A tungsten needle is a slender metal rod made of tungsten, commonly used as a heating element in vacuum tubes, electronic devices, high-temperature furnaces, etc. The crystal structure of a tungsten needle depends on its preparation method and purity.
Pure tungsten needles have a body-centered cubic (bcc) structure, which is common in metals, such as chromium, manganese, iron, etc. At high temperatures, pure tungsten needles exhibit good strength and toughness while having a high melting point and low vapor pressure.
However, if impurities or defects are present in the tungsten needle, its crystal structure may change. For example, when a tungsten needle contains carbon impurities, a wc (tungsten carbide) phase may be formed, which is a compound with a hexagonal structure. The wc phase has a higher melting point and lower vapor pressure, so it is often used in heating elements of high-temperature furnaces.
In addition, after high-temperature treatment, the tungsten needle may undergo grain growth or phase transformation, resulting in changes in its crystal structure. For example, after prolonged exposure to high temperatures, tungsten needles may undergo creep phenomena, which is elongation or bending in the axial direction. This phenomenon may be related to grain boundary slip, dislocation creep, etc.
In addition to the bcc structure and the wc phase of the hexagonal structure, tungsten needles may also form other compounds or amorphous structures. For example, under certain conditions, tungsten needles may form compounds such as w2c and w3c, which have different crystal structures and physical properties. In addition, if the tungsten needle is subjected to strong mechanical or thermal stress, an amorphous structure or a nanocrystalline structure may be formed.
In summary, the crystal structure of a tungsten needle depends on its preparation method and purity. Pure tungsten needles have a bcc structure, but other compounds or amorphous structures may be formed when impurities or defects are present. These different crystal structures and physical properties have an important impact on the performance and applications of tungsten needles.
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