Mechanisms and kinetics of MgB2 synthesis from boron fibers

Superconducting MgB₂ fibers were synthesized through the reaction of liquid magnesium with 140 μm diameter boron fibers. Fiber reaction occurs by two concurrent mechanisms. First, concentric MgB₂, MgB₄ and MgB₇ shells grow radially into the B fibers. Diffusion modeling provides rate constants and effective diffusion coefficients for the borides and their activation energies. Second, radial cracks form in the MgB₄ or MgB₇ shells, allowing for Mg ingress into the fibers and diffusional growth of MgB₂ wedges in the radial and circumferential directions. Combining both shell and wedge MgB₂ growth mechanisms into a single model provides predictions of overall MgB₂ reaction kinetics as a function of time and temperature, which are in good agreement with in situ X-ray diffraction measurements performed at 885–1025 °C.     

Load partitioning between ferrite and cementite during elasto-plastic deformation of an ultrahigh-carbon steel

An ultrahigh-carbon steel was heat-treated to form an in situ composite consisting of a fine-grained ferritic matrix with 34 vol.% submicron spheroidized cementite particles. Volume-averaged lattice elastic strains for various crystallographic planes of the α-Fe and Fe₃C phases were measured by synchrotron X-ray diffraction for a range of uniaxial tensile stresses up to 1 GPa. In the elastic range of steel deformation, no load transfer occurs between matrix and particles because both phases have nearly equivalent elastic properties. In the steel plastic range after Lüders band propagation, marked load transfer takes place from the ductile α-Fe matrix to the elastic Fe₃C particles. Reasonable agreement is achieved between phase lattice strains as experimentally measured and as computed using finite-element modeling.     

Synthesis of nanocrystalline magnesium diboride (MgB2) metallic superconductor by mechano-chemical reaction and post-annealing

MgB₂ is recently discovered superconducting compound with a record-breaking transition temperature (T_c = 39 K) for a conventional metallic superconductor. Nanocrystallinity can improve its electrical properties by strong pinning. In the present work we report the results of the synthesis of nanocrystalline MgB₂ superconducting compound by mechano-chemical reaction followed by post-annealing. The first stage of synthesis was carried out from elemental crystalline Mg and amorphous B powders by controlled mechanical alloying (CMA) in the magneto-mill Uni-Ball-Mill 5. X-ray diffraction studies reveal that the nucleation of small amount of MgB₂ is initiated after two-step milling for combined 100 h under protective helium gas. Further reaction to form MgB₂ is accelerated by subsequent annealing of the milled powder at various temperatures. X-ray diffraction shows formation of a well-developed nanocrystalline MgB₂ after annealing of the pre-nucleated powder at the 630–650 °C range for a few hours.     

Hydrogen sorption properties of MgH2–LiBH4 composites

A detailed analysis of the reaction mechanism of the reactive hydride composite (RHC) MgH₂ + 2LiBH₄ ↔ MgB₂ + 2LiH + 4H₂ was performed using high-pressure differential scanning calorimetry (HP-DSC) measurements and in situ synchrotron powder X-ray diffraction (XRD) measurements along with kinetic investigations using a Sievert-type apparatus. For the desorption the following two-step reaction has been observed: MgH₂ + 2LiBH₄ ↔ Mg + 2LiBH₄ + H₂ ↔ MgB₂ + 2LiH + 4H₂. However, this reaction is kinetically restricted and proceeds only at elevated temperatures. In contrast to the desorption reaction, LiBH₄ and MgH₂ are found to form simultaneously under fairly moderate conditions of 50 bar hydrogen pressure in the temperature range of 250–300°C. As found in pure light metal hydrides, significant improvement of sorption kinetics is possible if suitable additives are used.     

Tensile creep of directionally solidified NiAl–9Mo in situ composites

Well-aligned Mo fiber-reinforced NiAl in situ composites were produced by specially controlled directional solidification. The creep behavior parallel to the growth direction was studied in static tensile tests at temperatures between 900 °C and 1200 °C. A steady-state creep rate of 10^?6 s^?1 was measured at 1100 °C under an initial applied tensile stress of 150 MPa. Compared to binary NiAl and previously investigated NiAl–Mo eutectics with irregularly oriented Mo fibers, this value demonstrates a remarkably improved creep resistance in NiAl–Mo with well-aligned unidirectional Mo fibers. A high-resolution transmission electron microscope investigation of the NiAl/Mo interface revealed a clean semi-coherent boundary between NiAl and Mo, which enabled an effective load transfer from the NiAl matrix to the Mo fibers, and thus leads to the remarkably increased creep strength. The stress exponent, n, was found to be between 3.5 and 5, dependent on temperature. The activation energy for creep, Q_c, was measured to be 291 ± 19 kJ mol^–1, which is close to the value for self-diffusion in binary NiAl. Transmission electron microscopy observations substantiated that creep occurred by dislocation climb in the NiAl matrix. The Mo fiber was found to behave in a quasi-rigid manner during creep. A creep model for fiber-reinforced metal matrix composites was applied for an in-depth understanding of the mechanical behavior of the individual components and their contribution to the creep strength of the composite.     

Effect of sintering process on the microstructures and properties of in situ TiB2–TiC reinforced steel matrix composites produced by spark plasma sintering

Spark plasma sintering (SPS) is a new technique to rapidly produce metal matrix composites (MMCs), but there is little work on the production of TiB₂–TiC reinforced steel matrix composites by SPS. In this work, in situ TiB₂–TiC particulates reinforced steel matrix composites have been successfully produced using cheap ferrotitanium and boron carbide powders by SPS technique. The effect of sintering process on the densification, hardness and phase evolution of the composite is investigated. The results show that when the composite is sintered at 1050 °C for 5 min, the maximum densification and hardness are 99.2% and 83.8 HRA, respectively. The phase evolution of the composite during sintering indicates that the in situ TiB₂–TiC reinforcements are formed by a hybrid formation mechanism containing solid–solid diffusion reaction and solid–liquid solution-precipitation reaction. The microstructure investigation reveals that fine TiB₂–TiC particulates with a size of ～2 μm are homogeneously distributed in the steel matrix. The TiB₂–TiC/Fe composites possess excellent wear resistance under the condition of dry sliding with heavy loads.     

Formation of TiAl–Ti2AlC in situ composites by combustion synthesis

Preparation of TiAl–Ti₂AlC in situ composites with a broad range of composition was conducted by self-propagating high-temperature synthesis (SHS) with compressed samples from the mixture of elemental powders. When compared with SHS formation of monolithic TiAl, the addition of carbon particles to the Ti–Al powder mixture enhances the sustainability of the reaction. It was found that no prior heating was required for the samples prepared to produce the composites containing more than 20 mol% Ti₂AlC, in contrast to the need of preheating at 200 °C for single-phase TiAl formation. This is attributed to the fact that formation of Ti₂AlC is more exothermic than that of TiAl. As a result, the combustion temperature and combustion wave velocity increase with the content of Ti₂AlC formed in the TiAl–Ti₂AlC composite, and approach the values associated with formation of single-phase Ti₂AlC when considerable amounts of Ti₂AlC are yielded. The XRD analysis of the end products confirms formation of TiAl–Ti₂AlC in situ composites. Moreover, simultaneous formation of Ti₂AlC promotes the phase evolution of the aluminide compounds. That is, the secondary aluminide phase, Ti₃Al, was no longer detected in the TiAl–matrix composites containing Ti₂AlC of 30 mol% or above.     

In situ X-ray synchrotron diffraction study of MgB2 synthesis from elemental powders

The kinetics of MgB₂ synthesis is studied in situ by synchrotron X-ray diffraction, using pressed compacts of 200–400 μm magnesium powders mixed with three types of submicrometer, amorphous, high-purity boron powders. Reaction times for commercially available and plasma-synthesized boron powders decreases from 100 to 2 min as temperature increases from 670 to 900 °C. They can be described by diffusion-controlled models of a reacting sphere with kinetics characterized by diffusion coefficients increasing with temperature from 2 × 10⁻¹⁷ to 3 × 10⁻¹⁶ m² s⁻¹, with activation energies of 123–143 kJ mol⁻¹. Plasma-synthesized boron powders doped with 7.4 at.% carbon show no significant differences in reaction kinetics as compared to undoped powders.     

High-pressure synthesized nanostructural magnesium diboride-based materials for superconductive electromotors,generators and pumps

Additions of Zr can increase critical current density (j_c) of high-pressure synthesized MgB₂ (HPS-MgB₂) in the same manner as additions of Ta or Ti, i.e. due to the absorption of impurity hydrogen (to form ZrH₂). The formation in HPS-MgB₂ of ZrB₂ phase at higher synthesis temperatures (about 950 °C) does not result in the j_c increase. Some increase in j_c of HPS-MgB₂ at 10 K in the fields higher than 8 T was observed when nano-SiC was added. The additions of Zr, Ta or Ti can prevent the harmful MgH₂ impurity phase from appearing and may prevent hydrogen from being introduced into the material structure and besides, their presence in HPS-MgB₂ promotes the formation of a higher amount of Mg–B (most likely MgB₂) inclusions in the Mg–B–O material “matrix” that in turn leads to the increase of j_c in magnetic fields. The high level of superconductive (SC) and mechanical characteristics attained for HPS-MgB₂ and the possibility to manufacture large samples make its application in the superconductive electromotors, generators, pumps, etc., very promising.     

An in situ bulk Zr58Al9Ni9Cu14Nb10 quasicrystal-glass composite with superior room temperature mechanical properties

An in situ bulk Zr₅₈Al₉Ni₉Cu₁₄Nb₁₀ quasicrystal-glass composite has been fabricated by means of copper mould casting. The microstructure and constituent phases of the alloy composite have been analyzed by using X-ray diffraction, transmission electron microscopy and high-resolution transmission electron microscopy. Icosahedral quasicrystals were found to be the majority phase and the grain size is in half-μm scale. In between the I-phase grains is a glassy phase. Optical microscopy and scanning electron microscopy revealed that the as-cast alloys were pore-free. The microhardness of the composite is about 5.90 ± 0.30 GPa. The room temperature compression stress–true strain curve exhibits a 2% elastic deformation up to failure, and a maximum fracture stress of 1850 MPa at a quasi-static loading rate of 4.4 × 10⁻⁴ s⁻¹. The mechanical property is superior to the early developed quasicrystal alloys, and is comparable to Zr-based bulk metallic glasses and their nanocomposites. The quasicrystal-glass composite exhibits basically a brittle fracture mode at room temperature.     

Elastic properties of hard cobalt boride composite nanoparticles

This paper reports on the determination of elastic and hardness properties of Co–B composite nanoparticles (CNP). Co boride materials is usually known for their functional properties (hydrogen catalysis, magnetism, corrosion, biomedics), but nanoscale dimensions also bring significant mechanical properties. In situ compression tests of 70–150 nm core–shell silica-coated Co₂B CNP (Composite nanoparticles) were performed for the first time with a nanoindenter in the load range 30–300 μN. The CNP modulus is comparable with the bulk material (E_CNP = 159–166 GPa), but the hardness is as much as 5 times higher (～4.5 ± 1.0 GPA). Both modulus and hardness (to a lesser extent) are found to increase with the applied pressure. The paper first addresses the limitations of ordinary contact analysis intended for single-phase NP, and then presents a hybrid Oliver–Pharr strategy suitable for CNP, where numerical modeling overcomes issues related to anisotropy and heterogenety of the composite nanostructure that hinder the direct application of basic contact models. An alternative regression-based approach for estimating modulus and hardness is also considered for comparison. The importance of the model selection for the contact area A for accurate modulus and hardness results is emphasized. Besides typical Hertzian, geometrical and cylindrical area models, a new one is formulated from a “rigid-sphere” approximation, which turned out to perform best and consistently in this study, on a par with the cylindrical model. Finally, evidence of the magnetic nature of CNP and, unexpectedly, reverse plasticity is provided.     

Nanofibers doped with a phosphorescent iridium complex: Synthesis,characterization, and photophysical property study

In this article, we report a novel Ir(III) complex of Ir(acac)(F-BT)₂ (acac = pentane-2,4-dione, and F-BT = 2-(2-fluorophenyl)benzo[d]thiazole), including its synthesis, characterization, crystal structure, and electronic nature. Data suggest that Ir(acac)(F-BT)₂ molecules crystallize in monoclinic system with double molecules per unit cell, and the onset electronic transition is assigned as a character of metal-to-ligand-charge-transfer. By doping Ir(acac)(F-BT)₂ into a polymer matrix of poly(vinylpyrrolidone) (PVP), we investigate the morphology, emissive character, excited state lifetime, and photostability of the resulting composite nanofibers, as well as those of pure sample. Data comparison suggests that the emission of the composite nanofibers shows a blue shift tendency, the excited state lifetime and photostability of the composite nanofibers become longer and better than those of pure Ir(acac)(F-BT)₂. The rigid framework provided by PVP matrix is responsible for above improvements.     

Conducting polymer embedded with nanoferrite and titanium dioxide nanoparticles for microwave absorption

The present paper deals with the synthesis of conducting ferrimagnetic polyaniline nanocomposite embedded with γ-Fe₂O₃ (9–12 nm) and titanium dioxide (70–90 nm) nanoparticles via a micro-emulsion polymerization. The microwave absorption properties of nanocomposite in 12.4–18 GHz (Ku-band) frequency range shows shielding effectiveness due to absorption (SE_A) value of ?45 dB, which is much higher than polyaniline composite with iron oxide and polyaniline–TiO₂ composites. The higher EMI shielding is mainly arising due to combined effect of γ-Fe₂O₃ and TiO₂ that leads to more dielectric and magnetic losses which consequently contributed to higher values of shielding effectiveness. XRD analysis of the nanocomposite reveals the incorporation of nanoparticles in the conducting polymer matrix while the thermal gravimetric analysis (TGA) demonstrates that the nanocomposite is stable up to 250 °C.     

Effect of thermal cycling on the expansion behavior of Al/SiCp composite

The coefficient of thermal expansion (CTE) and accumulated plastic strain of the pure aluminum matrix composite containing 50% SiC particles (Al/SiC_p) during thermal cycling (within temperature range 298–573 K) were investigated. The composite was produced by infiltrating liquid aluminum into a preform made by SiC particles with an average diameter of 14 μm. Experiment results showed that the relationship between the CTE of Al/SiC_p and temperature is nonlinear; CTE could reach a maximum value at about 530 K. The theoretical accumulated plastic strain of Al/SiC_p composites during thermal cycling has also been calculated and compared with the experimental results.     

Measurement of scratch-induced residual stress within SiC grains in ZrB2–SiC composite using micro-Raman spectroscopy

An analytical framework for determination of scratch-induced residual stress within SiC grains of ZrB₂–SiC composite is developed. Using a “secular equation” that relates strain to Raman-peak shift for zinc-blende structures and the concept of sliding blister field model for scratch-induced residual stress, explicit expressions are derived for residual stress calculation in terms of phonon deformation potentials and Raman peak shift. It is determined that, in the as-processed composite, thermal expansion coefficient mismatch between ZrB₂ and SiC induces compressive residual stress of 1.731 GPa within the SiC grains and a tensile tangential stress of 1.126 GPa at the ZrB₂–SiC interfaces. With increasing scratch loads, the residual stress within the SiC grains becomes tensile and increases in magnitude with scratch load. At a scratch load of 250 mN, the calculated residual stress in SiC was 2.6 GPa. Despite this high value, no fracture was observed in SiC grains, which has been rationalized based on fracture strength calculations from Griffith theory.     

Enhanced microwave absorption properties in polyaniline and nano-ferrite composite in X-band

Highly conducting polyaniline (PANI) nanocomposite with Mn_0.2Ni_0.4Zn_0.4Fe₂O₄ ferrite was prepared by mechanical blending. The present work reports the EMI shielding characteristics of the ferrite-Pani nanocomposite with different thickness. The saturation magnetization (Ms) for pure MnNiZn ferrite (52 emu/g) and composite (41 emu/g) was measured by VSM at room temperature. The crystalline size of MnNiZn ferrite was found in the range of 25–30 nm as analyzed by TEM and XRD. The complex permittivity, permeability and shielding effectiveness of the composite for different thicknesses were measured in the 8–12 GHz (X-band) frequency range. The composite of 2.5 mm thickness has shown high shielding effectiveness (49.2 dB) due to absorption (SE_A). The high value of SE_A suggests that this composite can be used as a promising absorbing material for X-band frequency range.     

Nanoconfinement significantly improves the thermodynamics and kinetics of co-infiltrated 2LiBH4–LiAlH4 composites: Stable reversibility of hydrogen absorption/resorption

A uniformly distributed composite of 2LiBH₄–LiAlH₄ was successfully nanoconfined in mesoporous carbon scaffolds by using the solvent-mediated infiltration technique. The onset dehydrogenation temperatures of LiAlH₄ and LiBH₄ in the infiltrated 2LiBH₄–LiAlH₄ composite are decreased to ～80 and ～230 °C, respectively, and are 40 and 145 °C lower for their post-milled counterparts. Isothermal measurements reveal that ～10 wt.% H₂ could be released from the nanoconfined 2LiBH₄–LiAlH₄ composite at 300 °C within 300 min, while less than 4 wt.% H₂ was released with respect to the post-milled mixture, even at 350 °C. Moreover, by taking advantage of both nanoconfinement and thermodynamic destabilization, the release of toxic diborane from LiBH₄ was successfully suppressed. The dehydrogenation mechanism reveals that, under the structure-directing effects of carbon supports, the decomposition of the well-distributed 2LiBH₄–LiAlH₄ composite favors the formation of AlB₂ instead of the thermodynamically stable Li₂B₁₂H₁₂, which has been verified to play a crucial role in enhancing the hydrogenation of the 2LiBH₄–LiAlH₄ composite. In combination with the extra LiH supplied by the in situ decomposition of nanoconfined LiAlH₄, the thus-tailored thermodynamics and kinetics of the 2LiBH₄–LiAlH₄ composite endow it with significantly advanced reversible hydrogen storage properties, with a stable reversibility without apparent degradation after seven dehydrogenation/rehydrogenation cycles.     

Deformation behavior of Zr55.9Cu18.6Ta8Al7.5Ni10 bulk metallic glass matrix composite in the supercooled liquid region

A Zr_55.9Cu_18.6Ta₈Al_7.5Ni₁₀ bulk metallic glass (BMG) composite with an amorphous matrix reinforced by micro-scale particles of Ta-rich solid solution was prepared by copper-mold casting. Isothermal compression tests of the BMG composite were carried out in the range from glass transition temperature (∼673 K) to onset crystallization temperature (∼769 K) determined by differential scanning calorimetry (DSC). The compressive deformation behavior of the BMG composite in the supercooled region was investigated at strain rates ranging from 1 × 10⁻³ s⁻¹ to 8 × 10⁻² s⁻¹. It was found that both the strain rate and test temperature significantly affect the stress–strain behavior of the BMG composite in the supercooled liquid region. The alloy exhibited Newtonian behavior at low strain rates but became non-Newtonian at high strain rates. The largest compressive strain of 0.8 was achieved at a strain rate of 1 × 10⁻³ s⁻¹ at 713 K. The strain rate change method was employed to obtain the strain rate sensitivity (m). The deformation mechanism was discussed in terms of the transition state theory based on the free volume.     

In situ synthesis,microstructure, mechanical properties and thermal shock resistance of (ZrB2 + SiC)/Zr2[Al(Si)]4C5 composites

Dense (ZrB₂ + SiC)/Zr₂[Al(Si)]₄C₅ composites with adjustable content of (ZrB₂ + SiC) reinforcements (0–30 vol.%) were prepared by in situ hot-pressing. The microstructure, room and high temperature mechanical and thermal physical properties, as well as thermal shock resistance of the composites were investigated and compared with monolithic Zr₂[Al(Si)]₄C₅ ceramic. ZrB₂ and SiC incorporated by in situ reaction significantly improve the mechanical properties of Z₂[Al(Si)]₄C₅ by the synergistic action of many mechanisms including particulate reinforcement, crack deflection, branching, bridging, “self-reinforced” microstructure and grain-refinement. With (ZrB₂ + SiC) content increasing, the flexural strength, toughness and Vickers hardness show a nearly linear increase from 353 to 621 MPa, 3.88 to 7.85 MPa·m^1/2, and 11.7 to 16.7 GPa, respectively. Especially, the 30 vol.% (ZrB₂ + SiC)/Zr₂[Al(Si)]₄C₅ composite retains a high modulus up to 1511 °C (357 GPa, 86% of that at 25 °C) and superior strength (404 MPa) at 1300 °C in air. The composite shows higher thermal conductivity (25–1200 °C) and excellent thermal shock resistance at ΔT up to 550 °C. Superior properties render the composites a promising prospect as ultra-high-temperature ceramics.     

Preparation,microstructure characterisation and corrosion behaviour of directionally grown TaSi2 fibre arrays with sharp tip fabricated by selective wet etching

Directionally solidified Si–TaSi₂ eutectic composite is selectively etched to fabricate well aligned TaSi₂ tip array for field emission application. The effect of etching parameters on TaSi₂ tip structure, and its corrosion behaviour in HNO₃/HF solution are investigated. At the optimised condition (HNO₃/HF = 5 for 50 min), sharp TaSi₂ tips with curvature radius of 18 nm and homogenous distribution are obtained, which greatly improves the figure of merit associated with field emission current. The TaSi₂ fibre presents smaller dissolution rate in the HNO₃/HF solution than Si matrix. The formation mechanism of etching pit on the Si matrix surface is also discussed.