Heating experiments, temperature calibration methods

An important prerequisite for the reproducibility of NMR experiments at elevated temperatures is the accurate determination of the temperature inside the sample volume of the probe. Often, there is a systematic error in the temperature displayed by the controller unit. Therefore, methods for the temperature calibration of MAS NMR probes under various working conditions, such as various heating rates, sample spinning frequencies, etc., are required for experiments at elevated temperatures. 

Temperature calibration Temperature calibration is of particular concern because the temperature measured by the thermocouple which is not in contact with the sample could cause a thermal lag of few degrees. Unless an experiment is carried out isother-mally on a very small sample, this lag will vary with temperature, progress of the reactions, furnace atmosphere, heating rate and geometry of the system. Hence, it is very important to calibrate the TGA under conditions which would be reproduced in actual experiments. The most appropriate methods of temperature calibration of TGA recommended by ICTAC are discussed in the following. 

Experiments were performed in tlie SIMULAR calorimeter using the power compensation method of calorimetry (note that it can also be used in the heat flow mode). In this case, the jacket temperature was held at conditions, which always maintain a temperature difference ( 20°C) below the reactor solution. A calibration heater was used to... 

A marginal but very important application of the drop calorimetric method is that it also allows enthalpies of vaporization or sublimation [162,169] to be determined with very small samples. The procedure is similar to that described for the calibration with iodine—which indeed is a sublimation experiment. Other methods to determine vaporization or sublimation enthalpies using heat flow calorimeters have been described [170-172], Although they may provide more accurate data, the drop method is often preferred due to the simplicity of the experimental procedure and to the inexpensive additional hardware required. The drop method can also be used to measure heat capacities of solids or liquids above ambient temperature [1,173],... 

Analogously to the dynamic method, the energy equivalent of the calorimeter, k.Q, can be obtained by performing calibration experiments in the isothermal mode of operation, using electrically generated heat or the fusion of substances with well-known A us//. Recommendations for the calibration of the temperature scale of DSC instruments for isothermal operation have also been published [254,270]. 

Use of the a-relaxation of amorphous polymers as a standard A simple method to calibrate the temperature is to compare the loss peaks related to the glass-transition dynamics (a-relaxation) of pure amorphous polymers, obtained in a well calibrated apparatus, with the data obtained in the paratus in which the temperature is not calibrated. The calibration tests in the uncorrected equipment must be done under similar conditions (e.g. heating rate) to the ones that are to be performed in future experiments using that equipment. This method was tested using polycarbonate films [30], where a global comparison between the different strategies proposed in this section is also discussed. 

It is seen that the calibration constant disappears, which assumes that it is constant over the experimental conditions. The calculation is carried out using dedicated software. In some circumstances the crucible used for the sample may have to be different from that used for the calibrant. This means that a correction will be required to take into account the difference between the heat capacity of the two crucibles - readily calculated with sufficient accuracy. Measurements can be made at a series of temperatures but are meaningful only within the quasi-steady-state region of the experiment. The specific heat capacity of sapphire has been listed by ASTM in connection with the standard test method E 1269 (1999) for determining specific heat capacity by differential scanning calorimetry. 

This phenomenon is clearly observed as well in MC as in DSC experiments. Differences in the actual values can be due to the differences in the methods and calibration. However, the measured in MC experiments show the lowest values whereas the decrease of I m proceeds faster with increasing magnetite content/ZFe304 than in thawing at ambient. This is in accordance with the hypothesis of heat transfer by temperature gradient at the interphase between the particle surface and the surrounding. 

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