Thermobalance

Experimental factors. In the previous section it was stated that the precise temperature regions for each reaction of the thermal decomposition of copper sulphate pentahydrate is dependent upon experimental conditions. When a variety of commercial thermobalances became available in the early 1960s it was soon realised that a wide range of factors could influence the results obtained. Reviews of these factors have been made by Simons and Newkirk30 and by Coats and Redfern31 as a basis for establishing criteria necessary to obtain meaningful and reproducible results. 

Since thermogravimetry is a dynamic technique, convection currents arising in a furnace will cause a continuous change in the gas atmosphere. The exact nature of this change further depends upon the furnace characteristics so that widely differing thermogravimetric data may be obtained from different designs of thermobalance. 

When using a modern thermobalance which incorporates an electronic microbalance requiring small sample weights, the following operating precautions should be noted. 

The following experiments are designed to make the operator familiar with the use of the thermobalance. 

A. The thermal decomposition of calcium oxalate monohydrate. This determination may be carried out on any standard thermobalance. In all cases the manufacturer s handbook should be consulted for full detailed instructions for operating the instrument. 

Steam gasification and combustion kinetics of gingko nut shell in a Thermobalance Reactor... 

The TGA system was a Perkin-Elmer TGS-2 thermobalance with System 4 controller. Sample mass was 2 to 4 mgs with a N2 flow of 30 cc/min. Samples were initially held at 110°C for 10 minutes to remove moisture and residual air, then heated at a rate of 150°C/min to the desired temperature set by the controller. TGA data from the initial four minutes once the target pyrolysis temperature was reached was not used to calculate rate constants in order to avoid temperature lag complications. Reaction temperature remained steady and was within 2°C of the desired temperature. The actual observed pyrolysis temperature was used to calculate activation parameters. The dimensionless "weight/mass" Me was calculated using Equation 1. Instead of calculating Mr by extrapolation of the isothermal plot to infinity, Mr was determined by heating each sample/additive to 550°C under N2. This method was used because cellulose TGA rates have been shown to follow Arrhenius plots (4,8,10-12,15,16,19,23,26,31). Thus, Mr at infinity should be the same regardless of the isothermal pyrolysis temperature. A few duplicate runs were made to insure that the results were reproducible and not affected by sample size and/or mass. The Me values were calculated at 4-minute intervals to give 14 data points per run. These values were then used to... 

A sample of white plastic tape was placed in a thermobalance and heated at 10°C min-1 in nitrogen to give the thermogravimetric curve below. By careful measurement identify... 

The thermogram of bromocriptine mesilate, carried out on a Perkin Elmer TGS-1 thermobalance, is given in fig. 9. 

Fig. 2. Comparison of the classical Honda thermobalance with present, frequently used equipment...

The different types of thermobalances which are used today in research laboratories and in industry are usually bound to and designed for specific applications. They may either be commercially available instruments, or assembled from individual components or completely homemade6,7 13 1S. As an example, the set-up of a modern thermobalance is shown schematically in Fig. 5. This type of instrument with its... 

Depending on the instrument (thermobalance), the weight of the sample is followed and recorded within a specific sensitivity range. This means that one obtains the weight change expressed in a single weight curve. Some balances however allow... 

The number of influences in a thermobalance, which is a continuously weighing balance , is relatively large. To a great extent these are dependent on the construction of the balance. Normal macro balances are not affected by small vibrations if they are properly installed, regardless of whether this is on an upper or lower floor. Important for good reproducibility are good thermostated housing reproducible gas flow conditions... 

The weight signal is normally recorded in the manner indicated above. The first derivation can be performed with a derivative computer and is recorded simultaneously. Another way today is the use of a small or large programmable desk calculator which is connected to the thermobalance with an interface. The print-out of weight change per time unit gives the same information as the DTG-curve. 

Instructions on how to prepare samples for thermal analyses can be given only very generally since the variety and complexity of the problems involved makes it impossible to specify strict rules of procedure. In carrying out series of tests on any particular material, it is advisable to make this up in sufficient quantity and also to test it by other analytical methods such as X-ray and IR. All samples should be as far as possible homogeneous. Sample conditioning, i.e. adjustment to a predetermined degree of moisture content, may be carried out either before or after samples are weighed and loaded on the thermobalance. 

The procedure is to heat up a sample of these materials in the thermobalance. Figure 10 shows the DTA-curve and the evaluation of point A and B. These can be... 

Most of the thermobalances have built-in, simple heating programs which allow e.g. linear heating and cooling with different rates, change to isothermal conditions, or to cycle the temperature between two preselected values (Fig. 14 E). The application of the latter is to check the reversibility of certain decompositions, or the reproducibility of DTA-peaks. 

There are basically three possibilities for arranging the furnace which used is with a thermobalance. Above, below and beside the balance housing. The common arrangement (Fig. 15 A) today is placing the furnace above the balance. The advantages of this combination of balance and furnace are ... 

Figure 16 shows a schematic sketch of a thermobalance for TG, DTA and mass spectrometer measurements. - In the center is the thermobalance, enclosed in a vacuum-tight tank, with a thermostatically controlled water jacket. The reaction chamber (R) is surrounded by the furnace and is clearly separated from the balance housing by a diffusion baffle. Diffusion pumps (K) evacuate the balance housing and the reaction chamber. 

This means that even very rapid reactions (detonations) may be followed on the thermobalance. Range, or single-mass recording is possible thus, the pressure behavior of a specific mass, take for instance water, M/el8, or CO, M/e28, can be traced all over the complete decomposition range simultaneously with the thermal decomposition. 

The Fig. 19 shows schematically a typical instrumental set-up in which a thermobalance is combined with a high temperature X-ray camera and with a quadrupole mass spectrometer. 

A special film holder allows transportation of the film with various rates. Time marks are printed automatically on the film for correlating the X-ray patterns to specific times and temperatures in the TMBA curves. The temperature program of the X-ray camera furnace is regulated by the thermobalance heating control system. Up to the maximum temperature of 1200 °C usual heating rates can be varied from 0.2 to 4 °C/min. The temperature of the impact plates can be held constant between room temperature and 450 °C and is recorded during the... 

Fig. 19. Instrumental set-up of thermobalance, X-ray high temperature camera and mass spectrometer...

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