What Is Differential Thermal Analysis?

Differential Thermal Analysis (DTA), referred to as differential thermal analysis, is a technique for measuring the temperature difference and temperature relationship between a test substance and a reference substance under a program-controlled temperature. At a certain temperature during the heating or cooling process, the substance is often accompanied by physical and chemical changes of endothermic or exothermic effects, such as physical changes such as crystal form conversion, boiling, sublimation, evaporation, and melting, as well as redox, decomposition, dehydration and decomposition Away from chemical changes. There are other physical changes such as glass transitions. Although no thermal effect occurs, some physical properties such as hot melt will also change. At this time, the substance does not necessarily change, but the temperature must change. Differential thermal analysis is a technique established on the basis of such properties as matter. Differential thermal analysis is an accurate measurement and recording of the various physical-chemical phenomena mentioned above during the heating process. Therefore, it is widely used to measure the characteristic temperature and heat absorbed or emitted during the thermal reaction of materials, and it is also widely used in the research and production of various departments such as geology, metallurgy, petroleum, building materials, and chemical industry.

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 are measured on the same instrument, and the differential thermal curve results obtained 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 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 ^[2] .

Choice of Atmosphere and Pressure for Differential Thermal Analysis

Atmosphere and pressure can affect the equilibrium temperature and peak shape of the sample's chemical reactions and physical changes. Therefore, an appropriate atmosphere and pressure must be selected according to the nature of the sample. Some samples are susceptible to oxidation and can be filled with inert gases such as N ₂ and Ne.

Influence and selection of heating rate in differential thermal analysis

The heating rate not only affects the position of the peak temperature, but also the size of the peak area. Generally, the peak area becomes larger and the peak becomes sharper at a faster heating rate. However, the rapid temperature rise rate causes the sample decomposition to deviate from the equilibrium condition to a large extent, so it is easy to drift the baseline. The main reason is that two adjacent peaks overlap and the resolution is reduced. Slower heating rate, small baseline drift, make the system close to equilibrium conditions, get broad and shallow peaks, can also better separate the two adjacent peaks, so the resolution is high. However, the measurement time is long, and the sensitivity of the instrument is high. Generally, it is appropriate to select 8 / min 12 / min.

Pretreatment and dosage of differential thermal analysis samples

The large amount of sample can easily overlap two adjacent peaks and reduce 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 damage the crystallinity of the sample. For samples that easily decompose to produce gas, the particles should be larger. The particle, packing and tightness of the reference should be consistent with the sample to reduce baseline drift.

Selection of Reference Materials for Differential Thermal Analysis

To obtain a smooth baseline, the choice of reference is 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 similar to the sample as possible during the entire heating process. Commonly used-Al ₂ O ₃ or calcined magnesium oxide (MgO) or quartz sand as 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 from each other, it can be solved by diluting the sample, mainly to reduce the severity of the reaction. If gas is generated during the sample heating process, a large amount of gas can be reduced to avoid the test. Run out. The selected diluent must not have any chemical or catalytic reaction with the sample. Commonly used diluents are SiC, iron powder, Fe ₂ O ₃ , glass beads Al ₂ O, etc.

Choice of Differential Thermal Analysis Paper Speed

Under the same experimental conditions, the same sample, such as fast paper feed speed, has a large peak area, but the peak shape is flat, and the error is small; the paper feed speed is small, and the peak area is small. Therefore, it is necessary to select an appropriate paper feeding 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 in the sample and reference. Commercially available differential calorimeters, the above factors have been fixed, but the self-assembled differential calorimeters need to consider these factors ^[3] .