10. Thermogravimetry (TG) or Thermogravimetric Analysis
Tiverios C. Vaimakis
Chemistry Department, University of Ioannina, P. O. Box 1186, Ioannina 45110, Greece
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.
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
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
The sample temperature, Ts, will usually lag behind the furnace temperature, Tf, and Ts. cannot be measured very readily