Thermobalance, controlled

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... 

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. 

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. 22. Thermobalance for vapor pressure measurements. Schematic drawing of experimental equipment. A-Knudsen cell B-cold trap C-Ionization gauge D-Balance and housing E-Diffusion pumps F-Thermostatically controlled reaction chamber...

The zinc hydroxide carbonate sample (Merck) was weighed on the thermobalance and linearly heated or cooled in the water vapor furnace. The H20-C02 atmosphere was generated by a flow of C02 through the water vaporizer into the sample chamber. The condensed water flows back into the flask, the C02 leaves through the gas outlet. An additional flow of C02 through the balance prevents any water condensation in the balance chamber or on the sample holder. The tests could only be started after both C02 gas flows were adjusted to a constant rate and the vaporizer showed a constant return flow of condensed water. Flowmeters were used to adjust and control the gas flow rates. 

Thermal dehydroxylation of FeOOH has been studied both in vacuum and under various atmospheres. Kinetic studies of these transformations must be carried out under vacuum (Giovanoli Briitsch, 1974) and at a constant temperature. The temperature at which a phase transformation occurs, however, is determined by increasing the temperature of the sample in a controlled manner, i.e. by using a thermobalance (DTA or TGA method, see Ghap. 7). Mechanical and mechanochemical dehydroxylation of FeOOH at room temperature can also be achieved by grinding. 

The basic approach is to consider the problem in two parts. Firstly, the reaction of a single particle with a plentiful excess of the gaseous reactant is studied. A common technique is to suspend the particle from the arm of a thermobalance in a stream of gas at a carefully controlled temperature the course of the reaction is followed through the change in weight with time. From the results a suitable kinetic model may be developed for the progress of the reaction within a single particle. Included in this model will be a description of any mass transfer resistances associated with the reaction and of how the reaction is affected by concentration of the reactant present in the gas phase. 

Figure 1. The Ferkin-Elmer laboratory for thermal analysis. From left to right the DSC-1B differential scanning calorimeter with evolved gas analyzer, the TGS-1 thermobalance (top to bottom), the recorder chart control, model UU-1 temperature programmer control, and model TMS-1 control unit. At right is the model TMS-1 thermomechanical analyzer.

Accurate temperature calibration using the ASTM temperature standards [131, 132] is common practice for DSC and DTA. Calibration of thermobalances is more cumbersome. The key to proper use of TGA is to recognise that the decomposition temperatures measured are procedural and dependent on both sample and instrument related parameters [30]. Considerable experimental control must be exercised at all stages of the technique to ensure adequate reproducibility on a comparative basis. For (intralaboratory) standardisation purposes it is absolutely required to respect and report a number of measurement variables. ICTA recommendations should be followed [133-135] and should accompany the TG record. During the course of experiments the optimum conditions should be standardised and maintained within a given series of samples. Affolter and coworkers [136] have described interlaboratory tests on thermal analysis of polymers. 

One use here will be for a controlled atmosphere thermobalance I am building. Some of the atmospheres I plan to use will be quite noxious. 1 11 need exhaust some of the time but I do not want the thermobalance in a hood. 

In thermogravimetry (TG or TGA) the change in sample mass is determined as a function of temperature and/or time. The instrument is a thermobalance that permits the continuous weighing of a sample as a function of time. The sample holder and a reference holder are bounded to each side of a microbalance. The sample holder is in a furnace, without direct contact with the sample, the temperature of which is controlled by a temperature programer. The balance part is maintained at a constant temperature. The instrument is able to record the mass loss or gain of the sample as a function of temperature and time [m = /( )]. Most instruments also record the DTG curve, which is the rate of the mass change dm/dt = f(T). 

Figure 12-7 Schematic of thermobalance A beam B sample cup and holder C counterweight D lamp and photodiodes E coil F magnet G control amplifier H tare calculator 1 amplifier and J recorder. (Courtesy of Mettler Toledo, Inc., Columbus, OH.)...

The possibility of performing sample controlled thermomagnetometry is illustrated by an SCTM experiment on ICTAC nickel, with a permanent magnet placed above the thermobalance furnace (Fig. 9). 

Several alternative geometric arrangements, involving different relative dispositions of the components, have been successfully used in thermobalances, in which the essential feature is that the sample is suspended in the temperature-controlled zone. Typical designs of apparatus capable of making mass measurements of heated samples are described in References 4-8, 10, 12, and 16. 

In thermogravimetry the variation of the mass of a sample with temperature is monitored in a thermobalance comprising a microbalance, a furnace and a temperature controller. This apparatus allows the calculation of the fractional loss of mass and thus the stoichiometry of a solvate by TGA (Caira, 1998) (Figure 10-6). 

The loading of moisture- or oxygen-sensitive samples into the Mettler thermobalance sample holder is conveniently carried out by use of the controlled atmosphere enclosure shown in Figure 3.9 (3). The sample is introduced, via an enclosed sample holder, into the enclosure and loaded into the furnace chamber after the controlled atmosphere has been introduced. After loading, the furnace chamber is closed and the enclosure is removed. 

An atmosphere control system for the thermobalance, as described by Williams (37), permits the rapid changing of the dynamic gas atmosphere between oxygen, hydrogen, and argon. 

The obvious advantage of the automated thermobalance system over existing instruments is the ability to determine the mass-loss curves of eight successive samples. Operation of the instrument is completely automatic, and once the cycle is begun the instrument does not require the attention of the operator until the eighth sample curve is completed. The instrument should find use for the routine TG examination of a large number of samples, each to be studied under identical thermal conditions. Because the system is completely automated, data reduction or control by a small digital computer could easily be accomplished (see Chapter 14.)... 

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).

Zitomer (67) was the first to describe the coupling of a thermobalance to a time-of-flight mass spectrometer and a magnetic sector mass spectrometer. This technique eliminated the practice of collecting or trapping fractions for subsequent analysis and also permitted careful control of the furnace atmosphere. One of the important features of the TG-MS system is its relatively short dead time, that is, the time between product evolution and introduction into the mass spectrometer ion source. Under proper flow conditions, this time is of the order of seconds. There is also less probability of the formation of secondary reaction that can lead to products other than those initially evolved. 

Fig. 2.11 Computerized control of a thermobalance. (Reproduced from [27] with permission of the Royal Society of Chemistry).

FIGURE 31-1 Thermobalance components. The balance beam is shown as A. The sample cup and holder are 0 C is a counterweight. /) is a lamp and photodiodes, E is a magnetic coil, and F is a permanent magnet. The computer data-acquisition, data-processing, and control systems are components G, //. and /. Component J is the printer and display unit. (Courtesy of Mettler-Toledo.)... 

The similarities of TG and DTA are obviously great, at least instrumentally. As a consequence, many commercial instruments are designed to perform both types of analysis the heating device, temperature-control unit, atmospheric control, and recording device are essentially used in common and are contained in a single control unit, only the thermobalance and DTA sample compartments being separate. 

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