Curie temperature standards for thermogravimetry: the effect of magnetic field strength and comparison with melting point standards using Ni and Pb

Abstract

Thermogravimetric (TG) runs were made with a Ni sample over a wide range of heating and cooling rates using an electromagnet at three different field strengths. The extrapolated onsets and endpoints of the weight change induced by the magnetic field gradient were obtained from both TG and the differential curves (DTG). The onset upon heating and endpoint on cooling are very clearly dependent upon magnetic field strength and hence unsuitable for temperature calibration using this method. The endpoint upon heating and onset upon cooling, however, showed only a slight dependence upon the cube root of the magnetic field strength. Observed values of T c showed a linear dependence upon heating or cooling rates which extrapolated to a common point at zero rate. The value of T c derived from extrapolation to zero rate and then zero field is 339.15°C, which is 16°C below the literature value. Simultaneous measurements of the melting point ( T m) of Pb and the T m of Ni using a different furnace/thermocouple/magnet combination indicated a value of T m = 311.8°C and T c = 343.1°C. The value of T m is 15.7°C below the value from the International Temperature Scale. The value of T c is uncorrected to zero magnetic field strength, which would probably lower it several degrees. It would appear that both methods of temperature calibration are suitable for TG and the choice is best determined by the convenience with the particular apparatus involved and the temperature range desired.