Kinetics analysis for the sintering of bulk MgB<Subscript>2</Subscript> superconductor

The pertaining kinetic characteristics during the sintering of bulk polycrystalline MgB₂ superconductors is essentially important for the improvement of properties. Here Differential Thermal Analysis was adopted to record the heat effect during the preparation of bulk MgB₂ samples. The reaction between Mg and B powders starts before the melting point of pure Mg and the evolution for the fractions of MgB₂ were determined as a function of sintering temperatures. After fitting with different kinetic mechanism functions assumed, the sintering process of bulk MgB₂ superconductors was attributed to a solid-state interface-reaction controlled mechanism with an apparent activation energy of 4.54 × 10⁵ J mol⁻¹. Combined with microstructural observations by scanning electron microscopy and phase identification by X-ray diffraction, the formation process of MgB₂ phase was classified into two different stages: (i) solid-solid reaction stage, in which Mg and B powder starts to react and the growth of MgB₂ grain is restricted by the pinning effects of pores; (ii) solid–liquid reaction stage, in which the molten Mg melt promotes the reaction process and the regular hexagon bulk MgB₂ grain forms in a solution-reprecipitation and growth mode.     

Sintering mechanism of Ag-doped MgB<Subscript>2</Subscript> superconductor from low temperature to high temperature

Combined with thermal analysis and phase identification, the sintering process of Ag-doped MgB₂ superconductor was investigated. It is found that the Ag doping could form Mg–Ag liquid through the eutectic reaction at low temperature (about 470 °C) and then obviously accelerated the formation of MgB₂ phase. Moreover, a sintering model is also proposed to illustrate the liquid activated sintering mechanism present in the sintering process of Ag-doped MgB₂ samples. The sintering model is supposed to provide theoretic guidance for optimizing the sintering condition in the synthesis of doped MgB₂ superconductors.     

Formation of MgO whiskers on the surface of bulk MgB<Subscript>2</Subscript> superconductors during in situ sintering

Magnesium oxide (MgO) whiskers (with diameters of about 60–80 nm) formed on the surface of bulk polycrystalline MgB₂ superconductor at a relative low temperature (720 °C) during in situ sintering process. The reaction between Mg and B powders begins at a temperature below melting point of Mg and maintains till about 750 °C. The residual Mg powders evaporate and react with trace oxygen to form MgO vapor as the temperature exceeds the melting temperature of Mg and a low supersaturation is required for the growth of MgO whiskers. The preformed MgB₂ and MgO crystals act as substrates and the melted Mg powders on the surface of them serve as catalysts during the growth process of MgO whiskers. The growth process of MgO whiskers is dominated by a self-catalytic vapor–liquid–solid (VLS) mechanism.     

Observation of Flux Jump in (MgB2)0.96Ni0.04 Superconductor Doped with Milled Ni powders

Bulk (MgB₂)_0.96Ni_0.04 samples doping with premilled Ni powders were sintered at 750 °C for 30 min. During sintering, liquid Mg–Ni phase prompts solid–solid reaction between Mg and B and the size of milled Ni powder determines the final distribution of the secondary MgNi_2.5B₂ phase in the sintered samples. A flux jump was observed in the (MgB₂)_0.96Ni_0.04 samples doping with Ni powders. Recognized from the measured superconductive properties, smaller-sized Ni powders can provide more effective flux pinning centers and thus improve the performance of the critical current density.     

The mechanism of accelerated phase formation of MgB2 by Cu-doping during low-temperature sintering

Thermal analysis, phase identification and microstructure observation were adopted to explore the accelerating sintering effect of Cu-doping on the low-temperature formation of MgB₂ below Mg melting. It is found that the Cu-doping sintering of MgB₂ bulk followed an activated sintering mechanism and thus obviously promoted the reaction between Mg and B forming bulk MgB₂. Accordingly, dense MgB₂ bulks with excellent T_c and J_c were successfully synthesized by the Cu-doping sintering at 575 °C for only 5 h. Further, a sintering model was proposed to illuminate the Cu-doping activated sintering mechanism. It is found that during the Cu-doping activated sintering process, the local Mg-Cu liquid generates firstly and then segregates to the interface between Mg and B particles, which can wet Mg particles and provide a rapid transport for the diffusion of neighboring Mg atoms into B particles resulting in the accelerated formation of MgB₂ phase.     

Effect of Sn-doping on the sintering process of MgB<Subscript>2</Subscript> and its superconductive properties

Bulk (Mg_1.02B₂)_1−x Sn _x samples (x = 0.0, 0.01, 0.03 and 0.05) were synthesized by in situ sintering at 850 °C for one hour. Based on the phase identification and microstructure observation, the Mg₂Sn and Sn impurities are found as the main impurities in Sn-doped samples. According to the magnetization measurements, the low doping level of Sn was observed to have small influence on the grain connectivity, and thus a high critical current density was maintained at low field. However, the values of the critical current density at high field in the Sn-doped samples show a little decrease.     

Effect of nano-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> doping on formation and superconductivity of bulk MgB<Subscript>2</Subscript>

Bulk materials of MgB₂ have been prepared with the stoichiometry of MgB₂(Al₂O₃) _x (x = 0, 2, 5, 10 and 20% nano-Al₂O₃ powders), by using solid-state reaction route. All samples were sintered at 750 °C for 30 min in a calorimeter to monitor the sintering reaction process. It is found that the onset temperatures of reaction between Mg and B powders increase significantly with increasing the amount of Al₂O₃. However, the reaction time is shortened for the nano-Al₂O₃ powders can effectively activate the reaction as a catalyst. The critical transition temperature decreases from 38.5 to 31.6 K, and the corresponding temperature window becomes narrow (less than 2.6 K). Furthermore, the amount of MgO impurity was found to increase with the increase of Al₂O₃, which probably indicates that partial Mg was replaced by Al.     

The Effect of Cu Addition on the Phase Formation and Critical Current Density in the Sugar Doped MgB2 Superconductor

With the aim of improving the critical current density (J _c ) in the MgB₂ superconductor, minor Cu (3?at%) was doped to the MgB₂ samples in-situ sintered with Mg powder and sugar-coated amorphous B powder. Combined with thermal analysis, phase identification, microstructure observation and J _c measurement, the effect of minor Cu addition on the sintering mechanism, microstructure and critical current density of sugar-doped MgB₂ superconductors were investigated. It is found that the minor Cu addition could obviously accelerated the MgB₂ phase formation and improve the growth of MgB₂ grains during the sintering process of sugar-doped MgB₂ due to the appearance of Mg?CCu liquid at low sintering temperature. On the other hand, the Mg?CCu liquid hindered the reactive C released from sugar entering in the MgB₂ crystal lattice. Hence, the connectivity between MgB₂ grains was improved accompanying with the C substitution for B is decreased. At 20?K, the J _c of co-doped samples at low fields was further increased whereas it is decreased at high fields, compared with the only sugar-doped samples.     

The Effects of Structural,Thermal, and Magnetic Properties of Hexylbenzene-Doped MgB<Subscript>2</Subscript> Superconductor

The effects of the amount of hexylbenzene additive (C₁₂H₁₈) on the structural, thermal, and magnetic properties of MgB₂ superconductor are examined in this study. Pure and hexylbenzene-doped MgB₂ bulk samples were produced with in situ solid-state reaction method. X-ray diffraction patterns of MgB₂ doped with MgB₂ and hexylbenzene at different ratios were determined to have MgB₂ as the main phase and consisted of a small amount of MgO. Pure and different ratios of hexylbenzene-doped Mg and B starting powders were heat-treated by a differential scanning calorimeter between room temperature and 800 °C. It was determined from the differential scanning calorimetry curves obtained that the first exothermic peak pointed the MgB₂ phase emerging with a solid–solid (Mg–B) reaction, and this temperature shifted towards the low temperatures as the hexylbenzene addition rates increased. It was observed that there was dependency to the applied field in all samples from the ac susceptibility measurements as a function of the temperature in pure and hexylbenzene-doped MgB₂ superconductor materials, and shift towards the lower temperatures in T _c, superconducting transition temperature, with increasing content. It was observed that the changes occurred in in-phase (\(\chi ^{\prime })\) and out-off-phase (\(\chi ^{\prime \prime }\)) components of ac susceptibility both weakened the MgB₂ phase structure of hexylbenzene content and, as a result of this, led to changes in the pinning mechanism.     

Synthesis and sintering of nano-sized BaSnO<Subscript>3</Subscript> powders containing BaGeO<Subscript>3</Subscript>

The formation of solid solutions of the type [Ba(HOC₂H₄OH)₄][Sn_1−x Ge _x (OC₂H₄O)₃] as BaSn_1−x /Ge _x O₃ precursor and the phase evolution during its thermal decomposition are described in this paper. The 1,2-ethanediolato complexes can be decomposed to nano-sized BaSn_1−x /Ge _x O₃ preceramic powders. Samples with x = 0.05 consist of only a Ba(Sn,Ge)O₃ phase, whereas powders with x = 0.15 and 0.25 show diffraction patterns of both the Ba(Sn,Ge)O₃ and BaGeO₃ phase. The sintering behaviour was investigated on powders with a BaGeO₃ content of 5 and 15 mol%. These powders show a specific surface area of 15.4–15.9 m²/g and were obtained from calcination above 800 °C. The addition of BaGeO₃ reduced the sintering temperature of the ceramics drastically. BaSn_0.95Ge_0.05O₃ ceramics with a relative density of at least 90% can be obtained by sintering at 1150 °C for 1 h. The ceramic bodies reveal a fine microstructure with cubical-shaped grains between 0.25 and 0.6 μm. For dense ceramics, the sintering temperature could be reduced down to 1090 °C, when the soaking time was extended up to 10 h.     

Formation and properties of SnO–MgO–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses

A new glass system SnO–MgO–P₂O₅ with low viscosity has been developed by a melt-quenching method. Formation, thermal properties, and chemical durability of these glasses have been investigated. For a constant P₂O₅ concentration, the glass formation ability is enhanced with the increasing Sn/(Sn + Mg) ratio. The glasses exhibit low glass transition temperature (T _g = 270–400 °C), low dilatometric softening temperature (T _DS = 290–420 °C), and high thermal expansion coefficient (CTE = 110–160 × 10⁻⁷ K⁻¹). With the increasing Sn/(Sn + Mg) ratio, T _g and T _DS decrease, and CTE increases. When Sn/(Sn + Mg) ratio is varied, the relationship between chemical durability and thermal properties of the present glasses is not consistent with what expected in general cases. It is noted that the glasses with 32–32.5 mol% P₂O₅ exhibit excellent chemical durability and tunable T _g, T _DS, and CTE (by varying Sn/(Sn + Mg) ratio).     

Excellent Jc in the low-temperature sintered MgB2 superconductors consisted of uncompleted MgB2 phase and residual Mg

Though low-temperature sintering of MgB₂ superconductors below Mg melting point can effectively depress volatility of Mg and increase sintering density, its development was limited in recent years for the reason that it usually took very long time to form complete MgB₂ phase with excellent J_c at low temperature. In present work, significantly improved J_c was surprisingly obtained in the MgB₂ samples sintered at 575 °C for only 5 h after short-time ball milling, even though formation of MgB₂ phase is not completed and lots of residual Mg is still present in these samples. The grain connectivity in prepared samples is obviously improved compared to referred MgB₂ sample sintered at high temperature, which is responsible for the improvement of J_c. The method developed in present work seems bring a new opportunity for the development of low-cost practical MgB₂ superconductors with improved J_c without using expensive nanometer-size dopants or high-temperature sintering.     

Fabrication and magnetoresistivit of ex-situ processed MgB<Subscript>2</Subscript>/Fe monofilament tapes without any intermediate annealing

We have fabricated MgB₂/Fe monofilament wires and tapes by a powder-in tube (PIT) technique, using an ex-situ process without any intermediate annealing. MgB₂/Fe monofilament tapes were annealed at 650–1,050°C for 60 min and 950°C for 30–240 min. We have investigated the effect of annealing temperatures and times on the formation of MgB₂ phase, activation energy, temperature dependence of irreversibility field H _irr(T) and upper critical field H _c2(T), transition temperature (T _c), lattice parameters (a and c), full width at half maximum, crystallinity, resistivity, residual resistivity ratio, active cross-sectional area fraction and critical current densities. We observed that the activation energies of the MgB₂/Fe monofilament samples increased with increasing annealing temperature up to 950°C and with increasing annealing time up to 60 min while it decreased with increasing magnetic field. For the MgB₂/Fe monofilament tape, the slope of the H _c2–T and H _irr–T curves decreased with increasing annealing temperature from 850 to 950°C as well as with increasing annealing time from 30 to 60 min. The transport and microstructure investigations show that T _c, J _c and microstructure properties are remarkably enhanced with increasing annealing temperature. The highest value of critical current density is obtained for the sample annealed at 950°C for 60 min. The J _c and T _c^offset values of the sample annealed at 950°C for 60 min were found to be 260.43 A/cm² at 20 and 38.1 K, respectively.     

Monofilamentary In Situ Fe/MgB<Subscript>2</Subscript> Superconducting Wires Fabricated by Pellet-in-Tube Method

Thin monofilamentary Fe/MgB₂ superconducting wires without barriers are investigated by means of electrical transport measurements and surface and structural analysis methods. Small diameter wires are fabricated by pellet-in-tube method (PeIT) to obtain a high uniform initial filling density and heat treated as a function of various sintering temperatures and times. The results are discussed in terms of the grain connectivity, Fe₂B phase formation, and the relation between wire diameter and sintering conditions. We suggest that PeIT has a crucial importance to achieve homogeneous initial filling density, which leads to the fabrication of uniform long-length MgB₂ wires.     

Nano-Ni addition to MgB<Subscript>2</Subscript>: effects on the superconducting properties

Samples of MgB₂ pure phase, with Ni nanoparticles addition, were prepared using a solid diffusion reaction method. Clearly, the Ni nanoparticles act as effective pinning centers and enhance the critical current density values, especially for a sample with 0.5%Ni. A negligible amount of Ni diffuses inside the MgB₂ grains, thus having a small effect on the transition temperature, which remains around 37.5 K.     

Ex Situ Spark Plasma Sintering of Short Powder-in-Tube MgB<Subscript>2</Subscript> Tapes with Open and Closed Ends

Short powder-in-tube tapes of MgB₂ in the Fe sheath were fabricated by ex situ route from a commercial powder containing some free Mg and MgO impurity phases. The final heat treatment was performed by spark plasma sintering (SPS). Tapes were with open (OT) or closed (CT) endings. Closed endings were made by folding and pressing. The MgB₂ core of the OT sample has shown a higher low-field critical current density, a higher maximum pinning force, a slightly higher disorder, smaller average MgB₂ crystallite size, a weak contact between Fe and MgB₂ core, and more macro-flux jumps. The upper and irreversibility fields were similar for OT and CT samples. In the center of the MgB₂ cores, the detected impurity phase is MgO, while at the interface with Fe, MgB₄ also occurs. Impurity phases found at interface, MgO and MgB₄, are present in the center of the bulk SPSed samples. Reactions and pinning-force-related parameters are discussed with respect to Mg behavior influenced by condition of endings. It is inferred that the presence of free Mg in the raw MgB₂ powder has an important contribution to observed differences, and its removal or control is recommended.     

Low temperature preparation of the Zn<Subscript>2</Subscript>SiO<Subscript>4</Subscript> ceramics with the addition of BaO and B<Subscript>2</Subscript>O<Subscript>3</Subscript>

The Zn₂SiO₄ ceramics with the addition of BaO and B₂O₃ are fabricated by traditional solid-state preparation process at a sintering temperature of 900 °C. The introduction of BaO and B₂O₃ to the binary system ZnO-SiO₂ is achieved by adding 10 and 20 wt. % flux BB to the mixed ZnO-SiO₂ ceramic powders pre-sintered at 1,100 °C, respectively. The chemical composition of the flux BB (50 wt.%BaO-50 wt.% B₂O₃) is located at a liquid phase zone with a temperature range of about 869–900 °C in the binary diagram BaO-B₂O₃. In addition, the introduction of BaO and B₂O₃ to the binary system ZnO-SiO₂ is also achieved by the means of a chemical combination of H₂SiO₃, H₃BO₃, ZnO and Ba(OH)₂·8H₂O, which can result in the formation of the hydrated barium borates with low melting characteristics. In turn, by the liquid sintering aid of the barium borate melts, the preparation process of the Zn₂SiO₄ ceramics can be further simplified. In the two preparation methods, the Zn₂SiO₄ ceramics with the 1.5–2.0 ZnO/SiO₂ molar ratios and the addition of a 10 wt. % flux BB can show good dielectric properties whereas the bending strength mainly depends on the microstructure of the Zn₂SiO₄ ceramics and SiO₂ content in the composition of the specimen.     

Effect of MnO<Subscript>2</Subscript> on chemical interaction between CaO and Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

We have studied the effect of manganese dioxide (5–20 wt %) on the formation of calcium monoaluminate in the CaO-Al₂O₃ system during solid-state sintering of five oxide mixtures corresponding to the known calcium aluminates at temperatures from 1100 to 1400°C. X-ray diffraction examination indicated the formation of calcium manganates. The mixture of the calcium manganates melts at 1330°C, promoting the sintering process and the reaction between the calcium oxide and alumina and raising the yield of calcium monoaluminate as the dominant phase. The calcium manganates are shown to be nonreactive with the forming calcium aluminates. The X-ray diffraction results are supported by scanning electron microscopy data for a number of samples.     

Liquid-Phase Shock-Assisted Consolidation of Superconducting MgB<Subscript>2</Subscript> Composites

An original two-stage liquid-phase hot explosive compaction (HEC) procedure of Mg-B precursors above 900 ^°C provides the formation of superconductivity MgB₂ phase in the whole volume of billets with maximal T _c = 38.5 K without any further sintering. The liquid-phase HEC strongly increases the solid-state reaction rate similar to photostimulation, but in this case, due to the high penetrating capability of shock waves in a whole volume of cylindrical billets and consolidation of MgB₂ precursors near to theoretical density allows one to produce bulk, long-body cylindrical samples important for a number practical applications.     

Effects of transition metal oxide and Ni addition on the hydrogen-storage properties of Mg

Mg-based hydrogen-storage materials with the compositions of Mg–10 wt%oxide (oxide = Cr₂O₃, Fe₂O₃, MnO, and SiO₂) and Mg–xFe₂O₃–yNi were prepared by reactive mechanical grinding (RMG). Taking into consideration the hydriding and dehydriding rates and the cost of materials, Fe₂O₃ prepared by spray conversion is an appropriate oxide additive to Mg. Mg–5 wt%Fe₂O₃–15 wt%Ni exhibited the best hydrogen-storage performance among the Mg–xFe₂O₃–yNi hydrogen materials. It stored 5.47 wt%H under 1.2 MPa H₂ for 60 min and released 5.42 wt%H under 0.1 MPa H₂ for 15 min at 593 K. The addition of Fe₂O₃ and Ni to Mg by the RMG shortens the diffusion distances through the reduction of the particle size of Mg. These additives are also considered to facilitate nucleation by creating many defects on the surface and in the interior of Mg. The added Fe₂O₃ and Ni themselves may also act as active sites for the nucleation. Ni forms the Mg₂Ni phase by a reaction with Mg, and Fe appears from the reduction of Fe₂O₃ by hydrogen after hydriding–dehydriding cycling.