Zinc oxide thermal conductivity

An activator in rubber compounds containing organic accelerators. In polychloroprene, zinc oxide is considered to be the accelerator rather than the activator. The use of zinc oxide as a reinforcing agent and as a white colouring agent is obsolescent. Zinc oxide is manufactured by either the French (or indirect) process or by the American (or direct) process. It can be used as a filler to impart high thermal conductivity. 

In most semiconductors, there are, in addition to the allowed energy levels for electrons in the conduction and filled bands of the ideal crystal, discrete levels with energies in the forbidden gap which correspond to electrons localized at impurity atoms or imperfections. In zinc oxide, such levels arise when there are excess zinc atoms located interstitially in the lattice. At very low temperatures the interstitial zinc is in the form of neutral atoms. However, the ionization energy of the interstitial atoms in the crystal is small and at room temperature most are singly ionized, their electrons being thermally excited into the conduction band. These electrons give rise to the observed A-type conductivity. 

Zinc oxide has many uses. By far the most important is in the rubber industry. Almost half the world s ZnO is used as an activator for vulcanization accelerators in natural and synthetic rubber. The reactivity of the ZnO is a function of its specific surface area, but is also influenced by the presence of impurities such as lead and sulfates. The ZnO also ensures good durability of the vulcanized rubber, and increases its thermal conductivity. The ZnO content is usually 2-5%. 

Table 1.1. Abundance of the metal in the earths s crust, optical band gap Es (d direct i indirect) [23,24], crystal structure and lattice parameters a and c [23,24], density, thermal conductivity k, thermal expansion coefficient at room temperature a [25-27], piezoelectric stress ea, e3i, eis and strain d33, dn, dig coefficients [28], electromechanical coupling factors IC33, ksi, fcis [29], static e(0) and optical e(oo) dielectric constants [23,30,31] (see also Sect. 3.3, Table 3.3), melting temperature of the compound Tm and of the metal Tm(metal), temperature Tvp at which the metal has a vapor pressure of 10 3 Pa, heat of formation AH per formula unit [32] of zinc oxide in comparison to other TCOs and to silicon...

One specimen was irradiated with ultraviolet light, and its conductivity increased simultaneously, however, a decrease in conductivity was observed for the other specimen which had been shielded from the radiation. The immediate conclusion is that oxygen has been transferred from the irradiated specimen to the other one. This phenomenon of oxygen release from ZnO under the stimulus of irradiation has since been described by a number of investigators 48, 108-113), several of whom have studied the process manometrically. The conditions under which the build-up of pressure occurs are such that photolysis of zinc oxide can be ruled out, and at the same time several types of test have established that the effect is not simply thermal. It is clear that we are dealing here with genuine photodesorption. 

Properties Gray, amorphous powder (can be prepared as crystals). Sublimes at 1900C, d 3.44, bulk d 70-75 lb/cu ft depending on mesh, Mohs hardness 9+, thermal conductivity 10.83 Btu/in/sq ft/hr/F (400-2400F). Resistant to oxidation, various corrosive media, molten aluminum, zinc, lead, and tin soluble in hydrogen fluoride. 

Special considerations in conductive appheations, special conductive blacks must be employed fillers influence chemical degradation reactions in silox- zinc oxide was found to increase thermal resistance of... 

The nonstoichiometry of an oxide strongly depends on the presence of alio-valent impurities and dopants affect the number of thermal defects in non-stoichiometric oxides and their electrical conductivity because the solutes have a valence other than the atoms they replace. This is illustrated in the case of lithium and chromium doping of nickel and zinc oxides in equilibrium with gaseous oxygen. 

Fluoroelastomers are blended with fluorosilicones and other high-temperature polymers to meet engine compartment environments and cost/performance balance. Fine-particle sihca increases hardness, red iron oxide improves heat resistance, and zinc oxide improves thermal conductivity. Hardness ranges from about Shore 35 A to 70 A. Fluorosilicones are resistant to nonpolar and nominally polar solvents, diesel and jet fuel, and gasoline, but not to solvents such as ketones and esters. 

Thermal grease uses a hydrocarbon oil or silicone as a base and is filled with a thermally conductive material such as aluminum oxide or zinc oxide. The typical thermal grease used in production applications has a thermal conductivity of approximately 1.0 W/m K. The typical thickness is 0.001-0.003 in. Newer thermal greases have thermal conductivities as high as 16 W/m K [15,24]. 

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