Hexagonal Fibres Research Articles

Microstructural evolution of polymer-derived hexagonal boron nitride fibres under high-temperature stretching

Microstructural evolution of polymer-derived hexagonal boron nitride fibres under high-temperature stretching

Spatiotemporal multiplexing based on hexagonal multicore optical fibres

Based on a genetic algorithm, we have solved the problem of finding the parameters of optical Gaussian pulses which make their efficient nonlinear combining possible in one of the peripheral cores of a 7-core hexagonal fibre. Two approaches based on individual selection of peak powers and field phases of the pulses launched into the fibre are considered. The found regimes of Gaussian pulse combining open up new possibilities for the development of devices for controlling optical radiation.

Hexagonal Ferrite Fibres and Nanofibres

Hexagonal 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 BaFe12O19and SrFe12O19Z-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.

ChemInform Abstract: Hexagonal Ferrites: A Review of the Synthesis, Properties and Applications of Hexaferrite Ceramics

AbstractReview: 560 refs.

A solution for transverse thermal conductivity of composites with quadratic or hexagonal unidirectional fibres

Abstract A solution for the transverse thermal conductivity (K e ) of unidirectional fibre arrays, quadratic and hexagonal, is developed analytically. The solution integrates the thermal conductivity of fibre (K f ) and fluid (K m ) based on electricity analogy without thermal contact resistance (R c ) at the fibre/fluid interface. The expression K e is a function of K f and K m , as well as of the fibre volume fraction (V f ). In this article, K e values of four composites were predicted and verified by computational fluid dynamics (CFD) simulations. The results showed good agreement when the ratio of K f /K m is close to 1. An increase in the ratio or V f gives poorer agreement owing to the local temperature gradient at the fibre/fluid interface. CFD simulation also showed that K e is decreasing as the R c value increases.

Magnetic properties of randomly oriented BaM, SrM, Co2Y, Co2Z and Co2W hexagonal ferrite fibres

Abstract The microstructure and magnetic properties of randomly oriented BaFe 12 O 19 , SrFe 12 O 19 , Ba 2 Co 2 Fe 12 O 22 , Ba 3 Co 2 Fe 24 O 41 , Ba 3 Ca 0.3 Co 2 Fe 24 O 41 and BaCo 2 Fe 16 O 27 hexaferrite fibres were characterised. 2D and 3D AFM and MFM images were taken of a single BaM fibre. Magnetic properties of random ferrite fibres compared well to expected values for polycrystalline ceramics. The little-characterised Co 2 W ferrite was found to have M s and H c similar to that of Co 2 Z. Relatively small applied fields of H c 2 W fibres had a high M r / M s ratio of 0.56, (greater than M ferrites), despite being very magnetically soft (low coercivity), due to the unusual “lobed” shape of their hysteresis loop, which was attributed to their fibrous nature, and elongated growth of the grains along the fibre axis. Co 2 Z had the lowest H c of all the ferroxplana fibres.

Fibre cell organization in crystalline lenses

This review qualitatively and quantitatively compares the gross shape and size of lenses from different species as a function of their fibre cell organization. Grossly, all vertebrate lenses are asymmetrical, oblate spheroids with size and spheroidicity that varies considerably between species. Correlative LM and SEM analysis of the basic structural element of lenses, the fibre cell, shows that the average equatorial fibre width and thickness is relatively constant between most species. This indicates that inter-species differences in lens size is primarily a function of fibre number. Comparable analysis demonstrates that lens spheroidicity is due at least in part, to differences in the average anterior and posterior fibre end segment thickness in relation to that at the equator. In addition, the above analysis, supplemented by 3D-CAD reconstructions, illustrates how lifelong lens growth produces crescent fibres, that become arranged in age-related, concentric growth shells overlaid in slightly imprecise register. The reason for the non-exact registering of growth shells is that, while the vast majority of fibres are hexagonal in cross-section, a very small minority are pentagonal in cross-section and of inconstant width and thickness. Hexagonal and pentagonal fibres are required because the increase in the circumference of successive growth shells is frequently less than the widths of hexagonal fibres. Thus, lens growth is likely accomplished by a combination of the addition of successive growth shells containing more fibres, as well as by the addition of growth shells containing equal numbers of fibres that are incrementally wider as a function of radial location. Finally, SEM analysis, supplemented by 3D-CAD reconstructions, highlights the fact that the end-to-end arrangement of fibres within growth shells, the suture patterns, is not identical in all vertebrate lenses. This is important because lens optical quality is directly related to lens suture type and a negative influence of sutures on lens optical quality increases with age and as a result of some ocular surgeries (vitrectomy and trabeculectomy). These facts suggest that future research efforts should be directed at determining the factor or factors that influence or direct the differences in fibre shape, size and organization in branched and unbranched suture lenses.

Scattering of light from atmospheric ice analogues

New analogues replacing atmospheric ice crystals in light scattering measurements have been developed. These include thin, hexagonal glass fibres and both simple and complex microcrystals resembling cirrus ice, including columns, plates, rosettes and aggregates with a variety of sizes. Results of angle-dependent scattering measurements on the fibres and on levitated crystals are presented, including phase functions, polarization properties and the asymmetry parameter.

A halide free route to the manufacture of microstructurally improved M ferrite (BaFe12O19 and SrFe12O19) fibres

Abstract In hexagonal ferrite precursor fibres halide was found to be retained up to high temperatures, delaying the formation of the BaM (BaFe12O19) and SrM (SrFe12O19) phases, and resulting in a product with a grain size of 0.5–1 μm. This paper reports a halide free route for these fibres, using a nitrate stabilised sol precursor, from which the M phases formed at lower temperatures with smaller grain sizes. This sol was characterised and compared to the halide based sol, the evolution of the crystalline phases was studied and the hysteresis loops of the M ferrite fibres assessed by VSM. It was shown that retained halide inhibited the formation of the ferrite phase, as both SrM and BaM formed at a temperature 200 °C lower than in the halide containing precursor fibres. The grain sizes were accordingly lower, the SrM grains being below 100 nm, although the crystallite sizes were only slightly smaller than those reported for the halide containing fibres fired to 1000 °C, and the magnetic properties were also similar. The BaM fibres had Ms=58.4 kA m2 g−1 and Hc=401 kA m−1, while the SrM fibres had Ms=65.0 kA m2 g−1 and Hc=440 kA m−1, comparable to small grained polycrystalline powdered samples. The M ferrite phase was formed directly from haematite, with no BaFe2O4 observed as an intermediate, and the pure ferrites were made from stoichiometric precursors.

Decomposition, shrinkage and evolution with temperature of aligned hexagonal ferrite fibres

The decomposition, shrinkage and evolution of aligned gel fibres with the stoichiometric compositions for BaM, SrM and Co 2Z were investigated over a range of temperatures. SrM and BaM began to form by 750 and 800°C respectively, giving pure SrM and BaM phases at 900 and 1000°C. The Z phase formed at 1250°C, and only after the full crystallisation of the M and Y phases at 1000°C. Pure phase fibres of all ferrites were produced from stoichiometric mixes, unlike standard ceramic preparations. The fibres shrank by up to 22–23% at 1200°C, with no loss of alignment, and were appeared sintered (~97%) by this point. The formation of the M ferrite phases seemed to occur at unexpectedly high temperatures compared to previous work on bulk sol-gel M ferrites. Halides were retained in the fibre over 800°C, and ferrite formation was delayed until the halides were lost.

The synthesis and characterisation of Co2X (Ba2Co2Fe28O46) and Co2U (Ba4Co2Fe36O60) ferrite fibres, manufactured from a sol-gel process

The 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 M s = 45.0 emu g−1 and H c = 0.085 T, and the Co2U fibres having M s = 51.5 emu g−1 and H c = 0.059 T.

SiC polycrystalline fibre and its fibre-bonded ceramic

A type of SiC polycrystalline fibre (SA fibre) obtained by sintering an amorphous Si-Al-C-O fibre precursor at 1900 ° C in Argon atmosphere is described. A sintered SiC fibre-bonded ceramic (SA-Tyrannohex), which was synthesized by hot-pressing piled sheets of the amorphous Si-Al-C-O fibre precursor prepared from an organosilicon polymer, is also introduced. The former, SA fibre, which contains a very small amount of aluminium, has a high tensile strength and modulus, and shows no degradation of strength or change in composition on heating up to 2000 °C in inert atmosphere and up to 1000 °C in air for a time longer than 2000 hours. Moreover, SA fibre shows better high-temperature creep resistance in air at 1300 °C than commercial counterparts. We also found that the mechanical properties of the fibre were markedly improved by a decrease in the fibre diameter. On the other hand, the latter, SA-Tyrannohex, consists of a highly ordered, close-packed structures of very fine hexagonal columnar fibres with a thin interfacial carbon layer. The interior of the fibre element was composed of sintered SiC crystals. This material shows high strength, fibrous fracture behaviour, excellent high-temperature properties, and high thermal conductivity (even at temperatures over 1000 °C). It was also found that the initial strength of the SA-Tyrannohex, which was prepared from thinner starting fibre, was perfectly preserved up to 1700 °C in air.

A method for the preparation of aligned fibre samples for magnetic measurement using VSM

Ceramic 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.

Use of natural topaz as a transporting agent in the vapor growth of potassium tungsten oxide bronze fibres

Single crystal fibres with thickness >6 μm and length ∼3 mm were obtained by calcining the mixtures of natural topaz (Al 2SiO 4(F 0.75,(OH) 0.25) 2), tungsten trioxide (WO 3), and potassium tungstate (K 2WO 4). The single crystal fibres were characterised by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and electron probe microanalysis (EMPA). It was found that the fibres exhibited a hexagonal crystalline structure and possessed the nonstoichiometry of K 0.25WO 3.11. It was believed that the vapours of SiF 4, HF, and H 2O from the decomposition of the natural topaz were responsible for the chemical transport of the WO 3 and the K 2WO 4. The subsequent chemical reaction of the transported vapour species resulted in the formation of the hexagonal tungsten oxide bronze fibres.

Aligned hexagonal ferrite fibres of Co2W, BaCo2Fe16O27 produced from an aqueous sol–gel process

Gel fibres of Co2W, BaCo2Fe16O27, were blow spun from an aqueous inorganic sol and collected as an aligned tow blanket with an alignment of over 90%. Upon calcination up to 1300°C, the ceramic fibres were shown by X-ray diffraction to form single-phase crystalline Co2W at 1200°C, an unusually low temperature for this compound to form as the major phase. The formation of Co2W was also accompanied by sudden grain growth, forming crystallites over 10 μm in length. While this reduced the mechanical strength of the fibres, they were still suitable for use in a composite material.

Quadratic eigenmaps between spheres

A quadratic formf:Sm →Sn between spheres is separable if, up to isometries on the source and the range, the components off are pure or mixed quadratic polynomials. The space parametrizing the separated quadratic eigenmapsf is shown here to fiber over a semi-algebraic set with each fiber a finite-dimensional compact convex body. Form = 3, this gives a new description of the parameter space of all quadratic eigenmapsf:S3 →Sm as a fibration over an ‘inflated tetrahedron’ and generic hexagonal fibres.

Laser Raman spectrum of isotactic polybutene-1 fibres

Detailed polarized laser Raman spectra of isotactic hexagonal polybutene-1 fibres are presented. The use of different scattering geometries enables us to determine unambiguously the symmetry of Raman bonds from 300 to 3000 cm −1. The comparison between calculated and observed intensities enables us to test the individuality of CH 2 and CH 3 group vibrations and shows that some bands, more particularly those located between 800 and 1200 cm −1, can only be explained by the coupling of skeleton vibrations with side group vibrations.

Anisotropy of hexagonal ferrites with M, W and Y structure containing Fe 3+ and Fe 2+ as magnetic ions

Since 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 BaFe12O19 (BaM or barium ferrite), SrFe12O19 (SrM or strontium ferrite), and cobalt–titanium substituted M ferrite, Sr- or BaFe12−2xCoxTixO19 (CoTiM).•Z-type ferrites (Ba3Me2Fe24O41) such as Ba3Co2Fe24O41, or Co2Z.•Y-type ferrites (Ba2Me2Fe12O22), such as Ba2Co2Fe12O22, or Co2Y.•W-type ferrites (BaMe2Fe16O27), such as BaCo2Fe16O27, or Co2W.•X-type ferrites (Ba2Me2Fe28O46), such as Ba2Co2Fe28O46, or Co2X.•U-type ferrites (Ba4Me2Fe36O60), such as Ba4Co2Fe36O60, or Co2U .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 Ba2Mg2Fe12O22 Y ferrite at cryogenic temperatures, and now Sr3Co2Fe24O41 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 The microwave properties of hexagonal ferrites, 8 Magnetoelectric (ME), multiferroic (MF) and dielectric properties of hexaferrites, 9 Hexaferrite composites, 10 Hexagonal ferrite fibres, 11 Nanoscale hexagonal ferrite particles, ceramics and powders 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.