Synthesis and sintering of oxide ceramic powders by the thermal decomposition and combustion of metal nitrate-urea mixture

(NZP) (or NaZr$sb2$P$sb3$O$sb{12}$ structure type) compounds, silicates, and other oxide powders were prepared by the thermal decomposition and combustion of metal nitrate-urea mixtures within 5 minutes in a furnace preheated at 500$spcirc$C. Other starting materials such as metal acetates and alkoxides instead of metal nitrates were also found effective for the synthesis of the oxides. It is evident that most oxide ceramic powders can be prepared using suitable metal salts and appropriate amounts of urea and ammonium nitrate by this technique. The as-prepared powders were porous agglomerates with irregular shape, and composed of numerous spherical particles of 0.1-0.2 $mu$m in size. Average agglomerate size ranged from 3 to 8 $mu$m for (NZP) powders and 10 to 18 $mu$m for aluminosilicate powders. The specific surface area of the prepared powder is a function of batch weight of starting materials, the amount of urea and ammonium nitrate, and the amount of water. Usually, the powders prepared by the thermal decomposition had much higher surface area (150-300 m$sp2$/g) than the combustion-synthesized powders. The sinterability of (NZP) powders prepared by both methods was comparable to that of the sol-gel derived powders. For instance, the bulk density for Ca$sb{0.5}$Sr$sb{0.5}$Zr$sb4$P$sb6$O$sb{24}$ ceramics sintered at 1350$spcirc$C for 6 h was 99% of theoretical. $alpha$-cordierite (2MgO$cdot$2Al$sb2$O$sb3{cdot}$5SiO$sb2$) and mullite (3Al$sb2$O$sb3{cdot}$2SiO$sb2$) powders prepared by the thermal decomposition and combustion process had lower sinterability than sol-gel derived powders. The lower sinterability is mainly attributed to the hard agglomeration of the aluminosilicate powders. It is believed that the sintered density would increase if the degree of agglomeration were reduced.;The advantages of this method to prepare oxide powders include its much lower furnace temperature and shorter reaction time. In addition, the prepared powders are homogeneous and highly reactive because of their high surface area. The powders can be sintered to higher density at lower temperature than those prepared by the solid state reaction method.