Grain growth in Nio–MgO and its dependence on faceting and the equilibrium crystal shape

The impact of faceting on grain growth was approached by model experiments in NiO–MgO. Grain growth rates were found to be 10 times higher in NiO compared to MgO. As the self-diffusion acoefficients differ by a factor of 250, grain growth in NiO is unexpectedly slow compared to MgO. Recently, the movement of steps was identified as the atomic mechanism of grain boundary migration. According to the equilibrium crystal shape, grain boundaries in NiO are more faceted. The faceted grain boundaries of NiO have fewer steps at the grain boundaries resulting in a relatively lower mobility

Equilibrium and kinetic shapes of grains in polycrystals

The equilibrium crystal shape is a convex shape bound by the lowest energy interfaces. In many polycrystalline microstructures created by grain growth, the observed distribution of grain boundary planes appears to be dominated at low driving forces (after long grain growth times) by the planes present in the equilibrium crystal shape. However, at earlier stages of grain growth, it is expected that kinetic effects will play an important role in grain boundary motion and morphology. Analogous to the equilibrium crystal shape, the kinetic crystal shape of seed crystals growing from a liquid at higher supersaturations is bound by the slowest growing orientations. This study presents an equivalent construction for grain boundaries in polycrystals and uses it to determine the kinetic crystal shape for strontium titanate as a function of temperature. Relative grain boundary mobilities for strontium titanate for the low energy crystallographic orientations from seeded polycrystal experiments are used to calculate the kinetic crystal shapes as a function of temperature and annealing atmosphere. The kinetic crystal shapes are then compared to the morphologies and orientations of the interfaces of the growing seed crystals, and to the equilibrium crystal shapes, as we

Note: Thermal analog to atomic force microscopy force-displacement measurements for nanoscale interfacial contact resistance

Thermal diffusion measurements on polymethylmethacrylate-coated Si substrates using heated atomic force microscopy tips were performed to determine the contact resistance between an organic thin film and Si. The measurement methodology presented demonstrates how the thermal contrast signal obtained during a force-displacement ramp is used to quantify the resistance to heat transfer through an internal interface. The results also delineate the interrogation thickness beyond which thermal diffusion in the organic thin film is not affected appreciably by the underlying substrate

Domain switching mechanisms in polycrystalline ferroelectrics with asymmetric hysteretic behavior

A numerical method is presented to predict the effect of microstructure on the local polarization switching of bulk ferroelectric ceramics. The model shows that a built-in electromechanical field develops in a ferroelectric material as a result of the spatial coupling of the grains and the direct physical coupling between the thermomechanical and electromechanical properties of a bulk ceramic material. The built-in fields that result from the thermomechanically induced grain-grain electromechanical interactions result in the appearance of four microstructural switching mechanisms: (1) simple switching, where the c-axes of ferroelectric domains will align with the direction of the applied macroscopic electric field by starting from the core of each grain; (2) grain boundary induced switching, where the domain's switching response will initiate at grain corners and boundaries as a result of the polarization and stress that is locally generated from the strong anisotropy of the dielectric permittivity and the local piezoelectric contributions to polarization from the surrounding material; (3) negative poling, where abutting ferroelectric domains of opposite polarity actively oppose domain switching by increasing their degree of tetragonality by interacting with the surrounding domains that have already switched to align with the applied electrostatic field. Finally, (4) domain reswitching mechanism is observed at very large applied electric fields, and is characterized by the appearance of polarization domain reversals events in the direction of their originally unswitched state. This mechanism is a consequence of the competition between the macroscopic applied electric field, and the induced electric field that results from the neighboring domains (or grains) interactions. The model shows that these built-in electromechanical fields and mesoscale mechanisms contribute to the asymmetry of the macroscopic hysteretic behavior in poled samples. Furthermore, below a material-dependent operating temperature, the predicted built-in electric fields can potentially drive the aging and electrical fatigue of the system to further skew the shape of the hysteresis loops

Effect of Texture on Temperature-Dependent Properties of K0.5Na0.5NbO3 Modified Bi1/2Na1/2TiO3-xBaTiO(3)

Textured (1-x-y)Bi1/2Na1/2TiO3-xBaTiO(3)-yK(0.5)Na(0.5)NbO(3) (BNT-100xBT-100yKNN) ceramics with a {001} pseudocubic (pc) orientation were fabricated by templated grain growth using Bi1/2Na1/2TiO3 templates. Temperature-dependent electromechanical results demonstrate that the strain response of templated BNT-xBT-yKNN ceramics is stable from room temperature (RT) to 125 degrees C. The temperature-dependent strain and polarization response are compared to randomly oriented ceramics, for BNT-100xBT-2KNN (0.05 <= x <= 0.07). Textured BNT-7BT-2KNN reached a maximum 0.47% strain response at 5 kV/mm, an almost 50% increase compared to randomly oriented BNT-7BT-2KNN. Over the temperature range RT-125 degrees C, the strain response of templated BNT-6BT-2KNN degraded from 0.38% to 0.22% (-42.1%) compared to 0.37% to 0.18% (-51.4%) for randomly oriented ceramics. The temperature-dependent strain response suggests that templated BNT-100xBT-100yKNN ceramics are well suited for elevated temperature applications.close0

Stress, temperature and electric field effects in the lead-free (Ba,Ca)(Ti,Zr)O3 piezoelectric system

The large signal strain response as a function of uniaxial compressive stress, electric field and temperature is investigated for compositions across the morphotropic phase boundary in the (Ba,Ca)(Ti,Zr)O3 ferroelectric system. The largest piezoelectric coefficient in terms of unipolar strain divided by the maximum applied field, Su/EmaxSu/Emax, is 1540 pm V−1, which clearly exceeds the piezoelectric response of most lead zirconate titanate materials. The extraordinarily large piezoelectric properties occur in the vicinity of the morphotropic phase boundary region on the rhombohedral side of the phase diagram. In this material, an electric threshold field is observed that is required to overcome the stress-induced domain clamping and obtain a measurable strain response. Moreover, the study reveals that careful selection of composition, stress and field amplitude allow for large signal piezoelectric coefficients of over 740 pm V−1 in the temperature range of 25–75 °C. The extraordinarily large unipolar strain response can be assigned to an electric field-controlled regime, in which the unipolar compressive stress induces non-180° domain switching perpendicular to the applied electric field. During electrical loading, the electric field can realign these domains back into the parallel direction, maximizing non-180° domain switching and enhancing unipolar strain