Characterization of construction materials

Dr. Piyush Chaunsali

Department of Civil Engineering
Indian Institute of Technology, Madras

Lecture No - 24
Thermal Analysis- Part 2

(Refer Slide Time: 00:13)

So, in DTA, we know that we are measuring the difference in temperature. First I will go over
the techniques, and then we will look at the applications. Differential Scanning Calorimetry is a
thermoanalytical technique in which the difference in the amount of heat required to increase the
temperature of a sample and reference are measured, as a function of temperature.

So here what we are measuring is the difference in the amount of heat, to increase the
temperature of sample and reference are measured as a function of temperature. As you heat it
up, your temperature will change, then you want to minimize that. So you are measuring the heat
supplied. Suppose you need heat suppose it is endothermic reaction, it is taking heat, you supply
heat.
(Refer Slide Time: 01:17)
So let us look at what are the common types of Differential Scanning Calorimeters. So, one is
Power-compensation DSC. Idea is here we have a separate sample and reference temperature
sensors and furnaces. So you will see two separate sample and reference temperature sensors and
furnaces, they are separate. And, in this case Differential thermal power is measured.

In the Heat flux DSC, the sample and reference are put together, that is, they are not separate.
And we are measuring the temperature difference. And then we are converting into thermal
power. See eventually in DSC you are interested in measuring the thermal power. So in this case
we are measuring differential temperature or temperature difference, and then converting it to
differential thermal power. In Power compensated DSC, we are directly measuring the thermal
power.
(Refer Slide Time: 02:35)
Now you must be wondering, what the difference between DSC and DTA is. As I explained in
DTA, you have a temperature difference ΔT, plotted against the temperature.
ΔT = TS - TR
Where TS is the sample temperature and TR is the reference temperature.

Here (upper graph in slide), we have power, , so here we are directly measuring the

power. That is the difference between DSC and the DTA.

(Refer Slide Time: 03:18)

So, let’s see a few examples of DSC. So, here is a typical DSC curve for polyethylene
terephthalate. So, what happens when you increase the temperature to the heat flow? This is the
temperature increase. So you see some drop (65°C). Then, the curve stays like this (till 110°C).
Then you see the peak, this is exothermic peak. Then you see endothermic peak (starting from
255°C). Exothermic means you are releasing heat, endothermic means you are using heat.

So it can tell you what is happening, that is the point with this DSC curve. So, basically this
small drop here tells you that it’s the glass transition temperature range between 60 to 70°C. That
is the glass transition happening. At 110°C, this is because of the crystallization, and we know
crystallization is exothermic. So, it tells you what kind of reactions is taking place here.

At 255°C, then you have melting, melting will need heat. So yo u see endothermic peak. So, it
can give you a lot of information about the reactions.
(Refer Slide Time: 04:55)

Now we will go towards the quantitative; how we can use it to measure things. So, before we do
that, let us look at a schematic representation of DSC curve. So again, you always see when you
talk about DSC, heat flow is plotted against temperature. Heat flow is plotted on Y-axis and
temperature on X-axis. Now, you see this exothermic peak. So now the question comes what
temperature should we use?

You see, Ti is initial temperature, you see that (in Graph in slide) beyond Ti you see that things
are changing. It is not flat anymore and the slope is changing.
Then you have this linear portion. Now, if you draw a line tangential to the linear portion, and
tangential to your initial portion. The point where it intersects is called Te, which is extrapolated
onset temperature.

See, Ti is initial temperature or you can say it as onset temperature. Things are changing now,
now Te is extrapolated onset temperature I will come back to it, why it is important? But for now
you can see that we are extrapolating, as we are drawing a tangent here (initial), and then we are
drawing a tangent on this curve. And where it is intersecting, that is Te. Tm is the peak
temperature, at the peak. T final (Tf) is this one, where it has now become flat. These are some of
the characteristics.
(Refer Slide Time: 06:52)

Now if you look at the effect of heating rate, for an example. Whatever we discussed earlier, like
sample size, heating rate, all that will affect the DSC curve also, because it is basically same, and
the response is going to be very similar.

In this case, we are measuring the heat. So, if you look at the effect of heating rate on the DSC
curve you see, as we saw earlier, as you increase the heating rate, the peaks become broader and
deeper.
That is what you are seeing here. So 1°C, 3°C per minute; what is happening? Peaks are
becoming broader and deeper. What you notice is that your peak temperature is changing, but
extrapolated onset temperature does not change. So that is the usefulness. So efficacy of
extrapolated onset temperature, compared to peak temperature, because it does not change with
heating rate.

See here, I mean, extrapolated onset temperature will be somewhere here, extrapolated means
you extrapolate, and you take tangent at this initial regime. And then you draw a tangent. So,
there is not much change. If you see the peak temperature has changed significantly. So there is
some usefulness in using extrapolated onset temperature, because it is unaffected or is not as
much affected as your peak temperature, when you are using different heating rates.
(Refer Slide Time: 08:34)

Question is how do we get this extrapolated onset temperature. So, so let us look at this plot
(drawing in slide – Refer Figure 1), so you have this exothermic peak. So, you have the flat
region where nothing is happening.
 Figure 1 Extrapolated onset tempe rature

Things are changing, suppose this is the point Ti when the slope starts changing. Now you are in
this linear regime, assume this is a linear part, and now you can draw a tangent here. Now you
can initially you have, you draw a tangent here (flat region), so it will intersect at some point that
can be taken as extrapolated. So it is an extrapolated onset temperature. So your onset
temperature is Ti, but this is extrapolated.
(Refer Slide Time: 09:27)

So, heat capacity can be calculated. Basically we are calculating the power. And if you integrate,
that is, if you calculate the peak area, you will be able to calculate the heat capacity. Enthalpies
of transitions and transformations - any transformation, or transition which happens, we can
calculate associated enthalpies, because basically we can measure the heat - that is the idea.

Purity - if you are talking about the impurity in something, we can see we can measure the
purity, degree of purity. And also, chemical kinetics, thermal conductivity.
(Refer Slide Time: 10:10)

Let us look at how do you calculate heat of transition? Something is happening, when something
is transforming or transitioning. We want to know what the heat of transition is. So basically, this
is a simple equation:

Where ΔH is the heat of transition reaction

m is the mass of reactive sample
K is the calibration coefficient
A is the curve peak area.
Here you can calculate the area under the peak, for this particular transition. The calibration
coefficient, you can get for particular instrument, if you know heat of reaction of a material. So
you use standards, where you know what the heat of reaction is. Then you can get the calibration
coefficient, you know ‘K’. Now you do your experiments, you can calculate A, A is the peak
area, and then based on the mass, you can calculate ΔH. So this equation gives you heat of
transition.
(Refer Slide Time: 11:14)

Measurement of purity - here is an example; the figure shows the DSC curves for the melting of
benzoic acid containing increasing amounts of an impurity. So, you have benzoic acid and as we
increase the amount of impurity, how it affects your DSC. So is what do you see here? ‘c’ is 97%
purity, ‘b’ curve is for 99% purity and ‘a’ is for 99.9% purity.

See how the purity affects your DSC curve. So, the po int is, this DSC measurement can be used
to assess the purity. So the curve shows an increase in the width of peaks – basically, the
temperature range over which the melting takes place. If it is purer, the peaks will become
narrower. So, idea is you can use the DSC curves and get an idea about the purity, if you are
thinking about assessing the purity in your sample.
(Refer Slide Time: 12:41)
The third technique which we will talk in this thermal analysis will be Thermogravimetry. It is
very commonly used. So what does thermogravimetry mean? In thermogravimetry, we measure
the change in mass. In DTA, we are measuring change in temperature between sample and
reference. In DSC, we are measuring the heat between sample and reference. Here we are
measuring the mass change as you increase the temperature.

So basically we are determining the mass loss or gain, depending on the reaction as a function of
temperature and/or time. And this measured mass- loss curve can provide information on:
 Changes in sample composition - what kind of changes is happening?
 Thermal stability, how stable the sample is.
 The kinetic parameters for chemical reactions in the sample.
(Refer Slide Time: 13:46)

Again, these are the physical and chemical processes, which can be studied using
thermogravimetry, because there is some mass change. If you are interested in mass change, then
you can use thermogravimetry. So gas adsorption, desorption, phase transition, decomposition,
when something is decomposing with some mass loss, chemisorption. So, these are the
phenomena causing mass changes. As long as there is mass change, we can use
thermogravimetry to capture them.
(Refer Slide Time: 14:21)

So weight loss can happen when you have decomposition, when something is decomposing.
Simple example one can think of is, this also decomposition right?
CaCO3 is converting into calcium oxide and carbon dioxide.
Evaporation - you are losing moisture, loss of volatiles.
Reduction, Desorption. These are the typical processes, where we have weight loss.

Similarly you can have weight gain also. If there’s oxidation, suppose you have a material which
can oxidize, then you will see weight gain. Absorption also. So these are all processes where you
can have either, weight loss, or weight gain, and as long as there is a weight loss, like change in
the mass, thermogravimetry can be used. And obviously these processes are kinetic, that is, there
is a rate at which they occur.

They won’t happen immediately, so kinetics plays a role, so how fast a reaction takes place. So
that also has to be kept in mind.
(Refer Slide Time: 15:32)

So typical, TG instrument, Thermogravimetry, either use TG or TGA, when you refer to papers,
often times, you will see people using TG or TGA for this technique. So this is a TGA or TG
instrument. So basically, you have a sample pan here. Next to sample pan here, you have a
thermocouple, because we are interested in measuring temperature, because we are heating the
sample, and then we are measuring the weight.
So you see you have a big component here, it is called thermobalance. You want to make sure
that you are able to measure the small weight change. So, thermobalance is here. You have also a
furnace. Now you are heating the sample in a controlled environment, and so there is a furnace.

And you can purge it, depending on the use. You can use nitrogen you can use other gases. For
cement, we use nitrogen because we do not want it to react with the atmosphere. So there is a
way to change the purge gas.
(Refer Slide Time: 16:47)

So what kind of temperature-time programs are we talking about? So you can either linearly
increase the temperature, so it is like a constant heating rate, temperature is increasing linearly
with time. Or you could have gradually isothermic, keeping temperature constant, and then you
are ramping it up, keeping it constant, ramping it up. Another could be, just go to a particular
temperature and keep it there. It is isothermic.

You understand the difference. These are the ways you can change the temperature of the
furnace. So experimentally, we are talking about sample size of about milligrams. 1-100
milligrams can be used easily, and usually for cement, we use 20-30 milligrams, that is the
amount we use. And heating to cooling rate, depending on the application, but you can vary it
from 1 to 50 °C/ minute. In one minute you can change temperature up to 50 °C easily.
(Refer Slide Time: 17:55)

Typically we go 1100 °C. So when we talk about the mass loss, so here we have a mass- loss
curve. So this is the calcium hydroxide, what happens when you heat Ca(OH)2 ?
Ca(OH)2 → CaO + H2O
At some point of time it will lose water. You see here (Figure in slide – TG curve), up to around
400°C you do not see any change, and beyond 400°C, you see change, and then nothing happens.
So that tells you there is a dehydration, it is losing water or dehydroxylation.

We are talking about now Derivative thermogravimetry. Derivative thermogravimetry is nothing

but first derivative of your mass-change vs. temperature curve. So you have this mass-change vs.
temperature curve. Now if I take the derivative of this, it will give me the second plot, which is
here (DTG curve).

Basically DTG curve presents the information in more visually accessible form; it is easier to see
peaks. Basically, there is no new data, you are re-plotting the same data, but you are just taking
the first derivative. Now I can see, up to here it is flat, so I can just measure this, I can find out
what is the temperature range where it is losing water.

So it just gives you information in a more visually accessible form, so whenever you see data in
literature (in papers), you often see TG with DTG. And area under the DTG curve is directly
proportional to the mass-change. If you can calculate the area under the curve, it will give you
the mass-change. Here is an example of calcium hydroxide dehydration.
(Refer Slide Time: 20:10)

One example – let us look at the decomposition of calcium oxalate monohydrate (CaC2 O4 .H2O).

So, it has three stages. First, it loses water. Secondly, it loses carbon monoxide (CO).
Thirdly, it uses CO2 . So, we will be able to capture that using TGA, that is what it shows. When
you look at the curve, you see the first stage where you have a loss of water; second stage
corresponds to loss of CO. Third stage corresponds to loss of CO 2 . And this is the decomposition
of calcium oxalate monohydrate in nitrogen (N2 ) environment. So, that’s the significance or
power of this TG. Depending on the reactions when they occur, you will be able to capture it as
long as there is a mass change. If there is no mass change, you won’t be able to use this
technique. If you have phase transition, where there is no mass change, then thermogravimetry
will not be useful, and then you will have to use DTA or DSC.
(Refer Slide Time: 21:26)
Factors affecting TG curve, it is very important to pay attention to these factors.
 Heating rate affects your TG curve - higher rates lead to better-defined, narrower peaks,
but also higher observed dehydration and dehydroxylation temperatures.
 Sample mass - higher quantities of a phase lead to a seemingly higher dehydration
temperature and to a broader peaks.
 Furnace atmosphere - that will depend on the type of reactions, suppose you are using
TG for cement paste, and you have a CO2 , then you will have carbonation forming
CaCO3 .
So these are the things which affect your TG curve.
(Refer Slide Time: 22:11)
Okay, we will stop here, in the next class; we will go over the application of thermal analysis to
study construction materials. Thank you.