Zinc Oxide Properties
Occurrence – Zinc oxide (zincite) rarely occurs in nature, particularly in a crystalline form. It is usually orange/red in color due to manganese impurity. The best known locale is the Sterling Hill – Franklin mining area of New Jersey, USA. http://franklin-sterlinghill.com
Crystal Structure - Zinc oxide has the wurtzite hexagonal crystal structure. Commercial zinc oxides show this crystal structure under electron microscopic examination. The precise shape of the crystal depends on the method of formation. In regular zinc oxide these vary between acicular needles and plate shaped crystals. Zinc oxide can be induced to form a very large variety of crystalline shapes using specialized deposition methods, this is a very active area of research.
Molecular Weight – 81.37
Color – Pure microcrystalline zinc oxide is white. Single crystal zinc oxide is colorless. Zinc oxide turns lemon yellow on heating and reverts to white on cooling.
Relative Density – 5.607
Melting Point – Zinc oxide sublimes at atmospheric pressure at temperatures over 1200C. Under high pressure a melting point of 1975C has been estimated.
Vapor Pressure @ 1500C = 12mm.
Refractive Index – w = 2.004, e = 2.020
Heat of Sublimation between 1350C and 1500C – 129 Kcal/mole (vapor not disassociated) and 193 Kcal/mole (vapor associated).
Heat Capacity, Cp = 9.62 cal/deg/mole @ 25C
Heat, deltaH = -83.25Kcal/deg/mole @25C
Free energy of formation, deltaF = -76.1 Kcal/degC/mole @ 25C
Entropy, S = 10.43 cal/degC/mole
Coefficient of Thermal Expansion = 4 x 10(-6)/degC
Electrical Conductivity – In its normal form, zinc oxide is an n type semiconductor, ie conductivity by electrons. This conductivity is thought to be due to a stoichiometric excess of zinc ions which occupy interstitial locations in the crystal lattice. By doping with other elements to replace either the zinc or the oxygen, the conductivity can be varied over a very wide range. Even with controlled doping it has proven to be very difficult to reproducibly make p type zinc oxide. For many semiconductor devices it is necessary to make pn junctions, it is only recently that p type zinc oxide has been made successfully.
Rectification – Single crystals of zinc oxide can act as rectifiers. This is the origin of ‘crystal sets’, very early radio receivers.
Optical Properties – Zinc oxide is transparent to visible light but strongly absorbs ultra violet light below 3655 A. The absorption is stronger than other white pigments. In the visible region wavelengths, regular zinc oxide appears white, however, rutile and anatase titanium dioxide have a higher reactive index and hence has a superior opacity.
The band gap energy (between valence and conducting bands) is 3.2 ev, this corresponds to the energy of 3655 A photons. Under ultra violet light zinc oxide is photoconductive. This was the basis of the historical Electrofax photocopying system.
The combination of optical and semiconductor properties make doped zinc oxide a candidate for new generations of devices. Solar cells require a transparent conductive coating, indium tin oxide and doped zinc oxide are the best materials.
Zinc oxide is amphoteric, that is it reacts with both acids and alkalis.
With acid it reacts to form familiar compound such as zinc sulfate. With alkali it forms zincates.
The following reaction is extremely important in zinc pyrometallurgy-
ZnO + CO = Zn + CO2
The equilibrium is at around 1,000C, at lower temperatures the zinc oxide plus carbon monoxide side is favored. In the early days of zinc blast furnace development, the surprising step of allowing limited air ingress to the zinc vapor was successful because the exothermic zinc/oxygen reaction pushed the equilibrium to the right.
Zinc oxide reacts with fatty acids such as stearic directly by mixing and heating the components above the acid melting point.
Commercial zinc oxide shows a measurable but low level of water solubility, 0.005 g/litre.
Zinc oxide exposed to air absorbs both water vapor and carbon dioxide. This results in the formation of basic zinc carbonate.
Zinc oxide also undergoes solid state reactions (calcination) at moderately elevated temperatures. Under electron microscopic examination, commercial zinc oxide frequently shows groups of particles that are conjoined, rather than loosely agglomerated. Zinc oxide calcines with other oxides such as silica and magnesia.