Вплив термоциклування на електрофізичні властивості термоелектродних матеріалів залізо-константан і стабільність термопари З-Кн

Abstract

Досліджено вплив циклічної зміни температури при нагріванні до 400…800 К і охолодженні зі швидкістю 104 К/с до кімнатної температури на електроопір, термо-ЕРС та мікроструктуру термоелектродних матеріалів залізо і константан, а також на стабільність показів термопари залізо-константан (ТЗ-Кн). Похибка вимірювання, викликана термоциклуванням, представлена залежно від вимірюваної температури і числа термоциклів. При температурах вище 500оС вона може перевищувати допустимі значення, що вимагає переградуювання термопари ТЗ-Кн.

Investigations concerning the improvement of the engineering units operation control, nuclear power plants in particular, have become of great importance especially after the repeated explosions in the reactor halls in Japan, which are likely to be caused by the decrease of the reactors hardware maintenance system control, thermoshock damages in the thermo-electrode wires in particular. In the paper the effect of cyclic temperature changes on electrical resistance, thermal electromotive force and microstructure of the thermocouple materials iron and constantan (TIC), as well as on the stability of thermocouple iron – constantan when heated to 400 ... 800 K and cooled at a rate 104 K/s to the room temperature has been investigated. It is found that thermal cycling (50 cycles) in an inert environment during cooling from temperatures (873 K) increases the constant electric resistance by 3,5 % and that of iron by 29 %. Structural changes of the thermocouple wire after thermoshocks in the air testify the formation of microcracks of thermal origin on the surface of electrodes and increase of grain size. While cycling in the argon only slight growth of grains and partial decomposition of austenite in the iron thermal electrodes was observed . The dependencies of the deviations of thermal electromotive force of the thermal electrodes on the number of cycles in the environment (argon, air) during calibration in the range of 293 ... 900 K are presented. According to the results an alloy constantan (-100 mV) is more sensitive to thermal cycling (heating in argon 50 cycles), for technically pure iron complex dependencies are observed: up to 15 cycles electronegative deviations that with the increase to 50 cycles are transformed into electropositive (+50 mV), are recorded. Thermal cycling of thermal electrodes in the air causes deviation of thermal electromotive force in 2 – 6 times higher, than after heating in argon. It is shown that thermal cycling (50 cycles, heating in argon) will cause the deviation of iron – constantan values in minus 2,6º C, similar number of cycles while heating in the air will change iron – constantan values relatively NSC in minus 6,5º C.