting powder consists of grains

  of silver obtained by chemical reduction.

  Physico-chemical parameters are as follows: Morphology: aggregates of 5 to 7 x 10-6 m on average,

up to 25 x 10-6 m.

Specific surface area: 1 m 2 / g.

Experimental conditions

Device according to Figure 3.

Carrier gas: argon, flow rate 5 l / min.

  Rotation speed of the propeller: 3000 rpm.

  Oven temperature: 12000 C. Results: The particles are perfectly spherical,

  the particle size is spread from 2 to 5 x 10-6 m.

Example 5

  The starting powder consists of copper grains obtained by thermal decomposition at 3000 ° C.

  under nitrogen of a pulverulent copper formate.

  Morphology: aggregates up to 50 x 10-6 m

Specific surface area 0.8 m 2 / g.

  Conditions for obtaining spherical powders

c:

Device according to Figure 3.

Carrier gas: argon.

  Rotation speed of the propeller: 3000 rpm.

  Oven temperature: 13000 C. Results:

  The particles are perfectly spherical.

  The average particle size is 8 x 10-6 m and the

specific surface area of 0.2 m 2 / g.

Claims (6)

  1) Powders of metals and binary and ternary alloys of metals in the form of spherical and compact microcrystalline grains, characterized in that the average grain diameter is between 0.1 x 10-6 m and less than 5 x 10- 6 m and whose particle size is between 0.01 x 10-6 m at 25 x -6 m, preferably from 0.1 x 10-6 m to less than 10 x -6 m.   2) Powders according to claim 1, characterized   characterized in that they comprise precious metals or binary or ternary alloys of precious metals, preferably silver, gold, palladium or platinum or their alloys.   3) A method for producing metal and binary and ternary metal powders in the form of spherical and compact microcrystalline grains whose particle size is between 0.01 x -6 m and approximately 50 x 10-6 m, this process comprising introducing the metals or their alloys in the form of non-spherical grains into an oven to heat them to a temperature above the melting temperature and cooling of the spherical grains obtained to solidify them, characterized in that it uses as the starting material a metal or alloy powder or the powders of   the alloy, in the form of non-microcrystalline grains   spherical, with a specific surface area larger than that of the powders to be produced, this starting material is suspended (cloud of powder) in an inert carrier gas, this suspension is introduced into a tubular furnace with external heating, the suspension after passing through the oven heating zone and then recovering the   suspension powder using known methods.   4) Process according to claim 3, characterized in that the temperature of the heating zone of the oven is set to a temperature of 100 to   250 ° C higher than the melting temperature.   5) Process for the manufacture of precious metal powders or binary or ternary alloys of precious metals in the form of spherical and compact microcrystalline grains having an average diameter of between 0.1 x 10-6 m and less than 5 x 10- 5 m and whose particle size is between 0.01 x 10-6 m to x 10-6 m, preferably 0.1 x 10-6 m to less than   x 10-6 m according to any one of claims 3   or 4, characterized in that the starting material used is a powder of the metal or alloy obtained by chemical synthesis, preferably by chemical reduction of metal salts and optionally followed by a thermomechanical treatment.   6) Apparatus for carrying out the process for producing metal and binary alloy and metal ternary alloy powders in the form of spherical and compact microcrystalline grains whose particle size is between 0.01 × 10 -6 m and 50 x 10-6 m from powders of metals or their alloys in the form of non-spherical microcrystalline grains with a specific surface area greater than that of the powders to be manufactured, characterized by the suspension of the powder (1) and comprising a tubular furnace (8), one or more slurry cooling devices (11), a powder recovery device (13) for separating the powder from the carrier gas, said device (1) is composed of a sealed enclosure (2), a powder introduction system (3), at least one inlet (4) of the carrier gas, a device (5, 6) for intimately mixing the powder with the gas and an outlet (7) which are t joined with the inlet (9a) of the tube (9) of the tubular furnace (8) or an area (9b) of the tube (9) is surrounded by one or more heating devices (10), the device or devices (11) are arranged in or around a portion of the tube (9) between the heating zone (9b) and the outlet (9c) of the tube and / or optionally in or around a chimney ( 12) between the outlet (9c) and the recovery (13).