Lecture 6 Dta & Dsc01

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  • 8/14/2019 Lecture 6 Dta & Dsc01


    Lecture 6 DTA & DSC 01 1




    The Differential Thermal Analysis (DTA) and Differential Scanning

    Calorimetry (DSC) have always used to study the same phenomena.

    Differential Thermal Analysis (DTA) is a thermal technique where the

    sample temperature is compared with the temperature of a reference

    (an inert material) and the temperature difference is recorded as a

    function of the sample, reference or furnace temperature while the

    sample is being heated or cooled at a constant rate.

    The temperature change is detected by a differential method as shown

    in Figure 6.1.

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    Figure 6.1 The basic concept of DTA system

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    The DTA method is very useful for the characterization of materials that willundergo physical or chemical changes by the heat effect without the mass


    The temperature change of the sample is due to either an endothermic orexothermic reaction or enthalpy change.

    The related processes include: a phase change


    melting crystal structural inversion

    destruction of the crystal lattice




    dehydration reaction

    other reactions.

    The phase change, dehydration, reduction and some of the decompositionprocesses produce endothermic effect, while crystallization, oxidation and

    some of the decomposition reactions give exothermic effects.

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    Comparison between the thermal method and the differential thermal

    analysis are shown in Figure 6.2

    The Normal Thermal Method:

    In curves (a) and (b) the

    temperature is measured as a

    function of the furnace

    temperature or time, while the

    sample is continuously heated at a

    constant rate of temperature


    No enthalpy change is indicated by

    the curve (a), but an endothermic

    and exothermic enthalpy change is

    indicated by the curves in (b).

    Subsequently no deviation in curve

    (a), but there are deviation in

    curves (b): the change in Ti is dueto exothermic or endothermic


    These changes subside at Tf and

    the sample temperature returns to

    that of the furnace temperature.

    The Differential Thermal Method:

    In curve (c) the difference between the sample

    temperature and that of the reference (TsTr) ismeasured as a function of the system

    temperature, T.

    At Ti, the curve deviates from horizontal and

    forms an upward peak (exothermic) or

    downward peak (endothermic).

    The final reaction temperature Tf, does not occur

    at the top of the peak, but lies on the higher

    temperature side of the relevant peak. Its exactposition depends on the instrumental system


    Therefore, in a differential thermal method, a

    very small temperature change can be detected

    easily and the peak area is proportional to the

    enthalpy change (H) and the sample mass.

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    DTA curve is a plot of the temperature difference (T= Ts - Tr) versus the temperature

    (T). Endothermic process is shown as a downward peak, while exothermic process

    is shown as an upward peak (Figure 6.3)

    Figure 6.3



    Four types of transitions

    detected by DTA method:

    I second order transition,

    where a change in the

    horizontal line is detected

    (e.g. glass transition);

    II a sharp endothermic curve

    due to the fusion or melting


    III a broad endothermic curve

    due to a decomposition or

    dissociation process;

    IV an exothermic curve due to

    the crystalline phase


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    Two approaches in the temperature sensing system of DTA (Figure 6.4):

    Temperature Sensor in DTA

    1. Classic DTA: the thermocouple is immersed into the sample and the reference materials

    2. Heat flux DTA: the thermocouple is placed outside the sample and the reference


    Figure 6.4

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    Differential Scanning Calorimetry (DSC)

    DSC is a technique in which the difference in the heat flow (power) to the sample (container) and the

    reference (container) is detected versus time or temperature while the sample is continuously heated

    in a certain atmosphere

    i) Power Compensated DSC: the sample and the reference are heated separately by

    using different heaters and the temperature difference between the sample and the

    reference is maintained zero while the electrical power needed to maintain that both

    temperatures are the same (P= d(Q/dt) is measured (Figure 6.5)

    Two Types of DSC:

    Figure 6.5

    The heat absorbed by the sample per unit time is

    compensated accurately by the difference in the

    electrical power (P) supplied to the heaters

    P = dq / dt = . (CSCR)

    The peak area is, therefore, directlyproportional to the value ofH. In the power

    compensated DSC the calibration constant

    needed to convert the peak area into joule is

    the electricalconversion factor.

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    The Effect of High Temperature

    Most DSC systems are designed to measure heat capacity and H attemperatures of less than 700 oC. At higher temperatures, radiation (which

    increases with the temperature rise) becomes more significant.

    Most DTA systems work on the principle of heat conductivity at lowertemperatures where the heat radiation is low. At higher temperatures the heat

    transition from its source to the sample takes place at faster rate and if the

    thermocouple is placed below the sample holder, the system becomes less


    When a sample undergoes a reaction, there is a temperature gradient in the

    sample. In order to obtain a meaningful calorimetric data, the sample size should

    be minimum to reduce the heat gradient in the sample. However, if the sample

    consists of different materials it is usually difficult to obtain a sufficiently small butrepresentative sample.

    The Effect of the Sample Size

    Power Compensated DSC

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    ii) Heat Flux DSC: the sample and the reference are heated using the sameheat source and the temperature difference T is measured (Figure 6.6). The

    signal is converted into the power difference Pand this value is plotted versus

    temperature or time

    Figure 6.6 Heat flux DSC (Source: Haines F 3.4)

    The DSC curve is a plot of P versus

    temperature (or time). Since an endothermic

    process involves absorption of more heat by the

    sample than the reference, the process is

    indicated by the upward peak.

    In a heat flux DSC system thethermocouple is placed under the

    sample and the reference containers

    (Figure 6.8 (a)-(e)). The base line

    variation is due to a change in the heat

    capacity of the sample and it is not

    influenced by other properties of the

    sample. The DSC system can be usedfor the determination of heat capacity

    of the material. However, since the

    base line variation is influenced by the

    physical properties of the sample

    container material, the instrument

    needs to be calibrated using a

    standard material.

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    The calibration constant Kis a thermal factor (varies with temperature) that is

    used to convert the peak area into joule.

    The main components of the DTA/DSC system is shown in Figure 6.7.

    The area under the peak is directly proportional to the heat of reaction:

    H = KTdt = K. (peak area)


    The main components ot the DTA/DSC system:

    The DTA/DSC detector and amplifier

    The furnace and temperature sensor

    Computer and temperature programmer

    Recorder, plotter or any data collection device

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    The DTA/DSC Detector

    In the DCS/DTA method, a thermocouple is used to detect the temperature change while the sample

    being heated under controlled conditions.

    For a low temperature process, a copperconstantan or cromel-alumel thermocouple may be used,

    while heating at high temperature or under corrosive atmosphere the Pt-Pt/13%Rh thermocouple isused.

    The choice of thermocouple that can be used for a DSC/DTA system is shown in Figure 6.8.

    A DTA/DSC experiment normally uses between 10 20 mg of sample.

    A sample container may also be specially designed for use in high pressure thermal experiment. For

    example in thepressure DSC(PDSC) technique, the sample container can should be able to stand

    up to 70 atm pressure. Hence, reactions that produce gas at high temperature can be studied.

    For example in the stability study of accelerated oxidative experiment by using a PDSC to a cooking

    oil under high oxygen pressure or under catalytic reduction of organic compound by using hydrogen.

    The Sample Container

    Various materials may be used to make the sample plate and crucible, but most of the sample

    container for use at lower temperatures (less than the melting point of Al: 660o

    C), are made fromaluminum.

    Platinum or ceramic may be used as the materials for the sample holder when the experiments

    involve heating at higher temperatures. However, the thermal conductivity difference must be taken

    into account in making a choice of the sample holder to be used.

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    Reference Materials

    DTA and DSC are defined as the differential techniques because the behaviour

    of the sample is compared with the behaviour of a reference.

    All thermal properties that could be involved in the thermal analysis experiment

    must be taken into account.

    For example, emissivity of the sample might change when the sample changed

    phase, or react or change in colour.

    Among the commonly used materials for the reference are calcined alumina

    Al2O3 or carborundum, SiC.

    Sometimes the sample is diluted with the reference material.

    In this case, the sample does not react with the reference material

    Dilution of the sample with the reference material will normally improves the

    base line and the the shape of the DTA peak.

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    Figure 6.9: The normal DTA or DSC curve for a melting material

    During the melting process,

    completion of the reaction is

    indicated by the tip of the


    Curve A indicates the curve

    that is obtained experimentallyreturns gradually to the

    baseline (curve A)

    Theoretically, the curve (peak)

    should return immediately to

    the baseline (curve B)

    Active area

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    Figure 6.10 A change in the temperature detected by the thermocouple

    before and after the thermal reaction which is due to a change in the

    thermal conductivity of the sample material (Source: Haines F 3.7)

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    The Second Type Theory: The Relationsh ip BetweenTand The Sample Temperature

    This theory takes into account various

    relationship between the temperaturedifference T and the sample

    temperature at certain times in order to

    obtain the activation energy and the

    order of the reaction.

    The theory, however, does not take into

    account the effect of heat transfer

    There exists a relationship between the

    activearea of the DTA peak thermogramand the quantity of heat involved.

    The active area of the DTA/DSC curve

    evolved from the thermal reaction process,

    while the other areas are produced as the

    T returns to the baseline after thereaction has completed (point E in Figure


    The shape of the curve (curve B) is

    exponential due to the influence of natural

    heating or cooling process and it depends

    on the arrangement of the sample holder.

    Therefore, the plot of ln(T) vs time

    beyond the point E of the curve A will belinear.

    The active area is the area that beginsfrom the point where the curve starts to

    the point E of the curve.

    The relationship between the active area

    and the reaction heat is shown by the

    following equation:

    Heat = A(active area)

    whereA = slope of the plot ln(T) vs time

    The theory assumes that the physical

    properties of the sample and the reference

    do not change during the thermal


    Therefore, the curve region after the point

    E is influenced by the instrumental system


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    For melting, crystal change and zero order reactions:

    The active region of the DTA curve (related to the reaction) ends near the tip of the

    DTA peak

    A portion of the DTA curve exponentially returns to the baseline which is influenced by

    the instrumental parameters

    Therefore, the end of the reaction and theA factor can be precisely determined.

    For reactions other than melting, the Tposition before the curve returns to thebaseline when the reaction has been completed will depend on the type of the


    However, if the reaction is controlled by a different type mechanism, the reaction ends up

    while the Tgets near to the baseline.

    Under such conditions, determination of the constantA would be difficult.

    The DTA curve that ends up at high temperature might contain a mixture of reaction

    mechanisms and the final portion of the curve is exponential, hence, differ from one device

    to another.

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    Since the tip of the DTA peak might

    not really represent the reaction

    process difficulty could arise if two

    close consecutive reactions

    produce two overlapping peaks orcompletely combined peak.

    i) due to the structural inversion of silica at

    573 oC

    ii) due to the crystal transition of potassium

    sulphate at 583 oC.

    The ability to separate such

    overlapping peaks indicate the

    resolution of the instrument.

    For example: a mixture of silica and potassium

    sulphate show two peaks:

    Most DTA theory assume that:

    a) during the thermal reaction, the physical properties of the material does not change

    and it does not depend on temperature,

    a) the baseline is the same before and after the reaction.

    However, most DTA/DSC curves do not have the same baseline before and after

    the reaction because physical changes always occur during the thermal reaction.

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    The baseline change of the DTA/DSC

    curve will affect:

    calculation of the area under the DTA


    the magnitude of error in the areacalculation which depends on the peak

    size: the smaller the peak the larger the


    the actual determination of the baseline

    If the baseline is changed, estimation of the

    peak area may be carried out by first drawing a

    straight line across the peak from where the

    peak leaves the baseline to the point where the

    peak meets the baseline again (Figure 6.11(a)) The result is that the measured peak area

    tends to become larger

    Two baselines approach may also be used as

    shown in Figure 6.11(b).

    Figure 6.11 Estimation of thearea under the DTA/DSC curve by

    constructing a baseline before andafter the reaction as shown by the

    dotted lines



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    The instrument temperature must be properly calibrated in order to obtain accurate data.

    For the DSC system, sensitivity of the calorimetry need to be calibrated.

    Some of the reference materials recommended by the ICTAC (International Confederation for

    Thermal Analysis and Calorimetry) for the calibration of DTA/DSC system is given in Table 6.1.

    The DSC thermogram of the melting process of pure indium metal (99.999%) is

    shown in Figure 6.12.

    The melting point 156.6 oC is obtained by determining the intersection point Te.

    The area marked under the peak, AS in the Figure is used for the determination of

    calorimetry sensitivity constant, K:

    K = HS . mS /AS

    Where HS = the fusion enthalpy of indium (28.71 J/g)

    mS = mass of the indium sample (g)

    AS = peak area (cm2)

    K = calorimetric sensitivity (J/cm2)

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    Figure 6.12 DSC peak for indium melting process which shows the determination ofintersection point Te (6 mg, 10 K/min, nitrogen) (Source: Haines: F 3.11)

    Calibration for a wide operational temperature

    range using various materials allows

    determination of relationship between K and


    Example: pure indium sample 6.68 mg gives a

    peak with area of 21.94 cm2. Calculate the value

    ofKat 156 oC.

    Answer: K = (28.70 x 6.68 x 10-3) / 21.94

    = 8.74 x 10-3 J / cm2

    Note !

    The DTA or DSC curve depends on the conditions of the sample

    and the instrument:

    Heating Rate Sample (chemical properties, purity, origin) Sample mass (volume, packing, distribution, dissociation) Crucible (material, geometry) Atmosphere (gas, static, flowing)