Thermobalance, controlled atmosphere

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. 

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. 

Thermogravimetric Analysis (TGA) or Thermogravimetry (TG) is defined as followed by ICTAC A technique in which the mass of the sample is recorded versus time or temperature while the temperature of the sample is programmed, in a controlled atmosphere. The instrument is called a thermogravimetric analyzer (TGA) or a thermobalance. [17]. 

A thermobalance is built around a highly sensitive balance module, a furnace and a controlled atmosphere cabinet. Different types of balance detectors are available but the most commonly used is based on the principle of the null position balance. An example is given in Fig. 2.29. 

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. 

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. 

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. 

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. 

The transition used to calibrate the temperature scale of a thermobalance should have the following properties [1] (i) the width of the transition should be as narrow as possible and have a small energy of transformation (ii) the transition should be reversible so that the same reference sample can be used several times to check and optimize the calibration (iii) the temperature of the transition should be independent of the atmospheric composition and pressure, and unaffected by the presence of other standard materials so that a multi-point calibration can be achieved in a single run and (iv) the transition should be readily observable using standard reference materials in the milligram mass range. Transitions or decompositions which involve the loss of volatile products are usually irreversible and controlled by kinetic factors, and are unsuitable for temperature calibration. Dehydration reactions are also unsuitable because the transition width is strongly influenced by the atmospheric conditions. 

TG curves are recorded using a thermobalance. The principle elements of a thermobalance are an electronic microbalance, a furnace, a temperature programmer, an atmospheric controller and an instrument for simultaneously recording the output from these devices. A schematic illustration of a thermobalance is shown in Figure 2.3. Usually a computer stores a set of mass and... 

All thermobalances effectively utilize baffling to reduce the thermal convection currents that would otherwise disturb the measurement of mass and heat the balance compartment. Optimization of the atmospheric flow pattern and the baffling are critical to reduction of thermal noise and drift in the measurement of the mass. Many models will impose a water-cooled plate between the balance compartment and the furnace. When long-term stability is required, it is advisable to consider actually thermostatting the balance compartment. Since heat normally rises, vertical configurations are most susceptible. The horizontal arrangement leads to less interference from the flow patterns arising from control of the atmosphere. 

Absorption and desorption of water in and out of a polymeric material can be readily monitored in a TGA that has a nitrogen gas stream whose humidity level is adjustable. Some instrument manufacturers sell accessories to control humidity. But the moisture level of the gas can also be controlled by the relative flow of wet and dry nitrogen gas lines that are mixed before they enter the thermobalance. Using this kind of experimental approach, it was found that the desorption of 0.53% by mass water from an epoxy (DGEBA) required about 600 min of storage at 23 °C in the 0% relative humidity nitrogen atmosphere of the TGA before equilibrium was reached (Fig. 3.15). 

As the crucible is open for the TGA test, the atmosphere around the sample has to be carefully controlled, especially in the case of catalysts. Different gas options are available to work in a thermobalance ... 

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