Papers by Keyword: Anelasticity

Abstract: Anelastic and dielectric relaxation measurements have been carried out on poled and unpoled samples of the ferroelectric perovskite (Na_0.5Bi_0.5)_1−xBa_xTiO₃ (NBT-xBT), with composition in the range between pure NBT and the morphotropic phase boundary, 0 ≤ x ≤ 0.08. The complex elastic compliance spectra contain clear indications of both the rhombohedral/tetragonal and tetragonal/cubic transitions, allowing the determination of the phase diagram, which is difficult to obtain by diffraction techniques due to the very low distortions in both the tetragonal and rhombohedral phases and to the structural disorder in the Na/Bi sublattice. An extensive study is made for concentrations in the region of the morphotropic boundary (x ∼ 0.06) in order to find possible signatures of monoclinic phase, as for the case of PbZr_1-xTi_xO₃ (PZT). The main features in the anelastic curves are compared with those in the dielectric spectra and are tentatively related to different modes of octahedral rotations and polar cation shifts.

Abstract: Anelastic and dielectric spectroscopy measurements are presented, which, together with previous measurements [1], clarify some controversial aspects of the phase diagram of PbZr_1xTi_xO₃ close to the border with the antiferroelectric (AFE) phase, and at the morphotropic phase boundary (MPB). No evidence is found of a border separating monoclinic (M) from rhombohedral (R) phases, supporting recent structural studies according to which the two phases coexist, with the fraction of M prevailing near the MPB. A large frequency independent softening at the MPB indicates a genuine M phase over only finely twinned R phase. A new phase transition is found in both the anelastic and dielectric spectra at x = 0.1, at a temperature T_IT between the Curie temperature T_C and the boundary T_T to the phases with tilted octahedra. Such a diffuse transition is interpreted as onset of disordered tilts, which finally become ordered below T_T. In this manner, the phase diagram of PZT is rationalised with respect to the common tendency of perovskites to undergo tilting when the mismatch between the cation sizes exceeds a threshold.

Abstract: Anelastic relaxation measurements were performed in a Nb-46wt%Ti alloy, in the temperature range of 300 to 700 K, using a torsion pendulum operating at an oscillating frequency near 2.0 Hz. The samples were measured in different conditions: cold worked, annealed in ultra-high vacuum and doped with several quantities of nitrogen. The relaxation spectra obtained were resolved into their component peaks, corresponding to the different kinds of interaction of the interstitial solutes with the metallic matrix. The relaxation parameters of each process were calculated using Debye’s elementary peaks.

Abstract: The effect of heating temperature and pulling rate on Young's modulus of steel FAS390Q was studied. The results show that the relationship between Young's modulus and reciprocal of absolute temperature follows exponent change, and Young's modulus and pulling rate follows power function relation (power index for 0.189). Crystal vacancy is one of the most important reasons for elasticity phenomenon of steels, which results in the fact that the Young's modulus is affected by pulling rate. Based on the statistical analysis, the mathematical model between Young's modulus of high temperature and pulling rate is established.

Abstract: From neutron diffraction it is known that the BaCeO₃ perovskite undergoes a sequence of phase transformations from high temperature cubic C to rhombohedral R, to orthorhombic O1 (Imma) and to orthorhombic O2 (Pnma). Doping Y³⁺ on the Ce⁴⁺ site introduces charge compensating O vacancies (V_O) that may be partially filled with OH complexes with exposition to H₂O, so making the material an ionic conductor. Anelastic relaxation experiments have been carried out on samples doped with 2%Y and 10%Y; the real part s’(T) of the complex elastic compliance presents softenings at the transitions, and the loss s’’/s’ curves allow the content of V_O and H to be monitored. Doping has a strong effect on the temperature of the Pnma/R transition: with 10%Y in the fully hydrated state T_O1-R increases up to 750 K while after full outgassing falls to 500 K, meaning that the introduction of ~5% V_O shifts the transition of 250 K. While the effect of cation substitution on the transitions temperature is easily explained in terms of simple arguments usually valid for perovskites based on bond length considerations, the remarkable stabilization of the R phase by V_O requires to take into account the anomalous sequence of phase transitions of undoped BaCeO₃, where the R structure transforms into orthorhombic Pnma on cooling with the loss of an octahedral tilt system.

Abstract: The perovskite (Na_1/2Bi_1/2)TiO₃ (NBT) undergoes a series of structural and polar transitions starting from the high temperature paraelectric phase: tetragonal paraelectric, tetragonal antiferroelectric, rhombohedral antiferroelectric and finally rhombohedral ferroelectric, according to neutron diffraction and dielectric spectroscopy. In solid solution with BaTiO₃ (BT) the ferroelectric phase changes from rhombohedral to tetragonal, at the so-called morphotropic phase boundary, and the phases at higher temperature become ill-defined, also because of the large lattice disorder induced by the coexistence of differently charged cations in the same sublattice. Combined dielectric and anelastic spectroscopy measurements are presented, which clarify some issues related to the phase transitions in NBT-BT. The influence of Ba substitution on the tetragonal antiferroelectric phase is determined for the first time, and the possibility that a monoclinic phase, although with very short coherence length, exists near the morphotropic phase boundary is discussed in view of a large maximum of the elastic compliance.

Abstract: Composites made from compacted powders blends of Al with different Fe contents were produced and characterised with respect to mechanical and induction heating properties. Mechanical spectroscopy and hardness measurements were employed to follow the evolution of Young modulus and internal friction after ageing. It was found that above a critical iron content (>30% of the volume) a percolation network of Fe grains is obtained inside the specimen and the induction heating characteristics become comparable with those of ferritic steel samples.

Abstract: The discovery of the superconductivity of MgB2 was of great importance, because this material is one of the few known binary compounds and has one of the highest critical temperatures (39° K). As MgB2 is a granular compound, it is fundamentally important to understand the mechanisms of the interaction of the defects and the crystalline lattice, in addition to the eventual processes involving the grain boundaries that compose the material. In this sense, the mechanical spectroscopy measurements constitute a powerful tool for this study, because through them we can obtain important information about phase transitions, the behavior of interstitial or substitutional elements, dislocations, grain boundaries, diffusion, instabilities, and other imperfections of the lattice. For this paper, the samples were prepared using the PIT method and were characterized by density, X-ray diffraction, scanning electron microscopy, electric resistivity, magnetization, and mechanical spectroscopy. The samples were measured in their as-cast condition and after an ultra-high-vacuum heat treatment. The results showed complex spectra, in which were identified relaxation processes due to dislocation movement, interaction among interstitial elements and dislocations, auto-diffusion, and movement of grain boundaries. Some of these processes disappeared with the heat treatment.

Abstract: The purpose of this research is to investigate the micro-mechanism of anelastic behavior of metallic glass. The changes of atomic arrangement in Cu50Zr50 metallic glass were observed directly by EXAFS method during tensile deformation. The transmission-mode EXAFS measurements around Zr K-edge and Cu K-edge of the sample in various amounts of tensile deformation conditions were done. The interatomic distance between Cu-Zr increased, and that between Zr-Zr decreased simultaneously, with the amount of tensile deformation increase of the sample at first stage of deformation. When the atomic distances reached to certain values respectively, they recovered to the initial values of no tensile deformation condition at first stage, and then the atomic distances changed as same manner as the first stage of deformation, respectively. This is the first observation on the change of atomic arrangement during tensile deformation of metallic glass.

Abstract: Non-elastic strain behavior was investigated for several different zirconia ceramics and a possible mechanism for anelasticity was discussed. Anelastic strain was detected in zirconia ceramics irrespective of the crystallographic phase and its productivity depended on the particular kind of dopant additive. It was found that the anelastic properties could be significantly influenced by the level of oxygen vacancy in the matrix, and that the anelastic strain might be produced by a slight shift of ionic species. In order to investigate the effect of anelasticity on mechanical properties on zirconia ceramics, the tensile strength was investigated for a wide range of strain rates. The obviously unique strain rate dependence was observed only in the materials having anelastic properties. It was assumed that anelasticity could be efficient at improving the tensile strength.