Large decalibrations in Ta-sheathed, Al 2O 3-insulated Pt/Rh thermocouple assemblies during heating to 1330 °C

Abstract

Tantalum-sheathed, Al 2O 3-insulated Pt 90Rh 10/Pt (type S) and Pt 70-Rh 30 /Pt 94Rh 6 (type B) thermocouple assemblies were unstable when heated above 1100 °C. Decalibrations of −152 °C were measured when one lot of type S assemblies were heated to 1330 °C. The type B assemblies were more stable and decalibrated only −11 °C during heating to 1330 °C. The total time above 1100 °C was 2.5 h. After the assemblies had been heated to 1330 °C, measurements with a moving temperature gradient technique showed that the Seebeck coefficient, which produces the e.m.f. of a thermocouple, changed −86% and −4.5% in the type S and type B assemblies, respectively. Metallographie and ion microprobe mass analyses showed that reactions had occurred within the thermocouple assemblies at the Pt-Al 2O 3 and Pt/Rh-Al 2 O 3 interfaces, resulting in solution of up to 27 at.% Al in the thermocouple wires. Aluminum concentrations were less in the Pt/Rh thermocouple wires than in the Pt thermocouple wires and were inversely related to the Rh content of the wires. Magnesium and Si, in concentrations about 1000 times lower than Al, were found in the thermocouple wires. The origin of these impurities was the MgO and SiO 2 impurities in the Al 2O 3 insulation. Aluminum was not found in the Ta sheath; however, an O concentration gradient did exist in the Ta sheath, with the higher O concentration at the Ta-Al 2O 3 interface. The work of others suggests that Pt can reduce Al 2O 3 in low O environments, forming the compound AlPt 3. The thermochemistry of the reaction 3 Pt + 1 2 Al 2 O 3 + 6 10 Ta → AlPt 3 + 3 10 Ta 2 O 5 is discussed to support the proposed reaction mechanism. It is concluded that type S and type B thermocouples insulated with Al 2O 3 and sheathed in Ta are inherently unstable above 1100 °C. References are cited to extend this conclusion to Pt/Rh thermocouples insulated with other oxides, such as MgO, and sheathed with other O-reactive metals, such as Nb.