*2.5. Etching Method to Test the Cross-Sectional Morphology of the Samples*

The eight groups of prepared samples were cut into strips with a size of <sup>10</sup> × <sup>2</sup> × 2 mm3, and they were placed in liquid nitrogen to freeze for 5 min. Then, the test strips were taken out with tweezers and broken in the middle. The broken samples were completely immersed in

chloroform for 72 h (the samples were taken out and replaced with a new chloroform solvent every 24 h to completely dissolve the etched MVSR phase). After 72 h, the samples were taken out from the chloroform solvent and placed in a fume hood for 12 h to allow the solvent on the surface of the samples to evaporate completely. Finally, the cross-sections were sprayed with gold, and we observed them with a scanning electron microscope (SEM) to obtain the secondary electron images.
