Tammann temperature

Oxidizer Formula Melting point, °C Melting point, °K Tammann temperature, °C... 

If this is the approximate temperature where diffusion becomes probable, it is therefore also die temperature where a chemical reaction between a good oxidi2er and a mobile, reactive fuel becomes possible. This is a very important point from a safety standpoint - the potential for a reaction may exist at surprisingly low temperatures, especially with sulfur or organic fuels present. Table 5.2 lists the Tammann temperatures of some of the common oxidizers. The low temperatures shown for potassium chlorate and potassium nitrate may well account for the large number of mysterious, accidental ignitions that have occurred with compositions containing these materials. 

Since, however, the melting point of nickel oxide is about 2600°K., its Tammann temperature should lie around 1000°C., and it is hard to believe that ionic motion could be so rapid at 300°C. as to reach equilibrium throughout the crystal in so short a time. A more recent investigation by Engell and Hauffe (37) confirms this impression. 

Table 1. Surface free energies, three-dimensional melting points (7jncit) and Tammann temperatures (7Yain) of oxides [14].

Table 2. Surface free energies and Tammann temperatures of selected metals [113] and their oxides [14].

Baker [27] observed mobilization of small particles of several metal oxides on graphite at a temperature (the so-called mobility temperature) that was identical to the Tammann temperature. Thus, at least in systems exhibiting relatively weak interactions between active phase and support surface, particle mobility may be induced at this temperature. The particle migration may perhaps be described as a floating of the active phase particle on the liquid-like surface layer. 

Catalytic molecular surface species may undergo drastic changes in their structure in the presence of reactants. For example, polymeric clusters may transform into highly distorted monomeric species. A crystalline phase may become mobile at its Tammann temperature, as shown by Raman spectroscopy, and it may spread over oxide supports driven by the reduction of the overall surface free energy. Reactive environments trigger many structural transformations, exemplified by particle sintering, dispersion of bulk phases, segregation of surface species into bulk phases, and solid-state reactions between supported oxides and supports. 

This explanation concerning the role of oxide catalysts in the gasification does not hold for temperatures below the Tammann temperature. In the kinetic studies of gasification of coal without an activator, it is frequently assumed that the reaction is not catalytic but is induced by direct gas-solid... 

The surface free energy of thin foils can be determined at temperatures between the melting point and approaching the Tammann temperature of the metal using the zero creep/laser interferometer technique. The use of the laser interferometer allows smaller sample strains to be measured with a higher level of confidence. 

Baker pioneered direct measurements of the onset of particle mobility on substrates using controlled atmosphere electron microscopy (17). He has pointed out the close relationship between the onset of particle motion as determined in his studies and the Tammann temperature (18). It is important to establish whether melting temperature decreases monotonically with particle size as indicated by the data of Buffat and Borel (7) or is equal to the Tammann temperature as hypothesized by Baker (18). 

This mobility point coincides with the Tammann temperature of the material and in some cases can be used as a Tinger-print" to predict the identity of metal or metal oxide particles. 

This is a purely empirical relationship based on Tammann s discovery in 1932 (ref. 78) that there was a minimum temperature at which a solid would undergo a solid-solid interaction. Since that time other workers reported that the rates of sintering of oxides increased markedly at about half the value of the absolute melting temperature. It was also found that under these circumstances defects in the surface of a solid became mobile enabling the surface migration of ions to take place. It should be realized that the Tammann temperature does not represent a discontinuity of behavior, but rather a temperature in the vicinity of which a rapid change in the rate of motion of ions or atoms occurs. 

The ramifications of this phenomenon on the sintering characteristics of a system where there are only weak interaction forces between the particles and the support become distinct. At temperatures below the Tammann temperature particle growth takes place by the atomic migration mode, and, when this temperature is exceeded, the dominant mode of sintering switches to crystallite migration. 

Surface-free energies of several oxides, which bear relevance in the present context as either supports or active oxides, are summarized in Table 1 together with their bulk melting points T u and Tammann temperatures Tj = 0.5 T g,j. 

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