Phase transformations of δAl2O3 (Saffil) fibres during their interaction with molten MgLi alloys

The phase transformation of Al₂O₃ occurring in Saffil fibres during their infiltration with molten Mg-8 wt% Li alloy was studied by secondary ion mass spectroscopy, X-ray diffraction and infrared spectroscopy methods. It has been shown, that lithium penetrates very quickly into the whole fibre volume, attaining up to Li/Al 0.25–0.30 ion ratio. The metastable spinel-like compound, (Li), was formed by incorporation of Li⁺ ions into the Al₂O₃ lattice in which the basic spinel structure unit has been assigned by the formula Al₈ [Al(_40–x)/3(_8–2x )/3]Li _x ]O₃₂. During long-term annealing, a further transformation (Li) LiAl₅O₈ proceeded, and LiAlO₂ aluminate was also identified in Saffil fibres with high Li/Al concentration ratio values. In parallel with lithium, magnesium also penetrated the Saffil fibres within an infiltration period; however, the incorporation of magnesium into the spinel lattice has not been observed.     

Metal matrix composite reinforced with co-continuous Al–Al2O3 fibres produced in situ by reaction of an aluminium matrix with fossil SiO2 spicules

Abstract

Aluminium metal matrix composite (MMC) reinforced with fossil silica fibres was produced by a powder metallurgy extrusion route. The fibres, silica rich spicules known as spongilites, come from abundant natural geological deposits in Brazil. Further processing the MMC by heat treating at high temperatures converts the silica fibres into an interlaced (Al–Si)/Al₂O₃ microcomposite structure, retaining the original fibre morphology. The new co-continuous microstructure of the fibres is a result of a reduction or displacement reaction, where the silicon released from the silica reduction forms at first a liquid Al–Si phase around the fibres and later on diffuses into the solid matrix. The fine internal microstructure of the fibres was studied by FEG-SEM and optical microscopy both on polished and fractured surfaces. Tensile properties of the MMC before and after heat treatment were measured, showing a considerable increase in UTS. Analysis of the fracture surface of the heat treated ruptured specimens showed necking (bridge formation) in the metallic phase of the fibre and no visible pullout.     

Characterization of SiO2–Na2O–Fe2O 3–CaO–P2O 5–B 2O 3 glass ceramics

Bioactivity and magnetic properties were investigated in glass and glass ceramics based on the SiO₂–Na₂O–Fe₂O₃–CaO–P₂O₅–B₂O₃ system to find their suitability as thermoseed for hyperthermia treatment of cancer. The effect of change in compositions on bioactivity was examined in simulated body fluids. The glass ceramic samples exhibit Na₃CaSi₃O₈ and Na_3-XFe_XPO₄ phases. After dipping the glass ceramic samples in simulated body fluids silica hydrogel first forms, followed by an amorphous calcium phosphate layer. Magnetic and microwave resonance experiments further demonstrate the potential of these glass ceramics for possible use in hyperthermia.     

The formation ofβ-SiC fibres with SiO2-C-NaF(AlF3) components

The formation of-SiC fibres with SiO₂-C-NaF(AIF₃) components was investigated. It was found that the formation of a longer-SiC fibre was governed by the mole ratio of C/SiO₂ or C/NaF. Using a mole ratio for C/SiO₂ or C/NaF of 3 or more,-SiC fibres of length 3 mm were prepared in a closed system. On the other hand, short-SiC fibres were obtained in an open system.-SiC fibres prepared under the various experimental conditions were stable when heated in a high-concentration acidic solution such as HCl or H₂SO₄, and in an alkaline solution such as NaOH.     

Corrosion investigation of zinc−aluminum alloy matrix (ZA-27) reinforced with alumina (Al2O3) and fly ash

In this study, zinc?aluminum alloy (ZA-27) matrix composites reinforced by different weight fractions of fly ash or alumina (Al₂O₃) were produced using the traditional stir casting technique. The corrosion behaviors of both unreinforced alloy and reinforced composites were examined using direct current polarization (DCP) test in a simulated sea solution (3.5 wt.% NaCl). Scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) were used to examine the morphology of the composites’ surface before and after corrosion tests. The results of corrosion revealed that reinforcing ZA-27 alloy by fly ash or Al₂O₃ particles decreases its tendency to uniform corrosion due to the formation of weak microgalvanic couple between matrix and reinforcement particles. The fly ash and alumina (Al₂O₃) particles have protected the matrix material from pits formation at early stage of polarization. However, once these pits are formed, they grow faster. Positive hysteresis of the polarization curves implies that the salt layer breakdown and matrix dissolution overshadow surface passivation during the reverse scan. The electrochemical results are consistent with the pits’ morphology of the corroded composite. Composites with fly ash reinforcements have autocatalytic pits, whereas composites with alumina (Al₂O₃) reinforcements have shallow pits.     

Case study of dry mixing of δ-alumina (‘Saffil’) and silicon carbide (‘Nicalon’) fibres with a rapidly solidified Al-20Si-X powder

Dry mixing of -alumina and SiC short fibres with a powder metallurgy processed aluminium alloy was performed aiming to provide a homogeneous powder material for subsequent consolidation. The major problem was to deagglomerate the clustered fibres, realizing that fibre aspect ratio should not be sacrificed to too large an extent during this first processing step. The mixing was tackled by co-processing the materials in a cylindrical tumbler operating either with or without ceramic balls. This comes down to applying two differing intensities of mechanical action. Particle size, shape, hardness and specific surface area of the materials were measured after ball milling, which was a successful deagglomeration process only for the -alumina fibre.     

Enhancing the physical,optical and shielding properties for ternary Sb2O3–B2O3–K2O glasses

Ternary Sb₂O₃–B₂O₃–K₂O glass system, with general composition of x Sb₂O₃–(70-x) B₂O₃–30 K₂O (where x?=?0, 10, 20, 30, 40, 50) were prepared using melt-quenching technique. Structures of these glasses were investigated using XRD analysis and FT-IR spectroscopy. The optical properties were investigated using the UV–Vis NIR JASCO (Model V-670) Double Beam Spectrophotometer. Different physical parameters, such as density (ρ), molar volume (V_M), oxygen molar volume (V_O) and oxygen packing density values (OPD) have been estimated. Also, the Gamma radiation shielding ability have been characterized for the investigated glasses using Phy-X/PSD software in the photon energy range (0.015–15 MeV). XRD analysis confirm the amorphous nature of the prepared glasses. The optical energy gap of SBK glasses is decreasing from 4 to 2.63 eV with increasing Sb₂O₃ content while refractive index is increasing from 2.17 to 2.5 because of increasing the non-bridging oxygens (NBOs) in the glass matrix. The molar refractivity (R_m), molar polarizability (α_m) and the third-order nonlinear optical susceptibility values χ³ have been calculated, their values are found to increase with increasing Sb₂O₃ content. Glass density and molar volume values for the SBK glasses increase with increasing Sb₂O₃ content. Increasing the Sb₂O₃ concentration enhance the radiation shielding features of the prepared glasses against gamma rays and neutrons. Hence, the mass attenuation coefficient (MAC) increases from 8.6 to 35.4 cm²/gm while the half value layer (HVL) decreases from 0.036 to 0.0046 cm at 0.015 MeV as the Sb₂O₃ concentration increases from 0 to 50 mol%. The fast neutron cross section of the six investigated SBK glasses are (0.087, 0.092, 0.095, 0.091, 0.094 and 0.092 cm^?1), respectively.

     

Hot pressing effect on (Bi0·25Sb0·75)2Te3 mechanical and thermoelectric properties

(Bi_0·25Sb_0·75)₂Te₃ thermoelectric material is a well known p type of compound that has higher figure of merit than other stoichiometries. The crystal of this compound was prepared, pulverized in a particle size ratio of 64% with a mesh of 80 (200 μm²) and 36% with a mesh of 60 (250 μm²). The powder was sintered in a heat up to 350–500°C under pressure of 500 MPa (hot pressing). To find out the temperature effects on thermal conductivity of the sample it was systematically investigated in nano-scale intrinsic structures by systems of X-ray diffraction, scanning electron microscopy and, for only once successful attempt, atomic force microscopy. The acquired images ensured to show homogeneous structures for hot pressed samples. In terms of thermal conductivity and with regard to the figure of merit (Z), optimum sintering temperature hovers at around 500°C, which leads to a maximum Z value of around 1·53 K^?1.     

Complexes with substituted 2,5-dihydroxy-p-benzoquinones: AE[C6(C2H5)2O4]·3H2O (AE=Ca,Sr, Ba)

Single crystals of AE[C₆(C₂H₅)₂O₄]·3H₂O belonging to space group P2₁2₁2₁ were grown in aqueous silicagel (AE=Ca, Sr) and in aqueous solution (AE=Ba), respectively. AE²⁺ is coordinated by four oxygen atoms of two bis-chelating [C₆(C₂H₅)₂O₄]²⁻ ions, thus forming infinite, corrugated chains extending along [010]. The coordination polyhedron is completed by additional water molecules. The coordination sphere is different for the three different AE²⁺ ions yielding CN increasing from Ca²⁺ to Ba²⁺. The principle features of the crystal structure, however, remain unchanged. Adjacent chains are interlinked by hydrogen bonds between water molecules of the coordination sphere and the oxygen atoms of the [C₆(C₂H₅)₂O₄]²⁻ ion.     

Hydrothermal synthesis of mixed phosphates of neodymium and alkaline metals (Me2O·Nd2O3·4P2O5)

The phase formation in the system Me₂O-Nd₂O₃-P₂O₅-H₂O (where Me = Li, Na, K, Rb and Cs) has been studied under hydrothermal conditions in the temperature and pressure range 300 to 700 C and 0.5 to 600 atm, respectively, using vitreous carbon glass liners. A composition diagram showing the possible fields of crystallization of different phases under equilibrium conditions is given. AB-diagrams of fields of crystallization of different phases in the system investigated are given. These diagrams are in correspondence with the theoretical composition diagram. The advantages and disadvantages of different methods employed in the growth of MeNd-phosphates are discussed. The crystals obtained were characterized by various methods.     

Modified mixed oxide perovskites 0.7Sr(Al1/2B1/2) O3·0.3LaAlO3 and 0.7Sr(Al1/2B1/2) O3·0.3NdGaO3 (B=Ta5+ or Nb5+) for high-T c superconductor substrate applications

Single-crystal fibres of modified strontium aluminium tantalum oxide (1-x)Sr(Al_1/2Ta_1/2) O₃·xLaAlO₃(SAT-LA) and (1-x)Sr(Al_1/2Ta_1/2)O₃·xNdGaO₃ (SAT·NG), and modified strontium aluminum niobium oxide (1-x)Sr(Al_1/2Nb_1/2)O₃·xNdGaO₃(SAT·NG) and (1-x)Sr (Al_1/2Nb_1/2)O₃·xLaAlO₃ (SAN·LA) were grown using a laser-heated pedestal growth technique. 0.7SAT·0.3LA grows congruently and retains a twin free simple cubic perovskite structure (as the SAT) when cooled down to room temperature. 0.7SAT·0.3LA crystals have a moderate dielectric constant ( = 21.7) and low dielectric loss (tan = 7.5 × 10^–5) at 10 kHz and 90 K. The reduction problem of Ta⁵⁺ is eliminated (which is common in the case of SAT growth). 0.7SAT·0.3NG and 0.7SAN·0.3NG have lower melting temperatures and crystal growth is easier. NdGaO₃ addition to the SAT and SAN enhances the potential of SAT and SAN as large-area substrates for high-T _c superconductor growth. However, the dielectric constants increased from -12 to -16(0.7SAT·0.3NG) and from 18 to 23 (0.7SAN·0.3NG) as a result of NdGaO₃ incorporation.     

Complexes with substituted 2,5-dihydroxy-p-benzoquinones: The inclusion compounds [Y(H2O)3]2 (C6Cl2O4)3·6.6H2O and [Y(H2O)3]2 (C6Br2O4)3·6H2O

Single crystal of [Y(H₂O)₃]₂ (C₆Br₂O₄)₃·6H₂O and [Y(H₂O)₃]₂ (C₆Cl₂O₄)₃·6.6H₂O were grown in aqueous silicagel. The compounds are in principle isostructural. In Y chloranilate one additional water site is occupied as verified by X-ray single crystal structure analysis. Y³⁺ is nine-coordinated by three water molecules and six oxygen atoms of the bischelating (C₆X₂O₄)²⁻ ions (XCl, Br). The coordination polyhedron is an only slightly distorted tri-capped trigonal prism. The connection of Y³⁺ with the dianions leads to infinite, corrugated layers. The layer stacking yields cage-like cavities in which water molecules are accomodated. Hydrogen bonds interlink adjacent layers. Further hydrogen bonds involve the entrapped water molecules. DSC measurements indicated a complicated dehydration process which caused right at the start destruction of the single crystals.     

Effect of P2O5, B2O3 and PbO on the sinterability of β.quartz solid solution and gahnite glass-ceramics

The effect of P₂O₅, B₂O₃ and PbO additives on the sintering and crystallization behaviour of .quartz solid solution and gahnite glass-ceramics were investigated. According to our results, only the addition of PbO to glasses produces sinterable glass-ceramics containing .quartz solid solution gahnite as the sole crystalline phases, respectively. The nucleation mechanism of the glasses were determined by the Ozawa equation and their activation energy for crystallization were determined by the modified form of the Kissinger, Matusita and Marotta equations. The results indicate that the better sinterability of the lead bearing .q._ss glass-ceramic than the lead free one could be explained by changes of bulk to surface nucleation and increase of its activation energy for crystallization. At the other hand, it could not be observed any differences between the nucleation mechanism and the activation energy for crystallization of the lead bearing and lead free gahnite glass-ceramics. Then the differences in sinterability of the two glass-ceramics could not be explained by comparison of theirs activation energy for crystallization.     

Microwave dielectric properties and low temperature sintering of Ba3Ti4?x(Mg1/3Nb2/3)xNb4O21 ceramics with BaCu(B2O5) addition

Microwave dielectric ceramics of Ba₃Ti_4?x(Mg_1/3Nb_2/3)_xNb₄O₂₁ solid solutions (BTMNN-x, x?=?0–4) were prepared via the conventional solid-state reaction method. The X-ray powder diffraction analysis revealed that the BTMNN-x ceramics formed complete solid solutions with hexagonal structure. The dielectric constant (ε_r) and the temperature coefficient of the resonant frequency (τ_f) of BTMNN-x ceramics decreased with the increase of x, while the quality factor (Q?×?f) enhanced with increasing the substitution content. In addition, a small amount of BaCu(B₂O₅) (BCB) additive can effectively lower the sintering temperature of BTMNN ceramics. The 1.5?wt% BCB doped BTMNN-2 ceramics can be sintered at 950?°C and have good microwave dielectric properties of ε_r?=?50, Q?×?f?=?10,500?GHz and τ_f?=?18?ppm/°C, which makes it possible to be a promising candidate for mid-permittivity low temperature co-fired ceramic materials.     

Soft mechanochemical synthesis of MgFe2O4 nanoparticles from the mixture of α-Fe2O3 with Mg(OH)2 and Fe(OH)3 with Mg(OH)2

Abstract

The influence of long term soft milling of a mixture of (1) Mg(OH)₂ and α-Fe₂O₃ and (2) Mg(OH)₂ and Fe(OH)₃ powders in a planetary ball mill on the reaction synthesis of nanosized MgFe₂O₄ ferrites was studied. Soft mechanochemical reaction leading to formation of the MgFe₂O₄ spinel phase was followed by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and magnetisation measurements. The spinel phase formation was first observed after 5 h of milling and its formation was completed after 15 h in case (2). The synthesised MgFe₂O₄ ferrite had a nanocrystalline structure with a crystallite size of about 10 and 15 nm respectively for cases (1) and (2). Measurements after 15 h of milling show magnetisation values of 15·23 and 10·14 J T^–1 kg^–1 respectively for cases (1) and (2).