1：Experimental Design and Method Selection:

A two-stage synthesis process was used, combining microwave plasma nitridation as a pre-synthesis step at low temperature (550°C) to form an AlN shell, followed by thermal nitridation at higher temperatures (850-1050°C) for complete conversion. This design leverages the advantages of both plasma and thermal techniques to avoid agglomeration and enhance nitridation efficiency.

2：Sample Selection and Data Sources:

Aluminum powder (purity 99.8%, particle size 2-5 μm) and NH4Cl powder (purity 99.99%, particle size 1-4 μm) were used. Samples were prepared as ingots by compressing at 2000 psi.

3：8%, particle size 2-5 μm) and NH4Cl powder (purity 99%, particle size 1-4 μm) were used. Samples were prepared as ingots by compressing at 2000 psi. List of Experimental Equipment and Materials:

3. List of Experimental Equipment and Materials: Equipment includes a microwave plasma-nitridation system (magnetron YJ-1600, waveguide WR340), high-temperature furnace, X-ray diffractometer (XRD RINT-2000), thermogravimetric analyzer (TGA SDT Q600), scanning electron microscope (SEM S3000N), and optical emission spectrometer (OES HR 4000CG). Materials include N2 gas, Al powder, and NH4Cl powder.

4：Experimental Procedures and Operational Workflow:

First stage: Plasma nitridation at 550°C for 2-10 min with 1200 W power, N2 flow rates of 3 slm (axial) and 9 slm (swirl). Second stage: Thermal nitridation in a furnace with heating rate of 15°C/min to 850-1050°C, held for 1 h with N2 flow rate of 1 slm. Samples were analyzed using XRD, TGA, SEM, and OES.

5：Data Analysis Methods:

XRD for crystal structure analysis, TGA for AlN content calculation using a specific equation, SEM for particle size and morphology, and OES for active species measurement.