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Grain Shape (grain + shape)

Distribution by Scientific Domains
Polymers and Materials Science42%
Engineering28%

Selected Abstracts

Dependence of Grain Shape and Growth on PT Content in PbO-Excess PMN,PT

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2006
Chang Eui Seo
When Pb(Mg1/3Nb2/3)O3 (PMN) with 40 wt% excess PbO is sintered at 1050°C, the grains grow normally and have flat {100} surface segments with round edges. As PbTiO3 (PT) is added up to 8 mol%, the grain growth is still nearly normal, characterized by time invariant normalized size distributions, but the grain edges become more sharply curved. This normal growth behavior is consistent with relatively small surface step free energy as indicated by the round grain edges. As the PT content is increased above 10,35 mol%, the grain edges become sharp and abnormal grain growth occurs. This is consistent with the step free energy increasing with the PT content as indicated by the sharpening grain edges. [source]

Influence of particle shape and angularity on the behaviour of granular materials: a numerical analysis

INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 14 2003
C. Nouguier-Lehon
Abstract This paper analyses the influence of grain shape and angularity on the behaviour of granular materials from a two-dimensional analysis by means of a discrete element method (Contact Dynamics). Different shapes of grains have been studied (circular, isotropic polygonal and elongated polygonal shapes) as well as different initial states (density) and directions of loading with respect to the initial fabric. Simulations of biaxial tests clearly show that the behaviour of samples with isotropic particles can be dissociated from that of samples with anisotropic particles. Indeed, for isotropic particles, angularity just tends to strengthen the behaviour of samples and slow down either local or global phenomena. One of the main results concerns the existence of a critical state for isotropic grains characterized by an angle of friction at the critical state, a critical void ratio and also a critical anisotropy. This critical state seems meaningless for elongated grains and the behaviour of samples generated with such particles is highly dependent on the direction of loading with respect to the initial fabric. The study of local variables related to fabric and particle orientation gives more information. In particular, the coincidence of the principal axes of the fabric tensor with those of the stress tensor is sudden for isotropic particles. On the contrary, this process is gradually initiated for elongated particles. Copyright © 2003 John Wiley & Sons, Ltd. [source]

Grain Growth and Twin Formation in 0.74PMN·0.26PT

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2002
Jay S. Wallace
The mechanisms controlling normal and exaggerated grain growth in lead magnesium niobate,lead titanate (PMN,PT) ceramics have been investigated by varying the PbO-based liquid-phase volume fraction from 0.03 to 0.6 and sintering temperature from 900° to 1100°C. There is a transition in matrix grain growth rate and matrix grain shape with liquid fraction; samples with liquid volume fractions less than ,0.15 show relatively small equiaxed grains resulting from grain-to-grain impingement. Samples with higher liquid fractions show significantly larger, facetted, cube-shaped grains, whose size is independent of liquid fraction, indicating that a surface nucleation rate mechanism controls growth in this regime. Exaggerated grains were found in the high liquid fraction samples. Electron backscatter diffraction showed that all of the exaggerated grains contained 60°,111, twins but none of the normal matrix grains contained twins. The reentrant angles in the twinned grains give them a growth advantage over untwinned grains, resulting in a population of exaggerated grains. [source]

Nucleation, growth, and phase transformation mechanism of Ge2Sb2Te5 thin films

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 11 2009
Eun Tae Kim
Abstract The nucleation, growth, and metastable-to-stable phase transformation behavior of Ge2Sb2Te5 thin film were studied by means of transmission electron microscopy. The results confirm that nucleation begins at the surface of the film and proceeds by growth of grains through the thickness of film. Also, the result provides further grounds to the suggestion that some of the Ge2Sb2Te5 grains grow with an elongated grain shape to reduce interface energy between crystalline and amorphous phases at the initial stage of crystallization. We find edge dislocations in the metastable-to-stable phase transition region. We think these edge dislocations can explain the metastable-to-stable phase transformation mechanism as a result of vacancy diffusion. [source]

Effects of Structural Variation on the Photocatalytic Performance of Hydrothermally Synthesized BiVO4,

ADVANCED FUNCTIONAL MATERIALS, Issue 16 2006
J. Yu
Abstract Highly crystalline monoclinic scheelite BiVO4 powders are synthesized from aqueous Bi(NO3)3 and NH4VO3 solutions over a wide range of pH by a hydrothermal process. BiVO4 powders with various morphologies, surface textures, and grain shapes are selectively synthesized by adjusting the pH. The dependence of the Raman peak position and intensity on the synthesis conditions indicates that the symmetry distortions in the local structure of the synthesized BiVO4 are affected by the preparation conditions. These variations in the local structure result in the modification of the electronic structure of BiVO4, which results in a blue-shift in the UV-vis absorption spectrum of hydrothermally synthesized BiVO4 in comparison with a well-crystallized sample prepared by homogeneous coprecipitation. The photocatalytic activities for O2 evolution from an aqueous AgNO3 solution under visible-light irradiation are strongly dependent on the pH used in the synthesis. The differences in the photocatalytic activities between BiVO4 samples prepared under various conditions is attributed to the degree of structural distortion, leading to differences in the mobility of photogenerated holes formed in the valence band, which consists of Bi,6s and O,2p orbitals. [source]

Geometrical modeling of granular structures in two and three dimensions.

INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 4 2009
Application to nanostructures
Abstract A granular structure can be modeled by a parallelepiped containing spherical balls in three dimensions or by a rectangle filled with disks in two dimensions. These grains (spherical balls or disks) are separated by interfaces called grain boundaries and their size correspond to a size distribution, which is obtained experimentally. The geometrical modeling of such a structure consists in determining the repartition of the set of disjoint grains according to these specifications. In this paper, a new constructive algorithm based on an advancing-front approach, usually used in the context of mesh generation, is proposed. This algorithm is nearly linear in complexity, robust and fast in both two and three dimensions. Enhancements in computing time and density are observed and reported via comparisons with existing methods. Moreover, we propose a method to transform spherical balls (disks) into polyhedral (polygonal) cells similar to the real grain shapes. Examples of nanostructure modeling in two and three dimensions are presented. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Diffraction line profiles from polydisperse crystalline systems

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 5 2001
Paolo Scardi
Diffraction patterns for polydisperse systems of crystalline grains of cubic materials were calculated considering some common grain shapes: sphere, cube, tetrahedron and octahedron. Analytical expressions for the Fourier transforms and corresponding column-length distributions were calculated for the various crystal shapes considering two representative examples of size-distribution functions: lognormal and Poisson. Results are illustrated by means of pattern simulations for a f.c.c. material. Line-broadening anisotropy owing to the different crystal shapes is discussed. The proposed approach is quite general and can be used for any given crystallite shape and different distribution functions; moreover, the Fourier transform formalism allows the introduction in the line-profile expression of other contributions to line broadening in a relatively easy and straightforward way. [source]