Zinc oxide

Zinc oxide is a chemical compound with the formula ZnO. It is nearly insoluble in water but soluble in acids and 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.

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Applications

Medical applications

Zinc oxide lozenges are a popular over-the-counter cold remedy, but numerous studies have failed to demonstrate any significant effect.^[1] Zinc oxide in a mixture with about 0.5% iron(III) oxide (Fe₂O₃) is called calamine and is used in calamine lotion. There are also two minerals, zincite and hemimorphite, which have been called calamine historically (see: calamine (mineral)). When mixed with eugenol, zinc oxide eugenol forms which has restorative and prosthodontic applications in dentistry.

Zinc peroxide, ZnO₂ .½ H₂O, is a white to yellow powder that is used in antiseptic ointments.

As a pigment

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 absorbs 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). It is the broadest spectrum UVA and UVB absorber that is approved for use as a sunscreen by FDA, and is completely photostable. Additionally, since zinc oxide has antimicrobial and antifungal activities, it is the number one active ingredient recommended by pediatricians for the treatment of diaper rash.

Rubber manufacture

Zinc oxide and stearic acid are ingredients in the commercial manufacture of rubber goods. A 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.

Electronic materials

ZnO is a semiconductor with a direct bandgap energy of 3.37 eV at room temperature. The most common applications are in laser diodes and light emitting diodes since it has an exciton and biexciton energies of 60 meV and 15 meV, respectively. It is expected that this exciton properties of ZnO will be improved further by epitaxy.

Most ZnO has n-type character, even in the absence of intentional doping. Native defects such as oxygen vacancies or zinc interstitials are often assumed to be the origin of this, but the subject remains controversial^[2]. An alternative explanation has been proposed, based on theoretical calculations, that unintentional substitutional hydrogen impurities are responsible^[3].

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 aluminum, indium, or excess zinc^[4]. p-type doping is difficult and is currently an active area of research, with arsenic as the leading candidate dopant^[5]. Thin-film solar cells, LCD and flat panel displays are typical applications of this material. Appropriately doped ZnO may be 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 sputter deposition 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.

ZnO has been observed to act as a chemical reagent for Friedel-Craft Acylation Reaction.

Pyroelectric coefficients

• Primary pyroelectric coefficient: -6.8 µC/m²·K
• Secondary pyroelectric coefficient: -2.5 µC/m²·K
• Total pyroelectric coefficient: -9.4 µC/m²·K

Production methods

Zinc oxide is produced by two main processes:^{[citation needed]}

French process

Metallic zinc is melted in a graphite crucible and vaporized above 907 °C. Zinc vapor instantaneously reacts with the oxygen in the air to give ZnO, accompanied by a drop in its temperature and bright luminescence. Zinc oxide particles are transported into a cooling duct and collected in a bag house. This indirect method is commonly known as the French process (FP) which was popularised by LeClaire (France) in 1844. A typical FP, zinc oxide normally consists of agglomerated zinc oxide particles with an average size of 0.1 micrometres to a few micrometres. By weight, most of the world's zinc oxide is manufactured via French process and major applications involve industries related to rubber, varistors, suncreens, paints, healthcare, and poultry nutrients. Recent developments involve acicular nanostructures (rods, wires, tripods, tetrapods, plates) synthesized using a modified French process known as catalyst-free combust-oxidized mesh (CFCOM) process. Acicular nanostructures usually have micrometre-length nanorods with nanometric diameters (below 100 nm).

The so-called direct method is related to the FP. In this process, zinc ores or roasted sulfide concentrates are mixed with coal. In a reduction furnace, ore is reduced to metallic zinc and the vaporized zinc is allowed to react with oxygen to form zinc oxide.

American process

In this process ore of zinc (zinc ash) is dissolved (as ZnCl₂) and precipitated with alkali. Zinc oxide made from this process is known as "Active Zinc Oxide"

Safety

Fumes of zinc oxide are generated when melting brass, because the melting point of brass is close to the boiling point of zinc.^[6] Exposure to zinc oxide in the air (also while welding) can result in a nervous malady called metal fume fever.^[7]

References

1. ^ Caruso TJ, Prober CG, Gwaltney JM (2007). "Treatment of naturally acquired common colds with zinc: a structured review". Clin. Infect. Dis. 45 (5): 569-74. doi:10.1086/520031. PMID 17682990.
2. ^ Look, D.C.; Hemsky, J.W.; Sizelove, J.R. (1999). "Residual Native Shallow Donor in ZnO". Physical Review Letters 82 (12): 2552-2555. doi:10.1103/PhysRevLett.82.2552.
3. ^ Janotti, A.; Van De Walle, C.G. (2007). "Hydrogen multicentre bonds.". Nat Mater 6 (1): 44-7. Retrieved on 2007-10-26.
4. ^ Hirschwald, W.H. (1985). "Zinc oxide: an outstanding example of a binary compound semiconductor". Accounts of Chemical Research 18 (8): 228-234. Retrieved on 2007-10-26.
5. ^ Braunstein, G.; Muraviev, A.; Saxena, H.; Dhere, N.; Richter, V.; Kalish, R. (2005). "p type doping of zinc oxide by arsenic ion implantation". Applied Physics Letters 87: 192103. Retrieved on 2007-10-26.
6. ^ The Safetey of Zinc Casting
7. ^ Zinc and Cadmium

• Physics Today, volume 58, No. 8, p.33, August 2005.
• "Introduction and recovery of point defects in electron-irradiated ZnO", Tuomisto, F., Saarinen, K., Look, D.C., and Farlow, G.C., Physical Review B 72, 085206 (2005).
• "Nanostructure of ZnO fabricated via French Process and its correlation to electrical properties of semiconducting varistors", S.Mahmud, M.J.Abdullah, J.Chong, A.K.Mohamad, M.Z.Zakaria, J. Syn. React. Inorg., Met-org. and Nano-metal Chem., 36, pp155-159 (2006).