Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
1998-07-30
2001-02-06
Dunn, Tom (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C423S606000, C423S608000
Reexamination Certificate
active
06183716
ABSTRACT:
FIELD OF THE INVENTION
The present invention concerns a method for making negative thermal expansion (NTE) materials, particularly molybdate and tungstate NTE materials, and compositions and devices comprising such materials.
BACKGROUND OF THE INVENTION
Negative thermal expansion materials are unique in that they expand upon cooling and contract upon heating. By combining negative thermal expansion materials with other common materials, such as ceramics, compositions can be formed which resist expansion on heating and contraction upon cooling. Negative thermal expansion materials also are useful for adjusting the thermal expansion of a composition to match that of another material which it contacts. Particularly useful are NTE materials which expand or contract isotropically, i.e, to substantially the same extent in all dimensions.
A number of U.S. patents concern negative thermal expansion materials and methods for making such materials, including Arthur Sleight's U.S. Pat. Nos. 5,322,559 and 5,514,360, which are incorporated herein by reference. Sleight's '360 patent describes a method for making zirconium and hafnium tungstates. The method involves heating mixtures comprising appropriate amounts of starting reagents. But, the temperatures required to produce single-phase compounds have heretofore been higher than 1,100° C. Sleight's '360 patent states that “the production of single-phase compounds generally has required heating temperatures to be at least as high as about 1165° C. [and] typically should be from about 1165° C. to about 1250° C. . . .” Sleight's '360 patent, column 3, lines 17-19. Example 5 of Sleight's '360 supports this conclusion. The material produced according to Example 5 of Sleight's '360 patent includes both crystalline ZrW
2
O
8
and WO
3
.
Several literature reports also discuss the production of zirconium and hafnium compounds. See, for example, (1) Clearfield and Blessing's “The preparation of a Crystalline Basic Zirconium Tungstate,”
J. Inorg. Nucl. Chem
., 36:1174-1176 (1974); and (2) S. Palitsyna et al.'s “Synthesis and Some Properties of Basic Crystalline Hafnium,”
Bulletin of the Academy of Sciences, U.S.S.R., Division of Chemical Sciences
, 26:611-613 (1977). Clearfield et al. teach a method for synthesizing ZrW
2
O
7
(OH)
2
(H
2
O)
2
by combining zirconium oxychloride octahydrate (ZrOCl
2
.8H
2
O) with sodium tungstate (Na
2
WO
4
.2H
2
O), followed by heating the precipitate and mother liquor formed by the combination. The solution is acidified and refluxed for “several days.” Clearfield et al., supra, p. 1175. The solid is then collected and washed with hydrochloric acid to remove sodium ion. Id.
Palitsyna et al. describe the synthesis of a hafnium tungstate by combining hafnium oxychloride octahydrate (HfOCl
2
.8H
2
O) with sodium tungstate dihydrate (Na
2
WO
4
.2H
2
O). The hydrate formed by this reaction and subsequent workup is then heated to temperatures greater than 500° C. (presumably celsius; the publication does not say) to form the crystalline hafnium tungstate.
Several problems have been identified with methods developed prior to the present invention for making zirconium and hafnium tungstates. First, such methods produce substantially single-phase compounds only at elevated temperatures, and generally only after long heating periods, which are significant drawbacks to developing an efficient, commercially viable process. Second, previous methods limited the number of different NTE compounds that could be made because they allowed only for the substitution of hafnium for zirconium; no other elemental substitutions apparently have been possible using prior synthetic methodologies. And, prior methods ostensibly designed to make zirconium and hafnium tungstates do not reproducibly produce such compounds. In fact, commercial preparations sold as zirconium tungstate have been found to include little or no zirconium tungstate.
Based on the above, it is apparent that there is a need for a new method for reproducibly forming known negative thermal expansion materials as substantially single-phase compounds at substantially reduced temperatures relative to known methods. There also is a need for a method which allows for the synthesis of novel negative thermal expansion materials.
SUMMARY OF THE INVENTION
The present invention concerns a new method for making molybdate and tungstate NTE materials that solves many of the problems identified for prior synthetic methodologies. First, substantially single-phase crystalline compounds can be made according to the present method at temperatures much lower than that of known methods, such as at temperatures within the range of from about 500° C. to about 700° C. As used herein, the phrase “substantially single phase” means that compounds produced by the method have purities of 95% by weight or greater. Second, the present method can be used to form new classes of negative thermal expansion materials, particularly molybdate and tungstate compounds, that apparently could not be made by previous methods. Third, the time required to produce desired compounds is substantially decreased by the present method.
One novel feature of the present invention is the recognition that sodium-ion-containing reagents contribute significantly to the problems identified for prior synthetic methods. As a result, reagents containing sodium ions are not used to practice the present method and this has been found to substantially increase the efficiency and reproducibility of the method.
One embodiment of the present method comprises making compounds having the formula
A
4
+
M
2
6
+
O
8
,
where A
4+
is Zr
4+
, Hf
4+
, or mixtures thereof, and M
6+
is Mo
6+
, W
6+
, or mixtures thereof. The method first comprises heating an acidic, liquid mixture comprising stoichiometric amounts of (1) a soluble source of Zr
4+
, Hf
4+
, or mixtures thereof, and (2) a sodium-ion-free tungstate salt, a sodium-ion-free molybdate salt, or mixtures thereof, to form a solid fraction. This first heating step generally comprises refluxing the liquid mixture. But, the liquid reaction mixture also can be heated at pressures significantly greater than ambient in closed systems, such as Parr bombs. Working embodiments of the present method have heated the liquid reaction mixture in closed systems to pressures within the range of from about 200 psi to about 300 psi, with 250 psi being a currently preferred pressure. Heating the reaction mixture at pressures greater than ambient substantially reduces the reaction time. For example, the liquid reaction mixture must be heated for about 48 hours at ambient pressures, or pressures close to ambient, for the reaction to proceed substantially to completion. Heating the same reaction mixture in a closed system reduces the reaction time to about 4-5 hours.
Generally, but not necessarily, the liquid solution comprises an aqueous solution. Furthermore, working embodiments of the method predominantly have used ammonium tungstate as the sodium-ion-free tungstate salt, ammonium molybdate as the sodium-ion-free molybdate salt, and oxyhalides or oxynitrates to provide the soluble source of Zr
4+
and/or Hf
4+
.
The solid fraction produced in the first heating step is separated from the liquid mixture and heated sufficiently to form compounds having the formula
A
4
+
M
2
6
+
O
8
.
The temperature range for heating the solid fraction to form desired NTE compounds is within the range of from about 500° C. to less than about 700° C., and more preferably is within the temperature range of from about 500° C. to about 600° C.
A currently preferred method for making compounds according to the formula
A
4
+
M
2
6
+
O
8
comprises first forming an aqueous mixture that includes appropriate amounts of (1) a zirconium oxyhalide, a zirconium oxynitrate, a hafnium oxyhalide, a hafnium oxynitrate, or mixtures thereof, and (2) ammonium tungstate, ammonium molybdate, or
Closmann Claire E.
Haygarth John C.
Sleight Arthur W.
Upadhyayula Kameswari
Dunn Tom
Klarquist Sparkman Campbell & Leigh & Whinston, LLP
Nguyen Cam N.
State of Oregon acting by and through the State Board of Higher
LandOfFree
Solution method for making molybdate and tungstate negative... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Solution method for making molybdate and tungstate negative..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Solution method for making molybdate and tungstate negative... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2587005