1,720,964 research outputs found
Reduction of PbO loss in PZT-cobalt ferrite composites through quite-fast sintering and its quantification by means of XRD analysis
Full text linkNowadays, considerable efforts have been devoted to design and control the fabrication of multifunctional materials in order to fulfil the needs of modern technology for novel sensors, microwave devices, energy harvesting, photovoltaic technologies, solid-state refrigeration, data storage recording technologies and multiferroic random access multistate memories (MFRAM) [1]. Particulate ceramic composites are low cost, simple production technology, higher strain mediated magnetoelectric coupling (since electric order phase/magnetic phase interface density can be higher) and easy control of electrical and magnetic properties if the ferroelectric phase and the ferromagnetic one are mixed in a favourable proportion under the percolation threshold of the ferromagnetic phase. A great research effort is in progress to improve the fabrication of PZT-CoFe2O4 (PZT-CF) composites due to the excellent piezoelectric properties showed by the PZT material class and the large magnetostrictive coefficient of the CF. Unfortunately, during the sintering process particulate PZT-CF composites, side reactions do occur that are detrimental to the properties of the so-obtained material. In this study, we have avoided such reactions and PbO loss by setting a quite-fast sintering process [2]. The extent of PbO loss was determined by means of XRD analysis of the densified samples taking into account the amount of ZrO2 and the variations of the perovskite's tetragonality [2]. The calculated PbO loss values are in agreement with the final density and the microstructure of PZT-CF composites. In particular, microstructural characterization showed that CF grain size distribution can be mono- or bi-modal, and CF overgrowth was found to affect the coercivity of the material [3]. [1] M. M. Vopson, Fundamentals of Multiferroic Materials and Their Possible Applications. Crit. Rev. Solid State 4:40 (2015) 223-250 doi:10.1080/10408436.2014.992584 [2] P. Galizia, et al., PZT-cobalt ferrite particulate composites: Densification and lead loss controlled by quite-fast sintering. J. Eur. Ceram. Soc. (2016). doi:10.1016/j.jeurceramsoc.2016.08.025 [3] P. Galizia, C. Baldisserri, C. Capiani, C. Galassi, Multiple parallel twinning overgrowth in nanostructured dense cobalt ferrite. Mater. Design 109 (2016) 19-26. doi:10.1016/j.matdes.2016.07.05
Heterostructured ceramic materials based on PZTN-CFO compounds
Full text linkMultiferroic composites are currently one of the hot research topics [1]. Particulate ceramic composites have the advantages of low cost, simple production technology, good magnetoelectric effect and easy control of electrical and magnetic properties if the ferroelectric phase (generally a perovskite) and the ferromagnetic one (a ferrite with spinel structure) are mixed in a favourable proportion under the percolation threshold of the ferromagnetic phase. A great research effort is in progress to improve the fabrication of PZT-CoFe2O4 (PZT-CF) composites due to the excellent piezoelectric properties showed by the PZT material class and the large magnetostrictive coefficient of the CF. Unfortunately unwanted reactions occur during densification of PZT-CF materials at 1100-1200 °C. They are promoted by initial PbO loss that is calculated through XRD analysis, considering the amount of ZrO2 and variation of perovskite's tetragonality. The resulting titania reacts with CF to form cobalt titanate [2]. The microstructure of the composites at 26-81 mol % CF content was thoroughly investigated; the CF grain size distribution can be mono- or bi-modal and overgrowth [3] occurs. By setting a quite-fast sintering full densification and prevention of unwanted reactions was achieved for the PZT:CF 74:26 composites. The high coercivity (789 Oe) shown by these composites is correlated to the good dispersion of euhedral CF grains (250 nm) in the PZT matrix that is affected as well by limited grain growth (240 nm). [1] G. Schileo, Recent developments in ceramic multiferroic composites based on core/shell and other heterostructures obtained by sol-gel routes. Prog. Solid State Ch. 41 (2013) 87-98. [2] P. Galizia, et al., PZT-cobalt ferrite particulate composites: Densification and lead loss controlled by quite-fast sintering. J. Eur. Ceram. Soc. (2016). doi:10.1016/j.jeurceramsoc.2016.08.025 [3] P. Galizia, C. Baldisserri, C. Capiani, C. Galassi Multiple parallel twinning overgrowth in nanostructured dense cobalt ferrite. Mater. Design 109 (2016) 19-26. doi:10.1016/j.matdes.2016.07.05
Milling and quite-fast sintering as key production steps to obtain fully dense PZTN-CF particulate composites
No full textIn the near future (within ten years) magnetoelectric multiferroics could be implemented into the emerging technologies such as wireless power, internet of things, machine-to-machine communication services, mesh network, etc. Remarkable efforts have been done to develop laminated bi-layer and multilayer multiferroic composites as bulk or thin films. These structures lead to remarkable magneto-electric coupling coefficients of a few Volts / cm∙Oe because the ferroic layer is a "full dielectric" which can be completely poled in the conventional way. On the other hand in the particulate ceramic composites the requirement for "full dielectric" is no longer applicable, since the ferroic phases are fully separated within the composite. The strengths of particulate ceramic composites are low cost, simple production technology, higher strain mediated magneto-electric coupling (since electric order phase/magnetic phase interface density can be higher) and easy control of electrical and magnetic properties if the ferroelectric phase (generally a perovskite) and the ferromagnetic one (a ferrite with spinel structure) are mixed in a favourable proportion under the percolation threshold of the ferromagnetic phase. A great research effort is in progress to improve the fabrication of PZT-CoFe2O4 (PZT- CF) composites in order to avoid the unwanted reactions, which occur during densification of PZT-CF materials at 1100-1200 °C, and to achieve the electric saturation during the poling. Up to date, by setting a quite-fast sintering, full densification and prevention of unwanted reactions were achieved for the PZT:CF 74:26 composites,1 but achieving electric saturation is still a challenge. Further important results were: the understanding that the main cause of reactions is the PbO loss;1 the proposal of an equation to calculate the PbO loss through XRD analysis, considering the amount of ZrO2 and variation of perovskite's tetragonality;1 and the ability to design the ceramic process (milling of the CF powers in particularly) to control the CF grain size distribution, which can be mono- or bi-modal, and overgrowth.1,2 References 1. P. Galizia, C.E. Ciomaga, L. Mitoseriu and C. Galassi, "PZT-cobalt ferrite particulate composites: Densification and lead loss controlled by quite-fast sintering", J. Eur. Ceram. Soc., 37, pp. 161-168, 2016. 2. P. Galizia, C. Baldisserri, C. Capiani and C. Galassi, "Multiple parallel twinning overgrowth in nanostructured dense cobalt ferrite", Mater. Design, 109, pp. 19-26, 2016
Structure analysis of cobalt ferrite/titania-silica composite
Full text linkMagnetodielectric bulk composite of a magnetic phase in dielectric matrix have been studied. Silica coated titania powder was produced by elctrocoagulation and used as dielectric matrix; while the cobalt ferrite powder was selected as magnetic filler. This study aims at tailoring the magnetic and dielectric phases and their interfaces in order to obtain new electromagnetic properties. The microstructure of sintered titaniasilica/cobalt ferrite composites has been related to compositional modifications in terms of silica/titania weight ratio and titania-silica/cobalt ferrite volume ratio. The crystalline structure was studied by XRD analysis supported also by EDS investigationand image analysis performed on the electron micrographs. The image analysis supported the XRD and EDS interpretations, and contributed to evaluate the effective volume contentof the phases after heat treatmen
Multiple parallel twinning overgrowth in nanostructured dense cobalt ferrite
No full textCobalt ferrite powders were synthesized by solid state reaction of the nanosized oxides at different temperatures. The highly aggregated powders were milled, and the aggregate size was reduced from 25 - 40 μm to 12-20 μm, depending on the milling time. A correlation between milling media diameter and final granulometry, and an unexpected calcination temperature effect on the milling efficiency were found. Highly homogenous green bodies and fully dense materials were produced for the first time after conventional sintering. The crystallite size depends primarily on the heating conditions and decreases from 50 - 70 nm to 27-13 nm. Under the same sintering conditions, particle morphology and crystallite size control the final grain shape, producing twinned grains with increased multiple parallel twinning overgrowth for the finer powders. The sintered cobalt ferrite ceramics show a relative density of 96-99%. The higher the planar faults density and grain size, the lower is the induced magnetization due to increased domain walls pinning. The variation of initial susceptibility was explained by extending the Globus model to the case where the domain walls are pinned at twinning boundaries. A linear correlation between multiparallel-twinned grains fraction and initial susceptibility was foun
Structure analysis of cobalt ferrite/titania composite
Full text linkMagnetodielectric bulk composite of a magnetic phase and a dielectric one were tailored to tune the macroscopic properties of permittivity and permeability. The designing of heterostructure formed by magnetic and dielectric counterparts offers a versatile route for the production of isotropic composite material with unusual electromagnetic properties. This study investigates the microstructure of sintered titania (TO)/cobalt ferrite (CFO) composites related to compositional modifications. The crystalline structure was studied through the comparison of theXRD patterns withthe EDS analysis and the results of the image analysis done on the electron micrographs. The image analysis wasfundamental to verifythe XRD and EDS interpretations, and to measure the effective volume contentof the phasesafter heat treatment.Several TO/CFO ratios from 0.7 to 11.8were analyzed. The formation oftheternary compound (FCTO) was foundonly in the composite with the TO/CFO ratio equal or bigger than
Novel magnetodielectric cobalt ferrite-titania-silica ceramic composites with tunable dielectric properties
No full textThe cobalt ferrite (CF)-titania (TiO2)-silica (SiO2) system has been studied to produce new ceramic composites by conventional solid state reaction. The microstructure of the sintered CF-TiO2-SiO2 mixture has been related to compositional modifications in terms of SiO2/TiO2 weight ratio keeping constant the CF weight percentage. Microstructural characterization of the sintered bodies was performed in order to understand microstructure evolution, and to quantify the phases volume fraction. The final compositions after sintering differ significantly from the starting ones as a consequence of the reaction of titania with the ferrite, and the formation of the ilmenite-type CoTiO3. Four different distributed phases are present, depending on the starting SiO2/TiO2 weight ratio. The complex permittivity dispersion of ceramic composites was investigated and correlated to their microstructure. Lastly, CF-SiO2 magneto-dielectric composites are suggested as possible candidates for high frequency applications as miniaturized antennas
Study of the role of porosity on the functional properties of (Ba,Sr)TiO<inf>3</inf> ceramics
No full textThe role of porosity on the low and high field dielectric properties was studied in Ba0.70Sr0.30TiO3 ceramics with various porosity levels obtained by using lamellar graphite as sacrificial template. The permittivity decreases with increasing porosity, from around ε ∼ 7690 (dense ceramic) down to 380 (ceramic with 29% porosity), while preserving the Curie temperature at about 35 °C. The effective permittivity was discussed by using Effective Medium Approximation and Finite Element Method approaches. The role of porosity and specific microstructural characteristics induced by the pore forming addition, from closed porosity (0–3) towards a combined (0–3, 2–2) with lamellar-type of microstructures for the most porous ceramics was taken into consideration. All the investigated ceramics preserve a high level of tunability as in the dense material, irrespective of the porosity level, while the zero field permittivity was decreased to a few hundreds
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
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