American scientists have discovered that irradiation of a ceramic carbide of silicon ions leads to an increase in the concentration of carbon atoms near the grain boundaries of the material. Previously it was thought that this process of segregation of the atoms under the influence of radiation is more typical for metals and alloys, not ceramic materials. Grain boundaries in polycrystals perform the role of binding “cement”, so changing their chemical composition can significantly affect material properties. Now scientists can accurately predict the result of the effects of radiation on the properties of ceramics and to create resistant to the aggressive conditions of ceramic materials. Article published in the journal Nature Materials.
The process of radiation-induced segregation is the distribution of atoms under the influence of ionizing radiation. Bombarding a substance particles (e.g., ions or neutrons) knock out atoms from their seats, forming a pair of point defects of the crystal lattice. Knocked out of the host crystal lattice, the atom becomes interstitial, and the empty node where it was — in the vacancy. These defects can recombine or migrate through the crystal to the drains, which perform the role of, for example, grain boundaries in polycrystals. The presence of such boundaries in the material significantly affects its physical properties, mechanical strength, corrosion and radiation resistance.
Through the process of segregation under the action of radiation near the grain boundaries can be formed, the excess or deficiency of a specific element substance that leads to local change of material properties in these areas. This effect is known since the 1970s and is well studied in many alloys of metals, but ceramic materials it is practically not considered.
To fill this gap, Isabela Szlufarska (Izabela Szlufarska) from the University of Wisconsin-Madison and his colleagues irradiated the samples of ceramic SiC silicon carbide ions of carbon and silicon. The material they have chosen as a typical representative of ceramic with a covalent bond between atoms. In addition, silicon carbide is considered as a material for use in nuclear power and creating microelectromechanical devices to work in difficult conditions.
To determine the chemical composition near the grain boundaries and monitor the process of segregation, the researchers used a scanning transmission electron microscope and spectroscopy characteristic energy loss of electrons. In the crystal structure of silicon carbide generated from high-purity initial components, the concentration of carbon atoms equal to about 50 atomic percent. However, in the SiC samples that were not exposed, the researchers found a lack of carbon atoms near the grain boundaries. Such a deviation from the expected values they connected with the fact that grain boundaries have a less ordered structure, containing a greater number of defects than in the bulk.