Energy Transfer Mechanism in Pr-Doped SrF$_{2}$ Crystals
Results of investigation of energy transfer mechanism in SrF2 crystals doped with Pr3+ ions are presented. Temperature dependences of 5d-4f and exciton emission in this material under X-ray excitation, and thermostimulated luminescence (TSL) in temperature range 80-300 K are studied. It has been established that in SrF2-Pr3+ crystals the consecutive electron-hole capture mechanism of energy transfer takes place. Also we found that hole centers made a contribution to energy transfer from primary electrons and holes to Pr3+ ions.
M centres in alkaline earth fluorides
We have investigated the origins of the α and β bands in additively coloured CaF2, which were described by Mollwo (1934). We have shown that the α band is a superposition of bands arising from the F centre, the M centre and higher F aggregate centres and that the β band is the analogue of the M band in the alkali halides. To avoid confusion with the α and β bands in the alkali halides, which have an excitonic origin, we rename Mollwo’s α and β bands the S and M bands. The M centre in CaF2 consists of two nearest neighbour F centres and hence is alined along <100>. The structure of the Mcentre was determined through the anisotropy of its optical absorption and fluorescence. The lifetime of the M centre fluorescence has been measured. The thermal orientation of the M centre in its ground state has been investigated. The optical absorption of the M centre in SrF2and BaF2 has also been investigated.
EFFECT OF SAMPLE PREPARATION ON WATER TREE RESISTANCE OF XLPE/SEBS BLENDS
The effects of an aromatic polymer, such as poly[styrene-b-(ethylene-co-butylene)b-styrene] (SEBS), and its preparation processes on water tree resistance, morphology, crystallinity, dielectric constant and dielectric loss tangent of crosslinked polyethylene (XLPE)/SEBS and XLPE/SEBS/ethylene vinyl acetate (EVA) blends were investigated. The XLPE/SEBS samples were characterized by means of differential scanning calorimetry, scanning electron microscopy, dielectric spectral analysis and the accelerated water treeing experiment. The results clearly showed that although SEBS was partially compatible with LDPE and it was dispersed evenly in XLPE matrix, the blends demonstrated excellent dielectric performances, such as dielectric constant and dissipation factor. The crystallinity of XLPE/SEBS blends was decreased with increase in SEBS content and it was further decreased with the addition of EVA. SEBS could effectively improve the water-tree resistance of XLPE and both SEBS and EVA were found to improve the water tree resistance of XLPE synergistically as expected. The differences in sample preparation processes could change the phase morphology and dielectric performance of the blends, and could also influence the dispersion of SEBS in XLPE matrix as well as the crystallinity of the blends. Different preparation processes led to different water tree resistance behaviours of the samples. The improvement of SEBS dispersion in XLPE matrix helped to improve the water tree resistance of XLPE.