Results 1 - 10 of 2050
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[en] A structural and electromechanical investigation has been performed on (1-x)BaTiO3–(x)BiInO3 in the region 0.03 ≤ x ≤ 0.12. A gradual structural phase transition has been observed where the structure changes from tetragonal (P4mm) and passes through two regions of coexisting phases: (1) P4mm + R3m in the range 0.03 ≤ x ≤ 0.075 and (2) Pm3̄m + R3m for 0.10 ≤ x ≤ 0.12. The properties also transition from ferroelectric (x ≤ 0.03) to relaxor ferroelectric (x ≥ 0.05) as the dielectric permittivity maximum becomes temperature and frequency dependent. This transition was also confirmed via polarization-electric field measurements as well as strain-electric field measurements. At the critical composition of x = 0.065, a moderate strain of ~0.104% and an effective piezoelectric coefficient (d∗33) of 260 pm/V were observed. Finally, the original purpose of this study was to demonstrate the polarization extension mechanism as predicted in the literature, but due to the ferroelectric to relaxor transition, this mechanism was not found to be present in this system. However, this demonstrates that BaTiO3-based lead-free ceramics could be modified to obtain enhanced electromechanical properties for actuator applications.
[en] In this study, recent experimental and modeling studies in nanolayered metal/ceramic composites are reviewed, with focus on the mechanical behaviors of metal/nitrides interfaces. The experimental and modeling studies of the slip systems in bulk TiN are reviewed first. Then, the experimental studies of interfaces, including co-deformation mechanism by micropillar compression tests, in situ TEM straining tests for the dynamic process of the co-deformation, thickness-dependent fracture behavior, and interrelationship among the interfacial bonding, microstructure, and mechanical response, are reviewed for the specific material systems of Al/TiN and Cu/TiN multilayers at nanoscale. The modeling studies reviewed cover first-principles density functional theory-based modeling, atomistic molecular dynamics simulations, and mesoscale modeling of nanolayered composites using discrete dislocation dynamics. The phase transformation between zinc-blende and wurtzite AlN phases in Al/AlN multilayers at nanoscale is also reviewed. Finally, a summary and perspective of possible research directions and challenges are given.
[en] This paper presents an extensive set of measurements from complementary surface analysis techniques performed for characterizing deuterium retention in carbon fiber composite SepcarbNB31 subsequently to a deuterium beam exposition: adsorption isotherm measurements and Hg porosimetry to characterize the porous network, coupled deuterium beam exposure and in situ μ-NRA analysis to follow the dynamics of the penetration of the retained deuterium in the material, TEM and Raman microscopy to investigate the change in the material structure induced by ion irradiation, and finally TPD to estimate the respective proportion of deuterium retained in the surface layer and in the bulk material. The parameters extracted from this set of characterizations are introduced in a 2D Monte Carlo model which couples the propagation of deuterium and eroded carbon atoms in a network of rectilinear pores and plasma-surface interaction parameters from the SRIM code. The simulations reproduce satisfactorily the main features of the measured deuterium concentration profile and behavior of the retained deuterium amount with incident fluence, validating the physical picture of deuterium retention in CFC as due to the simultaneous implantation of the incident ions in the few 10-nm-depth layer the closest to the surface and penetration of atoms deeper in the material (several tens of microns) through the pore network. (authors)
[en] This work reports detailed investigations for the preparation of nanostructured titania powders by a solvent-free sol-gel-derived process, operated in supercritical CO2 (SC-CO2) at high pressures (10-30 MPa) and large range of temperatures (373-823 K). Depending on the processing temperature, the reaction between Ti(OiPr)4 and water performed in a single supercritical phase led to the formation of either amorphous (Ti(OH)4 - titanium hydroxide) or crystalline (TiO2 - titanium dioxide) nanostructured particles. Crystalline (anatase) mesoporous powders with high specific surface area were obtained directly in CO2 solvent under supercritical conditions at temperatures as low as 523 K. The effect of hydrodynamic key process parameters such as stirring and water injection rate on both powder morphology and aggregation degree was also investigated in details. The optimized TiO2 anatase powders exhibited attractive photocatalytic activity, with high potential for the degradation of water pollutants. (authors)
[en] Here, using a newly developed embedded-atom-method potential for Mg–Nb, the semi-coherent Mg/Nb interface with the Kurdjumov–Sachs orientation relationship is studied. Atomistic simulations have been carried out to understand the shear strength of the interface, as well as the interaction between lattice glide dislocations and the interface. The interface shear mechanisms are dependent on the shear loading directions, through either interface sliding between Mg and Nb atomic layers or nucleation and gliding of Shockley partial dislocations in between the first two atomic planes in Mg at the interface. The shear strength for the Mg/Nb interface is found to be generally high, in the range of 0.9–1.3 GPa depending on the shear direction. As a consequence, the extents of dislocation core spread into the interface are considerably small, especially when compared to the case of other “weak” interfaces such as the Cu/Nb interface.
[en] The purpose of this work is to study the effect of prior plastic deformation on the precipitation mechanisms of MgxSiy in an AA6061 alloy. Differential scanning calorimetry (DSC), tensile testing and transmission electron microscopy (TEM) were used to characterize the precipitation sequence in samples that were isothermally aged at 180 C with and without prior plastic deformation. Compressively deforming the AA6061 alloy by 4% caused a shift in the exo-thermal precipitation peaks to lower temperatures (DSC) and enabled the peak strength condition to be reached after a shorter aging period, revealing that plastic deformation accelerated the precipitation kinetics. TEM analysis determined that the accelerated precipitation kinetics in the deformed material was due to heterogeneous precipitation of the Q' phase along dislocation lines and a modification of the precipitation sequence with the L, C and Q' phases dominating over the β'' phase (which is dominated in the non-deformed material). Additionally, the formation of β' was largely suppressed by plastic deformation. (authors)
[en] We compare the influence of using either molecular or colloidal precursors on the synthesis of a ceramic material containing SiC and porous carbon. Remarkably, the temperature of synthesis for crystalline SiC is independent of the route chosen. The excess carbon in the initial mixture is the source of the excess porous carbon that binds to the crystalline domains of SiC in the final products. Interestingly, increasing the initial area of surface contact between carbon and silicon in the ceramic precursor results in different porosities in the 'meso' range. Simultaneous control of the size and the relative amounts of Si and C in the precursors allows control to be exerted over the nature and texture of the final powders. A simple and general mechanism is herein proposed to explain the evolution of the surface area as a function of the volume fraction of residual carbon in the synthesised ceramic. (authors)
[en] β-SiC nano-precipitates can be patterned in crystalline silicon with an almost mono-modal size distribution by simultaneous-dual-beam of C"+ and Si"+ ion implantations at 550 C. Their shape appears as spherical (average diameter ∼4-5 nm), and they are in epitaxial relationship with the crystalline silicon matrix. The narrow size distribution follows the left wing of the carbon distribution where the nuclear ion stopping, and thus the point defect generation rate is largest. This observation allows us to conclude that the induced damage act as sinks for C atoms leading to the SiC nano-precipitates formation centered at the maximum of the simulated damage distribution. The nuclear reaction analysis, X-ray diffraction, Raman spectroscopy, and transmission electron microscopy techniques were used to characterize the samples. (authors)
[en] Industrial borosilicate glasses containing fission products and minor actinides can be subjected to structural damage caused mainly by α-self-irradiation effects. In this field of glasses under extreme conditions, we present an X-ray Absorption Near-Edge Structure investigation of two six-oxide borosilicate curium-doped glasses (based on the International Simplified Glass (ISG) composition). The first sample is an 8-year ISG damaged glass, which has already accumulated an α-decay dose greater than 6.1018 α g-1, a value corresponding to a damaged but stabilized structural state. The second sample results from annealing of the latter ISG damaged glass. Three species, Cm, Pu and Zr were probed at L3-edge, L3-edge and K-edge, respectively. From the experimental results, Cm and Pu species appear respectively in +3 and +4 oxidation states in both glasses. No Cm local environment changes are observed. In contrast, a small variation in Pu local environment appears between the damaged and annealed glasses, reflecting a possible coordination variation or Pu-Zr substitution. A more drastic effect appears for Zr local environment, where a sevenfold coordinated site grows over time under α-self-irradiation effects, at the expense of the initial major sixfold site of symmetry. Moreover, annealing the damaged glass does not permit to retrieve a similar structural state to the one of a just melted curium-doped ISG. (authors)
[en] In this paper, the effects of biaxial mean stress, mainly contributed by the weld residual stress, and thermal loading conditions on cracking directions and damage in high cycle thermal fatigue crazing subjected to variable amplitude loadings are investigated by a combined analytical and computational approach. The cracking directions are related to the orientation of the critical plane defined by the maximum damage. Analytical solutions of the critical plane orientation under constant amplitude biaxial tension/compression loadings are first derived and then employed to study the effects of biaxial mean stress on the critical plane orientation. The critical plane orientation appears to strongly depend on the dominant direction defined by the larger maximum stress. The developed analytical solution of the critical plane orientation and the analytical solution of the thermal stress from the literature are employed to study the effects of thermal loading conditions on the critical plane orientation. The critical plane orientation does not seem to significantly depend on the frequency, the amplitude and the mean value of the fluid temperature fluctuations, and the heat transfer film coefficient between the fluid and the pipe wall. The critical plane orientation under variable amplitude loadings is also studied, and an approximate solution is proposed for convenient engineering applications. The critical plane orientations are used to partially explain the observed cracking directions in the high cycle thermal fatigue crazing in the old residual heat removal system of a nuclear power plant. Finally, the effects of biaxial mean stress and thermal loading conditions on the fatigue crack initiation life are discussed. (authors)