Magnesium activation energy

The kinetics of anionic ring opening polymerization of caprolactam initiated by iso-phthaloyl-bis-caprolactam and catalyzed by caprolactam-magnesium-bromide satisfactorily fit Malkin s autocatalytic model below 50 percent conversion. The calculated value of the overall apparent activation energy for this system is 30.2kJ/mol versus about 70kJ/mol for Na/hexamethylene-l,6,-bis-carbamidocaprolactam as the initiator/catalyst system. 

In support of that explanation, X-ray analysis of the catalyst after use indicated the presence of MgO. Hence, the catalytically active phase was finely divided copper in intimate contact with magnesia, quasi as carrier. The same phenomenon was observed with the Zintl-phase alloys of silver and magnesium. Such catalysts were then deliberately prepared by coprecipitation of copper and silver oxides with magnesium hydroxide, followed by dehydration and reduction. Table I shows that these supported catalysts had the same activation energies as those formed by in situ decomposition of copper and silver alloys with magnesium. 

It is interesting to note that in comparison to these rapid, low-energy, second-order exchange processes Witanowski and Roberts have shown that inversion of bis(neohexyl)magnesium is a relatively slow first-order process with an activation energy of 20 kcal/mole (154). It therefore appears that exchange and inversion go through different mechanisms. 

Processes (b) and (c) are limited by diffusion and heat removal. The activation energies of these processes are low. Process (a) involves common chemical reactions and is improbable at low temperatures ( 80 K). Indeed, as already mentioned, only the most active organic halides with weakened carbon-halogen bonds react with magnesium immediately in the course of condensation. Therefore, only the aggregation and stabilization processes are actually important. Let us consider them in the light of quantum-chemical calculations. 

The Hall effect provides a measure of the net carrier concentration of the dopants. Depending on the depth of the dopants, the activation of the impurity can be very much reduced. For example, Mg in GaN forms a level at 250 meV above the valence band, and the percentage of activation of the magnesium atoms at room temperature is about 1%. DLTS provides a measure of the deep states within the bandgap of the semiconductor. However, it only provides the activation energy and the impurity concentration, and it does not give the exact nature of the impurity concerned. Implantation experiments are required to correlate known impurities with the energy levels measured by DLTS. 

Both magnesium and beryllium have been used to make p-type GaN however, there is an unintentional incorporation of oxygen and silicon in the GaN growth process. . p-Type materials are possible with beryllium or vanadium (in the Ga position with approximate activation energy of 0.236 eV), magnesium or vanadium (in the Ga position with approximate activation energy of 0.236 eV), and zinc or vanadium (in the Ga position with approximate activation energy of 0.232 eV). There are a number of deep-level acceptors. 

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