Calibration enthalpy

Few materials exist that have accurately known enthalpies and that are also available with traceable certification. Many textbooks will quote heat of fusion values for materials, and it is often possible to obtain high-purity quantities of these materials. Yet it is very difficult to obtain high-purity materials that have traceable and certified enthalpy values. Again, LGCs provide indium that is certified with respect to both enthalpy and temperature. This material can be used, in a single experiment, to 

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Proper calibration of the DSC instruments is crucial. The basis of the enthalpy calibration is generally the enthalpy of fusion of a standard material [21,22], but electrical calibration is an alternative. A resistor is placed in or attached to the calorimeter cell and heat peaks are produced by electrical means just before and after a comparable effect caused by the sample. The different heat transfer conditions during calibration and measurement put limits on the improvement. DSCs are usually limited to temperatures from liquid nitrogen to 873 K, but recent instrumentation with maximum temperatures close to 1800 K is now commercially available. The accuracy of these instruments depends heavily on the instrumentation, on the calibration procedures, on the type of measurements to be performed, on the temperature regime and on the... 

A change in the flow rate of the gas used to purge the DSC can have several effects. First, it is possible that it will change the temperature and enthalpy calibration. The magnitude of this variation will vary from one type of DSC design to another as some instruments preheat the purge gas prior to its entering the DSC cell. Second, for experiments where a volatile substance is evolved from the sample when it is heated, the DSC peak shape will be affected by the speed at which the volatile substance is removed. 

Full temperature range calibration Enthalpy calibration Baseline calibration Check flow meters and gas bottles Back-up data and methods Monthly... 

DSC measurements were performed on a TA Instruments 2190 DSC with temperature and enthalpy calibrations performed using an indium reference. Experiments were performed under a nitrogen atmosphere with a flow rate of 50ml/min. Extruded granules of nanocomposites were heated at 2°C/min to 250°C, held isothermally for 5mins, then cooled to room temperature at 2°C/min. All samples were heated twice, first to examine the properties post extrusion and secondly to examine the preferred crystal structures with slow cooling. 

Temperature modulated DSC requires the same baseline, temperature and enthalpy calibration as conventional DSC (see Sections 3.1 and 3.2). Measuring the heat capacity is a necessary First step in calculating the reversing and nonreversing heat flows, so a heat capacity calibration must be performed for TMDSC. Calibration parameters such as sample vessel type, purge gas, heating rate, modulation amplitude and period must be identical to those used in subsequent experiments. 

Reference material sets which are certified by the International Confederation for Thermal Analysis and Calorimetry (ICTAC) are available through the US National Institute of Standards and Testing (NIST), and are listed in Appendix 2.2. High-purity metals and organic compounds including polymers have been certified. If the standard reference material must be dispensed with a syringe into the sample vessel (for example cyclohexane), care must be taken to ensure that only one droplet is formed in the sample vessel. Multiple transition peaks will be observed if there is more than one droplet present. The transition temperatures listed in Appendix 2.2 are the statistical mean values of measurements made in a number of laboratories and institutes. The ICTAC reference materials are certified for temperature calibration only and not for enthalpy calibration. The reference temperatures in Appendix 2.1 should be used if very accurate calibration of the instrument is required. In order to determine the heat capacity Cp ) of a sample, sapphire (a-alumina, AI2 O3) is used as a standard reference material. The Cp of... 

Calibration of TA instrumentation and development of standards for calibration continue to be administered by ICTAC in conjunction with ASTM. The standardization Committee of ICTAC has certified a range of materials for temperature calibration of TA systems, and in addition, standards for calibration of mass (known as Class M Standards ) are available for this purpose. A range of certified reference materials are available for enthalpy calibration in DSC. Temperature calibration for TMA and dynamic mechanical analysis (DMA) is effected by using disks of pure metals (silver, aluminum, and tin) separated by alumina disks. Load or force calibration for DMA is a complex process involving the use of calibrated weights. Temperature calibration for DETA is effected by measuring the melting transition of benzoic acid and dielectric calibration is... 

Table 1.5 Melting points and enthalpies enthalpy calibration reference materials [53]... 

Temperature and enthalpy calibration of DSC curve. Calibration is carried out using the DSC curve determined under the same conditions as in step 2 for high-purity indium (purity >99.99%), and the leading slope of the indium is used to determine the instantaneous sample temperature during melting taking into account the heat-resistance correction and the heat of fusion of indium (28.41 J g ) used to calibrate the enthalpy of the DSC curve (mjcm" ). 

Fig. 4. Typical enthalpy calibration curve for DSC. Salt (Solidi-solid2 standard) Metal (melting standard) (14).

Today s calorimeters have a furnace calibration routine that is separate from the temperature and enthalpy calibration routines. Follow the manu-... 

A general overview of temperature calibration and enthalpy calibration (heat of fusion) is best if DSC operational basics are understood before calibration is begun. 

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