Methods of Thermal Analysis / Differential Thermal Analysis (DTA)
Returning to my tasks in steem after two months absent (without publishing scientific content) but nevertheless always awaiting the SteemSTEM community reading and supporting fantastic content, I would like to resume my series of thermal analysis and its different methods to characterize materials.
A brief summary of the above...
I mentioned that heat can cause very significant changes in the physical properties of a material, due of course to the variation in temperature which is what this group of techniques is all about. Some of these changes can be the atoms within the crystalline network, crystalline structure, changes in their initial chemical composition, melting point, in turn also some materials can have the particularity of solidifying or oxidizing, all this due to temperature changes.
Therefore, we can say that thermal analysis refers to the physical and chemical changes that a certain sample may undergo when subjected to a certain temperature, i.e. when the sample is heated or cooled with special equipment that gradually controls the temperature supplied.
Through this type of thermal analysis we can obtain values about certain properties of the material that can change at the moment of being submitted to an analysis by a type of method or specific technique; this could be its weight or the compressibility module, difference between the properties of a sample when these are submitted to changes in its temperature.
The quality control of a product is very important for a company and the thermal analysis offers you a very specific study to be able to obtain viable results with respect to the quality of a product. For example, if a company wants to manufacture a product with X material, first before the product goes on sale to the consumer must go through a very thorough quality control, in order to create the right product and without failures. For example, if we buy a reclining or swivel chair and this in a few days breaks for no reason, or at the time we want to spin its bearings are broken and detached, this means that the material with which this product was manufactured is deficient quality or simply did not perform a proper quality control. The error comes mainly in the design of course, but this is due to the poor processing of the materials with which the product was manufactured, is for the importance of determining whether a material has the right properties for use. Here is the importance of selecting the right materials and thus optimizing the production processes necessary for their manufacture.
Some examples of application are the identification of polymers by their transition temperature, melting and crystallization temperatures, chemical composition analysis and decomposition behavior as. Other processes in the determination of mechanical parameters such as the coefficient of thermal expansion or its modulus.
The purpose of this article is to show you a group of techniques that are used for thermal analysis, including thermogravimetry (TA), which was explained earlier in a delivery, you can see by entering here the Differential Thermal Analysis (DTA) of which I will speak next, the Differential Scanning Calorimetry (DSC), among others, which can also be coupled with each other and form a technique as a whole.
Let's go into matter and talk about one of the best known techniques mainly for determining the melting point and transitions phases in materials, which are very important when it comes to synthesize in my case a semiconductor compound. In this case I am going to show you a small introduction on the differential thermal analysis and perhaps later on to be able to share material on the analysis of thermographs that is a little more complex and the content can be much more extensive.
Differential Thermal Analysis
It is a very important and powerful technique for material scientists. Through this method it is possible to determine reactions and different phase transitions.
In this method we measure the temperature difference during the constant heating rate between the sample to be analyzed and a totally inert material that serves as a reference.
There is a great variety of equipment dedicated to the analysis by means of this method, some more sophisticated than others but that present a basic principle that all equipment must contain independently of its design or how sophisticated it is the same.
DTA system developed by @iamphysical
Let's describe the next experimental configuration as follows:
It is a relatively simple system that must contain any DTA apparatus to carry out the differential thermal analysis of any sample, first of all it must be made up of a kind of metallic block totally covered or we could also call it "furnace", this must be totally covered and not present contact with the external environment, in the same way it must not undergo any transformation in the range of temperatures that is going to be studied at the moment of the sweeping of the sample. Inside the oven there should be two capsules that are generally made of silver (Ag), in which two samples should be placed, in one the one to be analysed and in the other capsule the reference material. These capsules are placed side by side without any contact (see figure) on top of a ceramic block, which serves as a heat absorber.
An important point to emphasize is that the reference material must be inert, it must not undergo changes or undergo transformation in any temperature range. In both capsules a separate thermocouple must be connected, both are connected outside the oven by means of some cables, this union allows the recording of the difference of temperature between both capsules during the heating and at the same time registers the difference with respect to the time.
DTA experimental setup
This connection tends to present a null voltage when the sample does not react with the effect of the supplied temperature, on all heat, then a straight line register is manifested. But the total sweep depends basically on the configuration that we make with the measuring equipment. On the surface of both capsules the temperature is restricted, i.e. no heat is detected so the heat flow is completely out of the sample. All this brings as a consequence a result that is derived from the baseline thermogram that may cause a lack of reliable results at the time of the final study of the thermal effect.
What do we get from a differential thermal analysis?
It actually helps a lot to complement studies of different materials. In my case, the importance that DTA has arises from the need to carry out a more exhaustive study on the compound that we are characterizing, for example, to characterize structurally the normal semiconductors and the substitutional alloys of the systems of compounds with vacancies, for which a complementary technique known as Differential Thermal Analysis (DTA) is used, it is first necessary to carry out previous studies such as X-ray diffraction. In this study we can mainly determine the crystalline structure of a semiconductor material, then we can find its network parameters and obtain a diffraction pattern that is being like the "fingerprint" of the material that allows us to verify the presence or absence of secondary phases in a material. In general the result of the presence or not of these phases is made mainly to compounds of 2 or more chemical elements, because when synthesizing several elements with different or equal chemical proportion can bring about changes during their synthesis, that is to say the mixture of the elements supplied can be found in greater or smaller concentration in an interval of the diffraction pattern.
An example is the following capture of an image obtained from a crystallographic analysis program. At the top we have a new semiconductor compound, i.e. the material we want to analyze that has no record. And at the bottom is the diffraction pattern of another material of the same family of elements but with a different proportion that is recorded in the data of the program or library of SEARCH MATCH.
Difference patterns of the Search Match crystallographic program
In this case I proceed to compare both diffraction patterns and observe in detail each diffraction peak and verify if there is a similarity and if they are in the same measurement interval. If after observing in detail I find a peak in my new element that is not in the reference pattern can assume that this latent appearance of a secondary phase in my new material, this would be a new discovery.
It is here where the complementary technique of Differential Thermal Analysis (DTA) is used in the ingots of different normal semiconductors and in compounds with cation voids, which allows us to confirm the appearance of secondary phases through the phase diagram for semiconductor compounds.
The Nb–Sn phase diagram. CC BY 4.0
In a future publication I will explain in more detail the study of the phase diagram by means of the DTA.
Some applications
As I just mentioned, differential thermal analysis gives you a unique record of a "fingerprint" material that serves as identification of the material for life, ie every time we want to consult about X material and see what is its melting point or phase states at a certain temperature, we can look for the record of the phase diagram of the material and thus be able to consult what is necessary for further studies.
DTA is widely used in pharmaceuticals, in the manufacture of medicines, both solid and liquid.
Its use is also essential in the food industry when it comes to analyzing each ingredient or material that contains a food and create a unique food, because each has its phase and to make a correct mixture without errors, we must know each of them.
DTA is also very important for building engineers, or industries that process building materials such as cement, blocks, clay, etc. The same is true for the mining industry, where materials are extracted that are then potentially used in the construction industry.
Similarly in the oil industry for which it is necessary to know the phases of the hydrocarbon and then use the crude oil in different applications of derivatives thereof.
Another important fact that I found on the web is that it is used a lot in the archaeological area to identify samples of different fossils, the DTA curves play a very important role in this study because using DTA you can get phase diagram lines of liquids and solids.
In general, DTA determines phase changes, crystallization of materials, melting points at certain temperatures and sublimation of materials.
Cover image source:
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Video credits @gtg
Understanding the phase diagrams of different materials is crucial for many fields, especially when one start thinking about building an experiment in the best working conditions.
That's absolutely true and for us in the semiconductor area we should have a small notebook in our shirt pocket where it contains the phase diagram of each compound :P If you don't ask @iamphysical :D
There is also the phase diagram of QCD that one tries to understand for a while. ;)
It's a fundamental piece for everyone. Thank you very much for reading my content
My pleasure! You know that I read a lot around here ;)
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