G.W.H. Höhne

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.

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.

Dynamic behaviour of power compensated differential scanning calorimeters. Part 1. DSC as a linear system

Abstract The power compensated differential scanning calorimetry (DSC) can be considered as a linear system to a first approximation. Thus the theory of linear response is applicable to describe the apparatus. An outline of this mathematical tool is given. It is shown how the apparatus and the sample and their thermal connection influence the output signal, and how to obtain the true heat flow from the measured curve with the aid of the Greens function of the apparatus and of the sample.

Chip-calorimeter for small samples

A method for measuring heat capacities of small samples using a chip-calorimeter and a heat pulse technique is described. The theoretical background to calculate the heat transport properties and the heat capacity of the sample from the pulse response function is given. Problems and potentials of the method are discussed. An example is given.

Dynamic behaviour of power compensated differential scanning calorimeters: Part 2. The signal flow

Abstract The dynamic behaviour of the measured signal of a power compensated DSC is studied in terms of linear response taking into account the measuring system, the sample, the reference, the control circuits, the heat transport coefficients and the heat losses. As a result it is shown that the apparatus (or Greens) function of the DSC (including sample and heat transfer) is different from that necessary to desmear the sample heat flow. The quality of the extracted Greens function is discussed for different measuring conditions.

Work, heat and stored energy in compressive plastic deformation of glassy polymers

Abstract For a number of organic glassy polymers with different chemical structures and immiscible blends, the mechanical work expended for deformation, the plastic deformation heat and the internal energy stored in deformed samples (stored energy of cold work) have been measured using different experimental techniques. A comparison of the measured quantities is made and suggestions concerning their nature are discussed. It is found that the fraction of expended mechanical work transformed into stored internal energy upon deformation is very high for all studied systems. This reflects the non-isostructural character of inelastic deformation in glassy polymers. A two-stage deformation mechanism is introduced and the experimental results are analyzed in a framework of the suggested mechanism.

High pressure DSC investigations on n-alkanes, n-alkane mixtures and polyethylene

Abstract A high pressure cell for a power-compensated DSC is presented, allowing measurements at pressures up to 500 MPa. With the aid of this apparatus, n -alkanes, binary n -alkane mixtures, and two linear polyethylenes have been investigated. The fusion temperature and the heat of fusion have been measured for different n -alkanes in the pressure region between normal pressure and up to 500 MPa. For five eutectic and non-eutectic binary mixtures, the liquidus and solidus temperatures were determined. It is possible to describe the mixing behaviour of n -alkanes with the one-parameter approximation derived by Porter. The increase of the Porter parameter with pressure implies decreasing miscibility at higher pressures. From the Porter parameter the critical point can be calculated. Thus, it can be seen that n -alkanes which build mixed crystals at normal pressure demix at higher pressures. From these results, it can be concluded that the same is true for polyethylene.

Dynamic behaviour of power-compensated differential scanning calorimeters. Part 4. The influence of changes in material properties *

Abstract The influence of sample material properties on the smearing of measured curves has been proved experimentally. As shown by model calculations, changes in material properties at the glass transition result in a change in Greens function for the liquid state compared to that for the glassy state. From experimental investigations, moreover, it follows that these effects are overlapped by influences of the change in the thermal contact between sample and pan. It was found that change in the thermal contact influences the determined Greens function more than changes in material properties do. The error in the desmeared curves, determined using a Greens function obtained via other materials properties, is within the limits of accuracy of the measurements. Therefore the Greens function estimated from the switch-on as well as from the switch-off behaviour can be used to desmear the whole measured curve, if the material properties do not change markedly, as is the case during the glass transition of polymer materials.

Round robin test on the kinetic evaluation of a complex solid state reaction from 13 European laboratories. Part 2. Kinetic DSC-analysis

Abstract In 1994 the decomposition of calciumoxalate monohydrate as a three-step reaction has been investigated by 13 labs using the TG-method. The authors working group performed objective kinetic evaluations of 144 TG data sets obtained from the same charge of substance in different laboratories. A non-linear optimization procedure was applied for different reaction models to perform single and overall optimizations. The results were analyzed critically on the basis of real model concepts for the course of the dehydration, the CO-split off and of the CO 2 -split off, respectively. The results allow an optimistic assessment for the application of kinetic procedures to solid state reactions with well-known chemical course investigated by TG.