1,721,185 research outputs found
Aligned Co2Z hexagonal ferrite fibers
No full textThe hexagonal ferrites, also know as hexaferrites, have become massively important materials commercially and technologically, accounting for the bulk of the total magnetic materials manufactured globally, and they have a multitude of uses and applications. They all have a magnetocrystalline anisotropy; that is the induced magnetization has a preferred orientation within the crystal structure. They can be divided into two main groups: those with an easy axis of magnetization, the uniaxial hexaferrites, and those with an easy plane (or cone) of magnetization, known as the ferroxplana or hexaplana ferrites. There is currently increasing interest in composite materials containing hexaferrite fibers. It had been predicted that properties such as thermal and electrical conductivity, and magnetic, electrical and optical behavior will be enhanced in material in fibrous form. This is because a continuous fine fiber can be considered as effectively one-dimensional, and it does not behave as a homogeneously distributed solid. Although the intrinsic magnetization of the material is unaffected, the effective magnetization of an aligned fiber sample should be greater when a field is applied parallel with fiber alignment compared to when applied perpendicularly to fiber alignment. This feature was first demonstrated by the author for aligned hexaferrite fibers in 2006, who was also the first to synthesize fibers of BaM, SrM, Co2Y, Co2Z, Co2W, Co2X and Co2U hexaferrites, as micron-scale (3-10 mm) continuous fibers in both random and aligned blankets. The magnetic properties of aligned Co2Z (Ba3Co 2Fe24O41) hexaferrite microfibers are presented in this paper. It is shown that unlike with the uniaxial M ferrite fibers, the ferroxplana Co2Z ferrite fibers do not shown any significant alignment affects with direction of applied field. They have high Ms values in all orientations. This is attributed to the microstructure of these fibers, in which the hexagonal plates are stacked and oriented parallel to the fiber axis. This appears to counteract the expected effects of fiber alignment on measured Ms values
Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics
No full textSince their discovery in the 1950s there has been an increasing degree of interest in the hexagonal ferrites, also know as hexaferrites, which is still growing exponentially today. These have become massively important materials commercially and technologically, accounting for the bulk of the total magnetic materials manufactured globally, and they have a multitude of uses and applications. As well as their use as permanent magnets, common applications are as magnetic recording and data storage materials, and as components in electrical devices, particularly those operating at microwave/GHz frequencies. The important members of the hexaferrite family are shown below, where Me = a small 2+ ion such as cobalt, nickel or zinc, and Ba can be substituted by Sr: M-type ferrites, such as BaFe 12O 19 (BaM or barium ferrite), SrFe 12O 19 (SrM or strontium ferrite), and cobalt-titanium substituted M ferrite, Sr- or BaFe 12-2xCo xTi xO 19 (CoTiM).Z-type ferrites (Ba 3Me 2Fe 24O 41) such as Ba 3Co 2Fe 24O 41, or Co 2Z.Y-type ferrites (Ba 2Me 2Fe 12O 22), such as Ba 2Co 2Fe 12O 22, or Co 2Y.W-type ferrites (BaMe 2Fe 16O 27), such as BaCo 2Fe 16O 27, or Co 2W.X-type ferrites (Ba 2Me 2Fe 28O 46), such as Ba 2Co 2Fe 28O 46, or Co 2X.U-type ferrites (Ba 4Me 2Fe 36O 60), such as Ba 4Co 2Fe 36O 60, or Co 2U. The best known hexagonal ferrites are those containing barium and cobalt as divalent cations, but many variations of these and hexaferrites containing other cations (substituted or doped) will also be discussed, especially M, W, Z and Y ferrites containing strontium, zinc, nickel and magnesium. The hexagonal ferrites are all ferrimagnetic materials, and their magnetic properties are intrinsically linked to their crystalline structures. They all have a magnetocrystalline anisotropy (MCA), that is the induced magnetisation has a preferred orientation within the crystal structure. They can be divided into two main groups: those with an easy axis of magnetisation, the uniaxial hexaferrites, and those with an easy plane (or cone) of magnetisation, known as the ferroxplana or hexaplana ferrites. The structure, synthesis, solid state chemistry and magnetic properties of the ferrites shall be discussed here. This review will focus on the synthesis and properties of bulk ceramic ferrites. This is because the depth of research into thin film hexaferrites is enough for a review of its own. There has been an explosion of interest in hexaferrites in the last decade for more exotic applications. This is particularly true as electronic components for mobile and wireless communications at microwave/GHz frequencies, electromagnetic wave absorbers for EMC, RAM and stealth technologies (especially the X and U ferrites), and as composite materials. There is also a clear recent interest in nanotechnology, the development of nanofibres and fibre orientation and alignment effects in hexaferrite fibres, and composites with carbon nanotubes (CNT). One of the most exciting developments has been the discovery of single phase magnetoelectric/multiferroic hexaferrites, firstly Ba 2Mg 2Fe 12O 22 Y ferrite at cryogenic temperatures, and now Sr 3Co 2Fe 24O 41 Z ferrite at room temperature. Several M, Y, Z and U ferrites have now been characterised as room temperature multiferroics, and are discussed here. Current developments in all these key areas will be discussed in detail in Sections 7-11 of this review, and for this reason now is the appropriate time for a fresh and critical appraisal of the synthesis, properties and applications of hexagonal ferrites. © 2012 Elsevier Ltd. All rights reserved
Combinatorial bulk ceramic magnetoelectric composite libraries of strontium hexaferrite and barium titanate
No full textBulk ceramic combinatorial libraries were produced via a novel, high-throughput (HT) process, in the form of polycrystalline strips with a gradient composition along the length of the library. Step gradient ceramic composite libraries with 10 mol % steps of SrFe 12O 19-BaTiO 3 (SrM-BT) were made and characterized using HT methods, as a proof of principle of the combinatorial bulk ceramic process, and sintered via HT thermal processing. It was found that the SrM-BT libraries sintered at 1175 °C had the optimum morphology and density. The compositional, electrical and magnetic properties of this library were analyzed, and it was found that the SrM and BT phases did not react and remained discrete. The combinatorial synthesis method produced a relatively linear variation in composition. The magnetization of the library followed the measured compositions very well, as did the low frequency permittivity values of most compositions in the library. However, with high SrM content of ≥80 mol %, the samples became increasingly conductive, and no reliable dielectric measurements could be made. Such conductivity would also greatly inhibit any ferroelectricity and magnetoelectric coupling with these composites with high levels of the SrM hexagonal ferrite. © 2012 American Chemical Society
Method for the preparation of aligned fibre samples for magnetic measurement using VSM
No full textCeramic vibrating sample magnenometer (VSM) samples are usually in the form of a sintered monolith, a sintered pressed pellet, or a powder set in a resin or wax. The problem of mounting an aligned fibre sample, so that it retains alignment whilst remaining fixed in place as it is vibrated in a large magnetic field, was addressed for the first time. The most successful sample preparation method involved the formation of a flat, square composite sample through impregnation of the fibre with a fast-setting resin. Such aligned fibre samples had a fibre volume fraction of 3-4%, but random fibre samples were also prepared, forming a much denser composite with typically 10 times as much fibre in the sample. The magnetic properties of a range of polycrystalline hexagonal ferrite fibres prepared by this method, and the effects of fibre alignment upon those properties, are currently being published in a series of articles
The synthesis, properties, and applications of columbite niobates (M 2+Nb2O6): A critical review
No full textThe binary niobate ceramics, with the formula M2+Nb 2O6 where M2+=Ca, Mg, or a transition metal (TM), have the orthorhombic columbite structure. The best-known members of this group are zinc niobate (ZnNb2O6) and magnesium niobate (MgNb2O6), but Ca, Co, Ni, Mn, Cu, Cd, and Fe 2+ cations can also be included in the columbite structure. The TM columbite niobates have been found to sinter at temperatures of 1100°-1200°C, much lower than the complex perovskites, and this can be lowered even more when Cu2+ is used. The best columbite niobates have Q × f values similar to those of BaZn0.33Nb 0.67O3, and all have εr between 17 and 25 and negative τf values of <-80 ppm/°C. There is a growing interest in the columbites as microwave dielectric ceramics, due to their lower processing temperatures, less complicated processing due to the simple chemistry of the binary compounds, and the lower cost of niobium compared with tantalum, and with incorporation of Cu2+ they are approaching low-temperature cofired ceramics (LTCC) temperatures. They have also been investigated combined with other dielectric ceramics (to compensate for the negative τf values), and with additives to lower sintering for LTCC. Furthermore, MgNb 2O6 is in wide use as a precursor to synthesize single phase PMN (Pb(Mg1/3Nb2/3)O3) in the "columbite" process, and NiNb2O6 is being increasingly investigated as a catalyst for splitting water and organic compounds. CoNb2O6 and other columbites have interesting magnetic properties, and CaNb2O6 and CdNb 2O6 have useful optical properties. This review covers the various means of synthesis of these ceramics, and the effects of processing upon structural, physical, electronic, and optical properties. This review will concentrate on the dielectric properties and applications, as this is the greatest area of interest, but will also cover other properties and applications of these ceramics. All available reported microwave dielectric data for columbites is compiled, compared, and assessed. © 2009 The American Ceramic Society
Magnetic Properties of Aligned Co2Z Hexagonal Z-Ferrite Fibers
No full textHexaferrites have become massively important materials commercially and technologically, accounting for the bulk of total magnetic materials manufactured globally, with a multitude of uses and applications. There is currently increasing interest in composite materials containing hexaferrite fibers. It had been predicted that properties such as thermal, magnetic, electrical, and optical behavior will be enhanced in material in fibrous form, because a continuous fine fiber can be considered as effectively one dimensional, and does not behave as a homogeneously distributed solid. Aligned Co(2)Z (Ba3Co2Fe24O41) hexaferrite microfibers were found to have high magnetization values in all orientations and low coercivity. However, unlike the uniaxial M ferrite fibers previously reported, the ferroxplana Co(2)Z fibers do not show any significant alignment affects with direction of applied field. This is attributed to the microstructure of these fibers, in which the hexagonal plates are stacked and oriented parallel to the fiber axis, counteracting the expected effects of fiber alignment on measured magnetic saturation (M-s) values
Hexagonal ferrite fibres and nanofibres
No full textHexagonal ferrites, or hexaferrites, are hugely important materials commercially and technologically, with common applications as permanent magnets, magnetic recording and data storage media, components in electrical devices operating at wireless frequencies, and as GHz electromagnetic wave absorbers for EMC, RAM and stealth technologies. Hexaferrites are all ferrimagnetic materials, and their magnetic properties are intrinsically linked to their crystalline structures, all having a strong magnetocrystalline anisotropy; that is the induced magnetisation has a preferred orientation within the crystal structure. They can be divided into two main groups: those with an easy axis of magnetisation (known as uniaxial), the hard hexaferrites, and those with an easy plane (or cone) of magnetisation (known as ferroxplana or hexaplana), soft ferrites. The common hexaferrite members are: « M-type ferrites, such as BaFe12O19 and SrFe12O19« Z-type ferrites (Ba3Me2Fe24O41) « Y-type ferrites (Ba2Me2Fe12O22) « W-type ferrites (BaMe2Fe16O27) « X-type ferrites (Ba2Me2Fe28O46) « U-type ferrites (Ba4Me2Fe36O60) where Me = a small 2+ ion such as cobalt, nickel or zinc, and Ba can be fully substituted by Sr. Generally, the M ferrites are hard, the Y, Z and U ferrites are soft, and the W and X ferrites can very between these two extremes, but all have large magnetisation (M) values. There is currently increasing interest in composite materials containing hexaferrite fibres. It had been predicted that properties such as thermal and electrical conductivity, and magnetic, electrical and optical behaviour will be enhanced in material in fibrous form. This is because a continuous fine fibre can be considered as effectively one-dimensional, and it does not behave as a homogeneously distributed solid. Although the intrinsic magnetisation of the material is unaffected, the effective magnetisation of an aligned fibre sample should be greater when a field is applied parallel with fibre alignment compared to when applied perpendicularly to fibre alignment. This feature was first demonstrated by the author for aligned hexaferrite fibres in 2006. This chapter will deal with progress in the manufacture and properties of hexaferrite fibres, from the first syntheses of BaM, SrM, Co2Y, Co2Z, Co2W, Co2X and Co2U micron-scale fibres by the author 12-15 years ago, to recent developments in M ferrite hollow fibres and nanofibres, and hexaferrite-coated CNTs (carbon nanotubes).The relative properties of all reported hexaferrite fibres are compared and summarised at the end of this chapter. © (2016) Trans Tech Publications, Switzerland
The synthesis and characterisation of Co2X (Ba2Co2Fe28O46) and Co2U (Ba4Co2Fe36O60) ferrite fibres, manufactured from a sol-gel process
No full textThe X and U phases of hexagonal ferrites are notoriously difficult to make as pure materials, and what little data there is published regarding their physical or magnetic properties relates only to single crystals. Stoichiometric Co2X and Co2U fibres were made from a sol-gel based process, and the characteristics of the sols and fibres were studied using PCS, XRD, SEM and VSM. The evolution of the X and U fibres was studied, and shown to be similar that of the W and Z fibres whose structures they respectively resemble. Co2X and Co2U had been formed at 1200°C, and the morphology of the fibres was different from those of the W and Z phases. The magnetic properties, again while resembling the W and Z phases, were different to any of the other hexagonal ferrite fibres. The magnetic hysteresis loops of both fibres were magnetically as was, the Co2X fibres having Ms = 45.0 emu g-1 and Hc = 0.085 T, and the Co2U fibres having Ms = 51.5 emu g-1 and Hc = 0.059 T. © 2001 Kluwer Academic Publishers
Ecoceramics
No full textEnvironmentally conscious ceramics (ecoceramics) are a new class of biomimetic/biomorphic material that can be manufactured from renewable resources, such as wood from sustainable sources or wood wastes. The idea is to manufacture ceramics with the microstructure of wood. The wood is pyrolysed to convert it into carbon, which is nanoporous, but maintains the wood’s microstructure and morphology. It is possible to infiltrate this carbon matrix with a ceramic precursor, and then heat it to burn out the carbon and form the ceramic
Comparison of low and high pressure infiltration regimes on the density and highly porous microstructure of ceria ecoceramics made from sustainable cork templates
Full text linkCork templates were used to produce lightweight bulk biomimetic ecoceramic (environmentally conscious ceramic) monoliths. Bulk/monolithic ceramics are vital for many applications, i.e. energy materials and fuel cells. Using simple and flexible, aqueous green-chemistry procedures, for the first time the influence of infiltration regime, number of infiltration cycles and sintering temperature on ecoceramic density and microstructure was studied. This lightweight three-dimensionally ordered macroporous (3DOM) CeO2 preserved the hexagonal cellular structure of cork, but unlike the wood, the rear cell walls were open, greatly increasing open porosity. Higher sintering temperatures (1600 instead of 1000 °C) were required to produce cm size monolithic ecoceramics mechanically strong enough to be handled. The infiltration regime and number of infiltration cycles affected density and porosity. Lower infiltration pressure led to higher porosity ecoceramics (3.3–5.7%), which may favour catalytic performance, showing the possibility of tailoring porosity and specific surface area by modifying the number of infiltration cycles
- …
