Key factors and choices affecting differential thermal analysis of instrumentation - News - Global IC Trade Starts Here.

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Differential thermal analysis is simple to operate, but in practice it is often found that the same sample is measured on different instruments, or different people are measuring on the same instrument, and the resulting differential thermal curve results are different. The peak temperature, shape, area, and peak size will vary. The main reason is that heat is caused by many factors and the heat transfer is 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 chemical and physical changes of the sample. Therefore, it is necessary to select an appropriate atmosphere and pressure according to the nature of the sample, and some samples are easily oxidized, and an inert gas such as N2 or Ne can be introduced.

2. The effect and choice of heating rate

The heating rate not only affects the position of the peak temperature, but also affects the size of the peak area. Generally, at a faster heating rate, the peak area becomes larger and the peak becomes sharper. However, the rapid rate of temperature rise causes the sample to decompose out of equilibrium conditions to a large extent, and thus the baseline is easily drifted. More importantly, the two adjacent peaks overlap and the resolution decreases. The slower temperature rise rate, the lower baseline drift, the system close to the equilibrium condition, the wide and shallow peaks are obtained, and the adjacent two peaks can be better separated, so the resolution is high. However, the long measurement time requires high sensitivity of the instrument. In general, it is preferable to select 8 degrees·min-1 to 12 degrees·min-1.

3. Pretreatment and dosage of samples

The large amount of sample is easy to make the adjacent two peaks overlap, which reduces the resolution. Generally reduce the amount 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 destroy the crystallinity of the sample. For samples that are easily decomposed to produce gas, the particles should be larger. The particles, loading and tightness of the reference should be consistent with the sample to reduce baseline drift.

4. Paper speed selection

Under the same experimental conditions, the same sample has a fast paper feed 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 an appropriate paper feed speed according to different samples. 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 reference. Commercially available differential thermal meters, the above factors have been fixed, but the self-assembled differential thermal instrument should consider these factors.

5. Selection of reference materials

To achieve a smooth baseline, the choice of reference is important. The reference material is required to undergo no change during heating or cooling. The specific heat, thermal conductivity and particle size of the reference material are as consistent or similar as possible to the sample throughout the heating process.

Alpha-alumina Al2O3) or calcined magnesia (MgO) or quartz sand is commonly used as a reference. If the analysis sample is metal, metal 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 there is gas generated during the heating of the sample, the gas can be reduced in a large amount to avoid the test. The sample rushed out. The selected diluent cannot have any chemical reaction or catalytic reaction with the sample. The commonly used diluents are SiC, iron powder, Fe2O3, glass beads Al2O and the like.