THE SOLID-SOLID PHASE TRANSITION OF LITHIUM SULFATE FOR USE IN SELF-CALIBRATING THERMOCOUPLES

摘要

The most common fixed-point cells make use of the order of a kg of pure metal undergoing a liquid-solid phase transition. They are generally implemented using the techniques recommended for the ITS-90. Substances which do not have the high thermal conductivity and thermal diffusivity of metals, or which having a powder-like structure undergo a solid-solid transition, are expected to perform poorly using these techniques. Such is the case of lithium sulfate (Li_2SO_4), commercially available in pure form (>99.99 %) as a white powder with different crystallite-size distributions (typically from 0.5 μm to 400 μm). At about 578°C and associated to an enthalpy change of 26 kJ/mol this substance is subject to a reversible first-order phase transition from monoclinic to FCC crystal-structure. When used in the form of a regular fixed-point cell, the phase transition leads to a poor transition plateau with a range of several K. On the other hand, used as a miniature fixed-point cell, it provides a thermometric reference point with a 2σ reproducibility of about +-40 mK. The miniature fixed-point cells described in this paper are cylindrical capsules made of graphite (10 mm long, 3 mm diameter) containing about 20 mg of Li_2SO_4. Miniatures cells of this type, with very pure metals (In, Sn, Zn, etc.), are currently being used at INTI to approximate the ITS-90, and are also used to make self-calibrating thermocouples for laboratory and industrial applications. Construction details, associated equipment, measuring techniques, and typical results are discussed.