Materials Science Forum Vols. 702-703

Abstract: Abrasive Water jet cutting (AWJ) is a process used to cut materials using a jet of pressurized water. These water jets are capable of cutting a variety of materials like superalloys, titanium alloys, stainless steel, tool steels, ceramics, armor tool and granite. In the present study an attempt has been made to investigate microstructural changes taking place in the sub-surface as a result of AWJ cutting using cross-sectional Transmission Electron Microscopy (TEM). TEM analysis shows that the sub-surface of the AWJ machined sample surface experienced severe deformations. The surface microstructure is observed to contain extremely fine sized grains, going down from sub-micrometer to nanometer range. The selected area diffraction pattern analysis also confirms the presence of extremely fine grained microstructure.

Abstract: Our research investigates the correlations between domain texture and microstructural features, including crystallographic texture in bulk and thin film polycrystalline materials to understand the development of piezoelectric and other anisotropic properties in a number of rapidly evolving lead free piezoelectric materials. We investigate approaches to understanding polarization distributions by starting from polarization measurements. In addition, 2D and 3D microstructural simulations are carried out in all types of ferroelectrics to rationalize and then engineer their equilibrium and kinetic response. This paper discusses recent findings associated with bulk piezoelectricity, phase stability, and ferroelastic and ferroelectric domain motion for materials such as Ba(Ti_0.8Zr_0.2)O₃-x(Ba_0.7Ca_0.3)TiO₃ (BZT-BCT) and Bi_0.5Na_0.5TiO₃ (BNT). Conventional and synchrotron-based x-ray diffraction, electron and optical microscopy and piezoelectric characterization techniques are employed to assess texture, both as a function of poling and temperature. The coupling between microstructure and the inherent directional biases fundamental to piezoelectric and ferroelectric performance enable consideration of orientation and anisotropy in systems with unique constraints.

Abstract: Thin films of hematite find extensive applications in photoelectrochemistry, photocatalysis, and gas sensors. c-axis oriented hematite films have been directly grown on c-plane sapphire substrate using chemical method via hydrolysis of ferric cations. X-ray diffraction (XRD) reveals that the crystalline phases of the films and corresponding sediment produced in the solution were α-Fe₂O₃ and pure β-FeOOH, demonstrating the promotion of nucleation of hematite on sapphire substrate as a result of lowered interface energy. Phi-scan results indicate that the hematite films are grown with (0001) planes parallel to c-plane of Al₂O₃. Scanning electron microscopic observation shows that the hematite films are composed of pyramid-shaped nanocrystals with smooth surface facets.

Abstract: A stochastic optimization procedure incorporating a continuum modelling is used to identify the optimal texture (orientation distribution) parameters of ferroelectrics (FEs) for piezoelectric applications. FE polycrystals differ significantly from single crystals because of the presence of variously oriented crystallites. The orientation of FE crystals plays a critical role in the anisotropy of their piezoelectric properties. The set of combination of variables, known as solution space, which dictates the texture of crystallites is unlimited. Crystallographic orientation in FEs is characterised through Euler angles . The macroscopic properties of a ceramic FE, differ significantly from those of single crystals mainly due to the imperfect alignment of the crystallographic axes of the constituent domains or crystallites. This suggests that piezoelectric properties can be tailored by a proper choice of the parameters which control the orientation distribution. Nevertheless, this choice is complicated and it is impossible to analyze all possible combinations of the distribution parameters or the angles themselves. Stochastic optimization combined with a generalized Monte Carlo scheme optimizes the objective functions, the effective piezoelectric coefficients . Objective functions are calculated using the homogenization method at each orientation configuration chosen by the optimization algorithm. A modified simulated annealing is employed to describe the stochastic optimization. Here we have simulated the texture of polycrystals using a simple model with a Gaussian distribution. Optimal design variables that enhance the macroscopic piezoelectricity are identified.

Abstract: A description of recent work performed by collaboration among the CIMAV Crystal Physics Group, the ILL Diffraction Group and the GEC Nanotechnology Education and Research Centre (NERC), regarding structure-electromagnetic properties relationships, is given. Structure analysis puts emphasis on thin films texture characterization. The new software package ANAELU, for texture analysis via two-dimensional (2-D) diffraction detection, is described. Crystallographic texture plays a significant role on ferroic and multiferroic bulk and nano-structured materials properties. With the objective of estimating effective values for polycrystal dielectric, piezoelectric, elastic and magnetoelectric coefficients, the Voigt, Reuss and Hill approximations are systematized in an extended version of program SAMZ.

Abstract: Ferroelectric BiFeO₃ (BFO) is potential candidate for future generation of FeRAM due to its large polarization. However, BFO is very sensitive to secondary phase formation during synthesis because of volatility issues related to Bismuth. Investigation of the microstructure for phase purity is the key as impurities can destroy the desired properties. We have used backscattered electron diffraction to study the microstructure of BFO ceramic. The EBSD results provide a direct evidence of the appearance of secondary phase that XRD could not be detected in XRD.

Abstract: Homo-epitaxial (100) single crystal diamond films of different isotopic (13C) composition were synthesized by micro-wave plasma chemical vapor deposition (MPCVD). Undoped 12C (100) films were also synthesized at different thicknesses. The growth surface of 13C doped showed a sequential morphological change from flat surface to stepped or ledge surface and coarsening of the steps/ledges with the increase of 13C-isotope concentration. As the growth proceeds via surface steps/ledges and coarsening of these steps/ledges, further increase in imperfection of the grown CVD single crystal diamonds observed with the development of non-(100) orientations. In case of undoped 12C samples, as the thickness increased the in-grain misorientation got reduced.