Pulsed cathodoluminescence of nanoscale aluminum oxide with different phase compositions

The methods of pulsed cathodoluminescence have been used to study compacted powders and ceramics containing different phases of aluminum oxide. An intensive luminescence of the samples under study in the visible, NIR, and UV regions of the spectrum has been found. The luminescence bands are very broad and include a few components. The number of the bands depends on the phase composition of the samples. The oxygen vacancies, which capture one or two electrons, produce luminescence centers in the near UV region. The most probable in the visible region is the luminescence of aggregate defects, impurities, and surface centers.     

Simulation of the superlinearity of dose characteristics of thermoluminescence of anion-defective aluminum oxide

The specific features revealed in the superlinearity of dose dependences of thermoluminescence of anion-defective aluminum oxide single crystals have been considered theoretically in terms of the model of an interactive trap system. The model explains the decrease in the degree of superlinearity at a low heating rate, as well as with an increase in the dose increment, the occupancy of deep traps, and the sensitivity of the crystals to radiation due to the increase in the concentration of luminescence centers. The obtained results indicate a widening of the possibilities of the model for interpreting experimental data in the investigation of the dose characteristics of thermoluminescence of the studied crystals.     

Electronic structure of phosphate glasses with a complex oxygen sublattice structure

Calculations of the electronic structure of phosphate glasses are performed in the MO LCAO x _α approximation discrete variation method. On the basis of an analysis of theoretical and experimental electronic spectra of the system BeO-P₂O₅ regularities are found in the formation of the valence band of alkaline-earth phosphate glasses with different types of anion sublattice. Data on the electronic structure are used to refine the models of short-range order; in particular, the possibility of oxygen in the threefold coordination state is confirmed. With the features of the spectrum of electronic states taken into account, localization of charge carriers, the nature of the optical transitions, and hole-transport phenomena are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1366–1372 (August 1997)     

Luminescent defects in nanostructured silica

The spectral and kinetic properties of excited states of luminescent defects (oxygen-deficient centers) in SiO₂ ceramics are studied using pulsed cathodoluminescence and time-resolved photoluminescence. It is found that, in nanostructured samples prepared by thermal decomposition of polysilazane in air, there can exist modifications of oxygen-deficient centers in the form of surface analogs of either neutral oxygen monovacancies ≡Si-Si≡ (≡Ge-Ge≡) or twofold-coordinated silicon atoms =Si: (=Ge:). Photoluminescence of these centers is efficiently excited in the optical absorption bands of E′_s surface centers and silicon clusters ≡SiSiSi≡ and can be associated with the intercenter energy transfer in the course of their nonradiative relaxation. The photoluminescence and excitation spectra indicate thermally induced conversion of different types of oxygen-deficient centers. The specific features of the thermally induced changes in the luminescence characteristics of the defects due to the transformation of the structure of the silica samples from amorphous to partially crystalline are revealed from analyzing the spectral composition and decay kinetics of pulsed cathodoluminescence.     

Phototransferred thermoluminescence in anion-defect α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> crystals

Thermally stimulated migration of charge carriers from trapping centers responsible for the main thermoluminescence peak at 450 K to a deep trap was experimentally observed in anion-defect α-Al₂O₃ crystals. Optically induced transport of carriers from the deep trap to the main trap was also observed to cause phototransferred thermoluminescence at 450 K. The results obtained experimentally confirm the adequacy of the basic assumptions made in the model of interactive interaction between main and deep traps in the crystals under study, as well as substantiate the method of repeated obtaining of dosimetric data.     

Mechanism of formation of nonlinearity in the dose dependence of the thermoluminescence output for anion-defect α-Al2O3 crystals

The mechanism of formation of nonlinearity in the dose dependence of the thermoluminescence output was investigated for anion-defect α-Al₂O₃ single crystals. The experiments revealed a new specific feature: the nonlinearity of the dose dependence of the thermoluminescence output depends on the heating rate of crystals during recording of thermoluminescence. It was established that this effect and other thermoluminescence features studied earlier for anion-defect α-Al₂O₃ crystals are adequately described in the framework of a unified mechanism associated with the interactive interaction of dosimetric and deep traps. Recommendations are given for a decrease in the range of the superlinear portion in the dose dependence of the thermoluminescence output for the crystals under investigation.     

Electron relaxation and transport in nanostructured and bulk silica

Processes of ballistic and hot electron relaxation in extended bulk as well as nanostructured silica have been analyzed by means of a phonon-based scattering model and respective Monte-Carlo computer simulation. Optical as well as acoustic phonons are taken into account. Trajectories of electrons and their energy attenuation in nanostructured silica are additionally affected by scattering processes at the grain boundaries between the nanoparticles, i.e. by surface phonon as well as potential scattering. Moreover, a flatter conduction band and a higher effective electron mass have been taken into account too. According to these calculations, electrons with an initial energy of several eV, but still below the valence band ionization threshold, were thermalized in 50–300 fs increasing with the silica grain size from 1 nm up to bulk material. The electron emission probabilities over the surface barrier into vacuum are extended up to depths of 60–100 nm, respectively, increasing with enhancement by an electric field.     

Study on using high injection voltage and spiral inflector in the central region of VEC

A feasibility study of using high injection voltage and spiral inflector in the present heavy ion central region of VEC is described. Conditions necessary for waist-to-waist transfer of the ion beam in the hole lens of main magnet have been obtained. The results of orbit studies of a variety of heavy ions indicate that a spiral inflector can be used in the central region.     

Probability that a material will undergo cavitation erosion

The probability for cavitation erosion has been obtained as a function of the time, the parameters of the external effect, and the physicomechanical properties of the material. The behavior of unstable steels is predicted theoretically for prolonged use under intense cavitation conditions. It is shown that the destruction of unstable materials having a relatively large number of intermediate unstable phases during plastic deformation is relatively improbable under otherwise equal conditions. A normal distribution law is established experimentally for the cavitation pressure-amplitude pulsations.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 12, No. 6, pp. 95–102, June, 1969.The authors thank N. V. Nesterov for assistance in preparing the experiments.