Process for synthesizing niobium dioxide and mixed metal oxides containing niobium

Niobium dioxide is produced by heating particulate niobium nitride in an oxygen atmosphere at a temperature above the melting point of niobium dioxide. The product obtained predominantly contains niobium dioxide.

BACKGROUND OF THE INVENTION 
Oxides of niobium and processes for making them and their derivatives have 
received relatively little attention for use as refractory materials. This 
is principally because Nb.sub.2 O.sub.5, the most commonly encountered 
oxide, is not highly refractory. It melts at 1450 degrees Celsius and 
suffers considerable loss of oxygen near this temperature. Other oxides of 
niobium, e.g., NbO and NbO.sub.2, have received relatively little 
attention because they are difficult to prepare. As a result, the use of 
NbO.sub.2 as a novel starting material for any binary and higher-order 
mixtures with other oxides has not been extensive. Because of this, 
difficulty there has been limited use of such materials. There is need, 
therefore, for a better procedure for preparing NbO.sub.2 in order to 
better exploit the use of oxides of niobium and its derivatives. 
Heretofore, the obtaining of NbO.sub.2 has been achieved in a variety of 
ways such as the reduction of Nb.sub.2 O.sub.5 by hydrogen, which reaction 
is also reversible. Also, by heating Nb.sub.2 O.sub.5 in a current of 
argon at 1150 degrees Celsius, the dissociation of the pentoxide is 
appreciable. For use in X-ray studies, single crystals of NbO.sub.2 have 
been prepared by melting pressed tablets of Nb.sub.2 O.sub.5 and metal 
powder, correctly proportioned, in an electric arc furnace. 
Niobium dioxide prepared as described is known to have a tetragonal crystal 
structure as its most common polymorph, a variant of the rutile structure 
of TiO.sub.2 Zircon, ZrSiO.sub.4, is also a tetragonal oxide material, 
which structure is useful in several applications. Therefore, it may be 
projected useful applications would be found for NbO.sub.2 provided it 
were more directly and economically available. 
Due to its complicated electronic structure, it has been used or proposed 
in such applications as secondary lithium batteries, fuel cell catalyst, 
photoelectrochemical cell anode, and as a thermochromic material. 
Japanese Patent 89-128355 describes the use of NbO.sub.2 in batteries which 
exhibit a high capacity and a long cycle life. The use of NbO.sub.2 is 
also described in PCT number 87-07422 for an interference film component 
in a magneto-optical recording disk. Some of these uses include NbO.sub.2 
alone or in combination with other metals or metal oxides. Nevertheless, 
mixtures of NbO.sub.2 with other oxides have not been studied extensively 
since it is first necessary to produce NbO.sub.2 as a powder, then to 
combine it with another material and heat it to elevated temperatures for 
prolonged times to generate a new material. There is, therefore, a need 
for a process which generates NbO.sub.2 at a high thermal plateau, so that 
it can react readily. 
PRIOR ART 
In the monograph by F. Fairbrother entitled "The Chemistry of Niobium and 
Tantalum", Elsevier Publishing Co., Amsterdam, 1967, p. 23-25, are given 
several ways to prepare NbO.sub.2 : 
i) Nb.sub.2 O.sub.5 +H.sub.2 and high temperature, 800.degree. 
C.-1350.degree. C.; 
ii) Nb.sub.2 O.sub.5 +in argon stream at 1150.degree. C.; 
iii) Arc melting of a mixture of Nb.sub.2 O.sub.5 +Nb. 
In "Gmelins Handbuch der Anorganischen Chemie", Teil Bl, "Niob", #49, p. 
33-34, 1970, are listed further methods: 
i) Nb.sub.2 O.sub.5 +CaH.sub.2 ; 
ii) hydrogen reduction of alkali metal niobates; 
iii) careful oxidation of Nb. 
All these methods except the arc melting, have in common that they generate 
a powder. The arc melting has been conducted on a very small scale for the 
purpose of generating a few small crystals suitable for single crystal 
X-ray structure determination. Such specialized methods for producing 
NbO.sub.2 have limited the detailed study of and commercial interest in 
the material. 
It is, therefore, one object of the present invention to provide an 
efficient, safe, and economical process for preparing niobium dioxide. 
It is another object of this invention to provide a process for preparing 
NbO.sub.2 in other than a powdered form i.e. a densified and consolidated 
form obviating the need of sintering. 
It is yet another object of this invention to provide a process for 
generating NbO.sub.2 at a high thermal plateau so that there can be a 
rapid reaction of NbO.sub.2 with other materials mediated by the presence 
of a liquid phase. 
These and other objects are achieved by the present invention directed to 
novel processes for the synthesis of NbO.sub.2 as more fully described 
herein.

The following are specific examples of the preparation of niobium dioxide 
and reactions with niobium oxide according to the present invention: 
EXAMPLE 1 
A small quantity of NbN powder, -200 mesh, obtained from the methods 
described in copending U.S. Ser. No. 721,887 and U.S. Ser. No. 721,884 
both filed Jun. 27, 1991 was placed inside a section of open-ended quartz 
tubing. A stream of oxygen was directed over the NbN powder and a gas 
torch applied to the underside of the tube for a few seconds. The powder 
ignited with first a pink, then a brilliant white light and regions were 
seen to coalesce into molten spherical globules. After cooling, these 
globules were recovered as consolidated shiny black solids. Surfaces which 
had been in contact with the quartz were shiny and conformed to the tubing 
shape. Since the product of ordinary passive air oxidation is a loose, 
ivory-colored powder of Nb.sub.2 O.sub.5, it was immediately evident that 
an unexpected result had been achieved. An X-ray diffraction pattern 
revealed the solids to be a mixture of NbO.sub.2 (tetragonal modification, 
JCPDS card 9-235), NbO.sub.2 (monoclinic modification, JCPDS card 19-859) 
and NbO.sub.1.92 (tetragonal, JCPDS 34-672). All of these are reduced 
phases relative to the expected Nb.sub.2 O.sub.5. 
EXAMPLE II 
NbN (1.00 g) and ZrO.sub.2 (1.00 g) were mixed and treated in a manner 
similar to that in Example I. The sample recovered was partially melted. 
The X-ray powder diffraction pattern of NbZr.sub.6 O.sub.17 accounted for 
all but three faint lines. The phase diagram of the system ZrO.sub.2 
-Nb.sub.2 O.sub.5 (Phase diagrams for ceramists, FIG. 4457) shows that a 
temperature of at least 1435 degrees Celsius must have been reached. 
EXAMPLE III 
A sample of the NbO.sub.2 obtained was subjected to heating in air at 1000 
degrees Celsius. After 20.5 hours, its weight gain was only 11% of that 
expected for the conversion of NbO.sub.2 to Nb.sub.2 O.sub.5. This shows 
the high oxidation resistance of the consolidated product form available 
from this invention. 
SUMMARY OF THE INVENTION 
Niobium Nitride (NbN) when heated to a sufficiently high temperature in the 
presence of a stream of oxygen gas will exothermically react with the 
oxygen to produce a massive consolidated black product which predominately 
contains niobium dioxide (NbO.sub.2) 
DETAILED DESCRIPTION OF THE INVENTION 
It has been observed that heating niobium nitride in air will form niobium 
pentoxide (Nb.sub.2 O.sub.5). If this oxidation is conducted in passively 
circulating air, a slow conversion of the niobium nitride occurs producing 
a yellow colored, voluminous product. If the powdered mass is slowly 
tumbled during the reaction, glowing areas are observed briefly. 
Surprisingly, however, when niobium nitride powder is burned in a stream of 
oxygen the combustion of the niobium nitride takes a completely different 
course. Instead of the pink-orange glow of the slow combustion in air, 
brilliant bursts of white light are observed. In addition, small regions 
of the powdered mass sinter together to form globules of white-hot liquid. 
The molten material, when cooled, forms massive, shiny consolidated black 
chunks. These chunks were found to contain one or more of the forms of 
niobium dioxide. 
Further, it has been discovered that niobium nitride can be mixed with 
other compounds prior to oxidation as described herein to form products 
previously only available after extensive ceramic synthesis, i.e. solid 
state reactions. 
The oxidation described herein forms the dioxide at a high temperature or 
high thermal plateau and in the molten state. Niobium dioxide has a 
melting point of 1915.degree. C. and at or above this temperature its 
intimate contact with other reactants can produce a wide variety of mixed 
oxides. For example, mixing powdered niobium nitride and zirconium oxide 
(ZrO.sub.2) will, under oxygen burning at high temperatures, produce the 
liquid niobium dioxide which will react with the ZrO.sub.2 to form mixed 
oxide products which are only otherwise available from firing mixtures of 
the unreactive oxide to produce products by diffusion. 
The niobium nitride starting material can be obtained from any conventional 
process for the production of the nitride. Preferably, the nitride is 
produced according to the process described in co-pending U.S. patent 
application Ser. No. 908,787, filed Jul. 6, 1992, of James A. Sommers and 
Lloyd J. Fenwick, the same inventor as herein, the disclosure of which is 
incorporated herein in its entirety by reference. 
The process of the present invention can be practiced in a manner of 
different than described herein, it only being necessary for the niobium 
nitride to be contacted with an atmosphere containing a sufficient 
concentration of oxygen at a high enough temperature to produce an 
exothermic reaction and the melt formation of niobium dioxide.