Thermal Methods of Analysis

Thermal Analysis

• Thermal analysis is a branch of materials

science where the properties of materials

are studied as they change with

temperature.

Thermoanalytical Methods

• Thermogravimetry (TGA): 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

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

Thermogravimetry Analysis (TGA)

The mass of a sample is measured

as a function of the time and temperature.

A highly sensitive balance monitors the weight

loss of a sample (verses time and temperature).

Thermal Analysis

• TGA can provide information about

physical phenomena like vaporization,

sublimation, absorption, and desorption.

• Also chemical phenomena including

dehydration, decomposition and oxidation

and reduction reactions.

Material Characteristics

• From decomposition and degradation

patterns.

• Organic content and inorganic content

(ash).

• Especially useful for the study of polymers,

thermosets, plastics, coatings, paints

Material Characteristics

• TGA requires a high degree of precision in

measuring mass change (accurate

balance), programmable temperature and

temperature change over time.

• Therefore need precision balance,

programmable furnace with constant or

programmable heating rate.

Thermogravimetry Analyser

Microgram

Balance

Sample

crucible

Temperature programable

furnace

Mass Change (TGA)

Thermogram

Polytetrafluoroethylene

(teflon)

Calcium oxalate monohydrate (12.51 mg)

CO2 (39.75%,3.85 mg)

H2O, (11.68%, 1.46 mg)

CO (18.32%,2.92 mg)

Weight decreases as lose H2O, CO and CO2

TGA

Filler Content in Polymers

Instrumentation

Main components:

•Sensitive analytical balance

•Furnace

•Temperature programming unit

•Recorder

•Sample holder (thermally isolated)

Thermogravimetry (TGA)

Evolved Gas Analysis (EGA)

TGA-FT-IR• A Thermogravimetric Analyzer

(TGA) combined with an Infrared Spectrometer (TG-IR).

• Heating a sample on the TGA, will release volatile materials or generate combustion components as it burns.

• The components can be identified in the IR cell.

• This technique is most useful when the evolved gases are known small compounds such as water, carbon dioxide or common solvents which have characteristic IR spectra.

TGAIR

Evolved Gas Analysis (EGA)

TGA-MS• The combination of a TGA with

a MS allows you to detect very low levels of impurities in real time.

• Heating a sample on the TGA, the sample will release volatile materials or generate combustion components as it burns.

• These gases are transferred to the MS. This technique is most useful when the evolved gases or breakdown products are known in advance but are few in number.Mass-spec TGA

Evolved Gas Analysis (EGA)

TGA-MS

Evolved Gas Analysis (EGA)

TGA-MS

Calcium oxalate monohydrate (12.51 mg)

CO2 (39.75%,3.85 mg)

Mass = 44

H2O, (11.68%, 1.46 mg) mass = 18

CO (18.32%,2.92 mg), mass = 28

Weight decreases as lose H2O, CO and CO2

TGA-MS?

TGA-MS

Evolved Gas Analysis (EGA)

TGA-GC-MS• Heating a sample on the TGA

causes gases to be released.

• These gases are then transferred to the GC where the components can be separated and the peaks identified by the MS.

• Because of its ability to detect very low levels of material in complex mixtures, the TG-GC/MS is a powerful tool for quality control, safety, and product development.GC-MS

TGA

Differential Thermal Analysis

(DTA)

Sample Reference

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 be used to study thermal properties and phase changes (fusion, vaporization, sublimation, desorption, some chemical reactions) which do not lead to a change in enthalpy.

Differential thermal analysis (DTA),

In analytical chemistry, a technique for identifying and quantitatively analyzing

the chemical composition of substances by observing the thermal behaviour of

a sample as it is heated. The technique is based on the fact that as a

substance is heated, it 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. Moreover, 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.

Differential Thermal Analysis (DTA)

Absorbs Heat

Releases Heat

Absorbs Heat

Thermogram

area

Differential Thermal Analysis (DTA)

• 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.

Differential thermographS= sample being measured

R= reference inert material

Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry (DSC)

• DSC analysis operates by determining how much energy is required to heat a pan containing a sample compared to a reference pan.

• If the sample undergoes an endothermic (absorbs heat) or exothermic (gives off heat) reaction, the sample pan will require more or less energy to increase the temperature at the same rate as the reference pan.

• By measuring the difference between the energy applied to each heater as the temperature is increased, the energy consumed or released by the sample can be determined.

Differential Scanning Calorimeter

Sample Reference

Thermocouple

Heater

Control

Detector

Differential Scanning

Calorimeter

Differential Scanning Calorimetry is the most widely used

thermal analysis technique in the world. DSC measures the heat

flow in materials and provides information about phase changes,

such as amorphous and crystalline transitions (glass transition

temperature, melting point, crystallization point, crystallinity) as

well as chemical changes (aging, degradation, chemical

reactions, and thermal history). The data can be used to identify

materials, determine specific heat capacity of materials, degree

of cure, and to characterize the materials for their thermal

performance

Differential Scanning Calorimetry (DSC)

• DSC can determine phase transitions of

materials. These phase transitions include:

• Melting Point (Tm)

• Glass Transition Temperature (Tg)

• Energy Absorbed (Hm) while melting

• Crystallization Point (Tc)

• Energy Released (Hc) during crystallization

∆H(enthalpy) = K (constant) x A(area)

Crystalization

Temperature

Heat

Flow

(or

Flux)

Melting

Glass transition

(Softening)

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

temperature or versus time.

• There are two different conventions: exothermic reactions in the sample shown with a positive or negative peak, depending on the kind of technology used in the experiment.

• This curve can be used to calculate enthalpies of transitions. This is done by integrating the peak corresponding to a given transition.

• ∆H is the enthalpy of transition, K is the calorimetric constant, and A is the area under the curve. ∆H = K*A

• The calorimetric constant will vary from instrument to instrument, and can be determined by analyzing a well-characterized sample with known enthalpies of transition

Measures heat directly

Enthalpy

• Enthalpy is a measure of the total energy of a thermodynamic system.

• The unit of measurement for enthalpy in the International System of Units(SI) is the joule, but other historical, conventional units are still in use, such as the British thermal unit and the calorie.

• The total enthalpy, H, of a system cannot be measured directly. Enthalpy itself is a thermodynamic potential, so in order to measure the enthalpy of a system, we measure is the change in enthalpy, ΔH.

• The change ΔH is positive in endothermic reactions, and negative in heat-releasing exothermic processes.

• For processes under constant pressure, ΔH is equal to the change in the internal energy of the system, plus the work that the system has done on its surroundings.

Two types of methods used in DSC:

i) Power-compensated DSC

Sample and reference temperatures are kept equal

by heating with separate heaters while external

heat is increased or decreased linearly.

ii) Heat flux DSC

Difference in heat flux into the reference and sample is

measured as sample temperature is increased or

decreased linearly

Heat flux DSC

- One furnace system

- Sample and reference cups (Al) are placed on elevated

platforms on thermoelectric disc.

- Heat flows to the sample and reference

- Differential heat flow is monitored by chromel

thermocouples

- Differential heat flow directly proportional to output

Heat Flux DSC Cell Design

Thermocouples

Heater

Sample Reference

Furnace

DSC analysis to determine the spreading

qualities and wear qualities of lipsticks

Temperature

0-50 +50

Heat flux

DSC

Heat flow

Power-compensated DSC

- Two independent furnaces

- Furnaces are small, allow about 1 g of sample, for

rapid heating, cooling and equilibration.

- Furnaces are embedded in a large heat sink

- Sample and reference holders are equipped with Pt

resistance thermometers to continuously monitor

temperature (T) values.

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

for differential T control.

Power Compensated DSC

Sample

Reference

Heating block

Furnace

Temperature

sensor

DSC applications

DSC applications

- Acetaminophen exists in three polymorphic forms, but only two of which

can be readily isolated. Form I (monoclinic) is the commercially marketed

version.

- Form II(orthorhombic), however, has distinct advantages in its tableting

properties due to its plasticity, which enables direct compression without

binders .

- The crystal structures of both forms are known. Form II has proven difficult

to make on a large scale,

- Form II can be made on a small scale, either by slow cooling of the Form I

melt or by recrystallization.

- Characterization of the two forms has revealed that Form I is more stable

than Form II under ambient temperature conditions.

Polymorphic Forms

Form I

(Mp 169 o C)

Endothermic

Simultaneous TGA/DTA

• Simultaneous TGA/DTA measures both heat flow and weight changes (TGA) in a material as a function of temperature or time in a controlled atmosphere.

• Simultaneous measurement of these two material properties can help interpretation of the results.

• The complementary information obtained allows differentiation between endothermic and exothermic events with no associated weight loss (e.g., melting and crystallization) and those that involve a weight loss (e.g., degradation).

DTA/TGA

A plot of TG/DTA for the thermal behavior of YBaCuO 2, 4, 8

DTA/TGA

Some important application areas:

-Compositional analysis (quality control)

-Thermal and oxidative stability (food expirary dating)

-Product life time

-Filler content of materials (quality control)

-Moisture and volatile content

-Effects of reactive atmospheres on materials

-Decomposition kinetics (protein stability)

TGA/DSC

Mettler TGA/DSC

• The TGA/DSC 3+ uses a TGA balance from the

worldwide leader in weighing technology with position-

independent weighing, automatic internal calibration

weights, a wide measurement range, the best minimum

weight performance and the highest weighing accuracy

and precision.

• It allows users to analyze a wide variety of sample types

up to 1600 °C.

• A complementary DSC heat flow sensor simultaneously

detects thermal events such as melting and

crystallization in addition to providing accurate and

precise transition temperatures.

TGA/DCS/MS