Affecting the key factors of differential thermal analysis of instruments and their selection

Differential thermal analysis is simple, but in practice, it is often found that the same sample is measured on different instruments, or different people measure on the same instrument. The resulting differential thermal curve results are different. The peak temperature, shape, area, and peak size all change. The main reason is that heat is related to many factors and the heat transfer conditions are more complicated. In general, one is the instrument and the other is the sample. Although there are many influencing factors, good reproducibility can still be achieved by strictly controlling certain conditions.

(1) Choice of atmosphere and pressure

Atmosphere and pressure can affect the equilibrium temperature and peak shape of the sample 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 inert gases such as N2 and Ne can be introduced.

(2) Effect and selection of heating rate

The heating rate not only affects the position of the peak temperature, but also affects the size of the peak area. In general, the peak area becomes larger and the peak becomes sharper at a faster heating rate. However, the rate of rapid heating increases the extent to which the sample decomposes away from the equilibrium conditions, making it easy to drift the baseline. More importantly, it may cause the adjacent two peaks to overlap and the resolution decreases. The slower rate of temperature rise, the smaller baseline drift, the closer the system to equilibrium conditions, the wider and shallower peaks, and the better separation of the adjacent two peaks, resulting in higher resolution. However, the long measurement time requires high sensitivity of the instrument. In general, 8 degrees min-1 to 12 degrees min-1 are suitable.

(3) Sample Pretreatment and Dosage

Large amount of sample, easy to overlap the adjacent two peaks, reducing the resolution. Generally reduce the dosage as much as possible up to milligrams. The particle size of the sample is between 100 mesh and 200 mesh. Small particles can improve the heat conduction conditions, but too fine may destroy the crystallinity of the sample. For samples that are easily decomposed to generate gas, the particles should be larger. The granules, loading conditions and tightness of the reference should be consistent with the sample to reduce baseline drift.

(4) The choice of reference

For a smooth baseline, the choice of reference is important. It is required that the reference material does not undergo any change during heating or cooling. The specific heat, thermal conductivity, and particle size of the reference material should be the same as or similar to the sample during the entire temperature increase process.

Commonly used α-alumina Al2O3) or calcined magnesia (MgO) or quartz sand are used as reference materials. If the analysis sample is metal, metallic nickel 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 to solve the problem, mainly to reduce the severity of the reaction; if gas is generated during the heating of the sample, it can reduce the amount of gas, so as to avoid the test Like out. The selected diluent can not have any chemical reaction or catalytic reaction with the sample. Commonly used diluents include SiC, iron powder, Fe2O3, and glass beads Al2O.

(5) Selection of paper speed

Under the same experimental conditions, the same sample, such as the speed of paper feeding, the area of ​​the peak is large, but the shape of the peak is flat, the error is small; the speed of paper feeding is small and the area of ​​the peak is small. Therefore, according to different samples to choose the appropriate speed of paper. The choice of different conditions will affect the differential heat curve, in addition to the above there are many factors, such as the sample tube material, size and shape, the thermocouple material and the location of the thermocouple inserted in the sample and reference. For a commercially available differential heat meter, all of the above factors have been fixed, but these factors must be taken into consideration when assembling the differential heat meter.