Single-layer graphite is a very magical material, it can conduct electricity, and its strength is hundreds of times that of steel. However, recent testing of single-layer graphite samples by scientists has shown that although this material may be the strongest material in existence today, it is also very fragile, just like ceramics. Why? The reason is that it breaks easily.
A team of scientists from Rice University and Georgia Institute of Technology in the United States tested small pieces of "double-layer" graphite-two pure carbon flakes equivalent to a single atom thick stacked on top of each other. Scientists used concentrated ion beams to illuminate them so that they produced tiny cracks. This graphite was then dragged and twisted to observe the rate at which these cracks expanded until the material finally broke.
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"Single-layer graphite is very sensitive to the presence of cracks. In contrast, if there is a small crack in steel, it is not particularly dangerous. Steel has a huge resistance to crack expansion. Single-layer graphite is more like window glass." Research Lead author Ting Zhu, associate professor of the Department of Mechanical Engineering at Georgia Institute of Technology, said.
The measurement of a material ’s ability to resist cracking—that is, fracture toughness—not only includes tensile strength—that is, the possibility of a material breaking when it is stretched, it also measures when a particular material is twisted , The "punishment" it can endure before breaking. For example, metal is malleable, you need to twist and bend repeatedly to break a spoon. Glass can resist twisting, but it does not have ductility, so if the twisting force or tensile force exceeds a certain limit, it will quickly break. Even a small crack is enough to cause the glass to shatter.
A study conducted by Professor Zhu and Rice University ’s Jun Lou found that cracked single-layer graphite is 10 times more likely to fracture than steel, and at this time its fracture toughness is closer to alumina or silicon carbide Based ceramics. The relatively low fracture toughness means that a small crack in a single layer of graphite is enough to destroy it. Such small cracks are probably the result of naturally occurring graphite.
There are several ways to manufacture graphite, including chemical vapor deposition, that is, carbon vapor is gradually cooled and carbon atoms stay on the surface, and exfoliation, which is to put graphite exfoliated from a single layer of graphite into a solvent. The graphite layer produced by the first method can be large, but it is not perfect. The lattice of carbon atoms produced by this method will have very small defects-one atom may be missing everywhere. If a single layer of graphite is used as a conductor or semiconductor, this defect will not cause much impact. However, if it is used in mechanical applications, such as manufacturing flexible displays or improving the structural strength of other materials, the impact of such defects cannot be ignored.
The perfect single-layer graphite can withstand a pressure of 100 GPa before breaking, which is equivalent to about 1,000,000 times the atmospheric pressure. But the fracture strength of the defective single-layer graphite is only about one hundred thousandth of the former. It can only withstand a pressure of 4 MPa, which is 40 times the standard atmospheric pressure. This experiment is not only very important for the study of single-layer graphite, it may provide a very important reference for other studies, because other materials with two-dimensional structures may also have similar characteristics. "The modeling method used in this study may be used to study other 2D materials, such as aluminum disulfide or boron nitride," Professor Zhu said. The research was published in the journal Nature Communications on April 29.
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