1：Experimental Design and Method Selection:

The study employed molten salt synthesis (MSS) to synthesize h-BN at lower temperatures. The rationale was to use a eutectic NaCl-KCl salt mixture to promote reaction kinetics and mass transfer. Theoretical models included thermodynamic and kinetic considerations for reaction mechanisms.

2：Sample Selection and Data Sources:

Reactants were boric acid (H3BO3) and ammonium chloride (NH4Cl) with molar ratios varied from 2:1 to 1:

3：The salt mixture was NaCl-KCl in a

1 molar ratio. Samples were prepared by mixing powders and heat-treating in a nitrogen atmosphere.

4：List of Experimental Equipment and Materials:

Equipment included a tube furnace (Model 1000K, CM Furnaces), X-ray diffractometer (PANalytical X'Pert Diffractometer), FTIR spectrometer (Mattson Instruments Galaxy 5000), FE-SEM (Zeiss-Sigma), EDS detector (Oxford-XmaX80), and TEM (FEI Tecnai G2 F20). Materials included H3BO3 (purity >99.9%, U.S. Borax, Inc.), NH4Cl (purity >99%, Fisher Scientific), NaCl, KCl, methanol, and alumina crucibles.

5：0). Materials included H3BO3 (purity >9%, U.S. Borax, Inc.), NH4Cl (purity >99%, Fisher Scientific), NaCl, KCl, methanol, and alumina crucibles. Experimental Procedures and Operational Workflow:

4. Experimental Procedures and Operational Workflow: Reactant and salt mixtures were layered in an alumina crucible and heated in a tube furnace under nitrogen flow at 10 °C/min to temperatures of 800-1000 °C with dwell times of 1-6 hours. After cooling, products were washed with hot deionized water, filtered, and dried at 110 °C for characterization.

6：Data Analysis Methods:

XRD was used for phase identification, crystallinity, and crystallite size calculation using the Scherrer equation. FTIR identified chemical bonds. SEM and TEM analyzed morphology and microstructure. EDS provided elemental composition.