Specialized metallurgical processes – compositions for use therei – Compositions – Consolidated metal powder compositions
Reexamination Certificate
2000-06-12
2002-10-15
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Consolidated metal powder compositions
C075S245000, C075S255000, C075S348000, C075S351000, C075S369000
Reexamination Certificate
active
06464750
ABSTRACT:
DESCRIPTION
The invention relates to a method of preparing metal powders, particularly iron powder.
The term “metal powder” covers both pure metal powders and powders of metal alloys.
More precisely, the invention is concerned with a method of preparing metal powders, in particular, iron powder by thermal reduction of transition metal or rare earth metal oxalates or oxides that result from the decomposition of transition metal or rare earth metal oxalates, for example, iron oxalates or iron oxides resulting from iron oxalates, these oxalates or oxides being constituted by particles with needle-like morphology having a size, more precisely a length that is specific.
The metal powder, for example the prepared iron powder has a spongy and filament-like structure that equals or indeed exceeds the specifications current for such a metal powder, for example such an iron powder, which makes it particularly suitable for numerous uses, in particular, for use in heating compositions for thermo-piles, when they make use of iron powder.
In addition, the invention relates to metal compacts, unstoved or annealed, obtained by application of compacting pressure to the metal powders according to the invention, possibly accompanied by partial sintering.
The invention relates to numerous applications or uses that require the use through pressing of one or more metal powders, pure or alloyed, which may or may not be associated later with other components enclosed in the porosity of the compact.
Among these applications or uses, one may mention, for example, the manufacture of pyrotechnic mixtures comprising iron powder according to the invention and an oxidizing agent, porous electrodes used notably in electrical energy accumulators, such as electro-chemical Ni—Cd cells or Ni-metal hydride cells etc., soft or hard magnetic materials, such as armatures for electric motors, and permanent magnets, catalyst supports, in which a catalytic powder is enclosed within a porous metal structure, filters that are able to make use of the ferro-magnetic properties of certain metals for magnetic separation and, more generally, mechanical components of complex shape which can be produced by a simple pressing operation, avoiding having to carry out complicated machining operations.
The technological field of the invention can therefore be defined as that of metal powders and their preparation, as well as unstoved or annealed metal compacts prepared by pressing these powders, this pressing operation being possibly accompanied by partial sintering.
Such compacts find uses in numerous sectors of industry, in which they must have high mechanical strength.
The compacts prepared from known metal powders have mechanical strengths that are markedly inadequate.
In a preferred manner, which is non-limitative, the invention is more particularly set in the field of thermo-piles.
In particular, the invention relates to a heating pyrotechnic composition comprising the metal powder, in particular iron powder, according to the invention, as well as the thermo-pile comprising this heating pyrotechnic composition.
Thermo-piles are non-rechargeable cells, inert before initiation, which can be stored without any maintenance, sometimes for more than 20 years while remaining usable at any time, with a response time that can be less than three tenths of a second.
Thermo-piles find increasing use in all fields where there is a need for immediately available energy, in a reliable manner, even after a very long storage time.
Thermo-piles are thus mainly used in the aeronautics and space industries and also in all emergency services that require such a source of energy for example, in the nuclear industry, the oil industry and commercial and warehousing buildings etc.
A compacted composition of iron/potassium perchlorate is the traditional heating pyrotechnic composition for thermo-piles.
In effect, this composition has shown a clear superiority in comparison with so-called “paper” zirconium/barium chromate used in the past as a heating pyrotechnic composition for thermo-piles.
The performance of these thermo-piles is directly linked to the properties of this composition and in particular to the microstructure of the iron powder.
The iron powder most commonly used up to now in iron/potassium perchlorate heating compositions for thermo-piles is the powder sold under the name of NX-1000 powder by the Company PFIZER METAL and Composite Products of Walkingford Connecticut, USA or by the Company AMETEKO Specialty Metal Products Division.
This iron powder meets all the following specifications which an iron powder for thermo-piles must fulfill: a sponge-like structure, a total iron content greater than 95%, an elemental iron content greater than 89%, a specific surface Sw (m
2
/g) greater than 0.5 m
2
/g, a loss with hydrogen less than 3%, a Scott density of from 0.8 to 1.15, a Fischer sub-granulometry of from 1.5 to 3.5 &mgr;m, a granulometry in which more than 70% of the particles pass a sieve with a 44 &mgr;m opening and less than 1% of the particles are retained on a sieve with a 150 &mgr;m opening, a “Green force” or compact resistance (determined in accordance with standard ASTM B312-56T) greater than 35 MPa for a forming pressure of 276 MPa.
Numerous methods have been described in the literature to prepare iron powder, but none of these methods permits the preparation of iron powder that can be used in pyrotechnic heating compositions for thermo-piles and that can meet the specifications given above.
Document U.S. Pat. No. 4,414,021 relates to a method of preparing iron powder suitable for use in iron-potassium perchlorate pyrotechnic heating mixtures for thermo-piles.
This method comprises the preparation of a homogeneous dense precipitate of iron hydroxide by precipitation from an aqueous solution of a ferric salt, formic and sulfuric acids, ammonium hydroxide and urea as precipitation agent.
The dense precipitate obtained is then reduced by hydrogen at 650 to 900° C. for 0.5 to 2 hours to prepare the iron powder which has a spongy structure and which meets the specifications given above.
The precipitate obtained in the first stage of the method is an iron oxyhydroxide which generally has a formula FeOOH. In effect, if ferric sulfate is used in the aqueous solution, the salt formed is Fe
3
(SO
4
)
2
(OH)
5
.2H
2
O which subsequently gives an iron powder having a sulfur content that is excessive for use in a thermo-pile.
The shape and the morphology of the powder produced from the precipitate has not been made clear.
In example 5, this document studies the reduction of various iron based compounds by hydrogen.
Among the twelve compounds tested in fifty tests, only five, following reduction by hydrogen, gave iron powders suitable for use in a thermo-pile, that is to say, powders having properties equivalent to iron powder NX-1000 from PFIZER® (AMETEK®). Apart from the precipitates already mentioned, these compounds are various ferric nitrates and oxides.
Ferrous oxalate, the morphology of which is not described, has also been subjected to reduction by hydrogen, in four different tests. This compound gives an iron powder with a spongy structure, but according to the inventors, this iron powder prepared from ferrous oxalate is not suitable for use in a thermo-pile and is not analogous to the NX-1000 powder from PFIZER® (AMETEK®).
The method described in the document U.S. Pat. No. 4,414,021, though it apparently enables one to prepare an iron powder that meets the specifications given above, still has numerous disadvantages, for example, since no specification is given concerning the morphology of “good” precursors, no controls on them can be carried out before the reduction stage, which can lead to random results in relation to the properties of the iron powders produced according to the method of document U.S. Pat. No. 4,414,021.
Similarly, as has already been mentioned above, in the more general case of the preparation of metal powders, the metal powders prepared at present by the known methods give, when pressed, compacts, the strength
Carles Valérie
Rousset Abel
Tailhades Philippe
ASB Aerospatiale Batteries
Burns Doane Swecker & Mathis L.L.P.
Mai Ngoclan
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