Ostwald-Volmer rules

Actually, a spontaneous nucleation of c-BN from a degradation of Li3BN2 will not occur due to the fact that the less dense phase will nucleate first according to the Ostwald-Volmer rule, which is a general rule of thumb for the kinetic behavior of reactions [237]. Thus, seeding with c-BN can overcome this problem. Hence, this approach seems to be well suited to develop a low-pressure synthesis of c-BN. 

The position marked B at 4 GPa and temperatures between 1775 and 2075 K, gives the condition under which well faceted diamonds of about 200 pm diameter have been crystallized using specially treated phenolic resins as the source of carbon and molten cobalt as solvent [30]. The notable point of this particular crystallization is that it occurred well into the graphite stable region of Fig. 7 and is thus an example of the metastable growth of diamond . An explanation for this may be found in consideration of the relative solubilities of phenolic resins and diamond in molten cobalt allowing dissolution of one metastable form and precipitation of another, namely diamond. This is an example where rules governing the transitions between metastable states, such as the Ostwald and Ostwald-Volmer rules, can be applied [31]. 

By making use of Volmer s equations some attempts have been made by Becker and Doring (1935), Stranski and Totomanov (1933), and Davey (1993) to explain the Rule in kinetic terms. In doing this, it becomes apparent that the situation is by no means as clear cut as Ostwald might have us believe. Figure 2.11 shows the three possible simultaneous solutions of the nucleation equations which indicate that by careful control of the occurrence domain there may be conditions in which the nucleation rates of two polymorphic forms are equal, and hence their appearance probabilities are nearly equal. Under such conditions we might expect the polymorphs to crystallize concomitantly (see Section 3.3). In other cases, there is a clearer distinction between kinetic and thermodynamic crystallization conditions, and that distinction may be utilized to selectively obtain or prevent the crystallization of a particular polymorph. 

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