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The Forefront of Space Science

The Liquid Boron Turned Out to Be a Semiconductor
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About the Boron

Boron (B) is the fifth element in the periodic table of the elements. Boron is light but tough with a high melting point, and in space explorations, researches are being conducted to use it as a kind of heat-resistant material on the rockets. Since boron is easy to mine, it has been used by people for a long time. The borides, such as the boron oxide (B2O3) used as the base material of glassware and the boron nitride or boron carbide used as abrasives, are all important materials widely used in industries. Currently, there are as many as 6 kinds of allotropes (simple substances formed by the same element but with different properties since the atoms are bonded together in a different manner) of boron known. In all elements, it is the second most next to sulfur (S), and there may be new allotropes under ultimate environments such as high temperature and high pressure in future researches.

In the periodic table, the elements are mainly classified as metals and nonmetals (semiconductors and insulators). According to the Dictionary of Physics and Chemistry, a metal is a substance that usually has a shiny appearance, is a good conductor of electricity and heat, and is usually capable of being shaped when in solid state. In the microscopic viewpoint, we can describe a metal as a substance with its valence electrons inside moving around freely. The properties of substances are determined by the behavior of their valence electrons. Sometimes when the electrons possess interaction with each other, we will find magnetism in the substance. How valence electrons move around inside is an important criterion to presume the properties of substances, so almost all periodic tables are color coded to make it easy to know whether the elements are metals or nonmetals. Such classification is as well important for liquids so that we can study the liquid state of most of the stable elements.

Figure 1
Figure 1. Periodic table of the elements. [Click for large image]

Boron and silicon (Si) are located at the dividing line between metals and nonmetals. It is known that for such elements, the liquid properties are quite different from the solid properties. The elements such as silicon, carbon and germanium, which are typically semiconductors in the solid form, will become metals when melted. Similarly, the semiconductor boron is considered to be a metal when melted. However, the melting point of boron is as high as 2077C, and furthermore, the reactivity of the melted boron (boron melt) is so high that is would interfere during experiments. As a result, it had been an important issue in material science that whether the boron could become a metal on melting.

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