1,138 research outputs found
Structures, phase transitions and microwave dielectric properties of the 6H perovskites Ba3BSb2O9, B=Mg, Ca, Sr, Ba
No full textWe present a complete temperature-composition phase diagram for Ba3BSb2O9, B=Mg, Ca, Sr, Ba, along with their electrical behavior as a function of B. These compounds have long been recognized as 6H-type perovskites, but (with the exception of B=Mg) their exact structures and properties were unknown due to their low symmetries, temperature-dependent phase transitions, and difficulties in synthesizing pure samples. The full range of possible space group symmetries is observed, from ideal hexagonal P63/mmc to monoclinic C2/c to triclinic P over(1, -). Direct second-order transitions between these phases are plausible according to group theory, and no evidence was seen for any further intermediate phases. The phase diagram with respect to temperature and the effective ionic radius of B is remarkably symmetrical for B=Mg, Ca, and Sr. For B=Ba, a first-order phase transition to a locally distorted phase allows a metastable hexagonal phase to persist to lower temperatures than expected before decomposing around 600 K. Electrical measurements revealed that dielectric permittivity corrected for porosity does not change significantly as a function of B and is in a good agreement with the values predicted by the Clausius-Mossotti equation. © 2008 Elsevier Inc. All rights reserved
A behavior-oriented model for long-term coastal profile evolution: validation, identification, and prediction
No full textStructure and microwave dielectric properties of La(Mg0.5Ti0.5)O3-CaTiO3 system
No full text(1 - x)La(Mg0.5Ti0.5)O3 (LMT)-xCaTiO3 (CT) [0 < x < 1] ceramics were prepared from powder obtained by a nonconventional chemical route based on the Pechini method. The crystal structure of the microwave dielectric ceramics has been refined by Rietveld method using X-ray powder diffraction data. LMT and CT were found to form a solid solution over the whole compositional range. The 0.9LMT-0.1CT composition was refined using P21/n space group, which allows taking into account B-site ordering. The compounds having x ≥ 0.3 were found to be disordered and were refined using Pbnm space group. Microstructure evolution was also analysed. Dielectric characterization at microwave frequencies was performed on the LMT-CT ceramics. The permittivity and the temperature coefficient of resonant frequency of the solid solutions showed a non-linear variation with composition. The quality factor demonstrates a considerable decrease with the increase of CT content. © 2003 Elsevier Ltd. All rights reserved
Microstructure and residual stress evolution during cyclic elastoplastic deformation of AISI316L fabricated via laser powder bed fusion
No full textIn metal additive manufacturing (MAM), microstructural properties such as texture, residual stresses, and dislocation density have emerged as key factors ruling the resulting mechanical performances. In this study, cylindrical AISI 316L specimens, fabricated with laser powder bed fusion (LPBF), were tested under cyclic elastoplastic (EP) deformation using a constant strain amplitude to highlight the evolution of residual stresses (RS), dislocation density and texture with increasing number of EP cycles, N, across the hardening-softening (H–S) transition stage, in the attempt to find correlations between relevant microstructural parameters and macroscopic properties. The structural and microstructural analysis is carried out through whole powder pattern modeling (WPPM) of neutron diffraction (ND) data and Electron Back-Scattering Diffraction (EBSD) analysis. The H–S transition is found to occur within 7–9 cycles, with RS fading out already after 5 cycles. Across the H–S transition, the trend of the maximum tensile stress correlates closely with the trend of WPPM-calculated total dislocation density, suggesting a major role of dislocations’ characteristics in the evolution of macroscopic mechanical properties. EBSD analysis reveals the rearrangement of geometrically necessary dislocations (GND) into cellular structures, and moderate grain refinement, which are deemed to be responsible for the quick fading of RS in the very early stage of EP loading. ND-based texture analysis reveals a (220) preferential orientation retained throughout the EP tests but with orientation density functions (ODFs) changing non-monotonically with N, suggesting preliminary partial randomization of grains around the deformation axis followed by the recovery of crystallographic anisotropy and more localized ODFs
Structure-property relations in xBaTiO3-(1-x)La(Mg1/2Ti1/2)O3 solid solutions
No full textThe microwave dielectric properties and microstructures of compounds in the solid solution series xBaTiO3-(1-x)La(Mg1/2-Ti1/2)O3 (BTLMT) have been investigated. The structural phase transitions that occur as a function of x have been studied and are related to changes in the dielectric properties. For compounds where x ≤ 0.1, X-ray diffraction (XRD) showed evidence of 1:1 ordering between Mg and Ti cations. For x ≤ 0.3, XRD and electron diffraction revealed that compounds were tilted in both antiphase and in-phase. However, for 0.3 < x < 0.7, only antiphase tilting was present. The temperature coefficient of resonant frequency (τf) vs the relative permittivity (εr) was linear until x = 0.5 at which point in the solid solution the transition to a nontilted structure resulted in nonlinear behavior. τf values close to zero (-2 ppm/°C) were achieved at x = 0.5 (εr ∼ 60), which had a quality factor (Q·fo) of 9600 GHz
Towards automatic analytic evaluation of massive Feynman diagrams
No full textAvdeev LV, Fleischer J, Kalmykov MY, Tentyukov M. Towards automatic analytic evaluation of massive Feynman diagrams. In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT. Vol 389. ELSEVIER SCIENCE BV; 1997: 343-346.A method to calculate two-loop self-energy diagrams of the Standard Model is demonstrated. A direct physical application is the calculation of the two-loop electroweak contribution to the anomalous magnetic moment of the muon 1/2(g-2)(mu). Presently we confine ourselves to a ''toy'' model with only mu, gamma and a scalar particle (Higgs). The algorithm is implemented as a package of computer programs in FORM. For generating and automatically evaluating any number of two-loop self-energy diagrams, a special C-program has been written. This program creates the initial FORM-expression for every diagram generated by QGRAF, executes the corresponding subroutines and sums up the final results
Magnetic properties of CeM1.5M’0.5Ge4O12 (M = Mn, Co; M’ = Ni, Cu)
No full textA study of the solid solutions CeM2-xM’xGe4O12 (M = Mn, Co; M’ = Ni, Cu) by X-ray diffraction and magnetometry is described. For M = Co the introduction of Ni and Cu was possible for x ≤ 0.5; for M = Mn attempts to introduce Cu were unsuccessful but CeMn1.5Ni0.5GeO4 was obtained as a single phase. These compositions crystallise in the space group P4/nbm with a ~9.76, c ~4.85 Å. They all order antiferromagnetically with TN< 10 K. The magnetic structures adopted by CeMn1.5Ni1.5Ge4O12 and CeCo1.5Cu0.5Ge4O12 were shown by neutron diffraction to be the same as those adopted by CeMn2Ge4O12 and CeCo2Ge4O12, respectively. The in-field metamagnetic transition of the latter to a weakly ferromagnetic phase was shown to persist in the Cu-doped composition. The available data suggest that the anisotropy associated with the Co2+ cation plays a dominant role in determining the magnetic behaviour of these compounds
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