A New Generation of Methods for Obtaining Metal–Ceramic Nanocomposites with Specific Sizes of Metal Nanocrystallites Stable at Elevated Temperatures and Testing the Chemical Properties of the Obtained Nanomaterials

The starting material for this research was a metal–ceramic nanocomposite containing nanocrystalline iron with an average nanocrystallite size equal to 23 nm (based on X-Ray Diffraction; a specific surface area of 9 m2/g by the BET method) and a nanocrystallite size distribution standard deviation σ = 15 nm, promoted with hardly reducible oxides (Al2O3, CaO, K2O in total, max. 10 wt%), obtained by melting magnetite with promoter oxides at 1600 °C and reducing the resulting alloy with hydrogen at 500 °C. This material was then oxidized in a controlled manner with water vapor at 425 or 500 °C to achieve different oxidation degrees. Metallic iron remaining in the samples after the oxidizing step was removed by two-stage acid etching. Promoters introduced into the melt ensured the stability of the nanocomposite structure at elevated temperatures. After etching, the iron oxide was reduced with hydrogen at 375 or 500 °C. A series of nanocrystalline iron samples with different nanocrystallite sizes (in the range from 18 to 35 nm; specific surface areas decreased from 32 to 16 m2/g with increasing nanocrystallite size) and a narrowed nanocrystallite size distribution standard deviation σ = 3–5 nm was synthesized, which was then tested in the process of nitriding (at 375 and 500 °C), carburizing (400–550 °C), and oxidation (at 425 and 500 °C). The progress and rate of these reactions were measured in a differential tubular reactor with thermogravimetric measurement of mass changes in the solid sample and catharometric measurement of hydrogen concentration in the gas phase. The scalability of the proposed method was also investigated by conducting measurements on 1, 10, and 100 g samples. The effect of nanocrystallite size on the chemical properties of the tested samples was observed. The nanocomposite samples containing the smallest iron nanocrystallite sizes were found to be the most active in the nitriding reaction and catalytic decomposition of ammonia. All the tested modified samples were at least several times more active in the decomposition of ammonia than the unmodified sample. The practical effect of our work is the presentation and use of a new, more precise method for obtaining nanocrystallites of specific sizes.