DSC_3

Differential Scanning Calorimeter

(DSC)

The technique was developed by E.S. Watson and

M.J. O'Neill in 1962, and introduced commercially

at the 1963 Pittsburgh Conference on Analytical

Chemistry and Applied Spectroscopy.

The first adiabatic differential scanning calorimeter

that could be used in biochemistry was developed

by P.L. Privalov and D.R. Monaselidze in 1964.

The term DSC was coined to describe this

instrument which measures energy directly and

allows precise measurements of heat capacity.

Introduction

Differential scanning calorimetry or DSC is a

thermoanalytical technique in which the difference in

the amount of heat required to increase the

temperature of a sample and reference is measured

as a function of temperature.

Both the sample and reference are maintained at nearly the same temperature throughout the experiment.

What is Differential Scanning Calorimetry ?

“Measures temperatures and heat flows associated with thermal transitions in a material”

“These are techniques by which the difference in heat flow to or from a sample and to or from a reference is monitored as a function of temperature or time, while the sample is subjected to a controlled temperature program”

DSC

Generally, the temperature program for a DSC analysis is designed such that the sample holder temperature increases linearly as a function of time.

The reference sample should have a well-defined heat capacity over the range of temperatures to be scanned.

A DSC is a twin instrument, comprising of individual sample and reference calorimeters within a common thermal enclosure, the cell.

Instrumentation

Each calorimeter will measure heat flow as a function

of temperature or time. The two calorimeters are

assumed to be identical. The term “calorimeters”

refers to each raised platform and its respective

sensor.

The output of the DSC is the difference between the

heat flows measured by each of the calorimeters.

The cell is heated at a linear heating rate. The sensor

body is made out of constantan (an alloy usually of

55% Cu and 45% Ni), consisting of a thick flat base

and a pair of raised platforms where the sample pan

and reference pan are positioned for the analysis.

The thin side (wall) of the sensor creates the

thermal resistance. This constantan material

provides good thermal conduction to the sample

and the reference.

As the temperature of the furnace is changed, heat

is transferred from the silver base of the enclosure

to the sensor body and thus to the pans. The

temperature of the furnace is controlled by the

refrigerated cooling system (RCS).

Thermocouples on the underside of each platform

are used to measure the temperature of the

sample and the reference. A third detector is used

to measure the temperature of the sensor base.

Pans are made from gold, copper, aluminium,

graphite and platinum or alodine-aluminium.

The lids can either provide a complete enclosure

or can contain a pin hole to release the pressure

that builds up inside, due to evaporation of water as

the temperature is increased.

Most samples can be run in non-hermetically sealed

pans either uncovered, or crimped with an aligned

or inverted cover. Atmospheric interaction is

optimised by using an open (uncovered) pan.

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SELECTING CRUCIBLES FOR DSC

DSC pans & lids are available in aluminum, gold,

platinum, graphite, and stainless steel versions.

They can be used under a variety of temperature

and pressure conditions.

Samples can be run in the standard DSC mode in open pans, pressed or hermetically sealed pans/lids .

Sample Pans

When a sample undergoes a physical transformation such as a phase transition, more or less heat will need to flow to it than to the reference (typically an empty sample pan) to maintain both at the same temperature. Whether more or less heat must flow to the sample depends on whether the process is exothermic or endothermic.

For example, as a solid sample melts to a liquid it will require more heat flowing to the sample to increase its temperature at the same rate as the reference.

This is due to the absorption of heat by the sample as it undergoes the endothermic phase transition from solid to liquid.

Principle of Operation

Likewise, as the sample undergoes exothermic processes (such as crystallization) less heat is required to raise the sample temperature.

By observing the difference in heat flow between the sample and reference, differential scanning calorimeters are able to measure the amount of heat absorbed or released during such transitions.

DSC may also be used to observe more subtle phase changes, such as glass transitions.

Differential Scanning Calorimetry

• Differences in heat flow are measured as a function of temperature

Sample and reference

Energy difference measured

• Power Compensated DSC

Two furnaces

Heat flow evaluated

Difference in power input monitored

In watts

• Useful for glass transition temperatures

• Purity of drug samples

DSC • Constant Heating Rate

o Initial Temp

o Final Temp

o Heating Rate (°C/min)

• Data

o Heat flow to sample minus

Heat flow to reference Vs

Time (Temp.)

• Measures heat of

crystallization

Polymer without weight change in this temperature range

Detection of Phase Transitions

The basic principle underlying this technique is that when the sample undergoes a physical transformation such as phase transitions, more or less heat will need to flow to it than the reference to maintain both at the same temperature.

Whether less or more heat must flow to the sample depends on whether the process is exothermic or endothermic.

For example, as a solid sample melts to a liquid it will

require more heat flowing to the sample to increase its

temperature at the same rate as the reference.

This is due to the absorption of heat by the sample as it

undergoes the endothermic phase transition from

solid to liquid.

Likewise, as the sample undergoes exothermic

processes (such as crystallization) less heat is

required to raise the sample temperature.

By observing the difference in heat flow between the

sample and reference, differential scanning

calorimeters are able to measure the amount of heat

absorbed or released during such transitions.

DSC may also be used to observe more subtle physical changes, such as glass transitions. It is widely used in industrial settings as a quality control instrument due to its applicability in evaluating sample purity and for studying polymer curing.

The method of differential scanning calorimetry (DSC) allows the measurement of the energy flow to and from a sample during a temperature controlled program.

The sample is prepared in DSC capsules, which can be either sealed or open, and then submitted to a temperature program.

The actual measurement is performed against a control, which normally is an empty DSC capsule, and the energy required to keep both sample and control at the same temperature is plotted vs. the temperature. The resulting curve is called a thermogram.

DSC thermogram with definitions of onset temperature (Tonset),

peak temperature (Tpeak) and offset temperature (Toffset).

DSC provides rapid and precise determinations of transition

temperatures using minimum amounts of a sample.

Common temperature measurements include the following:

Melting

Glass Transition

Thermal Stability

Oxidation Onset

Cure Onset

Crystallization

Polymorphic Transition

Liquid Crystal

Protein Denaturation

TYPICAL DSC TRACES

Differential scanning calorimetry can be used to

measure a number of characteristic properties of

a sample.

Using this technique it is possible to observe fusion

and crystallization events as well as glass transition temperatures Tg. DSC can also be used

to study oxidation, as well as other chemical

reactions.

Glass transitions may occur as the temperature of

an amorphous solid is increased. These transitions

appear as a step in the baseline of the recorded

DSC signal. This is due to the sample undergoing a

change in heat capacity; no formal phase change

occurs.

As the temperature increases, an amorphous solid will

become less viscous. At some point the molecules may

obtain enough freedom of motion to spontaneously

arrange themselves into a crystalline form. This is known

as the crystallization temperature (Tc). This transition

from amorphous solid to crystalline solid is an exothermic

process, and results in a peak in the DSC signal.

As the temperature increases the sample eventually

reaches its melting temperature (Tm). The melting

process results in an endothermic peak in the DSC curve.

The ability to determine transition temperatures and

enthalpies makes DSC a valuable tool in producing

phase diagrams for various chemical systems.

This composite shows typical shapes for

the main transitions observed in DSC.

An alternative technique, which shares much

in common with DSC, is differential thermal

analysis (DTA).

In this technique it is the heat flow to the

sample and reference that remains the same

rather than the temperature.

Differential Thermal Analysis (DTA)

When the sample and reference are heated

identically, phase changes and other thermal

processes cause a difference in temperature

between the sample and reference.

Both DSC and DTA provide similar information.

Difference of DSC and DTA:

DSC measures the energy required to keep both the

reference and the sample at the same temperature,

whereas

DTA measures the difference in temperature

between the sample and the reference when they

are both put under the same heat.

Differential Thermal Analysis (DTA):

Measures temperature difference (dT)

between the sample and the reference

Differential Scanning Calorimetry (DSC):

Measures heat flow difference (dH/dT)

between the sample and the reference

Difference of DTA and DSC

Both techniques are concerned with the measurement of energy changes in materials.

They are the most generally applicable of all TA methods since every physical or chemical change involves a change in energy.

COMPARISON BETWEEN THERMAL ANALYSIS AND DIFFERENTIAL THERMAL ANALYSIS

Applications of DSC

• Heat capacity and heat of fusion

• Phase changes

• Glass transitions temperature

• Melting and crystallization behavior

• Heat of melting and crystallization

• Purity

• Compatibility

• Oxidative stability

• Polymorphism

• Thermal stability

• Cure / or Cure Kinetics

• With the new DSC offers the users of thermal

analysis a solution with a secure future that exceeds

today's demands

• The development of DTA and DSC instruments

made it possible to determine values faster, more

conveniently and at a lower cost

• Today's instrument is operated by a computer data

station which determines both the transition

temperature and the enthalpy of the transition.

Determining dH values involves drawing the best

base line

• The instrument must be calibrated. This is usually

done with indium since metals can often be

obtained with a higher purity than organic

materials

• However, highly purified liquid crystals have also

been used for calibration. Then an accurately

weighed sample is placed in an aluminum pan,

crimped tightly closed with a sealing press and

placed in the sample chamber of the DSC

instrument.

• Temperature calibration is carried out by running standard materials, usually very pure metals with accurately known melting points

• Energy calibration – using either known heat of fusion for metals (indium) or known heat capacities (e.g. synthetic sapphire – corrundum or aluminium oxide)

• Gas flow inside the furnace:

o To sweep away volatiles

oTo provide required atmosphere

o To assist in heat transfer

ADVANTAGES OF DTA/DSC

• Rapidity of determination – a wide

range of T (minute or hours)

• Small sample size – several mg to

several hundred mg

• Versatility – solids or liquids

• Simplicity and ease of procedure and data analysis

• Applicable to cooling and measurements under high pressure

• Ability to study many different types of chemical reactions

…..ADVANTAGES OF DTA/DSC

DISADVANTAGES OF DTA/DSC

• Relatively low accuracy and precision, 5 – 10% in most cases

• Inability to be used to determine the enthalpy of overlapping reactions

• The need for calibration over the entire temperature range

• Inaccuracies in determining peak areas due to baseline change

Tutorial 4.1

1. What is differential scanning calorimetry (DSC) ?

2. Explain the difference between DSC and DTA.

3. Propose FIVE (5) applications of DSC.

DSC curves

The result of a DSC experiment is a curve of heat flux

Vs temperature or Vs time.

This curve can be used to calculate enthalpies of

transitions. This is done by integrating the peak

corresponding to a given transition.

Figure 2: The area under the curve of a thermal event gives its enthalpy.

The area under the curve of any event in the thermogram is proportional

to the enthalpy of this event. After suitable calibration with standard

materials with known thermal events and their related enthalpy, it is

possible to utilise the integral of a thermal event and obtain very accurate

information about the energetics of thermal events.

Direct integration will give this enthalpy as energy per sample mass

(Joule per gram, J g-1). For better comparison, this value is transformed

to give energy per moles by multiplication with the molar mass. This

value is then given as kJ mol-1 and allows the comparison of enthalpies

for corresponding events detached from the actual measured sample.

The slope of the thermogram baseline gives the heat capacity of

the sample. As this characteristic will change with temperature,

the baseline normally increases with increasing temperature.

where ∆H is the enthalpy of transition, K is the

calorimetric constant, and A is the area under the curve.

The calorimetric constant will vary from instrument to

instrument, and can be determined by analyzing a well-

characterized sample with known enthalpies of transition.

∆H = KA

It can be shown that the enthalpy of transition can be

expressed using the following equation:

How to interpret DSC curve ?

Examples of DSC curve

The solid-state reaction of mullite formation is highly endothermic reaction:

DSC curves of prepared powder of undoped and doped mullite.

The DSC curves reveal that the presence of Fe in the reaction mixture affects

the drop in temperature of mullitization reaction from 1250 to 1150ºC.

It could be ascribed to the existence of Fe which can stabilize the mullite lattice.

The phase transformations are evidenced above 900ºC and between 1100 and

1300ºC.

The 1st endothermic process probably corresponds to the

formation of transitional γ-alumina phase, denoted as a spinel

phase, while the 2nd endothermic peak represents the process of

mullite formation.

Journal of Alloys and Compounds Volume 527, 25 June 2012, Pages 5–9

DSC, ESR and optical absorption studies of Cu2+ ion

doped in boro cadmium tellurite glasses

P. Gayathri Pavani , M. Prasad, V. Chandra Mouli

Differential scanning calorimetry (DSC) is used to

characterize the glasses and to determine glass transition

temperature (Tg), which is useful in suggesting structural

changes that takes place by the compositional changes. This is

because Tg is very sensitive to any change of the coordination

number of the network-forming atoms and also to the

formation of non-bridging oxygen.

Fig. 4: DSC thermogram and inset representing glass transition temperature with CdO mol.%.

Typical DSC plot of boro cadmium tellurite glass system (C3 sample) is shown in Fig. 4.

The inset of Fig. 4 shows variation of glass transition temperature with CdO mol.%.

The transformation to a glass does not take place at on, strictly defined temperature, but

within a temperature range, representing the transformation region. exo

The single endothermic glass transition peak indicates the

homogeneity of the glass. Tg represents the ‘strength’ or

‘rigidity’ of the glass structure.

The glass transition temperature increases with CdO content

which indicates that network connectivity increase. This

result is consistent with molar volume data, which shows that

the network becomes more compact on increasing CdO

concentration.