Heterogenite material for making submicron cobalt metal powders

This application is a division of copending application Ser. No. 09/652,485, filed Aug. 31, 2000 which is incorporated herein by reference.

This invention relates to cobalt compounds and methods of making cobalt powders. In particular, this invention relates to HCoO 2 (heterogenite) and methods for making submicron cobalt powders.

Submicron particle size (<1 μm) cobalt powders exhibit distinct advantageous properties over larger, micron size (>1 μm) powders. For example, in tungsten carbide applications, submicron size cobalt particles greatly improve sinterability and mass transport at temperatures significantly below the sintering temperatures required for micron-size cobalt powders. Typically, submicron cobalt metal powder is made via thermal decomposition of cobalt oxalate (CoC 2 O 4 ). However, this method is not very efficient as CoC 2 O 4 contains only 40 wt. % Co compared to 53.6 wt. % Co in Co(OH) 3 . Hence, it would be advantageous to use a compound having a higher cobalt content such as cobalt hydroxide to make submicron cobalt powders.

U.S. Pat. No. 4,218,240 teaches the conversion of Co(OH) 3 to a micron-size cobalt metal powder having a FSSS value of 1.3-1.4 μm. Impure CoCl 2 in solution is converted to Co(NH 3 ) 6 Cl 3 which is then converted to Co(OH) 3 by decomposition with NaOH. Cobalt powder is produced by direct reduction of the wet cobalt hydroxide in hydrogen at 300-500° C. Although the patent describes air drying of the Co(OH) 3 at 100° C. to form a hydrated cobaltic oxide, it also teaches that air drying is not necessary prior to reducing the cobalt containing precipitate. No submicron powders are described.

It is an object of the invention to obviate the disadvantages of the prior art.

It is another object of the invention to provide a high-cobalt-content precursor material for making submicron cobalt metal powders.

In accordance with the objects of this invention, there is provided a heterogenite powder having a surface area of at least about 90 m 2 /g.

FIG. 1 is a TG-DTA curve of a room-temperature-dried Co(OH) 3 precipitate.

FIG. 2 is an SEM photomicrograph of the heterogenite powder of this invention.

FIG. 3 is an SEM photomicrograph of the submicron cobalt powder made from the heterogenite powder of this invention.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings.

It was discovered that a high surface area HCoO 2 (heterogenite) material could be made by drying a wet cobalt hydroxide precipitate at a moderate temperature of 110° C. The surface area as measured by the conventional BET method was at least about 90 m 2 /g and preferably about 90 m 2 /g to about 110 m 2 /g. The high surface area HCoO 2 is then be reduced under a hydrogen-containing atmosphere to form submicron-size cobalt metal powders. Preferably, the HCoO 2 is reduced at a temperature from about 325° C. to about 425° C. for about 0.5 hours to about 2.0 hours. The particle size of the resultant cobalt metal powders ranges from 0.4 to 0.9 μm as determined by Fisher Sub-Sieve Sizer (FSSS). The HCoO 2 contains about 64 wt. % Co.

A wet cobalt hydroxide precipitate may be made by decomposing Co(NH 3 ) 6 Cl 3 with NaOH as described in U.S. Pat. No. 4,218,240. The wet cobalt hydroxide, Co(OH) 3 , which contains about 30-32 weight percent (wt. %) Co, may be represented by the chemical formula Co(OH) 3 .4H 2 O. After drying at room temperature, it converts to a compound which may be represented as HCoO 2 .xH 2 O where 0<x<1. The room-temperature-dried Co(OH) 3 precipitate is a layered compound wherein water molecules reside between layers of HCoO 2 . After heating at 110° C., the room-temperature-dried Co(OH) 3 precipitate looses all of its water molecules and converts to HCoO 2 . This change is illustrated in FIG. 1 which is a TG-DTA curve of a room-temperature-dried Co(OH) 3 precipitate. The water loss from the room-temperature-dried Co(OH) 3 precipitate is marked by an endothermic peak in the differential thermal analysis (DTA) curve at about 110° C. Upon further heating to greater than above 150° C., the HCoO 2 converts to cobalt oxide, Co 3 O 4 .

The following non-limiting examples are presented.