Calculation of energies of radiative tunneling transitions between defects in alkali halides

The procedure for calculating the energies of radiative tunneling transition between spatially separated electron and hole centres in ionic crystals based on semiempirical INDO calculation of a pair of close defects and on a further account of electronic polarization of a crystal due to defects has been presented. Pairs of an electron, F, and hole (H, V_k, V₂) centres in KCl and LiF crystals are studied. The influence of close defect interaction and of polarization upon emitted photon energy has also been considered. It is shown that due to polarization this energy varies with a distance between defects in a more complicated way than $\text{e}{}^2\text{?R}$ shown in semiconductors.     

The theory of diffusion-limited recombination of donor—acceptor pairs

The asymptotics of the quasi-steady kinetics of diffusion-limited recombination of donor-acceptor (DA) pairs has been developed. The theory is based on the probability of DA pair recombination according to the relation w₀ exp [-(r/r_B)], where w₀ is constant, r is the separation between defects and r_B is the half Bohr radius of the more diffuse wavefunction. The reaction rate equation (for the thermal activation mechanism) includes the reaction constant as follows: k = 4πr_B[E_a/kT + ln (w₀r^2_B/D₀) + 1.54 + 2 K₀ (α)], α = 2r₀(w₀/D)$\text{1}\text{2}\text{,}$ where E_a is the activation energy, and K₀, I₀ are the modified Bessel functions. The quasi-steady recombination radius is introduced. The theory developed has been compared with the zone one. The interpretation of the activation energy and frequency factor have been presented for the diffusion model. An extension of the theory developed by taking into account Coulomb interaction between defects has also been presented. The applicability of the theory to the recombination of V_k centers with electronics centers is alkali halides has been shown. It is assumed that the theory is applicable to other centers of hole nature (H, proton, low symmetry exciton i.e. V_k + electron).     

Atomic and electronic structure of the corundum (0001) surface: comparison with surface spectroscopies

The electronic structure and geometry of the Al-terminated corundum (0001) surface were studied using a slab model within the ab-initio Hartree-Fock technique. The distance between the top Al plane and the next O basal plane is found to be considerably reduced on relaxation (by 0.57 Å, i.e. by 68% of the corresponding interlayer distance in the bulk). An interpretation of experimental photoelectron spectra (UPS He I) and metastable impact electron spectra (MIES) is given using the calculated total density of states of the slab and the projections to the atoms, atomic orbitals, and He 1s floating atomic orbital at different positions above the surface. Calculated projected densities of states exhibit a strong dependence on the relaxation of surface atoms. The good agreement of simulated and experimental UPS and MIES spectra supports the correctness of calculated surface relaxation.     

Temperature and impurity concentration dependences of the efficiency of Frenkel defect accumulation in alkali halide crystals

A simple phenomenological model of the efficiency of accumulation of F centers in alkali halide crystals as affected by temperature and homologous cation impurity is considered. The conclusion is drawn that the temperature dependence is probably due to elastic interaction of primary Frenkel defects — F , H centres affecting their diffusion-controlled separation (similar to metals) whereas the impurity concentration dependence is due to the tunneling recombination of F , H_A pairs.     

Critical detonation diameters of highly dispersed energetic substances

The relationship between the critical detonation diameters of energetic substances and their specific surface area was revealed from the results of studying the detonation ability of a series of energetic substances [pentaerythritol tetranitrate (TEN), cyclotrimethylenetrinitramine (RDX), cyclotetramethylenetetranitramine (HMX), 2,4,6-trinitrophenylmethylnitramine (tetryl), benzotrifuroxan (BTF), hexanitrohexaazaisowurtzitane (CL-20), dinitrodifurazanylfuroxan (DNTF), dinitrodifurazanylfurazan (NTF), 1,1-diamino-2,2-dinitroethene (FOX-7)]. A mathematical expression for this relationship was suggested. The critical detonation diameters of highly dispersed (micrometer particles) energetic substances were determined by experiments and calculations. These diameters are in the range 0.06–0.8 mm, which makes these substances suitable for use in aerospace engineering, priming systems, detonation manifolds, and explosion logic devices.     

Preparation of microplastics with polysulfone and montmorillonite by impregnating aramid and glass fibers

Microplastics have been prepared by impregnating aramid and glass fibers with of polysulfone melts of various viscosities unfilled and filled with montmorillonite. A minimum of the melt viscosity has been found at a montmorillonite content of 1 wt %. Radial-transverse impregnation of multifilament fiber by a thermoplastic melt has been studied by the gravimetric method. Introduction into the polysulfone melt up to 1% of the filler at the initial stage of impregnation have almost no effect on the rate of the process; however, a decrease in it is observed as compared subsequently to impregnation with the melt containing no filler. It has been shown that torsional rigidity of microplastics, along with weight change, characterizes the degree of fiber impregnation.     

The electronic properties of an oxygen vacancy at ZrO(2)-terminated (001) surfaces of a cubic PbZrO(3): computer simulations from the first principles

Combining B3PW hybrid exchange-correlation functional within the density functional theory (DFT) and a supercell model, we calculated from the first principles the electronic structure of both ideal PbZrO(3) (001) surface (with ZrO(2)- and PbO-terminations) and a neutral oxygen vacancy also called the F center. The atomic relaxation and electronic density redistributions are discussed. Thermodynamic analysis of pure surfaces indicates that ZrO(2) termination is energetically more favorable than PbO-termination. The O vacancy on the ZrO(2)-surface attracts approximately 0.3 e (0.7 e in the bulk PbZrO(3)), while the remaining electron density from the missing O(2-) ion is localized mostly on atoms nearest to a vacancy. The calculated defect formation energy is smaller than in the bulk which should lead to the vacancy segregation to the surface. Unlike Ti-based perovskites, the vacancy-induced (deep) energy level lies in PbZrO(3) in the middle of the band gap.     

Peculiarities of diffusion-controlled recombination kinetics at long time and/or for great initial reagent concentrations

The earlier developed generalized theory of bimolecular reactions between reagents (defects) in solids and liquids, based on a more complete than the standard one decoupling of the hierarchy of equations for many-point densities, is used for detailed calculations of theA+BAB reaction kinetics at long time (great reaction depth). The effects of great initial concentrations and relative diffusion coefficients upon the reaction kinetics are also studied. The earlier predicted lowering of the reaction rate due to a dynamical aggregation of similar (non-interacting) reagents is confirmed. The greater the initial reagent concentration and if one kind of reagents (A's orB's) are immobile, the more pronounced is the aggregation effect.One of the authors (E. K.) is greatly indebted to the Institute of Physics of Charles University for warm hospitality during his visit in autumn 1983 and Dr. L. Skála for the help in computer calculations at the early stage of this work.     

Optimization of the Component Concentrations in Solutions of Synthetic Detergents

The detergency of formulations composed of organic and inorganic components that form the basis of synthetic detergents was studied. Mathematical design of experiment—method of orthogonal Latin rectangles, was applied to determine the component concentrations corresponding to the maximal detergency. Thus obtained concentrations of the detergent components in aqueous solutions can be used for optimization of the detergent formulation.