Specialized metallurgical processes – compositions for use therei – Processes – Producing solid particulate free metal directly from liquid...
Reexamination Certificate
2002-06-11
2004-10-19
Wyszomierski, George (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing solid particulate free metal directly from liquid...
C075S339000, C264S012000
Reexamination Certificate
active
06805726
ABSTRACT:
The invention relates to a process for producing a powder by atomizing a jet of molten metal by means of an atomization gas which comes into contact with the jet.
The prior art has disclosed what are known as melt atomization processes for the production of fine metallic powders with particle sizes of less than 100 &mgr;m. Processes of this type are disclosed, for example, in DE 35 33 964 C1 and DE-A 1 758 844. Melt atomization processes are based on the principle that a molten jet or film flowing out of a vessel is finely atomized by an atomization gas which comes into contact with the jet or film at a high velocity. In practice, for this purpose the metallic melt and the atomization gas are combined via an external mixing nozzle.
DE 26 39 107 C2 has disclosed a process for producing a composite powder based on silver/cadmium oxide. In this process, a metal melt is atomized using an unreactive, i.e. inert, gas. Then, the powder which has been produced by the atomization is oxidized in a second step.
DE 39 13 649 A1 and DE-A 24 00 026 describe a process for the production of metallic powders from a metal melt by gas atomization. Inert gas is used as atomization gas. DE 28 18 720 discloses a process in which a metal melt is atomized, likewise under protective gas. The metal melt is atomized into metal particles by means of water.
DE-A 24 46 698 describes a process for producing a two-layer contact. In this process, AgNiO powder is obtained by atomization of an AgNi melt. The AgNi powder produced by atomization is oxidized in air for approximately 1 hour after atomization at 800° C.
It is known from DE 39 13 649 C2 for the atomization gas to be preheated in order to increase the gas discharge velocity. Increasing the gas discharge velocity makes it possible to produce finer particles. The known process relates to the production of metallic powders, with an inert gas being used as atomization gas.
DE 40 23 278 A1 has disclosed a process for producing metal oxide powders. In this process, oxygen acts on the molten jet as atomization gas. The atomization gas is taken directly from a high-pressure tank. It is in the liquefied or cold state. A drawback of this process is that the particles are only superficially oxidized, this being sufficient for their use, for example, as a pigment. An only partially reacted powder of this type is unsuitable in particular as a starting material for the production of a metal with predetermined electrical, mechanical, chemical and/or physical properties.
It is an object of the invention to provide a process for producing a predetermined chemical compound in the form of fine particles. A further aim of the invention consists in particular in describing a process which is as efficient as possible for the production of a homogeneous powder for producing electrical contacts.
The invention provides a process for producing a powder by atomizing a jet of molten metal by means of an atomization gas which comes into contact with the jet, in which process
a) the atomization gas used is a reactive gas, so that a compound is formed by reaction of the atomization gas with the metal or an alloying constituent thereof, and
b) the temperature of the atomization gas and the cooling rate are set in such a way that the metal or the alloying constituent thereof is substantially completely converted into the compound in one step.
By means of the process according to the invention, it is possible to produce a compound in the form of fine particles, the particles having a homogeneous composition through their entire cross section. The particles are converted into a compound of predetermined stoicheometry by the reactive gas as early as during the atomization. The conversion reaction is, for example, an oxidation reaction. It is also possible, under certain circumstances, to produce spherical particles from an alloy, at least one alloying constituent being present in the form of a compound and this alloying constituent being substantially homogeneously distributed through the particle cross section. With the process according to the invention, it is possible in particular to provide powders for the production of metallic contacts with good electrical conductivity combined, at the same time, with a high resistance to electrical erosion and abrasive wear and a reduced tendency to weld together.
According to one configuration, the temperature of the atomization gas is set in such a way that, when it comes into contact with the jet, the temperature is at least 0.3 times the melting point in ° C. of the metal.
The temperature of the atomization gas is advantageously greater than or equal to the melting point in ° C. of the metal. At the selected temperature, the atomization gas effects complete reaction with the metal or an alloying constituent thereof.
According to a further configuration, the cooling rate is less than 10
2
K/s. The above feature also contributes to complete reaction of the atomization gas with the metal or the alloying constituent.
The compound is advantageously a metal oxide, metal nitride or metal carbide. Accordingly, it is expedient for the atomization gas used to be air, nitrogen, ammonia, oxygen, carbon-containing gas or a mixture thereof. It is also possible for noble gas to be added to the atomization gas or the mixture.
According to a further configuration, the metal may be an alloy formed at least from a first and a second alloying constituent. The first alloying constituent may be held in the molten state in a first vessel, and the second alloying constituent may be held in the molten state in a second vessel, it being possible for the first and second alloying constituents to be mixed in a mixer pipe which leads to the melt outlet opening. Particularly for the production of powders for producing electrically conductive materials for electrical switching contacts, it is expedient for the melt of the first alloying constituent to be saturated with oxygen. In this case, the second alloying constituent can be melted under inert conditions. The first alloying constituent may be silver or copper, the second alloying constituent expediently being tin, indium, bismuth or a mixture thereof.
When the process according to the invention is applied to an alloy having the alloying constituents described above, and when oxygen is used as atomization gas, predominantly fine, spherical particles are formed, which contain a homogeneous distribution of, for example, tin oxide precipitations in a matrix formed from silver. This makes it possible to produce an electrically conductive component, for example by sintering or hot pressing of a powder of this type. The particularly homogeneous distribution of tin oxide in the matrix reduces the likelihood of the material becoming welded to contacts produced therefrom as a result of a spark-over; it has an excellent stability with respect to the electrical erosion and abrasive wear.
It has proven expedient to set the throughput of atomization gas to 1 to 10 m
3
/min [s.t.p.], the term [s.t.p.] being understood as meaning standard temperature and pressure. The admission pressure of the atomization gas can be set to 1 to 50 bar.
A particularly low cooling rate can be achieved by guiding the metal in a substantially horizontal trajectory at least from time to time during the reaction. For this purpose, and to set the cooling rate, atomization gas can be guided onto the particle stream downstream of the melt outlet opening.
According to a further configuration, the powder is cooled after the compound has been formed. For this purpose, the powder may be acted on by gas, liquefied gas or water. It may be applied either in countercurrent or in the direction of flow. The gas used may be air, oxygen, nitrogen, noble gas or a mixture thereof.
The process according to the invention is explained in more detail below with reference to exemplary embodiments.
A. Production of Powders with a Mean Grain Size of ≦10 &mgr;m.
To produce a compound in the form of a fine, sinter-active powder, the temperature of the atomi
Applikations - und Technikzentrum fur Energieverfahrens- Umvelt-
Rankin, Hill Porter & Clark LLP
Wyszomierski George
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