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

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Differential thermal analysis (DTA) is relatively straightforward to perform, but in practice, it's common to observe variations in the resulting DTA curves when the same sample is analyzed on different instruments or by different operators using the same instrument. These differences can manifest in the peak temperature, shape, area, and intensity of the peaks. The main causes are the complex nature of heat generation and transfer during the process. Typically, the primary factors are the instrument itself and the sample preparation. However, with proper control over certain conditions, good reproducibility can still be achieved.

1. Selection of Atmosphere and Pressure

The atmosphere and pressure can significantly influence the equilibrium temperature and the shape of the peaks in the DTA curve. It’s essential to choose an appropriate atmosphere based on the sample's properties. For example, if a sample is prone to oxidation, an inert gas like nitrogen (N₂) or neon (Ne) should be used to prevent unwanted reactions.

2. Heating Rate Effects and Selection

The heating rate has a major impact on both the position of the peak temperature and the peak area. A faster heating rate generally leads to a larger peak area and a sharper peak. However, this can cause the sample to decompose out of equilibrium, leading to baseline drift. Additionally, adjacent peaks may overlap, reducing resolution. On the other hand, a slower heating rate results in a more stable baseline, closer to equilibrium conditions, and wider, shallower peaks that are easier to distinguish. This improves resolution, although it increases measurement time and requires a more sensitive instrument. A typical range for heating rate is between 8°C/min and 12°C/min.

3. Sample Pretreatment and Dosage

Using a large amount of sample can lead to overlapping peaks, which reduces resolution. It’s best to use as little as possible, ideally in the milligram range. The particle size of the sample should be around 100–200 mesh. Smaller particles improve thermal conductivity, but overly fine particles may damage the sample's crystallinity. For samples that release gas upon decomposition, larger particles are preferred. The reference material should have similar particle size, loading, and compactness to the sample to minimize baseline drift.

4. Paper Speed Selection

The paper speed affects the appearance of the DTA curve. A faster paper speed results in a larger peak area but a flatter peak shape, reducing error. A slower paper speed produces a smaller peak area but a more defined peak. Therefore, the paper speed should be selected based on the specific characteristics of the sample. Other factors, such as the material and dimensions of the sample crucible, the thermocouple type, and its placement, also play a role. Commercial DTA instruments often fix these variables, but for self-assembled systems, careful attention must be given to these details.

5. Reference Material Selection

Choosing the right reference material is crucial for obtaining a smooth baseline. The reference should remain unchanged during heating and cooling. Its specific heat, thermal conductivity, and particle size should closely match those of the sample throughout the entire heating process. Common reference materials include alpha-alumina (Al₂O₃), calcined magnesia (MgO), and quartz sand. If the sample is metallic, nickel powder can also serve as a reference. If there's a significant difference in thermal properties between the sample and the reference, dilution of the sample may be necessary to reduce the reaction intensity. If gas is released during heating, using a diluent can help prevent the sample from being expelled. The diluent should not react chemically or catalyze the sample. Common diluents include silicon carbide (SiC), iron powder, Fe₂O₃, and glass beads made of Al₂O₃.