E. Gmelin

The caloric calibration of scanning calorimeters

Abstract The present recommendation of the GEFTA working group “Calibration of Scanning Calorimeters” allows a precise heat and heat flow rate calibration of scanning calorimeters, largely independent of instrumental, sample-related and experimental parameters. Electric energy, electric power, heats of transition and heat capacities of suitable calibration substances are used for calibration. The measuring method, measuring and evaluation procedure, calibration materials, significant influencing factors, sources of error and detailed examples are presented for these calibration methods. Besides specific problems of heat measurement (interpolation of the baseline for the peak area determination) and heat capacity measurement (interpolation between initial and final isotherms, determination of the true heating rate of the sample, thermal lag of the sample), general aspects (thermodynamic fundamentals, difference between heat and heat flow rate calibration factor, weighing procedure) are also discussed.

The temperature calibration of scanning calorimeters: Part 2. Calibration substances

The recommendation for temperature calibration consists of two parts. Part 1 (see section 2.3) presented a correct method for the calibration of scanning calorimeters (DSC) and of instruments for differential thermal analysis (DTA), irrespective of the instrument type. The present paper recommends calibration substances for the range 120-1350 K. Sections 2 and 3 indicate the documents to be taken into account and define the most important terms used. Section 4 consists of general requirements to be met by the calibration substances, a list of the substances and explanations with respect to their use. Basically, such materials have been selected as calibration substances which define fixed points of the International Temperature Scale of 1990 (ITS-go). Added to these are fixed point materials from the previously valid International Practical Temperature Scale of 1968 (IPTS-68, see References in Table 1). Moreover, two substances are recommended for temperature ranges for which no suitable fixed point material exists, and the respective uncertainty of measurement is stated.

Thermal boundary resistance of mechanical contacts between solids at sub-ambient temperatures

The experimental values of the thermal boundary resistance occurring at interfaces between two solids at sub-ambient temperatures are reviewed. New data are presented in the temperature range from 4 K to 300 K for the thermal resistance between different metals (Cu, stainless steel), interlayered by various cryogenic bonding agents (Apiezon-N, Cryocon grease, In and InGa), or mechanically connected (dry) contacts. Depending on the contact materials, the thermal conductance varies between and at room temperature, and decreases approximately linearly by one order of magnitude between 200 K and 20 K. Our experimental data agree well with the data reported in the literature for the temperature range below 4 K and measurements near room temperature.

Temperature, heat and heat flow rate calibration of differential scanning calorimeters

Abstract This contribution discusses basic problems and fundamental requirements for a universally applicable, unified and metrologically correct calibration of differential scanning calorimeters (DSC). The essential features of previously published recommendations of the German Society for Thermal Analysis (GEFTA) [see Hohne et al., Thermochim. Acta 160 (1990) 1–12; Cammenga et al., Thermochim. Acta 219 (1993) 333–343; Sarge et al., Thermochim. Acta 247 (1994) 129–168; J. Thermal Anal. 49 (1997) 1125–1134] developed for a correct and instrument-independent calibration of temperature, heat and heat flow rate of DSC, are outlined.

Magnetic, electric and thermal properties of La0.7Ca0.3Mn1-xFexO3 compounds

Magnetization, resistivity and specific heat of La0.7Ca0.3Mn1-xFexO3 (0.0 less than or equal to x less than or equal to 0.12) are investigated between 77 and 300 K. The substitution of Fe for Mn does not alter the lattice constants and structural symmetry of the parent compound significantly, but results in a reduction of the magnetization, an increase of the resistivity, decreases of the magnetic and metal-insulter transition temperatures. The substitution also introduces inhomogeneities in the compounds. The experimental results indicate that Fe ions exist in the form of trivalent ions in the compounds, acting as trapping centers and thus blocking the site-percolation path of the e(g) electrons

The heat capacity of MgCr2O4, FeCr2O4, and Cr2O3 at low temperatures and derived thermodynamic properties

Abstract The heat capacity of synthetic eskolaite, Cr2O3, and of the synthetic spinels magnesiochromite, MgCr2O4, and chromite, FeCr2O4 were measured from 1.5 K to 340 K. For MgCr2O4, a substantial magnetic contribution to the entropy is revealed by a sharp peak in the heat capacity curve at 12.55 ± 0.05 K, which indicates the transition to antiferromagnetic long-range order. Integration of the heat capacity curve yields a value of 118.3 ± 1.2 J/(mol·K) for the standard entropy at 298.15 K, which is in excellent agreement with that calculated from phase equilibria studies on the reaction MgCr2O4+ SiO2 = Cr2O3 + MgSiO3. The new calorimetric results for Cr2O3 indicate a standard entropy at 298.15 K of 82.8 ± 0.8 J/(mol·K). The measurements for FeCr2O4 show three distinct heat capacity anomalies, one of which (peaking at 36.5 ± 0.2 K) was missed by previous low temperature heat capacity measurements, which only extend down to 53 K. Integration of the heat capacity curve yields a value for the standard entropy at 298.15 K of 152.2 ± 3.0 J/(mol·K) for FeCrO4, some 6 J/ (mol·K) greater than the previous calorimetric value. These low-temperature heat capacity data were combined with high-temperature heat content measurements from the literature to derive heat capacity equations for all three phases to 1800 K. The resulting heat capacity equations were then used to extract revised recommended values of the standard enthalpies of formation and entropies of MgCr2O4 and Cr2O3 from phase equilibrium data. For FeCr2O4, the phase equilibrium data are of dubious accuracy, the enthalpy of formation is only approximate

Temperature, heat and heat flow rate calibration of scanning calorimeters in the cooling mode

Abstract The current publication continues the series of recommendations of the ‘Calibration’ Working Group of the German Society for Thermal Analysis (GEFTA) for temperature, heat and heat flow rate calibration of scanning calorimeters. It deals with calibration in the cooling mode. The procedures to be applied are essentially identical to those applied in the heating mode. Due to the general occurrence of supercooling for first-order phase transitions during cooling, liquid crystals and substances with higher-order phase transitions are recommended for temperature calibration. Substances with weak supercooling or substances for which the temperature dependence of the transformation enthalpy is known are recommended for heat calibration. The thermodynamic fundamentals relevant to the temperature dependence of phase transition enthalpies and phase transition temperatures are discussed. Detailed examples make it easy to follow the recommendations.

Metamagnetic transition and magnetocaloric effect in ErCo2

The magnetization MH(T) and the specific heat capacity cP,H(T) of the ErCo2 intermetallic compound were measured in the temperature range 5 - 100 K and in 0, 7 or 14 T applied field, respectively. A clear first-order phase transition is found at the magnetic ordering of the Er sublattice. While for order-disorder transitions in simple ferromagnets there is a good agreement between magnetocaloric performance predicted on the basis of magnetization measurements compared to calorimetric measurements, it is necessary to investigate whether the agreement is still present for materials with more complex transitions (e.g. order - order, metamagnetic, first order etc). From the magnetization data the magnetic entropy change at the transition was calculated using the Maxwell relations. From the cP,H(T) measurements both the magnetic entropy change and the adiabatic temperature change were calculated and compared to values obtained from MH(T) and to the values calculated by the usual approximative expressions. The agreement is less good than in the case of second-order phase transitions. The discrepancy is interpreted in terms of the theory of first-order/metamagnetic transitions showing that the boundary conditions used in the derivation of the approximative formulae for simple ferromagnetic materials are not appropriate for more complex transitions as in ErCo2.

Sub-micrometer thermal physics – An overview on SThM techniques

Abstract The historical development and present state of the art of scanning thermal microscopy (SThM) – temperature profiling, thermography and other measurements of thermal parameters – in the sub-μm range and the relevant thermal sensors are reviewed. The paper proposes a classification scheme for the various experimental arrangements of STM and AFM based SThMs. Limitations of the SThM technique and future prospects are briefly discussed.

Classical temperature-modulated calorimetry: A review

Abstract The historical, methodological and technical development and the applications of temperature-modulated calorimetry (also known as AC calorimetry) are reviewed over the last three decades, up to about 1992. Modulation calorimetry (MC) is compared with other calorimetric methods. Particular emphasis is given to temperature scanning and frequency-dependent heat capacity experiments. Recent advances in MC, in particular the combination of differential scanning calorimetry (DSC) and modulation technique - the temperature-modulated DSC (TMDSC) - is discussed in view of measured dynamic specific heat. The theoretical models worked out before 1990 are briefly described. The limiting boundary conditions for dynamic calorimetry are critically discussed. Some selections of experimental setups for MC, selected among about 200 citations given here, exemplify by their characteristic experimental parameters the broad variety of existing possibilities to carry out temperature-modulated calorimetric experiments.