The operation of differential thermal analysis is simple, but in actual work, it is often found that the same sample is measured on different instruments, or different people measure on the same instrument. The maximum temperature, shape, area and peak size of the peak will change to some extent. The main reason is that the heat is related to many factors and the heat transfer situation is more complicated. 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) Choice of atmosphere and pressure
The atmosphere and pressure can affect the equilibrium temperature and peak shape of the chemical reactions and physical changes of the sample. Therefore, it is necessary to select the appropriate atmosphere and pressure according to the nature of the sample. Some samples are easily oxidized and inert gases such as N2 and Ne can be introduced.
(2) Influence and selection of heating rate
The heating rate affects not only the location of the peak temperature, but also the size of the peak area. Generally speaking, the peak area becomes larger and the peak becomes sharper at a faster heating rate. However, the rapid heating rate makes the decomposition of the sample deviate from the equilibrium condition to a large extent, so the baseline drifts easily. More importantly, the two adjacent peaks may overlap and the resolution may decrease. The slower heating rate and the smaller baseline drift make the system close to equilibrium conditions and obtain broad and shallow peaks. It can also make the two adjacent peaks better separated, so the resolution is high. However, the long measurement time requires high sensitivity of the instrument. In general, it is appropriate to select 8 degrees Â· min-1 ï½ž 12 degrees Â· min-1.
(3) Sample pretreatment and dosage
The large amount of sample makes it easy to overlap two adjacent peaks, which reduces the resolution. Generally reduce the dosage as much as possible, up to milligrams. The particle size of the sample is about 100 mesh to 200 mesh. Small particles can improve the thermal conductivity, but too fine may damage the crystallinity of the sample. For samples that easily decompose to produce gas, the particles should be larger. The particles, packing and tightness of the reference substance should be consistent with the sample to reduce the drift of the baseline.
(4) Selection of reference substance
To obtain a stable baseline, the choice of reference is very important. It is required that the reference material does not change during heating or cooling, and the specific heat, thermal conductivity, and particle size of the reference material should be as close as possible to or similar to the sample as much as possible throughout the temperature increase process.
Commonly used Î±-alumina (Al2O3) or calcined magnesium oxide (MgO) or quartz sand as a reference. 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 are very different, it can be solved by diluting the sample, mainly to reduce the severity of the reaction; if gas is generated during the heating of the sample, a large amount of gas can be reduced to avoid the test Kind of rushed out. The selected diluent cannot have any chemical reaction or catalytic reaction with the sample. Commonly used diluents are SiC, iron powder, Fe2O3, glass beads Al2O, etc.
(5) Paper speed selection
Under the same experimental conditions, the same sample has a fast paper speed and a large peak area, but the peak shape is flat and the error is small; the paper speed is small and the peak area is small. Therefore, according to different samples to choose the appropriate paper speed. The choice of different conditions will affect the differential thermal curve. In addition to the above, there are many factors, such as the material, size and shape of the sample tube, the material of the thermocouple, and the position of the thermocouple inserted in the sample and the reference. All the above factors have been fixed for commercially available differential calorimeters, but these factors must be taken into account for the self-assembled differential calorimeters.
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