## 2 Melting and Solidification Experiment of BiFeO\({}_{3}\) Materials  
In this paper, the melting and solidification experiments of bismuth ferrite (BiFeO\({}_{3}\)) were conducted in HTMSEC. The HTMSEC is shown in Figure 1.  
The X-ray transmission imaging module relies on the different thicknesses and X-ray absorption contrast of the material sample itself to analyze its material structure. Materials with different chemical compositions absorb X-rays with different degrees of effectiveness [13]. In this paper, the transmission imaging of BiFeO\({}_{3}\) materials was examined. Since the melting point of BiFeO\({}_{3}\) is 960 \({}^{\circ}\)C, it is not possible to obtain high-definition X-ray images using close-range transmission imaging. The molten BiFeO\({}_{3}\) sample should be kept at a certain distance from the X-ray source and the detector [14; 15; 16]. The imaging principles are illustrated in Figure 2.  
Figure 1: Schematic diagram of HTMSEC (adapted from [11]). HTMSEC mainly includes a high-temperature furnace module, a sample management module, an X-ray transmission imaging module, a control module, and other subsystems (adapted from [12]). The X-ray sample box unit consists of a sample unit for placing the sample, a heating unit, an insulation unit, and a seal between the lid and the box [17]. The X-ray Sample Cartridge is mainly composed of a top cover plate and a bottom plate with a transparent rectangular window to facilitate the observation of experimental phenomena (Figure 3a). The sample heating unit consists of a metal heating wire and a high-temperature-resistant ceramic plate with equally spaced grooves (Figure 3b). The metal heating wires are wound around the grooves of the high-temperature-resistant ceramic plate to form a distribution pattern with varying spacing patterns [17]. X-ray transmission imaging technology makes use of coherent scattering resulting from the interactions between X-rays regularly arranged atoms inside the melt, so as to observe the structure and interface of the metal melt during melting and solidification. In this way, information on the atomic arrangement of the melt can be obtained, and the characteristic parameters reflecting the structure can be derived. The sparse and dense winding of the heating wire, the design of the insulation unit, and the electronically controlled heating make it easier to attain a controlled temperature gradient. By heating some parts of the sample at high temperatures, the sample is subjected to different temperature gradients, and a semi-molten and semi-solidified state can be achieved. Afterward, the experimental process is observed through the distinctive densities of the same substance in different states and the principles of X-ray transmission imaging [17]. The melting point of pure BiFeO\({}_{3}\) is about 960 \({}^{\circ}\)C. Co-solvent addition can reduce the melting point up to as low as 800 \({}^{\circ}\)C. By utilizing this property, low-melting-point BiFeO\({}_{3}\) is placed on the left and the high-melting-point counterpart is placed on the right. Upon heating, the low-melting-point BiFeO\({}_{3}\) becomes molten, and the solid-liquid interface appears. The solid-liquid interfaces of the unmelted and molten BiFeO\({}_{3}\) can be observed using the X-ray transmission imaging module. The heating process is shown in Figure 4.  
Figure 3: (a) Schematic diagram of the top cover and the lower plate of the X-ray Sample Cartridge; (b) The high-temperature-resistant ceramic plate. Figure 2: Schematic diagram of the X-ray transmission imaging module. After visualizing the data obtained by the X-ray transmission imaging module, the images of the melting and solidification processes of BiFeO\({}_{3}\) are obtained as shown in Figure 5. Due to the low contrast of the image obtained by the X-ray transmission imaging module, the solid-liquid interface is unclear, and it is not possible to accurately determine the morphological transformations of BiFeO\({}_{3}\) during melting and solidification based on the image.