Self generated atmosphere

The most common crucible form in the laboratory is the cylindrical form (see Fig. 8 b). The size with respect to volume depends mainly on the expected weight change and on the homogeneity of the studied sample. For these types of crucibles lids are often used which do not close the liner hermetically, but rather influence the temperature homogeneity and the equilibrium of reaction by the self-generated atmosphere. 

Figure 3. In-situ ND results lor LiBD4 - MgD2 sample (0.3 1 ratio) ball milled for 1 h and heated in a sealed vessel, i.e. under a dynamic vacuum (top) and under a self-generated atmosphere.

Figure 17.7 shows the evolving ND patterns for two experiments the first experiment was the decomposition of the 1 0.3 mixture in an evacuated sealed vessel, i.e. decomposition under a self-generated deuterium atmosphere and the second experiment was the decomposition of the same material but under a dynamic vacuum. As the sample was heated for the first experiment (with a self-generated deuterium atmosphere) the first change was the... 

Many of the problems involved in obtaining reproducible TG curves of a sample can be solved by using the technique of self-generated atmospheres. This technique is discussed later in this chapter. 

A self-generated atmosphere is one which is composed of the gaseous decomposition products of the reaction and which is in intimate contact with the sample by virtue of the type of sample holder employed. The thermogravimetry of various compounds in such an atmosphere is of importance because of the reproducibility of the composition of the atmosphere. 

The most important aspect of the technique is in the design of the sample holders, two of which are described in Chapter 3. Other sample-holder designs have been described by Newkirk (57, 58). The evolution of sample holders for use in self-generated atmospheres is illustrated in Figure 2.33. A listing of the different types of compounds that have been studied by the self-generated atmosphere technique is shown in Table 2.1 (57). 

The primary influence of the use of the self-generated atmosphere technique is to increase the pressure of the gas evolution to I atm, which gives rise to favorable thermodynamic, physical, and kinetic effects. The control is not exact, and some investigators consider the use of the technique to be a makeshift or last resort (57). In many instances, however, precise atmosphere control is not available, or is difficult or impossible, because the reaction products are complex or unknown. The technique has a sound theoretical basis and, in such instances, would seem to be a good choice for the initial TG study of a complex solid-gas system. 

Table 2.1, Compounds Studied by the Self-generated atmosphere Technique (57)... 

The thermal decomposition of manganese ) acetate 4-hydrate is discussed (57X to illustrate the use of the self-generated atmosphere. A two-step decomposition sequence has been proposed, similar to the following ... 

The effect of the self-generated atmosphere, curve B, is to increase the initial mass-loss temperature, 7J, and to decrease the reaction interval, Tf — T. The increase in % has the beneficial effect of eliminating the initial mass-loss at room temperature with its resulting uncertainty about the starting point of the curve. The inflection point during loss of water is located at about 135°C and a mass fraction of 0.9. The second stage of mass-loss... 

A rather dramatic effect of the use of the self-generated atmosphere technique plus the quasistatic heating-rate mode is shown in Figure 2.35. Paulik and Paulik (21) found that the thermal decomposition of Ni(NH3)6CI2 dissociated in three separate steps, at 180, 320, and 360°C, respectively. The slight overshoot of the curve during the first dissociation reaction was said to be due to an induction period caused by delayed nucleus formation. The sample holder employed is described in Chapter 3. 

The advantages and the limitations of the self-generated atmosphere technique have been described by Newkirk (57) they are the following. 

Thermogravimetry in self-generated atmospheres may be useful for studies of the following ... 

Figure 3.7. Sample holders for self-generated atmospheres (7-10).

Various sample holders, several of which are illustrated in Figure 3.21, have been described (40) and their applications to specific problems discussed. The labyrinth, self-generated atmosphere type of sample holder has previously been examined (Chapter 2.)... 

Figure 6.20. Sample holders for constant pressure in a self-generated atmosphere J701-...

An identification of the sample atmosphere by pressure, composition, and purity, and by whether the atmosphere is self-generated or dynamic through or over the sample. The flow rate, total volume, construction, and temperature of the system between the sample and detector should be given, together with an estimate of the time delay within this system. 

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