Showing papers in "Journal of Materials Science in 1999"

Effect of crystalline phase, orientation and temperature on the dielectric properties of poly (vinylidene fluoride) (PVDF)

Abstract: The effect of crystalline phase, uniaxial drawing and temperature on the real (e′) and imaginary (e″) parts of the relative complex permittivity of poly (vinylidene fluoride) (PVDF) was studied in the frequency range between 102 and 106 Hz. Samples containing predominantly α and β phases, or a mixture of these, were obtained by crystallization from a DMF solution at different temperatures. α phase samples were also obtained from melt crystallization and from commercial films supplied by Bemberg Folien. Different molecular orientations were obtained by uniaxial drawing of α and β phase samples. The results showed that the crystalline phase exerts strong influence on the values of e′ and e″, indicating that the αa relaxation process, associated with the glass transition of PVDF, is not exclsively related to the amorphous region of the polymer. An interphase region, which maintains the conformational characteristics of the crystalline regions, should influence the process decisively. The molecular orientation increased the values of e′ for both PVDF phases and modified its dependence with temperature over the whole frequency range studied. The influence of the crystallization and molecular orientation conditions on the dc electric conductivity (σdc) were also verified. The value of σdc was slightly higher for samples crystallized from solution at the lowest temperature and decreased with draw ratio.

Surface modification and adhesion mechanisms in woodfiber-polypropylene composites

Abstract: The interfacial adhesion between wood fiber and thermoplastic matrix polymer plays an important role in determining the performance of wood-polymer composites. The objectives of this research were to elucidate the interaction between the anhydride groups of maleated polypropylene (MAPP) and hydroxyl groups of wood fiber, and to clarify the mechanisms responsible for the interfacial adhesion between wood fiber and polypropylene matrix. The modification techniques used were bulk treatment in a thermokinetic reactive processor and solution coating in xylene. FT-IR was used to identify the nature of bonds between wood fiber and MAPP. IGC and wood veneer pull-out test was used to estimate the interfacial adhesion. Mechanical properties of injection molded woodfiber-polypropylene composites were also determined and compared with the results of esterification reaction and interfacial adhesion tests. Confocal Microscopy was employed to observe the morphology at the wood fiber-polypropylene interface, and the dispersion and orientation of wood fiber in the polypropylene matrix, respectively. The effectiveness of MAPP to improve the mechanical properties (particularly the tensile strength) of the composites was attributed to the compatibilization effect which is accomplished by reducing the total wood fiber surface free energy, improving the polymer matrix impregnation, improving fiber dispersion, improving fiber orientation, and enhancing the interfacial adhesion through mechanical interlocking. There was no conclusive evidence of the effects of ester links on the mechanical properties of the composites.

Review Processing and mechanical properties of fine-grained magnesium alloys

Abstract: Magnesium alloys are promising light structural materials. The present paper focuses on fine-grained magnesium-based materials. Grain refinement is attained by hot working without additional treatments. Also, a very small grain size of less than 1 μm is obtained by equal channel angular extrusion. A good combination of high strength and high ductility at room temperature is attained by grain refinement. Furthermore, fine-grained magnesium-based materials exhibit superplastic behavior at high stain rates (≥10−1 s−1) or low temperatures (≤473 K). These point out the importance of grain refinement to process magnesium-based materials with excellent mechanical properties.

Interfacial and mechanical properties of environment-friendly green composites made from pineapple fibers and poly(hydroxybutyrate-co-valerate) resin

Abstract: Physical and tensile properties of pineapple fibers were characterized. Tensile properties of pineapple fibers, like most natural fibers, showed a large variation. The average interfacial shear strength between the pineapple fiber and poly(hydroxybutyrate-co-valerate) (PHBV) was 8.23 MPa as measured by the microbond technique. Scanning electron microscopy (SEM) photomicrographs of the microbond specimens revealed an adhesive failure of the interface. Fully degradable and environment-friendly “green” composites were prepared by combining pineapple fibers and PHBV with 20 and 30% weight content of fibers placed in a 0°/90°/0° fiber arrangement. Tensile and flexural properties of these “green” composites were compared with different types of wood specimens. Even though tensile and flexural strength and moduli of these “green” composites were lower than those of some wood specimens tested in grain direction, they were significantly higher than those of wood specimens tested in perpendicular to grain direction. Compared to PHBV virgin resin, both tensile and flexural strength and moduli of these “green” composites were significantly higher. SEM photomicrographs of the fracture surface of the “green” composites, in tensile mode, showed partial fiber pull-out indicating weak bonding between the fiber and the matrix.

Superionic solid: composite electrolyte phase – an overview

Abstract: A general overview on the field of solid state ionics, including materials and transport property, is presented. Superionic systems in the composite electrolyte phase are discussed in great detail. Possible theoretical models suggested to understand the ion-transport mechanism in these systems are reviewed extensively.

The effect of SiO2 addition in super-hydrophilic property of TiO2 photocatalyst

Abstract: The relation between the effect and the amount of SiO2 addition on photo-generated hydrophilicity of TiO2 thin film, was investigated by measuring the X-ray diffraction, the microstructure, the specific surface area and the TG-DTA. It was found that the optimum range existed in the amount of SiO2 addition to TiO2, 10–30 mol % SiO2 addition was most effective for contact angle of water. The SiO2 addition less than 30 mol % has a suppressive effect on the transformation of anatase to rutile and on the crystal growth of anatase in calcination, and it has large surface area. With the consequence that the photocatalytic activity of TiO2 and the capability of holding absorbed water which increases during UV irradiation improved.

The effect of calcination on the microstructural characteristics and photoreactivity of Degussa P-25 TiO2

Abstract: Changes in the microstructural characteristics of Degussa P-25 titania as a result of calcination have been studied using XRD, BET and TEM. The photocatalytic activities of the samples were also examined using the degradation of phenol as a model reaction. The results indicate firstly that calcination significantly affects both microstructural characteristics and photoactivity and secondly that there is an apparent relationship between photocatalytic activity and certain microstructural characteristics. Over the range of calcination temperatures and durations studied, the sample calcined at 923 K for 3 hours revealed the highest photoreactivity, which can be ascribed to an improvement in crystallinity on calcination. The increase in the rutile content and grain growth caused by the calcination at higher temperatures were observed to decrease the photocatalytic activities of the TiO2 samples.

X-ray diffraction characterisation of low temperature plasma nitrided austenitic stainless steels

Abstract: The nitrided layers produced by low temperature (400–500 °C) plasma nitriding on austenitic stainless steels, AISI 316, 304 and 321, have been characterised by X-ray diffraction, in conjunction with metallographic and chemical composition profile analysis. The thin, hard and corrosion resistant layers exhibited similar X-ray diffraction patterns, but the positions of the major diffraction peaks varied with nitriding temperature and nitrogen concentration profile. The low temperature nitrided layers are predominantly composed of a phase with a face centred cubic (fcc) structure, which is named “S” phase. However, the positions of the diffraction peaks from the “S” phase deviated in a systematic way from those for an ideal fcc lattice. Detailed analysis of the deviation suggested that very high compressive residual stresses and stacking faults were formed in the layers, resulting in a highly distorted and disordered fcc structure. The lattice parameter of the distorted and disordered “S” phase was found to increase with increasing nitrogen concentration.

Dense Ti3SiC2 prepared by reactive HIP

Abstract: The dense polycrystalline Ti3SiC2 has been synthesized by reactive HIPing of Ti, SiC and C powders The bulk material with the highest Ti3SiC2 content about 97 vol % was obtained when treated at 1500°C, 40 MPa for 30 min The density was 99% of the theoretical value The Ti3SiC2 grains had the columnar and plate-like shapes The grains were well boned to form a network structure Many stacking faults were observed along the (001) plane of Ti3SiC2 The Vickers hardness, Young's modulus, flexural strength and fracture toughness were 4 GPa, 283 GPa, 410 MPa and 112 MPa m1/2, respectively The Ti3SiC2 was stable up to 1100°C in air The electrical resistivity was 27 × 10−7 Ω·m at room temperature The resistivity increased linearly with the increasing temperature It may be attributed to a second order phase transition The Seebeck coefficient was from 4 to 20 μV/K in the temperature range 300–1200 K It seems that Ti3SiC2 is semi-metallic with hole carriers from this small positive value

Fatigue damage mechanisms in unidirectional carbon-fibre-reinforced plastics

Abstract: The fatigue life behaviour and the underlying micromechanisms have been studied in two different Types of unidirectional carbon-fibre-reinforced plastics loaded in tension-tension along the fibre direction. The carbon fibres (AS4) were the same in the two composite systems. One thermoplastic matrix (polyetheretherketone, PEEK) and one thermosetting matrix (epoxy toughened with a thermoplastic additive) were used. The macroscopic fatigue behaviour was characterised by fatigue life diagrams. Surface replicas were taken intermittently during the course of the fatigue tests to monitor the active fatigue damage micromechanisms. The thermoset based composite showed a higher fatigue resistance with few microcracks initiated at distributed fibre breaks growing at a decelerating rate. The thermoplastic composite had a more pronounced fatigue degradation with a steeper fatigue life curve, which was caused by widespread propagating debonds and matrix cracks. The use of a tougher and more ductile matrix results in an inferior fatigue life performance, due to a more widely distributed accumulation of damage that propagates at a higher rate.

Chemically deposited zinc oxide thin film gas sensor

Abstract: Zinc oxide (ZnO) thin films were prepared by a low cost chemical deposition technique using sodium zincate bath. Structural characterizations by X-ray diffraction technique (XRD) and scanning electron microscopy (SEM) indicate the formation of ZnO films, containing 0.05–0.50 μm size crystallites, with preferred c-axis orientation. The electrical conductance of the ZnO films became stable and reproducible in the 300–450 K temperature range after repeated thermal cyclings in air. Palladium sensitised ZnO films were exposed to toxic and combustible gases e.g., hydrogen (H2), liquid petroleum gas (LPG), methane (CH4) and hydrogen sulphide (H2S) at a minimum operating temperature of 150 °C; which was well below the normal operating temperature range of 200–400 °C, typically reported in literature for ceramic gas sensors. The response of the ZnO thin film sensors at 150 °C, was found to be significant, even for parts per million level concentrations of CH4 (50 ppm) and H2S (15 ppm).

In situ ESEM study of the deformation of elementary flax fibres

Abstract: The deformation behaviour of single elementary flax fibres was investigated in an ESEM, using a modified loop test. Plastic deformation starts on the compressive side of the loop, whereas fibre failure occurs on the tensile side of the loop. The primary and the secondary cell wall show a different deformation behaviour. The primary cell wall breaks in a brittle manner, whereas in the secondary cell wall, due to its fibrillar nature, a coarse crack grows, bridged by fibrils. The secondary cell wall was found to split relatively easily along the length direction, indicating that the lateral strength of the fibre is lower than its tensile strength, which also accounts for the lower compressive strength of the fibre compared to its tensile strength.

Preparation of nanostructured tin oxide using a sol-gel process based on tin tetrachloride and ethylene glycol

Abstract: A sol-gel process starting with tin tetrachloride and ethylene glycol as precursors, has been successfully used to prepare nanostructured tin oxide powders. The molecular structure evolution during the process has been identified using infrared spectroscopy and the underlying reaction mechanisms of the sol-gel process are proposed. Results suggest that the -OHCH2CH2OH- prevent Cl− ions from access to tin ions due to steric effect and hence increase the stability of the sol solution. Ethylene glycol functions not only as a complexion agent to form a polymer network but also as a spacer to modulate the distance between metal ions, preventing metal oxide particles from aggregation during earlier stages of organics removal. Further, conversion of xerogel to tin oxide are studied using thermogravimetric analysis, X-ray diffraction, and electron microscopy. It is found that cassiterite begins to form at a temperature as low as 250 °C when organics start to burn off. However, nanocrystalline tin oxide powders are formed only after the chemically bonded hydroxyl groups are completely removed at about 600 °C.

Comprehensive study of the degradation of an intumescent EVA-based material during combustion

Abstract: In this work, we have studied the evolution of an flame retardant intumescent EVA-based material during combustion. The formulation uses an original concept of the laboratory, i.e., use of char forming polymer (here the polyamide-6) as carbonisation agent associated to a carbonisation catalyst in an intumescent formulation. The fire behavior was modelled using the cone calorimeter and we have explained the evolution of the rate of heat release curve in terms of chemistry of the system using FTIR and solid state NMR.

Parameters controlling solidification of molten wax droplets falling on a solid surface

Abstract: An experimental study was done to identify parameters that determine the shape of splats formed by droplets of paraffin wax impacting and freezing on a polished aluminum surface. Impact velocity was varied from 0.5 to 2.7 m/s and surface temperature from 23 to 73 °C. Droplet impact was photographed, and the splat diameter and liquid-solid contact angle measured from photographs. A simple energy conservation model was used to predict the maximum extent of droplet spread and the rate of droplet solidification. The extent of droplet solidification was found to be too small to affect droplet impact dynamics. Photographs showed liquid recoiling in the droplet center following impact on a cold surface (23 °C); the height of recoil diminished if either substrate temperature or impact velocity was increased. Droplet recoil was attributed to surface tension pulling back the periphery of the splat. Reducing the surface temperature increased surface tension, promoting recoil. At sufficiently large impact velocities droplets fragmented. A model based on the Rayleigh-Taylor instability was used to predict the number of satellite droplets that broke loose after impact.

Luminescence quenching in ZnS nanoparticles due to Fe and Ni doping

Abstract: Nanoparticles of zinc sulphide have been synthesized by a chemical method Mercaptoethanol is used to passivate the surface of the particles Under certain conditions highly luminescent particles emitting blue light at ∼425 nm can be synthesized This blue light emission in nanoparticles of zinc sulphide is observed to be completely quenched when doped with iron or nickel metal ions

Synthesis and properties of Mn-Zn ferrite ferrofluids

Abstract: A Mn-Zn ferrite ferrofluid is produced by chemical synthesis. Two different types of ferrofluids, according to the type of carrier liquid, are synthesized: an aqueous cationic ferrofluid and a surfacted hydrocarbon-based one. Ferrite particles are characterized by using several techniques: X-ray diffraction, transmission electronic microscopy, IR-spectroscopy, thermogravimetry, magnetization measurements and chemical analysis. Particles size depends on the synthesis parameters and can be partly controlled by choosing the type of the coprecipitating base. Increasing of the Zn concentration leads to smaller size of synthesized particles, as well as effects the content of associated water. Magnetization of the ferrofluid significantly decreases when the degree of Zn substitution exceeds 0.5.

Characterization of bitumens modified with SEBS, EVA and EBA polymers

Abstract: A laboratory evaluation of the modified bitumens containing styrene-ethylene-butylene-styrene (SEBS), ethylene vinyl acetate (EVA) and ethylene butyl acrylate (EBA) copolymers is presented The morphology, storage stability and rheological and ageing properties of the modified binders were studied using fluorescence microscopy, dynamic mechanical analysis, creep test (bending beam rheometer) and conventional methods The results indicated that the morphology and storage stability of the modified binders were largely dependent on the polymer content and were influenced by the characteristics of the base bitumens and the polymers At a low polymer content (3% by weight), the modified binders showed dispersed polymer particles in a continuous bitumen matrix At a sufficiently high polymer content (6% by weight), a continuous polymer phase was observed Regardless of the nature of the two phases, the storage stability of the modified binders decreased as polymer content increased Polymer modification improved bitumen rheological properties such as increased elastic responses at high temperatures and reduced creep stiffness at low temperatures The degree of improvement generally increased with polymer content, but varied with bitumen source/grade and polymer type Polymer modification also influenced bitumen ageing properties Evaluation of ageing effect was dependent on testing conditions (eg temperature and frequency)

The microstructure and mechanical properties of TiC and TiB2-reinforced cast metal matrix composites

Abstract: TiC and TiB2 particles have been spontaneously incorporated into commercial purity aluminum melts through the use of a K-Al-F-based liquid flux that removes the oxide layer from the surface of the melt. The combination of spontaneous particle entry and close crystal structure matching in the Al-TiB2 and Al-TiC systems, results in low particle-solid interfacial energies and the generation of good spatial distributions of the reinforcing phase in the solidified composite castings. The reinforcement distribution is largely insensitive to the cooling rate of the melt and the majority of the particles are located within the grains. Modulus increases after TiC and TiB2 particle additions are greater than those for Al2O3 and SiC. It is thought that interfacial bonding is enhanced in the TiC and TiB2 systems due to wetting of the reinforcement by the liquid and particle engulfment into the solid phase. TiC-reinforced composites exhibit higher stiffnesses and ductilities than TiB2-reinforced composites. This has been attributed to stronger interfacial bonding in the Al-TiC system, due to the increased tendency for nucleation of solid on the particle surfaces.

The photoelectrochemistry of transition metal-ion-doped TiO2 nanocrystalline electrodes and higher solar cell conversion efficiency based on Zn2+-doped TiO2 electrode

Abstract: Metal-ion-doped TiO2 nanoparticles were prepared with hydrothermal method. The change of photocurrents at different electrode potentials and wavelengths of incident light showed two different characteristics for various transition metal-ion-doped TiO2 electrodes. In Zn2+ and Cd2+-doped TiO2 electrodes, a characteristic of n-type semiconductor was observed and the incident photon to conversion efficiency (IPCE) were larger than that of pure TiO2 electrode at the thickness of electrode film of 0.5 μm when the content of doped metal ion was less than 0.5%. The effect of the thickness of films on IPCE was also investigated. The IPCE of pure TiO2 electrode was strongly dependent on the thickness of films. The change tendency of the IPCE for Zn2+-doped TiO2 (0.5% Zn2+) electrodes with its thickness was different from that of pure TiO2. In Fe3+, Co2+, Ni2+, Cr3+ and V5+-doped TiO2 electrodes, a phenomenon of p-n conversion was observed. The difference of photoresponse and the value of photocurrents are dependent on the doping method and concentration of the doped metal ions. The maximum conversion efficiency of RuL2(SCN)2-sensitized Zn2+-doped TiO2 solar cell (1.01%) was larger than that of RuL2(SCN)2-sensitized pure TiO2 solar cell (0.82%) at the same conditions when 0.5 mol · l−1 (CH3)4N · I + 0.05 mol · l−1 I2 in propylene carbonate solution was used as electrolyte.

Characterization of polyaniline by cyclic voltammetry and UV-Vis absorption spectroscopy

Abstract: Characterization of polyaniline (PANI) by electrochemical and spectroscopic techniques are presented. The electrochemical investigation reveals the influence of the dopant acid on the kinetic parameters of polyaniline films: exchange current density (I0) and the cathodic transfer coefficient (αc). UV-Vis spectra of thin films of polyaniline in N-metil pyrrolidine solutions indicate the increase of the absorbance with the number of deposition cycles, N. The inhibition of polymer degradation in thick films was confirmed by these spectra.

Preparation, structure and properties of uniaxially oriented polyethylene-silver nanocomposites

Abstract: Uniaxially oriented composites of high-density polyethylene and silver nanoparticles were prepared using solution-casting, melt-extrusion and solid-state drawing techniques. The absorption spectrum in the visible wavelength range of the drawn nanocomposites was observed to strongly depend on the polarisation direction of the incident light. For instance, the nanocomposites appear bright yellow or red when the vibration direction of linearly polarised light is perpendicular or parallel, respectively, to the drawing axis. The optical anisotropy of the drawn nanocomposites originates from uniaxially oriented, pearl-necklace type of arrays of nanoparticles of high aspect ratios. The absorption spectrum of the nanocomposites can be shifted to higher wavelengths using appropriate annealing procedures. The annealing results in an increased size of the primary silver particles, due to Ostwald ripening, and consequently a range of polarisation-dependent colours can be generated in the drawn nanocomposites.

Effect of Mg on the ageing behaviour of Al-Si-Cu 319 type aluminium casting alloys

Abstract: The present study was performed on primary A319.2 alloy to investigate the effect of magnesium addition as well as other melt treatment parameters such as Sr modification and grain refinement on the heat treatment behaviour of the alloy. The results show that increasing the Mg content in A319.2 up to 0.45% considerably enchances the alloy response to heat treatment in the T5 and T6 tempers, more particularly, the T6 temper. Modification of the high-Mg version of 319 alloy with Sr in amounts of ∼350 ppm results in a marked amount of porosity formation which counteracts the beneficial effect of the modification, leading to a noticeable decline in the alloy strength. Grain refining the Sr-modified (A319.2 + 0.45% Mg) alloy produces sounder castings and, hence, an identical ageing response to that offered by unmodified high-Mg alloys. The properties, however, are more consistent. Addition of Mg (∼0.45%) leads to the precipitation of coarse particles of Al5Mg8Si6Cu2. Modification with Sr tends to cause severe segregation of both Cu-containing intermetallics, i.e., Al2Cu and Al5Mg8Si6Cu2 in areas away from the growing Al-Si eutectic regions. Thus, their dissolution rates are fairly sluggish upon solutionizing at 505 °C. Increasing the solutionizing temperature would lead to incipient melting of the phases and, hence, a catastrophic failure. Fracture of intermetallic phases in the interdendritic regions is mostly brittle, with the formation of microcracks at the Si, Cu, Fe-base intermetallics and aluminium interfaces. Fracture of the α-aluminium is always ductile. Hardening during ageing occurs by cooperative precipitation of Al2Cu and Mg2Si phase particles.

FTIR spectral characterization of thin film coatings of oleic acid on glasses: I. Coatings on glasses from ethyl alcohol

Abstract: Oleic acid was coated on soda lime–silicate, soda-silicate, and silica glasses from ethyl alcohol The coating properties of oleic acid were related to the structure, composition, and dissolution properties of the glasses The bonding mechanism between the organic coating and the glass surface for each different composition of the glass was revealed by analysis of the diffuse reflectance infrared Fourier transform (DRIFT) spectra The metal ions on the soda lime–silicate glass surface produced metal-(Na-, Ca-, and Al-) oleates through the formation of metal–carboxylate complexes Two different structural types were present for calcium-oleate Some of the oleic acid coated undissociatively on the glass surface through hydrogen bonding Oleic acid reacted with the surface of the soda-silicate glass, which possessed less Na2O content than the soda lime–silicate glass did, completely dissociating, producing only sodium-oleate species because of the high diffusion and dissolution rates of Na+ ions Oleic acid formed only hydrogen bonds with silanol groups on silica glass The coated organic layer on soda lime–silicate glass possessed a more ordered and compact structure than either on silica glass or in pure oleic acid The metal ions on the soda lime–silicate glass surface strongly coordinated to COO− ions from the oleic acid and made the alkyl chains of the oleates more rigid and oriented, consequently causing the coated layer to be more ordered and compact This structural result caused the dimeric pairs of COOH groups of undissociatively adsorbed oleic acid molecules to be closer, possessing stronger hydrogen bonds than occurring on the silica glass surface These results suggest that the composition of the glass is one of the most important factors for determining the coating mechanism involving oleic acid

Use of zirconium diboride-copper as an electrode in plasma applications

Abstract: Frequent replacement of electrodes, due to their high wear rate, is an undesired feature of most thermal plasma processes. Hence, the discovery of a high spark-resistive tool, ZrB2-Cu, is of interest. Performance evaluation of this metal matrix ceramic (MMC) employed electrical discharge machining (EDM), where steel is used as the cathode workpiece and the MMC is used as the anode tool. Compared with the performance of copper and graphite tools, ZrB2-Cu yields the highest workpiece removal rate,; and the lowest tool wear rate at high plasma heat flux conditions, resulting in an extremely low wear ratio. Energy dispersive spectroscopy shows deposition of workpiece materials (Fe, Cr, Ni and S) on the ZrB2-Cu surface after EDM. This is due to the difference between the surface temperature of the tool and the workpiece. Scanning electron microscopy and elemental mapping analysis reveal that the composite electrode erodes by a combination of dominant evaporation and melting of the metal phase, negligible melting and thermal spalling in the ceramic phase, quick refreezing of the metal phase back to the surface, and deposition of the workpiece (steel) on the tool surface. Most of the heat is conducted through the Cu phase, reducing thermal stress in the ceramic phase. This causes lower surface temperatures for the molten ZrB2 matrix; hence, the Cu tends to refreeze quickly near the surrounding ceramic matrix.

Influence of the α/β-SiC phase transformation on microstructural development and mechanical properties of liquid phase sintered silicon carbide

Abstract: The transformation kinetics and microstructural development of liquid phase sintered silicon carbide ceramics (LPS-SiC) are investigated. Complete densification is achieved by pressureless and gas pressure sintering in argon and nitrogen atmospheres with Y2O3 and AlN as sintering additives. Studies of the phase transformation from β to α-SiC reveals a dependency on the initial β-content and the sintering atmosphere. The transformation rate decreases with an increasing β-content in the starting powder and in presence of nitrogen. The transformation is completely supressed for pure β-SiC starting powders when the additive system consists of 10.34 wt % Y2O3 and 2.95 wt % AlN. Materials without phase transformation showed a homogeneous microstructure with equiaxed grains, whereas microstructures with elongated grains were developed from SiC powders with a high initial α/β-ratio (> 1 : 9) when phase transformation occurs. Since liquid phase sintered silicon carbide reveals predominantly an intergranular fracture mode, the grain size and shape has a significant influence on the mechanical properties. The toughness of materials with platelet-like grains is about twice as high as for materials with equiaxed grains. Materials exhibiting elongated microstructures show also a higher bending strength after post-HIPing.

Effect of casting and homogenizing treatment conditions on the formation of Al–Fe–Si intermetallic compounds in 6063 Al–Mg–Si alloys

Abstract: The effect of casting and homogenizing treatment conditions on the formation of several Al–Fe–Si intermetallic compounds in 6063 aluminum alloy was investigated using X-ray diffraction and transmission electron microscopy (TEM) The four kinds of alloys containing 01 to 05 mass% Fe were melted and then cooled at three different cooling rates ranging from 006 to 50 K/s, following the homogenization at 858 K for 54 ks and 2400 ks The Al–Fe–Si compound particles were extracted from the alloy ingots using the thermal phenol method The as-cast 01 mass% Fe ingot obtained at the casting cooling rate of 006 K/s had a largest amount of the β phase among the ingots investigated When this ingot was homogenized at 858 K for 54 ks and 2400 ks, the amount of the β phase decreased, while that of the α′ phase increased On the other hand, the as-cast 05 mass% Fe ingot obtained at the casting cooling rate of 50 K/s had the largest amount of the α phase among the ingots investigated When this ingot was homogenized at 858 K for 54 ks, a large amount of the α phase remained However, the homogenization at 858 K for 2400 ks resulted in the transformation of the α phase to the α′ phase The main phase in the as-cast 02 mass% Fe ingot obtained at the casting cooling rate of 5 K/s, close to the industrial cooling rates, was the β phase The β phase gradually decreased, and the relative amounts of the α and α′ phases increased during homogenization at 858 K for 54 ks Furthermore, almost all of the Al–Fe–Si intermetallic compounds were transformed into the α′ phase in the ingots homogenized at 858 K for 2400 ks

Kinetics of thermal decomposition of hydroxyapatite bioceramics

Abstract: The mechanism and kinetics of thermal decomposition of injection moulded hydroxyapatite ceramics were studied over the temperature range of 1473–1758 K. At temperatures below 1473 K the sintering and transformation of hydroxyapatite to hydroxyoxyapatite proceeded to a conversion degree of 0.4 to 0.5. At temperatures between 1473 and 1758 K the hydroxyapatite was decomposed to α-TCP, H2O and CaO. The decomposition of HOA started on the surface of the HOA ceramics. The rate of increase in the thickness of the reaction products (α-TCP) was described by the parabolic law. The kinetic analysis of the time dependence of HOA conversion to TCP by means of the J-M-A-J-K equation also showed that the thermal decomposition of HOA ceramics was controlled by diffusion of water from the reaction zone to the surface of the ceramic sample. The activation energy of the thermal decomposition of HOA ceramics amounted to 283.5 kJ/mol.

Influence of the preparation conditions on the synthesis of high surface area SiC for use as a heterogeneous catalyst support

Abstract: The influence of different parameters (temperature, duration and SiO source) on the synthesis of silicon carbide SiC according to the gas-solid reaction between SiO vapors and activated charcoal was investigated. The material obtained retained the general shape of the activated charcoal, which is an advantage because of the difficulty in post shaping SiC, due to the high strength of the material. High temperature (>1250 °C) and long reaction duration led to a high C* → SiC conversion but with a relatively low surface area (20–25 m2 · g−1) due to sintering via the surface diffusion phenomenon. The combination of a lower reaction temperature (1200 °C), longer reaction duration (15 h) and high (Si + SiO2)/C* weight ratio allowed SiC to be obtained with a surface area of around 50 m2 · g−1, which can be used as a support material for heterogeneous catalysis.

Effect of microstructure on the mechanical properties and fracture of commercial hypoeutectic Al-Si alloy modified with Na, Sb and Sr

Abstract: The microstructure, hardness, tensile properties and fracture have been studied for the non-modified and modified aluminium (Al) silicon (Si) commercial hypoeutectic alloy. Three modifiers were used being sodium (Na), antimony (Sb) and strontium (Sr). The Sb-modified structure revealed small plate-like Si morphology. The Na and Sr-modified structures exhibited fibrous Si. A slight increase in the hardness values (HV) due to modification was observed. A general increase in the tensile properties was observed due to modification. The tensile properties of the sand mould Sr-modified alloy were significantly higher than those of the Na-modified alloy by 12.7% in proof stress, 16.3% in ductility and 33.3% in toughness. For the metal mould ingots the increase in tensile properties of Sr-modified alloy were respectively: 16.7%, 32.5% and 41.7% compared to a Na-modified alloy. Optical fractography on longitudinal sections near the fracture surfaces of the modified alloys revealed that the crack propagates in the eutectic thus, circumventing the Al-dendrites. The dimple and smooth ripple patterns observed by scanning electron microscope (SEM) on the fracture surface of the Na and Sr-modified alloys suggest a transgranular type of fracture across the grains of the eutectic matrix.