Piezoelectric Voltage Constant and Sensitivity Enhancements Through Phase Boundary Structure Control of Lead-free (K,Na)NbO3-based ceramics

The piezoelectric voltage constant (g33) is a material parameter critical to piezoelectric voltage-type sensors for detecting vibrations or strains. Here, we report a lead-free (K,Na)NbO3 (KNN)-based piezoelectric accelerometer with voltage sensitivity enhanced by taking advantage of a high g33. To achieve a high g33, the magnitudes of piezoelectric charge constant d33 and dielectric permittivity er of KNN were best coupled by manipulating the intrinsic polymorphic phase boundaries effectively with the help of Bi-based perovskite oxide additives. For the KNN composition that derives benefit from the combination of er and d33, the value of g33 was found to be 46.9 ´ 10-3 V·m/N, which is significantly higher than those (20 - 30 ´ 10−3 V·m/N) found in well-known polycrystalline lead-based ceramics including commercial Pb(Zr,Ti)O3 (PZT). Finally, the accelerometer sensor prototype built using the modified KNN composition demonstrated higher voltage sensitivity (183 mV/g) when measuring vibrations, showing a 29% increase against the PZT-based sensor (142 mV/g).

Charged domain boundaries stabilized by translational symmetry breaking in the hybrid improper ferroelectric Ca3–xSrxTi2O7

Charged domain walls and boundaries in ferroelectric materials display distinct phenomena, such as an increased conductivity due to the accumulation of bound charges. Here, we report the electron microscopy observations of atomic-scale arrangements at charged domain boundaries in the hybrid improper ferroelectric Ca2.46Sr0.54Ti2O7. Like in the prototype improper ferroelectric YMnO3, we find that charged domain boundaries in Ca2.46Sr0.54Ti2O7 correspond to out-of-phase boundaries, which separate adjacent domains with a fractional translational shift of the unit cell. In addition, our results show that strontium ions are located at charged domain boundaries. The out-of-phase boundary structure may decrease the polarization charge at the boundary because of the ferrielectric nature of Ca2.46Sr0.54Ti2O7, thereby promoting the stabilization of the charged state. By combining atomic-resolution imaging and density-functional theory calculations, this study proposes an unexplored stabilization mechanism of charged domain boundaries and structural defects accompanying out-of-phase translational shifts.

General null asymptotics and superrotation-compatible configuration spaces in d ≥ 4

We address the problem of consistent Campiglia-Laddha superrotations in d > 4 by solving Bondi-Sachs gauge vacuum Einstein equations at the non-linear level with the most general boundary conditions preserving the null nature of infinity. We discuss how to generalise the boundary structure to make the configuration space compatible with supertanslation-like and superrotation-like transformations. One possibility requires the time-independent boundary metric on the cuts of "Image missing" to be non-Einstein, while the other sticks to Einstein but time-dependent metrics. Both are novel features with respect to the four dimensional case, where time-dependence of the two-dimensional cross-sectional metric is not required and the Einstein condition is trivially satisfied. Other cases are also discussed. These conditions imply that the configuration spaces are not asymptotically flat in the standard sense. We discuss the implications on the construction of the phase space and the relationship with soft scattering theorems. We show that in even spacetime dimensions, the initial data compatible with such asymptotic symmetries produce maximally polyhomogeneous expansions of the metric and we advance a potential interpretation of this structure in terms of AdS/CFT and realizations of Ricci-flat holography.

INVESTIGATION OF THE CAUSES OF UNSATISFACTORY RESULTS OF ULTRASOUND CONTROL OF EXPERIMENTAL FORGES FROM AUSTENITE HIGH-ALLOY STEEL

The purpose of the work is to establish the main causes of defects by ultrasonic testing (UST) of experimental forgings made of high-alloyed austenitic steel 08Kh18N10T on the basis of comprehensive comparative studies of samples of defective and suitable forgings. Techniques. Chemical analysis of forgings was performed on a high-sensitivity spectrometer "SPECTROMAX" company "SPECTRO", Germany; studies of macro- and microstructure (liquation heterogeneity, grain boundary structure, non-metallic inclusions, excess phases, etc.) were carried by the methods: metallographic and electron microscopic with micro-X-ray spectral analysis; tests of forgings for durability against intergranular corrosion (IGC) were carried out by the methods of AMU, GOST 6032. Results. It was established that experimental forgings rejected by UZK, in contrast to suitable forgings, were characterized by: the presence of areas of liquation inhomogeneity of steel, including high content of δ-ferrite; local defects of the macrostructure in the form of flocs; areas with anomalous multi-grained microstructure with a grain size of –2 to 8 points according to GOST 5639; release of chromium carbides at the boundaries of austenitic grains; susceptibility to intergranular corrosion (ICC). All forgings contained non-metallic inclusions within the permissible limits according to GOST 1778, as well as titanium carbides. Scientific novelty. For the first time on the basis of complex researches it is proved that the main reason of unsatisfactory results of UZK of experimental forgings from high-alloyed austenitic steel, is the anomalous multigrain structure connected with the unstable temperature-deformation mode of forging of ingots. Practical significance. Recommendations for improving the structure and improving the quality characteristics of industrial forgings made of high-alloy austenitic steels in terms of industrial production have been developed.

Study of Micro-Upsetting by Finite Element Simulation Based on Crystal Plasticity and Grain Boundary Strengthening Theories

The properties of individual grains affect the mechanical behaviors and response of materials in micro-scaled deformation, viz., microforming, and there are unknown phenomena and deformation behaviors existing and limiting the wide application of microforming due to size effect. In this paper, a composite model combining crystal plasticity and grain boundary strengthening theories was developed for numerical investigation into the effect of grain boundaries on the plastic deformation of copper micro-upsetting. By comparing the results with and without grain-boundary structure, it is revealed that grain boundaries, which act as the barriers of crystal slip, result in the enhanced flow stress and the discontinuous distribution of stress and strain. The grain size effect is also considered in this research, and the results show the coarse-grained material reduces the flow stress and enhances the inhomogeneous deformation.

Molecular Dynamics Study on Nano-Friction and Wear Mechanism of Nickel-Based Polycrystalline Superalloy Coating

In this work, molecular dynamics simulations are employed to study the nanotribological process of nickel-based polycrystalline superalloy coating. A series of simulations were carried out using the method of repeated friction to explore the influence of frictional force, friction coefficient, grinding groove morphology, wear scar depth, debris flow direction, subsurface damage degree and evolution of defects during the nano-friction process. In addition, the change mechanism of different grain sizes on wear scar depth, frictional force, friction coefficient, and internal damage in the repeated friction process is also explored. The results show that the frictional force is related to the direction of the dislocation slip, and that the friction coefficient change is related to the number of repeated frictions. Moreover, it is observed that the grinding ball has a shunting effect on the formed wear debris atoms, and the shunt point is located at the maximum horizontal radius. We reveal that the grain boundary structure has a strengthening effect. When the grinding ball rubs to the grain boundary, the nucleation of dislocation defects inside the workpiece is obviously hindered by it. Simultaneously, we also find that the closer the subsurface is to the bottom of the grinding ball, the greater the degree of damage to the workpiece by friction. Furthermore, with the grain size decreases that the material begins to soften, resulting in a decrease of frictional force, friction coefficient, and smaller defects are formed inside the workpiece. The research of this work can better clarify the microscopic mechanism of the polycrystalline friction process.