Sintering of 6H(α)-SiC and 3C(β)-SiC powders with B4C and C additives

The sintering behavior of three fine industrial SiC powders (two 6H()-type and one 3C()-type) has been comparatively investigated. The powders were pressureless sintered with B₄C and C additives between 1950°C and 2250°C in a high temperature dilatometer with flowing Ar atmosphere. The densification and shrinkage rate curves, polytype content, and grain growth were correlated with physical and chemical characteristics of starting powders. One of 6H()-type powders presented good sinterability only after extensive milling, even though it presented small average particle size, narrow particle size distribution and high specific surface area. The main difference in densification behavior among powders was the narrower shrinkage rate curve of -SiC powder, with its maximum shifted to higher temperature. Grain growth and phase transformation simultaneously occurred. In -SiC, 6H polytype partially transformed to 4H. This transformation was favored by aluminum impurity and resulted in a microstructure with more elongated grains. In -SiC, 3C transformed mainly to 6H, 15R and 4H, introducing many stacking faults which resulted in elongated SiC grains.     

Mullite formation from xerogels of (0.84–2.2) Al2O3·1SiO2

Xerogels of (0.84–2.2) Al₂O₃·1SiO₂ prepared by chemical coprecipitation of Al(NO₃)₃·9H₂O and Si(OC₂H₅)₄ experience three thermal reaction paths for mullite formation. Those with pseudoboehmite are found to form mullite via the paths of either Al-Si spinel mullite transformation or -Al₂O₃ -Al₂O₃ + amorphous SiO₂ mullite, depending upon the ratios of Al₂O₃/SiO₂. Higher SiO₂ content may prefer the former reaction. Xerogels composed of bayerite form mullite via the route -Al₂O₃ -Al₂O₃ + amorphous SiO₂ mullite. Mullite thus formed exhibits a different crystal size, being 20–25 nm for that obtained from pseudoboehmite and around 37 nm for bayerite. The highest yield of mullite formation may be achieved with xerogels of pseudoboehmite with the stoichiometric mullite compositions, 3Al₂O₃·SiO₂.     

Pressureless sintering of β-SiC with Al2O3 additions

-SiC was pressureless sintered to 98% theoretical density using Al₂O₃ as a liquid-phase forming additive. The reaction between SiC and Al₂O₃ which results in gaseous products, was inhibited by using a pressurized CO gas or, alternatively, a sealed crucible. The densification behaviour and microstructural development of this material are described. The microstructure consists of fine elongated -SiC grains (maximum length 10 m and width 2–3 m) in a matrix of fine equi-axed grains (2–3 m) and plate-like grains (2–5m). The densification behaviour, composition and phases in the sintered product were studied as a function of the sintering parameters and the initial composition. Typically, 50% of the -phase was transformed to the -phase.     

Processing and Properties of Al2O3-ZrO2(3Y)-SiC Nanocomposites

本文研究了非均相沉淀法制备 Al     

Microstructural characterization of amorphous SiO2 wrapped β-SiC nanowhiskers

Wrapped composite nanowhiskers were prepared during the carbothermal reduction of silica. SEM, TEM and HREM analyses have showed that each wrapped composite nanowhisker consists of an internal β-SiC nanowhisker with a uniform amorphous SiO₂ wrapper on the outside surface.     

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.     

Sintering and crystallization of (SrO·SiO2)-(SrO·Al2O3·2SiO2) glass-ceramics

In the ternary SrO-Al₂O₃-SiO₂ system the pseudobinary join composition of 50 wt % SrO·SiO₂–50 wt % SrO·Al₂O₃·2SiO₂ (SS-SA2S) showed a glass melting temperature of 1500 °C and a crystallization peak temperature of 1100 °C. The (SS-SA2S) glass-ceramic pellets prepared by cold pressing and pressureless sintering, showed very low porosity. The (SS-SA2S) glass-ceramics containing B₂O₃ and those containing B₂O₃ and TiO₂ revealed crystallization peak temperatures of 1000 °C and unexpectedly high porosity. By applying Kissinger analyses to the DTA data the activation energy values for crystallization of the three glass-ceramics were determined to range from 196 to 255 kJ/mol. The Ozawa analyses on the DTA data gave the Avrami parameter values at 3.69 to 3.95. The X-ray diffraction (XRD) patterns from the three glass-ceramics revealed formation of the equilibrium crystalline phases of SrO·SiO₂ and SrO·Al₂O₃·2SiO₂ (monocelsian).     

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.     

Enhancement of β phase crystals formation with the use of nanofillers in PVDF films and fibres

Nanoclay and carbon nanotubes (CNT) have been in focus recently as means of enhancing β phase crystals formation in poly(vinylidene fluoride)(PVDF). Dominantly, the so-far work has been carried out on films/thin sheets filled with nanoclay. It has been found, mainly from combined XRD and DSC data, that nanoclay influences the PVDF structure, and particularly the β phase crystals formation is enhanced. Results published by various groups are in fairly good agreement. There are no results for nanoclay filled melt-spun PVDF fibres.The influence of CNT on PVDF structure has been less studied. XRD data indicating an enhancing role of multi-wall carbon nanotubes (MWNT) on β phase crystals formation in solution compounded PVDF films are available. Published results for MWNT/PVDF films are not in good agreement. The only study into single-wall carbon nanotube (SWNT)/PVDF has been made on electrospun nanofibres.We explore above findings towards melt-spun nanofilled PVDF fibres. We present new results obtained by us for melt-spun PVDF fibres containing non-functionalized and amino-functionalized double-wall carbon nanotubes (DWNT). The key finding is that amino-DWNT can influence the β to α polymorphic balance.     

Bone formation following implantation of fibrous calcium compounds (β−Ca(PO3)2, CaCO3(aragonite)) into bone marrow

Bone formation around three types of fibrous calcium-containing crystals has been examined histologically using rats. The implanted materials are (i) calcium metaphosphate (–Ca(PO₃)₂) fibers having aspect ratios of 15–80 with 2–20 m in diameter, (ii) –Ca(PO₃)₂) fibers surface-modified using dilute NaOH and (iii) calcium carbonate (CaCO₃; aragonite phase) whiskers having aspect ratios of 15–40 with 0.5–3 m in diameter. –Ca(PO₃)₂ fibers show a mechanically high strength with a low modulus of elasticity, and the surface-modified fibers have a thin layer consisting of a calcium orthophosphate phase. CaCO₃ whiskers were used for comparison reasons. The materials were implanted for 4, 8, and 12 weeks into bone defects created in the bone marrow of rat tibiae. Cancellous bone formation was observed around –Ca(PO₃)₂ fibers, the surface-modified fibers and CaCO₃ whiskers after implantation for 12, 4 and 4 weeks, respectively. CaCO₃ whiskers were scarcely observed after 12 weeks for resorbing. The calcium phosphate fibrous materials show combined advantages of mechanically high strength for toughening a matrix phase and biological activities; thus, these materials may prove to be useful for novel applications in the biomedical field.     

Influence of stabilizers on Na-β′′-Al2O3 phase formation in Li2O(MgO)-Na2O-Al2O3 ternary systems

The influences of stabilizers on - and -Al2O3 phase formations in Li2O(MgO)-Na2O-Al2O3 systems were investigated. When stabilized with 4MgCO3Mg(OH)25H2O, most of the -Al2O3 phase formed below 1200°C and further - to -Al2O3 transformation with an increase of temperature was not observed. On the other hand, when stabilized with Li2CO3,-Al2O3 formation occurred by two steps. First, -Al2O3 was partly formed below 1200°C, and, second, noticeable transformation from -Al2O3 to -Al2O3 occurred at higher temperature ranges. It was shown that transient eutectic liquid in the Li2O-Na2O-Al2O3 system promoted the - to -Al2O3 transformation at higher temperatures. Uniform distribution of both Mg2+ and Li+ stabilizing ions enhanced -Al2O3 formation at low temperatures. In the Li-stabilized systems, however, homogeneous distribution of Li+ ions hindered both the formation of transient eutectic liquid and the second - to -Al2O3 phase transformation at high temperatures.     

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).     

The effect of Ge implantation dose on the optical properties of Ge nanocrystals in SiO(2)

Ge nanocrystallites (Ge-nc) embedded in a SiO(2) matrix are investigated using Raman spectroscopy, photoluminescence and Fourier transform infrared spectroscopy. The samples were prepared by ion implantation with different implantation doses (0.5, 0.8, 1, 2, 3 and 4) × 10(16)?cm(-2) using 250?keV energy. After implantation, the samples were annealed at 1000?°C in a forming gas atmosphere for 1?h. All samples show a broad Raman spectrum centred at w≈304?cm(-1) with a slight shift depending on the implantation doses. The Raman intensity also depends on the Ge(74+) dose. A maximum photoluminescence intensity is observed for the sample implanted at room temperature with a dose of 2 × 10(16)?cm(-2) at 3.2?eV. Infrared spectroscopy shows that the SiO(2) films moved off stoichiometry due to Ge(74+) ion implantation, and Ge oxides are formed in it. This result is shown as a reduction of GeO(x) at exactly the dose corresponding to the maximum blue-violet PL emission and the largest Raman emission at 304?cm(-1). Finally, the Raman spectra were fitted with a theoretical expression to evaluate the average size, full-width at half-maximum (FWHM) and dispersion of Ge-nc size.     

Micelle-mediated synthesis of single-crystalline β(3C)-SiC fibers via emulsion electrospinning

Submicroscale SiC fiber mats were prepared by the electrospinning of an oil-in-water(O/W) precursor emulsion, a subsequent thermal curing treatment, and calcination at 1600 °C. Low-molecular-weight PCS micelles entrapped within an aqueous PVP matrix played an important role in forming the continuous and dense core structure, resulting in pure SiC fibers. The manipulation of SiC fiber diameters could be obtained via control of the micellar PCS concentration (10-30 wt %), enabling the production of dense and highly crystallized SiC fiber architectures with diameters ranging from 200 to 350 nm.     

The behavior of osteoblast-like cells on various substrates with functional blocking of integrin-β1 and integrin-β3

This study was designed to examine the influence of integrin subunit-β1 and subunit-β3 on the behavior of primary osteoblast-like cells, cultured on calcium phosphate (CaP)-coated and non coated titanium (Ti). Osteoblast-like cells were incubated with specific monoclonal antibodies against integrin-β1 and integrin-β3 to block the integrin function. Subsequently, cells were seeded on Ti discs, either non coated or provided with a 2 μm carbonated hydroxyapatite coating using Electrostatic Spray Deposition. Results showed that on CaP coatings, cellular attachment was decreased after a pre-treatment with either anti-integrin-β1 or anti-integrin-β3 antibodies. On Ti, cell adhesion was only slightly affected after a pre-treatment with anti-integrin-β3 antibodies. Scanning electron microscopy showed that on both types of substrate, cellular morphology was not changed after a pre-treatment with either antibody. With quantitative PCR, it was shown for both substrates that mRNA expression of integrin-β1 was increased after a pre-treatment with either anti-integrin-β1 or anti-integrin-β3 antibodies. Furthermore, after a pre-treatment with either antibody, mRNA expression of integrin-β3 and ALP was decreased, on both types of substrate. In conclusion, osteoblast-like cells have the ability to compensate to great extent for the blocking strategy as applied here. Still, integrin-β1 and β3 seem to play different roles in attachment, proliferation, and differentiation of osteoblast-like cells, and responses on CaP-coated substrates differ to non coated Ti. Furthermore, the influence on ALP expression suggests involvement of both integrin subunits in signal transduction for cellular differentiation.     

Stabilisation of crosslinked ultra-high molecular weight polyethylene (UHMW-PE)-acetabular components with α-tocopherol

A stabilisation of crosslinked ultra-high molecular weight polyethylene (UHMW-PE) with α-tocopherol (vitamin E) used for endoprostheses can increase its resistance against oxidative degradation remarkably. However, the method used for conventional UHMW-PE of adding α-tocopherol to the UHMW-PE powder before processing can not be applied for crosslinked UHMW-PE, since the α-tocopherol hinders the crosslinking process, which would be accompanied by a heavy degradation of this vitamin. The α-tocopherol has therefore to be added after the crosslinking process. This paper presents two methods for a stabilisation of finished products with α-tocopherol. In method 1, UHMW-PE-cubes (20 × 20 × 20 mm³) were stored in pure α-tocopherol under inert atmosphere at temperatures from 100^∘C to 150^∘C resulting in a high mass fraction of α-tocopherol in the edge zones. For further homogenisation, the cubes were stored in inert atmosphere at temperatures from 160^∘C to 200^∘C. In method 2, supercritical CO₂ was used to incorporate the vitamin into the UHMW-PE. In an autoclave vessel, the cubes were treated with α-tocopherol dissolved in supercritical CO₂ for several hours at temperatures from 100^∘C to 170^∘C. In both cases, the mass fraction of α-tocopherol was detected with the help of a FTIR-microscope. Both methods are well suited to stabilise crosslinked UHMW-PE with α-tocopherol. A stabilisation of the sensitive edge layer as well as a nearly homogenous distribution with varying α-tocopherol content may be realised by varying the process parameters. Using method 2, standard hip cups were stabilized nearly homogeneously with varying mass fraction of α-tocopherol. No oxidation of the UHMW-PE could be detected by infrared spectroscopy (FTIR) and HPLC studies showed a very low degradation of the α-tocopherol for both processes. In partial fulfilment of a Ph.D. (Dr.mont.) thesis at the University of Leoben In partial fulfilment of a diploma thesis at the University of Leoben     

A novel Ca3(PO4)2–CaCO3 support mixture for the CVD synthesis of roughened MWCNT-carbon fibres

Multiwalled carbon nanotubes (MWCNTs) with unusual rough surfaces (including pits) have been synthesised by the chemical vapour decomposition process of acetylene using a novel Ca₃(PO₄)₂–CaCO₃ support mixture. An Fe–Co bimetallic mixture (50?:?50, w/w) was impregnated (5?wt% loading) onto either Ca₃(PO₄)₂ to give <1% MWCNTs or CaCO₃ to give high yields of MWCNTs with smooth surfaces. Mixtures of Ca₃(PO₄)₂–CaCO₃ (0/100–100/0) yielded tubes with very rough surfaces (t?=?30?min, 1?h) and the CNT yield increased with respect to the amount of CaCO₃ in the support mixture. The inner walls of the CNTs possessed a regular orientation of crystalline graphite sheets (3–5?nm), while the outer surface of the CNTs had a thick, rough, compact layer (～30?nm) of carbon with a random orientation of graphite sheets. These induced extremely rough surfaces of the CNTs could serve as ‘docking stations’ that can harbour a variety of metal catalyst particles for industrially important applications.     

Hydrothermal synthesis of Ca2(SiO3)(OH)2 microbelts and their transformation to β-Ca2SiO4 microbelts by microwave heating

Calcium silicates possess potential applications in biomedical fields such as drug delivery and bone tissue regeneration owing to their comparatively good bioactivity, biocompatibility and biodegradability. In this paper, we report the preparation of monoclinic β-Ca₂SiO₄ microbelts by microwave thermal transformation of Ca₂(SiO₃)(OH)₂ microbelts at 670 °C for 2 h. Ca₂(SiO₃)(OH)₂ microbelts are successfully used as both the precursor source material and the template for the preparation of β-Ca₂SiO₄ microbelts. The morphology and size of Ca₂(SiO₃)(OH)₂ microbelts can be well preserved during the microwave thermal transformation process. Ca₂(SiO₃)(OH)₂ microbelts are synthesized using Na₂SiO₃?9H₂O, NaOH and Ca(NO₃)₂?4H₂O in the solvent of water by a hydrothermal method at 200 °C for 24 h. The products are characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM).     

Hot pressing ‘window’ for (P2O5, B2O3)-containing magnesium aluminosilicate reinforced with SiC fibres

Unidirectional magnesium aluminosilicate (MAS) glass-ceramic matrix/silicon carbide (SiC) fibre composites were produced, employing the slurry infiltration-hot pressing technique. The densification and crystallisation behaviour of the (P₂O₅, B₂O₃)-containing MAS glass-ceramic matrix was studied in depth. The major result of the matrix study was the construction of a two-dimensional hot pressing ‘window’ within which MAS/SiC composites were successfully processed. The ‘window’ dimensions, namely temperature and time, were shown to depend on each other. This revised version was published online in September 2006 with corrections to the Cover Date.