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


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that 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. \u392y 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. \u39d\u3bf 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 t\u3bf 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 t\u3bf 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 t\u3bf 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. \u391 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 \u2192Ag20 \u2192 
2Ag+1/2O2)· 
The buoyancy force FB is equal to (VSC + VS + VA)\u2022\u3c1gas)\u2022g, while the 
measurement signal as a function of temperature is: 
 
 SMP\u2022g = ((mSC + mS + mA) - (VSC + VS + VA)\u2022\u3c1gas)\u2022g. (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 \u3c1gas - density of gas. 
The evaluation of a single TG curve is depicted in Fig. 6. The reactions 
corresponding t\u3bf 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): 
 [ ]
\uf8fa
\uf8fa
\uf8fb
\uf8f9
\uf8ef
\uf8ef
\uf8f0
\uf8ee
\uf8f7\uf8f7
\uf8f8
\uf8f6
\uf8ec\uf8ec
\uf8ed
\uf8eb \u2212\u22c5\u22c5+
\u2212\u22c5=
Asym
Hwd/)Posx(Asym21ln2lnexpAmply
2
 (3) 
 
where: Ampl \u2013 peak amplitude, Asym \u2013asymmetry of the peak, Pos \u2013 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.