Melting Point Of Mg Research Articles

Atomistic modeling of Mg-Al-Zn solid–liquid interfacial free energy

In this study anisotropic solid–liquid interfacial energy, γ, of Mg-Al-Zn system is evaluated based on capillary fluctuation method. To investigate effects of temperature and solute compositions on γ, five simulation cases, Cases I-V, are designed and divided into two group, Group I consist of Cases I-III for evaluation on solute compositions, while Group II consist of Cases I, IV and V for evaluation on temperatures. Interfacial energy stiffness of six differently oriented interfaces is evaluated, average interfacial energy, γ0, and anisotropic parameters of the interfaces are obtained. This study determined melting point of Mg as 964 ± 5 K, which matches the standard value of 923 K. Evaluation on γ0, and γ in high symmetric orientations γBasal, γPrismaticIA and γPrismaticIIA suggests, for Cases I-V, lower γ0 values within 15.74–18.19 mJ/m2 compared with elemental Mg could be related with the insufficiency of selected interatomic potential to describe the solid–liquid interface, and relation of γBasal>γPrismaticIIA>γPrismaticIA was identified. Comparisons within Group I indicate the increase of relative average equilibrium composition, c¯i/c¯j, will improve solute adsorption of element i on interface, which causes γ0 , γBasal, γPrismaticIA and γPrismaticIIA to decrease. As for Group II, temperature increase will result in decline of γ0 , γBasal, γPrismaticIA and γPrismaticIIA, similar trend was found in Al-Sm alloy systems. Primary dendrite growth orientations for Cases I-V were determined as [0001], analysis shown increase in either temperature or c¯i/c¯j stabilize this preference further towards [0001].

Oxidation kinetics of magnesium particles determined by isothermal and non-isothermal methods of thermogravimetric analysis

Knowledge of the oxidation kinetics of magnesium (Mg) particles is important for the development of advanced propulsion and power systems based on combustion of metals. However, reports on the high-temperature oxidation of Mg powders in oxygen are contradictory. In the present work, the oxidation of spherical and non-spherical (flakes) Mg particles in O2 flow was investigated using isothermal and non-isothermal thermogravimetry. Three fractions of the spherical powder were tested, with average sizes about 30, 60, and 300 μm. The oxidation of all powders was complete at temperatures lower than the melting point of Mg, 650 °C. The Friedman analysis of non-isothermal and isothermal thermogravimetric (TG) data has shown that the activation energy exhibits only small fluctuations during a large part of the oxidation process, which indicates that the process can be modeled as a single-step reaction. Model-based analysis has shown that the Avrami-Erofeev model of simultaneous nucleation and growth provides the best fit with the experimental isothermal and non-isothermal TG curves. Scanning electron microscopy of the oxidized spherical particles has shown submicron grains, which is consistent with the Avrami-Erofeev model. The Mampel-Delmon analysis of the isothermal TG data has shown that for the 30 and 60 μm particles, the nucleation is relatively slow, leading to the sigmoidal TG curve, described by the three-dimensional Avrami-Erofeev equation. An increase in the particle size decreases the dimension of this equation, and for the 300 μm particles and flakes, the entire process can be considered as a growth of a non-protective oxide layer. The apparent activation energy of Mg oxidation is likely in the range of 200 – 230 kJ/mol, independently of the particle size and shape.

Processing of porous β-type Ti74Nb26 alloys for biomedical applications

Ti74Nb26 alloys with porosities in the range of 45–68% were produced employing Mg space holder technique combined with hot pressing using elemental Ti, Nb and Mg powders. Powder mixtures were hot pressed at 600 °C (below the melting point of Mg, 650 °C) for 1 h under a constant pressure of 50 MPa and flowing argon gas. After hot pressing samples were sintered in a vertical furnace at 1200 °C (above the boiling temperature of Mg, 1090 °C) for 4 h under flowing inert argon gas atmosphere. Evaporation and removal of Mg particles from the samples were carried out during sintering simultaneously. X-ray diffraction and scanning electron microscopy analysis revealed that β phase was the main phase in the microstructure of all the samples. A small amount of α and undissolved pure Nb were also observed. Room temperature Young’s moduli and compressive strength of the specimens increased with decreasing porosity and were in the range of 1.6–14 GPa and 14–136 MPa, respectively. The mechanical properties of the 45%, 54% and 59% porous alloys were sufficient for both cortical and cancellous bone replacement applications. The alloy with a porosity content of 68% demonstrated mechanical properties suitable only for the cancellous bone implants.

Functionally gradient magnesium-based composite for temporary orthopaedic implant with improved corrosion resistance and osteogenic properties

Magnesium (Mg) is a potential alternative for conventional orthopaedic implant materials owing to its biodegradation behavior and physical characteristics similar to natural human bone. Due to its biomimetic mechanical attributes, Mg in orthopaedic applications could reduce the risk of the ‘stress shielding effect’. However, the major limitation of Mg is its high in-vivo corrosion rate. Thermal sprayed coatings of osteoconductive ceramics like hydroxyapatite (HA) have been explored as a potential solution, albeit with limited success due to the low melting point of Mg, which restricts the ease of fabricating surface-adherent ceramic coating. The present study focuses on overcoming this limitation through a Mg-HA functionally gradient material (FGM) system, which is expected to provide a highly corrosion-resistant surface and uniform mechanical integrity throughout the structure. In addition to corrosion resistance, the FGM system has improved biocompatibility and osteoconductivity without compromising its mechanical stability. The FGM, with a compositional gradient of Mg-HA composite, consisting of Mg at the core and increasing HA towards the outer layer, has been fabricated through spark plasma sintering. Mechanical properties of the overall structure were better than those of the best individual composite. More importantly, corrosion resistance of the FGM structure was significantly improved (~154%) as compared to individual composites. In addition, alkaline phosphatase activity, osteogenic gene expression and cell viability, all pertaining to efficient osteogenic differentiation, were enhanced for FGM and 15 wt% HA reinforced composites. These observations suggest that the FGM structure is promising for temporary biodegradable orthopaedic implants.

Study on thermophysical performance of Mg–Bi–Sn phase-change alloys for high temperature thermal energy storage

Mg-based alloys are potential high temperature phase change materials for thermal energy storage due to their high melting enthalpy, high thermal conductivity and excellent compatibility with Fe-based containment materials. This work mainly focuses on studying the microstructure and thermophysical characterization of Mg–Bi–Sn alloys phase change materials with 4 different components and their thermal energy storage performance. The experimental results show that the Mg–33Bi–17Sn, Mg–39Bi–17Sn and Mg–45Bi–17Sn alloys mainly consist of primary α-Mg phase and α-Mg + Mg2Sn + Mg3Bi2 eutectic structure, and the Mg–33Bi–23Sn alloy mainly consist of primary α-Mg phase, primary Mg2Sn + Mg3Bi2 phases and α-Mg + Mg2Sn + Mg3Bi2 eutectic structure. Their melting enthalpies are respectively 18.5 J/g, 168.8 J/g, 106.3 J/g and 140.3 J/g, while their melting temperatures are about 515 °C–525 °C. As from the results, the Mg–39Bi–17Sn alloy obtains the highest melting enthalpy, which attributes to its higher proportion of eutectic α-Mg + Mg2Sn + Mg3Bi2 structure. However, the Mg–45Bi–17Sn alloy shows the highest thermal conductivity. In addition, the melting point of Mg–39Bi–17Sn alloys increase by about 1.8 °C, and the melting enthalpy decreases by about 4.2% after 300 thermal cycling. Based on all results, the Mg–39Bi–17Sn alloy shows with great thermalphsycial performance and is expected to be used as thermal energy storage material.

Influence of the lamella structure and high isostatic pressure on the critical current density in in situ MgB2 wires without a barrier

In this article, the significant impact of a lamellar (layered) structure and a high isostatic pressure on the normal state resistance (Rn), critical temperature (Tc), irreversible magnetic field (Birr), upper magnetic field (Bc2) and critical current densities (Jc) at 4.2 K and 20 K was presented. Our research showed that annealing at temperatures in the range of 630 °C–680 °C (above the melting point of Mg) at atmospheric pressure (0.1 MPa) did not create a lamellar (layered) structure. This led to low Tc, Jc and Birr and high Rn. The analysis, made by using scanning electron microscopy (SEM), showed that the annealing temperature increased up to 700 °C under a pressure of 0.1 MPa, which created a lamellar structure. This led to significant growth of Tc, Jc and Birr and a slight increase of Rn. Moreover, the measurements showed that annealing at temperatures from 630 °C to 700 °C did not change the Bc2 value. In comparison to pressureless heat treatment, annealing under the high isostatic pressure of 1.1 GPa obtained a lamellar structure with layers of lower thickness and higher density. This led to significant increases in Jc and Birr and a visible reduction of Rn. SEM analysis showed that the increase of isostatic pressure up to 0.3 GPa created a lamellar structure with thicker layers and lower density. This microstructure led to lower Jc and Birr and significantly higher Rn. On the other hand, the SEM analysis showed that annealing under 0.8 GPa did not cause the formation of a layered structure, and as a result, it led to significant reductions in Birr and Jc (4.2 K and 20 K) and higher Rn. The increase of the isostatic pressure from 0.1 MPa to 1.1 GPa did not affect Tc (B = 0 T) and Bc2. The results indicated that the layered structure obtained a high density of pinning centers, which were particularly effective at higher magnetic fields. Jc of 100 A/mm2 in 8 T at 4.2 K was obtained in in situ undoped MgB2 wires after annealing at 700 °C for 40 min under an isostatic pressure of 1.1 GPa.

Infiltration behavior of molten Mg and its influence on microstructural evolution in SiC-doped MgB2 wires prepared by internal Mg diffusion process

We elucidate the microstructural evolution in MgB2 wires fabricated at two different temperatures by the internal Mg diffusion (IMD) method via multiple-scale microstructural observations. The critical current (Ic) of MgB2 wire was enhanced by the IMD method; however, the Ic of MgB2 was not sufficiently high to allow its practical applications. To prepare MgB2 by the IMD method, molten Mg is required to infiltrate the gaps between the B grains and penetrate each B grain. Here, we suggest that controlling the infiltration and penetration behaviors of Mg are key to enhancing the Ic of MgB2 fabricated by the IMD method. One reason for the decrease in Ic is the existence of residual B grains in the MgB2 crystalline region, which has an area fraction of 25% even in the MgB2 wire exhibiting the highest Ic. The amount of residual B grains, which formed due to the insufficient penetration distance of Mg atoms in the B grain, decreased after high-temperature treatment. However, the microstructure of the MgB2 wire fabricated at a temperature higher than the melting point of Mg was non-uniform because Mg infiltrated the gap between the B grains rapidly and non-uniformly. Thus, optimization of heat treatment temperature can effectively control the infiltration and penetration of Mg in the B grains, yielding uniform microstructures.

Synthesis and characterisation of ultra-hard and lightweight AlMgB14–xTiB2 composites for wear-resistance and ballistic protection

As an alternative to mechanical alloying, high temperature synthesis (HTS) of ultra-hard, super-abrasive AlMgB14 was performed under normal pressure. The reaction mixture consisted of elemental Al and B, whereas Mg was added in the form of a Mgprecursor which liberates elemental magnesium approximately 400 ºC above the melting point of Mg, in this way reducing its evaporation during heating-up. 95 wt % conversion to AlMgB14 and 5 wt % to MgAl2O4 was achieved. The synthesized AlMgB14 baseline powder, as well as mixtures of AlMgB14 consisting of 30, 50 and 70 wt% of TiB2, were hot pressed to near theoretical density. The various samples produced were characterized for microstructure and hardness. A microhardness of 29.4GPa in hot pressed AlMgB14 and a maximum Vickers hardness of 30.2 GPa in hot pressed samples of AlMgB14 reinforced with 70 wt% of TiB2 particles (d50=4,1µm) was achieved. Future project milestones necessary for achieving a higher AlMgB14 reaction yield, reducing the MgAl2O4 content and producing sinter-active AlMgB14 powder, as well as hot pressed composites processing improvement for gaining maximum hardness are also presented.

Thermoelectric properties of Sb-doped Mg2(Si0.95Ge0.05) synthesized by spark plasma sintering

Abstract Magnesium silicide (Mg 2 Si) has recently attracted much interest as an n -type thermoelectric (TE) material for converting waste heat into electric power. The objective of this work was to reveal a mechanism to increase the thermoelectric properties of Mg 2 Si by Sb-doping and Ge-doping. Generally, Mg 2 Si is synthesized by all-molten method. However, this synthesis method has some problems, for example, mass defects of Mg for Mg 2 Si composition caused because the melting point of Mg 2 Si is very close to the boiling point of Mg. In this study, we tried to synthesize high purity Mg 2 Si from Mg and Si alloyed with a dopant (Sb, Ge) by spark plasma sintering (SPS). The Ge-doped samples had a higher ZT value than the ZT value of the Sb-doped sample of the same concentration without Ge because the thermal conductivity of the former was lower. The maximum ZT value of Sb0.23 at%-doped Mg 2 (Si 0.995 Ge 0.05 ) was 0.74 at 756 K.

Microstructural evolution of a focused ion beam fabricated Mg nanopillar at high temperatures: Defect annihilation and sublimation

Microstructural evolution of focused ion beam machined Mg nanopillars was investigated by in situ heating experiments in a transmission electron microscope. The dislocation loops generated by a Ga+ ion beam were annihilated at around half of the melting point of Mg. At a higher temperature (673 K), the sublimation of Mg occurred due to the reduced stability of Mg under the vacuum environment. In the course of sublimation, the Ga-rich liquid was formed and stabilized the structural instability of moving solid–vapor interface.

Synthesis, Crystal Structure, Thermal Decomposition, and 11B MAS NMR Characterization of Mg(BH4)2(NH3BH3)2

A metal borohydride–ammonia borane complex, Mg(BH4)2(NH3BH3)2 was synthesized via a solid-state reaction between Mg(BH4)2 and NH3BH3. Different mechanochemical reaction mechanisms are observed, since Mg(BH4)2(NH3BH3)2 is obtained from α-Mg(BH4)2, whereas a mixture of Mg(BH4)2(NH3BH3)2, NH3BH3, and amorphous Mg(BH4)2 is obtained from γ-Mg(BH4)2. The crystal structure of Mg(BH4)2(NH3BH3)2 has been determined by powder X-ray diffraction and optimized by first-principles calculations. The borohydride groups act as terminal ligands, and molecular complexes are linked via strong dihydrogen bonds (<2.0 A), which may contribute to the high melting point of Mg(BH4)2(NH3BH3)2 found to be ∼48 °C in contrast to those for other molecular metal borohydrides. Precise values for the 11B quadrupole coupling parameters and isotropic chemical shifts are reported for the two NH3BH3 sites and two BH4– sites in Mg(BH4)2(NH3BH3)2 from 11B MAS NMR spectra of the central and satellite transitions and MQMAS NMR. The 11B quadrupole...

DTA/TG study of tungsten oxide and ammonium tungstate reduction by (Mg + C) combined reducers at non-isothermal conditions

In this work the reaction mechanism in the WO3–Mg/C systems and ammonium paratungstate (APT)–Mg/C systems are studied. As reducer magnesium, carbon or combinations of both are explored. It is shown that in the WO3–Mg system the reduction undergoes by solid–solid mechanism before melting Mg, where metallic tungsten and MgO are formed. Unlike this system, in the WO3–C system mainly WOx (<880°C) and WO2 (>960°C) and small amount W is formed. In the WO3–Mg–C ternary system reduction temperature shifts to higher temperature range and depends on amount of carbon. Similar to WO3–Mg system, APT–Mg reaction starts and completes in the solid state. Thus, firstly the APT decomposes, then reduction of formed WO3 takes place at ~600°C yielding W and MgO. Likewise to WO3–Mg system adding carbon into APT–Mg mixture shifts reduction temperature to even higher temperature zone which can exceed melting point of Mg and further reduction undergoes with molten magnesium. It is shown that the reduction products are MgO and W.

Thermochemical recycling of hydrolyzed NaBH4. Part I: In-situ and ex-situ evaluations

This paper presents a combined application of in-situ and ex-situ evaluations of products obtained by thermochemical recycling process of NaBH4 from NaBO2–Mg–H2 ternary system. In-situ yield evaluation according to on-line pressure measurements of hydrogen gas, although already applied by some authors, is presented here with an innovative analysis which offers a thorough comprehension of the NaBH4 regeneration process making feasible the qualitative and quantitative estimations of product and by-product. On the basis of in-situ observations, NaBH4 formation in presence of NaBO2–Mg–H2 ternary system initiates slowly from 400 °C and accelerates above 550 °C to the melting point of Mg at 650 °C. MgH2 is significantly produced between 370 °C and 450 °C in both heating and cooling. The amounts of these products produced at the different temperatures are clearly detectable by hydrogen pressure drops. Additionally, ex-situ evaluations by titration of NaBH4 have also been performed in order to confirm the correct interpretation of the experimental data.

Synthesis, microstructure and mechanical properties of porous MgZn scaffolds

Magnesium alloys have been intensively studied as biodegradable implant materials, as their mechanical properties render them promising candidates for bone tissue engineering applications. In the present work, porous Mg–4wt% Zn and Mg–6wt% Zn scaffolds were prepared using a powder metallurgy process. The effects of the porosity and Zn content on the microstructure and the mechanical properties of the fabricated scaffolds were studied. The above mentioned fabrication process involved sequential stages of mixing and compression of Mg and Zn powders with carbamide materials as space-holder particles followed by sintering the green compacts at different temperatures below the melting point of Mg. The results indicate that the porous MgZn specimens with a porosity and pore size of approximately 21–36% and 150–400μm, respectively, could have enhanced mechanical properties comparable with those of cancellous bone. In addition, an increase in the amount of Zn in the applied alloy gives rise to a significant refinement of magnesium grain size and an improvement in the mechanical properties, such as the compression strength, of the porous MgZn specimens. Furthermore, according to the results, the porous MgZn alloy could be considered one of the most promising scaffold materials for hard tissue regeneration.

Low-temperature Magnesiothermic Synthesis of Mesoporous Silicon Carbide from an MCM-48/Polyacrylamide Nanocomposite Precursor

Mesoporous silicon carbide with high specific surface area was successfully synthesized from an MCM-48/polyacrylamide nanocomposite precursor in the temperature range of 550–600 °C (below the melting point of Mg) by means of a magnesiothermic reduction process. The MCM-48/polyacrylamide precursor nanocomposite was prepared by in-situ polymerization of acrylamide monomer in the presence of mesoporous MCM-48 synthesized by sol-gel method. The physicochemical properties and microstructures of the nanocomposite precursor and the low-temperature SiC product were characterized by X-ray diffraction (XRD), differential scanning calorimetry-thermo gravimetric analysis (DSC-TGA), transmission electron microscopy (TEM) and N2 adsorption–desorption. TEM micrographs and Brunauer–Emmett–Teller (BET) gas adsorption studies showed that the SiC powder was nanocrystalline and had a specific surface area of 330 m2/g and a mesoporosity in the range of 2–10 nm. The presence of an exothermic peak in the DSC trace corresponds to the self-combustion process of the SiC magnesiothermic synthesis. The results also show that the carbon in excess to that required to produce SiC plays a role in the reduction of the SiO2. The mechanism of magnesiothermic synthesis of mesoporous SiC is discussed.

Plasma arc Synthesis of Nano-Boron Towards the Synthesis of MgB2 Superconductors

Since the discovery of superconductivity in the tempting binary intermetallic compound MgB2, the solid-state synthesis technique is highly dominated by the usage of amorphous boron as one of the precursor powders. The formation of MgB2 phase proceeds through the diffusion of Mg into B powder mainly driven by the low melting point of Mg as compared to B. Once the nucleation is achieved, the progress of polycrystalline MgB2 phase occurs due to the out diffusion of boron through the MgB2 layer and by the inward diffusion of Mg. This growth is impeded due to the presence of certain oxide phases or formation of Mg deficient phases. It is speculated that the probability for the inclusion of Mg(B)–O phases is higher for crystalline boron precursor as compared to the amorphous B. Thus, the use of nanosized amorphous boron may lead to larger nucleation centers, smaller grain size and consequently higher packing density in the polycrystalline MgB2, which will in turn provide optimum superconducting properties. Hence an attempt to synthesize amorphous nano-boron powders is presented. Plasma arc discharge technique was successfully employed to produce nano-boron powder. The XPS analysis was carried out to inveterate the formation of boron. The as-synthesized powder had a uniform average particle size distribution of around 20 nm as confirmed by TEM measurements. The selected area electron diffraction pattern composed of diffused ring clearly depicts the amorphous nature of boron powder.

Microstructures and critical currents of single- and multi-filamentaryMgB2 superconducting wires fabricated by an internal Mg diffusion process

A single-filament wire and 7- and 19-filament wires ofMgB2 superconductor were fabricated by an internal Mg diffusion (IMD) process. The wire issheathed by a Cu–Ni alloy and each filament is composed of an outermost Ta, an intermediateB + SiC powder layer and an Mg core at the center. Despite the large total area reduction, the crosssections of all wires show uniform deformation of the composite. During the subsequentheat treatment, a reacted layer with a dense composite structure composed of aMgB2 matrix and fine particles is formed by Mg liquid infiltration and the reaction with theB + SiC powder. For all wires,the highest transport Ic was obtained at furnace temperatures of 640–645 °C, which is just below the melting point of Mg. In the single-filament wire, a fairly large amount ofB + SiC remains outside the reacted layer, while the residualB + SiC is much reduced in the multi-filamentary wires, resulting in higherIc, than that of the single-filament wire. However, theJc, estimatedfor the reacted layer is not so different between the wires. When the heat treatment temperature exceeds650 °C, theIc value rapidly decreases, although the volume fraction of theMgB2 detected continues to increase. It is observed that the thickness of the reactedlayer formed at higher temperatures becomes significantly inhomogeneous,which is thought to be responsible for the deterioration of transportIc values. Thehighest Jc(layer) estimated for the reacted layer is as high as9.9 × 104 A cm − 2 at 4.2 K and10 T and 3.3 × 105 A cm − 2 at 20 K and 1 T achieved for the multi-filamentary wires. TheJc(core) estimated for the area including the hole and remnant B is about1/3 ofthe Jc(layer). From good workability of the composite and excellentJc values, it is expected that the IMD process can compete in terms of practical wirefabrication with the conventional powder-in-tube (PIT) process.

On the Stability of Mg Nanograins to Coarsening after Repeated Melting

Herein we report on the extraordinary thermal stability of approximately 35 nm Mg-nanograins that constitute the matrix of a Ti(2)AlC-Mg composite that has previously been shown to have excellent mechanical properties. The microstructure is so stable that heating the composite three times to 700 degrees C, which is 50 degrees C over the melting point of Mg, not only resulted in the repeated melting of the Mg, but surprisingly and within the resolution of our differential scanning calorimeter, did not lead to any coarsening. The reduction in the Mg melting point due to the nanograins was approximately 50 degrees C. X-ray diffraction and neutron spectroscopy results suggest that thin, amorphous, and/or poorly crystallized rutile, anatase, and/or magnesia layers separate the Mg nanograins and prevent them from coarsening. Clearly that layer is thin enough, and thus mechanically robust enough, to survive the melting and solidification stresses encountered during cycling. Annealing in hydrogen at 250 degrees C for 20 h, also did not seem to alter the grain size significantly.

High Critical Current Density ${\rm MgB}_{2}/{\rm Fe}$ Multicore Wires Fabricated by an Internal Mg Diffusion Process

We fabricated multicore MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Fe wires with much higher packing density and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> than powder-in-tube (PIT)-processed wires by internal Mg diffusion (IMD). Mg cores in the composite were uniformly cold worked into fine filaments by room-temperature groove rolling and drawing. After cold drawing, short specimens were heat treated at various temperatures from 550 to 800degC. During heat treatment, Mg reacted with a B(+SiC) layer forming a dense reacted layer inside an Fe sheath, which is composed of a MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> major phase and some minor impurity phases. Interestingly, the 7-core wire added with 5 mol% SiC heat treated at 600degC below the Mg melting point showed an apparent reaction forming a MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer and a fairly large amount of unreacted Mg inside a reacted MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer, suggesting that such formation is induced by solid-state reaction at the interface. On the other hand, almost all samples heat treated above 650degC showed MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> phase formation by typical liquid Mg infiltration producing a hollow at the center. The highest <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> at 8 T and 4.2 K obtained for the multicore wire was 1.1 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> achieved by the addition of 5 mol% SiC and heat treatment at 600degC for 1 hr, which is slightly higher than the highest <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the monocore wire (1.0 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) achieved for the wire added with 5 mol% SiC and heat treated at 670degC for 3 hr. The temperature of 600degC was lower than the melting point of Mg, suggesting that solid-solid reaction also forms a MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer in IMD. The small B layer thickness made it possible for Mg atoms to diffuse throughout the B layer at this low temperature.

Fabrication of seven-core multi-filamentaryMgB2 wires with high critical current density by an internal Mg diffusion process

We found that the reaction between a Mg core and a B powder layer in aninternal Mg diffusion (IMD)-processed multi-filamentary wire can proceedrapidly even at a furnace temperature lower than the melting point of Mg(650 °C), resulting in the formation of a reacted layer with a fine composite structure and, hence,excellent in-field critical current properties. The multi-filamentary wire is composed of anoutermost Cu–Ni sheath and seven filaments with a Ta sheath, a Mg core, andB+SiC powder filled in the space between the Ta sheath and the Mg core. Heat treatment at645 °C for 1 h produced a reacted layer with dense composite structure along the innerwall of the Ta sheath and a hole at the center of each core. This reactionprobably initiated from the heat generation at the B/Mg interface, resulting in atemperature rise of the Mg core and the occurrence of liquid Mg infiltration. TheJc value at 4.2 K for thereacted layer exceeds 105 cm−2 at 9 T, which is the highest reported so far forMgB2 wire, including powder-in-tube (PIT)-processed wires. These results indicate that the IMDprocess can compete in terms of practical wire fabrication with the conventional PITprocess.