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Zinc oxide is a chemical compound with formula ZnO. It is nearly insoluble in water but soluble in acids or alkalis. It occurs as white hexagonal crystals or a white powder commonly known as zinc white. It remains white when exposed to hydrogen sulfide or ultraviolet light. Crystalline zinc oxide exhibits the piezoelectric effect and is thermochromic (it will change colour from white to yellow when heated, and back again when cooled down). Zinc oxide decomposes into zinc vapor and oxygen at around 1975°C. High-quality single-crystalline ZnO is almost transparent. Zinc oxide occurs in nature as the mineral zincite.
Applications Zinc oxide in a mixture with a about 0.5% iron(III) oxide (Fe2O3) is called calamine and is used in calamine lotion. There are also two minerals, smithsonite and hemimorphite, which have been called calamine historically (see: calamine (mineral). Zinc peroxide, ZnO2 .½ H2O, is a white to yellow powder that is used in antiseptic ointments. Zinc white is used as a pigment in paints and is more opaque than lithopone, but less opaque than titanium dioxide. It is also used in coatings for paper. Chinese white is a special grade of zinc white used in artists' pigments. Because it reflects both UVA and UVB rays of ultraviolet light, zinc oxide can be used in ointments, creams, and lotions to protect against sunburn and other damage to the skin caused by ultraviolet light (see sunscreen). Zinc oxide and stearic acid are important ingredients in the commercial manufacture of rubber goods. A proper mixture of these two compounds allows a quicker and more controllable rubber cure. Zinc oxide can also be used as a filler in some rubber mixtures. Zinc oxide is a semiconductor with a direct band gap of 3.37 eV (368 nm at room temperature, deep violet/borderline UV). A common application is in gas sensors. As of 2003, it has been utilized in recent research to build blue LEDs and transparent TFTs. ''n''-type doped films are often used in thin film technology, where zinc oxide serves as a TCO (transparent conducting oxide). n-type doping is possible by introduction of hydrogen or zinc interstitial defects. Oxygen vacancies are no longer considered to contribute to the n-type conductivity because these levels are too deep in the band gap. ''p''-type doping is difficult and is currently an active area of research. Thin-film solar cells, LCD and flat panel displays are typical applications of this material. Zinc oxide is transparent and conductive, and can therefore be used as a transparent electrode. Indium tin oxide (ITO) is another transparent conducting oxide often used in microelectronics. ZnO has also been considered for spintronics applications because of theoretical predictions of room temperature ferromagnetism. Unsubstantiated reports of ferromagnetism have been made, but presence of dilute magnetic semiconductors remains a large unanswered question in physics. ZnO layers are mainly deposited by sputtering and chemical vapor deposition (CVD). The latter method allows the growth of a rough layer, which can diffuse the incoming light by scattering, increasing the efficiency of solar cells. Recently, ZnO has been observed to act as a chemical reagent for Friedel-Craft Acylation Reaction. Pyroelectric coefficient Production Methods Zinc oxide is produced by oxidation of metallic zinc vapor at elevated temperatures. There are two methods. Direct method and indirect method. Indirect method Metallic zinc is vaporized in suitable containers. Zinc vapor reacts with oxygen that is in the air to form zinc oxide. Direct method Zinc ores or roasted sulfide concentrates are mixed with coal. In a reduction furnace, ore is reduced to metallic zinc and vaporized zinc reacts with oxygen to form zinc oxide. For more information and specifications zinc oxide | |||||||||||||||
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