Note

Feasibility study on the determination of accurate temperature values for the ICTA certified reference materials: potassium chromate

${\mathrm{Ca}}_2{\mathrm{Fe}}_2{\mathrm{O}}_5$ shows a first-order phase transition from space group Pnma to a modulated structure described in superspace group $\mathit{Imma}(00\gamma )s00$. The transition starts at 947 K during heating and the reverse transition in cooling conditions starts at 923 K, as determined from differential thermal analysis. In-situ high-temperature single-crystal X-ray diffraction experiments revealed a phase coexistence over a range of ca. 25 K. Data collected in this range show a good fit to a combined model of the two simultaneously occurring phases, which was refined using the software Jana2006. In-situ high-temperature transmission electron microscopy (selected area diffraction, dark field and high resolution images) was utilized to prove the existence of Pnma and $\mathit{Imma}(00\gamma )s00$ domains within the temperature range of the phase coexistence. Remaining planar defects were observed within the Pnma phase at temperatures below the phase transition. From the analysis of high-resolution electron micrographs, these defects were proven to be antiphase boundaries with a displacement vector of $\mathbf{R}=\frac{1}{2}[111]$.

The heat capacity of nickel ferrite was measured as a function of temperature from 50 to 1200 °C using a differential scanning calorimeter. A thermal anomaly was observed at 584.9 °C, the expected Curie temperature, T_C. The observed behavior was interpreted by recognizing the sum of three contributions: (1) lattice (vibrational), (2) a spin wave (magnetic) component and (3) a λ-transition (antiferromagnetic–paramagnetic transition) at the Curie temperature. The first was modeled using vibrational frequencies derived from an experimentally-based IR absorption spectrum, while the second was modeled using a spin wave analysis that provided a T^3/2 dependency in the low-temperature limit, but incorporated an exchange interaction between cation spins in the octahedral and tetrahedral sites at elevated temperatures, as first suggested by Grimes [15]. The λ-transition was fitted to an Inden-type model which consisted of two truncated power law series in dimensionless temperature (T/T_C). Exponential equality (m=n=7) was observed below and above T_C, indicating symmetry about the Curie temperature. Application of the methodology to existing heat capacity data for other transition metal ferrites (AFe₂O₄, A=Fe, Co) revealed nearly the same exponential equality, i.e., m=n=5.

This chapter describes calibration and standardization in disc. Particular emphasis is placed on calibration schemes that lead to true thermodynamic properties, rather than arbitrary quantities that may not even be reproducible between two different types of instrument. Quantitative data can only be obtained after calibration, under conditions that approach those of the unknown as closely as possible, using materials with well-defined properties. The calibration of differential scanning calorimeters is not a difficult procedure. A range of materials is available for temperatures from about 250 to 700 K, but the lower and (especially) upper temperature ranges are less well served. The differential scanning calorimeters (DSC) signal is a differential temperature or power as a function of time or temperature (abscissa). The aim of any calibration procedure is to define the temperature scale for a given set of experimental conditions and to fix the ordinate so that it refers to known heat flow rates, either directly or in combination with the abscissa, as an area that corresponds to a known quantity of heat.

The quantitative performance of differential scanning calorimeters is reviewed. Temperature calibration is discussed in terms of an isothermal correction plus a contribution from thermal lag, this can be derived from individual curves and is valid in both, heating and cooling. It is emphasised that baselines that are drawn to thermal events, such as melting and transition phenomena, must have thermodynamic significance and a general procedure is suggested. When this is used, a power compensation calorimeter calibrated for heat-capacity work can reproduce heats of fusion and transition for a diverse range of materials to better than 1%.