## 1 Introduction  
Two-dimensional flaky alumina has the advantages of good acid and alkali resistance, a high melting point, robust temperature resistance, high electrical resistance, and satisfactory thermal conductivity [1], which is generally used as thermally conductive fillers and coatings [2], ceramic toughening agents [3], polishing abrasives, plastics, and rubber additives. Large-sized ultra-thin flaky alumina features a super-large diameter-to-thickness ratio [4], high glossiness, and good surface adsorption [5]. When flaky alumina is combined with spherical alumina, the heat-conducting pathways of the gel system can be optimized, effectively improving the heat conduction value and meeting the requirements of thermal interface materials for heat conduction fillers [6]. The primary processes for preparing flaky alumina include the molten salt, hydrothermal, high-temperature solid-state, and sol-gel methods [7]. Among these methods, molten salt synthesis, which is developed relatively early, is a mature process for preparing large-sized flaky alumina. However, this process is costly, complicated, and not environmentally friendly. Moreover, the particle size of the finished products is mostly below 10 \(\upmu\)m, but there is a demand for thermally conductive fillers with alumina with a size greater than 15 \(\upmu\)m to be on the market. The other two common methods, hydrothermal [8] and sol-gel [9], yield a flakey alumina particle size of merely about 2-5 \(\upmu\)m, failing to meet the needs of thermally conductive fillers. In contrast, the high-temperature solid-state method can obtain flaky alumina with a maximum size of 70-120 \(\upmu\)m. However, these products tend to have a subpar diameter-to-thickness ratio, dispersibility, and surface glossiness. In the conventional process, the particle size of precursors, e.g., hydrogen aluminum, boehmite, and alumina sol, is tiny, which is also why the hydrothermal and sol-gel approaches fail to produce large-sized flaky alumina. The new process in this study combined low-content molten salt ions with the gas-phase transport effect of the intermediate AlOF generated by the calcination of fluorides, obtaining large-sized flaky \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) with a size of 10-30 \(\upmu\)m, a thickness of 200-500 nm, and a diameter-to-thickness ratio of over 100. The innovation of this process lies in the Ostwald ripening effect of inorganic molten salts in the molten salt process, which effectively enveloped the precursor raw materials. The rising temperature promoted the nucleation and growth of flaky alumina, making it less prone to agglomeration. The prepared flaky alumina was well dispersed, with no large interlocking structures between the flakes. The additional fluorine (F) elements from the fluorides, particularly aluminum fluoride, predominantly adsorb on the (0001) surface of alumina [10], thereby reducing its surface energy. The driving force needed to generate a new (0001) plane of equal area is less than that required for the (1010) plane. When combined with the anisotropy of alumina crystal growth, this results in a faster growth rate for the (0001) plane. Additionally, water molecules produced during the thermal decomposition of the precursor react to form HF gas, which, in the presence of alumina, yields the intermediate compound AlOF. Through the gas-phase transport of AlOF, the flakes continuously fused and grew on the (1010) plane. This process completely transformed the materials into flaky alumina at 900 \({}^{\circ}\)C (but with small particle sizes and uneven thicknesses) and formed large-sized flake \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) with particle sizes ranging from 10 to 30 \(\upmu\)m, thicknesses ranging from 200 to 500 nm, and diameter-to-thickness ratios ranging from 80 to 150 at 1100 \({}^{\circ}\)C.  
The new process utilizes just a tenth of the inorganic molten salt typically required by the traditional molten salt method.