| abstract |
A method for making negative thermal expansion materials having the formulawhere A4+ is Zr4+, Hf4+, or mixtures thereof, and M6+ is Mo6+, W6+, or mixtures thereof, is described. The method comprises first forming an acidic, liquid mixture comprising stoichiometric amounts of (1) a soluble source of Zr4+, Hf4+, or mixtures thereof, and (2) a sodium-ion-free tungstate salt, a sodium-ion-free molybdate salt, or mixtures thereof. This liquid mixture is then heated, either at reflux temperature or in a closed system at pressures greater than ambient, to produce a solid fraction. The solid fraction is separated from the liquid mixture and heated a second time at a temperature and for a period of time sufficient to form compounds having the formulaThe solid fraction generally is heated within the temperature range of from about 500° C. to less than about 700° C. An alternative embodiment of the method involves adding a fuel material to the materials listed above before the final heating step. A currently preferred fuel material is urea. The fuel material apparently undergoes an exothermic reaction that provides additional energy to help produce the desired compound. The use of the fuel has substantially reduced the reaction time, such as to less than an hour, and in certain embodiments the first heating cycle has been completed in about fifteen minutes. Novel negative thermal expansion materials made by the method also are described. Such novel negative thermal expansion materials have the formulawhere A4+ and A'4+ are Hf4+ or Zr4+, M6+ and M'6+ are W6+ or Mo6+, X is from 0 to 1, and Y is from about 0.05 to <=2. |