 In this module we have been looking at several characterization techniques which are useful for materials characterization. One of the most prolific characterization methods which has found useful application in a variety of areas and mainly focusing on material chemistry is thermal analysis. It is also called as thermo analytical methods broadly because it has now become a family of convenient techniques merged together it is not a single technique, but it has included many related techniques to itself and just spread into a big field. Why I want to single out and give a separate lecture on thermal analysis is that there are more than three journals, international journals which are floated on just thermal analysis and those are nearly three decades old and still several fascinating studies are coming up. So, this is a very useful technique which has proved to prove very useful both to chemist, physicist and material scientist. I call this as a very convenient technique because it is upper double and most of the labs can have this instrument and the chemical processes that happens during heating of a sample can be actually mapped both qualitatively and quantitatively. Therefore, you this is both a qualitative measurement as well as qualitative measurement. So, what this thermal analysis means it is a measurement of changes in physical properties of a substance as a function of temperature when the sample is undergoing a controlled temperature program. So, you can actually do this variety of events that are happening you can try to trace using a isothermal heating also suppose I do not have a thermal analysis instrument the best thing that I can do is freeze out on a particular temperature and then try to map the changes that are happening, but we can also do a controlled temperature program which can take the sample from room temperature to a known temperature and whatever that happens is nothing but the property which you can map it in the y axis. So, in the x axis it is always temperature and in y axis it can be anything that is coming out of such a process and output can these days be mapped very conveniently using a computer therefore, you can analyze the data in a very sophisticated way. So, it is a fairly simple technique, but the amount of information that one can get if you can transcend our imagination. A lot of references are particular about this technique has been taken from this website therefore, I request you to note this there are also groups working in UK and especially virus.com which gives you useful information about it. Next it is another company which has brought out commercial thermo gravimetric instruments therefore, it is good to view all this. So, what properties are measured we can see here first regarding the property and the corresponding technique and the abbreviation that is used. So, it is useful to get used to it if there is only a change in the mass then you call this as thermo gravimetric usually called as TG or it is also called as TGA thermo gravimetric analysis. If it is to do with temperature then you call this as differential thermal analysis and this is nothing but DTA you look at the thermal changes in a differential pattern and when we say differential pattern you always have a standard sample and with respect to that what are all the thermal effects that are happening you try to study in a differential way. The same thing to do with the thermo gravimetric also you can try to map this as a differential plot. So, even if there are minute variations you can pinpoint that very clearly by differential TG. Suppose there is a heat flow or heat exchange that is happening then you call this as DSC it is calorimetry. As you know calorimetry means measurement of heat and therefore, you can also conveniently determine the heat of crystallization, heat of fusion, heat of transition or any enthalpy change that is associated with different chemical process you can try to do that it is a very popular method called DSC used by material scientist. And sometimes we miss out on very very important mechanistic issues if you let go some of the gases which are coming out of the process. So, in such cases it is also useful if you have a TG instrument and trap the gases that are coming out and those gases can be in a sealed tube. So, that it is only the gases which can be mounted into a infrared spectrophotometer and that will give you exactly the details of what the gases that are coming out. So, your predictions are not just quantitative, but you can also have a mapping of what fractions that are coming out. So, it is called evolved gas detection or evolved gas analysis. So, you can analyze that using spectrophotometry. So, as you see here there are multiple events that are happening in just heating a sample and you can try to map several issues out of it. There are three which I want to pick up and that one little bit more on the instrumentation and the principle. So, this of all the thermal analysis measurements that are listed, thermal analysis a group of techniques in which a physical property is measured as a function of temperature while the sample is subjected to a predefined heating or cooling program. So, as I pointed out I am going to single out these three methods. Differential thermal analysis is nothing but you are measuring the difference between the sample and a inert reference material usually you are measuring the delta T and it is measured as both are subjected to identical heat treatments. The reference as well as the sample are heated under same thermal conditions or same temperature and the difference between these two is noted. Therefore, your reference material has to be inert to this temperature program. In other words it should be stable with no physical or chemical changes happening to that particular solid. Therefore, whatever that is happening the difference between the reference and sample will solely be the manifestation of that sample. Differential scanning color imagery here we are not measuring the difference in temperature between sample and the reference, but we are actually going to maintain same temperature for the sample and the reference irrespective of whatever changes that is happening to the sample. Therefore, if the sample is undergoing a particular change whether it is a heat releasing or a heat absorbing reaction then the temperature will all the time be maintained such a way the process will map how much of heat is absorbed or how much of heat is evolved during the reaction. So, this is not the reaction the temperature difference between sample and the reference it is the heat flow whether it is taken by the system or given out by the system because you are maintaining both the reference and the sample temperature at the same time. And in thermo grammatical analysis it is merely a change in the mass of a sample on heat. So, these are the primary differences, but if you look at the complementary nature the first two are complementary because they almost give the same information, but BAC is more a quantifying picture, DTA is more a qualitative picture and TG is a very different analysis altogether. So, basic principles of thermal analysis mainly revolves around the instrumentation. So, modern instrumentation used for thermal analysis consists of four parts. So, if you think of a TG instrument you should know that there is a sample and a specific sample folder you do not just dump it into any container a specific sample folder is there and relevance of that I will emphasize shortly now. And then you have very sensitive sensors which mostly we call it as thermocouples and then an enclosure in which the experimental parameters can be controlled. For example, we can do the heating of the sample using a heating element or a furnace sophisticated thermal analysis measurements are using IR based heating. You do not have a heating furnace, but you just using IR heating so that you can concentrate on the sample or the center of the sample. So, several sophistications are there with respect to the nature by which we can heat and then of course a computer to map and analyze the whole thing. When you consider a thermal event that can happen in a particular sample there are several issues that are undergoing in a sample decomposition. I would like to list it out so that when we look at specific examples you will see how much of information that we can get out of a simple heating protocol. Now, one is phase transition a liquid to solid or solid to gas you can map the phase transition adsorption or desorption that can happen in heterogeneous catalysis can also be evaluated with a sensitive thermal analytical probe melting that is fusion or sublimation both can be mapped very clearly using DT and TSE. And thermal decomposition solid A going to solid B plus C, C may not be a solid it can be a gaseous protein. So, thermal analysis can play a vital role. Radiolytic decomposition or simple glass transition where you can think about the solid going to a liquid phase or a amorphous going to a crystalline phase all this can be mapped. Oxidation and combustion reactions can be mapped very clearly. Heterogeneous catalysis as I said double decomposition addition and dehydration or desolvation reaction. So, many chemical effects are there when we heat a sample and all this can be usefully mapped. Let us come little bit to understand what a simple thermal gravimetric analysis is. We all know in the postgraduate or undergraduate level we have done nickel DMG complex dimethylgleroxane which is red precipitate and this is a very useful but a time consuming and a very laborious gravimetric exercise. But the same material can be very usefully and easily determined using thermogravimetric analysis with a modern instrumentation. I will tell you basically what this instrument is all about in the top part of this picture you can see is a balance it is a micro balance which is sensitive with the 5 digit sensitivity. So, any change in milligram quantity can be usefully mapped. Usually the samples are in milligram quantity therefore, you need to have 5 digit precision. So, typically it is a metal of balance which we use and this is in this balance you actually suspend the sample here and the sample is kept in between furnace wall. So, you can actually do the heating here and depending on the weight intake or weight loss the information can be fed from the micro balance as a result you can see what is the weight loss against the particular decomposition process that is going on. And it is not that simple as we see here in this case the lower chamber you can apply vacuum and then there are facilities by which you can actually purge the sample flow of either argon or air atmosphere. So, that whatever reaction that is happening suppose there is gas evolution it is not exactly concentrated here and it will be flushed out periodically. So, a inert gas is usually flushed or we can even have air if the sample needs air we can flush sample and we can try to map it. This is a typical TG analysis curve which is used and the most referred and quoted example is calcium oxalate and as you would see a typical TG curve looks like this where you see a bend here and then another bend and another bend. We call this as thermo gravimetric steps and each step has something to convey therefore, if there is one step and then there is another step and there is another step we call this as a three step decomposition. And each step might relate to a different physical change and a chemical reaction I will show you some of the example of this in the upcoming slides. So, in thermo presence allows monitoring of sample weight as a function of temperature now before we run any sample two things we need to do one you need to do a weight calibration another one you need to do a temperature calibration weight calibration you can do with calibrated weights if you want temperature calibration usually you try to take a ferromagnetic sample and take it beyond the ferromagnetic transition for example nigga and then you can calibrate your temperature whether the reaction as the transition is exactly happening then you can do the correction terms. Usually bio and C effects are very very crucial when we do TG therefore, this bio and C effects are largely manifested with the sample quantity or with the temperature gradients or the purging rate you can just flush the sample with whatever atmosphere you want you cannot try to bubble vigorous flow of this gases all this will cause bio and C effect as a result it will largely affect your results. Differential thermal analysis on the other hand is that to cup cavity ok in the next slide I will show you that advantages instruments can be used at very high temperature for differential thermal analysis you can go up to 1000 kelvin comfortably and it is very sensitive to measure and there is also flexibility in using the sample size and whichever form you want. Characteristic transition or reaction temperatures can be accurately determined and one of the disadvantages in DTA could be the uncertainty of heats of fusion heats of transition and heats of reaction and you can involve a error of 20 to 50 percent therefore, while determining the enthalpy of reactions of transition usually we resort to differential scanning calorimetry rather than DTA but DTA also gives essentially same inputs differential thermal analysis that way you have a alumina block here and in the alumina block you actually have a sample pan and you also have a reference pan and both are connected to the same thermocouple platinum rhodium or chromol alumina thermocouple and you purge it with either vacuum or inert gas now the difference in the temperature between the sample and the reference will determine your delta D. So, that will be plotted against the temperature sample holders are usually aluminum because it is very easy to handle less expensive and you wouldn't like to use the aluminum cups again you would rather discard so that it is free from contamination sensitive sensors are needed for measuring the difference because you are using my milligram quantities platinum rhodium is a very useful one because it shows a linear curve in the high temperature from room temperature up to 1000 Kelvin there is linearity therefore, you can use this so calibration becomes useful these are joined together with the differential temperature controller therefore, whatever you get is a differential block now furnace is usually aluminum block as I mentioned earlier and the temperature controller controls the furnace temperature. The sample atmosphere is important in determining the reaction that that takes place DTA can record either in the presence of oxygen or in inner condition for example, this is a typical DTA plot that is coming out of instrument and you would see there is a small hump here which is actually recorded as a endopic endothermic peak and there is another one which is actually called there exothermic peak but what you would see here this is a endothermic peak relating to the physical change that is happening where calcium oxalate loses the water molecule and therefore, dehydration is an endothermic peak but this is not crucial as much as the second peak is concerned in the second peak it is a exo peak which involves a unhydrated calcium oxalate going to calcium carbonate ok but this exo peak can miss out if I am going to heat the sample in nitrogen because in nitrogen it does not get the required amount of oxygen to decompose into calcium carbonate therefore, it goes as a endopic in other words it will come out like this. So, these two peaks are not important for atmosphere but this middle peak becomes important because it will determine whether it is endo or exo. So, in DTA it is very very important to know in which atmosphere you did it. So, purging a purging gas is not just for mere purging but it also affects the chemical process that is happening therefore, we need to know for sure. Again the last step is a endothermic reaction where calcium carbonate goes to calcium oxide whether you do it in air or in nitrogen it will give the same. So, the middle peak is determined by the nature of purging gas. In differential scanning colorometry the main difference from DTA is that the sample and reference are both maintained at temperature predetermined by the program. So, at every temperature during the temperature programmed reaction suppose the sample is at 100 then both the sample and the reference will be maintained at 100 but during the process of the sample change whatever that is happening that will be recorded either as a endo or exo peak. This is the main difference between DTA and DAC. During a thermal event in the sample the system will transform the transfer heat to or from the sample plant to maintain the same temperature. So, transfer of heat will be happening in order to keep the sample and reference at the same temperature. There are two ways that you can achieve this either using a power compensation method or using a heat flux method I will show that in the next two slides. Power compensation method you actually have two different alumina blocks to heat the plants. So, you have a sample plant in a different heating block you have the reference in a different heating block and they both are connected to the thermocouple where your delta T is actually maintained at 0 delta T is equal to 0. So, in this case your platinum resistance thermocouple is what you use separate sensors and heaters for the sample and the reference then the differential thermal power is supplied to the heaters to maintain the temperature of the sample and the reference at the program value. So, these are the main issues when we talk about power compensation DAC. When you think of a heat flux DAC you actually have the same block to heat both your sample and reference and this is actually mounted onto a constant that is a very sensitive heating block and you actually have two type of sensors there one is chromal constant thermocouple this is for maintaining the differential heat flow. Whereas chromal element thermocouple is there. So, you have both the thermocouples coming together the chromal constant for differential flow and the chromal element thermocouple for measuring the individual sample temperatures. So, two different sensors are there in a heat flux DAC unlike the other case. So, and you the primary difference here is you have one block for heating both the samples temperature difference between the sample and reference is converted to differential thermal power which is nothing, but D delta Q by DT. So, this is what is measured which is supplied to the heaters to maintain the temperature of the sample and the reference of the same value. So, this is the primary difference. Now, I will take specific examples for thermogrammetry and tell how TG can be used and what are all the protocol that we need to follow. As I told you it is a heating with the thermo balance therefore you are essentially measuring the percentage weight loss as a function of temperature. The optimum conditions in measuring TG comes from taking only few milligrams and this few milligrams of samples should essentially have a good support on the pan. In other words the effective contact area between the pan and the sample has to be maximum because you are playing with very few milligram and thin layer of sample can be should be uniformly spread throughout the sample and then it should be a open sample container. You cannot close the container whereas in the case of DAC you need to actually close it. It cannot be operated without open sample container mainly because if there is any evolving gas it does not really create any burst. Inert gas flow is recommended and then slow heating rate. These are important principles for TG. So, there are several sample cups which can be used. Quartz you can use because you can go comfortably up to 1000 k. Copper and nickel they are very sensitive therefore only for reactions where you anticipate the sample to undergo chemical reactions below 200 degrees or so. You can use these samples. Convenience is much more cheaper compared to other ones. So, you can actually do a one time reaction and discard it. So, copper samples can also be used. Aluminum samples are available but more expensive for sensitivity aspects you use aluminum and platinum boats. Small sample masses and low heating rates increase the resolution but at the expense of sensitivity. Suppose you use very fast heating rate and a large amount of sample you increase the sensitivity but the resolution may be missing. I will show one of the example how you need to make a compromise. So, depending on the nature of the sample you need to optimize on the sample weight. Typically 3 to 20 milligram is taken and there are several fan configurations that are there. You can have a sealed one also but with a lot of holes so that gases can escape. Same material in configuration should be used for both the sample and the reference. You cannot use two different sample holders. Material should be completely covered to the bottom of the pan so that a good thermal contact is there and to avoid overfilling the pan to minimize thermal lag we need to measure that less amount of sample is there so that sample does not jump out and create any temperature like that. As I told you earlier TG involves instrument calibration and weight calibration is needed as well as the QD temperature calibration is needed. Today there are many instruments where QD temperature calibration is not needed because you can zero it and adjust the error for temperature. So, but there are several standards which are available like aluminal, nickel, permaloid and iron. All these are having the QD points at the specific temperatures so you can use them for internal calibrations. So, it is important to have these calibrations made before you start. Typical TG curves are like this where you can have a flat one. The line is flat what do you mean then there is no change there is no problem but if there is a small change in the baseline then that can amount to a glass transition or it could be a desorption and drying a small amount of adsorbed gases or water can be escaping so that can be easily mapped to a small change in the slope and suppose there is a clear step like this then you can think about a single stage or single step decomposition. If it is a multi-step decomposition then you see curve 4 and you have some steps there but which is not resolved then you may have multi-step decomposition because of the heating rate you do not see it more pronounced. There is another way you suddenly see the curve increasing and reaches a plateau then it is an atmospheric reaction which means there is an uptake of air or oxygen to the sample or it could be increasing and then it is going down which could be like your 6 interacting with the atmosphere but decomposes at higher temperature. All these are possible in a TG curve as I told you if it is just one step one stage decomposition then it is very easy for us to map but if there is a curve like this you really do not know because it is not very well resolved at sometimes so you do not know what is this but you can easily map this slope and this slope because there is a clear plateau so in such case you actually transfer the data TG data and you make a differential plot so differential TG will tell you how many crucial steps are there for example this single-step decomposition means in DTG will give you only one maximum and this is not XO or NDO we should not confuse this with DTA this is not NDO XOP this is just a differential plot so you can know for sure that this is the midpoint of such a transition. Suppose this is a multi-stage decomposition then you have essentially 4 maxima so 4 different things are happening so this DTG is more useful if you have a multi-step reaction or many processes are going through such a reaction. Some more applications of TG accurate definition of conditions for drying analytical precipitates can be noted thermal stability of different materials for example drugs conditions of polymer degradation metal oxidation metal combustion can be noted fingerprint minerals or you can identify polymers for example this is a DTG TG curve of various polymers as you see this is poly methyl methacrylate showing this trace polyvinyl chloride shows like this low density polyethylene or teflon pdfp they all show a different weight loss which means you can understand the thermal stability of such polymers using a simple thermogravimetric protocol and if you are actually doing this process for a particular sample it is good to involve all the exercise where you have a combined measurement that is TG, DTG and DTA so that all the parameters can be evaluated not just the weight loss but also find out what is the nature of such chemical reaction so typically this is your DTA plot which shows several valleys with each one amounts to a particular phenomena that is happening in etria alumina soil so there may be many useful informations coming if you have a combined technique I will show some more examples of that in the next few slides so one of the first ever multiple technique or combined approach that was in the form of instrument was TG, DTG, DTA now we also have a combined technique with DAC which is coming in the modern instruments I am going to give you some examples from our own work where TG has proved useful in some of the lectures in the in this present course I have touched on this same example but I will just highlight this with respect to TG as this lecture is mainly on thermo gravimetric first I will try to show you how alloys can be mapped then hydrogen bonding can be evaluated using thermal technique and how in OLED application this can be used and also in magnetic materials this is one of the useful work where a new cobalt ion alloy precursor was made and this was converted into cobalt ferrite and it is very difficult to prepare this COFP to alloy so how do we know that we have made this alloy if you actually do a XRD you can clearly see that this is the X-ray pattern of the alloy precursor it is a very reactive alloy precursor but if you actually heat the sample then it gets converted to cobalt ferrite now one should know whether it is truly the alloy which is getting converted to the ion oxide or how much of this cobalt ferrite is getting converted to ion oxide this is the place where TG comes into picture this is the thermo gravimetric curve which clearly shows that there is nearly 21% of a gain in the weight so instead of losing weight here you see an uptake which means COFP2 is getting converted to COFP2O4 by taking atmospheric air as a result you see nearly a 20% weight loss but if you really quantify this result you will find out that around 1 to 2% or 1 to 5% so to say of cobalt ferrite is already present in the sample which cannot be detected by the X-ray and nearly 95% of the alloy is in actual COFP2O4 this is a very important information to know whether any ferrite precipitates are already there in the sample even before conversion from alloy to oxide so such fine details you can try to get it and this is another important work where we found hydrogen bonding was evaluated mainly from a TG diagram this is a Benz imidazole molecule which is attached to a phenyl ring in 1 to 2 position it is attached so you would see this imidazole can either be intramolecularly hydrogen bonded or intramolecularly hydrogen bonded in this case there is a intramolecular hydrogen bonding in this case intramolecular hydrogen bonding you can clearly see between these two isomers the TG pattern is very different the one which is actually hydrogen bonded with other molecules intramolecular hydrogen bonding shows a very high decompotion temperature compared to intramolecular hydrogen bonding so such information you can easily find out but without this information it would have been a partial justification of hydrogen bonding if we had just shown the change in the PL emission between a solid for the intramolecular hydrogen bonded and the intramolecular hydrogen bonded samples so TG can provide a vital information and same in the case of old applications as you know several molecules are made several molecules are there and each of this molecules are actually deposited in this layers but we are actually looking for making new organic molecules when we make this sort of a old phase one of the important criteria is it has to be amorphous so one of the important criteria in this old fabrication is when you make this organic films it should not crystallize if it crystallizes then the electrical connectivity will be lost and as a result you need to make amorphous films of this organic molecules to provide a electrical continuity so when you actually deal with polymers you don't have this problem because when you evaporate this organic molecules they form essentially a very good connectivity because they are amorphous but when you go for crystalline organic materials when you fabricate this there will be electrical discontinuity as a result the device operation will fail even after a first cycle it cannot be sustained so one of the problems involved in organic LEDs is to make thermally as well as electrochemical they should be a more rugged molecule rather it should withstand several heating cycles or several current cycles so the emphasis is to actually make bigger organic molecules bigger the molecule more the molecular weight less the crystallization temperature in other words you can actually improve on the decomposition temperature or the melting temperature so if the melting temperature is pushed further then you can actually make a much better films so for that matter if you can take benzene tetra carboxylic di-anhydrate as your starting material you can try to put two amino phenol and using this two amino phenol you can essentially make a compound like this as you would see that the di-anhydrate is now substituted with four such Benz thysol molecules so what you are essentially doing is instead of just adding one molecule to this you are substituting four as a result you make a bulky organic molecule and as you would see from the tg pattern here typically one or two Benz thysol substituted molecules will have the melting point somewhere here but you can see that the melting point and the subsequent decomposition temperature is pushed by at least 150 degrees so this is a single step melting point which involves a decomposition somewhere around 550 degrees so these are molecules which are recommended for OLED application because they will not crystallize easily and spoil the device application and you would see here this is the DTG pattern which is corresponding to the single step decomposition here and this curve is nothing but your differential thermal analysis curve DTA which first shows some glass transition and then it shows melting and then the decomposition so all these are easily mapped in the combined technique so it is very useful to analyze and to recommend the sort of materials that you need in one of the modules I have quoted this example where simple TG can actually bring about a great amount of insight into mechanism that is of fundamental as well as application oriented studies in this there was a confusion about the magnetism that was happening at room temperature manganese doped ring oxide and this is the paper published in Nature Materials in the year 2006 sorry 2005 so you can refer to this particular paper to understand how simple TG analysis can be used to resolve such a mysterious behavior in the samples as you would know zinc oxide is a semiconductor it is not a magnetic compound but just substituting 2% of manganese magnet itself is not a magnetic ion further and you can still see a room temperature look so in that case it was thought that a new system has emerged in for device applications called as dilute magnetic semiconductors but later it was understood that it is not a substitution oriented magnetism but it is some other impurity which is giving giving the origin for this magnetic curve and what was finally used to resolve this mystery was the TG plot here as you would see a simple thermogravimetry to make a composite of 2% M102 grounded with zinc oxide finally went on to prove that it is not manganese which is substituting in zinc oxide rather it is some amount of zinc which is going into manganese oxide in other words in Mn2O3 phase which is actually responsible for such a magnetic order so it is very very vital although it looks simple not many would even call this as a prima facie an important characterization tool but you would see that a simple technique can really alter confusion that prevails in the scientific community therefore TG mapping is very important now I will take you through some issues related to differential scanning colorimetry because here you can quantify more results as I told you there is a basic difference between DTA and DST here temperature differences are measured whereas here the heat flow from or out of the system is actually measured applications of both the techniques are similar whereas DST is more popular because it can give you understanding on to the heat exchange that is happening whereas DT is conveniently used because you can go down go up to very high temperatures in DST one of the things that you need to do is calibration you cannot just take the result at its face value and do the quantification so the base line correction is very important for example if you get an endothermic peak like this this is the way you try to deconvolve the peaks where you try to draw this base line and then you do these peak corrections such a way that the area under the curve is a measure of the amount of heat that is either taken or released so essentially you can determine the heat capacity or you can determine the delta H of fusion or delta H of combustion all this can be maximized therefore base line correction or the estimation of the area under the peak is very very important so evaluation of the thermal resistance of the sample and reference sensors need to be taken care it's a two step process the temperature difference of the two empty crucibles have to be first measured and then the thermal response is then acquired for a standard material specially using sapphire sapphire because melting point is above 1700 degree C therefore you use sapphire as the sample calibrant amplified DAC signal is automatically varied with temperature to maintain a constant calorimetric sense duty with temperature DST calibration can be done with a variety of calibrants for example you can if you are dealing with high temperature samples you can use metals and if your sample is metal then you can use metal as well some inorganics are also good for at calibrants KNO3 potassium perchlorate organics for example polystyrene benzoic acid and resin can be used and one of the important point is the calibrants should be of high purity they should be thermally stable they should not undergo changes with the light they should be non-hygroscopic which is much much more important and it should not react with the fan or with the atmosphere that is purged with there are typical features that can be understood from a DAC curve for a polymeric system for example if you take sulphur pyridine so many useful information that you can get number one you see a small change in the slope which actually refers to glass transition that is step A and in sequence B it is actually a crystallization from a melt to a crystallization which is a phase transition and then here you actually have a melting of metastable modification that is happening in event B which is the end of thermic peak and then the melting of a stable modification which is another end of thermic peak so you start with one sample and then you actually take it through crystallization after crystallization it goes into a metastable form which is nothing but your phase transition which is your even C where you do not seem anything particular but then you see again two successive end of thermic peak that relates to two different metastable phases before the sample finally decomposes so end of thermic events are melting, sublimation, solid-solid transition, desolvation, chemical reactions, exothermic peaks are crystallization, decomposition and the chemical reactions. Baseline shifts are mostly due to glass transition as you see here. Baseline shifts are very very milder and therefore you need to be very careful while analyzing it but you should also understand any ups and downs in the DAC curve does not mean something is happening for example several artifacts can be recognized both in the exo peak as well as end of peak. In the exo peak if the sample topples over in the pan then you get a fluctuation like this or sample pan is distorted or if you have a mechanical shock of the measuring cell or if the flow of air is leaked out then you would see this sort of noise coming up similarly in the end of peak if you see some sort of a plateau like this it is not exo or end though actually it is electrical spikes and spikes can come like this you also have burst of the pan. So several artifacts can come in a typical DAC which we should be watchful and in DAC predominantly you are talking about the thermal stability where you can analyze several of these events which are happening or you can measure the heat flow depending on the nature of the peak or you can look at the chemical purity you can look at the phase transitions all these information you can get from a DAC curve. If you are looking at a typical endothermic peak which is a transition temperature actually the onset of this has to be linear if your peak is to be assessed it can also be a non-linear curve it can be more Gaussian in such case it tells something different. So this is the onset of the transition and this is the peak of the transition in this case if it is a melting then this is how it is done and if your sample is pure then there will be a linearity here if your sample is impure for example then you would see the onset is actually more with a curvature that means the sample is impure and if suppose there is some other eutectic impurity that is there it will also show off like another small peak. If there is a endothermic peak that is melting followed by a exothermic decompotion it would come out like this if it is endothermic decompotion followed by melting then it would come out like this and in that case actually glass transitions although they are look very minimal one can actually try to resolve to find out whether there is a linearity in this slope which indicates there is a glass transition and in such case if you do the baseline correction you will be able to see what is the enthalpy that is involved in this glass transition this is one of our own work where you take amorphous alloys of cobalt platinum and if you try to heat that sample you can map what sort of transformation that is happening for example if you do this forward run there is a glass transition which amounts to a good endothermic peak after that there is a exothermic peak which actually corresponds to crystallization which is a FCC to FCT transformation so it is a exothermic peak so when you cool this sample again you see the same thing happening where FCT is now reversible to FCC so this phase transition is a reversible one therefore on the cooling you would see a reversible curve but once you go again for the next run you can see because of the previous you know sweep the area of this curve has tremendously reduced which means most of the sample has got crystallized so this much information you can get about amorphous alloys about its glass transition about the crystallization process and whether it is a reversible or a irreversible transformation so all this information you can get from DAC curve enthalpy of fusion for example you can try to measure it could be a multiple thermal steps or it could be a single melting step and the delta H of fusion can be measured and I will also show one more example how purity can be determined for example if you take benzoic acid depending on the purity of this samples the endothermic peak will actually shift so that will tell you the sort of purity of your sample so this is a good measure by which you can measure the purity and lastly I will touch upon the heating rate suppose you heat the sample at 0.5 degree then you get a single peak and then one peak here if you increase the sample heating rate you can see same transformation but it is actually giving a very different protocol so to obtain thermal events close to the true thermodynamic value the recommended heating rates are 1 to 5 degree per minute that is what is recommended and it has to be done at a very very slow or purging rate of your air or organ atmosphere otherwise same process but you get a confusing DAC curves lastly I would also like to touch upon this issue that TG can be combined with FTIR or it can be combined with mass peak so you can evaluate the evolved gases for example calcium oxalic carbonate heated carbon dioxide comes carbon dioxide can be measured using IRC so all this multiple techniques or hyphenated techniques can be studied using this protocol so in that way evolved gas analysis your and your gas detection can be achieved using a mass spectra or it can be connected to an online GC gas chromatography so best practices of thermal analysis use small sample size good thermal contact between sample and sensing device proper sample encapsulation when you are talking about DAC and starting temperature well below expected transition temperature slow scanning speeds proper instrument calibration and then purging gas should not be corrosive and avoid decomposition in the DAC so these are the best practices with which one can get very useful information there are several groups both in chemistry and physics where they use the thermogrammetry essentially to resolve many many fascinating features that happen during daily research activities