Thermal conductivity and expansion of PbF2 single crystals

In order to check a phenomenon of the negative correlation between ionic and thermal conductivities of solid substances, we studied the thermal conductivity and expansion of cubic PbF₂ single crystals at 50–300 and 5.6–317 K, respectively. We found that lead difluoride had a thermal expansion coefficient α that was equal to (28.5 ± 0.3)10⁻⁶ K⁻¹ at 300 K, and a thermal conductivity coefficient k(T) was equal to 1.40 ± 0.07 W/(m K) at the same temperature. Thus, the thermal conductivity for PbF₂ is the lowest among fluorite-type MF₂ (M = Ca, Sr, Ba, Cd, Pb) thermal conductivities, whereas its fluoride-ion conductivity is the highest one among MF₂ (M = Ca, Sr, Ba, Cd, Pb) ionic conductors.

     

The influence of local phonon modes in a wide-band matrix on the tunnel current-voltage characteristics of quasi-zero-dimensional structures

Experimental results on the visualization of the density of states in InAs/GaSa(001) quantum dots that were obtained by tunnel atomic-force microscopy in an ultrahigh vacuum are presented. A one-dimensional (1D) model of dissipative quantum tunneling is proposed for describing experimental current-voltage characteristics of a tunnel contact between an atomic force microscope probe and the surface of InAs/GaAs (001) quantum dots. It was found that the influence of two local modes of the wide-band matrix on the probability of 1D dissipative tunneling leads to the appearance of several randomly spaced peaks in the field dependence. It was shown that the theoretical dependence agrees qualitatively with experimental the current-voltage characteristic of the atomic force microscope tip and the surface of InAs/GaAs(001) quantum dots.     

Studying the energy dependence of intrinsic conversion efficiency of single crystal scintillators under X-ray excitation

Single crystal scintilators are used in various radiation detectors applications. The efficiency of the crystal can be determined by the Detector Optical Gain (DOG) defined as the ratio of the emitted optical photon flux over the incident radiation photons flux. A parameter affecting DOG is the intrinsic conversion efficiency (n _C ) giving the percentage of the X-ray photon power converted to optical photon power. n _C is considered a constant value for X-ray energies in the order of keV although a non-proportional behavior has been reported. In this work an analytical model, has been utilized to single crystals scintillators GSO:Ce, LSO:Ce and LYSO:Ce to examine whether the intrinsic conversion efficiency shows non proportional behavior under X-ray excitation. DOG was theoretically calculated as a function of the incident X-ray spectrum, the X-ray absorption efficiency, the energy of the produced optical photons and the light transmission efficiency. The theoretical DOG values were compared with experimental data obtained by irradiating the crystals with X-rays at tube voltages from 50 to 140 kV and by measuring the light energy flux emitted from the irradiated screen. An initial value for n _C (calculated from literature data) was assumed for the X-ray tube voltage of 50 kV. For higher X-ray tube voltages the optical photon propagation phenomena was assumed constant and any deviations between experimental and theoretical data were associated with changes in the intrinsic conversion efficiency. The experimental errors were below 7% for each experimental setup. The behavior of n _C values for LSO:Ce and LYSO:Ce were found very similar, i.e., ranging with values from 0.089 at 50 kV to 0.015 at 140 kV, while for GSO:Ce, n _C demonstrated a peak at 80 kV.     

Improvement of piezoelectric transformer performances using SSHI and SSHI-max methods

The piezoelectric transformers reach densities of power more significant than their magnetic counterparts. However, one of the principal factors limiting the density of power is the acceptable maximum deformation by material constituting the transformer. The heating of the piezoelectric transformers is mainly of mechanical origin. This heating generates a degradation of the characteristics which in its turn generates an additional heating being able to lead to a phenomenon of thermal avalanche. In this work, two nonlinear methods [synchronized switch harvesting on inductor (SSHI) and SSHI-max] have been explored to improve the performance of the Rosen transformer basing on the tension generated by the secondary so as to increase the capacity of mechanic-electric conversion. The simulation results show that SSHI and SSHI-max techniques significantly increase the capacity of mechanic-electric conversion of inserts stuck on a vibrating structure and consequently, the power recovered in electric form. The comparative results of voltage gain, efficiency and the transmitted power of the transformer, before and after SSHI-max and SSHI control are given. These ones indicated that the two nonlinear techniques are promising as applications to improve the performances of the piezo-transformers.     

First-principles study of structural, electronic and magnetic properties in Cd1−xFexS diluted magnetic semiconductors

In this paper, we report theoretical investigations of structural, electronic and magnetic properties of ordered dilute ferromagnetic semiconductors Cd_1−xFe_xS with x=0.25, 0.5 and 0.75 in zinc blende (B3) phase using all-electron full-potential linear muffin tin orbital (FP-LMTO) calculations within the density functional theory and the generalized gradient approximation. The analysis of band structures, density of states, total energy, exchange interactions and magnetic moments reveals that both the alloys may exhibit a half-metallic ferromagnetism character. The value of calculated magnetic moment per Fe impurity atom is found to be 4 μ_B. Moreover, we found that p-d hybridization reduces the local magnetic moment of Fe from its free space charge value of 4 μ_B and produces small local magnetic moments on Cd and S sites.     

Spectroscopical splitting of Cu ion energy levels in magnesium lead fluoro silicate glasses

Homogeneous 40.0 MgO-(10-x) PbF₂-50.0 SiO₂: x CuO glasses were prepared using melt-quenching technique under controlled conditions. Spectroscopic studies (UV-vis absorption, ESR, FT-IR) are carried out for these glasses. One broad characteristic visible absorption band is observed around 700-850 nm in these glasses, the optical band gap decreases as the content of the CuO increases in the glass network up to 0.7 mol % then reversal trend is observed. ESR spectra of all these glasses show resonance peaks characteristic of Cu²⁺ ions and hyperfine splitting is resolved with increasing the CuO content in the glass network. From the observed ESR spectra, the spin-Hamiltonian parameters have been evaluated and indicate that Cu²⁺ ions have octahedral coordination with a strong tetragonal distortion in these glasses. By correlating ESR and optical absorption data, the molecular orbital coefficients have been evaluated. FT-IR spectra give important information about the nature of bonds in the glass matrix. The density of the glasses is also measured and is found to decrease with the increase CuO contents in the glass matrix. The physical parameters along with spectroscopic parameters are measured.     

Spectral studies of Erbium doped heavy metal borophosphate glass systems

The ternary lead bismuth magnesium borophosphate glass system (LBMBPE) with molar concentrations of (50-x) PbO-xBi₂O₃-25MgHPO₄-24B₂O₃-1Er₂O₃ (x=10, 20, 30 and 40) was prepared using Melt quenching technique. The amorphous nature of these glass samples were confirmed with XRD studies, The spectral data from the optical absorption studies was employed to compute the spectroscopic parameters such as Racah coefficients (E¹, E² and E³), spin-orbit coupling (ξ_4f), configuration interaction factor (α) and Judd-Ofelt intensity parameters (Ω_λ, λ=2, 4 and 6). The IR spectra exhibit the presence of vibrational modes of phosphates, borate radicals, bismuth, lead and magnesium ions. The Judd-Ofelt parameterization indicates the covalency and vibrationonic frequencies of the ligands with rare earth ions. The radiative parameters such as radiative transition probabilities (A), the total radiative transitional probabilities (A_T), radiative life times (τ_R), branching ratios (β) and absorption cross sections (∑) were computed for certain lasing levels. The effect of compositional changes on the optical band gap is also reported. The glass systems thus developed indicate their potential lasing candidature.     

Simulations of nematic homopolymer melts using particle-based models with interactions expressed through collective variables

We develop a hybrid Monte Carlo approach for modelling nematic liquid crystals of homopolymer melts. The polymer architecture is described with a discrete worm-like chain model. A quadratic density functional accounts for the limited compressibility of the liquid, while an additional quadratic functional of the local orientation tensor of the segments captures the nematic ordering. The approach can efficiently address large systems parametrized according to volumetric and conformational properties, representative of real polymeric materials. The results of the simulations regarding the influence of the molecular weight on the isotropic-nematic transition are compared to predictions from a Landau-de Gennes free energy expansion. The formation of the nematic phase is addressed within Rouse-like dynamics, realized using the current model.     

The effect of the boundary conditions on in-plane and out-of-plane stress field in three dimensional plates weakened by free-clamped V-notches

Dealing with the material microstructure an analytical multiscale model has recently been developed by Sih. Physically, the different orders of the stress singularities are related to the different constraints associated with the defect thought as a microscopic V-notch at the tip of the main crack. Irregularities of the material microstructure tend to curl the crack tip being the clamped-free boundary conditions the more realistic and general representation of what occurs on the microscopic V-notch. As a result, mixed mode conditions are always present along the V-notch bisector line.It is known for a long time that at the antisymmetric (mode II) stress distribution ahead of the crack tip generates a coupled out-of-plane singular mode. Recent theoretical and numerical analyses have demonstrated that this out-of-plane mode due to three-dimensional effects occurs also in the case of large V-notches where the mode II stress field is no longer singular. In addition, when the notch opening angle is non-zero, the three-dimensional singular stress state is strongly influenced by the plate thickness.The aim of this paper is to investigate the effect of free-fixed boundary conditions along the notch edges in three dimensional plates weakened by pointed V-notches and quantify the intensity of the out-of-plane singularity occurring under this constraint configuration. For the sake of simplicity a macronotch is considered rather than a micronotch. A synthesis of the magnitude of the stress state through the plate thickness is carried out by using the mean value of the strain energy density over a given control volume embracing the notch tip. The capability of the strain energy density to capture all the combined effects due to the out-of-plane mode make it a powerful parameter at every scale levels.