Development of super- and sub-critical water annealing processes

• Annealing processes utilizing super- and sub-critical water with metal-salts were invented.
• Larger Scherrer sizes of Ni and TiO2 powders were obtained with less agglomeration.
• Metal-salts for lessening agglomerations also contributed crystallite diameter increments.
• Around water critical point was best for large crystallite diameter and less agglomeration.
• Pre-treatment of ultrasonic dispersion contributed to lessen agglomeration.

To achieve higher crystallite diameter (i.e. Scherrer size) with no simultaneous agglomeration of inorganic powders, thermal treatment fields of super- and sub-critical water with water-soluble metal-salts have been applied as new annealing processes. Operating temperatures were selected as 600, 660 and 720 K, taking into consideration that the water critical temperature is 647.3 K. As representative test metal and metal-oxide powders, Ni and TiO2 were selected. For adding metal-salts, NaCl and KNO3 were chosen, because they are common and typical in terms of including and excluding oxygen atoms. The crystallite diameters of Ni and TiO2 powders could be further increased in super- and sub-critical water compared to the conventional annealing case under the air atmosphere. Adding metal-salts contributed quite unexpectedly to larger crystallite diameters of the powders as well as their lesser agglomerations as expected. Furthermore, ultrasonic dispersing pre-treatment was found beneficial for reducing the agglomeration of the powders.

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