Carbonyl iron silicide powder

Details Carbonyl iron silicide powder Carbonyl iron silicide powder

1999-07-21

2001-10-30

Sample, David R. (Department: 1755)

Chemistry of inorganic compounds

Silicon or compound thereof

Oxygen containing

C423S417000, C423S594120, C148S513000, C075S233000, C075S252000, C075S361000

Reexamination Certificate

active

06309620

ABSTRACT:

The present invention relates to carbonyl iron suicide powders and a process for preparing them.
Finely divided iron powders in which a low proportion of secondary constituents is important are required for many applications in powder metallurgy, electro-technology, in chemistry and pharmacy. For certain applications, a defined silicon content of the powders is desirable in addition to the high purity. Thus, it is known that the addition of silicon influences the magnetic properties of iron powders. Furthermore, the catalytic action of iron-silicon alloys, for example in the hydrogenation of CO in the Fischer-Tropsch process, is known from the literature. In addition, iron-silicon alloys are more resistant to environmental influences than is pure iron.
High-purity, silicon-containing iron powders can be obtained by gas-phase reaction of volatile iron and silicon compounds. Such a process is described in DE-A 197 16 882 which has earlier priority but is not a prior publication. A further possible method is preparation from the elements.
The preparation of high-purity, silicon-containing iron powders from the elements is today frequently carried out by mechanical alloying, e.g. by milling the raw materials in ball mills. Mechanical alloying processes are described in V. E. Martin et al., IEEE Trans. Magn. 26 (1990), pages 2223-25 and in M. Abdellaoui et al., J. Phys. IV (1992), C3, pages 73-78. Disadvantages of these processes are the very long processing times which extend from tens to hundreds of hours.
Thermal processes for preparing iron-silicon alloys are described, for example, in DD-A 228 673 and EP-A 0 335 213. In these processes, element and alloy powders are pressed and sintered under high pressure. These processes are not directed at the preparation of powders; rather, finished shaped bodies such as magnetic cores and thermocouples are obtained.
T. H. Song et al., J. Mater. Sci. Lett. 14 (1995), pages 1715-17, describe the preparation of iron-silicon alloys in a “self-propagating high-temperature synthesis”. Here, a mixture of iron and silicon powders together with 20% by weight of KNO
3
is pressed at 100 MPa to form tablets and is subsequently ignited in an electric arc discharge. The process is complicated in chemical engineering terms and is not without hazards because of the use of KNO
3
.
It is an object of the present invention to prepare iron-silicon alloy powders which are suitable for many applications from the elements by a simple and inexpensive process.
We have found that this object is achieved by a process for preparing carbonyl iron suicide by heat treatment of an iron/silicon mixture comprising
a) from 20 to 99.9% by weight of finely divided carbonyl iron and
b) from 0.1 to 80% by weight of silicon powder,
where the sum of the components a) and b) is 100% by weight.
For the purposes of the present invention carbonyl iron silicide is iron suicide which is obtainable by alloying carbonyl iron powder with silicon.
In the process of the present invention, alloying is carried out by heat treatment of a mixture of finely divided carbonyl iron powder and silicon powder.
For the purposes of the present invention, finely divided carbonyl iron encompasses carbonyl iron powder and carbonyl iron whiskers.
Carbonyl iron powder and carbonyl iron whiskers can be obtained by known methods by thermal decomposition of iron pentacarbonyl in the gas phase, e.g. as described in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A 14, page 599, or in DE-A 34 28 121 or in DE-A 39 40 347, and consist of particularly pure metallic iron. The high purity of the powder or whiskers is a result of the high purity of the iron pentacarbonyl. Depending on the decomposition conditions (pressure, temperature), powder or whiskers are formed.
Carbonyl iron powder is a gray, finely divided powder which is made up of metallic iron having a low content of secondary constituents and consists essentially of spherical particles having a mean particle diameter of up to 10 &mgr;m.
In the process of the present invention, it is possible to use mechanically hard, unreduced carbonyl iron powders or mechanically soft, reduced carbonyl iron powders.
The unreduced carbonyl iron powders preferably used in the process of the present invention have an iron content of >97% by weight, a carbon content of <1.0% by weight, a nitrogen content of <1.0% by weight and an oxygen content of <0.5% by weight. The mean particle diameter of the powder particles is preferably from 1 to 10 &mgr;m, particularly preferably from 1.5 to 5.0 &mgr;m, and their specific surface area (BET) is preferably from 0.2 to 2.5 m
2
/g.
The reduced carbonyl iron powders preferably used in the process of the present invention have an iron content of >99.5% by weight, a carbon content of <0.05% by weight, a nitrogen content of <0.1% by weight and an oxygen content of <0.6% by weight. The mean particle diameter of the powder particles is preferably 1-8 &mgr;m, particularly preferably 2-8 &mgr;m. The specific surface area of the powder particles is preferably 0.2-2.5m
2
/g.
Carbonyl iron whiskers are very fine, polycrystalline iron threads. The carbonyl iron whiskers preferably used in the process of the present invention consist of thread-like assemblies of spheres having sphere diameters of 0.1-1 &mgr;m, with the threads having different lengths and being able to form balls, and have an iron content of >83.0% by weight, a carbon content of <8.0% by weight, a nitrogen content of <4.0% by weight and an oxygen content of <7.0% by weight.
The carbonyl iron powders and whiskers preferably used in the process of the present invention have a very low content of extraneous metals which is usually below the detection limit of atomic absorption analysis and is due to the preparation from the very pure starting compound iron pentacarbonyl. The carbonyl iron powders contain, inter alia, the following proportions of further, extraneous elements: nickel<100 ppm, chromium<150 ppm, molybdenum<20 ppm, arsenic<2 ppm, lead<10 ppm, cadmium<1 ppm, copper<5 ppm, manganese<10 ppm, mercury<1 ppm, sulfur<10 ppm, silicon<10 ppm and zinc<10 ppm.
Preference is given to using carbonyl iron powders in the process of the present invention.
As finely divided silicon, commercial silicon powder can be used in the process of the present invention. Preference is given to using a silicon powder of high purity. The silicon content of the silicon powders used is generally >95% by weight, preferably >98% by weight, particularly preferably >99% by weight. The proportion of extraneous metals (apart from iron) is generally <2% by weight, preferably <1% by weight, particularly preferably <0.5% by weight. The mean particle size of the silicon powders used is generally <1000 &mgr;m, preferably <500 &mgr;m.
The carbonyl iron silicide is prepared by intimate mixing of the finely divided carbonyl iron with the silicon powder and subsequent heat treatment. The proportion of silicon in the mixture is generally from 0.1 to 80% by weight, preferably from 0.5 to 30% by weight, particularly preferably from 1 to 20% by weight. The heat treatment is generally carried out by heating the iron/silicon mixture to a reaction temperature of generally ≧700° C., preferably ≧900° C., particularly preferably from 900° C. to 1100° C., in particular 1000° C., over a period of from 20 to 80 minutes, preferably from 30 to 60 minutes, and holding it at this temperature for a period of from 80 to 160 minutes, preferably from 100 to 140 minutes.
In a further embodiment, the iron/silicon mixture comprises carbonyl iron and silicon and additionally
c) elemental copper and/or at least one copper compound, elemental aluminum and/or at least one aluminum compound or elemental cobalt and/or at least one cobalt compound, in proportions of from 0.01 to 5% by weight, preferably from 0.1 to 2% by weight, based on the sum of the components a), b) and c). Preferred copper compounds are copper(II) salts

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