A combination of ab initio molecular dynamic simulations and fully relaxed total energy calculations is used to predict that molecular CO2 should transform to nonmolecular carbonate phases based on CO4 tetrahedra at pressures in the range of 35 to 60 gigapascals. The simulation suggests a variety of competing phases, with a more facile transformation of the molecular phase at high temperatures. Thermodynamically, the most stable carbonate phase at high pressure is predicted to be isostructural to SiO2 alpha-quartz (low quartz). A class of carbonates, involving special arrangements of CO4 tetrahedra, is found to be more stable than all the other silica-like polymorphs.
The mechanisms of charge injection, transport and trapping in
low-density, high-density and cross-linked polyethylene (LDPE, HDPE and XLPE)
are investigated in this paper through charging-discharging current
measurements and space-charge observations. The conductivity of LDPE is much
larger than that of XLPE and HDPE. The threshold for space-charge accumulation
and that for a space-charge-limited current mechanism, coinciding for the same
material, are almost identical for LDPE and HDPE, while the threshold of XLPE
is higher. However, HDPE accumulates more charge than the other two materials.
The depolarization space-charge curves and the conduction current versus field
characteristics indicate that the mobility of LDPE is larger than that of XLPE
and HDPE, which supports the significant difference in conductivity. The lower
mobility, as well as the nature, depth and density of trap sites, can explain
the difference in space-charge accumulation and thresholds.
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