A hard mystery solved

Science News, Oct 2, 1993

Why are silicon and other substances that are known as covalent solids so much harder and more brittle than pure metals? This has long mystified materials scientists. In fact, John J. Gilman has pondered the phenomenon for 40 years.

Scientists know that covalent solids are particularly hard because electrons in them pair up to form tight bonds. And one indication of silicons hardness is that dislocation lines in the crystal structure move very slowly through it. A dislocation is a line in the crystal where the atoms are not arranged perfectly-like a big wrinkle in the middle of a rug. A dislocation moves when stress is applied. When it moves, the crystal deforms plastically -- that is, one part of it slides over another and the crystal gradually deforms without shattering or cracking. But no one had ever explained adequately why dislocation lines move slowly, says Gilman, a materials scientist at the Lawrence Berkeley (Calif.) Laboratory,

A material as important as silicon -to which computer chips, solar cells, and other electronic devices owe their existence deserves better, Gilman thought.

He found the explanation by analyzing how silicons electronic structure changes when a dislocation line moves, he reports in the Sept. 10 SCIENCE. Other scientists had looked at electrons' general mechanical properties but not at their arrangement and behavior, he says.

Gilman argues that kinks along a dislocation line determine the rate of the line's movement. For the line to move, the kinks have to separate the paired electrons in front of them. Then the line moves through the electrons, and the electrons close up behind it.

The strength of the electrons' bonds depends on the size of the gap between the energy levels of the electrons that are bonded and those that are not bonded. The wider the gap, the stronger the bonds and, therefore, the harder the material.

"This suggests that the kink mobility is directly related to the electronic structure," Gilman reports.

Showing how the electronic structure affects kink mobility enabled Gilman to calculate the amount of stress needed to form the kinks, break up electron pairs, and move the kinks. This is a measure of how much silicon resists being plastically deformed.

More recently, Gilman has found that the reasons for silicons hardness apply to other covalent solids, including silicon carbide, which is used in abrasives.