An Investigation Of The Thermoelectric Inhomogeneity Of Dilute Magnetic Cu Fe Thermocouple Wire

The inhomogeneity of thermoelectrode,which can seriously affect measurement accuracy, is the primary parameter to represent the stability and reproducibility of thermocouple and also one of the main measures to evaluate the quality of thermocouples.The dilute magnetic copper-iron alloy is the only material that can replace the dilute magnetic gold-iron alloy which is very expensive,as cryogenic thermocouple whose working temperature range is between 4 Kelvin and 273 Kelvin.Therefore there are important practical value and economic benefits to popularize the use of dilute magnetic copper-iron thermocouple by improving the performance and quality of this material.The research was supported by National Natural Science Fund(Grant No. 51276070),an applied fundamental research on thermoelectric force of dilute magnatiel copper-iron alloys at low temperature. This paper studies the inhomogeneity of thermoelements made of dilute magnetic CuFe alloy. A set of test equippment is designed and constructed for the experiment based on the double-medium method. The thermoelectric potential of inhomogeneity of long CuFe( Cu+0.13% at Fe)enamel-covered wires?bare wires,and such of nichrome wires are measured.The influence of the test equipment itself on the experiment is evaluated first. Then the results are analyzed with statistical methods. Through these methods, coupled with microstructure studies,it is found the inhomogeneity of wires made of different materials consisits of two parts,one is the slowly changing part in the nearby points,the other one is the outburst points.The NiCr wires are the most homogeneous in the three wires for its mature manufacturing process.The CuFe bare wires present weak periodicity,this is because of nonuniform annealing temperature after the alloy was drewn out into a long wire and the wire was wound into a roll.The CuFe enamel-covered wires' inhomogeneity is weaker than the The CuFe bare wire,and the periodicity has disappeared.This shows the insulating process and the low temperature annealing after insulating process has reduced the inhomogeneity of bare wire. The slowly changing part of inhomogeneity,which can be reduced as a whole by low temperature annealing,is because of morphological differences between grains in the alloy microstructure.The outburst points in the test results is because of the vacancies in the alloy microstructure,whose influence can be weakened but not eliminated by the low temperature annealing.With this analysis method,it is posssible to reveal the characters and the source of inhomogeneity of thermoelements,thus providing a way to improve the processing technic of CuFe thermocouple and to promote the use of the dilute magnetic copper-iron thermocouple,even to creat a system of quality control for the manufacture of thermocouples. This system can also be used for other thermocouples.