Thermogravimetry (TG) or Thermogravimetric Analysis (TGA) (Tiverios C. Vaimakis)
DisciplinaAnálise Térmica29 materiais • 85 seguidores
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.