Low-temperature Na4Ti5O12 from X-ray and neutron powder diffraction data

The crystal structure of the low-temperature Na₄Ti₅O₁₂ (tetra­sodium penta­titanium dodeca­oxide) phase has been solved and refined from X-ray and neutron powder diffraction data at 295 K. The structure is trigonal, space group P3, with Z = 1, although it is pseudo-centrosymmetric. The O and Na atoms form a distorted close-packed structure, where Ti atoms occupy octahedral sites.     

A behavior-oriented model for long-term coastal profile evolution: Validation,identification, and prediction

A behavior-oriented diffusion model, governing the time evolution of the cross-shore position of coastal profiles, is studied. Here, two time-independent, space-varying coefficients, which embody the relevant physical properties, are identified simultaneously. Two sets of real data, the first measured over 10 years at Duck, in NC (USA), the second obtained over 39 years measurements at Delfland (Holland), have been processed numerically by a suitable “inversion algorithm”, earlier developed by the authors. This is based on the minimization of a certain cost functional in order to identify both coefficients. The numerical results, obtained by solving the diffusion equation with the so-determined coefficients, favorably agree with the real data, which fact validates and calibrates somehow the diffusion model under investigation. A short-term prediction is finally obtained for coastal profiles, using such a model.     

Investigation of the conditions of efficient I*<Subscript>2</Subscript> (342 nm) luminescence in a barrier discharge in a Kr-I<Subscript>2</Subscript> mixture

The energy and spectral characteristics of a barrier discharge in a mixture of iodine vapor with krypton have been investigated theoretically and experimentally. The emission spectrum consists of the single I*₂ molecular band D′ → A′ peaking at 342 nm, the iodine resonance line at 206.2 nm, and the group of bands corresponding to iodine emission. The highest intensity of the I*₂ (342 nm) band was obtained at a mixture pressure near 450 Torr. A mean output power and an efficiency of 550 mW and 1.6%, respectively, have been achieved. It is shown that, for the I₂ barrier-discharge excilamp, the homogeneous, rather than filamentary form of discharge glow is optimal from the viewpoint of the highest mean output power. The maximal calculated value of the emission efficiency for the 342-nm band was 5%. The main processes determining energy losses in plasma have been found, and ways to increase the efficiency of emission in the D′ → A′ band of the I*₂ molecule have been proposed.     

Optical characteristics of plasma of I*<Subscript>2</Subscript>, Cl*<Subscript>2</Subscript>, Br*<Subscript>2</Subscript> halogen dimer barrier-discharge excilamps

Barrier-discharge excilamps operating in homonuclear chlorine, bromine, and iodine and their mixtures with inert gases have been studied. The spectral and energy characteristics of the barrier-discharge plasma have been obtained. The conditions have been determined at which the band D′ → A′ predominates in the spectra of molecules I*₂ (342 nm), Cl*₂ (257.8 nm), and Br*₂ (291 nm). The efficiencies of I₂, Cl₂, and Br₂ excilamps were found to be 1.6, 2, and 3.8%.     

Heat transfer in bubble layers at elevated pressures

Heat transfer with steam condensation under moderate pressure on the surface of a horizontal tube immersed in a bubbling layer was experimentally investigated. A copper test section 16 mm in outer diameter and 400 mm in length was placed in a bubbling column 455 mm in diameter. Experiments were made under pressures of 0.14–0.8 MPa, with void fraction 0.04–0.23, vapor superficial velocities 0.05–0.42 m/s, liquid-wall temperature differences 47–105 K, and heat flux densities 0.12–0.8 MW/m². The heat transfer process in the bubbling layer is shown to be of a high intensity: with moderate values of steam content, heat transfer coefficients reach 12–14 kW/(m² · K). Data obtained showed that the known correlations do not consider the influence of pressure on heat transfer. For the first time, data on radial steam content distribution under pressures higher than atmospheric were obtained by an electroprobe method. A table of experimental data is presented.     

Heat transfer in bubble layers at high pressures

An original experimental investigation of heat transfer with steam condensation on a surface of a horizontal cooled tube immersed in a bubbling layer was carried out. A copper test section 16 mm in diameter and 285 mm in length was placed in a bubbling column 295 mm in diameter. Experiments were made under a pressure of 0.72-3.8 MPa with volume steam content 0-0.18, steam superficial velocities 0-0.18 m/s, and liquid-wall temperature difference 38–106 K. The heat transfer process in a bubbling layer under high pressures is shown to be of considerably intensity; with moderate values of steam content heat transfer coefficients reach 10–12 kW/(m²·K). The use of the known correlations assumed for the case of air bubbling under atmospheric pressure results in systematically underestimating heat transfer by 30–80%. Data were obtained on heat transfer with film condensation of steam and natural convection of subcooled water at high temperature differences outside the range investigated earlier. Experimental data table is appended.     

The ferroelectric phase of CdTiO3: A powder neutron diffraction study

The synthesis of bulk samples of polycrystalline CdTiO₃ in both the rhombohedral ilmenite and orthorhombic perovskite forms is described and the structures of these have been refined using powder neutron diffraction data. This involved the preparation of samples enriched in ¹¹⁴Cd. Cooling perovskite-type CdTiO₃ to 4 K induces a ferroelectric phase transition, with the neutron data suggesting the low temperature structure is in Pna2₁. Mode analysis shows the polar mode to be dominant at low temperatures. The ilmenite-structure of CdTiO₃ is compared with that of ZnTiO₃. The refined scattering length of the ¹¹⁴Cd is estimated to be 5.56 fm. Attempts to dope CdTiO₃ with Ca and Sr are described.     

Compaction and Consolidation of Aramid and Composite Fibers. 2. Consolidation of Composite Fibers

The transverse compaction and consolidation of composite fibers obtained from blends of rigid-chain aromatic polyamides and a thermoplastic polycaproamide PA-6 are investigated with the aim to predict the fiber behavior during their compression and processing into plastics. Introduction of the aliphatic polyamide PA-6 into aramid fibers considerably increases their transverse compliance and promotes their sintering. A method for calculating the viscosity of the thermoplastic in the interfiber space from the ratio between the volumetric rate of compaction and the porosity of the material is proposed. It is found that the effective viscosity of the PA-6 melt, during its flow in the thin interfibrillar layers under compression, grows with decrease in its content in the composite fibers.