Chemistry of inorganic compounds – Nitrogen or compound thereof – Binary compound
Reexamination Certificate
1998-04-27
2001-03-06
Langel, Wayne (Department: 1754)
Chemistry of inorganic compounds
Nitrogen or compound thereof
Binary compound
C423S439000, C423S440000
Reexamination Certificate
active
06197272
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing transition metal carbides and/or transition metal carbonitrides and the use thereof together with novel transition metal xerogels.
2. Description of the Prior Art
Transition metal carbides are because of their great hardness of major importance for the hard metal industry. They are used in particular for the manufacture of cutting tools. Hardness and wear resistance increase, moreover, with decreasing grain size of the carbides. The manufacture of particularly fine-particle carbides, in particular those with grain sizes of less than 1 &mgr;m, is therefore of great interest.
The manufacture of transition metal carbides from their oxides by solid-phase carburization with graphite at temperatures of between 1400° and 2000° C. is known. Sub-&mgr; carbides are obtainable in this manner only by laborious grindings of the products or by the use of expensive, extremely finely-divided educts.
WC powders with mean particle sizes of less than about 0.5 &mgr;m cannot be prepared in this manner, and some novel methods of preparation have therefore been developed in recent times.
In DE-A 4 214 725 a gas-phase process is described with which extremely finely-divided particles for non-oxide ceramics can be produced. This process is highly complicated in equipment terms, however, as is also the process described in WO 91/07244 for the manufacture of WC/Co composites by the spray drying of co-precipitated W/Co salts and subsequent reduction to the metal powders with downstream carburization to the carbidic WC-Co mixed phase.
In U.S. Pat. No. 5,372,797 the carburization of tungsten trioxide in a reactive gas atmosphere consisting of hydrogen and methane is described.
In EP-A 0 239 301 the manufacture of carbide precursors of the 4th to 6th sub-groups by transesterification of the metal alkoxides with polyols and subsequent precursor pyrolysis to the desired carbides is described.
In JP-A 56-155 013 a similar method is described, with the difference that the metals can be used not only as alkoxides, but also in the form of their halides and oxohalides.
The known solutions have the following disadvantages: The conventional method of the solid-phase carburization of metals or metal oxides with graphite powder requires as a rule temperatures of more than 1400° C. and yields products with mean particle sizes of more than 1 &mgr;m, which have to be worked up with a correspondingly high amount of grinding and sorting. In addition, extensive grindings in general impair the powder properties.
The carburization in a reactive gas atmosphere yields as a rule sub-stoichiometric products.
Precursors from alkoxides are too expensive for hard metal powders, since the alkoxides required are not available commercially in large amounts and their manufacture is complicated. Precursors containing halides are not very suitable on environmental grounds.
The object of the invention was therefore to provide a simple process which is suited to manufacturing sub-s carbides and carbonitrides of the 5th and 6th sub-groups with mean particle sizes of less than 0.5 &mgr;m on a large scale and at a modest price from commercially available, low-cost educts and thereby to meet the increasing requirements as to their finely divided character.
SUMMARY OF THE INVENTION
It was found that transition metal carbides and/or carbonitrides of the 5th and 6th subgroups with a mean particle diameter of <0.5 &mgr;m can be manufactured if at least one transition metal oxide is reacted with at least one organic compound comprising at least two OH groups, surplus portions of the organic compound are then drawn off and the product so obtained is pyrolyzed in an inert gas atmosphere and/or vacuum, or, in order to obtain carbonitrides, under a nitrogen-containing atmosphere.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention therefore provides a process for manufacturing transition metal carbides and/or carbonitrides of the 5th and 6th sub-groups with a mean particle diameter of <0.5 &mgr;m, according to which at least one transition metal oxide is reacted in at least an organic compound containing at least two OH groups, preferably at temperatures between room temperature and the boiling point of the mixture, i.e. preferably between 20 and 300° C., surplus portions of the organic compound are then drawn off and the product so obtained is pyrolyzed preferably at temperatures between 300° C. and the sintering temperature, i.e. between 300 and 4000° C., preferably between 600 and 2000° C., particularly preferably between 900 and 1300° C., of the respective carbide in an inert gas atmosphere and/or vacuum, or, in order to obtain carbonitrides, under a nitrogen atmosphere.
The pressure in the inert gas or nitrogen atmosphere comes to 100 mbar to 5 bar, particularly preferably 500 mbar to 1.2 bar.
By vacuum is meant here preferably a pressure range from 100 to 0.01 mbar, preferably 30 to 0.1 mbar.
In a preferred embodiment of the invention the pressure during the pyrolysis comes to between 0.01 mbar and 5 bar, preferably 0.1 mbar and 1.2 bar.
The transition metal carbide and/or carbonitride of the 5th and 6th sub-groups manufactured by the process according to the invention is preferably vanadium, molybdenum and/or tungsten carbide and/or vanadium carbonitride.
The term carbides covers here both the stoichiometric compounds such as e.g. WC, W
2
C, Mo
2
C and the compounds in their respective homogeneity range. By homogeneity range of the respective compound is to be understood the range in the associated phase diagram in which the compound is present as a single phase.
The mean particle diameter <0.5 &mgr;m was determined here by the analysis of TEM exposures. The number average is involved here.
The transition metal oxide used in the process according to the invention is preferably an inorganic polyacid or an inorganic oxide.
The metal compounds of the 5th and 6th sub-groups designated as polyacids can be e.g. iso- or heteropolyacids (see e.g. Römpp, 9th edition, Georg Thieme Verlag, pp. 1789 and 2073). Preferably isopolyacids are used, particularly preferably their ammonium salts, more particularly preferably ammonium metavanadate, ammonium heptamolybdate and ammonium paratungstenate.
In a preferred embodiment of the invention there are used as transition metal oxides mixtures of polyacids of various metals, such as e.g. a mixture of ammonium metavanadate and ammonium paratungstenate.
The organic compounds used in the process according to the invention that comprise at least two OH groups are preferably compounds which comprise 2≦n≦10 OH groups.
There are used as compounds containing OH groups preferably di- or trivalent alcohols such as e.g. ethylene glycol, propanediol, butanediol, glycerol or alternatively mixtures of the latter and compounds with more than two OH groups, such as e.g. pentaerythritol sugar alcohols such as sorbitol or mannitol, saccharose etc. Particularly preferred are mixtures of a divalent alcohol and a sugar or sugar alcohol, more particularly preferred are ones of ethylene glycol and sorbitol.
In a further embodiment of the process according to the invention the reaction takes place preferably in a solvent. Also acting as solvent here, in addition to the organic compounds comprising at least 2 OH groups, can be e.g. water or sodium hydroxide solution. Preferably, however, the organic compound comprising the OH groups is used as solvent, which organic compound also acts as a reactant.
In the event of only one organic compound comprising at least 2 OH groups being used in the process according to the invention, the latter is preferably added in an amount such that the transition metal oxide dissolves therein.
In a second embodiment the transition metal oxide is initially dissolved in an organic compound with two OH groups, preferably a diol, and reacted therewith. The solvent is removed and the residue is purified by washing or recrystallisation. The powder obtained is then again dissolved in a s
Passing Gerd
Perchenek Nils
Womelsdorf Hermann-Jens
Bayer AG
Connolly Bove & Lodge & Hutz LLP
Langel Wayne
LandOfFree
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