Thermogravimetry (TG) or Thermogravimetric Analysis (TGA) (Tiverios C. Vaimakis)

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10. 
Thermogravimetry (TG) or Thermogravimetric Analysis 
(TGA) 
 
Tiverios C. Vaimakis 
Chemistry Department, University of Ioannina, P. O. Box 1186, Ioannina 45110, Greece 
 
Introduction 
Thermogravimetry (TG). A technique whereby the weight of a substance, in an 
environment heated or cooled at a controlled rate, is recorded as a function of time or 
temperature. Thus, the data obtained from a TG experiment are displayed as a thermal 
curve with an ordinate display having units of weight (or weight percent) and the 
abscissa may be in units of either temperature or time. [The abbreviation TG has been 
used, but should be avoided, so that it is not confused with Tg (glass transition 
temperature)]. Many types of materials can be characterized by techniques of 
thermogravimetry, and there are numerous applications of TG for materials 
characterization by the quantitative weight losses that occur in specified temperature 
regions of the TG thermal curve (see Table 1). 
In most TG studies, mass loss is read directly in units of weight percent of the 
original sample quantity. The results from thermogravimetric analysis may be 
presented by (1) mass versus temperature (or time) curves, referred to as 
Thermogravimetric curve, or (2) rate of mass loss versus temperature curve, referred to 
as Derivative Thermogravimetric (DTG). The results of a TG experiment may be used, 
in many cases, as "compositional analysis". A common example of this is the 
assignment of moisture content of polymers and coals. Another example would be the 
determination of residual solvent in many pharmaceutical compounds. The 
determination of ash value or ash residues also fall into this category since the 
remaining weight is read directly as weight or weight percent. Also, by using the 
techniques of TG, can determine the purity of a mineral, inorganic compound, or 
organic material. 
TGA can be used to evaluate the thermal stability of a material. In a desired 
temperature range, if species is thermally stable, there will be no observed mass 
change. TGA also gives the upper use temperature of a material. 
 
Table 1. Processes accompanied by weight change 
Process Weight gain Weight loss 
Adsorption  
Absorption  
Desorption  
Drying  
Dehydration  
Desolvation  
Vaporisation  
Decomposition  
Solid-solid reactions  
Solid-gas reactions   
Oxidation  
 
Measurements of changes in sample mass with temperature are made using a 
thermobalance. Thermogravimetric analysis relies on high degree of precision in three 
measurements: mass change, temperature, and temperature change. Therefore, the 
basic instrumental requirements for TGA are a precision balance with a pan loaded 
with the sample, and a programmable furnace. The furnace can be programmed either 
for a constant heating rate, or for heating to acquire a constant mass loss with time. 
The atmosphere in the sample chamber may be purged with an inert gas to prevent 
oxidation or other undesired reactions. The balance should be in a suitably enclosed 
system so that the atmosphere can be controlled (Fig. 1). 
 
 
Figure 1 Α schematic thermobalance instrumentation. 
 
The TGA instrument continuously weighs a sample as it is heated. TGA analytic 
technique may couple with FTIR and Mass spectrometry gas analysis. As the 
temperature increases, various components of the sample are decomposed and the 
volatile products can be measured. This technique called Evolved gas analysis (EGA), 
and could determine the nature and/or amount of volatile product or products formed 
during thermal analysis. The recording is the corresponding curve of species as 
ordinate against either t or T as abscissa. 
 
The balance 
Several types of balance mechanism are possible. These include beam, spring, 
cantilever and torsion balances. Some operate οn measurements of deflection, while 
others operate in null mode. Null-point weighing mechanisms are favored in TG as 
they ensure that the sample remains in the same zone of the furnace irrespective of 
changes in mass. 
Various sensors have been used to detect deviations of the balance beam from the 
null-position. Some of them use an electro- optical device with a shutter attached tο 
the balance beam. The shutter partly blocks the light path between a lamp and a 
photocell. Movement of the beam alters the light intensity οn the photocell and the 
amplified output from the photocell is used tο restore the balance tο the null-point 
and, at the same time, is a measure of the mass change. The restoring mechanism is 
electromagnetic. The beam has a ribbon suspension and a small coil at the fulcrum, 
located ίη the field of a permanent magnet. Ρrοvision is also usually made for 
electrical tarring and for scale expansion το give an output of mass loss as a 
percentage of the original sample mass. 
Use of the piezoelectric effect in certain crystals (usual1y quartz) for measuring 
the mass of material deposited or condensed on a crystal face is wel1 documented. 
There are basically two ways in which such crystals can be used in TG studies. 
Usually, the sample may be heated separately in one part of a reaction chamber and 
the face of a crystal which is held at a suitably low temperature. Changes in the 
amount of material deposited οn the crystal surface show up as changes of frequency 
of oscillation of the crystal, which is usual1y excited in a conventional series 
resonance circuit. The observed change in frequency depends on the value of the 
frequency itself and the mass and area of the coating ου the crystal face. Mass changes 
of as little as 10-12 g can be detected. 
The output signal may be differentiated electronically to give a derivative 
thermogravimetric (DTG) curve and represent the rate of mass change. 
 
DTG signal = dm/dt (1) 
 
When the TG curve is stable the DTG curve is fitted in zero line. The peak maximum 
temperature is corresponded with the inflection point of TG curve. 
 
Figure 2. TG curve and corresponding DTG curve. 
 
Heating the sample 
In most conventional thermobalances, there are three main variations in the 
position of the sample relative ιο the furnace, as they are depicted in Fig. 3. The 
furnace is normally an electrical resistive heater and may also, as shown, be within the 
balance housing, part of the housing, or external tο the housing. It should have a 
uniform hot-zone of reasonable length and not affect the balance mechanism through 
radiation or convection. Transfer of heat tο the balance mechanism should be 
minimized by inc1usion of radiation shields and convection baffles. Heating by 
radiation becomes significant only at high temperatures in such furnaces, but 
alternative heating systems, using either infrared or microwave radiation, have been 
considered. For infrared heating the light from several halogen lamps is focused onto 
the sample by means of elliptic or parabolic reflectors. 
 
 
Figure 3. Alternative arrangements of furnace. 
 
The atmosphere 
Thermobalances are normally housed in glass or ceramic systems, to allow for 
operation at pressure range varying from high vacuum (< 10-4 Pa) to high pressure (> 
3000 kPa), of inert, oxidizing, reducing or corrosive gases. A correction should be 
made for buoyancy arising from lack of symmetry ΔV in the weighing system. The 
mass of displaced gas is m= PΜΔV/RT (where Ρ is the pressure and Μ the molar mass 
volume). The buoyancy thus depends not only of the ΔV, but also of the pressure, 
temperature and nature of the gas. Attempts may be made to reduce ΔV, or a 
correction may be applied by heating an inert sample under similar conditions to those 
to be used in the study of the sample of interest. 
At atmospheric pressure, the atmosphere can be static or flowing. Α flowing 
atmosphere has the advantagesthat it: (i) reduces condensation of reaction products on 
cooler parts of the weighing mechanism; (ii) removes out corrosive products; (iii) 
reduces secondary reactions; and (iv) acts as a coolant for the balance mechanism. The 
balance mechanism should, however, not be disturbed by the gas flow. 
The atmosphere affects on the noise level of TG traces. The use of dense carrier 
gases at high pressures in hot zones with large temperature gradients gives the most 
noise. Noise levels also increase as the radius of the hangdown tube increases. 
Thermal convection, and hence noise, can be reduced by introducing a low density 
gas, such as helium. Alternatively, and more practically, baffles and radiation shield 
can be introduced in the hangdown tube (Fig. 4). 
 
 
Figure 4. Reduction of convection effects by use of baffles or radiation shields in the 
hangdown tube. 
 
 
The sample 
Solids with similar chemical composition, have structural differences in the solid, 
such as the defect content, the porosity and the surface properties, which are dependent 
on the way in which the sample is prepared and treated after preparation. So the samples 
may have considerable differences in their behavior on heating. For example, 
significant different behavior will generally be observed for single crystals compared to 
finely ground powders of the same compound. 
As the amount of sample used increases, several problems arise. The temperature 
of the sample becomes non-uniform through slow heat transfer and through either self-
heating or self-cooling as reaction occurs. Also exchange of gas within the surrounding 
atmosphere is reduced. These factors may lead to irreproducibility. Small sample 
masses also protect the apparatus in the event of explosion or deflagration. The sample 
should be powdered where possible and spread thinly and uniformly in the container 
 
Calibration 
The sample temperature, Ts, will usually lag behind the furnace temperature, Tf, 
and Ts. cannot be measured very readily without interfering with the weighing 
process. The lag, Tf-Ts, may be as much as 30°C, depending upon the operating 
conditions. Temperature is measured usually by thermocouple and it is necessary to 
have separate thermocouples for measurement of Ts and for furnace regulation. 
One method of temperature calibration uses the Curie points. A ferromagnetic 
material loses its ferromagnetism at a characteristic temperature known as the Curie 
point. If a magnet is positioned below the ferromagnetic material (Fig. 4), at 
temperatures below the Curie point, the total downward force on the sample is the sum 
of the sample weight and the magnetic force. At the Curie point the magnetic force is 
zero and an apparent mass loses is observed. Βy using several ferromagnetic materials, 
a multi-point temperature calibration may be obtained. 
 Figure 4. Curie-point method of temperature calibration 
 
TGA temperature calibration is commonly accomplished using melting point or 
phase transformation of standards materials (see Table 1). 
TGA weight calibration is most modern thermobalance is very simple. In the 
software, there is a corresponding calibration procedure using standard weights. 
 
 
Table 1. Calibration Materials and Calibrate Temperature (°C) 
Material Temperature (oC) Material Temperature (oC) 
Biphenyl 69.3 Hg -38.8 
Benzil 94.5 Ga 29.8 
Benzoic Acid 122.4 In 156.6 
Diphenylacetic Acid 147.3 Sn 231.9 
Anisic Acid 183.3 Bi 271.4 
2-Chloroanthraquinone 209.6 Pb 327.5 
 Zn 419.6 
 CsCl 476.0 
 Al 660.3 
 Ag 961.9 
 
Interpretation of TG and DTG curves 
Actual TG curves obtained may be classified into various types as illustrated in 
Fig. 5. Possible interpretations are as follows. 
Type (i) curve. The weight sample is stable over the temperature range 
considered. Νο information is obtained, however, on whether solid phase transitions, 
such as melting, polymerization or other reactions involving no volatile products have 
occurred. 
Type (ii) curve. The rapid initial mass loss observed, is characteristic of 
desorption or drying. The buoyancy phenomenon is observed. 
Type (iii) curve represents decomposition of the sample in a single stage. The 
curve may be used tο determine the stoichiometry of the reaction, and to investigate 
the kinetics of reaction. 
Type (iv) curve indicates multi-stage decomposition with relatively stable 
intermediates. The curve may be used tο determine the stoichiometry and to 
investigate the kinetics of reaction, for all stages. 
Type (v) curve also represents multi-stage decomposition, but in this example 
stable intermediates are not formed and little information for the stages can be 
obtained. At lower heating rates, type (v) curves may tend tο resemble type (iv) one, 
while at high heating rates both type (iv) and type (v) curves may resemble type (iii) 
curves and hence the detail information for stages is lost. 
Type (vi) curve. The weight sample is increased as a result of reaction of the 
sample with the surrounding atmosphere. Α typical example would be the oxidation of 
a metal sample. 
Type (vii) curve. This is a characteristic TG curve representing an oxidation 
reaction which decomposes again at higher temperatures (e.g. 2Ag+1/202 →Ag20 → 
2Ag+1/2O2)· 
The buoyancy force FB is equal to (VSC + VS + VA)•ρgas)•g, while the 
measurement signal as a function of temperature is: 
 
 SMP•g = ((mSC + mS + mA) - (VSC + VS + VA)•ρgas)•g. (2) 
 
where: mA - mass of adsorbed gas, mSC - mass of sample container, mS - mass of 
sample, VA - volume of adsorbed gas , VSC - volume of sample container, VS - volume 
of sample, and ρgas - density of gas. 
The evaluation of a single TG curve is depicted in Fig. 6. The reactions 
corresponding tο the mass losses can best be determined, or confirmed, by 
simultaneous evolved gas analysis (EGA). For example, in Fig. 7, the appearance of 
traces of H2O, CO2 and CO in the evolved gases would indicate the onset of 
crystallized water removal and carbonate decomposition of CaC2O4.H2O. 
 
 
 
 
Figure 5. Main types of 
thermogravimetric (TG) curves. 
Figure 6. The evaluation of a single TG 
curve. 
 
 
Figure 7. The TG and mass 
spectrometry curves of CaC2O4.H2O 
decomposition. 
Figure 8. TG curve for multi-stage 
decomposition and corresponding DTG 
curve. 
 
Resolution of stages of more complex TG curves can be improved by recording 
DTG curves (Fig. 8). If the peaks of DTG are overlapped, we can use special software 
for deconvolution of them. The DTG curves usually have an asymmetric Gaussian 
distribution profile (Fraser-Suzuki profile) which is depicted from the equation 
(NETZSCH Separation of Peaks software): 
 [ ]













 −⋅⋅+
−⋅=
Asym
Hwd/)Posx(Asym21ln2lnexpAmply
2
 (3) 
 
where: Ampl – peak amplitude, Asym –asymmetry of the peak, Pos – peak position 
(temperature), Hwd - the observed peak width at half maximum peak height. 
For example, the thermal decomposition of calcium deficient hydroxyapatite with 
empirical type: Ca9.90(HPO4)0.10(PO4)5.90(OH)1.90·2.72H2O, is depicted in Fig. 9 and 
the corresponding peak deconvolution of DTG curve is depicted in the Fig. 10. The 
output result includes the peak area and the mass loss, as well as the optimum 
parameters of the single peaks. 
0 200 400 600 800 1000 1200 1400
 
TG
DTG
 
Temperature, oC
 
0 200 400 600 800 1000 1200 1400
-0,06
-0,05
-0,04
-0,03
-0,02
-0,01
0,00
 DTG
 Sum
76
54
3
2
1
dM
/d
t, 
%
/m
in
Temperature, oC 
Figure 9. TG and DTG curves of 
hydroxyapatite. 
Figure 10. The peak separation of 
hydroxyapatite DTG curve.Another way to isolate the various reactions that occur at similar temperatures 
and produce more accurate results is simplifies quasi-isothermal thermogravimetry 
mode. Fig. 11 shows the principles of quasi-isothermal thermogravimetry. In these 
measurements, the heating at a constant rate when there is no weight change. When a 
weight change and the DTG signal passes over the upper limit threshold, the 
temperature temporarily stops rising. At this temperature, an isothermal plateau is 
maintained and the weight change is measured. When, in this plateau, the DTG signal 
passes below the lower limit threshold (weight change stops), the heating again 
continues to rise until the next weight change. Fig. 12 shows the thermal dehydration 
of CuSO4.5H2O using quasi-isothermal thermogravimetry mode. 
 
Figure 11. The principles of quasi- Figure 12. The dehydration of CuSO4.5H2O. 
isothermal thermogravimetry mode. 
 
 
Influences of experimental conditions on TG/DTG curves 
The properties of the system and the characteristics of samples influence the 
experimental TG curves (see Table 2) 
 
Table 2. The experimental conditions influence on TG/DTG curves 
 Thermal Analysis Setup Characteristics of the sample 
a) Reaction Atmosphere 
b) Size and shape of the oven 
c) Sample holder material 
d) Sample holder geometry 
e) Heating rate 
f) Thermocouple (wire diameter) 
g) Thermocouple location 
h) Response time 
 
a) Particle size 
b) Thermal conductivity 
c) Thermal capacity 
d) Packing density of particles 
 (powder, pill, tablet) 
e) Sample expansion and shrinking 
f) Sample mass 
g) Inert filler 
h) Degree of crystallinity 
 
Examples 
a) Effect of heating rate 
The decomposition of PTFE at various heating rates is depicted in Fig. 13. From the Fig. 13, 
we can conclude: (Ti)F > (Ti)S, (Tf)F > (Tf)S and (Tf - Ti)F > (Tf - Ti)S, where Ti and Tf - initial 
and final temperature of decomposition, and F and S subscripts indicate the fast and slow heating 
rates. 
 
 
Figure 13. The TG curves of PTFE, heated at 2.5, 5, 10 and 20 °C/min in nitrogen. 
 
Table 3. The effect of heating rate on DTG peaks. 
 Heating rate 
 Slow Fast 
Effect 
Little base-line drift Base-line drift may be appreciable 
Near-equilibrium conditions Conditions far from equalibrium 
Broad shallow peaks on 
 DTG/t curves 
Large narrow peaks on 
 DTG/t curves 
Sharp small peaks on DTG/T curves Large broad peaks on DTG/T curves 
Long time per determination Short time per determination 
 
b) Influence of external gas flow 
The gas flow, above sample, removes the produced volatile product and affect 
on the decomposition of sample. The decomposition of СаС2О4∙nН2О takes place 
according the reaction (4) in three stages: 
СаС2О4∙nН2О→ СаС2О4 + nН2О → СаСО3 +1/2 СО2 → СаО + СО2 (4) 
The three stages are clear when dry air flow is used. In static atmosphere the two first 
stages are overlapped and occur at higher temperature range (see Fig 14). 
 
Figure 14. Decomposition of CaC2O4·nH2O in an open crucible in a dry air flow (red 
curve) and in a static atmosphere (green curve). 
 
c) Influence of external atmosphere 
The external atmosphere may effect on the decomposition equilibrium. In the 
case of calcite decomposition (Fig. 15) in vacuum, the produced CO2 is removed and 
the decomposition takes place at lower temperature range than dry air atmosphere. On 
the contrary, the presence of CO2 in atmosphere, displays the decomposition at higher 
temperature range. 
 
Figure 15. TG curves of CaCO3 decomposition in vacuum, dry air and CO2 
atmosphere. 
 
The TG curves may be used for quantitative analysis of materials. For example, 
in the thermal composition of a rubber sample, if initially the atmosphere is inert gas 
(N2) two stages are observed which are attributed to removal of remained oil and the 
charring of polymer, respectively. When the atmosphere is changed into air, the burn 
of carbon takes place while the residue solid is ash. 
 
 
Figure 16. TG curve of rubber. 
 
Applications of TGA 
Ability of TG to generate fundamental quantitative data from almost any class of 
materials, has led to its widespread use in every field of science and technology. Key 
application areas are listed below: 
►Thermal Stability: related materials can be compared at elevated temperatures under 
the required atmosphere. The TG curve can help to elucidate decomposition 
mechanisms. 
►Material characterization: TG and DTG curves can be used to "fingerprint" 
materials for identification or quality control. 
►Compositional analysis: by careful choice of temperature programming and gaseous 
environment, many complex materials or mixtures may be analyzed by selectively 
decomposing or removing their components. This approach is regularly used to 
analyze e.g. filler content in polymers; carbon black in oils; ash and carbon in coals, 
and the moisture content of many substances. 
►Simulation of industrial processes: the thermobalance furnace may be thought of as 
a mini-reactor, with the ability to mimic the conditions in some types of industrial 
reactor. 
►Kinetic Studies: a variety of methods exist for analyzing the kinetic features of all 
types of weight loss or gain, either with a view to predictive studies, or to 
understanding the controlling chemistry. Values of activation energies, obtained in this 
way, have been used to extrapolate to conditions of very slow reaction at low 
temperatures and to very fast reaction at high temperatures. 
►Corrosion studies: TG provides an excellent means of studying oxidation, or 
reaction with other reactive gases or vapors. 
► Material preparation: The mass losses define the stages, and the conditions of 
temperature (and surrounding atmosphere) necessary for preparation of the anhydrous 
compounds, or intermediate hydrates, can be established immediately. 
 
 
References 
 
1. Brown M. Ε., Introduction to Thermal Analysis, Techniques and applications, 
1998, CHAPMAN AND HALL New York 
2. Brown, Μ. Ε., Dollimore, D. and Galwey, Α. Κ. (1980) Reactions in the Solid 
State,Comprehensive Chemical Kinetics, Vol. 22, (eds. C. Η. Bamford and C. F. Η. 
Tipper), Elsevier, Amsterdam Czanderna, Α. W. and Wolsky, S. Ρ. (1980) 
Microweighing in Vacuum and Controlled Environments, Elsevier, Amsterdam. 
3. Keattch, C. J. and Dol1imore, D. (1975) An Introduction tο 
Thermogravimetry, Heyden, London, 2nd edn. 
4. Daniels, Τ. (1973) Thermal Analysis, Kogan Page, London. 
5. Duval, C. (1963) Inorganic Thermogrαvimetric Analysis, Elsevier, 
Amsterdam, 2nd edn 
6. J. Sestak, Thermophysical properties of solids, Academia, Prague, 1984 
7. Moukhina E., J. Therm Anal. Calorim. (2012) 109: 1203-1214 
8. http://www.netzsch.com