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Measurements based on dynamic relationships between temperature and

• Mass• Volume• Heat of reaction

Methods:

• Thermogravimetry (TG)• Differential thermal analysis (DTA)• Differential scanning calorimetry (DSC)

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Thermal AnalysisThermal Gravimetric Analysis (TG)

- Measure change in weight during heating or cooling

Differential Thermal analysis (DTA)- Measure temperature difference between the

sample and reference.

Differential Scanning Calorimetry (DSC)- Measure heat absorbed or liberated during

heating or cooling

Thermoanalytical Methods• Thermogravimetry (TG): recording of sample weight changes during

controlled temperature programs (dynamical or isothermal)

• Differential thermoanalysis (DTA): recording of temperature difference between sample and reference crucible during controlled temperature programs. After calibration the heat flux into/out of the sample (reaction or phase transition enthalpy) can be calculated.

• Differential scanning calorimetry (DSC):– heat flux DSC: measurement of temperature difference between

sample and reference similar to DTA, but with higher sensitivity

– power compensated DSC: sample and reference are kept at the same temperature, the difference in the necessary heating power is recorded.

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Thermogravimetry (TG)Thermogravimetry (TG)

The mass of a sample is measured

as a function of the temperature,

which increases linearly (e.g. from RT to 1200oC)

Calcium oxalate monohydrate (12.51 mg)

CO2 (39.75%,3.85 mg)

H2O, (11.68%, 1.46 mg)

CO (18.32%,2.92 mg)

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Mass Change

Instrumentation

Main components:

•Sensitive analytical balance•Furnace•Temperature programming unit•Recorder•Sample holder (thermally isolated)

Thermogravimetry (TG)Thermogravimetry (TG)

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The change in mass is proportional to the amplified difference of current between two photodiodes recorded as the beam moves.

Furnace:

• Generally programmed linearly (5oC/min to 25oC/min)• Temperature monitored by a thermocouple placed

close to sample• Insulation and coating of furnace necessary

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Most important application areas:

-Compositional analysis

-Thermal and oxidative stability

-Product lifetime

-Filler content of materials

-Moisture and volatile content

-Effects of reactive atmospheres on materials

-Decomposition kinetics

Nylon Bristles of Toothbrush

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TGA degradation for two different HDPE bottles

Filler Content in Polymers

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Differential thermal analysis (DTA)Differential thermal analysis (DTA)

• Heat absorbed or released by a chemical system isrecorded by measuring the temperature differencebetween the system and an inert reference.

•The temperatures of system and reference are increased at the same constant rate.

Differential Thermal analysis (DTA)

1. DTA involves heating or cooling a test sample and an inert reference under identical conditions, while recording any temperature difference between the sample and reference.

2. This differential temperature is then plotted against time, or against temperature. Changes in the sample which lead to the absorption or evolution of heat can be detected relative to the inert reference.

3. DTA can therefore be used to study thermal properties and phase changes which do not lead to a change in enthalpy.

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• Differential thermal analysis (DTA), is a technique for identifying and quantitatively analyzing the chemical composition of substances by observing the thermal behaviour of a sample as it is heated

• and undergoes reactions and phase changes that involve absorption or emission of heat.

• In DTA the temperature of the test material is measured relative to that of an adjacent inert material.

• A thermocouple imbedded in the test piece and another in the inert material are connected so that any differential temperatures generated during the heating cycle are graphically recorded as a series of peaks on a moving chart.

• The amount of heat involved and temperature at which these changes take place are characteristic of individual elements or compounds; identification of a substance, therefore, is accomplished by comparing DTA curves obtained from the unknown with those of known elements or compounds.

• The amount of a substance present in a composite sample will be related to the area under the peaks in the graph, and this amount can be determined by comparing the area of a characteristic peak with areas from a series of standard samples analyzed under identical conditions.

• The DTA technique is widely used for identifying minerals and mineral mixtures.

Figure 31.7

Differential thermal analysisDifferential thermal analysis

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Aspects of a differential thermogramAspects of a differential thermogram

Minima: sample becomes cooler than reference becausereactions are endothermic.

Maxima: sample becomes warmer than reference,reaction is exothermic.

Endothermic processes: fusion, vaporization, sublimation,desorption, some chemical reactions.

Differential Thermal Analysis• A DTA consists of a sample holder comprising

thermocouples, sample containers and a ceramic or metallic block; a furnace; a temperature programmer; and a recording system.

• The key feature is the existence of two thermocouples connected to a voltmeter. One thermocouple is placed in an inert material such as Al2O3, while the other is placed in a sample of the material under study.

• As the temperature is increased, there will be a brief deflection of the voltmeter if the sample is undergoing a phase transition. This occurs because the input of heat will raise the temperature of the inert substance, but be incorporated as latent heat in the material changing phase

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Differential thermographDifferential thermograph

Peak areas: Peak areas: depend upon m, �H, geometry, conductivity.

kHGm

A∆−=

G = calibration factor based on sample geometryk = constant related to thermal conductivity of sample

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Differential scanning calorimetry (DSC)Differential scanning calorimetry (DSC)

• Heat is added to the sample or to the reference so thatthey both remain at identical temperatures.

•The amount of added heat is recorded.

Differential Scanning Calorimeter

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�H(enthalpy) = K (constant)A(area)

Thermogram obtained by Thermogram obtained by DSCDSC

Minima (negative peaks): system cools down, must be given heat to keep same temperature as reference.

Maxima (positive peaks): system warms up, reference mustbe given heat to keep up with sample.

Endothermic processes: fusion, vaporization, sublimation,desorption, some chemical reactions.

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Two types of methods used in DSC:Two types of methods used in DSC:

i) Power-compensated DSC

Sample and reference heated by separate heaters such thattemperatures are kept equal while increased or decreasedlinearly.

ii) Heat flux DSC

Difference in heat flux into the reference and sample is measured as sample T is increased or decreased linearly.

PowerPower--compensated DSC:compensated DSC:

- Two independent furnaces- Furnaces are small, allow about 1 g of sample, for rapidheating, cooling and equilibration.

- Furnaces are embedded in a large T-controlled heat sink- Sample and reference holders are equipped with Pt resistance thermometers to continuously monitor T values.

Two control circuits, one for average T control, the other for differential T control.

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Heat flux DSC:Heat flux DSC:

- Heat flows in the sample and reference- A thermoelectric disc is used for this purpose- Sample and reference cups (Al) are placed on elevatedplatforms on disc

- Differential heat flow is monitored by chromelthermocouples

- Differential heat flow directly proportional to output- Difference between both thermocouple junctions

Heat flux DSC cell

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Simultaneous DSC-TGA

• Simultaneous DSC-TGA employs a dual horizontal beam design, which allows for the difference in temperature and weight to be constantly measured.

Differential Thermal Analysis