Metastable character

The selective reaction of K with the a-rh boron is explained by the metastable character of this variety at high T and by the existence in the structure of weak, three-center B—B bonds that correspond to particularly long B — B distances (2.03 X 10 pm a normal covalent B—B bond is 1.76 X 10 pm). 

The data given above definitely prove also the metastable character of the CBF/NBF transition and provide the experimental base for further quantitative explanation of this process. The transition to NBF can be regarded as a transition to bi-dimensional state. For instance, the process of grey film/NBF transition in films stabilised by non-ionic surfactants and CBF/NBF transition in films stabilised by ionic surfactants can be presented as a nucleation process of a new phase. So far this approach is applied only to analyse the stability of NBF (see Section 3.4.4). 

One drawback of amorphous catalysts is their metastable character, which can hinder their high-temperature use. There are methods, how-... 

A disadvantage is the metastable character of the amorphous alloys. As a function of temperature and time amorphous alloys tend to transform into the corresponding crystalline phases. This will be discussed in more detail in section 6. 

A disadvantage of amorphous alloys is their metastable character which makes them transform into the stable crystalline state as a function of temperature and time. In calorimetric experiments the amorphous-to-crystalline transition is revealed by an exothermic heat effect. Typical traces obtained using a differential scanning calorimeter are shown for amorphous Gd064Co0 36 in fig. 51. The dependence of the crystallization temperature Tx on the heating rate s implies that there is a risk of crystallization taking place even at room temperature after an extended period (s - 0). This is particularly likely when Tx is rather low, and it may have consequences for practical applications. 

A kind of an equilibrium situation seemed to be reached first after 1440 hours annealing at 260°C. The Tm-value then increased from 280°C to 358°C and the Hf(m)-value increased from 2 J/g to 17 J/g, see Figure 10.5. These results clearly illustrated the metastable character of the Tm transition but did not contribute to a further improvement of an DSC detection method of this LCP system in a PP matrix due to the possible long sample preparation time required. 

From the chemical point of view it seems to be of particular interest that further (metastable) tellurium subhalides can be obtained by hydrothermal syntheses in acid solutions. In the hydrothermal work on the crystal growth of arsenic telluro-iodides a pure binary tellurium subiodide with the composition Tcsl was obtained as well as a ternary phase with only small amounts of arsenic (AsT Ij The composition Tejl was checked by chemical as well as energy-dispersive X-ray analysis. Crystals of the new tellurium subiodide are shown in Fig. 43 a crystal structure analysis failed because the quality of the crystals was not suitable for single crystal investigation. During annealing of the subiodide a stable phase combination a-Tel -H Te was formed which indicated the metastable character of TesL... 

As a result of the metastable character of many superconductors, ion implantation has proved to be advantageous for producing various types of superconductors. Ion implantation is especially appropriate for investigating concentration-dependent properties, since the concentration can be systematically varied on the same sample and the measured changes attributed to the implanted atom. By this method, it is possible to exceed the equilibrium solubility limits. A number of new superconductors have been made by ion implantation. 

Figure 11-2 shows the kinetics of crystallization in the presence of a P40 in an Al-free reaction mixture. To follow the kinetics, crystallization was carried out in a 5 liter-autoclave that could be probed directly. It is seen that crystallization is accelerated with increasing temperature, when compared to syntheses listed in Tables 11-2 to 11-4. Furthermore, this experiment emphasized the metastable character of the new layer silicates. Quartz instead of cristobalite is observed as the final product of the crystallization sequences in the Al-free cases. Crystallization starts at 4 hours and is completed after 9-10 hours. Quartz appears as a product component after 11-12 hours. The growth of quartz ceases after 40-44 hours. From the gradient of the crystallization curve, a growth rate of 15% per hour is determined for this novel silicate under these conditions. 

Figure 13.3 (a) <2-3 > is a stable stoichiometric compound up to its non congruent melting point at Tpi. (b) <1-1> has a metastable character when compared to < 2-3 > it also shows non-congruent fusion at TP2. 

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