Fabrication of carbon nanofiber(CNF)-dispersed Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites by pulsed electric-current pressure sintering and their mechanical and electrical properties

Dense Al₂O₃-based composites (≥99.0% of theoretical) dispersed with carbon nanofibers (CNFs) were fabricated using the pulsed electric-current pressure sintering (PECPS) for 5 min at 1300°C and 30 MPa in a vacuum. The dispersion of CNFs into the matrix depended much on the particle size of the starting Al₂O₃ powders. Mechanical properties of the composites were evaluated in relation with their microstructures; high values of three-point bending strength σ_b (∼800 MPa) and fracture toughness K _IC (∼5 MPa·m^1/2) were attained at the composition of CNF/Al₂O₃ = 5:95 vol%, which σ_b and K _IC values were ∼25% and ∼5%, respectively, higher than those of monolithic Al₂O₃. This might be due to the small Al₂O₃ grains (1.6 μm) of dense sintered compacts compared with that (4.4 μm) for the pure Al₂O₃ ceramics, resulting from the suppression of grain growth during sintering induced by uniformly dispersed CNFs in the matrix. Electrical resistivity of CNF/Al₂O₃ composites decreased rapidly from >10¹⁵ to ∼2.1 × 10⁻² Ωm (5vol%CNF addition), suggesting the machinability of Al₂O₃-based composites by electrical discharge machining.

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Microstructure and mechanical properties of B6O-B4C sintered composites prepared under high pressure

Sintered composites in the B₆O-xB₄C (x = 0–40 vol%) system were prepared under high pressure and high temperature conditions (3–5 GPa, 1500–1800°C) from the mixture of in-laboratory synthesized B₆O powder and commercially available B₄C powder. Relationship among the formed phases, microstructures and mechanical properties of the sintered composites was investigated as a function of sintering conditions and added B₄C content. Microhardness of the sintered composite was found to increase with treatment temperature up to 1800°C, while fracture toughness decreased slightly. Maximum microhardness of Hv 46 GPa was obtained from B₆O-30vol%B₄C sintered composite under the sintering conditions of 4 GPa, 1700°C and 20 min.     

Preparation and investigation of Al–4 wt % B<Subscript>4</Subscript>C nanocomposite powders using mechanical milling

Boron carbide nanoparticles were produced using commercially available boron carbide powder (0·8 μm). Mechanical milling was used to synthesize Al nanostructured powder in a planetary ball-mill under argon atmosphere up to 20 h. The same process was applied for Al–4 wt % B₄C nanocomposite powders to explore the role of nanosize reinforcements on mechanical milling stages. Scanning electron microscopy (SEM) analysis as well as apparent density measurements were used to optimize the milling time needed for completion of the mechanical milling process. The results show that the addition of boron carbide particles accelerate the milling process, leading to a faster work hardening rate and fracture of aluminum matrix. FE-SEM images show that distribution of boron carbide particles in aluminum matrix reaches a full homogeneity when steady state takes place. The better distribution of reinforcement throughout the matrix would increase hardness of the powder. To study the compressibility of milled powder, modified heckel equation was used to consider the pressure effect on yield strength as well as reinforcing role of B₄C particles. For better distribution of reinforcement throughout the matrix, r, modified heckel equation was used to consider the pressure effect on yield strength as well as reinforcing role of B₄C particles.     

Si-incorporated alumina phases formed out of diphasic mullite gels

The diphasic mullite gel forms o-mullite on heating via intermediate spinel phase. Characterization of the latter phase with various physico-chemical techniques is concisely reviewed. It is noticeable that XRD intensity of both the amorphous scattering band and the diffraction peak of Al–Si spinel phase changes during each step of transformation processes of diphasic gel. Accordingly, the integrated area of the intensity peak of amorphous band and that of Al–Si spinel phase generated during heating diphasic gels were measured by XRD technique with the help of X’Pert Graphics and Profit softwares. The amount of free SiO₂ (A) content present at various stages of heating diphasic gels was estimated by classical alkali leaching study standardized earlier. The results show that diphasic gel which forms an aluminosilicate (A) phase initially by dehydration and dehydroxylation, subsequently crystallizes to Al–Si spinel phase. In consequence, the ratio of XRD peak of spinal phase to that of amorphous band increases in the temperature range of 600–1000 °C. This study confirms the earlier view of incorporation of silica into the alumina structure with formation of Al–Si spinal phase. Complementary alkali leaching study indicates the existence of non-crystalline silica-rich aluminous phase other than free non-crystalline silica during heating diphasic gel at ~1000 °C.

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Investigation on in situ Al0.5FeSi0.5/Al composites prepared by transient liquid phase sintering

In situ Al_0.5FeSi_0.5/Al composites were prepared by transient liquid-phase sintering. The hardness and wear resistance of the composites were investigated with an XHV-1000 microhardness tester and an M-2000 wear tester. Results show that with increased sintering temperature and holding time, the in situ needle-like reinforcement is transformed into short, bar-like, massive particles. At a sintering temperature of 510 °C and holding time of 4 h, the reinforcement consists of short, bar-like Al_0.5FeSi_0.5; moreover, the hardness of the in situ Al_0.5FeSi_0.5/Al composites peaks to a value eight times that of pure aluminum and 2.5 times that of Al–Si alloy. Accordingly, the wear resistance of the composites is the highest, i.e., 6.6 that of pure Al and 4.5 times that of Al–Si alloy.     

Effects of sintering temperature and addition of Fe and B4C on hardness and wear resistance of diamond reinforced metal matrix composites

The effects of sintering temperature and addition of Fe instead of Co into the matrix composition on the mechanical properties of diamond-reinforced MMC’s have been studied. Diamond-reinforced MMC’s based on Fe-Co compositions with and without boron carbide (B₄C) have been processed. Three different matrix composites (with different Fe/Co ratios) have been produced with and without B₄C at a pressure of 25 MPa and sintered in N₂ at various temperatures (800, 900, and 1000°C). After sintering, mechanical properties of the resultant composites have been studied and the results discussed. Addition of B₄C has been found to improve the hardness and wear resistance of the composites. Optical microscopy, SEM and EDS have been used to examine the microstructure and surface of the synthesized composites.     

High pressure consolidation of B6O-diamond mixtures

Sintered composites in the B₆O-xdiamond (x= 0–80 vol%) system were prepared under high pressure and high temperature conditions (3–5 GPa, 1400–1800°C) from the mixture of in-laboratory synthesized B₆O powder and commercially available diamond powder with various grain sizes (<0.25, 0.5–3, and 5–10 m). Relationship among the formed phases, microstructures, and mechanical properties of the sintered composites was investigated as a function of sintering conditions, added diamond content, and grain size of diamond. Sintered composites were obtained as the B₆O-diamond mixed phases when using diamond with grain sizes greater than 0.5 m, while the partial formation of the diamond-like carbon was observed when using diamond grain sizes less than 0.25 m. Microhardness of the sintered composite was found to increase with treatment temperature and pressure, and the fracture toughness slightly decreased. A maximum microhardness of H _v57 GPa was measured in the B₆O-60 vol% diamond (grain size < 0.25 m) sintered composite under the sintering conditions of 5 GPa, 1700°C and 20 min.     

Influence of coal tar pitch coating on the properties of micro and nano SiC incorporated carbon–ceramic composites

Carbon-micro or nano silicon carbide–boron carbide (C-micro or nanoSiC–B₄C) composites were prepared by heating the mixtures of green coke and carbon black as carbon source, boron carbide and silicon at temperature of 1,400 °C. Green coke reacts with silicon to give micron sized silicon carbide while the reaction between silicon and carbon black gives nano silicon carbide in the resulting carbon–ceramic composites. The green coke was coated with a suitable coal tar pitch material and used to develop carbon-(micro or nano) silicon carbide–boron carbide composites in a separate lot. The composites were characterized for various properties including oxidation resistance. It was observed that both types of composites made from uncoated as well as pitch-coated green coke exhibited good oxidation resistance at 800–1,200 °C. The density and bending strength of composites developed with pitch-coated green coke improved significantly due to the enhanced binding of the constituents by the pitch.     

Synthesis and characterization of high surface area silicon carbide by dynamic vacuum carbothermal reduction

Silicon carbide (SiC) precursor was obtained by sol–gel used tetraethoxysilane as silicon source and saccharose as carbon source, and then the precursor was used to prepare SiC by carbothermal reduction under dynamic vacuum condition. The samples were characterized by X-ray diffraction, scanning electron microscope, and low-temperature nitrogen adsorption–desorption measurement. The results showed that the carbothermal temperature for synthesizing SiC needed to be at 1,100 °C under dynamic vacuum. At this temperature, the obtained sample is composed of agglomerated regular grains with size ranging from 20 to 40 nm and has a high surface area of 167 m²/g and the main pore size center at 5.3 nm.

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Morphology and constitution of the compound layer formed on nitrided Fe–4wt.%V alloy

The microstructure of the compound (“white”) layer formed on the surface of Fe–4wt.%V alloy, by nitriding in a gas mixture of ammonia and hydrogen at 580 °C, has been investigated by employing light and scanning electron microscopy, X-ray diffraction and electron probe microanalysis. The compound layer is dominantly composed of γ^|-Fe₄N nitride. Quantitative analysis of the composition data demonstrated that V is present in the compound layer as VN precipitates, i.e. V is not taken up significantly in (Fe, V) nitrides. A mechanism for compound-layer formation has been proposed.

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Low-Temperature Synthesis of Boron Carbide Ceramics

This study has been the first to demonstrate the possibility of producing boron carbide ceramics from coarse (D = 25–150 μm) B₄C powder (which is impossible to sinter by conventional methods) through infiltration with molten silicon and subsequent treatment within the field of the controlled temperature gradient. This produces yields a composite ceramics B₄C–SiC–Si with a hardness of 26 to 35 GPa and a splitting tensile strength of 110 to 170 MPa. The influence of the velocity of movement of the temperature gradient on the structure, phase composition, and properties of the prepared composites has been studied.     

Microstructures and microwave dielectric properties of low-temperature sintered Ca2Zn4Ti15O36 ceramics

Low-temperature sintered Ca₂Zn₄Ti₁₅O₃₆ microwave dielectric ceramic was prepared by conventional solid state reaction method. The influences from V₂O₅ addition on the sintering behavior, crystalline phases, microstructures and microwave dielectric properties were investigated. The crystalline phases and microstructures of Ca₂Zn₄Ti₁₅O₃₆ ceramic with V₂O₅ addition were investigated by X-ray diffraction, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). V₂O₅ addition lowered the sintering temperature of Ca₂Zn₄Ti₁₅O₃₆ ceramics from 1140 °C to 930 °C. Ca₂Zn₄Ti₁₅O₃₆ ceramic with 5wt% V₂O₅ addition could be densified well at 930 °C, and showed good microwave dielectric properties of ε_r ~ 46, Q × f ~ 13400 GHz, and temperature coefficient of resonant frequency (τ_f) ~ 164 ppm/°C.

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SiC matrix composites containing transition metal boride particulates internally synthesised by carbothermal reaction

SiC matrix composites reinforced with the various borides of the transition metals in group IV a-VI a, which were synthesized from the transition metal oxide, boron carbide and carbon mixed with SiC powder. Dense composites containing boride particulates of titanium, zirconium, niobium and chromium were prepared through reactive hot-pressing. The morphology of the internally synthesized boride particles reflected that of the starting oxide powders. SiC-NbB₂ composites with four-point flexural strength of 500 to 600 MPa and better oxidation resistance than SiC-TiB₂ were prepared even through pressureless sintering process. Pressureless-sintered and HIPed SiC-20 vol% NbB₂ exhibited the four-point flexural strength of 760 MPa at 20 °C and 820 MPa at 1400 °C.     

Influence of processing on the microstructure of Cu–8Cr–4Nb

The particle-strengthened Cu–8 at.%Cr–4 at.%Nb alloy is processed by consolidation of atomized powders followed by extrusion to obtain bars and rolling to produce sheets. Comparison of copper matrix grain and second-phase particle structures in both extruded and rolled Cu–8Cr–4Nb was performed. Extruded material displayed locally banded arrangements of Cr₂Nb particles, while the distribution of particles was more uniform in rolled material. Mean Cr₂Nb particle sizes were found to be essentially the same for both processing methods. Non-spherical particles in the extruded alloy showed some preferred orientation, whereas the rolled material displayed a more uniform particle orientation distribution. Extruded material exhibited a dual grain size distribution with smaller grains in banded regions. The mean grain size of 1.36 μm in extruded material was larger than the 0.65 μm grain size of rolled material. A [101] texture was evident in extruded material, whereas the rolled material was only slightly textured along the [001] and [111] directions. The processing differences for the rolled and extruded forms give rise to different microstructures and hence higher creep strength for the extruded material in the temperature range of 773–923 K.

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Melt synthesis of oxide phosphors with K2NiF4 structures: CaLa1? x Eu x GaO4

In order to synthesize compounds of various Perovskite-related structures, we have utilized a novel “melt synthesis technique” for phosphors rather than the conventional solid state reaction techniques. The solid state reactions require multi-step processes of heating/cooling with intermediate grindings to make homogeneous samples. However, for the melt synthesis, it is possible to make a homogeneous sample in a single step within a short period of time (1–60 s) due to the liquid phase reaction in the molten samples, which were melted by strong light radiation in an imaging furnace. In this study, we have prepared a red-phosphor CaLaGaO₄:Eu³⁺ which has a perovskite—related layered K₂NiF₄ structure. Well-crystallized CaLa_1−x Eu _x GaO₄ samples with the K₂NiF₄ structure have been obtained up to x = 0.25, but there was the formation of an olivine phase when x = 0.5–1.0. The red emission at 618 nm increased with the increasing value of x up to x = 0.25.

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In situ boron carbide–titanium diboride composites prepared by mechanical milling and subsequent Spark Plasma Sintering

Boron carbide–titanium diboride composites were synthesized and consolidated by Spark Plasma Sintering (SPS) of mechanically milled elemental powder mixtures. The phase and microstructure evolution of the composites during sintering in the 1,200–1,700 °C temperature range was studied. With increasing sintering temperature, the phase formation of the samples was completed well before full density was achieved. The distribution of titanium diboride in the sintered samples was significantly improved with increasing milling time of the Ti–B–C powder mixtures. A bulk composite material of nearly full density, fine uniform microstructure, and increased fracture toughness was obtained by SPS at 1,700 °C. The grain size of boron carbide and titanium diboride in this material was 5–7 and 1–2 μm, respectively.     

Microstructure and stresses in a keatite solid–solution glass–ceramic

The microstructure of a translucent keatite solid–solution glass–ceramic (keatite s.s.) of the LAS-system (Li₂O–Al₂O₃–SiO₂) has been analyzed with SEM, AFM, XRF, XRD, and TEM. The glass–ceramic consists mainly of keatite s.s. with minor secondary phases such as zirconium titanate, gahnite and probably rutile. Furthermore the resistance to temperature differences (RTD) of this glass–ceramic was investigated. It is shown that, in spite of the relatively high coefficient of thermal expansion (CTE) of about 1 × 10⁻⁶ K⁻¹, an improved RTD can be achieved by special ceramization treatment. With this, compressive stresses in the first 100 μm to 150 μm are induced. These stresses can presumably be contributed to a difference in CTE between the surface-near zone and the bulk. Said CTE difference is caused by chemical gradients of CTE-relevant elements, such as Zn, K, and supposedly additional alkali elements such as Li. These stresses are useful to increase the strength and application range of glass–ceramics based on keatite s.s.

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Amorphous and nano crystalline phase formation in Ni<Subscript>2</Subscript>MnGa ferromagnetic shape memory alloy synthesized by melt spinning

Melt spinning technique was used to synthesize Ni₂MnGa ferromagnetic shape memory alloy ribbons. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) analysis of the ribbon synthesized at lower wheel speed (20 m/s) reveal the formation of very fine clusters of austenitic phase of Ni₂MnGa. However at higher wheel speed (30 m/s) the formation of martensite and nanoparticles of Ni₂MnGa with a size range of 10–20 nm in the amorphous matrix is observed. Also an amorphous phase was observed at higher wheel speed in some areas of the ribbon. Annealing (1000 °C, 1 h) of the ribbon synthesized at higher wheel speed resulted in martensite and γ (gamma) phases. Amorphous phase, Ni₂MnGa nanoparticles, and the martensite phase are analyzed in detail.

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A review of sealing technologies applicable to solid oxide electrolysis cells

This article reviews designs and materials investigated for various seals in high temperature solid oxide fuel cell “stacks” and how they might be implemented in solid oxide electrolysis cells that decompose steam into hydrogen and oxygen. Materials include metals, glasses, glass–ceramics, cements, and composites. Sealing designs include rigid seals, compressive seals, and compliant seals.

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Effect of boron carbide particle size and volume fraction of TiB–TiC reinforcement on steady state creep behaviour of PM processed titanium matrix composites

Abstract

TiB–TiC reinforced titanium matrix composites (TMCs) were synthesised through pressureless sintering of titanium and boron carbide (B₄C) powder compacts. Effect of boron carbide (B₄C) particle size and volume fraction of TiB–TiC reinforcement on steady state compression creep behaviour of composites was investigated in the temperature range of 773–873 K. The creep rates of composites are found to be about two orders of magnitude lower than those of unreinforced titanium. The creep rates further lowered with decrease in size of B₄C particles (from 16 to 3 μm) used in preparation of composites as well as with increase in volume fraction of the TiB–TiC reinforcement from 10 to 30 vol.%. By using the concept of effective stress as well as incorporation of load transfer and substructural strengthening effect produced by the reinforcement into analysis, the entire creep data of Ti and the composites can be made to merge on to a single line within a scatter band of factor of 2–3 and can be represented by a unified power-law equation.