Lead oxide thermal conductivity

Finally, it is to be expected that the evaporation coefficient of a very stable compound, such as alumina, which has a large heat of sublimation resulting from the decomposition into the elements, will be low. Since the heat of evaporation must be drawn from the surface, in die case of a substance widr a low thermal conductivity such as an oxide, the resultant cooling of the surface may lead to a temperature gradient in and immediately below the surface. This will lower die evaporation rate compared to that which is calculated from the apparent, bulk, temperature of the evaporating sample as observed by optical pyromeuy, and thus lead to an apparently low free surface vaporization coefficient. This is probably die case in the evaporation of alumina in a vacuum. 

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%. 

Calcination can be carried out by placing the mixed powders in shallow saggers in a batch or continuous kiln. The saggers may need to be closed if any of the constituents are volatile, as is the case with lead oxide. The container surfaces in immediate contact with the powder must not react with it both to avoid contamination and to permit reuse of the sagger. The thermal conductivity of powdered materials is always low, so that a sufficiently uniform temperature can only be obtained through a depth of a few centimetres when the period at maximum temperature is, as is usual, only 1 or 2h. 

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. 

Compounds containing fillers are usually more sensitive to thermal decomposition due to the acceleration of thermo-oxidative reactions by a number of additives at elevated temperatures. Fillers could allow sintering compounds at lower temperatures due to an increase in the conductivity of the part. For example, a metal-filled PTFE compound (steel, lead, or bronze) has a significantly higher thermal conductivity than PTFE, which leads to rapid heating of the part. 

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