The operation of differential thermal analysis is simple, but in practical work, it is often found that the same sample is measured on different instruments, or different samples are measured on the same instrument, and the results of the differential thermal curve of industrial electrical appliances are different. The high temperature, shape, area, and size of the peaks will all change. The primary reason for this is that heat is related to many factors, and the heat transfer conditions are relatively complex. Generally speaking, one is the instrument and the other is the sample. Although there are many influencing factors, as long as certain conditions are strictly controlled, good reproducibility can still be obtained.
(1) Selection of atmosphere and pressure
Atmosphere and pressure can affect the equilibrium temperature and peak shape of the chemical reaction and physical changes in the sample. Therefore, it is necessary to select the appropriate atmosphere and pressure according to the nature of the sample, and some samples are easy to oxidize and can be introduced into inert gases such as N2 and Ne.
(2) The influence and selection of the heating rate
The warming rate not only affects the orientation of the peak temperature, but also affects the size of the peak area. However, the fast heating rate makes the sample differentiation violate the equilibrium condition to a large extent, so it is easy to drift the baseline. More importantly, it can lead to two adjacent peaks stacking and reduced resolution. The slower warming rate and small baseline drift make the system close to the equilibrium condition, and the wide and shallow peaks are obtained, which can also make the two adjacent peaks better separated, so the resolution is high. However, the measurement time requires the high sensitivity of the instrument. Under normal conditions, it is advisable to choose 8 degrees · min-1 - 12 degrees · min-1.
(3) Pretreatment and dosage of samples
The large amount of sample is large, which makes it easy to stack two adjacent peaks, which reduces the resolution. In general, reduce the dosage as much as possible,* as much as possible. The particle size of the sample is around 100 mesh - 200 mesh, and the small particles can improve the thermal conductivity conditions, but too fine may damage the crystallinity of the sample. For samples that are prone to differentiation and gas, the particles should be larger. The particles, packing condition and compactness of the specific material should be the same as that of the specimen to reduce the drift of the baseline.
(4) Selection of references
To obtain a smooth baseline, the selection of references is important. It is required that the reference material does not change in any way during the heating or cooling process, and the specific heat, thermal conductivity and particle size of the reference material are as common or similar to the sample as possible during the whole heating process. α-aluminum oxide Al2O3) or calcined magnesium oxide (MgO) or quartz sand are commonly used as references. If the analysis sample is metal, nickel metal powder can also be used as a reference. If the thermal properties of the sample and the reference material are far apart, the sample can be diluted, and the first thing is to reduce the intensity of the reaction; If there is gas generation during the heating process of the specimen, it can reduce the large amount of gas and avoid the specimen from flushing out. The selected diluent can not have any chemical reaction or catalytic reaction with the sample, and the commonly used diluents are SiC, iron powder, Fe2O3, glass beads Al2O, etc.
(5) Selection of paper speed
Under the same test conditions, the same sample has a fast paper walking speed and a large peak area, but the shape of the peak is flat and the error is small. The paper feed rate is small, and the peak area is small. Therefore, it is necessary to select the appropriate paper feeding speed according to different samples. In addition to the above, there are many factors such as the profile, size and shape of the sample tube, the material of the thermocouple, and the orientation of the thermocouple inserted into the specimen and reference. The above elements have been fixed in the commercially available differential calorimeter, but some elements should be considered in the differential calorimeter assembled by yourself.
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