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 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 n 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 t the balance beam. The shutter partly blocks the light path between a lamp and a photocell. Movement of the beam alters the light intensity n the photocell and the amplified output from the photocell is used t restore the balance t 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 the field of a permanent magnet. rvision is also usually made for electrical tarring and for scale expansion 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 n 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 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 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 t 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 t 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 V in the weighing system. The mass of displaced gas is m= PV/RT (where is the pressure and the molar mass volume). The buoyancy thus depends not only of the V, but also of the pressure, temperature and nature of the gas. Attempts may be made to reduce V, 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. flowing atmosphere has the advantages 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 30C, 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. y 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. 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 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 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 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. 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)gas)g, while the measurement signal as a function of temperature is:

SMPg = ((mSC + mS + mA) - (VSC + VS + VA)gas)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 gas - density of gas.

The evaluation of a single TG curve is depicted in Fig. 6. The reactions corresponding t 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):

[ ]

+=

AsymHwd/)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)1.902.72H2O, 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

7654

3

21

dM/d

t, %

/min

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 24n2 takes place according the reaction (4) in three stages:

24n2 24 + n2 3 +1/2 2 + 2 (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 CaC2O4nH2O 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. ., Introduction to Thermal Analysis, Techniques and applications, 1998, CHAPMAN AND HALL New York

2. Brown, . ., Dollimore, D. and Galwey, . . (1980) Reactions in the Solid State,Comprehensive Chemical Kinetics, Vol. 22, (eds. C. . Bamford and C. F. . Tipper), Elsevier, Amsterdam Czanderna, . W. and Wolsky, S. . (1980)

Microweighing in Vacuum and Controlled Environments, Elsevier, Amsterdam. 3. Keattch, C. J. and Dol1imore, D. (1975) An Introduction t

Thermogravimetry, Heyden, London, 2nd edn. 4. Daniels, . (1973) Thermal Analysis, Kogan Page, London. 5. Duval, C. (1963) Inorganic Thermogrvimetric 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