This brief tutorial and somewhat personal review discusses general experimental aspects and several examples of how thermomagnetometry
has proven valuable in a variety of studies. The three areas of focus concern 1) determination of the magnetic transition
temperature to characterize the material or to serve subsequently as a potential temperature standard for thermogravimetry;
2) detection of magnetic reactants, intermediates, or products during reactions and 3) following the reaction rates for such
reactions. A broad range of materials is examined, metals, ceramics, catalysts, minerals, and inorganic complexes.
The goal of this research is to prepare a series of alloys having sharp, reproducible magnetic transitions for calibrating
temperature in thermogravimetry from the magnetic transition temperature of pure cobalt (1121°C) to below room temperature.
Alloys in the Ni-Co and Ni-Cu systems were prepared by the thermal decomposition of coprecipitated oxalates in argon. The
alloys were subsequently annealed under 5% hydrogen.
Magnetic transition temperatures were measured using simultaneous thermomagnetometry/differential thermal analysis. Transition
temperatures were corrected using well known meltingpoint standards. Magnetic transition temperatures along with precision
are reported as a function of composition.
Kinetics of the oxidation of magnetite (Fe3O4) to hematite (a-Fe2O3) are studied in air using simultaneous TG/DSC. The mechanism is complex and the differences between the kinetic conclusions
and Arrhenius parameters based on either TG or DSC are discussed. As in our previous work on CaCO3 , the determination of a satisfactory baseline for the DSC results adds considerable uncertainty to those kinetic results.
Consequently the calculations based on the TG data are considered superior. Solid state reactivity varies from one source
of material to another and the results are compared for two different commercial samples of magnetite, both presumably prepared
by wet chemical methods. These materials are much more reactive than the material studied previously , which had been coarsened
and refined at high temperatures. In that earlier study, the metastable spinel, g-Fe2O3, was formed as an intermediate in the oxidation to the final stable form, a-Fe2O3. The exothermic reaction of the gamma to alpha form of the product during the oxidation process destroys the direct comparison
between the TG and DSC results, since the former only detects the change in mass of the sample and not the crystallographic
transformation. The TG results, however, represent the true oxidation process without superposition of the structural aspects.
The thermal decomposition of CaCO3was studied using simultaneous TG/DSC for two different ranges of particle size from the same source and a physical mixture
of each. The difference in kinetic behavior was as expected qualitatively, but significantly different quantitatively. In
addition, the mixture did not behave as a simple combination of its end members. These discrepancies are attributed to the
problems associated with mass and thermal transport. The TG data again proved easier to fit than the DSC data.
Authors:P. K. Gallagher, E. M. Gyorgy and W. R. Jones
The reduction of NiO by H2 was followed by conventional thermogravimetry and a new evolved gas analysis approach which follows the course of the reaction by measuring the H2O content of the gas stream. Excellent correspondence is observed between the two techniques for simultaneous measurements. Heating rates between 0.5 and 10° min−1 shift the temperature of the reaction as does changing the surface area of the NiO. These shifts are discussed in terms of the Neel temperature (TN) of NiO and the thermal history of the sample. No correlation between reaction rate andTN is observed under dynamic conditions. Preheating the sample in vacuum at 130° has a marked effect on shape of the DTG and EGA curves.
Authors:E. Charsley, C. Earnest, P. Gallagher and M. Richardson
The ICTAC Committee on Standardisation has formed a Task Group to investigate the suitability of the ICTAC Certified Reference
Materials for DTA, covering the temperature range 450°–1100°C, for accurate temperature calibration purposes and to evaluate
their potential as enthalpy calibrants for DTA and DSC equipment. This paper reports the results of preliminary round-robin
studies on barium carbonate and strontium carbonate, using a dual-point calibration method based on the melting points of
aluminium and gold. In addition the fusion of ICTAC potassium sulphate has been investigated as a possible calibration transition.
Authors:P. K. Gallagher, J. P. Sanders, P. M. Woodward and I. N. Lokuhewa
Summary Simultaneous TG/DSC and high temperature X-ray diffraction studies were performed on the system SrCO3-Fe2O3 in an atmosphere of CO2 from room temperature to 1300°C. SrCO3 decomposes and reacts simultaneously with the Fe2O3 beginning around 850°C. A transient iron rich phase is formed initially, which soon diminishes to yield a mixture of perovskite SrFeO3-x and the iron rich phase Sr4Fe6O13. Upon cooling the perovskite phase predominantly orders into the brownmillerite structure Sr2Fe2O5. There does not appear to be a Hedvall effect associated with the first order phase transformation in SrCO3 at 927°C.