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 measurements 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 situation is 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 achieved.
(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 differentiation of the sample deviate from the equilibrium condition to a large extent, so it is easy to drift the baseline. More importantly, it may result in two adjacent peaks stacked on top of each other, resulting in a decrease in resolution. The slower warming rate and the small baseline drift make the system close to the equilibrium conditions, and obtain wide and shallow peaks, which can also make the two adjacent peaks better separated, so the resolution is high. However, at the time of measurement, the sensitivity of the instrument is high. 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. Generally, reduce the dosage as much as possible,* as much as milligrams. The particle size of the sample is about 100-200 mesh, and the small particles can improve the thermal conductivity conditions, but too fine may destroy the crystallinity of the sample. For samples that are prone to differentiation and gas, the particles should be larger. The particles, filling condition and compactness of the specific material should be consistent with the specimen to reduce the drift of the baseline.
(4) Selection of references
To achieve 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 may be consistent or similar to that of the sample 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 method of diluting the sample can be used, first of all, 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 experimental 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 walking 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 installed by yourself.
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