NPL routinely calibrates radiation thermometers to 3000°C. To achieve this the output of the instrument under test is compared to a reference thermometer viewing the same source. However, because the defining fixed-point of ITS-90 used to calibrate the reference thermometer (the freezing point of gold) is some 2000°C below the upper limit, the uncertainty due to scale realisation is rather large. To reduce the uncertainty it is desirable to have higher temperature fixed-points near the upper limit of scale realisation. This paper describes the current state of research at NPL in developing such fixed-points. These are based on metal-carbon binary alloys that have an invariant eutectic reaction, and therefore are expected, under equilibrium conditions, to have well-defined melts and freezes. In addition, since the eutectic reaction takes place at a unique temperature, independent of the carbon concentration within the sample, fixed-points based on such alloys can be used with graphite crucibles. At NPL blackbody fixed-point cavities using metal-carbon eutectic alloys have been constructed, and the melt and freeze behaviour of a number of metal-carbon eutectic alloys assessed. Initial findings indicate that the crucibles are robust, and the melts and freezes are repeatable at a level of less than 100 mK. The fixed-points are stable to within the uncertainty of the measuring instrument, and show promise as a means of reducing uncertainties in scale realisation. The construction methods, tests and results obtained with these sources, together with their implications, are discussed.
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