Zinc oxide is an oxide of zinc that is hardly soluble in water and soluble in acids and bases

- May 10, 2018-

Zinc oxide is an oxide of zinc that is hardly soluble in water and soluble in acids and bases. It is a white solid, it is also called zinc white. It can be obtained by burning zinc or roasting sphalerite (zinc sulfide). In nature, zinc oxide is the main component of mineral zincite. Although there are two methods of manufacturing artificial zinc oxide: oxidizing or baking zinc ore from pure zinc. Zinc oxide is used as an additive in a variety of materials and products including plastics, ceramics, glass, cement, lubricants [2], paints, ointments, adhesives, caulking materials, pigments, foods (zinc supplements), batteries , ferrite materials, flame retardant materials [3] and medical emergency bandages.

Zinc oxide crystals have three structures: a hexagonal wurtzite structure, a cubic sphalerite structure, and a rare NaCl-octahedral structure. The wurtzite structure has the highest stability among the three and is therefore the most common. The cubic sphalerite structure can be obtained by gradually generating zinc oxide on the surface. In both crystals, each zinc or oxygen atom forms a tetrahedral structure centered on its neighboring atoms. The octahedral structure was only observed under high pressure conditions of 10 billion Pascals.

The wurtzite structure and the sphalerite structure have central symmetry, but no axial symmetry. The symmetry property of the crystals makes the wurtzite structure have a piezoelectric effect and a coke hot spot effect, and the sphalerite structure has a piezoelectric effect.

The wurtzite structure has a spot group of 6 mm (indicated by an international symbol) and the space group is P63mc. In the lattice constants, a = 3.25 angstroms, c = 5.2 angstroms; the c/a ratio is approximately 1.60, close to the ideal hexagonal ratio of 1.633. In semiconductor materials, zinc and oxygen are mostly ionically bonded, which is one of the reasons for their high piezoelectricity.

Zinc oxide has a hardness of about 4.5 and is a relatively soft material. The elastic constant of zinc oxide is smaller than that of a III-V group semiconductor material such as gallium nitride. Zinc oxide has good thermal stability and thermal conductivity, and has a high boiling point and a low thermal expansion coefficient, and is useful in ceramic materials.

Zinc oxide has the highest piezoelectric tensor in various semiconductor materials with tetrahedral structures. This property makes zinc oxide one of the most important materials for mechanical electro-mechanical coupling.

At room temperature, the energy band gap of zinc oxide is about 3.3 eV, so pure zinc oxide is colorless and transparent. The high energy band gap is a feature that zinc oxide brings about high breakdown voltage, strong ability to maintain electric field, small electronic noise, and high power capability. Zinc oxide mixed with a certain proportion of magnesium oxide or cadmium oxide will change the band gap between 3-4 eV.

Zinc oxide has the characteristic of an N-type semiconductor even if it is not doped with any other substance. The characteristics of N-type semiconductors have been considered to be related to the non-integral properties of compound atoms, and the study of pure zinc oxide has become a counterexample. The use of halogens such as aluminum, gallium, indium, etc., or halogens such as chlorine and iodine can adjust the N-type semiconductor performance. However, there are certain difficulties in making zinc oxide into P-type semiconductors. The additives that can be used include alkali metal elements such as lithium, sodium, and potassium, elements such as nitrogen, phosphorus, and arsenic, Group V main elements, and metals such as copper and silver, all of which need to be used under special conditions.