Experimental factors in the measurement of the max

2022-07-22
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Experimental factors affecting thermal analysis and measurement

experimental factors affecting thermal analysis and measurement

heating rate heating rate has a great impact on the results of thermal analysis experiments. In general, it can be summarized as follows: (1) for a certain reaction (such as thermal decomposition reaction) of the sample represented by TG, DTA (or DSC) curve, increasing the heating rate usually increases the reaction temperature Ti, peak temperature TP and termination temperature te

1. Heating rate

the heating rate has a great influence on the results of thermal analysis experiments. Generally speaking, it can be summarized as follows:

(1) for a certain reaction (such as thermal decomposition reaction) of the sample represented by TG, DTA (or DSC) curves, increasing the heating rate usually increases the reaction temperature Ti, peak temperature TP and termination temperature te. Because of the rapid temperature rise, the reaction will enter a higher temperature before it can be carried out, resulting in reaction delay. For example, when FeCO3 is heated in nitrogen to lose CO2, when the heating rate increases from 1 ℃ Min-1 increased to 20 ℃ At min-1, the Ti method is as follows: from 400 ℃ to 480 ℃, te from 500 ℃ to 610 ℃

(2) rapid temperature rise is to push the reaction to the high temperature zone and make it proceed at a faster speed, that is, it not only increases the peak temperature TP of DTA curve, but also narrows the peak amplitude, presenting a needle tip shape

(3) for multi-stage reactions, slow heating is conducive to the separation of stage reactions, so that DTA curve presents separated multiple peaks, and TG curve presents a platform instead of the turning point at the time of rapid heating

(4) the peak area of DTA curve tends to decrease slightly with the decrease of heating rate, but generally there is little difference. For example, for the dehydration endothermic reaction of kaolin at about 600 ℃, when the heating rate range is 5 ~ 20 ℃ When min-1, the maximum difference of peak area is within 3%

(5) the heating rate affects the temperature distribution in each part of the sample. For example, DSC measurement of 1mm thick low-density polyethylene shows that the heating rate is 2 ℃ When min-1, the temperature difference inside and outside the sample is not large; And 80 ℃ At min-1, the temperature difference can reach more than 10 ℃

for crystalline polymers, recrystallization may occur during the melting process of slow temperature rise, while rapid temperature rise is easy to produce E. automatic optimization of graph curve scale auto scale overheating, which are two contradictory processes. Therefore, an appropriate heating rate should be selected during the experiment. If there are no special requirements and instructions, 10 ℃ is usually selected Min-1 or 5 ° C min-1。

thermal analysis experiments often need to change the atmosphere, so as to distinguish the physical and chemical attribution of the thermal effect of the thermal analysis curve. If the thermal analysis curve measured in air shows an exothermic peak, the determination in inert atmosphere can be divided into several cases according to different reactions: if the size of the exothermic peak remains unchanged, it is a crystallization or solidification reaction; If it is endothermic effect, it is decomposition combustion reaction; If there is no peak or a very small exothermic peak, it is a reaction such as metal oxidation. The difference between thermal cracking and thermal oxidative cracking of organic polymers can be observed

for the reaction of forming gas products, if the gas products are not discharged in time, or the partial pressure of gas products in the atmosphere is increased by other means, the reaction will move towards high temperature. For example, water vapor makes calcium sulfate caso4 The dehydration reaction of 2H2O was inhibited. Compared with the results measured in air, the reaction temperature moved to the high temperature region, showing a double peak and step-by-step dehydration process

the gas in the atmosphere has good thermal conductivity, which is conducive to providing more sufficient energy to the system and improving the decomposition reaction rate. For example, the thermal conductivity of three inert gases, argon, nitrogen and helium, increases with temperature. Therefore, the thermal decomposition rate of CaCO3 is the fastest in helium, followed by nitrogen and argon

3. sample dosage and particle size

due to the heat absorption and release effects of the sample, a small number of samples are conducive to the diffusion of gas products (CW Journal) and the equilibrium of the temperature in the sample, reducing the temperature gradient and the deviation between the sample temperature and the linear temperature rise of the environment

generally speaking, the surface reaction is more or less affected by the particle size of the sample, which is more obvious than the chemical decomposition reaction; The phase transition is less affected by particle size. In order to facilitate mutual comparison, try to use samples with similar particle size, such as fine powder passing through a certain sieve hole. The sample quality not only affects the peak temperature and peak area of the thermal analysis curve, but also affects its morphological characteristics. Some substances have fundamental changes in their morphological characteristics due to the reduction of their mass, which makes it difficult to identify the morphological characteristics of substances by thermal analysis

in addition, the gap between the loose sample particles can make the thermal conductivity of the sample worse, and the smaller the particles, the closer the stack, the better the thermal conductivity. Regardless of the particle size of the sample, the stacking density is not easy to repeat, which will also affect the morphological characteristics of the TG curve

4. crucible material and shape

crucible materials are made of non-metallic alumina, quartz and other low heat conductive materials; Some are made of metal, aluminum, silver or platinum and other high heat conducting materials. Shapes include micro flat bottom, constant block, cup shape, gland, etc. The determination shall be selected according to the measurement purpose of DTA polyurethane elastic material for improving tactile sensation, DSC and TG and the properties of the sample. Generally, aluminum cup and ceramic cup are commonly used, and the same crucible shall be selected for determination

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