A new type of ceramics based on the phases of fullerite derivatives and boron carbide B
4
C is obtained. The material is synthesized at a temperature of 1500 K and a relatively low pressure of 4 GPa; ...it has a high hardness of 45 GPa and fracture toughness of 15 MPa m
1/2
.
The EPR spectra of Gd3+ ion replacing Y3+ or rare‐earth (RE) ions in aluminum borates REAl3(BO3)4 (RE = Y3+, Eu3+, Tm3+) have been studied. The spin‐Hamiltonian parameters of an impurity Gd3+ ion in ...these crystals were determined. The basic atom coordinates in the structure of TmAl 3 (BO3)4 crystal have been established. Distortions of the Gd3+ ion nearest environment were analyzed within the framework of the superposition model of zero‐field splitting. The b20 parameter change in isomorphic aluminum borates is shown to be determined by change of the angle between the Gd3+O2− bond and the C3‐axis.
Yttrium oxide has promising characteristics such as chemical stability and a porous structure for various high-temperature applications in catalysis and chemical engineering. The prediction of the ...structural properties of Y2O3 presents a computational challenge.
In this study, we implemented a phase-field approach to obtain a precise description of the Y2O3 sintering process over a wide range of temperatures. In the phase-field method, the microstructure is described by a system of continuous variables that model Y2O3 crystallites, where the microstructure interfaces have a finite width over which the material transfers.
The experiments on stepwise sintering process were carried out and the obtained data on the textural and morphological properties of Y2O3 particles were used to calibrate and validate the numerical model. The evolution of the specific surface area and pore volume for the pores ranging from 3 to 70 nm and the rate of growth of Y2O3 crystallites during sintering of Y2O3 grains were effectively predicted. The obtained model indicates that a stepwise increase in the calcination temperature from 600 to 900 and 1200 °C decreases the surface area of the materials from 54 to 15 and 5 m2/g, respectively.
This study can be used to predict the textural properties of yttrium oxide during the sintering of porous ceramics and for the exploitation of catalyst systems.
Aims.The existence of dark matter can be proved in an astrophysical context by the discovery of a system in which the observed baryons and the inferred dark matter are spatially segregated, such as ...the bullet cluster (1E0657-558). The full descriptions of the dark matter halo and X-ray gas substructure motions are necessary to forecast the location of the dark halo from X-ray maps, which can be confirmed by the detection of a galaxy concentration or by gravitational lensing. Methods.We present an analytical hydrodynamic model to determine the distance between the X-ray and dark-matter components and the Mach number of the merger shock. Results.An approximate solution is given for the problem of the substructure propagation in merging clusters. A new method to predict the position of a dark matter halo in clusters, where there is a separation between the X-ray gas and the dark halo, is proposed and applied to the clusters 1E0657-558 and Abell 1763.
Characteristics of a reference optical frequency spectrum formed in a circuit with a continuous laser and an amplitude modulator with a high modulation index are investigated via numerical modeling. ...The possibility of forming microwave oscillations of a desired frequency is demonstrated, and the generation of a broadband microwave oscillation at the modulation of two or more lines selected from the optical frequency spectrum is studied.
A method for generating an optical frequency comb in an optoelectronic oscillator is proposed and investigated. It is based on the modulation expansion of a laser line using large modulation indices. ...In the proposed scheme, a double-microwave-input amplitude modulator is used in a single-loop optoelectronic oscillator instead of a usual amplitude modulator, and an optical output is arranged after it. The circuit characteristics and parameters of the formed optical frequency comb and microwave oscillation are investigated using numerical simulation, which allows us to study highly nonlinear and multimode regimes. The advantage of the scheme is an easy control of all parameters of the optical frequency comb by changing the frequency of the CW laser, the average frequency of the microwave bandpass filter, and the amplitudes of the modulating signals applied to the modulator arms.
Dwarf spheroidal galaxy satellites of the Milky Way are prime targets for indirect detection of dark matter with gamma rays due to their proximity, high dark matter content, and absence of nonthermal ...emission processes. Recently, the Dark Energy Survey (DES) revealed the existence of new ultrafaint dwarf spheroidal galaxies in the southern-hemisphere sky, therefore ideally located for ground-based observations with the imaging atmospheric Cherenkov telescope array H.E.S.S. We present a search for very-high-energy ( E ≳ 100 GeV ) gamma-ray emission using H.E.S.S. observations carried out recently towards Reticulum II, Tucana II, Tucana III, Tucana IV, and Grus II satellites. No significant very-high-energy gamma-ray excess is found from the observations on any individual object nor in the combined analysis of all the datasets. Using the most recent modeling of the dark matter distribution in the dwarf galaxy halo, we compute for the first time on DES satellites individual and combined constraints from Cherenkov telescope observations on the annihilation cross section of dark matter particles in the form of Weakly Interacting Massive Particles. The combined 95% C.L. observed upper limits reach ⟨ σ v ⟩ ≃ 1 × 10−23 cm3 s−1 in the W+W− channel and 4 × 10−26 cm3 s−1 in the γ γ channels for a dark matter mass of 1.5 TeV. The H.E.S.S. constraints well complement the results from Fermi-LAT, HAWC, MAGIC, and VERITAS and are currently the most stringent in the γγ channels in the multi-GeV/multi-TeV mass range.
In this work we investigated Raman spectra of a number of industrial grades of polyvinylchloride powder and films, prepared from solutions in tetrahydrofuran and acetophenone. The number and spectral ...characteristics of the Raman lines in the spectral regions of the C-Cl stretching vibrations and in the region of the C-H and CH2 stretching vibrations were evaluated.
We have studied polarization-dependent broad-band dielectric response of single-crystalline Ti-substituted barium hexaferrites BaTixFe12-xO19, x=0.64., 0.75, 1.1, in the frequency range from 10 to ...8000 cm−1 and at temperatures from 5 to 300 K. In addition to a rich set of phonon resonances located at frequencies from 90 to 800 cm−1 we observed an unusual low-energy response (terahertz region, 10-100 cm−1) stemming from two different effects: relaxational process and second order spin-orbit coupling in Fe2+ ground state, that may set up new prospects for doped hexaferrites in terahertz technologies.
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