Zinc oxide densification

Magnesium oxide is always blended with the zinc oxide prior to ignition. Magnesium oxide promotes densification of the zinc oxide, preserves its whiteness and renders the sintered powder easier to pulverize (Crowell, 1929). The sintered mixed oxide has been shown to contain zinc oxide and a solid solution of zinc oxide in magnesium oxide (Zhuravlev, Volfson Sheveleva, 1950). Specific surface area is reduced compared with that of pure zinc oxide and cements prepared from the mixed oxides are stronger (Crowell, 1929 Zhuravlev, Volfson Sheveleva, 1950). 

As in the case of MgO, the powder is precalcined at temperatures of 1000-1350°C to reduce the surface area of the particles [22]. The surface area is reduced by partial elimination of porosity of individual grains and by grain growth resulting from consolidation of amorphous content. The presence of MgO and silica in zinc oxide promotes densification by forming a solid solution of ZnO with these oxides [7,23]. In addition to this densification, compounds of lower solubility formed by the solid solutions, such as zinc silicates, are likely to reduce the overall solubility of zinc oxide. 

Zinc oxide has a fairly high vapor pressure for commonly-used sintering temperatures above half the melting point, so that coarsening due to vapor transport can reduce the densification rate. Discuss how the changes in the particle size, applied pressure, and temperature described in Problem 8.10 will influence the rate of vapor transport. 

It is important when choosing the additives to consider what effect they will have on the properties of the sintered body or whether they can be totally removed by oxidation at high temperatures. For instance, when making a piece of zirconium tin titanate, a dielectric resonator material whose densification is promoted by the addition of zinc oxide, it is sensible to consider adding a lubricant such as zinc stearate but not one such as calcium stearate which could have adverse effects on the dielectric losses of the sintered body. Also it would be acceptable to use an ammonium based deflocculant, e.g. Dispex A40, but not a sodium based one, e.g. Dispex N40. In the first instance it will be possible to remove all of the dispersant at a suitably high temperature but in the second sodium ions will not be removed by burning off. The sodium ions would probably increase the losses of the dielectric resonators. 

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