Views: 0 Author: Site Editor Publish Time: 2023-02-24 Origin: Site
Tungsten or tungsten,is a chemical element with symbol W and atomic number 74.Tungsten is a rare metal that occurs naturally on Earth,almost exclusively in the form of compounds with other elements.It was identified as a new element in 1781 and was first isolated as a metal in 1783.Its important ores include scheelite and wolframite,the latter giving the element its nickname.This free element is known for its robustness,not least because it has the highest melting point of all known elements except carbon (which sublimes at atmospheric pressure),at 3,422 °C (6,192 °F; 3,695 K).It also has a maximum boiling point of 5,930 °C (10,706 °F; 6,203 K).It has a density of 19.30 grams per cubic centimeter (0.697 lb/in3),comparable to uranium and gold, and much higher (about 1.7 times) than lead.Polycrystalline tungsten is inherently a brittle and hard material (under standard conditions, when unbonded) and is therefore difficult to process into a metal.However, pure single crystal tungsten is more ductile and can be cut with a hard steel hacksaw.Tungsten is found in many alloys that have a variety of applications,including incandescent light bulb filaments,X-ray tubes, electrodes in gas tungsten arc welding,superalloys,and radiation shielding.Tungsten's hardness and high density make it suitable for military applications in armor-piercing projectiles.Tungsten compounds are often used as industrial catalysts.Tungsten is the only metal in the third transition series known to be present in biomolecules and is found in a few bacteria and archaea.However,tungsten interferes with molybdenum and copper metabolism and is somewhat toxic to most animal life forms.
Tungsten in its raw form is a hard, steel-gray metal that is often brittle and difficult to machine.Purified single crystal tungsten retains its hardness (more than that of many steels) and is ductile enough to be easily machined.It is formed by forging,drawing or extrusion, but more commonly by sintering.Of all the pure metals,tungsten has the highest melting point (3,422 °C, 6,192 °F),the lowest vapor pressure (temperatures above 1,650 °C, 3,000 °F),and the highest tensile strength.Although carbon remains solid at higher temperatures than tungsten,carbon sublimes rather than melts at atmospheric pressure,so it has no melting point.Furthermore,the most stable crystalline phase of tungsten does not exhibit any high-pressure-induced structural transitions up to pressures of at least 364 GPa.Tungsten has the lowest coefficient of thermal expansion of all pure metals.The low thermal expansion,high melting point, and tensile strength of tungsten originate from the strong metallic bonds formed by 5d electrons between tungsten atoms.Alloying small amounts of tungsten with steel can greatly improve its toughness.Tungsten exists in two main crystal forms: alpha and beta.The former has a body-centered cubic structure and is the more stable form.The structure of the β phase is called A15 cubic;it is metastable,but it can coexist with the α phase at ambient conditions due to non-equilibrium synthesis or impurity stabilization.In contrast to the α phase,which crystallizes in equidistant grains,the β phase exhibits a columnar habit.The α phase has one-third the resistivity and a much lower superconducting transition temperature TC:ca relative to the β phase. 0.015 K vs.1-4 K;mixing the two phases allows intermediate TC values.The TC value can also be increased by alloying tungsten with another metal (for example,W-Tc is 7.9 K).This tungsten alloy is sometimes used in low-temperature superconducting circuits.
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