Key factors affecting differential thermal analysis of instruments and their selection

Differential thermal analysis is simple to operate, 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, and the differential thermal curve results are different. The highest temperature, shape, area and peak size of the peak will change to a certain extent. The main reason is that heat is related to many factors, and the heat transfer situation is relatively complicated. Generally speaking, the first is the instrument and the second 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

Atmosphere and pressure can affect the equilibrium temperature and peak shape of the sample's chemical reaction and physical change. Therefore, the appropriate atmosphere and pressure must be selected according to the nature of the sample. Some samples are easily oxidized and can be introduced with inert gases such as N2 and Ne.

2. Influence and selection of heating rate

The heating rate not only affects the position 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, a fast heating rate will cause the sample decomposition to deviate from the equilibrium condition, which can easily cause the baseline to drift. More importantly, it may cause the two adjacent peaks to overlap and the resolution to decrease. A slower heating rate will result in a smaller baseline drift, which will bring the system closer to the equilibrium condition, resulting in a wide and shallow peak, and can also better separate the two adjacent peaks, thus achieving a high resolution. However, the measurement time is long, and the instrument needs to be highly sensitive. Generally, 8 degrees·min-1 to 12 degrees·min-1 is appropriate.

3. Sample pretreatment and dosage

A large amount of sample can easily cause two adjacent peaks to overlap, reducing the resolution. Generally, the amount should be reduced as much as possible, up to milligrams. The particle size of the sample is about 100-200 mesh. Small particles can improve thermal conductivity, but too fine particles may destroy the crystallinity of the sample. For samples that are easy to decompose and produce gas, the particles should be larger. The particles, filling conditions and compactness of the reference material should be consistent with those of the sample to reduce the drift of the baseline.

4. Paper speed selection

Under the same experimental conditions, for the same sample, if the paper feed speed is fast, the peak area is large, but the peak shape is flat and the error is small; if the paper feed rate is slow, the peak area is small. Therefore, it is necessary to select the appropriate paper feed speed according to different samples. The selection of different conditions will affect the differential thermal curve. In addition to the above, there are many other 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 reference. For commercially available differential thermal instruments, the above factors are fixed, but these factors must be considered for self-assembled differential thermal instruments.

5. Selection of reference materials

To obtain a stable baseline, the selection of the reference material is very important. It is required that the reference material does not change during the heating or cooling process, and the specific heat, thermal conductivity and particle size of the reference material are as consistent or close as possible to those of the sample during the entire heating process.

α-alumina (Al2O3) or calcined magnesium oxide (MgO) or quartz sand is often used as a reference. If the sample to be analyzed is metal, metal nickel powder can also be used as a reference. If the thermal properties of the sample and the reference are very different, the sample can be diluted to solve the problem, mainly to reduce the intensity of the reaction; if gas is generated during the heating of the sample, the large amount of gas can be reduced to avoid the sample from rushing out. The selected diluent cannot have any chemical reaction or catalytic reaction with the sample. Commonly used diluents include SIC, iron powder, Fe2O3, glass beads, Al2O, etc.