Thermobalances Thermocouple

As previously mentioned, in thermogravimetry the mass-change of the sample is continuously recorded as a function of temperature. The temperature, in this definition, may be that of the furnace chamber, the temperature near the sample (i.e., in close contact with the sample container), or the temperature of the sample. These three sources of temperature detection are shown in Figure 3.10. In (a), the thermocouple is near the sample container but not in contact with it. There is a correlation between ihe temperature of the container and that detected by the thermocouple, but the thermocouple will either lead or lag behind the sample lemperature, depending on the thermochemistry of the reaction. Most thermobalances use this type of... 

Figure 3.10. Location of temperature detection thermocouple in a thermobalance (12).

The calibration of the temperature of the furnace and/or sample chamber has been discussed by Stewart (12) and Norem et al. (14, 15). Stewart (12) used a conventional thermobalance which contained a thermocouple mounted external to the sample, while Norem et al. (14, 15) calibrated a furnace which used a resistance element for lemperature detection. 

The hangdown tube and furnace assembly used by Etter and Smith (47) is illustrated in Figure 3.31. A small platinum resistance heater located inside a relatively short quartz sample tube is used as the furnace. The temperature-sensing thermocouple is located inside the furnace chamber with entry into the chamber from the bottom. This design is similar to that of the Perkin-Elmer thermobalance. 

Figure 3.32. An automated thermobalance, (a) Balance, furnace, and sample changer mechanism (ft) furnace and sample holder (50). ( ) A. gas flow-meter B. furnace C. sample-holder disk D. cooling fan E, Cahn Model rtl recording balance F. balance platform, (ft) A. gas inlet tube B, thermocouples C. furnace heater windings and insulation D. sample container E. sample-holder disk F. ceramic sample probe.

Figure 3.34. Schematic diagram of the high-pressure thermobalance enclosure. A, end plate with threaded opening for gas inlet fitting B, Buna-N O-ring C. pressure cell D. high-pressure connector for control cable E, balance movement F. furnace chamber G. furnace thermocouple H, furnace heater wire in Marinite insulation J. hexdrive bolts K, end plate with threaded opening for gas outlet fitting (68).

The system is designed to collect up to six channels of analog information, as a function of time, from each thermobalance. Nominal collection rate is one data set logged alternately from each instrument every 5 sec for a per instrument rate of six sets per min. Data acquired from the two thermobalances is converted to actual units, such as temperature in °C. and so on. and stored in two arrays of 100 data sets, with one array being assigned to each instrument The conversion of the thermocouple EMF into temperature is based on two polynomials, one for the PtRhlO%-Pt system and the other for NiCr-Ni and stored in the data acquisition program. When the two arrays are filled, they are automatically recorded on tape. 

Identification of the thermobalance, including the location of the temperature-measuring thermocouple. 

In many TG experiments, the temperature of the furnace is raised at a constant rate. This type of experiment is referred to as non-isothermal, scanning or rising temperature. An alternative experimental technique is available, and is often used in kinetic studies. Instead of raising the temperature at a constant rate, the temperature is held constant and the mass loss (or mass gain) observed at this fixed temperature. The results are then presented as mass loss against time, t. In practice the sample has to be placed on the thermobalance and the furnace at first left away from the sample. The furnace is then run up to the required temperature and left to stabilise. When the furnace temperature is constant at the required value, the furnace has to be moved quickly around the sample. There are a number of difficulties with this technique. The sample, crucible, thermocouple and cradle have to move rapidly from room temperature to the experimental temperature. They all have a finite thermal capacity, so cannot heat instantaneously. There is a thermal lag while the sample temperature rises. The first part of this rise does not matter, because the reaction being studied will not occur rapidly at lower temperatures. However, as the reaction temperature is approached, some reaction will... 

A TG-DSC instrument capable of sub-ambient temperature operation has been available for a number of years, and is especially valuable in the study of systems containing moisture, or other volatiles. In this case, a DSC heat flux plate was adapted to allow it to be suspended from the thermobalance beam, instead of the pair of thermocouples shown in Figure 1. A completely different approach to TG-DSC is taken by SETARAM, in their TG-DSC 111 instrument. The sample and reference... 

The heart of the thermogravimetric analyzer is the thermobalance, which is capable of measuring the sample mass as a function of temperature and time. The relationship between the components of a thermobalance varies from one instrument to another. A schematic representation as shown in Figs. 3.1a and 3.1b indicates typical thermocouple placements relative to the sample. The three standard sample and furnace positions relative to the balance are depicted in Fig. 3.1a. Figure 3.2 shows actual examples of currently available commercial instruments. 

Figure 3.2. Three examples of commercial thermobalances, including typical thermocouple placement (a) a top-loading model (courtesy of Netzsch Instruments) (b) a side loading model (courtesy of Mettler-Toledo) (c) a bottom-loading model (courtesy of TA Instruments).

Adsorption or reaction under corrosive gas, such as CO, NH3 but also halogens (chlorine, fluorine), can be investigated using the TGA technique but caution has to be taken to avoid the contact between the corrosive gas and any metallic part of the thermobalance (especially the thermocouple). 

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