1,721,131 research outputs found
Second-phase assisted formation of {111} twins in barium titanate
No full textWe investigated the formation mechanism of {111} twins in BaTiO(3) through systematic studies of the effects of various processing parameters on the {111} twin formation, in particular, excess TiO(2), initial particle sizes of raw materials and sintering atmosphere (either oxidizing or reducing). Our study on the effect of excess TiO(2) revealed that the second-phase Ba(6)Ti(17)O(40) particles provided the nucleation sites of the {111) twins. The study on the initial particle size showed that the {111} twins formed during the growth of BaTiO(3) grains around the Ba(6)Ti(17)O(40) particles. The twins, however, Formed only when the Ba(6)Ti(17)O(40) interface and the BaTiO(3) grain boundaries were faceted in an oxidizing atmosphere. In a reducing atmosphere, the Ba(6)Ti(17)O(40) interface and the grain boundaries were defaceted trough), and no (111) twins formed. Based on these experimental observations, we proposed a second-phase (Ba(6)Ti(17)O(40)) assisted formation mechanism of (111} twins in BaTiO(3). The proposed mechanism explained well, all of the observed effects of the processing parameters. The mechanism may also account for the formation of annealing twins in single-phase systems with faceted grain boundaries. (C) 2001 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc
{111} twin formation and abnormal grain growth in barium strontium titanate
No full textTwo series of experiments were performed to study the experimental conditions for the formation of {111} twins and related microstructures in barium strontium titanate ((Ba,Sr)TiO3). In the first series, the phase equilibria in the BaTiO3-SrTiO3-TiO2 system were determined. XRD and WDS analysis, done in the BaTiO3-rich region, of 45(Ba,Sr)TiO3-10TiO(2) samples annealed at 1250degreesC for 200 h in air showed that (Ba,Sr)TiO3 was in equilibrium with Ba6Ti17O40 (B6T17) and Ba4Ti13O30 phases with strontium solubility (Sr/(Ba + Sr)) of similar to0.02 and 0.20, respectively. In the second series the micro-structures of samples consisting of a mixture of (Ba,Sr)TiO3 and 2.0 mol% TiO2, were observed after sintering at 1250degreesC for 100 h in air. {111} twins formed only in the samples with faceted B,T,, second phase particles, similar to the case of BaTiO3. In these samples, abnormal grain growth occurred in the presence of the {111} twins. In contrast, no {111} twins formed and no abnormal grain growth occurred in the samples containing second phase particles other than B6T17. With an increased substitution of strontium for barium, the aspect ratio of abnormal grains containing {111} twin lamellae was reduced. This result was attributed to a reduction in the relative stability of the {111} planes with the strontium substitution
Control of (111) twin formation and abnormal grain growth in BaTiO3
No full textTo investigate the experimental conditions for {111} twin formation and abnormal grain growth in BaTiO3, various powder compacts with and without excess TiO2 were sintered either in an oxidizing atmosphere (air) or in a reducing atmosphere (95N(2)-5H(2)). In all the samples sintered in 95N(2)-5H(2), no twins formed and no abnormal grain growth occurred. On the other hand, when the samples with excess TiO2 were sintered in air, abnormal grain growth occurred and all the abnormal grains contained {111} twins. X-ray diffraction analysis showed that, during sintering, the excess TiO2 formed a Ba6Ti17O40 phase with the space group A2/a in air and a Ba6Ti17O40-x phase with the space group C in 95N(2)-5H(2). These results show that the {111} twins form and the abnormal grain growth occurs only when the powder compact contains excess TiO2 to form a Ba6Ti17O40 phase and is sintered in an oxidizing atmosphere. The results may also suggest that the Ba6Ti17O40 phase acts as the nucleation site of {111} twins. The suggestion has been further confirmed via a TEM observation
Grain boundary faceting and abnormal grain growth in BaTiO3
No full textWhen 0.1 mol% TiO2-excess BaTiO3 was sintered below the eutectic temperature in air, abnormally large grains formed in the fine matrix BaTiO3 grains. The abnormal grains contained {111} twin lamellae, while the matrix grains did not. A TEM observation revealed that almost all the grain boundaries were faceted. On the other hand, however, when the air-sintered sample with faceted grain boundaries was annealed in H-2, the faceted boundaries became defaceted, and the growth of abnormal grains was suppressed while the growth of the matrix grains was enhanced, showing normal grain growth behavior. In addition, the abnormal grains that had been elongated along their twin lamella grew rather isotropically, irrespective of the presence of {111} twins. It appears therefore that {111} twins appear to enhance the growth of the abnormal grains along the twin lamellae only when the grain boundary is faceted. After reannealing the H-2-annealed sample in air, however, the grain growth behavior and grain boundary structure were found to recover those observed in the air-sintered sample. From these observations, it is concluded that abnormal growth of BaTiO3 grains observed is related to grain boundary faceting and that boundary faceting is a necessary condition for abnormal grain growth. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.
Core-shell structure formation in Nb2O5-doped SrTiO3 by oxygen partial pressure change
No full textA core-shell structure was observed in SrTiO3 doped with 1.2 mol% of Nb2O5, after sintering in a reducing atmosphere (5H(2)-95N(2)) and then in an oxidizing atmosphere lair). In undoped and Al2O3-doped SrTiO3 specimens, no core-shell structure formed after the same sintering treatments as those for SrTiO3 doped with 1.2 mol% of Nb2O5. The measured chemical compositions of the core and shell regions of 1.2-mol%-Nb2O5-doped SrTiO3 grains showed that the Sr/(Ti + Nb) ratio of the shell regions grown in air was similar to 1% less than that of core regions grown in 5H(2)-95N(2), which was in good agreement with a value predicted by available defect equations. Therefore, the observed core-shell structure is thought to result from the formation of strontium vacancies in an oxidizing atmosphere.
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
