Calcium oxide thermal conductivity

A common problem in boilers is the occurrence of calcium oxide build-up on the heating elements. This is not a corrosion problem in itself, because it is caused by a chemical reaction in the water at high temperatures. However, a scale deposit present on a metal surface may cause corrosion under the deposit. This type of underdeposit corrosion can be aggravated when corrosive species such as sulfides and/or chlorides are present in the water. While scale deposits reduce the thermal conductivity of the steel, and thereby increase energy costs, corrosion of the heating element can lead to a catastrophic tubing failure, which requires costly repairs. 

Typical mineral fillers like calcium carbonate, talc, kaolin, mica, silica, and wollastonite all have thermal conductivities an order of magnitude higher than that of polymers. However, specialty fillers are used to achieve better thermal conductivities. Examples include alumina, beryllium oxide, boron nitride (cubic and hexagonal), graphite, carbon nanotubes, metals, and the best of all, diamond. 

Pure AIN powders are sintered with sintering agents, typically oxides of yttrium or calcium. These additions degrade the thermal conductivity. In addition, moisture and many chemicals attack pure AIN substrates. As protection against these elements, and to promote adhesion to thick-fihn inks, a thin oxide layer is usually grown on the surface. Consequently, the thermal conductivity of AIN can vary widely from its maximum value, reported to be 320 W/(m and is typically in the range of 170 to 200 W/(m K). B. B. Poole of... 

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