On crystal lattice parameters of graphite-like phases of the B-C system

The structure of graphite-like BC _x phases (x = 1, 1.5, 3, 4, 32) has been studied using conventional X-ray diffraction. The results have been obtained, which unambiguously point to turbostratic (one-dimensionally disordered) structure of all phases under study. The crystal lattice parameters, sizes of coherent scattering domains, and microstrain values have been defined, which have allowed us to find a correlation between the structure and stoichiometry of the phases synthesized at the same temperature.     

Effect of dopants on the microstructure and lattice parameters of lead zirconate-titanate ceramics

The effect of dopants on the microstructure and lattice parameters of Pb_0.94Sr_0.06(Zr_0.53Ti_0.47)O₃ ceramics was studied. Small amounts (0 to 0.3 wt%) of Cr₂O₃, MnO₂, Co₂O₃ and U₃O₈, were used as dopants. In a few compositions two oxides were used, namely U₃O₈ with Cr₂O₃ as well as MnO₂ with Cr₂O₃ X-ray diffraction investigation of powdered samples showed that all the compositions (both modified and unmodified) were of perovskite type with tetragonal symmetry. The Curie temperature was decreased with the decrease of tetragonal distortion indicated by the axial ratio,c ₀/a ₀, in all cases. The average grain sizes for the doped and undoped compositions were determined from SEM photomicrographs of polished and acid-etched surfaces using the linear intercept technique. The average grain size for the undoped composition was found to be 10.7m. The addition of a small amount of dopants controlled the grain growth to give rise to the average grain size of 6.4 to 9.5m, which appeared to have some effect on the improvement of the piezoelectric properties of the ceramics.Communication No. 4123 from the National Chemical Laboratory, Poona, India.     

Electron density changes at the crystal lattice sites upon superconducting phase transition

It is established that complex copper oxides exhibit a relationship between a change in the electron density at the metal crystal lattice site and the temperature (T _c) of a phase transition to the superconducting state. In crystals containing two structurally inequivalent positions of copper atoms, a change in the electron density upon the transition is different for these lattice sites. In particular, there is a limiting value of the electron density variation upon the superconducting transition, which is different for Cu(1) and Cu(2) lattice sites and corresponds to the two different minimum values of the correlation length. The electron density changes at the sites of a Kronig-Penney crystal lattice upon the superconducting transition are calculated. It is established that the transition from normal to superconducting phase is accompanied by an increase in the charge density at the center of the unit cell. This growth increases with T _c, which agrees with the Mössbauer spectroscopy data.     

Orientation-dependent forces between flux lines and crystal lattice in pure niobium

Torque measurements were performed with cylindrical niobium crystals. Due to the very small pinning of the high-purity material, we were able to study a torque that tries to align the flux lines along special directions of the crystal lattice.     

Full-field phase error compensation method based on relationship between unwrapped phase and phase error

The three-dimensional reconstruction in phase-measuring profilometry (PMP) usually involves the phase error caused by the gamma effect of the projector. In this study, the relationship between the unwrapped phase and the phase error of every pixel is analysed, and an effective full-field phase error compensation method based on this relationship is proposed for the reduction of every pixel error. In our optimized PMP system, the full-field phase error can be detected by directly fitting the unwrapped phase of the reference plane. In addition, the relationship between the unwrapped phase and the phase error can be established by creating a phase-error lookup table for the phase error compensation of every pixel. The experimental results demonstrate the effectiveness of the proposed method in practical PMP, and the measurement errors can be reduced by a factor of least 10.     

Lidar inelastic multiple-scattering parameters of cirrus particle ensembles determined with geometrical-optics crystal phase functions

Multiple-scattering correction factors for cirrus particle extinction coefficients measured with Raman and high spectral resolution lidars are calculated with a radiative-transfer model. Cirrus particle-ensemble phase functions are computed from single-crystal phase functions derived in a geometrical-optics approximation. Seven crystal types are considered. In cirrus clouds with height-independent particle extinction coefficients the general pattern of the multiple-scattering parameters has a steep onset at cloud base with values of 0.5-0.7 followed by a gradual and monotonic decrease to 0.1-0.2 at cloud top. The larger the scattering particles are, the more gradual is the rate of decrease. Multiple-scattering parameters of complex crystals and of imperfect hexagonal columns and plates can be well approximated by those of projected-area equivalent ice spheres, whereas perfect hexagonal crystals show values as much as 70% higher than those of spheres. The dependencies of the multiple-scattering parameters on cirrus particle spectrum, base height, and geometric depth and on the lidar parameters laser wavelength and receiver field of view, are discussed, and a set of multiple-scattering parameter profiles for the correction of extinction measurements in homogeneous cirrus is provided.     

The effect of phase transition in quartz single crystal on its contact angle with lead melt

The temperature dependence of the contact angle between a quartz single crystal and lead melt has been studied in a temperature interval of 350–700°C. It is established that the contact angle exhibits a jumplike change at the point of phase transition in the crystal.     

On the ferroelastic nature of the scheelite-to-fergusonite phase transition in orthotungstates and orthomolybdates

New evidence supporting the ferroelastic nature of the pressure-induced scheelite-to-fergusonite phase transition in ABO₄ orthotungstates and orthomolybdates (A = Ca, Sr, Ba, Pb, Eu and B = W, Mo) has been obtained from the analysis of Raman spectroscopy data. In the studied scheelite-type compounds, one external translational mode of B_g symmetry in the scheelite phase softens up to the transition pressure and then becomes a hard A_g mode in the fergusonite high-pressure phase. However, other scheelite-type compounds not undergoing the scheelite-to-fergusonite transition do not show softening of the B_g mode. The reported results have allowed us to establish a relationship between the square of the soft-mode frequency at ambient pressure and the transition pressure in the series PbWO₄, BaWO₄, EuWO₄, SrWO₄, and CaWO₄ and also in the series PbMoO₄, BaMoO₄, SrMoO₄, and CaMoO₄. This fact is in accordance with the soft-mode theory of second-order phase transitions. The conclusions drawn confirm a previous characterization of the phase transition obtained from the Landau theory.     

Continuum model of two-dimensional crystal lattice of metamaterials

The continuum models of two-dimensional crystal lattice of metamaterial are investigated in this paper. The equivalent classical continuum requires the introduction of frequency-dependent mass density that becomes negative or infinite near the resonance frequency. In order to avoid the frequency-dependent mass density and make the dispersive characteristic of the elastic waves propagating in the equivalent continuum approximating that of lattice wave in two-dimensional crystal lattice of metamaterial, three kinds of continuum models, namely, the multiple displacements continuum model, the strain gradient continuum model and the nonlocal strain gradient continuum model, are investigated. It is found that the nonlocal gradient continuum model may better represent the dispersive relation and the bandgap characteristics of the metamaterial by the appropriate selection of nonlocal parameters.     

Energy transfer between lead sulfide quantum dots in the liquid phase

Förster resonant energy transfer (FRET) from smaller sized PbS Quantum Dots (QDs) to larger sized ones occurs in the liquid phase when adjacent QDs are brought in close proximity by the bridging action of the two SH groups in aromatic or aliphatic dithiols. Signatures of FRET were observed by the lowering of the fluorescence peak corresponding to the smaller particle group and intensification of the peak corresponding to the larger particle group in a mixture of the two in the liquid phase. The suspensions of the two QDs size distributions were mixed in a ratio such that total surface area of the smaller QDs was equal to that of the larger QDs. A 10–13% size deviation in each PbS QDs group also allows this phenomenon to be observed in these samples and is manifested by red-shift and broadening of the fluorescence peak. The ratio of the absorption peak intensity corresponding to the two groups of PbS QDs in the mixture remains the same.     

Phase-modulated ellipsometry for probing the temperature-induced phase transition in ruthenium-doped lead zinc niobate-lead titanate single crystal

Phase-modulated ellipsometry was applied to measure changes in the refractive index of pure and ruthenium (Ru)-doped 0.9Pb (Zn_1/3Nb_2/3)O₃ (PZN)-0.1PbTiO₃ (PT) during the heating process in real time. Both samples were heated from room temperature to 200 °C in a thermally insulated chamber. In both samples, the phase transitions were observed to change from tetragonal to cubic. The temperature region at which the phase transition (Curie region) of Ru-doped 0.9PZN-0.1PT occurred not only broadened but also shifted to a lower temperature. The refractive indices were extremely stable in this region, meaning that Ru-doped 0.9PZN-0.1PT is a more favorable medium for the fabrication of optical memories.     

Interplay between crystal phase purity and radial growth in InP nanowires

The interplay between crystal phase purity and radial growth in InP nanowires is investigated. By modifying the growth rate and V/III ratio, regions of high or low stacking fault density can be controllably introduced into wurtzite nanowires. It is found that regions with high stacking fault density encourage radial growth. Through careful choice of growth conditions pure wurtzite InP nanowires are then grown which exhibit narrow 4.2?K photoluminescence linewidths of 3.7?meV at 1.490?meV, and no evidence of emission related to stacking faults or zincblende insertions.     

Growth of unidirectional potassium dihydrogen orthophosphate single crystal by SR method and its characterization

Unidirectional <100> potassium dihydrogen orthophosphate (KDP) single crystals were grown for the first time by Sankaranarayanan-Ramasamy (SR) method. The <100> oriented seed crystals were mounted at the bottom of the glass ampoules and the crystal of 15 mm diameter, 65 mm height were grown by SR method. The grown crystals were characterized by high-resolution X-ray diffraction (HRXRD), etching studies. The HRXRD analysis indicates that the crystalline perfection is excellent without having any very low angle internal structural grain boundaries. Dislocation density is less in SR grown KDP compared to conventional method grown KDP.     

The relationship between fracture surface roughness and fatigue load parameters

A procedure was sought for estimating fatigue loading information from roughness characteristics of fracture surfaces. Topographs of fracture surfaces produced in compact tension specimens of a titanium alloy in load-shedding and monotonically increasing ΔK tests were analyzed with a fast Fourier transform. The resulting elevation power spectrum density (EPSD) curve was correlated with the stress intensity range, ΔK, and the stress ratio, R.A plot of (EPSD)^1/2 vs ΔK showed three distinct regions, possibly reflecting the competition between surface roughening and surface flattening from partial crack closure as ΔK increased. The plot further indicated the effect of the stress ratio.Although the influence of crack closure complicated the relationship among (EPSD)^1/2, ΔK, and R, the EPSD curves and (EPSD)^1/2 values systematically changed as a function of ΔK and R. These systematic changes may serve as a base for extracting fatigue loading information from roughness characteristics of fracture surfaces.     

The relationship between ductility and material parameters for dual-phase steel

A modified Crussard-Jaoul analysis has been employed to describe the strain hardening behaviour (the ln(d/d) versus In curves) of a 1020 dual-phase steel with quenching and quenching+tempering treatments and with different predeformations, which demonstrated that this dual-phase steel exhibits two stages of strain-hardening in the range of plastic deformation. An analysis of instability for dual-phase steel is also presented and the relationship between the maximum uniform strain and the material parameters is proposed, which shows good agreement with the experimental results for the present 1020 dual-phase steel and other dual-phase steels.     

Debye-Waller parameters of lead sulphide powders

The Debye-Waller parameters, the effective valueB _eff, the dynamic componentB _d and the static componentB _s, of lead sulphide powders with different particle sizes were determined for the bulk and for each constituent atom, lead and sulphur. The effective values for each atom,B _eff (Pb) andB _eff (S), were linearly related to that for the bulk,B _eff (PbS). The effective value and the dynamic component for the bulk,B _eff (PbS) andB _d(PbS), were almost equal to those for lead,B _eff(Pb) andB _d (Pb), suggesting a small contribution of sulphur atoms to Debye-Waller parameters for the bulk. The mean square displacement of the bulk, u ²_Pbs, was expressed by the atomic mass averaged mean square displacements ;u ²;_Pb and u ²_s in PbS crystal. The Debye temperature _D calculated fromB _d (PbS) increased with the decrease inB _s(PbS), and was extrapolated to the _D-value of the single crystal atB _s(PbS) = 0. A similar relation between _D andB _s were found for lead atom, where _D (Pb) was a little bit smaller than _D (PbS).