Test for Thermal Conductivity of Whiteware Ceramics ASTM C408
ASTM C408 is used to determine the Thermal Conductivity of Whiteware Ceramics at the temperature range of 100 to 300°F (40 to 150°C). The thermal conductivity of a material is a measure of its ability to conduct heat. The values are stated in SI units.
ASTM C408 provides information about the ability of ceramics to conduct and dissipate heat. This data is used in manufacturing, quality control, and selection of ceramic materials.
Thermal shock is a mechanical load due to a rapid change of temperature. At some point, this stress can exceed the tensile strength of the material, cracking the ceramic. ASTM C408 provides information about the resistance of a ceramic sample to thermal shock. At some point, this stress can exceed the tensile strength of the material, causing a crack to form.
The plane faces of the specimen are metalized with a platinum alloy or silver metallic glaze so that the sample can be easily soft soldered. The metalized specimen is then soft soldered between the upper and lower thermodes. After soldering, the thermodes are polished to reduce their emissivity. After the test specimen has been satisfactorily soldered to the thermodes, the upper thermode is heated by the heater element and a temperature gradient is established.
A steady-state condition is attained when the heat input equals the heat flowing down the thermode. After steady-state conditions have been reached, the emf values of the four thermocouples are recorded. Three such sets of readings are taken at each steady-state point and the three calculated thermal conductivity values must agree with each other within 1 % to be acceptable.
The ceramic test specimens used in the thermal conductivity instrument should be cylindrical, formed by normal ceramic procedures such as slip-casting, dry-pressing, or extrusion.
The thermal conductivity is calculated as follows:
Ks = (KCuACutCuXs)AstsXCu
Ks = Thermal conductivity of the test specimen KCu = Thermal conductivity of the copper thermode ACu = Cross-sectional area of the thermode tCu = Temperature difference in °F (or °C) between thermocouple Nos. 1 and 2, Xs = Length of the specimen As = Coss-sectional area of the specimen ts = Temperature difference in °F (or °C) between thermocouple Nos. 2 and 3, XCu = Distance (or centimeters) between thermocouple Nos. 1 and 2.