1,720,960 research outputs found
Processing of (BaSr)Fe12O19 for antenna miniaturization
Full text linkHexaferrites of M-type at high-frequency are investigated, owing to their higher ferro-magnetic resonances as compared to traditional magnetic materials, thus ensuring a less dispersive behavior of the material at microwaves frequencies. Here we present the experimental work concerned with the ceramic process for the production of the barium-strontium hexaferrite (BSFO). The critical issue in the production of this material is to obtain a fully dense and homogeneous microstructure. These aspects have been addressed by introducing powder grinding processes, and by varying the cold consolidation conditions of the powders. A number of samples of the material have been produced, both for morphological and microstructural characterization and a demonstrator antenna, onto which conductive patches were applied, and the radiating properties of which were tested. Barium-strontium hexaferrite as supporting material for antennas - while allowing substantial device miniaturization - displays relatively high dielectric and magnetic losse
Preparation and dielectric investigations of Ba0.60Sr0.40TiO3 ferroelectric ceramics with different degree of porosity, presenting
No full textBy introducing a controlled porosity into the ceramic systems, the properties can be improved and make the material proper for being used as high-frequency ultrasonic transducers or for tunable applications. The tailored porosity exposes special properties and characteristics of the materials, which might be completely different than their conventional dense equivalents. The aim of this study was to characterize (Ba,Sr)TiO3 ceramics with different porosity levels. Powders with the composition Ba0.60Sr0.40TiO3 (BST) were prepared following the mixed oxide method by solid-state reaction at a sintering temperature of 1450°C for 2h. The BST ceramics with various porosities have been prepared by addition of lamellar graphite in concentration of 10, 20 and 35%. The effect of addition of graphite on dielectric properties was investigated. We noticed that the increasing of the amount of graphite increases the graded porosity and leads to a low mechanical quality factor. The relative density of the obtained samples varies from values around 95% for the dense ceramic to 66% for porous ceramics. Graphite was burned out through the sintering process and this resulted in a modification of morphology, microstructure and pore density of the ceramic product. The porous samples produced with homogeneous or graded porosity show a strong difference of the dielectric properties and microstructure in comparison with the dense ceramic samples. The role of anisotropic porosity on the functional properties on the dielectric properties is discussed. Microstructural, morphological and dielectric investigations have been performed and data were compared with other literature studies. From SEM analysis it was found that with addition of graphite as pore forming the samples present a fracture mode transformation from intragranular fracture to an intergranular fracture and a decreasing of grain size of BST. Furthermore, as BST grain size becomes smaller, the volume of grain boundary increases and the crack prefers spreading along grain boundary to splitting the strengthened grain. The Impedance Spectroscopy analyse was performed in the temperature range of (21-200)°C and frequency of 20Hz–2MHz. The dielectric investigations have shown that the dielectric constant of the Ba0.60Sr0.40TiO3 (BST) ceramics with different degree of porosity, decrease and remains constant at higher frequency, indicating dielectric dispersion. The frequency dependence of the real and imaginary part of permittivity present a few characteristic relaxations which appear in the specific frequency ranges: (i) the increasing of the real and imaginary parts of permittivity at low frequencies below 100Hz for the compositions with x=10, 20 and 35% graphite is most probably associated to the thermally activated space charge effects (Maxwell–Wagner phenomena) and (ii) anomalies of the imaginary part of permittivity above 103Hz, for samples with x=10 and 20% porosity, which seems to be related to a Debye-type relaxation process. The dielectric properties of the investigated ceramics were characterized and correlated with their microstructure. From dielectric constant dependence with temperature, a shift of Curie temperature of ceramics was observed and possible reasons for these effects were interpreted. Acknowledgements: This work was financially supported by CNCS-UEFISCDI projects PNII-ID-PCE-2011-30745 and by ERASMUS placement programme between Univ. “Al. I. Cuza” Iasi & ISTEC-CNR Faenza, Italy. The collaboration in frame of the COST Action MP0904 is highly acknowledgements
Porous ceramics structured for bone-cartilage implants.
No full textThere are cases of damage to articular joints in which not only a reconstruction of the cartilage is necessary but also replacement of the underlying subchondral bone portion, the thickness of which can be as much as 1-2 mm. Are-construction of the articular surface must take into consideration the particular structure of the bone in the epiphysis position, characterised by an outward fine porosity. The structure of this external porosity does not allow cell migration from the inside to the outside of the bone surface although, at the same time, it does allow good permeation of physiological fluid, flowing out from the inside the bone. This physiological fluid transports oxygen, sugars, proteins and other molecules in order to supply nourishment to the cells (chondrocytes) which populate the layer of cartilage that overhangs the surface of the bone plate. Outgoing from the cartilage layer, the flux of physiological fluid sweeps away carbon dioxide and any other metabolic or catabolic products of the cells. The surface microporosity further serves to offer rootage to cartilaginous proteins for their natural mechanical retention in the site. Since a patent aimed at producing thin ceramic layers with a gradient of porosity (decreasing from one principal face to the opposite) has been applied for, these layers are going to be tested to evaluate the degree to which they meet the requirements. Preliminary tests carried out on samples at different but homogeneous porosity have emphasised that cell permeation of osteoblast cultures is practically stopped after some few tenths microns. The osteoblasts are shown to settle in the interior of the larger pores. They remodel the surrounding walls of the pores in which they lodge and start the production of bone nodules which are useful, for in vivo applications, in producing the desired tissue continuity between the interior of the ceramic and the underlying bone tissue, thus promoting a definitive firm setting. A cytological test, performed one week after cells had seeded on one side of the ceramic samples, has shown a provisional response towards the lodging material. In fact the cells assumed a globular shape (tending to flattening), but displaying cytoplasmatic processes of cellular interconnection. Measurements of the vitality, population and penetration into the cell pores inside the ceramic were carried out. On the basis of the results, the proposed ceramic devices with a gradient of porosity indicate their suitability for the intended function. Successive experimental steps will concern in vivo applications, simulating a repair intervention for an articular arthrosis
A glance at processing-microstructure-property relationships for magnetoelectric particulate PZT-CFO composites
Full text linkIn this work, we investigated the processing-microstructure-property relationships for magnetoelectric (ME) particulate composites consisting of hard ferromagnetic CoFe2O4 (CFO) particles dispersed in a Nb-doped PbZrxTi1-xO3 (PZT) soft ferroelectric matrix. Several preparation steps, namely PZT powder calcination, PZT-CFO mixture milling and composite sintering were tailored and a range of microstructures was obtained. These included open and closed porosities up to full densification, PZT matrices with decreasing grain size across the submicron range down to the nanoscale and well dispersed CFO particles with bimodal size distributions consisting of submicron and micron sized components with varying weights. All samples could be poled under a fixed DC electric field of 4 kV/mm and the dielectric, piezoelectric and elastic coefficients were obtained and are discussed in relation to the microstructure. Remarkably, materials with nanostructured PZT matrices and open porosity showed piezoelectric charge coefficients comparable with fully dense composites with coarsened microstructure and larger voltage coefficients. Besides, the piezoelectric response of dense materials increased with the size of the CFO particles. This suggests a role of the conductive magnetic inclusions in promoting poling. Magnetoelectric coefficients were obtained and are discussed in relation to densification, piezoelectric matrix microstructure and particle size of the magnetic component. The largest magnetoelectric coefficient ff33 of 1.37 mV cm-1 Oe-1 was obtained for submicron sized CFO particles, when closed porosity was reached, even if PZT grain size remained in the nanoscale
Additive Manufacturing of Piezoelectric Niobium-Doped Lead Zirconate Titanate (PZT-N) by Binder Jetting
No full textBinder jetting is an emerging indirect additive manufacturing technique for ceramic materials, which could be employed to produce application-oriented designed components impossible to achieve with traditional processes and featuring enhanced performances. In our study, niobium-doped lead zirconate titanate (PZT-N) powder, usually processed through the standard press-and-sinter route, was employed as the raw material. First, the powder was characterized in terms of granulometry and flowability to assess its suitability for the printing process. Then, shaping by binder jetting was studied, and the effect of three levels of binder saturation (75–90–105%) on the green bodies was assessed. Finally, the microstructure of the sintered samples was studied using SEM, to investigate the effect of thermal treatments on the grain size distribution and residual porosity (~40%). The piezoelectric properties were measured and compared to those of conventionally processed material. The piezoelectric charge and voltage constants (d33 and g33) were evaluated to determine the possible use of printed parts as porous piezoelectric components to be exploited in hydrophones in the direct mode
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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