Acicular Particles Research Articles

Influence of Mn-dopant on the properties of α-FeOOH particles precipitated in highly alkaline media

The effects of Mn-dopant on the formation of solid solutions α-(Fe, Mn)OOH in dependence on the initial concentration ratio r = [Mn]/([Mn] + [Fe]), as well as on the size and morphology of the corresponding particles were investigated using Mössbauer and FT-IR spectroscopies, high-resolution scanning electron microscopy (FE SEM) and an energy dispersive X-ray analyser (EDS). The value of the hyperfine magnetic field of 34.9 T, as recorded for the reference α-FeOOH sample at RT, decreased linearly up to 21.4 T for sample with r = 0.1667. Only a paramagnetic doublet at RT was recorded for sample with r = 0.2308, a ferrite phase was additionally found for r = 0.3333. Fe–OH bending IR bands, δ OH and γ OH, were influenced by the Mn-substitution as manifested through their gradual shifts. FE SEM micrographs showed a great elongation of the starting acicular particles along the c-axis with an increase in Mn-doping. For r = 0.1667 and 0.2308 star-shaped and dendritic twin α-(Fe, Mn)OOH particles were observed. The length of these α-(Fe, Mn)OOH particles decreased, whereas their width increased. The α-Fe 2O 3 phase was not detected in any of the samples prepared.

Preparation and morphology of anisometric KSr 2Nb 5O 15 particles

Anisometric KSr 2Nb 5O 15 (KSN) particles were synthesized by conventional mixed-oxide (CMO) method and molten salt synthesis (MSS) method, respectively. In MSS method, preparation of KSN was carried out in both SrCO 3–Nb 2O 5–K 2CO 3–KCl system (system I) and SrNb 2O 6–Nb 2O 5–KCl system (system II). The results revealed that the single-phase KSN obtained by CMO method was clumped and without obvious acicular morphology, making it difficult to use in texturing processes. In system I, though acicular KSN particles were synthesized by controlling the amount of KCl salt, they were easily contaminated by the presence of blade-like Sr 2Nb 2O 7. In contrast, pure KSN particles with high aspect ratio and uniform size were successfully synthesized by controlling the ratio of SrNb 2O 6/Nb 2O 5 in the new system (system II). The obtained KSN particles which are 5–30 μm in length and 2–4 μm in diameter are ideal templates for fabricating textured ceramics with tungsten bronze structure. Growth mechanisms in MSS method were also proposed in this work.

The stable orientations of the net magnetic moment within single-domain particles: Experimental evidence for a range of stable states and implications for rock magnetism and palaeomagnetism

An idealised stable uniaxial single-domain (SD) particle permits only two possible stable positions in which the magnetic moment can lie, either closely parallel or anti-parallel to the particle long (easy) axis. In real acicular SD particles, which have generally been regarded as uniaxial, this implicit two state feature has never been challenged, whilst there has been considerable debate concerning the mechanism of moment reversal between the two states. We present experimental results suggesting that acicular SD particles may actually have a range of several quantifiable stable (or metastable) orientations of the net magnetic moment. In order to help explain our experimental observations we present a new simple model of acicular SD particles, which gives quantitative predictions verified by further experiments. The model also appears to be relevant to other SD particle morphologies and crystal structures (such as hematite). A possible physical basis for our model in acicular particles may lie in non-uniform SD structures (such as the flower or vortex states). Small variations in the non-uniform SD structures available to a particle might allow a range of stable positions of the net moment. The results have several implications for rock magnetism and palaeomagnetism. Firstly, the new model can quantitatively account for several previously unexplained diverse phenomena exhibited by real acicular SD particles. These include the acquisition of gyroremanences and field-impressed anisotropy in dilute dispersions of such particles, as well as observations of transverse components of remanence in individual acicular SD particles. All these phenomena are theoretically impossible in idealised uniaxial SD particles. Interestingly, it appears that these phenomena could now be used to quantify the deviation of real acicular SD particles from ideal uniaxial behaviour and also, therefore, the deviation from a uniform SD structure. In hematite, observations of large field-impressed anisotropy appear to be quantitatively explained by the available positions of the moment in the basal plane. Secondly, computations of the ancient field vector and palaeointensity from remanence anisotropy techniques would not only be controlled by the shape and distribution of the particles, but also by the range of possible stable orientations of the net moment within each SD particle. Laboratory analogue remanences (and, we suspect, natural remanences) would be influenced by the range of possible stable moment positions. Quantifying the range of these stable moment positions, upon acquisition of laboratory or natural remanences, should lead to improved methods of computing the ancient field direction and palaeointensity in anisotropic rocks.

Microwave Effect for Synthesis of TiO2 Particles by Self-Hydrolysis of TiOCl2

TiO 2 nanoparticles were synthesized by self-hydrolysis of TiOCl 2 using conventional and microwave heating methods. By XRD analysis, rutile and anatase phases were detected at the early stage of reaction. In conventional heating method, the XRD peaks of rutile became stronger with time and the roundish agglomerates surrounded by acicular particles were observed by SEM. The initial product was confirmed by TEM to consist of small acicular particles assigned to rutile and fine nanoparticles assigned to hydrous TiO 2 sol having anatase-like structure. As the reaction time was longer, the amount of acicular particles increased. These results suggest that TiO 2 sol formed at the early stage by hydrolysis of TiOCl 2 and the crystal growth of rutile progressed by dissolution-reprecipitation of TiO 2 sol. In order to synthesize TiO 2 nanoparticles, microwave was applied to this system. The microwave irradiation shortened the period of the precipitation because microwave can quickly heat up the aqueous solution, but retarded the crystal growth of rutile, leading to size reduction of TiO 2 particles identified a hydrous TiO 2 phase.

磁気記録媒体用超微小ヘマタイト粒子の焼成状態と非磁性基体の表面平滑性との関係

Non-magnetic particles such as acicular hematite particles with finer size and higher dispersibility are required to produce multi-layered magnetic recording media with better performance. The specific surface area and primary particle size of non-magnetic acicular hematite particles were controlled by adjusting the heating conditions in the dehydration process. A sintering-parameter (SBET/STEM) was defined as the ratio of specific surface area measured by B. E. T. method to the specific surface area calculated from the particle size determined by TEM observation. The lower surface roughness of non-magnetic substrate is used to express the dispersibility of hematite particles. For 1.0≤SBET/STEM≤1.4, the dispersibility of hematite particles was much better in non-magnetic lacquer. When the optimum heating condition was applied to make hematite particles, the lower surface roughness was attained by using finer particles.

Phase transition control of nanostructuredTiO2 powders with additions of various metal chlorides

Formation behavior and photo-oxidation abilities of nanostructuredTiO2 powders were investigated through a direct crystallization from aqueousTiOCl2 solutions containing various metal chlorides at100 °C. Theobtained TiO2 powders without any additives and those withNi2+,Fe3+ and Nb5+ ions added, which have a similar positive ionic radius toTi4+, were mainly crystallized with rutile phase, whereas those withAl3+ and Zr4+ ions added, which have a quite different positive ionic radius, were mainlycrystallized with anatase phase. On the other hand, the secondary particles in theTiO2 powder consisted of acicular and spherical primary particles, corresponding to rutile andanatase phases, respectively. From these results, it seems that the positive ionicradius of the additives would affect phase formation as well as morphology ofTiO2 precipitates.Among the TiO2 powders prepared, Ni-added powder, which consisted mainly of rutile phase with a smallamount of anatase phase, showed excellent photocatalytic ability in decomposition of4-chlorophenol.

Photonic decomposition of ultrafine, rutile phasedTiO2 powder in aqueous 4-chlorophenol solutions

The redox properties of a homogeneously precipitatedTiO2 rutile powder with aBET surface area of ∼200 m2 g−1, consisting of acicular primary particles, were characterized using a photocatalyticreaction in aqueous 4-chlorophenol, Cu- and Pb-EDTA solutions underultraviolet irradiation. Results were compared to those of commercial P25TiO2 powder with spherical primary particles as well as those of home-made anataseTiO2 powderwith ∼200 m2 g−1 BET surface area. Here, the anatase powder also includes mainlythe primary particles very similar to the acicular shapes of the rutileTiO2 powder. The rutile powder showed the fastest decomposition rate and the largestamount of product in the photoredox, compared with the anatase or P25 powder,while the anatase powder unexpectedly showed the slowest rate and the smallestamount in the same experiments regardless of having almost the same surface area.The excellent photoredox abilities of this rutile powder appear to be due to aspecific powder preparation method, for example a homogeneous precipitationleading to direct crystallization from the solution, regardless of their crystallinestructures, even when the powders have similar particle shapes and surface areas.

The influence of Zn-dopant on the precipitation of α-FeOOH in highly alkaline media

The influence of Zn-dopant on the precipitation of α-FeOOH in highly alkaline media was monitored by X-ray diffraction (XRD), 57Fe Mössbauer and Fourier transform infrared (FT-IR) spectroscopies and field emission scanning electron microscopy (FE SEM). Acicular and monodisperse α-FeOOH particles were precipitated at a very high pH by adding a tetramethylammonium hydroxide solution to an aqueous solution of FeCl 3. The XRD analysis of the samples precipitated in the presence of Zn 2+ ions showed the formation of solid solutions of α-(Fe, Zn)OOH up to a concentration ratio r = [Zn]/([Zn] + [Fe]) = 0.0909. ZnFe 2O 4 was additionally formed in the precipitate for r = 0.1111, whereas the three phases α-FeOOH, α-Fe 2O 3 and ZnFe 2O 4 were formed for r = 0.1304. In the corresponding FT-IR spectra, the Fe OH and Fe O stretching bands were sensitive to the Zn 2+ substitution, whereas the Fe OH bending bands of α-FeOOH at 892 and 796 cm −1 were almost insensitive. The Mössbauer spectra showed a high sensitivity to the formation of α-(Fe, Zn)OOH solid solutions which were monitored on the basis of a decrease in B hf values in dependence on Zn-doping. A strictly linear decrease in B hf for α-FeOOH doped with Zn 2+ ions was measured up to r = 0.0291, whereas for r = 0.0476 and higher there was a deviation from linearity. The presence of α-(Fe, Zn)OOH, α-Fe 2O 3 and ZnFe 2O 4 phases in the samples was determined quantitatively by Mössbauer spectroscopy. Likewise, Mössbauer spectroscopy did not show any formation of the solid solutions of α-Fe 2O 3 with Zn 2+ ions. FE SEM showed a strong effect of Zn-doping on the elongation of acicular α-FeOOH particles (∼500–700 nm in length) up to r = 0.1111. For r = 0.1304 the sizes of ZnFe 2O 4 particles were around 30–50 nm, and those of α-Fe 2O 3 particles were around 500 nm, whereas a relatively small number of very elongated α-(Fe, Zn)OOH particles was observed. A possible mechanism of the formation of α-(Fe, Zn)OOH, α-Fe 2O 3 and ZnFe 2O 4 particles was suggested.

Influence of ruthenium ions on the precipitation of α-FeOOH, α-Fe 2O 3 and Fe 3O 4 in highly alkaline media

The influence of ruthenium ions on the precipitation of goethite (α-FeOOH), α-Fe 2O 3 and Fe 3O 4 in highly alkaline media was investigated by 57Fe Mössbauer and FT-IR spectroscopies, thermal field emission scanning electron microscope (FE SEM) and EDS. The presence of Ru-dopant strongly affected the precipitation of α-FeOOH at highly alkaline pH, i.e. the formation of α-Fe 2O 3 was also noticed. A decrease of hyperfine magnetic field (HMF) at RT from 35.1 T (undoped α-FeOOH) to 31.3 T for sample with [Ru]/([Ru] + [Fe]) = 0.0196 was assigned to the incorporation of ruthenium ions into the α-FeOOH structure. Mössbauer spectroscopy showed the formation of stoichiometric Fe 3O 4 for [Ru]/([Ru] + [Fe]) = 0.0291–0.0909. α-Fe 2O 3 and Fe 3O 4 did not show a tendency to the formation of solid solutions with ruthenium ions. FE SEM observations of the samples showed that reference α-FeOOH sample contained acicular particles of good uniformity, which increased the length up to ∼5 times with increase of concentration of ruthenium ions. On the other hand, large octahedral Fe 3O 4 crystals (particles) were associated with small particles of ruthenium (hydrous) oxide with a size in the range ∼100 nm or less. A possible catalytic action of ruthenium that created reduction conditions for Fe 3+ ions and formation of Fe 2+ ions for precipitation of Fe 3O 4 was discussed.

Dielectric anisotropy and sinterability improvement of Ba 4Nd 9.33Ti 18O 54 textured ceramics

The 〈0 0 1〉-textured Ba 4Nd 9.33Ti 18O 54 (BNdT) ceramics were fabricated by templated grain growth process. Acicular particles prepared using K 2SO 4 molten salt were used as template and aligned by tape casting in the equiaxed BNdT powder. Crystalline phases of sintered specimen exhibited { h k 0} and {0 0 1} development in the plane of parallel and perpendicular to the casting direction, respectively. The formation of a slight amount of secondary phase in the template particles and the compositional deviation inhibited the densification and texture development for BNdT. To improve the sinterability, the composition shifted from BNdT to Ba 2Ti 9O 20 (B2T9) rich side slightly in BaO Nd 2O 3 TiO 2 ternary system, which enables to promote a liquid phase during the sintering, was examined as a matrix phase. Resultant ceramics including B2T9-rich matrix phase displayed high density and large dielectric anisotropy. Temperature dependence of dielectric constant showed negative and positive behavior between the direction of parallel and perpendicular to the 〈0 0 1〉-textured BNdT. Near-zero temperature coefficient of resonant frequency ( τ f) in TE 0 1 1 mode was obtained in the textured ceramics with the degree of {0 0 1} orientation of approximately 0.37 on the disk plane. The exceptional temperature behavior of BNdT made it possible to control the τ f over the wide range by the combination of dielectric anisotropy.

Preparation of Nanostructured TiO2 Powders with Additions of Various Metal–Chlorides for Efficient Photocatalyst

Preparation of nanostructured TiO2 powders for efficient photocatalyst was investigated through a direct crystallization from aqueous TiOCl2 solutions containing various metal–chlorides. The TiO2 powders without any additives and those added with Ni2+, Fe3+ and Nb5+ions, respectively, which has a similar cationic radius to Ti4+, were mainly crystallized to rutile phase, whereas those added with Al3+ and Zr4+ ions, which have a quite different positive ionic radius, were mainly crystallized to anatase phase. On the other hand, the secondary particles in the TiO2 powder consisted of acicular and spherical primary particles corresponding to rutile and anatase phases, respectively. From these results, it seems that the cationic radius of the additives could affect phase formation as well as morphology of TiO2 precipitates. Among the TiO2 powders prepared, Ni2+ doped powder, showing mainly rutile phase, had excellent photocatalytic ability in decomposition of 4-chlorophenol.

Influence of Ni-dopant on the properties of synthetic goethite

The influence of Ni-dopant on the properties of α-FeOOH was investigated by XRD, FT-IR, 57Fe Mössbauer spectroscopy and transmission electron microscopy. α-FeOOH was synthesized at a highly alkaline pH by precipitation from the FeCl 3 solution with the addition of tetramethylammonium hydroxide and autoclaving at 160 °C. The samples doped with Ni 2+ ions were precipitated in the same way, but in the presence of varying concentrations of NiCl 2. Solid solutions, having the structure of α-FeOOH, were observed in samples with the Ni/Fe ratio up to 0.05. Upon increasing the amount of Ni-dopant the XRD lines were gradually broadened. The sample with the ratio Ni/Fe = 0.10 showed NiFe 2O 4, besides the dominant phase having the structure type of α-FeOOH. Shifts of IR bands at 892 and 796 cm −1 were not observed in all samples doped with Ni. For the ratio Ni/Fe = 0.10, the IR bands centered at 631 and 404 cm −1 were significantly broadened. RT Mössbauer spectrum of undoped α-FeOOH and Ni-doped α-FeOOH showed distributions of hyperfine magnetic fields. B hf decreased from 35.1 T for an undoped α-FeOOH to 32.1 T for α-FeOOH containing Ni 2+ ions (Ni/Fe = 0.05). The saturation of the α-FeOOH structure with Ni 2+ ions in amounts higher than ∼5 mol% was also observed by Mössbauer spectroscopy. The particle size (length) of acicular α-FeOOH particles with a maximum in the interval 180–220 nm was slightly decreased with Ni-doping, but the distribution of the length/width ratio showed no change, having a maximum at 4–5. TEM photographs additionally showed small populations of cubic-shaped or pseudocubic particles of ∼10 nm in size for the ratio Ni/Fe = 0.05 and about 10–20 nm in size for the ratio Ni/Fe = 0.10. These particles were assigned to NiFe 2O 4.

Crystallisation behaviour, microstructure and magnetic properties of BaO–Fe 2O 3–B 2O 3–SiO 2 glass ceramics

Differential thermal analysis, X-ray diffractometry and scanning electron microscopy with energy dispersive spectroscopy were used to study the crystallisation of glass ribbons with a composition of 35% BaO, 35%Fe 2O 3, 20%B 2O 3 and 10% SiO 2 (mol%) prepared via cooling the melts between steel rollers. The magnetic properties of specimens were also measured with a vibrating sample magnetometer. Heat treatment of glass specimens both in the form of bulk (ribbon) or pressed powders at their respective DTA exo-peak temperatures of 722 and 702 °C led to the formation of BaFe 12O 19 and BaB 2O 4 as the only crystalline phases. The pressed powders showed the finest and the broadest size distribution of acicular and platelet particles in the 0.1–2.5 μm range. The glass ribbons heat treated in one-stage exhibited the most uniform distribution of platelet particles of 0.9 μm mean size, while a two-stage heat treatment led to a bimodal size distribution of platelet particles. All specimens showed nearly identical values of saturation magnetisation of M s = 25–27 emu/g while the coercivity values varied between H c = 2800–3800 Oe.

Adsorption of Acicular Particles at Liquid−Fluid Interfaces and the Influence of the Line Tension

In this paper, the adsorption energy of an acicular (prolate and cylindrical) particle onto a liquid-fluid interface and the effect of the line tension are investigated. The results show that, without line tension, acicular particles always prefer to lie flat in the plane of the interface. However, line tension plays a significant role in determining the adsorption of an acicular particle. First, the line tension creates an energy barrier for the adsorption of particles onto an interface. The planar configuration has a larger energy barrier due to the longer contact line. Therefore, the particles prefer to enter the interface in a homeotropic configuration and then rearrange to a planar configuration or an oblique configuration with a small tilt angle. Second, for prolate particles, an energy maximum occurs at some tilt angles when the line tension is large. Therefore, once the prolate particle is adsorbed on the interface in a homeotropic configuration or with a larger tilt angle, it must conquer an energy barrier to rearrange to a planar configuration. For cylindrical particles, when the line tension is higher, the planar configuration will not be the most energy-favorable configuration. The cylindrical particles prefer to stay in the interface with a small tilt angle.

The influence of edge sites on the development of surface charge on goethite nanoparticles: A molecular dynamics investigation

Large-scale molecular simulation of proton accumulations were carried out on (i) (110) and (021) slabs immersed in aqueous solution and (ii) a series of model goethite nanoparticles of dimension 2 to 8 nm with systematically varying acicularity and (110)/(021) surface areas. In the slab systems, the (021) surface exhibits 15% more proton charge per unit area than the (110) surface. In the particulate systems, the acicular particles having the highest (110)/(021) ratio accumulate the most charge, opposite to the trend expected from the slab simulations, indicating that, at length scales on the order of 10 nm, the slab results are not a good indicator of the overall charging behavior of the particles. The primary reason for the discrepancy between the particulate systems and slab systems is the preferential accumulation of protons at acute (110)–(110) intersections. Charge accumulates preferentially in this region because excess proton charge at an asperity is more effectively solvated than at a flat interface.

Influence of temperature, [Ca 2+], Ca/P ratio and ultrasonic power on the crystallinity and morphology of hydroxyapatite nanoparticles prepared with a novel ultrasonic precipitation method

Nanocrystalline hydroxyapatite was prepared by a precipitation method with the aid of ultrasonic irradiation using Ca(NO 3) 2 and NH 4H 2PO 4 as source material and carbamide (NH 2CONH 2) as precipitator. The influence of Ca/P molar ratio, precipitation temperature, concentration of Ca 2+ ([Ca 2+]) and ultrasonic power on the crystallinity of the nanopowder were systematically investigated by XRD analysis. The size of the as-prepared particles was analyzed using TEM and XRD methods. The results revealed that the monophase hydroxyapatite could be obtained at the following technological conditions: [Ca 2+] = 0.01–0.1 mol/L, ultrasonic power = 300 W, Ca/P (mol) = 1.2–2.5 and T = 313–353 K. In addition, the acicular and spherical particles could be prepared at different ultrasonic powers of 300 and 200 W, respectively.

Texture effects in particulate media

In this paper, we report on a study of the switching field distribution of the particles in a tape that has been fabricated with different easy axes orientations along its length. The samples are composed of acicular metal particles dispersed within binders and solvents coated onto a plastic film. We observed a broadening of the switching field distribution in samples with a wide easy axes distribution.

Inception of shear flow for ferromagnetic dispersions

Rheological data are presented for shear flows of concentrated dispersions of acicular ferromagnetic particles; such dispersions are widely used in the manufacture of data storage materials. In steady shear flow these dispersions are strongly shear thinning. Upon the inception of shear flow, they show stress overshoots for both the shear stress and the primary normal stress difference. The data can be rationalized in terms of a temporary junction network, resulting from the strong magnetic interactions between the particles, if the network consists of relatively weak magnetic links connecting strongly bound flocs of particles. Application of a shear deformation results in disruption of the initially sample spanning network. The overshoot in the shear stress occurs when the flocs become arranged in the shear planes. The overshoot in the primary normal stress difference occurs as a result of deformation of the strongly bound flocs.

Tensile behavior of friction-stir-welded A356-T6/Al 6061-T651 bi-alloy plate

Abnormally low tensile ductility has often been reported for the friction-stir-welded (FSWed) dissimilar metals. The mechanism(s) for such a low tensile ductility has, however, not been established. In the present study, the tensile behavior of FSWed A356-T6/Al 6061-T651 bi-alloy plate was studied to understand the underlying mechanism for the reduced tensile ductility with the friction stir welding of dissimilar metals based on thorough micrographic and fractographic observations. The present study also demonstrated that the tensile ductility of the friction-stir-welded A356-T6/Al 6061-T651 bi-alloy specimen was substantially lower than that of the weighted mean value of the uni-alloy counterparts, including A356-T6 and Al 6061-T651 alloys. Interestingly, a relatively large number of acicular shaped Si particles were observed locally in the FSWed bi-alloy specimens compared to the dominantly globular shaped particles in the FSWed uni-alloy counterpart. Moreover, these acicular shaped Si particles were found to be mostly aligned parallel to the tool-rotating direction. Such agglomerated areas of the preferentially oriented, acicular Si particles in the present study appeared to serve as initiation sites for the tensile fracture and eventually caused low tensile ductility.

Energy Dissipation in a Solid Dispersion System of Single-Domain Ferromagnetic Particles under the Action of a Variable Linearly Polarized Field

The heat power released as a result of magnetic hysteresis in a system of small ferromagnetic particles suspended in a solid matrix acted upon by a linearly polarized magnetic field has been determined theoretically and experimentally. The calculations were done with the use of a model of noninteracting, uniformly magnetized particles with uniaxial anisotropy. Acicular particles of gamma iron oxide were used in the experiments. It is shown that the heat power released as a result of magnetic hysteresis may measure several megawatts per cubic meter, which makes it possible to use disperse magnetic systems as volumetric heaters.