Sample records for crystal faces
from WorldWideScience.org

Sample records 1 - 3 shown.



1

Eixo quinário: um elemento de simetria presente em moléculas e quasi-cristais - um experimento didático em cristalografia/ Quinary axis: an element of symmetry that occurs in molecules and quasicrystals - a didactic experiment of crystallography

Neves, Paulo Cesar Pereira das; Freitas, Darcson Vieira de; Baptista, Luis Andre
2010-01-01

Resumo em inglês Axis of quinary symmetry occur in molecular symmetry, as in the case of fullerenes, and in crystalline symmetry, in the quasicrystals. Minerals with pentagonal faces do not have this element of symmetry, as the pyrite (FeS2) which shows a ridge that is different from the other ones, in any face of the crystal. The purpose of this paper is to demonstrate conceptual differences between pyritohedron and regular pentagonal dodecahedron symmetries, discussing students' difficu (mais) lties to identify them. Also is proposed a didactic experiment with spatial models of the above-mentioned forms and the demonstration of its symmetries in clinographic projections.

Scientific Electronic Library Online (Portuguese)

2

Crescimento de cristais de rubi e safira pelo método do fluxo/ Growth of ruby and sapphire crystals by the flux method

Ribeiro, Gislene da Silva; Prado, Rogério Junqueira
2010-01-01

Resumo em inglês This work reports the growth of corundum crystals by the flux method. The main objective was the evaluation of versatility, effectiveness and real possibilities of the flux method to the synthesis and doping of monocrystals with impurities of particular interest. In this work the chosen impurities were i) Cr and ii) Fe and Ti, aiming the synthesis of rubies and sapphires, respectively. The crystals were grown by heating a mixture of Al2O3:Cr or Al2O3:Fe:Ti and flux (MoO3) (mais) . The maximum crystal size obtained was 1.0 mm, all transparent, presenting well developed faces, bipiramidal hexagonal shape, and showing a typical red (ruby) and/or light blue (sapphire) color. EDX and XPD experiments were performed in order to characterize some of the synthesized crystals. All crystallized specimens presented the α-alumina atomic structure.

Scientific Electronic Library Online (Portuguese)

3

Indução de estruturações esféricas ou similares durante a cristalização da água por processos físicos ou químicos/ Induction of spherical or similar structures during the water crystallization by physical or chemical process

Carvalho, Fabiano de
2007-06-01

Resumo em português Estruturar cristais de gelo é atrativo devido à possibilidade de diminuir os danos causados pelo congelamento ao mínimo. A ocorrência de agulhas ou esqueletos danosos às células significa que foram desiguais os crescimentos das faces do cristal. Objetivou-se com este trabalho induzir a formação de sementes cristalinas menores, arredondadas ou com alto grau de simetria, para aumentar a probabilidade de crescimentos geometricamente semelhantes à forma original e qu (mais) e elimine crescimentos desiguais nas faces do cristal de gelo. Foram usadas: 1) substâncias anfifílicas, para afetar a estruturação cristalina habitual da água e 2) substâncias com a possibilidade de formação de ligações de hidrogênio nas diversas direções e formar arranjos simétricos. Sob a influência do dodecil sulfato sódico (SDS), um tensoativo, os cristais de gelo cristalizaram na forma de agregados cristalinos esféricos. Cristais esferulíticos foram obtidos sob a influência dos aminoácidos valina, histidina, glicina e do sal sulfato de alumínio e amônio. Cristais poliédricos com faces curvas foram conseguidos sob a influência dos tensoativos Triton ® X 100, Tween ® 80 e de um sal biliar (ácido quenodeoxicólico). O melhor resultado foi obtido sob a influência da floroglucina. Esse trifenol produziu cristais pequenos, com extremidades arredondadas e em todas as camadas da amostra. Resumo em inglês The possibility of ice crystal structuring is attractive because it is possible to reach some structures of reduced dimensions capable of decrease the freezing damage to the minimal. The occurrences of needles or skeletons harmful to the frozen tissues demonstrate that the crystal face grown were unequal. The objective of the research is to the induce formation of smaller crystalline seeds, spherical or with a high symmetry level to increase the probability of original fo (mais) rm. It was used: 1) anphiphilic substances, to affect the usual crystalline structure of the water; 2) substances with hydrogen bond possibility containing water in all directions in order to arrange symmetrical structures. Under the influence of sodium dodecyl sulfate (SDS), a surface-active agent, the ice crystals composed of spherical crystalline aggregates. Spherulitic crystals were obtained under the influence of valina amino acid, histidine, glicine and salt sulfate aluminum and ammonia. Polyhedric crystals with curve faces were obtained with the influence of the surface-active agent Triton ® X 100, Tween ® 80 and of the bile salt the quenodeoxicolic acid. The best result was obtained under the influence of phoroglucine. This of the triphenol was capable of producing small crystals with rounded extremities and in all stratum of the sample. In all the order solutions the spherical crystals appears in the superior stratum only.

Scientific Electronic Library Online (Portuguese)