High pressure sintering of non-oxide materials

Pure materials of AIN, alpha-Si3N4 and TiC, without additives were sintered at 800 C to 1400 C under the pressures of 30 kbar and 50 kbar for 0.5 hours. The maximum density of sintered bodies for the cited materials was nearly 100% for AIN, 98% for TiC and 96% for alpha-Si3N4

Glassy dynamics in thin films of polystyrene

Glassy dynamics was investigated for thin films of atactic polystyrene by complex electric capacitance measurements using dielectric relaxation spectroscopy. During the isothermal aging process the real part of the electric capacitance increased with time, whereas the imaginary part decreased with time. It follows that the aging time dependences of real and imaginary parts of the electric capacitance were primarily associated with change in volume (film thickness) and dielectric permittivity, respectively. Further, dielectric permittivity showed memory and rejuvenation effects in a similar manner to those observed for poly(methyl methacrylate) thin films. On the other hand, volume did not show a strong rejuvenation effect.Comment: 7 pages, 7 figures. Phys. Rev. E (in press

Charge state of vacancy defects in Eu-doped GaN

Eu ions have been doped into GaN in order to achieve red luminescence under current injection, where coupling between the Eu ions and intrinsic defects such as vacancies are known to play an important role. However, the charge state of the vacancies and the impact it would have on the optical and magnetic properties of the Eu ions have not been explored. Through a combination of first-principle calculations and experimental results, the influence of the charge state of the defect environment surrounding the Eu ions has been investigated. We have identified two Eu centers that are related through the charge state of a local vacancy defect. These two centers were found to exhibit a mutual metastability, such that each center can be excited in one configuration and emit as the other. This metastability was found to be dependent on temperature and the wavelength of the excitation laser. Furthermore, one of these centers was found to have an effective magnetic g factor that is substantially larger than what is expected for an isolated Eu3+ ion and is explained by a change in the charge state of the defect environment around the Eu. This prediction could also offer a new explanation for the saturation magnetization previously observed in GaN : Eu and other GaN: RE systems.112Ysciescopu

Charge state of vacancy defects in Eu-doped GaN

Eu ions have been doped into GaN in order to achieve red luminescence under current injection, where coupling between the Eu ions and intrinsic defects such as vacancies are known to play an important role. However, the charge state of the vacancies and the impact it would have on the optical and magnetic properties of the Eu ions have not been explored. Through a combination of first-principle calculations and experimental results, the influence of the charge state of the defect environment surrounding the Eu ions has been investigated. We have identified two Eu centers that are related through the charge state of a local vacancy defect. These two centers were found to exhibit a mutual metastability, such that each center can be excited in one configuration and emit as the other. This metastability was found to be dependent on temperature and the wavelength of the excitation laser. Furthermore, one of these centers was found to have an effective magnetic g factor that is substantially larger than what is expected for an isolated Eu3+ ion and is explained by a change in the charge state of the defect environment around the Eu. This prediction could also offer a new explanation for the saturation magnetization previously observed in GaN : Eu and other GaN:RE systems

Theoretical study of kinks on screw dislocation in silicon

Theoretical calculations of the structure, formation and migration of kinks on a non-dissociated screw dislocation in silicon have been carried out using density functional theory calculations as well as calculations based on interatomic potential functions. The results show that the structure of a single kink is characterized by a narrow core and highly stretched bonds between some of the atoms. The formation energy of a single kink ranges from 0.9 to 1.36 eV, and is of the same order as that for kinks on partial dislocations. However, the kinks migrate almost freely along the line of an undissociated dislocation unlike what is found for partial dislocations. The effect of stress has also been investigated in order to compare with previous silicon deformation experiments which have been carried out at low temperature and high stress. The energy barrier associated with the formation of a stable kink pair becomes as low as 0.65 eV for an applied stress on the order of 1 GPa, indicating that displacements of screw dislocations likely occur via thermally activated formation of kink pairs at room temperature