|Thermal analysis & calorimetry FAQs|
D2. I am willing to use DSC for measuring the specific heat capacity of solid samples. Do I need to use a particular standard?
D3. May I use any type of inert gas for sweeping of the DSC detector?
D4. When is it necessary to use a tight closed crucible in a DSC test?
D5. I wish to investigate a liquid sample above 100°C with the DSC technique. Are there some recommendations to follow?
D6. How can I increase my DSC signal?
D7. What is an exothermic process?
D8. What is an endothermic process?
D9. Which endothermic processes can I investigate with thermal analysis techniques?
D10. Which exothermic processes can I investigate with thermal analysis techniques?
D11. Why is the test method so important for measurement of Tg?
D12. What types of thermal analyser are available for measuring the glass transition temperature of a sample?
D13. I am seeing a baseline drift in my DSC equipment and so cannot provide a good interpretation of my DSC curve. What can I do?
D14. I have two non-separated peaks on my DSC curve. Is there a way to separate them?
D15. I need to calibrate the mass variation obtained with my TGA equipment. What types of standard are available to do this?
It is recommended to use pure metal standards (99.999% pure) for the temperature and heat calibration of DSC and DTA. According to the manufacturers of thermal analysis equipment, one or more standards are needed. For selecting the standards and the providers, see the "Reference Materials" section on this website. Inorganic materials can also be used but with caution.
Yes, for such a measurement sapphire has to be used as a standard. (See the "Reference materials" section on this website.) The Cp determination using DSC requires a run of three different tests as follows: 1- two pans empty, 2- one pan with the sample, the reference pan remaining empty, 3- one pan with the standard, the reference pan remaining empty. The pans need to have the same weight, and an identical heating rate has to be used for the three runs. See the "Standards" section for Cp determination.
Yes, it is possible to use argon, nitrogen or helium. However, depending on your DSC detector type (especially heat flux plate DSC), the thermal conductivity of the inert gas will affect the calibration of your DSC. For example, helium is more thermally conductive than argon. It will improve the DSC peak resolution, but it will decrease the DSC sensitivity. So it is recommended to calibrate your DSC with the inert gas you will be selecting.
Most DSC tests are run in a closed crucible so as to guarantee good thermal contact between the sample and detector. In some cases, the gas pressure (due to water vapour, gas from decomposition or sublimation) will increase inside the crucible and will cause leakage or even blowing of the cover. To prevent these problems and guarantee a good measurement, tightly closed crucibles are required.
For such an investigation, it is recommended to use tight crucibles that can resist the internal pressure generated by the liquid vapour. If the crucible is not tight, there is a risk of leaking, thus distorting the DSC signal. When a tight crucible is used, it is necessary to first check that it can resist a high internal vapour pressure. If the pressure is too high, there is a risk of bursting the crucible and damaging the DSC detector. I am measuring a very small thermal effect.
To increase the DSC signal increase the sample mass (though limited by the crucible size) and/or increase the heating rate (though at the risk of also increasing the drift of the baseline). If unsuccessful or not possible with the current set-up, use a calorimetric technique adapted for investigation of large mass samples.
An exothermic process is one that gives off heat. This heat is transferred to the surroundings.
An endothermic process is one that absorbs heat from the surroundings.D9. Which endothermic processes can I investigate with thermal analysis techniques?
It is necessary to distinguish between endothermic processes that occur with or without a change in mass. Where there is mass change, DTA, DSC and TGA are the preferred approach. This would apply to processes that occur with gas emission, such as dehydration, dehydroxylation, evaporation, sublimation and pyrolysis. For endothermic processes without mass change - and this would include melting, glass transition, first- and second order phase transitions and denaturation - DTA and DSC are the preferred techniques.
It is necessary to distinguish between exothermic processes that occur with or without a change in mass. Where there is mass change, DTA, DSC and TGA are the preferred approach. This would apply to processes that occur with gas emission or absorption, such as in oxidation, combustion, decomposition, hydrogenation, reduction and polycondensation. For exothermic processes without mass change - and this would include curing, crystallisation and phase transitions - DTA and DSC are the preferred techniques.
D11. Why is the test method so important for measurement of Tg?
The Glass Transition Temperature, Tg, can be measured by thermal analysis instruments. These can be DSC (Differential Scanning Calorimetry), TMA (Thermal Mechanical Analysis) or DMA (Dynamic Mechanical Analysis). Because each method focuses on different properties affected by this transition, different Tg values are obtained. In general, the Tg values obtained with the different methods are in increasing order: TMA < DSC < DMA. Also important is the heating rate used for these tests: the faster the heating rate, the higher the Tg. It is therefore difficult to compare Tg values of products from different suppliers if the test method and the heating rate are not mentioned or described properly.
Different thermal analysis techniques are available for determining the glass transition temperature, namely: DSC (raID variation of the heat capacity of the sample), TMA (raID variation of the coefficient of dilatation of the sample), DMA (variation of the viscoelastic properties of the sample).
First it is necessary to know the cause of the baseline drift, which can be due to the instrument itself, the sample, a displacement of the crucible on the detector or leakage from the crucible. As there can be many different experimental causes, the first test to run is a blank test and that means using two empty crucibles and running a test under the same conditions as when the sample was present. This will give you the instrument baseline without a sample. If this baseline is significantly different from the one you obtained previously, then you need to repeat the first test and see if the problem is due to the sample or the crucible. If the blank baseline is similar to your first test, then you need to check the detector of your instrument.
A better separation of the peaks can be obtained by any of the following means: reducing the mass of the sample (thus decreasing the thermal gradient within it), by pressing the cover down to the bottom of the crucible (to obtain a better thermal contact), by reducing the heating rate (thus increasing the time for separation), or by using a conductive sweeping gas (for example, helium is more conductive than nitrogen or argon).
There are no standards available for TGA calibration. It is only possible to calibrate the balance using calibrated masses. However, it is possible to use some materials with known mass variations, such as copper sulphate pentahydrate or calcium oxalate, to check for good operation of the thermobalance. The latter materials are generally agreed as working standards but do not use the first mass loss, corresponding to the first hydrate.
Temperature calibration of the TGA requires the use of a metal that is 99.99% pure and magnetic. A magnet has to be adjusted on the TGA furnace. If the TGA is combined with a DTA or a DSC detector, see the DTA or DSC temperature calibration.