ASTM C408-88 (2020) pdf free. Standard Test Method for Thermal Conductivity of Whiteware Ceramics.
The ceramic test specimens used in the thermal conductivity instrument shall be accurately ground cylinders.Thespecimens may be formed using normal ceramic procedures ofslip-casting, dry-pressing, or extrusion.
Metallize the plane faces of the specimen to be evalu-ated with a fired-on platinum alloy,silver，or other metallicglaze that may be readily soft soldered. Then soft solder themetallized specimen into position between the upper and lowerthermodes by placing solder preforms between the specimenand thermode and applying heat to the thermode. Uponcompletion of this soldering operation, polish the thermodes toreduce their emissivity.
After the test specimen has been satisfactorily solderedto the thermodes, and positioned in the apparatus, close off thesystem and evacuate it to approximately 1 um of mercury.Thenapply heat to the upper thermode by means of the heaterelement and establish a temperature gradient along the copperand along the specimen.When the heat input equals the heatflowing down the thermode, steady-state conditions have beenattained. For practical purposes，when the temperature of thethermocouple nearest the heater element changes at some rate smaller than 0.05 °F (0.03°C)/min,steady-state is consideredattained.After steady-state conditions have been reached, readthe emf of the four thermocouples in the following order: 1,2,3,4,3,2, and 1,No. 1 thermocouple being the one nearest theheater element.Then convert the average emf for each ther-mocouple to temperature for use in the calculations.Take threesuch sets of readings at each steady-state point.The threecalculated thermal conductivity values must agree with eachother within 1 % to be acceptable.
The heating unit,shown in detail inFig. 1， shall consist of a 500-W nichrome element helicallysupported by ceramic insulators inside a 2-in. (50.8 mm)diameter brass cylinder. This brass cylinder shall be highlypolished to serve also as a reflector, since heat transmission tothe upper thermode is primarily by radiation. Line voltage tothe heater circuit shall be held constant to within 0.1 % bymeans of an electronic voltage regulator，and power input tothe heater shall be controlled by a variable transformer.
The thermodes to which the ceramic testspecimen is soldered shall be machined from electrolyticallydeposited pure copper to tolerances of 0.001 in.(0.03 mm)on diameters. Thermodes of diameters from 0.250 to 0.500 in.(6.50 to 12.70 mm) may be used, but in all cases the diametersof the specimen,and that portion of the upper and lowerthermodes incorporating the thermocouple shall be equal indiameter to within 0.001 in.(0.03 mm). The section of theupper thermode above the upper cooling jacket shall be coatedwith carbon black to increase its emissivity.ASTM C408 pdf download.