2.2.2. Scanning Electron Microscopy Tests

The specimen was rapidly frozen with liquid nitrogen (−190 ◦C) and vacuum-pumped for 24 h with a vacuum freeze-drying apparatus to ensure the specimen reached the dry state when the water of the specimen was completely sublimated. The dry specimens were then sliced into suitable sheet sizes, and scanning electron microscope (SEM) tests were performed at different magnifications (×50, ×100, ×200, ×500, ×1000, ×2000, ×5000, and ×10,000) using a JBM-7500F field emission SEM (JEOL, Japan). Only four suitable magnification images (×200, ×2000, ×5000, and ×10,000) were selected to fully present the results.
