Weight-loss kinetics

Grain boundary phase elimination kinetics. Weight loss by grain boundary elimination was 1930°C in different settings under nitrogen gas flow. 

The protection potential can be evaluated kinetically for such cases [10,21]. It is assumed that the concentration of metal ions on the metal surface is Cq. The weight loss rate follows from the first law of diffusion ... 

In actual experiments we do not usually observe directly the desorbed amount, but rather the derived read-out quantities, as is the time dependence of the pressure in most cases. In a closed system, this pressure is obviously a monotonously increasing function of time. In a flow or pumped system, the pressure-time dependence can exert a maximum, which is a function of the maximum desorption rate, but need not necessarily occur at the same time due to the effect of the pumping speed S. If there are particles on the surface which require different activation energies Ed for their desorption, several maxima (peaks) appear on the time curve of the recorded quantity reflecting the desorption process (total or partial pressure, weight loss). Thereby, the so-called desorption spectrum arises. It is naturally advantageous to evaluate the required kinetic parameters of the desorption processes from the primarily registered read-out curves, particularly from their maxima which are the best defined points. 

Figure 5. Typical kinetic plots for 0(3P)-induced weight loss in various polybutadienes and polyalkenamers.

Atrophy of the thymus is a consistent finding in mammals poisoned by 2,3,7,8-TCDD, and suppression of thymus-dependent cellular immunity, particularly in young animals, may contribute to their death. Although the mechanisms of 2,3,7,8-TCDD toxicity are unclear, research areas include the role of thyroid hormones (Rozman et al. 1984) interference with plasma membrane functions (Matsumura 1983) alterations in ligand receptors (Vickers et al. 1985) the causes of hypophagia (reduced desire for food) and subsequent attempts to alter or reverse the pattern of weight loss (Courtney et al. 1978 Seefeld et al. 1984 Seefeld and Peterson 1984) and excretion kinetics of biotransformed metabolites (Koshakji et al. 1984). 

While the rate of decomposition in air was more rapid than in nitrogen, the kinetics of oxidative decomposition cannot be estimated reliably by isothermal weight loss because ofthe possibility of competing oxidative weight gain process. For this reason the kinetics were not estimated from the available data... 

Fig. 4.24. Single TG thermogram of a 10°C per min scan of a hot-melt coating formulation, showing the percent weight loss conversion points for building the kinetic model.

A number of studies have been performed investigating the kinetics of the decrease in mechanical properties. Stamm (1956), in a study of the heat treatment of wood under a variety of conditions, plotted the logarithm of strength loss against linear treatment time and found linear relationships (as with weight loss), showing that the decrease in mechanical properties also obeyed first order rate laws. 

The kinetic models all allow for evaporation-condensation to be a significant mechanism in surface reconstruction. In particular, as noted earlier, it was frequently suggested that a metal or metal oxide would evaporate preferentially from certain planes, leading to a surface (presumably equilibrium) consisting of planes with the lowest evaporation rates. Net weight loss was anticipated. Yet, no evidence of weight loss is available from the early literature. 

In the case of the rich shale, the low temperature weight loss, on the average, is 25.0% and (as are all kinetic data) is based on thermobalance data rather than on chemical analysis. There was no way to identify the components in this 25% from the data taken. 

It is interesting that the loss of sulfur does not correspond to a steady decrease in activity. It is well possible that the leaching of sulfur results in a structure resembling that of Raney Ni. The kinetic pattern is in fact the same. Some remedies have been proposed to slow down such a weight loss. For instance, it has been shown [151] that a small addition of MoS2 co-deposited with NiSx in a Ni matrix can apparently alleviate the problem. 

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