With the rapid development of new thermal management materials, a demand for high quality AlN powders becomes increasingly urgent. The liquid phase precursor method is regarded as a crucial approach for producing high purity AlN nano-powder due to its advantage of homogeneous mixing in molecular level. Therefore, a study on the liquid phase precursor method and the effect of additives within the system on the precursor pyrolysis, liquid phase regulation, and nitridation kinetics is essential for improving the purity, reaction efficiency, and structural uniformity of sized AlN nano-powder. In this study, AlN nano-powder was synthesized by a liquid phase precursor method at low temperatures, and the role, reaction pathways, and regulatory mechanisms of yttrium additives in the precursor system were systematically investigated. This work could provide significant insights toward enhancing powder purity, achieving controlled crystal growth, and optimizing structural homogeneity.

Aluminum chloride hexahydrate was used as an aluminum source and dicyandiamide and melamine as nitrogen sources. Yttrium acetate tetrahydrate acted as a soluble yttrium additive, and methanol was selected as a solvent. The aluminum and nitrogen sources were dissolved in methanol at an N/Al molar ratio of 4, and yttrium was added at 0-3% (calculated as Y₂O₃ in the final AlN). After magnetic stirring for 30 min, a uniform precursor solution was obtained. The solution was dried at 85 ℃ for 12 h, and then ground into a white precursor powder. The precursor powders were placed in covered alumina crucibles and heated in a tube furnace under flowing N₂ to 1100 ℃ at 10 ℃/min, followed by a 3h dwell, to synthesize AlN powders with different yttrium contents. The thermal behavior of yttrium acetate and precursor powders was determined in N₂ by thermogravimetric analysis-differential scanning calorimetry (TG-DSC) from room temperature to 1200 ℃. The phase compositions were examined by X-ray diffraction (XRD, Cu K_α, 20°-80°). The particle morphology of the synthesized powder was determined by field-emission scanning electron microscopy, using an accelerating voltage of 20 kV. The microstructure of the particles in the powder was analyzed by transmission electron microscopy (TEM). The elemental composition and distribution were characterized by energy dispersive spectroscopy (EDS), while the crystalline phase and crystal structure were determined by selected area electron diffraction (SAED).

The liquid phase dispersion combined with a low temperature calcination effectively reduces the synthesis temperature. Compared with the conventional carbothermal reduction method, the Al—Cl bond exhibits a bond strength of approximately 174 kJ/mol, which is lower than that of the Al—O bond, resulting in a reduced activation energy for the reaction. As high nitrogen solid molecules, cyanamide species can release high concentrations of highly reactive nitrogen in-situ on the surface and inside the aluminum precursor, facilitating the completion of the nitridation reaction at a lower temperature for a shorter time. The aluminum source, nitrogen sources, and yttrium additives are mixed and contacted more uniformly due to the excellent dispersibility of the liquid phase, thereby further enhancing both the nitridation rate and its uniformity. The results indicate that the introduction of a small amount of yttrium additive promotes the refinement and uniformity of AlN particles. As the yttrium content increases from 0% to 3%, the AlN particles gradually evolve toward a quasi-spherical morphology, accompanied by improved sphericity and reduced particle size. The average AlN grain size decreases from 74 nm without yttrium addition to 40 nm at 3% yttrium addition. A similar grain-size reduction trend is also obtained based on calculations using the Scherrer equation from the XRD results. The added yttrium additive is uniformly distributed throughout the AlN samples and does not compromise the lattice purity of AlN. No lattice expansion associated with impurity incorporation or solid solution formation occurrs in either the lattice parameters or interplanar spacings.

At the synthesis temperature of 1100 ℃ that could be lower than that required for the carbothermal reduction method, the precursor effectively transformed into crystalline AlN regardless of whether yttrium additives introduced. The addition of a small amount of yttrium effectively promoted the refinement and uniformity of AlN particles, resulting in synthesized AlN powders with relatively uniform particle size and morphology. Yttrium containing compounds had an inhibitory effect on the AlN grain growth, leading to a further refinement of AlN grains in nanoscale. The average grain size of AlN decreased from 74 nm without yttrium addition to 62 nm at 1% yttrium content, and further to 40 nm at 3% yttrium content. The incorporation of yttrium additives did not compromise the lattice purity of AlN during synthesis. The interplanar spacing of AlN nanocrystals remained consistently at 0.270 nm at all yttrium concentrations, corresponding to the plane (100) of hexagonal AlN. In addition, no significant increase occurred in the a-axis, c-axis, c/a ratio, or unit cell volume of AlN nanocrystals with different yttrium contents, confirming the absence of lattice expansion due to the impurity incorporation or the solid-solution formation.