Using Pyrometric Cones
The most common problem encountered in initial attempts to make superconductors in a modestly equipped laboratory are problems due to inaccurate pyrometers (temperature indicators) on the laboratory furnace or kiln. We have found that it is not unusual for pyrometers, especially on pottery kilns, to read lower than the actual chamber temperature when operating in the 1000 degree Celsius range. A significant pyrometer error will cause you to use an actual firing temperature that may be high enough to melt the ceramic YCBO (yttrium-barium-copper-oxide) and cause it to stick to the alumina dish.
Note: All temperatures listed in these instructions are in degrees Celsius. If your temperature indicator is in Fahrenheit degrees, be sure to make the appropriate conversions.
We highly recommend obtaining from a pottery shop self-supporting pyrometric cone number 06. Be sure to get cone 06 and not cone 6. These cones begin to melt in the 991 to 999 degree Celsius temperature range. The cones provide an accurate calibration reference for pyrometers on kilns and furnaces used for making ceramic superconductors.
One of these cones may be placed upright in an empty chamber to check the calibration of the pyrometer before firing the superconducting ceramic. Place the cone in the same position in the chamber where you intend to place the superconducting material. Leave the furnace set for a pyrometer reading of 975 degrees Celsius for several hours. If the cone melts appreciably (so that the cone forms an angle of less than 45 degrees with the bottom of the furnace), then the pyrometer is giving you a falsely low indication of true chamber temperature. (We have yet to encounter a pyrometer with significant error in the other direction in this temperature range, but you may want to also check to make sure that the cone does begin to melt at an indicated temperature of 1000 degrees Celsius.)
(If you are in too much of a hurry to make the "empty chamber" test, and if you are reasonably confident that your pyrometer has an accuracy of at least 5 percent, the cone may be placed in the chamber with the powder during the first "calcination" firing. This firing should be done at 925 to 950 degrees Celsius, but a pyrometer error at this point will usually result only in a hard-to-regrind mass. A melt is unlikely on the first firing unless the pyrometer has a considerable error.)
If the cone does melt while your pyrometer reads 950 degrees Celsius or less, calibrate your pyrometer before continuing. It is not necessary to buy a new pyrometer. Just know exactly how far off your pyrometer is (when it is in this temperature range).
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